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.

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Hydrogen-based direct reduction (HyDR) of iron ores has attracted immense attention and is considered a forerunner technology for sustainable ironmaking. It has a high potential to mitigate CO ₂ emissions in the steel industry, which accounts today for ~ 8–10% of all global CO ₂ emissions. Direct reduction produces highly porous sponge iron via natural-gas-based or gasified-coal-based reducing agents that contain hydrogen and organic molecules. Commercial technologies usually operate at elevated pressure, e.g., the MIDREX process at 2 bar and the HyL/Energiron process at 6–8 bar. However, the impact of H ₂ pressure on reduction kinetics and microstructure evolution of hematite pellets during hydrogen-based direct reduction has not been well understood. Here, we present a study about the influence of H ₂ pressure on the reduction kinetics of hematite pellets with pure H ₂ at 700 °C at various pressures, i.e., 1, 10, and 100 bar under static gas exposure, and 1.3 and 50 bar under dynamic gas exposure. The microstructure of the reduced pellets was characterized by combining X-ray diffraction and scanning electron microscopy equipped with electron backscatter diffraction. The results provide new insights into the critical role of H ₂ pressure in the hydrogen-based direct reduction process and establish a direction for future furnace design and process optimization. Graphical Abstract: (Figure presented.)

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Hydrogen-based reduction of iron ores is the key technology for future sustainable ironmaking, to mitigate the CO2 burden from the steel industry, accounting for ~7–8% of all global emissions. However, using hydrogen as a reductant prompts concerns about hydrogen embrittlement in steel products. This raises the question of how much hydrogen remains from green ironmaking in the metal produced. We answer this question here by quantifying the amount of hydrogen in iron produced via two hydrogen-based ironmaking processes, namely, direct reduction and plasma smelting reduction. Results suggest no threat of hydrogen embrittlement resulting from using hydrogen in green steel production.@en