Factors Affecting Methane Emission and Their Implications Regarding the Selection of Control Techniques.
Conf on Underground Mng Env 10/27-29/71 Univ of Missouri Rolla Missouri, 1972 :229-240
The best thing that can be said of the methane problem is that most of the gas produced during the coalification stage is gone. Tens of thousands of cubic feet of methane are produced in those reactions that transform vegetation into a ton of coal, and on the average only a few hundred cubic feet of methane remain. Even this amount creates enormous problems. A ton of coal is approximately 25 cubic feet; this means about 10 cubic feet of gas for 1 cubic foot of coal. It has been said that mining of deep coal seams is like digging out a low-grade gas well. Coal adsorbs gas like a low-grade activated charcoal. Because of its basic graphitic structure it contains an extensive network of fine pores, called micropores. These micropores are extremely small; diameters have been estimated in the 5 to 10 angstrom range. As a result, coal, like charcoal, has a large internal surface. One hundred square meters per gram is a typical figure for coal. Most of the gas in coal is adsorbed on this internal surface under pressure. These pressures can range from just a few psi upwards. Gas pressures as high as 650 psi have been measured in U.S. coalbeds, and the Russians have seen over a thousand. It has been known for a long time that the pressure, and consequently the amount of gas, correlates with the depth of the coalbed more than with anything else, and that shallow mines have less gas. Of course there are other factors: for instance, the coalbed around an older mine has been draining into the mine for some time and therefore is at lower pressure and contains less gas. Fac
Conf. on Underground Mng. Env., 10/27-29/71, Univ. of Missouri, Rolla, Mo., 1972, PP. 229-240