Details of methane sorption properties of some Indian coals ranging in rank from high-volatile bituminous C (0.62% Ro max) to medium-volatile bituminous coal (1.46% Ro max) were investigated to determine the major factors influencing gas adsorption capacity and desorption rate. Variables studied included moisture content, mineral-matter content, rank, and coal type (maceral composition).
Adsorption isotherm analysis of dry coals showed that adsorption capacity followed a second-order polynomial trend with rank. Equilibrium moist samples showed a linear increase in adsorption capacity with rank and had a significantly reduced adsorption capacity compared to the dry coals, with the reduction being related to the moisture content. Mineral matter acted as a simple diluent to the gas adsorption capacity of the coals and was found to be nonadsorbent. Adsorption capacity (moist) was reduced by 0.32 cm3/g (10 ft3 gas/t) for every 1% increase in the ash yield.
Bright and dull coal lithotypes showed strong separation in their adsorption capacities on an as analyzed basis, with the bright coals adsorbing greater quantities of gas. On a dmmf basis, however, no relationship was observed between coal type and gas storage capacity. Comparison of maceral composition with adsorption capacity (dry) confirmed this observation.
Effective diffusivity (De) of methane through the coal was seen to be affected by coal type and rank. Effective diffusivity, De, decreased as rank increased, which is related to the increasing microporous nature of the coal. Bulk coals tested had 2-3 times larger effective diffusivities than bright coals, and dull coals had intermediate rates. The larger De values for the bulk coals may be related to the presence of collodetrinite coupled with mineral matter, which acts as higher permeability pathways for the gas compared with the high-ash dull coals dominated by inertinite macerals.
Differences were noted with similar-age (Permian) Australian coals, where methane adsorption capacities were larger and coal type had a stronger influence on adsorption capacity.