Reverse osmosis (RO) systems provide power plant owners and operators with reliable and proven water treatment solutions. However, the development and maintenance of reverse osmosis systems requires a complete understanding of the water supply and technical capabilities of the facility. The information presented here helps plant engineers design and optimize reverse osmosis systems to meet their needs.
There is no such thing as pure water in nature. All water in its natural state contains varying amounts of dissolved and suspended solids. Osmosis is the process in which water flows from a low concentration solution to a high concentration solution through a semi permeable membrane. This normal osmotic flow can be reverse osmotic (reverse osmosis) by applying water pressure to a more concentrated (contaminated) solution to produce purified water.
Types of water needed in Power plants
Boiler storage water
Boiler water supply is water that must be as close to absolute purity as possible in order to absorb heat. Depending on the boiler pressure, treated water can range from simple ion exchange to advanced double-pass reverse osmosis systems followed by electrical ionization. Pouring poor quality water into a boiler can cause serious problems. The boiler heats water and evaporates it into very hot steam. Impurities such as minerals and gases in the water can reduce the efficiency of this process, wear generators that take in evaporated water, and corrode equipment exposed to it. For this reason, it is very important to make sure that the water used for boiler water supply is as pure as possible.
If the reverse osmosis system cannot achieve the required purity on its own, deionization unit works most effectively to reach the limits of the purest water. Double-pass reverse osmosis systems followed by electronic ionization (EDI) are recommended for high-pressure boiler applications.
Ultrapure water absorbs a large amount of heat, so it is suitable for dissipating heat from the generator and transporting it to the generator. Many types of motors and generators store a lot of heat because they manipulate so much heat that they can literally destroy themselves inside out. Like car radiators, ultrapure water absorbs and dissipates heat, preventing the generator from damaging itself.
Condensate Polishing System
The Contaminated Polishing System is used as an effective water treatment strategy to save on electricity, energy and chemical costs. These systems are also used to extend the life and performance of boiler systems. This is done by reducing the amount of contaminants in the condensate return water that can damage expensive boiler systems. The condensate polishing system is fitted with an ion exchange polishing machine that uses a resin specially developed for very hot applications.
Problems of RO: Semi Permeable Membrane
There is no such thing as a complete semi permeable membrane. A small amount of dissolved salt can also diffuse, which results in a lower concentration compared to the water supply value. The benefits of reverse osmosis (RO) technology need to be fully understood when treating water for power generation, especially as it has the potential to reduce operational and maintenance costs. For most water sources, RO is the cheapest way to remove most dissolved salts.
The term "total dissolved solids" (TDS) refers to most inorganic salts present in solution. Salts exist as cations (mainly calcium, magnesium, sodium, potassium) and anions (mainly bicarbonates, chlorides, sulfates, nitrates). These positively and negatively charged ions can flow electrically, thus determining the conductivity of water as a measure of TDS concentration. Pure water is a conductor with poor electricity.
For systems originally built solely on ion exchange, the addition of RO can reduce the need for chemical regeneration by more than one-twentieth. You can also consider removing the renewable system altogether. The upstream RO removes most of the dissolved salt, allowing the abrasive ion exchange system to be economically replaced by a service desalting bed that is chemically regenerated by an external water company. Alternatively, it can be replaced with electronic ionization (EDI). The EDI unit uses electricity to continuously regenerate the ion exchange resin.
Some new and existing plants require the removal of dissolved salts from the wastewater stream prior to drainage. A powerful reverse osmosis system allows water to be reused within the system. The concentrated salt stream remaining after RO treatment can be transported more economically to areas that can be treated more environmentally. Alternatively, it can be vaporized or otherwise disposed of.
The political and regulatory benefits of installing a Zero Liquid Discharge (ZLD) facility can offset some of the cost of capital and operating costs. However, the great economics of RO operations can only be achieved if the system and its upstream processing components are properly designed, operated, and maintained.
The most important role of pretreatment is to protect RO from non-conforming substances. With the polyamide thin film RO membranes commonly used today, the biggest problem is the removal or destruction of chlorine or other potentially oxidizing compounds. This membrane has very low resistance to free chlorine (present in many municipal water sources) and little resistance to chloramines (other municipal water sources). The success of new RO membrane systems is often directly related to their pretreatment.