| OCR Text |
Show of these species, which in turn depend on various characteristics of the combustion system. The fuel-NO mechanism proceeds through the formation of H C N from which amine species are subsequently formed. These amine species finally react according to the combustion conditions to form either N O or nitrogen molecules: <NO (fuel-lean conditions) N2 (fuel-rich conditions) Thus, changes in HCN concentration engendered by sulphur would be of crucial importance. This was indicated by Corley and Wendt5 , who obtained increases of both N O and H C N in the presence of S 0 2 in fuel-rich systems, whereas concentrations of N 2 decreased, suggesting that S 0 2 interacts with the cyanide-amine sub-system. Decreases in the concentrations of H C N have also been observed recently by Nimmo et al. u along with increases of fuel-NO, which may account for a shift of N species from H C N to NO. Pfefferle and Churchill 15 obtained decreases of fuel-NOx emissions caused by H2S, both experimentally and numerically. Numerical modelling showed that indirect radical rearrangements are important, with relevant reactions in which N S radicals (for which experimental evidence has been found 11 ) formed at fuel-rich conditions can reduce N O to N2. Although the numerical model suggested that diminished concentrations of O and O H radicals were responsible for the NOx reductions, the calculated concentrations were lower than those recorded experimentally. Key reactions in the N-S interaction processes were thought to be missing from the model. Jeffries and Crosley 11 found that N S radicals can recombine with NHj, as well as take part in oxidation reactions with NO, 02, O, OH, N 2 0 and N02. Other possible direct reactions between sulphur and nitrogenous species have also been investigated. Wendt and Sternling 21 performed computer calculations of the oxidation of S 0 2 to S 0 3 via N O catalysis through the following mechanism 1,e: NO + 02 T-> N03 1 N 0 3 + S 0 2 r ^ S 0 3 + N 0 2 2 However, they found that at NO concentrations typical of post-flame stack gases SO3 was formed in negligible amounts via this mechanism. One important fact to bear in mind is that, although S 0 2 is predominant in flames, a variety of other sulphur species can be found in fuel-rich conditions. For instance, in H2/02/S02 flames 23 S O is formed in the reaction zone of the flame; subsequent decay of its concentration leads to renewed formation of reduced species such as H2S, S H and S2, the latter being the most important species after S02. At long residence times H2S becomes the most important species, with concentrations higher than those of S02. 2 |
