Thermochemical reduction of iron oxide powders with hydrogen: Review of selected thermal analysis studies.

• 176 thermal analyses of the hydrogen reduction of iron oxide powders reviewed. • Results highly sensitive to water presence and particle characteristics. • Kinetic models of varying complexity proposed, none supersede others. [Display omitted] The reduction of iron oxide with hydrogen is a much-studied research topic, whose interest is growing even more with the emergence of new applications, for example the hydrogen-based direct reduction or for the production of a carbon-free chemical energy carrier. But the numerous works on this topic reveal the great disparity in the authors' findings, especially regarding the reduction of powders. And while many authors point out this issue, no attempt has been made to converge towards a common understanding of this heterogeneous thermochemical conversion. The endeavor of the present review is to identify the points of consensus, and where discrepancies exist, to explain them. The first part starts with a revision of the latest recommendations on the thermodynamics of iron and its oxides, easing the further comprehension of the reduction process. Then, twelve publications meeting specific criteria on the sample type and reducing agents are systematically confronted. The types of experiments and major experimental conditions have been listed, leading to the identification of typical profiles. Furthermore, chemical pathways are proposed based on these observations and supported by various analytical measurements. Finally, the multiple mathematical approaches to derive kinetic models are compared and discussed. This present review points out the need for appropriate experimental conditions to derive the intrinsic chemistry of the reduction, such as the limitation of water vapor, and also emphasizes the need for more detailed chemical mechanisms. [ABSTRACT FROM AUTHOR]