March 15, 2008

Benefits of evaporating FGD purge water

William A. Shaw, PE, HPD

Making salt water

Many of the challenges of designing an FGD wastewater treatment process based on evaporation are related to the basic chemistry profile of wastewater. Calcium and magnesium chloride salts are difficult to crystallize and highly corrosive.

To avoid both problems, FGD wastewater can be pretreated using conventional lime-soda ash softening. Slaked lime (calcium hydroxide, Ca(OH)₂) and soda ash (sodium carbonate, Na₂CO₃) are used as reagents in the pretreatment step along with small quantities of ferric chloride and polyelectrolyte to enhance the separation of suspended solids. Adding lime slurry to the wastewater increases the calcium ion concentration, which reduces the gypsum supersaturation and in turn the scaling potential of the wastewater. It also raises the pH so that magnesium hydroxide precipitates. Soda ash is added in a separate reaction tank to precipitate the calcium as calcium carbonate. The net result of the softening process: Sodium ions are substituted for most of the calcium and magnesium ions, so the softened FGD wastewater becomes mainly an aqueous solution of sodium chloride.

Another possibility is to use a conventional water-softening process, producing a ZLD system similar to those currently in use at power plants to treat cooling tower blowdown. Sodium chloride can be readily crystallized in a conventional forced-circulation crystallizer. Although salt is corrosive, its use requires less use of noble materials of construction such as super-austenitic and super-duplex stainless steels.

Future system design

Although power plants have used evaporation for decades to eliminate their discharges of cooling tower blowdown, the evaporation of FGD purge water is a much more recent application with only a few installations and very little operating experience. However, some evaporator suppliers have experience in evaporating calcium, magnesium, and sodium salts that is directly applicable to the evaporation of FGD wastewater. Evaporation is a treatment method that will be considered more often in the future, given the inevitable tightening of limits on air and water discharges from all power plants, the size of the existing U.S. coal-fired fleet, and the prospects for its future expansion.

A plant's decision about which treatment method to use must take into account its capital and operating costs and other site-specific factors. Due to the current dearth of ZLD system operating experience on FGD wastewater, plant designers, builders, and owners considering a ZLD process should closely examine a prospective supplier's bench and pilot-scale testing capabilities for calcium, magnesium, and sodium chloride evaporators. A robust system is also important to look for. Do your homework before you buy to avoid costly equipment failures and even more costly losses of generation revenue.

—William A. Shaw, PE (bill.shaw@veoliawater.com) is a senior process engineer for HPD, a Veolia Water Solutions & Technologies company based in Plainfield, Ill.