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

Abstract

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 N₂ and 5.5 MPa for CO₂ on anthracite from China using a volumetric method. The excess adsorption for N₂ 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 N₂ : 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 N₂. 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 N₂ 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.