Coal

Coal is a combustible sedimentary rock formed in stratified deposits known as coal seams. Composed predominantly of carbon, with varying proportions of hydrogen, sulfur, oxygen and nitrogen, it represents one of the most important fossil fuels in human history. Coal originated from the slow geological transformation of plant matter into peat and subsequently into coal under conditions of heat and pressure over millions of years.

Geological Formation and Coalification

The process that converts dead vegetation into coal is known as coalification. During the late Carboniferous and Permian periods, large expanses of low-lying tropical wetlands—often termed coal forests—covered extensive regions of the Earth. In these oxygen-poor, waterlogged environments, plant matter accumulated and was preserved as peat rather than decomposing fully. Subsequent burial by sediment layers exposed the peat to elevated temperature and pressure over geological time, driving off water, methane and carbon dioxide while increasing the relative carbon content.
Different grades of coal formed depending on the extent of burial and thermal conditions:

• Lignite (brown coal): produced under relatively mild conditions.
• Sub-bituminous and bituminous coal: formed under higher temperatures.
• Anthracite (hard or black coal): formed under the greatest degrees of heat and pressure.

Temperature was the most significant factor in determining coal rank, with anthracite requiring exceptionally high temperatures to form. Although coal deposits are found across geological periods, approximately 90 per cent of known coal beds were formed during the Carboniferous and Permian times, coinciding paradoxically with a global icehouse phase. Glacially induced sea-level changes exposed vast continental shelves and river deltas, creating ideal environments for peat formation.
The end of this major coal-forming interval, sometimes referred to as the coal gap, followed the Permian–Triassic extinction event. Factors contributing to the earlier rapid accumulation of coal include high atmospheric oxygen levels that fostered frequent wildfires, elevated carbon dioxide concentrations stimulating plant growth and the structure of Carboniferous forests, which featured fast-growing lycophyte trees. The evolution of lignin-rich woody tissues also played a role; for millions of years fungi and bacteria lacked the enzymes required to effectively decompose lignin, allowing woody material to accumulate and be preserved. This changed when lignin-degrading fungal enzymes evolved roughly 200 million years ago, reducing the later prevalence of large-scale coal deposits.
Coal also occurs in Precambrian rocks predating land plants, where the deposits are interpreted to originate from algal material. Coal seams are often associated with cyclothems, sedimentary cycles linked to glacially driven sea-level fluctuations that alternately exposed and flooded low-lying environments.

Chemical Transformations During Coalification

The organic components of peat—principally cellulose, hemicellulose and lignin—undergo profound chemical changes as coal matures. Modern peat is dominated by lignin, with only small to moderate amounts of cellulose and hemicellulose. Through carbonization, the oxygen and hydrogen content of the material decreases as water, carbon dioxide and methane are expelled.
Key reactions include:

• Dehydration, which removes water molecules and begins the reduction of oxygen and hydrogen content.
• Decarboxylation, which releases carbon dioxide and lowers oxygen levels.
• Demethanation, which removes hydrogen-rich methane during later stages of maturation.

As coal progresses toward higher ranks, its molecular structure shifts from aliphatic chains to increasingly aromatic and polyaromatic rings, leading to structures resembling graphite. Physical changes accompany these chemical transformations, such as decreasing pore size and increasing carbon density.

Macerals and Coal Composition

Coal contains distinct organic constituents known as macerals, which are analogous to minerals in inorganic rocks. These derive from different parts of the original plant material and vary in appearance, chemical behaviour and reactivity. The proportion of macerals influences coal properties such as combustion efficiency, coking potential and industrial applicability.
Bituminous coal typically contains around 84 per cent carbon, with small amounts of hydrogen, oxygen, nitrogen and sulfur. The low oxygen content indicates extensive carbonization, reflecting the degree to which coal has progressed along the coalification spectrum.

Historical Use and Industrial Significance

Coal has been used as a fuel for thousands of years, though large-scale exploitation began with the Industrial Revolution. The introduction of steam engines greatly increased coal demand, propelling industrial growth, urbanisation and the development of heavy industry. Even in the early twenty-first century, coal remains a major global energy source.
In 2020 coal provided roughly one-quarter of the world’s primary energy and over one-third of global electricity generation. It is also used in metallurgical processes such as iron and steel production. China is the largest consumer and importer of coal, accounting for nearly half of worldwide production, followed by India. Major coal exporters include Indonesia, Australia and Russia.
Global coal consumption in 2022 reached approximately 8.3 billion tonnes, reflecting persistent dependence on the fuel despite widespread recognition of its environmental impacts.

Environmental Impacts and Climate Considerations

Coal mining and combustion have substantial environmental consequences. The extraction process can cause habitat destruction, soil degradation and water pollution, while the burning of coal releases particulate pollutants linked to respiratory illness and premature death.
Coal is the single largest anthropogenic source of carbon dioxide emissions. In 2020 approximately fourteen billion tonnes of CO₂ were emitted from coal combustion, representing about 40 per cent of emissions from fossil fuels and over one-quarter of total global greenhouse gas emissions. This makes coal a central focus in international climate policy and energy transition strategies.
Many countries have implemented coal phase-out programmes aimed at reducing or eliminating coal-fired power generation. The United Nations has urged governments to cease the construction of new coal-fired power plants as part of global decarbonisation efforts.

Etymology

The English word coal derives from Old English col, rooted in the Proto-Germanic term kulan and ultimately from the Proto-Indo-European root meaning “live coal”. Related words appear across the Germanic languages, and the Irish cognate stems from the same ancient linguistic origin.

Factors Influencing Coal Formation and Distribution

Several geological and biological factors contributed to the extensive coal beds of the late Paleozoic era:

• Climate: The central Pangean Mountains created a monsoonal climate with year-round precipitation conducive to peat preservation.
• Fire regimes: Elevated oxygen levels promoted widespread wildfires and charcoal formation, which resisted decay.
• Biological evolution: The delayed evolution of lignin-degrading organisms allowed plant material to accumulate faster than it could be decomposed.
• Tectonics and sea-level fluctuations: Glacial cycles repeatedly flooded and exposed continental shelves, generating the alternating layers observed in cyclothems.