Poultry Manure: The New Frontier for Anaerobic Digestion

摘要

The science of anaerobic digestion is well understood, but practical applications in animal agriculture are still limited in the United States. Small-scale “gobar-gas” units have been in operation in rural India for several decades and primarily generate gas that can be used for cooking and lighting. Along with hundreds of on-farm systems in Europe, Denmark and Germany also have some very large centralized digesters that take manure from multiple farms. The common thread between most digesters, large or small, is that they use dairy manure, or in some cases swine manure, as the substrate for digestion. A new on-farm digester that came on-line in Spring 2007 uses broiler poultry litter to generate methane that is used to heat the chicken houses and generate electricity. The farmer has an interconnect agreement with the local utility to sell the excess electricity. This digester is located in south-central Mississippi and is considered to be the first on-farm unit of its kind in North America. Digester developers have largely ignored poultry litter, especially from broiler chickens, as a viable substrate for digestion. However, data show that biogas generation potential from litter is comparable to that of dairy manure, if not better. Broiler litter also poses a significant environmental challenge for U.S. poultry producers in areas ranging from Arkansas – Oklahoma to Maryland – Delaware. Using anaerobic digestion to process litter provides numerous benefits that include energy generation, environmental hazards mitigation, liquid fertilizer generation, and marketable soil amendment solids. Another potentially lucrative revenue stream for the digester owners is the ability to qualify for carbon credits since each ton of methane generated qualifies for 18 tons of carbon credits as per data available through the Chicago Climate Exchange. A cash flow model has been developed that includes various revenue streams and credits and capital and O&M costs that shows a simple payback of less than five years for an on-farm poultry litter digester system. There is also a growing interest in integrating large-scale digesters with ethanol plants as evidenced by a recent project in Nebraska. This is a highly symbiotic relationship where the digester supplies utility-scale gas and electricity to the ethanol plant and receives ethanol process byproducts that serve as co-digestion feedstocks along with the dairy manure. Given the spatial concentration of poultry farms and the relative ease of transporting broiler litter, utility-scale poultry litter digesters will be well suited for such integrated operations.