Steel production plays an important role in the world’s global transition toward sustainable practices, since it is one of the largest contributor to CO2 emissions in the manufacturing sector, accounting for 8~10% of global CO2 emissions. Hydrogen-based direct reduction (HyDR) of iron ores has attracted immense attention as a pioneering solution for sustainable ironmaking owing to its high technology readiness level (TRL: 6-8). HyDR is a multistep solid-gas reaction that involves the reduction of hematite to magnetite, followed by wüstite, and finally metallic iron. Adjusting the reduction parameters, such as temperature, reducing gas composition, hydrogen gas pressure, as well as the pellet's chemical composition and microstructural characteristics (e.g., porosity fraction and pore connectivity), are crucial in determining reduction kinetics and have been the focus of extensive research for decades. While prior studies have investigated the impact of hydrogen gas pressure on the reaction kinetics, the underlying reaction mechanisms governing individual reaction steps and microstructural evolution remain poorly understood. In addition, the chemistry of the final product is of great importance since even a small amount of diffusible hydrogen (e.g., in ppm level) retention in the final product (e.g., high-strength steel) can result in catastrophic failure, known as hydrogen embrittlement. Therefore, the influence of the reduction parameters (e.g., hydrogen pressure) on the reaction mechanisms and quality of the final product needs to be better understood to optimize the process efficiency and downstream processes following HyDR of iron ore. The question of “How hydrogen gas pressure affects the reduction kinetics and microstructural evolution of hematite pellets” was addressed. We investigated the reduction kinetics of hematite pellets with pure hydrogen at 700 °C at various pressures, i.e., 1, 10, and 100 bar under static gas exposure, and 1.3 and 50 bar under dynamic gas exposure. The microstructure of the reduced pellets was characterized by combining X-ray diffraction and scanning electron microscopy equipped with electron backscatter diffraction. The findings revealed that higher hydrogen gas pressure significantly enhances reduction kinetics. The microstructure of the metallic iron transformed from a dense metallic iron layer formed over the oxide surface at 1 bar to porous metallic iron at 100 bar under static conditions. Simultaneously, the pore morphology transitioned from large, elongated shapes at 1 bar to fine, random shapes at 100 bar under dynamic conditions. In addition, questions of “How does hydrogen gas pressure affect the individual reactions of HyDR AbstractVIIiron ore?” and “Does the solid-gas reaction mechanism change with hydrogen gas pressure?” were explored using in-situ synchrotron high-energy X-ray diffraction during HyDR to monitor real-time phase transformations of iron ore under hydrogen gas pressures of 1, 10, 50, and 100 bar. Last but not least, deploying hydrogen in ironmaking raises critical questions about its impact on material properties: How much hydrogen remains in green steel, and will it cause hydrogen embrittlement? To address this, hydrogen content was quantified in iron produced via hydrogen-HyDR and hydrogen plasma smelting reduction (HPSR) using thermal desorption spectroscopy (TDS) and hot extraction. HyDR iron contained 39.90±9.00 wppm hydrogen, which dropped to 1.46±0.50 wppm after melting in an arc furnace, while HPSR iron contained 0.98±0.50 wppm. Hydrogen trapping sites and energies in direct reduced iron were identified using TDS and microstructure analysis. Comparing hydrogen levels in green steel with conventional processes suggests that hydrogen-based methods do not promote hydrogen embrittlement. The results of this dissertation demonstrate that using hydrogen as a reducing agent to reduce iron ore does not lead to hydrogen embrittlement in the final product. Furthermore, these findings offer valuable insights into the critical influence of hydrogen pressure on the reduction kinetics and microstructural evolution of iron ore during the HyDR process. An increasing H2 pressure increases the partial pressure of H2, which promotes faster reduction kinetics. This fact should be considered for the design of industrial reactors. These insights shed light on advancements in furnace design, e.g., hydrogen gas pressure, and process optimization.
Özge Özgün (Wed,) studied this question.