Combustion of Synthetic Fuels 9780841207738, 9780841210387, 0-8412-0773-9

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Publication Date: April 29, 1983 | doi: 10.1021/bk-1983-0217.fw001

Combustion of Synthetic Fuels

Publication Date: April 29, 1983 | doi: 10.1021/bk-1983-0217.fw001

Combustion of Synthetic Fuels William Bartok, EDITOR Exxon Research and

Publication Date: April 29, 1983 | doi: 10.1021/bk-1983-0217.fw001

Engineering Company

Based on a symposium sponsored by the ACS Division of Petroleum Chemistry at the 183rd Meeting of the American Chemical Society, Las Vegas, Nevada, March 28-April 2, 1982

ACS SYMPOSIUM SERIES 217

AMERICAN

CHEMICAL

WASHINGTON, D.C. 1983

SOCIETY

Publication Date: April 29, 1983 | doi: 10.1021/bk-1983-0217.fw001

Library of Congress Cataloging in Publication Data Combustion of synthetic fuels. (ACS symposium series, ISSN 0097-6156; 217) Includes index. Contents: "An overview of synthetic fuel combustion: issues and research activities / A.A. Boni . . . [et al.] — characteristics of typical synthetic fuel components" / R. B. Edelman, R. C. Farmer, and T.-S. Wang — "An experimental study of synthetic fuel atomization characteristics" / R. G. Oeding and W. D. Bachalo— [etc.] 1. Combustion — Congresses. 2. Synthetic fuels — Congresses. I. Bartok, William, 1930. II. American Chemical Society. Division of Petroleum Chemistry. III. Series. QD516.C6155 1983 ISBN 0-8412-0773-9

621.402*3

83-2822

Copyright © 1983 American Chemical Society All Rights Reserved. The appearance of the code at the bottom of the first page of each article in this volume indicates the copyright owner's consent that reprographic copies of the article 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. 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 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. PRINTED IN THE UNITED STATES OF

AMERICA

Society Library 16th St. N. w.

1155

Washington, D. C.

20038

Publication Date: April 29, 1983 | doi: 10.1021/bk-1983-0217.fw001

ACS Symposium Series M . Joan Comstock, Series Editor

Advisory Board

David L. Allara

Robert Ory

Robert Baker

Geoffrey D. Parfitt

Donald D. Dollberg

Theodore Provder

Brian M. Harney

Charles N. Satterfield

W. Jeffrey Howe

Dennis Schuetzle

Herbert D. Kaesz

Davis L. Temple, Jr.

Marvin Margoshes

Charles S. Tuesday

Donald E. Moreland

C. Grant Willson

Publication Date: April 29, 1983 | doi: 10.1021/bk-1983-0217.fw001

FOREWORD The ACS SYMPOSIUM SERIES was founded in 1974 to provide

a medium for publishing symposia quickly in book form. The format of the Series parallels that of the continuing ADVANCES IN CHEMISTRY SERIES except that in order to save time the papers are not typeset but are reproduced as they are submitted by the authors in camera-ready form. Papers are reviewed under the supervision of the Editors with the assistance of the Series Advisory Board and are selected to maintain the integrity of the symposia; however, verbatim reproductions of previously published papers are not accepted. Both reviews and reports of research are acceptable since symposia may embrace both types of presentation.

Publication Date: April 29, 1983 | doi: 10.1021/bk-1983-0217.pr001

PREFACE W ITH T H E INEXORABLE DEPLETION of premium fossil fuels, oil and gas, it appears that synthetic fuels derived from coal, shale, and tar sands will become part of the overall energy supply in the United States. Synthetic fuels comprise an array of different products including coal derived liquids and gases, shale oil, and methanol. In comparison to conventional fuels, the principal changes that will be introduced by the advent of synthetic fuels affect their production, refining, and end utilization. These are interrelated issues because the end utilization imposes product-quality requirements to which synthetic fuel properties should conform; conversely, it may be possible in certain instances to modify the design of combustion hardware to accommodate properties peculiar to synthetic fuels. This alternative would be particularly attractive from the standpoint of energy efficiency, thus decreasing the need for costly hydrogenative refining steps. In the utilization of synthetic fuels, the key issues are impact on performance, equipment integrity, and emission characteristics of combustion hardware. Emissions of oxides of nitrogen and soot are the most actively researched emission problems for continuous combustion systems, which range from burners to gas turbine combustors. This volume provides an overview of current fundamental and applied combustion research studies that address the use of synthetic fuels. The main emphasis in these studies is on the combustion of liquid fuels, ranging from research on spray atomization to pilot-scale testing of the combustion of synthetic fuels. I wish to thank the contributing authors for their efforts and to acknowledge the help received from Jack Fisher in the early stages of organizing the ACS symposium. W I L L I A M BARTOK

Exxon Research and Engineering Company Linden, NJ 07036 December 1982

ix

1

Publication Date: April 29, 1983 | doi: 10.1021/bk-1983-0217.ch001

Research

Issues and

Technology-An

Overview

A. A. BONI and R. B. EDELMAN Science Applications, Inc., La Jolla,CA92038 D. BIENSTOCK U.S. Department of Energy, Pittsburgh Energy Technology Center, Pittsburgh,PA15236 J. FISCHER Argonne National Laboratory, Argonne,IL60439 The need to conserve energy and to control pollu­ tant emissions while at the same time introducing a new generation of fuels derived from coal, oil shale and tar sands is introducing severe re­ quirements on the design and retrofit of combustion equipment. The different chemical and physical properties of these synthetic fuels leads to substantial differences in their combustion characteristics and emissions. In particular there is the potential for increased soot formation, higher ΝO emissions, increased and modified radiation and heat-load distribution, and increased contamination and fouling of combustion and heat transfer surfaces when compared to more conven­ tional fuels. Staged combustion techniques to simultaneously control ΝO and soot production are being developed. However, various burner, boiler and furnace configurations are involved in practical applications and they each have different aerodynamic flow patterns and turbulence character­ istics. These flow field characteristics couple with the fuel physical and chemical properties in controlling the efficiency, emissions and fuel flexibility characteristics of practical systems. The U. S. Department of Energy, Advanced Research & Technology Development Program in Direct Utiliza­ tion, AR&TD (DU), is providing the scientific and technical information for improved, expanded, and accelerated utilization of synthetic fuels in the generic utility and industrial market sectors. In the present paper, we review the current under­ standing of synfuel combustion, and present an overview of the AR&TD (DU) program. x

x

0097-6156/83/0217-0001$08.25/0 © 1983 American Chemical Society

Publication Date: April 29, 1983 | doi: 10.1021/bk-1983-0217.ch001

2

COMBUSTION OF SYNTHETIC FUELS

With the r e d u c t i o n i n the a v a i l a b i l i t y of c o n v e n t i o n a l hydrocarbons f o r f u e l s i n the t r a n s p o r t a t i o n , u t i l i t y , and i n d u s t r i a l s e c t o r s , t h e r e i s a need t o i n c l u d e f u e l s produced from low hydrogen-to-carbon r a t i o sources, such as c o a l , o i l s h a l e , and tar sands. V a r i o u s processes are being developed t o produce c o a l d e r i v e d l i q u i d s , s o l i d s and gases, o i l from s h a l e , and heavy o i l s from t a r sands. I t has been e s t a b l i s h e d that the cost and energy i n t e n s i v e requirements to r e f i n e these syncrudes t o a hydrogencarbon r a t i o and b o i l i n g range more t y p i c a l of c o n v e n t i o n a l f u e l s i s v e r y l a r g e (1_). Therefore, there i s a l a r g e economic d r i v i n g f o r c e f o r the d e s i g n , development, and implementation of combust i o n equipment capable of burning s y n t h e t i c f u e l s of w i d e l y varyi n g p r o p e r t i e s i n a t h e r m a l l y e f f i c i e n t and environmentally acceptable manner. C o n c u r r e n t l y , the need t o conserve energy and to c o n t r o l p o l l u t a n t emissions i s a l s o f o r c i n g improvements i n combustion e f f i c i e n c y and r e d u c t i o n s i n p o l l u t a n t emissions of e x i s t i n g energyc o n v e r s i o n devices u s i n g present-day f u e l s i n c l u d i n g heavy and residual oils. The r e q u i r e m e n t s on t h e d e s i g n of c o m b u s t i o n equipment to meet these o b j e c t i v e s w i l l be severe and w i l l demand s u b s t a n t i a l improvements i n our a b i l i t y t o understand the combust i o n process and i t s c o n t r o l l i n g parameters. Many recent s t u d i e s have considered the combustion of s y n t h e t i c f u e l s , c . f . B l a c k , et a l . ( 2 ) , Bowman and B i r k e l a n d ( 3 ) , E n g l a n d , e t a l . ( 4 ) , and M u z i o , et a l . (5)« The problem i s that current combustor t e c h nology has evolved s l o w l y , i s based upon e m p i r i c a l methods, and contains l i t t l e consideration f o r f u e l f l e x i b i l i t y . The s i t u a t i o n i s p a r t i c u l a r l y acute now because of the present u n c e r t a i n t i e s i n f u e l s u p p l i e s and t h e c o r r e s p o n d i n g u n c e r t a i n t i e s i n design f o r f u e l f l e x i b i l i t y . Because of these u n c e r t a i n t i e s , equipment manufacturers and i n d u s t r i a l users are c u r r e n t l y r e l u c t a n t t o make the necessary investments r e q u i r e d f o r e i t h e r r e t r o f i t t i n g or manufacturing new equipment designed s p e c i f i c a l l y for synthetic l i q u i d fuels. There i s a near term need f o r e x i s t i n g equipment to u t i l i z e s y n t h e t i c f u e l s and low grade r e s i d u a l f u e l s that have many of t h e same combustion problems. A l s o , there i s a longer term need to d e v e l o p new and advanced equipment t o meet t h e r o l e t h e s e f u e l s may p l a y i n the f u t u r e . Because of the preponderance of e x i s t i n g combustion equipment i n p l a c e i t i s necessary to modify c u r r e n t b u r n e r d e v i c e s and systems f o r s y n t h e t i c f u e l s u s e . U n t i l r e c e n t l y , petroleum-based f u e l s have been both p l e n t i f u l and cheap, and design p r a c t i c e has not had to c o n s i d e r the impact of f u e l t y p e . Improvements t h a t have e v o l v e d have been of mechanical design r a t h e r than aerothermochemical. T h i s i s no l o n g e r s u f f i c i e n t and a b e t t e r understanding of the e f f e c t of s y n t h e t i c versus c o n v e n t i o n a l f u e l p r o p e r t i e s on combustion process c o n t r o l i s r e q u i r e d . Through the understanding of the performance of e x i s t i n g hardware and of the e f f e c t of f u e l types ( c o n v e n t i o n a l and s y n t h e t i c ) , design c r i t e r i a f o r modifying cur-

1.

BONI ET AL.

Research Issues and Technology

3

rent systems can be e s t a b l i s h e d . Moreover, t h i s understanding of t h e e f f e c t of f u e l type on the combustion process forms the b a s i s f o r new concept development and w i l l c o n t r i b u t e t o the upgrading of design procedures through a r e d u c t i o n i n the l e v e l o f e m p i r i cism u n d e r l y i n g c u r r e n t design methodologies*

Publication Date: April 29, 1983 | doi: 10.1021/bk-1983-0217.ch001

T e c h n i c a l Issues R e l a t e d t o Combustion of Synfuels The p h y s i c a l and chemical p r o p e r t i e s of s y n t h e t i c crudes are d i f f e r e n t from those of petroleum* Increased NO and soot prod u c t i o n are the p r i n c i p a l problems of the combustion of s y n t h e t i c f u e l s , and c o n t r o l concepts f o r these two problems a r e i n conflict. F u e l - r i c h combustion decreases NO but augments soot prod u c t i o n , w h i l e f u e l - l e a n combustion decreases (and can e l i m i n a t e ) soot p r o d u c t i o n but augments NO emissions. Moreover, c o n t r o l procedures can a f f e c t combustion e f f i c i e n c y and h e a t - t r a n s f e r d i s t r i b u t i o n t o the chamber s u r f a c e s . Table I , taken from Grumer ( 6 ) , i l l u s t r a t e s some s p e c i f i c r e l e v a n t p r o p e r t i e s of s y n t h e t i c l i q u i d f u e l s and petroleum-based f u e l s . The p r i n c i p a l d i f f e r ences between these f u e l s as r e l a t e d t o t h e i r combustion behavior are summarized i n Table I I . I n the f o l l o w i n g d i s c u s s i o n , we c o n s i d e r these p r o p e r t y d i f ferences and i l l u s t r a t e t h e i r e f f e c t on the combustion process and combustor peformance by use of data a v a i l a b l e i n the l i t e r a ture. The higher aromatic content and the lower hydrogen-to-carbon r a t i o are chemical p r o p e r t i e s which combine t o promote the increased formation of soot and other r e l a t e d combustion problems. F i g u r e 1, t a k e n from N a e g e l i ( 7 ) , i l l u s t r a t e s t h e c o r r e l a t i o n of i n c r e a s e d smoke emission w i t h r e d u c t i o n i n H/C r a t i o f o r measurements on a T63 gas t u r b i n e combustor o p e r a t i n g on aromatic-doped petroleum f u e l s . S i m i l a r r e s u l t s have been r e p o r t e d by P i l l s b u r y , e t a l . (8_, _9). The i n c r e a s e d soot formation i s r e s p o n s i b l e f o r the i n c r e a s e d l u m i n o s i t y and corresponding enhanced t h e r m a l r a d i a t i o n from s y n f u e l f l a m e s , c . f . F i g u r e 2, again taken from N a e g e l i ( 7 ) . These r e s u l t s and those reported by P i l l s b u r y , e t a l . (8_,_9) i n d i c a t e the success i n u s i n g the H/C r a t i o o f the f u e l t o c o r r e l a t e the s o o t i n g tendency and the enhanced thermal r a d i a t i o n which occur f o r low H/C r a t i o f u e l s . The sharp i n c r e a s e of exhaust smoke when the H/C i s reduced below 2 i s s i g n i f i c a n t , because s y n f u e l s made from c o a l may approach a H/C r a t i o o f 1.2 whereas p e t r o l e u m f u e l s have a H/C r a t i o o f about 2. From a h e a t - t r a n s f e r p o i n t of view, the h i g h soot concent r a t i o n s r e s u l t i n g from the combustion of s y n t h e t i c f u e l s w i l l t e n d t o cause b o t h h i g h e r r a d i a t i o n h e a t i n g and more s e v e r e f o u l i n g of h e a t - t r a n s f e r s u r f a c e s . Depending on the soot conc e n t r a t i o n and temperature of the combustion gases, as much as 95 p e r c e n t of the t o t a l heat t r a n s f e r i n a furnace o r a gas t u r b i n e combustor may take p l a c e due t o r a d i a t i o n ; Sarofim ( 1 0 ) . The

19,000

GROSS H E A T O F COMB., B T U / L B

12.2 0.29 0.57 3.3

H Y D R O G E N , WT %

NITROGEN, W T %

SULFUR, W T %

O X Y G E N , WT %

*Grumer, Reference 6.

!

I I I i l l I I I

Η

oo

en

Η

Ν

so

CM ON

οο ci

rH

- H CM"

σ\νθιη}

§ s s " a s

22°

ι r

CM

C l ON VO C M

Ν

r ^ i n v o r ^

ssss

ssss

Ν soin

u

**!

ΓG οCM ουc ο ~ υ „ c

S |-5

x o - ^ m m

O V O O N - *

Γ Η ^ - Ι Γ Η Ι Η

m

Ν

m en m ci f i

η H «î H c i m c i en

H ο Ν ^ ο » Î ci ci

η C M η in ci π ci ci

0 0 0

o o o d o d o

01 H vO

ON H vO

ON r-t N O C I C I CM

C 0 (11) 2

2

6.

125

Global Flame Kinetics

LEVY ET A L .

H CH

+ 1/2

2

+

4

0

2 0

-> H 0

+

2

(12)

2

2

C0

+

2

2H 0

(13)

2

For the computation of the r e a c t i o n rate i n t e g r a l that appears i n the expression f o r S , the f o l l o w i n g two-step model was de­ v i s e d (confirmed by the measured concentration p r o f i l e s ) : L

1.

In a f i r s t step, CH4 i s depleted to form CO and H 2 O , and H 0 i s formed a l s o by o x i d a t i o n of the H i n the mixture : 2

2

CH

H

Publication Date: April 29, 1983 | doi: 10.1021/bk-1983-0217.ch006

2.

+

4

2 0

+ 1/2

2

+

2

0

CO +

1/2

0

2

+

2H 0

(14)

2

(15)

H 0 2

2

In a second s t e p , CO (both from the CH4 r e a c t i o n and that i n i t i a l l y present i n the mixture) r e a c t s to form C0 : 2

(CO)

1

(CO)"

+ 1/2

0

+

0

1/2

-> ( C 0 )

2

F

(16)

2

M

·> ( C 0 )

2

(17)

2

For the r e a c t i o n r a t e s of Equations 14 to 17 g l o b a l expres­ sions from the l i t e r a t u r e were adopted. For r e a c t i o n s 14, 16, and 17, the o v e r a l l c o r r e l a t i o n s of Dryer and Glassman ( 1 ) were used, expressing r e s p e c t i v e l y the methane disappearance r a t e , the rate of r e a c t i o n of carbon monoxide with oxygen i n the presence of water, and the appearance rate of carbon d i o x i d e i n the methane-oxygen r e a c t i o n : d [ C H

~

4

]

β

_ dMl Î ^ i -

=

i n

1 0

1 0

1 3 . 2[

14.6

R R U

C 1

Ί

0.7

V

Γ Λ

Ο Ί

02 . 8

^Ο· ]

[ C O ] [ H 2 0 ]

0.5

^ - " [ C O J ^ O I

0

[ 0 2 ]

-

5

e x

, 48,000.

P(

0.25

^ ]

0

-

2

ε

5

, 18 . 1 F T

§ϊ—)» ( )

χ

ρ

^ 4 0 ^ ,

^

4

exp ( - - ^ 2 0 )

(20)

For r e a c t i o n 15, the only expression found i n the l i t e r a t u r e f o r the g l o b a l rate of water formation was the one by Fenimore and Jones (6) 2

8[H ]

dt

[CO]

d[H 0]

2

d[C0 ] 2

dt

(21)

and t h i s was used i n conjunction with Equation 19 assuming d[C0 ] f_ = - d[C0] , so dt dt~ o

= 8 χ 10

1 4

that

6

5

· [Η ][Η 0]°· [0 }°· 2

2

2

2 5

βχρ(-

4 0

Q 0 Q

> ) RT

(22)

126

COMBUSTION OF SYNTHETIC FUELS

When using Equation 22, a non-zero concentration of water has to be assumed even i f no water i s i n i t i a l l y present i n the r e a c t a n t s . Best r e s u l t s were obtained by taking [H2O] = [ Η ] + 2[CH4], i . e . , the t o t a l water e v e n t u a l l y to be i n the combustion products under o x i d i z i n g c o n d i t i o n s , although part of i t i s produced by d e p l e t i o n of hydrogen. T h i s can be j u s t i f i e d by n o t i n g that Equation 7 was derived f o r r a t e expressions i n which both reactants were depleted during combustion, and assumed that the reactant c o n c e n t r a t i o n i n the combustion zone i s constant and equal to a < a and ^ ef f u b ^ < b^. When one of the species appearing i n the r a t e expres­ 2

£ £

e

s i o n a c t u a l l y increases

Publication Date: April 29, 1983 | doi: 10.1021/bk-1983-0217.ch006

that i n t r o d u c t i o n of i t s

during the r e a c t i o n , i t i s

plausible

i n i t i a l value makes Equation 7 i n t o l e r ­

ably s m a l l . A l t e r n a t i v e expressions f o r the g l o b a l rates of Reactions 14 and 16 were t r i e d while developing the model. For the CO CO2 conversion (Reaction 16) the o v e r a l l c o r r e l a t i o n d e r i v e d by Howard et a l . (2) was i n i t i a l l y used, but w i t h t h i s the c a l c u l a t e d values of S L were considerably lower than the measured ones. For the methane disappearance r a t e (Reaction 14) the c o r r e l a t i o n s proposed by Westbrook and Dryer (7) were t r i e d , and these gave r e s u l t s n e g l i g i b l y d i f f e r e n t from those obtained by Equation 18. V a l i d a t i o n of the G l o b a l Rates E x p r e s s i o n s . In order to v a l i d a t e the g l o b a l r a t e expressions employed i n the model, temperature and concentration p r o f i l e s determined by probing the flames on a f l a t flame burner were s t u d i e d . A t t e n t i o n was con­ centrated on Flames Β and C . The experimental p r o f i l e s were smoothed, and the s t a b l e species net r e a c t i o n r a t e s were d e t e r ­ mined using the laminar f l a t - f l a m e equation described i n d e t a i l by F r i s t r o m and Westenberg (3) and summarized i n Reference (8). A p l o t of the l o g a r i t h m ^ of Aexp(-E/RT) f o r three of the four rate expressions used i s shown i n Figures 3, 4, and 5 (for Equa­ t i o n s 18, 19, and 22, r e s p e c t i v e l y ) . I n i t i a l l y , an attempt was made to develop o r i g i n a l g l o b a l r a t e expressions f o r Reactions 14 to 16 from these r a t e d a t a . It soon became c l e a r , however, that the number of experimental p o i n t s was too few to allow the attainment of t h i s g o a l . More­ over, s i n c e a ternary system was being analyzed, the concentra­ t i o n p r o f i l e s had an i n t r i c a t e form which made numerical d i f f e r ­ e n t i a t i o n to r e t r i e v e the r a t e s somewhat i n a c c u r a t e . I t was therefore decided to use these r a t e data to check the o v e r a l l r a t e expressions derived by other authors and used i n the present model. I t i s apparent that the adopted c o r r e l a t i o n s represent i n an acceptable way the experimental data at h i g h temperatures. The best agreement i s obtained f o r the CO d e p l e t i o n (Equation 19), while f o r H2O formation and CH4 disappearance, the agreement i s less satisfactory. Given however, the r e l a t i v e l y small number of

Figure 3. Global r a t e constant v s . 1/ t for carbon monoxide o x i d a t i o n r e a c t i o n (Eq. 19).

Figure 4. Global rate constant v s . 1/T for water formation r e a c t i o n (Eq. 22).

Publication Date: April 29, 1983 | doi: 10.1021/bk-1983-0217.ch006

128

COMBUSTION OF SYNTHETIC FUELS

Publication Date: April 29, 1983 | doi: 10.1021/bk-1983-0217.ch006

• Flame Β Ο Flame C

•

Fenimore & Jones (1959) + Dryer & Glassman (1973) l_l I

6

1

1

I

7

8

9 1 0 " 1 . 1 1.2 1.3 1.4 1.5-10

l_J

I

I

I

I

3

l/T

[K" ] 1

Figure 5. Global rate constant v s . formation r e a c t i o n ( E q . 22).

l / T f o r water

6.

LEVY E T AL.

Global Flame Kinetics

129

experimental data p o i n t s , i t should be deemed s a t i s f a c t o r y that they f a l l around the used c o r r e l a t i o n s (which are p l o t t e d w i t h i n t h e i r claimed range of v a l i d i t y ) . Results and D i s c u s s i o n . The burning v e l o c i t y was c a l c u l a t e d by the model described above f o r a number of d i f f e r e n t gas mix­ tures burning at s t o i c h i o m e t r i c c o n d i t i o n s . Table 3 presents the compositions of the v a r i o u s gas mixtures s t u d i e d . Each mixture i s c h a r a c t e r i z e d by a mixture number MN and a mixture r a t i o R. The mixture r a t i o R i s a volume concentration of index f u e l (CO + H ) r e l a t i v e to the sum of index and l i m i t f u e l s , where the l i m i t f u e l i s CO + CH4 or H + C H 4 . Mixtures having the same value of MN y i e l d the same composition of combustion p r o ­ ducts and a d i a b a t i c flame temperature when burning s t o i c h i o m e t r i c a l l y with a i r . This choice was made i n order to assess whether a d i a b a t i c flame temperature and f i n a l composition were s i g n i f i c a n t f a c t o r s i n e x p l a i n i n g d i f f e r e n c e s of behavior f o r d i f f e r e n t f u e l compositions. A d d i t i o n a l d e t a i l s on the s e l e c t ­ ion of gas mixtures composition can be found i n Reference (9). The a d i a b a t i c flame temperatures Tf were c a l c u l a t e d by the computer code NASA SP 273. λ and c_ were computed by the c o r r e ­ l a t i o n s of Mansouri and Heywood (10;. The c a l c u l a t e d values of S were compared w i t h the e x p e r i ­ mental ones obtained f o r the same mistures by measurements i n a wedge-shaped flame. A p l o t of c a l c u l a t e d versus measured S L i s shown i n Figure 6. I t i s c l e a r that the model p r e d i c t s c o r r e c t l y the change i n S L that i s to be expected from a change i n mixture composition, w h i l e the c a l c u l a t e d values are s a t i s f a c t o r i l y c l o s e to the measured ones. The b e t t e r f i t between c a l c u l a t e d and p r e d i c t e d values here as compared w i t h the r a t e c o r r e l a t i o n i n Figures 3, 4, and 5 a l s o r e f l e c t s the b e t t e r f i t near s t o i c h ­ i o m e t r i c c o n d i t i o n s and at higher flame temperature (11). 2

Publication Date: April 29, 1983 | doi: 10.1021/bk-1983-0217.ch006

2

L

Conclusions Measurements of temperature and c o n c e n t r a t i o n i n C O - H 2 - C H 4 (or n a t u r a l gas) flames were c a r r i e d out. Rate p r o f i l e s were developed f o r two excess a i r and two s l i g h t l y f u e l - r i c h flames as a f u n c t i o n of temperature. S u b s t i t u t i o n of n a t u r a l gas f o r methane does not b r i n g about a marked change i n the o v e r a l l r e a c t i v i t y of these systems. A p p l i c a t i o n of a modified theory a n a l y s i s to these m u l t i p l e - f u e l flame mixtures allows one to s a t i s f a c t o r i l y c o r r e l a t e c a l c u l a t e d values of the burning v e l o c i t y with measured v a l u e s .

130

COMBUSTION OF SYNTHETIC FUELS

TABLE 3.

Publication Date: April 29, 1983 | doi: 10.1021/bk-1983-0217.ch006

Mixture Number MN

Mixture Ratio R

COMPOSITION OF THE GAS MIXTURES STUDIED (mol f r a c t i o n s )

CO

H

2

CH4

C0

2

0

2

N

2

1

1.0 0.50 0.0

0.590 0.566 0.542

0.295 0.147 0.0

0.0 0.096 0.193

0.115 0.151 0.187

0.0 0.039 0.078

0.0 0.0 0.0

2

1.0 0.50 0.0

0.295 0.147 0.0

0.590 0.373 0.156

0.0 0.205 0.410

0.115 0.191 0.268

0.0 0.083 0.166

0.0 0.0 0.0

3

1.0 0.50 0.0

0.442 0.360 0.277

0.442 0.221 0.0

0.0 0.171 0.342

0.115 0.179 0.243

0.0 0.069 0.138

0.0 0.0 0.0

4

1.0 0.67 0.50 0.33 0.0

0.257 0.249 0.245 0.240 0.232

0.128 0.085 0.064 0.043 0.0

0.0 0.028 0.042 0.055 0.083

0.115 0.131 0.140 0.148 0.164

0.0 0.0 0.0 0.0 0.0

0.500 0.507 0.511 0.514 0.521

5

1.0 0.50 0.0

0.128 0.064 0.0

0.257 0.162 0.067

0.0 0.087 0.173

0.115 0.167 0.218

0.0 0.0 0.0

0.500 0.521 0.541

6

1.0 0.50 0.33

0.192 0.155 0.148

0.192 0.096 0.064

0.0 0.073 0.097

0.115 0.158 0.173

0.0 0.0 0.0

0.500 0.518 0.523

(CO + H ) R =/ "(CO + H ) + ([CO or H ] + C H 4 ) 2

2

LEVY E T A L .

Global Flame Kinetics

Publication Date: April 29, 1983 | doi: 10.1021/bk-1983-0217.ch006

6.

Figure 6. C o r r e l a t i o n of c a l c u l a t e d v s . measured burning v e l o c i t i e s .

131

COMBUSTION OF SYNTHETIC FUELS

132 Acknowledgment This of Energy debted to Equation

paper i s based on work conducted under U . S . Department Contract No. DE-AC22-75ET10653. The authors are i n ­ Dr. John R. Overley for working out the expression for 2.

Literature Cited

Publication Date: April 29, 1983 | doi: 10.1021/bk-1983-0217.ch006

1.

Dryer, F. L . , and Glassman, I., 14th Symp. (Int.) on Combust. 987 (1973). 2. Howard, J. B., Williams, G. C., and Fine, D. H., 14th Symp. (int.) on Combust., 975 (1973). 3. Fristrom, R. Μ., and Westenberg, Α. Α., Flame Structure, McGraw-Hill (1965). 4. Semenov, Ν. N., NACA TM 1026 (1942). 5. Evans, M. W., Chem. Reviews, 51, 363 (1952). 6. Fenimore, C. P., and Jones, G. W., J. Phys. Chem., 63, 1834 (1959). 7. Westbrook, C. Κ., and Dryer, F. L . , The Combust. Inst. CSS 1981 Spring Meeting; UCRL-84943 Preprint. 8. Levy, Α., Overley, J. R., and Merryman, E. L . , Battelle Topical Report, Contract No. (ERDA) Ε(49-18)-2406, July 26, 1977. 9. Ball, D. Α., Putnam, Α. Α., Radharkrishman, E . , and Levy, Α., Battelle Topical Report, Contract No. (ERDA) E(49-18)-2406, July 26, 1977. 10. Mansouri, S. Η., and Heywood, J. B., Combust. Sci. Technol., 23, 251 (1980). 11. Westbrook, C. Κ., and Dryer, F. L . , Combust. Sci. Technol., 27, 31 (1981). RECEIVED October 25, 1982

7 Continuous Combustion Systems A Study of Fuel Nitrogen Conversion in Jet-Stirred Combustors

Publication Date: April 29, 1983 | doi: 10.1021/bk-1983-0217.ch007

R. M. KOWALIK and L. A. RUTH Exxon Research and Engineering Company, Linden, ΝJ 07036

Results from laboratory jet-stirred combustor experiments suggest that the conversion of fuel­ -bound nitrogen to total fixed nitrogen (TFN=NO+HCN+ NH ) in fuel-rich mixtures is strongly related to the concentration of unburned hydrocarbons (HC's) within the combustor. Most conversion trends with equiva­ lence ratio, residence time, and combustor type may be explained in terms of the effects of these vari­ ables on HC concentrations. Changes in these vari­ ables which reduce HC's generally reduce the degree of fuel nitrogen conversion. Fuel type (aliphatic vs aromatic) effects on conversion appear to be most pronounced for very rich, short residence time condi­ tions. At these conditions toluene/pyridine mixtures produce less ΤFN and more soot than similar isooctane/pyridine mixtures. This trend may be related to an interaction between soot and HCN. 3

Synthetic l i q u i d f u e l s derived from c o a l and shale w i l l d i f f e r i n some c h a r a c t e r i s t i c s from conventional f u e l s derived from petroleum. For example, l i q u i d synfuels are expected to contain s i g n i f i c a n t l y higher l e v e l s of aromatic hydrocarbons, e s p e c i a l l y f o r c o a l - d e r i v e d f u e l s , and higher l e v e l s of bound n i t r o g e n . These d i f f e r e n c e s can a f f e c t the combustion system accepting such f u e l s i n important ways. In continuous combus­ t o r s , i . e . gas t u r b i n e s , the increased aromatics content of c o a l - d e r i v e d f u e l s i s expected to promote the formation of soot which, i n t u r n , w i l l increase r a d i a t i o n to the combustor l i n e r , r a i s e l i n e r temperature, and p o s s i b l y r e s u l t i n shortened s e r ­ vice l i f e . Deposit formation and the emission of smoke are other p o t e n t i a l e f f e c t s which are cause f o r concern. Higher n i t r o g e n l e v e l s i n synfuels are expected to show up as increased emissions of N 0 (NO+NO2). An e a r l i e r paper presented r e s u l t s of an experimental study on the e f f e c t of aromatics and combustor X

0097-6156/83/0217-0133$06.00/0 © 1983 American Chemical Society

134

COMBUSTION OF SYNTHETIC FUELS

operating c o n d i t i o n s on soot formation (JO . T h i s paper focuses on the e f f e c t of increased f u e l n i t r o g e n and aromatics on the emission of Ν 0 . N 0 can be formed e i t h e r from atmospheric n i t r o g e n ("thermal" N 0 ) or from the o x i d a t i o n of n i t r o g e n compounds present i n the f u e l ("fuel" N 0 ) . The r a t e of formation of thermal N 0 i s very s e n s i t i v e to temperature, and techniques which have been d e v e l ­ oped to c o n t r o l t h i s type of N 0 are based l a r g e l y on l i m i t i n g peak flame temperatures. The formation of f u e l N 0 , by c o n t r a s t , i s much l e s s dependent on temperature, and methods to c o n t r o l thermal N 0 are g e n e r a l l y i n e f f e c t i v e f o r f u e l N 0 . Conven­ t i o n a l petroleum-derived d i s t i l l a t e f u e l s are low enough i n n i t r o g e n so that thermal N 0 predominates, and e x i s t i n g c o n t r o l techniques are u s u a l l y adequate to keep N 0 emissions w i t h i n acceptable l e v e l s . However, t y p i c a l l i q u i d f u e l s derived from c o a l and shale may have n i t r o g e n concentrations that are an order of magnitude higher than those i n petroleum-derived d i s t i l l a t e s and, f o r such f u e l s , the N 0 due to f u e l n i t r o g e n u s u a l l y p r e ­ dominates. Although extensive treatment to remove f u e l n i t r o g e n at the r e f i n e r y would e l i m i n a t e any p o t e n t i a l emission problem owing to f u e l N 0 , i t would almost c e r t a i n l y be cheaper and more energy e f f i c i e n t to modify the combustor and/or combustion c o n d i ­ t i o n s to minimize f u e l N 0 emissions. I t i s toward t h i s l a t t e r end that t h i s research i s d i r e c t e d . χ

X

X

X

X

X

X

X

X

X

Publication Date: April 29, 1983 | doi: 10.1021/bk-1983-0217.ch007

X

X

X

X

Although gas t u r b i n e combustion systems operate with o v e r a l l a i r / f u e l r a t i o s which are q u i t e f u e l - l e a n , perhaps three times s t o i c h i o m e t r i c , s t a b i l i z a t i o n of the combustion process r e q u i r e s that a p o r t i o n of the combustor, the primary zone, operate s t o i c h i o m e t r i c or f u e l - r i c h . Under f u e l - l e a n c o n d i t i o n s , f u e l bound n i t r o g e n can be converted d i r e c t l y to N 0 . Under f u e l - r i c h c o n d i t i o n s , fuel-bound n i t r o g e n can be converted to HCN and NH3 i n a d d i t i o n to N 0 . Of course, i n e i t h e r case, the most d e s i r a b l e product of converted f u e l n i t r o g e n would be molecular n i t r o g e n , N2. The sum of the gaseous f i x e d n i t r o g e n species (excluding N 2 ) i s c a l l e d t o t a l f i x e d n i t r o g e n , or ΤFN. Under f u e l - r i c h c o n d i ­ t i o n s , ΤFN c o n s i s t s p r i m a r i l y of N 0 , HCN, and N H 3 . It should be appreciated that i n any two-stage combustion p r o c e s s , i t i s v i t a l to minimize the formation of TFN i n the f u e l - r i c h primary stage because HCN and N H 3 , i f formed, can be o x i d i z e d to N 0 i n the f u e l - l e a n secondary stage. Thus, any strategy for minimizing the emission of N 0 from the combustion of high n i t r o g e n f u e l s i n gas turbines must, i n the f u e l - r i c h primary zone, minimize the conver­ s i o n of f u e l n i t r o g e n to TFN. A f u r t h e r point i s that the e q u i l i b r i u m l e v e l s of TFN under f u e l - r i c h combustion c o n d i t i o n s are very low. The s t a b l e form of n i t r o g e n i s N 2 . In p r a c t i c a l combustors, however, e q u i l i b r i u m i s not a t t a i n e d because of the slow rates of both chemical and p h y s i ­ c a l (mixing) processes. The chemical processes c o n s i s t of r e a c ­ t i o n s convering f u e l - n i t r o g e n species to N 2 , the r e a c t i o n of NO with hydrocarbon species to form HCN, and the subsequent slow conversion of HCN to N 2 . X

X

X

X

X

7.

KOWALIK AND RUTH

Continuous Combustion Systems

135

In t h i s paper we report on f a c t o r s which a f f e c t the convers i o n of f u e l n i t r o g e n to TFN i n l a b o r a t o r y j e t - s t i r r e d combustors which serve to simulate the primary zone i n a gas t u r b i n e . The independent v a r i a b l e s i n the experiments were f u e l type ( a l i p h a t i c isooctane v s . aromatic t o l u e n e ) , equivalence r a t i o ( f u e l - t o oxygen r a t i o of combustor feed d i v i d e d by s t o i c h i o m e t r i c f u e l - t o oxygen r a t i o ) , average gas residence time i n the combustor, and method of f u e l i n j e c t i o n i n t o the combustor (prevaporized and premixed with a i r v s . d i r e c t l i q u i d s p r a y ) . Combustion temperature was kept constant at about 1900K i n a l l experiments. Pyrid i n e , C5,H5N, was added to the f u e l s to provide a f u e l - n i t r o g e n c o n c e n t r a t i o n of one percent by weight.

Publication Date: April 29, 1983 | doi: 10.1021/bk-1983-0217.ch007

Experimental Combustors. The f u e l n i t r o g e n conversion experiments were conducted i n two Exxon j e t - s t i r r e d combustors: the J e t - S t i r r e d Combustor (JSC) and the L i q u i d F u e l J e t - S t i r r e d Combustor (LFJSC). Both combustors operate at atmospheric pressure and temperatures of 1600-2000K. Figure 1 contains a schematic diagram of the J S C . Homogeneous a i r / f u e l mixtures enter t h i s combustor through f o r t y 0.5 mm diameter j e t s p o s i t i o n e d near the r a d i a l center of a r e f r a c t o r y - l i n e d h e m i s p h e r i c a l r e a c t i o n zone. Fuels are p r e v a p o r i z e d and preheated to 575K p r i o r to mixing with a i r , which i s also preheated to 575K. T o t a l a i r plus f u e l flow rates are chosen such that near sonic i n j e c t i o n j e t v e l o c i t i e s are obtained to v i g o r o u s l y s t i r the contents of the r e a c t i o n zone and produce mixtures of e s s e n t i a l l y uniform temperature and composition. Combustor temperatures are i n f e r r e d from a thermocouple mounted i n one of the r a d i a l exhaust p o r t s ; samples f o r composition analyses are withdrawn w i t h a hot water-cooled s t a i n l e s s s t e e l probe i n s e r t e d through another exhaust p o r t . A d d i t i o n a l thermocouples are l o c a t e d i n the r e f r a c t o r y l i n i n g and on the s t e e l s h e l l to o b t a i n temperatures f o r estimates of combustor heat l o s ses. Two d i f f e r e n t s i z e r e a c t i o n zones were used i n the f u e l n i trogen conversion experiments; one had a 5.08cm i n s i d e diameter; the other had a 7.62cm i n s i d e diameter. Outside diameters of the two combustor modules were e q u a l . D e t a i l s of the c o n s t r u c t i o n of the JSC and i t s a i r and f u e l supply systems may be found i n Reference (2) . The LFJSC has a s p h e r i c a l j e t - s t i r r e d zone (diameter = 5 . 0 8 cm) followed by a c y l i n d r i c a l plug flow zone (diameter = 2.2 cm; length - 7.6 cm); both zones are r e f r a c t o r y l i n e d . Primary comb u s t i o n a i r enters the j e t - s t i r r e d zone through two nozzles p o s i t i o n e d 1 8 0 ° a p a r t . A set of four 1.1 mm diameter a i r j e t s from each nozzle i s aimed towards the corners of a cube imagined to s i t w i t h i n the s p h e r i c a l zone. One set of a i r j e t s i s r o t a t e d 4 5 ° with respect to the other to allow the opposing j e t s to mesh r a t h e r than to c o l l i d e . Flow r a t e s are chosen to produce near

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136

COMBUSTION

Figure 1.

OF SYNTHETIC

J e t - S t i r r e d Combustor

FUELS

7.

KOWALIK AND RUTH

Continuous Combustion Systems

137

Publication Date: April 29, 1983 | doi: 10.1021/bk-1983-0217.ch007

sonic i n j e c t i o n v e l o c i t i e s . F u e l sprays enter the combustor at two p o s i t i o n s 9 0 ° from the a i r n o z z l e s . A i r atomizing nozzles obtained from Spraying Systems Company produce the f u e l s p r a y s . A l l a i r and f u e l i n l e t streams enter the LFJSC at room tempera­ ture. Exhaust gas samples are withdrawn from the combustor w i t h a hot water-cooled s t a i n l e s s s t e e l probe at the end of the plug flow zone. A thermocouple i s a l s o i n s e r t e d near the center of the s p h e r i c a l r e a c t i o n zone f o r temperature measurements. In the f u e l n i t r o g e n conversion experiments approximately 30% of the t o t a l combustion a i r was d i r e c t e d through the f u e l n o z z l e s . Sauter mean diameters of the corresponding f u e l sprays were e s t i ­ mated to be of the order of 5 ym. A d d i t i o n a l d e t a i l s of the LEJSC c o n s t r u c t i o n and instrumentation may be found i n References (1) and (3). Gas A n a l y s i s . Gas samples from both combustors are analyzed with a common instrument t r a i n . Sample gases c o l l e c t e d i n the probes are t r a n s f e r r e d through e l e c t r i c a l l y heated Τ efIon l i n e s to a 400K oven. Within the oven samples are s e q u e n t i a l l y d i r e c ­ ted to four separate c o n d i t i o n i n g / a n a l y s i s streams. The f i r s t stream e x i t s the oven and passes through a c o l d water trap (~ 10°C) to remove most of the combustor water. I t then proceeds to four conventional gas analyzers f o r measurements of CO and C0£ (nondispersive i n f r a r e d ) , O2 (amperometric), and H 2 (gas chromatograph) c o n c e n t r a t i o n s . The second stream i s d i l u t e d i n s i d e the oven with 400K n i t r o g e n ( d i l u t i o n r a t i o « 20:1) and t r a n s ­ f e r r e d v i a e l e c t r i c a l l y heated s t a i n l e s s s t e e l l i n e s to a flame i o n i z a t i o n hydrocarbon analyzer f o r measurements of t o t a l u n burned hydrocarbon (HC) concentrations ( v o l . % as methane). The n i t r o g e n d i l u t i o n i s employed to keep HC concentrations w i t h i n the l i n e a r range of our instrument. The t h i r d a n a l y s i s stream i s drawn through a p a i r of soot c o l l e c t i o n f i l t e r s i n the oven and subsequently sent to a c o l d water trap and a p o s i t i v e d i s ­ placement wet t e s t meter. Soot concentrations are then computed as the weight of soot c o l l e c t e d per measured volume of sample gas flow. The f i n a l a n a l y s i s stream i s d i l u t e d i n the oven with 400K a i r ( d i l u t i o n r a t i o * 10:1) and t r a n s f e r r e d i n unheated T e f l o n l i n e s to a c a t a l y t i c converter/chemiluminescent analyzer for measurements of NO and t o t a l f i x e d n i t r o g e n (TFN) concentra­ tions. NO concentrations are obtained d i r e c t l y from the chemiluminescent a n a l y z e r ; TFN concentrations are obtained by passing the sample, a f t e r d i l u t i o n with a i r , through a c a t a l y t i c r e a c t o r which converts a l l TFN species to NO and then measuring the NO with the chemiluminescent a n a l y z e r . The r e a c t o r u t i l i z e s a platinum c a t a l y s t and c a r e f u l l y c o n t r o l l e d temperatures and pressures to achieve near 100% conversion of TFN species to NO (4). A d d i t i o n of a i r to the sample assures an o x i d i z i n g atmos­ phere i n the c a t a l y t i c r e a c t o r and prevents water condensation w i t h i n the unheated sample l i n e s .

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COMBUSTION OF SYNTHETIC FUELS

Publication Date: April 29, 1983 | doi: 10.1021/bk-1983-0217.ch007

Results P r i n c i p a l r e s u l t s from the f u e l n i t r o g e n conversion e x p e r i ­ ments with p y r i d i n e doped isooctane and toluene are summarized i n T a b l e s I and I I . In these t a b l e s JSC-S and J S C - L r e f e r to the 5.08 and 7.62 cm diameter v e r s i o n s of the J S C , r e s p e c t i v e l y ; φ i s the o v e r a l l equivalence r a t i o of the mixtures; τ i s the average residence time of the gases i n the combustor computed as the com­ bustor volume d i v i d e d by the r e a c t a n t s ' volumetric flow r a t e at 1900K; TFN i s the weight % of f u e l n i t r o g e n emitted as TFN; NO i s the volume % of TFN emitted as NO; HC i s the volume % of unburned hydrocarbons i n the combustion gases (as C H 4 ) , and SOOT i s weight % of f u e l carbon c o l l e c t e d as soot. The TFN values may a l s o be i n t e r p r e t e d as the approximate weight % of f u e l n i t r o g e n converted to TFN s i n c e a d d i t i o n a l experiments with undoped toluene and i s o octane suggested that "thermal" TFN c o n t r i b u t i o n s were g e n e r a l l y of the order of 20% of the "thermal" plus " f u e l " TFN v a l u e s . With t h i s q u a l i f i c a t i o n , the term " f u e l n i t r o g e n conversion" i s a p p l i e d to "thermal" plus " f u e l " TFN values throughout the balance of the paper. The data i n the t a b l e s are averages from two consecutive sequences of measurements. T y p i c a l r e p e a t a b i l i t y between measure­ ments was ±10%. Soot c o n c e n t r a t i o n measurements were made during a l l of the LFJSC runs and during the f u e l r i c h ( φ = 1 . 6 , 1 . 8 ) , short residence time (3,6 ms) toluene runs i n the J S C . Previous e x p e r i ­ ments suggested that measurable q u a n t i t i e s of soot would probably not be produced at leaner c o n d i t i o n s or longer residence times i n the J S C . Flames f o r the r i c h e s t ( φ = 1 . 8 ) toluene mixtures i n the JSC at 8 and 10 ms residence times, however, appeared s l i g h t l y yellow i n d i c a t i n g the p o s s i b i l i t y of a p p r e c i a b l e soot c o n c e n t r a ­ tions. Soot y i e l d s from the isooctane mixtures i n the LFJSC were l e s s than 0.01%. During the J SC experiments oxygen concentrations i n the " a i r " were v a r i e d to maintain i n d i c a t e d thermocouple temperatures of approximately 1900K. V a r i a t i o n s from 1900K were g e n e r a l l y l e s s than 25K, except f o r the two r i c h e s t ( φ = 1 . 8 ) isooctane mixtures at 10 and 20 ms residence times. For these runs oxygen concen­ t r a t i o n s were l i m i t e d by the gas supply system, and i n d i c a t e d temperatures were approximately 1800K. For the LFJSC experiments, oxygen concentrations were s e l e c t e d to provide s p e c i f i c a d i a b a t i c flame temperatures s i n c e i n d i c a t e d thermocouple temperatures appeared to be a f f e c t e d by the impingement of the f u e l sprays on the thermocouple bead. The a d i a b a t i c flame temperatures were 2400K f o r the toluene mixtures and 2300K f o r the isooctane mix­ t u r e s . These temperatures were approximately equal to c o r r e s ­ ponding a d i a b a t i c flame temperatures of toluene and isooctane mixtures run i n the J S C at an equivalence r a t i o of 1.2 and a residence time of 6 ms. Heat l o s s e s estimated from the JSC s h e l l and r e f r a c t o r y temperatures d i d not vary s i g n i f i c a n t l y as f u e l s and flow rates (residence times) were changed w i t h i n each com-

7.

KOWALIK AND RUTH

Table I.

Combustor

φ

Results from f u e l n i t r o g e n conversion experiments - isooctane f u e l . τ (ms)

TFN , % of

NO

rsc

Publication Date: April 29, 1983 | doi: 10.1021/bk-1983-0217.ch007

1.2 1.4 1.6 1.8 1.2 1.4 1.6 1.8 1.2 1.4 1.6 1.2 1.4 1.6 1.8 1.2 1.4 1.6 1.8 1.2 1.4 1.6 1.8 1.2 1.4 1.6

3 3 3 3 6 6 6 6 8 8 8 8 8 8 8 10 10 10 10 20 20 20 20 6 6 6

65 88 83 96 93 66 41 62 87 72 51 88 69 62 74 73 53 50 77 73 46 40 38 67 56 61

o f v

s

4uel N JSC-S

139

Continuous Combustion Systems

;

4 FN 74 28 12 5 65 74 52 9 77 72 60 53 43 21 11 55 53 37 5 69 62 66 24 55 46 27

;

HC ,vol,

% x

^as C H ' 4

0.03 0.59 1.97 4.34

M H

I

ρ

190

COMBUSTION O F S Y N T H E T I C

staged conditions were much l e s s s e n s i t i v e to f u e l n i t r o g e n cont e n t , and above 0.4 percent were almost independent of n i t r o g e n content. Under these c o n d i t i o n s , with heat e x t r a c t i o n r e s u l t i n g i n w a l l heat f l u x of approximately 38.8 k J / M - s e c (12,300 B t u / h r / f t ) i n the flame zone, most of the s y n t h e t i c f u e l s produced p a r t i c u l a t e emissions which were comparable to those produced by the conventional petroleum f u e l s . The s y n t h o i l , however, produced much higher emissions than the other f u e l s , which l i k e l y r e s u l t s from both the high ash content of the f u e l (1.56 percent) and poor atomization due to the high v i s c o s i t y . P a r t i c u l a t e emissions were also not s e n s i t i v e to o v e r a l l excess oxygen l e v e l s above 2 percent. Figure 1 a l s o shows that p a r t i c u l a t e emissions from a l l f u e l s increase s t e a d i l y as primary zone stoichiometry i s reduced. The increase i n emissions, compared to normal s i n g l e staged combustion, i s not s u b s t a n t i a l l y d i f f e r e n t for synfuels and petroleum derived fuels. This i n d i c a t e s that a l l f u e l s respond i n a s i m i l a r fashion to the a p p l i c a t i o n of staged combustion. While d e t a i l e d SEM analyses of the sampled p a r t i c u l a t e have not been c a r r i e d out, microscopic examination of the samples reveals some d i f f e r e n c e s between the p a r t i c u l a t e matter from d i f ferent f u e l s . P a r t i c u l a t e produced by combustion of the petroleum r e s i d u a l No. 6 o i l i s comprised l a r g e l y of cenospheres with agglomerated c l u s t e r s of p a r t i c l e s of a smaller s i z e . The l a r g e cenospheres appear to be r e l a t e d to the atomization and v a p o r i z a t i o n c h a r a c t e r i s t i c s of the No. 6 o i l , while the smaller p a r t i c l e s are c h a r a c t e r i s t i c of soot (.5,6). The p a r t i c u l a t e matter from the d i e s e l o i l , the shale DFM and the SRC-II f u e l s , on the other hand, c o n s i s t s p r i m a r i l y of c l u s t e r s of small soot p a r t i c l e s . For the SRC-II heavy d i s t i l l a t e f u e l , some cenospheres were a l s o observed, though much fewer than for the No. 6 o i l . These trends can be seen to some extent i n the comparison of Bacharach smoke number measurements and t o t a l p a r t i c u l a t e emissions i n Figure 2. Smoke number may be considered to be r e l a t e d ( a l b e i t q u a l i t a t i v e l y ) to the emission of f i n e r p a r t i c u l a t e matter. For the experimental conditions i n v e s t i g a t e d , smoke numbers were g e n e r a l l y low f o r a l l f u e l s except the SRC-II heavy d i s t i l l a t e and s y n t h o i l . The r e s u l t s show, however, that smoke number increases more s t r o n g l y for the c o a l derived l i q u i d f u e l s as the t o t a l p a r t i c u l a t e emission increases as a r e s u l t of staged combustion. This increase i s most s t r o n g l y marked f o r the SRC-II heavy d i s t i l l a t e f u e l . The screening studies tend to show that there are no substant i a l d i f f e r e n c e s between synfuels and petroleum derived l i q u i d f u e l s i n terms of N0 and t o t a l p a r t i c u l a t e emissions. Both types of f u e l appear to respond i n a s i m i l a r way to the a p p l i c a t i o n of staged combustion and to changes i n atomization parameters (not discussed i n the present paper). 2

2

Publication Date: April 29, 1983 | doi: 10.1021/bk-1983-0217.ch010

FUELS

X

10.

KRAMLICH ET A L .

Publication Date: April 29, 1983 | doi: 10.1021/bk-1983-0217.ch010

Droplet

Soot Formation

191

Studies

Experimental. To f u r t h e r understand the process of d r o p l e t combustion and p a r t i c u l a t e formation, a more fundamental study of the e f f e c t s of droplet s i z e , l o c a l stoichiometry and gas-droplet r e l a t i v e v e l o c i t y has been c a r r i e d out. This work made use of a c o n t r o l l e d flow v a r i a b l e s l i p r e a c t o r i n which the combustion of d r o p l e t streams can be examined under w e l l defined c o n d i t i o n s . The r e a c t o r (7) c o n s i s t s of 5 by 28 cm f l a t flame burner downfired i n t o a chimney of s i m i l a r dimensions, f i t t e d with Vycor windows for o p t i c a l access. Access ports f o r d r o p l e t i n j e c t i o n and sample probing are p r o v i d e d . As i l l u s t r a t e d i n Figure 3, f u e l d r o p l e t s are normally i n j e c t e d b a l l i s t i c a l l y across the face of the burner. The droplets could be i n j e c t e d as a t i g h t l y c o l l i m a t e d stream which behaved as an almost c y l i n d r i c a l source of f u e l vapors for an attached d i f f u s i o n flame. This r e s u l t e d i n a w e l l d e f i n e d , laminar vapor sheet which i s separated from the d r o p l e t s by the gas flow; the general appearance i s shown i n the f i g u r e . Droplets were generated by the v i b r a t i n g o r i f i c e technique (8) i n an arrangement by which the d r o p l e t s could be dispersed s u f f i c i e n t l y to c o n t r o l d r o p l e t - d r o p l e t spacing. Droplet diameter and spacing were v e r i f i e d by high r e s o l u t i o n spark shadowgraphs. Operating conditions were s e l e c t e d to minimize the appearance of s a t e l l i t e droplets. Soot samples were obtained by use of a nitrogen-quench, porous-walled probe and Nucleopore f i l t e r s (7). Gas phase hydro­ carbons were c o l l e c t e d by the porous probe as batch samples and analyzed by standard FID gas chromatography. Thermal measurements included gas temperature by r a d i a t i o n - c o r r e c t e d bare wire thermo­ couple, and soot temperature by Kurlbaum r e v e r s a l (9,J-0) and two c o l o r pyrometry (11). Results. The f u e l s studied i n t h i s program have been described p r e v i o u s l y , and the ranges of the combustion v a r i a b l e s i n v e s t i g a t e d were as f o l l o w s : d r o p l e t diameter 66-190 ym, s l i p v e l o c i t y 300-1200 cm/sec; gas temperature < 1500 K; and s t o i c h i o m ­ e t r y 0.5 < Φ < 1.5. I n i t i a l drop s i z e was determined by the o r i ­ f i c e employed i n the d r o p l e t generator and was v e r i f i e d through the use of L D A / v i s i b i l i t y measurements and photography. Slip v e l o c i t y was c a l c u l a t e d as the vector sum of the LDA measured d r o p l e t v e l o c i t y and the hot gas v e l o c i t y . Two extremes of d r o p l e t d i s p e r s i o n mode were i n v e s t i g a t e d : non-dispersed where the d r o p l e t stream remained t i g h t l y c o l l i m a t e d as i t traversed the r e a c t o r ; and d i s p e r s e d , where the d r o p l e t s were spread across the r e a c t o r but there was no side w a l l impinge­ ment. This approach allowed some c o n t r o l of d r o p l e t - d r o p l e t spacing and i n t e r a c t i o n from a h i g h l y c o l l i m a t e d stream to a con­ d i t i o n where i n d i v i d u a l d r o p l e t behavior was c l e a r l y evident. For a l l f u e l s t e s t e d , the o v e r a l l appearance of the flame produced by the c o l l i m a t e d d r o p l e t stream was of a narrow, l u m i ­ nous sheet of soot forming below the d r o p l e t stream as shown

Publication Date: April 29, 1983 | doi: 10.1021/bk-1983-0217.ch010

192

COMBUSTION OF S Y N T H E T I C

—

1 1 3 % E X C E S S 0 S O N I C O R E 0 5 2

1

1

FUELS

1

2

-

-

—

—

1 II 1

D

Figure 2.

5

,

I

• -

ι •

Ο I

I

1 0 1 5 2 0 P A R T I C U L A T Em g / M J

ι 2 5

Bacharach smoke number and t o t a l p a r t i c u l a t e emissions.

Publication Date: April 29, 1983 | doi: 10.1021/bk-1983-0217.ch010

K R A M L i C H ET AL.

193

Soot Formation

u ο •H

% Xi CU

crj

ο

•H

CO

Pu ω rH

Ρ*

ο

M TJ M-l Ο

CO

ο

t ο

ω ΟΟ •Η

Pu

Publication Date: April 29, 1983 | doi: 10.1021/bk-1983-0217.ch010

194

COMBUSTION

O F SYNTHETIC

FUELS

schematically i n Figure 3. The emission i n t e n s i t y was greatest d i r e c t l y under the d r o p l e t s and was increased by excess oxygen i n the f r e e stream. Under f u e l - l e a n c o n d i t i o n s , the sheet had sharp, w e l l - d e f i n e d edges and tended to c o l l a p s e i n t o a ribbon near the bottom of the burner. Under f u e l - r i c h c o n d i t i o n s , the boundaries were d i f f u s e and no c o l l a p s i n g was n o t i c e d . Figure 3 i s a composite of photographs and scanning e l e c t r o n micrographs of impacted samples at v a r i o u s p o s i t i o n s w i t h i n the soot sheet f o r SRC-II middle d i s t i l l a t e f u e l o i l d r o p l e t s . Soot f i r s t becomes v i s i b l e i n the wake behind d r o p l e t s as d i f f u s e t r a i l s composed of f i n e l y d i v i d e d m a t e r i a l . This soot band r a p i d l y c o l l a p s e s i n t o l o n g , small-diameter f i l a m e n t s . The v i s i b l e sheet i s p r i m a r i l y made up of these h o r i z o n t a l filaments that can grow to lengths greater than one m i l l i m e t e r . High m a g n i f i c a t i o n photographs of the filaments r e v e a l that they are composed of very f i n e (500 A) s p h e r i c a l p a r t i c l e s . Such filament formation i s s t r i k i n g l y s i m i l a r to that observed p r e v i o u s l y i n d r o p l e t wakes (12) and both i n laminar (13,jL4) and turbulent (15) p u l v e r i z e d c o a l flames. The f a r edge of the soot sheet i s v i s u a l l y charact e r i z e d as b r i g h t flashes of r a d i a t i o n shown i n the streak photograph (top l e f t corner of F i g u r e 3) which terminates the d r o p l e t trajectories. The termination of d r o p l e t t r a j e c t o r i e s by a "micro-explosion" of t h i s nature was observed f o r a l l of the s y n t h e t i c f u e l s and f u e l blends t e s t e d , but d i d not occur for the petroleum derived No. 6 o i l . With t h i s l a t t e r f u e l , d r o p l e t s were seen to burn to e x t i n c t i o n and to r e s u l t i n the formation of a carbonaceous r e s i d u e , u s u a l l y i n the form of cenospheres. The termination of i n d i v i d u a l d r o p l e t s was observed, t h e r e f o r e , to be s t r o n g l y dependent upon f u e l type and could be c h a r a c t e r i z e d by three d i s t i n c t types of behavior: 1) l a r g e (1 mm) micro-explosions with a d i s t i n c t l y d i r e c t i o n a l behavior (SRC process donor solvent b l e n d ) , 2) smaller micro-explosions (SRC-II middle, heavy, middle/ heavy b l e n d , DFM), and 3) carbonaceous residue formation (Indonesian-Malaysian No. 6) without m i c r o - e x p l o s i o n s . High speed photographs show the micro-explosions to occur i n a time span much f a s t e r than the camera framing r a t e (1/5000 s e c ) . In an attempt to modify the observed d r o p l e t b e h a v i o r , a b r i e f q u a l i t a t i v e i n v e s t i g a t i o n was c a r r i e d out with blends of SRC-II heavy d i s t i l l a t e and pure heptane. The o b j e c t i v e was to enhance d r o p l e t d i s r u p t i v e combustion as a means of reducing e f f e c t i v e d r o p l e t s i z e and hence soot formation. With these fuels v i s i b l e d r o p l e t fragmentation was found to occur throughout the d r o p l e t stream. The fragmentation produced new d r o p l e t s on d i f ferent t r a j e c t o r i e s ; these i n turn were terminated by small d i s r u p t i o n s , as described above. Three blends were used: 60/40, 80/20, and 90/10. Secondary atomization was observed f o r a l l three, although the v i o l e n c e of the a c t i v i t y was n o t i c e a b l y reduced as the heptane content of the blend became s m a l l e r . This secondary atomization was a completely d i f f e r e n t process than the

Publication Date: April 29, 1983 | doi: 10.1021/bk-1983-0217.ch010

10.

KRAMLICH ET A L .

195

Soot Formation

terminal m i c r o - e x p l o s i o n . Secondary atomization has been p r e v i ­ ously noted i n the l i t e r a t u r e (16) and involves i n t e r n a l d r o p l e t boiling. In c o n t r a s t , t e r m i n a l micro-explosions occur a f t e r a s i g n i f i c a n t amount of m a t e r i a l has been removed, are extremely r a p i d , have a d i s t i n c t d i r e c t i o n a l c h a r a c t e r , and leave no observ­ able fragments. A p o s s i b l e mechanism involves the formation of a s o l i d coke surface on the d r o p l e t followed by r u p t u r e . In a d d i t i o n to v i s u a l observations of a q u a l i t a t i v e n a t u r e , a number of d e t a i l e d sampling traverses were made to determine p a r t i c u l a t e and gaseous species concentrations w i t h i n the soot sheet. The nature of the sampling system employed r e s u l t s i n the sampled s o l i d m a t e r i a l being composed l a r g e l y of soot since ceno­ spheres and heavy s o l i d m a t e r i a l follow the d r o p l e t t r a j e c t o r i e s and are not captured by the probe. The soot c o n c e n t r a t i o n , hydrocarbon species and soot temper­ ature i n the immediate v i c i n i t y of the d r o p l e t d i s p l a y e d c l o s e l y coupled behavior. Hydrocarbon s p e c i e s , l i s t e d i n approximate order of

c o n c e n t r a t i o n were

C2H2,

CH4, C2H4,

C2H6

and Ο β ' ε .

Reso­

l u t i o n of the propane and propylene peaks was not p o s s i b l e under current GC procedure, and these concentrations are reported merely as C 3 compounds. The hydrocarbons were found to decay i n the same time frame as the growth of the soot c o n c e n t r a t i o n . The zone of chemical a c t i v i t y , defined as where the vaporized hydrocarbon p r o ­ ducts react to form s o o t , i s approximately 2 cm, which corresponds to 13 msec, a f t e r which the soot concentration decays due appar­ e n t l y to o x i d a t i o n . The soot temperature was found to exceed the gas temperature as measured by thermocouples i n the absence of d r o p l e t i n j e c t i o n but decayed at a s i m i l a r r a t e . This i s a t t r i b u t e d to bulk heating e f f e c t s associated with the l o c a l i z e d burning of vaporized m a t e r i ­ al. A d e t a i l e d d i f f u s i o n flame c a l c u l a t i o n f o r a c y l i n d r i c a l source of reactants and r e l a t i v e v e l o c i t y on the same order as these experimental d a t a , i n d i c a t e that t h i s bulk heating e f f e c t i s reasonable. Both d i f f u s i o n a l flame c a l c u l a t i o n s and d e t a i l e d s p a t i a l mapping i n d i c a t e that the nondispersed i n j e c t i o n mode produces a vapor cloud that i s c h a r a c t e r i z e d by d i f f u s i o n a l l y c o n t r o l l e d com­ bustion and bulk heating while s u b j e c t i n g the d r o p l e t s to near isothermal c o n d i t i o n s . The soot produced i n t h i s cloud i s s t r o n g ­ l y influenced by bulk d i f f u s i o n l i m i t a t i o n s and as such represents a bulk soot formation extreme. I t was found that f u e l changes had l i t t l e e f f e c t on the o v e r a l l soot y i e l d due to t h i s d i f f u s i o n con­ trol. Lower gas temperatures and r i c h e r conditions were found to favor soot formation under bulk sooting c o n d i t i o n s , probably due to a decrease i n the o x i d a t i o n r a t e of the soot. In the dispersed mode, the d r o p l e t stream was aerodynamically dispersed to permit the sooting behavior of i n d i v i d u a l d r o p l e t s to be i n v e s t i g a t e d . The v i s u a l appearance of the flame produced was s t r i k i n g l y d i f f e r e n t from the nondispersed flame. Instead of a sheet of luminous r a d i a t i o n down the center of the burner, the

Publication Date: April 29, 1983 | doi: 10.1021/bk-1983-0217.ch010

196

COMBUSTION O F S Y N T H E T I C

FUELS

flame now was only streaks of l i g h t apparently associated with individual droplets. These streaks were broader than expected f o r d r o p l e t s alone because of the associated soot formation. Under lean c o n d i t i o n s , the d r o p l e t / s o o t streaks became sharp, w e l l defined thinner streaks while under r i c h c o n d i t i o n s , the streaks were d i f f u s e bands. Again, as i n the case for nondispersed i n j e c t i o n with the SRC-II middle d i s t i l l a t e , the streaks were observed to terminate with a b r i g h t f l a s h of r a d i a t i o n . Because the phenomenon was not masked by the h i g h l y r a d i a n t soot sheet, the micro-explosions were c l e a r l y v i s i b l e and were observed to r e s u l t i n a luminous cloud s e v e r a l times bigger than the d r o p l e t streak. A l s o , i t could be seen that every streak ended with such a micro-explosion. Soot concentrations were much smaller f o r the dispersed mode than f o r the non-dispersed mode under s i m i l a r c o n d i t i o n s . For example, the soot concentration was found to be approximately 700 mg/m3 f o r 190 ym SRC-II middle d i s t i l l a t e d r o p l e t s under f u e l r i c h (φ = 1.33), dispersed c o n d i t i o n s . Under s i m i l a r non-dispersed c o n d i t i o n s , the soot concentration was of the order 10^ mg/m3. The y i e l d of soot f o r the dispersed i n j e c t i o n mode was found to be a f f e c t e d by gas stoichiometry i n the opposite d i r e c t i o n to the e f f e c t on non-dispersed i n j e c t i o n . As seen i n Figure 4 f o r the SRC-II heavy d i s t i l l a t e , the soot concentration was increased by leaner o v e r a l l gas c o n d i t i o n s . Such measurements are c o n s i s ­ tent with observations of attached flames and higher temperatures w i t h i n the d r o p l e t flame under leaner conditions (1,2) both of which can promote d r o p l e t soot. These are also c o n s i s t e n t with p h y s i c a l observation made i n t h i s study. Soot concentrations were found a l s o to be s e n s i t i v e to f u e l type f o r dispersed c o n d i t i o n s despite the i n s e n s i t i v i t y observed under the bulk d i f f u s i o n c o n t r o l l e d nondispersed i n j e c t i o n mode. For 190 ym f u e l o i l d r o p l e t s under r i c h c o n d i t i o n s (φ = 1.33), the soot y i e l d could be d i r e c t l y r e l a t e d to the carbon to hydrogen r a t i o of the f u e l (Figure 5 ) . The soot concentration was found to increase by an order of magnitude with a f a c t o r of approximately two i n the C:H r a t i o . I t i s d i f f i c u l t to r e l a t e these r e s u l t s d i r e c t l y to the p a r t i c u l a t e measurements made i n the screening studies since the sampling system used i n these l a t t e r experiments cannot d i s t i n g u i s h between soot and other carbonaceous p a r t i c u ­ late. There i s , however, good q u a l i t a t i v e agreement i n that s o o t ­ ing tendencies (smoke number) are higher i n the tunnel furnace f o r the c o a l derived l i q u i d s (SRC-II heavy d i s t i l l a t e i n p a r t i c u l a r ) although i n terms of t o t a l emission, t h i s i s masked by the high cenosphere formation with the No. 6 o i l .

10.

KRAMLICH ET A L .

~

197

Soot Formation

3 0 0

c

§1 5S Publication Date: April 29, 1983 | doi: 10.1021/bk-1983-0217.ch010

3:

Φ Œ

200

Φ

ο ° 1 0 0

.4

. 6

. 8

Equivalence

1.0

1 . 2

1.4

Ratio

Figure 4. E f f e c t of stoichiometry on soot concentration f o r dispersed 190 ym SRC-II heavy d i s t i l l a t e d r o p l e t s .

Figure 5. E f f e c t of C:H r a t i o on f u e l type on soot concentration for 190 ym dispersed d r o p l e t s (φ = 1.33).

198

COMBUSTION OF SYNTHETIC

FUELS

Conclusions

Publication Date: April 29, 1983 | doi: 10.1021/bk-1983-0217.ch010

In s p i t e of the l a r g e d i f f e r e n c e s seen i n the behavior of d i f f e r e n t f u e l s at the d r o p l e t l e v e l , the f u e l screening s t u d i e s , c a r r i e d out i n a more r e a l i s t i c spray flame environment, suggest that there are no s u b s t a n t i a l d i f f e r e n c e s between synfuels and conventional petroleum derived l i q u i d f u e l s . Both s t u d i e s i n d i ­ c a t e , however, that soot production i s higher f o r the c o a l derived l i q u i d s , p a r t i c u l a r l y under staged combustion c o n d i t i o n s . In an a c t u a l combustor, longer residence time at high temperature and excess oxygen w i l l be a v a i l a b l e compared to the d r o p l e t studies r e a c t o r , such that some of the trends observed w i l l be masked by soot o x i d a t i o n i n l a t e r stages of combustion. Acknowledgmen ts This research was funded by the U . S . Department of Energy (Contract DE-AC22-80PC30298). The P r o j e c t Manager was J . H i c k e r son and the T e c h n i c a l Monitor was J . F i s c h e r . The authors wish to acknowledge a l s o the experimental work c a r r i e d out by Y . Kwan and u s e f u l c o n t r i b u t i o n s from W. R. Seeker and G. S. Samuelson i n formulating the t e c h n i c a l approach.

Literature Cited 1. 2.

Sjogren, A. 14th Symp. (Int.) on Comb. 1973, p. 919. Nakanishi, K.; Kadota, T.; and Hiroyasu, H. Comb, and Flame 1981, 40, 247. 3. England, G. C.; Heap, M. P.; Pershing, D. W.; Nihart, R. K.; and Martin, G. B. 18th Symp. (Int.) on Comb. 1981, p. 163. 4. England, G. C.; Kramlich, J.; Kwan, Y.; and Payne, R. "Third Quarterly Technical Progress Report," D0E/PC/30298-T3, NTIS DE81028391, July 1981. 5. England, G. C.; Kramlich, J.; Kwan, Y.; and Payne, R. "Fifth Quarterly Technical Progress Report," D0E/PC/30298-T5 (avail­ able NTIS). 6. Belli, R.; Paoli, L.; and Tarli, R. Paper presented at the 2nd Joint Meeting of the Chemistry and Pulverized Fuel Panel, IFRF Doc. No. F25/ha/6, September 1975. 7. Kramlich, J. C.; Samuelsen, G. S.; and Seeker, W. R. WSS/CI81-52, presented at the Fall Meeting of the Western States Section of the Combustion Institute, Tempe, Arizona, 1981. 8. Berglund, R. N.; and Liu, Β. Y. H. Env. Sci. and Tech. 1973, 147. 9. Gaydon, A. G.; and Wolfhard, H. G. "Flames." 3rd Edition, Chapman and Hall, 1970. 10. Sangiovanni, J. J.; and Dodge, L. G. 17th Symp. (Int.) on Comb. 1979, p. 455.

10.

11. 12. 13. 14. 15.

Publication Date: April 29, 1983 | doi: 10.1021/bk-1983-0217.ch010

16.

KRAMLICH ET AL.

Soot Formation

199

Seeker, W. R.; Samuelsen, G. S.; Kramlich, J. C.; and Heap, M. P. Volume III, Section 4, EPA Final Report 68-02-2631, 1981. Natarajan, R.; and Brzustowski, T. A. Comb. Sci. and Tech. 1970, 2, 259. McLean, W. J.; Hardesty, D. R.; and Pohl, J. H. 18th Symp. (Int.) on Comb. 1981, p. 1239. Seeker, W. R.; Samuelsen, G. S.; Heap, M. P.; and Trolinger, J. D. 18th Symp. (Int.) on Comb. 1981, p. 1213. Szpindler, G. D. CSIRO Investigation Report 381R, Sydney Laboratory, 1970. Lasheras, J. C.; Fernandez-Pello, A. C.; and Dryer, F. L. 18th Symp. (Int.) on Comb. 1981, p. 293.

RECEIVED October 20, 1982

11 E f f e c t of L i q u e f a c t i o n P r o c e s s i n g C o n d i t i o n s

Publication Date: April 29, 1983 | doi: 10.1021/bk-1983-0217.ch011

on C o m b u s t i o n C h a r a c t e r i s t i c s of S o l v e n t - R e f i n e d C o a l R. W. BORIO, G. J. GOETZ, T. C. LAO, Α. Κ. MEHTA, and N. Y. NSAKALA Combustion Engineering, Inc., Kreisinger Development Laboratory, Windsor, CT 06095 W. C. ROVESTI Electric Power Research Institute, Palo Alto, CA 94303 SRC-I processing has been performed using three variations in the manner in which mineral matter and unconverted coal are separated from the hot coal liquid. These processes are the Pressure Filtration Deashing (PFD), Anti-Solvent Deashing (ASD), and Critical Solvent Deashing (CSD). Since these processing conditions may influence the combustion of SRC-I solids produced, an experimental program was carried out at both the bench and pilot plant scale to determine the influence of processing (i.e., solids separation method) and com­ bustion conditions on carbon burnout of these three SRC's. Included in this study was an examination of NO emissions (particularly for the CSD and PFD SRC'S) with the objective of attaining low ΝO emissions without adversely affecting combustion efficiency. Reactivity andNO emissions results from the SRC testing were compared with those obtained from two coals that were previously tested and used as reference coals. One of these coals was a high reactivity Wyoming sub­ -bituminous coal and the other was a low reactivity Kentucky high volatile bituminous coal. X

X

X

The Solvent Refined Coal-I (SRC-I) process (1) provides a way in which coal, by way of direct hydrogenative liquefaction, can be transformed into an environmentally clean fuel for the electric u t i l i t i e s . Earlier tests (_2, _3> A» 1 ) pulverized SRC-I solid fuel (SRC), while considered successful, indicated the need for concern in two areas: carbon in the fly ash and nitrogen oxides (Ν0 ) emissions. Although good combustion efficiencies (generally greater than 98%) were attained there was a substantial amount of carbon in the particulates (generally greater than 60%). This w i l l pose a collection problem i f an electrostatic precipitator (ESP) is envisioned for particulate collection because of the very low resistivity imparted by carbon. In addition, the high nitrogen contents (1.8-1.9%) of SRC indicate that there is a potential for high Ν0 emissions. 0097-6156/83/0217-0201 $06.00/0 © 1983 American Chemical Society w i t h

χ

χ

COMBUSTION O F S Y N T H E T I C F U E L S

202

SRC has been produced using three d i f f e r e n t schemes for sep­ a r a t i n g the m i n e r a l matter and unconverted c o a l from the hot c o a l liquid. These schemes are designated as Pressure F i l t r a t i o n De­ ashing (PFD, 2) A n t i - S o l v e n t Deashing (ASD, 6) and C r i t i c a l Solvent Deashing (CSD, ^7). As these processing conditions may i n f l u e n c e the combustion of SRC s o l i d s produced, Combustion E n g i n e e r i n g , under a contract with EPRI, conducted an experimental program to determine the i n f l u e n c e of processing ( i . e . , s o l i d s s e p a r a t i o n method) and combustion operating conditions on carbon burnout of PFD, ASD, and CSD SRC. Included i n t h i s study was an examination of Ν 0 emissions ( p a r t i c u l a r l y f o r the CSD and PFD SRC) with the o b j e c t i v e of a t t a i n i n g low N 0 emissions without adversely a f f e c t i n g combustion e f f i c i e n c y . R e a c t i v i t y and Ν 0 emissions r e s u l t s from the SRC t e s t i n g were compared with those obtained from two p r e ­ v i o u s l y tested reference c o a l s , a low r e a c t i v i t y Kentucky high v o l a t i l e bituminous c o a l (KHB) and a high r e a c t i v i t y Wyoming subbituminous c o a l (WSB). The primary o b j e c t i v e of t h i s study was to determine the i n ­ fluence of SRC-I processing ( i . e . , s o l i d s separation) and combus­ t i o n operating conditions on carbon burnout under combustion conditions s i m u l a t i n g those achievable i n b o i l e r s o r i g i n a l l y de­ signed f o r c o a l f i r i n g . The secondary o b j e c t i v e was to examine combustion operating conditions that r e s u l t e d i n low Ν 0 emissions while simultaneously achieving high carbon burnout. The primary research t o o l s used i n t h i s program were C - E s Drop Tube Furnace System (DTFS), a bench s c a l e entrained laminar flow furnace and the C o n t r o l l e d Mixing H i s t o r y Furnace (CMHF), a p i l o t s c a l e entrained plug flow furnace. Both the DTFS and CMHF by v i r t u e of t h e i r a b i l i t y to r e s o l v e combustion time i n t o distance along t h e i r r e s p e c t i v e furnace lengths were used to examine carbon burnout phenomena associated with the SRC and reference c o a l s . In a d d i t i o n , the CMHF by v i r t u e of i t s staged combustion c a p a b i l i t i e s was used e x t e n s i v e l y to evaluate Ν 0 emissions and to e s t a b l i s h conditions conducive to low Ν 0 . χ

y

Publication Date: April 29, 1983 | doi: 10.1021/bk-1983-0217.ch011

χ

χ

f

χ

χ

RESEARCH FACILITIES AND PROCEDURES A number of standard and s p e c i a l bench s c a l e t e s t s along w i t h the Drop Tube Furnace System (DTFS) and p i l o t s c a l e C o n t r o l l e d Mixing H i s t o r y Furnace (CMHF) were employed i n t h i s program. Standard t e s t s consisted of proximate, u l t i m a t e , higher heating v a l u e , ash composition, ash f u s i b i l i t y temperatures, Hardgrove g r i n d a b i l i t y , and screen analyses. S p e c i a l bench s c a l e c h a r a c t e r ­ i z a t i o n t e s t s consisted of micro-proximate a n a l y s i s and m i c r o ultimate a n a l y s i s (C, Η, N ) ; micro-proximate and m i c r o - u l t i m a t e analyses were performed on p a r t i c u l a t e samples c o l l e c t e d from varying stages of combustion i n the DTFS and CMHF. In a d d i t i o n , s e l e c t e d samples of SRC and chars from p a r t i a l combustion or p y r o l y s i s of the SRC were submitted f o r Thermo-Gravimetric analyses. Thermo-Gravimetric Analyses were performed on ASTM v o l a t i l e matter char residues ground to -200 mesh. Thes^ residues £ 4 - 5 mg) were heated i n n i t r o g e n and then burned i s o t h e r m a l l y (700 C) i n a i r .

11.

BORIO E T A L .

203

Liquefaction Processing Conditions

The Drop Tube Furnace System (DTFS) c o n s i s t s e s s e n t i a l l y of an e l e c t r i c a l l y heated 2 inch I . D . χ 18 inch long furnace where f u e l (1 gm/min) and preheated secondary gas ( a i r or i n e r t s ) are introduced. The h i s t o r y of combustion i s monitored by s o l i d s / g a s sampling at various points along the length of the furnace. The p i l o t s c a l e C o n t r o l l e d Mixing H i s t o r y Furnace (0.5 χ 10 Btu/hr) i s based on the p r i n c i p l e of plug flow which r e s o l v e s time i n t o distance along the length of the furnace. By sampling at different ports along the length of the furnace, i t i s p o s s i b l e to examine the burnout and Ν 0 formation h i s t o r y of a f u e l . The CMHF also has f l e x i b i l i t y f o r c o n t r o l l i n g the primary and secondary a i r / f u e l r a t i o s and f o r delaying and/or staging secondary a i r i n t r o d u c t i o n (at any of seven l e v e l s along the length of the furnace). 6

Publication Date: April 29, 1983 | doi: 10.1021/bk-1983-0217.ch011

χ

EXPERIMENTAL PROGRAM The t e s t program was set up i n three phases: bench s c a l e , DTFS, and CMHF. Bench s c a l e and DTFS t e s t s were performed on a l l three f u e l s , while the CMHF t e s t s were performed only on the CSD and PFD SRC f u e l s . The low melting temperatures of the SRC r e s u l t e d i n pluggage of both DTFS and CMHF f u e l i n j e c t i o n systems. Special water cooled f u e l i n j e c t o r s were f a b r i c a t e d to a l l e v i a t e t h i s problem. Testing i n the DTFS involved examining the e f f e c t s of furnace w a l l temperature, p a r t i c l e s i z e , and combustion medium on burnout. More extensive t e s t i n g was conducted on the CSD SRC sample i n both the DTFS and CMHF as recommended by EPRI. In the CMHF the e f f e c t of two stage combustion was examined. S p e c i f i c a l l y , f i r s t stage s t o i c h i o m e t r y , f i r s t stage residence time, and o v e r a l l excess a i r upon burnout and NO formation of the CSD SRC sample were examined. Based on the CSD SRC r e s u l t s , a l i m i t e d t e s t matrix was e s t a b l i s h e d for the PFD SRC sample to examine the e f f e c t s of f i r s t stage s t o i c h ­ iometry and o v e r a l l excess a i r on burnout and Ν 0 . A plug flow char combustion model was used to p r e d i c t the com­ b u s t i o n e f f i c i e n c i e s of SRC under simulated commercial b o i l e r operating c o n d i t i o n s . Inputs were based on the v o l a t i l e y i e l d s and char c h a r a c t e r i s t i c s measured i n the CMHF. χ

RESULTS Fuel Analyses A n a l y t i c a l r e s u l t s (Table 1) show that SRC have very high v o l a t i l e matter and n i t r o g e n contents (52-60% and 1.8-1.9%, respec­ t i v e l y , on a d r y - a s h - f r e e b a s i s ) and very low moisture and ash contents (0.1-0.3%, a s - r e c e i v e d b a s i s i n each c a s e ) . The Higher Heating Values for the SRC (15,920-16,115 B t u / l b , d r y - a s h - f r e e b a s i s ) are much higher than those of reference c o a l s (13,290 and 14,110 B t u / l b f o r the WSB and KHB c o a l s , r e s p e c t i v e l y ) .

39.4

39.4

15860

1210

Higher Heating Value, (HHV), Btu/lb

F1aamab111ty Index °F

15920 1270

16050

16115

1270

15880

15940

1040

10900

13290

100.0

1170

12730

14110

100.0 100.0 100.0 100.0 100.0

100.0

100.0

100.0

100.0

Total

8.6 14.9

0.3

-

0.1

-

0.3

Ash

-

12.5 11.3 21.1 17.3 3.0

3.1

3.4

3.4

2.9

2.9

Oxygen (D1ff.)

1.4 1.3 1.3 1.1 1.9

1.9

1.8

1.8

1.9

Nitrogen

0.8 0.7 0.4 0.3 0.7

0.7

1.0

1.0

1.9

59.4

88.5

88.1

87.7

87.3

1.0

1.0

Sulfur

88.4

88.0

Carbon

4.9 80.4 72.5

4.8 72.4

3.9

5.9

5.9

6.1

6.1

5.8

5.8

Hydrogen

w

1.2 4.4

3.1

-

0.1

Moisture (Total)

-

100.0

0.1

100.0

-

100.0

0.3

Ultimate, Ut. Percent

100.0

100.0

100.0

100.0

100.0

100.0

100.0

Total

8.6

37.3 62.7 56.6

59.8

49.0

39.6 14.9

40.0

33.6

40.2

1.2

as-rec. daf

33.0

daf

Kentucky High Vol. Bit. Coal

60.4

3.1

as-rec.

0.3

48.2

0.1 60.2

59.8

0.3

as-rec. daf

59.6

daf

Wyoming Subbltumlnous Coal

0.1

Fixed Carbon

51.8

as-rec.

Ant1 Solvent Deashed SRC

0.3

48.0

Volatile Matter

daf

Critical Solvent Deashed SRC

Ash

0.1

51.6

Moisture (Total)

Proximate, Ut. Percent

Analysis

Pressure Filtered Deashed SRC

Table I ANALYSES OF SRC AND REFERENCE COALS

Publication Date: April 29, 1983 | doi: 10.1021/bk-1983-0217.ch011

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Η

w

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Η

ο

δ

Η

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11.

Liquefaction Processing Conditions

BORIO E T A L .

liée

Publication Date: April 29, 1983 | doi: 10.1021/bk-1983-0217.ch011

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Ο

Ξ

S S

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CM CM

m CM

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