A lignite boiler hot primary air steam preheating system

Abstract

This invention belongs to the field of thermal power generation and discloses a steam preheating system for the primary air of a lignite boiler, including a steam matching system, a primary air heating system, and a feedwater heating system. This invention achieves stable full-load control of the primary air temperature in a lignite boiler above 350℃, solving the drying output problem of medium-speed mill pulverizing systems in lignite boiler applications. This invention is applicable to lignite power plant boilers of various capacities equipped with medium-speed mill positive pressure direct-fired pulverizing systems. It is suitable for the overall design of boiler systems in newly built lignite power plants, as well as for energy-saving upgrades and retrofits of the primary air systems in existing lignite power plant boilers. It is used to stably increase the inlet temperature of the primary air from the lignite boiler mill, ensuring the drying output and coal powder fineness of the medium-speed mill pulverizing system, while optimizing the steam supply of the boiler feedwater heating system, thus improving the overall operational economy and safety stability of thermal power generating units.

Description

Technical Field

[0001] This invention belongs to the field of thermal power generation, specifically a hot primary air steam preheating system for lignite boilers. Background Technology

[0002] In the existing design of large-scale coal-fired power plant boilers, the medium-speed mill positive pressure direct-fired pulverizing system has become the mainstream pulverizing solution for 300MW and above capacity thermal power units due to its significant advantages such as compact structure, reliable operation, low power consumption, and fast adjustment response, accounting for more than 80% of applications in my country's thermal power industry. However, my country's coal resources endow it with abundant lignite reserves, accounting for about 13% of the country's total coal reserves, mainly distributed in Inner Mongolia, Yunnan, Heilongjiang and other regions, and is one of my country's important power coals. Lignite has typical characteristics such as high moisture content (usually 30%-60%, and some young lignite can even reach more than 70%), low calorific value (generally 12-17 MJ / kg), high volatile matter, and easy spontaneous combustion. When using a medium-speed mill positive pressure direct-fired pulverizing system, there is often a serious problem of insufficient drying output. Insufficient drying capacity not only directly leads to a decrease in the actual output of the coal mill, failing to meet the pulverized coal demand for full-load boiler operation, but also causes excessive moisture content and unqualified fineness of the pulverized coal. This, in turn, results in unstable combustion in the furnace, upward shift of the flame center, increased boiler flue gas temperature, reduced thermal efficiency, and increased NOx emissions. x A series of problems, such as increased emissions, have severely restricted the widespread application of medium-speed grinding systems in lignite power plant boilers.

[0003] To ensure the drying output and pulverized coal fineness of the coal mill, industry standards such as the "Technical Regulations for Design Calculation of Pulverizing Systems in Thermal Power Plants" (DL / T 5145-2012) clearly require that the primary air temperature at the inlet of the coal mill in a lignite boiler must reach above 350℃. However, due to limitations in the inlet flue gas temperature of the boiler denitrification system, the tolerance limit of the metal wall temperature of the air preheater, and the boiler exhaust temperature control requirements, simply optimizing the boiler body and air preheater design can only raise the hot primary air temperature at the air preheater outlet to around 300℃, which is insufficient to meet the stringent requirements for lignite drying. Therefore, an additional primary air preheating device must be added between the air preheater and the coal mill to further increase the hot primary air temperature.

[0004] In the existing technology, there are two main technical routes to increase the temperature of hot primary air, and both have related patent technologies disclosed, but both have inherent defects that are difficult to overcome: The first method is high-temperature flue gas heating technology at the boiler tail end. This involves adding a flue gas-air tubular heat exchanger to the boiler tail flue to heat the primary air using the high-temperature flue gas from the economizer outlet or denitrification inlet. While this method can utilize waste heat from the flue gas, it has drawbacks such as system complexity, large space requirements for equipment layout, susceptibility of the heat exchanger to corrosion and wear from fly ash in the flue gas, and high operating and maintenance costs. Furthermore, due to design limitations on boiler exhaust temperature, the air temperature increase is limited, typically only raising the primary air temperature by 50-80℃, making it difficult to stably reach the requirement of above 350℃. Moreover, when the boiler is operating at low load, the flue gas temperature decreases, further deteriorating the heating effect.

[0005] The second method is turbine extraction or boiler reheat steam heating technology, which is currently the most researched and applied technology in the industry, with several related patents already published. Beijing Lan'aidi Electric Technology Co., Ltd.'s publication number CN111167590A proposes a coal mill capacity expansion and efficiency improvement device based on lignite. This device draws high-temperature superheated steam from the turbine and connects it to a steam heat exchanger, which heats the intake air of the coal mill's primary air chamber. While this solution can improve the primary air temperature to some extent, it only uses turbine extraction as a single heat source. During low-load operation, the extraction pressure and temperature drop significantly, failing to meet the air temperature requirement of over 350℃. More importantly, the steam after heat exchange directly enters the condensate drain system, failing to effectively recover a large amount of waste heat. Furthermore, this system operates independently of the boiler feedwater heating system, and the additional turbine extraction reduces the amount of steam originally used for the high-pressure heater, leading to a decrease in feedwater temperature and a reduction in overall unit thermal efficiency of approximately 0.5%-1%.

[0006] The Inner Mongolia Shangdu Power Generation Co., Ltd., in its publication number CN213777663U, discloses a reheat steam-heated primary air lignite supercritical boiler. This method increases the temperature of the hot primary air at the preheater outlet by adding a heater to the hot primary air duct and extracting some of the reheater outlet steam. While this scheme utilizes the advantage of higher reheat steam temperatures and can basically meet the air temperature requirements under rated load, the lack of a pressure matching device results in inherently low reheat steam pressure. This pressure further decreases at low loads, leading to poor drainage in the heat exchanger and potentially causing water hammer, pipe vibration, and other safety hazards. Furthermore, the reheat steam after heat exchange is directly discharged into the condenser, with all its latent heat being carried away by the circulating water, resulting in significant energy waste. Moreover, it cannot complement the feedwater heating system and instead increases the unit's cooling losses.

[0007] A patent application (CN116697390A) from China Energy Engineering Group Guangdong Electric Power Design Institute Co., Ltd. proposes a hot primary air heating system for a lignite boiler. This system draws a portion of steam from the high-temperature reheat steam pipeline at the reheat steam boiler outlet into a steam heater to heat the hot primary air duct at the air preheater outlet. However, this scheme also uses a single reheat steam source, resulting in insufficient steam parameters under low load conditions. Furthermore, the system design does not consider the utilization of waste heat from the steam after heat exchange; the steam, after being cooled by the primary air heat exchanger, directly enters the condensate expansion tank, wasting high-quality thermal energy and increasing the power plant's makeup water rate and water treatment costs. In addition, this system is completely independent of the boiler feedwater heating system, preventing the cascade utilization of steam resources and hindering the improvement of the unit's overall energy efficiency.

[0008] Regarding steam pressure regulation, CN108534126A, published by China Datang Corporation Science and Technology Research Institute Co., Ltd., discloses a boiler-turbine waste heat coupling utilization system with a pressure matcher. This system uses the pressure matcher to mix the extraction and exhaust steam from the low-pressure cylinder of the steam turbine, outputting intermediate-pressure steam for heating the cold primary air. While this solution incorporates pressure matcher technology, its application scenario is cold primary air heating, aiming to solve the problem of insufficient steam pressure in winter heaters. This is completely different from the requirements of this invention for high-temperature hot primary air above 350℃ in lignite boilers. Furthermore, its steam source is low-pressure extraction and exhaust steam, which cannot provide a sufficiently high temperature to meet the requirements of lignite drying. Additionally, this system also does not connect the heat-exchanged steam to the feedwater heating system, resulting in low waste heat utilization efficiency.

[0009] In summary, both existing flue gas heating and steam heating technologies have their own insurmountable shortcomings. The most prominent and unresolved common problem is the lack of adequate consideration for the utilization of waste heat from the heated steam and the inability to organically integrate it with the boiler feedwater heating system. In all the aforementioned steam heating schemes, the steam after heat exchange is either directly discharged into the condenser or enters the condensate drain system, resulting in the ineffective recovery of a large amount of latent and sensible heat, leading to serious energy waste. Furthermore, these systems operate independently of the boiler's original feedwater heating system. The additional high-quality steam extracted crowds out the steam resources originally used for the high-pressure heater, causing a decrease in feedwater temperature, an increase in boiler flue gas temperature, and a decline in the overall thermal efficiency of the unit. In addition, the single steam source design makes it impossible to guarantee sufficient steam parameters under low-load conditions, failing to meet the coal mill inlet air temperature requirements and also resulting in problems such as poor condensate drainage and significant safety hazards. Therefore, there is an urgent need to develop a lignite boiler hot primary air preheating system that can stably provide high-temperature primary air above 350℃ under full load conditions, has a simple system structure, small footprint, reliable operation, and can realize the cascade utilization of steam waste heat, organically integrate with the boiler feedwater heating system, and not reduce the overall thermal efficiency of the unit. Summary of the Invention

[0010] To address the aforementioned shortcomings in the existing technology, this invention aims to provide a hot primary air steam preheating system for lignite boilers. This system addresses the problems of insufficient hot primary air temperature, poor drainage of the steam heating system under low load conditions, and low energy utilization in existing lignite boilers. Simultaneously, it ensures stable boiler feedwater temperature and improves the overall economic efficiency of the unit.

[0011] To achieve the above objectives, the technical solution adopted by the present invention is as follows: A hot primary air steam preheating system for a lignite boiler includes a steam matching unit, a hot primary air heating unit, and a feedwater heating unit, which are connected by pipelines. The steam matching unit is located inside the boiler and includes a screen-type superheater, a final-stage superheater, a final-stage reheater, a low-temperature reheater, a boiler water system, and a pressure matching device. The screen-type superheater, final-stage superheater, final-stage reheater, low-temperature reheater, and boiler water system are connected by pipelines. The screen-type superheater and the final-stage superheater are connected sequentially by pipelines. A high-pressure superheated steam pipeline connects the pipeline between the outlet of the screen-type superheater and the inlet of the final-stage superheater. The end of the high-pressure superheated steam pipeline furthest from the outlet of the screen-type superheater and the connecting pipeline between the final-stage superheater and the pressure matching device is connected to the main steam inlet of the pressure matching device. The ejector steam inlet of the pressure matching device connects to a reheated steam pipeline, and the end of the reheated steam pipeline furthest from the pressure matching device is connected to the connecting pipeline between the low-temperature reheater and the final-stage reheater. The primary air heating unit includes a primary air fan, an air preheater, a primary air heat exchanger, a coal mill, and a mixing heating steam pipeline. The primary air fan's blower outlet is connected to the air preheater's inlet via a pipeline; the air preheater's outlet is connected to the primary air heat exchanger's air inlet via a pipeline; the primary air heat exchanger's air outlet is connected to the coal mill's inlet; and the primary air heat exchanger's steam inlet is connected to the outlet of the pressure matching device in the steam matching unit via the mixing heating steam pipeline. The feedwater heating system includes a high-pressure cylinder, an intermediate-pressure cylinder, a deaerator, a feedwater pump, a first high-pressure heater, a second high-pressure heater, a third high-pressure heater, and a turbine extraction heating pipeline. The inlet of the high-pressure cylinder is connected to the final-stage superheater in the steam matching unit via a pipeline. The first exhaust port of the high-pressure cylinder is connected to the inlet of the low-temperature reheater in the steam matching unit via a pipeline. The second exhaust port of the high-pressure cylinder is connected to the steam inlet of the first high-pressure heater via a pipeline. The inlet of the intermediate-pressure cylinder is connected to the outlet of the final-stage reheater in the steam matching unit via a pipeline. The first exhaust port of the intermediate-pressure cylinder is connected to the steam inlet of the third high-pressure heater via a pipeline. The second exhaust port of the intermediate-pressure cylinder is connected to the deaerator and the feedwater pump in sequence via a pipeline. The outlet of the feedwater pump is connected to the inlet of the third high-pressure heater. The connecting pipeline between the high-pressure cylinder and the low-temperature reheater in the steam matching unit is connected to the turbine extraction heating pipeline. The end of the turbine extraction heating pipeline furthest from the connecting pipeline between the high-pressure cylinder and the low-temperature reheater in the steam matching unit is connected to the steam inlet of the second high-pressure heater.

[0012] As a limitation of the present invention, a high-pressure superheated steam pipeline control valve is provided on the high-pressure superheated steam pipeline, and a high-pressure primary air heat exchanger outlet temperature measuring point is provided on the air outlet pipeline of the primary air heat exchanger; the high-pressure superheated steam pipeline control valve and the high-pressure primary air heat exchanger outlet temperature measuring point are electrically connected to form a closed-loop interlocking control circuit.

[0013] As a further limitation of the present invention, the control logic of the closed-loop interlocking control circuit is as follows: when the temperature detected by the outlet temperature measuring point of the hot primary air heat exchanger is lower than the set value, the opening degree of the high-pressure superheated steam pipeline control valve is increased; when the detected temperature is higher than the set value, the opening degree of the high-pressure superheated steam pipeline control valve is decreased.

[0014] As another limitation of the present invention, the mixed heating steam pipeline and the steam turbine extraction heating pipeline are connected; the portion of the mixed heating steam pipeline near the steam outlet of the primary air heat exchanger is provided with a mixed heating steam pipeline control valve and a mixed heating steam pipeline safety valve.

[0015] As a further limitation of the present invention, the set pressure of the safety valve for the mixed heating steam pipeline is equal to the design pressure of the turbine extraction heating pipeline.

[0016] As a third limitation of the present invention, a high-pressure heater exhaust pipe check valve is provided on the part of the steam turbine exhaust heating pipe near the second high-pressure heater. The flow direction of the high-pressure heater exhaust pipe check valve is from the steam turbine exhaust heating pipe away from the connecting pipe between the high-pressure cylinder and the low-temperature reheater in the steam matching unit to the second high-pressure heater.

[0017] As a fourth limitation of the present invention, the pressure matching device is a steam jet type pressure matching device, wherein the pressure of the mixed steam at its outlet is higher than the steam pressure in the reheat steam pipeline, and the difference between the pressure of the mixed steam and the steam pressure in the turbine extraction heating pipeline is not greater than 0.2 MPa.

[0018] As a fifth limitation of the present invention, the primary air heat exchanger is a shell-and-tube heat exchanger, with air flowing through the tubes and steam flowing through the shell; the primary air heat exchanger is arranged in a single row or a double row.

[0019] As another limitation of the invention, the system is used in lignite power plant boilers equipped with medium-speed mill positive pressure direct-fired pulverizing systems.

[0020] By adopting the above-described technical solution, the beneficial effects achieved by this invention compared to the prior art are as follows: (1) This invention relates to a hot primary air steam preheating system for lignite boilers. By using a pressure matching device to mix and pressurize high-pressure superheated steam and reheated steam, even when the unit is running at low load, the mixed steam can be guaranteed to have sufficient pressure and temperature, thereby ensuring that the hot primary air temperature can stably reach above 350°C, and completely solving the problem of insufficient drying output of the medium-speed grinding system of lignite boilers.

[0021] (2) This invention relates to a hot primary air steam preheating system for a lignite boiler. The mixed steam pressure is higher than the reheat steam pressure and is basically matched with the turbine extraction pressure. The steam after heat exchange can flow smoothly into the high-pressure heater. There is no risk of poor drainage or water hammer, which improves the safety and reliability of the system operation.

[0022] (3) This invention relates to a steam preheating system for primary air in a lignite boiler, which introduces the waste heat of steam after heating the primary air into a high-pressure heater for heating feedwater, thereby realizing the cascade utilization of steam energy; at the same time, the mixed heating steam pipeline is connected to the turbine extraction pipeline to form a steam adaptive regulation loop; when the amount of heating steam is insufficient, it is supplemented by turbine extraction; when the amount of heating steam is excessive, the excess steam can be returned to the high-pressure cylinder exhaust pipeline through the turbine extraction pipeline, ensuring the stability of the feedwater temperature at the outlet of the high-pressure heater and improving the overall thermal efficiency of the unit.

[0023] (4) This invention relates to a hot primary air steam preheating system for lignite boilers. It does not require the addition of complex flue gas heat exchange equipment. It only requires the addition of a pressure matching device and a hot primary air heat exchanger to the existing steam system and air system. The equipment layout is flexible, the footprint is small, and the modification and construction costs are low. It is suitable for the modification of new lignite power plant boilers and existing lignite boilers.

[0024] In summary, this invention addresses the core shortcomings of existing lignite boiler hot primary air preheating technologies, such as poor temperature stability under full load, poor drainage at low loads, low energy utilization, and system complexity. It innovatively proposes a steam-air coupling heating scheme based on a pressure matcher. Through high-pressure superheated steam and reheated steam mixing and pressurization technology, organic integration of hot primary air heating and feedwater heating systems, and adaptive steam flow regulation technology, it achieves stable full-load control of the hot primary air temperature in lignite boilers above 350℃, completely solving the bottleneck problem of drying output in medium-speed grinding systems applied to lignite boilers. This invention not only significantly improves the safety and reliability of system operation and achieves efficient cascade utilization of steam energy, but also greatly reduces system construction and modification costs. It fills the gap in existing technology for stable heating of hot primary air under low load conditions in lignite boilers, providing key technical support for the clean and efficient utilization of lignite resources.

[0025] This invention is applicable to lignite power plant boilers of various capacities equipped with medium-speed mill positive pressure direct-fired pulverizing systems. It is suitable for the overall design of boiler systems in newly built lignite power plants, as well as for the energy-saving upgrade and renovation of the hot primary air system of existing lignite power plant boilers. It is used to steadily increase the inlet hot primary air temperature of lignite boiler pulverizers, ensure the drying output and coal powder fineness of the medium-speed mill pulverizing system, and optimize the steam supply of the boiler feedwater heating system, thereby improving the overall operating economy and safety stability of thermal power generating units. Attached Figure Description

[0026] The present invention will now be described in further detail with reference to the accompanying drawings and specific embodiments.

[0027] Figure 1 This is a schematic diagram of the system structure according to an embodiment of the present invention.

[0028] In the diagram: 1. Pressure matching device; 2. Hot primary air heat exchanger; 3. Second high-pressure heater; 4. High-pressure superheated steam pipeline control valve; 5. Mixed heating steam pipeline control valve; 6. Mixed heating steam pipeline safety valve; 7. Hot primary air heat exchanger outlet temperature measuring point; 8. High-pressure heater exhaust pipeline check valve; 9. First high-pressure heater; 10. Third high-pressure heater; 11. High-pressure superheated steam pipeline; 12. Reheat steam pipeline; 13. Mixed heating steam pipeline; 14. Steam turbine exhaust heating pipeline; 15. Coal mill; 16. Air preheater; 17. Primary air fan; 18. Feed water pump; 19. Deaerator; 20. Intermediate pressure cylinder; 21. High pressure cylinder; 22. Screen-type superheater; 23. Final stage superheater; 24. Final stage reheater; 25. Low-temperature reheater; 26. Boiler water system. Detailed Implementation

[0029] The preferred embodiments of the present invention will be described below with reference to the accompanying drawings. It should be understood that the preferred embodiments described herein are for illustrative and understanding purposes only and are not intended to limit the scope of the invention.

[0030] Example 1: A primary air steam preheating system for a lignite boiler like Figure 1As shown, this embodiment relates to a primary air steam preheating system for a lignite boiler, which is fully integrated into the boiler-turbine thermal cycle system of a thermal power plant. It includes a pressure matching device 1, a primary air heat exchanger 2, a second high-pressure heater 3, a high-pressure superheated steam pipeline control valve 4, a mixed heating steam pipeline control valve 5, a mixed heating steam pipeline safety valve 6, a primary air heat exchanger outlet temperature measuring point 7, a high-pressure heater extraction pipeline check valve 8, a first high-pressure heater 9, a third high-pressure heater 10, a high-pressure superheated steam pipeline 11, a reheated steam pipeline 12, a mixed heating steam pipeline 13, a turbine extraction heating pipeline 14, a coal mill 15, an air preheater 16, a primary air fan 17, a feedwater pump 18, a deaerator 19, a turbine intermediate-pressure cylinder 20, a turbine high-pressure cylinder 21, a boiler screen-type superheater 22, a boiler final-stage superheater 23, a boiler final-stage reheater 24, a boiler low-temperature reheater 25, and a boiler water system 26. This system is organically coupled with three parts: a steam matching subsystem, a hot primary air heating subsystem, and a feedwater heating subsystem. It also seamlessly connects with the boiler's steam-water system, turbine thermal system, and pulverizing system. Without interfering with the normal operation of the power plant's main thermal cycle, it achieves a stable increase in hot primary air temperature and the cascade utilization of steam energy. The boiler's steam generation and circulation serve as the energy source for this system. The basic process is as follows: saturated steam generated by the boiler water system 26 flows sequentially through the boiler screen superheater 22 and the boiler's final superheater 23, where it is heated to high-pressure superheated steam with parameters of approximately 16-18 MPa and 540°C. This superheated steam is then sent to the turbine's high-pressure cylinder 21 through the main steam pipeline to perform work. The exhaust steam from the turbine's high-pressure cylinder 21 (with parameters of approximately 3-4 MPa and 320°C) is sent to the boiler's low-temperature reheater 25 through a cold reheat pipeline. After being reheated by the boiler's final reheater 24, it is sent to the turbine's intermediate-pressure cylinder 20 through a hot reheat pipeline to continue performing work. The steam matching subsystem extracts two streams of steam from the middle of the boiler body steam cycle for mixing and pressurization. Specifically, the connection is as follows: the high-pressure superheated steam pipeline 11 is led out from the main steam pipeline between the outlet of the boiler screen superheater 22 and the inlet of the final stage superheater 23, and is connected to the main steam inlet of the pressure matching device 1 after passing through the high-pressure superheated steam pipeline control valve 4; the reheat steam pipeline 12 is led out from the cold reheat pipeline between the outlet of the boiler low-temperature reheater 25 and the inlet of the final stage reheater 24, and is connected to the ejector steam inlet of the pressure matching device 1; the outlet of the pressure matching device 1 is connected to the steam inlet of the hot primary air heat exchanger 2 through the mixed heating steam pipeline 13.The hot primary air heating subsystem is responsible for heating the preheated primary air to the required drying temperature of the coal mill. The process is as follows: the cold air from the outlet of the primary air fan 17 is sent into the air preheater 16, and after exchanging heat with the flue gas at the tail of the boiler, the temperature rises to 280~300℃, forming the preheated primary air; the preheated primary air is connected to the air inlet of the hot primary air heat exchanger 2, and after efficient countercurrent heat exchange with the high-temperature mixed steam in the shell side, it is sent into the coal mill 15 through the air outlet pipe; the hot primary air heat exchanger outlet temperature measuring point 7 is installed on the air outlet pipe of the hot primary air heat exchanger 2 to monitor the inlet air temperature of the coal mill 15 in real time. The feedwater heating subsystem is used to recover the waste heat of the steam after heat exchange and to achieve adaptive regulation of the steam flow rate. Its connection is as follows: feedwater from the deaerator 19 outlet is pressurized by the feedwater pump 18 and flows sequentially through the third high-pressure heater 10, the second high-pressure heater 3, and the first high-pressure heater 9, being heated stage by stage before being sent to the boiler water system 26; the steam outlet of the hot primary air heat exchanger 2 is connected to the second steam inlet of the first high-pressure heater 9 via the mixing and heating steam pipeline 13; the exhaust pipe of the turbine high-pressure cylinder 21 leads out to the turbine extraction heating pipeline 14, and after passing through the check valve 8 of the high-pressure heater extraction pipeline, it is connected to the first steam inlet of the first high-pressure heater 9; the mixing... The heating steam pipeline 13 is connected to the turbine extraction heating pipeline 14 at the steam inlet side of the first high-pressure heater 9, forming a steam flow adaptive regulation loop. A mixing heating steam pipeline control valve 5 and a mixing heating steam pipeline safety valve 6 are sequentially installed in series on the section of the mixing heating steam pipeline 13 located between the hot primary air heat exchanger 2 and the first high-pressure heater 9. The condensate from the first high-pressure heater 9 is connected to the steam side of the second high-pressure heater 3, the condensate from the second high-pressure heater 3 is connected to the steam side of the third high-pressure heater 10, and the condensate from the third high-pressure heater 10 is returned to the deaerator 19, achieving condensate flow by gravity at each stage, further improving energy utilization. Furthermore, the high-pressure superheated steam pipeline control valve 4 and the hot primary air heat exchanger outlet temperature measuring point 7 are both connected to the unit's DCS control system, forming a closed-loop interlocking control loop, which can adjust the input flow of high-pressure superheated steam in real time according to the inlet air temperature of the coal mill 15.

[0031] When the hot primary air steam preheating system involved in this embodiment is working, it works in coordination with the main thermal system of the power plant to complete three core processes in sequence: steam matching and pressurization, hot primary air heating, feedwater heating and steam adaptive regulation, and achieves real-time stabilization of air temperature through closed-loop automatic control. First, the steam matching and pressurization process is carried out: High-pressure superheated steam (parameters approximately 16~18MPa, 540℃) from the outlet of the boiler screen superheater 22 enters the main steam inlet of the pressure matching device 1 through the high-pressure superheated steam pipeline 11 as working steam. Reheated steam (parameters approximately 3~4MPa, 320℃) from the outlet of the boiler low-temperature reheater 25 enters the ejector steam inlet of the pressure matching device 1 through the reheated steam pipeline 12. The high-pressure superheated steam expands and accelerates in the Laval nozzle of the pressure matching device 1 to form a supersonic jet. A low-pressure zone is formed at the nozzle outlet, which draws the reheated steam into the mixing chamber. After sufficient energy and momentum exchange, the two are decelerated and pressurized in the diffusion chamber to form mixed steam with parameters approximately 4~5MPa and 420~450℃. The pressure of this mixed steam is higher than that of the reheated steam, and the pressure difference with the exhaust pressure of the turbine high-pressure cylinder 21 is no more than 0.2MPa. This lays the pressure foundation for the smooth flow of subsequent steam and the utilization of waste heat, fundamentally solving the problem of insufficient pressure under low load in traditional extraction heating systems. The pressurized mixed steam then enters the hot primary air heating process: the mixed steam enters the shell side of the hot primary air heat exchanger 2 through the mixed heating steam pipe 13, and performs efficient countercurrent heat exchange with the preheated primary air (temperature of about 280~300℃) from the air preheater 16 in the tube side; after heat exchange, the primary air temperature rises steadily to 350~380℃, and is sent to the coal mill 15 through the air outlet pipe, meeting the drying and pulverizing process requirements of high moisture lignite, and completely solving the bottleneck problem of insufficient drying output of the medium speed mill pulverizing system; after the mixed steam releases latent heat and sensible heat, the temperature drops to 250~280℃, and flows out from the steam outlet of the hot primary air heat exchanger 2, entering the next stage of waste heat utilization.After heat exchange and cooling, the steam enters the feedwater heating and steam adaptive regulation process: it enters the first high-pressure heater 9 through the mixing heating steam pipe 13, and mixes with the exhaust gas (parameters approximately 4~4.5MPa, 330~350℃) from the turbine high-pressure cylinder 21 exhaust pipe, jointly heating the boiler feedwater in the tube side; the mixing heating steam pipe control valve 5 is used to finely regulate the steam pressure entering the first high-pressure heater 9, making it precisely match the turbine exhaust gas pressure, ensuring stable mixing without significant pressure fluctuations; since the mixing heating steam pipe 13 and the turbine exhaust gas heating pipe 14 are connected to the steam inlet side of the first high-pressure heater 9, the system can automatically achieve adaptive balance of steam flow without the need for additional complex adjustment settings. The steam-saving device is designed to ensure that when the amount of steam required for heating the primary air decreases and the amount of steam entering the first high-pressure heater 9 is excessive, the excess steam can be returned to the exhaust pipe of the turbine high-pressure cylinder 21 through the turbine extraction pipe 14 to participate in the main thermodynamic cycle. When the amount of steam required for heating the primary air increases and the amount of steam entering the first high-pressure heater 9 is insufficient, the exhaust steam of the turbine high-pressure cylinder 21 is automatically replenished to the first high-pressure heater 9, thereby ensuring that the outlet feedwater temperature of the first high-pressure heater 9 remains stable at the design value and avoiding the impact of steam fluctuations on the overall thermal efficiency of the unit. At the same time, the condensate from the first high-pressure heater 9 flows by gravity to the second high-pressure heater 3 and the third high-pressure heater 10, and finally returns to the deaerator 19, realizing the full-process cascade utilization of steam energy. Throughout the entire process, the system continuously adjusts the inlet air temperature of the coal mill 15 through closed-loop automatic control logic: the outlet temperature measuring point 7 of the hot primary air heat exchanger collects the inlet air temperature data of the coal mill 15 in real time and transmits the temperature signal to the DCS control system; when the detected temperature is lower than the set value (e.g., 350℃), the control system outputs a control signal to increase the opening of the high-pressure superheated steam pipeline control valve 4, increasing the input flow of high-pressure superheated steam, thereby increasing the temperature and total flow of the mixed steam, so that the hot primary air temperature quickly rises back to the set range; when the detected temperature is higher than the set value, the control system outputs a control signal to decrease the opening of the high-pressure superheated steam pipeline control valve 4, reducing the temperature and total flow of the mixed steam, so that the hot primary air temperature falls back to the set range, ensuring that the drying output of the pulverizing system remains stable.

[0032] In this embodiment, the pressure matching device 1 adopts a steam jet structure with no moving parts, ensuring reliable operation and minimal maintenance. It can stably achieve steam pressurization within the unit's 30%~100% rated load range, overcoming the limitation of insufficient pressure at low loads in traditional steam turbine extraction heating systems. The hot primary air heat exchanger 2 adopts a shell-and-tube structure, with air flowing through the tubes and steam through the shell. It has the advantages of high heat transfer efficiency, low airflow resistance, and resistance to high temperature and pressure. It can be flexibly arranged in single or double rows according to the boiler capacity and site layout conditions. In the double-row arrangement, maintenance of equipment on one side can be carried out without affecting the overall system operation, significantly improving the system's standby and continuous operation reliability. In terms of safety protection, the set pressure of the safety valve 6 in the mixed heating steam pipeline is strictly equal to the design pressure of the turbine extraction heating pipeline 14. When the pressure in the pipeline exceeds the set pressure due to abnormal operating conditions such as valve misoperation or system failure, the safety valve automatically opens to release pressure, preventing damage to the pipeline and equipment due to overpressure. The check valve 8 in the high-pressure heater extraction pipeline adopts a swing-type structure, which has the characteristics of rapid opening and closing and good sealing performance. It can effectively prevent the high-temperature and high-pressure steam in the high-pressure heater from flowing back into the turbine under abnormal operating conditions such as turbine load shedding and emergency shutdown, avoiding the occurrence of serious accidents such as turbine overspeed and ensuring the safe operation of the turbine main equipment.

[0033] It should be noted that the above description is merely a preferred embodiment of the present invention and is not intended to limit the present invention. Although the present invention has been described in detail with reference to the above embodiments, those skilled in the art can still modify the technical solutions described in the above embodiments or make equivalent substitutions for some of the technical features. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A hot primary air steam preheating system for a lignite boiler, characterized in that: It includes a steam matching unit, a hot primary air heating unit, and a water supply heating unit, which are connected by pipelines; The steam matching unit is located inside the boiler and includes a screen-type superheater, a final-stage superheater, a final-stage reheater, a low-temperature reheater, a boiler water system, and a pressure matching device. The screen-type superheater, the final-stage superheater, the final-stage reheater, the low-temperature reheater, and the boiler water system are connected by pipelines. The screen-type superheater and the final-stage superheater are sequentially connected by pipelines. A high-pressure superheated steam pipeline connects the pipeline between the outlet of the screen-type superheater and the inlet of the final-stage superheater. One end of the high-pressure superheated steam pipeline, away from the connecting pipeline between the screen-type superheater and the final-stage superheater, is connected to the main steam inlet of the pressure matching device. The ejector steam inlet of the pressure matching device is connected to a reheated steam pipeline. One end of the reheated steam pipeline, away from the pressure matching device, is connected to the connecting pipeline between the low-temperature reheater and the final-stage reheater. The primary air heating unit includes a primary air fan, an air preheater, a primary air heat exchanger, a coal mill, and a mixing heating steam pipeline; wherein, the blower of the primary air fan is connected to the inlet of the air preheater via a pipeline; the outlet of the air preheater is connected to the air inlet of the primary air heat exchanger via a pipeline, and the air outlet of the primary air heat exchanger is connected to the inlet of the coal mill; the steam inlet of the primary air heat exchanger is connected to the outlet of the pressure matching device in the steam matching unit via the mixing heating steam pipeline; The feedwater heating unit includes a high-pressure cylinder, an intermediate-pressure cylinder, a deaerator, a feedwater pump, a first high-pressure heater, a second high-pressure heater, a third high-pressure heater, and a steam turbine extraction heating pipeline. The inlet of the high-pressure cylinder is connected to the final-stage superheater in the steam matching unit via a pipeline; the first exhaust port of the high-pressure cylinder is connected to the inlet of the low-temperature reheater in the steam matching unit via a pipeline; and the second exhaust port of the high-pressure cylinder is connected to the steam inlet of the first high-pressure heater via a pipeline. The inlet of the intermediate-pressure cylinder is connected to the final-stage reheater in the steam matching unit via a pipeline. The outlet is connected, and the first exhaust port of the intermediate pressure cylinder is connected to the steam inlet of the third high-pressure heater through a pipeline. The second exhaust port of the intermediate pressure cylinder is connected to the deaerator and the feed water pump in sequence through a pipeline. The exhaust port of the feed water pump is connected to the inlet of the third high-pressure heater. The connecting pipeline between the high-pressure cylinder and the low-temperature reheater in the steam matching unit is connected to a turbine extraction heating pipeline. One end of the turbine extraction heating pipeline away from the connecting pipeline between the high-pressure cylinder and the low-temperature reheater in the steam matching unit is connected to the steam inlet of the second high-pressure heater.

2. The lignite boiler hot primary air steam preheating system according to claim 1, characterized in that: A high-pressure superheated steam pipeline control valve is installed on the high-pressure superheated steam pipeline, and a high-pressure primary air heat exchanger outlet temperature measuring point is installed on the air outlet pipeline of the high-pressure primary air heat exchanger; the high-pressure superheated steam pipeline control valve is electrically connected to the high-pressure primary air heat exchanger outlet temperature measuring point to form a closed-loop interlocking control circuit.

3. The lignite boiler hot primary air steam preheating system according to claim 2, characterized in that: The control logic of the closed-loop interlocking control circuit is as follows: when the temperature detected by the outlet temperature measuring point of the primary air heat exchanger is lower than the set value, the opening degree of the high-pressure superheated steam pipeline control valve is increased; when the detected temperature is higher than the set value, the opening degree of the high-pressure superheated steam pipeline control valve is decreased.

4. The lignite boiler hot primary air steam preheating system according to claim 1, characterized in that: The mixed heating steam pipeline is connected to the steam turbine extraction heating pipeline; the portion of the mixed heating steam pipeline near the steam outlet of the primary air heat exchanger is equipped with a mixed heating steam pipeline control valve and a mixed heating steam pipeline safety valve.

5. The lignite boiler hot primary air steam preheating system according to claim 4, characterized in that: The set pressure of the safety valve for the mixed heating steam pipeline is equal to the design pressure of the turbine extraction heating pipeline.

6. The lignite boiler hot primary air steam preheating system according to claim 1, characterized in that: A high-pressure heater exhaust pipe check valve is installed on the steam turbine exhaust heating pipe. The flow direction of the high-pressure heater exhaust pipe check valve is from the steam turbine exhaust heating pipe away from the connecting pipe between the high-pressure cylinder and the low-temperature reheater in the steam matching unit to the second high-pressure heater.

7. The lignite boiler hot primary air steam preheating system according to claim 1, characterized in that: The pressure matching device is a steam jet type pressure matching device, whose outlet mixed steam pressure is higher than the steam pressure in the reheat steam pipeline, and the difference between the steam pressure in the turbine extraction heating pipeline and the pressure in the reheat steam pipeline is not greater than 0.2 MPa.

8. The lignite boiler hot primary air steam preheating system according to claim 1, characterized in that: The primary air heat exchanger is a shell-and-tube heat exchanger, with air flowing through the tubes and steam flowing through the shell; the primary air heat exchanger can be arranged in a single row or a double row.

9. The lignite boiler hot primary air steam preheating system according to any one of claims 1 to 8, characterized in that: The lignite boiler hot primary air steam preheating system is used in lignite power plant boilers equipped with medium-speed mill positive pressure direct-fired pulverizing systems.

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