Oxidation of iron particles for energy storage: Experiments in a fluidized bed reactor and kinetic modeling

Fluidized bed reactors could be used for combustion of iron powder, a promising recyclable metal fuel. The modeling and development of such reactors will be facilitated if a simple kinetic model for the oxidation of iron particles in air is available. However, there are discrepancies in the literature on the oxidation of iron powders, including different kinetic models developed to describe the experimental data obtained. In the present study, isothermal oxidation of iron particles (size: 50–70 µm) in an O 2 /N 2 (20:80 mole ratio) mixture at temperatures of 500–800 °C was studied using a laboratory-scale fluidized bed reactor and a thermogravimetric analyzer (TGA). X-ray diffraction analysis of the oxidized samples has shown that with increasing the temperature, the content of magnetite decreases, while that of hematite increases. Scanning electron microscopy revealed a porous surface of the oxidized particles. The mass gain profiles obtained in both fluidized bed and TGA show a rapid first stage and a slow second stage. The profiles were used in a model-fitting analysis that assumed a two-stage conversion of a spherical iron particle into a hematite one and inward transport of oxygen through the growing oxide layer. The use of a three-dimensional diffusion-controlled reaction function resulted in an excellent correlation between the simulated and experimental mass gain profiles. The values of the apparent activation energy obtained for fluidized bed at temperatures of 600–800 °C are close to the literature data on the activation energy of flat iron oxidation.