Design – Granite Ridge Energy

Engineered solution improves SCR performance

Granite Ridge Energy
Owned by Granite Ridge Energy LLC
Operated by NAES Corp

Challenge.  The problem, as described by industry veterans Jim Carlton, president, 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 set out to determine the cause and correct the issue.

Solution. GRE 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 (Fig 3). 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.

Solution. The OEM’s engineer created a model (Fig 4) 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 5).

   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 6). The 80-ft-tall x 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 International Inc, Louisville, during the facility’s most recent planned fall outage.

Results. 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.