Modeling Test of Combustion Cavity Growth during Underground Coal Gasification in the Early Stage of Ignition

The growth parameters of the underground coal gasification (UCG) combustion cavity are important for the regulation of its gasification process. The irregular cavities formed in the early stages of ignition can affect the stability of the gasification process. In this study, a heat-solid coupling model is used to determine the combustion cavity boundary at the early stage of coal seam ignition to simulate the movement of the combustion cavity boundary by indirectly inheriting the coal seam temperature. It reveals the evolution of the temperature field, stress field, and plasticity zone at the combustion cavity boundary at the early stage of ignition in the UCG process and compares with the ex situ small-scale experiments. The simulation results show that in the early stage of ignition, the temperature transfer to the top of the coal seam and the direction of the gasification agent outlet pipeline is faster, while the transfer rate to the direction of the gasification agent inlet pipeline is slower. The main stresses are mainly distributed in the left and right sides of the combustion cavity and gradually increase directly above. The plastic zone is mainly distributed directly above the combustion cavity and arc-shaped plastic zones. The experimental results show that the temperature directly above the combustion cavity is higher than in the other directions, and the ash layer hinders the temperature transfer to the bottom. Therefore, the combustion cavity has a longer elliptical shape in the upper part, which is consistent with the simulation results. The model better reveals the extension law of the combustion cavity at the early stage of UCG ignition and provides theoretical guidance for the study of combustion cavity formation.