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Show 17 The objective of the modeling task on the demonstration project was to provide a validated tool that could be used to assist the commercialization of the new burner technology. To validate the models, the predictions with the original cell burners and the LNCB@ burners were compared with measurements taken during the baseline and post-retrofit testing at two boiler loads (Latham, et aI., 1994). Initially, flow and combustion ~odeling were performed before the retrofit to determine the impact of the LNCB@ burners on furnace performance. Lower combustion temperatures were found implying lower thermal NO; insignificant increases in unburned carbon were predicted; and more uniform distributions of furnace exit gas flow and temperature were predicted. Although the predicted average furnace exit gas temperature (FEGT) increased slightly by 4K (7°F), the predictions showed less side-to-side variation and had lower peak-to-minimum values. The model predictions did indicate high carbon monoxide (CO) concentrations below the burners. However, the predicted concentrations above the burners and at the furnace exit were similar to the preretrofit predictions and measurements. Following startup, preliminary post-retrofit testing confirmed the high CO concentrations below the burners. Changes to burner and air port flow control settings could alter the CO concentrations in the hopper, but could not reduce them to acceptable levels. CO measurements were taken through an observation port in the hopper for three different sets of operating conditions and compared well with model predictions. The CO predictions and measurements near the right side wall are shown in Figure 17. Consequently, the models were used to investigate schemes to mitigate the high CO levels. The models were used to identify an arrangement for the burners and air ports that eliminated the high CO concentrations below the burners, but maintained the combustion efficiency and furnace heat transfer characteristics (Latham, et aI., 1992). The original and new arrangements are both shown in Figure 18. The new arrangement was achieved by inverting alternating cells on the lower row. Following installation of the new arrangement, the postretrofit testing resumed. The testing confirmed that the CO concentrations in the furnace hopper had been reduced to pre-retrofit levels of about 1000 ppm. The predicted CO concentrations near the wall for the installed arrangement are shown in Figure 19. The overall objectives of the demonstration program were achieved. The average NOx emission rate for full load with all mills in service was reduced by 55%. NOx reductions exceeded 50% over the entire load range. The overall operation of Unit No.4 was unchanged. The FEGT at full load was about 6 K (1 O°F) lower than the baseline value. The reheat steam temperature was controllable to 814 K (1005°F) at full and intermediate loads. The unburned carbon (UBC) loss increased slightly, but averaged 1.2% (flyash catch) atfuilioadwith all mills in service. However, overall boiler thermal efficiency was unchanged. The effect of the increased UBC was offset by a decrease in dry gas loss due to a lower economizer gas outlet temperature and operation at lower excess air levels. Localized corrosion rates measured on bare SA-213T2 boiler tubes taken from a corrosion test panel on the furnace side wall in the burner zone show corrosion rates that are not significantly |
