Modeling and Assessment of Ammonia Direct Reduction for Decarbonizing Iron Production

Ammonia's favorable properties for transport and storage make it a promising alternative to hydrogen as a low-carbon reductant for green steel, particularly in regions lacking renewable energy. Despite several experimental investigations, the systematic evaluation of iron ore reduction by ammonia in the shaft furnace is still challenging, as there is no comprehensive mathematical model to simulate its chemical and physical performance. In this study, a one-dimensional plug-flow model was first developed to describe the ammonia reduction process in an industrial-scale shaft furnace with counter-current gas and solid flows. The kinetics of chemical conversions, including ammonia decomposition, direct reduction by ammonia and hydrogen, and iron nitridation, were incorporated into the model, along with mass and heat transfer, within the framework of a grain model at the pellet scale. Calibrated by TGA experimental data, the effects of process conditions and gas recycling were systematically investigated through numerical simulation. The results indicate that the process of ammonia-based reduction of iron ore is strongly endothermic, making intensive heat supply essential. A high inlet gas temperature above 900 °C is favorable to the operation of the shaft furnace with a high metallization rate and a low nitridation rate. By introducing a purge stream, excess nitrogen and water vapor can be removed from the system, enabling gas recycling and the reuse of ammonia and hydrogen from the off-gases; this achieves a metallization rate exceeding 0.90 with a nitridation rate below 0.10. By comparing it with hydrogen-based reduction of iron ore, this work provides a deep understanding of ammonia reduction and offers valuable guidance for industrial-scale reactor design and supply chain analysis.