Water Technologies & Solutions technical paper

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off-site condensate resin regeneration by service contract Authors: Authors: G.L. Bartley, Tennessee Valley Authority and R.T. Taylor, SUEZ

summary

At four fossil stations, condensate polisher throughput increased by 27-37% after initiating off-site resin regeneration. This service improved cycle chemistry, lowered cost of treatment, eliminated lost generation from acid, and caustic ingress, and eliminated hazardous regenerant chemicals from the sites. Contract regeneration also provides the unique capability to adjust resin ratios at each regeneration to optimize polisher performance based on cycle chemistry.

introduction

The Tennessee Valley Authority (TVA) is the largest electricity generator in the world. TVA identified and targeted water treatment systems as an area to reduce power production costs. Objectives were established to improve cycle chemistry and lower the overall cost of treatment by improving condensate polisher and makeup water treatment performance.

Aging and inefficient regeneration systems were a major contributor to plant forced outages (by regenerant chemical ingress into operating units), and escalating plant operations and maintenance (O&M) expense. In addition, TVA’s Board of Directors established “Environmental Leadership” as one of the company’s primary corporate goals in 1993.1 Hazardous regenerant chemicals and the corresponding wastewater discharge were identified as a potential source for adverse environmental impacts and liabilities. This combination of drivers led TVA management to direct the elimination of hazardous regenerant chemicals, and associated equipment, from TVA fossil plants. After evaluating options, Tennessee Valley Authority entered into a

Partnering Agreement with SUEZ, a global high purity water treatment company (the Service Company), to achieve these goals.

For makeup water treatment, a combination of electric and pressure driven membrane technologies was selected to process raw water and purify to ultrapure water quality without hazardous chemicals. The service provider’s membrane-based DeltaFlow* system is the primary makeup system serving the TVA fossil and nuclear system. The DeltaFlow system combines three membrane separation technologies: reverse osmosis (RO), gas transfer membranes (GTM*), and electro deionization (EDI), into a continuous, reliable ultrapure water treatment system. The Service Company provides makeup water to the TVA system under a Design, Build, Own, Operate and Maintain (DBOOM) service contract (e.g. outsourcing). Details regarding the high purity makeup systems have been presented in prior work.2,3,4

A more challenging and unprecedented goal required elimination of hazardous regenerant chemicals used for the deep bed condensate polishing systems. For bead resin systems, the only feasible solution was to perform the condensate resin regenerations at an external facility. In October 1998, TVA commenced off-site regeneration services, and their fossil system became hazardous regenerant chemical-free. This paper introduces off-site condensate resin regeneration by service contract, and presents the results from 24 months of service.

off-site regeneration

Conventional deep bed condensate polishing resin requires chemical reactivation prior to reuse. This function has traditionally been performed in situ at individual generating stations. Off-site regeneration

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services merely shift this chemical reactivation procedure to a remote regeneration facility.

To support multiple generating units in different geographic regions, four prerequisites were required for efficient off-site regeneration services: 1. Condensate resin standardization 2. A method for resin transportation 3. An external regeneration facility 4. Modifications to existing polisher systems

resin standardization.

Although not a strict prerequisite, resin standardization was a practical requirement. TVA realized numerous benefits by selection of one condensate polishing resin manufacturer and type: • Simplified the project logistics • Eliminated the possibility of resin cross-

contamination between types • Provided consistent operating results • Lowered cost of treatment • Eliminated the need to maintain and manage an

inventory of multiple manufacturers and resin types • Provided the flexibility to divert shipments, if

necessary, between multiple generating stations

TVA selected a uniform particle size (UPS), 10% DVB crosslinked, cellular strong acid cation resin, and a macro-porous, Type 1, strong base anion. TVA chose this combination after determining it gave the best results in field performance in their units. The initial service charge delivered to each facility was a 1:1 volumetric resin ratio (approximate 2:1 equivalent cation to anion capacity).

transportation vehicle

Off-site regeneration requires the exhausted resin be moved from the service vessel into an exhausted resin storage vessel, or transferred directly to a transportation unit for delivery to the centralized regeneration facility. The bulk resin is transported by two stainless steel Heil type tank trailers. Co-designed by TVA and the Service Company, the initial unit has volume capacity of 1500 ft3 (42 m3), but highway weight limitations restrict transport volume to approximately 1200 ft3 (34 m3). Isolated compartments permit the unit to carry both “clean” and “dirty” resin without

intermixing. These compartments allow one trailer to service multiple plants in a single trip. Isolated compartments also enhance resin transfer and aid in complete resin removal.

Each compartment is equipped to receive air or water pressure (or both) to fluidize and move the resin. A custom underdrain design facilitates resin movement, and allows the sluice water from a “dirty” bed to drain to waste. The prototype Heil trailer was purchased and transported by TVA. Several design advances were incorporated into the second trailer, which was purchased by the Service Company (Figure 1). Modifications included increasing vessel pressure rating from 14 psi to 25 psi, adding independent rupture disks for each compartment, and design changes for improved compartment drainage. Provisions were also made to heat the bottom cone area during transportation.

Figure 1: Transportation Vehicle

regeneration facility

The St. Peters Regional Plant, located 15 miles northwest of St. Louis, MO, began operation in January 1999 (Figure 2). The 48,000 ft2 (4500 m2) facility has more than 25,000 ft2 (2300 m2) dedicated to ion exchange resin processing and regeneration. The majority of resin regeneration performed at the facility is to support the Service Company’s fleet of mobile demineralizer equipment. The facility is equipped with segregated and dedicated equipment for cleaning, separation, and regeneration of condensate polishing resin.

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Figure 2: St. Peters Regional Plant

Upon arrival, exhausted resins are physically inspected prior to processing to detect the presence of crud or other resin foulants. This inspection identifies required cleaning steps prior to regeneration. The exhausted resin is then transferred from the transportation vehicle to resin cleaning equipment. This equipment is custom designed and built to optimize physical and chemical resin cleaning. Proprietary separation techniques prevent or minimize resin cross contamination, which minimizes ion leakage and achieves a high purity effluent. The separated cation and anion segments are transferred to dedicated cation and anion regeneration vessels for chemical reactivation. After regeneration to the H+/OH- cycle, the resins are mixed and transferred to clean resin storage, or directly to the transportation vehicle.

An inventory of replacement resin is maintained at the service provider’s facility. All hazardous regenerant chemical ordering, storage, handling, neutralization, and treated wastewater discharge is managed and permitted by the Service Company.

required plant modifications

The two required plant modifications were design and installation of bulk resin storage, and a resin transfer system:

bulk storage

It is a practical requirement for the generating station to install ample storage for regenerated and exhausted resin. Ultimately, the required or desired storage capacity is a plant decision. The bed exhaustion schedule, response record of the service provider, regeneration and transportation time, and the total resin “float” volume all merit consideration in this decision on an individual plant basis. Economics and risk assessment also play a major role to determine the required safety margin to protect against a condenser leak. Undoubtedly, spare charges of regenerated resin stored at the owner’s facility provide the best protection against unexpected problems.

TVA operates with a minimum one spare charge of regenerated resin onsite per operating unit (Table 1). At least one additional bed is in transportation or regeneration at the service provider’s regeneration facility. As is typical, the plants originally had little condensate resin storage capacity. To achieve the desired storage volume, TVA retrofitted existing vessels, and installed some new storage vessels.

Table 1: Condensate Polisher Operational Data

resin transfer system.

Design changes were required for the transportation vehicle loading and unloading area, referred to as the “transfer area.” Plant modifications include a supply of the following to the boundary of the transfer area: • Pressurized demineralized water • Pressurized oil-free plant air • Transfer piping for regenerated resin • Transfer piping for exhausted resin • Wastewater piping The Service Company personnel trained TVA plant operators in pneumatic and hydraulic transfer

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techniques to sluice resin to and from the transport trailer and the appropriate storage vessels. Transfer distances vary 300 to 400 feet (91 to 122 meters), at up to 100 feet (30 meters) elevation. The typical time to unload a trailer is one to two hours, including set-up.

TVA plants utilizing off-site condensate resin regeneration services

The commercial dates for TVA’s eleven fossil plants range from the early 1950s to the early 1970s. Condensate polishing equipment was not included with any of TVA’s drum-type fossil units, while all of the once through steam generator (OTSG) designs are equipped with condensate polishing equipment. OTSG pressures range from subcritical 2,400 psi to 3600 psi, and plant generating capacity ranges from 825 MW to 2500 MW. The four TVA fossil generating stations, which include six generating units, with deep bed condensate polishers are: 1. Bull Run Fossil, Clinton, TN 2. Colbert Fossil, Tuscumbia, AL 3. Cumberland Fossil, Cumberland City, TN 4. Paradise Fossil, Drakesboro, KY

The original plant designs for the condensate polishers use “naked” mixed beds, except during startup where pre-filters are used. The four plants did not traditionally use any special techniques or equipment to improve resin separation.

At Paradise and Colbert Fossil, the resins were backwashed, separated, regenerated, mixed, and rinsed in a single external regeneration vessel. Cumberland and Bull Run Fossil used conventional external separation and regeneration equipment.

The storage volume added to each fossil plant for regenerated and exhausted resin storage is detailed in Table 1. Installed resin storage was 33% to 50% of condensate resin service volume. Paradise and Cumberland fossil each have two generating units per plant; thus, the larger resin service volumes.

results

Since commencement of off-site condensate resin regeneration services in October 1998, more than 96,000 cubic feet of condensate polishing resin has been regenerated at the service provider’s facility.

The data in Figure 3 shows the average bed throughput has increased between 27% to 37% after initiating off-site regeneration. The increase in production capacity per cubic foot of mixed bed resin is detailed in Figure 4.

Figure 3: Bed Throughput

Figure 4: Bed Throughput per Cubic Foot

Greater than 420 bed exchanges have been performed at the four generating stations. Typical exchange frequency currently averages 10 beds per month, system wide. Since establishing routine service, only one bed was removed from service because of control set points. Analysis revealed that several cubic feet of exhausted resin had been left in the service vessel, which caused the bed to not rinse down to specification. During the first 18 months of operations, the balance of the beds was changed out according to schedule. As

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confidence in this process increased, the beds were allowed to run to end points, which reduced the overall number of regenerations.

The new process has improved the consistency of the polished condensate quality. Previously, on-site regenerations were frequently repeated when rinse down specifications could not be met, and most service runs were erratic in throughput and quality. After initiating the new process, bed rinse down time to service specifications are normally less than two minutes, bed throughput has increased, and effluent quality is stable and predictable (Table 2).

Table 2: Polisher Effluent Quality

TVA has traditionally practiced tight cycle chemistry in their fossil system. The strict criteria for taking a bed out of service remained unchanged after initiation of contract regeneration services: • Sodium (Na+) 0.5 ppb • Specific conductivity 0.1 micromho/cm

discussion

Flexibility to tailor resin ratios to match plant performance is a major advantage to contract services. At start, all four plants used the 1:1 volumetric resin ratio. Colbert and Paradise currently operate with the original 1:1 resin ratio. Bed throughputs have increased 27% and 33% respectively after commencement of off-site resin regeneration. While the 27% increased bed throughput at Colbert is a primary result off-site regeneration, the 33% improvement in polisher throughput at Paradise is a result of improved makeup water and off-site resin regeneration.

The tight condensers and consistent high purity makeup supply has made ammonia the major ionic challenge to the resin at Bull Run and Cumberland. Bull Run has also converted to oxygenated feedwater treatment pH levels, which reduces the ammonia loading. This has enabled these two plants to adjust to a 2:1 cation to anion volumetric ratio to further extend bed throughput. As shown in Figure 3 and 4, off-site regeneration, with the cation heavy beds

at Bull Run and Cumberland, has increased bed throughput 33% and 37%, respectively. This capability to adjust resin ratios to match plant operational conditions is not readily feasible with conventional fixed on-site regeneration equipment.

Increased resin regeneration efficiency and minimal cross contamination achieved by off-site regeneration is a major contributor for the increased bed throughput. It is worth restating that the majority of the regenerations performed at the service provider’s plant are to support other segments of the service business. The result is specialized, highly trained personnel whose core function is to clean, separate, and regenerate ion exchange resin.

Shifting the resin regeneration responsibility to their water treatment partner enabled TVA to demolish the old regeneration equipment. The hazardous regenerant chemical storage, and transfer equipment, maze of valve nests, and all ancillary equipment were eliminated, as was the associated O&M expense. Outsourcing resin regeneration also freed TVA operators to focus on their core skill set of achieving efficient power plant operations. In addition, performing resin regeneration at a remote location reduced each plant’s fresh water intake, and treated wastewater discharge.

economic influences

Off-site condensate resin regeneration services are not confined to retrofit of existing plants. This service presents many advantages to designers and owners of new “grassroots” plants. The cost for off-site resin regeneration can be classified into three categories: capital, fixed operational, and variable operational costs.

For a retrofit, additional capital is required to install resin storage vessels, construct the plant resin transfer system, and trailer transfer area. For a new plant, off-site regeneration services will reduce the initial capital costs to build the plant by eliminating the equipment associated with hazardous regenerant chemical delivery, storage, transfer, and use. Large wastewater treatment systems may also be scaled down, or in some cases, eliminated.

Operational costs can be generically considered as fixed and variable costs. The fixed operational costs are the transportation expense and the regeneration expense. For TVA, the distance from the central

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regeneration facility to the user plants ranges from 250 to 480 miles. The variable costs that determine the unit cost of polished condensate are the bed throughput and foulant cleaning requirements. Bed throughput is influenced by many different factors: • Quality requirements • Operational cycle (H+/OH- vs. NH4

+/OH-) • Ionic loading (boiler chemistry and condenser

leaks) • Bed design (SAC/MB vs. “naked” MB) • Particulate (crud) load and other foulants

The steam generator chemistry and resin ionic form have the largest influence on bed throughput. The low ammonia concentration associated with oxygenated treatment (OT) make polishers operating on this treatment program an ideal candidate for off-site regeneration services. TVA generating stations practice partial oxygenated treatment in the OTSG. Feedwater oxygenation treatment is regulated between 50-150 ppb at the economizer inlet, with ammonia to control pH. The low-pressure heaters are not oxygenated. The average ammonia challenge to the condensate polisher is 0.1 ppm at a pH between 8.5-8.8. TVA operates their condensate polishers in the H+/OH- cycle, and does not run past the ammonia break.

Polishers that operate past the ammonia break can also be a strong candidate for service. The high regeneration frequency that results from an elevated pH treatment program and H+/OH- resin cycle operation can challenge project economics.

Resin fouling leads to a reduction in bed throughput and requires additional treatment to restore resin condition. Treatment can consist of laborious scouring, foulant specific cleaning procedures, and may dictate elevated regenerant levels to restore resin operating capacity. TVA protects the mixed bed polishing resin from high crud loading at unit start-up with a coated pre-filter. This pretreatment step minimizes resin crud fouling and saves on the variable cost of treatment.

summary

The data demonstrates that off-site condensate resin regeneration is a qualified alternative to conventional condensate polisher design and operation. At four fossil stations, condensate polisher throughput increased by 27% to 37% after initiating off-site resin regeneration. Contract services achieved the goals to

increase quality, improve cycle chemistry, lower the cost of treatment, and eliminated forced plant outages, and lost generation from acid and caustic ingress. Unfortunately, competition spurred by deregulation prevented the authors from presenting a detailed economic evaluation, but it can be stated the project does provide a payback to TVA.

Contract regeneration provides the unique capability to adjust resin ratios to optimize polisher performance and throughput based on cycle chemistry.

Off-site condensate resin regeneration, coupled with advanced membrane treatment for makeup water, enables a plant (or utility) to completely eliminate use of hazardous regenerant chemicals. This allows the user to focus their capital, managerial and operational resources on efficient power production, not water. Outsourcing resin regeneration also mitigates any corporate environmental liabilities and adverse effects of uncertain future wastewater discharge regulations.

Indeed, this concept is in its infancy, but there is strong industry interest in this service. A successful application requires more than a typical vendor/customer relationship, it must be approached as a strong partnership with good communication and a high degree of trust.

This paper was presented at IWC-00-04.

references

1. Bartley, G.L., “Capital and Service Contract Reverse Osmosis Systems at TVA’s Fossil Plants,” 57th International Water Conference Proceedings, Pittsburgh, PA, October 1996, pp. 672-679.

2. Ibid.

3. Bartley, G.L., Taylor, R.T., “Outsourcing Eliminates Hazardous Regenerant Chemicals at TVA,” 19th Annual Electric Utility Workshop at the University of Illinois, Champaign, IL May 1999.

4. Wagner, T.C., “Installation and Startup of Outsourced DeltaFlow Systems at TVA’s Sequoyah and Watts Bar Nuclear Facilities,” 58th International Water Conference, Pittsburgh, PA, October 1997.