A boiler flue gas waste heat recovery and utilization system

Abstract

This invention discloses a boiler flue gas waste heat recovery and utilization system, belonging to the field of heat exchange. It addresses the problems of high pressure requirements for heat exchange pipelines and poor equipment reliability in existing technologies. The system includes a first heat exchanger, a second heat exchanger, a third heat exchanger, a flue gas pipeline, an air pipeline, and a liquid pipeline. The inlet of the flue gas pipeline connects to the boiler exhaust port; the inlet of the air pipeline connects to external air, and the outlet of the air pipeline connects to external heat-using equipment; the liquid pipeline is connected to the third heat exchanger and is used to introduce a heat exchange liquid. The flue gas pipeline connects sequentially to the first and second heat exchangers along the flue gas transport direction; the air pipeline connects sequentially to the second, third, and first heat exchangers along the air transport direction; and the liquid pipeline connects to the third heat exchanger along the liquid transport direction. This invention reduces the pressure requirements for the heat exchange pipelines and improves equipment reliability.

Description

Technical Field

[0001] This invention relates to the field of heat exchange, and in particular to a boiler flue gas waste heat recovery and utilization system. Background Technology

[0002] Boiler flue gas waste heat recovery is an important means to improve energy utilization and reduce energy consumption in thermal power generation and industrial thermal systems. Existing heat exchangers mainly recover and utilize flue gas waste heat through heat transfer between different media.

[0003] In existing waste heat recovery technologies, patent CN207146343U discloses a corrosion-resistant modular waste heat boiler system that combines a rotary regenerative air preheater. It uses water and flue gas for heat exchange. Since the convective heat transfer coefficient of water is much greater than that of flue gas, the heat exchange resistance is mainly concentrated on the flue gas side. The heat exchange wall temperature is closer to the water temperature inside the tube. Therefore, the heat exchange wall temperature is mainly dominated by the water temperature.

[0004] The critical liquefaction temperature of ammonium bisulfate is 147℃. To prevent the wall temperature from falling below 147℃, which could cause the ammonium bisulfate component in the flue gas to liquefy, adhere, deposit, corrode, and clog the heat exchange surface, the aforementioned heat exchange system must maintain the water body in a relatively high temperature range. Considering the aforementioned heat exchange characteristics, the heat exchange wall temperature changes synchronously with the water temperature. To raise the heat exchange wall temperature and avoid corrosion risks, the water temperature inside the pipe must be increased.

[0005] Under normal pressure, the saturated boiling point of water is only 100℃. Water boils and vaporizes at 100℃, and the temperature cannot rise further, making it difficult to meet the 147℃ wall temperature control requirement. Since the boiling point of water increases with pressure, the above system requires increasing the water pressure within the pipeline to push the boiling point above 100℃ to achieve the 147℃ wall temperature control requirement. However, pressurizing the water necessitates raising the pipeline pressure design standards and increasing equipment manufacturing costs. Furthermore, high-pressure operation can easily cause cracks in pipeline welds and leaks, leading to a higher equipment failure rate. If a leak occurs, the leaked water reacts with acidic components in the flue gas to generate corrosive substances, easily causing corrosion and blockage of the flue and related equipment. Simultaneously, flue gas dust and acidic corrosive impurities seep into the closed-loop pipeline, contaminating the heat exchange water, causing water quality deterioration and pipe scaling. The contaminated water cannot meet the requirements of external heating equipment, resulting in water waste. Summary of the Invention

[0006] The purpose of this invention is to provide a boiler flue gas waste heat recovery and utilization system, which solves the problem in the prior art that when water directly exchanges heat with flue gas, it is necessary to maintain the water at a high temperature, which leads to high pressure design requirements for heat exchange pipelines and poor equipment reliability. This invention reduces the pressure design requirements for heat exchange pipelines and improves equipment reliability.

[0007] To achieve the above objectives, the present invention adopts the following technical solution: a boiler flue gas waste heat recovery and utilization system, comprising a first heat exchanger, a second heat exchanger and a third heat exchanger; The flue gas duct is connected in sequence to the first heat exchanger and the second heat exchanger along the flue gas conveying direction. An air duct connects sequentially to the second heat exchanger, the third heat exchanger, and the first heat exchanger along the air delivery direction. A liquid pipeline, connected to the third heat exchanger, is used to introduce heat exchange liquid; The inlet of the flue gas duct is used to connect to the boiler exhaust port, and the outlet of the flue gas duct is used to connect to external flue gas treatment equipment. The air duct has an inlet for connecting to external air and an outlet for connecting to external heating equipment.

[0008] By adopting the above technical solution, the present invention has the following advantages: High-temperature flue gas first enters the first heat exchanger along the flue gas pipe to complete the initial heat exchange, releasing most of the heat and lowering its temperature to form low-temperature waste heat flue gas, which then flows along the flue gas pipe into the second heat exchanger for further heat exchange. External low-temperature air first enters the second heat exchanger along the air pipe, absorbing heat from the low-temperature waste heat flue gas to achieve a temperature increase. The small temperature difference between the two effectively avoids the formation of localized low-temperature regions and reduces the liquefaction and precipitation of ammonium bisulfate. The heated air then enters the third heat exchanger, where it absorbs excess heat from the air using a liquid medium with strong heat absorption capacity, making fuller use of the heat and adjusting the air temperature to a more reasonable range.

[0009] Air, after being conditioned by the third heat exchanger, is sent to the first heat exchanger via an air duct to participate in heat exchange. In this device, the liquid only exchanges heat with the air in the third heat exchanger; the first and second heat exchangers do not contain any liquid medium. Compared to existing schemes that directly exchange heat between flue gas and water, this application achieves targeted improvements in two aspects: the heat exchange medium path and the wall temperature control logic. First, in terms of the medium contact path, existing technologies use direct heat exchange between water and flue gas. Once the water leaks, it will react with the flue gas to generate a corrosive medium, causing equipment corrosion, heat exchange medium contamination, and liquid deterioration, which seriously affects the operational stability of the equipment. This application confines liquid heat exchange within the third heat exchanger, while the first and second heat exchangers use heat exchange between flue gas and air throughout the entire process, isolating the contact path between the liquid and flue gas. This not only fully preserves the excellent heat exchange performance of the liquid, but also eliminates the risk of corrosion and medium contamination caused by liquid leakage from the root, ensuring the reliability of equipment operation.

[0010] Secondly, regarding wall temperature control and pressure design, existing technologies require a heat exchange wall temperature ≥147℃ to prevent ammonium bisulfate liquefaction. Since the convective heat transfer coefficient on the water side is much higher than on the flue gas side, the heat exchange wall temperature is closer to the water temperature. Existing technologies rely primarily on water temperature for heat exchange wall temperature. However, the boiling point of water at atmospheric pressure is only 100℃, necessitating pressurization to raise the saturated boiling point to meet temperature requirements. This leads to higher pressure standards for pipelines and equipment, increased manufacturing costs, and the increased risk of weld cracking and media leakage under high pressure conditions, further amplifying the risk of corrosion failure. The heat exchange wall temperatures of the first and second heat exchangers in this application... The temperature of the air exchange walls is determined by a weighted average of the thermal resistances of the media on both sides, namely flue gas and air. Since air is a gaseous medium with no phase change boiling point constraint, the required heat exchange wall temperature can be met without pressurizing the air ducts and the first and second heat exchangers. This reduces the internal operating pressure of the system, minimizing the risk of weld cracking and media leakage, lowering the equipment failure rate, and improving equipment reliability. Furthermore, it eliminates the need for a high-pressure structural design, effectively reducing equipment processing difficulty and manufacturing costs. Even if a leak occurs in the air duct, it is merely a physical mixing of air and flue gas, rather than the formation of corrosive substances from contact between water and flue gas, thus eliminating the risk of corrosion and media contamination. After heat exchange, the air and liquid can be separately transported to external heat-using equipment, achieving staged heat recovery and further improving the overall thermal energy utilization rate of the system.

[0011] Furthermore, the air duct is provided with a first temperature sensor, a controller, and a first switching valve between the third heat exchanger and the first heat exchanger. The first temperature sensor is used to detect the air temperature entering the first heat exchanger. The first temperature sensor is signal-connected to the controller, and the controller is signal-connected to the first switching valve. The controller is used to control the opening and closing of the first switching valve according to the detected temperature, so that the air temperature entering the first heat exchanger is maintained within a set first temperature range. This avoids the formation of ammonium bisulfate liquefaction deposits on the heat exchange wall as much as possible, and stabilizes the internal medium pressure of the air duct, maintaining the duct in a low-pressure operating state.

[0012] Using the aforementioned technical solution, the first temperature sensor can collect the temperature of the air about to enter the first heat exchanger in real time and transmit the monitoring signal to the controller. The controller dynamically adjusts the opening and closing state of the first switching valve according to the measured temperature, so that the temperature of the air entering the first heat exchanger is stabilized within the set range, suppressing the liquefaction and precipitation of ammonium bisulfate as much as possible, while keeping the air in a lower and more reasonable temperature range, reducing the thermal expansion deformation of the air, reducing the pressure load on the air pipeline, ensuring the temperature control effect of the heat exchange wall, and improving the overall operational reliability of the equipment.

[0013] Furthermore, the first temperature range is 60°C to 80°C.

[0014] Using the aforementioned technical solution, if the air temperature entering the first heat exchanger is below 60℃, the heat exchanger wall surface is prone to entering the critical temperature range for ammonium bisulfate liquefaction, causing ammonium bisulfate liquefaction and deposition, blocking the internal channels of the first heat exchanger. If the air temperature is above 80℃, it will not only reduce the temperature difference between flue gas and air, decreasing the overall heat exchange efficiency, but also increase the air temperature rise, raising the expansion pressure of the medium inside the air pipeline and increasing the pressure load on the air pipeline. Limiting the air temperature to the range of 60℃-80℃ can minimize the problem of ammonium bisulfate liquefaction and deposition on the heat exchanger wall surface, reduce the amount of thermal expansion of the air by relying on the lower air operating temperature, reduce the pressure on the air pipeline, lower the design and processing requirements of the air pipeline, reduce the risk of air pipeline leakage, and further improve the reliability of equipment operation.

[0015] Furthermore, the liquid pipeline includes an outlet pipeline and an inlet pipeline. The two ends of the outlet pipeline and the two ends of the inlet pipeline are respectively connected to a third heat exchanger and an external heat-using device to form a circulation loop. At least one of the inlet pipeline and the outlet pipeline is equipped with a circulation pump that drives the liquid to circulate in the circulation loop. The controller is signal-connected to the circulation pump and is used to control the operation of the circulation pump.

[0016] By adopting the aforementioned technical solution, the controller can adjust the operating status of the circulating pump in real time based on the air temperature data fed back by the first temperature sensor, thereby changing the liquid circulation flow rate, dynamically adjusting the heat exchange capacity of the third heat exchanger, and thus correcting the air temperature, keeping the air within a lower and more reasonable temperature range, reducing the thermal expansion deformation of the air, reducing the pressure load on the air pipeline, simultaneously ensuring the temperature control effect of the heat exchange wall, and improving the overall operational reliability of the equipment.

[0017] Furthermore, the air duct entering the second heat exchanger is equipped with a fan, and the controller is signal-connected to the fan for controlling the operation of the fan.

[0018] Through the above technical solution, the controller adjusts the fan operation status based on the measured temperature signal fed back by the first temperature sensor, changes the air delivery flow rate and velocity, thereby adjusting the heat absorbed by the ambient temperature air in the second heat exchanger, and thus changing the outlet air temperature of the second heat exchanger. Temperature intervention is completed at the front end of the air heat exchange, and the temperature of the air that finally enters the first heat exchanger is adjusted from the source to stabilize it within the first temperature range. This also avoids excessive air heating, keeps the air at a low temperature, reduces the expansion pressure of the medium in the air pipeline, and ensures that the heat exchange wall temperature of the second heat exchanger meets the standard.

[0019] Furthermore, a second temperature sensor is installed on the air duct between the second and third heat exchangers. The second temperature sensor is signal-connected to the controller. The controller controls the operation of the fan according to the temperature detected by the second temperature sensor, so that the air temperature entering the third heat exchanger is maintained within a set second temperature range.

[0020] Through the above technical solution, the second temperature sensor monitors the air temperature delivered by the second heat exchanger in real time and feeds it back to the controller. The controller adjusts the operating status of the fan and changes the air flow rate and velocity accordingly to adjust the heat absorption of the air, so that the temperature of the air entering the third heat exchanger is stably maintained within the set second temperature range. This avoids the air temperature entering the third heat exchanger from being too low, thereby ensuring a sufficient heat exchange temperature difference between the air and the liquid. This allows the third heat exchanger to complete its heat regulation operation normally, laying a solid foundation for more precise control of the air temperature entering the first heat exchanger. It also avoids the air temperature entering the third heat exchanger from being too high, ensuring the stable operation of the air pipeline and the third heat exchanger, and reducing the pressure requirements on the air pipeline.

[0021] Furthermore, the second temperature range is 85°C to 110°C.

[0022] Through the above technical solution, if the air temperature entering the third heat exchanger is below 85℃, the temperature will further decrease after heat exchange between the air and the heat exchange liquid, making it easy for the final air temperature sent to the first heat exchanger to fall below the lower limit of the first temperature range, causing the heat exchanger wall surface of the first heat exchanger to enter the critical temperature range for ammonium bisulfate liquefaction and precipitation. If the temperature is above 110℃, the air carries too much heat, easily exceeding the temperature control load of the third heat exchanger, making it difficult to reduce the air temperature to the first temperature range, and increasing the expansion pressure of the medium inside the air pipeline, thus increasing the pressure load on the air pipeline. The second temperature range ensures, as much as possible, a sufficient temperature difference between the air and the liquid to enable the third heat exchanger to successfully complete temperature control operations, while also matching the downstream temperature control requirements as much as possible, limiting the air temperature rise, maintaining low-pressure air operation, reducing the pressure requirements of the air pipeline structure, and further ensuring that the air temperature sent to the first heat exchanger remains stable within the first temperature range.

[0023] Furthermore, the liquid outlet pipe and the liquid inlet pipe are respectively connected to the same side of the third heat exchanger.

[0024] By adopting the aforementioned technical solution, the length of liquid pipelines is reduced, the overall liquid pipeline assembly structure is simplified, the assembly and processing costs of liquid pipelines are reduced, the number of liquid pipeline splicing interfaces is reduced, the risk of liquid pipeline leakage is reduced, and the requirements for low leakage and high reliability of equipment operation are met.

[0025] Furthermore, the inlet and outlet of the air duct and flue gas duct are located on different sides of the second heat exchanger, respectively.

[0026] With the above technical solutions, if the air duct and flue gas duct inlets and outlets are arranged on the same side, the hot and cold media can easily converge and disturb each other at close range. This may cause the temperature of part of the heat exchange wall inside the second heat exchanger to drop below the dew point temperature of ammonium bisulfate, thereby causing ammonium bisulfate to precipitate and deposit on the heat exchange wall. Usually, additional thermal insulation structures are required for protection. This application arranges the air duct and flue gas duct ports on separate sides and increases the distance between them, which can avoid the formation of local low-temperature areas due to close convergence of hot and cold media, and minimize the problem of ammonium bisulfate precipitation and deposition on the heat exchange wall of the second heat exchanger due to excessively low temperature.

[0027] Furthermore, the flue gas duct is equipped with a second switching valve, which is located between the first heat exchanger and the second heat exchanger.

[0028] Through the above technical solution, the second switching valve can control the opening and closing of the flue gas passage between the first heat exchanger and the second heat exchanger; during equipment maintenance, the flue gas flow is blocked to ensure operational safety, and during normal operation, the flue gas passage is kept unobstructed, stabilizing the heat exchange conditions of the media on both sides, maintaining the temperature of the heat exchange wall of the second heat exchanger and the operating pressure of the air pipeline are stable, and ensuring the long-term reliable operation of the equipment. Attached Figure Description

[0029] The present invention will be further described below with reference to the accompanying drawings: Figure 1 This is a schematic diagram of the boiler flue gas waste heat recovery and utilization system in this invention; Figure 2 This is a detailed structural diagram of the boiler flue gas waste heat recovery and utilization system in this invention.

[0030] In the diagram, 10 is the first heat exchanger; 20 is the second heat exchanger; 30 is the third heat exchanger; 40 is the flue gas duct; 50 is the air duct; 60 is the liquid duct; 600 is the liquid outlet duct; 601 is the liquid inlet duct; 61 is the circulating pump; 70 is the first temperature sensor; 71 is the first switching valve; 72 is the fan; 73 is the second temperature sensor; and 74 is the second switching valve. Detailed Implementation

[0031] 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.

[0032] The terms “first,” “second,” “third,” “fourth,” etc., as used in this invention (if applicable), are used to distinguish similar objects and are not necessarily used to describe a specific order or sequence. It should be understood that such data can be interchanged where appropriate so that embodiments of the invention described herein can be implemented in orders other than those illustrated or described herein.

[0033] It should be understood that in the various embodiments of the present invention, the number of each process does not imply the order of execution. The execution order of each process should be determined by its function and internal logic, and should not constitute any limitation on the implementation process of the embodiments of the present invention.

[0034] It should be understood that in this invention, "comprising" and "having," and any variations thereof, are intended to cover non-exclusive inclusion, for example, a process, method, system, product, or device that includes a series of steps or units is not necessarily limited to those steps or units explicitly listed, but may include other steps or units not explicitly listed or inherent to such process, method, product, or device.

[0035] It should be understood that in this invention, "multiple" refers to two or more. "And / or" is merely a description of the relationship between related objects, indicating that three relationships can exist. For example, X and / or Y can represent: X alone, X and Y simultaneously, or Y alone. The character " / " generally indicates that the preceding and following related objects are in an "or" relationship. "Contains X, Y, and Z", "Contains X, Y, and Z" means that all three X, Y, and Z are contained; "Contains X, Y, or Z" means that one of X, Y, and Z is contained; "Contains X, Y, and / or Z" means that any one, two, or three of X, Y, and Z are contained.

[0036] The technical solution of the present invention will be described in detail below with reference to specific embodiments. The following specific embodiments may be combined or substituted with each other according to the actual situation, and the same or similar concepts or processes may not be described again in some embodiments.

[0037] like Figure 1As shown, this invention provides a boiler flue gas waste heat recovery and utilization system, including a first heat exchanger 10, a second heat exchanger 20, a third heat exchanger 30, a flue gas duct 40, an air duct 50, and a liquid duct 60. The inlet of the flue gas duct 40 is connected to the boiler exhaust port, and the outlet of the flue gas duct 40 is connected to external flue gas treatment equipment; the inlet of the air duct 50 is connected to external air, and the outlet of the air duct 50 is connected to external heat-using equipment; the liquid duct 60 is connected to the third heat exchanger 30, the inlet of the liquid duct 60 is connected to external liquid, and the outlet of the liquid duct 60 is connected to the external heat-using equipment, for introducing heat exchange liquid. The flue gas duct 40 sequentially connects to the first heat exchanger 10 and the second heat exchanger 20 along the flue gas conveying direction; the air duct 50 sequentially connects to the second heat exchanger 20, the third heat exchanger 30, and the first heat exchanger 10 along the air conveying direction; the liquid duct 60 connects to the third heat exchanger 30 along the liquid conveying direction.

[0038] High-temperature flue gas first enters the first heat exchanger 10 along flue gas duct 40 for initial heat exchange, releasing most of its heat and lowering its temperature to form low-temperature waste heat flue gas, which then flows along flue gas duct 40 into the second heat exchanger 20 for further heat exchange. Meanwhile, low-temperature external air enters the second heat exchanger 20 along air duct 50, absorbing heat from the low-temperature waste heat flue gas to raise its temperature. The small temperature difference between the two effectively avoids the formation of localized low-temperature zones and reduces the liquefaction and precipitation of ammonium bisulfate. The heated air then enters the third heat exchanger 30, where a liquid medium with strong heat absorption capacity absorbs excess heat from the air, making fuller use of the heat and regulating the air temperature to a more reasonable range.

[0039] Air, after being conditioned by the third heat exchanger 30, is sent to the first heat exchanger 10 via air duct 50 to participate in heat exchange. In this device, the liquid only exchanges heat with the air within the third heat exchanger 30; the first heat exchanger 10 and the second heat exchanger 20 do not have any liquid medium. Compared to existing schemes that directly exchange heat between flue gas and water, this application achieves targeted improvements in two aspects: the heat exchange medium path and the wall temperature control logic. First, in terms of the medium contact path, the existing technology uses direct heat exchange between water and flue gas. Once the water leaks, it will react with the flue gas to generate a corrosive medium, causing equipment corrosion, heat exchange medium contamination, and liquid deterioration, which seriously affects the stability of equipment operation. This application limits liquid heat exchange to the third heat exchanger 30, and the first heat exchanger 10 and the second heat exchanger 20 use heat exchange between flue gas and air throughout the entire process, isolating the contact path between liquid and flue gas. This not only fully preserves the excellent heat exchange performance of liquid, but also eliminates the risk of corrosion and medium contamination caused by liquid leakage from the root, ensuring the reliability of equipment operation.

[0040] Secondly, regarding wall temperature control and pressure design, existing technologies require a heat exchange wall temperature ≥147℃ to prevent ammonium bisulfate liquefaction. Since the convective heat transfer coefficient on the water side is much higher than on the flue gas side, the heat exchange wall temperature is closer to the water temperature. Existing technologies rely primarily on water temperature for heat exchange wall temperature control. However, the boiling point of water at atmospheric pressure is only 100℃, necessitating pressurization to raise the saturated boiling point to meet temperature requirements. This leads to higher pressure standards for pipelines and equipment, increased manufacturing costs, and a greater risk of weld cracking and media leakage under high pressure, further amplifying the risk of corrosion failure. In this application, the heat exchange wall temperature of the first heat exchanger 10 and the second heat exchanger 20 is controlled by… The weighted average thermal resistance of the media on both sides of the flue gas and air determines the temperature of the heat exchange walls. Since air is a gaseous medium with no phase change boiling point constraint, the required heat exchange wall temperature can be met without pressurizing the air duct 50 and the first and second heat exchangers 10. This reduces the internal operating pressure of the system, minimizing the risk of weld cracking and media leakage, lowering the equipment failure rate, and improving equipment reliability. Furthermore, it eliminates the need for a high-pressure structural design, effectively reducing equipment processing difficulty and manufacturing costs. Even if the air duct 50 leaks, it only results in physical mixing of air and flue gas, rather than water contacting the flue gas to produce corrosive substances, thus posing no risk of corrosion or media contamination. After heat exchange, the air and liquid can be separately transported to external heat-using equipment, achieving staged heat recovery and further improving the overall thermal energy utilization rate of the system.

[0041] It should be noted that the first heat exchanger 10 is a rotary air preheater. The rotary air preheater is equipped with a rotating heat storage rotor and densely arranged heat storage corrugated plates. Due to the limitations of rotor rotation heat exchange and heat storage corrugated plate arrangement, the rotary air preheater has the structural characteristics of fine internal medium flow channels, complex internal cavity structure, a large number of matching sealing air ducts, small overall structural pressure margin, and complex end face and air duct sealing nodes.

[0042] Considering the structural characteristics of the rotary air preheater, if the liquid directly exchanges heat with the flue gas in the rotary air preheater, the liquid needs to be pressurized to break through its boiling point and reach the required high temperature. Given the rotary air preheater's fine flow channels, multiple sealed air ducts, and low-pressure shell structure, this would drastically increase the overall pressure load on the rotor, cavity shell, connecting pipelines, and various sealing nodes. This would significantly raise the pressure design standards and manufacturing difficulty of the rotary air preheater's shell and rotor air ducts. Coupled with the numerous sealing nodes inherent in the rotary air preheater, the overall risk of medium leakage would be greatly increased. Once the liquid leaks from the sealing nodes or pipeline interfaces and comes into contact with the flue gas, a chemical reaction occurs, generating corrosive impurities that directly erode the heat storage corrugated plates and heat exchange walls. Simultaneously, it induces the precipitation of ammonium bisulfate, which blocks the fine heat storage flow channels inside the rotary air preheater and hinders the normal rotation of the rotor.

[0043] This application uses air as the heat exchange medium in conjunction with a rotary air preheater to complete flue gas heat exchange. This is more suitable for the rotary air preheater's structure, which has a fine flow channel, many sealing points, and low pressure margin. Air can reach the required temperature without pressurization, and the rotary air preheater operates at a lower pressure, effectively reducing the overall pressure design requirements of the rotary air preheater, air pipe 50, and flue gas pipe 40, simplifying the manufacturing process, and simultaneously reducing the risk of leakage at the sealing points of air pipe 50, flue gas pipe 40, and rotary air preheater. At the same time, air has more stable chemical properties. Even if air leaks from the sealing points of the rotary air preheater or the interfaces of air pipe 50, the air and flue gas only undergo physical mixing, without chemical reaction or the generation of corrosive impurities. It will not corrode the heat storage rotor and heat exchange wall of the rotary air preheater, nor will it further aggravate the liquefaction and deposition of ammonium bisulfate on the heat exchange wall, thus minimizing the risk of flow channel blockage and rotor jamming in the rotary air preheater.

[0044] The second heat exchanger 20 is a plate-type air heater, which has a more compact overall structure, faster medium heat exchange response, and simpler air duct sealing structure, making it more suitable for air medium heat exchange and allowing for rapid front-end air temperature control. The third heat exchanger 30 is an air cooler with a high degree of heat exchange module integration. It can independently complete gas-liquid isolation heat exchange, with the liquid medium only enclosed inside the air cooler. The heat exchange chamber has strong isolation and can specifically absorb excess heat from the air to complete air temperature regulation, resulting in a higher heat recovery and utilization rate.

[0045] It should be noted that external flue gas treatment equipment includes desulfurization towers, denitrification devices, dust collectors, and flue gas chimneys, as well as other tail-end flue gas purification and emission equipment; external heat-using equipment includes boilers, air heaters, heating equipment, industrial heat exchange equipment, and hot water recycling equipment, as well as other heat energy utilization equipment. The inlet medium of air duct 50 is not limited to ambient temperature air; other gaseous heat exchange media, such as nitrogen, can also be introduced. The selected gaseous medium must meet the following requirements: after leakage, it must not chemically react with the flue gas to generate corrosive substances or harmful impurities, avoiding corrosion, damage, and media contamination of air duct 50, flue gas duct 40, and the first heat exchanger 10. Liquid duct 60 can be introduced with liquid media such as water and heat transfer oil, which have excellent heat exchange performance. The appropriate medium can be flexibly matched according to the actual on-site working conditions and heat exchange temperature requirements.

[0046] like Figure 2As shown, to achieve precise and controllable air temperature entering the first heat exchanger 10 and ensure stable and efficient operation of the equipment, a first temperature sensor 70, a controller, and a first switching valve 71 are installed on the air duct 50 between the third heat exchanger 30 and the first heat exchanger 10. The first temperature sensor 70 detects the air temperature entering the first heat exchanger 10 and can collect the air temperature about to enter the first heat exchanger 10 in real time. The first temperature sensor 70 is connected to the controller, and the controller is connected to the first switching valve 71. The controller controls the opening and closing of the first switching valve 71 based on the detected temperature, so that the air temperature entering the first heat exchanger 10 is maintained within a set first temperature range. This minimizes the liquefaction and precipitation of ammonium bisulfate, reduces channel blockage, reduces thermal expansion deformation of the air, lowers the pressure load on the air duct 50, simultaneously ensures the temperature control effect of the heat exchange wall, and improves the overall operational reliability of the equipment.

[0047] The lowest temperature of the heat exchange wall of the first heat exchanger 10 is located at the air inlet, i.e., the connecting end of the first heat exchanger 10 in the air duct 50. This temperature is calculated by weighting the flue gas temperature and the inlet air temperature using the thermal resistance of the media on both sides. Under normal operating conditions, the flue gas temperature is higher than the critical liquefaction temperature of ammonium bisulfate, which is 147°C. Since both flue gas and air are gaseous media, the convective heat transfer coefficients on both sides are on the same order of magnitude, and the heat exchange wall temperature always lies between the flue gas temperature and the inlet air temperature. Based on this heat transfer characteristic, when the inlet air temperature is not lower than 60°C, the lowest temperature of the cold end wall after weighting by thermal resistance can be stably maintained above 147°C, meeting the heat exchange wall temperature requirements for preventing ammonium bisulfate liquefaction.

[0048] If the air temperature entering the first heat exchanger 10 is below 60°C, the heat exchange wall of the first heat exchanger 10 is prone to falling into the critical temperature range of ammonium bisulfate liquefaction, causing ammonium bisulfate liquefaction and deposition, which blocks the internal channels of the first heat exchanger 10. If the air temperature is above 80°C, it will not only reduce the temperature difference between flue gas and air, reducing the overall heat exchange efficiency, but also increase the air temperature rise, increase the expansion pressure of the medium inside the air pipe 50, and increase the pressure load on the air pipe 50. Therefore, in this application, the first temperature range is 60°C to 80°C. This ensures that the cold end temperature of the heat exchange wall is stably higher than the critical temperature of ammonium bisulfate liquefaction, minimizing the problem of ammonium bisulfate liquefaction and deposition on the heat exchange wall. It also reduces the thermal expansion of the air by relying on the lower air operating temperature, reducing the pressure on the air pipe 50, lowering the pressure design standard of the air pipe 50, reducing the risk of leakage in the air pipe 50, and further improving the reliability of equipment operation.

[0049] To maintain the air temperature entering the first heat exchanger 10 within a set first temperature range, the liquid pipeline 60 includes an outlet pipeline 600 and an inlet pipeline 601. The two ends of the outlet pipeline 600 and the two ends of the inlet pipeline 601 are respectively connected to the third heat exchanger 30 and an external heat-using device to form a circulation loop. At least one of the inlet pipeline 601 and the outlet pipeline 600 is equipped with a circulation pump 61 that drives the liquid to circulate in the circulation loop. The controller is signal-connected to the circulation pump 61. The controller can adjust the operating state of the circulation pump 61 in real time according to the air temperature data fed back by the first temperature sensor 70, thereby changing the liquid circulation flow rate, dynamically adjusting the heat exchange capacity of the third heat exchanger 30, and thus correcting the air temperature, further ensuring that the air entering the first heat exchanger 10 is stable within the set first temperature range.

[0050] The specific adjustment logic is as follows: when the first temperature sensor 70 detects that the intake air temperature is below 60°C, the controller controls the speed of the circulation pump 61 to be reduced, or even stops the operation of the circulation pump 61, to reduce the amount of liquid circulating into the third heat exchanger 30, reduce the heat loss of the air in the third heat exchanger 30, and realize the air temperature recovery; when the first temperature sensor 70 detects that the intake air temperature is above 80°C, the controller controls the speed of the circulation pump 61 to be increased, increase the liquid circulation flow rate, improve the heat exchange and heat dissipation capacity of the third heat exchanger 30, reduce the air temperature, and stabilize the air temperature within the set first temperature range.

[0051] Furthermore, the air duct 50 entering the second heat exchanger 20 is equipped with a fan 72. The controller is connected to the fan 72 for signal control. The controller adjusts the operating state of the fan 72 based on the measured temperature signal fed back by the first temperature sensor 70, thereby changing the air delivery flow rate and velocity. This adjusts the heat absorbed by the ambient temperature air in the second heat exchanger 20, thereby changing the outlet air temperature of the second heat exchanger 20. Temperature intervention is completed at the front end of the air heat exchange, ensuring that the air temperature entering the first heat exchanger 10 is stable within the first temperature range from the source. It can also prevent the air from overheating, keep the air at a low temperature, reduce the expansion pressure of the medium in the air duct 50, and ensure that the heat exchange wall temperature of the second heat exchanger 20 meets the standard, reducing the problem of ammonium bisulfate liquefaction and deposition on the heat exchange wall of the second heat exchanger 20.

[0052] The specific adjustment logic is as follows: When the first temperature sensor 70 detects that the air temperature entering the first heat exchanger 10 is lower than 60°C, the controller controls the fan 72 to reduce the airflow and velocity entering the second heat exchanger 20, so that the heat of the fixed flue gas is exchanged with less air, increasing the heat absorption of a single air stream, thereby raising the outlet air temperature of the second heat exchanger 20; when the first temperature sensor 70 detects that the air temperature entering the first heat exchanger 10 is higher than 80°C, the controller controls the fan 72 to increase the airflow and velocity entering the second heat exchanger 20, so that the heat of the fixed flue gas is distributed to more air medium, reducing the heat absorption of a single air stream, thereby lowering the outlet air temperature of the second heat exchanger 20, and achieving front-end cooling regulation of the first heat exchanger 10.

[0053] It should be noted that, in addition to adjusting the air intake by changing the speed of fan 72, multiple fans can be configured, and the air intake can be changed by controlling the number of fans in operation to adapt to the heat exchange and temperature control requirements under different working conditions.

[0054] It should be noted that this application allows for flexible selection of control methods based on actual operating conditions. The controller can independently adjust the operating parameters of either the fan 72 or the circulating pump 61, or it can control both to adjust in tandem, significantly improving the response speed and accuracy of temperature control. For example, when the intake air temperature is detected to be below 60°C and the temperature difference is large, the controller can simultaneously reduce the speed of both the circulating pump 61 and the fan 72. Through a dual control method of reducing heat absorption by the air at the front end and reducing heat dissipation by the air at the back end, the intake air temperature can be quickly raised back to the first temperature range.

[0055] Furthermore, a second temperature sensor 73 is installed on the air duct 50 between the second heat exchanger 20 and the third heat exchanger 30. The second temperature sensor 73 is connected to the controller signal and monitors the air temperature delivered by the second heat exchanger 20 in real time and feeds it back to the controller. The controller controls the operation of the fan 72 according to the temperature detected by the second temperature sensor 73, changing the air flow rate and velocity to adjust the heat absorption of the air, so that the air temperature entering the third heat exchanger 30 is kept within the set second temperature range. This avoids the air temperature entering the third heat exchanger 30 being too low, thereby ensuring a sufficient heat exchange temperature difference between the air and the liquid, allowing the third heat exchanger 30 to complete the heat regulation operation normally. This lays a solid foundation for more precise control of the air temperature entering the first heat exchanger 10, and also avoids the air temperature entering the third heat exchanger 30 being too high, ensuring the stable operation of the air duct 50 and the third heat exchanger 30, and reducing the pressure requirements of the air duct 50.

[0056] If the air temperature entering the third heat exchanger 30 is below 85°C, the air temperature will further decrease after heat exchange with the heat exchange liquid, making it easy for the final air temperature sent to the first heat exchanger 10 to fall below the lower limit of the first temperature range. This would cause the heat exchange wall of the first heat exchanger 10 to fall into the critical temperature range for ammonium bisulfate liquefaction and precipitation. If the air temperature entering the third heat exchanger 30 is above 110°C, the air carries too much heat, easily exceeding the temperature control load of the third heat exchanger 30, making it difficult to lower the air temperature to the first temperature range. Therefore, in this application, the second temperature range is 85°C to 110°C. This ensures that the air and liquid have a sufficient heat exchange temperature difference, allowing the third heat exchanger 30 to successfully complete the temperature control operation. It also matches the downstream temperature control requirements as much as possible, ensuring that the final air temperature sent to the first heat exchanger 10 is stable within the first temperature range, maintaining low-pressure air operation, reducing the pressure requirements of the air pipeline 50 structure, and further ensuring that the air temperature sent to the first heat exchanger 10 is stable within the first temperature range. It should be noted that the lower limit of the second temperature range needs to be higher than the upper limit of the first temperature range, so that after the air is cooled by the third heat exchanger 30, it can fall into the first temperature range. This avoids the air temperature from falling below the lower limit of the first temperature range due to insufficient heat exchange, and also prevents the precipitation and deposition of ammonium bisulfate on the heat exchange wall of the first heat exchanger 10 due to insufficient temperature. It also keeps the liquid temperature in the outlet pipe 600 at a lower level, reducing the pressure load on the outlet pipe 600 and reducing the risk of deformation and leakage of the outlet pipe 600.

[0057] The specific adjustment logic is as follows: When the second temperature sensor 73 detects that the air temperature entering the third heat exchanger 30 is lower than 85°C, the controller controls the fan 72 to reduce the air flow rate and velocity, reduce the air heat exchange volume, increase the heat absorption of a single stream of air, raise the outlet air temperature of the second heat exchanger 20, and make the inlet air temperature rise back to above 85°C; when the second temperature sensor 73 detects that the air temperature entering the third heat exchanger 30 is higher than 110°C, the controller controls the fan 72 to increase the air flow rate and velocity, distribute the heat of the flue gas to more air medium, reduce the temperature rise of a single stream of air, and make the outlet air temperature of the second heat exchanger 20 drop back to below 110°C, thereby continuously maintaining the air temperature entering the third heat exchanger 30 stably within the pre-temperature range of 85°C to 110°C.

[0058] It should be noted that this application incorporates a hierarchical priority control logic. When the temperature detected by the first temperature sensor 70 exceeds the first temperature range of 60℃ to 80℃, and simultaneously the temperature detected by the second temperature sensor 73 exceeds the second temperature range of 85℃ to 110℃, the controller prioritizes the operation of the fan 72 based on the detection data of the second temperature sensor 73. First, the outlet air temperature of the second heat exchanger 20 is corrected by adjusting the fan speed 72, ensuring that the air temperature entering the third heat exchanger 30 meets the set requirements of the second temperature range, guaranteeing stable front-end heat exchange conditions and ensuring that the third heat exchanger 30 has adequate basic heat exchange conditions. After the front-end air temperature stabilizes within the second temperature range, the controller switches to the detection data of the first temperature sensor 70 as the control benchmark, precisely matching the back-end air intake requirements, ultimately maintaining the air temperature entering the first heat exchanger 10 stably within the first temperature range. This minimizes temperature control conflicts between the front and back ends, progressively ensuring the accuracy of layered temperature control and improving the operational stability and reliability of the entire temperature control system.

[0059] If the inlet and outlet of the air duct 50 and the flue gas duct 40 are arranged on the same side, the hot and cold media can easily converge and disturb each other, which may cause the temperature of part of the heat exchange wall inside the second heat exchanger 20 to drop below the dew point temperature of ammonium bisulfate. This can lead to the precipitation and deposition of ammonium bisulfate on the heat exchange wall of the second heat exchanger 20, which usually requires additional insulation structures for protection. Therefore, in this application, the inlet and outlet of the air duct 50 and the flue gas duct 40 are located on different sides of the second heat exchanger 20, increasing the distance between the inlet and outlet, avoiding local low temperature problems as much as possible, eliminating the need for additional insulation components, simplifying the structure, reducing costs, and effectively suppressing the formation of ammonium bisulfate.

[0060] Furthermore, the inlet and outlet of the air duct 50 are located on different sides of the third heat exchanger 30, such as the left and right sides. This structure allows air to pass laterally through the internal heat exchange area of ​​the third heat exchanger 30, extending the air heat exchange path and ensuring more thorough and uniform heat exchange between air and liquid. This effectively improves the temperature regulation accuracy and heat exchange uniformity of the third heat exchanger 30, ensuring stable air temperature control. Simultaneously, the liquid outlet duct 600 and liquid inlet duct 601 are centrally connected to the same side of the third heat exchanger 30, as shown in the lower side layout. Compared to a distributed pipe layout, this significantly shortens the laying length of the liquid pipe 60, reduces pipe transition interfaces and bends, and results in a compact and orderly arrangement of the overall liquid pipe 60, effectively reducing the overall space occupied by the equipment. At the same time, the centralized arrangement of the liquid pipes 60 on the same side avoids the problem of messy and intertwined liquid pipes, greatly facilitating daily inspections, troubleshooting, disassembly, replacement, and maintenance of the liquid pipes 60, circulating pump 61, and other supporting components. This effectively reduces the difficulty of equipment operation and maintenance and improves the convenience of later maintenance.

[0061] For ease of control, the flue gas duct 40 is equipped with a second switching valve 74, located between the first heat exchanger 10 and the second heat exchanger 20. The second switching valve 74 controls the opening and closing of the flue gas passage between the first heat exchanger 10 and the second heat exchanger 20. During maintenance, it blocks the flue gas to ensure safety, while maintaining unobstructed passage during normal operation, adapting to different operating conditions. The flue gas temperature between the first heat exchanger 10 and the second heat exchanger 20 in the flue gas duct 40 is typically between 180℃ and 190℃, while the flue gas temperature discharged from the second heat exchanger 20 in the flue gas duct 40 is typically between 85℃ and 100℃, ensuring sufficient heat recovery.

[0062] Flue gas process as follows Figure 1 and Figure 2 As indicated by the middle arrow: The high-temperature flue gas generated by the boiler first enters the first heat exchanger 10 through the flue gas duct 40, where it undergoes initial heat exchange with the temperature-adjusted, higher-temperature air, releasing most of the flue gas heat and achieving flue gas cooling. The cooled, low-temperature waste heat flue gas continues to flow along the flue gas duct 40 and enters the second heat exchanger 20, where it undergoes secondary heat exchange with ambient-temperature air, further recovering the waste heat of the flue gas. After being cooled by two stages of heat exchange, the low-temperature flue gas is finally transported along the flue gas duct 40 to external flue gas treatment equipment such as the desulfurization tower, denitrification device, dust collector, and flue gas chimney, completing purification and achieving emission standards, realizing the step-by-step recovery and efficient utilization of flue gas waste heat.

[0063] Air flow such as Figure 1 and Figure 2 As indicated by the middle arrow: External ambient temperature air enters the system through the air duct 50 inlet, is pressurized by the fan 72, and first enters the second heat exchanger 20. There, it undergoes gas-to-gas heat exchange with the low-temperature waste heat flue gas, absorbing heat to achieve initial temperature rise and maintain a stable air temperature within the second temperature range of 85℃-110℃. After being heated, the air flows through the second temperature sensor 73 for pre-temperature monitoring and then enters the third heat exchanger 30 for precise gas-liquid heat exchange with the liquid heat exchange medium. This removes excess heat from the air, precisely regulating its temperature to the first temperature range of 60℃-80℃. The regulated air is then sent to the first heat exchanger 10 after temperature measurement by the first temperature sensor 70, where it exchanges heat with the high-temperature flue gas, further absorbing waste heat. Finally, the high-temperature air, having completed all heat exchange, is transported through the air duct 50 outlet to external heat-using equipment such as boilers, air heaters, and heating systems, achieving heat energy recovery and reuse. The entire air process employs a staged heat exchange and step-by-step temperature control mode to suppress ammonium bisulfate precipitation and blockage, ensuring heat exchange efficiency.

[0064] Liquid process such as Figure 1 and Figure 2As indicated by the middle arrow: Driven by the circulating pump 61, water flows into the third heat exchanger 30 through the liquid inlet of the liquid pipe 60, where it undergoes gas-liquid heat exchange with the high-temperature air output from the second heat exchanger 20, absorbing excess heat from the air and achieving air cooling. The heated water is then transported along the liquid outlet pipe 600 to external heat-consuming equipment such as industrial heat exchange equipment, hot water reuse equipment, and factory heating equipment to release heat and complete the heat energy supply. After releasing heat and cooling, the water flows back to the third heat exchanger 30 through the liquid inlet pipe 601, completing water recycling. Simultaneously, the controller can dynamically adjust the speed of the circulating pump 61 and the liquid circulation flow rate based on the air temperature signals from both the upstream and downstream ends to match the air temperature control requirements and ensure system operational stability.

[0065] Understandably, in other embodiments, the circulating pump can be installed separately on the inlet or outlet pipe, depending on the actual installation requirements, and both can realize the functions of liquid circulation drive and heat exchange regulation.

[0066] In addition to the preferred embodiments described above, the present invention has other embodiments. All other embodiments obtained by those skilled in the art based on the embodiments of the present invention without creative effort are within the scope of protection claimed by the present invention.

Claims

1. A boiler flue gas waste heat recovery and utilization system, characterized in that: It includes a first heat exchanger (10), a second heat exchanger (20) and a third heat exchanger (30); The flue gas duct (40) is connected in sequence to the first heat exchanger (10) and the second heat exchanger (20) along the flue gas conveying direction; An air duct (50) is connected in sequence to a second heat exchanger (20), a third heat exchanger (30), and a first heat exchanger (10) along the air transport direction; A liquid pipe (60) is connected to a third heat exchanger (30) for introducing heat exchange liquid; The inlet of the flue gas duct (40) is used to connect to the boiler exhaust port, and the outlet of the flue gas duct (40) is used to connect to external flue gas treatment equipment. The inlet of the air duct (50) is used to connect to external air, and the outlet of the air duct (50) is used to connect to external heating equipment.

2. The boiler flue gas waste heat recovery and utilization system according to claim 1, characterized in that, The air duct (50) is provided with a first temperature sensor (70), a controller and a first switching valve (71) between the third heat exchanger (30) and the first heat exchanger (10). The first temperature sensor (70) is used to detect the air temperature entering the first heat exchanger (10). The first temperature sensor (70) is signal-connected to the controller, and the controller is signal-connected to the first switching valve (71). The controller is used to control the opening and closing of the first switching valve (71) according to the detected temperature so that the air temperature entering the first heat exchanger (10) is maintained within a set first temperature range.

3. The boiler flue gas waste heat recovery and utilization system according to claim 2, characterized in that, The first temperature range is 60°C to 80°C.

4. The boiler flue gas waste heat recovery and utilization system according to claim 2, characterized in that, The liquid pipeline (60) includes an outlet pipeline (600) and an inlet pipeline (601). The two ends of the outlet pipeline (600) and the two ends of the inlet pipeline (601) are respectively connected to a third heat exchanger (30) and an external heat-using device to form a circulation loop. At least one of the inlet pipeline (601) and the outlet pipeline (600) is provided with a circulation pump (61) to drive the liquid to circulate in the circulation loop. The controller is signal-connected to the circulation pump (61) and is used to control the operation of the circulation pump (61).

5. The boiler flue gas waste heat recovery and utilization system according to claim 2, characterized in that, An air duct (50) leading into the second heat exchanger (20) is equipped with a fan (72), and the controller is signal-connected to the fan (72) for controlling the operation of the fan (72).

6. The boiler flue gas waste heat recovery and utilization system according to claim 5, characterized in that, A second temperature sensor (73) is provided on the air duct (50) between the second heat exchanger (20) and the third heat exchanger (30). The second temperature sensor (73) is connected to the controller. The controller controls the operation of the fan (72) according to the temperature detected by the second temperature sensor (73) so that the air temperature entering the third heat exchanger (30) is maintained within a set second temperature range.

7. The boiler flue gas waste heat recovery and utilization system according to claim 6, characterized in that, The second temperature range is 85°C to 110°C.

8. The boiler flue gas waste heat recovery and utilization system according to claim 4, characterized in that, The liquid outlet pipe (600) and the liquid inlet pipe (601) are respectively connected to the same side of the third heat exchanger (30).

9. The boiler flue gas waste heat recovery and utilization system according to claim 1, characterized in that, The inlets and outlets of the air duct (50) and flue gas duct (40) are located on different sides of the second heat exchanger (20).

10. The boiler flue gas waste heat recovery and utilization system according to claim 1, characterized in that, The flue gas duct (40) is provided with a second switching valve (74), which is located between the first heat exchanger (10) and the second heat exchanger (20).

Images