Nitrogen oxide formation mechanism in iron dust flames

Nitrogen oxide formation in a freely propagating iron dust flame is studied. An Eulerian-Lagrangian framework is employed to model iron dust combustion, with detailed gas phase chemistry to model nitrogen oxide formation. Nitrogen oxide formation in an iron flame using dry air is found to be very small due to the low concentration of oxygen radicals. However, even for a small fraction of water vapor (X_H2O<0.5%) in the mixture, hydroperoxyl (HO₂) is formed, which accelerates the dissociation of molecular oxygen and increases the nitrogen oxide formation rate. This demonstrates that water vapor, present in the air, should be considered when studying nitrogen oxide formation in iron dust combusters. In iron flames, the particle temperature can significantly exceed the gas temperature. Using a posteriori analysis, we show that nitrogen oxide formation inside the particle boundary layer is not a concern. Finally, the nitrogen oxide formation in an iron dust flame is compared to the formation in methane and hydrogen flames. We find that iron dust flames produce less nitrogen oxide per unit of heat produced.