Molecular simulation of CH4/CO2/H2O competitive adsorption on low rank coal vitrinite

Abstract

The competitive adsorptions of CH₄/CO₂/H₂O on coal vitrinite (DV-8, C₂₁₄H₁₈₀O₂₄N₂) were computed based on density function theory (DFT) and grand canonical Monte Carlo (GCMC). The adsorption process reaches the saturation state after adsorbing 17 CH₄s, 22 CO₂s, and 35 H₂Os per C₂₁₄H₁₈₀O₂₄N₂ respectively. The optimal configurations of CH₄–vitrinite, CO₂–vitrinite, and H₂O–vitrinite respectively manifest as aromatic¹/T²/rT³ (1 adsorption location, 2 adsorption sites and T here represents sites above the carbon atom and the heteroatom, 3 adsorption orientation and rT here means the orientations of three hydrogen atoms pointing to vitrinite), aromatic/T/v (v represents the orientations perpendicular to the plane of vitrinite), and aromatic/rV/T (rV represents an oxygen atom pointing to the vitrinite surface). The GCMC results show that high temperature is not conducive to the vitrinite's adsorption of adsorbates and the adsorption capacity order is H₂O > CO₂ > CH₄ (263–363 K) in the one-component, binary, and ternary adsorbate systems. The optimal configurations of vitrinite are similar to graphite/graphene, while ΔE is significantly lower than graphite/graphene. Simulation data are in good agreement with the experimental results.