THE EARLY STAGES OF THE COMBUSTION OF PULVERIZED COAL AT HIGH-TEMPERATURES .1. THE KINETICS OF DEVOLATILIZATION

Fine coal particles have been included in the gas supplies to fuel-lean flat flames of H2 and O2, diluted with either Ar or N2. Initial coal particle size was carefully controlled and less than 106 μm. Measurements of [CO2] were made along the axes of these O2-rich flames for residence times up to 4 ms. During this period the coal initially heats up rapidly and devolatilizes, with the first volatiles burning on the surface of each particle. After {reversed tilde equals} 1 ms the volatiles are produced sufficiently quickly that they burn away from the surface of a coal particle. At this stage (termed here "ignition") the flame emits the yellow radiation due to soot, which is formed during the pyrolysis of volatiles after leaving a coal particle and right up to combustion occurring in the diffusion flame around each particle. Measurements have been made of these "ignition times," as well as of CO2 concentrations at various residence times. A mathematical model of these processes, involving devolatilization, combustion of volatiles in the O2-rich flame gases, heat transfer to each coal particle, and mass transfer of volatiles, combustion products, and O2, has been formulated. This model is capable of predicting the measured "ignition times" and also [CO2] as a function of residence time. In particular, [CO2] and its rate of change with time was found not to vary with particle size for Illinois No. 6 coal in certain flames. This emerges as a feature of the model, which is based on early devolatilization occurring as one simple first-order chemical reaction. Its rate coefficient [A exp(- E RT)] is found to be such that A = 3.0 ± 0.8 × 103s-1 and E = 23 ± 5 kJ/mol. © 1990.