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Show GPU GENCO - Seward Station A Hybrid SNCR/SCR system has been designed, constructed, and installed for G PU GENCO, at Seward Station, Boiler #15. This unit is a Combustion Engineering, coal burning, tangentially fired boiler rated at 148 M W gross electrical output, Figure 2. Current minimum load is 106 M W g , but it may become necessary to operate at loads as low as 74 M W g (50% MCR). A commercial NOxOUT® SNCR had been previously installed at Seward Station. The system provided the required NOx reduction from a 1990 baseline of approximately 0.78 lb/106 Btu to 0.45 lb/106 Btu with less than 5 ppmv slip. High concentrations of S02, and therefore S03, as well as cool air pre-heater exit temperatures combined to make this installation particularly sensitive to ammonium salt formation. It is currently being operated at reduced efficiency ( approximately 0.5 lb/106 Btu ) to produce less than 2 ppmv ammonia slip. As much as 7 5 % of the chemical is injected into the furnace where utilization is relatively low. The remaining chemical is injected behind the super heater tubes, above the arch, through multi-nozzle lances that provide excellent chemical distribution and extremely high chemical utilization. This unit is an excellent candidate for hybrid SNCR/SCR reduction. Using the cooler alone in limited testing, deep reductions in NOx have been possible with decreased chemical flow and reasonable ammonia slip ( at or below 20 ppmv ). A small in-duct SCR reactor was necessary to remove 9 0 % of the ammonia slip and provide additional NOx reduction. SCR experience on coal fired units has been limited but recent independent testing has shown that new catalyst formulations are able to withstand the harsh environment. Seward Station Cascade Design Construction has been completed for the Seward Station Hybrid SNCR/SCR commercial demonstration and testing is currently under way. Available space for two small reactor vessels, one in each of the right and left side ducts, was located between the economizer hoppers and the air pre-heater inlets. The duct design was completed to provide the proper average inlet and outlet conditions as specified by the catalyst vendors selected. Two independent catalyst vendors were selected. A static mixing grid and turning vanes have been utilized to decrease the known and predicted gas and solid flow imbalances in the unit. Design of these duct internals was completed using both computational fluid dynamic (CFD) and cold-flow models. CFD techniques were used to model the high temperature gases, Figures 3 and 4. The virtual environment permits non-intrusive measurement and an evaluation of a wide variety of duct configurations. Cold-flow modeling, however, can more closely approximate the actual geometry and provides important measurements necessary for scale-up. Both modeling tools have provided valuable insight into the design of constrained-space reactor vessels. The catalytic rate of S03 generation is particularly important in this case because current air pre-heater sensitivities to ammonium salt formation. The catalyst vendors have specified minimum operating temperatures above which ammonium salt formation and deposition on the catalyst face will be avoided. Rue gas mixing and turning vanes have been designed to reduce temperature variations and eliminate localized cool spots. Maximum performance in a full-scale SCR requires uniform ammonia to NOx ratios across the face of the catalyst. The ammonia slip to the SCR in a C A S C A D E system, however, will be significantly lower than the NOx at all points in the flow. Performance degradation due to variations in N H 3 concentration will, therefore, be greatly reduced. More importantly, any ammonia slip at or below design maximums will be significantly reduced. Page 5 |