5. Conclusions

1. The numerical model was able to predict emissions of NO from fuel M1 and Orimulsion at = 0.833 of the same order of magnitude as those obtained experimentally. However, the model was not accurate at calculating emissions at other equivalence ratios, where the calculated values were much larger than those observed experimentally. This results can be seen in Table 11.

2. The model predicted low emissions of NO₂ at fuel-lean equivalence ratios. However, it failed to predict the high exhaust concentrations of NO₂ measured from fuel M1 at = 1.000 and 1.200. This inaccuracy was attributed to the model not including hydroperoxyl radicals (RO₂) and their reactions with NO, which can enhance the formation of NO₂ in stoichiometric and fuel-rich systems.

3. The concentrations of sulphur dioxide calculated by the numerical model were in good agreement with the values obtained experimentally. These concentrations also increased with the equivalence ratio, as lower amounts of combustion air were present.

4. The model confirmed the transformation of SO₂ into reduced sulphurous species in oxygen-deficient environments. The reduction was also observed experimentally.

5. On addition of SO₂ to simulate fuel-S the numerical model calculated reductions of NO from fuel M1 in fuel-lean conditions. The reduction calculated was larger than that obtained experimentally. Although not accurate, the model also yielded reductions of NO concentrations in stoichiometric conditions.

6. Reductions of NO concentrations were also calculated from Orimulsion in fuel-lean conditions. The percentage reduction calculated by the model was similar to that obtained experimentally in the drop tube furnace on addition of various amounts of SO₂.

7. The reductions of NO emissions were partly attributed to the radical recombination of O and OH radicals performed by SO₂, which increased the concentrations of nitrogenous species such as HCN and those in the amine subsystem.

8. In addition, increased concentrations of NS radicals were found on addition of sulphur dioxide, which makes the reduction of NO by reaction of NS radicals viable.

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