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 gasification has multiple beneficial uses such as aggregate in concrete, aggregate in asphalt for road construction, grit in abrasive blasting, roofing granules, etc. CO is of paramount importance in global climate change. Ash is formed in gasification from inorganic impurities in the coal. Some of these impurities react to form microscopic solids which can be suspended in the syngas produced by gasification. Typically coal contains anywhere from 0.2 to 5 percent sulfur by dry weight, which converts to HS and COS in the gasifiers due to the high temperatures and low oxygen levels. These "acid gases" are removed from the syngas