Pure methane, carbon dioxide, and nitrogen adsorption on anthracite from China over a wide range of pressures and temperatures: experiments and modeling

The adsorption isotherms and kinetics characteristics were investigated at 294 K, 311 K, 333 K, and 353 K with pressures up to 18 MPa for CH4 and N-2 and 5.5 MPa for CO2 on anthracite from China using a volumetric method. The excess adsorption for N-2 belongs to the type I isotherm, while the adsorption capacity for CH4 and CO2 initially increased, followed by a sequence decrease with increasing pressure. The preferential ratio of the maximum in the excess adsorption for N-2 : CH4 : CO2 are 1 : 1.1 : 1.6, 1 : 1.3 : 1.8, 1 : 1.4 : 1.9, and 1 : 1.2 : 1.6 at 294 K, 311 K, 333 K, and 353 K, respectively. In addition, the excess adsorption capacity was predicted using the Langmuir + k and simplified Ono-Kondo lattice models. The Langmuir + k monolayer model has higher accuracy in modeling pure gas adsorption on coal, especially for CH4 and N-2. Moreover, the pressure decay method was used to analyse the adsorption kinetics of gases on coal. It is observed from the kinetics data that the adsorption rate and the effective diffusivity increase with the increasing pressure for CH4 at 0.55-6.97 MPa and N-2 at 0.63-10.05 MPa. However, for CO2, an increase in pressure reduces the adsorption rate and the effective diffusivity at 0.11-3.96 MPa due to intensive gas molecule-molecule collisions and the strong coal matrix adsorption swelling. The adsorption rate and the effective diffusivity with temperature are similar for the three gases, which increase with increasing temperature. Through verification of the experimental data, the diffusion model can be used to model the kinetics data of gases on coal under low and medium pressure conditions.