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Biomass
Biomass fuel sources
Most any biological material can be burned for fuel. Much of the world depends on such fuels for cooking and heat. Fuels used for commercial-scale electricity production, however, must be available in large quantities at a reasonable cost. Most of today's biomass fuels are in the form of wood-processing residues, which are the various bits and pieces of wood left over from lumber processing and pulp and paper mills. Second to this is "in-forest residue," which are the tops, limbs, and other pieces left over from forest harvesting. The third major type of biomass fuel now in use is agricultural residues, which includes myriad fuels left over from or associated with agricultural production. Examples include pits, shells, stalks, and prunings. In some cases, electricity production is a solution to a disposal problem: these wastes take up a lot of room, open burning is highly polluting and usually prohibited, and putting them in a landfill is expensive. According to the Department of Energy estimates, there are roughly 7 to 8 GW of biomass electricity generating capacity in the United States (excluding municipal solid waste and landfill gas).
One challenge in biomass burning is fuel variability. Biomass fuels, unlike fossil fuels, can vary considerably in moisture content, amount of trace contaminants, energy density (Btus per pound), and other quantities. Although direct-firing steam systems are relatively tolerant of fuel fluctuations, they do make it difficult to control emissions and to operate the plant at maximum efficiency.
A conceptually distinct fuel is MSW, sometimes called "refuse-derived fuel". Some don't consider MSW to be biomass because some of the solid waste is not of biological origin (such as plastic bags and some types of construction debris). According to industry sources, there are about 100 waste-to-energy plants in the United States with an electric generating capacity of 2,800 MW.
Landfill methane is often classified as a biomass fuel. Decomposition of organic waste in landfills results in emissions of "landfill gas," which is typically about 50 percent methane (CH4). (The rest is mostly CO2 , plus traces of volatile organic compounds and other gases.) This mix can be burned in a turbine or engine, yielding electricity and/or process heat. According to EPA estimates, there are now about 330 such systems now operating in the United States.
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