Research on the expansion law of combustion cavity in deep coal seam underground gasification based on multi-physics field simulations and response surface method

The combustion cavity expansion is the unique feature of underground gasification, and it could cause fluctuation in temperature field, combustible gas composition and calorific value, so the proper cavity expansion is essential to the safe, stable and continuous operation of gasification process. In this paper, a 3D model of underground gasification of deep coal seam with horizontal channel was constructed using COMSOL Multiphysics, and the effects of gas flow rate, oxygen concentration and gasification pressure on the three-dimensional expansion of a single-cycle combustion cavity were studied. The simulation results show the optimal parameter condition is gas flow rate of 0.45 m/s, oxygen concentration of 60 % and gasification pressure of 8.0 MPa. Furthermore, by applying the Box-Behnken design-response surface method coupled with variance analysis and goodness-of-fit test, the response surface model and regression equation between both coal consumption and three-dimensional expansion of the single-cycle combustion cavity with key gasification parameters, as well as the order of influence of control parameters were established and ascertained. The results indicate the variable with the highest impact on consumption volume, height and width expansion is gasification pressure, while gas flow rate poses the greatest influence on axial expansion. The relevant results can provide theoretical basis for the stable operation of deep gasification process.