| OCR Text |
Show -7- ejector, so that the gas (CHi radicals) has sufficient residence time to mix with, th~ primary zone products (NOx) to create the intermediate species which is requIre for the rate controlling step of the reburning process where HCN ~ NH3 + NO ~ N2. It is proposed that the high velocity (100 m/s) by-pass air creates local stoichiometries which are much higher than those indicated by the theoretical approach. It is also possible that the by-pass air shrouds the natural gas which inhibits mixing and thus the reburn mechanism cannot be fully implemented. It can therefore be deduced that the by-pass air velocity is another NOx controlling parameter. With a by-pass air velocity of 100 ml s, the issuing jet controls the overall reaction scheme and NOx concentrations in this mode of operation is essentially governed by the position of the tertiary air. With the bypass air velocity reduced to 26 mis, NOx is now controlled by the tertiary air velocity i.e. low velocities prevent the entrainment of primary / reburn products with the tertiary air so that mixing is delayed. In this mode the position of the tertiary air plays a less vital role, although best performance was observed when the air was introduced externally. Effect of reburn fuel fraction on NOx Another parameter investigated was the influence of the reburn fuel fraction (Rff). Figure 6 shows that increasing the Rff has the desired effect of decreasing NOx as a result of the increased productivity of CHi radicals, which effectively improves the first stage of the reburning mechanism where NO + CHi -> HeN. Furthermore, Figure 6 directly compares the technique of internal fuel staging with I classical' reburning with reburn fuel fractions in the range 5 % _ 30 0 / 0 • It is clear that NOx reduction with internal fuel staging is as good as the classical technique in the Rff range of 5 - 20 %. With a Rff of 30 %, the classical technique, using a six hole nozzle shows overall the best performance with flue gas NOx levels of ,... 100 ppm. NOx levels with internal fuel staging at this Rff are ,... 30 % higher and in fact shows very little difference from reburning with an Rff of 20 %. |
