Treatment system and method for denitration urea hydrolysis wastewater of thermal power plant

Abstract

The invention provides a thermal power plant denitration urea hydrolysis wastewater treatment system and method, and the method comprises the following steps: 1, pumping wastewater discharged by a urea hydrolyzer into a steam stripping tower, introducing steam into the steam stripping tower to carry out steam stripping treatment, adjusting the pH value of the wastewater to be alkaline, and under high-temperature and alkaline conditions, carrying out steam stripping treatment on the wastewater; ammonia nitrogen in the wastewater is blown off and recovered by steam, and urea in the wastewater is hydrolyzed at the same time; 2) recovering ammonia gas generated by steam stripping and returning the ammonia gas to the urea hydrolyzer to serve as a reaction raw material; and feeding the effluent subjected to steam stripping treatment into a biochemical treatment system for deep treatment, and discharging or recycling the effluent after reaching the standard. According to the treatment method, the technical effects of effectively recycling resources, reducing the subsequent treatment load, being low in operation cost and stable in treatment effect can be achieved, recycling and treatment are ingeniously combined, steam stripping serves as a resource recycling unit and also serves as an efficient pretreatment unit, and the operation cost is greatly reduced.

Description

Technical Field

[0001] This invention belongs to the field of industrial wastewater treatment technology, specifically relating to a method and system for treating high-concentration urea and ammonia nitrogen wastewater generated by the urea hydrolysis unit in a selective catalytic reduction (SCR) denitrification system of a thermal power plant. Background Technology

[0002] Urea hydrolysis is a crucial technology for preparing the denitrification reducing agent (ammonia) in the SCR flue gas denitrification system of thermal power plants. Urea hydrolysis is a complex chemical process. Ideally, urea would completely decompose into ammonia (NH3) and carbon dioxide (CO2), serving as the denitrification reducing agent. However, in actual operation, the reaction is often incomplete, requiring the system to periodically or continuously discharge a certain amount of process wastewater to prevent solids such as biuret from crystallizing and clogging reactors and pipelines. Simultaneously, during equipment maintenance and shutdowns, flushing water is needed, which also contains high concentrations of urea and its derivatives and is treated as wastewater from the urea hydrolysis system. This type of wastewater is characterized by high temperature, high urea concentration, and high concentrations of ammonia nitrogen and total nitrogen. Although the COD is high, the main component, biuret, is a recalcitrant biodegradable substance, resulting in a low BOD / COD ratio and poor biodegradability. Direct use of conventional biological treatment methods is extremely inefficient, classifying it as difficult-to-treat industrial wastewater.

[0003] Currently, the main methods for treating this type of wastewater are:

[0004] 1. Chemical oxidation method: This method uses strong oxidants such as sodium hypochlorite and ozone to directly oxidize urea and ammonia nitrogen into nitrogen gas. Although effective, this method requires large amounts of reagents, resulting in extremely high operating costs, and may also produce secondary pollution such as chlorinated organic compounds.

[0005] 2. Direct dilution and biological treatment: High-concentration wastewater is diluted significantly before entering the biological treatment system of the power plant's wastewater treatment station (such as the A / O process). However, this method has significant drawbacks: First, urea molecules in the wastewater inhibit the microorganisms in the biological system; second, urea slowly hydrolyzes in the biological tank, causing drastic fluctuations in the system's pH value, severely affecting the activity of nitrifying and denitrifying bacteria, resulting in low treatment efficiency or even system collapse; finally, large-scale dilution increases water consumption and the load on subsequent treatment facilities.

[0006] 3. Direct reuse: Some solutions attempt to return wastewater to the front end of the hydrolyzer. However, when the wastewater contains impurity ions or system corrosion products, it can cause salt and pollutants to accumulate in the hydrolyzer, potentially leading to equipment corrosion, pipe scaling, and catalyst poisoning, thus affecting the stable operation of the main process.

[0007] Therefore, there is an urgent need in this field for a new technological solution that can efficiently, economically, and stably treat urea hydrolysis wastewater and achieve resource recycling. Summary of the Invention

[0008] The purpose of this invention is to overcome the shortcomings of the prior art and provide a treatment system and method for denitrification urea hydrolysis wastewater from thermal power plants that can effectively recover resources, reduce subsequent treatment load, have low operating costs, and provide stable treatment results.

[0009] To achieve the above objectives, the present invention adopts the following technical solution:

[0010] A method for treating urea hydrolysis wastewater from a thermal power plant includes the following steps:

[0011] S1: Discharge the wastewater from the urea hydrolyzer and the maintenance flushing wastewater into the urea wastewater pool, and introduce it into the steam stripping tower through the urea wastewater pump.

[0012] S2: Steam is introduced into the steam stripping tower. Under high temperature and alkaline conditions, the free ammonia in the wastewater is stripped off, and the residual urea in the wastewater undergoes a hydrolysis reaction to generate ammonia and carbon dioxide.

[0013] S3: The ammonia mixture generated at the top of the steam stripping tower in step S2 is condensed by an ammonia water condenser to obtain high-concentration ammonia water, and the high-concentration ammonia water is recycled to the urea hydrolyzer for use as raw material.

[0014] S4: The effluent from the steam stripping tower in step S2 is cooled by the discharge cooler and then sent to the biochemical treatment system for further treatment. After meeting the standards, it is discharged or reused.

[0015] Preferably, in step S2, the operating temperature of the steam stripping tower is controlled at 105℃~130℃, and the pH value inside the tower is maintained at 9~11.

[0016] Furthermore, the recovered ammonia gas is converted into ammonia water by condensation and then returned to the urea hydrolyzer.

[0017] Furthermore, the biochemical treatment system employs an A / O process or an A / O process. 2 / O process.

[0018] A treatment system for denitrification urea hydrolysis wastewater from a thermal power plant, comprising:

[0019] The steam stripping tower has its inlet used to receive wastewater discharged from the urea hydrolyzer.

[0020] A steam supply unit for introducing steam into the lower air inlet of a steam stripping tower;

[0021] The ammonia recovery unit is used to recover the ammonia gas discharged from the top outlet of the steam stripping tower and return it to the urea hydrolyzer as a reaction feedstock.

[0022] The biochemical treatment unit has its inlet connected to the bottom outlet of the steam stripping tower.

[0023] Furthermore, the ammonia recovery unit includes an ammonia water condenser and an ammonia water storage tank connected in sequence. The inlet of the ammonia water condenser is connected to the top outlet of the steam stripping tower. The liquid ammonia water condensed by the ammonia water condenser is stored in the ammonia water storage tank. The outlet of the ammonia water storage tank is connected to the inlet of the urea hydrolyzer through a reflux pipe.

[0024] Furthermore, it also includes an outlet cooler disposed between the steam stripping tower and the biochemical treatment unit.

[0025] Furthermore, the steam stripping tower adopts a packed tower structure. A liquid distributor is installed in the top of the packed tower. The packed tower is filled with two layers of spaced packing. A redistributor is installed between the two packing layers to collect the liquid flowing down from the upper packing layer and redistribute it evenly, preventing the liquid from gradually concentrating on the tower wall as it flows towards the bottom of the tower. An air inlet is provided at the bottom of the packed tower and connected to the steam supply unit through a pipeline.

[0026] The wastewater outlet of the urea hydrolyzer is connected to the liquid distributor at the top of the packed tower via a pipeline. The wastewater is evenly sprayed onto the packing layer inside the packed tower through the liquid distributor and flows downward under the action of gravity. High-temperature steam is introduced into the air inlet at the bottom of the packed tower.

[0027] Furthermore, the biochemical treatment system includes an anoxic tank, an aerobic tank, and a sedimentation tank connected in sequence.

[0028] Furthermore, it also includes a urea wastewater tank, a urea wastewater pump, and a PLC control system. The wastewater discharged from the urea hydrolyzer is stored in the urea wastewater tank, and the urea wastewater pump is used to transport the wastewater in the urea wastewater tank to the steam stripping tower for steam stripping treatment.

[0029] The urea wastewater tank is equipped with a level gauge, which is connected to the urea wastewater pump and steam supply unit via a PLC control system. The PLC control system collects the level signal of the urea wastewater tank monitored by the level gauge. When the collected level signal is high, the PLC control system starts the urea wastewater pump, and the wastewater enters the steam stripping tower through the urea wastewater pump. At the same time, the PLC control system starts the steam supply unit to deliver steam. When the collected level signal is low, the PLC control system stops the urea wastewater pump and the steam supply unit.

[0030] Furthermore, the outlet pipe of the urea wastewater pump is connected to a NaOH solution replenishment mechanism, and a pH sensor is installed at the inlet of the steam stripping tower. The pH sensor is connected to the NaOH solution replenishment mechanism via a PLC control system. The PLC control system collects the pH value of the wastewater monitored by the pH sensor and performs feedback adjustment on the NaOH solution replenishment mechanism to adjust the pH of the wastewater entering the steam stripping tower to a set range.

[0031] The NaOH solution replenishment mechanism is existing technology, for example, Chinese patent CN107433116A.

[0032] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0033] 1) Achieved efficient resource recovery: Through steam stripping process, ammonia nitrogen in wastewater is recovered in the form of ammonia water and returned to the urea hydrolyzer as raw material, realizing the transformation of waste into treasure, reducing the raw material consumption of the denitrification system, and conforming to the concept of circular economy.

[0034] 2) Solved the problem of urea inhibition in biochemical systems: Innovatively utilizing the high-temperature alkaline conditions required for steam stripping, residual urea was simultaneously and completely hydrolyzed, converting it into easily stripped ammonia. This resulted in extremely low urea content in the wastewater entering the subsequent biochemical treatment unit, eliminating the inhibition of microorganisms and pH impact caused by urea, and ensuring the long-term stable operation of the biochemical system.

[0035] 3) Significant synergistic effect and low processing cost: This invention cleverly combines "recycling" and "processing," with steam stripping serving as both a resource recovery unit and a highly efficient pretreatment unit. Compared to expensive chemical oxidation methods, this method mainly consumes steam (which is readily available in power plants), significantly reducing operating costs.

[0036] 4) The process is simple and easy to implement: The system can be seamlessly connected with the existing urea hydrolysis unit and sewage treatment station of thermal power plant, with low modification difficulty and high return on investment. Attached Figure Description

[0037] Figure 1 This is a schematic diagram of the structure of the wastewater treatment system for denitrification urea hydrolysis in thermal power plants according to the present invention. Detailed Implementation

[0038] The present invention will be further described below with reference to specific embodiments, but the scope of protection of the present invention is not limited thereto.

[0039] Example: Control Figure 1

[0040] A treatment system for denitrification urea hydrolysis wastewater from a thermal power plant includes a urea hydrolyzer 1, a urea wastewater pool 2, a urea wastewater pump 3, a steam stripping tower 4, a steam supply unit 5, an ammonia water condenser 6, an ammonia water storage tank 7, a discharge cooler 8, a biochemical treatment system 9, and a control system 10.

[0041] Wastewater discharged from urea hydrolyzer 1 enters urea wastewater pool 2. Urea wastewater pump 3 pumps the wastewater in urea wastewater pool 2 into the inlet of steam stripping tower 4. Steam is introduced into the lower air inlet of steam stripping tower 4 through steam supply unit 5.

[0042] The ammonia gas discharged from the top outlet of the steam stripping tower 4 is condensed by the ammonia water condenser 6, and the condensed liquid ammonia water is stored in the ammonia water storage tank 7. The outlet of the ammonia water storage tank 7 is connected to the inlet of the urea hydrolyzer 1 through a reflux pipe.

[0043] After being treated by the steam stripping tower 4, the effluent from the tower bottom is cooled by the discharge cooler 8 and then sent to the biochemical treatment system 9 for further treatment.

[0044] In this application, the steam stripping tower 4 adopts a packed tower structure, with double-layer anti-aging wire mesh corrugated packing inside, providing a huge specific surface area to promote vapor-liquid mass transfer; a liquid distributor is set at the top of the packing layer to evenly distribute the feed water across the entire cross-section of the packing layer; a redistributor is set between the two packing layers to collect the liquid flowing down from the upper packing layer and redistribute it evenly, preventing the liquid from gradually concentrating on the tower wall as it flows towards the bottom of the tower; a porous support plate is set at the bottom of the packing layer to support the weight of the packing while ensuring that the gas phase can pass through smoothly.

[0045] Wastewater enters from the top of steam stripping tower 4, is evenly sprayed onto the packing material by a liquid distributor, and flows downwards under gravity. High-temperature steam enters from the bottom of steam stripping tower 4 and flows upwards countercurrently. On the surface of the packing material, the wastewater forms an extremely thin liquid film, making full contact with the rising steam. The heat provided by the steam disrupts the solubility equilibrium of ammonia in the water, carrying ammonia gas out of the liquid phase and discharging it from the top of the tower. The purified water, after ammonia removal, is discharged from the bottom of the tower.

[0046] The control system 10 adopts a PLC control system. A level gauge is installed in the urea wastewater tank 2. The level gauge is connected to the urea wastewater pump 3 and the steam supply unit 5 through the PLC control system. The PLC control system collects the level signal of the urea wastewater tank 2 monitored by the level gauge. When the collected level signal is high, the PLC control system feeds back to start the urea wastewater pump 3. The wastewater enters the steam stripping tower 4 through the urea wastewater pump 3. At the same time, the PLC control system feeds back to start the steam supply unit 5 to supply steam into the steam stripping tower 4. When the collected level signal is low, the PLC control system feeds back to stop the urea wastewater pump and the steam supply unit, and the system enters the standby state.

[0047] The outlet pipe of the urea wastewater pump 3 is connected to a NaOH solution replenishment mechanism, and a pH sensor is installed at the inlet of the steam stripping tower 4. The pH sensor is connected to the NaOH solution replenishment mechanism via a PLC control system. The PLC control system collects the pH value of the wastewater monitored by the pH sensor and performs feedback adjustment on the NaOH solution replenishment mechanism to adjust the pH of the wastewater entering the steam stripping tower 4 to the set range.

[0048] Adding alkali to the inlet pipe of steam stripping tower 4 to adjust the pH has the advantages of being convenient to adjust and having good controllability.

[0049] Example 1:

[0050] A 2x30MW thermal power plant's urea hydrolysis system discharges 5m³ of wastewater. 3 / d, urea concentration is 5000mg / L, ammonia nitrogen concentration is 8000mg / L, temperature is 80℃.

[0051] The wastewater is treated according to the following steps:

[0052] S1: Wastewater first enters urea wastewater pool 2 for temporary storage.

[0053] S2: Subsequently, the wastewater is pumped into the feed inlet of the steam stripping tower 4 via the urea wastewater pump 3. At the same time, steam (approximately 0.4 MPa pressure) drawn from the power plant auxiliary steam supply unit 5 is introduced from the bottom of the tower.

[0054] S3: Control the temperature inside the steam stripping tower 4 at 120℃, and add NaOH solution dropwise into the steam stripping tower 4 or the inlet pipe via a dosing pump to maintain the pH value of the wastewater at 10.5. Under these conditions, the urea in the wastewater rapidly hydrolyzes, and the ammonia gas produced by hydrolysis, along with the original free ammonia, is stripped away by the rising steam flow.

[0055] S4: The ammonia-water vapor mixture discharged from the top of the steam stripping tower 4 enters the ammonia water condenser 6. After being condensed by circulating cooling water, ammonia water with a concentration of about 15% is obtained and stored in the ammonia water storage tank 7. Then, it is precisely returned to the feeding system of the urea hydrolyzer 1 by a metering pump.

[0056] The effluent from the bottom of the steam stripping tower 4 (measured urea concentration <10mg / L, ammonia nitrogen concentration <100mg / L), after being stripped, is cooled to below 35℃ by the discharge cooler 8 before entering the power plant's existing A / O biological treatment system 9 for deep denitrification. The A / O biological treatment system 9 includes an anoxic tank, an aerobic tank, and a sedimentation tank connected in sequence. Its treatment conditions are: a 4-hour retention time in the anoxic tank to provide sufficient time for denitrification, where heterotrophic denitrifying bacteria remove nitrate nitrogen (NO3). --N) is reduced to nitrogen gas (N2); the residence time in the aerobic tank is 9 hours, providing sufficient time for nitrifying bacteria to completely oxidize ammonia nitrogen, which is then converted into nitrogen gas (N2) by autotrophic nitrifying bacteria. + -N) is oxidized to nitrate nitrogen (NO3) - -N). The sludge concentration in the aerobic tank is controlled at 4000 mg / L; the dissolved oxygen in the anoxic tank is controlled at <0.5 mg / L, and the dissolved oxygen in the aerobic tank is controlled at 3.0 mg / L. Especially at the end of the aerobic tank, sufficient DO (usually not less than 1.5-2.0 mg / L) must be ensured to ensure that ammonia nitrogen is completely oxidized. After treatment by the A / O biological treatment system, the final effluent ammonia nitrogen is <5 mg / L and total nitrogen is <15 mg / L, stably meeting the discharge or reuse standards.

[0057] The contents described in this specification are merely an enumeration of the implementation forms of the inventive concept, and the scope of protection of this invention should not be regarded as limited to the specific forms described in the embodiments.

Claims

1. A method for treating urea hydrolysis wastewater from a thermal power plant, characterized in that, Includes the following steps: Step 1: The wastewater discharged from the urea hydrolyzer is pumped into the steam stripping tower. Steam is introduced into the steam stripping tower for steam stripping treatment to adjust the pH of the wastewater to alkaline. Under high temperature and alkaline conditions, the ammonia nitrogen in the wastewater is stripped off and recovered by the steam, while the urea in the wastewater is hydrolyzed. Step 2: The ammonia gas generated by steam stripping is recovered and returned to the urea hydrolyzer as a reaction raw material; and the effluent after steam stripping is sent to the biochemical treatment system for further treatment, and discharged or reused after meeting the standards.

2. The method for treating urea hydrolysis wastewater from a thermal power plant as described in claim 1, characterized in that, The operating temperature for the steam stripping process is 105-130℃.

3. The method for treating urea hydrolysis wastewater from a thermal power plant as described in claim 1, characterized in that, During the steam stripping process, the pH value of the wastewater is controlled to be between 9 and 11.

4. The method for treating urea hydrolysis wastewater from a thermal power plant as described in claim 1, characterized in that, Ammonia gas is recovered by condensing it into ammonia water and then returning it to the urea hydrolyzer.

5. The method for treating urea hydrolysis wastewater from a thermal power plant as described in claim 1, characterized in that, The biochemical treatment system employs an A / O process or A... 2 / O process.

6. A processing system for implementing the method according to any one of claims 1-5, characterized in that, include: The steam stripping tower (4) has an inlet for receiving wastewater discharged from the urea hydrolyzer (1); Steam supply unit (5) is used to supply steam to the lower air inlet of steam stripping tower (4); The ammonia recovery unit is used to recover the ammonia discharged from the top outlet of the steam stripping tower (4) and return it to the urea hydrolyzer as a reaction feedstock. The inlet of the biochemical treatment unit (9) is connected to the bottom outlet of the steam stripping tower (4).

7. The system as described in claim 6, characterized in that, The ammonia recovery unit includes an ammonia water condenser (6) and an ammonia water storage tank (7) connected in sequence. The air inlet of the ammonia water condenser (6) is connected to the top air outlet of the steam stripping tower (4). The liquid ammonia water condensed by the ammonia water condenser (6) is stored in the ammonia water storage tank (7). The liquid outlet of the ammonia water storage tank (7) is connected to the feed inlet of the urea hydrolyzer (1) through a reflux pipe. The discharge cooler (8) is located between the steam stripping tower (4) and the biochemical treatment unit (9).

8. The system as described in claim 6, characterized in that, The steam stripping tower (4) adopts a packed tower structure. A liquid distributor is installed in the top of the packed tower. The packed tower is filled with two layers of spaced packing. A redistributor is installed between the two packing layers to collect the liquid flowing down from the upper packing layer and redistribute it evenly, preventing the liquid from gradually concentrating on the tower wall as it flows towards the bottom of the tower. An air inlet is provided at the bottom of the packed tower and connected to the steam supply unit (5) through a pipeline. The wastewater outlet of the urea hydrolyzer (1) is connected to the liquid distributor at the top of the packed tower through a pipeline. The wastewater is evenly sprayed onto the packing layer in the packed tower through the liquid distributor and flows downward under the action of gravity. High-temperature steam is introduced into the air inlet at the bottom of the packed tower.

9. The system as described in claim 6, characterized in that, The biochemical treatment system includes an anoxic tank, an aerobic tank, and a sedimentation tank connected in sequence.

10. The system as described in claim 6, characterized in that, It also includes a urea wastewater tank (2), a urea wastewater pump (3) and a PLC control system. The wastewater discharged from the urea hydrolyzer (1) is stored in the urea wastewater tank (2). The urea wastewater pump (3) is used to transport the wastewater in the urea wastewater tank (2) to the steam stripping tower (4) for steam stripping treatment. The urea wastewater tank (2) is equipped with a level gauge. The level gauge is connected to the urea wastewater pump (3) and the steam supply unit (5) via a PLC control system. The PLC control system collects the level signal of the urea wastewater tank (2) monitored by the level gauge. When the collected level signal is high, the PLC control system feeds back to start the urea wastewater pump (3). The wastewater enters the steam stripping tower through the urea wastewater pump (3). At the same time, the PLC control system feeds back to start the steam supply unit (5) to deliver steam. When the collected level signal is low, the PLC control system feeds back to stop the urea wastewater pump (3) and the steam supply unit (5).

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