Acoustic emission (AE)–based study on low-temperature pyrolysis characteristics of long flame coal

Investigating crack formation and propagation in the low-temperature pyrolysis of coal is beneficial for optimising the pyrolysis process and improving the coal utilisation rate. In this study, the low-temperature pyrolysis processes of long flame coal at different temperatures (150–450 ℃) were monitored by acoustic emission (AE) technology. In addition, the effects of temperature and holding time on crack development were also analysed. The results demonstrated that 300 ℃ was a threshold for depicting low-temperature pyrolysis characteristics of long flame coal. The AE signals were weak before 300 ℃ and improved with an increase in temperature. Next, the AE signals increased significantly and were negatively correlated with the temperature. According to the generation development model proposed in this paper, the increase in temperature led to the rapid development of the primary cracks before 300 ℃, which were then slowed down. Notably, the secondary cracks began to expand after 300 ℃. The AE b-values reflected different crack development modes at 350 ~ 450 ℃. The increasing pyrolysis temperature and holding time were conducive to the development of secondary cracks. The rapid decrease and then sudden disappearance of b-values could be regarded as the criterion of macro failure of coal samples.