OCR Text |
Show t N O formation would be impeded with less N radicals being formed, whilst its recombination would be promoted by forming N 2 with greater amounts of NHj by: NH + NO^N2 + OH 31 Examples of radical recombination were reported by Tang et al. in fuel-lean hexane flames doped with N as pyridine and S as tertiary butyl mercaptan, and Chen et al. 4 , who found large decreases of O H radicals when S 0 2 was added to a propane fuel-lean flame, allegedly by reactions8: OH + S02 + M T=^ HOS02 + M 32 H O S 0 2 + O H <-* H 202 + S 0 2 33 In the present work the largest increase of NO emissions caused by addition of sulphur was seen in Orimulsion. Although in small amounts, it has been suggested that the introduction of water in a combustion system leads to the enhanced production of O H radicals by means of7: H20 + H ^ H2 + OH 34 H2 0 + O > O H + O H 35 Larger amounts of OH radicals would displace the radical pool towards lower concentrations of H and O radicals, whilst higher concentrations of O H could be counteracted partly by recombination by S02. However, this would lead to increased concentrations of N 0 2 , which did not occur from Orimulsion. If H and O radicals are more effective than O H in the formation of NHi and NO, respectively, their recombination could cause a larger proportional effect on these species. 2. Stoichiometric conditions (q> = 1.00) The fact that NO emissions decrease whilst those of N 0 2 increase to a larger extent shows that the mechanisms of N O formation and N 0 2 disappearance are affected by sulphur in separate ways. Figure 8 shows the change of the [NO]/[N02] ratio with increasing amounts of S 0 2 in stoichiometric conditions. Whilst N 0 2 is reduced by reaction with H radicals, N O is formed in reactions with O and O H radicals. As the recombination of these radicals, promoted by increasing amounts of S02, progresses N O will not be formed to such a large extent and 125 T 1 / | 105-- ' E / ^* a / ~.~-' --•-•MI 3s 95^ ^ - * r --•--G1 z [?.~' -*-G2 75 -I 1 1 1 1 0.0 0.5 1.0 1.5 2.0 Fuel-S added, % Figure 10: Experimental readings of NOx from experiments with increasing amounts of sulphur at 9 = 1.00 13 |