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Gas drainage
measures to extract and collect mine gas or gas-air mixture from underground excavations and remove it to the surface. Gas is extracted through pipes running throughout the mine, or via boreholes connecting the mine to the surface.
Gas drainage reduces the amount of methane reaching the mine-works from coal seams or rocks, thereby simplifying ventilation and entirely removing or significantly reducing downtime in working (extractive) areas because of a build-up of gas. It allows electricity to be used in gassy shafts in place of less efficient pneumatic energy, increases labour productivity and safety of mining in gassy shafts, and in some circumstances entirely or partially averts unusual gas shows such as gas bleeding, sudden emissions of coal and gas, etc.
Gas drainage systems consist of degassing excavations or drill holes; gas pipelines with protective devices; decontamination equipment; and regulating, registering and protective equipment and devices. If the captured gas is to be used, a pipeline to the consumer is also needed; this equipment is placed at the surface, and consists of vacuum pumps or rotary blowers with non-sparking blades to propel gas around the degassing system, drives to propel the blowers and regulatory and control apparatus for the machines and drives.
The application of gas drainage has led to the development of new techniques for mining gas-bearing coal seams; given the production of associated methane, an integrated approach has to be taken to developing some gas-bearing deposits. Gas drainage takes three principal forms: preliminary gas drainage of the coal seams to be developed; gas drainage of adjacent coal seams; and removal of concentrated methane-air mixture from worked-out areas. Preliminary gas drainage takes place before development of a coal seam begins, and is done by drilling parallel boreholes of 80-120 mm in diameter to a depth of 100-250 m at intervals of 10-25 m. Each gas drain is connected via a water separator to the mine's gas pipeline network. Gas is pumped out of the coal seam using vacuum suction of 13.5-27 kN/m2 (100-200 mm Hg) over an extended period of time (more than 100-150 days).
Gas drainage of adjacent coal seams uses the partial relieving of rock pressure on the seams to help release any methane that they have absorbed. In this case, boreholes are drilled from the mine-working to seams immediately above and below the seam to be developed, at distances less than the effective gas-drainage radius. These boreholes are then connected to the pipeline network. When draining worked-out areas, the areas are thoroughly isolated from active mine-workings using brattices and airtight channels (made, for example, of rock and various sealants); the mine's gas pipelines are then used to extract gas with a high concentration of methane concentrated in the spaces formed between rockfalls.
Industrial gas drainage was first used in 1943 in the Rurhgebiet in Germany, and in the Soviet Union in the Donbass region in 1952.
