Merits of Combined Cycle HRSG Condensate Polishing

By Brad Buecker

Combined cycle power generation continues to be a leading technology for replacement of coal-fired units. But combined cycle plants, which typically are started up and shut down regularly, present special difficulties when it comes to chemistry control in heat recovery steam generators (HRSGs). This particularly applies to HRSGs that employ phosphate treatment for boiler water (evaporator) chemistry control.

The issue can be considerably simplified by using alternatives to phosphate treatment, but this may place the unit at much greater peril from corrosion and damage caused by impurity in-leakage. Condensate polishing is a technique that provides improved protection to HRSGs, and simplifies chemistry treatment and control. However, a condensate polisher is often never considered or is eliminated in the design phase of a project as a method to save some upfront cost.

A Review of Some Basic Issues

High-pressure steam generators require pristine water at all times, as even small amounts of impurities such as chloride, sulfate, or the hardness elements can cause corrosion and scale formation. An example that the author has used for years to highlight this issue comes from a small, 1,250 psi power boiler that was operated for a period of time (three weeks) with a known condenser leak. Even though the lab staff worked diligently to maintain boiler water chemistry (phosphate-based) within recommended control ranges, the infusion of impurities induced waterwall tube under-deposit acid corrosion and hydrogen damage that eventually required a complete replacement of all boiler tubes. Needless to say, the small amount of money gained by continued power sales was lost many times over by costs to replace most of the steam generator. Numerous other instances of steam generator damage due to impurity ingress have been documented over the decades.

A critical aspect of HRSG chemistry, and especially in the water-touched condensate/feedwater and boiler water piping, is pH, or more accurately, maintaining mildly alkaline conditions in the circuits to minimize general corrosion. For condensate/feedwater, the recommended chemistry control program is known as all-volatile treatment oxidizing [AVT(O)], in which basicity is maintained with ammonia feed (pH range of 9.6-10.0 for the most common type of HRSG), combined with a small residual concentration of dissolved oxygen (D.O.). The oxygen comes from the (normal) slight air in-leakage at the condenser. At times the D.O. concentration may need to be enhanced with supplemental oxygen injection to the feedwater.

Note: For those readers who may still be unaware, unless the condensate/feedwater system contains copper alloys (extremely rare to non-existent in HRSGs), an oxygen scavenger should never be employed as part of the chemistry program. This topic is regularly covered at the annual Electric Utility Chemistry Workshop and in publications from the Electric Power Research Institute (EPRI), the International Association for the Properties of Water and Steam (IAPWS), and others. The author included an update of this issue in his paper at the 2017 Power-Gen International conference.

The alkalinity induced by ammonia feed to the condensate will carry through to the boiler, but has little value in the event of impurity ingress, e.g., a condenser tube leak. So, common for years has been feed of tri-sodium phosphate (TSP) to boilers to establish more permanent alkalinity. The primary chemical reaction is:

Na3PO4 + H2O ⇔ NaH2PO4 + NaOH

The caustic alkalinity (NaOH) generated by this treatment helps to counteract the influence of the (usually) small concentrations of chloride and sulfate that may enter the condensate.

A considerable problem with tri-sodium phosphate is the variable solubility of this chemical with temperature.

The graph clearly shows that solubility increases as temperatures rise to 300o F, but then declines rapidly, and is virtually negligible at the temperatures of high-pressure boilers. Years ago, utility chemists and researchers began to realize that this solubility issue leads to significant difficulties. As a unit comes up in load, the TSP precipitates leaving very little residual in the boiler water. The common name given to this deposition is “hideout.” Then, if boiler load is reduced or especially if the unit comes off line, the TSP re-dissolves. The deposition/dissolution pattern causes swings in boiler water chemistry, including pH, and can make chemistry quite difficult to control. Even at base-loaded plants, hideout presents difficulties and though the plant may have plenty of on-line instrumentation to monitor boiler water conditions, much extra effort may be required to control chemistry. Also well-known now is that phosphate can directly react with boiler tube metal, apart from other corrosion that might develop due to impurities.

These difficulties are why some chemists, and most notably those at existing coal plants, have switched to caustic as the chemical for boiler water pH control. However, caustic treatment requires very careful monitoring as iron is an amphoteric material, meaning that the steel will corrode at high pH as well as low. The recommended free caustic concentration in high-pressure steam generators is 1 part-per-million (ppm). For plants without trained personnel, caustic control may be problematic.

Condensate Polishing to the Rescue

A method to avoid these issues, simplify chemistry treatment, and most importantly protect the steam generators is full-flow condensate polishing. A polisher provides a good buffer from impurity ingress that could otherwise cause significant to severe damage in the steam generator. Not to be forgotten is that feedwater is typically utilized for steam attemperation, and thus impurities will directly enter the steam and turbine during upset conditions. A polisher may pay for itself in preventing just one upset let alone multiple upsets over the life of the plant.

Another important aspect of polishing that can be particularly valuable, and especially at what are often lightly-staffed combined cycle plants, is simplified control of HRSG water/steam chemistry. We have already noted that the alkalinity generated by ammonia feed to the condensate system will carry over into the steam generator. But, impurity ingress almost immediately destroys the chemistry. Eliminate the chances of boiler water contamination with condensate polishing, and now the AVT(O) condensate/feedwater program becomes acceptable for the HRSG evaporators.

Space limitations prevent a full-blown discussion of condensate polishing processes, but two types are available, deep-bed and powdered resin. Both utilize ion exchange to remove impurities. In deep-bed polishers, as the name implies, condensate flows through vessels containing a bed of mixed cation and anion resin. Cations such as sodium, magnesium, and calcium are exchanged for hydrogen ions (H+). Anions are exchanged for hydroxyl ions (OH-). The reaction of H+ and OH- produces water.

A primary advantage of deep-bed polishers is that the resin can be regenerated and used over and over. The figure below outlines the general schematic of a deep-bed polisher.

A typical design might have two or three vessels in active service, with one standby unit of freshly-regenerated resin. In the design shown above, when the resin exhausts in a process vessel it is sluiced to the cation regeneration vessel where first the cation and anion resins are separated. The separation process is rather straightforward and relies on the density difference between the two resins, which, with the use of mild water agitation, induces the anion resin to settle as a layer on top of the cation resin. Once this process is complete, the anion resin is sluiced to its own separate regeneration vessel. Note: Some designs may have a single regeneration vessel in which, after separation, the cation and anion resin are regenerated in the same vessel. This requires additional piping and controls to keep the regenerants from contacting the contacting the opposite resins.

The common regenerants for cation and anion resin are sulfuric acid and caustic, respectively; diluted to just a few percent active concentration for the regeneration. A potential drawback to on-site regeneration at minimally-staffed combined cycle plants is having trained or available personnel to perform the regenerations and monitor polisher performance. A number of the major water treatment companies now offer off-site regeneration services, and this is an option to consider. This possibility reduces or eliminates on-site storage and handling, and associated risks, of concentrated sulfuric acid and caustic.

The other variety of polisher is the powdered-resin type, in which very fine resin is applied to candle-type filter elements, usually on top of a pre-coat material. The resin is identical to that in deep-bed polishers, but is ground to a very small size. Powdered-resin polishers provide fine particulate filtration, but the resin is only functional for a relatively short period of time. When the resin has reached the end of life (often due to a differential pressure limit, not resin exhaustion), it is removed from the filters, is discarded, and a fresh coat applied. A general schematic of one type (bottom filter support) of powdered-resin polisher is shown below.

In general, powdered-resin polishers are less expensive capital cost-wise than the deep-bed version, but of course the resin is not reused in the former but is discarded upon exhaustion. Thus, operating costs may be higher, although deep-bed operating costs are influenced by what have historically been large fluctuations in sulfuric acid and caustic prices.

The discussion above leads into an aspect that can have a significant influence on the selection of condensate polisher type. We have previously noted that AVT(O) is the preferred condensate/feedwater chemistry program for most HRSGs. Ammonia raises pH via the following reaction:

NH3 + H2O ⇔ NH4+ + OH-

But, ammonium ions (NH4+) like other cations are removed by polisher cation exchange resin. Thus, a powdered-resin polisher, with a lower resin volume, will exhaust on ammonia much more quickly than a deep-bed resin. It should be noted though that some condensate polishers, particularly of the deep-bed type, are operated beyond the ammonia break until sodium begins to break through. Fundamentally, ammonium-form operation is not harmful because, after all, ammonia is the condensate/feedwater pH conditioning agent. However, ammonium-form operation may allow slightly higher leakage of other cations, most notably sodium, although units have been successfully operated in ammonium-form.

A Quick Note About Units with Air-Cooled Condensers

The previous discussion was based on applications for HRSGs with water-cooled condensers, where even small condenser tube leaks can seriously impact evaporator and steam chemistry. But air-cooled condensers (ACCs) are increasingly being chosen for new plants. Condensate polishing would, in this author’s opinion, arguably be overkill for ACC-equipped units, although issues with in-leakage of air, of course containing carbon dioxide, must be considered. Regardless, condensate filtration is an absolute minimum requirement for ACC-equipped units. Air-cooled condensers are enormous in size, as a huge surface area is needed for steam cooling because air has a much lower density and lower heat capacity than water. On the steam-side of an ACC, much of the vast carbon steel surface is exposed to the phenomenon of two-phase flow-accelerated corrosion, which generates considerable iron oxide particulate in the condensate. These particles must not be allowed to transport to the steam generators where they would deposit on boiler internals. Condensate filters are available at significantly lower cost than condensate polishers for this service.

Conclusion

Many examples have shown how impurity ingress to high-pressure steam generators can cause serious to catastrophic corrosion, even at plants with trained personnel and on-line chemistry monitoring. The common “lean and mean” approach to staffing at combined cycle plants makes these facilities even more susceptible to upsets. Condensate polishing provides an added layer of protection to the unit(s), and eases burdens on boiler water chemistry control.

Brad Buecker is a senior process specialist in the Water Technologies group of Kiewit Engineering Group Inc.