By Dave Rigby, ELGA Process Water, UK
The modern power station boiler is the result of two centuries of development in materials and fabrication techniques. Parson’s first turbine was fed with steam from a Lancashire boiler operating at around 5 bar, with a furnace heat flux of 50 kw/m2. Today 100 bar boilers are commonplace while furnace heat fluxes have risen above 500 kW/m2. This could not have happened without advances in water treatment technology that have almost eliminated waterside problems such as silica carry-over, scale and corrosion. At such high heat fluxes the deposition of even a trace amount of scale or corrosion products on the heat transfer surfaces can be enough to cause a tube failure.
Lancashire boilers were fed with water softened by the then innovative Clarke’s process (lime softening). By comparison the modern make-up water treatment plant, using technologies like ion exchange and membrane separation, produces water with conductivity 0.1 µS/cm and silica less than 20 ppb. Such high purity water is not produced easily, but without it the generators stop turning. This means that the make-up water treatment plant is arguably the most critical item in the power station. With the advent of supercritical boilers and their requirement for “once-through” operation and even higher purity, this is ever more the case.
Raw water characteristics
The cost of producing high purity boiler make-up water is a significant contributor to the overall costs of power generation and the water treatment plant is normally designed to minimize chemical and power consumption. To achieve these lower operating costs, water treatment plant designers will need to know as much as possible about the quality of the raw water they have to treat.
For example, alkaline borehole waters with relatively high levels of dissolved salts are normally most efficiently treated by ion exchange demineralization using stratified beds of weak and strong cation and anion exchange resins, with intermediate degassing to remove the carbon dioxide generated by cation exchange. Lowland river waters usually have similar levels of dissolved salts but lower alkalinity, favouring reverse osmosis. Waters from moorland sources, on the other hand, usually have low levels of dissolved salts but contain high levels of natural organic matter. If these waters are treated by ion exchange the resin beds will be smaller than those required for a similar flow of borehole or lowland river water, but pre-treatment will be required to remove the organic matter, which would otherwise foul anion exchange resins or reverse osmosis membranes. Some smaller rivers are “flashy”, showing rapid variations in quality as they react to rainfall run-off, whilst lake waters may have seasonal problems with colloidal silica – the result of algal growth. Knowing the characteristics of raw water means that the designer can select the most appropriate process route and make sure that the various unit operations are correctly sized for the duty. But what happens if the quality of the water supply changes so that the make-up water treatment plant cannot treat it economically – or even at all?
Changing water quality
In the past many power stations took their water from municipal supplies – indeed many still do – and this was a guarantee of consistent quality. Some mains water supplies, particularly those derived from lakes and reservoirs, have always been affected by seasonal variations in organics and silica, but these changes are predictable and can be easily controlled by techniques like brine washing or chemical cleaning.
Now, however, things are changing. Municipal water supplies are treated to meet drinking water quality standards and the increasing pressure for compliance with the latest standards has resulted in increased costs as water works install new technologies to remove nitrates, arsenic, bromates and so on. As a result some power stations have opted for lower cost, private water supplies from wells and boreholes, re-using its discharged cooling water or even taking treated sewage works effluent. These sources are usually considerably higher in total dissolved solids (TDS) and other contaminants than the water for which their make-up water treatment plant was originally designed.
Even those power stations that still use municipal water supplies have found that compliance with drinking water quality standards is no guarantee of consistent quality. Most water suppliers have water from a variety of sources at their disposal and, depending on availability of resources and demand, these different water supplies are often blended or may even replace one with another. A change of source from river to borehole or vice versa can result in a sudden and dramatic change in the TDS of the water supplied to a power station.
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. As often as not, the most troublesome elements are those like barium and strontium that are not normally monitored. Even at trace levels these metals can cause severe scaling problems in reverse osmosis and ion exchange systems.
In the UK for example, the problem has been further exacerbated by a succession of dry winters, which are currently causing water supply problems across the south of the country. In some areas this has resulted in a fall in the water table allowing seawater to intrude into boreholes causing problems of increased salinity. It was this that posed particular problems to one of Scottish Power’s power station.
Shoreham, a 400 MW CCGT power station located on the south coast of England
Shoreham, a 400 MW CCGT power station in Brighton, on the south coast of England, 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 15 m3/h of high purity boiler make-up water with maximum conductivity 0.2 µS/cm supplied by a two-stream ion exchange demineralization plant.
The raw water that supplies the demineralization plant is drawn from boreholes close to the coast, and normally has a conductivity of around 500 µS/cm. Before the demineralization 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 a polishing mixed bed. The cation and anion exchange units use counter flow, packed bed regeneration technology and deliver demineralized water of typically 0.5 µS/cm conductivity and the mixed beds typically produce make-up water with conductivity less than 0.1 µS/cm.
The succession of dry winters, however, has allowed seawater to intrude into the boreholes at Shoreham power station, which in turn has caused problems for the site chemist, Keith Carter. He installed an on-line sodium monitor, and observed that the borehole water composition was quite clearly linked to tides. So the raw water was not only becoming more saline – the salinity was varying during the day depending on the tide. The demineralization plant was designed to produce 120 m3 of treated water between regenerations, but the deteriorating water quality reduced this to less than 40 m3 and also caused an increase in treated water conductivity. The increase in sodium concentration ultimately led to a complete failure of the demineralization plant.
Mobile water treatment solutions
The answer to the immediate problem turned out to be the AQUAMOVE MODI, a mobile ion exchange water treatment system from ELGA Process Water.
Mobile water treatment plants provide solutions for both short and long term boiler feedwater quality problems
The AQUAMOVE MODI units are installed in standard 40-foot trailers, which are insulated and supplied with heating, lighting and all necessary safety equipment, so are simply connected to the water supply. 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 disposing of regenerant chemicals.
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 demineralization plant, but the project was going to take several months. For a longer-term rental ELGA Process Water provides its AQUAMOVE MORO containerized 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 system provided the stopgap until ELGA Process Water built and commissioned the new permanent reverse osmosis system.
Scottish Power recognized that its water quality problem was a long-term one and invested in plant to handle the higher TDS, but often the changes in water quality are temporary so investment in permanent plant is inappropriate. Even if the investment could be justified, very often the quality changes are sudden and there is no time to procure and install permanent plant. To solve these short-term problems, an increasing number of power stations are making contingency plans using services like ELGA Process Water’s PreACT. When a power station registers with the service, a team of specialists surveys the water treatment facilities to identify potential problem areas, prepare a risk analysis and draw up a contingency plan detailing what site engineers should do in the event of a failure. If the contingency plan involves the deployment of a mobile treatment plant, full details of the temporary plant location, connection points, pipe sizes and so on are filed so that the correct equipment and fittings can be despatched first time, and can be installed using a pre-agreed method statement. That means minimum delay on site and a rapid return to full generating capacity. Once the raw water quality is back to normal the mobile unit is simply returned to the supplier, making it a highly cost-effective option.