Regulation of underground coal gasification by supercritical CO2 modification: synergistic evolution mechanism of pores, surface, and chemical states

Developing deep coal resources is often hindered by technical challenges and low chemical reactivity. Underground coal gasification, a key approach for exploiting medium to deep coal seams, suffers from low reaction efficiency. In this study, the regulatory mechanism of supercritical CO 2 pre-treatment on gasification performance was investigated using multi-scale characterization. Results show that supercritical CO 2 exposure transformed the coal surface from dense to loose, prompting the development of numerous multi-scale pores and fractures. Specifically, the micropore volume and specific surface area increased from 0.0364 cm 3 /g and 168.231 m 2 /g to 0.0365 cm 3 /g and 169.455 m 2 /g, respectively. Similarly, the mesopore volume and specific surface area rose from 0.0190 cm 3 /g and 3.717 m 2 /g to 0.0198 cm 3 /g and 4.113 m 2 /g. During high-temperature simulated gasification, the raw and modified coal exhibited similar initial weight loss (9.19% and 9.38%, respectively). However, during the medium-temperature pyrolysis stage (200–600 °C), the modified coal showed a higher cumulative weight loss (23.68%) than the raw coal (17.02%). Consequently, the residual carbon rate at 1000 °C decreased from 18.06% to 15.84%, indicating that supercritical CO 2 modification promotes earlier volatile release and more complete gasification. Microscopic chemical evolution analysis indicated that the oxygen-containing functional groups were removed in the sequence: carboxyl group–ether bond/hydroxyl group–carbonyl group. The total content of these functional groups in the modified coal increased from 26.18% at 25 °C to 48.50% at 550 °C, before dropping to 34.24% at 600 °C. Concurrently, the proportion of aromatic carbon (C–C/C=C) reached 80.37%, notably higher than that of the raw coal (55.21%). In summary, supercritical CO 2 promotes volatile release, lowers gasification activation energy, and enhances overall reactivity. This is achieved through the synergistic regulation of pore structure, surface oxygenated intermediates, and carbon skeleton reorganization, establishing a cohesive “pore–surface–chemical state” regulatory framework.