Rank-Dependent Moisture Effects onMethane Adsorption in Coals

Moisture significantly influences methane (CH4) adsorption in coal seams, but its rank-dependent mechanisms remain poorly understood at the molecular scale. In this study, we integrated high-pressure CH4 adsorption experiments with molecular simulation to address the moisture effect across low-, medium-, and high-rank coals. Experimental results reveal that moisture reduces the methane Langmuir adsorption capacity (VL) by 29.2% for the low-rank, 24.0% for the medium-rank, and 17.2% for the high-rank coal sample. Simulation results demonstrate that moisture inhibits CH4 adsorption through two pathways: competitive displacement of CH4 molecules and physical blockage of pore throats by water clusters. For the low-rank coal model, water binds to oxygen functional groups via hydrogen bonds, displacing CH4 from O to N atoms. For the medium-rank coal model, water interacts strongly with oxygen and nitrogen functional groups through hydrogen bonds to occupy adsorption sites of CH4. For the high-rank coal model, with fewer oxygen functional groups compared with the low-rank, water attaches closely to O atoms. This pattern is distinct from that in the low-rank coal model, where water scatters due to the widespread O atoms. These findings provide a nanoscale explanation for the rank-dependent moisture effect on CH4 adsorption in coals.