Coal is a combustible black or brownish-black sedimentary rock, formed as rock strata called coal seams. Coal is mostly carbon with variable amounts of other elements; chiefly hydrogen, sulfur, oxygen, and nitrogen.
The extraction and use of coal causes many premature deaths and much illness. Coal damages the environment; including by climate change as it is the largest anthropogenic source of carbon dioxide, 14 Gtin 2016 which is 40% of the total fossil fuel emissions. As part of the worldwide energy transition many countries have stopped using or use less coal.
The largest consumer and importer of coal is China. And China mines almost half the world's coal, followed by India with about a tenth. Australia accounts for about a third of world coal exports followed by Indonesia and Russia.
At various times in the geologic past, the Earth had dense forests in low-lying wetland areas. Due to natural processes such as flooding, these forests were buried underneath soil. As more and more soil deposited over them, they were compressed. The temperature also rose as they sank deeper and deeper. As the process continued the plant matter was protected from biodegradation and oxidation, usually by mud or acidic water. This trapped the carbon in immense peat bogs that were eventually covered and deeply buried by sediments. Under high pressure and high temperature, dead vegetation was slowly converted to coal. The conversion of dead vegetation into coal is called coalification. Coalification starts with dead plant matter decaying into peat. Then over millions of years 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 CarboniferousPeriod 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 have originated from residues of algae.
Sometimes coal seams (also known as coal beds) are interbedded with other sediments in a cyclothem.
Lignite, or brown coal, the lowest rank of coal, used almost exclusively as fuel for electric power generation
Jet, a compact form of lignite, sometimes polished; used as an ornamental stone since the Upper Palaeolithic
Sub-bituminous coal, whose properties range between those of lignite and those of bituminous coal, is used primarily as fuel for steam-electric power generation.
Bituminous coal, a dense sedimentary rock, usually black, but sometimes dark brown, often with well-defined bands of bright and dull material It is used primarily as fuel in steam-electric power generation and to make coke.
Anthracite, the highest rank of coal is a harder, glossy black coal used primarily for residential and commercial space heating.
Graphite is one of the more difficult coals[citation needed] to ignite and not commonly used as fuel; it is most used in pencils, or powdered for lubrication.
Cannel coal (sometimes called "candle coal") is a variety of fine-grained, high-rank coal with significant hydrogen content, which consists primarily of liptinite.
There are several international standards for coal. The classification of coal is generally based on the content of volatiles. However the most important distinction is between thermal coal (also known as steam coal), which is burnt to generate electricity via steam; and metallurgical coal (also known as coking coal), which is burnt at high temperature to make steel.
Hilt's law is a geological observation that (within a small area) the deeper the coal is found, the higher its rank (or grade). It applies if the thermal gradient is entirely vertical; however, metamorphism may cause lateral changes of rank, irrespective of depth.
