New insights into iron fuel combustion: integrated in-situ and ex-situ diagnostics of ignition delay, melting–oxidation, disruptive phenomena and nanoparticle sizing

Iron powders are attracting growing interest as recyclable energy carriers, offering high-temperature heat release during combustion and the potential for a carbon-free, closed energy cycle. However, key aspects remain insufficiently characterised, including single-particle combustion times over broad size ranges and the formation, size and concentration of nanoparticles during combustion. This study provides the first experimental investigation of irregular iron particles up to 250 μm in size. Combustion experiments were conducted using a controlled gas-supply, a tri-concentric tube burner with a motorised powder injector, and a quartz drop tube leading to a stainless-steel chamber for by-product collection and gas sampling. In-situ diagnostics employed a high-speed camera and a photomultiplier tube (PMT) module, while nanoparticle sizing used an aerodynamic particle sizer (APS). Ex-situ characterisation was performed by scanning electron microscopy (SEM). PMT data combined with particle-size analysis yielded new correlations for ignition delay and liquid-phase oxidation times. Ignition delay follows a second-order polynomial relationship, in contrast to the power-law behaviour reported for spherical particles, while liquid-phase oxidation shows simultaneous melting and oxidation and may be more accurately termed the melting-oxidation phase. Particle growth rates during this phase indicated oxidation rates of approximately 10–20 μm/ms. At later stages, oxide-shell rupture led to the ejection of molten nanoparticles, producing a bright secondary oxidation phase beyond the particle surface. SEM micrographs revealed a variety of disruptive events, including inter-particle collisions, impacts with the surrounding quartz tube, partial oxidation, micro-explosions and the development of surface cavities. Real-time APS measurements of exhaust emissions further demonstrated a unimodal nanoparticle distribution with a peak at 583 nm and evidence suggesting the presence of sub-500 nm particles.