Combustion of Synthetic Fuels - American Chemical Society

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 sh...
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Research

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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

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

Bartok; Combustion of Synthetic Fuels ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

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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-

Bartok; Combustion of Synthetic Fuels ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

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BONI ET AL.

Research Issues and Technology

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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* 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

Bartok; Combustion of Synthetic Fuels ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

Bartok; Combustion of Synthetic Fuels ACS Symposium Series; American Chemical Society: Washington, DC, 1983. 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.