Fossil-fuelled (coal, oil, gas) and nuclear power stations produce electricity with turbines powered with high-pressure steam. A schematic representation of this steam circuit is shown in the picture figure below. After going through the turbine, the steam is condensed and recycled.
To avoid deposits on the turbine blades and corrosion in the steam circuit, the steam must be extremely pure. However, being continuously recycled, the condensate collects corrosion and erosion products from the boiler and pipework, as shown in the figure. The contaminants in the condensate must have a concentration of a few µg/L (ppb) or less. Therefore, the condensate, in many power stations, is treated with ion exchange resins, ion exchange being the only process capable of achieving these low residual values.
Many new power stations are being built, particularly in emerging countries, so that the number of condensate polishing project has increased tremendously since the beginning of the 21st Century. Whilst ion exchange processes for water demineralization were mainly developed in Europe, the champions of condensate polishing design are largely American.
The condensate polisher must fulfill two simultaneous duties:
- It must remove the suspended solids (mostly metal oxides) resulting from corrosion and erosion.
- It must also remove any dissolved solids originating from the make-up water, possible leaks of the condenser, or from regeneration of the ion exchange resins.
This means that the polisher must perform filtration and demineralization at the same time.
Most power stations use ammonia or amines to condition the water and steam circuit to reduce corrosion. As a result, the ionic load on the condensate polishing resin is largely cationic, and ammonium ions are removed together with lower concentrations of cations and anions in the condensate polisher.
In view of the low salinity of the water to be treated, mixed bed polishers are used in over 90 % of the cases. These units are usually designed for a high specific flow rates. In large power stations, the flow rate through each condensate polishing unit is often 200 to 800 m3/h. When the operating pressure is not too high, cylindrical vessels are used, but at 4 MPa (40 bar) or above, spherical vessels are necessary to keep the shell thickness within reasonable limits.
In most cases, regeneration is external. This is done because:
To avoid accidental ingress of regenerate chemicals in the water and steam circuit;
To design the operating unit without internals, and with a low bed depth producing a relatively low-pressure drop, whilst the regeneration station is designed with narrower columns and a high bed depth facilitating resin separation.
The ability to change the cation/anion resin ratio is an important feature of Externally Regenerated Condensate Polishers. This is a significant advantage as condensate quality changes, for example when a condenser develops a chronic tube leak.
External regeneration produces a more highly polished condensate quality than In situ, because it reduces cross-contamination. It also offers added security, since chemicals are not injected into a service vessel directly connected to the process.
The principle of external regeneration is shown below. Exhausted resin is transferred hydraulically from the operating unit to the regeneration station, and a fresh regenerated resin charge is transferred back immediately.
The regeneration station has typically three columns, cation regeneration tower used for backwash, cleaning, and separation of the mixed resins and in addition to regeneration of the cation exchanger. An Anion regeneration tower for regeneration of the anion exchanger, and lastly mixing and storage vessel for mixing, fast rinse and storage of the regenerated resin.
Benefits of Condensate Polishing:
The benefits of Condensate Polishing are related to the improvement in the steam cycle, and water chemistry that it provides. Some of the benefits are long term while others are of immediate nature. The Condensate Polisher use will result in following benefits. The use Condensate Polishers have the below benefits:
- Improves unit reliability & availability
- Reduces frequency of Boiler chemical cleaning
- Reduces number of Boiler tube failures
- Reduces Turbine maintenance related to steam purity
- Allows faster start-ups and helps to achieve full load quickly.
- Provides protection against condenser tube leaks
- Improves Turbine efficiency from due to fewer turbines deposits
- Allows continued operation of unit through peak demand even during minor condenser leakage.
- Reduces replacement power costs caused by water chemistry related outages.
- Reduces boiler blowdown with a resultant decrease in make-up water requirements & energy losses.
- Lower fuel costs related to higher heat transfer efficiency associated with cleaner tubes.
These units are used to polish condensate at extremely high flow rates in power generation applications where low dissolved solids concentration is are an issue. They filter out corrosion products (CRUD) and remove trace hardness. In the event of a condenser leak, they provide short-term protection (enough time to shut down the boiler).
They are typically installed to stand alone in the power cycle loop and operate at pressures as high as 700 psi. In some cases, an Externally Regenerated Cation Polisher may be placed in front of the Condensate Polisher.