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Coal - Ranks and Uses
One of the world’s major sources of energy is coal. Coal is used to produce more than half of all the electrical energy that is generated and used in the United States. It is a very complex and diverse energy resource that can vary greatly, even within the same deposit. In general, there are four basic varieties of coal, which are the result of geologic forces having altered plant material in different ways. These varieties descended from the first stage in the formation of coal: the creation of peat or partially decomposed plant material.
Lignite: Increased pressures and heat from overlying strata causes buried peat to dry and harden into lignite. Lignite is a brownish-black coal with generally high moisture and ash content and lower heating value. However, it is an important form of energy for generating electricity.
Sub-bituminous Coal: Under still more pressure, some lignite changes into sub-bituminous coal. This is a dull black coal with a higher heating value than lignite that is used primarily for generating electricity and for space heating. If there are high volumes of volatile gases this coal may be sold to create coke to be used in steel making.
Bituminous Coal: Even greater pressure results in the creation of bituminous, or “soft” coal. This is the type most commonly used for electric power generation in the U.S. It has a higher heating value than either lignite or subbituminous, but less than that of anthracite.
Anthracite: Sometimes also called “hard coal,” anthracite forms from bituminous coal when great pressures developed in folded rock strata during the creation of mountain ranges. Anthracite has the highest energy content of all coals and is used for space heating and generating electricity.