OCR Text |
Show performance of the ESP was not affected from the baseline efficiency of 89% . Carbon in ash was also not affected, being 4 . 7% and 5 .4% for the baseline and urea injection ashes respectively. Figure 11 shows the size distribution of isokinetically collected ashes for the baseline coal and urea injection by the use of a Pilot Mark 3 Cascade Impactor. The mass median diameter of the baseline ash is 8 ~m compared to 5 pm for the urea injection ash. According to these results urea injection is not expected to pose any problem for the electrostatic precipitator. SUMMARY AND CONCLUSIONS Aqueous solutions of ammonium based compounds injected into the 640 MJ/h pilot scale furnace are very effective for reducing NOx emissions from flue gases. In this study, urea, ammonium carbonate and ammonia were evaluated. A 1.7% S eastern U.S. bituminous coal was burned giving rise to an initial NOx concentration of 550 ppm. A high sulphur (2.8%) coal from Nova Scotia with an initial NOx concentration of 537 ppm was also tested. The results of the study are summarized below: • Nozzle characteristics (type of atomizer, droplet size distribution and momentum) are important parameters for determining NOx capture efficiency. • Injection temperature and mode of injection was found to be important in controlling NOx emissions during additive injection. An optimum injection temperature of 1100°C was found for all the additives with urea removing 90%, ammonia removing 84% and ammonium carbonate removing 80% of the NOx • • NOx capture by urea and ammonium carbonate increases with increasing stoichiometry (NSR) up to 1. 7 and up to 2.0 for ammonia. At a stoichiometry of 1.7 to 2.0, 90% NOx removal was achieved with urea, and 80% with ammonium carbonate. At a stoichiometry of 2.0, 84% of NOx removal was achieved with ammonia. Utilizations for urea, ammonium carbonate and ammonia were: 53%, 40% and 42% respectively. • NOx capture by urea injection decreases with decreasing NOx level in the flue gas. At 550 and 867 ppm NOx in flue gas, 90% NOx can be removed at an NSR of 1.7. For a concentration of 347 ppm NOx in the flue gas at the same stoichiometry, NOx capture drops to 60%. • Ash resistivities remained unchanged from a baseline level of 107 ohm.cm and the performance of the electrostatic precipitator was not affected during urea injection. • No fouling or slagging was observed in the furnace on heat exchanger surfaces during additive injection. ACKNOWLEDGEMENT The work described in this paper was funded by the Envirorunental Protection Department of the Central Production Services Division of Ontario Hydro. The authors wish to acknowledge their contribution and funding for the test program. |