Coal is classified into four main types, or ranks: anthracite, bituminous, subbituminous, and lignite. The ranking depends on the types and amounts of carbon the coal contains and on the amount of heat energy the coal can produce. The rank of a coal deposit is determined by the amount of pressure and heat that acted on the plants over time.
It 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 2019. All of the anthracite mines in the United States are in northeastern Pennsylvania. In the United States, anthracite is mainly used by the metals industry.
Bituminous coal: (can be divided into two types: thermal and metallurgical)
It 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 48% of total U.S. coal production in 2019.
Bituminous coal is used to generate electricity and is an important fuel and raw material for making coking coal or use in the iron and steel industry. Bituminous coal was produced in at least 19 states in 2019, but five states accounted for about 75% of total bituminous production: West Virginia (27.5%), Pennsylvania (14.0%), Illinois (13.5%), Kentucky (10.6%), and Indiana (9.3%).
It 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. About 44% of total U.S. coal production in 2019 was subbituminous and about 88% was produced in Wyoming and 9% in Montana. The remainder was produced in Alaska, Colorado and New Mexico.
It 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. Lignite accounted for 8% of total U.S. coal production in 2019. About 51% was mined in North Dakota and about 41% was mined in Texas. The other 9% was produced in Louisiana, Mississippi, and Montana. Lignite is mostly used to generate electricity. A facility in North Dakota also converts lignite to synthetic natural gas that is sent in natural gas pipelines to consumers in the eastern United States.
Hard vs. Soft: Coal falls into two main categories: hard (Anthracite and Bituminous) and soft (Subbituminous coal and Lignite).
Soft coal is also known as brown coal or lignite. China produces more hard coal than any other country by a factor of about three. The whopping 3,162 million metric tons of hard coal produced by China dwarfs the output of the second and third-ranked producers—the U.S. at 932 million metric tons and India at 538 million metric tons.
Germany and Indonesia nearly tie for the honor of top honors in the production of soft brown coal. These countries dug up 169 million and 163 million metric tons respectively.
Coking vs. Steam:
Coking coal, also known as metallurgical coal, has low sulfur and phosphorus content and can withstand high heat. Coking coal is fed into ovens and subjected to oxygen-free pyrolysis, a process that heats the coal to approximately 1,100 degrees Celsius, melting it and driving off any volatile compounds and impurities to leave pure carbon. The hot, purified, liquefied carbon solidifies into lumps called "coke" that can be fed into a blast furnace along with iron ore and limestone to produce steel.
Steam coal, also known as thermal coal, is suitable for electric power production. Steam coal is ground into a fine powder that burns quickly at high heat and is used in power plants to heat water in boilers that run steam turbines. It also may be used to provide space heating for homes and businesses.