6. Conclusions

1. NO is formed rapidly from both fuels at all equivalence ratios in the first 150 mm from the atomiser nozzle by means of the fuel-NO mechanism. The amounts of fuel-N converted to NO were dependent on the stoichiometry and the furnace wall temperature. Although N₂O was also observed, the role of the prompt-NO mechanism was deemed to be of minor relevance.

2. As a result of the rapid formation of NO, significant amounts of NO₂ are also formed at short distances in stoichiometric and fuel-rich conditions. NO₂ is thought to be formed by means of the HO₂ and RO₂ mechanisms.

3. The extent of NO formation at longer distances depends mainly on the stoichiometry of the combustion system and the furnace wall (flame) temperature. Large amounts of NO are formed in fuel-lean conditions. The conversion of fuel-N into NO is greatest under these conditions. The fuel with the larger fuel-N content yields greater concentrations of NO, whereas conversion is higher for the fuel with lower fuel-N content. Raising the furnace wall temperature increases the conversion of fuel-N into NO.

4. Although significant amounts are formed, NO and NO₂ are emitted in small amounts in stoichiometric conditions. The concentrations of both species are reduced at relatively long distances from the atomiser nozzle.

5. Small amounts of NO are emitted at = 1.200. NO₂ constitutes the largest nitrogenous emission in these conditions. NO₂ is thought to be formed by the reaction of NO with hydroperoxyl radicals (RO₂) that stem from unburned hydrocarbons. Although further work is needed at 1,200 °C furnace wall temperature, the conversion of fuel-N into NO₂ does not show a dependence on the furnace wall temperature nor the fuel-N content.

6. No significant amounts of thermal-NO were found even in the most favourable conditions, ie 1,200 °C furnace wall temperature and = 0.714.

7. The measured concentrations of SO₂ rise with the equivalence ratio due to the reduced volume of combustion gases associated with high equivalence ratios.

8. In fuel-lean conditions the formation and emission of SO₂ seems independent from the furnace wall temperature.

9. In some cases the fuel-S conversion is slightly lower at = 0.714 than at = 0.833, which may indicate the formation of SO₃.

10. At stoichiometric and fuel-rich equivalence ratios the formation of SO₂ reaches a maximum and decreases subsequently. Larger decreases were found at the higher furnace wall temperatures. SO₂ is reduced into species such as COS, SO and H₂S in low concentrations of oxygen.

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