Experimental Study of Lignite Structure Evolution Characteristics and Mechanisms under Thermal-Mechanical Co-function

in-situ thermal upgrading modification technology is of great significance to lignite utilisation cleanly. it is an extremely complex multi-field coupling process. Therefore, it is necessary to study the physical properties of lignite under the thermo-mechanical coupling function. in this paper, the lignite pore evolution characteristics under thermal-mechanical co-function have been obtained at different scales based on experimental results. The mechanisms also have been deeply studied. The results indicated that lignite total porosity first increased and then decreased as the temperature increased from 23°C to 400°C under the triaxial stress of 7 MPa. The maximum value of 21.64% for the total porosity of lignite was observed at 200°C. Macropores were dominant when the temperature was lower than 100°C, while visible pores were dominant when at temperatures ranging from 100~400°C. The thermal weight loss and deformation characteristics of lignite were further studied using a thermal-mechanical testing platform. The weight loss and deformation process could be divided into three stages, namely the slow, rapid, and relatively slow stages. After being continuously pyrolysed for 5 hours at a temperature of 400°C, the maximum weight loss rate of lignite was 52.38%, the maximum axial linear strain was 11.12%, and the maximum irrecoverable radial strain was 18.79%. The maximum axial thermal deformation coefficient of lignite was −2.63×10 −4 ℃ −1 at a temperature of 289°C. Macro-deformation and component loss were the main mechanisms of lignite structure evolution.