er moves along the length of the face sat upon the armoured face conveyor, driving through a chainless haulage system (which resembles a ruggedised rack and pinion system especially developed for mining),Previous to chainless haulage systems, a heavy duty chain was run the length of the coal face for the shearer to pull itself along the face. The shearer moves at a speed of 10-30m/min depending on cutting conditions. The AFC on which the shearer sits, is placed in front of the powered roof supports, and the shearing action of the rotating drums cutting into the coal seam, disintegrates the coal this being loaded onto the AFC. The coal is removed from the coal face by a scraper chain conveyor called the AFC to the main gate. Here it is loaded onto a conveyor belt and transported to the surface, usually via a network of conveyor belts.
- At the main gate the coal is usually reduced in size in a crusher, and loaded onto the first conveyor belt by the beam stage loader (BSL).
As the shearer removes the coal, the AFC is snaked over behind the shearer and the powered roof supports move forward into the newly created cavity. As mining progresses and the entire longwall progresses through the seam, the goaf increases. This goaf collapses under the weight of the overlying strata. The strata approximately 2.5 times the thickness of the coal seam removed collapses and the beds above settle onto the collapsed goaf. This collapsing can lower surface height considerably, causing serious problems like changing the course of rivers and severely damage building foundations.
The advantages of longwall mining include:
- better resource recovery (about 80% compared with about 60 percent for room and pillar method).
- safety of the miners enhanced by being under the hydraulic roof supports when they are extracting coal.
The disadvantages of longwall mining include:
- surface subsidence, which may considerably alter the landscape above the mine which can damage natural or man-made structures or features.
- deterioration in groundwater quality, as it can become more brackish with increased sulfate levels.
- disruption of ecosystems and loss of stream and wetland functions.
- shock waves on the surface.
According to the Center for Public Integrity's study of southwestern Pennsylvania, the following damages have occurred:
- Due to subsidence, homes suffer shattered foundations, crooked roofs, and cracked plaster. As of September, 2008, 1,819 property owners had reported longwall damages since the state of Pennsylvania began documenting such complaints.
- Longwall is permanently lowering the area’s water table and draining its aquifers; state regulators have reported damages to 23 stretches over 97 miles of mined streams.
There is still the possibility of coal mining accidents, as roofs can collapse, and the process of longwall mining stirs up harmful coal dust and explosive methane, thought to be the cause of the explosion at Massey Energy's Upper Big Branch Mine in April 2010, which killed 29 miners.
How much land is disturbed by longwall mining?
Estimate for 2009: 13,235 acres
Background and calculations:
Longwall mining, a form of underground mining designed to completely remove underground coal seams, results in land subsidence over large areas. As documented by reports describing subsidence impacts in Pennsylvania and elsewhere, longwall mining produces serious impacts to buildings, surface water supplies, aquifers. In 2009, longwall mining accounted for 166.4 million tons of coal, or 50% of U.S. underground coal production. This included 101.2 million tons in Appalachia, 12.2 million tons in the Illinois basin, and 53.1 million tons in the West. According to the Energy Information Administration, longwall mine seam heights average 71 inches in Appalachia, 86 inches in the Illinois Basin, and 127 inches in the West. Assuming 1800 tons of recoverable coal per acre foot, the amount of surface area affected by longwall mining in 2009 was 13,235 acres.
TABLE 4: Longwall coal production, area disturbed, and land intensity by region (2009)
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