The early stages of the combustion of pulverized coal at high temperatures I: The kinetics of devolatilization

Abstract

Fine coal particles have been included in the gas supplies to fuel-lean flat flames of H 2 and O 2, diluted with either Ar or N 2. Initial coal particle size was carefully controlled and less than 106 μm. Measurements of [CO 2] were made along the axes of these O 2-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 ⋍ 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 CO 2 concentrations at various residence times. A mathematical model of these processes, involving devolatilization, combustion of volatiles in the O 2-rich flame gases, heat transfer to each coal particle, and mass transfer of volatiles, combustion products, and O 2, has been formulated. This model is capable of predicting the measured “ignition times” and also [CO 2] as a function of residence time. In particular, [CO 2] 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 × 10 3s −1 and E = 23 ± 5 kJ/mol.