Multiphysics-informed multi-objective optimization of hydrogen–carbon synergistic reduction of hematite pellets: Mechanistic insights and process decision strategies

To support deep decarbonization of the steel sector, hydrogen–carbon synergistic reduction has emerged as a key enabling technology. However, the nonlinear couplings linking process variables to reduction performance remain insufficiently understood. This study develops a physics-grounded optimization framework that integrates transient multiphysics modeling, response surface methodology, NSGA-II-based Pareto optimization, and TOPSIS–LINMAP decision-making for single hematite pellets reduced in H₂–CO atmospheres. The multiphysics model incorporates multi-step reduction kinetics, coupled molecular–Knudsen diffusion, non-isothermal heat transfer, and reduction-driven pore-structure evolution. The response surfaces show that pellet radius and reducing-gas temperature dominate the reduction time, pellet radius and the H₂-to-CO molar ratio govern the signed cumulative net reaction heat, and the H₂-to-CO molar ratio and inlet velocity strongly affect reducing-gas utilization efficiency. A high-efficiency operating window is identified at 1250–1270 K and a moderate H₂-to-CO molar ratio of 2.5–3.7, where rapid reaction kinetics and CO-assisted heat compensation are well balanced. Increasing inlet velocity enhances mass transfer but shortens gas residence time, revealing a trade-off between transport intensification and reaction sufficiency. Relative to the center-point baseline, the multiphysics-verified TOPSIS and LINMAP schemes substantially improve overall reduction performance: they shorten the time required to reach 95% reduction by approximately 37.7% and 39.3%, reduce the net heat absorption demand by approximately 65.2% and 68.7%, and improve reducing-gas utilization efficiency by approximately 65.5% and 61.0%, respectively. The proposed framework provides an interpretable, mechanism-resolved, and decision-oriented route toward cleaner and more efficient H₂–CO direct reduction.