Steve Cisek of WorleyParsons and Tom Eckstein of B&W talked about the wet FGD retrofit at Progress Energy Asheville 1 (200-MW) and 2, (194-MW), respectively. A wet FGD system designed for a minimum of 97 percent SO₂ reduction (when firing 3.08 percent S coal) was placed in service for Unit 1 in November of 2005 and for Unit 2 in May of 2006. A wet chimney with one common concrete stack enclosing two fiberglass flues was also installed. The project team included WorleyParsons, B&W and Fluor. The project was completed in 26 months and one of the two units was producing electricity at all times.

Minimizing single source failures was a primary consideration in systems engineering and equipment redundancy. Therefore each unit has its own absorber tower and associated support systems. The common systems are for limestone handling and preparation, secondary dewatering, gypsum handling, and the various water and air systems. Stebbins ceramic lined absorber towers (33 feet in diameter), two vertimills for grinding limestone, and centrifuges for gypsum dewatering are used. The flue gas passes through the perforated stainless steel absorber tray and encounters four absorber spray levels (three operating and one spare). Each spray level is supplied by a dedicated absorber recycle pump.

The primary wastewater treatment system consists of a clarifier for solids removal, redundant filter presses and pH control using acid and caustic metering systems. It also mixes the absorber tower blowdown with dilution water to lower the chlorides concentration and temperature to a level acceptable to the secondary treatment system, which is a constructed wetlands treatment system (CWTS). The CWTS was designed to treat approximately 0.54 MGD at the average projected FGD wastewater flow of 375 gpm. Total treatment area is 6.5 acres with a hydraulic retention time of eight days. The CWTS will achieve the treatment discharge objectives for mercury (0.00063 mg/l) and selenium (0.263 mg/l). See the article in the February 2006 FGD & DeNOx Newsletter for details on the Duke CWTS (also based on the Clemson University model).