A moving heat source framework for automatic expansion modeling of the gasification cavity in UCG and its application

Underground coal gasification (UCG) is a promising technology for the clean and efficient in-situ utilization of coal resources, and realizing this potential requires accurate characterization of cavity evolution, as cavity morphology directly influences gasification stability, groundwater contamination, and land subsidence. However, existing methods remain limited in coupling automatic cavity boundary evolution with the thermo-mechanical response of surrounding strata, particularly for the steeply inclined coal seams. To bridge the gap, this paper proposed a 3D thermo-mechanically coupled framework incorporating a Moving Heat Source (MHS) algorithm to quantify the cavity–strata interaction processes, then validated its effectiveness using data from the Ulanqab UCG test site, and then applies it to explore, in a preliminary manner, the influence of coal seam inclination on UCG. Results show that: (1) the modeling framework can successfully simulate the commonly observed hyperbolic and trapezoidal isolation coal pillars formed during UCG in horizontal coal seams, and can explain why these two types of pillar geometries occur; (2) a transition in cavity morphology is observed with increasing inclination, evolving from a curved hourglass shape at 25° to a slanted parallelogram at 55°; (3) the cavity morphology transition is accompanied by increasingly asymmetric stress redistribution and a shift toward shear-dominated failure during gasification, suggesting that the cavity width-to-pillar width ratio should be re-examined in practical engineering design. The proposed framework is therefore quite helpful for dynamic simulation of UCG cavity growth and provides a process-level decision-support tool for cleaner UCG design by linking cavity evolution, pillar instability, and environmental risk prevention.