A method for resource utilization of circulating water cooling sewage from thermal power plants

Abstract

The present invention provides a method for resource utilization of circulating cooling water wastewater from thermal power plants, belonging to the field of environmental protection technology. The method first involves softening the circulating cooling water with lime-soda to remove temporary calcium and magnesium ions, and then transferring the resulting softened sludge to a desulfurization system for use as a desulfurizer. The effluent from the lime-soda softening process is then passed through an ultrafiltration system to further remove suspended solids, producing ultrafiltration product water. The ultrafiltration concentrate and backwash effluent are then recycled to the front end of the lime-soda softening process for solid-liquid separation. The ultrafiltration product water is then passed through a sulfate adsorption device. The adsorption filtrate is then concentrated through reverse osmosis, and the reverse osmosis product water is recycled to the circulating cooling water system. The reverse osmosis concentrate is then passed through a bipolar membrane system to produce hydrochloric acid and sodium hydroxide solution. After the sulfate adsorption device is saturated, a portion of the reverse osmosis concentrate is introduced into the sulfate adsorption device, and the desorbed liquid is recycled to the desulfurization system. The sulfate ions in the desorbed liquid combine with the calcium ions in the desulfurizer to form gypsum. This method minimizes acid and alkali consumption and waste salt production in thermal power plants.

Description

Technical Field

[0001] The present invention relates to the technical field of water treatment, and in particular to a method for resource utilization of circulating water cooling sewage in a thermal power plant. Background Art

[0002] The amount of water used and discharged by a thermal power plant's circulating water system accounts for 80% to 90% of the total water consumption and total discharge of circulating cooling thermal power plants. Saving water and reducing emissions from the circulating water system are key water conservation and wastewater management efforts for these thermal power plants. Currently, most circulating cooling power plants have increased their concentration ratios by strengthening raw water pretreatment, optimizing water stabilizers, and upgrading condenser pipes. Circulating water wastewater is reused for desulfurization, coal transportation, and slag removal, but a large amount of excess circulating water wastewater still needs to be discharged. Furthermore, some power plants have adopted reverse osmosis technology to desalinate and reuse circulating water wastewater. However, these plants all suffer from issues such as incomplete pretreatment, severe membrane fouling, low system recovery rates, frequent chemical cleaning, and shortened membrane life. Furthermore, the resulting brine cannot be reused. As TDS requirements for discharged wastewater become increasingly stringent in various regions, the difficulties in treating circulating water wastewater are becoming increasingly apparent.

[0003] High salt content and high hardness are the main water quality characteristics of circulating wastewater from thermal power plants. Desalination treatment is required for reuse. Currently, reverse osmosis and ion exchange are commonly used desalination methods in industry.

[0004] Reverse osmosis and electrodialysis are commonly used membrane separation technologies, characterized by simple equipment and high separation coefficients. However, electrodialysis is susceptible to interference and contamination from charged substances in the solution, and its ability to remove only specific substances limits its application. In contrast, reverse osmosis is unaffected by charged substances and can remove the vast majority of inorganic and organic matter in solution. Furthermore, the properties of domestically produced membranes have reached a relatively advanced level, leading to its increasing application.

[0005] Ion exchange is one of the most widely used desalination methods, and data shows that many companies use this technology in wastewater treatment. While it utilizes simple equipment and is relatively inexpensive, it requires frequent resin regeneration, which consumes large amounts of regeneration fluid. This not only significantly increases costs but also generates significant waste, causing significant environmental pollution.

[0006] Currently, there are two main treatment options for circulating wastewater from thermal power plants: UF+RO double membrane process and lime softening+ion exchange+RO process.

[0007] 1. UF+RO double membrane method

[0008] Currently, this technology is the most widely used process for recycling wastewater from circulating water. Most thermal power plants utilize this technology to treat circulating wastewater and recover fresh water for use as make-up water. Typically, due to the significant evaporation of water during the operation of the circulating cooling system, the salinity of the circulating cooling water increases, necessitating a double-membrane desalination process. The main process involves allowing the wastewater to settle for a period of time before pouring it into a clear water tank. It is then pumped through a fiber filter and ultrafiltration system, followed by reverse osmosis treatment. After desalination, the water can be used as make-up water for circulating water, improving water resource utilization efficiency.

[0009] 2. Lime softening + ion exchange softening + RO treatment process

[0010] For the recycling and treatment of circulating sewage with high hardness, chemical treatment methods are needed to reduce the hardness of the water before RO desalination. This circulating sewage treatment lime softening technology and ion exchange softening technology can not only improve the reverse osmosis effect, but also increase the sewage reuse rate.

[0011] The existing circulating sewage treatment technology has achieved an improvement in the water utilization rate of thermal power plants. However, because it does not take into account the resource disposal of ionic substances, a large amount of concentrated brine is produced. This part of the concentrated brine cannot be fully utilized due to its strong corrosiveness and high scaling risk. As the country's control over industrial waste salt becomes increasingly strict, both discharge and zero-discharge treatment of wastewater have caused thermal power companies to face great environmental pressure and operational risks. Summary of the Invention

[0012] To solve the above problems, the present invention discloses a method for resource utilization of circulating water cooling wastewater in thermal power plants, which can effectively recycle and utilize the main substances such as water, chloride ions, sodium ions, sulfate ions, etc. in the circulating cooling wastewater, realize the resource utilization of the circulating cooling wastewater, and minimize the acid and alkali consumption and waste salt production of the thermal power plant.

[0013] To achieve the above object, the technical solution of the present invention is as follows:

[0014] A method for resource utilization of circulating water cooling wastewater in a thermal power plant, the method comprising the following steps:

[0015] Step 1: The circulating cooling wastewater is softened with lime soda to remove temporary calcium and magnesium ions, and the resulting softened sludge is transported to the desulfurization system for use as a desulfurizer;

[0016] Step 2: The effluent softened by lime-soda in step 1 is further filtered through an ultrafiltration system to remove suspended solids to obtain ultrafiltration water. The ultrafiltration concentrated water and backwash discharge water are recycled to the front end of the lime-soda softening process to achieve solid-liquid separation;

[0017] Step 3: The ultrafiltration water passes through the sulfate adsorption device, where the sulfate ions in the water are adsorbed by the ion exchange resin. The adsorption filtrate produced by the sulfate adsorption device contains more than 97% of the total dissolved solid mass percentage of sodium chloride.

[0018] Step 4: The adsorption filtrate is concentrated by reverse osmosis, and the mass concentration of sodium chloride is increased to more than 5%. The reverse osmosis produced fresh water is recycled and reused in the circulating cooling water system;

[0019] Step 5: The reverse osmosis concentrated water passes through a bipolar membrane system to produce hydrochloric acid and sodium hydroxide solutions, obtaining 3-4% hydrochloric acid and 5-6% sodium hydroxide, which are used for regeneration of the power plant ion exchange system and pH adjustment of industrial wastewater; the bipolar membrane fresh water is returned to the reverse osmosis system for further concentration treatment;

[0020] Step 6: After the sulfate adsorption device is saturated with adsorption, part of the reverse osmosis concentrated water is introduced into the sulfate adsorption device. At the same time, auxiliary direct current is used to enhance the desorption effect, so that the ion exchange resin in the sulfate adsorption device is converted from sulfate type to chloride type. The concentration ratio of sodium sulfate and sodium chloride in the desorption liquid is controlled at above 2:1. The desorption liquid is recovered to the desulfurization system, and the sulfate ions in the desorption liquid combine with the calcium ions in the desulfurizer to form gypsum.

[0021] Furthermore, the total dissolved solids in the circulating cooling wastewater in step 1 after being softened by lime soda are controlled within a range of 1-5 g / L, and the calcium and magnesium ion concentrations are within a range of 0.1-3 g / L.

[0022] Furthermore, the specific method of lime soda softening in step 1 is: adding lime in the first stage, controlling the pH value of the solution to 10.3, and the reaction time to 0.5 to 1 hour; adding sodium carbonate in the second stage, and the reaction time to 0.5 to 1 hour; controlling the concentration of calcium and magnesium ions in the effluent to be no higher than 50 mg / L, and setting a sedimentation tank in the lime soda softening system to achieve preliminary solid-liquid separation, and the SS of the effluent from the sedimentation tank does not exceed 20 mg / L; adding hydrochloric acid to the effluent from the sedimentation tank to adjust the pH value to 7 to 8.

[0023] Furthermore, in the ultrafiltration system described in step 2, a cross-flow filtration operation mode is adopted, and an ultrafiltration membrane with an accuracy of 0.02-0.05 mm is selected.

[0024] Furthermore, in the sulfate adsorption device described in step 3, a weak base anion exchange resin or a strong base anion exchange resin adsorbs sulfate, the operating flow rate is 20-40 m / h, and the sodium sulfate concentration of the adsorption filtrate is controlled below 0.5 g / L.

[0025] Furthermore, in the sulfate adsorption device described in step 6, the sulfate desorption stock solution adopts concentrated water with a sodium chloride content of 10% from the reverse osmosis system, the desorption flow rate is 4-8 m / h, and the auxiliary electric drive desorption is used to enhance the desorption effect, and the auxiliary electric desorption current density is 200-300 A / m 2, control the sodium sulfate concentration of the desorption solution to 2%~3%, and the sodium chloride concentration to 1%~1.5%.

[0026] Furthermore, the reverse osmosis influent sodium chloride concentration ranges from 1 to 5 g / L, the desalination rate is ≥ 97%, and the reverse osmosis concentrated water sodium chloride concentration is controlled at 5 to 6%.

[0027] Furthermore, the bipolar membrane system in step 5 uses a homogeneous ion exchange membrane, the influent sodium chloride concentration is 5-6%, the produced hydrochloric acid concentration is 3-4%, the sodium hydroxide concentration is 5-6%, and the operating current density is 300-500A / m 2 The membrane surface flow rate is 3-5 cm / s. The sodium chloride concentration of the fresh water discharged from the bipolar membrane system is 2-3%, which is recycled to the reverse osmosis system for further concentration treatment.

[0028] Compared with the prior art, the present invention has the following beneficial effects:

[0029] 1. The present invention achieves purification of sodium chloride solution in circulating cooling wastewater by using sulfate ion selective adsorption and desorption technology, creating favorable conditions for the stable operation of a bipolar membrane system using the sodium chloride component in the circulating cooling wastewater as a raw material, and effectively ensuring the quality stability and reliability of hydrochloric acid and sodium hydroxide produced by the circulating cooling wastewater treatment system of a thermal power plant.

[0030] 2. The present invention uses a high-concentration, high-purity sodium chloride solution as the desorption stock solution and assists in the application of electric-driven desorption technology to achieve accurate control of the sodium sulfate-sodium chloride concentration ratio of the desorption solution, thereby minimizing the amount of chloride ions discharged into the desulfurization system. The sulfate in the desorption solution is eventually precipitated in the form of gypsum and sold as a product, thereby minimizing the output of solid waste.

[0031] 3. The application of the electric-driven desorption technology in the present invention reduces the reagent consumption in the desorption process and improves the desorption efficiency.

[0032] 4. This invention combines sulfate ion-selective ion exchange adsorption technology with electrically driven assisted desorption technology to purify sodium chloride from circulating cooling wastewater. This technology minimizes the risk of system scaling and ensures the operational stability of the subsequent reverse osmosis concentration system. The concentration of the sodium chloride solution (reverse osmosis concentrate) is controlled above 5%, meeting the requirements for producing hydrochloric acid and sodium hydroxide using bipolar membranes. Using sodium chloride solution as the desorption stock solution, the sulfate / chloride mass ratio in the desorption solution is controlled to be above 2:1, achieving resourceful utilization of sulfate ions. BRIEF DESCRIPTION OF THE DRAWINGS

[0033] Figure 1 It is a process flow chart of the present invention. DETAILED DESCRIPTION

[0034] The technical solutions provided by the present invention will be described in detail below with reference to specific embodiments. It should be understood that the following specific embodiments are only used to illustrate the present invention and are not used to limit the scope of the present invention.

[0035] like Figure 1 As shown, the present invention provides a method for resource utilization of circulating water cooling wastewater from a thermal power plant, the method comprising the following steps:

[0036] Step 1: The circulating cooling wastewater is softened with lime soda to remove temporary calcium and magnesium ions, and the resulting softened sludge is transported to the desulfurization system for use as a desulfurizer;

[0037] Step 2: The effluent softened by lime-soda in step 1 is further filtered through an ultrafiltration system to remove suspended solids to obtain ultrafiltration water. The ultrafiltration concentrated water and backwash discharge water are recycled to the front end of the lime-soda softening process to achieve solid-liquid separation;

[0038] Step 3: The ultrafiltration water passes through the sulfate adsorption device, where the sulfate ions in the water are adsorbed by the ion exchange resin. The adsorption filtrate produced by the sulfate adsorption device contains more than 97% of the total dissolved solid mass percentage of sodium chloride.

[0039] Step 4: The adsorption filtrate is concentrated by reverse osmosis, and the mass concentration of sodium chloride is increased to more than 5%. The reverse osmosis produced fresh water is recycled and reused in the circulating cooling water system;

[0040] Step 5: The reverse osmosis concentrated water passes through a bipolar membrane system to produce hydrochloric acid and sodium hydroxide solutions, obtaining 3-4% hydrochloric acid and 5-6% sodium hydroxide, which are used for regeneration of the power plant ion exchange system and pH adjustment of industrial wastewater; the bipolar membrane fresh water is returned to the reverse osmosis system for further concentration treatment;

[0041] Step 6: After the sulfate adsorption device is saturated with adsorption, part of the reverse osmosis concentrated water is introduced into the sulfate adsorption device. At the same time, auxiliary direct current is used to enhance the desorption effect, so that the ion exchange resin in the sulfate adsorption device is converted from sulfate type to chloride type. The concentration ratio of sodium sulfate and sodium chloride in the desorption liquid is controlled at above 2:1. The desorption liquid is recovered to the desulfurization system, and the sulfate ions in the desorption liquid combine with the calcium ions in the desulfurizer to form gypsum.

[0042] In step 1 of this embodiment, the total dissolved solids in the system inlet water are controlled in the range of 1-5 g / L, and the calcium and magnesium ion concentrations are in the range of 0.1-3 g / L.

[0043] In this embodiment, lime-soda softening is performed in the first stage, with lime added to control the solution pH to 10.3 and a reaction time of 0.5 to 1 hour. Sodium carbonate is added in the second stage, with a reaction time of 0.5 to 1 hour, to control the effluent calcium and magnesium ion concentrations to no more than 50 mg / L. A sedimentation tank is included in the lime-soda softening system for preliminary solid-liquid separation, with the effluent SS not exceeding 20 mg / L. Hydrochloric acid is added to the effluent to adjust the pH to 7 to 8. The sedimentation tank sludge, primarily composed of calcium carbonate, is transported to the desulfurization system for reuse as a desulfurizer.

[0044] The filtration accuracy of the ultrafiltration membrane in this embodiment is selected to be 0.02~0.05mm to improve the solid-liquid separation efficiency. The ultrafiltration system adopts a cross-flow filtration operation mode with an overall recovery rate of more than 90%. The ultrafiltration concentrate and backwash wastewater are recycled to the lime soda softening unit to achieve further solid-liquid separation.

[0045] The sulfate adsorption system of this embodiment uses a weak-base or strong-base anion exchange resin to adsorb sulfate. The normal operating flow rate is 20-40 m / h, and the sodium sulfate concentration in the adsorption filtrate is controlled below 0.5 g / L. The sodium sulfate desorption stock solution uses concentrated water with a 10% sodium chloride content from a reverse osmosis system. Assisted electric desorption enhances the desorption effect, controlling the sodium sulfate concentration in the desorption solution to 2%-3% and the sodium chloride concentration to 1%-1.5%. This low sodium chloride concentration reduces the amount of chloride ions consumed by the desulfurization system while maximizing sulfate ion recovery.

[0046] The desorption flow rate of the sulfate adsorption system of this embodiment is 4-8 m / h. An anion exchange membrane is set at the anode for auxiliary electrodesorption, and a cation exchange membrane is set at the cathode to maximize the sulfate concentration of the desorption solution. The sulfate concentration of the desorption solution is adjusted by the reflux ratio of the desorption solution.

[0047] The auxiliary electric desorption current density of this embodiment is 200~300A / m 2 .

[0048] In this embodiment, the reverse osmosis inlet sodium chloride concentration ranges from 1 to 5 g / L, with a desalination rate of ≥97%. The reverse osmosis concentrate sodium chloride concentration is controlled at 5 to 6%. When the inlet sodium chloride concentration is low, the reverse osmosis concentrate is partially returned to the inlet to further increase the concentration ratio, ensure the concentrate flow rate, and prevent concentration polarization. The reverse osmosis product water is reused as circulating cooling water.

[0049] In this embodiment, the reverse osmosis brine return is automatically adjusted by detecting the brine conductivity. When the brine conductivity is less than 60ms / cm, all the brine is returned to the reverse osmosis water inlet tank. When the brine conductivity is ≥60ms / cm, the brine is discharged to ensure that the concentration of the sodium chloride solution in the system is within the set range.

[0050] The bipolar membrane system of this embodiment uses a homogeneous ion exchange membrane, the influent sodium chloride concentration is 5-6%, the produced hydrochloric acid concentration is 3-4%, the sodium hydroxide concentration is 5-6%, and the operating current density is 300-500A / m 2 , membrane surface flow rate 3~5cm / s.

[0051] The sodium chloride concentration of the fresh water discharged from the bipolar membrane system of this embodiment is 2-3%, which is recycled to the reverse osmosis system for further concentration treatment.

[0052] It should be noted that the above content merely illustrates the technical idea of ​​the present invention and cannot be used to limit the scope of protection of the present invention. For ordinary technicians in this technical field, several improvements and modifications can be made without departing from the principles of the present invention. These improvements and modifications all fall within the scope of protection of the claims of the present invention.

Claims

1. A method for resource utilization of circulating water cooling wastewater in thermal power plants, characterized in that: The method comprises the following steps: Step 1: The circulating cooling wastewater is softened with lime soda to remove calcium and magnesium ions, and the resulting softened sludge is transported to the desulfurization system for use as a desulfurizer; Step 2: The effluent softened by lime-soda in step 1 is further filtered through an ultrafiltration system to remove suspended solids to obtain ultrafiltration water. The ultrafiltration concentrated water and backwash discharge water are recycled to the front end of the lime-soda softening process to achieve solid-liquid separation; Step 3: The ultrafiltration water passes through the sulfate adsorption device, where the sulfate ions in the water are adsorbed by the ion exchange resin. The adsorption filtrate produced by the sulfate adsorption device contains more than 97% of the total dissolved solid mass percentage of sodium chloride. Step 4: The adsorption filtrate is concentrated by reverse osmosis, and the mass concentration of sodium chloride is increased to more than 5%. The reverse osmosis produced fresh water is recycled and reused in the circulating cooling water system; Step 5: The reverse osmosis concentrated water passes through a bipolar membrane system to produce hydrochloric acid and sodium hydroxide solutions, obtaining 3-4% hydrochloric acid and 5-6% sodium hydroxide, which are used for regeneration of the power plant ion exchange system and pH adjustment of industrial wastewater; The bipolar membrane fresh water flows back to the reverse osmosis system for further concentration treatment; Step 6: After the sulfate adsorption device is saturated with adsorption, part of the reverse osmosis concentrated water is introduced into the sulfate adsorption device. At the same time, auxiliary direct current is used to enhance the desorption effect, so that the ion exchange resin in the sulfate adsorption device is converted from sulfate type to chloride type. The concentration ratio of sodium sulfate and sodium chloride in the desorption liquid is controlled at above 2:

1. The desorption liquid is recovered to the desulfurization system, and the sulfate ions in the desorption liquid combine with the calcium ions in the desulfurizer to form gypsum.

2. The method for resource utilization of circulating water cooling sewage from a thermal power plant according to claim 1, characterized in that: After the circulating cooling wastewater in step 1 is softened by lime soda, the total dissolved solids are controlled in the range of 1~5g / L, and the calcium and magnesium ion concentrations are in the range of 0.1~3g / L.

3. The method for resource utilization of circulating water cooling sewage from a thermal power plant according to claim 1, characterized in that: The specific method of lime soda softening in step 1 is: adding lime in the first stage, controlling the pH value of the solution to 10.3, and the reaction time to 0.5 to 1 hour; adding sodium carbonate in the second stage, and the reaction time to 0.5 to 1 hour; controlling the concentration of calcium and magnesium ions in the effluent to be no higher than 50 mg / L, and setting a sedimentation tank in the lime soda softening system to achieve preliminary solid-liquid separation, and the SS of the effluent from the sedimentation tank does not exceed 20 mg / L; adding hydrochloric acid to the effluent from the sedimentation tank to adjust the pH value to 7 to 8.

4. The method for resource utilization of circulating water cooling wastewater from a thermal power plant according to claim 1, characterized in that: In the ultrafiltration system described in step 2, a cross-flow filtration operation mode is adopted, and an ultrafiltration membrane with an accuracy of 0.02-0.05 mm is selected.

5. The method for resource utilization of circulating water cooling wastewater in thermal power plants according to claim 1, characterized in that: In the sulfate adsorption device described in step 3, weak base anion exchange resin or strong base anion exchange resin adsorbs sulfate, the operating flow rate is 20-40 m / h, and the sodium sulfate concentration of the adsorption filtrate is controlled below 0.5 g / L.

6. The method for resource utilization of circulating water cooling wastewater in thermal power plants according to claim 1, characterized in that: In the sulfate adsorption device described in step 6, the sulfate desorption stock solution adopts concentrated water with a sodium chloride content of 10% from the reverse osmosis system, and the desorption flow rate is 4-8m / h.

7. The method for resource utilization of circulating water cooling wastewater from a thermal power plant according to claim 6, characterized in that: The sulfate desorption also assists the electric driven desorption to enhance the desorption effect, and the auxiliary electric desorption current density is 200~300A / m 2 , control the sodium sulfate concentration of the desorption solution to 2%~3%, and the sodium chloride concentration to 1%~1.5%.

8. The method for resource utilization of circulating water cooling wastewater from a thermal power plant according to claim 1, characterized in that: The reverse osmosis influent sodium chloride concentration ranges from 1 to 5 g / L, the desalination rate is ≥97%, and the reverse osmosis concentrated water sodium chloride concentration is controlled at 5 to 6%.

9. The method for resource utilization of circulating water cooling wastewater from a thermal power plant according to claim 1, characterized in that: The bipolar membrane system in step 5 uses a homogeneous ion exchange membrane, the influent sodium chloride concentration is 5-6%, the produced hydrochloric acid concentration is 3-4%, the sodium hydroxide concentration is 5-6%, and the operating current density is 300-500A / m 2 , membrane surface flow rate 3~5cm / s.

10. The method for resource utilization of circulating water cooling wastewater in thermal power plants according to claim 1, characterized in that: The sodium chloride concentration of the fresh water discharged from the bipolar membrane system is 2~3%, which is recycled to the reverse osmosis system for further concentration treatment.

Images