Application of synchrotron ATR-FTIR microspectroscopy for chemical characterization of bituminous coals treated with supercritical CO₂

Injection of CO₂ into deep coal seams has offered great potential to sequester CO₂ and while simultaneously enhancing coalbed methane recovery. Unlike other sequestration options, CO₂ sequestration in coal is still infancy and requires fundamental knowledge to understand physical and chemical phenomena that happen after CO₂ injection in coal. The injected CO₂ exists in supercritical state (ScCO₂) under reservoir conditions and has great potential to interact with inorganic and organic matters in coal. One of many questions that still need to be answered is how coal structure changes from a chemical perspective upon CO₂ exposure. Synchrotron ATR-FTIR microspectroscopy was used to investigate associated changes of the functional groups of high-volatile and low-volatile bituminous coals, before and after processing with ScCO₂ for a period of 1 month. With increasing rank of coal samples, the synchrotron ATR-FTIR spectra showed an increasing aromaticity (i.e. ratio of ν(C–H)_aromatic to ν(C–H)_aliphatic). After being subjected to ScCO₂ treatment, the synchrotron ATR-FTIR spectra revealed an increased degree of aromaticity in high-volatile bituminous coal and low-volatile bituminous coal by 16.7% and 21.1%, respectively, due to the loss of aliphatic C–H functional groups. Most of the literature reported that CO₂ tends to physically adsorb onto pore surface of coal without chemical reactions. The results of this study provide an evidence of chemical reactions, i.e. mobilization of alkanes due to the ScCO₂ treatment. The breakage of the aliphatic C–H bonds increases the degree of aromaticity in the macromolecular structure after ScCO₂ exposure, which is analogous to heating treatment of coal. Such findings further reinforce the concept that the ScCO₂ acts as a good solvent and can increase macromolecular mobility of coal when it enters the macromolecular structure.