Low Subcritical CO₂ Adsorption-Desorption Behavior of Intact Bituminous Coal Cores Extracted from a Shallow Coal Seam

This study focuses on improving fundamental understanding of low, subcritical CO₂ adsorption-desorption behavior of bituminous coals with the aim to evaluate the utility of shallow-depth coal seams for safe and effective CO₂ storage. Comprehensive data and a detailed description of coal-CO₂ interactions, e.g., adsorption, desorption, and hysteresis behavior of intact bituminous coals at CO₂ pressures <0.5 MPa, are limited. Manometric sorption experiments were performed on coal cores (50 mm dia. and 30- or 60-mm length) obtained from a 30 m deep coal seam located at the Upper Silesian Basin in Poland. Experimental results revealed that the adsorption capacities were correlated to void volume and equilibrium time under low-pressure injection (0.5 MPa). The positive deviation, observed in the hysteresis of adsorption-desorption isotherm patterns, and the increased sample mass at the end of the tests suggested CO₂ pore diffusion and condensation. This behavior is vital for assessing low-pressure CO₂ injection and storage capabilities of shallow coal seams where confining pressure is much lower than that of the deeper seams. Overall, CO₂ adsorption depicts a type II adsorption isotherm and a type H3 hysteresis pattern of the IUPAC classification. Experimental results fitted better to the Brunauer-Emmett-Teller model than the Langmuir isotherm model. CO₂ adsorption behavior of intact cores was also evaluated by characteristic curves. It was found that Curve I favored physical forces, i.e., the presence of van der Waals/London dispersion forces to describe the coal-CO₂ interactions. However, analysis of Curve II indicated that the changing pressure-volume behavior of CO₂ in the adsorbed phase, under low equilibrium pressures, cannot be ignored.