Coal

• 1. Introduction

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  Coal is a combustible black or brownish-black sedimentary rock, and in 2020 it supplied about a quarter of the world's primary energy and over a third of its electricity. In 2024 a record amount of it was burnt. That same year, burning it released over fifteen billion tonnes of carbon dioxide, more than a quarter of all global greenhouse gas emissions. This is a substance with two faces. One face powered the steam engine and the Industrial Revolution. The other is named the largest single source of the carbon dioxide driving climate change. How does decayed plant matter become a black rock that warms homes and the planet at once? Why did almost all of it form during just two stretches of deep time? And why, with consumption still climbing, do forecasters expect it to peak before the decade is out? The answers run from prehistoric swamps to a settlement on top of Landek Hill, and from a brazier on a Roman altar to loss-making power stations in China.

• 2. Born In Ancient Swamps

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  About 90% of all coal beds were laid down in the Carboniferous and Permian periods, and the process began in low-lying wetlands the source calls coal forests. Dead plant matter was protected from oxidation by mud or acidic water and turned into peat. Buried deep by sediment over millions of years, that peat lost its water, methane and carbon dioxide, and its proportion of carbon climbed. The grade depended on the maximum pressure and temperature reached. Lignite, also called brown coal, formed under mild conditions; with rising heat came sub-bituminous coal, then bituminous, then anthracite, the hard black coal. Of the factors involved, temperature matters far more than pressure or the time spent buried.

  The great coal age coincided, paradoxically, with the Late Paleozoic icehouse, a time of global glaciation. The drop in sea level exposed continental shelves once underwater, and erosion built wide river deltas. These vast wetlands were ideal for coal. High oxygen levels above 30% fed wildfires that made charcoal indigestible to decomposers, while high carbon dioxide promoted plant growth. Carboniferous forests included lycophyte trees whose determinate growth meant carbon was not locked in living heartwood for long.

  One theory holds that about 360 million years ago some plants evolved the ability to make lignin, a polymer that hardened their cellulose stems into wood and gave rise to the first trees. Bacteria and fungi could not yet break lignin down, so the wood was buried rather than decayed, becoming coal. About 300 million years ago fungi developed that ability, ending the main era of coal formation. Some researchers point instead to climatic and tectonic causes, yet phylogenetic reconstruction of ancestral enzymes supports lignin-degrading fungi appearing roughly 200 million years ago.

  The Central Pangean Mountains, an enormous range running along the equator, reached their greatest elevation near this time. Climate modeling suggests they helped deposit vast quantities of late Carboniferous coal by creating year-round heavy precipitation with no dry season, the condition needed to preserve peat in coal swamps. Coal even appears in Precambrian strata that predate land plants, presumed to have come from the residues of algae.

• 3. The Chemistry Of Carbon

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  Bituminous coal has a composition, on a dry, ash-free weight basis, of about 84.4% carbon, 5.4% hydrogen, 6.7% oxygen, 1.7% nitrogen, and 1.8% sulfur. That low oxygen figure tells the story of how coal forms, because coalification strips away most of the oxygen and much of the hydrogen in a process called carbonization. By contrast, the lignin in its precursor wood was about 54% carbon, 6% hydrogen and 30% oxygen.

  Carbonization runs along three main reactions. Dehydration removes water molecules, decarboxylation removes carbon dioxide, and demethanation removes methane. In the source's formulas, R stands for the remainder of a cellulose or lignin molecule. Dehydration and decarboxylation happen early; demethanation begins only after the coal has reached bituminous rank. As the process advances, aliphatic compounds convert to aromatic ones, and aromatic rings fuse into polyaromatic structures that increasingly resemble graphene, the building block of graphite.

  Within coal, coalified plant parts called macerals keep the shape and some properties of the original plant. Vitrinite derives from woody parts, lipinite from spores and algae, and inertite from woody parts burnt in prehistoric times. Huminite is a precursor that coalification replaces with shiny vitrinite. The sulfur content can vary from less than 1% to as much as 4%, and most of it is bound into organosulfur compounds within the hydrocarbon matrix. Some coal also carries inorganic sulfur as iron pyrite, a dense mineral that can be removed by froth flotation.

• 4. Ranks And Varieties

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  Anthracite is the highest rank of coal, a harder, glossy black coal used mainly for residential and commercial space heating. Below it sit bituminous coal, often banded with bright and dull material and used to make coke; sub-bituminous coal, used chiefly for steam-electric power; and lignite, the lowest rank and the most harmful to health when burned. The fuel value rises with carbon content, so anthracite leads, followed by bituminous, then lignite and brown coal. Some anthracite deposits hold pure carbon as graphite, a coal so hard to ignite it ends up in pencils or as a powdered lubricant.

  The most important practical split is between thermal coal, burnt to generate electricity via steam, and metallurgical or coking coal, burnt at high temperature to make steel. Cannel coal, sometimes called candle coal, is a fine-grained, high-rank variety rich in hydrogen and made mostly of liptinite. Bituminous coal was known as steam coal in the UK, where it once raised steam in locomotives and ships.

  Hilt's law captures a tidy pattern: within a small area, the deeper the coal lies, the higher its rank. It holds when the thermal gradient is vertical, but metamorphism can shift rank sideways regardless of depth. In the Madrid coal field of New Mexico, some seams were turned to anthracite by contact with an igneous sill while the rest stayed bituminous.

• 5. From Stone Age To Steam

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  The oldest intentional use of black coal was documented at Ostrava, Petřkovice, in an older Stone Age settlement on top of Landek Hill. Radiocarbon dating places that site between 25,000 and 23,000 years BC. In China, Neolithic inhabitants near Shenyang were carving ornaments from black lignite by 4000 BC, and coal from the Fushun mine was used to smelt copper as early as 1000 BC. Marco Polo, who traveled to China in the 13th century, described coal as black stones that burn like logs and noted it was so plentiful people could take three hot baths a week.

  In Britain, outcrop coal was used during the Bronze Age, between 3000 and 2000 BC, where it formed part of funeral pyres. The Romans exploited coal across the major coalfields of England and Wales by the end of the second century AD. Coal cinders turn up in the hearths of villas and forts in Northumberland, dated to around AD 400. At Aquae Sulis, modern Bath, writers marveled at a permanent brazier of coal kept burning on the altar of Minerva.

  Coal came to be called seacoal in the 13th century, because so much of it washed up on the shore from exposed seams. The London wharf where it arrived was named Seacoal Lane in a charter granted by King Henry III in 1253. Between 1257 and 1259, coal from Newcastle upon Tyne was shipped to London for the smiths and lime-burners building Westminster Abbey. As easy surface sources ran low, miners turned to shaft mining and adits, and the alternative name pitcoal marked coal that came from mines.

  A wholesale reliance on coal for the home hearth probably did not exist until a switch in fuels happened in London in the late sixteenth and early seventeenth centuries. Historian Ruth Goodman has traced the socioeconomic effects of that switch and argued its importance in shaping the industrial adoption of coal has been underappreciated. In 1700, five-sixths of the world's coal was mined in Britain, and without it the country would have run out of suitable watermill sites by the 1830s. In 1947 some 750,000 miners worked in Britain, yet the last deep coal mine in the UK closed in 2015.

• 6. What Coal Becomes

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  In 2022-68% of global coal use went to electricity generation, where thermal coal is pulverized and burned in a furnace with a boiler. The heat turns boiler water to steam, the steam spins turbines, and the turbines turn generators. The thermodynamic efficiency runs between about 25% and 50%, depending on treatment, turbine technology and the plant's age. A few integrated gasification combined cycle plants gasify the coal into syngas first, burning it more cleanly; when used for combined heat and power, overall efficiency can reach as much as 94%.

  Coke is the carbonaceous residue at the heart of steelmaking. Metallurgical coal is baked in an oven without oxygen at temperatures as high as 1,000 degrees Celsius, driving off volatiles and fusing the fixed carbon and ash. In a blast furnace the carbon monoxide from burning coke reduces hematite to iron, and grey, porous coke carries a heating value of 29.6 megajoules per kilogram. Byproducts can include coal tar, ammonia, light oils, and coal gas.

  Chemicals have come from coal since the 1940s, chiefly by gasifying it into syngas. From that syngas flow methanol, hydrogen and carbon monoxide, the building blocks for olefins, acetic acid, formaldehyde, ammonia, urea and more. Because the same products can be made from natural gas and petroleum, industry uses whichever feedstock is cheapest, so interest in coal rises when oil and gas prices climb.

  Coal can also be turned directly into synthetic gasoline or diesel by hydrogenation or carbonization, though liquefaction emits more carbon dioxide than refining crude oil. Through the Fischer-Tropsch process, syngas becomes transportation fuels, a route the Sasol chemical company of South Africa used to make fuels and chemicals from coal. State owned China Energy Investment runs a coal liquefaction plant and plans to build two more.

• 7. The Global Trade

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  China mines almost half the world's coal and is the largest consumer and importer, followed by India with about a tenth. About 8,000 megatonnes are produced annually, roughly 90% hard coal and 10% lignite, with just over half coming from underground mines. The industry employs almost 2.7 million workers. Underground mining brings more accidents than surface mining, and most deaths are thought to occur in China, where there were 375 coal mining related deaths in 2017.

  Indonesia was the largest exporter by volume in 2022, shipping 471 megatonnes for a 34% share of global exports, followed by Australia with 344 and Russia with 224. That year China, India and Japan were the biggest importers, taking in 301, 228 and 184 megatonnes. As Europe transitions to renewables and sanctions Russia over its invasion of Ukraine, Russia is steering its coal exports from Europe toward Asia.

  China used 4,520 megatonnes of coal in 2022, more than half the global total, with India, the European Union and the United States next at 1,162, 461 and 455. Because China has almost always driven the global growth in coal demand, international market trends hinge on Chinese energy policy. Metallurgical coal prices stay volatile and far above thermal coal, since coking coal must be lower in sulfur and needs more cleaning. In some countries new onshore wind or solar already costs less than power from existing coal plants, and India's new plants are uneconomic even when subsidized.

• 8. The Cost Of Burning

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  Globally coal is estimated to cause 800,000 premature deaths every year, mostly in India and China. Its smokestacks emit nitrogen oxides, sulfur dioxide, particulate pollution and heavy metals, and the deadly London smog was driven primarily by heavy coal use. Burning coal is a major source of sulfur dioxide, which forms PM2.5 particulates, the most dangerous form of air pollution. The illnesses linked to it include asthma, strokes, heart attacks, mercury poisoning and lung cancer. Annual health costs in Europe from coal-fired electricity are estimated at up to 43 billion euros.

  Breathing coal dust causes coalworker's pneumoconiosis, or black lung, named because the dust turns the lungs black. In the US alone an estimated 1,500 former coal industry employees die each year from breathing mine dust. Coal also produces hundreds of millions of tons of ash and waste, including fly ash that holds mercury, uranium, thorium and arsenic. Around 10% of coal is ash, and because it carries uranium and thorium, the United Nations Scientific Committee on the Effects of Atomic Radiation estimates coal plants release far more radioactivity than nuclear plants.

  The largest and longest-term effect is carbon dioxide. In 2016 world gross carbon dioxide emissions from coal reached 14.5 gigatonnes, and coal-fired plants were the single largest contributor to the growth in global emissions in 2018. Each megawatt-hour of coal-fired electricity emits around a tonne of carbon dioxide, double the roughly 500 kilograms from a natural gas plant. Underground fires add to the toll: Brennender Berg has been burning since 1668 and still burns in the 21st century, while coal fires in China consume an estimated 120 million tons a year.

  The United Nations Secretary General asked governments to stop building new coal plants by 2020, and the Glasgow Climate Pact agreed to phase down coal. Since the mid-1980s the term clean coal has shifted meaning, from scrubbers that fought acid rain to carbon capture and storage. Yet between 1972 and 2017-98% of plans to add carbon capture failed by 2021, and as of 2024 it operates at only four coal power plants and one gas plant worldwide. The IPCC considers fossil fuels unabated unless interventions capture 90% or more of emissions.