Conversion of biomass into methane gas

Biomass 6 (1984) 85-92

Conversion of Biomass into Methane Gas* Aziz Shiralipour and Paul H. Smith Department of Microbiology and Cell Science, Institute of Food and Agricultural Sciences, University of Florida, Gainesville, Florida 32611, USA (Received: 27 May, 1984)

ABSTRA CT Land, marine and agricultural residues were subjected to a bioassay for an ultimate methane yield. Bioassays were performed in 250 ml serum bottles, incubated at 35°C. Methane yields were calculated from the percent methane in the gases formed and the total volume of gas produced. Methane yields from woody plants were lower in general than from other plant resource groups. High methane yields were obtained from several aquatic plants, some crop residues, and some root and tuber plants. Because o f potentially high biomass productivity and high methane yields, water hyacinth (Eichhornia crassipes (Mart.) Solms ) and Napiergrass (Pennisetum purpureum L. ) were selected for intensive study. Methane yield varied among different groups, various species within each group and different parts of the same plant species. Treatment o f plants with various nutrients, especially N, during the growth period and the age of the plants at harvesting time affected the methane production. Key words: bioassay, volatile solid, methane yield, energy crop.

INTRODUCTION Biological conversion of biomass into methane gas has received increasing attention in recent years. 1,2 Most digesters used for the conversion of biomass into methane gas are experimental or small-scale. The development of new technologies should make it possible to design and con* Contributions of the Florida Agricultural Experiment Stations, Journal Series Number 5339. 85 Biornass 0144-4565/84/$03.30-© Elsevier Applied Science Publishers Ltd, England, 1984. Printed in Great Britain

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struct economically feasible, large-scale anaerobic digesters for the production of methane gas. Florida produces only about 18% of the energy it consumes.3 Because of its dependence on outside sources of energy, it is very vulnerable to energy shortages. Fortunately, Florida has abundant rainfall and solar energy, and has a long growing season. It is a major producer of forest products and has wetlands and open freshwaters where aquatic plants grow vigorously. These factors make Florida attractive as a site for research on methane production from biomass. To investigate new technologies, a program 'Methane from Biomass and Waste', was established by the Gas Research Institute and the Institute of Food and Agricultural Sciences, University of Florida. The overall objective of the program is to develop integrated processes, including production, harvesting and conversion to commercially produce methane gas from decentralized biomass resources on a local or regional basis. Investigators involved in the production projects of this program provide land, marine and waste biomass feedstocks to a centralized bioconversion laboratory. These feedstocks are subjected to a bioassay for methane yield, to aid in developing ultimate yield projections from the plant materials and relate the gas production to biological, physical and chemical properties of the biomass. The bioassay procedure is a modification of the procedure for monitoring biochemical methane potential. 4 MATERIALS AND METHODS Measurements of dry weight and volatile solids All plant samples, except for ligno-cellulosic samples, were ground into small pieces using a food processor. Ligno-cellulosic samples were dried at 60°C and then ground in a Christy-Norris hammer mill (1 mm mesh). Ground samples were dried in an oven at 105°C for 16 h after which they were weighed. Volatile solids (VS) were determined after the samples were ashed at 550°C for 3 h. Sample

preparation

An equivalent of 0.20 g volatile solids of plant material was transferred to a 250 ml serum bottle, and the volume was brought to 80 ml by

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addition of deionized water. A mixture of a defined medium and a liquor from a digester (seed inoculum) was added to bring the total volume to 100 ml. Defined medium The defined medium is the solution described by Owens e t al. 4 modified to include NiC12 (0.18 g liter-l), 2-mercaptoethansulfonic acid (0.1 g liter -x) and NaHCO3 (14.2 g liter-a). The final pH of the medium was adjusted to 7-4 with 5N NaOH. Seed inoculum

Five-L-digesters were established and maintained at 35°C with a 20 day retention time and a feed containing 5% volatile solids. The feed consisted of 30% high protein animal feed and 70% dried Bermuda grass. The pH was maintained at 7.0. Prior to each bioassay 200 ml of the digester liquor was removed and held at 35°C for 48 h. It was then mixed with 1800 ml of defined medium. A volume of 20 ml of this mixture was added to plant suspension in each serum bottle. During the bioassay, oxygen was excluded from glassware, tubing and the solutions using a deoxygenated 80% N2:20% CO2 gas mixture. The serum bottles containing the bioassay material were flushed with 80% N2:20% CO2 gas mixture, sealed with butyl rubber stoppers and aluminum retainers. The bottles were then transferred to a 35°C incubator. Gas analysis Plant samples were assayed for methane production every 2 to 5 weeks depending on the type of biomass and the length of the fermentation activity. The biogas pressure produced inside the serum bottles was measured using an electronic manometer (Setra Systems, Inc., Model: 300C). Methane concentration in the biogas was measured using gas chromatographs equipped with thermal conductivity detectors and electronic integrators. These measurements were then used to calculate methane production, as std* m 3 kg -1 VS added. * Methane production was calculated at 15.5°C and 1 atm according to the standard of gas industries.

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RESULTS AND DISCUSSION To date (15 October, 1983) over 1000 samples of plant material have been received and assayed in triplicate for methane yields. Some of these samples were tested more than once, for a total of 1025 bioassays. Of these, 688 have been completed and 337 are in process. Methane production from 24 samples of ligno-cellulosic (woody) plants ranged from 0.02 to 0.27 std m 3 kg q VS added (Table 1). Poplar was the best methane producer among the woody plants assayed. Methane yields from woody plants were lower in general than from other plant resource groups. Pretreatments or new digestion procedures will be needed if these biomass materials are to be used. Experiments are underway to examine the effects of various pretreatments. Methane production among samples from several freshwater aquatic plants generally was high. The range among 203 samples assayed was from 0.07 to 0.43 std m 3 kg -1 VS added (Table 1). The best methane production was from water hyacinth (Eichhornia crassipes (Mart.) Solms) shoots; however, roots from this plant showed a relatively low level of gas production. By nutritional s and genetic manipulations it is possible to produce water hyacinth with a higher shoot-to-root ratio for more methane production. Methane yields generally were higher among floating species than among submerged or emergent samples. Water hyacinth, a floating

TABLE 1

Methane Production of Different Biomass Resources Plant resources groups

Ligno-cellulosics Freshwater aquatics Forage and grasses Root and tuber Marine Crop residue

Number of samples

24 203 153 86 57 180

Methane yield fstdm3 kgq VS added)a Average

Range

0.13 0.22 0.24 0.33 0-21 0.27

0.02-0.27 0.07-0.43 0.07-0.41 0.19-0-43 0.08-0.38 0.08-0.53

a Value for each sample is the mean of three replicates.

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aquatic, was selected as one of the species for intensive investigation in the 'Methane from Biomass and Waste Program' because of its great productivityS, 6 and high potential methane production (Table 1). Forage and grass samples were generally intermediate in methane production. Methane yields among 49 samples assayed ranged from 0.07 to 0.41 std m 3 kg -l VS added (Table 1). Sugar sorghum (Sorghum bicolor) was the highest methane producer in this group. Napiergrass or elephantgrass (Pennisetum purpureum L.), another species chosen for intensive investigation, was selected for methane because of high biomass productivity 7 and relatively high potential convertability (up to 0.32 std m 3 k g -1 VS added) to methane. Roots and tubers from plants possessing these storage organs were among the highest methane producers. Methane production for 86 samples assayed ranged from 0-19 to 0.43 std m 3 kg -1 VS added (Table 1). Morado sweet potato (Ipomoea batatas) tuber was the best methane producer in this group. Methane production among 57 marine samples ranged from 0.08 to 0.38 std m 3 kg -1 VS added (Table 1). Gracilaria tikvahiae, a red alga, was the highest producer of methane gas in this group. Some vegetable crop residues were among the highest methane producers. Methane production in this group ranged from 0.08 to 0.53 s t d m 3 kg -1 VS added (Table 1). Fruits were the highest methane producers (0.31 to 0.51 s t d m 3 k g -1 VS added) while structural or support roots in sharp contrast to storage roots were relatively lower producers (generally less than 0.25 std m 3 kg -1 VS added). Methane production varied among different groups, various species within each group (Table 1) and different parts of the same plant species (Table 2). Although methane yields for shoots were generally higher than structural roots, for all species assayed, the situation was reversed with storage root and tuber samples. For example, methane yields in water hyacinth shoots were higher than in roots, while gas yields for sweet potato shoots were lower than in tubers (Table 2). This variation in methane production could be attributed to the differences in chemical composition of the biomass. Age of the plants at harvesting time affected the methane yield (Table 3). Young tissues produced more methane than the older tissues probably because younger tissues are less lignified. Treatment of plants with various nutrient elements, especially nitrogen in nutrient-deficient environments, during the growth period

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TABLE 2 Methane Production of Different Plant Parts of Water Hyacinth and Sweet Potato

Spec&s or plant part

Number of samples

Water hyacinth shoots Water hyacinth roots Sweet potato shoots Sweet potato tubers

9 9 6 6

Methane yield (std m3 kg-1 VS added)a Average

Range

0.32 O.18 0.23 0-35

0.26-0.43 0.13-0.24 0.19-0.26 0.31-0.43

a Value for each sample is the mean of three replicates.

TABLE 3 Effect of Age on Methane Production of Napiergrass

Age {days)

120 180 330

Number of samples

3 5 6

Methane yield (stdma kg-1 VS added)a Average

Range

0.31 0.26 0-24

0.31-0.32 0.24-0.26 0.23-0.24

a Value for each sample is the mean of three replicates.

increased the methane production. Addition of N urea [CO(NH2)2] to the. growth media consisting of 10% Hoagland's solution minus N s increased the gas production (Table 4). It has been shown s that N fertilization substantially increases the productivity of the water hyacinth. Change in productivity appears to be associated with an alteration of protein metabolism. 9'1° This suggests that increase in protein content of plant tissue due to N fertilization may be a factor which favors methane production. To reach a definitive conclusion with regard to fermentability of a particular species, bioassays from plants and plant parts grown under the range of potential environments and growth stages are essential.

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TABLE 4

Effect of Nitrogen Treatment on Methane Production of Water Hyacinth after 11 Weeks Incubation N concentration (mg liter-l)

0 1

5 10 a

Methane yieM (std m 3 kg-1 VS added) a Root

Shoot

Entire plant

0.11 0.13 0-13 0.16

0.15 0.21 0.25 0.28

0.12 0-16

0.17 0-22

Each value is the mean of three replicates.

In selecting an energy crop, competition with agricultural products for the use of land, e c o n o m y of planting, maintenance, harvesting, transportation to conversion site, etc. must be carefully evaluated. Excluding these factors the two most important factors to be considered are productivity and convertability of the plant species. High productivity and convertability are highly desirable characteristics for an energy crop. Crop species possessing low productivity and/or low convertibility characteristics may be improved by using new biotechnology procedures. ACKNOWLEDGMENTS The technical assistance of Ms Phyllis Hansen and Mr F. M. Bordeaux throughout the course of this study is gratefully acknowledged. This research is part of the joint program between the Gas Research Institute and the Institute of F o o d and Agricultural Sciences, University of Florida. REFERENCES 1. Chynoweth, D. P., Dolence, D. A., Ghosh, S., Henry, M. P., Jerger, D. E. & Srivastava, V. J. (1982). Kinetics and advanced digester design for anaerobic digestion of water hyacinth and primary sludge. Biotech. Bioeng. Syrup., 12, 381-9.

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2. Clausen, E. C., Sitton, O. C. & Gaddy, J. L. (1979). Biological production of methane from energy crops. Biotech. Bioeng., XXI, 1209-19. 3. Florida State Energy Office (1977). Forecasts of future supply and demand of energy in Florida, Department of Administration, State of Florida. 4. Owens, W. F., Stuckey, D. C., Healy, J. B., Young, L. Y. & McCarty, P. L. (1979). Bioassay for monitoring biochemical methane potential and aerobic toxicity, WaterRes., 13,405-92. 5. Shiralipour, A., Garrard, L. A. & Hailer, W. T. (1981). Nitrogen source, biomass production, and phosphours uptake in waterhyacinth. J. Aquat. Plant Manage., 19, 40-3. 6. Ryther, J. J., Williams, L. D., Hanisak, M. D., Stenberg, R. W. & DeBusk, T. A. (1979). Biomass production by marine and freshwater plants. Proc. Third Annual Biomass Energy Systems Conference, Golden, pp. 13-23. 7. Prine, G. M. & Mislevy, P. (1983). Grass and herbaceous plants for biomass. Proc. Soil. and Crop Sci. Soc. of Fla., 42, 8-12. 8. Hoagland, D. R. & Arnon, D. I. (1950). The water culture method for growing plants without soil, Berkeley, Calif. Agric. Exp. Sta. Circ. 347. 9. Dhindsa, R. S. & Bewley, D. (1977). Water stress and protein synthesis. V. Protein synthesis, protein stability, and membrane permeability in droughtsensitive and drought-tolerant moss. Plant Physiol., 59,295-300. 10. Shiralipour, A., Harris, H. C. & West, S. H. (1969). Boron deficiency and amino acid and protein contents of peanut leaves. Crop Sci., 9,455-6.