By Brad Buecker, Contributing Editor
The Second Law of Thermodynamics clearly states it is impossible to use heat solely to generate work in a cyclic process. Even in the most efficient cycles, some heat must be exhausted for the cycle to operate. In power plants, that means the primary heat sink is the water-cooled or air-cooled steam condenser. Similar heat exchange is often required at large, non-utility plants as well. Waterside scaling or fouling seriously impedes heat transfer in condensers, and can literally increase large plant fuel costs by hundreds to thousands of dollars per day.1 This article touches upon some of the important fundamentals of cooling water treatment.
Except along the coasts, the typical makeup water for cooling systems comes from a lake, river or, sometimes, groundwater for plants with recirculating cooling systems; that is, systems with cooling towers. Regardless of the makeup source, at plants with cooling towers the transfer of heat from the warm condenser water is primarily due to evaporation in the tower. Of course, this causes the concentration of impurities in the bulk water to cycle upwards. Blowdown is the common method to prevent excess dissolved solids buildups. But by itself blowdown typically cannot prevent scale formation, as several compounds will exceed saturation level and precipitate on condenser tubes and other system internals. Some of the most common scale-forming compounds, along with their heat transfer coefficients, are shown on page 52, along with the coefficients for several common condenser tube materials.
Table 1 shows how insulating the typical hardness compounds as compared to the metals they may coat. (The table also shows why copper alloys were once almost universally selected for condenser tube material: Thick-walled tubes could be used for structural integrity, but excellent heat transfer was still possible.) Conversely, other foulants such as silica compounds and slime from microbiological deposits can be even worse than the hardness deposits regarding heat transfer.
In the absence of any treatment programs, the primary scale-forming reaction produces calcium carbonate deposits.
Ca+2 + 2HCO3– + heat → CaCO3+ CO2 + H2O
Unlike many salts, calcium carbonate is inversely soluble with temperature. Thus as temperatures rise in a condenser or heat exchanger, the potential for CaCO3 scaling increases.
A straightforward and often used method to reduce the potential for calcium carbonate scaling is to treat the makeup with sulfuric acid (H2SO4), where the hydrogen ions (H+) from the acid convert the bicarbonate to carbon dioxide (CO2). The gas exits from the cooling tower.
H+ + HCO3– → CO2 + H2O
However, acid feed alone may not eliminate scaling. Upsets of acid feed can lead to corrosion or scaling, and sulfuric acid introduces sulfate ions (SO4-2) to the water. Over time, this may lead to calcium sulfate deposition. Accordingly, supplemental chemicals are normally used in cooling tower programs to further minimize scale formation.
An early treatment method was feed of phosphates, either ortho-phosphate (PO4-2) or phosphate complexes that would revert to ortho-phosphate, to precipitate calcium as [Ca3(PO4)2]. These programs also provided corrosion protection because phosphate will react with ferrous ions (Fe+2) produced at anodic sites to form a protective barrier, while [Ca3(PO4)2] precipitates in the local alkaline environment at cathodic sites. Zinc was a common corrosion protection supplement, as zinc ions will also precipitate (as zinc hydroxide [Zn(OH)2] at cathodic sites) thus enhancing the barrier film.
However, even small upsets in phosphate programs can cause severe calcium phosphate fouling. Accordingly, treatment methods evolved to more forgiving methodologies, where, in many cases, the backbone of these programs is an organic phosphate or phosphates (phosphonates) with a supplemental polymer to sequester and modify the crystal structure of scale-forming ions and compounds.
Phosphonates primarily function as crystal modifiers and attach to crystals as they form, and thus alter their structure and reduce the crystals’ adhesiveness to condenser tubes and other surfaces.
The phosphonates shown in Figure 1 (and their cousins) have been around for years and have worked well in a large number of applications, although excess phosphonate feed can result in calcium-phosphonate scale formation and fouling of heat transfer surfaces.
Figure 1 Two common phosphonates, HEDP and PBTC, for cooling water treatment
Also, many of the early phosphonates are susceptible to rapid degradation by oxidizing biocides such as chlorine, bromine, chlorine dioxide and so on. A newer phosphonate, phosphino succinc oligomer (PSO), offers excellent calcium carbonate scale inhibition combined with good resistance to degradation. Often in phosphonate programs, a few parts-per-million (ppm) of ortho-phosphate [PO4] are used for both cathodic and anodic corrosion inhibition, as outlined earlier. However, at the higher pH of these treatments, a strong possibility exists for calcium phosphate [Ca3(PO4)2] scale formation. Thus, a small dosage of organic polymer is frequently used in the formulations to keep calcium phosphate in suspension, such that it leaves the cooling system in the blowdown. Modern treatments may use or rely on co- or ter-polymersin which active groups include carboxyl and sulfonic acid ions (COO- and SO3-, respectively) and amide OC-NH2. These products are designed to control a variety of sales. A thorough evaluation of the cooling water is necessary when considering and implementing any of these programs.
Some facilities will not allow the use of phosphate-containing compounds, either inorganic or organic, due to the fact that phosphate is a nutrient for microbes, including algae. In these circumstances, an all-polymer program (perhaps with acid treatment of the makeup) may be the only alternative to maintain reasonable cycles of concentration in the tower. Polyacrylate and related compounds (polymers with COO- groups) can be effective at controlling CaCO3 deposition in these situations. In once-through systems, a well-monitored biocide program along with a polyacrylate can often keep condenser tubes in good condition through the warm weather months.
Microbiological Fouling Control
In my 20 years of direct experience at two coal-fired power plants and a manufacturing facility, the most common cause of reduced heat transfer in condensers and auxiliary heat exchangers was microbiological fouling.
Cooling systems provide an ideal environmentwarm and wetfor microbes. Bacteria will grow in condensers and cooling tower fill, fungi on and in cooling tower wood and algae on wet 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: Use oxygen in the metabolic process
- Anaerobic: Live in oxygen-free environments and use other sources, for example, sulfates, nitrates, or other donors for their energy supply
- Facultative: Can live in aerobic or anaerobic environments.
A problem with microbes, particularly many 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 any microbiological treatment program 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.
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.
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. The major difficulty with bleach is that the product contains a low concentration of sodium hydroxide, thus when it is injected into the cooling water stream it raises the pH, if by only a small amount.
A popular 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.
Another factor in favor of bromine is that it does not react irreversibly with ammonia or amines. The primary disadvantages are that an extra chemical is needed and feed systems are a bit more complex than for bleach alone.
Chlorine dioxide (ClO2) has found some application as an oxidant for two primary reasons. Its killing power is not affected by pH, and it does not form halogenated organic compounds. Also, chlorine dioxide is more effective in attacking established bio-deposits. However, ClO2 is unstable and must be generated on-site, a common method being reaction of sodium chlorite (NaClO2) and chlorine in a slipstream fed to the cooling water.
Costs are usually several times higher than for straight halogen treatment.
Other oxidants that have been tested for cooling water include hydrogen peroxide (H2O2) and ozone (O3), but the short lifespan and tendency of these chemicals to escape from solution in the cooling tower typically make them ineffective in large cooling water systems.
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 2 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.
1. B. Buecker, “Steam Generation Thermodynamics 101”; Power Engineering, pp. 106-114, Vol. 112, No. 11, November 2008.
2. Properties of Some Metals and Alloys, The International Nickel Co., 1982.
Author: Brad Buecker is a contributing editor for Power Engineering.
Temporary Fabric Structures
By Beth B. Wilson, Mahaffey Fabric Structures
The need for temporary space is at an all-time high in today’s rapidly changing environment. That need is increased by weather delays and ever greater demands for worker productivity that can impact a plant’s bottom line.
Whether due to frigid temperatures in northern climates or the extreme heat in the South, working conditions can be the key factor in deadline setbacks. Productivity is boosted when the work can be completed in conditions made comfortable by heating or air conditioning, in shaded environments, and without the threat of inclement weather.
Warehouse and storage space at AEP power plant in Fulton, Arkansas. Photos courtesy of Mahaffey Fabric Structures
Problems brought about due to extreme weather conditions can be addressed by keeping employees sheltered from whatever Mother Nature throws your way. Temporary, portable fabric structures can help a company continue generating revenue during plant improvement projects.
Furthermore, power plants generally lack lunch and dining facilities, which can also factor into workers efficiency. When employees have to completely stop what they’re doing, get in their vehicles and drive to find a place to eat, the workday becomes increasingly shorter. Temporary tents and structures are the ideal solution for on-site dining facilities. Dining facilities can be built to serve a few dozen persons to more than 3,000. Temporary fabric structures can be equipped with such optional features as climate control, insulated steel hard sides, and an array of flooring options. Mahaffey can even provide full-service food preparation and cooking areas, as well as on-site catering personnel.
Because they can be easily transported to various locations when needed, these structures are economical. Speaking of economics, money to build additional storage and warehouse space is scarce these days. Temporary structures serve as an economic alternative to steel storage buildings, which can be a significant advantage today.
Not only can these structures protect workers from the elements and provide a comfortable dining area, they also present a way to control interior temperature and humidity. Large clear span structures can be outfitted with flooring, lighting, personnel or roll-up doors, tables and chairs for dining, multiple HVAC units, and power distribution via the grid or on-site sources.
Another potential cost saving can be achieved by installing a translucent fabric roof to let in natural light, saving the expense of an elaborate lighting system. To retain heat or cool air inside, install insulated hard sides for optimal climate control, regardless of what the outside temperature is.
Mahaffey’s Mega Series structures are built to withstand extreme winds of up to 120 miles per hour, combined with snow loads of 30 lb/square feet. The Tension Series, or MTS, structures are fully-engineered to meet and exceed snow and wind loads, while also having a superior snow-shedding capability for those in snow country.
Mahaffey is currently providing two large structures for Shaw Power Group, who was awarded a contract by Southwestern Electric Power Company (SWEPCO), a unit of AEP for its 600 MW power plant in Fulton, Arkansas. Shaw is using two 100’ X 150’ structures as temporary warehouses for storage of equipment and parts needed for the project. A third structure is being used as a dining facility for workers.
Mahaffey also recently provided a large warehouse facility for Bechtel. The warehouse was needed for six to eight months. Mahaffey provided five full-engineered 10m dining facilities for the plant for Fluor at a ConocoPhillips Refinery in Roxana, Illinois. Nine temporarily fabric structures were eventually used on the project. The project is set to be up for more than two years. Mahaffey is also providing two large structures for S&B Engineers in Houston, Texas, at their Petro Logistics Plant. The first, a 66’ X 115’ structure is being used as a warehouse facility. The second a 100’ X 66’ structureis being used as an on-site lunch tent.
Beth B. Wilson is the Marketing Manager for Mahaffey Fabric Structures.