Coal As an Energy Source

Coal varies in the amount of heat energy it contains. There are four types of coal, and each type differs due to the geologic forces acting on the coal.

(Click here to see the coal reserves throughout the United States.)

Lignite: Pressure and heat from overlying strata caused buried peat to dry and harden. Lignite is a brownish-black coal with high moisture and ash content and a lower heat energy. Lignite mines are located in Texas, North Dakota, Montana, and Louisiana.
Subbituminous: More pressure caused lignite to become the next rank of coal--subbituminous. This is a dull, black coal with higher heat energy than lignite. Most subbituminous reserves are located in Montana, Wyoming, Colorado, New Mexico, Washington, and Alaska.
Bituminous: Even greater pressure resulted in the formation of bituminous or "soft" coal. It has a higher heat energy than lignite or subbituminous, but less than anthracite. Bituminous coal is chiefly mined in Appalachia and the Midwest.
Anthracite: Sometimes called "hard coal", anthracite was formed from bituminous coal when grat pressures developed in folded fock strata during the creation of mountain ranges. This occurred in only limited geographic areas--primarily the Appalachian region of Pennsylvania. Anthracite has the highested heat energy.
All the coal in Wyoming is bituminous or sub-bituminous.

The amount of heat energy that a coal reserve contains is measured in "British Thermal Units (BTU)". A BTU is the amount of energy required to raise the temperature of one pound of water one degree Fahrenheit. The heat energy of coal ranges from 13 million BTU per ton for lignite to 30 million BTU per ton for some bituminous coal. The average is 22 million BTU per ton.

Coal As an Energy Source

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