5.1.3. Formation of NOX beyond 100 mm

5.1.3.a. Fuel-lean conditions (j = 0.714 and 0.833)
The formation of nitric oxide beyond 100 mm from the atomiser is extensive in fuel-lean conditions. The concentrations measured are greater for the fuel with higher fuel-N content, namely M1, than for fuel G1. However, calculations of the conversion of fuel-N into NO yield opposite results, with higher conversion for fuel G1. This can be observed in Figure 45. Previously, other researchers have reported an inverse relationship between conversion to NO and fuel-N content (Fenimore (1972), Williams (1979)) . Increasing the furnace wall temperature causes an increase in the conversion of fuel-N into NO, larger between 900 and 1,100 °C than between 1,100 and 1,200 °C. In fact, small decreases of the conversion were observed from 1,100 to 1,200 °C for fuel M1.

Although the measured concentrations of NO at j = 0.714 do not differ greatly from those at j = 0.833 the values of the conversion are greater at j = 0.714, possibly due to the higher availability of oxidising species in larger amounts of oxygen. The presence of oxidising species is ensured at both fuel-lean equivalence ratios, as can be seen in Figures 70 through to 75, where residual oxygen can reach 4 % at j = 0.714. It seems logical that with higher concentrations of oxidising species (ie lower equivalence ratio in fuel-lean conditions) N radicals are more likely to find an oxidising species to undergo NO formation. Also, although no information is available about flame temperature in the drop tube furnace at j = 0.714, it is predictable that it will be higher than at j = 0.833, which may add to the larger conversion.

Another effect caused by the furnace wall temperature is an increase of the rate of formation between 900 and 1,100 °C furnace wall temperature, which can be observed by comparing Figures 39 through to 41. Higher flame temperatures increase the rate of formation of fuel-NO by increasing the rates of fuel evaporation and nitrogen release, as well as by accelerating the reactions of the fuel-NO mechanism.

Eventually NO readings at fuel-lean conditions stabilise at approximately 300-350 mm. At j = 0.833 and 350 mm from the atomiser the flame temperature has decreased from its maximum value (1,691 °C measured at 201 mm) to approximately 1,300 °C. The concentration of oxygen has already reached the values that will be exhausted, as can be seen in Figures 70 and 71. At this stage the fuel-NO process is extinguished due to the long time elapsed since injection, although fuel-N may have not been transformed totally into NO.

The formation of NO2 is low in fuel-lean conditions. Although small amounts of NO2 are detected at very short distances from the atomiser, they disappear subsequently and no nitrogen dioxide is eventually emitted. NO2 is likely to be formed at short distances (low temperature and existing O2) via the HO2 and the RO2 mechanisms.

Experimental results reported by Merryman and Levy (Merryman and Levy (1974)) from CH4 flames doped with pyridine as N-containing agent showed early formation of NO2, whose disappearance gives place to NO. This phenomenon was explained in terms of the reaction of NO2 with O radicals:

NO2 + O ==> NO + O2 reac 30

which is a fast reaction. Concentrations of O radicals increase as those of O2 decrease and the temperature increases early in the flame. Simultaneously, O radicals contribute to the formation of NO via amine oxidation reactions, ie:

N + oxidant (O) ==> NO reac 91

which explains the high levels of NO and negligible concentrations of NO2 obtained in fuel-lean conditions.

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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