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
Coal molten metal. Scrap steel can be recycled in an electric arc furnace; and an alternative to making iron by smelting is direct reduced iron, where any carbonaceous fuel can be used to make sponge or pelletised iron. To lessen carbon dioxide emissions in future hydrogen might be used as the reducing agent and biomass or waste as the source of carbon. Coal gasification, as part of an integrated gasification combined cycle (IGCC) coal-fired power station, is used to produce syngas, a mixture of carbon monoxide (CO) and the hydrogen (H) gas to fire gas turbines to produce electricity. Syngas can