The quality and variability of raw water sources can have a significant impact on the efficiency, reliability, performance and hence cost of power generation. Richard Kitson and Vinod Ramachandran explain how effective water purification can help to reduce plant operating costs.

Richard Kitson & Vinod Ramachandran, Ondeo Industrial Solutions, UK

The power generation sector throughout Europe is currently operating against a background of rapid political, climatic and demographic change, and is under increasing pressure to reduce operating costs and improve output efficiencies, while meeting the demands of ever more stringent safety and environmental legislation.

The European power generation sector must meet stringent safety and environmental legislation
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Reaching these often conflicting goals demand continuous improvements in operating processes and procedures, for example, enhancing the ratio of power output to raw materials and energy consumed, and in the flexibility and speed with which existing plant can be modified or new plant brought online.

In many areas, this had led to a focus on smaller, more versatile power plant and, increasingly, to the development of generating technologies that either optimize the efficiency of existing gas, coal or nuclear systems, or make use of renewable or alternative forms of fuel, for baseload and local or peak generation duties. These developments range from the introduction of compact or modular nuclear power reactors and advanced gas-cooled systems, to circulating fluidized bed power stations, multi-fuel power plant and combined heat and power (CHP) stations such as those found in many regions such as Scandinavia and Benelux.

In almost every case, water is an essential raw material and is used as liquid, steam and superheated steam in a wide variety of plant applications, from basic cleaning and cooling duties through to boiler feed, driving of turbines and, in CHP systems, for industrial processes and district heating schemes.

Importance of raw water quality

Regardless of its form, the quality and variability of raw water sources can have a significant impact on the efficiency, reliability, performance and hence the cost of a power generation plant. In particular, the presence of dissolved, suspended and organic material in feed streams can cause bio-fouling, corrosion and the build up of scale in downstream systems; typically, contaminants can include colloidal silica and salts, such as chlorides and sulphates.

Ultimately, high levels of impurities and contamination in water will directly affect station operating costs, by reducing the efficiency with which systems such as boilers, turbines and heat generation equipment function, with a corresponding increase in both the frequency of plant maintenance and the consumption of energy and other raw materials.

In addition, factors such as the rising cost of raw water drawn from municipal supplies and, in dryer regions, the competition for scarce or dwindling water sources, are now beginning to have a significant effect on the operating overhead of many smaller power plants. As a result, there is a growing interest both in recycling and the use of alternative water sources that often include river and seawater, or waste streams from other process plant, such as the outfall from sewage works.

The Need For Purity

Although it has been accepted for many years that raw water sources need to be purified and treated before they can be used for plant duties, neither the consequences of substandard water purification, or the need for exceptionally high levels of purity in critical applications are widely understood. In addition, it is not generally appreciated that the quality of raw water sources, even those drawn from municipal supplies can vary considerably, with wide fluctuations in turbidity, levels of dissolved solids and organic loading affecting the subsequent performance of downstream water treatment equipment.

Because of the improvements in water treatment technology, it is now possible to deliver highly efficient, purpose built solutions using a combination of technologies for raw water treatment. The level of treatment required is driven by operating parameters of the process equipment. For example, the water quality required in a boiler depends on the type of boiler, and the level of treatment varies accordingly.

Water purity is either shown as a measure of resistivity or, more commonly within the power sector, conductivity. Typically, water used in high pressure boiler feed streams is purified to a level of less than 0.2 microSiemens per cm, with purification systems being designed to remove dissolved and suspended matter, plus the bacteria that can cause the growth of algae on filter media.

Holistic approach to water treatment

The treatment of water for cooling systems concentrates on the reduction of suspended matter, chloride, sulphates and the reduction of bacteria in cooling systems. The treatment processes are then followed by chemical conditioning systems that will control the pH and dissolved oxygen content of the feed water for boilers, and the pH bacteriological quality and scaling potential of the cooling water. It is essential that the treatment systems and the conditioning systems are viewed as a whole to achieve the desired results.

The processes used to produce pure or ultra-pure water tend to be similar, regardless of the size or nature of the power generation plant. Traditionally, these have involved a sequence of treatment operations that include chemical dosing, filtration through particulate and activated carbon media, cation and anion exchange units, deaeration to remove dissolved gases, and mixed bed ion exchange policies. In addition, polisher ion exchange units or condensate polishers are often used between the steam condenser and the feed water heaters.

Feed water purification systems need to be carefully operated and maintained
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As new and lesser quality raw water sources are exploited, membrane technologies such as reverse osmosis (RO) are becoming a more familiar sight at power plants. This technology has been well proven in other applications and can reliably treat lesser quality water as well as delivering whole life costs and environment benefits.

RO works by passing pressurized feed water through semi-permeable membranes to remove up to 98 per cent of the minerals or salts contained in the pre-treated water supply, together with silica and organic compounds and over 99 per cent of bacteria. If higher purity water is required, then a double pass RO system can be used, with the feed water being passed through a sequence of membranes.

It is also worth noting that recent developments in membrane technology also allow membrane degassers with hydrophobic hollow fibres to be used instead of, or as well as, the conventional heat, degassing towers and chemical scavengers, which are needed to remove the dissolved oxygen present in the water. Dissolved oxygen and, to a lesser degree, dissolved carbon dioxide are among the principal causes of corrosion on the internal surfaces of boilers and ancillary systems, causing failures and forming deposits of metallic oxides.

The hydrophobic nature of the latest membrane materials prevent liquid from penetrating the membrane, but allows gases to diffuse through. By using purified water on one side of the membrane and applying air, vacuum or nitrogen to the other side, oxygen is drawn across the membrane and out of the purified water stream.

A carefully designed RO membrane plant will normally operate at an efficiency of up to 75 per cent recovery, depending on the feed water quality and system configuration. This is, however, relying on the use of effective pre-treatment systems; indeed, it should be noted that the efficiency and reliability of even the best RO plant will rapidly decline if there is insufficient provision for pre-treating the feed stream, especially in areas where feed water has high levels of organic contamination, suspended matter or hardness.

To optimize the performance and longevity of the RO plant it is, therefore, necessary to take a holistic approach to the construction, operation and maintenance of the entire treatment plant and to consider both pre-treatment and RO is as a combined system or process.

A similar approach needs to be taken when building CHP power stations. Because these stations effectively ‘lose’ all the boiler feed water because it is exported as steam for use in industrial process, they will use larger quantities of raw water. Quantities of 400-500 m3/h can be required. Again this treated water has to be of high purity, generally better than 0.2 microSiemens, to ensure that the output steam feed is of appropriate quality for the steam turbines.

To achieve this level of purity, often from sources of raw water or waste steams that are of indifferent and variable quality, depends on water purification systems that are carefully operated and maintained. In turn, this requires close co-operation with the system supplier, to ensure both that the specification of the water purification system for the power generation plant and the requirements of the receiving stream systems are fully considered.

Effective water teatment makes economic sense

A water purification system that is correctly specified, operated and managed can have a significant impact on the overall running costs of power plant of all sizes, helping to ensure that maintenance, fuel and feed water costs are minimized.

In particular, the ability to control the quality of feed water and thus the effects that it can have throughout the plant, gives engineers and business managers alike the opportunity to face rapidly changing economic, environmental and legislative challenges in an increasingly competitive global economy.