OCR Text |
Show As expected, the best results were achieved under staged conditions with a very lean mixture in the first stage, ~=0.6, and a fuel-rich mixture, ~=1.4, in the second stage (see Cases 3 and 6). Relative flow rates in each stage were varied between Cases 3 and 6 to get the desired overall equivalence ratios. Cases 1 and 4 are single-stage burning conditions operating at ~=0.87 and 1.0, respectively. Note that NOx levels of 23 and 36 ppmv, dry, were obtained compared to 10 and 20 ppmv, dry, respectively, for staged burning at the same overall equivalence ratios. Cases 2 and 5 are two-stage burning conditions with fuelrich combustion in the first stage and fuel-lean in the second, resulting in overall equivalence ratios of 0.87 and 1.0, respectively. The formation of NOx was in the 35 to 38 ppmv dry range even with excessive CO emissions (1.6 to >2.5%). Note that cases 3 and 6 not only had the lowest NOx emissions but also the lowest CO levels. These results indicate that staged burning with fuel-lean equivalence ratios in the first stage and fuel-rich equivalence ratios in the second stage offers a significant advantage over single-stage combustion at equivalent overall equivalence ratios. The reason for these results is the fact that thermal NOx formation resulting from Zeldovich mechanisms is retarded in the first stage due to the low flame temperature achieved with high excess air conditions. In the second stage, a fuel-rich mixture is added, but formation of NOx from the cyano mechanism is retarded due to the combined effects of the unreacted oxygen (at a reduced concentration) from the first stage and the effect of radiant heat transfer lowering the flame temperature at the exit end of the ceramic porous matrix tube. Nondetectable levels of CO for Cases 3 and 6 indicate good combustion characteristics even at a stoichiometric fuel air ratios (~=1.0). SUMMARY AND CONCLUSIONS An experimental investigation of premixed, staged combustion stabilized within a porous media ceramic burner was performed. Emissions of NOx were determined at equivalence ratios of 0.6 to 1.5, the axial temperature profile for two-stage combustion was determined, and a comparison of NOx and CO emissions for singlestage and two-stage (rich-lean and lean-rich) operation was -13- |