Role of moisture on gas sorption capacity and kinetics of Coal: Novel experimental and modeling insights

Abstract

Water in coal pores, whether as gas or condensed can impact the CO₂ and CH₄ sorption behavior during Carbon Capture, Utilization, and Storage (CCUS) and Enhanced Coal Bed Methane (ECBM) processes. Most of the studies so far use dry or moisture equilibrated coal samples to assess water’s impact on CO₂/CH₄ sorption. Experiments with the dry coal samples could overestimate the net adsorption due to not accounting for the moisture while moisture equilibrated coal samples capture the impact of moisture to some extent but lacks insights into the dynamic and competitive sorption processes. To address these limitations, we devised controlled RH systems using salt solutions (CRHS) experiments, offering a realistic and dynamic approach to study water–gas interactions in coal. CRHS experiment models the simultaneous sorption of water vapor and gases on coal surfaces under controlled RH systems. The experiments were performed up to < 9.5 MPa and < 5.2 MPa for CO₂ and CH₄, respectively. The results demonstrate that vapor’s presence reduces CH₄ and CO₂ adsorption capacity from 25.36 g/kg to 18.72 g/kg and from 55.94 g/kg to 37.71 g/kg, respectively, highlighting the stronger inhibitory effect of water on CO₂/CH₄ sorption. Additionally, a decrease in heat of adsorption (ΔQ) from 17.38 kJ/mol to 15.34 kJ/mol for CH₄ and from 15.56 kJ/mol to 14.93 kJ/mol for CO₂ indicates weakened van der Waals interactions due to moisture. Overall, findings signifies that the CRHS effectively demonstrated that the presence of moisture not only reduces the gas adsorption capacity but also alters the sorption kinetics.