Molecular concept and experimental evidence of competitive adsorption of H2O, CO2 and CH4 on organic material

Unconventional gas, such as shale gas or coalbed methane offers an attractive low-carbon solution and furthermore provides possibilities for CO₂-storage and coevally for enhanced gas recovery. In order to better understand gas and water interaction with organic matter (natural coal) of different maturity we developed a molecular concept with experimental and literature support for sorption of CH₄, CO₂ and H ₂O on organic material over a broad range of thermal maturity (0.5-3.3% vitrinite reflectance). We present a conceptual model to explain CO₂ and CH₄ sorption in the presence of water on coal with varying coal maturity (from subbituminous to anthracite). Adsorption experiments have been performed on coals of different maturity at temperatures between 303 and 350 K, pressures up to 20 MPa and under dry and moisture-equilibrated conditions. With increasing coal maturity we find for both gases a linear sorption capacity trend for the moist and a more parabolic trend for the dry coal samples. Based on the differences in CH₄ and CO₂ sorption capacity on coals of different maturity as a function of moisture content we infer that oxygen-containing functional groups represent the primary sorption sites for which CO₂ or CH₄ compete with water molecules. The competitive interaction turns out to be a volumetric displacement independent of the gas type. A pore blocking mechanism could not be confirmed. Adsorbed molecules on anthracite are mobile within the adsorbed phase at low surface coverage. Additionally, restrictions in translational and vibrational movements of the sorbed gas molecules induced by adsorbed water molecules are observed. Therefore we conclude that sorbed molecules are more localised when water is present in the adsorbed phase, whereas at high surface coverage, the thermodynamic properties of adsorbed molecules are dominated by adsorbate-adsorbate interactions.