Ignition and combustion of a single iron particle with impurities in hot post-flame gas flow

Micron-sized iron particles are promising carbon-free fuels. Efforts have been made to understand the combustion process of pure iron particles. However, it is not well-established how important are the impurities and their influence on combustion. The ignition and combustion characteristics of iron particles with 6.18 wt.% silicon were studied both by online optical diagnostics and offline scanning electron microscope (SEM) and X-ray Diffraction (XRD) analysis in this work. It is found that the ignition delay time is proportional to dⁿ with n=1.23∼1.56, indicating that the oxidation reaction is unlikely solely limited by internal ionic diffusion before ignition. Compared with pure iron particles, the silicon-containing iron particle has a lower peak temperature and less oxidized combustion products. The intensity-rising time in the first intensity peak stage scales (1/X_O2)ⁿ with n=0.80or1.38, which implies an external diffusion controlled combustion regime. Silicon in the oxide layer likely reduces the internal diffusion rate of oxygen, which becomes a rate-limiting step after the first intensity peak. Meanwhile, the surface tension also decreases due to the silicon content, which may facilitate the gas bubble nucleation and burst. The particle could partially change back to the external diffusion regime when the oxide layer is broken by the bubble expansion. This mechanism allows the reaction rate to increase and particle temperature to rise again. This explanation is verified by plenty of holes in the combustion products as well as the fact that the rising time in the large peak stage is proportional to (1/X_O2)ⁿ with n=0.37or0.32. With the increase in the temperature of the ambient gas flow, evaporation was observed. The high ambient temperature is hypothesized to be a key factor to trigger the evaporation of silica.