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What Are the Four Types of Coal & How Are They Formed?

Coal is a fossilized plant material and derives from lush vegetation. The best sources for coal are the tropical forests that grew in the United States and other countries some 300 million years ago. The vegetation died and accumulated in swamps where it formed peat, a spongy material that is a precursor to coal. Over millions of years sediments covered and compacted the peat bogs. The coal hardened as more sediment, or overburden, deposited over it. Coal's ranking depends on how the original plant matter transformed into carbon and the percentage of total carbon remaining. The rank also depends on the coal's heating value, ash production, mechanical strength and presence of sulfur and any other impurities. Coal's calorific value is the heat produced by burning it to produce carbon dioxide, sulfur dioxide, nitrogen and water vapor. Coal is mined in 26 U.S. states and is the country's most abundant fossil fuel.

Lignite
- Lignite is the youngest coal in geological terms with between 25 and 35 percent carbon content and 35 percent water content. Most lignite occurs in geological formations between 50 and 70 million years old. Also called brown coal because of its color, its main use is in electricity generation. About half of the world's coal reserves are lignite. The world's largest lignite producers are Germany, Russia and the United States. In the United States most lignite reserves occur in Texas, Montana, North Dakota and the Gulf Coast region. Only 7 percent of United States coal production is lignite.
Subbituminous
- This is a coal with between 35 and 45 percent carbon. It has less moisture than lignite but also has low sulfur content, a quality that makes it attractive for clean burning. The largest United States subbituminous coal reserves are located in the West and in Alaska. Subbituminous coal accounts for about 46 percent of United States coal production.
Bituminous
- The most widespread coal resource in the United States, bituminous coal is used mainly in the power generation industry and in the manufacture of coke for steel production. Its carbon content is between 45 and 86 percent. Bituminous coal reserves occur mostly east of the Mississippi River and in the Midwestern states of Ohio and Illinois. About half of United States coal production is bituminous.
Anthracite
- Anthracite holds between 86 and 98 percent carbon and is the hardest coal type. Its main use is in home heating because it does not produce volatile gases. The largest anthracite reserves in the United States are in Pennsylvania and the northeast. Only 0.5 percent of coal mined in the United States is anthracite.

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