March 15, 2008

Benefits of evaporating FGD purge water

William A. Shaw, PE, HPD

Next, separate the vapor

Because power plant wastewaters (including FGD purge water) are often saturated with gypsum, the brine concentrator is “seeded” with calcium sulfate to prevent the deposition of low-solubility calcium salts on the tube surfaces as scale. Scaling is prevented by preferential precipitation of low-solubility calcium salts on the seed crystals rather than on the tubes (Figure 4). In a falling-film evaporator, a vapor separator with an integral mist eliminator is attached to the lower end of the evaporator's tube bundles to help separate the water vapor from the wastewater (Figure 5). The two subsystems ensure that only clean, dry vapor enters the compressor, reducing the risk of its erosion and corrosion by droplets of brine carried over with the vapor. Mist elimination also ensures the distillate's purity. A well-designed falling-film evaporator will produce distillate with less than 5 ppm TDS.

4. Scale buster. Seeding a falling-film evaporator avoids deposition of calcium salts on the tube surfaces of a brine concentrator. Source: HPD

5. Corrosion controller. Eliminating the mist from the vapor separated by a falling-film evaporator dries the input to the compressor, reducing the potential for corrosion and erosion. Source: HPD

The falling-film brine concentrator with seeding and MVR can now be integrated into a system (Figure 6). A plate heat exchanger recovers heat from the outgoing distillate, making it available to preheat the feed. The feed is typically acidified to eliminate residual alkalinity, and a deaerator is included to remove CO₂ and dissolved oxygen from the feed. That reduces the potential for corrosion and scaling in the evaporator vessel.

6. A complete ZLD system. Here, the brine concentrator evaporator of Figure 5 is integrated with seeding and MVR subsystems. The system produces concentrated brine for crystallization. Source: HPD

Then, concentrate the solids

Forced-circulation crystallizers (Figure 7) are the type most often used in power plant wastewater applications. Condensing steam normally provides the energy to evaporate water from waste brine, but in this type of evaporator the brine is forced through the heater tubes by a recirculation pump at high velocity. Flooding the tubes prevents evaporation from occurring inside them. The high velocity of the brine and the suppression of boiling in the tubes prevent scale from forming on the crystallizer's tube surfaces.

7. Crystal clear. A forced-circulation crystallizer with MVR and solids dewatering uses a compressor and recirculation to produce recovered water and solids for disposal. The feed at bottom left comes from the brine concentrator of Figure 6. Source: HPD

Evaporation occurs when the circulating brine flash boils in a separate vapor body. Nucleation and growth of salt crystals that have exceeded their solubility also occur there. As in the evaporator, a mist eliminator installed in the vapor body separates droplets of brine from the water vapor, protecting the compressor from corrosion and erosion and ensuring the purity of the distillate.

Depending on the TDS concentration maintained in the scrubber, a falling-film brine concentrator can typically concentrate FGD purge water five to 10 times before running up against the limitations imposed by the elevation of boiling point and the solubility limits of the sodium salts. The system reduces the water content of the wastewater by 80% to 90%, so for every 100 gpm of feed, 10 to 20 gpm of concentrated brine is discharged and 80 to 90 gpm of distilled water is produced for reuse in the power plant.

Ordinarily, the concentrated brine would be sent to a forced-circulation crystallizer to evaporate the remaining water and precipitate and dewater the solid salts. However, the solubility of the calcium and magnesium chloride salts that dominate the content of FGD wastewater is so high that it is usually impractical to try to precipitate these salts in a forced-circulation crystallizer. It can be done, but the boiling point of the solution is too high to use MVR, so high-pressure steam must be supplied. What's more, because the calcium and magnesium chloride are acidic salts that are extremely corrosive at the temperatures and concentrations required to crystallize them, any crystallizer equipment that comes in contact with the brine must be made of very expensive noble alloys such as palladium-alloyed titanium and high nickel-chrome-molybdenum alloys.

There are some cases in which a brine crystallizer may not be economically feasible. Some power plants have considered evaporating their FGD wastewater by a brine concentrator and disposing of the resulting concentrate stream on-site either along with ash or in a separate surface impoundment. Others have considered using spray drying to remove the remaining moisture from the concentrate, producing a dry product suitable for disposal in a landfill or coal mine. It should be noted that because a spray dryer will require a fuel oil or natural gas supply, it also will likely need an air permit. In addition, the dried salt residuals produced are strongly hygroscopic, so they must be bagged quickly to keep them from absorbing moisture from the atmosphere before sending them off for disposal. Other methods of drying the brine concentrator blowdown are possible. They include the use of flakers, prilling towers, and other methods common to the production of calcium and magnesium chloride salts.