Iron as recyclable energy carrier: Feasibility study and kinetic analysis of iron oxide reduction

Carbon-free and sustainable energy storage solutions are required to mitigate climate change. One possible solution, especially for stationary applications, could be the storage of energy in metal fuels. Energy can be stored through reduction of the oxide with green hydrogen and be released by combustion. In this work a feasibility study for iron as possible metal fuel considering the complete energy cycle is conducted. On the basis of equilibrium calculations it could be shown that the power-to-power efficiency of the iron/iron oxide cycle is 27 %. As technology development requires a more detailed description of both the reduction and the oxidation, a first outlook is given on the kinetic analysis of the reduction of iron oxides with hydrogen. Thermogravimetric experiments using Fe$_2$O$_3$, Fe$_3$O$_4$ and FeO indicate a three-step process for the reduction. The maximum reduction rate can be achieved with a hydrogen content of 25 %. Based on the experimental results a reaction mechanism and accompanied kinetic data were developed for description of Fe$_2$O$_3$ reduction with H$_2$ under varying experimental conditions.