Combustion system for a pure combustion high basicity coal boiler

Abstract

The present application relates to the technical field of boiler combustion, and discloses a combustion system of a pure-burning high-alkali coal boiler, a primary air fan is provided with a primary air volume adjusting valve, an air inlet end of a primary air pipe faces an outlet of a venturi pipe, the primary air pipe is separated from the venturi pipe, the primary air pipe is provided with a rotational flow nozzle, an inner secondary air pipe is connected with the venturi pipe, the primary air pipe is connected with the inner secondary air pipe, an inner secondary air volume adjusting valve is arranged between the inner secondary air pipe and a secondary air fan, an outer secondary air volume adjusting valve is arranged between an outer secondary air pipe and the secondary air fan, the outer secondary air pipe is connected with a tail flue of the boiler through a recirculation air fan, a circulating air volume adjusting valve is arranged between the outer secondary air pipe and the recirculation air fan, the inner secondary air pipe is provided with an inner rotational flow baffle, the outer secondary air pipe is provided with an outer rotational flow baffle and an outer secondary air door; the present application can solve the problems that the pure-burning high-alkali coal boiler is prone to ash deposition and slagging when running at high load, and the NO x emission is high and the combustion stability is poor when running at low load.

Description

Technical Field

[0001] This invention relates to the field of boiler combustion technology, and in particular to a combustion system for a boiler that burns only high-alkali coal. Background Technology

[0002] For boiler systems that burn only high-alkali coal, the high alkali metal content in the coal will lower the ash melting point and affect the formation of nitrogen oxides. When the boiler is running at high load, it will cause serious problems such as fouling and slagging.

[0003] Currently, to alleviate the problem of ash accumulation and slagging in boilers burning high-alkali coal, one main approach is to blend high-alkali coal with anti-slagging additives such as kaolin. Existing boiler systems burning high-alkali coal use kaolin as an anti-slagging additive, but kaolin is expensive and its grindability coefficient differs significantly from that of high-alkali coal. The pulverized coal fed into the boiler after grinding has an inconsistent particle size with the kaolin. Furthermore, current boilers are required to burn only high-alkali coal, making the blending method unsuitable.

[0004] Another major approach is to reduce the furnace heat load to lower the flue gas temperature within the furnace, such as through layered burner arrangement, flame segmentation technology, and rich-lean combustion technology. Reducing the furnace heat load is generally achievable for newly built boilers; for example, layered burner arrangement requires modification during boiler design. However, for existing boiler systems, modification is difficult, costly, and difficult to implement, and cannot meet the requirements for operation under variable load conditions in boilers burning only high-alkali coal. Existing fuel staged combustion technologies mostly directly send secondary fuel into the reburning zone, making it difficult to achieve ultra-low NO₂ levels. x The emissions are poorly controlled, and the applicability to variable load and ultra-low load scenarios is limited. Furthermore, the combination of fuel staging and air staging technologies is not suitable for combustion requirements under different loads. Existing rich-lean combustion technologies can reduce nitrogen oxide formation through rich-lean separation combustion, but combustion stability must be considered when separating the rich and lean sides, making it difficult to adjust combustion under different loads. Existing flue gas recirculation technologies can improve the temperature and flow fields of the combustion chamber by controlling combustion temperature and oxygen concentration, thereby reducing nitrogen oxide formation. However, controlling the flue gas recirculation flow rate under variable load conditions is very difficult, requiring consideration of the flue gas recirculation method and the enhancement of flue gas recirculation combustion during recirculation.

[0005] Existing high-alkali coal boilers operating at low loads increase the excess air coefficient to ensure stable combustion and complete fuel combustion. However, excess air can promote NO production. x The generation of NO from boilers x Emissions have increased significantly. Summary of the Invention

[0006] The purpose of this invention is to provide a combustion system for a boiler that burns only high-alkali coal, which can solve the problems of ash and slag accumulation in the boiler under high load and NO emission during low load operation. x The problem is high emissions and poor combustion stability.

[0007] To achieve the above objectives, the present invention provides a combustion system for a pure high-alkali coal-fired boiler, comprising a burner, a primary air fan, a secondary air fan, and a recirculation fan. The burner includes a Venturi tube, a primary air duct, an inner secondary air duct, and an outer secondary air duct. The primary air fan is connected to the inlet of the Venturi tube via a connecting pipe. A primary air volume regulating valve is provided between the primary air fan and the connecting pipe. The air inlet of the primary air duct faces the outlet of the Venturi tube, and the primary air duct is separate from the Venturi tube. The primary air duct is equipped with an adjustable swirl nozzle. The inner secondary air duct is connected to the venturi tube, the primary air duct is connected to the inner secondary air duct, an inner secondary air volume regulating valve is provided between the inner secondary air duct and the secondary air fan, an outer secondary air volume regulating valve is provided between the outer secondary air duct and the secondary air fan, the outer secondary air duct is connected to the tail flue of the boiler through the recirculation fan, a recirculation air volume regulating valve is provided between the outer secondary air duct and the recirculation fan, an inner swirl baffle is provided on the inner secondary air duct, and an adjustable outer swirl baffle and an outer secondary air damper are provided on the outer secondary air duct. When the boiler is running at high load, the opening of the primary air volume regulating valve is increased, the swirl nozzle is adjusted to direct flow mode, the external secondary air volume regulating valve is opened, the external swirl baffle is adjusted to swirl mode, the external secondary damper is opened, and the opening of the circulating air volume regulating valve is decreased. When the boiler is running at low load, the opening of the primary air volume regulating valve is reduced, the opening of the internal secondary air volume regulating valve is increased, the swirl nozzle is adjusted to swirl mode, the external swirl baffle is adjusted to direct flow mode, the opening of the external secondary air damper is reduced, the opening of the circulating air volume regulating valve is increased, and the external secondary air volume regulating valve is closed.

[0008] As a preferred embodiment of the present invention, the air inlet of the primary air duct is provided with an expansion port, the end of the expansion port near the outlet of the Venturi tube is the small diameter end, and the end of the expansion port away from the outlet of the Venturi tube is the large diameter end.

[0009] As a preferred embodiment of the present invention, the primary air duct is provided with a first concentration probe for detecting the coal powder concentration of the primary air, and the inner secondary air duct is provided with a second concentration probe for detecting the coal powder concentration of the secondary air.

[0010] As a preferred embodiment of the present invention, the primary air fan provides primary air to the burner, and the secondary air fan provides secondary air to the burner. During combustion, the primary air accounts for 15% to 30% of the total air volume, and the secondary air accounts for 70% to 85% of the total air volume.

[0011] As a preferred embodiment of the present invention, when the boiler is running at high load, the amount of recirculated air supplied by the recirculation fan in the external secondary air duct is 10% to 20% of the total amount of external secondary air in the external secondary air duct.

[0012] As a preferred embodiment of the present invention, the downward swing angle of the burner nozzle during high-load operation of the boiler is greater than the downward swing angle of the burner nozzle during low-load operation of the boiler.

[0013] As a preferred embodiment of the present invention, when the boiler is running at high load, the nozzle of the burner swings downward at an angle of 30° to 40°; when the boiler is running at low load, the nozzle of the burner swings downward at an angle of 20° to 30°.

[0014] As a preferred embodiment of the present invention, when the boiler is running at high load, after the concentration separation by the Venturi tube, 50% of the primary air and 85% of the pulverized coal enter the primary air duct, and the remaining 50% of the primary air and 15% of the pulverized coal enter the inner secondary air duct.

[0015] As a preferred embodiment of the present invention, when the boiler is running at low load, after the concentration separation by the Venturi tube, 40% of the primary air and 75% of the pulverized coal enter the primary air duct, and the remaining 60% of the primary air and 25% of the pulverized coal enter the inner secondary air duct.

[0016] As a preferred embodiment of the present invention, the flue gas recirculation rate is 10%~15% when the boiler is running at high load.

[0017] Compared with the prior art, the combustion system of a pure high-alkali coal boiler according to an embodiment of the present invention has the following advantages: This invention utilizes the compression effect of the throat and the expansion effect of the expansion section behind the throat of a Venturi tube to separate the pulverized coal-air mixture into a high-concentration pulverized coal airflow in the middle and a low-concentration pulverized coal airflow on the outer side. The concentrated pulverized coal airflow enters the primary air duct, and the low-concentration pulverized coal airflow enters the inner secondary air duct. The primary air volume regulating valve and the inner secondary air volume regulating valve can regulate the flow rate and volume of the primary air and the inner secondary air. The recirculation fan extracts the tail flue gas and sends it back to the outer secondary air duct of the burner, reducing the temperature and oxygen content, thereby suppressing NO. x This generates and alleviates the serious problem of ash accumulation and slagging in boilers burning high-alkali coal under high load; During high-load boiler operation, increasing the primary air velocity by widening the primary air volume regulating valve and adjusting the swirl nozzles of the primary air duct to direct flow significantly improves the jet stiffness of the pulverized coal airflow, preventing ash and slag buildup at the swirl nozzles of high-alkali coal. Opening the external secondary air damper and regulating valve, and adjusting the external swirl baffle to swirl flow, creates a reasonable air grading flow field, allowing the external secondary air to envelop the primary air in a swirling manner, preventing the pulverized coal airflow from directly scouring the furnace wall and thus reducing ash and slag buildup on the furnace wall. Reducing the circulating air volume regulating valve avoids excessive recirculated flue gas affecting the combustion of unburned fuel. During high-load boiler operation, the increased boiler heat load and unstable combustion problem are alleviated by adjusting the primary air velocity and recirculated flue gas volume ratio to reduce the regional heat load, thereby adjusting the oxygen concentration and temperature field and mitigating ash and slag buildup. When the boiler is operating at low load, the primary air velocity is reduced by decreasing the opening of the primary air volume regulating valve, while the internal secondary air velocity is increased by increasing the opening of the internal secondary air volume regulating valve. This enhances the static pressure difference regulation between the primary and internal secondary air ducts, resulting in more thorough separation of rich and lean air and increasing the pulverized coal concentration in the primary air duct. This ensures stable combustion at low load while reducing NO₂ levels. x Emissions; by adjusting the swirl nozzle of the primary air duct to swirl mode, adjusting the external swirl baffle to direct flow mode, reducing the opening of the external secondary air damper, increasing the opening of the circulating air volume regulating valve, and closing the external secondary air volume regulating valve, the external secondary air damper opening can be reduced and the external secondary air velocity increased. This allows the primary air ejected from the swirl nozzle to act as internal secondary air, and the internal secondary air ejected from the internal secondary air duct to act as external secondary air. All of the external secondary air is recirculated flue gas, creating a strong reducing atmosphere, significantly reducing the oxygen concentration and temperature in the combustion zone, and deeply suppressing NO. x This process generates nitrogen oxides, enabling stable low-NOx combustion under low load conditions. When the boiler operates at low load, the reduced heat load alleviates ash and slagging problems. By increasing the degree of rich-lean separation and reducing the combustion zone, combustion stability is ensured, while simultaneously reducing NOx emissions. x The generation of . Attached Figure Description

[0018] To more clearly illustrate the technical solutions of the embodiments of the present invention, the accompanying drawings of the embodiments will be briefly described below.

[0019] Figure 1 A schematic diagram of the combustion system of a pure high-alkali coal-fired boiler provided by the present invention; Figure 2 This is a schematic diagram of the burner provided by the present invention; In the diagram, the components are: burner 1; Venturi tube 11; primary air duct 12; primary air volume regulating valve 121; swirl nozzle 122; expansion port 123; first concentration probe 124; inner secondary air duct 13; inner secondary air volume regulating valve 131; inner swirl baffle 132; second concentration probe 133; outer secondary air duct 14; outer secondary air volume regulating valve 141; outer swirl baffle 142; outer secondary air damper 143; primary air fan 2; secondary air fan 3; recirculation fan 4; recirculation air volume regulating valve 41; connecting pipe 5; coal feeder 51; burnout air nozzle 6; burnout air fan 61. Detailed Implementation

[0020] The specific embodiments of the present invention will be described in further detail below with reference to the accompanying drawings and examples. The following examples are for illustrative purposes only and are not intended to limit the scope of the invention.

[0021] In the description of this invention, it should be understood that the terms "upper", "lower", "left", "right", "front", "rear", "top", "bottom", etc., indicate the orientation or positional relationship based on the orientation or positional relationship shown in the accompanying drawings. 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. Therefore, they should not be construed as limitations on this invention.

[0022] like Figures 1 to 2As shown, a combustion system for a pure high-alkali coal-fired boiler according to a preferred embodiment of the present invention includes a burner 1, a primary air fan 2, a secondary air fan 3, and a recirculation fan 4. The burner 1 and a burnout air nozzle 6 are arranged in the boiler furnace. The burnout air nozzle 6 is connected to the burnout fan 61. The burner 1 includes a Venturi tube 11, a primary air duct 12, an inner secondary air duct 13, and an outer secondary air duct 14. The primary air fan 2 is connected to the Venturi tube 11 via a connecting pipe 5 for supplying pulverized coal. The primary air duct 12 is connected to the inlet of the connecting pipe 5. A primary air volume regulating valve 121 is provided between the primary air fan 2 and the connecting pipe 5. A coal feeder 51 is connected to the connecting pipe 5. The primary air duct 12, the inner secondary air duct 13, and the outer secondary air duct 14 are coaxially mounted from the inside to the outside. The air inlet end of the primary air duct 12 faces the outlet of the venturi tube 11, and the primary air duct 12 is separate from the venturi tube 11. The primary air duct 12 is provided with an adjustable swirl nozzle 122. The angle of the swirl blades in the swirl nozzle 122 can be adjusted. The inner secondary air duct 13 is connected to the outlet of the venturi tube 11. The primary air duct 12 is connected to the inner secondary air duct 13. An inner secondary air volume regulating valve 131 is provided between the inner secondary air duct 13 and the secondary air fan 3. An outer secondary air volume regulating valve 141 is provided between the outer secondary air duct 14 and the secondary air fan 3. The outer secondary air duct 14 is connected to the tail flue of the boiler through the recirculation fan 4. A recirculation air volume regulating valve 41 is provided between the outer secondary air duct 14 and the recirculation fan 4. An inner swirl baffle 132 is provided on the inner secondary air duct 13. An adjustable outer swirl baffle 142 and an outer secondary air damper 143 are provided on the outer secondary air duct 14. The angle of the swirl blades in the outer swirl baffle 142 can be adjusted. The outer secondary air damper 143 is located behind the outer swirl baffle 142 along the flow direction of the outer secondary air. When the boiler is running at high load, the opening of the primary air volume regulating valve 121 is increased, the swirl nozzle 122 is adjusted to direct flow mode, the external secondary air volume regulating valve 141 is opened, the external swirl baffle 142 is adjusted to swirl mode, the external secondary damper 143 is opened, and the opening of the circulating air volume regulating valve 41 is decreased. When the boiler is operating at low load, the opening of the primary air volume regulating valve 121 is reduced, the opening of the internal secondary air volume regulating valve 131 is increased, the swirl nozzle 122 is adjusted to swirl operation, the external swirl baffle 142 is adjusted to direct flow operation, the opening of the external secondary air damper 143 is reduced, the opening of the circulating air volume regulating valve 41 is increased, and the external secondary air volume regulating valve 141 is closed. The internal primary air volume regulating valve 121 is always open during boiler operation.

[0023] This invention utilizes the compression effect of the throat of the Venturi tube 11 and the expansion effect of the expansion section behind the throat to separate the pulverized coal-air mixture into a high-concentration pulverized coal airflow in the middle and a low-concentration pulverized coal airflow on the outside. The concentrated pulverized coal airflow enters the primary air duct 12, and the low-concentration pulverized coal airflow enters the inner secondary air duct 13. The primary air volume regulating valve 121 and the inner secondary air volume regulating valve 131 can regulate the flow rate and flow of the primary air and the inner secondary air. The recirculation fan 4 extracts the tail flue gas and sends it back to the outer secondary air duct 14 of the burner 1 to reduce the temperature and oxygen content, thereby inhibiting the formation of NOx and alleviating the serious problem of ash accumulation and slagging under high load in boilers burning high-alkali coal. When the boiler is operating at high load, the primary air velocity is increased by increasing the opening of the primary air volume regulating valve 121, and the swirl nozzle 122 of the primary air duct 12 is adjusted to direct flow mode, which significantly improves the jet stiffness of the pulverized coal airflow and prevents ash and slag accumulation at the swirl nozzle 122 of high-alkali coal. The external secondary air damper 143 and the external secondary air volume regulating valve 141 are opened, and the external swirl baffle 142 is adjusted to swirl mode to form a reasonable air classification flow field, so that the external secondary air wraps around the primary air in a swirling manner, preventing the pulverized coal airflow from directly scouring the furnace wall, thereby reducing ash and slag accumulation on the furnace wall. The circulating air volume regulating valve 41 is reduced to avoid excessive recirculated flue gas affecting the combustion effect of unburned fuel. When the boiler is operating at high load, the problem of unstable combustion due to increased boiler heat load is alleviated. The regional heat load is reduced by adjusting the ratio of primary air velocity and recirculated flue gas volume, thereby adjusting the oxygen concentration and temperature field and alleviating ash and slag accumulation. When the boiler is operating at low load, the primary air velocity is reduced by decreasing the opening of the primary air volume regulating valve 121, while the internal secondary air velocity is increased by increasing the opening of the internal secondary air volume regulating valve 131. This enhances the static pressure difference regulation between the primary air duct 12 and the internal secondary air duct 13, resulting in more thorough separation of rich and lean coal and increasing the pulverized coal concentration in the primary air duct 12. This ensures stable combustion at low load while reducing NO₂ levels. x Emissions; by adjusting the swirl nozzle 122 of the primary air duct 12 to swirl mode, adjusting the outer swirl baffle 142 to direct flow mode, reducing the opening of the outer secondary air damper 143, increasing the opening of the circulating air volume regulating valve 41, and closing the outer secondary air volume regulating valve 141, the opening of the outer secondary air damper 143 can reduce and increase the outer secondary air velocity, so that the primary air ejected from the swirl nozzle 122 acts as the inner secondary air, and the inner secondary air ejected from the inner secondary air duct 13 acts as the outer secondary air. The outer secondary air is all recirculated flue gas, creating a strong reducing atmosphere, significantly reducing the oxygen concentration and temperature in the combustion zone, and deeply suppressing NO. x This process generates nitrogen oxides, enabling stable low-NOx combustion under low load conditions. When the boiler operates at low load, the reduced heat load alleviates ash and slagging problems. By increasing the degree of rich-lean separation and reducing the combustion zone, combustion stability is ensured, while simultaneously reducing NOx emissions. x The generation of .

[0024] For example, such as Figure 2 As shown, the air inlet of the primary air duct 12 is provided with an expansion port 123. The end of the expansion port 123 near the outlet of the Venturi tube 11 is a small-diameter end, and the end of the expansion port 123 away from the outlet of the Venturi tube 11 is a large-diameter end. After the concentrated coal powder airflow is separated by the Venturi tube 11, it enters the primary air duct 12 after the flow velocity is reduced by the expansion port 123. This avoids boundary layer separation and eddy current generation, significantly reduces local resistance loss, achieves efficient static pressure recovery, and maintains a high static pressure level in the primary air duct 12.

[0025] For example, such as Figure 2 As shown, the primary air duct 12 is equipped with a first concentration probe 124 for detecting the coal powder concentration of the primary air, and the inner secondary air duct 13 is equipped with a second concentration probe 133 for detecting the coal powder concentration of the secondary air. By measuring the coal powder concentration of the primary air and the inner secondary air after concentration separation using the primary air concentration probe and the inner secondary air concentration probe, real-time monitoring and feedback adjustment of the coal powder concentration on the concentrated side and the diluted side after concentration separation can be achieved. The air volume of the primary air and the inner secondary air can be adjusted according to the coal powder concentration results. When adjusting the air volume, the excess air coefficient in the inner secondary air duct 13 must be greater than 1 (oxidizing atmosphere), while the excess air coefficient in the primary air duct 12 must be less than 1 (reducing atmosphere).

[0026] Specifically, the primary air fan 2 provides primary air to the burner 1, and the secondary air fan 3 provides secondary air to the burner 1. During combustion, the primary air accounts for 15% to 30% of the total air volume, and the secondary air accounts for 70% to 85% of the total air volume, thereby achieving a reasonable graded distribution of air during the combustion process.

[0027] For example, during high-load operation of the boiler, the recirculated air supplied by the recirculation fan 4 in the external secondary air duct 14 accounts for 10% to 20% of the total external secondary air volume in the external secondary air duct 14. The external secondary air also serves to provide the air required for combustion of unburned parts, avoiding the impact on the combustion effect of unburned fuel due to an excessively high proportion of recirculated flue gas. This achieves NO reduction while ensuring high-load combustion efficiency. x Effective control of emissions.

[0028] For example, the downward swing angle of the nozzle of burner 1 during high-load boiler operation is greater than that during low-load boiler operation; specifically, during high-load boiler operation, the downward swing angle of the nozzle of burner 1 is 30°~40°; during low-load boiler operation, the downward swing angle of the nozzle of burner 1 is 20°~30°. The larger downward swing angle at high load can prolong the residence time of pulverized coal in the lower part of the furnace, reduce the heat load in the upper part of the furnace, effectively reduce the near-wall flue gas temperature, and alleviate the problem of ash accumulation and slagging on the high-temperature wall surface of high-alkali coal; the smaller downward swing angle at low load can increase the local heat load in the burner 1 area, ensuring combustion stability and efficiency under low load. The nozzle of burner 1 can be understood as the outlet end structure of all the primary air duct 12, inner secondary air duct 13, and outer secondary air duct 14.

[0029] In this embodiment, during high-load boiler operation, after the venturi tube 11 separates the primary air and 85% of the pulverized coal, 50% of the primary air and 85% of the pulverized coal enter the primary air duct 12, while the remaining 50% of the primary air and 15% of the pulverized coal enter the inner secondary air duct 13. This creates a dense phase airflow with a high pulverized coal concentration (rich fuel combustion) within the primary air duct 12, which is beneficial for maintaining stable ignition and combustion. Simultaneously, a depleted phase airflow with a low pulverized coal concentration is formed within the inner secondary air duct 13. Through staged fuel combustion, NO is effectively suppressed. x The generation of primary air and the airflow distribution ratio of the two air ducts are conducive to maintaining the static pressure balance inside the burner 1. When the boiler is running at low load, after the rich and lean separation by the Venturi tube 11, 40% of the primary air and 75% of the pulverized coal enter the primary air duct 12, and the remaining 60% of the primary air and 25% of the pulverized coal enter the inner secondary air duct 13. This achieves enhanced rich and lean separation under low load, increases the pulverized coal concentration entering the primary air duct 12, and increases the pulverized coal concentration and heat load in the combustion zone at low load. This effectively solves the problem of unstable combustion at low load. At the same time, by increasing the proportion of primary air entering the inner secondary air duct 13, the disturbance and mixing of flue gas in the later stage of combustion is enhanced, promoting complete combustion.

[0030] In this embodiment, when the boiler is running at high load, the flue gas recirculation rate is 10%~15%. The flue gas recirculation rate refers to the proportion of the total amount of flue gas extracted from the tail flue to the total amount of flue gas in the boiler. Since the total amount of flue gas in the boiler is large at high load, if the flue gas recirculation rate is too high, it will easily lead to a large amount of unburned carbon that cannot be burned completely and unstable combustion, thus ensuring combustion stability.

[0031] In the description of this invention, it should be noted that, unless otherwise explicitly specified and limited, the terms "connected" and "linked" 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.

[0032] The above description is only a preferred embodiment of the present invention. It should be noted that for those skilled in the art, several improvements and substitutions can be made without departing from the technical principles of the present invention, and these improvements and substitutions should also be considered within the scope of protection of the present invention.

Claims

1. A combustion system for a boiler burning only high-alkali coal, characterized in that, The system includes a burner, a primary air fan, a secondary air fan, and a recirculation fan. The burner includes a venturi tube, a primary air duct, an inner secondary air duct, and an outer secondary air duct. The primary air fan is connected to the inlet of the venturi tube via a connecting pipe for supplying pulverized coal. A primary air volume regulating valve is provided between the primary air fan and the connecting pipe. The air inlet of the primary air duct faces the outlet of the venturi tube and is separate from the venturi tube. The primary air duct is equipped with an adjustable swirl nozzle. The inner secondary air duct is connected to the venturi tube... The system is connected to the inner secondary air duct, and an inner secondary air volume regulating valve is provided between the inner secondary air duct and the secondary air fan. An outer secondary air volume regulating valve is provided between the outer secondary air duct and the secondary air fan. The outer secondary air duct is connected to the tail flue of the boiler through the recirculation fan. A recirculation air volume regulating valve is provided between the outer secondary air duct and the recirculation fan. An inner swirl baffle is provided on the inner secondary air duct, and an adjustable outer swirl baffle and an outer secondary air damper are provided on the outer secondary air duct. When the boiler is running at high load, the opening of the primary air volume regulating valve is increased, the swirl nozzle is adjusted to direct flow mode, the external secondary air volume regulating valve is opened, the external swirl baffle is adjusted to swirl mode, the external secondary damper is opened, and the opening of the circulating air volume regulating valve is decreased. When the boiler is running at low load, the opening of the primary air volume regulating valve is reduced, the opening of the internal secondary air volume regulating valve is increased, the swirl nozzle is adjusted to swirl mode, the external swirl baffle is adjusted to direct flow mode, the opening of the external secondary air damper is reduced, the opening of the circulating air volume regulating valve is increased, and the external secondary air volume regulating valve is closed.

2. The combustion system of the pure high-alkali coal-fired boiler as described in claim 1, characterized in that, The air inlet of the primary air duct is provided with an expansion port. The end of the expansion port near the outlet of the Venturi tube is the small-diameter end, and the end of the expansion port away from the outlet of the Venturi tube is the large-diameter end.

3. The combustion system of the pure high-alkali coal-fired boiler as described in claim 1, characterized in that, The primary air duct is equipped with a first concentration probe for detecting the coal powder concentration of the primary air, and the inner secondary air duct is equipped with a second concentration probe for detecting the coal powder concentration of the secondary air.

4. The combustion system of the pure high-alkali coal-fired boiler as described in claim 1, characterized in that, The primary air fan provides primary air to the burner, and the secondary air fan provides secondary air to the burner. During combustion, the primary air accounts for 15% to 30% of the total air volume, and the secondary air accounts for 70% to 85% of the total air volume.

5. The combustion system of the pure high-alkali coal-fired boiler as described in claim 1, characterized in that, When the boiler is operating at high load, the amount of recirculated air supplied by the recirculation fan in the external secondary air duct is 10% to 20% of the total amount of external secondary air in the external secondary air duct.

6. The combustion system of the pure high-alkali coal-fired boiler as described in claim 1, characterized in that, The burner nozzle swings downward at a greater angle when the boiler is operating at high load than when the boiler is operating at low load.

7. The combustion system of the pure high-alkali coal-fired boiler as described in claim 6, characterized in that, When the boiler is operating at high load, the burner nozzle swings downward at an angle of 30° to 40°; when the boiler is operating at low load, the burner nozzle swings downward at an angle of 20° to 30°.

8. The combustion system of the pure high-alkali coal-fired boiler as described in claim 1, characterized in that, When the boiler is running at high load, after the concentration separation in the Venturi tube, 50% of the primary air and 85% of the pulverized coal enter the primary air duct, and the remaining 50% of the primary air and 15% of the pulverized coal enter the inner secondary air duct.

9. The combustion system of a pure high-alkali coal-fired boiler as described in claim 1, characterized in that, When the boiler is running at low load, after the venturi tube separates the primary air and pulverized coal, 40% of the primary air and 75% of the pulverized coal enter the primary air duct, and the remaining 60% of the primary air and 25% of the pulverized coal enter the inner secondary air duct.

10. The combustion system of a pure high-alkali coal-fired boiler as described in claim 1, characterized in that, When the boiler is operating at high load, the flue gas recirculation rate is 10%~15%.

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