Effects of Supercritical CO2 Treatment Temperature on Functional Groups and Pore Structure of Coals

The buried depth of a coal seam determines the temperature at which CO2 and coal interact. To better understand CO2 sequestration, the pore structure and organic functional groups of coal treated with different ScCO2 temperatures were studied. In this study, three different rank coals were treated with ScCO2 at different temperatures under 8 MPa for 96 h in a geochemical reactor. The changes in pore structure and chemical structure of coal after ScCO2 treatment were analyzed using mercury intrusion porosimetry, attenuated total reflection Fourier transform infra-red spectroscopy, fractal theory, and curve fitting. The results show that the enhancement effect of ScCO2 on pore structure of coal becomes less significant as the increase of buried depth. In most of the treated coal samples, the variation proportion of mesopores decreased and the variation proportion of macropores increased. In the relatively higher rank coals, the degree of condensation (DOC) of aromatic rings decreased after treatment with ScCO2. The DOC values showed a U-shape relationship with temperature, and the aromaticity showed a downward trend with increasing temperature. The chemical structural changes in the relatively lower rank coal sample were complex. These findings will provide an understanding of mechanisms relevant to CO2 sequestration with enhanced coalbed methane recovery under different geothermal gradients and for different ranks of coal.