Modeling the oxidation of iron microparticles during the reactive cooling phase

Iron and its oxides have been proposed as energy carriers for a carbon-free, circular energy economy. Focusing on the energy release step of the cycle, this work is concerned with the combustion of iron particles in air. In this context, the accurate prediction of oxidation time scales of individual iron microparticles represent the fundamental building block of high-fidelity models for iron dust flames. While a lot of progress has been made in modeling the particle burnout up to the stoichiometry of FeO, the further oxidation to Fe3O4 in a ’reactive cooling’ phase still requires research. During reactive cooling, the heat released by the (molten) iron oxide particle is insufficient to balance the heat losses and sustain the high particle temperatures but the temperature decrease is significantly slower than for an inert cooling process. So far, the rate limiting mechanism during this oxidation phase is not well understood. Based on the thermodynamic equilibrium of the liquid Fe/O system, a new model for this reactive cooling phase is proposed. At high temperatures, transport and chemical reactions are fast and the calculated vapor pressure of oxygen at the liquid interface controls the particle oxidation, which causes the particle to arrive at higher oxidation states with decreasing temperatures. Eventually arriving at its solidification temperature, the particle’s final oxidation state corresponds to Fe3O4. The modeling approach is integrated into a point-particle model, including the transition from diffusion-controlled burnout to the equilibrium-controlled reactive cooling. The latter is determined by heat transfer to the environment, heat release from the slow reaction, and the limited oxygen capacity of the liquid Fe/O phase at a given temperature. Calculated temperature evolution profiles of oxidizing iron microparticles show very good agreement with several independent experimental measurements for the reactive cooling phase.