Apparent Kinetics for Direct Oxidation of Iron Particles Determined from Tests at High Temperature in a Flat Flame Reactor

The results from experiments with 63–75 μm iron particles burning in high temperature gases in a flat flame reactor with 4–16% O2 are presented and compared with calculations, with the main goal of determining the vicinity of the measured oxidation rates to the external diffusion limit. The simulations consider oxygen diffusion through the particle’s boundary layer and encompass internal processes (diffusion, liquid convection, reaction, etc.) in the form of apparent kinetics in the outer surface of the particle. The main magnitudes monitored experimentally are particle temperature and especially its oxidation degree, based on the mass of oxygen absorbed; both are determined at different heights along the reactor, which results in detailed profiles with distance traveled or residence time. The assumption of combustion in the (external) diffusion limit notably overestimates the oxidation rate and the peak temperature of the particles. On the contrary, the introduction of different, progressively slower apparent kinetics for each oxidation stage, with Fe–O ratios corresponding to Fe → FeO → Fe3O4 → Fe2O3, results in an excellent agreement to the experimental data, particularly regarding the oxidation curves. The same kinetics are shown to fit equally well previous results obtained in tests with 75–90 μm, recently published by the authors.