Ultra Pure Water treatment for Power Plants
Power generation’s increasing demand for Ultra Pure boiler feed water and pure water injection systems for NOx reduction.
Water purification is vital to combat the scaling and corrosion that would otherwise severely damage steam and power generation systems. The potential problems escalate with the operating temperatures and pressures of today’s power stations and the low quality of the water supplies they often need to rely on.
Pure Water Group offers a wide range of purification systems and technical solutions to assure power generators of reliable and consistent water quality at the lowest possible cost. Typically, our equipment is used to produce Ultra Pure feed water for boilers and NOx-reducing water injection systems for gas turbines.
Thermal power stations have always relied on effective water treatment to reduce the incidence of fouling, erosion, corrosion and scaling but the water quality requirements are increasing. As the design of supercritical boilers and steam turbines evolves to improve efficiency and reduce atmospheric emissions, the demands on water purification systems are becoming more rigorous and complex. This is mainly because the adverse effects of ionic and other contaminants are accelerated by higher working temperature and pressure.
However, with the ability to remove dissolved minerals down to trace levels, technology such as Electro Deionization (EDI) is keeping pace with the rate of change in global power plant design. The use of EDI is integrally linked to Reverse Osmosis (RO) performance and systems must be carefully designed so that the two technologies, together meet the water quality requirements of any particular power project. In this way, demineralization using EDI is able to support the energy savings, reduced carbon dioxide emissions and lower operating costs now demanded in the power generation sector.
Boiler feed water treatment system
What does a boiler feed water treatment system typically remove?
Steam-cycle power plants are increasingly sited on coasts and estuaries in order to provide the large volume of water required for steam raising and cooling. The seawater or brackish water intake therefore has a high mineral content, calling for a thorough process of pre-treatment and purification. Examples of the dissolved minerals found in the influent water include sodium chloride, sodium chlorate, magnesium sulphate, calcium carbonate, calcium sulphate, magnesium carbonate and silica.
These compounds behave differently when heated. For instance, bicarbonates of calcium and magnesium in solution produce carbon dioxide and insoluble carbonates. The carbonates may precipitate directly on boiler surfaces and can also form a sludge. Sulphates also generally precipitate directly to produce a hard deposit but for many decades, silica has been considered the most deleterious to high pressure steam plant.
Silica is a highly soluble impurity and can be present in the steam as the result of boiler water carryover, or it may enter the steam in a volatile form, behaving like a gas. As boiler operating pressures increase, there is a corresponding increase in the tendency for silica to be carried into the steam.
When carried with steam to the turbines, silica can become supersaturated as steam expansion occurs. An exceedingly hard, glass-like film is deposited on the turbine blades, causing instability and reducing their ability to transfer heat. This type of scaling can be severely damaging, causing loss of efficiency and output.
In boilers, the major problem that silica scaling causes is tube failure, as a result of localised overheating. This is because deposits act as an insulating layer and an excessive build up prevents efficient heat transfer through the tube to the water. Consequently the tube wall overheats and ultimately it can fail. Tube overheating also occurs if deposits accumulate to form an obstruction, leading to water starvation. In addition, any significant scaling is likely to encourage corrosion at the interface between silica and metal. Whether by promoting corrosion, overheating, or both mechanisms, deposits cause unscheduled outages, increased cleaning time and expenses. Overall operating efficiency is reduced, resulting in higher fuel consumption.
What’s included in a basic boiler feed water treatment system?
Pre-treatment and demineralization regimes vary according to the characteristics of the incoming water supply – whether it is municipal water, groundwater, or surface water withdrawals such as brackish, estuarine water.
In power generation, systems often broadly comprise:
- Water softening
- Dual media filtration
- Carbon adsorption filtration
- Reverse Osmosis
- Membrane Degassing (CO2 removal)
- De-Oxygenation (GTMDO)
In the past, mixed bed ion exchange vessels were normally prescribed for demineralization but these are relatively large-scale assemblies, with chemical feed and waste streams associated with resin regeneration.The EDI process has evolved into the preferred polishing demineralization technology. It is almost always used downstream of a reverse osmosis system because EDI devices are susceptible to hardness scaling, organic fouling and physical plugging by particulates and colloids. It is now common practice for power plants to use RO as a roughing demineralizer to remove the bulk of the mineral, organic and particulate contaminants following basic filtration and pre-treatment. Also, RO removes organics that could foul the ion exchange resins in the EDI modules and it removes particulates that could clog the narrow flow channels in the resin compartments or the resin bed itself.
Increased flow rates achieved by recent EDI innovations have reduced the number of modules required to achieve the high flow rates required by power generation. This also provides significant capital systems cost and savings and simplifies overall design of water systems by reducing the amount of power supplies, piping, frames and assembly time.
Silica and Supercritical steam generation
Metallurgical advancements have led to ‘supercritical’ steam generators, operating at pressures above which water turns instantaneously to steam, becoming the new standard for efficient power plant. Unlike many subcritical boilers, they do not have a steam drum separating the water and steam, which therefore increases the opportunity for impurities to be carried onto turbine surfaces.
Higher steam temperatures in supercritical units result in a smaller differential temperature for heat transfer than is normal for subcritical boilers. So in order to produce the equivalent thermal transfer, larger surface areas are required, exposing more metal to potential scaling and corrosion.
NOx formation and pure water injection in power plants
NOx gases – individually nitrous oxide (NO) and nitrogen dioxide (NO2) – are products of fossil fuel combustion with harmful effects on the atmosphere. They contribute to smog and acid rain, while indirectly acting as greenhouse gases.
Pure Water Group builds demineralization systems for water injection, as well as for boiler feed water. In NOx reduction applications, for example in a gas turbine cogeneration plant, Ultra Pure water is injected through the fuel jets at a closely controlled rate. As the fuel ignites and burns, the deionized water turns to steam, absorbing energy as it changes phase and lowering the combustion temperature. This has the result of significantly cutting the amount of NOx gas produced.
Ultra Pure water injection is one of several strategies for NOx pollution abatement:
- Water or steam injection. Injecting a small amount of water or steam into the immediate vicinity of the flame will lower the flame temperature and reduce the local oxygen concentration. The result is to decrease the formation of thermal and fuel-bound NOx
- Fuel switching
- Flue-gas recirculation (FGR)
- Low NOx burners• Derating
- Staged combustion• Fuel re-burning
- Reduced-oxygen concentration
- Selective catalytic reduction (SCR)
Pure Water Group water treatment at Power Plants
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Thanks to our extensive experience in water treatment solutions for a wide range of applications, we developed essential insights into the EDI, MD and EDR technologies and how they meet specific requirements ‘in the field’. This is reflected in the quality of technical/commercial support, training, service and documentation we provide.
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