Cogeneration, Combined Cycle, Water Treatment

Keeping up with Water/Steam and Chemistry Issues at CC and Co-Gen Plants: Part 3

(Editor’s Note: This is third in a three-part series)

Part 2 of this series examined several critical aspects of steam generation chemistry for modern high-pressure units.  Equally as important is proper treatment of cooling water for those many plants that still have steam surface condensers and other water-cooled heat exchangers.  Scaling, corrosion, and microbiological fouling can cause loss of heat transfer and equipment failure, sometimes resulting in staggering costs to the plant.  And, as the figure below graphically illustrates, any of the three mechanisms can influence the others.

Figure 1.  The fundamental cooling water control triangle.

Furthermore, due in large part to environmental regulations related to protection of aquatic creatures, once-through cooling systems have, for new plant construction, virtually disappeared in favor of cooling towers, or in some cases, air-cooled condensers (ACC).  Cooling towers by their very nature increase the complexity of treatment programs due to the fact that impurities in the water “cycle up” in concentration because of evaporation from the tower.  Compounding treatment issues is that discharge regulations are becoming more stringent to reduce environmental impacts.  Often included in new permits are limits for phosphorus and metals such as zinc.  These factors have been a strong driver towards cooling water treatment evolution.

Initial Evolution – From Simple to Complex

In the middle of the last century, the method to protect cooling system components was based on chromate chemistry for corrosion protection, with sulfuric acid feed for scale control.  The chemistry is quite straightforward.  In most natural waters, and especially when cycled up in a cooling tower, the primary water chemistry issue (apart from microbiological fouling) is calcium carbonate (CaCO3) deposition.

This equation illustrates the inverse solubility of CaCO3 as a function of temperature.  Obviously, cooling water temperature rises in heat exchangers, thus making exchanger tubes or plates susceptible to CaCO3 deposition.   The acid-chromate program inhibited scaling by reaction of sulfuric acid with bicarbonate ions (HCO3) to convert the ions to CO2, which escape as gas.  A typical pH control range was at or near 6.5 to 7.0.  The second compound in the formulation, disodium chromate (Na2Cr2O7), provides chromate ions that react with carbon steel to form a pseudo-stainless steel layer that can be quite protective.

However, in the 1970s dawning recognition of the toxicity of hexavalent chromium led to a ban on chromium discharge to the environment, which essentially eliminated chromate treatment for open cooling water systems.  The replacement program was quite different, with a key concept being operation at an alkaline pH to assist with corrosion control.

Figure 2.  General influence of pH on the corrosion rate of iron.

A typical formulation includes inorganic phosphates and polyphosphates, an organic phosphate (aka phosphonate), a small polymer dosage, and often a small concentration of zinc.  Precipitation of salts such as calcium phosphate, iron phosphate, zinc hydroxide and others at anodic and cathodic sites on metals assist in corrosion control, while the phosphonates and polymers, via crystal modification and ion sequestration, help to control scale formation.  However, the multiple chemistries within these formulations can be rather complex, making control a sometimes difficult prospect.  For example, overfeed of phosphate and phosphonate can lead to excessive calcium salt deposition.  In fact, in the early days of these programs, calcium phosphate scaling became a significant problem.  Additionally, phosphonates exhibit varying degrees of susceptibility to degradation from thermal and chemical stresses, where oxidizing biocides in particular may cause component breakdown.

All these points aside, a major factor that has influenced development of new scale/corrosion inhibitor chemistry are issues related to phosphorus (and zinc) discharge.  Phosphorus is the limiting nutrient for algae growth in surface waters, and toxic algae blooms have increased in frequency and location.  Most recently, severe problems have occurred in Florida.

Figure 3.  Algae bloom infests Florida marina.  Source:  www.news.wgcu.org.

While phosphorus, and nitrogen, enter various watersheds as the byproducts of agricultural applications, point sources such as cooling tower blowdown are easier to control.  And naturally, focus has zeroed in on the phosphate/phosphonate programs that replaced chromate.  Much research has gone into development of non-phosphorus (non-P) programs, and through this research, scientists have formulated chemistries that not only are effective at scale control, but do a better job of corrosion control than phosphate/phosphonate treatment.  One such program is known as FlexPro®, which in more generic terms is known as “reactive polyhydroxy starch inhibitor” (RPSI).  Unlike the phosphate/phosphonate programs that rely on deposition of reaction products to inhibit current flow at corrosion sites, the RPSI program forms a direct barrier on the metal surface.  In one instance at a large industrial complex in the southeastern United States, RPSI replaced previous polyphosphate and then zinc chemistry. Carbon steel corrosion rates have been reduced from 0.2–0.25 mm per year to 0.0025–0.0075 mm per year.  The change from polyphosphate to zinc and then to RPSI chemistry was in large measure influenced by problems with severe algae formation in a clarifier and recycle pond at the plant. The removal of phosphate from the water eliminated the algae blooms.  The improved corrosion protection was the obvious second major benefit to the change in program.

Don’t Forget About Microbiological Control

Microbiological fouling is often the most problematic issue in cooling water systems.  Even with fresh water as the makeup source, fouling can occur rapidly, but now many plants are utilizing alternative, lower-quality water for plant makeup.  Supplies such as secondary wastewater treatment plant effluent have the potential to introduce large quantities of both nutrients (nitrogen species and phosphorus) and food (organic compounds) to cooling water, thereby increasing the potential for fouling.  If organisms settle on heat exchanger and other cooling system surfaces, some will produce a protective slime layer.  Other organisms will then grow and multiply underneath this layer to form very complex microbiological colonies.  The colonies and the slime layer collect silt and other particulates from the water to generate deposits that greatly restrict heat transfer and can also lead to under-deposit corrosion and microbiologically-influenced corrosion (MIC) of metals.

Figure 4.  Microbiological slime and “mud” on the tubesheet of a heat exchanger.

For decades, some form of oxidizing biocide has constituted the core treatment for microbiological control.  Gaseous chlorine was once the common choice due to low cost, but safety concerns influenced movement towards safer compounds such as liquid bleach.  However, because many makeup water supplies are mildly alkaline, and the most common cooling tower treatment programs also operate in a basic pH range, chlorine can be rendered somewhat ineffective.  Compounds such as monochloramine (NH2Cl) and monobromamine (NH2Br) have exhibited strong results in attacking settled microbiological colonies.  The efficiency appears due to the fact that being weaker oxidizers than chlorine or bromine, they are not consumed by the slime layer but rather penetrate to the microorganisms underneath to kill them.  Improvements also continue with regard to stabilized oxidizer feed.  In this chemistry, a stabilizing compound or compounds allows gradual release of the oxidizers.  The formulation is often used in conjunction with a biodispersant/surfactant that disrupts the protective slime layer and allows the oxidizer to better contact the microorganisms underneath.

A key point with any program, whether it be control of scale/corrosion or microbiological fouling is to have the system in place and operating properly at system startup.  Once deposits form, they can be very difficult to remove without some type of mechanical cleaning or other laborious procedure.  One aspect in this regard has been development of traced chemistries that allow sensors to accurately measure chemical concentrations in the circulating water and in the chemical feed stream to the cooling system.  Also, instrumentation for monitoring standard operating parameters is much improved, and allows for accurate, continuous, on-line readings of critical data, including:

·         pH

·         Specific conductivity

·         Oxidation-reduction potential (ORP) and/or residual oxidizing biocide

·         Corrosion rate

·         Scaling potential

·         Biofouling potential

·         Tank levels

Figure 5.  A compact cooling water monitoring unit.

The unit shown above contains the necessary instruments to accurately monitor important aspects of cooling water chemistry.  It includes corrosion coupons, instrument probes, flow regulators, and other devices.

Part I of the series on crucial water/steam chemistry issues.

About the author: Brad Buecker is Senior Technical Publicist with ChemTreat. He has 35 years of experience in or affiliated with the power industry, much of it in steam generation chemistry, water treatment, air quality control, and results engineering positions with City Water, Light & Power (Springfield, Illinois) and Kansas City Power & Light Company’s La Cygne, Kansas station. Most recently he was a technical specialist with Kiewit Engineering Group Inc.  Buecker has a B.S. in chemistry from Iowa State University with additional course work in fluid mechanics, energy and materials balances, and advanced inorganic chemistry.  He may be reached at [email protected].