Using Thermodynamics and Microstructure to Mitigate Overfitting in Pellet Reduction Models

Abstract

Direct reduction of iron involves complex, multiscale phenomena, encompassing solid-state phase transformations and gas transport through pores that must be accurately represented for predictive industrial implementation. Here, we present a thermodynamically sound pellet-scale model that describes these mechanisms and can serve as a foundation for improving the understanding of pellet reduction kinetics in H₂/CO-containing atmospheres. The model assumes that the gas phase remains in thermodynamic equilibrium instantly adjusting to any changes in composition. This reduces the number of fitting parameters drastically compared to other existing models, while maintaining a strict thermodynamic upper bound estimate. A driving force term is included in the reaction rate equation ensuring that the three iron oxide reduction steps and the formation of graphite and cementite in carbon-containing gases occur only if they are thermodynamically possible. It is demonstrated that fitting kinetic parameters based on conversion degree data alone leads to overfitting. This is true both for existing models and the model introduced here, despite the fact that the latter contains fewer parameters. To overcome this overfitting problem, spatially resolved microstructural data at key reduction stages can be considered, as shown here for recently reported data for a pellet reduced in H₂ atmosphere.