Coal is a combustible sedimentary rock formed from ancient plant material that was buried, compressed, and chemically altered over millions of years. It remains one of the most widely used fuels on the planet, generating a substantial share of the world’s electricity while simultaneously being the single largest source of carbon dioxide emissions from the energy sector. Understanding coal means understanding how it forms, why it comes in different grades, what happens when we burn it, and why phasing it out has proven so politically and economically difficult.
How Coal Forms
Coal begins as dead plant matter in waterlogged environments. When trees, ferns, and reeds die in swamps where the water table stays high and oxygen is scarce, the plant material does not fully decompose. Instead of breaking down into gases and water as it would in open air, the organic matter accumulates in a reducing environment that favors preservation, gradually building up thick layers of peat.1Elsevier. Chapter 5 Diagenes of Coal (Coalification) Over geological time, sediment buries the peat. Rising pressure and heat from the overlying rock squeeze out moisture and concentrate the carbon content, transforming the peat into progressively harder and more energy-dense forms of coal. This transformation, called coalification, can span tens of millions of years.
Ranks of Coal
Not all coal is the same. Geologists classify it into ranks based on how far along the coalification process has gone, and rank determines nearly everything about how the coal behaves when you try to burn it.
At the low end sits lignite, sometimes called brown coal. Lignite still retains high moisture and ash content and delivers relatively low heat per kilogram. It formed under moderate burial pressure and has not been squeezed hard enough to drive off most of its water. At the high end is anthracite, or hard coal, which formed under the intense pressures found in folded rock strata during mountain-building events. Anthracite has the highest heating value and the lowest moisture and ash of any coal type.2Elsevier. An overview of coal rank influence on ignition and combustion phenomena at the particle level Between these extremes lie sub-bituminous and bituminous coals, which account for the bulk of coal burned in power plants worldwide.
Rank also influences the coal’s chemical makeup beyond simple carbon content. Trace elements like molybdenum, chromium, selenium, and vanadium can be concentrated in certain coals, often bound to the organic matter itself rather than sitting in mineral grains.3Fuel. Modes of occurrence of highly-elevated trace elements in superhigh-organic-sulfur coals The sulfur content varies dramatically too. Some coals are loaded with pyrite, an iron sulfide mineral, and the trace elements arsenic, nickel, and iron tend to tag along with it.4International Journal of Coal Geology. Concentration and distribution of trace elements in some coals from Northern China High-sulfur coals produce more sulfur dioxide when burned and create bigger headaches for mine sites after they close, a problem discussed below.
How Coal Is Mined
Coal extraction falls into two broad categories: underground mining and surface mining. Underground mines use tunnels to reach coal seams buried deep beneath the surface. Surface mining strips away the overlying rock and soil to expose shallower seams. The choice depends on how deep the coal sits and on local geology.
The most environmentally controversial form of surface mining is mountaintop removal, practiced extensively in the Appalachian region of the eastern United States. Upper-elevation forests are cleared, topsoil is stripped, and explosives break up rock to reach the buried coal. The leftover rock, called spoil, is pushed into adjacent valleys, burying streams underneath.5Science. Mountaintop mining consequences Research in Central Appalachia has shown that these changes to stream hydrology, channel shape, and water quality cause severe impairment of stream ecological health.6Frontiers in Water. Mountaintop removal coal mining impacts on structural and functional indicators in Central Appalachian streams
Even after a mine closes, the environmental damage can continue for decades. When pyrite in mine waste reacts with water and air, it produces acid mine drainage, an acidic runoff loaded with dissolved metals. Acid mine drainage is considered one of the major pollution concerns worldwide associated with mining.7PubMed. A review: The formation, prevention, and remediation of acid mine drainage The pyrite embedded in coal itself is often more vulnerable to atmospheric oxidation than standalone pyrite minerals, which can make the acid problem worse at coal sites specifically.8International Journal of Coal Science & Technology. Aqueous oxidation of coal-associated pyrite and standard pyrite mineral towards understanding the depyritization kinetics and acid formations
Burning Coal for Electricity
Most coal mined globally goes into power stations, where it is crushed and burned to produce steam that drives turbines. A conventional coal plant converts roughly a third of the fuel’s energy into electricity, with the rest lost as waste heat. More advanced designs push that efficiency higher. Ultra-supercritical plants, which operate at higher temperatures and pressures, improve boiler efficiency by about two percentage points and boost overall power generation efficiency by roughly seven percentage points compared to conventional units, while also cutting carbon emissions per unit of power produced.9Applied Thermal Engineering. Comprehensive energy efficiency analysis of ultra-supercritical thermal power units
Even at its most efficient, coal-fired electricity is the most carbon-intensive form of large-scale power generation. A harmonized review of life-cycle studies found that coal plants emit a median of about 1,000 grams of CO₂-equivalent per kilowatt-hour, with an interquartile range spanning roughly 890 to 1,130 grams.10Journal of Industrial Ecology. Life Cycle Greenhouse Gas Emissions of Coal‐Fired Electricity Generation For comparison, life-cycle emissions from natural gas are about a third lower than coal’s.11PubMed. Life-cycle greenhouse gas emissions of shale gas, natural gas, coal, and petroleum
Air Pollution Beyond CO₂
Carbon dioxide gets the most attention, but coal combustion also releases sulfur dioxide, nitrogen oxides, and particulate matter. Sulfur dioxide contributes to acid rain and respiratory problems; nitrogen oxides feed smog and ground-level ozone. The amount of each pollutant depends heavily on the coal type. Bituminous coals with high sulfur content produce far more sulfur dioxide than low-sulfur lignites, though the gap in nitrogen oxide emissions between the two is smaller.12Fuel. Curtailing the generation of sulfur dioxide and nitrogen oxide emissions by blending and oxy-combustion of coals One strategy explored in research is co-firing high-sulfur bituminous coal with alkali-rich lignite. The lignite ash acts as a natural sulfur sorbent, absorbing sulfur dioxide beyond what a simple proportional blend would suggest.13Fuel. Curtailing the generation of sulfur dioxide and nitrogen oxide emissions by blending and oxy-combustion of coals
Health Effects on Miners and Nearby Communities
The health toll of coal falls hardest on the people who dig it out of the ground. Inhaling coal mine dust causes a spectrum of lung diseases collectively known as coal mine dust lung disease, which includes coal workers’ pneumoconiosis (black lung), silicosis, mixed-dust pneumoconiosis, dust-related diffuse fibrosis, and chronic obstructive pulmonary disease.14PubMed Central. Respiratory Diseases Caused by Coal Mine Dust A systematic review covering more than 650,000 coal miners across Asia found that about 9% had black lung disease, roughly four times the global average. Risk factors included years of dust exposure, age, smoking, the type and size of the mine, and weak occupational health oversight.15Safety and Health at Work. Black Lung Disease Among Coal Miners in Asia: A Systematic Review
Communities near coal power plants also face health risks. A study comparing adults living near a coal-burning plant to a control group found that those near the plant were about five times as likely to report respiratory symptoms, roughly two and a half times as likely to report gum problems, and over three times as likely to report skin symptoms.16PubMed. Health symptoms among adults living near a coal-burning power plant While a single study like this does not prove causation on its own, it fits a broader pattern in the epidemiological literature linking coal plant emissions to respiratory and other health complaints.
Methane Leaking from Coal Mines
Coal seams trap methane, a potent greenhouse gas, within their structure. When mining disturbs the seam, that methane escapes. What surprises many people is that the problem does not end when a mine shuts down. Projections suggest that by 2100, methane emissions from the growing number of abandoned mines could increase by a factor of eight, rising faster than emissions from active operations. Even under aggressive mitigation scenarios, abandoned mines continue leaking methane through the century.17Journal of Cleaner Production. Global methane emissions from coal mining to continue growing even with declining coal production
For active mines, the scale of methane release can be tricky to pin down. Satellite measurements of one underground mine in the United States found methane emissions averaging about 64 kilograms per hour, yet the mine fell below the federal reporting threshold and did not report any emissions that year.18ACS EST Air. Methane Emissions from Underground Coal Mines That gap between actual emissions and what gets reported is a recurring theme in greenhouse gas accounting for the coal sector. When U.S. federal lease coal production has been analyzed over time, fugitive methane emissions account for a small percentage of total life-cycle greenhouse gases, around 1.3% on average, but even small percentages of a very large number add up.19Earth’s Future. Waning Greenhouse Gas Emissions From U.S. Federal Lease Coal Production by the Mid‐21st Century
Carbon Capture and “Clean Coal” Technologies
The phrase “clean coal” usually refers to technologies designed to reduce coal’s emissions rather than to any property of the coal itself. The most talked-about approach is carbon capture and storage, which strips CO₂ from a plant’s exhaust and pumps it underground. The core challenge is the energy penalty: capturing the CO₂ requires a lot of energy, which means the plant has to burn more coal to produce the same amount of electricity. Early reviews pegged the efficiency penalty at about 10 percentage points, with roughly two-thirds of that penalty coming from the capture process itself.20Applied Energy. A review of efficiency penalty in a coal-fired power plant with post-combustion CO2 capture A reassessment found the penalty could be substantially larger, ranging from about 11 to 23 percentage points depending on how the steam requirements of the capture unit are met, which would push electricity costs up by roughly five to eight cents per kilowatt-hour over uncontrolled plants.21PubMed. Reassessing the Efficiency Penalty from Carbon Capture in Coal-Fired Power Plants
Integrated gasification combined cycle, or IGCC, represents an alternative design philosophy. Instead of burning coal directly, an IGCC plant gasifies it into a synthetic gas (syngas), cleans the syngas, and then burns it in a gas turbine. Simulations of IGCC plants with sorbent-based CO₂ capture have shown net efficiencies approaching 46%, well above what conventional capture-equipped plants achieve.22Applied Energy. Economic assessment of integrated coal gasification combined cycle with sorbent CO2 capture Even without carbon capture, optimized IGCC designs have achieved thermal efficiencies around 45% with CO₂ emissions of about 698 kilograms per megawatt-hour and very low sulfur dioxide output.23Computers & Chemical Engineering. Integrated gasification combined cycle (IGCC) process simulation and optimization The catch is cost and complexity. Few commercial-scale IGCC plants operate worldwide, and the economics have generally not kept pace with the rapidly falling costs of wind and solar.
Coal Beyond Electricity
Power generation dominates coal consumption, but the rock has other industrial lives. Coking coal, a form of bituminous coal with specific properties, is essential for steelmaking. When heated in the absence of air, it becomes coke, which serves as both a fuel and a chemical reducing agent in blast furnaces. There is currently no widely deployed substitute for coke in conventional steel production, which is why coking coal occupies a different economic niche from thermal coal used in power plants.
Coal can also be converted into liquid fuels through a process called coal liquefaction. Indirect liquefaction gasifies the coal to produce syngas, which is then converted into fuels like methanol and dimethyl ether over catalysts.24Energy for Sustainable Development. Synthetic fuel production by indirect coal liquefaction Direct liquefaction dissolves coal in a solvent under heat and pressure to yield liquid hydrocarbons. When end-use efficiency is factored in, the two approaches wind up with comparable overall system efficiencies, but they differ in environmental performance. Some indirect liquefaction-derived fuels can outperform crude-oil-derived fuels on both air-pollutant and greenhouse gas emissions, while direct-liquefaction fuels cannot make that claim.25Energy for Sustainable Development. A comparison of direct and indirect liquefaction technologies for making fluid fuels from coal
A more recent area of interest is recovering rare earth elements from coal fly ash, the fine particulate residue left after combustion. Rare earths are critical for electronics, magnets, and green-energy technologies, and fly ash concentrates them during burning. Some of the highest rare earth contents show up in fly ash from Appalachian Basin coals, particularly those influenced by volcanic ash input. But extraction remains difficult. Leaching studies show that eastern U.S. coal fly ash, despite being richer in rare earths, tends to give them up less readily than western U.S. samples.26USGS Publications Warehouse. Rare Earth Elements in coal fly ash and their potential recovery Turning fly ash into a commercial rare earth source still requires advances in concentration and purification technology.
Coal’s Role in Industrialization
Coal’s influence on human civilization is hard to overstate. The relationship between coal deposits and urban growth during the Industrial Revolution was not coincidental. An economic analysis of European cities from 1750 to 1900 found that proximity to coal had a strong influence on city population size from 1800 onward, with the estimated coal effect explaining roughly 60% of European urban population growth over that period.27National Bureau of Economic Research. Coal and the European Industrial Revolution Coal powered the steam engines, iron furnaces, and railways that defined industrialization, and the geography of coal deposits shaped which cities boomed and which stayed small.
Why Phasing Out Coal Is So Complicated
If coal is the dirtiest major fuel, why is it still so widely used? The short answer is economics, infrastructure, and politics. A systematic review of coal phase-out experiences around the world found that most transitions are driven by rising coal production costs, falling prices for alternative energy, or local concerns about air pollution.28Environmental Research Letters. Coal transitions—part 1: a systematic map and review of case study learnings from regional, national, and local coal phase-out experiences Policymakers typically rely on regulatory tools to push the transition forward and compensation schemes or investment plans to manage the social fallout.
That social fallout is real and unevenly distributed. In regions where coal mining is a major employer, mine closures can devastate local economies. Research from a coal city in Inner Mongolia, China, illustrates the disparities. Workers at state-owned mines received job reallocation and retirement plans. Workers at private mines, many of whom were rural migrants, were laid off without adequate compensation or government assistance. The government did not consider supporting private coal workers to be its responsibility, a gap rooted in broader structural inequalities between rural and urban populations.29Energy Research & Social Science. Political economy of just transition: Disparate impact of coal mine closure on state-owned and private coal workers in Inner Mongolia, China Treating all coal workers as a single group, the researchers argue, masks these inequalities and creates a false sense of fairness.
Global Trade and Geopolitical Disruptions
Coal is one of the most heavily traded dry bulk commodities, and the shipping networks that carry it are surprisingly fragile. An analysis of global dry bulk shipping found that coal trade networks are highly concentrated around a small number of key export ports, with strong core-periphery structures. External shocks hit coal trade hard in unexpected ways: the COVID-19 pandemic triggered a major reorganization of coal trade communities, reshuffling who bought from whom in ways that outlasted the initial disruption.30Transportation Research Part E: Logistics and Transportation Review. Trade dynamics of the global dry bulk shipping network Geopolitical events like sanctions and trade disputes can similarly redirect coal flows, stranding some exporters while enriching others.
Climate modeling of coal phase-out scenarios adds another layer. Whether countries pursue a swift exit from coal or delay action until after 2030 matters enormously for near-term emissions but less so for cumulative emissions by mid-century, since delayed scenarios tend to require steeper cuts later. The near-term mitigation costs of phasing out coal are generally lower for pathways that start early, though long-term costs converge across most scenarios.31Environmental Research Letters. Coal transitions—part 2: phase-out dynamics in global long-term mitigation scenarios In practical terms, starting the transition sooner spreads the economic adjustment over a longer period and avoids the whiplash of abrupt change.