Reduction and Combustion of Iron Powder for Energy Storage: The Influence of Micro-Structure

Cyclic reduction and combustion of iron powder is emerging as a promising method for seasonal storage and intercontinental transport of renewable energy. When renewable energy is in surplus, iron-oxide powder can be reacted with green hydrogen to produce “energized” iron powder. When the energy needed again, the iron powder can be reacted/combusted with oxygen in air, thereby releasing a tremendous amount of heat.<br/>Both the reduction (in solid-state) and the combustion processes (in liquid state) result in changes in the micro-structure and morphology of the powder, which in turn influences the performance of the cycle. The reduction process produces porous particles (sponge iron), with a reduction-temperature-dependent pore size. The resulting effective surface area influences the ignitability and safety of the powder. The subsequent combustion process produces a combination of (1) spherical particles with hollow (open and closed) cores and (2) “tulip”-shaped hollow shells. The closed cores are likely the result of shrinkage cavities, while the “tulip”-shaped shells are most likely caused by micro-explosions observed during combustion. All combusted particles show cracks and preferred crystal orientations, in turn influencing the reduction process.<br/>In this work combusted and reduced powder is analyzed using various material characterization techniques, such as scanning electron microscopy (SEM), X-ray diffraction (XRD), particle size analysis, and X-ray computed tomography (μCT). The micro-scale findings are coupled to macro-scale phenomena observed during the combustion and reduction processes.