|This article is part of the Global Coal Mine Tracker, a project of Global Energy Monitor.|
Coal mining is the process of extracting a coal reserve from the ground. As a commodity, coal is valued for its energy content and since the nineteenth century has been widely used to generate electricity and as a fuel source for the steel and cement industries.
Coal mining has a long history of negative environmental impacts, health effects for surrounding communities and workers, and contributes heavily to global environmental crises, such as poor air quality and climate change.
Coal has been one of the first fossil fuels to be phased out of various parts of the global energy economy.
Before a mining project begins, it requires feasibility studies to justify investment. While there is no international agreement on the terminology for each stage of development and there is no agreed standard for quality or accuracy, the Australian Mining Industry (AusIMM) "Cost Estimation Handbook" provides a set of standards that have become widely used internationally.
The first evaluations of a property consider the potential costs and revenues of a new mine, often by examining comparative mines in the surrounding area. This market analysis forms the basis of a decision to commence a scoping study. At this stage, an individual or company may apply for a mining license for a nominal fee to explore a specific area.
A scoping or concept study is carried out prior to exploration, usually as the basis for acquiring exploration areas or securing exploration funding. These studies are based on very limited information or speculative assumptions and directed at the potential of the property. They set minimum targets for grade and tonnage. The estimation accuracy may be 30 to 35%, but at this stage the investment risk may be relatively small. The scoping study may also include information on land-use planning, such as ownership and means of access.
There are three common reasons for carrying out pre-feasibility studies: 1) to attract a buyer to the project or to attract a joint venture partner to raise the required capital, 2) as part of due diligence process of potential purchasers, 3) to provide a justification for proceeding to a final feasibility study.
The pre-feasibility study considers a range of mining and processing options and varying production rates, with the options narrowed down to one or two in each area. The study accuracy runs from 20% to 25%.
The main features of the pre-feasibility study:
- Mineral reserve estimate
- Preliminary studies on geotechnical, environmental, and infrastructure requirements.
- Mine designs based on a resource model.
- Cost estimates based on factored or comparative prices.
The study process is often iterative and several increasingly detailed studies may be undertaken before committing to a final feasibility study. At this stage it is possible to detail any additional work, such further drilling, metallurgical tests, site investigations, etc. The results of the pre-feasibility study are used to justify expenditure on a final feasibility study. The results are often the first hard project information that is seen by corporate decision makers.
Final feasibility studies
The final feasibility study is usually based on the most attractive alternative for the project. The aim of the study is to remove all significant uncertainties and to present the relevant information with back up material in a concise and accessible way.
The final feasibility study has three objectives:
- To provide a basis for detailed design and construction.
- To demonstrate within a reasonable confidence that the project can be constructed and operated in a technically sound and economically viable manner.
- To enable the raising of finance for the project from banks or other sources.
Over all, technical and economic feasibility are evaluated based on the following: regional geological 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; groundwater conditions; availability of labor and materials; coal purchaser requirements in terms of tonnage, quality, and destination; and capital investment requirements.
The level of engineering discussed in a feasibility study is well short of that required for construction, so a further period of detailed engineering design follows after the project has been approved. This usually continues during construction and only ends when production is imminent.
At the forefront of the mine permitting process is an environmental review process consisting of preparation of an Environmental Impact Statement (EIS).
Methods of extraction
Coal mining has had many developments over the recent years, from an early history of humans tunneling, digging, and manually extracting the coal on carts to contemporary methods of open pit and long wall mining. Today, technological advancements have made coal mining in many parts of the world 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.
Today, coal extraction methods vary depending on whether the mine is an underground mine or a surface mine. 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 180 ft, are usually surface mined; coal seams at depths of 180 to 300 ft are usually deep mined. (In the Powder River Basin, coal is mined with the open pit methods at depths of 200 ft, owing to thickness of the seam).
When coal seams are near the surface, it may be economical to extract the coal using open-pit mining (also known as opencast mining). Opencast 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. 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.
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. 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 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, which may be drilled and blasted or otherwise loaded onto trucks or conveyors for transport to the coal preparation 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 equipment to be used depends on geological conditions (e.g. to remove overburden that is loose or unconsolidated, a bucket wheel excavator might be the most productive).
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 15 to 20 ft 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 removal 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 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.
When coal seams are too deep underground for opencast mining, they require underground mining, a method that currently accounts for about 60 percent of world coal production. There are six principal methods of underground mining.
Longwall mining accounts for about 50 percent of underground production. The longwall shearer has a face of 1000 ft 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.
This method 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 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. Some continuous miners can bolt and rock dust the face 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 incorporation of bolting makes the machines wider, and therefore, less maneuverable.
Room and pillar mining
Room and pillar 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 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 (retreat mining).
Blast mining or conventional mining
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 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.
Shortwall mining currently accounts for less than 1 percent of deep coal production, involves the use of a continuous mining machine with movable roof supports, similar to longwall. The continuous miner shears coal panels 150-200 ft wide and more than a half-mile long, having regard to factors such as geological strata.
Retreat mining 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.
After extraction, most coals require washing in a coal preparation plant, also known as a coal handling or benefaction. A tipple or wash plant is a facility that washes the soil and rock from the coal and crushes it into graded sized chunks, stockpiles grades preparing it for transport to market, and loads coal into rail cars, barges, or ships.
The more waste material removed from coal, the lower its total ash content and the greater its market value and the lower its transportation costs.
Run of mine (ROM) coal
The coal delivered from the mine that reports to the coal preparation plant is called Run of Mine (ROM) coal. This is the raw material for the coal preparation, and consists of coal, rocks, middlings, minerals, and contamination. Contamination is usually introduced by the mining process and may include machine parts, used consumables and parts of ground engaging tools. ROM coal can have a large variability of moisture and maximum particle size. The washed product is referred to as "salable" coal or "marketable" coal.
Coal is mined commercially in over 50 countries. 7,921 Mt of coal were produced in 2019, a 70% increase over the 20 years since 1999. In 2018, the world production of brown coal and 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 USA offset by increases from China, Indonesia and Australia.
The top coal mining nations:
|United States||640 Mt|
|South Africa||264 Mt|
Globally coal mining is highly concentrated in certain jurisdictions, concentrating much of the social and economic impacts of the industry. Globally the industry directly employs over 7 million workers, which create millions of indirect jobs. In 2018 coal production, reserves, miners, and major coal-producing regions for China, India, The US, and Australia. Together, these countries account for 70% of global annual coal production. This table includes jurisdictions which are the top coal-producing provinces/states, responsible for over 85% of each country's coal production.
|Country||Coal production (million tonnes)||Coal reserves (million tonnes)||Coal miners (thousands)||Top producing provinces or states||% of national production covered|
|China||3349||138 819||6110||Shanxi, Inner Mongolia, Shaanxi, Anhui, Heilongjiang, Xinjiang, Shandong, Henan, Guizhou||90%|
|India||717||97 728||485||Chhattisgarh, Jharkhand, Orissa, Madhya Pradesh, Telangana||85%|
|United States||701||250 916||52||Wyoming, West Virginia, Pennsylvania, Illinois, Kentucky, Texas, Montana, Indiana, North Dakota||90%|
|Australia||478||144 818||50||New South Wales, Queensland, Victoria||99%|
Safety and Hazards
Dangers to miners
Coal mining is dangerous activity and the list of mining disasters is a long one. In the US alone, more than 100,000 coal miners were killed in accidents in the twentieth century, 90 percent of the fatalities occurring in the first half of the century. More than 3,200 died in 1907 alone. Open cut hazards are principally mine wall failures and vehicle collisions; underground mining hazards include suffocation, gas poisoning, roof collapse, rock burst, outbursts, and gas explosions. However, in some developing countries, many miners continue to die from direct accidents or through adverse health consequences from working under poor conditions.
China has the highest number of coal mining related deaths in the world, with official statistics claiming that 6,027 deaths occurred in 2004. To compare, 28 deaths were reported in the US in the same year.
Coal production in China is twice that in the US, while the number of coal miners is around 50 times that of the US, making deaths in coal mines in China 4 times as common per worker (108 times as common per unit output) as in the US.
Black Lung Disease
Chronic lung diseases, such as pneumoconiosis (black lung) remain common in miners, leading to reduced life expectancy. There are 4,000 new cases of black lung every year in the US (4 percent of workers annually) and 10,000 new cases every year in China (0.2 percent of workers). The use of water sprays in mining equipment reduces the risk to miners' lungs.
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, and is formed as a result of corrosion in enclosed spaces so removing oxygen from the atmosphere.
- Afterdamp: similar to black damp, after damp consists of carbon monoxide, carbon dioxide, and nitrogen and forms after a mine explosion.
- Firedamp: consists of mostly methane, a highly flammable gas that explodes between 5% and 15% – at 25% it causes asphyxiation.
- Stink damp: so named for the rotten egg smell of the hydrogen sulfide gas, stink damp can explode and is also very toxic.
- White damp: air containing carbon monoxide which is toxic, even at low concentrations
Noise is also a contributing factor to potential adverse effects on coal miners' health. Exposure to excessive noise can lead to noise-induced hearing loss. Hearing loss developed as a result of occupational exposures is coined occupational hearing loss. To protect miners' hearing, the US Mine Safety and Health Administration's (MSHA) guidelines for noise place a Permissible Exposure Limit (PEL) for noise at 90 dBA time-weighted over 8 hours. A lower cutoff, 85 dBA, is set for a worker to fall into the MSHA Action Level which dictates that workers be placed into hearing conservation programs.
Noise exposures vary depending on the method of extraction: a study has found that among surface coal mine operations, dragline equipment produced the loudest sound at a range of 88–112 dBA. Within longwall sections, stageloaders used to transport coal from the mining face and shearers used for extraction represent some of the highest noise exposures. Auxiliary fans (up to 120 dBA), continuous mining machines (up to 109 dBA), and roof bolters (up to 103 dBA) represent some of the noisiest equipment within continuous mining sections. Exposures to noise exceeding 90 dBA can lead to adverse effects on workers' hearing.
Environmental impacts of coal
There are numerous damaging environmental impacts of coal that occur through its mining, preparation, combustion, waste storage, and transport. On this see Environmental impacts of coal.
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