OCR Text |
Show 3. Oxidation of NHi (where i = 0, 1, 2) to N O 4. Reduction of N O to N 2 by reaction with NHi in recirculation zones Direct interactions between N intermediates and SO such as in reactions 8 and 9 were deemed to be of secondary importance: Model calculations did not show major variations when they were included. Also a decrease of N atoms was noticed, which caused lower reduction of NO. Both nitric oxide and S 0 2 were found to compete for hydrocarbon fragments, which inhibited the formation of H C N and its path of NO reduction. The stoichiometry of the combustion mixture is also an important parameter: Wendt et al.20 investigated the effect of local stoichiometry in turbulent diffusion flames of distillate oil. Sulphur was effective at increasing N O (up to 40 % ) as long as mixing was poor and local fuel-rich conditions occurred. Tang et al.16 used a refractory burner to burn hexane doped with varying amounts of tertiary butyl mercaptan (fuel-S) and pyridine (fuel-N). N O x formed from pyridine was reduced by sulphur at all conditions of stoichiometry, but the extent of the reduction was largely independent of the amounts of sulphur added. On the contrary, Hampartsoumian and Nimmo 1 showed that fuel-NO increased as the concentration of sulphur in the flame was increased at all stoichiometries. Burning a variety of complex fuels, they found that the increase of fuel-NO emissions was directly proportional to the amount of N and S in fuel. Staging of the combustion air produced a more pronounced N O enhancement when the conditions in the primary zone became fuel-rich. Reactions of sulphur with N species were proposed to explain the increase of fuel-NO caused by fuel-sulphur: N + SO <-* NO + S NO + S^=^NS + 0 NS + 0 <-* SO + N SH + NH <-* NS + H2 N + NS ^ N2 + S N + SH ^ NS + H 3 6 7 20 21 22 the latter three being able to remove amine species by N H r N O reduction reactions, which in turn may lead to higher N O concentrations. Graville 9 published the only report on N-S interactions in heavy fuel oils to date. The release of nitrogen and sulphur from these fuels is gradual, and they can enter the N O x formation or reduction processes at a later stage. This feature has a strong potential for large N O x reductions, and also shows the importance of the residence time in these processes. In his experimental work, Graville reported reductions of N O x emissions between 20 and 30 % on addition of 1 % S as thianthrene (fuel-S) to a residual fuel oil. The efficacy of sulphur as a N O x suppresser was diminished as the fuel-N content increased. Conversely, addition of light N and S additives did not cause change of N O x concentrations as the early release of N and S could not contribute to the post-flame NOx reduction processes. His results, partly predicted by means of numerical modelling, provided evidence of the competition of oxygen-containing 5 |