OCR Text |
Show Another mechanism is the withdrawal of H radicals that contribute to the disappearance of N 0 2 , possibly by S02 radical recombination. Chen et al. 4 deemed radical recombination unlikely in fuel-rich propane flames as the product, H202, is unstable. However, enhanced H2 02 concentrations in the vertical tube furnace can lead to higher H 0 2 concentrations, for instance, by reaction with O H radicals. This could, in turn, promote the formation of N 0 2 in the low-temperature, early stages of the flame at the cost of nitric oxide. A practical application that could -2 O 15.0- 12.5- 10.0- 7.5- 5.0- 2.5- 001 _-•_. Ml --•--G1 - * - G 2 4 i ^y i y* / / £~ . •= 1 .• -- " _-^~ <* *"- ••-- 1 1 0.0 0.5 1.0 Fuel-S added, % 1.5 2.0 Figure 12: Percentage change of N O x emissions at q> = 1.20 with increasing amounts of sulphur be derived from the enhancement of N 0 2 concentrations by sulphur is that it may facilitate N O x removal, as nitrogen dioxide can be removed by gas desulphurisation equipment. Conclusions The present study has shown that emissions of fuel-NOx can be modified by the presence of sulphur in the combustion system. The effect of sulphur on N O and N 0 2 emissions, and their combined effect on N O x emissions, w a s assessed at fuel-lean, stoichiometric and fuel-rich equivalence ratios. Table 2 summarises the qualitative effect of the addition of S02-gaS. Equivalence ratio (9) 0.833 1.00 1.20 NO i t N02 t t Total NOx I t t Table 2: Qualitative effect of SO2 addition on emissions of nitrogenous species according to the equivalence ratio (font size denotes relative extent of interactions) This Table suggests a pattern for the influence of sulphur on fuel-NOx emissions whereby emissions of N O are decreased at fuel-lean conditions and those of N 0 2 are increased at fuel-rich conditions. In stoichiometric mixtures the effects on these species are opposed, the resulting effect on total-NOx emissions being determined by the larger change. Several mechanisms may explain decreases of N O emissions at fuel-lean conditions, such as the intercombination of S 0 2 and N O to form S 0 3 and N02, decreases in radical availability by recombination which can also shift the amine 15 |