Iron powder can be a sustainable alternative to fossil fuels in power supply due to its high energy density and abundance. Iron powder releases energy through exothermic oxidation (combustion), and stores back energy through its subsequent hydrogen-based reduction, establishing a circular loop for renewable energy supply. Hydrogen-based direct reduction is also gaining global momentum as possible future backbone technology for sustainable iron and steel production, with the aim to replace blast furnaces. Here, we investigate the microstructural formation mechanisms and reduction kinetics behind hydrogen-based direct reduction of combusted iron powder at moderate temperatures (400–500 °C) using thermogravimetry, ex-situ X-ray diffraction, scanning electron microscopy coupled with energy dispersive spectroscopy and electron backscatter diffraction, as well as in-situ high-energy X-ray diffraction. The influence of pre-oxidation treatment was studied by reducing both as-combusted iron powder (50 % magnetite and 50 % hematite) and the same powder after pre-oxidation (100 % hematite). A gas diffusion-limited reaction was obtained during the in-situ high-energy X-ray diffraction experiment, with successive hematite and magnetite reduction, and a strong increase in reduction kinetics with initial hematite content. Faster reduction kinetics were obtained during the thermogravimetry experiment, with simultaneous hematite and magnetite reduction. In this case, the reduction reaction was limited by a mix of phase boundary and nucleation and growth models, as analyzed by multi-step model fitting methods as well as by microstructural investigation. When not limited by gas diffusion, the pre-oxidation treatment showed almost no influence on the reduction time but a strong effect on the final microstructure of the reduced powder.

To successfully transition from fossil-fuel to sustainable carbon-free energy carriers, a safe, stable and high-density energy storage technology is required. The combustion of iron powders seems very promising in this regard. Yet, little is known about their in-process morphological and microstructural evolution, which are critical features for the circularity of the concept, especially the subsequent reduction of the combusted oxide powders back to iron. Here, we investigated two iron powder combustion pathways, one in air and one with the assistance of a propane pilot flame. Both processes resulted in spherical hollow particles composed of a complex microstructure of wüstite, magnetite and/or hematite. Partial evaporation is indicated by the observation of nanoparticles on the micro-sized combustion products. The associated gas production inside the liquid droplet could be the origin of the internal porosity and micro-explosion events. Cracking at the end of the combustion process results in mostly open porosity, which is favorable for the subsequent reduction process. With this study, we aim to open the perspective of iron metal fuel from macroscopic combustion analysis towards a better understanding of the underlying microscopic thermodynamic, kinetic, microstructural and thermomechanical mechanisms.