Thermal Conversion of Solid Wastes and Biomass - American

25 Pyrolysis of Agricultural Residues in a Rotary Kiln R. H. ARSENAULT, M. A. GRANDBOIS, E. CHORNET, and G. E. TIMBERS1

Downloaded by EMORY UNIV on June 16, 2014 | http://pubs.acs.org Publication Date: August 29, 1980 | doi: 10.1021/bk-1980-0130.ch025

Sandwell Beak Research Group, 6870 Goreway Drive, Mississauga, Ontario, Canada L4V 1L9

The conversion of agricultural residues to more amenable forms of energy such as gas, oil or char shows good potential both on the large and small scale. This paper is concerned with the pyrolysis of agricultural residues using a rotary kiln to potentially supply the energy needs of one or a few farm operations. The rotary kiln pyrolysis unit used in these experiments was originally used for studies in peat conversion directed by E. Chornet at the University of Sherbrooke, Québec (Campion, 1978). The scope of work in a first phase of the project included the adaptation of this peat pyrolysis system for conversion of agricultural residues, preliminary testing to identify operating problems, characterization of two feedstocks (oat straw and corn stover) and finally, experiments to determine product yield and composition at various operating conditions. The work was performed by the Sandwell Beak Research Group and supported by Agriculture Canada under contract number AERD 8-1802. This first phase lasted one year and results are presented in this paper. The second phase of the work is now underway detailing the pyrolysis study and some gasification and based on these two phases, design criteria for small scale pyrolytic conversion of agricultural biomass will be recommended. Description of Process A schematic diagram of the process is shown in Figure 1. The feed was chopped to 5-10 cm in a forage harvester and air dried to less than 10% moisture content before use. The feeder system consisted of a funnel-shaped 0.2 m3 capacity hopper with chain-driven mixer arms, emptying onto a 5 cm diameter screw 'Current Address: Engineering and Statistical Research Institute, Agriculture Canada, Ottawa, Ontario, Canada 0-8412-0565-5/80/47-130-337$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.

338

THERMAL

C O N V E R S I O N O F SOLID W A S T E S A N D BIOMASS

CZO/


3

p/zETzeATMeuT

EXTERNAL

CXHAUST

GAS£$

(FOR PROCSSS HEATING)

ZEACTOZ

TU&e(3.Hm

*0J€>

m

PZOPAK/E

&UJZMEZ3

I.Ο.) gas TA/ZS GASCS

V/ATÇ8. WASH WATEJZS

5C&ue>&E€ CLSAU GAS

FILTE ft

SOU Ο

FUEL

Figure 1.

N2> CH*+ y > CO^and C^Hy and the remainder of the gas was f l a r e d . Char samples were analysed f o r C, H, 0, S, Ν and mois ture content (moisture content by oven drying at 378 K); the remainder was stored i n l a r g e p l a s t i c bags. The tars were not analysed since v e r y small amounts were produced. The feed and char were weighed and gas production was con t i n u o u s l y monitored with a test meter to permit mass and energy balances. The water product was not determined, however, be cause of condensation problems i n the scrubber. 2

co

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


340


25.

ARSENAULT ET A L .

Pyrolysis of Agricultural Residues

Heat inputs (propane) and outputs were monitored.

(combustion

exhaust

341 gases)

Results


Feedstock C h a r a c t e r i s t i c s . Feedstock c h a r a c t e r i s t i c s are presented i n Table I . Results o f the metal analysis w i l l be considered i n Phase I I o f the p r o j e c t . Calorific v a l u e s were approximated using Dulong's formula and not a c t u a l l y measured. The oat straw had a lower ash content than the stover and a correspondingly higher c a l o r i f i c v a l u e . C a l o r i f i c v a l u e s , cal culated on a d r y b a s i s and i n c l u d i n g the l a t e n t heat of water vapour i n the products o f combustion (high heat v a l u e ) , were 17.9 MJ/kg f o r the straw and 16.0 MJ/kg f o r the s t o v e r . Table I : Feedstock C h a r a c t e r i s t i c s Type of Feed :

Straw Mean S.D.

5

1

Stover Mean S.D.

b

z

Ultimate A n a l y s i s - % wt 0.4 43.2 Carbon 0.7 48.1 5.2 0.1 Hydrogen 0.1 6.1 0.2 34.0 Oxygen 1.0 41.6 0.01 0.25 Sulphur 0.01 0.05 2.8 0.1 Nitrogen 0.4 0.1 16.0 0.3 0.4 C a l o r i f i c v a l u e (MJ/kg) 17.9 Proximate Analysis--% wt 1.4 7.8 Moisture 0.7 5.4 0.6 Ash 0.4 14.6 3.8 0.4 0.4 Fixed c a r b o n 0.7 0.1 77.2 0.9 V o l a t i l e matter 0.6 90.1 1 From the a n a l y s i s o f 5 d i f f e r e n t samples f o r each feedstock 2 Moisture f r e e 3 High heat value approximated using Dulong's formula h- Calculated by d i f f e r e n c e ( 1 0 0 % - % a s h - % m o i s t u r e - % v o l a t i l e matter) 5 S.D. = Standard d e v i a t i o n 3

4

Product Y i e l d and Composition. Product y i e l d and composi t i o n d i d not v a r y s i g n i f i c a n t l y within the range of residence times s t u d i e s . Feed rate d i d not have a n o t i c e a b l e effect e i t h e r , although f u r t h e r experiments a t 10 kg/h are r e q u i r e d . The system output was l i m i t e d by the feeding system used. I t i s l i k e l y that the k i l n could convert more m a t e r i a l since conver sion was completed w i t h i n 10 minutes. This was i n d i c a t e d by the f a c t that gas production f e l l to zero w i t h i n 5 to 10 minutes a f t e r feeding was stopped. As w e l l , during previous experiments with peat (Campion, 1978), feed r a t e s up to 18 kg/h were used. Product mass y i e l d s are shown i n Figure 3. Char y i e l d s de creased almost by a f a c t o r of 2 from 870 Κ to 1170 K; converse-




342

Figure 3.

Mass product/feed ratios ((

) char; (

) gas)


25.

ARSENAULT

ET AL.


343

l y , gas y i e l d s doubled. Char y i e l d s were greater f o r stover than f o r straw by about 20%; gas y i e l d s were lower by the same amount. Gas production at 1020 Κ averaged 0.55 m /kg f o r straw and 0.44 m /kg f o r s t o v e r . I t was s u r p r i s i n g to f i n d that t a r y i e l d s were only around 1% or l e s s . Apparently the l a r g e mole cules c o n s t i t u t i n g the tar are e a s i l y broken down i n secondary p y r o l y s i s and g a s i f i c a t i o n s t a g e s . The primary p y r o l y s i s stage where t a r s are produced w i l l be studied i n a bench s c a l e r e t o r t i n Phase I I to v e r i f y the secondary p y r o l y s i s and g a s i f i c a t i o n assumptions· Results o f gas composition a n a l y s i s are presented i n Figure 4 f o r the straw experiments and Figure 5 f o r the stover experiments. As expected, C 0 content fell c o n s i d e r a b l y (by about 1/2) when temperature was increased from 870 Κ to 1170 K. CO and H 2 increased with temperature and CH^ content was a max imum at 1020 K. Although the C f r a c t i o n was not analysed f o r the i n d i v i d u a l components, approximate molecular weights and c a l o r i f i c values were used to c a l c u l a t e the s p e c i f i c gravity and c a l o r i f i c values o f the gases. These were quite s i m i l a r f o r both product gases (straw and s t o v e r ) . C a l o r i f i c v a l u e s averag ed 12 MJ/m . Higher c a l o r i f i c values around 14 MJ/m were ob tained a t 1020 Κ where CHi+ production was g r e a t e s t . Specific g r a v i t y v a r i e d from 1.1 a t 870 Κ to 0.8 at 1170 K. Results o f the char analyses are given i n Table I I . They are only presented f o r two s p e c i f i c experiments since the char a c t e r i s t i c s d i d not v a r y c o n s i d e r a b l y with r e a c t o r temperature or residence times. The c a l o r i f i c value of the straw char was higher than that of the stover char by about 20%. The c a l o r i f i c value of the straw char on a u n i t weight b a s i s was about 1.6 times g r e a t e r than the straw feed and the c a l o r i f i c value of the stover char was about 1.4 times g r e a t e r than the stover feed. Further c h a r a c t e r i z a t i o n of the char w i l l be c a r r i e d out i n Phase I I . Of p a r t i c u l a r i n t e r e s t are the burning c h a r a c t e r i s t i c s , h y g r o s c o p i c i t y , ash and a c i d i n s o l u b l e ash content and measured c a l o r i f i c v a l u e . 3

3


2

2

3

Table I I :

Feed Straw

3

Results o f char a n a l y s i s f o r p y r o l y s i s o f straw and stover at 1020 Κ and 0.5 hour residence time

C 74.9

Elemental A n a l y s i s o f Char ( a i r d r i e d ) (% by weight) Η Ashl 0 S Ν 2.4

7.0

0.20

1.8

13.7

Calculated Calorific V a l u e 2 MJ/kg 27.6

Stover 60.0 1.7 21.1 9.2 25.7 0.28 3.1 1 Calculated by d i f f e r e n c e (100% - %C, H, 0, S, N) 2 Dulong's formula Average f o r a l l experiments: straw char - 27 MJ/kg stover char - 23 MJ/kg



344


50-»

ZEACTOe. TEMPE£ATU£.e Figure 4.

(K)

Gas composition from straw pyrolysis—Series Π (Note: heavy com ponents not considered)


25.

ARSENAULT

ET

AL.

345



5θ—ι

370

I020 SEACTOZ

TEMPERATURE

U70 (Κ)

Figure 5. Gas composition from corn stover pyrolysis—Series III (Note: heavy components not considered)




346

Energy Data, Figure 6 gives the energy product/feed r a t i o s , that i s , the energy content found i n the gas or char for a u n i t q u a n t i t y of feed. At 870 Κ there i s almost 3 times more energy i n the char than i n the gas. However, a t 1170 Κ the energy i n the char and gas are about equal using stover feed stock and approximately twice as much i n the gas as i n the char using straw feedstock. Therefore, i f char i s the d e s i r e d prod uct, r e a c t o r temperatures should be low (870 Κ ) , but i f gas i s the d e s i r e d product, r e a c t o r temperatures should be higher (1020 Κ or 1170 K ) . Mass balances averaged 85%. Losses i n the operation were mainly due to the i n a b i l i t y to adequately condense and trap p y r o l y s i s waters. Experiments of longer d u r a t i o n would improve the mass b a l a n c e s . Energy i n the char and gas averaged 77% of the gross cal o r i f i c value of the feedstock. In these experiments, energy to pyrolyse the feedstock was supplied by propane. About 100 MJ were required to heat the k i l n to 870 K. Another 121 MJ/h were required to maintain t h i s temperature and 189 MJ/h were r e q u i r ed to maintain a temperature of 1170 K. About 90% of t h i s heat was l o s t i n the exhaust and i n an a c t u a l system most could be recovered f o r process h e a t i n g . C a l c u l a t i o n s were made to pre d i c t thermal e f f i c i e n c i e s of a s e l f - s u s t a i n i n g system where a p o r t i o n of the product i s r e c y c l e d and combusted to provide process heat; thermal e f f i c i e n c i e s of about 65% were p r e d i c t e d . Product Uses The most convenient a p p l i c a t i o n f o r the f u e l s obtained would be to provide s u b s t i t u t e heat f o r d i r e c t use on the pro ducing farm or w i t h i n short d i s t a n c e s r e q u i r i n g a minimum of t r a n s p o r t a t i o n . The char has a s i m i l a r heat output to bitumin ous c o a l on a mass b a s i s , while the gas has about one-third the value of n a t u r a l gas. R e l a t i v e l y l i t t l e c a p i t a l would be r e quired to modify e x i s t i n g equipment f o r burning p y r o l y s i s prod u c t s . The p o s s i b i l i t y of mixing ground char and f u e l o i l will be considered i n Phase I I . To provide some i n d i c a t i o n of the use p o t e n t i a l i n Eastern Canada, the f o l l o w i n g example was developed f o r an Eastern Canadian farm producing 50 ha of g r a i n corn per year. Assume the mean production given by Southwell and Rothwell (1977) of 5604 kg g r a i n corn/ha at 28% moisture and 4700 kg stover/ha. Assume as w e l l that one-half the stover can s a f e l y be harvested without damaging the s o i l or i n c r e a s i n g winter r u n o f f . Con v e r t i n g a l l of the harvested stover to p y r o l y s i s products and using a 10 kg/h, s e l f - s u s t a i n i n g process would y i e l d 1.5 χ 10 MJ/yr. About 20% to 30% of the energy produced may be required to dry the feedstock, depending on the year and h a r v e s t i n g con d i t i o n s . About 20% of the output would supply adequate f u e l f o r an average farm r e s i d e n c e , l e a v i n g about 50% of the output f o r

6



ARSENAULT ET AL.


370

Figure 6.

I02Q

Energy product/feed ratios ((

—I η'70 ) char; (

)

American Chemical Society Library 1155 16th St. N. W. In Thermal Conversion of Solid Wastes Biomass; Jones, J., et al.; Washington, D. C.and20036 ACS Symposium Series; American Chemical Society: Washington, DC, 1980.


348


other p o t e n t i a l on-farm a p p l i c a t i o n s such as g r a i n drying and farm h e a t i n g . Other uses may i n c l u d e replacement of hydrocarbon fuels with char i n engines equipped with g a s i f i e r s . The gas may also have an a p p l i c a t i o n as an engine f u e l although storage may be expensive· I n t e g r a t i o n of crop-residue p y r o l y s i s into the work routine on the farm would d i f f e r depending on the crop residue a v a i l a b l e , the end-use o b j e c t i v e , the kind of farm, and the weather zone i n which the farm i s l o c a t e d . A 10 kg/h unit would r e q u i r e w e l l over a year to process the crop residue from the 50 ha farm discussed e a r l i e r . A 50 kg/h k i l n operating continuously would process the same amount i n 100 days. The r e s u l t i n g char could be stored more e a s i l y than the feedstock. Economic A n a l y s i s An elementary economic a n a l y s i s of the s i t u a t i o n follows. We have seen that a t y p i c a l 50 ha farm could y i e l d about 10 MJ of energy i n the form of high energy char or low energy gas. Compared to f u e l o i l at (U.S.) $ 0 . 1 1 / l i t r e (U.S. values $0.45/ g a l l o n ) t h i s would have a value of about (U.S.) $3,400. Design c r i t e r i a f o r the small scale pyrolytic conversion of a g r i c u l t u r a l biomass w i l l be proposed i n Phase I I , keeping t h i s f i g u r e i n mind. Increased expenditures f o r h a r v e s t i n g , t r a n s p o r t , storage of residue f u e l , and equipment m o d i f i c a t i o n s required to use the product w i l l have to be c o n s i d e r e d . There may a l s o be economies of scale i n v o l v e d . A co-operative of a few farms w i t h i n a small radius of each other may be more prac t i c a l . Larger western farms ( t y p i c a l l y 500 ha) should also be considered - the s c a l e up may prove advantageous· 6

Comparison with Peat P y r o l y s i s It i s i n t e r e s t i n g to compare these experiments with p r e v i ous experiments on peat p y r o l y s i s using the same u n i t . Ground sod peat with a degree of decomposition of 27% (Baturin formula - B a t u r i n , 1975; H4 on the Von Post s c a l e ) was a i r d r i e d to 12% moisture content and fed i n at r a t e s up to 18 kg/h (Campion, 1978). Y i e l d s and c a l o r i f i c values at r e a c t i o n temperatures of 1120 Κ are compared with stover and straw p y r o l y s i s at 1170 Κ i n Table I I I . Mass y i e l d s are s i m i l a r f o r the d i f f e r e n t feedstocks but c a l o r i f i c values f o r both feed and products are somewhat higher f o r peat. C l e a r l y , a l l three feedstocks show good potential. Depending on l o c a t i o n , a farm could conceivably use peat as feedstock when a g r i c u l t u r a l residue was not a v a i l a b l e .


25.

ARSENAULT ET AL.

Table III:


Comparison of straw, stover and peat pyrolysis Type of Feed:


349

Oat Straw1

Corn Stover1

Sod Peat2

Feedstock Calorific value (MJ/kg)

17.9

16.2

20.1

Char Yield (%) Calorific Value (MJ/kg)

22 26.0

30 24.0

30 26.9

Thermal Conversion of Solid Wastes and Biomass - American

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