Coal the heat and pressure of deep burial causes the loss of water, methane and carbon dioxide and an increase in the proportion of carbon. Thus first lignite (also called "brown coal"), then sub-bituminous coal, bituminous coal, and lastly anthracite (also called "hard coal" or "black coal") may be formed. The wide, shallow seas of the Carboniferous Period provided ideal conditions for coal formation, although coal is known from most geological periods. The exception is the coal gap in the Permian–Triassic extinction event, where coal is rare. Coal is known from Precambrian strata, which predate land plants—this coal is presumed to
Metallurgical coal Metallurgical coal Metallurgical coal is a grade of low-ash, low-sulfur and low-phosphorus coal that can be used to produce high grade coke. Coke is an essential fuel and reactant in the blast furnace process for primary steelmaking. The demand for metallurgical coal is highly coupled to the demand for steel. Primary steelmaking companies will often have a division that produces coal for coking, to ensure stable and low-cost supply. Metallurgical coal is distinguished by the strong low-density coke produced when the coal is heated in a low-oxygen environment to reduce mineral impurities (e.g. less sulfur, phosphorus). On heating, the coal