Deformation characteristics and structural evolution mechanism of bituminous coal under thermo-hydro-mechanical-chemical coupling

Studying the thermal deformation characteristics of coal under thermo-hydro-mechanical-chemical coupling is crucial for analysing roof stability, surface subsidence, and heat injection channel formation during the in situ pyrolysis mining of coal. Therefore, in this study, the deformation characteristics, gas production characteristics, and pore structure evolution as well as strength of bituminous coal during carbonisation and steam pyrolysis were explored. The results show that: 1) coal sample deformation can be divided into two stages: softening and pyrolysis deformation. Softening deformation depends solely on temperature, and pyrolysis deformation is affected by the severity of coal pyrolysis; 2) the deformation of bituminous coal and the evolution of pore and crack structures influence each other. Deformation promotes changes in pore and crack structures, and these structural changes serve as the internal drivers of deformation; 3) below 400°C, steam injection accelerates the softening deformation of bituminous coal. Meanwhile, above 400°C, deformation intensifies considerably under steam pyrolysis conditions owing to increased gas production of the coal sample compared to that during carbonisation. [Received: August 12, 2024; Accepted: April 9, 2025]