Bituminous Coal Sorption Characteristics and Its Modeling of the Main Coal Seam Gas Component in the Huaibei Coalfield, China

Knowledge of the gas sorption and permeability characteristics of a coal provides an essential basis for the evaluation of coalbed methane reserves and their recoverability. Thus, the gas excess sorption capacities of the main gas component of coal seam gas (CSG) in bituminous coal samples derived from the Xutuan Coal Mine in the Huaibei Coalfield, in the Anhui Province of China, were measured using a volumetric method. The results showed that under the same equilibrium pressure, the order of excess sorption capacity was CO2 > CH4 > N-2. Furthermore, the sorption capacity ratios of coal from the Xutuan Mine for CO2, CH4, and N-2 were approximately 6.0:2.3:1. It was also demonstrated that the sorption capacity during depressurization was always larger than that of the adsorption process, which is indicative of desorption hysteresis. The behaviors of three adsorption models, Langmuir, BET, and D-R, all of which include two parameters, are considered in this paper. The different gas sorption measurement data were fitted by the three models. For the bituminous coal samples, the fits of the D-R equation of all three different gases are higher than 0.99, the fits of the Langmuir equation are higher than 0.985, while the fits of the BET equation for CH4 and N-2 absorption are higher than 0.95. However, the fits of the BET equation for CO2 absorption are only about 0.5. Coal sorption has an apparent influence on coal permeability; the permeability of the same coal sample to N2, CH4, and CO2 gases was tested and analyzed. The result shows that the permeability of CO2 was found to be lower than that of other coal seam gas constituents, CH4 and N-2, due to their different adsorption abilities.