Theoretical elucidation of the hindering effect of oxide-layer growth on the ignition of iron particles

Abstract

This study investigates the hindering effect of a growing oxide layer on the ignition behavior of fine iron particles, focusing on scenarios where the gas temperature is below the critical ignition temperature ($T_{\mathrm{ign}}$). The analysis is grounded in a thermophysical model integrating solid-phase iron oxidation mechanism and heat and mass transfer between the particle and surrounding gas. The results reveal that the growth of an oxide layer under subcritical temperatures has a hindering effect, significantly raising the $T_{\mathrm{ign}}$ required for thermal runaway. The effect is more pronounced for smaller particles, with the required $T_{\mathrm{ign}}$ increasing by 80–250 K over a residence time of three seconds for particle sizes ranging from 200 to 50 µm, respectively. These findings provide a theoretical framework linking ignition mechanism to the heating process experienced by an iron particle in a practical combustor, offering insights for improving ignition rates through tailored heating conditions.

Novelty and significance statement

This study theoretically identifies the primary reason why a significant number of iron particles fail to ignite in practical combustors—an issue previously reported by developers of iron-powder combustion technologies in industry, yet insufficiently addressed by academic researchers. It elucidates the hindering effect of oxide-layer growth, caused by inadequate heating rates, on the ignition propensity of iron particles—a mechanism suggested in the work of Mi et al. (2022), but still not fully understood within the Metal-enabled Cycle of Renewable Energy (MeCRE) community. By establishing a theoretical framework, this brief communication provides guidance for the design of iron-powder combustors, emphasizing the need for sufficient heating rates to improve ignition reliability and overall combustion efficiency.