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Show NOZZLE CHARACTERISTICS Atomizer type, atomizing air pressure and addition rate of the reagents are important parameters for determining NOx capture efficiency by additive injection. High atomizing air pressure improves the quality of atomization and promotes an early release of the reagent to react with NOx; it also increases the momentum of the droplets leading to enhanced penetration and mixing with the flue gas stream. Two high pressure twin fluid atomizers were tested; one from Turbotak and the other from National Research Council of Canada (NRC). The mass median diameters (MMD) of droplets for the two nozzles were 17 and 8.6 pm's respectively. The NRC nozzle provided a wider droplet particle size range and could be operated at higher pressures (100 psig compared to 70 psig for the Turbotak) . The comparative performance for the two nozzles is shown in Figure 7 while injecting a 13.5% aqueous urea solution into the pilot combustor. The NRC nozzle performs much better at lower stoichiometries than the Turbotak, for example, 80% NOx capture at a stoichiometry of 1.2 is achieved with the NRC nozzle compared to 67% for the Turbotak nozzle. Corresponding urea utilizations are 65 and 56%. The Turbotak nozzle however performs slightly better at stoichiometries greater than 1.6. The NRC nozzle generally showed better performance which is due mainly to the atomizer design allowing for better penetration and mixing with the combustion gas. EFFECT OF INJECTION TEMPERATURE After having a proper nozzle to inject the additives, injection temperature was found to be the most important parameter affecting NOx removal and additive utilization. Therefore gas temperatures were measured using a water cooled suction pyrometer, to assure that stratification in combustion gas temperatures does not exist. As expected, temperatures near the wall tended to be lower than temperatures towards the centre of the furnace due to the cooling jackets behind the refractory wall (see Figure 6). However, no significant stratification in gas temperatures was observed and furnace temperature distribution in the middle of the furnace was found to be fairly uniform. NOx removal by additives as a function of varying gas temperatures is shown in Table 2 and plotted in Figure 8. Additives injected include: a 13.5% aqueous solution of urea, a 16.1% aqueous solution of ammonium carbonate and a 5.6% aqueous solution of ammonia while burning a 1.7% S US bituminous coal. The ammonia and ammonium carbonate solutions were injected cocurrently with the flue gas at a normalized stoichiometric ratio - 2.0 (NSR - moles additives injected to the theoretical moles required to remove 100% of the NOx). Urea was injected both cocurrently and countercurrently at an NSR of 3.0 and 2.0 and was also tested with the 2.8% S Nova Scotia coal. The highest NOx capture was obtained by the cocurrent injection mode at temperatures ranging between 1100 - 1200°C for all three additives: urea removing 90% NOx• ammonium carbonate removing 80% NOx and ammonia removing 84% NOx from the flue gas at an additive NOx stoichiometry of 2.0. From Table 2 it is seen that with the Nova Scotia coal with an initial NOx concentration of 537 ppm and injecting a 13.5% urea solution cocurrently at a NSR - 2.0, 83% NOx capture was achieved. |