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POWERGEN Updates
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By Brad Buecker, SAMCO Technologies
Introduction
Renewable resources now provide a significant portion of U.S. electricity needs. However, issues related to renewable energy storage capacity, a projected large nationwide load increase (much of it due to data center growth), grid stability, political influences, and others will continue to require traditional power generation. Some utilities are delaying the decommissioning of remaining coal plants, but new simple- and combined cycle power generating units are expected to fill much of the need for dispatchable generation. Additionally, support for small modular nuclear reactors appears to be gaining momentum. While no crystal ball exists to calculate the exact pathway of these developments, it is apparent that high-purity makeup water production will remain a critical process for much of the power generation industry. And, of course, purified makeup is required for many other industries including cogeneration, semi-conductor, pharmaceutical, the list goes on and on.
Kevin Clark and his POWERGEN staff recognize the importance of these issues and are working to reintegrate water/steam treatment topics into the conference. At POWERGEN 2025, he offered a an O&M Knowledge Hub slot for a colleague and me to discuss modern makeup water treatment methods for combined cycle heat recovery steam generators (HRSGs) and cogeneration units.1 The positive response to our presentation signaled that a significant group of POWERGEN attendees recognized the importance of well-controlled HRSG water/steam chemistry. Outages from chemistry-induced failures can be enormously expensive and, in some cases, threaten employee safety. This two-part series highlights a number of important topics from the presentation, and it provides additional insights regarding modern makeup water treatment methods.
Makeup Water Parameters
High-pressure steam generators for power production require makeup water with low part-per-billion (ppb) concentrations of impurities. A common makeup water treatment effluent guideline is shown below.
To understand how this purity may be obtained, let us briefly examine the evolution of treatment technology over the decades, in a discussion that also offers practical information for those Power Engineering readers at co-generation and industrial steam plants
As power boiler technology progressed and boiler size/efficiency increased from the 1920s into the middle of the last century,2 the development of synthetic ion exchange resins greatly improved high-purity water production capabilities.
Figure 1. Closeup photo of ion exchange resin beads. This Photo by Unknown Author is licensed under CC BY-SA. One of the first applications (which continues to this day) of ion exchange was basic sodium softening for lower-pressure steam generators, as these units can tolerate moderate concentrations of most dissolved ions, apart from the hardness ions, calcium and magnesium. Figure 2 below is an extract taken from the recent revision of the American Society of Mechanical Engineers (ASME) industrial boiler water guidelines.3 (The complete guidelines are available from the ASME at very reasonable cost and should be in the library of any industrial plant with steam generators.)
Figure 2. Data extracted from Table 1, Reference 3 – “Suggested Water Chemistry Targets Industrial Water Tube with Superheater.” Note the very low feedwater hardness limits for all cases. The most common backbone for ion exchange resins is polystyrene cross-linked with divinylbenzene. The typical active group is the sulfonate molecule (SO3–). Figure 3 outlines the basic structure of softener resin, where, in this case, each active site contains an exchangeable sodium ion. Just one resin bead may have up to 4 x 1019 active sites.4
Figure 3. Basic schematic of the IX molecular structure showing cross-linkage of polystyrene with divinylbenzene along with active sites containing sodium ions. The fundamental exchange chemistry is:
Figure 4. Basic softener reactions. The “R” in R-SO3-Na+ symbolizes the organic backbone of the resin beads. Note the equilibrium reaction arrows that indicate greater attraction (affinity) of the dissolved ions, and especially the divalent cations, Ca2+, Mg2+, to the resin over sodium. As a softener processes water, the resin accumulates hardness and other cations until reaching exhaustion. The standard procedure is to take the unit out of service, followed by backwash and then regeneration with a brine solution. The concentrated brine drives the exchange reaction in the opposite direction to restore most of the resin capacity for the next run.
Figure 5. Multiple softener vessels with feed/backwash piping that provide redundancy for softened water production. A modern sodium softening configuration is shown in Figure 5.
Sodium softening is still very common for lower-pressure (≤600 psi) industrial steam generators. Systems frequently include a downstream forced draft decarbonator or dealkalizing unit to remove bicarbonate alkalinity (HCO3-), which otherwise can decompose to carbon dioxide in the boiler and carry over with steam. The CO2 will then dissolve in condensate to lower the pH and induce corrosion of carbon steel condensate return piping.
It has been this author’s direct experience, combined with accounts from numerous water treatment colleagues, that industrial plant personnel too often focus on process engineering and chemistry to the neglect of softener operation and maintenance. This oversight leads to hardness carryover and scale formation in boilers, with tube failures a frequent outcome.
Figure 6. Boiler tube bulges and blisters caused by internal scale formation and overheating.5 With this background information in place, we will now examine makeup treatment developments for high-pressure utility steam generators.
High-Pressure Makeup Treatment
In the middle of the last century, ion exchange became the “go-to” process for producing high-purity makeup. While various IX system configurations emerged, the most popular design became:
Strong Acid Cation (SAC) –> Strong Base Anion (SBA) –> Mixed Bed
Figure 7 outlines the basic structure of SAC resin. The reader will note the same organic backbone as sodium softening resins, but with hydrogen ions (H+) replacing sodium ions at the active sites
Figure 7. Basic schematic of SAC resin. Cations exchange with hydrogen ions. The mildly acidified effluent flows to the SBA vessel, in which (the somewhat more complex) active groups hold hydroxide ions (OH–). Here, the anionic impurities are exchanged. The final net reaction is:
H+ + OH– –> H2O (1)
This basic two-step process can remove most dissolved mineral ions from the feed:
However, SAC/SBA alone cannot produce water of the purity shown in Table 1, so a mixed-bed polisher became standard for many systems. As the name implies, the MB vessel contains intermingled SAC and SBA resins.
As with sodium softeners, typical for these early units (and some remaining systems today) was on-site regeneration. Common regenerant concentrations are:
· SAC: Concentrated sulfuric acid (93%-98%) diluted to 4%. Sometimes a multistage regeneration process, starting off at say 2% acid concentration, may be needed to prevent calcium sulfate (CaSO4) precipitation in the resin.
· SBA: Concentrated sodium hydroxide (50%) diluted to 4%.
Regeneration is usually performed within the ion exchange vessel, where accurate regenerant concentration and flow rate measurements are necessary to expose the resins to the required regenerant mass. (A typical specification has units of lb/ft3 or metric equivalents.) One major design improvement was evolution from co-current to counter-current regeneration, which significantly improved the efficiency of the process. Space limitations prevent an in-depth discussion, but details are available in Reference 6. This reference also discusses mixed-bed regeneration, where the density difference between cation and anion resins induces, via water sluicing, resin separation, whereupon each can be regenerated and then remixed.
As suggested above, ion exchange was a marvelous advancement for producing the makeup required for high-pressure steam generators. However, even with relatively pristine waters as the feed source, IX train run times may be limited to a few hours before regeneration is required. Short run times can be problematic with regard to operational flexibility, regeneration chemical costs, and personnel safety in handling dangerous chemicals. (Properly located and well-maintained safety shower stations and other safety equipment are top priority.) The maturation of membrane technologies, and especially reverse osmosis (RO), have greatly changed the landscape of makeup water treatment, as we will now review.
Reverse Osmosis to the Rescue
As RO technology evolved and matured through the middle and end of the last century, water treatment experts realized that retrofitting a reverse osmosis unit ahead of an IX demineralizer could greatly extend IX resin run lengths and reduce chemical usage.
Most modern RO systems are of the spiral-wound design in which the membrane along with spacer sheets are wrapped around a perforated core. Each assembly is known as an “element”.
Figure 8. Cutaway view of an RO element. Common element dimensions are 8” in diameter by 40” in length. The surface area of a single membrane has increased from an original 365 ft2 design to 400 ft2 or even greater. Multiple elements (typically five or six per pressure vessel) are assembled in series, with multiple pressure vessels needed for normal industrial applications.
Figure 9. A compact, skid-mounted RO. Source: SAMCO Technologies. Feed enters the front end of each element and passes along the feedwater carrier, where the feed pressure forces water through the membrane. This process is known as crossflow filtration. The purified water (permeate) flows to the central core, while the increasingly concentrated feedwater (known as concentrate or reject) exits the element. Brine seals prevent feedwater from short-circuiting the elements, and anti-telescoping devices inhibit the water pressure from pushing the membrane and spacer sheets out of the element.
A key aspect of crossflow filtration is that the reject impurity concentration continually increases as the feed passes along the elements. This concentration increase has a large influence on membrane performance and selection of scale control chemistry, as will be discussed in Part 2.
A fundamental, single-pass RO configuration is shown below.
Figure 10. Schematic of a single-pass, two-stage RO unit, where the first stage reject is treated in a second stage. The second stage reject goes to waste. For normal surface and ground waters, each stage will produce approximately 50% purified water (permeate) and 50% reject. Thus, the overall feed-to-permeate conversion of a standard two-stage RO is 75%. Just half the membranes are needed for the second stage as for the first stage.
Modern RO membranes can remove over 99% of dissolved ions. Membrane flux rate (gallons per square foot per day or metric equivalents) is a critical design factor for the number of membranes and pressure vessels required per application. Flux rates are influenced by feed water chemistry. Reference 7 offers additional details.
For high-pressure steam generators and other high-purity water applications, the two-pass RO design shown below is common.
Figure 11. A basic two-pass RO schematic, where each pass has two stages. In the two-pass configuration, the permeate from the first pass is further treated in the second pass to produce water of even greater purity. Note that the second-pass reject is of high enough quality that it can be recycled to the RO inlet rather than discharged to waste. Also note the (typically small) caustic feed to the first pass permeate stream. Caustic will convert any free CO2 in the permeate to bicarbonate alkalinity, which is then removed in the second pass.
At many coal-fired power plants in the 1980s and 1990s, including both of this author’s former plants, management installed RO ahead of existing demineralizers. But now, as combined cycle power generation has replaced coal, membrane technologies are frequently included in the makeup system design phase, with ion exchange serving in a polishing role. Figure 12 outlines a fundamental configuration.
Figure 12. A now common makeup design for combined cycle power plants, with micro- or ultrafiltration for upstream suspended solids removal, RO, and portable mixed-bed “bottles” for RO permeate polishing, where an outside contractor regularly swaps out an exhausted bottle and regenerates the resin off-site. Less common but popular at some facilities is electrodeionization (EDI) for polishing. Look for additional discussion on EDI in a future article. Critical for reliable operation of RO units is well-designed and operated pre-treatment, with conscientious monitoring of process chemistry and physical parameters. We will explore these issues in Part 2, along with the often overlooked need of having comprehensive raw water chemistry data as the backbone of makeup system design.
Conclusion
This article and the upcoming second part of the series serve as a follow-up to a well-received POWERGEN 2025 O&M Knowledge Hub presentation. POWERGEN 2026 (January 20-22, 2026, San Antonio, Texas) promises to expand the discussion of steam generation, cooling water, and makeup water treatment technologies. Many new personnel are being exposed to these issues and need good sources of information to reliably operate their plants.
Disclaimer
This article offers general information and should not serve as a design specification. Every project has unique aspects that must be individually evaluated by experts from reputable water treatment and engineering firms. Also, any issues that could potentially have an environmental influence, for example, wastewater discharge from a proposed makeup, process, or cooling water treatment system, must be presented to and approved by the proper environmental regulators during the project design phase.
References
1. Buecker, B., and E. Sylvester, “Modern Makeup Water Treatment Methods for Combined Cycle, Co-Gen, and Other Energy Industries”; O&M Knowledge Hub, POWERGEN25, February 12, 2025, Dallas, Texas.
2. Kitto, J.B., and S.C. Stultz, eds., Steam/its generation and use. 41st edition, The Babcock & Wilcox Company, Barberton, Ohio, 2005.
3. Consensus on Operating Practices for the Control of Feedwater and Boiler Water Chemistry in Modern Industrial Boilers, The American Society of Mechanical Engineers, New York, NY, 2021.
4. Personal conversations with Ed Sylvester, ChemTreat.
5. S. Shulder and B. Buecker, “Combined Cycle and Co-Generation Water/Steam Chemistry Control”; pre-workshop seminar to the 40th Annual Electric Utility Chemistry Workshop, June 7-9, 2022, Champaign, Illinois.
6. D. Owens, Practical Principles of Ion Exchange Water Treatment, Tall Oaks Publishing, Littleton, Colorado, 1995.
7. W. Byrne, Reverse Osmosis: A Practical Guide for Industrial Users, Tall Oaks Publishing, Littleton, Colorado, 2002.
About the Author: Brad Buecker currently serves as Senior Technical Consultant with SAMCO Technologies. He is also the owner of Buecker & Associates, LLC, which provides independent technical writing/marketing services. Buecker has many years of experience in or supporting 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 (now Evergy) La Cygne, Kansas, station. Additionally, his background includes eleven years with two engineering firms, Burns & McDonnell and Kiewit, and he spent two years as acting water/wastewater supervisor at a chemical plant. 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 has authored or co-authored over 300 articles for various technical trade magazines, and he has written three books on power plant chemistry and air pollution control. He is a member of the ACS, AIChE, AIST, ASME, AWT, CTI, and he is active with Power-Gen International and the International Water Conference. He can be reached at bueckerb@samcotechnologies.com and beakertoo@aol.com.
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