Multi-scale mechanisms governing temperature field propagation and gas release in water-bearing coal seam in the context of in-situ enhanced combustion

Efficient and safe in-situ enhanced combustion of coal seams (IECC) underpins in-situ coal chemical fluidization and artificial geothermal reservoir construction. As an inherent characteristic of coal seams, water significantly influences temperature field dynamics and gas composition during combustion. This study employed TG-FTIR and DSC, and a custom-built ex-situ system to reveal the mechanisms governing the synergistic evolution of temperature propagation, gas release, and exhaust gas explosivity under typical water-bearing conditions in IECC. The results show a non-monotonic dependence on moisture content: both combustion intensity and gas release increase first and then decrease as moisture rises. At moderate moisture levels, rapid evaporation produces water vapor that participates in high-temperature reactions, promotes radical chemistry, and enhances the coupled diffusion and contact of O 2 and H 2 O in pores, thereby strengthening heat release and product escape. Optimal performance occurs at 24% moisture, yielding a maximum propagation rate of 167 mm·h -1 , while intermittent evaporation can introduce early-stage temperature fluctuations. Based on temperature–time integration, the relative thermal contributions of the drying/pyrolysis, reduction, and oxidation zones follow an approximate 1:4:6 pattern, with 24% moisture shifting the balance toward a higher oxidation contribution and lower pyrolysis heat demand. Combustible gases (H 2 , CH 4 , and CO) are released predominantly within the first 2 h after ignition and peak near 24% moisture. Composition-based screening using theoretical criteria indicates that the exhaust gas mixture may enter a potentially explosive envelope within the initial 1.5 h, highlighting the need for early-stage monitoring of both total combustibles and residual oxygen. These findings clarify the moisture-regulated combustion mechanism and provide guidance for efficient operation and safety control of IECC.