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Coal mining
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Coal mining is the extraction or removal of coal from the earth by mining. When coal is used for fuel in power generation it is referred to as steaming or thermal coal. Coal that is used to create coke for steel manufacturing is referred to as coking or metallurgical coal.
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Coal mining is the extraction or removal of coal from the earth by mining. When coal is used for fuel in power generation it is referred to as steaming or thermal coal. Coal that is used to create coke for steel manufacturing is referred to as coking or metallurgical coal. In the United States, United Kingdom, and South Africa, a coal mine and its accompanying structures are collectively known as a colliery. In Australia, 'colliery' usually only refers to an underground coal mine.
History
The oldest continuously worked deep-mine in the UK and possibly the world is Tower Colliery at the northern end of the South Wales valleys in the heart of the South Wales coalfield. This colliery was started in 1805 and at the end of the 20th century it was bought out by its miners rather than being allowed to be closed. Tower Colliery was finally closed on the 25th January 2008.
Coal mining was evident in colonial America in the early 1700s and the first commercial coal mines in the United States were started around 1730 in Midlothian, Virginia, near Richmond, Virginia.
In the 1880s, Coal-cutting machines became available (prior to that, coal was mined underground by hand using a pick and shovel.)
By 1912, surface mining was under way with steam shovels specifically designed for coal mining.
Methods of extraction
The most economical method of coal extraction from coal seams depends on the depth and quality of the seams, and also the geology and environmental factors of the area being mined. Coal mining processes are generally differentiated by whether they operate on the surface or underground. Many coals extracted from both surface and underground mines require washing in a coal preparation plant.
Coal is mined only where technically feasible and economically justifiable. Evaluation of technical and economic feasibility of a potential mine requires consideration of many factors: regional geologic conditions; overburden 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; ground water conditions; availability of labor and materials; coal purchaser requirements in terms of tonnage, quality, and destination; and capital investment requirements.
There are two basic methods of mining a coal reserve: surface mining and deep underground mining. The choice of mining method depends primarily on depth of burial and thickness of the coal seam. Seams relatively close to the surface, at depths less than approximately , are usually surface mined. Coals that occur at depths of 180 to are usually deep mined but, in some cases, surface mining techniques can be used. For example, some western U.S. coals that occur at depths in excess of are mined by open pit methods, due to thickness of the seam (60-90 feet). Coals occurring below are usually deep mined.
Modern surface mining
When coal seams are near the surface, it may be economical to extract the coal using open cut (also referred to as open cast, open pit, or strip) mining methods. Open cast coal mining recovers a greater proportion of the coal deposit than underground methods, as more of the coal seams in the strata may be exploited. Opencast coal mines can cover many square kilometers.
Most open cast mines in the United States extract bituminous coal. In Australia and South Africa open cast mining is used for both thermal and metallurgical coals. In South Wales open casting for steam coal and anthracite is practiced. Surface mining accounts for around 80% of production in Australia, while in the USA it is used for about 67% of production. Globally, about 40% of coal production involves surface mining.
Area Mining
The most usual surface mining method for coal is strip or area mining. Strip mining exposes the coal by removing the overburden (the earth above the coal seam(s)) in long cuts or strips. The spoil from the first strip is deposited in an area outside the planned mining area. Spoil from subsequent cuts is deposited as fill in the previous cut after coal has been removed. Usually, the process is to drill the strip of overburden next to the previously mined strip. The drill holes are filled with explosives and blasted. The overburden is then removed using large earthmoving equipment such as draglines, shovel and trucks, excavator 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.
The suitability of equipment is governed by geologic conditions. For example, to remove overburden that is loose or unconsolidated, a bucket wheel excavator might be most productive. The life of some area mines may be more than 50 years.
Contour Mining
The contour mining method consists of removing overburden from the seam in a pattern following the contours along a ridge or around a 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 15 to 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 on 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 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 ridge and hill tops 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, which method currently accounts for about 60% 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, is commonly started. This is when miners remove the coal in the pillars, thereby recovering as much coal from the coal seam as possible. A work area that is 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 five principal underground mining methods:
- Longwall mining accounts for about 50% 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 on to a 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% coal recovery rate when surrounding geology allows their use.
- Continuous mining utilizes a machine with a large rotating steel drum equipped with tungsten carbide teeth 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 “rooms” or work areas cut into the coalbed—it can mine as much as five tons of coal a minute, more than a non-mechanised miner of the 1920s would produce in an entire day. Continuous miners account for about 45% of underground coal production. Conveyors 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.
- Blast mining is an older practice that uses explosives such as dynamite to break up the coal seam, after which the coal is gathered and loaded on to 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% of total underground production in the U.S. today.
- Shortwall mining, a method currently accounting for less than 1% of deep coal production, involves the use of a continuous mining machine with moveable roof supports, similar to longwall. The continuous miner shears coal panels 150-200 feet wide and more than a half-mile long, having regard to factors such as geological strata.
- Retreat mining is a method in which the pilliars or coal ribs used to hold up the mine roof are extracted; allowing the mine roof to collape as you retreat. This is one of the most dangerous forms of mining owing to imperfect predictability of when the ceiling will collapse and possibly crush or trap workers in the mine.
Production
Coal is mined commercially in over 50 countries. Over 7,036 Mt/yr of hard coal is currently produced, a substantial increase over the past 25 years. In 2006, the world production of brown coal and lignite was slightly over 1,000 Mt, with Germany the world’s largest brown coal producer at 194.4 Mt, and China second at 100.6 Mt.
Coal production has grown fastest in Asia, while Europe has declined. The top coal mining nations (figures in brackets are 2007 estimate of total coal production in millions of short tons) are:
Most coal production is used in the country of origin, with around 16% of hard coal production being exported.
Global coal production is expected to reach 7,000 Mt/yr in 2030, with China accounting for most of this increase. Steam coal production is projected to reach around 5,200 Mt/yr; coking coal 620 Mt/yr; and brown coal 1,200 Mt/yr.
Coal reserves are available in almost every country worldwide, with recoverable reserves in around 70 countries. At current production levels, proven coal reserves are estimated to last 147 years.
Modern mining
Technological advancements have made coal mining today more productive than it has ever been. To keep up with technology and to extract coal as efficiently as possible modern mining personnel must be highly skilled and well trained in the use of complex, state-of-the-art instruments and equipment. Future coal miners have to be highly educated and many jobs require four-year college degrees. Computer knowledge has also become greatly valued within the industry as most of the machines and safety monitors are computerized.
In the United States, the increase in technology has significantly decreased the mining workforce from 335,000 coal miners working at 7,200 mines fifty years ago to 104,824 miners working in fewer than 2,000 mines today. As some might see this as a sign that coal is a declining industry its advances has reported an 83% increase of production from 1970 to 2004
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Dangers to miners
Historically, coal mining has been a very dangerous activity and the list of historical coal mining disasters is a long one. Open cut hazards are principally mine wall failures and vehicle collisions; underground mining hazards include suffocation, gas poisoning, roof collapse and gas explosions. Most of these risks can be greatly reduced in modern mines, and multiple fatality incidents are now rare in some parts of the developed world.
However, in lesser developed countries and some developed countries, many miners continue to die annually, either through direct accidents in coal mines or through adverse health consequences from working under poor conditions. China, in particular, has the highest number of coal mining related deaths in the world, with official statistic 6,027 deaths in 2004. To compare, the USA reported 28 deaths in the same year. Coal production in China is twice that of the United States, while the number of coal miners is around 50 times that of the USA, making deaths in coal mines in China 4 times as common per worker (108 times as common per unit output) as in the USA.
When compared to industrial countries such as China, the U.S. fatality rate is low. However in 2006 fatal work injuries among U.S. miners doubled from the previous year, totaling 47. These figures can in part be attributed to the Sago Mine disaster. The recent mine accident in Utah's Crandall Canyon Mine, where nine miners were killed and six entombed, speaks to the increase in occupational risks faced by U.S. miners.
Chronic lung diseases, such as pneumoconiosis (black lung) were once common in miners, leading to reduced life expectancy. In some mining countries black lung is still common, with 4000 new cases of black lung every year in the USA (4% of workers annually) and 10 000 new cases every year in China (0.2% of workers). Rates may be higher than reported in some regions.
Build-ups of a hazardous gas are known as damps, possibly from the German word "Dampf" which means steam or vapor:
- Black damp: a mixture of carbon dioxide and nitrogen in a mine can cause suffocation.
- After damp: similar to black damp, an after damp consists of carbon dioxide and nitrogen and forms after a mine explosion.
- Fire damp: consists of mostly methane, a flammable gas.
- Stink damp: so named for the rotten egg smell of the sulfur, a stink damp can explode.
- White damp: air containing carbon monoxide which is toxic, even at low concentrations
Safer times in modern mining
Improvements in mining methods (e.g. longwall mining), hazardous gas monitoring (such as safety-lamps or more modern electronic gas monitors), gas drainage, and ventilation have reduced many of the risks of rock falls, explosions, and unhealthy air quality. Statistical analyses performed by the U.S. Department of Labor’s Mine Safety and Health Administration (MSHA) show that between 1990 and 2004, the industry cut the rate of injuries by more than half and fatalities by two-thirds. However, according to the Bureau of Labor Statistics, mining remains the second most dangerous occupation in America.
Environmental impacts
Coal mining can result in a number of adverse effects on the environment. Surface mining of coal completely eliminates existing vegetation, destroys the genetic soil profile, displaces or destroys wildlife and habitat, degrades air quality, alters current land uses, and to some extent permanently changes the general topography of the area mined, This often results in a scarred landscape with no scenic value, though rehabilitation can mitigate some of these concerns.
Mine tailing dumps produce acid mine drainage which can seep into waterways and aquifers, with consequences on ecological and human health. If underground mine tunnels collapse, this can cause subsidence of land surfaces. During actual mining operations, the potent greenhouse gas, methane, may be released into the air. And by the movement, storage, and redistribution of soil, the community of microorganisms and nutrient cycling processes can be disrupted.
Coal mining by country
Australia
Coal is mined in every state of Australia as well as the Northern Territory. It is mostly used to generate electricity, and 75% of annual coal production is exported, mostly to eastern Asia. Coal provides about 85% of Australia's electricity production. In 2007, 428 million short tons of coal was mined in Australia.
China
The People's Republic of China is by far the largest producer of coal in the world, producing over 2.8 billion tons of coal in 2007, or approximately 39.8 percent of all coal produced in the world during that year. For comparison, the second largest producer, the United States, produced a bit more than 1.1 billion tons in 2007. An estimated 5 million people work in China's coal-mining industry. As many as 20,000 miners die in accidents each year.
Most Chinese mines are deep underground and do not produce the surface disruption typical of strip mines. Although there is some evidence of reclamation of mined land for use as parks, China does not require extensive reclamation and is creating significant acreages of abandoned mined land which is unsuitable for agriculture or other human uses, and inhospitable to indigenous wildlife. Chinese underground mines often experience severe surface subsidence (6-12 meters), negatively impacting farmland because it no longer drains well. China uses some subsidence areas for aquaculture ponds but has more than they need for that purpose. Reclamation of subsided ground is a significant problem in China.
Because most Chinese coal is for domestic consumption and is burned with little or no air pollution control equipment, it contributes greatly to visible smoke and severe air pollution in industrial areas using coal for fuel. Air pollution control equipment is being installed on some plants, but there are unconfirmed reports it is only turned on when inspectors visit. China's CO2 emissions may increase 30% in 2008 due to increased coal combustion.
Colombia
Some of the world's largest coal reserves are located in South America, and an opencast mine at Cerrejón in Colombia is one of the world's largest open pit mines. Output of the mine in 2004 was 24.9 million tons (compared to total global hard coal production of 4,600 million tons). Cerrejón contributed about half of Colombia's coal exports of 52 million tons that year, with Colombia ranked sixth among major coal exporting nations. The company planned to expand production to 32 million tons by 2008.
The company has its own 150km standard-gauge railroad, connecting the mine to its coal-loading terminal at Puerto Bolívar on the Caribbean coast. There are two 120-car unit trains, each carrying 12,000 tons of coal per trip. The round-trip time for each train, including loading and unloading, is about 12 hours. The coal facilities at the port are capable of loading 4,800 tons per hour on to vessels of up to 175,000 tons of dead weight. The mine, railroad and port operate 24 hours per day.
Cerrejón directly employs 4,600 workers, with a further 3,800 employed by contractors.
The reserves at Cerrejón are low-sulfur, low-ash, bituminous coal. The coal is mostly used for electric power generation, with some also used in steel manufacture. The surface mineable reserves for the current contract are 330 million tons. However, total proven reserves to a depth of 300 metres are 3,000 million tons.
United States
The American share of world coal production remained steady at about 20% from 1980 to 2005, at about 1 billion short tons per year.
In a conference with the West Virginia Coal Association President George W. Bush said that there is no more reliable source of electricity than coal and put coal at center of US energy independence .
Ukraine
More than 90% of Ukraine’s coal production is produced from the Donets Basin. The country's coal industry employs about 500,000 people.
Other coal business
See also
Further reading
- Daniel Burns. The modern practice of coal mining (1907)
- Chirons, Nicholas P. Coal Age Handbook of Coal Surface Mining (ISBN 0-07-011458-7)
- Hamilton, Michael S. Mining Environmental Policy: Comparing Indonesia and the USA (Burlington, VT: Ashgate, 2005). (ISBN 0-7546-4493-6).
- Hayes, Geoffrey. Coal Mining (2004), 32 pp
- Hughes. Herbert W, A Text-Book of Mining: For the use of colliery managers and others (London, many editions 1892-1917), the standard British textbook for its era.
- Charles V. Nielsen and George F. Richardson. 1982 Keystone Coal Industry Manual (1982)
- Saleem H. Ali. Minding our Minerals, 2006.
- A.K. Srivastava. Coal Mining Industry in India (1998) (ISBN 81-7100-076-2)
- James Tonge. The principles and practice of coal mining (1906)
External links
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- - overview and graphic of coal mining methods
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- - Petroleum and Coal
- - educational resource on longwall mining
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