A method and system for removing heavy metals and ultra-clean flue gas treatment

Abstract

This invention discloses a method and system for removing heavy metals and ultra-clean flue gas treatment, relating to the field of flue gas treatment technology. The method includes the following steps: S1, boiler flue gas is pre-desulfurized using sodium method, and after washing with NaHCO3 to remove impurities, gas phase a and mother liquor a are obtained. SO3 and other indicators in gas phase a are detected; S2, when gas phase a reaches the preset target, it is sent to the ammonia desulfurization section, where it is washed with dilute ammonia and other substances and bleached to obtain compliant vented gas phase b and mother liquor b; S3, NH3 and CO2 are passed through mother liquor a to react and separate NaHCO3 and mother liquor c; S4, mother liquor b and c are frozen and crystallized, and the crystals are sent to a double decomposition reaction device, while mother liquor d is sent to an ammonia stripping device; S5, the ammonia stripping product is returned to the double decomposition reaction device, and mother liquor d is processed to produce ammonium sulfate. The remaining liquid is recycled, solving the problems of existing desulfurization processes that are difficult to achieve the expected requirements, as well as the low comprehensive utilization rate and insufficient added value of their by-products.

Description

Technical Field

[0001] This invention relates to the field of flue gas treatment technology, specifically to a method and system for removing heavy metals and ultra-clean flue gas treatment. Background Technology

[0002] Ammonia-based flue gas desulfurization refers to a technology system that uses ammonia (NH3) as an absorbent to remove sulfur dioxide (SO2) from industrial flue gas. This technology is widely used in flue gas purification in industries such as power, chemical, and metallurgy. Its core function is to reduce sulfur dioxide emissions and curb environmental problems such as acid rain. The underlying principle is to utilize the chemical reaction mechanism between ammonia and sulfur dioxide to convert harmful components in industrial waste gas into recyclable byproducts (such as ammonium sulfate), thereby achieving the circular economy goal of "turning waste into treasure." Currently, this technology faces two major challenges in industrial applications: Firstly, the issue of ammonia slip control urgently needs to be addressed. According to HJ2001-2018, "General Technical Specification for Ammonia-based Flue Gas Desulfurization Engineering," the ammonia slip concentration during industrial denitrification processes must be strictly controlled at 8 mg / m³. 3 Within a certain range. However, in the practice of ammonia-based desulfurization in scenarios such as boiler flue gas, the phenomenon of flue gas tailing is common, and in extreme cases, it can even lead to "ammonium sulfate rain" pollution. The main cause is that excessive ammonia escape has not been effectively controlled. This secondary pollution problem not only affects the surrounding atmospheric environment, but also threatens the stable operation of subsequent treatment equipment. Secondly, there are technical bottlenecks in meeting the quality standards of the byproduct ammonium sulfate. According to the T / CPCIF0006—2017 standard for ammonium sulfate byproducts of ammonia desulfurization, its nitrogen (N) mass fraction (dry basis) must be ≥20.0%, which meets the requirements for fertilizer-grade ammonium sulfate (Type II). However, the standard stipulates that the fluoride (F) content must be ≤1%, significantly higher than the 500 mg / kg limit in GBT535-2020 for fertilizer-grade ammonium sulfate. More critically, current industry standards do not limit the total amount of thiocyanate ions and polycyclic aromatic hydrocarbons, making it difficult for actually produced ammonium sulfate to meet the quality requirements of fertilizer-grade products. Furthermore, in complex industrial flue gas treatment processes, the content of heavy metals such as mercury and arsenic often exceeds the limits specified in the T / CPCIF0006—2017 standard, further exacerbating the technical difficulties in the resource utilization of byproducts. The aforementioned technical bottlenecks not only make it difficult for ammonia-based desulfurization processes to meet increasingly stringent environmental emission standards, but also hinder the effective realization of the economic benefits of this technology route due to its low comprehensive utilization rate of by-products and insufficient added value. Breakthroughs in precise ammonia escape control technology and deep purification processes for by-products have become key research directions for promoting the large-scale application of ammonia-based desulfurization technology. Summary of the Invention

[0003] The purpose of this invention is to solve the problems of existing desulfurization processes, such as difficulty in achieving expected results, low comprehensive utilization rate of by-products, and insufficient added value, which lead to high processing costs. This invention proposes a method for removing heavy metals and ultra-clean flue gas treatment, which can effectively improve the above-mentioned technical problems.

[0004] This invention is achieved through the following technical solution: A method for removing heavy metals and ultra-cleaning flue gas includes the following steps: S1. Collect boiler flue gas and send it to the sodium pre-desulfurization section of the desulfurization island. Add NaHCO3, use the desulfurization liquid to circulate and wash the flue gas, and remove dust and heavy metals through a filtration device to obtain gas phase a and Na-rich gas phase a. + SO4 2- NO3 - The mother liquor a was analyzed, and the SO3 and NO in the gas phase a were detected. x Contents of HCl, HF, and heavy metals such as Hg; S2, When the SO3 content in gas phase a is ≤20mg / Nm 3 HCl content ≤20mg / Nm 3 HF content ≤20mg / Nm 3 NO x Content ≤30mg / Nm 3 Hg content ≤ 0.005 mg / Nm 3 At that time, NO x The content of low-priced heavy metals is based on thresholds set with reference to national or local emission standards. Gas phase a is then sent to the ammonia desulfurization section of the desulfurization island, where ammonia gas, ammonia water, or ammonium salts (preferably about 5%~20% dilute ammonia water) are added. The desulfurization liquid continues to circulate and wash gas phase a, which then undergoes a whitening treatment to obtain purified gas phase b rich in NH4. + SO4 2- The mother liquor b of the ions was used to detect particulate matter, SO2, and NO in the gas. x After the content of [specific substance] reaches the standard, it is vented. S3. Transfer the mother liquor a from step S1 to a double decomposition reaction apparatus, introduce NH3 and CO2 or add NH4HCO3, control the temperature in the apparatus to 33~45℃, the pH value to 8.5~9.5, the pressure to 0~8000kPa, and react for 20~40 min to obtain Na-rich solution. + HCO3 - NH4 + SO4 2- The mixed solution was then centrifuged and dried to separate the NaHCO3 product, leaving a Na-rich residue. + NH4 + SO42- HCO3 - Mother liquor c; S4. Send the mother liquor b and mother liquor c from steps S2 and S3 into the freezing device, control the temperature in the freezing device to -5~5℃, freeze and crystallize, send the precipitated crystals to the double decomposition reaction device, and send the remaining mother liquor d to the ammonia stripping device. S5. By adjusting the temperature and / or pressure in the ammonia stripping unit (e.g., at atmospheric pressure, the temperature is set to 90~105℃), the stripped ammonia and CO2 gases are transported to the double decomposition reaction unit. When NH4... + When the mass percentage concentration reaches 12-16%, the temperature is reduced to 50-85℃, the solid is concentrated by cyclone separation and centrifugation, and then dried to obtain (NH4)2SO4 product. The remaining liquid is sent to the metathesis reaction device.

[0005] Furthermore, in step S4, the refrigeration unit includes two stages, with an ammonia stripping unit connected to the rear end of each stage. The operation method is as follows: S4.1. The mother liquor b and mother liquor c from steps S2 and S3 are sent to the first-stage freezing device and the temperature is controlled at -5~5℃ for freezing treatment (preferably -5~0℃). The separated NH4HCO3 and Na2SO4·10H2O are sent to the double decomposition reaction device, and the mother liquor d1 is sent to the ammonia stripping device I. By adjusting the temperature and / or pressure of the ammonia stripping device I (for example, under normal pressure, the temperature is set to 98~105℃, and under negative pressure, the temperature can be set to a lower temperature), the goal is to distill out ammonia gas and CO2 gas. The distilled ammonia gas and CO2 gas are sent to the double decomposition reaction device, and the remaining mother liquor d2 is used. S4.2. The mother liquor d2 is sent to the second-stage refrigeration unit, where the temperature is controlled at -5~5℃ for freezing treatment. The separated Na2SO4·(NH4)2SO4·4H2O is sent to the first-stage refrigeration unit, and the remaining mother liquor d is sent to the ammonium sulfate evaporation unit. By controlling the temperature and / or pressure of the ammonium sulfate evaporation unit, the water in the solution is evaporated. When NH4... + When the mass percentage concentration reaches 12-16%, the temperature is reduced to 50-85℃, and after cyclone concentration, centrifugal separation, and drying, ammonium sulfate product is obtained. The remaining liquid is sent to the second-stage refrigeration unit.

[0006] Furthermore, in step S2, when the sulfate concentration in mother liquor b reaches a certain level, mother liquor b is removed and, according to its concentration, sent to the double decomposition reaction apparatus, the first-stage refrigeration apparatus, the second-stage refrigeration apparatus, or the ammonium sulfate evaporation apparatus. When NH4 in the double decomposition reaction apparatus +When the concentration falls below the preset normal operating threshold, a portion of mother liquor b is sent to the metathesis reaction unit. Under normal circumstances, mother liquor b does not need to be sent to the metathesis reaction unit during this step. It is only sent when there is an anomaly in the mother liquor of the metathesis reaction or subsequent mother liquors (such as NH4). + Only when the concentration is too low should mother liquor b (ammonium sulfate solution) be sent into the system for adjustment; When the mass percentage concentration of (NH4)2SO4 in the mother liquor c separated by the metathesis reaction is <24%, part of the mother liquor b is sent to the primary freeze crystallization unit. When the mass percentage concentration of (NH4)2SO4 in mother liquor d is <30%, part of mother liquor b will be evaporated using an ammonium sulfate evaporation device.

[0007] Furthermore, in step S2, when the particulate matter in gas b is ≤5 mg / Nm³ 3 SO2 ≤ 10 mg / Nm 3 NO x ≤20mg / Nm 3 To perform a short-selling operation.

[0008] Furthermore, in step S1, desulfurizing agent is continuously supplied to the sodium pre-desulfurization section to control the pH value of the desulfurization liquid between 6.2 and 7.2, and oxygen is introduced to remove SO3. 2- NO2 - Ions oxidized to SO4 2- NO3 - ion.

[0009] Furthermore, the desulfurizing agent is one or both of dilute ammonia or dilute sodium bicarbonate solution.

[0010] Furthermore, the dilute ammonia solution is ammonia solution with a mass percentage concentration of 1-18%, and the dilute sodium bicarbonate is sodium bicarbonate solution with a mass percentage concentration of 7-14%.

[0011] Furthermore, in step S3, a portion of the separated sodium bicarbonate stream is transported to the sodium pre-desulfurization section as a desulfurizing agent.

[0012] Furthermore, in step S2, the whitening process includes defogging, three-stage water washing, and magnetic energy whitening treatment.

[0013] A system for removing heavy metals and ultra-clean flue gas treatment includes: The sodium pre-desulfurization section has a flue gas inlet connected to a flue gas inlet pipe. The sodium pre-desulfurization section is equipped with a desulfurizing agent supply pipe, a pH sensor, a NaHCO3 supply pipe and an oxygen supply pipe. The sodium pre-desulfurization section is equipped with a desulfurization liquid circulation and washing assembly. A filtration unit, connected in series in the mother liquor discharge pipeline of the sodium pre-desulfurization section, is used to remove dust and heavy metals from the mother liquor, outputting a solution rich in Na. + SO4 2- NO3 - Mother liquor a; The first gas phase detection device is connected to the gas phase outlet of the sodium pre-desulfurization section and is used to detect the content of target ions in gas phase a. The flue gas inlet of the ammonia desulfurization section is connected to the gas phase outlet of the sodium pre-desulfurization section. The ammonia desulfurization section is equipped with an ammonia / ammonium salt supply pipe and a desulfurization liquid circulation and washing assembly. The whitening device has its inlet connected to the gas phase outlet of the ammonia desulfurization section. The whitening device includes a demisting component, a three-stage water washing component and a magnetic energy whitening component connected in sequence, which are used to obtain the purified gas phase b. The second gas phase detection device is connected to the outlet of the whitening device and is used to detect smoke, SO2, and NO in gas phase b. x The content is controlled to be vented from gas phase b when the detection standard is met. The double decomposition reaction device has its first feed inlet connected to the mother liquor outlet of the filtration device. The double decomposition reaction device is equipped with a temperature sensor, a pH sensor, and a pressure sensor, and is connected to a reagent supply pipe for conveying NH3 and CO2, or for conveying NH4HCO3. The NaHCO3 separation assembly includes a centrifuge and a drying unit connected in sequence. Its inlet is connected to the solution outlet of the metathesis reaction apparatus, used to separate NaHCO3 product. The mother liquor outlet of the NaHCO3 separation assembly is used to output Na-rich liquid. + NH4 + SO4 2- HCO3 - Mother liquor c; The first-stage refrigeration unit has a first feed inlet connected to the mother liquor outlet of the NaHCO3 separation component to receive mother liquor c, and a second feed inlet connected to the mother liquor outlet of the ammonia desulfurization section. The first-stage refrigeration unit is equipped with a temperature sensor and a solid-liquid separation component. The crystallization outlet of the solid-liquid separation component is connected to the second feed inlet of the metathesis reaction device. The ammonia stripping unit has its inlet connected to the mother liquor outlet of the first-stage refrigeration unit. The ammonia stripping unit is equipped with a temperature sensor, and its gas phase outlet is connected to the third inlet of the metathesis reaction unit through a gas pipe. The second-stage refrigeration unit has its inlet connected to the mother liquor outlet of the ammonia stripping unit. The second-stage refrigeration unit is equipped with a temperature sensor and a solid-liquid separation component. The crystallization outlet of the solid-liquid separation component is connected to the third inlet of the first-stage refrigeration unit. The ammonium sulfate evaporator has its inlet connected to the mother liquor outlet of the second-stage refrigeration unit. The ammonium sulfate evaporator is equipped with a temperature sensor and also has a cyclone concentration component, a centrifugal separation component, and a drying component. The product outlet of the ammonium sulfate evaporator is used to output (NH4)2SO4 product, and its residual liquid outlet is connected to the second-stage refrigeration unit.

[0014] Furthermore, the mother liquor outlet of the ammonia desulfurization section is used to output liquor rich in NH4. + SO4 2- The ion mother liquor b also includes a mother liquor b diversion control unit and a mother liquor concentration detection device III; The mother liquor concentration detection device Ⅲ is connected to the mother liquor outlet of the ammonia desulfurization section and is used to detect the mass percentage concentration of Na2SO4 and (NH4)2SO4 in mother liquor b. The mother liquor b diversion control unit is electrically connected to the valves on the pipelines connecting the mother liquor concentration detection device III, the mother liquor outlet of the ammonia desulfurization section, the first inlet of the metathesis reaction unit, the inlet of the first-stage refrigeration unit, the inlet of the second-stage refrigeration unit, and the inlet of the ammonium sulfate evaporation unit. The control logic of the mother liquor b diversion control unit is as follows: When NH4 in the double decomposition reaction apparatus + When the concentration is lower than the preset normal operating threshold, part of the mother liquor b will be diverted to the first feed inlet of the metathesis reaction device; When the mass percentage concentration of (NH4)2SO4 in the mother liquor c separated by the metathesis reaction is <24%, part of the mother liquor b will be diverted to the feed inlet of the first-stage refrigeration unit. When the mass percentage concentration of (NH4)2SO4 in mother liquor d is [24%, 30%), a portion of mother liquor b is diverted to the inlet of the second-stage refrigeration unit or the inlet of the ammonium sulfate evaporation unit.

[0015] Furthermore, the mother liquor outlet of the ammonia desulfurization section is connected to the first inlet of the metathesis reaction unit, the inlet of the first-stage refrigeration unit, the inlet of the second-stage refrigeration unit, and the inlet of the ammonium sulfate evaporation unit via pipes I, II, III, and IV. Regulating valves I, II, III, and IV are respectively installed on the connections between pipes I, II, III, and IV. When NH4 in the double decomposition reaction apparatus + When the concentration is lower than the preset normal operating threshold, open regulating valve VII and close regulating valves II and III to divert part of the mother liquor b to the first feed inlet of the double decomposition reaction device; When the mass percentage concentration of (NH4)2SO4 in the mother liquor c separated by the metathesis reaction is <24%, open regulating valve II and close regulating valves VII and III to divert part of the mother liquor b to the feed inlet of the first-stage refrigeration unit. When the mass percentage concentration of (NH4)2SO4 in mother liquor d is [24%, 30%), part of the mother liquor b is diverted to the inlet of the second-stage refrigeration unit or the inlet of the ammonium sulfate evaporation unit by opening regulating valve III or regulating valve IV and closing regulating valves VII and II.

[0016] Furthermore, the NaHCO3 product outlet of the NaHCO3 separation component is also connected to the desulfurizing agent supply device of the sodium pre-desulfurization section via a pipeline, which is used to transport part of the NaHCO3 to the sodium pre-desulfurization section as a desulfurizing agent.

[0017] Furthermore, the filtration device is one of a bag filter, a cartridge filter, or a filter press, and the filtration accuracy of the filtration device is 25~100μm.

[0018] Furthermore, the metathesis reaction device is a tank reactor with a stirring structure; the first-stage refrigeration device and the second-stage refrigeration device are fully mixed crystallizers or DTB crystallizers; the ammonia stripping device is an evaporation device with packing or trays.

[0019] Furthermore, a regulating valve V is installed on the flue gas inlet pipe, and a regulating valve VI is installed on the NaHCO3 supply pipe. The first gas phase detection device is connected to the regulating valve V and the regulating valve VI respectively.

[0020] Compared with the prior art, the present invention has the following advantages and beneficial effects: I. In this invention, the collected flue gas is treated in a sodium pre-desulfurization section to remove dust and heavy metals, and then further desulfurized in an ammonia desulfurization section. The gas phase is then treated with water washing, demisting, and whitening processes. The dust content in the treated gas is ≤5mg / Nm³. 3 SO2 ≤ 10 mg / Nm 3 NO x ≤20mg / Nm 3 The emission standards are met, and ammonia escape is ≤3mg / m³. 3 The range.

[0021] Second, in this invention, by combining it with the "sodium sulfate to alkali" process, the purpose of purifying ammonium sulfate is achieved, so that the ammonium sulfate produced meets the GBT535-2020 "Fertilizer Grade Ammonium Sulfate" standard, and the ammonium sulfate product can be sold as a qualified fertilizer, which greatly improves the technical and economic efficiency of the equipment.

[0022] Third, in this invention, the method for removing heavy metals and ultra-cleaning flue gas can be used to initially introduce a portion of Na2SO4, which helps the entire system quickly reach reaction equilibrium and significantly shortens the product discharge time. Attached Figure Description

[0023] Figure 1 This is a system flowchart related to the method of the present invention.

[0024] Figure 2 This is a system flowchart related to another embodiment of the method of the present invention.

[0025] Figure 3 This is a schematic diagram of the structure of Example 3.

[0026] The components include: 1. Sodium-based pre-desulfurization section; 2. Flue gas inlet pipe; 3. Gas pipe; 4. pH sensor; 5. NaHCO3 supply pipe; 6. Oxygen supply pipe; 7. Circulation pipeline; 8. Filtration device; 9. First gas phase detection device; 10. Ammonia-based desulfurization section; 11. Ammonia / ammonium salt supply pipe; 12. Whitening device; 13. Second gas phase detection device; 14. Metathesis reaction device; 15. Temperature sensor; 16. Pressure sensor; 17. Reagent supply pipe; 18. NaHCO3 separation component; 19. First-stage refrigeration unit; 20. Ammonium sulfate inlet pipe; 21. Ammonia stripping device; 22. Second-stage refrigeration unit; 23. 24. Ammonium sulfate evaporation device; 25. Mother liquor concentration detection device III; 26. Pipeline I; 27. Pipeline II; 28. Pipeline III; 29. ​​Pipeline IV; 30. Regulating valve I; 31. Regulating valve II; 32. Regulating valve III; 33. Regulating valve IV; 34. Regulating valve V; 35. Regulating valve VI; 36. Sodium bicarbonate product extraction pipeline; 37. Pipeline V; 38. Regulating valve VII; 12.1. Demisting assembly; 12.2. Three-stage water washing assembly; 12.3. Magnetic energy whitening assembly; 18.1. Centrifuge device; 18.2. Drying device; 23.1. Cyclone concentration assembly; 23.2. Centrifugal separation assembly; 23.3. Drying assembly. Detailed Implementation

[0027] The present invention will be further described in detail below with reference to embodiments, but the implementation of the present invention is not limited thereto.

[0028] Example 1 This embodiment combines Figure 1 The present invention describes a method for removing heavy metals and for ultra-cleaning flue gas, comprising the following steps: Step 1: Collect boiler flue gas and send it to the sodium pre-desulfurization section of the desulfurization island. Add NaHCO3 and use the desulfurization liquid to circulate and wash the flue gas. Then, filter the gas to remove dust and heavy metals, obtaining gaseous phase a and Na-rich gas. + SO4 2- NO3 - The mother liquor a was analyzed, and the SO3 and NO in the gas phase a were detected. x Content of HCl, HF and low-valent heavy metals (such as Hg).

[0029] In this sodium-based pre-desulfurization section, the pH value of the desulfurization solution is controlled between 6.2 and 7.2, and oxygen is introduced to remove SO3. 2- NO2 - Ions oxidized to SO4 2- NO3 - ion.

[0030] The desulfurizing agent is one or both of dilute ammonia water or dilute sodium bicarbonate solution, preferably ammonia water with a mass percentage concentration of 1-18% or sodium bicarbonate solution with a mass percentage concentration of 7-14%.

[0031] In this step, the filtration device is one of a bag filter, a cartridge filter, or a filter press, and the filtration accuracy of the filtration device is preferably 25~100μm.

[0032] Step 2: When the SO3 content in gas phase a is ≤20 mg / Nm 3 HCl content ≤20mg / Nm 3 HF content ≤20mg / Nm 3 NO x Content ≤30mg / Nm 3 Hg content ≤ 0.005 mg / Nm 3 Time (NO) x The content of low-priced heavy metals is based on the threshold set with reference to national or local emission standards. Then, gas phase a is sent to the ammonia desulfurization section of the desulfurization island, and ammonia gas or ammonium salt is introduced. The desulfurization liquid is used to continue to circulate and wash gas phase a, and after whitening treatment, purified gas phase b and gas phase rich in NH4 are obtained. + SO4 2- The mother liquor b of the ions was used to detect particulate matter, SO2, and NO in the gas. x Once the content meets the standard, the mixture is vented.

[0033] This step involves defogging, three-stage water washing, and magnetic defogging treatment.

[0034] In this step, when the particulate matter in gas b is ≤5mg / Nm³ 3 SO2 ≤ 10 mg / Nm 3 NO x ≤20mg / Nm 3 To perform a short-selling operation.

[0035] Step 3: Transfer the mother liquor a from step S1 to a double decomposition reaction apparatus, introduce NH3 and CO2 or add NH4HCO3, control the temperature in the apparatus to 33~45℃, the pH value to 8.5~9.5, the pressure to 0~8000kPa, and react for 20~40 min to obtain a product rich in Na. + HCO3- NH4 + SO4 2- The mixed solution was then centrifuged and dried to separate the NaHCO3 product, leaving a Na-rich residue. + NH4 + SO4 2- HCO3 - Mother liquor c.

[0036] In this step, the metathesis reaction device is a tank reactor with a stirring structure. Preferably, the NaHCO3 stream separated in this step is transported to the sodium pre-desulfurization section as a desulfurizing agent, realizing internal circulation of the stream and reducing the supply of external reagents.

[0037] Step 4: Send mother liquor b and mother liquor c from steps S2 and S3 into a freezing device, control the temperature in the freezing device to -5~5℃, freeze and crystallize, send the precipitated crystals to the double decomposition reaction device, and send the remaining mother liquor d to the ammonia stripping device.

[0038] In this step, the freezing device is a DTB crystallizer; the ammonia stripping device is a packed evaporator.

[0039] Step 5: The temperature in the ammonia stripping unit is set to 90~105℃ (atmospheric pressure). The stripped ammonia and CO2 gases are transported to the double decomposition reaction unit. When NH4... + When the mass percentage concentration reaches 12-16%, the temperature is reduced to 50-85℃, the solid is concentrated by cyclone separation and centrifugation, and then dried to obtain (NH4)2SO4 product. The remaining liquid is sent to the metathesis reaction device.

[0040] In practice, the preset temperature of the ammonia stripping device is related to the pressure in the device. Under negative pressure, the temperature can be set to a lower temperature. The purpose is to strip out ammonia and CO2 gas and send the gas to the double decomposition reaction device.

[0041] Example 2 To facilitate public understanding of this solution, this embodiment uses a preferred method for removing heavy metals and achieving ultra-clean flue gas treatment as an example to further illustrate the solution. (Reference) Figure 2 This includes the following steps: Step 1: Collect boiler flue gas and send it to the sodium pre-desulfurization section of the desulfurization island. Add NaHCO3 and use the desulfurization liquid to circulate and wash the flue gas. Then, filter the gas to remove dust and heavy metals, obtaining gaseous phase a and Na-rich gas. + SO4 2- NO3 - The mother liquor a was analyzed, and the SO3 and NO in the gas phase a were detected. x Content of HCl, HF and low-priced heavy metals (such as Hg).

[0042] In this sodium-based pre-desulfurization section, the pH value of the desulfurization solution is controlled between 6.2 and 7.2, and oxygen is introduced to remove SO3. 2- NO2 - Ions oxidized to SO4 2- NO3 - ion.

[0043] The desulfurizing agent is one or both of dilute ammonia water or dilute sodium bicarbonate solution, preferably ammonia water with a mass percentage concentration of 1-18% or sodium bicarbonate solution with a mass percentage concentration of 7-14%.

[0044] In this step, the filtration device can be selected from bag filters, cartridge filters, or filter presses, and the filtration accuracy of the filtration device is preferably 25~100μm. In this embodiment, a bag filter with a filtration accuracy of 100μm is used, which can ensure filtration accuracy while providing filtration efficiency.

[0045] Step 2: When the SO3 content in gas phase a is ≤20 mg / Nm 3 HCl content ≤20mg / Nm 3 HF content ≤20mg / Nm 3 NO x Content ≤30mg / Nm 3 Hg content ≤ 0.005 mg / Nm 3 Time (NO) x The content of low-priced heavy metals is based on the threshold set with reference to national or local emission standards. Then, gas phase a is sent to the ammonia desulfurization section of the desulfurization island, and ammonia gas or ammonium salt is introduced. The desulfurization liquid is used to continue to circulate and wash gas phase a, and after whitening treatment, purified gas phase b and gas phase rich in NH4 are obtained. + SO4 2- The mother liquor b of the ions was used to detect particulate matter, SO2, and NO in the gas. x Once the content meets the standard, the mixture is vented.

[0046] This step involves defogging, three-stage water washing, and magnetic defogging treatment.

[0047] Once the sulfate concentration in mother liquor b reaches a certain level, mother liquor b is removed and, depending on its concentration, sent to the double decomposition reaction unit, the first-stage refrigeration unit, the second-stage refrigeration unit, or the ammonium sulfate evaporation unit. When NH4 in the double decomposition reaction apparatus + When the concentration is below the preset normal operating threshold, that is, when NH4 in the metathesis reaction device... + If the concentration is not up to standard, send mother liquor b into the double decomposition reaction apparatus to adjust the NH4. +Concentration, proceed to step three; When the mass percentage concentration of (NH4)2SO4 in the mother liquor c separated by the metathesis reaction is <24%, part of the mother liquor b is sent to the first-stage cryogenic crystallization unit to proceed to step four. When the mass percentage concentration of (NH4)2SO4 in mother liquor d is [24%, 30%), part of mother liquor b is evaporated with ammonium sulfate and then proceeds to step five.

[0048] In this step, when the particulate matter in gas b is ≤5mg / Nm³ 3 SO2 ≤ 10 mg / Nm 3 NO x ≤20mg / Nm 3 To perform a short-selling operation.

[0049] Step 3: Transfer the mother liquor a from Step 1 to a double decomposition reaction apparatus, introduce NH3 and CO2 or add NH4HCO3 (in this embodiment, NH4HCO3 reagent is directly introduced), control the temperature in the apparatus to be 35~38℃, the pH value to be between 8.5~9.5, the pressure to be 0~50kPa, and react for 20~40min to obtain a mixed solution rich in NaHCO3 and Na2SO4. Then, centrifuge and dry the mixed solution to separate the NaHCO3 product, leaving the Na2SO4-rich product. + NH4 + SO4 2- HCO3 - Mother liquor c.

[0050] In this step, the metathesis reaction device is a tank reactor with a stirring structure. Preferably, the NaHCO3 stream separated in this step is transported to the sodium pre-desulfurization section as a desulfurizing agent, realizing internal circulation of the stream and reducing the supply of external reagents.

[0051] Step 4: Send the mother liquor b from Step 2 and the mother liquor c from Step 3 into the first-stage freezing device. Control the temperature in the freezing device to 0℃ (generally -5~5℃ is preferred). Freeze and crystallize. The primary crystals of NH4HCO3 and Na2SO4·10H2O separated are sent back to the double decomposition reaction device. The remaining mother liquor d1 is sent to the ammonia stripping device. The temperature of the ammonia stripping device is set to 98~105℃ (under normal pressure). The distilled ammonia gas and CO2 gas are sent to the double decomposition reaction device. The remaining mother liquor d2.

[0052] In this step, the freezing device is a DTB crystallizer; the ammonia stripping device is an evaporator with trays.

[0053] Step 5: Send the mother liquor d2 to the second-stage refrigeration unit, control the temperature of the unit at 0℃ (generally -5~5℃ is preferred), and perform freezing treatment. The secondary crystals of Na2SO4·(NH4)2SO4·4H2O separated are sent back to the first-stage refrigeration unit. The remaining mother liquor d is sent to the ammonium sulfate evaporation unit, control the temperature of the ammonium sulfate evaporation unit at 90~98℃ (under normal pressure), and evaporate the water in the solution. The evaporated water is sent back to the second-stage refrigeration unit to obtain the ammonium sulfate-rich stream e.

[0054] Step Six: When NH4 in logistics e + When the mass percentage concentration reaches 12-16%, the temperature is reduced to 50-85℃, and then concentrated by cyclone, separated by centrifugation, and dried to obtain ammonium sulfate product. The remaining liquid is sent to the metathesis reaction device.

[0055] Example 3 This embodiment proposes a superior system for removing heavy metals and for ultra-clean flue gas treatment, with reference to... Figure 3 ,include: The sodium pre-desulfurization section 1 has a flue gas inlet connected to a flue gas inlet pipe 2. The sodium pre-desulfurization section 1 is equipped with a desulfurizing agent supply pipe 3, a pH sensor 4, a NaHCO3 supply pipe 5, and an oxygen supply pipe 6. The sodium pre-desulfurization section 1 is equipped with a desulfurization liquid circulation and washing assembly. Filter device 8 is connected in series in the mother liquor discharge pipeline of sodium pre-desulfurization section 1. Filter device 8 is used to remove dust and heavy metals from the mother liquor, and outputs a solution rich in Na. + SO4 2- NO3 - Mother liquor a; The first gas phase detection device 9 is connected to the gas phase outlet of the sodium pre-desulfurization section 1 and is used to detect the content of target ions in gas phase a. The flue gas inlet of the ammonia desulfurization section 10 is connected to the gas phase outlet of the sodium pre-desulfurization section 1. The ammonia desulfurization section 10 is equipped with an ammonia / ammonium salt supply pipe 11 and a desulfurization liquid circulation and washing assembly. The whitening device 12 has its inlet connected to the gas phase outlet of the ammonia desulfurization section 10. The whitening device 12 includes a demisting component 12.1, a three-stage water washing component 12.2 and a magnetic energy whitening component 12.3 connected in sequence, which are used to obtain the purified gas phase b. The second gas phase detection device 13 is connected to the outlet of the whitening device 12 and is used to detect smoke, SO2, and NO in gas phase b. x The content is controlled to be vented from gas phase b when the detection standard is met. The metathesis reaction device 14 has its first feed inlet connected to the mother liquor outlet of the filter device 8. The metathesis reaction device 14 is equipped with a temperature sensor 15, a pH sensor 4, a pressure sensor 16, and is connected to a reagent supply pipe 17 for conveying NH3 and CO2, or conveying NH4HCO3. The NaHCO3 separation component 18 includes a centrifuge device 18.1 and a drying device 18.2 connected in sequence. Its inlet is connected to the solution outlet of the metathesis reaction device 14, and it is used to separate NaHCO3 product. The mother liquor outlet of the NaHCO3 separation component 18 is used to output Na-rich liquid. + NH4 + SO4 2- HCO3 - Mother liquor c; The NaHCO3 separation component 18 is connected to the sodium bicarbonate product extraction pipeline 35. A portion of the concentrated NaHCO3 solution obtained after processing by the NaHCO3 separation component 18 is sent back to the sodium pre-desulfurization section 1 through the circulation pipeline 7 to be used as desulfurization liquid to wash the flue gas. The first-stage refrigeration unit 19 has a first feed inlet connected to the mother liquor outlet of the NaHCO3 separation component 18 to receive mother liquor c, and a second feed inlet connected to the mother liquor outlet of the ammonia desulfurization section 10. The first-stage refrigeration unit 19 is equipped with a temperature sensor 15 and a solid-liquid separation component. The crystallization outlet of the solid-liquid separation component is connected to the second feed inlet of the metathesis reaction device 14. The ammonia stripping device 21 has its inlet connected to the mother liquor outlet of the first-stage refrigeration device 19. The ammonia stripping device 21 is equipped with a temperature sensor 15, and its gas phase outlet is connected to the third inlet of the metathesis reaction device 14. The second-stage refrigeration unit 22 has its inlet connected to the mother liquor outlet of the ammonia stripping unit 21. The second-stage refrigeration unit 22 is equipped with a temperature sensor 15 and a solid-liquid separation component. The crystallization outlet of the solid-liquid separation component is connected to the third inlet of the first-stage refrigeration unit 19. The ammonium sulfate evaporator 23 has its inlet connected to the mother liquor outlet of the second-stage refrigeration unit 22. The ammonium sulfate evaporator 23 is equipped with a temperature sensor 15. The ammonium sulfate evaporator 23 is also equipped with a cyclone concentration component, a centrifugal separation component and a drying component. The product outlet of the ammonium sulfate evaporator 23 is used to output (NH4)2SO4 product, and its residual liquid outlet is connected to the second-stage refrigeration unit 22.

[0056] Preferably, the mother liquor outlet of the ammonia desulfurization section 10 is used to output liquor rich in NH4. + SO4 2- The mother liquor b of the ions also includes a mother liquor b diversion control unit and a mother liquor concentration detection device Ⅲ24; The mother liquor concentration detection device Ⅲ24 is connected to the mother liquor outlet of the ammonia desulfurization section 10 and is used to detect the mass percentage concentration of Na2SO4 and (NH4)2SO4 in mother liquor b. The mother liquor b diversion control unit is electrically connected to the valves on the pipelines connecting the mother liquor concentration detection device Ⅲ24, the mother liquor outlet of the ammonia desulfurization section 10, the first feed inlet of the double decomposition reaction device 14, the feed inlet of the first-stage refrigeration device 19, the feed inlet of the second-stage refrigeration device 22, and the feed inlet of the ammonium sulfate evaporation device 23. The control logic of the mother liquor b diversion control unit is as follows: When NH4 in the double decomposition reaction apparatus 14 + When the concentration is lower than the preset normal operating threshold, part of the mother liquor b will be diverted to the first feed port of the metathesis reaction device 14; When the mass percentage concentration of (NH4)2SO4 in the mother liquor c separated by the metathesis reaction is <24%, part of the mother liquor b is diverted to the feed inlet of the first-stage refrigeration unit 19. When the mass percentage concentration of (NH4)2SO4 in mother liquor d is [24%, 30%), a portion of mother liquor b is diverted to the inlet of the second-stage refrigeration unit 22 or the inlet of the ammonium sulfate evaporation unit 23.

[0057] Preferably, the mother liquor outlet of the ammonia desulfurization section 10 is connected to the first inlet of the metathesis reaction device 14, the inlet of the first-stage refrigeration device 19, the inlet of the second-stage refrigeration device 22, and the inlet of the ammonium sulfate evaporation device 23 via pipes I25, II26, III27, and IV28, respectively. Regulating valves I29, II30, III31, and IV32 are respectively installed on the connections of pipes I25, II26, III27, and IV28. When NH4 in the double decomposition reaction apparatus 14 + When the concentration is lower than the preset normal operating threshold, open the regulating valve VII 37, close the regulating valve II 30 and the regulating valve III 31, and divert part of the mother liquor b to the first feed port of the double decomposition reaction device 14; When the mass percentage concentration of (NH4)2SO4 in the mother liquor c separated by the metathesis reaction is <24%, open the regulating valve II 30, close the regulating valve VII 37 and the regulating valve III 31, and divert part of the mother liquor b to the feed inlet of the first-stage refrigeration unit 19. When the mass percentage concentration of (NH4)2SO4 in mother liquor d is [24%, 30%), open regulating valve III 31 or regulating valve IV 32, close regulating valve VII 37 and regulating valve II 30, and divert part of mother liquor b to the inlet of the second-stage refrigeration unit 22 or the inlet of the ammonium sulfate evaporation unit 23.

[0058] Preferably, the NaHCO3 product outlet of the NaHCO3 separation component 18 is also connected to the desulfurizing agent supply device of the sodium pre-desulfurization section 1 via a pipeline, for transporting part of the NaHCO3 to the sodium pre-desulfurization section 1 as a desulfurizing agent.

[0059] Preferably, the filtration device 8 is one of a bag filter, a cartridge filter, or a filter press, and the filtration accuracy of the filtration device 8 is 25~100μm.

[0060] Preferably, the metathesis reaction device is a tank reactor with a stirring structure; the first-stage refrigeration device 19 and the second-stage refrigeration device 22 are DTB crystallizers; and the ammonia stripping device 21 is an evaporation device with packing material.

[0061] Preferably, the flue gas inlet pipe 2 is equipped with a regulating valve V33, the NaHCO3 supply pipe 5 is equipped with a regulating valve VI34, and the first gas phase detection device 9 is connected to the regulating valve V33 and the regulating valve VI34 respectively.

[0062] Preferably, if ammonium sulfate product is not obtained after the system has been running for a long time, a certain amount of pure ammonium sulfate product can be added from the double decomposition reaction device 20 connected to the double decomposition reaction device 14 to promote the system to quickly achieve stable and continuous operation.

[0063] The above description is merely a preferred embodiment of the present invention and is not intended to limit the present invention in any way. Any simple modifications or equivalent changes made to the above embodiments based on the technical essence of the present invention shall fall within the protection scope of the present invention.

Claims

1. A method for removing heavy metals and ultra-cleaning flue gas, characterized in that, Includes the following steps: S1. Collect boiler flue gas and send it to the sodium pre-desulfurization section of the desulfurization island. Add NaHCO3, use the desulfurization liquid to circulate and wash the flue gas, and remove dust and heavy metals through a filtration device to obtain gas phase a and Na-rich gas phase a. + SO4 2- NO3 - The mother liquor a was analyzed, and the SO3 and NO in the gas phase a were detected. x Content of HCl, HF and low-valent heavy metals; S2, when NO in gas phase a x The content of low-priced heavy metals is lower than the preset value, and the content of SO3, HCl and HF is ≤20mg / Nm 3 Then, gas phase a is sent to the ammonia desulfurization section of the desulfurization island, and ammonia gas, ammonia water, or ammonium salts are introduced. The desulfurization liquid continues to circulate and wash gas phase a, and after whitening treatment, purified gas phase b and gas phase rich in NH4 are obtained. + SO4 2- The mother liquor b of the ions was used to detect particulate matter, SO2, and NO in the gas. x After the content of [specific substance] reaches the standard, it is vented. S3. Transfer the mother liquor a from step S1 to a double decomposition reaction apparatus, introduce NH3 and CO2 or add NH4HCO3, and control the temperature in the apparatus to 33~45℃, the pH value to 8.5~9.5, and the pressure to 0~8000kPa. The reaction yields a product rich in Na. + HCO3 - NH4 + SO4 2- The mixed solution was then centrifuged and dried to separate the NaHCO3 product, leaving a Na-rich residue. + NH4 + SO4 2- HCO3 - Mother liquor c; S4. Send the mother liquor b and mother liquor c from steps S2 and S3 into the freezing device, control the temperature in the freezing device to -5~5℃, freeze and crystallize, send the precipitated crystals to the double decomposition reaction device, and send the remaining mother liquor d to the ammonia stripping device. S5. By adjusting the temperature and / or pressure in the ammonia stripping unit, the stripped ammonia and CO2 gases are transported to the double decomposition reaction unit. When NH4... + When the mass percentage concentration reaches 12-16%, the temperature is reduced to 50-85℃, the solid is concentrated by cyclone, separated by centrifugation, and dried to obtain (NH4)2SO4 product. The remaining liquid is sent to the metathesis reaction device.

2. The method for removing heavy metals and ultra-clean flue gas according to claim 1, characterized in that: In step S4, the refrigeration unit consists of two stages, with an ammonia stripping unit connected to the rear end of each stage. The operation method is as follows: S4.

1. The mother liquor b and mother liquor c from steps S2 and S3 are sent to the first-stage freezing device and the temperature is controlled at -5~5℃ for freezing treatment. The separated NH4HCO3 and Na2SO4·10H2O are sent to the double decomposition reaction device, and the mother liquor d1 is sent to the ammonia stripping device I. By adjusting the temperature and / or pressure of the ammonia stripping device I, the stripped ammonia gas and CO2 gas are transported to the double decomposition reaction device, and the remaining mother liquor d2 is used. S4.

2. The mother liquor d2 is sent to the second-stage refrigeration unit, where the temperature is controlled at -5~5℃ for freezing treatment. The separated Na2SO4·(NH4)2SO4·4H2O is sent to the first-stage refrigeration unit, and the remaining mother liquor d is sent to the ammonium sulfate evaporation unit. By controlling the temperature and / or pressure of the ammonium sulfate evaporation unit, the water in the solution is evaporated. When NH4... + When the mass percentage concentration reaches 12-16%, the temperature is reduced to 50-85℃, and after cyclone concentration, centrifugal separation, and drying, ammonium sulfate product is obtained. The remaining liquid is sent to the second-stage refrigeration unit.

3. The method for removing heavy metals and ultra-clean flue gas according to claim 2, characterized in that: In step S2, once the sulfate concentration in mother liquor b reaches a certain level, mother liquor b is removed and, depending on its concentration, sent to the double decomposition reaction apparatus, the first-stage refrigeration apparatus, the second-stage refrigeration apparatus, or the ammonium sulfate evaporation apparatus. When NH4 in the double decomposition reaction apparatus + When the concentration is lower than the preset normal operating threshold, part of the mother liquor b will be sent to the double decomposition reaction device; When the mass percentage concentration of (NH4)2SO4 in the mother liquor c separated by the metathesis reaction is <24%, part of the mother liquor b is sent to the primary freeze crystallization unit. When the mass percentage concentration of (NH4)2SO4 in mother liquor d is [24%, 30%), part of the mother liquor b will be evaporated using an ammonium sulfate evaporation device.

4. The method for removing heavy metals and ultra-cleaning flue gas according to claim 1, characterized in that: In step S2, when the particulate matter in gas b is ≤5mg / Nm³ 3 SO2 ≤ 10 mg / Nm 3 NO x ≤20mg / Nm 3 To perform a short-selling operation.

5. The method for removing heavy metals and ultra-clean flue gas according to claim 1, characterized in that: In step S1, desulfurizing agent is continuously supplied to the sodium pre-desulfurization section to control the pH value of the desulfurization liquid between 6.0 and 7.5, and oxygen is introduced to remove SO3. 2- NO2 - Ions oxidized to SO4 2- NO3 - ion.

6. The method for removing heavy metals and ultra-clean flue gas according to claim 5, characterized in that: The desulfurizing agent is one or both of dilute ammonia or dilute sodium bicarbonate solution.

7. The method for removing heavy metals and ultra-clean flue gas according to claim 6, characterized in that: The dilute ammonia solution is ammonia solution with a mass percentage concentration of 1-18%, and the dilute sodium bicarbonate is sodium bicarbonate solution with a mass percentage concentration of 7-14%.

8. The method for removing heavy metals and ultra-clean flue gas according to claim 1, characterized in that: In step S3, a portion of the separated sodium bicarbonate stream is transported to the sodium pre-desulfurization section as a desulfurizing agent.

9. The method for removing heavy metals and ultra-clean flue gas according to claim 1, characterized in that: In step S2, the whitening process includes defogging, three-stage water washing, and magnetic whitening treatment.

10. A system for removing heavy metals and ultra-clean flue gas treatment, characterized in that, include: The sodium pre-desulfurization section (1) has a flue gas inlet connected to a flue gas inlet pipe (2). The sodium pre-desulfurization section (1) is equipped with a desulfurizing agent supply pipe (3), a pH sensor (4), a NaHCO3 supply pipe (5) and an oxygen supply pipe (6). The sodium pre-desulfurization section (1) is equipped with a desulfurization liquid circulation washing assembly. The filter device (8) is connected in series to the mother liquor discharge pipeline of the sodium pre-desulfurization section (1). The filter device (8) is used to remove dust and heavy metals from the mother liquor and output a solution rich in Na. + SO4 2- NO3 - Mother liquor a; The first gas phase detection device (9) is connected to the gas phase outlet of the sodium pre-desulfurization section (1) and is used to detect the content of target ions in gas phase a. The flue gas inlet of the ammonia desulfurization section (10) is connected to the gas phase outlet of the sodium pre-desulfurization section (1). The ammonia desulfurization section (10) is equipped with an ammonia / ammonium salt supply pipe (11) and a desulfurization liquid circulation washing assembly. The whitening device (12) has its inlet connected to the gas phase outlet of the ammonia desulfurization section (10). The whitening device (12) includes a demisting component (12.1), a three-stage water washing component (12.2), and a magnetic energy whitening component (12.3) connected in sequence to obtain the purified gas phase b. The second gas phase detection device (13) is connected to the outlet of the whitening device (12) and is used to detect smoke, SO2, and NO in gas phase b. x The content is controlled to be vented from gas phase b when the detection standard is met. The metathesis reaction device (14) has its first feed inlet connected to the mother liquor outlet of the filter device (8). The metathesis reaction device (14) is equipped with a temperature sensor (15), a pH sensor (4), a pressure sensor (16), and is connected to a reagent supply pipe (17) for conveying NH3 and CO2, or conveying NH4HCO3. The NaHCO3 separation component (18) includes a centrifuge (18.1) and a drying device (18.2) connected in sequence. Its inlet is connected to the solution outlet of the metathesis reaction device (14) for separating NaHCO3 product. The mother liquor outlet of the NaHCO3 separation component (18) is used to output Na-rich products. + Mother liquor c; The first-stage refrigeration unit (19) has its first inlet connected to the mother liquor outlet of the NaHCO3 separation component (18) to receive mother liquor c, and its second inlet connected to the mother liquor outlet of the ammonia desulfurization section (10). The first-stage refrigeration unit (19) is equipped with a temperature sensor (15) and a solid-liquid separation component. The crystallization outlet of the solid-liquid separation component is connected to the second inlet of the metathesis reaction device (14). The ammonia stripping device (21) has its inlet connected to the mother liquor outlet of the first-stage refrigeration device (19). The ammonia stripping device (21) is equipped with a temperature sensor (15), and its gas phase outlet is connected to the third inlet of the metathesis reaction device (14) through a gas pipe (3). The second-stage refrigeration unit (22) has its inlet connected to the mother liquor outlet of the ammonia stripping unit (21). The second-stage refrigeration unit (22) is equipped with a temperature sensor (15) and a solid-liquid separation component. The crystallization outlet of the solid-liquid separation component is connected to the third inlet of the first-stage refrigeration unit (19). The ammonium sulfate evaporator (23) has its feed inlet connected to the mother liquor outlet of the second-stage refrigeration unit (22). The ammonium sulfate evaporator (23) is equipped with a temperature sensor (15). The ammonium sulfate evaporator (23) is also equipped with a cyclone concentration component (23.1), a centrifugal separation component (23.2), and a drying component (23.3). The product outlet of the ammonium sulfate evaporator (23) is used to output (NH4)2SO4 product, and its residual liquid outlet is connected to the second-stage refrigeration unit (22).

11. The system according to claim 10, characterized in that: The mother liquor outlet of the ammonia desulfurization section (10) is used to output NH4-rich liquid. + SO4 2- The ion mother liquor b also includes a mother liquor b diversion control unit and a mother liquor concentration detection device Ⅲ (24). The mother liquor concentration detection device Ⅲ (24) is connected to the mother liquor outlet of the ammonia desulfurization section (10) and is used to detect the mass percentage concentration of (NH4)2SO4 in mother liquor b. The mother liquor b diversion control unit is electrically connected to the valves on the pipelines connecting the mother liquor concentration detection device III (24), the mother liquor outlet of the ammonia desulfurization section (10), the first feed inlet of the metathesis reaction device (14), the feed inlet of the first-stage refrigeration device (19), the feed inlet of the second-stage refrigeration device (22), and the feed inlet of the ammonium sulfate evaporation device (23), and the control logic of the mother liquor b diversion control unit is as follows: When NH4 in the double decomposition reaction apparatus (14) + When the concentration is lower than the preset normal operating threshold, part of the mother liquor b will be diverted to the first feed port of the metathesis reaction device (14); When the mass percentage concentration of (NH4)2SO4 in the mother liquor c separated by the metathesis reaction is <24%, part of the mother liquor b is diverted to the feed inlet of the first-stage refrigeration unit (19); When the mass percentage concentration of (NH4)2SO4 in mother liquor d is [24%, 30%), a portion of mother liquor b is diverted to the inlet of the second-stage refrigeration unit (22) or the inlet of the ammonium sulfate evaporation unit (23).

12. The system according to claim 11, characterized in that: The mother liquor outlet of the ammonia desulfurization section (10) is connected to the first feed inlet of the metathesis reaction device (14), the feed inlet of the first-stage refrigeration device (19), the feed inlet of the second-stage refrigeration device (22), and the feed inlet of the ammonium sulfate evaporation device (23) through pipes I (25), II (26), III (27), and IV (28). Regulating valves I (29), II (30), III (31), and IV (32) are respectively installed on the pipes I (25), II (26), III (27), and IV (28). When NH4 in the double decomposition reaction apparatus (14) + If the concentration is lower than the preset normal operating threshold, open regulating valve VII (37), close regulating valve II (30) and regulating valve III (31) to divert part of the mother liquor b to the first feed port of the double decomposition reaction device (14); When the mass percentage concentration of (NH4)2SO4 in the mother liquor c separated by the metathesis reaction is <24%, open the regulating valve II (30), close the regulating valve VII (37) and the regulating valve III (31), and divert part of the mother liquor b to the feed inlet of the first-stage refrigeration unit (19); When the mass percentage concentration of (NH4)2SO4 in mother liquor d is [24%, 30%), part of the mother liquor b is diverted to the inlet of the second-stage refrigeration unit (22) or the inlet of the ammonium sulfate evaporation unit (23). Then, the regulating valve III (31) or regulating valve IV (32) is opened, and the regulating valve VII (37) and regulating valve II (30) are closed.

13. The system according to claim 10, characterized in that, The NaHCO3 product outlet of the NaHCO3 separation component (18) is also connected to the desulfurizing agent supply device of the sodium pre-desulfurization section (1) through a pipeline, which is used to transport part of the NaHCO3 to the sodium pre-desulfurization section (1) as a desulfurizing agent.

14. The system according to claim 10, characterized in that: The filter device (8) is one of the following: bag filter, cartridge filter, or filter press. The filtration accuracy of the filter device (8) is 25~100μm.

15. The system according to claim 10, characterized in that: The metathesis reaction device is a fully mixed crystallizer or an Oslo crystallizer; the first-stage refrigeration unit (19) and the second-stage refrigeration unit (22) are fully mixed crystallizers or DTB crystallizers; the ammonia stripping unit (21) is an evaporation device with packing or trays.

16. The system according to claim 10, characterized in that: A regulating valve V (33) is provided on the flue gas inlet pipe (2), and a regulating valve VI (34) is provided on the NaHCO3 supply pipe (5). The first gas phase detection device (9) is connected to the regulating valve V (33) and the regulating valve VI (34) respectively.

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