Diffusion of methane from threecoals ranging in rank from anthracite to HVA bituminous has been studied at initial methane pressures up to about 2.76 MPa (400 psi). Unsteadystate diffusion conditions existed, the methane pressure within the coal particle decreasing with time while the methane pressure outside the particles remained at atmospheric. The diffusion parameter D112/ro increased with increasing methane concentration at high values of methane sorption. Diffusion was activated but the exact magnitude of the activation energy is uncertain owing to the suspected contribution of the heat of sorption to the temperature coefficient.
Dli2/rn increased with decreasing particle size of coal studied, but ro is clearly less than the particle radius Methane is present in coal measures, and methane pressures as high as 3.55 MPa have been encountered in some U.S. coal seamsi. The amount of methane released from a coal seam depends on the nature of the coal, as well as on the geological and mining conditions.
It has been suggested by Cervik' that most of the methane released into U.S. mines comes from within the intact coal bed. 1n situ methane permeability measurements on three coal beds indicate that the nature of the coal has a more important effect than overburden pressure on permeability3. Laboratory data on the sorption characteristics of methane by different coals are expected to provide the inputs to the different mathematical models for the estimation of methane release from coal seams. Information provided by such models is not only important in contributing to a better understanding of the undesirable release of methane during mining operations but also to a better evaluation of the possibility of using degasification of coal beds as a commercial source of pipeline gas.
EXPERIMENTAL
Rates of desorption have been measured at three temperatures (0, 25 and SO'C) with three coals in the pressure range 1.14-2.52
MPa. In addition, the effect of particle size on diffusion has been followed on one coal. Analytical data on the coals studied are shown in Table 1. PSOC-177, 135, and 171 can be classified as anthracite, mediumvolatile bituminous, and HVA bituminous respectively.