Recycling Iron Power as an Energy Carrier: Investigating Material Properties Through Cycles of Combustion and Reduction

The world’s energy system is transforming from a fossil fuel-based energy infrastructure to a society supported by sustainable energy sources, like solar and wind energy. The production of sustainable energy is quite intermittent, causing a great demand for additional capacity of local energy storage. We propose a low-cost alternative for high density energy storage: iron powder. Recently iron powder has been proposed as an energy carrier for its high energy density, ease to be stored, and for its fully CO2-free combustion process. Renewable electricity can be used to reduce iron oxide powder into iron, which can be densely stored under ambient conditions and combusted whenever and wherever energy is needed, providing fully renewable heat. This research determines the potential of the entire iron fuel cycle, in terms of the material properties.<br/>Iron oxide powder is combusted and reduced over 6 cycles, where each partial cycle had the same conditions. Reduction experiments are performed inside a lab-scale reactor using a hydrogen flow at elevated temperatures of 400 to 600 °C. High temperatures are beneficial from a kinetic point of view. However, previous studies showed de-fluidization due to sticking of the powder above 600 °C. The reactor and heating furnace are designed for a full control of operating conditions. The powder is assessed in terms of particle size, morphology, porosity, composition and reduction degree after each partial cycle. We observe that the particle size distribution shifts to larger size already after the first combustion process. Nevertheless, the size distribution remains approximately the same over multiple cycles. This indicates that the powder can be used in the cyclic process without intermediate treatments (e.g sieving).<br/>A trade-off between a high reduction degree and absence of solid sticking behavior is identified at the temperature of 590 °C. The results of the XRD show that reduction conversion to iron is larger than 80% for each cycle. Sticking is observed in the powder after combustion, which is assumed to take place within the collection part of the process.The losses due to the handling of the powder through each step of the cycle is negligible. Iron powder is therefore a good candidate to be used as energy carrier in the iron fuel cycle.