OCR Text |
Show THE NEED FOR BETTER CONTROLS AND SAFETY As is evident from the presented results, flame stability remains the greatest challenge at ultra low N O x levels. The N O x emissions reductions with increased levels of F G R c o m e with a price. Figure 3 and Figure 4 illustrate conceptually the narrowing of the flame stability margins with increasing levels of F G R and lower N O x for the full-scale prototype burner. If the burner controls set for the target emission level of 5 ppm, the allowable deviations in percentages of the excess air and F G R shall be limited dynamically to 6 to 7 % from the set point. This creates problems, especially at turn down, even for the burner as shown on Figure 2 with widened stability margins. At 1 2 % load, for example, it means, that the inaccuracy in the absolute amount of F G R shall be better than 2 % of its maximum flow at high fire. The accuracy in the air flow shall be even higher, about 1 % of the air flow at high fire. That requires equipping the burner package with new high dynamic accuracy combustion controls that w e were developing for applications of the new burner. Utilizing of high percentages of FGR for NOx control also presents challenges to the burner management system, as reliable flame scanning becomes difficult. Flame radiation to a typical U V sensor is attenuated by water and C02. At the same time, weak signals to IR scanners due to low flame temperatures and possible interference from the furnace background radiation are a problem. The effective result of the decrease in the U V radiation reaching the scanner from the ultra low N O x flame is that the gain on the scanner must be increased to maintain the flame scanning signal. This increased gain results in high signal to noise ratios and the potential of detecting a marginal flame conditions. Additional work must be invested into flame scanners for ultra low N O x burners to alleviate these problems. 9 |