The carbon-based reduction of iron oxide for iron and steel production accounts for more than 7% of worldwide CO2 emissions. In contrast, the hydrogen-based direct reduction of iron oxide (HyDRI) has garnered significant interest due to its potential to significantly decrease CO2 emissions, as water is produced as a by-product. The HyDRI involves three phase transformations: (1) Fe2O3 to Fe3O4, (ii) Fe3O4 to FeO, and (iii) FeO to alpha-iron. Of these, transformation (iii) is nearly an order of magnitude slower than others [1]. Moreover, the significant porosity arising from the sintering of fine ores into pellets and its subsequent evolution during these reactions and transformations occurring in HyDRI play a crucial role in governing the mass transport of species, including oxygen, hydrogen, and water, as well as the phase transformations. Consequently, these porosity-related phenomena exert a substantial influence on the reaction kinetics of the reduction process [2].

A multiphysics phase-field model is developed utilizing the principles of chemo-mechanics theory. The model incorporates phase transformation, chemical reactions, diffusion, vacancy production, and clustering leading to nanopore formation and coarsening of porous microstructures. The developed PF model can successfully simulate porosity evolution in classical core-shell type iron oxide pellets and samples with complex microstructures characterized using electron microscopy. The findings indicate that the evolution of porosity is sensitive to local reaction kinetics. Furthermore, the presence of accumulated water within closed pores has been observed to impede the reaction progress.

References:

1. Kim et al., Acta Mater., 212, 116933, 2021.

2. X. Zhou et al., Phys. Rev. Lett., 130(16), 168001, 2023