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Show 14 The three reburning systems designs that were evaluated are shown in Figure 13. The results of the mixing evaluations are shown in Figure 14. The modeling results indicated that 80% of the mass flow reached substoichiometric conditions in the reburn zone with four reburning burners, as opposed to only 60% with three reburning burners. With four overfire air ports, the penetration and mixing was sufficiently good in that all flow reached oxidizing conditions before exiting the furnace. The system with four reburning burners and four overfire air ports was selected for installation. Predicted flow patterns and stoichiometry distributions for the installed arrangement are shown in Figure 15. The reburning burner flow penetrates far enough to maintain flame stability without impinging on the target wall. In contrast to the baseline flow field (see Figure 12), more of the flow passes upward near the front wall of the furnace, and no large recirculation zone is present. The overall objectives of the demonstration program were achieved. The nominal NOx reductions were 55% when firing an Illinois bituminous coal and 61 % when firing a Powder River Basin coal. The furnace exit gas temperature (FEGT) was 56 to 83 K (100 to 150°F) lower than the baseline values with the Illinois bituminous coal atfuilioad. The decrease in FEGTwas reflected by a 50% decrease in superheater and reheater attemperator flows. No change in FEGT was measured at 75% load, but the FEGT was 28 to 42 K (50 to 75°F) higher at 50% load. Combustion and heat transfer modeling were used later to investigate these changes in FEGT. The models were able to predict the trends that were observed with the bituminous coal and provided insight to the causes that could not be accounted for by the flue gas recirculation, alone. The changes in heat transfer were due to the altered flow patterns and radiation properties in the upper furnace that occurred as a result of reburning. In general, these changes that occurred were beneficial to furnace operation. The unburned carbon efficiency loss (UBCL) increased slightly by 0.1 % at full load with the bituminous coal. Ultrasonic thickness (UT) and destructive examinations of rear wall 3 Reburn Burners 3 Overfire Air Ports Air Port '0 0 0 ~ I.. -I E 3m C) a: Reburn Burner .. ... "- • • -... ... ...... ... ... ... ... , ,,,," " , ...... , " I' I"''' " " I' , ...... " " • • • I" I'" • • I. • .' " '., ' ••• ." '., '... " ~'~i~~:\ ..... '" .' (view of rear wall) (ij ~ ~ (1) -oJ 3 Rebum Burners 4 Overfire Air Ports o 0 o 0 U 2m 3rn I" ~I • • • ... ......... ,' ..... '. , ...... '" ," .... '" • • • I" t I •• I • " " .. ' " " '... " .. '... ,,' Figure 13 Reburning NOx Control Systems Designs Evaluated 4 Reburn Burners 4 Overfire Air Ports o 0 o o U 2m |