System analysis of direct reduced iron production with elevated levels of hydrogen in the shaft furnace

• Progressive transition towards green steelmaking with mixture of H 2 and NG. • Recycling the shaft furnace top gas via steam methane reformer. • Evaluating substitution and addition pathways for the transition from NG to H 2 . • Increasing H 2 in the feed and the recycling rate improves the metallization degree. • Higher gas pressure boosts the reduction process but lowers the methane conversion. The transition toward hydrogen-based direct reduction of iron ore is gaining increased attention as a key pathway for decarbonizing the steel industry. However, full transition to hydrogen-based iron ore reduction faces challenges that may hinder its adoption by steelmakers, e.g. low utilization degree of hydrogen, the reactions’ endothermic nature and the high price of green hydrogen. This paper explores computationally a system consisting of direct reduction shaft furnace and external gas handling units, using a mix of green hydrogen and natural gas in the feed. Detailed models of the process units are developed in Aspen Plus software, including a kinetic model of steam methane reformer to simulate the operation of an industrial-scale plant under increased levels of hydrogen in the feed gas to the process. The results indicate that increasing hydrogen level in the fresh feed can reduce CO 2 emissions by about two thirds, still keeping the metallization degree of the direct reduced iron above 90%. Recycling the top gas leads to an elevated CO fraction in the reducing gas and higher metallization rate of iron, but it may reduce the conversion efficiency of methane in the reformer, so there are trade-offs between different objectives. It was found that higher process gas pressure is beneficial for iron ore reduction, resulting in higher metallization rate and lower pressure drop, but it lowers the methane conversion. The paper provides guidelines for possible operation states of direct reduction process to support smooth transition towards carbon-lean steelmaking.