Image via WikipediaThe water source of power plants
Water in rivers and lakes comes from rain, it has lower hardness and salt content than underground water, it is not stable and easy influenced by seasons and surroundings. Water in lakes and reservoirs is similar with that in rivers, it has lower turbidity than that in rivers because of the lower flowing characters. But it is good for microorganism growing.
Water in sea has large quantity of dissolved salt, such as sodium chloride, because of the long time evaporation and concentration.
The classification of water in power plants
Water in power plant has different characters and functions, they can be classified as: raw water, make-up water, boiler water, condensate water, drain water, cooling water, saturated steam and superheated steam.
Main method of make-up water treatment
Natural water contains impurities that will lead to scale deposit, corrosion, salt deposits in the thermal system, and consequently threat the safety and economics of the power plant.
The organics, suspensions, colloid and part of the hardness can be eliminated by chlorination, coagulants and clarification. These methods are usually the first step of water treatment in power plants, we call it pretreatment. The dissolved solid can be eliminated by ion exchange, electrolysis, reverse osmosis and distillation. The dissolved gas can be eliminated by thermal method, such as deaerator, chemical method and gas-liquid exchange.
The Pretreatment of Make-up Water Filtration
When an electricity factory uses the tap water as the water resource, the pretreatment system is simple. It is like this flow: tap water cleaning water activated carbon filter cation exchanger anion exchanger mixing bed demineralized water tank main building.
Even with some clarifiers operating at optimum levels under anticipated conditions, additional solids-removal equipment is required downstream. It serves to reduce the remaining suspended solids, colloids and remnant chloride to the parts-per-billion level required for the boiler operation.
Filtration is a process of placing a pervious barrier across flowing water to remove suspensible matter. It can be accomplished simply through mechanical straining at the surface of the barrier or by removal throughout the depth of the medium.
Proper performance does not begin until a layer of silt and bacterial matter has developed in the top inch or so of sand. This unit then produces exceedingly clear water, but at a rate no faster than 0.008m/s is reached. The upper layer, where all the filtering action takes place, is then scraped off, washed externally, and restored to the bed.
There are many factors to have influence on the process, such as filter medium, filter speed, water head loss, flow uniformity and reverse-flow backwash.
Filter medium, such as active carbon, is the basic unit of filtration. Choose of the proper medium is important to running efficiently. The medium must have these characters: good stable chemical character, enough mechanical strength, proper and uniform size and proper medium layer height.
If the speed of filtration is too slow, the output is limited. On the contrary, it will lead bad outlet water quality, higher loss of water head and short operating cycle.
Water head loss is an important parameter to judge whether the filter is scraped off. When the water head loss reaches to a certain value the filter should be put into outage and cleaned by reverse-flow backwash.
There are two types filter used in power plants: gravity type, such as valveless filter, and pressure type.
The Demineralization Treatment of the Make-up water
The ion-exchange demineralization treatment is that the cation-exchange resin exchange the cation in water to H+, the anion-exchange resin exchange the anion to OHˉ,and the undissociated water molecules are the end product. By the way the ion exchangers are named not for what they are but rather for what they do. There are different demineralization treatment systems with strong and weak electrolyte ion to adapt different demands of water quality such as primary demineralization treatment system and secondary demineralization treatment system (mixing bed).
Primary demineralization treatment system
The individual cation-exchange resin bed is the ion-exchange unit which the water encounters first. The cations, such as sodium and calcium, are removed by exchange with hydrogen ions from the resin.
The individual anion-exchange resin bed is the second ion-exchange unit the water encounters. The anions, such as silicate and carbonate, are removed by exchange with hydroxylions from the resin.
The aerating tower is used for eliminating the dissoluble gas from water, such as carbon dioxide and oxygen.
To get high quality make-up water, the mixing bed is needed. The cation and anion resin are mixed thoroughly in one bed with a certain proportion. The exchange of both ions happens simultaneously, and the outcome ions interact to form undissociated water molecules. The mixing bed produces high quality water which electrical conductivity approaches ideal pure water.
Makeup water system and its devices in Phase II of Kuching Power Station
According to the quality of the water resource and the requirement of the steam and water by the unit, the flow of the makeup water system is:
Tap water→clarified water tank→clarified water pump→active carbon filter→cation exchanger→anion exchanger →mixed bed→demineralized water→demineralized water pump→main building
The technical process of the treatment system is the same as in Phase I.
In Phase I of Kuching Power Station there are two active carbon filters and two demineralized devices with output of 26m /h. An active carbon filter and a set of demineralized devices are added in Phase II, to satisfy the requirement of the makeup water quantity by 4 units in the station. The parameters of the new supply devices are the same as that in Phase I, its output is 26 m /h; the designed operation period of the cation-exchanger and anion-exchanger is 41 hours, the operation period of the mixing exchanger is 13 days; cation resin adopts 30% of hydrochloric acid regenerating, and anion resin adopts 50% of sodium hydroxide regenerating.
The extended water treatment system possesses 3 sets of devices with two sets operating and one as standby.
The devices supplied in Phase II are as follows:
An active carbon filter; a cation-exchanger; an anion-exchanger; a mixing bed
Two demineralized water tank with each capacity of 150 m
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