Similar gas problems can also be encountered after the closure of a coal mine, since gas may continue to be emitted from the cracks and crevices created in the ground by mining activities. The resulting drop in gas pressure leads to the desorption of the adsorbed gas. This effect may be compared to a champagne bottle: when the cork is removed, the carbonic acid bubbles until none is left. The mine gas escapes into the atmosphere through the ventilation pipes remaining in many shafts. It also migrates into the environment through the overlying rock, since it is impossible to seal a disused pit to make it completely gastight. This methanous gas is released into the atmosphere in a cold state.
The potential danger due to the diffuse emission of this mine gas and its main constituent methane (25 to 60 percent) is reduced by efficiently using it for power generation. The gas can be used to replace conventional fuels, preventing the emission of methane into the atmosphere. The use of combined heat and power stations for the generation of electrical energy from mine gas is an established practice, and will continue to grow in importance in the future due to the free availability of fuels, the fact that problematic cold flaring is no longer necessary, and that in many instances the heat generated can also be used directly.
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