Transient numerical modeling of carbon particle ignition and oxidation

The chemically reacting flow field on and around the surface of an isolated carbon particle has been simulated numerically using a fully transient formulation. Computations were performed for a spherically symmetric system utilizing detailed transport properties, a comprehensive gas-phase reaction mechanism, and a five-step heterogeneous surface mechanism. The effects of surface regression on the combustion characteristics of carbon particles are investigated for two size ranges (for small particles, the diameters ranged from 100 to 300 μm and for large particles, the diameter is typified by 2000 μm). Particles were ignited by placing them in various gaseous oxidizing media at a temperature sufficiently high (approximately 1400 K) to initiate gas-phase oxidation of CO evolved from the particle surface. Results show the typical behavior commonly associated with diffusion-limited and chemical kinetically limited burning of "large" and "small" particles. However, the range of diameters over which both diffusion and kinetics affect the burning characteristics is large. The regression of the particle surface introduces an additional characteristic time to the problem. The order of this time for small particles is approximately that necessary to establish a reactive gas-phase boundary layer and causes burning predictions to depart substantially from those found for quasi-steady burning. For large particles, other transients result from effects similar to those of "accumulation" for liquid droplet gasification and burning. Finally, transient behavior departures from quasi-steadiness are obtained for a large range of surface kinetic rates that encompass modifications that could result if the particles were porous.