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 technology can sometimes be used as a supplement to postcombustion technologies to control emissions from coal-fueled boilers. Coal burnt as a solid fuel in coal power stations to generate electricity is called thermal coal. Coal is also used to produce very high temperatures through combustion. Efforts around the world to reduce the use of coal have led some regions to switch to natural gas and electricity from lower carbon sources. When coal is used for electricity generation, it is usually pulverized and then burned in a furnace with a boiler. The furnace heat converts boiler water to steam, which is