H2 and CH4 adsorption on coal: Insights from experiment and mathematical model

Underground hydrogen storage (UHS) plays a crucial role in promoting the large-scale adoption of hydrogen energy. Due to their high adsorption capacity and porosity, coal seams present a promising option for UHS. To investigate the adsorption characteristics of coal for methane (CH4) and hydrogen (H2) as well as analyze the pore structure of coal, several techniques were employed, including nitrogen (N2) adsorption, carbon dioxide (CO2) adsorption, and high-pressure mercury intrusion. In addition, isothermal adsorption and desorption experiments were performed for CH4 and H2 at temperatures ranging from 40 to 80 degrees C, under pressures from 0 to 18 MPa. The experimental data were fitted using a simplified local density (SLD) model combined with different equations of state to interpret the results. The adsorption results for CH4 showed that the excess adsorption increased and then decreased, while the excess adsorption of H2 showed an approximately linear increase with increasing pressure. The SLD model combined with different equations of state indicates that the peaks of adsorbed phase density of both CH4 and H2 occur at the molecular center of the first adsorbed layer, and this position does not vary with pressure or temperature. Both H2 and CH4 molecules are preferentially adsorbed in micropores. The molar density and absolute adsorption of CH4 increased with increasing pressure, but the growth rate gradually became slower. The molar density of the adsorbed phase and the absolute adsorption of H2 increase approximately linearly with increasing pressure. The ratio of the average density of the adsorbed phase to the bulk phase density is relatively constant during H2 adsorption; thus, adsorption plus compression for hydrogen storage could be a much more efficient strategy to store hydrogen than compression alone.