Reducibility of high-grade pellets directly reduced in hydrogen atmosphere: Modeling and experimental procedure

The paper analyzes the behavior of high-grade pellets specially developed for hydrogen direct reduction (HDRI). The reducibility of these pellets depends largely on their composition, porosity, pore structure and distribution. X-ray tomographic analyzes of the unreduced pellets showed different degrees of porosity as well as different pore sizes and distributions in each observed pellet. This prompted us to investigate the reduction behavior of each individual pellet at 1000 °C and 1 bar. The effect of composition on the reduction kinetics was analyzed using HSC software to isolate the effect of composition from the porosity structure of the pellets. After reduction, X-ray tomographic observations enabled the measurement of porosity variation in each pellet studied. The porosity variation data was used to validate a finite element model developed to analyze porosity evolution using COMSOL Multiphysics. The study found that larger pores have higher activity and a tendency to coalesce, forming interconnected networks that allow for better gas and heat diffusion. The results showed that porosity increased from approximately 30 % to approximately 65 % after reduction, and the close agreement between the experimental data and the FEM simulations confirmed the accuracy of the model. It was found that the presence of CaO and MgO increased the porosity and thus improved the reducibility, while the inhibitory effects of SiO2 and Al2O3 were minimized. These results contribute significantly to the optimization of pellet composition and structure for efficient and uniform reduction of iron oxides in hydrogen atmospheres.