Coal Mining Technologies
Coal mining employs surface and underground methods to extract coal. A number of alternative technologies are associated with each method.
How Is Coal Mined?
Coal is mined using giant machines to remove the coal from the ground. There are two basic methods to remove coal: surface mining and underground mining. Surface mining is used when the coal is typically less than 200 feet below the surface. Giant machines are used to remove the top layers of soil and rock to expose the coal. The coal is excavated, and after the mining is complete, the soil and rock are returned to fill the mine. The area is then revegetated and can be used for other purposes, such as cropland, wildlife habitat, recreation, commercial, or industrial use. This method is used most frequently in the United States because much of the coal resource base is near the surface and it is less expensive than underground mining.
Underground mining is used when the coal is buried several hundred feet below the surface or more. Some mines can extend to depths of more than 1,000 feet. Miners use heavy machinery to cut out the coal and rely on conveyor systems to transport the coal to the surface. Some underground mines require elevator shafts to move miners and coal to and from the surface.
What Are the Different Types of Surface Mines?
Strip and auger mining are the two most common surface methods of extracting coal in the United States. Open-pit mining is sometimes used in thick shallow-lying western coal seams.
Strip mining is accomplished by two techniques, area stripping and contour stripping. Where coalbeds are relatively flat and near the surface, as in much of the Western United States, area stripping is the dominant technique. In area strip mining, overlying material is removed from a seam of coal in long, narrow bands, or strips, followed by removal of the exposed coal. With the exception of the first cut, overburden from each cut is discarded in the previous cut from which the coal has been removed. These parallel cuts continue across the coal seam until the thickness of the overburden becomes too great to be removed economically or until the end of the coal seam or property is reached. Both single and multiple seams, near the surface, can be mined in this manner.
Overburden removal is usually accomplished in the United States with draglines and shovels. Much of the overburden contains layers of shale, limestone, or sandstone and must be blasted before it can be removed. After the overburden is removed, coal is usually drilled and blasted, then loaded into coal haulers with either a shovel or front-end loader.
Contour stripping is practiced on steep terrain mostly in the Appalachian Coal Region. The method consists of removing overburden from the coalbed with the first cut at or near the outcrop, and proceeding around the hillside. Overburden is stacked along the outer edge of the bench. After the uncovered bed is removed (successive cuts, usually two or three) are made until the depth of the overburden becomes too great for economical recovery of the coal. Contour mining creates a shelf or bench on the side of the hill. On the inside, it is bordered by the highwall, ranging in height from a few feet to more than 100 feet, and on the outer side, by a high ridge of spoil. Equipment used for contour stripping is smaller in size and load capacity than that used for area stripping. Bulldozers and front-end loaders are often used for overburden removal at these operations.
In the eastern United States, auger mining is used on hillside terrain. It requires a surface cut (removal of overburden and a portion of the coalbed) to allow the auger access to the bed. It is also used to recover part of the coal left from underground mining. In the western United States, auger mining is used in conjunction with strip mining. Coal mining by the auger method entails boring horizontal or near-horizontal holes in an exposed face of the coal, and loading the coal removed by the auger. Single, dual, or triple auger heads can remove up to 90 inches of coal for a distance of over 200 feet. Augering is generally used to supplement recovery at contour or strip mines when the overburden becomes too great to be economically removed. It is also used where the terrain is too steep for overburden removal and where recovery by underground methods would be impractical or unsafe.
In open-pit mining, overburden is removed and placed outside the mining area. The overburden can be removed with either scrapers or shovels loading into trucks. Mining begins by drilling and blasting waste rock to expose the coal seams, excavating additional overburden, and removing and transporting the coal. The pit increases in size and depth as mining progresses, and it is unusual that overburden, once removed, is ever returned to the pit. The open pit method is sometimes used in coal mining where numerous seams lie parallel to each other and outcrop on relatively flat terrain.
Mountaintop Removal Mining
Mountaintop removal mining (MTR), often referred to as mountaintop mining/valley fills, is a form of surface mining that involves extreme topographic change to the summit or summit ridge of a mountain. It is most closely associated with coal mining in the Appalachian Mountains, located in the eastern United States. The process involves the removal of up to 1,000 vertical feet of overburden to expose underlying coal seams. The overburden is often scraped into the adjacent drainage valleys in what is called a valley fill.
Because of its destructive nature, MTR is controversial and is protested by environmentalists, local residents, and others. Controversy over the practice stems from both the extreme topographical and ecological changes that the mining site undergoes, as well as from the storage of waste material generated from the mining and processing of the coal. It is also essentially impossible to recontour the site to pre-mine conditions during reclamation. Waivers are often given to mining companies in these cases to use the leveled area for alternate industrial uses. Proponents of MTR point to its efficiency, job creation, and increase of flat land in areas where there is often little.
The practice of mountaintop removal mining is not recommended and will not be discussed further in this section.
What Are the Different Types of Underground Mines?
Access to coal deposits for underground mines is provided by (1) drifts which are cut horizontally into a hill; (2) slopes which are cut at an angle from the valley bottom into the hill where the coal is located; and (3) shafts which are cut straight down deep into the surface by way of a vertical shaft with an elevator to reach the coal seam.
In underground mining, after the initial development has gained access to the coalbed, one of three methods is commonly used to extract the coal: room-and-pillar, longwall, or shortwall.
Room-and-pillar mining has been used in the United States longer than any other underground method. Mining is accomplished by driving entries off the panel entries. As mining advances, rooms are excavated in the coal seam; the strata above the seam are supported by pillars of coal left in place. After a block panel or section has been mined, part of the coal in the pillars can be recovered as a retreat is made toward a main entry. Since about 1950, continuous mining using electric-powered machines to bore, dig, or rip the coal from the working face has largely replaced conventional mining, which involved undercutting, drilling, placing explosives, and blasting to extract the coal. Coal is either loaded directly into shuttle cars by the machine or in a separate operation. So-called continuous mining is interrupted by stops to support the roof, await shuttle cars, advance power and water supplies, and service the equipment.
Longwall mining is used most efficiently in uniform coal seams of medium height (42 to 60 inches). As in the room–and-pillar method, longwall mining starts with sets of entries cut into the panel areas. The difference in the technique lies in the distance between these sets of entries and the method used to extract intervening coal. Longwall blocks range from 300 to 600 feet wide and are sometimes a mile long. The longwall machine laterally shears or plows coal from the entire face, transports the fallen coal by an advancing conveyor to a secondary haulage conveyor, reverses direction at the end of a cut, and supports the roof in the area of the face by a self-advancing system of hydraulic jacks. Over 80% of the entire coal face can be removed with this method. The roof is allowed to cave behind the advancing work areas; the roof is occasionally blasted to ensure a controlled cave-in rate and to reduce overburden pressure on the coalbed being mined.
The shortwall method of mining coal is best described as a method similar to longwall mining with two exceptions. The blocks of panels are smaller, usually ranging from 100 to 150 feet wide and 300 feet long and the coal is cut with a continuous miner and is loaded into shuttle cars.
Where Does the Coal Go After It Is Mined?
After coal is removed, it typically goes on a conveyor belt to a preparation plant that is located at the mining site. The plant cleans and processes coal to remove dirt, rock, ash, sulfur, and other unwanted materials, increasing the heating value of the coal. The coal may also be sorted by quality and size.
Once the coal is processed it is shipped to wherever it is needed, typically by rail, but also by truck or barge or even a coal-slurry pipeline. Transportation methods depend on the distance to be traveled and access to existing transportation systems.