Design and hierarchical control strategy of a solar SMR–hydrogen metallurgy system for direct reduced iron production

Hydrogen-based ironmaking offers a promising route for carbon emission reduction, and integrating solar energy into the reforming process further enhances this potential. In this study, a novel solar-driven steam methane reforming–hydrogen metallurgy integrated system was proposed and comprehensively evaluated through thermodynamic, economic, and emission analyses. The results were then compared with those of conventional blast furnace and methane-reforming hydrogen metallurgy systems. The operating boundaries were determined through off-design analysis of the shaft furnace, which informed the development of a hierarchical two-stage control strategy designed to maintain stable direct reduced iron production under solar-irradiation fluctuations. The control framework regulated the reducing-gas flow rate, composition, and temperature, and included two SMR control modes—reaction-temperature control and flow-rate control. Results show that the solar-driven configuration achieves 54.5 % and 25.4 % lower carbon emissions per tonne of iron than the blast furnace and methane-reforming systems, respectively. Although the levelized cost of product is 7.9 % higher than that of the blast furnace and 1.6 % higher than that of the methane-reforming system, it becomes economically competitive when the carbon price exceeds 54.9 USD·t⁻¹. Dynamic simulations under step-change and real solar-irradiation conditions demonstrate that temperature control achieves faster stabilization, 18.8 % lower cumulative production deviation, and 3.2 % and 2.9 % higher average energy and exergy efficiencies than flow-rate control. This work provides an effective approach for system design and dynamic control, offering a feasible reference for integrating renewable energy into metallurgical processes.