Power from Wastes via Steam Gasification - ACS Symposium Series

13 Power from Wastes via Steam Gasification

Downloaded by UNIV OF CALIFORNIA SAN DIEGO on January 20, 2017 | http://pubs.acs.org Publication Date: June 1, 1978 | doi: 10.1021/bk-1978-0076.ch013

J. A. COFFMAN and R. H. HOOVERMAN Wright-Malta Corporation, Ballston Spa, NY 12020

The Wright-Malta system had i t s genesis i n a wideranging a n a l y t i c a l and conceptual study into ways and means of generating power from " d i f f i c u l t " fuels l i k e s o l i d waste and sewage sludge. The system was born complete, rather than growing from research findings or e x i s t i n g product l i n e s . It has changed in d e t a i l with refinement i n conceptual design, and will undoubtedly change again as the p i l o t plant is designed, b u i l t , and operated. However, the basic rotary k i l n g a s i f i e r - g a s turbine-recuperator-feedback system and c y c l e have weathered many reviews and appear to be sound and v i a b l e . Projected Power Plants An a r t i s t ' s conception of a single unit plant i s shown i n Figure 1. For perspective, i t might be noted that the pressurized (300 psi) rotary kiln is projected to be 5 ft. in diameter and 150 f t . in length. The plant would consume 200 tons/day of s o l i d waste and 300 tons/day of l i q u i d (97% water) sewage sludge and produce 10 megawatts of e l e c t r i c power. Plant operation can perhaps best be described by following material through it. Solid waste is delivered by compactor trucks, i s coarsely shredded but not c l a s s i f i e d , passes through lock hoppers to a metering auger where i t mixes with sewage sludge and alkaline catalyst. In the k i l n , the material rotates and tumbles, moving forward at about 3 ft/minute. During i t s slow steady t r a v e l to the hot ( 1 1 0 0 ° F ) end of the k i l n , the organic m a t e r i a l , mostly c e l l u l o s i c , p a r t i a l l y dries and pyrolyzes into gases, v o l a t i l e l i q u i d s , and char. The l i q u i d s and tars then steam-reform into a d d i t i o n a l gas; the char undergoes catalyzed steam g a s i f i c a t i o n to y i e l d still more gas. The 0-8412-0434-9/78/47-076-252$05.50/0 © 1978 American Chemical Society Jones and Radding; Solid Wastes and Residues ACS Symposium Series; American Chemical Society: Washington, DC, 1978.


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Figure 1.

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Single-unit power plant

Jones and Radding; Solid Wastes and Residues ACS Symposium Series; American Chemical Society: Washington, DC, 1978.


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r e s i d u e , m o s t l y cans, broken g l a s s , and o t h e r i n o r g a n i c d e b r i s , i s d i s c h a r g e d through a r o t a t i n g t r i a d of l o c k v a l v e s ; the steam-laden f u e l gas (125 B t u / s c f ) e x i t s through a f i l t e r and a s m a l l d i a m e t e r r o t a r y s e a l assembly. The whole p r o c e s s i s c a l l e d steam gasi f i c a t i o n and d i f f e r s from o r d i n a r y g a s i f i c a t i o n i n the absence o f o x i d a t i o n . Most o f the hot f u e l gas f l o w s t o the combustion chamber of the gas t u r b i n e , burns, d r i v e s the t u r b i n e g e n e r a t o r , and g i v e s up i t s r e s i d u a l heat i n a heat recovery b o i l e r (recuperator). Steam so r a i s e d and super-heated i s super-heated f u r t h e r i n a g a s - f i r e d u n i t at the hot end of the k i l n , and then, f l o w i n g through passages i n the k i l n w a l l , g i v e s most o f i t s heat t o the g a s i f i c a t i o n p r o c e s s . The condensate e x i t s the k i l n a t the c o o l end, g i v e s i t s r e m a i n i n g heat t o the incoming sewage s l u d g e , and then r e t u r n s t o the recuperator. Recovery o f i r o n and s t e e l from the r e s i d u e i s made easy by the absence o f b u l k y o r g a n i c m a t e r i a l . Cans a r e prime s c r a p : c l e a n , u n o x i d i z e d , d e t i n n e d . The remainder of the r e s i d u e i s u s a b l e as dense, s t e r i l e f i l l f o r c o n s t r u c t i o n purposes. A W-M p l a n t o f t h i s s i z e would s e r v e the homes, commercial e s t a b l i s h m e n t s , and i n d u s t r i e s o f a commun i t y of about 40,000 p e o p l e , consuming t h e i r s o l i d , l i q u i d , and hazardous wastes, and p r o v i d i n g o n e - q u a r t e r of t h e i r e l e c t r i c i t y . U n i t s as s m a l l as o n e - t e n t h t h i s s i z e would s t i l l be p r a c t i c a l i f a t t a c h e d t o p l a n t s h a v i n g o p e r a t i n g and maintenance crews w i t h a p p r o p r i a t e skills. L a r g e r c i t i e s would use g r o u p i n g s of k i l n s and turbines operating i n p a r a l l e l . S t i l l larger c i t i e s would have s e v e r a l p l a n t s s t r a t e g i c a l l y l o c a t e d t o keep waste haulage t o a minimum. New York C i t y , f o r examp l e , might have t h i r t y 1000 ton/day 50 MWe plants tucked i n t o a p p r o p r i a t e s p o t s i n a l l f i v e boroughs. Such d e c e n t r a l i z e d g e n e r a t i o n of power and d i s p o s a l o f waste i s more e c o n o m i c a l , e f f i c i e n t , and e n v i r o n m e n t a l l y a t t r a c t i v e than huge r e g i o n a l systems. Power C y c l e In F i g u r e 2 i s shown the W-M power c y c l e f o r a 10 MWe p l a n t f u e l e d by 200 ton/day of s o l i d waste and 300 ton/day of sewage s l u d g e . Two major flow streams are evident: t h e primary power stream c o n s i s t i n g of s o l i d waste and s l u d g e , hot f u e l gas, v e r y hot combustion p r o d u c t s , and somewhat c o o l e r , but s t i l l hot, exhaust gases; and the feedback l o o p of s u p e r h e a t e d steam and water which r e t u r n s r e c o v e r e d exhaust heat t o the


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gasifier. The w i d t h o f the c h a n n e l s denotes the magn i t u d e o f energy f l o w through each p a r t o f the system. The diagram i s l a r g e l y s e l f - e x p l a n a t o r y , p a r t i c u l a r l y when c o n s i d e r e d i n c o n j u n c t i o n w i t h the p l a n t d e s c r i p t i o n i n the p r e v i o u s s e c t i o n , but these comments might be made. The abundance of steam i n the hot f u e l gas e x i t i n g the k i l n means t h a t a l e s s than n o r mal amount of d i l u t i o n a i r i s r e q u i r e d i n the combustor. T h i s makes p o s s i b l e a s m a l l e r compressor and l a r g e r g e n e r a t o r than normal, e.g., a 7.5 MW t u r b i n e d r i v i n g a 5 MW compressor and a K> MWe g e n e r a t o r . The t u r b i n e i s r e a l l y a p a r t i a l steam t u r b i n e s i n c e i t d e r i v e s a s i g n i f i c a n t p o r t i o n of i t s power from the steam r a i s e d i n the k i l n . Thermodynamically, the c y c l e i s s i m i l a r t o t h a t i n the combined gas t u r b i n e / s t e a m t u r b i n e system and the e f f i c i e n c i e s a r e s i m i l a r , about 40%. F o r comparison, i t might be noted t h a t steam p l a n t s f u e l e d by p e t r o l e u m have e f f i c i e n c i e s of 32-35%. The c y c l e i s a t t r a c t i v e as drawn but s t i l l has room f o r improvement. One change f o r the b e t t e r might be t o p l a c e the s l u d g e p r e - h e a t e r i n the lower end of the t u r b i n e exhaust, t h e r e b y e l i m i n a t i n g a s t e p i n the t r a n s f e r of heat and p u l l i n g the exhaust temperature down by, say, a n o t h e r 50°F. I t i s p r o b a b l e , too, t h a t f u r t h e r development work w i l l l e a d t o r e d u c t i o n i n the energy c o n t e n t of the k i l n r e s i d u e , from 200 MBtu/day t o , say, 100 MBtu/day. The c y c l e e f f i c i e n c y c a l c u l a b l e from the diagram i s c o n s e r v a t i v e because i t i s based on p r e s e n t e x p e r i m e n t a l d a t a and performance c h a r a c t e r i s t i c s of p r e s e n t t u r b i n e s and does not take i n t o account improvements t h a t a r e l i k e l y t o r e s u l t as the system matures. Expérimentâtion-MinikiIn W-M r e s e a r c h on steam g a s i f i c a t i o n o f o r g a n i c s o l i d m a t e r i a l s has been performed i n two t y p e s o f equipment: a b a t c h - t y p e m i n i a t u r e k i l n ( m i n i k i l n ) and a c o n t i n u o u s f e e d biomass g a s i f i e r ( b i o g a s s e r ) . The m i n i k i l n r e s e a r c h a p p a r a t u s i s shown i n F i g u r e 3, i n s t a l l e d i n the t e s t bay of the g a s i f i c a t i o n l a b . It i s , i n e f f e c t , a r o t a t i n g autoclave, intended to s i m u l a t e , by t u m b l i n g a c t i o n , g r a d u a l i n c r e a s e i n temp e r a t u r e , and change i n gas c o m p o s i t i o n , the slow t r a v e r s e o f a charge of m a t e r i a l through a l o n g , p r e s surized rotary k i l n . There i s p r o v i s i o n f o r steam i n j e c t i o n d u r i n g the c o u r s e of a run and c o n t r o l l e d r e l e a s e of gas. There a l s o i s p r o v i s i o n (not shown) f o r c o n d e n s a t i o n of water and o r g a n i c l i q u i d s from the e f f l u e n t gas, and a n a l y s i s of t h e f i x e d gas by chroma-



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tograph. F o r p e r s p e c t i v e , i t might be noted t h a t t h e i n n e r b a r r e l o f t h e m i n i k i l n i s about 3 | f t . i n l e n g t h by 1 f t . i n d i a m e t e r . In a t y p i c a l run, 10 l b s o f c o a r s e l y shredded s o l i d waste, 2 g a l l o n s o f l i q u i d s l u d g e ( i n m i l k c a r t o n s ) , and \ l b o f sodium c a r b o n a t e a r e p l a c e d i n t h e minikiln. The door i s b o l t e d on, t h e thermocouple l e a d s a t t a c h e d , and r o t a t i o n and h e a t i n g s t a r t e d . Steam i s i n j e c t e d as needed t o h o l d t h e d e s i r e d waterto-waste' r a t i o . Waste d e c o m p o s i t i o n s t a r t s a t about 350°F and r e a c h e s i t s peak a t about 500°F ( m o s t l y CO2, CO, and H 0 ) . Methane s t a r t s e v o l v i n g a t about 500°F, hydrogen a l i t t l e h i g h e r ; bv 900°F, t h e f l o w o f CO has stopped. From 1000 t o 1100°F, t h e gas i s m o s t l y hydrogen and CO2 i n a r a t i o o f about n i n e t o f o u r . 2

4CH

5

(char)+8H 04C0 +9H 2

2

2

D u r i n g t h e p e r i o d from about 400 t o 900°F, l i q u i d s and t a r s a r e a l s o b e i n g e v o l v e d and a r e caught i n t h e condenser. I t i s e s t i m a t e d t h a t perhaps a h a l f o f t h e o r g a n i c waste i s v o l a t i l i z e d as c o n d e n s a b l e l i q u i d s . And so t h e gas a n a l y s i s does not g i v e a t r u e p i c t u r e o f the c o m p o s i t i o n t h a t would be r e a l i z e d i n a c o n t i n u ous p r o c e s s , where gases from s t e a m - r e f o r m i n g o f t h e l i q u i d v o l a t i l e s would combine w i t h t h o s e from t h e i n i t i a l p y r o l y s i s and t h e c h a r g a s i f i c a t i o n . An a p p r o x i m a t i o n was, however, o b t a i n e d by r u n n i n g the m i n i k i l n as a bomb w i t h no r e l e a s e o f gas, s t a r t i n g w i t h a s u f f i c i e n t l y s m a l l charge o f wood, water, and c a t a l y s t so t h a t p r e s s u r e l i m i t s were not exceeded. In a r u n t o peak p r e s s u r e and temperature o f 400 p s i and 1100°F, t h e gas had t h i s c o m p o s i t i o n : CH. 4 C Hg e t c . 2

18% 8% 28% 1% 45%

Note t h a t t h e CO had almost e n t i r e l y water g a s - s h i f t e d t o C 0 , but t h a t t h e methane and r i c h ethane were unaffected. The l i q u i d s and t a r s , which must have been p r e s e n t a t i n t e r m e d i a t e t e m p e r a t u r e s , had steamreformed c l e a n l y , j u d g i n g by t h e f a c t t h a t t h e r e was no soot i n t h e r e s i d u e and i t s weight was s l i g h t l y l e s s than normal. On t h e graph i n F i g u r e 4 a r e shown c h a r w e i g h t s , 2



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Figure 3. Minikiln research apparatus



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as p e r c e n t a g e s o f d r y paper s t a r t i n g w e i g h t s , a f t e r r e a c t i o n w i t h water/steam t o t h e temperatures shown. Note t h a t the weight, which i s d r o p p i n g s t e e p l y w i t h paper d e c o m p o s i t i o n , s t a r t s t o l e v e l o f f , then drops a g a i n w i t h the onset o f c h a r g a s i f i c a t i o n . Note a l s o t h a t t h e c a t a l y s t e x e r t s an i n f l u e n c e from the lowest temperature. I t i s p r o b a b l e t h a t the c a t a l y s t o p e r a t e s by s e v e r a l mechanisms: h y d r o l y s i s a t the lowest temp e r a t u r e s , t h e n d e c a r b o x y l a t i o n and d e h y d r o x y l a t i o n , then r e a c t i o n w i t h b e n z y l i c a c i d i t y , and f i n a l l y , r e a c t i o n w i t h f r e e r a d i c a l s i n the c h a r and promotion of r i n g opening by steam. F i g u r e 5 shows r e s i d u a l c h a r weight of wood c h i p s a f t e r h e a t i n g t o 1100°F a t the p r e s s u r e s shown. Again the e f f e c t of the c a t a l y s t i s marked and apparent from the lowest steam p r e s s u r e , and the c u r v e s t i l l seems t o have a s u b s t a n t i a l s l o p e a t 400 p s i . The p r e s s u r e and temperature c u r v e s appear t o be independent, and t h i s g i v e s the d e s i g n e r some degree o f freedom t o p i c k the c o m b i n a t i o n o f p r e s s u r e , temperature, f e e d r a t e , and r e s i d u e which i s most e c o n o m i c a l . E x p e r i m e n t a l work on c o a l showed that both l i g n i t e and bituminous a r e s u b s t a n t i a l l y more r e f r a c t o r y than i s s o l i d waste. There was some r e a c t i o n o f c o a l w i t h steam, but under c o n d i t i o n s where waste was e s s e n t i a l l y consumed, the r e s i d u e s from both types of c o a l were o n l y about o n e - t h i r d l e s s than i f they had been pyrolyzed. More s t r e n u o u s c o n d i t i o n s may s u f f i c e , and i f they do so, c o a l can be c o n s i d e r e d as a s u p p l e m e n t a l f u e l i n a W-M waste-to-power system. S e v e r a l experiments on p o l y c h l o r i n a t e d b i p h e n y l s (PCB's) were made t o determine the e f f i c a c y o f W-M r e a c t i o n c o n d i t i o n s f o r d e s t r o y i n g PCB s i n municipal and i n d u s t r i a l wastes. Bomb runs were employed t o prevent the escape o f any m a t e r i a l ; t y p i c a l charges were e q u a l p a r t s by weight of PCB's, water, paper, and sodium c a r b o n a t e i n amounts s u f f i c i e n t t o g e n e r a t e about 400 p s i a t 1100°F. The d a t a a r e shown i n F i g u r e 6. I t i s e v i d e n t t h a t a time-temperature o f about an hour a t 1100°F i s n e c e s s a r y and s u f f i c i e n t t o complete the PCB d e c o m p o s i t i o n . By i n f e r e n c e , s i n c e P C B s a r e among the most s t a b l e o r g a n i c m a t e r i a l s known, any hazardous o r g a n i c waste would be consumed i n a W-M p l a n t a t l i t t l e c o s t and no d e t r i m e n t t o the e n v i r o n ment . One o t h e r f i n d i n g might be mentioned b e f o r e l e a v ing the m i n i k i l n work. The apparent a c t i v a t i o n energy f o r char-steam g a s i f i c a t i o n a t 200-400 p s i i n the temperature range o f 1050-1100°F was c a l c u l a t e d t o be o n l y o n e - h a l f (30 k c a l / m o l e ) t h a t a t one atmosphere T

T



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, pyrolysis at

, I

ι

I5(latm)

,

I

,

200

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1 400

I atm

STEAM P R E S S U R E - p s i

Figure 5.

Residual char of wood as function of pressure: peak temperature 1100° F


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Figure 6.

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Residual PCB vs. temperature


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(60 k c a l / m o l e ) . I t i s p r o b a b l e t h a t t h e lowered a c t i v a t i o n energy i s due t o the presence o f a condensed aqueous f i l m on the c h a r p a r t i c l e s , which promotes r e a c t i o n between water and c h a r . I t i s probable, too, that the hygroscopic nature of the c a t a l y s t helps t o c r e a t e t h e condensed phase and s u s t a i n i t t o h i g h e r temperatures.


Expérimentâtion-Biogasser R e s e a r c h on g a s i f i c a t i o n o f biomass has product i o n o f f u e l gas as i t s u l t i m a t e purpose, and m a t e r i a l t r a n s p o r t w i t h i n the g a s i f i e r i s by auger r a t h e r than k i l n action. However, the c h e m i s t r y i s e s s e n t i a l l y the same, and the e x p e r i m e n t a l f i n d i n g s a r e d i r e c t l y a p p l i c a b l e t o the waste-to-power program. The b i o g a s s e r i s shown i n the f i n a l s t a g e s o f i t s c o n s t r u c t i o n i n F i g u r e 7. In the upper f o r e g r o u n d i s the f u e l r e s e r v o i r , the i n s u l a t e d r e a c t o r w i t h p r o t r u d i n g thermocouple c o n n e c t i o n s s l a n t s back through the c e n t e r o f the p i c t u r e , and i n the background a r e the r e s i d u e t r a p and gas r e l e a s e v a l v e s . The condens e r and g a s / l i q u i d s e p a r a t o r a r e a l o n g the lower right-hand s i d e of the r e a c t o r . For perspective, i t might be noted t h a t the biomass s l u r r y r e s e r v o i r i s about 6 i n . ID and 6 f t . i n h e i g h t , the s l a n t e d r e a c t o r i s 1.75 i n . ID and 10 f t . i n l e n g t h , and the r e s i d u e t r a p i s about 3 i n . ID and 3 f t . i n h e i g h t . The equipment i s s t a i n l e s s s t e e l t h r o u g h o u t . I t s o p e r a t i o n can be d e s c r i b e d by moving through the sequence of a t y p i c a l r u n . The cap and p i s t o n a r e l i f t e d from the f u e l r e s e r v o i r , the r e s e r v o i r i s f i l l e d w i t h sawdust o r o t h e r shredded biomass, water, and c a t a l y s t , and the p i s t o n and cap r e p l a c e d . Then t h e h e a t e r s a r e turned on, the h y d r a u l i c pump d r i v i n g the p i s t o n down i s s t a r t e d as i s t h e auger d r i v e motor, and the s l u r r y moves s l o w l y down out o f the r e s e r v o i r and up the s l o p e o f the r e a c t o r . Temperatures r i s e u n t i l the d e s i r e d p r o f i l e i s reached, t y p i c a l l y 100°F a t the f e e d end and 1200°F a t t h e t r a p and r e d u c i n g v a l v e ; the p r e s s u r e r i s e s t o the d e s i r e d f i g u r e (up t o 3200 p s i ) by e v o l u t i o n o f gas and f o r m a t i o n o f steam, and then t h a t p r e s s u r e i s m a i n t a i n e d by c o n t r o l l e d r e l e a s e o f gas t o t h e condenser. As t h e gas i s r e l e a s e d , the condensate i s m o n i t o r e d f o r l i q u i d s and t a r s , and the non-condensing gas i s a n a l y z e d by chromatograph. At the end o f the r u n , the r e a c t o r and t r a p a r e examined f o r r e s i d u e . In s a t i s f a c t o r y s t e a d y s t a t e o p e r a t i o n , the g a s i f i c a t i o n i s r e a s o n a b l y complete, the condensate c o n t a i n s l i t t l e o r g a n i c l i q u i d o r t a r ,


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Figure 7.

Biogasser in itsfinalstages of construction


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and

the r e s i d u e i s e s s e n t i a l l y i n o r g a n i c . In the graph of F i g u r e 8 a r e shown data on r e s i dues as a f u n c t i o n of p r e s s u r e from the b i o g a s s e r ( t r i a n g l e s ) and the m i n i k i l n ( c i r c l e s ) . The m i n i k i l n data show how the c h a r r e s i d u e d e c r e a s e s r a p i d l y w i t h p r e s s u r e i n the range of 0-400 p s i and how the e f f e c t of p r e s s u r e i s g r e a t e r i n the presence of added N 2C03 catalyst. The r e s i d u e s from b i o g a s s e r runs at one atmosphere and 300 p s i l i e s l i g h t l y below the c o r r e s ponding m i n i k i l n c u r v e s because the b i o g a s s e r was o p e r a t e d a t a s l i g h t l y h i g h e r temperature; otherwise the b i o g a s s e r data seem c o n s i s t e n t w i t h the m i n i k i l n d a t a i n t h i s r e g i o n and show a c a t a l y s t e f f e c t of the same magnitude. By 600 p s i , however, the e f f e c t of the c a t a l y s t seems t o have d i s a p p e a r e d ; the r e s i d u e from the c a t a l y z e d b i o g a s s e r run i s as l a r g e as t h a t from the unc a t a l y z e d run. And a t 1000 p s i , the c h a r r e s i d u e i s c l e a r l y h e a v i e r than those at lower p r e s s u r e s . The exact shape of the c u r v e i s not w e l l - d e f i n e d by t h e s e experiments because of d i f f i c u l t i e s i n m a i n t a i n i n g the same peak temperatures i n runs a t d i f f e r e n t p r e s s u r e s . N e v e r t h e l e s s , i t i s e v i d e n t t h a t the t r e n d s i n the l o w e r - p r e s s u r e m i n i k i l n data may not be e x t r a p o l a t e d to h i g h e r p r e s s u r e s . Q u a l i t a t i v e l y , t h e r e i s a l s o a change i n the n a t u r e of t h e r e s i d u e at h i g h e r p r e s s u r e s . In both the m i n i k i l n and b i o g a s s e r runs a t p r e s s u r e s up t o 300-400 p s i , the c h a r r e s i d u e i s l o o s e and g r a n u l a r . In the b i o g a s s e r runs a t 600 and 1000 p s i , much of the r e s i d u e i s s t u c k t o g e t h e r i n hard lumps. T h i s type of c a k i n g and i n c r e a s e i n r e s i d u e become p r o g r e s s i v e l y more apparent as the p r e s s u r e i n c r e a s e s , a t l e a s t t o 3200 p s i . The o b s e r v a t i o n s suggest t h a t t h e r e a r e two p r e s s u r e i n f l u e n c e s working i n o p p o s i t e d i r e c t i o n s . In the lower range, i n c r e a s e i n p r e s s u r e promotes r e a c t i o n by i n c r e a s i n g the c o n c e n t r a t i o n of steam and f o r m a t i o n of aqueous f i l m s . But above 300-400 p s i , t h i s e f f e c t i s swamped out by an i n c r e a s i n g tendency toward f o r m a t i o n o f c o k e - l i k e m a t e r i a l , presumably from c h e m i c a l c o n d e n s a t i o n of v o l a t i l i z e d l i q u i d intermediates. The l a t t e r e f f e c t i s c o n s i s t e n t w i t h LeChatelier s principle. I t i s c o n c l u d e d t h a t f o r maximum c o n v e r s i o n of s o l i d s t o f u e l gas, t h e r e i s an optimum i n r e a c t o r p r e s s u r e i n the neighborhood o f 300-400 p s i . T h i s f i n d i n g i s s i g n i f i c a n t t o f u r t h e r development of the W-M p r o c e s s , i n t h a t i t i s now e s t a b l i s h e d t h a t t h e r e i s no advantage i n g o i n g t o h i g h e r r e a c t o r p r e s s u r e .


a

T



COFFMAN

AND

15%

Steam Gasification

HOOVERMAN

h

J

Ο

.

I

200

,

I

400

•

I

600

.

ι 800

.

Ι

1000

PRESSURE - psig

Figure 8.

Residual wood char vs. pressure, minikiln and biogasser



266

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RESIDUES

I t i s f o r t u n a t e t h a t the optimum p r e s s u r e c o i n c i d e s w i t h t h a t deemed most d e s i r a b l e f o r k i l n o p e r a t i o n f o r other reasons: gas t u r b i n e o p e r a t i o n , l o n g gas d w e l l time i n the k i l n , low p a r t i c u l a t e , heat t r a n s f e r , e t c . Gas c o m p o s i t i o n d a t a from the b i o g a s s e r s u p p o r t e a r l i e r work i n the m i n i k i l n , the p r o p o r t i o n s of hydrogen, methane, c a r b o n d i o x i d e , e t c . b e i n g s i m i l a r t o those i n m i n i k i l n bomb r u n s . Under c o n d i t i o n s o f e s s e n t i a l l y complete c o n v e r s i o n o f s o l i d s t o f u e l gas (1150°F, 300 p s i ) , the f i x e d gases had an i n t e g r a t e d hydrogen-to-oxygen r a t i o of two t o one, the same as i n c e l l u l o s e and water. A t l e s s than complete c o n v e r s i o n , the r a t i o f e l l below t h i s f i g u r e , because the oxygen tends t o come o f f f i r s t , as CO2 and C0> and the hydrogen tends t o s t a y behind i n the c h a r and l i q u i d s . Use o f t h i s gas measuring and i n t e g r a t i n g t e c h n i q u e enabled the g a t h e r i n g o f much more d a t a on t h e degree of c o n v e r s i o n v e r s u s temperature and p r e s s u r e than would have been p o s s i b l e by measure of r e s i d u e w e i g h t s . The graph i n F i g u r e 9 shows the hydrogen t o oxygen r a t i o (times J) as a f u n c t i o n o f r e a c t o r t e m p e r a t u r e s . I t i s t o be noted t h a t the temperatures a r e maximum w a l l temperatures as measured by thermocouples embedded i n the r e a c t o r w a l l . I t i s e s t i m a t e d t h a t the c o r r e s p o n d i n g gas temperatures a r e about 200°F lower. The bend i n the c u r v e , t h e r e f o r e , comes a t about 1150°F, c o r r e s p o n d i n g to complete g a s i f i c a t i o n as judged by r e s i d u e and c o n d e n s a t e . C a l c u l a t i o n s have been made from heats o f format i o n t o determine e q u i l i b r i u m gas c o m p o s i t i o n s as a f u n c t i o n o f water/wood r a t i o , temperature, and p r e s sure. These c a l c u l a t e d e q u i l i b r i a have been found t o c o r r e s p o n d r e a s o n a b l y w e l l t o e x p e r i m e n t a l composit i o n s above 1150°F; below t h a t temperature, the exp e r i m e n t a l c o m p o s i t i o n s d e v i a t e i n f a v o r of the f i r s t formed p r o d u c t s . T h i s f i n d i n g has t h e o r e t i c a l s i g n i f i c a n c e but l i t t l e p r a c t i c a l u t i l i t y i n the k i l n power system where parameters l i k e water-to-waste r a t i o a r e f i x e d by c y c l e c o n s i d e r a t i o n s . However, i n the b i o mass g a s i f i c a t i o n system, the water/wood r a t i o can be v a r i e d t o maximize the u t i l i t y o f the product gas t o ward i t s i n t e n d e d use: medium B t u f u e l gas, s y n t h e s i s of methanol o r ammonia, o r methanation t o p i p e l i n e q u a l i t y gas. Pilot

Plant

Gasifier

The t r a n s i t i o n from bench s c a l e r e s e a r c h t o p i l o t p l a n t w i l l be a c c o m p l i s h e d i n s t a g e s , the f i r s t b e i n g g a s i f i e r d e s i g n . A s c h e m a t i c i s shown i n F i g u r e 10.



COFFMAN

A N D HOOVERMAN

900

1000

Steam Gasification

1100

MAXIMUM WALL

Figure 9.

1200

1300

1400

1500

e

TEMP.- F

The hydrogen to oxygen ratio as a function of reactor temperatures



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3>


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269


The p i l o t p l a n t g a s i f i e r i s p r o j e c t e d t o be 60 f t . i n l e n g t h and 2\ f t . i n d i a m e t e r at the c o o l end, and thus has about o n e - t e n t h the volume of the p r o d u c t i o n unit described e a r l i e r . I t s nominal throughput i s 20 tons/day of s o l i d waste and 30 tons/day o f l i q u i d sewage s l u d g e , and i t s product gas, consumed i n a gas t u r b i n e , would g e n e r a t e 1 MWe. There a r e s i g n i f i c a n t d i f f e r e n c e s between t h i s d e s i g n and t h a t d e s c r i b e d e a r l i e r , t h e s e r e f l e c t i n g the c o n t i n u i n g e v o l u t i o n o f the W-M system. Feed. G r a v i t y - f e e d l o c k hoppers have been d i s carded i n f a v o r of a p o s i t i v e p i s t o n - f e e d system. C o a r s e l y shredded but u n c l a s s i f i e d s o l i d waste i s d e l i v e r e d from the l e f t by w e i g h i n g conveyor and dep o s i t e d i n t o the open t h r o a t of a c y l i n d r i c a l c a v i t y . The m a t e r i a l i s tamped l i g h t l y i n t o p l a c e and the c y l i n d e r s e a l e d w i t h the tamping p i s t o n . Then the i n n e r s t r u c t u r e r o t a t e s n i n e t y degrees, and the waste i s pushed from the c a v i t y i n t o the k i l n by a second piston. At the c o m p l e t i o n of t h i s p i s t o n s t r o k e , the i n n e r s t r u c t u r e b a c k - r o t a t e s to t h e v e r t i c a l p o s i t i o n , the tamping p i s t o n l i f t s , the i n n e r p i s t o n drops, and the c y c l e i s r e p e a t e d . L i q u i d s l u d g e i s pumped cont i n u o u s l y through i t s p r e - h e a t e r i n t o the t h r o a t of the k i l n , m i x i n g t h e r e w i t h the s o l i d waste. Homogen i z a t i o n w i l l o c c u r i n the f i r s t few f e e t of the k i l n . The f e e d system i s h e l d i n p l a c e a g a i n s t k i l n p r e s s u r e by a t h r u s t rod, t h r u s t r i n g assembly working a g a i n s t the f r o n t f l a n g e of the k i l n , and r e q u i r e s no external bracing. The feed support s t r u c t u r e i s mounted on r o l l e r s so t h a t the feed system can be unb o l t e d and moved away f o r maintenance of the f r o n t r o t a t i n g s e a l s , which a r e i n s i d e the t h r u s t r i n g and independent of i t . K i l n Support. The f i x e d p o i n t f o r the k i l n i s e s t a b l i s h e d through the t h r u s t assembly a t the f r o n t end. T h i s means t h a t the f r o n t end t r u n n i o n r o l l assembly and the m i d d l e one a r e n o n - f l a n g e d and nontracking . The e n t i r e n o n - r o t a t i n g assembly a t the hot end of the k i l n i s mounted on a support c a r r i a g e on r o l l e r s . I t s p o s i t i o n r e l a t i v e t o the k i l n i s determined by having t h i s end of the k i l n s u p p o r t e d by a t r a c k i n g trunnion. Movement o f the support c a r r i a g e w i t h t h e r mal e x p a n s i o n and c o n t r a c t i o n o f the k i l n means no a l l o w a n c e f o r a x i a l t h e r m a l motion i s r e q u i r e d i n the r o t a t i n g s e a l assembly.



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Stepped K i l n . The o u t e r b a r r e l o f the k i l n has a u n i f o r m , s l i g h t l y g r e a t e r than 2\ f t . diameter t h r o u g h out i t s l e n g t h . The i n n e r s h e l l , which c o n t a i n s t h e waste and on which t h e steam h e a t i n g tubes a r e wrapped, i s 2^ f t . i n d i a m e t e r f o r t h e f i r s t 30 f t . , then dec r e a s e s t o 2 f t . f o r t h e next 15 f t . , and d e c r e a s e s f u r t h e r t o \\ f t . f o r the f i n a l 15 f t . The space between the steam c o i l and t h e o u t e r b a r r e l i s p r e s s u r e e q u a l i z e d and f i l l e d w i t h t h e r m a l i n s u l a t i o n . This reduces the temperature o f the o u t e r b a r r e l a t t h e hot end t o about t w o - t h i r d s o f what i t would be i f the b a r r e l were immediately a d j a c e n t t o t h e steam t u b e s . T h i s means t h a t the b a r r e l can more r e a d i l y c a r r y i t s p r e s s u r e l o a d and can be l e s s massive than i f t h e i n n e r s h e l l were not stepped down A n o t h e r b e n e f i t o f t h e stepped d e s i g n i s the slowing o f the forward movement of the m a t e r i a l as the diameter decreases. Much of the m a t e r i a l b u l k w i l l have been l o s t by the time i t reaches t h e m i d - p o i n t , and t h e k i l n w i l l not dam up a t the s t e p . The r e s u l t i n g i n c r e a s e i n d w e l l time w i l l a i d i n c o m p l e t i n g steam g a s i f i c a t i o n of the char. Steam H e a t i n g . The steam h e a t i n g system w i l l o p e r a t e a t 600 p s i , a p p r o x i m a t e l y t w i c e k i l n gas p r e s sure. T h i s w i l l ensure an adequate temperature d i f f e r e n t i a l between the condensing zone i n t h e steam c o i l s and t h e e v a p o r a t i n g zone w i t h i n the k i l n . The b o i l e r and s u p e r h e a t e r w i l l be f i r e d by a p o r t i o n o f the gas from the k i l n , i n c l u d i n g the low p r e s s u r e gas from t h e dump v a l v e s ; t h e remainder w i l l be f l a r e d . Gas w i l l l e a v e and steam w i l l e n t e r the r o t a t i n g s t r u c t u r e through a s m a l l diameter s t u f f i n g box s e a l , o f t h e type c o n s t r u c t e d f o r t h e m i n i k i l n and proved s a t i s f a c t o r y t h e r e . The condensate w i l l e x i t t h e k i l n through the c e n t e r o f t h e f a c e s e a l a t the f r o n t end and, by v i r t u e o f i t s h i g h e r p r e s s u r e , p r o v i d e a s s u r ance a g a i n s t gas leakage. Gas F i l t e r . Because o f the h i g h h u m i d i t y i n t h e k i l n , the r e l a t i v e l y low gas v e l o c i t y , and t h e g e n t l e t u m b l i n g a c t i o n , l i t t l e entrainment o f p a r t i c u l a t e i n the gas i s a n t i c i p a t e d . N e v e r t h e l e s s , a gas f i l t e r i s provided i n the e x i t l i n e . I t has a convex f a c e which i s c o n t i n u o u s l y wiped from behind as i t r o t a t e s by a j e t o f steam from a s t a t i o n a r y f i n g e r , shaped t o the contour of the f i l t e r . D i r t thus d i s l o d g e d f a l l s i n t o the d i s c h a r g e c a v i t y .



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Residue D i s c h a r g e . The dump v a l v e s a r e f u l l - b o r e b a l l v a l v e s a r d the c h a n n e l s above and between t h e v a l v e s have the same d i a m e t e r as t h e channels* w i t h i n the v a l v e s ; t h e r e a r e no s h o u l d e r s on which m a t e r i a l w i l l lodge. The v a l v e s swing w i t h the k i l n . At the bottom o f the f i r s t swing, t h e t o p v a l v e opens and the accumulated m a t e r i a l drops through the v a l v e s i n t o the c a v i t y between them. Then the v a l v e c l o s e s , and as i t swings, t h e c a v i t y i s s l o w l y d r a i n e d o f i t s f u e l g a s . At t h e bottom o f t h e second swing, t h e bottom v a l v e opens and drops the r e s i d u e i n t o t h e r e c e i v i n g container. Then t h e v a l v e c l o s e s and t h e c y c l e i s r e peated . Controls. The c o n t r o l system w i l l be designed f o r manual o r a u t o m a t i c o p e r a t i o n F o r automatic operat i o n , t h e p r e d i c t i v e c o n t r o l l e r w i l l be programmed t o maintain the apparatus w i t h i n pre-set l i m i t s . The o p e r a t o r w i l l have the a b i l i t y a t any time t o o v e r r i d e the p r e d i c t i v e c o n t r o l l e r o r t o r e - s e t i t s l i m i t s . Because o f t h e slow t r a v e r s e o f m a t e r i a l through t h e k i l n and hence the s l u g g i s h n e s s o f response t o changes i n q u a l i t y and q u a n t i t y o f feed m a t e r i a l , the c o n t r o l system w i l l have a n t i c i p a t o r y c h a r a c t e r , s e n s i n g the r a t e o f change as w e l l as the a c t u a l v a l u e s o f c o n t r o l parameters. When o p e r a t e d w i t h a gas t u r b i n e i n l a t e r phases of development, t h e g a s i f i e r p r e s s u r e w i l l be s e t about 50 p s i h i g h e r than t h a t r e q u i r e d a t the t u r b i n e combustion chamber i n l e t . The k i l n thus becomes a s m a l l surge tank, and i t s p r e s s u r e f l u c t u a t i o n s w i l l m i l i t a t e a g a i n s t changes i n f u e l v a l u e o f t h e waste b e i n g f e l t as s u r g e s on the power l i n e . (That, and a smart o p e r a t o r . ) Economics Economic a n a l y s i s work t o date assumes u t i l i t y ownership and o p e r a t i o n , w i t h d i s t r i b u t i o n o f t h e g e n e r a t e d power through t h e u t i l i t y g r i d . (A W-M p l a n t i s t o be thought o f as a power p l a n t f u e l e d by waste r a t h e r than a waste d i s p o s a l p l a n t w i t h e l e c t r i c i t y as a b y - p r o d u c t . ) The m u n i c i p a l i t y would have r e s p o n s i b i l i t y f o r c o l l e c t i o n and d e l i v e r y o f s o l i d waste t o the p l a n t , d e l i v e r y o f l i q u i d sewage sludge, and removal and d i s p o s a l o f p l a n t r e s i d u e s . Putting the i n t e r f a c e between u t i l i t y and m u n i c i p a l i t y a t t h e u n l o a d i n g dock would seem t o be t h e most r e a s o n a b l e a l t e r n a t i v e because i t p r e s e r v e s t h e t r a d i t i o n a l r o l e of each. Other arrangements a r e p o s s i b l e , o f c o u r s e :



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m u n i c i p a l o p e r a t i o n and d i s t r i b u t i o n o f power, munic i p a l o p e r a t i o n w i t h s a l e o f power t o t h e u t i l i t y , t h i r d p a r t y o p e r a t i o n , e t c . They would e n t a i l d i f f e r ent assumptions r e g a r d i n g t a x and i n t e r e s t r a t e s , and would y i e l d d i f f e r e n t r e s u l t s . The c a p i t a l c o s t o f t h e b a s i c 10 MWe, 200 ton/day s o l i d waste, 300 ton/day sewage s l u d g e p l a n t i s e s t i mated t o be $9 m i l l i o n o r $900/KW. The a n n u a l d i r e c t o p e r a t i n g c o s t i s e s t i m a t e d a t $850,000. To t h i s must be added d e p r e c i a t i o n , i n t e r e s t on d e p r e c i a b l e i n v e s t ment, l o c a l t a x e s , and income t a x e s . I f one assumes t i p p i n g c h a r g e s o f $8/ton f o r s o l i d waste and $10/ton f o r l i q u i d sewage s l u d g e , and v a l u e s o f 2, 3, and 4

Power from Wastes via Steam Gasification - ACS Symposium Series

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