Thermal-hydro-chemical-mechanical alteration of coal pores in underground coal gasification

Underground coal gasification (UCG) is being considered an economically and environmentally sustainable clean coal technology which converts coal in-situ into combustible gases. It involves a number of complex physiochemical processes, e.g., drying and evaporation, thermal expansion, pyrolysis, compressional shrinkage, gasification and combustion. As a result, coal pores constantly undergo drastic changes in the course of UCG. This work aims to shed light upon the complex changes of coal structure in the context of UCG through modelling. A 3-D UCG model was constructed using very fine grids, and several important indicators regarding pore variations were studied, i.e., weight loss of coal, solid concentrations, porosity and permeability. The modelling results were comprehensively compared with experimental data, and a good agreement in trend was achieved. It shows that coal pores alteration is associated with the various phenomena occurred in UCG, and four distinct characteristics can be categorized in the indicator curves. Furthermore, coal pores variation with respect to coal gasification and combustion, not covered in the experimental works, was also revealed. The findings would serve as a helpful guide for process design and optimization.