Cyclic reduction of combusted iron powder: A study on the material properties and conversion reaction in the iron fuel cycle

Abstract

Recently, iron powder has been proposed as a high energy density, easily storable, and CO2-free energy carrier. During the iron combustion, thermal energy is released as heat. The combusted products, the iron oxide particles, are captured and cyclically reduced back into iron powder in a process that is powered by renewable energy. Each combustion step, followed by a reduction, constitutes one cycle in the process. Previous studies predominantly focused on the reduction and combustion as individual steps. This work investigates the impact of cyclic combustion-reduction on material properties, with the specific focus on the reduction of iron oxides using hydrogen under fluidisation conditions. The powder is combusted and reduced for 3 cycles under the same conditions. Afterwards the properties of the powder and the reaction conversion are characterized. The physical techniques used for size and shape analysis include laser diffraction particle size analysis and scanning electron microscopy (SEM). For structure and chemistry, X-ray diffraction (XRD) and energy dispersive X-ray (EDX) analysis are also employed. To study the effect of the reduction conditions on the cycle and on the material properties of the powder, different experiments are conducted using 550 and 575°C as reduction temperatures. The higher conversion is observed at 575°C. During the first combustion particles resulted in large agglomerates and required a manual grinding step before being sent to the next reduction. However, the particle size distribution remains relatively stable in all subsequent cycles. The results suggest that the powder can be effectively utilized in the iron fuel cycle without requiring additional intermediate treatments, such as grinding and sieving after each cycle. This aspect highlights the potential feasibility and simplicity of implementing the cyclic combustion-reduction process for practical applications of iron powder as an energy carrier.