In-bed char combustion of Australian coals in PFBC. 3. Secondary fragmentation

Secondary fragmentation occurs during the char combustion stage of coal combustion in fluidised beds and was investigated at pressure in a bench-scale PFBC rig for five Australian black coals. The critical diameter of char particles above which secondary fragmentation occurred was 2 mm for all the coals studied. In contrast with previous work [1 and 2] at atmospheric pressure, secondary fragmentation was found to occur during the early stage of char combustion, explaining its influence on char burnout and in-bed char loading in PFBC.

Two normal distribution functions were found to describe the secondary fragmentation behaviour in PFBC. The first determined randomly which particle broke up, and the second predicted the size distribution of fragments. Neither char properties nor oxygen concentration significantly affected the parameters in these two statistical functions. However, the rank of the parent coal correlated with the ratio of maximum number of char fragments to initial number of char particles. This secondary fragmentation behaviour should be included in mathematical modelling of char combustion in PFBC to predict the char burnout correctly.