Air distribution system based on secondary air and flue gas recirculation and boiler

Abstract

This application relates to an air distribution system and boiler based on secondary air and flue gas recirculation. The air distribution system includes a secondary air fan; an air preheater connected to one end of the secondary air fan; a first damper connected to the other end of the air preheater; a secondary air nozzle disposed on the boiler and connected to the other end of the first damper; a second damper connected to the flue between the economizer and the semi-dry reaction tower; a recirculation fan connected to the other end of the second damper for absorbing flue gas from the flue; a third damper connected to the other end of the recirculation fan; a recirculation flue gas nozzle disposed on the boiler and connected to the other end of the third damper; and a fourth damper connected to the other end of the air preheater, and the other end of the fourth damper connected to both the third damper and the recirculation fan. This system can reduce NOx generation while simultaneously reducing boiler exhaust losses, improving boiler efficiency, and overcoming the corrosion problem of the recirculation flue gas duct.

Description

Technical Field

[0001] This application relates to the technical field of boilers, and in particular to an air distribution system and boiler based on secondary air and flue gas recirculation. Background Technology

[0002] Boiler combustion controls nitrogen oxide emissions primarily through two methods: reducing nitrogen oxide formation and reducing nitrogen oxides through the reaction of denitrification agents. Flue gas recirculation is an effective low-NOx combustion technology that can reduce nitrogen oxide formation during the combustion phase.

[0003] Existing technology CN105222138A discloses a secondary air system scheme. This scheme utilizes a secondary air fan to extract economizer outlet flue gas and ambient air, mixes them in different proportions, and then blows them into the furnace as secondary air to provide a low-oxygen environment for flue gas combustion in this area, achieving low-NOx combustion. However, this scheme requires mixing high-temperature flue gas of approximately 200°C with cold air of 25°C in the air duct. When the proportion of flue gas is low, the temperature of the mixed gas is low, which easily leads to low-temperature corrosion, causing corrosion failure of the air duct and the fan.

[0004] Existing technologies CN108543419A and CN109764337A disclose a flue gas recirculation scheme. This scheme mainly uses a flue gas recirculation fan to extract high-temperature flue gas after the induced draft fan and transport it into the furnace, creating a local low-oxygen environment in this area and reducing NOx formation. However, because the flue gas temperature after the induced draft fan is low, low-temperature corrosion can easily occur when there is a leak in the negative pressure section of the recirculation flue gas pipeline, or when the system is shut down, leading to rust and perforation of the pipeline and fan, affecting the effective operation of the system. Secondly, this scheme results in a slightly higher volume of flue gas entering the flue gas treatment system than projects without flue gas recirculation, thus increasing the load on the flue gas treatment system. Furthermore, this scheme has a relatively small impact on boiler efficiency, essentially maintaining the same efficiency as boilers without flue gas recirculation. Utility Model Content

[0005] In order to reduce NOx emissions, improve boiler efficiency, and effectively avoid low-temperature corrosion of recirculated flue gas ducts, this application provides an air distribution system, air distribution method, and boiler based on secondary air and flue gas recirculation.

[0006] Specifically, this application provides an air distribution system based on secondary air and flue gas recirculation for low-NOx, high-efficiency combustion in a boiler, wherein the boiler includes an economizer and a semi-dry reaction tower; the air distribution system includes:

[0007] Secondary air fans are used to extract air;

[0008] An air preheater, one end of which is connected to the secondary air fan, is used to heat the air;

[0009] The first damper is connected at one end to the other end of the air preheater;

[0010] A secondary air nozzle is installed on the boiler and connected to the other end of the first air damper;

[0011] The second damper is connected at one end to the flue between the economizer and the semi-dry reaction tower;

[0012] A recirculation fan, one end of which is connected to the other end of the second damper, is used to draw in the flue gas;

[0013] The third damper is connected at one end to the other end of the recirculation fan;

[0014] A recirculated flue gas nozzle is installed on the boiler and connected to the other end of the third damper; and

[0015] The fourth damper is connected at one end to the other end of the air preheater and at the other end to one end of the third damper and the other end of the recirculation fan.

[0016] As a preferred embodiment, the air preheater and the first damper are connected through a first channel, and one end of the fourth damper is connected to the first channel; the recirculation fan and the third damper are connected through a second channel, and the other end of the fourth damper is connected to the second channel.

[0017] As a preferred embodiment, the air preheater uses steam extracted from a steam turbine for heating.

[0018] As a preferred embodiment, the recirculated flue gas nozzle is located below the secondary air nozzle.

[0019] In addition, this application also provides a boiler, including an economizer and a semi-dry reaction tower; characterized in that it also includes an air distribution system based on secondary air and flue gas recirculation as described above.

[0020] Compared with the prior art, this application has the following beneficial effects:

[0021] This application sets up independent secondary air fans and flue gas recirculation fans, with the secondary air and recirculated flue gas entering the boiler through independent pipelines. The flue gas recirculation nozzle is located below the secondary air nozzle. The recirculated flue gas is drawn from the flue between the economizer and the reaction tower, pressurized by the recirculation fan, and injected into the boiler. The secondary air is drawn from the boiler room or waste pit, heated by the secondary air fan and air preheater, and then injected into the boiler. The secondary air duct at the outlet of the air preheater is connected to the flue gas duct at the outlet of the recirculation fan through a damper. When the heat load is high, the recirculation system and secondary air are used simultaneously to provide oxygen for combustion. When the heat load is low, the flue gas recirculation is stopped, the air preheater is put into operation, and the hot secondary air is injected into the furnace simultaneously through the secondary air nozzle and the recirculated flue gas nozzle to aid combustion. The flow rate is higher, the turbulence of the flue gas is stronger, and low-oxygen combustion can be achieved, reducing the formation of nitrogen oxides.

[0022] This application extracts flue gas from the economizer outlet for recirculation, reducing the amount of flue gas discharged from the boiler, reducing boiler flue gas losses by 10-15%, and improving boiler efficiency by 1-2%.

[0023] The recirculated flue gas in this application dilutes the heat of combustion, lowers the combustion temperature, and slows down or prevents boiler coking.

[0024] When the flue gas recirculation of this application is put into use, the temperature of the flue gas in the recirculation flue is about 200°C. Even if there is slight air leakage, low-temperature corrosion will not occur. When the flue gas recirculation is withdrawn, the pipe is filled with hot secondary air at 150~180°C. The flue gas remaining in the flue will not cause low-temperature corrosion to the flue, thus completely avoiding corrosion of the recirculation flue.

[0025] This application demonstrates that when the heat load decreases, using hot secondary air can achieve a higher secondary air velocity with the same secondary air volume, enhancing flue gas turbulence, ensuring complete combustion, and avoiding incomplete dioxin decomposition due to insufficient secondary air volume and turbulence at low loads. Furthermore, compared to increasing secondary air volume to enhance turbulence, increasing secondary air temperature to enhance turbulence also satisfies low-oxygen combustion requirements, reducing NOx formation. Simultaneously, the use of turbine-extracted steam for reheating also improves unit efficiency. Attached Figure Description

[0026] To more clearly illustrate the technical solutions in the embodiments of this application or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of this application. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0027] The structures, proportions, sizes, etc., shown in the accompanying drawings are only for the purpose of assisting those skilled in the art in understanding and reading the content disclosed in the specification, and are not intended to limit the implementation conditions of this application. Therefore, they have no substantial technical significance. Any modifications to the structure, changes in the proportions, or adjustments to the size should still fall within the scope of the technical content disclosed in this application, provided that they do not affect the effects and purposes that this application can produce.

[0028] Figure 1 This is a schematic diagram of the gas distribution system of this application;

[0029] Figure 2 This is a gas flow diagram of the gas distribution system of this application under flue gas recirculation operation conditions;

[0030] Figure 3 This is a diagram showing the gas flow direction of the purging flue gas during the exhaust of the gas distribution system of this application.

[0031] Figure 4 This is a diagram showing the gas flow direction of the purging flue gas after the flue gas recirculation system of this application has ended. Detailed Implementation

[0032] To enable those skilled in the art to better understand the present application, the technical solutions in the embodiments of the present application will be clearly and completely described below with reference to the accompanying drawings.

[0033] In the description of this application, it should be understood that the orientations or positional relationships indicated by terms, etc., are based on the orientations or positional relationships shown in the accompanying drawings and are only for the convenience of describing this application and simplifying the description, and are not intended to indicate or imply that the device, element, module, system, platform, or device referred to must have a specific orientation, or be constructed and operated in a specific orientation, and therefore should not be construed as a limitation of this application. The following description of this application is only to be understood as a description of individual embodiments of the technical solutions of this application. Other embodiments are not reflected in the following description, but this does not mean that this application excludes these other embodiments, nor is the technical solution of this application limited to the specific implementations described below, and the protection scope of this application is not limited to the specific implementations described below. Based on the embodiments in this application, all other embodiments obtained by those skilled in the art without creative effort should fall within the protection scope of this application.

[0034] It should be noted that if the terms "first," "second," etc., appear in the specification, claims, and accompanying drawings of this application, such descriptions are only used to distinguish similar objects and are not necessarily used to describe a specific order or sequence. It should be understood that such data can be interchanged where appropriate so that the embodiments of this application described herein can be implemented in orders other than those illustrated or described herein. Furthermore, the terms "comprising" and "having," and any variations thereof, are intended to cover non-exclusive inclusion; for example, a system, product, or device that comprises a series of units, modules, or components is not necessarily limited to those explicitly listed, but may include other components not explicitly listed or inherent to such systems, products, or devices.

[0035] The technical solution of this application will be further described below with reference to the accompanying drawings and specific embodiments.

[0036] In some embodiments, such as Figure 1 As shown, this application provides an air distribution system based on secondary air and flue gas recirculation for low-NOx and high-efficiency combustion in boiler 1. Boiler 1 includes economizer 5 and semi-dry reaction tower 4. The high-temperature flue gas generated by combustion in boiler 1 is cooled to about 190-210°C by the boiler and then enters the semi-dry reaction tower (4) through the flue 12 between economizer 5 and semi-dry reaction tower 4.

[0037] The air distribution system includes a secondary air fan 21 for drawing in air; an air preheater 22, one end of which is connected to the secondary air fan 21 for heating air; a first damper 23, one end of which is connected to the other end of the air preheater 22; a secondary air nozzle 24, which is mounted on the boiler 1 and connected to the other end of the first damper 23; a second damper 32, one end of which is connected to the flue duct 12 between the economizer 3 and the semi-dry reaction tower 4; a recirculation fan 33, one end of which is connected to the other end of the second damper 32 for absorbing flue gas from the flue duct 12; a third damper 34, one end of which is connected to the other end of the recirculation fan 33; a recirculated flue gas nozzle 35, which is mounted on the boiler 1 and connected to the other end of the third damper 34; and a fourth damper 36, one end of which is connected to the other end of the air preheater 22, and the other end of which is connected to both the third damper 34 and the other end of the recirculation fan 33.

[0038] Specifically, the secondary air fan 21 draws air from the environment, which passes through the air preheater 22, the first damper 23, and the secondary air nozzle 24 before entering the boiler 1 to provide combustion air. The recirculation fan 33 draws medium-temperature flue gas from the flue 12 between the economizer ash hopper 11 and the semi-dry reaction tower of the boiler 1, passes through the air intake tee 31, the second damper 32, the recirculation fan 33, and the third damper 34, and is injected into the boiler 1 from the recirculation flue gas nozzle 35 below the secondary air nozzle 24 to provide some combustion oxygen, create a low-oxygen combustion environment, and reduce NOx production.

[0039] The outlet of the secondary air preheater 22 and the outlet of the recirculating flue gas fan 33 are connected by a fourth damper 36. The flue gas recirculation intake tee 31 is located downstream of the economizer's ash hopper 11 to reduce dust entering the flue gas recirculation system. The recirculating fan 33 uses welded composite wear-resistant plates for dustproof and wear-resistant functions, and the air preheater 22 is heated by steam extracted from a steam turbine.

[0040] In some embodiments, the air preheater 22 and the first damper 23 are connected through a first channel, and one end of the fourth damper 36 is connected to the first channel; the recirculation fan 33 and the third damper 34 are connected through a second channel, and the other end of the fourth damper 36 is connected to the second channel.

[0041] In some embodiments, the recirculated flue gas nozzle 35 is disposed below the secondary air nozzle 24.

[0042] In some embodiments, this application also provides an air distribution method using the air distribution system based on secondary air and flue gas recirculation as described above, such as... Figure 2 The above includes:

[0043] Open the first air damper 23, the second air damper 32 and the third air damper 34;

[0044] Close the fourth air damper 36; and

[0045] Operate the secondary air fan 21, the air preheater 22, and the recirculation fan 33;

[0046] In this system, the air drawn by the secondary air fan 21 is heated by the air preheater 22, passes through the first damper 23, and is injected into the boiler 1 through the secondary air nozzle 24. The recirculation fan 33 draws in flue gas from the flue 12, passes through the third damper 34, and is injected into the boiler 1 through the recirculation flue gas nozzle 35. This method is for flue gas recirculation operation, where the boiler has a high heat load, and NOx emissions can be reduced through flue gas recirculation. At this time, the second damper 32, the third damper 34, and the first damper 23 are open, the secondary air fan 21, the air preheater 22, and the recirculation fan 33 are in operation, and the fourth damper 36 is closed. The recirculation fan 33 draws flue gas from the flue 12 back into the boiler 1, reducing exhaust losses, improving boiler 1 efficiency, reducing the amount of flue gas entering the flue gas treatment system, reducing the flue gas treatment load, and also reducing NOx generation.

[0047] In some embodiments, when the heat load in the boiler 1 is lower than a threshold, the fourth damper 36 is opened and the recirculation fan 33 is shut down; such as Figure 3 As shown.

[0048] In this method, the air drawn by the secondary air fan 21 is heated by the air preheater 22. The heated air is then injected into the boiler 1 via a first route through the secondary air nozzle 24, a second route through the recirculated flue gas nozzle 35, and a third route into the flue duct 12. The first route consists of the first damper 23 and the secondary air nozzle 24; the second route consists of the fourth damper 36, the third damper 34, and the recirculated flue gas nozzle 35; and the third route consists of the fourth damper 36, the recirculation fan 33, and the second damper 32. This method involves purging the flue gas when the flue gas recirculation process ends. Because the boiler's heat load decreases, the flue gas recirculation cannot achieve the goal of reducing NOx, therefore, the flue gas recirculation process needs to be terminated. The specific operation involves starting the air preheater 22 to provide secondary air temperature to 150-180℃, opening the fourth damper 36, keeping the first damper 23, second damper 32, fourth damper 36, and third damper 34 open, stopping the recirculation fan 33, and allowing the secondary air heated by the air preheater 22 to purge the recirculation flue gas system through the fourth damper 36, thus purging the high-temperature flue gas in the recirculation fan 33 and flue gas duct into the economizer 3 and the semi-dry reaction tower 4, preventing low-temperature corrosion caused by internal flue gas cooling after the flue gas recirculation system is shut down.

[0049] In some embodiments, the second damper 32 is closed after the flue is filled with heated air; such as Figure 4 As shown.

[0050] The air drawn by the secondary air fan 21 is heated by the air preheater 22, passes through the first damper 23, and is injected into the boiler by the secondary air nozzle 24. After passing through the fourth damper 36 and the third damper 34, it is injected into the boiler 1 by the recirculated flue gas nozzle 35. This method involves closing the second damper 32 after the flue gas recirculation system has exited operation and been shut down for purging. The recirculation fan 33 and flue duct 12 are filled with medium-temperature secondary air. The first damper 23, third damper 34, and fourth damper 36 remain open, and the air preheater is kept operational. The high-temperature secondary air is distributed by adjusting the openings of the first damper 23 and third damper 34, controlling the amount of secondary air injected into the boiler 1 from the secondary air nozzle 24 and recirculation nozzle 35. As the heat load decreases, the secondary air volume also decreases. Increasing the secondary air temperature increases the injection rate into the boiler, strengthens flue gas turbulence, and reduces the total air volume. This maintains a high combustion temperature while reducing the amount of oxygen in the combustion, achieving low-oxygen combustion. This meets the requirements for low-oxygen combustion and also appropriately reduces NOx formation. Since the air preheater uses steam extracted from the turbine for heating, it improves the reheating of the thermal system and also increases unit efficiency. Furthermore, because the recirculated flue gas system is filled with medium-temperature secondary air, low-temperature corrosion will not occur.

[0051] In some embodiments, this application also provides an air distribution method using the air distribution system based on secondary air and flue gas recirculation as described above, such as... Figure 3 As shown, it includes:

[0052] Open the first air damper 23, the second air damper 32, the third air damper 34 and the fourth air damper 36;

[0053] Shutting down the recirculation fan 33; and

[0054] Operate the secondary air fan 21 and the air preheater 22;

[0055] The air drawn by the secondary air fan 21 is heated by the air preheater 22. The heated air is injected into the boiler 1 via the secondary air nozzle 24 through a first route, into the boiler 1 via the recirculated flue gas nozzle 35 through a second route, and into the flue 12 via a third route. The first route consists of the first damper 23 and the secondary air nozzle 24; the second route consists of the fourth damper 36, the third damper 34, and the recirculated flue gas nozzle 35; and the third route consists of the fourth damper 36, the recirculated air fan 33, and the second damper 32.

[0056] In some embodiments, this application also provides an air distribution method using the air distribution system based on secondary air and flue gas recirculation as described above, such as... Figure 4 As shown, it includes:

[0057] Open the first air door 23, the third air door 34 and the fourth air door 36;

[0058] Close the second damper 32 and the recirculation fan 33; and

[0059] Operate the secondary air fan 21 and the air preheater 22;

[0060] The air drawn by the secondary air fan 21 is heated by the air preheater 22, passes through the first damper 23, and is injected into the boiler 1 by the secondary air nozzle 24. After passing through the fourth damper 36 and the third damper 34, it is injected into the boiler 1 by the recirculated flue gas nozzle 35.

[0061] In some embodiments, this application also provides a boiler 1, including an economizer 3 and a semi-dry reaction tower 4; characterized in that it further includes an air distribution system based on secondary air and flue gas recirculation as described above.

[0062] The technical features of the above embodiments can be combined in any way. For the sake of brevity, not all possible combinations of the technical features in the above embodiments are described. However, as long as there is no contradiction in the combination of these technical features, they should be considered to be within the scope of this specification.

[0063] The above-described embodiments are merely illustrative of several implementation methods of this application and are only used to illustrate the technical solutions of this application, not to limit it. Although this application has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some of the technical features. These modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the spirit and scope of the technical solutions of the embodiments of this application. For those skilled in the art, several variations and improvements can be made without departing from the concept of this application, and these all fall within the protection scope of this application.

Claims

1. A system for low-nitrogen and high-efficiency combustion of a boiler based on secondary air and flue gas recirculation, the boiler comprising an economizer and a semi-dry reaction tower; characterized in that, The air distribution system includes: Secondary air fans are used to extract air; An air preheater, one end of which is connected to the secondary air fan, is used to heat the air; The first damper is connected at one end to the other end of the air preheater; A secondary air nozzle is installed on the boiler and connected to the other end of the first air damper; The second damper is connected at one end to the flue between the economizer and the semi-dry reaction tower; A recirculation fan, one end of which is connected to the other end of the second damper, is used to draw in the flue gas; The third damper is connected at one end to the other end of the recirculation fan; A recirculated flue gas nozzle is installed on the boiler and connected to the other end of the third damper; and The fourth damper is connected at one end to the other end of the air preheater and at the other end to one end of the third damper and the other end of the recirculation fan.

2. The air distribution system based on secondary air and flue gas recirculation according to claim 1, characterized in that: The air preheater and the first damper are connected through a first channel, and one end of the fourth damper is connected to the first channel; the recirculation fan and the third damper are connected through a second channel, and the other end of the fourth damper is connected to the second channel.

3. The air distribution system based on secondary air and flue gas recirculation according to claim 1, characterized in that: The air preheater uses steam extracted from a steam turbine for heating.

4. The air distribution system based on secondary air and flue gas recirculation according to claim 1, characterized in that: The recirculated flue gas nozzle is located below the secondary air nozzle.

5. A boiler comprising an economizer and a semi-dry reaction tower; characterized in that It also includes the air distribution system based on secondary air and flue gas recirculation as described in any one of claims 1-4.

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