June 1, 2010

Improving SCR Performance on Simple-Cycle Combustion Turbines

By Terry McTernan, PE, Cormetech

Phase II: Process Modeling and Design Improvement

A computational fluid dynamic (CFD) study was conducted to both quantify the existing ammonia:NO_x distribution within the SCR reactor and to assess the potential design improvements. The existing AIG system consisted of 11 two-pronged lances designed with 22 single, 3/16-inch-diameter injection holes orientated to direct ammonia parallel to the flue gas flow, aimed directly at the SCR catalyst inlet.

Figure 5 illustrates the baseline ammonia distribution within the flue gas stream as predicted by the CFD analysis. The three panels show the ammonia mixing as it approaches the SCR catalyst—the gas flow is perpendicular to the page with each panel located downstream of the ammonia grid as indicated. This modeling indicates significant nonhomogeneity of ammonia in the flue gas at the SCR catalyst inlet. The original AIG design produced stratification, providing a ribbon-like effect where ammonia mixing was limited.

5. Out with the old AIG. CFD analysis of the existing ammonia injection grid found significant nonhomogeneity of ammonia in the flue gas at the SCR catalyst inlet. This stratification is seen by the ribbon-like effect (from front to back of the catalyst), where ammonia mixing was limited. Shown in the figure is the relative ammonia concentration in the flue gas. Each figure is a cross-section of the flue gas path measured downstream from the AIG. Source: Cormetech

The CFD model was then modified to incorporate improvements designed to optimize mixing by changing AIG parameters such as nozzle spacing, nozzle diameters, and spacing between lances. Figure 6 shows the predicted ammonia distribution for the improved design. The figure shows that the stratification has been eliminated and that the ammonia distribution at the SCR catalyst inlet was substantially improved.

6.    In with a new AIG. The CFD models found that a modified AIG design would significantly improve mixing of the flue gas with ammonia. The results found that the stratification (Figure 5) was eliminated and that the ammonia distribution at the SCR catalyst inlet was substantially improved. Source: Cormetech

Phase III: Catalyst Replacement and Optimization

As a result of the successful process modeling, recommendations were made for replacing the existing two-prong design AIG (Figure 7) with a three-prong design, as shown in Figure 8. The new lances also employed an optimized nozzle pattern and size. Cormetech also designed new sidewall baffles to eliminate bypass of untreated flue gas along the reactor sidewalls. These design recommendations were accepted by SHEC. The proposed modifications were installed on a single combustion turbine, Unit 3, to test and confirm the benefits of these changes.

7. Out with the two-prong lance. The original ammonia injection grid used two lances attached to a single ammonia supply header, a “two-pronged” design (Figure 2). Courtesy: Cormetech

8. In with a three-prong lance. The new “three-prong” design added an additional AIG lance on each header where two had been used previously (Figure 7). Sidewall baffles were also added to improve ammonia mixing with the flue gas. Courtesy: Cormetech

As a final step prior to implementation of the modeled changes, Cormetech first performed full-load NO_x reduction baseline testing. A test plan was developed to operate Unit 3 at full load and perform sampling across the duct cross section. The flue gas characterization was completed through a series of sampling measurements taken across the duct (side to side) and repeated at different elevations (vertically). The goal was to access the effective ammonia:NO_x molar ratio distribution, which is a key input parameter to a successful SCR when operating at high efficiency and low ammonia slip. Testing before modifications revealed, as suspected, a poor ammonia:NO_x distribution of approximately 30% RMS, further verifying the CFD model result and field observations.                                        

Unit 3 was then modified during the site’s planned fall maintenance outage. The AIG lances were replaced and sidewall baffles were installed. Two weeks after the outage was completed, flue gas testing was repeated. Results showed a significant ammonia:NO_x distribution improvement from the original 30% RMS to 7% RMS. The improvements to the ammonia injection system are estimated to save more than 11% in the annual operating costs for aqueous ammonia. Further, the improved ammonia:NO_x distribution will effectively extend the operating life of the SCR catalyst by allowing the margin previously needed to compensate for poor distribution to be used to compensate for catalyst deterioration over time.

Given the successful evaluation of the improved system, similar changes were made on the remaining three units during the regularly scheduled spring 2009 outage.

With the new high-performance catalyst and ammonia injection modifications complete, performance tests were conducted under full load and at design conditions (2.5 ppm outlet NO_x ). Operational testing confirmed outlet NOx levels trended at 2.5 ppm and the ammonia slip remained below 5 ppm, meeting all performance guarantees (Figure 9).

9.    Under the limit. Unit 3 performance test results with the new catalyst and AIG modification at full load are illustrated. Testing confirmed outlet NOx levels trending at 2.5 ppm and the ammonia slip remained below 5 ppm meeting all performance guarantees. Source: Cormetech

Following 2,660 hours of operation, the activity of the high-performance SCR catalyst continues to be exceptional, with NO_x reductions exceeding 92% at 5 ppm ammonia slip, showing no degradation in performance from the new condition. A second catalyst test is scheduled after approximately 5,000 hours of operation.