Rotational flow pulverized coal burner with main gas and exhaust gas separated and control method of rotational flow pulverized coal burner

Abstract

The invention provides a rotational flow pulverized coal burner with separated main exhaust gas and a control method thereof, and belongs to the field of pulverized coal burning equipment, the rotational flow pulverized coal burner comprises a burner elbow and a central air pipe, the burner elbow comprises a bent section and a straight pipe section, the straight pipe section is fixedly connected with the bent section, the other end of the straight pipe section is a burner nozzle, and the central air pipe is connected with the burner nozzle. The end, away from the straight pipe section, of the bent section is connected with a separation valve block, the end, away from the bent section, of the separation valve block is fixedly connected with a primary air pipeline for circulation of primary air powder, an inner cavity of the straight pipe section is provided with a multi-stage dense and thin separator, and the multi-stage dense and thin separator separates the primary air powder into dense pulverized coal airflow and thin pulverized coal airflow. Exhaust gas pipelines are arranged at the elbows of the combustors, and valves are arranged on the exhaust gas pipelines. The combustion stability, the burn-off rate and the combustion efficiency of the pulverized coal are improved, NOx emission is reduced, the overall energy consumption of the system is indirectly reduced, meanwhile, the transformation process is simple, and the cost is reduced.

Description

Technical Field

[0001] The invention relates to the field of pulverized coal combustion equipment, and in particular to a swirl pulverized coal burner with main exhaust gas separation and a control method thereof. Background Art

[0002] In my country's energy structure, thermal power generation still plays a very important role. Thermal power has long played a fundamental role in the power system, such as safe and uncertain power supply, peak load regulation, and centralized heating. The swirl pulverized coal burner is a pulverized coal combustion device widely used in current large-scale power station boilers. The characteristics of the traditional swirl pulverized coal burner are: the primary air carrying pulverized coal enters the furnace in a direct current or swirl manner, and the secondary air enters from the periphery of the primary air. The strong rotation of the airflow will form a low-pressure area in the center of the jet, and there are radial and axial pressure gradients, especially the reverse pressure gradient in the axial direction, which will suck the high-temperature flue gas in the center of the furnace back and heat the pulverized coal airflow.

[0003] As one of the core equipment of thermal power generating units, swirl pulverized coal burner must have better ignition and stable combustion performance, higher burnout rate, lower nitrogen oxide emission, and wider coal quality adaptability. This requires the main exhaust gas separation transformation of swirl pulverized coal burner. The current mainstream transformation scheme is to set up an external separation device on the swirl pulverized coal burner, and pre-separate the primary air through the swirl pulverized coal separator and the exhaust gas channel to reduce the primary air rate sent to the furnace. This transformation method has the following defects:

[0004] 1. High transformation cost: The transformation process is complicated, involving multiple links such as equipment purchase, installation and commissioning, and the initial investment is huge.

[0005] 2. The separation efficiency is not high: the mixing uniformity of coal powder and air in the main exhaust gas is not good, which affects the combustion efficiency and completeness, and the separation effect is not ideal.

[0006] 3. Increased energy consumption: In order to compensate for the insufficient separation efficiency, measures such as increasing the air supply volume and improving the coal grinding fineness are often taken, which indirectly increases the overall energy consumption of the system.

[0007] Therefore, we propose a swirl pulverized coal burner with main exhaust gas separation and a control method thereof. Summary of the invention

[0008] The object of the present invention is to provide a swirl pulverized coal burner with main exhaust gas separation and a control method thereof, so as to solve the problems raised in the above-mentioned background technology.

[0009] To achieve the above object, the present invention adopts the following technical solutions:

[0010] In a first aspect, the present invention provides a swirl pulverized coal burner with main exhaust gas separation, comprising a burner elbow and a central air duct arranged in the inner cavity of the burner elbow, wherein the burner elbow comprises a curved section and a straight pipe section, one end of the straight pipe section is fixedly connected to the curved section, the other end of the straight pipe section is a burner nozzle, the end of the curved section away from the straight pipe section is connected to a separation valve block, the end of the separation valve block away from the curved section is fixedly connected to a primary air duct for circulating primary air powder, and the primary air duct is formed by assembling a plurality of gradually converging and expanding pipes;

[0011] The inner cavity of the straight pipe section is provided with a multi-stage thick-lean separator, which separates the primary air powder into a thick coal powder airflow and a thin coal powder airflow, wherein the thin coal powder airflow is located in the axial center area of ​​the inner cavity of the straight pipe section, and the thick coal powder airflow is located in the area between the thin coal powder airflow and the inner surface of the straight pipe section;

[0012] An exhaust gas pipeline is provided at the elbow of the burner, and valves are provided on the exhaust gas pipelines. The exhaust gas pipelines include a first exhaust gas pipeline, a second exhaust gas pipeline and a third exhaust gas pipeline. The air inlet end of the first exhaust gas pipeline is connected to the inner cavity of the curved section, the air outlet end of the first exhaust gas pipeline is connected to the area where the light coal powder airflow is located, the air inlet end of the second exhaust gas pipeline is connected to the primary air pipeline, the air outlet end of the second exhaust gas pipeline is connected to the area where the rich coal powder airflow is located, the air inlet end of the third exhaust gas pipeline is connected to the air outlet end of the separation valve block, and the air outlet end of the third exhaust gas pipeline is connected to the inner cavity of the straight pipe section close to the curved section.

[0013] Furthermore, a separation baffle is provided at one end of the primary air duct inner cavity close to the separation valve block.

[0014] Furthermore, the valve includes a first valve, a second valve and a third valve, the first valve is arranged at the air inlet end of the first exhaust gas pipeline, the second valve is arranged at the air inlet end of the second exhaust gas pipeline, and the third valve is arranged at the air inlet end of the third exhaust gas pipeline.

[0015] Furthermore, the air inlet end of the central air duct is connected to a secondary air box, the air outlet end of the central air duct is connected to a furnace, and central air circulates in the central air duct.

[0016] Furthermore, the material of the inner wall of the burner elbow, the primary air duct and the exhaust gas duct is at least one of double-layer ceramic material and silicon carbide material.

[0017] In a second aspect, the present invention provides a control method for a swirl pulverized coal burner with main exhaust gas separation, using the swirl pulverized coal burner with main exhaust gas separation as described in the first aspect, the swirl pulverized coal burner is assembled in a boiler for use, and the control method includes:

[0018] When the boiler is in high-load operation, the first valve, the second valve and the third valve are closed to stop the primary air powder flowing in the primary air duct separated by the first exhaust gas duct, the second exhaust gas duct and the third exhaust gas duct, so that the combustion of the primary air powder is in a stable combustion state;

[0019] When the boiler is running at low load, the first valve, the second valve and the third valve are opened, the first exhaust gas duct, the second exhaust gas duct and the third exhaust gas duct separate the primary air powder circulating in the primary air duct, and the remaining primary air powder in the primary air duct is separated by a multi-stage thick-thin separator to form a distribution of thin inside and thick outside. The first exhaust gas duct, the second exhaust gas duct and the third exhaust gas duct increase the air intake of the primary air powder and improve the uniform distribution of the primary air powder with thin inside and thick outside, so that the combustion of the primary air powder is in a stable combustion state.

[0020] Compared with the prior art, the present invention has the following technical effects:

[0021] A primary air duct is formed by assembling a plurality of gradually contracting and expanding pipes, and primary air powder circulates in the primary air duct. The coal powder in the primary air powder is fully mixed by scaling pipes of different sizes, thereby improving the uniformity of coal powder carried by the primary air. A multi-stage thick-thin separator is arranged in the inner cavity of the straight pipe section of the burner elbow, and the primary air powder is separated by the multi-stage thick-thin separator to form a thick coal powder airflow and a thin coal powder airflow. The thin coal powder airflow is located in the center, and the thick coal powder airflow is located outside the thin coal powder airflow. The primary air powder forms a distribution of thin inside and thick outside, thereby achieving fine control of the coal powder airflow and improving the separation effect. A first exhaust gas duct, a second exhaust gas duct and a third exhaust gas duct are arranged, and the first exhaust gas duct, the second exhaust gas duct and the third exhaust gas duct separate the primary air powder. The third exhaust gas duct increases the primary air volume in the horizontal direction of the burner. The first exhaust gas duct and the second exhaust gas duct cooperate with the multi-stage thick-thin separator to further improve the uniformity of the distribution of the primary air powder inside and thick outside. On the whole, this burner improves the uniformity of coal powder through the primary air duct composed of gradually converging and expanding tubes, laying the foundation for the subsequent separation of thick and thin coal powder and optimization of the combustion process. A variety of separation measures are adopted later with good separation efficiency, which increases the uniformity of the distribution of light inside and thick outside of the primary air powder, improves the combustion stability, burnout rate and combustion efficiency of coal powder, reduces NOx emissions, and indirectly reduces the overall energy consumption of the system. At the same time, the transformation process is simple, with good safety and reliability, and reduces costs. BRIEF DESCRIPTION OF THE DRAWINGS

[0022] Figure 1 It is a structural schematic diagram of a burner elbow according to an embodiment of the present invention;

[0023] Figure 2 This is a schematic structural diagram of an exhaust gas pipeline according to an embodiment of the present invention;

[0024] Figure 3 It is a schematic structural diagram of a primary air duct according to an embodiment of the present invention.

[0025] Explanation of the reference numerals in the accompanying drawings: 1. a first-stage thick-thin separator, 2. a second-stage thick-thin separator, 3. a central air duct, 4. a burner elbow, 41. a curved section, 42. a straight pipe section, 5. a burner nozzle, 6. a first exhaust gas duct, 7. a separation valve block, 8. a second exhaust gas duct, 9. a third exhaust gas duct, 10. a first-stage gradually converging and expanding pipe for primary air, 11. a second-stage gradually converging and expanding pipe for primary air, 12. a separation baffle, 13 a primary air duct connection, 14. a first valve, 15. a second valve, 16. a third valve. DETAILED DESCRIPTION

[0026] The following will be combined with the drawings in the embodiments of the present invention to clearly and completely describe the technical solutions in the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work belong to the protection scope of the present invention.

[0027] In this article, terms such as "left, right, up, down, front, and back" are established based on the positional relationships shown in the drawings. Depending on the different drawings, the corresponding positional relationships may also change accordingly. Therefore, they cannot be understood as absolute limitations on the scope of protection.

[0028] Embodiment 1

[0029] See also Figures 1 to 3 The present embodiment provides a swirl pulverized coal burner with main exhaust gas separation, including a burner elbow 4 and a central air duct 3 arranged in the inner cavity of the burner elbow 4. The burner elbow 4 includes a curved section 41 and a straight pipe section 42, one end of the straight pipe section 42 is a burner nozzle 5, the other end of the straight pipe section 42 is fixedly connected to the curved section 41, and the end of the curved section 41 away from the straight pipe section 42 is fixedly connected to the separation valve block 7, and the separation valve block 7 can separate the primary air and pulverized coal, which is beneficial to the subsequent separation of the primary air and pulverized coal.

[0030] Specifically, one end of the separation valve block 7 away from the bending section 41 is fixedly connected to a primary air duct through which primary air powder circulates. The primary air duct is formed by assembling a plurality of gradually converging and expanding tubes. In the present embodiment, the primary air duct is formed by assembling a primary air first-stage gradually converging and expanding tube 10 and a primary air second-stage gradually converging and expanding tube 11. The reduced diameter of the primary air first-stage gradually converging and expanding tube 10 is smaller than the reduced diameter of the primary air second-stage gradually converging and expanding tube 11. The primary air powder circulates in the primary air duct. Through the scaling of the two stages with different sizes, the coal powder in the primary air powder is fully mixed, thereby improving the uniformity of the coal powder carried by the primary air.

[0031] Specifically, the inner cavity of the straight pipe section 42 is provided with a multi-stage thick-thin separator. In the present embodiment, the inner cavity of the straight pipe section 42 is provided with a two-stage thick-thin separator, and the two-stage thick-thin separator includes a primary thick-thin separator 1 and a secondary thick-thin separator 2. The primary air powder flows through the primary thick-thin separator 1 and the secondary thick-thin separator 2. A part of the primary air powder flows through the primary thick-thin separator 1 and the secondary thick-thin separator 2, and forms a light coal powder airflow after being separated by the primary thick-thin separator 1 and the secondary thick-thin separator 2. The light coal powder airflow is located in the axial center area of ​​the inner cavity of the straight pipe section 42. Another part of the primary air powder is gathered by the primary thick-thin separator 1 and the secondary thick-thin separator 2 to form a thick coal powder airflow. The thick coal powder airflow is located in the area between the light coal powder airflow and the inner surface of the straight pipe section 42. The primary air powder is separated by the primary thick-thin separator 1 and the secondary thick-thin separator 2 to form a light coal powder airflow outside. By setting up a two-stage thick-thin separator, the separation effect is improved, the primary air powder is finely controlled, and the uniform and reasonable distribution of the thick coal powder airflow and the thin coal powder airflow inside the burner is ensured. In addition, the staff can adjust the distance between the first-stage thick-thin separator 1 and the second-stage thick-thin separator 2 according to different coal types and combustion loads to achieve the best combustion state of the burner.

[0032] Specifically, an exhaust gas pipeline is provided at the burner elbow 4, and the exhaust gas pipeline includes a first exhaust gas pipeline 6, a second exhaust gas pipeline 8 and a third exhaust gas pipeline 9. The air inlet end of the first exhaust gas pipeline 6 is connected to the inner cavity of the curved section 41, the air inlet end of the first exhaust gas pipeline 6 is arranged on the concave side of the curved section, and the air inlet end of the first exhaust gas pipeline 6 is located above the separation valve block 7, the air outlet end of the first exhaust gas pipeline 6 is connected to the area where the light coal powder airflow is located, the air inlet end of the second exhaust gas pipeline 8 is connected to the primary air pipeline, the air outlet end of the second exhaust gas pipeline 8 is connected to the area where the rich coal powder airflow is located, the air inlet end of the third exhaust gas pipeline 9 is connected to the air outlet end of the separation valve block 7, and the air outlet end of the third exhaust gas pipeline 9 passes through the outer shell of the curved section 41 and extends to the inner cavity of the straight pipe section 42.

[0033] Specifically, the first exhaust gas duct 6, the second exhaust gas duct 8 and the third exhaust gas duct 9 can separate the primary air powder flowing in the primary air duct, and the primary air powder flows to the straight pipe section 42 through four paths. The primary air powder passing through the separation valve block 7 in the third exhaust gas duct 9 enters the inner cavity of the straight pipe section 42, which can increase the primary air volume in the horizontal direction of the burner. At the same time, it also avoids the problem of excessive air mixing causing the ignition heat of the coal powder airflow to increase, thereby causing difficulty in stable ignition and combustion. In addition, the third exhaust gas duct 9 ensures the flow rate of the primary air powder during low-load operation, effectively preventing problems such as coal powder deposition caused by too low coal powder airflow velocity after exhaust gas separation, and also plays an oxygen supplement role. The second exhaust gas duct 6 and the second exhaust gas duct 8 cooperate with the two-stage thick and thin separators, and the primary air powder in the primary air duct is separated by the first-stage thick and thin separator 1 and the second-stage thick and thin separator 2 to form a distribution of thin inside and thick outside. The primary air powder separated by the separation valve block 7 in the first exhaust gas duct 6 enters the area where the light coal powder airflow is located, maintaining the coal powder concentration in the center of the primary air powder and improving the uniformity of the distribution of the primary air powder inside and thick outside. The primary air powder not separated by the separation valve block 7 in the second exhaust gas duct 8 enters the area where the thick coal powder airflow is located on the lower side, improving the influence of the elbow on the primary air powder concentration in the area where the thick coal powder airflow is located on the lower side, and increasing the primary air powder concentration there to make it close to the primary air powder concentration in the area where the thick coal powder airflow is located on the upper side, thereby improving the uniformity of the distribution of the primary air powder inside and thick outside. Through the coordinated work of the three, the uniformity of the distribution of the light coal powder airflow and the thick coal powder airflow is ensured. In addition, the primary air powder in the first exhaust gas duct 6 enters the area where the light coal powder airflow is located, which helps to form a high-temperature flue gas recirculation area that is conducive to ignition, greatly reduces the primary air rate entering the furnace, and makes the airflow strongly mixed, improving the low-load stable combustion characteristics and the ignition characteristics under full load.

[0034] Specifically, on the whole, the burner improves the uniformity of coal powder through the primary air duct composed of gradually converging and expanding tubes, which is the key prerequisite for achieving subsequent thick and thin separation of coal powder and optimizing the combustion process. The primary air powder is separated by the third exhaust gas duct 9, which increases the primary air volume in the horizontal direction of the burner. The primary air powder is separated by the first exhaust gas duct 6 and the second exhaust gas duct 8, and the first exhaust gas duct 6, the second exhaust gas duct 8 and the two-stage thick and thin separators work together to separate the primary air powder into a thick coal powder airflow and a thin coal powder airflow. The separation is more thorough, and the uniformity of the distribution of the thin coal powder airflow and the thick coal powder airflow is improved. Through a variety of improvement measures, the burner improves the combustion stability, burnout rate and combustion efficiency of coal powder, reduces NOx emissions, and indirectly reduces the overall energy consumption of the system. At the same time, the transformation process is simple, the safety and reliability are good, and the cost is reduced.

[0035] Specifically, a separation baffle 12 is provided at one end of the primary air duct inner cavity close to the separation valve block 7, so as to further improve the uniformity of the coal powder in the primary air powder.

[0036] Specifically, valves are installed on the exhaust gas pipelines, including a first valve 14, a second valve 15 and a third valve 16. The first valve 14 is arranged at the air inlet end of the first exhaust gas pipeline 6, the second valve 15 is arranged at the air inlet end of the second exhaust gas pipeline 8, and the third valve 16 is arranged at the air inlet end of the third exhaust gas pipeline 9. The staff can adjust the valve opening and closing degree according to the coal type and combustion requirements, so as to meet different coal types and combustion requirements and achieve the purpose of optimizing the combustion process.

[0037] Specifically, the air inlet end of the central air duct 3 is connected to the secondary air box, and the air outlet end of the central air duct 3 is connected to the boiler furnace. The secondary air box injects the central air into the central air duct 3, and the central air flows in the central air duct 3 to the boiler furnace. The central air can adjust the position of the central recirculation zone of the burner, thereby controlling the ignition point and helping to obtain the best combustion conditions.

[0038] Specifically, the material of the burner elbow 4, the primary air duct and the inner wall of the exhaust gas duct is at least one of double-layer ceramic material and silicon carbide material, which can significantly improve the wear resistance, corrosion resistance, high temperature resistance and fluid transmission efficiency of the equipment, extend the service life, and improve the combustion efficiency.

[0039] Embodiment 2

[0040] This embodiment provides a control method for a swirl pulverized coal burner with main exhaust gas separation, using the swirl pulverized coal burner with main exhaust gas separation as described in Example 1, the swirl pulverized coal burner is assembled in a boiler for use, and the control method includes:

[0041] When the boiler is in high-load operation, the first valve 14, the second valve 15 and the third valve 16 are closed, and the primary air powder flowing in the primary air duct is stopped from being separated by the first exhaust gas duct 6, the second exhaust gas duct 8 and the third exhaust gas duct 9, and the primary air powder enters the boiler furnace through the primary air duct for stable combustion;

[0042] When the boiler is in low-load operation, the first valve 14, the second valve 15 and the third valve 16 are opened, and the first exhaust gas pipeline 6, the second exhaust gas pipeline 8 and the third exhaust gas pipeline 9 separate the primary air powder circulating in the primary air pipeline, and the primary air powder flows to the inner cavity of the straight pipe section 42 through four paths. The primary air powder in the primary air pipeline is separated into a concentrated coal powder airflow and a light coal powder airflow by a secondary thick-lean separator. The light coal powder airflow is located in the central area of ​​the inner cavity of the straight pipe section 42, and the concentrated coal powder airflow is located in the area between the light coal powder airflow and the inner surface of the straight pipe section 42. The primary air powder forms a distribution of light inside and concentrated outside. The primary air powder separated by the separation valve block 7 in the first exhaust gas pipeline 6 flows to the area where the light coal powder airflow is located, and the area is kept The concentration of the light coal powder airflow in the domain is improved, and the uniformity of the distribution of the primary air powder inside and the thick outside is improved. The primary air powder that has not been separated by the separation valve block 7 in the second exhaust gas duct 8 flows to the area where the thick coal powder airflow is located on the lower side, thereby increasing the primary air powder concentration there, making the thick coal powder airflow there similar to the thick coal powder airflow concentration in the area where the thick coal powder airflow is located on the upper side, thereby improving the uniformity of the distribution of the primary air powder inside and the thick outside. The primary air powder separated by the separation valve block 7 in the third exhaust gas duct 9 flows to the inner cavity of the straight pipe section 42, thereby increasing the primary air volume in the horizontal direction. The first exhaust gas duct 6, the second exhaust gas duct 8 and the third exhaust gas duct 9 increase the air intake of the primary air powder and improve the uniform distribution of the primary air powder, so that the primary air powder enters the boiler furnace for stable combustion.

[0043] The above embodiments are only to illustrate the basic principles and characteristics of the present invention, but are not limited to the above embodiments. It should be understood that for ordinary technicians in this field, various changes and modifications can be made to the present invention without departing from the spirit and scope of the present invention, and these changes and modifications fall within the scope of the present invention to be protected. The scope of protection of the present invention is defined by the attached claims and their equivalents.

Claims

1. A cyclone pulverized coal burner with main exhaust gas separation, characterized in that: The burner comprises a burner elbow (4) and a central air duct (3) arranged in the inner cavity of the burner elbow (4), wherein the burner elbow comprises a curved section (41) and a straight pipe section (42), one end of the straight pipe section (42) is fixedly connected to the curved section (41), the other end of the straight pipe section (42) is a burner nozzle (5), one end of the curved section (41) away from the straight pipe section (42) is fixedly connected to a separation valve block (7), one end of the separation valve block (7) away from the curved section (41) is fixedly connected to a primary air duct for circulating primary air powder, and the primary air duct is formed by assembling a plurality of gradually converging and expanding pipes; The inner cavity of the straight pipe section (42) is provided with a multi-stage thick-lean separator, which separates the primary air powder into a thick coal powder airflow and a thin coal powder airflow, wherein the thin coal powder airflow is located in the axial center area of ​​the inner cavity of the straight pipe section (42), and the thick coal powder airflow is located in the area between the thin coal powder airflow and the inner surface of the straight pipe section (42); An exhaust gas pipeline is provided at the burner elbow (4), and valves are provided on the exhaust gas pipelines. The exhaust gas pipelines include a first exhaust gas pipeline (6), a second exhaust gas pipeline (8) and a third exhaust gas pipeline (9). The air inlet end of the first exhaust gas pipeline (6) is connected to the inner cavity of the curved section (41), and the air outlet end of the first exhaust gas pipeline (6) is connected to the area where the light coal powder airflow is located. The air inlet end of the second exhaust gas pipeline (8) is connected to the primary air pipeline, and the air outlet end of the second exhaust gas pipeline (8) is connected to the area where the rich coal powder airflow is located. The air inlet end of the third exhaust gas pipeline (9) is connected to the air outlet end of the separation valve block (7), and the air outlet end of the third exhaust gas pipeline (9) is connected to the inner cavity of the straight pipe section (42) close to the curved section (41).

2. The swirl coal powder burner with main exhaust gas separation according to claim 1, characterized in that: A separation baffle (12) is provided at one end of the primary air duct inner cavity close to the separation valve block (7).

3. The swirl coal powder burner with main exhaust gas separation according to claim 1, characterized in that: The valves include a first valve (14), a second valve (15) and a third valve (16); the first valve (14) is arranged at the air inlet end of the first exhaust gas pipeline (6); the second valve (15) is arranged at the air inlet end of the second exhaust gas pipeline (8); and the third valve (16) is arranged at the air inlet end of the third exhaust gas pipeline (9).

4. The swirl coal powder burner with main exhaust gas separation according to claim 1, characterized in that: The air inlet end of the central air duct (3) is connected to a secondary air box, the air outlet end of the central air duct (3) is connected to a boiler furnace, and central air circulates in the central air duct (3).

5. The swirl coal powder burner with main exhaust gas separation according to claim 1, characterized in that: The burner elbow (4), the primary air duct and the inner wall of the exhaust gas duct are made of at least one of a double-layer ceramic material and a silicon carbide material.

6. The control method of the swirl pulverized coal burner with main exhaust gas separation according to any one of claims 1 to 5, characterized in that: The swirl pulverized coal burner is assembled in a boiler for use, and the control method includes: When the boiler is in high-load operation, the first valve (14), the second valve (15) and the third valve (16) are closed, and the separation of the primary air powder flowing in the primary air duct by the first exhaust gas duct (6), the second exhaust gas duct (8) and the third exhaust gas duct (9) is stopped, so that the combustion of the primary air powder is in a stable combustion state; When the boiler is in low-load operation, the first valve (14), the second valve (15) and the third valve (16) are opened, and the first exhaust gas pipeline (6), the second exhaust gas pipeline (8) and the third exhaust gas pipeline (9) separate the primary air powder circulating in the primary air pipeline. The remaining primary air powder in the primary air pipeline is separated by a multi-stage thick-thin separator to form a distribution of thin inside and thick outside. The first exhaust gas pipeline (6), the second exhaust gas pipeline (8) and the third exhaust gas pipeline (9) increase the air intake of the primary air powder and improve the uniform distribution of the thin inside and thick outside of the primary air powder, so that the combustion of the primary air powder is in a stable combustion state.

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