Combustion of Synthetic Fuels

0 downloads 0 Views 4MB Size Report
Apr 29, 1983 - efficiency, thus decreasing the need for costly hydrogenative refining steps. In the utilization of synthetic fuels, the key issues are impact on.
Publication Date: April 29, 1983 | doi: 10.1021/bk-1983-0217.fw001

Combustion of Synthetic Fuels

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

Combustion of Synthetic Fuels William Bartok, EDITOR Exxon Research and

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

Engineering Company

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

ACS SYMPOSIUM SERIES 217

AMERICAN

CHEMICAL

WASHINGTON, D.C. 1983

SOCIETY

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

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

621.402*3

83-2822

Copyright © 1983 American Chemical Society All Rights Reserved. The appearance of the code at the bottom of the first page of each article in this volume indicates the copyright owner's consent that reprographic copies of the article may be made for personal or internal use or for the personal or internal use of specific clients. This consent is given on the condition, however, that the copier pay the stated per copy fee through the Copyright Clearance Center, Inc. for copying beyond that permitted by Sections 107 or 108 of the U.S. Copyright Law. This consent does not extend to copying or transmission by any means—graphic or electronic—for any other purpose, such as for general distribution, for advertising or promotional purposes, for creating new collective work, for resale, or for information storage and retrieval systems. The copying fee for each chapter is indicated in the code at the bottom of the first page of the chapter. The citation of trade names and/or names of manufacturers in this publication is not to be construed as an endorsement or as approval by ACS of the commercial products or services referenced herein; nor should the mere reference herein to any drawing, specification, chemical process, or other data be regarded as a license or as a conveyance of any right or permission, to the holder, reader, or any other person or corporation, to manufacture, reproduce, use, or sell any patented invention or copyrighted work that may in any way be related thereto. PRINTED IN THE UNITED STATES OF

AMERICA

Society Library 16th St. N. w.

1155

Washington, D. C.

20038

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

ACS Symposium Series M . Joan Comstock, Series Editor

Advisory Board

David L. Allara

Robert Ory

Robert Baker

Geoffrey D. Parfitt

Donald D. Dollberg

Theodore Provder

Brian M. Harney

Charles N. Satterfield

W. Jeffrey Howe

Dennis Schuetzle

Herbert D. Kaesz

Davis L. Temple, Jr.

Marvin Margoshes

Charles S. Tuesday

Donald E. Moreland

C. Grant Willson

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

FOREWORD The ACS SYMPOSIUM SERIES was founded in 1974 to provide

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

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

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

Exxon Research and Engineering Company Linden, NJ 07036 December 1982

ix

1

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

Research

Issues and

Technology-An

Overview

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

x

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

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

2

COMBUSTION OF SYNTHETIC FUELS

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

1.

BONI ET AL.

Research Issues and Technology

3

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

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

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

19,000

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

12.2 0.29 0.57 3.3

H Y D R O G E N , WT %

NITROGEN, W T %

SULFUR, W T %

O X Y G E N , WT %

*Grumer, Reference 6.



+

4

CO

2

CHO

χ y ζ

co

OXIDATION C H 2

2

|_OH

2

H 0 2

C

L

H

2 4

J

J 0

IV

Γ

CO

ELEMENTARY

H ->•

H

2

+

C Η 0

STEPS TO COMPLETION

]

χ y ζ

L

H

2°2

OH CHO

J

H0

V C

°2.

2

CContinued on next page.)

1 zj

COMBUSTION OF SYNTHETIC FUELS

32

Publication Date: April 29, 1983 | doi: 10.1021/bk-1983-0217.ch002

TABLE I .

GENERIC QUASIGLOBAL MODEL (continued).

V

ALIPHATICS

SOOT

AROMATICS

FORMATION

INTERMEDIATES

SOOT

w

2 CO

VI

co SOOT

SOOT

+

CO 2

H0

C0

o

2

GASIFICATION

H

CH 2

4J

OH

BOUND NITROGEN AND/OR FUEL RICH NITROGEN CONVERSION

FUEL

VII

HCN

NO

NH

±

χ FORMATION

'V °2 Ν

NO +

THERMAL FIXATION

2.

EDELMAN E T AL.

Combustion and Emissions

33

process might be obtainable by extending the n o t i o n of a s i n g l e step o v e r a l l r e a c t i o n to a scheme which c h a r a c t e r i z e s the r e a c ­ t i o n s high up i n the chain with one or more "subglobal steps coupled to a set of b e t t e r known d e t a i l e d r e v e r s i b l e r e a c t i o n s to c h a r a c t e r i z e the k i n e t i c s processes at the lower end of the c h a i n . We introduced the term " q u a s i g l o b a l as a d e f i n i t i o n of t h i s type of k i n e t i c s modeling. That work l e d to the demonstration that f u e l - l e a n combustion and lean N 0 emissions f o r both long chain and c y c l i c type h y d r o ­ carbon f u e l s could be c h a r a c t e r i z e d by q u a s i g l o b a l k i n e t i c s 05, 6) . However, the need to address f u e l - r i c h combustion and wide o p e r a t ­ ing ranges and a p p l i c a t i o n s to cover r e s i d e n t i a l , commercial and i n d u s t r i a l u t i l i z a t i o n became apparent, p a r t i c u l a r l y i n view of the advent of s y n t h e t i c f u e l s . Following the suggestions of Edelman and Harsha ( 6 ) , the current extended q u a s i g l o b a l model was p o s t u l a t e d , Table I ( 7 ) . A s i g n i f i c a n t feature of t h i s model i s the d i r e c t coupling of the intermediate r a d i c a l s to the subglobal steps high up i n the c h a i n . To address the primary compositional c h a r a c t e r i s t i c s of c o a l d e r i v e d type s y n t h e t i c f u e l s , a neat aromatic hydrocarbon ( t o l u ­ ene) and a neat a l i p h a t i c hydrocarbon ( η - o c t a n e ) were s e l e c t e d f o r t h i s study. Although, i n c e r t a i n i n s t a n c e s , r a t e parameters can be e s t i ­ mated using c l a s s i c a l techniques the primary source f o r t h i s information i s e m p i r i c a l . T y p i c a l Arrhenius f i t s are employed even f o r the most d e t a i l e d k i n e t i c s and mechanism treatments that have been proposed f o r v a r i o u s chemical systems 08, 9) . The r e q u i r e d data f o r q u a s i g l o b a l model development i n c l u d e s : concentrations of major species i n c l u d i n g soot and nitrogenous s p e c i e s , r e a c t i o n temperature, heat l o s s , and pressure as f u n c ­ t i o n s of operating c o n d i t i o n s that c o n t r o l the progress of the o x i d a t i o n process. I d e a l l y , the source of t h i s information should be devoid of r a t e phenomena other than chemical k i n e t i c s . Turbu­ l e n t t r a n s p o r t , mixing and m u l t i - d i m e n s i o n a l i t y are such f a c t o r s that should be avoided or otherwise p r o p e r l y taken i n t o account i n order to o b t a i n v a l i d data f o r the development and v a l i d a t i o n of the chemical k i n e t i c s model. S t i r r e d r e a c t o r s , plug-flow r e a c ­ t o r s , shock-tubes, and laminar f l a t - f l a m e s are c o n c e p t u a l l y sources of k i n e t i c s d a t a . In combination, t h i s array of e x p e r i ­ mental t o o l s would provide a broad range of o p e r a t i n g c o n d i t i o n s u s e f u l i n developing and v a l i d a t i n g v a r i o u s aspects of the p o s t u ­ l a t e d mechanism. I n i t i a t i o n , propagation, b r a n c h i n g , quenching, and completion of r e a c t i o n can each be more s p e c i f i c a l l y d e l i n ­ eated according to the p a r t i c u l a r r e a c t o r s e l e c t e d . In the c u r ­ rent research a set of Exxon J e t - S t i r r e d Combustors were s e l e c t e d as the primary source of d a t a . These r e a c t o r s are based upon the Longwell design and have proven to be a p p r o p r i a t e f o r q u a s i g l o b a l model development and v a l i d a t i o n 0 5 ) . Furthermore, the intense backmixing c h a r a c t e r i s t i c of the J e t - S t i r r e d Combustor represents 11

11

Publication Date: April 29, 1983 | doi: 10.1021/bk-1983-0217.ch002

X

Publication Date: April 29, 1983 | doi: 10.1021/bk-1983-0217.ch002

34

COMBUSTION OF SYNTHETIC FUELS

the l i m i t i n g flow behavior of primary zones and flame s t a b i l i z a t i o n regions i n many p r a c t i c a l burners and combustors. A s e r i e s of experiments were performed by Exxon Research and Engineering C o . , and the r e s u l t s were u t i l i z e d i n the current modeling e f f o r t . Although the g l o b a l steps i n the q u a s i g l o b a l toluene and i s o - o c t a n e k i n e t i c s model were developed to represent these d a t a , the range of a p p l i c a b i l i t y of the model was f u r t h e r tested by analyzing k i n e t i c s data from shock-tube experiments. The g l o b a l r e a c t i o n s considered i n c l u d e the conversion by pure p y r o l y s i s of toluene to acetylene and the conversion of i s o octane to e t h y l e n e , o x i d a t i v e p y r o l y s i s of the acetylene and e t h y l e n e , and p a r t i a l o x i d a t i o n of the parent f u e l s and these h y d r o carbon intermediates to CO, H 2 , and H2O. The s p e c i f i c r e a c t i o n s and r a t e s f o r t h i s system are given i n Table I I . Soot formation i s assumed to be a f u n c t i o n of temperature and oxygen and p r e c u r sor c o n c e n t r a t i o n s . In the present study the soot precursors are taken to be acetylene and toluene, expressed as C2 hydrocarbons. Soot o x i d a t i o n i s modeled with the s e m i - e m p i r i c a l r e l a t i o n s h i p developed by Nagle and S t r i c k l a n d - C o n s t a b l e (10), which i n c l u d e s an e x p l i c i t dependence on soot s i z e , oxygen c o n c e n t r a t i o n , and temperature. The chemical equations and r a t e expressions f o r the o v e r a l l t o l u e n e / i s o - o c t a n e model are given i n Table I I . Note that the precursors that were used preclude i s o - o c t a n e from forming soot. This i s c o n s i s t e n t with the Exxon data but obviously not v a l i d f o r a l l i s o - o c t a n e combustion c o n d i t i o n s . The N 0 emissions submodel includes thermal f i x a t i o n r e a c t i o n s , nitrogen/hydrocarbon i n t e r a c t i o n r e a c t i o n s , hydrocarbon fragmentation r e a c t i o n s , and a g l o b a l fuel-bound n i t r o g e n decomposition r e a c t i o n . The i n t e r a c t i o n and fragmentation r e a c t i o n s developed by Levy (11) to r e p r e sent methane and C2 combustion were used i n t h i s study. These interactions include: FBN species decomposition to HCN, HCN conv e r s i o n r e a c t i o n s to NO which i n c l u d e r e a c t i o n s i n v o l v i n g NH-£ spec i e s as i n t e r m e d i a t e s , and r e a c t i o n s with hydrocarbon r a d i c a l s such as CH3, CH2, CH, and CHO. The c o n t r i b u t i o n of longer chain r a d i c a l s to these i n t e r a c t i o n routes to NO has not been e s t a b lished. The hydrocarbon fragmentation r e a c t i o n s convert the intermediate hydrocarbons i n t o the small hydrocarbon r a d i c a l s which are included i n the coupled N2/HC r e a c t i o n s e t . X

The g l o b a l r a t e s were determined by d i r e c t comparison of p r e d i c t i o n s , using s e l e c t e d sets of r a t e s , with the near a d i a b a t i c data obtained from the Exxon J e t - S t i r r e d Combustor ( 1 2 ) . The p a r t i c u l a r set of r a t e parameters r e s u l t i n g from t h i s process are given i n Table I I . A d e t a i l e d comparison between the p r e d i c t i o n s using the q u a s i g l o b a l k i n e t i c s model and the Exxon data are shown i n Figures 1 through 5. F i g u r e 1 compares the p r e d i c t e d temperature to experimental values over a range of equivalence r a t i o s f o r t o l uene with near a d i a b a t i c operation of the r e a c t o r . Species p r e d i c t i o n s f o r CO, CO2, O 2 , soot and unburned hydrocarbons are shown i n F i g u r e 2 f o r these same experiments. These species were measured by e x t r a c t i n g samples, and the l i m i t e d chemical analyses

2.

EDELMAN E T AL.

TABLE I I .

Combustion and Emissions

35

QUASIGLOBAL TOLUENE AND ISO-OCTANE MODEL

GLOBAL STEPS Reaction

7 8 * I

C

H

C

H

ï

+

2 2

H

2

1 Rate Expression

[ C H ] = - A [ C H ] exp {-E/RT} 7

Reaction

g

7

C Hg + OH - \

g

C H

7

2

+ H + \

2

0

2

2

Publication Date: April 29, 1983 | doi: 10.1021/bk-1983-0217.ch002

2 [ C H ] = - A [ C H ] [ O H ] exp {-E/RT}

Rate Expression

7

g

Reaction

C

7

H

7 8

+

g

1 ° 2 ~*

7

C

0

+

4

H

2

3 Rate Expression

5

[C *Hg] = - A [ C H g ] ° - [ 0 ] 7

7

C H

Reaction

2

Τ exp {-E/RT}

2

+ 6 OH ·»· 2 CO + 4 H 0

2

2

4 Rate Expression

[C *H23 = - A [ C H ] [ 0 H ] exp {-E/RT} 2

2

C H

Reaction

2

2

2

+ 2 OH •»• 2 CO + 2 H

2

5 Rate Expression

[ C H ] = - A [ C H ] [ 0 H ] exp {-E/RT} 2

2

2

Reaction

C

H

2

4

8 18 -

C

H

2 4

+

H

2

6 Rate Expression

Reaction

[ C g H ] = - A [ C g H ] exp {-E/RT} lg

CgH

1 8

18

+ OH -

4 C H 2

4

+ f

H + \ 2

0

2

7 Rate Expression

[ C g H ] = - A [ C H ] [ O H ] exp {-E/RT} lg

g

18

(Continued on next page.)

36

COMBUSTION OF SYNTHETIC FUELS

TABLE I I .

QUASIGLOBAL TOLUENE AND ISO-OCTANE MODEL

Reaction

C

H

8 18

+

4

8

°2

C

0

+

(continued).

9

H

2

X

P

8 Rate Expression

[ C

8*18

]

=

~

A [ C

H

] 0 , 5 [

8 18

]

°2

T

G

{

~

E

/

R

T

}

C H , + 8 OH + 6 H 0 + 2 CO 2 4 2

Reaction

0

o

9

Publication Date: April 29, 1983 | doi: 10.1021/bk-1983-0217.ch002

Rate Expression

[ C H ] = - A [ C H ] [ O H ] exp 2

2

4

Reaction

4

C H . + 2 OH 2 4

{-E/RT}

3 H + 2 CO 2

0

0

10 Rate Expression

[ C H ] = - A [ C H ] [ O H ] exp 2

4

2

|- C H

Reaction

?

4

+ C H

8

2

{-E/RT}

Ξ HC -> SOOT

2

11 b

1 , 8 1

[SOOT] = + A T [ H C ]

Rate Expression

Reaction

5

[ 0 r ° " e x p {-E/RT} 2

SOOT + °

2

->

C 0

2

12 Κ X

Rate Expression

WHERE:

[SOOT] = -12 Ρ

1

K

f o r i = Α, Β , Τ , Ζ

=

A

A

i

Θ

Χ

Ρ

ί"

= 6 C /ρ t

D

Ε ±

κ

/

· D Î5

τ

^

[surface

area of

soot/vol]

ο

Ρ

= p a r t i a l pressure of 0 « 2 Cg = mass s o o t / v o l ϋ

[atm]

l

= soot D

x>

2

X = £l + ^ / ( K ^ )J i

+vi

H/ H + H 0

2

2

2

+ OH *=> H 0 + H 0

2

2

29

H 0 + OH 0

2

2

2

+ H 0

2

+ OH

2

2 OH

2

2

2

+ H

2

2

HCHO Mechanism* 33

HCHO + OH ϊ ± H 0 + CHO

34

HCHO + Η •=> H + CHO

35

HCHO + Ο ϊ=> OH + CHO

36

CHO + OH ϊ± H 0 + CO

2

2

2

37

CHO + Η ϊ=> H

38

CHO + 0 ï±OH + CO

39

HCHO + H 0 «=* H ^ + CHO

40 41

+ CO

2

2

CHO + H 0 i ± 0 2

CHO + 0

2

+ HCHO

2

i=? H 0 + CO 2

42

HCHO + Μ ί± Η + CHO + M

43

CHO + M ;=? CO + H + M

4- Rates were taken from Reference 9. * Rates were taken from Reference 4.

EDELMAN E T A L .

Combustion and Emissions



JSC DATA

Ο

EST. Τ FROM MEASURED SPECIES

Publication Date: April 29, 1983 | doi: 10.1021/bk-1983-0217.ch002

EST. ERROR IN Τ PREDICTION

2000 h

1800 h Τ (Κ)

h

1600

1400

1200

Figure 1.

Toluene Combustion as Measured i n Exxon JSC and

as P r e d i c t e d .

COMBUSTION OF SYNTHETIC FUELS

— PREDICTION JSC DATA Δ CO •

C0



0

2

2

Publication Date: April 29, 1983 | doi: 10.1021/bk-1983-0217.ch002

MOLE %

— PREDICTION 4-

C2H2,

0

JSC DATA

MOLE %

J

0 0.8 0.6 SOOT (mg/t)

L

— PREDICTION

h

Ο JSC DATA

0.4

0.2

$.6

I , 0.8

I 1.0

L 1.2

1.4

1.6

1.8

2.0

Φ F i g u r e 2. as

Toluene Combustion as Measured i n Exxon JSC and

Predicted.

Publication Date: April 29, 1983 | doi: 10.1021/bk-1983-0217.ch002

2.

EDELMAN E T AL.

Combustion and Emissions

41

i n d i c a t e d the unburned hydrocarbons to be mostly a c e t y l e n e . The comparisons are e x c e l l e n t . Neither water nor hydrogen were mea­ sured i n t h i s s e r i e s . However, s i m i l a r comparisons were a l s o made for nominal 1900°K and 1700°K isothermal experiments f o r the same range of equivalence r a t i o s . Hydrogen was measured i n these experiments. Thus, hydrogen and water (by d i f f e r e n c e ) could a l s o be compared. For the 1900°K case, the comparisons were very s i m i ­ l a r to the a d i a b a t i c data shown. For the 1700°K case, the a c c u ­ racy of the measured gas temperature was q u e s t i o n a b l e . Near i s o ­ thermal o p e r a t i o n r e q u i r e d adjustment of the n i t r o g e n content i n the " a i r " stream. This mode of o p e r a t i o n d i d not always y i e l d a good comparison of temperatures c a l c u l a t e d from the energy balance using the measured species with the measured temperatures. Resi­ dence times f o r these comparisons were 3 msec and the pressure was 1 atm. D e t a i l e d comparisons f o r near a d i a b a t i c combustion of i s o octane are shown i n Figures 3 and 4. F i g u r e 3 shows temperature and F i g u r e 4 shows CO, CO2, 0 and unburned hydrocarbons as func­ t i o n s of equivalence r a t i o . The q u a l i t y of these comparisons i s very good and i s s i m i l a r to that obtained f o r the toluene e x p e r i ­ ments. Soot was not observed to form i n measurable q u a n t i t i e s f o r i s o - o c t a n e mixtures which could be s t a b l y burned i n the J e t S t i r r e d Combustor. The r a t e parameters f o r soot formation i n f u e l - r i c h toluene o x i d a t i o n were determined i n the same manner that was used f o r the s e l e c t i o n of the other g l o b a l r a t e s . F i g u r e 2 shows the compari­ son of the p r e d i c t i o n s with the data f o r pure toluene and F i g u r e 5 shows comparisons f o r blends of toluene and i s o - o c t a n e using the same r a t e parameters. The trends f o r the blends are proper, but the amount of soot formed i s somewhat u n d e r p r e d i c t e d . Since the soot o x i d a t i o n model depends on an assumed mean p a r t i c l e s i z e of 25 nm, a study of the e f f e c t of soot p a r t i c l e s i z e showed that the r e s u l t s were r e l a t i v e l y i n s e n s i t i v e to t h i s assumption and that under these operating c o n d i t i o n s only about 25 percent of the soot would be consumed by oxygen attack (12). This r e l a t i v e i n s e n s i t i v i t y suggested that both soot g a s i f i c a t i o n by the other species given i n Table I and s y n e r g i s t i c enhancement of soot formation from a l i p h a t i c s i n mixtures with aromatics should be considered i n our future work. Although the p r e d i c t i o n of Ν 0 emissions under lean and s t o i ­ chiometric combustion with the extended Z e l d o v i c h mechanism i s adequate f o r c e r t a i n a p p l i c a t i o n s , p r e d i c t i v e methods f o r f u e l s c o n t a i n i n g bound n i t r o g e n and f o r r i c h combustion c o n d i t i o n s r e q u i r e s u b s t a n t i a l improvement. However, the e a r l y s t u d i e s of Fenimore (13, 14) demonstrated the p o t e n t i a l importance of HCN and N H type species i n f u e l - n i t r o g e n i n t e r a c t i o n s . To i l l u s t r a t e the c r i t i c a l importance of the coupling of nitrogenous species r e a c ­ t i o n s i n r i c h combustion, p r e d i c t i o n s of NO emissions from r i c h i s o - o c t a n e combustion i n a j e t - s t i r r e d combustor are shown i n Table I I I . C hydrocarbon fragmentation and o x i d a t i o n creates 2

χ

i

2

COMBUSTION OF SYNTHETIC FUELS

PREDICTION MEASURED Τ WITH RADIATION CORRECTION •

PREDICTED POINTS

Ο

EST. Τ FROM MEASURED SPECIES

V

JSC DATA

Publication Date: April 29, 1983 | doi: 10.1021/bk-1983-0217.ch002

2100

2000

1900

1800

1700 Τ (Κ)I 1600

1500

1400

1300 1200 0.4

0.6

0.8

1.0

1.2

1.4

1.6

1.8

Φ F i g u r e 3. and as

Iso-Octane Combustion as Measured i n Exxon JSC

Predicted.

Combustion and Emissions

Publication Date: April 29, 1983 | doi: 10.1021/bk-1983-0217.ch002

EDELMAN E T A L .

Figure 4. and as

Iso-Octane Combustion as Measured i n Exxon JSC

Predicted.

Publication Date: April 29, 1983 | doi: 10.1021/bk-1983-0217.ch002

COMBUSTION OF SYNTHETIC FUELS

F i g u r e 5.

Soot Emissions From Toluene/Iso-Octane

Blends.

2.

EDELMAN E T AL.

TABLE I I I .

CONDITIONS:

Publication Date: April 29, 1983 | doi: 10.1021/bk-1983-0217.ch002

45

Combustion and Emissions

NO

EMISSIONS ANALYSIS

Rich i s o - o c t a n e combustion in a w e l l - s t i r r e d reactor; Residence time = 3 msec; Equivalence r a t i o =

1.8;

Pressure = 1 atm.

ISO-OCTANE QUASIGLOBAL MODEL WITH HYDROCARBON/ NITROGEN INTERACTION INTERACTIONS

ISO-OCTANE QUASIGLOBAL MODEL WITH HYDROCARBON/ NITROGEN INTERACTION AND C FRAGMENTATION AND OXIDATION REACTIONS

T(K)

2110

2106

NO

4.9E-5

7.5E-3

CO

18.7

16.7

OH

2.3E-2

9.1E-2

0

1.18E-3

3.6E-3

Ν

2.65E-8

3.17E-4

2

COMBUSTION OF SYNTHETIC FUELS

46

HCO, CH, C H and CH3 r a d i c a l s which modify the Η, 0 and OH r a d i c a l pool. These r a d i c a l s i n t e r a c t with nitrogenous compounds to form HCN and N % . These species r e a c t to form Ν and u l t i m a t e l y NO and N0 . Two cases are shown i n Table I I I ; the f i r s t represents r i c h combustion with the q u a s i g l o b a l i s o - o c t a n e model which y i e l d s HCO, Η, 0 and OH r a d i c a l s that i n t e r a c t with the nitrogenous s p e c i e s . The second case a l s o includes d e t a i l e d r e a c t i o n s i n v o l v i n g the fragmentation and o x i d a t i o n of C hydrocarbons to HCO and C % . Case 2 p r e d i c t s r e a l i s t i c NO l e v e l s f o r t h i s example. Without these hydrocarbon fragments i n c l u d e d , the NO l e v e l i s two orders of magnitude too low. This example demonstrates the extreme sen­ s i t i v i t y of the N0 emissions p r e d i c t i o n to hydrocarbon-NO i n t e r ­ a c t i o n s among species which do not a f f e c t the thermal s t a t e of the combustion or the major, s t a b l e products of combustion and sug­ gests that such i n t e r a c t i o n s r e q u i r e f u r t h e r i n v e s t i g a t i o n . The range of a p p l i c a b i l i t y of the q u a s i g l o b a l k i n e t i c s model was f u r t h e r i n v e s t i g a t e d by c o n s i d e r i n g other experiments which c h a r a c t e r i z e d d i f f e r e n t aspects of the combustion mechanism. Ini­ t i a t i o n processes, as measured with shock-tube experiments, were considered as a severe t e s t of the model which was developed with j e t - s t i r r e d combustor data which are c o n t r o l l e d p r i m a r i l y by recombination r e a c t i o n s that occur during the l a t t e r stages of combustion. Q u a s i g l o b a l toluene k i n e t i c s model p r e d i c t i o n s were compared with shock-tube i g n i t i o n delay data and with McLain and Jachimowski s (15) d e t a i l e d k i n e t i c s p r e d i c t i o n f o r these d e l a y s . This comparison i s shown i n F i g u r e 6; the agreement i s e x c e l l e n t . I g n i t i o n delay was defined as the time at which the pressure s t a r t e d to r a p i d l y increase from i t s i n i t i a l v a l u e . The p r e d i c t e d pressure r i s e was very r a p i d , so t h i s parameter was e a s i l y o b t a i n ­ able. S i m i l a r experimental data for r i c h e r combustion and over a wider range of pressure and d i l u e n t concentrations would be d e s i r ­ a b l e , yet the c o n d i t i o n s presented are s i g n i f i c a n t l y d i f f e r e n t from those of the J e t - S t i r r e d Combustor flow f o r which the g l o b a l k i n e t i c s r a t e s were determined. 2

2

2

Publication Date: April 29, 1983 | doi: 10.1021/bk-1983-0217.ch002

X

x

1

Conclusions Q u a s i g l o b a l k i n e t i c s models, which have p r e v i o u s l y been shown to represent lean and s t o i c h i o m e t r i c combustion of a v a r i e t y of hydrocarbon f u e l s , have been extended to represent lean and r i c h combustion of toluene and i s o - o c t a n e . The model p r e d i c t s the thermal s t a t e of the flow and emissions of CO, s o o t , and N 0 . The thermal s t a t e of the flow and the s t a b l e species were shown to be a c c u r a t e l y p r e d i c t e d f o r j e t - s t i r r e d combustor experiments. For r i c h combustion, hydrocarbon intermediates and soot are a d d i t i o n a l combustion products. The g l o b a l r e a c t i o n s and r a t e s were d e v e l ­ oped to represent n e a r - a d i a b a t i c j e t - s t i r r e d combustor data and were then v e r i f i e d by comparison to the near i s o - t h e r m a l j e t s t i r r e d combustor d a t a . Ν 0 emissions behavior was i n v e s t i g a t e d with the q u a s i g l o b a l k i n e t i c s model to represent r i c h combustion X

χ

2.

EDELMAN E T AL.

47

and an Ν Ο emissions model. Ν 0 emissions p r e d i c t i o n s are q u a l i ­ t a t i v e l y c o r r e c t , and r e s u l t s i n d i c a t e that more work needs to be done to d e s c r i b e the formation and i n t e r a c t i o n of hydrocarbon fragments such as HCO and CHJL with nitrogenous s p e c i e s . Since the j e t - s t i r r e d combustor represents only a l i m i t e d range of experimental c o n d i t i o n s , the extended q u a s i g l o b a l k i n e t ­ i c s model was a l s o used to p r e d i c t i g n i t i o n delay times f o r shocktube experiments f o r toluene. These p r e d i c t i o n s were i n e x c e l l e n t agreement with the experimental o b s e r v a t i o n s . Accurate r e p r e s e n ­ t a t i o n s of both the j e t - s t i r r e d combustor and shock-tube data are very encouraging with respect to the apparent g e n e r a l i t y o f f e r e d by q u a s i g l o b a l modeling. This q u a s i g l o b a l k i n e t i c s model f o r a r o ­ matic and a l i p h a t i c f u e l components represents a major i n i t i a l step i n d e s c r i b i n g the combustion behavior of a c t u a l s y n f u e l s . However, research i s i n progress to f u r t h e r examine the range of a p p l i c a b i l i t y of the extended q u a s i g l o b a l k i n e t i c s model and to develop analogous models f o r s y n f u e l s . Χ

Publication Date: April 29, 1983 | doi: 10.1021/bk-1983-0217.ch002

Combustion and Emissions

χ

Acknowledgment T h i s work was performed as a subcontract f o r Exxon Research and Engineering Company under the b a s i c contract DE-AC22-77-ET11313 f o r the P i t t s b u r g h Energy Technology Center, U . S. Depart­ ment of Energy.

Figure 6.

I g n i t i o n Delay Comparisons f o r Toluene.

American Chemical Society Library

48

Literature

COMBUSTION OF SYNTHETIC FUELS

Cited

Muzio, L . J.; Arand, J. K. "Combustion and Emissions Charac­ teristics of SRC-II Fuels"; WSS 80-12, Western States Section of The Combustion Institute, 1980. 2. Burke, F. P.; Winschel, R. Α.; Pochapsky, T. C. "Composition and Performance of Distillate Recycle Solvents From the SRC-I Process"; Fuel, 1981, 60, 562-572. 3. Becker, M.; Bendoraitis, J. G.; Bloch, M. G.; Cabal, Α. V.; Callen, R. B.; Green, L. Α.; Simpson, C. A. "Analytical Studies for the Η-Coal Process"; DOE FE-2676-1, Mobil Research and Development Corp., DOE, 1978. 4. Edelman, R. B . ; Fortune, O. F. "A Quasi-Global Chemical Kinetic Model for the Finite-Rate Combustion of Hydrocarbon Fuels With Application to Turbulent Burning and Mixing i n Hypersonic Engines and Nozzles"; AIAA Paper 69-86, AIAA, 1969. 5. Engleman, V. S.; Bartok, W.; Longwell, J. P . ; Edelman, R. B. "Experimental and Theoretical Studies of NO Formation i n a Jet-Stirred Combustor"; Fourteenth Symposium (International) on Combustion, 1973, 755-765. 6. Edelman, R. B . ; Harsha, P. T. "Laminar and Turbulent Gas Dynamics i n Combustors - Current Status"; Prog. Energy Combust. Sci., 1978, 4, 1-62. 7. Farmer, R. C.; Edelman, R. B . ; Wong, E. "Modeling Soot Emis­ sions i n Combustion Systems"; Particulate Carbon Formation During Combustion, 1980, GM Research Symposium. 8. Engleman, V. S. "Survey and Evaluation of Kinetic Data on Reactions i n Methane/Air Combustion"; Report No. EPA-600/276-003, 1976. 9. Westbrook, C. K. "An Analytical Study of the Shock Tube I g n i ­ tion of Mixtures of Methane and Ethane"; UCRL-81507, Lawrence Livermore Laboratory, 1978. 10. Nagle, J.; Strickland-Constable, R. F. Proc. F i f t h Carbon Conf., 1962, 1, 154-164. 11. Levy, J. M. "Modeling of Fuel-Nitrogen Chemistry i n Combus­ t i o n : The Influence of Hydrocarbons"; F i f t h EPA Fundamental Combustion Research Workshop, Newport Beach, CA, 1980. 12. Kowalik, R.; Ruth, L . Α.; Edelman, R. B . ; Wong, E.; Farmer, R. C. "Fundamental Characterization of Alternate Fuel Effects i n Continuous Combustion Systems"; DOE/ET/11313-1, 1981, DOE. 13. Fenimore, C. P. "Formation of N i t r i c Oxide From Fuel-Nitrogen i n Ethylene Flames"; Comb. and Flame, 1972, 19, p. 289. 14. Fenimore, C. P. "Reactions of Fuel-Nitrogen i n Rich Flames"; Comb. and Flame, 1976, 26, p. 249. 15. McLain, A. G . ; Jachimowski, C. J. "Chemical Kinetic Modeling of Benzene and Toluene Oxidation Behind Shock Waves"; NASA TP 1472, 1979, NASA. Publication Date: April 29, 1983 | doi: 10.1021/bk-1983-0217.ch002

1.

x

RECEIVED October 25, 1982

3 Synthetic F u e l Atomization Characteristics

Publication Date: April 29, 1983 | doi: 10.1021/bk-1983-0217.ch003

R. G. OEDING Spectron Development Laboratories, Inc., Costa Mesa, CA 92626 W. D. BACHALO Aerometrics, Inc., Mountain View, CA 94042

A visualization study of fuel atomization using a pulsed laser holography/photography tech­ nique indicates that basic spray formation processes are the same for both a coal-derived synthetic fuel (SRC-II) and comparable petroleum fuels (Νο. 2 and Νο. 6 grade). Measurements were made on both pressure swirl and air assisted atomizers in a cold spray facility having well controlled fuel temperature. Quality of the sprays formed with SRC-II was between that of the No. 2 and No. 6 fuel sprays and was consistent with measured fuel viscosity. Sauter mean droplet diameter (SMD) was found to correlate with fuel viscosity, atomization pressure, and fuel flow rate. For all three fuels, a smaller SMD could be obtained with the air assisted than with the pressure swirl atomizer. The substitution of synthetic liquid fuels, derived from coal and shale, for dwindling petroleum resources is a major element of the future energy picture. The utilization of these synthetic fuels in an energy-efficient and environmentallyacceptable manner requires an improved understanding of funda­ mental combustion processes. Specifically, the low hydrogen to carbon ratio and high fuel nitrogen content typical of these synthetic fuels requires modifications to conventional combus­ tion approaches for control of Ν0 and particulate emissions. Atomization is the initial phase of the spray combustion process and has a strong influence on subsequent combustion and pollutant formation mechanisms. Although a number of combus­ tion studies (J_~3) have been conducted utilizing coal-derived liquid fuels, very limited spray characterization data exists for these alternate fuels. This paper summarizes the results of an experimental study to visualize and compare spray formation processes χ

0097-6156/83/0217-0049$06.00/0 © 1983 American Chemical Society

50

COMBUSTION OF SYNTHETIC FUELS

a s s o c i a t e d with t y p i c a l atomizers using both a c o a l - d e r i v e d s y n t h e t i c f u e l (SRC-II middle d i s t i l l a t e ) and petroleum f u e l s (No. 2 and No. 6 grade f u e l o i l s ) . Emphasis i s placed on the q u a l i t a t i v e assessment of f u e l and atomizer influences on spray formation and q u a l i t y . Experimental

Publication Date: April 29, 1983 | doi: 10.1021/bk-1983-0217.ch003

Spray F a c i l i t y The experimental study was conducted i n a l a b o r a t o r y s c a l e c o l d spray f a c i l i t y . The experimental c o n f i g u r a t i o n , shown schematically i n Figure 1, consisted of a 61 cm (24 i n . ) diameter c y l i n d r i c a l spray chamber with a v e r t i c a l l y o r i e n t e d atomizer and spray, a f u e l supply system, and an exhaust system with l i q u i d / v a p o r removal. Windows for o p t i c a l access were mounted along a h o r i z o n t a l o p t i c a l a x i s . V a r i a b l e speed blowers placed upstream supplied a 10 m/s primary a i r flow through a 15.2 cm (6 i n . ) diameter tube surrounding the i n j e c t o r and a 0.5 m/s screen a i r to maintain a uniform flow w i t h i n the test s e c t i o n . Wire mesh f i l t e r s were used i n the base of the chamber to remove l i q u i d f u e l while the remaining f u e l v a p o r / a i r mixture was exhausted by a large v a r i a b l e speed blower through an e x t e r i o r mounted carbon f i l t e r . A n i t r o g e n p r e s s u r i z e d f u e l system provided f u e l to the atomizer at w e l l c o n t r o l l e d temperatures up to 2 4 0 ° F . Preheating was provided by a combination of r e s e r v o i r and l i n e heaters. Atomizer a i r was obtained from the l a b o r a t o r y a i r supply. Atomizers Both a i r a s s i s t e d and pressure s w i r l atomizers t y p i c a l of o i l burning systems were used i n t h i s study. The pressure s w i r l atomizers u t i l i z e d f u e l pressure and t a n g e n t i a l s l o t s to create a s w i r l flow w i t h i n an i n t e r n a l chamber and o r i f i c e . The f u e l emerged from the o r i f i c e as a c o n i c a l l i q u i d sheet and subsequently d i s i n t e g r a t e d i n t o a c o n i c a l spray. Delavan atomi z e r s i n three d i f f e r e n t s i z e s (nominal flow rates of 1, 2, and 5 gph), three d i f f e r e n t spray angles (45, 60, and 90 degrees) and with hollow cone spray patterns were i n v e s t i g a t e d . Nominal f u e l pressures considered were 50, 100, and 150 p s i g . The a i r a s s i s t e d atomizer s e l e c t e d was the Sonicore 052H. I t u t i l i z e s a high v e l o c i t y a i r jet to shear and f r a g ment the f u e l i n t e r n a l l y . The r e s u l t i n g two phase jet passed from the o r i f i c e i n t o an e x t e r n a l l y mounted resonator cap designed to produce a resonant sonic f i e l d and enhance atomization. F u e l pressures from 0 to 12 p s i g and a i r pressures from 3 to 20 p s i g were used to study both nominal and off-nominal operation of the atomizer. T h i s range of operating pressures

OPTICAL

AIR

£

S C R E E N

P R I M A R Y

Experimental

1.

T R A P )

Figure

E X H A U S T ( L I Q U I D / V A P O R

A I R A I R

R E S E R V O I R W I T H H E A T E R

H*J-L

F I L T E R

Configuration

A T O M I Z E R

L I N E

A — < ^ H O P

Publication Date: April 29, 1983 | doi: 10.1021/bk-1983-0217.ch003

52

COMBUSTION OF SYNTHETIC FUELS

r e s u l t e d i n f u e l flow rates (for No. 2 f u e l o i l ) 0.7 to 4.4 gph.

ranging from

Publication Date: April 29, 1983 | doi: 10.1021/bk-1983-0217.ch003

Fuels P h y s i c a l p r o p e r t i e s of the three t e s t f u e l s are presented i n Table I . Except f o r the surface tension of No. 6 f u e l o i l , which was a t y p i c a l v a l u e , a l l p r o p e r t i e s were measured for the s p e c i f i c samples t e s t e d . The primary d i f f e r e n c e s between the SRC-II middle d i s t i l l a t e and the No. 2 f u e l were the higher s p e c i f i c g r a v i t y , surface t e n s i o n , and v i s c o s i t y of the SRC-II. The No. 6 grade f u e l , a r e s i d u a l f u e l o i l , had a much higher v i s c o s i t y than e i t h e r of the d i s t i l l a t e f u e l s . Both the SRC-II and No. 2 f u e l o i l were sprayed at a nominal temperature of 8 0 ° F to simulate usage i n a non-preheat combustion system. The No. 6 f u e l o i l was sprayed at temperatures ranging from 1 5 0 ° to 2 4 0 ° F i n order to assess spray formation processes and spray q u a l i t y over a broad range of v i s c o s i t i e s . TABLE I PHYSICAL PROPERTIES OF TEST FUELS

Property

SRC-II Middle D i s t i l l a t e

No. 2 Fuel O i l

No. 6 Fuel O i l

0.97

0.86

0.94

Specific gravity at 15°C ( 6 0 ° F ) Surface tension at 27°C ( 8 0 ° F ) (Dyne/cm) Viscosity at 27°C

30.9

27.1

(35.0)

(cs) (80°F)

5.0

4.0

60°C ( 1 4 0 ° F )

2.1

1.8

70.4

99°C ( 2 1 0 ° F )

-

-

16.3

D i a g n o s t i c Technique The dynamic events a s s o c i a t e d with spray formation p r o cesses were observed instantaneously using pulsed l a s e r holography and photography. O p t i c a l holography i s a technique which s t o r e s , for l a t e r r e c o n s t r u c t i o n , a l l of the coherent o p t i c a l information that has passed through or i s r e f l e c t e d from a volume of i n t e r e s t . Holograms of f u e l sprays i n c l u d i n g atomizer, spray formation r e g i o n , and i n i t i a l spray f i e l d were obtained and reconstructed to provide three-dimensional images

Publication Date: April 29, 1983 | doi: 10.1021/bk-1983-0217.ch003

3.

OEDING AND BACHALO

Atomization Characteristics

s u i t a b l e for d e t a i l e d plane-by-plane study (and measurement). A pulsed ruby l a s e r was used to provide the necessary coherent illumination. The l a s e r pulse duration was 20 nanosec which was adequate to freeze the motion of the sprays to less than 1 μ . Double pulsed holograms were a l s o obtained (pulse separa­ t i o n s 5 and 10 psec) i n order to determine v e l o c i t y and observe p h y s i c a l changes. The basic arrangement and performance s p e c i ­ f i c a t i o n s for the general purpose holocamera used for t h i s study have been presented p r e v i o u s l y (4). In a d d i t i o n to holograms, high r e s o l u t i o n b a c k - l i g h t e d photographs with the same f i e l d diameter as the holograms were obtained by blocking the reference beam and imaging the object beam. Imaging o p t i c s provided a 3x p r e - m a g n i f i c a t i o n f o r both holograms and photo­ graphs. This r e s u l t e d i n a r e a l sample volume diameter of ap­ proximately 3.8 cm (1.5 i n . ) . P a r t i c l e s i z e data were obtained f o r s e l e c t e d t e s t c o n d i ­ tions d i r e c t l y from holograms and photographs. Spray measure­ ments were made at an a x i a l distance of approximately 2.5 cm (1 i n . ) from the i n j e c t o r where spray formation processes had been completed and s p h e r i c a l droplets formed. Observations and D i s c u s s i o n Pressure S w i r l Atomizer Spray formation processes associated with pressure s w i r l atomizers are i l l u s t r a t e d i n F i g u r e 2, a b a c k - l i g h t e d l a s e r photograph of a preheated No. 6 f u e l spray. F u e l pressure i s u t i l i z e d to produce, v i a a s w i r l chamber and o r i f i c e , a c o n i c a l l i q u i d sheet. As the sheet expands, unstable wave forms develop due to i n t e r a c t i o n with the atmosphere and surface tension forces leading to d i s i n t e g r a t i o n of the sheet. This "wavy sheet" mode of d i s i n t e g r a t i o n i s one of s e v e r a l i d e n ­ t i f i e d by F r a s e r (5) and Is the dominant mode observed i n t h i s study. The unstable wave c r e s t s e v e n t u a l l y blow out forming a complex network of l i q u i d ligaments or threads. Droplet formation occurs p r i m a r i l y through the surface tension and v i s c o s i t y dominated breakup of these l i q u i d threads due to symmetric (or d i l a t i o n a l ) waves as described by R a y l e i g h (6) f o r i n v i s c i d l i q u i d s and by Weber (7_) for viscous f l u i d s . F i g u r e 3 shows the double pulsed image of the d r o p l e t formation process for No. 2 and SRC-II f u e l sprays under i d e n t i c a l atomizer c o n d i t i o n s . These two photographs i l l u s t r a t e t y p i c a l d i f f e r e n c e s seen between these two f u e l s . For the No. 2 f u e l spray, droplet formation from l i q u i d threads was r a p i d with t y p i c a l wave length-to-diameter r a t i o s c l o s e to a value of 4.5 as p r e d i c t e d by R a y l e i g h . D r o p l e t formation was slower and l e s s o r d e r l y f o r the SRC-II with long ligament extending into the i n i t i a l spray r e g i o n . While the s l i g h t l y higher surface tension of the SRC-II should enhance

53

Publication Date: April 29, 1983 | doi: 10.1021/bk-1983-0217.ch003

COMBUSTION OF SYNTHETIC FUELS

CM U

*

o) 00 Ν •H

Ή Ç0

So « 2 eu

·

3 W ο CO CO

Ο



Ι

Λ

Ν

«

Η

vO

sO

ON ON ΟΙ Ο

m .3· ON ON r H Ο ON Ο

ο

ο

H

H

m

N O O

H

c i CM CM CM CM r H C l

Ô cô so m

r«» vO

H

H

H

\ C Ν 00 CM CM CM C l

H Ο OMfl C l C l CM C l

ON 00 Ο vO C l C l ST C l

75

4.8

5.5

900

1100

L

1

l

96.8

u

0.0

74.7

87.7

q

S

i

d

7.9

0.0

l

94.5

20.5

o

( 3 )

G

a

23.8

25.4

20.8

7%

0.0

60.4

76.2

96.0

Liquid

4.0

s

(76.2)

14.5

3.0

0.0

Solid

( 3 , 4 )

33.9

33.2

(24.6)

4.6

Gas

( 1 )

0.0

56.2

72.1

95.4

Liquid

20%

66.1

10.6 ( 3 )

( 1 )

0.0 (3.4)

Solid

Based on averages o f data at c o n d i t i o n s of higher and lower oxygen c o n c e n t r a t i o n s . Based on assumption that no s o l i d s remained i n tube at high oxygen l e v e l s . S o l i d s caught i n condenser traps combined with s o l i d s l e f t on r e a c t o r w a l l . By d i f f e r e n c e .

73Z

=

(33.7)

(0.0)

(0.0)

0.0

Solid

_ _ _

0.0

49.9

71.7

93.7

Liquid

66.3

50.1

28.3

6.3

Gas

NORMALIZED AVERAGE RECOVERIES OF SOLIDS, LIQUIDS AND GASES FROM PYROLYSIS AND OXIDATIVE-PYROLYSIS OF SRC II DISTILLATE FRACTIONS

_ = _ _ _ _ = _ _ = ^ _ _ _ _ _ _ _ _ _ _ _ _ _ _

4.4

700

(1) (2) (3) (4)

3.2

S

500

a

Q%

G

d

8 e n

S t

°Temp,°*C

Av. Percent of

TABLE I I I .

Publication Date: April 29, 1983 | doi: 10.1021/bk-1983-0217.ch004

( 2 , 3 )

( 2 )

( 2 )

Η Ο

Η

oa

Q 5ζ Q *1

SH

g

ON

4.

LONGANBACH E T AL.

Bench-Scale Pyrolysis-Oxidation Studies

11

Publication Date: April 29, 1983 | doi: 10.1021/bk-1983-0217.ch004

decrease i n s u l f u r content from an average of 0.21 weight p e r cent i n the d i s t i l l a t i o n cuts to 0.06 weight percent i n the l i q u i d products. Other chemical changes were s m a l l . When the temperature was increased from 500°C to 1 1 0 0 ° C , s e v e r a l changes occurred i n each of the d i s t i l l a t e f r a c t i o n s . Aromaticity. The r a t i o of aromatic carbon to t o t a l carbon increased from an average of 0.53 i n the s t a r t i n g m a t e r i a l and 0.56 at 500°C to 0.83 at 7 0 0 ° C , 0.95 at 9 0 0 ° C , and 0.99 at 1100°C (for the s o l i d s ) . The number of aromatic r i n g s increased p r o p o r tionally. There was almost uniform behavior among the d i f f e r e n t b o i l i n g point f r a c t i o n s , as shown i n F i g u r e 3. A r o m a t i z a t i o n of the r i n g systems i s an important r e a c t i o n during p y r o l y s i s . Figure 4 shows the r e l a t i o n s h i p between a r o m a t i c i t y of the l i q u i d products at each temperature and the s t o i c h i o m e t r i c amount of oxygen a v a i l a b l e during o x i d a t i v e p y r o l y s i s . A r o m a t i c i t y does not appear to increase s i g n i f i c a n t l y with i n c r e a s i n g concentration of oxygen at any temperature. The number of aromatic r i n g s u b s t i t u e n t s and the average s u b s t i t u e n t length decreased with i n c r e a s i n g p y r o l y s i s temperature as shown i n Figures 5 and 6, r e s p e c t i v e l y . These data i n c l u d e a l i p h a t i c r i n g s attached to aromatic r i n g s at two p o i n t s , as i n t e t r a l i n - t y p e molecules. The numbers of s u b s t i t u e n t s decreased from approximately three per r i n g i n the s t a r t i n g m a t e r i a l and at 500°C to approximately one per r i n g at 9 0 0 ° C . The a l i p h a t i c s u b s t i t u e n t s are very s h o r t , ranging from an average of l e s s than two carbon atoms at low temperatures to l e s s than one carbon atom per s u b s t i t u e n t ( i n c l u d i n g phenolic f u n c t i o n a l groups) at 9 0 0 ° C . Data could not be obtained on the s o l i d s produced at 1 1 0 0 ° C . Molecular Weight. Aromatization of hydroaromatic s t r u c t u r e s , r a t h e r than removal of a l i p h a t i c s u b s t i t u e n t s , was suggested because the weight per molecule remained approximately constant with i n c r e a s i n g temperature up to 9 0 0 ° C . The s h i f t to s o l i d products at 1 1 0 0 ° C probably represents a s i g n i f i c a n t i n c r e a s e i n molecular weight. The r e l a t i o n s h i p between molecular weight and f r a c t i o n of s t o i c h i o m e t r i c oxygen i s shown i n F i g u r e 7. At 500 and 7 0 0 ° C , molecular weight d e c l i n e d but at 900°C the molecular weight i n creased with i n c r e a s i n g f r a c t i o n of s t o i c h i o m e t r i c oxygen. Oxidat i o n (the a d d i t i o n of oxygen) appears to be the cause of the molecular weight i n c r e a s e . No data could be obtained at 1 1 0 0 ° C . Elemental Composition. The weight percent of carbon was roughly constant from 5 0 0 ° t o 900°C but jumped s i g n i f i c a n t l y at 1 1 0 0 ° C , as shown i n F i g u r e 8. The 1 1 0 0 ° C sample was c o l l e c t e d i n the condensers and was probably obtained i n approximately the same residence time as the samples obtained at the lower temperatures. Hydrogen dropped to l e s s than 1 weight percent and oxygen (by d i f f e r e n c e ) a l s o dropped to about 1 weight percent at 1 1 0 0 ° C .

Publication Date: April 29, 1983 | doi: 10.1021/bk-1983-0217.ch004

COMBUSTION OF SYNTHETIC FUELS

È * β ι ê β

Ο

Ο /

_

» _»

Ο (Η .



LEGEND • = MD-2 o=MD-3 Δ = MD-4 ο = HD-2

αθ

200.0

400.0 600.0 800.0 P y ro lys is T e m p e r a t u r e , C

1000.0

1200.0

β

FIGURE 3. AROMATICITY OF LIQUID PRODUCTS VERSUS PYROLYSIS TEMPERATURE

LONGANBACH ET AL.

Publication Date: April 29, 1983 | doi: 10.1021/bk-1983-0217.ch004

ν

Bench-Scale Pyrolysis-Oxidation Studies

-

LEGEND • = 500 °C ο = 700 °C Δ =900°C ο = 1100 -C

0.0

02

0.4

0.6

—r— 0.8

1.0

12

F r a c t i o n S t o i c h i o m e t r i c Oxygen FIGURE 4. AROMATICITY VERSUS FRACTION OF STOICHIOMETRIC OXYGEN

80

COMBUSTION OF SYNTHETIC FUELS

Δ

Ο

Ο

MD-2



MD-3

Ο MD-4

Publication Date: April 29, 1983 | doi: 10.1021/bk-1983-0217.ch004

Δ

HD-2

Ο

h-

ο

ο

• \

Δ

\

\

\

\

\

\

\

\

\8 •

Starting Material

500

700

900

Λ

Pyrolytis Temperature, C

FIGURE 5. NUMBER OF AROMATIC RING SUBSTITUENTS (R ) VERSUS PYROLYSIS TEMPERATURE S

4.

LONGANBACH E T AL.

81

Bench-Scale Pyrolysis-Oxidation Studies

Publication Date: April 29, 1983 | doi: 10.1021/bk-1983-0217.ch004

Z5

Ο zol 0

15

a

O

MD-2



MD-3

Δ

HD-2

ζ) MD-4

_



\

1.0

• 0.51

Δ Starting Material

500

700 Pyrolysis Temperature, C

FIGURE 6. SUBSTITUENT LENGTH (n) VERSUS PYROLYSIS TEMPERATURE

900

Publication Date: April 29, 1983 | doi: 10.1021/bk-1983-0217.ch004

COMBUSTION OF SYNTHETIC FUELS

LEGEND • = 500°C o = 700°C Δ =900°C

0.0

02

OA

αβ

αβ

F r a c t i o n S t o i c h i o m e t r i c Oxygen FIGURE 7. MOLECULAR WEIGHT VERSUS FRACTION OF STOICHIOMETRIC OXYGEN

LONGANBACH E T A L .

Bench-Scale Pyrolysis-Oxidation Studies

83

Publication Date: April 29, 1983 | doi: 10.1021/bk-1983-0217.ch004

4.

200.0

400.0

600.0

800.0

1000.0

1200.0

P y r o l y s i s Temperature, °C FIGURE 8. CARBON, HYDROGEN AND OXYGEN CONTENT VERSUS PYROLYSIS TEMPERATURE

Publication Date: April 29, 1983 | doi: 10.1021/bk-1983-0217.ch004

84

COMBUSTION OF SYNTHETIC FUELS

Compounds containing oxygen and hydrogen are destroyed between 9 0 0 ° a n d 1 1 0 0 ° C at the residence times used i n these experiments (about 7 seconds). A h i g h l y carbonaceous m a t e r i a l , presumably with a g r a p h i t i c s t r u c t u r e , was obtained at 1 1 0 0 ° C . There appeared to be l i t t l e d i f f e r e n c e i n the carbonaceous products obtained at 1 1 0 0 ° C from the d i f f e r e n t SRC I I d i s t i l l a t i o n f r a c tions . There was a s i g n i f i c a n t i n c r e a s e i n the c o n c e n t r a t i o n of other elements, presumably oxygen, up to 9 0 0 ° C . The i n c r e a s e i n oxygen must r e s u l t from p r e f e r e n t i a l r e t e n t i o n i n the l i q u i d products of the oxygen i n the SRC I I d i s t i l l a t i o n c u t s . There i s no other source of oxygen i n the system. T h i s i s not s u r p r i s i n g s i n c e the n o n l i q u i d products c o n s i s t of soot (carbon) and l i g h t gases ( p r i m a r i l y methane). Most or a l l of the oxygen i n the SRC II d i s t i l l a t i o n cuts i s p h e n o l i c . The p h e n o l i c s u b s t i t u e n t apparently i n h i b i t s soot formation, p o s s i b l y by quenching free r a d i c a l s formed thermally during p y r o l y s i s . The hydrogen d i s t r i b u t i o n i s shown i n more d e t a i l versus temperature i n F i g u r e 9. The average number of hydrogen atoms per molecule decreased and a l i p h a t i c hydrogen decreased to no more than one per molecule at 9 0 0 ° C . There was an i n c r e a s e i n the number of aromatic hydrogens per molecule. The changes i n elemental composition of the o x i d a t i o n p y r o l y s i s l i q u i d s w i t h i n c r e a s i n g f r a c t i o n of s t o i c h i o m e t r i c oxygen are shown i n F i g u r e s 10-12. Carbon and hydrogen d e c l i n e d s l i g h t l y while oxygen content i n c r e a s e d . This i s apparently due to oxygen i n c o r p o r a t i o n i n the molecules by o x i d a t i o n . A mechanism such as the one described by Santoro and Glassman, i n which dihydroxy benzenes are i n t e r m e d i a t e s , i s i n agreement with t h i s observation (4). Unsubstituted Aromatics. The analyses i n d i c a t e d that the mixture was s i m p l i f i e d during p y r o l y s i s u n t i l i t c o n s i s t e d p r i m a r i l y of unsubstituted aromatics such as benzene, naphthalene, phenanthrene, and pyrene. Of course, aromatization i s not the only mechanism o c c u r r i n g s i n c e l i q u i d products with both higher and lower molecular weights than the s t a r t i n g l i q u i d s , as w e l l as gases and carbon, were formed. However, s u b s t a n t i a l amounts of the unsubstituted aromatics were found, as shown i n F i g u r e 13. At 900°C more than 60 percent of each l i q u i d product was unsubstituted aromatics. The amounts of one, two, and three r i n g molecules ( i . e . , benzene, naphthalene, and phenenthrene) v a r i e d with the molecular weight of the s t a r t i n g material. For example, the l i q u i d product of MD-3 at 900°C was more than 50 percent benzene while naphthalene was more than 30 percent of the l i q u i d product from MD-4 at 9 0 0 ° C . These u n s u b s t i tuted aromatics are more thermally s t a b l e than s u b s t i t u t e d aromatic molecules and can be considered soot precursors i n staged combustion processes.

LONGANBACH E T AL.

Bench-Scale Pyrolysis-Oxidation Studies

Publication Date: April 29, 1983 | doi: 10.1021/bk-1983-0217.ch004

4.

FIGURE 9. HYDROGEN DISTRIBUTION VERSUS PYROLYSIS TEMPERATURE

85

Publication Date: April 29, 1983 | doi: 10.1021/bk-1983-0217.ch004

COMBUSTION OF SYNTHETIC FUELS

FIGURE 10. CARBON COMPOSITION VERSUS FRACTION OF STOICHIOMETRIC OXYGEN

Bench-Scale Pyrolysis-Oxidation Studies

Publication Date: April 29, 1983 | doi: 10.1021/bk-1983-0217.ch004

LONGANBACH ET AL.

FIGURE 11. HYDROGEN COMPOSITION VERSUS FRACTION OF STOICHIOMETRIC OXYGEN

Publication Date: April 29, 1983 | doi: 10.1021/bk-1983-0217.ch004

COMBUSTION OF SYNTHETIC FUELS

FIGURE 12. OXYGEN COMPOSITION VERSUS FRACTION OF STOICHIOMETRIC OXYGEN

LONGANBACH ET AL.

Bench-Scale Pyrolysis-Oxidation Studies

89

Publication Date: April 29, 1983 | doi: 10.1021/bk-1983-0217.ch004

4.

FIGURE 13. UNSUBSTITUTED AROMATICS IN THE LIQUID PRODUCT VERSUS PYROLYSIS TEMPERATURE

90

COMBUSTION OF SYNTHETIC FUELS

Publication Date: April 29, 1983 | doi: 10.1021/bk-1983-0217.ch004

F i g u r e 14 shows the amounts of unsubstituted aromatics i n the l i q u i d products versus f r a c t i o n of s t o i c h i o m e t r i c oxygen present. At the highest concentrations of oxygen and 9 0 0 ° C , the amounts of unsubstituted aromatics decreased, apparently due to the formation of molecules with oxygen s u b s t i t u t e d on the aromatic r i n g s . Nitrogen D i s t r i b u t i o n . As shown i n F i g u r e 15, t o t a l n i t r o g e n was unchanged from room temperature to 5 0 0 ° C , was p r e f e r e n t i a l l y r e t a i n e d i n the unpyrolyzed l i q u i d s between 500 and 700°C and was destroyed at an i n c r e a s i n g r a t e from 700 to 1 1 0 0 ° C . There was a conversion of b a s i c to nonbasic n i t r o g e n below 500°C and b a s i c n i t r o g e n appeared to be l e s s s t a b l e than nonbasic n i t r o g e n at a l l temperatures. I t i s c u r r e n t l y thought that b a s i c n i t r o g e n i s p r i m a r i l y p y r i d i n e and q u i n o l i n e - t y p e compounds, which are r e l a ­ t i v e l y s t a b l e , and nonbasic n i t r o g e n i s p y r o l e and s i m i l a r com­ pounds, which are g e n e r a l l y l e s s s t a b l e . These r e s u l t s suggest that the assumed n i t r o g e n types are i n c o r r e c t . F i g u r e 16 i n d i c a t e s that n i t r o g e n d i s t r i b u t i o n remains approximately constant as the f r a c t i o n of s t o i c h i o m e t r i c oxygen i s increased. Summary The o x i d a t i v e p y r o l y s i s r e a c t i o n s of SRC I I d i s t i l l a t i o n cuts b o i l i n g from 150°C to 350°C are s i m i l a r . Aromatization occurs as temperature i s i n c r e a s e d , r e s u l t i n g i n n e a r l y constant carbon c o n c e n t r a t i o n and molecular weight from 500°C to 900°C with a s i g n i f i c a n t l o s s of t o t a l hydrogen (although aromatic hydrogen increases) and a s i g n i f i c a n t increase i n oxygen concen­ tration. This i n d i c a t e s that phenolic oxygen i s a r e l a t i v e l y s t a b l e f u n c t i o n a l group, while short a l i p h a t i c side chains are l o s t to produce l a r g e amounts of unsubstituted aromatics. These are presumed to be soot precursors i n staged combustion. Nonbasic n i t r o g e n i s more s t a b l e than b a s i c n i t r o g e n . Total n i t r o g e n content increases up to 700°C although a s i g n i f i c a n t i n t e r c o n v e r s i o n of b a s i c to nonbasic n i t r o g e n occurs around 5 0 0 ° C . The s t a b l e nonbasic n i t r o g e n compounds may be the major source of Ν 0 emissions from the second stage of staged combustion. As the percent s t o i c h i o m e t r i c oxygen i s i n c r e a s e d , a r o m a t i c i t y remains constant. Carbon and hydrogen content and the con­ c e n t r a t i o n of unsubstituted aromatics decrease as oxygen i s added by o x i d a t i o n . Thus, the most l i k e l y s u r v i v o r s of f u e l - r i c h , f i r s t - s t a g e p y r o l y s i s are unsubstituted and oxygen s u b s t i t u t e d aromatics and nonbasic n i t r o g e n compounds. These are the precursors to soot and Ν 0 formation during o x y g e n - r i c h , second-stage combustion. χ

χ

Publication Date: April 29, 1983 | doi: 10.1021/bk-1983-0217.ch004

4.

LONGANBACH ET AL.

0.0

02

91

Bench-Scale Pyrolysis-Oxidation Studies

0.4

0.6

OA

1.0

XZ

F r a c t i o n S t o i c h i o m e t r i c Oxygen FIGURE 14. CONCENTRATION OF UNSUBSTITUTED AROMATICS IN THE LIQUID PRODUCT VERSUS FRACTION OF STOICHIOMETRIC OXYGEN

Publication Date: April 29, 1983 | doi: 10.1021/bk-1983-0217.ch004

COMBUSTION OF SYNTHETIC FUELS

40O0

600.0

800.0

P y r o l y s i s T e m p e r a t u r e , °C

1200.0

FIGURE 15. NITROGEN DISTRIBUTION VERSUS PYROLYSIS TEMPERATURE

LONGANBACH E T AL.

Bench-Scale Pyrolysis-Oxidation Studies

93

Publication Date: April 29, 1983 | doi: 10.1021/bk-1983-0217.ch004

4.

FIGURE 16. NITROGEN DISTRIBUTION VERSUS FRACTION OF STOICHIOMETRIC OXYGEN (Total Ν , Basic Ν )

94

COMBUSTION OF SYNTHETIC FUELS

Acknowledgment Samples of SRC II naphtha, middle d i s t i l l a t e , and heavy d i s ­ t i l l a t e s were provided by Mr. David Schmalzer of the P i t t s b u r g and Midway Coal Mining Company. Support by the United States Department of Energy, Contract No. DE-AC22-80PC-30502, i s g r a t e f u l l y acknowledged.

Literature Cited

Publication Date: April 29, 1983 | doi: 10.1021/bk-1983-0217.ch004

1.

Black, C.H.; Chin, H.H.; Fischer, J.; Clinch, J.D. "Review and Analysis of Spray Combustion as Related to Alternative Fuels"; Argonne National Laboratory; Report No. ANL-79-77 prepared for the U.S. Department of Energy; Contract No. W-31-109-Eng-38. 2. Moore R.T.; McCutchan, P.; Young, D.A.; Anal. Chem. 1951; 23 (11), 1639-1641. 3. Schwager, I.; Farmanian, P.Α.; Yen, T.F. "Analytical Chemistry of Liquid Fuel Sources", ACS Advances in Chemistry Series 170; American Chemical Society: Washington, D.C. 1978; Chapter 5. 4. Santoro, R.J.; Glassman, I.; Combustion Science and Technology 1979; 19, 161-164. RECEIVED

October 29, 1982

5 Nonequilibrium Distillation Effects in Vaporizing Droplet Streams

Publication Date: April 29, 1983 | doi: 10.1021/bk-1983-0217.ch005

S. HANSON, J. M. BEER, and A. F. SAROFIM Massachusetts Institute of Technology, Department of Chemical Engineering, Cambridge, MA 02139

The combustion of fossil fuels in the United States will be complicated by an increased reliance on fuels with high nitrogen and low hydrogen content. The problem will be aggravated by the displacement of petroleum-based fuels by coal-derived synthetic fuels and shale oil. Previous studies on the contribution of fuel nitrogen to ΝO emissions (1,2) used specific combustion systems and fuels doped with fuel nitrogen. These experiments provided useful information for the systems studied and indicated that the most effective met­ hod for limiting the formation of NO from fuel nitrogen is staged combustion. Staged combustion takes advantage of the fact that fuel nitrogen in the gas phase is strongly dependent on the oxygen available. This method consists of a fuel-rich first stage, in which the fuel nitrogen is released and converted mostly to N , followed by a second stage in which the remaining oxygen is added to complete the combustion. The process requires that the fuel nitrogen be evolved into the gas phase prior to exiting the first stage, and with sufficient residence time to participate in the gas phase reactions that form N . Therefore, the objective of this study has been to determine the rate of nitrogen evolution from vaporizing fuel droplets. x

x

2

2

In order to obtain detailed information on the evolution of fuel nitrogen, a laminar flow drop tube furnace, able to simulate conditions found in actual combustion systems, was adopted for this study. The nitrogen in fuels consists of complex, mostly heterocy­ clic compounds. In petroleum crudes, these include pyrroles, in­ doles, isoquinolines, acridines, and porphyrins. During refining most of these concentrate in the heavy resin and asphaltene frac­ tions, which might suggest their relatively late release in the 0097-6156/83/0217-0095$06.00/0 © 1983 American Chemical Society

96

COMBUSTION OF SYNTHETIC FUELS

Publication Date: April 29, 1983 | doi: 10.1021/bk-1983-0217.ch005

combustion of r e s i d u a l f u e l o i l s . However, the e v o l u t i o n of n i t r o gen from a heavy f u e l o i l d r o p l e t during i t s v a p o r i z a t i o n i n a flame d i f f e r s s i g n i f i c a n t l y from the r e s u l t s of e q u i l i b r i u m d i s t i l l a t i o n obtained under slow h e a t i n g c o n d i t i o n s at atmospheric pressure. The e v o l u t i o n of a p a r t i c u l a r compound i s determined by the r a t e of d r o p l e t v a p o r i z a t i o n , the r e l a t i v e v o l a t i l i t y and d i f f u s i v i t y i n the l i q u i d phase of the compound i n question. Because of the complex composition of f u e l o i l s , i t i s d i f f i c u l t to separate the r e l a t i v e importance of these e f f e c t s s i n c e the measured n i t r o gen e v o l u t i o n i s a summation of many f r a c t i o n s , each of which i s i n f l u e n c e d to a d i f f e r e n t extent. In order to r e v e a l the r o l e of d i f f u s i o n and r e l a t i v e v o l a t i l i t y i n the e v o l u t i o n of f u e l n i t r o gen, a complementary set of experiments were performed using ndodecane doped w i t h p y r i d i n e , q u i n o l i n e or a c r i d i n e . In the f o l l o w i n g , experimental r e s u l t s are presented of the e v o l u t i o n of f u e l n i t r o g e n during e q u i l i b r i u m d i s t i l l a t i o n and i n e r t p y r o l y s i s of d r o p l e t arrays i n the laminar flow furnace f o r three f u e l o i l s and a doped model f u e l .

EXPERIMENTAL The experimental furnace i s a v e r t i c a l l y o r i e n t e d laminar f l o w drop tube furnace having a 30 cm long uniformly hot t e s t sect i o n w i t h o p t i c a l access. The f u e l d r o p l e t array i s introduced on the l o g i t u d i n a l a x i s c o n c u r r e n t l y w i t h the ambient gas. The dropl e t stream i s i n t e r r u p t e d at s e v e r a l p o i n t s i n i t s t r a j e c t o r y by a sampling probe i n s e r t e d a x i a l l y from the base of the furnace. The probe quenches and t r a n s p o r t s the e n t i r e f l o w to a sampling t r a i n which recovers the f u e l d r o p l e t residue f o r a n a l y s i s . The above process i s repeated at s e v e r a l furnace temperatures f o r each f u e l . A d e t a i l e d d e s c r i p t i o n of the system i s to be found i n references (3) and (4).

RESULTS Table 1 presents the f u e l p r o p e r t i e s and composition of the f u e l s s t u d i e d , namely, an Indo-Malaysian r e s i d u a l petroleum f u e l , a Gulf #6 petroleum f u e l , and a raw Paraho shale o i l . These f u e l s a l l have a C/H atomic r a t i o of about 0.6 and have s i m i l a r b o i l i n g curves. However, when one considers the n i t r o g e n content, d i f f e r ences i n composition and behavior emerge. The Indo-Malaysian r e s i d u a l petroleum f u e l contains the l e a s t n i t r o g e n , 0.25% by weight; the raw Paraho shale o i l contains the most, 2.15% by weight. The Gulf #6 petroleum f u e l resembles the Indo-Malaysian petroleum f u e l more c l o s e l y i n source and composition, having 0.44% by weight n i -

5.

HANSON E T AL.

TABLE 1:

Publication Date: April 29, 1983 | doi: 10.1021/bk-1983-0217.ch005

97

Nonequilibrium Distillation Effects

API G r a v i t y at 1 5 ° C

FUEL OIL ANALYSIS

Raw Paraho Shale O i l

Indo-Malaysian Petroleum

Gulf #6 Petroleum

37

21.8

13.2

100 27 12

300 50 18

49

99

68

27

16

4.5

Heat of Combustion Gross B t u / l b Net Btu/lb

19,400 18,240

19,070 17,980

18,400 17,260

Elemental A n a l y s i s Carbon % Hydrogen % Nitrogen % Sulfur % Ash % Oxygen %

83.55 11.69 2.15 0.74 0.09 1.65

V i s c o s i t y ssu at 40°C ssu at 70°C ssu at 100°C F l a s h Point

e

C

Pour Point °C

87.08 12.05 0.25 0.23 0.036 0.48

88.29 12.31 0.44 · χ

V 1300 Κ Ο 1400Κ

COAST

NO. 6 P E T R O L E U M

1.3 1.2 1.1 1.0

IΝ D O - M A L A Y SI A Ν NO. 6

PETROLEUM

1.3

oc
( C 0 )

2

F

(16)

2

M

·> ( C 0 )

2

(17)

2

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

~

4

]

β

_ dMl Î ^ i -

=

i n

1 0

1 0

1 3 . 2[

14.6

R R U

C 1

Ί

0.7

V

Γ Λ

Ο Ί

02 . 8

^Ο· ]

[ C O ] [ H 2 0 ]

0.5

^ - " [ C O J ^ O I

0

[ 0 2 ]

-

5

e x

, 48,000.

P(

0.25

^ ]

0

-

2

ε

5

, 18 . 1 F T

§ϊ—)» ( )

χ

ρ

^ 4 0 ^ ,

^

4

exp ( - - ^ 2 0 )

(20)

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

8[H ]

dt

[CO]

d[H 0]

2

d[C0 ] 2

dt

(21)

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

= 8 χ 10

1 4

that

6

5

· [Η ][Η 0]°· [0 }°· 2

2

2

2 5

βχρ(-

4 0

Q 0 Q

> ) RT

(22)

126

COMBUSTION OF SYNTHETIC FUELS

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

£ £

e

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

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

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

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

plausible

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

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

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

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

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

128

COMBUSTION OF SYNTHETIC FUELS

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

• Flame Β Ο Flame C



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

6

1

1

I

7

8

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

l_J

I

I

I

I

3

l/T

[K" ] 1

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

l / T f o r water

6.

LEVY E T AL.

Global Flame Kinetics

129

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

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

2

L

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

130

COMBUSTION OF SYNTHETIC FUELS

TABLE 3.

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

Mixture Number MN

Mixture Ratio R

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

CO

H

2

CH4

C0

2

0

2

N

2

1

1.0 0.50 0.0

0.590 0.566 0.542

0.295 0.147 0.0

0.0 0.096 0.193

0.115 0.151 0.187

0.0 0.039 0.078

0.0 0.0 0.0

2

1.0 0.50 0.0

0.295 0.147 0.0

0.590 0.373 0.156

0.0 0.205 0.410

0.115 0.191 0.268

0.0 0.083 0.166

0.0 0.0 0.0

3

1.0 0.50 0.0

0.442 0.360 0.277

0.442 0.221 0.0

0.0 0.171 0.342

0.115 0.179 0.243

0.0 0.069 0.138

0.0 0.0 0.0

4

1.0 0.67 0.50 0.33 0.0

0.257 0.249 0.245 0.240 0.232

0.128 0.085 0.064 0.043 0.0

0.0 0.028 0.042 0.055 0.083

0.115 0.131 0.140 0.148 0.164

0.0 0.0 0.0 0.0 0.0

0.500 0.507 0.511 0.514 0.521

5

1.0 0.50 0.0

0.128 0.064 0.0

0.257 0.162 0.067

0.0 0.087 0.173

0.115 0.167 0.218

0.0 0.0 0.0

0.500 0.521 0.541

6

1.0 0.50 0.33

0.192 0.155 0.148

0.192 0.096 0.064

0.0 0.073 0.097

0.115 0.158 0.173

0.0 0.0 0.0

0.500 0.518 0.523

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

2

LEVY E T A L .

Global Flame Kinetics

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

6.

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

131

COMBUSTION OF SYNTHETIC FUELS

132 Acknowledgment This of Energy debted to Equation

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

Literature Cited

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

1.

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

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

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

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

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

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

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

134

COMBUSTION OF SYNTHETIC FUELS

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

X

X

X

X

X

X

X

X

X

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

X

X

X

X

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

X

X

X

X

7.

KOWALIK AND RUTH

Continuous Combustion Systems

135

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

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

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

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

136

COMBUSTION

Figure 1.

OF SYNTHETIC

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

FUELS

7.

KOWALIK AND RUTH

Continuous Combustion Systems

137

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

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

138

COMBUSTION OF SYNTHETIC FUELS

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

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

7.

KOWALIK AND RUTH

Table I.

Combustor

φ

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

TFN , % of

NO

rsc

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

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

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

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

o f v

s

4uel N JSC-S

139

Continuous Combustion Systems

;

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

;

HC ,vol,

% x

^as C H ' 4

0.03 0.59 1.97 4.34

-

SI CO

μ! C

6

c

£ ω

LU ,«

Ο ce

5 =

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

LU H . Ο

> I

\

- V

20 L

1

7^

ι

10

15

20

RESIDENCE TIME (ms) Figure for

4.

TFN Emissions and HC Concentration v s .

Residence Time

Prevaporized Toluene Fuel

1.4

1.6

1.8

EQUIVALENCE RATIO φ Figure 5. TFN Emissions v s . Equivalence Ratio f o r Prevaporized Isooctane F u e l at Two Residence Times

COMBUSTION OF SYNTHETIC FUELS

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

146

ISOOCTANE τ = 6 ms

Q

JSC-S LFJSC #

OD •

J 2

1.2

1.4

1.6

1.8

EQUIVALENCE RATIO Φ Figure 6. TFN Emissions and HC Concentration v s . Equivalence Ratio f o r L i q u i d Spray and Prevaporized Isooctane F u e l

7.

147

Continuous Combustion Systems

KOWALIK AND RUTH

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

100 TOLUENE τ = 8 ms JSC-S O D

80

JSC-L

-|

# • Ζ

ζ ο

g^ co " c

Ο αΓ 60 oc £

CO —

il LU

^

ζ ° 40 I

20

J1

1.2

1.4

1.6

1.8

EQUIVALENCE RATIO Φ Figure 7. TFN Emissions and HC Concentration v s . Equivalence Ratio i n Two Combustors of D i f f e r e n t Size f o r Prevaporized Τ oluene F u e l

American Chemical Society Library 1155 16th St. N. W. Washington, D. C. 20030

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

148

COMBUSTION OF SYNTHETIC FUELS

smaller J S C ; however, u n c e r t a i n t i e s i n the data preclude any f u r t h e r d i s t i n c t i o n between the two l i k e l y causes of the d i f ­ ferences i n r e s u l t s . Mixing arguments f o l l o w i n g E v a n g e l i s t a , et a l . (10) do not appear to e x p l a i n observed d i f f e r e n c e s s i n c e r e l a t i v e mixing i n t e n s i t i e s at f i x e d residence time should have been l a r g e r i n the l a r g e r J SC and s i n c e one would normally expect l a r g e r HC concentrations with smaller mixing i n t e n s i t i e s . The observed d i f f e r e n c e s among the data obtained at s i m i l a r JSC and LFJSC operating c o n d i t i o n s suggest that comparisons of data from the v a r i o u s combustors must allow f o r p o s s i b l e e f f e c t s of combustor type i n the r e s u l t s . Extensions of the r e s u l t s to l a r g e r , commercial s i z e gas t u r b i n e combustors or to s i m i l a r l a b o r a t o r y combustors should a l s o be made w i t h extreme c a u t i o n . Q u a l i t a t i v e trends such as the dependence of f u e l n i t r o g e n con­ v e r s i o n on HC concentrations should be v a l i d w i t h i n a given combustor; however, q u a n t i t a t i v e numerical r e s u l t s may depend s t r o n g l y on s p e c i f i c combustor c h a r a c t e r i s t i c s . For example, HC concentrations f o r T=10ms toluene runs i n the l a r g e r J S C , were l a r g e r than HC concentrations f o r x=6ms toluene runs i n the smaller J S C , even though the general trend w i t h i n each combus­ tor was smaller HC concentrations f o r longer residence times. Summary and Conclusions Results from experiments i n three l a b o r a t o r y j e t - s t i r r e d combustors suggest that the conversion of fuel-bound n i t r o g e n to t o t a l f i x e d n i t r o g e n CTFN) i n f u e l - r i c h mixtures i s s t r o n g l y r e l a t e d to the c o n c e n t r a t i o n of unburned hydrocarbons (HC's) w i t h i n the combustor. As mixture equivalence r a t i o s increased from 1.2 to 1.8, the f r a c t i o n of f u e l n i t r o g e n converted to TFN u s u a l l y decreased to a minimum and then i n c r e a s e d . The minimum l e v e l of conversion g e n e r a l l y occurred at an equivalence r a t i o near that at which a p p r e c i a b l e HC's f i r s t appeared i n the exhaust gases. V a r i a t i o n s of residence time and combustor type which reduced H C s also reduced the degree of f u e l n i t r o g e n convers ion. F u e l type ( a l i p h a t i c v s . aromatic) d i f f e r e n c e s were most p r o ­ nounced at very r i c h ( φ = 1 . 8 ) , short residence time ( τ < 10ms) conditions. At these c o n d i t i o n s t o l u e n e / p y r i d i n e mixtures p r o ­ duced l e s s TFN and more soot than s i m i l a r i s o o c t a n e / p y r i d i n e mix­ tures. These trends suggested a r e l a t i o n s h i p between soot p r o ­ duction and f u e l n i t r o g e n c o n v e r s i o n , p o s s i b l y a heterogeneous i n t e r a c t i o n between soot and HCN. f

Acknowledgment T h i s research was conducted f o r the U . S. Department of Energy as part of Contract No. DE-AC22-77ET11313. Funding f o r the f u e l n i t r o g e n conversion experiments was provided by Exxon Research and Engineering Company.

7. KOWALIK AND RUTH

Continuous Combustion Systems 149

Literature Cited 1. 2.

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

3.

4. 5. 6. 7. 8. 9. 10.

Kowalik, R. M.; Ruth, L. A. "Particulate Carbon: Formation During Combustion"; Siegla, D. C.; Smith, G. W.; Ed.; Plenum Press: New York, 1981, p. 285. Blazowski, W. S.; Edelman, R. B.; Harsha, P. T. "Fundamental Characterization of Alternate Fuel Effects in Continuous Com­ bustion Systems", Summary Τechnical Progress Report for the period 15 August 1977 - 14 August 1978 under Department of Energy Contract EC-77-C-03-1543, September, 1978. Blazowski, W. S.; Edelman, R. B.; Wong, E. "Fundamental Characterization of Alternate Fuel Effects in Continuous Combustion Systems", Summary Τechnical Progress Report for the period 15 August 1978 - 31 January 1980 under Department of Energy Contract DE-AC03-77ET11313, February, 1980. Hardy, J . E.; Knarr, J. J. J. Air Pollut. Control Assoc. 1982, 32, 376. Blazowski, W. S.; Sarofim, A. F.; Keck, J. C. J. Eng. Power 1981, 103, 43. Seeker, W. R.; Samuelson, G. S.; Heap, M. P.; Trolinger, J. P. "18th Symp. (Int.) on Combustion"; The Combustion Insti­ tute: Pittsburgh, 1981; p. 1213. Lanier, W. S. unpublished work. DeSoete, G. G., Riv. Combust. 1981, 35, 1. Wendt, J. O. L . , personal communication. Evangelista, J . J.; Shinnar, R.; Katz, J.; "12th Symp. (Int.) on Combustion"; The Combustion Institute: Pittsburgh, 1969; p. 901.

RECEIVED November 2, 1982

8 Synthetic F u e l Character Effects on a

Rich-Lean

Publication Date: April 29, 1983 | doi: 10.1021/bk-1983-0217.ch008

Gas Turbine Combustor LEONARD C. ANGELLO and WILLIAM C. ROVESTI Electric Power Research Institute, Palo Alto, CA 94303 THOMAS J. ROSFJORD United Technologies Research Center, East Hartford, CT 06108 RICHARD A. SEDERQUIST United Technologies Corp., South Windsor, CT 06074 Five fuels including No. 2 fuel oil, SRC II, Η-Coal, and EDS middle distillates, and hydrotreated Paraho shale oil residual were tested in a subscale 5-inch diameter, staged rich-lean com­ bustor at conditions representative of baseload and part power settings of 30-MW utility combustion turbine. A minimumNO emission level corrected to 15% oxygen of approximately 35 ppmv was attained for all the fuels despite fuel bound nitrogen levels of up to 0.8 percent by weight. Smoke emissions did depend on fuel properties and ranged between a SAE Smoke Number of 20 to 45 at baseload operation. Indication of increased smoke and liner heating with reduced fuel hydrogen content was observed, although the indicated trends were not as consistent as those for lean combustors. x

Rich-lean combustion systems are a recent generic c l a s s of s t a t i o n a r y gas turbine combustors capable of low Ν 0 emission performance w i t h f u e l s c o n t a i n i n g high concentrations of n i t r o ­ gen. Several r i c h - l e a n combustor designs are c u r r e n t l y under development by u t i l i t y gas turbine manufacturers as part of the ongoing DOE/NASA Low Ν 0 Heavy Fuel Combustor Concepts Program (1)· As i l l u s t r a t e d i n Figure 1 the r i c h - l e a n combustor concept i s s i m i l a r to the f u e l staging technique used i n b o i l e r com­ b u s t i o n systems f o r c o n t r o l l i n g Ν 0 emissions from f u e l s con­ t a i n i n g high f u e l - n i t r o g e n . In b r i e f , a small amount of primary a i r i s mixed w i t h the f u e l i n the head-end of a r i c h lean combustor. This creates a f u e l r i c h combustion zone t o r e l e a s e n i t r o g e n from f u e l s c o n t a i n i n g n i t r o g e n compounds and maximizes the e a r l y formation of molecular n i t r o g e n . This r i c h burn step i s followed by the r a p i d i n t r o d u c t i o n of secondary a i r χ

χ

χ

0097-6156/83/0217-0151$06.50/0 © 1983 American Chemical Society

152

COMBUSTION OF SYNTHETIC FUELS

to achieve complete combustion of unburned hydrocarbons and carbon monoxide under f u e l l e a n c o n d i t i o n s to minimize the formation of thermal Ν 0 · The combustion process i s optimized i n the r i c h stage to minimize the formation of Ν 0 and molecules such as NHg and HCN which would convert r e a d i l y to Ν 0 i n the lean stage. S u f f i c i e n t residence time i n the lean stage assures complete combustion of even poor q u a l i t y f u e l s . The purpose of t h i s paper i s to present the r e s u l t s of an experimental program sponsored by the E l e c t r i c Power Research I n s t i t u t e (EPRI). The purpose of the e f f o r t was to determine, by subscale combustor r i g t e s t and data a n a l y s i s , the e f f e c t s of s y n t h e t i c f u e l property v a r i a t i o n s on the emissions, performance, and d u r a b i l i t y c h a r a c t e r i s t i c s of r i c h - l e a n combustion systems. Fuel property v a r i a t i o n s were i n v e s t i g a t e d by t e s t i n g f i v e f u e l s i n c l u d i n g No. 2 petroleum d i s t i l l a t e f u e l , c o a l - d e r i v e d middle d i s t i l l a t e f u e l s produced by the SCR-II, HCoal and EDS processes, and a shale o i l r e s i d u a l f u e l . Tests with these f u e l s provided data f o r r e a l i s t i c ranges of f u e l v i s c o s i t y , and hydrogen and n i t r o g e n content. Tests were performed at conditions r e p r e s e n t a t i v e of f u l l - and p a r t i a l power s e t t i n g s of a 30-MW u t i l i t y combustion t u r b i n e . χ

χ

Publication Date: April 29, 1983 | doi: 10.1021/bk-1983-0217.ch008

χ

Test

Program

Test F a c i l i t y . The experimental program to document the consequences of burning f u e l s that possess chemical and/or p h y s i c a l p r o p e r t i e s which d i f f e r from f u e l s i n common use was conducted i n the t e s t f a c i l i t y shown s c h e m a t i c a l l y i n Figure 2. The t o t a l a i r f l o w was supplied to the t e s t c e l l by a p o s i t i v e displacement compressor, metered by a c a l i b r a t e d v e n t u r i and heated i n an e l e c t r i c a l r e s i s t a n c e - t y p e heater. The a i r f l o w which e x i t e d the heater was d i v i d e d i n t o a primary a i r f l o w , which f e d the r i c h - s t a g e combustor, and a secondary a i r f l o w which was i n j e c t e d through the combustor quench s e c t i o n . V a r i a t i o n s i n the primary-secondary a i r f l o w s p l i t were achieved by a c t u a t i n g a pneumatic c o n t r o l valve located i n the primary a i r l i n e ; a high temperature gate valve l o c a t e d i n the secondary a i r l i n e provided the supply system pressure drop necessary f o r c o n t r o l . A c a l i b r a t e d v e n t u r i was located i n the primary l i n e to meter the primary a i r f l o w and hence permit c a l c u l a t i o n of the r i c h combustor equivalence r a t i o . The secondary a i r f l o w rate was c a l c u l a t e d as the d i f f e r e n c e of the t o t a l and the primary a i r f l o w r a t e s . The model combustor used i n the present study was a copy o f one c o n f i g u r a t i o n ( C o n f i g u r a t i o n 2C) evaluated under the DOE/NASA Low Ν 0 Heavy Fuel Combustor Concepts Program, and c o n s i s t e d of four component s e c t i o n s : f u e l preparation, f u e l r i c h combustion, a i r quench, and f u e l - l e a n combustion s e c t i o n s ( 2 ) . A l l of the t e s t f u e l was i n j e c t e d i n t o a r i c h combustion χ

8.

ANGELLO E T A L .

Rich-Lean

153

Gas Turbine Combustor

Secondary airflow Fuel — Primary—airflow ν

>~ 1.4-1.6

0-0.3

ν

Publication Date: April 29, 1983 | doi: 10.1021/bk-1983-0217.ch008

y Fuel preparation

Figure

1.

Fuel-rich combustor

Elements

Rapid quench

Fuel-lean combustor

o f Rich-Lean Staged

Combustor

AIR S U P P L Y

VALVE TO CONTROL AIRFLOW

EXIT

SPLIT

COOLED COMBUSTOR SECTIONS

PRIMARY AIRFLOW VENTURI

Figure

2.

Synthetic

Fuel

Combustor R i g

INSTRUMENTATION

Publication Date: April 29, 1983 | doi: 10.1021/bk-1983-0217.ch008

154

COMBUSTION OF SYNTHETIC FUELS

chamber through the f u e l p r e p a r a t i o n s e c t i o n . This s e c t i o n consisted of a s i n g l e , a i r - a s s i s t f u e l i n j e c t o r which was c e n t r a l l y mounted i n an annular, vane-type s w i r l e r ; the s w i r l e r nozzle assembly was recessed approximately 1.2 inches from the r i c h combustor i n l e t . Several i n j e c t o r models were evaluated during the combustor shakedown t e s t s ; excessive smoke l e v e l s or nozzle f a i l u r e ( e . g . , l o s s of nozzle resonator cap) were experienced with most. Acceptable o p e r a t i o n was a t t a i n e d with use of a D e l a v a n ® s w i r l a i r i n j e c t o r ; a l l reported data were acquired using t h i s i n j e c t o r . The nozzle a s s i s t a i r f l o w was metered with a c a l i b r a t e d v e n t u r i and was included i n c a l c u l a t i o n of the primary and t o t a l a i r f l o w r a t e s . Most of the data were acquired with an a i r - a s s i s t pressure of 300 p s i at the f u e l i n j e c t o r ; l i m i t e d t e s t s were performed to i n v e s t i g a t e the i n f l u e n c e of higher or lower a s s i s t - a i r pressure l e v e l s . The f u e l - r i c h combustion chamber was a 5 - i n c h diameter c y l i n d r i c a l s e c t i o n , 11.0 inches l o n g , with 1 . 9 - i n c h long c o n i c a l s e c t i o n s at both the i n l e t and e x i t (Figure 3 ) . The e n t i r e chamber was double-jacketed to allow a nominal 40 GPM water coolant flow r a t e . An H / 0 t o r c h was incorporated i n the design to i g n i t e the burner. The quench s e c t i o n was a 3 - i n c h diameter c y l i n d r i c a l s e c t i o n , 3 inches l o n g , containing 16 s l o t s to permit the a d d i t i o n of the secondary a i r f l o w to the r i c h combustor e f f l u e n t . The f u e l - l e a n combustor consisted of a 1 0 . 6 - i n c h long c o n i c a l d i f f u s e r followed by a 5 - i n c h diameter c y l i n d r i c a l s e c t i o n to give an o v e r a l l l e n g t h of 18 inches from the quench s e c t i o n e x i t to the exhaust measurement plane (Figure 3 ) . This combustor was a l s o d o u b l e - j a c k e t ; the water coolant used f o r the r i c h burner was a l s o used f o r the f u e l - l e a n device. The combustor e x i t conditions were documented by a f i v e - p o r t ganged sampling probe, a t h r e e - p o i n t thermocouple rake and a smoke probe. The water-cooled sampling probe spanned the combustor diameter, and contained f i v e 0.034-inch diameter i n l e t orfices. The probe was designed to achieve an aerodynamic quick-quench of the captured streams i n order to minimize chemical r e a c t i o n w i t h i n the probe. The captured sample was t r a n s f e r r e d i n an e l e c t r i c a l l y - h e a t e d sample l i n e to an emission a n a l y s i s system capable of continuously monitoring the emissions of carbon monoxide, oxygen, carbon d i o x i d e , unburned hydrocarbons and oxides of n i t r o g e n . A water-cooled smoke probe was designed i n accordance with SAE ARP1179. The probe, which had a sample i n l e t diameter of 0.07 i n c h e s , was s i z e d to i s o k i n e t i c a l l y sample the gas stream at the baseload condition. Three PT6RH/PT30RH thermocouples were mounted on a water-cooled s t r u t with a vented r a d i a t i o n s h i e l d around each sensor. 2

2

8.

ANGELLO ET A L .

155

Rich-Lean Gas Turbine Combustor

Test Fuels

Publication Date: April 29, 1983 | doi: 10.1021/bk-1983-0217.ch008

F i v e f u e l s were i n v e s t i g a t e d i n c l u d i n g : No. 2 petroleum d i s t i l l a t e f u e l , coal-derived middle d i s t i l l a t e f u e l s produced by the SRC-II, Η-Coal and EDS processes and shale o i l r e s i d u a l f u e l . D e t a i l e d analyses of each t e s t f u e l are presented i n Table 1. The No. 2 petroleum d i s t i l l a t e f u e l and the c o a l derived l i q u i d s had s i m i l a r d i s t i l l a t i o n and v i s c o s i t y ranges but d i s p l a y e d s i g n i f i c a n t l y d i f f e r e n t l e v e l s of hydrogen and fuel-bound n i t r o g e n content. The hydrocarbon-type also showed considerable v a r i a t i o n , with excess of 75% of the SRC-II f u e l having an aromatic character. In a d d i t i o n the i n t i a l b o i l i n g point of the Η-Coal d i s t i l l a t e f u e l was somewhat lower than the other d i s t i l l a t e f u e l s .

93-9 ALL

Figure

3.

D I M E N S I O N S IN C E N T I M E T E R S

Subscale

Rich-Lean

Combustor

Configuration

*At 49°C **Net Heat of Combustion

Distillation I n i t i a l b o i l i n g point 10% 50% 90% End point recovery Recovery Residue F 424 510 605

wt% wt% wt% wt% vol% vol% vol%

Chemical P r o p e r t i e s Carbon Hydrogen Surfur Nitrogen Saturates Olefins Aromatics



F F Btu/lb es es

Unit

Physical Properties S p e c i f i c Gravity 40°C Pour Point F l a s h Point Gross H t . of Combustion Kinetmatic V i s c o s i t y 40C 100C

Property

360 400 466 536 660 98.5 1.5

87 12.95 0.25 0.02 64.8 0.7 34.5

0.841 -5 152 18280** 2.53 1.04

No. 2

TABLE 1

312 326 408 500 560 98.0 2.0

86.04 8.97 0.2495 0.75 19.3 2.9 77

0.9610 -40 164 17161 3.49 1.114

SRC-II

240 425 468 568 590 98.0 2.0

88.21 11.28 0.08 0.32 39 0.04 60.09

0.872 -45 90 17725 1.58 0.69

H-Coal

TEST FUEL ANALYSES

Publication Date: April 29, 1983 | doi: 10.1021/bk-1983-0217.ch008

390 616 747 848 596 98.5 1.5

88.12 11.31 0.01 0.02 40 0.1 60

0.894 -55 167 19266 2.40 0.95

EDS

855 94.0 6.0

480

— — —

86.7 12.69 0.02 0.46

0.863* 95 235 19350 12.9 3.77

Shale

8.

ANGELLO ET A L .

157

Rich-Lean Gas Turbine Combustor

The shale o i l r e s i d u a l had been hydrotreated to a s u b s t a n t i a l degree, providing i t with a hydrogen content very s i m i l a r to the No. 2 petroleum d i s t i l l a t e f u e l . The shale o i l r e s i d u a l f u e l had v i s c o s i t y c h a r a c t e r i s t i c s s i m i l a r to a viscous No. 4 petroleum d i s t i l l a t e f u e l . The n i t r o g e n content of the hydrotreated shale o i l r e s i d u a l was 0.49 weight p e r c e n t . Test Conditions Tests were performed over a matrix of conditions to e s t a b l i s h emissions and heat load c h a r a c t e r i s t i c s for each t e s t fuel. The t e s t s were s t r u c t u r e d to allow determination of both combustion tradeoffs ( e . g . , low Ν 0 emissions v s . low smoke emissions) and the i n f l u e n c e of varying s e l e c t e d f u e l p r o p e r t i e s ( e . g . , n i t r o g e n or hydrogen content) on the emission l e v e l s . Three categories of t e s t s were performed. The f i r s t category i n c l u d e d tests to determine the change i n combustor emissions as the primary combustor equivalence r a t i o ( φ ρ ) was v a r i e d between 1.0 and 1.8. These t e s t s were r e f e r r e d to as signature t e s t s , and were performed at both baseload and 50% power operating c o n d i t i o n s . For these t e s t s the t o t a l a i r f l o w and a i r f l o w s p l i t between the primary and secondary streams were held constant while the f u e l flow rate was v a r i e d . With t h i s technique, the primary combustor equivalence r a t i o was changed while h o l d i n g the residence time i n t h i s chamber n e a r l y constant. The second category of t e s t conditions included design point operation at peak, baseload and 70% load c o n d i t i o n s of an u t i l i t y gas t u r b i n e . These points were s e l e c t e d to match the t e s t points from the DOE/NASA Low Ν 0 Program allowing comparison with data from that program. The t h i r d category of t e s t c o n d i t i o n s focused on o f f - d e s i g n operation to i n v e s t i g a t e the i n f l u e n c e of r i c h combustor residence time and pressure on combustor emissions; o f f - d e s i g n t e s t s departed from baseload and 70% power design p o i n t s .

Publication Date: April 29, 1983 | doi: 10.1021/bk-1983-0217.ch008

χ

χ

Test Results Tests with No. 2 Petroleum D i s t i l l a t e F u e l . Tests were performed with No. 2 petroleum d i s t i l l a t e f u e l to e s t a b l i s h a b a s e l i n e for comparison with the other t e s t f u e l s . The Ν 0 emissions signature i s shown i n Figure 4 f o r operation at both the baseload and 70% power c o n d i t i o n s . The Ν 0 emissions s t a r t e d at very high l e v e l s for equivalence r a t i o s near u n i t y and decreased r a p i d l y to values l e s s than 50 ppmv for φρ>1.3. The minimum Ν 0 l e v e l of approximately 37 ppmv was equivalent f o r e i t h e r operating c o n d i t i o n and was achieved f o r both at φ ρ = 1 . 5 5 . E q u i l i b r i u m chemistry considerations would lead to the conclusion that the rapid d e c l i n e i n Ν 0 emissions was a r e s u l t of both the decreased flame temperature and reduced χ

χ

χ

χ

158

COMBUSTION O F SYNTHETIC F U E L S

oxygen concentration at the f u e l - r i c h operation c o n d i t i o n s . These e f f e c t s would stongly reduce thermal f i x a t i o n of molecular n i t r o g e n to Ν 0 . While these considerations o f f e r an explanation f o r the f i n a l l e v e l of Ν 0 achieved, i t must be r e a l i z e d that the f u e l was i n j e c t e d as a spray and not as a f u l l y - v a p o r i z e d a i r mixture. Hence for any equivalence r a t i o there were regions of near s t o i c h i o m e t r i c combustion with the attendant production of high l e v e l s of Ν 0 . Therefore, an a n n i h i l a t i o n mechanism must have been present to reduce these i n i t i a l l e v e l s to the very low Ν 0 emissions a t t a i n e d . The combustor smoke emissions v a r i e d with the primary zone equivalence r a t i o , as shown i n Figure 5. There was no apparent d i s t i n c t i o n between the smoke l e v e l s achieved at e i t h e r the baseload or the 70% operation c o n d i t i o n ; the data f e l l into a band with i n c r e a s i n g smoke production f o r higher primary combustor equivalence r a t i o . The t r a d e o f f of choosing e i t h e r to minimize Ν 0 emissions or smoke emissions i s shown i n Figure 6, which i s a c r o s s - p l o t of the previous two f i g u r e s . Operation at the threshold of v i s i b l e smoke (SAE Smoke Number=20) could be achieved with Ν 0 emissions l e s s than 50 ppmv; attempts to f u r t h e r reduce the smoke emissions to lower values would r e s u l t i n excessive Ν 0 emissions. One operating concern for a r i c h combustor i s the occurrence of high combustor w a l l temperatures. In a f u e l - r i c h combustor, a i r cannot be used to f i l m - c o o l the walls and other techniques ( e . g . , f i n cooling) must be employed. The temperature r i s e of the primary combustor coolant was measured and normalized to form a heat f l u x c o e f f i c i e n t which included both convective and r a d i a t i v e heat l o a d s . Figure 7 d i s p l a y s the dependence of t h i s heat f l u x c o e f f i c i e n t on primary combustor equivalence r a t i o . These data were acquired i n t e s t s i n which the combustor a i r f l o w was kept constant. I f convective heat t r a n s f e r were the dominant mechanism a constant heat f l u x c o e f f i c i e n t of approximately 25 B t u / f t -hr-deg F would be expected. The higher values of heat f l u x and i t s convex character i n d i c a t e that r a d i a t i v e heat t r a n s f e r was an important mechanism. Furthermore, there i s an apparent t r a d e o f f between the temperature of the r a d i â t i n g - m e d i u m and the e f f e c t i v e e m i s s i v i t y of the medium. That i s , at equivalence r a t i o s near u n i t y the gas temperatures would be maximum, but few carbon p a r t i c l e s have formed. As the equivalence r a t i o was i n c r e a s e d , the temperature decreased, but the number of r a d i a t i n g p a r t i c l e s began to increase r e s u l t i n g i n a net increase i n the r a d i a t i v e mechanism. At high equivalence r a t i o s , the temperature of the r a d i a t i n g p a r t i c l e s decreased s u f f i c i e n t l y to o v e r r i d e the abundance of r a d i a t i n g p a r t i c l e s and consequently the heat f l u x c o e f f i c i e n t diminished. Comparison with the previous figures reveals that the region of primary zone equivalence r a t i o d e s i r e d f o r low Ν 0 operation and acceptable smoke emissions χ

χ

χ

Publication Date: April 29, 1983 | doi: 10.1021/bk-1983-0217.ch008

χ

χ

χ

χ

χ

8.

ANGELLO

159

Rich-Lean Gas Turbine Combustor

ET A L .

Publication Date: April 29, 1983 | doi: 10.1021/bk-1983-0217.ch008

ENGINE POWER L E V E L

1.8

1.2

1.6

10

PRIMARY RICH Z O N E E Q U I V A L A N C E RATIO -

Figure NO

ρ

4.

Dependence on P r i m a r y

Equivalence Distillate

βθι-

φ

Ratio

for

No.

Combustor 2

Petroleum

Fuel

ENGINE POWER L E V E L Δ

PEAK

Ο

BASE

Ο

70%

SMOKE THRESHOLD

Δ

. _ O-

0.8

j 1.0

1.2

JL 1.4

1.6

PRIMARY RICH Z O N E E Q U I V A L A N C E RATIO -

Figure 5. Smoke D e p e n d e n c e o n P r i m a r y Equivalence Ratio for No. 2 Distillate Fuel

1.8 φ.

Conbustor Petroleum

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

2501-

Publication Date: April 29, 1983 | doi: 10.1021/bk-1983-0217.ch008

SMOKETHRESHOLD

0

20

40

10

M

SAE SMOKE NUMBER -

Figure 6. T r a d e o f f o n NO

and

Smoke

100

SN

93-15

Emissions

for

χ No.

2 Petroleum

Distillate

Fuel

Μι-

Μ

ENGINE POWER L E V E L

0.8

1.0 PRIMARY

1.2

1.4

Δ

PEAK

Ο

BASE

Figure 7. Average Heat Transfer t o Wall for No. 2 Petroleum

1.8

1.6

RICH Z O N E E Q U I V A L A N C E RATIO -

φ„

P r i m a r y Combustor D i s t i l l a t e Fuel

8.

ANGELLO ET AL.

Rich-Lean Gas Turbine Combustor

161

a l s o produced the maximum heat t r a n s f e r to the primary combustor walls. Test Results with Middle D i s t i l l a t e C o a l - D e r i v e d F u e l s . Tests were performed with middle d i s t i l l a t e f u e l s produced by the SCR-II, Η - C o a l , and EDS processes. The composition of these f u e l s represented s i g n i f i c a n t v a r i a t i o n s i n the fuel-hydrogen content, the f u e l - n i t r o g e n content, and the mix of c h a r a c t e r i s t i c hydrocarbon compounds. Ν 0 emissions signatures were obtained for the SCR-II, Η-Coal and EDS f u e l s at baseload and part-power t e s t conditions. For each f u e l the Ν 0 s t a r t e d at a very high l e v e l (greater than 200 ppmv) f o r equivalence r a t i o s near u n i t y and decreased r a p i d l y with i n c r e a s i n g equivalence r a t i o . For a l l three fuels the minimum Ν 0 l e v e l was independent of the simulated power c o n d i t i o n , with a l e v e l of 35 to 40 ppmv achieved f o r φ ρ = 1 . 5 5 . This minimum value i s most s i g n i f i c a n t for the SCR-II f u e l which contained 0.75 percent n i t r o g e n i n the fuel. If t h i s f u e l n i t r o g e n content were f u l l y o x i d i z e d , the Ν 0 l e v e l would approach 350 ppmv at baseload o p e r a t i o n . Again i t i s b e l i e v e d that high concentrations of Ν 0 are formed i n the r i c h combustor but that a n n i h i l a t i o n r e a c t i o n s are s u f f i c i e n t vigorous to reduce the emissions to the observed v a l u e s . The combustor smoke emission increased with primary combustor equivalence r a t i o . The data i n d i c a t e d a s i g n i f i c a n t d i f f e r e n c e between o p e r a t i o n at baseload and 50% power c o n d i t i o n s f o r the SCR-II and Η-Coal f u e l . For these two f u e l s , smoke emissions were s i g n i f i c a n t at high power o p e r a t i o n but n e a r l y non-existent for operation at pressures below 100 p s i . There was no systematic d i s t i n c t i o n between these operating c o n d i t i o n s f o r the data acquired using EDS f u e l ; the smoke l e v e l s were more c h a r a c t e r i s t i c of high power pressure o p e r a t i o n . The three f u e l s are compared i n Figure 8 to show the tradeoff i n choosing e i t h e r to minimize Ν 0 emissions or smoke emissions. It would be d e s i r a b l e to operate i n the lower l e f t - h a n d region of t h i s p l o t and hence a t t a i n low emissions of both s p e c i e s . The data acquired for Η-Coal and EDS i n d i c a t e the a b i l i t y to a t t a i n Ν 0 emissions l e s s than 50 ppmv f o r SAE SN=10. Higher emissions of e i t h e r species would have to be expected i f operating on SCR-II fuel. χ

Publication Date: April 29, 1983 | doi: 10.1021/bk-1983-0217.ch008

χ

χ

χ

χ

χ

χ

The average heat f l u x c o e f f i c i e n t to the primary combustor w a l l i s p l o t t e d f o r the f u e l s i n Figure 9. The r e s u l t s i n general d i s p l a y e d a convex character as was observed with NO. 2 fuel. The l e v e l of the heat t r a n s f e r c o e f f i c i e n t and i t s convex trend i n d i c a t e s the importance of r a d i a t i v e heat t r a n s f e r f o r these f u e l s . The maximum value of the c o e f f i c i e n t f o r SCR-II f u e l exceeded the maximum f o r other f u e l s by 30%. The hydrogen content of SCR-II was l e s s than that f o r the other f u e l s t e s t e d which apparently r e s u l t e d i n a more intense r a d i a t i n g medium.

Publication Date: April 29, 1983 | doi: 10.1021/bk-1983-0217.ch008

COMBUSTION OF SYNTHETIC

Figure 8. C o m p a r i s o n o f T r a d e - o f f o f NO a n d S m o k e Emissions at Baseload Conditions for Coal-Derived

Figure 9. Comparison o f Primary Combustor W a l l Heat Loading for Coal-Derived Fuels at Baseload

Fuel

FUELS

8.

Rich-Lean Gas Turbine Combustor

ANGELLO ET A L .

163

Again the maximum i s observed at primary combustor equivalence r a t i o s c l o s e to that desired for minimum Ν 0 emission o p e r a t i o n . χ

Test Results with Shale O i l Residual F u e l . Tests were performed with a shale o i l r e s i d u a l f u e l which had the v i s c o s i t y c h a r a c t e r i s t i c s of a heavy No. 4 petroleum d i s t i l l a t e f u e l . The t e s t s were performed a f t e r heating the f u e l to 160°F to reduce i t s v i s c o s i t y to 7 cs i n an attempt to enhance the f u e l atomization and v a p o r i z a t i o n process. Even with t h i s degree of h e a t i n g , t h i s f u e l has a v i s c o s i t y twice the l e v e l of other fuels t e s t e d . The Ν 0 signature at both baseload and 50% power conditions was s i m i l a r to the other fuels t e s t e d , reached a minimum of 40 ppmv at φ ρ = 1 . 5 . The corresponding smoke emissions were higher than f o r other fuels tested (Figure 10). These l e v e l s are a t t r i b u t e d to r e l a t i v e l y poor atomization because of the higher f u e l v i s c o s i t y . A s i n g l e data point i s shown for which the a i r - a s s i s t pressure was increased from 300 p s i to 500 p s i to improve the f u e l a t o m i z a t i o n . A s u b s t a n t i a l reduction i n smoke emissions was observed with the smoke number decreasing from SAE SN=40 to 3. The Ν 0 emissions increased from 32 to 50 ppmv with the enhanced a i r a s s i s t i n d i c a t i n g that the f u e l p r e p a r a t i o n process o f f e r s an emissions t r a d e o f f . In a l i m i t e d number of t e s t s , i t was determined that heating the a s s i s t a i r d i d not improve the smoke emissions. The Ν 0 l e v e l s obtained were independent of the operation of the primary combustor. Neither reducing the residence time nor changing the pressure l e v e l s i g n i f i c a n t l y a f f e c t e d the Ν 0 l e v e l s . The heat f l u x to the combustor w a l l was comparable to that f o r NO. 2 petroleum distillate fuel.

Publication Date: April 29, 1983 | doi: 10.1021/bk-1983-0217.ch008

χ

χ

χ

χ

D i s c u s s i o n of Test Results The t e s t data acquired were analyzed to determine the f u e l property e f f e c t s on the staged combustor performance. Influences on the emissions and the heat f l u x to the primary combustor w a l l are presented i n t h i s s e c t i o n . Comparisons with data acquired by Westinghouse E l e c t r i c Corporation i n another EPRI-sponsored c o n t r a c t u a l program (RP989-1) i n which s i m i l a r t e s t f u e l s were combusted i n a c o n v e n t i o n a l , lean combustor are also presented ( 3 ) · Ν 0 E m i s s i o n s . The n i t r o g e n content i n the d i s t i l l a t e f u e l s ranged from 0.0 to 0.75 wt%. The i n f l u e n c e of t h i s range on Ν 0 emissions i s d i s p l a y e d i n Figure 11. The values p l o t t e d correspond to the minimal Ν 0 l e v e l f o r each f u e l . Since the minima occurred over a small range of primary combustor equivalence r a t i o (1.5 dCN

οο

O

A» 1 ) pulverized SRC-I solid fuel (SRC), while considered successful, indicated the need for concern in two areas: carbon in the fly ash and nitrogen oxides (Ν0 ) emissions. Although good combustion efficiencies (generally greater than 98%) were attained there was a substantial amount of carbon in the particulates (generally greater than 60%). This w i l l pose a collection problem i f an electrostatic precipitator (ESP) is envisioned for particulate collection because of the very low resistivity imparted by carbon. In addition, the high nitrogen contents (1.8-1.9%) of SRC indicate that there is a potential for high Ν0 emissions. 0097-6156/83/0217-0201 $06.00/0 © 1983 American Chemical Society w i t h

χ

χ

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

202

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

y

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

χ

χ

f

χ

χ

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

11.

BORIO E T A L .

203

Liquefaction Processing Conditions

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

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

χ

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

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

39.4

39.4

15860

1210

Higher Heating Value, (HHV), Btu/lb

F1aamab111ty Index °F

15920 1270

16050

16115

1270

15880

15940

1040

10900

13290

100.0

1170

12730

14110

100.0 100.0 100.0 100.0 100.0

100.0

100.0

100.0

100.0

Total

8.6 14.9

0.3

-

0.1

-

0.3

Ash

-

12.5 11.3 21.1 17.3 3.0

3.1

3.4

3.4

2.9

2.9

Oxygen (D1ff.)

1.4 1.3 1.3 1.1 1.9

1.9

1.8

1.8

1.9

Nitrogen

0.8 0.7 0.4 0.3 0.7

0.7

1.0

1.0

1.9

59.4

88.5

88.1

87.7

87.3

1.0

1.0

Sulfur

88.4

88.0

Carbon

4.9 80.4 72.5

4.8 72.4

3.9

5.9

5.9

6.1

6.1

5.8

5.8

Hydrogen

w

1.2 4.4

3.1

-

0.1

Moisture (Total)

-

100.0

0.1

100.0

-

100.0

0.3

Ultimate, Ut. Percent

100.0

100.0

100.0

100.0

100.0

100.0

100.0

Total

8.6

37.3 62.7 56.6

59.8

49.0

39.6 14.9

40.0

33.6

40.2

1.2

as-rec. daf

33.0

daf

Kentucky High Vol. Bit. Coal

60.4

3.1

as-rec.

0.3

48.2

0.1 60.2

59.8

0.3

as-rec. daf

59.6

daf

Wyoming Subbltumlnous Coal

0.1

Fixed Carbon

51.8

as-rec.

Ant1 Solvent Deashed SRC

0.3

48.0

Volatile Matter

daf

Critical Solvent Deashed SRC

Ash

0.1

51.6

Moisture (Total)

Proximate, Ut. Percent

Analysis

Pressure Filtered Deashed SRC

Table I ANALYSES OF SRC AND REFERENCE COALS

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

£

w

Η

w

Χ

Η

ο

δ

Η

s

ο ο w

11.

Liquefaction Processing Conditions

BORIO E T A L .

liée

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

r-

d

Ο

Ξ

S S

8

CM CM

m CM

^ CM



O

206

COMBUSTION OF SYNTHETIC FUELS

Table II depicts the major compositional d i f f e r e n c e s between the CSD, PFD, and ASD SRC are i n the s o l u b l e f r a c t i o n s . The CSD SRC has s i g n i f i c a n t l y l e s s benzene i n s o l u b l e s ( i . e . , p r e - a s p h a l tenes) than do the other two SRC. This compositional d i f f e r e n c e may be r e s p o n s i b l e f o r the CSD SRC greater v o l a t i l i t y as discussed later i n this report. Both the CSD and ASD SRC have melting temperatures c a . 100 F lower than the PFD SRC, which could s i g n i f i c a n t l y a f f e c t the design of f u e l handling and i n j e c t i o n systems. TABLE II

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

ADDITIONAL ANALYSES* OF SRC

Analysis

Pressure Filtered Deashed SRC

Cricital Solvent Deashed SRC

Anti-Solvent Deashed SRC

Solvent E x t r a c t i o n s Oils Asphaltenes Benzene I n s o l . Calc. Total

WT% WT% WT% WT%

18.9 56.8 24.3 100.0

24.2 66.2 11.6 102.0

26.5 52.1 21.4 100.0

Vacuum D i s t i l l a t i o n - 1 0 0 0 ° F Fraction

WT%

2.7

5.8

11.0

Softening Point

°F

356

248

257

Fusion Point

°F

383

284

289

*Provided by EPRI

Physical Characteristics Pasting occurred on the r a c e , bowl, and b a l l s of the Hardgrove machine during the g r i n d a b i l i t y index determination on a l l three SRC.In each case an a d d i t i o n of as l i t t l e as 1% moisture (by weight) eliminated the pasting problem. The e f f e c t of moisture a d d i t i o n i s c l e a r l y shown i n F i g u r e 1. The HGI of the SRC are i n the 136-156 range, i n d i c a t i n g that these m a t e r i a l s are more e a s i l y ground than coals ( c o a l s HGI are t y p i c a l l y l e s s than 100). 1

Thermo-Gravimetric Char R e a c t i v i t i e s Thermo-gravimetric a n a l y s i s r e s u l t s are presented i n F i g u r e 2. They i n d i c a t e that: (1) the PFD SRC char i s r e l a t i v e l y more r e a c t i v e than the CSD and ASD chars; (2) PFD char r e a c t i v i t y i s between those of WSB and KHB c o a l chars; (3) CSD and ASD char r e a c t i v i t i e s are both comparable to that of the KHB c o a l char, but

BORio ET AL.

Liquefaction Processing Conditions

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

11.

Figure 1. Photographs of SRC-1 a f t e r g r i n d i n g i n the Hardgrove machine.

207

COMBUSTION OF SYNTHETIC

FUELS

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

208

TIME, MINUTES

Figure 2. Thermogravimetric burn-off curves i n a i r at 700°C f o r SRC and reference coal v o l a t i l e matter chars.

11.

BORio E T AL.

Liquefaction Processing Conditions

209

s u b s t a n t i a l l y higher than that of the a n t h r a c i t e char; and (4) the l i g n i t e char i s the most r e a c t i v e of a l l the chars under t h i s study. The TGA char r e a c t i v i t y data i n d i c a t e that the CSD SRC has a r e l a t i v e l y low r e a c t i v i t y ( s i m i l a r to KHB c o a l ) . However, the extremely high v o l a t i l e y i e l d s of the CSD SRC ( i l l u s t r a t e d i n DTFS and CMHF r e s u l t s below) leaves very l i t t l e char f o r subsequent burnout and r e s u l t s i n a high o v e r a l l combustion e f f i c i e n c y . P y r o l y s i s , Combustion and Ν 0 C h a r a c t e r i s t i c s

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

χ

Two reference coals and the CSD SRC were pyrolyzed i n an argon atmosphere i n the DTFS at 2 8 0 0 ° F furnace w a l l temperature (Figure 3) In a residence time of about 330 m i l l i s e c o n d s , Wyoming subbituminous (WSB) c o a l evolved about 50% of i t s mass as v o l a t i l e matter the Kentucky high v o l a t i l e bituminous (KHB) c o a l evolved about 53% v o l a t i l e matter and the CSD SRC evolved about 99% v o l a t i l e matter. These are much higher than the ASTM v o l a t i l e matter values of 40.2, 37.3, and 59.8% for the WSB, KHB, and CSD f u e l s , r e s p e c t i v e l y . The d i f f e r e n c e between these values i s f a r more pronounced f o r the SRC than for the c o a l s . Figures 4, 5, and 6 show the s o l i d conversion e f f i c i e n c i e s of the three SRC and the reference coals i n a i r i n the DTFS at three temperatures (furnace w a l l temperatures of 2500, 2700, and 2800 F ) . The CSD and PFD SRC and WSB reference c o a l achieved a high s o l i d conversion e f f i c i e n c y (>75%) i n l e s s than 50 m i l l i s e c o n d s , while the ASD SRC and the KHB reference c o a l r e s u l t e d i n lower i n i t i a l conversion e f f i c i e n c i e s , l e s s than 60%. The i n i t i a l high degree of conversion of the CSD and PFD SRC r e s u l t s i n r e l a t i v e l y low amounts of r e s i d u a l char to be burned i n the l a t t e r stages of combustion. The CSD SRC showed the highest i n i t i a l and o v e r a l l s o l i d conver­ s i o n e f f i c i e n c y of a l l the fuels s t u d i e d . The r e s u l t s f u r t h e r show that the s o l i d conversion e f f i c i e n c i e s increase more d r a m a t i c a l l y with temperature than with residence time i n c r e a s e s . For the CSD SRC s o l i d conversion e f f i c i e n c i e s increased from 91 to 95% at a furnace temperature of 2500 F , as the residence time increased from 0.05 second to 0.3 second. The corresponding e f f i c i e n c i e s at 2800 F were 97% and 99% r e s p e c t i v e l y i n d i c a t i n g the pronounced e f f e c t of higher temperatures. A r e l a t i v e comparison of the f i v e fuels (three SRC and two reference coals) i n d i c a t e s that the CSD SRC has a high s o l i d con­ v e r s i o n e f f i c i e n c y s i m i l a r to the Wyoming subbituminous c o a l (WSB). This WSB c o a l has been found to y i e l d high combustion e f f i c i e n c i e s in u t i l i t y boilers. The PFD SRC i s seen to be l e s s r e a c t i v e com­ pared to the CSD SRC and the WSB c o a l i n the e a r l y stages of com­ b u s t i o n , however, o v e r a l l s o l i d conversion e f f i c i e n c y d i f f e r e n c e s are reduced with i n c r e a s i n g residence time, e s p e c i a l l y at the higher temperatures s t u d i e d . The ASD SRC, however, i s seen to give lower s o l i d conversion e f f i c i e n c i e s than the marginal KHB reference c o a l at a l l three temperatures s t u d i e d . At a furnace

COMBUSTION

210

»

1

OF SYNTHETIC

I

FUELS

1

" Γ

100 ρ -

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

90 r -

80 ço CO

< j£

70

< Ο

a* ^ 60 υ ζ LU 5 50 LU .2 40 CO

T E S T CONDITIONS

>

TW = 2 8 0 0 ° F 30

R E G U L A R GRIND FUEL

20

T E S T No.

Ο W Y O M I N G S U B . BIT.

WSB-7

Π



K E N T U C K Y HIGH V O L . BIT. Δ CSDSRC

10

ι ι ιI 0.1

K

m H

B

7 7

" CSD-7

I • ι ι I • ι ι ' 0.2

0.3

0.4

0.5

RESIDENCE TIME, SEC

Figure at

TW

3.

DTFS p y r o l y s i s

= 2800OF.

e f f i c i e n c i e s of f u e l s

i n argon

_ |

BORIO E T A L .

211

Liquefaction Processing Conditions

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

11.

RESIDENCE TIME, SEC

Figure 4- DTFS s o l i d conversion e f f i c i e n c i e s a i r at TW = 2500°F.

of fuels i n

212

COMBUSTION

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

90

OF

SYNTHETIC

U

80 U

70

μ

60

μ

5ομ TESTNo.

FUEL 40

μ

30

μ

20

μ

T E S T CONDITIONS TO = 2 7 0 0 ° F

ίο

AVG. FURNACE μ

-I

0 ' 15 ± 1%

R E G U L A R GRIND

2

Ο

WSB C O A L

WSB-2



KHB COAL

KHB-2

Δ

CSD S R C

CSD-2



PFD SRC

PFD-2

ASD SRC

ASD-2



ι

-L

L 0.1

0.2 0.3 RESIDENCE TIME, SEC

0.4

ι

I 0.5

Figure 5. DTFS s o l i d conversion e f f i c i e n c i e s of fuels i n a i r at TW = 2700°F.

FUELS

BORio ET AL.

213

Liquefaction Processing Conditions

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

11.

0.2

0.3

RESIDENCE TIME, SEC

Figure 6. DTFS s o l i d conversion e f f i c i e n c i e s a i r at TW = 2800°F.

of fuels i n

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

214

temperature of 2 8 0 0 ° F s o l i d conversion e f f i c i e n c i e s at 0.3 second residence time are 99% f o r the CSD and WSB f u e l s , 97% f o r the PFD SRC, 90% f o r KHB, and 77% f o r the ASD SRC. Both the CSD and PFD SRC were studied i n the CMHF as w e l l as two reference c o a l s . The e f f e c t of primary stage stoichiometry on Ν 0 emissions i s shown i n F i g u r e 7. For CSD SRC the maximum Ν 0 r e d u c t i o n was obtained at an opt­ imum primary stage stoichiometry of about 40%. At an o v e r a l l excess a i r value of 20%, b a s e l i n e , unstaged emissions were 570 ppm compared to the optimum, staged Ν 0 emissions of 240 ppm ( a l l values corrected to 3% 0 ) . For PFD SRC the minimum Ν 0 (230 ppm) was a t t a i n e d at a primary stage stoichiometry of about 55% at the same o v e r a l l excess a i r . The SRC, due to t h e i r r e l a t i v e l y high f u e l n i t r o g e n contents, have a high Ν 0 formation p o t e n t i a l under conventional f i r i n g c o n d i t i o n s . However, staging the combustion a i r can r e s u l t i n acceptably low Ν 0 emissions without j e o p a r d i z i n g t h e i r combustion e f f i c i e n c i e s . Varying the primary stage residence time (Figure 8) showed the importance of p r o v i d i n g a s u f f i c i e n t time i n the s u b s t o i c h i o metric primary stage f o r achieving low Ν 0 . The o v e r a l l s o l i d conversion e f f i c i e n c i e s were unaffected by changes i n t h i s p a r a ­ meter. Ν 0 was found to increase s l i g h t l y with i n c r e a s i n g excess a i r for a l l f u e l s t e s t e d i n the CMHF (Figure 9 ) . The r a t e of increase i n Ν 0 was small and decreased with i n c r e a s i n g excess a i r . Ν 0 increased from 245 ppm to only 288 ppm as excess a i r increased from 0% to 35% f o r CSD SRC. This was not unexpected since a l l t e s t s were run under optimum low Ν 0 primary stage s t o i c h i o m e t r i e s . V a r i a t i o n i n excess a i r from 0 to 35% under optimized conditions was found to have l i t t l e i n f l u e n c e on the o v e r a l l s o l i d conversion e f f i c i e n c y i n the CMHF. This i s probably because SRC e x h i b i t s high p y r o l y s i s e f f i c i e n c i e s with low amounts of r e s i d u a l chars being produced f o r subsequent burnout and because primary stage conditions had been optimized. χ

χ

χ

2

χ

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

χ

χ

χ

χ

χ

χ

γ

Mathematical M o d e l l i n g In order to e x t r a p o l a t e the l a b o r a t o r y r e s u l t s to the f i e l d and to make semiquantitative p r e d i c t i o n s , an in-house computer model was used. Chemical r e a c t i o n rate constants were derived by matching the data from the C o n t r o l l e d Mixing H i s t o r y Furnace to the model p r e d i c t i o n s . The d e v o l a t i l i z a t i o n phase was not modeled since v o l a t i l e matter r e l e a s e and subsequent combustion occurs very r a p i d l y and would not s i g n i f i c a n t l y impact the accuracy of the mathematical model p r e d i c t i o n s . The " o v e r a l l " s o l i d conver­ s i o n e f f i c i e n c y at a given residence time was obtained by adding both the simulated char combustion e f f i c i e n c y and the average p y r o l y s i s e f f i c i e n c y (found i n the primary stage of the CMHF).

10

,1

1 30

1

Figure 7.

1 20

τ

60

ι

1

70

ι

1

KHB COAL Δ 80

ι

stoichiometry on Ν 0 χ emissions.

90

100

WSBCOAL Ο

ι

PFD-SRC

φ

ι

1

1

PRIMARY STAGE STOICHIOMETRY, %

50

ι

1

E f f e c t of primary stage

1 40

1

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

110

ι

r

I 120

216

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

COMBUSTION OF SYNTHETIC

0.5

1.0

1.5

2.0

PRIMARY STAGE RESIDENCE TIME, SEC

Figure 8. E f f e c t of primary stage residence time on ΝΟχ emissions.

FUELS

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

ι—*