Coal (mineral resource)

Coal is a sedimentary rock composed of altered plant debris. Its principal uses are for fueling steam power plants, as a source of coke for smelting metals, and for space heating and industrial process heating. Synthetic gas and oil can be manufactured from coal on a large scale.

Organic Matter

Coal is a heterogeneous mixture of large, complex, organic molecules. It is mostly carbon but contains significant amounts of hydrogen, nitrogen, sulfur, and water. Coal is derived from plant debris that accumulated as peat (plant remains in which decay and oxidation have ceased). When covered by sediments, peat begins to lose its water and more volatile organic compounds. It also compacts and progressively becomes more chemically stable. Thereafter, peat may successively alter to lignite, bituminous coal, anthracite, or graphite, as deeper burial, deformation of the earth’s crust, and igneous intrusion increase temperature and pressure.

Enzymes, insects, oxygen, fungi, and bacteria convert plant debris to peat. If left unchecked, they can quickly destroy the deposit. Thus, permanent accumulation is limited to situations where oxygen is excluded and accumulated organic waste products prevent further decay. Rapid plant growth, deposition in stagnant water, and cold temperatures promote peat accumulation. Bacteria, the principal agents of decay, operate under a wide range of acidity and aeration; eventually, they remove oxygen and raise acidity so that decay stops.

Peat is a mixture of degraded plant tissue in humic acid jelly. All protoplasm, chlorophyll, and oil have decayed. Carbohydrates have been seriously attacked: First, starch, then cellulose, and finally lignin are destroyed. Epidermal tissue, seed coats, pigments, cuticles, spore and pollen coats, waxes, and resins are most durable, but they occur in relatively small amounts. Thus, peat is dominated by lignin, the most resistant carbohydrate, with an enhanced proportion of durable tissues.

Types of Coal

The degree to which coal has been chemically modified after burial is called its rank. Lignites range from brown coal, which closely resembles peat but has been buried, to black or dark brown lignite, which is similar to higher-ranking coal. Lignite is partially soluble in ammonia. Its resins and waxes dissolve in organic solvents. Water content is high, and there generally is less than 78 percent carbon and more than 15 percent oxygen on an ash-free basis. Woody structure may be obvious and well preserved. Lignite yields less than 8,300 British thermal units (Btu) per pound.

Further compression and heating progressively convert lignite to subbituminous coal. Fibrous, woody structure gradually disappears, color darkens, the coal becomes denser and harder, water content goes down, and carbon content increases. There is a pronounced decrease in alkali solubility and susceptibility to oxidation. Subbituminous coal, ranging from 8,300 to 13,000 Btu per pound, still weathers significantly and is subject to spontaneous combustion. Like lignite, subbituminous coal burns to powdery ash.

Bituminous coals range from 46 to 86 percent fixed carbon and from 11,000 to about 15,000 Btu per pound. They burn to fused or “agglomerating” ash, resist weathering, and do not spontaneously ignite. Anthracite ranges from 86 to 92 percent fixed carbon, having lost almost all water and volatiles. Additionally, it is nonagglomerating, and heating values are about 12,500 to 15,000 Btu per pound.

Unlike most rocks, which are formed from minerals, coal is said to be formed from “macerals”—recognizable varieties of plant debris. Just as minerals can occur as families of numerous specific minerals, macerals occur as “group macerals” that in turn are composed of specific materials. Coal is composed of the group macerals vitrain, a shiny black material with a glassy luster; durain, a dull black, granular material; clairain, a laminated, glossy black material; and fusain, a dull black, powdery material. Bright coal is dominated by vitrain. Banded coal, which is the most abundant, is dominated by clairain. Dull coals are mostly durain, and fusain is referred to as mineral charcoal. Microscopic study reveals that macerals are themselves composed of numerous other materials. Materials derived from woody or cortical tissues are called vitrinite or fusinite. Vitrinite is dominant in vitrain and the “bright” laminae of clairain. Fusinite characterizes durain and the “dull” laminae in clairain. Other macerals include exinite, coalified spores and plant cuticles; resinite, fossil resin and wax; sclerotinite, fungal sclerotia; and alginite, fossil algal remains. Micrinite is unidentified vegetal material. The exact chemical composition of coal is hard to determine because the large, complex, organic molecules in coal break down under attempts to separate them and likewise under analysis. The molecular composition of many derivative molecules, however, is known.

Mineral Matter

Mineral matter in coal includes all admixed minerals as well as inorganic elements in the coal itself. The organic elements that form the organic matter in the coal—carbon, hydrogen, oxygen, nitrogen, and sulfur—also occur in compounds, such as iron sulfide, which are part of the mineral matter. Ash is altered mineral matter that remains after the coal is burned and is not synonymous with “mineral matter.” Carbonates such as calcite (calcium carbonate) lose their carbon dioxide. Sulfides such as pyrite (iron sulfide) break down to yield sulfur dioxide. Clay minerals lose their water and are altered drastically in molecular structure. The minerals and inorganic elements in the coal react with one another when the coal is burned to produce an ash of mixed oxides, silicates, and glass.

Clays are the most abundant inorganic minerals in coal. Some clay is washed into the coal swamp, but much arises from chemical reactions occurring in the peat and coal during and after coalification. Sulfides generally are half as abundant as clay, with the iron sulfides—pyrite and marcasite—being the most widespread. Sulfides of zinc and lead also may be abundant. Pyrite and marcasite may originate during plant decay and coalification as hydrogen sulfide generated from organic sulfur combines with iron. Hydrogen sulfide also may result from the decay of marine organisms as areas of swamp are invaded by the sea, thus producing more pyrite. Coals associated with marine rocks generally have higher sulfur content than those coals from wholly alluvial deposits. Carbonates of calcium (calcite), iron (siderite), and magnesium (dolomite) generally are half as abundant as sulfides. Quartz is ubiquitous, ranging from small amounts to as much as one-fifth of the mineral matter. More than thirty additional minerals have been noted as abundant or common in coal.

Trace elements such as zinc, cadmium, mercury, copper, lead, arsenic, antimony, and selenium are associated with sulfides. Others, such as aluminum, titanium, potassium, sodium, zirconium, beryllium, and yttrium, are associated with mineral grains washed into the swamp. Still others find their way into the peat within plant tissues or later are concentrated from waters circulating through either the peat swamp or the coal seam. These elements include germanium, beryllium, gallium, titanium, boron, vanadium, nickel, chromium, cobalt, yttrium, copper, tin, lanthanum, and zinc. Coal ash has been a source of germanium and vanadium, and both uranium and barium have been mined from some coal seams. Selenium, gallium, zinc, and lead occurrences in coal have been investigated as possible sources of these metals, and—given the harm caused by their release in coal mining and coal combustion—the recovery of these metals in pollution control may be feasible.

Current Coal Formation

Peats form today in two very different environments. Poor drainage in areas of recent glaciation, coupled with low temperature, facilitates peat formation in high latitudes. In warm temperate and tropical regions that are poorly drained, vigorous forests may produce peat. Good examples also can be found in coastal plains and shoreline deposits such as the Dismal Swamp and Everglades, alluvial plains such as the Mississippi Delta, and tropical alluvial plains such as the upper Amazon Basin. Most of the high-latitude peat is unlikely to be incorporated in major sedimentary accumulations and, therefore, is unlikely to become coal. Other modern peats, however, are an extension of the sort of peat accumulation that occurred in the geologic past and provide a guide to understanding coal formation.

Coal can form in two major settings. Autochthonous coal formed where plants grew and died and was transported by rivers to deltas and coastal lagoons. The Red River of Louisiana once contained a good example of an allochthonous coal environment in the Great Raft, a vast tangle of floating vegetation that completely covered the surface of the river. The Great Raft no longer exists, as it was broken up to open the river for navigation during the nineteenth century.

Individual coal beds or seams may be very widespread or of limited extent. The Illinois #2 Coal, for example, is recognizable from western Kentucky to northeastern Oklahoma. Other coal beds cover only a few square kilometers. Coals range from a few millimeters to more than 100 meters thick. They generally are tabular but may be interrupted by filled stream channels or rolls, which are protuberances of overlying rock that apparently sagged into the coal while the original peat was still in a soft state. Thin layers of clay (splits) and cracks filled with clay (clay veins) interfere with mining and dilute mined coal with extraneous rock. Coal beds may be subhorizontal or steeply inclined, depending on deformation in the area. They also may be continuous or offset by faults—fractures of the crust along which movement has occurred. Inclination or interruption of the bed interferes with mining.

Despite a high energy content, anthracite coal is used less frequently than coals of lower rank. Because it has been metamorphosed, anthracite frequently occurs in folded rocks that make mining difficult and expensive. It also burns at such a high temperature its use requires special furnaces. If anthracite is metamorphosed still further, it becomes graphite, which is useless as a fuel but has numerous industrial uses. Contrary to popular misconception, diamonds do not form from the heating of coal. Coal is never buried deeply enough to produce diamond; the carbon in diamonds comes from the earth’s mantle.

Testing

The distribution and character of coal beds are determined by standard field geologic methods. Surface exposures are plotted on maps, and their geometric orientation is recorded. Thereafter, coal beds are projected geometrically into the subsurface. Wherever possible, their position is verified in wells and mine shafts so that the full regional extent, depth, and attitude of the coal are illustrated. If detailed information is required for mining, specially drilled test holes will be utilized to locate channel fillings and faults interrupting the coal, and to determine changes in thickness and quality. These test holes make it possible to plan mining efficiently. In addition, the relationship of the coal to rocks above and below will be investigated so that mining methods may be adjusted to potential geologic hazards, such as caving roofs and incursion of underground water.

Standardized, practical tests define quality and/or suitability for specific uses. Burning coal samples under controlled conditions at 750 degrees Celsius produces a standard ASTM International ash, which defines the total ash content and its nature. Coals with low-ash content are preferred. The character of the ash—agglomerating produces a glassy clinker, and nonagglomerating produces a powder—determines the type of grate on which the coal can be burned. The heating value of the coal, figured on an ash-free basis, is determined by controlled combustion in a calorimeter: a closed “bomb” fitted with temperature sensors. The amount and quality of volatile materials are analyzed by carbonizing the coal at a standard temperature in a closed vessel and measuring the amount and kinds of substances driven out. These data are required to classify the coal according to rank and grade, and to fix its value in the market.

Analysis

More precise analytical techniques and tools are employed in research, as opposed to routine coal testing. The physical composition of the coal may be determined by microscopic study of thin sections (slices of coal mounted on glass and reduced to a thickness that allows for the transmission of light), or by examining polished surfaces with a reflecting microscope. In this way, the components of the coal may be distinguished and examined separately. More detailed study may utilize either transmission or scanning electron microscopy. Coal rank also may be determined very precisely by means of measuring reflectance—in this case, the amount of light reflected from polished vitrinite.

Mineral matter in the coal, as distinguished from ash, may be examined directly by these techniques but also may be recovered from the coal employing a low-temperature asher. The asher is an electronic device that vaporizes combustible materials without significantly raising the temperature of the sample. In this way, clays, carbonates, sulfides, and other minerals that are significantly altered by heat are delivered for study in their original state.

The elemental chemical analysis of coal provides information as to its composition at a level of limited value to coal investigators. The organic compounds in coal, however, suffer substantial alteration under most analytical techniques. The volatiles recovered by coal carbonization in the absence of oxygen may be separated by distillation, but they are not the compounds originally present in the coal. Solid organic materials may be selectively dissolved and the resultant materials subjected to organic analysis, but, again, these materials are not the ones that were present in the original coal. In spite of these limitations, coal investigators gather some conception of the original chemistry of the coal and develop information applicable to coal utilization. X-ray diffraction and other spectroscopic techniques have begun to uncover the structure of coal molecules without altering them. Concepts of coal molecular structure, however, remain rudimentary.

Fuel Source

Coal is a major source of heat energy and a significant source of organic compounds of practical use—from drugs to plastics. It fueled the Industrial Revolution and, therefore, is responsible for modern industrial society. Coal was the principal source of energy until World War I, after which its use declined under competition from oil and natural gas. In 2022, coal provided approximately one third of the world’s electricity, based on International Energy Agency data. Practices such as mountaintop removal and increased automation have facilitated coal production while also reducing the number of workers required. In contrast to oil and gas, the reserves of which are more limited and for which production is consequently expected to peak sooner, coal reserves appear to be abundant. High-quality coal will become increasingly difficult to access, however. Analysts anticipate that with lower-cost shale oil and gas and renewable alternatives such as solar energy available, coal consumption will drop. Nonetheless, in 2022, global coal-fired generation reached an all-time high.

Challenges

In spite of its large reserves, coal presents several significant problems. Coal is a solid fuel and inappropriate for use in domestic heating and vehicular transport. Therefore, scientists undertook substantial research into converting coal into liquid and gaseous fuel; however, the high cost of commercial production prevented synthetic oil and gas from competing successfully with low-cost traditional oil and gas production.

Furthermore, coal combustion produces gaseous and solid wastes that must be managed. Sulfides are converted to metallic oxides and sulfur dioxide, which combine with water in the atmosphere to produce acid rain. This precipitation kills plants and destroys aquatic life. The carbon in coal, as well as in wood or hydrocarbons, combines with oxygen to form carbon dioxide when burned. Enough carbon dioxide has already been produced by burning wood, coal, and hydrocarbons to change the global atmospheric composition. As a result, the global climate is changing, even though ultimate results are not yet predictable. Heavy metals, such as lead, cadmium, arsenic, and mercury, are also released in small amounts into either the atmosphere or in ash. Mine hazards—gas and cave-ins especially—are geologically controlled. The subsidence of underground mines—in extent, timing, and ultimate cost to surface values—is geologically controlled as well. Additional concerns include strip-mining reclamation and mine waters entering both surface and subsurface water supplies.

Coal mining also poses human health hazards: emphysema or black lung disease among longtime miners; cancers among gasification or coke workers exposed to coal tar; and asthma, bronchitis, emphysema, or pneumonia from sulfur dioxide and nitrogen oxide emissions.

Principal Terms

British thermal unit (Btu): the amount of heat required to raise the temperature of one pound of water by 1 degree Fahrenheit at the temperature of maximum density for water (39 degrees Fahrenheit or 4 degrees Celsius)

cellulose: the substance forming the bulk of plant cell walls

fixed carbon: the solid, burnable material remaining after water, ash, and volatiles have been removed from coal

humic acid: organic matter extracted by alkalis from peat, coal, or decayed plant debris; it is black and acidic but unaffected by other acids or organic solvents

lignin: a family of compounds in plant cell walls, composed of an aromatic nucleus, a side chain with three carbon atoms, and hydroxyl and methoxyl groups, and the molecule that binds cellulose fibers together

molecule: the smallest entity of an element or compound retaining chemical identity with the substance in mass

organic molecules: molecules of carbon compounds produced in plants or animals, plus similar artificial compounds

volatiles: substances in coal that are capable of being gasified

Bibliography

Averitt, P. “Coal.” United States Mineral Resources, U.S. Geological Survey Professional Paper 820, pubs.usgs.gov/bul/1412/report.pdf. Accessed 28 Feb. 2025.

"Coal." International Energy Agency, 2025, www.iea.org/energy-system/fossil-fuels/coal. Accessed 28 Feb. 2025.

Dalverny, Louis E. Pyrite Leaching from Coal and Coal Waste. U.S. Department of Energy, 1996.

Evans Pritchard, Ambrose. "International Energy Agency Sees 'Peak Coal' as Demand for Fossil Fuel Crumbles in China." The Telegraph, 19 Dec. 2015, www.telegraph.co.uk/finance/economics/12058456/IEA-sees-peak-coal-as-demand-crumbles-in-China.html. Accessed 28 Feb. 2025.

Freese, Barbara. Coal: A Human History. Basic Books, 2016.

Galloway, William E., and David K. Hobday. Terrigenous Clastic Depositional Systems Applications to Fossil Fuel and Groundwater Resources. Reprint 2d ed., Springer Berlin, 2013.

Gayer, Rodney A., and Jierai Peesek, editors. European Coal Geology and Technology. Geological Society, 2007.

Heinberg, Richard. Blackout: Coal, Climate and the Last Energy Crisis. Gabriola Island, B.C., 2009.

Inman, Mason. "Mining the Truth on Coal Supplies." National Geographic, 9 Sept. 2010, www.nationalgeographic.com/history/article/100908-energy-peak-coal. Accessed 28 Feb. 2025.

Keefer, Robert F., and Kenneth S. Sajwan. Trace Elements in Coal and Coal Combustion Residues. Lewis Publishers, 1993.

Qi Ye, and Jiaqi Lu. "The End of Coal-Fired Growth in China." Brookings, 4 Aug. 2016, www.brookings.edu/blog/up-front/2016/08/04/the-end-of-coal-fired-growth-in-china. Accessed 28 Feb. 2025.

Stewart, W. N. Paleobotany and the Evolution of Plants. 2d ed., Cambridge UP, 2010.

Swaine, Dalway J. Trace Elements in Coal. Butterworth, 1990.

Thomas, Larry. Coal Geology. 3rd ed., John Wiley & Sons, 2020.

U.S. Department of Labor, Mine Safety and Health Administration. Coal Mining. U.S. Department of Labor, Mine Safety and Health Administration, 1997.