U.S. Biogas Energy Potential
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The process of creating energy through anaerobic digestion (AD) of organic materials (aka “biogas”) is not a new concept, but in light of climate change, it may be one with a lot of unrealized potential for both renewable energy and reducing emissions of the potent GHG methane. In the report titled Biogas Opportunities Roadmap, the agency looks at ways to voluntarily develop biogas systems, building on efforts by the nation’s dairy industry that were initiated in 2008 with the goal of reducing GHG emissions by 25 percent by 2020.
Looking at the energy side of biogas, the report notes, the United States has more than 2,000 sites now producing biogas, which is just a fraction of the more than 11,000 additional sites that could be deployed. Compared to the landfill gas energy industry sector, which has grown over the past 4 decades, the biogas industry as a whole has not matured at the same rate. For example, while there are 630 landfill gas sites now producing more than 2,000 megawatts (MW) of installed electricity generation capacity, only 239 manure-based units are in operation.
The report notes, however, that while early biogas systems were designed to utilize just one feedstock, such as manure, new technologies are enabling systems that can accept a variety of feedstocks (i.e., manure, food wastes, and wastewater biosolids). This ability increases the recognition of organic wastes as a resource, rather than a problem, not just on farms but also in communities and for industrial applications. In addition, renewable energy from biogas “delivers a continuous source of energy with a very high capacity factor” rather than intermittently like solar and wind.
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At the state level, the report says 37 states now recognize biogas in their renewable energy goals, and federal agencies have a target of using 20 percent renewable energy in their total electricity use by 2020. Commercially proven biogas energy uses include:
- Thermal applications: Biogas is used directly on-site to heat digesters and buildings/maintenance shops, to fuel boilers or kilns, and to generate heat or steam.
- Power generation: Electricity is produced through an internal combustion engine, gas turbine, or microturbine for on-site use or sale to the electric grid. Combined heat and power (CHP) systems also increase system energy efficiency by producing heat and electricity at the same time for heating, cooling, dehumidification, or other uses.
- Industrial applications use biogas to offset use of fossil fuels. For example, industries such as sugar refineries, distilleries, dairies, and paper mills generate processing and wastewater that can be digested directly on-site, resulting in biogas to fuel on-site in boilers, cement/brick/pottery kilns, sludge dryers, infrared heaters, paint shop oven burners, tunnel furnaces, process heaters, and blacksmithing forges, or other direct thermal applications.
- Biomethane injection: Upgraded and refined biogas, or renewable natural gas (RNG), can be injected into existing natural gas networks.
- Vehicle fuels: Upgraded biogas can be converted to various vehicle fuels, including compressed natural gas, liquefied natural gas, hydrogen, and liquid transportation fuels.
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Considering methane emissions reductions, the report notes that livestock manure operations alone account for 26 percent of GHG emissions from all livestock sources and that the 239 livestock biogas systems now operating reduce annual methane emissions by 2 million metric tons of carbon dioxide equivalent (MMT CO2e) while providing energy to power 70,000 average homes. In total, the report estimates that potential biogas systems (manure, landfill, and wastewater combined) could reach 13,008, which, if fully realized, “could provide 41 billion kWh/year [kilowatt-hours per year] of electricity from 654 billion cubic feet of biogas/year. This is enough energy to power more than 3 million U.S. homes for one year or to produce the equivalent of 2.5 billion gallons of gasoline for vehicles.”