Combustion and Fragmentation Characteristics of Single Wet Coal Slime Particle

The combustion and fragmentation characteristics of wet coal slime particle are different from that of dried coal particle. In this paper, an understanding of the combustion process of wet coal slime particle through the combined results of the morphology characteristics, mass changes, and gas release pattern is provided. A tube furnace and high-speed camera were used to observe morphological changes of the combustion process of wet coal slime. For comparison, the mass change and gas release pattern of wet coal slime was monitored by a thermogravimetric analyzer and mass spectrometer (TG/MS). The four combustion stages of coal slime were observed for the first time: (1) drying and evaporation, (2) devolatilization and fragmentation, (3) volatile homogeneous combustion, and (4) char heterogeneous combustion. The water content in wet coal slime greatly affected the fragmentation of coal slime, it could build up a great internal pressure when evaporated, which caused the primary fragmentation to occur earlier and be more severe than traditional coals. The temperature and diameter of the slime were considered as factors for the occurrence of fragmentation and the level of it. A larger diameter with a higher furnace temperature resulted in a more severe fragmentation of the wet coal slime. The smaller coal slime at the lower temperatures did not experience the fragmentation stage. Here, the fragmentation and non-fragmentation zones of wet coal slime at different temperatures and different paticle diameters are given, which could be a guide for the combustion of coal slime in CFB(Circulating Fluidized Bed). Also, for larger diameters and lower setting temperatures, the ignition delay time and burnout time of wet coal slime became longer. The gas release pattern of wet coal slime during the combustion process was analyzed, and it was determined that it could be affected greatly by fragmentation. A model of the drying and evaporation stage of wet coal slime was established for the first time by using COMSOL Multiphysics. The results of the theoretical calculations show that there were two evaporation rate peaks of water when fragmentation occurred, which theoretically explained the early stage of the combustion process of wet coal slime, and the results were in agreement with the experiments.