A semidetailed model of primary fragmentation of coal

Abstract A mathematical model has been developed to predict the primary fragmentation of coal particles under a wide range of heating conditions as a consequence of thermal stress and of overpressure generated by volatiles. The model calculates the temperature and devolatilization profiles within the particle during heat up, the stress profiles that arise within the particle as a consequence of temperature gradients and volatiles pressure, the probability, timing and mode of fragmentation of the particle based on Weibull theory [13] . Gradual transition from the chemico-physical properties of the original coal to those of the incipient char is allowed for. The model is used to interpret experimental results of primary fragmentation under different heat treatment conditions. Results show that different fragmentation patterns are possible according to initial particle size and heating severity. Under conditions typical of PF combustion, i.e. small particle size and very high temperature and heating rate, the main fragmentation pattern is exfoliation of the outer shell under the action of thermal shock which produces a multitude of fine particles. Under conditions typical of fixed/moving bed reactors, i.e. larger particles and lower temperature/heating rate, fragmentation occurs mainly at an inner radial position, close to the particle center, under the cooperative action of thermal stress and volatile overpressure, which produces a limited number of relatively coarse fragments. In some situations, such as for large particles and severe heating conditions, multiple successive events of exfoliation and fragmentation at the particle center are possible.