OCR Text |
Show One possible path for N O reduction in the post-flame region is the recycle with CH and C H 2 to give HCN. Is this path affected by sulphur compounds? Tseregounis and Smith 17 found large increases in fuel-NO formation when S02 was added to premixed, laminar, fuel-rich H2/C2N2 flames. Results in this hydrocarbon-free flame ruled out the inhibition of the recycling process between N O and hydrocarbons by the addition of a sulphur compound. In addition, their calculations showed that direct interactions between fuel-N and fuel-S of the kind N + SO *-* NO + S 3 were not plausible as this would lead to a decrease in NO concentrations in fuel-rich conditions. The relatively low importance of this kind of direct interaction was also assessed by Chen et al. *, as discussed in the following paragraphs. The opposite conclusions however were obtained experimentally ana numerically by Wendt ef al. * in studies of rich, moist, CO/Ar/02 flames doped with C2N2 The absence of H radicals excluded radical recombination as a means of NOx reduction in this case. Although the results could not be extrapolated to hydrocarbon flames, the authors attributed reductions of N O in the post-flame zone to the action of the reverse Zeldovich mechanism only: N + NO r-* N2 + O 4 the N radicals being formed in: O + NO r-* N + 02 5 Direct interactions of sulphur species with nitrogenous species were proposed to explain accelerated reduction of N O and formation of N2: N + NO NS SO <-* N O + + S *-» N S + + 0 v=^ S O + S 0 N 3 6 7 This work was continued by Corley and Wendt5 in fuel-rich CH4/He/02 flames doped with C2N2 and S02. Increased N O concentrations were considered to be caused by the direct interaction of N and S O radicals in reaction 3. H C N also increased whereas N2 decreased. Reaction 3 was complemented by interactions of reduced sulphur species in the cyanide and amine subsystems, such as: CN + SO > NCO + S 8 NH + S O > H N O + S 9 Detailed work performed by Tseregounis and Smith 18 showed that S02 decreased the concentrations of most radicals (O, O H and H) by almost 50 % in near-stoichiometric C2N2/H2/02/Ar flames. S 0 2 increased the N O formation rate in the initial 3 |