On the combustion of Fe-Al and Fe-Si powders for sustainable energy storage

Abstract

Recently, metal fuels have been proposed as sustainable energy carriers with high energy density. In particular, iron powder holds great promise due to its relatively low cost, safety, abundance, and easy retrofit of power plants. So far, research and development have been focused on the use of pure Fe powder for this sustainable fuel application. Yet, the use of impure Fe coming from low-cost Fe sources such as recycled scraps would strongly increase the economic competitiveness of iron fuel compared to fossil fuels or other high-energy density carriers such as ammonia. The influence of these impurities on the combustion process has however not yet been studied in detail. In this work, the combustion behaviour of Fe-Al and Fe-Si powders will be presented, with a focus on their ignition process and on their microstructural and chemical evolution after combustion. Chemically homogeneous metallic powders with different concentrations of Al and Si (up to 7 wt%) will be produced by casting followed by gas atomization. The ignition efficiency of each powder will be compared to that of pure Fe for identical combustion parameters, based on the relative amount of unoxidized, metallic phase in the combusted products measured by x-ray diffraction. The ignition temperature for each composition will be additionally measured using a solid particle ignition reactor, and linked in situ with the particle diameter using high-speed optical diagnostics. The ignition efficiency of each composition will be discussed in light of their solid-state oxidation reactions. Isothermal solid-state oxidation kinetics will be measured by thermogravimetry analysis (TGA) at different oxidation temperatures (700°C- 1000°C) and coupled with microstructural characterization to identify the limiting mechanisms governing the oxidation process. Advanced microstructural characterization of the combusted powders will also be presented to unveil the impact of Al and Si on the combustion behaviour of alloyed Fe powders. In particular, the global chemical, porosity and phase evolution will be characterized using Inductive Coupled Plasma (ICP), gas pycnometer, surface area BET measurements, and x-ray diffraction (XRD). The local microstructural, morphological and chemical evolution of the different phases in the micro- and nano-particles will be presented using scanning electron microscopy (SEM, STEM) coupled with Energy Dispersive Spectroscopy (EDS) and Electron Backscatter Diffraction (EBSD). The microstructural and chemical evolution of the powders will be discussed with thermodynamic simulations of the combustion process using Thermo-Calc