Thermal Conversion of Solid Wastes and Biomass - American

28 The Role of Catalysis in Wood Gasification D. P. C. FUNG1 and R. GRAHAM

Downloaded by MONASH UNIV on October 10, 2015 | http://pubs.acs.org Publication Date: August 29, 1980 | doi: 10.1021/bk-1980-0130.ch028

Forintek Corporation Canada, 800 Montreal Road, Ottawa, Ontario, Canada K1G 3Z5

Early in 1978, a program was initiated at the Eastern Forest Products Laboratory of the Forintek Canada Corporation to perform research and development on wood gasification. This research includes the identification of those parameters which significantly affect the yield and distribution of products, the rate of reaction, and overall process efficiency. Long-term objectives of Forintek are to demonstrate a viable wood gasification technology and to encourage the forest product industries to use woodbark residues as a substitute for rapidly depleting petroleum resources. Coal gasification technology was developed prior to that of wood and is somewhat applicable to the gasification of wood. As suggested by Hoffman (1), the development of coal gasification technology may be grouped into three generations. The first generation technology was developed in Germany before World War II for producing liquid and gaseous fuels from coal. This included simple pyrolysis processes (for char & o i l ) , rudimentary gas producers, fixed-bed hydrogenation, low and medium pressure oxygen gasification for synthesis gas production, and the development of the Fischer-Tropsch process for the production of hydrocarbons from synthesis gas. The second generation refers to the technical movements and modifications of coal gasifiers. The third generation can be regarded as the application of catalysis to gasification. Catalysis research should permit a maximum yield of combustible or synthesis gas at the expense of less desirable char and liquid products (at lowest possible operating temperatures). In turn, this would reduce several technical problems and would increase the process efficiency and throughput. The use of catalysis could also be made to pre-determine the final composition of the

Current address: CANMET, Department of Energy, Mines and Resources, 555 Booth Street, Ottawa, Ontario, Canada, K1A OG1

1

0-8412-0565-5/80/47-130-369$05.00/0 © 1980 American Chemical Society In Thermal Conversion of Solid Wastes and Biomass; Jones, J., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1980.

THERMAL CONVERSION OF SOLID WASTES AND BIOMASS

370

gas product depending on intended end use. Since c a t a l y s i s i s of such c u r r e n t i n t e r e s t , t h i s paper presents a review of some of the l i t e r a t u r e which deals with the c a t a l y t i c g a s i f i c a t i o n of biomass. I t a l s o summarizes the prel i m i n a r y r e s u l t s of a bench-scale f l u i d bed g a s i f i c a t i o n study which employed Canadian hybrid poplar. The e f f e c t of such c a t a l y s t s as potassium carbonate and calcium oxide on wood g a s i f i c a t i o n i s r e p o r t e d . Hybrid poplar was s e l e c t e d due to i t s r a p i d growth and, t h e r e f o r e , i t s relevance to the concept of wood energy plantations.


Inorganic S a l t s as G a s i f i c a t i o n C a t a l y s t s A l k a l i carbonates such as sodium carbonate and potassium c a r bonate were s t u d i e d i n t e n s i v e l y during e a r l y g a s i f i c a t i o n r e s e a r ch p r i o r to World War I I . White and Fox (2) noted that sodium carbonate increased g r a p h i t e g a s i f i c a t i o n r a t e s , increased net gas y i e l d s and a l t e r e d the gas product d i s t r i b u t i o n i n favor of carbon monoxide and hydrogen over carbon d i o x i d e and methane. They presented a r e a c t i o n mechanism f o r the carbonate c a t a l y s t i n the optimum temperature range of 800 to 1000 C:

C + CO

> Na C0 2

2C0

3

Gas y i e l d s using sodium carbonate at lower temperatures are simi l a r to those from the noncatalyzed r e a c t i o n at higher temperatures. Subsequent g r a p h i t e g a s i f i c a t i o n r e s e a r c h by White and h i s group (3,4) found potassium carbonate to be an e x c e l l e n t c a t a l y s t at an optimum p r o p o r t i o n of 20 percent (by weight) of the carbonaceous f u e l feed. White and Weiss (3} noted that a l k a l i carbonates a l s o c a t a l y z e d the water-gas s h i f t r e a c t i o n when steam was i n j e c t e d during g a s i f i c a t i o n :

CO + H 0 2

Alkali

> Carbonate

C0

2

+

H

2

In 1953, Lewis, G i l l i l a n d and H i p k i n (5) i n v e s t i g a t e d the r a t e of g a s i f i c a t i o n of wood c h a r c o a l c a t a l y z e d with potassium carbonate (10% by weight) at 650 C. The r e a c t i o n r a t e s a t 650 C with the c a t a l y s t were approximately equal to the non-catalyzed r a t e s a t 870°C. More recent r e s e a r c h has recognized a l k a l i carbonates as e f f i c i e n t c a t a l y s t s i n the p y r o l y s i s and g a s i f i c a t i o n of urban r e f u s e , c o a l , and biomass. o

In Thermal Conversion of Solid Wastes and Biomass; Jones, J., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1980.


28.

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Role of Catalysis in Wood Gasification

371

Prahacs (6) observed that spent p u l p i n g l i q u o r s t r e a t e d with a d d i t i o n a l sodium carbonate showed a s i g n i f i c a n t l y higher r a t e but not as r a p i d l y as those l i q u o r s treated with sodium carbonate. Magnesium oxide was found to i n h i b i t the c a t a l y t i c e f f e c t of sodium carbonate. Cox and co-workers (7, 8, 9) catalyzed the g a s i f i c a t i o n of wood, c o a l , paper, sludge, manure and m u n i c i p a l r e f u s e with potassium carbonate. They found that a l k a l i carbonates promoted the steam-carbon and steam-hydrocarbon r e a c t i o n s and concluded: ( i ) conversion r a t e s would be s i g n i f i c a n t l y l e s s at the same temperature i n the absence of potassium carbonate, ( i i ) the c a t a l y z e d products tend to be lower molec u l a r weight hydrocarbons, ( i i i ) the optimum a l k a l i carbonate/coal r a t i o i s about 27% by weight. Rai and Tran (10) employed potassium carbonate as a c a t a l y s t f o r D o u g l a s - f i r bark and p u l p i n g l i q u o r g a s i f i c a t i o n , and found that the c a t a l y z e d g a s i f i c a t i o n r a t e and gas y i e l d s were greater than f o r uncatalyzed r e a c t i o n s under otherwise i d e n t i c a l c o n d i t ions. A l k a l i carbonates e x h i b i t e d s e l e c t i v i t y towards the production of l i g h t hydrocarbons. They concluded that l i t t l e had been done to assess the p o t e n t i a l f o r a l k a l i carbonate c a t a l y s i s i n l a r g e - s c a l e g a s i f i c a t i o n and encouraged f u r t h e r r e s e a r c h i n this direction. A d d i t i o n a l l i t e r a t u r e by Cox, (8), A p p e l l (11), Hooverman (12), C a v a l i e r (13), Knight (14) and Love (15) serves to r e i n f o r c e the evidence that a l k a l i carbonates i n c r e a s e r e a c t i o n r a t e s and o v e r a l l gas y i e l d s i n the g a s i f i c a t i o n of biomass, c o a l and r e f u s e . The general consensus i s that a l k a l i carbonates c a t a l y z e the water-gas s h i f t and carbon-steam r e a c t i o n s . Laboratory Studies a t F o r i n t e k The l a b o r a t o r y experiments were designed f o r p r e l i m i n a r y work and f o r screening of c a t a l y s t s to be used i n our c u r r e n t c a t a l y t i c g a s i f i c a t i o n research program. R e s u l t s from these s t u d i e s are intended to i d e n t i f y those c a t a l y s t s which best p r o mote s y n t h e s i s gas production (H^/CO) and maximize r e d u c t i o n of char and l i q u i d y i e l d s . Feed M a t e r i a l F i v e year o l d Ontario hybrid poplar was used during the experiments. I t had a c a l o r i f i c value of 4700 c a l / g and was hammermilled to a s i z e range of 600 to 800 microns. The wood was then oven d r i e d p r i o r to g a s i f i c a t i o n .



372


Reactor Assembly The r e a c t o r assembly i s depicted i n F i g u r e 1 and c o n s i s t e d of a screw feeder/hopper, the r e a c t o r proper, furnace, and a condenser t r a i n . The r e a c t o r proper was made of 316 s t a i n l e s s s t e e l tubing (5 cm I.D. and 134 cm long) i n three detachable segments. The top segment supported the gas e x i t l i n e and contained a f i l t e r i n g medium (supported by a p e r f o r a t e d p l a t e ) to r e s t r i c t the movement of entrained f i n e s from the r e a c t o r to the condenser t r a i n . The middle segment was equipped with the screw feeder and hopper. The hopper was designed to c o n t a i n a 150 gm wood sample and could be r e a d i l y s e a l e d . Wood was added to the r e a c t o r at a steady r a t e f o r 20 minutes by means of the screw feeder. The base of the middle segment was connected to the bottom segment by f l a n g e s , between which a gas d i s t r i b u t o r g r i d was placed. The gas d i s t r i b u t o r had twenty holes (0.4 mm d i a meter) and was covered with a 74 micron wire mesh to prevent s o l i d p a r t i c l e s from plugging the g r i d h o l e s . The g r i d served to d i s t r i b u t e the n i t r o g e n f l u i d i z i n g gas evenly and a l s o supported a 15 cm bed of commercial c r a c k i n g c a t a l y s t ( s i l i c a / a l u m i n a ) . The c r a c k i n g c a t a l y s t was s e l e c t e d as a bed m a t e r i a l due to i t s i d e a l f l u i d i z i n g p r o p e r t i e s and not f o r i t s c a t a l y t i c p r o p e r t i e s . The n i t r o g e n gas was preheated to furnace temperature and was fed to the r e a c t o r through an i n l e t on the bottom segment. The r e a c t o r was heated by a tubular Lindberg furnace and the temperature was c o n t r o l l e d by a thermocouple feedback mechanism connected to the oven c o n t r o l u n i t . G a s i f i c a t i o n products were l e d along the e x i t gas l i n e from the r e a c t o r to four consecutive sealed condensers i n the recovery train. The f i r s t two condensers were immersed i n an i c e bath at 0 C and the l a s t two were immersed i n a -70 C dry ice/acetone mixture. The gas l i n e from the l a s t condenser was fed to a s e r i e s of sample bags where the gas could be stored f o r l a t e r chromatograph a n a l y s i s . Gas A n a l y s i s The gas composition was determined with a d u a l column Hewlett Packard gas chromatograph equipped with a thermal conducti v i t y d e t e c t o r . Chromasorb 102 and Molecular Sieve 5A columns with a helium c a r r i e r gas were employed f o r gas a n a l y s i s . Experimental Procedure Wood samples were prepared by dry mixing 150g of oven dry wood with 5 g of potassium carbonate or 5 g of calcium oxide. Uncatalyzed wood samples were a l s o used to e s t a b l i s h baseline c o n t r o l data.



28.

FUNG

AND GRAHAM


E-4


373


374


The r e a c t o r was placed i n the furnace and heating was i n i t i a t e d a t l e a s t one hour before the run. Immediately a f t e r heating was i n i t i a t e d , 300 grams of CBZ-1 c r a c k i n g c a t a l y s t (Davison Chemical Co.) was added to the r e a c t o r . Preheated n i t r o g e n was then introduced to the r e a c t o r a t a volumetric flow r a t e of 1.5 l i t e r s per minute. When the r e a c t o r and bed m a t e r i a l reached the d e s i r e d temperature (500 C), wood was fed continuously i n t o the r e a c t o r f o r a period of twenty minutes. Gas samples were c o l l e c t e d at 4, 8 and 12 minutes a f t e r the feeding was i n i t i a t e d . At the completion of the experiment (approx. 1.5 h r ) , the char was allowed to c o o l and was then removed and weighed along with the o r i g i n a l bed m a t e r i a l . Four p o r t i o n s (200 ml each) of acetone were used to f l u s h and remove the l i q u i d products from the condensers. The acetone mixture was then f i l t e r e d to c o l l e c t any entrained charcoal/ash f i n e s which were then accounted f o r i n the mass balance. The acetone was evaporated o f f a t reduced pressure and the remaining l i q u i d r e s i d u e (pyroligneous a c i d s and t a r s ) was weighed. The amount of gas product was then determined by d i f f e r e n c e . Where wood was treated with potassium carbonate or calcium oxide, i t was assumed that these two c a t a l y s t s could be accounted f o r i n the s o l i d r e s i d u e f o r purposes of determining the mass balance. This method should give a good approximate mass balance of the s o l i d , l i q u i d and gas products f o r comparative study purposes. Results and D i s c u s s i o n Table 1 summarizes the r e s u l t s of noncatalyzed hybrid poplar g a s i f i c a t i o n a t 500 C i n a f l u i d i z e d bed r e a c t o r . Approximately f o r t y f i v e percent (45%) of the wood was g a s i f i e d . Twenty f i v e percent (25%) was converted to a l i q u i d and t h i r t y percent (30%) remained as a s o l i d product. The g a s i f i c a t i o n reached a steady s t a t e by at l e a s t four minutes as i n d i c a t e d by the chemical composition of the gas. Samples c o l l e c t e d at 8 and 12 minutes had s i m i l a r gas composition. No hydrogen was detected i n the gas product. Carbon monoxide and carbon d i o x i d e were the major components with a small amount of methane and low molecular weight hydrocarbons. Table I I summarizes the c a t a l y z e d g a s i f i c a t i o n r e s u l t s of the hybrid poplar i n the presence of potassium carbonate and calcium oxide. Both of these c a t a l y s t s increased the l i q u i d and gas y i e l d at the expense of the s o l i d . Both c a t a l y s t s p r o moted a f i n a l product which was approximately f i f t y percent (50%) gaseous. The remainder of the product was t h i r t y percent (30%) l i q u i d and twenty percent (20%) s o l i d . I t appears that potassium carbonate enhances the formation of hydrogen and methane and reduces the formation of carbon d i o x i d e . Calcium oxide has a s i m i l a r but more pronounced e f f e c t .


FUNG

28.


AND GRAHAM

375

Table I


G a s i f i c a t i o n Results of Hybrid Poplar at 500°C i n a F l u i d Bed System

Run No.

Solid %

Liquid %

GasificGas ation % Time, min.

1

32

26

42

2

29

24

47

3

31

a

25

44

Nitrogen and oxygen f r e e

Gas Composition,% by Volume H CO CO CH C - C Hydrocarbons 2

4

0

56

38

4

2

8

0

51

40

4

5

12

0

51

40

6

3

4

0

54

38

5

3

8

0

55

35

6

4

12

0

55

30

10

5

4

0

53

39

5

3

8

0

56

32

8

4

12

0

55

37

5

3

90

90

90

basis.




376

Table I I G a s i f i c a t i o n Results of Hybrid Poplar with C a t a l y s t s a t 500°C

Catalyst Used

Solid %

Liquid %

Gas %

Gas Composition, % by volume H CO C0 CH. C -C.Hydro2 2 4 2 3 , carbons o

Control^

31

25

44

0

54

37

6

3

Potassium Carbonate

19

29

52

14

36

31

15

4

19

30

51

20

43

20

13

4

c Calcium Oxide

Nitrogen and oxygen f r e e

basis

Average of 5 experiments Average of 3 experiments


28.

FUNG AND GRAHAM

Role of Catalysis in Wood Gasification 311

Cox and his co-workers (9) reported that alkali carbonates increased the rate and yield of gas production of lodgepole pine at 550 C using nitrogen as a carrier gas during gasification. The hydrogen composition remained unchanged at two percent (2%) in both catalyzed and noncatalyzed runs. Our results with pot assium carbonate do not support this conclusion.


Conclusions: The results of some preliminary research indicate that cat alysts may be used to alter gas composition and gas yield. Spec ifically, potassium carbonate and calcium oxide appear to promote hydrogen production at the expense of the solid char product. The literature indicates that gasification rates may be significantly accelerated by employing such catalysts as alkali carbonates and metallic oxides. Results from the hybrid poplar gasification are encouraging and warrant additional research. Future catalytic work at Forintek will employ incremental temperatures ranging from 600 to 1000 C and will include a variety of catalysts and catalytic combinations. An attempt will be made to promote the production of the following gases which enjoy different end uses: (a) methane/hydrocarbon synthesis gas (H^/CO) (b) ammonia synthesis gas (H^/N^) (c) substitute natural gas (CH^) (d) producer gas for combustion (H 0 , CO, CH. C H Δ H n m C0 2 , N 2 ). Research will be conducted in the apparatus described in this paper, and also in larger fluidized bed (100,000 BTU/hr) and fixed bed downdraft (1 χ 10 BTU/hr) reactors. These two larger reactors better approximate commercial gasification chemistry in that the process heat is supplied internally by partial combustion of the biomass feedstock. Acknowledgement The authors wish to thank Mr. E.E. Doyle of Forintek for performing the gas analysis. References 1. 2. 3.

E.J. Hoffman, "Coal Conversion", Modern Printing Company, Laramie, Wyoming, 1968, pp 11-15. A.H. White, and D.A. Fox, Ind. Eng. Chem., 23 (3), 259 (1931). A.H. White, and C.B. Weiss, Ind. Eng. Chem. 26 (1), 83 (1934).


378 4. 5. 6. 7. 8.


9.

10. 11. 12. 13. 14. 15.


A.H. White and Q.W. Fleer, Ind. Eng. Chem., 28 (11), 1301 (1936). W.K. Lewis, E.R. Gilliland and H. Hipkin, Ind. Eng. Chem. 48 (8), 1697 (1953). S. Prahacs, Advances in Chemistry Series 69, Div. Fuel Chem., 152nd National Meeting of the American Chemical Society, Chicago, September 1967. J.O. Cox, G.W. Wilson and E.J. Hoffman, J. of Environmental Engineering Division, Amer. Chem. Soc., June 1974. W.G. Wilson, L.J. Sealock, Jr., F.C. Hoodmaker, R.W. Hoffman, J.L. Cox, and D.L. Stinson, Preprints Amer. Chem. Soc., Div. Fuel Chem. 18 (2), 29(1973). L.J. Sealock, Jr., R.J. Robertus, L.K. Mudge, D.H. Mitchell, and J.L. Cox, Papers Presented at the First World Conference on Future Sources of Organic Raw Materials, Toronto, July 1978. C. Rai and D.Q. Tran, Amer. Inst. Chem. Eng., Symp. Series 157, 72, 100 (1976). H.R. Appell, I. Wender, and R.D. Miller, U.S. Bureau of Mines, Tech. Prog. Report 25 (1970). R.H. Hooverman, and J.A. Coffman, Symposium on Clean Fuels from Biomass and Wastes, Institute Gas Technol. Conference, Orlando, January 1977. J.C. Cavalier and E. Chornet, Fuel, 56, 57 (1977). J.A. Knight, "Thermal Uses and Properties of Carbohydrates and Lignins", F. Shafizadeh, E . , Academic Press, New York, 1977. P. Love and R. Overend, Energy, Mines and Resources Canada, Report ER-78-1, Ottawa, Canada, 1977.

RECEIVED November 16,

1979.


Thermal Conversion of Solid Wastes and Biomass - American

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