Modelling the ignition of brown-coal particles

Abstract

The unsteady-state mass and energy equations have been set up for the case of a single particle of brown coal plunged into a hot gas stream. These equations have been solved, with the aid of a digital computer, to give temperature histories for particles 20, 100 and 1000 μm in diameter. Analysis of these histories reveals that although ignition sometimes occurs with an abrupt temperature-jump, it can also occur without it; the last may possibly be associated with ignition instability. Reduction of particle size and of oxygen partial pressure both render temperature-jump ignition less likely. Gas temperatures required for ignition of brown-coal particles are only 600–800 K, the larger particles igniting at lower temperatures; ignition times are typically 5 ms for a 20 μm particle and 50 ms for a 100 μm particle, and are more affected by gas temperature than by oxygen concentration. It is unlikely that radiation plays a major part in the ignition of pulverized brown coal; recirculation of hot combustion products probably dominates. It is concluded that the ignition stability of burners fired with pulverized brown coal could be improved by avoiding the dilution of the coal stream with water vapour or inert drying gas, and by promoting the recirculation of hot gases back to the burner inlets. While the methods used are general and applicable (with appropriate reaction rate data) to other materials, the results here given depend heavily on the high chemical reaction rates between brown coal and oxygen at relatively low temperatures and are specific.