Micron-sized iron particles as energy carrier: Cycling experiments in a fixed-bed reactor

Iron is a promising energy carrier with the potential to store substantial amounts of energy over extended time periods with minimal losses. For instance, the energy from green hydrogen sources can be used to reduce iron oxides, be stored or transported, and thus be regained by exothermic oxidation of the iron. This work explores the influence of oxygen partial pressure and temperature on the oxidation process in a fixed-bed reactor. Furthermore, the analysis extends to the reduction of oxidized iron particles at varying temperatures. The experimental findings highlight that both oxidation and reduction progress through the fixed-bed reactor as distinct reaction fronts. In the oxidation process, the speed of the reaction front increases with rising oxygen content and temperature, resulting in a higher reaction rate and a correspondingly increased heat release. Conversely, the reaction rate for reduction experiences a notable decrease for 600°C and 700°C. The reprocessability of the iron powder was validated for up to 16 cycles under the optimal reaction conditions established. Furthermore, it was demonstrated that the performance improves with an increasing number of cycles. This improvement is attributed to the formation of pores due to density changes and the subsequent creation of a larger surface area, mitigating the negative effects of sintering and agglomeration.