A self-adjusting method for removing heavy metals from flue gas

Abstract

The present invention provides a self-adjusting method for removing heavy metals from flue gas, which belongs to the field of pollutant removal. More specifically, the method comprises the following steps: passing the flue gas into a desulfurization absorption liquid with an oxidant added thereto to absorb SeO2 in the flue gas and oxidize it into SeO4 2‑ ; Pre-treat the desulfurization absorption liquid after absorbing the flue gas to obtain a desulfurization waste liquid, and then add a reducing agent to the desulfurization waste liquid to convert SeO4 2‑ Reducing to solid Se element; spraying the dried solid Se element into the flue gas to adsorb gaseous element Hg in the flue gas, thereby realizing self-adjusting removal of heavy metals from the flue gas. The present invention does not require the introduction of additional adsorption active components and carbon-based adsorbents, and has the advantages of low cost, environmental friendliness, and simple process. At the same time, the generated HgSe is extremely stable, has a low probability of secondary release, and has no environmental safety risks. It also effectively solves the problem of high selenium emissions faced by traditional desulfurization waste liquid.

Description

Technical Field

[0001] The present invention belongs to the field of pollutant removal, and more specifically, relates to a self-adjusting method for removing heavy metals from flue gas. Background Art

[0002] Heavy metals are one of the many environmental pollutants. Although their content is relatively low, they are highly toxic, have strong environmental mobility and accumulation in organisms, and are extremely harmful to the ecological environment. Coal-fired power plants are one of the main sources of heavy metals in the environment. Especially in my country, where coal consumption is high, there is an urgent need to strictly control the heavy metal emissions from coal-fired power plants.

[0003] Coal contains a variety of toxic heavy metal elements such as mercury, selenium, arsenic, lead, chromium, and cadmium. During high-temperature combustion, they will be released from the coal into the flue gas to varying degrees in the form of gas or particles. Among the above heavy metals, mercury and selenium are the most difficult to control because they are the most volatile and difficult to completely convert into particles through natural cooling of flue gas or adsorption of fly ash. For mercury, the saturated vapor pressure of elemental mercury is high (0.25 Pa at 25°C, which is equivalent to a saturated concentration of about 20 mg / m 3 , while the mercury content in coal-fired flue gas is 1-100 μg / m 3 The main form of mercury in coal-fired flue gas is elemental mercury vapor, which is difficult to dissolve in water and cannot be removed by the wet flue gas desulfurization system. Therefore, without the use of additional technical means, the existing pollutant control equipment in coal-fired power plants cannot effectively purify mercury. As for selenium, it is mainly present in coal-fired flue gas in the form of gaseous SeO 2 It exists and is soluble in water, so it can be captured by the wet flue gas desulfurization system. However, the captured selenium will partially exist in the desulfurization wastewater in the form of highly soluble selenate, which cannot be removed by conventional wastewater treatment processes, resulting in excessive discharge of selenium in the desulfurization wastewater. Therefore, the control of heavy metals such as mercury and selenium in coal-fired power plants is still severe and needs to be solved urgently.

[0004] The existing technologies for controlling the emission of heavy metal pollutants in coal-fired flue gas mainly include: (1) CN114797780A discloses a device and method for preparing an iron-based modified nano-biochar flue gas mercury adsorbent doped with multiple metals. Agricultural waste is used as raw material, and conventional chemical precipitation, microwave heating, multi-temperature zone pyrolysis, multiple metal multi-layer loading and biomass pyrolysis coking process are optimized and integrated. On the basis of selecting specific components for structural design, the mercury removal performance of the biochar adsorbent is greatly improved. This method can solve the problems of competitive adsorption, high cost and easy deactivation of existing activated carbon adsorbents. However, this method includes microwave modification, spraying, extraction, etc. The process is complicated and not suitable for industrial-scale production and application. In addition, the carbon-based adsorbent will increase the carbon content of the fly ash, reduce the quality of the fly ash, and affect the secondary utilization of the fly ash. (2) CN107824169A discloses an activated carbon fiber loaded with nano-selenium and its chemical preparation and application. Sodium selenite is dispersed in a polyvinyl alcohol aqueous solution with a mass concentration of 0.01-0.5%, stirred evenly, and then activated carbon fiber is added. The mixture is stirred evenly at room temperature, and finally ascorbic acid is added. The mixture is reacted at room temperature for 1 hour, filtered, and dried to obtain activated carbon fiber loaded with nano-selenium. The method has an initial mercury concentration of 10-30 μg / m 3 The mercury removal efficiency is about 95%-99%, which is a high mercury removal efficiency. However, this method requires the additional consumption of expensive chemical reagents such as sodium selenite and polyvinyl alcohol, which is costly and not suitable for industrial-scale production and application. (3) CN111892248A discloses a biological treatment system for removing pollutants from flue gas desulfurization wastewater. The desulfurization wastewater is first pumped to a pretreatment unit for pretreatment through a high-efficiency flocculation coupled softening process. The pretreated water is pumped to a primary anoxic / secondary anoxic / anaerobic / aerobic biofilm reactor of the biological treatment unit to remove selenium, ammonia nitrogen and sulfate from the desulfurization wastewater. The biologically produced water is discharged into a post-treatment unit for adsorption and interception to remove the biological treatment residues and meet the discharge standards. The biological treatment system can effectively remove selenium from the flue gas desulfurization wastewater and make the wastewater meet the discharge standards. However, the process is complicated and requires the consumption of a large amount of highly toxic chemical reagents, which is costly. In addition, the selenium eventually enters the mud and cannot be recycled. There is a risk of re-release in the subsequent disposal process. Research has found that the existing technologies for controlling heavy metal pollutants in coal-fired flue gas have the following main problems: (1) the need to introduce additional expensive / highly toxic chemical reagents, resulting in high technical costs; (2) the process flow is complex, with high requirements for operators, making industrial application difficult; and (3) it has a negative impact on other links in the coal-fired process, and the removal of by-products has potential environmental risks. Summary of the invention

[0005] In view of the defects of the prior art, the purpose of the present invention is to provide a self-adjusting method for removing heavy metals from flue gas, aiming to solve the problems that the existing control of heavy metal pollutants in coal-fired flue gas requires the introduction of a large amount of expensive chemical reagents, the process is complicated, and the removal of by-products has potential environmental risks.

[0006] To achieve the above object, the present invention provides a method for self-adjusting removal of heavy metals from flue gas, the method comprising the following steps:

[0007] S1 passes the flue gas into the desulfurization absorption liquid with added oxidant to absorb SeO in the flue gas. 2 and oxidized to SeO 4 2- ;

[0008] S2 pre-treats the desulfurization absorption liquid after absorbing the flue gas to obtain a desulfurization waste liquid, and then adds a reducing agent to the desulfurization waste liquid to reduce SeO 4 2- Reduced to solid Se element;

[0009] S3 sprays the dried solid Se element into the flue gas to absorb the gaseous element Hg in the flue gas, thereby achieving self-regulating removal of heavy metals in the flue gas.

[0010] As a further preferred embodiment, in step S1, the oxidant is Na 2 S 2 O 8 , KMnO 4 , NaClO, H 2 O 2 One or more of .

[0011] As a further preferred embodiment, in step S1, a catalytic ion is added to the desulfurization absorption liquid, and the catalytic ion is CuSO 4 , MnSO 4 , FeSO 4 One or more of .

[0012] As further preferred, in step S2, the pretreatment includes cyclone separation, neutralization, precipitation and flocculation.

[0013] As further preferred, in step S2, the reducing agent includes one or more of iron powder, zinc powder and aluminum powder, and the particle size of the reducing agent is 10 to 200 μm.

[0014] As a further preferred embodiment, in step S2, an auxiliary agent is added to the desulfurization waste liquid to maintain the reducing activity of the reducing agent, and the auxiliary agent is FeSO 4 、MnSO 4 One or both of the following.

[0015] As further preferred, in step S2, the pH of the desulfurization waste liquid to which the reducing agent is added is controlled between 5.8 and 8.5.

[0016] As further preferred, in step S3, the injection rate of solid Se element is 0.05-5 kg / h.

[0017] As further preferred, in step S3, the solid Se single substance is placed in an air or nitrogen atmosphere and dried at 50-80° C. for 0.5-2 h to obtain a dried solid Se single substance.

[0018] As a further preference, in step S3, the injection point of the solid Se element is located before the dust collector, and the smoke temperature at the injection point is 100-110°C.

[0019] In general, the above technical solution conceived by the present invention has the following advantages compared with the prior art:

[0020] Beneficial effects:

[0021] 1. Based on the existence forms, basic properties and treatment processes of selenium and mercury in actual coal-fired flue gas, the present invention uses a combination of physical / chemical methods to achieve directional regulation of selenium forms, and uses oxidants and reductants to convert the original gaseous SeO 2 It is converted into particulate elemental selenium and re-injected into the flue to achieve adsorption of gaseous elemental mercury, thereby achieving integrated self-adjusting removal of mercury and selenium in coal-fired flue gas. There is no need to introduce additional adsorption active components and carbon-based adsorbents. It has the advantages of low cost, environmental friendliness, simple process, etc., and will not have a negative impact on the quality of fly ash. At the same time, mercury and selenium in the flue gas eventually enter the coal-fired fly ash in the form of HgSe. HgSe has extremely strong stability and low probability of secondary release. It is mixed with fly ash in solid form, which is easier to manage than liquid form. It has low environmental mobility and no environmental safety risk for by-products. In addition, the present invention converts highly soluble selenium compounds in desulfurization waste liquid into particulate elemental selenium, separates it from the desulfurization waste liquid, and finally enters fly ash in a stable form, solving the problem of high selenium emissions faced by traditional desulfurization waste liquid. The treated wastewater can be directly discharged or reused harmlessly, effectively reducing the potential harm of coal-fired power plants to the surrounding environment.

[0022] 2. At the same time, the present invention optimizes the particle size of the reducing agent, thereby ensuring the reduction rate and mercury adsorption efficiency while avoiding the subsequent formation of fine-grained selenium-mercury particles, so that the generated selenium-mercury particles are not easy to penetrate from the dust collector and the wet flue gas desulfurization system, and effectively improve the overall removal rate;

[0023] 3. In addition, the present invention also optimizes the pH of the desulfurization waste liquid to which the reducing agent is added, so as to avoid the pH value being too high, resulting in insufficient liquid phase hydrogen ions, low electron transfer rate, weakened reducing ability of the reducing agent, low overall reduction efficiency, and extended extraction cycle. At the same time, it avoids the pH value being too low, resulting in sufficient hydrogen ions, excessive reactivity of the reducing agent, and direct reduction reaction with hydrogen ions to form hydrogen, resulting in waste of the reducing agent. BRIEF DESCRIPTION OF THE DRAWINGS

[0024] Figure 1 It is a schematic diagram of the application of the self-adjusting removal method of heavy metals from flue gas provided in an embodiment of the present invention.

[0025] Throughout the drawings, the same reference numerals are used to denote the same elements or structures, wherein:

[0026] 1- boiler, 2- selective catalytic reduction denitrification device, 3- dust collector, 4- wet flue gas desulfurization system, 5- chimney, 6- oxidant addition box, 7- hydrocyclone, 8- neutralization box, 9- sedimentation box, 10- flocculation box, 11- reduction extraction box, 12- drying chamber, 13- flue before dust collector. DETAILED DESCRIPTION

[0027] In order to make the purpose, technical solution and advantages of the present invention more clearly understood, the present invention is further described in detail below in conjunction with the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are only used to explain the present invention and are not intended to limit the present invention.

[0028] The present invention provides a method for self-adjusting removal of heavy metals from flue gas, the method comprising the following steps:

[0029] S1 passes the flue gas into the desulfurization absorption liquid with added oxidant, and the original gaseous selenium in the flue gas dissolves into the desulfurization absorption liquid and turns into SeO 3 2- Form exists, SeO 3 2- Oxidized to SeO under the action of oxidant 4 2- ;

[0030] S2 performs pre-treatment on the desulfurization absorption liquid after absorbing the flue gas, including cyclone separation, neutralization, precipitation and flocculation, so as to obtain desulfurization waste liquid, and then adds a reducing agent to the desulfurization waste liquid to reduce SeO 4 2- Reduced to solid Se element;

[0031] S3 sprays the dried solid Se element into the flue gas to absorb the gaseous element Hg in the flue gas, thereby achieving self-regulating removal of heavy metals in the flue gas.

[0032] Specifically, in step S1, the low concentration of gaseous SeO in the flue gas 2 From gas to liquid, the selenium-containing flue gas enters the wet flue gas desulfurization system. The dense and fine alkali droplets distributed in the tower come into contact with the flue gas, and the water-soluble SeO 2 Entering the absorption liquid through mass transfer, the reaction formula is as follows:

[0033] SeO 2 (g)→SeO 2 (aq)

[0034] SeO 2 (aq)+H 2 O→HSeO 3 -

[0035] HSeO 3 - +OH - →SeO 3 2- +H 2 O

[0036] The generated SeO 3 2- It can combine with some ions (such as iron ions) in the absorption liquid to form precipitation and enter the desulfurized gypsum. It cannot be separated and recovered later. It needs to be oxidized by an oxidant to SeO with higher solubility. 4 2- , exists completely in the liquid phase in ionic form.

[0037] In step S2, the selenium-containing desulfurization absorption liquid is first subjected to the steps of cyclone (separation of liquid absorption liquid and solid gypsum), neutralization (pH adjustment, separation of sulfate, fluoride, some heavy metal ions, etc.), precipitation (separation of some heavy metal ions, etc.), and flocculation (separation of insoluble suspended matter, etc.) to remove the main ion components in the absorption liquid. 4 2- Due to its high solubility, it cannot be removed by the above steps and will remain in the desulfurization wastewater and finally enter the reduction extraction tank. By adding a reducing agent to the reduction extraction tank, SeO 3 2- Reduced to elemental selenium.

[0038] In step S3, solid Se is sprayed into the flue to be evenly distributed and dispersed in the flue gas, and contact with the gaseous mercury in the flue gas. The strong affinity between Hg and Se is used to convert the gaseous mercury into solid HgSe. Subsequently, the gas-solid separation is achieved by using the dust collector of the coal-fired power plant, and the mercury is captured into the ash field together with the coal-fired fly ash. The specific reaction formula is as follows:

[0039] Se+Hg→HgSe.

[0040] The present invention targets gaseous elemental mercury. Group VIII elements, such as oxygen, sulfur, and selenium, have an oxidative adsorption effect on it. Under certain conditions, they can combine with elemental mercury to form substances such as HgO, HgS, and HgSe, converting mercury from gas to solid state, and then separating it from flue gas. Among them, elemental selenium has the strongest affinity with elemental mercury, has a fast binding rate with mercury, and has a high efficiency of mercury removal. Based on the strong affinity between elemental selenium and elemental mercury, the gaseous SeO2 originally present in the flue gas is converted into solid state mercury by the absorption-oxidation-reduction process. 2 It is converted into particulate elemental selenium, which is then re-injected into the flue to adsorb gaseous elemental mercury in the flue gas, and finally enters the coal-fired fly ash in the form of HgSe. The physical and chemical properties of heavy metal pollutants in coal-fired flue gas are utilized to convert difficult-to-control gaseous heavy metals into stable and easy-to-control particulate heavy metals, thereby achieving integrated self-regulating removal of mercury and selenium without the need to introduce additional adsorption active components, thereby effectively reducing heavy metal emissions from coal-fired power plants to the atmospheric environment, overcoming the problems of complex process flow, high cost, and large secondary pollution in existing heavy metal control technologies, and being easy to operate, low cost, and without producing negative effects. At the same time, the present invention converts highly soluble selenium compounds in desulfurization waste liquid into particulate elemental selenium, separates it from desulfurization waste liquid, and finally enters fly ash in a stable form, solving the problem of high selenium emissions faced by traditional desulfurization waste liquid. The treated wastewater can be directly discharged or reused harmlessly, reducing the potential harm of coal-fired power plants to the surrounding environment; in the present invention, the final destination of flue gas mercury and selenium is to enter coal-fired fly ash in the form of HgSe. On the one hand, HgSe has extremely strong stability and low probability of secondary release. On the other hand, HgSe is mixed with fly ash in solid form, which is easy to manage compared to liquid form, has low environmental mobility, and has no environmental safety risk for by-products. In addition, the present invention uses coal itself as a selenium source to prepare elemental selenium as a demercuration adsorbent, does not introduce carbon-based adsorbents into flue gas, and does not have a negative impact on the quality of fly ash.

[0041] Further, in step S1, the oxidant is Na 2 S 2 O 8 , KMnO 4 , NaClO, H 2 O 2 One or more of the following, the specific reaction formula is as follows:

[0042] SeO 3 2- +S 2 O 8 2- +H 2 O→SeO 4 2- +2SO 4 2-+2H +

[0043] 3SeO 3 2- +2MnO 4 - +2H + →3SeO 4 2- +2MnO 2 +H 2 O

[0044] SeO 3 2- +ClO - →SeO 4 2- +Cl -

[0045] SeO 3 2- +H 2 O 2 →SeO 4 2- +H 2 O

[0046] At the same time, catalytic ions can be added to the desulfurization absorption liquid, and the catalytic ions are CuSO 4 , MnSO 4 , FeSO 4 One or more of the above to catalyze SeO 3 2- of oxidation.

[0047] Further, in step S2, the reducing agent includes one or more of iron powder, zinc powder and aluminum powder, and the reducing agent is used as the core medium to make the soluble SeO 4 - The adsorption-reduction-dissolution process occurs on the surface of the reducing agent, forming dispersed elemental Se particles that adhere to the surface of the reducing agent. The reducing agent is an electron donor and a physical medium, realizing the transformation of selenium from a soluble chemical state to an insoluble elemental state. At the same time, an auxiliary agent can be added to the desulfurization waste liquid to maintain the reducing activity of the reducing agent and prevent the passivation of the reducing agent. The auxiliary agent is FeSO 4 、MnSO 4 When the reducing agent is iron powder, the specific reaction formula is as follows:

[0048] 2Fe+SeO 4 2- +2H + →Fe 2 O 3 +Se+H 2 O

[0049] Fe+SeO 4 2- +3Fe 2+ +2H 2 O→2Fe 2 O 3 +Se+4H +

[0050] 2Fe+2SeO 4 2- +3Mn 2+ +10H + →Fe 2 O 3 +2Se+3Mn 4+ +5H 2 O

[0051] The particle size of the reducing agent is 10-200 μm. The reducing agent serves as both the main electron donor for selenium reduction and the carrier of elemental selenium. The appropriate particle size range is the key to determining its reduction rate and mercury adsorption efficiency. If the particle size is too large, the surface area of ​​the reducing agent particles will be reduced under the same mass, resulting in a decrease in the contact area between the solid Se element and the gaseous mercury after being sprayed into the flue. Accordingly, if a higher mercury removal efficiency is to be achieved, the injection amount of the solid Se element needs to be increased, resulting in a decrease in the overall economic benefits of the technology. On the contrary, if the particle size is too small, the specific surface area of ​​the reducing agent increases, which makes the reducing agent too reactive and a large amount of the reducing agent will directly react with H + or H 2 O reacts to produce H 2 , resulting in a large amount of waste of reducing agent. On the other hand, the particle size of the solid Se element formed will be reduced. Although its adsorption area with gaseous mercury will increase and the binding efficiency will increase, the fine particles of Se-mercury can easily penetrate from the dust collector and wet flue gas desulfurization system, and still remain in the flue gas as fine particles and are eventually discharged into the atmosphere, resulting in a decrease in the overall removal rate.

[0052] In addition, HNO 3 , NaOH is used as a regulator to control the pH of the desulfurization wastewater with the added reducing agent between 5.8 and 8.5. If the pH value is too high, the liquid phase hydrogen ions are insufficient, the electron transfer rate is low, the reducing ability of the reducing agent is weakened, the overall reduction efficiency is low, and the extraction cycle is extended; on the contrary, if the pH value is too low, the number of hydrogen ions is sufficient, the reductant reaction activity is too strong, and a large amount of the reducing agent will react with the hydrogen ions to form H 2 , resulting in a waste of reducing agent.

[0053] Further, in step S3, the solid Se element is placed in an air or nitrogen atmosphere and dried at 50-80°C for 0.5-2h to obtain a dried solid Se element. The injection rate of the solid Se element is 0.05-5kg / h, which is mainly determined according to the load of the coal-fired power plant, the mercury content in the flue gas, the flue gas temperature at the injection point, etc. The mercury content in the flue gas and the demercuration efficiency of the solid Se element are determined based on the above parameters. If the addition rate of the solid Se element is too high, a large amount of solid Se element will not fully absorb mercury and will directly enter the coal-fired fly ash, resulting in a waste of resources; if the addition rate of the solid Se element is too low, the gaseous elemental mercury in the flue gas cannot be completely captured, resulting in a large amount of mercury emissions.

[0054] The injection point of solid Se element is located in front of the dust collector, and the optimal flue gas temperature range of the injection point is 100-110℃. After elemental selenium adsorbs gaseous mercury, it will exist in the form of mercury selenide particles. Only by separating it from the flue gas can selenium and mercury be truly removed. In the pollutant purification device of a coal-fired power plant, both the dust collector and the wet flue gas desulfurization system can capture particles, and the two are usually arranged in series, among which the dust collector has a higher removal effect. Elemental selenium is injected in front of the dust collector to fix gaseous mercury, and the dual effects of the dust collector and the wet flue gas desulfurization system can be used to capture particulate mercury selenide; if elemental selenium is injected after the dust collector and before the wet flue gas desulfurization system, the wet flue gas desulfurization system alone may not be able to completely remove particulate mercury selenide, and there is a risk of escape. In addition, considering the optimal adsorption temperature range of elemental selenium to gaseous mercury, the position in front of the dust collector and the flue gas temperature range of 100-110℃ is finally selected as the preferred elemental selenium injection point.

[0055] like Figure 1 As shown, the self-adjusting heavy metal removal method for coal-fired flue gas provided by the present invention is applied to a coal-fired power plant. The specific process is as follows: the flue gas generated by the combustion of the boiler 1 passes through the selective catalytic reduction desulfurization device 2, the dust collector 3, and the wet flue gas desulfurization system 4 in sequence and is discharged from the chimney 5, wherein the oxidant adding box 6 is connected to the wet flue gas desulfurization system 4 to add an oxidant to the desulfurization absorption liquid, so that SeO dissolved in the desulfurization absorption liquid is 3 2- Oxidized to SeO 4 2-The desulfurized absorption liquid is sequentially passed through a hydrocyclone 7, a neutralization tank 8, a sedimentation tank 9, and a flocculation tank 10 to separate gypsum from wastewater, remove heavy metals such as copper, lead, manganese, and chromium, and suspended solids, and obtain desulfurized waste liquid; the desulfurized waste liquid is sent to a reduction extraction tank 11, and a reducing agent is added to the reduction extraction tank 11, so that selenium is converted from a liquid ionic state to a solid elemental state, and the filtered solid elemental selenium is sent to a drying chamber 12 for drying; finally, the dried solid elemental selenium is uniformly sprayed into the flue 13 before the dust collector, and enters the coal-fired fly ash after combining with the gaseous elemental mercury in the flue gas, thereby realizing the self-adjusting removal of heavy metals in the coal-fired flue gas.

[0056] The self-regulating method for removing heavy metals from coal-fired flue gas provided by the present invention is suitable for the mainstream flue gas treatment technology (selective catalytic reduction device + dust collector + wet flue gas desulfurization system) and wastewater treatment technology (cyclone separation + neutralization + precipitation + flocculation) of existing coal-fired power plant pollutants. It has low investment cost for new equipment, strong applicability and low cost.

[0057] The technical solution provided by the present invention is further described below based on specific embodiments.

[0058] Example 1

[0059] Application object: A 1000MW coal-fired unit, the selenium content of the incoming coal is 1.2μg / g, the mercury content is 0.135μg / g, the sulfur content is 0.8%, the chlorine content is 0.08%, and the flue gas volume is 3150000m 3 / h (standard state), the pollutant control device is SCR system + electrostatic precipitator + wet flue gas desulfurization system, among which the wet flue gas desulfurization system adopts limestone-gypsum method and is equipped with forced oxidation fan.

[0060] Operation process: Use the dosing pump to add Na to the desulfurization absorption liquid. 2 S 2 O 8 and CuSO 4 , add iron powder and FeSO to the reduction extraction box 4 , where the average particle size of the iron powder is 50 μm, by adding NaOH and HNO 3 The pH value was maintained at 7.5±0.2. After a period of reaction, the reduction extract was filtered to obtain solid Se elemental substance, which was sent to the drying room. The drying room was air atmosphere, the drying temperature was 70°C, and the drying time was 0.6h. The smoke temperature of 102°C before the dust collector was selected, and the dried solid Se elemental substance was uniformly sprayed into the flue at a constant rate of 1.56kg / h.

[0061] After the elemental selenium is reduced and extracted in this embodiment and then re-injected into the flue before the dust collector, the selenium concentration at the outlet of the wet flue gas desulfurization system is 1.35 μg / m 3, mercury concentration is 0.12μg / m 3 The selenium concentration at the outlet of the desulfurization waste liquid treatment system is 0.56μg / L.

[0062] Example 2

[0063] Application object: A 900MW coal-fired unit, the coal entering the furnace contains 3.2μg / g of selenium, 0.423μg / g of mercury, 1.9% of sulfur, 0.42% of chlorine, and a flue gas volume of 2680000m 3 / h (standard state), the pollutant control device is SCR system + bag filter + wet flue gas desulfurization system, among which the wet flue gas desulfurization system adopts limestone-gypsum method and is equipped with forced oxidation fan.

[0064] Operation process: Since the coal entering the unit has a high chlorine content and the flue gas components are highly oxidizing, there is no need to add additional oxidants to the desulfurization absorption liquid. Add reagent iron powder and FeSO 4 , where the average particle size of the iron powder is 10 μm, by adding NaOH and HNO 3 The pH value is maintained at 6.8±0.2. After a period of reaction, the reduction extract is filtered to obtain solid Se elemental substance, which is sent to the drying room. The drying room is a gas atmosphere, the drying temperature is 65℃, and the drying time is 1h. The smoke temperature of 109℃ before the dust collector is selected, and the dried solid Se elemental substance is uniformly sprayed into the flue at a constant rate of 4.7kg / h.

[0065] After the elemental selenium was reduced and extracted in this embodiment and re-injected into the flue before the dust collector, the selenium concentration at the outlet of the wet flue gas desulfurization system was 2.68 μg / m 3 , mercury concentration is 0.35μg / m 3 The selenium concentration at the outlet of the desulfurization waste liquid treatment system is 0.38μg / L.

[0066] Example 3

[0067] Application object: A 660MW coal-fired unit, the coal entering the furnace contains 1.8μg / g of selenium, 0.036μg / g of mercury, 0.5% of sulfur, 0.12% of chlorine, and a flue gas volume of 1980000m 3 / h (standard state), the pollutant control device is SCR system + electrostatic precipitator + wet flue gas desulfurization system, among which the wet flue gas desulfurization system adopts limestone-gypsum method and is not equipped with forced oxidation fan.

[0068] Operation process: Use the dosing pump to add NaClO and MnSO to the desulfurization absorption liquid. 4 , add iron powder and FeSO to the reduction extraction box 4 , where the average particle size of the iron powder is 110 μm, by adding NaOH and HNO3 The pH value is maintained at 7.3±0.2. After a period of reaction, the reduction extract is filtered to obtain solid Se elemental substance, which is sent to the drying room. The drying room is air atmosphere, the drying temperature is 80℃, and the drying time is 0.7h. The smoke temperature of 105℃ before the dust collector is selected, and the dry solid Se elemental substance is uniformly sprayed into the flue at a constant rate of 0.08kg / h.

[0069] After the elemental selenium was reduced and extracted in this embodiment and re-injected into the flue before the dust collector, the selenium concentration at the outlet of the wet flue gas desulfurization system was 1.63 μg / m 3 , mercury concentration is 0.06μg / m 3 The selenium concentration at the outlet of the desulfurization waste liquid treatment system is 0.31μg / L.

[0070] Example 4

[0071] Application object: A 600MW coal-fired unit, the coal entering the furnace contains 0.8μg / g selenium, 0.368μg / g mercury, 1.2% sulfur, 0.16% chlorine, and 1810000m3 of flue gas. 3 / h (standard state), the pollutant control device is SCR system + electrostatic precipitator + wet flue gas desulfurization system, among which the wet flue gas desulfurization system adopts double alkali method and is equipped with forced oxidation fan.

[0072] Operation process: Use the dosing pump to add KMnO to the desulfurization absorption liquid. 4 and CuSO 4 , add zinc powder and MnSO to the reduction extraction box 4 , where the average particle size of zinc powder is 40 μm, by adding NaOH and HNO 3 The pH value was maintained at 8.4±0.2. After a period of reaction, the reduction extract was filtered to obtain solid Se elemental substance, which was sent to the drying room. The drying room was filled with nitrogen atmosphere, the drying temperature was 52°C, and the drying time was 1.5h. The smoke temperature of 105°C before the dust collector was selected, and the dried solid Se elemental substance was uniformly sprayed into the flue at a constant rate of 3.68kg / h.

[0073] After the elemental selenium is reduced and extracted in this embodiment and then re-injected into the flue before the dust collector, the selenium concentration at the outlet of the wet flue gas desulfurization system is 0.23 μg / m 3 , the mercury concentration is 0.31μg / m 3 The selenium concentration at the outlet of the desulfurization waste liquid treatment system is 0.68μg / L.

[0074] Example 5

[0075] Application object: A 300MW coal-fired unit, the coal entering the furnace contains 5.6μg / g of selenium, 0.753μg / g of mercury, 1.6% of sulfur, 0.18% of chlorine, and a flue gas volume of 960000m 3 / h (standard state), the pollutant control device is SCR system + electrostatic precipitator + wet flue gas desulfurization system, among which the wet flue gas desulfurization system adopts limestone-gypsum method and is equipped with forced oxidation fan.

[0076] Operation process: Use the dosing pump to add H2 into the desulfurization absorption liquid. 2 O 2 and FeSO 4 , add aluminum powder and MnSO to the reduction extraction box 4 , where the average particle size of aluminum powder is 190 μm, by adding NaOH and HNO 3 The pH value is maintained at 6.0±0.2. After a period of reaction, the reduction extract is filtered to obtain solid Se elemental substance, which is sent to the drying room. The drying room is air atmosphere, the drying temperature is 75℃, and the drying time is 1.8h. The smoke temperature of 101℃ before the dust collector is selected, and the dry solid Se elemental substance is uniformly sprayed into the flue at a constant rate of 2.45kg / h.

[0077] After the elemental selenium was reduced and extracted in this embodiment and re-injected into the flue before the dust collector, the selenium concentration at the outlet of the wet flue gas desulfurization system was 2.36 μg / m 3 , mercury concentration is 0.45μg / m 3 The selenium concentration at the outlet of the desulfurization waste liquid treatment system is 1.35μg / L.

[0078] In this way, through the process of self-regulating removal of mercury and selenium from coal-fired flue gas of the present invention, the self-binding and stabilization of mercury and selenium elements in coal can be achieved without introducing additional active substances, thereby reducing the emission of heavy metal pollutants in the flue gas and wastewater of coal-fired power plants. It has the advantages of low cost, simple process, high removal rate, and no environmental safety risks or other negative effects of by-products.

[0079] It will be easily understood by those skilled in the art that the above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions and improvements made within the spirit and principles of the present invention should be included in the protection scope of the present invention.

Claims

1. A self-adjusting method for removing heavy metals from flue gas, characterized in that: The method comprises the following steps: S1 passes the flue gas into the desulfurization absorption liquid with added oxidant to absorb SeO2 in the flue gas and oxidize it into SeO4 2- ; S2 pre-treats the desulfurization absorption liquid after absorbing the flue gas to obtain a desulfurization waste liquid, and then adds a reducing agent to the desulfurization waste liquid to convert SeO4 2- Reduced to solid Se element; S3 sprays the dried solid Se element into the flue gas to absorb the gaseous element Hg in the flue gas, thereby achieving self-regulating removal of heavy metals in the flue gas.

2. The method for self-adjusting heavy metal removal from flue gas according to claim 1, characterized in that: In step S1, the oxidant is one or more of Na2S2O8, KMnO4, NaClO, and H2O2.

3. The self-adjusting method for removing heavy metals from flue gas according to claim 1, characterized in that: In step S1, catalytic ions are also added to the desulfurization absorption liquid, and the catalytic ions are one or more of CuSO4, MnSO4, and FeSO4.

4. The method for self-adjusting removal of heavy metals from flue gas according to claim 1, characterized in that: In step S2, the pretreatment includes cyclone separation, neutralization, precipitation and flocculation.

5. The self-adjusting method for removing heavy metals from flue gas according to claim 1, characterized in that: In step S2, the reducing agent includes one or more of iron powder, zinc powder and aluminum powder, and the particle size of the reducing agent is 10 to 200 μm.

6. The self-adjusting method for removing heavy metals from flue gas according to claim 1, characterized in that: In step S2, an auxiliary agent is added to the desulfurization waste liquid to maintain the reducing activity of the reducing agent, and the auxiliary agent is one or both of FeSO4 and MnSO4.

7. The self-adjusting method for removing heavy metals from flue gas according to claim 1, characterized in that: In step S2, the pH of the desulfurization waste liquid to which the reducing agent is added is controlled between 5.8 and 8.

5.

8. The self-adjusting method for removing heavy metals from flue gas according to claim 1, characterized in that: In step S3, the injection rate of solid Se element is 0.05-5 kg / h.

9. The method for self-adjusting removal of heavy metals from flue gas according to claim 1, characterized in that: In step S3, the solid Se single substance is placed in air or nitrogen atmosphere and dried at 50-80° C. for 0.5-2 h to obtain a dried solid Se single substance.

10. The self-adjusting method for removing heavy metals from flue gas according to any one of claims 1 to 9, characterized in that: In step S3, the injection point of the solid Se element is located before the dust collector, and the smoke temperature at the injection point is 100-110°C.

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