DRYING AND DEVOLATILIZATION OF LARGE COAL PARTICLES UNDER COMBUSTION CONDITIONS

An experimental and modelling investigation was carried out on the drying and devolatilization (under combustion conditions) characteristics of low-rank coals for particle sizes of interest in fluidized bed combustion. Experiments were conducted on single particles, suspended by a flow of air in a quartz reactor, of low grade coal (Bowmans deposit) for an extensive range of operating conditions. The residence time of the coal particle in the reactor was varied to obtain time resolved histories on the drying and devolatilization behaviour under combustion conditions. To model drying and devolatilization under combustion conditions, the analysis was divided into pre-ignition and post-ignition stages. The pre-ignition stages corresponded to drying and devolatilization under pyrolysis conditions, while post-ignition stages were characterized by the formation of a diffusion flame around the coal particle. The energy feed back from the flame, at a higher temperature than the air, enhances the rate of subsequent drying and devolatilization. The model result, although in qualitative agreement with experimental data, indicate the necessity for more detailed modelling, including: a more rigorous estimate for the wet-dry interface temperature; and the effect of heterogeneous ignition and combustion of volatiles trapped in the solid matrix for larger particles (initial radius ≈5 mm) prior to the formation of the volatiles flame. © 1990.