By Christina Ho, Consolidated Edison of New York, Inc. and Jonathan Wood, Siemens Water Technologies
Con Edison’s East River Repowering Project (ERRP) increased the electric generating capacity of the East River complex from 300 MW to 660 MW and expanded its steam generating capacity from 2.7 to 5.7 million pounds/hour. This was accomplished by installing two dual-fuel combustion turbines, two heat recovery steam generators, a 6,730 gallon per minute (gpm) demineralization facility and 80,000 feet of process piping – all without increasing the station’s footprint in downtown Manhattan.
Con Edison elected to use reverse osmosis (RO) and continuous electrodeionization (CEDI) to provide the demineralized water required as makeup to the steam generators. In addition to being some of the latest technology available, this system also avoids the regular use of hazardous regeneration chemicals required by conventional ion exchange systems.
In this article, we will describe the design and construction of the entire makeup water system, including the challenges associated with placing the equipment in an operating powerhouse. Operating data from the first year of service will also be presented.
Con Edison undertook this repowering project in 2001 to ensure that the company’s steam system-the largest in the world-would continue to provide the environmental benefits of steam service year-round and to provide a new source of electric generation to New York City. The East River Station facility was selected because it had the needed space and allowed for the maximum use of Con Edison’s existing energy infrastructure. The new East River equipment replaces Con Edison’s Waterside Station and uses natural gas as its primary fuel. It also has the most up-to-date emissions control technology. Overall air quality in New York City will benefit as the project’s overall annual emissions will be significantly less than those of the Waterside Station it is replacing.
Washington Group International of Princeton, N.J., provided a feasibility study, licensing support, conceptual design and engineering and procurement services for the water treatment project with the input of several major water treatment equipment companies including USFilter (now Siemens Water Technologies) and GE Water Technologies.
Two water sources are available to the East River Station through the New York City potable water system: the Catskill/Delaware watershed and the Croton watershed. Although Catskill water is lower in total dissolved solids (TDS), the most likely available source for the plant was the Croton watershed. Therefore, the initial system design was based on the Croton watershed water analysis (Table 1) as this was felt to be the “worst case.” However, due to construction activities on the supply aqueduct, water from the Croton watershed has not been supplied and the system to date has been fed exclusively from the Catskill/Delaware watershed.
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One of the principal system design constraints was to minimize the storage of on-site bulk chemicals, as well as transportation of these chemicals by truck through New York City. Other system design constraints included limits on the water discharged to the East River (Table 2) and the product water quality specifications (Table 3).
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Designed to produce 6,730 gallons per minute of de-ionized water, the makeup water system is much larger than typical for a combined heat and power plant. The reason is that the New York City district heating system dates back to the late 1800’s and the steam distribution system was not designed for return of the steam condensate. Therefore, the units providing the steam supply to this system are 100 percent makeup.
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The water treatment system for the repowering project was designed to consist of two parallel trains, with each consisting of five parallel banks, or skids. Each train is composed of pre-filtration, chemical addition, reverse osmosis and continuous electrodeionization. A schematic diagram is shown in Figure 1 (page 24). The water treatment equipment was supplied by Siemens Water Technologies and installed by Slattery Skanska.
FIGURE 1 WATER TREATMENT SYSTEM LOCATION
A schematic showing the location of the water treatment system. One floor houses multi-media filtration, two floors house reverse osmosis skids and one floor houses the CEDI modules. Click here to enlarge image
Water Treatment Process
The raw water feed to the water treatment system is supplied by the raw water pumps that take suction from an 80,000-gallon raw water storage tank. Before it reaches the multi-media filters, the raw water passes through heat exchangers to provide a heat-sink for the units’ closed cooling loops. This results in seasonally varying supply temperatures to the water treatment system.
The pre-treatment section is composed of chemical treatment, as well as multi-media and cartridge filtration. A chemical feed system was provided to allow injection of polymer coagulant prior to the inlet of the multi-media filters. The treated raw water is passed through the multi-media filters to remove small particulate matter to a nominal size of 10 Î¼m. The filtered water is then injected with an antiscalant before it reaches the cartridge filters. The cartridge filters were designed to remove additional small particulates. Filtered water from the cartridge filters is treated with sodium bisulfite to keep the free chlorine in the water source from oxidizing the thin film composite polyamide membranes.
The water is then forwarded via the reverse osmosis feed pumps to the inlets of the RO banks. RO permeate flows to the inlet of the continuous electrodeionization banks while the RO reject and RO flush flow to the East River. RO permeate dump water from pre- and post-service flushing flows back to the raw water storage tank.
The CEDI system uses all-filled type modules and therefore does not require concentrate brine injection or concentrate recirculation. The CEDI product flows to the five demineralized water storage tanks while the CEDI reject flows to the inlet of the decarbonators. The decarbonated CEDI reject is recycled back to the raw water storage tank.
Backwash and rinse flows from the multi-media filter cleaning sequence are directed to an auto pulse filter (APF) system for treatment prior to discharge to the East River. These filters operate via a diatomaceous earth filtration aid. The APF discharges particulate matter to a sludge tank for trucking off-site.
Water treatment system control consists of two redundant Allen Bradley PLC 5/80B controllers with ethernet side cards. The operator interface consists of two 21-inch LCD graphic displays driven by two project computers (Dell desktop PCs). The computers run RSView32 supervisory control and data acquisition (SCADA) packages, displayed on the LCDs, which communicates to the PLC via DH+. The PLC communicates with the plant DCS via ethernet. Smart transmitters and controllers communicate with the plant asset management system, using the HART protocol.
The system makeup to the demineralized water silos is based on tank level. All five water silos are typically operated as one unit so the level is uniform across them. Control logic is “unitized” in that the skids each operate at nearly constant flow, but turn on or off based on the need for water. Units are rotated based on a first-in/first- out arrangement. Generally, both Train A and Train B will be in operation, but the number of skids operating per train varies with the demand.
One of the most unique aspects of the repowering project was the challenge of fitting two combined cycle units and their associated auxiliary systems into an existing structure. In terms of the water treatment system, two major aspects of equipment siting are particularly interesting: the location and construction of the main process equipment and the storage space for the demineralized water.
The location chosen for the water treatment system was the former footprint of a high-pressure boiler. The boiler had been retired and removed and structures were built in its place to support the load of the water treatment equipment. There is one floor of multi-media filtration, two floors of reverse osmosis skids and one floor for the CEDI modules, as shown in Figure 1. The water treatment flow path is from the ground floor elevation up to the 119’ 11” elevation where the CEDI modules are housed.
The footprint of the retired boiler extends from the ground floor elevation of 10’ 6” up to 218’ 6”. This portion of the building structure was built around 1950. Portions of this area still support sections of the building that are in use. Therefore, modifications had to account for the need that the structure support the appropriate existing loads.
One of the existing floors within the boiler space was removed completely, one floor was added at a different elevation and three existing floors were modified for the new equipment at the lower elevations. The total equipment weight added to the building in that area was less than one-third of the weight of what was removed. This reduced the stresses in the columns supporting the new water treatment plant. The design of the new and modified floors was such that it maintained the structure’s originally devised integrity.
In addition to the structural work required to prepare the area for the equipment, this portion of the repowering project had the most live equipment requiring relocation based on its proximity to the existing operating plant. The fact that the structures had to be modified at the same time that equipment needed to be landed made the construction process challenging. Before the floor structures were completed and closed, openings had to be left so that equipment could be rigged into place. To complete enough of the structural work and still be able to move the equipment into the appropriate areas, the openings could not be very large. Some of the equipment had to be rigged and lifted into the building sideways.
To provide storage capacity for demineralized water and ensure that a significant unit run time is available upon a failure of the water treatment plant, large storage vessels were required. Taking into consideration the space constraints, the solution was to utilize five retired coal silos.
The diameter of each tank is 23’ 10”. Each demineralized water storage tank is designed for a usable water storage capacity of 150,000 gallons. The total water storage for the five tanks is 750,000 gallons. This capacity enables approximately two hours of continuous full-load operation of the heat recovery steam generators, with no additional demineralized water supply to the tanks.
To repurpose the coal silos for use as water tanks, modifications had to be performed. Concrete repairs were made to any area identified during inspections. Each silo was constructed of original and repaired reinforced concrete, with an internal PVC liner and external reinforcing steel tendons around the circumference of the tank. These tendons were required because of the difference in density between the water and the pulverized coal. This ensured that the silo would withstand the stresses created by the volume of water to be stored.
Commissioning of the water treatment system began in early 2005 and commercial operation began in April of that year. The system was designed to be run by one operator with 24-hour coverage; however the equivalent of a second operator is often required during cleaning periods.
Table 4 summarizes the typical RO/CEDI system performance for the period of June 2005 through June 2006. This shows the considerable variation in temperature of the RO permeate (and thus the RO feed water). The variation in feed water temperature is important because it is seen to affect several operating parameters, most notably the RO permeate conductivity and CEDI electrical resistance. System monitoring thus requires separating the temperature effect from other potentially more harmful effects such as fouling or scaling.
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To date, the major issue with system operation has been the increase in the RO system feed-to-reject pressure drop, resulting in the need for periodic cleaning. The original estimated cleaning frequency supplied by Siemens Water Technologies was four to six times a year. While RO cleaning initially was required about every two months, recently it has been much more frequent. In early 2006, the cleaning frequency increased to once or twice per month. RO fouling appears via multiple-element autopsy to be the result of a combination of biofilm growth and passage of fine (<2 Î¼m) particulate material through the pre-treatment filtration section of the system. Some of the RO biofouling may have resulted from periods of RO inactivity during the first few months of system operation. This may be addressed by incorporating chemical disinfectants into the cleaning regimen or adding an on-line biocide injection system. In addition, the chemical control provided for the sodium bi-sulfite feed is flow-based only. Since no ability exists to trim the feed, there is frequently a bisulfite excess of 2 ppm to 8 ppm at the original design dosage. Better chemical control is being investigated and will be implemented by the station.
To date, the plant has operated without the addition of any coagulant or polymeric filter aid upstream of the multi-media filters. This is in accordance with the recommendations of the equipment supplier, which were based on pilot testing performed before system startup. The RO feed water (SDI15) silt density index, however, is approaching the maximum possible value for a five-minute test. Con Edison is therefore considering implementing polymer injection before the media filters. This will increase the rate of filter backwashing at the same time, so a balance must be struck between solids removal and backwash frequency.
CEDI system operation has been stable and has not yet required any chemical cleaning. After an initial stabilization period, during which instruments were calibrated and outputs scaled to provide the correct input to the data acquisition system, product water quality has been consistently less than 0.1 Î¼S/cm, less than 15 ppb silica and less than 3 ppb sodium. CEDI pressure drops and electrical resistance have been stable, once the effect of temperature is accounted for.
Construction of such a large demineralization system in an operating power plant in downtown Manhattan posed numerous challenges from locating and sizing equipment, to rigging, to scheduling. The RO/CEDI system for the East River Repowering Project has produced more than 750 million gallons of deionized water and has provided stable final product water quality, easily meeting the outlet water quality specifications. However, a significant amount of additional work needs to be done to improve the RO pretreatment, reduce the frequency of RO membrane cleaning and reverse the increase in resources and maintenance required to ensure that the system is able to maintain its consistent outlet water quality.
Additional maintenance has been performed on this water purification system. RO membranes were replaced in January and February 2007 due to an increase in salt passage. At about the same time, increased CEDI module electrical resistance led to a chemical cleaning of the CEDI system. An alkaline brine solution was found to be most effective for restoring the electrical resistance. The use of a polymeric filter aid has also been implemented upstream of the multi-media filters and is seen to be effective at reducing the RO feed water silt density index. This has resulted in less frequent RO membrane cleaning. In addition, dismantling the Waterside Station has been completed.
(1) Gifford, J. and Atnoor, D. An Innovative Approach to Continuous Electrodeionization Module and System Design for Power Applications, International Water Conference, Paper IWC-00-52, October 2000.
(2) Wood, J., Westberg, E. and Blackbourn, D., Field Experience with a New CEDI Module Design, International Water Conference, Paper IWC-03-37, October 2003.
This is an abridged and updated version of a paper presented as part of the 67th Annual International Water Conference, which took place in Pittsburgh, PA., October 22-26, 2006. For the full version with more detailed operating data, see the Official Conference Proceedings.
Authors: Christina Ho is a Chemical Engineer for Consolidated Edison, based at the East River Generating Station. Jonathan Wood is Director of Applications R&D for the Ionpure Products division of Siemens Water Technologies.