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Iron particle ignition inhibition by prior surface oxidation
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DOI：10.1016/j.combustflame.2026.115029.png)
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Abstract
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This paper reports on the ignition of powdered iron, a promising but understudied alternative to fossil fuels for power and heat generation. The specifics of iron particle ignition depend on many variables, one of which is the thickness of the pre-ignition oxide layers formed during storage and handling of this fuel. A thick oxide layer can delay or altogether prevent ignition of particles, which decreases fuel conversion efficiency. This investigation aims to empirically determine how the formation of oxidation layers on iron particles, either during their storage, or upon their injection into the hot environment of furnaces, affects their ignitability. Three different sized populations of iron particles were tested including two irregularly shaped (36–58 μm and 18–39 μm) and one spherical 45–58 μm. The effect of the oxidation layer forming during particle storage and particle heat up on their ignitability was investigated. First, iron particles were subjected to accelerated oxidation by keeping them in humid environments. Afterwards, they were injected into an externally heated drop tube furnace (DTF), operated at wall temperatures between 1000 K – 1450 K. Second, the air flow velocity in the furnace was also varied to investigate the effects of the particle temperature profiles and subsequent oxide growth during heat up in the DTF on their ignitability. The percentage of particles that ignited was determined by observing and linking the sizes and shapes of the final oxidized particles to the chemical analysis of the particle composition accomplished using energy-dispersive x-ray spectroscopy. Experiments examining oxidation through humidity and heating rates illustrated the importance of oxide growth for the combustion of iron powder. The particles which were pre-oxidized and formed outer oxide layers even thinner than 1 μm dropped in ignition attainment percentage and required gas temperatures at times over 100 K higher to ignite at similar percentages. Particles which were pre-oxidized with layers 1–2 μm experienced more significant shifts in ignition temperature. This aligns with simulation results which have shown a shift in ignition temperature once an oxide layer surpasses 0.3% of the diameter. Modifying airflow resulted in the ignition temperature of the iron particles changing by over 50 K depending on the magnitude of the heating rate. A previously published model was modified to simulate the experiments of spherical iron particles used in this work. Particle pre-oxidation and slow heating in the furnace were found to decrease the conversion efficiency of iron fuel, an important factor in designing and operating iron fuel furnaces.
Keywords:
Iron powder
Fuel cycle
Pre-oxidation layer
Ignition
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