Selective transport of CO2, CH4, and N2 in coals: insights from modeling of experimental gas adsorption data

Selective adsorption and transport of gases in coal are important for natural gas recovery and carbon sequestration in depleted coal seams for environmental remediation. Gases are stored in coal mainly in the adsorbed state. In this study, the interaction energies of adsorbates (CO2, CH4, and N2) and micropores with various widths are investigated using a slit-shape pore model. The experimental adsorption rate data of the three gases conducted on the same coal sample are numerically simulated using a bidisperse model and apparent diffusivities of each adsorbate in the macropore and micropore are determined. The results indicate that the relative adsorbate molecule size and pore structure play an important role in selective gas adsorption and diffusion in micropores. Generally, the microporous coals diffusion is activated and the apparent micropore diffusivities of gases in coal decrease strongly with increase in gas kinetic diameters. Apparent micropore diffusivity of CO2 is generally one or two order of magnitude higher than those of CH4 and N2 because their kinetic diameters have the relation: CO2 (0.33 nm)<N2 (0.36 nm)<CH4 (0.38 nm). In contrast to theoretical values, apparent macropore diffusivity of CO2 is also larger than those of CH4 and N2, suggesting that coal has an interconnected pore network but highly constricted by ultra micropores with width <∼0.6 nm. It is also found that the apparent diffusivity strongly decreases with an increase in gas pressure, which may be attributed to coal matrix swelling caused by gas adsorption. Therefore, rigorous modeling of gas recovery and production requires consideration of specific interaction of gas and coal matrix.