OCR Text |
Show stages, followed by a decrease in its consumption in the downstream regions of the flame, thus increasing fuel-NO emissions. Tseregounis and Smith suggested a mechanism of N O reduction in the post-flame zone which, because of its dependence on H radical concentrations, leads to the increase of N O emissions caused by S 0 2 (M denotes a third-body): NO + H + Mr-^HNO + M 10 HNO + Hr-^NH + O H 11 HNO + O H <-> N O + H 20 12 NH + N O ^ N 2 + OH 13 together with the recombination of H radicals by S02: H + S02 + M^HS02 + M 14 H + HS02 r-> S02 + H2 15 Tseregounis and Smith observed increases in the concentration of HCN only at slightly fuel-rich conditions, and no changes at other equivalence ratios. Increased HCN concentrations would decrease N O concentrations, which was not the effect observed. The fact that radical concentrations are lowered by the presence of sulphur was also observed in fuel-rich hydrogen flames by Chen et al.4. In a variety of equivalence ratios (mostly fuel-rich) the concentrations of H radicals were seen to decrease on addition of S02, thus lowering the formation of N H and N from N O via HNO. The reduction mechanism was similar to that reported by Tseregounis and Smith 18 , and was extended to incorporate reactions of amine species: NO + Nr-*N2 + 0 16 H N O + H T - > N O + H2 17 NH + H^-^N + H2 18 NH + O H ^ N + H20 19 Thus lower H concentrations would decrease the extent of NO reduction, leading to higher N O emissions. When experiments were carried out by Chen et al. in propane flames in a jet-stirred reactor, increases of NH3 and HCN with the addition of S02 were reported, which led to increased N O emissions. In relatively complex flames such as this it is likely that the processes for N O reduction are different according to the flame zone considered, which is a concept that was also included by Graville 9 . Chen et al. acknowledged four major routes for the reaction of N O in hydrocarbon flames: 1. Reaction of NO with free hydrocarbon radicals (NO + CHX <-* HCN + ...), predominant in the injection zone, where hydrocarbon concentrations are high 2. Reaction of cyano species to form N H and NH2, which is important in zones of high recirculation 4 |