Pyrolysis of Agricultural and Forest Wastes - ACS Publications

Chapter 24

Pyrolysis of Agricultural and Forest Wastes D. S. Scott, J. Piskorz, and D. St. A. G. Radlein

Downloaded by STANFORD UNIV GREEN LIBR on August 6, 2012 | http://pubs.acs.org Publication Date: December 4, 1992 | doi: 10.1021/bk-1992-0476.ch024

Department of Chemical Engineering, University of Waterloo, Waterloo, Ontario N2L 3G1, Canada

Pyrolysis of biomass materials has been practised for many years to prepare charcoal. However, in recent years, it has been shown that short residence time pyrolysis at high heating rates can give high conversions to liquid products. The liquid obtained in these processes can contain surprisingly large amounts of specific oxychemicals or anhydro sugars which are of industrial interest. The amounts of liquid products obtainable and the chemical characteristics of typical liquids from various biomass feedstocks are described. Modifications of the fast pyrolysis process can influence selectivity and different products can then be produced. Finally, some results of research on upgrading or conversion of bio-oil fractions to higher value products is described. It is concluded that fast pyrolysis can represent a direct route to the production of a variety of special chemical products. The pyrolysis of wood to produce gases, liquids and char is an ancient art. Only one hundred years ago, the process was the source of many basic organic chemicals such as acetone and methanol (wood alcohol). Pyrolysis is no longer a source of such chemicals, and has been practiced in recent years on an industrial scale primarily as a method to produce wood charcoal. By definition, pyrolysis implies decomposition by heating in the absence of oxygen. Traditionally, heating was done slowly at high temperatures, over long periods of time, to give maximum yields of charcoal. Only in recent years has some attention been paid to the effects associated with short time rapid heating of wood particles. Flash pyrolysis or flash hydropyrolysis (that is, pyrolysis in a hydrogen atmosphere) are terms usually used to describe processes with reaction times of only several seconds, or less. Flash

0097-6156/92A)476-0422$06.00/0 © 1992 American Chemical Society

In Emerging Technologies for Materials and Chemicals from Biomass; Rowell, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1992.


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pyrolysis i s often carried out at near atmospheric pressures, while hydropyrolysis commonly employs pressures to 20 MPa. Residence times of less than a few seconds with reaction at high temperatures require a reactor configuration capable of very high heating rates. Among the most appropriate designs are the entrained flow reactor, the f l u i d i z e d bed reactor and the ablative reactor. Nearly a l l f l a s h pyrolysis studies have employed one or the other of these reactor types. It has been found i n the past decade that "fast" or " f l a s h " pyrolysis processes can y i e l d a variety of products i n concentrations of economic interest. This chapter w i l l describe the nature of these products and discuss some aspects of t h e i r production. Fast Pyrolysis Processes. A l l f a s t pyrolysis processes have i n common a short residence time at high temperature f o r the v o l a t i l e decomposition products which y i e l d gases and condensable l i q u i d s . Generally, such processes exclude oxygen or other reactive gases, and i t i s only processes of this kind which w i l l be described. In addition to the residence time of the v o l a t i l e s , the temperature of reaction i s a c r i t i c a l variable i n f a s t pyrolysis. At low temperature, below 400°C, decomposition i s r e l a t i v e l y slow, and gas and char are the major products. At higher temperatures, 450° to 600 C, the amount of condensable l i q u i d product increases to a maximum and then decreases. Gas y i e l d increases steadily as temperature i s increased and above 650°C, i t w i l l become the major product. Therefore, depending on whether a char, a l i q u i d or a gas i s the p r i n c i p a l product desired, f a s t pyrolysis may operate anywhere i n the temperature range of 400° to 1200°C. Any process development must have an economic rationale, even though i t i s rather tentative, to j u s t i f y continued development. Novel energy conversion processes f o r biomass are no exception, and are characterized by a unique dependence on l o c a l environments and economics, raw material supply, technological factors and p o l i t i c s . In the present case, the overriding l i m i t a t i o n i s often the supply of raw material (biomass) which can be economically harvested and transported to a central conversion plant. The source area f o r harvesting can be estimated roughly as a c i r c l e with a radius of about 50 km. For readily gathered forest waste, t h i s might support a plant of 1000 dry tonnes per day throughput. For a single sawm i l l or pulpwood operation, waste available from logging might be from 100 to 1000 tonnes per day. None of this represents a large plant i n energy production terms, although 1000 tonnes of dry biomass per day i s a large scale forest or a g r i c u l t u r a l operation. The reasonable conclusion might be that a biomass conversion process would be most useful i f i t were a simple process, not c a p i t a l intensive, that could be operated e f f i c i e n t l y on a small scale and which was capable of producing higher value added chemicals rather than alternative fuels. Such a plant could then be s i t e d where raw material could be supplied at reasonable cost. Liquid products might be used or modified on s i t e , i f f e a s i b l e . If not, the l i q u i d s could be r e a d i l y transported to a central upgrading or r e f i n i n g plant. In the past ten years or so, a number of methods of achieving fast pyrolysis of biomass have been described. Some of these were developed primarily as g a s i f i c a t i o n processes to make synthesis gas



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or ethylene (1)(2). Biomass as a g a s i f i c a t i o n raw material has the considerable advantage of being nearly sulfur free. However, pyrol y t i c g a s i f i c a t i o n produces primarily carbon monoxide, carbon d i oxide, hydrogen and some methane and ethylene. High temperatures, normally 800 to 1000°C are required. Also, to obtain rapid heat ing to these high temperatures small p a r t i c l e s are also required which usually implies a high consumption of energy f o r grinding. As a r e s u l t , p y r o l y t i c g a s i f i c a t i o n without steam or oxygen addi t i o n i s usually not economically a t t r a c t i v e , although a good q u a l i ty synthesis gas can be made (3) and ethylene y i e l d s from 5% to 10% of the dry biomass fed have been achieved. High temperature fast pyrolysis of biomass has not yet, therefore, shown economic potential as a gas-producing i n d u s t r i a l process. A second approach to f l a s h pyrolysis has been described by Scott and Piskorz (4) (5) and Scott et a l . (6). In these publica tions, the development has been outlined of an atmospheric pressure f l a s h pyrolysis process u t i l i z i n g a f l u i d i z e d bed of s o l i d as heat c a r r i e r . The process studied has as a primary objective the deter mination of conditions f o r maximum y i e l d of l i q u i d s from biomass, p a r t i c u l a r l y forest materials. Results from a bench-scale unit (15 g/h feed rate) indicated that at apparent vapor residence times of about 0.5 s, organic l i q u i d y i e l d s of 60 to 70% on a moisture-free basis could be obtained from hardwoods such as aspen-poplar and maple. Lower but s t i l l high y i e l d s of organic l i q u i d s (40 to 60%) could be obtained from a g r i c u l t u r a l wastes such as wheat straw, corn stover, and bagasse. Bench-scale reaction conditions used a biomass p a r t i c l e size of -295 +104 μιη i n a nitrogen atmosphere over a temperature range of 400° to 650°C. In terms of c a l o r i f i c value and hydrogen-to-carbon r a t i o , the best q u a l i t y l i q u i d was obtained at conditions which also gave the maximum l i q u i d y i e l d . In view of the high y i e l d s of organic l i q u i d s obtained -- among the highest y i e l d s reported f o r a pyrolysis conversion process f o r biomass -- and the reasonable operating conditions, a larger scale continuous process unit was designed and constructed. The l i q u i d yields attained i n t h i s unit were as good as, or better than, those achieved i n the bench-scale unit. Detailed results and a descrip t i o n of the process have been given as well as a preliminary eco nomic analysis by Scott and Piskorz (5). More recently, Knight et a l . (7) described the operation of an entrained flow reactor f o r the production of l i q u i d s . A somewhat d i f f e r e n t upflow entrained pyrolyzer f o r the production of l i q u i d s from wood has been described by Beaumont (8). Kosstrin (9) has also used a f l u i d i z e d bed f o r thermal conversion of biomass to liquid. In general, processes f o r the attainment of high l i q u i d yields operate at much lower temperatures, commonly 450 to 550 C, than do processes to y i e l d gaseous product, but at about the same vapor residence times, about 200-700 ms. In a publication by Scott et a l . (10), i t was shown that any type of reactor capable of meeting c e r t a i n heat transfer rate c r i t e r i a would be expected to give results comparable to those ob tained with the f l u i d i z e d bed. Indeed, the "ablative" reactor developed by workers at SERI has recently been modified and oper ated at lower temperatures to produce higher l i q u i d y i e l d s (11). It has also been reported that high l i q u i d y i e l d s , e s p e c i a l l y at very short residence times, were obtained i n an entrained flow



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reactor (12). In addition, a vacuum pyrolysis unit which has slow heating rates but short v o l a t i l e s residence time has been reported to obtain good l i q u i d y i e l d s (13). Detailed analysis of the fast pyrolysis l i q u i d products obtained from wood or a g r i c u l t u r a l wastes as reported by Piskorz et a l . (14)(15) and Radlein et a l . (16)(17) have shown the presence of a number of simple organic oxychemicals i n s i g n i f i c a n t concentrations, up to 15% concentration i n some cases. These same researchers have reported a change i n s e l e c t i v i t y of the f a s t pyrolysis process by an appropriate pretreatment which can give high conversions of c e l l u l o s e to sugars (18). They have also described the upgrading of a l i g n i n f r a c t i o n (19), and additional products obtainable by use of a c a t a l y t i c f l u i d bed (20)(23). A l l of these alternative fast pyrolysis methods w i l l y i e l d l i q u i d s of d i f f e r e n t compositions from which high value chemicals could be extracted. With t h i s approach, biomass f a s t pyrolysis has now become a p o t e n t i a l l y economically a t t r a c t i v e alternative process i n the short term. In the next section, the products with possible commercial potential w i l l be discussed. Products from Pyrolysis Liquids Pyrolysis of Wood. The atmospheric pressure f l u i d i z e d bed fast pyrolysis process developed at the University of Waterloo (now known as the Waterloo Fast Pyrolysis Process or WFPP) has welldefined optimal conditions f o r maximum yields of l i q u i d s . As mentioned e a r l i e r , any reactor which can achieve these operating cond i t i o n s together with the required high rates of heat transfer can obtain similar l i q u i d y i e l d s . However, discussion w i l l be l i m i t e d to the f l u i d i z e d bed reactor as a t y p i c a l f a s t pyrolysis process, and s p e c i f i c a l l y to the WFPP f o r which a good deal of experimental data has been made available. A schematic of the WFPP as operated i n a p i l o t plant i s shown i n Figure 1. The t y p i c a l performance of the f l u i d bed reactor f o r sawdust i s shown i n Figure 2. The rather narrow temperature range (about 50°C) i n which maximum l i q u i d y i e l d s are obtainable (Figure 2) i s t y p i c a l of a l l fast pyrolysis processes and f o r a wide variety of biomass feedstocks. Fast pyr o l y s i s processes are now under active semi-commercial development. For example, two demonstration-scale plants using WFPP technology at a scale of f i v e dry tonnes per day of biomass feed w i l l shortly be i n operation. Similar scales of operation are reported to be planned or i n operation f o r other f a s t pyrolysis processes. The y i e l d s of gas, l i q u i d and s o l i d s f o r two t y p i c a l woods, a hardwood and a softwood, are shown i n Table I together with the gas composition (15). The l i q u i d y i e l d s on a moisture-free feed basis, are about 77% f o r both the spruce and the poplar. The properties of t h i s l i q u i d product are given i n Table I I . The l i q u i d i s t y p i c a l l y a dark brown homogeneous f l u i d of low v i s c o s i t y , with a high density and low pH. It has a high oxygen content of 37% to 40%, and although i t can be used quite s a t i s f a c t o r i l y as an alternative f u e l o i l , t h i s would represent, i n most circumstances, a low value application. Analysis of the p y r o l y t i c l i q u i d presents a much more interesting picture. A detailed analysis (by HPLC and GC) of the organic l i q u i d fractions f o r the spruce and poplar i s shown i n Table III


In Emerging Technologies for Materials and Chemicals from Biomass; Rowell, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1992. HOT WATER CONDENSEE

PILOT

PLANT

ICE WATER CONDENSER

TO VENT

Figure 1. Flow Diagram of a F l u i d i z e d Bed Fast P y r o l y s i s Process [the Waterloo Fast P y r o l y s i s Process (WFPP)].

PYROLYSIS


w O

S

Ο

9

ο a m S

H M

ON



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Figure 2. Organic Liquid, Gas and Char Yields from Poplar Wood with the WFPP F l u i d Bed Units.


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MATERIALS AND CHEMICALS FROM BIOMASS Table I. Pyrolysis Yields from Different Woods


White Spruce (Softwood)

Poplar (Hardwood)

Run# Temperature, °C

42 485

Moisture content, wr% Particle Top Size, um

7.0 7.0 7.0 6.2 1000 1000 1000 590

4.6 3.3 1000 590

Apparent Residence Time, s Feed Rate, kg/hr

0.70 2.07

0.65 1.91

0.62 1.58

0.55 2.24

0.48 1.85

0.46 0.05

63.1 10.7 16.3

66.5 11.6 12.2

66.1 11.1 12.3

65.8 9.3 12.1

66.2 10.7 11.8

65.7 12.2 7.7

0.4 4.16 3.38 0.34 0.16 0.02 0.03

0.02 3.82 3.37 0.38 0.17 0.03 0.04

0.01 4.01 2.69 0.43 0.16 0.05 0.06

0.02 5.32 6.30 0.48 0.20 0.04 0.09

0.01 4.44 5.75 0.37 0.13 0.05 0.08 0.19

0.09 3.10

8.1

7.8

7.4

12.4

11.0

10.8

97.8

97.7

96.7

99.7

99.8

96.4

Yields, wt% of m.f. wood Organic liquid Water Char Gas: H* CO CO, CH* cyi 4

Q Q Total Gas Overall recovery wt%, m.f.

43 500

45 520

27 500

59 504

A-2 497

—

5.34 4.78 0.41 0.19 —

Reproduced from ref. 15. Copyright 1988 American Chemical Society

Table II. Properties of Pyrolytic Liquids White Spruce

Poplar

Run#

42

43

45

59

Yields, wt% of wood as fed

75.2

79.2

78.5

77.6

Water content, wt% PH

21.9 2.1

22.4 2.1

21.8 23

18.6 2.4

1.22

1.22

53.5 6.6

54.0 6.8

Density, g/cc Elemental analysis, wt%, m.f. Carbon Hydrogen

Reproduced from ref. 15.

Copyright 1988

122 56.6 6.9

1.23 53.6 7.0

American Chemical Society


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Table Ι Π . Analysis of Liquid Products White Spruce


Run# Temperature

43 500

Poplar

A-2 504

Yields, wt % of feed, m.f. Organic Liquid 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15.

Oligosaccharides Cellobiosan Glucose Fructose & other hexoses Glyoxal Methylglyoxal Levoglucosan 1,6 anhydroglucofuranose Hydroxyacetaldehyde Formic Acid Formaldehyde Acetic Acid Ethylene Glycol Acetol Acetaldehyde

62.6

66.5

-

0.70 1.30 0.41 1.32 118 0.65 3.04 2.43 10.03 3.09 1.16 5.43 1.05 1.40 0.02

Water-Solubles - Total Above Pyrolytic Lignin

33.0 20.6

34.2 16.2

Amount not accounted for (losses, water soluble phenols, furans, etc.)

12.9

11.91

1.11 0.30 0.05

0.12

2.49 0.99 2.27 2.47

3.96

-

7.67 7.15

3.86 0.89 1.24

bv G.C. Methanol Furfural Methylfurfural Reproduced from r e f . 15. Copyright 1988

—

American Chemical Society



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(15). Note that about 52% to 55% of the l i q u i d has been i d e n t i f i e d as individual water-soluble components. A further 26% to 31% i s characterized as "pyrolytic l i g n i n " (the insoluble f r a c t i o n remaining a f t e r water extraction of the pyrolysis l i q u i d ) . This f r a c t i o n represents the compounds which are aromatic and l a r g e l y phenolic i n character, presumably deriving from the l i g n i n f r a c t i o n of the wood. The outstanding feature of these l i q u i d s i s the presence of r e l a t i v e l y large amounts of a few chemicals such as hydroxyacetaldehyde, formic acid, acetic acid, acetol and glyoxal. Some sugar and anhydrosugars are also present. It i s apparent that the pyroly s i s l i q u i d has a value as a chemical feedstock that may eclipse that of a l i q u i d fuel i f e f f i c i e n t separation and recovery methods can be developed. Pyrolysis of Pretreated Wood. Early work carried out by several workers including Shafizadeh and co-workers (21)(22) suggested that pretreatment of wood to remove hemicellulose and inorganic ash could result i n s i g n i f i c a n t increases i n the y i e l d s of anhydrosugars, p r i n c i p a l l y l e v o g l u c o s a n , d u r i n g slow p y r o l y s i s under vacuum. In our laboratory, samples of poplar wood and of steamexploded (Stake) cellulose were pretreated with a mild acid hydroly s i s to remove much of the hemicellulose f r a c t i o n (18). The pretreated wood was then subjected to fast pyrolysis at optimal WFPP conditions, and the r e s u l t i n g l i q u i d s showed a completely d i f f e r e n t composition from that f o r untreated wood. Two t y p i c a l results are shown i n Table IV (18). It i s clear that the pretreatment changed the thermal decomposition mechanism, from fragmentation of the c e l l u l o s e polymer to y i e l d carbonyl compounds of low molecular weight i n untreated wood or cellulose to a depolymerization mode which yielded mainly monomeric anhydrosugars from the pretreated samples. In t h i s l a t t e r case, removal of the p y r o l y t i c l i g n i n followed by mild acid hydrolysis y i e l d s glucose which i t has been shown recently i n our laboratory can be rapidly and quantitatively fermented to ethanol (18). On the other hand, i t should be r e a l ized that the anhydroglucose, levoglucosan, which i s a c h i r a l compound, i s i n i t s e l f a substance of considerable potential commerc i a l interest. Again, methods f o r recovery of this material on a bulk scale would allow i t s exploitation as a new and reasonably priced compound. An important concept a r i s i n g from t h i s work i s the r e a l i z a t i o n that thermal pyrolysis i s not necessarily a random and l a r g e l y uncontrollable reaction. In fact, s e l e c t i v i t y to hydroxyacetaldehyde production on the one hand, or f o r levoglucosan production on the other, can be as high as 80% f o r the cellulose conversion step. This kind of result suggests that many selected products may be possible i n fast pyrolysis given the appropriate reaction conditions or feed treatment. The use of a catalyst i n a f l u i d bed rather than sand, together with a hydrogen atmosphere, i s one such modification of the WFPP. Hydropyrolysis i n a C a t a l y t i c Bed If a nickel-alumina catalyst i s used i n the f l u i d bed WFPP reactor together with a hydrogen atmosphere, a high conversion of the carbon i n a biomass feed to methane can be achieved. Garg et a l .


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Table IV. Pyrolysis of raw and treated wood

Temperature, °C Vapor residence time, s Particle size, um Moisture, % Cellulose, % mf DP Ash, % mf Yields, % mf wood Organic liquid Water Char Gas Yields of tar components, % mf feed Oligosaccharides Cellobiosan Glucose Fructose (?) Glyoxal 1,6-Anhydroglucoruranose Levoglucosan Hydroxyacetaldehyde Formic acid Acetic acid Ethylene glycol Acetol Methylglyoxal Formaldehyde Pyrolytic lignin*** Totals % mf pyrolysis oil Sugars, glucose equivalent yield, % cellulose

Poplar Wood

Stake Cellulose

Untreated Pretreated

Untreated Pretreated

497 501 0.46 0.45 -590 -590 3.3 16.5 49.1 62.8

—

0.46

—

0.04

500 0.50 -1000 24.1 93.9 303 2.5

490 0.50 -1000 3.1 91.3 149 0.32

65.8 12.2 7.7 10.8

79.6 0.9 6.7 6.5

58.2 9.0 15.4 15.3

65.3 7.0 (19.0)* 3.0

0.7 1.3 0.4 1.31 2.18 2.43 3.04 10.03 3.09 5.43 1.05 1.40 0.65 1.16 16.2 51.5 78.3

1.19 5.68 1.89 3.89 0.11 4.50 30.42 0.37 1.42 0.17

ND 0 0 0 ND 0 0 17.1 7.4 8.5

(29.4)** 3.1 1.7 2.0 2.5 5.5 27.3 0.4 0.1 0.1

0.06 0.38 0.8 19.0 69.9 87.8

5.4 3.3

41.7 71.6

IJ0 7.0 71.1 0.4

20.4

83.4

0

67.5

*Over 90% of this water-soluble material held up in the outlet tube of the reactor. See below. **This includes the 19%reportedabove as 'char'. Since it was essentially all soluble in water and on hydrolysis with sulfuric acid showed only glucose by the HPLC analysis. ***Tyrolytic lignin' is the material precipitated by addition of water [5]. Reproduced with permission from reference 18 Copyright 1989 E l s e v i e r Science Publishers.


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MATERIALS AND CHEMICALS F R O M BIOMASS

(20) showed that 70% to 75% of the feed carbon can be converted to methane at the optimal WFPP pyrolysis conditions. Recent work i n our laboratory with an improved catalyst has attained 80% to 85% conversions to CH^. As the necessary hydrogen can be prepared from biomass, two or three stage overall g a s i f i c a t i o n reactions as shown by equations (1) and (2) (using c e l l u l o s e as a model compound) can become technically feasible without the use of steam or oxygen as gasifying agents by employing c a t a l y t i c hydropyrolysis followed by reforming (20). C


—

H

+

H

6 10°5 2° C H 0 + H0 6

1 Q

5

2

> 3 CH + 3 C0 > 6 CO + 6 H 4

2 2

(1) (2)

The use of other catalysts i n a hydrogen atmosphere can lead to much larger y i e l d s of l i g h t hydrocarbons than i s possible by high temperature f a s t thermal p y r o l y s i s . For example, Scott et a l . (23) have shown that 21% by weight of biomass fed can be d i r e c t l y converted to C -Cy hydrocarbons with 62% of this amount being C^+ hydrocarbons. The mechanisms of these l i g h t hydrocarbon-forming reactions from wood or c e l l u l o s e are not yet clear, and much further research i s required. 2

Upgrading of P y r o l y s i s Products The p y r o l y t i c l i q u i d s obtained from biomass are r i c h i n oxygen. As i s apparent from the previous discussion, t h i s oxygen i s present i n oxychemicals which may have themselves a high value. However, because of the demand f o r hydrocarbon transportation fuels or octane enhancers, many attempts have been made to "upgrade" the crude pyrolysis l i q u i d s to gasoline or d i e s e l o i l f r a c t i o n s . Two general approaches have been used, f i r s t to hydrogenate the crude pyrolysis l i q u i d , or secondly, to d i r e c t l y c a t a l y t i c a l l y hydrogenate or reform the pyrolysis vapors (usually by a dehydrat i o n catalyst) i n a second stage reactor. The f i r s t of these, d i r e c t hydrogénation, has been developed by E l l i o t t et a l . (24) and by E l l i o t t and Baker (25) f o r pyrolysis o i l s from peat using hydrotreating catalysts i n a two-stage process. About a 30% to 35% y i e l d of hydrocarbon l i q u i d s could be obtained. In our laboratory, Piskorz et a l . (19) have shown that by use of a non-isothermal plug flow packed bed hydrotreater 64% of the p y r o l y t i c l i g n i n f r a c t i o n can be converted to l i q u i d hydrocarbons i n the gasoline and d i e s e l o i l range. Therefore, the technology f o r production of a hydrocarbon transportation fuel from biomass has been achieved. The second approach, to process p y r o l y t i c vapors i n a second reactor has been described by Marshall (26) and by Diebold and S c a h i l l (27) using nickel catalysts or z e o l i t e catalysts. While some encouraging results have been achieved, the process i s not yet judged to be economic, although i t has the potential to be so (28). Use o f A g r i c u l t u r a l Wastes Much of the basic research i n l i q u e f y i n g biomass by fast pyrolysis or by other techniques has been carried out on wood. It i s possible, of course, to use any l i g n o c e l l u l o s i c material and to obtain similar results to those obtained with wood. However, there w i l l be differences i n overall y i e l d s of gas, l i q u i d and char as well as


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Table V. WFPP Pyrolysis of Agricultural Wastes Feedstock

Wheat Chaff

Sunflower Hulls

Flax Shives

Moisture, wt% Ash, wt% Panicle Size, mm

6.9 22.5 -1.0

11.1 4.0 -1.0

16.3 2.65 -2.0

e


Temp. C Yields, wt% mf Feed Gas Char Water Organic Liquid Total Recovery

515

500

500

* 15.9 15.7 15.7 51.0 100.2

18.7 9.4 9.4 44.4

17.7 13.7 13.7 42.4

98.8

96.8

* maf basis Reproduced with permission from reference Copyright 1990 D.S. Scott.

29 .

i n s p e c i f i c compounds because of the wide variations i n holocellul o s e / l i g n i n amounts, as well as due to the presence of other organi c or inorganic materials. A very wide variety of a g r i c u l t u r a l wastes are available i n p a r t i c u l a r l o c a l circumstances (e.g. straw, bagasse, nut s h e l l s , chaff etc.). Their use i n pyrolysis w i l l be dictated by both the supply and by the economics of alternative uses or of disposal. A considerable number of tests of various a g r i c u l t u r a l wastes have been reported f o r both pyrolysis to l i q u i d s and p y r o l y t i c gasification. Pyrolytic g a s i f i c a t i o n with p a r t i a l oxidation to y i e l d a low BTU gas or a char i s an o l d and well-established process with a variety of technologies, and has been developed on a l o c a l scale i n many third-world countries. Pure p y r o l y t i c g a s i f i cation of a g r i c u l t u r a l wastes i s much less common, and indeed i s not usually practised on an i n d u s t r i a l scale. Fast pyrolysis of a g r i c u l t u r a l wastes has been carried out i n our laboratories on a variety of materials. Some results are given i n Table V and Table VI to i l l u s t r a t e the differences and s i m i l a r i t i e s to wood as a feedstock. In general, l i q u i d y i e l d s are somewhat lower from a g r i c u l t u r a l biomass, but the chemical products and t h e i r y i e l d s r e l a t i v e to the amounts of c e l l u l o s e , hemicellulose and l i g n i n are similar. Hence, as a generalization, the y i e l d s of products obtainable may be roughly estimated from a knowledge of the amounts of each of the three main constituents. However, i t must be said that the chemical nature of the l i g n i n i n a g r i c u l t u r a l biomass, f o r example, grasses or straws, can be very d i f f e r e n t from that found i n woody biomass, and much research needs yet to be done to evaluate t h i s type of p y r o l y t i c l i g n i n .


434


Table VL Liquid Product Composition - Agricultural Wastes


Feedstock

Yields, wt%, mf Feed Cellobiosan Glucose Fructose (?) Glyoxal Levoglucosan Hydroxyacetaldehyde Formic Acid Formaldehyde Acetic Acid Ethylene Glycol Acetol Water Insoluble

Wheat Chaff

Sunflower Flax Hulls Smves

* 0.40 0.19 0.70 0.70 1.95 6.53 ND 1.30 6.11 0.93 3.20 15.1

0.06

0.11

0.12 0.07 0.33 0.78 1.01 ND 2.12 0.27 1.16 38.4

0.28 0.21 0.58 1.44 ND 0.41 2.86 ND 1.42 22J

—

—

*maf basis Reproduced with permission from reference Copyright 1990 D.S. Scott.

29 .

Summary Fast pyrolysis appears to have a promising future as a method of preparing a l i q u i d feedstock from which a variety of high value products can be recovered. In p a r t i c u l a r , a number of low molecul a r weight carbonyl compounds, and sugars, can be produced i n suff i c i e n t l y high y i e l d s to make t h e i r recovery as individual commerc i a l chemicals economically worthwhile. In addition, some of these compounds f o r example, levoglucosan, hydroxyacetaldehyde, and pyrol y t i c l i g n i n s , may have unique c h a r a c t e r i s t i c s which would lead to new market applications. The p y r o l y t i c l i q u i d s produced from biomass can be successfully upgraded to hydrocarbon f u e l s , but not yet i n an economical way. The use of catalysts, or of special pretreatments of biomass, can lead to changes i n s e l e c t i v i t y of the thermal decomposition process, and f a s t pyrolysis allows a surprising degree of control over these alternative pyrolysis mechanisms. Much research needs to be done, p a r t l y because of the wide variety of the feed materials available, but also because our knowledge of methods of recovery f o r the higher valued products of f a s t pyrolysis i s s t i l l i n i t s early stages. Acknowledgments The authors are pleased to acknowledge the f i n a n c i a l assistance of the Natural Sciences and Engineering Research Council and of the


24. SCOTT ET AL.


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Bioenergy Development Program of Energy, Mines and Resources Canada. The valuable assistance of Peter Majerski with experimental work and analysis was also much appreciated.


Literature Cited (1) Diebold, J.P.; Scahill, J.W. Proceedings of the 15th Biomass Thermo-Contractors' Meeting; CONF-830323, PNL-SA-11306; Pacific Northwest Laboratory: Richland, WA, 1983. (2) Mok, L.K.; Graham, R.G.; Overend, R.P.; Freel, B.A.; Bergougnou, M.A. In BioEnergy 84; Egneus, Η., Ellegard, A. Eds.; Elsevier: London, 1985; pp 23-30. (3) Kuester, J.L. In BioEnergy 84; Egneus, Η., Ellegard, Α., Eds.; Elsevier: London, 1985; pp 48-55. (4) Scott, D.S.; Piskorz, J. Can.J.Chem.Eng., 1982, 60, pp 666-674. (5) Scott, D.S.; Piskorz, J. Can.J.Chem.Eng., 1984, 62, pp 405-412. (6) Scott, D.S.; Piskorz, J. Ind.Eng.Chem.Process Des.Devel., 1985, 24, pp 581-588. (7) Knight, J.Α.; Gorton, C.W.; Stevens, D.J. In BioEnergy 84; Egneus, Η., Ellegard, Α., Eds.; Elsevier: London, 1985; pp 9-14. (8) Beaumont, 0. Ind.Eng.Chem. Process Des.Devel., 1984, 23, pp 637-641 (9) Kosstrin, H. Proc. Specialists Workshop on Fast Pyrolysis of Biomass; SERI/CP 622-1096; 1980; pp 105-121. (10) Scott, D.S.; Piskorz, J.; Bergougnou, M.A.; Graham, R.; Overend, R.P. I&EC Research, 1988, 27, pp 8-15. (11) Diebold, J . ; Scahill, J. In Pyrolysis Oils from Biomass; Soltes, E . J . , Milne, R.A., Eds.; ACS Symp. Series 376, American Chemical Society: Washington, DC, 1988, pp 31-40. See also Diebold, J . ; Power, A. In Research in Thermochemical Biomass Conversion; Bridgewater, A.V., Kuester, J . L . , Eds.; Elsevier: NY, 1988; pp 609-628. (12) Graham, R.G.; Freel, B.A.; Bergougnou, M. In Research in Thermochemical Biomass Conversion; Bridgewater, A.V., Kuester, J . L . , Eds.; Elsevier: London, 1988, pp 629-641. (13) Roy, C.; De Caumia, B.; Pakdel, H. In Research in Thermo chemical Biomass Conversion; Bridgewater, A.V., Kuester, J . L . , Eds.; Elsevier: London, 1988, pp 585-596. (14) Piskorz, J . ; Radlein, D.; Scott, D.S., J.Anal.Appl.Pyrolysis, 1986, 9, pp 121-137. (15) Piskorz, J.; Scott, D.S.; Radlein, D. In Pyrolysis Oils from Biomass; Soltes, E . J . , Milne, T.A., Eds.; ACS Symposium Series 376, American Chemical Society: Washington, DC, 1988, pp 167-178. (16) Radlein, D.St.A.G.; Grinshpun, Α.; Piskorz, J.; Scott, D.S. J.Anal.Appl.Pyrolysis, 1987, 12, pp 39-49. (17) Radlein, D.St.A.G.; Piskorz, J.; Scott, D.S. J. Anal. Appl. Pyrolysis, 1987, 12, pp 51-59. (18) Piskorz, J.; Radlein, D.St.A.G.; Scott, D.S.; Czernik, S. J. Anal.Appl.Pyrolysis, 1989, 16, pp 127-142. (19) Piskorz, J . ; Majerski, P.; Radlein, D.; Scott, D.S. Energy & Fuels, 1989, 3, pp 723-726. (20) Garg, M.; Piskorz, J . ; Scott, D.S.; Radlein, D. I&EC Research, 1988, 27, pp 256-264. See also US Patent 4 822 935, April, 1989. (21) Shafizadeh, F.; Stevenson, T.T. J.Appl.Polym.Sci., 1982, 27, pp 4577.



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(22) Shafizadeh, F.; Furneaux, R.H.; Cochran, T.G.; Scholl, J.P.; Sakai, Y. J.Appl.Polym.Sci., 1979, 23, 3525. (23) Scott, D.S,; Radlein, D.; Piskorz, J.; Mason, S.L. In Biomass for Energy and Industry; Grassi, G., Gosse, G., dos Santos, G., Eds.; Proc. 5th EC Conference; Elsevier: London, 1990, Vol. 2; pp 2.600-2.605. (24) Elliott, D.C.; Baker, E.G.; Piskorz, J.; Scott, D.S.; Solantausta, Y. Energy & Fuels, 1988, 2, pp 234-235. (25) Elliott, D.C.; Baker, E.G. In Energy from Biomass and Wastes X; Klass, D.L., Ed.; Inst, of Gas Technology: Chicago, IL, 1987, pp 765-784. (26) Marshall, A.J.; MASc Thesis, "Catalytic Conversion of Pyrolysis Oil in the Vapour Phase", Dept. of Chemical Engineering, University of Waterloo, Waterloo, Ontario, Canada, 1984. (27) Diebold, J.P.; Scahill, J.W. Energy Progress, 1988, 8, 59-65. (28) Beckman, D.; Elliott, D . C ; Covert, B. ; Hornell, C . ; Kjellstrom, B.; Ostman, A.; Solantausta, Y.; Tulenheimo, V. TechnoEconomic Assessment of Selected Biomass Liquefaction Processes; Final Report of IEA Cooperative Project Direct Biomass Liquefaction, Report 697, Tech. Research Centre of Finland, Espoo, Finland, 1990. (29) Piskorz, J.; Majerski, P.; Radlein, D.; Scott, D.S. Fast Pyrolysis of Some Agricultural and Industrial Materials; presented at the 8th Canadian Bioenergy R&D Seminar, Ottawa, Canada, October 23 & 24, 1990. RECEIVED May 20, 1991


Pyrolysis of Agricultural and Forest Wastes - ACS Publications

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