Combustion characteristics of single iron particles under ammonia co-firing conditions

Iron powder is regarded as a highly promising zero-carbon energy carrier, with combustion as its primary mode of energy release. However, iron dust flames exhibit poor stability, prompting the common practice of co-firing with hydrocarbon fuels to ensure stable combustion. This approach still yields carbon emissions. In pursuit of a fully zero-carbon iron-fuel cycle, the present work firstly investigates the combustion characteristics of single iron particles under ammonia co-firing conditions. Two distinct combustion behaviors, including micro-explosion and fragment release, are observed. The fragments are inferred to be the nanoparticle cloud based on 30k fps high-speed shadowgraphy. Under ammonia as the carrier gas, the micro-explosion probability of iron particles exceeds that observed with methane or nitrogen, significantly at oxygen mole fractions of 10.9%–20.4%. This phenomenon likely arises from iron nitride decomposition at the liquid iron (L1)–liquid iron oxide (L2) interface. Furthermore, the micro-explosion probability in ammonia/iron combustion decreases with increasing oxygen concentration. The micro-explosion delay time (MDT) is defined to quantify the effect of particle size on liquid-phase combustion under ammonia co-firing conditions. Further experimental results show that at higher oxygen concentrations, MDT is nearly proportional to the inverse of oxygen mass fraction, suggesting that particle oxidation is limited by external oxygen diffusion. However, at Y_O2 = 12.5%, MDT deviates from the linear correlation. In the low oxygen concentration cases, iron nitride may react with absorbed oxygen and impede the internal transport of oxygen, thereby constraining the oxidation rate of iron and delaying the formation of a complete core–shell structure. Overall, ammonia/iron co-firing technology shows great promise for regulating micro-explosions and represents a crucial step toward realizing a genuinely zero-carbon iron-fuel cycle. Novelty and Significance Statement The fundamental combustion characteristics of iron particles under ammonia co-firing conditions were first investigated in this work. The micro-explosion probability of iron particles in a hot ammonia environment is significantly high and decreases with increasing oxygen concentration in the bulk gas. The effects of particle size and ambient oxygen concentration on the iron particles combustion time under ammonia co-firing conditions were quantitatively analyzed. The potential mechanisms underlying the influence of ammonia on the micro-explosion of iron particles were discussed. The ammonia/iron co-firing technology offers a novel approach for achieving a truly zero-carbon iron-fuel cycle.