Underground coal mining:

Underground coal mining:

When coal seams obtained at a great depth, surface coal mining is not feasible; the extraction of coal is done by a method called underground mining. Underground mining currently accounts for about 60% of world coal production.

In underground coal mining, there are two system of working – (1) board and pillar method; (2) Longwall method.

A. In board and pillar method, all along the coal seam, coal pillars are formed by developing drives. Drives are supported by timbers. The coal pillars thus formed kept standing till the development is done upto the mine boundary in a certain portion of the block called ’district’. Once, full area is developed in several districts, the pillars are to be extracted by a method called ‘depillaring’.

B. In Longwall (retreat) method of coal mining, drives are made at the two periphery of a district till end and these two drives are joined by another drive. This drive becomes the coal production face, where continuous machines like shearer loader, chain conveyors etc., are put. The hydraulic supports are made at the face. This is a high productive method for coal extraction from underground coal mines.

C. In underground coal mines, methane gas present in the originally formed coal seam gets released in to the mine atmosphere during excavation of coal. Methane when mixed with air in the proportions between 5 and 14 % forms an explosive mixture. Hence, there is possibility of a methane-air mixture being dangerously formed in the working areas. In underground coal mines, there is also an additional risk as ‘coal-dust’, when mixed with air, can undergo a dust explosion when suitably ignited. Both methane-air and coal dust explosions cause loss of life and extensive damage to the underground mines. Therefore, avoidance of such incidences is to be observed essentially by taking suitable safety measures. Keeping proper ventilation is again very important aspect in underground mines. Sufficient ventilation air dilutes the noxious gases generated in the coal seams and also due to blasting operation.