Coal mining is the process of
extracting coal
Coal is a combustible black or brownish-black sedimentary rock, formed as rock strata called coal seams. Coal is mostly carbon with variable amounts of other elements, chiefly hydrogen, sulfur, oxygen, and nitrogen.
Coal is formed when dea ...
from the ground. Coal is valued for its
energy content
In physics, energy density is the amount of energy stored in a given system or region of space per unit volume. It is sometimes confused with energy per unit mass which is properly called specific energy or .
Often only the ''useful'' or extrac ...
and since the 1880s has been widely used to generate electricity.
Steel
Steel is an alloy made up of iron with added carbon to improve its strength and fracture resistance compared to other forms of iron. Many other elements may be present or added. Stainless steels that are corrosion- and oxidation-resistant ty ...
and
cement
A cement is a binder, a chemical substance used for construction that sets, hardens, and adheres to other materials to bind them together. Cement is seldom used on its own, but rather to bind sand and gravel ( aggregate) together. Cement mix ...
industries use coal as a fuel for extraction of iron from
iron ore
Iron ores are rocks and minerals from which metallic iron can be economically extracted. The ores are usually rich in iron oxides and vary in color from dark grey, bright yellow, or deep purple to rusty red. The iron is usually found in the fo ...
and for cement production. In the
United Kingdom
The United Kingdom of Great Britain and Northern Ireland, commonly known as the United Kingdom (UK) or Britain, is a country in Europe, off the north-western coast of the continental mainland. It comprises England, Scotland, Wales and North ...
and
South Africa
South Africa, officially the Republic of South Africa (RSA), is the southernmost country in Africa. It is bounded to the south by of coastline that stretch along the South Atlantic and Indian Oceans; to the north by the neighbouring countri ...
, a coal mine and its structures are a colliery, a coal mine is called a 'pit', and the above-ground structures are a '
pit head
A headframe (also known as a gallows frame, winding tower, hoist frame,Ernst, Dr.-Ing. Richard (1989). ''Wörterbuch der Industriellen Technik'' (5th ed.). Wiesbaden: Oscar Brandstetter, 1989. pit frame, shafthead frame, headgear, headstock o ...
'. In
Australia
Australia, officially the Commonwealth of Australia, is a Sovereign state, sovereign country comprising the mainland of the Australia (continent), Australian continent, the island of Tasmania, and numerous List of islands of Australia, sma ...
, "colliery" generally refers to an underground coal mine.
Coal mining has had many developments in recent years, from the early days of men tunneling, digging and manually extracting the coal on carts to large
open-cut
In civil engineering, a cut or cutting is where soil or rock from a relative rise along a route is removed. The term is also used in river management to speed a waterway's flow by short-cutting a meander.
Cuts are typically used in road, rail, ...
and
longwall
Longwall mining is a form of underground coal mining where a long wall of coal is mined in a single slice (typically thick). The longwall panel (the block of coal that is being mined) is typically long (but can be upto long) and wide.
Histor ...
mines. Mining at this scale requires the use of
draglines, trucks,
conveyor
A conveyor system is a common piece of mechanical handling equipment that moves materials from one location to another. Conveyors are especially useful in applications involving the transport of heavy or bulky materials. Conveyor systems allow ...
s,
hydraulic jack
A jack is a mechanical lifting device used to apply great forces or lift heavy loads. A mechanical jack employs a screw thread for lifting heavy equipment. A hydraulic jack uses hydraulic power. The most common form is a car jack, floor jack o ...
s and shearers.
The coal mining industry has a long history of significant negative
environmental impacts
Environmental issues are effects of human activity on the biophysical environment, most often of which are harmful effects that cause environmental degradation. Environmental protection is the practice of protecting the natural environment on t ...
on local ecosystems, health impacts on local communities and workers, and contributes heavily to the global environmental crises, such as
poor air quality and
climate change
In common usage, climate change describes global warming—the ongoing increase in global average temperature—and its effects on Earth's climate system. Climate change in a broader sense also includes previous long-term changes to E ...
. For these reasons, coal has been one of the first
fossil fuels to be phased out of various parts of the
global energy economy. The major coal producing countries, though, such as
China
China, officially the People's Republic of China (PRC), is a country in East Asia. It is the world's most populous country, with a population exceeding 1.4 billion, slightly ahead of India. China spans the equivalent of five time zones and ...
, Indonesia,
India
India, officially the Republic of India (Hindi: ), is a country in South Asia. It is the seventh-largest country by area, the second-most populous country, and the most populous democracy in the world. Bounded by the Indian Ocean on the so ...
and Australia, have not reached peak production, with production increases replacing falls in Europe and US
[ and proposed mines under development.][Ryan Driskell Tate, Christine Shearer, and Andiswa Matiknca]
"Deep Trouble: Tracking Global Coal Mine Proposals,"
Global Energy Monitor, June 2021
History
Methods of extraction
Coal extraction methods vary depending on whether the mine is an underground mine or a surface (also called open cast) mine. Additionally, coal seam thickness and geology are factors in the selection of a mining method. The most economical method of coal extraction for surface mines is the electric shovel or drag line. The most economical form of underground mining is the long wall, which involves using two spinning drums with carbide bits that runs along sections of the coal seam. Many coals extracted from both surface and underground mines require washing in a coal preparation plant
300px, A coal "washer" in Eastern Kentucky
A modern coal breaker in Mahanoy City, Pennsylvania combines washing, crushing, grading, sorting, stockpiling, and shipping in one facility built into a stockpile of anthracite coal below a mountain top ...
. Technical and economic feasibility are evaluated based on the following: regional geological conditions; overburden
In mining, overburden (also called waste or spoil) is the material that lies above an area that lends itself to economical exploitation, such as the rock, soil, and ecosystem that lies above a coal seam or ore body. Overburden is distinct from tai ...
characteristics; coal seam continuity, thickness, structure, quality, and depth; strength of materials above and below the seam for roof and floor conditions; topography (especially altitude and slope); climate; land ownership as it affects the availability of land for mining and access; surface drainage patterns; groundwater conditions; availability of labor and materials; coal purchaser requirements in terms of tonnage, quality, and destination; and capital investment requirements.
Surface mining and deep underground mining are the two basic methods of mining. The choice of mining method depends primarily on depth, density, overburden, and thickness of the coal seam; seams relatively close to the surface, at depths less than approximately , are usually surface mined.
Coal that occurs at depths of are usually deep mined, but in some cases surface mining techniques can be used. For example, some western U.S. coal that occur at depths in excess of are mined by the open pit methods, due to thickness of the seam . Coals occurring below are usually deep mined.[Christman, R.C., J. Haslbeck, B. Sedlik, W. Murray, and W. Wilson. 1980. ''Activities, effects and impacts of the coal fuel cycle for a 1,000-MWe electric power generating plant''. Washington, DC: U.S. Nuclear Regulatory Commission.] However, there are open pit mining operations working on coal seams up to below ground level, for instance Tagebau Hambach
The Tagebau Hambach is a large open-pit coal mine (german: Tagebau) in Niederzier and Elsdorf, North Rhine–Westphalia, Germany. It is operated by RWE and used for mining lignite.
The mine is on the site of the ancient Hambach Forest, whi ...
in Germany.
Surface mining
When coal seams are near the surface, it may be economical to extract the coal using open-cut
In civil engineering, a cut or cutting is where soil or rock from a relative rise along a route is removed. The term is also used in river management to speed a waterway's flow by short-cutting a meander.
Cuts are typically used in road, rail, ...
(also referred to as open-cast, open-pit, mountaintop removal or strip) mining methods. Opencast coal mining recovers a greater proportion of the coal deposit than underground methods, as more of the coal seams in the strata
In geology and related fields, a stratum ( : strata) is a layer of rock or sediment characterized by certain lithologic properties or attributes that distinguish it from adjacent layers from which it is separated by visible surfaces known as ei ...
may be exploited. This equipment can include the following: Draglines which operate by removing the overburden, power shovels, large trucks in which transport overburden and coal, bucket wheel excavators, and conveyors. In this mining method, explosives are first used in order to break through the surface or overburden, of the mining area. The overburden is then removed by draglines or by shovel and truck. Once the coal seam is exposed, it is drilled, fractured and thoroughly mined in strips. The coal is then loaded onto large trucks or conveyors for transport to either the coal preparation plant or directly to where it will be used.
Most open cast mines in the United States extract bituminous coal
Bituminous coal, or black coal, is a type of coal containing a tar-like substance called bitumen or asphalt. Its coloration can be black or sometimes dark brown; often there are well-defined bands of bright and dull material within the seams. It ...
. In Canada (BC), Australia and South Africa, open cast mining
Open-pit mining, also known as open-cast or open-cut mining and in larger contexts mega-mining, is a surface mining technique of extracting rock or minerals from the earth from an open-air pit, sometimes known as a borrow.
This form of mining ...
is used for both thermal
A thermal column (or thermal) is a rising mass of buoyant air, a convective current in the atmosphere, that transfers heat energy vertically. Thermals are created by the uneven heating of Earth's surface from solar radiation, and are an example ...
and metallurgical coal
Metallurgical coal or coking coal is a grade of coal that can be used to produce good-quality coke. Coke is an essential fuel and reactant in the blast furnace process for primary steelmaking. The demand for metallurgical coal is highly coupled ...
s. In New South Wales
)
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, image_map = New South Wales in Australia.svg
, map_caption = Location of New South Wales in AustraliaCoordinates:
, subdivision_type = Country
, subdivision_name = Australia
, established_title = Before federation
, es ...
open casting for steam coal and anthracite
Anthracite, also known as hard coal, and black coal, is a hard, compact variety of coal that has a submetallic luster. It has the highest carbon content, the fewest impurities, and the highest energy density of all types of coal and is the hig ...
is practiced. Surface mining accounts for around 80 percent of production in Australia, while in the US it is used for about 67 percent of production. Globally, about 40 percent of coal production involves surface mining.
Strip mining
Strip mining exposes coal by removing earth above each coal seam. This earth to be removed is referred to as 'overburden' and is removed in long strips.[Ryan Driskell Tate, "Places of Overburden: Strip Mining and Reclamation on the Northern Great Plains," ''The Greater Plains: Rethinking a Region's Environmental Histories'' (Lincoln: University of Nebraska Press, 2021).] The overburden from the first strip is deposited in an area outside the planned mining area and referred to as out-of-pit dumping. Overburden from subsequent strips is deposited in the void left from mining the coal and overburden from the previous strip. This is referred to as in-pit dumping.
It is often necessary to fragment the overburden by use of explosives. This is accomplished by drilling holes into the overburden, filling the holes with explosives, and detonating the explosive. The overburden is then removed, using large earth-moving equipment, such as dragline
A dragline excavator is a piece of heavy equipment used in civil engineering and surface mining.
Draglines fall into two broad categories: those that are based on standard, lifting cranes, and the heavy units which have to be built on-site. Mo ...
s, shovel
A shovel is a tool used for digging, lifting, and moving bulk materials, such as soil, coal, gravel, snow, sand, or ore.
Most shovels are hand tools consisting of a broad blade fixed to a medium-length handle. Shovel blades are usually made of ...
and trucks, excavator
Excavators are heavy construction equipment consisting of a boom, dipper (or stick), bucket and cab on a rotating platform known as the "house". The house sits atop an undercarriage with tracks or wheels. They are a natural progression fro ...
and trucks, or bucket-wheels and conveyors. This overburden is put into the previously mined (and now empty) strip. When all the overburden is removed, the
underlying coal seam will be exposed (a 'block' of coal). This block of coal may be drilled and blasted (if hard) or otherwise loaded onto trucks or conveyors for transport to the coal preparation (or wash) plant. Once this strip is empty of coal, the process is repeated with a new strip being created next to it. This method is most suitable for areas with flat terrain.
Equipment to be used depends on geological conditions. For example, to remove overburden that is loose or unconsolidated, a bucket wheel excavator might be the most productive. The life of some area mines may be more than 50 years.[U.S. Department of the Interior, Office of Surface Mining Reclamation and Enforcement (1987). ''Surface coal mining reclamation: 10 years of progress, 1977–1987''. Washington, D.C.: U.S. Government Printing Office.]
Contour mining
The contour mining method consists of removing overburden from the seam in a pattern following the contours along a ridge or around the hillside. This method is most commonly used in areas with rolling to steep terrain. It was once common to deposit the spoil on the downslope side of the bench thus created, but this method of spoil disposal consumed much additional land and created severe landslide and erosion problems. To alleviate these problems, a variety of methods were devised to use freshly cut overburden to refill mined-out areas. These haul-back or lateral movement methods generally consist of an initial cut with the spoil deposited downslope or at some other site and spoil from the second cut refilling the first. A ridge of undisturbed natural material wide is often intentionally left at the outer edge of the mined area. This barrier adds stability to the reclaimed slope by preventing spoil from slumping or sliding downhill.
The limitations of contour strip mining are both economic and technical. When the operation reaches a predetermined stripping ratio (tons of overburden/tons of coal), it is not profitable to continue. Depending on the equipment available, it may not be technically feasible to exceed a certain height of highwall. At this point, it is possible to produce more coal with the augering method in which spiral drills bore tunnels into a highwall laterally from the bench to extract coal without removing the overburden.
Mountaintop removal mining
Mountaintop coal mining is a surface mining practice involving removal of mountaintops to expose coal seams, and disposing of associated mining overburden in adjacent "valley fills." Valley fills occur in steep terrain where there are limited disposal alternatives.
Mountaintop removal
Mountaintop removal mining (MTR), also known as mountaintop mining (MTM), is a form of surface mining at the summit or summit ridge of a mountain. Coal seams are extracted from a mountain by removing the land, or overburden, above the seams. Thi ...
combines area and contour strip mining methods. In areas with rolling or steep terrain with a coal seam occurring near the top of a ridge or hill, the entire top is removed in a series of parallel cuts. Overburden is deposited in nearby valleys and hollows. This method usually leaves the ridge and hilltops as flattened plateaus. The process is highly controversial for the drastic changes in topography, the practice of creating ''head-of-hollow-fills'', or filling in valleys with mining debris, and for covering streams and disrupting ecosystems.
Spoil is placed at the head of a narrow, steep-sided valley or hollow. In preparation for filling this area, vegetation and soil are removed and a rock drain constructed down the middle of the area to be filled, where a natural drainage course previously existed. When the fill is completed, this underdrain will form a continuous water runoff system from the upper end of the valley to the lower end of the fill. Typical head-of-hollow fills are graded and terraced to create permanently stable slopes.
Underground mining
Most coal seams are too deep underground for opencast mining and require underground mining, a method that currently accounts for about 60 percent of world coal production. In deep mining, the room and pillar or bord and pillar method progresses along the seam, while pillars and timber are left standing to support the mine roof. Once room and pillar mines have been developed to a stopping point (limited by geology, ventilation, or economics), a supplementary version of room and pillar mining, termed second mining or retreat mining
Retreat mining is the removal of pillars in the underground mining technique known as room and pillar mining.
In the first phase of room and pillar mining, tunnels are advanced into the coal or ore body in a rectangular pattern resembling city s ...
, is commonly started. Miners remove the coal in the pillars, thereby recovering as much coal from the coal seam as possible. A work area involved in pillar extraction is called a pillar section.
Modern pillar sections use remote-controlled equipment, including large hydraulic mobile roof-supports, which can prevent cave-ins until the miners and their equipment have left a work area. The mobile roof supports are similar to a large dining-room table, but with hydraulic jacks for legs. After the large pillars of coal have been mined away, the mobile roof support's legs shorten and it is withdrawn to a safe area. The mine roof typically collapses once the mobile roof supports leave an area.
There are six principal methods of underground mining:
* Longwall mining
Longwall mining is a form of underground coal mining where a long wall of coal is mined in a single slice (typically thick). The longwall panel (the block of coal that is being mined) is typically long (but can be upto long) and wide.
Histor ...
accounts for about 50 percent of underground production. The longwall shearer has a face of or more. It is a sophisticated machine with a rotating drum that moves mechanically back and forth across a wide coal seam. The loosened coal falls onto an armored chain conveyor or pan line that takes the coal to the conveyor belt for removal from the work area. Longwall systems have their own hydraulic roof supports which advance with the machine as mining progresses. As the longwall mining equipment moves forward, overlying rock that is no longer supported by coal is allowed to fall behind the operation in a controlled manner. The supports make possible high levels of production and safety. Sensors detect how much coal remains in the seam while robotic controls enhance efficiency. Longwall systems allow a 60-to-100 percent coal recovery rate when surrounding geology allows their use. Once the coal is removed, usually 75 percent of the section, the roof is allowed to collapse in a safe manner.
* Continuous mining
Room and pillar or pillar and stall is a variant of breast stoping. It is a mining system in which the mined material is extracted across a horizontal plane, creating horizontal arrays of rooms and pillars. To do this, "rooms" of ore are dug out w ...
utilizes a Continuous Miner Machine with a large rotating steel drum equipped with tungsten carbide picks that scrape coal from the seam. Operating in a "room and pillar" (also known as "bord and pillar") system—where the mine is divided into a series of 20-to-30-foot (5–10 m) "rooms" or work areas cut into the coalbed—it can mine as much as 14 tons of coal a minute, more than a non-mechanised mine of the 1920s would produce in an entire day. Continuous miners account for about 45 percent of underground coal production. Conveyors
A conveyor belt is the carrying medium of a belt conveyor system (often shortened to belt conveyor). A belt conveyor system is one of many types of conveyor systems. A belt conveyor system consists of two or more pulleys (sometimes referred to ...
transport the removed coal from the seam. Remote-controlled continuous miners are used to work in a variety of difficult seams and conditions, and robotic versions controlled by computers are becoming increasingly common. Continuous mining is a misnomer, as room and pillar coal mining is very cyclical. In the US, one can generally cut up to around . This may be increased with MSHA permission. In South Africa, the limit may be as high as . After the cutting limit is reached, the continuous miner assembly is removed and the roof is supported by the use of a roof bolter, after which the face has to be serviced before it can be advanced again. During servicing, the "continuous" miner moves to another face. Some continuous miners can bolt and rock dust the face (two major components of servicing) while cutting coal, while a trained crew may be able to advance ventilation, to truly earn the "continuous" label. However, very few mines are able to achieve it. Most continuous mining machines in use in the US lack the ability to bolt and dust. This may partly be because the incorporation of bolting makes the machines wider, and therefore, less maneuverable.
* Room and pillar mining
Room and pillar or pillar and stall is a variant of breast stoping. It is a mining system in which the mined material is extracted across a horizontal plane, creating horizontal arrays of rooms and pillars. To do this, "rooms" of ore are dug out ...
consists of coal deposits that are mined by cutting a network of rooms into the coal seam. Pillars of coal are left behind in order to keep up the roof. The pillars can make up to forty percent of the total coal in the seam, however, where there was space to leave the head and floor coal there is evidence from recent open cast excavations that 18th-century operators used a variety of room and pillar techniques to remove 92 percent of the ''in situ'' coal. However, this can be extracted at a later stage (''see'' retreat mining
Retreat mining is the removal of pillars in the underground mining technique known as room and pillar mining.
In the first phase of room and pillar mining, tunnels are advanced into the coal or ore body in a rectangular pattern resembling city s ...
).
* Blast mining or conventional mining, is an older practice that uses explosive
An explosive (or explosive material) is a reactive substance that contains a great amount of potential energy that can produce an explosion if released suddenly, usually accompanied by the production of light, heat, sound, and pressure. An expl ...
s such as dynamite
Dynamite is an explosive made of nitroglycerin, sorbents (such as powdered shells or clay), and Stabilizer (chemistry), stabilizers. It was invented by the Swedish people, Swedish chemist and engineer Alfred Nobel in Geesthacht, Northern Germa ...
to break up the coal seam, after which the coal is gathered and loaded onto shuttle cars or conveyors for removal to a central loading area. This process consists of a series of operations that begins with "cutting" the coalbed so it will break easily when blasted with explosives. This type of mining accounts for less than 5 percent of total underground production in the US today.
* Retreat mining
Retreat mining is the removal of pillars in the underground mining technique known as room and pillar mining.
In the first phase of room and pillar mining, tunnels are advanced into the coal or ore body in a rectangular pattern resembling city s ...
is a method in which the pillars or coal ribs used to hold up the mine roof are extracted; allowing the mine roof to collapse as the mining works back towards the entrance. This is one of the most dangerous forms of mining, owing to imperfect predictability of when the roof will collapse and possibly crush or trap workers in the mine.
Production
Coal is mined commercially in over 50 countries. 7,921 million metric tons (Mt) of coal were produced in 2019, a 70% increase over the 20 years since 1999. In 2018, the world production of brown coal (lignite) was 803.2 Mt, with Germany the world's largest producer at 166.3 Mt. China is most likely the second largest producer and consumer of lignite globally although specific lignite production data is not made available.
Coal production has grown fastest in Asia, while Europe has declined. Since 2011, world coal production has been stable, with decreases in Europe and US offset by increases from China, Indonesia and Australia. The top coal mining nations are:
Economic impact
Globally coal mining is highly concentrated in certain jurisdictions, concentrating much of the social and economic impacts of the industry.[Table is extracted from ] Globally the industry directly employs over 7 million workers, which creates millions of indirect jobs. In many parts of the world, producers h