Kinetics of combustion of size-graded pulverized fuels in the temperature range 1200–2270°K

The rates of reaction of size-graded fractions of three pulverized fuels have been measured in the temperature range 1200–2270°K. The fuels, and the corresponding mass median sizes of the fractions, were: petroleum coke (4, 18, 67 and 77 μm); anthracite (42 and 72 μm); and char from a swelling bituminous coal (34 and 60 μm). For all three materials the combustion rates were found always to be less, and usually to be much less, than the limiting rate of oxygen diffusion to the particles. The chemical reaction rate coefficients (g/(cm2 sec atm O2), calculated from the measured rates corrected for external diffusion resistance, were found to be independent of particle size and burn-off over the range 0–95% burn-off. The particles of all three materials appeared to shrink as they burned, and to burn at substantially constant density. The temperature coefficient of reaction rate for each of the three materials corresponded to an apparent activation energy of about 17 kcal/mole. It is probable that combustion rates were limited by the combined effects of diffusion into the pores of the particles and chemical reaction on the pore walls. The results for anthracite and for bituminous-coal char agree well with those obtained by other workers, who have used precisely controlled conditions, notably Field [Combustion & Flame, 13, 237 (1969); 14, 237 (1970)], and Field and Roberts [BCURA Inf. Circ., 325 (1967)].