Insights into solid-gas conversion and cavity growth during Underground Coal Gasification (UCG) through Thermo-Hydraulic-Chemical (THC) modelling

This paper presents the application of a thermal-hydraulic-chemical model to simulate (1) a laboratory small-scale borehole combustion experiment using high-ash-content coal and (2) an ex-situ large-scale UCG experiment using low-moisture-content hard coal. The focus was to study temperature development, syngas composition, pore structure alteration of coal, and cavity growth pertinent to the UCG process. Both numerical simulations reproduced the temperature development trends along the gasification channel. Compared with the measured cavity with a horizontal length of 15 cm and a maximum vertical extension of 5.4 cm that was created in the 10-h-long borehole combustion experiment, a similar simulated cavity was formed in the first simulation, which had a horizontal length of 10 cm and a maximum vertical extension of 5.1–5.7 cm. This simulation indicated that the proposed model can be used to estimate the cavity growth with the process of UCG due to solid-gas conversion. In the second simulation, the simulated composition of syngas showed good agreement with the experimental results. It also revealed that fierce gasification and combustion mainly occurred close to the inlet, and thus created an L-shaped cavity in the 20-h-long UCG process. Moreover, the UCG process was distinctly divided into three stages from the perspective of solid loss and pore structure alteration at a representative node owing to thermal expansion, compressional shrinkage, pyrolysis, and gasification and combustion. The second stage of 3–11 h was the main period for combustion and gasification of the UCG process, namely, 70% of the solid mass was converted into syngas at temperatures above 1320 K, and the porosity increased linearly with time and the permeability showed exponential growth. Furthermore, a parametric sensitivity study based on the second simulation indicated that the simulated cavity growth of UCG was sensitive to the kinetics of char combustion, while the assumed pyrolyzed production ratios in the model were not of significance to the cavity growth during UCG. It is claimed that the coupled thermo-hydraulic-chemical model adopted in this paper provides novel insights into the UCG reactor's dynamic behavior, including solid-gas conversion and cavity growth