The combustion of metal fuels as energy carriers in a closed-cycle carbon-free process is a promising approach for reducing CO₂ emissions in the energy sector. For a possible large-scale implementation, the influence of process conditions on particle properties and vice versa has to be well understood. In this study, the influence of different fuel–air equivalence ratios on particle morphology, size and degree of oxidation in an iron–air model burner is investigated by means of small- and wide-angle X-ray scattering, laser diffraction analysis and electron microscopy. The results show a decrease in median particle size and an increase in the degree of oxidation for leaner combustion conditions. The difference of 1.94 μm in median particle size between lean and rich conditions is twentyfold greater than the expected amount and can be connected to an increased intensity of microexplosions and nanoparticle formation for oxygen-rich atmospheres. Furthermore, the influence of the process conditions on the fuel usage efficiency is investigated, yielding efficiencies of up to 0.93. Furthermore, by choosing a suitable particle size range of 1 to 10 μm, the amount of residual iron content can be minimized. The results emphasize that particle size plays a key role in optimizing this process for the future.

Iron is a very promising candidate for the use as a chemical energy carrier. For application it is crucial to understand the reaction behavior of iron and iron oxide powders in the micrometer range. Recent evidence shows that the oxidation behavior of iron powders differs from the well understood iron slab oxidation. As the reactions are fast, approaches using quantitative in-situ analytical methods are desired. Herein, we follow iron powder oxidation and iron oxide powder reduction using in-situ Fe-57 Mössbauer spectroscopy and in-situ X-ray diffraction. A time resolution of 30 min is achieved with Mössbauer spectroscopy even for iron powder with natural isotopic abundance (2.1 % Fe-57 isotope). We show that both methods are suitable and complementary, motivating future coherent approaches. By combining compositional results from Mössbauer spectroscopy and X-ray diffraction with morphological characterization, advanced mechanistic models for the processes during oxidation and reduction are proposed.