OXIDATION RATES OF CARBON PARTICLES IN METHANE AIR FLAMES

Small amounts of graphite were introduced into a laminar, flat, low-pressure, fuel-lean methane-air flame. The mean initial diameter of the particles was about 4-mu-m. The rate of oxidation of the particles was observed by optical means as they passed through the flame. In addition, measurements of particle and gas temperatures were used to estimate the heat released by the surface reaction. The results were found to be reasonably consistent with those found from more direct measurements of particle size. The study showed a very considerable enhancement (by a factor of up to 5) of the carbon oxidation rates in the main gaseous flame reaction zone as compared with the burned gas region, thus indicating the importance of the flame radicals O, OH, and H in controlling the oxidation. Consideration of the collision frequencies with the external surface area showed, however, that the bulk gas-phase concentrations of the radicals were insufficient for their direct diffusion and reaction to be able to account for the enhancement. At the same time consideration of the particle properties excludes porosity effects as a means of rate enhancement. An outline mechanism whereby the radicals catalyze attack by molecular oxygen is proposed. The CO arising from the char oxidation mechanism is simultaneously coupled with the other reactions involved in the CH4-air kinetic scheme. Numerical simulation shows the mechanism to be consistent with the observations if radical recombination at the particle surface is also included.