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. The ATTRA website has a section titled “Anaerobic Digesters and Other Biomass Options,” which identifies applicable publications and other resources. It is available at https://attra.ncat.org/attra-pub/farm_energy/biomass.html. On this page, you’ll find links to the following publications, which discuss various aspects of using anaerobic digesters: An Introduction to Bioenergy: Feedstocks, Processes, and Products. This publication discusses how farmers can utilize bioenergy to reduce dependency on petroleum-based energy sources. Micro-Scale Biogas Production: A Beginners Guide. An introduction to biogas, this publication gives the basics of small-scales anaerobic digester operations. Anaerobic Digestion of Animal Wastes: Factors to Consider. This publication discusses the benefits and constraints, as well as design considerations, of methane production through anaerobic digestion of animal waste. A couple good examples of digesters are in Tillamook, Oregon, and New Belgium Brewing in Fort Collins, Colorado. “In Tillamook: Turning (More) Cow Poop Into Power,” by Cassandra Profita for Ecotrope, the current digester is discussed, as well as plans for a more high-tech operation: http://ecotrope.opb.org/2012/03/in-tillamook-turning-more-cow-poop-into-power/#more-9970. New Belgium Brewing in Fort Collins, Colorado, uses an onsite water-processing plant to turn brewery waste into energy: www.newbelgium.com/culture/alternatively_empowered/sustainable-business-story/planet/energy-and-greenhouse-gas-emission.aspx. The effluent from the anaerobic digestion should be viewed as a resource. The liquids can be packaged and sold as liquid fertilizer, and the solids can be composted (note: uncomposted 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 by-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 may be removed by condensing in a conventional air cooled condensing unit or cooling tower. Carbon dioxide (CO2) 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. This is not an issue unless you want to store the gas for some length of time and don’t want to invest in the capital cost of gas-holding equipment for useless CO2.