Product category:
Pure water: UV and Membrane systems
News Release from: Elga Process Water | Subject: Boiler make-up water
Edited by the Processingtalk Editorial
Team on 20 September 2007
What's in the boiler make-up water?
Jeremy Wakeham looks at the variations common with raw water and the resulting impact on the production of power station high purity boiler make-up water, and the effects of falling water tables
The modern power station boiler is the result of two centuries of development in materials and fabrication techniques, but without similar advances in water treatment technology it could not exist! Current 100bar power station boiler technology has pushed furnace heat fluxes to more than 500kW/m2 and any deposition of scale or corrosion products on the heat transfer surfaces can be enough to cause a tube failure
This article was originally published on Processingtalk on 14 Sep 2007 at 8.00am (UK)
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This is why boiler make-up water is typically specified with conductivity 0.1uS/cm and silica less than 20ppb.
Such high purity water is not produced easily; and without it the generators fail.
The cost of producing high purity boiler make-up water is a significant part of the overall costs of generation: and the water treatment plant is normally designed to minimise chemical and power consumption.
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To achieve the minimum of operating costs, the design of the water treatment plant is commonly optimised to the quality of the water with which it will be supplied.
Alkaline borehole waters are usually most efficiently treated by ion exchange demineralisation using stratified beds of weak and strong cation and anion exchange resins with intermediate de-gassing to remove the carbon dioxide generated by cation exchange.
Thin waters from moorland sources often contain high levels of natural organic matter and usually require pre-treatment to remove this material which would otherwise foul anion exchange resins or reverse osmosis membranes.
If these waters are treated by ion exchange, the resin beds will be smaller than those required for a similar flow of lowland river water, with its higher total dissolved solids (TDS) concentration.
Lake waters may have seasonal problems with colloidal silica - the result of algal growth.
Thus the water quality will influence both the process route selected to treat the water and the sizing of the unit operations.
But what happens if the quality of the water supply changes? Many power stations are supplied with Towns Mains water, which is treated to meet drinking water quality standards.
However, this is no guarantee of consistent quality.
Pressure on water resources may force the water supplier to change the source of the water supply from river to borehole or vice versa, with a sudden and dramatic change in TDS.
This already happens from time to time but it will become more frequent as global warming places water resources under increasing stress, and as common carriage is introduced into the water supply industry.
The water still meets the drinking water regulations but the effect on a water treatment plant can be devastating: ion exchange plants designed for thin, upland waters may have insufficient capacity to treat thicker borehole waters whilst, those designed for hard, borehole waters may suffer from fouling by higher concentrations of natural organic matter in supplies derived from moorland reservoirs.
The Scottish Power Shoreham 400MW CCGT Power Station in the UK is an in-line, single-shaft combination of gas turbine generator and steam turbine with a seawater direct cooling system using the cooling water pumphouse and outfall structures of the old Brighton B station.
It consumes 15m3/h of high purity boiler make-up water with maximum conductivity 0.2uS/cm supplied from a two stream ion exchange demineralisation plant.
The raw water that supplies the demineralisation plant is drawn from boreholes close to the coast, and normally has a conductivity of around 500uS/cm.
Before the demineralisation plant was built, the designers assessed the borehole water quality and decided to include pre-treatment by activated carbon filtration followed by a cation exchange unit with a layered bed of weakly and strongly acidic cation resins, a degasser to remove carbon dioxide, an anion exchange unit with a layered bed of weakly and strongly basic anion resins and, finally, a polishing mixed bed.
The cation and anion exchange units use counter flow, packed bed regeneration technology and deliver demineralised water of, typically, 0.5uS/cm conductivity and the mixed beds typically produce make-up water with conductivity less than 0.1uS/cm.
The succession of dry winters is currently causing water supply problems in some parts of the UK, resulting in a fall in the water table.
In the case of Shoreham Power Station, this has allowed sea water to intrude into the boreholes - causing new problems for Station Chemist, Keith Carter.
He installed an on-line sodium monitor and this showed a wide variation in concentration that was quite clearly linked to tides.
So the raw water was not only getting more saline; the salinity was varying during the day depending on the tide.
The demineralisation plant was designed to produce 120m3 of treated water between regenerations, but the deteriorating water quality reduced this to less than 40m3 and also caused an increase in treated water conductivity.
The increase in sodium concentration ultimately led to a complete failure of the demineralisation plant.
The answer to that immediate problem was provided by Elga Process Water in the form of an Aquamove MODI mobile ion exchange water treatment system.
The Aquamove MODI units are installed in standard 40foot trailers, which are insulated and supplied with heating, lighting and all necessary safety equipment.
They just need to be connected to the water supply, and the Aquamove team takes care of that.
The MODI ion exchange units are not regenerated on site.
Once they are exhausted, the complete trailer is returned to the central regeneration facility, which means zero discharge on site and no problems of handling or disposal of regenerant chemicals so that on-site chemical systems can be completely isolated whilst maintenance or repair work is carried out.
It was clear to Scottish Power that the changing water supply was now a long-term problem for the Power Station, so they decided to install a reverse osmosis plant upstream of the existing demineralisation plant, but the project was going to take several months.
For the longer-term rental Elga Process Water mobilised Aquamove MORO containerised reverse osmosis system.
Like the MODI trailer, the MORO container comes with all of the necessary ancillaries and connections, but can be operated at site for days, weeks, or even years.
The Aquamove MORO system provided the stopgap until Elga Process Water built and commissioned the new permanent reverse osmosis system.
The simplicity of the Aquamove concept makes it very versatile with process options to suit almost any application.
This article was supplied by Jeremy Wakeham of Elga Process Water.
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