Circulating fluidized bed flue gas recirculation system and circulating method thereof

Abstract

The invention provides a circulating fluidized bed flue gas recirculation system and a circulating method thereof, and relates to the technical field of boiler flue gas treatment.The circulating fluidized bed flue gas recirculation system is characterized in that an outlet of a second pipeline is sequentially connected with a waste heat exchanger, a recirculation fan and a flow distributor, and two outlets of the flow distributor are a dense-phase zone and a dilute-phase zone respectively; the two branch pipelines of the dense-phase area are sequentially communicated with an electric proportional adjusting baffle, a warm air blower, a primary air fan and a preheater, the preheater is connected into an air chamber, an arc-shaped flow guide plate is arranged in the air chamber in the length direction, and a wear-resistant composite air cap is installed on an air distribution plate from the air chamber to the hearth; and stop valves are arranged on an outlet pipeline of the dilute-phase region, a third pipeline and a flue gas bypass, so that the problem that the denitration efficiency and the flue gas temperature regulation and control effect are poor under the low-load working condition is solved, and the purposes that the flue gas is injected into the dilute-phase region to directly dilute the oxygen concentration of the high-temperature region, meanwhile, reducing components are supplemented, NOx generation is inhibited, and the denitration efficiency is improved are achieved. And the technical effect of ultra-low emission requirements is met.

Description

Technical Field

[0001] This invention relates to the field of boiler flue gas treatment technology, and in particular to a circulating fluidized bed flue gas recirculation system and its circulation method. Background Technology

[0002] Circulating fluidized bed boilers have become the mainstream combustion equipment in coal-fired power plants due to their advantages such as wide fuel adaptability, high combustion efficiency, and low initial pollutant emissions. Flue gas recirculation technology is used to regulate the combustion atmosphere in the furnace and reduce nitrogen oxides (NOx). x This method, which generates and improves the stability of boilers under low load operation, is widely used in the ultra-low emission retrofit and energy-saving optimization of circulating fluidized bed boilers.

[0003] Chinese patent CN120926434A discloses a flue gas recirculation system for low-load circulating fluidized bed boilers. The system mainly includes a furnace, induced draft fan, recirculation fan, connecting pipes, air chamber, conventional carbon steel air caps, and preheating and heat exchange auxiliary equipment. The flue gas circulation path is as follows: flue gas from the induced draft fan outlet is drawn out through pipes, pressurized by the recirculation fan, and injected into the bottom of the dense phase zone of the furnace only through the air chamber, air distribution plate, and conventional air caps. The mixing ratio of recirculated flue gas and primary air relies solely on fan flow rate adjustment. The system lacks an independent flue gas waste heat recovery device, and the air chamber does not have a fluidization and homogenization guiding structure. Equipment shutdown and corrosion protection are achieved only through flue gas bypass and secondary hot air backflow.

[0004] Therefore, existing technologies suffer from a single recirculated flue gas injection point, concentrated only in the dense phase region of the furnace, and do not address NO. x Setting up an independent injection channel in the dilute phase region of the core generation area makes it impossible to accurately enhance the reducing atmosphere in the high-temperature core area, resulting in poor denitrification efficiency and flue gas temperature control under low load conditions. Summary of the Invention

[0005] The purpose of this invention is to provide a circulating fluidized bed flue gas recirculation system and method to alleviate the technical problems of poor denitrification efficiency and flue gas temperature control under low load conditions in the prior art.

[0006] The circulating fluidized bed flue gas recirculation system provided by the present invention includes: a furnace, a first pipe, an induced draft fan and a chimney arranged sequentially along the flue gas flow direction, the outlet of the induced draft fan being connected to a flue gas bypass through a second pipe, the flue gas bypass being connected to the first pipe as a shutdown protection, and a third pipe being connected to the second pipe as hot secondary air. The outlet of the second pipeline is connected in sequence to a waste heat exchanger, a recirculation fan, and a flow distributor. The two outlets of the flow distributor are the dense phase outlet pipeline and the dilute phase outlet pipeline, respectively. The outlet pipeline of the dense phase zone is equipped with two branch pipelines. Each branch pipeline is connected in sequence to an electric proportional regulating baffle, a warm air fan, a primary air fan, and a preheater. The preheater is connected to an air chamber. Inside the air chamber, an arc-shaped guide plate is arranged along the length direction. A wear-resistant composite air cap is installed on the air distribution plate from the air chamber to the furnace. The dilute phase zone outlet pipe is connected to the dilute phase zone of the furnace, and shut-off valves are installed on the dilute phase zone outlet pipe, the third pipe, and the flue gas bypass.

[0007] Furthermore, the waste heat exchanger is a finned tube flue gas-air heat exchanger, used to preheat the primary air using the waste heat of the recirculated flue gas.

[0008] Furthermore, the arc-shaped guide vanes are spaced 2 to 3 meters apart within the air chamber, dividing the air chamber into 3 to 4 independent fluidization zones; The connection point of the outlet pipe in the dilute phase zone is located 6 to 8 meters away from the air distribution plate inside the furnace, corresponding to a furnace temperature of 850℃ to 950℃.

[0009] Furthermore, the recirculation fan is connected to an inert gas storage tank.

[0010] The present invention provides a circulating fluidized bed flue gas recirculation method, which uses the above-mentioned recirculation system and includes the following steps: Step 1: Under low load conditions, a portion of the flue gas is drawn from the boiler induced draft fan outlet and introduced into the second pipe. Step 2: After the flue gas in the second pipeline is preheated by the waste heat exchanger, it is pressurized by the recirculation fan and then distributed to the dense phase zone outlet pipeline by the flow distributor at a ratio of 60% to 70% and to the dilute phase zone outlet pipeline at a ratio of 30% to 40%. Step 3: The flue gas from the outlet duct of the dense phase zone is mixed with the primary air after the ratio is controlled by the electric proportional adjustment baffle. It is then heated by the warm air blower, the primary air blower, and the preheater before being sent into the air chamber. Step 4: The mixed airflow in the air chamber is guided by the arc-shaped guide plate and then evenly injected into the dense phase zone of the furnace through the wear-resistant composite air cap; Step 5: After being regulated by the shut-off valve, the flue gas from the dilute phase zone outlet pipe is directly injected into the high-temperature core area of ​​the dilute phase zone in the furnace. Step 6: When the system is shut down, close the outlet pipe of the dilute phase zone and open the flue gas bypass and the secondary hot air backflow pipe of the third pipe to prevent condensate corrosion.

[0011] Furthermore, the calculation formula for the recirculated flue gas distribution in the dense phase zone outlet pipe and the dilute phase zone outlet pipe in step 2 is as follows: (1); (2); in, For dense phase region flow; For the dilute phase region flow; Total circulating flue gas flow rate; The proportion of flue gas in the dense phase region; The proportion of flue gas in the rare phase region.

[0012] Furthermore, the amount of waste heat recovered by the waste heat exchanger in step 2 is: (3); in, The specific heat at constant pressure of flue gas; The specific heat of air at constant pressure. For flue gas quality, For air quality, The value for flue gas cooling. This represents the air temperature rise value.

[0013] Furthermore, in step 3, the adjustment accuracy of the electric proportional regulating baffle is ±1°, and the calculation formula for the primary air in the electric proportional regulating baffle is: (4); in, The air volume required for theoretical combustion. The primary air excess coefficient is ≤1.05.

[0014] Furthermore, in step 4, the wear-resistant composite wind cap is made of silicon carbide chromium alloy, and the formula for calculating the wind speed at the small hole of the wear-resistant composite wind cap is: (5); in, is the total air volume of the air chamber; n is the number of air caps for a single boiler; The radius of a single hole is 5mm; The flow coefficient is 0.85.

[0015] Furthermore, step 6 includes the following specific steps: Step 6.1: Close the shut-off valve on the outlet pipe of the dilute phase zone to stop the injection of flue gas into the dilute phase zone; Arrangement 6.2: Open the shut-off valve on the flue gas bypass and close the valve on the second pipeline; open the shut-off valve on the third pipeline to allow secondary hot air to be introduced into the second pipeline to prevent flue gas condensate from corroding the pipeline and equipment.

[0016] Beneficial effects: The circulating fluidized bed flue gas recirculation system and method provided by this invention recovers waste heat from the recirculated flue gas through a pre-heat exchanger and achieves differentiated injection of flue gas into the dense phase zone and the dilute phase zone through a flow distributor. Combined with an electric proportional adjustment baffle, the mixing ratio of flue gas and primary air is precisely controlled. This can specifically enhance the reducing atmosphere in the NOx generation core zone of the furnace dilute phase zone, reducing NOx emissions at the source. At the same time, it can accurately adapt to high and low load conditions to stabilize the oxygen content in the furnace, effectively improve denitrification efficiency, reduce the heat load of the preheater and the energy consumption of the primary air fan, and improve the overall thermal efficiency of the boiler.

[0017] Furthermore, the arc-shaped baffle plate inside the air chamber of this invention, combined with the wear-resistant composite air cap, can optimize the uniformity of air volume distribution in the air chamber, solve the problems of bed material stratification, poor slag discharge, and bed pressure fluctuation. The wear-resistant composite air cap significantly reduces equipment wear caused by dusty flue gas and significantly extends service life. This invention also adds a finned tube waste heat exchanger before the recirculator inlet, using 120-150℃ recirculated flue gas to preheat the primary air, recover waste heat from the flue gas, reduce the power consumption of the primary air fan, and improve the system thermal efficiency. Attached Figure Description

[0018] To more clearly illustrate the specific embodiments of the present invention or the technical solutions in the prior art, the drawings used in the description of the specific embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are some embodiments of the present invention. For those skilled in the art, other drawings can be obtained from these drawings without creative effort.

[0019] Figure 1 This is a connection diagram of a circulating fluidized bed flue gas recirculation system provided in an embodiment of the present invention; Figure 2 A flowchart of a circulating fluidized bed flue gas reprocessing method provided in an embodiment of the present invention.

[0020] Icons: 1-Furnace; 2-First Pipeline; 3-Induced Draft Fan; 4-Chimney; 5-Second Pipeline; 6-Flue Gas Bypass; 7-Third Pipeline; 8-Waste Heat Exchanger; 9-Recirculation Fan; 10-Flow Distributor; 11-Dense Phase Zone Outlet Pipeline; 12-Dilute Phase Zone Outlet Pipeline; 13-Branch Pipeline; 14-Electric Proportional Adjustment Baffle; 15-Warm Air Fan; 16-Primary Air Fan; 17-Preheater; 18-Air Chamber; 19-Arc-Shaped Baffle Plate; 20-Wear-Resistant Composite Air Cap; 21-Inert Gas Storage Tank. Detailed Implementation

[0021] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. The components of the embodiments of the present invention described and shown in the accompanying drawings can generally be arranged and designed in various different configurations.

[0022] Therefore, the following detailed description of the embodiments of the invention provided in the accompanying drawings is not intended to limit the scope of the claimed invention, but merely to illustrate selected embodiments of the invention. All other embodiments obtained by those skilled in the art based on the embodiments of the invention without inventive effort are within the scope of protection of the invention.

[0023] It should be noted that similar labels and letters in the following figures indicate similar items. Therefore, once an item is defined in one figure, it does not need to be further defined and explained in subsequent figures.

[0024] In the description of this invention, it should be noted that the terms "center," "upper," "lower," "left," "right," "vertical," "horizontal," "inner," and "outer," etc., indicate the orientation or positional relationship based on the orientation or positional relationship shown in the accompanying drawings, or the orientation or positional relationship commonly used when the product of this invention is in use. They are only for the convenience of describing this invention and simplifying the description, and do not indicate or imply that the device or element 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 invention. In addition, the terms "first," "second," "third," etc., are only used to distinguish descriptions and should not be construed as indicating or implying relative importance.

[0025] Furthermore, terms such as "horizontal," "vertical," and "sag" do not imply that components must be absolutely horizontal or suspended, but rather that they can be slightly tilted. For example, "horizontal" simply means that its direction is more horizontal relative to "vertical," and does not mean that the structure must be completely horizontal, but can be slightly tilted.

[0026] In the description of this invention, it should also be noted that, unless otherwise explicitly specified and limited, the terms "set," "install," "connect," and "link" should be interpreted broadly. For example, they can refer to a fixed connection, a detachable connection, or an integral connection; they can refer to a mechanical connection or an electrical connection; they can refer to a direct connection or an indirect connection through an intermediate medium; and they can refer to the internal connection of two components. Those skilled in the art can understand the specific meaning of the above terms in this invention based on the specific circumstances.

[0027] The following detailed description of some embodiments of the present invention is provided in conjunction with the accompanying drawings. Unless otherwise specified, the following embodiments and features can be combined with each other.

[0028] Example 1 like Figure 1 As shown, the circulating fluidized bed flue gas recirculation system provided by the present invention includes: a furnace 1, a first pipe 2, an induced draft fan 3 and a chimney 4 arranged sequentially along the flue gas flow direction in the furnace 1, the outlet of the induced draft fan 3 is connected to a flue gas bypass 6 through a second pipe 5, the flue gas bypass 6 is connected to the first pipe 2 as a shutdown protection, and a third pipe 7 is also connected to the second pipe 5 as hot secondary air. The outlet of the second pipe 5 is connected in sequence to the waste heat exchanger 8, the recirculation fan 9 and the flow distributor 10. The two outlets of the flow distributor 10 are the dense phase outlet pipe 11 and the dilute phase outlet pipe 12, respectively. The dense phase zone outlet pipe 11 is provided with two branch pipes 13. Each branch pipe 13 is connected in sequence to an electric proportional adjustment baffle 14, a warm air fan 15, a primary air fan 16, and a preheater 17. The preheater 17 is connected to an air chamber 18. Inside the air chamber 18, an arc-shaped guide plate 19 is arranged along the length direction. A wear-resistant composite air cap 20 is installed on the air distribution plate from the air chamber 18 to the furnace 1. The dilute phase zone outlet pipe 12 is connected to the dilute phase zone of the furnace 1. The dilute phase zone outlet pipe 12, the third pipe 7, and the flue gas bypass 6 are all equipped with shut-off valves.

[0029] Specifically, the flue gas outlet of furnace 1 is connected sequentially to the first pipe 2, the induced draft fan 3, and the chimney 4 along the main flue gas flow direction. These three components constitute the main flue gas exhaust path of the boiler foundation, completing the routine emission of flue gas from the furnace. The outlet end of the induced draft fan 3 branches off to the flue gas bypass 6 through the second pipe 5. The flue gas bypass 6 serves as a dedicated protection pipeline for shutdown, with its end reconnected to the first pipe 2. This enables the flue gas bypass to flow back during shutdown and prevents pipeline corrosion. A third pipe 7 is also reserved on the second pipe 5 section as a dedicated hot secondary air delivery pipeline, used to deliver hot air to the recirculation pipeline and prevent flue gas condensate corrosion. In this invention, a waste heat exchanger 8, a recirculation fan 9, and a flow distributor 10 are sequentially connected in series along the flue gas flow direction at the flue gas outlet end of the second pipe 5. The flow distributor 10, as the core control component for flue gas diversion, has two independent outlets: a dense phase zone outlet pipe 11 for conveying gas to the dense phase zone of the furnace and a dilute phase zone outlet pipe 12 for conveying gas to the dilute phase zone of the furnace. Downstream of the dense phase zone outlet pipe 11, two symmetrically arranged branch pipes 13 are provided. Each branch pipe 13 is equipped with an electrically proportional regulating baffle 14 and a warm air supply along the flue gas flow direction. The machine 15, primary air fan 16 and preheater 17 are stably connected to the air chamber 18 at the bottom of the furnace 1. Multiple sets of arc-shaped guide plates 19 are arranged at equal intervals along the length of the air chamber 18. Wear-resistant composite air caps 20 are evenly installed at the connection position between the air chamber 18 and the air distribution plate of the furnace 1. The end of the dilute phase zone outlet pipe 12 is directly connected to the core position of the dilute phase zone inside the furnace 1. At the same time, the dilute phase zone outlet pipe 12, the third pipe 7 and the flue gas bypass 6 are all equipped with shut-off valves to control the on / off of the pipes and regulate the flow of the medium.

[0030] Relying on the waste heat exchanger 8 connected in series with the second pipeline 5, the low-temperature waste heat of the recirculated flue gas can be efficiently recovered and used to preheat the primary air, reducing the heat load of the preheater 17 and the power consumption of the primary air fan 16, and simultaneously improving the overall thermal efficiency of the system; the flow distributor 10 accurately distributes the recirculated flue gas to the dense phase zone outlet pipeline 11 and the dilute phase zone outlet pipeline 12, realizing differentiated flue gas injection in the dense phase zone and dilute phase zone, directly strengthening the reducing atmosphere in the NOx generation core zone and reducing the original emissions of nitrogen oxides; the electric proportional regulating baffle 14 on the branch pipeline 13 can precisely control the mixing ratio of recirculated flue gas and primary air, adapting to the oxygen control requirements under different load conditions, and solving the industry pain point of excessive primary air and inability to form a reducing atmosphere under low load conditions; the arc-shaped guide plate 19 in the air chamber 18, combined with the wear-resistant composite air cap 20, optimizes the fluidization uniformity inside the air chamber, reduces the wear rate of the air cap, and extends the service life of the equipment.

[0031] It should be noted that the main flow direction of the flue gas in this invention is furnace 1, first pipe 2, induced draft fan 3, and chimney 4. The flow direction of the recirculated flue gas is induced draft fan 3 outlet, second pipe 5, waste heat exchanger 8, recirculation fan 9, and flow distributor (divided into two paths); the flow direction in the dense phase zone is dense phase zone outlet pipe 11, two branch pipes 13, warm air fan 15, primary air fan 16, air chamber 18, wear-resistant composite air cap 20, and furnace 1 dense phase zone. The flow direction in the dilute phase zone is dilute phase zone outlet pipe 12 and furnace 1 dilute phase zone. The flow direction of the auxiliary process is divided into third pipe 7 (hot secondary air), second pipe 5; inert gas storage tank 21 to recirculation fan 9; and flue gas bypass 6 to first pipe 2.

[0032] It should be noted that existing technologies lack a proportional control device, and the proportion of recirculated flue gas depends on the fan flow rate adjustment. At low load (30% THA), the total primary air volume is still 15% higher than the combustion demand, and the oxygen content remains stable above 3.0%, failing to form a reducing atmosphere and exacerbating NOx formation. This invention adds an electrically operated proportional control baffle 14 at the pipe connection to control the proportion of recirculated flue gas (10-30% adjusted according to the load), reducing excessive primary air supply.

[0033] It should be noted that existing technologies use conventional carbon steel wind caps. When recirculated flue gas contains a small amount of dust, the wind velocity at the small holes of the wind cap exceeds 50 m / s after mixing with the primary air, resulting in a wear rate of 0.15 mm / 1000 h and a lifespan of only 2 years. This invention presents a silicon carbide-chromium alloy wear-resistant composite wind cap 20, which stabilizes the wind velocity within the optimal range. The new material has a hardness of HRC≥60, improving wear resistance by 5 times and extending lifespan.

[0034] Example 2 In an embodiment of the present invention, the waste heat exchanger 8 is a finned tube flue gas-air heat exchanger, used to preheat the primary air using the waste heat of the recirculated flue gas.

[0035] The arc-shaped guide vanes 19 are spaced 2 to 3 m apart in the air chamber 18, dividing the air chamber 18 into 3 to 4 independent fluidization zones; The access point of the dilute phase zone outlet pipe 12 is located inside the furnace 1 at a distance of 6 to 8 meters from the air distribution plate, corresponding to a temperature of 850°C to 950°C inside the furnace 1.

[0036] The recirculation fan 9 is connected to an inert gas storage tank 21.

[0037] Specifically, the waste heat exchanger 8 is a finned tube flue gas-air heat exchanger, located between the second pipe 5 and the recirculation fan 9, used to recover the waste heat of the recirculated flue gas and preheat the primary air; the inlet side of the recirculation fan 9 is connected to an inert gas storage 21, which can supplement inert gas into the recirculated flue gas according to the combustion control requirements; arc-shaped guide plates 19 are installed along the length of the air chamber 18, and the spacing of the arc-shaped guide plates 19 in the air chamber 18 is 2-3m, dividing the air chamber 18 into 3-4 independent fluidization zones; the flue gas output end of the dilute phase zone outlet pipe 12 is connected to the interior of the furnace 1, and its connection point is located in the furnace 1 at a distance of 6-8m from the air distribution plate, which corresponds to the high-temperature reaction zone of 850℃-950℃ in the furnace 1.

[0038] The waste heat exchanger 8 is set as a finned tube flue gas-air heat exchanger, which can efficiently recover the low-temperature waste heat of the recirculated flue gas, increase the primary air inlet temperature, reduce the heat load of the preheater 17 and the energy consumption of the primary air fan 16, and improve the overall thermal efficiency of the system. The arc-shaped guide plates 19 with a spacing of 2-3m in the air chamber 18 form an independent fluidization zone, which can make the airflow evenly distributed in the air chamber 18, improve the problem of uneven fluidization, improve the slag discharge efficiency and stabilize the furnace bed pressure. The dilute phase zone outlet pipe 12 can directly enhance the reducing atmosphere in this area, suppress the generation of nitrogen oxides from the source and improve the denitrification effect. The recirculation fan 9 is connected to the inert gas storage tank 21, which can flexibly adjust the oxygen concentration in the furnace to adapt to the low-NOx combustion requirements under special working conditions.

[0039] Example 3 like Figure 2 As shown, the circulating fluidized bed flue gas recirculation method provided by the present invention uses the recirculation system of Example 1 or 2 and includes the following steps: Step 1: Under low load conditions, a portion of the flue gas is drawn from the outlet of the induced draft fan 3 of the boiler and introduced into the second pipe 5; Specifically, when the boiler is operating at a low load of 30% THA (peak load), 10% to 30% of the total flue gas volume is drawn from the outlet of the boiler induced draft fan 3 for recirculation. The drawn flue gas is smoothly introduced into the second pipe 5 to form the main channel for recirculated flue gas. Under the premise of ensuring normal boiler exhaust and stable furnace negative pressure, a stable gas source is provided for subsequent waste heat recovery, flue gas circulation and furnace injection.

[0040] Step 2: After the flue gas in the second pipeline 5 is preheated by the waste heat exchanger 8, it is pressurized by the recirculation fan 9 and then distributed by the flow distributor 10 to the dense phase zone outlet pipeline 11 at 60% to 70% and to the dilute phase zone outlet pipeline 12 at 30% to 40%. The calculation formula for the recirculated flue gas distribution between the dense phase zone outlet pipe 11 and the dilute phase zone outlet pipe 12 is as follows: (1); (2); In the formula, For dense phase region flow; For the dilute phase region flow; Total circulating flue gas flow rate; The proportion of flue gas in the dense phase region; The proportion of flue gas in the rare phase region.

[0041] The waste heat recovery rate of waste heat exchanger 8 is: (3); In the formula, The specific heat at constant pressure of flue gas; The specific heat of air at constant pressure. For flue gas quality, For air quality, The value for flue gas cooling. This represents the air temperature rise value.

[0042] Specifically, the recirculated flue gas in the second pipe 5 preferentially flows through the waste heat exchanger 8, and completes the waste heat exchange between the flue gas and the primary air through the finned tube heat exchange structure, fully recovering the low-temperature waste heat of the recirculated flue gas. After the waste heat exchange, the flue gas is pressurized to the rated working air pressure by the recirculation fan 9 and then sent to the flow distributor 10 for graded diversion. The flow distributor 10 distributes the total recirculated flue gas to the dense phase zone outlet pipe 11 and the dilute phase zone outlet pipe 12 according to the proportion, realizing the differentiated directional transportation of the recirculated flue gas.

[0043] Step 3: The flue gas from the dense phase zone outlet pipe 11 is mixed with the primary air after the ratio is controlled by the electric proportional adjustment baffle 14, and then heated by the heater 15, the primary air fan 16, and the preheater 17 before being sent into the air chamber 18. The adjustment accuracy of the electrically operated proportional adjustment baffle 14 is ±1°, and the calculation formula for the primary air in the electrically operated proportional adjustment baffle 14 is: (4); In the formula, The air volume required for theoretical combustion. The primary air excess coefficient is ≤1.05.

[0044] Step 4: The mixed airflow in the air chamber 18 is guided by the arc-shaped guide plate 19 and uniformly injected into the dense phase zone of the furnace through the wear-resistant composite air cap 20. In step 4, the wear-resistant composite wind cap 20 is made of silicon carbide chromium alloy. The formula for calculating the wind speed through the small holes of the wear-resistant composite wind cap 20 is: (5); In the formula, is the total air volume of the air chamber; n is the number of air caps for a single boiler; The radius of a single hole is 5mm; The flow coefficient is 0.85.

[0045] Specifically, the mixed airflow fed into the air chamber 18 is divided into zones by the arc-shaped guide plate 19 inside the air chamber 18. The arc-shaped guide plate 19 divides the air chamber 18 into 3 to 4 independent fluidization zones, eliminating the problems of airflow eddies and deflection in the air chamber, so that the mixed airflow is evenly distributed and the flow velocity is stable in the air chamber 18. The mixed airflow after being evenly divided is evenly sprayed into the dense phase zone of the furnace 1 through the wear-resistant composite air cap 20 on the air distribution plate of the furnace 1, providing stable power for the fluidization of the bed material in the dense phase zone.

[0046] Step 5: After the flue gas from the dilute phase zone outlet pipe 12 is regulated by the shut-off valve, it is directly injected into the high-temperature core area of ​​the dilute phase zone of the furnace 1. Specifically, the recirculated flue gas delivered to the dilute phase zone outlet pipe 12 is directly injected into the high-temperature core area of ​​the dilute phase zone in the furnace 1 after the flow rate and on / off state are regulated by the shut-off valve on the pipeline. This specifically strengthens the reducing atmosphere in the NOx generation core area, suppresses the generation of nitrogen oxides from the source, and ensures that the boiler meets the ultra-low emission standards.

[0047] Step 6: When the system is shut down, close the dilute phase zone outlet pipe 12 and open the flue gas bypass 6 and the secondary hot air backflow pipe of the third pipe 7 to prevent condensate corrosion.

[0048] Step 6 includes the following specific steps: Step 6.1: Close the shut-off valve on the dilute phase zone outlet pipe 12 to stop the injection of flue gas into the dilute phase zone; Arrangement 6.2: Open the shut-off valve on the flue gas bypass 6 and close the valve on the second pipeline 5; open the shut-off valve on the third pipeline 7 to allow secondary hot air to be introduced into the second pipeline 5 to prevent flue gas condensate from corroding the pipeline and equipment.

[0049] Example 4 In this embodiment, taking the low-load flue gas recirculation of a 220t / h circulating fluidized bed boiler as an example, the dilute phase zone outlet pipe 12 is made of φ400mm Q235B material with a high-temperature resistant and anti-corrosion coating. The injection point is set on the side wall of the dilute phase zone of the furnace 1, 7m away from the air distribution plate. It is equipped with a DN400 shut-off valve with manual and electric linkage, and a flow distributor 10 with a rated flow of 40000m³ / h, so as to realize the distribution of recirculated flue gas into the dense phase zone (65%) and the dilute phase zone (35%).

[0050] All the original wear-resistant composite wind caps with 8 φ12mm small holes were replaced with wear-resistant wind caps made of silicon carbide-chromium alloy (SiC-Cr, HRC≥62). Each wind cap is equipped with 11 φ10mm small holes, with a wind cap height of 150mm, and a total of 800 caps, to ensure that the wind speed through the small holes is stable at 35-40m / s.

[0051] An electric proportional adjustment baffle 14 (adjustment angle 0-90°, accuracy ±1°, Q235B+ wear-resistant coating) is added at the connection between the dense phase zone outlet pipe 11 and the two branch pipes 13. A Venturi flow distributor 10 is added at the fourth pipe branch to accurately control the proportion of recirculated flue gas (25% when THA is 30% and 10% when THA is 100%).

[0052] A finned tube waste heat exchanger 8 is added before the inlet of the recirculation fan 9 in the second pipeline 5. The heat exchange area is 65m², the fin height is 15mm, the spacing is 10mm, and the material is 20G. It is used to preheat the primary air.

[0053] Arc-shaped guide vanes 19 are arranged along the length of the wind chamber 18 at a spacing of 2.5m and an arc radius of 1.8m (Q235B + anti-corrosion coating), dividing the wind chamber 18 into 3 independent zones, each zone corresponding to 267 wear-resistant composite wind caps 20.

[0054] Under low load conditions of 30% THA: Exhaust fan outlet 3 The flue gas (in standard condition, accounting for 25% of the total flue gas volume) enters the second duct 5, and is cooled by 15°C (from 135°C to 120°C) by the waste heat exchanger 8, and the primary air is preheated from 20°C to 30°C.

[0055] The recirculation fan 9 pressurizes the flue gas to 2.5 kPa and then sends it to the flow distributor 10, which divides it into two paths: Pipeline 11 enters the dense phase zone exit area. Enter the dilute phase zone exit pipeline 12.

[0056] The flue gas from the branch pipe 13 of the outlet pipe 11 in the dense phase zone is connected to the primary air via the electrically proportional regulating damper 14. The mixture (in standard state) is heated to 120°C by a warm air blower, then pressurized by a primary air blower 16 and heated to 220°C by a preheater 17 before being sent into the air chamber 18.

[0057] The mixed air in the air chamber is evenly distributed to three zones by the arc-shaped guide plate 19, and then passes through the wear-resistant composite air cap 20. The small-hole air velocity is injected into the dense phase zone of the furnace; the flue gas in the dilute phase zone is directly injected into the high-temperature core zone of 880℃ to enhance the reducing atmosphere.

[0058] During operation, monitor the NOx concentration. If an abnormality is detected, turn on the inert gas storage tank 21 and the recirculation fan 9 to replenish the reducing atmosphere.

[0059] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, and not to limit them; although the present invention 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 or all of the technical features; and these modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the scope of the technical solutions of the embodiments of the present invention.

Claims

1. A circulating fluidized bed flue gas recirculation system, comprising: The furnace (1) is provided with a first pipe (2), an induced draft fan (3), and a chimney (4) in sequence along the flue gas flow direction. The outlet of the induced draft fan (3) is connected to a flue gas bypass (6) through a second pipe (5). The flue gas bypass (6) is connected to the first pipe (2) as a shutdown protection device. A third pipe (7) is also connected to the second pipe (5) as a hot secondary air device. The outlet of the second pipe (5) is connected in sequence to a waste heat exchanger (8), a recirculation fan (9) and a flow distributor (10), and the two outlets of the flow distributor (10) are the dense phase outlet pipe (11) and the dilute phase outlet pipe (12). The dense phase zone outlet pipe (11) is provided with two branch pipes (13). Each branch pipe (13) is connected in sequence to an electric proportional adjustment baffle (14), a warm air fan (15), a primary air fan (16), and a preheater (17). The preheater (17) is connected to an air chamber (18). An arc-shaped guide plate (19) is arranged along the length direction inside the air chamber (18). A wear-resistant composite air cap (20) is installed on the air distribution plate from the air chamber (18) to the furnace (1). The dilute phase zone outlet pipe (12) is connected to the dilute phase zone of the furnace (1), and a shut-off valve is provided on the dilute phase zone outlet pipe (12), the third pipe (7) and the flue gas bypass (6).

2. The recycling system according to claim 1, characterized in that, The waste heat exchanger (8) is a finned tube flue gas air heat exchanger used to preheat primary air using the waste heat of recirculated flue gas.

3. The recycling system according to claim 1, characterized in that, The arc-shaped guide plates (19) are spaced 2 to 3 m apart in the air chamber (18), dividing the air chamber (18) into 3 to 4 independent fluidization zones; The access point of the rare phase zone outlet pipe (12) is located 6 to 8 m away from the air distribution plate inside the furnace (1), corresponding to a temperature of 850℃ to 950℃ inside the furnace (1).

4. The recycling system according to claim 1, characterized in that, The recirculation fan (9) is connected to an inert gas storage tank (21).

5. A method for recirculating flue gas in a circulating fluidized bed, characterized in that, This method uses the recycling system of any one of claims 1-4 and includes the following steps: Step 1: Under low load conditions, a portion of the flue gas is drawn from the outlet of the induced draft fan (3) of the boiler and introduced into the second pipe (5). Step 2: After the flue gas in the second pipeline (5) is preheated by the waste heat exchanger (8), it is pressurized by the recirculation fan (9) and then distributed by the flow distributor (10) to the dense phase zone outlet pipeline (11) at 60% to 70% and to the dilute phase zone outlet pipeline (12) at 30% to 40%. Step 3: The flue gas from the dense phase zone outlet pipe (11) is mixed with the primary air after the ratio is controlled by the electric proportional adjustment baffle (14), and then heated by the heater (15), the primary air fan (16), and the preheater (17) before being sent into the air chamber (18). Step 4: The mixed airflow in the air chamber (18) is guided by the arc-shaped guide plate (19) and uniformly injected into the dense phase zone of the furnace through the wear-resistant composite air cap (20); Step 5: The flue gas from the outlet pipe (12) of the dilute phase zone is directly injected into the high-temperature core area of ​​the dilute phase zone of the furnace (1) after being regulated by the shut-off valve; Step 6: When the system is shut down, close the outlet pipe (12) of the dilute phase zone and open the flue gas bypass (6) and the secondary hot air backflow pipe of the third pipe (7) to prevent condensate corrosion.

6. The recycling method according to claim 5, characterized in that, The calculation formula for the recirculated flue gas distribution in step 2 for the dense phase zone outlet pipe (11) and the dilute phase zone outlet pipe (12) is as follows: （1）； （2）； in, For dense phase region flow; For the dilute phase region flow; Total circulating flue gas flow rate; The proportion of flue gas in the dense phase region; This refers to the proportion of flue gas in the rare phase region.

7. The recycling method according to claim 5, characterized in that, The amount of waste heat recovered by the waste heat exchanger (8) in step 2 is: （3）； in, The specific heat at constant pressure of flue gas; The specific heat of air at constant pressure. For flue gas quality, For air quality, The value for flue gas cooling. This represents the air temperature rise value.

8. The recycling method according to claim 5, characterized in that, The adjustment accuracy of the electric proportional adjustment baffle (14) in step 3 is ±1°, and the calculation formula for the primary air in the electric proportional adjustment baffle (14) is: （4）； in, The air volume required for theoretical combustion. The primary air excess coefficient is ≤1.

05.

9. The recycling method according to claim 5, characterized in that, The wear-resistant composite wind cap (20) mentioned in step 4 is made of silicon carbide chromium alloy, and the formula for calculating the wind speed of the small hole of the wear-resistant composite wind cap (20) is: （5）； in, is the total air volume of the air chamber; n is the number of air caps for a single boiler; The radius of a single hole is 5mm; The flow coefficient is 0.

85.

10. The recycling method according to claim 5, characterized in that, The specific steps of step 6 include: Step 6.1: Close the shut-off valve on the dilute phase zone outlet pipe (12) to stop the injection of flue gas into the dilute phase zone; Arrangement 6.2: Open the shut-off valve on the flue gas bypass (6) and close the valve on the second pipeline (5); open the shut-off valve on the third pipeline (7) to allow secondary hot air to be introduced into the second pipeline (5) to prevent flue gas condensate from corroding the pipeline and equipment.

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