In-depth analysis of coal chemical structural properties response to flue gas saturation: perspective on long-term CO₂ sequestration

A comprehensive study on the chemical structural properties of coals after flue gas exposure is significant to consider long-term carbon dioxide (CO2) sequestration in deep and unmineable coal reservoirs. Two South African coals were exposed to a five-component synthetic flue gas typical of a coal-fired power plant for 90 days at 9.0 MPa pressure and 60°C temperature. Advanced characterisation techniques were used, including carbon-13 solid-state nuclear magnetic resonance spectroscopy (¹³C ^ssNMR), universal attenuated total reflectance-Fourier transform infrared (UATR-FTIR), field emission gun scanning electron microscopy with energy dispersive X-ray spectroscopy (FEG SEM-EDX), and wide-angle X-ray diffraction (WXRD) to capture the chemical structural changes. The results show weakened functional groups of –OH, out-of-plane aromatic C–H, aliphatic C–O, C–C, and C–H. There is structural deformation in the crystalline diameter and inter-layer spacing that is sorption-induced owing to the flue gas saturation, and the coals oxygen functionalities revealed notable changes.