H2, CH4 and CO2 adsorption on Cameo coal: Insights into the role of cushion gas in hydrogen geological storage

Abstract

Large-scale hydrogen underground storage is widely considered as an important solution for energy transition to mitigate global warming, however, our understanding on the various chemical and physical processes leading to H2 loss during geo-storage remains largely unknown. We conducted adsorption isotherms measurements of H2, CH4 and CO2 on Cretaceous Cameo coal at 35, 50 and 65 °C, to explore adsorption mechanisms of H2 and deciphering the effect of cushion gas (e.g., CH4, CO2) on H2 loss in the subsurface. The adsorbed amounts of H2 are just 12% and 6% of CH4 and CO2 at temperature of 35 °C and pressure of 2.3 MPa, respectively, and H2 adsorption behavior on Cameo coal can be described well by Langmuir monolayer theory. The extremely high Langmuir pressures (35–49 MPa) for H2 indicate very weak affinity with coal, compared to CH4 and CO2. Differences of Langmuir adsorption capacities suggest that the gas adsorption of H2 and CH4 are mainly by physical interactions while both physical and chemical bonding with coal structures occurs for CO2. The affinity with solid materials for the three gases can be quantitatively evaluated by the heat of adsorption, which was determined to be ∼9.2 kJ/mol for H2. Extrapolation of our experimental results to natural coal seams indicates that H2 presents much smaller adsorption amounts at depth up to 2000 m, corresponding to temperatures lower than 65 °C and pressure less than 30 MPa, geological conditions that are suitable for hydrogen underground storage. This study confirms that the injection of CH4 or CO2 as cushion gas can largely reduce the H2 loss by adsorption in the subsurface. Empirical calculation suggests H2 adsorption will be insignificant with chemical composition of CH4 above 8% or CO2 above 2% at the storage sites (e.g., abandoned mines, depleted coal seams). Our study lays a foundation on the understanding of the risk of H2 loss in large-scale hydrogen underground storage.