Recently, metal fuels have been proposed as sustainable energy carriers with high energy density. In particular, iron powder holds great promise due to its relatively low cost, safety, abundance, and easy retrofit of power plants. Heat is produced through the self-sufficient exothermic oxidation of iron particles, producing flames, and the iron particles can be regenerated from the combustion products using hydrogen-based direct reduction. A net CO2-free and low NOx emissions cycle of energy storing is obtained as long as the reduction process and hydrogen production are conducted using renewable energy. So far, research and development has been focused on the use of pure Fe powder for this sustainable fuel application. Yet, the price of Fe sources increases strongly with its purity, and the tolerance of iron powder to the presence of impurities is a crucial parameter for the economic and technologic feasibility of iron powder as metal fuel. This project investigates the potential of mill scale, produced during thermomechanical treatments of high-manganese steel, as a sustainable metal fuel. The influence of the impurities is assessed on both the reduction of the mill scale through hydrogen-based direct reduction, and on the combustion of the reduced mill scale in a thermally-ignited tornado burner.

Reduction was conducted at 400°C, 450°C, 500°C and 700°C under pure hydrogen by in-situ thermogravimetry. The positive influence of a pre-oxidation step at 700°C, 800°C and 900°C was also studied. Moreover, ex-situ microstructural investigations (SEM, EDX, XRD) were conducted at different stages of the reduction process to observe the microstructural, chemical and morphological evolution of the powders during the reduction process. The deconvolution of each phase transformation was conducted using in-situ high-energy x-ray diffraction under pure hydrogen at 500°C.

On the combustion side, this work focuses mostly on the microstructural, chemical and morphological analysis of the combusted powder using SEM, EDX, EBSD, XRD, granulometry and chemical analysis. An excellent oxidation efficiency was obtained, confirming the potential of mill scale as metal fuels. Moreover, the formation of an additional liquid phase in the final microstructure indicates a different phase transformation path as compared to the combustion of pure Fe. This study paves the way for the use of different low-value sources of iron for sustainable energy storage.