Power Engineering

Reverse Osmosis Supplies High Quality Water at Low Cost

By: Daniel J. Hilson, Osmonics Process Water Group,
Robert Lattomus and Keith Kehrer, Conectiv Energy

The primary goal of power plants is to generate and deliver power as dependably and efficiently as possible. While this goal may seem clear-cut, achieving it takes work. Variables, such as the role of high purity water, can significantly impact power plant efficiency and reliability.

Conectiv Energy, Wilmington, Del., provides reliable energy to wholesale and commercial customers primarily in the U.S. Mid-Atlantic region. Increased competitive pressures created by the deregulation of the power industry compelled Conectiv to examine ways to improve its water system design, and ultimately reduce operating costs. This need became particularly apparent when the company began analyzing the water treatment system at one of its power plants.

A Need for Improvement

Since 1951, the Edge Moor Power Station, Wilmington, Del., has burned coal, but over the years the plant has added new fuel sources, namely oil in 1973 and natural gas in the early 1990s. When steam is generated, contaminants in the feedwater can cause steam-side fouling, which in turn reduces boiler efficiency and reliability. In order to treat the boiler feedwater, Conectiv has used twin bed (cation and anion) deionization (DI) and mixed bed deionizers (MBDI) at the plant. However, to improve system performance and maximize productivity it was clear that changes to the plant's existing water treatment system were required.

Automatic controls for reverse osmosis pretreatment system. Photo courtesy of Conectiv Energy.
Click here to enlarge image

To reduce operating costs associated with water treatment at Edge Moor, in 1994 Conectiv explored the use of reverse osmosis (RO) pretreatment to their existing cation/anion DI and MBDI system. At that time, Conectiv installed an Osmonics' single-pass RO unit ahead of the DI system. The role of the RO unit was to pre-treat the water before it was sent to the twin bed and MBDI system.

Within months, Conectiv saw that the installation reduced operating costs, including labor and chemicals, as well as improving the quality of the boiler feedwater. Key to the cost savings was the reduction in the number of required regenerations, which went from 30 per month down to two per month. In less than 18 months, the new RO system had paid for itself in caustic savings alone.

While the twin bed DI in combination with the single-pass RO proved effective, it was not the ultimate solution Conectiv wanted. One concern was the age of the DI system and the need to spend $50,000 to replace the resins in the system. Although the regeneration cycles had been greatly reduced with the first RO, there was still a significant amount of labor and chemicals being used in regeneration and maintenance of the twin bed and MBDI beds. After evaluating the cost-effectiveness of the system, Conectiv decided to explore options other than refurbishing the old twin bed DI system.

Sticking with What Works

Based on the success of the first-pass RO and the decision to continue using the existing MBDI, Conectiv decided to install a second-pass RO from Osmonics in December 1998. Conectiv engineers were confident that adding another RO unit in conjunction with the MBDI system and eliminating the twin bed DI would provide consistent water quality to the MBDI system. It would also reduce operating and maintenance costs.

Conectiv Energy's Edge Moor Power Station. Photo courtesy of Conectiv Energy.
Click here to enlarge image

Installing a second-pass RO unit to complement the first-pass RO would provide high quality water that could be sent directly to the existing MBDI units without being treated by the twin bed DI system. At the same time, the system would allow Conectiv to keep the existing equipment that was in good condition.

Before installation, Osmonics determined that the following improvements were needed for a successful system retrofit: Build a larger RO unit in front of the existing RO, incorporate a degasifier between the two RO units, and install a heat exchanger for the RO feedwater.

System Design

In two-pass RO systems the first RO must be 20 to 25 percent larger than the second pass to eliminate the potential for fouling and to balance the system's flow rates. Knowing this, the existing RO, which was to become the second pass in the new configuration, was retrofitted to produce 190 gallons per minute (gpm) of final product water. To accommodate the feedwater needs of the second pass, the new RO unit was designed to produce 240 gpm of product water.

Click here to enlarge image

To reduce anion loading and help maximize the life of the MBDI, a forced-draft degasifier was installed between the RO units. As carbon dioxide (CO2) passes through the degasifier, it is removed by forcing air counter to the water flow. The forced-draft degasifier has a blower mounted in the bottom side of the tower to optimize the amount of air/water interface time. A transfer pump is used to transfer the water, which has fallen through the degasifier tower, to the next RO. By using the degasifier to remove CO2, Conectiv was able to optimize water system performance.

Since the plant's source water is treated river water supplied by the city, its temperature fluctuates seasonally. Because of differences in water viscosity at varying temperatures, changes in temperature can dramatically affect the output from an RO system. As a remedy, Conectiv opted to install a heat exchanger in front of the dual media filters to ensure that the RO feedwater is kept at a consistent temperature of 75 F. This not only improved output efficiency, but also helped to reduce the system's operating costs.

The Benefits

The second-pass RO system became functional in August 1999. Since its installation, Conectiv has seen a number of plant-wide benefits, including reduced maintenance, lower operating costs and decreased operator involvement.

Click here to enlarge image

By removing the twin bed DI system from service, Conectiv reduced the labor requirement for routine maintenance to only minutes per day. RO maintenance is primarily periodic cartridge filter changes and pump lubrication. The twin bed DI system was much older and required regular maintenance of the numerous actuated control valves.

The plant was also able to reduce the total number of system regenerations from two per month to six times a year. Combined, the plant was able to save $25,000/yr on maintenance. In addition, Conectiv is well on its way to meeting the goal of having the new equipment pay for itself in three years.

Another benefit of the new system is the decreased level of operator involvement. With the DI system, operators needed to be present throughout the regeneration process. Since the new RO systems incorporate a high level of automation, operators are no longer required to be physically present to control the machine. Instead of having someone monitor the regeneration process the operator now only has to monitor a few gauges and meters and record operating data.

One of the unexpected advantages of using RO pretreatment in the water system was the increased robustness and reliability of the system. This benefit was realized during a drought in the summer of 1999 following the installation. At that time, drought conditions caused a significant level of seawater intrusion into the river water, which in turn infiltrated fresh water creeks that serve as the feedwater supply. This put the water treatment systems at local power plants under extraordinary pressure.

For example, one plant using a conventional DI system with the same feedwater source was overwhelmed and had to spend more than $50,000 on rental water treatment equipment because their water plant couldn't handle the high levels of dissolved solids. Meanwhile, the Edge Moor Power Station water system that included the new RO system, continued to operate successfully with only a slight increase in regeneration frequency of the mixed bed demineralizer from monthly to weekly. This robustness saved Conectiv time and money and reinforced the value of the new system.

A system retrofit that began as a cost savings measure has proven to be a valuable enhancement to the plant's operation. Because of the success of the installation, Conectiv has incorporated RO into its new power plants. Today, the Edge Moor water treatment system continues to run smoothly and has far exceeded Conectiv's expectations.

Daniel J. Hilson is a manager of power sales for Osmonics Process Water Group. He has five years of experience with high purity water treatment systems. Hilson holds a BS degree in Chemical Engineering from the University of Notre Dame.

Robert Lattomus is a plant chemist at Conectiv's Edge Moor Power Station. He has 14 years of experience in the fossil power industry. Lattomus holds BS and MS degrees in Chemistry from Delaware State College.

Keith Kehrer is a project engineer at Conectiv's Edge Moor Power Station. He has 25 years of experience in the fossil power industry. Kehrer holds a BS degree in Chemistry from Widner University.

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