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Show E Q. Q. >< 0 Z 35 . 30 . Fig. 5. QLN Performance. 75,000 pph B&W Packaged Boiler .-~~--r-------~~~~~~----~------_350 300 250 E 200 Q. 25 . r-~~~--~---+--~~-+~~~~~~~ Q. 20 . 15 0 20 40 60 Load, 0/0 80 150 o.. 100 (.) 50 100 ENCOUNTERED DIFFICULTIES +NOx A#REF! .co With the increased number of applications we encountered several problems that are likely common to fuel staged burners. The first was high CO emissions in old narrow and leaky boilers. The second problem was low frequency pulsation. The problem with CO emissions was anticipated, as high concentrations of CO are present in the area around the flame and the furnace gas escapes through the gaps in the partition between the furnace and the convection section. The CO emissions were reaching sometime 1500 ppm at the stack, while readings at the rear of the furnace were below 50 ppm. The problem was typically solved, first, bv sealing the gaps between the water tubes, second, by adjusting the regime of the firing and '. by modifying some parts of the burner. Steps two and three, however, result in deviation fr . . ne optimum NOx performance. The problem with low frequency pulsation was not expected, as it did not occur during the test yard firing or within the initial group of applications. Yet it happened in some furnaces with different degree of severity, mostly at lower loads, and was difficult to overcome without a substantial increase of NOx. On some of the pulsation jobs the problem could be solved simply with reduced deviations in fuel to air ratio, like 02 trim. With the given simplistic controls, however, it would impose additional requirements to the tuning and maintenance and was never used as a permanent solution. A one dimensional acoustical analysis was performed in the attempt to identify applications that are prone to the pulsation. The system that was considered encompassed all the volumes and resistances to the flow of air and combustion products starting from combustion air fan up to the discharge into the atmosphere. Never the less, it was difficult to explain all the differences in the response of seemingly identical systems. Eventually an effective way was found to eliminate flame pulsation with the minimum impact on the burner performance and no additional requirements to the combustion controls. The controls still might be as simple as Single point positioning even for outdoor installations. 8 |
