Getting gas-turbine inlet air and exhaust gas to flow where and how you want it to in powerplant ductwork is akin to herding cats. This was brought to light once again while arranging entries for the CCJ’s annual Best Practices Awards judging.
The SCR performance issue and its solution, described below, is only one of many industry best practices that will be written up in the 1Q/2012 COMBINED CYCLE Journal. The top entries, in the opinion of the judges—the so-called BEST OF THE BEST—will be announced during a special awards ceremony at the upcoming CTOTF Spring Turbine Forum in Williamsburg, Va, April 15-19.
The problem, as described by industry veterans Jim Carlton, president, Granite Ridge Energy LLC, and Larry Hawk, plant engineer, Granite Ridge Energy, operated by NAES Corp, was that the plant had been challenged by an underperforming NOx catalyst since commissioning. However, routine destructive sampling of the catalyst showed reactivity at or above expectations. And testing confirmed that the ammonia injection grid was properly balanced. What to do?
The plant contracted the SCR OEM and an emissions testing company to conduct an online evaluation of the catalyst grid. Measurements of ammonia-to-NOx ratio, flue-gas velocity, and gas temperature were made upstream and downstream of the grid and at different distances from the side walls. Data collected revealed the following:
The ammonia/NOx ratio ranged from about 0.5 to 1.6, lowest near the side wall and highest at 5 ft from the wall.
Gas velocity ranged from 7.5 to 20 ft/sec, highest closest to the side wall and lowest about 4 ft from the wall.
Temperature ranged from 570F to 595F, again highest close to the side wall and lowest at about 4 ft from the wall.
1. Steel baffles were installed to prevent exhaust gas from bypassing around the catalyst grid
The OEM’s engineer created a model to help determine the best path forward to correction, and a two-phase implementation plan was created. The first step was to increase the as-designed catalyst to support steel baffling and thereby prevent gas from bypassing the catalyst grid (Fig 1).
This relatively low-cost activity was implemented in one of the plant’s two heat-recovery steam generators (HRSGs) in 2010, during a planned outage. Performance gain proved negligible.
The second step was to erect a perforated-plate baffle immediately upstream of the ammonia grid on each side of the HRSG (Fig 2). The 80-ft-tall by 3-ft-wide baffle was designed to have a local restriction of 40% but negligible effect on backpressure. The goal was to slow down exhaust-gas velocity at the side walls to allow proper catalytic conversion. Both HRSGs were equipped with the baffles, supplied by Vogt Power Aftermarket (Louisville, KY), during the facility’s most recent planned fall outage.
- 2. Perforated flow restrictor reduces exhaust-gas velocity close to the side wall with minimal impact on backpressure
Worked! The baffles’ control of flow characteristics immediately upstream of the SCR enabled an approximate 13% improvement in the ammonia/NOx ratio. Result: The facility is able to operate as intended with efficient control over it ammonia usage.
The editors are often told that plant design to site and operating conditions begins after commissioning. This apparently is one of those instances. EPC firms build plants with the best information available to designers—which sometimes involves educated guesses—and within the constraints of the bid spec. The responsibility of meeting the pro forma falls squarely in the lap of the plant staff.
Every plant has its idiosyncrasies and solutions are not universally successful. One facility manager told the editors that his plant was pushing backpressure alarm limits because of severe tube fouling and baffles were removed to keep the plant in service. Output was limited because some exhaust gas was bypassing the catalyst.
On the front end of the gas turbine, compressor fouling is often associated with inlet air bypassing the filters. In such situations, you can find air velocities at the sides of the inlet hood twice those in the middle of the filter array—much as Granite Ridge found on the exhaust side.
Further downstream in units equipped with evaporative coolers, leakage around the media bed can adversely impact performance because the entire inlet air stream is not cooled to the design point. Likewise, the performance of fogging systems depends significantly on varying spray-nozzle density with air velocity—more nozzles being required in sections of the flow stream with the highest velocities. For these kinds of analyses, computational fluid dynamics (CFD) can eliminate guesswork.
