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Show 6 Figure 4. The geometry is typical of gas- and oil-fired boilers. The stoichiometry distributions at a horizontal plane 4.6 m (15 ft) above the NOx ports are shown in Figure 5. The burner and overall furnace stoichiometries were 0.85 and 1.16, respectively. With ports only on the front wall, the overfire air does not penetrate far enough to mix with the substoichiometric combustion products near the rear wall, and the Us of 35 is quite high. With ports on both the front and rear walls and the air flow biased to the rear wall, the Us is 14 at this plane indicating considerably better mixing. This port configuration was applied in the retrofit of Southern California Edison's Mandalay Station Unit 1. The retrofit of this 215-MWe unit included 18 oil and gas B&W S-type burners, 12 overfire air ports, gas recirculation fans and ductwork, and controls and instrumentation (Sutherland and Waanders, 1992). NOx emissions were reduced to 105 ppm with oil firing and to 25 ppm with gas firing. The CO emissions of 15 ppm with oil and 45 ppm with gas are evidence that good mixing was achieved with the overfire air ports. Side Wall Air Ports Limiting the mixing between the air and coal in a low NOx burner typically results in a longer and narrower flame. Nevertheless, these burners must be installed in single wall-fired boilers that were designed for burners with shorter and "bushier" flames. Longer flames increase the possibility for flame impingement on the opposite wall. The impingement may increase slagging, corrosion, or unburned carbon losses due to flame quenching. The likelihood that the upper flames will impinge Front Elevation View ~ &~ &~ & 28.5 m ~ ~~ ~~ ~ ~ ~ ~~ ~ ~ ~ ~~ ~~ ~ 15.2m ---~.. I 4.6 m Above NOx Ports Side Elevation View NOx Ports Secondary Superheater 3m 1 -~ -- 3m J -- 3m 1 0 • 0 G 1.8m Figure 4 215-MWe Single Wall-Fired Utility Boiler NOx Port Burner CCWSwiri CWSwiri |