A general model for devolatilization of large coal particles

There have been several studies on the devolatilization of large coal particles under laboratory conditions simulating large-scale fluidized or fixed-bed combustion conditions, all of which demonstrate that the process of devolatization of large coal particles occurs over a much longer timescale than that of pulverized coals under the same conditions. This paper reports the development of a model for large coal particle devolatilization under fluidized and fixed-bed combustion conditions. The model combines a heat transfer model with a general and comprehensive coal devolatilization model, FG-DVC. It inherits the generality of FG-DVC and can be applied to coals of various types without extensive prior knowledge of the coals. This model includes a detailed treatment of coal pyrolysis reactions including the yields of individual gas species, tar yields, and tar molecular weight distributions. The changes in coal physical properties are modeled with the application of Merrick's coal physical property submodels for specific heat and thermal conductivity. Both the convective and radiative heat transfer between the gas phase and coal particles are considered and Gunn's correlation for the surface heat transfer coefficient is used. Model predictions are compared with the data of large coal particle devolatilization measured in fluidized beds and tubular reactors, for particle thermal response, total weight loss, and individual gas evolution rates. In general, the agreement with the data is very good for the particle center temperature and weight loss curves and is fair for the individual gas evolution curves.