Wall collision-induced fragmentation of coal particles in rapid pyrolysis: Effects of coal rank and impact angle

During rapid pyrolysis, the fragmentation effect induced by the collision of coal particles with walls is a key factor promoting the formation of fine particulate matter in tar. This study employed an integrated approach combining high-speed visualization systems and digital image processing techniques to systematically analyze the fragmentation characteristics of six coal particles of different ranks during interactions with walls at various impact angles. The results demonstrate a significant negative correlation between coal rank and particle fragmentation propensity. Among the coal types studied, Zhaotong (ZT) coal, a representative low-rank coal, displayed pronounced fragmentation behavior due to the rapid release of volatiles during pyrolysis, forming an easily fractured porous and fragile char matrix. Notably, compared to the high breakage ratio of 6.5 % observed during normal (0°) impact, the fragmentation probability of coal particles colliding with inclined walls significantly decreased (e.g., reduced to 4.3 % at 30° and further to 0.3 % at 75°). This fragmentation probability exhibited a clear decreasing trend with increasing impact angle. The collision behavior was further quantified by analyzing key parameters including the coefficient of restitution, impact angle, incident velocity, and rebound velocity. This systematic investigation provides important theoretical insights into the fundamental mechanisms governing coal particle behavior under fast pyrolysis conditions.