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Show -6- position 3 or 4 NOx levels were further reduced and in particular when A was less than 0.7. p 1000 • Rff=O% : type II Ilame • A.p=1.15 (standard bumer) 800 to- Rff=20% : RIv=58m1s (IFSB) o A.p=1.15 6 A.p=1.05 0 N 'il A.p=0.9 <f!. 600 M <> A.p=O.7 ~ 0 A.p=O.5 ~ "'0 'il 0 Ea. 400 a. 0 fi 'il 6 0 <> J( 0 Z 'il 200 0 0 0 I I I I o 1 234 none annulus int.(16 pipes) ext.(8 pipes) Overtired Air no swirl Tertiary air location Figure 5 Effect of tertiary air position and Ap on NOx This initial analysis shows that the primary stoichiometry and tertiary air position has some influence on the overall NOx levels. It is however pertinent to suggest that the rebum zone stoichiometry (combination of Ap and Rff), is the major contributor to the overall NOx reduction scheme. Effect of by-pass air velocity on NOx The rebum air, which was injected as by-pass air, had a velocity of 26 or 100 m/ s. For this study the primary stoichiometry was set at 0.9, a Rff of 20 0/0 (natural gas) was also imposed, giving an overall reburn zone stoichiometry of 0.78, together with a constant Rfv of 56 m/ s. With a by-pass air velocity of 100 m/s entering the primary combustion zone, it seems that the air acts like an ejector entraining the surrounding air /unbumt products into its core which results in the volatile nitrogen species being oxidised before the reburning mechanism can be activated. Furthermore, with the tertiary air at position 1 or 2, NOx levels reach their peak which may be attributed to a high and rapid degree of mixing of the tertiary air with the remaining residuals from the reburn zone. Moving the tertiary air to the outside of the main reaction zone (external air staging) results in a decrease in NOx as mixing of the tertiary air with the unburnt products is delayed which has the overall effect of decreasing the combustion intensity and hence local temperatures. Increasing the residence time of reactions in the rebum zone, by introducing the tertiary air downstream of the burner (as overfired air), improves the production rate of CHi radicals which in the rebuming mechanism is the major contributor to reducing NOx. Reducing the by-pass air velocity to 26 m/ s has the overall effect of reducing NOx. In this mode of operation the by-pass air no longer acts like an |