Natural gas hydrogen-doped low-nitrogen burner for boiler

Abstract

The invention relates to a natural gas hydrogen-doped low-nitrogen burner for a boiler, belongs to the technical field of burners, and aims to overcome the defect that an existing burner is excessive in pollutant emission in the using process. The natural gas hydrogen-doped low-nitrogen burner comprises a main burning nozzle, a sleeve and a micro-mixing on-duty nozzle, the sleeve and the micro-mixing on-duty nozzle are of a sleeved structure, and a combustion-supporting channel is formed between the sleeve and the micro-mixing on-duty nozzle; the multiple main combustion nozzles are distributed in the circumferential direction of the sleeve, first premixing cavities used for mixing fuel gas and flue gas are formed in the main combustion nozzles, the sleeve guides air into the combustion-supporting channel and the micro-mixing on-duty nozzles, and second premixing cavities of fuel gas cavities and air cavities are formed in the micro-mixing on-duty nozzles. An air outlet and a second premixing cavity communicated with the air cavity are machined in the end face of the cylinder, the upstream of the unit micro-mixing spray pipe is located in the air cavity, the downstream of the unit micro-mixing spray pipe is located in the second premixing cavity and communicated with the gas cavity, and an igniter is installed in the air outlet. According to the invention, the peak temperature and speed of flame are effectively reduced by enhancing mixing, so that the generation of NOx is inhibited.

Description

Technical Field

[0001] The invention belongs to the technical field of industrial boiler burners, and in particular relates to a natural gas hydrogen-blended low-nitrogen burner for boilers. Background Art

[0002] Natural gas is a primary energy source that has been vigorously developed during my country's energy transformation. By 2022, the production of natural gas will be 2200×10 8 m 3 , ranked fourth in the world, with a consumption of 3663×10 8 m 3 , ranking third in the world. Natural gas, a clean energy source that emerged after fossil fuels like coal, is now also facing constraints from global climate change. Hydrogen energy, with its renewable, energy-efficient, and environmentally friendly advantages, is attracting attention from many countries as a zero-carbon energy carrier.

[0003] Hydrogen has a wide flammability limit in air (4-75 vol%), low ignition energy (0.019 mJ), and low density (0.0899 kg / m 3 ), the adiabatic flame temperature is high (2380 K), and the laminar flame propagation speed is about 7 times that of methane. This makes it very easy for the flame propagation speed to exceed the fuel injection speed during combustion, significantly increasing the risk of flashback. Industrial burners usually give priority to non-premixed combustion because it can reduce the risk of flashback and explosion by injecting the fuel and oxidant separately. However, under this combustion mode, the fuel and oxidant easily reach a stoichiometric ratio in the local mixing area, causing the local flame temperature to approach the adiabatic temperature, resulting in the large-scale production of thermal NOx, which is also the main challenge facing the current application of hydrogen fuel.

[0004] The main technical challenge in hydrogen-blended burner design is to increase the hydrogen content in the fuel and reduce NOx emissions under safe operating conditions. Therefore, a technical concept is urgently needed to modify existing natural gas boiler burners to meet the requirements of hydrogen-blended combustion. Summary of the Invention

[0005] In order to overcome the defect of excessive pollutant emissions during use of existing burners, the present invention provides a natural gas hydrogen-blended low-nitrogen burner for boilers.

[0006] The technical solution adopted by the present invention is: a natural gas hydrogen-blended low-nitrogen burner for a boiler, comprising a main combustion nozzle, a sleeve and a micro-mixing service nozzle which are coaxially arranged from the outside to the inside, the sleeve and the micro-mixing service nozzle are a set structure which are connected with the air and gas source pipelines respectively, and the two are matched through a radial gap to form a combustion-supporting channel, a plurality of main combustion nozzles are evenly distributed along the circumference of the sleeve, the gas source is connected with each main combustion nozzle, a first premixing chamber is provided in the main combustion nozzle, the first premixing chamber is used to mix the gas flowing out of the main combustion nozzle with the flue gas inhaled from the outside of the main combustion nozzle and then spray it out; the air source is respectively introduced into the combustion-supporting channel and the micro-mixing service nozzle through the sleeve, the micro-mixing service nozzle is cylindrical, and its inner cavity is divided into two cavities, upstream and downstream, by a diversion partition, an annular gas cavity is provided in the upstream cavity, and an air guide channel connecting the upstream cavity and the downstream cavity is formed along the axial length direction of the outer wall of the inner ring of the gas cavity. The longitudinal section of the air guide channel is in the shape of a tapered trumpet, and its air outlet side is located in the downstream cavity; in the downstream cavity, the diversion baffle, the side wall of the shell and the end wall together enclose an air cavity, and an air outlet coaxial with the air guide channel and a second premixing cavity connected to the air cavity are respectively processed on the end face of the cylinder of the micro-mixing service nozzle, and an igniter is installed in the air outlet; the second premixing cavity is a plurality of through holes arranged in a circumferential array on the periphery of the air outlet, and a unit micro-mixing nozzle is arranged in each second premixing cavity, and the radial clearance between the second premixing cavity and the unit micro-mixing nozzle forms a unit nozzle annular gap; air enters the air cavity through the air guide channel, and the air after entering is respectively distributed to the air outlet and the second premixing cavity; gas is distributed to each unit micro-mixing nozzle through the gas cavity, and the gas flowing in the unit micro-mixing nozzle is ejected from the side wall of the unit micro-mixing nozzle at the second premixing cavity and mixed with the air and then ejected from the second premixing cavity.

[0007] Preferably, the micro-mixing service nozzle includes a cylindrical shell, and an array of injection holes for accommodating unit micro-mixing nozzles are processed on the downstream end face of the shell, and the injection holes are opened on the outer periphery of the air outlet. The second premixing chamber is the axial space from one end face to the other end face of the through hole of the injection hole.

[0008] Preferably, the fuel chamber, the air chamber and the second premixing chamber have the same axial spatial length.

[0009] Preferably, the unit micro-mixing nozzle is a straight tube of equal diameter, and small holes for outward spraying are circumferentially spaced apart at the downstream of the tube body away from the end surface.

[0010] Preferably, swirl blades are provided at intervals in the circumferential direction on the outer wall of the unit micro-mixing nozzle, and the swirl blades are fixedly arranged away from the small hole and close to the upstream side of the tube body.

[0011] Preferably, the main combustion nozzle is composed of a main combustion pipe and a sleeve that are installed in a set. The air inlet side of the main combustion pipe is connected to the gas source. The sleeve is composed of an air inlet constant diameter section, a necked section and an air outlet constant diameter section. The first premixing chamber is a cavity formed by the air inlet constant diameter section and the necked section. The outlet side of the main combustion pipe is installed in the first premixing chamber. An opening is provided on the outer wall of the first premixing chamber to connect the inside of the sleeve with the outside.

[0012] Preferably, the constricted section is a gradually converging-diverging composite variable diameter structure, and the nozzle of the main combustion pipe is located at the throat section of the composite variable diameter structure.

[0013] Preferably, the throat section is the minimum flow cross-section of the casing, and the axial length of the tapered section is smaller than that of the diverging section.

[0014] Preferably, the contraction shape of the main combustion pipe nozzle aperture is a lobe structure, and the number of lobes in the lobe structure is greater than three.

[0015] Preferably, the openings are circular with equal and uniform diameters.

[0016] The beneficial effects of the present invention are: 1. The swirl diffusion flame of a conventional duty burner exhibits a high equivalence ratio and large-scale combustion within the swirl shear layer, resulting in large localized high-temperature zones and high NOx emissions. Under low-load hydrogen-blended operation, swirl flame stability further deteriorates, posing a significant risk of flashback (nozzle ablation) or flameout. To overcome these drawbacks, the present invention abandons the traditional swirl structure and innovatively employs a micro-mixing nozzle array design. This design, through the coordinated action of multiple discretely distributed micro-mixing nozzles, decomposes a single, large-scale turbulent diffusion flame into numerous small, compact, and relatively independent turbulent diffusion flame units.

[0017] 2. This invention uses the designed independent unit micro-mixing nozzle as the service nozzle of the industrial burner. This design significantly reduces the local high temperature peak of the service diffusion flame, effectively suppressing the resulting thermal NOx generation; at the same time, it greatly improves the flame stability under low load and high hydrogen mixing ratio conditions, significantly reducing the risk of flashback.

[0018] 3. The main combustion nozzle of this invention utilizes a composite design featuring a lobed main combustion tube surrounded by a shrinking sleeve. The lobed structure disrupts the working fluid flow field at its lip, inducing enhanced vortex structures (such as horseshoe vortices), significantly promoting mixing of the fuel and flue gases. This not only enhances fuel-air mixing during the injection process and improves gas distribution uniformity at the sleeve nozzle, but also effectively reduces peak flame temperature and velocity through enhanced mixing, thereby suppressing NOx generation.

[0019] 4. The present invention adopts a modular integrated design, integrating the swirl micro-mixing nozzle and the main combustion nozzle into an independently detachable unit. During maintenance or replacement, the independent module can be conveniently disassembled and replaced, which greatly reduces the workload, modification costs and technical risks of project implementation and significantly improves the convenience of maintenance. BRIEF DESCRIPTION OF THE DRAWINGS

[0020] Figure 1 It is a structural schematic diagram of the present invention; Figure 2 for Figure 1 sectional view of Figure 3 for Figure 2 A cross-sectional view of the middle main burner nozzle; Figure 4 for Figure 3 Exploded diagram: Figure 5 for Figure 3 sectional view of Figure 6 for Figure 1 Cross-sectional view of the medium and micro mixing duty nozzle; Figure 7 for Figure 6 Schematic diagram of the structure of the medium unit micro-mixing nozzle; Figure 8 for Figure 7 sectional view of Among them: air 1, gas 2, flue gas 3, gas pipeline 4, first premixing chamber 5, main combustion nozzle 6, sleeve 7, micro-mixing service nozzle 8, main combustion pipe 9, sleeve 10, lobe structure 11, circular hole 12, fuel chamber 13, air chamber 14, second premixing chamber 15, air inlet 16, unit micro-mixing nozzle 17, air outlet 18, unit nozzle annular gap 19, swirl vane 20, small hole 21. DETAILED DESCRIPTION Example

[0021] A natural gas-hydrogen-blended, low-nitrogen burner for a boiler employs a radially staged arrangement. Its head is secured to the furnace wall via a flange and supplied with the required gas source by a gas supply pipeline. The gas supply pipeline comprises an air pipeline and a main gas pipeline. The gas in the main gas pipeline is a mixture of natural gas and hydrogen, which is branched to form multiple gas pipelines 4. The burner comprises a main burner nozzle 6, a sleeve 7, and a micro-mixing service nozzle 8. The sleeve 7 and micro-mixing service nozzle 8 are coaxially mounted on the air and gas pipelines 4, respectively. The radial gap between the sleeve 7 and the micro-mixing service nozzle 8 forms an air channel (i.e., a combustion-supporting channel) that provides the main burner nozzle 6 with the air required for combustion. The main burner nozzle 6 passes through the flange and is arranged on the outer periphery of the sleeve 7.

[0022] The main combustion nozzles 6 are uniformly arranged in a ring around the outer periphery of the sleeve 7. Preferably, the main combustion nozzles 6 are six in number and arranged in a hexagonal pattern. The micro-mixing pilot nozzle 8 is located at the center of the six main combustion nozzles 6 and is separated from them by the sleeve 7, thereby playing a role in stabilizing the flame.

[0023] The main combustion nozzles 6 are mainly composed of a sleeved main combustion tube 9 and a sleeve 10. The gas inlet side of each main combustion tube 9 is fixed to a flange plate and communicates with the gas pipeline 4. The nozzle aperture of the gas outlet side of the main combustion tube 9 is conical. The nozzle aperture is in the form of a wave lobe structure. The number of wave lobes is more than three. Preferably, the number of wave lobes is four. The thickness of the lip is thinner than the wall thickness of the main combustion tube 9. For example, the wall thickness of the main combustion tube 9 is 4 mm, and the thickness of the lip is 2 mm. Specifically, in the direction of gas flow, the main combustion tube 9 is composed of an equal-diameter section, a conical section, and a nozzle section in sequence.

[0024] The sleeve 10 is a variable cross-section passage. In the direction of gas flow, the sleeve 10 is composed of a gas inlet equal-diameter section, a converging section, and a gas outlet equal-diameter section. The cavity formed by the gas inlet equal-diameter section and the converging section is set as the first premixing cavity 5. The gas outlet side of the main combustion tube 9 is sleeved in the first premixing cavity 5. The converging section adopts a compound variable-diameter structure of gradual contraction and gradual expansion. The throat section is between the gradual contraction section and the gradual expansion section. The throat section is the smallest flow cross-section of the sleeve 10. The axial length of the gradual contraction section is smaller than that of the gradual expansion section. This structure is based on Bernoulli effect, forming a high-speed low-pressure zone at the throat section. At the same time, the axial flow passage is extended through the expansion section, so that the gas experiences a "contraction acceleration-expansion deceleration" process in the variable cross-section passage, effectively prolonging the residence time. When sleeved and installed, the wave lobe structure of the nozzle of the main combustion tube 9 is fixedly connected at the throat section of the converging section. A plurality of equal-diameter circular openings 12 are uniformly arranged on the outer wall of the gas inlet equal-diameter section and the gradual expansion section of the first premixing cavity 5. Preferably, eight openings 12 are arranged in the circumferential direction along the length direction. The radius of each opening 12 is 8 mm. For example, the inner diameters of the throat section of the sleeve 10 and the nozzle aperture of the main combustion tube 9 are 23.63 mm and 10.26 mm, respectively.

[0025] When the gas flows through the nozzle of the main combustion tube 9, it is disturbed by the wave lobe structure and induces enhanced vortex structures (such as horse-shoe vortex) at the lip. These vortices significantly strengthen the mixing of the gas and the flue gas in the shear layer, improving the uniformity of the gas distribution at the main combustion nozzle aperture. At the same time, the converging section of the outer sleeve 10 forms an ejector effect, which volatilizes the flue gas from the circumferential openings 12 into the sleeve. The introduced flue gas further mixes with the gas, diluting the local equivalence ratio, thereby effectively suppressing the local high-temperature peak in the region of the main combustion tube 9.

[0026] In the direction of gas flow, the upstream end of the micro-mixing pilot nozzle 8 communicates with the gas supply pipeline, and the downstream end surface of the micro-mixing pilot nozzle 8 is always flush with the end surface of the sleeve 7.

[0027] The micro-mixing service nozzle 8 includes a shell and a unit micro-mixing nozzle 17. A diversion partition is arranged in the shell. According to the direction of air flow, the partition divides the shell into two cavities, an upstream cavity and a downstream cavity.

[0028] Within the upstream cavity, an annular fuel cavity 13 is machined on the diversion baffle. The cross-section of the fuel cavity 13 gradually widens in the direction of airflow, and an air guide channel (i.e., a funnel-shaped trapping structure with a tapered trumpet-shaped longitudinal cross-section) is formed in the middle of the fuel cavity 13, communicating with the downstream cavity. The air guide channel is coaxially arranged with the shell, and the air inlet side of the air guide channel is connected to the diverted air, while the air outlet side of the air guide channel is located downstream. The air guide channel is cylindrical, and an air inlet 16 is provided on its outlet sidewall. The air inlet 16 is a plurality of densely distributed, uniformly circumferentially arranged small holes. By providing densely distributed, small air inlets 16, the uniformity of the air outflow can be improved, while also facilitating a more uniform passive distribution of subsequent air.

[0029] Within the downstream cavity, the diverter baffle, the sidewalls, and the inner end wall of the housing together enclose an air cavity 14, with the air inlet 16 located within the air cavity 14. An air outlet 18, coaxial with the air guide channel, is provided on the end face of the housing. This cylindrical air outlet 18 houses an igniter. An array of injection holes, connected to the air cavity 14, is arranged on the end face of the housing. These injection holes are arranged around the periphery of the air outlet 18. Both the air outlet 18 and the injection holes are connected to the air cavity 14, and the axial space between the injection holes from one end face of the through hole to the other is defined as the second premixing chamber 15.

[0030] The air inlet end of the unit micro-mixing nozzle 17 is inserted through the injection hole and sequentially passes through the second premixing chamber 15, the air chamber 14, and the diverter baffle before connecting to the gas chamber 13. The air outlet end of the unit micro-mixing nozzle 17 is located within the second premixing chamber 15, and its end is flush with the end surface of the shell. The radial distance between the unit micro-mixing nozzle 17 and the wall of the second premixing chamber 15 forms a unit nozzle annular gap 19.

[0031] The unit micro-mixing nozzle 17 is a straight tube of uniform diameter, for example, with an outer diameter of 12 mm and an inner diameter of 10 mm. The size of the second premixing chamber 15 is crucial for the adequate development of air after entry and the influence of the unit micro-mixing nozzle 17 on the mixing effect. Therefore, the air chamber 14 and the second premixing chamber 15 have the same axial length, for example, 100 mm. Preferably, the fuel chamber 13 also has the same axial length as the air chamber 14. The specific lengths can be adjusted proportionally based on the size of the chamber.

[0032] According to the direction of airflow, the outer wall of the unit micro-mixing nozzle 17 located in the second premixing chamber 15 is provided with swirl blades 20 and small holes 21 in sequence. The swirl blades 20 are arranged on the outer wall of the unit micro-mixing nozzle 17 at equal intervals in the circumferential direction. They are spaced apart from the small holes 21, preferably with a spacing of 20 mm. The radial length of the swirl blades 20 is the same as the size of the unit nozzle annular gap 19, both of which are 4 mm. There are six swirl blades 20, and the blade inclination angle of the swirl blades 20 is 45 degrees. By adjusting the inclination angle of the swirl blades 20, the swirl intensity can be precisely controlled, which is conducive to the realization of the body structure here and thus the flame stabilization effect. The small holes 21 are multiple equal-diameter circles, which are evenly arranged circumferentially at a distance of 10 mm from the downstream end face of the tube body. Preferably, the small holes 21 are eight with a radius of 1 mm. The position of the small holes 21 and the distance from the downstream end face determine the premixing length and mixing level of the gas and air.

[0033] The distance between each two unit micro-mixing nozzles 17 can be adjusted according to the size of the duty space located at the radially most central position of the burner. The specific size of the unit micro-mixing nozzles 17, the size of the unit nozzle annular gap 19, the opening size of the small hole 21 and the axial position can all be determined according to the gas composition.

[0034] The gas enters the gas chamber 13 through the gas pipeline 4 and is distributed to each unit micro-mixing nozzle 17, and is finally ejected from the small hole 21. At the same time, the upstream graded air is captured by the funnel structure and enters the air chamber 14 through the circular hole 16. The air is redistributed in the air chamber 14. Part of the air flows out through the outlet 18 as a cooling airflow to provide protective cooling for the igniter; the other part enters the second premixing chamber 15. After the airflow passes through the swirl blades 20, a rotating flow field is formed, and the swirl disturbance produces a turbulent vortex structure. After passing through the swirl blades 20, the air forms a partially premixed gas with the gas ejected from the small hole 21. The swirl blades 20 can significantly increase the mixing rate of the air and the gas ejected from the small hole 21. The premixed gas is ignited at the unit nozzle annular gap 19 downstream of the second premixing chamber 15. Through the coordinated operation of multiple unit micro-mixing nozzles 17 arranged in an array, the traditional single large-scale diffusion combustion mode is transformed into a small-scale partially premixed combustion mode with uniform spatial distribution. The partial premixing characteristics are used to avoid the risk of flashback and burner end ablation, and the local equivalence ratio and flame temperature are reduced through small-scale uniform combustion, thereby effectively suppressing NOx generation. Example

[0035] This embodiment is based on the first embodiment, and only optimizes the design of the unit micro-mixing nozzle 17. The other structures are the same as those of the first embodiment.

[0036] The swirl blade 20 is not provided on the outlet side of the unit micro-mixing nozzle 17 , and a swirl reflow zone can still be formed at the downstream end of the unit micro-mixing nozzle 17 , which also has a stable combustion effect.

[0037] The foregoing are merely preferred embodiments of the present invention. These embodiments are all different implementations based on the overall concept of the present invention. The scope of protection of the present invention is not limited thereto. Any changes or substitutions that can be easily conceived by a person skilled in the art within the technical scope disclosed in the present invention should be included within the scope of protection of the present invention. Therefore, the scope of protection of the present invention should be based on the scope of protection of the claims.

Claims

1. A natural gas hydrogen-blended low-nitrogen burner for boilers, characterized by: The invention comprises a main combustion nozzle (6), a sleeve (7) and a micro-mixing service nozzle (8) which are coaxially arranged from the outside to the inside. The sleeve (7) and the micro-mixing service nozzle (8) are set structures which are connected to the air and gas source pipelines respectively, and the two cooperate to form a combustion-supporting channel through a radial gap. A plurality of main combustion nozzles (6) are evenly distributed along the circumference of the sleeve (7). The gas source is connected to each main combustion nozzle (6). A first premixing chamber (5) is provided in the main combustion nozzle (6). The first premixing chamber (5) is used to mix the gas (2) flowing out of the main combustion nozzle (6) with the gas source. The main combustion nozzle (6) sucks in the flue gas (3) from the outside and mixes it before ejecting it; the air source is introduced into the combustion-supporting channel and the micro-mixing duty nozzle (8) through the sleeve (7), respectively. The micro-mixing duty nozzle (8) is cylindrical, and its inner cavity is divided into two cavities, upstream and downstream, by a diversion partition. An annular combustion chamber (13) is provided in the upstream cavity, and an air guide channel connecting the upstream cavity and the downstream cavity is formed along the axial length direction of the inner ring outer wall of the combustion chamber (13). The longitudinal section of the air guide channel is in the shape of a tapered trumpet, and its outlet side is located in the downstream cavity; in the downstream cavity, an annular combustion chamber (13) is provided. The air cavity (14) is enclosed by the flow dividing baffle, the side wall and the end wall of the shell. An air outlet (18) coaxial with the air guide channel and a second premixing cavity (15) communicating with the air cavity (14) are respectively processed on the end face of the cylinder of the micro-mixing duty nozzle (8). An igniter is installed in the air outlet (18); the second premixing cavity (15) is a plurality of through holes arranged in a circumferential array on the periphery of the air outlet (18), and a unit micro-mixing nozzle (17) is arranged in each second premixing cavity (15), and the second premixing cavity (15) is connected to the unit micro-mixing nozzle (17). The radial clearance of the elementary micro-mixing nozzle (17) is matched to form a unit nozzle annular gap (19); air (1) enters the air cavity (14) through the air guide channel, and the air (1) after entering is respectively distributed to the air outlet (18) and the second premixing cavity (15); the gas (2) is distributed to each unit micro-mixing nozzle (17) through the gas cavity (13), and the gas (2) flowing in the unit micro-mixing nozzle (17) is ejected from the side wall of the unit micro-mixing nozzle (17) at the second premixing cavity (15) and mixed with the air (1) and then ejected from the second premixing cavity (15).

2. A natural gas hydrogen-blended low-nitrogen burner for a boiler according to claim 1, characterized in that: The micro-mixing service nozzle (8) includes a cylindrical shell, and an array of injection holes for accommodating the unit micro-mixing nozzle (17) are processed on the downstream end face of the shell, and the injection holes are opened on the periphery of the air outlet (18). The second premixing chamber (15) is the axial space from one end face to the other end face of the injection hole.

3. A natural gas hydrogen-blended low-nitrogen burner for a boiler according to claim 1, characterized in that: The fuel cavity (13), the air cavity (14) and the second premixing cavity (15) have the same axial spatial length.

4. A natural gas hydrogen-blended low-nitrogen burner for a boiler according to any one of claims 1 to 3, characterized in that: The unit micro-mixing nozzle (17) is a straight tube of equal diameter, and small holes (21) for outward spraying are circumferentially spaced apart at the downstream of the tube body away from the end surface.

5. A natural gas hydrogen-blended low-nitrogen burner for a boiler according to claim 4, characterized in that: Swirl blades (20) are provided at intervals in the circumferential direction on the outer wall of the unit micro-mixing nozzle (17), and the swirl blades (20) are fixedly arranged at a position away from the small hole (21) and close to the upstream side of the tube body.

6. A natural gas hydrogen-blended low-nitrogen burner for a boiler according to claim 1, characterized in that: The main combustion nozzle (6) is composed of a main combustion pipe (9) and a sleeve (10) that are mounted together. The air inlet side of the main combustion pipe (9) is connected to a fuel gas source. The sleeve (10) is composed of an air inlet constant diameter section, a constricted section, and an air outlet constant diameter section. The first premixing chamber (5) is a cavity formed by the air inlet constant diameter section and the constricted section. The outlet side of the main combustion pipe (9) is mounted inside the first premixing chamber (5). An opening (12) is provided on the outer wall of the first premixing chamber (5) to connect the inside of the sleeve (10) with the outside.

7. A natural gas hydrogen-blended low-nitrogen burner for a boiler according to claim 6, characterized in that: The constricted section presents a gradually converging-expanding composite variable diameter structure, and the nozzle of the main combustion pipe (9) is located at the throat section of the composite variable diameter structure.

8. A natural gas hydrogen-blended low-nitrogen burner for a boiler according to claim 7, characterized in that: The throat section is the minimum flow cross-section of the sleeve (10), and the axial length of the tapered section is smaller than that of the diverging section.

9. A natural gas hydrogen-blended low-nitrogen burner for a boiler according to any one of claims 6 to 8, characterized in that: The contraction shape of the nozzle aperture of the main combustion pipe (9) is a lobe structure, and the number of lobes in the lobe structure is greater than three.

10. A natural gas hydrogen-blended low-nitrogen burner for a boiler according to claim 6, characterized in that: The opening (12) is a circle with equal diameters and uniform openings.

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