Dynamics of temperature-strength changes in the immediate roof and formation of the gasified cavity of an underground gasifier

This study aims to identify regularities in the evolution of the temperature field and the uniaxial compressive strength (UCS) of immediate roof rocks under the influence of chemical reaction zones along the combustion face. It also aims to determine how the goaf area forms and changes over time as a function of gasification duration, injection pressure of the blowing mixture, and coal seam thickness. The study was carried out using a laboratory UCG setup that reproduces combustion face advance and roof deformation. The temperature in the modelled immediate roof was recorded by sensors installed along the reaction channel. Siltstone samples taken from the immediate roof of seam n7n at the Mezhyrichanska mine (SE “Lvivvuhillia”, Ukraine) were thermally treated and tested in uniaxial compression using a KL 200/CE-Tecnotest press. The goaf geometry was determined from roof-subsidence reference sensors, graphical reconstruction of contours at different time instants, and area calculation by the trapezoidal rule with consideration of producer-gas composition and concentration. At 0.55 m above the seam, temperature in the oxidizing zone (0-9 m) increased from approximately 323 to 550°C, reached about 573°C in the transition zone (9-11 m), and decreased to ~200°C in the reducing zone (11-30 m). UCS varied along the combustion face with a maximum near the transition zone and a subsequent decrease in the reducing zone. An exponential relationship was observed in the oxidizing zone, whereas a logarithmic relationship was observed in the reducing zone. The goaf area changed nonlinearly, predominantly exponentially, with gasification duration and seam thickness. It was also associated with the injection-pressure regime of the blowing mixture and roof-caving manifestations. The identified relationships can be used to predict goaf parameters and to assess roof stability when substantiating controlled UCG operating regimes.