A black or brownish/black solid, combustible substance formed by the partial decomposition of vegetable matter without access to air. Types of coal are anthracite, subanthracite, bituminous, subbituminous and lignite; the categories are based on fixed carbon, volatile matter, coking properties and heating value. The contents of subbituminous and bituminous coal range from 16 million to 24 million Btu per ton and from 19 million to 30 million Btu per ton, respectively. Anthracite contains approximately 22 million to 28 million Btu per ton.
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