By: Steve Willis, Siemens-Ionpure Products, UK
Modern high-pressure boilers are very particular about the quality of water in contact with their tubes. The smallest speck of corrosion or accumulation of scale can initiate failure in a tube conducting extremely high heat fluxes. Not only are boiler surfaces at risk, but steam turbines can also suffer damage when silica carries over in the steam and plates out onto blade surfaces. So it has to be ultrapure water, and while purification systems exist that can meet the needs of even the latest supercritical boilers, the crucial attributes of treatment systems, such as efficiency, environmental impact, reliability and cost ensure that technological development continues apace. There is unrelenting competition between water technologists to claim the future of power plant water treatment.
Some challenges lie in dealing with the wide variation in feedwater quality available to power stations. Developments in membrane microfiltration, for example, have made real advances in water clarification, carbon dioxide (CO2) degassing and pre-treatment. However, it is at the demineralization stage that the treatment process is literally being ‘electrified’. Demineralization is the second of two final treatment steps, following clarification and pre-treatment.
Ion exchange technology, or deionization, is currently the only effective option for demineralization in power plants. Before exploring the current technology a brief review of the way final treatment and deionization has evolved is given below.
Move towards CEDI
First generation systems were based on chemically regenerated ion exchange resins, with a mixed bed deionization (MBDI) polishing stage preceded by two separate resin beds for cation and anion exchange (primary demineralization). Then as the need to remove organics increased, separate cation and anion units began to be replaced by reverse osmosis (RO).
Using RO’s membrane technology, second generation systems made a dramatic reduction in chemical usage but the MBDI stage remained. Finally there is the current generation final treatment regime, which totally eradicates chemical regeneration by following RO with continuous electrodeionization (CEDI).
The rise of CEDI in polishing demineralization parallels a global trend towards high-efficiency technologies that recycle and reuse materials. Above all, CEDI meets the need to reduce chemical usage cutting out high operating costs, meeting more stringent waste discharge limits and improving health and safety by minimizing bulk chemical handling.
Recent years have seen a 20-30 per cent increase in sales of CEDI systems and a 50 per cent drop in the cost of modules as the technology has developed, and a number of trends have become established. First, this electrically-powered purification method has been accepted as a replacement for existing mixed bed systems and is now specified as an option on most new deionization systems. Second, on plants that retain MBDI, CEDI modules are taking on the bulk of the work, extending the running time between resin regeneration. Finally, modules are being integrated with RO on combined technology skids that vastly reduce the plant footprint, when compared to traditional demineralization systems.
The link with RO is a vital one. CEDI cannot function effectively unless the feedwater has first passed through a RO stage. In fact the system may require a two-pass RO stage if the feed is particularly high in silica or sodium. However, a two-pass RO system clearly affects the cost comparison between MBDI and CEDI. Unsurprisingly, given the critical nature of water quality in power generation, capital and operating costs do not top the list of important factors. Market research by us indicates performance is the most important end-user criterion, followed by ease of use and maintenance. Lower down the order comes cost, customer support and factors, such as environmental impact.
When it comes to ease of use and maintenance, the benefits of CEDI are in many ways linked to the fact that it operates on a continuous basis. The equipment may need cleaning-in-place, typically on an annual basis, but does not require the regular chemical regeneration, downtime, waste neutralization and chemical handling associated with MBDI. The reject stream is pH neutral and can be reclaimed to feed the RO system.
The capital cost of a CEDI system is approximately equal to 1-1.25 times the cost of RO per m3/h of output. For smaller systems this is on level terms with MBDI, including regeneration skids. As the treatment plant size increases, CEDI does become more expensive to install, but where a neutralization system is required MBDI has the higher capital cost. Of greater importance, however, is the marked difference in operating costs. As acid and caustic costs continue to rise each year, the costs of operating a chemically regenerated MBDI plant rise with them. The only operating cost element for CEDI is electricity, which a system typically consumes at less than one third of the rate of a RO booster pump.
East River Repowering Project
As part of a repowering project at ConEdison’s East River Generating Station in New York, USA, a new on-site water treatment plant was constructed, which clearly demonstrates the benefits of CEDI technology.
The East River Generating Station located right in the heart of Manhattan Island
The repowering project expanded the steam generating capacity of the East River complex from 1200 to 2600 tonnes/h, and the electric generating capacity from 300 to 660 MW. This was accomplished through the installation of two dual-fuel combustion turbines, two heat-recovery steam generators, a 1530 m3/h demineralization facility and 24 500 m of process piping all without increasing the footprint of the station.
ConEdison elected to use single-pass RO and CEDI to provide the demineralized water required as make-up to the steam generators. Constituting some of the latest technology available, this system also avoids the regular transportation and use of hazardous regeneration chemicals.
Water System Design
ConEdison undertook this repowering project in 2001 to ensure that the steam system would continue to provide the environmental benefits of steam service year-round and to provide a new source of efficient power generation to New York City. The East River Station facility was selected for the project because it had the required space and allowed for the maximum use of existing energy infrastructure.
Two water sources were available to the East River Station through the New York City potable water system, via the Catskill/Delaware watershed and the Croton watershed. Although the Catskill water is lower in total dissolved solids the most likely available source for the plant was the Croton watershed. Therefore, the initial system design was based on the water analysis of the latter as this was felt to be the ‘worst case’. However, because of construction activities on the supply aqueduct, water from the Croton watershed has never been supplied, and the system has to date been fed exclusively from the Catskill/Delaware watershed.
One of the main 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 and the product water quality specifications.
Designed to produce 1530 m3/h of deionized water, the makeup water system is much larger than a typical system for a combined heat and power plant. The reason is that the New York City district heating system dates back to the late 1800s, and the steam distribution system was not designed for return of the steam condensate. Therefore, the units providing the steam supply to this system operate on 100 per cent makeup.
The water treatment system for the repowering project was designed to consist of two parallel trains, with each train consisting of five parallel banks, or skids. Each train comprises pre-filtration, chemical addition, RO and electrodeionization.
Water Treatment Process
The raw water feed to the water treatment system is supplied by the raw water pumps that take suction from a 300 000 litre raw water storage tank. Before it reaches the multimedia 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 is 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 microns. The filtered water is then injected with an anti-scalant before it reaches the cartridge filters, which are designed to remove additional small particulates. Filtered water from the cartridge filters is treated with sodium bisulphite to prevent the free chlorine in the water source from oxidizing the thin film composite polyamide membranes.
The water is then forwarded via the RO feed pumps to the inlets of the RO banks (Figure 2.) RO permeate flows to the inlet of the CEDI banks, while the RO reject and RO flush flows 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 degassers. The degassed CEDI reject is recycled back to the raw water storage tank.
The table below summarizes the system’s performace for the period of June 2005 to June 2006. To date, the only 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. Fouling of the RO appears via multiple-element autotopsy, to be the consequence of a combination of biofilm growth and the passage of fine (less than 2 microns) particulate material through the pre-treatment filtration section of the system. This may be addressed by incorporating chemical disinfectants in the cleaning regime, or by adding an on-line biocide injection system.
Initially, the plant operated without the addition of any coagulant or polymeric filter aid upstream of the multi-media filters. The silt density index (SDI15) of the RO feed water, however, was approaching the maximum possible value for a 5-minute test, and ConEdison therefore implemented polymer injection before the media filters. This increases the rate of filter backwashing at the same time, so a balance has to be struck between solids removal and backwash frequency.
The CEDI system operation has been stable. After a initial stabilization period, product water quality has been consistently less than 0.1 microSiemen/cm, less than15 parts per billion (ppb) silica and less than 3 ppb sodium.
The RO/CEDI system has now been in operation for two and a half years, and has produced almost 5 million tonnes of deionized water, easily meeting the outlet water quality specifications. However a significant amount of additional work needed to be done to improve the RO pre-treatment, to ensure that the system was able to maintain its consistent outlet water quality.
Consequently, RO membranes were replaced in January and February 2007 because of an increase in salt passage. At about the same time, an 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 resistance. The use of a polymeric filter aid has also been implemented upstream of the multi-media filters and is seen to be effective in reducing the RO feed water silt density index. This has resulted in less frequent RO membrane cleaning.