Thermogravimetric and microstructural analysis of the low-temperature oxidation of iron powders

• Iron micrometer particles predominantly oxidize as magnetite below 570 °C. • Hematite forms a thin layer around magnetite duplex layer, itself surrounding iron. • Despite strong sintering between particles, full oxidation is achievable. • An activation energy of 220 kJ/mol was found using the Kissinger-AkahiraSunose method. • Truncated Sestak and Berggren model accurately reproduces the set of TGA experiments. Metal fuels, particularly iron, hold great promise as a sustainable source for decarbonating heat or electricity supply. However, a comprehensive understanding of the physics governing the combustion of iron dust flames is currently lacking. In this study, we present a thorough kinetic investigation of the oxidation of iron micrometer particles. Thermogravimetric analysis experiments were conducted under isothermal conditions and with varying uniform heating rates, with air, and with pure oxygen, as well as with various Fe powders. Our results indicate that the oxidation of iron particles can achieve completion at relatively low temperatures, even before the appearance of wüstite. Microstructural analysis suggests an oxidation process under 570 °C dominated by the growth of a duplex magnetite layer, surrounded by a network of hematite ridges. In later stages of oxidation, an inter-particle continuous surface might form, hindering the contact between the iron material and the oxidizing agent. A kinetic analysis has been conducted using model-free and model-fitting methods. An approximate activation energy of 220 kJ/mol has been derived from the Kissinger-Akahira-Sunose method and it is shown that the truncated Sestak and Berggren model could fairly reproduce isothermal and dynamic thermogravimetric analysis experiments.