Multi-stage oxidation-reduction flue gas denitration system and method

Abstract

The invention provides a multistage oxidation-reduction flue gas denitration system and method, and belongs to the technical field of waste gas treatment. Comprising at least one stage of denitration assembly, the denitration assembly comprises an oxidation tower, a reduction tower and a controller, and the oxidation tower comprises a first tower body, an oxidation absorption liquid circulating water tank, a first circulating water pump and a first spraying nozzle system which are connected in sequence; the first circulating water pump is used for spraying the oxidation absorption liquid in the oxidation absorption liquid circulating water tank from the first spraying nozzle system in the first tower body. The flue gas to be treated is fed into the oxidation tower and the reduction tower, and the multiple stages of denitration assemblies can be connected in series according to purification requirements, so that compared with single oxidation absorption or single reduction absorption treatment, the flue gas is treated by a two-stage or multi-stage oxidation-reduction series process without an ozone generator and urea pyrolysis or hydrolysis, and the flue gas treatment efficiency is improved. The energy consumption is low, the treated flue gas NOx can meet the ultralow emission requirement, and secondary pollution such as ozone emission or ammonia escape is avoided.

Description

TECHNICAL FIELD

[0001] The present application belongs to the technical field of waste gas treatment, and particularly relates to a multi-stage oxidation-reduction flue gas denitration system and method. BACKGROUND

[0002] With the increasingly stringent environmental protection regulations in China, ultra-low emission of nitrogen oxides (NOx) in industrial flue gas has become a key problem that enterprises need to overcome. The existing wet flue gas denitration technology mainly uses an oxidizing agent to oxidize NO in the flue gas into high-valence nitrogen oxides (such as NO2, N2O3, etc.) that are easily soluble in water to achieve denitration. The selective catalytic reduction (SCR) technology is the most mature denitration scheme at present. However, when dealing with flue gas under complex conditions such as low temperature, high humidity, and large fluctuation of flue gas load, the activity of the catalyst is significantly reduced, resulting in a sharp decrease in denitration efficiency, which seriously affects the stability and service life of the catalytic system.

[0003] Chinese patent CN2012105891323 discloses a two-step oxidation-reduction flue gas denitration method. The flue gas denitration system is composed of an ozone oxidation device, an ozone generator, an ozone addition control system, a NO / NOx sensor, a reduction reaction tower, an absorption liquid recovery tank, a reducing agent absorption liquid circulating tank, a reducing agent solution preparation device, a reducing agent solution adding device, and a circulating water pump system. First, the flue gas to be treated is introduced into the ozone oxidation device, and part of the NO in the flue gas is oxidized to NO2 under the action of ozone. Then, the flue gas is transported to the reduction reaction tower and contacted with the reducing agent solution. Under the action of the reducing agent, most of the NO and NO2 in the flue gas are reduced to N2 to achieve the purpose of flue gas denitration purification. Although this method can achieve efficient purification of flue gas NOx, it uses an ozone generator to generate ozone as an oxidizing agent. Not only is the energy consumption of the oxidation equipment high, but also the ozone and NO in the flue gas are oxidized by gas-gas phase contact in the ozone oxidation device. The reaction time is limited, the utilization rate of ozone is not high, and the conversion rate of NO is limited. The reducing agent used in the subsequent reduction reaction tower is industrial-grade urea. Since urea does not directly react with NOx, it needs to be first pyrolyzed or hydrolyzed to generate NH3, which inevitably leads to ammonia escape. Not only does it affect the denitration effect, but it also causes secondary pollution problems. SUMMARY

[0004] (1) Technical problem to be solved In view of the deficiencies of the prior art, the purpose of the present application is to provide a multi-stage oxidation-reduction flue gas denitration system and method to solve the problems that the existing flue gas denitration system uses an ozone generator to generate ozone as an oxidizing agent, which not only has high energy consumption of the oxidation equipment, but also has low utilization rate of ozone and limited conversion rate of NO. Moreover, the reducing agent used in the subsequent reduction reaction tower is industrial-grade urea, which needs to be first pyrolyzed or hydrolyzed to generate NH3, which inevitably leads to ammonia escape, affecting the denitration effect and causing secondary pollution problems.

[0005] (2) Technical solution In order to solve the above technical problems, the application provides a multi-stage oxidation-reduction flue gas denitration system, which comprises at least one denitration assembly, the denitration assembly comprises an oxidation tower, a reduction tower and a controller, the oxidation tower comprises a first tower body, an oxidation absorption liquid circulating tank, a first circulating water pump and a first spray nozzle system which are connected in sequence, the first circulating water pump is used for spraying the oxidation absorption liquid in the oxidation absorption liquid circulating tank from the first spray nozzle system in the first tower body, the reduction tower comprises a second tower body, a reduction absorption liquid circulating tank, a second circulating water pump and a second spray nozzle system which are connected in sequence, the second circulating water pump is used for spraying the reduction absorption liquid in the reduction absorption liquid circulating tank from the second spray nozzle system in the second tower body, and the output end of the first tower body is communicated with the input end of the second tower body through a pipeline. A pH detection sensor is installed in the oxidation absorption liquid circulating tank and the reduction absorption liquid circulating tank, the pH detection sensor is used for detecting the pH value in the oxidation absorption liquid circulating tank and the reduction absorption liquid circulating tank, the oxidation absorption liquid in the oxidation absorption liquid circulating tank is any one of NaClO2, NaClO, H2O2, Fenton reagent, Fenton-like reagent and persulfate, and the reduction absorption liquid in the reduction absorption liquid circulating tank is any one of Na2S, Na2S2O3 and Na2SO3.

[0006] Preferably, the first tower body and the second tower body are any one of a spray tower, a packed tower and a plate tower.

[0007] Preferably, the first tower body and the second tower body each comprise a tower shell, a demister installed at the top of the tower shell, a plurality of layers of fillers installed in the middle of the tower shell, and a cyclone structure installed at the bottom of the tower shell, each layer of the fillers is arranged in a staggered and inclined manner, and the middle of each layer of the fillers is overlapped, the tower shell is provided with an air inlet pipe, a first sensor is installed in the air inlet pipe, a second sensor is installed in the inside of the tower shell, the first sensor and the second sensor each comprise a concentration sensor and an air flow sensor, the cyclone structure is located between the air inlet pipe and the fillers, and a side spray nozzle system is arranged on the side inner wall of the tower shell.

[0008] Preferably, the cyclone structure comprises a center block, a plurality of guide vanes are fixedly connected to the outer surface of the center block, the guide vanes are in an inclined spiral structure and form channels, and the other end of the guide vanes is fixedly connected to the inner wall of the tower shell.

[0009] Preferably, the first spray nozzle system and the second spray nozzle system each comprise a liquid inlet pipe, a plurality of branch pipes fixedly connected to the liquid inlet pipe, the other end of the branch pipes extends to the inside of the tower shell and is provided with a plurality of main nozzles, and a first flow control valve is installed on the branch pipe.

[0010] Preferably, the side spray nozzle system comprises a side pipe, a plurality of auxiliary nozzles are installed on the side pipe, a main pipe is fixedly connected to the bottom end of the side pipe, a second flow control valve is installed on the main pipe, and the main pipe and the other end of the liquid inlet pipe are connected with the first circulating water pump or the second circulating water pump respectively.

[0011] Preferably, a reflux pipe is installed between the tower shell of the first tower body and the oxidation absorption liquid circulating water tank and between the tower shell of the second tower body and the reduction absorption liquid circulating water tank, and a gas distribution plate with two upper and lower openings is installed in the inside of the tower shell.

[0012] Preferably, the first sensor, the second sensor, the first flow control valve and the second flow control valve are electrically connected with the controller.

[0013] Preferably, a multi-stage oxidation-reduction flue gas denitration method comprises the multi-stage oxidation-reduction flue gas denitration system described above, and the denitration method comprises the following steps: Step one, the flue gas to be treated is sent into the oxidation tower, the flue gas is in countercurrent contact with the oxidation absorption liquid sprayed by the first spray nozzle system in the oxidation tower, so that the NO in the flue gas is oxidized into NO2, and part of the NO2 is dissolved in the absorption liquid and is oxidized into nitrite and nitrate; Step two, the remaining NO2 in the flue gas enters the subsequent reduction tower and is in countercurrent contact with the reduction absorption liquid sprayed by the second spray nozzle system in the reduction tower, so that the NO2 is reduced into N2.

[0014] Preferably, the flue gas to be treated contains low-concentration nitrogen oxides NOx, the concentration is between 100-500 ppm, the liquid-gas ratio of the oxidation tower is 5 L / m 3 -20 L / m 3 , the concentration of the oxidation absorption liquid is 0.01-10 mmol / L, the pH is 2-6, the liquid-gas ratio of the reduction tower is 5 L / m 3 -20 L / m 3 , the concentration of the reduction absorption liquid is 0.1-20 mmol / L, and the pH is 8-13.

[0015] (3) Beneficial effects Compared with the prior art, the beneficial effects of the present application are: In the above scheme, the flue gas to be treated is sent into the oxidation tower, and the flue gas is in countercurrent contact with the oxidation absorption liquid sprayed by the first spray nozzle system in the oxidation tower, so that NO in the flue gas is oxidized to NO2, part of NO2 is dissolved in the absorption liquid and is oxidized to nitrite and nitrate, then the remaining NO2 in the flue gas enters the subsequent reduction tower, and is in countercurrent contact with the reduction absorption liquid sprayed by the second spray nozzle system in the reduction tower, so that NO2 is reduced to N2, and according to the purification requirements, multiple denitration assemblies can be connected in series, compared with single oxidation absorption or single reduction absorption treatment, the flue gas is treated by two-stage or multi-stage oxidation-reduction series process, and the treated flue gas NOx can meet the ultra-low emission requirements, and no secondary pollution such as ozone emission or ammonia escape occurs.

[0016] In the above scheme, the oxidation tower and the reduction tower are arranged as spray towers, packed towers or plate towers, and the gas and liquid are in countercurrent contact during treatment, the residence time of the flue gas in the tower is appropriate, NO and the oxidant and NO2 and the reducing agent can fully contact and react, the NOx conversion rate is high, not only the process operation is simple, the reaction conditions are mild, and the operation cost is low, but also the high-efficiency removal of NOx in the flue gas can be realized, and the secondary pollution is small.

[0017] In the above scheme, the cyclone structure is arranged on the inner bottom of the tower shell, on the one hand, the waste gas under atmospheric pressure enters the tower shell and first passes through the cyclone structure to buffer the waste gas, on the other hand, the cyclone structure guides the waste gas to produce the tendency of turbulence, and the packing in the tower shell is arranged in an inclined overlapping manner, the waste gas at the lower part of the inclined packing can form an isobaric air chamber to adjust the flow and impact force of the waste gas, thereby the consistency is improved, the uniformity of the waste gas is improved, and the contact time of the denitration agent and the waste gas is increased by the overlapping mode of the packing, part of the waste gas will rush to the inner wall of the tower shell after passing through the cyclone structure, therefore, the side spray nozzle system of the oxidation absorption liquid and the reduction absorption liquid is also arranged on the inner wall, and the waste gas rushing to the inner wall can be subjected to the denitration reaction for the first time. BRIEF DESCRIPTION OF DRAWINGS

[0018] Figure 1 The structure diagram of the oxidation tower and the reduction tower of the multi-stage oxidation-reduction flue gas denitration system and method.

[0019] Figure 2 The structure diagram of the tower body of the multi-stage oxidation-reduction flue gas denitration system and method.

[0020] Figure 3 The structure diagram of the cyclone structure of the multi-stage oxidation-reduction flue gas denitration system and method.

[0021] Figure 4 The performance curve of the first-stage oxidation tower for removing NO of the multi-stage oxidation-reduction flue gas denitration system and method.

[0022] Figure 5 The performance curve of the first reduction tower of the multi-stage oxidation-reduction flue gas denitration system and method for removing NO2.

[0023] The labels in the drawings are: 1, oxidation tower; 2, reduction tower; 3, controller; 4, first tower body; 5, oxidation absorption liquid circulating tank; 6, first circulating water pump; 7, first spraying nozzle system; 8, second tower body; 9, reduction absorption liquid circulating tank; 10, second circulating water pump; 11, second spraying nozzle system; 111, pH detection sensor; 12, tower shell; 13, demister; 14, tray; 15, cyclone structure; 16, gas inlet pipe; 17, first sensor; 18, second sensor; 19, gas distribution plate; 21, side spraying nozzle system; 22, liquid inlet pipe; 23, branch pipe; 24, main nozzle; 25, first flow control valve; 26, return pipe; 151, center block; 152, guide vane; 211, side pipe; 212, auxiliary nozzle; 213, main pipe; 214, second flow control valve. DETAILED DESCRIPTION

[0024] The embodiment of the present application provides a multi-stage oxidation-reduction flue gas denitration system, which comprises at least one denitration assembly, the denitration assembly comprises an oxidation tower 1, a reduction tower 2 and a controller 3, the oxidation tower 1 comprises a first tower body 4, an oxidation absorption liquid circulating tank 5, a first circulating water pump 6 and a first spraying nozzle system 7 which are sequentially connected, the first circulating water pump 6 is used for spraying the oxidation absorption liquid in the oxidation absorption liquid circulating tank 5 from the first spraying nozzle system 7 in the first tower body 4, the reduction tower 2 comprises a second tower body 8, a reduction absorption liquid circulating tank 9, a second circulating water pump 10 and a second spraying nozzle system 11 which are sequentially connected, the second circulating water pump 10 is used for spraying the reduction absorption liquid in the reduction absorption liquid circulating tank 9 from the second spraying nozzle system 11 in the second tower body 8, and the output end of the first tower body 4 is communicated with the input end of the second tower body 8 through a pipeline. The pH detection sensor 111 is installed in the oxidation absorption liquid circulating tank 5 and the reduction absorption liquid circulating tank 9, the pH detection sensor 111 is used for detecting the pH value in the oxidation absorption liquid circulating tank 5 and the reduction absorption liquid circulating tank 9, the oxidation absorption liquid in the oxidation absorption liquid circulating tank 5 is any one of NaClO2, NaClO, H2O2, Fenton reagent, Fenton-like reagent and persulfate, and the reduction absorption liquid in the reduction absorption liquid circulating tank 9 is any one of Na2S, Na2S2O3 and Na2SO3.

[0025] In the embodiment, the first tower body 4 and the second tower body 8 are any one of a spraying tower, a packed tower and a plate tower.

[0026] Since the exhaust gas has a large pressure when entering the tower, the exhaust gas has poor consistency and has a certain dispersing effect on the atomized denitration agent, in order to avoid affecting the denitration effect, as shown in Figure 1 and Figure 2 In the embodiment, the first tower body 4 and the second tower body 8 each include a tower shell 12, a demister 13 mounted at the top of the tower shell 12, a plurality of tower plates 14 mounted in the middle of the tower shell 12, and a cyclone structure 15 mounted at the bottom of the tower shell 12. Each tower plate 14 is arranged in a staggered and inclined manner, and the middle of each tower plate 14 overlaps. Each tower plate 14 is uniformly provided with a gas hole penetrating the tower plate (14). The tower shell 12 is provided with an air inlet pipe 16, and the first sensor 17 is mounted in the air inlet pipe 16. The second sensor 18 is mounted in the inside of the tower shell 12. The first sensor 17 and the second sensor 18 each include a concentration sensor and an air flow sensor. The cyclone structure 15 is located between the air inlet pipe 16 and the tower plate 14. The side wall of the tower shell 12 is provided with a side spray nozzle system 21.

[0027] By arranging the cyclone structure 15 at the bottom of the tower shell 12, on the one hand, the atmospheric pressure exhaust gas enters the tower shell 12 and first passes through the cyclone structure 15 to buffer the exhaust gas. On the other hand, the cyclone structure 15 guides the exhaust gas to produce a turbulent flow trend. The tower plates 14 in the tower shell 12 are arranged in an inclined and overlapping manner. The exhaust gas at the lower part of the inclined tower plate 14 can form an isobaric air chamber to adjust the exhaust gas flow and impact force, thereby improving the consistency and uniformity of the exhaust gas. At the same time, the overlapping arrangement of the tower plates 14 increases the contact time of the denitration agent and the exhaust gas. After the exhaust gas passes through the cyclone structure 15, part of the exhaust gas will rush to the inner wall of the tower shell 12. Therefore, the side spray nozzle system 21 of the oxidation and absorption liquid and the reduction and absorption liquid is arranged on the inner wall to perform the denitration reaction on the exhaust gas rushing to the inner wall at the first time.

[0028] As shown in Figure 2 and Figure 3 In the embodiment, the cyclone structure 15 includes a center block 151, and a plurality of guide vanes 152 are fixedly connected to the outer surface of the center block 151. The guide vanes 152 are inclined spiral structures and form channels. The other end of the guide vanes 152 is fixedly connected to the inner wall of the tower shell 12. The channels between the guide vanes 152 guide the exhaust gas to produce a turbulent flow trend. After the exhaust gas enters the tower shell 12, it first passes through the cyclone structure 15 to buffer the exhaust gas.

[0029] As shown in Figure 1 and Figure 2As shown, in the present embodiment, the first spray nozzle system 7 and the second spray nozzle system 11 each include a liquid inlet pipe 22, a plurality of branch pipes 23 fixedly connected to the liquid inlet pipe 22, the other end of the branch pipe 23 extending to the inside of the tower shell 12 and being provided with a plurality of main nozzles 24, and a first flow control valve 25 being installed on the branch pipe 23; a variable frequency motor plus a speed reducer gear can be used to drive the spray arm of the plurality of main nozzles 24, so that the plurality of main nozzles 24 can rotate and spray, the rotating speed of the motor can be controlled and adjusted by the controller 3, the plurality of main nozzles 24 and the plurality of branch pipes 23 can be arranged in multiple layers, 3-5 layers are arranged along the height direction of the tower, the interval is 1.5-2m, the length of each layer is adapted to the diameter of the tower, the main nozzles 24 adopt a spiral angle design, such as 30°, 45°, and a conical atomization area is formed when rotating, the overlapping rate of the atomization areas of adjacent main nozzles 24 is ≥80%, and there is no liquid distribution blind area.

[0030] As shown in Figure 1 and Figure 2 As shown, in the present embodiment, the side spray nozzle system 21 includes a side pipe 211, a plurality of auxiliary nozzles 212 being installed on the side pipe 211, a main pipe 213 being fixedly connected to the bottom end of the side pipe 211, a second flow control valve 214 being installed on the main pipe 213, the main pipe 213 and the other end of the liquid inlet pipe 22 being respectively connected to the first circulating water pump 6 or the second circulating water pump 10, the main nozzles 24 and the auxiliary nozzles 212 adopting an inverted conical flow channel to avoid impurity accumulation, and the material being selected from wear-resistant ceramic or polytetrafluoroethylene, which is corrosion-resistant and not easy to scale; By installing the first sensor 17 in the air inlet pipe 16 and the second sensor 18 in the inside of the tower shell 12, the first sensor 17 and the second sensor 18 each include a concentration sensor and an air flow sensor, the waste gas concentration and air flow speed data can be collected in real time during the denitration process, the sensor data is compared with the preset threshold value by using the controller 3, an adjustment instruction is output, the flow of the sprayed oxidation absorption liquid and reduction absorption liquid is controlled by controlling the flow of the first flow control valve 25 and the second flow control valve 214, the first flow control valve 25 and the second flow control valve 214 are electromagnetic flow valves, the liquid flow of the single-way spray pipe is controlled by using the electromagnetic flow valve, the response time is ≤0.5s, so that the closed loop is realized through sensing, feedback and adjustment, the absorption liquid is distributed as needed, and the precise control of the liquid amount is realized.

[0031] As shown in Figure 1 and Figure 2As shown, in the embodiment, a reflux pipe 26 is installed between the tower shell 12 of the first tower body 4 and the oxidation absorption liquid circulating tank 5 and between the tower shell 12 of the second tower body 8 and the reduction absorption liquid circulating tank 9, and a gas distribution plate 19 with two upper and lower openings is installed in the inside of the tower shell 12; the first sensor 17, the second sensor 18, the first flow control valve 25 and the second flow control valve 214 are electrically connected with the controller 3; the absorption liquid that is not completely reacted can be drawn back to the oxidation absorption liquid circulating tank 5 and the reduction absorption liquid circulating tank 9, so that the recycling rate is greater than or equal to 90%; a filter with a filtering precision of 100 microns is arranged at the outlet of the pump body to prevent impurities from entering the nozzle.

[0032] The application further provides a multi-stage oxidation-reduction flue gas denitration method, comprising the multi-stage oxidation-reduction flue gas denitration system. Step one, the flue gas to be treated is sent into the oxidation tower 1, the flue gas is in reverse contact with the oxidation absorption liquid sprayed by the first spray nozzle system 7 in the oxidation tower 1, so that the NO in the flue gas is oxidized into NO2, and part of the NO2 is dissolved in the absorption liquid and is oxidized into nitrite and nitrate; Step two, the remaining NO2 in the flue gas enters the subsequent reduction tower 2, and is in reverse contact with the reduction absorption liquid sprayed by the second spray nozzle system 11 in the reduction tower 2, so that the NO2 is reduced into N2.

[0033] In the embodiment, the flue gas to be treated contains low-concentration nitrogen oxides NOx, the concentration is between 100-500 ppm, the liquid-gas ratio of the oxidation tower 1 is 5 L / m 3 -20 L / m 3 , the concentration of the oxidation absorption liquid is 0.01-10 mmol / L, the pH is 2-6, the liquid-gas ratio of the reduction tower 2 is 5 L / m 3 -20 L / m 3 , the concentration of the reduction absorption liquid is 0.1-20 mmol / L, and the pH is 8-13.

[0034] All the technical features in the embodiment can be freely combined according to actual needs.

[0035] The above embodiment is a preferred implementation scheme of the application, in addition to this, the application can also be implemented in other ways, and any obvious replacement without departing from the technical scheme concept is within the protection scope of the application.

Claims

1. A multi-stage oxidation-reduction flue gas denitration system, characterized in that, The system comprises at least one denitration assembly, the denitration assembly comprises an oxidation tower (1), a reduction tower (2) and a controller (3), the oxidation tower (1) comprises a first tower body (4), an oxidation absorption liquid circulating tank (5), a first circulating water pump (6) and a first spray nozzle system (7) connected in sequence, the first circulating water pump (6) is used for spraying the oxidation absorption liquid in the oxidation absorption liquid circulating tank (5) from the first spray nozzle system (7) in the first tower body (4), the reduction tower (2) comprises a second tower body (8), a reduction absorption liquid circulating tank (9), a second circulating water pump (10) and a second spray nozzle system (11) connected in sequence, the second circulating water pump (10) is used for spraying the reduction absorption liquid in the reduction absorption liquid circulating tank (9) from the second spray nozzle system (11) in the second tower body (8), the output end of the first tower body (4) is communicated with the input end of the second tower body (8) through a pipeline; The pH detection sensor (111) is arranged in the oxidation absorption liquid circulating tank (5) and the reduction absorption liquid circulating tank (9), and is used for detecting the pH value in the oxidation absorption liquid circulating tank (5) and the reduction absorption liquid circulating tank (9), the oxidation absorption liquid in the oxidation absorption liquid circulating tank (5) is any one of NaClO2, NaClO, H2O2, Fenton reagent, Fenton-like reagent and persulfate, and the reduction absorption liquid in the reduction absorption liquid circulating tank (9) is any one of Na2S, Na2S2O3 and Na2SO3.

2. The multi-stage oxidation-reduction flue gas denitration system according to claim 1, characterized in that, The first tower body (4) and the second tower body (8) are any one of a spray tower, a packed tower and a plate tower.

3. The multi-stage oxidation-reduction flue gas denitration system according to claim 2, the first tower body (4) and the second tower body (8) each comprise a tower shell (12), a demister (13) arranged at the top of the tower shell (12), a plurality of tower plates (14) arranged at the middle of the tower shell (12) and a cyclone structure (15) arranged at the bottom of the tower shell (12), each of the tower plates (14) is arranged in a staggered and inclined manner, the middle of each of the tower plates (14) is overlapped, and each of the tower plates (14) is uniformly provided with air holes penetrating the tower plate (14), the tower shell (12) is provided with an air inlet pipe (16), the air inlet pipe (16) is provided with a first sensor (17), and the inside of the tower shell (12) is provided with a second sensor (18), the first sensor (17) and the second sensor (18) each comprise a concentration sensor and an air flow sensor, the cyclone structure (15) is located between the air inlet pipe (16) and the tower plate (14), and the side inner wall of the tower shell (12) is provided with a side spray nozzle system (21).

4. The multi-stage oxidation-reduction flue gas denitration system according to claim 3, characterized in that, The cyclone structure (15) comprises a center block (151), a plurality of guide vanes (152) are fixedly connected to the outer surface of the center block (151), the guide vanes (152) are in an inclined and spiral structure and form channels, and the other ends of the guide vanes (152) are fixedly connected to the inner wall of the tower shell (12).

5. The multi-stage oxidation-reduction flue gas denitration system according to claim 4, characterized in that, The first spray nozzle system (7) and the second spray nozzle system (11) each comprise a liquid inlet pipe (22), a plurality of branch pipes (23) fixedly connected with the liquid inlet pipe (22), the other ends of the branch pipes (23) extending to the inside of the tower shell (12) and being provided with a plurality of main nozzles (24), and the branch pipes (23) being provided with first flow control valves (25).

6. The multi-stage oxidation-reduction flue gas denitration system according to claim 5, characterized in that, The side spray nozzle system (21) comprises a side pipe (211) provided with a plurality of auxiliary nozzles (212), the bottom end of the side pipe (211) being fixedly connected with a main pipe (213), the main pipe (213) being provided with a second flow control valve (214), and the main pipe (213) and the other end of the liquid inlet pipe (22) being connected with the first circulating water pump (6) or the second circulating water pump (10) respectively.

7. The multi-stage oxidation-reduction flue gas denitration system according to claim 6, characterized in that, The tower shell (12) of the first tower body (4) and the circulating water tank (5) of the oxidation absorption liquid and the tower shell (12) of the second tower body (8) and the circulating water tank (9) of the reduction absorption liquid are each provided with a reflux pipe (26), and the inside of the tower shell (12) is provided with a gas distribution plate (19) with two upper and lower openings.

8. The multi-stage oxidation-reduction flue gas denitration system according to claim 7, characterized in that, The first sensor (17), the second sensor (18), the first flow control valve (25) and the second flow control valve (214) are electrically connected with the controller (3).

9. A multi-stage oxidation-reduction flue gas denitration method comprising the multi-stage oxidation-reduction flue gas denitration system according to any one of claims 1 to 8, characterized in that, The denitration method comprises the following steps: Step one, sending the flue gas to be treated into the oxidation tower (1), the flue gas being in counterflow contact with the oxidation absorption liquid sprayed by the first spray nozzle system (7) in the oxidation tower (1), so that the NO in the flue gas is oxidized into NO2, and part of the NO2 is dissolved in the absorption liquid and oxidized into nitrite and nitrate; Step two, the remaining NO2 in the flue gas entering the subsequent reduction tower (2) and being in counterflow contact with the reduction absorption liquid sprayed by the second spray nozzle system (11) in the reduction tower (2), so that the NO2 is reduced into N2.

10. The multi-stage oxidation-reduction flue gas denitration method according to claim 9, characterized by, The flue gas to be treated contains low concentration of nitrogen oxides NOx, the concentration is between 100-500 ppm, the liquid-gas ratio of the oxidation tower (1) is 5 L / m 3 -20 L / m 3 , the concentration of the oxidation absorption liquid is 0.01-10 mmol / L, the pH is 2-6, the liquid-gas ratio of the reduction tower (2) is 5 L / m 3 -20 L / m 3 , the concentration of the reduction absorption liquid is 0.1-20 mmol / L, the pH is 8-13.

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