Thermochemical integration of solid oxide electrolysis cells with direct reduced iron shaft Furnace: Primary energy consumption and exergy analyses

This study evaluates thermodynamic performance of alternative reducing-gas production pathways for hydrogen-based direct reduced iron (DRI) steelmaking using primary energy consumption and exergy analyses. Although emissions from steelmaking can be significantly reduced with natural gas-direct reduced iron plants, there are still several emissions challenges associated with this method of iron reduction. The technical feasibility of direct reduction of iron using higher concentrations of hydrogen as feedstock is reviewed for achieving lower emissions. Exergy and primary energy consumption analyses are performed to compare the efficiencies of four reducing gas generation methods: i) a steam reforming process using pipeline natural gas as a feedstock; ii) a shaft furnace coupled with a proton exchange membrane electrolysis cell (PEMEC) system producing hydrogen in an open-loop cycle; iii) a solid oxide electrolysis cell (SOEC) system that is thermally integrated with a shaft furnace performing steam electrolysis to produce hydrogen; and iv) a thermochemically integrated SOEC system performing co-electrolysis in a closed-loop cycle. The results show that electrochemical production of a syngas mixture comprised of 55 mol% H 2 and 35 mol% CO, achieved an 18% increase in the second-law efficiency when compared to thermochemical production of syngas using a reformer. In terms of hydrogen direct reduction of the iron ore, the exergy analysis shows that PEMEC and SOEC are nearly identical; however, the SOEC system requires 31% less primary energy with proper thermal integration. The results demonstrate that thermally and thermochemically integrated SOEC systems can significantly reduce primary energy consumption associated with reducing gas generation while improving second-law efficiency compared to conventional reforming-based reducing-gas production.