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
Pulverized coal-fired boiler pre-heated combustion air and forces it out of a nozzle similar in action to fuel being atomized by a fuel injector in modern cars. Under operating conditions, there is enough heat in the combustion zone to ignite all the incoming fuel. There are two methods of ash removal at furnace bottom: The fly ash is carried away with the flue gas and is separated in various hoppers in the path and finally in an ESP or a bag filter. Pulverized coal power plants are broken down into three categories; subcritical pulverized coal (SubCPC) plants, supercritical pulverized coal (SCPC) plants, and