Compartment Modeling for Flow Characterization of Underground Coal Gasification Cavity

Study on Effects of Thermal Resistance and Thermal Buoyancy on Oxygen Flow Patterns during Underground Coal Gasification

The controllable growth of the cavity is the basis by which underground coal gasification (UCG) can achieve stable production, and the oxygen flow path and velocity are important factors in determining the expansion rate of the cavity. In this paper, a mathematical model of UCG in horizontal channels was developed, and the effects of multiple factors, including temperature, pressure, flow velocity, and the size of the cavity, on the flow pattern, path, and velocity field distribution of oxygen in the cavity were investigated by using COMSOL Multiphysics software. The results showed that temperature and pressure were the influencing factors of thermal buoyancy. In the established model, oxygen formed a counter-clockwise air vortex at the gas injection port under normal pressure, and the range of the air vortex and the gas flow rate increased with the increase in temperature. In the high-temperature area located in the center of the cavity, the phenomenon of oxygen throttling occurred, and the oxygen flow velocity increased. When the maximum temperature in the cavity was over 850 °C, gas back-mixing occurred at the end of the cavity. Under pressurized conditions, the air vortex at the inlet and back-mixing phenomenon at the outlet disappeared. The flow velocity and the cross-sectional area of the cavity determined the thermal resistance. In the model, the flow velocity was between 0.045 and 0.40 m/s and there were both airflow vortices and back-mixing. In addition, with the expansion of the cavity, back-mixing progressively decreased at the outlet and the airflow vortex changed from counter-clockwise to clockwise.