July 24, 2008 Cows provide us with milk, meat, and leather jackets, but it’s possible that we’re turning our noses up at their most valuable offering to society. A study published in the Institute of Physics’ Environmental Research Letters section today claims that by converting livestock manure to biogas, the United States could reduce its greenhouse gas emissions and generate up to 108.8 billion kW h – 2.9% of the country’s total electricity requirement.

The bovine contribution to climate change is no secret, and the concept of harnessing their, er, emissions to provide power is not new. However, the journal paper, titled “Cow Power: The Energy and Emissions Benefits of Converting Manure to Biogas”, is the first study to examine in detail what the energy and environmental consequences would be.

The EPA and EIA estimate that between 57.1 and 117.9 million metric tonnes of carbon dioxide equivalents were emitted annually from undigested animal manure in 2005 and 2006. This accounts for up to 22% of the 536 million metric tonnes of carbon dioxide equivalent produced by the agricultural sector in the US – and the agricultural sector, in turn, is responsible for 7% of total national emissions. Most of the manure is disposed of in lagoons or left to decompose outside, where it releases methane and nitrous oxide, greenhouse gasses with, respectively, 21 and 310 times the global warming potential of carbon dioxide. Manure also emits ammonia, VOCs, hydrogen sulfide, and particulate matter, which can contaminate ground water and lead to eutrophication of the soil.

By subjecting manure to anaerobic digestion, a series of processes in which bacteria break down organic material into its basic elements, manure can be transformed into a rich biogas capable of replacing fossil fuel. While the combustion of the biogas would create CO2 emissions, the output is less than that from coal. The paper estimates that the conversion of manure to biogas could have an efficiency of 25% to 40% - which would create 68.0 to 108.8 billion kW h. While the US consumes 3.8 trillion kW h of electricity annually, the entire process would reduce the greenhouse gas emissions in energy production by up to 4%.

Authors of the paper, Dr. Michael E. Webber and Amanda D Cuellar from the University of Texas at Austin, write, "In light of the criticism that has been leveled against biofuels, biogas production from manure has the less-controversial benefit of reusing an existing waste source and has the potential to improve the environment. Nonetheless, the logistics of widespread biogas production, including feedstock and digestates transportation, must be determined at the local level to produce the most environmentally advantageous, economical, and energy efficient system."

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