Underground coal thermal treatment (UCTT) is an emerging technology for cleaner coal utilization, with the residual in-situ pyrolytic char offering promising potential for CO2 storage. To elucidate the dynamic evolution of coal pore structures during underground thermal treatment, this study systematically examined changes in pore structure, fractal characteristics, and evolution patterns across a temperature range of 0-600 ℃ using low-temperature CO2 adsorption, low-temperature N2 adsorption, and mercury intrusion porosimetry. The results show that as temperature rises, the total pore volume initially decreases and then increases, whereas the total specific surface area increases continuously. Specifically, micropore ( 50 nm) volume first decreases and then increases. With increasing temperature, the fractal dimensions D1 and D2, derived from nitrogen adsorption, initially decrease and then increase, whereas the D1 obtained from mercury intrusion porosimetry exhibits a continuous decline, indicating reduced surface roughness and complexity followed by recovery, except for macropores (> 140 nm) whose complexity consistently decreases. Based on the thermal evolution characteristics of coal, the pore evolution model can be categorized into three stages: Ro o o > 1.39% (450-600 ℃). A higher thermal treatment temperature (600 ℃) promotes the development of micropores and macropores, increases the total specific surface area, and enhances pore connectivity. These findings establish a theoretical basis for optimizing the operational parameters of UCTT.
Yang et al. (Thu,) studied this question.