Nanoparticle formation in the boundary layer of burning iron microparticles: Modeling and simulation

Iron powder is emerging as a promising carbon-free energy carrier that can be used fora clean iron-based energy cycle. The powder can be combusted with air to generate heat and power. Thereafter, the iron oxide powder is collected and regenerated by means of a thermochemical reduction with green H2 closing the loop. During the combustion, the formation of nanoparticles poses challenges in terms of particulate emissions and material losses. Nanoparticles are difficult to separate from the exhaust gases and are respirable, which is why a comprehensive understanding of their formation and how to avoid them is necessary. In this study, a model for nanoparticle formation is introduced, which is based on the condensation of supersaturated iron/iron oxide vapor to liquid nanoparticles in the boundary layer of the burning iron microparticle. Resolved boundary layer simulations of single iron microparticles are compared with recent in situ measurements to investigate the onset of nanoparticle formation and characteristics of the nanoparticle cloud that is formed close to the burning parent microparticle. Nanoparticle formation and nanoparticle cloud evolution are investigated, considering combustion and transport processes in the boundary layer. It is shown that the particle temperature is the most important parameter for nanoparticle formation and a correct prediction of particle temperature evolution is crucial for nanoparticle prediction. Further analysis identifies convection and thermophoresis as the primary transport processes for the nanoparticle cloud, while diffusiophoresis is negligible. Additionally, the sensitivity of nanoparticle formation to the evaporation model and reaction mechanism is evaluated.