Over the past decade, the pursuit of an ecological industrial revolution has had an impact on research in the field of chemical process engineering. The steel industry, which plays a pivotal role in current society both in economics and in terms of environmental impact, has long been stuck in a fossil-carbon-based production methodology that requires timely innovation. As an alternative, to the carbon route, hydrogen-based reduction of iron oxides has been explored over the last 60 years and lately at accelerated speed due to huge environmental impact of the present iron- and steel-making processes. The main objective of this review is to examine the extensive literature on experimental data and different modeling approaches, focusing on the use of hydrogen as a reducing agent of iron oxides under various operating conditions (e.g., temperature, composition of the reducing gas, etc.), for different structural properties (e.g., particle size, composition, etc.), and under different mechanistic and mathematical modeling assumptions. The large variation in experimental data and modeling interpretations collected over the years has led to a large scatter in the evaluation of kinetic parameters related to the reduction process. The average activation energies calculated for the individual reactive steps show significant deviations: hematite to magnetite Ea = 74.8 ± 49.0 kJ/mol, magnetite to wüstite Ea = 66.0 ± 57.2 kJ/mol and wüstite to iron Ea = 62.0 ± 43.9 kJ/mol. This scatter shows the need to further deepen the analysis in this area, starting from molecular and microscopic phenomena all the way to the scaleup into furnace scale.
