Primary fragmentation of a coal in fluidized bed combustion

Abstract

Primary fragmentation is related to the build up of pressure in the pore network of a coal particle during devolatilization. Experiments of primary fragmentation of a bituminous, non-swelling South African coal have been carried out by injecting irregular particles, spheres and cubes of size ranging between 1 and 15.5 mm in a basket-equipped fluidized bed combustor of 40 mm ID. Depending on the size of coal, fragmentation occurs with a different degree of probability and with the generation of fragments characterized by a variety of shapes. This may include up to eight different types of fragments for the coarser coal particles tested. Material shows an elastic-brittle behavior with fractures parallel to bedding, planes of coal. Separate experiments have been carried out in order to determine average tensile strength of coal at different temperatures and its dispersion around the average due to non-homogeneities of material. A model of primary fragmentation has been developed. It is made of a serial structure of four submodels which account for the heatup of coal particles in the bed, the kinetics of devolatilization, the volatile convective transport and build up of pressure in the particle pore network and, according to the statistical theory of Weibull, the fragmentation of particles when internal pressure force overcomes the resistance force of the material. The basic model assumes spherical symmetry of coal particles. A modified version considers that non-spherical fragments still rich in volatiles may experience further fragmentation as if they were fresh spheres of equivalent diameter. The study shows that effects of primary fragmentation may noticeably influence fluidized combustion behavior of coal tested particularly in the case of coarse feed. Probabilities of particle fragmentation, multiplication factors of the number of particles, average sizes of fragmented material predicted by the modified model are in good agreement with experimental results.