The effects of supercritical CO₂ on the chemical structure of bituminous coal and anthracite

The effects of supercritical CO₂(ScCO₂)on coal chemical structure play a critical role in CO₂ storage capacity in coal. In this study, CO₂ sequestration processes were replicated using a ScCO₂ geochemical reactor. Four coal samples with different ranks were exposed to ScCO₂ and water under 62.5℃ and 15 MPa. Fourier transform infrared spectroscopy and X-ray diffraction analysis were used to identify the chemical structure of the coal samples before and after ScCO₂-H₂O treatment. Then evolution mechanism of coal chemical structure was studied. The results show that after ScCO₂ treatment, the length of aliphatic hydrocarbon generally increased, and only the abundance of the aromatic hydrocarbon in the fat coal increased. The increase of the abundance of the oxygen-containing groups in coal samples was mainly contributed by the increase of hydrogen bonding groups, except for the anthracite whose abundance of the oxygen-containing groups was affected by the dissolution of low-molecular compounds. Swelling caused by ScCO₂ caused breakage of cross links between the aromatic layers in fat and lean coal, making the structure of crystallite loose. While the formation of C_Ar-C_Ar cross links in the aromatic layer increased the aromaticity and aromatic ring condensation of the fat and lean coal. The falling off of none polycyclic aromatic hydrocarbons in meagre coal and anthracite made the structure of crystallite more compact, and the increase of the length of the aliphatic hydrocarbon reduced the aromaticity and aromatic ring condensation of meagre coal and anthracite.