Coal Mining

Posted September 5th, 2011 by

Coal MiningCoal is a combustible black or brownish-black sedimentary rock normally occurring in rock strata in layers or veins called coal beds or coal seams. The harder forms, such as anthracite coal, can be regarded as metamorphic rock because of later exposure to elevated temperature and pressure. Coal is composed primarily of carbon along with variable quantities of other elements, chiefly hydrogen, with smaller quantities of sulfur, oxygen, and nitrogen.

How Coal Is Produced
Coal found close to the surface can be uncovered and removed by large machines in a process that is called surface mining. The development and use of large power equipment provided the impetus that moved surface mining into prominence, and during the 1970s it became the leading method of coal mining. Today’s surface mines are large, intensively engineered, and highly efficient mechanized operations. When an area is to be mined, topsoil and subsoil are removed first and set aside to be used later in reclaiming the land. Then specially designed machines-draglines, wheel excavators, or large shovels remove the rock and other material, call overburden, to expose the bed of coal. Smaller shovels load the coal into large trucks that remove the coal from the pit. Once the coal is removed, the area is reclaimed. First the overburden and then the soils are replaced and the area is restored as nearly as possible to its original contour. Vegetation currently suitable to the area is planted to anchor the soil and return the land to a natural, productive state. Reclaimed lands are a valuable resource that can support farm crops, provide new wildlife habitats, enhance recreational opportunities, and even serve as sites for commercial development.

The complete mining operation is scheduled so that as one area is being mined, another is being reclaimed where the coal was removed. Thus, even at the largest surface mines only a relatively small area is disturbed by active mining at any one time. Underground mining methods are used where the coal seam is too deep or the land too hilly for surface mining. Most underground mining takes place east of the Mississippi, especially in the Appalachian mountain states. Coal production was once dominated by underground mining methods, but the growth of coal mining in the West changed that. Now, only 40 percent of our coal comes from underground mines. Underground mines differ according to how the coal seam is situated with respect to the surface. If the coal deposit outcrops (appears at the surface) on a hillside, a drift mine can be driven horizontally into the seam. Where the bed of coal is relatively close to the surface, yet too deep to be recovered by surface mining, a slope mine can be constructed, with the mine shaft slanting down from the surface to the coal seam. The most common type is the shaft mine. To reach the coal, which may be as deep as 2,000 feet, vertical shafts are cut through the overburden to the coal bed, which is excavated by machines.

In deep mines, the seam is mined in carefully engineered patterns that keep as much as half of the coal in place to help support the roof of the active mining area. This “room and pillar” method requires that large columns of coal remain between mined-out areas, or rooms, which are created when the coal is mined, either by continuous mining machines or conventional methods.
The largest amount of coal taken from underground mines is produced using continuous miners. This machine has a large, rotating, drum-shaped cutting head studded with carbide-tipped teeth that break up the seam of coal. Large gathering arms on the machines scoop the coal directly onto a built-in conveyor for loading into waiting shuttle cars.

In conventional mining, a machine resembling an oversized chainsaw cuts into the coal. This gives the coal an area to expand into during blasting. Holes are drilled for explosives, which blast loose large chunks of coal. Machines called loaders scoop the coal onto conveyors which dump it into shuttle cars that haul the coal out through the shaft. This traditional method of mining accounts for about 11 percent of total production.

What Is Coal Used For?
Coal has the following uses:

1. Cooking
2. Heating buildings
3. Electricity generation
4. Heat source for the production of bricks and cement
5. Natural gas supplement
6. The manufacturing of paper, chemicals, steel, metal products, plastics, ceramics, fertilizers, and coal tar
7. Coke production for use in the steel industry

How Is Electricity Generated From Coal?
Coal is a fuel that is found in the ground. It is made of the remains of plants that died millions of years ago. Soil piled up on top of the remains and that weight compacted it into a more dense material, called coal. The energy in the coal came from the sun and was stored in the plants. When the coal is burned, it gives up that energy as heat. The coal’s heat energy can then be turned into electrical energy. This happens at a power plant. First the coal is mined and taken to a power plant. Then the coal is burned in a boil which causes the water in the boil pipes to become steam. Next, the steam travels through the pipe to the turbine and the steam spins the turbine blade, the spinning blades turn a shaft connected to the generator.

For an overview of the advantages and disadvantage of coal as an energy source, refer to our previous article, Coal Energy.

Image Credit: Citizendium

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