A DC-Swirl Phase Coupled Coal-fired Boiler Ultra-Low Load Stable Combustion System

Abstract

A direct-current / swirl-coupling combustion system for an ultra-low load stable combustion coal-fired boiler includes: a self-preheating stable combustion swirl burner, a direct-current burner, a secondary air nozzle, an air box, a burnout air nozzle, a boiler furnace, and a coal mill or pulverized coal silo. The self-preheating stable combustion swirl burner is installed vertically at the secondary air nozzle between two layers of direct-current burners, serving as an ignition source (at low load) or secondary air (at high load), and is arranged tangentially in the horizontal direction. There are six layers of direct-current burners, each corresponding to a coal mill, and all six layers of direct-current burners are placed at the four corners of the furnace in a tangential combustion manner. In this invention, the self-preheating stable combustion swirl burner and the direct-current burner operate in a coupled manner. This invention achieves ultra-low load stable combustion without requiring modifications to pressure-bearing components such as the boiler water-cooled walls during boiler low-load stable combustion retrofitting, while simultaneously achieving stable ignition of coals with poor combustion characteristics at low load, reducing nitrogen oxide emissions, and preventing slagging at the pre-combustion chamber outlet.

Description

Technical Field

[0001] This invention relates to the field of boiler combustion technology, specifically to a DC-swirl phase-coupled ultra-low load stable combustion system for coal-fired boilers. Background Technology

[0002] By modifying thermal power units to improve their flexibility, they can absorb more clean energy and reduce wind and solar power curtailment in various regions. During peak electricity demand periods or when renewable energy generation is unstable, thermal power units can fully utilize their operating capacity; during off-peak periods or when renewable energy generation is stable, they can reduce power generation, thus achieving deep peak shaving. The most critical issue in improving the flexibility of unit peak-shaving capacity through these modifications is achieving stable boiler operation under low or even ultra-low loads. For tangential boilers in existing coal-fired power units, the minimum operating load for stable operation is typically only 30%-50% of the rated load, and cannot be further reduced.

[0003] In conventional power plant boilers, some secondary air nozzles are equipped with ignition oil guns to inject oil for combustion support during low-load operation. When a self-preheating and stable combustion swirl burner is installed in a traditional coal-fired system, pulverized coal is heated at high temperature in the pre-combustion chamber, undergoing pyrolysis and gasification to transform into residual char and flammable volatile gases. The volatile gases ignite rapidly and ignite the residual char, thus stabilizing combustion. At the same time, it can replace the ignition oil gun during boiler start-up, achieving stable combustion at low loads and reducing heavy oil consumption.

[0004] Nitrogen oxides emitted during coal-fired power plant combustion are a major contributor to environmental pollution. Current standards stipulate that nitrogen oxide emission concentrations from coal-fired power plants must not exceed 50 mg / m³. 3 Using a self-preheating, stable-fire swirl burner is the most effective way to remove NOx at the source. Pulverized coal undergoes low-oxygen combustion in the pre-combustion chamber of the self-preheating, stable-fire swirl burner, creating a localized reducing atmosphere that reduces NOx to N2, thus lowering NOx emissions.

[0005] The combination of coal-fired boilers and self-preheating stable combustion swirl burners will greatly improve the power plant's deep peak shaving capacity and denitrification efficiency. However, it is not easy to install swirl burners on the basis of the original four-corner tangential once-through combustion system of the power plant boiler.

[0006] Application No. 202011148540.6 discloses an ultra-low load stable combustion preheating and decomposition combustion system and an ultra-low load operation method. Four preheating and decomposition swirl burners are installed on the front and rear walls of a tangentially circular boiler furnace, arranged in an opposing configuration. The nozzle center height of the swirl burners is the same as the nozzle height of the direct-flow burners in the same layer, and ultra-low load stable combustion is achieved through parallel connection of the two. However, the opposing arrangement of the preheating and decomposition swirl burners in this system easily disrupts the original aerodynamic field and furnace filling degree. Furthermore, the ejection effect of the swirl burners causes the original tangential circle to expand and adhere to the wall, as well as temperature field deviation. Application No. 202111606354.7 discloses an ultra-low load combustion device suitable for tangentially circular boilers. It adds a layer of low-capacity pulverized coal burners and a layer of secondary air nozzles sequentially above the original uppermost primary air burner, or replaces the original single-layer direct-flow burner with two layers of staged low-capacity pulverized coal burners, achieving long-term stable operation of the boiler at 15% rated load and below. This device uses a direct-flow burner instead of a swirl burner. For a single jet, the entrainment effect is small and there is no central backflow, which is detrimental to ignition. Therefore, the low-capacity pulverized coal burner arranged in a tangential circle is not suitable for single operation. When applying the above two methods, the swirl burner must be installed in the existing intact boiler furnace wall, which requires drilling holes or excavating holes in the side walls or the four corners of the furnace. The water-cooled walls inside the furnace wall, as pressure-bearing components, require tube winding or cutting, resulting in a large workload and high practical difficulty.

[0007] Furthermore, the quality of coal in my country varies greatly, and the coal used in power plant boilers is generally of poor quality. Adding a layer of swirl burner may not guarantee stable ignition of coals with poor combustion characteristics under low loads; high-alkali coal is prone to slagging at the pre-combustion chamber outlet and other parts during use, which adversely affects boiler performance. Summary of the Invention

[0008] To overcome the above technical problems, the present invention aims to provide a DC-swirl coupled coal-fired boiler ultra-low load stable combustion system. This system replaces one secondary air nozzle between the two layers of DC primary air nozzles in the original boiler, and installs a self-preheating stable combustion swirl burner tangentially at the four corners of the furnace, coupling it with the DC burner. Using this system to retrofit existing tangential coal-fired boilers achieves ultra-low load stable combustion without altering pressure-bearing components such as the boiler water-cooled walls. Simultaneously, it achieves stable ignition of coals with poor combustion characteristics at low loads, reduces nitrogen oxide emissions, and prevents slagging at the pre-combustion chamber outlet.

[0009] To achieve the above objectives, the technical solution adopted by the present invention is as follows:

[0010] A DC-swirl phase coupled coal-fired boiler ultra-low load stable combustion system includes: a self-preheating stable combustion swirl burner, a DC burner, a secondary air nozzle, a wind box, a burnout air nozzle, a boiler furnace, and a coal mill;

[0011] The self-preheating and stable combustion swirl burner 1 is installed vertically at the secondary air nozzle between two layers of direct-flow burners to serve as an ignition source (at low load) or secondary air (at high load), and is arranged in a tangential circle in the horizontal direction; the airflow moves along the tangential direction of the spiral after exiting the preheating and stable combustion swirl burner.

[0012] The DC burner consists of six layers, numbered A, B, C, D, E, and F from bottom to top. Each layer corresponds to a coal mill, and the six layers of DC burners are arranged in a tangential combustion manner at the four corners of the furnace.

[0013] The self-preheating and stable combustion swirl burner includes a primary air duct, a direct-flow inner secondary air duct, a swirl inner secondary air duct, an outer secondary air duct, and a pre-combustion chamber; the primary air duct of the self-preheating and stable combustion swirl burner is connected in parallel to the primary air duct of the direct-flow burner; the direct-flow inner secondary air duct, the swirl inner secondary air duct, and the outer secondary air duct of the self-preheating and stable combustion swirl burner are connected to the air box to realize the delivery of secondary air.

[0014] The DC burners are divided into DC burner one, DC burner two, DC burner three, DC burner four, DC burner five and DC burner six.

[0015] There are two options for the pulverized coal supply in the self-preheating and stable combustion swirl burner;

[0016] One approach is to connect the primary air duct in parallel to the branch pipe of the coal mill connected to the DC burner on the top floor (backup floor), and control the amount of pulverized coal supplied to the self-preheating stable combustion swirl burner by adjusting the coal pulverization rate of the coal mill.

[0017] Secondly, the primary air duct is directly connected to the medium-sized coal pulverized silo, and the amount of coal fed to the self-preheating stable combustion swirl burner is directly adjusted by changing the speed of the coal feeder, so as to respond quickly and stably to load changes.

[0018] The boiler furnace contains four self-preheating and stable combustion swirl burners per layer, with a total of n layers and 1≤n≤2. Each layer is located above the secondary air nozzle above the direct-flow burner in layer B or layer D.

[0019] The self-preheating and stable combustion swirl burner 1 consists of a central pipe, a primary air pipe, a direct-flow inner secondary air pipe, and a swirl inner secondary air pipe, arranged axially from the inside out, all of which are connected to the inlet of the pre-combustion chamber. The inlet pipe of the swirl inner secondary air pipe has several tangential swirl blades. The outer secondary air pipe is sleeved on the outer wall of the pre-combustion chamber, and the outlet of the outer secondary air pipe and the outlet of the pre-combustion chamber are both connected to the furnace.

[0020] The DC burner has secondary air nozzles on both the upper and lower sides of the primary air nozzle; the burnout air nozzle is located above the top secondary air nozzle.

[0021] An operation method for an ultra-low load stable combustion system of a DC-swirl phase coupled coal-fired boiler includes the following steps:

[0022] At low load, the primary air duct of the self-preheating and stable combustion swirl burner is opened; the direct-flow internal secondary air duct, the swirl internal secondary air duct, and the external secondary air duct are connected to the air box and secondary air is supplied; after the primary air pulverized coal is pyrolyzed and ignited at high temperature in the pre-combustion chamber, it is sent into the boiler furnace and combusted stably. At this time, the self-preheating and stable combustion swirl burner can provide a stable ignition heat source for the direct-flow burner.

[0023] When operating under normal load, the number of layers N and the opening degree of the DC burner are determined based on the boiler load, with 1≤N≤6. The primary air duct in the self-preheating and stable combustion swirl burner is closed, and the swirl inner secondary air duct, the DC inner secondary air duct and the outer secondary air duct operate normally to send air into the boiler furnace. At this time, the function of the self-preheating and stable combustion swirl burner is exactly the same as that of the secondary air nozzle.

[0024] The beneficial effects of this invention are:

[0025] In the ultra-low load stable combustion system of a direct-current-swirl coupled coal-fired boiler described in this invention, when the system operates under low load, the primary air of the self-preheating stable combustion swirl burner is activated. Pulverized coal is heated at high temperature in the pre-combustion chamber, undergoing pyrolysis and gasification to transform into residual char and flammable volatile gases. The flammable volatile gases ignite rapidly and ignite the residual char. The stable combustion of both within the furnace provides a stable ignition heat source for the direct-current burner, thereby achieving stable combustion at ultra-low load.

[0026] Furthermore, the self-preheating and stable combustion swirl burner of this invention can replace the ignition oil gun during the start-up process, reducing the consumption of heavy oil and lowering operating costs.

[0027] Furthermore, this invention can increase the number of self-preheating and stable combustion swirl burners, and by adding two layers, it can ensure stable combustion of coals with poor combustion characteristics, thus expanding the range of fuels. At the same time, installing an additional layer of self-preheating and stable combustion swirl burners at the top can increase the furnace outlet temperature, thereby increasing the steam temperature at the outlet and improving steam quality.

[0028] Furthermore, the self-preheating and stable combustion swirl burner of this invention does not affect the pressure-bearing components during installation, thus avoiding drilling holes in the furnace wall and large-scale modifications to the water-cooled wall, reducing construction difficulty.

[0029] Furthermore, this invention can control the swirl intensity and flue gas entrainment by adjusting the ratio of direct-flow internal secondary air to swirl internal secondary air, thereby controlling the pre-combustion chamber temperature; and by adjusting the ratio of internal to external secondary air, it ensures that the pre-combustion chamber is in a low-oxygen environment. Pulverized coal is pyrolyzed in the high-temperature, low-oxygen environment of the pre-combustion chamber, and NOx is reduced to N2 using a locally reducing atmosphere, effectively reducing NOx emissions.

[0030] Furthermore, the present invention can directly cool the pre-combustion chamber through the external secondary air around the pre-combustion chamber, while isolating the backflow of high-temperature flue gas around the pre-combustion chamber, thereby indirectly cooling the outlet of the pre-combustion chamber and preventing slagging at the nozzle of the pre-combustion chamber. Attached image description:

[0031] Figure 1 This is a schematic diagram of the overall structure of the boiler.

[0032] Figure 2 This is a schematic diagram of the structure of a boiler furnace with tangent circles at the four corners.

[0033] Figure 3 This is a schematic diagram of a self-preheating, stable-burning swirl burner.

[0034] Figure 4a , Figure 4b These are schematic diagrams of a staged air distribution coal combustion system with one and two layers of self-preheating and stable combustion swirl burners, respectively.

[0035] Figure 5 A schematic diagram of the combustion system in a staged air distribution coal-fired boiler.

[0036] Figure 6a , Figure 6b These are schematic diagrams of two combustion system structures in a coal-fired boiler with two-stage direct-flow burners.

[0037] Among them, 1 is a self-preheating stable combustion swirl burner, 2 is a direct-flow burner, 3 is a secondary air nozzle, 4 is a wind box, 5a is a compact burnout air nozzle, 5b is a separate burnout air nozzle, 6 is a boiler furnace, 7 is a coal mill, 8 is a central tube, 9 is a primary air duct, 10 is a direct-flow internal secondary air duct, 11 is a swirl internal secondary air duct, 12 is a tangential swirl blade, 13 is an external secondary air duct, 14 is a pre-combustion chamber, 15 is an ignition oil gun, and 16 is an igniter. Detailed Implementation

[0038] The present invention will be further described in detail below with reference to the embodiments.

[0039] like Figure 1 , Figure 2 , Figure 3 , Figure 4a , Figure 4b , Figure 5As shown in Figure 6, this invention provides a direct-current / swirl-coupling coal-fired boiler ultra-low load stable combustion system. This system replaces the original secondary air nozzle 3 in the coal-fired boiler with a self-preheating stable combustion swirl burner 1, coupling it with the direct-current burner 2. Specifically, at low loads, the self-preheating stable combustion swirl burner 1 acts as an ignition source, providing a high-temperature flame to the direct-current burner 2 to ignite its primary air. At higher loads, the self-preheating stable combustion swirl burner 1 supplies secondary air into the furnace, providing sufficient air for the primary air pulverized coal combustion of the direct-current burner 2, thus enabling the two to operate in synergy. This system ensures stable ignition and combustion of the boiler at below 30% of its rated load while minimizing construction work, achieving ultra-low load stable combustion.

[0040] refer to Figure 1 , Figure 2 The ultra-low load stable combustion system includes a self-preheating stable combustion swirl burner 1, a direct-flow burner 2, a secondary air nozzle 3, an air box 4, a burnout air nozzle 5, a boiler furnace 6, and a coal mill 7. In existing uniform air distribution coal-fired boilers, one or two secondary air nozzles 3 are evenly arranged between two primary air nozzles and are close to each other; in staged air distribution coal-fired boilers, the secondary air nozzles 3 are relatively concentrated and close to the lower part of the burner. In this invention, the self-preheating stable combustion swirl burner 1 directly replaces the original one layer of secondary air nozzles 3, and is placed vertically between two layers of direct-flow burners 2, and arranged in a tangential circle in the horizontal direction; after the pulverized coal gas flow is ejected from the self-preheating stable combustion swirl burner 1, it moves along the tangential direction of the spiral. Initially, the four radial annular airflows are very violently disturbed, and later a relatively large tangential circle is formed in the center of the furnace.

[0041] The DC burner 2 has six layers, which are designated as DC burner 2A, DC burner 2B, DC burner 2C, DC burner 2D, DC burner 2E, and DC burner 2F from bottom to top for ease of description. Each layer of DC burners is arranged in a tangential combustion pattern at the four corners of the furnace 6, and each corresponds to a coal mill 7. After the outlet airflows of the DC burners 2 at the four corners ignite, they intersect and ignite each other, and after being tangential to the imaginary circle, they rotate intensely in the furnace. The tangential circles produced by the two types of burners are conducive to complete combustion and uniform heat load distribution in the furnace. The tangential diameter of the self-preheating and stable combustion swirl burner 1 is relatively larger, which can improve the furnace filling degree, and the flames in the upper adjacent corners are closer to the jet root, making it easier to stably ignite under ultra-low load conditions and ignite the primary air and pulverized coal of the DC burner 2.

[0042] There are two methods for supplying pulverized coal to the self-preheating and stable combustion cyclone burner 1. One method is to connect the primary air duct 9 in parallel to a branch pipe of the coal mill 7 connected to the top-level (standby) DC burner 2F. The amount of pulverized coal supplied to the self-preheating and stable combustion cyclone burner 1 is controlled by adjusting the coal feed rate of the coal mill 7. The other method is to directly connect the primary air duct 9 to the intermediate storage pulverized coal silo. The amount of pulverized coal supplied to the self-preheating and stable combustion cyclone burner 1 can be directly adjusted by changing the feeder speed, providing a rapid and stable response to load changes. Simultaneously, the pulverized coal silo contains a large amount of pulverized coal, improving the reliability of the unit operation. The DC inner secondary air duct 11, the cyclone inner secondary air duct 12, and the outer secondary air duct 13 of the self-preheating and stable combustion cyclone burner 1 are all connected to the air box 4 to realize the delivery of secondary air.

[0043] refer to Figure 3 The self-preheating and stable combustion swirl burner 1 consists of a central pipe 8, a primary air pipe 9, a direct-flow internal secondary air pipe 11, and a swirl internal secondary air pipe 12, all of which are connected to the inlet of the pre-combustion chamber 14. The inlet pipe of the swirl internal secondary air pipe 12 has tangential swirl blades 10. The external secondary air pipe 13 is fitted on the outer wall of the pre-combustion chamber 14, and the outlet of the external secondary air pipe 13 and the outlet of the pre-combustion chamber 14 are both connected to the furnace 6.

[0044] During operation, the swirling internal secondary air injected through the swirling internal secondary air duct 12 entrains the high-temperature flue gas at the rear of the pre-combustion chamber 14, and the high-temperature heat storage in the pre-combustion chamber 14 promotes timely ignition of pulverized coal through reverse radiation. The external secondary air duct 13 injects direct-flow external secondary air into the furnace 6 at high speed, cooling the outlet of the pre-combustion chamber 14 and thus preventing or mitigating slagging. By adjusting the ratio of direct-flow internal secondary air to swirling internal secondary air, and the ratio of internal secondary air to external secondary air, the pulverized coal gas flow can be pyrolyzed and gasified in the high-temperature, low-oxygen environment of the pre-combustion chamber 14, effectively reducing NOx emissions.

[0045] When the boiler load decreases, the amount of coal burned and the furnace temperature drop, leading to unstable combustion. When the load drops to ultra-low levels below 30%, the once-through burner 2 cannot operate normally due to ignition difficulties. At this time, the self-preheating, stable-burning swirl burner 1, with its advantages of small heat capacity and low ignition heat, can ensure rapid and stable ignition of the pulverized coal. Combined with... Figure 1 , Figure 2 , Figure 3 It can be seen that when the self-preheating and stable combustion swirl burner 1 and the direct current burner 2 operate simultaneously under ultra-low load conditions, the mixed pulverized coal airflow of the self-preheating and stable combustion swirl burner 1 can stably burn and generate a high-temperature flame, providing superior ignition conditions for the adjacent primary air pulverized coal, thus enabling the direct current burner 1 to operate stably; the airflows ejected from the adjacent burners ignite each other, and the two burners operate in a direct current-swirl coupling mode to achieve stable combustion under ultra-low load.

[0046] The uniform air distribution method shown in Figure 4 is mostly used in boilers burning high-volatile bituminous coal and lignite. (Reference) Figure 4a When this invention is applied to a coal-fired boiler using uniform air distribution, the self-preheating and stable combustion swirl burner 1 can replace any one layer of secondary air nozzles 3 between the primary air nozzles of the AF layer direct-flow burner. During ultra-low load operation, the pulverized coal in the self-preheating and stable combustion swirl burner 1 undergoes high-temperature, low-oxygen pyrolysis gasification in the pre-combustion chamber 14, transforming into residual char and combustible volatile gases. The combustible volatile gases ignite rapidly and carry the residual char into the furnace, igniting the primary air pulverized coal ejected from the adjacent direct-flow burner 2 and stabilizing combustion, thus playing a role in stabilizing the flame of the small flare. (Reference) Figure 4b When this boiler burns coal with poor combustion characteristics, the number of layers n of the self-preheating and stable combustion swirl burner 1 is 2, that is, it replaces any two layers of secondary air nozzles 3 between the AF layers. The two layers of self-preheating and stable combustion swirl burners 1 provide two layers of high-temperature torches, which promote more intense combustion of the pulverized coal injected by the direct-flow burner 2, ensuring that even coal with poor combustion characteristics can be completely burned; installing an additional layer of self-preheating and stable combustion swirl burner 1 at the top can also increase the outlet temperature of the furnace 6, thereby increasing the steam temperature at the outlet and improving the steam quality.

[0047] Figure 5 The graded air distribution method described above is suitable for boilers burning low-volatile anthracite, lean coal, and low-quality bituminous coal. (Reference) Figure 5 When this invention is applied in a coal-fired boiler using staged air distribution, the self-preheating and stable combustion swirl burner 1 can replace any one layer of secondary air nozzle 3. The ignition gas flow delivered at high speed by the self-preheating and stable combustion swirl burner 1 is strongly mixed with the pulverized coal gas flow of the downstream direct-flow burner 1, increasing the diffusion speed. The stable combustion effect of the high-temperature flame promotes the vigorous combustion of pulverized coal and the rapid completion of the burnout process.

[0048] refer to Figure 6a When this invention is applied in a coal-fired boiler equipped with two layers of direct-flow burners 2, the self-preheating and stable-fire swirl burner 1 can replace the original secondary air nozzle 3 between the two layers of direct-flow burners 2; if the space at this secondary air nozzle is insufficient, such as Figure 6b As shown in Figure 6, the self-preheating and stable combustion swirl burner 1 can be placed above the top secondary air nozzle 3, arranged tangentially at the four corners of the furnace 6. The ultra-low load operation principle of the installation method shown in Figure 6 is similar to... Figure 4a The installation method shown is the same. Similarly, in coal-fired boilers with three, four, or five layers of DC burners 2, the installation method of the self-preheating and stable combustion swirl burner 1 is the same as that for two layers; coal-fired boilers with different numbers of DC burner layers can meet the boiler capacity requirements of different working scenarios.

[0049] Example 1:

[0050] When a tangential coal-fired boiler operates at or above 30% load, the number N (1≤N≤6) and opening degree of the direct-flow burners 2 are determined according to the magnitude of the working load, with one layer (usually F) reserved as a spare. The coal mill 7 and the air box 4 supply pulverized coal and air to the direct-flow burners 2, respectively. The primary air duct 9 of the self-preheating and stable-fire swirl burner 1 is closed; the opening degree of the swirl inner secondary air duct 12 is reduced or closed to maintain airflow rigidity; the direct-flow inner secondary air duct 11 and the outer secondary air duct 13 respectively send the direct-flow inner and outer secondary air into the furnace 6; no reaction occurs in the pre-combustion chamber 14. At this time, the function of the self-preheating and stable-fire swirl burner 1 is exactly the same as that of the secondary air nozzle 3 in a conventional power plant boiler.

[0051] Example 2:

[0052] When the tangential coal-fired boiler operates at less than 30% load, the primary air duct 9 of the self-preheating and stable combustion swirl burner 1 is opened, introducing a mixture of pulverized coal and primary air. The direct-flow inner secondary air duct 11, the swirl inner secondary air duct 12, and the outer secondary air duct 13 are connected to the air box 4, introducing secondary air. The pulverized coal is pyrolyzed and gasified in the high-temperature environment of the pre-combustion chamber 14, transforming into residual coke and flammable volatile gases. The highly concentrated flammable volatile gases in the pre-combustion chamber 14 ignite rapidly and carry the residual coke into the boiler furnace 6, mixing with the outer secondary air of the self-preheating and stable combustion swirl burner 1 and undergoing stable combustion in the direct-flow burner. During this process, the self-preheating and stable combustion swirl burner 1 provides a stable ignition heat source at the root of the adjacent direct-flow burner 2 above or below it, playing a role in stabilizing the flame of the small flare and ensuring the stable operation of the direct-flow burner 2 in that layer.

[0053] Example 3:

[0054] During the start-up process of the coal-fired boiler, all direct-current burners 2 are shut down first, and the self-preheating and stable-fire swirl burners 1 are started. At this time, the specific operation mode of the self-preheating and stable-fire swirl burners 2 is exactly the same as in Example 2. As the boiler load continues to increase, the direct-current burners 2 are turned on, and the self-preheating and stable-fire swirl burners 1 continue to operate normally. After the boiler load reaches 30% of the rated load, the primary air ducts 9 and the secondary air ducts 12 inside the swirl of all self-preheating and stable-fire swirl burners 1 can be shut down, and an appropriate number of direct-current burners 2 can be operated.

[0055] The above description is merely illustrative of the technical concept of the present invention and should not be construed as limiting the scope of protection of the present invention. Terms such as "one layer," "two layers," "above," "top," "arrangement," and "installation" used in the description of this invention should be taken in a broad sense and are not intended to indicate or imply a specific location, orientation, or installation method, and should not be construed as limiting the present invention. Any modifications made to the technical solution based on the technical concept of this invention, such as changing the two-layer DC burner in Figure 6 to three, four, or five layers, making the DC burner horizontal or vertical, or adding a concentrator to the swirl burner, etc., all fall within the scope of protection of the claims of this invention.

Claims

1. An operation method for a DC-swirl phase-coupled coal-fired boiler ultra-low load stable combustion system, characterized in that, The combustion system includes: a self-preheating and stable combustion swirl burner (1), a direct-flow burner (2) arranged in a tangential pattern at the four corners, a secondary air nozzle (3), a wind box (4), a burnout air nozzle (5), a boiler furnace (6), and a coal mill (7). The self-preheating and stable combustion swirl burner (1) is installed vertically at the secondary air nozzle (3) between the two layers of direct current burners (2). It acts as an ignition source at low load and as secondary air at high load. The self-preheating and stable combustion swirl burner (1) is arranged in a tangential manner at the four corners of the boiler furnace (6) in the horizontal direction. The self-preheating stable combustion swirl burner (1) includes a primary air duct (9), a direct-flow inner secondary air duct (11) connected to the air box (4), a swirl inner secondary air duct (12) and an outer secondary air duct (13), and a pre-combustion chamber (14). After exiting the preheating stable combustion swirl burner (1), the airflow moves along the tangential direction of the spiral. It is equipped with two operating modes: When the boiler is running at low load, the primary air duct (9) is opened, and the direct-flow inner secondary air duct (11), the swirl inner secondary air duct (12) and the outer secondary air duct (13) are connected to the air box (4) and secondary air is supplied. After the primary air pulverized coal is pyrolyzed and ignited in the pre-combustion chamber (14) at high temperature, it is sent into the boiler furnace (6) and burns stably. At this time, the self-preheating and stable combustion swirl burner (1) can provide a stable ignition heat source for the direct-flow burner (2). When operating under high load, the number of layers N and opening degree of the DC burner (2) are determined based on the size of the boiler load. 1≤N≤6. The primary air duct (9) is closed. The DC inner secondary air duct (11), the swirl inner secondary air duct (12) and the outer secondary air duct (13) operate normally to send air into the boiler furnace (6). At this time, the function of the self-preheating and stable combustion swirl burner (1) is exactly the same as that of the secondary air nozzle (3). The DC burner (2) has six layers, from bottom to top: A, B, C, D, E, F. Each layer corresponds to a coal mill (7). All six DC burners are arranged in a tangential combustion manner at the four corners of the boiler furnace (6).

2. The operation method of a DC-swirl phase-coupled coal-fired boiler ultra-low load stable combustion system according to claim 1, characterized in that, When the DC burner (2) has six layers, from bottom to top they are divided into DC burner one (2A), DC burner two (2B), DC burner three (2C), DC burner four (2D), DC burner five (2E) and DC burner six (2F), with each layer corresponding to a coal mill (7).

3. The operation method of a DC-swirl phase-coupled coal-fired boiler ultra-low load stable combustion system according to claim 2, characterized in that, The primary air duct (9) of the self-preheating stable combustion swirl burner (1) is connected in parallel to the primary air duct of the DC burner six (2F); its secondary air ducts, namely the DC inner secondary air duct (11), the swirl inner secondary air duct (12) and the outer secondary air duct (13), are all connected to the air box (4) to realize the delivery of secondary air.

4. The operation method of a DC-swirl phase-coupled coal-fired boiler ultra-low load stable combustion system according to claim 2, characterized in that, There are two options for the pulverized coal supply in the self-preheating stable combustion swirl burner (1); One is to connect the primary air duct (9) in parallel to the branch pipe of the coal mill (7) connected to the DC burner six (2F) on the top floor, i.e. the backup layer, and control the amount of coal powder supplied to the self-preheating stable combustion swirl burner (1) by adjusting the coal pulverizing amount of the coal mill (7). Secondly, the primary air duct (9) is directly connected to the medium-sized coal pulverized silo, and the amount of coal supplied to the self-preheating stable combustion swirl burner (1) is directly adjusted by changing the speed of the coal feeder, so as to respond quickly and stably to load changes.

5. The operation method of a DC-swirl phase-coupled coal-fired boiler ultra-low load stable combustion system according to claim 2, characterized in that, The boiler furnace (6) has four self-preheating and stable combustion swirl burners (1) arranged in each layer, with a total of n layers and 1≤n≤2; each layer is located at the secondary air nozzle (3) above the DC burner (2) in layer B or layer D.

6. The operation method of a DC-swirl phase-coupled coal-fired boiler ultra-low load stable combustion system according to claim 1, characterized in that, The self-preheating and stable combustion swirl burner (1) consists of a central pipe (8), a primary air pipe (9), a direct-flow internal secondary air pipe (11), and a swirl internal secondary air pipe (12) along the axial direction from the inside to the outside, and all of them are connected to the inlet of the pre-combustion chamber (14); the inlet pipe of the swirl internal secondary air pipe (12) has several tangential swirl blades (10); the external secondary air pipe (13) is fitted on the outer wall of the pre-combustion chamber (14), and the outlet of the external secondary air pipe (13) and the outlet of the pre-combustion chamber (14) are both connected to the boiler furnace (6).

7. The ultra-low load stable combustion system for a coal-fired boiler with DC-swirl phase coupling according to claim 1, characterized in that, The DC burner (2) has secondary air nozzles (3) on both the upper and lower sides of the primary air nozzle; the burnout air nozzle (5) is located above the top secondary air nozzle (3).

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