Microstructure evolution in bituminous-coal pyrolysis under in situ and stress-free conditions: a comparative study

A self-made triaxial testing machine with thermal-hydraulic-mechanical-chemical (THMC) coupling and a tubular heating furnace, combined with in situ (IS) micro-computed-tomography technology was utilized in this study. The evolution of pore-fissure (PF) structure parameters (porosity, PF scale distribution, effective PF volume ratio, and permeability) of bituminous coal under stress-free (SF) and IS conditions with temperature was investigated, and then the mechanism of experimental results was analyzed. Results showed that (1) under SF conditions, at 300-550 degrees C, the coal samples after pyrolysis are dominated by elongated large fissures, with PF structure parameters positively correlating with temperature. After 400 degrees C, the number of PFs increases, with most PFs having equivalent diameter (R) <= 100 mu m. (2) Under IS conditions, coal sample fissures are dominated by elongated large fissures at 300-350 degrees C and by holes at 350-600 degrees C. (3) Under IS conditions at 300-600 degrees C, the PF structure parameters of coal samples initially decrease with temperature and subsequently increase. The number of PFs fluctuates within a certain range, and the PF scale distribution dynamically shifts with temperature. (4) After 300 degrees C, the PF structure parameters of bituminous coal under SF and IS conditions show a bipolar distribution with temperature. Therefore, the weakening effect of stress on the PF structure of coal samples should not be overlooked during IS pyrolysis mining of coal bodies. In situ micro-CT scanning technology was developed.Microstructure evolution in bituminous-coal pyrolysis was studied under SF and IS conditions.Weakening mechanism in bituminous-coal pyrolysis was clarified under SF and IS conditions.