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Show 10 Injection rates are usually less than NINO = 2. SNCR typically uses ammonia, or cyanuri c ac i d as the reagent, a 1 though severa 1 vendors have deve 1 in proprietary reagents to improve levels of NOx reduction or to modify the effe ~Jve temperature window. NOx reductions using SNCR are typically: i~ the 40-60~ range, . . depending upon the uncontrolled NOx level, and the process · variables such as injection rate and available time at the required temperatures; some demonstrations have achieved 70% reductions. Disadvantages to SNCR include the formation of nitrous oxide (N20), ammonia slip (in which ammonia that does not react with NO is released), the deposition of ammonium bisulfates formed by the reaction of sulfur trioxide and ammonia, and the presence of a visible plume of ammonium chloride formed by the reaction of ammonia and hydrogen chloride. 17 SCR relies on reactions similar to those with SNCR, and also requires the injection of a nitrogen-containing chemical. However, SCR operates at significantly lower temperatures, relying on a catalyst to increase the reaction rate. SCR can ach i eve NOx reduct ions of up to 80% where the uncontro 11 ed NOx 1 eve 1 s are high (> 800 ppmv). Such high reduct ions s i gni fi cant 1y reduce the catalyst life, however, and in instances where large reductions are required, the usua 1 pract i ce is to use SCR as the fi na 1 step ina seri es of NOx reduct ion processes beginning with some form of combustion modification followed by the use of SCR. The SCR system then acts as a polishing system to bring NOx levels to the final desired level. SCR and SNCR are both applicable to sources in which the use of combustion modification methods is not feasible, or where combustion modifications cannot achieve the required NOx reductions alone. v. Conclusions Most of the NOx emission standards in the Pacific Rim countries discussed here are not "technology-based," requiring specific equipment to be installed. Rather, they are "emissions~based," with emission limits often set such that certain technologies can be used to achieve those limits. It may appear that setting a standard so that one or more specific technologies must be applied to achieve that standard is the same as requiring its installation, but there is an important difference. When technology-based standards are used, there is little, if any, room for improving the s~ate of the art beyond .minimizing costs and performance impacts. When emissions-Qased standards are set, however, there is considerable flexibility allowed in achieving those standards over the long term. This can allow new technologies to be developed and applied, which can significantly improve performance and reduce both capital and operating costs. o • |
