Flue gas recirculation system of circulating fluidized bed boiler

Abstract

The utility model relates to a flue gas recirculation system of a circulating fluidized bed boiler, which aims to solve the technical problems of high running oxygen content and high NOx emission concentration of the existing CFB boiler, and adopts the technical scheme that a flue gas guide connection point Y is arranged on a flue between an induced draft fan and a chimney, and is connected with a flue gas branch point F1 through a flue gas recirculation pipeline; and the primary air treatment system and the secondary air treatment system are branched into a primary air treatment pipeline and a secondary air treatment pipeline, the pipelines are symmetrically arranged about a flue gas branch point F1, each of the primary air treatment system and the secondary air treatment system comprises a fan, a recirculating flue gas adjusting valve and a fan heater, flue gas feeding points are arranged on the pipelines, and finally, the flue gas enters the air pre-heater along the primary air pipeline and the secondary air pipeline and returns to the boiler. According to the utility model, the operation oxygen amount of the boiler can be reduced, NOx up-to-standard emission can be assisted, the secondary air is synchronously added to a smoke reprocessing system, and under the condition that the operation oxygen amount is certain, the air volume of the secondary air is increased, the cooling effect of a low-load secondary air nozzle is improved, and the condition that the secondary air nozzle is ablated and damaged is solved.

Description

Technical Field

[0001] This utility model belongs to the field of flue gas denitrification technology, specifically relating to a circulating fluidized bed boiler flue gas recirculation system. Background Technology

[0002] In recent years, my country's new energy development has been rapid, with the installed capacity of hydropower, wind power, and solar power all ranking first in the world. The total power generation of renewable energy also ranks first in the world. New energy has the characteristics of randomness, intermittency, and instability. Therefore, the rapid development of new energy in my country has led to increasing difficulties in the consumption of new energy and the peak regulation of the power grid. Thermal power units are increasingly participating in deep peak regulation, especially circulating fluidized bed (CFB) boilers. However, CFB boilers are limited by the minimum fluidizing air volume and the minimum secondary air nozzle cooling air volume. The oxygen content is high when operating at low load, and it is difficult to stably meet the emission standards for nitrogen oxides. Combined with flue gas recirculation (FGR), the load of CFB boilers can be reduced to a very low level. Therefore, flue gas recirculation will definitely be recognized and put into use by more and more power plants.

[0003] However, under low-load conditions, the current CFB boiler operates with high oxygen levels, reaching 6% to 12%, resulting in high NOx emission concentrations. Furthermore, the separator temperature is low, leading to low SNCR denitrification efficiency and the inability to achieve ultra-low NOx emissions, which will have a certain impact on the atmospheric environment. At the same time, there are no effective means to adjust the large deviations in bed temperature and oxygen levels between the left and right sides of the CFB boiler. In addition, the low secondary air volume at low loads results in poor nozzle cooling and makes the boiler prone to ablation damage. Utility Model Content

[0004] The purpose of this invention is to solve the above problems and provide a circulating fluidized bed boiler flue gas recirculation system.

[0005] To solve the above-mentioned technical problems, the technical solution adopted by this utility model is as follows:

[0006] A circulating fluidized bed boiler flue gas recirculation system includes a boiler, a cyclone separator, an air preheater, a desulfurization tower, a dust collector, an induced draft fan, and a chimney. The boiler is connected to the cyclone separator, the cyclone separator is connected to the air preheater, the air preheater is connected to the desulfurization tower, the desulfurization tower is connected to the dust collector, the dust collector is connected to the induced draft fan, and the induced draft fan is connected to the chimney.

[0007] It also includes flue gas recirculation fans, primary air treatment systems, secondary air treatment systems, flue gas recirculation pipelines, and coal feeder sealing air systems;

[0008] A flue gas inlet point Y is provided on the flue between the induced draft fan and the chimney. The air inlet of the flue gas recirculation fan is connected to the flue gas inlet point Y through a flue gas recirculation pipeline. A recirculation flue gas shut-off valve is provided on the pipeline between the two. A flue gas branch point F1 is provided near the center line of the boiler. The flue gas branch point F1 is connected to the air outlet of the flue gas recirculation fan through a flue gas recirculation pipeline. The inner wall of the flue gas recirculation pipeline is provided with an anti-corrosion coating. Drain valves are installed at all low points of the pipeline and at intervals of 15 to 30 meters along the route.

[0009] The primary air treatment system includes primary air fan A, primary air fan B, warm air fan A, warm air fan B, recirculated flue gas regulating valve A, recirculated flue gas regulating valve B, cold primary air duct, and hot primary air duct. The flue gas recirculation duct splits into two paths after passing through flue gas branch point F1. One path connects sequentially to recirculated flue gas regulating valve A and the inlet of primary air fan A; the other path connects sequentially to recirculated flue gas regulating valve B and the inlet of primary air fan B. The outlets of primary air fan A and primary air fan B are connected by cold primary air... After the pipelines are connected and merged, they are connected to the cold primary air interface of the air preheater. One end of the hot primary air pipeline is connected to the hot primary air interface of the air preheater, and the other end is connected to the primary air box at the bottom of the boiler. A flue gas inlet point A-S1 is provided on the pipeline between the primary air fan A and the recirculation flue gas regulating valve A. The warm air fan A is connected to the flue gas inlet point A-S1. A flue gas inlet point B-S2 is provided on the pipeline between the primary air fan B and the recirculation flue gas regulating valve B. The warm air fan B is connected to the flue gas inlet point B-S2.

[0010] The secondary air treatment system includes secondary air fan A, secondary air fan B, warm air fan C, warm air fan D, recirculated flue gas regulating valve C, and recirculated flue gas regulating valve D. After passing through flue gas branch point F1, two flue gas recirculation pipelines branch off and then connect in parallel to form two branch lines. One branch line connects sequentially to the recirculated flue gas regulating valve C and the air inlet of secondary air fan A, while the other branch line connects sequentially to the recirculated flue gas regulating valve D and the air inlet of secondary air fan B. The air outlets of secondary air fan A and secondary air fan B are connected and merged through a cold secondary air pipeline, which then connects to the cold secondary air interface of the air preheater. One end of the hot secondary air duct is connected to the hot secondary air interface of the air preheater, and the other end is connected to the secondary air nozzle of the boiler. A flue gas inlet point C-S3 is provided on the duct between the secondary air fan A and the recirculating flue gas regulating valve C. The warm air fan C is connected to the flue gas inlet point C-S3. A flue gas inlet point D-S4 is provided on the duct between the secondary air fan B and the recirculating flue gas regulating valve D. The warm air fan D is connected to the flue gas inlet point D-S4. The heat source for the warm air fans A, B, C and D is steam or the primary and secondary hot air circulation heating produced by the boiler itself.

[0011] The coal feeder sealing air system includes a coal feeder sealing fan and a coal feeder. The outlet of the coal feeder sealing fan is connected to the coal feeder, and the coal feeder is connected to the coal feed port of the boiler. A sealing fan outlet shut-off valve is installed on the pipeline between the two. A branch pipe is split off from the combined cold primary air pipeline and connected to the pipeline between the coal feeder sealing fan outlet and the coal feeder. A sealing air shut-off valve is installed on the branch pipe.

[0012] Furthermore, a desulfurization flue gas recirculation pipeline is connected between the pipeline between the air preheater and the desulfurization tower and the pipeline between the induced draft fan and the chimney, and the flue gas inlet point Y is located on the desulfurization flue gas recirculation pipeline.

[0013] Furthermore, a recirculating flue gas flow meter A is installed on the pipeline between the flue gas inlet point A-S1 and the recirculating flue gas regulating valve A; a recirculating flue gas flow meter B is installed on the pipeline between the flue gas inlet point B-S2 and the recirculating flue gas regulating valve B; a recirculating flue gas flow meter C is installed on the pipeline between the flue gas inlet point C-S3 and the recirculating flue gas regulating valve C; and a recirculating flue gas flow meter D is installed on the pipeline between the flue gas inlet point D-S4 and the recirculating flue gas regulating valve D.

[0014] Furthermore, the recirculated flue gas shut-off valve is a butterfly valve with a soft seal or a slide gate valve with a clamping device.

[0015] Furthermore, the flue gas recirculation fan can be a single or dual variable frequency fan.

[0016] Furthermore, the coal feeder sealing fan operates at industrial frequency and adopts a one-in-one-backup structure.

[0017] Furthermore, the primary air treatment pipelines and secondary air treatment system pipelines branching off after the flue gas branch point F1 are symmetrically arranged with the flue gas branch point F1 as the center point.

[0018] Compared with the prior art, the beneficial effects of this utility model are:

[0019] 1. This utility model can reduce the oxygen content of boilers under low load conditions, help NOx emissions meet standards, and at the same time, the secondary air is simultaneously increased by the flue gas recirculation system. Under the condition of a certain operating oxygen content, the secondary air volume can be increased, the cooling effect of the secondary air nozzles under low load conditions can be improved, and the problem of secondary air nozzles being burned and damaged due to low cooling air volume under low load conditions can be solved.

[0020] 2. After the recirculated flue gas of this utility model enters the primary air treatment system, the low-oxygen flue gas replaces part of the original fresh air, which solves the problem of high operating oxygen caused by high fluidized air volume in circulating fluidized bed boilers under low load operating conditions.

[0021] 3. After the recirculated flue gas of this utility model is sent into the secondary air treatment system, the low-oxygen flue gas replaces part of the original fresh air. While reducing the operating oxygen content, it increases the secondary air flow rate, which solves the problems of small cooling air volume, difficult distribution, and easy sintering of nozzles in the secondary air nozzles of the circulating fluidized bed boiler under low load operating conditions, and ensures the safe operation of the unit with low oxygen and low nitrogen. Attached Figure Description

[0022] Figure 1 This is a schematic diagram of the structure of Embodiment 1 of the present utility model;

[0023] Figure 2 This is a schematic diagram of the structure of Embodiment 2 of the present invention;

[0024] In the diagram: 1. Boiler; 2. Cyclone separator; 3. Air preheater; 4. Desulfurization tower; 5. Dust collector; 6. Exhaust fan; 7. Chimney; 8. Flue gas recirculation fan; 9. Primary air fan A; 10. Primary air fan B; 11. Secondary air fan A; 12. Secondary air fan B; 13. Warm air fan A; 14. Warm air fan B; 15. Warm air fan C; 16. Warm air fan D; 17. Recirculated flue gas flow meter A; 18. Recirculated flue gas flow meter B; 19. Recirculated flue gas flow meter C; 20. Recirculated flue gas flow meter D; 21. Recirculated flue gas regulating valve A; 22. Recirculated flue gas regulating valve B; 2 3. Recirculating flue gas regulating valve C; 24. Recirculating flue gas regulating valve D; 25. Recirculating flue gas shut-off valve; 26. Flue gas recirculation pipeline; 27. Coal feeder sealing fan; 28. Sealing fan outlet shut-off valve; 29. ​​Sealing air shut-off valve; 30. Coal feeder; 31. Cold primary air pipeline; 32. Cold secondary air pipeline; 33. Hot secondary air pipeline; 34. Hot primary air pipeline; 35. Desulfurization flue gas recirculation pipeline; Y. Flue gas connection point; F1. Flue gas branch point; S1. Flue gas inlet point A; S2. Flue gas inlet point B; S3. Flue gas inlet point C; S4. Flue gas inlet point D. Detailed Implementation

[0025] The present invention will be further described below with reference to the accompanying drawings and embodiments. Example

[0026] like Figure 1 As shown, a circulating fluidized bed boiler flue gas recirculation system includes a boiler 1, a cyclone separator 2, an air preheater 3, a desulfurization tower 4, a dust collector 5, an induced draft fan 6, and a chimney 7. The boiler 1 is connected to the cyclone separator 2, the cyclone separator 2 is connected to the air preheater 3, the air preheater 3 is connected to the desulfurization tower 4, the desulfurization tower 4 is connected to the dust collector 5, the dust collector 5 is connected to the induced draft fan 6, and the induced draft fan 6 is connected to the chimney 7.

[0027] It also includes a flue gas recirculation fan 8, a primary air treatment system, a secondary air treatment system, a flue gas recirculation pipeline 26, and a coal feeder sealing air system. The inner wall of the flue gas recirculation pipeline 26 is provided with an anti-corrosion coating to achieve the pipeline's own anti-corrosion. Drain valves are installed at all low points of the pipeline and at intervals of 15-30 meters along the route to ensure timely discharge of condensate and avoid inevitable corrosion caused by long-term accumulation of condensate. The pipeline adopts a natural compensation method along the route to ensure the cleanliness of the recirculation flue. The flue gas recirculation fan 8 is a single variable frequency fan with a calculated flue gas recirculation flow rate of 110%.

[0028] A flue gas inlet point Y is provided on the flue between the induced draft fan 6 and the chimney 7. The air inlet of the flue gas recirculation fan 8 is connected to the flue gas inlet point Y through the flue gas recirculation pipeline 26. A recirculation flue gas shut-off valve 25 is provided on the pipeline between the two. The recirculation flue gas shut-off valve 25 is a slide gate valve with a clamping device, which can achieve zero leakage and prevent the flue gas recirculation fan 8 and the flue gas recirculation pipeline 26 from condensing due to the small amount of flue gas and heat loss during system shutdown, which would lead to equipment and pipeline corrosion. A flue gas branch point F1 is provided near the center line of the boiler 1. The flue gas branch point F1 is connected to the air outlet of the flue gas recirculation fan 8 through the flue gas recirculation pipeline 26.

[0029] The primary air treatment system includes primary air fan A-9, primary air fan B-10, warm air fan A-13, warm air fan B-14, recirculating flue gas regulating valve A-21, recirculating flue gas regulating valve B-22, cold primary air duct 31, and hot primary air duct 34. The flue gas recirculation duct 26 is symmetrically divided into two paths after passing through the flue gas branch point F1. One path is connected sequentially to the recirculating flue gas regulating valve A-21 and the air inlet of primary air fan A-9. The primary air fan A-9 and the recirculating... A flue gas inlet point A-S1 is provided on the pipeline between the flue gas regulating valve A-21 and the recirculating flue gas regulating valve A-21. The heater A-13 is connected to the flue gas inlet point A-S1. A recirculating flue gas flow meter A-17 is provided on the pipeline between the flue gas inlet point A-S1 and the recirculating flue gas regulating valve A-21. Another path is connected in sequence to the recirculating flue gas regulating valve B-22 and the air inlet of the primary air fan B-10. A flue gas meter is provided on the pipeline between the primary air fan B-10 and the recirculating flue gas regulating valve B-22. The flue gas inlet point B-S2 is connected to the warm air blower B-14. A recirculating flue gas flow meter B-18 is installed on the pipeline between the flue gas inlet point B-S2 and the recirculating flue gas regulating valve B-22. The outlets of the primary air blower A-9 and B-10 are connected and merged via a cold primary air pipeline 31, then connected to the cold primary air interface of the air preheater 3. One end of the hot primary air pipeline 34 is connected to the hot primary air interface of the air preheater 3, and the other end is connected to the boiler. The primary air box connection at the bottom of boiler 1 and the symmetrical arrangement of the pipelines facilitate the balanced regulation of the flue gas flow rate supplied to the primary air fans A-9, B-10, A-11, and B-12 on both sides of boiler 1. This is beneficial for the balanced control of oxygen content and bed temperature on both sides of the boiler, and avoids uneven oxygen content in the fans on both sides due to uneven flue gas supply, which can lead to uneven combustion and uneven stress on the boiler heating surface, thus causing unit operation safety issues.

[0030] The secondary air treatment system includes secondary air fan A-11, secondary air fan B-12, warm air fan C-15, warm air fan D-16, recirculating flue gas regulating valve C-23, and recirculating flue gas regulating valve D-24. Two flue gas recirculation pipelines 26 branch off from the flue gas branch point F1, each branching into two parallel branches. One branch connects sequentially to the inlet of recirculating flue gas regulating valve C-23 and secondary air fan A-11. A flue gas inlet point C-S3 is installed on the pipeline between secondary air fan A-11 and recirculating flue gas regulating valve C-23. Warm air fan C-15 is connected to flue gas inlet point C-S3. A recirculating flue gas flow meter C is installed on the pipeline between flue gas inlet point C-S3 and recirculating flue gas regulating valve C-23. -19, another branch is connected in sequence to the air inlet of the recirculating flue gas regulating valve D-24 and the secondary air fan B-12. The pipe between the secondary air fan B-12 and the recirculating flue gas regulating valve D-24 is equipped with a flue gas inlet point D-S4. The warm air fan D-16 is connected to the flue gas inlet point D-S4. The pipe between the flue gas inlet point D-S4 and the recirculating flue gas regulating valve D-24 is equipped with a recirculating flue gas flow meter D-20. The air outlets of the secondary air fan A-11 and the secondary air fan B-12 are connected and merged through the cold secondary air pipe 32, and then connected to the cold secondary air interface of the air preheater 3. One end of the hot secondary air pipe 33 is connected to the hot secondary air interface of the air preheater 3, and the other end is connected to the secondary air nozzle of the boiler 1.

[0031] The coal feeder sealing air system includes a coal feeder sealing fan 27 and a coal feeder 30. The coal feeder sealing fan 27 operates at industrial frequency and is used in a one-in-one-outstand configuration. The outlet of the coal feeder sealing fan 27 is connected to the coal feeder 30, and the coal feeder 30 is connected to the coal feed port of the boiler 1. A sealing fan outlet shut-off valve 28 is installed on the pipeline between the two. A branch pipe is branched off from the combined cold primary air pipeline 31 and connected to the pipeline between the outlet of the coal feeder sealing fan 27 and the coal feeder 30. A sealing air shut-off valve 29 is installed on the branch pipe.

[0032] The recirculated flue gas shut-off valve 25 can also be a butterfly valve with a soft seal.

[0033] The heat source for heaters A-13, B-14, C-15 and D-16 is steam or primary and secondary hot air circulation generated by boiler 1.

[0034] The flue gas recirculation fan 8 can also be a dual variable frequency fan, with the flue gas recirculation flow rate calculated at 60% and the fan head selected based on 120% of the design system resistance. Example

[0035] like Figure 2As shown, the difference between this embodiment and embodiment 1 is that a desulfurization flue gas recirculation pipeline 35 is connected between the pipeline between the air preheater 3 and the desulfurization tower 4 and the pipeline between the induced draft fan 6 and the chimney 7. The flue gas inlet point Y is set on the desulfurization flue gas recirculation pipeline 35. Other structures are the same.

[0036] The operation process of this utility model:

[0037] When this utility model is in operation, firstly, start the coal feeder sealing fan 27, open the sealing fan outlet shut-off valve 28, and close the sealing air shut-off valve 29. The coal feeder sealing fan 27 provides sufficient sealing air to the coal feeder 30 to prevent hot air backflow caused by excessive bed pressure at the bottom of boiler 1 and to avoid damage to the coal feeder 30. The coal feeder 30 supplies coal to boiler 1. When the ambient temperature is below 10~15℃, start the warm air blowers A-13, B-14, C-15, and D-16 to ensure that the flue gas / air mixture supplied to the primary air blower is adequate. Before operating the system, if the temperature is 20°C higher than the dew point temperature, carefully check to ensure that the recirculated flue gas regulating valves A-21, B-22, C-23, D-24, and 25 are closed. Then start the flue gas recirculation fan 8. After the operation is stable, gradually open the recirculated flue gas regulating valves A-21, B-22, C-23, and D-24, and finally open the recirculated flue gas shut-off valve 25.

[0038] After the system starts to operate stably, the generated flue gas is discharged from the chimney 7 through the flue along the flue after being driven by the induced draft fan 6. Part of the flue gas, under the action of the flue gas recirculation fan 8, enters the primary air treatment system and the secondary air treatment system from the flue gas inlet point Y along the flue gas recirculation pipeline 26 and through the flue gas branch point F1 respectively. After being treated by the primary air treatment system, the flue gas enters the air preheater 3 for heating along the cold primary air pipeline 31, and then enters the primary air box at the bottom of the boiler 1 through the hot primary air pipeline 34. It mainly performs bed fluidization and provides some oxygen for fuel combustion. After being treated by the primary air treatment system, the low oxygen flue gas replaces part of the original fresh air, which solves the problem of high operating oxygen caused by high fluidization air volume under low load operating conditions of the circulating fluidized bed boiler.

[0039] After being treated by the secondary air treatment system, the flue gas enters the air preheater 3 through the cold secondary air duct 32 for heating, and then enters the boiler 1 through the hot secondary air duct 33. It mainly serves as burnout air to supplement the oxygen in the furnace combustion, thereby improving boiler efficiency. After being treated by the secondary air treatment system, the low-oxygen flue gas replaces part of the original fresh air. While reducing the operating oxygen content, it increases the secondary air flow rate, which solves the problems of small cooling air volume, difficult distribution, and easy sintering of nozzles in the secondary air nozzles of the circulating fluidized bed boiler under low load operation conditions, and ensures the safe operation of the unit with low oxygen and low nitrogen.

[0040] After the initial cycle treatment by the system, the flue gas content generated in subsequent cycles is reduced, meeting emission standards.

[0041] After the system finishes running, the circulating flue gas shut-off valve 25 and the flue gas recirculation fan 8 are closed in sequence; the flue gas recirculation pipeline 26 is disconnected, and then the recirculating flue gas regulating valves A-21, B-22, C-23, and D-24 are closed one by one. The sealing air shut-off valve 29 is opened, and the warm air fans C-15 and D-16, the coal machine sealing fan 27, and the sealing fan outlet shut-off valve 28 are closed in sequence. At this time, the warm air fans A-13 and B-14 deliver fresh air to the cold primary air pipeline 31 through the flue gas inlet points A-S1 and B-S2, respectively, and deliver it to the coal feeder 30 after passing through the sealing air shut-off valve 29 along the branch.

Claims

1. A circulating fluidized bed boiler flue gas recirculation system, comprising a boiler (1), a cyclone separator (2), an air preheater (3), a desulfurization tower (4), a dust collector (5), an induced draft fan (6) and a chimney (7), the boiler (1) is connected with the cyclone separator (2), the cyclone separator (2) is connected with the air preheater (3), the air preheater (3) is connected with the desulfurization tower (4), the desulfurization tower (4) is connected with the dust collector (5), the dust collector (5) is connected with the induced draft fan (6), and the induced draft fan (6) is connected with the chimney (7); characterized in that It also includes a flue gas recirculation fan (8), a primary air treatment system, a secondary air treatment system, a flue gas recirculation pipeline (26) and a coal feeder sealing air system, a corrosion-resistant coating is arranged on the inner wall of the pipeline of the flue gas recirculation pipeline (26), and a drain valve is arranged at all low points of the pipeline and at intervals of 15-30 meters along the pipeline; A flue gas connection point (Y) is arranged on the flue between the induced draft fan (6) and the chimney (7), the air inlet of the flue gas recirculation fan (8) is connected with the flue gas connection point (Y) through the flue gas recirculation pipeline (26), a recirculated flue gas shutoff valve (25) is arranged on the pipeline between the two, a flue gas branch point (F1) is arranged on the ground near the center line of the boiler (1), and the flue gas branch point (F1) is connected with the air outlet of the flue gas recirculation fan (8) through the flue gas recirculation pipeline (26); The primary air treatment system comprises a primary air fan A (9), a primary air fan B (10), a warm air fan A (13), a warm air fan B (14), a recirculated flue gas regulating valve A (21), a recirculated flue gas regulating valve B (22), a cold primary air pipeline (31) and a hot primary air pipeline (34), the flue gas recirculation pipeline (26) is divided into two routes after the flue gas branch point (F1), one route is sequentially connected with the recirculated flue gas regulating valve A (21) and the air inlet of the primary air fan A (9), and the other route is sequentially connected with the recirculated flue gas regulating valve B (22) and the air inlet of the primary air fan B (10), the air outlets of the primary air fan A (9) and the primary air fan B (10) are connected and combined through the cold primary air pipeline (31) and then connected to the cold primary air interface of the air preheater (3), one end of the hot primary air pipeline (34) is connected to the hot primary air interface of the air preheater (3), and the other end is connected to the primary air box at the bottom of the boiler (1), a flue gas feeding point A (S1) is arranged on the pipeline between the primary air fan A (9) and the recirculated flue gas regulating valve A (21), the warm air fan A (13) is connected with the flue gas feeding point A (S1), a flue gas feeding point B (S2) is arranged on the pipeline between the primary air fan B (10) and the recirculated flue gas regulating valve B (22), and the warm air fan B (14) is connected with the flue gas feeding point B (S2). The secondary air treatment system comprises a secondary air fan A (11), a secondary air fan B (12), a warm air fan C (15), a warm air fan D (16), a recirculated flue gas regulating valve C (23) and a recirculated flue gas regulating valve D (24), two recirculated flue gas pipelines (26) are branched out after the flue gas branch point (F1) and are respectively branched out into two branches, one of the two branches is connected with the recirculated flue gas regulating valve C (23) and the air inlet of the secondary air fan A (11) in sequence, and the other branch is connected with the recirculated flue gas regulating valve D (24) and the air inlet of the secondary air fan B (12) in sequence, the air outlets of the secondary air fan A (11) and the secondary air fan B (12) are connected and combined through a cold secondary air pipeline (32) and are connected to a cold secondary air interface of an air preheater (3), a hot secondary air interface of the air preheater (3) is connected to a secondary air spout of a boiler (1) through a hot secondary air pipeline (33), a flue gas feeding point C (S3) is arranged on a pipeline between the secondary air fan A (11) and the recirculated flue gas regulating valve C (23), the warm air fan C (15) is connected to the flue gas feeding point C (S3), a flue gas feeding point D (S4) is arranged on a pipeline between the secondary air fan B (12) and the recirculated flue gas regulating valve D (24), the warm air fan D (16) is connected to the flue gas feeding point D (S4), and a heat source of the warm air fan A (13), the warm air fan B (14), the warm air fan C (15) and the warm air fan D (16) is steam or a primary air and secondary air hot air circulation heating produced by the boiler (1). The coal feeder sealing air system comprises a coal feeder sealing air fan (27) and a coal feeder (30), the air outlet of the coal feeder sealing air fan (27) is connected to the coal feeder (30), the coal feeder (30) is connected to a coal feeding port of the boiler (1), a sealing air fan outlet shutoff valve (28) is arranged on a pipeline between the two, a pipeline after being combined of a cold primary air pipeline (31) is branched into a branch pipeline connected to the pipeline between the air outlet of the coal feeder sealing air fan (27) and the coal feeder (30), and a sealing air shutoff valve (29) is arranged on the branch pipeline.

2. A flue gas recirculation system for a circulating fluidized bed boiler according to claim 1, characterized in that A desulfurized flue gas recirculation pipeline (35) is connected between a pipeline between the air preheater (3) and the desulfurizing tower (4) and a pipeline between the induced draft fan (6) and the chimney (7), and the flue gas induction point (Y) is arranged on the desulfurized flue gas recirculation pipeline (35).

3. A flue gas recirculation system for a circulating fluidized bed boiler according to claim 1 or 2, characterized in that A recirculated flue gas flow meter A (17) is arranged on a pipeline between the flue gas feeding point A (S1) and the recirculated flue gas regulating valve A (21), a recirculated flue gas flow meter B (18) is arranged on a pipeline between the flue gas feeding point B (S2) and the recirculated flue gas regulating valve B (22), a recirculated flue gas flow meter C (19) is arranged on a pipeline between the flue gas feeding point C (S3) and the recirculated flue gas regulating valve C (23), and a recirculated flue gas flow meter D (20) is arranged on a pipeline between the flue gas feeding point D (S4) and the recirculated flue gas regulating valve D (24).

4. A flue gas recirculation system for a circulating fluidized bed boiler according to claim 3, characterized in that The recirculated flue gas shutoff valve (25) is a butterfly valve with a soft seal or a plug valve with a clamping device.

5. A flue gas recirculation system for a circulating fluidized bed boiler according to claim 3, characterized in that The flue gas recirculation fan (8) is single or double variable frequency fan.

6. A flue gas recirculation system for a circulating fluidized bed boiler according to claim 3, characterized in that The coal feeder sealing fan (27) is operated at a working frequency and adopts a one-for-one standby structure.

7. A flue gas recirculation system for a circulating fluidized bed boiler according to claim 3, characterized in that The primary air treatment pipeline and the secondary air treatment system pipeline branched after the flue gas branch point (F1) are symmetrically arranged on the left and right sides of the flue gas branch point (F1) as a center point.

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