Hybrid Chemisorption–Physisorption of Subcritical CO₂ on Coals: Implications for Safe and Long-Term Underground CO₂ Sequestration

Low-pressure (LP) CO2 adsorption experiments were conducted on low-rank and high-rank coal samples at three different temperatures─273.15, 283.15, and 293.15 K. A hysteresis loop, although closed, was observed between the adsorption and desorption isotherms for all samples. The isosteric heat of adsorption (Qst) ranged from 39.6 to 60.9 kJ/mol (average 47.60 kJ/mol) and from 30.33 to 69.53 kJ/mol (average 43.85 kJ/mol) for two low-rank coal samples, suggesting the occurrence of both physisorption and chemisorption of CO2. In contrast, Qst values for three high-rank coal samples varied between 22.3 and 32.4 kJ/mol (average 25.6 kJ/mol), 20.8 and 30.4 kJ/mol (average 24.0 kJ/mol), and 24.6 and 38.7 kJ/mol (average 27.0 kJ/mol), indicating only CO2 physisorption. The adsorption process in low-rank coals was classified as weak chemisorption, while in high-rank coals, it was considered enhanced physisorption. A low-rank coal sample showed a remarkable absorption peak at wavenumbers 1000–1100 cm–1 in FTIR spectroscopy that was attributed to the hydroxyl (−OH) and −COOR groups in alcohols, phenols, and esters and Si–O–Si or Si–O–C groups in minerals. The strong interactions between these functional groups and the CO2 molecule were suggested as a possible reason for the weak chemisorption in low-rank coals. However, enhanced physisorption in high-rank coals was attributed to the relatively high intensity of other O-bearing groups at 1300 and 1600 cm–1 wavenumbers. This study demonstrates the occurrence of weak CO2 chemisorption in coals, characterized by adsorption energies exceeding the physisorption range, which has the potential to substantially enhance long-term storage safety.