Lignite - Sometimes called "brown coal," lignite is the youngest of the coal types, and has the lowest energy content, containing between 25 and 35 percent carbon. Lignite usually has not been subjected to the extreme temperatures and pressures typical of the higher energy content coal types. It is crumbly, has a high moisture content, and is typically burned in power plants for electrical production. About 20 lignite mines produce about 7 percent of total US produced coal, mostly in Texas and North Dakota.
Sub-bituminous - Sub-bituminous coal has higher energy content than lignite, containing between 35-45 percent carbon. The sub-bituminous coal found in the US is normally at least 100 million years old and represents 44 percent of total US coal production, with Wyoming being the largest producer.
Bituminous - Bituminous coal contains anywhere from 45 to 86 percent carbon, giving it an even higher heating value than sub-bituminous. High heat and pressure and between 100 and 300 million years are required to produce this type of coal. About half of the coal produced in the US falls in this category with West Virginia, Kentucky and Pennsylvania being the largest producers. Bituminous coal is used for electrical production and plays a large role in the steel and iron industries.
Anthracite - Anthracite contains 86-97 percent carbon but has a slightly lower heating value than does bituminous coal. It is rare in the U.S. (all known anthracite deposits are located in Pennsylvania) and only represents less than half of one percent of total U.S. coal production.1
The term low-rank coal is commonly used; this refers to the lowest heating value and higher moisture content (and lower cost) lignite and sub-bituminous coals.