Significant factors affecting devolatilization of fragmenting, non-swelling coals in fluidized bed combustion

Abstract

A theoretical study has been made to identify those variables having a statistically significant effect on the devolatilization of fragmenting coals. To achieve this objective, a simple mathematical model has been developed for prediction of the devolatilization time of fragmenting coals and the number of fragmentations during devolatilization in conditions relevant to fluidized bed combustion. It incorporates important phenomena occurring during coal devolatilization, namely heat transfer from the bed to the surface of the coal particle, heat transfer inside the devolatilizing coal particle, primary decomposition reactions, transport of released volatiles from the coal matrix to the surface of the coal particle, and devolatilization-induced fragmentation. The model has been used for a factorial evaluation to determine significant variables affecting the devolatilization of fragmenting coals. Eight factors and two responses were selected, the responses were devolatilization time and number of fragmentations. Results of the factorial evaluation show that for both responses, the devolatilization of 1–20 mm diameter coal particles at 1023–1223 K is predominantly governed by coal particle diameter. The devolatilization time is also significantly affected by coal thermal properties, and less significantly by bed temperature and reaction rate of devolatilization. In addition to coal particle diameter, the number of fragmentations is significantly affected by variables related to volatiles transport to the coal particle surface, i.e. the diameter of coal convective pores and the viscosity of volatiles.