Concerns over freshwater consumption have spurred changes in powerplant cooling technologies-some mandated by new laws. As a result, plant operators have had to substantially modify their chemical treatment programs for cooling water.

Unless equipped with an aircooled condenser, all steam plants have to deal with chemical treatment of the cooling water. The goals of a cooling water treatment program are the prevention of corrosion and deposits-the same as they are for chemical treatment of the steam side. However, that's where the similarities end, and the complications begin.
For starters, the causes of corrosion and deposits in a cooling system can be a combination of biological and inorganic forces, whose interactions are not always clearcut. Another complication is that the treatments to counteract each of these problems can be at odds with one another~ so what fixes one problem can create another. Furthermore, there are three interacting mechanisms: Corrosion in one area creates deposits; deposits are preferential sites for corrosion~ biofouling can cause both deposits and corrosion.
Finally, there are regulatory complexities when dealing with cooling water treatment, that don't enter into steam-cycle treatment concerns. Chromate, which was the corrosion inhibitor of choice for most cooling systems, is now banned in the US. And in many states, regulations mandate the use of reclaimed water, higher-cycle operation of cooling towers, or the eventual phase-out of once-through cooling systems.
Cooling towers themselves have experienced a variety of changes, many of which affect the water treatment programs. Noteworthy among the changes is the increasing use of film-type fill. In contrast to the older splash-type fill, film-type fill has much narrower passages for more intimate contact with water-which improves heattransfer capability, but increases the susceptibility to fouling.
This article covers several popular chemical treatments for cooling systems which are designed to prevent inorganic deposits, corrosion, and biological fouling. Most of this discussion will focus on the chemistry involved in recirculating cooling systems-those that use evaporation to cool the water before returning it to the plant for another cycle (Fig 1 ). Oncethrough cooling systems typically handle such large volumes of water that only minimal amounts of chemical treatment are economical. Recirculating systems are where many of the more complex chemical treatments are applied.
Chemical treatment in recirculating systems typically consists of polymers and phosphonates to prevent deposits, corrosion inhibitors for copper alloys and for steel, and one or more biocide treatments. All these chemicals need to be able to work together, each doing their job, without inhibiting the others from doing theirs.
Since cooling water treatment often constitutes the bulk of a plant's chemical treatment costs, choosing the right combination and optimizing the chemical treatment levels is important. Optimization can be thought of in three steps:

• Predicting what is needed.
• Measuring what is added.
• Monitoring performance.

Inorganic deposits
The formation of inorganic deposits is a function of the chemical composition of the makeup water, temperature of the water, and the number of cycles of concentration through the cooling tower (which in turn determine the blowdown rate). Increasing the cycles of concentration well beyond the level that is predicted to form deposits is the job of polymers and phosphonates.
Besides the dissolved species forming deposits, silt and organic material in the cooling water can also affect the chemistry. Silt can be in the makeup water or become entrained from dust in the air. It accumulates in the cooling water basin and in the condenser tubes where flow velocities and turbulence are low enough to allow it to settle. Silt not only acts as an insulator to prevent heat transfer, it also can support microbiological growth, protect it from attack by biocides, and act as a sponge for treatment chemicals-thus preventing them from doing their job.
Prevention of calciumcarbonate scale is typically the first order of business in cooling water treatment. However, depending on water source, other deposits can also be of major concern-such as calcium sulfate, calcium phosphate, and magnesium silicate.

What's needed?
Predicting when deposits will form in a dynamic cooling system is a difficult but necessary task in establishing the proper levels of treatment. Historically two common indices for predicting calcium carbonate precipitation were the Langelier Saturation Index (LSI) and the Ryzner Stability Index (RSI). Both were developed to predict the relative saturation of calcium carbonate using such readily measurable parameters as pH, calcium concentration, alkalinity, temperature, and conductivity. Both focus on calcium-carbonate precipitation only, and consider no other chemical species in the water or any interaction between these species and calcium carbonate.
A more recent scaling index is the Practical Scaling Index (PSI), which was developed by Puckorious & Associates Inc, Evergreen, Colo, using data collected from actual, operating cooling systems. Unfortunately, the PSI-like the earlier indices-is restricted to predicting calcium-carbonate precipitation only.
The latest approach uses software models, which have been developed and marketed for determining the relative saturation of all of the common cooling water deposits, not just calcium carbonate. Another benefit of software models is that they take into account a variety of subtle factors-such as the interaction between various ions in the solution that can affect the solubility. One of the most popular of these models is WaterCycle, distributed by French Creek Software, Kimberton, Pa (Fig 2).

Several standard indices predict when deposits will form, based on calcium-carbonate precipitation. Newer software packages, such as French Creek's WaterCycle, are designed to measure the relative saturation of all of the common cooling water deposits, not just calcium carbonate.
Predicting the saturation of calcium carbonate in cooling water is not the same as predicting when deposits will form. All prediction tools-including the most sophisticated software model-can only determine conditions in the bulk water. High Surface temperatures on the inner diameter of' condenser tubes and the presence of biofilm on a Surface can create localized conditions that are very different than the bulk water and therefore create deposits that otherwise would not form.

The two Ps'
Chemicals used by the industry to prevent the deposition of calcium carbonate fall into two broad classes: phosphonates and polymers. Phosphonates do not exactly prevent the precipitation of calcium carbonate, but they disrupt the formation of crystals so that they cannot adhere to each other to form hard deposits. For an analogy, think of the difference between a stack of bricks, and a pile of clay and sand. Since the phosphonates only interfere and do not react directly with the calcium carbonate, the amount required for treatment is relatively low. There are many different phosphonates used in cooling water treatment. Sorne of the most common are AMP, HEDP, and PBTC.
Similar to phosphonates, polymers can also act as dispersants and crystal modifiers. The dispersant quality of a polymer is important because it indicates the chernical's ability to keep silt and other deposits in Suspension, thus minimizing problems in the cooling tower fill. Each formulation has slightly different properties. Some are better chelants, others are good dispersants, and still others are better crystal modifiers.
The polymers used in cooling water treatments are categorized by the number of different molecules used in building the polymer chain. There are polymers made from a single compound, such as polyacrylates, and polymaleates that are in common use. Two, three, or more monomers can be polymerized together to produce compound polymers called copolymers, terpolymers, and even quad polymers. Common monomer building blocks include acrylic acid, acrylamide, maleic anhydride, and sulfonated organics.
The biggest enemy of the phosphonates are oxidizing biocides that break them down, not only preventing their proper functioning, but also stimulating algae growth. Restrictions on phosphorous discharge, already in place for some environmentally sensitive areas, are likely to become more prevalent as more waterways are designated as impaired.
One of the newest additions to the list of calcium-carbonate scale inhibitors are oligomers (very short chain-length polymers) based on succinic acid. Testing shows that they are excellent calcium-carbonate scale inhibitors, they are chlorine resistant, and-since they contain only carbon and oxygen-they don't add phosphate to the cooling water as they biodegrade.

Measuring results
The challenge with polymer and phosphonate programs has always been adding only what is needed for the changing conditions of the cooling system. In the past, testing for polymers and phosphonate residuals in the system was very difficult. The longest and most difficult analytical test at the plant was determining the total phosphate in the cooling water.
Ondeo-Nalco, Naperville, Ill, has overcome this challenge with its Trasar concept, which ties fluorescent molecules to the company's treatment products and measures the amount of product in the water with a flurometer. Other strategies have been developed that allow the plant to not only determine the amount of total product in the cooling water, but also the amount of "free" or unused chemical.

The real test to see if deposits are forming is to monitor condenser performance-either on an operating condenser or through a highly controlled model heat exchanger where the heattransfer coefficient is monitored continuously.

The two 'Cs'
Although stainless steel and titanium are becoming more Popular, the lion's share of condenser metallurgy is still accomplished with copper alloys. As a result, the prevention of copper corrosion remains a key task of cooling water treatment programs.
Primarily selected for their excellent heat-transfer properties and lower cost, the copper alloys also have biocidal characteristics because of the toxicity of copper ions. This toxicity, however, can be an issue if the powerplant discharges back to a river or take. Severe restrictions on copper in the cooling-water effluent have been placed on some plants, so minimizing copper corrosion is a priority not only to prevent tube leaks but to prevent a violation of the plant's environmental discharge limits.
Both benzotriazole (BZT) and tolytriazole (TTA) have been commonly used as yellow-metal corrosion inhibitors in cooling water (Fig 3). Azoles contain nitrogen and, like other nitrogen species-such as ammonia-the azoles complex some of the copper and create a protective film on the metal surface.
Unfortunately, oxidizing biocides most commonly, chlorine-react with the azole, thus damaging this film and creating a release of copper into the cooling water. Therefore constant application of BZT and TTA are required to maintain the protective film. Recently, new azoles have been developed that are more resistant to oxidizing biocides and create a more stable copper-azole film.

It may be stainless, but it's not perfect
Since the 1970s, many powerplants have moved away from conventional copper-alloy condensers, and begun using stainless steel and titanium tubes. The 300 series of stainless-steel alloys (typically 304 and 316) have been the most popular. But these have created problems of their own in some applications. In fact, plants that have switched to stainless steel often find that they have simply traded one problem for another.
One of the most interesting corrosion issues with stainless steel has to do with corrosion that is caused by manganese oxide-a phenomenon that causes severe pitting on the tubes. Though the mechanism is not completely understood, it appears that soluble manganese precipitates as manganese dioxide on the condenser tube surface. The manganese may be naturally Occurring in river or lake water, or in sediments. If sediments become anaerobic, the manganese in them can solubilize. The soluble manganese subsequently oxidizes and precipitates as manganese dioxide on stainless steel condenser tubes.
One possible explanation for the corrosion is that oxidizing biocides-such as chlorine-oxidize the manganese oxide to soluble permanganate. This destroys the passive layer on the stainless steel Lind creates cathodic and anodic areas that generate severe pitting corrosion.
Some researchers also theorize that biofilms themselves can concentrate manganese oxide. When the biofilm contains iron and manganese-oxidizing bacteria they can create manganeseoxide deposits on the tubing. These deposits may work in conjunction with sulfate-reducing bacteria, creating corrosion cells. Of particular concern is the pitting corrosion of stainless steel in seawater. The US Navy-which has more than a few condensers operating in seawater~--has done significant research in this area.
Measuring corrosion in cooling water is often done with coupon racks or linear polarization devices like the Corrator that can measure general corrosion and pitting. Electrochemical noise measurements (or ECN) are also finding use in measuring corrosion in cooling systems.

The two 'Bs'
Biofouling can have profound effects on the efficiency of heat transfer and corrosion in a cooling water system. The increasing use of industrial and municipal effluent as a makeup water source is great news for the bugs, as these waters can contain significant amounts of nitrogen, phosphate, and organic compounds necessary for biological growth. But it makes the job of keeping biofouling under control-the purpose of biocides-even more challenging.
It's important to remember that free-swimming or planktonic bacteria in the water do not cause microbiological fouling or corrosion. It is only when bacteria become fixed in one place (or sessile), that they become significant.
As bacteria settle on surfaces, they begin to form colonies, joined together by a gelatinous substance they manufacture called exopolysaccaride, or EPS. The bacteria and EPS form a layer called a biofilm.
Inside a biofilm, bacteria are shielded from changes in the bulk water conditions and, to a remarkable degree, shielded from attack by many common biocides. The biofilm creates its own environments including areas that are oxygen -deficient and areas of high and low pH. In an established biofilm there are many different species of bacteria, all finding or creating conditions that are optimum for their survival. The stickiness of the EPS also works to trap various nutrients and silt from the water. Biofilms may also take an active role in the formation of calcium carbonate and other deposits by providing a stable area for crystals to grow.
One aspect of biofilms that often is misunderstood is the relationship between the number and type of planktonic bacteria; that is to say, those that can be measured and counted by taking a sample of the cooling water, and the sessile bacteria involved in a biofilm. The fact is that studies have shown no correlation between the two.
As mentioned, biofilms create their own environment. Planktonic bacteria can enter a cooling system via the makeup water or drift, but they want to establish or become part of a biofilm community as soon as possible to increase their chance of survival. This misunderstanding leads people to judge a biocide's effectiveness by determining the reduction in planktonic bacteria. instead of measuring the chemical's direct effect on a biofilm.
Monitoring biofilin formation is not trivial. Systems designed for this purpose measure changes in heat transfer, pressure drop, or reduction in the intensity of light passing through a clear disk that can become fouled. At least one system, BioGeorge, has had success directly monitoring the activity of biofilms and effectiveness of biocide treatments in situ.
Biocides can be divided into two major groups, oxidizing and non-oxidizing. Oxidizing biocides include chlorine, bromine, bromochlorodimethylhydantoin-or BCDMH-chlorine dioxide, hydrogen peroxide, and ozone. Common non-oxidizing biocides include gluteraldehyde, isothiazalone, DBNPA, and methyl bisthiocyanate. Chlorine is still the most commonly used biocide. However, many powerplants in the US have abandoned their economical gaseous chlorine systems in favor of bleach or bleach/ bromide combinations. Though more expensive, the safety, regulatory, and public relations issues associated with 1-ton cylinders of chlorine were too much for most plants to bear.
When chlorine gas is added to water it reacts to create hydrochloric acid and hypochlorous acid, in equal proportions.
Bleach reacts in a similar fashion forming the hypochlorous acid as it reacts with water. The hypochlorous acid penetrates the cell membrane and is thought to interfere with the enzyme system of the cell, destroying it.
But depending on the pH of the water, hypochlorous acid can be quickly dissociated, forming hypochlorite ion, which is not as effective a biocide as the acid. If the cooling water PH rises from 7 to 8, the percentage of hypochlorous acid in chlorinated water drops from 70 to 20%, with a proportional decrease in biocide effectiveness.
Besides acting as a biocide, chlorine can react with any number of other organic and inorganic compounds. If the cooling water contains amines or ammonia, chloroamines are formed. These consume chlorine and thus increase the amount of chlorine required to produce the desired results. Chloroamines also can act as biocides; although they are not as effective as hypochlorous acid, they are less volatile and may survive longer in a cooling water.
Other than gaseous chlorine and bleach, some facilities use an electrolytic process and salt brine to directly generate the required amount of hypochlorite. Proponents of this approach claim that in addition to the hypochlorite, a variety of chemical species are generated in the electrolytic process that are effective but short- lived-such as chlorine dioxide, peroxides, and ozone that enhance the effectiveness of this treatment over a typical industrial bleach solution.
The legislated elimination of chromate as a corrosion inhibitor forced plants to raise the PH of cooling water to between 8 and 9. While better for corrosion prevention, operating at the higher pH levels made chlorination less effective.
The solution was to add bromide to the water and create hypobromous acid. While I . ust as effective a biocide as hypochlorous acid, the hypobromous acid remains primarily in its dissociated form in the more alkaline PH water. Hypobromous acid is created by a reaction of hypochlorous acid with a source of bromine, typically sodium bromide.
BCDMH is a bromine/chlorine donor than functions similarly to the chlorine/ bromide mixtures, with the exception that it is a single solid chemical. This ease-of-use is very important to some plants and outweighs the additional cost of the chemical.
Chlorine dioxide is a powerful oxidizer that is popular in industrial wastewater treatment application and some powerplants. Chlorine dioxide is unstable, and must be generated on site. In most generator systems, chlorine dioxide is produced by the reaction of sodium chlorite with chlorine gas or with bleach and hydrochloric acid. Chlorine dioxide is capable of oxidizing living organisms and organic compounds and it is not affected by the pH of the cooling water. Handling of additional wet and dry chemicals has limited its widespread use to specific applications.

Non-halogen options
For powerplants needing to get away from halogenated compounds altogether, there are oxygen-based biocides-such as hydrogen peroxide, peracetic acid, and ozone. All have a history of use in other industries such as drinking water or pulp and paper manufacturing and may have application in specific instances in cooling water. These compounds disperse very quickly and leave no environmental legacy. They tend to be very powerful but also very localized. Ozone has a low solubility in warm water, which limits its applicability in most cooling systems. Peroxide has been injected into individual cells in a cooling tower to clean film fill with very good results (POWER, May/June 200 1, p 76).

Non-oxidizing biocides
Non-oxidizing biocides tend to be used as supplements to an oxidizing biocide program-not a complete replacement because of the high cost of applying these chemicals at the levels needed. Non-oxidizing biocides also can be tuned to address a particular problem, such as blue-green algae, or macrofouling issues-such as clams.
For years the power industry has been fighting off Asiatic clams and zebra mussels and a host of native saltwater macrofouling species, which restrict the inlet structure flow and foul the waterboxes. Special paints, thermal treatments, and different chemicals have all been tried in the battle against the invaders. Simple chlorination is not particularly effective because the clams and mussels sense the chlorine and close up until the biocide has passed. Consequently oxidizing biocides require exposure periods of several days or weeks to be effective on most bivalves. However, certain non-oxidizing biocides are effective using only short exposures since they are not readily sensed by the bivalve and do not trigger an avoidance response.
Recently, a new invader has arrived the Green mussel. Native to the IndoPacific region of Asia, they were first found in the western hemisphere in 1990 in Trinidad and 1993 in Venezuela. Then in 1999 they were discovered in the inlet structures of Tampa Bay Electric Co plants. Their current range is between Tampa Bay and Charlotte Harbor in Florida. The US Geological Society (USGS) speculates that the Green mussel's range will continue to grow until limited by colder water temperatures.

Mechanical anti-fouling
Because of the environmental issues and cost associated with biocides, more plants are considering mechanically cleaning their condensers on line. In the past this meant the sponge-ball type of cleaners that were injected and captured by permanent equipment installed in cooling water piping, or brushes held in cages attached to the tube openings. The brushes make a single pass through a condenser tube when the water flow is reversed. More recently, such devices as "Sidtec rockets" have been used. They require very little equipment to inject and retrieve.
Some powerplants have used these devices periodically for 10 years, and report that they have completely eliminated the use of chemical biocides.

Acknowledgments
Thanks to Wayne Micheletti of Wayne Micheletti Inc, Charlottesvile, Va; Raymond Post of GEBetz Inc, Trevose, Pa;; and Rick Brundage of Ondeo Nalco, Naperville, Ill.



First published in Platts POWER, September 2002

ArticlesHomePrint this page