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Over the past two decades, a number of gaseous emissions regulations have impacted the specifications of fuel used by coal-fired electrical generators. In the United States, limits on sulfur-dioxide emissions have increased usage of lower sulfur coal, particularly coal from the Powder River Basin in Wyoming and the central Appalachian Basin in West Virginia and Kentucky. Utilities continue to burn higher sulfur coal, though, by blending, using emission credits, and installing pollution control devices such as flue-gas desulfurization (FGD) units. Installation of FGD units has the added benefit of producing FGD gypsum for the manufacture of wallboard. Over 3.3 million tons of FGD gypsum was used in wallboard manufacture in 2000 in the U.S. and the amount is expected to increase as stricter emission standards force more utilities to employ FGD.
Mercury emissions regulations will be the next air-quality issue faced by the U.S. coal and utility industries. The U.S. Environmental Protection Agency (EPA) estimates that more than 43 tons of mercury were emitted by coal-fired utilities in 1999, more than 50 percent of it by seven major coal-consuming states, led by Texas and Pennsylvania. Some regulation of mercury emissions will be proposed by the EPA in December 2003. Any restrictive regulation will further affect the fuel choice of utilities and could also impact the quality of coal combustion by-products. Higher sulfur coal, however, may actually benefit from such regulations, as beneficiation methods used to clean mercury from coal are likely to remove sulfur as well.
Pollution control technologies
"Clean Coal Technologies" include new pollution-control and power-generating processes that reduce hazardous air emissions and lower greenhouse gases. These technologies are expected to increase the attractiveness of coal as a fuel source for existing and new electrical generation plants. The U.S. Department of Energy's Vision 21 program, with numerous combustion options, is taking the lead in funding clean coal research through pilot studies with private companies.
Many research agencies around the world, particularly in Europe, are conducting carbon sequestration research, another promising pollution-control process. The U.S. Midcontinent Interactive Digital Carbon Atlas and Relational Database, a consortium of the state geological surveys of Illinois, Indiana, Kentucky, Ohio, and Kansas, is funded to inventory locations of large stationary carbon-dioxide sources, including coal-fired power plants, fertilizer plants, and steel mills, and to compile potential sequestration sites, such as unmineable coal beds and depleted oil and gas reservoirs. Some carbon dioxide is already being "sequestered" in oil and natural gas reservoirs, where it is used to flush out that part of the resource that cannot be removed by conventional recovery. Carbon dioxide can also be used to recover coalbed methane (CBM) that is adsorbed onto the surface of coal. Research shows that carbon dioxide may bond to coal more efficiently than hydrogen, thereby displacing the methane gas and pushing it out of the reservoir.
Coal and coalbed methane resources
Reliance on coal for electricity requires an understanding and accounting of coal and CBM resources. CBM currently accounts for about 8 percent of the total U.S. natural gas production; one of the hottest onshore gas plays in the conterminous United States are the subbituminous coals of the Powder River Basin. The discovery that low-rank coal can produce vast amounts of CBM has initiated assessments and exploration all over the world for economic gas deposits from low-rank coal fields. Currently, the U.S. Geological Survey (USGS) is assessing CBM resources in the United States, concentrating on the geologic controls of CBM occurrence and recoverability and the environmental implications of developing the resource, which include the gradual depletion of local water resources and concerns about the disposal of waters produced from CBM production.
The USGS, in cooperation with numerous state geological surveys, is completing a National Coal Resource Assessment. The agency and cooperators assessed the top-producing coal beds and zones in the nation's five coal-producing regions - the Rocky Mountains and Northern Great Plains, Colorado Plateau, Appalachian Basin, Gulf Coast, and Illinois Basin. The assessment has created publicly available databases and digital maps, which answer a variety of questions of importance to government, nongovernment, industry, and public decision-makers and provide interpretive information in a digital format on the geology and geochemistry of the most important U.S. coal resources. In the 27 coal fields assessed, the USGS estimates that about 1,600 bst of coal remain.
Many state geological surveys have created additional sets of digital coal resource maps, GIS map layers, and resource estimates. The West Virginia Geological and Economic Survey is undertaking perhaps the most ambitious assessment by compiling digital maps of all mined and unmined coal beds throughout that state.
Given current economic, environmental, and technological restrictions, the lower sulfur coal beds of the Powder River Basin, Colorado Plateau, and central Appalachian Basin will dominate U.S. coal production in the coming decades.
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