Exploration of critical moisture content of coal based on CH4, N2, and CO2 adsorption experiments

Two types of experiments were designed to systematically analyze the mechanisms regulating gas adsorption by the wet phase state of the coal body and the critical moisture. Water vapor adsorption experiments at variable relative humidity (0-97%RH ) were used to characterize the moisture state transitions. These were combined with isothermal adsorption experiments at 25°C and gas injection displacement experiments to determine the adsorption characteristics of single-component gases and post-injection displacement gases. The experimental data show that when the moisture content reaches the phase transition threshold (critical moisture content = 4.50% ), the pore water undergoes the transition from an adsorbed to a free state; the inhibitory effect of moisture content on gas adsorption diminishes with the increase of moisture content, and the rate of change in gas adsorption capacity ∆Q approaches zero near the critical moisture content. The critical moisture content values measured for the CH4/N2/CO2 system converge within the range of 4.50 − 4.95%. Following gas injection replacement, the total gas adsorption change ∆Q_mix approaches zero at the critical moisture content, consistent with the single-component system. This confirms the universality of critical moisture content determination for gases, where N2 and CO2 can serve as substitutes.