Low-temperature flue gas waste heat recovery system and method based on double-safety-area wall temperature control

Abstract

The invention discloses a low-temperature flue gas waste heat recovery system and method based on double-safety-area wall temperature control, and belongs to the technical field of industrial energy conservation and environmental protection. Aiming at the problems of low-temperature corrosion and low efficiency in deep waste heat recovery of acid-containing low-temperature flue gas, the invention provides a solution based on a corrosion control theory: a phase change heat exchanger is divided into a high-temperature section and a low-temperature section along a flue gas flow, and the high-temperature section and the low-temperature section are respectively connected with an independent steam pocket to form two independent phase change loops; through interlocking control of a wall temperature measuring point and a cold source adjusting unit, the operation pressure of the two loops is independently adjusted, and precise partition wall temperature regulation and control are achieved. The high-temperature loop operates in a first safe area (higher than the flue gas acid dew point). The cryogenic circuit operates in a second safe zone (corrosion valley zone). Materials are prepared according to different temperature gradients, so that the equipment cost is reduced. The problem of low-temperature corrosion of flue gas is solved, safe and deep cooling of the flue gas is realized, and the device has the outstanding effects of high heat exchange efficiency and remarkable reduction of equipment investment.

Description

Technical Field

[0001] This invention belongs to the field of industrial energy conservation and environmental protection technology, specifically relating to a low-temperature flue gas waste heat recovery system and method based on dual safety zone wall temperature control, and in particular to a phase change heat recovery process system and method based on corrosion control theory, which achieves efficient, safe and deep recovery of flue gas waste heat by setting and independently controlling the wall temperature safety zones in different zones. Background Technology

[0002] In the field of industrial waste heat recovery, especially for low-temperature flue gas at 230℃ and below, indirect heat exchangers (such as shell-and-tube heat exchangers and plate heat exchangers) are currently the most widely used and technologically mature heat exchange technology. Their principle is to indirectly transfer the heat of the flue gas to the working fluid (such as water or air) through the metal wall, resulting in a simple structure and high reliability. Despite their widespread application, traditional indirect heat exchange technology has the following problems when deeply recovering waste heat from acid-containing low-temperature flue gas: 1. Severe Low-Temperature Corrosion Problem: In indirect heat exchangers, the low-temperature working fluid to be heated enters from the inlet. The metal wall temperature of the inlet section of the heat exchange tubes is close to the inlet working fluid temperature, which is far below the acid dew point of the flue gas. Sulfuric acid vapor in the flue gas rapidly condenses into high-concentration liquid sulfuric acid on this low-temperature wall surface, undergoing an electrochemical corrosion reaction with the metal. This results in an extremely rapid corrosion rate in this area, making it a major site for equipment perforation and leakage.

[0003] 2. Inherently low heat exchange efficiency: Traditional indirect heat exchange technology mainly relies on convection and conduction for heat transfer. Under specific operating conditions of low-temperature flue gas waste heat recovery, due to the small heat transfer temperature difference (terminal difference), its overall heat transfer coefficient is generally very low, resulting in low overall heat exchange efficiency.

[0004] 3. Huge equipment investment: Due to the low energy grade of low-temperature flue gas and the small usable heat exchange temperature difference between it and the working fluid, coupled with the low heat transfer coefficient, an exceptionally large heat exchange area must be configured in order to cool the flue gas to the target exhaust temperature. Summary of the Invention

[0005] In view of this, the purpose of the present invention is to solve the above-mentioned defects of existing indirect heat exchange technology in recovering waste heat from acid-containing low-temperature flue gas, and to provide a low-temperature flue gas waste heat recovery system and method based on dual safety zone wall temperature control.

[0006] To achieve the above objectives, the present invention provides the following technical solution: A low-temperature flue gas waste heat recovery system based on dual safety zone wall temperature control includes a phase change heat exchanger. The phase change heat exchanger is divided into a high-temperature phase change loop and a low-temperature phase change loop along the flue gas flow direction. The high-temperature phase change loop is located on the flue gas inlet side, and the low-temperature phase change loop is located on the flue gas outlet side. Both the high-temperature phase change loop and the low-temperature phase change loop contain phase change working fluids, forming two independent phase change circulation loops. The two phase change circulation loops absorb heat from the high-temperature section and the low-temperature section of the flue gas through their internal phase change working fluids, respectively. The operating temperature of the phase change working fluid in the high-temperature phase change circuit is higher than the acid dew point of the flue gas, i.e., it is in the first safe zone; the operating temperature of the phase change working fluid in the low-temperature phase change circuit is within the corrosion trough range, i.e., it is in the second safe zone.

[0007] Furthermore, the phase change working fluid in the high-temperature phase change circuit and the low-temperature phase change circuit is the same; the operating pressure of the high-temperature phase change circuit is positive, and the corresponding saturation temperature of the phase change working fluid at this pressure is ≥115℃, so that the wall temperature of the heat exchange tube bundle in the high-temperature section operates in the first safe zone, avoiding low-temperature corrosion in the high-temperature section; the operating pressure of the low-temperature phase change circuit is negative, and the corresponding saturation temperature of the phase change working fluid at this pressure is 70~80℃, so that the wall temperature of the heat exchange tube bundle in the low-temperature section operates in the second safe zone, reducing the corrosion rate in the low-temperature section.

[0008] Furthermore, both phase change circulation loops include heat exchange tube bundles, condensate lower headers, steam upper headers, condensate outlet pipes, condensate preheating pipes, steam riser pipes, condensate downcomer pipes, and steam drums; The heat exchange tube bundle is arranged inside the phase change heat exchanger along the flue gas flow direction, and is used to transfer the heat of the flue gas to the liquid phase change working fluid inside the tubes through the tube walls; the upper steam header is located at the upper part of the heat exchange tube bundle and is connected to the upper end of the heat exchange tube bundle, and is used to collect the gaseous phase change working fluid that evaporates after the liquid phase change working fluid absorbs heat; the lower condensate header is located at the lower part of the heat exchange tube bundle and is connected to the lower end of the heat exchange tube bundle, and is used to distribute the preheated liquid phase change working fluid into the heat exchange tube bundle. One end of the steam riser is connected to the upper steam header, and the other end is connected to the steam drum. The steam drum is equipped with a cold source heat exchanger tube, and a cold source is installed inside the cold source heat exchanger tube. The gaseous phase change working fluid rises into the steam drum and releases heat and condenses on the outer wall of the cold source heat exchanger tube. The released latent heat is transferred to the cold source and carried out of the steam drum by the cold source. One end of the condensate outlet pipe is connected to the bottom of the steam drum, and the other end is connected to the inlet of the condensate preheating pipe. The condensate preheating pipe is located inside the upper steam header, and uses the gaseous phase change working fluid in the upper steam header to preheat the condensed low-temperature liquid phase change working fluid to the saturation temperature. One end of the condensate downcomer is connected to the outlet of the condensate preheating pipe, and the other end is connected to the condensate lower header, which is used to transport the preheated liquid phase change working fluid to the condensate lower header.

[0009] Furthermore, a cold source supply and regulation unit is connected to the cold source heat exchange tube of the steam drum to provide a cold source and regulate the flow rate.

[0010] Furthermore, it also includes a measurement and control system, which includes a temperature sensor and a control system. The temperature sensor is located on the steam riser or the condensate lower header and is used to measure the operating temperature of the working fluid. The temperature sensor and the cold source supply and regulation unit are all connected to the control system. The control system drives the cold source supply and regulation unit to adjust the cold source flow rate according to the feedback temperature of the temperature sensor, thereby controlling the condensation rate of the gaseous working fluid in the steam drum and adjusting the operating pressure of the steam drum to achieve the purpose of controlling the operating temperature of the working fluid, so that the wall temperature of the heat exchange tube bundle is always within the safe temperature range.

[0011] Furthermore, each of the two steam drums in the high-temperature phase change loop and the low-temperature phase change loop is independently connected to a cold source supply and regulation unit. The cold source supply and regulation unit includes a pump and a regulating valve. The pump is located on the cold source inlet side and provides power for cold source delivery. The regulating valve is located downstream of the pump and is connected to the inlet of the cold source heat exchange tube of the steam drum. The pump and the regulating valve work together to regulate the cold source flow rate and control the condensation rate of the gaseous phase change working fluid in the steam drum, thereby dynamically maintaining the operating pressure and operating temperature of the corresponding phase change loop.

[0012] Furthermore, the heat exchange tube bundle of the high-temperature phase change circuit is made of ND steel, which is suitable for working conditions where the wall temperature is higher than the acid dew point; the heat exchange tube bundle of the low-temperature phase change circuit is made of 2205 duplex stainless steel, which is suitable for humid flue gas environments where the wall temperature is in the corrosion valley zone.

[0013] Furthermore, the system is used for waste heat recovery from acidic low-temperature flue gas at 230℃ and below. The phase change working fluid and the cold source are both demineralized water, which is supplied independently by the cold source supply and regulation unit to realize the absorption of the latent heat of condensation of the phase change working fluid and the heating of the cold source.

[0014] A method for recovering waste heat from low-temperature flue gas based on dual safety zone wall temperature control is proposed. The system described above is used to recover waste heat from low-temperature flue gas. The low-temperature flue gas enters the phase change heat exchanger and flows through the high-temperature phase change circuit and the low-temperature phase change circuit in sequence. The heat is transferred to the phase change working fluid inside the tube through the tube wall, and the flue gas is gradually cooled to below 100°C before being discharged. The phase change working fluid circulates independently in the high-temperature phase change loop and the low-temperature phase change loop. The phase change working fluid is drawn from the bottom of the steam drum, enters the condensate preheating pipe through the condensate outlet pipe, and indirectly exchanges heat with the gaseous phase change working fluid in the upper steam header. After being preheated to near saturation temperature, it enters the lower condensate header through the condensate downcomer. The liquid phase change working fluid in the lower condensate header enters the heat exchange tube bundle to absorb heat from the flue gas and evaporates into steam. The gaseous phase change working fluid rises into the upper steam header, then enters the steam drum to condense and release heat. The condensed phase change working fluid is drawn from the bottom of the steam drum, completing the cycle. The cold source is divided into two paths. The steam drums of the high-temperature phase change circuit and the low-temperature phase change circuit are supplied and regulated by independent cold source supply and regulation units. The steam absorbs the latent heat of condensation of the phase change working fluid and is then output. The wall temperature is monitored by the measurement and control system, and the cold source flow is adjusted accordingly. The wall temperature of the high-temperature phase change circuit is independently controlled to be ≥115℃ and the wall temperature of the low-temperature phase change circuit is 70~80℃. This ensures that the wall temperature of the heat exchange tube bundle in the high-temperature section operates in the first safe zone to avoid low-temperature corrosion in the high-temperature section, and the wall temperature of the heat exchange tube bundle in the low-temperature section operates in the second safe zone to reduce the corrosion rate in the low-temperature section.

[0015] Furthermore, the high-temperature phase change circuit maintains positive pressure, and the low-temperature phase change circuit maintains negative pressure. By adjusting the flow rate of the cold source entering the steam drum, the condensation rate is controlled, thereby adjusting the operating pressure inside the steam drum and changing the saturation temperature of the working fluid accordingly, achieving precise control of the working fluid's operating temperature. When the wall temperature is higher than the set value, the corresponding cold source flow rate is increased to accelerate condensation and reduce the operating pressure inside the steam drum, causing the working fluid's saturation temperature to decrease accordingly until it reaches the set value. When the wall temperature is lower than the set value, the cold source flow rate is decreased to reduce condensation and increase the operating pressure inside the steam drum, causing the working fluid's saturation temperature to increase accordingly until it reaches the set value, achieving precise control of the wall temperature.

[0016] The beneficial effects of this invention are as follows: 1. Achieves safe and in-depth waste heat recovery: This invention utilizes a segmented system design based on the dual-safety-zone corrosion theory. It divides the phase change heat exchanger into two independent phase change loops along the flue gas flow direction: a high-temperature phase change loop and a low-temperature phase change loop. This ensures that different sections operate strictly within specific corrosion-safe zones. This successfully solves the technical challenge of low-temperature corrosion, enabling the flue gas to be safely and stably cooled to below 100°C or even lower. It recovers waste heat from the low-temperature zone, which is unusable with traditional technologies, significantly improving energy efficiency.

[0017] 2. Achieved efficient and compact equipment characteristics: This invention fully utilizes the advantages of phase change heat transfer technology, which can efficiently transfer heat under a small temperature difference of 5~10℃. Combined with optimized system design, the heat exchange area required by the equipment is greatly reduced under the same waste heat recovery capacity, making the system more compact and significantly reducing equipment manufacturing costs and space occupation.

[0018] 3. Ensures long-term, reliable, and safe operation: This invention achieves uniform and precise control of the heat exchange tube wall temperature through the dual protection of "condensate preheating" and "dual pressure regulation." The cold source flow regulation realizes feedback control, stabilizing the heat exchange tube bundle wall temperature in the high-temperature section in the first safe zone (above the flue gas acid dew point) and the heat exchange tube bundle wall temperature in the low-temperature section in the second safe zone (corrosion trough zone), greatly extending the service life and operational reliability of the core components of the equipment.

[0019] 4. Excellent technical and economic efficiency: This invention adopts a "gradient material configuration" strategy, which differentiates the use of ND steel and 2205 duplex stainless steel for the drastically different corrosion environments at high and low temperatures. It ensures safety margins in critical parts and saves costs in non-critical parts, achieving the optimal balance between safety and cost throughout the entire life cycle and improving the return on investment of the project.

[0020] Other advantages, objectives, and features of the invention will be set forth in part in the description which follows, and in part will be apparent to those skilled in the art from the following examination, or may be learned from practice of the invention. The objectives and other advantages of the invention can be realized and obtained through the following description. Attached Figure Description

[0021] To make the objectives, technical solutions, and advantages of the present invention clearer, the preferred embodiments of the present invention will be described in detail below with reference to the accompanying drawings, wherein: Figure 1 This is an overall schematic diagram of the low-temperature flue gas waste heat recovery system based on dual safety zone wall temperature control in this invention.

[0022] Figure 2 This is a flow chart of the various media involved in the phase change heat transfer process in this invention.

[0023] Reference numerals: 1. Phase changer; 2. High-temperature section heat exchanger tube bundle; 3. Low-temperature section heat exchanger tube bundle; 4. High-temperature steam drum; 5. Low-temperature steam drum; 6. High-temperature loop cold source regulating valve group; 7. High-temperature loop cold source pump; 8. Low-temperature loop cold source regulating valve group; 9. Low-temperature loop cold source pump; 10. Cold source heat exchange tube inside the steam drum; 11. Condensate outlet pipe; 12. Condensate preheating pipe; 13. Steam riser pipe; 14. Upper steam header; 15. Condensate downcomer pipe; 16. Lower condensate header; F1: Flue gas inlet; F2: Flue gas outlet; W1: High-temperature loop cold source inlet; W2: High-temperature loop heated water outlet; W3: Low-temperature loop cold source inlet; W4: Low-temperature loop heated water outlet. Detailed Implementation

[0024] The following specific examples illustrate the implementation of the present invention. Those skilled in the art can easily understand other advantages and effects of the present invention from the content disclosed in this specification. The present invention can also be implemented or applied through other different specific embodiments, and various details in this specification can be modified or changed based on different viewpoints and applications without departing from the spirit of the present invention. It should be noted that the illustrations provided in the following embodiments are only schematic representations of the basic concept of the present invention. Unless otherwise specified, the following embodiments and features can be combined with each other.

[0025] The accompanying drawings are for illustrative purposes only and are schematic diagrams, not actual pictures. They should not be construed as limiting the invention. To better illustrate the embodiments of the invention, some parts in the drawings may be omitted, enlarged, or reduced, and do not represent the actual product dimensions. It is understandable to those skilled in the art that some well-known structures and their descriptions may be omitted in the drawings.

[0026] In the accompanying drawings of the embodiments of the present invention, the same or similar reference numerals correspond to the same or similar components. In the description of the present invention, it should be understood that if terms such as "upper," "lower," "left," "right," "front," and "rear" indicate the orientation or positional relationship based on the orientation or positional relationship shown in the drawings, they are only for the convenience of describing the present invention and simplifying the description, and do not indicate or imply that the device or element referred to must have a specific orientation, or be constructed and operated in a specific orientation. Therefore, the terms used to describe positional relationships in the drawings are only for illustrative purposes and should not be construed as limiting the present invention. For those skilled in the art, the specific meaning of the above terms can be understood according to the specific circumstances.

[0027] Example 1 like Figures 1-2 The diagram illustrates a low-temperature flue gas waste heat recovery system based on dual-safety-zone wall temperature control, comprising a phase change heat exchanger 1. The phase change heat exchanger 1 is divided into high-temperature and low-temperature phase change loops along the flue gas flow direction. The high-temperature phase change loop is located at the flue gas inlet, and the low-temperature phase change loop is located at the flue gas outlet. Both the high-temperature and low-temperature phase change loops contain phase change working fluids, forming two independent phase change circulation loops. These two loops absorb heat from the high-temperature and low-temperature flue gas sections respectively through their internal phase change working fluids. The phase change working fluid in both the high-temperature and low-temperature phase change circuits is demineralized water. The high-temperature phase change circuit operates at a positive pressure, at which the corresponding phase change working fluid saturation temperature is ≥115℃, ensuring that the heat exchange tube bundle wall temperature in the high-temperature section operates within the first safe zone (above the flue gas acid dew point) and preventing low-temperature corrosion in the high-temperature section. The low-temperature phase change circuit operates at a negative pressure, at which the corresponding phase change working fluid saturation temperature is 70~80℃, ensuring that the heat exchange tube bundle wall temperature in the low-temperature section operates within the second safe zone (corrosion trough zone) and reducing the corrosion rate in the low-temperature section.

[0028] Both phase change circulation loops include heat exchange tube bundles, a lower condensate header 16, an upper steam header 14, a condensate outlet pipe 11, a condensate preheating pipe 12, a steam riser pipe 13, a condensate downcomer pipe 15, and a steam drum (high-temperature steam drum 4 or low-temperature steam drum 5). The heat exchange tube bundles are arranged in the phase change heat exchanger 1 along the flue gas flow direction to transfer heat from the flue gas to the liquid phase change working fluid inside the tubes through the tube walls. The upper steam header 14 is located above the heat exchange tube bundles and connected to the upper end of the heat exchange tube bundles, used to collect the gaseous working fluid that evaporates after the liquid phase change working fluid absorbs heat. The lower condensate header 16 is located below the heat exchange tube bundles and connected to the lower end of the heat exchange tube bundles, used to distribute the preheated liquid phase change working fluid into the heat exchange tube bundles. One end of the steam riser pipe 13 is connected to the upper steam header. The steam drum is connected to a steam drum at one end and a cold source heat exchange tube 10 inside. A cold source is located within the cold source heat exchange tube 10. Gaseous phase change working fluid rises into the steam drum and releases heat, condensing on the outer wall of the cold source heat exchange tube 10. The released latent heat is transferred to the cold source and carried out of the steam drum by the cold source. A condensate outlet pipe 11 is connected at one end to the bottom of the steam drum and at the other end to the inlet of a condensate preheating pipe 12, located inside the upper steam header 14. The preheating pipe 12 utilizes the gaseous phase change working fluid in the upper steam header 14 to preheat the condensed low-temperature liquid phase change working fluid to its saturation temperature. A condensate downcomer 15 is connected at one end to the outlet of the condensate preheating pipe 12 and at the other end to a lower condensate header 16, used to transport the preheated liquid phase change working fluid to the lower condensate header 16. A cold source supply and regulation unit is connected to the cold source heat exchange tube 10 of the steam drum to provide a cold source and regulate the flow rate.

[0029] This embodiment also includes a measurement and control system, which includes a temperature sensor and a control system. The temperature sensor is located on the steam riser pipe 13 or the condensate lower header 16 and is used to measure the operating temperature of the working fluid. The temperature sensor, the cold source supply and regulation unit are all connected to the control system. The control system drives the cold source supply and regulation unit to adjust the cold source flow rate according to the feedback temperature of the temperature sensor, controls the condensation rate of the gaseous working fluid in the steam drum, and then adjusts the operating pressure of the steam drum to achieve the purpose of controlling the operating temperature of the working fluid, so that the wall temperature of the heat exchange tube bundle is always within the safe temperature range.

[0030] Each of the two steam drums in the high-temperature phase change loop and the low-temperature phase change loop is independently connected to a cold source supply and regulation unit. The cold source supply and regulation unit includes a pump and a regulating valve (high-temperature loop cold source pump 7 + high-temperature loop cold source regulating valve group 6, low-temperature loop cold source pump 9 + low-temperature loop cold source regulating valve group 8). The pump is located on the cold source inlet side and provides cold source delivery power. The regulating valve is located downstream of the pump and is connected to the inlet of the cold source heat exchange tube 10 of the steam drum. The pump and the regulating valve work together to regulate the cold source flow rate and control the condensation rate of the gaseous phase change working fluid in the steam drum, thereby dynamically maintaining the operating pressure and operating temperature of the corresponding phase change loop.

[0031] The heat exchange tube bundle of the high-temperature phase change circuit is made of ND steel, suitable for operating conditions where the wall temperature is higher than the acid dew point; the heat exchange tube bundle of the low-temperature phase change circuit is made of 2205 duplex stainless steel, suitable for humid flue gas environments where the wall temperature is in the corrosion valley zone. This system is used for waste heat recovery from acid-containing low-temperature flue gas at 230℃ and below. The cold source is demineralized water, which is supplied independently by the cold source supply and regulation unit to achieve absorption of the latent heat of condensation of the phase change working fluid and heating of the cold source.

[0032] The system configuration and design parameters in this embodiment are as follows: Flue gas conditions: The treated flue gas is purified waste incineration flue gas that meets national emission standards, with a flow rate of 100,000 Nm³. 3 / h, inlet temperature 150℃, acid dew point temperature about 110℃, design target flue gas temperature is 100℃.

[0033] Cold source conditions: The medium to be heated is demineralized water with an initial temperature of 20℃ and a design total flow rate of 40 t / h.

[0034] Key design values: The high-temperature section wall temperature is controlled at 115℃, corresponding to a design pressure of approximately 0.07 MPaG (positive pressure) for the high-temperature steam drum 4; the low-temperature section wall temperature is controlled at 75℃, corresponding to a design pressure of approximately -0.06 MPaG (vacuum) for the low-temperature steam drum 5. This vacuum level is maintained by the equilibrium of condensation and evaporation of the phase change medium itself after sufficient venting during system startup.

[0035] Low-temperature flue gas flows from left to right through phase change heat exchanger 1 from flue gas inlet F1, and exchanges heat with high-temperature section heat exchange tube bundle 2 and low-temperature section heat exchange tube bundle 3 in sequence before cooling down and being discharged from flue gas outlet F2. The phase change working fluid circulates in two independent loops. The condensed low-temperature condensate is led out through condensate outlet pipe 11 and enters steam upper header 14. It exchanges heat indirectly with saturated steam in condensate preheating pipe 12 and is preheated to near saturation temperature. Then it is sent to condensate lower header 16 through condensate downcomer 15 and distributed to heat exchange tube bundles. It absorbs heat from the flue gas and evaporates into steam, which flows into steam upper header 14 and is then sent to steam drum through steam riser pipe 13. In the steam drum, it condenses and releases heat on the outer wall of cold source heat exchange tube 10, completing the cycle. The two cold sources enter the cold source heat exchange tubes 10 in the two steam drums through the cold source inlet W1 of the high-temperature circuit and the cold source inlet W3 of the low-temperature circuit, respectively. After absorbing the heat released by the condensation of the phase change working fluid, they are discharged from the heated water outlet W2 of the high-temperature circuit and the heated water outlet W4 of the high-temperature circuit through two independent pump and valve systems (cold source regulating valve group 6 of the high-temperature circuit + cold source pump 7 of the high-temperature circuit and cold source regulating valve group 8 of the low-temperature circuit + cold source pump 9 of the low-temperature circuit).

[0036] Example 2 This embodiment describes a low-temperature flue gas waste heat recovery method based on the low-temperature flue gas waste heat recovery system with dual safety zone wall temperature control in Embodiment 1. The method specifically includes the following steps: When starting up a waste incineration flue gas waste heat recovery project, demineralized water is first injected into both the high-temperature and low-temperature phase change circuits to the standard level. Flue gas is then introduced for heating, and the high-point vent valves of the circuits are opened simultaneously. After continuous steam is discharged, the valves are closed to ensure that there are no non-condensable gases in the system that could affect heat transfer and control.

[0037] The system is put into automatic control mode. The monitoring and control system monitors the wall temperature of the high-temperature heat exchange tube bundle 2 and the low-temperature heat exchange tube bundle 3 in real time. The control logic of the two loops is completely independent. The demineralized water is divided into two paths and enters the respective steam drums through the high-temperature loop cold source pump 7 + high-temperature loop cold source regulating valve group 6 and the low-temperature loop cold source pump 9 + low-temperature loop cold source regulating valve group 8, respectively. After being heated, the output can be combined.

[0038] High-temperature loop control: The high-temperature section wall temperature is the controlled object, with a setpoint of 115℃. When the measured wall temperature is higher than the setpoint, the control system instructs to increase the opening of the high-temperature loop cold source regulating valve group 6 or the operating frequency of the high-temperature loop cold source pump 7, allowing more low-temperature demineralized water to enter the high-temperature steam drum 4, accelerating internal steam condensation, resulting in a decrease in the steam drum operating pressure, and consequently, a decrease in the saturation temperature and wall temperature, returning to the setpoint. When the measured wall temperature is lower than the setpoint, the opening of the regulating valve or the operating frequency of the pump is reduced to decrease condensation, causing the pressure and temperature to rise again.

[0039] Low-temperature loop control: The controlled object is the wall temperature of the low-temperature section, with a setpoint of 75℃. The control principle is the same as that of the high-temperature loop. When the measured wall temperature is higher than the setpoint, the control system commands to increase the opening of the low-temperature loop cold source regulating valve group 8 or the operating frequency of the low-temperature loop cold source pump 9, allowing more low-temperature demineralized water to enter the low-temperature steam drum 5, accelerating internal steam condensation, resulting in a decrease in the steam drum operating pressure, and consequently, a decrease in the saturation temperature and wall temperature, returning to the setpoint. When the measured wall temperature is lower than the setpoint, the opening of the regulating valve or the operating frequency of the pump is reduced to decrease condensation, causing the pressure and temperature to rise again. The negative pressure in the low-temperature loop is maintained by the equilibrium of condensation and evaporation of the phase change working fluid after the exhaust is started.

[0040] Through the aforementioned closed-loop regulation, the cold source flow directly controls the condensation rate of the two steam drums, thereby independently and precisely maintaining the high-temperature phase change loop wall temperature at ≥115℃ (above the flue gas acid dew point) and the low-temperature phase change loop wall temperature at 70~80℃ (located in the corrosion trough zone), achieving dual-safety zone wall temperature control and ensuring safe cooling of the flue gas to below 100℃. The system requires no additional auxiliary means during the entire operation, achieving precise wall temperature control solely through flow regulation, greatly extending equipment lifespan.

[0041] Once the system is running stably, the flue gas flow rate is 100,000 Nm. 3 The waste incineration flue gas, with an inlet temperature of 150°C, is gradually cooled to 100°C before being discharged. This process can heat 40 t / h of demineralized water from 20°C to approximately 59°C, recovering a total heat of approximately 1.81 MW.

[0042] Compared with traditional indirect heat exchange technology, this embodiment has the following significant advantages: Deep recovery: Successfully overcomes the acid dew point corrosion barrier, safely reducing the flue gas temperature from the traditional 150℃ or above to 100℃, achieving full recovery of low-temperature waste heat.

[0043] High efficiency and compact design: Utilizing the high-efficiency heat transfer characteristics of small end difference in phase change heat transfer, the heat transfer area is reduced by 40%-50% compared to traditional shell-and-tube heat exchangers under the same heat transfer capacity, making the equipment more compact.

[0044] Safe and reliable: Through closed-loop control, the wall temperatures of the high and low temperature sections are actively stabilized at "dual safety zones" of 115℃ and 75℃ respectively, reducing the risk of corrosion and extending the equipment life.

[0045] Advantageous in terms of economy: The use of a gradient material configuration of "high temperature section ND steel + low temperature section 2205 stainless steel" significantly reduces the cost of key materials compared to the overall 2205 stainless steel solution.

[0046] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and are not intended to limit it. Although the present invention has been described in detail with reference to preferred embodiments, those skilled in the art should understand that modifications or equivalent substitutions can be made to the technical solutions of the present invention without departing from the spirit and scope of the present invention, and all such modifications or substitutions should be covered within the scope of the claims of the present invention.

Claims

1. A low-temperature flue gas waste heat recovery system based on dual-safety zone wall temperature control, characterized in that, It includes a phase change heat exchanger; the phase change heat exchanger is divided into a high-temperature phase change circuit and a low-temperature phase change circuit along the flue gas flow direction. The high-temperature phase change circuit is located on the flue gas inlet side and the low-temperature phase change circuit is located on the flue gas outlet side. Both the high-temperature phase change circuit and the low-temperature phase change circuit contain phase change working fluid, forming two independent phase change circulation circuits. The two phase change circulation circuits absorb heat from the high-temperature section and the low-temperature section of the flue gas through the internal phase change working fluid, respectively. The operating temperature of the phase change working fluid in the high-temperature phase change circuit is higher than the acid dew point of the flue gas, i.e., it is in the first safe zone; the operating temperature of the phase change working fluid in the low-temperature phase change circuit is within the corrosion trough range, i.e., it is in the second safe zone.

2. The system according to claim 1, characterized in that: The phase change working fluid in the high-temperature phase change circuit and the low-temperature phase change circuit is the same; the operating pressure of the high-temperature phase change circuit is positive, and the corresponding saturation temperature of the phase change working fluid at this pressure is ≥115℃, so that the wall temperature of the heat exchange tube bundle in the high-temperature section operates in the first safe zone, avoiding low-temperature corrosion in the high-temperature section; the operating pressure of the low-temperature phase change circuit is negative, and the corresponding saturation temperature of the phase change working fluid at this pressure is 70~80℃, so that the wall temperature of the heat exchange tube bundle in the low-temperature section operates in the second safe zone, reducing the corrosion rate in the low-temperature section.

3. The system according to claim 1, characterized in that: Both phase change circulation loops include heat exchanger tube bundles, condensate lower header, steam upper header, condensate outlet pipe, condensate preheating pipe, steam riser pipe, condensate downcomer pipe, and steam drum; The heat exchange tube bundle is arranged inside the phase change heat exchanger along the flue gas flow direction, and is used to transfer the heat of the flue gas to the liquid phase change working fluid inside the tubes through the tube walls; the upper steam header is located at the upper part of the heat exchange tube bundle and is connected to the upper end of the heat exchange tube bundle, and is used to collect the gaseous phase change working fluid that evaporates after the liquid phase change working fluid absorbs heat; the lower condensate header is located at the lower part of the heat exchange tube bundle and is connected to the lower end of the heat exchange tube bundle, and is used to distribute the preheated liquid phase change working fluid into the heat exchange tube bundle. One end of the steam riser is connected to the upper steam header, and the other end is connected to the steam drum. The steam drum is equipped with a cold source heat exchanger tube, and a cold source is installed inside the cold source heat exchanger tube. The gaseous phase change working fluid rises into the steam drum and releases heat and condenses on the outer wall of the cold source heat exchanger tube. The released latent heat is transferred to the cold source and carried out of the steam drum by the cold source. One end of the condensate outlet pipe is connected to the bottom of the steam drum, and the other end is connected to the inlet of the condensate preheating pipe. The condensate preheating pipe is located inside the upper steam header, and uses the gaseous phase change working fluid in the upper steam header to preheat the condensed low-temperature liquid phase change working fluid to the saturation temperature. One end of the condensate downcomer is connected to the outlet of the condensate preheating pipe, and the other end is connected to the condensate lower header, which is used to transport the preheated liquid phase change working fluid to the condensate lower header.

4. The system according to claim 3, characterized in that: The heat exchange tube of the steam drum is connected to a cold source supply and regulation unit, which is used to provide a cold source and regulate the flow rate.

5. The system according to claim 4, characterized in that: It also includes a measurement and control system, which includes a temperature sensor and a control system. The temperature sensor is located on the steam riser or the condensate lower header and is used to measure the operating temperature of the working fluid. The temperature sensor and the cold source supply and regulation unit are all connected to the control system. The control system drives the cold source supply and regulation unit to adjust the cold source flow rate according to the feedback temperature of the temperature sensor, thereby controlling the condensation rate of the gaseous working fluid in the steam drum and adjusting the operating pressure of the steam drum to achieve the purpose of controlling the operating temperature of the working fluid, so that the wall temperature of the heat exchange tube bundle is always within the safe temperature range.

6. The system according to claim 4, characterized in that: Each of the two steam drums in the high-temperature phase change circuit and the low-temperature phase change circuit is independently connected to a cold source supply and regulation unit. The cold source supply and regulation unit includes a pump and a regulating valve. The pump is located on the cold source inlet side and provides power for cold source delivery. The regulating valve is located downstream of the pump and is connected to the inlet of the cold source heat exchange tube of the steam drum. The pump and the regulating valve work together to regulate the cold source flow rate and control the condensation rate of the gaseous phase change working fluid in the steam drum, thereby dynamically maintaining the operating pressure and operating temperature of the corresponding phase change cycle circuit.

7. The system according to claim 3, characterized in that: The heat exchange tube bundle of the high-temperature phase change circuit is made of ND steel, which is suitable for working conditions where the wall temperature is higher than the acid dew point; the heat exchange tube bundle of the low-temperature phase change circuit is made of 2205 duplex stainless steel, which is suitable for humid flue gas environments where the wall temperature is in the corrosion valley zone.

8. The system according to claim 1, characterized in that: This system is used for waste heat recovery from acidic low-temperature flue gas at 230℃ and below. The phase change working fluid and the cold source are both demineralized water, which is supplied independently by the cold source supply and regulation unit to realize the absorption of the latent heat of condensation of the phase change working fluid and the heating of the cold source.

9. A method for recovering waste heat from low-temperature flue gas based on dual-safety-zone wall temperature control, characterized in that, Low-temperature flue gas waste heat recovery is performed using the system described in any one of claims 1 to 8; the low-temperature flue gas enters the phase change heat exchanger and flows sequentially through the high-temperature phase change circuit and the low-temperature phase change circuit, transferring heat to the phase change working fluid inside the tube through the tube wall, and the flue gas is gradually cooled to below 100°C before being discharged. The phase change working fluid circulates independently in the high-temperature phase change loop and the low-temperature phase change loop. The phase change working fluid is drawn from the bottom of the steam drum, enters the condensate preheating pipe through the condensate outlet pipe, and indirectly exchanges heat with the gaseous phase change working fluid in the upper steam header. After being preheated to near saturation temperature, it enters the lower condensate header through the condensate downcomer. The liquid phase change working fluid in the lower condensate header enters the heat exchange tube bundle to absorb heat from the flue gas and evaporates into steam. The gaseous phase change working fluid rises into the upper steam header, then enters the steam drum to condense and release heat. The condensed phase change working fluid is drawn from the bottom of the steam drum, completing the cycle. The cold source is divided into two paths. The steam drums of the high-temperature phase change circuit and the low-temperature phase change circuit are supplied and regulated by independent cold source supply and regulation units. The steam absorbs the latent heat of condensation of the phase change working fluid and is then output. The wall temperature is monitored by the measurement and control system, and the cold source flow is adjusted accordingly. The wall temperature of the high-temperature phase change circuit is independently controlled to be ≥115℃ and the wall temperature of the low-temperature phase change circuit is 70~80℃. This ensures that the wall temperature of the heat exchange tube bundle in the high-temperature section operates in the first safe zone to avoid low-temperature corrosion in the high-temperature section, and the wall temperature of the heat exchange tube bundle in the low-temperature section operates in the second safe zone to reduce the corrosion rate in the low-temperature section.

10. The method according to claim 9, characterized in that: The high-temperature phase change circuit maintains positive pressure, and the low-temperature phase change circuit maintains negative pressure. By adjusting the flow rate of the cold source entering the steam drum, the condensation rate is controlled, thereby adjusting the operating pressure inside the steam drum and changing the saturation temperature of the working fluid accordingly, achieving precise control of the working fluid's operating temperature. When the wall temperature is higher than the set value, the corresponding cold source flow rate is increased to accelerate condensation and reduce the operating pressure inside the steam drum, causing the working fluid's saturation temperature to decrease accordingly until it reaches the set value. When the wall temperature is lower than the set value, the cold source flow rate is decreased to reduce condensation and increase the operating pressure inside the steam drum, causing the working fluid's saturation temperature to increase accordingly until it reaches the set value, achieving precise control of the wall temperature.

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