Can I use livestock manure for methane-electric generation on my farm?
Answer: Most methane-electric generation is from animal waste. If you are accepting animal waste by truck, perhaps food waste or brewery waste can be trucked in as well. This will provide the desired variety of feedstock and will greatly increase biogas production. Manure works because it is readily available and cheap, and it naturally contains the anaerobic bacteria. Check out the ATTRA webpage, Anaerobic Digesters and Other Biomass Options, where applicable publications and other resources are listed to help you learn more on this topic. In addition, the following ATTRA publications discuss various aspects of a using anaerobic digesters: An Introduction to Bioenergy: Feedstocks, Processes, and ProductsThis publication discusses how farmers can utilize bioenergy to reduce dependency on petroleum-based energy sources. Micro-Scale Biogas Production: A Beginners GuideAn introduction to biogas, this publication gives the basics of small-scales anaerobic digester operations. Anaerobic Digestion of Animal Wastes: Factors to ConsiderThis publication discusses the benefits and constraints, as well as design considerations of methane production through anaerobic digestion of animal waste. The effluent from the anaerobic digestion should be perceived as a resource. The liquids can be packaged and sold as liquid fertilizer, and the solids can be composted (note: un-composted solid effluent from digesters “digestate” is not a very good fertilizer). Solids can be removed with an auger conveyor and run through a belt thickener and/or screw press to remove liquids. Composted digestate can be packaged and sold as fertilizer. Possible negative bi-products of methane production are the non-methane components of bio-gas: hydrogen sulfide, carbon dioxide, and water vapor. Removal of hydrogen sulfide is considered essential because the hydrogen sulfide will combine with products of combustion to make sulfuric acid, which will shorten the life of the generator. A thin film of aerobic bacteria can be grown on the surface of the manure inside the digester by allowing about 5% by volume of biogas of air into the headspace of the digester. This will decrease hydrogen sulfide gas significantly. The rest of it can be removed with an “iron sponge.” Moisture can be removed by condensing in a conventional air cooled condensing unit or cooling tower. Carbon dioxide can stay in the gas stage; it is un-reactive and will pass through the engine and end up in the exhaust. The effect of the CO2 is to decrease the calorific value of the biogas. Apparently, this is not a big deal unless you want to store the gas for some length of time, where you will not want to invest in the capital cost of gas holding equipment for useless CO2.