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 liquefaction well as from generation of necessary process heat and electricity inputs to the liquefaction reactors, thus releasing greenhouse gases that can contribute to anthropogenic global warming. Especially if coal liquefaction is conducted without any carbon capture and storage technologies. There are technically feasible low-emission configurations of CTL plants. High water consumption in the water-gas shift reaction or steam methane reforming is another adverse environmental effect. Pyrolysis of coal produces polycyclic aromatic hydrocarbons, which are known carcinogens. On the other hand, synthetic fuels produced by indirect coal liquefaction processes tend to be 'cleaner' than naturally occurring crudes, as heteroatom (e.g. sulfur)