September 15, 2008

Focus on O&M (September 2008)

WATER TREATMENT
Tackling substandard water sources

Boiler and cooling tower feedwater typically must be softened prior to use, usually with well-established processes for reducing water hardness. A growing problem for many power plant operators is maintaining adequate softening, clarification, and treated water quality while dealing with increasingly substandard source water quality. Many power plants also are being forced to adapt their process water systems so that they can use a fluctuating, off-spec water supply that is high in hardness, silica, and solids.

Luckily, innovative technologies are being developed to treat lower-quality water--and to do so within an existing or smaller footprint. For example, high-rate, small-footprint processes for water clarification, softening, and silica removal have been recently introduced to shorten retention times and provide greater flexibility in dealing with poor or varying source water quality.

Softening essentials

As the quality of source water decreases, the importance of efficient and effective water softening processes increases. Softening is essential for keeping the heating surfaces on the water side of boilers free of deposits. Various salts present in the water become less soluble as the temperature rises, and they precipitate out and adhere to the internal boiler and piping metal as soft or hard scale deposits. Dissolved minerals can precipitate out of the solution and deposit as scale on the heat exchange surfaces of evaporative cooling systems, just as in a boiler. Scaling on these surfaces decreases the rate of heat transfer, leading to increased operating costs and deterioration of equipment.

These accumulations can plug boiler tubes and piping, and can also decrease heat transfer to the point that the metal overheats, leading to costly tube failure. Contaminants can also volatilize in the steam and carry over into the turbines, creating maintenance problems and impairing turbine performance. In addition, embrittlement can occur when high caustic concentrations are trapped under scale deposits, causing cracks and leaks (see “Put a Lid on Chemical Costs,”). The pH of the water outside the scale deposits can register a safe 8.0 but be as high as 10.0 to 11.0 under the scale and directly on the metal.

The traditional water pretreatment method for softening entails the addition of lime to precipitate out the calcium, magnesium, and silica in the raw water supply. For boiler feedwater treatment, the exact process used depends on the boilers. Low-pressure boilers are less demanding, although some secondary treatment is typically applied after the lime precipitation stage. By contrast, high-pressure, supercritical boilers require high-purity, deionized water, requiring traditional softening with lime and other chemicals followed by polishing with reverse osmosis, cation and anion exchange, or electrodeionization (or all three processes in series).

Softening options

In the precipitation stage, quick or slaked lime is added to water in a rapid mix zone upstream from one or more clarifiers. Lime is added to reach the pH required for calcium and magnesium to form a solid precipitate. If the alkalinity of the source water is insufficient, an additional source of alkalinity (like soda ash) may be added to the water to further precipitate the calcium ions. The silica present in the water piggybacks with the magnesium, so in some cases an additional source of magnesium may be added to the water to help capture the silica. After the precipitation reaction, the solid precipitates formed are settled in a clarification system, where sludge is separated from the clarified water effluent. At this point, water intended for use in low-pressure boilers (below 450 psi) will typically flow through a closed sodium exchange vessel, where it contacts synthetic ion exchange resin. In this vessel, any remaining anions are exchanged for sodium.

High-pressure boilers (above 600 psi) require ultra-pure water that must be first pretreated with lime softening and then further processed through both cation and anion exchange beds. Calcium, magnesium, and sodium are exchanged for hydrogen ions in the cation beds, and sulfate, chloride, carbonate, and silica are exchanged for hydroxide ions in the anion beds.

Membrane processes are also often used to remove impurities. Reverse osmosis blocks salts, and electrodialysis with current reversal forces dissolved salts through cation- and anion-selective membranes. Following this series of demineralization treatments, some plants also further remove alkalinity from the water. The degree of “polish” achieved with these downstream processes is directly related to the effectiveness of the lime precipitation process. For example, synthetic resins deplete over time and the “cleaner” the water entering the exchange beds, the longer the interval is between regenerations. (See POWER, August 2008 for a discussion of how to select the right condensate treatment approach.)

In the past, lime softening was performed in batch systems by mixing the chemicals with raw water in a tank, allowing time for precipitation and settling, and then flowing off the clear water. Over time, this method has been improved by adding continuous-feed chemical dispensers to treat incoming water and using a separate tank for retention and settling, followed by continuous outflow of softened water from a third tank downstream.

The equipment configurations typically used today for lime softening haven’t really changed much in the past 50 years. Big, circular, solids contact or sludge blanket clarifiers are used, and the volumes that can be treated as well as the associated retention times are restrictive. Fluctuations in raw water quality and flow rate can often be difficult to accommodate. As the characteristics of the raw water change, operators must perform frequent adjustments to optimize their softening treatment.

With cold or warm lime softening methods, the flow rates of conventional systems are typically limited to less than 1.5 gpm/ft² of settling area, with a one-hour retention time. Plus, the water is not “finished” at this point and further precipitation occurs downstream. The hot lime softening method is only slightly better, with a typical flow rate of 1.7 to 2.0 gpm/ft², with a one-hour retention time.

Compact, high-rate softening

Recently introduced high-rate softening technologies can handle 12 to 32 gpm/ft² flow rates compared to the 1.5 to 2.0 gpm/ft² outputs from conventional lime softening methods. This increased capability substantially reduces the installation footprint required for the softening process (Figure 1), an important consideration where high-purity specifications demand intensive reverse osmosis or demineralization treatment downstream.

With these new treatment systems, designers have taken the principles of softening (reduction of hardness, alkalinity, silica, and other constituents in the water through lime and chemical additions) and have put them to work in a single compact treatment line. The increased overflow rates achieved by the systems translate into a higher tolerance for significant changes in raw water flow rates, compared to conventional softening processes.

Depending on the raw water’s properties, one of two high-rate technologies can be applied. The first softener system is designed to handle high flow rates with a rising velocity up to 12 gpm/ft². This system, the Multiflo Softener, can reduce high raw water hardness concentrations of 150 to 4,000 ppm to concentrations of 35 mg/l CaCO₃ and 50 mg/l magnesium in the clarified water (Figure 2). The other high-rate system, the Actiflo Softener, handles very high flow rates and has a maximum rising velocity of 32 gpm/ft². It reduces 150 to 500 ppm hardness concentrations to 35 mg/l CaCO₃ and 50 mg/l magnesium in the clarified water, and is specially adapted for silica removal (Figure 3). Both systems incorporate a new advanced precipitation reactor that promotes rapid growth of large crystals and helps increase settling velocity.

Both softening systems have a single treatment line that includes:

• A dynamic mixing zone stage that creates rapid mixing conditions to destabilize anions by adding metallic salts, if necessary.
• An enhanced precipitation reactor designed to improve the reaction of lime and carbonate ion with the hardness and natural alkalinity of water to form insoluble compounds.
• An accelerated flocculation stage to provide rapid development of large settleable flocs.
• A settling unit providing both gravity and enhanced lamella clarification. Sludge is recirculated and reinjected into the precipitation reactor to improve the precipitation kinetics and optimize chemical consumption.

The precipitation reactor is a critical component in the high-rate processes, and it allows for complete homogenization of water and chemicals. The proprietary design of the draft-tube reactor (named Turbomix) combines the advantages of plug flow and complete mixing. It reduces the reactor tank volume by suppressing dead zones and decreases reagent loss by suppressing short circuiting (Figure 4). Sludge is recirculated in the reactor so that large crystals form quickly and retention time is reduced. Recycling the sludge also increases the sludge waste concentration and thus reduces the volume of sludge for disposal.

The reactor immediately downstream from the precipitation reactor is used to slowly mix the precipitated particles with a flocculating polymer and promote the formation of large flocs. The Actiflo process includes the introduction of micro-sand at this point to act as a nucleus for the flocs. The ballasted flocs settle very rapidly and provide the system with its very high rising velocity. The sand is cleaned and recovered for reuse through the use of hydrocyclones. The final stage includes a clarification tank with lamella-enhanced settling.

By comparison, the Multiflo system does not use sand in the recirculation loop, but it can handle a higher solids loading than Actiflo and will provide sludge thickening in the lamella settling step. The choice between the two technologies depends upon the influent water flow rate and characteristics.

99% total hardness removal

A Multiflo Softening system was installed as part of a pilot demonstration to investigate technologies with the potential to ease demand on the Pecos watershed in New Mexico. The goal was to soften water sufficiently to prevent scaling on reverse osmosis membranes in a downstream treatment step. The raw water contained a total hardness (CaCO₃) of 226 mg/l.

Four reactors and one clarifier (all components of the Multiflo system) with associated chemical dosing systems were provided. Lime and soda ash were added in the first and second reactors to remove the calcium hardness. Caustic was added in the third reactor to remove the magnesium hardness and co-precipitate silica. An anionic polymer was added in the fourth reactor to enhance solids/liquid separation.

The solids generated through chemical precipitation were then removed in a clarifier. A portion of the clarifier sludge was recirculated to the first reactor to optimize lime usage and serve as a nucleus for precipitate formation. Total hardness was reduced to 22.5 mg/l with this process, representing a 99% reduction.

--Contributed by Marta Beltran-Perez (marta.beltran-perez@veoliawater.com) an application engineer for Multiflo/Actiflo, and Larry Gurnari (larry.gurnari@veoliawater.com) power industry market manager for N.A. Water Systems, a Veolia Water Solutions & Technologies company.

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