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Creating Fuel from Manure is a Hot Topic--AgainPhilip R. Goodrich, Associate Professor, University of Minnesota Department of Biosystems and Agricultural EngineeringThe old idea of extracting fuel from animal manure is taking on new life thanks to renewed interest in energy and in reducing greenhouse gases. Manure can be coaxed into producing biogas—mostly methane—through anaerobic digestion. This is nothing new but anaerobic digesters have come a long way since the 1970s. During the energy crisis then, consideration was given to anaerobic digestion of manure as a potential fuel source. Farmers participating in on-site research projects often found the systems disappointing. There was no support system to make the needed improvements or repairs. There were no incentives to reduce odors, and energy companies were unwilling to pay much for the energy produced. Today anaerobic digestion is being taken seriously because of increasing concerns about the environmental hazards of manure. The push to produce more renewable energy locally to reduce dependence on fossil fuels from distant places has also had an effect. There have been some significant improvements in the design, implementation, and support of anaerobic digestion including new materials, better monitoring devices, and more reliable methane to electricity conversion. How does the anaerobic digestion process work? In the controlled anaerobic digestion process, bacteria break down organic matter (manure is a great source) in an oxygen-free environment. Unlike an open lagoon or a manure storage tank, it is a closed system, so methane is captured and is, therefore, a usable fuel. The system also reduces odorous gases or vapors. The system is not magic and the manure is not destroyed. The outflow from an anaerobic digester contains all of the nutrients and water that went into the digester. Only energy has been extracted. Who may be most interested in anaerobic digesters?
What types of anaerobic digesters make sense for modern agriculture? Because of the differences in the manure produced from different animals, a system to make methane from dairy cows is quite different from a digester for manure from swine. For dairy cows, a plug-flow digester system works well for collecting and breaking down manure, and capturing the gas produced from this process. To describe the flow one can visualize a sausage grinder—when some material is forced in one end, an equal amount exits the other end. This works well when the solids content of the manure is 10-12% and is a viscous mixture which does not separate. The tank is usually built in the ground and is long and slim. The outflow may go into an outside storage pond to be held until the manure is recycled onto cropland. A completely mixed digester is better for swine manure. This type of digester could be compared to a blender. The material inside is mixed continuously. These have commonly been built using insulated upright circular tanks. The outflow is then recycled onto cropland. A third type of digester is a plastic-lined and covered pond. This digester works better in warmer climates than Minnesota or Wisconsin because there is a large area for heat loss in the winter time. This type also serves as the storage facility for the manure. It is more difficult to operate continuously and to clean out for disposal. A working digester demonstration in Minnesota A working digester has been built on the Haubenschild dairy in Princeton, Minnesota. The multiple benefits of the digester project at Haubenschild Farms were originally brought to the attention of several Minnesota governmental agencies with the help of the Onanegozie Resource Conservation and Development Council and The Minnesota Project. The Minnesota Office of Environmental Assistance, Minnesota Department of Commerce and Minnesota Department of Agriculture were all interested in anaerobic digesters. The federal AgSTAR program helped the family determine the type of digester to use and the initial feasibility of installing it. AgSTAR is a joint program of the U.S. Environmental Protection Agency, U.S. Department of Energy and U.S. Department of Agriculture. It is designed to provide technical assistance to farmers interested in building anaerobic manure digesters. With this advice, Dennis Haubenschild carefully selected an experienced digester engineer to develop the project. The 500 cows produce enough methane to generate about 2000 kWh per day of electricity, most of which is used on the farm. The remainder is sold to East Central Energy to power additional local homes. The dairy also saves $400 per month in heating costs by using waste heat from the generator. Other expected benefits are reduced odors from the manure and good returns from the manure applied to sandy soils used for growing corn and soybeans. More details on the project are available from the web sites www.misa.umn.edu/~mnproj/eg/ and www.ecenet.com/~hauby/farm-presentation.prz/odyframe.htm. What are the benefits of anaerobic digestion?
Lessons learned from the Haubenschild Project (listed by the Minnesota Project)
Resources on anaerobic digestion as listed by the Minnesota Project 1. Minnesota Project web site: http://www.misa.umn.edu/~mnproj/eg/ Contains links to many of the other useful sources of information. 2. AgSTAR provides assistance to farmers considering installing an anaerobic digester system. They can be reached by calling 1-800-95AgSTAR. The AgSTAR web site http://www.epa.gov/agstar contains information for farmers interested in installing a digester, including:
3. Methane Recovery from Animal Manures: The Current Opportunities Casebook. This report was published by the Department of Energy’s National Renewable Energy Lab and gives an overview of digester technology, as well as information on currently operating digesters in the United States. Available for download at the Minnesota Project web site. 4. Haubenschild Farms web site: www.ecenet.com/~hauby/farm-presentation.prz/odyframe.htm. This web site gives an overview of the farm business and how they approached the project. |
The information given in this publication is for educational purposes only. Reference to commercial products or trade names is made with the understanding that no discrimination is intended and no endorsement is implied.
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