A supersonic plasma laval nozzle ignition device and method for a fan mill lignite

Abstract

The application discloses a supersonic plasma Laval nozzle ignition device and method for a fan mill lignite, which comprises a coaxial cathode-anode assembly, a Laval nozzle section and a gradient composite ceramic lining. The coaxial cathode-anode assembly is provided with coaxial first and second cathodes and an anode, the anode is provided with a carrier air inlet, and the two-stage electrode and the anode enclose a two-stage series arc area. The Laval nozzle section is fixedly connected with the gas outlet of the anode, and is sequentially provided with a contraction section, a throat and an expansion section suitable for the characteristics of lignite pyrolysis along the flow direction. The inner wall of the nozzle metal base is provided with the gradient composite ceramic lining. The two-stage arc provides a supersonic gas source, the Laval nozzle prolongs the high-temperature jet, and the three-layer gradient lining buffers thermal stress and resists coal powder erosion, thereby solving the problems of low jet speed, poor ignition effect and easy peeling and wearing of the single-layer protective coating of the traditional single-stage arc, and effectively improving the lignite ignition stability and the service life of the equipment.

Description

Technical Field

[0001] This invention belongs to the technical field of plasma ignition equipment for thermal power plant boilers, specifically relating to a supersonic plasma Laval nozzle ignition device and method for lignite mills using fan mills. Background Technology

[0002] Lignite boilers in thermal power plants equipped with direct-fired pulverized coal systems using fan mills mostly rely on plasma generators for pulverized coal ignition and stable combustion at low loads. Traditional plasma ignition equipment uses only a single-stage electric arc heating method, resulting in a subsonic plasma jet with a short effective high-temperature zone. This leads to insufficient thermal cracking of the lignite pulverized coal, preventing rapid precipitation of volatiles and resulting in a low boiler ignition success rate. Conventional plasma anodes are typically only fitted with a straight cylindrical structure or a simple converging nozzle, making it difficult to generate supersonic airflow. Furthermore, the inner wall of the nozzle is mostly coated with only a single layer of ceramic protective layer, which is prone to cracking and peeling due to long-term erosion by high-temperature plasma and hard lignite pulverized coal. Consequently, the anode substrate suffers ablation and loss, resulting in frequent equipment maintenance and a short service life. Currently, similar technologies lack a two-stage series electric arc for gradual temperature increase, making it impossible to supply the high-temperature and high-pressure gas source required to form a supersonic jet. They also lack a Laval nozzle profile specifically designed to address the pyrolysis characteristics of lignite, and do not incorporate multi-layer gradient buffer linings to counteract thermal stress and erosion damage. Consequently, they are ill-suited to the ignition requirements of large-particle lignite pulverized coal output from fan mills, resulting in significant deficiencies in overall ignition stability and equipment lifespan. Summary of the Invention

[0003] The purpose of this invention is to provide a supersonic plasma Laval nozzle ignition device and method for lignite milling, which solves the technical defects of existing plasma ignition devices, such as the use of a single-stage electric arc that can only produce subsonic jets, insufficient pyrolysis of lignite powder and low ignition success rate, conventional nozzles that cannot generate supersonic airflow, single-layer ceramic protective coatings that are easily eroded and peeled off, resulting in substrate ablation and loss, lack of a two-stage series electric arc for step-by-step gas supply structure, failure to optimize nozzle profile according to lignite pyrolysis characteristics, lack of gradient buffer lining to resist thermal stress erosion, resulting in weak ignition stability, frequent equipment maintenance and short service life.

[0004] To achieve the above objectives, this application provides the following technical solution: The first aspect of this application provides a supersonic plasma Laval nozzle ignition device for a fan mill grinding lignite, comprising: A coaxial cathode and anode assembly includes a first cathode, a second cathode, and an anode arranged coaxially. The anode has a carrier air inlet. The first cathode and the second cathode are coaxially arranged along the anode axis. The first cathode and the second cathode respectively enclose the anode to form a two-stage series arc zone that is sequentially connected along the airflow direction. The Laval nozzle section is fixed to the gas flow outlet of the anode. The Laval nozzle section is provided with a converging section, a throat and an expanding section in sequence along the gas flow direction. The outline size of the expanding section is adapted to the pyrolysis characteristics of lignite to extend the high-temperature jet region. A gradient composite ceramic liner is disposed on the inner wall of the metal matrix of the Laval nozzle section. The gradient composite ceramic liner includes an adhesive layer, a stress buffer transition layer, and a high-temperature resistant and erosion-resistant working layer. The inner side of the adhesive layer is tightly fused to the inner wall of the metal matrix, the outer surface of the adhesive layer is fixed to the inner side of the stress buffer transition layer, and the outer side of the stress buffer transition layer is bonded to the inner side of the high-temperature resistant and erosion-resistant working layer. The adhesive layer, stress buffer transition layer, and high-temperature resistant and erosion-resistant working layer are formed by stacking the metal substrate in sequence towards the airflow side.

[0005] In one optional embodiment, the Laval nozzle section and the anode are connected by integral casting. The contraction section adopts a smooth curved surface structure; The contraction section smoothly transitions to the throat, and the throat cross-section is the minimum flow cross-section of the Laval nozzle section; The rear end of the throat is smoothly connected to the expansion section.

[0006] In one optional embodiment, the expansion section is provided with an expansion angle that matches the release pattern of lignite volatiles, and the expansion section is used to extend the length of the high-temperature zone in the core of the supersonic jet.

[0007] In one optional embodiment, the gradient composite ceramic liner is a metal-ceramic composite structure with a continuously varying gradient composition.

[0008] In one alternative embodiment, the stress buffer transition layer is made of a ceramic material with a continuously gradient composition. The stress buffer transition layer is seamlessly fused to the adhesive layer and the high-temperature resistant and erosion-resistant working layer at both ends.

[0009] In one optional embodiment, the high-temperature resistant and erosion-resistant working layer is made of a high-melting-point high-temperature resistant ceramic material; The inner wall surface of the high-temperature resistant and erosion-resistant working layer is a smooth, stepless, continuous curved surface. The high-temperature resistant and erosion-resistant working layer is in direct contact with high-temperature plasma and dust-laden coal powder airflow.

[0010] In one optional embodiment, both the first cathode and the second cathode are columnar electrode structures; The first cathode and the second cathode are arranged coaxially and equidistantly along the central axis of the anode. The flow channel of the two-stage series arc zone is continuously connected without interruption along the airflow direction of the carrier.

[0011] In one alternative embodiment, the anode is hollow to form an annular airflow channel; The carrier air inlet is connected to the annular airflow channel; The annular airflow channel is connected to two series-connected electric arc zones to achieve staged electric arc heating of the carrier air.

[0012] In one optional embodiment, the metal substrate is an integral nozzle substrate made of pure copper alloy. The outer wall of the metal substrate is provided with a water-cooled cooling channel.

[0013] A second aspect of this application provides a method for igniting a supersonic plasma Laval nozzle for lignite milling, the method employing the supersonic plasma Laval nozzle ignition device for lignite milling as described above, comprising: Carrier air is introduced into the annular airflow channel inside the anode through the carrier air inlet. The carrier air flows through two-stage series electric arc zones in sequence, and the carrier air is heated step by step by the two-stage electric arcs formed between the first cathode, the second cathode and the anode. The high-temperature airflow enters the integrated Laval nozzle section, and after being converged and accelerated in the contraction section and throttled in the throat, it is accelerated by the expansion section to form a supersonic plasma jet. The optimized expansion section profile is used to lengthen the high-temperature region of the jet. The supersonic jet directly impacts the lignite powder conveyed by the fan mill. The gradient composite ceramic lining resists the erosion of the coal powder and high-temperature ablation. The water-cooled flow channel on the outer wall of the metal substrate continuously cools the nozzle substrate, completing the lignite ignition and stable combustion operation.

[0014] Compared with the prior art, the present invention has the following beneficial effects: 1. A two-stage series-connected coaxial cathode and anode assembly, a Laval nozzle section, and a three-layer gradient composite ceramic liner are installed. The two-stage electric arc supplies a supersonic gas source, the nozzle elongates the high-temperature jet, and the multi-layer liner buffers thermal stress and coal powder erosion. This solves the defects of low jet velocity, poor ignition, and easy peeling and wear of single-stage electric arc, and improves the ignition stability of lignite and the overall service life of the device.

[0015] 2. The Laval nozzle section is integrally cast with the anode, and features a smoothly transitioning contraction section, throat, and expansion section structure. The integrated structure eliminates air leakage problems caused by assembly gaps, and the smooth, continuous surface reduces airflow resistance, ensuring stable airflow acceleration to form a supersonic jet. This overcomes the problems of poor air tightness, large airflow disturbance, and unstable jet formation associated with split nozzles.

[0016] 3. The expansion angle of the expansion section is set according to the release law of lignite volatiles. The optimized expansion section structure extends the high-temperature region of the supersonic jet core, which allows large-particle lignite pulverized coal to be fully heated and decompose volatiles. This solves the defects of the original nozzle with short high-temperature section, insufficient pyrolysis of pulverized coal, and low ignition success rate, and is suitable for lignite ignition conditions of fan mill.

[0017] 4. The gradient composite ceramic liner is set as a metal-ceramic structure with continuously varying composition. The difference in thermal expansion between layers is reduced according to the gradient change of material composition, which alleviates the problem of thermal stress concentration between layers under high temperature conditions, avoids cracking and falling off of the liner, and improves the technical shortcomings of easy peeling and failure of single-layer protective coating and easy erosion and burning of nozzle substrate.

[0018] 5. The stress buffer transition layer adopts a gradient ceramic material with seamless cladding of the bonding layer and working layer at both ends. Based on the deformation difference between the inner and outer layers, the transition layer disperses the high-temperature thermal stress layer by layer, eliminating the cracking and peeling of the interlayer interface. This solves the problems of traditional single-layer coating without stress buffer structure, easy whole-piece peeling when heated, and rapid failure of nozzle protection.

[0019] 6. The high-temperature resistant and erosion-resistant working layer is made of high-melting-point ceramic with a smooth and continuous curved inner wall. This working layer directly faces the plasma and coal powder erosion. The high-melting-point material can withstand high temperatures, and the smooth surface reduces coal powder adhesion and wear, effectively resisting media erosion and ablation, and solving the defects of easy rapid wear and high equipment maintenance frequency of the surface protection structure.

[0020] 7. Two columnar cathodes are arranged coaxially and equidistantly with the arc channel running through the entire length. By using the two-stage cathodes to sequentially ignite the arc and construct a series arc heating path, the carrier air can be heated step by step. This can generate the high-temperature and high-pressure airflow required for supersonic jets, solving the core problem that the heating capacity of a single-stage arc is limited, unable to provide sufficient heat source, and difficult to form a supersonic plasma jet.

[0021] 8. An annular airflow channel is set inside the anode to connect the carrier air inlet and the two-stage electric arc zone. The carrier air can pass through the two-stage electric arc to complete progressive heating. The airflow delivery path is regular and turbulent, and the heat absorption is sufficient and stable. This makes up for the shortcomings of single-stage heating, such as insufficient temperature rise and unstable gas source parameters, which cannot continuously maintain the supersonic jet working condition.

[0022] 9. The pure copper alloy integral metal substrate is adopted and a water-cooling channel is added to the outer wall. The copper substrate has excellent thermal conductivity. Combined with the water-cooling circuit, it continuously removes the excess heat conducted from the inner lining to the substrate, inhibits high-temperature creep and oxidation erosion of the substrate, and solves the defects of the nozzle substrate being easily deformed and damaged by long-term high temperature, resulting in short service life and frequent failures of the overall device. Attached Figure Description

[0023] To more clearly illustrate the technical solutions of the embodiments of the present invention, the accompanying drawings used in the embodiments will be briefly introduced below. It should be understood that the following drawings only show some embodiments of the present invention and should not be regarded as a limitation on the scope. For those skilled in the art, other related drawings can be obtained based on these drawings without creative effort.

[0024] Figure 1 A schematic diagram of an overall ignition device for a supersonic plasma Laval nozzle used in a fan mill for lignite production, provided by the present invention; In the figure: 1. First cathode; 2. Second cathode; 3. Anode; 4. Laval nozzle section; 5. Contraction section; 6. Throat; 7. Expansion section; 8. Gradient composite ceramic liner; 9. Metal matrix; 10. Carrier air inlet. Detailed Implementation

[0025] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. The components of the embodiments of the present invention described and shown in the accompanying drawings can generally be arranged and designed in various different configurations.

[0026] Therefore, the following detailed description of the embodiments of the invention provided in the accompanying drawings is not intended to limit the scope of the claimed invention, but merely to illustrate selected embodiments of the invention. All other embodiments obtained by those skilled in the art based on the embodiments of the invention without inventive effort are within the scope of protection of the invention.

[0027] It should be noted that similar labels and letters in the following figures indicate similar items. Therefore, once an item is defined in one figure, it does not need to be further defined and explained in subsequent figures.

[0028] Existing conventional plasma ignition devices have the following drawbacks: First, the electrode structure only features a single-stage arc heating structure, and the carrier air is only heated once. This results in low gas enthalpy and temperature upper limits, generating only subsonic plasma jets. The jets have weak shearing, crushing, and entrainment capabilities for pulverized coal, leading to insufficient thermal cracking of large-particle lignite, slow volatile matter release, and a low boiler ignition success rate. Second, the anode outlet is only equipped with a straight-tube nozzle or a simple contraction nozzle, lacking a complete contraction-throat-expansion Laval profile. This prevents the airflow from exceeding the speed of sound to form a supersonic jet, and the high-temperature section of the jet core is short, resulting in insufficient residence time of pulverized coal in the high-temperature region. The first problem is that the nozzle is prone to delayed ignition and low burnout rate. The second problem is that the nozzle is only coated with a single layer of ceramic protective coating. The difference in thermal expansion coefficient between the ceramic coating and the metal substrate is large. Under the cold and hot cycle conditions, the internal thermal stress is concentrated, and the coating is prone to cracking and peeling off in whole pieces. The metal substrate of the nozzle is directly exposed to the high temperature plasma and high speed dust-laden coal powder scouring environment, resulting in severe substrate ablation and wear. The equipment has a short maintenance cycle and high operation and maintenance costs. The third problem is that the existing nozzle profile is a general standardized design and has not been optimized for the pyrolysis kinetics and volatile matter release characteristics of lignite. It cannot match the special ignition conditions of coarse coal powder from the fan mill, resulting in poor equipment adaptability.

[0029] To address the technical deficiencies mentioned in the background section, this embodiment provides a supersonic plasma Laval nozzle ignition device and method for using a fan mill to grind lignite.

[0030] The present invention will now be described in further detail with reference to the accompanying drawings: like Figure 1 As shown, in a first aspect of the present invention, a coaxial dual-electrode liquid metal composite cooling system for a plasma generator is provided, comprising a coaxial cathode-anode assembly, including a first cathode 1, a second cathode 2, and an anode 3 arranged coaxially. The anode 3 has a carrier air inlet 10. The first cathode 1 and the second cathode 2 are coaxially arranged along the axial direction of the anode 3, and the first cathode 1 and the second cathode 2 respectively enclose the anode 3 to form a two-stage series arc zone that is sequentially connected along the airflow direction. A Laval nozzle section 4 is fixed to the airflow outlet of the anode 3, and the Laval nozzle section 4 is provided with a converging section 5, a throat 6, and an expanding section sequentially along the airflow direction. Section 7, the outline size of expansion section 7 is adapted to the pyrolysis characteristics of lignite to extend the high-temperature jet region; gradient composite ceramic liner 8, set on the inner wall of the metal substrate 9 of Laval nozzle section 4, the gradient composite ceramic liner 8 includes a bonding layer, a stress buffer transition layer and a high-temperature resistant and erosion-resistant working layer; the inner side of the bonding layer is tightly fused to the inner wall of the metal substrate 9, the outer surface of the bonding layer is fixed to the inner side of the stress buffer transition layer, and the outer side of the stress buffer transition layer is attached to the inner side of the high-temperature resistant and erosion-resistant working layer; wherein, the bonding layer, the stress buffer transition layer and the high-temperature resistant and erosion-resistant working layer are stacked in layers from the metal substrate 9 toward the airflow side.

[0031] In this embodiment, the coaxial cathode and anode assembly includes a first cathode 1, a second cathode 2, and an anode 3. The anode 3 is a hollow cylindrical integrated electrode structure. A carrier air inlet 10 is opened on the side wall of the anode 3. The carrier air inlet 10 serves as the only air intake channel for high-pressure and high-temperature carrier air. The external high-pressure air pipeline is directly and sealed to the carrier air inlet 10.

[0032] The first cathode 1 and the second cathode 2 are arranged coaxially along the central axis of the anode 3. Both extend forward along the internal cavity of the anode 3. The first cathode 1 is located upstream of the airflow, and the second cathode 2 is arranged downstream of the first cathode 1. The two are equidistant along the central axis without radial offset or axial misalignment.

[0033] The first-stage electric arc heating zone is formed by the inner walls of the first cathode 1 and the anode 3, and the second-stage electric arc heating zone is formed by the inner walls of the second cathode 2 and the anode 3. The two-stage series electric arc zones are continuously connected along the direction of the carrier wind, without any partitions or throttling baffles in between, so that the carrier wind can flow through the two-stage electric arc zones in sequence to complete the stage-by-stage heating.

[0034] The hollow area inside the anode 3 forms an annular airflow channel. The carrier air inlet 10 is directly connected to the annular airflow channel. The upstream of the annular airflow channel is connected to the external air source, and the downstream is connected to two-stage series electric arc zones. Finally, high-temperature plasma airflow is output from the airflow outlet at the front end of the anode 3.

[0035] The Laval nozzle section 4 is fixedly connected to the airflow outlet at the front end of the anode 3. The two are rigidly sealed by an integrated casting process, eliminating the gaps between the parts and ensuring that the high-temperature and high-pressure plasma gas flow is completely prevented from leaking, thus ensuring stable gas flow pressure and providing a pressure basis for supersonic acceleration.

[0036] The Laval nozzle section 4, along the flow direction of the carrier wind and plasma airflow, is divided into three continuous and smooth sections: the contraction section 5, the throat 6, and the expansion section 7. These three sections are seamlessly connected, without steps or abrupt turns. The contraction section 5 is located upstream of the airflow and directly connects to the airflow outlet of the anode 3. The contraction section 5 adopts a smooth, continuous curved surface, narrowing smoothly inward from the large-diameter inlet. The smallest diameter point at the front end of the contraction section 5 is the throat 6, which is the smallest cross-section in the entire Laval nozzle section 4 and is the key section for the airflow to reach supersonic speeds. The rear end of the throat 6 smoothly extends outward to connect to the expansion section 7. The expansion section 7 gradually expands its cross-section along the airflow direction, relying on cross-sectional expansion to achieve secondary acceleration of the airflow to supersonic speeds.

[0037] The expansion section 7 is set with a dedicated expansion angle that matches the release pattern of lignite volatiles. The angle of the expansion angle and the overall length of the expansion section are determined through repeated optimization and simulation of lignite pyrolysis kinetics. The optimized expansion section 7 can effectively lengthen the core high-temperature zone of the supersonic plasma jet and extend the residence time of coal powder particles in the high-temperature zone, which is suitable for the working condition of fully thermally desorbing volatiles from coarse lignite particles by the fan mill.

[0038] The outermost substrate of the Laval nozzle section 4 is a metal substrate 9, which is made of one-piece pure copper alloy. Copper alloy has excellent thermal conductivity and can quickly dissipate the high-temperature heat transferred by the inner lining. The outer wall of the metal substrate 9 is surrounded by an integrated water-cooled cooling channel. The external cooling water circulation pipeline is sealed and connected to the water-cooled channel to continuously remove the heat accumulated in the metal substrate 9, inhibit high-temperature oxidation and creep deformation of the substrate, and improve the overall equipment operation stability.

[0039] The inner wall of the metal substrate 9 is completely fitted with a gradient composite ceramic liner 8. The gradient composite ceramic liner 8 is a metal-ceramic composite structure with continuously varying composition. It is divided into three independent and seamlessly connected functional layers, which are stacked in sequence from the inner wall of the metal substrate 9 toward the internal airflow side.

[0040] The bonding layer is the first layer of the gradient composite ceramic liner 8 closest to the metal substrate 9. It is made of metal ceramic material with a thermal expansion coefficient similar to that of the pure copper alloy metal substrate 9. The inner surface of the bonding layer is tightly bonded to the inner wall of the metal substrate 9 through a high-temperature fusion process, without any delamination gaps. This greatly improves the interfacial bonding strength between the liner and the metal substrate 9, and prevents the liner from falling off completely under hot and cold cycles. The outer surface of the adhesive layer is seamlessly fused and bonded to the inner surface of the stress buffer transition layer. The stress buffer transition layer is made of ceramic material with a continuous gradient of composition. The material composition smoothly transitions from the side closer to the adhesive layer to the side closer to the working layer. This can gradually resolve the huge thermal expansion difference between the adhesive layer and the working layer, disperse the interlayer thermal stress under high temperature conditions, and prevent interlayer interface cracking and peeling. The outer surface of the stress buffer transition layer is completely bonded to the inner surface of the high-temperature resistant and erosion-resistant working layer. The high-temperature resistant and erosion-resistant working layer is the innermost layer of the gradient composite ceramic liner 8, which is in direct contact with the high-speed high-temperature plasma and dust-laden coal powder airflow inside.

[0041] The high-temperature resistant and erosion-resistant working layer is made of high-melting-point high-temperature resistant special ceramic material, which has excellent high-temperature resistance, erosion resistance and wear resistance. At the same time, the inner wall of the high-temperature resistant and erosion-resistant working layer is designed as a smooth, stepless continuous curved surface, which eliminates dead angles of airflow eddies, reduces coal powder particle deposition and local concentrated erosion wear, and further extends the overall service life of the lining.

[0042] The contraction section 5 adopts a smooth, continuous curved surface throughout, without straight lines, sharp angles, or abrupt steps. When the high-temperature plasma gas flows inside the contraction section 5, there are no local eddies or energy losses, and the airflow accelerates smoothly and continuously. The minimum cross-sectional position of the contraction section 5 smoothly transitions to the throat 6, which strictly serves as the minimum flow cross-section of the entire nozzle, precisely controlling the airflow to reach the sonic critical state at the throat 6. The rear end of the throat 6 extends smoothly outward, seamlessly connecting to the expansion section 7. The airflow smoothly transitions from the sonic state to the supersonic acceleration range, avoiding jet divergence and shortening of the high-temperature zone caused by airflow disturbance. This embodiment solves the defects of traditional split nozzles, such as poor airtightness, large airflow disturbance, and unstable supersonic jet formation.

[0043] The expansion section 7 is set with a fixed expansion angle that matches the release pattern of lignite volatiles. The expansion angle value and the overall axial length of the expansion section are determined based on the simulation results of lignite pyrolysis kinetics. It is customized for the working condition where the volatilization analysis of coarse lignite particles in the fan mill is slow and a longer high-temperature residence time is required.

[0044] The optimized expansion section 7 can significantly extend the length of the core high-temperature zone of the supersonic jet, greatly increasing the coverage of the high-temperature zone. When coarse lignite powder passes through the jet, it can fully absorb heat, quickly complete pyrolysis, and release a large amount of volatiles, solving the core defects of traditional nozzles such as short high-temperature section, insufficient coal powder pyrolysis, and low ignition success rate.

[0045] The gradient composite ceramic liner 8 adopts a metal-ceramic composite structure with continuously varying composition. The material composition of the three functional layers changes smoothly along the metal substrate 9 towards the inner wall of the airflow, eliminating abrupt interface changes between two layers and significantly reducing the concentration of thermal stress between layers. Compared to existing single ceramic coatings, the overall gradient structure can withstand frequent hot and cold start-stop cycles without cracking or peeling of the entire coating, thus improving the lifespan of the nozzle inner wall from the overall material level of the liner.

[0046] The stress buffer transition layer uses ceramic materials with a continuous gradient of composition throughout, and the material components smoothly transition from the adhesive layer side to the working layer side, achieving a gradual transition in the coefficient of thermal expansion. The interfaces on both sides of the stress buffer transition layer are bonded to the outer side of the adhesive layer and the inner side of the high-temperature resistant and erosion-resistant working layer using a high-temperature seamless cladding process. There are no air gaps or weak points between the layers, which can disperse the deformation differences of each layer under high-temperature operation layer by layer, completely eliminating interlayer interface cracking and delamination. This solves the technical shortcomings of traditional single-layer protective coatings, such as lack of stress buffer structure, rapid whole-piece detachment under heat, and rapid erosion and burning of the nozzle substrate.

[0047] The high-temperature resistant and erosion-resistant working layer is made of high-melting-point high-temperature resistant special ceramic material, whose own high-temperature resistance limit is far higher than the working temperature of plasma jet, and can withstand continuous radiation of high-temperature plasma at thousands of degrees Celsius for a long time. The inner wall surface of the high-temperature resistant and erosion-resistant working layer is processed into a smooth, stepless continuous curved surface to eliminate local airflow vortex dead angles and reduce the local accumulation and concentrated erosion of coal powder particles. The inner wall of the working layer is in direct contact with the internal high-temperature plasma and high-speed dust-laden coal powder airflow. Relying on its own high hardness and high melting point characteristics, it resists the continuous erosion of coal powder particles and high-temperature plasma ablation, which greatly reduces the wear rate of the nozzle inner wall and reduces the frequency of equipment downtime for maintenance.

[0048] Both the first cathode 1 and the second cathode 2 adopt a standard columnar electrode structure, with regular electrode shape, stable discharge end face, and stable arc ignition and maintenance. The first cathode 1 and the second cathode 2 are strictly coaxially and equidistantly arranged along the central axis of the anode 3, with no radial offset between the two electrodes. The two-stage series arc zones are continuously connected without interruption along the entire direction of carrier airflow, allowing the carrier air to pass through the two-stage arc heating zones in sequence, absorbing arc heat step by step and progressively increasing the gas temperature and enthalpy. Compared with the traditional single-stage arc structure, the two-stage series coaxial electrodes can significantly increase the overall temperature of the carrier air without increasing the power of a single electrode, providing a sufficient high-temperature and high-pressure gas source for generating supersonic jets in the Laval nozzle section 4, solving the core problem of insufficient heating capacity of single-stage arcs and the inability to provide the high-temperature gas source required for supersonic airflow.

[0049] The anode 3 has a hollow interior forming an annular airflow channel. The carrier air inlet 10 on the side wall is fully connected to the annular airflow channel. The external high-pressure carrier air enters the annular airflow channel evenly through the carrier air inlet 10. The annular airflow channel connects two series-connected electric arc zones along the airflow direction. After entering the anode 3, the carrier air is first evenly distributed in the annular channel, then smoothly enters the first-stage electric arc zone to complete the initial heating, and then flows into the second-stage electric arc zone without interruption to complete the secondary heating, realizing two-stage progressive electric arc heating of the carrier air. The annular channel is regular and has no turbulent dead zones, ensuring stable airflow delivery and uniform and sufficient heat absorption. It continuously outputs a stable high-temperature and high-pressure plasma airflow for the Laval nozzle section 4, compensating for the shortcomings of insufficient temperature rise in single-stage heating, large fluctuations in gas source parameters, and the inability to continuously and stably maintain supersonic jet conditions.

[0050] The metal substrate 9 is integrally formed from pure copper alloy. Pure copper alloy has a high thermal conductivity, which can quickly dissipate the high-temperature heat transferred from the gradient composite ceramic liner 8 to the substrate. An integrally formed surrounding water-cooled channel is located on the outer wall of the metal substrate 9. An external industrial cooling water circulation pipeline continuously supplies low-temperature cooling water to the water-cooled channel, continuously removing the heat accumulated in the metal substrate 9 and inhibiting high-temperature creep, oxidation, and ablation deformation. The water-cooling structure, combined with the high thermal conductivity copper alloy substrate, continuously reduces the overall operating temperature of the nozzle, preventing damage and deformation of the substrate under prolonged high temperatures, extending the service life of the entire nozzle component, and reducing the probability of equipment failure and downtime.

[0051] In a second aspect, the present invention provides a method for igniting a supersonic plasma Laval nozzle for lignite milling, the method employing the supersonic plasma Laval nozzle ignition device for lignite milling as described above, comprising: After the equipment is started, the external high-pressure blower delivers room-temperature high-pressure carrier air to the carrier air inlet 10. The high-pressure carrier air smoothly enters the annular airflow channel inside the anode 3 through the carrier air inlet 10. The annular airflow channel evenly distributes the carrier air to the upstream first-stage series arc zone. The high-pressure airflow passes through the first-stage arc formed between the first cathode 1 and the anode 3, completing the initial high-temperature heating, and the gas temperature and enthalpy value are initially increased. The high-temperature airflow after the initial heating flows directly into the downstream second-stage series arc zone without interruption, passes through the second-stage arc that is stably maintained between the second cathode 2 and the anode 3, and completes the second progressive heating. The carrier air is ionized and converted into high-temperature and high-pressure plasma airflow, reaching the temperature and pressure standards required for supersonic jet, and completing the gas source preparation.

[0052] The high-temperature plasma gas flow, fully heated by two-stage electric arcs, enters directly from the gas outlet at the front end of anode 3 into the interior of the integrated cast Laval nozzle section 4. The high-temperature plasma gas flow first passes through the smooth curved contraction section 5, where the flow cross-section continuously narrows, the gas flow continuously converges and accelerates, and the pressure increases simultaneously. The gas flow reaches the throat 6, which is the minimum flow cross-section of the nozzle, where the gas flow reaches the critical state of sonic velocity. After passing the throat 6, the gas flow enters the expansion section 7, which has been optimized by lignite pyrolysis kinetics. The flow cross-section of the expansion section 7 gradually expands, the gas flow continuously expands and accelerates, and finally forms a stable supersonic plasma jet at the outlet of the expansion section 7. Relying on the expansion angle and length parameters of the expansion section 7 that match the volatile matter release law of lignite, the core high-temperature zone of the supersonic jet is effectively lengthened, and the high-temperature coverage area is greatly improved.

[0053] The fan mill continuously conveys coarse lignite pulverized coal to the boiler burner area. The supersonic plasma jet generated by this device directly impacts and entrains the surrounding dust-laden lignite pulverized coal airflow at high speed. The supersonic jet has a strong shearing and crushing effect, which can shear and crush large particles of lignite pulverized coal into fine particles. At the same time, the elongated core high-temperature zone ensures that the pulverized coal particles are in a high-temperature environment for a long time. The volatile matter inside the lignite is rapidly and fully released. The volatile matter is rapidly ignited when it encounters the high-temperature plasma, completing the oil-free ignition of the boiler. When the unit is running at low load, it continuously and stably outputs the supersonic high-temperature jet to maintain stable combustion of pulverized coal in the furnace and achieve stable combustion at low load.

[0054] During the entire ignition and stable combustion operation of the device, the gradient composite ceramic lining 8 on the inner wall of the Laval nozzle section 4 continuously resists the scouring and abrasion of high-speed coal powder particles and the ablation of high-temperature plasma. It relies on the three-layer gradient structure to buffer the interlayer thermal stress and prevent the lining from cracking and peeling off. The water-cooled cooling channels surrounding the outer wall of the metal substrate 9 continuously supply circulating cooling water, which uninterruptedly removes the high-temperature heat accumulated in the metal substrate 9, controls the overall operating temperature of the nozzle, prevents the metal substrate from high-temperature oxidation and deformation damage, and enables the device to operate continuously and stably for a long time, reducing the frequency of equipment maintenance.

[0055] External high-pressure carrier air enters the annular airflow channel of anode 3 through carrier air inlet 10. Relying on the continuous series electric arc zone constructed by the first cathode 1 and the second cathode 2 coaxial cylindrical electrodes, the carrier air is heated in two stages, breaking through the upper temperature limit of traditional single-stage electric arc heating, thus producing a high-temperature, high-pressure plasma gas source that meets the conditions for supersonic acceleration. The high-temperature plasma gas flow enters the Laval nozzle section 4, which is integrally cast with anode 3. Relying on the smooth curve contraction section 5, the critical section throat 6, and the lignite-specific expansion section 7 to form a complete Laval profile, it propels subsonic... The high-temperature airflow is accelerated to supersonic speed, and the high-temperature zone of the jet core is lengthened by optimizing the expansion section 7, which enhances the volatile matter analysis of coarse-grained lignite. The inner wall of the Laval nozzle section 4 is equipped with a three-layer gradient composite ceramic liner 8, which buffers thermal stress layer by layer by layer through a bonding layer with gradually changing composition, a stress buffering transition layer, and a high-temperature resistant and erosion-resistant working layer, solving the problems of easy peeling of single-layer ceramic coating and rapid wear and erosion of the substrate. The outer wall of the pure copper alloy integrated metal substrate 9 is equipped with a water-cooled flow channel to continuously cool the nozzle substrate, further improving the nozzle's high-temperature resistance and long-term operation capability.

[0056] The entire device works synergistically from five dimensions: gas source heating, jet acceleration, high-temperature zone extension, inner wall protection, and substrate cooling. It is specifically adapted to the ignition conditions of coarse-particle lignite in a fan mill direct-fired pulverizing system. It completely solves a series of technical defects of traditional plasma ignition devices, such as low jet velocity, short high-temperature zone, insufficient coal powder pyrolysis, easy wear and damage of nozzles, short equipment life, and frequent maintenance. It can significantly improve the ignition success rate of lignite boilers and the stable combustion effect under low load, while reducing equipment operation and maintenance costs, and has extremely strong engineering application value.

[0057] The above are merely preferred embodiments of the present invention and are not intended to limit the invention. Various modifications and variations can be made to the present invention by those skilled in the art. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the scope of protection of the present invention.

Claims

1. A supersonic plasma Laval nozzle ignition device for a fan mill for lignite grinding, characterized in that, include: A coaxial cathode and anode assembly includes a first cathode, a second cathode, and an anode arranged coaxially. The anode has a carrier air inlet. The first cathode and the second cathode are coaxially arranged along the anode axis. The first cathode and the second cathode respectively enclose the anode to form a two-stage series arc zone that is sequentially connected along the airflow direction. The Laval nozzle section is fixed to the gas flow outlet of the anode. The Laval nozzle section is provided with a converging section, a throat and an expanding section in sequence along the gas flow direction. The outline size of the expanding section is adapted to the pyrolysis characteristics of lignite to extend the high-temperature jet region. A gradient composite ceramic liner is disposed on the inner wall of the metal matrix of the Laval nozzle section. The gradient composite ceramic liner includes an adhesive layer, a stress buffer transition layer, and a high-temperature resistant and erosion-resistant working layer. The inner side of the adhesive layer is tightly fused to the inner wall of the metal matrix, the outer surface of the adhesive layer is fixed to the inner side of the stress buffer transition layer, and the outer side of the stress buffer transition layer is bonded to the inner side of the high-temperature resistant and erosion-resistant working layer. The adhesive layer, stress buffer transition layer, and high-temperature resistant and erosion-resistant working layer are formed by stacking the metal substrate in sequence towards the airflow side.

2. The supersonic plasma Laval nozzle ignition device for lignite milling according to claim 1, characterized in that, The Laval nozzle section and the anode are connected by integral casting. The contraction section adopts a smooth curved surface structure; The contraction section smoothly transitions to the throat, and the throat cross-section is the minimum flow cross-section of the Laval nozzle section; The rear end of the throat is smoothly connected to the expansion section.

3. The supersonic plasma Laval nozzle ignition device for lignite milling according to claim 1, characterized in that, The expansion section is configured with an expansion angle that matches the release pattern of lignite volatiles, and the expansion section is used to extend the length of the high-temperature zone in the core of the supersonic jet.

4. The supersonic plasma Laval nozzle ignition device for lignite milling according to claim 1, characterized in that, The gradient composite ceramic liner is a metal-ceramic composite structure with a continuously varying gradient composition.

5. The supersonic plasma Laval nozzle ignition device for lignite milling according to claim 1, characterized in that, The stress buffer transition layer is made of a ceramic material with a continuously gradient composition. The stress buffer transition layer is seamlessly fused to the adhesive layer and the high-temperature resistant and erosion-resistant working layer at both ends.

6. The supersonic plasma Laval nozzle ignition device for lignite milling according to claim 5, characterized in that, The high-temperature resistant and erosion-resistant working layer is made of high-melting-point high-temperature resistant ceramic material. The inner wall surface of the high-temperature resistant and erosion-resistant working layer is a smooth, stepless, continuous curved surface. The high-temperature resistant and erosion-resistant working layer is in direct contact with high-temperature plasma and dust-laden coal powder airflow.

7. The supersonic plasma Laval nozzle ignition device for lignite milling according to claim 1, characterized in that, Both the first cathode and the second cathode have columnar electrode structures; The first cathode and the second cathode are arranged coaxially and equidistantly along the central axis of the anode. The flow channel of the two-stage series arc zone is continuously connected without interruption along the airflow direction of the carrier.

8. The supersonic plasma Laval nozzle ignition device for lignite milling according to claim 7, characterized in that, The hollow interior of the anode forms an annular airflow channel; The carrier air inlet is connected to the annular airflow channel; The annular airflow channel is connected to two series-connected electric arc zones to achieve staged electric arc heating of the carrier air.

9. The supersonic plasma Laval nozzle ignition device for lignite milling according to claim 1, characterized in that, The metal substrate is a one-piece nozzle substrate made of pure copper alloy. The outer wall of the metal substrate is provided with a water-cooled cooling channel.

10. A method for igniting lignite using a supersonic plasma Laval nozzle in a fan mill, characterized in that, The method employs the supersonic plasma Laval nozzle ignition device for fan milling lignite as described in any one of claims 1-9, comprising: S1. Carrier air is introduced into the annular airflow channel inside the anode through the carrier air inlet. The carrier air flows through two-stage series electric arc zones in sequence. The carrier air is heated step by step by the two-stage electric arcs formed between the first cathode, the second cathode and the anode. S2. The high-temperature airflow enters the integrated Laval nozzle section, and after being converged and accelerated in the contraction section and throttled in the throat, it is accelerated by the expansion section to form a supersonic plasma jet. The optimized expansion section profile is used to lengthen the high-temperature region of the jet. S3: The supersonic jet directly impacts the lignite powder conveyed by the fan mill. The gradient composite ceramic lining resists the erosion of the coal powder and high-temperature ablation. The water-cooled flow channel on the outer wall of the metal substrate continuously cools the nozzle substrate, completing the lignite ignition and stable combustion operation.

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