CN112915755B - System and method for jointly recovering sulfur dioxide in flue gas and removing nitrogen oxide - Google Patents

Abstract

The invention discloses a system and a method for jointly recovering sulfur dioxide in flue gas and removing nitrogen oxide. Oxidizing nitrogen oxide in the flue gas into nitrogen dioxide by ozone; the flue gas is absorbed by sodium sulfite and ammonium sulfite, sulfur dioxide is converted into bisulfite, and nitrogen oxide is converted into nitrate and nitrite; sulfur dioxide is enriched and recovered through evaporation and regeneration processes; nitrate and nitrite are reduced into ammonia nitrogen by an electrochemical reduction process, and the ammonia nitrogen returns to the absorption tower to be used as absorption liquid for recycling. Compared with the traditional sodium-alkali wet absorption method, the method utilizes the wet absorption and electrochemical reduction combined technology, removes nitrogen oxides in the flue gas while recovering sulfur dioxide, and recycles the sulfur dioxide as an absorbent; meanwhile, the problem of pipeline blockage caused by the fact that sodium sulfate as an oxidation by-product is easy to crystallize at low temperature is effectively solved by utilizing wet absorption of ammonium salt.

Description

System and method for jointly recovering sulfur dioxide in flue gas and removing nitrogen oxide

Technical Field

The invention belongs to the field of chemical industry and environmental protection, and relates to a system and a method for jointly recovering sulfur dioxide in flue gas and removing nitrogen oxide; in particular to a system and a method for recovering sulfur dioxide in flue gas and removing nitrogen oxide by combining sodium-ammonium double-alkali wet absorption and electrochemical reduction.

Background

Sulfur dioxide (SO) in the atmosphere₂) And Nitrogen Oxides (NO)_X) Is an important source of photochemical smog, acid rain and other environmental problems, can cause great harm to the environment and human health, and is an air pollutant of major concern. Therefore, the industrial flue gas can be discharged after being subjected to desulfurization and denitrification to reach the standard.

Flue gas desulfurization is mainly based on a high-concentration acid making technology and a low-concentration recovery method. NO_XIs mainly composed of NO₂And NO, where NO is more difficult to handle. In recent years, they have attracted more and more attention, and the emission standards set by countries and industries have become stricter. NO in general industrial fumes_XIn a lower concentration and with SO₂Coexisting and the temperature of the flue gas is low, so that the flue gas is difficult to remove by adopting the traditional reduction and denitrification technology. In view of the above requirements for flue gas desulfurization and denitration and the requirement for multi-pollutant cooperative control, the flue gas purification efficiency needs to be improved by combining new environmental protection requirements. In industry, SO in flue gas is removed conventionally₂The technologies of the method are mainly divided into a dry method, a wet method, a semi-dry method and the like, wherein the limestone-gypsum method, the circulating fluidized bed method and the like are widely applied and mature dry process methods, but the problems of non-ideal desulfurization efficiency, a plurality of byproducts and the like still exist. The wet desulphurization efficiency is higher, but the disadvantages of large occupied area and difficult secondary wastewater treatment exist. Removal of NO_XThe main method adopts a selective catalytic reduction denitration process, has high denitration efficiency, but has the problems of high operating cost, harsh conditions, high maintenance cost and the like.

Through the search of the prior art, the patent CN103203176A uses a composite oxidant to oxidize NO and uses a calcium-based absorbent to oxidize oxidation products and SO₂The absorption and removal are carried out, but the problems of large consumption of the oxidant and more desulphurization byproducts still exist; patent CN1660474A uses limestone to react with desulfurization solution by sodium-calcium double alkali method to regenerate the desulfurizer, but the treatment of the desulfurization byproduct calcium sulfate is still a problem; patent CN103990362A uses chlorine dioxide gas to oxidize NO, and combines wet method or semi-dry method to synergistically remove NO_XAnd SO₂But the preparation cost of the oxidant is high, and secondary pollution is easy to cause; in patent CN101347706A, soda ash, caustic soda or waste alkali is used as an absorbent, and carbide slag is used as a regenerant to desulfurize flue gas, so as to treat waste with waste, save energy and reduce emission, but sulfur dioxide in flue gas cannot be reused.

Generally speaking, wet absorption is still a desulfurization and denitrification method with high efficiency and wide application. The traditional sodium sulfite-sodium bisulfite absorption system has low cost and high efficiency, but oxidizes side reaction productsThe absorption capacity of the absorption liquid is easily reduced, and the pipeline is easily blocked under the low-temperature condition due to the low solubility of the sulfate. In addition, the process is directed to NO_XHas a limited removal effect.

Disclosure of Invention

The invention aims to provide a system and a method for recovering sulfur dioxide in flue gas and removing nitrogen oxide by combining sodium-ammonium double-alkali wet absorption and electrochemical reduction aiming at the defects of the existing flue gas combined desulfurization and denitrification process technology and combining the advantages of adjustable electrode performance and controllable reactant of the electrochemical reduction method, and simultaneously effectively solve the problem of pipeline blockage caused by low-temperature easy crystallization of oxidation by-product sulfate in the actual process.

The working process of the invention is as follows: nitrogen oxide in the ozone oxidized flue gas after dust removal is nitrogen dioxide, sodium sulfite and ammonium sulfite are used as absorbents to absorb sulfur dioxide and nitrogen oxide in the ozone oxidized flue gas, the sulfur dioxide is converted into sodium bisulfite, and the nitrogen oxide is converted into nitrate and nitrite; then the concentrated sulfur dioxide is released by evaporation and regeneration for enrichment and recovery; nitrate and nitrite are reduced into ammonium salt by virtue of electrochemical reduction reaction, the ammonium salt is recycled to the absorption tank to supplement the ammonium salt in the absorption liquid, and when the ammonium salt reaches a certain concentration, the ammonium salt is concentrated and discharged to be used as a nitrogen fertilizer.

In the system of the invention, nitrate/nitrite is electrochemically reduced into ammonium by an electrochemical method, the process is easy to implement compared with the process of reducing into nitrogen, no chloride ion and the like are added, and the reduction of the generated ammonium is not reduced after the ammonium is returned to an absorption cell₂The low-temperature solubility of the oxidation product sulfate radical is far lower than that of sodium sulfate, so that the outstanding problem that the low-temperature pipeline is easy to block in the actual process can be effectively solved. In the system, the ammonium radical has the effect that (1) the solubility of the oxidation product is far greater than that of sodium salt under the low-temperature condition, so that the problem of pipeline blockage is effectively solved; (2) ammonium radical being derived from NO_XThe absorption product of (2) does not need to be additionally added; (3) nitrate radical/nitrite radical is reduced into ammonium radical through electrochemical process, which is easy to realize and after the absorption tank is reusedCan realize NO_XThe cyclic utilization of the NO is realized on the basis of solving the practical process problem_XInnocent treatment and utilization; (4) ammonium does not affect SO₂And NO_XThe absorption effect of (1). In addition, excessive control on the concentration of sulfite and the pH value in the absorption liquid is not needed, because the concentration of sulfite influences the absorption capacity, the higher the concentration is, the larger the absorption capacity is; the method for controlling the pH to be alkaline is to add an alkaline solution, such as sodium hydroxide, and the process itself is a process for increasing the concentration of sulfite.

The purpose of the invention can be realized by the following scheme:

in a first aspect, the invention relates to a method for combined recovery of sulfur dioxide and removal of nitrogen oxides from flue gas, which combines a sodium-ammonium double-alkali wet absorption method and an electrochemical reduction method. The invention combines a sodium-ammonium double-alkali wet absorption desulfurization process method and an electrochemical reduction denitration method to realize the purposes of absorption, regeneration and recovery of sulfur dioxide and absorption, cyclic utilization and removal of nitrogen oxides.

As an embodiment of the present invention, the method comprises the steps of:

s1, oxidizing the dedusted flue gas by ozone to oxidize NO in the flue gas into NO₂；

S2, absorbing sulfur dioxide SO in the flue gas of the step S1 by using sulfite absorption liquid₂And nitrogen oxide NO_XFormation of bisulfite HSO₃ ^-Nitrate NO₃ ^-And nitrite NO₂ ^-A solution;

s3, evaporating and regenerating the sulfite absorption liquid after the sulfite absorption liquid is saturated, releasing and regenerating sulfur dioxide, and enriching and recovering; carrying out electrochemical reduction denitration on the evaporated and regenerated solution, wherein nitrate and nitrite are reduced into ammonia nitrogen;

s4, recycling the ammonia nitrogen-containing solution subjected to the reduction denitration treatment in the step S3 to the step S2 to be used as a supplementary absorption liquid for recycling; or when the concentration of the ammonium salt in the ammonia-containing nitrogen solution after the reduction denitration treatment in the step S3 exceeds 75%, discharging the ammonia-containing nitrogen solution out of the system. In the system of the present invention, the concentration of ammonium in the absorption liquid gradually increases as the reaction proceeds, but when the ammonium concentration is too high, ammonia slip is easily caused, and the absorption efficiency is lowered due to the decrease in pH.

As an embodiment of the present invention, in step S1, the ozone oxidation process is performed by an ozone generator discharging and oxidizing air or pure oxygen into ozone.

As an embodiment of the present invention, in step S2, the initial sulfite absorbing solution of the reaction is a sodium sulfite solution with a concentration of 10% to 20%; in the reaction process, the sulfite absorption liquid is a mixed liquid of sodium sulfite and ammonium sulfite; the mass ratio of the sodium sulfite to the ammonium sulfite in the mixed solution is 25-100%, and the mass ratio of the ammonium sulfite is 0-75%.

As an embodiment of the present invention, in step S2, the absorption process is carried out under ambient atmospheric pressure (101kPa) and at medium-low temperature (20 ℃ C. to 60 ℃ C.). Preferably, the absorption process is carried out at ambient atmospheric pressure and 40 ℃.

As an embodiment of the invention, in step S2, during the absorption process, part of the nitrite is oxidized into nitrate, and part of the sulfite absorption solution is absorbed by O in the flue gas₂Oxidation to the oxidation by-product sulfate.

As an embodiment of the present invention, in step S2, the absorption process is added with supplementary alkali solution, wherein the alkali solution is sodium hydroxide or sodium sulfite. When the treatment is carried out in step S3, sodium bisulfite is evaporated under reduced pressure to release SO₂Sodium sulfite is generated, and the part of the sodium sulfite is returned to an absorption tank (a desulfurization absorption tower) to be used as an absorbent; in addition, because the absorption liquid is partially oxidized due to the existence of oxygen in the flue gas, a supplementary alkali liquor is additionally added in the process, and the added alkali liquor is sodium hydroxide or sodium sulfite.

As an embodiment of the present invention, in step S3, the evaporation regeneration process is to regenerate sulfur dioxide by heating decomposition or reduced pressure evaporation process. The temperature for heating and decomposing is 95-100 ℃; the discharged solution after evaporation and regeneration is concentrated and used as fertilizer or building material.

As one embodiment of the present invention, in step S3, the recovered sulfur dioxide is enriched to produce high concentration sulfur dioxide or acid.

In step S3, the electrochemical reduction denitration process is performed in an electrolytic cell, wherein the anode is Ir-Ru/Ti-based inert electrode, the cathode is Ti-based metal oxide electrode, and the cathode and anode are separated by proton exchange membrane.

As an embodiment of the invention, the Ti-based metal oxide electrode is selected from Co₃O₄/Ti or Fe₂O₃a/Ti electrode.

As one embodiment of the present invention, the Ti-based metal oxide electrode is obtained by coating a metal oxide-supported thin film on a Ti substrate by a sol-gel method. The metal oxide is Co₃O₄Or Fe₂O₃。

In step S3, ammonia nitrogen in the solution in the evaporation and regeneration device is discharged after reaching a predetermined concentration. In the evaporation and regeneration device, oxidation byproducts sodium sulfate and ammonium sulfate generated in the absorption process and the electrochemical reduction process are discharged out of the system and are concentrated to be used as fertilizers, so that economic benefits are generated.

In a second aspect, the invention also relates to a system for jointly recovering sulfur dioxide in flue gas and removing nitrogen oxide, which comprises an ozone generator, a desulfurization absorption tower, an evaporation regenerator, an electrochemical reaction tank and an absorption liquid supplement tank which are sequentially connected; and the supplementary absorption liquid in the absorption liquid supplementary pool reflows to the desulfurization absorption tower through a pipeline.

Compared with the prior art, the invention has the following beneficial effects:

1) the invention forms a sodium sulfite-ammonium sulfite wet absorption system, reduces the crystallization temperature of the absorption system on the premise of ensuring the total absorption efficiency, and effectively solves the problem of pipeline blockage caused by low-temperature oxidation byproducts in the absorption process of the traditional process;

2) the invention utilizes the advantages of the electrochemical denitration method to recover SO in the traditional process₂While suckingCollecting NO_XAnd the desulfurization and denitrification can be realized at the same time by recycling.

Drawings

Other features, objects and advantages of the invention will become more apparent upon reading of the detailed description of non-limiting embodiments with reference to the following drawings:

FIG. 1 is a schematic overall flow chart of the method for jointly recovering sulfur dioxide and removing nitrogen oxides from flue gas according to the present invention;

wherein: 1. an ozone generator; 2. an absorption tower; 3. an evaporation regenerator; 4. an electrochemical reaction cell; 5. and an absorption liquid replenishing pool 6 and a condenser.

Detailed Description

The invention is described in detail below with reference to the figures and specific embodiments. The following examples, which are set forth to provide a detailed description of the invention and a detailed description of the operation, will help those skilled in the art to further understand the present invention. It should be noted that the scope of the present invention is not limited to the following embodiments, and that several modifications and improvements made on the premise of the idea of the present invention belong to the scope of the present invention.

Example 1

The embodiment relates to combined recovery of SO in flue gas₂And removing NO_XThe system comprises an ozone generator 1, an absorption tower 2, an evaporation regenerator 3, an electrochemical reaction tank 4 and an absorption liquid replenishing tank 5 which are connected in sequence as shown in figure 1;

the ozone generator 1 is provided with a flue gas inlet, and the flue gas after dust removal is conveyed to the ozone generator 1 for ozone oxidation treatment;

the absorption tower 2 is provided with a tail gas discharge port and a supplementary absorption liquid inlet, and supplementary absorption liquid in the absorption liquid supplementary tank 5 flows back to the absorption tower 2 through the supplementary absorption liquid inlet; the absorption liquid supplement tank 5 is also provided with an alkali liquid supplement device;

SO is arranged on the evaporation regenerator 3₂An outlet and an oxidation by-product outlet, evaporating the regenerated SO in the regenerator 3₂Via SO₂Outlet into SO₂Release line, said SO₂A condenser 6 is arranged on the releasing pipeline and is used for recovering and enriching the SO with high concentration₂A gas; and discharging the oxidation byproducts of the sodium sulfate and the ammonium sulfate out of the system through an oxidation byproduct outlet, and concentrating the oxidation byproducts to be used as fertilizer.

The combined recovery of SO in flue gas of the present embodiment₂And removing NO_XThe method comprises the following steps: the flue gas after dust removal is subjected to an ozone oxidation process to oxidize nitric oxide in the flue gas into nitrogen dioxide; absorbing sulfur dioxide and nitrogen oxides in the flue gas by using sulfite absorption liquid to generate bisulfite, nitrate and nitrite; after absorption saturation, the absorption liquid is subjected to evaporation regeneration, sulfur dioxide is released and regenerated, and enrichment recovery is carried out to prepare high-concentration sulfur dioxide or acid; carrying out electrochemical reduction on the evaporated and regenerated solution, and reducing nitrate and nitrite into ammonia nitrogen; the ammonia nitrogen is circulated to the absorption tank to supplement absorption liquid for recycling, and is discharged out of the system after reaching a certain concentration, so that the ammonia nitrogen is used as fertilizer to produce economic benefit.

The method comprises the following specific steps:

firstly, the flue gas after dust removal is subjected to an ozone oxidation process, and NO in the flue gas is oxidized into NO₂This step will be more favorable for NO_XThe absorbed liquid is absorbed into the liquid phase, and the ozone oxidation process is realized by oxidizing air or pure oxygen into ozone through the discharge of an ozone generator;

NO+O₃→NO₂+O₂

secondly, absorbing SO in the flue gas by sulfite absorption liquid₂And NO_X，SO₂With absorption liquid SO₃ ^2-Reaction to HSO₃ ^-，NO_XInto the liquid phase to form NO₃ ^-And NO₂ ^-，NO₂ ^-As the main component; meanwhile, in this process, part of NO₂ ^-Is oxidized to NO₃ ^-Part of SO₃ ^2-Absorption liquid is absorbed by O in flue gas₂Oxidation to by-product SO₄ ^2-(ii) a The initial absorption liquid is absorption liquid Na₂SO₃With NO₃ ^-And NO₂ ^-Reducing absorption liquid SO₃ ^2-Is Na₂SO₃And (NH)₄)₂SO₃The absorption process is carried out under the conditions of ambient atmospheric pressure and normal temperature;

SO₂+SO₃ ^2-+H₂O→2HSO₃ ^-

2NO₂+SO₃ ^2-+H₂O→SO₄ ^2-+2NO₂ ^-+2H⁺

2NO₂+HSO₃ ^-+H₂O→SO₄ ^2-+2NO₂ ^-+3H⁺

NO₂+NO+H₂O→2NO₂ ^-+2H⁺

3NO₂+H₂O→2NO₃ ^-+NO+2H⁺

2NO₂+H₂O→NO₂ ^-+NO₃ ^-+2H⁺

2SO₃ ^2-+O₂→2SO₄ ^2-

thirdly, the absorption liquid which is absorbed to saturation is evaporated and regenerated in an evaporation crystallization tower, and SO₂Released again and then is enriched and recovered for preparing high-concentration SO₂Gas or for acid production; the evaporation and regeneration process realizes SO through heating and reduced pressure evaporation processes₂Regeneration;

2HSO₃ ^-→SO₃ ^2-+SO₂+H₂O

fourthly, evaporating the regenerated solution for electrochemical reactionChemical reduction of NO₃ ^-And NO₂ ^-Is reduced to NH₄ ⁺(ii) a The electrochemical reduction denitration process is carried out in an electrolytic cell, wherein the anode of the electrolytic cell is an inert Ir-Ru/Ti-based electrode, and the cathode of the electrolytic cell is Co₃O₄/Ti、Fe₂O₃The negative electrode and the positive electrode are separated by a proton exchange membrane; the cathode Co₃O₄/Ti、Fe₂O₃the/Ti electrode is prepared by coating a Ti substrate with supported Co by a sol-gel method₃O₄And Fe₂O₃A film;

and fifthly, circulating the generated ammonia nitrogen in the electrochemical reaction tank to an absorption tank for supplementing as absorption liquid for recycling, thereby realizing the recycling of N, discharging oxidation byproducts sodium sulfate and ammonium sulfate generated in the absorption process and the electrochemical process out of a system after the ammonia nitrogen reaches a certain concentration, and using the ammonia nitrogen as a fertilizer after concentration to generate economic benefit.

Example 2

The purpose of this example is to consider the crystallization of oxidation by-products sodium sulfate and ammonium sulfate in the sulphurous acid system, and to compare the crystallization temperatures of the sodium sulfate and ammonium sulfate systems.

To 20g of deionized water was added 8g of sodium sulfate (28% by mass, near saturation), to which was added varying amounts of ammonium sulfate, and the crystallization temperature of the mixed sulfate solution was determined.

The crystallization test solution is placed in a water bath, a magnetic stirrer is used for accelerating the temperature transfer and crystallization of the solution, and a temperature sensor and a temperature transmitter are used for monitoring and recording the temperature of the solution to be analyzed. Meanwhile, deionized water was used as a blank comparison. Controlling the temperature of the water bath by a circulating condenser, gradually reducing the temperature of the water bath, and recording the crystallization condition of the mixed liquid. When the temperature of the mixed solution is not reduced and rises suddenly, the temperature of the system is increased due to the heat generated by the crystallization of the solution, and the moment is the crystallization moment. The temperature difference curve can be obtained through the temperature difference between the mixed liquid and the deionized water at each moment, so that the crystallization temperature is obtained.

The crystallization temperature when the mixed system contains more ammonium sulfate with different amounts is as follows:

from the above results, it can be seen that the addition of ammonium sulfate is advantageous for reducing the crystallization temperature of sodium sulfate, and more than 50% of ammonium sulfate can be dissolved under the same conditions, which also proves that the addition of ammonium sulfate can reduce the crystallization temperature and alleviate the problem of pipeline blockage caused by the oxidation by-product sulfate.

Example 3

In this example, the relationship between the quality and content of ammonium sulfate in a mixed system of sodium sulfate and ammonium sulfate and the crystallization temperature was examined.

The crystallization temperature of the mixed sulfate solution was measured by adding a mixed solution of 28%, 25%, 20%, 15%, 10% sodium sulfate and ammonium sulfate (28% being nearly saturated) to 20g of deionized water to change the ammonium sulfate content. The procedure was as in example 2, and the crystallization temperatures were as follows:

from the above results, it can be seen that the crystallization temperature gradually decreases as the ammonium sulfate content increases, regardless of the total mass fraction of the mixed solute. The significant reduction in crystallization temperature again demonstrates that ammonium salts in the system can reduce the crystallization temperature of the absorption liquid and reduce pipe plugging caused by crystallization.

Example 4

The purpose of this example is to investigate the sodium sulfite and ammonium sulfite solutions versus SO₂And NO_XThe absorption efficiency of (2).

SO pair of sodium sulfite and ammonium sulfite absorption liquid₂And NO_XThe absorption effect of the method is realized by liquid absorption, the mixed gas of NO, NO2 and SO2 is subjected to full absorption reaction by a bubbling absorption reactor through sodium sulfite and ammonium sulfite absorption liquid under the ambient atmospheric pressure, the absorption temperature is controlled to 40 ℃ in the absorption process, and the reaction condition is ensured to be consistentThe reaction proceeds sufficiently. Analyzing the gas concentration at the inlet and the outlet of the absorption cell in real time and calculating SO₂And NO_XGas absorption rate.

NO in mixed gas used in the invention mode₂、NO、SO₂And O₂The concentration ranges of (A) are 300, 50-100, 3000-10000ppm and 0-10% respectively; the total gas flow is 1L/min; the absorbent concentration was 5%.

After complete absorption, detecting and calculating SO of sodium sulfite and ammonium sulfite₂The absorption efficiencies of (A) and (B) were 98.18% and 98.47%, respectively, indicating that they have a good adsorption effect: (>98%); for NO₂The absorption efficiencies of (1) and (3) are respectively 98.38% and 93.44%, which shows that the removal efficiency is higher and meets the requirement of NO₂The removal requirement of (2); the absorption efficiency of NO is 68.89 percent and 69.18 percent respectively, and the removal efficiency is relative to NO₂Low, difficult to meet industry emission standards, and therefore ozone is required to oxidize NO to NO prior to absorption₂Can greatly increase NO_XThe absorption efficiency of (2).

Example 5

The purpose of this example is to investigate the SO in the Combined recovery flue gas₂And removing NO_XThe sulfite absorption solution used in the method of (1) to NO_XThe absorption effect of (1).

Continuously introducing 350ppm NO into 5% sulfite absorption solution₂And 50ppm NO, and after absorption for 270 minutes, the concentration of the absorbed product is 0.385g/L NO₃ ^-And 1.627g/L of NO₂ ^-. The results show that the absorption products of nitrogen oxides are mainly NO₂ ^-(majority) and NO₃ ^-，NO₃ ^-Is significantly lower than NO₂ ^-. While in the whole absorption system, NO₂ ^-Not in a stable state, part of it may be oxidized to NO₃ ^-. In addition, in the process from NO₃ ^-During the initial electrochemical reduction, the reduction product comprises NH₄ ⁺、N₂Etc. of NH₄ ⁺Is the most easily reduced product. When a reductive pathway is of interest, NO₃ ^-Is firstly electrifiedChemical reduction to NO₂ ^-And part of the possible NO, N₂O、N₂And finally is NH₄ ⁺The specific intermediate product depends on electrochemical reduction control, including electrode material, applied voltage, environment, etc. From NO₂ ^-Electrochemical reduction to NH₄ ⁺Specific from NO₃ ^-Electrochemical reduction to NH₄ ⁺More easily, where the charge of nitrogen is equal to +3, and NO₃ ^-The charge of the medium nitrogen is equal to + 5. Taking into account NO₃ ^-And NO₂ ^-From competing reactions of NO₂ ^-Due to its higher concentration and more favorable electrochemical potential, faster reaction rates are obtained. At the same time, NO₃ ^-It is also considered to be reduced at a sufficient voltage for the reduction reaction. Thus, NO₃ ^-Is the focus of the process.

Example 6

The purpose of this example is to investigate the SO in the Combined recovery flue gas₂And removing NO_XThe titanium-based metal oxide electrode adopted in the method has an electrochemical denitration effect.

The nitrate wastewater is treated by a commonly used titanium-based metal oxide electrode in an electrochemical reduction denitration process in an electrolytic bath. The object to be processed is C_NNaNO 100mg/L₃Solution containing 0.1mol/L of Na₂SO₃The volume of the wastewater is 100 mL.

Mixing Co by sol-gel method₃O₄、Fe₂O₃The coating is loaded on a Ti substrate and used as a cathode electrode, and the coating has good effect on the electrochemical reduction of nitrate; the anode of the electrolytic cell adopts an inert Ir-Ru/Ti-based electrode, and the cathode and the anode are separated by a proton exchange membrane; the voltage was set at 5V and the cathode and anode had a submerged area of 15cm²(5 cm. times.3 cm); the current is 0.06A-0.2A in the reaction process.

After 3h, sampling and analyzing, and determining the removal rate of nitrate nitrogen and the generation rate of ammonia nitrogen, wherein the results are as follows.

Metal oxide electrode	Co3O4/Ti	Fe2O3/Ti
			NO3 --N removal (%)	93.7	63.8
NH4 +-N production ratio (%)	85.6	49.6

As can be seen from the above results, Co₃O₄the/Ti cathode showed a nitrate removal efficiency of 93.67% which is better than that of Fe₂O₃Ti, while showing a specific Fe₂O₃Better ammonia production efficiency (85.56%) by Ti (49.59%). Both cathodes showed a total nitrogen removal efficiency of about 10%, which is present in the form of other gaseous or liquid nitrogen. Overall, both cathodes showed good NO₃ ^-Reducing power, NO₃ ^-Is mainly reduced to NH₄ ⁺Nitrogen, and a small part of NO₃ ^-And other forms of nitrogen. It offers the possibility of nitrogen conversion and recycling in the absorption system to achieve the purpose of denitrification. Co₃O₄Ti has satisfactory reduced NO₃ ^-Efficiency and NH of₄ ⁺Efficiency of generation, while SO₃ ^2-The presence of (A) has no inhibitory effect on the electrochemical reduction reaction, again demonstrating the feasibility of this approachAnd (4) sex.

Example 7

The purpose of this example is to recover SO in flue gas in combination with research₂And removing NO_XThe influence of continuous operation of the process on the denitration efficiency in the method is avoided.

In order to consider the requirement of continuous operation of the actual process, NO is intermittently added in the electrochemical denitration process₃ ^-N source, added every 2 hours at a concentration corresponding to the initial concentration of nitrate nitrogen, the concentration of each form of nitrogen and the variation of pH value throughout the reaction were examined. Every 2 hours, NO₃ ^-The N concentration is obviously reduced, and the interval removal efficiency is maintained to be more than 60% in the whole process; NH (NH)₄ ⁺The N concentration is obviously increased and the interval generation efficiency is maintained to be more than 55.2 percent in the whole process. When the interval time is extended to 3 hours, NO₃ ^-The N removal efficiency is improved to over 86.2 percent; NH (NH)₄ ⁺the-N generation efficiency rises to above 72%, and this efficiency will continue to rise over time. The whole denitration and ammonia nitrogen conversion efficiency in the whole process is maintained at a higher level.

The pH results during the process showed that the pH increased rapidly from an initial value of 9.26 to 12.45 after the start of the reaction due to the formation of ammonia, and then remained at a higher value of about 12.7-13.1 due to (NH)₄)₂SO₃Is a weak alkali salt, has buffering effect on the process, and the pH value of the system is generally stable.

The foregoing description of specific embodiments of the present invention has been presented. It is to be understood that the present invention is not limited to the specific embodiments described above, and that various changes and modifications may be made by one skilled in the art within the scope of the appended claims without departing from the spirit of the invention.

Claims (9)

1. A method for recovering sulfur dioxide in flue gas and removing nitrogen oxide is characterized in that a sodium-ammonium double-alkali wet absorption method and an electrochemical reduction method are used in a combined manner; the method comprises the following steps:

s1, oxidizing the dedusted flue gas with ozone to oxidize nitric oxide in the flue gas into nitrogen dioxide;

s2, absorbing sulfur dioxide and nitrogen oxides in the flue gas in the step S1 by using a sulfite absorption liquid to generate a solution of bisulfite, nitrate and nitrite;

s3, after being absorbed and saturated, the sulfite absorption liquid is subjected to reduced pressure evaporation and regeneration, wherein the bisulfite releases sulfur dioxide, and the sulfur dioxide is enriched and recovered; the solution after evaporation and regeneration enters an electrochemical reduction denitration process, wherein nitrate and nitrite are reduced into ammonia nitrogen;

s4, recycling the ammonia nitrogen-containing solution subjected to the reduction denitration treatment in the step S3 to the step S2 to be used as a supplementary absorption liquid for recycling; or when the concentration of the ammonium salt in the ammonia-containing nitrogen solution after the reduction denitration treatment in the step S3 exceeds 75%, discharging the ammonia-containing nitrogen solution out of the system.

2. The method for jointly recovering sulfur dioxide and removing nitrogen oxides in flue gas according to claim 1, wherein in step S2, the initial sulfite absorption solution of the reaction is sodium sulfite solution with a concentration of 10% -20%; in the reaction process, the sulfite absorption liquid is a mixed solution of sodium sulfite and ammonium sulfite; the mass ratio of the sodium sulfite to the ammonium sulfite in the mixed solution is 25-100%, and the mass ratio of the ammonium sulfite is 0-75%.

3. The method for jointly recovering sulfur dioxide and removing nitrogen oxides in flue gas according to claim 1, wherein in the step S2, the absorption process is performed under the conditions of ambient atmospheric pressure and medium-low temperature.

4. The method for jointly recovering sulfur dioxide and removing nitrogen oxides in flue gas according to claim 1, wherein in the step S2, in the absorption process, part of sulfite in the sulfite absorption solution is oxidized into sulfate which is an oxidation byproduct; part of the nitrite is further oxidized to nitrate.

5. The method for jointly recovering sulfur dioxide and removing nitrogen oxides in flue gas according to claim 1, wherein in the step S2, a supplementary alkali solution is added in the absorption process, and the alkali solution is sodium hydroxide or sodium sulfite.

6. The method for jointly recovering sulfur dioxide and removing nitrogen oxides in flue gas according to claim 1, wherein in step S3, the evaporation regeneration process is implemented by heating decomposition or reduced pressure evaporation process.

7. The method for combined recovery of sulfur dioxide and removal of nitrogen oxides in flue gas according to claim 1, wherein in step S3, the electrochemical reduction denitration process is performed in an electrolytic cell, wherein the anode is Ir-Ru/Ti-based inert electrode, the cathode is Ti-based metal oxide electrode, and the cathode and the anode are separated by a proton exchange membrane.

8. The combined sulfur dioxide and nitrogen oxide recovery process as recited in claim 7, wherein said Ti-based metal oxide electrode is selected from Co₃O₄/Ti or Fe₂O₃a/Ti electrode; obtained by coating a metal oxide-supporting thin film on a Ti substrate by a sol-gel method.

9. The system for jointly recovering the sulfur dioxide and removing the nitrogen oxides in the flue gas by applying the method as claimed in claim 1 is characterized by comprising an ozone generator, a desulfurization absorption tower, an evaporation regenerator, an electrochemical reaction tank and an absorption liquid replenishing tank which are connected in sequence; and the supplementary absorption liquid in the absorption liquid supplementary pool reflows to the desulfurization absorption tower through a pipeline.

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