By Brad Buecker
Combined cycle power plants have become in large measure the de facto replacement for the many coal plants that are being retired. A particular aspect of these new plants is that they are often minimally staffed with few if any personnel who are well trained in water/steam chemistry. Yet, failures due to inadequate water treatment and chemistry control, whether they be in the steam generator or cooling system, can cost a plant huge amounts in lost revenue and equipment repair if they cause failures and forced outages. With respect to cooling, no longer are plants being built with once-through cooling systems but rather the choice is either a cooling tower or air-cooled condenser. This article focuses on the former. Cooling system treatment programs have evolved substantially over the last several decades, and this evolution continues, but with increasing complexity.
Basic Cooling Tower Flow Path – 1
Cooling Tower Heat Transfer Basics
The figure below illustrates the fundamental flow path of a cooling tower, and indicates one set, of an obviously vast number, of conditions that are possible with regard to temperature and moisture content of the process streams. The data are included to outline important concepts of a cooling tower.
Note the large change not only in the temperature of the air moving through the tower but the relative humidity (RH). Typically, 65 to 85 percent of the cooling in a tower comes from evaporation of a small portion of return water to the tower. The remainder is by sensible heat transfer. While rather detailed mathematics and engineering are required to properly select the size and air-to-water flow ratio (more commonly known as the liquid-to-gas [l/g] ratio) of a cooling tower, several common equations are very functional for determining operating aspects of a system. Consider the example above, where we will select an evaporation ratio of 75 percent, which in the following equation is converted to decimal format as an evaporation factor (ƒ).
E = (R * ƒ * Range)/1000, where
E = Evaporation (gpm)
R = Circulating water rate (in this example, 150,000 gpm)
ƒ = Evaporation factor (in this example, 0.75)
Range = Temperature difference between hot return water and cold discharge (27oF)
So, the evaporation rate in this example is 3,037 gpm, which is approximately 2 percent of the circulating water rate.
As water evaporates in a cooling tower, dissolved solids (and suspended solids) remain behind. These solids concentrate and increase the scaling, corrosion, and fouling potential of the water. Chemical treatment programs are based on controlling all three of these mechanisms, which we will soon examine after first considering additional fundamental flow calculations.
Even with the best chemical treatment program, impurities in the cooling tower can only be allowed to accumulate to a certain level before the scaling potential overwhelms the chemical treatment. The amount to which the impurities are allowed to concentrate over those in the cooling system makeup water is known as the cycles of concentration (COC). The COC is controlled by periodic blowdown of a fraction of the cooling water, whose volume is replenished with makeup water. The relationship of blowdown to evaporation and COC is shown by the following equation.
BD = E/(COC – 1)
Blowdown Rate vs. COC – 2
A very small amount of water, often negligible with regard to these calculations, leaves the tower fan exhaust in the form of water droplets. This loss is known as drift (D), and can be considered as a very minor continuous blowdown. So, adding this factor to the equations above, the total makeup (MU) to a cooling tower is represented by the following equation.
MU = E + BD + D
In cooling towers with well-engineered mist eliminators, a common design drift rate is 0.0005% of the circulating water flow rate. So, for our example drift would amount to approximately 1 gpm, which is indeed negligible.
An important issue with regard to blowdown is that as COC increases the blowdown volume decreases inversely, as is shown in the following chart.
Fig. 2. Blowdown rate vs. COC for the example outlined above.
As is evident, the “law of diminishing returns” applies as the COC increases. A common COC range is 4 to 8. But if the tower is located in an arid environment or blowdown quantity is restricted, a higher COC may be mandated. However, this comes at a cost, as the higher COC means a greater concentration of dissolved solids in the cooling water, which increases the scaling and corrosion potential.
Modern Chemistry Control
In towers supplied by fresh water, and in the absence of any treatment, almost always the first scale to form would be calcium carbonate (CaCO3). Calcium ions (Ca2+) love to combine with bicarbonate ions (HCO3-), especially as temperatures rise in condensers and other heat exchangers.
Ca2+ + 2HCO3- + heat → CaCO3↓ + CO2↑ + H2O
So, in the middle of the last century a very common treatment program was utilized, which addressed both scaling and corrosion via a two-chemical process. The first step was feed of sulfuric acid to maintain a cooling water pH within a range of about 6.5 to 7.0. Acid converts bicarbonate to carbon dioxide, which of course escapes as a gas. This was coupled with feed of sodium dichromate to the water. Chromium forms a surface layer on carbon steel and gives it stainless steel-like qualities. In common vernacular, this program was almost a “no-brainer,” although upsets in acid feed could and did cause serious corrosion.
Two Common Phosphonates – 3
Unfortunately, chromate-based treatment can generate hexavalent chromium (Cr6+), which is toxic. The method essentially disappeared for all cooling systems, whether open or closed. The popular replacement until recently has been alkaline-based treatment, primarily relying on inorganic and organic phosphates (phosphonates), with a supplemental polymer to sequester and modify non-carbonate scale-formers, and perhaps a low dosage of a zinc salt.
At the typical pH range of these programs, low- to upper-8 range, corrosion is reduced, but also the various chemicals inhibit the corrosion reactions that occur at anodic and cathodic sites in metal.
Fig. 4. The effect of inhibitors on corrosion. Source: Reference 1.
The Effect of Inhibitors on Corrosion – 4
Common Polymer Building Blocks and Functional Groups – 5
Fouled Cooling Tower Film Fill – 6
The chemistry minimizes calcium carbonate formation, but unfortunately can induce calcium phosphate scaling without careful control. Prevention of this scale is a primary reason why a polymer is usually part of the chemical blend.
The major water treatment chemical companies have developed sophisticated monitoring and feed programs for the phosphate/phosphonate programs, but a new problem has arisen. Many receiving bodies of water in the country are now considered phosphorus-impaired due to the influence of phosphorus on the generation of toxic algae blooms. Thus, new plants may not be allowed to discharge any phosphorus-containing streams to these water bodies. Emerging are all-polymer programs, in which the polymer blends act as crystal modifiers and sequestering agents to keep scale-forming ions and crystals in suspension. The figure below illustrates the active sites on many of these polymers.
When choosing a cooling water treatment program, modeling software can be of great benefit. French Creek Software is a leader in this technology, and many of the major water treatment chemical vendors utilize the software for their programs.
Control of Microbiological Fouling
While scaling and corrosion are very important concerns in cooling systems, microbiological fouling often by far causes the most problems.
Cooling systems provide an ideal environment, warm and wet, for microbes to grow and establish colonies. Bacteria will grow in condensers and cooling tower fill, fungi on and in cooling tower wood, and algae on wetted cooling tower components exposed to sunlight. Biocide treatment is absolutely essential to maintain cooling system performance and integrity.
Bacteria are separated into the following three categories,
- Aerobic: Utilize oxygen in the metabolic process.
- Anaerobic: Live in oxygen-free environments and use other sources, i.e., sulfates, nitrates, or other donors for their energy supply.
- Facultative: Can live in aerobic or anaerobic environments.
Dissociation of HOCl as a Function of pH – 7
A problem with microbes, particularly bacteria, is that once they settle on a surface the organisms secrete a polysaccharide layer for protection. This film then will collect silt from the water, thus growing even thicker and further reducing heat transfer. Even though the bacteria at the surface may be aerobic, the secretion layer allows anaerobic bacteria underneath to flourish. These bugs in turn can generate acids and other harmful compounds that directly attack the metal. Microbial deposits also establish concentration cells, where the lack of oxygen underneath the deposit causes the locations to become anodic to other areas of exposed metal. Pitting is often a result, which can cause tube failure well before the expected lifetime of the material.
Fungi will attack cooling tower wood in an irreversible manner, which can eventually lead to structural failure. Algae will foul cooling tower spray decks, potentially leading to reduced performance and unsafe working locations.
The core of most microbiological treatment programs is feed of an oxidizing biocide to kill organisms before they can settle on condenser tube walls, cooling tower fill, and other locations. Chlorine was the workhorse for many years, where when gaseous chlorine is added to water the following reaction occurs.
Cl2 + H2O ⇔ HOCl + HCl
HOCl, hypochlorous acid, is the killing agent. The functionality and killing power of this compound are greatly affected by pH due to the equilibrium nature of HOCl in water.
HOCl ⇔ H+ + OCl-
OCl- is a much weaker biocide than HOCl, probably due to the fact that the charge on the OCl- ion does not allow it to penetrate cell walls. The killing efficiency of chlorine dramatically declines as the pH goes above 7.5. Thus, for the common alkaline scale/corrosion treatment programs, chlorine chemistry may not be efficient.
Chlorine demand is further affected by ammonia or amines in the water, which react irreversibly to form the much less potent chloramines. Due to safety concerns, liquid bleach (NaOCl) feed has replaced gaseous chlorine at many facilities.
A common alternative is bromine chemistry, where a chlorine oxidizer and a bromide salt, typically sodium bromide (NaBr), are blended in a makeup water stream and injected into the cooling water. The chemistry produces hypobromous acid (HOBr), which has similar killing powers to HOCl, but functions more effectively at alkaline pH.
Chlorine dioxide (ClO2) is becoming more popular for several reasons. Its killing power is not affected by pH, the chemical does not react with ammonia, and it does not form halogenated organic compounds. Also, chlorine dioxide is more effective in attacking established bio-deposits.
ClO2 is unstable and must be generated on-site. In the past, a common method was reaction of sodium chlorite (NaClO2) and chlorine in a slipstream fed to the cooling water.
2NaClO2 + Cl2 ® 2ClO2 + 2NaCl
However, this technique required storage of large quantities of hazardous chemicals, and was several times more expensive than bleach or even bromine treatment. Much improved technology is now available, with one design based on the following chemistry.
NaClO3 + 1/2 H2O2 + 1/2 H2SO4 → ClO2 + 1/2 O2 + 1/2 Na2SO4 + H2O
Sodium chlorate (NaClO3) is the core chemical rather than sodium chlorite.
A method to help control microbes is a supplemental feed of a non-oxidizing biocide. Typically, feed is needed on a temporary but regular basis, perhaps once per week. Table 1 outlines some of the properties of the most common non-oxidizers.
Careful evaluation of the microbial species in the cooling water is necessary to determine the most effective biocides. None of these chemicals should be used or even tested without approval from the appropriate regulating agency. They must fit in with the plant’s National Pollutant Discharge Elimination System (NPDES) guidelines.
As with all chemicals, safety is an absolutely critical issue when handling the non-oxidizers. Adherence to all handling guidelines and use of proper personal protective equipment is a must. Many of these chemicals will attack human cells as well as those of microbes.
Brad Buecker is a process specialist in the Process Engineering and Permitting group of Kiewit Engineering and Design Company.