Detailed physicochemical evolution of iron particles burnt under controlled, realistic conditions

Abstract

The combustion of pulverized iron has been studied experimentally in a flat flame reactor in a variety of conditions (high gas temperature, constant [O₂] within 4.1–16 %, 75–90 µm). Particle temperature profiles were measured in situ. Samples were collected through rapid cooling in N₂ at different residence times in these conditions, resulting in a very detailed characterization of their evolution in terms of internal structure, composition, size and mass. For the latter, a thermogravimetric method has been developed in order to determine the oxidation degree, i.e. the fraction of oxygen in each sample, with considerable advantages over e.g. X-ray diffraction. These new curves for mass vs. distance travelled (as well as the temperature profiles) show a clear gradation with [O₂], highlighting iron may indeed be seen as a ‘regular’ fuel and pointing to existing technologies for controlling its oxidation rate and temperature in a potential industrial facility. SEM and XRD give sound evidence for the existence of successive stages in the oxidation process, namely Fe → FeO → Fe₃O₄ → Fe₂O₃, with no overlapping between them. In the step Fe → FeO, two clear phases are observed, with a receding iron core surrounded by iron oxide and spontaneous emulsification of both phases. The particles steadily grow when they get oxidized. Statistically significant voids appear in the last stages of oxidation; some particles nearly double their size in these stages. At least two types of particle breakup were observed, but none of them affected noticeably the particle size distribution.