Green Ironmaking by Rapid Reduction of Iron Oxide with Ammonia

Steel will be essential to the energy transition, and decarbonizing steel production will therefore also be critical. Steel is found in electric vehicles, wind turbines, solar panel racking, geothermal plants, and electricity transmission towers—not to mention buildings, ships, and bridges. However, steel production is responsible for 7-8% of annual global CO₂ emissions, with half of this CO₂ coming from converting iron ore into iron.<br/>Ironmaking alone produces ca. 4% of annual CO₂ emissions, some 2×10⁹ metric tons/year (2 Gt/a). These CO₂ emissions result from reduction of iron oxides to iron metal with fossil fuel-derived CO, producing CO₂ as a chemical byproduct, and can only be eliminated through new chemistry. Using ammonia instead of coke—releasing only water and nitrogen as byproducts—could be a widely deployable method of decarbonizing industrial ironmaking, because ammonia carries hydrogen and can be transported easily worldwide.<br/>Direct reduction of iron oxide with ammonia has been reported previously,^1-3 but the process suffers from slow reaction kinetics and low utilization of ammonia. To date, the fastest and most efficient reduction of any iron oxide with ammonia was reported in 21 min at 750 °C, utilizing 8.9% of the ammonia on the ~1.7 mmol scale.³ Because the cost drivers of ironmaking processes are throughput (i.e., capital equipment utilization), ore costs, and fuel costs, the low utilization and slow rate provide critical barriers to the possibility of decarbonizing ironmaking using ammonia.<br/>A thermochemical assessment of the reaction highlighted high temperatures and high flow rates of pure NH₃ as key parameters to explore to enhance utilization; from a simple kinetics perspective, these conditions should also increase the reaction rate. Drawing on this analysis and subsequent experiments, we have reduced iron oxide to iron on the mmol scale in 1 minute while using 64% of the ammonia supplied. These results represent order of magnitude improvements in both rate and utilization. We will also discuss scale-up of the reaction, and translation of the process insights to pelletized ore, a common form used industrially.<br/><br/><b>REFERENCES </b><br/><br/>(1) Hosokai, S.; Kasiwaya, Y.; Matsui, K.; Okinaka, N.; Akiyama, T. Ironmaking with Ammonia at<br/>Low Temperature. <i>Environ. Sci. Technol.</i> <b>2011</b>, <i>45</i> (2), 821–826.<br/>https://doi.org/10.1021/es102910q.<br/>(2) Ma, Y.; Bae, J. W.; Kim, S.-H.; Jovičević-Klug, M.; Li, K.; Vogel, D.; Ponge, D.; Rohwerder, M.; Gault, B.; Raabe, D. Reducing Iron Oxide with Ammonia: A Sustainable Path to Green Steel. <i>Adv. Sci.</i> <b>2023</b>, <i>10</i> (16), 2300111. https://doi.org/10.1002/advs.202300111.<br/>(3) Iwamoto, I.; Kurniawan, A.; Hasegawa, H.; Kashiwaya, Y.; Nomura, T.; Akiyama, T. Reduction Behaviors and Generated Phases of Iron Ores Using Ammonia as Reducing Agent. <i>ISIJ Int.</i> <b>2022</b>, <i>62</i> (12), 2483–2490. https://doi.org/10.2355/isijinternational.ISIJINT-2022-155.