| OCR Text |
Show 12 A16SE738.T by injection configuration, NOx setpoint and boiler conditions with regard to excess O2, as defmed in Table 1. Other factors impacting the operation of the SNCR system during the longtenn demonstration are listed, chronologically, in Table 2. It should be noted that limitations imposed by the design of the temporary injection system restricted the ability of the process to respond rapidly to changes in boiler conditions. Some of these limitations can be minimized in a full scale commercial installation. Though conditions throughout the test were not always representative of target operating conditions, the SNCR system was able to demonstrate overall reductions in NOr Such reductions were more significant at lower load, but, on average, indicate that a 30 % reduction in NOx should be achievable under dispatch conditions (Figure 5) while simultaneously maintaining NH3 slip at a level which minimizes air heater fouling. From an average baseline of 1.1 lb/~tu during the test, the system could generally average NOx emissions around set points of 0.75-0.82 lb/~tu . During these periods, the chemical usage corresponded to an average NSR of 0.7. Ammonia Slip Ammonia is a by-product of the decomposition of urea at elevated temperatures. Ammonia that has not reacted by the time the flue gas leaves the higher-temperature sections of the boiler is referred to as ammonia slip. This residual ammonia can react with S03 downstream of the economizer to fonn ammonium sulfate «NH4)2S04) and ammonium bisulfate (NH4HS04). It can also physically adsorb onto the flyash. Any remaining ammonia will exit with the flue gas. An upper limit of 10 ppm (dry 7 % O2) was targeted for NH3 slip at the economizer outlet. To hold slip below this level, upper limits were placed on the reagent injection rates based on results of the short-tenn test conducted in May of 1993 and based on results of tests taken as the long tenn demonstration proceeded. These upper limits were generally responsible for operations above NOx setpoint. Reference to Figure 5 shows that during injection Periods 4,6 and 7 (0.68Ibs/~tu setpoint) approximately 50% of the ammonia tests above 110 MW, the setpoint was not achieved. Ammonia samples collected during the test ranged from 1 ppm to over 20 ppm (dry, 7 % O2). Since these samples represent an average for the gas collection period, the range for instantaneous ammonia concentrations is unknown but would likely be wider. This wide range is due in large part from the inability to correlate average baseline NOx to load and O2 (which established the minimum and maximum pump rates). Ammonia samples were drawn from the center of each duct; however, a nine point traverse was perfonned in each duct to ascertain the distribution of NH3 across the economizer ducts and the representativeness of the center sampling location. The test was perfonned during constant boiler conditions at a fixed load of 120 MW. The urea injection system was on automatic control with a setpoint of 0.61 lb/~tu, however, actual NOx values varied from 0.71 to 0.84 lb/~tu. |
