Temperature and phase evolution of individual iron and iron-oxide particles during combustion and hydrogen plasma reduction

This study investigates the temperature and solid–liquid–vapor phase transitions of iron and iron-oxide particles during oxidation and reduction, respectively. In iron combustion, particles are injected into a laminar premixed methane/air flame, and their temperature history is measured by high-speed multi-band RGB pyrometry and near-infrared hyperspectral emission spectroscopy. The agreement between the two temperature measurements obtained in distinct spectral regions supports the validity of the gray-body assumption, i.e., spectrally and temperature-independent emissivity, under the present conditions. The RGB pyrometry diagnostic is applied in an optically accessible argon/hydrogen microwave plasma reactor to monitor iron-oxide particles during in-flight reduction. The particles exhibit rapid heating to temperatures exceeding the boiling point of iron, accompanied by luminous vapor clouds indicative of partial vaporization and plasma-driven reactions. A comparison with the plasma emission spectra reveals that the gas-phase luminescence originates from atomic iron vapor. The average particle temperature decreases rapidly both with radial distance from the reactor centerline and in the downstream direction. During subsequent cooling, the temperature exhibits a plateau associated with the solid–liquid or crystal phase formation. In this stage, particles occasionally undergo thermomechanical breakup. The particle size and feed-gas composition strongly influence the particle temperature history and must therefore be carefully controlled to achieve efficient reduction while avoiding excessive vaporization and mass loss. Novelty and significance statement While most studies in the metal-fuel community focus on iron particle oxidation, the reverse pathway of iron-oxide reduction has received far less attention. This work combines optical pyrometry and spectroscopic diagnostics to resolve the in-flight thermal evolution of individual particles in both a premixed flame and a hydrogen microwave plasma. The measurements enable a detailed analysis of particle temperature evolution, vaporization and associated luminescent gas-phase iron species, particle breakup events, and temperature plateaus indicating phase transitions, as well as the influence of operating parameters on particle behavior. These results provide direct insight into plasma–particle interactions and the thermal mechanisms governing plasma-assisted iron-oxide reduction, relevant to metal-fuel cycles and hydrogen-based low-carbon metallurgy.