Reverse Osmosis (RO) Technology In Power Plant Water Treatment
Chemical water treatment process of power plant
Chemical water treatment system of power plant I. The necessity of chemical water treatment is seen from the quality standard of water supply
The following are the boiler feed water quality standards:
Total hardness (umol/L), dissolved oxygen (μg/L), electrical conductivity (us/cm), silica (μg/L), PH (25 ℃ ℃), carbon dioxide (μg/L) standard ≤30 <50 10 <20 8.8~9.2<20
Using deep well water source, the water quality exceeding the standard most seriously is the total hardness, which refers to the average molar concentration of calcium ions (Ca2+) and magnesium ions (Mg2+) in the solution. Molar concentration refers to the number of millimoles of solute content per liter of solution. For example, the atomic weight of Ca is 40, and the mass of 1mol of Ca2+ is 80g(its chemical meaning is :1 molca 2+ contains 6.02x1023 calcium ions). If 1 solution contains 1g of Ca2+, then its molar concentration is 1/80=0.0125mol/L=12.5mmol/L.
Poor water quality, especially calcium, magnesium, sodium and silicate ions exceeding the standard, will cause the following hazards to thermal equipment:
1.Scaling of thermal equipment: if the water quality into the boiler or other heat exchanger is poor, after a period of operation, some solid attachments will be generated on the heating surface in contact with water. This phenomenon is called scaling, and these solid attachments are called scale. Because the thermal conductivity of scale is hundreds of times worse than that of metal, and these scale is easily generated in the boiler tube with high heat load, so the scale is very harmful to the boiler (or heat exchanger); It can make the temperature of the metal pipe wall in the scale part too high, cause the metal strength to decrease, so that under the action of the pressure in the pipe, there will be local deformation of the pipe, bulge, and even cause serious accidents such as tube explosion. Scaling not only endangers safe operation, but also greatly reduces the economy of power plants. For example, when there is 1mm thick scale in the economizer of the boiler of a thermal power plant, the fuel consumption is 1.5%~2.0% more than the original. Therefore, effectively preventing or reducing scaling will produce great economic benefits. In addition, the water quality of the circulating water is poor, and the scaling in the condenser of the steam turbine will lead to the reduction of the vacuum degree of the condenser, thus reducing the thermal efficiency and output of the steam turbine. The scaling of the superheater will make the steam temperature not reach the design value, which will reduce the economy of the whole thermal system. After scaling of thermal equipment, cleaning work must be carried out in time, which will shut down the equipment and reduce the annual utilization hours of the equipment; In addition, the workload and cost of maintenance should be increased.
2. Corrosion of thermal equipment and its system: the metal of thermal equipment in power plant is often in contact with water. If the water quality is poor, it will cause metal corrosion, such as water supply pipeline, coal saver, evaporator, heater, superheater and heat exchange pipe of steam turbine condenser, will be corroded due to poor water quality. Corrosion not only shortens the service life of the equipment itself, but also causes economic losses. Moreover, the corrosion product is transferred into the water, which increases the impurities in the water, thus aggravating the scaling process on the heating surface of high heat load, and the scaling will accelerate the scaling corrosion of the furnace tube. This vicious cycle can quickly lead to tube bursts and other accidents.
3. Salt accumulation in the circulation part of superheater and steam turbine: poor water quality will also make the steam dissolve and carry impurities (mainly Na+ and HSi03- ions) increase, these impurities will be deposited in the circulation part of steam, such as superheater and steam turbine, this phenomenon is called salt accumulation. The salt accumulation in the superheater tube can cause the metal tube wall to overheat or even burst. The valve will be closed loosely due to salt accumulation, and the salt accumulation in the steam turbine will greatly reduce the output and efficiency of the steam turbine. Even a small amount of salt accumulation will significantly increase the resistance of steam circulation, so that the output of the steam turbine will decrease. When the salt accumulation of steam turbine is serious, it will also increase the load of thrust bearing and bend the separator, resulting in accident shutdown.
In short, the high hardness of water supply, indicating that the content of calcium and magnesium ions is large, easy to cause the boiler each heating surface, drum and pipe wall scaling and corrosion, light affect the conduction of heat, heavy cause boiler tube burst, water impurities carried by steam to the superheater and steam turbine, it will cause salt accumulation in the steam flow part, causing further harm.
PH value is an index to judge the acidity and alkalinity of water quality, PH value =-l0g(hydrogen ion concentration in solution, mol/L). The content of both H+ and OH- in pure water is 1x10-7mol/L, so the PH=7. If acid is dissolved in water, such as hydrochloric acid HCI,H+ concentration will increase, the greater the H+ concentration, the smaller the PH value, PH<7 is acidic water quality; If alkali is dissolved in water, such as sodium hydroxide NaOH, the H+ concentration will decrease, the metal sodium ion concentration will increase, the smaller the H+ concentration, the greater the metal ion concentration, the greater the PH value, PH>7 is alkaline water quality.
Water treated by chemical method (ion exchange) shows weak alkaline (PH =8.8~9.2). Weak acidic water is corrosive to metals; Using weak alkaline water, has the advantage of passivating steel and copper surface, so that it is not easy to be corroded, and prevent iron scale and copper scale from forming on the surface of boiler and heat exchanger.
The process of water treatment
The water treatment process is divided into two major components, the first part is the physical softening water process, the second part is the chemical desalination process.
Physical softened water process: raw water (also known as raw water) from the plant water supply network, through quartz sand filter, activated carbon filter to remove solid particles and suspended impurities in the raw water, called clarified water; The clarified water is then removed by reverse osmosis device to remove most of the calcium and magnesium ions and become softened water.
Chemical desalination process: the soft water through the carbon removal device, remove the carbon dioxide in the water (strictly said to be HC03-), and then through the mixed bed, remove the residual calcium, magnesium, sodium, silicate and other harmful ions in the water, become the desalination, that is, the boiler supply water, stored in the desalination water tank, and then the desalination pump into the deaerator, and finally the boiler drum through the feed pump.
Reverse osmosis technology in power plant water treatment
Reverse osmosis mainly refers to the use of membrane separation technology to treat water, which has the characteristics of high desalination rate, strong applicability and environmental protection, and has been widely used in many industries. The core of reverse osmosis technology application lies in reverse osmosis membrane, which is made of a kind of polymer material and has selective semi-permeable film. Under the action of external pressure, the water in the solution can form a selective permeation phenomenon with some components, and then realize the purpose of purification, separation and concentration. The application of reverse osmosis technology in the water treatment of power plants can get better results, and realize the saving of water resources and environmental protection. This paper firstly states the principle and characteristics of reverse osmosis membrane technology, then analyzes the practical application of reverse osmosis technology in power plant water treatment, and finally discusses the application matters for attention of reverse osmosis technology.
Principle of reverse osmosis
Reverse osmosis is the use of sufficient pressure to let the solvent in the solution through the reverse osmosis membrane, and then separated out, the direction is opposite to the direction of osmosis, should be used to separate, purify and concentrate the solution with higher pressure than the reverse osmosis method. Because the pore size of reverse osmosis membrane is particularly small, its application can be very good at removing dissolved salts and colloids, bacteria, viruses and some organic matter in water. The most important separation object of reverse osmosis membrane is the ion in the solution, and the effective removal of salt in water can be achieved without the application of any chemical substances, and the salt removal rate can reach more than 98 percent.
Characteristics of reverse osmosis technology
Reverse osmosis technology is the application of the principle of reverse osmosis to achieve the purification and concentration of the solution, it has the separation characteristics of the great ding it has the following characteristics:
(1) The degree of automation presented by reverse osmosis technology is higher, and the energy consumption generated by it is lower in various methods. The main reason is that the driving force applied in the water treatment process is the pressure of water. Under the condition of room temperature and no phase change, it can be liked to separate the solvent and solute, the loss of active components is very small, and it is very suitable for the separation and concentration of heat-sensitive substances. Compared with the phase change separation method, the energy consumption is lower.
② There is no need to take regeneration measures, because the treatment process is a physical reaction, will not be applied to chemical substances, the product will not be contaminated.
(3) The properties of reverse osmosis membrane and its stability, in the application process will not appear phase changes, is carried out under normal temperature conditions, and the removal rate of impurities is very high.
(4) Reverse osmosis equipment can realize the application of a variety of raw water, the overall structure of the equipment is relatively simple, and the operation is more convenient and highly adaptable, the processing scale has a certain flexibility, and whether continuous operation or intermittent operation can be.
⑤ Can achieve better economic benefits. The cost of reverse osmosis system in operation is very low and the investment can be recovered in a short time.
Practical application of reverse osmosis technology in power plant water treatment
1.Recycling and utilization of circulating cooling sewage
The circulating cooling water used in thermal power plants accounts for about 70% of the total water consumption of power plants, so its recycling and utilization has very important practical significance, which can realize the saving of limited water resources. In recent years, the national requirements for environmental protection are gradually increasing, and the setting of related indicators for wastewater discharge is becoming more and more strict, which leads to a significant increase in the cost of power plants in the process of wastewater treatment. The application of reverse osmosis technology can realize the reuse of wastewater. Combined with the actual operation of various equipment in the power plant, the water obtained by reverse osmosis technology can be used in the supplementary water of circulating cooling water, and has the characteristics of safety and reliability. After the use of reverse osmosis technology, the water quality of the circulating water has been significantly improved, the turbidity has been greatly reduced, and the water supplement has also been significantly reduced. However, at present, reverse osmosis technology will produce a large cost for water treatment, and the capital investment is significantly greater than the method of water purification from natural water body. However, because it can treat wastewater at the same time, the investment of environmental costs can be reduced, and the water resources also form a certain saving, so the comprehensive cost is more obvious. Achieved a high degree of unity of economic benefits, social benefits and environmental benefits.
2.Waste liquid treatment of boiler pickling
Based on the research of simulation experiment on the treatment of pickling waste liquid in the power plant, the author compares and analyzes the treatment effect of low pressure composite membrane, cellulose acetate membrane and seawater membrane by using reverse osmosis technology and circulation mode, and then obtains the following conclusions: among the three reverse osmosis membranes, seawater membrane has the best performance. Therefore, the most suitable for reverse osmosis treatment of boiler pickling waste liquid is seawater membrane, the application of treatment is the way of circulation. Through the application of reverse osmosis technology in the treatment of boiler pickling waste liquid in power plant, it can achieve very good results and achieve the expected goal. The best way to deal with the waste liquid of citric acid in the boiler is: after the waste liquid of citric acid is first concentrated by reverse osmosis, it can be discharged or recycled. After removing iron, it is spray dried, and then the recovery of sodium citrate salt is realized. The application of the treatment technology can well solve the environmental pollution caused by the acid washing waste liquid in the boiler, and has very good social and economic benefits.3.Comprehensive treatment of wastewater
The comprehensive treatment of waste water in power plants is a systematic project, which mainly includes two important parts: waste water recovery and treatment. Reverse osmosis technology is applied in the process of waste water treatment, and the recovered domestic sewage, condensed water, acid and alkali waste water and site flushing water, etc. Their mixed water is basically acidic. After the weak acid treatment, it can be realized to reverse osmosis treatment, and the water source after this treatment can be directly applied. The application of this method not only reduces the water demand of the electric field, but also is very beneficial to the recycling of water resources in the power plant, and then enables the enterprise to achieve sustainable development.
Precautions for the application of reverse osmosis technology
Selection of device
When selecting the original reverse osmosis membrane, the characteristics of the inlet water quality should be taken into account. When it is applied in the treatment of wastewater, the anti-pollution membrane should be used, or some other pollution treatment measures should be used. The designed water temperature has a great influence on the water yield. The water quantity of the membrane element should be configured to ensure that the water yield can reach the designed amount when operating in the designed lowest water temperature environment. When A conventional REVERSE OSMOSIS WATER treatment device is designed for use, THE maximum inlet pressure for the initial operation of the REVERSE osmosis body should be less than 1.5 MPA. In the design and application of the reverse osmosis device for seawater desalination, the maximum inlet pressure presented by the initial operation of the reverse osmosis body is less than 6.9 MPA. The filtration speed designed for the filter element should not be too large. If it can be in normal operation for a long time, the replacement cycle of the filter should not be higher than three months.
Performance parameters of reverse osmosis device during operation
According TO THE ANALYSIS of the conventional REVERSE OSMOsis problem, the operating parameters (desalination rate and recovery rate, etc.) of the REVERSE OSMOsis unit in operation should meet the requirements of the contract. Generally, the desalination rate should be greater than 98 percent and the recovery rate should be greater than 75 percent in the first year. The water production should meet the national standards under certain water temperature conditions, and the valve switch should be more flexible.
In all, the electric power industry is the basic industry that provides high quality electric energy for People's Daily life, which is of great practical significance to the improvement of people's living standards and economic growth. The application of reverse osmosis technology in power plant water treatment has formed a good effect, that is, reducing the occurrence of environmental pollution, but also to achieve the saving of water resources. The reverse osmosis device technology is combined with the actual situation of power plant water treatment, and then the cost of materials applied by reverse osmosis technology is reduced, the universal application of reverse osmosis technology in power plant is realized, and the double harvest of economic and social benefits of power plant is realized.