Micro-Explosion mechanism of iron hybrid Methane-Air premixed flames

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Compared to gaseous hydrocarbon flame, a metal hybrid flame has higher energy and even has a higher flame temperature. This study explored the micro-explosion mechanisms of iron particles in methane-air premixed flames. Four different particle sizes were used in this experiment, that is 2–10 μm, 50 μm, 100 μm, and 150 μm. A conical methane-air premixed flame was fixed in a stoichiometric condition and doped with iron particles at various feeding rates. The 50- and 100-μm iron particles had a 20% and 500% higher micro-explosion probability than the 2–10-μm particles. The micro-explosion probability would increase with the increase of particle size. In addition, the micro-explosion probability of the 2–10 μm iron particles was 125% higher under 10% CO addition than under no CO addition. CO plays a critical role in increasing the micro-explosion probability of iron particles smaller than 50 µm.Regarding the mechanism of the micro-explosion of iron particles, the oxidizer diffuses into iron particles and produces iron oxide in a thin shell. However, limited oxygen heterogeneously reacts over the surface of the iron particles and produces Fe3O4. Fe3O4 comprises FeO and Fe2O3. However, when CO reacts Fe3O4, this reaction makes Fe3O4 redox into fresh Fe particles, whereas it will lead to FeO while Fe reacts with O2 predominantly. The presence of FeO brings about iron particles blinking and shrinking right after passing the flame front. When CO was dominated in the reaction, it gave rise to the formation of Fe(CO)5 gas bubbles in the film of iron particles. During the agglomeration of the bubbles, incorporating Fe(CO)5, CO, O2, and N2, the iron particles expand gradually and become hollow spherical particles eventually. However, under special conditions, this particle expansion will deteriorate the micro-explosion of the iron particles.