OCR Text |
Show - 10- 3.2 Reburn fuel mixing The effect of reburn fuel mixing on NOx reduction was found to be related to ARB. At ARB of 0.87-0.94 (M1, M3 and M5), there was little influence of mixing on NOx reduction for a given tertiary air arrangement. With tertiary air injected through Pipes "1,2", the NOx concentration was reduced by approximately 40% from 850-1000 ppm in the precombustor to 450-620 ppm in the flue, Figure 7. Mixing was more significant at a lower ARB. Tests M2, M4 and M6 (ARB: 0.77-0.84) had NOx emissions that were 50 to 130 ppm lower at the high mixing level. The improved mixing appeared to be most important for M6 which had the highest Ap and the lowest fRB. The NOx emissions were reduced from 470 to 330 ppm. Differences in the effect of mixing at high and low ARB on NOx emissions could be partly related to the availability of O2 in the reburn zone. Process conditions which resulted in O2 concentration of less than 0.5 % throughout the reburn zone had lower NOx emissions. With a high ARB (M1, M3 and MS), it was not possible to attain O2 concentration of less than 0.5 % with either the low or the high mixing. In these cases, the mixing could not influence the reduction of NOx emissions. In M5 at low mixing, O2 concentration of 1-1.5 % and trace amounts of CO were measured in the region around 0.4-0.6 m horizontal position, Figure 8. In the high mixing case, 1 % O2 remained throughout the forward flow. Improved reburn fuel mixing was not sufficient to lower the O2 concentration throughout the flow and NOx emissions were approximately 600 ppm for early tertiary air injection and 510 ppm for delayed tertiary air injection. With a low ARB (M2, M6 and M7), high mixing resulted in 0.1-0.5 % O2 throughout the reburn zone and lower NOx emissions. In M6 at low mixing, 0.8-1.4 % O2 was measured at 0.4-0.6 m horizontal position, Figure 9, however at high mixing, 0.2 % O2 was measured across most of the reburn zone exit. The improved reburn fuel mixing lowered the NOx emissions from 470 to 330 ppm for early tertiary air addition and 350 to 260 ppm for delayed tertiary air addition. Mixing was also important for flames with low O2 concentration in the reburn zone. In M2, 0-0.5% O2 was measured throughout the reburn zone for low and high mixing. Despite the low O2 in both cases, the NOx emissions were reduced from 430 to 360 ppm with higher reburn fuel mixing for early tertiary air injection and 340 to 250 ppm for delayed tertiary air injection. Thus, mixing was important to increase reactions between NOx, hydrocarbon fragments and N-species intennediates as well as to reduce the local O2 concentration. 3.3 Delayed tertiary air addition NOx emissions were reduced when tertiary air addition was delayed. With tertiary air injected through Pipes "2,4", the NOx decreased from 800 to 250 ppm as ARB decreased from 1.04 to 0.77. At the lowest ARB, these emissions were approximately 100 ppm less than with tertiary air injected through Pipes" 1 ,2". Delayed tertiary air injection was studied in detail for M7 at a primary stoichiometry of 1.0 and 20% reburn fuel fraction. The NOx emissions were measured with tertiary air injected through several combinations of pipes from Pipes "1,3" to Pipes "3,4". This allowed the separation distance between reburn guns and tertiary air to be adjusted from 1.5 to 4.2 m and nominal reburn zone gas residence time to be increased from 0.4 to 2.0 s. The temperature at the point of tertiary air injection decreased from 1400°C at Pipe "1" to 1100°C at Pipe "4". |