Nitric Oxide Pathways in Surface-Flame Radiant Burners

Abstract

<p>Nitrogen oxide (NO_x) formation in surface-flame burners is studied. Surface-flame burners are typically made of metal fibers, ceramic fibers, or ceramic foam and provide radiant flux with low pollutant emissions. A one-dimensional model represents combustion on and within the porous medium using multistep chemistry, separate gas and energy equations, and a radiatively participating porous medium. We describe experimental measurements of NO_x profiles above a surface-flame burner and compare them to model predictions. The model predicts NO_x concentration with reasonable success. Deviations between model and experiment are primarily the result of heat loss in the experiment that is not considered in the model. Reaction rate analysis is performed to identify the chemical kinetic source of NO in the flame. Zeldovich NO is significant only at the highest firing rate studied (600 kW/m², ϕ = 0.9), where it is responsible for 50-60% of the total NO. At the lower firing rates (200 and 300 kW/m², ϕ = 0.9), where total NO is low, nearly all of the NO is formed in the flame front. Zeldovich NO accounts for 20-30% percent of the total NO, the Fenimore pathway accounts for less than 10% of the NO, and 50-75% percent of the NO is formed through the NNH, N₂O and other paths. Sensitivity analysis shows that NO production in the flame front is most sensitive to NNH+O = NH+NO, with CH+N₂ = HCN+N having the second highest sensitivity coefficient. At the lower firing rates NO emission is insensitive to porous medium properties, while at the high firing rate NO emission is slightly sensitive to porous medium properties.</p>