The steel industry currently faces significant challenges. These are enormous environmental impacts, material usage, energy consumption, and byproducts of the processes. [1] In 2020, the total crude steel production was 1869 million tons (Mt). [2] The conventional blast furnace-basic oxygen furnace (BF-BOF) route produces 70% of the global crude steel, [3] utilizing coke to produce iron by reducing iron ore, which emits high amounts of CO 2 in the environment. Steel production contributed 6.7% of the global anthropogenic emissions of CO 2 in 2020, and the average emission is estimated to be 1.8 t CO 2 / per t steel. [4] This corresponds to %3.46 Gt CO 2 per year, eventually leading to even higher annual emissions in 2050 due to the growing steel demand. Given this high level of CO 2 emissions, steel production cannot rely on continuous process optimization but instead, needs fundamental technological changes to meet future emission limitations.Under these circumstances, the direct reduction (DR) process of iron oxide is a promising technology to reduce the CO 2 emissions where hematite (Fe 2 O 3 ) is reduced by syngas (H 2 /CO-mixture) or even with pure hydrogen (H 2 ). Many mathematical models have been proposed in the literature to predict the gaseous reduction behavior of iron oxide. However, due to a large number of influencing factors in the direct reduction process, for example, gas composition, operating pressure, temperature, or solid material characteristics (porosity, tortuosity, grain size, gangue contents, mineralogy, etc.), uncertainty is relatively high. Researchers usually try to balance between model simplicity and accuracy (agreement with experimental data). To avoid the inherent complexity of the direct reduction process, McKewan [5] developed a simple one-interface shrinking core model by considering the interface chemical reaction as the rate-limiting step. They concluded while validating against their data that diffusion and chemical reaction should be considered simultaneously. Tsay et al. [6] developed a model based on a three-interface shrinking core model for H 2 /CO mixtures, considering the same diffusivities and mass transfers for reactants and products. However, this approach has the evident weakness of assuming a distinct sharp interface between different solid oxides. A transient isothermal model based on the grain model has been proposed by Valipour et al. [7] to simulate a porous hematite pellet's thermal and kinetic behavior in a syngas environment. They showed how models not considering the film resistances deviate systematically from the experimental data.Aside from the fundamental mathematical approach, the chemical kinetics of reduction of iron oxide with H 2 /CO mixtures has not been adequately investigated, especially the carbon deposition phenomena. Turkdogan et al. [8] studied the kinetics of direct reduction process of iron oxide using hydrogen. They found out that the transformation process of hematite to iron is mediated by magnetite (Fe 3 O 4 ) and wüstite (Fe (1Ày) O).
The direct reduction of iron ore pellets with syngas or hydrogen is a promising technology to reduce the CO2 emissions of the iron and steel industry. The conversion rate of single iron ore pellets to iron is extensively investigated. In most of these studies, a shrinking core model is employed to reproduce the experimental observations. However, this model presents an inherent bias by assuming a sharp separation between a fully converted region and a fully unreacted one. Herein the present study, an improved porous solid model is proposed. This model solves the mass balances of the individual gas species and the solid ones assuming spherical symmetry. The governing equations, the main algorithm, and validation cases are presented. The present model also offers wide flexibility to incorporate complex phenomena such as porosity changes or carbon deposition. Furthermore, the proposed model is integrated into a computational fluid dynamics (CFD) environment. It is verified that identical input parameters yield almost identical results in both frameworks, opening the gate toward reliable CFD simulations of industrial‐scale reactors.
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