Clean Energy from Waste and Coal - American Chemical Society

Chapter 3

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

Downloaded by CORNELL UNIV on July 25, 2016 | http://pubs.acs.org Publication Date: December 23, 1992 | doi: 10.1021/bk-1992-0515.ch003

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

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

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Recovery of Ethanol from Municipal Solid Waste

ACKERSON ET AL. Table I .


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

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

F*W»r

Fall

Winter

Spring

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

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

42.2 0.4 24.1 10.2 9.7 3.6 2.7 1.5 4.2

36.5 14.4 20.8 8.8 8.2 3.1 2.2 1.2 3.7

Average

37.4 13.9 20.0 9.8 8.4 3.1 2.2 1.2 3.4

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

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

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CLEAN ENERGY FROM WASTE AND COAL


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

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

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

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

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

44.8 36.5 36.7 41.3 33.7 61.0 47.0

24.8 10.4 15.1 14.9 13.5 21.0 12.0



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Recovery of Ethanol from Municipal Solid Waste 31

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

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




MSW

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


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

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

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

Acid

Hydrolyzates

Concentration g/L

Prehydrolyzate Xylose Glucose Hydrolyzate Xylose Glucose Combined Xylose Glucose

Yield g/100q

9.5 18.5

8.0 16.0

0.0 67.8

0.0 44.0 8.0 60.0

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


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

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


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

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

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

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

Acid Concentration

Rate o f Formation/Decomposition

Glucose

2N 3N 4N

0.0053 0.0090 0.0074

Xylose

2N 3N 4N

0.0257 0.0402 0.0374

Sugar

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


3. ACKERSON ET AL.



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

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

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

Amino Acids

NH3PO3)

Amino A c i d s and NH3(P03)

Yeast Extract

16

15.9

21.9

27.3

97.5

23

19.3

24.9

35.8

97.5

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



36


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


3.

Recovery of EthanolfromMunicipal Solid Waste

ACKERSON ET AL.


100

0

10

20

30

Fermentation

40 Time

50

60

(hrs)

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

200 180 φ-

s

0

160 ,r À

•

50

g/L

140

ο

100

g/L

a

150

g/L

δ

200

g/L

12θφ\ 100

\

Λ

80

> ο

60

\

^

\

\ ο

\

40 20 0 0

20

40

60

Time

80

100

120

(min)

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

37


38


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



3.

ACKERSON ET AL.

Recovery of EthanolfromMunicipal Solid Waste

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

A.

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

C a p i t a l Cost

MUU

Clean Energy from Waste and Coal - American Chemical Society

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