摘要：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 and reduce
greenhouse gas (GHG) emissions. In the US, livestock agriculture produces over one
billion tons of manure annually on a renewable basis. Most of this manure is
disposed of in lagoons or stored outdoors to decompose. Such disposal methods emit
methane and nitrous oxide, two important GHGs with 21 and 310 times the global
warming potential of carbon dioxide, respectively. In total, GHG emissions from the
agricultural sector in the US amounted to 536 million metric tons (MMT) of carbon
dioxide equivalent, or 7% of the total US emissions in 2005. Of this agricultural
contribution, 51 to 118 MMT of carbon dioxide equivalent resulted from livestock
manure emissions alone, with trends showing this contribution increasing from
1990 to 2005. Thus, limiting GHG emissions from manure represents a valuable
starting point for mitigating agricultural contributions to global climate change. Anaerobic digestion, a process that converts manure to methane-rich biogas, can lower
GHG emissions from manure significantly. Using biogas as a substitute for other fossil fuels,
such as coal for electricity generation, replaces two GHG sources—manure and
coal combustion—with a less carbon-intensive source, namely biogas combustion. The biogas energy potential was calculated using values for the amount of
biogas energy that can be produced per animal unit (defined as 1000 pounds
of animal) per day and the number of animal units in the US. The 95 million
animal units in the country could produce nearly 1 quad of renewable energy
per year, amounting to approximately 1% of the US 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 US. Replacing coal and manure GHG emissions
with the emissions from biogas would produce 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 US.
