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 all enter the circulating cooling water, causing corrosion and scaling of equipment and pipelines in the circulating cooling water system, reducing the heat transfer efficiency of heat exchangers, reducing the cross section of water, and even causing corrosion and perforation of equipment and pipelines.
Scaling, corrosion and microbial reproduction in circulating cooling water system are interrelated. Dirt and microbial mucus can cause corrosion under scale, and corrosion products form dirt. To solve these problems in circulating cooling water system, comprehensive treatment must be carried out.
By using water quality stabilization technology combined with physical and chemical treatment to control and improve water quality, the problems of corrosion, scaling and biological scaling in circulating cooling water system can be effectively solved, and good water-saving and energy-saving benefits can be achieved. Ozone products have been widely used in domestic electronics, power, beverages, pharmaceuticals and other industries, and the quality has reached the level of the same industry abroad in the 1990s. The comparative benefit of input-output ratio is: 1: 2- 1: 10, which can save energy, improve equipment utilization efficiency, prolong equipment service life and operation safety, and reduce environmental pollution.
Ozone can be used as the only treatment agent to replace other cooling water treatment agents, which can scale, inhibit corrosion and sterilize, make the cooling water system run at high concentration multiple or even zero emission, save water and energy and protect water resources; At the same time, ozone cooling water treatment has no environmental pollution. The application of ozone in circulating water treatment has been successful abroad, but it is still blank in China. Industrial (power plant) circulating cooling water treatment system
I. Overview
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 all enter the circulating cooling water, causing corrosion and scaling of equipment and pipelines in the circulating cooling water system, reducing the heat transfer efficiency of heat exchangers, reducing the cross section of water, and even causing corrosion and perforation of equipment and pipelines.
Scaling, corrosion and microbial reproduction in circulating cooling water system are interrelated. Dirt and microbial mucus can cause corrosion under scale, and corrosion products form dirt. To solve these problems in circulating cooling water system, comprehensive treatment must be carried out.
By using water quality stabilization technology combined with physical and chemical treatment to control and improve water quality, the problems of corrosion, scaling and biological scaling in circulating cooling water system can be effectively solved, and good water-saving and energy-saving benefits can be achieved. Ozone products have been widely used in domestic electronics, power, beverages, pharmaceuticals and other industries, and the quality has reached the level of the same industry abroad in the 1990s. The comparative benefit of input-output ratio is: 1: 2- 1: 10, which saves energy, improves equipment use efficiency, prolongs equipment service life and operation safety, and reduces environmental pollution.
China is a water-deficient country, because according to international standards, the annual water supply per person is below 1000 tons. At present, China's water shortage exceeds 1000 billion cubic meters. The per capita water resources in many areas are close to the world-famous water shortage country Israel. This is the case in the Loess Plateau. China is listed as one of the countries lacking water in the world. Especially in the vast areas in the north and west, water shortage is particularly serious. The water shortage caused by surface water pollution in southeast China is also very serious. Among the 670 large and medium-sized cities in China, 400 cities are short of water to varying degrees. Among them, 1 10 city is seriously short of water. The latest data shows that the per capita water resources in Beijing are currently less than 300 cubic meters. Faced with such a severe water shortage situation, the industrial water consumption in China is amazing. Mainly due to the low reuse rate of industrial water. The reuse rate of industrial water is only 20-30%. Only one third of that of developed countries. If 1 ton steel is produced, the water consumption of China is more than 6 times of the international advanced level, and the water consumption of China 1 ton paper is more than 3 times of the international advanced level. Other industries also waste water resources. Saving water has become a top priority in our country, and the problem of water shortage will seriously restrict the sustainable development of our economy in this century, and will also cause social and environmental problems such as the deterioration of ecological environment, the deterioration of human settlements and the increasingly prominent contradiction of water resources. In order to save water, the state is formulating and implementing some specific measures and policies to encourage water conservation and improve water reuse and sewage treatment and reuse. China will gradually implement measures such as rationing water supply, raising water prices and imposing fines on excessive water use. According to relevant data, before 2006, the water price in northern China will be raised to 4 yuan/ton for civil water and 6 yuan/ton for industrial water, and limited water supply will be implemented for different industries, and the excess will be fined 6 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 and drainage for each unit is gradually increasing rapidly. However, as far as the cooling circulating water process of power plant 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 and play an important role in reducing costs and increasing efficiency. In this case, it is obviously necessary to do a good job in water articles. 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 the general trend for enterprises to improve efficiency, clean production, save energy and reduce environmental pollution.
Second, the technical background and significance
Circulating cooling water is the main project of industrial water use. In petrochemical, electric power, steel and metallurgy industries, the consumption of circulating cooling water accounts for 50-90% of the total water consumption of enterprises. Because of the different salt content in 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 300,000 kW condensing unit, the circulating cooling water should reach about 33,000 tons/hour. Assuming that the salt content of raw water is 1000mg/L and the concentration coefficient is 3, the concentrated water discharge of circulating cooling water is about 6-8 ‰, that is, 198-264 m3/h, and the new water to be replenished is equal to the loss of drainage and evaporation.
Due to the limitation of concentration multiple, a certain amount of concentrated water must be discharged from circulating cooling water during operation, and a certain amount of new water must be supplemented. 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.
Three. Present situation and existing problems of circulating cooling water
Circulating cooling water is pumped to all users in the cooling system. After heat exchange, the 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 in the process of gas-water contact. When the water temperature drops to meet the requirements of cooling water, it will continue to be recycled.
When air overflows from the top of the tower, it takes away water vapor and increases the ion content in circulating water. Therefore, it is necessary to replenish fresh water and discharge concentrated water to keep the salt content at a certain concentration, thus ensuring the normal operation of the whole system. The amount of supplementary 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 of circulating water to supplementary water is the concentration multiple of 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.
Cooling water circulates continuously in the circulation system. Due to the increase of water temperature, the change of flow and evaporation, and the concentration of various inorganic ions and organic substances, the cooling tower and cooling pool are exposed to sunlight, wind and rain, dust and sundries, and the comprehensive effect 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 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, with poor thermal conductivity (≤ 1. 16W/(m.K), compared with 45W/(m.K) for general steel. Therefore, scaling will reduce the heat transfer efficiency of the heat exchanger. In severe cases, it will block the heat exchanger, increase the system resistance, reduce the efficiency of water pumps and cooling towers, increase production energy consumption, reduce output, accelerate local corrosion, and even cause abnormal shutdown.
2, equipment corrosion
In the circulating cooling water system, a large number of devices are made of metal. If circulating cooling water is used for a long time, corrosion and perforation will occur. This is caused by many factors, 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. Leakage caused by corrosion and perforation of equipment pipe wall, or leakage of process medium into cooling water, resulting in material loss and water pollution; Or the cooling water seeps into the process medium, which affects the product quality, causes economic losses and affects the safety production.
3. Microbial growth and sludge
In circulating water, the concentration of nutrients, the rise of water temperature and the irradiation of sunlight create conditions for the rapid reproduction of bacteria and algae. Mucus secreted by bacteria makes dust and impurities floating in water stick together with chemical deposits, forming deposits attached to the heat transfer surface, that is, biological mucus 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 production to stop and clean up. To sum up, after long-term recycling of cooling water, it will inevitably bring about scaling, corrosion and microbial growth. Only by solving these three problems can we stabilize production, save resources and energy, thus reducing environmental pollution and improving economic benefits.
Fourth, the current situation of circulating cooling water treatment technology
1, proportional control
Calcium carbonate scale is the most easily generated scale in circulating water system, and controlling scale is to prevent calcium carbonate precipitation. There are roughly the following methods.
(1) The calcium and magnesium ions 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 without forming scale. At present, there are two commonly used softening methods:
One is ion exchange resin method, which is suitable for circulating water system with small supplementary water. The second is lime softening method, that is, adding lime makes CaCO3 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 PH value and stabilize bicarbonate.
Adding acid (usually sulfuric acid) to circulating water or introducing CO2 gas to lower the PH value, so that the lower balance moves to the left, and the bicarbonate is in a stable state.
ca(HCO 3)2 = = = = = = CaCO3+H2O+CO2
At present, the method of adding acid is still adopted, and the key is to control the amount of acid added, otherwise too much acid will accelerate the corrosion of equipment. When introducing CO2 gas, 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, blocking the filler and forming 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 source.
(3) adding scale inhibitor
Adding scale inhibitor to circulating water can destroy the crystal growth process of CaCO3, thus controlling the formation of scale. At present, the commonly used scale inhibitors include polyphosphate, organic polyphosphonic acid, organic phosphate, polyacrylate, etc., which is also the most widely used scale prevention method.
2. Fouling control
To control fouling, efforts can be made in the following aspects:
(1) Pretreatment of make-up water to reduce turbidity.
(2) Do a good job in circulating water quality treatment.
⑶ Adding dispersant can disperse the attached mud and impurities into particles and suspend them in water, which will flow with the water without precipitation, thus reducing the influence of dirt on heat transfer, and some suspended substances can also be discharged with sewage.
(4) Adding bypass filter equipment If bypass filter equipment is added to the system and the bypass flow and turbidity of the inlet and outlet bypass equipment are well controlled, the turbidity of the system can be kept within the control index during long-term operation and the formation of dirt can be reduced.
3. Control of metal corrosion in circulating cooling water system
There are four common methods to control metal corrosion in circulating cooling water system:
(1) Adding Corrosion Inhibitor Corrosion Inhibitor is an additive for inhibiting metal corrosion in corrosive media. Its dosage is small, it will not change the properties of corrosive media, and it 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, commonly used corrosion inhibitors include silicate, molybdate, zinc salt, phosphate, polyphosphate, organic polyphosphonic acid, mercaptobenzothiazole (MBT), benzotriazole (BTA), methylbenzotriazole (TTA), ferrous sulfate and so on. In order to alleviate the pressure of environmental eutrophication, the latter organic phosphonate and low phosphorus corrosion inhibitor are preferred at present.
⑵ Increasing the PH value of circulating water increases the tendency of forming oxide protective film on metal surface, which is easy to passivate and is beneficial to control the corrosion of equipment. The open circulating cooling water system usually improves the PH value by aeration in the cooling tower. When CO2 in water and air reaches equilibrium, 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 increases; The corrosion rate of equipment has decreased, but it still can't meet the requirements; Some common corrosion inhibitors have 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-terpolymer, organic polyphosphate-polymer dispersant-pyrrole, polyol phosphate-acrylic polymer, HEDP-PMA and so on. The compound formula of these water treatment agents can play a comprehensive role in descaling and anticorrosion. Because of its synergistic or synergistic effect, it has more remarkable effect than the single action of a single agent, which is also the development trend of corrosion inhibitors.
⑶ 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.
⑷ Coating anticorrosion coating is to protect equipment from corrosion through shielding, corrosion inhibition, cathodic protection and PH buffering of anticorrosion coating.
4. Microbial control of circulating cooling water system
Second, the 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. In the open circulating water cooling system, water needs to be in direct contact with air, which is divided into water surface cooling, spray cooling pool cooling and cooling tower cooling according to the different contact modes between water and air.
Open circulating water cooling system can be divided into the following three categories:
1. pressure reflux circulating cooling system
Generally, the water quality of this circulating water system is not polluted, and only a small amount of water lost in the process of recycling is replenished. Make-up water can flow into the cold water pool or the lower part of the cooling structure. The cold water pool can also be located under the cooling tower and merged with the water collection pool.
Make-up water → cold pool → circulating pump house
↑ ↓
Cooling tower/workshop or cooling equipment
Pressure reflux circulating cooling system