Bound forms of methane in the porous system of coal

The total quantity of methane present in a coal bed at temperatures above the critical temperature was divided into four basic forms of bonding: the part adsorbed in micropores, the part adsorbed on the meso- and macropores surface, the part freely moving in the pore space which is not filled with water, and finally the part dissolved in the water present in the pores. The sum of the partial volumes mentioned was interpreted as the theoretical isotherm of the total amount of methane occluded in coal; for its evaluation Dubinin's low-pressure isotherm extrapolated to the range of true in situ pressures and the volume and surface of meso- and macropores were used. Isotherms computed in this way were in very good agreement with experimental high-pressure isotherms measured up to 10 MPa. For the estimation of methane amount occluded in coal under real mining conditions the theoretical isotherms will be modified by the introduction of correction terms characterizing the changes of the porous system of coal due to water content and overburden pressure. It has been shown that the presence of water content has on the restriction of methane amount a greater effect than the overburden pressure. (C) 1997 Elsevier Science Ltd.