Mineral Matter and Ash in Coal 9780841209596, 9780841211339, 0-8412-0959-6

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Publication Date: April 2, 1986 | doi: 10.1021/bk-1986-0301.fw001

Mineral Matter and Ash in Coal

Publication Date: April 2, 1986 | doi: 10.1021/bk-1986-0301.fw001

ACS

SYMPOSIUM

SERIES

301

Mineral Matter and Ash in Coal Karl S. Vorres, EDITOR

Publication Date: April 2, 1986 | doi: 10.1021/bk-1986-0301.fw001

Argonne National Laboratory

Developed from a symposium sponsored by the Division of Fuel Chemistry at the 188th Meeting of the American Chemical Society, Philadelphia, Pennsylvania, August 26-31, 1984

American Chemical Society, Washington, DC 1986

Library of Congress Cataloging-in-Publication Data Mineral matter and ash in coal. ( A C S symposium series, ISSN 0097-6156; 301)

Publication Date: April 2, 1986 | doi: 10.1021/bk-1986-0301.fw001

"Developed from a symposium sponsored by the Division of Fuel Chemistry at the 188th meeting of the American Chemical Society, Philadelphia, Pennsylvania, August 26-31, 1984." Bibliography: p. Includes index. 1. Coal—Analysis—Congresses. I. Vorres, Karl Spyros, 1937. II. American Chemical Society. Division of Fuel Chemistry. III. Series. TP325.M567 1986 I S B N 0-8412-0959-6

662.6'22

86-3556

Copyright © 1986 American Chemical Society All Rights Reserved. The appearance of the code at the bottom of the first page of each chapter in this volume indicates the copyright owner's consent that reprographic copies of the chapter may be made for personal or internal use or for the personal or internal use of specific clients. This consent is given on the condition, however, that the copier pay the stated per copy fee through the Copyright Clearance Center, Inc., 27 Congress Street, Salem, MA 01970, for copying beyond that permitted by Sections 107 or 108 of the U.S. Copyright Law. This consent does not extend to copying or transmission by any means—graphic or electronic—for any other purpose, such as for general distribution, for advertising or promotional purposes, for creating a new collective work, for resale, or for information storage and retrieval systems. The copying fee for each chapter is indicated in the code at the bottom of the first page of the chapter. The citation of trade names and/or names of manufacturers in this publication is not to be construed as an endorsement or as approval by ACS of the commercial products or services referenced herein; nor should the mere reference herein to any drawing, specification, chemical process, or other data be regarded as a license or as a conveyance of any right or permission, to the holder, reader, or any other person or corporation, to manufacture, reproduce, use, or sell any patented invention or copyrighted work that may in any way be related thereto. Registered names, trademarks, etc., used in this publication, even without specific indication thereof, are not to be considered unprotected by law. PRINTED IN T H E UNITED STATES OF A M E R I C A

ACS Symposium Series M . Joan Comstock, Series Editor

Publication Date: April 2, 1986 | doi: 10.1021/bk-1986-0301.fw001

Advisory

Board

Harvey W. Blanch

Donald E. Moreland

University of California—Berkeley

U S D A , Agricultural Research Service

Alan Elzerman

W. H. Norton

Clemson University

J . T. Baker Chemical Company

John W. Finley

James C. Randall

Nabisco Brands, Inc.

Exxon Chemical Company

Marye Anne Fox

W. D. Shults

The University of Texas—Austin

Oak Ridge National Laboratory

Martin L. Gorbaty

Geoffrey K. Smith

Exxon Research and Engineering C o .

Rohm & Haas C o .

Roland F. Hirsch

Charles S.Tuesday

U.S. Department of Energy

General Motors Research Laboratory

Rudolph J . Marcus

Douglas B. Walters

Consultant, Computers & Chemistry Research

National Institute of Environmental Health

Vincent D. McGinniss

C. Grant Willson

Battelle Columbus Laboratories

I B M Research Department

FOREWORD T h e A C S S Y M P O S I U M S E R I E S was founded i n 1974 to provide a m e d i u m for

p u b l i s h i n g symposia q u i c k l y i n b o o k

form.

The

Publication Date: April 2, 1986 | doi: 10.1021/bk-1986-0301.fw001

format o f the Series parallels that of the c o n t i n u i n g A D V A N C E S I N C H E M I S T R Y S E R I E S except that, i n order to save time, the papers are not typeset but are reproduced as they are submitted by the authors i n camera-ready f o r m . Papers are reviewed under the supervision o f the E d i t o r s w i t h the assistance of the Series A d v i s o r y B o a r d a n d are selected to m a i n t a i n the integrity of the symposia; however, v e r b a t i m reproductions o f previously p u b lished papers are not accepted.

B o t h reviews a n d reports

research are acceptable, because symposia may embrace types o f presentation.

of

both

PREFACE

Publication Date: April 2, 1986 | doi: 10.1021/bk-1986-0301.pr001

^N^INERAL C O N S T I T U E N T S O F C O A L a n d the changes these minerals undergo o n heating in different environments are of scientific and commercial interest. T h i s interest warrants a t h o r o u g h discussion of the nature of mineral matter in coal. T h i s volume is the product of a s y m p o s i u m given at the 188th N a t i o n a l M e e t i n g o f the A m e r i c a n C h e m i c a l Society. A s is p r o b a b l y the case for m a n y symposia, the s y m p o s i u m u p o n w h i c h this b o o k is based came as a result of a number o f discussions a m o n g some of the session chairmen and the s y m p o s i u m c h a i r m a n over a period o f years. M a n y of the D i v i s i o n o f F u e l Chemistry's symposia dealt w i t h a number of aspects of coal, but none in recent m e m o r y have dealt w i t h the m i n e r a l matter in a comprehensive manner. T h i s significant constituent affects almost every application of this a b u n d a n t resource. F r o m these thoughts came a n interest in developing a s y m p o s i u m that w o u l d cover a l l of the important aspects in sufficient detail so that the current t h i n k i n g i n the field c o u l d be reasonably represented. In a d d i t i o n , it was intended that the whole subject be developed in a logical manner, assuming that there is a logical manner. W i t h these goals in m i n d , the matter o f organizing i n d i v i d u a l sessions led to the a p p r o a c h of inviting a speaker to provide a n introduction to each session, such that the novice in this part o f the s y m p o s i u m activity c o u l d be q u i c k l y introduced to the area and be able to follow the talks in that session. T h e speakers that followed w o u l d then talk about particular topics o f significance. T h e sequence o f session topics was chosen to be similar to that w h i c h coUld be used in a text o n the subject of the chemistry of mineral matter and ash i n c o a l . A n introductory session described what the mineral matter is, where mineral matter comes f r o m , the chemical constituents that are present, the manner in w h i c h the constituents vary a m o n g the different coal deposits, a n d the special nature of some of the deposits, such as volcanic materials. T h e second session dealt w i t h the effects of high temperatures o n m i n e r a l matter a n d the conversion of that material to a variety of forms dependent o n the temperature a n d history, such as slag, deposits, fly ash, or just ash. T h i s session was intended to deal w i t h the observations o f the properties at high temperatures in the laboratory and the efforts to correlate these observations w i t h some knowledge of the c o m p o s i t i o n of the mineral matter or ash. XI

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T h e third session dealt with efforts to reverse the perspective and begin w i t h the materials that are present i n the m i n e r a l matter a n d then try to predict the behavior that w o u l d be observed. A wide range of properties are of interest, a n d a number of papers covered t h e r m o d y n a m i c properties a n d also the physical properties related to flow a n d t h e r m a l effects such as conductivity. T h e fourth session l o o k e d at the question " W h a t does the owner or operator of large equipment that consumes c o a l observe because of the mineral matter?" Because most of the coal consumed i n the United States is used to generate steam i n electric power generation facilities, the major purpose of the c o a l is to provide heat energy i n boilers. T h e problems are associated w i t h the a c c u m u l a t i o n of deposits o n the walls or tubes inside the boiler. In some cases these materials are hot a n d flow f r o m the walls as a slag, whereas i n other cases the mineral matter undergoes a series of changes leading to the f o r m a t i o n of deposits o n the tubes. T h e fifth session l o o k e d at the possibility that there may be some desirable aspects associated w i t h the mineral matter i n the coal. T h e mineral matter may be a catalyst for some o f the current o r future uses of c o a l . Specifically, the mineral matter c o u l d have some effect o n c o m b u s t i o n a n d also o n future synthetic fuels efforts that c o u l d provide either gaseous or liquid fuels. T h e final session responded to the question " I f the mineral matter is i n the c o a l , what can be done to remove the mineral matter?" T h i s session dealt w i t h a n u m b e r o f techniques to physically remove the m i n e r a l matter i n processes called c o a l cleaning. These processes involve crushing a n d i n some cases pulverizing to fine a n d even ultrafine sizes to permit the liberation of the m i n e r a l matter f r o m the coal. A n efficient separation requires removal of m i n e r a l matter w i t h a m i n i m u m r e m o v a l o f the desirable combustible material. O n e of the goals for this v o l u m e is to provide the type and quality of chapters that w i l l endure or stand the "test of t i m e " such that the b o o k w i l l be cited for years to come. It is hoped that the efforts that have gone into this w o r k w i l l achieve this goal. A s i n any large u n d e r t a k i n g such as this b o o k , m a n y people are i n v o l v e d . Expressions of thanks a n d a p p r e c i a t i o n go to m a n y w h o helped p l a n the i n d i v i d u a l sessions, the session c h a i r m e n . E a c h o f the authors deserves thanks for c o n t r i b u t i n g to the i n d i v i d u a l presentations a n d to the further revisions that led to the chapters i n this b o o k . T h a n k s also go to the m a n y reviewers w h o read a n d commented o n the manuscripts to b r i n g out a d d i t i o n a l points for the clarity a n d i m p r o v e d q u a l i t y o f the manuscripts. T h e A m e r i c a n C h e m i c a l Society staff have been very helpful in their many ways.

xii

A c k n o w l e d g m e n t is made to the d o n o r s of T h e P e t r o l e u m Research F u n d , administered by the A m e r i c a n C h e m i c a l Society, for p a r t i a l travel support for K . C . M i l l s a n d E r i c h R a a s k . A d d i t i o n a l thanks go to Beth M u s t a r i a n d the late Katherine A r c h a m beault for their t y p i n g assistance. M y wife, N a n c y , deserves appreciation for patience t h r o u g h m a n y hours of paperwork w i t h less than the usual attention that she is accustomed to having. Last, a n d by no means least, I thank the U . S . D e p a r t m e n t o f Energy, Office o f Basic Energy Science, C h e m i c a l Sciences D i v i s i o n , whose support has made m y participation possible.

Publication Date: April 2, 1986 | doi: 10.1021/bk-1986-0301.pr001

KARL S. VORRES Chemistry Division Argonne National Laboratory A r g o n n e , IL 60439 N o v e m b e r 4, 1985

xiii

1 Chemistry of Mineral Matter and Ash in Coal: A n Overview Karl S. Vorres

Publication Date: April 2, 1986 | doi: 10.1021/bk-1986-0301.ch001

Chemistry Division, Argonne National Laboratory, Argonne, IL 60439

Coal contains significant and variable amounts of largely incombustible mineral matter. This mineral matter primarily includes clays, shales, pyrite, quartz, calcite and lesser amounts of other materials, depending on the chemical and mineralogical composition. It occurs in many forms and sizes, which may be seen by the naked eye or occur in micron-sized particles that require an optical or electron microscope to observe. Coal is usually burned in combustion equipment, liberating hot gases and also heating the mineral matter to temperatures of 2000°F and above. The different forms of the mineral matter can and do interact to bring about new chemical materials with a variety of properties. This volume describes the nature of the mineral matter, its occurrence, composition, sources, and the effects of heating this material in terms of the chemical and physical properties of mineral matter and deposits. These effects are observed both experimentally and in the operation of boilers and other coal utilization equipment. The properties may be predicted with suitable models and a certain amount of data from simpler systems. The deposition of mineral matter or ash in boilers is a major concern in the design of coal-burning equipment and the major reason for the forced outages of these units. Under certain conditions, there can be beneficial effects from certain kinds of mineral matter in the coal, and these are explored. Finally, there are approaches that can be taken to reduce the mineral matter content of the coal from the original values. The extent of the reduction depends on the nature of the coal, the need for reduction, and the premium price that can be obtained for the cleaned coal. T h i s volume i s d i v i d e d i n t o s i x s e c t i o n s , c o r r e s p o n d i n g t o the s i x s e s s i o n s i n the Symposium on C h e m i s t r y of M i n e r a l M a t t e r and Ash i n Coal presented t o the F u e l C h e m i s t r y Division at the American Chemical S o c i e t y N a t i o n a l Meeting in Philadelphia i n August, 1984. These s i x s e s s i o n s were i n t e n d e d t o p r o v i d e a forum f o r the s i x major a r e a s i n d i c a t e d above. Each s e c t i o n c o n t a i n s c h a p t e r s d e a l i n g w i t h the s u b j e c t a r e a . The n a t u r e o f the m i n e r a l matter i n c o a l i s d i s c u s s e d i n the f i r s t s e c t i o n . The second

0097-6156/ 86/ 0301 -0001 $06.00/ 0 © 1986 American Chemical Society

Publication Date: April 2, 1986 | doi: 10.1021/bk-1986-0301.ch001

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M A T T E R A N D A S H IN C O A L

s e c t i o n examines the e x p e r i m e n t a l i n f o r m a t i o n a v a i l a b l e from a v a r i e t y of h i g h temperature measurements on the c o a l a s h . By c o n t r a s t , the t h i r d s e c t i o n examines the approaches t o p r e d i c t the p r o p e r t i e s of h i g h temperature c o a l ash from a knowledge of the c h e m i c a l c o m p o s i t i o n and an u n d e r s t a n d i n g of the thermodynamics of these and similar systems. The fourth section looks at the problems caused by the m i n e r a l m a t t e r t h a t are b e i n g e x p e r i e n c e d by the owners and o p e r a t o r s of c o a l - b u r n i n g equipment. These problems a r e p r i m a r i l y r e l a t e d t o the d e p o s i t i o n of c o a l ash and the i n t e r a c t i o n s of the m i n e r a l matter t o p r o v i d e m a t e r i a l of d i f f e r e n t c o m p o s i t i o n s than t h e o r i g i n a l m i n e r a l s p e c i e s . The f i f t h s e c t i o n examines the p o t e n t i a l b e n e f i t s d e r i v e d from the presence of the m i n e r a l m a t t e r i n the c o a l . Some of the s p e c i e s may c a t a l y z e c e r t a i n c o a l c o n v e r s i o n p r o c e s s e s such as l i q u e f a c t i o n . Other s p e c i e s , such as sodium, may c a t a l y z e combustion i t s e l f . Finally, s i n c e the m i n e r a l matter i s g e n e r a l l y not d e s i r e d , a number o f u t i l i t i e s a r e w i l l i n g t o pay a premium p r i c e t o have the m i n e r a l matter c o n t e n t r e d u c e d . The c l o s i n g s e c t i o n e x p l o r e s t e c h n o l o g y t o remove the m i n e r a l matter from c o a l . Taken t o g e t h e r t h e s e t o p i c s s h o u l d p r o v i d e an u n d e r s t a n d i n g of the n a t u r e of the m i n e r a l m a t t e r and ash. In a d d i t i o n the study p r o v i d e s i n s i g h t s i n t o the c u r r e n t methods of d e a l i n g w i t h the i n o r g a n i c c o n s t i t u e n t s of c o a l and suggests u s e f u l d i r e c t i o n s f o r f u r t h e r research. The n a t u r e of the m i n e r a l matter i n c o a l i s d i s c u s s e d i n the i n i t i a l chapters. The m i n e r a l matter p r e s e n t i n c o a l i n c l u d e s the m i n e r a l matter p r e s e n t i n the l i v i n g p l a n t s which were a l t e r e d o v e r time t o produce the c o a l m a t e r i a l . In a d d i t i o n , m i n e r a l matter has been added through the e f f e c t s of s u b s i d e n c e and the subsequent a d d i t i o n of sedimentary m a t e r i a l , and the a c c u m u l a t i o n of a i r b o r n e dust or o t h e r i n o r g a n i c m a t e r i a l . V o l c a n i c e r u p t i o n s have c o n t r i b u t e d s u b s t a n t i a l m i n e r a l matter i n l i m i t e d a r e a s . Furthermore, during the c o a l i f i c a t i o n p r o c e s s , water has u s u a l l y p e r c o l a t e d through the c o a l seams and p r o v i d e d a d d i t i o n a l m i n e r a l matter as w e l l as a l t e r e d the matter t h a t was p r e s e n t . The n a t u r e of the c o a l m i n i n g p r o c e s s i s such t h a t some of the f l o o r and r o o f of the seam may be i n c l u d e d i n the p r o d u c t w i t h normal use of the m i n i n g equipment. The m i n e r a l m a t t e r c o n t e n t can then v a r y depending not o n l y on the m a t e r i a l p r e s e n t i n the c o a l but a l s o depending on the m a t e r i a l d i r e c t l y above o r below the seam and the c a r e t a k e n i n m i n i n g . The e f f e c t s of the m i n e r a l matter depend on the c h e m i c a l and m i n e r a l o g i c a l composition. Many s t a n d a r d a n a l y t i c a l t e c h n i q u e s a r e a v a i l a b l e t o q u a n t i f y the elements p r e s e n t i n the m i n e r a l s p e c i e s . These i n c l u d e s i l i c o n , aluminum, i r o n , c a l c i u m , magnesium, sodium, p o t a s s i u m , t i t a n i u m and o t h e r s . The elements a r e u s u a l l y r e p o r t e d as o x i d e s , because the o x i d e a n i o n i s the predominant one i n f l y ash. However, the m i n e r a l s p e c i e s a r e not u s u a l l y s i m p l e o x i d e s , but v e r y f r e q u e n t l y a r e t i e d up i n the d i f f e r e n t m i n e r a l forms as more complex a l u m i n o s i l i c a t e s o r o t h e r s p e c i e s as i n d i c a t e d above. A v a r i e t y of t e c h n i q u e s i s used t o i d e n t i f y the m i n e r a l m a t t e r .

1.

VORRES

Mineral

Matter

and Ash in Coal

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U s u a l l y low-temperature a s h i n g (LTA) t e c h n i q u e s a r e used t o remove the c o m b u s t i b l e matter t o p r o v i d e a sample of m i n e r a l m a t e r i a l which has undergone m i n i m a l change from that i n the original coal. The LTA t e c h n i q u e i n v o l v e s the use of a lowp r e s s u r e , low-temperature oxygen plasma t o o x i d i z e the c o m b u s t i b l e m a t e r i a l away from the g r a i n s of m i n e r a l matter i n the c o a l . Even though this t e c h n i q u e i s the most g e n t l e i n common use, some changes have been o b s e r v e d i n the m i n e r a l matter when LTA has been used. The p r e s e n c e of p y r i t e and c a l c i t e i n the c o a l sample has l e d t o the o b s e r v a t i o n of some s u l f a t e d c a l c i u m s p e c i e s . In the normal p r e p a r a t i o n o f a c o a l ash sample f o r a n a l y s i s , the c o a l i s h e a t e d to 700-750°C f o r s u f f i c i e n t time to burn o f f the carbonaceous m a t t e r . These temperatures and times a r e s u f f i c i e n t to p e r m i t i n t e r a c t i o n among the more r e a c t i v e s p e c i e s , and l e a d t o changes from the m a t e r i a l s which have been o b s e r v e d from LTA s t u d i e s on the same c o a l . Pétrographie e x a m i n a t i o n under the m i c r o s c o p e can p r o v i d e identification of the m a t e r i a l , and can be complemented with t e c h n i q u e s such as x - r a y d i f f r a c t i o n (XRD) t o i d e n t i f y c r y s t a l l i n e minerals. The q u a n t i t a t i v e measure of the r e l a t i v e amounts of the m i n e r a l s i s a somewhat c o n t r o v e r s i a l s u b j e c t due t o some o f the l i m i t a t i o n s o f the t e c h n i q u e s . However the comparison of the r e s u l t s from a number of l a b o r a t o r i e s has l e d t o the c o n c l u s i o n t h a t no one XRD method i s s u p e r i o r t o a l l o t h e r s . Repeated attempts t o compare r e s u l t s between d i f f e r e n t l a b o r a t o r i e s has l e d to the consensus t h a t the t e c h n i q u e i s s e m i - q u a n t i t a t i v e . The t y p e s o f m i n e r a l matter found i n a p a r t i c u l a r d e p o s i t depend on the geography o f the d e p o s i t . Those d e p o s i t s found i n t h e e a s t e r n p a r t of the U n i t e d S t a t e s have m i n e r a l m a t t e r which i s r i c h i n c l a y , q u a r t z and p y r i t e . As a r e s u l t u t i l i t i e s which burn e a s t e r n h i g h e r - s u l f u r c o a l s must now use equipment w h i c h can reduce the s u l f u r o x i d e s which a r e r e l e a s e d . The d e p o s i t s i n the w e s t e r n p a r t s o f the U.S. f r e q u e n t l y have m i n e r a l matter c h a r a c t e r i z e d by h i g h c a l c i t e and sodium and lower c l a y and p y r i t e c o n t e n t s . The low s u l f u r c o n t e n t and some i n h e r e n t a b i l i t y t o c a p t u r e l i b e r a t e d s u l f u r o x i d e s w i t h c a l c i u m compounds has l e d t o the use of the low s u l f u r w e s t e r n c o a l s f o r a growing p a r t o f the U.S. power g e n e r a t i o n market. The c o m p o s i t i o n o f the m i n e r a l m a t t e r i n a p a r t i c u l a r c o a l seam i s affected by the geologic conditions surrounding the deposit. A marine, b r a c k i s h o r f r e s h w a t e r environment w i l l a l t e r the c h e m i s t r y of the d e p o s i t and the n a t u r e of the m i n e r a l s p e c i e s w h i c h a r e found w i t h the c o a l . Higher s u l f u r contents are u s u a l l y a s s o c i a t e d w i t h the p r e s e n c e of a marine environment d u r i n g p a r t o f the c o a l i f i c a t i o n p r o c e s s . The p r o p e r t i e s o f c o a l ash r e f l e c t the changes t h a t have t a k e n p l a c e i n the m i n e r a l matter through some h e a t i n g p r o c e s s . As a r e s u l t o f h e a t i n g the d i f f e r e n t m i n e r a l forms may have undergone decomposition (as with carbonates, h y d r a t e s ) , or solid-state

3

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reaction (as q u a r t z w i t h o t h e r s p e c i e s t o form silicates) or heterogeneous r e a c t i o n (as p y r i t e w i t h a i r t o form i r o n o x i d e s ) . The changes i n c h e m i c a l c o m p o s i t i o n cause changes i n the p h y s i c a l p r o p e r t i e s as w e l l . The r e a c t i o n s do not a l l p r o c e e d a t the same rate. In g e n e r a l the t h e r m a l d e c o m p o s i t i o n r e a c t i o n s w i l l p r o c e e d rapidly. The heterogeneous r e a c t i o n s proceed at an i n t e r m e d i a t e r a t e , which may v a r y w i t h time i f a l a y e r w i t h low p e r m e a b i l i t y f o r the r e a c t i n g gas i s g e n e r a t e d . The r a t e w i l l d e c r e a s e w i t h time under t h e s e c i r c u m s t a n c e s . The slowest r e a c t i o n s i n v o l v e the c o n v e r s i o n of two s o l i d phases to some new m a t e r i a l . T h i s r e a c t i o n p r o c e e d s through the d i f f u s i o n o f some m o b i l e s p e c i e s a c r o s s the i n t e r f a c e between two a d j a c e n t p a r t i c l e s . T h i s r e a c t i o n can be augmented by the f o r m a t i o n o f a l i q u i d phase which can a c t as a solvent for a reactive species. The a l k a l i e s can be r e l e a s e d i f they a r e i n an a c t i v e form and a c t as a f l u x , o r agent f o r the f o r m a t i o n of a lower m e l t i n g p o i n t phase. The r e a c t i o n s w i l l p r o c e e d measurably f a s t e r under t h e s e c o n d i t i o n s . The m e l t i n g p r o p e r t i e s and v i s c o s i t y of molten c o a l ash have been the s u b j e c t o f c o n t i n u i n g work. They a r e not a simple f u n c t i o n of the c o m p o s i t i o n . The p r o p e r t i e s a r e used i n a t t e m p t i n g t o p r e d i c t the problems t h a t might be e n c o u n t e r e d w i t h a p a r t i c u l a r c o a l i n an o p e r a t i n g b o i l e r w i t h a s p e c i f i c d e s i g n . A number o f c o r r e l a t i o n s have been developed, but have had l i m i t e d a p p l i c a tion. One of the problems has been a l a c k of complete u n d e r s t a n d i n g o f the r o l e o f the a c i d s and b a s e s . These terms a r e used in many of the correlations of the melting behavior with c o m p o s i t i o n of the c o a l ash. The acids include the oxides of silicon, aluminum and titanium. The bases i n c l u d e the o x i d e s of i r o n ( i r o n i s m o s t l y p r e s e n t as b a s i c FeO), c a l c i u m , magnesium, sodium and potassium. Taken t o g e t h e r these elements make up v i r t u a l l y a l l of the c a t i o n s found i n the h i g h temperature m i n e r a l m a t t e r . The m o l t e n c o a l ash i s a polymer of the a c i d i c s p e c i e s s i l i c a and a l u m i n a . The average m o l e c u l a r weight of the p o l y m e r i c s p e c i e s depends on the p r e s e n c e of c h a i n t e r m i n a t i n g s p e c i e s or b a s e s . The t e r m i n a t i n g s p e c i e s i s t h e o x i d e i o n a s s o c i a t e d w i t h the base. The v i s c o s i t y depends on the average molecular s i z e o f the polymer. F o r example the a d d i t i o n o f a base t o an a c i d i c e a s t e r n c o a l ash w i l l reduce the viscosity over a p e r i o d o f time as the p o l y m e r i c s p e c i e s a r e g r a d u a l l y reduced i n s i z e . A r e a s o n a b l y complete u n d e r s t a n d i n g o f the b e h a v i o r o f m o l t e n s l a g s a l s o r e q u i r e s an u n d e r s t a n d i n g o f the c o m p o s i t i o n , gaseous environment ( o x i d i z i n g o r r e d u c i n g ) , l o s s of v o l a t i l e s p e c i e s , e x i s t e n c e of s e v e r a l i m m i s c i b l e l i q u i d phases, t h e r m a l h i s t o r y and i n t e r a c t i o n w i t h the c o n t a i n e r . Workers a r e d e v e l o p i n g the t h e o r e t i c a l u n d e r s t a n d i n g o f the b e h a v i o r of molten c o a l ash systems. T h i s a c t i v i t y proceeds from a c o n s i d e r a t i o n o f the energy e f f e c t s upon m i x i n g the c o n s t i t u e n t o x i d e s and a d e s c r i p t i o n of the n o n - i d e a l b e h a v i o r of systems containing polymers of v a r y i n g s i z e . The models of polymer c h e m i s t r y are b e i n g a p p l i e d t o these h i g h e r temperature systems.

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Publication Date: April 2, 1986 | doi: 10.1021/bk-1986-0301.ch001

The h i g h temperature d a t a a v a i l a b l e on c o a l s l a g s i s l i m i t e d , and the work r e q u i r e d t o o b t a i n a d d i t i o n a l d a t a i s e x p e n s i v e and time consuming. T h e r e f o r e i t has become more d e s i r a b l e t o d e v e l o p s u i t a b l e models o f h i g h temperature b e h a v i o r t o a v o i d the time and cost involved i n acquiring additional information. The models are needed f o r r a p i d e s t i m a t i o n of p r o p e r t i e s and t e s t i n g of o p e r a t i n g s t r a t e g i e s t o p e r m i t c o n t i n u a l o p e r a t i o n o f a b o i l e r w i t h new f u e l s or a f u e l w i t h v a r y i n g m i n e r a l matter c o m p o s i t i o n s . The work done w i t h c o a l ash i n v o l v e s s i l i c a - r i c h systems w h i c h are not unique to c o a l . The same types of m a t e r i a l s a r e found i n s t e e l - m a k i n g s l a g s , v o l c a n i c magmas, and some f l u x e s , g l a s s e s and refractories. The b e h a v i o r of these systems i s , not s u r p r i s i n g l y , s i m i l a r t o and c o n s i s t e n t w i t h the b e h a v i o r of c o a l s l a g s . The d a t a on these systems may then be used t o t e s t models which have been d e v e l o p e d f o r c o a l ash. S i m i l a r l y the u n d e r s t a n d i n g of c o a l ash systems can be augmented by u s i n g models d e v e l o p e d f o r t h e s e other materials. Work w i t h f l u x e s demonstrates the a b i l i t y o f m a t e r i a l s c o n t a i n i n g f l u o r i d e t o form l o w - m e l t i n g p h a s e s . This i s c o n s i s t e n t w i t h the h y p o t h e s i s of the base as an a n i o n donor, and the a b i l i t y of the a n i o n s t o d i m i n i s h the s i z e of the s i l i c a - t y p e polymer i n m e l t s c o n t a i n i n g t h e s e s p e c i e s . Other work w i t h v o l c a n i c magmas has demonstrated the s i g n i f i cance of the c r y s t a l l i t e s which appear as the m o l t e n m a t e r i a l cools. A sharp transition i n the A r r h e n i u s p l o t of l o g o f v i s c o s i t y v e r s u s r e c i p r o c a l a b s o l u t e temperature was o b s e r v e d as the volume of c r y s t a l l i t e s approached 30% of the t o t a l volume. This transition has been o b s e r v e d i n c o a l ash systems. The c o m p o s i t i o n of the s o l i d phases or c r y s t a l l i t e s can be i n t e r p r e t e d by comparison w i t h phase e q u i l i b r i u m diagrams of r e l a t e d s i m p l e r systems. The continuing efforts to generate electric power more efficiently have l e d t o f u r t h e r s t u d i e s of magnetohydrodynamic power g e n e r a t i o n . T h i s approach i n v o l v e s the use o f an i o n i z a b l e vapor which i s passed through a s t r o n g m a g n e t i c f i e l d a t h i g h velocities. The c u r r e n t produced i n t h i s way i s used t o augment normal steam g e n e r a t i o n . The e f f i c i e n c y depends on the amount o f p o t a s s i u m vapor t h a t can be m a i n t a i n e d i n the system i n c o n t a c t w i t h the s l a g d e r i v e d from the c o a l b e i n g burned. S t u d i e s of t h e s e s l a g s have added t o the u n d e r s t a n d i n g o f the i n t e r a c t i o n s of a l k a l i e s with slags. The problems w i t h the commercial use of c o a l a r e u s u a l l y due to the m i n e r a l matter p r e s e n t i n the c o a l . Over 80% of the c o a l used i n the U n i t e d S t a t e s i s burned i n b o i l e r s t o g e n e r a t e steam f o r e l e c t r i c power p r o d u c t i o n . In the combustion p r o c e s s , the m i n e r a l matter i s r e l e a s e d and i n a few seconds t r a v e r s e s the f u r n a c e and subsequent p a r t s on the way t o the smokestack. I f most o f the m i n e r a l m a t t e r has a r e l a t i v e l y h i g h ash f u s i o n temperature, t h i s m a t e r i a l , termed f l y a s h , moves t h r o u g h the f u r n a c e p a r t s t o some equipment d e s i g n e d to remove the f i n e p a r t i c l e s . I f the ash f u s i o n temperature i s not r e l a t i v e l y h i g h then the s m a l l p a r t i c l e s

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become s o f t and s t i c k y . They can impinge on the w a l l s and the tubes of the f u r n a c e c a v i t y to produce d e p o s i t s . The p r e s e n c e o f t h e f l y a s h and d e p o s i t s r e d u c e s heat t r a n s f e r and steam g e n e r a t i o n , impedes gas f l o w t h r o u g h the f u r n a c e c a v i t y , causes p h y s i c a l damage t o the p a r t s , and c o r r o d e s as w e l l as e r o d e s i n t e r n a l p a r t s of the furnace.

Publication Date: April 2, 1986 | doi: 10.1021/bk-1986-0301.ch001

The problems w i t h c o a l ash a r e most o f t e n a s s o c i a t e d w i t h deposits i n b o i l e r s . The s t i c k y m a t e r i a l may accumulate on the walls of the large furnace cavity. This process i s u s u a l l y r e f e r r e d to as s l a g g i n g . The l a r g e f u r n a c e s have suspended tubes i n them. D e p o s i t s form and grow on t h e s e tubes i n a p r o c e s s c a l l e d fouling. Sometimes t h e s e d e p o s i t s grow to a p o i n t at which the o p e r a t i o n of the b o i l e r must be t e r m i n a t e d , c a l l e d a f o r c e d outage, f o r c l e a n i n g of the w a l l s and/or t u b e s . The a c c u m u l a t i o n o f t h e s e d e p o s i t s may be p a r t i a l l y due to d e f o r m a t i o n and a d h e s i o n of s o f t , h i g h l y v i s c o u s p a r t i c l e s as they impinge on hot m e t a l s u r f a c e s . S t u d i e s have shown t h a t an i n i t i a l p r o c e s s i n v o l v i n g the f o r m a t i o n of a low m e l t i n g phase on the s u r f a c e of tubes p r o v i d e s a k i n d o f g l u e which p e r m i t s d e p o s i t s to accumulate. The low m e l t i n g m a t e r i a l s t h a t are a s s o c i a t e d w i t h the f o r m a t i o n of the i n i t i a l l a y e r s i n c l u d e a l k a l i s u l f a t e s and i r o n or aluminum s u l f a t e s . A s u c c e s s i o n of s t a g e s seems to be i n v o l v e d i n the formation of the hard deposits w h i c h can not be easily removed. The p r e s e n c e of two i m m i s c i b l e l i q u i d phases d e r i v e d from o x i d e and s u l f a t e m a t e r i a l s has been o b s e r v e d , i n d i c a t i n g m u l t i p l e mechanisms f o r d e p o s i t formation. A v a r i e t y of t e c h n i q u e s have been used t o model the d e p o s i t i o n processes. F r e q u e n t l y a s m a l l f u r n a c e w i l l be b u i l t to provide c o n t r o l l e d combustion and c a r e f u l m o n i t o r i n g o f the conditions. The c o a l i s burned and the ash i s a l l o w e d to impinge on t u b e s . The i n c r e a s e i n d e p o s i t weight i s n o t e d as a f u n c t i o n o f time under different conditions. T h i s approach has been h e l p f u l and i s u s e f u l i f v a r i a t i o n s i n ash c h e m i s t r y are the most s i g n i f i c a n t v a r i a b l e s . In some f u r n a c e s , the aerodynamics a r e a l s o q u i t e i m p o r t a n t and cannot u s u a l l y be s c a l e d . I t i s a l s o p o s s i b l e t o model the e f f e c t s of i m p i n g i n g a s h - l i k e p a r t i c l e s by s u b s t i t u t i n g g l a s s e s of known f u s i o n or v i s c o s i t y p r o p e r t i e s f o r the c o a l ash type m a t e r i a l s . As was i n d i c a t e d above, the n a t u r e of the m i n e r a l s p e c i e s quite important i n d e t e r m i n i n g whether a g i v e n coal w i l l difficult to burn continually in a particular boiler. r e a c t i o n s between the s p e c i e s and the speed w i t h which they form new low m e l t i n g phases, or r e l e a s e v o l a t i l e s p e c i e s which f o r m low m e l t i n g s p e c i e s , u s u a l l y d e t e r m i n e the s e v e r i t y o f problems.

is be The can can the

To d a t e , no u n i v e r s a l l y a c c e p t e d t o o l to p r e d i c t f o u l i n g and s l a g g i n g from c o a l ash c o m p o s i t i o n s has been d e v e l o p e d . Informat i o n about the performance of p a r t i c u l a r c o a l s i s f r e q u e n t l y not w i d e l y a v a i l a b l e f o r a v a r i e t y of r e a s o n s . When the i n f o r m a t i o n i s available from both commercial and small scale studies then

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Publication Date: April 2, 1986 | doi: 10.1021/bk-1986-0301.ch001

c o r r e l a t i o n s can be proposed between performance i n the s m a l l and large s c a l e equipment. The use of these correlations, when a c c u r a t e , p e r m i t the d e s i g n e r o r o p e r a t o r to a v o i d problems b e f o r e they a r e a l l o w e d to o c c u r . S t a t i s t i c a l s t u d i e s have been employed t o e s t a b l i s h w h i c h ones a r e the most i m p o r t a n t . C o a l ash i s not always a d e l e t e r i o u s m a t e r i a l f o r a p r o c e s s . In coal liquefaction, i t has been o b s e r v e d t h a t the r a t e i s i n c r e a s e d i n the p r e s e n c e o f p y r i t e . In g a s i f i c a t i o n , the r a t e i s i n c r e a s e d i n the p r e s e n c e of a l k a l i e s . There i s l i m i t e d data a v a i l a b l e on the e f f e c t s of m a t e r i a l s on combustion. Although i n t e r e s t i n s y n t h e t i c f u e l s from c o a l i s q u i t e l i m i t e d a t p r e s e n t , t h e r e i s an i n t e r e s t i n d e v e l o p i n g the t e c h n i c a l c a p a b i l i t y to permit the production of more premium fuel types from less d e s i r a b l e ones. The c o n v e r s i o n of s o l i d c o a l to l i q u i d f u e l s has been a v e r y demanding p r o c e s s i n terms o f the p r e s s u r e s and, t o some e x t e n t , the temperatures t h a t have been used. C a t a l y s t s have been r e q u i r e d i n a l l c a s e s . The c a t a l y s t s have been p o i s o n e d by the s u l f u r and o t h e r s p e c i e s i n the m i n e r a l m a t t e r . As a r e s u l t , c a t a l y s t c o s t s and replacement r a t e s can be q u i t e h i g h . A cheap, n a t u r a l l y o c c u r r i n g c a t a l y s t t h a t came w i t h the c o a l would be o f significant interest. P y r i t e seems to be such a m a t e r i a l . F i n a l l y , one can ask "What can be done to remove the m i n e r a l m a t t e r from the c o a l ? " The answer i s t h a t s e v e r a l p r o c e s s e s can be used to reduce the m i n e r a l m a t t e r . The e x t e n t and c o s t of removal u s u a l l y d i c t a t e the c l e a n i n g p r o c e s s t h a t i s chosen. Generally, a l a r g e p a r t of the c o a l t h a t i s mined f o r e l e c t r i c power p r o d u c t i o n i s " c l e a n e d " o r p h y s i c a l l y p r o c e s s e d to remove p a r t o f the m i n e r a l matter by i n i t i a l l y c r u s h i n g the c o a l and then d e n s i t y s e p a r a t i o n o f the c o a l from the m i n e r a l m a t t e r . T h i s i s e s p e c i a l l y t r u e when the run-of-mine c o a l c o n t a i n s more than about 15% m i n e r a l m a t t e r . In the p h y s i c a l s e p a r a t i o n a dense f r a c t i o n and l i g h t f r a c t i o n , o r s e v e r a l of t h e s e , a r e o b t a i n e d . S i n c e some c o m b u s t i b l e matter i s p r e s e n t i n the denser f r a c t i o n s , the p r o c e s s r e s u l t s i n an energy loss. The quality of the s e p a r a t i o n a l s o depends upon the d i f f e r e n c e s i n the s u r f a c e s p r o p e r t i e s o f the c o a l and m i n e r a l matter as w e l l as the d e n s i t y d i f f e r e n c e s . The m i n e r a l s p e c i e s and c l a y s can v a r y i n such a way t h a t the t y p i c a l c l e a n i n g p r o c e s s s t e p s a p p a r e n t l y f u n c t i o n i n c o n s i s t e n t l y and r e j e c t a s i g n i f i c a n t amount o f c o m b u s t i b l e m a t e r i a l w i t h the m i n e r a l f r a c t i o n s . In o r d e r to m i n i m i z e the l o s s , i t i s n e c e s s a r y to c r u s h and g r i n d the c o a l i n such a way t h a t t h e r e a r e p a r t i c l e s o f m i n e r a l matter f r e e from a t t a c h e d coal. The complete s e p a r a t i o n u s u a l l y r e q u i r e s g r i n d i n g t o s m a l l e r and s m a l l e r p a r t i c l e s i z e s . T h i s concept has been c a r r i e d to v e r y f i n e g r i n d i n g , c a p a b l e of g i v i n g p a r t i c l e s f i n e r than 15 m i c r o n s . M i n e r a l m a t t e r s e p a r a t i o n has improved but becomes v e r y s e n s i t i v e t o c o a l and m i n e r a l s u r f a c e p r o p e r t i e s . Improved c l e a n i n g has y i e l d e d c o a l w i t h ash c o n c e n t r a t i o n s o f the o r d e r of a t e n t h o f a per c e n t . C o a l c l e a n i n g i s based on a knowledge o f the m i n e r a l s p e c i e s t h a t must be removed. The c l e a n i n g p l a n t i s a s e r i e s of c i r c u i t s t h a t s e p a r a t e d i f f e r e n t f r a c t i o n s and p r o v i d e f o r r e c o m b i n a t i o n o f

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Publication Date: April 2, 1986 | doi: 10.1021/bk-1986-0301.ch001

c e r t a i n ones t o maximize the y i e l d o f a q u a l i t y p r o d u c t , and m i n i m i z e l o s s e s of c o m b u s t i b l e m a t e r i a l . The performance of the c i r c u i t s w i l l change as the f e e d streams v a r y . The a c t of c l e a n i n g t a k e s out some o f the m i n e r a l m a t t e r , l e a v i n g a p o r t i o n of the m i n e r a l matter y e t to be removed. T h i s r e m a i n i n g matter may be e n r i c h e d or d e p l e t e d i n c e r t a i n p a r t s of the o r i g i n a l matter and l e a d t o performance d i f f e r e n t than t h a t which would be e x p e c t e d on the b a s i s of the o r i g i n a l m a t e r i a l . There i s a need t o remain aware of the c o m p o s i t i o n o f the d i f f e r e n t streams i n the c l e a n i n g plant. The a n a l y t i c a l t e c h n i q u e s t h a t have been used a r e d i f f i c u l t t o c a r r y out q u i c k l y enough and c h e a p l y enough f o r easy m o d i f i c a t i o n of the c i r c u i t s . R e c e n t l y the t e c h n i q u e of automated image a n a l y s i s has been d e v e l o p e d so t h a t i t may find application in s o l v i n g t h i s problem. Chemical treatment o f c o a l has a l s o been used to f u r t h e r reduce the m i n e r a l matter c o n t e n t . U s u a l l y the p h y s i c a l t e c h n i q u e s mentioned e a r l i e r a r e used to reduce the i n o r g a n i c s u l f u r s p e c i e s ( m o s t l y p y r i t e ) t o a v e r y low l e v e l . Since there i s a great c o n c e r n about the s u l f u r c o n t e n t s o f c o a l , and the o r g a n i c s u l f u r content exceeds the p e r m i s s i b l e s t a n d a r d s f o r many h i g h - s u l f u r c o a l s , n o n - p h y s i c a l or chemical techniques have been t r i e d to f u r t h e r reduce the s u l f u r c o n t e n t . Strong c a u s t i c reagents are used most o f t e n t o remove the more r e f r a c t o r y s u l f u r forms. The removal of b o t h m i n e r a l matter and s u l f u r s p e c i e s t o v e r y low v a l u e s would p r o v i d e premium s o l i d f u e l s and p o s s i b l y new chemical feedstocks. S e v e r a l t e c h n i q u e s are b e i n g e x p l o r e d t o achieve these g o a l s . The m i n e r a l matter i n a p h y s i c a l l y c l e a n e d c o a l can be f u r t h e r reduced by the s o l u b i l i z a t i o n of the a l u m i n o s i l i c a t e minerals. T h i s can t e c h n i c a l l y be a c c o m p l i s h e d w i t h the use of a l k a l i n e and then a c i d t r e a t m e n t s . A v a r i e t y of s t u d i e s a r e under way t o d e f i n e the c o n d i t i o n s r e q u i r e d f o r e f f e c t i v e removal of the m i n e r a l matter and e s t a b l i s h the amount o f s u l f u r r e d u c t i o n t h a t can be a c c o m p l i s h e d . O t h e r s i n v o l v e the use o f f i n e g r i n d i n g to l i b e r a t e the c o a l from the m i n e r a l m a t t e r . Then an agglomérant i s used to s e p a r a t e the c o a l matter from the aqueous phase cont a i n i n g suspended m i n e r a l m a t t e r . A new approach uses microwave energy t o s e l e c t i v e l y decompose the c l a y s i n t o s p e c i e s t h a t can be s o l u b i l i z e d and removed. S t i l l another t e c h n i q u e i n v o l v e s t r e a t ment w i t h carbon d i o x i d e to reduce the p a r t i c l e s i z e and permit the l i b e r a t i o n of the m i n e r a l m a t t e r . Over the next few y e a r s these w i l l be s t u d i e d f u r t h e r and i t i s hoped t h a t c o a l w i l l become available i n a form w i t h l e s s of t h e s e i n t e r e s t i n g , but not e n t i r e l y desirable mineral species. I t i s hoped t h a t t h i s volume w i l l s e r v e as a f u r t h e r s t i m u l u s to workers i n and o u t s i d e of the f i e l d of c o a l m i n e r a l matter and ash c h e m i s t r y . We a l l b e n e f i t from the work o f o t h e r s t o extend our c u r r e n t knowledge and can h e l p make c o a l not o n l y an abundant f u e l , but a l s o a more c o n v e n i e n t and d e s i r a b l e r e s o u r c e as w e l l . R E C E I V E D November 12, 1985

2 Mineral Matter in Illinois and Other U.S. Coals Richard D . Harvey and Rodney R. Ruch

Publication Date: April 2, 1986 | doi: 10.1021/bk-1986-0301.ch002

Illinois State Geological Survey, Champaign, IL 61820

This review (1) describes and classifies the geological, physical, and chemical occurrences of mineral matters in coal; (2) summarizes analytical methods used to characterize the mineral matter; and (3) compares the mineral-matter composition of Illinois coals with that of other U.S. coals. In Illinois coals the organic constituents generally vary vertically within seams by a factor of 2 to 3; many elements also vary laterally within the same seam. Similarly, variations are substantial from one seam to another within the Illinois Basin. There is a considerable overlap, however, in compositional ranges of coals from different U.S. basins. On a regional basis, the material variability of many elements, such as sulfur and chlorine, is the result of geological processes. Low-sulfur coals mined in many western states, southern West Virginia, southwestern Virginia, and eastern Kentucky underlie strata deposited from fresh water; whereas many high-sulfur seams underlie strata deposited from sulfate-bearing brackish or marine waters. Because of the high variability of coal, samples from all prospective deposits need to be analyzed. Methods for analysis of inorganic elements in coal are generally precise, but methods for quantitative analysis of mineral phases need to be improved.

M i n e r a l m a t t e r i s g e n e r a l l y c o n s i d e r e d t o b e t h e sum t o t a l o f a l l i n o r g a n i c m i n e r a l s ( d i s c r e t e p h a s e s ) and e l e m e n t s t h a t a r e present i n coal ( J J . Thus, a l l elements i n coal except o r g a n i c a l l y c o m b i n e d C , Η, 0 , N , a n d S a r e c l a s s i f i e d b y t h i s d e f i n i t i o n as m i n e r a l m a t t e r . T h i s a d e q u a t e l y c l a s s i f i e s most i n o r g a n i c e l e m e n t s i n c o a l s , t h o s e t h a t a r e s t r u c t u a l l y bound w i t h i n v a r i o u s m i n e r a l s , b u t some o t h e r e l e m e n t s a r e a l s o combined i n t h e o r g a n i c m a t t e r . F o r bituminous coals such e l e m e n t s i n c l u d e B, B e , B r , G e , S b , and V ( 2 - 5 ) ; f o r l o w e r r a n k c o a l s much o f t h e Ca i s c o m b i n e d a s a n o r g a n i c s a l t . In 0097-6156/ 86/ 0301 -0010508.75/ 0 © 1986 American Chemical Society

2.

HARVEY AND

RUCH

Publication Date: April 2, 1986 | doi: 10.1021/bk-1986-0301.ch002

a d d i t i o n , many o t h e r

Mineral

Matter

elements

in U.S.

11

Coals

are combined

in

both the

organic

and t h e m i n e r a l m a t t e r f r a c t i o n s . The c h e m i s t r y o f c o a l i s f u r t h e r c o m p l i c a t e d by t h e f a c t t h a t t h e o r g a n i c e l e m e n t s , e x c e p t n i t r o g e n , a l s o o c c u r i n one o r more m i n e r a l c o n s t i t u e n t s : carbon in carbonates; hydrogen in a b s o r b e d w a t e r and a s h y d r o x i d e i n c l a y m i n e r a l s ; o x y g e n i n w a t e r , q u a r t z and o t h e r s i m p l e o x i d e s , c l a y m i n e r a l s , c a r b o n a t e s , s u l f a t e s , and o t h e r t r a c e m i n e r a l s ; and s u l f u r i n p y r i t e , r e l a t e d s u l f i d e s , and s u l f a t e s . F o r s c i e n t i f i c and t e c h n o l o g i c a l p u r p o s e s , i t i s i m p o r t a n t t o know t h e o r g a n i c i n o r g a n i c a s s o c i a t i o n s of the elements in coals being studied or used i n i n d u s t r i a l p r o c e s s e s . C h a r a c t e r i z a t i o n of the mineral matter in coal i s important f o r a number o f r e a s o n s . B e c a u s e o f t h e way c o a l m u s t be m i n e d , t h e m i n e - r u n p r o d u c t a l w a y s i n c l u d e s some r o c k m a t e r i a l o t h e r than c o a l , thereby c o n t r i b u t i n g mineral m a t t e r to the mine product. M i n e r s o f a t h i n seam u s u a l l y h a v e t o r e m o v e some r o o f and f l o o r s t r a t a . Irregular bodies of roof shale or other rocks f r e q u e n t l y o c c u r w i t h i n c o a l seams and i n e v i t a b l y become p a r t o f t h e mine p r o d u c t . Some c o a l seams g r a d e u p w a r d s t o s h a l e c o a l o r c o a l y s h a l e s so t h a t t h e r e i s no c l e a r - c u t b o u n d a r y b e t w e e n c o a l and t h e r o o f r o c k s . In a d d i t i o n , many c o a l seams c o n t a i n o n e o r more m i n e r a l p a r t i n g s o f v a r i o u s t h i c k n e s s so t h a t m i n e r s must i n c l u d e t h e s e i n t h e mine p r o d u c t . It i s best f o r mining and s a l e s p u r p o s e s i f t h e a m o u n t s , t y p e s , and c h a r a c t e r i s t i c s o f t h e s e r o c k m a t e r i a l s be known p r i o r t o m i n i n g i n o r d e r t o p r o p e r l y d e s i g n t h e m i n e and p r e p a r a t i o n p l a n t . W h i l e i t h a s l o n g b e e n known t h a t m i n e r a l m a t t e r i s a n i m p o r t a n t d e s i g n f a c t o r f o r u t i l i t y and o t h e r l a r g e b o i l e r s (6-_ S) consumer p r e s s u r e f o r low e l e c t r i c b i l l s c o u p l e d w i t h more s t r i n g e n t regulations of s u l f u r emissions are causing u t i l i t i e s t o seek more e f f i c i e n t o p e r a t i o n o f e x i s t i n g c o a l - f i r e d power plants. T h u s , t h e r e i s r e n e w e d i n t e r e s t and c o n c e r n w i t h s u c h p r o b l e m s a s s l a g g i n g and f o u l i n g o f b o i l e r s ( 9 - 1 6 ) . Slagging i s the accumulation of mineral matter in a fused and h a r d e n e d f o r m on t h e w a l l s and o t h e r s u r f a c e s i n t h e f u r n a c e of b o i l e r s . S l a g g i n g r e d u c e s h e a t t r a n s f e r and c a u s e s e x c e s s i v e t e m p e r a t u r e o f t h e e x h a u s t gases as t h e y e x i t t h e f u r n a c e . The n e t e f f e c t o f s l a g g i n g i s t o add m a i n t e n a n c e c o s t s f o r b o i l e r o p e r a t i o n a s i t damages t h e m e t a l p a r t s on w h i c h t h e s l a g forms. B l o c k s o f s l a g h a v e b e e n known t o g r o w t o o v e r 5 f e e t t h i c k i n t h e t o p o f f u r n a c e s and e v e n t u a l l y t o f a l l o f t h e i r own w e i g h t , k n o c k i n g out t h e b o t t o m of t h e f u r n a c e . A v a i l a b i l i t y of t h e b o i l e r f o r g e n e r a t i n g e l e c t r i c power i s s i g n i f i c a n t l y r e d u c e d by s e v e r e s l a g g i n g . F o u l i n g i s t h e a c c u m u l a t i o n o f m i n e r a l - d e r i v e d a s h on t h e s u p e r h e a t e r and r e h e a t e r t u b e s i n t h e c o n v e c t i v e ( h e a t e x c h a n g e r ) s e c t i o n downstream from the f u r n a c e . Fouling r e s t r i c t s t h e f l o w o f e x h a u s t g a s e s and i m p e d e s h e a t t r a n s f e r t h r o u g h s u p e r h e a t e r t u b e w a l l s and t h e r e b y r e d u c e s t h e amount o f steam g e n e r a t e d . A b r a s i o n of metal p a r t s i n s i d e the b o i l e r i s a t h i r d d e l e t e r i o u s e f f e c t o f a s h p a r t i c l e s p r o d u c e d by some c o a l s . Ash p a r t i c l e s t h a t are e s p e c i a l l y hard w i l l cause e x c e s s i v e a b r a s i o n 9

12

M I N E R A L M A T T E R A N D A S H IN C O A L

Publication Date: April 2, 1986 | doi: 10.1021/bk-1986-0301.ch002

when t h e y

i m p i n g e on t h e

furnace

and b o i l e r

surfaces.

At

plants

where a b r a s i o n i s s e r i o u s , i t c a u s e s f r e q u e n t s h u t - d o w n s . A f o u r t h i m p o r t a n t f a c t o r t h a t i n c r e a s e s o u t a g e s and maintenance costs i s the c o r r o s i v e e f f e c t s of a l k a l i elements, e s p e c i a l l y sodium, which i s associated with c h l o r i n e in c e r t a i n coals. The s p e c i f i c damage t o b o i l e r s b y c h l o r i n e i s n o t w e l l known, but t h i s m i n e r a l c o n s t i t u e n t c l e a r l y causes e x c e s s i v e rates of corrosion to coal handling equipment. T h i r t y - f o u r m i n o r and t r a c e e l e m e n t s a r e o f p o t e n t i a l environmental concern (17). S u l f u r i s the element of major c o n c e r n due t o i t s a b u n d a n c e i n f l u e g a s e s f r o m some c o a l b u r n i n g p l a n t s and i t s s u b s e q u e n t c o n t r i b u t i o n t o " a c i d r a i n . " S u l f u r as a c i d i c i o n s o f s u l f a t e c a n a l s o c o n t r i b u t e t o p o l l u t i o n o f s u r f a c e w a t e r and g r o u n d w a t e r . Other elements of g r e a t e s t c o n c e r n a r e A s , B, C d , P b , H g , Mo, and S e . With the e x c e p t i o n o f Β and S e , t h e s e e l e m e n t s a r e s t r o n g l y a s s o c i a t e d w i t h m i n e r a l m a t t e r i n t h e c o a l and a r e c o n c e n t r a t e d i n w a s t e p i l e s from coal p r e p a r a t i o n p l a n t s . I f the waste d i s p o s a l site i s n o t c o n s t r u c t e d as a c l o s e d s y s t e m , p o l l u t i o n o f n e a r b y groundwater i s p o s s i b l e . B o r o n and Se may c o n t r i b u t e t o t h e p o l l u t i o n r i s k as t h e y a r e a s s o c i a t e d w i t h b o t h m i n e r a l and organic components. On t h e o t h e r h a n d , c e r t a i n c o a l - m i n e w a s t e s h a v e p o t e n t i a l f o r r e c o v e r y o f v a l u a b l e m e t a l s s u c h a s z i n c and cadmium ( 1 8 ) . G e o l o g i c a l s t u d i e s of the d i s t r i b u t i o n (or v a r i a t i o n ) of m i n e r a l m a t t e r i n a l o c a l a r e a , s u c h as a l a r g e m i n e , as w e l l a s r e g i o n a l l y o v e r the e x t e n t o f a commercial seam, e n a b l e u s e f u l p r e d i c t i o n s o f t h e c o n c e n t r a t i o n s o f some i n o r g a n i c e l e m e n t s important to coal q u a l i t y . G e o l o g i c a l p r o c e s s e s t h a t formed the seam o p e r a t e d o v e r e x t e n s i v e a r e a s ; t h e r e f o r e , p a t t e r n s o f e l e m e n t a l d i s t r i b u t i o n e n a b l e q u a l i t y p a r a m e t e r s o f t h e c o a l bed t o be p r e d i c t e d i n some u n e x p l o r e d a r e a s ( 5 , 1 9 - 2 4 ) .

Coal

Geology

C o a l i s a brown t o b l a c k , c o m b u s t i b l e , s e d i m e n t a r y r o c k composed p r i n c i p a l l y o f c o n s o l i d a t e d and c h e m i c a l l y a l t e r e d p l a n t r e m a i n s (25)(Figure 1). The o r i g i n a l p l a n t s g r e w a b u n d a n t l y i n a n c i e n t swamps and t h e i r r e m a i n s a c c u m u l a t e d a s p e a t on t h e swamp f l o o r , m o s t l y under w a t e r , which r e s t r i c t e d d e c o m p o s i t i o n of the p l a n t debris. Commercial t h i c k n e s s e s o f c o a l a r e t h o u g h t t o have r e q u i r e d p o s s i b l y a c e n t u r y o r more o f u n u s u a l l y c o n s i s t e n t c l i m a t i c and h y d r o l o g i e c o n d i t i o n s t o a l l o w f o r t h e d e p o s i t i o n of the required t h i c k n e s s of peat. S t u d i e s h a v e shown t h a t p e a t : c o a l ( b i t u m i n o u s ) t h i c k n e s s r a t i o s r a n g e f r o m a b o u t 3:1 t o 30:1 ( 2 6 ) . C o T l seams i n t h e U n i t e d S t a t e s t h a t a r e o f m i n a b l e t h i c k n e s s and q u a l i t y r a n g e i n a g e f r o m t h e P e n n s y l v a n i a n t o t h e Tertiary geologic periods (Figure 2). A rough e s t i m a t e suggests t h a t t w o - t h i r d s to t h r e e - q u a r t e r s of the coal produced t o date i n t h e U n i t e d S t a t e s has b e e n b i t u m i n o u s c o a l f r o m P e n n s y l v a n i a n s t r a t a m i n e d f r o m seams i n t h e A p p a l a c h i a n , I l l i n o i s , a n d Western I n t e r i o r Basins ( F i g u r e 3 ) . Most o f t h e r e m a i n d e r has b e e n l i g n i t e and s u b b i t u m i n o u s c o a l m i n e d f r o m T e r t i a r y and

RUCH

Mineral

Matter

in U.S.

Coals

Publication Date: April 2, 1986 | doi: 10.1021/bk-1986-0301.ch002

HARVEY AND

0 5 i_i

50 ι

μπη

F i g u r e 1. T h i n s e c t i o n o f a c o a l i n t r a n s m i t t e d l i g h t s l i c e d normal t o b e d d i n g s h o w i n g l a y e r e d s t r u c t u r e o f p l a n t d e b r i s and d i s s e m i n a t e d g r a i n s ( 2 - 4 ym) o f q u a r t z (Q) a n d p y r i t e ( Ρ ) .

MINERAL

Yrs (106)

Geologic Period

Subbituminous

Lignite

M A T T E R A N D A S H IN C O A L

Bituminous Medium volatile

High volatile

Low volatile

Anthra­ cite

Tertiary western

50-

:··.·:··Λ w e s t e r n states

100-

Cretaceous

150-

I

Jurassic

Publication Date: April 2, 1986 | doi: 10.1021/bk-1986-0301.ch002

200Triassic

VA

Permian

ΤΧ,ΟΗ, PA, W V

250-

300-

Pennyslvanian

350-

Mississippian

East o f G r e a t Plains rWxWidely scattered a n d l i m i t e d coals:

F i g u r e 2. G e o l o g i c age ( p e r i o d ) and rank N o r t h A m e r i c a , a f t e r Cady ( 2 7 ) .

Figure

3. C o a l f i e l d s

of the United

of coal

States

deposits

(28,29).

in

2.

HARVEY A N D RUCH

Mineral

Matter

in U.S. Coals

Publication Date: April 2, 1986 | doi: 10.1021/bk-1986-0301.ch002

C r e t a c e o u s s t r a t a i n t h e n o r t h e r n Great P l a i n s , t h e Rocky M o u n t a i n s ( i n c l u d i n g c e n t r a l Utah and n o r t h e a s t e r n A r i z o n a ) , W a s h i n g t o n , T e x a s and A l a s k a . In r e c e n t y e a r s p r o d u c t i o n f r o m t h e s e T e r t i a r y and C r e t a c e o u s d e p o s i t s h a s g r e a t l y i n c r e a s e d . F o r e x a m p l e , between 1972 and 1982 t h e p r o d u c t i o n f r o m t h e s e d e p o s i t s i n Wyoming i n c r e a s e d f r o m an a n n u a l t o n n a g e o f a b o u t 10 m i l l i o n t o n e a r l y 110 m i l l i o n , w h i l e p r o d u c t i o n i n I l l i n o i s d e c r e a s e d f r o m a b o u t 6 5 t o 61 m i l l i o n t o n s . O r i g i n o f Coal Seams. By a n a l o g y t o p r e s e n t - d a y d e p o s i t s and t o the l i t h o l o g i e character of associated s t r a t a , the ancient coal seams f o r m e d i n v a s t p e a t s w a m p s . The swamps w e r e l o c a t e d on coastal p l a i n s , often o n , or adjacent t o , major r i v e r d e l t a s . Some o f t h e swamps m i g h t h a v e b e e n 2 0 0 t o 4 0 0 m i l e s a c r o s s . O v e r a p e r i o d o f many c e n t u r i e s t h e swamp l a n d s l o w l y s u b s i d e d w h i l e m a i n t a i n i n g a dense f o r e s t a t o r near sea l e v e l . Layer upon l a y e r o f f a l l e n v e g e t a t i o n was c o n v e r t e d t o p e a t . Mineral m a t t e r was i n t r o d u c e d i n t o t h e swamps, m a i n l y by r i v e r s , a s s u s p e n d e d mud and d i s s o l v e d i o n i c s p e c i e s d u r i n g s t o r m s t h a t f l o o d e d t h e swamp, b y s e a w a t e r d u r i n g w i n d s t o r m s , a n d by d i s t a n t v o l c a n i c e r u p t i o n s t h a t r a i n e d d u s t o n t o t h e swamp. Some i n o r g a n i c e l e m e n t s w e r e d r a w n i n t o p l a n t t i s s u e s t h r o u g h t h e i r r o o t s f r o m t h e swamp w a t e r and p e a t y s o i l ; t h i s m a t e r i a l i s known a s " i n h e r e n t " m i n e r a l m a t t e r ( 3 0 ) i n t h e f i n a l coal. U l t i m a t e l y t h e swamp was d r o w n e d b y r i v e r s o r s e a w a t e r f r o m w h i c h s a n d and mud w e r e d e p o s i t e d o n t o t h e d y i n g f o r e s t . Peats c o v e r e d w i t h s e a w a t e r were i n j e c t e d w i t h s u l f a t e , b u t t h o s e c o v e r e d by t h i c k a n d i m p e r m e a b l e r i v e r muds ( f r e s h w a t e r ) r e m a i n e d more o r l e s s f r e e o f s u l f a t e . M o r e o f t e n t h a n n o t , o n c e a p e a t swamp was e s t a b l i s h e d on a c o a s t a l p l a i n o r d e l t a and l a t e r c o v e r e d b y s e d i m e n t s d u e t o l a n d s u b s i d e n c e , a s e c o n d swamp was e s t a b l i s h e d i n t h e same r e g i o n b y c y c l i c c h a n g e s i n s e a l e v e l and r e - e s t a b l i s h m e n t o f swamp-type v e g e t a t i o n . In t h i s w a y , m u l t i p l e l a y e r s o f p e a t a n d s u b s e q u e n t l y o f c o a l were f o r m e d . Coal i f i c a t i o n . The t r a n s f o r m a t i o n o f p e a t i n t o c o a l was a s l o w and c o m p l e x p r o c e s s t h a t b e g a n w i t h b a c t e r i a l r e d u c t i o n o f p l a n t d e b r i s d u r i n g t h e p e a t s t a g e ( F i g u r e 4 ) . As t h e p e a t was p r o g r e s s i v e l y b u r i e d u n d e r h u n d r e d s o f f e e t o f s e d i m e n t s , i t was c o m p a c t e d ; i t s i n h e r e n t m o i s t u r e was p r o g r e s s i v e l y f o r c e d o u t b y t h e p r e s s u r e o f o v e r l y i n g s e d i m e n t s and b y h e a t f r o m t h e e a r t h ' s interior. M e t h a n e and C 0 w e r e r e l e a s e d o r e n t r a p p e d w i t h i n pores o f t h e compacted o r g a n i c d e b r i s . Several chemical p r o c e s s e s b r i e f l y d e s c r i b e d as p e a t i f i c a t i o n , h u m i f i c a t i o n , g e l i f i c a t i o n , and v i t r i f i c a t i o n , a l t e r e d t h e r o o t s , b a r k , and o t h e r c e l l u l o s e and l i g n i n b e a r i n g d e b r i s t o v i t r i n i t e - - t h e m o s t abundant maceral c o m p r i s i n g b i t u m i n o u s c o a l s ( F i g u r e 4 ) . P l a n t s p o r e s , c u t i c u l e s , and r e s i n s a n d o t h e r l i p i d c o m p o n e n t s w e r e i n c o r p o r a t e d i n t o p e a t and t h e s e a r e c l a s s i f i e d a s t h e l i p t i n i t e group of macérais. Various oxidized (charred or f u s i n i z e d ) d e b r i s a r e c l a s s i f i e d as i n e r t i n i t e s ( F i g u r e 4 ) . With i n c r e a s i n g c o a l i f i c a t i o n ( r a n k ) , t h e s e m a c é r a i s become i n c r e a s i n g l y e n r i c h e d i n c a r b o n so t h a t a l l macérais i n a n t h r a c i t e h a v e e s s e n t i a l l y t h e same p r o p e r t i e s a s i n e r t i n i t e . 2

> ζ

ζ o

MACERAL COMPONENTS

Increasing carbon content

SPORES, L E A F CUTICLES, ROOTS, LIMBS. BARK . RESINS, A L G A E , ETC. , , C E L L U L O S E . LIGNIN

95%lo MOISTURE

v/tineral g r a i n s suspended in w a t e r or atmosphere

3

90%lo MINERAL COMPONENTS

Epigenesis

Deposition solution^ Syngenesis

Moisture and ionic compounds, including sulfur, d i s s o l v e d in w a t e r — IONS

INORGANIC COMPONENTS

Figure 4. Diagramatic representation of c o a l i f i c a t i o n processes i n v o l v i n g t h e c o n v e r s i o n o f p l a n t d e b r i s t o v a r i o u s macérais, and t h o s e i n v o l v i n g m o i s t u r e a n d g e n e t i c p r o c e s s e s o f m i n e r a l matter.

Swamps, s o m e lakes and lagoons

T r e e s , s h r u b s , reeds, grasses, algae, f u n g i , b a c t e r i a

ORGANIC COMPONENTS

Publication Date: April 2, 1986 | doi: 10.1021/bk-1986-0301.ch002

η

Χ

CO

>

> Ζ α

25 H m

r

>

73

2.

HARVEY A N D RUCH

Mineral

Matter

in U.S.

Coals

Because o f t h e c o m p l e x i t y o f the coal i f i c a t i o n p r o c e s s e s , d i f f e r e n t measures are used t o d e f i n e d i f f e r e n t l e v e l s o f rank ( T a b l e I ) : h i g h m o i s t u r e , l o w h e a t i n g v a l u e , and n o n a g g l o m e r a t i n g c h a r a c t e r o f t h e c o a l d e f i n e t h e rank (group) w i t h i n t h e l i g n i t e and s u b b i t u m i n o u s c l a s s e s ; a n d v o l a t i l e m a t t e r ( o r f i x e d carbon) d e f i n e t h e v a r i o u s groups o f rank i n t h e b i t u m i n o u s and a n t h r a c i t e c l a s s e s . In a d d i t i o n t o t h e s e p r o p e r t i e s , the r e f l e c t a n c e of v i t r i n i t e , carbon content of the c o a l ( d r y , m i n e r a l m a t t e r f r e e ) , and some o t h e r p r o p e r t i e s change p r o p o r t i o n a t e l y as rank i n c r e a s e s (Table I ) .

Publication Date: April 2, 1986 | doi: 10.1021/bk-1986-0301.ch002

Mineral

Occurrences

i n Coal

Seams

Over 125 d i f f e r e n t m i n e r a l s have been r e p o r t e d i n c o a l ( 3 2 ) ; h o w e v e r , o n l y about 25 have been r e c o g n i z e d as o c c u r r i n g i n s i g n i f i c a n t amounts ( T a b l e I I ) . M i n e r a l s i n c o a l o c c u r as d i s c r e t e g r a i n s o r f l a k e s o r a g g r e g a t e s i n one o f f i v e p h y s i c a l modes ( 3 1 ) : ( 1 ) as m i c r o s c o p i c a l l y d i s s e m i n a t e d i n c l u s i o n s w i t h i n macérais; (2) as l a y e r s o r p a r t i n g s w h e r e i n f i n e - g r a i n e d c l a y m i n e r a l s u s u a l l y p r e d o m i n a t e ; ( 3 ) as n o d u l e s i n c l u d i n g l e n t i c u l a r and s p h e r i c a l c o n c r e t i o n s ; ( 4 ) a s f i s s u r e s , i n c l u d i n g c l e a t and o t h e r f r a c t u r e o r v o i d f i l l i n g s ; ( 5 ) a s r o c k f r a g m e n t s — m e g a s c o p i c masses o f rock m a t e r i a l found w i t h i n t h e c o a l bed as a r e s u l t o f f a u l t i n g , s l u m p i n g , o r r e l a t e d d i s t u r b a n c e s . T h i s c l a s s i f i c a t i o n i s u s e f u l i n t h a t t h e removal o f m i n e r a l s f r o m c o a l i n p r e p a r a t i o n p l a n t s i s s t r o n g l y i n f l u e n c e d by t h e m i n e r a l ' s p h y s i c a l mode o f o c c u r r e n c e . Fine-grained quartz, c l a y , and p y r i t e d i s s e m i n a t e d w i t h i n m a c é r a i s a r e l e a s t s u s c e p t i b l e t o removal by p h y s i c a l c l e a n i n g m e t h o d s ; whereas r o c k f r a g m e n t s and m i n e r a l s i n l a y e r e d , n o d u l a r , and f i s s u r e modes b r e a k f r e e and a r e more e a s i l y r e m o v e d . Origin of Mineral Matter. A genetic c l a s s i f i c a t i o n of mineral m a t t e r p r o v i d e s t h e b e s t means t o p r e d i c t t h e q u a l i t y o f c o a l i n a r e a s a h e a d o f m i n i n g and e x p l o r a t i o n . We c o n s i d e r t h r e e m a i n g e n e t i c p r o c e s s e s t o be r e s p o n s i b l e f o r m i n e r a l s i n c o a l : d e t r i t a l d e p o s i t i o n , syngenesis, or epigenesis (see the r i g h t s i d e o f F i g u r e 4 ) . D e t r i t a l g r a i n s were i n t r o d u c e d i n t o t h e swamp by r i v e r s , t i d a l ( s t o r m ) w a v e s , a n d w i n d ( v o l c a n i c dusts). S y n g e n e t i c m i n e r a l s were formed d u r i n g t h e peat s t a g e o f c o a l f o r m a t i o n , b e f o r e t h e p e a t was "1 U n i f i e d " t o l i g n i t e . T h r e e d i f f e r e n t s u b c l a s s e s o f s y n g e n e t i c m i n e r a l s c a n be d i s t i n g u i s h e d on t h e o r e t i c a l g r o u n d s . ( a ) m i n e r a l s formed by c r y s t a l l i z a t i o n o f i n o r g a n i c e l e m e n t s t h a t were i n c o r p o r a t e d i n t o t h e t i s s u e s o f the l i v i n g plants (the inherent mineral matter referred to above). ( b ) m i n e r a l s f o r m e d by b a c t e r i a l r e d u c t i o n o f a q u e o u s s u l f a t e s by c r y s t a l g r o w t h i n m i c r o s c o p i c p o r e s within the plant remains. (c) m i n e r a l i z a t i o n around n u c l e i or o t h e r c e n t e r s , forming nodules; these include coal b a l l s ( p e r m i n e r a l i z e d p e a t ) and o t h e r p l a n t r e p l a c e m e n t forms. It i s r a r e l y p o s s i b l e t o u n e q u i v o c a l l y d i s t i n g u i s h type (a)

17

\

Meta-anthracite

Anthracite

Semianthracite

L o w volatile bit. c o a l

M e d . volatile bit. c o a l

1

1 I

Reproduced with permission from Ref. 31. Copyright 1984,

> 14.000

/

No

• Yes

No

No

No

Agglomerating

Plenum Press.

>32.5

>32.5

30.2-32.5

13.000-14.000*

H i g h volatile Β bit. c o a l >14,000

26.7-30.2 Ϊ

10.500-13.000*

H i g h volatile C bit. c o a l

H i g h volatile A bit. c o a l

24.4-26.7 J

19.3-22.1 j 22.1-24.4

9.500-10.500*

Subbituminous C coal

14.6-19 3 1

10.500-11.500*

8.300-9.500*

Lignite A

Subbituminous A coal

6.300-8.300*

Lignite Β

-14.6

Subbituminous Β coal

Undefined-6.300*

" A i r dried. » W e l l - s u i t e d for rank d i s c r i m i n a t i o n in range indicated. ' Moderately well-suited for rank d i s c r i m i n a t i o n .

Anthracitic

Bituminous

Subbituminous

Lignite

class

1

(moist, m m f)

group

ASTM

7.0-9.3

MJ/kg (moist.mmf)

Btu/lb

ASTM

H e a t i n g value

%R

22-31*

1-2»

i-r >94'

90-97' 0.5-2 2.2-5.0*

2-8*

1-3

2-4' >4.5»

2-4»

3-5' 89-93'

m 73

H

r

>

m 73

00

3

10

4

10

2

2

2

3

4

10

4

4

3

2

2

2

3

3

Mn,Ce,Sr,U K.Mg

Na, Sr, Pb,

Mn, Ti Ca

—

Mn, Ti

__

Mn, Zn, Sr

As, Co, Cu, A other chalcophile elements

Na.Ca.Fe.Li ,Ti ,Mn,F h other lithophile elements

Common Moderate Moderate Moderate Rare Rare Moderate Rare

Common Cjmmon Common

Trace Trace Trace Minor Minor Trace Trace Trace

Abundant Minor-trace Trace

Abundant Trace Mi nor

Ν 0,L D D,F F D D D

D, L, Ν Ν D

Ν,F N,L Ν

d,s(?)

d,s(?) s d(?)

e,s s ,e s ,e

s ,e s(?) s(?) e e

n,N,F D(?) η F F

Variable Trace Trace Minor-trace Trace

Rare-common Rare-moderate Rare Rare Rare Rare-common Moderate Rare

d,e,s(' d

L,F L

Abundant Moderate

d,s(?)

d,s(?)

Common Rare

D,L

0,L

Abundant

Abundant

Common

1

Chief occurrences Genetic Physical*

Common

Concentration in min. matter

Frequency of occurrence in coal seams

*n-disseminated, L-layers (partings), N-nodules, F-fissures (cleat). First listed is the most common occurrence. **d-detrital, s-syngenetic, e-epigenetic, w-weathering. First listed is the most common occurrence; ? indicates there is divergence of opinion about the genetic occurrence.

5

4

4

4

4

2

FeOOH K(Nz)AlSi 0o ZrSi0 CaS0 .2H 0 BaS0 FeS0 .H 0 Ca (P0 ) (F,Cl,0H) NaCl

2

3

Others goethite/1imonite feldspar zi rcon sulfates: gypsum barite szomolnokite apatite halite

3

SiOo

3

3

2

Fe 0 /Fe 0 Ti0

2

CaC0 CaMg[C0 ) FeC0

x

FeS (isometric) FeSÔ (orthorhombic) Fe S Zn S PbS/CuFeS

2

a

2

Al Si 0.(0H) Mg|AlfATSi 0 (OH)

2

Al Si 0 (0H) -H 0

2

KAl (AlSi 0 )(OH)

Composition

Carbonates cal cite dolomite (ankerite) siderite Oxides quartz magneti te/hemati te rutile A anatase

kaolinite group chlorite Sulfides pyrite marcasite pyrrhotite sphalerite galena/chalcopyrite

Mineral Clay minerals i l l i t e (sericite) smectite (including mixed layered)

Common minor and trace element associations

Table II. Minerals Frequently Occurring in Coals, Their Stoichometric Compositions, Relative Abundances, and Modes of Occurrences

Publication Date: April 2, 1986 | doi: 10.1021/bk-1986-0301.ch002

>

Ci

Co

3

s

ί

C η χ

70

D

> ζ

m -
Ο

ο 500 Ι

0.000

1000 h

5.000

10.000 15.000 ENERGY (KEV)

20.000

e

Fe Pyrite

3 Ο

Kaolinite

500

Fe

0.000

5.000

10.000

15.000

ENERGY ( K E V ) Figure

2.

Continued.

20.000

Minerals

136

M I N E R A L M A T T E R A N D A S H IN C O A L

T a b l e IV.

Subjects of Supporting

Publication Date: April 2, 1986 | doi: 10.1021/bk-1986-0301.ch010

Assemblage Calcite-kaolinite Calcite-quartz Pyrite-quartz Pyrite-quartz Pyrite-calcite Pyrite-kaolinite Pyrite-kaolinite Pyrite-montmorillonite Pyrite-montmorillonite Py.-calc.-kao. Py.-cale.-mont. Py.-cale.-mont. Note:

Atmosphere inert inert oxidizing reducing reducing oxidizing reducing inert reducing reducing inert reducing

Experiments Τ

(°C) 1400 1400 1200 1200 1200 1200 1200 800 800 1200 800 800

Duration 60 60 30 30 30 30 30 15 15 30 15 15

(min)

The i n d i c a t e d temperature was the s t e a d y - s t a t e temperature f o r the t r i a l . The i n d i c a t e d d u r a t i o n was the p e r i o d o f time f o r which t h a t s t e a d y - s t a t e temperature was m a i n t a i n e d .

D i s c u s s i o n and C o n c l u s i o n s I t i s apparent t h a t the phenomena d e s c r i b e d here a r e not complete p r o c e s s e s t e r m i n a t i n g i n e q u i l i b r i u m assemblages. The times o f r e a c t i o n are too s h o r t f o r many o f the p r o d u c t s o f s i l i c a t e s such as c l a y s and q u a r t z t o come to thermodynamic e q u i l i b r i u m a t the new temperature. That t h i s i s i n d e e d the case i n o p e r a t i o n of powerp l a n t b o i l e r s i s o b v i o u s from the c o n s i d e r a t i o n o f the amount o f g l a s s found i n f u r n a c e s l a g s and f l y - a s h . I l l i t e and m o n t m o r i l l o n i t e a r e s i m i l a r i n s t r u c t u r e and d i f f e r s l i g h t l y from k a o l i n i t e i n t h i s r e g a r d . The f i r s t two a r e composed of two s i l i c o n - o x y g e n l a y e r s p e r o c t a h e d r a l l a y e r c o n t a i n i n g i r o n , magnesium and aluminum and i n k a o l i n i t e the r a t i o o f t e t r a h e d r a l and o c t a h e d r a l l a y e r s i s 1. In c l a y s t h e r m a l m o d i f i c a t i o n o c c u r s a t lower temperature t h a n s i l i c a because the bonds formed between the A l , Fe, and Mg atoms and oxygen a r e weaker than the S i - 0 bonds. C l a y s , t h e r e f o r e , a r e e x p e c t e d to be more r e a c t i v e than the s i l i c a crystals. There i s e v i d e n c e of d r a s t i c s t r u c t u r a l m o d i f i c a t i o n s , i f not v i t r i f i c a t i o n , i n the c l a y m i n e r a l domains d u r i n g t h e r m a l t r e a t ­ ment and t h a t i r o n e n t e r s the r e g i o n . F u r t h e r i n q u i r i e s are i n p r o g r e s s t o answer t h e s e q u e s t i o n s . C a l c i t e appears l a r g e l y i n e r t a t temperatures a p p r o a c h i n g 600°C i n the p r e s e n c e of some o f the c l a y m i n e r a l s and i n e r t u n t i l about 900°C i n t h e i r absence. The cause o f t h i s f l u x i n g i s not w e l l under­ s t o o d a t t h i s time, but i n v e s t i g a t i o n s a r e p l a n n e d t o e x p l a i n t h i s behavior. At whatever temperature, the r e a c t i o n o b s e r v e d i s the d e c o m p o s i t i o n o f c a l c i t e to l i m e (CaO) and c a r b o n d i o x i d e . The e x t e n t to which c a r b o n d i o x i d e i n f l u e n c e s f u r t h e r r e a c t i o n i s n o t known but must be c o n s i d e r e d as an i m p o r t a n t s t e p i n a complete explanation. L i k e c a l c i t e , p y r i t e i s q u i t e r e a c t i v e and i t s t h e r m a l b e h a v i o r i s i n f l u e n c e d by the p r e s e n c e o f c l a y m i n e r a l s . The i n i t i a l r e a c t i o n temperature o f p y r i t e a l o n e o r i n the p r e s e n c e o f c a l c i t e a l o n e i s to

10.

BIGGS A N D LINDSAY

High-Temperattire

Interactions

among Minerals

137

produce p y r r h o t i t e , ( F e ^ ^ S ) and t r o i l i t e ( F e S ) . The l o s s o f s u l f u r i s o b v i o u s and c o n t i n u e s over an a p p r e c i a b l e temperature range. The most i m p o r t a n t r e a c t i o n p r o d u c t s a r e those o f t h e i r o n enrichment o f t h e c l a y m i n e r a l r e s i d u e s , p r o b a b l y a p r e c u r s o r o f t h e i r o n o x i d e and g l a s s m i x t u r e s commonly o b s e r v e d i n s l a g s and f l y a s h e s , and t h e f o r m a t i o n o f t h e s u l f i d e o f c a l c i u m , o l d h a m i t e . Oldhamite i s o b s e r v e d i n a l l experiments where c a l c i t e and p y r i t e i n t e r a c t , whereas a n h y d r i t e i s o b s e r v e d o n l y where they have r e a c t e d i n t h e p r e s e n c e o f an o x y g e n - r i c h atmosphere. T h i s behavior suggests t h a t o l d h a m i t e , formed i n t h e r e d u c i n g p a r t o f a f l a m e , i s a n e c e s ­ sary p r e c u r s o r t o the formation o f a n h y d r i t e .

Publication Date: April 2, 1986 | doi: 10.1021/bk-1986-0301.ch010

Acknowledgments T h i s m a t e r i a l i s based, i n p a r t , on work s u p p o r t e d by t h e U.S. Bureau o f Mines, Department o f t h e I n t e r i o r , under Grant No. Gl106002. Any o p i n i o n s , f i n d i n g s , and c o n c l u s i o n s o r recommendations e x p r e s s e d i n t h i s p u b l i c a t i o n a r e those o f t h e a u t h o r s and do n o t n e c e s s a r i l y r e f l e c t t h e views o f t h e U.S. Bureau o f Mines, Department of t h e I n t e r i o r . Work on o t h e r p a r t s o f t h i s p u b l i c a t i o n was performed i n Ames L a b o r a t o r y and was s u p p o r t e d by t h e A s s i s t a n t S e c r e t a r y o f F o s s i l Energy, D i v i s i o n o f C o a l U t i l i z a t i o n t h r o u g h t h e P i t t s b u r g h Energy Technology C e n t e r , C o a l P r e p a r a t i o n Branch. Ames L a b o r a t o r y i s o p e r a t e d f o r t h e U.S. Department o f Energy by Iowa S t a t e U n i v e r s i t y under C o n t r a c t W-7405-Eng-82.

Literature Cited 1. 2. 3. 4. 5. 6. 7. 8. 9.

Mukkherjee, D. K.; Chowdhury, B. Fuel 1976, 55, 4-8. Wakely, L. D.; Davis, Α.; Jenkins, R. G.; Mitchell, G. D.; Walker, P. L., Jr. Fuel 1979, 58, 379-385. Gray, D. Fuel 1978, 57, 213-216. Moody, A, H.; Langan, D. D. Combustion 1935, 6, 13-20. Gauger, A. W. Procedures of the American Society of Testing Materials 1937, Part I, 376-401. Palmenburg, O. W. Industrial and Engineering Chemistry 1939, 31, 1058-1059. Electric Power Research Institute (EPRI) Report CS-1418, Research Project 736; prepared by Battelle, Columbus Labora­ tories, 1980. Huggins, F. E . ; Kosmack, D. Α.; Huffman, G. P. Fuel 1981, 60, 577-584. Gluskoter, H. J . Fuel 1965, 44, 285-291.

RECEIVED June 13, 1985

11 Flame Vitrification and Sintering Characteristics of Silicate Ash Erich Raask

Publication Date: April 2, 1986 | doi: 10.1021/bk-1986-0301.ch011

Technical Planning and Research Division, Central Electricity Generating Board, Leatherhead, Surrey, United Kingdom

Silicate species constitute the bulk of the mineral matter in most coals, and the formation of boiler deposits depends largely on the physical and pyrochemical changes of the ash residue constituents. In this work the mode of occurrence of coal silicate minerals, and the flame induced vitrification and sodium initiated sintering mechanisms have been studied. The pulverized coal flame temperature is sufficiently high to vitrify the quartz particles. On cooling some devitrification occurs and the rate of sintering depends largely on the ratio of glassy phase to crystalline species in the ash. The flame volatile sodium captured by the vitrified silicate particles can initiate the coalescence of deposited ash by viscous flow and the rate of sintering is markedly increased by the alkali-metal dissolved in the glassy phase. The flame i m p r i n t e d c h a r a c t e r i s t i c s o f p u l v e r i z e d c o a l ash r e l e v a n t to b o i l e r s l a g g i n g , c o r r o s i o n and e r o s i o n have been d i s c u s s e d p r e v i o u s l y (1,2). S i l i c a t e m i n e r a l s c o n s t i t u t e between 60 and 90 p e r c e n t o f ash i n most c o a l s and b o i l e r d e p o s i t s a r e l a r g e l y made up from the s i l i c i o u s i m p u r i t y c o n s t i t u e n t s . T h i s work s e t s o u t f i r s t t o examine t h e mode o f o c c u r r e n c e o f the s i l i c a t e m i n e r a l s p e c i e s i n c o a l f o l l o w e d by a c h a r a c t e r i z a t i o n assessment o f the flame v i t r i f i e d and sodium e n r i c h e d s i l i c a t e ash p a r t i c l e s . The ash s i n t e r i n g s t u d i e s a r e l i m i t e d t o i n v e s t i g a t i o n s o f the r o l e o f sodium i n i n i t i a t i n g and s u s t a i n i n g the bond f o r m i n g r e a c t i o n s t o the f o r m a t i o n o f b o i l e r d e p o s i t s . Silica

( Q u a r t z ) and S i l i c a t e M i n e r a l

Species

i n Coal

The q u a r t z and a l u m i n o - s i l i c a t e s p e c i e s found i n most c o a l s c o n s t i t u t e t h e b u l k o f combustion ash r e s i d u e . The a l u m i n o - s i l i c a t e s i n c l u d e m u s c o v i t e and i l l i t e which c o n t a i n p o t a s s i u m , and k a o l i n i t e species (3-6). The s i l i c a (S1O2) and a l u m i n a (AI2O3) as d e t e r m i n e d by c h e m i c a l a n a l y s i s a r e p r e s e n t i n a l u m i n o - s i l i c a t e s on an average 0097-6156/ 86/ 0301 -0138S06.00/ 0 © 1986 American Chemical Society

11.

RAASK

Characteristics

of Silicate

139

Ash

w e i g h t r a t i o o f 1.5 t o 1 as r e p o r t e d by Dixon e t a l . ( 6 ) . The excess of s i l i c a r e p r e s e n t s the amount o f q u a r t z i n c o a l m i n e r a l m a t t e r :

(sio )

(sio ) - 1, 5 ( A 1

2

2

t

2

0 )

(1)

3

Where ( S i 0 ) q , ( S i 0 ) c and (A1 0^) denote r e s p e c t i v e l y the q u a r t z , t o t a l s i l i c a and alumina c o n t e n t s o f a s h . An approximate amount o f p o t a s s i u m a l u m i n o - s i l i c a t e s i n c o a l m i n e r a l m a t t e r can be o b t a i n e d from the p o t a s s i u m o x i d e ( K 0 ) c o n t e n t of a s h . The amount o f n o n - s i l i c a t e p o t a s s i u m s p e c i e s i s s m a l l i n most c o a l s and t h e s i l i c a t e m i n e r a l s c o n t a i n on average 11 p e r c e n t K 0 by w e i g h t ( 6 ) . Thus the p o t a s s i u m a l u m i n o - s i l i c a t e c o n t e n t o f c o a l mineral matter ( K ^ L - S I L ) w e i g h t p e r cent i s : 2

2

2

2

2

KO

Publication Date: April 2, 1986 | doi: 10.1021/bk-1986-0301.ch011

Κ

A L - S I L

c m

=

9

·

1

κ

(2)

2 °

where K 0 denotes the p o t a s s i u m o x i d e c o n t e n t o f a s h . The t o t a l amount o f s i l i c a t e m i n e r a l s e q u a l s a p p r o x i m a t e l y the sum o f S1O2, A 1 0 ^ and K 0 i n ash, and an e s t i m a t e o f k a o l i n i t e s p e c i e s i s thus g i v e n b y : 2

2

Kaolinite

2

=

(Si0

2

+ A1 0 2

3

+ K 0 ) - (Quartz + P o t . 2

Silicates) (3)

T a b l e I g i v e s the S i 0 , A 1 0 ^ and K 0 c o n t e n t s o f some US and B r i t i s h b i t u m i n o u s c o a l asnes (4,7) w h i c h were used t o c a l c u l a t e the approximate amounts o f q u a r t z , p o t a s s i u m a l u m i n o - s i l i c a t e and k a o l i n i t e species i n the mineral matter. 2

Table I .

2

2

E s t i m a t e d Amounts o f S i l i c a t e S p e c i e s i n Bituminous Coal M i n e r a l Matter Mineral species (weight per cent)

Ash c o n s t i t u e n t s (weight p e r cent of ash) Type o f c o a l Si0

o

A 1

2°3

KoO

Quartz

P o t . alum, silicates

Kaolinite

Low silica

British U.S.

31.1 29.2

18.1 14.2

1.2 1.5

3.9 7.9

10.9 13.6

26.2 23.6

Medium silica

British U.S.

46.5 46.6

22.8 27.8

2.8 1.1

12.3 4.9

25.5 10.0

34.3 60.6

High silica

British U.S.

55.5 56.5

30.0 32.2

2.7 2.6

10.5 8.0

24.5 23.6

53.2 59.7

T a b l e I shows t h a t the k a o l i n i t e s p e c i e s c o n s t i t u t e up t o 60 p e r c e n t o f the c o a l m i n e r a l m a t t e r . The amount o f p o t a s s i u m a l u m i n o - s i l i c a t e s , c h i e f l y m u s c o v i t e and i l l i t e i s between 10 and 25 p e r c e n t , and the q u a r t z c o n t e n t i s u s u a l l y below 12 p e r c e n t . The a l u m i n o - s i l i c a t e minerals contain frequently i r o n , calcium, magnesium and sodium as p a r t replacement f o r p o t a s s i u m and p a r t l y

Publication Date: April 2, 1986 | doi: 10.1021/bk-1986-0301.ch011

140

M I N E R A L M A T T E R A N D A S H IN C O A L

i n c o r p o r a t e d i n the k a o l i n i t e s t r u c t u r e . A l s o , the s i l i c a t e m i n e r a l s o c c u r as h y d r a t e d s p e c i e s w i t h the i n h e r e n t water c o n t e n t o f between 2 to 5 p e r c e n t , thus the s i l i c i o u s m i n e r a l c o n t e n t s are l i k e l y to be about 5 p e r c e n t h i g h e r than those g i v e n i n T a b l e I . The s i l i c a and a l u m i n a c o n t e n t s o f the f i r s t two samples a r e e x c e p t i o n a l l y low f o r bituminous c o a l ashes. The u s u a l c o n c e n t r a t i o n range o f s i l i c a i s 35 t o 55 p e r c e n t and t h a t o f alumina i s 20 to 30 p e r c e n t , t h u s the a l u m i n o - s i l i c a t e s p e c i e s t o g e t h e r w i t h q u a r t z c o n s t i t u t e between 60 t o 90 per cent o f b i t u m i n u s c o a l m i n e r a l m a t t e r . The s i l i c a t e s p e c i e s o c c u r i n c o a l c h i e f l y as s e p a r a t e s t r a t a and l a r g e p a r t i c l e i n c l u s i o n s , and t h i s mode o f o c c u r r e n c e i s termed the " a d v e n t i t i o u s " m i n e r a l m a t t e r . F i g u r e l a shows a t y p i c a l sample o f the a d v e n t i t i o u s s i l i c a t e m i n e r a l p a r t i c l e s , d e n s i t y s e p a r a t e d from p u l v e r i z e d c o a l . The d e n s i t y s e p a r a t i o n t e c h n i q u e does n o t remove the s m a l l s i l i c a t e p a r t i c l e s , c h i e f l y a l u m i n o - s i l i c a t e s p e c i e s , the " i n h e r e n t " m i n e r a l matter, i n the c o a l substance ( F i g u r e l b ) . The ash c o n t e n t o f b i t u m i n o u s c o a l s d e l i v e r e d to u t i l i t y power s t a t i o n i s u s u a l l y between 10 and 25 p e r cent ( 4 , 8 ) . About 25 p e r c e n t o f the ash i s p r e s e n t i n the form o f i n h e r e n t m i n e r a l m a t t e r o f d i s p e r s e d s m a l l p a r t i c l e s and a l s o as m i n e r a l elements r e a c t e d w i t h the c o a l s u b s t a n c e . The m i n e r a l elements can be h e l d i n the c o a l s u b s t a n c e as o r g a n o - m e t a l l i c s a l t s , and a l s o as a r e s u l t o f m o l e c u l a r a d s o r p t i o n and c o - v a l e n t b o n d i n g . The m i n e r a l s p e c i e s d i s s o l v e d i n c o a l pore w a t e r , c h i e f l y c h l o r i d e s can a l s o be c o n s i d e r e d as p a r t o f the i n h e r e n t matter. The l i g n i t e s and sub-bituminous c o a l s can have a h i g h f r a c t i o n o f the m i n e r a l elements, c h i e f l y sodium, c a l c i u m and a l s o aluminium and i r o n c h e m i c a l l y combined i n the f u e l s u b s t a n c e (9,10). The c h e m i c a l r e a c t i v i t y and p o r o s i t y o f the f u e l m a t r i x decrease w i t h the i n c r e a s e o f c o a l age from l i g n i t e t o b i t u m i n o u s rank. The l o s s o f c a r b o x y l , h y d r o x y l and quinone b o n d i n g s i t e s i n the f u e l m a t r i x r e s u l t s i n a low " c h e m i c a l " m i n e r a l m a t t e r c o n t e n t of bituminous c o a l s . C h l o r i d e i n C o a l Pore and

Seam Water

C h l o r i d e m i n e r a l s a r e r a r e l y found i n c o a l i n the form o f s o l i d s p e c i e s because o f h i g h s o l u b i l i t y o f sodium, c a l c i u m and t r a c e m e t a l c h l o r i d e s i n coal s t r a t a waters. The " i n h e r e n t " water c o n t e n t o f c o a l i s r e l a t e d t o i t s p o r o s i t y and thus the m o i s t u r e c o n t e n t o f l i g n i t e d e p o s i t s can exceed 40 p e r cent d e c r e a s i n g to below 5 p e r cent i n f u l l y b i t u m i n o u s c o a l s ( 1 1 ) . C h l o r i d e s , c h i e f l y a s s o c i a t e d w i t h sodium and c a l c i u m c o n s t i t u t e the b u l k o f w a t e r - s o l u b l e m a t t e r i n B r i t i s h b i t u m i n o u s c o a l s ( 1 2 ) . S k i p s e y (13) has found t h a t the d i s t r i b u t i o n o f c h l o r i n e c o a l s was c l o s e l y r e l a t e d t o the s a l i n i t y o f mine w a t e r s . Hypersaline b r i n e s with concentrations of d i s s o l v e d s o l i d s up to 200 kg m"** o c c u r i n s e v e r a l o f the B r i t i s h C o a l f i e l d s . The mode o f f o r m a t i o n o f h y p e r s a l i n e b r i n e s has been d i s c u s s e d by the o s m o t i c f i l t r a t i o n through c l a y and s h a l e d e p o s i t s . The s a l i n i t y o f the b r i n e ground waters i n c r e a s e s w i t h depth and when they are i n c o n t a c t w i t h f u e l b e a r i n g s t r a t a , c o r r e s p o n d i n g l y more c h l o r i d e i s taken up by the f u e l . However, a c c o r d i n g t o S k i p s e y (13) the h i g h rank b i t u m i n o u s c o a l s because o f t h e i r low p o r o s i t y are unable t o take up l a r g e amounts o f the c h l o r i d e and a s s o c i a t e d c a t i o n s , and the c h l o r i n e c o n t e n t r a r e l y exceeds 0.2 p e r c e n t . The

11.

RAASK

Characteristics

of Silicate

141

Ash

c h l o r i n e c o n t e n t of low rank b i t u m i n o u s c o a l s can r e a c h one p e r cent and c o r r e s p o n d i n g l y the sodium f r a c t i o n a s s o c i a t e d w i t h c h l o r i n e w i l l amount up t o 0.4 p e r cent o f c o a l . That i s , the ash from a h i g h c h l o r i n e c o a l can c o n t a i n up t o 3 per cent o f flame v o l a t i l e sodium. The c h l o r i n e c o n t e n t o f l i g n i t e s and sub-bituminous c o a l s i s u s u a l l y low, below 0.1 p e r c e n t , and sodium i s h e l d c h i e f l y i n the f u e l s u b s t a n c e i n the form of organo-metal components (9,10). A l l c o a l s c o n t a i n some sodium combined i n the a l u m i n o - s i l i c a t e s p e c i e s which w i l l remain l a r g e l y i n v o l a t i l e i n the f l a m e . The r a t i o o f the s i l i c a t e sodium to n o n - s i l i c a t e sodium v a r i e s o v e r a wide r a n g e . The a l k a l i - m e t a l i s p r e s e n t c h i e f l y i n the s i l i c a t e s i n low c h l o r i n e bituminous c o a l s . In the h i g h c h l o r i n e b i t u m i n o u s c o a l s and i n many l i g n i t e s and sub-bituminous c o a l s i t i s p r e s e n t m a i n l y i n a flame v o l a t i l e form.

Publication Date: April 2, 1986 | doi: 10.1021/bk-1986-0301.ch011

Flame V i t r i f i c a t i o n o f S i l i c a

Minerals

A c h a r a c t e r i s t i c f e a t u r e of flame h e a t e d ash i s t h a t the p a r t i c l e s are s p h e r i c a l i n shape as shown i n F i g u r e 2. The t r a n s f o r m a t i o n o f the a n g u l a r s i l i c a t e m i n e r a l p a r t i c l e s i n p u l v e r i z e d c o a l to s p h e r i c a l p a r t i c l e ash i s a r e s u l t of the s u r f a c e t e n s i o n f o r c e a c t i n g on the v i t r i f i e d s p e c i e s . The s t r e s s ( f ) on a n o n - s p h e r i c a l s u r f a c e s e c t i o n of the p a r t i c l e i s : f

=

2γ/ρ

(4)

where γ i s the s u r f a c e t e n s i o n of g l a s s y s i l i c a t e and ρ i s the radius of curvature. I t i s e v i d e n t from E q u a t i o n (4) t h a t the s t r e s s i s i n v e r s e l y p r o p o r t i o n a l to the r a d i u s o f c u r v a t u r e and thus the s m a l l sharp-edged p a r t i c l e s are f i r s t to take a s p h e r i c a l form. F r e n k e l (15) has shown t h a t time ( t ) r e q u i r e d t o t r a n s f o r m an a n g u l a r p a r t i c l e to sphere i s g i v e n to f i r s t a p p r o x i m a t i o n by: -t/Z

r

=

r

/CN (5)

ζ

=

4irnr /γ ο

(6)

ο

where

and r i s the d i s t a n c e of a p o i n t on the o r i g i n a l s u r f a c e from the c e n t e r o f a sphere of e q u i v a l e n t volume h a v i n g r a d i u s r , η i s the v i s c o s i t y and γ i s the s u r f a c e t e n s i o n . E q u a t i o n (5) can be used t o c a l c u l a t e the approximate time r e q u i r e d f o r a p a r t i c l e to assume a s p h e r i c a l shape when the s u r f a c e t e n s i o n , v i s c o s i t y , s i z e and i n i t i a l shape o f p a r t i c l e are known. A l t e r n a t i v e l y , an e s t i m a t e of the v i s c o s i t y f o r the change t o take p l a c e , can be made when the r e s i d e n c e time o f p a r t i c l e s at a g i v e n temperature i s known. T a b l e I g i v e s the c a l c u l a t e d v a l u e s o f v i s c o s i t y when the time f o r the change i s one second. I t was assumed t h a t the t h i c k n e s s o f moving s u r f a c e l a y e r was about ten p e r cent o f the r a d i u s , and the s u r f a c e t e n s i o n o f f u s e d ash was t a k e n Q

-ι

to be

0.32

N m

'

as measured p r e v i o u s l y

(16).

142

MINERAL

Table I I .

MATTER

A N D A S H IN C O A L

Calculated V i s c o s i t i e s f o r Spheridization of D i f f e r e n t Size S i l i c a t e P a r t i c l e s

r a d i u s (μπι) Viscosity (N s m )

χ

7

2

5

x

1 Q

6

2

>

5

x

1 Q

5

2

.5 x 10

4

2.3 χ 1 0

3

Publication Date: April 2, 1986 | doi: 10.1021/bk-1986-0301.ch011

Δ

T a b l e I I shows t h a t the s m a l l i r r e g u l a r l y shaped p a r t i c l e s t r a n s f o r m t o spheres i n c o a l flame when the v i s c o s i t y o f the m a t e r i a l i s s e v e r a l o r d e r s h i g h e r than t h a t r e q u i r e d f o r b u l k f l o w under g r a v i t y , which i s about 25 N s m~^. A l a b o r a t o r y t e c h n i q u e was used to d e t e r m i n e t h e minimum temperature a t w h i c h c o a l m i n e r a l s p e c i e s are t r a n s f o r m e d to s p h e r i c a l shapes ( 1 7 ) . P a r t i c l e s o f 10 t o 200 ym i n d i a m e t e r were i n t r o d u c e d i n t o a gas s t r e a m and then p a s s e d t h r o u g h a v e r t i c a l furnace. The temperature o f the f u r n a c e was v a r i e d from 1175 t o 2025 Κ and was measured by a r a d i a t i o n pyrometer and by thermocouples p l a c e d i n the f u r n a c e . The r e s i d e n c e time o f p a r t i c l e s i n the f u r n a c e was between 0.2 and 0.5 s e c . depending on the p a r t i c l e size. F i g u r e 3a shows a s u r f a c e - f u s e d s i l i c a t e p a r t i c l e h e a t e d t o a temperature some 25 Κ lower than t h a t r e q u i r e d f o r i t s s p h e r i d i z a t i o n . F i g u r e 3b shows a s p h e r i d i z e d p a r t i c l e h e a t e d i n the l a b o r a t o r y furnace. F i g u r e 4 shows the temperature range a t w h i c h the shape change o f d i f f e r e n t c o a l m i n e r a l p a r t i c l e s o c c u r r e d . The c h l o r i t e m i n e r a l c o n t a i n s some q u a r t z and the two s p e c i e s s p h e r i d i z e d a t markedly d i f f e r e n t temperatures as shown by c u r v e s D^ and D2. The t e m p e r a t u r e o f m i n e r a l p a r t i c l e s i n the p u l v e r i z e d c o a l flame exceeds 1800 Κ ( F i g u r e 5 ) , and i t i s t h e r e f o r e t o be e x p e c t e d t h a t a l l p a r t i c l e s w i t h the e x c e p t i o n o f l a r g e s i z e q u a r t z w i l l v i t r i f y and change t o s p h e r i c a l shapes. F i g u r e 6a shows a s u r f a c e - f u s e d b u t n o n - s p h e r i c a l q u a r t z p a r t i c l e found i n a sample o f f l y ash c a p t u r e d i n the e l e c t r o s t a t i c p r e c i p i t a t o r . O c c a s i o n a l l y e l o n g a t e d e l l i p o s o i d a l p a r t i c l e s o f a l u m i n o - s i l i c a t e s ( F i g u r e 6b) can be found i n the ash i n d i c a t i n g t h a t t h e h i g h temperature r e s i d e n c e time was too s h o r t f o r complete s p h e r i d i z a t i o n . However, the m a j o r i t y o f the ash p a r t i c l e s appear t o be s p h e r i c a l as shown i n F i g u r e 2. The s p h e r i c a l s i l i c a t e ash p a r t i c l e s , when viewed a t c l o s e - u p range appear t o h o s t a l a r g e number o f sub-micron p a r t i c l e s a t the surface (Figure 6c). The m i c r o i d s c o u l d be s i l i c a t e c r y s t a l l o i d s p r e c i p i t a t e d from the v i t r i f i e d phase o r s u l p h a t e fume p a r t i c l e s formed from the n o n - s i l i c a t e c o a l m i n e r a l s ( 1 8 ) . The l a t t e r a r e s o l u b l e i n a d i l u t e a c i d (HC1) s o l u t i o n and F i g u r e 6d shows the a c i d etched p a r t i c l e s . C l e a r l y , most o f the m i c r o i d p a r t i c l e s were d i s s o l v e d and the l e a c h s o l u t i o n c o n t a i n e d sodium and p o t a s s i u m sulphates. A n o t h e r d i a g n o s t i c t e s t f o r s i l i c a t e a s h i s to t r e a t the p a r t i c l e s w i t h h y d r o f l u o r i c (HF) a c i d s o l u t i o n (18-20). The a c i d w i l l d i s s o l v e the g l a s s y phase r e v e a l i n g s k e l e t o n s o f c r y s t a l l i n e s p e c i e s w h i c h may be i n the form o f m u l l i t e n e e d l e s ( F i g u r e 6e) o r q u a r t z c r y s t a l l o i d s ( F i g u r e 6 f ) . The r a t i o o f the g l a s s y phase t o c r y s t a l l i n e s p e c i e s v a r i e s from p a r t i c l e t o p a r t i c l e depending on

Publication Date: April 2, 1986 | doi: 10.1021/bk-1986-0301.ch011

RAASK

Characteristics

of Silicate

Ash

F i g u r e 1. Mineral matter i n c o a l . inherent white p a r t i c l e s .

Figure 2.

Figure

3.

particles.

Surface

fused

(a) A d v e n t i t i o u s ;

Pulverized fuel

(a) and

(b)

ash.

spheoidized

(b)

silicate

144

M I N E R A L MATTER A N D A S H IN C O A L

Publication Date: April 2, 1986 | doi: 10.1021/bk-1986-0301.ch011

2100

25

50

75

100

PARTICLE RADIUS, μιπ F i g u r e 4. S p h e r i c a l shape t r a n s f o r m a t i o n of g r a n u l a r m i n e r a l s i n h o t gas streams: A, i l l i t e ; B, m u s c o v i t e ; C, n a t i v e q u a r t z ; D, c h l o r i t e . Reproduced w i t h p e r m i s s i o n from r e f e r e n c e 2. C o p y r i g h t 1985 I t e m i s p h e r e P u b l i s h i n g Corp.

11.

RAASK

Characteristics

145

of Silicate Ash

2000

Publication Date: April 2, 1986 | doi: 10.1021/bk-1986-0301.ch011

UJ ce

ce UJ CL Σ UJ

ιοοο I -

10

20

TIME, s F i g u r e 5. Temperature/time p l o t f o r ash p a r t i c l e s i n a 500 MW pulverized coal f i r e d b o i l e r : 0.1 ym ( t o p curve t o 100 \\m (lower curve) s i z e s . A, combustion and heat exchange chambers; B, e l e c t r o s t a t i c p r e c i p i t a t o r s and chimney. Reproduced w i t h p e r m i s s i o n from r e f e r e n c e 2. C o p y r i g h t 1985 Hemisphere P u b l i s h i n g Corp.

M I N E R A L M A T T E R A N D A S H IN C O A L

Publication Date: April 2, 1986 | doi: 10.1021/bk-1986-0301.ch011

146

Figure 6. D i a g n o s t i c f e a t u r e s of flame heated ash. a, unfused q u a r t z p a r t i c l e ; b, e l o n g a t e d s i l i c a t e p a r t i c l e s ; c, m i c r o i d s on ash; d, a c i d c l e a n e d ash; e, m u l l i t e n e e d l e s i n ash; and f , quartz c r y s t a l l o i d s .

11.

RAASK

Characteristics

147

of Silicate Ash

the o r i g i n a l c o m p o s i t i o n o f the s i l i c a t e m i n e r a l s , the c a p t u r e o f v o l a t i l e sodium and the r a t e o f c o o l i n g o f f l u e gas borne a s h . The flame i m p r i n t e d c h a r a c t e r i s t i c s o f s i l i c a t e m i n e r a l s p e c i e s from the p o i n t o f view o f subsequent s i n t e r i n g a r e summarized i n T a b l e I I I . Table I I I .

Vitrification

Particle

Publication Date: April 2, 1986 | doi: 10.1021/bk-1986-0301.ch011

Constituent species

Quartz Kaolinite Potassium alumino­ silicates

and R e c r y s t a l l i z a t i o n o f S i l i c a t e s

vitrification

Temperature range (K)

Extent

1700 t o 1900 1600 t o 1700 1400 t o 1600

Medium High High

Recrystallization tendency

Low High Low

Glass content

Medium Medium High

The r e l a t i v e amount o f c o a l m i n e r a l q u a r t z s u r v i v i n g i n the p u l v e r i z e d f u e l flame depends on the p a r t i c l e s i z e and temperature. In the i n t e n s e combustion o f c y c l o n e f i r e d b o i l e r s the flame temperature exceeds 2000 Κ and the q u a r t z p a r t i c l e s o f a l l s i z e s w i l l vitrify. Some q u a r t z p a r t i c l e s i n the c r y s t a l l i n e form w i l l s u r v i v e the flame t r e a t m e n t i n p u l v e r i z e d c o a l f i r e d b o i l e r s and the ash may c o n t a i n 25 p e r c e n t o f the o r i g i n a l c o a l q u a r t z i n the c r y s t a l l i n e form ( 2 1 ) . The k a o l i n i t e m i n e r a l s p e c i e s i n c o a l c o n t a i n some sodium, c a l c i u m and i r o n i n the c r y s t a l l i n e s t r u c t u r e (6) and the p r e s e n c e of f l u x i n g m e t a l s enhances v i t r i f i c a t i o n o f the flame h e a t e d particles. The h i g h temperature c r y s t a l l i n e form o f k a o l i n i t e s p e c i e s i s m u l l i t e and the c h a r a c t e r i s t i c n e e d l e shapes o f m u l l i t e ( F i g u r e 6e) a r e f r e q u e n t l y found i n l a r g e , above 5 ym d i a m e t e r particles. The m u l l i t e n e e d l e c r y s t a l s i n ash a r e always embedded i n a g l a s s y phase o f the l a r g e p a r t i c l e s and i t appears t h a t the s m a l l , below 5 ym d i a m e t e r p a r t i c l e s o f the flame h e a t e d k a o l i n i t e s p e c i e s a r e n o t e x t e n s i v e l y r e c r y s t a l l i z e d on c o o l i n g . The c r y s t a l l i n e s p e c i e s o f i l l i t e and m u s c o v i t e a r e n o t found i n the flame h e a t e d ash and thus i t i s l i k e l y t h a t the p o t a s s i u m a l u m i n o ­ s i l i c a t e s remain on c o o l i n g l a r g e l y i n the form o f g l a s s y p a r t i c l e s . The i n h e r e n t s i l i c a t e ash ( F i g u r e l b ) w i l l c o a l e s c e on combustion f i r s t to a s i n t e r e d m a t r i x i n s i d e the b u r n i n g c o a l p a r t i c l e and a l s o to s m a l l s l a g g l o b u l e s a t the s u r f a c e o f coke residue. F i g u r e 7a shows the s l a g g l o b u l e s on a coke p a r t i c l e s e p a r a t e d from p u l v e r i z e d c o a l ash and F i g u r e 7b shows a l a c e s k e l e t o n o f s i n t e r e d ash i n a n o t h e r coke p a r t i c l e r e v e a l e d a f t e r combustion a t 900 K. D u r i n g combustion o f the m i n e r a l r i c h c o a l p a r t i c l e s i n t h e p u l v e r i z e d f u e l flame, ash e n v e l o p e s may be c r e a t e d which can take the form o f censopheres as shown i n F i g u r e 7c and d. The gas b u b b l e e v o l u t i o n l e a d i n g t o cenosphere f o r m a t i o n (16,22) and f l y ash u s u a l l y c o n t a i n s between 0.1 and 2 p e r c e n t by w e i g h t o f the l i g h t ­ w e i g h t a s h . The m i n e r a l r i c h c o a l p a r t i c l e s may l e a v e t h e combustion a s h r e s i d u e a l s o i n the form o f p l e r o s p h e r e ( s p h e r e s -

M I N E R A L M A T T E R A N D A S H IN C O A L

Publication Date: April 2, 1986 | doi: 10.1021/bk-1986-0301.ch011

148

F i g u r e 7. C o a l e s c e n c e p r o d u c t s of i n h e r e n t ash i n flame. a, ash p a r t i c l e s on coke; b, ash s k e l e t o n i n coke; c, cenospheres; d, f r a c t u r e s cenosphere; and e, p l e r o s p h e r e .

11.

RAASK

Characteristics

of Silicate

149

Ash

i n s i d e - s p h e r e ) as shown i n F i g u r e 7e. The above examples show t h a t the i n h e r e n t s i l i c a ash p a r t i c l e s undergo e x t e n s i v e c o a l e s c e n c e by s i n t e r i n g and s l a g g i n g d u r i n g combustion o f the h o s t c o a l p a r t i c l e s However, the a d v e n t i t i o u s ash r e t a i n the p a r t i c l e i d e n t i t y i n the flame and the p r o c e s s e s o f s i n t e r i n g and s l a g g i n g take p l a c e a f t e r d e p o s i t i o n on b o i l e r tubes. T r a n s f e r o f Flame V o l a t i l e Sodium to

Silicates

Publication Date: April 2, 1986 | doi: 10.1021/bk-1986-0301.ch011

The c o a l sodium o r i g i n a l l y p r e s e n t as c h l o r i d e and o r g a n o - m e t a l l i c compounds i s r a p i d l y v o l a t i l i z e d i n the p u l v e r i z e d c o a l flame ( 2 3 ) . S u b s e q u e n t l y the v o l a t i l e s p e c i e s are p a r t l y d i s s o l v e d i n the s u r f a c e l a y e r o f flame h e a t e d s i l i c a t e p a r t i c l e s and p a r t l y s u l p h a t e d i n the f l u e gas ( 8 ) . The f o r m a t i o n o f sodium s u l p h a t e can p r o c e e d v i a two routes : Route 1 - In the F l u e

Gas

+ S0 + | 0J+2HC1 + N a S 0 — : vapour phase r e a c t i o n s

|2Na

+ H0 2

2X + /„ 2NaX O X T

v

2

2

v

Route 2 - At the S u r f a c e Ash

2X + 2NaX

V

a

P

o

2

u

r

Na S0 fume . •% particles 2

4

Particles

-, • + H qJ->2HCl + N a 0 ^ N a 0 + S 0

|2Na

4

2

phase r e a c t i o n s

2

Reactions

2

1°2 + | Na S0 2

a t ash

4

surface

Route 1 f o r g e n e s i s o f sodium s u l p h a t e fume can be d e s c r i b e d as the n o n - c a p t i v e f o r m a t i o n and r o u t e 2 as the c a p t i v e f o r m a t i o n . Some p o t a s s i u m s u l p h a t e can a l s o be formed v i a the two r o u t e s . P o t a s s i u m i s p r e s e n t i n c o a l c h i e f l y i n the form o f p o t a s s i u m a l u m i n o - s i l i c a t e s ( T a b l e I) and a l a r g e p a r t of the a l k a l i - m e t a l w i l l remain i n v o l a t i l e i n the flame h e a t e d s i l i c a t e p a r t i c l e s . Some 5 t o 20 p e r c e n t o f the p o t a s s i u m i s r e l e a s e d f o r s u l p h a t i o n (24) which takes p l a c e p a r t l y at the s u r f a c e o f the p a r e n t p a r t i c l e s (25) and p a r t l y v i a the v o l a t i l i z a t i o n r o u t e s as d e s c r i b e d above. However, sodium s u l p h a t e c o n t e n t o f f l y ash h e a t e d i n p u l v e r i z e d c o a l flame, and chimney s o l i d s i s always h i g h e r than t h a t of p o t a s s i u m s u l p h a t e . The d i s t r i b u t i o n o f the flame v o l a t i l e sodium between the ash s i l i c a t e and s u l p h a t e phases i s markedly i n f l u e n c e d by the temperature and r e s i d e n c e time o f the ash p a r t i c l e s i n the f l a m e . The h i g h temperature o f l a r g e b o i l e r flame reduces the v i s c o s i t y o f v i t r i f i e d s i l i c a t e p a r t i c l e s and as a r e s u l t a l a r g e f r a c t i o n o f the v o l a t i l e sodium i s d i s s o l v e d i n the s i l i c a t e phase. On average 60 p e r c e n t o f the sodium i s d i s s o l v e d i n the s i l i c a t e ash p a r t i c l e s ^ the remainder b e i n g p r e s e n t as s u l p h a t e fume p a r t i c l e s i n the f l u e gas ( 8 ) . The

Mechanism and Measurements o f Sodium Enhanced

Sintering

The f o r m a t i o n o f s i n t e r e d ash d e p o s i t s on b o i l e r tubes r e q u i r e s f i r s t a c l o s e , m o l e c u l a r d i s t a n c e c o n t a c t between the p a r t i c l e s f o l l o w e d by a growth o f p a r t i c l e - t o - p a r t i c l e b r i d g e s c h i e f l y by v i s c o u s f l o w . Sodium s u l p h a t e phase t o g e t h e r w i t h some p o t a s s i u m s u l p h a t e may p l a y

MINERAL

150

MATTER

A N D A S H IN C O A L

a s i g n i f i c a n t r o l e i n the i n i t i a l s t a g e o f s i n t e r i n g by b r i n g i n g t h e s i l i c a t e p a r t i c l e s t o g e t h e r as a r e s u l t o f s u r f a c e t e n s i o n . Sodium s u l p h a t e m e l t s a t 1157 Κ b u t mixed a l k a l i - m e t a l s u l p h a t e s can form a m o l t e n phase a t lower temperatures ( 2 6 ) . Once the c l o s e c o n t a c t between the s i l i c a t e p a r t i c l e s has been e s t a b l i s h e d a v i s c o u s f l o w o f the p a r t i c l e s u r f a c e l a y e r can commence and the s i n t e r bonds a r e e s t a b l i s h e d a c c o r d i n g t o E q u a t i o n 7, as d i s c u s s e d by F r e n k e l ( 1 5 ) :

^

Publication Date: April 2, 1986 | doi: 10.1021/bk-1986-0301.ch011

r

m

2

h

i

(7) U

2nr

)

where χ i s the r a d i u s o f neck growth between the s p h e r i c a l p a r t i c l e s o f r a d i u s r , γ i s the s u r f a c e t e n s i o n , η i s the v i s c o s i t y o f f u s e d ash, and t i s the t i m e . The ( x / r ) ^ r a t i o can be t a k e n as a c r i t e r i o n o f the degree o f s i n t e r i n g , i . e . t h e s t r e n g t h o f b o i l e r d e p o s i t ( s ) d e v e l o p e d i n time t , t h a t i s :

s

and

the r a t e o f d e p o s i t s t r e n g t h development i s : ds_ dt

=

3ky 2nr

/q\ K

J

where k i s a c o n s t a n t . E q u a t i o n (9) shows t h a t the r a t e o f a s h s i n t e r i n g , i . e . the development o f c o h e s i v e s t r e n g t h o f a d e p o s i t m a t r i x i s p r o p o r t i o n a l to the s u r f a c e t e n s i o n and i n v e r s e l y p r o p o r t i o n a l t o the v i s c o s i t y . The s u r f a c e t e n s i o n and p a r t i c l e s i z e a r e n o t markedly changed by d i s s o l u t i o n o f sodium, i r o n o r c a l c i u m o x i d e s i n the g l a s s y phase o f s i l i c a t e a s h . However, t h e v i s c o s i t y i s markedly changed by the oxides. I n p a r t i c u l a r , an e n r i c h m e n t o f sodium i n the s u r f a c e l a y e r o f t h e s i l i c a t e ash p a r t i c l e s can l e a d t o a h i g h r a t e o f s i n t e r i n g . Some o f the flame v o l a t i l e sodium i s d i s s o l v e d i n the v i t r i f i e d s i l i c a t e a s h p a r t i c l e s b e f o r e d e p o s i t i o n and an a d d i t i o n a l amount o f sodium i s t r a n s f e r r e d from the s u l p h a t e t o s i l i c a t e phases d u r i n g sintering. The r e a c t i o n between sodium s u l p h a t e and s i l i c a t e s a t a s h s i n t e r i n g temperatures has been m o n i t o r e d by t h e r m o - g r a v i m e t r i c measurements. Some o f t h e r e s u l t s a r e g i v e n i n T a b l e IV. Table

IV.

Weight Loss o f S u l p h a t e

Loss i n i t i a t i o n temperature (K)

Na S0, 2 4 o

1425

v

CaSO^

Na S0, .. Kaolin

Na S0, 2 4 Ash

CaSO^

1085

1175

>1525

2

Sample

and S u l p h a t e / S i l i c a t e M i x t u r e s

o

Kaolin 1275

CaSO. 4 Ash 1275

Anhydrous s u l p h a t e samples and the s u l p h a t e / s i l i c a t e m i x t u r e s (50 p e r c e n t by w e i g h t s u l p h a t e ) were h e a t e d i n a i r a t the r a t e o f 6 Κ per m i n u t e .

11.

RAASK

The sulphate l o s s due

Characteristics

Publication Date: April 2, 1986 | doi: 10.1021/bk-1986-0301.ch011

Ash

151

r e s u l t s i n T a b l e IV show the r e a c t i o n between sodium and k a o l i n commenced a t 1085 Κ as i n d i c a t e d by the weight t o r e l e a s e o f SO2 and SO3:

Al 0 .2Si0 2

of Silicate

3

2

+ xNa S0 2

4

-> x N a O . A l 0 . 2 S i 0 2

2

3

2

+ x ( S 0 , S0 )+ 2

3

(10)

A t y p i c a l b i t u m i n o u s c o a l ash r e q u i r e d a h i g h e r temperature o f 1175 Κ f o r the s u l p h a t e d e c o m p o s i t i o n r e a c t i o n , i n d i c a t i n g t h a t the ash s i l i c a t e s p e c i e s were l e s s r e a c t i v e than k a o l i n m i n e r a l o f s m a l l ( Ο

System · , Δ 2-A

FeO - A l 0

- Si0

2

FeO - N a 0 - S i 0

2

2

Ο 2-B A 2-C -

3

2

FeO - CaO - S i 0

2

5

I

I Q|

I I

I

I

• 2-D

Na 0 - A l 0

•

CaO - A l 0

2

2-E

2

2

I I I '

'

ι

« I

3

3

- Si0 - Si0

» '

2

2

J J

'

Molar basicity Figure 4. Measured s u l f i d e c a p a c i t i e s o f c a n d i d a t e s l a g s . B a s i c i t y c a l c u l a t e d from c h e m i c a l a n a l y s e s . P o i n t s denoted by W/P2O5 and W/B2O3, r e p r e s e n t s l a g s i n which 5% S i 0 was r e p l a c e d by 5% o f t h e s e o x i d e s . P o i n t denoted by w/CaO i s f o r s l a g t o which 5% C a F was added, but a l l f l u o r i n e was l o s t d u r i n g e x p e r i m e n t . P o i n t denoted by w/MgO f o r s l a g t o which 12% MgO was added. 2

2

178

M I N E R A L M A T T E R A N D A S H IN C O A L

Publication Date: April 2, 1986 | doi: 10.1021/bk-1986-0301.ch013

glass. The o t h e r phase had a m e t a l l i c appearance and w i l l be r e f e r r e d t o as t h e "matte phase". The s u l f i d e c a p a c i t i e s f o r t h e s e s l a g s s h o u l d be c o n s i d e r e d as " a p p a r e n t " because t h e s u l f i d e c a p a c i t y i s d e f i n e d f o r a s i n g l e l i q u i d phase. The matte phase i n a l l o f t h e s e double-phased s l a g s c o n t a i n e d 27-31% S, w h i l e t h e g l a s s phase c o n t a i n e d from 0.2 t o 13% S. X-ray d i f f r a c t i o n a n a l y s e s showed t h e g l a s s phase t o be amorphous and t h e matte phase t o c o n t a i n FeS and FeS2* Discussion. S l a g c o m p o s i t i o n s 2 - A - l , 2-A-7, 2-B-2, 2 - C - l , 2-D-2, and 2 - E - l were c l o s e s t t o t h e c o a l ash c o m p o s i t i o n g i v e n i n T a b l e I , c o n t a i n i n g 25-38% a d d i t i v e . As seen from T a b l e IV, l o g Cg ranged from a p p r o x i m a t e l y -3.8 t o -5.5 at 1100°C. U s i n g t h e b a s i c i t y o f t h e ash c a l c u l a t e d from T a b l e I and t h e d a t a shown i n F i g u r e 4, t h e s u l f i d e c a p a c i t y f o r pure ash i s e s t i m a t e d t o be a p p r o x i m a t e l y l o g Cg = -5.2. This i s quite low as compared t o r e s u l t s o b t a i n e d f o r s l a g s c o n t a i n i n g s i g n i f i c a n t quantities of additives. As w i l l be demonstrated l a t e r i n t h e r e p o r t , s u l f u r c a p t u r e d by c o a l ash s l a g w i t h t h i s s u l f i d e c a p a c i t y would be i n s i g n i f i c a n t even at v e r y f a v o r a b l e c o n d i t i o n s - v e r y low oxygen p o t e n t i a l and low t e m p e r a t u r e . There i s a g e n e r a l c o r r e l a t i o n between s u l f i d e c a p a c i t y and b a s i c i t y f o r a g i v e n system, as shown i n F i g u r e 4. There i s a s h a r p drop i n s u l f i d e c a p a c i t y between b a s i c i t i e s o f 1.0 t o 0.5, which c o r r e s p o n d s t o t h e m e t a s i l i c a t e t o d i s i l i c a t e c o m p o s i t i o n s in a binary s i l i c a t e . F o r a g i v e n b a s i c i t y , systems 2-A (FeO) and 2-C (FeO, CaO) have s i g n i f i c a n t l y h i g h e r s u l f i d e c a p a c i t i e s t h a n systems 2-D (Na20) and 2-E (CaO), so t h a t f o r a g i v e n b a s i c i t y , FeO i s s u p e r i o r t o CaO and Na20 as an a d d i t i v e . T h i s i s not what would be expected c o n s i d e r i n g t h e s t a n d a r d f r e e e n e r g i e s o f f o r m a t i o n o f t h e s u l f i d e s and o x i d e s o f Fe, Ca, and Na. C o n s i d e r i n g standard f r e e energies f o r the formation of m e t a l s u l f i d e s from m e t a l o x i d e s , FeO and CaO s h o u l d be a p p r o x i m a t e l y e q u i v a l e n t d e s u l f u r i z e r s and N a 0 s h o u l d be superior. However, s l a g s a r e f a r from i d e a l s o l u t i o n s because o f t h e s t r o n g i n t e r a c t i o n s among s p e c i e s — p a r t i c u l a r l y with Si0 . T h i s i s why e x p e r i m e n t a l measurements o f s u l f i d e c a p a c i t i e s were needed. F r e e energy o f m i x i n g d a t a (24) f o r N a 0 , CaO, and FeO b i n a r y s i l i c a t e s show t h a t t h e c h e m i c a l i n t e r a c t i o n w i t h s i l i c a d e c r e a s e s i n t h e o r d e r N a 0 , CaO, FeO, and f o r a g i v e n b a s i c i t y , t h e a c t i v i t y o f t h e b a s i c o x i d e i n t h e s i l i c a t e s i n c r e a s e i n t h e o r d e r Na20, CaO, and FeO. On t h i s b a s i s , FeO s h o u l d be a b e t t e r d e s u l f u r i z e r t h a n CaO or N a 0 . T h i s i s c o n s i s t e n t with the present r e s u l t s . Not s u r p r i s i n g l y , t h e m e t a l o x i d e - s i l i c a i n t e r a c t i o n i s a major f a c t o r i n t h e d e s u l f u r i z a t i o n a b i l i t y of the s l a g . S e v e r a l m o d i f i c a t i o n s o f s l a g s based on t h e F e O - A l 0 3 - S i 0 2 system were t e s t e d t o determine i f a l e s s e x p e n s i v e a d d i t i v e c o u l d be s u b s t i t u t e d f o r some o f t h e i r o n o r i f a d d i t i v e s c o u l d be used t o reduce l i q u i d u s t e m p e r a t u r e s . F i g u r e 4 shows t h a t r e p l a c i n g a p o r t i o n o f t h e i r o n o x i d e i n s l a g s of the F e O - A l 0 3 - S i 0 system w i t h CaO (5 wt %) or MgO (12 wt %) had no e f f e c t on t h e s u l f i d e c a p a c i t i e s . Results f o r c o m p o s i t i o n 2 - C - l a l s o support t h i s c o n c l u s i o n , because f o r 2

2

2

2

2

2

2

2

DEYOUNG

Sulfur Solubility

179

in Slags

t h i s c o m p o s i t i o n a p p r o x i m a t e l y 14% o f t h e FeO o f an e q u i v a l e n t c o m p o s i t i o n i n t h e F e O - A l 0 3 - S i 0 2 system was r e p l a c e d by CaO, w i t h o n l y a s l i g h t d e c r e a s e i n s u l f i d e c a p a c i t y . Replacement o f p o r t i o n s o f t h e S 1 O 2 i n F e O - A l 2 0 3 - S i 0 s l a g s by B 0 3 or P 0 5 has no e f f e c t on s u l f i d e c a p a c i t i e s ; t h e r e f o r e , these a d d i t i v e s are p o t e n t i a l l y u s e f u l f o r reducing s l a g l i q u i d u s temperatures. F i g u r e 5 compares some r e s u l t s o f t h i s study t o t h o s e found i n t h e l i t e r a t u r e f o r s i m i l a r s l a g s at h i g h e r t e m p e r a t u r e s . These l i t e r a t u r e d a t a were a d j u s t e d t o a b a s i c i t y o f 1.65 u s i n g d a t a g i v e n i n F i g u r e 4. The d a t a from t h i s study a r e q u i t e consistent with the l i t e r a t u r e data. The l i n e a r i t y o f t h e s u l f i d e c a p a c i t y w i t h i n v e r s e temperature i s c o n s i s t e n t w i t h t h e theoretical relationship, 2

2

Publication Date: April 2, 1986 | doi: 10.1021/bk-1986-0301.ch013

2

2

L J ^ i

=

- AH° /R * H^o/R - H? /* e

eS

(3)

Ο

where Δ Η 1/2 and

.

i s t h e s t a n d a r d e n t h a l p y change f o r t h e S (g)

+ FeO

HÎ?

and

2

λ

FeO

1/2

=

H*J

0 ( g ) + FeS 2

reaction, (4)

„

FeS

a r e t h e p a r t i a l molar e n t h a l p i e s o f m i x i n g o f FeO and FeS i n t h e slag. T h i s r e l a t i o n s h i p can be d e r i v e d from E q u a t i o n 2, t h e e q u i l i b r i u m c o n s t a n t f o r E q u a t i o n 4, and t h e G i b b s - H e l m h o l t z equation. Most o f t h e s l a g s t e s t e d were found t o have been p a r t i a l l y or c o m p l e t e l y melted at 1100°C. However, at 1000°C, a l l s l a g s from t h e F e O - A l 0 3 - S i 0 2 system were not m o l t e n . It is thought t h a t f o r t h e s e c o m p o s i t i o n s t h e e n t i r e a d d i t i v e r e a c t e d w i t h s u l f u r s p e c i e s i n t h e atmosphere w h i l e none r e a c t e d w i t h t h e S 1 O 2 or w i t h o t h e r components i n t h e ash. Hence, s u l f i d e c a p a c i t i e s measured from t h i s experiment are not t r u e s u l f i d e c a p a c i t i e s of the s l a g s . Another p o i n t which s u p p o r t s t h i s i s t h a t t h e "measured" s u l f i d e c a p a c i t i e s at 1000°C are g r e a t e r t h a n t h o s e at 1100°C, w h i l e F i g u r e 5 shows t h e o p p o s i t e t r e n d for r e s u l t s f o r molten s l a g s . A l s o , o t h e r l i t e r a t u r e d a t a show t h a t s u l f i d e c a p a c i t i e s g e n e r a l l y i n c r e a s e w i t h temperature. T h i s p o i n t s out an i n h e r e n t d i s a d v a n t a g e i n u s i n g a c o a l ash s l a g f o r d e s u l f u r i z a t i o n . When s i l i c a r e a c t s w i t h t h e d e s u l f u r i z i n g agent, e.g., l i m e , t h e e f f e c t i v e n e s s o f t h e d e s u l f u r i z i n g compound i s g r e a t l y r e d u c e d . Hence, i t i s d e s i r a b l e t o d e s i g n a d e s u l f u r i z i n g combustor i n which t h e ash does not r e a c t w i t h t h e d e s u l f u r i z i n g m a t e r i a l . 2

E v a l u a t i o n Of A P i l o t

Combustor

Calculations. The measured s u l f i d e c a p a c i t i e s were used t o e s t i m a t e s u l f u r e m i s s i o n s from a s t a g e d , s l a g g i n g , c y c l o n e combustor o p e r a t i n g c l o s e t o e q u i l i b r i u m , and t o determine t h e

180

MINERAL

MATTER

A N D A S H IN

COAL

Publication Date: April 2, 1986 | doi: 10.1021/bk-1986-0301.ch013

e f f e c t s o f v a r i o u s o p e r a t i n g v a r i a b l e s on the s u l f u r r e m o v a l . To c a l c u l a t e s u l f u r e m i s s i o n s , an e q u a t i o n f o r g a s - s l a g c h e m i c a l e q u i l i b r i u m f o r s u l f u r ( E q u a t i o n 2) and a mass b a l a n c e f o r s u l f u r are s o l v e d s i m u l t a n e o u s l y . F i r s t , t h e e q u i l i b r i u m gas c o m p o s i t i o n s were c a l c u l a t e d f o r t h e combustion o f c o a l w i t h a i r f o r a range o f s u l f u r c o n c e n t r a t i o n s i n t h e c o a l . T h i s was done u s i n g A l c o a ' s C h e m i c a l E q u i l i b r i u m Computer Program ( 2 3 ) . Next, the c o n c e n t r a t i o n s o f s u l f u r i n t h e s l a g s f o r e q u i l i b r i u m w i t h t h e combustion gases were c a l c u l a t e d . F i n a l l y , t h e q u a n t i t y o f a d d i t i v e s needed t o o b t a i n t h e s e c o m p o s i t i o n s were c a l c u l a t e d from s u l f u r mass b a l a n c e s . Results. F i g u r e 6 shows an example o f t h e r e s u l t s f o r t h e s e c a l c u l a t i o n s , f o r combustion w i t h 55% o f s t o i c h i o m e t r i c a i r ( s t a g e 1) at 1100°C. T h i s i s an i s o t h e r m a l c a l c u l a t i o n , i . e . , b o t h s l a g and gas t e m p e r a t u r e s are assumed t o be 1100°C. O b v i o u s l y , t h i s cannot o c c u r i n p r a c t i c e , but t h e r e s u l t s o f t h e c a l c u l a t i o n s h o u l d p r o v i d e an upper bound f o r s u l f u r r e m o v a l . A r e a s o n a b l e g o a l f o r the s u l f u r c a p t u r e , c o n s i d e r i n g p r o j e c t i o n s o f f u t u r e EPA r e g u l a t i o n s , i s 70%. The s l a g mass can v a r y between 85 and 350 g/kg c o a l ( t h e upper l i m i t was e s t a b l i s h e d from a heat b a l a n c e f o r A l c o a ' s p i l o t combustor), so t h e n e c e s s a r y l o g Cg f o r a 70% s u l f u r removal i s between -2.75 and -3.3. A s u l f i d e c a p a c i t y of l o g C « -3.3 at 1100°C was o b t a i n e d f o r c e r t a i n s l a g s based on t h e FeO-Al203-Si02 system, e.g., c o m p o s i t i o n s 2-A-3 or 2-A-10. T h i s shows t h a t 70% s u l f u r removal i s t h e r m o d y n a m i c a l l y p o s s i b l e . As t h e combustion s t o i c h i o m e t r y i s d e c r e a s e d , t h e c u r v e s i n F i g u r e s 6 are r o t a t e d c o u n t e r c l o c k w i s e about t h e o r i g i n , i . e . , t h e s u l f u r removal i s i n c r e a s e d . An i n c r e a s e i n temperature w i l l have t h e o p p o s i t e e f f e c t . The c u r v e s are r o t a t e d c l o c k w i s e about t h e o r i g i n . However, f o r a p a r t i c u l a r s l a g c o m p o s i t i o n t h e s u l f i d e c a p a c i t y i n c r e a s e s w i t h t e m p e r a t u r e , as shown i n F i g u r e 5. The net r e s u l t o f t h e two o p p o s i n g e f f e c t s ( u s i n g t h e temperature b e h a v i o r shown i n F i g u r e 5) i s t h a t t h e s u l f u r removal d e c r e a s e s w i t h i n c r e a s i n g t e m p e r a t u r e . I n t h e range o f c o a l - s u l f u r c o n t e n t s i n v e s t i g a t e d , 2-6%, t h e f r a c t i o n o f s u l f u r removed by s l a g does not change w i t h s u l f u r content i n t h e coal. The t o t a l s u l f u r e m i t t e d i n c r e a s e s w i t h i n c r e a s i n g s u l f u r c o n c e n t r a t i o n i n t h e c o a l , but the s u l f u r removal by t h e s l a g also increases. s

A f i n a l p o i n t t o note r e g a r d i n g s u l f u r removal i s t h a t as the c o n c e n t r a t i o n o f hydrogen i n t h e combustion gases i s d e c r e a s e d , t h e s u l f u r removal by t h e s l a g w i l l i n c r e a s e . This i s due t o t h e h i g h s t a b i l i t y o f t h e h y d r o g e n - s u l f u r s p e c i e s , such as H S ( g ) , as compared t o t h e c a r b o n - s u l f u r s p e c i e s , such as COS. Thus, d r y i n g and c h a r r i n g o f c o a l would s i g n i f i c a n t l y i n c r e a s e t h e t h e o r e t i c a l removal o f s u l f u r by t h e s l a g . These c a l c u l a t i o n s assume g a s - s l a g e q u i l i b r i u m w i t h r e s p e c t to sulfur. T h i s i s p r o b a b l y o n l y approached at t h e g a s - s l a g s u r f a c e near t h e e x i t o f t h e f i r s t s t a g e . At t h e e n t r a n c e end o f t h e combustor, the c o n d i t i o n s would p r o b a b l y be more o x i d i z i n g t h a n c o n d i t i o n s c a l c u l a t e d from the o v e r a l l combustion s t o i c h i o m e t r y , ff and t h u s s u l f u r s o l u b i l i t y i n t h e s l a g would 2

9

DE YOUNG

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in Slags

-T(°C) 1600

1400

I

I

1200 I I

I

I

1000 I

Molar basicity = 1.65 -

-2

—

• Present study • Fincham & Richardson.

-3

Publication Date: April 2, 1986 | doi: 10.1021/bk-1986-0301.ch013

log C

8

= 2.55 - 8330/T

-4 I

I

I

I

5

I

I

I

I

I

I

6

I

I

I I

7

8

l/T x 1 0 ( K" ) 4

e

1

F i g u r e 5. Comparison o f r e s u l t s from t h i s study t o t h o s e o f Fincham and R i c h a r d s o n (6) f o r an i r o n s i l i c a t e w i t h molar b a s i c i t y o f 1.65· Data from Fincham and R i c h a r d s o n were f o r pure i r o n s i l i c a t e s w h i l e t h e s e from t h e p r e s e n t study c o n t a i n e d some c o a l a s h .

0 Ε 0

I

I

I

I

I

I

1—1

100 200 300 400 500 600 700 Slag mass, g/kg coal

F i g u r e 6. E q u i l i b r i u m s u l f u r removal by s l a g f o r a combustor o p e r a t i n g w i t h L o v e r i d g e Seam (West V i r g i n i a ) coal.

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182

M I N E R A L M A T T E R A N D A S H IN C O A L

be l e s s t h a n t h a t c a l c u l a t e d . At some depth below t h e s l a g s u r f a c e near t h i s e n t r a n c e end o f t h e f i r s t s t a g e , t h e c o n d i t i o n s would be more r e d u c i n g t h a n t h o s e c a l c u l a t e d from t h e o v e r a l l combustion s t o i c h i o m e t r y . T h i s would r e s u l t i n increased s u l f u r s o l u b i l i t y . The a c t u a l combustion p r o c e s s and s u l f u r removal p r o c e s s e s a r e q u i t e complex, and t h e extent o f s u l f u r removal w i l l depend on t h e combustion k i n e t i c s . For example, c o n s i d e r two extreme s i t u a t i o n s . I n one, where most c o a l i s combusted a f t e r i t h i t s t h e s l a g g e d w a l l , s u l f u r removal s h o u l d be r e l a t i v e l y good. I n t h e o t h e r extreme, where a l l t h e c o a l i s combusted b e f o r e i t r e a c h e s t h e s l a g g e d w a l l , s u l f u r removal would be r e l a t i v e l y poor because i t would be dependent on mass t r a n s p o r t through t h e gas phase, and t h e gas has a r e l a t i v e l y short residence time. In summary, t h e k i n e t i c s o f t h e combustion p r o c e s s i s important w i t h r e g a r d t o s u l f u r removal. The k i n e t i c s must be c o n s i d e r e d e i t h e r by m o d e l l i n g or e x p e r i m e n t a t i o n b e f o r e a f i n a l judgment on d e s u l f u r i z a t i o n i n a s l a g g i n g , c y c l o n e combustor can be made. The r e s u l t s o f t h i s study show t h a t i t i s theoretically possible. Conclusions S u l f i d e c a p a c i t y measurements o f r e l a t i v e l y low m e l t i n g ( a p p r o x i m a t e l y 1100°C i n most c a s e s ) s l a g s based on t h e FeO-Al203-SiC>2, FeO-Na20-Si0 , FeO-CaO-Si0 , N a 2 0 - A l 0 3 - S i 0 , and C a O - A l 2 0 3 - S i 0 systems but composed o f c o a l ash + a d d i t i v e s , have shown t h a t t h e F e O - A l 0 3 - S i 0 - b a s e d s l a g s had t h e h i g h e s t s u l f i d e c a p a c i t i e s . For a given b a s i c i t y , t h e s u l f i d e c a p a c i t i e s c o u l d be ranked i n t h e f o l l o w i n g o r d e r : FeO-Al 03-Si0 > FeO-CaO-Si0 > FeO-Na20-Si02 > CaO-Al203-Si02 > N a 0 - A l 0 3 - S i 0 2 . The c h e m i c a l i n t e r a c t i o n o f t h e b a s i c o x i d e s w i t h s i l i c a appears t o be a dominant f a c t o r c o n t r o l l i n g t h e s u l f i d e c a p a c i t y . There was good c o r r e l a t i o n between s u l f i d e c a p a c i t y and s l a g b a s i c i t y , and s u l f i d e c a p a c i t i e s i n c r e a s e d with temperature. C a l c u l a t i o n s o f t h e e q u i l i b r i u m s u l f u r removal f o r a commercial combustor u s i n g t h e measured s u l f i d e c a p a c i t i e s , showed t h a t i t was t h e o r e t i c a l l y p o s s i b l e t o remove 70% o r more of t h e s u l f u r i n c o a l . The s u l f u r removal i n c r e a s e s w i t h d e c r e a s i n g temperature, d e c r e a s i n g combustion s t o i c h i o m e t r y i n t h e f i r s t s t a g e o f t h e b u r n e r , i n c r e a s i n g s l a g f l o w , and d e c r e a s i n g c o n t e n t o f hydrogen i n t h e f u e l . T h i s work showed t h a t a s l a g g i n g , c y c l o n e combustor can remove s u l f u r i n t o t h e s l a g , but k i n e t i c m o d e l l i n g and/or e x p e r i m e n t a t i o n i s needed t o prove whether o r not t h e concept w i l l work. 2

2

2

2

2

2

2

2

2

2

2

2

Acknowledgment T h i s work was sponsored by t h e U.S. Department o f Energy under C o n t r a c t No. DE-ACO7-78CS40037, " P u l v e r i z e d C o a l F i r i n g o f Aluminum M e l t i n g F u r n a c e s . "

13.

DEYOUNG

Sulfur Solubility

in Slags

183

Publication Date: April 2, 1986 | doi: 10.1021/bk-1986-0301.ch013

Literature Cited 1. Ε. M. Levin, C. R. Robbins, and H. F. McMurdie: Phase Diagrams for Ceramists, American Ceramic Society, Columbus, OH (1964). 2. Ε. M. Levin, C. R. Robbins, and H. F. McMurdie: Phase Diagrams for Ceramists, 1969 Supplement, American Ceramic Society, Columbus, OH (1969). 3. Ε. M. Levin and H. F. McMurdie: Phase Diagram for Ceramists, 1975 Supplement, American Ceramic Society, Columbus, OH (1975). 4. R. S. Roth, T. Negas, and L. P. Cook: Phase Diagrams for Ceramists, Volume IV, American Ceramic Society, Columbus, OH (1981). 5. G. R. St. Pierre and J. Chipman: Trans. AIME, v. 206, pp. 1474-1483 (1956). 6. C.J.B. Finchman and F. D. Richardson: Proc. R. Soc. Series A223, 40 (1954) and J. Iron Steel Ins., 178, 4 (1954). 7. F. D. Richardson: Physical Chemistry of Melts in Metallurgy, v. 2, pp. 291-304, Academic Press, London (1974). 8. P. T. Carter and T. G. McFarlane: J. Iron Steel Inst., 185, 54 (1957). 9. K. P. Abraham, M. W. Davies, and F. D. Richardson: J. Iron Steel Inst., v. 196, pp. 309-312 (1960). 10. K. P. Abraham and F. D. Richardson: J. Iron Steel Inst., v. 196, pp. 313-317 (1960). 11. E . W. Dewing and F. D. Richardson: J. Iron Steel Inst., v. 194, 194, pp. 446-450 (1960). 12. R. A. Sharma and F. D. Richardson: J. Iron Steel Inst., v. 198, pp. 386-390 (1961). 13. R. A. Sharma and F. D. Richardson: J. Iron Steel Inst., v. 200, pp. 373-379 (1962). 14. G.J.W. Kor and F. D. Richardson: J. Iron Steel Inst. v. 206, pp. 700-704 (1968). 15. G.J.W. Kor and F. D. Richardson: Trans. AIME, v. 245, pp. 319-327 (1969). 16. R. J. Hawkins, S. G. Meherali, and M. W. Davis: J. Iron Steel Inst., v. 209, pp. 646-657 (1971). 17. A. Bronson and G. R. St. Pierre, Met. Trans. Β, v. 10B, pp. 375-380 (1979). 18. A. Bronson and G.R. St. Pierre: Met. Trans. Β, v. 12B, pp. 729-731 (1981). 19. H. B. Bell: Can. Met. Quart., v. 20 (2), pp. 169-179 (1981). 20. J. D. Shim and S. Ban-ya: Tetsu-to-Hagane, v. 68, pp. 251-60 (1982). 21. S. D. Brown, R. J. Roxburgh, I. Ghita, and H. B. Bell: Ironmaking and Steelmaking, v. 9, pp. 163-67 (1982). 22. I. Ghita and H. B. Bell: Ironmaking and Steelmaking, v. 9, pp. 239-43 (1982). 23. W. E . Wahnsiedler: "Chemical Equilibrium Package Users Manual," Alcoa Laboratories Report No. 7-78-25. 24. D. R. Gaskell: Metallurgical Treatises, J. K. Tien and J. F. E l l i o t t , ed., p. 60, TMS-AIME, Warrendale, PA (1981). RECEIVED July 23, 1985

14 Analyses of Thermodynamic Properties of Molten Slags Milton Blander and Arthur D . Pelton

Publication Date: April 2, 1986 | doi: 10.1021/bk-1986-0301.ch014

1

2

1

Argonne National Laboratory, Argonne, IL 60439

2

Ecole Polytechnique, Université de Montreal, Montreal, PQ H 3 C 3A7, Canada

A novel technique has been developed for the simultaneous analysis of all thermodynamic data (including liquidus phase diagrams, activities of components, free energies of formation of compounds, volatilities, miscibility gaps, etc.) on highly ordered binary solutions such as silicates. The results for a number of silicate and aluminate solutions lead to an accurate description of all reliable solution data on a given binary and, because of the nature of the equations used for the analysis, they also provide a reliable means for interpolating or extrapolating outside the range of the data. When silica is the only acid component, a simple combining rule appears to lead to good predictions of the properties of multicomponent systems based solely on data for the binary systems. Concepts developed for molten salts lead to correlations of our results with fundamental physical properties and interactions of ions in terms of ionic radii, charge, polarizabilities, dispersion interactions, and ligandfieldeffects. Silica based slag systems are highly ordered liquids which have been a difficult class of materials on which to perform thermodynamic analyses. (1-5) T o our knowledge, no satisfactory, self-consistent prior method of analysis has been developed for systems as ordered and complex as silicates which incorporates all known data in a meaningful way. In this paper, we discuss the results of a method of analysis which permits one to simultaneously analyze a large amount of different types of data on binary systems. The calculations lead to a small set of parameters which permit one to calculate the thermodynamic properties of slag solutions as a function of temperature and composition. T h e thermodynamic self-consistency and the form of the equations used provide some confidence in the use of the results for interpolations and extrapolations outside the range of data. In addition, for systems in which silica is the only acid constituent, we propose a theoretically justified combining rule to calculate the properties of ternary systems based solely on data for the three subsidiary binaries. The results are in good agreement with available data. The ionic nature of molten silicates suggests that many of the theories and correlations developed for molten salts (6) can be applied to the development of correlations

0097-6156/ 86/ 0301 -0186506.00/ 0 © 1986 American Chemical Society

14.

Thermodynamic

BLANDER AND PELTON

Properties

of Molten

Slags

187

between the relative magnitudes of the deviations from ideal solution behavior in terms of ionic radii, charges, polarizabilities, dispersion interactions and ligand field effects. Calculational Method (7) T h e molar free energy of mixing, AG

of a silicate is represented by the expression

m

AG

= Σ

m

RTXilnXi

+ Σ

$

RTXilnv

= AG%

eal

+ G

(1)

E

t

where X,· is the mole fraction of component i, 7,·, the activity coefficient, represents deviations from ideal solution behavior of component i, AG\*

eal

of mixing of a hypothetical ideal solution and G

is the molar free energy

is the molar excess free energy of

E

mixing which represents the deviations from ideality of the molar free energy of solution. The conventional representation of Ιη7,· and G

is a power series in mole fractions.

B

Publication Date: April 2, 1986 | doi: 10.1021/bk-1986-0301.ch014

T h e complexity of ordered solutions would require a very long power series in order to obtain a reasonable representation of their properties. This arises from the tendency of such solutions to have a " V " shaped dependence of the enthalpy of mixing and an " m " shaped dependence of the entropy of mixing on concentration. (1) T h e fitting of data using a long polynomial will generally be poor and ambiguous in such systems. In order to obtain reasonable fits, one must use equations which inherently have the concentration and temperature dependence of ordered solutions built in. We have deduced a set of equations with such properties based on empirical modifications of the quasi-chemical theory. (7^8) For binary systems the excess free energy of mixing is given by the expression

G

E

=

(aX

t

+

Λ

bX )

K-\

2

+ 2y

x

vAK

K-\

+ l)

+ 2y

2

(2)

υο(Κ + 1 )

where X,· is the mole fraction of component i, and the values of a and b are equal to 0.34435 times the cationic charge of components 1 and 2, respectively. T h e quantities y - are equivalent fractions which for component 2 is given by the expression y = bX2/(aXi + b X ) and Κ is given by the equation t

2

2

Κ =

1 + 4yiy l**p(W /RT) 2

l2

- l] / 1

2

(3)

where W is an energy which is takeo to be dependent on temperature and is written as a polynominal in concentration J 2

Wi2=

Σ (

* -

Λ

Τ

β

* ) ^

(4)

where no more than four values of k chosen between 1 and 7 were found to be needed and where we generally take the component 2 to be more acid than component 1. A l l these quantities define the number of the 1-1, 2-2, and 1-2 bonds in the quasichemical model, n , n 2, and n i . T h e total number of bonds to each component 1 cation is 2a and to component 2, 2b so that if n, is the number of moles of component i, then n

2

2

απχ = n

n

+ n /2 12

(5)

188

MINERAL

6fl

Π22 +

=

2

and if the fraction of ij bonds, X,*y =

n#/ Σ

n

MATTER

A N D A S H IN C O A L

Λ12/2 »i> ^

n e n

Xii=yi-Xi2/2

(6)

and the distribution of bonds is governed by the equation

(X\ lX X ) 2

xl

=

22

4cxp(-W /RT)

=

12

a\

2

(7)

Publication Date: April 2, 1986 | doi: 10.1021/bk-1986-0301.ch014

T h e solutions to Eqs. (6) and (7) can be used to calculate the molar excess free energy from the equation

^

-

r

f

^

+ IXift,^

(8)

which is equivalent to E q . (2). T h e equations are somewhat more complex for ternary systems with the two equations of E q . (6) replaced by the three equations

Xn=Vi-Xi2/2-JW2 X

2

2

=

y

2

-X

1

/2-X 3/2

2

2

Xzz =

y*-Xis/2-X s/2 2

where now, for example, y 3 = cXs/(aXi + b X equations replacing E q . (7) of the form

(XyXiiXjj)

(9)

-

4

+ cXj). In addition there are three

2

4cxp(-W /RT)

=

i}

(10)

By substituting E q . (9) into E q . (10), one obtains three simultaneous equations with three unknowns which can be solved numerically. T h e power series representation of the three values of W,y can be approximated in several ways. For systems with only one acid component (e.g., silica) and with the remaining components being basic, a logical representation is the "asymmetric approximation." If we choose S1O2 to be component 1, then we set

W12 — c + ciKi + c y\ + · · · 2

0

H i r

S

=

(11)

cO + c' y -r-c yî + - . . 1

1

2

and

^23 =

^0 + ^ 2 3

+#23 +

· "

where t,y = yj/(y% + yy). T h e coefficients c - contain a constant and a temperature dependent term. t

14.

BLANDER AND

Thermodynamic

PELTON

Properties

of Molten

189

Slags

T h e molar excess free energy of the ternary system is given by the expression

g R

= a tn*±

+ cX tn^f

+ bX tn^f

Xl

2

(12)

z

y\

T

vl

vl

W i t h the equations we have given, one can calculate the properties of a ternary system based on the properties of the three subsidiary binaries. This approximation leads to very good representations of ternary data. (8) For binary systems, the parameters hy and sy are deduced from a complex optimization procedure which performs a global and simultaneous analysis of all thermodynamic data on a system. This includes liquidus phase diagrams, activity data, data on miscibility gaps, enthalpies of fusion, free energies of formation of compounds, etc. T h e small set of resultant parameters (seven at most in the systems considered including

Publication Date: April 2, 1986 | doi: 10.1021/bk-1986-0301.ch014

temperature coefficients) are then used to recalculate the input data to double check the accuracy of the curve fitting procedure as well as the efficacy of the use of the equations for representing the data. The results were generally very good. For multicomponent systems, we used the asymmetric approximation given in E q . ( l l ) . When silica is the only acid component, the predictions based on this approximation were in very good agreement with measured data. (8) A partial theoretical justification for such a method can be based on theories for ternary systems. (8-9) Results of Thermodynamic Analyses We have performed analyses of ten of the fifteen binary systems and six of the twenty ternary systems containing the components M g O , FeO, C a O , N a 0 , A l O s , and S i 0 . 2

(7^8) ( T a b l e

2

2

I) Table I Systems Which Have Been Analyzed Binary Systems CaO-Si0

2

FeO-Si0

2

MgO-Si0

Ternary Systems

CaO-A10i.5

CaO-A10!. -Si0

NaO .5-AlOi.5

NaO .5-CaO-SiO

MgO-FeO

NaOo.s-AlOLs-SiOjî

5

0

0

2

NaO .s-SiO

2

MgO-CaO

CaO-FeO-Si0

AlO .s-SiO

2

CaO-FeO

CaO-MgO-Si0

0

0

MgO-FeO-Si0

2

2

2

2

2

We illustrate our calculations for one ternary system below. T h e analysis of the three binary subsystems and the ternary system C a O - F e O - S i 0 was performed using 2

as input the liquidus phase diagram, (10)

activities of C a O , (11)

free energies of formation of C a S i O j and C a S i 0 4 , (13) 2

and S i 0

2

(12) , the

and the miscibility gap

in the C a O - S i 0 system, measured activities of FeO in the C a O - F e O system, (15) 2

the activities of FeO, (16-18) the phase diagram (19) of F e S i 0 (13) 2

4

(M) and

and the free energy of formation

in the F e O - S i 0 system. To illustrate some of the results, we exhibit 2

(1) the calculated phase diagram of the F e O - S i 0 system in Fig. 1 along with measured 2

values of the invariant points and (2) a comparison of activities of "FeO" measured in the iron saturated molten F e O - S i 0 system with calculated values in Fig. 2. 2

Using our asymmetric combining rules, the data for the binary systems were combined and led to the results for ternary systems given in Fig. 3; this figure illustrates

190

MINERAL MATTER AND ASH IN COAL

2 0 0 0

1

1900

___(

1600

—

1

1

) V A L U E S

1

F R O M

1

ELLIOTT a

I

ι

1 7 0 0

ι \

/ "

ι

G L E I S E R

1723 (1723)

/

1 6 7 8(Ι69Θ)

W

/ 0 . 5 6

=j

/

1 6 0 0—

S

1 5 0 0

£

1 4 0 0

/

1379 ^

^

\

1 4 7 0( 1 4 7 0 )

—

/ ( 0 . 4 2 )

(1369) M U A N

—

(0.97)_

/ 0 . 4 3 2

/(I379)R0BIE

1217 1300

0.955

(0.55)

—

/

(I2I7ÎR0BIE/

—

Nil205) MUAN/

Publication Date: April 2, 1986 | doi: 10.1021/bk-1986-0301.ch014

1200

1100

—

I I 7 7 ( I I 7 7 ) \ ^ 11 0.236 (0.235)

1

J

1179( 1 1 7 8 )

—

0.362 (0.3715)

1

WEIGHT

1

1

1

1

F R A C T I O N -

Figure 1. Calculated phase diagram of the F e O - S i 0 system. Numbers in parentheses are measured values from Muan and Osborne (1Q) and Robie, et al. (13) 2

S I 0

2

(WEIGHT

PERCENT)

Figure 2. Activities of F e O measured in iron saturated molten FeO-Si02 at 1325°C ( Δ ) (17) , 1785°C (O) (10), 1880°C ( O ) ( 1 £ ) , and 196O°C (•) (1£) . T h e two solid lines represent calculated points at 1325°C and 1880°C. T h e filled circles along one solid line represent individual calculated points and the three filled circles labeled I960, 1880, and 1785 represent calculated points at three temperatures and fixed composition which illustrate the calculated temperature dependence. u

n

Publication Date: April 2, 1986 | doi: 10.1021/bk-1986-0301.ch014

14.

BLANDER AND

PELTON

Thermodynamic

Properties

of Molten

Slags

191

Figure 3. Activities of FeO in iron saturated C a O - F e O - S i 0 at 1550°C. Dashed lines 2

are from Timucin and Morris ( 2 Q ) and the solid lines represent our calculations.

192

M I N E R A L MATTER A N D A S H IN C O A L

the correspondence between calculated and measured values (2Q) of the activities of FeO in the CaO-FeO-Si02 system. T h e differences are well within the uncertainties in the measurements. We find that this method essentially permits us to make predictions in ternary systems based solely on data for the three subsidiary binary systems for cases in which silica is the only acid component. When alumina and silica were both present, a more complex representation was necessary. T h e good correspondence of calculations with the complex concentration dependence of activities in the CaO-FeO-Si02 system illustrates the fact that our equations properly take into account the kinds of ternary interaction terms known to exist in such systems. (8,9,21)

This feature lends confidence in the use of our equations for

predictions in multicomponent systems (containing only silica as an acid component) based solely upon the subsidiary binaries. If, as it appears, this is generally true, our method provides an important predictive capability.

Publication Date: April 2, 1986 | doi: 10.1021/bk-1986-0301.ch014

Correlations of Properties Theories and concepts which have been developed for molten salt solutions can be used to correlate the thermodynamic properties of silicates. (fi) Coulomb interactions lead to a dependence of thermodynamic functions on the inverse of the cation-anion interatomic distance. Thus, by analogy with molten salts, one would expect a linear dependence of the magnitudes of free energies of mixing on this parameter which is in a direction such that negative deviations from ideality increase in the order Li"*", Na+, K , R b , C s , and M g , C a , S r , and Ba++. In addition, solutions with monovalent alkali oxides should exhibit more negative deviations from ideality than those with divalent alkaline earth oxides. The polarizability of oxide anions leads to an additional contribution with a similar dependence on cations. T h e magnitude of cationcation dispersion interactions are related to the polarizabilities, ionization potentials and interaction distances. Thus, the dissolution of oxides of cations with large dispersion interactions leads to a loss of this negative energy and hence to a positive contribution to deviations from ideality. In addition ligand field effects for divalent transition metals tend to contribute to negative deviations from ideal solution behavior in molten salts with monovalent cations. The effective charge of Si is greater than two and one would thus expect a positive contribution to deviations from ideal solution behavior from this source. The effect for M n which has a half filled shell for example, should be much less positive than for F e . +

+

+

+ +

+ +

+ +

2 +

2 +

The data for testing these influences on solution behavior are too sparse to reach quantitative conclusions. However, the general trends are in the right direction. Measured deviations from ideality of silicates with divalent oxides become more negative (or less positive) in the order Fe+ , M n + , P b , M g * , C a . (1,7,9,22) In this view, ligand field effects lead to F e preceding M n and M n preceding even M g which has a smaller radius; dispersion interactions lead to P b preceding even Mg "*" even though its radius is even larger than Ca "** and Sr "*"; finally, coulomb and polarization interactions lead to Mg "*" preceding Ca "*". W i t h careful measurements of a larger number of binary silicate systems, it should be possible to develop useful correlations and a means of making reasonable predictions of the magnitudes of thermodynamic properties of silicates. 2

2

2 +

2 +

2

2 +

2 +

2 +

2 +

2

2

2 +

2

2

2

Conclusions There are several significant conclusions which can be reached. 1. We have performed analyses of thermodynamic data on binary silicate systems

14.

BLANDER AND PELTON

Thermodynamic Properties of Molten Slags

193

which lead to a unique and accurate mathematical representation of their known properties. 2. Our use of equations which have the properties of ordered liquids built in appears to have the innate capability for representing a mass of different types of data on binary systems measured in various ranges of temperature and composition. This result lends confidence in the use of our analyses for interpolations and extrapolations outside the range of measurements to calculate thermodynamic properties at unmeasured temperatures and compositions. 3. We can theoretically justify an "asymmetric combining rule which, for cases in which silica is the only acid component, leads to a priori predictions for ternary systems based on data for the three subsidiary binaries. It appears likely that such predictions would be valid for multicomponent systems.

Publication Date: April 2, 1986 | doi: 10.1021/bk-1986-0301.ch014

11

4. A preliminary examination of themodynamic data on silicates indicates that correlations developed for molten salts may be useful in understanding and ultimately in predicting magnitudes of the thermodynamic solution properties of silicates. Acknowledgments Work performed under the auspices the Morgantown Energy Technology Center of the U. S. Department of Energy. References 1. Lin, P.L.; Pelton, A. D. Metall. Trans. 1979, 10B, 667-676. 2. Masson, C. R.; Smith, I. B.; Whiteway, S. G. Can. J. Chem. 1977, 48, 1456-1464; Proc. 11th International Congress on Glass, J. Gotz, Ed., Prague, 1977, Vol. 1, pp. 3-41. 3. Gaskell, D. R. Metall. Trans. 1977, 8B , 131-145. 4. Fincham, C. J . B.; Richarson, F. D. Proc. Roy. Soc. 1954, 223, 40. 5. Toop, G. W.; Samis, C. S. Trans. TMS-AIME 1962, 224, 878-887. 6. Blander, M. "Thermodynamic Properties of Molten Salt Solutions," Molten Salt Chemistry , Blander, M., Ed., Interscience, N.Y., 1964, pp. 127-237. 7. Blander, M.; Pelton, A. D. "Computer Assisted Analyses of the Thermodynamic Properties of Slags in Coal Combustion Systems," ANL/FE-83-19, Argonne Na­ tional Laboratory, Argonne, IL, 1983 Available from NTIS, U.S. Dept. of Com­ merce, Washington, D.C. 8. Pelton, A. D.; Blander, M. "Computer Assisted Analysis of the Thermodynamic Properties and Phase Diagrams of Slags" in Proc. Second Int'l. Symp. on Metal­ lurgical Slags and Fluxes, Fine, Η. Α.; Gaskell, D. R., eds., TMS-AIME, Warrendale PA, 1984, pps 281-294; Blander, M.; Pelton, A. D. "Analyses and Predictions of the Thermodynamic Properties of Multicomponent Silicates", ibid, pps 295304 9. Saboungi, M.-L.; Blander, M. J. Chem. Phys. 1975, 63, 212-220; J. Amer. Ceram. Soc. 1975, 58, 1-7. 10. Muan, A; Osborne, E. F. "Phase Equilbria Among Oxides in Steelmaking," Addison-Wesley Publishing Co., Reading, Mass. 1965.

194

MINERAL MATTER AND ASH IN COAL

11. Sharma, R. Α.; Richardson, F. D. J . Iron Steel Inst. 1962, 200, 373; 1961, 198, 308. 12. Rein, R. H.; Chipman, J., Trans. AIME 1965, 233, 415-425. 13. Robie, R. Α.; Hemingway, B. S.; Fisher, J . R. "Thermodynamic Properties of Minerals and Related Substances at 298.15 Κ and 1 Bar Pressure and at Higher Temperatures," Geologic Survey Bulletin 1452, U.S. Govt. Printing Office, Wash­ ington, D.C. 1978. 14. Ol'shanskii, Ya. I.,Dokl. Akad. Nauk SSSR 1951, 76, 94; Tewhey, J. D.; Hess, P. C. Phys. Chem. Glasses 1979, 20, 41-53. 15. Elliott, J. F. Trans. AIME 1955, 203, 485. 16. Distin, P. Α.; Whiteway, S. G.; Masson, C. R. Can. Met. quart. 1971, 10, 73.

Publication Date: April 2, 1986 | doi: 10.1021/bk-1986-0301.ch014

17. Schuhmann, R; Ensio, P. J . Metals 1951, 3, 401. 18. Bodsworth, C. J . Iron and Steel Inst., London 1959, 193, 13. 19. Levin, Ε. M.; Robbins, C. R.; McMurdie, H. F. "Phase Diagrams for Ceramists," Am. Ceram. Soc., Columbus, Ohio 1964; Supplement, 1969; Ε. M. Levin and H. F. McMurdie, Supplement 1975. 20. Timucin, M.; Morris, A. E. Met. Trans. 1970, 1, 19. 21. Blander, M., "Some Fundamental Concepts in the Chemistry of Molten Salts" in Molten Salts, G. Mamantov, ed., Marcel Dekker, New York 1969. 22. Gaye, H.; Riboud, P. "Donnèes Experimentales sur les Activitès des Constituants de Laitiers," IRSID PCM-RE 337, St. Germain-en-Laye, France 1976. RECEIVED January 24, 1986

15 Estimation of Physicochemical Properties of Coal Slags and Ashes K. C. Mills

Publication Date: April 2, 1986 | doi: 10.1021/bk-1986-0301.ch015

National Physical Laboratory, Teddington, Middlesex TW11 OLW, United Kingdom

The various methods available for estimating the melting range, viscosity, density, surface tension, heat capacity, enthalpy, thermal conductivity, absorption coefficient and emissivity of coal slags and ashes are reviewed and evaluated. New routines for estimating the density, surface tension, heat capacity and enthalpy of coal slags from their chemical composition are presented and assessed. It was concluded that the distribution of iron in the slag between free Fe, FeO and Fe O was very important as virtually every physical property of the slag was greatly influenced by this factor. 2

3

S l a g s formed d u r i n g t h e g a s i f i c a t i o n o f c o a l u s u a l l y c o n t a i n S 1 O 2 , A I 2 O 3 , i r o n o x i d e s j CaO, N a 2 0 and K 0 w i t h minor amounts o f v a r i o u s other oxides. A knowledge o f t h e p h y s i c o - c h e m i c a l p r o p e r t i e s o f t h e s e s l a g s c a n improve t h e c o n t r o l o f t h e p r o c e s s eg. t h e amount of f l u x required t o b r i n g the s l a g v i s c o s i t y t o a l e v e l s u i t a b l e f o r t a p p i n g c a n be c a l c u l a t e d from v i s c o s i t y - c o m p o s i t i o n r e l a t i o n s . P h y s i c a l p r o p e r t y d a t a f o r t h e c o a l s l a g s c a n a l s o improve p r o c e s s d e s i g n by p r o v i d i n g i n p u t v a l u e s f o r m a t h e m a t i c a l models o f t h e p r o c e s s eg. t h e r m a l p r o p e r t i e s o f t h e s l a g s a r e needed f o r h e a t b a l a n c e c a l c u l a t i o n s . There i s an a p p r e c i a b l e v a r i a t i o n i n t h e c o m p o s i t i o n o f s l a g s formed from v a r i o u s c o a l s and even from d i f f e r e n t b a t c h e s o f t h e same s t o c k on o c c a s i o n s and t h e s e composit i o n a l v a r i a t i o n s can give r i s e t o c o n s i d e r a b l e d i f f e r e n c e s i n the physical properties. As t h e c h e m i c a l a n a l y s i s i s f r e q u e n t l y a v a i l a b l e on a r o u t i n e b a s i s i t would be p a r t i c u l a r l y d e s i r a b l e t o have r e l i a b l e models f o r t h e p r e d i c t i o n o f p h y s i c o - c h e m i c a l p r o p e r t i e s from t h e i r c h e m i c a l c o m p o s i t i o n . F u r t h e r m o r e such models would have t h e f u r t h e r advantage t h a t t h e need f o r arduous i n t e r p o l a t i o n s on p s e u d o - t e r n a r y p l o t s f o r s l a g s , which a r e r e a l l y multicomponent and i n t e r a c t i v e systems, would be e l i m i n a t e d . 2

This chapter not subject to U.S. copyright. Published 1986, American Chemical Society

Publication Date: April 2, 1986 | doi: 10.1021/bk-1986-0301.ch015

196

MINERAL MATTER AND ASH IN COAL

The p r o p e r t i e s o f s l a g s a r e dependent n o t o n l y upon c h e m i c a l c o m p o s i t i o n , b u t upon o t h e r f a c t o r s a l s o . The most pronounced d e v i a t i o n s from a d d i t i v i t y r u l e s based on c o m p o s i t i o n a r i s e i n t h e e s t i m a t i o n o f t h o s e p r o p e r t i e s which i n v o l v e i o n i c t r a n s p o r t eg. e l e c t r i c a l c o n d u c t i v i t y . However s u r f a c e t e n s i o n v a l u e s e s t i m a t e d from a d d i t i v i t y r u l e s a r e f r e q u e n t l y i n e r r o r as b u l k thermo­ dynamic p r o p e r t i e s do n o t a p p l y a t s u r f a c e s . Furthermore, v i r t u a l l y a l l t h e p h y s i c a l p r o p e r t i e s o f s l a g s a r e , t o some e x t e n t , dependent upon t h e s t r u c t u r e o f t h e s l a g ( v i z . t h e l e n g t h o f s i l i c a t e c h a i n s , degree o f c r y s t a l l i n i t y e t c . ) thus e s t i m a t i o n p r o c e d u r e s have t o accommodate t h e s e s t r u c t u r a l f a c t o r s , where p o s s i b l e . There i s o n l y a l i m i t e d amount o f p h y s i c a l p r o p e r t y d a t a a v a i l a b l e f o r c o a l s l a g s and c o n s e q u e n t l y i t has been n e c e s s a r y t o examine a much b r o a d e r range o f s i l i c a t e s i n c l u d i n g magmatic l i q u i d s and t h o s e s l a g s e n c o u n t e r e d i n s t e e l m a k i n g , g l a s s m a k i n g and nonferrous processes. Thus t h e models d e s c r i b e d here s h o u l d have a much wider range o f a p p l i c a b i l i t y . A c r i t i c a l e v a l u a t i o n o f t h e e x t a n t p h y s i c a l p r o p e r t y d a t a has shown t h a t v i r t u a l l y e v e r y p h y s i c a l p r o p e r t y i s markedly dependent upon t h e d i s t r i b u t i o n o f i r o n i n t h e s l a g between FeO, Fe203 and free iron. F r e q u e n t l y t h e s e d i s t r i b u t i o n s a r e n o t r e p o r t e d and a p p r o p r i a t e v a l u e s cannot r e a d i l y be p r e d i c t e d s i n c e t h e o x i d a t i o n s t a t e depends upon ( i ) t h e p a r t i a l p r e s s u r e o f oxygen, P Q ^ (ϋ) temperature (JL) » * ( i i i ) the nature o f the other oxides present, thus CaO, Na20 and K 0 ^ ^ i n c r e a s e t h e amount o f F e 0 while S 1 O 2 i n c r e a s e s t h e amount o f FeO. Even when t h e i r o n d i s t r i b u t i o n s a r e g i v e n t h e v a l u e s a r e v u l n e r a b l e t o e r r o r owing t o d i f f i c u l t i e s i n c h e m i c a l a n a l y s i s and t h e p o s s i b i l i t y o f some r e d i s t r i b u t i o n o f the Fe/Fe / F e ^ r a t i o s d u r i n g t h e quench. N e v e r t h e l e s s i t i s s t r o n g l y recommended t h a t a l l f u t u r e p h y s i c a l p r o p e r t y d e t e r m i n ­ a t i o n s on c o a l s l a g s s h o u l d be accompanied by c h e m i c a l a n a l y s i s f o r F e ( f r e e ) , FeO and F e 2 0 on t h e quenched specimen. 2

T

a n c

2

2 +

2

3

+

3

Models f o r E s t i m a t i n g P h y s i c o - C h e m i c a l P r o p e r t i e s . M e l t i n g Range E m p i r i c a l r u l e s have been f o r m u l a t e d f o r t h e e s t i m a t i o n o f m e l t i n g range on t h e b a s i s o f t h e b a s i c i t y o f t h e s l a g o r ash. How­ e v e r t h e v a r i o u s c o n s t a n t s used i n t h e c a l c u l a t i o n s a r e a p p l i c a b l e t o v e r y narrow c o m p o s i t i o n a l ranges and thus a l a r g e number o f c o n s t a n t s a r e r e q u i r e d t o r e p r e s e n t t h e s l a g s formed from d i f f e r e n t coals. A more u n i v e r s a l approach has been adopted by Gaye^Z^^) who e x p r e s s e d , t h e dependency o f l i q u i d u s temperature, T u q , upon c h e m i c a l c o m p o s i t i o n as p o l y n o m i a l s f o r each o f t h e phases ( o r compounds) formed by t h e s l a g . The maximum c a l c u l a t e d v a l u e o f ^ l i q f ° "the v a r i o u s phases i s t h e T ^ value. Good agreement was o b t a i n e d between t h e c a l c u l a t e d and e x p e r i m e n t a l v a l u e s f o r T ^ q f o r the systems, S i 0 + A I 2 O 3 + MgO + FeO and S 1 O 2 + A 1 0 + MgO+CaQ. However development o f t h i s model t o c o v e r t h e multicomponent c o a l s l a g s c o u l d prove d i f f i c u l t . r

q

2

2

3

15.

Physicochemical

MILLS

Properties

197

of Slags and Ashes

A second model due t o Gaye (9.) would appear t o o f f e r more promise; i t i s based on the Kapoor-Frohberg model f o r the e s t i m ­ a t i o n o f a c t i v i t i e s and assumes t h a t b o t h a c i d i c and b a s i c o x i d e s are made up o f s y m m e t r i c a l c e l l s and t h e s e i n t e r a c t t o form assymmetrical c e l l s . The a c t i v i t i e s o f the v a r i o u s o x i d e s c a l c u l a t e d f o r q u a t e r n a r y s l a g s w i t h t h i s model a r e i n e x c e l l e n t agreement w i t h t h o s e determined e x p e r i m e n t a l l y . V a l u e s o f T ^ q f o r a g i v e n com­ p o s i t i o n can be d e r i v e d by d e t e r m i n i n g the temperature a t which the s o l i d phase formed has an a c t i v i t y o f one. The model has been developed f o r systems based on seven components, S 1 O 2 1 A I 2 O 3 , CaO, MgO, FeO, Fe203 and MnO. The model w i l l have t o be e n l a r g e d t o i n c l u d e N a 0 e t c b e f o r e i t can be used f o r the r e l i a b l e e s t i m a t i o n o f T ^ i q o f c o a l s l a g s b u t i t would seem t o have c o n s i d e r a b l e promise and has the d e c i d e d advantage t h a t a c t i v i t y d a t a f o r the v a r i o u s component o x i d e s a r e a l s o p r o d u c e d .

Publication Date: April 2, 1986 | doi: 10.1021/bk-1986-0301.ch015

2

V i s c o s i t y ( η ) . S e v e r a l models have been r e p o r t e d f o r the e s t i m a t i o n o f v i s c o s i t i e s (η) o f s i l i c a t e m e l t s t o c o v e r t h e c o m p o s i t i o n a l ranges o f g l a s s e s ( i 2 > i i ) s t e e l m a k i n g s l a g s (12*14), magmas^~~IS) and c o a l s l a g s and a s h e s ( 1 2 ~ 2 4 ) . The temperature (T) dependence o f the v i s c o s i t y i s e x p r e s s e d i n the form o f the A r r h e n i u s r e l a t i o n ­ s h i p ( e q u a t i o n 1) o r the F r e n k e l r e l a t i o n s h i p ( e q u a t i o n 2 which i s sometimes known as the Weymann e q u a t i o n ) where A and Β a r e con­ s t a n t s , Ε i s the a c t i v a t i o n energy and R i s the Gas C o n s t a n t . η= η=

A exp AT exp

(E/RT) (B/RT)

(1) (2)

E s t i m a t e d v i s c o s i t i e s have been c a l c u l a t e d u s i n g t h e s e v a r i o u s models and the c l o s e s t agreement w i t h e x p e r i m e n t a l v a l u e s was o b t a i n e d w i t h the models due t o Riboud e t a l and U r b a i n e t a l (Γ7)(18). These e s t i m a t i o n p r o c e d u r e s use the F r e n k e l r e l a t i o n s h i p and thus t h e i r s u p e r i o r i t y may be l a r g e l y due t o the use o f e q u a t i o n (2). The model due t o S c h o b e r t which i n v o l v e s pétrographie c l a s s i f i c a t i o n has n o t been a s s e s s e d and t h i s p r o c e d u r e may p r o v i d e r e l i a b l e e s t i m a t e s o f v i s c o s i t y f o r c o a l s l a g s but c o u l d not be a p p l i e d t o s l a g s c o v e r i n g a wide range o f c o m p o s i t i o n . Thus e f f o r t i n the p r e s e n t s t u d y was f o c u s s e d p r e d o m i n a n t l y on the Riboud and U r b a i n models. Model due t o Riboud e t a l . The s l a g c o n s t i t u e n t s a r e c l a s s i f i e d i n f i v e d i f f e r e n t c a t e g o r i e s i n t h i s model. The mole f r a c t i o n s (x) f o r those c a t e g o r i e s b e i n g g i v e n by (i) x("Si0 ) = x(Si0 ) + X(P0 ) + x(Ti0 ) + x(Zr0 ) (ii) x("CaO") = x(CaO) + x(MgO) + x(FeO) + x ( F e 0 ) (iii) x(Al 0 ) (iv) x ( C a F ) and (v) x("Na 0") = x(Na 0) + x ( K 0 ) . The parameters A and Β o f e q u a t i o n (2) are c a l c u l a t e d from the mole f r a c t i o n o f the f i v e c a t e g o r i e s by u s i n g e q u a t i o n s (3) and (4) and the M

2

2

2 i 5

2

2

l e 5

2

3

2

2

2

2

A = exp(-19.81+1.73x("CaO")+5.82x(CaF )+7.02x(Na 0)-33.76x(A1 0 )(3) Β =+31140-23896x("Ca0")-46356x(CaF )-39159x("Na 0")+68833x(Al 0 )(4) v i s c o s i t y f o r the temperatures i n q u e s t i o n by use o f e q u a t i o n 2. 2

2

2

2

2

3

2

3

198

MINERAL MATTER AND ASH IN COAL

Model due t o U r b a i n e t a l ^ Q ) . In t h i s model the parameters A and Β are c a l c u l a t e d by d i v i d i n g the s l a g c o n s t i t u e n t s i n t o t h r e e c a t e g o r i e s ( i ) " g l a s s f o r m e r s " , X Q = x ( S i 0 ) + x(P205) ( i i ) " m o d i f i e r s " , x = x(CaO) + x(MgO) + x ( N a 0 ) + x(K20) + 3x(CaF2) + x(FeO) + x(MnO) + 2 x ( T i 0 ) + 2 x ( Z r 0 2 ) and ( i i i ) "amphoterics", x = x(Al 0 ) + x(Fe 0 ) + x(B 0 ). 2

M

2

2

A

2

3

2

3

2

3

However we c o n s i d e r t h a t F e 0 behaves more l i k e a m o d i f i e r than an amphoteric and i n our r e v i s e d programme 1.5 x ( F e 0 ^ 5 ) has been added t o xjyj and x ( F e 0 ) removed from x^. "Normalized" values X Q * and xjyj* and x^* a r e o b t a i n e d by d i v i d i n g the mole f r a c t i o n s , X Q , X and x by the term (1 + 2 x ( C a F ) + 0.5 x ( F e 0 i . 5 ) + x(Ti0 ) + x(Zr0 )). U r b a i n proposed t h a t the parameter Β was i n f l u e n c e d b o t h by the r a t i o , β = xjJj/Ujyj + x j ) and by xg. The parameter Β can be e x p r e s s e d i n the form o f e q u a t i o n (5) where B-p B and B can be o b t a i n e d by e q u a t i o n ( 6 ) . 2

3

e

2

3

M

A

2

2

2

Publication Date: April 2, 1986 | doi: 10.1021/bk-1986-0301.ch015

2

Β = B

B.

o

Β

+

= a.

χ

χ

£

b.p

+

+

B (x*)

+

c.p

2

2

+

B (x*)

3

3

(5)

3

2

(6)

B , B^ B and B can be c a l c u l a t e d from the e q u a t i o n s l i s t e d i n T a b l e I and t h e s e parameters are then i n t r o d u c e d i n t o e q u a t i o n (5) t o c a l c u l a t e B. The parameter A can be c a l c u l a t e d from Β by e q u a t i o n (7) and the v i s c o s i t y o f the s l a g ( i n PaS) can then be determined u s i n g e q u a t i o n ( 8 ) . 0

2

3

- InA = 0.2693 Β + 11.6725 η

= 0.1

Table I. Β

The =

AT exp

(10

3

B/T)

relationship of B

13.8

(7)

+ 39.9355 β

Q >

(8) Βχ, B

2

- 44.049

ο Β

ι

B

2

B

3

= 30.481

-

-

3

w i t h the f u n c t i o n β

2 pι 2

117.1505 (J + 129.9978

=-40.9429 + 234.0486 ρ\ - 300.04 = 60.7619

and B

β

ι

ρ

153.9276 {J\ + 211.1616

2

2 β

M o d i f i c a t i o n s t o the U r b a i n Model. U r b a i n — . has r e c e n t l y m o d i f i e d the model t o c a l c u l a t e s e p a r a t e Β v a l u e s f o r d i f f e r e n t i n d i v i d u a l m o d i f i e r s , CaO, MgO and MnO. The g l o b a l Β v a l u e f o r a s l a g c o n ­ t a i n i n g a l l t h r e e o x i d e s can be d e r i v e d u s i n g e q u a t i o n (9)

x(Ca0)B(Ca0) + x(Mg0)B(Mg0) + x(Mn0)B(Mn0) Β(global)=

(9) x(Ca0) + x(Mg0) + x(Mn0)

15.

MILLS

Physicochemical

Properties

199

of Slags and Ashes \ -L J , Ι Ο )

Assessment o f t h e V i s c o s i t y Models. These two models have"— —- been used t o c a l c u l a t e t h e v i s c o s i t i e s o f s l a g s w i t h w i d e l y - v a r y i n g c o m p o s i t i o n s and i t has been found t h a t b o t h g i v e v a l u e s which agree w e l l w i t h experiment. The model o f U r b a i n g i v e s a s l i g h t l y b e t t e r f i t than t h e Riboud model. The d i s c r e p a n c i e s between t h e e x p e r i ­ mental v a l u e s and t h e p r e d i c t e d v a l u e s a r e o f t h e o r d e r o f + 30% which a r e o f s i m i l a r magnitude t o t h e e x p e r i m e n t a l u n c e r t a i n t i e s f o r v i s c o s i t y measurements. Density ( p ) (25) R e c e n t l y Keene — has r e p o r t e d t h a t t h e d e n s i t y a t 1673K o f molten s l a g s c a n be o b t a i n e d w i t h i n + 5% u s i n g t h e e q u a t i o n ( 1 0 ) p = 2.49 + 0.012 (% FeO + % F e ^ + % MnO + % NiO)

(10)

Publication Date: April 2, 1986 | doi: 10.1021/bk-1986-0301.ch015

gem An a d d i t i v e method f o r t h e e s t i m a t i o n o f d e n s i t i e s (p) i n s l a g s has been w i d e l y used f o r some time (26,27). i t h i s method, t h e molar volume, V, c a n be o b t a i n e d from e q u a t i o n s (11) and (12) below where Μ, χ and 7 a r e t h e m o l e c u l a r weight, mole f r a c t i o n and t h e p a r t i a l molar volume, r e s p e c t i v e l y , and 1, 2 and 3 denote t h e v a r i o u s oxide c o n s t i t u e n t s o f the s l a g . n

V = Μ χ χ

V = x

+ M x

χ

l V l

2

x V

+

2

2

+ M x / ρ

2

3

+

x V 3

(11)

3

(12)

3

The p a r t i a l molar volume i s u s u a l l y assumed t o be e q u a l t o t h e molar volume o f t h e pure component ( V ° ) . B o t t i n g a and W e i l l (.£§) produced a s e r i e s o f v a l u e s o f V f o r v a r i o u s o x i d e s assuming a c o n s t a n t v a l u e f o r V ( S i 0 2 ) and i t was c l a i m e d t h a t good e s t i m a t i o n s o f t h e d e n s i t y c o u l d be o b t a i n e d f o r c o m p o s i t i o n s c o n t a i n i n g between 40 and 80% S i 0 « Two more r e c e n t s t u d i e s ( J L ? ) ( 3 0 ) a l s o c o n c l u d e d t h a t V ( S 1 O 2 ) was independent o f c o m p o s i t i o n and have r e v i s e d the V v a l u e s f o r t h e v a r i o u s o x i d e s . However i t has been p o i n t e d o u t (31f32) t h a t t h e d e n s i t y o f t h e s l a g i s a l s o r e l a t e d t o i t s structure. S i l i c a t e s l a g s c o n t a i n a m i x t u r e o f c h a i n s , r i n g s and b a s i c s i l i c a t e u n i t s , which a r e dependent upon t h e s i l i c a con­ c e n t r a t i o n and upon t h e n a t u r e o f t h e c a t i o n s p r e s e n t . Thus t h e d e n s i t i e s o f s i l i c a t e slags estimated using a constant value f o r V ( S i 0 ) w i l l be s u b j e c t t o e r r o r as t h e arrangement o f t h e s e s i l i c a t e chains v a r i e s with s i l i c a concentration. F u r t h e r m o r e Grau and M a s s o n ( ^ J ) p o i n t e d o u t t h a t f o r t h e s e r i e s , M0, M S i 0 , M S i 0 7 , t h e p a r t i a l molar volume o f S i 0 i s not constant. They c a l c u l a t e d a Δ ν term f o r t h e d i f f e r e n c e s between any two members o f t h e s e r i e s and i n t h i s way, c a l c u l a t e d v a l u e s were d e r i v e d f o r t h e systems FeO + S i 0 , PbO + S i 0 , FeO + MnO + S i 0 and FeO + CaO + S i 0 , f o r c o m p o s i t i o n s i n t h e range Si02 1·0· However t h i s method i s n o t s u i t a b l e f o r c a l c u l a t i n g d e n s i t i e s o f multicomponent systems. Very r e c e n t l y , B o t t i n g a e t al(33.) have p r e s e n t e d a model i n which t h e p a r t i a l molar volumes o f a l u m i n a - s i l i c a t e l i q u i d s were c o n s i d e r e d t o be composition-dependent. 2

2

2

4

3

2

2

2

2

x

=

t

o

2

2

200

MINERAL MATTER AND ASH IN COAL

New Model f o r C a l c u l a t i n g the D e n s i t i e s o f S l a g s . S l a g s c o n t a i n i n g S i 0 , A l ° 3 and P 2 O 5 c o n s i s t o f c h a i n s , r i n g s and complexes which are dependent upon the amount and n a t u r e o f the c a t i o n s p r e s e n t . Thus i t i s n e c e s s a r y t o make the p a r t i a l molar volumes dependent upon c o m p o s i t i o n f o r o x i d e s o f t h i s t y p e . I f i n a b i n a r y system, e q u a t i o n (12) were a p p l i c a b l e and i f = V± i e . V i s independent o f c o m p o s i t i o n , t h e n the c u r v e o f V as a f u n c t i o n o f c o m p o s i t i o n w i l l be t h a t shown by the s o l i d l i n e i n F i g u r e l a and the two x^V^ c o n ­ t r i b u t i o n s by the d o t t e d l i n e s . I f we now c o n s i d e r a b i n a r y s i l i c a t e system the molar volume (V) would have the form shown as a s o l i d l i n e i n F i g u r e l b . I t i s r e a s o n a b l e t o assume t h a t V(MyO) i s independent o f c o m p o s i t i o n and would have the form o f x i V ^ i n F i g u r e l b . The parameter x can be d e r i v e d f o r S i 0 by use o f e q u a t i o n (13) below. 2

2

v

2

x V

Publication Date: April 2, 1986 | doi: 10.1021/bk-1986-0301.ch015

2

Thus X 2

2

2

2

= V - x V

2

w i l l have the form o f the c u r v e shown i n F i g u r e l b .

V

1

(13)

1

I t i s p o s s i b l e t o d e r i v e xV f o r S i 0 i n t e r n a r y and q u a t e r n a r y s l a g s by u s i n g e q u a t i o n ( 1 4 ) . V a l u e s f o r xV ( S i 0 ) have been d e r i v e d u s i n g e x p e r i m e n t a l d e n s i t y d a t a f o r the systems, FeO + S i 0 , CaO + S i 0 , MnO + S i 0 , N a 0 + S i 0 , K 0 + S i 0 and CaO + FeO + S i 0 and a r e p l o t t e d a g a i n s t x ( S i 0 ) i n F i g u r e 2. I t can be seen from t h i s f i g u r e t h a t t h e r e i s e x c e l l e n t agreement between the x V ( S i 0 ) c a l c u l a t e d from d i f f e r e n t s o u r c e s , w i t h the e x c e p t i o n o f t h a t f o r the MnO + S i 0 system. However the r e l i a b i l i t y o f the e x p e r i m e n t a l d a t a f o r t h i s system, have been q u e s t i o n e d p r e v i o u s l y . From t h i s c u r v e f o r x V ( S i 0 ) we can d e r i v e the r e l a t i o n s h i p , V ( S i 0 ) = 19.55 + 7.966.x(Si0 )· The recommended v a l u e s f o r V f o r the v a r i o u s o x i d e s a t 1500 °C a r e g i v e n i n T a b l e I I . 2

2

2

2

2

2

2

2

2

2

2

2

2

2

2

2

xV(Si0 ) 2

= V - x V 1

1

- x V 2

2

- x V 3

3

(

1

4

)

V a l u e s f o r xV ( A l 0 3 ) were determined i n a s i m i l a r manner by u s i n g e x p e r i m e n t a l d e n s i t y d a t a f o r the systems, CaO + A l 0 3 , C a F + A 1 0 , S i 0 + A 1 0 , MgO + CaO + A 1 0 and MnO + S i 0 + A 1 0 The xV ( A 1 0 ) r e s u l t s are p l o t t e d i n F i g u r e 3 and the r e l a t i o n s h i p V ( A 1 0 ) = 28.31 + 32 x ( A l 0 ) - 31.45 χ 2 ( Α 1 0 ) was d e r i v e d from t h i s curve. There a r e few e x p e r i m e n t a l d a t a f o r the d e n s i t y o f phosphate s l a g s but x V ( P 0 ) v a l u e s were d e r i v e d from d a t a f o r the systems CaO + FeO + P 2 O 5 and N a 0 + P 2 O 5 . A c o n s t a n t v a l u e o f V = 65.7 cm3 m o l o b t a i n e d from the s e l e c t e d l i n e a r r e l a t i o n s h i p . 2

2

2

3

2

2

2

2

3

2

3

2

2

2

3

3

3

2

2

3

2

3

5

- 1

2

In o r d e r t o p r o v i d e a temperature c o e f f i c i e n t , the temperature dependencies o f the molar volumes (dV/dT) o f many s l a g systems were examined and a mean v a l u e o f 0.01% K was adopted. - 1

was

Publication Date: April 2, 1986 | doi: 10.1021/bk-1986-0301.ch015

MILLS Physicochemical Properties of Slags and Ashes

202

MINERAL MATTER AND

ASH IN COAL

T a b l e I I . Recommended v a l u e s f o r the p a r t i a l molar volumes, o f v a r i o u s s l a g c o n s t i t u e n t s a t 1500 °C Si0 A1 0 CaO MgO 2

2

3

19.55 28.31 20.7 16.1

+ 7.966 x ( S i 0 2 ) + 32 χ ( Α 1 0 ) - 3 1 . 4 5 χ 2

2

(A1 0 ) 2

3

FeO Fe 0 2

3

MnO Na 0 2

Units

Publication Date: April 2, 1986 | doi: 10.1021/bk-1986-0301.ch015

3

o f V = cm

mol

=10

m

15.8 38.4 15.6 33

CaF P 2 Ti0 2

2

V,

31.3 O 5 65.7 24

mol

Assessment o f d e n s i t y models. An a n a l y s i s o f the u n c e r t a i n t i e s a s s o c i a t e d w i t h the e s t i m a t i o n o f d e n s i t i e s w i t h t h i s model has not y e t been completed. However on the b a s i s o f t h o s e v a l u e s o b t a i n e d so f a r the s t a n d a r d d e v i a t i o n o f the f a c t o r ( p e s t " pexpt/ pexpt) i s between 1 and 2% and l e s s than t h a t r e c o r d e d u s i n g the method due to B o t t i n g a et a l (J£). The e x p e r i m e n t a l u n c e r t a i n t i e s a s s o c ­ i a t e d w i t h d e n s i t y measurements f o r s l a g s a r e c a . + 2%. Surface

Tension

(Y)

Methods f o r e s t i m a t i n g the s u r f a c e t e n s i o n o f ^ s l a g s based on the a d d i t i o n o f the p a r t i a l molar c o n t r i b u t i o n s (Y) o f the i n d i v i d u a l c o n s t i t u e n t s have been r e p o r t e d by Appen (34) by B o n i and Derge (35) and by P o p e l ( 3 6 ) . A l l t h e s e methods make use o f e q u a t i o n (15) where 1, 2, 3 e t c denote the v a r i o u s s l a g c o n s t i t u e n t s . Υ

= χ

1

Ϋ

+

1

χ

Ϋ

2

2

+

χ

3

Ϋ

3

+

(15)

V a l u e s o f Y i a r e o f t e n t a k e n t o be the s u r f a c e t e n s i o n o f the pure components, γ ° , and have a l s o been o b t a i n e d by i t e r a t i v e p r o c e d u r e s . F i g u r e 4a shows a t y p i c a l c l o t o f Y as a f u n c t i o n o f χ f o r a b i n a r y s l a g and the i n d i v i d u a l X i Y i c o n t r i b u t i o n s have a l s o been i n c l u d e d . These methods work w e l l f o r c e r t a i n s l a g m i x t u r e s but break down when s u r f a c e - a c t i v e c o n s t i t u e n t s , such as Ρ 2 Ο 5 » are p r e s e n t . These components m i g r a t e p r e f e r e n t i a l l y t o the s u r f a c e and cause a s h a r p d e c r e a s e i n the s u r f a c e t e n s i o n and c o n s e q u e n t l y o n l y v e r y s m a l l c o n c e n t r a t i o n s a r e r e q u i r e d t o cause an a p p r e c i a b l e d e c r e a s e i n Y. Thus some u n r e p o r t e d o r u n d e t e c t e d i m p u r i t y c o u l d have a marked e f f e c t on the s u r f a c e t e n s i o n o f the s l a g and t h e r e b y produce an a p p a r e n t e r r o r i n the v a l u e e s t i m a t e d by the model. In t h i s r e s p e c t s u r f a c e t e n s i o n d i f f e r s from a l l the o t h e r p h y s i c a l p r o p e r t i e s which are e s s e n t i a l l y bulk p r o p e r t i e s . New Model f o r C a l c u l a t i n g the S u r f a c e T e n s i o n o f S l a g s . F i g u r e 4a shows the s u r f a c e t e n s i o n o f two s l a g c o n s t i t u e n t s which a r e n o t s u r f a c e a c t i v e . F o r a b i n a r y m i x t u r e w i t h one s u r f a c e a c t i v e component the s u r f a c e t e n s i o n - c o m p o s i t i o n r e l a t i o n s h i p w i l l have the form o f t h a t shown i n F i g u r e 4b where 2 denotes the s u r f a c e - a c t i v e c o n s t i t u e n t . J^f we assume t h a t χ Y f o r component 1 i s u n a f f e c t e d , t h e n the term Y x , p a r t i a l molar c o n t r i b u t i o n o f the s u r f a c e t

2

2

n

e

Physicochemical

Properties

of Slags and Ashes

Publication Date: April 2, 1986 | doi: 10.1021/bk-1986-0301.ch015

MILLS

2 X

a n

Figure 4 The c o m p o s i t i o n dependence of Ύ, /Y^> d 2^2 ^ y ^- 8 systems w i t h (a) n o n - s u r f a c e - a c t i v e c o n s t i t u e n t s and (b) one s u r f a c e - a c t i v e c o n s t i t u e n t .

X

i

n

a

r

s

a

204

MINERAL MATTER AND

ASH IN COAL

a c t i v e m a t e r i a l . can be c a l c u l a t e d by e q u a t i o n (16) below. The term ( X 2 Y 2 ) can s i m i l a r l y be c a l c u l a t e d f o r t e r n a r y and q u a t e r n a r y systems p r o v i d i n g t h e r e i s o n l y one s u r f a c e a c t i v e component.

xY = Υ -Χ Υ 2

2

χ

(16)

λ

The c o m p o s i t i o n a l dependence o f the ( x Y ) term is^shown i n F i g u r e 4b and i t s h o u l d be n o t e d t h a t as x —+ 1 then χ Υ — * γ . V a l u e s o f (xY) f o r v a r i o u s s u r f a c e a c t i v e m a t e r i a l s d e r i v e d from e x p e r i m e n t a l s u r f a c e t e n s i o n d a t a are shown i n F i g u r e 5. i t i s p o s s i b l e to deal w i t h the c o m p o s i t i o n a l dependence o f t h e s e (xY). V a l u e s by c o n ­ s i d e r i n g two c u r v e s v i z . one o p e r a t i n g up t o the p o i n t Ν and the o t h e r r e p r e s e n t i n g v a l u e s o f χ where χ > N . The p a r t i a l s u r f a c e t e n s i o n f o r n o n - s u r f a c e a c t i v e c o n s t i t u e n t s i s shown i n T a b l e I I I and the e q u a t i o n s f o r c a l c u l a t i n g Y f o r s u r f a c e a c t i v e components and v a l u e s o f Ν a r e g i v e n i n T a b l e IV. Thexe i s a c o n s i d e r a b l e d i s c r e p a n c y i n the r e l a t i o n s h i p s between x Y * 2 2°3 o b t a i n e d from e x p e r i m e n t a l d a t a on two d i f f e r e n t systems, and a mean v a l u e has been adopted u n t i l f u r t h e r d a t a become a v a i l a b l e . 2

2

Publication Date: April 2, 1986 | doi: 10.1021/bk-1986-0301.ch015

2

2

a n (

2

Table

Oxide Y /mNm-1 Table

2

2

2

5

2

3

2

3

-2 -0.8

2

A1 0 655 2

a t 1500

x-jYi f o r x
N

χ χ χ χ χ χ χ χ

The r e p o r t e d v a l u e s o f ( d Y / d T ) f o r v a r i o u s s l a g systems were examined and a mean v a l u e o f -0.15 mN m K was a p p l i e d as a temperature c o e f f i c i e n t . -1

- 1

Assessment o f the Model. The s t a n d a r d d e v i a t i o n o f the f a c t o r ( ( Y e s t - Y e x p t ) / y e x p t ) was ca.+ 10%. Undoubtedly* much o f the u n c e r t a i n t y a r i s e s from e x p e r i m e n t a l e r r o r s , where the e f f e c t o f u n r e p o r t e d s u r f a c e a c t i v e i m p u r i t i e s and the n a t u r e o f the gaseous atmosphere c o u l d have a marked e f f e c t on the v a l u e o f s u r f a c e tension. Another major s o u r c e o f e r r o r i s the amount o f F e 0 3 p r e s e n t i n the s l a g , and t h i s i n v e s t i g a t i o n has shown c l e a r l y t h a t Fe 0 i s v e r y s u r f a c e a c t i v e . Few i n v e s t i g a t o r s r e p o r t the (Fe3+/Fe +) r a t i o which i s dependent upon, ( ΐ ) Ρ θ 2 , ( ϋ ) Τ and ( i i i ) the 2

2

3

2

Publication Date: April 2, 1986 | doi: 10.1021/bk-1986-0301.ch015

Figure 5 The c o m p o s i t i o n a l dependence o f x i ? i , various surface active constituents.

for

MINERAL MATTER AND ASH IN COAL

206

n a t u r e and amount o f o t h e r o x i d e s p r e s e n t so even, when quoted, the r a t i o may be i n e r r o r . Thus, i f a d e c r e a s e i n Y i s r e c o r d e d when N a 0 i s added t o the s l a g , i t i s q u e s t i o n a b l e whether t h i s d e c r e a s e i s due t o an i n c r e a s e i n F e 0 3 c o n t e n t o r t o the s u r f a c e a c t i v i t y o f the N a 0 . In t h i s i n v e s t i g a t i o n , attempts were made t o a d j u s t Y f o r an i n c r e a s e i n F e 0 3 c o n t e n t b u t the e f f e c t o f b a s i c o x i d e s on the r a t i o i s n o t w e l l documented and some e r r o r may r e s u l t . A major u n r e s o l v e d problem c o n c e r n s the s i t u a t i o n where the s l a g c o n t a i n s more than one s u r f a c e a c t i v e component. It i s possible t h a t t h e r e i s some c o m p e t i t i o n f o r s i t e s on the s u r f a c e and hence the d e c r e a s e i n ^ would n o t be as sharp as t h a t c a l c u l a t e d from the summation o f ( X A Y A + B ^ B ) where A and Β denote s u r f a c e a c t i v e constituents. In t h i s c a s e , t h e model may o v e r e s t i m a t e the d e c r e a s e i n Y . There a r e no e x t a n t d a t a t o c o n f i r m t h i s p o s s i b i l i t y , and ( N a 0 + K 0 ) has been i n c l u d e d as a s i n g l e c o n t r i b u t i o n i n the model. 2

2

2

2

X

Publication Date: April 2, 1986 | doi: 10.1021/bk-1986-0301.ch015

2

2

Thermal P r o p e r t i e s The computations of the t h e r m a l l o s s e s i n the c o n v e r t e r by c o n d u c t i v e and r a d i a t i v e p r o c e s s e s r e q u i r e knowledge o f the f o l l o w i n g thermal p r o p e r t i e s , heat c a p a c i t y , enthalpy, thermal c o n d u c t i v i t y , a b s o r p t i o n c o e f f i c i e n t and e m i s s i v i t y . Heat C a p a c i t y C^ and E n t h a l p y

(H^ -

H 2

98^

When a s i l i c a t e l i q u i d i s c o o l e d the s t r u c t u r e o f the s o l i d formed i s dependent upon the c o o l i n g r a t e and the t h e r m a l h i s t o r y o f the sample. C o n s i d e r a l i q u i d a t a temperature c o r r e s p o n d i n g t o the p o i n t C i n F i g u r e 6, a r a p i d quench w i l l produce a g l a s s and the e n t h a l p y e v o l v e d w i l l f o l l o w the p a t h CLGA. By c o n t r a s t , a v e r y slow c o o l i n g r a t e w i l l r e s u l t i n the f o r m a t i o n o f a c r y s t a l l i n e s l a g , t h e e n t h a l p y e v o l u t i o n f o l l o w i n g the p a t h CL DB. I t w i l l be n o t e d from F i g u r e 6 t h a t ( H - H g ) c r y s t = ( H - H g ) g l a s s + A , where ^ e n t h a l p y o f the endothermic t r a n s f o r m a t i o n o f crystal—"glass). The C v a l u e s f o r the v a r i o u s phases can be summarized a s : Cp(crystal) = C ( g l a s s ) < C (supercooled l i q u i d ) = C (liquid) H

T

H

v

i

t

i

s

t

n

2

8

T

2

v

i

t

8

e

p

p

p

p

I t can be seen from F i g u r e 6 t h a t a t the g l a s s temperature ( T ) t h e r e i s a sudden i n c r e a s e i n C (ACpS ) as the g l a s s t r a n s ­ forms i n t o a s u p e r c o o l e d l i q u i d . Drop c a l o r i m e t r y s t u d i e s on the g l a s s phase a t temperatures between T ^ and T ^ w i l l produce p r o ­ g r e s s i v e l y more c r y s t a l l i z a t i o n as i s approached and c o n ­ s e q u e n t l y the (H-p - H g ) - Τ r e l a t i o n s h i p w i l l be s i m i l a r t o t h a t d e p i c t e d by the d o t s i n F i g u r e 6 and n o t the p a t h , AGLC; the magni­ tude o f the apparent e n t h a l p y o f f u s i o n (^H^ ) w i l l be dependent upon the f r a c t i o n o f the sample c r y s t a l l i z e d d u r i n g a n n e a l i n g a t temperatures between T and T . The C v a l u e s f o r t h e l i q u i d and s u p e r c o o l e d l i q u i d s have been r e p o r t e d t o be c o n s t a n t and i n d e p e n ­ dent o f temperature(.22). I t f o l l o w s from the t r i a n g l e GEL i n F i g u r e 6 that C (T - T ) = AH . Thus, i t i s p o s s i b l e t o e s t i m a t e the e n t h a l p y o f a s l a g w i t h a g l a s s y s t r u c t u r e from e s t i m a t e s o f p ( g l a s s ) , C ( l i q ) and T However the e s t i m a t i o n o f T is 1

g l

p

g

2

q

8

us

g l

l i q

g l

p

f u s

p

l i q

g l

c

p

g l

g l

15.

Physicochemical

MILLS

Properties

of Slags and Ashes

207

d i f f i c u l t as i t c a n v a r y between 700 and 1100 Κ and the v a r i o u s e s t i m a t i o n r u l e s s u g g e s t e d a r e known t o be prone t o a p p r e c i a b l e errors (38). I n s p e c t i o n o f l i t e r a t u r e d a t a (,2§» 2.9) i n d i c a t e s t h a t the g l a s s t r a n s f o r m a t i o n o c c u r s when Cp a t t a i n s a v a l u e o f c a . 1.1 J K ~ g " ; t h i s r u l e has been used i n t h e development o f t h e f o l l o w i n g model f o r t h e e s t i m a t i o n o f (H

3.5 and values of k can be c a l c u l a t e d u s i n g e q u a t i o n (21), where G i s the S t e f a n - B o l t z m a n n c o n s t a n t . In r e c e n t y e a r s f o r m u l a e have been

Publication Date: April 2, 1986 | doi: 10.1021/bk-1986-0301.ch015

a

n

(

a

p

7

s

7

2

1

1

R

R

R

R

R

2

k

3

_ 16 η Τ σ " 3a

R

u

i

;

p r o p o s e d f o r the c a l c u l a t i o n o f kR f o r o p t i c a l l y t h i n conditions. The a b s o r p t i o n c o e f f i c i e n t {oc) i s markedly dependent upon the amounts o f F e + and M n p r e s e n t i n the s l a g s a t l e v e l s o f FeO .< 5% the f o l l o w i n g r e l a t i o n s h i p can be s u p p l i e d , aicm" ) = ll(%Fe0). Temperature appears t o have l i t t l e e f f e c t on the a b s o r p t i o n c o e f f i c i e n t o f g l a s s e s but the a b s o r p t i o n o f magmas have been r e p o r t e d t o i n c r e a s e w i t h i n c r e a s i n g t e m p e r a t u r e » ˧) C r y s t a l l i z a t i o n o f the s l a g w i l l r e s u l t i n a l a r g e i n c r e a s e i n the a b s o r p t i o n (or e x t i n c t i o n ) c o e f f i c i e n t which c o u l d reduce k to v i r t u a l l y zero. 2

2 +

1

β

R

15.

MILLS

Physicochemical

Properties

211

of Slags and Ashes

E m i s s i v i t y (ε) The e m i s s i v i t y (ε) o f a s e m i - t r a n s p a r e n t medium i s a b u l k p r o p e r t y i n c o n t r a s t t o ε (metal) which i s s o l e l y dependent upon t h e s u r f a c e . On t h e b a s i s o f t h e s p e c t r a l and t o t a l normal e m i s s i v i t y d a t a r e p o r t e d f o r c o a l and m e t a l l u r g i c a l s l a g s , t h e v a l u e ε = 0.8 + 0.1 can be adopted f o r c o a l s l a g s i n t h e range (1100 - 1900 K ) . CONCLUSIONS

Publication Date: April 2, 1986 | doi: 10.1021/bk-1986-0301.ch015

1) E s t i m a t i o n p r o c e d u r e s based on t h e c h e m i c a l c o m p o s i t i o n o f t h e s l a g have now been d e v e l o p e d f o r t h e p r e d i c t i o n o f v i s c o s i t y , s u r f a c e t e n s i o n , d e n s i t y and h e a t c a p a c i t y . 2) The a c c u r a c y o f t h e s e e s t i m a t i o n r o u t i n e s f o r some p h y s i c a l p r o p e r t i e s ( e g - v i s c o s i t y , s u r f a c e t e n s i o n ) c a n be improved when more r e l i a b l e e x p e r i m e n t a l d a t a become a v a i l a b l e . 3) has and the of

The d i s t r i b u t i o n o f i r o n i n t h e s l a g a pronounced e f f e c t on v i r t u a l l y a l l i t i s recommended t h a t e x p e r i m e n t a l s l a g s h o u l d always be accompanied by iron.

between F e , FeO and F e 0 3 the p h y s i c a l p r o p e r t i e s data f o r the p r o p e r t i e s o f values f o r the d i s t r i b u t i o n 2

4) The development o f models f o r t h e p r e d i c t i o n o f some p h y s i c a l p r o p e r t i e s (thermal c o n d u c t i v i t i e s , a b s o r p t i o n c o e f f i c i e n t ) i s r e s t r i c t e d by t h e l i m i t e d amount o f e x p e r i m e n t a l d a t a a v a i l a b l e .

REFERENCES 1)

DARKEN, L. S; and GURRY, R. W. J . Am. Chem. Soc. 1946, 68, 798.

2)

LARSON, H.; CHIPMAN, J .

3)

BANYA, S.; SHIM, J . D.

4)

SONDREAL, Ε. Α.; ELLMAN, R. C. Fusibility of ash from lignite and its correlation with ash composition. US Energy Res. and Develop. Admin. GFERC/RI-75-1, Pittsburgh, 1975.

5)

WINEGARTNER, E. C.; RHODES, B. T.

6)

BRYERS, R. W.; TAYLOR, T. E.

7)

STEILER, J . M. Commission of the European Communities Research Contract, 7210 CA/3/3/03. Thermodynamic data for steelmaking. 1981, Eur N7820, BP 1003, Luxembourg 1982.

8)

GAYE, H.

9)

GAYE, H. Proc. Metallurgical Conference, Centenary of Teaching of Metallurgy, held at Strathclyde University, 25/26 June 1984.

J . Metals, 1953, Sept., 1089. Canad. Metall. Q., 1982, 21, 319.

J . Eng. Power, 1975, 97, 395.

J . Eng. Power, 1976, 98, 528.

Ironmaking and Steelmaking, 1984,

11, 67.

212

MINERAL MATTER AND ASH IN COAL

Publication Date: April 2, 1986 | doi: 10.1021/bk-1986-0301.ch015

10) OCHOTIN, M. W. Steklo Keram.,

1954, 11, 7.

11)

LYON, K. C. J. Res. Natl. Bur Stand., 1974, 78, 497.

12)

McCAULEY, W. D. and APELIAN, D. 20, 247.

13)

RIBOUD, P. V.; ROUX, Y; LUCAS, L. D.; GAYE, H. Fachber. Huttenpraxis Metalveiterverarb., 1981, 19, 859.

14)

MAIRY, B. Private communication, Centre Recherches Metallurgique, Liege, Belgium.

15)

BOTTINGA, Y.; WEILL, D. F.

16)

SHAW, H. Am. J. S c i . , 1972, 272, 870.

17)

URBAIN, G.; CAMBIER, F . ; DELETTER, M.; ANSEAU, M. R. Brit. Ceram. Soc., 1981, 80, 139.

18)

URBAIN, G. private communication, CNRS, Laboratoire de Ultrs­ -Refractaires, Odeillo, France, Jan. 1983.

19)

WATT, J. D.; FEREDAY, D. J. Instr. Fuel, 1969, 338, 99.

20)

ΒOKAMP, Inst. of Gas Technol. Preparation of a coal conversion systems Technical data Book. Energy Res. and Dev. Admin. Report Fe-1730-21 (1976).

21)

CAPPS, W.; KAUFFMAN, D. Natl. Bur. Stand. Quart. Prog. Report. to Office of Coal Res., Dept. Interior, 30/9/74.

22)

SCHOBERT, Η. H. Div. Fuel Chem. Prep., 1977, 22, 143.

23)

SCHOBERT, H. H.; WITTHOEFFT, C. 5, 157.

24)

HENSLEE, S.P.; KELSEY, P. V. EGG-FM-6049, Sept. 1982.

25)

KEENE, B. J. National Physical Laboratory Report, DMA(D)75, Feb. 1984.

26)

MOREY, G. H. 1954, p 217.

27)

HUGGINS, M L; SUN, K. H. J. Am. Ceram. Soc. 1948, 26, 4.

28)

BOTTINGA, H.; WEILL, D. E.

29)

NELSON, S. A.; CARMICHAEL, I.S.E. 1979, 71, 117.

Canad. Metall. Quart., 1981,

Am. J. S c i . , 1972, 272, 438.

Trans. J.

Fuel Proc. Technol., 1981,

Jr. US Dept. of Energy Report,

"Properties of Glasses", Reinhold, New York,

Am. J. Sci.,1972, 272, 438. Contrib. Mineral Petrol.

Publication Date: April 2, 1986 | doi: 10.1021/bk-1986-0301.ch015

15.

MILLS

Physicochemical

Properties of Slags and Ashes

213

30)

MO, Y.; CARMICHAEL, I.S.E.; RIVERS, M.; STEBBINS; J; Mineralog. Mag., 1982, 45, 237.

31)

GRAU, A.E.; MASSON, C. R. Canad. Metall. Quart., 1976, 15, 367.

32)

LEE, Y. E . , GASKELL, D. R. Metall. Trans., 1974, 5, 853.

33)

BOTTINGA, Y.; WEILL, D. E.; RICHET, P. Acta. 1982, 46, 909.

34)

ΑΡΡΕΝ, Α. Α.; SHISHKOV, Κ. Α.; KAYALOVA, S. S. Khim. 1952, 26, 1131.

35)

BONI, R. E.; DERGE, G. J. Metals, 1956, 8, 53.

36)

POPEL, S. E. "Metallurgy of slags and their use in building." 1962, p 67.

37)

RICHET, P.; BOTTINGA, Y. Geochim, Cosmochim Acta, 1984, 44, 1533.

38)

BACON, C. R. Am. J. Sci., 1977, 277, 109.

39)

MOYNIHAN, C. T.; EASTEAL, S; A. J.; TRAN, D. C.; WILDER, J. Α., DONOVAN, E. P. J. Am. Ceram. Soc., 1976, 59, 137.

40)

CARMICHAEL, I. S. E.; NICHOLLS, J.; SPERA, F. J.; WOOD, B. J.; NELSON, S. A. Phil. Trans. Roy. Soc. (London), 1977, A286, 373.

41)

RHINES, J. R.; MILLS, K. C.; PUTLAND, F. H. High Temp-High Pressure in press.

42)

MOREY, G. W. "Properties of Glass," Reinhold, New York, 1954, p 217.

43)

ABAKOVA, I. G.; SERGEEV, O. A. "Thermophysical properties of glasses." Proc. of Metrological Inst. of USSR, No. 129 (189), Moscow-Leningrad, 1971, p 13.

44)

MISNAR, A. "Thermal conductivity of solid, gases and composite materials." Leningrad, 1968.

45)

VAVILOU, Y. V.; KOMAROV, V. E.; TABUNOVA, W. A. Phys. Chem. Glasses, 1982, 8, 472.

46)

GIBBY, R. L.; BATES, J. L. Proc. 10th Thermal Conductivity Conf. held Newton, Mass. Sept. 1970, p IV-7,8.

47)

RHINES, J. R.; MILLS, K. C. unpublished work, 1984.

48)

FINE, Η. Α.; ENGH, T.; ELLIOTT, J. F. Metal. Trans. B. 1976, 7B, 277.

Geochim Cosmochim. Zhur. Fiz.

214

MINERAL M A T T E R A N D A S H IN COAL

49)

NAUMAN, J.; FOO, G.; ELLIOTT, J. F. Copper." Chapter 12, p 237.

50)

STEELE, F. N.; DOUGLAS, R. W. Phys. Chem. Glasses, 1965, 6, 246.

51)

FUKAO, Y.; MITZUTANI, H.; UYEDA, S. Interiors, 168, 1, 57.

52)

ARONSON, J. R.; BELLOTI, L. H.; ECKROAD, S. W.; EMSLIE, A. G.; McCONNEL, R. K.; THUNA, P. C.; von. J. Geophys. Res., 1970, 75, 3443.

Publication Date: April 2, 1986 | doi: 10.1021/bk-1986-0301.ch015

RECEIVED June 17, 1985

"Extractive Metallurgy of

Phys. Earth Planet

16 Viscosity of Fluxes for the Continuous Casting of Steel W. L . McCauley and D . Apelian

Publication Date: April 2, 1986 | doi: 10.1021/bk-1986-0301.ch016

Materials Engineering, Drexel University, Philadelphia,PA19104

The effects of composition and temperature on the viscosity of oxide melts suitable for use as casting fluxes is discussed. A relation to express the tem­ perature dependence of viscosity for oxide melts based on the Clausius-Clapeyron Equation is evaluated, v i z . , lnη = C + C /T + C lnT. This relation produces a better description of the temperature dependence of fused oxides than the more familiar Arrhenius equation. 1

2

3

Mold f l u x e s a r e r o u t i n e l y used i n b o t h c o n t i n u o u s c a s t i n g and bottom pouring of s t e e l . These f l u x e s a r e g e n e r a l l y c a l c i u m s i l i c a t e based c o m p o s i t i o n s w i t h a l k a l i o x i d e s [ ( L i , Na, K ) 0 ] and f l u o r i d e s [ C a F , NaF] added as f l u i d i z e r s . The c o m p o s i t i o n s f r e q u e n t l y use f l y a s h as t h e base m a t e r i a l because i t p r o v i d e s a s i g n i f i c a n t c o n c e n t r a t i o n of s i l i c a i n a p r e f u s e d form e a s i l y d i s s o l v e d as t h e powder m e l t s on the l i q u i d s t e e l . A v a r i e t y o f p r o p e r t i e s o f t h e f l u x must be c o n t r o l l e d , i n c l u d ­ i n g f u s i o n c h a r a c t e r i s t i c s ( f u s i o n temperature range and s i n t e r i n g c h a r a c t e r i s t i c s ) , i n s u l a t i o n c h a r a c t e r i s t i c s , flow p r o p e r t i e s of the powder, v i s c o s i t y o f t h e m o l t e n f l u x , and n o n - m e t a l l i c a b s o r p t i o n ability. The v i s c o s i t y i n f l u e n c e s t h e consumption r a t e o f f l u x , heat t r a n s f e r i n t h e mold, and n o n - m e t a l l i c d i s s o l u t i o n r a t e , and has been the s u b j e c t o f p u b l i s h e d and u n p u b l i s h e d work o v e r t h e l a s t t e n y e a r s . In t h i s p a p e r , r e c e n t work on t h e v i s c o s i t y o f mold f l u x compo­ s i t i o n s i s r e v i e w e d , and a r e l a t i o n t o d e s c r i b e t h e temperature de­ pendence o f v i s c o s i t y i s d i s c u s s e d . T h i s r e l a t i o n i s based on t h e C l a u s i u s - C l a p e y r o n E q u a t i o n and was o r i g i n a l l y d e v e l o p e d by K i r c h o f f and Rankine t o d e s c r i b e t h e temperature dependence o f v a p o r p r e s s u r e . 2

2

P r e v i o u s Work S e v e r a l r e c e n t p u b l i c a t i o n s have d i s c u s s e d t h e e f f e c t s o f composi­ t i o n a l v a r i a b l e s on t h e v i s c o s i t y o f o x i d e m e l t s f o r use as mold fluxes. L a n y i (1) measured t h e v i s c o s i t y o f s e v e r a l c o n t i n u o u s c a s t i n g f l u x e s and found t h a t t h e v i s c o s i t y c o r r e l a t e d w e l l w i t h t h e combined s i l i c a and alumina c o n t e n t o f t h e f l u x . The f l u x e s

0097-6156/ 86/ 0301 -0215506.00/ 0 © 1986 American Chemical Society

216

MINERAL MATTER AND ASH IN COAL

e v a l u a t e d c o n t a i n e d 30 t o 55% S i 0 + A 1 0 , 20 t o 45% CaO + MgO, 0 t o 8% FeO, 15 t o 35% R 0 + F, and 0 t o 5% B 0 . The v i s c o s i t y a t 1300°C ranged from 0.06 t o 3.2 Pa*s. L a n y i was a l s o a b l e t o c o r r e l a t e t h e a c t i v a t i o n energy f o r v i s c o u s f l o w i n the A r r h e n i u s E q u a t i o n w i t h the f l u x b a s i c i t y r a t i o and v i s c o s i t y . McCauley and A p e l i a n (2) measured the v i s c o s i t y o f m i x t u r e s con­ t a i n i n g o n l y S i 0 , A 1 0 , CaO, N a 0 , and C a F , i n an attempt t o i s o ­ l a t e the e f f e c t s o f g l a s s network m o d i f i e r s and network b r e a k e r s i n mold f l u x type c o m p o s i t i o n s . V i s c o s i t y a t 1300°C ranged from 0.1 t o 2.8 Pa»s. T h e i r r e s u l t s showed t h a t the f l u x v i s c o s i t y c o u l d be ex­ p r e s s e d as a q u a d r a t i c f u n c t i o n o f the r a t i o o f network f o r m i n g c a t ­ i o n s ( s i l i c o n and aluminum) t o a n i o n s (oxygen and f l u o r i n e ) . Riboud, e t a l . (3) examined a s l i g h t l y b r o a d e r range o f c o n ­ t r o l l e d c h e m i s t r i e s than McCauley and A p e l i a n as w e l l as s e v e r a l i n d u s t r i a l f l u x e s s i m i l a r t o t h o s e used by L a n y i . Riboud used t h e Frenkel r e l a t i o n 2

2

2

Publication Date: April 2, 1986 | doi: 10.1021/bk-1986-0301.ch016

2

3

2

2

3

2

3

2

η = A Τ exp(B/T)

(1)

to d e s c r i b e b o t h the c o m p o s i t i o n a l and temperature e f f e c t s on v i s ­ c o s i t y , η. I n E q u a t i o n 1, the v i s c o s i t y p a r a m e t e r s , A and B, a r e e x p r e s s e d i n terms o f the c o m p o s i t i o n a l v a l u e s . T h i s r e l a t i o n y i e l d s a p r e d i c t e d v i s c o s i t y w i t h i n 20% o f the measured v a l u e s o v e r a wide c o m p o s i t i o n range. N i c h o l s (4) measured the v i s c o s i t y o f s e v e r a l bottom p o u r i n g f l u x compositions. These f l u x e s c o n t a i n e d o n l y 5 t o 20% a l k a l i o x i d e s and f l u o r i d e s and 50 t o 70% S i 0 + A 1 0 . V i s c o s i t y ranged from 5 t o 80 Pa-s a t 1500°C, much h i g h e r than c o n t i n u o u s c a s t i n g fluxes. A l t h o u g h the c o r r e l a t i o n f a i l s f o r the lower v i s c o s i t y f l u x e s , N i c h o l s a l s o showed good c o r r e l a t i o n f o r v i s c o s i t y w i t h the s i l i c a content. The r e s u l t s i n d i c a t e t h a t , f o r mold f l u x o x i d e c o m p o s i t i o n s , the v i s c o s i t y i s dependent on the q u a n t i t y o f network f o r m i n g o x i d e s p r e s e n t , p r i n c i p a l l y s i l i c a and a l u m i n a . T h i s i s demonstrated by the r e s u l t s o f McCauley (2) i n F i g u r e 1. In t h i s c a s e , i t i s the r a t i o of network f o r m i n g i o n s t o t o t a l a n i o n c o n c e n t r a t i o n . However, as shown i n F i g u r e 2, the v i s c o s i t y / r e c i p r o c a l temperature r e l a t i o n s h i p i s not l i n e a r and cannot be a d e q u a t e l y r e p r e s e n t e d by the A r r h e n i u s E q u a t i o n over a wide temperature range. 2

2

3

V i s c o s i t y v s . Temperature V i s c o s i t y can be c o n s i d e r e d as a measure o f the ease o f movement o f molecules i n a l i q u i d undergoing shear. S e v e r a l f a c t o r s may i n f l u ­ ence t h i s ease o f movement i n c l u d i n g m o l e c u l e s i z e and i n t e r m o l e c u l a r a t t r a c t i o n , but a major f a c t o r i s the amount o f space a v a i l a b l e between the m o l e c u l e s , hence, the v a r i e t y o f models i n c o r p o r a t i n g a f r e e volume term. The C l a u s i u s - C l a p e y r o n e q u a t i o n r e l a t e s p r e s s u r e w i t h tempera­ t u r e , e n t h a l p y , and volume, and has been used t o d e v e l o p semit h e o r e t i c a l e x p r e s s i o n s o f vapor p r e s s u r e 05). Many p r o p e r t i e s , i n ­ c l u d i n g v i s c o s i t y , can be r e l a t e d t o an energy b a r r i e r , f r e e volume and temperature. The attempt h e r e i s t o e x p r e s s v i s c o s i t y i n the form o f the C l a u s i u s - C l a p e y r o n e q u a t i o n .

Viscosity of Casting

MCCAULEY AND APELIAN

Publication Date: April 2, 1986 | doi: 10.1021/bk-1986-0301.ch016

0

L_LL_I

I

Q29

0.27

I

I

1

0.31

0.33

1

Fluxes

1

1

1

0.35

1

ί 0 39

0.37

Κι

F i g u r e 1. V i s c o s i t y a t 1500°C as a f u n c t i o n o f network ions f o r the f l u x e s i n Table I .

1500

1400

ι

1

ι

5.5

6.0

TEMPERATURE C C ) 1300 1200

1

MOO

1

6.5

forming

1 — I

7.0

73

RECIPROCAL TEMPERATURE (κ'κΚΓ ) 4

F i g u r e 2. T y p i c a l v i s c o s i t y r e s u l t s v s . r e c i p r o c a l f o r some o f t h e f l u x e s l i s t e d i n T a b l e I .

temperature

218

MINERAL MATTER AND ASH IN COAL

The dP dT

C l a u s i u s - C l a p e y r o n e q u a t i o n can be w r i t t e n ΔΗ TAV

=

=

AH T(V-V )

(2)

0

where Ρ, T, and ΔΗ have t h e i r u s u a l meaning. F o r t h i s d i s c u s s i o n , Δν i s a measure o f f r e e volume o r t h e d i f f e r e n c e between t h e volume a t temperature and t h e volume a t some s t a n d a r d s t a t e , e.g., a t abso­ l u t e z e r o o r s o l i d volume a t t h e m e l t i n g p o i n t . E q u a t i o n 2 can be r e w r i t t e n as d(l,nP) d(l/T)

=

AH RAz

m V

;

where Δζ = PV/RT - PV /RT

Publication Date: April 2, 1986 | doi: 10.1021/bk-1986-0301.ch016

Q

Expanding ΔΗ t o t h e s e r i e s form and i n t e g r a t i n g w i t h r e s p e c t t o 1/T yields £nP = I

[ a - ^ψ- + b£nT + dT + |τ

2

+....

J

(4)

I f t h e h i g h e r o r d e r terms a r e i g n o r e d , t h e e x p r e s s i o n reduces t o

£nP = A - γ + C£nT

(5)

Such a d e r i v a t i o n was o r i g i n a l l y developed and used by K i r c h o f f [1858] and Rankine [1849] (5) t o e x p r e s s t h e temperature dependence of v a p o r p r e s s u r e . I t was a l s o s u c c e s s f u l l y used by Brostow (6) t o e x p r e s s the temperature dependence o f t h e i s o t h e r m a l c o m p r e s s i b i l i t y o f a wide v a r i e t y o f o r g a n i c l i q u i d s , some m e t a l l i c l i q u i d s and water. By a s i m i l a r a n a l o g y , we have used i t t o e x p r e s s t h e v i s c o s i ­ t y o f l i q u i d mold f l u x e s . Regression A n a l y s i s The v i s c o s i t y o f twenty f l u x c o m p o s i t i o n s determined e a r l i e r (2) were used t o e v a l u a t e t h e K i r c h o f f - R a n k i n e E q u a t i o n . The composi­ t i o n s o f t h e s e f l u x e s a r e g i v e n i n T a b l e I w i t h a summary o f t h e v i s c o s i t i e s g i v e n i n T a b l e I I . The f l u x v i s c o s i t y d a t a was f i t t e d to t h e K i r c h o f f - R a n k i n e e q u a t i o n as

and

Q η = e x p ( C ! + -y

+ C £nT)

to the Arrhenius

Equation

3

η = A exp(E/RT) u s i n g t h e Marquardt method o f n o n - l i n e a r l e a s t squares the S t a t i s t i c a l A n a l y s i s Systems [SAS] program package r e s u l t s o f the r e g r e s s i o n are given i n Table I I I , with d e v i a t i o n and an average d i f f e r e n c e between c a l c u l a t e d values.

(6)

(7) regression i n (7_) . The the standard and measured

34.8 34.4 34.5 34.6 34.6 34.7 26.7 30.7 31.2 35.2 33.5 38.8 41.9 48.0 46.8 30.6 30.0 39.6 32.4 39.1

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20

2

Si0

Flux

10.2 9.8 10.0 10.1 10.3 10.1 9.8 8.9 10.4 10.3 10.4 10.4 10.6 10.6 10.4 10.0 10.2 10.4 10.4 10.4

2

A1 0 3

CaO 7.6 8.4 7.6 7.8 8.0 7.2 11.7 2.3 7.6 3.3 11.5 3.5 7.8 3.3 5.5 7.0 8.2 4.7 11.7 1.7

2

10.7 10.9 11.0 10.8 10.9 10.8 14.4 12.9 5.7 5.7 15.1 15.1 6.8 6.5 10.7 10.3 18.6 4.0 10.8 11.3

2

CaF 0.9 0.7 0.7 0.6 0.7 0.6 0.6 0.7 0.8 0.9 0.5 0.7 0.7 0.8 0.6 1.0 0.8 1.0 0.7 0.9

MgO

Fluxes - F r i t

Na 0

Experimental

32.7 32.1 32.7 32.5 32.3 33.0 31.8 32.6 40.3 40.8 24.9 26.2 29.5 28.6 22.2 36.2 27.8 35.7 30.4 32.6

Table I.

0.6 0.6 0.7 0.6 0.5 0.6 1.9 8.8 0.7 0.8 0.4 0.9 0.9 1.4 2.7 2.1 1.6 1.3 1.1 1.2

Zr0 2

Composition,

Publication Date: April 2, 1986 | doi: 10.1021/bk-1986-0301.ch016

97.5 96.9 97.2 97.0 97.3 97.0 96.9 96.9 96.7 97.0 96.3 95.6 98.2 99.2 98.9 97.2 97.2 96.7 97.5 97.2

Total

wt%

0.94 0.93 0.95 0.94 0.93 0.95 1.19 1.06 1.29 1.16 0.77 0.67 0.77 0.60 0.47 1.18 0.93 0.90 0.94 0.83

V-ratio

MINERAL MATTER AND ASH IN COAL

220

Publication Date: April 2, 1986 | doi: 10.1021/bk-1986-0301.ch016

Table I I .

Flux

V i s c o s i t y at 1300°C, Ns π Γ

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20

0.395 0.310 0.340 0.485 0.290 0.510 0.110 NA* 0.280 6.00 0.270 0.930 1.15 2.80 2.40 7.00 0.160 1.40 0.250 1.40

*Not

2

Summary o f F l u x V i s c o s i t i e s Viscosity at 1400°C, Ns π Γ

2

Viscosity at 1500°C, Ns n f

2

0.135 0.112 0.128 0.125 0.088 0.122 0.035 NA* 0.080 0.180 0.114 0.270 0.280 0.710 0.725 0.090 0.059 0.380 0.094 0.410

0.230 0.175 0.205 0.235 0.190 0.230 0.065 NA* 0.160 0.360 0.150 0.460 0.530 1.30 1.30 0.170 0.115 0.670 0.130 0.720

available

In some c a s e s , v i z . , F l u x e s 5, 6 and 13 i n T a b l e I I I , t h e s i g n s o f t h e c o e f f i c i e n t s a r e r e v e r s e d , and a concave downward c u r v e i s generated. T h i s i s most l i k e l y caused by t h e r e g r e s s i o n b e i n g t r a p p e d a t a l o c a l minimum i n t h e d a t a and assuming convergence a t that p o i n t . I t i s r e q u i r e d f o r these cases t h a t the s i z e o f the r e ­ g r e s s i o n s t e p s h o u l d be i n c r e a s e d t o a v o i d t h e l o c a l minima, which SAS does n o t a l l o w . A l s o , t h e r e may n o t be enough d a t a p o i n t s t o expand t h e r e g r e s s i o n s t e p as i s p r o b a b l y t r u e f o r F l u x e s 6 and 13. For t h e m a j o r i t y o f f l u x e s e v a l u a t e d , t h e s t a n d a r d d e v i a t i o n , s, and t h e average p e r c e n t v a r i a t i o n , Δ%, i s lower f o r t h e K i r c h o f f Rankine f i t t e d e q u a t i o n v s . t h e A r r h e n i u s E q u a t i o n , i n d i c a t i n g a b e t t e r f i t of the experimental data. The d i f f e r e n c e i s most p r o ­ nounced f o r t h o s e f l u x e s where t h e n o n l i n e a r i t y o f t h e e x p e r i m e n t a l &ηη v s . 1/T d a t a i s g r e a t e s t . Discussion When t h e n o n l i n e a r i t y o f t h e l o g v i s c o s i t y v s . r e c i p r o c a l tempera­ t u r e d a t a was f i r s t o b s e r v e d , t e s t s were made t o i n s u r e t h a t t h e c u r v a t u r e was r e a l and n o t an a r t i f a c t o f t h e e x p e r i m e n t a l a p p a r a t u s . H y s t e r e s i s c u r v e s and c o n s t a n t temperature f o r extended time t e s t s showed t h a t t h e n o n l i n e a r i t y was n o t caused f ;ovolatilization a l k a l i o r f l u o r i d e c o n s t i t u e n t s o r from t h e r m a l d e v i a t i o n s i n t h e furnace setup. I t was found t h a t t h e o b s e r v e d c u r v a t u r e o f t h e d a t a was n o t an a r t i f a c t and r e p r e s e n t e d t h e t r u e p h y s i c a l b e h a v i o r o f the m a t e r i a l s . The a p p l i c a t i o n o f t h e K i r c h o f f - R a n k i n e e q u a t i o n 0

Δ%

n

tt

ttt

1

8.65E-3 2.66E-2 1.48E-2 1.62E-1

1.43E-2 6.00E-2 5.19E-2 2.26E-1 1.32E-1 _

nexp

ncalc

η exp

χ

1

0

Q

= number o f o b s e r v a t i o n s

=

t

7.03E-3 1.99E-2 8.92E-3 1.82E-2 2.15E-2 2.33E-2 1.31E-2 1.22E-2 _

s

Equation

e(î?exp - n c a l c ) / ( n - l )

2

33783 36304 30095 41748 29493 41814 41439 30736 36000* 32510 45110 40700 48579 40568 30000* 32981 39880 32313 50240

8.831E-6 2.704E-6 2.332E-6 8.016E-7 2.528E-5 8.079E-7 1.963E-7 1.496E-5 7.5E-3 7.820E-6 4.828E-7 2.661E-6 5.488E-8 5.761E-6 2.0E-2 5.469E-6 4.126E-6 7.393E-6 1.493E-7

1 2 3 4 5 6 7 9 10 11 12 13 14 15 16 17 18 19 20

=

(cal/mole)

(Pa s)

Flux

t

Ε

Andrade-Arrhenius

A

Table

5.05 3.01 7.39 16.46

-

2

51341 68183 47516 71633 -10764 -5309 297516 321853 1735752 63473 62253 -13879 212141 60668 8456727 105664 56382 85334 116706

C 3

20.502 32.337 20.792 31.502 -17.290 -16.447 179.394 192.942 1019.0 31.935 27.155 -21.843 116.012 26.660 5071.8 56.221 22.457 43.656 60.246

c

Kirchoff-Rankine

Analysis Results

-184.35 -282.59 -184.30 -278.11 132.94 123.80 -1511.91 -1626.10 -8603.16 -276.14 -239.51 169.80 -987.72 -233.90 -42701.6 -483.18 -200.79 -377.12 -517.30

Ci

Regression

3.15 2.95 4.69 13.00 5.57

-

2.40 7.98 2.08 2.73 4.14 5.41 7.01 3.15

III.

Publication Date: April 2, 1986 | doi: 10.1021/bk-1986-0301.ch016

6.36E-3 1.27E-2 5.68E-3 1.67E-2 2.10E-2 2.29E-2 5.72E-3 1.11E-2 2.83E-2 5.81E-3 3.35E-2 5.00E-2 9.85E-2 6.26E-2 8.69E-2 1.60E-3 1.66E-2 8.69E-3 5.32E-2

s

Equation

*

n 12 6 6 6 14 7 5 6 18 7 12 8 11 10 17 5 6 7 8

ttt

estimated

1.94 3.47 1.42 3.20 3.91 5.68 3.91 3.62 3.97 1.52 1.10 3.89 5.30 3.07 15.8 0.84 1.84 4.89 4.46

Δ%

222

MINERAL MATTER AND ASH IN COAL

produced a more accurate description of the temperature dependence of viscosity. Additional work on liquid metals, simple chloride salts and some small molecule organic liquids (8) indicates that the advantage of the Kirchoff-Rankine equation over the Andrade-Arrhenius equation improves as the size of the melt species increases. The improvement in the description of viscosity vs. temperature for metals and simple salts (e.g., NaCl and BiCl ) is not great, but for materials with larger melt species (e.g., silicate melts and organic liquids), there is a distinct improvement. 2

Publication Date: April 2, 1986 | doi: 10.1021/bk-1986-0301.ch016

Summary The evaluation of the viscosity of mold fluxes has shown that the viscosity is primarily controlled by the concentration of network forming oxides, particularly the s i l i c a content. It has also been demonstrated that the temperature dependence of viscosity can be expressed by the relation, £ηη = C + C /T + C £nT, derived from the Clausius-Clapeyron Equation. This relation produces a better de­ scription of viscosity vs. temperature than the more familiar Arrhenius Equation. x

2

3

Literature Cited 1. 2. 3. 4. 5. 6. 7. 8.

Lanyi, M. D. M.S. Thesis, University of Cincinnati, 1980. McCauley, W. L.; Apelian, D. Canadian Metallurgical Quarterly, 1984, 20, 247-262. Riboud, P. V.; Roux, Y . ; Lucas, L. D.; Gaye, H. Fach. Huttenpraxis Metalveiterverarbeitung 1981, 19, 1-8. Nichols, M. W.; Lingras, A. P.; Apelian, D. 2nd Int. Sym. on Metallurgical Slags and Fluxes, Fine, Η. Α.; Gaskell, D. R.; Eds.; TMS-AIME, 1984, pp. 235-251. G. W. Thomson, Chemical Review, 1946, 38, 1-39. W. Brostow and P. Maynadier, High Temperature Science, 1979, 11, 7-21. SAS User's Guide, SAS Institute, Inc., Cary, North Carolina, 1979. McCauley, W. L.; Apelian, D. 2nd Int. Sym. on Metallurgical Slags and Fluxes, Fine, Η. Α.; Gaskell, D. R.; Eds.; TMS-AIME, 1984, pp. 925-947.

RECEIVED August 5, 1985

17 Rheological Properties of Molten Kilauea Iki Basalt Containing Suspended Crystals H . C . Weed, F. J. Ryerson, and A . J. Piwinskii

Publication Date: April 2, 1986 | doi: 10.1021/bk-1986-0301.ch017

University of California, Lawrence Livermore National Laboratory, Livermore, CA 94550

In order to model the flow behavior of molten silicate suspensions such as magmas and slags, the rheological behavior must be known as a function of the concentration of suspended crystals, melt composition, and external conditions. We have determined the viscosity and crystallization sequence for a Kilauea Iki basalt between 1250°C and 1149°C at 100 kPa total pressure and fO corresponding to the quartz-fayalite-magnetite buffer in an iron-saturated Pt30Rh rotating cup. viscometer of the Couette type. The apparent viscosity varies from 9 to 879 Pa.s. The concentration of suspended crystals varies from 18 volume percent at 1250°C to 59 volume percent at 1149°C. The molten silicate suspension shows power-law behavior: 2

log

|τ | yx

= A + A log |du/dx|, o

l

where τ is the shear stress and (du/dx) the shear rate. Since A ≤ 1, the apparent viscosity decreases with increasing shear rate and the system is pseudoplastic. yx

l

In o r d e r t o u n d e r s t a n d the f l o w b e h a v i o r o f m o l t e n s i l i c a t e s c o n t a i n i n g suspended c r y s t a l s , we need t o know t h e r h e o l o g i c a l p r o p e r t i e s o f t h e system as a f u n c t i o n o f volume f r a c t i o n o f t h e suspended c r y s t a l l i n e phases a t a p p r o p r i a t e t e m p e r a t u r e s , oxygen f u g a c i t i e s and m e l t c o m p o s i t i o n s . Because o f t h e w i d e s p r e a d o c c u r e n c e o f s i l i c a t e s , t h i s approach c a n be a p p l i e d t o magma t r a n s p o r t d u r i n g v o l c a n i c e r u p t i o n s , l a r g e s c a l e c o n v e c t i v e and m i x i n g p r o c e s s e s i n magmatic systems, and f o u l i n g o f i n t e r n a l b o i l e r s u r f a c e s by c o a l a s h s l a g s i n p l a n t s b u r n i n g p u l v e r i z e d coal. The f i r s t system we have s t u d i e d i s a b a s a l t i c l a v a from K i l a u e a I k i , H a w a i i , f o r which we have determined t h e c r y s t a l l i z a t i o n sequence and t h e dynamic v i s c o s i t y . 0097-6156/ 86/0301 -0223S06.00/0 © 1986 American Chemical Society

224

MINERAL MATTER AND ASH IN COAL

E x p e r i m e n t a l Methods

Publication Date: April 2, 1986 | doi: 10.1021/bk-1986-0301.ch017

Starting Material. The s t a r t i n g m a t e r i a l f o r t h e s e experiments i s K i l a u e a I k i b a s a l t fragments which have been ground i n a t u n g s t e n c a r b i d e s h a t t e r b o x t o -100 mesh and homogenized by m i x i n g w i t h a p a d d l e and t h e n t u m b l i n g f o r 1.5 h o u r s . T h i s m a t e r i a l was then f u s e d i n a i r a t 1400°C f o r about 3.3 h o u r s , quenched i n d e i o n i z e d water, r i n s e d w i t h a c e t o n e , and d r i e d under vacuum (15 ym) a t 110°C f o r 70 h o u r s . The d r i e d m a t e r i a l was ground i n a boron c a r b i d e m o r t a r and p e s t l e and s t o r e d i n a screw cap g l a s s b o t t l e b e f o r e use. L i g h t m i c r o s c o p i c e x a m i n a t i o n shows t h a t i t i s c l e a r , f r e e o f opaque i n c l u s i o n s , and i s o t r o p i c under c r o s s e d niçois; i t i s t h e r e f o r e presumed t o be g l a s s . C r y s t a l l i z a t i o n Sequence D e t e r m i n a t i o n . The s t a r t i n g m a t e r i a l f o r the d e t e r m i n a t i o n o f the c r y s t a l l i z a t i o n sequence was the homogenized -100 mesh r o c k powder b e f o r e f u s i o n . Samples were i n the form o f s p h e r i c a l beads suspended by s u r f a c e t e n s i o n i n p l a t i n u m l o o p s as d e s c r i b e d by Grove e t a l . (JL). They were p r e p a r e d by p r e s s i n g about 190 mg o f powder i n t o a c y l i n d r i c a l p e l l e t w i t h two o r t h r e e drops o f e t h a n o l as a b i n d e r . The d i a m e t e r o f the p l a t i n u m w i r e was 0.25 mm and the d i a m e t e r o f t h e l o o p about 3 mm. The p e l l e t was f u s e d t o the l o o p by r e s i s t i v e h e a t i n g o f the p l a t i n u m and t h e n suspended from a p l a t i n u m b r i d g e w i r e by an AI2O3 c o n n e c t o r which i n s u l a t e d i t e l e c t r i c a l l y from the b r i d g e w i r e . The b r i d g e w i r e was c o n n e c t e d a c r o s s two p l a t i n u m l e a d s which suspended the e n t i r e assembly from the top end f i t t i n g o f the v e r t i c a l f u r n a c e tube. Oxygen f u g a c i t y ( f 0 ) was c o n t r o l l e d by a d j u s t i n g the m i x i n g r a t i o o f a CO/CO2 gas m i x t u r e f l o w i n g through the f u r n a c e tube; i t was m a i n t a i n e d a t the q u a r t z - f a y a l i t e - m a g n e t i t e (QFM) b u f f e r (2, 3) and was m o n i t o r e d by r e c o r d i n g the EMF from a s o l i d s t a t e 0 s e n s o r i n the form o f a C a O - s t a b i l i z e d Z r 0 tube which extended from the lower end o f the f u r n a c e tube i n t o the h o t zone next t o the sample ( 4 ) . Pure 0 a t 110 kPa (1 atm) was c i r c u l a t e d i n s i d e the Z r 0 tube as the r e f e r e n c e gas. The CO was Matheson C. P. grade o r e q u i v a l e n t ; the C 0 was Matheson Coleman Instrument grade o r e q u i v a l e n t . Sample temperatures were measured by a Pt/10Rh thermocouple i n the hot zone; the thermocouple had been c a l i b r a t e d a t the g o l d p o i n t (1063°C). Run times v a r i e d from 77 hours a t 1270°C t o 738 hours a t 1130°C. The sample was quenched i n d e i o n i z e d water by e l e c t r i c a l l y f u s i n g the b r i d g e w i r e , which dropped the sample out o f the f u r n a c e tube. The quenched sample was mounted and p o l i s h e d f o r a n a l y s i s . The e l e m e n t a l c o m p o s i t i o n and the phases p r e s e n t were d e t e r m i n e d by wavelength d i s p e r s i v e X-ray a n a l y s i s w i t h a f u l l y automated JEOL 733 Superprobe. The weight p e r c e n t a g e s o f the phases p r e s e n t were d e t e r m i n e d from c o n s t r a i n e d l e a s t squares a n a l y s i s o f the b u l k c o m p o s i t i o n and the c o m p o s i t i o n s of the i n d i v i d u a l phases i n each experiment. Weight p e r c e n t a g e s were c o n v e r t e d t o volume p e r c e n t a g e s u s i n g e s t i m a t e d d e n s i t i e s f o r the m e l t and c r y s t a l l i n e phases. The c a l c u l a t e d volume p e r c e n t a g e s were compared w i t h t h o s e d e t e r m i n e d by r e f l e c t e d l i g h t p o i n t counts (1000 p t s ) o f s e l e c t e d e x p e r i m e n t s . The r e s u l t s o f the two methods a r e i n good agreement ( F i g u r e 1 ) . The s i z e d i s t r i b u t i o n was not d e t e r m i n e d . 2

2

2

2

2

2

17.

WEED ET AL.

Rheological

Properties

of Molten

Kilauea

Iki Basalt

225

Viscometry. The s t a r t i n g m a t e r i a l f o r t h e v i s c o s i t y d e t e r m i n a t i o n s was t h e f u s e d and r e - g r o u n d g l a s s d e s c r i b e d above. The v i s c o m e t e r was o f t h e r o t a t i n g cup C o u e t t e type i n which t h e t o r q u e and temperature were measured a t t h e bob (J>). The cup was c y l i n d r i c a l i n shape w i t h a r a d i u s o f 10.0 mm and a h e m i s p h e r i c a l bottom. The l e n g t h o f t h e c y l i n d r i c a l p o r t i o n was 32.5 mm. The bob was t h e same shape, w i t h a r a d i u s o f 8.25 mm and an o v e r a l l l e n g t h o f 88.1 mm. They were a l i g n e d w i t h t h e hemispheres c o n c e n t r i c by means o f x-y-z micrometer a d j u s t i n g screws on t h e framework w h i c h s u p p o r t e d t h e bob. They were f a b r i c a t e d from Pt/30Rh a l l o y , w i t h an F e - r i c h s u r f a c e produced by h e a t i n g w i t h a m e l t (25 weight p e r c e n t Fe3Û4, 75 weight p e r c e n t Na2SiÛ3) under f 0 c o r r e s p o n d i n g t o t h e i r o n - w u s t i t e b u f f e r (_3). T h i s i s i n t e n d e d t o m i n i m i z e Fe t r a n s p o r t from t h e K i l a u e a I k i m e l t t o t h e bob and cup (6). Oxygen f u g a c i t y was c o n t r o l l e d by p a s s i n g CO/CO2 gas m i x t u r e s t h r o u g h t h e i n n e r sample t u b e , which was i s o l a t e d from t h e r e s t o f t h e f u r n a c e by w a t e r - c o o l e d r o t a t a b l e mercury s e a l s a t t h e i n l e t and o u t l e t ends. Flow r a t e s were c o n t r o l l e d by v e r n i e r t h r o t t l e v a l v e s and b a l l - t y p e f l o w m e t e r s ; t h e meter s e t t i n g s were c a l c u l a t e d from t h e m a n u f a c t u r e r ' s p u b l i s h e d c u r v e s (_7) · The CO/CO2 r a t i o was a d j u s t e d t o g i v e an f02 c o r r e s p o n d i n g t o an e x t r a p o l a t i o n o f t h e q u a r t z - f a y a l i t e - m a g n e t i t e (QFM) b u f f e r (_2_3)· The t o t a l f l o w r a t e was 581 cm^/min c o r r e s p o n d i n g t o a l i n e a r v e l o c i t y o f 0.9 cm/sec a t room temperature and p r e s s u r e ; t h i s had been shown t o be l a r g e enough t o a v o i d unmixing o f t h e gases due t o t h e r m a l d i f f u s i o n e f f e c t s ( 8 ) . The CO was Matheson C. P. grade o r e q u i v a l e n t ; t h e CO2 was Matheson Coleman Instrument grade o r equivalent. Temperature was measured by a Pt6Rh/Pt30Rh thermocouple which had been c a l i b r a t e d a g a i n s t a pyrometer and s t a n d a r d lamp, w i t h a p p r o p r i a t e c o r r e c t i o n s f o r i n t e n s i t y l o s s e s due t o windows and a r e f l e c t i n g p r i s m i n t h e o p t i c a l t r a i n . The a c c u r a c y o f t h e temperature measurement was e s t i m a t e d t o be + 2°C (9). Torque was measured by a c a l i b r a t e d t o r s i o n b a r i n which t h e a n g u l a r d e f l e c t i o n and t o r q u e were r e a d from f o u r s t r a i n gauges mounted on t h e f a c e s o f t h e b a r and c o n n e c t e d as a Wheatstone bridge. The u n i t used i n t h e s e measurements had a range o f + 0.353 N.m (+ 50 o z . i n ) and d i g i t a l r e a d o u t t o 7.1 χ 10""^ N.m (0.1 oz.in). The tachometer was a t o o t h e d wheel and magnetic p i c k u p w i t h a range o f + 999 r e v / m i n , r e a d a b l e t o 1 r e v / m i n .

Publication Date: April 2, 1986 | doi: 10.1021/bk-1986-0301.ch017

2

}

T h r e e t y p e s o f v i s c o m e t r y e x p e r i m e n t s were p e r f o r m e d : c a l i b r a t i o n measurements on a s t a n d a r d o i l t o d e t e r m i n e t h e e f f e c t i v e l e n g t h o f t h e c y l i n d r i c a l column o f l i q u i d , i s o t h e r m a l runs on t h e b a s a l t m e l t s , and p o l y t h e r m a l e x p l o r a t o r y runs on t h e b a s a l t m e l t s . C a l i b r a t i o n measurements were performed on B r o o k f i e l d o i l ( μ = 1 0 5 Pa.s) a t room temperature and under l a b o r a t o r y atmosphere. The e f f e c t i v e l e n g t h was d e t e r m i n e d o v e r a range which i n c l u d e d t h e l e n g t h o f t h e m e l t samples. Rotation speed was v a r i e d from 0 t o + 220 rev/min i n 40 rev/min s t e p s , w i t h i n c r e a s i n g speed v a l u e s a t even m u l t i p l e s o f 20 rev/min and d e c r e a s i n g speed v a l u e s a t odd m u l t i p l e s o f 20 r e v / m i n . The p o s i t i v e r o t a t i o n a l d i r e c t i o n was s e l e c t e d f i r s t . A t l e a s t two t o r q u e r e a d i n g s were taken a t each v a l u e o f t h e r o t a t i o n a l speed. The same p r o c e d u r e was f o l l o w e d d u r i n g i s o t h e r m a l runs on t h e b a s a l t m e l t s , e x c e p t t h a t t h e temperature was n o t e d f o r each t o r q u e ο

MINERAL MATTER AND ASH IN COAL

226

measurement a t c o n s t a n t r o t a t i o n a l speed i n o r d e r t o m o n i t o r t h e temperature i n c r e a s e due t o v i s c o u s energy d i s s i p a t i o n . The m e l t sample weight was about 6.65 g, which w i t h an assumed d e n s i t y o f 2.78 g/cnr* c o r r e s p o n d e d t o a g e o m e t r i c a l l e n g t h o f 14.7 mm f o r the c y l i n d r i c a l p a r t o f t h e sample. The temperature s e t t i n g s were approached from above by g o i n g from room temperature t o 1260°C o r 1270°C a t 100°C/hr t o 150°C/hr, then c o o l i n g t h e system a t about l°C/min u n t i l t h e d e s i r e d temperature was r e a c h e d . The e f f e c t o f c h a n g i n g t h e h e a t i n g and c o o l i n g r a t e s on t h e v i s c o s i t y was n o t explored. P o l y t h e r m a l runs were made a c o n s t a n t r o t a t i o n a l speed, u s u a l l y 90 t o 100 rev/min, and were used t o e x p l o r e t h e temperature range o v e r which s u i t a b l e t o r q u e r e a d i n g s c o u l d be o b t a i n e d .

Publication Date: April 2, 1986 | doi: 10.1021/bk-1986-0301.ch017

R e s u l t s and D i s c u s s i o n C r y s t a l l i z a t i o n Sequence. T a b l e I shows the major element b u l k c o m p o s i t i o n o f t h e s t a r t i n g m a t e r i a l s used i n o u r e x p e r i m e n t s and t h o s e o f Shaw e t a l . ( 1 0 ) . K i l a u e a I k i b a s a l t c o n t a i n s l e s s AI2O3 and CaO than t h e i r m a t e r i a l , and much more MgO. Both a r e w i t h i n t h e normal range f o r b a s a l t i c l a v a s . Table I.

O x i d e , Wt.%

2

3

2

3

-

2

p o 2

K 0 Ti0 MnO

5

2

2

TOTAL *A11

Kilauea I k i 46.29 10.44 17.90 11.34*

S1O2

A1 0 MgO FeO Fe 0 CaO Na 0

Analyses of S t a r t i n g

8.49 1.84 0.22 0.40 1.89 0.19 99.90

Material

Shaw e t a l . U 0 ) 50.14 13.37 8.20 10.13 1.21 10.80 2.32 0.27 0.53 2.63 0.17 99.77

i r o n as FeO

The e x p e r i m e n t a l r e s u l t s o b t a i n e d a t t h e QFM b u f f e r a r e l i s t e d i n T a b l e I I . and t y p e f o r m u l a s f o r t h e v a r i o u s m i n e r a l s e r i e s i n T a b l e III. O l i v i n e and chrome s p i n e l a r e t h e o n l y c r y s t a l l i n e phases which o c c u r between 1240°C and 1179°C; c l i n o p y r o x e n e and p l a g i o c l a s e f e l d s p a r c r y s t a l l i z e a t a p p r o x i m a t e l y 1170°C. The c o n c e n t r a t i o n o f c r y s t a l s i n c r e a s e s from about 22 weight p e r c e n t t o about 28 weight p e r c e n t between 1250°C and 1180°C. The l i q u i d l i n e o f d e s c e n t i s c h a r a c t e r i z e d by a s l i g h t S1O2, AI2O3 and a l k a l i enrichment and an FeO and MgO d e p l e t i o n . The volume p e r c e n t a g e o f m e l t as a f u n c t i o n o f temperature i s shown i n F i g u r e 1. The b r e a k i n s l o p e a t a p p r o x i m a t e l y 1170°C c o r r e s p o n d s t o t h e appearance o f c l i n o p y r o x e n e and p l a g i o c l a s e f e l d s p a r (see Table I I . ) . The volume p e r c e n t a g e o f m e l t , V , i s g i v e n by E q u a t i o n s 1 and 2: m

17.

WEED ET AL.

Rheological

Properties

of Molten

Kilauea

Table I I . Results o f Selected K i l a u e a I k i Liquidus

Publication Date: April 2, 1986 | doi: 10.1021/bk-1986-0301.ch017

1

Exp 1 No.

Time (Hrs)

Temp

14

93.0

1240

9

189.5

1230

8

24.0

1219

10

290.0

1209

12

289.0

1189

13

364.0

1179

16

380.0

1170

19

400.0

1160

20

400.0

1149

a) b) c)

Experiments

Wt % Melt

Vol % Melt

Experiment Products

CO

227

Iki Basalt

3

o l i v i n e , chrome spinel, glass o l i v i n e , chrome spinel, glass o l i v i n e , chrome spinel, glass o l i v i n e , chrome spinel, glass o l i v i n e , chrome spinel, glass o l i v i n e , chrome spinel, glass o l i v i n e , chrome spinel, clino­ pyroxene, p l a g i o ­ clase, glass ο1iν i n e , chrome spinel, clino­ pyroxene, p l a g i o ­ clase, glass o l i v i n e , chrome spinel, clino­ pyroxene, p l a g i o ­ clase, glass

78.1

71.8 80. 2

b

76.1

C

77.3

74.8

76.2

74.2

63.8

72.5

71.8

71.5

69.6

68.5

54.7

53.5

40.7

49.8

Volume p e r c e n t o f m e l t was d e t e r m i n e d by a 1000 p o i n t mode on metallograph. Weight p e r c e n t o f m e l t was d e t e r m i n e d by c o n s t r a i n e d l e a s t squares a n a l y s i s o f phase c o m p o s i t i o n s . Volume p e r c e n t g l a s s was d e t e r m i n e d by an 850 p o i n t mode on metallograph.

Table I I I .

Type Formulae

Name Olivine Chrome S p i n e l

Series

Type Formula (Mg, F e ) S 1 O 4 AB2O4

A = Mg, F e Β = Al, Fe Clinopyroxene

f o r Mineral

2 +

3 +

2 +

, Zn, M n , N i , Mn , Cr 3 +

ABS12O5 2 +

A = Mg, F e , Ca, Na Β = Mg, F e , A l Ranges from NaAlSi3U3 2 +

Plagioclase

to CaAl2Si 0g 2

MINERAL MATTER AND ASH IN COAL

228

V

m

(01+Chsp) = 0.157 V

= 1.36

m

T(°C)

T(°C)

- 114.0, where T(°C) > 1170,

- 1522.7, where T(°C)

(1)

< 1170.

(2)

E x t r a p o l a t i o n o f E q u a t i o n 1 t o V =100 c o r r e s p o n d s t o T=1360°C f o r the l i q u i d u s . E x t r a p o l a t i o n o f E q u a t i o n 3 t o V =0 y i e l d s T=1119°C f o r the d i s a p p e a r a n c e o f l i q u i d , the s o l i d u s t e m p e r a t u r e . T h i s i s a complex system f o r which complete phase diagrams a r e not a v a i l a b l e ; p s e u d o t e r n a r y diagrams such as those p r e s e n t e d by Grove e t a l . (_1) f o r s i m i l a r c o m p o s i t i o n s a r e g e n e r a l l y a p p l i c a b l e t o t h i s composition. m

Publication Date: April 2, 1986 | doi: 10.1021/bk-1986-0301.ch017

m

Viscometry. D u r i n g v i s c o s i t y measurements s i g m o i d a l t o r q u e v e r s u s r o t a t i o n speed c u r v e s a r e o b t a i n e d a t a l l i n v e s t i g a t e d temperatures. The c u r v e s a r e l i n e a r a t r o t a t i o n speeds l e s s than 0.4 r e v s / s e c , w i t h a p o s i t i v e s l o p e . The c u r v e s become concave toward the r o t a t i o n speed a x i s a t h i g h e r r o t a t i o n r a t e s , i n d i c a t i n g p s e u d o p l a s t i c b e h a v i o r ( 1 1 ) . T h i s b e h a v i o r becomes more pronounced a t low temperature as the c o n c e n t r a t i o n o f suspended c r y s t a l s increases. A l s o , a t a g i v e n r o t a t i o n a l speed, the o b s e r v e d t o r q u e appears t o be s l i g h t l y lower f o r n e g a t i v e r o t a t i o n than f o r positive rotation. This indicates s l i g h t thixotropic behavior of t h e sample, s i n c e p o s i t i v e r o t a t i o n i s used f i r s t d u r i n g experiments. At temperatures n e a r 1149°C, t o r q u e measurements a r e more e r r a t i c than a t h i g h e r t e m p e r a t u r e s . T h i s may be due t o s e g r e g a t i o n o f c r y s t a l s which causes the sample t o r o t a t e i n t e r m i t t a n t l y as a r i g i d body, o r due t o l o c a l i z e d r e m e l t i n g o f c r y s t a l s by v i s c o u s h e a t i n g which i n c r e a s e s a t lower t e m p e r a t u r e s . We have a n a l y z e d the r e s u l t s i n d i c a t e d by E q u a t i o n 3: log

Ιτ

I

= Α

+ A

χ

2

i n terms of the power

law

l o g Idu/dxl

(3)

where I T y I i s the a b s o l u t e v a l u e o f the shear s t r e s s and Idu/dxl the a b s o l u t e v a l u e o f the s h e a r r a t e , c a l c u l a t e d by a m o d i f i c a t i o n o f the method o f K r i e g e r and E l r o d (12) which a p p l i e s t o non-Newtonian systems. The a p p a r e n t v i s c o s i t y , μ , i s X

μ

= T /(du/dx) y x

(4)

The f l o w c u r v e , F i g . 2, i s a p l o t o f E q u a t i o n 3 showing e x p e r i m e n t a l r e s u l t s o b t a i n e d a t the 1186°C i s o t h e r m . Fig. 3 i s a log-log plot o f a p p a r e n t v i s c o s i t y as a f u n c t i o n o f s h e a r r a t e a t the same temperature. The apparent v i s c o s i t y d e c r e a s e s w i t h i n c r e a s i n g s h e a r r a t e , which i s c h a r a c t e r i s t i c f o r p s e u d o p l a s t i c systems (11). The l o g a r i t h m o f the v i s c o s i t y a t u n i t s h e a r r a t e , l o g μ , i s c a l c u l a t e d from E q u a t i o n 3: 0

log

μ

0

= Αχ

(5)

T a b l e IV g i v e s μ as a f u n c t i o n o f t e m p e r a t u r e . It v a r i e s from 9 Pa.s a t 1249°C t o 879 Pa.s a t 1149°C. The a c c u r a c y i s e s t i m a t e d as + 15% above 1170°C; a t 1149°C, where the system shows e r r a t i c b e h a v i o r , the a c c u r a c y i s e s t i m a t e d as + 50%. 0

Rheological

Publication Date: April 2, 1986 | doi: 10.1021/bk-1986-0301.ch017

WEED ET AL.

Properties

of Molten

1200

Kilauea

Iki Basalt

1150

T,°C F i g u r e 1. Volume p e r c e n t and weight p e r c e n t m e l t as a f u n c t i o n o f temperature f o r K i l a u e a I k i b a s a l t . V e r t i c a l bars i n d i c a t e two s t a n d a r d d e v i a t i o n s .

2.5 Ii

1.0

ι

I

I

1.4 log

ι

ι

ι

'

»

1.8 (Shear Rate,

•

'

t

'

2.2 sec

F i g u r e 2. L e a s t squares f i t o f l o g (Shear S t r e s s ) v s . l o g (Shear R a t e ) f o r K i l a u e a I k i b a s a l t a t 1186°C. V e r t i c a l b a r i n d i c a t e s two s t a n d a r d d e v i a t i o n s .

MINERAL MATTER AND ASH IN COAL

230

T a b l e IV. Apparent A c t i v a t i o n E n e r g i e s from L e a s t Squares A n a l y s i s o f l o g μ v s . 1/(T,K) f o r K i l a u e a I k i B a s a l t , Halemaumau B a s a l t , and C o a l S l a g X 0

System

Kilauea I k i

Publication Date: April 2, 1986 | doi: 10.1021/bk-1986-0301.ch017

Halemaumau Coal Slag X

T,°C

1249-1170 1170-1150 1300-1158 1158-1125 1482-1330 1330-1260

Apparent A c t i v a t i o n Energy K c a l mol""l

Standard D e v i a t i o n L e a s t Squares F i t

123 + 10 452 + 21 8 6 + 9 635 + 64 5 3 + 1 424 + 27

.09 .04 .12 .18 .013 .14

The r e s u l t s o f l e a s t - s q u a r e s a n a l y s e s o f l o g y v s . (1/T,K) a r e shown i n F i g u r e 4 and T a b l e V f o r t h i s i n v e s t i g a t i o n on K i l a u e a I k i b a s a l t , f o r t h e work o f Shaw on Halemaumau b a s a l t (1£,13) and f o r Corey's r e p o r t on C o a l S l a g X ( 1 4 ) . The K i l a u e a I k i d a t a show a sharp i n c r e a s e i n s l o p e a t 1170°C as i n d i c a t e d by the l i m i t i n g s t r a i g h t l i n e s above and below t h i s t e m p e r a t u r e . Below 1170°C, a p p r e c i a b l e c r y s t a l l i z a t i o n o c c u r s as shown i n T a b l e II and F i g . 1, and the system shows s t r o n g l y p s e u d o p l a s t i c behavior. Shaw's r e s u l t s on Halemaumau b a s a l t ( 1 3 ) , which i s a H a w a i i a n b a s a l t q u i t e s i m i l a r t o K i l a u e a I k i , show a sharp i n c r e a s e i n s l o p e a t 1158°C. T h i s system shows p s e u d o p l a s t i c b e h a v i o r a t 1125°C and Newtonian b e h a v i o r a t h i g h e r temperatures. The r e s u l t s o f Corey (14) on C o a l S l a g X i n d i c a t e a sharp change i n r h e o l o g i c a l b e h a v i o r a t 1330°C. H i s o r i g i n a l paper g i v e s no d e t a i l s on t h e c a l c u l a t i o n o f t h e v i s c o s i t y r e s u l t s , and no i n f o r m a t i o n on the c r y s t a l l i z a t i o n sequence o f the c o a l s l a g on which they were obtained. The s i m i l a r i t y o f t h e b a s a l t and c o a l s l a g d a t a i n d i c a t e s t h a t r h e o l o g i c a l b e h a v i o r o f the c o a l s l a g may be a f f e c t e d by suspended c r y s t a l l i n e m a t e r i a l a t temperatures n e a r 1330°C. A r e c e n t study on s l a g g i n g i n l a r g e c o a l - b u r n i n g f u r n a c e s c o r r e l a t e s the s l a g g i n g b e h a v i o r w i t h ash c o m p o s i t i o n , a s h p a r t i c l e morphology, and c a l c u l a t e d c r i t i c a l v i s c o s i t y temperature T ( 1 5 ) . The methods o f the p r e s e n t i n v e s t i g a t i o n c a n be a p p l i e d i n o r d e r t o measure the c r i t i c a l v i s c o s i t y temperatures and c o m p o s i t i o n s d i r e c t l y , which s h o u l d improve the e x p e r i m e n t a l b a s i s f o r the c o r r e l a t i o n . Q

c v

Publication Date: April 2, 1986 | doi: 10.1021/bk-1986-0301.ch017

WEED ET AL.

Rheological

1 16 I 1.0

ι

Properties

ι

ι

ι 1.4

log

of Molten

1—ι

1

Kilauea

Iki Basalt

»—J

1

"

1.8 (Shear Rate,

1—

2.2 sec

)

F i g u r e 3. Log (Apparent V i s c o s i t y ) v s . l o g (Shear R a t e ) f o r K i l a u e a I k i b a s a l t a t 1186°C. V e r t i c a l b a r i n d i c a t e s two standard d e v i a t i o n s .

0 0 ' 5.6

1

'

1

1

1

1

1

1

— —

6.0

1

1

1

6.4

1

—

1

1

— — 6.8

1

— ' 7.2

H

10 /(T,K) F i g u r e 4. L e a s t squares a n a l y s i s o f l o g ( V i s c o s i t y ) v s . r e c i p r o c a l temperature. P r e s e n t s t u d y O ; Shaw ( 1 3 ) Δ ; Corey (14) • . V e r t i c a l b a r s i n d i c a t e two s t a n d a r d d e v i a t i o n s .

232

MINERAL MATTER AND ASH IN COAL

T a b l e V· Apparent V i s c o s i t y a t U n i t Shear Rate ( μ ) v s . Temperature (t,°C) f o r K i l a u e a I k i B a s a l t i c L a v a 0

Publication Date: April 2, 1986 | doi: 10.1021/bk-1986-0301.ch017

Run No. 57 63 58 64 59 65 60 66 61 62 67 68

T,°C 1249 1249 1235 1229 1220 1208 1201 1188 1186 1171 1170 1149

μ, Pa.s 13 9 9 14 15 25 38 51 52 71 94 879

I n b o t h t h e s t u d i e s on b a s a l t s , t h e b r e a k s i n t h e s l o p e o f the l o g μ v s . (1/T,K) c u r v e s o c c u r a t 20 t o 30 volume p e r c e n t o f suspended c r y s t a l s . The non-Newtonian b e h a v i o r o f t h e s e m o l t e n s i l i c a t e s u s p e n s i o n s appears t o a r i s e from the i n c r e a s i n g c o n c e n t r a t i o n o f suspended c r y s t a l s i n t h e m e l t . T h i s s u g g e s t s t h a t i n m o d e l i n g f l u i d f l o w i n s i l i c a t e systems, power law b e h a v i o r s h o u l d be c o n s i d e r e d when t h e suspended c r y s t a l c o n c e n t r a t i o n exceeds 20 volume p e r c e n t . 0

Acknowledgments T h i s work was performed under t h e a u s p i c e s o f t h e U.S. Department o f Energy by t h e Lawrence L i v e r m o r e N a t i o n a l L a b o r a t o r y under c o n t r a c t number W-7405-ENG-48.

Literature Cited 1. 2.

3. 4.

5.

Grove, T. L.; Gerlach, D. C . ; Sando, T. W. Contrib. Mineral. Petrol. 1982, 80, 160-182. Deines, P; Nafziger, R. H . ; Ulmer, G. C . ; Woermann, E. "Temperature-Oxygen Fugacity Tables for Selected Gas Mixtures in the C-H-O System at One Atmosphere Total Pressure"; Bulletin, Earth and Mineral Sciences Experiment Station, No. 88, College of Earth and Mineral Sciences, The Pennsylvania State University, University Park, PA, 1971. Huebner, J . S. In "Research Techniques for High Pressure and High Temperature"; Ulmer, G. C . , Ed.; Springer-Verlag, New York, 1971; pp. 123-177. Williams, Richard J.; Mullins, O. "A System Using Solid Ceramic Oxygen Electrolyte Cells to Measure Oxygen Fugacities in Gas-Mixing Systems"; Technical Memorandum TMX-58167, Lyndon B. Johnson Space Center, Houston, TX 77058 1976. Weed, H. C . ; Dibley, L.; Piwinskii, A. J . "A High-Temperature Viscometer for Use at 100 kPa"; UCRL-52477, Lawrence Livermore National Laboratory, Livermore, CA 94550, 1978.

17. WEED ET AL.

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6. 7.

Rheological Properties of Molten Kilauea Iki Basalt

233

Grove, T. L. Contrib. Mineral. Petrol. 1981, 78, 298-304. "The F and Ρ Tri-flat (Low Flow Rate) Variable Area Flowmeter Handbook; Application, Sizing, and Calibration Prediction Data", Catalog 10A9010, Fischer and Porter Co., Hatboro, Pa, Pub. 13317, September, 1959. 8. Darken, L. S.; Gurry, R. W. J. Am. Chem. Soc., 1945, 67, 1398-1412. 9. Weed, H. C.; Piwinskii, A. J.; Dibley, L. L. "Experimental Study of the Dynamic Viscosity of Some Silicate Melts to 1953K at 150 kPa", UCRL-52757, Lawrence Livermore National Laboratory, Livermore, CA, 94550, 1979, p. 2. 10. Shaw, H. R.; Wright, T. L . ; Peck, D. L . ; Okamura, R. Amer. Jour. Sci. 1968, 266, 225-64. 11. Skelland, A. H. P. "Non-Newtonian Flow and Heat Transfer"; J. Wiley and Sons, New York, 1967, pp. 5-12. 12. Krieger, I.; Elrod, H. J. Appl. Phys. 1953, 14, 134-6. 13. Shaw, H. R. Jour. Petrology 1969, 10, 510-535. 14. Corey, R. C. "Measurement and Significance of the Flow Properties of Coal-Ash Slag"; Bulletin No. 618, U. S. Bureau of Mines 1964, pp. 1-64. 15. Hazard, H. R. "Influence of Coal Mineral Matter on Slagging of Utility Boilers"; EPRI CS-1418, Project 736, Final Report, Electric Power Research Institute, Palo Alto, CA 94304, 1980. RECEIVED October 24, 1985

18 Crystallization of Coal Ash Melts D . P. Kalmanovitch and J. Williamson 1

Publication Date: April 2, 1986 | doi: 10.1021/bk-1986-0301.ch018

Department of Metallurgy and Materials Science, Imperial College, London SW7, England

When coal is burnt in pulverized fuel (p. f.). u t i l i t y boilers the inorganic material inherent to the fuel may form deposits on the heat absorbing surfaces causing significant reduction in thermal efficiency. This study has involved the investigation of the crystallization of coal ash melts and the relationship to the formation and growth of these troublesome deposits. The crystallization behaviour of compositions in the systems CaO-Al O -SiO , CaO-MgO-Al O -SiO and CaO-Al O -SiO -iron oxide relevant to coal ash compositions has been studied. The observations have been compared with the devitrification behaviour of both eastern and western type coal ashes determined under laboratory conditions. Results show that the crystallization of ash melts is well represented by the system CaO-FeO-Al O -SiO . The quaternary system has been constructed from literature data as planes of constant FeO content (5-30 wt% at 5% increments) and used to predict the behaviour of boiler deposits. 2

2

3

3

2

2

3

2

3

2

3

2

The formation of ash deposits within c o a l - f i r e d b o i l e r s may cause serious operating problems and reduction i n thermal e f f i c i e n c y . These deposits vary i n nature from f r i a b l e , s l i g h t l y sintered f o u l i n g to dense semi-vitreous slags. U t i l i t y b o i l e r designers and operators use a v a r i e t y of methods (1) to ascertain various design c r i t e r i a or the l i k e l i h o o d of an ash to form deposits (it's slagging propensity). Chief amongst these methods i s the standard ash fusion test (2) in which a coal ash i s heated at a given rate i n a i r and a m i l d l y reducing atmosphere while the temperatures a t which various

Current address: Energy Research Center, University of North Dakota, Grand Forks, N D 58202.

1

0097-6156/ 86/ 0301 -O234$06.50/ 0 © 1986 American Chemical Society

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KALMANOVITCH AND

WILLIAMSON

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of Coal Ash

Melts

235

d e g r e e s o f d e f o r m a t i o n o f a cone o f ash a r e r e c o r d e d . T h i s method has been known t o g i v e i n a c c u r a t e i n d i c a t i o n s o f s l a g g i n g p r o p e n s i t y due m a i n l y t o t h e s u b j e c t i v e n a t u r e o f t h e t e s t and t h e f a c t t h a t t h e t e c h n i q u e does n o t d u p l i c a t e the t h e r m a l h i s t o r y e x p e r i e n c e d b y c o a l m i n e r a l m a t t e r i n a p u l v e r i z e d f u e l (p. f.) b o i l e r . V a r i o u s l a b o r a t o r y s t u d i e s , e.g. t h o s e o f P a d i a (3) and Raask (4) h a v e shown t h a t s i g n i f i c a n t f u s i o n o f c o a l m i n e r a l m a t t e r (the p r e c u r s o r t o ash) o c c u r s w i t h i n a v e r y s h o r t time-frame a t temperatures a n a l o g o u s t o t h o s e i n t h e combustion zone o f a b o i l e r (about 1550-1650 C). T h i s i s c o n f i r m e d by m i c r o s c o p i c s t u d i e s o f p. f . f l y - a s h . The r e s i d e n c e t i m e s o f a s h p a r t i c l e s i n u t i l i t y b o i l e r systems has been e s t i m a t e d by Raask t o be 2 seconds w i t h a temperature g r a d i e n t r a n g i n g from 1650 t o 300 C. The s t u d i e s show t h a t t h e c h a r a c t e r i s t i c n a t u r e o f f l y ash, t h a t o f h o l l o w cenospheres i s due t o t h e f o r m a t i o n o f a s i g n i f i c a n t amount o f l i q u i d phase w i t h i n t h e 2s time frame. T h i s i s f u r t h e r c o n f i r m e d b y m i n e r a l o g i c a l a n a l y s i s o f t h e p a r t i c l e s which r e v e a l s an amorphous phase ( g l a s s ) and sometimes t h e p r e s e n c e o f a r e f r a c t o r y phase which had c r y s t a l l i z e d from a l i q u i d . Both f e a t u r e s are not products o f the thermal decomposition of i n d i v i d u a l c o a l m i n e r a l p a r t i c l e s (4,5,6). Raask (4) has shown t h a t t h e growth o f d e p o s i t s i s p r e d o m i n a n t l y v i a s i n t e r i n g by v i s c o u s f l o w . The r a t e o f i n c r e a s e i n s t r e n g t h (s) i.e. growth o f t h e d e p o s i t b e i n g g i v e n by: ds

3yk

dt

2 nr

Where γ i s t h e s u r f a c e t e n s i o n c o e f f i c i e n t , k a c o n s t a n t , r t h e i n i t i a l r a d i u s o f t h e p a r t i c l e and η the v i s c o s i t y of the l i q u i d phase. I t c a n be seen t h a t t h e n a t u r e o f a d e p o s i t can be d e s c r i b e d by t h e degree o f s i n t e r i n g which has t a k e n p l a c e . F o r a g i v e n c o a l ash t h e p a r a m e t e r s r and γ a r e e f f e c t i v e l y c o n s t a n t w i t h r e s p e c t t o the v a r i a b i l i t y of the v i s c o s i t y . T h e r e f o r e t h e f a c t o r s which a f f e c t the r h e o l o g i c a l behaviour w i l l determine t o a g r e a t extent t h e r a t e o f growth by v i s c o u s f l o w . F o r homogeneous m e l t s t h e d e t e r m i n i n g f a c t o r s a r e temperature and c h e m i c a l c o m p o s i t i o n . Lauf (6) has o b s e r v e d t h a t t h e amount o f f l y a s h p a r t i c l e s e m i t t e d a t t h e o u t l e t o f v a r i o u s b o i l e r s was i n v e r s e l y p r o p o r t i o n a l t o t h e c a l c u l a t e d v i s c o s i t y ( u s i n g a method based on c h e m i c a l composition) a t a g i v e n temperature. T h i s q u a l i t a t i v e l y confirms the r e l a t i o n above. A f a c t o r which has n o t r e c e i v e d a t t e n t i o n i n t h e l i t e r a t u r e i s t h e c r y s t a l l i z a t i o n o f t h e a s h p a r t i c l e s and/or t h e d e p o s i t s a l r e a d y p r e s e n t . W i t h d e v i t r i f i c a t i o n o f a phase from a homogeneous m e l t t h e c o m p o s i t i o n o f t h e l i q u i d phase w i l l change depending on t h e p r e c i p i t a t i n g phase and t h e d e g r e e o f c r y s t a l l i z a t i o n . This i n t u r n w i l l d i r e c t l y a f f e c t t h e v i s c o s i t y and t h e r e f o r e t h e r a t e o f growth o f t h e d e p o s i t s . Thus i t w i l l be o f g r e a t v a l u e t o be a b l e t o p r e d i c t t o some e x t e n t the c r y s t a l l i z a t i o n behaviour of c o a l ash m e l t s . For s i m p l i c i t y i t i s n e c e s s a r y t o c o n s i d e r t h a t c r y s t a l l i z a t i o n w i l l be from a homogeneous m e l t . The d a t a may be a p p l i e d t o t h e phenomena o f b o i l e r s l a g g i n g by u s i n g an a c c u r a t e model o f t h e v i s c o s i t y o f ash m e l t s based on c h e m i c a l c o m p o s i t i o n . The main aim o f t h e s t u d y has been t o o b t a i n r e l e v a n t c r y s t a l l i z a t i o n d a t a o f c o a l ashes and

M I N E R A L M A T T E R A N D A S H IN C O A L

236

to model the behaviour so as to be able to predict the d e v i t r i f i c a t i o n of a given ash. Coal ash i s u s u a l l y described as a mixture of up to 11 oxide components and though various investigators (7) have attempted to s i m p l i f y the system by using certain empirical equivalences the authors chose to use the major three or four components to model the behaviour. For eastern type coal ashes the major components are s i l i c a , alumina, iron oxide and lime, whereas for western type ashes the major components are s i l i c a , alumina, lime and iron oxide or magnesia. In both cases these components may comprise 90 wt% or more of the t o t a l composition of the ash. Sanyal and Williamson (8) have shown that the i n i t i a l c r y s t a l l i z a t i o n of two western type ashes of low iron oxide contents (5% or less) could be described by the normalized composition on the ternary equilibrium diagram of the system CaO-Al 0 -Si0 . That i s , the c r y s t a l l i z a t i o n behaviour of the ashes was the same as that which would be expected i f the remaining components were not present. The study presented here has extended the investigation to the d e v i t r i f i c a t i o n of an eastern type as w e l l as western type ashes and the c r y s t a l l i z a t i o n of compositions i n the systems CaO-Al-O^-SiO^r CaO-MgO-Al 03-Si0 , and C a O - A l 0 - S i 0 - i r o n oxide r e l e v a n t to c o a l ashes. The study of the ternary and quaternary systems was to determine which governed the behaviour of ashes most adequately. The r e s u l t s have been used to look at the behaviour of ash melts i n r e l a t i o n to the phenomena of b o i l e r deposits.

Publication Date: April 2, 1986 | doi: 10.1021/bk-1986-0301.ch018

2

3

2

2

2

3

2

2

Experimental C r y s t a l l i z a t i o n of ash melts: The three ashes studied were supplied by Babcock Power Ltd. (U. K.) and the chemical compositions are l i s t e d i n Table I. Ashes 1 and 2 were of western type from c o a l s f i r e d at Matla Power Station, South A f r i c a whereas ash 3 was of eastern type from a coal f i r e d at Drax Power Station, U. K.. The roasted ashes (treated at 800 C overnight) were fused at 1500-1550 C for up to 1 hr. i n Pt envelopes i n an e l e c t r i c muffle furnace. C r y s t a l l i z a t i o n of the melts was induced by reducing the temperature of the furnace to a given l e v e l and l e a v i n g for a given time. The process was repeated over a range of temperatures and times for each of the ashes. The samples were analyzed by pétrographie microscopy and powder X-ray d i f f r a c t i o n .

Table I.

SiO.

Chemical composition of coal ashes studied

Ash 1 43.1 27.8 5.1 17.3 3.8 0.7

wt% Ash 2 51.2 29.0 4.8 9.5 2.6 0.6

Ash 3 54.5 26.4 9.4 1.5 1.6 1.2

18.

KALMANOVITCH A N D WILLIAMSON

Crystallization

of Coal Ash Melts

237

C r y s t a l l i z a t i o n of model compositions: A series of f i v e CaO, and SiO^ compositions were chosen (designated A, B, C, D and E) to be of relevance to coal ashes. Composition A was the normalized composition of ash 1 whereas that of Β corresponded to that of an ash studied by Sanyal and Williamson (8). The compositions have been l i s t e d i n Table II. Homogeneous glasses were prepared for each of the compositions by fusing the correct mixture of components i n a Pt c r u c i b l e at 1550 C. C r y s t a l l i z a t i o n of each g l a s s was determined by remelting a small sample i n a Pt envelope at 1550 C for 0.5 hr. and reducing the temperature to a given l e v e l for a given time. The quenched samples were analyzed using pétrographie microscopy and X-ray d i f f r a c t i o n . AI9O3

Publication Date: April 2, 1986 | doi: 10.1021/bk-1986-0301.ch018

Table I I .

Composition of CaO-Al 0 --Si0 glasses. 2

Glass

sio

A Β C D Ε

49.0 37.0 41.5 44.0 49.0

2

Composition A1 0 2

3

2

wt% CaO

3

19.5 38.0 31.5 27.5 16.0

31.5 25.0 27.0 28.5 35.0

The c r y s t a l l i z a t i o n of glasses i n the system CaO-MgO-Al^^-SiO^ was studied as follows: The f i v e ternary compositions A-E were used as the basis of two series of glasses with 5 and 10 wt% MgO. This covers the usual range of magnesia content i n coal ashes. The compositions are l i s t e d i n Table III. The preparation of the glasses, the determination of the c r y s t a l l i z a t i o n behaviour and analysis of quenched samples were analogous to the methods used for the ternary compositions.

Table III. Composition of CaO-MgO-Al 0 -Si0 glasses. 2

Glass

Si0

am5 bm5 cm5 dm5 em5 aml0 bml0 cml0 dml0 eml0

47.0 35.0 39.0 42.0 47.0 44.1 33.3 37.6 39.6 44.1

2

Composition A1 0 2

30.0 24.0 26.0 27.0 33.0 28.4 22.5 24.3 25.6 31.5

3

3

wt% CaO 18.0 36.0 30.0 26.0 15.0 17.5 34.2 28.3 24.8 14.4

2

MgO 5.0 5.0 5.0 5.0 5.0 10.0 10.0 10.0 10.0 10.0

Iron can e x i s t i n three oxidation states but i n a modern p. f.

MINERAL MATTER AND ASH IN COAL

238

b o i l e r only the states of ferrous and f e r r i c are observed. Unlike magnesia, the range of iron oxide content i n coal ashes i s quite wide; some eastern type ashes may have an iron oxide content of 50% though 15 - 20 % i s more common. Therefore the study of the system C a 0 - A l 0 - S i 0 - i r o n oxide must take into account both the wide range of iron oxide contents and the v a r i a b l e oxidation states. The f i v e ternary compositions A-E were used as the basis of two series of compositions with 5 and 10 wt% equivalent FeO. A further two series of glasses were prepared based on the compositions A, Β and Ε with 15 and 20 wt% equivalent FeO. The compositions are l i s t e d i n Table IV. Glasses were prepared for each composition by mixing the correct proportions (with CaCO^ as the source of CaO and iron [II] oxalate, 994* 2° °urce of FeO) and fusing i n a Pt c r u c i b l e at Γ500-1550 C i n an e l e c t r i c muffle furnace. C r y s t a l l i z a t i o n of the homogeneous glasses was performed i n a v e r t i c a l tube furnace with a c o n t r o l l e d atmosphere (C0 /H 1:1 v/v with a t o t a l flow rate of 200 ml per min.). This i s the same atmosphere used i n the ash fusion test (2). The quenched sample was analyzed by pétrographie microscopy, powder X-ray d i f f r a c t i o n and chemical analysis (to determine the ferrous/total iron ratio) using the method of Hey (9). 2

3

Publication Date: April 2, 1986 | doi: 10.1021/bk-1986-0301.ch018

F e C

2

2 H

a s

t h e

s

2

2

Composition of Ca0-Al 0 --Si0 -iron oxide glasses.

Table IV.

2

sio

Glass

2

47.0 af5 bf5 35.0 39.0 cf5 42.0 df5 47.0 ef5 44.0 afl0 bfl0 33.3 37.4 cfl0 dfl0 39.5 44.0 efl0 afl5 41.6 bfl5 31.5 efl5 41.6 af20 39.2 bf20 29.6 ef20 39.2 *equivalent FeO

3

Composition A1 0 2

30.0 24.0 26.0 27.0 33.0 28.4 22.4 24.2 25.6 31.6 26.8 21.2 29.8 25.2 20.0 28.0

3

2

wt% CaO

FeO* 5.0 5.0 5.0 5.0 5.0 10.0 10.0 10.0 10.0 10.0 15.0 15.0 15.0 20.0 20.0 20.0

18.0 36.0 30.0 26.0 15.0 17.6 34.3 28.4 24.8 14.4 16.6 32.3 13.6 15.6 30.4 12.8

Results C r y s t a l l i z a t i o n of ash melts: The r e s u l t s of the c r y s t a l l i z a t i o n of the three ash melts studied are l i s t e d i n Table V. The only phase observed to p r e c i p i t a t e for the western type ashes was anorthite (Ca0.Alo0 .2Si0 ) despite lengthy treatment. The eastern type ash showed d i f f e r e n t behaviour with m u l l i t e (3Al 0 .2Si0 ) as the 3

2

2

3

2

18.

KALMANOVITCHA N D WILLIAMSON

Crystallization

of Coal Ash

Melts

239

primary phase. A secondary phase of hematite (FeC>3) was detected in a sample treated at 1250 C but o p t i c a l a n a l y s i s revealed that t h i s phase was only on the surface (i.e. the air-melt interface). The true secondary phase was observed i n samples treated i n the temperature range 1200-1100 C and was determined to be a member of the iron s p i n e l s o l i d s o l u t i o n series; magnetite (FeO.Fe20 )hercynite (FeO.A^O^). Hematite was again observed i n samples quenched from temperatures below 1100 C (at the surface of the sample). 2

3

Publication Date: April 2, 1986 | doi: 10.1021/bk-1986-0301.ch018

Table V. C r y s t a l l i z a t i o n behaviour of coal ash melts. C r y s t a l l i z a t i o n treatment* Temp./ C Time/h. 1390 Ash 1 80 1150 180 1100 42 22 1050 950 80 900 24 Ash 2 1300 1 1200 10 24 1100 1000 15 1400 Ash 3 18 1350 2 1200 20 1200 40 4 1170 1100 26 2 1050

C r y s t a l l i n e phases observed. glass anorthite anorthite anorthite anorthite anorthite anorthite anorthite anorthite anorthite mullite m u l l i t e + hematite m u l l i t e + iron spinel mullite + iron spinel m u l l i t e + iron spinel m u l l i t e + iron spinel m u l l i t e + hematite

* C r y s t a l l i z a t i o n treatment a f t e r fusion a t 1450 - 1550 C.

C r y s t a l l i z a t i o n of ternary compositions: The observed c r y s t a l l i z a t i o n behaviour o f the f i v e ternary glasses studied has been compiled i n Table VI. The l i q u i d u s temperature included i n the Table was obtained from the equilibrium phase diagram of the system for each of the compositions (11). The temperature of appearance of the secondary or t e r t i a r y phase i s that o f the highest treatment temperature at which that phase was observed. Only the g l a s s Β was observed to c r y s t a l l i z e to the three components expected from the phase diagram,i.e. the primary phase of gehlenite (2CaO.Al2O3.SiO2), followed by anorthite and c y c l o w o l l a s t o n i t e (aCaO.SiO?). The remaining glasses only c r y s t a l l i z e d anorthite despite lengthy treatment at temperatures near their respective eutectics. C r y s t a l l i z a t i o n of CaO-MgO-A^O^-S^ glasses: The r e s u l t s of the c r y s t a l l i z a t i o n studies o f the quaternary glasses have been compiled in Table VII. The liquidus temperature was determined experimentally by extensive series of quenching runs. The temperature of appearance of the secondary and t e r t i a r y phase i s the highest temperature a t which the respective phase was observed. The phase m e l i l i t e i s a s o l i d s o l u t i o n series between gehlenite and

3

1520 1450 1390 1450 1510

Liquidus Temp/C Anorthite Gehlenite Anorthite Anorthite Anorthite

Primary Phase

2

n.o. Anorthite n.o. n.o. n.o.

1100 76 1200 60 1100 72 1100 105 1100 72

2

Secondary Phase

Temp. Sec.* h. C

60

-

-

—

_ 1200

Temp. Tert.* h. C

glasses.

n.o. Cyclowoll. n.o. n.o. n.o.

Tertiary Phase

2

2

3

2

3

2

* Temperature of appearance of subsequent pahse a f t e r temperature treatment listed. n.o. - not observed Anorthite - CaO.Al 0 .2Si0 , Gehlenite - 2CaO.Al 0 .Si0 , Cyclowoll. - cyclowollastonite CaO.Si0 (high - temperature polymorph)

A Β C D Ε

Glass

2

Table VI. C r y s t a l l i z a t i o n behaviour observed for the C a O - A l 0 - S i 0

Publication Date: April 2, 1986 | doi: 10.1021/bk-1986-0301.ch018

1450 1420 1350 1410 1470 1420 1460 1450 1440 1455

am5 bm5 cm5 dm5 em5 aml0 bml0 cml0 dml0 eml0

Anorthite Melilite Anorthite Anorthite Anorthite Anorthite Spinel Spinel Spinel Spinel

Primary Phase

1360 1300 1200 1350 1400

-1350

1300 1300

_ n.o. Anorthite Melilite n.o. Spinel Spinel Melilite Anorthite Anorthite Anorthite

Temp. Sec. Secondary Phase/ C Phase

-

1200

-1200

1300

-1200

1200 1200

_

Temp. Tert. Phase/ C

n.o. Spinel Spinel n.o. Cordierite Melilite n.o. Melilite Melilite n.o.

Tertiary Phase

2

3

2

3

2

2

3

M e l i l i t e : S o l i d s o l u t i o n gehlenite (2CaO.Alo0 .Si0 ) - akermanite (2CaO.Mg0.2Si0 ). C o r d i e r i t e : 2Mg0.2Al 0 .5Si0 . Spinel: MgO.Al 0 . n.o. - not observed

2

Liquidus Temp/ C

Glass

Table VII. Observed c r y s t a l l i z a t i o n behaviour of CaO-MgO-Al203-Si02 glasses.

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MINERAL MATTER AND ASH IN COAL

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akermanite (2Ca0.Mg0.2Si0 ). Of the glasses containing 5% MgO o n l y bm5, cm5 and em5 c r y s t a l l i z e d to three components. Glasses am5 and dm5 only precipitated anorthite. A l l the glasses containing 10% MgO were observed to have a primary c r y s t a l l i n e phase of spinel (Mg0.Al 03) except aml0 where the primary phase was anorthite. The glasses bml0 and aml0 only precipitated the primary and secondary phases whereas the remaining glasses c r y s t a l l i z e d to three components. There was no evidence of a fourth phase i n any of the treated samples. C r y s t a l l i z a t i o n of CaO-Al 03-Si0 -iron oxide glasses: The d e v i t r i f i c a t i o n behaviour of the iron oxide glasses has been compiled i n Table VIII. The liquidus temperature indicated i s the highest treatment temperature at which a c r y s t a l l i n e phase was or was not present (hence the inequality). The temperature of appearance of subsequent phases i s analogous to that described above. While only anorthite was observed to c r y s t a l l i z e for the g l a s s af5, the other glasses precipitated secondary and t e r t i a r y phases and those of c f l 0 and b f l 5 c r y s t a l l i z e d to four components. The calcium s i l i c a t e phase determined i n some of the quenched samples was either c y c l o w o l l a s t o n i t e or the low temperature polymorph w o l l a s t o n i t e depending on the heat treatment. The g l a s s bf20 showed anomalous behaviour: the i n i t i a l secondary phase observed was iron wollastonite, (CaO,FeO).Si0 but at lower temperatures, l e s s than 1100 C anorthite and o l i v i n e (CaO.FeO.SiO^) were present with the primary phase of m e l i l i t e . A f u l l description of the r e s u l t s may be found elsewhere (10). The r e s u l t s of the analysis of the ferrous/total iron r a t i o showed wide variation. The value was found to be dependent on the composition of the melt, temperature treatment and c r y s t a l l i z a t i o n behaviour. Nevertheless there was a s i g n i f i c a n t amount of f e r r i c iron present i n most of the samples. 2

2

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2

2

2

Discussion To be able to c o r r e l a t e the c r y s t a l l i z a t i o n behaviour of the ash melts i t i s necessary to describe the d e v i t r i f i c a t i o n of the model compositions. In the case of the ternary glasses the c r y s t a l l i z a t i o n behaviour can be understood from the equilibrium phase diagram, Figure 1. The compositions have been p l o t t e d on the diagram and the i n i t i a l c r y s t a l l i z a t i o n paths drawn. The c r y s t a l l i z a t i o n path i s the extrapolation of the l i n e connecting the composition of the primary phase and the composition of the g l a s s to a given point on the diagram within the primary phase f i e l d , u s u a l l y an isotherm. The path indicates the change in composition of the l i q u i d phase as c r y s t a l l i z a t i o n proceeds and can be used to evaluate the proportion of l i q u i d and s o l i d phases at a given temperature. For more d e t a i l s see Levin et a l . (11). As can be seen, the primary phase predicted from the diagram agrees with the observed behaviour of the ternary glasses. The reluctance of the secondary phase to c r y s t a l l i z e for glasses A and Ε can be understood from the c r y s t a l l i z a t i o n path. As c r y s t a l l i z a t i o n of anorthite proceeds the residual l i q u i d phase becomes enriched i n s i l i c a , becoming more viscous and stable towards d e v i t r i f i c a t i o n . Glasses C and D were expected to c r y s t a l l i z e to two or more components. The observed behaviour of o n l y the primary phase p r e c i p i t a t i n g may be because

f

>1480 >1300 >1450 >1400 1400 >1400 1350 1300 JSi0 ) - nay take up iron in solid solution; iron spinel - solid solution EeOEe£> - E e O A l ^ ; fayalite (aEedSiC^); iron wollast. [(CaO EeO).Si02]; calcium silicate (wollastonite or cyclo^llastenite) ; olivine (CaO.EeO.SiC^).

af5 bf5 cf5 df5 ef5 afL0 bfL0 cfl0 dfL0 efl0 afl5 bfl5' e£15 af20 bf20 ef20

Glass Liquidas Tenp/C

Table VIII. Observed crystallization behaviour for the CaOAl^-eiO^-iron oxide glasses.

Publication Date: April 2, 1986 | doi: 10.1021/bk-1986-0301.ch018

Figure

1. System

CaO-Al^C^-SiC^.

Publication Date: April 2, 1986 | doi: 10.1021/bk-1986-0301.ch018

ζ η

ac

C/)

>

α

ζ

>

H

m

25

> r

ζm

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KALMANOVITCH AND WILLIAMSON

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245

i n s u f f i c i e n t time was allowed for the samples to reach equilibrium. Glass Β has a r e l a t i v e l y high lime content and although c r y s t a l l i z a t i o n of gehlenite would produce a l i q u i d phase enriched i n s i l i c a the l i q u i d would be expected to be r e l a t i v e l y f l u i d compared to those of the other compositions. The c l o s e proximity of the composition to the boundary and eutectic would tend to prevent any enrichment of viscous components i n the l i q u i d phase from having a profound effect. Therefore the c r y s t a l l i z a t i o n of g l a s s Β to three components would not be unexpected. The c r y s t a l l i z a t i o n behaviour of the glasses containing MgO i s more d i f f i c u l t to describe. Unlike ternary systems, four component phase diagrams are properly represented by a regular tetrahedron with each apex representing 100% of each component. The c r y s t a l l i z a t i o n path i s therefore i n three dimensions which i s sometimes d i f f i c u l t to v i s u a l i z e . The method of representation chosen to describe the observed behaviour i s as planes of constant MgO content in the regions of interest to t h i s study. Figures 2, 3, and 4 are planes of the quaternary system at 5, 10 and 15 wt% l e v e l s r e s p e c t i v e l y and have been compiled from various sources (10,12,13,14). Analysis of the planes indicates the profound e f f e c t magnesia has on the primary phase f i e l d s . The 5% plane contains magnesia bearing phases such as s p i n e l and pyroxene (a s o l i d s o l u t i o n series between diopside, CaO.Mg0.2Si02 and enstatite, MgO.Si0 ). On the 10% MgO plane the s p i n e l f i e l d i s very dominant and the primary phase f i e l d s of the base ternary system (i.e. CaOAI2O3-S1O2) have a very minor presence. The 15% MgO plane, Figure 4 i s predominantly composed of magnesia bearing phases with s p i n e l being dominant. The composition of the glasses studied have been p l o t t e d on the appropriate plane. The phase diagram not only c o r r e c t l y predicts the primary c r y s t a l l i z a t i o n observed for the quaternary glasses but a l s o indicates what subsequent d e v i t r i f i c a t i o n may take place. While i t i s too complex to discuss here i t i s p o s s i b l e to c a l c u l a t e the change i n composition of the l i q u i d phase with c r y s t a l l i z a t i o n for multi-component systems. For the purposes of t h i s study i t i s only necessary to q u a l i t a t i v e l y indicate the c r y s t a l l i z a t i o n path. With reference to the 5% MgO plane and compositions f a l l i n g i n the primary f i e l d of anorthite i t i s c l e a r that as the phase p r e c i p i t a t e s the l i q u i d must become p r o p o r t i o n a l l y richer in magnesia. Therefore the c r y s t a l l i z a t i o n path must be towards the MgO apex of the system. The actual d i r e c t i o n of the path w i l l be dependent on the o r i g i n a l composition. As the 10% plane comprises mainly MgO containing phases the secondary phase would be expected to be a magnesia bearing phase. The reverse s i t u a t i o n i s a l s o true, for compositions on the 10% plane and i n the primary phase f i e l d of s p i n e l the secondary phase would be expected to comprise components of the base ternary systems. This behaviour was observed with the compositions studied in t h i s investigation with the exceptions of those samples which c r y s t a l l i z e d anorthite only. The behaviour of these glasses can be explained in terms of the v i s c o s i t y of the melt hindering further c r y s t a l l i z a t i o n . This generalization of c r y s t a l l i z a t i o n of quaternary systems must be used with care e s p e c i a l l y when the p r e c i p i t a t i n g phase may be a s o l i d solution. Under the laboratory conditions used to study the glasses containing iron oxide both f e r r i c and ferrous iron would be expected 2

MINERAL MATTER AND ASH IN COAL

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246

Figure

2. System C a 0 - M g 0 - A l 0 o - S i 0 , 2

2

5% MgO

plane.

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MINERAL MATTER AND ASH IN COAL

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248

to be present. Therefore the system i s a f i v e component or quinary. These systems are extremely d i f f i c u l t to represent and i t i s therefore necessary to make s i m p l i f i c a t i o n s to obtain a c l e a r e r understanding of the data. With reference to the r e s u l t s obtained, Table VIII, i t can be seen that when an iron-bearing phase was observed the form of iron was ferrous, e.g. f a y a l i t e , 2Fe0.Si02« (There was some evidence of Fe^C^ i n s o l i d solution with the iron s p i n e l phase.). Therefore as phase diagrams represent the c r y s t a l l i n e phases which are i n equilibrium with a l i q u i d phase i t i s possible to describe the system C a O - A ^ C ^ - S ^ - i r o n oxide as a quaternary. I t i s assumed that a l l the iron i s present as FeO. Using t h i s approximation planes of constant FeO content were c o n s t r u c t e d from a number of sources (15, 16) a t 5, 10, 15, 20, 25, and 30% l e v e l s (Figures 5-10). In comparison to the analogous system containing MgO i t can be seen that FeO does not have such a s i g n i f i c a n t e f f e c t on the phase assemblages. Both the 5 and 10% planes are remarkably s i m i l a r to the base ternary system. There are no s i g n i f i c a n t primary phase f i e l d s containing iron oxide u n t i l the 15% plane. Even at the 30% l e v e l anorthite i s s t i l l present as a primary phase. To confirm the approximation used i n deciding to use the quaternary system i t i s necessary to check the r e s u l t s of t h i s study. The compositions studied (with the iron oxide taken to be FeO) have been p l o t t e d on the appropriate plane. For a l l the compositions studied the system c o r r e c t l y predicts the primary phase observed. Furthermore, as with the system above the c r y s t a l l i z a t i o n of subsequent phases can be explained by using the diagrams. There were however some e f f e c t s due to the presence of f e r r i c iron observed i n the study (10). An important aspect to t h i s study was the determination of the system which most adequately describes the c r y s t a l l i z a t i o n of the c o a l ash melts. The compositions of the three ashes studied have been normalized to the required number of components and p l o t t e d on the appropriate phase diagram. Normalization must be treated with care because, while only the major three or four components are assumed to be present, minor components may have a disproportionate e f f e c t on the c r y s t a l l i z a t i o n . In the case of the eastern type ash the low CaO and MgO content reduced the need for p l o t t i n g on the corresponding diagram. The observed primary c r y s t a l l i n e phase i s compared to that predicted for the normalized compositions i n Table IX. For ash 1 a l l the systems c o r r e c t l y predicted the c r y s t a l l i z a t i o n of anorthite. However for ash 2 only the system CaO-FeO-Aln03-Si02 predicted that the observed primary phase would be anorthite. This system a l s o predicted that the primary phase for ash 3 would be m u l l i t e . From the r e s u l t s of the c r y s t a l l i z a t i o n of the model compositions the secondary phase of iron spinel would not be unexpected. The system can a l s o explain i n part why o n l y anorthite was observed to c r y s t a l l i z e for the western type ashes. For example, for ash 1 the normalized composition l i e s on the 5% FeO plane, with reference to the planes of higher FeO content i t can be seen that s i g n i f i c a n t c r y s t a l l i z a t i o n of anorthite must occur before a secondary phase would be expected to precipitate. Therefore the system CaO-FeO-Al Oo-Si0 appears to govern the c r y s t a l l i z a t i o n behaviour of ash melts determined under laboratory conditions. 2

2

CaO

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NO

θ5

oo

MINERAL MATTER AND ASH IN COAL

250

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Si0

Figure

2

6. System C a O - F e O - A l C U - S i 0 , 2

2

Si0

Figure

2

7. System C a O - F e O - A l 0 - S i 0 , 2

3

10% FeO p l a n e .

2

15% FeO p l a n e .

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18.

KALMANOVITCH AND WILLIAMSON

Figure

8.

System

Crystallization

of Coal Ash Melts

CaO-FeO-Al 0 -Si0 , 2

3

2

20% FeO

plane.

251

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18.

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WILLIAMSON

Crystallization

of Coal Ash

Melts

253

Table IX. Comparison of observed c r y s t a l l i z a t i o n of ash melts with that predicted from relevant phase diagrams. Ash

1 2 3

Observed primary phase

Predicted primary phase System 3 System 1 System 2 Anorthite Mullite n. d.

Anorthite Anorthite Mullite

Anorthite Mullite n. d.

Anorthite Anorthite Mullite

System 1; CaO-Al 03-Si0 . System 2; CaO-MgO-Al 03-Si0 . System 3; CaO-FeO-Al 0^-Si0 . 2

2

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2

2

2

2

To be able to r e l a t e the r e s u l t s of t h i s study to the phenomena of b o i l e r deposits i t i s necessary to look at the mineralogy of deposits. Table X i s a comparison of the c r y s t a l l i n e phases observed in deposits from s i x u t i l i t y b o i l e r s with the primary phase predicted from the quaternary system. Three deposits are of western type and three are of eastern type. The normalized compositions of the deposits have been included in the Table and p l o t t e d on the appropriate plane of the equilibrium system. In the case of the western type deposits the quaternary system c o r r e c t l y predicts the primary phase. This was a l s o the case for the eastern type deposits except for Ironbridge where iron spinel was the primary phase whereas anorthite was predicted. This anomaly may be due to the presence of s i g n i f i c a n t amount of f e r r i c iron and the e f f e c t of minor components. Nevertheless the system CaO-FeO-Al 0^-Si0 appears to govern the c r y s t a l l i z a t i o n of coal ash deposits to a s i g n i f i c a n t extent. The a b i l i t y to predict to some degree the c r y s t a l l i z a t i o n behaviour of an ash melt may g i v e a v a l u a b l e insight into the deposition p o t e n t i a l of the ash. The growth of deposits has been shown above to be predominantly v i a a mechanism of viscous flow. For an ash melt c r y s t a l l i z i n g the composition of the residual l i q u i d phase w i l l change depending on; the o r i g i n a l composition, the c r y s t a l l i z i n g phases and the degree of c r y s t a l l i z a t i o n . The e f f e c t of various components on the r h e o l o g i c a l behaviour of a melt have not been r i g o r o u s l y determined. However i t i s known that "acidic" components such as s i l i c a and f e r r i c oxide increase v i s c o s i t y whereas "basic" components e.g. the a l k a l i and a l k a l i earth oxides decrease the v i s c o s i t y of a melt. Therefore an ash melt which becomes r e l a t i v e l y enriched i n acid as c r y s t a l l i z a t i o n proceeds w i l l produce a viscous melt. Conversely i f the residual l i q u i d phase becomes enriched i n basic oxides the l i q u i d phase w i l l become more f l u i d . From the model developed by Raask (4) the melt with the viscous l i q u i d phase w i l l produce a weaker deposit than the melt with a f l u i d residual l i q u i d phase. The strength of the deposit i s a measure of the degree of s i n t e r i n g and therefore indicates the rate of growth. D e t a i l s of the preliminary development of a method to determine the slagging propensity of an ash based on high temperature e q u i l i b r i a and the r e s u l t s of t h i s study may be found elsewhere (17). 2

2

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254 MINERAL MATTER AND ASH IN COAL

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Crystallization

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255

Conclusions While coal ash i s a complex mixture of mineral components the c r y s t a l l i z a t i o n behaviour of homogeneous ash melts has been shown to be governed by the major oxide components. The equilibrium system CaO-FeO-A^O^-S^ has been shown to be a b l e to model the i n i t i a l c r y s t a l l i z a t i o n behaviour of ash melts. Furthermore the system a l s o appears to govern the c r y s t a l l i z a t i o n of b o i l e r deposits to a s i g n i f i c a n t extent. The a p p l i c a b i l i t y of the use of phase e q u i l i b r i a data to the phenomena of b o i l e r deposits has been indicated.

Publication Date: April 2, 1986 | doi: 10.1021/bk-1986-0301.ch018

Acknowledgments D. Kalmanovitch g r a t e f u l l y acknowledges the award of a SERC (U. K.) CASE Studentship (1980-1983) and the sponsorship provided by Babcock Power Ltd. U. K.. The f a c i l i t i e s and support given by the Combustion and Carbonization Research Laboratory of Energy Mines & Resources Canada through an NSERC V i s i t i n g Fellowship (1983-1985) to D. K. i s a l s o acknowledged.

References 1. Winegartner, E.C.; "Coal fouling and slagging parameters". A.S.M.E. Special Publication, 1974. 2. B r i t i s h Standard 1016, Pt. 15, 1970. 3. Padia, A.S.; DSc Thesis, MIT, 1976. 4. Raask, E.; J. Eng. Power; 1982, 104, 856-66 5. Raask, E. and Goetz, L.; J. Inst. Energy, 1981, 54, 163. 6. Lauf, R.J.; F u e l , 1981,60, 1177-79 7. Barrett, E.P.; "Chemistry of Coal Utilization", Ed. E. Lowry, vol. 1, 1945, Ch. 15. 8. Sanyal, A. and Williamson, J.; J. Inst. Energy, 1981, 54, 158-62 9. Hey, M.H.; Min. Mag., 1941, 26, 116-8. 10. Kalmanovitch, D. P.; Ph. D. Thesis, U n i v e r s i t y of London, 1983. 11. Levin, E.M., Robbins, C.R., McMurdie, H.F., "Phase diagrams for ceramists." Amer. Ceram. Soc., Columbus, 1964, Introductory chapter. 12. Gutt, W. and R u s s e l l , A. D.; Jnl. Mat. S c i . , 1977, 12, 1869-79. 13. Cavalier, G. and Sandrea-Deudon, M.; Rev. de Metallurgie, 1960, 57, 1143-57. 14. Prince, A. T.; J. Am. Ceram. Soc., 1942, 25, 241-74 15. Schairer, J. F.; J. Am. Ceram. Soc., 1954, 37, 402-9 16. Muan, A. and Osborn, E. F.; "Phase diagrams for ceramists" Ed. Levin, E.M. et al., Amer. Ceram. Soc., Columbus, 1964, 288. 17. Kalmanovitch, D. P. and Williamson, J.; Presented at the 3rd Engineering Foundation Conf. on Slagging & Fouling due to Impurities in Combustion Gases. July 1984, Copper Mountain, Denver. 18. Tufte, P.H. and Beckering, W.A.; A proposed mechanism for ash fouling burning Great Plains lignite, ASME Meeting, Nov. 17 -22, 1974, New York, No. 74-WA/CD-3. RECEIVED June 21, 1985

19 Heat Transfer and Thermal Conductivity of Coal Slags K. C . Mills

Publication Date: April 2, 1986 | doi: 10.1021/bk-1986-0301.ch019

National Physical Laboratory, Teddington, Middlesex TW11 OLW, United Kingdom

The various factors affecting the complex heat transfer processes which occur in semi-transparent media like slags are discussed. This information has been used to evaluate the validity of the various experimental methods available for measuring the thermal conductivity of slags. A critical assessment of published experimental data for slags and magmas covering a wide composition range has been carried out. This review of thermal conductivity data showed that both the (Fe /Fe ) ratio and the crystallinity of the slag greatly influence the amount of heat transferred through the slag by radiation conduction. The heat transferred across the solid slag formed on the walls of the vessel has been shown to be dependent upon the thermal resistance of the slag/metal interface which, in turn, appears to be dependent upon the mineralogical constitution of the slag. 2+

3+

D u r i n g t h e g a s i f i c a t i o n o f c o a l , both molten and s o l i d s l a g s a r e formed i n the converter, and t h e heat transfer within the g a s i f i c a t i o n chamber i s governed t o a l a r g e e x t e n t by t h e t h e r m a l p r o p e r t i e s o f t h e s l a g phase. Thus i n o r d e r t o c a r r y o u t e i t h e r h e a t b a l a n c e o r m o d e l l i n g c a l c u l a t i o n s i t i s n e c e s s a r y t o have r e l i a b l e d a t a f o r t h e t h e r m a l p r o p e r t i e s o f b o t h s o l i d and l i q u i d c o a l s l a g s . However, the thermal transfer mechanisms i n high temperature p r o c e s s e s i n v o l v i n g s l a g s a r e e x c e e d i n g l y complex s i n c e h e a t c a n be t r a n s p o r t e d by c o n v e c t i o n , r a d i a t i o n and t h e r m a l c o n d u c t i o n . I n o r d e r t o s i m p l i f y t h e a n a l y s i s o f t h e combined c o n d u c t i v e - r a d i a t i v e t h e r m a l f l u x i t i s n e c e s s a r y t o r e s o r t t o one o f t h e v a r i o u s heat t r a n s f e r models a v a i l a b l e . The most w i d e l y - u s e d model i s the d i f f u s i o n approximation i n which t h e t h e r m a l f l u x i s used t o d e r i v e an e f f e c t i v e o r t o t a l t h e r m a l c o n d u c t i v i t y ( ^ ) which i s , i n t u r n , c o n s i d e r e d t o be made up o f c o n t r i b u t i o n s from ( i ) the thermal ("phonon") c o n d u c t i v i t y , k , ( i i ) r a d i a t i o n c o n d u c t i v i t y , k and e f f

c

This chapter not subject to U.S. copyright. Published 1986, American Chemical Society

R

19.

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Heat Transfer and Conductivity

of Coal Slags

257

( i i i ) electronic conductivity, k . Heat b a l a n c e c a l c u l a t i o n s must t a k e a c c o u n t o f each o f t h e s e t h % r m a l t r a n s p o r t mechanisms and t h i s is p a r t i c u l a r l y important as the boundary conditions in the e x p e r i m e n t a l d e t e r m i n a t i o n o f the t h e r m a l c o n d u c t i v i t y o f a s l a g may v a r y a p p r e c i a b l y from t h o s e a p p l y i n g t o the s l a g i n the g a s i f i e r . Thus i t i s n e c e s s a r y , where p o s s i b l e , t o determine the i n d i v i d u a l c o n t r i b u t i o n s t o the t o t a l h e a t f l u x f o r each c o n d i t i o n . In the p r e s e n t paper the f a c t o r s a f f e c t i n g the t h e r m a l c o n d u c t i v i t y o f s l a g s will be examined and the various methods available for the measurement o f t h e r m a l c o n d u c t i v i t i e s w i l l be assessed. Finally, e x p e r i m e n t a l d a t a f o r the t h e r m a l c o n d u c t i v i t y o f s l a g s , g l a s s e s and magmas w i l l be e v a l u a t e d t o p r o v i d e a r e l i a b l e d a t a base f o r the t h e r m a l c o n d u c t i v i t y o f s l a g s , and t o d e t e r m i n e the l i k e l y e f f e c t s o f v a r i a t i o n s i n c h e m i c a l c o m p o s i t i o n upon v a l u e s f o r c o a l s l a g s .

Publication Date: April 2, 1986 | doi: 10.1021/bk-1986-0301.ch019

Thermal c o n d u c t i o n mechanisms Thermal "phonon" c o n d u c t i v i t y (k ) . Heat i s t r a n s f e r r e d through a medium by phonons, which a r e q u a ^ a o f energy a s s o c i a t e d w i t h each mode o f v i b r a t i o n i n the sample. T h i s mode o f c o n d u c t i o n i s t h u s r e f e r r e d t o as t h e r m a l , phonon o r l a t t i c e c o n d u c t i o n . S c a t t e r i n g o f the phonons c a u s e s a d e c r e a s e i n the t h e r m a l c o n d u c t i v i t y which i s t h e r e f o r e s e n s i t i v e t o the s t r u c t u r e o f the sample. S c a t t e r i n g o f t h e phonons can o c c u r by c o l l i s i o n s o f the phonons w i t h one a n o t h e r , o r by impact w i t h g r a i n b o u n d a r i e s o r c r y s t a l i m p e r f e c t i o n s , such as p o r e s . Thus a l o w - d e n s i t y , h i g h l y - p o r o u s m a t e r i a l w i l l have a low t h e r m a l c o n d u c t i v i t y . In g l a s s y , n o n - c r y s t a l l i n e m a t e r i a l s i t has been s u g g e s t e d —* that thermal conductivity decreases as the d i s o r d e r i n g o f the s i l i c a t e network i n c r e a s e s . Radiation

conductivity

(k^.

Measurements o f t h e t h e r m a l c o n d u c t i v i t y o f g l a s s e s were found t o be dependent upon the t h i c k n e s s o f the specimens used, and t h i s i s shown i n F i g u r e 1. T h i s b e h a v i o u r was a s c r i b e d t o the c o n t r i b u t i o n from r a d i a t i o n c o n d u c t i v i t y , k , which can o c c u r i n s e m i - t r a n s p a r e n t media l i k e s l a g s and g l a s s e s . R a d i a t i o n c o n d u c t i v i t y o c c u r s by a mechanism involving absorption and emittance o f r a d i a n t energy by various s e c t i o n s through the medium. C o n s i d e r a t h i n s e c t i o n i n the s l a g , r a d i a n t energy absorbed by the s e c t i o n w i l l cause i t t o i n c r e a s e i n temperature and c o n s e q u e n t l y r a d i a n t h e a t w i l l be e m i t t e d t o c o o l e r s e c t i o n s . T h i s p r o c e s s can o c c u r c o n t i n u o u s l y t h r o u g h the medium and i t i s o b v i o u s t h a t the energy t r a n s f e r r e d i n t h i s way w i l l i n c r e a s e w i t h i n c r e a s i n g number o f s e c t i o n s ( i e . i n c r e a s i n g t h i c k n e s s ) until the p o i n t i s reached where k a t t a i n s a c o n s t a n t v a l u e . At t h i s p o i n t the s l a g i s s a i d t o be " o p t i c a l l y t h i c k , and this i s usually c o n s i d e r e d t o o c c u r when α d > 3 . 5 , where α and d a r e the a b s o r p t i o n c o e f f i c i e n t and t h i c k n e s s o f the s l a g , r e s p e c t i v e l y . At h i g h t e m p e r a t u r e , r a d i a t i o n c o n d u c t i o n can be the predominant mode o f h e a t t r a n s f e r , eg. i n g l a s s m a k i n g more than 90J o f the thermal transfer occurs by radiation conduction. The radiation c o n d u c t i v i t y can be c a l c u l a t e d f o r an o p t i c a l l y - t h i c k sample i f steady-state conditions apply and i f i t is assumed that the absorption c o e f f i c i e n t of the medium, α , i s independent of R

R

1 1

258

MINERAL MATTER AND ASH IN COAL

wavelength, λ , i e . grey-body c o n d i t i o n s o b t a i n . F o r t h e s e c o n d i t i o n s k can be c a l c u l a t e d by use o f E q u a t i o n 1, where σ and η a r e t h e Scefan-Boltzmann c o n s t a n t and r e f r a c t i v e i n d e x , r e s p e c t i v e l y .

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.

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;

Values o f k cannot e a s i l y be c a l c u l a t e d f o r o p t i c a l l y - t h i n samples ( 3 . 5 ) . Hence i n c r e a s i n g α has the e f f e c t o f d e c r e a s i n g k„ and d e c r e a s i n g t h e depth a t which a s l a g becomes o p t i c a l l y t h i c k , i f a p a r t i c u l a r s l a g sample were o p t i c a l l y t h i n , t h e s e two f a c t o r s would operate in opposition to one another. The absorption c o e f f i c i e n t i s markedly dependent upon t h e amounts o f FeO and MnO p r e s e n t i n the s l a g w ; a l t h o u g h Fe 0^ a b s o r b s i n f r a - r e d r a d i a t i o n , i t s e f f e c t on α i s much l e s s pronounced than t h a t o f FeO. An e m p i r i c a l r u l e has been d e r i v e d ^ f o r g l a s s e s c o n t a i n i n g l e s s than 5% FeO, from which t h e a b s o r p t i o n c o e f f i c i e n t a t room temperature i s g i v e n by the r e l a t i o n s h i p , α = 11. (% FeO). A b a s i c assumption adopted i n d e r i v i n g E q u a t i o n 1 was t h a t α was independent o f wavelength; however, i n p r a c t i c e t h e s p e c t r a l a b s o r p t i o n c o e f f i c i e n t ( α ) v a r i e s w i t h t h e wavelength (λ) as shown i n F i g u r e 2 f o r a g l a s s c o n t a i n i n g c a . 5% F e O ^ . I t can be seen from t h i s f i g u r e t h a t t h e r e i s s t r o n g a b s o r p t i o n by FeO a t c a . 1 jam and by S i 0 a t c a . 4.4 um. At h i g h temperatures t h i s r e s t r i c t s a b s o r p t i o n by t h e s l a g t o a "window", i n t h e wavelength range 1-4.4 pm. However, even w i t h i n t h i s wavelength band t h e r e i s some v a r i a t i o n i n α and the a v e r a g e a b s o r p t i o n coefficient,α i s determined by w e i g h t i n g o f the α values. χ

2

λ

9

m

χ

The a v e r a g e a b s o r p t i o n c o e f f i c i e n t , α , can be a f f e c t e d by t e m p e r a t u r e i n two d i f f e r e n t ways. F i r s t l y , ftie a b s o r p t i o n spectrum, i e , ( α ) , can change markedly w i t h temperature and c o n s e q u e n t l y a l t e r t h e v a l u e o f α . S e c o n d l y , even i f t h e a b s o r p t i o n spectrum i s u n a f f e c t e d by t e m p e r a t u r e , α would c o n t i n u e t o be a f u n c t i o n o f temperature because t h e wavefength d i s t r i b u t i o n used i n d e r i v i n g α i s i t s e l f a f u n c t i o n o f t e m p e r a t u r e . T h i s can be seen i n F i g u r e 3 where t h e f r a c t i o n o f t o t a l energy e m i t t e d i n t h e "window" 1-4.4 p i c o n s t i t u t e s 61.1%, 79.5$ and 81.9% o f t h e t o t a l energy e m i t t e d a t 1073 K, 1573 Κ and 1773 K, r e s p e c t i v e l y . I n a s i m i l a r manner, the various α v a l u e s o f the spectrum will have t o be weighted d i f f e r e n t l y i n the c a l c u l a t i o n o f α f o r t h e t h r e e temperatures i n question. λ

χ

m

I t can be seen from F i g u r e 2 t h a t α increases with increasing temperature,, and s i m i l a r b e h a v i o u r has i e e n o b s e r v e d i n r o c k s and m i n e r a l s ^ ' ^ ^ . By c o n t r a s t , the a b s o r p t i o n c o e f f i c i e n t s (a ) o f amber g l a s s have been found t o d e c r e a s e w i t h i n c r e a s i n g t e m p e r a t u r e . ffi

Heat Transfer and Conductivity

of Coal

Slags

Publication Date: April 2, 1986 | doi: 10.1021/bk-1986-0301.ch019

MILLS

F i g u r e 2. The wavelength dependence o f a g l a s s 67% S i 0 +16% N a 0 + 9% (FeO + F e 0 ) . 2

2

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Publication Date: April 2, 1986 | doi: 10.1021/bk-1986-0301.ch019

MINERAL MATTER AND ASH IN COAL

1

2

3

4

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λ/μπι

F i g u r e 3. Wavelength d i s t r i b u t i o n e m i s s i o n f o r a b l a c k body.

o f t h e s o u r c e energy

19.

Heat Transfer and Conductivity

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261

of Coal Slags

Extinction coefficient (E). In s o l i d s , radiant energy can be s c a t t e r e d by g r a i n b o u n d a r i e s , p o r e s and c r a c k s i n the m a t e r i a l . I n t h e s e c a s e s , i t i s n e c e s s a r y t o use the e x t i n c t i o n c o e f f i c i e n t (E) which i s g i v e n by the r e l a t i o n s h i p Ε = α + s, where s i s t h e scattering coefficient. E l e c t r o n i c c o n d u c t i v i t y (k ^ l * I t has been r e p o r t e d t h a t g l a s s e s which c o n t a i n s i g n i f i c a n t c o n c e n t r a t i o n s o f Fe i o n s behave i n a s i m i l a r manner t o s e m i - c o n d u c t o r s and hence t h e r m a l c o n d u c t i o n v i a c o n d u c t i o n e l e c t r o n s , h o l e s , e t c c o u l d be s i g n i f i c a n t , a c c o r d i n g t o F i n e e t a l ^ . L i t t l e i s known o f t h i s mechanism i n r e l a t i o n t o the h e a t t r a n s f e r i n s l a g s and c o n s e q u e n t l y the c o n t r i b u t i o n o f k . t o t h e measured t h e r m a l c o n d u c t i v i t i e s has been i g n o r e d i n t h i s r e v i e w . j

T o t a l thermal c o n d u c t i v i t y ( k ) _ . In p r a c t i c e , the r a d i a t i o n and c o n d u c t i o n c o n t r i b u t i o n s t o trie h e a t f l u x (Q) a r e i n t e r a c t i v e , and t h e i n t e r p r e t a t i o n o f the combined c o n d u c t i v e - r a d i a t i v e heat t r a n s f e r i s complex. V a r i o u s models have t h e r e f o r e been proposed t o s i m p i f y t h e t h e o r y o f t h e heat t r a n s f e r p r o c e s s . One w i d e l y - u s e d model i s t h e d i f f u s i o n a p p r o x i m a t i o n which assumes t h a t the heat f l u x (Q) i s g i v e n by E q u a t i o n 2, where k i s t h e e f f e c t i v e thermal c o n d u c t i v i t y and i s d e f i n e d by E q u a t i o n 3 where χ i s the d i s t a n c e . G a r d o n ^ has p o i n t e d out t h a t t h i s model o n l y a p p l i e s s t r i c t l y when ( i ) k is s m a l l and ( i i ) α d > 8.

Publication Date: April 2, 1986 | doi: 10.1021/bk-1986-0301.ch019

f f

Q k

e f f

=

-k

=

k

c

(dT/dx)

e f f

+

k

(2) (3)

R

E x p e r i m e n t a l Methods f o r D e t e r m i n i n g Thermal C o n d u c t i v i t y The experimental methods available for measuring thermal c o n d u c t i v i t i e s a r e summarised below; more d e t a i l e d , r e v i e w s .of the experimental techniques are available e l s e w h e r e ^ ' — ' ϋ-' — . The techniques f o r the d e t e r m i n a t i o n o f thermal c o n d u c t i v i t y can be d i v i d e d i n t o t h r e e c l a s s e s : ( i ) s t e a d y - s t a t e methods, ( i i ) non-steady s t a t e methods, and ( i i i ) i n d i r e c t methods. The s t e a d y - s t a t e methods usually yield k v a l u e s and the non-steady s t a t e t e c h n i q u e s u s u a l l y produce t h e r m a l ûiffusivity (a ) v a l u e s , which can be c o n v e r t e d t o t h e r m a l c o n d u c t i v i t y v a l u e s by use o f E q u a t i o n 4, where Ρ and C a r e the d e n s i t y and heat c a p a c i t y r e s p e c t i v e l y o f the s l a g . p

k

=

a. C . Ρ

(4)

S t e a d y - s t a t e methods. These methods a l l y i e l d ^ . v a l u e s p r o v i d e d t h a t the specimen i s o p t i c a l l y t h i c k . In the l i n e a r h e a t - f l o w method two d i s c - s h a p e d specimens a r e p l a c e d on e i t h e r s i d e o f an electrically-heated plate and the temperature profiles across the samples are monitored by thermocouples s i t e d on b o t h f a c e s o f t h e specimens. The a p p a r a t u s i s w e l l i n s u l a t e d t o m i n i m i s e heat l o s s e s . In some v e r s i o n s o f t h i s method, the t o t a l heat f l u x p a s s i n g through t h e samples i s determined e f {

262

MINERAL MATTER AND ASH IN COAL

by c a l o r i m e t e r s i n c o n t a c t w i t h t h e specimens. When h i g h - t e m p e r a t u r e measurements a r e rénuired, t h i s t e c h n i q u e i s u s u a l l y o p e r a t e d a s a c o m p a r a t i v e method — . In t h e r a d i a l h e a t - f l o w method t h e specimen i s i n t h e shape o f a h o l l o w c y l i n d e r , which i s p o s i t i o n e d i n t h e a n n u l u s between two c o a x i a l c y l i n d e r s w i t h t h e i n t e r n a l c y l i n d e r a c t i n g a s a r a d i a l heat s o u r c e * - * ' . The temperature p r o f i l e a c r o s s t h e specimen i s determined by thermocouples p l a c e d on t h e i n s i d e w a l l s o f t h e two c y l i n d e r s . T h i s method r e q u i r e s a l a r g e i s o t h e r m a l zone i n t h e f u r n a c e , which i s d i f f i c u l t t o a c h i e v e a t h i g h t e m p e r a t u r e s . When t h i s t e c h n i q u e i s used f o r measurements on l i q u i d s , e r r o r s can o c c u r from c o n v e c t i v e heat t r a n s f e r .

Publication Date: April 2, 1986 | doi: 10.1021/bk-1986-0301.ch019

1

Non-steady s t a t e methods. I n t h e r a d i a l wave method t h e s l a g i s p l a c e d i n a c y l i n d r i c a l c r u c i b l e s i t u a t e d i n t h e i s o t h e r m a l zone o f a f u r n a c e , and thermocouples a r e l o c a t e d on t h e w a l l s and a l o n g t h e g e o m e t r i c a x i s o f t h e c r u c i b l e ( t h e s l a g ) . The o u t s i d e w a l l o f t h e c r u c i b l e i s t h e n s u b j e c t e d t o v a r i a t i o n o f temperature and the v a r i a t i o n i n temperature o f t h e c e n t r a l thermocouple i s m o n i t o r e d . There i s a phase s h i f t between t h e i n p u t and o u t p u t which i s r e l a t e d t o t h e thermal diffusivity o f the s l a g . In the m o d i f i c a t i o n of this a p p a r a t u s used by E l l i o t t and co-workers ~ the p e r i o d i c v a r i a t i o n i n t e m p e r a t u r e i s produced i n a w i r e r u n n i n g a l o n g t h e c e n t r a l a x i s of t h e c y l i n d r i c a l c r u c i b l e , and t h e phase s h i f t i s measured by thermocouples l o c a t e d on t h e w a l l s o f t h e c r u c i b l e . The t h e r m a l d i f f u s i v i t y v a l u e s o b t a i n e d w i t h t h i s method may be v u l n e r a b l e t o e r r o r s a r i s i n g from c o n v e c t i v e heat t r a n s f e r . In t h e modulated beam method t h e specimen i s i n t h e form o f a d i s c , which i s m a i n t a i n e d a t a c o n s t a n t temperature, w h i l s t t h e f r o n t f a c e i s s u b j e c t e d t o a l a s e r beam which produces a p e r i o d i c v a r i a t i o n i n temperature o f c o n s t a n t f r e q u e n c y . The phase s h i f t between t h i s i n p u t and t h e s i g n a l from a temperature s e n s o r i n c o n t a c t w i t h t h e back f a c e i s d e t e r m i n e d . By c a r r y i n g o u t measurements a t two o r more f r e q u e n c i e s , S c h a t z and Simmons ~ were a b l e t o d e r i v e v a l u e s o f b o t h a and t h e e x t i n c t i o n c o e f f i c i e n t . I f t h i s method were a p p l i e d t o measurements i n l i q u i d s , i t t o o would be prone t o e r r o r s caused by convection. The l a s e r p u l s e m e t h o d ^ ' ffi when a p p l i e d t o s o l i d s uses a d i s c - s h a p e d s l a g specimen c o a t e d w i t h m e t a l l i c f i l m s on both p l a n a r s u r f a c e s . A l a s e r p u l s e i s d i r e c t e d on t o the f r o n t f a c e o f the specimen and t h e temperature o f t h e back face i s monitored c o n t i n u o u s l y . The maximum temperature i n c r e a s e o f t h e back face (ΔΤ^ ) u s u a l l y o c c u r s a f t e r c a . 10 seconds, and a , may be computed from t h e time t a k e n ( t ) f o r t h e back f a c e t o a t t a i n a temperature r i s e o f (0.5Δ T * ) , The method has a l s o been a p p l i e d t o measurements on l i q u i d s l a g s — ' — which were c o n t a i n e d i n A l 0. o r BN c r u c i b l e s . The major advantage o f t h i s t e c h n i q u e i s t h a t t h e s h o r t d u r a t i o n o f the experiment m i n i m i s e s t h e e r r o r s due t o c o n v e c t i o n . The major d i s a d v a n t a g e i s t h a t t h e maximum specimen t h i c k n e s s i s about 0.4 cm, and c o n s e q u e n t l y o p t i c a l l y - t h i c k c o n d i t i o n s o n l y a p p l y when t h e e x t i n c t i o n c o e f f i c i e n t i s g r e a t e r than 9 cm" . A second d i s a d v a n t a g e i s t h a t t h e l a s e r p u l s e method i s a t r a n s i e n t t e c h n i q u e and k cannot be c a l c u l a t e d by E q u a t i o n 1, which i s a p p l i c a b l e t o s t e a a y - s t a t e c o n d i t i o n s ; a t t h e p r e s e n t time no f o r m u l a e e x i s t f o r t h e c a l c u l a t i o n e f t {

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19.

MILLS

Heat Transfer and Conductivity

263

Slags

of k f o r t h i s method. Thus t h i s t e c h n i q u e i s most u s e f u l when a p p l i e d t o specimens which have ( i ) v e r y s m a l l v a l u e s o f « d ( i e . α d 45%) and (

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Heat Transfer and Conductivity

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1000

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Publication Date: April 2, 1986 | doi: 10.1021/bk-1986-0301.ch019

(b)

Ο

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F i g u r e 10. (a) Thermal c o n d u c t i v i t y (b) t h e r m a l d i f f u s i v i t y o f coal slags; , Taylor; — — — , Vargaftik; upper and lower l i m i t s o f a v a l u e s r e p o r t e d by Gibby and B a t e s .

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1600

T/ C e

F i g u r e 11. The thermal c o n d u c t i v i t y o f v a r i o u s s l a g s , g l a s s e and m i n e r a l s : O , M g O - A l ^ ; T, S S U O - 2 S i 0 ; X, 2MgO*Si0 ; ^ CaF ~based slags; Na 0 + S i 0 ; , continuousc a s t i n g s l a g s ; and - · · - , c o a l s l a g s . 2

2

2

2

2

MINERAL MATTER AND ASH IN COAL

274

s m a l l . However k i n c r e a s e s d r a m a t i c a l l y w i t h temperature and even a s l a g w i t h a r e l a t i v e l y h i g h a b s o r p t i o n c o e f f i c i e n t o f 100 cm" would g i v e r i s e t o a c o n t r i b u t i o n o f k o f 0.4 Wm Κ a t 1800 K. H o w e v e r a s t h e a b s o r p t i o n c o e f f i c i e n t i s v e r y dependent upon t h e (Fe ) concentration the k value w i l l be dependent upon t h e various f a c t o r s a f f e c t i n g the t F e /Fe^ ) r a t i o i n the s l a g v i z . the ratio i n c r e a s e s with; ( i ) i n c r e a s i n g temperature; ( i i ) decreasing p(0 ); ( i i i ) i n c r e a s i n g SiOp and T i 0 c o n t e n t s and d e c r e a s i n g CaO, NapO and KpO c o n t e n t s i n t h e s l a g . T h i s r e v i e w has r e v e a l e d t h e u r g e n t neecr f o r a b s o r p t i o n c o e f f i c i e n t d a t a f o r c o a l s l a g s a t h i g h temperatures and f o r i n f o r m a t i o n r e l a t i n g t h e a b s o r p t i o n c o e f f i c i e n t t o t h e FeO c o n t e n t o f t h e s l a g . The h e a t t r a n s f e r p r o c e s s i n t h e c o a l g a s i f i e r can a l s o be a f f e c t e d by t h e l a y e r o f s l a g which l i n e s t h e w a l l s o f t h e g a s i f i e r . R e c e n t l y , G r i e v e s o n and B a g h a ^ — have developed a s i m p l e experiment f o r measuring t h e t h e r m a l f l u x (Q) i n v a r i o u s s l a g s used i n t h e c o n t i n u o u s c a s t i n g o f s t e e l . A w a t e r - c o o l e d , copper f i n g e r i s lowered i n t o a c r u c i b l e c o n t a i n i n g molten i r o n covered with a l a y e r o f s l a g and a l a y e r o f s o l i d i f i e d s l a g forms around t h e c o l d f i n g e r . The t h e r m a l f l u x i s determined by measuring t h e temperature r i s e o f t h e c o o l i n g water f l o w i n g through t h e copper f i n g e r . I t was found t h a t t h e heat f l u x was r e l a t e d t o ; ( i ) t h e t h i c k n e s s o f t h e s l a g l a y e r ; and ( i i ) the thermal r e s i s t a n c e o f the Cu/slag interface. The t h i c k n e s s o f t h e s l a g l a y e r i s , i n t u r n , dependent upon t h e v i s c o s i t y o f t h e s l a g and upon o t h e r f a c t o r s d e t e r m i n i n g t h e "melt back" o f t h e slag layer. Grieveson and B a g h a ^ — observed t h a t the thermal r e s i s t a n c e o f t h e C u / s l a g i n t e r f a c e appeared t o be r e l a t e d t o ; ( i ) t h e m i n e r a l o g i c a l c o n s t i t u t i o n o f t h e s l a g : and ( i i ) t h e s t r e n g t h o f t h e a d h e s i v e bond between t h e copper and t h e s l a g eg. t h e h e a t f l u x , Q, r e c o r d e d w i t h s l a g s from CaO.SiO phase f i e l d , which g i v e s good C u / s l a g a d h e s i o n , i s g r e a t e r than t h e Q r e c o r d e d f o r s l a g s from CaO.Al 0 .2SiO phase field with poor Cu/slag adhesion. Thus r e l a t i v e l y s i m p l e experiments l i k e t h e s e s i m u l a t i o n t e s t s c a n p r o v i d e a valuable insight into the f a c t o r s affecting heat transfer mechanisms o c c u r r i n g i n i n d u s t r i a l p r o c e s s e s . R

2+

+

Publication Date: April 2, 1986 | doi: 10.1021/bk-1986-0301.ch019

2

Conclusions (i) E x p e r i m e n t a l d a t a f o r t h e t h e r m a l c o n d u c t i v i t i e s o f s l a g s must be c a r e f u l l y a n a l y s e d t o e s t a b l i s h t h e boundary c o n d i t i o n s o f the experiment ( e g * o p t i c a l t h i c k n e s s o f t h e specimen, magnitude o f k e t c ) . T h i s e v a l u a t i o n o f the d a t a a l l o w s one t o determine the s u i t a b i l i t y o f a s p e c i f i c thermal c o n d u c t i v i t y value f o r subsequent use i n heat b a l a n c e c a l c u l a t i o n s f o r the g a s i f i e r . (ii) The t h e r m a l c o n d u c t i v i t i e s o f c o a l s l a g s a r e n o t v e r y dependent upon t h e c h e m i c a l c o m p o s i t i o n o f t h e s l a g . ( i i i ) The t h e r m a l c o n d u c t i v i t y o f t h e s l a g i s dependent upon t h e degree o f c r y s t a l l i s a t i o n and c o n s e q u e n t l y upon t h e t h e r m a l h i s t o r y o f t h e specimen; t h e t h e r m a l c o n d u c t i v i t y o f the c r y s t a l l i n e phase i s g r e a t e r than t h a t o f t h e g l a s s y phase. (iv)

The r a d i a t i o n c o n d u c t i o n , k i s p r i n c i p a l l y determined by t h e magnitude o f t h e a b s o r p t i o n ( o r e x t i n c t i o n ) c o e f f i c i e n t . As t h e a b s o r p t i o n c o e f f i c i e n t o f t h e s l a g i s l a r g e l y dependent upon t h e (Fe ) c o n c e n t r a t i o n i n t h e s l a g , i t w i l l a l s o be dependent upon the f a c t o r s affecting t h e (Fe / F e ) ratio v i z . +

+

19.

MILLS

Heat Transfer and Conductivity

of Coal

Slags

275

temperature, p(0 ) and the SiO,,, CaO and Na O contents of the slag. * Experimental data are required f o r the absorption c o e f f i c i e n t s of coal slags at high temperatures so that the relationship between α and the FeO content can be established. Heat transfer i n the coal g a s i f i e r w i l l be p a r t i a l l y dependent upon the thermal resistance of the slag/wall interface and t h i s , i n turn, w i l l be dependent upon the mineralogical constitution of the slag adjacent to the wall. p

d

(v)

(vi)

Acknowledgements

Publication Date: April 2, 1986 | doi: 10.1021/bk-1986-0301.ch019

Valuable discussions with B. J. Keene, (National Physical Laboratory) Professor P. Grieveson (Imperial College) and Dr. R. Taylor (UMIST) are g r a t e f u l l y acknowledged.

References (1) (2) (3) (4) (5) (6) (7) (8) (9) (10) (11) (12) (13) (14) (15)

(16)

Ammar, M.M; Gharib, S; Halawa, M M; El Badry, Κ; Ghoneim, Ν A; Batal, Ν A; E L, Batal, H A. J . Non-Cryst. Solids 1982, 53, 165. Steele, F.N; Douglas, R.W. Phys. Chem. Glasses 1965, 6, 246. Blazek, A; Endrys, J . Review of thermal conductivity data in glass, Part II Thermal conductivity at high temperatures published Intl. Commission on Glass, 1983. Fukao, Y, Mitzutani, H; Uyeda, S. Phys. Earth Planet Interiors 1968, 1, 57. Aronson, J.R; Belloti, L.H; Ecroad, S.W; Emslie, A.G; McConnel, R.K; Thuna, P.C von. J . Geophys. Res. 1970, 75, 3443. Schatz, J . F ; Simmons, G. J. Geophys. Res. 1972, 77, 6966. Fine, H.A; Engh, T; E l l i o t t , J . F . Metall. Trans B, 1976, 7B, 277. Gardon, R. paper presented at 2nd Intl. Thermal Conductivity Conference, Ottawa, 1962, 167. Gardon, R. Review of thermal conductivity data in glass, Part I Thermal conductivity at low and moderate temperatures published Intl. Commission on Glass, 1983. Touloukian, Y.S; Powell, R.W; Ho C.Y; Klemens, P.G. "Thermophysical Properties of Matter", volumes 1 (1970) and volume 10 (1973) published by IFI/Plenum, New York. Tye, R.P, "Thermal conductivity", volumes 1 and 2, 1969, published by Academic Press, New York. Kingery, W.D; Francl, J ; Coble, R.L; Vasilos, T. J . Amer. Ceram. Soc., 1954, 37, 107. Ogino, K; Nishiwaki, A; Yamamoto, K; Hama, S. Paper presented at Intl. Symp. Phys. Chem. Steelmaking, Toronto, 1982, III-33. Nauman, J ; Foo, G; E l l i o t t , J F. Extractive Metallurgy of Copper, Chapter 12, 237. Gibby, R.L; Bates, J . L . 10th Thermal Conductivity Conference, held Newton, Mass., Sept. 1970, IV-7,8. See also Bates, J . L . Final Report to National Science Foundation, Grant GI-44100 Properties of Molten coal slags relating to open cycle MHD, Dec. 1975. Taylor, R; Mills, K.C. Arch. f. Eisenhuttenw., 1982, 53, 55.

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276

(31)

MINERAL MATTER AND ASH IN COAL

(17) Taylor, R; Edwards, R. cited by Mills, K.C and Grieveson, P, a paper presented at the Centenary Conference 1984, "Perspectives in Metallury" Sheffield University, July 1984. (18) Saito, A. Bull. Jap. Soc. Mech. Engr., 1980, 23, 1459. (19) de Castro, C.A.N; Li, S.F.Y; Maitland, G.C; Wakeham, W.A. in press Intl. J . Thermophys. (1984). (20) Mills, K.C; Powell, J.S; Bryant, J.W; Keene, B.J. Canad. Metall. Q., 1981, 20, 93. (21) Susa, M; Nagata, K; Goto, K.S. Trans. Iron Steel Inst. Japan, 1982 22, B42. (22) Mitchell, A; Wadier, J . F . Canad. Metall. Q, 1981, 20, 373. (23) Powell, J.S; Mills, K.C. Paper presented at the Ninth European Conference on Thermophysical Properties held in Manchester, September 1984. (24) Mills, K.C. Paper entitled "Estimation of physico-chemical properties of coal slags and ashes" to be presented at this conference. (25) Osinovskikh, L . L ; Kochetov, N.N; Bratchikov, S.G. Trudy Urals N-I Chern. Met., 1968 (8), 71. (26) Rudkin, R.L. Report US AF., ASD-TDR-62-24 II. (27) Keene, B.J; Mills, K.C. Arch f Eisenhuttenw. 1981, 52, 311. (28) Rafalovich, I.M; Denisova, I.A. Fiz. Khim Rasplav. Schlakov, 1970, 181. (29) Olusanya, A. "The Fundamental Properties of Continuous Casting Fluxes" PhD Thesis, Imperial College, London, 1983. (30) Mills, K.C; Grieveson, P; Olusanya, A; Bagha, S; Taylor, R. Paper entitled "Thermal and Physico-chemical properties of continuous-casting slags" to be presented at Ninth European Conference on Thermophysical Properties to be held in Manchester, Sept. 1984. Nagata, K; Goto, K.S. Private communication, Tokyo Inst. of Technology, 1984. (32) Powell, J.S; Mills, K.C. Unpublished thermal conductivity data on casting powders, National Physical Laboratory, 1984. (33) Ohmiya, S; Tacke, K.H; Sshwerdtfeger, K. Ironmaking and Steelmaking, 1983, 10, 24. (34) Varkaftik, N.B; Oleshchuk, O.N. Teploenergetika, 1955, 4, 13. (35) Ischenko, K.D. Met. Kobosokhim, 1970, (21), 82. (36) Kawada, K. Bull. Earthquake Res. Inst., 1966, 44, 1071. (37) Murase, T; McBirney, A.R. Science, 1970, 170, 16S. (38) Grieveson, P; Bagha, S cited by Mills, K.C; Grieveson, Ρ in a paper presented at the Centenary Conference 1984, "Perspectives in Metallurgy", Sheffield University, July 1984. RECEIVED June 17, 1985

20 Solid-Liquid-Vapor Interactions in Alkali-Rich Coal Slags L. P. Cook and J. W. Hastie

Publication Date: April 2, 1986 | doi: 10.1021/bk-1986-0301.ch020

National Bureau of Standards, Gaithersburg, MD 20899

Volcanic ash that falls into a coal-forming environment stands a good chance of being preserved and contributing to the mineral matter in coal. Such layers are relatively common in some coals although they are commonly not recognized as volcanic in origin even by geologists. The original material usually consists of fine-grained glass, crystals, and rock fragments, but there is a great variety in texture and composition. After burial the glass, in particular, is readily altered so that with increasing geologic age secondary clay minerals become abundant and evidence of the volcanic origin becomes less obvious. Kaolinite is most abundant, but smectite is not uncommon; occasionally unusual minerals occur, such as aluminum phosphates with notable amounts of Sr and rare earths. In North America, volcanic ash layers are most important in western coals of Tertiary and Cretaceous ages; they are generally uncommon in eastern coals or those of greater age. Sodium and p o t a s s i u m a r e i m p o r t a n t c o n s t i t u e n t s o f t h e c l a y m i n e r a l s found i n most c o a l s . As t h e c o a l i s combusted t h e s e m e t a l s may be v a p o r i z e d , t r a n s p o r t e d and r e a b s o r b e d by s l a g i n t h e c o o l e r p o r t i o n s of t h e system, l e a d i n g t o t h e p r o d u c t i o n o f s l a g s h a v i n g c o n c e n t r a t i o n s of a l k a l i e s s e v e r a l times t h a t o f t h e p r i m a r y m i n e r a l m a t t e r . Without doubt t h e most marked c o n c e n t r a t i o n s o c c u r i n s l a g s from magnetohydrodynamic g e n e r a t o r s , where p o t a s s i u m s a l t s a r e d e l i b e r a t e l y added t o t h e combustion gases t o enhance e l e c t r i c a l c o n d u c t i v i t y o f the plasma. S l a g s from MHD g e n e r a t o r s have K2O c o n c e n t r a t i o n s a p p r o a c h i n g 20 wt%. The c o r r o s i v e e f f e c t s o f t h e s e h i g h a l k a l i s l a g s on c e r a m i c components o f combustion systems a r e w e l l known. C o r r o s i o n a r i s e s from t h e f a c t t h a t such s l a g s a r e good s o l v e n t s f o r a wide range o f materials. Furthermore, when many c e r a m i c s come i n t o c o n t a c t w i t h h i g h a l k a l i s l a g s , d e s t r u c t i v e r e a c t i o n s p r o d u c i n g new s o l i d s may occur. F o r example, a l u m i n a , a w i d e l y used r e f r a c t o r y , may r e a c t t o produce NaAlSiOi+j K A l S i O ^ o r b e t a a l u m i n a , depending upon t h e This chapter not subject to U.S. copyright. Published 1986, American Chemical Society

MINERAL MATTER AND

Publication Date: April 2, 1986 | doi: 10.1021/bk-1986-0301.ch020

278

ASH IN COAL

a c t i v i t i e s of s i l i c a and the a l k a l i e s . I n most s i t u a t i o n s , r e a c t i o n s of t h i s t y p e would r e s u l t i n l o s s of s t r u c t u r a l i n t e g r i t y of the ceramic. F o r t h e s e and r e l a t e d r e a s o n s , t h e r e i s need f o r d e t a i l e d knowl e d g e o f t h e p h y s i c a l c h e m i s t r y o f h i g h a l k a l i c o a l ash - d e r i v e d slags. NBS has an ongoing t h e o r e t i c a l and e x p e r i m e n t a l program t o s y s t e m a t i c a l l y d e t e r m i n e t h e n a t u r e of s o l i d - l i q u i d - v a p o r e q u i l i b r i a in high a l k a l i coal slags. G i v e n the wide v a r i a b i l i t y o f c o a l s l a g , t h i s n e c e s s i t a t e s a c l o s e i n t e r a c t i o n between t h e o r y and experiment, i f s i g n i f i c a n t p r o g r e s s i s t o be made. E x p e r i m e n t a l l y , t h r e e p r i n c i p a l methods a r e b e i n g u t i l i z e d . A p p l i c a t i o n o f t h e h i g h temperature quenching method, w i t h e x a m i n a t i o n of r e s u l t s by x - r a y d i f f r a c t i o n and e l e c t r o n m i c r o p r o b e methods, i s f a c i l i t a t e d by the f a c t t h a t most s i l i c a t e m e l t s quench r e a d i l y t o g l a s s e s , p r e s e r v i n g t h e t e x t u r a l and c h e m i c a l r e l a t i o n s h i p s which p r e v a i l e d under e q u i l i b r i u m a t h i g h t e m p e r a t u r e s . On the o t h e r hand, the r e l a t i v e l y slow k i n e t i c s makes n e c e s s a r y g r e a t c a r e i n the d e t e r m i n a t i o n o f a l k a l i v a p o r p r e s s u r e s by the Knudsen e f f u s i o n / m a s s s p e c t r o m e t r i c method. N o n e t h e l e s s the t e c h n i q u e has been used s u c c e s s f u l l y a t NBS i n d e t e r m i n i n g v a p o r p r e s s u r e s by c l o s e l y c o r r e l a t i n g e f f u s i o n experiments w i t h on-going quench e x p e r i m e n t s . S i m i l a r l y , the a p p l i c a t i o n o f t h e t h i r d p r i n c i p a l e x p e r i m e n t a l method, h i g h temperature d i f f e r e n t i a l t h e r m a l - t h e r m o g r a v i m e t r i c a n a l y s i s , r e q u i r e s a degree of c a u t i o n . There has l o n g been i n t e r e s t i n the development of models f o r t h e p r e d i c t i o n of c o a l s l a g phase e q u i l i b r i a . W h i l e s i l i c a t e phase diagrams of l i m i t e d c o m p o s i t i o n a l range have been s u c c e s s f u l l y modeled, a s i n g l e model f o r a c c u r a t e p r e d i c t i o n of s l a g phase e q u i l i b r i a i n g e n e r a l w i l l r e q u i r e t h a t c o n s i d e r a b l y more p r o g r e s s be made n o t o n l y i n our u n d e r s t a n d i n g o f the s t r u c t u r a l c h e m i s t r y of s l a g s but a l s o i n the a v a i l a b i l i t y o f thermochemical d a t a needed f o r such models. P r o g r e s s t o d a t e i s r e l a t e d t o the r e a l i z a t i o n that t r e a t m e n t of s i l i c a t e l i q u i d s as p o l y m e r i z e d m e l t s may be n e c e s s a r y f o r v e r y p r e c i s e p r e d i c t i o n o f phase r e l a t i o n s h i p s . A l s o important i s the d i s c o v e r y t h a t a l k a l i a c t i v i t i e s can be modeled o v e r a wide range of c o m p o s i t i o n s by t r e a t i n g s l a g s as composed o f m i x t u r e s of complex m i n e r a l m e l t s such as C a A l S i 0 , K A l S i O ^ , NaAlSÎ308, e t c . Thus c o a l s l a g s , w h i l e n o t c h e m i c a l l y i d e a l m i x t u r e s o f the o x i d e components, appear t o be much more i d e a l w i t h r e s p e c t t o a c h o i c e of more complex components. 2

Phase E q u i l i b r i a

2

8

i n Coal Slags

C o a l S l a g as a 7-Component System. The v a r i a b i l i t y of c o a l a s h comp o s i t i o n i s d i r e c t l y r e l a t e d t o v a r i a t i o n s i n the p r o p o r t i o n s of m i n e r a l i m p u r i t i e s such as S i 0 , C a C 0 , C a M g ( C 0 ) , C a S 0 i - 2 H 0 , F e 0 , F e S , and the c l a y m i n e r a l s w h i c h comprise a complex group of h y d r a t e d a l k a l i a l u m i n o s i l i c a t e s . C o a l ashes may v a r y w i d e l y i n t h e i r c o n t e n t s o f i r o n , c a l c i u m and magnesium, but do not v a r y as g r e a t l y i n the amount o f s i l i c a and a l u m i n a they c o n t a i n . I n f a c t , of the 323 c o a l a s h a n a l y s e s r e p o r t e d i n U.S. Bureau of Mines B u l l . 567 ( 1 ) , the g r e a t m a j o r i t y have a S i 0 / ( S i 0 + A l 0 ) mole r a t i o between .67 and . 8 0 , w i t h a w e l l d e f i n e d maximum n e a r .75 (2). The b u l k chemistry of the ash i s r e l a t e d to the c o n d i t i o n s of formation of t h e c o a l . I n g e n e r a l l i g n i t e s and subbituminous c o a l s of the w e s t e r n U.S.A. a r e h i g h i n c a l c i u m w h i l e b i t u m i n o u s c o a l s of the 2

2

3

3

3

2

+

2

2

2

2

3

2

20.

COOK AND

Solid-Liquid-

HASTIE

Vapor Interactions

279

in Slags

e a s t e r n U.S.A. c o n t a i n more i r o n . Table I gives t y p i c a l analyses f o r ashes from t h e s e c o a l s , and i n c l u d e s f o r comparison an a n a l y s i s of c o a l s l a g from a magnetohydrodynamic g e n e r a t o r . From t h i s t a b l e , i t can be seen t h a t , i n g e n e r a l , c o a l s l a g must be r e g a r d e d as a seven component s u b s t a n c e , i f minor c o n s t i t u e n t s such as T 1 O 2 and P 2 O 5 a r e i g n o r e d and i f s u l f u r i s assumed to v a p o r i z e a t h i g h temperature. To account f o r t h e f a c t t h a t F e 0 3 r e d u c e s p a r t i a l l y to FeO w i t h i n c r e a s i n g temperature would r e q u i r e an added component. However t h e number may be r e d u c e d a g a i n t o seven i f the oxygen p a r t i a l p r e s s u r e i s i n c l u d e d as an i n t e n s i v e v a r i a b l e a l o n g w i t h temperature and c o m p o s i t i o n . By d o i n g t h i s the need t o s p e c i f y b o t h FeO and Fe2Û3 i n d e f i n i n g the b u l k c o m p o s i t i o n i s e l i m i n a t e d ; t h e s e a r e r e p l a c e d by FeO , where χ i s d e t e r m i n e d by the oxygen p a r t i a l pressure. 2

Publication Date: April 2, 1986 | doi: 10.1021/bk-1986-0301.ch020

Table I.

T y p i c a l C o a l Ash A n a l y s e s

(3) Montana C o a l — K 0 Na 0 CaO MgO A1 0 Fe 0 Si0 Ti0 2

2

3

2

3

Illinois

0.4 0.4 11.9 3.9 21.4 10.0 42.5 0.8 0.3 8.1

2

2

2

P2O5 S0 3

(wt % ) .

C o a l ^

MHD

S l a g ^

20.2 0.5 3.9 1.1 12.4 14.7 48.3 0.5

1.4 1.6 8.2 0.8 16.2 23.7 37.5 0.8 0.1 8.9

0.2

R e p r e s e n t a t i o n of S o l i d - L i q u i d - V a p o r E q u i l i b r i a . Ready v i s u a l i z a t i o n o f a range o f phenomena i s one of the a t t r i b u t e s making phase diagrams i n d i s p e n s a b l e i n u n d e r s t a n d i n g t h e c h e m i s t r y of heterogeneous systems. However, a l t h o u g h advanced m u l t i d i m e n s i o n a l p r o j e c t i v e methods have been d e r i v e d f o r t h e r e p r e s e n t a t i o n o f n-component systems ( 5 ) , t h e s e do n o t i n g e n e r a l l e a d t o e a s i l y v i s u a l i z e d diagrams. Thus f o r the seven component c o a l s l a g system, a l t e r n a t i v e methods must be u s e d . I t i s u s e f u l t o s u b d i v i d e t h e s l a g system i n t o s m a l l e r systems. The system A l 2 0 3 - S i 0 i s perhaps the most fundamental system f o r a l l s l a g s , and t o t h i s one may t h i n k of a d d i n g p r o g r e s s i v e l y c o m b i n a t i o n s of t h e a l k a l i e s and CaO, MgO and F e O , u n t i l the d e s i r e d degree of complexity i s reached. C o n s t i t u e n t systems making up the s l a g system a r e summarized i n T a b l e I I . Data a r e a v a i l a b l e f o r p a r t s of many of t h e s e systems (6-10), but as the number o f components i n c r e a s e s , d a t a become p r o g r e s s i v e l y fewer. At NBS, e x p e r i m e n t a l work i s p r e s e n t l y c o n c e n t r a t i n g on the system K 0 - C a 0 - A l 2 0 3 - S i 0 . This along with K20-FeO -Al 0 -Si02 and K20-Mg0-Al203-Si02, forms the b a s i s f o r m o d e l i n g p o t a s s i u m - r i c h slags. The approach has been t o e s t a b l i s h s u b s o l i d u s e q u i l i b r i a ( F i g u r e 1) and t h e n t o combine t h e s e d a t a w i t h l i t e r a t u r e thermochemical data v i a s o l i d s t a t e r e a c t i o n s of the type 2

x

2

2

A(s) + B(s)

C ( s ) + V(s)

x

+ 2K(g)

+ 1/2

0 (g) 2

2

3

2

2

(+K 0 +Na 0)

2

(+Na 0)

2

(+κ ο)

2

2

2

2

2

3

2

3

K 0-Na 0-Al 0 -Si0

2

(+K 0 +Na 0)

2

3

Na 0-Al 0 -Si0

2

2

(+Na 0)

2

2

2

K 0-Al 0 -Si0

3

(+K 0)

2

2

2

2

2

2

3

2

2

2

3

2

2

2

3

2

X

2

2

2

3

2

2

3

2

K 0-Na 0-CaO -FeO - A 1 0 -Si0

2

Na 0-CaO-FeO -Al 0 -Si0

2

K 0-CaO-FeO -Al 0 -Si0

2

2

2

2

3

3

2

2

2

2

3

K 0-Na O-MgO -FeO - A 1 0 -Si0

2

2

Na 0-Mg0-Fe0 -Al 0 -Si0

2

K 0-MgO-FeO -Al 0 -Si0 *

2

2

K 0-Na 0-Mg0 -Al 0 -Si0

2

2

3

2

3

3

Μςΰ-FeO - A 1 0 -Si0

2

2

2

Na 0-MgO-Al 0 -Si0

2

K 0-Mg0-Al 0 -Si0

CaO-FeO - A 1 0 -Si0

3

3

3

(+MgO +FeO ) χ

2

2

3

2

MgO-Al 0 -Si0

(+MgO)

(+CaO +FeO ) χ

2

K 0-Na 0-Ca0 -Al 0 -Si0

2

2

2

Na 0-CaO-Al 0 -Si0

2

3

K 0-Ca0-Al 0 -Si0

2

CaO-Al 0 -Si0

(+Ca0)

2

X

2

3

X

2

2

-A1 0 3

2

3

2

2

x

2

2

2

3

3

3

2

2

2

2

2

3

3

2

2

3

2

2

2

3

K 0-Na 0-CaO -MgO-Al 0 -Si0

2

2

Na 0-CaO-MgO -Al 0 -Si0

2

K 0-Ca0-Mg0 -Al 0 -Si0

2

(+CaO +MgO)

System.

CaO-MgO-Al 0 -Si0

(COAL SLAG SYSTEM)

2

x

2

K 0-Na 0-Ca0-Mg0 -FeO -Al 0 -Si0

2

x

X

Na 0-Ca0-Mg0 -FeO -Al 0 -Si0

2

3

2

3

2

K 0-Ca0-Mg0 -FeO -Al 0 -Si0

2

(+CaO +MgO +FeO ) χ

2

CaO-MgO-FeO -Al 0 -Si0

2

K 0-Na 0-Fe0 -Al 0 -Si0

22

Ntfa a 00-I -Fe0 - A 1 0 -Si0

2

K 0-FeO -Si0

X

FeO - A l 0 - S i 0

x

(+FeO )

Breakdown o f Lower Order Systems C o m p r i s i n g 7-Component C o a l S l a g

Al 0 -Si0 (BASE SYSTEM)

Table I I .

Publication Date: April 2, 1986 | doi: 10.1021/bk-1986-0301.ch020

2

ο

α >

>

73

—i

m

r

>

73

m

20.

COOK AND

Solid-Liquid-

HASTIE

Vapor Interactions

281

in Slags

where A , B, C and V d e n o t e c r y s t a l l i n e s o l i d s . At the temperature o f minimum m e l t i n g , such c a l c u l a t i o n s p r o v i d e a d i r e c t l i n k between t h e phase diagram and t h e measurements of p o t a s s i u m v a p o r p r e s s u r e w h i c h a r e independent o f any s o l u t i o n model ( F i g u r e 2 ) . T h i s a p p r o a c h a i d s g r e a t l y i n the d e t e r m i n a t i o n of an i n t e r n a l l y c o n s i s t e n t s e t o f thermochemical d a t a . As t h e systems i n v e s t i g a t e d become more complex (more components), o t h e r t e c h n i q u e s may be used t o r e d u c e t h e number of v a r i a b l e s , so t h a t r e s u l t s can be p o r t r a y e d g r a p h i c a l l y . For example, i n p r i n c i p l e phase e q u i l i b r i a a t 1 atm i n the system K 2 0 - C a O - F e O - A l 0 3 - S i 0 2 c o u l d be p o r t r a y e d i n t h r e e d i m e n s i o n s g r a p h i c a l l y as a t e t r a h e d r a l diagram a t c o n s t a n t y or P , Τ and Ρ . Another way of r e d u c i n g K 2 U K 2 O U 2 x

v

Λ

T7

Λ

Λ

the d i m e n s i o n a l i t y o f t h e r e p r e s e n t a t i o n a l problem i s t o d e a l w i t h s a t u r a t i o n s u r f a c e s - t h i s i s a c t u a l l y a form of p r o j e c t i o n . For example by c o n s i d e r i n g t h e e q u i l i b r i a i n which A 1 0 participated as a phase, t h e need t o use A I 2 O 3 as a r e p r e s e n t a t i o n a l component would be e l i m i n a t e d . 2

Publication Date: April 2, 1986 | doi: 10.1021/bk-1986-0301.ch020

2

3

R o l e o f P o l y m e r i z a t i o n Theory. One o f t h e major problems i n p r e ­ d i c t i o n and c a l c u l a t i o n o f phase e q u i l i b r i a i s the f o r m u l a t i o n o f a c c u r a t e e x p r e s s i o n s f o r t h e f r e e energy of m i x i n g o f s i l i c a t e m e l t s . R e l a t i v e l y few c a l o r i m e t r i c measurements a r e a v a i l a b l e , and hence t h e importance o f sound methods o f e s t i m a t i o n and p r e d i c t i o n o f m i x i n g d a t a t o w i t h i n t h e r e q u i r e d degree o f a c c u r a c y . Polymer t h e o r y , extended i n t h e I 9 6 0 s t o i n c l u d e s i l i c a t e m e l t s by Masson (11) and o t h e r s h o l d s promise. I t i s perhaps t h e o n l y g e n e r a l t h e o r y f o r s i l i c a t e m e l t s w h i c h d e a l s q u a n t i t a t i v e l y w i t h the problem of m e l t structure. T h i s has been u s e d , w i t h a s u r p r i s i n g degree of s u c c e s s , t o c a l c u l a t e phase e q u i l i b r i a i n b i n a r y o x i d e systems (12,13). P r e l i m i n a r y c a l c u l a t i o n s on multicomponent s l a g s have shown t h a t polymer t h e o r y , when t r e a t e d i n a q u a s i c h e m i c a l f a s h i o n , i s h i g h l y f l e x i b l e , and can accommodate seven component l i q u i d i m m i s c i b i l i t y by making r e l a t i v e l y few e s t i m a t e s and assumptions ( F i g u r e 3 ) . However, a t t e m p t s t o f i t l i q u i d u s s u r f a c e s i n the system K 0 - C a 0 A I 2 O 3 - S 1 O 2 have met w i t h o n l y p a r t i a l s u c c e s s . Further a p p l i c a t i o n s and e x t e n s i o n s o f polymer t h e o r y i n t h i s q u a t e r n a r y system a r e hampered by a l a c k o f e x p e r i m e n t a l d a t a , and an attempt i s b e i n g made t o r e c t i f y t h i s s i t u a t i o n . 1

2

S o l u t i o n Model f o r t h e P r e d i c t i o n of A l k a l i Vapor

Pressures

B a s i s o f t h e Model. The model employed f o r p r e d i c t i o n o f vapor p r e s s u r e s i n multicomponent c o a l s l a g s has been o u t l i n e d i n ( 1 4 ) . B r i e f l y , l a r g e n e g a t i v e d e v i a t i o n s from i d e a l thermodynamic a c t i v i t y b e h a v i o r a r e a t t r i b u t e d t o t h e f o r m a t i o n of complex l i q u i d s and s o l i d s ( a c t u a l components) such as K 2 S 1 O 3 , K A l S i O ^ , e t c . The f r e e e n e r g i e s o f f o r m a t i o n (AGf) a r e e i t h e r known or can be e s t i m a t e d f o r t h e s e l i q u i d s and s o l i d s . By m i n i m i z i n g the t o t a l system f r e e energy, one can c a l c u l a t e t h e e q u i l i b r i u m c o m p o s i t i o n w i t h r e s p e c t t o t h e s e components. Thus, f o r i n s t a n c e the mole f r a c t i o n o f K 0 p r e s e n t ( X * r Q J ) i n e q u i l i b r i u m w i t h K 2 S 1 O 3 , and o t h e r complex l i q u i d s (and S o l i d s ) c o n t a i n i n g K 0 , i s known. As has been shown p r e v i o u s l y f o r t h e t e r n a r y systems, the component a c t i v i t i e s c a n , t o 2

K

2

MINERAL MATTER AND ASH IN COAL

282

MOL.%

CaO

κ ο

Publication Date: April 2, 1986 | doi: 10.1021/bk-1986-0301.ch020

2

F i g u r e 1. E x p e r i m e n t a l l y d e t e r m i n e d s u b s o l i d u s phase r e l a t i o n s i n t h e system K 2 0 - C a 0 - A l 0 - S i 0 2 . c s = C a S i O t r d= Si0 c s = Ca Si 0 ksp = K A l S i 0 wo = C a S i 0 l c = KAlSi 0 ge = C a A l S i 0 k l s= KAIS1O4 an = C a A l S i 0 mu = 3 A l 0 - 2 S i 0 cag = C a A l i 0 i g cor = A1 0 2

3

2

3

2

2

7

2

3

2

3

2

8

7

2

2

2

2200

+

2

3

1600

2

2

8

2

6

3

3

1200

Τ (Κ) F i g u r e 2. C a l c u l a t e d p o t a s s i a p r e s s u r e s f o r s o l i d assemblages in K 0-Ca0-Al 0 -Si0 ( s e e F i g u r e 1 ) . Thermochemical d a t a used were f r o m ( 1 9 ) . Dashed l i n e s w i t h l a b e l s i n p a r e n t h e s e s i n v o l v e incongruently melting s o l i d s . A l l e q u i l i b r i a are metastable above t h e minimum m e l t i n g t e m p e r a t u r e s . 2

2

3

2

2

20.

Solid-Liquid-

COOK AND HASTIE

Vapor interactions

in Slags

283

Publication Date: April 2, 1986 | doi: 10.1021/bk-1986-0301.ch020

0.00 (Γ

- 2.00 h

0.00

0.50 X

(KAI0

2

1.00

+ Si0 ) 2

F i g u r e 3. C a l c u l a t e d f r e e energy o f m i x i n g a l o n g a c o m p o s i t i o n a l v e c t o r i n t h e system K 0-CaO-MgO-FeO - A l 0 - S i 0 p a s s i n g t h r o u g h e x p e r i m e n t a l l y d e t e r m i n e d c o m p o s i t i o n s o f i m m i s c i b l e m e l t s (F and S ) . C a l c u l a t i o n s were made u s i n g t h e q u a s i c h e m i c a l m e l t p o l y m e r i z a t i o n t h e o r y ( 2 3 ) . The c o m p o s i t i o n s o f p r e d i c t e d and o b s e r v e d i m m i s c i b l e m e l t s c a n be made t o agree r e a s o n a b l y w e l l by adjustments i n p o l y m e r i z a t i o n e q u i l i b r i u m c o n s t a n t s and i n the r a t i o ( F e / F e * + F e ) . 2

+ 3

3

2

+ 2

3

2

MINERAL MATTER AND ASH IN COAL

284

a good a p p r o x i m a t i o n , be equated t o t h e s e mole f r a c t i o n q u a n t i t i e s (15). From t h i s assumption i t a l s o f o l l o w s t h a t p o t a s s i u m p a r t i a l p r e s s u r e s c a n be o b t a i n e d from t h e r e l a t i o n s h i p

0.4 •Χ*

Κ [K 0] ρ Λ

Ί

2

where K i s t h e s t o i c h i o m e t r i c d i s s o c i a t i o n c o n s t a n t f o r pure K 0 ( l i q u i d o r s o l i d ) t o Κ and 0 . I n t h e f o l l o w i n g d i s c u s s i o n t h e model i s t e s t e d by comparing p r e d i c t e d d a t a determined i n t h i s manner with experimental values. Thermodynamic a c t i v i t i e s and phase com­ p o s i t i o n s were a l s o c a l c u l a t e d u s i n g t h i s model. The e x p e r i m e n t a l p

2

Publication Date: April 2, 1986 | doi: 10.1021/bk-1986-0301.ch020

2

TEMPERATURE Κ

-,

KAISio 2CaO

\ —

•

4

Si0

2

Al 0

2CaO

N

2

Si0

Ν. \ V \ \

3

2

2 < LU

MODEL

cr D c/>

CO

CO CO CN

Τ3

cd

fi

C ·Η

Φ

6

φ

\

ΜΗ

60

Ο sT sT CN CN CN

oo sr oo

CN

CN

CN

O Ν

O O

O vu

oo oo sr

CN CN

CN

Ο Ο Ο

μ ST

Pu

o

r H

rfi

te

re

ω

•H H CO

cd r-l

4-1

ο 4-1 CO α, cd rÛ φ

Q

CJN

CO CN

CN CN

Φ

o

CTJ

sr

CN

Cd

cd

C

sr o> σ>

4-J

H

X)

S

6 0

Φ ΜΗ

ω 4-1

Φ

ftf ω u

CO CO cd

Τ)

Φ

> ω CO •H B e •u ctj Φ o

ils

Publication Date: April 2, 1986 | doi: 10.1021/bk-1986-0301.ch024

00

S

φ

CO

Φ Ο çd

π

φ

s

cd

cd

U rH 3 ΜΗ

φ

ο

ΙΟ 00 rH vO H vO

sr m m

sr m in

N O 00 vO rH vO

sr in io

t φ

cd

ο

Φ fi fi H

Furnace

Publication Date: April 2, 1986 | doi: 10.1021/bk-1986-0301.ch024

MINERAL MATTER AND ASH IN COAL

F i g u r e 9. SEM p h o t o m i c r o g r a p h s o f i r o n - r i c h d e p o s i t base p a r t i c l e s c o l l e c t e d from t h e Keystone c o a l a t flame temperatures of: (A) 1470°C, and (B) 1500°C.

Publication Date: April 2, 1986 | doi: 10.1021/bk-1986-0301.ch024

ABBOTT AND AUSTIN

Slag Deposit

Initiation

Using a Drop-Tube

Furnace

343

F i g u r e 10. SEM p h o t o m i c r o g r a p h o f i r o n - r i c h d e p o s i t base p a r t i c l e s c o l l e c t e d from the Keystone c o a l a t a flame temperature o f 1560°C.

Publication Date: April 2, 1986 | doi: 10.1021/bk-1986-0301.ch024

MINERAL MATTER AND ASH IN COAL

F i g u r e 11. SEM photomicrographs o f i r o n - r i c h d e p o s i t base p a r t i c l e s c o l l e c t e d a t a flame temperature o f 1510°C from: (A) Montour c o a l , and (B) T u n n e l t o n c o a l .

0.24

340

feed rate g/min

0.24

p.c.

310

Substrate temperature °C

2.9

2.9

g

Total feed

Relative build-up rates mg/min

1.0 9.8

Deposit mass mg

12 118

22

2

Wt. % ASTM ΗΤΑ

Table VIII. D e p o s i t Furnace Mass and R e l a t i v e B u i l d - U p Rates f o r K e y s t o n e C o a l a t a F u r n a c e Temperature o f 1500°C a t Two D i f f e r e n t S u b s t r a t e S u r f a c e Temperatures ( D e p o s i t i o n Zone Gas Temperature, 1100°C; S u b s t r a t e , 1040 Carbon S t e e l ) .

Publication Date: April 2, 1986 | doi: 10.1021/bk-1986-0301.ch024

346

MINERAL MATTER AND ASH IN COAL

T a b l e IX. D e p o s i t Mass and R e l a t i v e B u i l d - U p Rates f o r the P u l v e r i z e r R e j e c t s a t a F u r n a c e Temperature o f 1515°C ( D e p o s i t i o n Zone Gas Temperature, 1100°C; S u b s t r a t e , 1040 c a r b o n s t e e l ; s u r f a c e temperature, 324°C)

Description

Publication Date: April 2, 1986 | doi: 10.1021/bk-1986-0301.ch024

Total deposit S i n t e r e d mass D e p o s i t base

D e p o s i t mass

359 319 40

Relative build-up rates mg/min 239 213 26

o f t h e base d e p o s i t p a r t i c l e s was v i r t u a l l y t h e same as t h o s e c o l l e c t e d from the t h r e e steam c o a l s . However, when the c o l l e c t o r probe was removed a f t e r so s h o r t a time p e r i o d , t h e r e was a d i s t i n c t s u l f u r odor, i n d i c a t i n g t h a t s u l f u r was e s c a p i n g from t h e d e p o s i t a t a r e l a t i v e l y r a p i d r a t e , even though i t was c o m p a r a t i v e l y c o o l i n temperature ( l e s s than 3 0 0 ° C ) . F i g u r e 12 shows a p e r p e n d i c u l a r view o f the d e p o s i t - s t e e l o x i d e i n t e r f a c e removed from the substrate surface. The average i n t e r f a c e c o m p o s i t i o n from x - r a y f l u o r e s c e n c e showed t h a t t h e r e was a s i g n i f i c a n t amount o f s u l f u r i n the d e p o s i t i n g p a r t i c l e s . A n a l y s i s o f the s u b s t r a t e s u r f a c e from which t h e d e p o s i t was removed showed r e g i o n s which c o n t a i n e d s u l f u r and sometimes t r a c e s o f p o t a s s i u m , c a l c i u m and s i l i c o n . Thus b o n d i n g o f t h e p a r t i c l e s t o t h e s u r f a c e appeared t o i n v o l v e c h e m i c a l t r a n s f e r from the p a r t i c l e t o the s t e e l s u b s t r a t e . T a b l e X g i v e s t h e r e l a t i v e b u i l d - u p r a t e s f o r two samples o f the Decker c o a l : an u n t r e a t e d sample, and a hydrogen exchanged sample i n which most o f the i o n exchange c a t i o n s i n c l u d i n g C a ^ and N a were r e p l a c e d w i t h H+" by a c i d washing. The d e p o s i t i o n r a t e s were somewhat l o w e r than t h e t h r e e P e n n s y l v a n i a c o a l s a t t h e same f u r n a c e temperature. The exchangeable c a t i o n s removed from the u n t r e a t e d sample appeared to p l a y a s i g n i f i c a n t r o l e i n d e p o s i t build-up. The s i n t e r e d m a t e r i a l (yellow-brown i n c o l o r ) c o l l e c t e d from t h e u n t r e a t e d c o a l was r e l a t i v e l y s t r o n g l y bonded t o g e t h e r , r e q u i r i n g a f o r c e o f 20 p s i t o b r e a k i t up. The s i n t e r ( c o r a l c o l o r e d ) from the acid-washed c o a l broke a p a r t w h i l e removing t h e s u b s t r a t e from t h e c o l l e c t o r probe. See F i g u r e s 7A and 13A f o r photos o f ash d e p o s i t s formed from u n t r e a t e d and acid-washed Decker c o a l samples, r e s p e c t i v e l y . The base l a y e r o f p a r t i c l e s from the Decker c o a l were t h e same i r o n - r i c h drops seen i n t h e P e n n s y l v a n i a c o a l d e p o s i t s (see Figure 7B). The c o n c e n t r a t i o n o f t h e s e p a r t i c l e s on the subs t r a t e s u r f a c e d e c r e a s e d when t h e c a t i o n s were removed from t h e coal. Compare F i g u r e s 7B w i t h 13B. The t o t a l p y r i t e concent r a t i o n i n t h e u n t r e a t e d and acid-washed c o a l samples were t h e same, 0.3 weight p e r c e n t i n b o t h i n s t a n c e s . Thus, i r o n - r i c h p a r t i c l e s may a l s o i n i t i a t e s l a g d e p o s i t s from Western c o a l s , even though t h e p y r i t e c o n c e n t r a t i o n i n the c o a l i s r e l a t i v e l y low. The c o n c e n t r a t i o n o f t h e exchangeable c a t i o n s appeared t o i n f l u e n c e the d e p o s i t i o n b e h a v i o r o f t h e i r o n - r i c h d r o p l e t s , a l t h o u g h the e x a c t mechanism o f t h i s e f f e c t i s unknown. +

+

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ABBOTT AND AUSTIN

Slag Deposit

Initiation

Using a Drop-Tube

Furnace

F i g u r e 12. P e r p e n d i c u l a r view o f i r o n - r i c h d e p o s i t base o x i d e s c a l e i n t e r f a c e f o r d e p o s i t c o l l e c t e d from p u l v e r i z e r r e j e c t s (SEM p h o t o m i c r o g r a p h ) .

Untreated c o a l sample

Acid-washed c o a l sample

1.

2.

Test

p.c.

0.24

0.28

feed rate g/min 5.6

4.8

20

g

feed

Total

20

Time, min

25

82

Deposit mass mg

1.3

4.1

Relative build-up rates mg/min

D e p o s i t Mass and R e l a t i v e B u i l d - U p Rates f o r t h e Decker Samples a t a F u r n a c e Temperatrue o f 1500±5°C ( D e p o s i t i o n Zone Gas Temperature, 1050°C; S u b s t r a t e , C r o l o y 1/2; S u r f a c e Temperature, 4 2 5 ± 5 ° C ) .

T a b l e X.

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24

34

Wt. % ASTM ΗΤΑ

Slag Deposit initiation

Using a Drop-Tube

Furnace

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ABBOTT AND AUSTIN

F i g u r e 13. Ash d e p o s i t c o l l e c t e d from the acid-washed Cecker c o a l on C r o l o y 1/2 s t e e l s u b s t r a t e : (A) T o t a l d e p o s i t s t r u c t u r e on a m a c r o s c a l e , and (B) O p t i c a l p h o t o m i c r o g r a p h showing i r o n rich particles.

349

350

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D i s c u s s i o n and

MINERAL MATTER AND

ASH IN COAL

Conclusions

The mechanism of d e p o s i t f o r m a t i o n f o r the t h r e e Pennsylvania steam c o a l s and the Decker (Montana) s u b - b i t u m i n o u s c o a l appeared t o be as f o l l o w s : (1) i r o n - r i c h m o l t e n s l a g drops formed from p y r i t e - r i c h m i n e r a l p a r t i c l e s i n the p.c. bonded t o the o x i d i z e d b o i l e r s t e e l s u b s t r a t e ; (2) c o n c u r r e n t l y a l a y e r o f f i n e p a r t i c l e s ( l e s s t h a n 3 ym) formed a t h i n l a y e r on the s t e e l coupon s u r f a c e ; (3) the i n i t i a l l a y e r of i r o n - r i c h drops t h e n p h y s i c a l l y t r a p p e d o r i n t e r a c t e d w i t h other a s h p a r t i c l e s r e a c h i n g the s u b s t a t e s u r f a c e , and the b u i l d - u p r a t e i n c r e a s e d as d e p o s i t i o n became l e s s d i s c r i m i n a t o r y w i t h r e s p e c t t o ash p a r t i c l e compostion o r s i z e ; (4) f i n a l l y , as the ash d e p o s i t grew and the temperature i n c r e a s e d f u r t h e r from the s u b s t r a t e s u r f a c e , s i n t e r i n g o c c u r r e d between d e p o s i t e d p a r t i c l e s u n t i l a semi-molten mass formed i n the o u t e r most r e g i o n . P r e f e r e n t i a l d e p o s i t i o n of i r o n - r i c h s l a g s has been s u g g e s t e d by o t h e r i n v e s t i g a t o r s (18,19) and was a l s o o b s e r v e d i n a s m a l l s c a l e p.c. t e s t combustor on an a i r - c o o l e d medium c a r b o n s t e e l probe (20). The f i n e p a r t i c l e l a y e r formed on the s u b s t r a t e s u r f a c e i s due t o c o n d e n s a t i o n on or t h e r m a l d i f f u s i o n t o the r e l a t i v e l y c o l d (300-350°C) s t e e l s u b s t r a t e (21). Its role i n d e p o s i t i n i t i a t i o n i s not y e t known. The weakest p o i n t i n the d e p o s i t o c c u r r e d i n the zone between the i n i t i a t i n g i r o n - r i c h p a r t i c l e s and the s i n t e r e d m a t e r i a l . This a l l o w e d removal o f the d e p o s i t down t o the i n i t i a l l a y e r by b r u s h ing with a f i n e b r i s t l e p a i n t brush or blowing with a high v e l o c i t y a i r j e t (25-30 m/sec). The f i n e p a r t i c l e l a y e r was removed by u s i n g d o u b l e - s i d e d s t i c k y tape. The s t r o n g l y bonded, i r o n - r i c h p a r t i c l e s had t o be s h e a r e d from the s u b s t r a t e s u r f a c e w i t h a razor blade. This suggests that conventional sootblowing techn i q u e s i n a u t i l i t y b o i l e r cannot remove a s t r o n g l y bonded i n i t i a t i n g l a y e r formed from s l a g d r o p l e t s bonded t o the s u r f a c e o r f i n e p a r t i c l e s d e p o s i t e d by c o n d e n s a t i o n o r t h e r m a l d i f f u s i o n and h e l d by Van der Waals f o r c e s . At h i g h e r flame temperatures the f l y ash p a r t i c l e s i z e appeared t o be r e d u c e d , presumably due t o a g r e a t e r degree o f b r e a k i n g up of i n d i v i d u a l b u r n i n g c o a l p a r t i c l e s , and the base o r i r o n - r i c h p a r t i c l e s and o v e r a l l d e p o s i t b u i d l - u p r a t e s were i n c r e a s e d . The Keystone c o a l i n p a r t i c u l a r gave drops which wet the s u r f a c e b e t t e r a t h i g h e r flame t e m p e r a t u r e s . The Keystone c o a l formed the most e x t e n s i v e d e p o s i t base of the t h r e e c o a l s a t the h i g h e s t flame temperature. The i r o n - r i c h p a r t i c l e s adhered to the o x i d i z e d s t e e l substrates a t temperatures as low as 310° C. S u r p r i s i n g l y , the same l a y e r o f i r o n - r i c h base d e p o s i t p a r t i c l e s was a l s o o b s e r v e d w i t h the low p y r i t e Decker c o a l . However, d e p o s i t i n i t i a t i o n and b u i l d - u p was r e d u c e d when the c o n c e n t r a t i o n of i o n - e x c h a n g e a b l e c a t i o n s i n the c o a l was r e d u c e d . S l a g d e p o s i t i n i t i a t i o n and b u i l d - u p were s e n s i t i v e t o b o t h flame and s t e e l s u b s t r a t e s u r f a c e t e m p e r a t u r e s , even though the gas temperature o f d e p o s i t i o n was h e l d c o n s t a n t . This suggests at l e a s t q u a l i t a t i v e agreement between r e s u l t s from the d r o p - t u b e f u r n a c e and t h o s e from the s t i c k i n g a p p a r a t u s (1,5). I t i s hypothes i z e d t h a t a h i g h e r flame ( m e l t i n g ) temperature g i v e s a more homo-

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24.

ABBOTT AND

AUSTIN

Slag Deposit Initiation

Using a Drop-Tube

Furnace

351

geneous s l a g drop, which remains as a v i s c o u s , s t i c k y , s u p e r c o o l e d g l a s s a t d e p o s i t i o n t e m p e r a t u r e s , whereas i n h o m o g e n e i t i e s i n the m e l t a t lower temperatures a c t as n u c l e i t o g i v e more c r y s t a l l i z a ­ t i o n on c o o l i n g . The h i g h c o n c e n t r a t i o n o f s u l f u r a t the d e p o s i t s t e e l i n t e r f a c e has been o b s e r v e d i n p.c. u t i l i t y b o i l e r s (22,23). The p r i n c i p a l q u e s t i o n s posed by the r e s u l t s of t h i s i n v e s t i ­ g a t i o n were: (1) can a s h d e p o s i t s be formed i n the d r o p - t u b e f u r n a c e i n the absence o f i r o n - c o n t a i n i n g m i n e r a l s o r s l a g d r o p s ? ; (2) what r o l e , i f any, does the f i n e p a r t i c l e l a y e r p l a y i n d e p o s i t f o r m a t i o n ? ; (3) i s t h e r e any s i g n i f i c a n c e t o the h i g h s u l f u r con­ c e n t r a t i o n a t the d e p o s i t i n t e r f a c e and what i s the mechanism by w h i c h t h i s s u l f u r enrichment o c c u r s : does i t o r i g i n a t e from p y r i t e o r from c o n d e n s i n g s u l f a t e ? ; (4) what i n f l u e n c e do a l k a l i s have on s l a g d e p o s i t i n i t i a t i o n and b u i l d - u p ? To answer t h e s e q u e s t i o n s , i t i s p l a n n e d t o t e s t s y n t h e t i c c o a l / m i n e r a l m i x t u r e s of c o n t r o l l e d c o m p o s i t i o n p r e p a r e d by d i s p e r s i n g f i n e l y ground m i n e r a l s i n l i q u i d o r g a n i c polymer f o l l o w e d by s e t t i n g and s i z e r e d u c t i o n t o p.c. g r i n d . Acknowledgments T h i s s t u d y was funded by a g r a n t from the Department o f Energy, C o n t r a c t No. DE-FG22-80PC-30199. We would l i k e t o thank Babcock and W i l c o x A l l i a n c e R e s e a r c h C e n t e r ( A l l i a n c e , Ohio) f o r s u p p l y i n g the C r o l o y 1/2 s t e e l f o r the t e s t s u b s t r a t e s .

References 1. 2. 3. 4. 5.

6. 7. 8. 9. 10. 11. 12. 13.

Moza, A. K.; Austin, L. G. Fuel 1981, 60, 1057-64. Abbott, M. F.; Moza, A. K.; Austin, L. G. Fuel 1981, 60, 1065-72. Moza, A. K.; Austin, L. G. Fuel 1982, 61, 161-65. Abbott, M. F.; Austin, L. G. Fuel 1982, 61, 765-70. Abbott, M. F.; Conn, R. E.; Austin, L. G. "Studies on Slag Deposit Formation in Pulverized Coal Combustors. 5. The Effect of Flame Temperature, Thermal Cycling of the Steel Substrate, and Time on the Adhesion of Slag Drops to Oxidized Boiler Steels" Fuel, accepted. Abbott, M. F.; Austin, L. G. ibid 6. The Sticking Behavior of Slag from Three Pennsylvania Steam Coals" Fuel, accepted. Moza, A. K.; Strickler, D. W.; Austin, L. G., Proc. of Annual SEM Symp. Chicago, Il., 1979. Mims, C. Α.; Neville, M.; Quann, R. T.; Sarofim, A. F. Proc. from the National AIChE Mtg., Boston, Ma., Aug. 1979. Sarofim, A. F.; Howard, J. B.; Padia, A. S. Comb. Sci. and Tech., 1977, 16, 187-204. Winegartner, E. J.; Lin, C. J. ASME Paper No. 79-WA/Fu-1, Dec. 1979. Rees, D. "An Assessment of Pulverized Coal Fired Combustor Performance" DOE Combustion Contractors Mtg., Pittsburgh, Pa., March 1981. Moza, A. K. Fouling and Slagging Resulting from Impurities in Combustion Gases, Eng. Foundation, NY, NY, 1983, 231-44. Shiomoto, G. H.; Muzio, L. J.; Mansour, M. N. ibid, 363-73.

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14. Hamor, R. J.; Smith, I. W. Fuel 1971, 50, 394-404. 15. Altenkirch, R. E.; Peck, R. E.; Chen, L. S. Powder Tech. 1978, 20, 189-96. 16. Pigford, R. L. Chem. Eng. Prog. Symp. Series 17, 1955, 51, 79-92. 17. Bird, R. B.; Stewart, W. E.; Lightfoot, Ε. N. Transp. Phenomena John Wiley and Sons, Inc., 1950. 18. Borio, R. W.; Narcisco, R. R. J. of Eng. for Power 1979, 101(4), 500. 19. Bryers, R. W. J. of Eng. for Power 1979, 101, 506. 20. Austin, L. G.; Abbott, M. F.; Kinneman, W. P. ASME 83-JPGC­ -Pwr-42, Sept. 1983. 21. Rosner, D. E.; Gokoglu, S.; Israel, R. Fouling and Slagging Resulting from Impurities in Combustion Gases, 1983, Eng. Found., N.Y., N.Y., 235-56. 22. Hazard, H. R. Final Report, EPRI CS-1418, June 1980. 23. Fessler, R. R.; Skidmore, A. J.; Hazzard, H. R.; Dimmer, J. P. ASME Paper No. 79-WA/CD-1 1979. RECEIVED June 24, 1985

25 Influence of Segregated Mineral Matter in Coal on Slagging Richard W. Bryers

Publication Date: April 2, 1986 | doi: 10.1021/bk-1986-0301.ch025

Foster Wheeler Development Corporation, Livingston, NJ 07039

The mineral content of any given rank of coal is a key factor in sizing and designing a steam generator or reactor. The mineral content becomes even more important as the premium solid fuels are consumed, leaving reserves with continually increasing mineral concentrations and lower quality ash. The problem of dealing with lower quality ash in coal is compounded by the increase in size of steam generators and refinements imposed by economic constraints. Empirical indices, based on coal ash chemistry and ASTM ash fusion temperatures or viscosities, are presently used to rank coals according to the fireside behavior of the mineral matter. Unfortunately, the indices are only marginally satisfactory, as they do not relate to operating or design parameters and frequently are based on a coal ash chemistry quite different from that deposited on the furnace wall. Frequently, different coals with identical ash chemistry produce decidedly different slagging conditions in steam generators of identical design operated in the same mode. Variations in composition of the slag, when compared with the coal ash, suggest specific minerals are being selectively deposited on furnace walls depending upon their specific gravity, size, composition, and physicochemical properties. It is quite apparent there is a need for a better understanding of the impact of mineral composition, its size, and its association with other minerals and carbonaceous matter on fireside deposits. S l a g g i n g and F o u l i n g The m i n e r a l s i n c o a l a r e c o n v e r t e d t o a s h d u r i n g combustion. The p o r t i o n o f t h e f l y ash i m p a c t i n g on heat t r a n s f e r s u r f a c e s , which i s r e t a i n e d as d e p o s i t s , depends on t h e p a r t i c l e s i z e , i t s c h e m i s t r y and i t s p h y s i c o c h e m i c a l b e h a v i o r d u r i n g combustion i n t h e steam gene r a t o r f u r n a c e , as w e l l as subsequent c o o l i n g i n t h e c o n v e c t i v e heat 0097-6156/ 86/ 0301 -0353506.50/ 0 © 1986 American Chemical Society

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recovery section. The type o f d e p o s i t formed f a l l s i n t o one o f two c a t e g o r i e s — f u r n a c e s l a g , o r bonded c o n v e c t i v e heat t r a n s f e r depos­ its. S l a g i s d e f i n e d as a m o l t e n ash d e p o s i t e d on f u r n a c e w a l l s i n zones s u b j e c t e d t o r a d i a n t heat t r a n s f e r , as shown i n F i g u r e 1. O c c a s i o n a l l y , when c o a l m o i s t u r e l e v e l s approach 50 t o 70 p e r c e n t and the flame temperature becomes e x c e p t i o n a l l y low, bonded d e p o s i t s i n s t e a d o f s l a g form on f u r n a c e w a l l s . F o u l i n g o c c u r s i n the convec­ t i v e heat t r a n s f e r zones. The p r o d u c t o f f o u l i n g i s a bonded d e p o s i t c o n s i s t i n g o f an aggregate o f dry p a r t i c u l a t e m a t t e r bound t o g e t h e r by a m o l t e n phase t h a t has wetted a d j a c e n t p a r t i c u l a t e and subse­ q u e n t l y become f r o z e n . The bonded d e p o s i t i s f r e q u e n t l y i n i t i a t e d by a melt on the tube s i d e l a y e r s o f d e p o s i t formed by the condensa­ t i o n o f s u b s t a n c e s , such as a l k a l i s u l f a t e , w i t h low vapor p r e s s u r e s and low m e l t i n g temperatures. Bonding o r s i n t e r i n g may a l s o o c c u r as a r e s u l t o f v i s c o u s o r p l a s t i c f l o w between p a r t i c l e s , a gas phase r e a c t i o n , s o l u t i o n and p r e c i p i t a t i o n , o r d i f f u s i o n t r a n s f e r between adjacent p a r t i c l e s . On o c c a s i o n , when the f u r n a c e e x p e r i e n c e s a temperature e x c u r s i o n , s l a g g i n g w i l l o c c u r i n the h i g h temperature gas p a s s e s . Mineral Matter

i n Coal

M i n e r a l s o c c u r r i n g i n c o a l , which are r e s p o n s i b l e f o r f i r e s i d e depos­ i t s , may be c l a s s i f i e d i n t o f i v e main groups. These i n c l u d e s h a l e , c l a y , s u l f u r , and c a r b o n a t e s . The f i f t h group i n c l u d e s a c c e s s o r y m i n e r a l s such as q u a r t z and minor c o n s t i t u e n t s l i k e the f e l d s p a r s [1]. S h a l e , u s u a l l y the r e s u l t o f the c o n s o l i d a t i o n o f mud, silt, and c l a y , c o n s i s t s o f many m i n e r a l s i n c l u d i n g i l l i t e and m u s c o v i t e - these are forms o f m i c a . K a o l i n i t e i s the most common c l a y m a t e r i a l [1]. The s u l f u r minerals include p y r i t e s with some m a r c a s i t e . M a r c a s i t e has the same c h e m i c a l c o m p o s i t i o n as p y r i t e s but a d i f f e r ­ ent m i n e r a l o g i c a l s t r u c t u r e . S u l f u r i s a l s o p r e s e n t as o r g a n i c mat­ ter and o c c a s i o n a l l y as sulfate. The l a t t e r u s u a l l y occurs i n weathered c o a l such as i n o u t c r o p s . The amount o f s u l f a t e s u l f u r i n c o a l i s g e n e r a l l y l e s s than 0.01 p e r c e n t . G e n e r a l l y , 60 p e r c e n t o f the s u l f u r i n c o a l o c c u r s as p y r i t e , p a r t i c u l a r l y when the s u l f u r c o n c e n t r a t i o n i s low. At h i g h e r concen­ t r a t i o n s i t may run as much as 70 t o 90 p e r c e n t . The m i n e r a l p y r i t e o c c u r s i n c o a l i n d i s c r e t e p a r t i c l e s i n a wide v a r i e t y of shapes and s i z e s . The p r i n c i p a l forms are [ 1 - 6 ] :

ο ο ο

ο ο

Rounded masses c a l l e d s u l f u r b a l l s or n o d u l e s an i n c h o r more i n s i z e . Lens-shaped masses which are thought to be f l a t t e n e d s u l f u r balls. V e r t i c a l , i n c l i n e d v e i n s or f i s s u r e s f i l l e d with p y r i t e r a n g i n g i n t h i c k n e s s from t h i n f l a k e s up t o s e v e r a l i n c h e s thick. S m a l l , d i s c o n t i n u o u s v e i n l e t s o f p y r i t e s , a number o f which sometimes r a d i a t e from a common c e n t e r , S m a l l p a r t i c l e s , 2μ, o r v e i n l e t s d i s s e m i n a t e d i n the c o a l . M i c r o s c o p i c p y r i t e o c c u r s i n f i v e b a s i c morphology t y p e s :

F i g u r e 1. P i l o t Plant Slagging to a F u l l - S c a l e Steam G e n e r a t o r

FULL SCALE STEAM GENERATOR

and

Fouling

Combustor

Compared

PILOT SCALE PLANT

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356

MINERAL MATTER AND ASH IN COAL

Publication Date: April 2, 1986 | doi: 10.1021/bk-1986-0301.ch025

(a) framboids; (b) i s o l a t e d , well-defined crystals; ( c ) n o n s p h e r i c a l aggregates o f w e l l - d e f i n e d c r y s t a l s ; (d) i r r e g u l a r shapes; and ( e ) f r a c t u r e d f i l l i n g s [ 7 ] . The term, framboid, i s d e r i v e d from the F r e n c h word f o r r a s p b e r r y and thus r e f e r s t o n a t u r a l l y o c c u r r i n g s p h e r o i d a l c l u s t e r s o f hundreds o f c u b i c o r o c t a h e d r o n a l c r y s t a l s o f p y r i t e s [ 9 ] . All c o a l s c o n t a i n some o f the t h i r d and f i f t h forms o f p y r i t e s , and some c o a l s c o n t a i n a l l f i v e o f t h e p r i n c i p a l forms [6, 8,9]. The carbonates are mainly c a l c i t e , dolomite, or s i d e r i t e . The o c c u r r e n c e o f c a l c i t e i s f r e q u e n t l y b i m o d a l . Some c a l c i t e o c c u r s as i n h e r e n t a s h , w h i l e o t h e r c a l c i t e appears as t h i n l a y e r s i n c l e a t s and f i s s u r e s . I r o n can be p r e s e n t i n s m a l l q u a n t i t i e s as h e m a t i t e , a n k o r i t e , and i n some o f the c l a y m i n e r a l s such as i l l i t e . In a d d i t i o n t o the more common m i n e r a l s , s i l i c a i s p r e s e n t sometimes as sand p a r t i c l e s o r q u a r t z . The a l k a l i e s a r e sometimes found as c h l o r i d e s o r as s u l f a t e s but p r o b a b l y most o f t e n as f e l d s p a r s , t y p i c a l l y o r t h o c l a s e and a l b i t e . I n the case o f l i g n i t e s , u n l i k e bituminous and subbituminous, sodium i s n o t p r e s e n t as a m i n e r a l but i s p r o b a b l y d i s t r i b u t e d throughout the l i g n i t e as the sodium s a l t o f a h y d r o x y l group o r a c a r b o x y l i c a c i d group i n humic a c i d . C a l c i u m , l i k e sodium, i s bound o r g a n i c a l l y t o humic a c i d . Therefore, i t too i s u n i f o r m l y d i s t r i b u t e d i n the sample [ 1 0 ] . The term, " m i n e r a l m a t t e r " , u s u a l l y a p p l i e s t o a l l i n o r g a n i c , noncarbonaceous m a t e r i a l i n the c o a l and i n c l u d e s those i n o r g a n i c elements which may o c c u r i n o r g a n i c c o m b i n a t i o n . P h y s i c a l l y , the i n o r g a n i c m a t t e r can be d i v i d e d i n t o two g r o u p s - - i n h e r e n t m i n e r a l m a t t e r and e x t r a n e o u s m i n e r a l m a t t e r . Inherent m i n e r a l matter o r i g i n a t e s as p a r t o f the growing p l a n t l i f e from which c o a l was formed. Under the c i r c u m s t a n c e s , i t has a u n i f o r m d i s t r i b u t i o n w i t h i n the coal. I n h e r e n t m i n e r a l m a t t e r seldom exceeds 2 t o 3 p e r c e n t o f the coal [12]. Extraneous m i n e r a l matter g e n e r a l l y c o n s i s t s o f l a r g e b i t s and p i e c e s o f i n o r g a n i c m a t e r i a l t y p i c a l o f the s u r r o u n d i n g g e o l o g y . In some cases the extraneous m a t t e r i s so f i n e l y d i v i d e d and u n i f o r m l y d i s p e r s e d w i t h i n the c o a l i t behaves as i n h e r e n t m i n e r a l matter. C o a l p r e p a r a t i o n can s e p a r a t e some o f the extraneous a s h from the c o a l s u b s t a n c e , but i t seldoms removes any o f the i n h e r e n t mine r a l matter. The p h y s i c a l d i f f e r e n c e s between i n h e r e n t and extraneous ash are important n o t o n l y t o those i n t e r e s t e d i n c l e a n i n g c o a l b u t a l s o to those concerned w i t h the f i r e s i d e b e h a v i o r o f c o a l a s h . I n h e r e n t m a t e r i a l i s so i n t i m a t e l y mixed w i t h c o a l t h a t i t s thermal h i s t o r y i s l i n k e d t o the combustion o f the c o a l p a r t i c l e i n which i t i s contained. T h e r e f o r e , i t w i l l most l i k e l y r e a c h a temperature i n excess o f the gas i n the immediate s u r r o u n d i n g s . The c l o s e p r o x i m i t y o f each s p e c i e s w i t h e v e r y o t h e r s p e c i e s p e r m i t s c h e m i c a l r e a c t i o n and p h y s i c a l changes t o o c c u r so r a p i d l y t h a t the subsequent ash particles formed w i l l behave as a s i n g l e m a t e r i a l whose c o m p o s i t i o n i s d e f i n e d by the m i x t u r e o f m i n e r a l s c o n t a i n e d w i t h i n the c o a l particle. The atmosphere under which the i n d i v i d u a l t r a n s f o r m a t i o n s take p l a c e w i l l , no doubt, approach a r e d u c i n g environment. Figure 2 i l l u s t r a t e s a model o f the c o a l and m i n e r a l m a t t e r as f e d t o the combustor and the f a t e o f t h e m i n e r a l s a f t e r combustion [ 1 3 ] .

BRYERS

Segregated Mineral

Matter

influence

HEAT

on

T R A N S F E R

Publication Date: April 2, 1986 | doi: 10.1021/bk-1986-0301.ch025

/ / / / / /

H E A T AND

Slagging

T R A N S F E R

t

/(

SURFACES

f

t

/ / '

/

SURFACES

REFRACTORY

F i g u r e 2. Fate of M i n e r a l M a t t e r i n C o a l During Combustion, as Proposed by Dr. S a r o f i m . Reproduced w i t h p e r m i s s i o n from r e f e r e n c e 23. C o p y r i g h t 1982 The Combustion I n s t i t u t e .

358

MINERAL MATTER AND ASH IN COAL

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E x t r a n e o u s m a t e r i a l s can behave as d i s c r e t e m i n e r a l p a r t i c l e s comprised o f a s i n g l e s p e c i e s o r a m u l t i p l i c i t y o f s p e c i e s . As a l r e a d y i n d i c a t e d , a p o r t i o n o f t h i s m a t e r i a l may be so finely d i v i d e d i t can behave as i n h e r e n t m i n e r a l m a t t e r . D u r i n g combustion the l a r g e r p a r t i c l e s respond i n d i v i d u a l l y t o the r i s i n g temperature of the environment. In the absence o f carbon o r o t h e r e x o t h e r m i c r e a c t a n t s , the p a r t i c l e s h o u l d always be a t a temperature somewhat l e s s than the l o c a l gas temperature. However, the p a r t i c l e s may be s u b j e c t e d t o e i t h e r r e d u c i n g o r o x i d i z i n g c o n d i t i o n s . As each p a r t i c l e r i s e s i n temperature, i t l o s e s water o f h y d r a t i o n , e v o l v e s gas, becomes o x i d i z e d o r reduced, and e v e n t u a l l y s i n t e r s o r m e l t s , depending on i t s p a r t i c u l a r c o m p o s i t i o n o r temperature l e v e l . I t i s e v i d e n t , then, t h a t t h e r e can be a g r e a t d i f f e r e n c e i n the f i n a l s t a t e o f each p a r t i c l e , depending upon i t s c o m p o s i t i o n and whether i t i s i n h e r e n t o r e x t r a n e o u s ash. F i g u r e 3 summarizes the phase t r a n s f o r m a t i o n s which pure m i n e r a l m a t t e r commonly found i n c o a l undergoes d u r i n g h e a t i n g [12-18]. S i n c e t h i s d a t a was d e v e l oped p r i m a r i l y by m i n e r a l o g i s t s p e r f o r m i n g d i f f e r e n t i a l thermal a n a l y s i s under a i r a t slow h e a t i n g r a t e s , i t must be used o n l y as a g u i d e l i n e f o r p r e d i c t i n g the t h e r m a l b e h a v i o r o f m i n e r a l s i n c o a l . Thermal s h o c k i n g these m i n e r a l s i n the presence o f carbon and o t h e r m i n e r a l forms a t v e r y h i g h t e m p e r a t u r e s , no doubt, w i l l a l t e r some of t h e s e t r a n s f o r m a t i o n s and may d e f e r o t h e r s u n t i l postcombustion d e p o s i t i o n on heat t r a n s f e r s u r f a c e s . C l a y s and S h a l e . The m e l t i n g temperatures o f most pure m i n e r a l s are i n the v i c i n i t y o f o r g r e a t l y exceed the maximum flame temperat u r e e n c o u n t e r e d d u r i n g combustion. T h e r e f o r e , the f u s e d s p h e r o i d a l f l y ash, g e n e r a t e d from the m i n e r a l m a t t e r i n c o a l , p r i m a r i l y forms as the r e s u l t o f the f l u x i n g a c t i o n between pure m i n e r a l s c o n t a i n e d w i t h i n each p a r t i c l e . I l l i t e and b i o t i t e appear t o be an e x c e p t i o n . Both m i n e r a l s c o n t a i n s m a l l c o n c e n t r a t i o n s o f i r o n and p o t a s s i u m and form a g l a s s y phase a t 950°C and 1100°C, r e s p e c t i v e l y . Depending upon i t s f l u i d i t y , t h i s g l a s s y phase c o u l d be r e s p o n s i b l e f o r s u r f a c e d e f o r m a t i o n a t a r e l a t i v e l y low temperature and p r o v i d e the n e c e s s a r y sticking p o t e n t i a l t o p r e v e n t r e e n t r a i n m e n t upon c o n t a c t i n g heat transfer surfaces. Low-temperature ash o f a g r a v i t y f r a c t i o n cont a i n i n g i l l i t e was h e a t e d i n a t h e r m a l a n a l y z e r under a i r t o 600 and 1000°C, r e s p e c t i v e l y , and compared t o the low-temperature ash of a g r a v i t y f r a c t i o n v o i d o f i l l i t e . The s c a n n i n g e l e c t r o n photom i c r o g r a p h s , a p p e a r i n g i n F i g u r e 4, i n d i c a t e the m i n e r a l s c o n t a i n i n g i l l i t e d i d , i n d e e d , show s i g n s o f the f o r m a t i o n o f a m e l t . Quartz. The i n h e r e n t s i l i c a r e t a i n e d i n the char as q u a r t z o r s i l i c a r e l e a s e d from k a o l i n i t e and i l l i t e a t low temperatures ( i . e . , 950°C) i s p a r t i a l l y reduced t o s i l i c a monoxide. U n l i k e s i l i c a , which b o i l s at 2230°C, s i l i c a monoxide m e l t s a t 1420°C and b o i l s a t 2600°C [ 1 9 ] . The vapor p r e s s u r e o f s i l i c o n i s l o w - - i n the range o f temperatures e x p e r i e n c e d d u r i n g combustion. Honig r e p o r t s v a l u e s r a n g i n g from 0.01m Hg a t 1157°C t o lm Hg p r e s s u r e a t 1852°C [19-21]. The p r e s e n c e of o t h e r m i n e r a l m a t t e r and carbonaceous m a t e r i a l appears t o a l t e r vapor p r e s s u r e s u b s t a n t i a l l y . When a m i x t u r e o f a l u m i n o s i l i c a t e and g r a p h i t e was h e a t e d , the v o l a t i l i z a t i o n o f s i l i c o n monoxide began at about 1150°C and reached a maximum a t 1400°C. Mackowsky r e p o r t s t h a t v o l a t i l i z a t i o n o f s i l i c o n monoxide s t a r t s a t about 1649°C i n

500 °C , , — 2 ^ 0

925°C

3

—^Si0

2

(

,—^Si0

2

3

2

2

(cristobaUte)

* Si0

e

I400°C

' I300°C

II00°C

I050°C

class**

amorphous

2

zsfa )| X

mul (3AI 0,.

from middle layers of lattice.

2

3

3t

•S, S0

4

2

2

e

970 C Fe

Fe (partial melt)

—«-S

I liquid glass]

700 C e

I500°C I glass «• olivinel

y Temperatures soneatlve to mineral and carbon impurities^

Fe 0 liquid I600°C

e

525 C*

2

FeS, Fe (S0 ) Fe0| (partial melt)

475°C" —«»S,S0

JE 1 IQxidizinq

I Pyrites |

"Formed from alkali and S i O , from thel outer layers of lattice.

I200°C

2

3

2Si0 )

2

(3AI 0 -

4SÎ0Î)

2

mullite

(Κ 0·ΑΙ 0,· 2

leucite

~H glass formed h

(MaOFejOj)

4

(Fe 0 ) OR

(SiOj) 3

spinel

j lattice destroyed at IIOO°Cr quartz

' F o r m e d from oluminum and magnésium

I liquid qloss

(AljOj-MgO)

spinel*

e

950 C

iMuscovitTl

2

Q

3

)

e

SiO* liquid 1723 C

950° C

inversion

Temperature

I liquid glass |

I500°C

(AI

|only glass + corundum

I400°C

Ialumina or spinel I

I050°C

lattice destroyed at: 940°-980°C

F i g u r e 3. Phase T r a n s f o r m a t i o n of Some M i n e r a l M a t t e r Commonly Found i n C o a l . Reproduced w i t h p e r m i s s i o n from r e f e r e n c e 19. C o p y r i g h t 1984 VGB.

CaO liquid 2570 C

1000° to II00°C

I Calcite |

Other members in kaolinite group, Ί (Le. liveeite and meta halloysito) are thought to undergo similar changes J

[

May not be completely liquid until 1800°C.

I

2

[mullite ^ A l g O ^ 2Si0 ) |

o

H400 C

fi-1 mullite-type phoseÎAfeOs · SiO^f

II00°C

2

1 silicon spinel (2Al 0 · 3Si0 )|

(clay lattice destroyed) ,

2

1 metakaolinite ( Α Ι 0 · 2Si02)|

Illite

Publication Date: April 2, 1986 | doi: 10.1021/bk-1986-0301.ch025

i I"

ο a

t

I.

J?

73 ·< m 73

CO

F i g u r e 4. SEM P h o t o m i c r o g r a p h s S h o w i n g t h e I m p a c t o f I l l i t e o n t h e T h e r m a l B e h a v i o r o f Low T e m p e r a t u r e A s h . Reproduced w i t h p e r m i s s i o n f r o m r e f e r e n c e 19. C o p y r i g h t 1984 VGB.

Publication Date: April 2, 1986 | doi: 10.1021/bk-1986-0301.ch025

n

Ο >

ζ

OC

>

D

>

H m

r

>

ζ m

s

25.

BRYERS

Segregated Mineral

Matter

Influence

on

Slagging

361

the p r e s e n c e o f c a r b o n a t e s and c l a y s and r e a c h e s a maximum a t 1704°C [22]. In the p r e s e n c e o f p y r i t e o r m e t a l l i c i r o n , v o l a t i l i z a t i o n b e g i n s a t about 1560°C and c o n t i n u e s a t a r a p i d r a t e as the tempera­ t u r e r i s e s u n t i l p r a c t i c a l l y a l l the s i l i c a i n the m i n e r a l i s v o l a ­ tilized. S a r o f i m has shown t h a t about 1.5 t o 2.0 p e r c e n t o f the ASTM ash i n b i t u m i n o u s c o a l s v o l a t i l i z e [ 2 3 ] . A p p r o x i m a t e l y 35 t o 40 p e r c e n t o f the v o l a t i l i z e d m a t e r i a l was s i l i c a . The next l a r g e s t component was iron. Extraneous q u a r t z appears t o be relatively innocuous u n l e s s contaminated by F e 0 , CaO, o r K 0 .

Publication Date: April 2, 1986 | doi: 10.1021/bk-1986-0301.ch025

2

3

2

Pyrites. The d e c o m p o s i t i o n o f p y r i t e has been examined by numerous investigators under oxidizing, neutral, or reducing environments [1,11,16,17]. TGA, r a t h e r than DTA, has been used by most i n v e s t i g a ­ t o r s ( F i g u r e 5 ) . A c q u i s i t i o n of r e p r e s e n t a t i v e data i s d i f f i c u l t , as the d e c o m p o s i t i o n p r o c e s s i s complex and s e n s i t i v e to many v a r i a b l e s i n c l u d i n g the c h e m i c a l c o m p o s i t i o n of p y r i t e s , i t s g r a i n s i z e , i t s o r i g i n , and the p r e s e n c e of a d v e n t i t i o u s i m p u r i t i e s , the c o m p o s i t i o n of the l o c a l environment, and d i f f u s i o n r a t e s through s u l f a t e d l a y e r s Under o x i d i z i n g c o n d i t i o n s i t i s b e l i e v e d t h a t p y r i t e s decompose d i r e c t l y t o an i r o n o x i d e and S 0 o r S 0 , o r i r o n s u l f a t e and S 0 , depending upon the f i n a l temperature l e v e l . Under r e d u c i n g c o n d i ­ t i o n s p y r r h o t i t e and e i t h e r s u l f u r o r hydrogen s u l f i d e form. Com­ plete reduction results i n e l e m e n t a l i r o n and c a r b o n disulfide. There i s a l s o a p o s s i b i l i t y t h a t p y r r h o t i t e may form under o x i d i z i n g c o n d i t i o n s as an i n t e r m e d i a t e s t e p i n the p r e s e n c e o f s u f f i c i e n t a d v e n t i t i o u s carbon. Pure p y r i t e s i g n i t e a t about 500°C i n the t h e r ­ mal a n a l y z e r a t 20°C/min and burn out by 550°C i n a s i n g l e - s t e p p r o ­ c e s s , as shown i n F i g u r e 6. Pure p y r i t e s do i g n i t e as r e a d i l y as b i t u m i n o u s c o a l ; however, the burnout time i s comparable. Although p y r r h o t i t e i g n i t e s r e a d i l y , i t s r e q u i r e s as much time as a n t h r a c i t e to complete combustion. Pyrites containing small quantities of a d v e n t i t i o u s c a r b o n , as might be found i n the -1.80+2.85 g r a v i t y f r a c t i o n , appear t o form p y r r h o t i t e d e f e r r i n g burnout u n t i l 800°C. W i t h i n the combustor the problem i s compounded by the f a c t that p y r i t e p a r t i c l e s do not s h r i n k d u r i n g the combustion p r o c e s s as do c o a l p a r t i c l e s , and hence t h e i r burnout time i s extended. The burn­ out time o f p a r t i c l e s i n e x c e s s o f 40μ appears t o exceed the r e s i ­ dence time a v a i l a b l e i n most combustors. 2

3

2

TGA r e v e a l s d e c o m p o s i t i o n r a t e s but t e l l s l i t t l e about the p h y s i c a l s t a t e d u r i n g the combustion p r o c e s s . Phase diagrams f o r the Fe-S-0 and FeS-FeO system, r e p r e s e n t i n g t r a n s i t o r y s t a t e s a t the p a r t i c l e s u r f a c e , imply the f o r m a t i o n o f a temporary melt a t low t e m p e r a t u r e s . SEM p h o t o m i c r o g r a p h s , a p p e a r i n g i n F i g u r e 6, o f pure p y r i t e s h e a t e d t o 600°C, 800°C, and 1000°C under r e d u c i n g c o n d i ­ t i o n s , c l e a r l y r e v e a l the f o r m a t i o n o f a melt a t temperatures as low a t 600°C. Large p a r t i c l e s o f p a r t i a l l y - s p e n t p y r i t e s , which may be m o l t e n on c o n t a c t w i t h the heat t r a n s f e r s u r f a c e , complete the o x i d a t i o n p r o c e s s i n s i t u , forming a s o l i d f u s e d d e p o s i t w i t h a v e r y h i g h m e l t i n g temperature. An e x a m i n a t i o n o f the t h e r m a l b e h a v i o r o f the m i n e r a l m a t t e r i n c o a l i n d i c a t e s the m i n e r a l o r i g i n o f the elements found i n c o a l ash and t h e i r j u x t a p o s i t i o n w i t h r e g a r d t o each o t h e r , as w e l l as o t h e r m i n e r a l forms, and determines t h e i r p h y s i c a l f a t e d u r i n g com­ bustion. As i n d i c a t e d i n F i g u r e 2, the p h y s i c a l s t a t e ( i . e . , vapor or s o l i d ) and the s i z e o f s o l i d i f i e d ash w i l l determine the mode

Publication Date: April 2, 1986 | doi: 10.1021/bk-1986-0301.ch025

362 MINERAL MATTER AND ASH IN COAL

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RHLD 20 0.382 0.064 0.073 0.260 0.057 0.160 0.004 0.000 1.000

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Publication Date: April 2, 1986 | doi: 10.1021/bk-1986-0301.ch027

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Publication Date: April 2, 1986 | doi: 10.1021/bk-1986-0301.ch027

404

MINERAL MATTER AND ASH IN COAL

I l l u m i n a t i n g r e s u l t s a r e p o s s i b l e even when o n l y s m a l l q u a n t i t i e s (20 - 40 g p e r t e s t ) o f s u b s t a n c e s may be a v a i l a b l e from p r o s p e c t sampling i n the e x p l o r a t i o n of new d e p o s i t s . At the same time, t h i s t e s t e q u i p m e n t o f f e r s i m p o r t a n t i n f o r m a t i o n on t h e mechanism of deposit formation. Adherence of ash p a r t i c l e s to the h e a t i n g s u r f a c e s i s a maximum when impact o c c u r s a f t e r a combustion p a t h i n w h i c h t h e end o f t h e b u r n - o u t of v o l a t i l e components produces the highest temperature o f c o a l g r a i n s o v e r t h e s u r r o u n d i n g gas atmosphere. Weser ( 2 0 ) i m p r o v e d t h e F i e l d t u b e by i n s t a l l i n g thermophotographic equipment f o r measurement of p a r t i c l e temperature p r i o r t o d e p o s i t i o n . Temperature d i f f e r e n c e s between s o l i d m a t t e r and f l u e gas of up t o 400 Κ were found u s i n g t h i s equipment. U s e o f t h e F i e l d tube f o r d e t e r m i n i n g the performance o f c o a l s i s l i m i t e d by t h e s h o r t r e s i d e n c e time of 0.3 seconds (2.1 m) and the i n a b i l i t y t o use p a r t i c l e s l a r g e r than 0.14 mm. T h i s l i m i t s the e x t e n t o f p a r t i c l e s i z e c h a n g e s and t h e s t r u c t u r e of the m i n e r a l substance r e l a t i v e t o p u l v e r i z e d f u e l s or c o a r s e - c r u s h e d brown c o a l s used i n u t i l i t y b o i l e r s . T h i s d i f f e r e n c e p r o b a b l y has an e f f e c t on the f o r m a t i o n o f d e p o s i t s . A newly i n t r o d u c e d model c y c l o n e f i r i n g u n i t ( F i g u r e 6) p e r m i t s a l o n g e r r e t e n t i o n t i m e i n t h e c e n t r i f u g a l f i e l d so t h a t normal p u l v e r i z e d f u e l s and g r a n u l a r m a t e r i a l s w i t h maximum g r a i n d i a m e t e r of 3 mm can be b u r n t . A q u a r t z tube w i t h diameter o f 100 mm permits direct o b s e r v a t i o n o f t h e p r o c e s s w i t h normal c o a l . The c y c l o n e f u r n a c e p r o d u c e s s t e a d y combustion a t gas temperature of 700°C but can a l s o be e x t e n d e d t o a temperature l e v e l of more than 1000°C. Combustion of s a l t c o a l s i s d i f f i c u l t to observe because the a l k a l i n e s vaporize from the f u e l and condense on the c o l d e r q u a r t z tube w a l l t o f o r m an o p a q u e l a y e r . T h i s p r o b l e m i s p a r t i a l l y overcome by i n s e r t i n g a probe which measures gas and w a l l temperature and a l l o w s t h e s l a g g i n g p r o c e s s t o be determined as a f u n c t i o n of temperature by w e i g h i n g t h e d e p o s i t . Measurement of d e p o s i t hardness s u p p l i e s a d d i t i o n a l i n f o r m a t i o n on the c h a r a c t e r of the c l i n k e r formed. The c y c l o n e s e p a r a t o r b e h i n d t h e r e a c t o r a l l o w s an a n a l y s i s o f s t r u c t u r a l changes which occur i n the f i n e s t g r a i n . The t e s t r e s u l t s o b t a i n e d i n d i c a t e t h e c y c l o n e r e a c t o r can be improved by a u t o m a t i o n ( f o r e a s i e r o p e r a t i o n ) and by s t a n d a r d i z a t i o n t o r e s u l t i n a p i e c e o f equipment c a p a b l e of e v a l u a t i n g s l a g g i n g t e n d e n c i e s . Boiler

Processes

O p e r a t i o n w i t h the e x p e r i m e n t a l f u r n a c e s has shown t h a t the m a t e r i a l p r o p e r t i e s o f c o a l or ash a r e o n l y p a r t o f the c l i n k e r i n g p r o c e s s e s . The g e o m e t r i c shape of the f l a m e , r e a c t i o n c h a r a c t e r i s t i c s and heat t r a n s m i s s i o n p r o c e s s e s n e a r t h e w a l l o f the c o r r e s p o n d i n g h e a t i n g s u r f a c e s and on i n d i v i d u a l p a r t i c l e s a r e c o u p l e d w i t h m e l t i n g , v a p o r i z a t i o n and c o n d e n s a t i o n p r o c e s s e s of m i n e r a l s u b s t a n c e t o form deposits. The p a r a m e t e r s shown i n F i g u r e 7 i n f l u e n c e t h e mechanism of d e p o s i t f o r m a t i o n and must be c o n s i d e r e d i n t h e e v a l u a t i o n o f p r o c e s s e s e x p e c t e d of r e d u c i n g f u r n a c e a v a i l a b i l i t y . The p r o c e s s e s must be c o n s i d e r e d t o be d i r e c t l y dependent on the p r o p e r t i e s of the fuel. The m a j o r i n f l u e n c e s on d e p o s i t s i n p u l v e r i z e d f u e l f i r i n g s

Publication Date: April 2, 1986 | doi: 10.1021/bk-1986-0301.ch027

ALTMANN Prediction of Slagging and Fouling Tendencies

F i g u r e 7. on d e p o s i t

I n f l u e n c e of f l o w p a t t e r n s and formation.

temperature

Publication Date: April 2, 1986 | doi: 10.1021/bk-1986-0301.ch027

gradients

Publication Date: April 2, 1986 | doi: 10.1021/bk-1986-0301.ch027

27.

ALTMANN

Prediction

of Slagging and Fouling

Tendencies

407

r e s u l t f r o m t h e a e r o d y n a m i c a l p r o p e r t i e s o f the flame j e t s , t h e i r i n f l u e n c e s o n o n e a n o t h e r and t h e i r s p a t i a l p o s i t i o n s i n t h e combustion chamber. Numeric c a l c u l a t i o n of three-dimensional flows i n b o i l e r s are s t i l l d i f f i c u l t , b u t n o n - i s o t h e r m a l c o m b u s t i o n chamber m o d e l s o p e r a t e d w i t h a t e s t f u e l o f a c e t y l e n e and n i t r o g e n (Altmann ( 2 1 ) ) c a n o f f e r q u a l i t a t i v e i n f o r m a t i o n on c o n d i t i o n s l e a d i n g t o s l a g g i n g and f o u l i n g . L a r g e t e m p e r a t u r e f l u c t u a t i o n s and p r o f i l e s a f f e c t o v e r h e a t i n g and c l i n k e r i n g ; j e t d e f l e c t i o n to the wall i n combination with high temperatures of r e a c t i n g p u l v e r i z e d fuel p a r t i c l e s can i n i t i a t e d e p o s i t i o n i n t h e combustion chamber; i n t e n s e t u r b u l e n c e n e a r t h e w a l l i n h i b i t s p a r t i c l e c o o l i n g below the c r i t i c a l adhesion temperature. I n f o r m a t i o n w h i c h c a n be d e r i v e d f r o m e x p e r i m e n t s on t h e t h e r m a l b e h a v i o r o f t h e a s h - m i n e r a l s u b s t a n c e a n d on t h e a c t u a l mechanism of d e p o s i t s i s , d e s p i t e a l l imperfections, s u i t a b l e f o r d e t e r m i n i n g t h e a p p l i c a b i l i t y and a p p l i c a t i o n l i m i t s o f f u e l s and s e l e c t i o n o f a l t e r n a t i v e combustion systems and t h e i r d e s i g n . Conclusion T h i s paper d e v e l o p e d new s t a t i s t i c a l methods f o r p r e d i c t i n g s l a g g i n g and f o u l i n g o f brown c o a l s . The method u s e s an e m p i r i c a l r e l a t i o n s h i p b e t w e e n m o d i f i e d a s h c o m p o s i t i o n s and o b s e r v a t i o n s o f s l a g g i n g a n d f o u l i n g p e r f o r m a n c e i n b o i l e r s and p i l o t s c a l e combustors. The a s h c o m p o s i t i o n i s m o d i f i e d by c a l c u l a t i n g t h e a s h c o n s t i t u e n t s on a S i 0 2 f r e e b a s i s ; q u a r t z i s r e m o v e d f r o m t h e a n a l y s i s s i n c e i t does n o t p a r t i c i p a t e i n s l a g g i n g o r f o u l i n g . The v a l u e o f q u a r t z used t o c o r r e c t t h i s ash c o m p o s i t i o n i s derived u s i n g a parameter which i s a f u n c t i o n o f t h e s u l f u r and i r o n c o n t e n t o f t h e a s h . The performance e s t i m a t e d u s i n g the new i n d i c e s agreed much b e t t e r w i t h o b s e r v e d b o i l e r p e r f o r m a n c e t h a n t h a t e s t i m a t e d from a v a i l a b l e i n d i c e s . H o w e v e r , i n d i c e s r e m a i n e m p i r i c a l and p r e d i c t e d and o b s e r v e d performance may d i s a g r e e . P i l o t s c a l e c o m b u s t i o n t e c h n i q u e s were a l s o used t o e s t i m a t e t h e p e r f o r m a n c e o f brown c o a l s . The F i e l d tube i s l i m i t e d f o r e s t i m a t i n g p e r f o r m a n c e o f brown c o a l s and was supplemented by a c y c l o n e combustor. The c y c l o n e combustor a l l o w s combustion o f t h e l a r g e r p a r t i c l e s w i t h l o n g e r r e s i d e n c e times t y p i c a l o f b r o w n - c o a l fired boilers.

Literature Cited 1.

11th World Energy Conference 1980, Survey of Energy Resources Part A.

2.

Radmacher, W.

Brennst.-Chemie, 1949 30, Nr. 21/22, 377-384.

3. Gibb, W. H. The Slagging Characteristics of Coal Ashes by Viscosity and S i n t e r i n g Measurements, VGB-Konferenz " R a u c h g a s s e i t i g e Korrosionen und Verschmutzungen i n Warmekraftwerken" Essen, FRG 1977. 4. Zalkind, I. Ja.; Trojankin, Ja. W. Feuerbestandige Stoffe und Schlacken in der Energietechnik Moskow, USSR, 1953.

Publication Date: April 2, 1986 | doi: 10.1021/bk-1986-0301.ch027

408

MINERAL MATTER AND ASH IN COAL

5.

Lustigova, M. Tavitelnost popela a tvorba nanosu, Vyuzivani Popelnatych Hnedych Uhli V Energetice Karlovy Varg 1978, 37-44.

6.

A t t i g , R. C . , Duzy, A. F. Coal Ash Deposition Studies and Application to Boiler Design, Amer. Power Conf., 1969, 290.

7.

Winegartner, E . C . ; Rhodes, B. T. J. of Engineering for Power July 1975, 395-436.

8.

Garner, L . J. J. Inst. Fuel 40, 1967 107-111.

9.

Schneider, W. Der Einsatz des Verbrennungs-rohres nach FIELD zur Ermittlung der Verschlackungs- und Ansatz- neigung von Braunkohlenaschen in Staubfeuerungen, Wiss.Z.TU Dresden 30, 1981, 6, 15-19.

10.

Lautenschlager, F. W. Braunkohle, 1976, 6, 206-214.

11.

Below, S. Ju.; R u n d g i n , Ju. A . Prognose der Verschmutzungsneigung f e s t e r Brennstoffe an D a m p f e r z e u g e r h e i z f l a c h e n mit f e s t e n Ablagerungen, Tagungsbericht Tallinn, 1980, Sektion 1 Teil A, 145-150.

12.

Kluge, K. H., B e t r i e b s v e r h a l t e n Salzkohle, in preparation.

13.

Pomrehn, H.; Rodegast, M. Untersuchungsmethoden zur Charakterisierung von Kohlen I. Analysen von Kohlen und Aschen AdW-Informationen aus Wissenschaft und Technik Berlin 1981.

14.

Jahns, H . ; Schinkel, W., Energietechnik 1979, 29, 12, 464-469.

15.

Beising, R., Die Minerale der niederrheinischen Braunkohle und ihr Verhalten bel der Verbrennung in Kraftwerken, N. Jahrbuch f. Mineralogie, Abhandlungen, 1971, 117, 96-115.

16.

Ots, A. A. The Processes in Steam Generators in Burning Oil Shale and Kansk-Achinsk Basin Coals, Moscow "Energia" 1977. Savic, D.; Opik, I. Fouling and Corrosion in Steam Generators, Beograd 1980.

17.

von Dampferzeugern fur

18.

Schneider, W. Untersuchungen zur Bewertung und Vorhersage des Verschlackungsver-haltens von Brennstoffen in Dampferzeuger-Feuerungen, Energietechnik, 1983, 33, 7, 254-256.

19.

Boross, L.; Horvath, F.; Voros, F . Reaktionskinetik und Verschlackungs-eigenschaften von minderwertigen Braunkohlen bei Kohlenstaubfeuerung, Forschungsbericht VEIKI Budapest 1979.

20.

Weser, A. Fotoradiografie und Thermografie als MeBverfahren in der V e r b r e n n u n g s f o r s c h u n g , E n e r g i e t e c h n i k 1984 ( i n preparation).

21.

Altmann, W. Nichtisotherme Modellierung von Flammen und Flammensystemen in Feuerraumen (in preparation).

RECEIVED October 24, 1985

28 Mineral Matter Catalysis of Coal Conversion Thomas D . Padrick and Barry Granoff

Publication Date: April 2, 1986 | doi: 10.1021/bk-1986-0301.ch028

Sandia National Laboratories, Albuquerque, NM 87185

Since the 1920s, a variety of studies have investigated the effects of mineral matter on coal conversion processes. This interest accelerated greatly about 10 years ago. We have followed the development of this field from those investigations that screened the effects of various mineral types on coal conversion processes to those investigations that studied the detailed mechanism of a specific mineral on either coal gasification or coal liquefaction. As an outgrowth of this field of study, we have witnessed the development of both catalytic coal gasification and slurry phase catalytic coal liquefactions. We will review the field of mineral matter effects on coal processing and discuss the impact on the coal conversion industry.

In general, a c a t a l y s t c o n s i s t s of a support (alumina, s i l i c a , s i l i c a - a l u m i n a ) , a n a c t i v e c a t a l y t i c m e t a l and, i n some c a s e s , a promoter ( 1 ) . The s u p p o r t i s u s u a l l y a h i g h - s u r f a c e - a r e a (up t o s e v e r a l hundred square meters p e r gram) porous s o l i d . Gamma a l u m i n a , f o r example, has a s u r f a c e a r e a o f 100-300 n r V g . The s u p p o r t i s n o t n e c e s s a r i l y i n e r t and may p l a y a s i g n i f i c a n t r o l e i n c h e m i s o r p t i o n and o x i d a t i o n s t a t e c o n t r o l . The a c t i v e m e t a l , which may be d e p o s i t e d by s e v e r a l t e c h n i q u e s , i s h i g h l y d i s p e r s e d on t h e s u p p o r t . The promoter i s an a d d i t i v e t h a t c a n i n c r e a s e t h e a c t i v i t y o f t h e m e t a l and/or m a i n t a i n t h e p h y s i c a l c h a r a c t e r i s t i c s of the support. A t y p i c a l example i s Co-Mo/AI2O3, which i s used as a hydrodesulfurization catalyst. I n t h i s case, alumina i s t h e s u p p o r t , Mo i s t h e c a t a l y t i c a l l y a c t i v e m e t a l and Co i s t h e promoter. I n a c t u a l p r a c t i c e , t h e s e c a t a l y s t s must f i r s t be s u l f i d e d to convert the metals t o metal s u l f i d e s . I t i s believed that these metal s u l f i d e s a r e the c a t a l y t i c a l l y a c t i v e species f o r hydrodesulfurization (2). The need f o r c l e a n l i q u i d f u e l s from c o a l has l e d t o t h e development o f c a t a l y s t s t h a t had o r i g i n a l l y been used f o r 0097-6156/86/ 0301 -0410S06.00/ 0 © 1986 American Chemical Society

Publication Date: April 2, 1986 | doi: 10.1021/bk-1986-0301.ch028

28.

PADRICK AND

GRANOFF

Mineral

Matter

Catalysis of Coal Conversion

411

p e t r o l e u m r e f i n i n g . In d i r e c t c o a l l i q u e f a c t i o n , p u l v e r i z e d c o a l i s mixed w i t h a c o a l - d e r i v e d s o l v e n t and t r e a t e d w i t h hydrogen a t h i g h t e m p e r a t u r e s (400-450°C) and p r e s s u r e s (1500-3000 p s i ) Ç 3 ) . At some s t a g e i n the c o n v e r s i o n p r o c e s s , h y d r o t r e a t i n g c a t a l y s t s a r e used t o improve the d i s t i l l a t e y i e l d , i n c r e a s e the hydrogen c o n t e n t and reduce the heteroatom (S, N, 0) c o n t e n t of the l i q u i d products. Since coals contain a l u m i n o s i l i c a t e s ( c l a y s ) , metal s u l f i d e s ( e . g . , p y r i t e ) and numerous m e t a l - c o n t a i n i n g s p e c i e s ( 4 ) , i t would seem r e a s o n a b l e t o propose t h a t a n a t u r a l l y o c c u r r i n g c a t a l y s t system e x i s t s w i t h i n the i n h e r e n t m i n e r a l m a t t e r I n c o a l . This c o n c e p t has been e x p l o r e d s i n c e the 1920's and s e v e r a l studies have f o c u s e d on the c a t a l y t i c e f f e c t s o f m i n e r a l m a t t e r on c o a l conversion (5). In r e c e n t y e a r s , we have w i t n e s s e d an i n c r e a s e i n the l e v e l o f c o a l r e s e a r c h and the development o f new c o a l u t i l i z a t i o n processes. In p a r a l l e l w i t h t h i s a c t i v i t y , t h e r e have been r e p o r t s on the e f f e c t s o f c o a l m i n e r a l s on c o a l l i q u e f a c t i o n , c o a l g a s i f i c a t i o n , i n - s i t u c o a l g a s i f i c a t i o n , and o t h e r a r e a s o f c o a l u t i l i z a t i o n . ( 5 - 8 ) . The terms m i n e r a l s , m i n e r a l m a t t e r , and a s h w i l l be used synonymously. An attempt w i l l not be made t o r e v i e w the e f f e c t s o f a l l c l a s s e s o f m i n e r a l s , but w i l l o n l y c o n s i d e r t h o s e m i n e r a l s which have shown a l a r g e e f f e c t on c o a l conversion processes.

Coal

Liquefaction

The Germans used c o a l l i q u e f a c t i o n on a commercial s c a l e from 1930 t o the end o f the second World War. They found t h a t a c a t a l y s t c o u l d enhance l i q u i d y i e l d s and h e l p remove h e t e r o a t o m s . The B e r g i u s p r o c e s s used an i r o n oxide-aluminum c a t a l y s t a t a 2-3% by c o a l weight c o n c e n t r a t i o n . I n r e c e n t y e a r s , i t has been r e a l i z e d t h a t m i n e r a l m a t t e r p l a y s an i m p o r t a n t r o l e i n c o a l l i q u e f a c t i o n ( 9 - 1 1 ) , s i m i l a r t o the r o l e of the added c a t a l y s t i n the B e r g i u s p r o c e s s . Several e x p e r i m e n t a l t e c h n i q u e s have been used t o s t u d y the e f f e c t s o f m i n e r a l s on c o a l l i q u e f a c t i o n and t o i d e n t i f y the s p e c i f i c c a t a l y t i c phase ( 1 2 ) . Most s t u d i e s (12-14) s t r o n g l y imply t h a t the i r o n s u l f i d e s a r e the most a c t i v e s p e c i e s , and the o t h e r m i n e r a l s appear to have l i t t l e e f f e c t on enhancement o f l i q u i d y i e l d or q u a l i t y . The s p e c i f i c r o l e o f p y r i t e (FeS2) as a c a t a l y s t has been under i n v e s t i g a t i o n s i n c e p y r i t e was i d e n t i f i e d as the most a c t i v e inherent mineral for coal l i q u e f a c t i o n . Under l i q u e f a c t i o n c o n d i t i o n s , FeS2 i s t r a n s f o r m e d i n t o a n o n s t o i c h i o m e t r i c i r o n s u l f i d e , F e i - x S (0 C

U 3 Pu

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