March 15, 2008

Benefits of evaporating FGD purge water

William A. Shaw, PE, HPD

Conventional treatment limitations

The traditional way to treat FGD wastewater is to discharge the liquid effluent of a limestone-gypsum system into a natural watercourse. In this process, wastewater from the FGD loop is fed into a series of reactor tanks where its heavy metals are precipitated by hydroxide/sulfide following the addition of lime, organosulfide, and ferric chloride. Two precipitation/flocculation stages are usually included due to the wide variation in the optimum pH values for precipitation of the metals. If selenium, nitrates, and organics are present in the purge stream, it will probably require biological treatment prior to discharge. Such treatment methods can reduce the concentration of suspended solids and metals, acidity, and oxygen demand, but they do not reduce levels of chloride or total dissolved solids (TDS).

However, physical, chemical, and biological treatment methods may not be able to reduce wastewater concentrations to the parts-per-trillion levels required for discharge of some chemical species such as mercury as their discharge limits become more stringent. When conventional treatment methods cannot produce an effluent that complies with the plant's discharge permit, evaporation of the purge stream should be considered.

Evaporation is an appealing FGD wastewater treatment method because, in theory, it can completely separate all dissolved species (benign, hazardous, or toxic) from the water, producing a stable solid that can be disposed of in a landfill. If the high-quality distilled water produced by the process is reused in the power plant, there will be zero discharge of wastewater to the environment.

First, reduce the volume

Falling-film evaporators (also called brine concentrators) have been used for many years to substantially reduce the volume of wastewater discharged from power plants. Often, evaporative crystallizers are added to achieve zero liquid discharge (ZLD). Such ZLD systems have historically been used to eliminate discharges of cooling tower blowdown and demineralizer wastes. Recently, several ZLD systems also have been installed to eliminate scrubber blowdown from wet FGD systems (Figure 2).

2. Benign effluents. This typical zero-liquid-discharge (ZLD) system at a combined-cycle plant produces only reusable water and a solid cake suitable for disposal in a landfill. Courtesy: HPD

Evaporators and crystallizers operate by transferring latent heat from condensing steam across a tube surface to cause a liquid at its boiling temperature to partially vaporize. But because the steam cycle in a power plant is in precise balance, steam is usually not available for use by a ZLD system. Instead, most power plant ZLD systems use an electric motor-driven heat pump in a technique called mechanical vapor recompression (MVR) to drive the evaporation process (Figure 3).

3. Steam substitute. Mechanical vapor recompression (MVR) is used to drive the evaporation process in a ZLD system when an external steam supply is not available. Source: HPD

External steam, often from a dedicated start-up electric boiler, is used only at cold start-up to heat the equipment and bring the wastewater to the boiling temperature. Once the wastewater begins to boil inside the evaporator, an electrically driven compressor is started. The water vapor evaporated from the wastewater is compressed, raising its pressure and temperature. The hotter compressed water vapor flows to the heating side of the evaporator tubes, where it condenses and transfers its latent heat across the tube wall. That causes more water to evaporate, completing the cycle. The condensed water evaporated from the wastewater is pumped from the evaporator as distilled water (distillate).

Falling-film evaporators/brine concentrators have vertical tube bundles that use some sort of device to distribute the wastewater at its boiling temperature as a thin film around the inside surfaces of the tubes. This film flows uniformly down the entire length of the tube, and as heat is transferred across the tube wall from the condensing steam on the other side, the film boils. Water vapor is released into the core of the tube and mixes and flows with the liquid film.