Coal
Coal is a combustible black or brownish-black sedimentary rock, formed as layers called coal seams. Coal is mostly carbon with variable amounts of other elements, chiefly hydrogen, sulfur, oxygen, and nitrogen. It is a fossil fuel, formed when plants decay into peat which is converted into coal by the heat and pressure of deep burial over millions of years. Vast deposits formed from wetlands called coal forests that covered much of the tropics during the late Carboniferous and early Permian.
Coal is used primarily as a fuel. While coal has been known and used for thousands of years, its usage was limited until the Industrial Revolution. With the invention of the steam engine, coal consumption increased. In 2020, coal supplied about a quarter of the world's primary energy and over a third of its electricity. Some iron and steel-making and other industrial processes burn coal.
The extraction and burning of coal damages the environment and human health, causing premature death and illness, and is the largest source of carbon dioxide contributing to climate change. Over fifteen billion tonnes of carbon dioxide were emitted by burning coal in 2024, which was more than a quarter of total global greenhouse gas emissions.As part of worldwide energy transition, many countries have reduced or eliminated their use of coal power. The United Nations Secretary General asked governments to stop building new coal plants by 2020.
A record amount of coal was burnt in 2024, but consumption is expected to peak before 2030. To meet the Paris Agreement target of keeping global warming below coal use needs to halve from 2020 to 2030, and "phasing down" coal was agreed upon in the Glasgow Climate Pact.
The largest consumer and importer of coal is China, which mines almost half the world's coal, followed by India with about a tenth. Indonesia and Australia export the most, followed by Russia.
Etymology
The word originally took the form col in Old English, from reconstructed Proto-Germanic *kula, from Proto-Indo-European root *g''u-lo-'' "live coal". Germanic cognates include the Old Frisian kole, Middle Dutch cole, Dutch kool, Old High German chol, German Kohle and Old Norse kol. Irish gual is also a cognate via the Indo-European root.Formation of coal
The conversion of dead vegetation into coal is called coalification. At various times in the geologic past, the Earth had dense forests in low-lying areas. In these wetlands, the process of coalification began when dead plant matter was protected from oxidation, usually by mud or acidic water, and was converted into peat. The resulting peat bogs, which trapped immense amounts of carbon, were eventually deeply buried by sediments. Then, over millions of years, the heat and pressure of deep burial caused the loss of water, methane and carbon dioxide and increased the proportion of carbon. The grade of coal produced depended on the maximum pressure and temperature reached, with lignite produced under relatively mild conditions, and sub-bituminous coal, bituminous coal, or anthracite coal produced in turn with increasing temperature and pressure.Of the factors involved in coalification, temperature is much more important than either pressure or time of burial. Subbituminous coal can form at temperatures as low as while anthracite requires a temperature of at least.
Although coal is known from most geologic periods, 90% of all coal beds were deposited in the Carboniferous and Permian periods. Paradoxically, this was during the Late Paleozoic icehouse, a time of global glaciation. However, the drop in global sea level accompanying the glaciation exposed continental shelves that had previously been submerged, and to these were added wide river deltas produced by increased erosion due to the drop in base level. These widespread areas of wetlands provided ideal conditions for coal formation. The rapid formation of coal ended with the coal gap in the Permian–Triassic extinction event, where coal is rare.
Favorable geography alone does not explain the extensive Carboniferous coal beds. Other factors contributing to rapid coal deposition were high oxygen levels, above 30%, that promoted intense wildfires and formation of charcoal that was all but indigestible by decomposing organisms; high carbon dioxide levels that promoted plant growth; and the nature of Carboniferous forests, which included lycophyte trees whose determinate growth meant that carbon was not tied up in heartwood of living trees for long periods.
One theory suggested that about 360 million years ago, some plants evolved the ability to produce lignin, a complex polymer that made their cellulose stems much harder and more woody. The ability to produce lignin led to the evolution of the first trees. But bacteria and fungi did not immediately evolve the ability to decompose lignin, so the wood did not fully decay but became buried under sediment, eventually turning into coal. About 300 million years ago, mushrooms and other fungi developed this ability, ending the main coal-formation period of earth's history. Although some authors pointed at some evidence of lignin degradation during the Carboniferous, and suggested that climatic and tectonic factors were a more plausible explanation, reconstruction of ancestral enzymes by phylogenetic analysis corroborated a hypothesis that lignin degrading enzymes appeared in fungi approximately 200 MYa.
One likely tectonic factor was the Central Pangean Mountains, an enormous range running along the equator that reached its greatest elevation near this time. Climate modeling suggests that the Central Pangean Mountains contributed to the deposition of vast quantities of coal in the late Carboniferous. The mountains created an area of year-round heavy precipitation, with no dry season typical of a monsoon climate. This is necessary for the preservation of peat in coal swamps.
Coal is known from Precambrian strata, which predate land plants. This coal is presumed to have originated from residues of algae.
Sometimes coal seams are interbedded with other sediments in a cyclothem. Cyclothems are thought to have their origin in glacial cycles that produced fluctuations in sea level, which alternately exposed and then flooded large areas of continental shelf.
Chemistry of coalification
The woody tissue of plants is composed mainly of cellulose, hemicellulose, and lignin. Modern peat is mostly lignin, with a content of cellulose and hemicellulose ranging from 5% to 40%. Various other organic compounds, such as waxes and nitrogen- and sulfur-containing compounds, are also present. Lignin has a weight composition of about 54% carbon, 6% hydrogen, and 30% oxygen, while cellulose has a weight composition of about 44% carbon, 6% hydrogen, and 49% oxygen. Bituminous coal has a composition of about 84.4% carbon, 5.4% hydrogen, 6.7% oxygen, 1.7% nitrogen, and 1.8% sulfur, on a weight basis. The low oxygen content of coal shows that coalification removed most of the oxygen and much of the hydrogen a process called carbonization.Carbonization proceeds primarily by dehydration, decarboxylation, and demethanation. Dehydration removes water molecules from the maturing coal via reactions such as
Decarboxylation removes carbon dioxide from the maturing coal:
while demethanation proceeds by reaction such as
In these formulas, R represents the remainder of a cellulose or lignin molecule to which the reacting groups are attached.
Dehydration and decarboxylation take place early in coalification, while demethanation begins only after the coal has already reached bituminous rank. The effect of decarboxylation is to reduce the percentage of oxygen, while demethanation reduces the percentage of hydrogen. Dehydration does both, and reduces the saturation of the carbon backbone.
As carbonization proceeds, aliphatic compounds convert to aromatic compounds. Similarly, aromatic rings fuse into polyaromatic compounds. The structure increasingly resembles graphene, the structural element of graphite.
Chemical changes are accompanied by physical changes, such as decrease in average pore size.
Macerals
Macerals are coalified plant parts that retain the morphology and some properties of the original plant. In many coals, individual macerals can be identified visually. Some macerals include:- vitrinite, derived from woody parts
- lipinite, derived from spores and algae
- inertite, derived from woody parts that had been burnt in prehistoric times
- huminite, a precursor to vitrinite.
Types
As geological processes apply pressure to dead biotic material over time, under suitable conditions, its metamorphic grade or rank increases successively into:- Peat, a precursor of coal
- Lignite, or brown coal, the lowest rank of coal, most harmful to health when burned, used almost exclusively as fuel for electric power generation
- 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. Known as steam coal in the UK, and historically used to raise steam in steam locomotives and ships
- Anthracite coal, the highest rank of coal, is a harder, glossy black coal used primarily for residential and commercial space heating.
- Graphite, a difficult to ignite coal that is used mostly in pencils, or in powdered form for lubrication.
- Cannel coal, a variety of fine-grained, high-rank coal with significant hydrogen content, that consists primarily of liptinite. It is related to boghead coal.
Hilt's law is a geological observation that the deeper the coal is found, the higher its rank. It applies if the thermal gradient is entirely vertical; however, metamorphism may cause lateral changes of rank, irrespective of depth. For example, some of the coal seams of the Madrid, New Mexico coal field were partially converted to anthracite by contact metamorphism from an igneous sill while the remainder of the seams remained as bituminous coal.