Process integration of turquoise hydrogen via natural gas pyrolysis for blast furnace ironmaking: techno-economic viability and CO₂ mitigation

Efforts to decarbonize the steel sector primarily follow two pathways: the use of alternative low-carbon fuels (e.g., hydrogen, ammonia) for blast furnace (BF)-based ironmaking, and the adoption of electrified processes utilizing direct reduced iron in electric arc furnace-based ironmaking. In this study, synergistic process integration is proposed for hydrogen-based BF ironmaking, and its techno-economic and environmental impacts are assessed. Turquoise hydrogen, produced via natural gas pyrolysis, is designed across four cases to examine how variations in injection temperature and hydrogen purity affect the balance among process design, economic performance, and CO₂ mitigation potential. Heat supply strategies, including hydrogen purification units, are also considered. Each case is evaluated in terms of energy consumption, BF injection performance, economic feasibility, and environmental impact. The findings reveal that Case A achieved the highest energy efficiency of 60.4%, while Case D showed the lowest at 47.6%. Regarding BF performance, increasing the injection temperature of high-purity H₂ improved the H₂-to-coke replacement ratio from 1.10 to 1.46 kg_coke/Nm³-gas, enabling a significantly higher H₂ injection rate of up to 41 kg_H2/tHM. Economically, the integration proved highly competitive due to the solid carbon byproduct; Case D achieved the most favorable unit production cost (UPC) of − 0.29 US$/kg-gas, compared to 0.016 US$/kg-gas for Case A. Environmentally, Case D also demonstrated the superior sustainability profile with a net-negative CO₂ emission of − 7.43 kg CO₂-eq./kg-gas. Overall, the proposed integration of turquoise H₂ with BF ironmaking demonstrates strong economic and environmental performance. A remaining challenge is determining the optimal degree of hydrogen purification for alternative applications within the ironmaking process.