OCR Text |
Show 16 A16SE738.T Figure 6 illustrates the NH3 distribution in each duct. In the east duct, the NH3 concentration varied from 1.9 ppm to 10.3 ppm, with an average concentration of 6.1 ppm. The ammonia concentration in the west duct was similar and varied from 3.5 to 12.6 ppm, with an average of 6.4 ppm. Both ducts showed high concentrations toward the outer sides of the ducts and decreased concentration toward the center of the unit. The average concentration at the center of the duct, which was the sampling location used for standard tests, was 6.4 ppm in the west duct and 3.0 ppm in the east duct. In addition to the NH3 samples taken at the economizer outlet, a number of tests were also performed at the stack. All of the NH3 concentrations at the stack were less than 1 ppm @ 7% O2, which was the same as that found during the short-term test. These low concentrations were consistent with reaction and adsorption of NH3 in the air heater and onto the flyash. S03 EMISSIONS S03 is formed during combustion of fuels containing sulfur. Its presence impacts SNCR operation, because S03 will react with NH3 to form ammonium sulfate «NH4)2S04) and ammonium bisulfate (NH4HS04). The formation of ammonium salts is a function of NH3 and S03 concentrations as well as residence time, and occurs at temperatures below SOoo-F, as is commonly encountered in the air heater. S03 can also react with oxides of condensible metals (Na, Mg, K, etc.) to form metal sulfates. These reactions tend to occur around 1000°F which corresponds to the region upstream of the economizer. Ammonium sulfate is a powdery aerosol which can lead to increased opacity, and can be the predominant product when NH3 is present in excess of S03. In contrast, ammonium bisulfate forms a sticky, semi-liquid material that binds to surfaces and can cause corrosion and fouling. Ammonium bisulfate is the dominant product in cases where the S03 concentration is greater than NH3• Based on the results of testing, S03 was found to vary from generally low levels (less than 10 ppm) typical of cyclone operation to levels five times greater. These higher levels would be expected to favor the formation of the bisulfate and would suggest that the 10 ppm limit of ammonia slip targeted during the test would be much too high. This was indeed the case. Air Heater Fouling Air heater fouling occurred four times during the long-term demonstration. A chronology of air heater washes over SNCR operating is shown in Figure 7. Water washes are typically performed when the furnace pressure approaches 20 in. H20 at full load, which corresponds to pressure drops of 9 to lOin. H20 across the air heaters. The average number of SNCR system operating hours before water washing was required was 274 hours as compared to 1,038 hours during unit operation without chemical injection. The fouling of the air heaters resulted from |