Biomass as a Nonfossil Fuel Source - American Chemical Society

8 Liquid Hydrocarbon Fuels from Biomass

Downloaded by NANYANG TECH UNIV LIB on November 1, 2014 | http://pubs.acs.org Publication Date: January 29, 1981 | doi: 10.1021/bk-1981-0144.ch008

JAMES L. KUESTER College of Engineering and Applied Sciences, Arizona State University, Tempe, A Z 85281

Considerable attention is currently being focused on development of synthetic liquid fuels from non-petroleum feedstocks. The major feedstock candidates are coal, oil shale and biomass. Potential products include alcohol fuels, alcohol-gasoline blends and equivalents of commercial materials currently derived from petroleum (kerosine. diesel, jet fuel, high octane gasoline, etc.). Various advantages and disadvantages of coal and oil shale feedstocks vs. biomass are listed in Table I. It seems obvious that coal and oil shale will have the largest short-term impact due to the large quantities available. Initially, biomass use will concentrate on waste streams, (industrial, agricultural, urban) and surplus agricultural crops. Longer term, agricultural and forest residues (not currently collected) and energy crops will play an increasingly important role. Eventual widespread use of renewable biomass sources appears inevitable (1,2) with the main question being primarily one of timing. Several approaches have been proposed for the production of liquid fuels from biomass. Alcohol production via fermentation is state-of-the-art technology for specific feedstocks (grain etc.). The use of non-food sources (urban refuse, industrial wastes, etc.) is not fully developed. Processing times are on the order of days however for biological conversion. Non-biological methods fall into two categories: (1) direct liquefaction, and (2) indirect liquefaction. Both involve a thermal conversion step. Direct liquefaction

0097-6156/81/0144-0163$05.50/0 © 1981 American Chemical Society

In Biomass as a Nonfossil Fuel Source; Klass, D.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.


164

B I O M A S S AS A N O N F O S S I L F U E L

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processes (3, 4) a t t e m p t t o p r o d u c e a p r o d u c t w i t h o u t g o i n g t h r o u g h t h e gas phase. The p r o d u c t h o w e v e r c o n t a i n s an a p p r e c i a b l e a m o u n t of o x y g e n a t e d c o m p o u n d s t h u s leading t o q u a l i t y a n d s t a b i l i t y p r o b l e m s . Current research in t h i s area is a i m e d at e l i m i n a t i n g t h e o x y g e n . Process c o n d i t i o n s appear severe (3). Indirect l i q u e f a c t i o n m e t h o d s c o n v e r t t h e biomass t o a synthesis gas a n d t h e n r e c o m b i n e c o m p o n e n t s t o f o r m q u a l i t y h y d r o c a r b o n fuel p r o d u c t s free of o x y g e n a t e d c o m p o u n d s . Three a p p r o a c h e s have been M o b i l , Naval W e a p o n s Center (China Lake), a n d A r i z o n a State University. T h e M o b i l process (5) c o n v e r t s m e t h a n o l t o h i g h o c t a n e gasoline via a c a t a l y t i c process at m i l d o p e r a t i n g c o n d i t i o n s . T h e m e t h a n o l is synthesized f r o m c a r b o n m o n o x i d e a n d h y d r o g e n w h i c h c a n be o b t a i n e d f r o m fossil fuels or biomass via g a s i f i c a t i o n . T h e Naval W e a p o n s Center a p p r o a c h (6) t h e r m a l l y polymerizes olefins (primarily ethylene). T h e olefins are separated f r o m t h e b i o m a s s g a s i f i c a t i o n s t r e a m . T h e A r i z o n a State U n i v e r s i t y process c o n v e r t s t h e h y d r o g e n , c a r b o n m o n o x i d e a n d olefins p r o d u c e d in a g a s i f i c a t i o n step t o paraffinic fuels (e.g. diesel, jet, kerosine) via a c a t a l y t i c reactor at m i l d o p e r a t i n g c o n d i t i o n s . Prior separation of t h e u n r e a c t i v e gas c o m p o n e n t s (ethane, m e t h a n e , c a r b o n d i o x i d e , etc.) is n o t r e q u i r e d . If a h i g h o c t a n e gasoline is d e s i r e d , t h e paraffinic fuels are passed t h r o u g h a c o n v e n t i o n a l c a t a l y t i c reformer. T h e present s t a t u s a n d f u t u r e p r o j e c t i o n s of t h e A r i z o n a State University process are d e s c r i b e d in t h i s paper. PROCESS DESCRIPTION T h e basic c o n v e r s i o n s c h e m e is d e p i c t e d in Figure I a n d t h e processing e q u i p m e n t is s h o w n in Figure II. T h e s y s t e m is o p e r a t e d c o n t i n u o u s l y a n d is a s i m u l a t o r of c o m m e n ç a i scale processing for t h e m o s t part. T h u s e q u i p m e n t a n d p r o c e d u r e d e v e l o p m e n t has a c c o m p a n i e d f a c t o r and o p t i m i z a t i o n studies. T h e s y s t e m is c o n v e n i e n t l y d i v i d e d i n t o t w o s e c t i o n s : (1) g a s i f i c a t i o n , a n d (2) l i q u i d fuels synthesis. G a s i f i c a t i o n . T h e pyrolysis reactor c o n s i s t s of a f l u i d i z e d b e d w h e r e t h e solids-fluidized m e d i u m is either inert (e.g. sand) or a catalyst in t h e 6 0 - 1 2 0 m e s h range. T h e bed d i a m e t e r is 10 in. w i t h a l e n g t h of 4 ft. T h e f l u i d i z e d bed zone is a p p r o x i m a t e l y 18 in. A s e c o n d i d e n t i c a l f l u i d i z e d bed serves as t h e solids heater a n d c a t a l y s t regenerator w i t h c o n t i n o u s transfer b e t w e e n t h e t w o vessels. T h e p r i n c i p l e is i d e n t i c a l t o t h a t e m p l o y e d in t h e c a t a l y t i c c r a c k i n g of p e t r o l e u m , a process t h a t has b e e n s u c c e s s f u l l y e m p l o y e d by p e t r o l e u m refiners since t h e 1940's. T h e a d v a n t a g e s of t h e s y s t e m are e f f i c i e n t heat transfer, e f f e c t i v e t e m p e r a t u r e c o n t r o l , c o n t i n o u s catalyst r e g e n e r a t i o n , a n d t h e p r o d u c t i o n of h i g h q u a l i t y gas free of c o m b u s t i o n


8.

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165

Liquid Hydrocarbon Fuels

co cp

A

C 0

2


FTSCHER;TROPSCH | ρ [REFORMER [—

GASOLINE

CELLULOSE - | PYROLYSB | ALIPHATIC HYDROCARBONS

ALCOHOL. WATER

Figure 1.

Basic chemical conversion

t TAP.

ASH >—>f

CHAR.

REGENERATOR

KCACTOW

ens

TAP

£1 REACTOR -COOLER •GAS

AM

1

TRANSFER COOPS

I TRAPS

1

PYROLYSIS GASOLINE SYNTHESIS Figure 2.

Conversion system schematic


166

BIOMASS AS A NONFOSSIL F U E L

SOURCE


p r o d u c t s . It is a n t i c i p a t e d t h a t t h e final c o n f i g u r a t i o n w i l l i n c l u d e t h e use of a c o n t i n u o u s l y regenerated c r a c k i n g c a t a l y s t w i t h s t e a m a d d i t i o n t o t h e pyrolysis reactor (to p r o m o t e t h e w a t e r gas shift reaction). If s t e a m a d d i t i o n w e r e used w i t h o u t c o n t i n o u s catalyst r e g n e r a t i o n , a s t e a m preheater p r o b a b l y w o u l d replace t h e s e c o n d f l u i d i z e d b e d , i.e., no solids loop transfer w o u l d be e m p l o y e d . Feedstock is c o n t i n o u s l y f e d i n t o t h e pyrolysis reactor via a s c r e w feeder. T h e b e d is o p e r a t e d at a c o n t r o l l e d t e m p e r a t u r e in t h e 6 0 0 - 8 0 0 ° C range a n d a s l i g h t positive pressure (0-5 psig). Residence t i m e s are in t h e 0-5 s e c o n d range. A t t h e s e c o n d i t i o n s , t h e f e e d s t o c k is flashed t o a gas. L o w e r t e m p e r a t u r e s p r o m o t e tar f o r m a t i o n . T h e m a x i m u m t e m p e r a t u r e is l i m i t e d by m e t a l l u r g y c o n s t r a i n t s (distributor plates, transfer loops). W o r k t o d a t e has been l i m i t e d t o d r y . f i n e l y g r o u n d f e e d s t o c k s free of i n o r g a n i c matter. W e t feeds s h o u l d n o t present a p r o b l e m f r o m a c h e m i s t r y v i e w p o i n t , b u t t h e o p e r a t i o n a l p r o b l e m of reliable c o n t i n u o u s f e e d i n g w o u l d have t o be d e m o n s t r a t e d . A p u m p e d w a t e r - s l u r r y feed m i g h t be preferable t o a w a t e r w e t cake. Particle size is l i m i t e d b y t h e scale of t h e e q u i p m e n t t o a b o u t 1/4 in. m a x i m u m . T h u s , larger particles or pellets are feasible at a larger scale a l t h o u g h particle heat transfer c o n s i d e r a t i o n s m a y be a l i m i t i n g factor. Inorganics s u c h as m e t a l s a n d glass f r o m u r b a n uefuse w o u l d have t o be r e m o v e d f r o m t h e f e e d s t o c k prior t o t h e g a s i f i c a t i o n step. T h e pyrolysis gas is passed t h r o u g h an o v e r h e a d s y s t e m c o n s i s t i n g of a c y c l o n e separator (to r e m o v e char a n d o t h e r solid particles), a w a t e r - f e e d s c r u b b i n g s y s t e m (to cool t h e gas a n d r e m o v e c o n d e n s i b l e s ) , a n d compressor. From t h e c o m p r e s s o r d i s c h a r g e , t h e gas can be split t o t h e pyrolysis reactor (fluidizing a n d sparge gas) a n d t h e liquid fuels synthesis s y s t e m . T h e use of a s t e a m - f l u i d i z e d pyrolysis s y s t e m w i t h o u t pyrolysis gas recycle is also b e i n g i n v e s t i g a t e d . Storage t a n k s are available t o c o n t a i n t h e g e n e r a t e d pyrolysis gas a n d a l l o w for o p e r a t i o n of t h e l i q u i d fuels synthesis s y s t e m i n d e p e n d e n t of t h e g a s i f i c a t i o n s y s t e m if desired. T h e fluidized bed regenerator (or s t e a m preheater) w o u l d be f u e l e d w i t h air a n d recycle c h a r plus off gases f r o m t h e l i q u i d fuels synthesis reactors o n a larger scale. In t h e research u n i t , p r o p a n e a n d o x y g e n are used w i t h o u t recycle of c h a r f r o m t h e c y c l o n e or d o w n s t r e a m gases. S o m e c h a r u n d o u b t e d l y c i r c u l a t e s in t h e transfer loops a n d coke is b u r n e d off t h e c a t a l y s t (if used) in t h e regenerator. The o v e r h e a d s y s t e m for t h e regenerator consists of a c y c l o n e separator a n d s c r u b b e r w i t h c o m b u s t i o n gases v e n t e d to the atmosphere.



8.

KUESTER


167

L i q u i d F u e l s S y n t h e s i s . T h e first step in t h e l i q u i d fuels synthesis s y s t e m is a f l u i d i z e d b e d c a t a l y t i c reactor (2 i n . x 6 ft.) c o n t a i n i n g a m o d i f i e d Fischer T r o p s c h t y p e c a t a l y s t (cobalt-alumina). Raw pyrolysis gas ( w i t h o u t gas separation) is fed t o t h i s s y s t e m at m i l d o p e r a t i n g c o n d i t i o n s (e.g., 2 5 0 - 3 0 0 ° C , 125 psig, 18 s e c o n d s residence t i m e ) . T h e fluidized m o d e is e m p l o y e d t o a c h i e v e t e m p e r a t u r e c o n t r o l w i t h t h e s i g n i f i c a n t e x o t h e r m i c heat of reaction. C o n t i n o u s regeneration is n o t used. T h e reaction is largely s e l f - s u s t a i n i n g . T h e gas p r o d u c t is c o o l e d in a c o n d e n s o r a n d t w o liquid phases result — a h y d r o c a r b o n phase a n d an a l c o h o l - w a t e r phase. T h e relative a m o u n t s of these phases are d e p e n d e n t o n t h e o p e r a t i n g c o n d i t i o n s a n d c a n vary f r o m 1 0 0 % h y d r o c a r b o n s t o 1 0 0 % w a t e r - a l c o h o l phase. The h y d r o c a r b o n phase is paraffinic in nature w i t h s o m e b r a n c h e d c o m p o u n d s , olefins a n d a r o m a t i c s . T h e w a t e r - a l c o h o l phase is essentially a binary of n o r m a l p r o p a n o l a n d w a t e r w i t h t h e a l c o h o l c o n t e n t as h i g h as 15 w t %. T h e h y d r o c a r b o n phase is similar t o JP-4 j e t fuel. A s i m p l e d i s t i l l a t i o n w i l l isolate a kerosine-diesel fuel t y p e f r a c t i o n . A h i g h o c t a n e gasoline is readily a c h i e v e d b y passing t h e Fischer T r o p s c h o r g a n i c phase t h r o u g h a c o n v e n t i o n a l c a t a l y t i c reformer (2 i n . x 2 ft.). A l t e r n a t i v e l y , a c o m p o s i t e catalyst is b e i n g e x p l o r e d in t h e Fischer T r o p s c h step t o p r o d u c e a h i g h o c t a n e p r o d u c t directly. Off gases are g e n e r a t e d f r o m b o t h t h e Fischer T r o p s c h a n d r e f o r m i n g steps. Q u a l i t y is very h i g h ( 6 0 0 - 2 5 0 0 Btu/SCF) w i t h a heavy c o n c e n t r a t i o n of l o w m o l e c u l a r w e i g h t paraffins. O n a c o m m e r c i a l scale, these gases w o u l d be r e c y c l e d back t o t h e g a s i f i c a t i o n s y s t e m . Photos of t h e c o n v e r s i o n s y s t e m laboratory are s h o w n in Figure III. GASIFICATION STUDIES Gasification studies c o m p l e t e d or in progress are listed in Table II. T h e original studies w e r e o n a 4 - i n . d i a m e t e r fluidized b e d w i t h paper c h i p s feedstock. A t e m p e r a t u r e a n d feed rate factorial s t u d y w i t h sand as t h e f l u i d i z e d m e d i a revealed t h a t t h e best c o n d i t i o n s w i t h regard t o gas phase yields a n d c o m p o s i t i o n w e r e at t h e u p p e r i m p o s e d c o n s t r a i n t s of t e m p e r a t u r e a n d feed rate (Figure IV. Table III). In order t o a c h i e v e h i g h e r t e m p e r a t u r e s a n d feed rates, t h e 4 - i n . beds w e r e replaced b y 10-in. beds. W i t h t h e original 4 - i n . beds. 1 0 - 6 0 fluidized solid particles w e r e used. W i t h t h i s size particle, t h e t e m p a r a t u r e differential b e t w e e n t h e solids heater a n d pyrolysis reactor w a s a b o u t 3 0 0 ° F . T h u s , t h e pyrolysis reactor t e m p e r a t u r e w a s l i m i t e d t o a b o u t 1 5 0 0 ° F (since c o m b u s t o r t e m p e r a t u r e s a b o v e 1 8 0 0 ° F can result in m e l t i n g of m e t a l parts). T o r e d u c e t h e t e m p e r a t u r e differential


168

FUEL

SOURCE


B I O M A S S AS A N O N F O S S I L

Figure 3.

Conversion laboratory: a, conversion equipment; b, control room; c, analytical equipment


8.

KUESTER

T a b l e I.


169

S Y N T H E T I C L I Q U I D FUELS FEEDSTOCK C O M P A R I S O N Biomass

Coal, Oil Shale Downloaded by NANYANG TECH UNIV LIB on November 1, 2014 | http://pubs.acs.org Publication Date: January 29, 1981 | doi: 10.1021/bk-1981-0144.ch008

Advantages:

1. large q u a n t i t i e s 2. l o w o x y g e n c o n t e n t

1. r e n e w a b l e 2. h i g h h y d r o g e n c o n t e n t 3. l o w c o n t a m i n a t i o n (sulfur, etc) 4. m i n i m a l e n v i r o m e n t a l p r o b l e m s (collection, processing)

Disadvantages:

1. n o n r e n e w a b l e 1. land use c o m p e t i t i o n 2. l o w h y d r o g e n c o n t e n t 2. h i g h o x y g e n c o n t e n t 3. h i g h c o n t a m i n a t i o n (sulfur, etc) 4. e n v i r o n m e n t a l p r o b lems ( m i n i n g , processing)

Table II. GASIFICATION STUDIES 1.

BASE O N 4 - i n . BEDS -

2.

TEMPERATURE, FEED RATE STUDY O N 4 - i n . BEDS -

COMPLETED

3.

BASE O N 10-in. BEDS -

COMPLETED

CONTINUING

4.

W A T E R G A S SHIFT C A T A L Y S T O N 10-in. BEDS -

5. 6. 7. 8. 9.

CRACKING C A T A L Y S T O N 10-in. BEDS - CONTINUING ONE PASS S T E A M O N 10-in. BEDS - STARTING M A T H E M A T I C A L M O D E L S - CONTINUING FEEDSTOCK CHARACTERIZATION - CONTINUING E N V I R O N M E N T A L ASSESSMENT - CONTINUING

CONTINUING


170

BIOMASS AS A NONFOSSIL F U E L

SOURCE


a n d a l l o w for desired h i g h e r pyrolysis t e m p e r a t u r e s , a smaller solid p a r t i c l e is called for ( 6 0 - 1 2 0 mesh). T h u s , t h e larger d i a m e t e r beds w e r e installed t o p r e v e n t solids b l o w o v e r a n d m a i n t a i n transfer loop s u r g e c a p a c i t y for decreased bed h e i g h t s . A r e s u l t i n g t e m p e r a t u r e d i f f e r e n t i a l of 100°F w a s a c h i e v e d . A larger c a p a c i t y c o m p r e s s o r w a s also installed t o h a n d l e t h e e x p e c t e d increase in gas phase yields. Studies are in progress o n t h i s revised s y s t e m t o explore t h e n e w range of t e m p e r a t u r e a n d feed rate. A d d i t i o n a l s t u d i e s in progress i n c l u d e a s u r v e y of w a t e r gas shift a n d c r a c k i n g catalysts, o n e pass s t e a m effect (no pyrolysis recycle) a n d a s u r v e y of v a r i o u s feedstocks. C o m m e r c i a l w a t e r gas shift c a t a l y s t s are l i m i t e d t o a m a x i m u m t e m p e r a t u r e of a b o u t 9 0 0 ° F a n d t h u s are n o t a p p r o p r i a t e for f l u i d i z a t i o n at t h e t e m p e r a t u r e s u n d e r i n v e s t i g a t i o n . A f i x e d b e d in t h e o v e r h e a d s y s t e m h o w e v e r w i t h s t e a m feed s i g n i f i c a n t l y altered t h e p r o d u c t c o m p o s i t i o n (Table IV) as p r e d i c t e d : CO + H 0 - > C 0 2

2

+ H

2

Use of c r a c k i n g c a t a l y s t appears h i g h l y desirable t o p r o m o t e t h e s e c o n d a r y reactions i n v o l v i n g t h e d e c o m p o s i t i o n of t a r s : gases cellulose

— tars — • s e c o n d a r y p r o d u c t s

There is s o m e i n d i c a t i o n t h a t an i m p r o v e m e n t in gas phase c o m p o s i t i o n can be a c h i e v e d as w e l l as an increase in gas phase yields a n d a decrease in tar residue. T h e use of s t e a m is required t o p r o m o t e t h e w a t e r gas shift reaction. T h u s t h e o p t i o n exists t o fluidize c o m p l e t e l y w i t h s t e a m a n d n o t recycle pyrolysis gas. T h e effect o f t h i s o p t i o n is u n d e r s t u d y . Feedstocks t h a t have been t e s t e d on t h e s y s t e m i n c l u d e a preprocessed m u n i c i p a l refuse (Eco-Fuel II, C o m b u s t i o n E q u i p m e n t Associates), kelp residue (Kelco Co.), paper c h i p s , s y n t h e t i c p o l y m e r (polyethylene), a n d q u a y u l e bagasse (Centro d e I n v e s t i g a c i o n e n Q u i m i c a A p l i c a d a , Saltillo. Coahuila, M e x i c o ) . T y p i c a l f e e d s t o c k analysis f o r s e l e c t e d materials is s h o w n in T a b l e V. T h e paper c h i p s , Eco-Fuel II a n d g u a y a l e bagasse appear t o be similar in p e r f o r m a n c e . T h e kelp residue w a s u n a c c e p t a b l e d u e t o t h e large a m o u n t of inert filter aid in t h e material. P o l y e t h y l e n e w a s s p e c t a c u l a r in p e r f o r m a n c e (high olefins, l o w o x y g e n a t e d c o m p o u n d s ) . H o w e v e r , a c o m m e r c i a l scale w a s t e s u p p l y of s y n t h e t i c p o l y m e r is not realistic.


8.

KUESTER

171


Table III. PYROLYSIS GAS YIELDS (lb g a s / l b f e e d x 1 0 0 ) Run

H

2

C H

co

2

4

C Hg 2

CH

C0

4

TOTAL

2


2

No. 1 Base No. 2 Base No. 3 Base +1.+1

1.11 1.42

12.9 14.6

+1.-1

1.15 1.61 1.36

10.8 12.9 9.4

-1.+1 - 1 . - 1

0.50 0.78

10.9 17.1

3.38

1.11

3.15 2.86

1.21

5.90 2.21 1.28 1.27

0.98 0.94 0.32 0.61 0.84

4.91 5.57

42.0 45.3

65.4

4.68 6.90 5.32 2.27

40.9 54.2 36.7 30.2

61.3 82.4 55.4

2.69

38.6

71.2

45.7 61.3

Table IV. W A T E R O A S SHIFT C A T A L Y S T Mole% H CO C0 Other 2

2

Base

Catalytic

14 58 6 22

46 14 23 17

Table V. FEEDSTOCK A N A L Y S I S (wt%)

ECO F u e l II

Paper Chips

c H

38.0 4.9

41.7

0 Ν

32.0

Biomass as a Nonfossil Fuel Source - American Chemical Society

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