Thermodynamic analysis of the reduction performance of iron oxide using H2 and CO gas mixtures

• 50% H2 and 50% CO optimal for different conditions. • The lower enthalpy of CO favors the initial reduction; H2 improves the later phases. • CO excels at low temperatures, H2 at high temperatures. • CaO and MgO improve the porosity and reducibility of the pellets. • H 2 processes demand more energy due to strong hydrogen–oxygen affinity. In this study, a comprehensive thermodynamic analysis of the direct reduction of iron oxide pellets was carried out, focusing on the quantitative influence of the reducing gas components (carbon monoxide, CO, and hydrogen, H 2 ) and the pellet composition (including additives such as CaO, SiO 2 and MgO). In a temperature range from 0 to 1000 °C at 1 bar pressure, the study showed a different efficiency of CO and H 2 in the reduction of iron oxides. At lower temperatures (200–600 °C), CO showed higher efficiency and achieved up to 15% higher equilibrium Fe content due to its lower enthalpy requirement and rapid reduction of Fe 2 O 3 to Fe 3 O 4 . Conversely, H 2 achieved up to 25% higher Fe reduction efficiency at higher temperatures (>700 °C), favored by its smaller molecule size and higher diffusivity, which improved gas penetration and accelerated the reduction of Fe 3 O 4 and FeO to metallic Fe. Mechanistically, the superior performance of H 2 at elevated temperatures is due to its higher affinity for oxygen and the improved diffusion rates that overcome the endothermic nature of the reduction process. Furthermore, the addition of CaO and MgO improved the reducibility of the pellets by modifying the porous structure, lowering the activation energy and increasing the gas permeability. Conversely, SiO 2 hindered the reduction by forming stable fayalite (Fe 2 SiO 4 ), which increased the enthalpy requirement. These results quantitatively explain the temperature-dependent role of the reducing gas components and the mechanisms underlying their effects. They provide a basis for optimizing gas composition and pellet designs to improve energy efficiency and environmental performance in industrial direct reduction processes.