Lignite, commonly referred to as brown coal, contains 25%-35% carbon and is a relatively young coal deposit with high moisture content. It is the lowest rank of coal with the lowest energy content, used almost exclusively as fuel for electric power generation. Lignite accounts for about 9% of the U.S. coal reserve base and is mined in Montana, Texas and North Dakota.
Sub-bituminous coal contains 35-45% carbon, with properties ranging between lignite to bituminous coal. Sub-bituminous coal is primarily used as fuel in steam-electric power generation; secondly it is an important fuel in cement manufacturing and a source of light aromatic hydrocarbons for the chemical synthesis industry. U.S. sub-bituminous coal is typically at least 100 million years old, making up about 37% of the reserve base and produced predominately in Montana and Wyoming.
Bituminous coal is dense black / dark brown mineral, with well-defined bands of bright and dull material containing 45-86% carbon. The principal uses of bituminous coal are for steam-electric power generation and in steel production. Coking coal or metallurgical coal is used in the manufacturing of iron and steel. Bituminous coal is between100 to 300 million years old, accounting for 53% of the U.S. reserve base, and is concentrated primarily east of the Mississippi River, with the greatest amounts in Illinois, Kentucky and West Virginia.
Anthracite, a harder, glossy, black coal containing 86-97% carbon, is the highest rank of coal. Anthracite is the “cleanest” burning rank of coal; because of this characteristic, it was a valuable domestic fuel for space heating, but reserves are now largely exhausted. Today, Anthracite, accounts for only 1.5 % of U.S. reserve base located almost entirely in northeastern Pennsylvania.
Meta-anthracite, commonly known as graphite, is technically the highest rank of coal, but is difficult to ignite and therefore not used as fuel.