Cow Power: The Energy and Air Quality Benefits of Converting Manure to Biogas

This report consists of a top-level aggregate analysis of the total potential for converting livestock manure into a domestic renewable fuel source (biogas) that could be used to help states meet renewable portfolio standard requirements, reduce greenhouse gas (GHG) emissions, and improve air quality. In the U.S. livestock produces over one billion tons of manure annually. Typical disposal methods for animal manure allow for the emission of ammonia, particulate matter, unpleasant odors, volatile organic compounds (VOCs) and a variety of other air pollutants, which can damage the environment and pose risks to animal and human health. These disposal methods also allow for the uncontrolled emission of methane and nitrous oxide, two potent greenhouse gases (GHGs), with 21 and 310 times the global warming potential of carbon dioxide, respectively. Annually, 51 to 118 million metric tons (MMT) of carbon dioxide equivalent are emitted from livestock manure alone. Because air quality problems are on the rise, and trends show the contribution to GHG emissions from manure increasing from 1990 to 2005, limiting emissions from manure represents a valuable starting point for improving air quality and mitigating global climate change. Through anaerobic digestion, a well-known and time-tested process, animal manure is converted to methane rich biogas and many of the emissions of common manure management practices are avoided. Biogas is useful in a variety of applications such as a coal or natural gas substitute in electricity generation, which is the scenario considered in this analysis. Using values for the amount of biogas that can be produced per animal unit (defined as 1000 pounds of animal) per day and the number of animal units in the U.S., the authors determined that the 95 million animal units in the country could produce nearly 1 quad of renewable energy per year, approximately 1% of the U.S. total energy consumption. Converting the biogas into electricity using standard microturbines could produce 88 ± 20 billion kWh, or 2.4 ± 0.6% of annual electricity consumption in the U.S. Such an approach replaces two relatively dirty energy sources—manure and coal combustion—with a relatively cleaner source, biogas combustion, thereby yielding a net potential GHG emissions reduction of 99 ± 59 million metric tons or 3.9 ± 2.3% of the annual GHG emissions from electricity generation in the U. S. In addition, this approach has the potential to eliminate 0.4 ± 0.09 MMT of sulfur dioxide emissions, 0.1 ± 0.03 MMT of NOx, 0.002 MMT of mercury, and 0.69 ± 0.12 MMT of particulate matter from coal combustion. When natural gas is displaced, biogas use could eliminate 0.083 ± 0.019 of NOx and 0.006 ± 0.001 of SO2 while biogas combustion would emit approximately 0.068 MMT of NOx, 0.007 MMT of carbon monoxide, 0.005 MMT of particulate matter, 0.002 MMT of sulfur dioxide and 0.002 MMT of VOCs.