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.
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.
In Power Generation, Systems often Broadly Comprise
- Water softening
- Dual media filtration
- Carbon adsorption filtration
- Reverse Osmosis
- Membrane Degassing (CO2 removal)
- De-Oxygenation (GTMDO)
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)