4.2.3. NO_X

In fuel-lean conditions ( = 0.833) there was a small 2 ppm-wet decrease in the concentration of NO₂ at short distances when SO₂ was added. This is the result of opposed effects encountered by NO and NO₂. Whilst nitric oxide is decreased by sulphur, nitrogen dioxide is increased. As the concentration of NO₂ falls at longer distances the decreasing effect of S on NO prevails. The final outcome was a moderate decrease of total NO_X of approximately 10 ppm-wet over a total 267 ppm-wet in absence of SO₂.

Figure 150: Experimental readings of NO_X from fuel M1 at three equivalence ratios, S addition and selected sampling distances. Solid symbols: air; open symbols: addition of 2.0 % S

In stoichiometric conditions the final effect was similar, although it evolved at a later stage in the gas path. No net change in NO_X concentration was seen in the first 200 mm of the sampling path. At 100 mm there is no change in the concentrations of NO and NO₂ as an effect from SO₂ addition. Thereafter, sulphur decreases nitric oxide whereas nitrogen dioxide is increased. As the concentration of NO_X peaked and decreased at longer distances from the atomiser, both effects intensify. The result on NO_X is dictated by the larger decrease of NO. A decrease of 9 ppm-wet was measured on exhaust.

Marked increases of total NO_X concentrations were obtained in fuel-rich conditions, although not from the earliest stages investigated. Whereas a small decrease of 4 ppm-wet was registered at 100 mm from the atomiser, a pronounced increase was obtained subsequently, mainly as a result of the large increase of NO₂ caused by the addition of sulphur dioxide.

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