Hydrogen reduction of combusted iron powder: Role of water vapor in reaction kinetics

Hydrogen-based reduction of iron oxides offers a promising, low-carbon alternative to hydrocarbon fuels in steel production and plays a critical role in the Iron Power Cycle, an emerging technology for renewable energy storage and transport. However, despite its practical significance, research on hydrogen reduction in the presence of water vapor remains limited, especially for high-purity, micron-sized iron oxide powders produced from iron combustion. In this study, we investigate the reduction kinetics of combusted iron powder using thermogravimetric analysis (TGA) at three temperatures: 500 ° C, 700 ° C and 800 ° C and partial pressure ratios ( Δ P H 2 O / Δ P H 2 ) ranging from 0.0 to 0.7. The results indicate that the conversion rate increases with temperature, and water vapor hinders the reduction process, particularly at lower temperatures. This emphasizes the necessity of removing water vapor during iron oxide reduction to guarantee high conversion. The reduction process is divided into distinct phases, with rate-limiting mechanisms in each phase analyzed using the Avrami constant ( n ). Kinetic parameters, including activation energy ( E a ) and the overall water vapor reaction order ( m ), are determined using the isoconversional method. We propose a modified multi-step kinetic model to extract the rate constant ( k app ), Avrami constant ( n ), and water vapor reaction order ( m ) for each step of the reduction pathway. The results are crucial for optimizing hydrogen-based ironmaking processes in sustainable steel production and for the Iron Power Cycle. • Water vapor reduces iron oxide conversion, strongest effect observed at 500 ° C. • Avrami analyses reveal kinetic transitions and shifts in rate-controlling mechanisms. • E a varies with conversion and water vapor, limiting isoconversional reliability. • A multi-step NAG model describes reduction processes well across temperatures.