OCR Text |
Show amount of N O formed, but somehow dependent on the amount of dopant added. However, other characteristics of the fuel may play an important role, as shown by the larger reductions of N O from Orimulsion, a fuel with a large content of water (30 % ) . It is likely that the experimental results obtained are the result of various chemical mechanisms operating concurrently or consecutively as conditions develop along the gas path. In the early stages of the atomisation cone, fuel-rich pockets may be formed where S 0 2 can be reduced to other sulphurous species - the equivalence ratio in this fuel-air cone was calculated to be 2.05, although it may be higher in isolated pockets. Reduced sulphur species may decrease the concentrations of nitric oxide via reactions such as those proposed by Chagger et al. SH + NO > NS + OH 23 2 NS > N2 + S2 24 NS + N O > N2 + S O 25 As the fuel-air mixture becomes more lean the interconversion of NO to N02 may play an important role 1 . This route leads to the simultaneous decrease of N O and increase of N 0 2 concentrations. At low concentrations N O can be oxidised by S 0 2 to form N 0 2 and S 0 3 in the following reactions: NO + 02 f* N03 26 N 0 3 + S 0 2 <-> N 0 2 + S 0 3 27 The optimum temperature range is between 700 and 900 °C , which can be encountered in the injection zone of the vertical tube furnace. Sulphur trioxide can later be readily transformed again to S 0 2 at temperatures above 1,000 °C 6 . Formation of S03 can partly explain the reduction of the exhaust concentration of oxygen observed on addition of S 0 2 in fuel-lean conditions. The decrease is more sustained and pronounced for Orimulsion, which also presents the highest concentrations of residual 02 among all fuels investigated at cp = 0.833. In zones of high radical super-equilibrium concentrations radical recombination is another likely mechanism involving S02, which can cause a decrease of the concentrations of O H and H radicals 18, . It takes place by the following reactions: X+S02 + M?-*XS02 + M 28 X + Y S 0 2 ^ S 0 2 + XY 29 where X= O, OH or H and Y = O or H. The formation of NO via the fuel-NO mechanism is thus curtailed in the early stages of the flame by the lower amounts of O H radicals available. Radical recombination can also alter the amine reaction subsystem 17 by shifting reaction 30 to the left: NHJ+X^NHM+HX 30 12 |