INVESTIGATION OF COMPETITIVE ADSORPTION AND DIFFUSION BEHAVIORS OF CO₂/CH₄/N₂/H₂O AFFECTED BY COAL RANKS IN SLIT STRUCTURES

In this work, different slit structures of coals (brown coal, bituminous coal, and anthracite coal) with various ranks are established at a molecular level. Based on the grand canonical Monte Carlo (GCMC) and molecular dynamics (MD) methods, the effects of coal rank on CO2, CH4, N-2, and H2O multi-component gases adsorption and diffusion in slit structures are investigated. The influence of coal rank on the swelling ratio caused by adsorption and diffusion in different slit structures is discussed for the first time. It is shown that the adsorption capacity of CO2, CH4, N-2, and H2O in different slit structures follows the order of H2O > CO2> CH4 >N-2, whereas the diffusion coefficient follows H2O < CO2 < CH4 < N-2. With the growth of coal rank, the adsorption capacity and diffusion coefficients of CO2, CH4, N-2, and H2O increase, meanwhile, the adsorption selectivity coefficients of CH4/CO2, N-2/CH4, and CO2/H2O rise, following the order of anthracite coal > bituminous coal > brown coal. In addition, both the adsorption and diffusion swelling ratio reduce with the decrease of coal rank. The present molecular investigation of competitive adsorption and diffusion characteristics of multi-component gases can provide theoretical basis for efficient exploitation of coalbed methane.