Methane Recovery from Coal Bed Gas Using Modified Activated Carbons: A Combined Method for Assessing the Role of Functional Groups

Chemical modification at room temperature on adsorbents is a cost-effective and convenient way of enhancing the methane recovery from the coal bed gas. In this study, modifications on coconut-shell-based activated carbons (ACs) by aqueous ammonia and KOH agents were investigated and their performances on CH4/N2 separation were evaluated through equilibrium and dynamic adsorption tests. The roles of the oxygen- and nitrogen-containing groups in selective adsorption for CH4 over N2 on modified ACs were mainly discussed on the basis of the results of the Fourier transform infrared spectroscopy (FTIR) characterization and the density functional theory (DFT) calculation. With this combined method, the practical changes in AC surface chemistry during modifications and the theoretical adsorption energies of CH4 and N2 over graphene models were obtained. Aqueous ammonia was shown to improve equilibrium and dynamic selectivities of CH4 by 11.7 and 14.9%, respectively, likely as a result of the introductions of amine and amide capable of differentiating the adsorption energy of CH4 and N2 on ACs. However, KOH modification reduced the selectivity of CH4 as a result of the decreases in amine and hydroxyl, despite an increased CH4 adsorption capacity likely as a result of the decreased epoxy and hydroxyl.