Brad will be co-delivering a presentation, Modern Makeup Water Treatment Methods for Combined Cycle, Cogen and Other Energy Industries, on February 12, 2025, at 10:00 – 10:45 a.m. at the O&M Knowledge Hub at POWERGEN International. POWERGEN is February 11-13 at the Kay Bailey Hutchison Convention Center in Dallas. Register for the POWERGEN technical conference program here.
As is well-known, combined-cycle power generation has replaced many retired coal-fired power plants. Given the uncertainties that still influence renewable energy development, combined-cycle power will likely be a significant part of the energy mix for years. But a trend that has emerged in the industry, and which this author observed for years while reviewing combined-cycle water treatment specifications, is that many plant owners strive to operate as “lean and mean” as possible.
An easy target for upfront cost cutting is water/steam chemistry staff, who are needed to minimize chemistry-related problems. Even just one evaporator tube failure from a chemistry upset can offset these staffing costs, while recurring failures are enormously expensive. Yet, some plant owners seem to have the mindset that combined-cycle operation is as basic as that for simple cycle units. This article summarizes some of the most important water/steam treatment issues that can affect HRSG performance, and it illustrates why knowledgeable personnel are critical for reliable operation.
Let’s start at the beginning
Conventional utility boilers (those that remain) and combined-cycle HRSGs require high-purity makeup, as what may seem to be just slight contamination can cause corrosion or scale formation in the steam generator. A common makeup system arrangement for modern combined-cycle facilities is micro- or ultrafiltration for raw water particulate removal, followed by reverse osmosis (RO) for bulk demineralization, followed by ion exchange (IX) polishing (or electrodeionization) to produce the final, purified product.
I have encountered situations where plant managers thought the makeup system could be set up so that a unit operator could simply flip a switch and walk away, leaving the system unattended for long periods. Yes, modern technology has made makeup water production more reliable, but this equipment still needs regular oversight. Several of the numerous items that require attention include:
- Fluctuations in raw water suspended solids or ingress of large debris can foul filtration equipment and block flow, potentially leading to system shutdown and no water production.1
- Even with properly treated makeup and a solid anti-scalant program, RO membranes accumulate solids over time. Someone on the plant staff needs to conscientiously monitor RO performance and evaluate instrument readings from the RO influent and effluent streams to periodically schedule and perform an off-line cleaning. RO membranes can become irreversibly fouled if not cleaned when the data indicates.
- Microbiological fouling of membranes and other equipment such as RO inlet filters is a common problem.
- An IX polishing unit will operate for a long period given that the RO has removed most ions, but eventually the resin will exhaust. If the staff does not swap out the exhausted resin vessel for a fresh unit, impurities travel to the HRSG.
Condensate/feedwater issues
The most frequent source of impurity ingress to a boiler or HRSG is a leak in the steam surface condenser (unless the unit has an air-cooled condenser). Impurities such as chloride and sulfate can concentrate under boiler tube deposits and induce localized corrosion. Severe upsets have been known to cause tube failures within weeks or even days. Yet, a mindset that has been difficult to correct is convincing plant management, when a leak is detected, to shut down the unit and plug the leaking condenser tube.
Another mindset that has been difficult to overcome is the use of oxygen scavengers as part of the condensate/feedwater treatment program. When I entered the power industry in 1981, at a coal-fired utility, accepted wisdom was that all dissolved oxygen (D.O.) must be removed from the feedwater, as otherwise severe oxygen attack of both carbon steel components and copper alloy feedwater heater tubes could result. This was especially true of copper alloys.
Accordingly, all units were equipped with a mechanical deaerator, which, if operating properly could reduce D.O. to 7 parts-per-billion. Chemical treatment programs included ammonia (or an alkalizing amine) for pH control with supplemental feed of an oxygen scavenger/reducing agent to remove all remaining D.O. Then, in 1986, a feedwater line rupture at a nuclear plant killed four employees. A number of similar failures at coal plants have caused additional fatalities over the years. The failures are generated by a phenomenon known as flow-accelerated corrosion (FAC), which is in large measure induced by oxygen scavengers and the reducing environment produced by these compounds.
But even before the first FAC failure occurred, chemistry had been developed (for supercritical units in Europe) in which, where the units had high-purity makeup and no copper alloys in the feedwater system, some dissolved oxygen was required to form a tight, protective oxide layer on metal surfaces. Research groups such as the Electric Power Research Institute (EPRI) and others developed related chemistry for drum units, and the programs have evolved (again for units with high-purity makeup and condensate, and no copper alloys in the feedwater system, which includes virtually all HRSGs), oxygen scavengers should not be used. Details are available in Reference 3, which can be downloaded at no cost, and I summarized many of the major points in a series for Power Engineering.4 As part of the requirement for a D.O. residual in the feedwater, it is not uncommon for deaerator vents to be closed during normal operation, with only an occasional, brief valve opening to vent non-condensable gases.4
The various forms of oxygenated feedwater chemistry represent a concept that some plant managers do not grasp, with HRSGs around the world still on oxygen scavenger programs that are no doubt inducing FAC in the feedwater systems.
HRSG evaporator treatment
Common for combined-cycle power plants are multi-pressure HRSGs. The chemistry regimes for each evaporator circuit are different, and for non-chemists, understanding the differences may be very challenging. A notable example is the design known as the feed forward low-pressure type, where the low-pressure (LP) evaporator essentially serves as a feedwater heater for the intermediate-pressure (IP) and high-pressure (HP) evaporators. Phosphate treatment is permissible in the IP and HP evaporators but cannot be employed in the LP evaporator because a portion of this water serves for steam attemperation. Additionally, the phosphate control ranges are not identical for the IP and HP evaporators due to the pressure differences and the potential influence on phosphate deposition.5
Another important point is that the dissolved oxygen which serves for chemistry control in the feedwater system will typically escape with the steam in the LP evaporator. This oxygen loss can establish locations for FAC in the IP and HP economizers. Supplemental oxygen injection may be required at these spots to minimize the FAC potential.3, 6
Online chemistry monitoring
Some chemistry upsets, e.g., a condenser leak, can induce prompt and severe damage in the boiler if not detected quickly. Online monitoring systems are ideal for this purpose, but they may not be included for budgetary purposes. Or, as this author has directly observed, the plant may not have the staff to operate and maintain the system or interpret the data from the instruments. Just one or two chemistry-induced failures will offset the savings from not having this equipment.
Don’t neglect shutdown and layup protection
I have observed situations in which plant management and technical personnel pay decent attention to chemistry during normal operation but leave units standing full of water and unprotected during shutdowns. Air ingress at various points in the system can establish serious localized corrosion. The attack also produces corrosion products that transport to the evaporators. The particulates can then form deposits that act as sites for under-deposit corrosion.
Frequent shutdowns, perhaps even on a daily basis, are common for combined-cycle plants and especially for those units that ramp up and down to follow load fluctuations from renewable sources. Techniques such as nitrogen capping, warm, dehydrated air circulation of the LP turbine, and makeup water D.O. removal can mitigate these issues.7 But again, the techniques require extra money and staffing support, which may be discarded per budgetary constraints.
Conclusion
Despite protestations by some from a strong environmental viewpoint, it seems that combined-cycle power generation will represent a sizable portion of energy production for the foreseeable future. Grid reliability is a key issue in this argument. Accordingly, plant reliability is also critical, in which issues related to water/steam chemistry monitoring and control are an integral feature.
References
- B. Buecker, “Membrane Methods for Makeup Pretreatment”; Power Engineering, Vol. 108, No. 3, March 2005.
- S. Shulder and B. Buecker, “Combined Cycle and Co-Generation Water/Steam Chemistry Control”; pre-workshop seminar for the 40th Annual Electric Utility Chemistry Workshop, June 6-8, 2022, Champaign, Illinois.
- Guidelines for Control of Flow-Accelerated Corrosion in Fossil and Combined Cycle Power Plants, EPRI Technical Report 3002011569, the Electric Power Research Institute, Palo Alto, California, 2017.
- B. Buecker, “HRSG Steam Generation Issues: Reemphasizing the Importance of FAC Corrosion Control, Parts 1-4”; Power Engineering, September-October 2022.
- International Association for the Properties of Water and Steam, Technical Guidance Document: Phosphate and NaOH treatments for the steam-water circuits of drum boilers of fossil and combined cycle/HRSG power plants (2015).
- J.B. Smith and D.M. Craven, “Supplemental Oxygen for All-Volatile Treatment under Oxidizing Conditions”; PPCHEM Journal, 2024, Vol. 6.
- B. Buecker and D. Dixon, “Combined-Cycle HRSG Shutdown, Layup, and Startup Chemistry Control”; Power Engineering, August 2012.
About the Author: Brad Buecker is president of Buecker & Associates, LLC, consulting and technical writing/marketing. Most recently he served as a senior technical publicist with ChemTreat, Inc. He 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, Ill.) and Kansas City Power & Light Company’s (now Evergy) La Cygne, Kan., station. His work has also included 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 250 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, the Electric Utility Chemistry Workshop planning committee, and he is active with the International Water Conference and POWERGEN International.
