After the circulating cooling water is used, the Ca2+, Mg2+, Cl-, SO42- plasma, dissolved solids and suspended solids in the water increase correspondingly, and pollutants in the air, such as dust, sundries, soluble gases and leakage of heat exchanger materials, can enter the circulating cooling water, causing corrosion and scaling of equipment and pipelines in the circulating cooling water system, resulting in lower heat transfer efficiency of the heat exchanger and reduced water-passing section.
Scaling, corrosion and microbial reproduction are interrelated in the circulating cooling water system. Dirt and microbial slime can cause corrosion under scale, and corrosion products form dirt. To solve these problems in the circulating cooling water system, comprehensive treatment must be carried out.
water quality stabilization technology is adopted to control and improve the water quality by combining physical and chemical treatment, so that corrosion, scaling and biological fouling in the circulating cooling water system can be effectively solved, thus achieving good benefits of water saving and energy saving. Ozone products have been widely used in domestic electronics, electric power, beverage and pharmaceutical industries, and the quality has reached the level of the same industry abroad in the 199s. The comparable benefits of input-output ratio are: more than 1: 2-1: 1, saving energy, improving equipment utilization efficiency, prolonging equipment service life and operation safety, and reducing environmental pollution.
Ozone can be used as the only treatment agent to replace other cooling water treatment agents. It can scale, inhibit corrosion and sterilize, and make the cooling water system run at high concentration factor or even zero pollution discharge, thus saving water and energy and protecting water resources. At the same time, ozone cooling water treatment does not have any environmental pollution. The application of ozone in circulating water treatment abroad has been successful, but this field is blank in China. Industrial (power plant) circulating cooling water treatment system
I. Overview
After the circulating cooling water is used, the Ca2+, Mg2+, Cl- and SO42- plasma, dissolved solids and suspended solids in the water increase correspondingly, and pollutants in the air, such as dust, sundries, soluble gases and material leakage of heat exchangers, can enter the circulating cooling water, causing corrosion and scaling of equipment and pipelines in the circulating cooling water system.
Scaling, corrosion and microbial reproduction are interrelated in the circulating cooling water system. Dirt and microbial slime can cause corrosion under scale, and corrosion products form dirt. To solve these problems in the circulating cooling water system, comprehensive treatment must be carried out.
water quality stabilization technology is adopted to control and improve the water quality by combining physical and chemical treatment, so that corrosion, scaling and biological fouling in the circulating cooling water system can be effectively solved, thus achieving good benefits of water saving and energy saving. Ozone products have been widely used in domestic electronics, electric power, beverage and pharmaceutical industries, and the quality has reached the level of the same industry abroad in the 199s. The comparable benefits of input-output ratio are: more than 1: 2-1: 1, saving energy, improving equipment use efficiency, prolonging equipment service life and operating safety, and reducing environmental pollution.
China is a water-deficient country, because according to international standards, a water supply of less than 1, tons per person per year is a water-deficient country. At present, the water shortage in China is over 1 billion cubic meters. The per capita water resources in many areas are close to those in Israel, a world-famous water shortage country. This is the case in the Loess Plateau. China is listed as one of the water-poor countries in the world. Especially in the vast areas of the north and west, the water shortage is particularly serious. The water shortage caused by surface water pollution in southeast China is also very serious. Among the 67 large and medium-sized cities in China, 4 cities are short of water to varying degrees. Among them, 11 cities are seriously short of water. According to the latest data, the per capita water resources in Beijing is currently less than 3 cubic meters. Faced with such a severe situation of water shortage, China's industrial water consumption is staggering. Mainly due to the low reuse rate of industrial water. The reuse rate of industrial water is only 2-3%. Only one third of that of developed countries. If one ton of steel is produced, the water consumption of China is more than 1-6 times that of the international advanced level, and that of China is more than 3 times that of the international advanced level. Water resources are also wasted in other industries. Saving water has become a top priority for our country, and the problem of water shortage will also seriously restrict the sustainable economic development of our country in this century, and will also cause social and environmental problems such as ecological environment degradation, human settlements deterioration, and increasingly prominent contradiction over water. In order to save water, the country is formulating and implementing some specific measures and policies to encourage water conservation, improve water reuse and sewage treatment and reuse. China will gradually implement measures such as rationing water supply, raising water prices and imposing fines for excessive water use. According to relevant information, before 26, the water price in the northern region will be raised to 4 yuan/ton for civil water and 6 yuan/ton for industrial water, and water supply will be rationed for different industries, and the excess will be fined six times. This will force all industries and residents to raise their awareness of water conservation.
at present, it is a foregone conclusion that the cost of water consumption and drainage in various units is gradually increasing rapidly. However, as far as the cooling circulating water process in power plants is concerned, although it will be improved continuously, it is impossible to make revolutionary progress in a short time because it is a mature process for many years. It seems that it is difficult for the main process to tap the potential to play an important role in reducing costs and improving efficiency. In this case, it is obviously necessary to make water articles, and only from the perspective of improving the economic benefits of enterprises, it will also have a multiplier effect. Therefore, the reuse of sewage after advanced treatment has become a general trend for enterprises to improve efficiency, clean production, save energy and reduce environmental pollution.
ii. technical background and significance
circulating cooling water is a major item of industrial water consumption. In petrochemical, electric power, steel, metallurgy and other industries, the consumption of circulating cooling water accounts for 5-9% of the total water consumption of enterprises. Because of the different salt content in the raw water, a certain amount of concentrated water must be discharged when the circulating cooling water is concentrated to a certain multiple, and new water must be added. For a 3, KW condensing unit, the circulating cooling water volume should reach about 33, tons/hour. Assuming that the salt content in the raw water is 1mg/L and the concentration factor is 3, the concentrated water discharge of the circulating cooling water is about 6-8 ‰, that is, 198-264 m3/h, and the new water to be supplemented is equal to the drainage and evaporation loss, and the supplementary water volume is about 2-2.6% of the circulating water volume.
due to the restriction of concentration multiple, circulating cooling water must discharge a certain amount of concentrated water and replenish a certain amount of new water during operation. Control the salt content, PH value, organic matter concentration and suspended matter content in cooling water within a reasonable allowable range. It is of great significance to treat and reuse this part of concentrated water discharge. It can not only improve the reuse rate of water and save water resources, but also greatly improve the overall situation of circulating cooling water.
III. Present situation and existing problems of circulating cooling water
Circulating cooling water is pumped to all users in the cooling system, and after heat exchange, its temperature rises and is sent to the cooling tower for cooling. In the cooling tower, hot water is sprayed downward from the top of the tower into water drops or water films, while air flows reversely or horizontally, and heat exchange is carried out during the gas-water contact. When the water temperature drops to meet the requirements of cooling water, it will continue to be recycled.
When the air overflows from the top of the tower, it takes away the water vapor, which increases the ion content in the circulating water. Therefore, it is necessary to replenish fresh water and discharge concentrated water to maintain the salt content at a certain concentration, thus ensuring the normal operation of the whole system. The amount of make-up water should make up for the water loss caused by evaporation, wind blowing (including splashing and entrainment) and sewage discharge. The ratio of salt content in circulating water to make-up water is the concentration multiple of the circulating water system. In a certain circulating cooling water system, as long as the salt content of make-up water is changed, the concentration multiple of the circulating cooling water system can be changed, and improving the concentration multiple is the key to ensure the economic operation of the whole circulating cooling water system.
The cooling water is continuously recycled in the circulation system. Due to the rising water temperature, the change of flow rate, evaporation and the concentration of various inorganic ions and organic substances, the cooling tower and cooling pool are exposed to sunlight, wind and rain, the entry of dust and sundries, and the combined effects of equipment structure and materials, many problems will occur.
1. Scale adhesion
In the circulating cooling water system, the concentration of bicarbonate increases with evaporation and concentration. When its concentration reaches a supersaturated state, or when the water temperature on the heat transfer surface rises, it will decompose to generate carbonate and deposit on the heat transfer surface, forming dense slightly soluble salt scale, which has poor thermal conductivity (≤1.16W/(m.K), compared with 45W/(m.K) for steel). Therefore, the scale adhesion will reduce the heat transfer efficiency of the heat exchanger, and in severe cases, it will block the heat exchanger, increase the system resistance, decrease the efficiency of water pump and cooling tower, increase the production energy consumption, decrease the output, accelerate local corrosion, and even cause abnormal shutdown.
2, equipment corrosion
In the circulating cooling water system, a large number of equipment are made of metal, and corrosion perforation will occur if circulating cooling water is used for a long time. This is caused by many factors, mainly including: electrochemical corrosion caused by dissolved oxygen in cooling water; Corrosion caused by harmful ions (Cl- and SO42-); Corrosion caused by microorganisms (anaerobic bacteria, iron bacteria), etc. Corrosion and perforation of equipment pipe wall will cause leakage, or process medium will leak into cooling water, which will lose materials and pollute water; Or cooling water seeps into the process medium, affecting product quality, causing economic losses and affecting safety production.
3. Microbial growth and slime
In circulating water, the concentration of nutrients, rising water temperature and sunlight create conditions for the rapid propagation of bacteria and algae. Mucus secreted by bacteria makes dust impurities and chemical deposits floating in water stick together, forming deposits attached to the heat transfer surface, that is, biological slime or soft scale. Sticky mud adhesion will cause corrosion, reduce cooling water flow, and then reduce cooling efficiency; In severe cases, the pipeline will be blocked, forcing the production to stop and clean. To sum up, after long-term recycling of cooling water, it will inevitably bring scaling, corrosion and microbial growth problems. Only by solving these three problems can we stabilize production, save resources and energy, thus reducing environmental pollution and improving economic benefits.
iv. current situation of circulating cooling water treatment technology
1. scale control
calcium carbonate scale is the most easily generated scale in circulating water system, and scale control is to prevent calcium carbonate precipitation. There are roughly the following methods.
(1) The calcium and magnesium ions that are removed from the make-up cooling water will be softened before the make-up water enters the circulating water system, and Ca2+ and Mg2+ will be removed, so no scale will be formed. At present, there are two commonly used softening methods: < P > One is ion exchange resin method, which is suitable for circulating water systems with small supplementary water; The second is lime softening method, that is, adding lime to make CaCO3 precipitate generated by the reaction of Ca(HCO3)2 precipitate in advance. This method has low cost and is suitable for circulating cooling water system with high calcium content in raw water (especially scaling raw water with temporary hardness) and large supplementary water.
⑵ add acid or introduce CO2 gas to lower the PH value and stabilize bicarbonate
add acid (usually sulfuric acid) or introduce CO2 gas into circulating water to lower the PH value, so that the following equilibrium shifts to the left and bicarbonate is in a stable state.
Ca (HCO3) 2 = = = = = CaCO3+H2O+CO2
Acid addition method is still used at present. The key is to control the amount of acid addition, otherwise too much acid will accelerate the corrosion of equipment. When CO2 gas is introduced, the PH value should also be well controlled, otherwise, when circulating water passes through the cooling tower, CaCO3 will crystallize in the tower due to the overflow of CO2, which will block the packing and form the phenomenon of calcium scale transfer. This method is still worth popularizing in some chemical fertilizer plants, chemical plants and power plants with CO2 gas sources.
(3) adding scale inhibitor
adding scale inhibitor in circulating water destroys the crystallization growth process of CaCO3, so as to control the formation of scale. At present, the commonly used scale inhibitors include polyphosphate, organic polyphosphonic acid, organic phosphate ester, polyacrylate and so on, which is also the most widely used method to control scale.
2. Control of fouling
To control fouling, efforts can be made in the following aspects:
① Pretreatment of make-up water to reduce turbidity
② Good treatment of circulating water quality
③ Adding dispersant can disperse the adhered sludge and impurities into particles and suspend them in water, which will flow with the water without deposition, thus reducing the influence of fouling on heat transfer, and some suspended substances can be discharged with sewage.
(4) Adding by-pass filtration equipment If by-pass filtration equipment is added to the system, and the by-pass flow rate and turbidity of inlet and outlet by-pass equipment are well controlled, the turbidity of the system under long-term operation can be kept within the control index, and the formation of dirt can be reduced.
3. Control of metal corrosion in circulating cooling water system
There are four commonly used methods to control metal corrosion in circulating cooling water system:
(1) Adding corrosion inhibitor is an additive used to inhibit metal corrosion in corrosive medium, which does not change the properties of corrosive medium, and does not need special equipment or surface treatment. Therefore, the use of corrosion inhibitor is a metal protection measure with high economic benefit and strong adaptability. In the open circulating water system, the commonly used corrosion inhibitors are silicate, molybdate, zinc salt, phosphate, polyphosphate, organic polyphosphonic acid, mercaptobenzothiazole (MBT), benzotriazole (BTA), methylbenzotriazole (TTA), ferrous sulfate, etc. In order to alleviate the pressure of environmental eutrophication, the latter organic phosphonates and low phosphorus corrosion inhibitors are more inclined to be used at present.
(2) Increasing the PH value of circulating water increases the tendency of forming oxidation protective film on metal surface, which is easy to passivate, thus being beneficial to control equipment corrosion. The open circulating cooling water system usually increases the PH value by aeration in the cooling tower. When CO2 in water and air reaches balance, the PH value of water is about 8.5. After increasing the PH value of circulating water, it will inevitably bring some problems: the scaling tendency of circulating water will increase; The corrosion rate of equipment has decreased, but it still cannot meet the requirements; Some common corrosion inhibitors failed. At present, it can be solved by adding compound corrosion inhibitors specially developed for alkaline cooling water treatment, such as polyphosphate-zinc salt-phosphonate-dispersant, polyphosphate-orthophosphate-phosphonate-ternary polymer, organic polyphosphonic acid-polymer dispersant-azole, polyol phosphate-acrylic polymer, HEDP-PMA and so on. The compound formula of these water treatment agents can play a comprehensive role in scale removal and corrosion prevention. Because of the synergistic or synergistic effect, it has a more significant effect than the single action of a single agent, which is also the development trend of corrosion inhibitors.
(3) heat exchangers made of corrosion-resistant materials, such as polypropylene heat exchangers or graphite modified polypropylene heat exchangers, are rarely used because of their poor heat exchange effect.
(4) coating with anti-corrosion coating protects the equipment from corrosion through shielding, corrosion inhibition, cathodic protection and PH buffering of anti-corrosion coating.
4. microbial control of circulating cooling water system
2. system technology
circulating water cooling is usually divided into closed circulating water cooling system and open circulating water cooling system. In the closed circulating water cooling system, water is in closed circulation, and the cooling of water is not in direct contact with air. open wide