The effect of surface reactions on the prediction of NO(x) conversion efficiency in a porous burner

It is known that the presence of solid surfaces can strongly influence gas-phase reaction systems. This effect arises as a result of heterogeneous reactions between gas-phase radicals and surfaces. The rate of radical termination at a surface is strongly influenced by both the nature of the surface and the conditions, if any, under which the surface has been pre-treated. Given the large surface-to-volume ratios that are characteristic of porous burners it is imperative that these surface reactions, leading to termination of gas-phase radicals, be considered when modelling combustion, and the formation of minor species, in porous burners.The impact of surface reactions on the predicted NOX conversion within a porous burner is modelled using a short-cut approach. Limiting cases which either ignore radical termination or assume that radical termination proceeds at the mass-transfer-limited rate are considered. For intermediate cases, an effective reaction rate that includes the combined effects of mass transfer and surface reaction is assumed. The current modelling predictions are compared with our previous work on NOX conversion in a porous burner, in which the effects of surface reactions were neglected. For these data, an effective rate of radical loss at the burner surface equivalent to 8×10-4 times the mass-transfer limited rate is found to give best agreement. Comparing the assumed effective rate of H radical loss at the burner surface with the estimated surface collision rate suggests that H atoms recombine with an efficiency of 1×10-5, which is in good agreement with recent measurements on silica and Pyrex surfaces. Despite very low radical recombination efficiencies, the overall rate of surface recombination is sufficiently large to markedly influence the model predictions for this system at φ≤1.3.The impact of surface reaction rate, different equivalence ratios, NO initial concentration and pathways is also investigated. It is found that under slightly fuel-rich conditions (φ≤1.3), surface reaction rate has the maximum impact while for higher equivalence ratios (φ>1.3) the effect is minimal. It is also found that the presence of surface reactions significantly impacts on the NOX reduction efficiency for mixtures with low NO concentration. These differences are believed to be related to the NO conversion pathways under these different conditions. For conditions where the NO conversion mainly follows a pathway: NO⇒HNO⇒NH⇒N2, the presence of surface reactions has the greatest effect on NOx predictions whereas for conditions where the NO conversion follows a pathway: NO⇒HCNO⇒HNCO⇒NH2⇒NH3, the presence of surface reactions has a minimal impact on NOx predictions. © 2013.Shahrooz Afsharvahid, Pedro N. Alvarado, Peter J. Ashman, Bassam B. Dall