Systematic investigation on the effect of particle size on low-rank coal spontaneous combustion under various extrinsic conditions

Understanding the effect of particle size is vital for studying the low-temperature self-heating characteristics of coal. However, the particle size effect within coals with varying self-heating propensities and extrinsic conditions (temperature and humidity) is still unknown. Three types of Powder River Basin coal samples with three particle sizes (<75 µm, 75–212 µm, and >212 µm) were subject to the modified R70 tests under humid oxygen flow of different temperatures and humidity levels. Characteristics parameters, such as peak temperature, the time to reach thermal runaway, the R70 index, and dynamic wetting heat percentage (DWHP) were determined and systematically analyzed. The results demonstrated that the peak temperature of coal was reached faster as the particle size decreased in the original R70 test. The results of modified R70 tests indicated that the self-heating process of low-rank coal was more sensitive to particle size at lower temperatures. Furthermore, the particle size had a more significant impact on the self-heating process of coals with higher self-heating propensity. In the water cutoff experiments, the finer coal particles received a greater catalyzing effect from oxygen, confirmed by the greater experiment time reduction rate. For the contribution of wetting heat, the DWHP revealed that the coarse coal particles and the coals with lower self-heating propensity absorbed relatively more condensation heat, thus their self-heating process was more dependent on the external heat source.