OCR Text |
Show comparable NO at SR^ < 0.8 and lower NO for SR^ > 0.8. For the SR^ > 0.8 fuel-staging cases, the fuel-rich-stage residence time (T2) is less than the classical-staging fuel-rich residence time (TJ). In these fuel-staging cases, less of the combustor is operated under fuel-rich conditions for equivalent NO reduction. In summary, compared to classical staging, fuel staging is beneficial in regions where SR2 < 1 and SRi >^ 1. Stage Residence Time A residence time comparison between fuel staging and classical staging was made in figure 5. Figure 6 presents the same fuel-staging data compared to advanced air staging at stations C and F (see figure 3 for a definition of staging locations). In this comparison the classical-staging first-stage residence time (T]_, classical) is equal to the fuel-staging second-stage residence time (T2> fuel staging). This comparison shows that, for equivalent fuel-rich stoichiometry, SR = 0.8, and residence time, the classical-staging case gives a 40 percent reduction from baseline NO levels, whereas the fuel-staging case gives a 60 percent reduction from the baseline NO level. As shown in figure 6, to achieve the equivalent NO reduction, the first-stage SR^ must be reduced to almost 0.4 in classical staging. Apparently, the NO decay process with fuel staging is more rapid than it is with classical staging. This reduction of NO with residence time is more clearly shown in figure 7. It should be noted that, for the SR2 = 0.9 case, the level and reduction of NO with residence time is insensitive to first-stage stoichiometry, SR^. Also for the SRj • 1.2 case, the fuel-rich second stage (SR2 • 0.9) reduces the first-stage NO (expected to be 800 ppm) to exit levels of 500 ppm NO in roughly 0.33 sec second-stage residence time. Second-stage residence times of about 1 sec at these conditions reduces NO below 400 ppm. However, the major part of the reduction occurs for second-stage residence times of 0.33 sec or lower. 8-14 |