OCR Text |
Show 3.0 0 "- "- ~ ...... ...... .9 ~ N ..... ...... ..D ____ ---'-' CO N 0 0 0 .N,., 2.0 ~ ~ ~ ~ 0 0 ......, ......, >aaE.. 1.0 NOx 1.0 agE-. x 0 0 (0. ) Z 0.0 0 20 40 60 80 100 Nozzle Velocity, m/sec Figure 6. EFFECT OF NOZZLE VELOCITY ON EMISSIONS measurements that were performed in the cold model. Therefore, the optimal main nozzle velocity can be recommended to be 50 to 60 mls for this typical burner configuration. When controlling the nozzle velocity within the optimized range, excellent performance of the test combustor was accomplished. Effect of Excess Air It is obvious that NOx emissions are reduced with an increase in excess air at a given firing rate because it lowers the combustion temperature. In contrast, CO emissions increase with excess air because of the decrease in both combustion temperature and residence time. Figure 7 shows NO, NOx, and CO emissions at different excess air at a 322-kW firing rate. It is seen that NOx was reduced from 2.2 to 0.6 vppm and CO was increased from 1.6 to 2.8 vppm as excess air was increased from 57% to 80%. Correspondingly, the combustion temperature was decreased from 24600 to 22500 F. The NO and NOx data also show that most of the NOx emissions (70% to 90%) were contributed by NO. The NO and NOx values were directly measured by the chemiluminescent analyzer, N02 values were determined only by difference; results show a range of 0.1 to 0.3 vppm for the test conditions employed. Therefore, the data presented in Figure 7 demonstrate that ultra-low emissions have been achieved. The maximum excess air is limited for flame stabilization. At a lower load operation, the maximum excess air can be increased as high as 110%. The combustion temperature is correspondingly decreased to 20000F, and NOx emissions can be reduced to as low as 0.3 vppm, as shown in Figure 4. Effect of Firing Rate (Turndown) Turndown tests were conducted for all five configurations. At each level of the firing rate, excess air was gradually increased to reduce NOx formation until an unstable flame appeared. The overall firing rates were varied from 12 to 500 kW with the main burner operation. The flame stabilizer was operated alone at very low overall firing rates ranging from 9 |