  Metals can serve as carbon-free energy carriers, e. g. in innovative
metal-metal oxide cycles as proposed by Bergthorson (Prog. Energy Combust.
Sci., 2018). Iron powder is a suitable candidate since it can be oxidized with
air. Nevertheless, the combustion of iron powder in air is challenging
especially with respect to flame stabilization which depends on the particle
size distribution among other factors. Models for the prediction of reaction
front speed in iron-air suspensions can contribute to overcoming this
challenge. To this end, three different models for iron particle oxidation are
integrated into a laminar flame solver for simulating reaction fronts. The
scientific objective of this work is to elucidate the influence of
polydispersity on the reaction front speed, which is still not satisfactorily
understood. In a systematic approach, cases with successively increasing
complexity are considered: From single particle combustion to iron-air
suspensions prescribing binary particle size distributions (PSDs), generic
PSDs, and a PSD measured for a real iron powder sample. The simulations show
that, dependent on the PSD, particles undergo thermochemical conversion in a
sequential manner according to their size and every particle fraction exhibits
an individual combustion environment. The local environment can be leaner or
richer than the overall iron-to-air ratio would suggest and can be very
different from single particle experiments. The contribution of individual
particle fractions to the overall reaction front speed depends on its ranking
within the PSD. The study further demonstrates, that although the three
particle models show good agreement for single particle combustion, they lead
to very different reaction front speeds. This is due to the different ignition
behavior predicted by the particle models, which is shown to strongly influence
the reaction front characteristics.
