Adsorption selectivity of confined CH4-CO2 mixture in coal: Influence of physicochemical structure and thermodynamic competitive mechanism

Efficient adsorptive separation of CH₄-CO₂ mixture in coal nanopores is important for CO₂-enhanced coalbed (CO₂-ECBM) recovery and CO₂ sequestration. However, The understanding of the adsorption selective behavior and its controlling mechanisms is still limited. The low-pressure N₂/CO₂ adsorption, Fourier transform infrared spectroscopy, and gravimetric dynamic adsorption measurements were conducted on coals with six metamorphisms to characterize the micropore structural, chemical properties and CH₄-CO₂ mixture adsorption isotherms. The results show that the selectivity for coal samples at maximum vitrinite reflectance R_o,max ≥ 1.78 % shows an asymmetric inverse parabola trend with the increase of pressure, while it follows a general convex decreasing tendency for coal samples at R_o,max < 1.78 %. Compared with chemical structures, the micropore volume primarily determines the adsorption selectivity, while the super-micropore volume displays the highest negative correlation with selective coefficients. Two adsorption competitive mechanisms are summarized from thermodynamics: For low-rank coal, the decreased $q_{\text{st}}^{{\text{CO}}_{\text{2}}}/q_{\text{st}}^{{\text{CH}}_{\text{4}}}$ but with values much greater than 1 leads to a monotonous decrease in selectivity; For high-rank coal, $q_{\text{st}}^{{\text{CO}}_{\text{2}}}/q_{\text{st}}^{{\text{CH}}_{\text{4}}}$ is slightly larger than 1, CO₂ filling in ultra-micropore compensates for the competitive adsorption on medium and super-micropore surfaces at the low-pressure range, resulting in the initial enhancement of the inverse parabola-curved adsorption selectivity.