Biomass 17 (1988) 251-263

Effect of Particle Size on Biogas Generation from Biomass Residues

Sudhi r K. Sharma, a I. M. Mishra , b M. P. Sha rma a & J. S. Saini c

"Alternate Hydro Energy Centre, hDepartment of Chemical Engineering, 'Department of Mechanical Engineering, University of Roorkee, Roorkee-247667, India

(Received 6 June 1988; revised version received 28 September 1988; accepted 10 October 1988)

A B S T R A C T

The effect of particle size of seven agricultural and forest residues used as feedstock for biogas generation through anaerobic digestion were investi- gated in batch digesters at 37°C. Out of five particle sizes (0"088, 0"40, 1"0, 6"0 and 30"0 mm), maximum quantity of biogas was produced from raw materials of 0"088 and 0"40 mm particles. For succulent materials such as leaves, large particles could be used. However, for other materials such as straws, large particles could decrease the gas production more effectively compared to those of leaves. Methane content of biogas was between 67 and 73% (v/v) for all materials except for straws which were around 60%.

The results support the concept that a physical pretreatment such as grinding could significantly reduce the volume of digester required, without decreasing biogas production, particle size.

Key words: agricultural residues, methane, biogas, anaerobic digestion, Mirabilis, Ipomoea fistulosa, banana peeling.

INT R ODUC TION

The anaerobic digestion of cellulose-containing residues and wastes of agricultural, forest, municipal or animal origin holds promise as a dual method of methane generation and residue treatment/pollution control. Formation of gaseous fuel by anaerobic digestion of organic residues is attractive as this process can utilize feedstock of a high moisture content and can operate at low ambient temperatures. One of the major problems in utilizing these materials is their low digestibility. Bio- degradability or digestibility can be increased by physical, chemical, and

251 Biomass 0144-4565/89/S03.50 - © 1989 Elsevier Science Publishers Ltd, England. Printed in Great Britain

252 s. K. Sharma, I. M. Mishra, M. P. Sharma, J. S. Saini

high temperature pretreatment. ~ Physical pretreatment includes size reduction and preincubation with water. 2 The beneficial effect of size reduction on digestion has been reported. 3'~ Particles (0.40 mm) of Ipomoea fistulosa plant stem produced 98% more biogas than did 6.0 mm particles 5 and predigested cattle dung, for example, produced 17-19% more biogas as compared with fresh cattle dung. 6

Many types of agricultural and forest residues such as banana peelings and leaves, 7,8 Mirabilis leaves, 9 rice straw j° and Ipomoeafistulosa leaves 5 are not used as cattle feed for a number of reasons. However, these could be used for biogas generation.

This paper reports the effect of particle size of influent on the efficiency of biogas digesters operating on agricultural and forest residues. The objective of our research was to enhance the gas produc- tion and to determine the optimum particle size of each residue fed to the digesters. Identification and control of various factors which affect the rate of methane yield are also covered.

MATERIALS AND METHODS

Collection of raw materials

Seven raw materials were collected from the surroundings of the University campus:

1. Wheat straw (WS), ( Triticum aestivum). 2. Rice straw (RS), (Oryza sativa). 3. Mirabilis leaves (ML), ( Mirabilis jalapa Linn.). 4. Cauliflower leaves (CFL), (Brassica oleraea, variety -- Botritis). 5. Ipomoea fistulosa leaves (IFL), (Ipomoea fistulosa Mar. Ex.

Choisy). 6. Dhub grass (DG), (Cynodon dactylon), (also called Bermuda grass). 7. Banana peeling (BP), (Musa paradisica).

These materials were first dried in the sun and then in an oven at 90°C for evaporating the remaining moisture.

Experiments

Experiments were conducted in batch-type modified aspirator bottles of 5-1itre capacity, equipped with temperature control, pH electrode, stirrer, gas outlet and sampling system, material feeding and withdrawal

Effect of particle s&e on biogas generation 253

system. The raw material was ground in a portable grinder (Belco) and strained through IS sieves of different particle sizes (0"088, 0.40, 1-0, 6.0 ram). The under-sized particles were stored and used for feed preparation. Thus, the sieved particles used in each digester were not of uniform size but a mixture of particles in the size (dp) range 0 < dp < 0'088, 0.088 < dp < 0.40, 0.40 < dp < 1.0, 1.0 < dp < 6.0 mm, respectively. For convenience, the sizes of the sieved particles are to be denoted as 0.088, 0.40, 1.0 and 6.0 mm, respectively. The ground material of a known size range was mixed with water to give a slurry of 8% (w/v) total solid concentration. Two litres of this slurry were seeded with effluent slurry, obtained from a domestic digester operating on cattle dung with 55 days retention time, in 1:1 ratio (v/v). All the digesters were operated at 37 _+ 0.5°C for 8 weeks. Mechanical stirring of substrate was done for 2 min every 3 h at 100 rpm using shaft with three propeller blades. The digester slurry pH was monitored and controlled between 6.75 and 7.25 using calcium hydroxide (Glaxo). Two identical digesters were operated for each particle size of a given feedstock. Details of digesters and raw material particle sizes are given below:

Set I.

Set II.

Set III.

Set IV.

Set V.

Set VI.

Set VII.

Digesters A1, A2, A3 and A4 were fed with WS of particle size 0.088, 0.40, 1.0 and 6-(I mm, respectively, whereas digester A5 received WS of size approximately 30 x 5 mm (length x diameter). Digesters B1, B2, B3, B4 and B5 were similar to set I but the material was RS instead of WS. ML of particle size 0.088, 0.40, 1-0 and 6.0 mm were fed to digesters C1, C2, C3 and C4, respectively. Digester C5 received unshredded leaves of size approximately 80 x 50 mm (length x width). Digesters D1, D2, D3 and D4 were fed CFL of particle size 0.088, 0"40, 1.0 and 6"0 mm while unshredded leaves of approximately 150 x 100 mm (length xwidth) were fed to the fifth digester D5. Digesters E l , E2, E3, E4 and E5 were fed IFL of similar particle size as that of set IV digesters. DG of particle size 0.088, 0.40, 1.0 and 6"0 mm were fed to digesters F1, F2, F3 and F4, respectively. Long clippings (30 mm) of DG were also anaerobically digested in digester F5. Digesters G1, G2, G3 and G4 received BP of particle size 0"088, 0"40, 1"0 and 6"0 ram, respectively, whereas digester G5 was fed unshredded BP of 30 x 10 mm (length x width).

254 S. K. Sharma, L M. Mishra, M. P. Sharma, J. S. Saini

Analytical

Moisture, volatile solids (VS), carbon, nitrogen, protein, cellulose and lignin contents of the raw materials were determined as per standard analytical methods described in our earlier papers?. ~ Biogas generated from the digesters was measured by the water displacement method and its composition was determined with the AIMIL-NUCON gas chromatograph with porapak Q column and nitrogen as a carrier gas at room temperature, pH of the substrate was measured by digital pH meter (Control Dynamics, model APX 175E).

RESULTS AND DISCUSSION

Characteristics of raw materials

Characterization of all raw materials (Table 1 ) showed that volatile solids were above 80% in most of the materials. CFL had the highest quantity of protein (24.7%, w/w) whereas BP was richest in cellulose content with a value of 61.4%, among all the raw materials. Lignin was found to be low in all raw materials except ML and IFL. Lignin/cellulose ratio is normally used to infer the degree of digestibility of raw materials. It was found that IFL had the highest lignin cellulose ratio, 0.448, followed by ML (0"390) whereas the rest of the raw materials had values less than 0"330. Ethanol-benzene-hot water solubility was found to be highest for IFL followed by ML, CFL and BE For the rest of the raw materials, the solubility was very low.

Performance of digesters

The average rate per week of biogas production (average of duplicate digesters for each particle size and each feedstock) and total biogas production (Table 2 and Fig. 1 ) for all feedstock increased with decreas- ing particle size. As the feedstock used in duplicate digesters for each particle remained the same and similar operating conditions were maintained during the digestion period, the biogas production rates were almost the same, giving maximum standard deviation of + 6.65%.

For the fifth digester of each feedstock having the largest particle dimensions, the gas production rate was lower in the first 2 weeks, reached its maximum in the 3rd, 4th or 5th week (depending on the feed- stock) and then decreased thereafter. In the case of WS, RS, ML and IFL, the gas production for the digesters with particle sizes 0.088, 0.40, 1.0

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258 S. K. Sharma, I. M. Mishra, M. P. Sharma, J. S. Saini

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PARTICLE SIZE (ram)

Fig. 1. Total biogas production versus particle size. m, Wheat straw; tz, rice straw; • , Mirabilis leaves; o, cauliflower leaves; A, lpomoea fistulosa leaves; o, dhub grass;

o, banana peeling.

and 6.0 mm was higher in the first week only; thereafter it decreased. For BP and CFL highest production rate was noticed in the second week. For DG the gas production was not linear for digesters F4 and F5. There were sudden fluctuations in gas production. Digesters F 1, F2 and F3 fed with finely ground grass particles had almost uniform gas production with a difference of 65% in total gas production observed at particle sizes of 0"088 and 30.0 mm. For the WS feedstock, digester A1 produced 54% more gas than A5, whereas in the case of the RS feedstock, digester B 1 gave 51% more gas as compared to digester B5.

The cellulose in the ligno-cellulosic polymeric form is not totally available for bacterial attack. Lignin surrounds the cellulose crystalline structure forming a 'seal' and protects the cellulose from easy hydroly- sis. ~2 Lignin itself is very resistant to degradation by anaerobic diges- tion.] 3. J4 Thus, grinding resulted in small particle size and consequently high surface area for the cellulosic materials. It enhanced the suscepti- bility of cellulose to bacterial and enzymatic attack, deformed the crystal lattice and reduced the degree of polymerization.~5

In the case of large particle size, especially for WS, RS, DG and BP, the material was solubilized in a longer time and also less surface area was available for bacterial degradation. Subsequently there was less gas production, but for ML, CFL and IFL the gas production was not much

Effect of particle size on biogas generation 2 5 9

affected when unshredded leaves were used. This may be either due to the high protein content and succulent and non-fibrous nature of the feedstock or due to lower C/N ratio (< 30). Optimum C/N ratio for methane production is in the vicinity of 30.16 The development of the methanogenic microflora is impaired if the C/N ratio exceeds 50: 1.17 Regardless of C/N ratio, many materials are found to be resistant to biodegradation. 18 This situation was observed in the case of DG feed- stock. The hydrolysis of ligno-celluloses is dependent on lignin to cellu- lose ratio and the digestibility of grasses is highly correlated with the lignin contenC 7

The digesters run on particle size 0"088 mm for feedstock ML, CFL, IFL and BP were found to produce, respectively, 16.9, 18.2, 10.9 and 50.2% more biogas than the digesters run on the largest particles of the respective feedstocks. Comparative analysis of total gas production for each particle size of each feedstock could be made from Fig. 1. Since the fifth digester of each feedstock was run on particles of differing dimen- sions, these values are not shown in the figure. It is evident that biogas production was highest for CFL followed by IFL, RS, BP, ML, WS and DG, respectively. CFL produced almost two times more gas than did DG.

Statistical analysis of the gas production rate (ANOVA) was done to analyse and see if there was significant difference between the gas produced for each particle size in duplicate digesters and also among the particle sizes for each feedstock. It was found that for all the feedstocks, the calculated values of F at 5 and 1% significance level were very large as compared to tabulated F values. Thus, it is discerned that the particle size has significant effect on gas production rate for each feedstock.

Reduction of cellulose, total solids and volatile solids was found to be lower for DG among all the seven materials and higher for RS, CFL and IFL (Table 3). In the anaerobic digestion process, cellulolytic bacteria constitute a small fraction of total acidogenic population. Cellulose reduction increased with the decrease in particle size for each feedstock. The maximum increase in cellulose reduction between smallest and largest particle sizes was observed for RS (13"5%), followed by WS (11.6%), BP (8.5%), DG (7-9%), CFL (6"5%), ME (5"6%)and IFL (4.6%). Reduction of higher quantities in any feedstock does not imply higher biogas or methane production. This is witnessed in the case of RS, CFL, IFL and BP.

No definite conclusions can be drawn for the effect of total solids (TS) and VS reduction on the biogas and methane production, though higher TS and VS reduction should give a higher quantity of gas. The percen- tage of methane in biogas obtained from feedstock WS, RS, ML, CFL,

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262 s. K. Sharma, 1. M. Mishra, M. P. Sharma, J. S. Saini

IFL, DG and BP for all particle sizes, was 59, 60, 69, 67, 71, 69 and 73% (v/v), respectively. Total methane production was highest for 0"088 mm particle size of IFL followed by CFL, BP, RS, ML, WS and DG, whereas CFL gave highest biogas quantity followed by IFL, RS and BP. This is only true for particle sizes 0-088, 0.40, 1-0 and 6.0 mm, and not for the feedstocks in the fifth digesters, as the particle dimensions were not similar.

CONCLUSIONS

(1) Particle size of influent for biogas digesters operating on agri- cultural or forest residues is an important parameter. Particle sizes of 0.088 and 0.40 mm produced an almost equal quantity of gas, thus grinding below 0.40 mm would seem to be uneconomical. In the case of succulent leaves such as ML, CFL, IFL, etc., full leaves can also be used without shredding.

(2) The highest quantity of methane (362 litres kg -j of TS) was produced by 0.088 mm particles of IFL followed by CFL (348 litres kg- ~ of TS) and BP (334 litres kg- ]TS) of the same particle size. Except for WS and RS, all the feedstocks produced methane- rich biogas ( > 67% (v/v)).

(3) Except for DG, the digestibility of all the raw materials used was quite satisfactory, so these can be used as a substitute to cattle dung for biogas generation.

ACKNOWLEDGEMENT

The authors acknowledge the generous financial support provided by the State Bank of India in the form of the 'State Bank Chair of Appro- priate Energy Technology for Rural Development'.

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Effect of particle size on b iogas generation 263

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