A SNCR and low-temperature SCR coupled flue gas denitrification system and its use method

Abstract

The invention discloses a SNCR and low-temperature SCR coupled flue gas denitration system and a method for using the same. The system comprises a tail flue, a flue gas heater, a low-temperature SCR reactor, a main spray gun and an ammonia water tank, wherein the main spray gun is connected to an outlet of the ammonia water tank through an ammonia water pump, the main spray gun is arranged in a front flue of the tail flue, the main spray gun is used to spray ammonia water into the front flue to perform an SNCR reaction, the flue gas outlet of the tail flue is connected to a flue gas inlet of a flue gas heater, the flue gas outlet of the flue gas heater is connected to a flue gas inlet of a low-temperature SCR reactor, the low-temperature SCR reactor is used to utilize escaped ammonia in the SNCR reaction as a denitration reducing agent to perform a low-temperature SCR reaction, and the SNCR and low-temperature SCR processes are coupled for denitration at a relatively low flue temperature to achieve NOx emission compliance under all operating conditions. The invention can save denitration cost, has certain flexibility and reliability, and the system is simple in device, has a small amount of boiler body modification, and is easy to implement and promote.

Description

Technical Field

[0001] The invention belongs to the technical field of air pollution control and environmental protection, and particularly relates to a SNCR and low-temperature SCR coupled flue gas denitration system and a use method thereof. Background Art

[0002] As my country's economy and living standards continue to improve, people will pay more and more attention to the environment. Environmental pollution caused by industrial production mainly based on coal burning is a restrictive factor in my country's industrial development. The tail gas produced by coal combustion contains smoke, sulfur dioxide (SO2), nitrogen oxides (NO x ) and carbon dioxide (CO2) and other pollutants and greenhouse gases have caused serious environmental problems and are important issues that need to be solved urgently for the sustainable development of my country's economy. At present, the main means of controlling NOx emissions from coal-fired boilers in power plants, boilers burning biomass and domestic waste, and industrial boilers / kilns are low-nitrogen combustion in boilers and SCR (selective catalytic reduction) denitrification technology, SNCR (selective non-catalytic reduction) denitrification technology, etc. The SNCR process system is simple and the investment cost is low, but due to the reaction conditions, the denitrification efficiency is low; the SCR denitrification process has high denitrification efficiency but a complex system and high investment cost. When applied to the transformation of old boilers, it is often necessary to transform the heating surface and flue at the rear of the boiler due to problems such as SCR reaction temperature conditions and insufficient layout space. The workload is large and the investment cost is further increased. In addition, when the boiler is under low load conditions, the flue gas temperature level is low, and the denitrification efficiency is difficult to meet. In order to ensure that the NOx concentration during flue gas emissions meets environmental protection standards and achieve ultra-low emissions, new technical means need to be invented. Summary of the invention

[0003] In order to solve the above problems, the present invention provides a SNCR and low-temperature SCR coupled flue gas denitrification system and a method of using the same, so as to improve the denitrification efficiency and achieve ultra-low NOx emissions under all operating conditions.

[0004] The present invention provides a SNCR and low-temperature SCR coupled flue gas denitrification system, comprising a tail flue, a flue gas heater, a low-temperature SCR reactor, a main spray gun and an ammonia water tank, wherein the main spray gun is connected to the outlet of the ammonia water tank through an ammonia water pump, the main spray gun is arranged in the front flue of the tail flue, the main spray gun is used to spray ammonia water into the front flue to carry out an SNCR reaction, the flue gas outlet of the tail flue is connected to the flue gas inlet of the flue gas heater, the flue gas outlet of the flue gas heater is connected to the flue gas inlet of the low-temperature SCR reactor, and the low-temperature SCR reactor is used to use the escaped ammonia in the SNCR reaction as a denitrification reducing agent to carry out a low-temperature SCR reaction.

[0005] Furthermore, a first ammonia valve and a first ammonia check valve are arranged between the ammonia tank and the ammonia pump; the air inlet of the main spray gun is connected to the air outlet of the air compressor, and a compressed air valve is arranged between the main spray gun and the air compressor; the main spray gun is connected to the air outlet of the primary fan, and the cooling air outlet is used to send out the cold air generated by the primary fan to cool the main spray gun.

[0006] Furthermore, the main spray gun, the first ammonia water valve, the first ammonia water check valve and the ammonia water pump are all made of corrosion-resistant materials.

[0007] Furthermore, a dust collector and an induced draft fan are provided between the tail flue and the low-temperature SCR reactor, the flue gas outlet of the tail flue is connected to the flue gas inlet of the dust collector, the flue gas outlet of the dust collector is connected to the flue gas inlet of the induced draft fan, the dust collector is used to remove smoke dust in the flue gas; the outlet of the induced draft fan is connected to the flue gas inlet of the flue gas heater, the flue gas outlet of the flue gas heater is connected to the flue gas inlet of the low-temperature SCR reactor, the flue gas heater is used to ensure the temperature level of the flue gas; the flue gas outlet of the low-temperature SCR reactor is connected to the chimney.

[0008] Furthermore, a superheater and an air preheater are provided in the tail flue, the outlet of the superheater is connected to the steam inlet of the flue gas heater through a steam distribution valve, the steam outlet of the flue gas heater is connected to a condenser, and the condenser is used to recover steam resources; the air preheater is connected to the air outlet of the primary fan, and the air preheater is used to heat the primary cold air generated by the primary fan.

[0009] Furthermore, the outlet of the superheater is also connected to the steam inlet of the steam turbine through a steam distribution valve, the steam outlet of the steam turbine is connected to the condenser, and the steam turbine is also connected to a generator.

[0010] Furthermore, an auxiliary spray gun is arranged in the front flue of the low-temperature SCR reactor, and the auxiliary spray gun is connected to the outlet of the ammonia water tank through an auxiliary ammonia water pump, and a second ammonia water valve and a second ammonia water check valve are arranged between the auxiliary spray gun and the ammonia water tank; the air inlet of the auxiliary spray gun is connected to the air outlet of the auxiliary air compressor, and an auxiliary compressed air valve is arranged between the auxiliary spray gun and the auxiliary air compressor, and the auxiliary spray gun is used to spray ammonia into the low-temperature SCR reactor to supplement the denitrification reducing agent.

[0011] Furthermore, the auxiliary spray gun, the second ammonia water valve, the second ammonia water check valve and the auxiliary ammonia water pump are all made of corrosion-resistant materials.

[0012] Furthermore, a catalyst is arranged in the low-temperature SCR reactor, the catalyst arrangement plane is perpendicular to the flue gas flow direction, and the catalyst needs to be arranged in a vertical flue.

[0013] The present invention also provides a method for using a SNCR and low-temperature SCR coupled flue gas denitrification system, comprising: ammonia water in an ammonia water tank enters a main spray gun through an ammonia water pump, air enters the main spray gun after being compressed by an air compressor, the compressed air atomizes the ammonia water and then sprays it from the main spray gun into a front flue of a tail flue to perform an SNCR reaction, the outlet of a primary fan is connected to the main spray gun, and the primary fan is used to send out cold air to cool the main spray gun; a superheater is arranged in the tail flue, and the steam outlet of the superheater is connected to the steam inlet of a flue gas heater; the flue gas after the SNCR reaction enters a dust collector from the outlet of the tail flue to remove smoke dust, and the smoke after smoke dust removal is led to the flue gas heater by an induced draft fan, and then enters The flue gas entering the flue gas heater exchanges heat with the steam from the heater to absorb heat, and the heated flue gas enters the low-temperature SCR reactor, and the escaped ammonia escaping from the SNCR reaction is used as a denitrification reducing agent for low-temperature SCR reaction, and the NOx in the flue gas is reduced to N2; when the escaped ammonia is insufficient, the ammonia water in the ammonia water tank enters the auxiliary spray gun through the auxiliary ammonia water pump, and the air enters the auxiliary spray gun after being compressed by the auxiliary air compressor. The compressed air atomizes the ammonia water and then sprays it from the auxiliary spray gun into the low-temperature SCR reactor for auxiliary ammonia spraying, and the NOx in the flue gas is reduced to N2; the reduced N2 is discharged through a chimney connected to the flue gas outlet of the low-temperature SCR reactor.

[0014] Compared with the prior art, the present invention has at least the following beneficial effects:

[0015] The SNCR and low-temperature SCR coupled denitrification system provided by the present invention can effectively ensure that the NOx concentration at the chimney outlet meets the standard under all operating conditions; since a low-temperature SCR reactor is arranged, the denitrification system is not affected by the boiler combustion conditions, and can also control flue gas NOx emissions at the initial start-up of the boiler and when the load is increased or decreased, thereby achieving ultra-low NOx emissions under all operating conditions.

[0016] Furthermore, the SNCR and low-temperature SCR coupled denitrification system provided by the present invention first utilizes the SNCR denitrification reaction occurring in the flue before the tail flue to preliminarily remove NOx, and then passes the escaped ammonia of the SNCR denitrification reaction into the low-temperature SCR reactor for further denitrification. The system is flexible and reliable in operation and saves denitrification costs.

[0017] Furthermore, the present invention provides an auxiliary spray gun at the low-temperature SCR reactor. When the escaped ammonia is insufficient, the auxiliary spray gun can be activated to perform auxiliary ammonia spraying to supplement the denitrification reducing agent required for the low-temperature SCR reaction, thereby ensuring that the flue gas entering the low-temperature SCR reactor can undergo sufficient denitrification reaction.

[0018] Furthermore, the present invention connects the steam turbine and the flue gas heater respectively through a steam distribution valve after the superheater, wherein the steam turbine is connected to a generator. When the steam flows through the steam turbine to expand and do work, it drives the generator to rotate and generate electricity, thereby realizing multi-effect utilization of energy. The steam after doing work is drawn out from the steam outlet of the steam turbine and enters the condenser for condensation; after heating the flue gas, the steam in the flue gas heater is passed into the condenser from the steam outlet of the flue gas heater, and the condenser plays a role in recovering steam resources.

[0019] Furthermore, in the present invention, the flue gas heat absorption is adjusted by adjusting the steam flow entering the flue gas heater, the flue gas temperature is changed, the steam amount is increased at low load, and the steam amount is reduced at high load or even the outlet of the steam distribution valve in the direction of the flue gas heater is closed, thereby ensuring the flue gas temperature level and the denitrification efficiency of the low-temperature SCR denitrification equipment. At the same time, the steam generated by the superheater is used as the heat source, and there is no need to input an additional heat source, so the system economy is better.

[0020] Furthermore, in the present invention, the low-temperature SCR device is arranged after the dust collector, and the dust concentration of the flue gas flowing through the SCR device is low, thereby reducing the dust accumulation and catalyst poisoning of the low-temperature SCR reactor and extending the service life of the catalyst; in addition, the present invention arranges the low-temperature SCR device after the air preheater, which is beneficial to avoid the problem of ammonium bisulfate blockage and corrosion of the air preheater caused by SO3 catalyzed by the SCR catalyst.

[0021] Furthermore, the catalyst layout plane in the low-temperature SCR reactor of the present invention must be arranged perpendicular to the flue gas flow direction, and the catalyst needs to be arranged in a vertical flue, thereby avoiding problems such as catalyst ash accumulation and blockage due to inertia when the catalyst is arranged in a horizontal flue, thereby increasing the service life of the catalyst and extending the unit operation cycle. BRIEF DESCRIPTION OF THE DRAWINGS

[0022] Figure 1 It is a SNCR and low-temperature SCR coupled flue gas denitrification system;

[0023] Among them, 1 represents the tail flue; 2 represents the superheater; 3 represents the steam distribution valve; 4 represents the steam turbine; 5 represents the generator; 6 represents the condenser; 7 represents the dust collector; 8 represents the induced draft fan; 9 represents the flue gas heater; 10 represents the low-temperature SCR reactor; 11 represents the chimney; 121 represents the second ammonia valve; 131 represents the second ammonia check valve; 122 represents the auxiliary compressed air valve; 123 represents the first ammonia valve; 132 represents the first ammonia check valve; 124 represents the compressed air valve; 141 represents the auxiliary ammonia pump; 142 represents the auxiliary air compressor; 143 represents the ammonia pump; 144 represents the air compressor; 151 represents the auxiliary spray gun; 152 represents the main spray gun; 16 represents the ammonia tank; 17 represents the primary fan; 18 represents the cold air distribution valve; 19 represents the air preheater. DETAILED DESCRIPTION

[0024] The following is a detailed description of a SNCR coupled low-temperature SCR flue gas denitrification system and a method of using the system proposed by the present invention in conjunction with the accompanying drawings.

[0025] The present invention provides an SNCR coupled low-temperature SCR flue gas denitrification system, wherein a main spray gun 152 is connected to the outlet of an ammonia water tank 16 through an ammonia water pump 143, and a first ammonia water valve 123 and a first ammonia water check valve 132 are provided between the ammonia water tank 16 and the ammonia water pump 143; an air inlet of the main spray gun 152 is connected to an air outlet of an air compressor 144, and a compressed air valve 124 is provided in a compressed air passage; the main spray gun 152 is connected to the air outlet of a primary fan 17 through a cold air distribution valve 18, and the cold air distribution valve 18 is provided with a cooling air outlet, and the cooling air outlet is used to send the cold air generated by the primary fan to cool the main spray gun 152;

[0026] Furthermore, the main spray gun 152 is arranged in the front flue of the tail flue 1, and the main spray gun 152 sprays ammonia water into the front flue, and the ammonia water reacts with the flue gas in the front flue for SNCR reaction;

[0027] In the present invention, a superheater 2 is provided in the upper part of the tail flue 1, and an air preheater 19 is provided in the lower part. The outlet of the superheater 2 is connected to the steam inlet of the flue gas heater 9 through the steam distribution valve 3, and the steam outlet of the flue gas heater 9 is connected to the condenser 6. The condenser 6 is used to recover steam resources; the air preheater 19 is connected to the air outlet of the primary fan 17 through the cold air distribution valve 18, and the cold air distribution valve 18 is also provided with a primary cold air outlet. The primary fan 17 generates primary cold air which is introduced into the air preheater 19 through the primary cold air outlet of the cold air distribution valve 18, and the air preheater 19 heats the primary cold air.

[0028] Furthermore, the outlet of the superheater 2 is also connected to the steam inlet of the steam turbine 4 through the steam distribution valve 3 , the steam outlet of the steam turbine 4 is connected to the condenser 6 , and the steam turbine 4 is also connected to the generator 5 .

[0029] In the present invention, the flue gas outlet of the tail flue 1 is connected to the flue gas inlet of the dust collector 7, and the flue gas outlet of the dust collector 7 is connected to the flue gas inlet of the induced draft fan 8. The dust collector 7 can remove smoke dust in the flue gas; the outlet of the induced draft fan 8 is connected to the flue gas inlet of the flue gas heater 9, and the flue gas outlet of the flue gas heater 9 is connected to the flue gas inlet of the low-temperature SCR reactor 10. The low-temperature SCR reactor 10 uses the escaped ammonia in the SNCR reaction occurring in the flue before the tail flue as a denitrification reducing agent to perform a low-temperature SCR reaction. The flue gas outlet of the low-temperature SCR reactor 10 is connected to the chimney 11, and the flue gas after denitrification is discharged from the flue gas denitrification system.

[0030] In the present invention, an auxiliary spray gun 151 is also arranged in the front flue of the low-temperature SCR reactor 10. The auxiliary spray gun 151 is connected to the outlet of the ammonia water tank 16 through the auxiliary ammonia water pump 141. A second ammonia water valve 121 and a second ammonia water check valve 131 are arranged between the auxiliary spray gun 151 and the ammonia water tank 16; the air inlet of the auxiliary spray gun 151 is connected to the air outlet of the auxiliary air compressor 142, and an auxiliary compressed air valve 122 is arranged on the auxiliary compressed air channel. When the escape ammonia from the SNCR reaction occurring in the front flue of the tail flue is insufficient, ammonia can be sprayed into the low-temperature SCR reactor 10 through the auxiliary spray gun 151 to supplement the denitrification reducing agent.

[0031] Furthermore, a catalyst is arranged in the low-temperature SCR reactor 10, the catalyst arrangement plane is perpendicular to the flue gas flow direction, and the catalyst is arranged in the vertical flue.

[0032] The present invention also provides a method for using a SNCR and low-temperature SCR coupled flue gas denitrification system. The ammonia water in the ammonia water tank 16 enters the main spray gun 152 through the ammonia water pump 143, and the air is compressed by the air compressor 144 and then passed into the main spray gun 152. The compressed air atomizes the ammonia water and then sprays it into the front flue of the tail flue 1 from the main spray gun 152. The atomized ammonia water and the flue gas in the front flue of the tail flue 1 undergo SNCR reaction. The outlet of the primary fan 17 is connected to the main spray gun 152, and the primary fan 17 sends out cold air to cool the main spray gun 152. A superheater 2 and an air preheater 19 are provided in the tail flue 1, and the steam outlet of the superheater 2 is connected to the steam inlet of the flue gas heater 9. The flue gas after the SNCR reaction in the front flue of the tail flue enters the dust collector 7 from the outlet of the tail flue 1 to remove the smoke dust, and the smoke after the smoke dust is removed is led to the smoke heater 9 by the induced draft fan 8. The smoke entering the smoke heater 9 The flue gas absorbs heat by heat exchange with the steam from the heater 2, and the heated flue gas enters the low-temperature SCR reactor 10 for low-temperature SCR reaction. The low-temperature SCR reactor 10 uses the escaped ammonia escaped from the SNCR reaction in the front flue of the tail flue as a denitrification reducing agent to carry out low-temperature SCR reaction, and NOx in the flue gas is reduced to N2 in the low-temperature SCR reactor 10; when the escaped ammonia is insufficient, the ammonia water in the ammonia water tank 16 enters the auxiliary ammonia water pipeline through the auxiliary ammonia water pump 141 and then enters the auxiliary spray gun 151, and the air is compressed by the auxiliary air compressor 142 and then enters the auxiliary spray gun 151 through the compressed air pipeline. The compressed air atomizes the ammonia water and then sprays it into the low-temperature SCR reactor 10 from the auxiliary spray gun 151 for auxiliary ammonia spraying, and the NOx in the flue gas is reduced to N2; the reduced N2 is discharged through the chimney 11 connected to the flue gas outlet of the low-temperature SCR reactor 10.

[0033] As attached Figure 1 As shown, the specific denitration method in this embodiment is as follows:

[0034] In this embodiment, the ammonia water in the ammonia water tank enters the main spray gun through the ammonia water pump, and the air enters the main spray gun after being compressed by the air compressor. The compressed air atomizes the ammonia water and then sprays it from the main spray gun into the front flue of the tail flue 1 to perform the SNCR reaction. The outlet of the primary fan is connected to the inlet of the cold air distribution valve, and the cooling air outlet of the cold air distribution valve is connected to the cooling air channel of the main spray gun. Since the smoke temperature here is relatively high, the primary fan is used to send out cold air to cool the main spray gun;

[0035] Here the SNCR denitrification reaction equation is:

[0036] 4NH3+4NO+O2→4N2+6H2O

[0037] 4NH3+2NO+2O2→3N2+6H2O

[0038] 8NH3+6NO2→7N2+12H2O

[0039] A superheater is arranged in the tail flue, and the steam outlet of the superheater is connected to the steam inlet of the flue gas heater; the flue gas after the SNCR reaction enters the dust collector from the outlet of the tail flue to remove the smoke dust, and the smoke-dust-removed flue gas is led to the flue gas heater by the induced draft fan, and absorbs heat by heat exchange with the steam from the heater. The heated flue gas enters the low-temperature SCR reactor, and the escaped ammonia that escapes from the SNCR reaction without reaction is used as a denitrification reducing agent to carry out a selective catalytic reduction reaction in the low-temperature SCR reactor, and the NOx in the flue gas is reduced to N2; when the escaped ammonia is insufficient, ammonia water and compressed air enter the auxiliary spray gun, and the compressed air atomizes the ammonia water and then sprays it from the auxiliary spray gun into the low-temperature SCR reactor for auxiliary ammonia spraying, and the NOx in the flue gas is reduced to N2;

[0040] The SCR denitration reaction equation occurring in the low-temperature SCR reactor is as follows:

[0041]

[0042]

[0043]

[0044] The N2 generated by the denitrification reaction is discharged through a chimney connected to the flue gas outlet of the low-temperature SCR reactor;

[0045] The outlet of the superheater is also connected to the steam inlet of the steam turbine through a steam distribution valve. The steam flows through the steam turbine to expand and do work, driving the generator to rotate and generate electricity. The steam after doing work is drawn out from the steam outlet of the steam turbine and enters the condenser for condensation, thereby recovering steam resources.

[0046] The denitrification agent used in the present invention is ammonia water, which can actually be replaced by urea solution.

[0047] In the present invention, the first ammonia valve, the second ammonia valve, the first ammonia check valve, the second ammonia check valve, the ammonia pump, the auxiliary ammonia pump, the main spray gun and the auxiliary spray gun are all made of corrosion-resistant materials, including but not limited to stainless steel 316L.

Claims

1. A SNCR and low-temperature SCR coupled flue gas denitrification system, characterized in that: The invention comprises a tail flue (1), a flue gas heater (9), a low-temperature SCR reactor (10), a main spray gun (152) and an ammonia water tank (16), wherein the main spray gun (152) is connected to an outlet of the ammonia water tank (16) via an ammonia water pump (143), the main spray gun (152) is arranged in a front flue of the tail flue (1), and the main spray gun (152) is used to spray ammonia water into the front flue to carry out a SNCR reaction; the flue gas outlet of the tail flue (1) is connected to a flue gas inlet of the flue gas heater (9), the flue gas outlet of the flue gas heater (9) is connected to a flue gas inlet of the low-temperature SCR reactor (10), and the low-temperature SCR reactor (10) is used to use escaped ammonia in the SNCR reaction as a denitrification reducing agent to carry out a low-temperature SCR reaction; A dust collector (7) and an induced draft fan (8) are further arranged between the tail flue (1) and the low-temperature SCR reactor (10); the flue gas outlet of the tail flue (1) is connected to the flue gas inlet of the dust collector (7), the flue gas outlet of the dust collector (7) is connected to the flue gas inlet of the induced draft fan (8), the dust collector (7) is used to remove smoke dust from the flue gas; the outlet of the induced draft fan (8) is connected to the flue gas inlet of a flue gas heater (9), the flue gas outlet of the flue gas heater (9) is connected to the flue gas inlet of the low-temperature SCR reactor (10), the flue gas heater (9) is used to ensure the temperature level of the flue gas; the flue gas outlet of the flue gas of the low-temperature SCR reactor (10) is connected to the chimney (11); A superheater (2) and an air preheater (19) are provided in the tail flue (1); the outlet of the superheater (2) is connected to the steam inlet of the flue gas heater (9) through a steam distribution valve (3); the steam outlet of the flue gas heater (9) is connected to a condenser (6); and the condenser (6) is used to recover steam resources; the air preheater (19) is connected to the air outlet of the primary fan (17); and the air preheater (19) is used to heat the primary cold air generated by the primary fan (17); The outlet of the superheater (2) is also connected to the steam inlet of the steam turbine (4) through a steam distribution valve (3); the steam outlet of the steam turbine (4) is connected to a condenser (6); and the steam turbine (4) is also connected to a generator (5).

2. The SNCR and low-temperature SCR coupled flue gas denitrification system according to claim 1, characterized in that: A first ammonia valve (123) and a first ammonia check valve (132) are provided between the ammonia tank (16) and the ammonia pump (143); an air inlet of the main spray gun (152) is connected to an air outlet of an air compressor (144), and a compressed air valve (124) is provided between the main spray gun (152) and the air compressor (144); the main spray gun (152) is connected to an air outlet of a primary fan (17), and the primary fan is used to send out generated cold air to cool the main spray gun (152).

3. The SNCR and low-temperature SCR coupled flue gas denitrification system according to claim 2, characterized in that: The main spray gun (152), the first ammonia water valve (123), the first ammonia water check valve (132) and the ammonia water pump (143) are all made of corrosion-resistant materials.

4. The SNCR and low-temperature SCR coupled flue gas denitrification system according to claim 1, characterized in that: An auxiliary spray gun (151) is arranged in the front flue of the low-temperature SCR reactor (10); the auxiliary spray gun (151) is connected to the outlet of the ammonia water tank (16) via an auxiliary ammonia water pump (141); a second ammonia water valve (121) and a second ammonia water check valve (131) are arranged between the auxiliary spray gun (151) and the ammonia water tank (16); an air inlet of the auxiliary spray gun (151) is connected to an air outlet of an auxiliary air compressor (142); an auxiliary compressed air valve (122) is arranged between the auxiliary spray gun (151) and the auxiliary air compressor (142); the auxiliary spray gun (151) is used to spray ammonia into the low-temperature SCR reactor (10) to supplement a denitration reducing agent.

5. The SNCR and low-temperature SCR coupled flue gas denitrification system according to claim 4, characterized in that: The auxiliary spray gun (151), the second ammonia water valve (121), the second ammonia water check valve (131) and the auxiliary ammonia water pump (141) are all made of corrosion-resistant materials.

6. The SNCR and low-temperature SCR coupled flue gas denitrification system according to claim 1, characterized in that: A catalyst is arranged in the low-temperature SCR reactor (10), the catalyst arrangement plane is perpendicular to the flue gas flow direction, and the catalyst needs to be arranged in a vertical flue.

7. The method for using the SNCR and low-temperature SCR coupled flue gas denitrification system according to claim 1, characterized in that: Ammonia water in the ammonia water tank (16) enters the main spray gun (152) through the ammonia water pump (143), and air enters the main spray gun (152) after being compressed by the air compressor (144). The compressed air atomizes the ammonia water and then sprays it from the main spray gun (152) into the front flue of the tail flue (1) to perform the SNCR reaction. The outlet of the primary fan (17) is connected to the main spray gun (152), and the primary fan (17) is used to send out cooling air to cool the main spray gun (152). A superheater (2) is arranged in the tail flue (1), and the steam outlet of the superheater (2) is connected to the steam inlet of the flue gas heater (9). The flue gas after the SNCR reaction enters the dust collector (7) from the outlet of the tail flue (1) to remove the smoke dust. The smoke after the smoke dust is removed is led to the smoke heater (9) by the induced draft fan (8) and enters the smoke heating system. The flue gas from the reactor (9) undergoes heat exchange with the steam from the superheater (2) to absorb heat, and the heated flue gas enters the low-temperature SCR reactor (10). The low-temperature SCR reaction is carried out using the escaped ammonia from the SNCR reaction as a denitrification reducing agent, and NOx in the flue gas is reduced to N2. When the escaped ammonia is insufficient, the ammonia water in the ammonia water tank (16) enters the auxiliary spray gun (151) through the auxiliary ammonia water pump (141), and the air enters the auxiliary spray gun (151) after being compressed by the auxiliary air compressor (142). The compressed air atomizes the ammonia water and then sprays it from the auxiliary spray gun (151) into the low-temperature SCR reactor (10) for auxiliary ammonia spraying, and NOx in the flue gas is reduced to N2. The reduced N2 is discharged through a chimney (11) connected to the flue gas outlet of the low-temperature SCR reactor (10).

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