Revealing contradictory reduction behavior in shaft furnaces at increasing H2 contents: Combined roles of thermodynamic equilibrium differences and gas flow rate

• Roles of H 2 in shaft furnaces are studied across H 2 :CO = 0:100–100:0. • The study uses our recently developed CFD process model. • The reported contradictory reduction behavior is successfully reproduced. • Thermodynamic data and gas flow rate together account for the reduction behavior. • The observed phenomena are explained in terms of internal states. Major iron and steel companies have identified the H 2 -based shaft furnace (SF) as a key decarbonization technology for future development. Its continued advancement aims to achieve full H 2 operation and broaden the applicability of ores with different grades. However, published studies report contradictory trends in SF reduction performance with increasing H 2 content, leading to significant confusion about the role of H 2 . The underlying causes remain unclear, and the influence of ore properties on SF performance has received limited investigation. In this work, using our recently developed CFD model for industrial SFs, the influence of ore properties on SF reduction behavior is investigated under various H 2 contents. Differences in ore properties focus on thermodynamic equilibrium differences, as documented in two well-known databases, NIST and FactSage. The results show that thermodynamic equilibrium differences can modify the contributions of high- and low-temperature reduction, leading to inconsistent trends in metallization as H 2 content increases. The gas flow rate is also a contributing factor, as it alters the H 2 :CO range in which the thermodynamic disadvantage of H 2 reduction hinders reduction. These findings provide insight into how ore properties and the reducing gas flow rate modulate in-furnace H 2 reduction and highlight the need for reliable ore characterization to properly assess H 2 -based SF performance.