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Coal is a combustible sedimentary rock composed mostly of carbon and hydrocarbons. It is the most abundant fossil fuel produced in the United States, but it is a nonrenewable resource. The energy in coal comes from the energy stored by plants that lived hundreds of millions of years ago in swampy forests. Over time, layers of dead plants at the bottom of the swamps were covered by layers of water and dirt, trapping the energy of the dead plants. The heat and pressure from the top layers turned the plant remains into coal.

Coal is a complex resource and can vary in composition even within the same deposit. Generally, there are four different types or ranking levels of coal, each with differences in energy output as a result of increased pressurization, heat, and time.

Lignite – This is a brownish-black coal with high moisture and ash content, which has the lowest heating value of the four types of coal. It is considered an “immature” coal that is still soft. It is used for generating electricity.

Subbituminous coal – This is a dull black coal with a higher heating value than lignite, and is used principally for electricity and space heating.

Bituminous coal – This is the most common type in the United States, accounting for over 50% of the demonstrated reserve base. It is the most commonly used type of coal for electric power generation in the United States. It is a dark, hard coal that has a higher heating value than lignite and subbituminous coal, but a lower heating value than anthracite.

Anthracite – This is also known as "hard coal" that was formed from bituminous coal under increased pressures in rock strata during the creation of mountain ranges. In the United States, it is located primarily in the Appalachian region of Pennsylvania. It is very hard and shiny. This type of coal is the most compact and therefore, has the highest energy content of the four levels of coal. It is used for space heating and generating electricity. It makes up only 1.5% of the demonstrated reserve base for coal in the United States.