Ammonia-based direct reduction of iron ores: modelling and experimental assessment

The iron and steel industry is a strategic sector of the EU economy, producing a material that is crucial to most of the EU’s industrial ecosystems while being responsible for approximately 5% of the EU’s total CO₂ emissions. Investment decisions made in the next decade must align with the EU’s climate targets to avoid the risk of stranded assets and locking in CO₂ emissions beyond 2050. The industry is focusing on developing direct reduced iron (DRI) technology, which currently relies on natural gas for reducing iron ore and represents 5% of global steel production outside the EU, because the application of green reducing agents in direct reduction furnaces is one of the most promising strategies for reducing CO₂ emissions in steel production. In fact, hydrogen has the potential to replace fossil fuels in the reduction of iron oxide pellets to pure iron, thus eliminating CO₂ emissions during the most carbon-intensive stage of production. However, the low volumetric energy density of hydrogen presents challenges for its transport and storage; it requires compression or liquefaction, which are costly and energy-intensive processes, and even high-pressure tanks add significant weight and raise safety concerns due to potential leaks and boil-off losses. Alternatively, ammonia, a hydrogen carrier, could be directly fed into the furnace, where its decomposition into hydrogen and nitrogen is promoted by high temperatures and catalyzed by iron in the shaft furnace. This approach not only leverages ammonia's transport advantages but also bypasses the costly ammonia-to-hydrogen conversion, as the cracking occurs within the furnace environment. However, the endothermic nature of ammonia decomposition adds further to the already substantial thermal load required for hydrogen-based reduction, making its management crucial for efficient furnace operation. To address the technical implications of ammonia integration, a kinetic model of the shaft furnace using the Unreacted Shrinking Core (URC) model was developed to evaluate the effects of ammonia on temperature profiles and mass flow rates. Additionally, an experimental study on the porosity evolution of iron oxide phases during hydrogen reduction was conducted, accurately representing hydrogen diffusion within the pores of iron oxide in determining the reaction rate in the URC model. Ultimately, direct use of ammonia in iron oxide reduction processes offers a compelling opportunity to combine CO₂ emission reduction with improved economic feasibility in steel production.