3.6. N2O

3.6.1. Fuel-lean conditions (j = 0.833)


Figure 146: Experimental readings of N2O from experiments of sulphur addition from all fuels investigated at j = 0.833

Addition of SO2 dopant did not seem to have a large effect on N2O emissions from any of the fuels studied at fuel-lean equivalence ratio (see Figure 146).

Emissions from Orimulsion reached 14 ppm in absence of SO2, whereas 12.8 ppm were measured when the dopant was added. However, both values are within the range of instrumental error.

Other fuels studied showed lower emissions of N2O. Fuel M1 produced 4.6 ppm without addition of SO2. Fuel G1 produced 4.9 ppm of N2O in similar conditions, whereas 5.4 ppm were formed from fuel G2. Minimum changes in N2O emissions were observed when SO2 was added to the combustion of these fuels. Only fuel G2 showed a somewhat significant decrease of 0.8 ppm on addition of 1 % fuel-S and greater.

3.6.2. Stoichiometric conditions (j = 1.000)

Readings of N2O at stoichiometric conditions were negative from the three fuels studied. This is likely to be due to large amounts of CO formed in these conditions, which interact with the N2O gas analyser by lowering its output. Since the actual amounts of carbon monoxide cannot be established as they lay were greater than the CO gas analyser detection limit (1 %), the real values of N2O concentrations cannot be calculated. A graph showing all experimental readings of N2O at stoichiometric conditions is shown in Figure 147.


Figure 147: Experimental readings of N2O from experiments of sulphur addition from the fuels investigated at j = 1.000

The instrumental readings of N2O decreased on addition of sulphur dopant in most cases. The rate of the decrease was inversely proportional to the initial sulphur content of the fuels. Thus, fuel M1 (3.59 % fuel-S) had its readings decreased from -5 ppm at a rate of 1.6 ppm per 1 % fuel-S added. Measurements from fuel G1 (2.20 % fuel-S) decreased from -26.9 ppm at a rate of 1.9 ppm per 1 % fuel-S added. Finally, values from fuel G2 (1.54 % fuel-S) decreased from -15.8 ppm at a rate of 2.8 ppm per 1 % fuel-S added.

However, the origin of these decreases remains unclear since the exact amounts of CO emitted are not known. Thus, the apparent decreases of N2O may be due either to the direct action of SO2 or to lower amounts of CO formed, also as a result of the addition of SO2. Although the CO and N2O gas analysers are equipped with a compensation setting, this is not sufficient to account for the interaction caused by very large amounts carbon monoxide, above 1 %.

3.6.3. Fuel-rich conditions (j = 1.200)

The readings of N2O at fuel-rich conditions showed a similar scenario to that at stoichiometric conditions. Moderate decreases were registered on addition of SO2 from all fuels (approximately 3 ppm on addition of 2 % S as SO2). However, as in stoichiometric conditions, the reason for the decrease has not been established as measurements of carbon monoxide exceeded the instrumental scale.
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Pollutant formation and interaction in the combustion of heavy liquid fuels
Luis Javier Molero de Blas, PhD thesis, University of London, 1998
© Luis Javier Molero de Blas