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Show platinum catalysts at atmospheric pressure, the reaction mixtures for which this behavior was observed were characterized by high NH3 concentrations typically encountered in nitric acid manufacture. These NH3 concentration levels are much higher than the levels studied in the laboratory program. It is not unexpected for reaction kinetics to change when the reaction conditions are varied (e.g., NOx conversion in SCR of NOx by NH3 is independent of inlet NOx concentration for concentrations greater than -150 ppm). Field Trials The objectives of this field program were to evaluate the NOx reduction capability of the SCR catalyst; characterize NH3 , NOx , temperature, and velocity profiles in and out of the SCR; and evaluate the performance of the AIG. Following an overall SCR system performance evaluation at the natural gas-fired cogeneration facility, the prototype probe system was used as a diagnostic tool to determine whether the less-than-optimal performance of the SCR system was due to AIG design/operating problems resulting in maldistribution of NH3 across the catalyst surface. Figure 6 illustrates a side view of sample ports and the relative locations where sampling was conducted. Figure 7 shows a duct cross-section view identifying the NH3 measurement locations at the SCR inlet. (Measurements were limited by the number of sampling ports available and access to the unit.) Figure 8 presents normalized NH3 and NOx concentration profiles for one of the AIG nozzles upstream of the SCR catalyst. The NOx concentration profile is flat while the NH3 concentrations (approximately two feet downstream of the nozzle and one foot upstream of the catalyst) indicate limited dispersion of the NH3 away from the nozzle. In Figure 9, peaks in NH3 levels are observed at sample points corresponding to NH3 injection nozzle locations. In addition, nozzle pluggage is observed at two locations . Similar behavior was also observed in traverses conducted from top to bottom of the unit. Figure 10 shows the NH3 distribution at the SCR outlet, indicating that the NH3 maldistribution is not corrected by passing through the catalyst bed . Based on the results of this laboratory and field investigation , and other non-invasive tests on the unit utilizing the plant's monitors, the site's AIG problems were diagnosed. AIG design changes were specified, and the AIG was modified at the next scheduled major plant outage. This redesign has resulted in improved SCR system performance, lower NH3 consumption, lower gas turbine water injection, and a delayed need for catalyst replacement . UPCOMING PLANS Phase II plans are focused on completing the Phase I laboratory study (including additional NH3 conversion test runs on the initial Phase I prototype system, and more rigorous kinetics/mass transfer analysis), improving the conversion efficiency of the stainless steel reactor, investigating other reactor materials and preparation techniques, determining effects of moisture and NO on NH3 conversion measurements, and evaluating loss of catalytic activity over time . In addition, design improvements will be made to the Phase I probe based on the Phase I field trials. Investigation of the Phase II probe system performance at high NH3 and NOx concentration levels that are characteristic of SCR and SNCR applications on boilers/incinerators 8 |
