Evolution of pore structure in coal during underground thermal treatment: an experimental investigation

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 (< 2 nm) volume and specific surface area increase continuously; mesopores (2-50 nm) volume and specific surface area gradually decrease; macropores (> 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: R_o < 0.54% (30-350 ℃), 0.54% < R_o < 1.39% (350-450 ℃), and R_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.