In situ mapping of iron oxidation in laminar metal fuel flames using X-ray absorption spectroscopy

Metal fuels such as iron are promising carbon-free energy carriers for a sustainable energy system, where energy release occurs via combustion in metal flames. Apart from the design of the appropriate burners, another challenge is to control the oxidation process and gain direct insight into the kinetics and the mechanism of metal oxidation in flames. Optimizing the combustion process to release the stored energy is crucial, but in situ analysis of metal flames remains a real chemical engineering and physical chemistry challenge. Here, we demonstrate an in situ approach with synchrotron X-ray radiation allowing identification and quantification of iron (oxide) phases during the combustion process that is adaptable for various combustion modes and conditions. Using quick scanning X-ray absorption spectroscopy, we were able to track the oxidation state of iron and thus the structure along the visual flame cross-section. Strong gradients in oxidation state and phase composition across and beyond the flame front were found. The flame front determined by visualization with an optical camera corresponds to the formation of FeO, while the main combustion product Fe$_3$O$_4$ was mainly formed outside the visible flame zone.