Uses of coal

Where Found

While coal has been found on every principal continent of the Earth, regional distribution is restricted to and metamorphosed sedimentary terrains of Upper Devonian age and younger (that is, the last 365 million years of geologic history). As a result of this geologic association, most of the coal of the world are found in the Northern Hemisphere continents of Asia, Europe, and North America. However, there are reasonably large reserves in Australia, South Africa, and Colombia.

Primary Uses

Coal was historically used as a domestic fuel for the heating of homes, and more than 26 percent of the coal mined globally is a primary source of energy. Worldwide, 41 percent of coal mined is burned in power plants, principally for the generation of electricity. Another significant use is in the manufacture of coke, an improved carbon-content derivative of coal employed in the production of steel. Lesser amounts of coal are used in the direct heating of homes, for a variety of industrial purposes, and, increasingly, in liquefaction and gasification processes whereby coal is converted to liquid and gaseous forms of hydrocarbon fuels.

Technical Definition

Coal is a general term encompassing a variety of combustible sedimentary and rocks containing altered and fossilized terrestrial plant remains in excess of 50 percent by weight and more than 70 percent by volume. Categories of coal differ in relative amounts of moisture, volatile matter, fixed carbon, and degree of compaction of the original carbonaceous material. Coal is commonly termed a fossil fuel. Categories of coal include peat (a coal precursor), lignite, bituminous coal, subbituminous coal, and anthracite.

Peat. Peat, an unconsolidated accumulation of partly decomposed plant material, has an approximate carbon content of 20 percent. In many classification schemes, peat is listed as the initial stage of coal formation. Moisture content is quite high, at least at the 75 percent level. When dry, peat has an oxygen content of about 30 percent, is flammable, and will freely but inefficiently burn slowly and steadily for months at a low-heat-content value of 5,400 British thermal units (Btus) per pound.

Lignite. Lignite, or brown coal, is brownish-black in color, banded and jointed, and subject to spontaneous combustion. Carbon content ranges from 25 to 35 percent. With a moisture content around 40 percent, it will readily disintegrate after drying in the open air. Because lignite has a maximum calorific value of 8,300 Btus, it is classed as a low-heating-value coal.

Bituminous coal. Deeper burial with even higher temperatures and pressures gradually transforms lignite into bituminous coal, a dense, dusty, brittle, well-jointed, dark brown to black fuel that burns readily with a smoky yellow flame. Calorific value ranges from 10,500 to 15,500 Btus per pound, and carbon content varies from 45 to 86 percent. Moisture content is as low as 5 percent, but heating value is high.

Subbituminous coal. The subbituminous class of coal is intermediate between lignite and bituminous and has characteristics of both. Little woody matter is visible. It splits parallel to bedding but generally lacks the jointing of bituminous coal. It burns clean but with a relatively low heating value.

Anthracite. Anthracite is jet black in color, has a high luster, is very hard and dust free, and breaks with a conchoidal fracture. Carbon content ranges from 86 to 98 percent. It is slow to ignite; burns with a short blue flame without smoke; and, with a calorific value in excess of 14,000 Btus per pound, is a high heating fuel.

Description, Distribution, and Forms

Coal is a found on all seven continents and is in commercial production on all but the continent of Antarctica. The top-ten producers of coal in 2008 were China, the United States, Australia, India, South Africa, Russia, Indonesia, Poland, Kazakhstan, and Colombia. In 1992, a reserve of more than 185 billion metric tons of lignite was discovered in Pakistan, but the cost of production and lack of infrastructure have prevented development or accurate analysis of the field.

The quantitative distribution of coal is more difficult to determine than its geographic distribution. Estimates indicate that total world coal resources, defined as coal reserves and other deposits that are not economically recoverable plus inferred future discoveries, are on the order of 9 trillion metric tons and exist in every country. Of this amount, estimates of world coal reserves, defined as those deposits that have been measured, evaluated, and can be extracted profitably under existing technologic and economic conditions, are approximately 900 billion metric tons and are found in about seventy countries. If the latter figure is accepted as reasonable, world reserves can be divided into two categories, with about three-quarters composed of anthracite and bituminous coals and about one-quarter composed of lignite. When the lignite reserves in Pakistan can be reliably analyzed, these figures will change.

On a country-to-country comparison, the United States possesses the greatest amount of total world coal reserves. Geographic distribution in the United States is divided into five coal provinces, incorporating at least thirty-three states. These are termed the Appalachian or Eastern, the Interior, the Gulf, the Rocky Mountain, and the Northern Great Plains coal provinces.

The Eastern (Appalachian) province, stretching along the flanks of the Appalachian Mountains from northern Pennsylvania into central Georgia, contains approximately 40 percent of the bituminous coal reserves of the United States as well as the principal reserve of anthracite rank coal on the continent. Within the Interior province, bituminous coals are divided among the Michigan, Illinois, and Western Interior basins, the latter located in Iowa, Missouri, Kansas, and Oklahoma. Lignite is the chief coal found in the Gulf province, situated in Mississippi, northern Louisiana, and coastal Texas. Rocky Mountain bituminous and subbituminous deposits are scattered throughout at least five states from Wyoming south into Arizona and New Mexico. Lignite and bituminous coals constitute the Northern Great Plains province of Montana and portions of North and South Dakota.

Coal is mined in twenty-one states, but ten states alone contain 90 percent of the total US reserves. These are, in order of increasing reserve tonnage, Indiana, Texas, Colorado, Ohio, Kentucky, Pennsylvania, West Virginia, Wyoming, Illinois, and Montana. Montana contains a full 25 percent of US coal reserves.

Small reserves of relatively low-grade coal are known in the Pacific Northwest region. Significant amounts of coal have been discovered in Alaska, but difficulty in mining and great distance to markets cause these deposits to be classed as resources and not economic reserves.

Following the United States, the countries with the largest coal reserves are Russia, China, India, Australia, and South Africa. Estimates indicate that global coal reserves will last some 130 to 150 years at current production levels, although by some methods of calculation that period is significantly shorter.

The more common and ordinary coals are of vascular vegetable origin, formed from the compaction and induration of accumulated remains of plants that once grew in extensive swamp and coastal marsh areas. These deposits are classed as humic coals consisting of organic matter that has passed through the peat, or earliest coal formation, stage. A variety of humic coals are known.

The swamp-water environment in which humic coals form must be deficient in dissolved oxygen, the presence of which would ordinarily cause decay of the plant tissue. Under such near-stagnant conditions plant remains are preserved, while the presence of hydrogen sulfide discourages the presence of organisms that feed on dead vegetation. Analog environments under which coal is presently forming are found within the Atchafalaya swamp of coastal Louisiana and the many peat-producing regions of Ireland. A layer of peat in excess of 2 meters in thickness and covering more than 5,000 square kilometers is present in the Dismal Swamp of coastal North Carolina and Virginia.

The sapropelic class of coal, relatively uncommon in distribution and composed of fossil algae and spores, is formed through partial decomposition of organic matter by organisms within oxygen-deficient lakes and ponds. Sapropelic coals are subdivided into boghead (algae origin) and cannel (spore origin) deposits.

The vegetable origin of coal has been accepted since 1825 and is convincingly evidenced by the identification of more than three thousand freshwater plant species in coal beds of Carboniferous (360 to 286 million years ago) age. The common association of root structures and even upright stumps with layers of coal indicates that the parent plant material grew and accumulated in place.

Detailed geologic studies of rock sequences that lie immediately above and below coal deposits indicate that most coals were formed in coastal regions affected by long-term sea-level cycles characterized by transgressing (advancing) and regressing (retreating) shorelines. Such a sequence of rock deposited during a single advance and retreat of the shoreline, termed a “cyclothem,” typically contains nonmarine strata separated from overlying marine strata by a single layer of coal. In sections of the Interior coal province, a minimum of fifty cyclothems have been recognized, some of which can be traced across thousands of square kilometers. Such repetition in a rock sequence is most advantageous to the economics of a coal region, creating a situation in which a vertical mine shaft could penetrate scores of layers of coal.

The formation of coal is a long-term geologic process. Coal cannot therefore be considered a renewable resource, even though it is formed from plant matter. Studies have suggested that 1 meter of low-rank coal requires approximately ten thousand years of plant growth, accumulation, biologic reduction, and compaction to develop. Using these time lines, the 3-meter-thick Pittsburgh coal bed, underlying 39,000 square kilometers of Pennsylvania, developed over a period of thirty thousand years, while the 26-meter-thick bed of coal found at Adaville, Wyoming, required approximately one-quarter of a million years to develop.

Coal formation favors conditions under which plant growth is abundant and conditions for organic are favorable. Such climates range from subtropical to cold, with the ideal being classed as temperate. Tropical swamps produce an abundance of plant matter but very high bacterial activity, resulting in low production of peat. Modern peats are developing in temperate to cold climate regions, such as Canada and Ireland, where abundant precipitation ensures fast plant growth while relatively cool temperatures diminish the effectiveness of decay-promoting bacteria.

The first coal provinces began to form with the evolution of cellulose-rich land plants. One of the earliest known coal deposits, of Upper Devonian age (approximately 365 million years ago), is found on Buren Island, Norway. Between the Devonian period and today every geologic period is represented by at least some coal somewhere in the world. Certain periods of time, however, are significant coal-forming ages.

During the Carboniferous and Permian periods (360 to 245 million years ago) widespread development of fern and scale tree growth set the stage for the formation of the Appalachian coal province and the coal districts of the United Kingdom, Russia, and Manchuria. Coal volumes formed during these periods of geologic time constitute approximately 65 percent of present world reserves. The remaining reserves, developed mainly over the past 200 million years, formed in swamps consisting of angiosperm (flowering) plants. The reserves of the Rocky Mountain province and those of central Europe are representative of these younger coals.

After dead land-plant matter has accumulated and slowly begun to compact, biochemical decomposition, rising temperature, and rising pressure all contribute to the lengthy process of altering visible plant debris into various ranks of coal. With the advent of the Industrial Revolution there was a need for a system of classification defining in detail the various types of coals. Up to the beginning of the nineteenth century, coal was divided into three rudimentary classes, determined by appearance: bright coal, black coal, and brown coal. Through the decades, other schemes involving various parameters were introduced, including oxygen content, percent of residue remaining after the burning of coal, ratio of carbon to volatile matter content, or analysis of fixed carbon content and calorific value (heat-generating ability).

In 1937, a classification of coal rank using fixed carbon and Btu content was adopted by the American Standards Association. Adaptations of this scheme are still in use, listing the steps of progressive increase in coal rank as lignite (brown coal), subbituminous, bituminous (soft coal), subanthracite, and anthracite (hard coal). Some classification schemes also list peat as the lowest rank of coal. (Technically speaking, peat is not a coal; rather, it is a fuel and a precursor to coal.)

Coalification is the geologic process whereby plant material is altered into differing ranks of coal by geochemical and diagenetic change. With an increase in rank, chemical changes involve an increase in carbon content accompanied by a decrease in hydrogen and oxygen. Correspondingly, diagenesis involves an increase in and calorific value, and a progressive decrease in moisture. At all ranks, common impurities include sulfur, silt and particles, and silica.

US reserves are found mainly in eleven northeastern counties in Pennsylvania. Subanthracite coal has characteristics intermediate between bituminous and anthracite.

Bedded and compacted coal layers are geologically considered to be rocks. Lignite and bituminous ranks are classed as organic sedimentary rocks. Anthracite, formed when bituminous beds of coal are subjected to the folding and regional deformation affiliated with mountain building processes, is listed as a metamorphic rock. Because peat is not consolidated or compacted, it is classed as an organic sediment. Graphite, a naturally occurring crystalline form of almost pure carbon, is occasionally associated with anthracite. While it can occur as the result of high-temperature alteration of anthracite, its chemical purity and common association with crystalline rock causes it to be listed as a mineral.

History

Considering the importance of coal to modern society, it is somewhat surprising that the production of this commodity played only a minor role in pre-Industrial Revolution (that is, prior to the middle eighteenth century) history. The origins of coal use date back at least several thousands of years, as evidenced by the discovery of flint axes embedded in layered coal in central England. These primitive tools have been attributed to Neolithic (New Age, c. 6000-2000 BCE) open-pit mining. The Chinese were acquainted early with the value of coal, using it in the making of porcelain. Coal cinders found in Roman-era walls in association with implements of similar age suggest the use of coal for heating purposes prior to the colonization of England by the Saxons.

The philosopher Theophrastus (c. 372-287 BCE), noted as the academic successor to Aristotle and the author of many studies on plants, called coal anthrax, a Greek word later used in the naming of anthracite coal. Later, the Anglo-Saxon term col, probably derived from the Latin caulis, meaning plant stalk, evolved into “cole” prior to the emergence of the modern spelling some three centuries ago.

With the decline of forests in England by the thirteenth century, coal began to assume a significant role. The first coal-mining charter was granted the freemen of Newcastle in 1239. This early burning of coal, however, because of its propensity to befoul the atmosphere, was banned in 1306 by King Edward I. King Edward III reversed this ban and again granted the Newcastle freemen a coal-mining license, whereby this town soon became the center of the first important coal-mining district.

Coal mining was initiated in North America near Richmond, Virginia, in 1748. A decade later, coal-mining activities had moved to the rich deposits around Pittsburgh, Pennsylvania. The spread of the Industrial Revolution, invention of the iron-smelting process, and improvement of the steam engine guaranteed the classification of coal as an industrial staple. With the development of the steam-driven electric generator in the last decade of the nineteenth century, coal became the dominant fuel. A century later, world coal production exceeded 4.5 billion metric tons and constituted some 26 percent of world energy production on a Btu basis. In the early twenty-first century, with the rapid growth of the Chinese economy, China passed the United States as the top producer of coal.

Obtaining Coal

Coal has been produced by two common methods: underground (or deep) mining and surface (or strip) mining. Underground mining requires the digging of extensive systems of tunnels and passages within and along the coal layers. These openings are connected to the surface so the coal can be removed. Prior to the development of the gigantic machinery necessary to open-pit mining, deep mining was the industry norm. This early period was characterized by labor-intensive pick-and-shovel work in cramped mine passages. Constant dangers included the collapse of ceilings and methane gas explosions.

Today, augers and drilling machinery supplement manpower to a large extent, and mine safety and health regulations have greatly reduced the annual death toll. The common method of underground extraction involves initial removal of about 50 percent of the coal, leaving a series of pillars to support the mine roof. As reserves are exhausted, the mine is gradually abandoned after removal of some or all of the pillars. Another modern underground-mining technique, with a coal removal rate approaching 100 percent, involves the use of an integrated rotary cutting machine and conveyer belt.

Surface mining of coal, accounting for about 40 percent of global production, is a multiple-step process. First, the material must be removed, allowing exposure of the coal. The coal is then mined by means of various types of surface machinery, ranging from bulldozers to gigantic power shovels. Finally, after removal of all the coal, the overburden is used to fill in the excavated trench and the area is restored to its natural topography and vegetation. Economics usually determine whether underground or open-pit techniques are preferable in a given situation. Generally, if the ratio of overburden to coal thickness does not exceed twenty to one, surface mining is more profitable.

In the Appalachian coal province, coal-mining technique is closely related to geology. In tightly folded regions of West Virginia, the steeply dipping coal beds are mostly mined underground. To the northwest, folds become gentler, and both deep- and surface-mining methods are used. In the Interior province, is the most common process. In the Rocky Mountain area, where many thick coal beds lie close to the surface, strip mining again predominates, although a few underground mines are present.

With increased concern regarding the state of the natural Earth environment, and with federal passage of the Coal Mine Health and Safety Act (1969) and the Clean Air Acts (US), the mining of coal in the United States has undergone both geographic and extraction-technology changes. Because the Rocky Mountain province coals, while lower grade than eastern coals, contain lower percentages of sulfur, the center of US production has gradually shifted westward. The burning of high-sulfur coals releases sulfur dioxide into the atmosphere; it is a significant contributor to acid rain.

Western coals are often contained within layers thicker than those found in the east, are shallow in depth, and can be found under large areas—all conditions amenable to surface mining. As a result, the state of Wyoming, with a 1995 production of 240 million metric tons of low-sulfur coal that is burned in more than twenty-four states in the generation of electricity, became the leading US coal producer.

Coal mining has played an integral role in the development of the industrialized world, and this role should continue well into the future. Reserve additions continue to closely equal losses due to mining, and at current levels of production estimates indicate that there is enough recoverable coal globally for some 130 to 150 years of future production.

Uses of Coal

Historically, coal has been industry’s fuel of choice. Those countries in possession of sufficient coal reserves have risen commercially, while those less endowed with this resource—or lacking it altogether—have turned to agriculture or stagnated in development. The top exporters of coal are Australia, Indonesia, Russia, Colombia, South Africa, China, and the United States. The top importers are Japan, South Korea, Taiwan, India, the United Kingdom, China, and Germany.

Different ranks of coal are employed for different purposes. In the middle of the twentieth century, it was common to see separate listings of coking, gas, steam, fuel, and domestic coals. Each had its specific uses. Domestic coal could not yield excessive smoke, while coal for locomotives had to raise steam quickly and not produce too high an ash content. Immediately after World War II, fuel coal use in the United States, representing 78 percent of annual production, was divided into steam raising (29 percent), railway transportation (23 percent), domestic (17 percent), electric generation (6 percent), and bunker coal (3 percent). The remaining 22 percent was employed in the production of pig iron (10 percent), steel (7 percent), and gas (5 percent). Fifty years later, more than 80 percent of the approximately 900 million metric tons of coal produced annually in the United States was used in the generation of electricity. Industrial consumption of coal, particularly in the production of coke for the steel and iron manufacturing industry, is the second most important use. Globally, 13 percent of hard coal production is used by the steel industry. Some 70 percent of global steel production depends on coal. Additional industrial groups that use coal include food processing, paper, glass, cement, and stone. Coal produces more energy than any other fuel, more than natural gas, crude oil, nuclear, and renewable fuels.

The drying of malted barley by peat fires has long been important in giving Scotch whiskey its smoky flavor. Peat has also been increasingly employed as a soil conditioner. While expensive to produce, the conversion of intermediate ranks of coal into liquid (coal oil) and gaseous (coal gas) forms of hydrocarbon fuels will become more economically viable, especially during times of increase in the value of crude oil and reserves.

New uses of coal are constantly being explored and tested. Two promising techniques are the mixing of water with powdered coal to make a that can be burned as a liquid fuel and the underground extraction of coal-bed methane (firedamp). Interest in the latter by-product as an accessible and clean-burning fuel is especially high in Appalachian province localities distant from conventional gas resources.

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U.S. Department of Energy

Coal.