Influence of high-pressure CO2 exposure on adsorption kinetics of methane and CO2 on coals

This work was performed to address the effect of high-pressure CO2 exposure on methane and CO2 adsorption kinetics behavior on various rank coals. The adsorption kinetics curves of methane and CO2 on coal samples before and after CO2 exposure were measured under 45 °C and approximately 0.41 MPa. The possible reasons for CO2 exposure dependence of adsorption kinetics of methane and CO2 were also studied. The results show that CO2 exposure causes a decrease in both methane and CO2 diffusion-adsorption rates on coals as indicated by both macropore and micropore apparent diffusion coefficients. The decreasing trend of diffusion-adsorption rate is more evident for CO2 than methane on coals after CO2 exposure. Further investigations indicate that the effect of CO2 exposure on adsorption kinetics of methane and CO2 is related to both surface chemistry and pore structure of coals. On the one hand, pore structure analyses indicate that CO2 exposure causes a slight decrease in micropore and meso/macropore of coals. On the other hand, CO2 exposure leads to a decrease in the oxygen-containing functional groups mainly including carbonyl (CO) and carboxyl (COOH) of coals. The oxygen-containing functional groups on coal surface benefit the diffusion and adsorption of CO2. However, this effect is opposite for methane. Thus, it is concluded that the alterations of both pore structure and oxygen-containing functional groups due to CO2 exposure contribute to the decrease of CO2 diffusion-adsorption rate. With regard to methane, the influence of pore structure on adsorption kinetics behavior is superior to the oxygen-containing functional groups, which accounts for the decrease of the adsorption and diffusion rate. The design of practical CO2-ECBM process needs to consider the effect of CO2 exposure on methane and CO2 diffusion and adsorption within coals.