coalCoal is a fossil fuel - a sedimentary organic rock that contains more than 50 percent carbonaceous material by weight and is composed largely of carbon, hydrogen, oxygen, nitrogen, and sulfur, with small amounts of other materials ranging from aluminum to zirconium. Its beginning was in the form of ancient plants that grew in swamps millions of years ago. Geological processes working over vast spans of time compressed and altered the plant remains, increasing the percentage of carbon present and thereby producing various ranks of coal.​

Coal Ranking

The four recognized ranks of coal in the U.S. classification scheme are: anthracite, bituminous coal, sub-bituminous coal, and lignite. In the United States coal rank is classified according to its heating value, its fixed carbon and volatile matter content and, to some extent, its caking characteristics during combustion.

Anthracite: Used primarily for residential and commercial space heating, anthracite is the highest rank of coal. Its hard, brittle, lustrous black texture contains a high percentage of fixed carbon and low percentage of volatile matter. It is often called "hard coal." Freshly-mined anthracite usually contains less than 15 percent moisture content. Its typical heat content is 15 million Btu per short ton or less.

Bituminous:Dense coal of black or dark brown color and usually has well-defined bands of contrasting bright and dull material. It is used primarily to generate steam electric power. Substantial quantities are used for power and heat applications in manufacturing as well as to make coke. In active U.S. mining regions, bituminous coal is the most abundant coal. It has a a moisture content of less than 20 percent and a heat content range between 21 to 30 million Btu per ton in a moist, mineral-matter-free basis. Of the four ranks, bituminous coal accounts for over half (51 percent) of the demonstrated reserve base. Bituminous coal is concentrated primarily east of the Mississippi River, with the greatest amounts in Illinois, Kentucky, and West Virginia.

Sub-bituminous: This coal can be dull, dark brown to black in color and soft and crumbly as one quality, to bright, jet black, hard and strong. Used primarily as fuel for steam-electric power generation, subbituminous coal has properties ranging from the properties of lignite or bituminous coal. The heat content of sub-bituminous coal consumed in the U.S. ranges from 17 to 18 million Btu per ton on a moist, mineral-matter-free basis. All sub-bituminous coal (38 percent of the demonstrated reserve base) is west of the Mississippi, with most of it in Montana and Wyoming.

Lignite: This coal's brownish-black color has a high moisture content and is the lowest rank coal. It is often called "brown coal" and is used almost entirely as fuel for steam-electric power generation. It accounts for slightly less than 10 percent of the demonstrated reserve base and is found mostly in Montana, Texas, and North Dakota.

Coal Deposits

The United States contains vast deposits of coal more extensive than those of natural gas and petroleum, the other major fossil fuels. If total estimated recoverable reserves of the major fossil fuels are compared on the basis of heat content, about three percent of reserves are crude oil, about four percent are natural gas, and over 90 percent are coal.

Total U.S. coal resources in the ground are estimated to be four trillion tons, of which 1.7 trillion tons are identified resources. Identified resources include the demonstrated reserve base, which comprises coal resources that have been mapped within specified levels of reliability and accuracy. They occur in coal beds meeting minimum criteria of thickness and depth from the surface generally required for economic mining under current technologies. The U.S. demonstrated reserve base contains an estimated 470 billion short tons. Because of property rights, land use conflicts, and physical and environmental restrictions, some coal in the demonstrated reserve base may not be available and accessible for mining.​​​​

The actual proportion of mineable coal resources that can be recovered from undisturbed deposits varies from less than 40 percent in some underground mines to more than 90 percent at some surface mines. In some underground mines, much of the coal may be left untouched as pillars needed to prevent surface collapse. Geologic features, such as folding, faulting, and interlayered rock strata, can mean a reduction in the amount of coal that can be recovered at both underground and surface mines.

New Uses for Coal

Continuing research is underway to develop and increase the beneficial uses and products of coal. Efforts to develop efficient conversion of coal-derived synthesis gas to clean-burning alcohol fuels and fuel extenders are underway at the National Research Center for Coal and Energy (NRCCE) at West Virginia University.

The "energy park" study to produce a magnitude of fuels and chemical feed stocks in order to minimize production costs is another focus of the Center. Producing coal-derived fuels with the enormous abundance of domestic coal may be helpful in reducing our nation's dependence on foreign petroleum.

Clean Coal Technology

In 1985, when the U.S. and Canada saw that their rivers, lakes, forests, and buildings were being damaged by "acid rain," both countries agreed to a partnership program for their governments, several U.S. states, and private companies to develop new, cleaner coal-burning technologies. This "Clean Coal Technology Program" develops methods to clean coal, to clean the sulfur from coal's combustion gases (called "scrubbers"), to find better ways to remove nitrogen oxides ("NOx") from the flue gases of coal burners, as well as to develop other coal-burning technologies such as coal gasification.

Additional Information

The U.S. Department of Energy: Coal
The U.S. Energy Information Administration: Coal overview
The Maryland Department of the Environment: Coal Mining in Maryland

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