What is coal?

Coal is a sedimentary deposit containing carbon that is readily combustible. It is formed from plant remains that have been compacted, hardened, chemically altered, and metamorphosed by heat and pressure over geologic time. Coal is classified into four main types, or ranks, depending on the amount of carbon it contains and on the amount of heat energy the coal can produce The greater the pressure and heat experienced for the longer the time, the higher the ranking. The four types are:

• Anthracite contains 86%–97% carbon and generally has the highest heating value of all ranks of coal. Anthracite accounted for less than 1% of the coal mined in the United States in 2022. All anthracite mines in the United States are in northeastern Pennsylvania. In the United States, anthracite is mainly used by the metals industry.
• Bituminous coal contains 45%–86% carbon. Bituminous coal in the United States is between 100 million and 300 million years old. Bituminous coal is the most abundant rank of coal found in the United States, and it accounted for about 46% of total U.S. coal production in 2022. Bituminous coal is used to generate electricity and is an important fuel and raw material for making coking coal for the iron and steel industry. Bituminous coal is produced primarily in the Appalacian Mountains, with about one-third coming from West Virginia, but also in the states of Illinois, Pennsylania, Kentucky, and Indiana.
• Subbituminous coal typically contains 35%–45% carbon, and it has a lower heating value than bituminous coal. Most subbituminous coal in the United States is at least 100 million years old. In 2022, subbituminous coal accounted for about 46% of total U.S. coal production. Almost all subbituminous coal is produced in Wyoming.
• Lignite contains 25%–35% carbon and has the lowest energy content of all coal ranks. Lignite coal deposits tend to be relatively young and were not subjected to extreme heat or pressure. Lignite is crumbly and has high moisture content, which contributes to its low heating value. In 2022, five states produced lignite, which accounted for 8% of total U.S. coal production. Lignite is primarily produced in North Dakota and Texas.

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