Clean Energy from Waste and Coal [First ed.] 9780841225145, 9780841213715, 0-8412-2514-1

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Publication Date: December 23, 1992 | doi: 10.1021/bk-1992-0515.fw001

Clean Energy from Waste and Coal

In Clean Energy from Waste and Coal; Khan, M.; ACS Symposium Series; American Chemical Society: Washington, DC, 1992.

Publication Date: December 23, 1992 | doi: 10.1021/bk-1992-0515.fw001

In Clean Energy from Waste and Coal; Khan, M.; ACS Symposium Series; American Chemical Society: Washington, DC, 1992.

ACS

SYMPOSIUM

SERIES

Publication Date: December 23, 1992 | doi: 10.1021/bk-1992-0515.fw001

Clean Energy from Waste and Coal M. Rashid Khan, EDITOR Texaco, Inc.

Developed from a symposium sponsored by the Division of Fuel Chemistry at the 202nd National Meeting of the American Chemical Society, New York, New York, August 25-30, 1991

American Chemical Society, Washington, DC 1992

In Clean Energy from Waste and Coal; Khan, M.; ACS Symposium Series; American Chemical Society: Washington, DC, 1992.

515

Library of Congress Cataloging-in-Publication Data Clean energy from waste and coal / editor.

Publication Date: December 23, 1992 | doi: 10.1021/bk-1992-0515.fw001

p.

M . Rashid Khan,

cm.—(ACS symposium series, ISSN 0097-6156; 515)

"Developed from a symposium sponsored by the Division of Fuel Chemistry at the 202nd National Meeting of the American Chemical Society, New York, New York, August 25-30, 1991." Includes bibliographical references and indexes. ISBN 0-8412-2514-1 1. Waste Congresses.

products

as

fuel—Congresses.

2. Coal—

I. Khan, M . Rashid. II. Series. TP360.C55 1992 333.79'38—dc20

92-38386 CIP

The paper used in this publication meets the minimum requirements of American National Standard for Information Sciences—Permanence of Paper for Printed Library Materials, ANSI Z39.48-1984. Copyright © 1993 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, M A 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 U N I T E D STATES O F A M E R I C A

In Clean Energy from Waste and Coal; Khan, M.; ACS Symposium Series; American Chemical Society: Washington, DC, 1992.

1993 Advisory Board ACS Symposium Series M. Joan Comstock, Series Editor V. Dean Adams Tennessee Technological University

Publication Date: December 23, 1992 | doi: 10.1021/bk-1992-0515.fw001

Robert J. Alaimo Procter & Gamble Pharmaceuticals, Inc. Mark Arnold University of Iowa David Baker University of Tennessee Arindam Bose Pfizer Central Research Robert F. Brady, Jr. Naval Research Laboratory Margaret A. Cavanaugh National Science Foundation Dennis W. Hess Lehigh University

Bonnie Lawlor Institute for Scientific Information Douglas R. Lloyd The University of Texas at Austin Robert McGorrin Kraft General Foods Julius J. Menn Plant Sciences Institute, U.S. Department of Agriculture Vincent Pecoraro University of Michigan Marshall Phillips Delmont Laboratories George W. Roberts North Carolina State University A. Truman Schwartz Macalaster College

Hiroshi Ito IBM Almaden Research Center

John R. Shapley University of Illinois at Urbana—Champaign

Madeleine M. Joullie University of Pennsylvania

Peter Willett University of Sheffield (England)

Gretchen S. Kohl Dow-Corning Corporation

In Clean Energy from Waste and Coal; Khan, M.; ACS Symposium Series; American Chemical Society: Washington, DC, 1992.

Publication Date: December 23, 1992 | doi: 10.1021/bk-1992-0515.fw001

Foreword l H E A C S SYMPOSIUM SERIES was first published in 1974 to provide a mechanism for publishing symposia quickly in book form. The purpose of this series is to publish comprehensive books developed from symposia, which are usually "snapshots in time" of the current research being done on a topic, plus some review material on the topic. For this reason, it is necessary that the papers be published as quickly as possible. Before a symposium-based book is put under contract, the proposed table of contents is reviewed for appropriateness to the topic and for comprehensiveness of the collection. Some papers are excluded at this point, and others are added to round out the scope of the volume. In addition, a draft of each paper is peer-reviewed prior to final acceptance or rejection. This anonymous review process is supervised by the organizers) of the symposium, who become the editor(s) of the book. The authors then revise their papers according to the recommendations of both the reviewers and the editors, prepare camera-ready copy, and submit the final papers to the editors, who check that all necessary revisions have been made. As a rule, only original research papers and original review papers are included in the volumes. Verbatim reproductions of previously published papers are not accepted. M. Joan Comstock Series Editor

In Clean Energy from Waste and Coal; Khan, M.; ACS Symposium Series; American Chemical Society: Washington, DC, 1992.

Publication Date: December 23, 1992 | doi: 10.1021/bk-1992-0515.pr001

Preface ± H E U S E OR DISPOSAL O F O U R ENORMOUS WASTE RESOURCES in an efficient and environmentally acceptable way constitutes one of the major challenges of our time. Currently, nearly half the petroleum consumed in the United States annually is imported. The billions of dollars the U.S. spends for imported oil underlines the magnitude of this country's energy needs and rate of consumption. Both domestic consumption and dependence on imported energy are expected to grow. Therefore, conversion of our wastes into clean energy is a significant—and urgent—technical challenge for our scientists. More than 160 million tons of garbage are produced in the United States every year, about two-thirds of a ton per person. In our culture of planned obsolescence, even durable goods find their way to the junkyard. It is often more convenient and economical to throw away clocks and cabinets than to repair them, as is generally done in other nations. Use of solid wastes, agricultural residues, and trees through thermal conversion processes have been practiced to a varying extent in many parts of the world. Energy shortages and environmental issues during the past decades, however, have introduced new perspectives in developing energy resources from these waste materials. We need to develop environmentally acceptable and economical waste-based fuel forms by appropriate pre- and posttreatment. The objective of this book is to identify problems and opportunities in deriving clean energy from waste. The following wastes are considered: municipal solid waste, sludge, biomass, plastics, and tires. The chapters can be divided into two broad categories: (a) fundamental and applied aspects of waste to energy conversion and (b) the characterization, use, and disposal of byproduct ash. In many chapters, co-utilization of coal and waste have been considered. No book of this sort can hope to be comprehensive; we have tried to present an interdisciplinary treatment of some of the major topics to stimulate scientific collaboration.

xi In Clean Energy from Waste and Coal; Khan, M.; ACS Symposium Series; American Chemical Society: Washington, DC, 1992.

This book would not have been possible without the dedicated work of the authors, reviewers, and the A C S Books Department staff. The efforts of Rhonda Bitterli, Cheryl Shanks, and Bruce Hawkins of the A C S are greatly appreciated. M . RASHID KHAN Texaco, Inc. Beacon, N Y 12508

Publication Date: December 23, 1992 | doi: 10.1021/bk-1992-0515.pr001

October 1, 1992

xii In Clean Energy from Waste and Coal; Khan, M.; ACS Symposium Series; American Chemical Society: Washington, DC, 1992.

Chapter 1

Clean Energy from Waste Introduction 1

2

M. Rashid Khan and Kenneth E. Daugherty 1

Research and Development Department, Texaco, Inc., P.O. Box 509, Beacon, NY 12508 Chemistry Department, University of North Texas, Denton, TX 76203

Publication Date: December 23, 1992 | doi: 10.1021/bk-1992-0515.ch001

2

I t has been stated that the North Americans are short on energy but long on wastes. On a per capita basis, the United States i s the largest consumer of energy but the greatest producer of waste i n the world. The US c i t i e s dispose of about half a m i l l i o n tons of municipal s o l i d waste (MSW) d a i l y (about 95% of which i s l a n d - f i l l e d ) , while importing over 15 m i l l i o n barrels of crude d a i l y . Over 80% of the c i t i e s are expected to run out of l a n d f i l l s by the end of t h i s decade. The enormity of the problem has been depicted by the story of a garbage-laden barge from NY crossing international waterways i n desperate search of a dump-site. The US uses about 5% of its MSW to generate e l e c t r i c i t y , while the other i n d u s t r i a l nations have been doing so at a much larger scale for a longer time. For example, Japan uses 26% of its waste i n 361 plants. The use by other nations are as follows: Germany 35%; Sweden 51%; Switzerland over 75%. The slow growth i n the waste utilization facilities i n the US is primarily a r e s u l t of concern over a i r pollution generated from these facilities. In addition to MSW problems, to the treatment plants i n the US, the t o t a l d a i l y flow of wastewater i s expected to increase from a current 28 billion gallons to about 33 billion gallons by the year 2000. Much of the sewage, present i n the wastewater, and i n d u s t r i a l sludges had been t r a d i t i o n a l l y l a n d f i l l e d or ocean dumped. Over a decade ago, the US embraced several waste-toenergy f a c i l i t i e s , but plant f a i l u r e s and operational d i f f i c u l t i e s were the norm. Years l a t e r , the technology i s now more mature but the problem still exists i n terms of public perception and governmental support. There are b a r r i e r s on both s c i e n t i f i c and s o c i a l / i n s t i t u t i o n a l aspects. For example, i n response to mounting pressures on municipalities to meet increasingly stringent

0097-6156/93/0515-0001$06.00/0 © 1993 American Chemical Society In Clean Energy from Waste and Coal; Khan, M.; ACS Symposium Series; American Chemical Society: Washington, DC, 1992.

Publication Date: December 23, 1992 | doi: 10.1021/bk-1992-0515.ch001

2

CLEAN ENERGY FROM WASTE AND COAL environmental r e g u l a t i o n s by i n s t a l l i n g expensive c o n t r o l equipment, many simply decided t o r e t i r e t h e i r f a c i l i t i e s t o "moth-ball" s t a t u s . When an o r g a n i z a t i o n plans a waste-to-energy p l a n t , the community t y p i c a l l y responds by expressing the "not-in-my-backyard" syndrome. There are s e r i o u s concerns over the p o t e n t i a l r e l e a s e of some products of incomplete combustion (products such as benzopyrene, d i o x i n , and benzofurans) and emission of v o l a t i l e metals from i n c i n e r a t o r s . Numerous s t u d i e s have been i n i t i a t e d by the US DOE, EPA and v a r i o u s i n d u s t r i e s f o r i d e n t i f y i n g ways t o m i t i g a t e these problems. Energy recovery from waste has r e c e i v e d broad technical acceptability as new and advanced technologies demonstrate t h e i r c a p a b i l i t y . For example, i n c o n t r a s t t o d i r e c t combustion, advanced g a s i f i c a t i o n t e c h n o l o g i e s can be used t o produce a c l e a n s y n t h e s i s gas from waste. The s y n t h e s i s gas can be combusted i n advanced gas t u r b i n e s t o generate e l e c t r i c i t y without c o n t r i b u t i n g t o air pollution. Coal represents 90% of the U.S. proven r e s e r v e s of f o s s i l f u e l s . In many cases, both waste and c o a l are used f o r energy generation. C o - u t i l i z a t i o n of waste and c o a l o f f e r s many t e c h n i c a l and economic advantages not achieved by p r o c e s s i n g waste alone. Thus, i n s e v e r a l chapters of t h i s book, the use of c o a l has been considered along with waste. The objective of this book, contributed by i n t e r n a t i o n a l l y recognized experts, i s t o present the s t a t e - o f - t h e - a r t data r e l a t e d t o waste-to-clean energy processes. The major wastes of i n t e r e s t are: sewage and i n d u s t r i a l sludge, municipal s o l i d waste, t i r e s , biomass, p l a s t i c s and polymeric wastes. The reader w i l l recognize t h a t s e v e r a l other important wastes ( r e f i n e r y waste, b l a c k l i q u o r , e t c . ) , may w e l l serve as e x c e l l e n t sources of energy, but are not considered here. Sewage and I n d u s t r i a l Sludge Dumping of sludge i n the Boston Harbor became a hot debate i s s u e during the 1988 p r e s i d e n t i a l e l e c t i o n , although a r e l a t i v e l y small p o r t i o n of sludge produced i s ocean dumped (Table I) . Major c i t i e s i n the E a s t e r n US (New York C i t y , Boston, and m u n i c i p a l i t i e s l o c a t e d i n major c o u n t i e s such as Westchester and Suffex) had been dumping sewage sludge i n the ocean. Treatment p l a n t s i n the US produce about 7 m i l l i o n tons of dry sewage sludge d a i l y . About 20% of the sludge produced i s i n c i n e r a t e d and about 60% of i t i s l a n d f i l l e d , while only a small p o r t i o n of the o v e r a l l production i s ocean dumped. The EPA, however, has imposed a s t i f f penalty f o r ocean dumping of sludge. A f t e r January, 1992, the penalty f o r ocean dumping was i n c r e a s e d t o $600 per dry ton of sludge i n the E a s t e r n US. The a l t e r n a t i v e s of ocean dumping, namely l a n d f i l l i n g or incineration, are meeting strong opposition for environmental reasons.

In Clean Energy from Waste and Coal; Khan, M.; ACS Symposium Series; American Chemical Society: Washington, DC, 1992.

1. KHAN & DAUGHERTY Table I .

Clean Energy from Waste: Introduction

D i s p o s a l and End Use of Sludge

Method

(dry ton/y) Amount

Landf i 1 1 i n g (dump ing) Incineration Land a p p l i c a t i o n (usage) Ocean d i s p o s a l Distribution/marketing Other

3,094 1,541 1,424 498 455 541

Percent 40.8 20.3 18.8 6.6 6.0 7.7

Publication Date: December 23, 1992 | doi: 10.1021/bk-1992-0515.ch001

SOURCE: Adapted from réf. 1. Before sludge can be burned or g a s i f i e d autogeneously (without a u x i l i a r y f u e l s ) some l e v e l of water removal i s needed. The major t e c h n i c a l steps i n v o l v e d f o r the use of sewage sludge as a source of energy are the f o l l o w i n g : dewatering, c o n d i t i o n i n g , and p r o c e s s i n g t o recover energy by combustion or g a s i f i c a t i o n . For example, processes u s i n g the m u l t i p l e hearth furnace, f l u i d - b e d combustor, or one of the many g a s i f i c a t i o n t e c h n o l o g i e s can be r e a d i l y used t o e x t r a c t energy from sludge. Primary sewage sludge i s g e n e r a l l y thickened by g r a v i t y , but the sludge can a l s o be mechanically dewatered t o produce a r e l a t i v e l y dry cake. Secondary sludge i s generated by the b i o l o g i c a l treatment processes. The t y p i c a l heat content of the primary sludge i s about 7000 Btu/lb, which i s g r e a t e r than t h a t of the secondary sludge which c o n t a i n s a h e a t i n g value of about 4000 B t u / l b . The s o l i d s content of the primary sludge i s low, g e n e r a l l y lower than 10 percent. The types of water t h a t can be a s s o c i a t e d with a p a r t i c l e of sewage sludge can be classified as follows: water bound to oxygenated f u n c t i o n a l groups and/or c a t i o n s , c a p i l l a r y or micropore bound water, monolayer water, and bulk or f r e e water. Polymeric a d d i t i v e s are g e n e r a l l y added t o agglomerate the sludge p a r t i c l e s and thereby water removal is f a c i l i t a t e d . About 3 t o 8 l b s of polymer i s used per ton of d i s s o l v e d s o l i d s . Polymer usage i s not only expensive, but added polymers may a l s o adversely a f f e c t the s l u r r y i n g and pumping c h a r a c t e r i s t i c s of the dewatered sludge. The major technology c u r r e n t l y i n use i s i n c i n e r a t i o n . Land usage of sludge, however, may r e q u i r e some c o n d i t i o n i n g of sludge t o l e s s e n the b i o l o g i c a l a c t i v i t y . Pyrolysis based processes apply r e l a t i v e l y high temperatures t o produce a l i q u i d f u e l from sewage sludge. However, no such process has ever been commercialized. Because of concerns r e l a t e d to environmental i s s u e s , i n c i n e r a t i o n has become an unpopular o p t i o n t o dispose of sludge. In p a r t i c u l a r , the f a t e of heavy metals present i n sludge d u r i n g i n c i n e r a t i o n i s not w e l l known. There are concerns

In Clean Energy from Waste and Coal; Khan, M.; ACS Symposium Series; American Chemical Society: Washington, DC, 1992.

3

4

CLEAN ENERGY FROM WASTE AND COAL regarding the r e l e a s e of some heavy metals i n t o the atmosphere. Wet a i r o x i d a t i o n processes serve t o dewater the sludge but do not address the i s s u e of u l t i m a t e d i s p o s a l of sludge or the metals present i n i t . During g a s i f i c a t i o n , however, the organics present i n sludge are converted t o u s e f u l s y n t h e s i s gas (CO & H ) while the minerals present i n the feed are converted i n t o r i g i d nonleachable s l a g p o t e n t i a l l y u s e f u l i n the m a t e r i a l s industry. The major t e c h n i c a l i s s u e s a s s o c i a t e d i n processes aimed at e x t r a c t i n g energy from sewage and i n d u s t r i a l sludges are the f o l l o w i n g : (a) low-cost dewatering o p e r a t i o n ; (b) improvement of the rheological and pumpability of c o n c e n t r a t i o n sludge ( c o n t a i n i n g >20% s o l i d s ) ; (c) nature, type and f a t e of i n o r g a n i c s present i n sludge d u r i n g advanced p r o c e s s i n g ; (d) improvements i n emission and odor c o n t r o l during sludge p r o c e s s i n g . Some i n t e r e s t i n g papers on sewage sludge can be found i n r e f e r e n c e s 1-5. Details of the i s s u e s are beyond the scope of t h i s chapter.

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2

M u n i c i p a l S o l i d Waste M u n i c i p a l S o l i d Waste (MSW) i s one of the l e a s t used byproduct resources i n the United S t a t e s (6,7). Until r e c e n t l y the common p r a c t i c e f o r d i s p o s a l of, MSW was dumping. The dumping s i t e s were u s u a l l y e i t h e r the ocean or an open p i t (8,9). The d i s p o s a l of r e f u s e i s an i n c r e a s i n g concern of m u n i c i p a l i t i e s and s t a t e governments throughout the United S t a t e s . In the year 1990, i t was estimated t h a t 160-200 m i l l i o n tons of MSW was disposed from the r e s i d e n t i a l , commercial, and institutional s e c t o r s and i s i n c r e a s i n g y e a r l y (9) . Each ton of municipal s o l i d waste i s e q u i v a l e n t i n energy content t o more than a b a r r e l of o i l (10). C i t i e s are running out of space f o r l a n d f i l l s . One of the a t t r a c t i v e s o l u t i o n s t o l a n d f i l l s i s i n c i n e r a t i o n . In the e a r l y 1970s, environmental concern began t o r i s e causing c i t i z e n s t o become i n c r e a s i n g l y c a u t i o u s of r e s i d i n g near l a n d f i l l s i t e s . Due t o a i r p o l l u t i o n , the garbage or MSW was no longer burned. At t h a t time there were many l a n d f i l l s , and new l a n d f i l l s i t e s were a v a i l a b l e f o r d i s p o s a l of garbage. Those l a n d f i l l s are e i t h e r f u l l or becoming f u l l , and new l a n d f i l l s are expensive and d i f f i c u l t to s i t e . Americans dispose of 80-90 percent of t h e i r MSW into l a n d f i l l s f i l l i n g them very q u i c k l y . The l a n d f i l l s i n America have been reduced from 10,000 i n 1980 t o 6,500 i n 1988, t o l e s s than 5,000 i n 1992 (11,12,13). Not only i s the a i r p o l l u t e d because of MSW, but the ground water i s p o l l u t e d as w e l l when the garbage decomposes. Changes i n the weather and r a i n f a l l are major f a c t o r s c o n t r i b u t i n g to garbage decomposition. I t has been estimated t h a t water p o l l u t e d today w i l l be a f f e c t e d f o r hundreds of years (14).

In Clean Energy from Waste and Coal; Khan, M.; ACS Symposium Series; American Chemical Society: Washington, DC, 1992.

1. KHAN & DAUGHERTY

Clean Energy from Waste: Introduction

5

Publication Date: December 23, 1992 | doi: 10.1021/bk-1992-0515.ch001

In 1989, the Environmental P r o t e c t i o n Agency (EPA) proposed r e g u l a t i o n s f o r s t r i c t e r c o n t r o l of new and preexisting l a n d f i l l s . These measures, which went i n t o e f f e c t i n 1991, w i l l help i n s o l v i n g the problem, but are expensive. I t i s estimated t h a t i t w i l l c o s t over 800 m i l l i o n d o l l a r s per year t o implement these methods nationwide. The r e g u l a t i o n s and controls include monitoring ground water f o r contamination, a l l o w i n g f o r the c o n t r o l l e d escape of methane which forms as the garbage decomposes, and permanently s e a l i n g l a n d f i l l s a f t e r they are f i l l e d (15). Sources of M u n i c i p a l S o l i d Waste. M u n i c i p a l r e f u s e i s a heterogeneous mixture of organic and i n o r g a n i c wastes d i s c a r d e d by homes, schools, h o s p i t a l s , and a v a r i e t y of other sources i n the community. The major c o n t r i b u t o r s to s o l i d waste are (16): a) b) c) d) e)

Domestic: s i n g l e and m u l t i p l e d w e l l i n g s Commercial: o f f i c e s and r e t a i l s t o r e s Entertainment c e n t e r s : r e s t a u r a n t s , h o t e l s and motels, and s e r v i c e s t a t i o n s Institutional: schools, h o s p i t a l s and municipal buildings M u n i c i p a l s e r v i c e s : d e m o l i t i o n and c o n s t r u c t i o n , s t r e e t and a l l e y c l e a n i n g , landscaping, catch b a s i n c l e a n i n g , parks and beaches, and waste treatment residues

Content of M u n i c i p a l S o l i d Waste. M u n i c i p a l s o l i d waste i s an aggregate mixture of waste m a t e r i a l s t h a t can be c l a s s i f i e d as an organic f r a c t i o n , an i n o r g a n i c f r a c t i o n , and moisture. The organic f r a c t i o n , which makes up t o 30% of the waste, i s p r i m a r i l y c e l l u l o s e (wood f i b e r s ) . It i s considered a major source f o r energy recovery. The i n o r g a n i c f r a c t i o n i s noncombustible. I t can be e i t h e r r e c y c l a b l e or a f t e r combustion c o n s t i t u t e s the ash residue. Table I I shows the summary of the chemical c h a r a c t e r i z a t i o n s of MSW (17). Another aspect or o b j e c t i v e of many of the recovery processes of MSW i s t o u t i l i z e i t s thermal energy. The heat content of MSW i s important. The heat content of the a s - r e c e i v e d r e f u s e can reach 3,500 t o 5,500 Btu/pound (16). A r e d u c t i o n of the moisture or i n e r t contents w i l l i n c r e a s e the heat content. Decreased q u a n t i t i e s of p l a s t i c s w i l l a l s o decrease the heat content of MSW (18). There are many s o l u t i o n s t h a t have been proposed f o r the problem of growing l a n d f i l l s and the i n c r e a s e i n MSW. These s o l u t i o n s i n c l u d e mass burning, burying, r e c y c l i n g , and use as an energy source u s i n g Refuse Derived Fuel (RDF) . The analyses shown i n Table I I were developed by one of the authors during experimental work on the Denton, Texas l a n d f i l l d u r i n g 1980-81 (17). The composition was based on repeated sampling and t e s t i n g of the Denton l a n d f i l l .

In Clean Energy from Waste and Coal; Khan, M.; ACS Symposium Series; American Chemical Society: Washington, DC, 1992.

6

CLEAN ENERGY FROM WASTE AND COAL

Table I I . a.

Combustible Paper Plastic Wood Garden Waste Food Waste Rubber Leather

b. Publication Date: December 23, 1992 | doi: 10.1021/bk-1992-0515.ch001

M u n i c i p a l S o l i d Waste Composition free) i n Denton, Texas - 1981

Non-Combustible Glass/Ceramic/ Stone Ferrous Aluminum Industrial/ Commercial Residual Dirt

(moisture

52% 14% 5% 4% 3% 1% 1%

9% 6% 2% 2% 1%

SOURCE: Reprinted from ref. 17. Table I I I presents r e s u l t s o f the manufacturing operations of t h e Eden P r a i r i e R e c y c l i n g p l a n t i n Eden P r a i r i e , Minnesota ( D i v i s i o n o f Green I s l e Environmental S e r v i c e s , Inc.)The data presented are r e c y c l i n g data f o r the months o f June, J u l y and August o f 1992 (19). Conversion o r O x i d a t i o n o f M u n i c i p a l S o l i d Waste. Burning MSW not only reduces the volume o f garbage by 80% but may a l s o provide usable energy. MSW can be used i n t h r e e d i f f e r e n t ways: 1.

D i r e c t combustion, mass burn: the MSW i s fed i n t o the furnace through a moving g r a t e where t h e temperature reaches 2400°F. The problems with the mass burn i n c i n e r a t o r s are the c o s t o f t h e i n c i n e r a t i o n f a c i l i t y and the emissions.

2.

Conversion o f MSW i n t o l i q u i d o r gaseous f u e l by means o f g a s i f i c a t i o n , p y r o l y s i s , biodégradation, or hydrogénation. The l i q u i d o r gaseous fuel produced can then be e a s i l y c o f i r e d with c o a l o r oil.

3.

Burning o f the combustible p o r t i o n o f MSW, Refuse Derived Fuel (RDF), after separating the incombustible p o r t i o n .

The incombustible p o r t i o n o f MSW and t h e ash a r e d i s c a r d e d i n l a n d f i l l s which c r e a t e a new problem, namely pollution. Ash c o n t a i n s some organic c o n s t i t u e n t s and some t r a c e elements a t d i f f e r e n t levels. Ash i s considered hazardous i f the l e v e l s o f t o x i c c o n s t i t u e n t s are h i g h . In Clean Energy from Waste and Coal; Khan, M.; ACS Symposium Series; American Chemical Society: Washington, DC, 1992.

1. KHAN & DAUGHERTY Table I I I .

7

Clean EnergyfromWaste: Introduction

Eden P r a i r i e R e c y c l i n g , Inc. (19) 1992 Tons Total June July August Tons

Total £

Publication Date: December 23, 1992 | doi: 10.1021/bk-1992-0515.ch001

Recyclables Aluminum Metal Plastic Corrugated Ferrous M t l Scrap Metal

16.40 0.00 3.21 170.81 114.87 107.18

14.11 0.00 3.12 191.47 95.51 91.37

10. 73 0. 00 5. 54 121. 24 102. 14 79. 56

41. 24 0. 00 11. 87 483. 52 312. 52 278. 11

TOTALS

412.47

395.58

319. 21

1127. 26

4. 00

2982.16

3376.23

2563. 49

8921.88

31. 62

0.15 0. 00 0. 04 1.71 1. 11 0. 99

Fuel RDF

Composted/Incinerated/Other 4091.09

3170.64

2286. 46

9548. 19

33. 84

Heavies Rej e c t s

1714.97 496.55

3246.49 609.67

1992. 34 557. 72

6953. 80 1663. 94

24. 65 5. 90

TOTALS

2211.52

3856.16

2550. 06

8617. 74

30. 55

7719.22 28215.07

100.01

Landfilled

TOTAL TONS

9697.24 10798.61

Table IV shows t h a t the use o f MSW as a source o f renewable energy i s expected t o grow a t a r a t e h i g h e r than 8% between 1989 and 2010 (20). L a n d f i l l i n g MSW. S a n i t a r y l a n d f i l l s were i n c r e a s e d i n 1976, when the Resource Conservation and Recovery A c t (RCRA) gave the EPA the a u t h o r i t y t o c l o s e open l a n d f i l l s and upgrade the q u a l i t y o f s a n i t a r y l a n d f i l l s . Sanitary l a n d f i l l s are t y p i c a l l y huge depressions l i n e d with c l a y to minimize leakage of p o l l u t a n t s i n t o the groundwater. Heavy equipment i s used t o spread the MSW out and compress i t every day. A f t e r the l a n d f i l l has been packed t o c a p a c i t y , a l a y e r of d i r t and/or p l a s t i c i s used t o cover the day's h a u l .

In Clean Energy from Waste and Coal; Khan, M.; ACS Symposium Series; American Chemical Society: Washington, DC, 1992.

CLEAN ENERGY FROM WASTE AND COAL

8

Publication Date: December 23, 1992 | doi: 10.1021/bk-1992-0515.ch001

Table IV. Renewable Energy ( Q u a d r i l l i o n Btu per Year) E l e c t r i c i t y and Non-Electric

1989

2010

Electricity Capability (gigawatts) Convent. Hydropower Geothermal MSW Biomass/Other Waste S o l a r Thermal Solar Photovoltaic Wind Total

75.48 2.47 1.98 5.31 .33 .00 1.93 87.51

78.46 10.65 10.81 8.88 1.78 .01 5.30 115.90

0.2 7.2 8.4 2.5 8.4 2.1 4.9 1.3

Generation ( b i l l i o n Kilowatthours) 276.90 Convt. Hydropower 15.05 Geothermal 13.31 MSW 29.54 Biomass/Other Waste .69 S o l a r Thermal .00 Solar Photovoltaic 3.38 Wind 338.90 Total

314.80 78.52 74.22 49.55 5.06 .00 12.97 535.10

.6 8.2 8.5 2.5 10.0 2.2 6.6 2.2

Consumption/Displacement Convent. Hydropower 2.88 .16 Geothermal .20 MSW .20 Biomass/Other Waste .01 S o l a r Thermal .00 Solar Photovoltaic .04 Wind 3.48 Total

3.27 .82 1.27 .33 .05 .00 .13 5.88

.6 8.2 9.2 2.5 10.0 2.3 6.6 2.5

N o n - E l e c t r i c Renewable Energy R e s i d e n t i a l , Commercial & I n d u s t r i a l .00 .00 Hydropower Geothermal .00 .39 4.54 2.63 Biofuels .54 .05 S o l a r Thermal .00 .00 Solar Photovoltaic .00 .00 Wind

Annual Growth 1989-2010 %

— —

2.6 11.6 — —

Transportation Ethanol Total

.07 2.75

.14 5.62

3.5 3.5

T o t a l Renewable Energy

6.23

11.50

3.0

SOURCE: Reprinted from ref. 20. In Clean Energy from Waste and Coal; Khan, M.; ACS Symposium Series; American Chemical Society: Washington, DC, 1992.

1. KHAN & DAUGHERTY

Clean Energy from Waste: Introduction

9

S a n i t a r y l a n d f i l l operators f o l l o w s t r i c t g u i d e l i n e s . They c o n t r o l and monitor methane gas generation, s u r f a c e water r u n o f f , and groundwater contamination by the landfill.

Publication Date: December 23, 1992 | doi: 10.1021/bk-1992-0515.ch001

Refuse Derived

Fuel

Refuse Derived Fuel (RDF), shredded MSW with most g l a s s and metals removed, i s an a t t r a c t i v e s o l u t i o n s i n c e i t a l s o addresses another problem a f f e c t i n g the United S t a t e s : d e p l e t i n g energy reserves. One t o n o f RDF has the energy e q u i v a l e n t of one b a r r e l o f o i l (21). RDF has a 7,000 t o 8,000 Btu/pound heat content. The powdered RDF, e m b r i t t l e d and p u l v e r i z e d refuse, w i l l even have a higher heat content of over 8,500 Btu/pound (16). RDF Technology. The s t a r t i n g m a t e r i a l o f RDF i s MSW. The exact composition o f MSW v a r i e s according t o the area, the time o f the year i t was c o l l e c t e d , and the make-up o f t h a t p a r t i c u l a r community. RDF r e f e r s t o the heterogenous mixture of the combustible p o r t i o n of MSW (22). The concept of RDF has e x i s t e d s i n c e the e a r l y 1970s (23) . There are seven forms of RDF t h a t have been d e f i n e d as d e s c r i b e d i n Table V (22) . RDF i s commonly used i n two forms, f l u f f (RDF-1) and d e n s i f i e d RDF (RDF-5; dRDF). There are s e v e r a l problems with using RDF-1 t h a t make i t l e s s a t t r a c t i v e , such as: i t i s hard t o handle, i t i s u s u a l l y burned i n suspension: and o f t e n causes problems i n ash handling s i n c e much o f i t remains unburned (14) . On the other hand, the main b e n e f i t s o f u s i n g RDF r a t h e r than raw r e f u s e a r e : *

RDF when p r o p e r l y processed, can be s t o r e d f o r an extended p e r i o d of time

*

RDF technology allows f o r the recovery of s a l e a b l e material

*

RDF can be combusted i n a wide range o f e x i s t i n g b o i l e r s , f l u i d i z e d bed combustors, g a s i f i e r s , and cement and b r i c k k i l n s

*

RDF can a l s o be used as a feedstock d i g e s t e r s t o produce methane gas

*

RDF can e a s i l y be transported from one l o c a t i o n t o another

*

RDF can be burned on a supplemental other f u e l , such as c o a l o r wood

f o r anaerobic

basis

In Clean Energy from Waste and Coal; Khan, M.; ACS Symposium Series; American Chemical Society: Washington, DC, 1992.

with

10

CLEAN ENERGY FROM WASTE AND COAL

Publication Date: December 23, 1992 | doi: 10.1021/bk-1992-0515.ch001

Table V.

Types o f Refuse Derived F u e l

RDF-1

waste used as f u e l i n a s - d i s c a r d e d form

RDF-2

waste processed t o coarse p a r t i c l e without f e r r o u s metal s e p a r a t i o n

RDF-3

shredded f u e l d e r i v e d from MSW t h a t has been processed t o remove metals, g l a s s and other i n o r g a n i c m a t e r i a l s (95 wt% passes 50-mm square mesh)

RDF-4

combustible waste processed i n t o powder form (95 wt% passes 10 mesh)

RDF-5

combustible waste d e n s i f i e d (compressed) i n t o a form o f p e l l e t s , s l u g s , c u b i t s o r b r i q u e t t e s (dRDF)

RDF-6

combustible waste processed i n t o l i q u i d

RDF-7

combustible waste processed i n t o gaseous f u e l

s i z e with o r

fuel

SOURCE: Adapted from ref. 22. *

RDF i s more homogeneous, y i e l d i n g low v a r i a b i l i t y i n f u e l c h a r a c t e r i s t i c s , thereby making combustion c o n t r o l e a s i e r t o implement. I t a l s o burns more evenly a t a higher s u s t a i n e d temperature.

*

RDF has a lower percentage o f unburnable r e s i d u a l s such as metals and g l a s s , and t h i s has a higher heat content per u n i t weight than does unprocessed s o l i d waste.

*

RDF when burned i n a dedicated b o i l e r has a g r e a t e r thermal e f f i c i e n c y (8-10 percent g r e a t e r ) .

*

RDF can have a b e n e f i c i a l e f f e c t on a i r emissions and ash r e s i d u e , compared t o burning MSW.

In order t o e f f e c t i v e l y u t i l i z e the combustible p o r t i o n of MSW, known as RDF, i t i s necessary t o d e n s i f y t h e RDF i n order t o t r a n s p o r t i t economically and e a s i l y . It is then c a l l e d D e n s i f i e d Refuse Derived Fuel (dRDF). T h i s d e n s i f i c a t i o n step can i n c r e a s e the d e n s i t y o f RDF from 2 t o 3 pounds p e r c u b i c foot t o 40 o r more pounds p e r c u b i c f o o t (10). I f dRDF i s going t o be s t o r e d f o r a p e r i o d o f time longer than s e v e r a l days, a b i n d e r must a l s o be added. Calcium hydroxide (Ca(0H) ), which has been proven t o be the best binder, i s o f t e n added t o RDF before d e n s i f i c a t i o n . The b i n d e r delays b i o l o g i c a l and 2

In Clean Energy from Waste and Coal; Khan, M.; ACS Symposium Series; American Chemical Society: Washington, DC, 1992.

1. KHAN & DAUGHERTY

Clean Energy from Waste: Introduction

chemical degradation f o r years and reduces S 0 other emissions by c o f i r i n g with c o a l (12,23).

X/

11

N0 and X

Plastics

Publication Date: December 23, 1992 | doi: 10.1021/bk-1992-0515.ch001

P l a s t i c s , u n r e c y c l a b l e and r e c y c l a b l e , make up 9% o f MSW by weight and approximately 19% by volume. In 1987, over 57 m i l l i o n pounds o f p l a s t i c s were s o l d i n t h e US. Although p l a s t i c s have been r e c y c l e d f o r over a decade, only a l i t t l e over 1% o f the p l a s t i c s produced a r e r e c y c l e d . Even with a l a r g e i n c r e a s e i n r e c y c l i n g , there i s c o n s i d e r a b l e room f o r research and development t o f i n d ways t o d e r i v e c l e a n energy from p l a s t i c s . Governments have taken o r are c o n s i d e r i n g t a k i n g a c t i o n s t h a t a f f e c t p l a s t i c r e c y c l i n g . A s i g n i f i c a n t one i n v o l v e s the banning o f c e r t a i n p l a s t i c s o u t r i g h t . One t a r g e t o f such laws i s polystyrene, the foamed, i n s u l a t i n g p l a s t i c t h a t i s most o f t e n used f o r c o f f e e cups and f a s t food packaging. Some areas have banned p o l y s t y r e n e packing; others have banned polystyrene foam made with c h l o r o f l u o r o c a r b o n s as a blowing agent. The US Congress has been c o n s i d e r i n g measures t h a t would encourage r e c y c l i n g by p l a c i n g a t a x on the use o f v i r g i n m a t e r i a l s . I f i n d u s t r i e s could s u c c e s s f u l l y demonstrate t h e usage o f these m a t e r i a l s as a feedstock f o r making u s e f u l chemicals or energy, then the urgency t o ban making them would be reduced. Once the t e c h n i c a l i s s u e s o f feed c o l l e c t i o n and p r e p a r a t i o n were addressed, these wastes c o u l d be used as a source o f c l e a n energy. V i r t u a l l y each type o f p l a s t i c has unique chemical and physical properties. These qualities e f f e c t the r e c y c l a b i l i t y o f the p l a s t i c s and the use o f r e c y c l e d plastics. For example, polyurethane mixed i n t o a batch of polyethylene t e r e p h t h a l a t e (PET) can g r e a t l y reduce the s t r e n g t h o f the PET and render i t u n s u i t a b l e f o r many a p p l i c a t i o n s . That means, the r e c y c l e r s must separate the d i f f e r e n t types o f p l a s t i c s before doing anything with them. T h i s a l s o shows t h a t t h e r e a r e l i m i t s on t h e r e c y c l a b i l i t y of p l a s t i c s . With many p l a s t i c s being u n r e c y c l a b l e , the p r o c e s s i n g r e q u i r e d f o r r e c y c l i n g o f t e n being l i m i t i n g , and t h e f a c t t h a t p l a s t i c s have a high energy content (18,000 Btu/lb.) , there e x i s t s an opportunity f o r p l a s t i c waste as a f u e l . The current options for plastics, i n addition to r e c y c l i n g , are combustion ( i n c i n e r a t i o n ) and l a n d f i l l i n g , both o f which are unpopular. S u c c e s s f u l use o f p l a s t i c s as a feedstock f o r a process such as g a s i f i c a t i o n would help t o reduce l a n d f i l l i n g and i n c i n e r a t i o n and would provide a c l e a n source o f energy.

In Clean Energy from Waste and Coal; Khan, M.; ACS Symposium Series; American Chemical Society: Washington, DC, 1992.

12

CLEAN ENERGY FROM WASTE AND COAL

Other Wastes There are numerous waste m a t e r i a l s which can be considered as p o t e n t i a l c l e a n energy sources, i n a d d i t i o n t o the sludge, MSW and p l a s t i c s considered above. Tires, for example, are being d i s c a r d e d i n the United S t a t e s a t the r a t e of about one t i r e per person per year. T i r e rubber has an energy content of about 18,000 Btu per pound but has some environmental concerns from a combustion perspective.

Publication Date: December 23, 1992 | doi: 10.1021/bk-1992-0515.ch001

Concluding Remarks Each waste m a t e r i a l i s a problem r e q u i r i n g extensive study and assessment of sound technology. Nor are these problems mutually e x c l u s i v e : as ( f o r example) t i r e s and biomass are commonly encountered i n MSW. However, each major waste area has the p o t e n t i a l of becoming a wastet o - c l e a n energy process. Through the d e d i c a t i o n and e f f o r t s of recognized experts i n t h i s book and elsewhere, t e c h n o l o g i e s w i l l be developed t o produce c l e a n energy from wastes. L i t e r a t u r e Cited 1.

2. 3.

4. 5. 6.

7. 8.

"Economic Analysis of Sewage Sludge Disposal A l t e r n a t i v e s " , Proc. of National Conference on Municipal Sewage Sludge Treatment Plant Management, May, 1987. (This table was revised) Chow, V.T., Eliassen, R., and L i n s l e y , R.K., "Wastewater Engineering: Treatment Disposal Reuse", McGraw-Hill, 1979. "National Sewage Sludge Survey: A v a i l a b i l i t y of Information and Data, and Anticipated Impact on Proposed Regulations; Proposed Rule", Federal Register, Vol. 55, No. 218, 40 CFR Part 503, November 9, 1990. Zang, R.B. and Khan, M.R., " G a s i f i c a t i o n of Sewage Sludges", presented to New York Water P o l l u t i o n Control Association Annual Meeting, January, 1991. Makansi, J . , "Power from Sludge", Power, February, 1984. Attili, B., "Particle Size D i s t r i b u t i o n and Qualitative/Quantitative Analysis of Trace Metals i n the Combustion of Gas and F l y Ash of Coal/Refuse Derived Fuel", Ph.D. D i s s e r t a t i o n , U n i v e r s i t y of North Texas, Denton, Texas, December, 1991. H a s s e l r i i s , Floyd, Refuse Derived Fuel Processing, Butterworths Publishers, Boston, 1983. H i l l , R., Daugherty, K.E., Zhao, B. and Brooks, C., " C o f i r i n g of Refuse Derived Fuel i n a Cement K i l n " , International Symposium on Cement Industry Solutions to Waste Management, Calgary, Canada, October, 1992.

In Clean Energy from Waste and Coal; Khan, M.; ACS Symposium Series; American Chemical Society: Washington, DC, 1992.

1. KHAN & DAUGHERTY 9.

10. 11.

Publication Date: December 23, 1992 | doi: 10.1021/bk-1992-0515.ch001

12.

13. 14. 15. 16. 17. 18.

19. 20. 21. 22. 23.

Clean Energy from Waste: Introduction

13

Ohlsson, Ο., and Daugherty, K.E., "Results of Emissions i n F u l l Scale Co-Combustion Test of Binder Enhanced d-RDF P e l l e t s and High Sulfur Coal", presented at A i r and Waste Management Association Forum 90 i n Pittsburgh, Pennsylvania, 1990. Daugherty, K.E., Refuse Derived Fuel, Monthly reports to the US Department of Energy, University of North Texas, Denton, Texas, 1984-1986. Rice, F., Fortune, A p r i l 11, 1988, 177, pp. 96100. H i l l , R. and Daugherty K.E., "Binder Enhanced Refuse Derived Fuel - Possible Emission Reductions", accepted f o r publication by S o l i d Waste and Power, Kansas City, Missouri, publication slated f o r November/December, 1992. Reuter, Inc., Annual Report, 1991. Diegmueller, Κ., Insight, 1986, 12, pp. 16-17. Johnson, P., McGee, K.T., USA Today, August 26, 1988. Hecht, N., Design P r i n c i p l e s i n Resource Recovery Engineering, Ann Arbor Science, Butterworth Publishers; Boston, 1983, pp. 23-33. Daugherty, K.E., "RDF As An Alternative Source of Energy f o r Brick Kilns", Department of Energy, DOE/CS/24311-1, December, 1982. Gershman, Brickner, and Bratton, Inc., Small Scale Municipal S o l i d Waste Energy Recovery Systems, Van Nostrand Reinhold Company, New York, 1986, pp. 420. Personal communication between Dr. Kenneth Daugherty and Mr. John S c h i l l i n g , Assistant Plant Manager of EPR, Inc. on September 11, 1992. US Department of Energy, Energy Information Administration/Annual Energy Outlook 1991. Carpenter, Β., Windows, Spring 1988, pp. 8-10. A l t e r , Η., Material Recovery from Municipal S o l i d Waste, Marcel Dekker, Inc., New York, 1983, pp. 181-190. Daugherty, K.E., "An I d e n t i f i c a t i o n of Potential Binding Agents f o r Densified Fuel Preparation from Municipal S o l i d Waste, Phase 1, F i n a l Report", Argonne National Laboratory, Argonne, I l l i n o i s , 1988.

RECEIVED October 6, 1992

In Clean Energy from Waste and Coal; Khan, M.; ACS Symposium Series; American Chemical Society: Washington, DC, 1992.

Chapter 2

Efficient and Economical Energy Recovery from Waste by Cofiring with Coal Charles R. McGowin and Evan E. Hughes

Publication Date: December 23, 1992 | doi: 10.1021/bk-1992-0515.ch002

Electric Power Research Institute, 3412 Hillview Avenue, P.O. Box 10412, Palo Alto, CA 94303

Cofiring fuels derived from municipal and nonhazardous industrial wastes with coal in industrial and utility boilers is an efficient and cost-effectivemethod of energy recovery from wastes in many cases. Waste fuels such as scrap tires, tire-derived fuel, refuse-derived fuel, paper mill sludge, sewage sludge, sawdust, wood, and industrial waste can be cofired with coal in many stoker, pulverized coal, cyclone, and fluidized bed boilers with only minimal modifications and with minimal impacts on environmental emissions and plant safety. Waste cofiring with coal usually exhibits a higher waste-to-energy conversion efficiency than 100 percent waste firing in dedicated waste-to-energy plants, because coal-firedplants typically operate at higher steam pressures and temperatures and therefore higher steam-cycle and thermal efficiencies than dedicated plants. In addition, waste cofiring generally requires a much lower incremental capital investment than waste firing in a dedicated waste-to-energy facility. Both factors can contribute to a lower breakeven waste disposal cost or tipping fee for waste fuel cofiring with coal than for dedicated plants. This economic advantage should be highest for low-volume, low heating-value fuels, such as municipal solid waste and sewage sludge, and lowest for high-volume, higher quality fuels, such as scrap tires. In response to the environmental crisis in the U.S. created by the growing volume of municipal and industrial wastes and declining availability of landfill disposal sites, many urban communities are developing integrated waste management plans to both reduce the volume of the wastes sent to landfill and recover valuable raw materials and energy as steam and/or electricity. Most integrated waste management plans involve a combination of recycling, composting, waste-to-energy technology, and landfilling of residues, applied in sequence. Waste-to-energy plants will therefore be the last stop for the large portion of the waste stream that is not recyclable. The waste fuels burned in waste-to energy plants are derived from a variety of sources, including residential and commercial refuse, sewage sludge, automotive tires, urban demolition wastes, agricultural wastes, wood waste from forestry operations and lumber mills, paper mill sludge, and other industrial wastes. Most waste-to-energy

0097-6156/93/0515-0014$06.00/0 © 1993 American Chemical Society

In Clean Energy from Waste and Coal; Khan, M.; ACS Symposium Series; American Chemical Society: Washington, DC, 1992.

2. McGOWlN & HUGHES

Energy Recovery from Waste by Cofiring with Coal

facilities use dedicated waste-to-energy technology designed to efficiently recover energy as steam or electricity while controlling environmental emissions. In addition, several existing fuel combustion facilities have been retrofitted to burn waste fuels either alone or in combination with coal or oil, including industrial and utility boilers and cement kilns. This paper addresses the current and future generation of waste fuels in the U.S., waste fuel properties, alternate waste-to-energy technologies, and energy conversion efficiencies and costs of the alternate technologies. To represent the range of waste fuels available, the discussion focuses on scrap tires, refuse-derived fuel, wood waste, and sewage sludge.

Publication Date: December 23, 1992 | doi: 10.1021/bk-1992-0515.ch002

Annual Generation and Properties of Waste Fuels Of the 185 million tons of municipal wastes and 200 million waste tires generated each year, it is estimated that up to 25 percent could be recycled and reused, leaving about 75 percent for disposal in landfills and waste-to-energy facilities. The remaining 140 million tons of MSW contains the energy equivalent of 58 million tons of bituminous coal and would be sufficient to provide 11 thousand megawatts of generating capacity. The corresponding figures for waste tires are 1.9 million tons of coal and 640 M W of generating capacity. Currently, about 16 percent of the municipal solid waste stream and a small fraction of the waste tires are processed in waste-to-energy facilities, and these fractions are expected to grow significantly by the year 2000, perhaps to 40 percent. At the same time, annual waste generation can be expected to grow at about two percent per year (7). Typical fuel properties are presented in Table I for tire chips, refuse-derived fuel, wood waste, and sewage sludge (2,3). The rubber tire chips are made by shearing waste tires to one-inch top size and removing as much of the steel belt and bead material as possible by magnetic separation, producing a fuel containing 1.2 percent sulfur, 14.8 percent ash, and 12,500-14,500 Btu/lb. The refuse-derived fuel (RDF) is produced from municipal solid waste by shredding, screening, and magnetic separation and contains 0.2 percent sulfur, 12 percent ash, 24 percent moisture, and 5,900 Btu/lb. The wood waste is a mixture of chipped forest residue, bark, and milling waste, containing 0.02 percent sulfur, 0.7 percent ash, 39 percent moisture, and 5,140 Btu/lb. The municipal sewage sludge has been dewatered to 86.2 percent moisture content in a centrifuge and contains 6.8 percent ash. Due to its high moisture content, the high heating value is only 464 Btu/lb and the low heating value is negative (-484 Btu/lb). Table I. %S: % Ash: % Moisture: Btu/lb:

Coal and Waste Fuel Properties

WVCoal 0.85 16.04 6.60 11,680

Tire Chips 1.19 14.76 8.55 13,500

RDF 0.20 12.0 24.0 5,900

Wood Chips 0.02 0.74 39.10 5,139

Sewage Sludge 0.07 6.8 86.2 464

SOURCE: Adapted from ref. 2.

Due to their low sulfur contents, cofiring wood chips or RDF with coal would reduce emissions of sulfur dioxide (S02) relative to those for coal firing. However, as discussed below, the high moisture contents of wood chips and R D F lead to degradation of power plant efficiency and performance. Relative to heat content, tire chips and sewage sludge have moderate sulfur contents, and cofiring with coal can either increase or decrease S02 emissions, depending on the coal sulfur content.

In Clean Energy from Waste and Coal; Khan, M.; ACS Symposium Series; American Chemical Society: Washington, DC, 1992.

15

16

CLEAN ENERGY FROM WASTE AND COAL

Waste-to-Energy Technologies

Publication Date: December 23, 1992 | doi: 10.1021/bk-1992-0515.ch002

Several methods are available for energy recovery from waste fuels, including construction of dedicated waste-to-energy facilities and conversion of existing coalfired facilities to burn the waste fuels. Most dedicated waste-to-energy facilities use mass-burn, stoker firing, or fluidized bed combustion technology to burn the waste fuels (4,5). In mass burn plants (Figure 1), waste fuels are typically burned in a refractory lined furnace or on a sloping reciprocating grate without prior size reduction or processing to remove noncombustibles. Stoker-fired and fluidized bed plants typically require at least some size reduction to avoid plugging the fuel handling and injection equipment (4). In Japan, several fluidized bed units are burning industrial and municipal wastes without significant size reduction (4,6). Many existing facilities, including industrial boilers, cement kilns, and coal-fired power plants, can be converted to burn waste fuels either alone or in combination with other fuels such as coal. Figure 2 is a schematic of a utility boiler converted for R D F cofiring. Currently, RDF is cofired with coal at one cyclone and three pulverized coal power plants (7), cofired with wood at two fluidized bed plants (8,9), and fired alone at three converted utility stoker-fired plants (10). Whole tires are cofired with coal at one wet-bottom pulverized coal plant, and tire-derived fuel has been cofired with coal at four cyclone-fired plants and at three stoker-fired plants (77). In addition, one utility has cofired pulverized wood chips with coal (72). It should be noted, however, that cofiring alternate fuels is not always technically feasible and that the maximum heat input fraction is often limited by practical or economic considerations. Factors that need to be considered include the boiler type (pulverized coal, cyclone-fired, stoker-fired, or fluidized bed), operating and performance limitations on coal, and required fuel specifications. Pulverized coal boilers are designed to burn finely-ground coal in suspension. Most must be modified with a bottom dump grate to handle solid fuels such as RDF and tire and wood chips, although, as mentioned earlier, whole tires have been successfully tested in a slagging wet-bottom boiler without a dump grate (77). Cyclone-fired units burn the solid fuels in external cyclone burners, and because they remove most of the ash as a molten slag in the burners, a bottom dump grate is not required. It is unlikely that more than 20 percent of the heat input could be provided by alternate fuels in either case, due to adverse impacts on burner performance and heat release and absorption profiles in the boiler. Stoker-fired and fluidized bed combustion units would be generally more suited to cofiring alternate fuels at higher heat input fractions, subject to the operating and performance limitations discussed below. Some utility boilers may exhibit operating and performance limitations on the net generating capacity while firing coal (13,14). These include the boiler convection pass flue gas velocity, ID fan capacity, electrostatic precipitator performance, ash handling capacity, and boiler tube slagging and fouling. In this case, cofiring low grade fuels such as RDF, wood chips, or sewage sludge would likely result in a loss of net generating capacity, even at low heat input fractions. This is because cofiring increases flue gas and bottom ash volumes and worsens any existing slagging and fouling problems. These boilers would not be suitable for cofiring alternate fuels. The degree of fuel processing, which affects fuel particle size and ash, moisture, and heat contents, can also determine the feasibility of cofiring waste fuels. High ash, glass, and metal contents can lead to increased boiler slagging and fouling in pulverized coal boilers and plugging and jamming of moving grates in stoker-fired boilers (13).

In Clean Energy from Waste and Coal; Khan, M.; ACS Symposium Series; American Chemical Society: Washington, DC, 1992.

In Clean Energy from Waste and Coal; Khan, M.; ACS Symposium Series; American Chemical Society: Washington, DC, 1992.

Figure 1. Schematic of Mass Burn Waste-to-Energy Facility (Reproduced with permission from Ref. 77. Copyright 1992, Wheelabrator Environmental Systems.)

Publication Date: December 23, 1992 | doi: 10.1021/bk-1992-0515.ch002

In Clean Energy from Waste and Coal; Khan, M.; ACS Symposium Series; American Chemical Society: Washington, DC, 1992.

RDF

RECEIVING

RECLAIM,

Boiler

RDF FIRING

Figure 2. Schematic of Electric Utility RDF Cofiring System (Reproduced with permission from Ref. 14. Copyright 1988, Electric Power Research Institute.)

METERING A N D BOILER FEED

RDF S T O R A G E ,

Publication Date: December 23, 1992 | doi: 10.1021/bk-1992-0515.ch002

2. McGOWIN & HUGHES

Energy Recovery from Waste by Cofiring with Coal

Large particles can jam and plug fuel feeding and ash removal systems and cause poor combustion efficiency. Increased processing improves fuel quality and uniformity, and reduces the possibility of fuel handling and other problems.

Publication Date: December 23, 1992 | doi: 10.1021/bk-1992-0515.ch002

Environmental Emissions and Plant Safety Environmental emissions in air, water, and solid effluents from waste-to-energy facilities depend on both the chemical analysis of major components as well as the trace metal contents of the waste fuels and the emissions control equipment used by the facility. As shown in Table 1, most of the waste fuels have very low to moderate sulfur contents relative to coal and thus are not significant producers of sulfur dioxide during combustion. However, many of the wastes contain trace metals and can produce hazardous substances during combustion. For example municipal refuse is known to contain variable levels of lead, cadmium, and mercury, and chlorine derived from salt, plastics, and bleached paper can form polychlorinated dibenzo dioxins and furans under certain conditions involving poor fuel/air mixing and insufficient combustion air. Modern dedicated waste-to-energy and coal-fired power plants are typically designed to effectively control carbon monoxide, acid gas, trace metal, and organic emissions to levels below state and federal requirement emissions limits. Wet or dry scrubbers and fabric filters or electrostatic precipitators are used to control acid gas (S02 and HC1) and particulate/trace metal emissions. Nitrogen oxide emissions are typically controlled using low-NOx burners, combustion modification, or ammonia thermal de-NOx systems. Carbon monoxide and trace organic emissions are controlled by designing the boiler for "good combustion practice." Waste fuel cofiring, however, can either decrease, increase, or have little impact on emissions relative to those for 100 percent coal firing. For example, sulfur dioxide emissions typically decrease when cofiring low-sulfur fuels such as RDF and wood, but may increase or decrease when cofiring moderate-sulfur fuels such as tire chips, depending on the coal sulfur content. Nitrogen oxide emissions also decrease for most waste fuels due to low nitrogen contents relative to coal and to the impact of high fuel moisture contents on flame temperature and thermal NOx formation. Particulate emissions often increase slightly for units with electrostatic precipitators due to increased flue gas volumes and fly ash resistivity for many waste fuels. It is interesting to note that no dioxin and furan emissions have been detected during stack emissions testing of utility boilers cofiring coal and refuse-derived fuel. The primary safety concern with waste fuel cofiring is fire prevention in waste fuel, receiving, storage, and reclaim areas, particularly with highly combustible wastes such as scrap tires and dried refuse and wood. Overall Energy Efficiency and Costs To illustrate the overall energy efficiency and economics of waste energy recovery, simplified examples of waste fuel/coal cofired and dedicated waste-to-energy plants are presented below. These examples have been refined since they were first reported in 1991 (75). Four waste fuels are considered for each system: tire-derived fuel, RDF, wood, and sewage sludge for the retrofitted coal/waste cofired plants; and whole tires, RDF, wood, and municipal solid waste (MSW) for the dedicated plants. In addition to the coal and waste fuel properties listed in Table I, major assumptions include:

In Clean Energy from Waste and Coal; Khan, M.; ACS Symposium Series; American Chemical Society: Washington, DC, 1992.

19

Publication Date: December 23, 1992 | doi: 10.1021/bk-1992-0515.ch002

20

CLEAN ENERGY FROM WASTE AND COAL

•

Waste fuel/coal-cofired plants: 250 M W pulverized coal power plant - West Virginia bituminous coal - Steam conditions: 2400 psia/1000 deg F superheat/1000 deg F reheat - Wet limestone flue gas desulfurization and cold-side electrostatic precipitator Retrofit fuel receiving, storage, and pneumatic transport systems and dump grate installed above ash hopper in furnace - 20% heat input from TDF, RDF, and wood waste; 2% heat input from sewage sludge.

•

Dedicated waste-to-energy plants: Same annual waste fuel consumption rates as cofired plants Steam conditions: 900 psi/830 deg F.

•

Reference date of capital and annual cost estimates: December 1990.

•

Busbar energy cost includes fixed and variable O & M , coal, and capital charges (16.5%/yr fixed charge rate).

•

Waste fuels available at zero cost.

•

Zero credit for recovered byproducts such as aluminum and steel.

For waste fuel cofiring, the analysis is based in part on waste fuel cofiring performance and cost estimates reported previously (13). The boiler efficiencies, heat rates, thermal efficiencies, fuel consumption rates, and busbar energy costs were estimated using a revised version of the R D F C O A L RDF Cofiring Boiler Performance Model (14). R D F C O A L predicts the impact of RDF cofiring on boiler efficiency, net heat rate, unit derating, fuel consumption, flue gas volume and composition, and bottom and flyash production as functions of the fraction of fuel heat input provided by RDF. For the dedicated wood-fired plant case, the performance and cost data are based on reported estimates (16). For the whole-tire, RDF, and MSW-fired plant cases, the data were estimated using the EPRI Waste-to-Energy Screening Guide and software (5). The WTE Screening Guide presents performance and cost data for alternate waste-toenergy technologies, and the software was used to prepare the estimates for the RDF/stoker fired and MSW mass burn plant cases. The estimates for the whole tirefired dedicated plants were derived by scaling the MSW mass burn data, based on waste fuel throughput, flue gas volume, and gross M W generating capacity. Table II and Figures 3 to 6 compare the energy recovery inefficiencies, annual waste fuel consumption rates, and incremental capital and annual costs for the cofired and dedicated waste-to-energy plants. Boiler and Thermal Efficiencies. Energy conversion efficiency is measured by the fraction of energy in the fuel converted to steam and/or electricity, expressed as boiler efficiency and thermal efficiency, respectively. Figure 3 compares the predicted overall boiler efficiencies as functions of waste fuel heat input fractions for utility boilers cofiring three waste fuels with coal, including tirederived fuel, refuse-derived fuel and wood (13). As the waste fuel heat input fraction increases, the boiler efficiencies for RDF and wood cofiring decline significantly from 89 percent at zero percent heat input (100 percent coal firing) to 78 and 75 percent, respectively, at 100 percent heat input. This results primarily from the high moisture contents and higher excess air requirements of the waste fuels, which increase the dry

In Clean Energy from Waste and Coal; Khan, M.; ACS Symposium Series; American Chemical Society: Washington, DC, 1992.

2. McGOWIN & HUGHES

Table II.

Energy Recovery from Waste by Cofiring with Coal

Performance and Economic Comparison of Waste Fuel-Cofired and Dedicated Power Plants (December, 1990 Dollars) 250 MW Waste Fuel/Coal-Cofired Power Plant

Assumptions: West Virginia Bituminous Coal @ $1.60/MBtu, 65% Capacity Factor

Publication Date: December 23, 1992 | doi: 10.1021/bk-1992-0515.ch002

TireDerived Fuel Waste Fuel Performance: Heat Input 20% Tons/Day 434 Tons/Year 103,000 Thermal Efficiency 34.2% Net Btu/kWh 9,997 Incremental Total Capital Requirement: $/Ton/Day Waste 22,000 Breakeven Fuel Payment: $/MBtu $/Ton Waste

$0.03 $0.74

Breakeven Tipping Fee ($/Ton Waste): Processing Cost $20.00 Less Fuel Payment (0.74) B.E. Tipping Fee $19.26 Sensitivity Range ($2.50 to $0.50/MBtu Coal) $-4 to 48

RDF

Wood

Sewage Sludge

20% 1018 241,000 30.5% 11,175

20% 1177 279,000 29.6% 11,536

2% 1335 317,000 8.5% 40,140

9500

8200

900

-$0.17 -1.96 (RDF) -1.65 (MSW)

-$0.13 -1.37

-$4.78 -4.44

$40.00 (MSW) (-1.65) (MSW) $41.65 (MSW)

$10.00 (-1.37) $11.37

$0.00 (-4.44) $4.44

$34 to 51

$4 to 20

$6 to 2

Dedicated Waste Fuel-Fired Power Plant Assumptions: Electricity Sale @ $0.05/kWh, 80-85% Capacity Factor Waste Fuel Performance: Tons/Day Tons/Year Net Capacity (MW) Thermal Efficiency Net Btu/kWh Total Capital Requirement: $/Ton/Day Waste $/kW Net

Whole Tires

RDF

Wood

MSW

332 103,000 27.1 24.8% 13,800

827 241,000 25.4 21.4% 16,000

900 279,000 22.5 19.9% 17,100

927 288,000 22.0 20.2% 16,900

$258,000 $3200

$164,000 $5300

$54,000 $2150

$117,000 $4960

$151.90 (36.90) $115.00

$38.80 (30.00) $8.80

$102.90 (28.50) $74.40

$-21 to 21

$46 to 86

Breakeven Tipping Fee ($/Ton Waste): Processing Cost $205.40 Less Electricity Rev. (97.90) B.E. Tipping Fee $107.50 Sensitivity Range ($0.10 to $0.03/kWh) $9 to 147

$78 to 130

In Clean Energy from Waste and Coal; Khan, M.; ACS Symposium Series; American Chemical Society: Washington, DC, 1992.

21

22

CLEAN ENERGY FROM WASTE AND COAL

100%

Tire-C erived Fuel

Publication Date: December 23, 1992 | doi: 10.1021/bk-1992-0515.ch002

Refuse-Dt rived Fuel

W tx>d Waste 70%

0%

10%

20%

30%

40%

50%

60%

70%

80%

90%

100%

Fraction Heat Input from Waste Fuel

Figure 3.

Cofiring

Boiler Efficiency for Waste Fuel/Coal Cofiring

Dedicated

Tire-Derived Fuel

Figure 4.

Cofiring

Dedicated

Refuse-Derived Fuel

Cofiring

Dedicated

Wood Waste

Total Efficiency Loss: Waste Fuel to Electricity

In Clean Energy from Waste and Coal; Khan, M.; ACS Symposium Series; American Chemical Society: Washington, DC, 1992.

Publication Date: December 23, 1992 | doi: 10.1021/bk-1992-0515.ch002

2. McGOWIN & HUGHES

Cofiring

Energy Recovery from Waste by Cofiring with Coal

Dedicated

Tire-Derived Fuel

Cofiring

Dedicated

Cofiring

Dedicated

Wood Waste

Refuse-Derived Fuel

Figure 5. Total Capital Requirement for Waste Fuel Energy Recovery Incremental Impact of Waste Processing (1990 $)

250

T

Cofiring

Dedicated

Tire-Derived Fuel

Figure 6.

Cofiring

Dedicated

Cofiring

Refuse-Derived Fuel

Dedicated

Wood Waste

Total Waste Processing Cost

Coal @ $1.60/MBtu, Electricity @ $0.05/kWh, 1990 $

In Clean Energy from Waste and Coal; Khan, M.; ACS Symposium Series; American Chemical Society: Washington, DC, 1992.

23

24

CLEAN ENERGY FROM WASTE AND COAL

Publication Date: December 23, 1992 | doi: 10.1021/bk-1992-0515.ch002

gas and moisture loss components of the boiler efficiency loss. For tire-derived fuel which has low moisture and high heat contents like coal, the boiler efficiency declines slowly to 86 percent at 100 percent heat input from tires. Thus, waste fuel cofiring with coal can be expected to provide higher boiler and steam conversion efficiency than 100 percent waste fuel firing, which in turn contributes to higher overall electricity conversion efficiency in most cases, as discussed further below. The waste fuel thermal efficiency is a function of boiler efficiency, gross turbine heat rate, and auxiliary power consumption. Modern coal-fired power plants typically operate at higher steam conditions than dedicated waste-to-energy plants (e.g. 2400 psi/1000 F superheated steam with one reheat to 1000 F vs. 900 psi/830 F superheated steam) and have lower auxiliary power requirements (8% vs. 11%). As a result, the waste fuel thermal efficiency can be expected to be higher for the cofired plants than for the dedicated plants, which is confirmed by the data in Table Hand Figure 4. For tires, RDF, and wood waste, the waste fuel thermal efficiencies range between 30 and 34 percent for the cofired plants and 20 and 25 percent for the dedicated plants. Waste Fuel Consumption. Annual waste fuel consumption is proportional to capacity factor and inversely proportional to fuel heat content and thermal efficiency. The 250 M W coal plant operates at 65 percent capacity factor, and the waste fuel consumption ranges between 434 tons/day (103,000 tons/year) for tire-derived fuel and 1177 tons/day (279,000 tons/year) for wood waste, based on 20 percent heat input from the waste fuel. Sewage sludge is cofired at 2 percent of total heat input, and the annual sludge consumption is 1335 tons/day (317,000 tons/year). The dedicated waste-to-energy facilities are sized to consume the same annual quantities of waste fuels, while operating at 80 to 85 percent capacity factor. As shown in Table II, net generating capacities range between 22 and 27 MW, and waste fuel consumption varies between 332 tons/day (103,000 tons/year) for whole tires and 900 tons/day (279,000 tons/year) for waste wood. The MSW-fired mass burn plant consumes 927 tons/day (288,000 tons/year) of unprocessed MSW. Total Capital Requirement. Total capital requirement includes direct and indirect field erection costs, as well as the costs of engineering and home office services, project and process contingencies, escalation, interest during construction, preproduction and startup, inventory, and land. Table II and Figure 5 illustrate that total capital requirements for waste fuel energy conversion are significantly lower for the waste-fuel/coal cofired plants ($900 to $22,000/ton/day) than for the dedicated waste-to-energy plants ($54,000 to $258,000/ton/day). Breakeven Fuel Price. The breakeven fuel price for the waste fuel-cofired plants is the price that results in no change in the cost of power generation relative to the unconverted coal-fired plant. The breakeven fuel price is $0.74/ton for tire-derived fuel cofiring, and is negative for the other fuels (-1.37 to -4.44 $/ton), i.e., the utility is paid to take the fuel. This occurs, because the incremental capital and O & M charges exceed the coal savings for RDF and wood cofiring. Sewage sludge cofiring actually increases coal consumption due to its large negative impact on boiler efficiency, and the fuel credit therefore decreases with increasing coal price. Breakeven Tipping Fee. The breakeven tipping fee represents the charge for waste disposal required to balance total processing costs and total revenues derived from tipping fees and sale of recovered energy and byproducts. As shown in Table II and Figure 6, at $1.60/MBtu coal purchase price, the breakeven tipping fee for the waste fuel-cofired plants ranges between $11.40 and $41.65/ton waste. For the dedicated plants, at $0.05/kWh electricity sale price, the breakeven tipping fee ranges between $8.80 and $115.00/ton waste. Because these breakeven tipping fee estimates are quite

In Clean Energy from Waste and Coal; Khan, M.; ACS Symposium Series; American Chemical Society: Washington, DC, 1992.

2. McGOWIN & HUGHES

Energy Recovery from Waste by Cofiring with Coal

sensitive to the assumed coal and electricity prices, the tipping fee ranges were also estimated for coal prices between $0.50 and 2.50/MBtu and for electricity prices between $0.03 and $0.10/kWh (Table II). Even for the wide ranges of coal and electricity prices, the cofired plants offer generally lower breakeven tipping fees than the dedicated plants. Note that the breakeven tipping fee range in Table II is reversed for sewage sludge cofiring ($6 to 2/ton) due to the inverse relationship between the fuel credit and coal price described in the previous paragraph.

Publication Date: December 23, 1992 | doi: 10.1021/bk-1992-0515.ch002

Discussion Although the estimated breakeven tipping fees are lowest for waste fuel cofiring in utility boilers, there are several institutional and economic factors that create barriers to implementing such projects. Regulated utilities typically pass on fuel savings to the rate payer as part of the rate making process and thus do not share directly in the economic benefits of waste fuel cofiring. Thus there is little incentive for a utility to participate other than to provide a service to the community and reduce landfill requirements. Economic dispatch of the utility system may also limit the hours when the power plant is available to consume waste to the point that the plant must operate at a higher rate and incur an economic dispatch penalty in order to consume all of the waste. Other important factors include the uncertainties created by potential environmental emissions from the waste fuels, and the separate and sometimes conflicting interests of the utilities, waste haulers, and municipalities. Clearly, a mechanism needs to be developed to share the financial and other risks as well as the economic benefits of waste fuel cofiring among all participants. Conclusions •

Significant and growing quantities of alternate fuels are available for partial replacement of coal in steam and power generation.

•

Cofiring waste fuels with coal in retrofitted coal-fired power plants and other industrial boilers offers the potential for higher energy recovery efficiency and lower breakeven waste tipping fees than dedicated waste-to-energy plants.

•

Institutional constraints may limited cofiring of waste fuels in the future, unless the fuel supplier and user develop a mechanism to share the profits as well as the risks.

Literature Cited 1.

2. 3.

4.

5.

Hughes, Evan E., "An Overview of EPRI Research on Waste-to-Energy," Proceedings: 1989 Conference on Municipal Solid Waste as Utility Fuel, EPRI GS-6994, February 1991, pp. 1-9 to 1-34. Alternative Fuel Firing in an Atmospheric Fluidized-Bed Combustion Boiler, EPRI CS-4023, June 1985. Murphy, Michael L., "Fluidized Bed Combustion of Rubber Tire Chips: Demonstration of the Technical and Environmental Feasibility", presented at the IGT/CBETS Energy from Biomass and Wastes XI Conference, Lake Buena Vista, Florida, March 1987. Howe, W. C. and C. R. McGowin, "Fluidized Bed Combustion of Alternate Fuels: Pilot and Commercial Plant Experience," Proceedings: 1991 International Conference on Fluidized Bed Combustion, ASME, Montreal, Canada, April 1991, pp. 935-946. Waste-to-Energy Screening Guide, draft final report, EPRI Project 2190-5, January 1991.

In Clean Energy from Waste and Coal; Khan, M.; ACS Symposium Series; American Chemical Society: Washington, DC, 1992.

25

26 6. 7.

8.

9.

Publication Date: December 23, 1992 | doi: 10.1021/bk-1992-0515.ch002

10.

11. 12. 13. 14. 15.

16.

17.

CLEAN ENERGY F R O M WASTE AND

COAL

Makansi, Jason, "Ebara internally circulating fluidized bed (ICFB) technology", Power, January 1990, pp 75-76. McGowin, C. R., "Guidelines for Cofiring Refuse-Derived Fuel in Electric Utility Boilers", Proceedings: 1989 Conference on Municipal Solid Waste as a Utility Fuel, EPRI GS-6994, February 1991, p. 2-47 to 2-66. Zylkowski, Jerome R. and Rudy J. Schmidt, "Waste Fuel Firing in Atmospheric Fluidized Bed Retrofit Boilers," Proceedings: 1988 Seminar on Fluidized Bed Technology for Utility Applications, EPRI GS-6118, February 1989, p. 2-47 to 2-61. Coleville, Erik E. and Patrick D. McCarty, "Repowering of the Tacoma Steam Plant No. 2 with Fluidized Bed Combustors Fired on RDF, Wood, and Coal," presented at Power-Gen '88 Conference, Orlando, Florida, December 1988. Follett, R. E. and M. J. Fritsch, "Two Years of RDF Firing in Converted Stoker Boilers," Proceedings: 1989 Conference on Municipal Solid Waste as a Utility Fuel, EPRI GS-6994, February 1991, pp. 3-25 to 3-44. Proceedings: 1991 Conference on Waste Tires as a Utility Fuel, EPRI GS-7538, September 1991. "Hurricane Hugo Wood Chips Making Electricity," The Logger and Lumberman, May 1990. McGowin,C. R., "Alternate Fuel Cofiring in Utility Boilers", Conference Proceedings: Waste Tires as a Utility Fuel, EPRI GS-7538, September 1991. Guidelines for Cofiring Refuse-Derived Fuel in Electric Utility Boilers, Vol. 2: Engineering Evaluation Guidelines, EPRI CS-5754, June 1988. McGowin, C. R. and Ε. E. Hughes, "Coal and Waste Fuel Cofiring in Industrial and Utility Applications", Proceedings: Eighth Annual International Pittsburgh Coal Conference, Pittsburgh, PA, October 14-18, 1991, pp. 853-858, p. 93. Hollenbacher, Ralph, "Biomass Combustion Technologies in the United States", presented at the USDOE/EPRI/NREL Biomass Combustion Conference, Reno, Nevada, January 1992. "Wheelabrator Environmental Systems, Inc. Refuse-to-Energy System", Wheelabrator Environmental Systems, Hampton, NH, January, 1992.

RECEIVED August 14, 1992

In Clean Energy from Waste and Coal; Khan, M.; ACS Symposium Series; American Chemical Society: Washington, DC, 1992.

Chapter 3

Recovery of Ethanol from Municipal Solid Waste M. D. Ackerson, E. C. Clausen, and J. L. Gaddy

Publication Date: December 23, 1992 | doi: 10.1021/bk-1992-0515.ch003

Department of Chemical Engineering, University of Arkansas, Fayetteville, AR 72701

Methods for disposal of MSW that reduce the potential for groundwater or air pollution will be essential in the near future. Seventy percent of MSW consists of paper, food waste, yard waste, wood and textiles. These lignocellulosic components may be hydrolyzed to sugars with mineral acids, and the sugars may be subsequently fermented to ethanol or other industrial chemicals. This chapter presents data on the hydrolysis of the lignocellulosic fraction of MSW with concentrated HCl and the fermentation of the sugars to ethanol. Yields, kinetics, and rates are presented and discussed. Design and economic projections for a commercial facility to produce 20 MM gallons of ethanol per year are developed. Novel concepts to enhance the economics are discussed. The United States generates about 200 million tons of MSW annually, or about 4 pounds per capita per day (1). The average composition of MSW is given in Table I, and varies slightly with the season (2). This material has traditionally been disposed of in landfills. However, recent environmental concerns over ground water pollution, leaching into waterways, and even air pollution, as well as increasing costs, have resulted in this technology becoming unacceptable in most areas. Few new landfills are being approved, and the average remaining life of operating landfills is only about five years. Alternatives to landfilling include incineration, composting, anaerobic digestion, and recycling. Incineration can result in energy recovery as steam. However, concerns over hazardous components in exhaust gases and high capital and operating costs detract from this alternative. Large areas required for composting and the ultimate use or disposal of compost with high metals content makes this technology uncertain. Very slow reaction rates and large reactors for anaerobic digestion makes this technology uneconomical at present. 0097-6156/93/0515-0028$06.00/0 © 1993 American Chemical Society In Clean Energy from Waste and Coal; Khan, M.; ACS Symposium Series; American Chemical Society: Washington, DC, 1992.

3.

Recovery of Ethanol from Municipal Solid Waste

ACKERSON ET AL. Table I .

Publication Date: December 23, 1992 | doi: 10.1021/bk-1992-0515.ch003

Category Paper Y a r d Waste Glass Metal Wood Textiles L e a t h e r & Rubber Plastics Miscellaneous

M u n i c i p a l S o l i d Waste C o m p o s i t i o n (Weight P e r c e n t as D i s c a r d e d )

F*W»r

Fall

Winter

Spring

31..0 27..1 17..7 7..5 7..0 2..6 1..8 1..1 3..1

38..9 6..2 22..7 9..6 9..1 3..4 2..5 1..4 4..0

42.2 0.4 24.1 10.2 9.7 3.6 2.7 1.5 4.2

36.5 14.4 20.8 8.8 8.2 3.1 2.2 1.2 3.7

Average

37.4 13.9 20.0 9.8 8.4 3.1 2.2 1.2 3.4

R e c y c l i n g o f g l a s s , m e t a l s , p l a s t i c s , and paper reduces the q u a n t i t y o f m a t e r i a l t o be l a n d f i l l e d by about 60 p e r c e n t , as seen from T a b l e I . Most s t a t e s have d e c i d e d t h a t r e c y c l i n g o f f e r s the b e s t s o l u t i o n t o the e n v i r o n m e n t a l concerns a s s o c i a t e d w i t h s o l i d waste d i s p o s a l and many have implemented r e g u l a t i o n s f o r c u r b s i d e s e g r e g a t i o n o f r e c y c l a b l e components. Markets f o r r e c y c l e d aluminum and s t e e l are w e l l e s t a b l i s h e d , however, markets f o r r e c y c l e d paper, g l a s s , and p l a s t i c s a r e n o t w e l l developed. Low p r i c e s ( n e g a t i v e i n some areas f o r p a p e r ) w i l l impede the application of recycling. Production. The U n i t e d S t a t e s c u r r e n t l y i m p o r t s about h a l f o f i t s crude o i l and must produce another 120 b i l l i o n g a l l o n s o f l i q u i d f u e l s a n n u a l l y t o become energy s e l f s u f f i c i e n t . E t h a n o l can be produced from l i g n o c e l l u l o s i c m a t t e r , l i k e paper, by h y d r o l y s i s o f the p o l y s a c c h a r i d e s t o s u g a r s , w h i c h can be fermented i n t o e t h a n o l . T h i s t e c h n o l o g y would e n a b l e the use o f the e n t i r e c a r b o h y d r a t e f r a c t i o n o f MSW (paper, y a r d and f o o d waste, wood and t e x t i l e s ) , w h i c h c o n s t i t u t e s 75 p e r c e n t o f the t o t a l , i n t o a u s e f u l and v a l u a b l e p r o d u c t . E t h a n o l can be b l e n d e d w i t h g a s o l i n e and, c u r r e n t l y , n e a r l y one b i l l i o n g a l l o n s o f e t h a n o l , p r i m a r i l y made from c o r n , a r e used as a t r a n s p o r t a t i o n f u e l i n t h i s c o u n t r y . The p o t e n t i a l market ( a t 10 p e r c e n t a l c o h o l ) i s 10 b i l l i o n g a l l o n s per y e a r . B l e n d i n g o f e t h a n o l w i t h g a s o l i n e reduces e m i s s i o n s and i n c r e a s e s the octane r a t i n g . Some s t a t e s , l i k e C a l i f o r n i a and Colorado where a i r q u a l i t y has degraded s e r i o u s l y i n m e t r o p o l i t a n a r e a s , a r e mandating the use o f alcohol fuels. The purpose o f t h i s paper i s t o d e s c r i b e a p r o c e s s f o r c o n v e r t i n g the l i g n o c e l l u l o s i c f r a c t i o n o f MSW i n t o e t h a n o l . The r e s i d u e i s c o n t a c t e d w i t h c o n c e n t r a t e d m i n e r a l a c i d a t room temperature t o g i v e t h e o r e t i c a l y i e l d s o f monomeric s u g a r s , w h i c h a r e r e a d i l y fermented i n t o e t h a n o l . Procedures t o g i v e h i g h sugar c o n c e n t r a t i o n s a r e d e s c r i b e d . Data f o r f e r m e n t a t i o n i n i m m o b i l i z e d c e l l columns i n a few minutes a r e p r e s e n t e d . The economics o f t h i s p r o c e s s i s then developed and key economic parameters i d e n t i f i e d . Alcohol

In Clean Energy from Waste and Coal; Khan, M.; ACS Symposium Series; American Chemical Society: Washington, DC, 1992.

29

30

CLEAN ENERGY FROM WASTE AND COAL

Publication Date: December 23, 1992 | doi: 10.1021/bk-1992-0515.ch003

HYDROLYSIS/ETHANOL PRODUCTION The h y d r o l y s i s o f biomass t o sugars and f e r m e n t a t i o n o f g l u c o s e t o e t h a n o l a r e t e c h n o l o g i e s t h a t have been commercial around the w o r l d f o r many y e a r s . The U. S. produced up t o 600 m i l l i o n g a l l o n s o f e t h a n o l p e r y e a r b y f e r m e n t a t i o n d u r i n g W o r l d War I I . A l s o , the Germans produced f u e l e t h a n o l from wood b y h y d r o l y s i s and f e r m e n t a t i o n d u r i n g W o r l d War I I . Today, B r a z i l produces most o f i t s l i q u i d f u e l from sugar cane. I t has been known f o r n e a r l y two c e n t u r i e s t h a t c e l l u l o s e c o u l d be c o n v e r t e d i n t o sugars by the a c t i o n o f m i n e r a l a c i d s ( 3 ) . The p r o c e s s became commercial e a r l y i n t h i s c e n t u r y w i t h d i l u t e a c i d p l a n t s b u i l t i n Georgetown, South C a r o l i n a , and F u l l e r t o n , L o u i s i a n a t o produce 2-3 m i l l i o n g a l l o n s o f e t h a n o l p e r y e a r from wood ( 4 ) . These p l a n t s o p e r a t e d through the end o f W o r l d War I when wood sugars c o u l d not complete w i t h cheap b y - p r o d u c t molasses from cane. D u r i n g W o r l d War I I , the Germans developed a p e r c o l a t i o n p r o c e s s and b u i l t 20 p l a n t s f o r the p r o d u c t i o n o f f u e l a l c o h o l from wood ( 5 ) . S i m i l a r p l a n t s were a l s o b u i l t i n S w i t z e r l a n d , Sweden, C h i n a , R u s s i a , and K o r e a . I n attempts t o produce e t h a n o l f o r b u t a d i e n e rubber p r o d u c t i o n , the U n i t e d S t a t e s b u i l t a 4 m i l l i o n g a l l o n p e r y e a r wood h y d r o l y s i s t o e t h a n o l f a c i l i t y i n S p r i n g f i e l d , Oregon i n 1944. The Germans a l s o o p e r a t e d p l a n t s based upon the B e r g i u s c o n c e n t r a t e d h y d r o c h l o r i c a c i d t e c h n o l o g y a t Mannheim and Regensburg d u r i n g World War I I ( 6 ) . Concentrated s u l f u r i c a c i d p l a n t s were a l s o o p e r a t e d i n I t a l y ( G i o r d a n i Leone) and J a p a n (Hokkaido) ( 7 ) . Most o f these f a c i l i t i e s were c l o s e d a f t e r W o r l d War I I w i t h the development o f p r o c e s s e s t o produce e t h a n o l from p e t r o l e u m . However, about 40 p e r c o l a t i o n p l a n t s a r e s t i l l o p e r a t e d i n R u s s i a today. Hydrolysis Technology. Biomass m a t e r i a l s a r e c o m p r i s e d o f t h r e e major components: h e m i c e l l u l o s e , c e l l u l o s e , and l i g n i n . The c o m p o s i t i o n o f v a r i o u s biomass m a t e r i a l s i s shown i n T a b l e I I . As n o t e d , most o f these m a t e r i a l s c o n t a i n 50-70 p e r c e n t c a r b o h y d r a t e ( h e m i c e l l u l o s e and c e l l u l o s e ) . These p o l y s a c c h a r i d e s can be h y d r o l y z e d t o monomeric s u g a r s , w h i c h can be c o n v e r t e d b y microorganisms i n t o f u e l s o r chemicals. The l i g n i n cannot be h y d r o l y z e d , b u t has a h i g h h e a t i n g v a l u e and can be u s e d as a source o f f u e l . From T a b l e I I , most o f the MSW biomass i s cellulose. Table I I .

Material Tanbark Oak Com Stover Red C l o v e r Hay Bagasse Oat H u l l s Newspaper P r o c e s s e d MSW

The C o m p o s i t i o n

o f S e l e c t e d Biomass M a t e r i a l s

P e r c e n t Dry Weight o f M a t e r i a l Lignin Cellulose Hemicellulose 19.6 28.1 20.6 20.4 20.5 16.0 25.0

44.8 36.5 36.7 41.3 33.7 61.0 47.0

24.8 10.4 15.1 14.9 13.5 21.0 12.0

In Clean Energy from Waste and Coal; Khan, M.; ACS Symposium Series; American Chemical Society: Washington, DC, 1992.

Publication Date: December 23, 1992 | doi: 10.1021/bk-1992-0515.ch003

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ACKERSON ET AL.

Recovery of Ethanol from Municipal Solid Waste 31

The c a r b o h y d r a t e h y d r o l y s i s can be c a r r i e d o u t by c o n t a c t w i t h c e l l u l a s e o r x y l a n a s e enzymes, o r by t r e a t m e n t w i t h m i n e r a l a c i d s . Enzymatic h y d r o l y s i s has the advantage o f o p e r a t i n g a t m i l d c o n d i t i o n s and p r o d u c i n g a h i g h - q u a l i t y sugar p r o d u c t . However, the e n z y m a t i c r e a c t i o n s a r e q u i t e s l o w (30 hour r e t e n t i o n t i m e ) , and the biomass must be p r e t r e a t e d w i t h c a u s t i c o r a c i d t o improve t h e y i e l d s and k i n e t i c s . The expense o f p r e t r e a t m e n t and enzyme p r o d u c t i o n , and the l a r g e r e a c t o r s r e q u i r e d make t h i s an uneconomical a l t e r n a t i v e . A c i d h y d r o l y s i s i s a much more r a p i d r e a c t i o n and v a r i o u s c o m b i n a t i o n s o f temperature and a c i d c o n c e n t r a t i o n may be used. Two methods o f a c i d h y d r o l y s i s have been s t u d i e d and d e v e l o p e d : a h i g h t e m p e r a t u r e , d i l u t e a c i d p r o c e s s (8,9) and a low t e m p e r a t u r e , c o n c e n t r a t e d a c i d p r o c e s s (10,11). F o r example, complete c o n v e r s i o n o f the h e m i c e l l u l o s e and c e l l u l o s e i n c o r n s t o v e r i n t o monomeric s u g a r s and sugar d e g r a d a t i o n p r o d u c t s r e q u i r e s m i n e r a l a c i d c o n c e n t r a t i o n s o f 2N a t temperatures o f 100-200 C ( 1 2 ) . However, a c i d c o n c e n t r a t i o n s o f 10-14N y i e l d complete c o n v e r s i o n s a t room temperature (30°C). A t h i g h t e m p e r a t u r e s , x y l o s e degrades t o f u r f u r a l and g l u c o s e degrades t o 5-hydroxymethyl f u r f u r a l (HMF), w h i c h a r e b o t h t o x i c to m i c r o o r g a n i s m s . Y i e l d s from d i l u t e a c i d p r o c e s s e s a r e t y p i c a l l y o n l y 50-60 p e r c e n t o f t h e o r e t i c a l because o f sugar l o s s e s by d e g r a d a t i o n and r e v e r s e p o l y m e r i z a t i o n a t h i g h t e m p e r a t u r e s . A l s o , equipment c o r r o s i o n a t h i g h t e m p e r a t u r e s i s a s e r i o u s problem. Work i n our l a b o r a t o r i e s has f o c u s e d a t t e n t i o n on c o n c e n t r a t e d a c i d p r o c e s s e s which produce t h e o r e t i c a l y i e l d s a t low t e m p e r a t u r e s . However, s i n c e h i g h a c i d c o n c e n t r a t i o n s a r e used, a c i d r e c o v e r y i s an economic n e c e s s i t y ( 1 0 ) . S t u d i e s i n our l a b o r a t o r i e s have r e s u l t e d i n b o t h s i n g l e s t e p and two-step h y d r o l y s i s p r o c e s s e s , u s i n g c o n c e n t r a t e d m i n e r a l a c i d s , w h i c h r e s u l t i n n e a r l y 100 p e r c e n t y i e l d s o f sugars from h e m i c e l l u l o s e and c e l l u l o s e . The r e a c t i o n s a r e conducted a t room temperature, without s i g n i f i c a n t degradation or r e v e r s e p o l y m e r i z a t i o n (11-13). An a c i d r e c o v e r y p r o c e s s has been d e v e l o p e d and t e s t e d , y i e l d i n g an energy e f f i c i e n t method o f s e p a r a t i n g sugar and a c i d ( 1 4 ) . The r e s u l t i n g sugar s o l u t i o n has been s u c c e s s f u l l y fermented t o e t h a n o l and o t h e r c h e m i c a l s w i t h o u t pretreatment (15). e

Process Description. F i g u r e 1 shows the proposed p r o c e s s f o r t h e a c i d h y d r o l y s i s o f MSW c o n s i s t i n g s i m p l y o f a mixed r e a c t o r where a c i d and MSW a r e c o n t a c t e d a t a c o n s t a n t temperature. The u n c o n v e r t e d s o l i d s ( l i g n i n and ash) a r e s e p a r a t e d by f i l t r a t i o n , washed, and used as f u e l . A c i d and sugars a r e s e p a r a t e d and the a c i d r e t u r n e d t o the r e a c t o r . I f d e s i r a b l e t o s e p a r a t e the s u g a r s , the h e m i c e l l u l o s e , w h i c h degrades a t m i l d e r c o n d i t i o n s , may be f i r s t h y d r o l y z e d t o produce a m i x t u r e o f f i v e and s i x c a r b o n s u g a r s . The s o l i d s from t h i s f i r s t s t a g e r e a c t o r are s e p a r a t e d and c o n t a c t e d w i t h a c i d i n a second s t e p t o h y d r o l y z e the c e l l u l o s e . Only s i x c a r b o n s u g a r s are o b t a i n e d from c e l l u l o s e i n t h i s second s t a g e . T h i s two s t e p h y d r o l y s i s g i v e s two streams; a x y l o s e r i c h p r e h y d r o l y z a t e and a g l u c o s e r i c h h y d r o l y z a t e ; and may be used where sugar s e p a r a t i o n

In Clean Energy from Waste and Coal; Khan, M.; ACS Symposium Series; American Chemical Society: Washington, DC, 1992.

Publication Date: December 23, 1992 | doi: 10.1021/bk-1992-0515.ch003

CLEAN ENERGY FROM WASTE AND COAL

MSW

F i g u r e 1. Schematic o f A c i d H y d r o l y s i s . (Reproduced w i t h p e r m i s s i o n from Energy from Biomass and Waste XV, I n s t i t u t e of Gas Technology, 1991. C o p y r i g h t 1991 M i c h a e l D. Ackerson.)

In Clean Energy from Waste and Coal; Khan, M.; ACS Symposium Series; American Chemical Society: Washington, DC, 1992.

3.

ACKERSON ET AL.

Recovery of Ethanol from Municipal Solid Waste 33

Publication Date: December 23, 1992 | doi: 10.1021/bk-1992-0515.ch003

i s d e s i r a b l e . I n the u s u a l case, as w i t h MSW, s t e p p r o c e s s w i l l be p r e f e r r e d .

the s i m p l e r s i n g l e

Hydrolysis Conditions. The two major f a c t o r s w h i c h c o n t r o l the h y d r o l y s i s r e a c t i o n s are temperature and a c i d c o n c e n t r a t i o n . S t u d i e s i n our l a b o r a t o r i e s have been made t o d e f i n e the a p p r o p r i a t e c o n d i t i o n s t o maximize r e a c t i o n r a t e s and y i e l d s . Sugar d e g r a d a t i o n i s promoted more a t h i g h temperature t h a n a t h i g h a c i d c o n c e n t r a t i o n . A l s o , f a s t r a t e s of h y d r o l y s i s are a c h i e v e d a t a c i d c o n c e n t r a t i o n s e x c e e d i n g 12N. T h e r e f o r e , the b e s t c o n d i t i o n s are a h i g h a c i d c o n c e n t r a t i o n (80 p e r c e n t H2SO4 o r 41 p e r c e n t HC1) and a m i l d temperature (~40°C). The sugar c o n c e n t r a t i o n s and y i e l d s from a t y p i c a l h y d r o l y s i s o f MSW from our l a b o r a t o r i e s are g i v e n i n T a b l e I I I (11). The p r e h y d r o l y s i s s t e p y i e l d s 8 p e r c e n t o f the i n i t i a l MSW as x y l o s e . The combined y i e l d o f g l u c o s e i s 60 p e r c e n t . These y i e l d s r e p r e s e n t n e a r l y complete c o n v e r s i o n o f h e m i c e l l u l o s e and c e l l u l o s e to sugars. However, v e r y d i l u t e (3-7 p e r c e n t ) sugar s o l u t i o n s r e s u l t from these r e a c t i o n s . Feedstock Preparation. I n o r d e r t o speed up the h y d r o l y s i s r e a c t i o n s , the s i z e o f the biomass p a r t i c l e s must be r e d u c e d t o i n c r e a s e the a c c e s s i b i l i t y t o the p o l y m e r i c s t r u c t u r e . A h i g h solids concentration i s desirable since this concentration c o n t r o l s the sugar c o n c e n t r a t i o n and the s i z e o f the h y d r o l y s i s and f e r m e n t a t i o n equipment. The s i z e o f the p a r t i c l e s a l s o a f f e c t s the f l u i d i t y o f the s o l i d s / a c i d s l u r r y . I t i s desirable to m a i n t a i n f l u i d i t y o f the s l u r r y t o promote mass t r a n s f e r and t o f a c i l i t a t e pumping and m i x i n g . T h e r e f o r e , the p a r t i c l e s i z e i s an i m p o r t a n t v a r i a b l e i n the biomass c o n v e r s i o n p r o c e s s . T a b l e I I I . MSW

Acid

Hydrolyzates

Concentration g/L

Prehydrolyzate Xylose Glucose Hydrolyzate Xylose Glucose Combined Xylose Glucose

Yield g/100q

9.5 18.5

8.0 16.0

0.0 67.8

0.0 44.0 8.0 60.0

T a b l e IV g i v e s the maximum s o l i d s c o n c e n t r a t i o n t o m a i n t a i n f l u i d i t y o f the s l u r r y , as a f u n c t i o n o f p a r t i c l e s i z e . A maximum c o n c e n t r a t i o n o f about 10 p e r c e n t i s p o s s i b l e w i t h p a r t i c l e s i z e s l e s s t h a n 40 mesh. G r i n d i n g t o 20 mesh g i v e s a p a r t i c l e s i z e d i s t r i b u t i o n i n w h i c h 90 p e r c e n t o f the m a t e r i a l i s l e s s t h a n 40 mesh. T h e r e f o r e , g r i n d i n g biomass t o pass 20 mesh g i v e s the a p p r o p r i a t e s i z e and produces the maximum p o s s i b l e s l u r r y c o n c e n t r a t i o n . A l s o , g r i n d i n g t o s m a l l e r s i z e s does n o t improve the r e a c t i o n r a t e .

In Clean Energy from Waste and Coal; Khan, M.; ACS Symposium Series; American Chemical Society: Washington, DC, 1992.

34

CLEAN ENERGY F R O M WASTE AND COAL T a b l e IV.

Maximum S o l i d s C o n c e n t r a t i o n f o r F l u i d S l u r r y Mesh Range ( S i e v e Nos.)

Publication Date: December 23, 1992 | doi: 10.1021/bk-1992-0515.ch003

0 1 12 - 20 20 - 30 30 - 40 40 - 45 45 - 70 70 - 100 100+

Solids Concentration vt. % 4.6 4.6 8.4 8.2 10.2 10.4 10.6 10.8

Sugar Decomposition. The f e r m e n t a b i l i t y o f t h e sugars i s dependent upon t h e sugar d e c o m p o s i t i o n t h a t o c c u r s d u r i n g h y d r o l y s i s . X y l o s e decomposes t o f u r f u r a l and hexoses decompose to HMF, w h i c h a r e b o t h t o x i c t o y e a s t . T o l e r a n c e c a n o f t e n be d e v e l o p e d , and t o x i c i t y i s d i f f i c u l t t o d e f i n e . However, t h e t o x i c l i m i t o f f u r f u r a l on a l c o h o l y e a s t i s r e p o r t e d t o be 0.03 t o 0.046 p e r c e n t ( 1 6 ) . HMF i s r e p o r t e d t o i n h i b i t y e a s t growth a t 0.5 p e r c e n t , and a l c o h o l p r o d u c t i o n i s i n h i b i t e d a t 0.2 p e r c e n t (17). The r a t e o f d e c o m p o s i t i o n o f x y l o s e t o f u r f u r a l and hexoses t o HMF have been s t u d i e d a t v a r y i n g sugar c o n c e n t r a t i o n s . U s i n g t h e method o f i n i t i a l r a t e s , these r e a c t i o n s were found t o be f i r s t o r d e r . The r a t i o o f r a t e c o n s t a n t s f o r d e c o m p o s i t i o n t o f o r m a t i o n a r e g i v e n i n T a b l e V. These r a t i o s a r e s m a l l , and subsequent c a l c u l a t i o n s and experiments show t h a t t h e r a t e o f HMF appearance i s i n s i g n i f i c a n t . However, t h e r a t e o f f u r f u r a l appearance c o u l d reach t o x i c l i m i t s , e s p e c i a l l y i f a c i d r e c y c l e i s u t i l i z e d . T a b l e V.

R a t i o o f F i r s t Order Rate C o n s t a n t s f o r Sugar D e c o m p o s i t i o n t o F o r m a t i o n Under Prehydrolysis Conditions

Acid Concentration

Rate o f Formation/Decomposition

Glucose

2N 3N 4N

0.0053 0.0090 0.0074

Xylose

2N 3N 4N

0.0257 0.0402 0.0374

Sugar

Hydrolyzate Fermentâtion/Ethanol Production G l u c o s e may be fermented t o e t h a n o l e f f i c i e n t l y by t h e y e a s t Saccharomyces cerevisiae, o r the bacterium

In Clean Energy from Waste and Coal; Khan, M.; ACS Symposium Series; American Chemical Society: Washington, DC, 1992.

3. ACKERSON ET AL.

Recovery of Ethanol from Municipal Solid Waste 35

Publication Date: December 23, 1992 | doi: 10.1021/bk-1992-0515.ch003

Zymomonas mob il is ( 1 8 ) . B a t c h f e r m e n t a t i o n experiments were c a r r i e d o u t t o compare t h e p r o d u c t i o n r a t e s o f e t h a n o l from h y d r o l y z a t e s and s y n t h e t i c g l u c o s e . Saccharomyces cerevisiae (ATCC 24860) was used i n t h e s t u d y . As shown i n F i g u r e 2, i d e n t i c a l r e s u l t s were found when f e r m e n t i n g s y n t h e t i c g l u c o s e and h y d r o l y z a t e . E t h a n o l y i e l d s were a l s o n e a r l y i d e n t i c a l . As n o t e d i n T a b l e V I , t h e f e r m e n t a t i o n proceeded w e l l i n t h e presence o f a s m a l l amount (0.25 p e r c e n t ) y e a s t e x t r a c t , w h i c h c a n be o b t a i n e d by r e c y c l e . A l m o s t t o t a l c o n v e r s i o n o f sugars i s o b t a i n e d i n o n l y 16 h o u r s . The c o n c e n t r a t i o n s o f f u r f u r a l and HMF i n t h e h y d r o l y z a t e s were found t o be n e g l i g i b l e . These low l e v e l s o f b y p r o d u c t s a r e b e l i e v e d t o be t h e major r e a s o n f o r t h i s h i g h l y e f f i c i e n t fermentation. Table V I .

Hydrolyzate Fermentation to Ethanol P e r c e n t Sugar U t i l i z a t i o n Hydrolyzate

W i t h V i t a m i n s and W i t h Y e a s t E x t r a c t Fermentation Time (hrs)

Amino Acids

NH3PO3)

Amino A c i d s and NH3(P03)

Yeast Extract

16

15.9

21.9

27.3

97.5

23

19.3

24.9

35.8

97.5

X y l o s e f e r m e n t a t i o n i s much more d i f f i c u l t , and t h e x y l o s e might be used as a source o f energy f o r g e n e r a t i n g steam and power. However, f u t u r e p o s s i b i l i t i e s f o r x y l o s e f e r m e n t a t i o n w i l l improve t h e economics. Recent work w i t h Pachysolen tannophilus shows promise f o r x y l o s e c o n v e r s i o n t o e t h a n o l (19) b u t , a t p r e s e n t , t h i s t e c h n o l o g y i s n o t f u l l y developed. E t h a n o l may a l s o be produced by c o n v e r t i n g x y l o s e t o x y l u l o s e , f o l l o w e d by fermentation w i t h yeast (20). Continuous F e r m e n t a t i o n . The s t a n d a r d t e c h n o l o g y f o r f e r m e n t i n g sugars t o e t h a n o l i s i n b a t c h v e s s e l s . B a t c h f e r m e n t a t i o n i s u s e d so t h a t c o n t a m i n a t i o n and m u t a t i o n c a n be c o n t r o l l e d . S t e r i l i z a t i o n between b a t c h e s and t h e use o f a f r e s h inoculum i n s u r e e f f i c i e n t f e r m e n t a t i o n . However, most b a t c h a l c o h o l f e r m e n t a t i o n s a r e d e s i g n e d f o r t h i r t y h o u r ( o r more) r e a c t i o n t i m e , w h i c h r e s u l t s i n v e r y l a r g e and e x p e n s i v e r e a c t o r s . The r e a c t o r s i z e c a n be reduced s u b s t a n t i a l l y by u s i n g c o n t i n u o u s f l o w f e r m e n t e r s . When f e r m e n t i n g a c i d h y d r o l y z a t e s , the problems w i t h m a i n t a i n i n g s t e r i l e c o n d i t i o n s a r e s u b s t a n t i a l l y reduced, s i n c e t h e s u b s t r a t e has been s t e r i l i z e d by c o n t a c t w i t h the a c i d . T h e r e f o r e , t h e use o f c o n t i n u o u s f e r m e n t a t i o n i s a n a t u r a l a p p l i c a t i o n f o r p r o d u c i n g a l c o h o l from MSW h y d r o l y z a t e s . A number o f c o n t i n u o u s f e r m e n t a t i o n schemes have been s t u d i e d , i n c l u d i n g t h e CSTR ( 2 1 ) , c e l l r e c y c l e r e a c t o r ( 2 2 ) , f l a s h f e r m e n t a t i o n (23) and i m m o b i l i z e d c e l l r e a c t o r s (24,25). I m m o b i l i z e d c e l l r e a c t o r s (ICR) show p o t e n t i a l i n s u b s t a n t i a l l y

In Clean Energy from Waste and Coal; Khan, M.; ACS Symposium Series; American Chemical Society: Washington, DC, 1992.

Publication Date: December 23, 1992 | doi: 10.1021/bk-1992-0515.ch003

36

CLEAN ENERGY FROM WASTE AND COAL

d e c r e a s i n g r e a c t o r s i z e and d e c r e a s i n g s u b s t r a t e and p r o d u c t i n h i b i t i o n (25-28). R e a c t i o n r a t e s f o r e t h a n o l p r o d u c t i o n i n a n i m m o b i l i z e d c e l l r e a c t o r a r e as h i g h as 10 times the v a l u e s o b t a i n e d i n a s t i r r e d tank r e a c t o r ( 2 4 ) . A wide v a r i e t y o f i m m o b i l i z a t i o n t e c h n i q u e s have been employed, i n c l u d i n g c r o s s l i n k i n g , entrapment, and c o v a l e n t b o n d i n g ( 2 5 ) . Data a r e g i v e n i n F i g u r e 3 f o r l a b o r a t o r y columns w i t h i m m o b i l i z e d S. cerevisiae. The g l u c o s e p r o f i l e s a r e g i v e n f o r i n i t i a l sugar c o n c e n t r a t i o n s from 50-200 g/L. As n o t e d , 90 p e r c e n t c o n v e r s i o n i s a c h i e v e d i n one hour o r l e s s . P r o d u c t i v i t i e s t o a c h i e v e 99 p e r c e n t c o n v e r s i o n were about 40 g/Lh r , o r about an o r d e r o f magnitude g r e a t e r t h a n the CSTR and 60 times more than the b a t c h r e a c t o r . Furthermore, a l c o h o l i n h i b i t i o n and t o x i c i t y t o e i t h e r i n h i b i t o r s i s reduced i n the ICR. The volume o f the ICR f o r MSW h y d r o l y z a t e f e r m e n t a t i o n i s about 5 p e r c e n t t h a t o f the b a t c h fermenter and s u b s t a n t i a l c a p i t a l savings r e s u l t . I n c r e a s i n g the Sugar C o n c e n t r a t i o n Perhaps the s i n g l e most i m p o r t a n t f a c t o r i n the economics o f t h i s p r o c e s s i s the sugar c o n c e n t r a t i o n t h a t r e s u l t s from a c i d h y d r o l y s i s . D i l u t e c o n c e n t r a t i o n s i n c r e a s e b o t h the equipment s i z e and the energy r e q u i r e d f o r p u r i f i c a t i o n . Methods t o i n c r e a s e the sugar and e t h a n o l c o n c e n t r a t i o n s have been developed. S o l i d s C o n c e n t r a t i o n . The u l t i m a t e sugar and a l c o h o l c o n c e n t r a t i o n s a r e d i r e c t f u n c t i o n s o f the i n i t i a l s o l i d s c o n c e n t r a t i o n i n the h y d r o l y s i s . S i n c e f l u i d i t y i n a s t i r r e d r e a c t o r i s a r e q u i r e m e n t , a 10 p e r c e n t m i x t u r e has been c o n s i d e r e d maximum. T h e r e f o r e , the r e s u l t a n t sugar c o n c e n t r a t i o n s have been o n l y 2-7 p e r c e n t and a l c o h o l c o n c e n t r a t i o n s o n l y h a l f a s much. I f the l i m i t i n g f a c t o r i s c o n s i d e r e d t o be f l u i d i t y i n the r e a c t o r , i n s t e a d o f the f e e d m i x t u r e , the f e e d c o n c e n t r a t i o n c o u l d be i n c r e a s e d b y r o u g h l y the r e c i p r o c a l o f one minus the s o l i d s c o n v e r s i o n i n the r e a c t o r . Of c o u r s e , s o l i d s and l i q u i d w o u l d have t o be f e d s e p a r a t e l y , w h i c h c o u l d a l s o save equipment c o s t . For biomass, c o n t a i n i n g 75 p e r c e n t c a r b o h y d r a t e , the r e a c t o r s i z e c o u l d be reduced b y 75 p e r c e n t . A t t e n d a n t r e d u c t i o n s w o u l d a l s o r e s u l t i n the f i l t r a t i o n and washing u n i t s . E q u a l l y i m p o r t a n t a r e the r e s u l t a n t i n c r e a s e s i n sugar c o n c e n t r a t i o n s . The g l u c o s e c o n c e n t r a t i o n would be q u a d r u p l e d t o about 280 g/L (28 p e r c e n t ) . Energy and equipment c o s t s i n the f e r m e n t a t i o n a r e a would be reduced p r o p o r t i o n a t e l y . T h i s s i m p l e a l t e r a t i o n i n the p r o c e s s has a p r o f o u n d impact on the economics. I t i s e s t i m a t e d t h a t the c a p i t a l c o s t reduced b y 40 p e r c e n t i n the h y d r o l y s i s and a c i d r e c o v e r y s e c t i o n s and 60 p e r c e n t i n the f e r m e n t a t i o n and u t i l i t i e s a r e a s . Furthermore, the energy r e q u i r e m e n t s f o r d i s t i l l a t i o n a r e reduced b y 60 p e r c e n t . Acid Recycle. A n o t h e r method t o i n c r e a s e the sugar c o n c e n t r a t i o n i s t o r e c y c l e a p o r t i o n o f the f i l t r a t e ( a c i d and sugar s o l u t i o n ) i n the h y d r o l y s i s s t e p . The a c i d would c a t a l y z e f u r t h e r p o l y s a c c h a r i d e h y d r o l y s i s t o i n c r e a s e the sugar c o n c e n t r a t i o n . Of c o u r s e , r e c y c l e o f the sugars would a l s o i n c r e a s e the p o s s i b l e d e g r a d a t i o n t o f u r f u r a l and HMF.

In Clean Energy from Waste and Coal; Khan, M.; ACS Symposium Series; American Chemical Society: Washington, DC, 1992.

3.

Recovery of EthanolfromMunicipal Solid Waste

ACKERSON ET AL.

Publication Date: December 23, 1992 | doi: 10.1021/bk-1992-0515.ch003

100

0

10

20

30

Fermentation

40 Time

50

60

(hrs)

F i g u r e 2. F e r m e n t a t i o n of H y d r o l y z a t e and S y n t h e t i c G l u c o s e . (Reproduced w i t h p e r m i s s i o n from Energy from Biomass and Waste XV, I n s t i t u t e of Gas Technology, 1991. C o p y r i g h t 1991 M i c h a e l D. Ackerson.)

200 180 φ-

s

0

160 ,r À

•

50

g/L

140

ο

100

g/L

a

150

g/L

δ

200

g/L

12θφ\ 100

\

Λ

80

> ο

60

\

^

\

\ ο

\

40 20 0 0

20

40

60

Time

80

100

120

(min)

F i g u r e 3. G l u c o s e P r o f i l e i n the ICR. (Reproduced w i t h p e r ­ m i s s i o n from Energy from Biomass and Waste XV, I n s t i t u t e of Gas Technology, 1991. C o p y r i g h t 1991 M i c h a e l D. Ackerson.) In Clean Energy from Waste and Coal; Khan, M.; ACS Symposium Series; American Chemical Society: Washington, DC, 1992.

37

Publication Date: December 23, 1992 | doi: 10.1021/bk-1992-0515.ch003

38

CLEAN ENERGY FROM WASTE AND COAL

Experiments have been conducted t o determine t h e enhancement p o s s i b l e w i t h a c i d r e c y c l e . V a r i o u s amounts o f t h e a c i d and sugar s o l u t i o n from t h e f i l t r a t i o n were r e c y c l e d t o determine t h e r e s u l t i n g sugar and b y - p r o d u c t c o n c e n t r a t i o n s . A c i d and s o l i d s c o n c e n t r a t i o n s and temperatures were k e p t c o n s t a n t . These experiments have shown t h a t t h e sugar c o n c e n t r a t i o n s c a n be i n c r e a s e d s i x f o l d a t t o t a l r e c y c l e . I t s h o u l d be n o t e d t h a t n o t a l l t h e f i l t r a t e c a n be r e c y c l e d , s i n c e a p o r t i o n adheres t o t h e solids i n f i l t r a t i o n . I n o r d e r t o m i n i m i z e sugar d e c o m p o s i t i o n , a r e c y c l e f r a c t i o n o f 50 p e r c e n t has been used, w h i c h r e s u l t s i n d o u b l i n g t h e sugar c o m p o s i t i o n , w i t h o u t s i g n i f i c a n t f u r f u r a l o r HMF l e v e l s . The e f f e c t o f a c i d r e c y c l e on t h e economics i s s i g n i f i c a n t . A r e c y c l e r a t e o f 50 p e r c e n t , c o u p l e d w i t h h i g h s o l i d s c o n c e n t r a t i o n s , w o u l d r e s u l t i n a x y l o s e c o n c e n t r a t i o n o f 15 p e r c e n t and a g l u c o s e c o n c e n t r a t i o n o f over 50 p e r c e n t c o u l d be achieved. P r a c t i c a l l y , sugar c o n c e n t r a t i o n s s h o u l d n o t exceed 25 p e r c e n t , so a s m a l l e r r e c y c l e f r a c t i o n i s r e q u i r e d . I t s h o u l d be n o t e d t h a t these c o n c e n t r a t i o n s have been a c h i e v e d i n t h e l a b o r a t o r y , w h i l e m a i n t a i n i n g f u r f u r a l and HMF l e s s t h a n 0.05 p e r c e n t . These h i g h c o n c e n t r a t i o n s reduce t h e equipment s i z e i n the a c i d r e c o v e r y s e c t i o n by 50 p e r c e n t and i n t h e f e r m e n t a t i o n s e c t i o n b y a n o t h e r 60 p e r c e n t . Energy consumption i s a l s o reduced a n o t h e r 60 p e r c e n t . Acid Recovery. A c i d r e c o v e r y i s e s s e n t i a l when u s i n g c o n c e n t r a t e d a c i d h y d r o l y s i s . P r o c e s s e s f o r r e c o v e r y o f b o t h h y d r o c h l o r i c and s u l f u r i c a c i d s have been developed. A number o f p o s s i b l e r e c o v e r y schemes were examined, i n c l u d i n g e l e c t r o d i a l y s i s , d i s t i l l a t i o n , etc. The r e c o v e r y t e c h n o l o g y t h a t has been s e l e c t e d i s b a s e d upon s o l v e n t e x t r a c t i o n . S o l v e n t s have been i d e n t i f i e d t h a t e x t r a c t HC1 and H2SO4 from t h e aqueous sugar s o l u t i o n s . Near complete a c i d r e c o v e r y i s p o s s i b l e and s o l v e n t l o s s e s a r e m i n i m i z e d . F o r HC1, t h e a c i d i s s e p a r a t e d from t h e s o l v e n t b y d i s t i l l a t i o n , and the s o l v e n t r e c y c l e d . A hexane wash o f t h e sugar s o l u t i o n i s u s e d to r e c o v e r t r a c e q u a n t i t i e s o f s o l v e n t , and hexane i s s e p a r a t e d b y d i s t i l l a t i o n f o r recycle. Some s o l v e n t i s l o s t i n t h e p r o c e s s ; however, t h e l o s s e s a r e q u i t e s m a l l and s o l v e n t replacement c o s t s a r e o n l y $0.02 p e r g a l l o n o f a l c o h o l . A c i d l o s s e s a r e m i n i m i z e d and a c i d c o s t s a r e $0,025 p e r g a l l o n o f a l c o h o l . The t o t a l h e a t r e q u i r e m e n t f o r s o l v e n t and a c i d r e c o v e r y i s l o w and amounts t o l e s s t h a n $0.05 p e r g a l l o n o f a l c o h o l . As shown l a t e r , t h e energy c o s t may be r e c o v e r e d from t h e l i g n i n and x y l o s e streams. ECONOMIC PROJECTIONS To i l l u s t r a t e t h e economics o f t h i s p r o c e s s , a d e s i g n h a s been p e r f o r m e d f o r a f a c i l i t y t o c o n v e r t MSW i n t o 20 m i l l i o n g a l l o n s per year o f e t h a n o l , u t i l i z i n g the a c i d h y d r o l y s i s procedures p r e v i o u s l y d e s c r i b e d . The c a p i t a l and o p e r a t i n g c o s t s a r e summarized i n T a b l e V I I . MSW w o u l d be c o l l e c t e d and d e l i v e r e d t o t h e p l a n t s i t e as

In Clean Energy from Waste and Coal; Khan, M.; ACS Symposium Series; American Chemical Society: Washington, DC, 1992.

Publication Date: December 23, 1992 | doi: 10.1021/bk-1992-0515.ch003

3.

ACKERSON ET AL.

Recovery of EthanolfromMunicipal Solid Waste

needed. F e e d s t o c k p r e p a r a t i o n c o n s i s t s o f p l a s t i c , m e t a l and g l a s s r e m o v a l , s h r e d d i n g , g r i n d i n g and c o n v e y i n g t o the r e a c t o r s . The c o s t o f the removal o f g l a s s and m e t a l s i s n o t i n c l u d e d i n the f e e d p r o c e s s i n g c o s t , as r e p o r t s i n d i c a t e t h a t r e s a l e o f t h e s e m a t e r i a l s w i l l o f f s e t the c a p i t a l and o p e r a t i n g c o s t s o f separation. The h y d r o l y s i s s e c t i o n , as shown i n F i g u r e 1, c o n s i s t s of continuous r e a c t o r s . A c i d r e s i s t a n t m a t e r i a l s of c o n s t r u c t i o n are n e c e s s a r y f o r t h i s equipment. Ethanol f e r m e n t a t i o n i n the ICR and t y p i c a l d i s t i l l a t i o n u n i t s are i n c l u d e d . The t o t a l c a p i t a l c o s t f o r t h i s p l a n t i s $35 m i l l i o n , i n c l u d i n g a l l u t i l i t i e s , s t o r a g e and o f f s i t e f a c i l i t i e s . The a n n u a l o p e r a t i n g c o s t s are a l s o shown i n T a b l e V I I . These c o s t s a r e a l s o g i v e n on the b a s i s o f u n i t p r o d u c t i o n o f a l c o h o l . As mentioned p r e v i o u s l y , no c o s t i s i n c l u d e d f o r MSW. A lignin b o i l e r i s used t o reduce the energy r e q u i r e m e n t s , and energy c o s t s are o n l y $0.08 p e r g a l l o n . F i x e d charges a r e computed as a p e r c e n t a g e o f the c a p i t a l i n v e s t m e n t and t o t a l $5.6 m i l l i o n p e r year. The p r e s e n t e t h a n o l p r i c e o f $1.50 p e r g a l l o n w i l l g e n e r a t e revenues o f $30 m i l l i o n and y i e l d a p r e - t a x p r o f i t o f $18.5 p e r y e a r ( $ . 9 3 / g a l ) o r 53 p e r c e n t per y e a r . I t s h o u l d be n o t e d t h a t t h i s p r o c e s s does n o t i n c l u d e u t i l i z a t i o n o f the pentose stream. A c i d r e c o v e r y i s i n c l u d e d , b u t f e r m e n t a t i o n o f the x y l o s e i s not p r o v i d e d . X y l o s e c o u l d be fermented t o a l c o h o l , a c i d s o r o t h e r v a l u a b l e c h e m i c a l s , w h i c h would improve the economics. However, s i n c e t h i s t e c h n o l o g y i s n o t p e r f e c t e d , such p r o d u c t s have not been i n c l u d e d . Table V I I .

A.

Economics o f 20 M i l l i o n G a l l o n Per Y e a r Ethanol F a c i l i t y

C a p i t a l Cost

MUU