• Pore water storage characteristics significantly influence methane adsorption in coal, with effects varying by coal rank.
• During water diffusion, adsorbed water penetrates micropores, with brown coal retaining more adsorbed water than anthracite.
• High-rank coal (anthracite) shows a marked decrease in methane adsorption capacity (Langmuir volume reduction of 21.22 cm3/g) during both water diffusion and addition phases.
• Molecular dynamics simulations reveal that hydroxyl groups enhance electrostatic interactions, increasing steric hindrance and inhibiting hydrogen bond formation, correlating with pore water content.