Coal has been a prominent source of power generation for more than hundred years now. It is also widely used in many other industries. Coal also serves as fuel in the extraction of iron from iron ore and for the production of cement. Due to a rise in demand for coal, many mining techniques have evolved to extract the fossil fuel. A large-scale mining of coal requires the use of draglines, trucks, conveyors, hydraulic jacks and shearers. Depending on their properties, coal is mainly divided into four types such as peat, lignite, bituminous and anthracite. The age and depth at which they are buried under the ground define the properties of a particular type of coal. Formed from decaying plants, peat is considered as a precursor to coal. Also known as brown coal, lignite possesses least concentration of carbon and is considered to be the lowest grade coal.
As mining of coal depends on the depth and quality of the coal seams, different techniques are used to dig the deposits. Surface mining and deep underground mining are the two main methods of mining coal. Depth, density, and thickness of the coal seam are the factors that are taken into account while selecting the mining method for coal extraction. However, many coals extracted by using these two methods need washing in a coal preparation plant. Surface mining: The mining technique is used when the coal seam is present near the earth’s surface. The techniques followed in surface mining include open-pit mining, strip mining and mountaintop removal. While open-pit mining involves extraction of coal from an open pit in the ground, strip mining includes stripping surface layers to reach coal deposits. Contour strip mining, area strip mining and auger mining are other techniques followed in surface mining of coal.
In case of strip mining, coal seams are reached by removing the surface above them. The surface, which is referred to as overburden, is removed in long strips in this coal mining technique. The overburden taken out from the first strip is deposited outside the mining place. This process is referred to as out-of-pit dumping. The void resulting from the first strip and the coal mining is used to deposit overburden from the subsequent strips.
Contour strip mining and area strip mining are the two forms of strip mining, which is often used to extract coal and lignite. In contour strip mining, bulldozers or power shovels are deployed to take away huge amounts of ground material in terraced strips for extracting coal from a seam on a hill. Area strip mining is commonly used on flat terrain to extract deposits spread over a large area.
Mountaintop removal technique involves removal of the mountaintop overlying the coal seam. The rock layers present above the seam will be broken up by using explosives. In recent years, the technique has found a wide usage in the Appalachian coal fields of West Virginia, Kentucky, Virginia and Tennessee in the US.
Underground mining: The mining technique is used to reach coal seams present deep underground. Underground mining is more widely used to extract coal compared to opencast mining, as many coal seams are said to be found deep below the earth surface. The underground mining includes six type of mining methods that include longwall mining, continuous mining, room and pillar mining, blast mining, shortwall mining and retreat mining.
In room and pillar mining, coal deposits are mined by creating a network of rooms into the coal seam. Pillars of coal are left at the site to support the roof.
In case of continuous mining, a Continuous Miner Machine is used to break off the coal from the seam. The coal removed from the seam will be later transported using conveyors. The machine operates in a room and pillar system, mining several tons of coal in a minute. Continuous miners account for a significant part of coal production in underground mining.
Developed as an alternative for room and pillar mining, Longwall mining is an advanced technique of underground mining to extract coal. It is used to mine a long wall of coal that is a few kilometres long and some hundred meters wide. Longwall mining technique is found to have better resource recovery rate compared to room and wall mining.