A flue gas waste heat recovery multistage heat exchanger heat pipe control method and system

Abstract

The application provides a flue gas waste heat recovery multistage heat exchanger heat pipe control method and system, and relates to the technical field of flue gas waste heat recovery.The method comprises the following steps: obtaining the change rate of the gas industrial boiler load instruction;obtaining the thermal inertia parameters of each stage of the series heat exchanger;according to the change rate and the thermal inertia parameters, determining the cooling medium flow pre-adjustment amount of each stage of the heat exchanger;according to the cooling medium flow pre-adjustment amount, adjusting the cooling medium flow of each stage of the heat exchanger, and combining the closed loop feedback control of each stage of the heat exchanger to correct the cooling medium flow.The method of the application aims to solve the problem of dynamic response mismatch caused by the thermal inertia difference of each stage of the heat exchanger in the flue gas waste heat recovery system under the condition of rapid change of the gas industrial boiler load, ensures the continuous effectiveness of the latent heat recovery function, improves the overall thermal efficiency of the system, significantly reduces the thermal stress risk of the equipment, and improves the operation stability.

Description

Technical Field

[0001] This invention relates to the field of flue gas waste heat recovery technology, and more specifically, to a method and system for thermal management of a multi-stage heat exchanger for flue gas waste heat recovery. Background Technology

[0002] During the operation of gas-fired industrial boilers, the discharged high-temperature flue gas contains enormous thermal energy, which has significant recovery value. To achieve cascaded energy utilization, flue gas waste heat recovery systems are typically designed as multi-stage series heat exchanger structures. The flue gas flows sequentially through high-temperature heat exchangers (e.g., superheaters, reheaters), medium-temperature heat exchangers (e.g., evaporators, high-pressure economizers), and low-temperature heat exchangers (e.g., low-pressure economizers, condensate heaters). Each stage of the heat exchanger operates under specific design conditions and heat exchange objectives. For example, high-temperature heat exchangers primarily generate high-quality steam, while low-temperature heat exchangers aim to maximize the recovery of waste heat from the flue gas, reducing the exhaust temperature to the design value. Especially in the condensation heat exchanger, latent heat in the flue gas must also be recovered. The control objective of the entire waste heat recovery system is to coordinate and regulate the operation of each stage of the heat exchanger while ensuring operational safety, thereby maximizing overall heat exchange efficiency.

[0003] However, in actual operation, the operating load of gas-fired industrial boilers needs to be adjusted frequently and rapidly according to actual commands. When a gas-fired industrial boiler receives a load reduction command, its fuel supply will decrease accordingly, causing the flow rate and temperature of its exhaust gas to drop rapidly in a short period of time. This drastic dynamic change poses a significant challenge to the coordinated control of the waste heat recovery system.

[0004] The challenge stems from the significant differences in thermal inertia among the various stages of heat exchangers. Due to variations in structure, materials, internal heat exchange medium capacity, and flow conditions, the response speeds to temperature changes differ considerably among these stages. For example, high-temperature superheaters, which contain numerous metal pipes and water / steam as the working fluid, typically exhibit high thermal inertia and a relatively slow temperature response; conversely, low-temperature economizers or condensing heat exchangers, with narrower pipes and lower water flow rates, have lower thermal inertia and respond to temperature changes much faster.

[0005] When the load suddenly drops and the flue gas inlet parameters change drastically, existing conventional control systems that independently adjust the outlet parameters of each stage of heat exchanger will reveal their inherent limitations. Specifically, the low-temperature heat exchanger in the system, due to its low thermal inertia, will quickly sense the decrease in inlet flue gas temperature. In order to maintain its own outlet water temperature setpoint, its independent controller will immediately reduce the flow rate of its internal circulating water to adapt to the new flue gas temperature conditions. From the perspective of local control logic, this adjustment action is rapid and reasonable.

[0006] However, simultaneously, the upstream high-temperature heat exchanger, due to its enormous thermal inertia, maintains a high temperature level due to its massive metal structure and internal working fluid, resulting in a very slow temperature decrease. Although the total flue gas flow rate has decreased, the flue gas flowing through the high-temperature heat exchanger continues to be heated, albeit with a reduced heat exchange capacity. Crucially, because the downstream low-temperature heat exchanger prematurely and significantly reduced its own heat exchange capacity (achieved by reducing the cooling water flow rate), the temperature of the flue gas flowing from the high-temperature section to the low-temperature section did not decrease synchronously as expected.

[0007] This creates a unique "temperature mismatch" phenomenon during the dynamic transition phase: for the low-temperature heat exchanger, the flue gas temperature it receives is far higher than what it should be operating stably under the current load. For example, a low-temperature heat exchanger designed to handle 120°C inlet flue gas may face flue gas temperatures as high as 150°C during a sudden load drop transition. This undesigned overheated flue gas will trigger a series of chain problems. First, it directly undermines the heat exchange objective of the low-temperature section. If this stage of heat exchanger is a condensation heat exchanger used to recover the latent heat of flue gas condensation, the excessively high flue gas temperature will prevent it from reaching the flue gas dew point temperature, causing the latent heat recovery function to completely fail, thus significantly reducing the overall thermal efficiency of the gas-fired industrial boiler. Second, this undesigned temperature shock may cause additional thermal stress to the materials and structure of the low-temperature heat exchanger. Repeated occurrences of such conditions over a long period will accelerate equipment fatigue and shorten its service life. Meanwhile, in order to cope with abnormally high inlet flue gas temperatures, the control system in the low-temperature section may take extreme adjustment actions, such as opening the circulating water flow to the maximum, which may cause oscillations in the hydraulic system and further affect the stability of the heating network or demineralized water system.

[0008] Therefore, under specific operating conditions with drastic load fluctuations, the lack of predictability and coordination in independent control between heat exchangers at each level will lead to disordered energy transfer within the system due to the inherent differences in thermal inertia. This will not only fail to achieve the goal of synergistic optimization, but will also cause energy waste and equipment operation risks. Summary of the Invention

[0009] The purpose of this invention is to provide a thermal management method and system for multi-stage heat exchangers in flue gas waste heat recovery. This method addresses the dynamic response mismatch caused by differences in thermal inertia among heat exchangers at different stages in a flue gas waste heat recovery system under rapidly changing load conditions in gas-fired industrial boilers. It maintains a coordinated temperature gradient, prevents abnormal increases in the inlet flue gas temperature of downstream heat exchangers, ensures the continuous effectiveness of latent heat recovery, improves the overall thermal efficiency of the system, significantly reduces the risk of thermal stress on the equipment, and enhances operational stability.

[0010] In a first aspect, the present invention provides a thermal management method for a multi-stage heat exchanger for flue gas waste heat recovery, applicable to gas-fired industrial boilers, comprising the following steps:

[0011] Obtain the rate of change of load commands for gas-fired industrial boilers;

[0012] Obtain the thermal inertia parameters of each series heat exchanger in the flue gas waste heat recovery system;

[0013] Based on the rate of change of the load command and the thermal inertia parameters of each heat exchanger, the pre-adjustment amount of the cooling medium flow rate for each heat exchanger is determined. Specifically, for any two heat exchangers, the heat exchanger with lower thermal inertia has a lower pre-adjustment amount of cooling medium flow rate compared to the heat exchanger with higher thermal inertia, so that the reduction of the cooling medium flow rate of the heat exchanger with lower thermal inertia is limited, while the reduction of the cooling medium flow rate of the heat exchanger with higher thermal inertia is increased.

[0014] Based on the pre-adjustment amount of the cooling medium flow rate, the cooling medium flow rate of each stage of the heat exchanger is adjusted, and combined with the closed-loop feedback control of each stage of the heat exchanger, the cooling medium flow rate is corrected to achieve thermal management of the multi-stage heat exchanger.

[0015] The present invention provides a multi-stage heat exchanger thermal management method for flue gas waste heat recovery. This method addresses the problems of existing control methods based on independent adjustment of each heat exchanger's outlet parameters. During sudden load drops, downstream heat exchangers, with their faster response, prematurely reduce their heat exchange capacity, while upstream heat exchangers, with their slower response, experience delayed cooling. This leads to abnormally high flue gas temperatures flowing to downstream heat exchangers during the transition period, resulting in downstream heat exchangers failing to achieve their design heat exchange targets, overall system thermal efficiency losses, excessive equipment thermal stress, and operational instability. The invention introduces a feedforward collaborative control strategy based on the load command change rate of the gas-fired industrial boiler. This strategy does not rely on the actual lag change in flue gas parameters but rather anticipates the load change trend and, combined with the inherent thermal inertia characteristics of each heat exchanger, calculates and pre-adjusts the cooling medium flow rate or bypass valve opening of each heat exchanger before significant changes in flue gas parameters occur. Through this predictive adjustment, coordinated action of each heat exchanger during the dynamic process is achieved, maintaining a reasonable flue gas temperature gradient, preventing abnormal increases in downstream heat exchanger inlet temperature, and ensuring efficient and stable system operation under transitional conditions.

[0016] Secondly, this invention provides a multi-stage heat exchanger thermal management system for flue gas waste heat recovery, applied to gas-fired industrial boilers, comprising:

[0017] The first acquisition module is used to acquire the rate of change of the load command of the gas-fired industrial boiler.

[0018] The second acquisition module is used to acquire the thermal inertia parameters of the series heat exchangers at each stage in the flue gas waste heat recovery system;

[0019] The determination module is used to determine the pre-adjustment amount of the cooling medium flow rate of each heat exchanger based on the rate of change of the load command and the thermal inertia parameters of each heat exchanger. Specifically, for any two heat exchangers, the heat exchanger with lower thermal inertia has a lower pre-adjustment amount of the cooling medium flow rate than the heat exchanger with higher thermal inertia, so that the heat exchanger with lower thermal inertia is limited in the reduction of its cooling medium flow rate, and the heat exchanger with higher thermal inertia is allowed to reduce its cooling medium flow rate by a larger margin.

[0020] The control module is used to adjust the cooling medium flow rate of each stage of the heat exchanger according to the pre-adjustment amount of the cooling medium flow rate, and to correct the cooling medium flow rate in combination with the closed-loop feedback control of each stage of the heat exchanger, so as to realize thermal management and control of multi-stage heat exchangers.

[0021] As can be seen from the above, the multi-stage heat exchanger thermal management method for flue gas waste heat recovery provided by this invention introduces feedforward collaborative control based on the load change rate. It pre-calculates and adjusts the cooling medium flow rate of each stage of the heat exchanger, breaking the inherent lag of independent responses from each stage and making the adjustment actions of each stage of the heat exchanger more coordinated in time. Simultaneously, by limiting the reduction in cooling medium flow rate of the low-temperature stage heat exchanger and appropriately advancing or increasing the adjustment of the high-temperature stage heat exchanger with greater thermal inertia, it ensures that the flue gas temperature maintains a coordinated decreasing gradient during load reduction, avoiding abnormal increases in downstream inlet flue gas temperature. This ensures that the low-temperature stage heat exchanger can effectively reduce the flue gas temperature below the dew point under dynamic operating conditions, guaranteeing the continuous effectiveness of the flue gas condensation latent heat recovery function, reducing heat loss caused by dynamic mismatch of each stage of the heat exchanger, and thus improving the overall thermal efficiency of the system under transitional operating conditions. Finally, the predictive adjustment reduces the undesigned temperature shocks and drastic fluctuations experienced by each stage of the heat exchanger, reduces the thermal stress on the equipment, avoids extreme adjustment actions, improves the operational stability of the system, and helps extend the service life of the equipment.

[0022] Other features and advantages of the invention will be set forth in the following description, and will be apparent in part from the description, or may be learned by practicing embodiments of the invention. The objects and other advantages of the invention may be realized and obtained by means of the structures particularly pointed out in the written description and the accompanying drawings. Attached Figure Description

[0023] Figure 1 This is a flowchart of a multi-stage heat exchanger thermal management method for flue gas waste heat recovery provided in an embodiment of the present invention.

[0024] Figure 2 This is a schematic diagram of an evaporator structure in an embodiment of the present invention.

[0025] Figure 3 This is a schematic diagram of a condenser structure in an embodiment of the present invention.

[0026] Figure 4 This is a schematic diagram of a heat exchanger structure for dynamic modeling in an embodiment of the present invention.

[0027] Figure 5 This is a schematic diagram of a multi-stage heat exchanger thermal control system for flue gas waste heat recovery provided in an embodiment of the present invention.

[0028] Label Explanation:

[0029] 100. First acquisition module; 200. Second acquisition module; 300. Determination module; 400. Control module. Detailed Implementation

[0030] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. The components of the embodiments of the present invention described and shown in the accompanying drawings can generally be arranged and designed in various different configurations. Therefore, the following detailed description of the embodiments of the present invention provided in the accompanying drawings is not intended to limit the scope of the claimed invention, but merely to illustrate selected embodiments of the invention. All other embodiments obtained by those skilled in the art based on the embodiments of the present invention without inventive effort are within the scope of protection of the present invention.

[0031] It should be noted that similar reference numerals and letters in the following figures indicate similar items; therefore, once an item is defined in one figure, it does not need to be further defined and explained in subsequent figures. Furthermore, in the description of this invention, terms such as "first," "second," etc., are used only to distinguish descriptions and should not be construed as indicating or implying relative importance.

[0032] Reference Appendix Figure 1 This invention provides a thermal control method for a multi-stage heat exchanger for flue gas waste heat recovery, applicable to gas-fired industrial boilers, comprising the following steps:

[0033] Obtain the rate of change of load commands for gas-fired industrial boilers;

[0034] Obtain the thermal inertia parameters of each series heat exchanger in the flue gas waste heat recovery system;

[0035] Based on the rate of change of the load command and the thermal inertia parameters of each heat exchanger, the pre-adjustment amount of the cooling medium flow rate for each heat exchanger is determined. Specifically, for any two heat exchangers, the heat exchanger with lower thermal inertia has a lower pre-adjustment amount of cooling medium flow rate compared to the heat exchanger with higher thermal inertia, so that the reduction of the cooling medium flow rate of the heat exchanger with lower thermal inertia is limited, while the reduction of the cooling medium flow rate of the heat exchanger with higher thermal inertia is increased.

[0036] Based on the pre-adjustment amount of the cooling medium flow rate, the cooling medium flow rate of each stage of the heat exchanger is adjusted, and combined with the closed-loop feedback control of each stage of the heat exchanger, the cooling medium flow rate is corrected to achieve thermal management of the multi-stage heat exchanger.

[0037] The rate of change of load command for a gas-fired industrial boiler refers to the speed at which the external load demand signal received by the boiler changes over time. It can be obtained by differentiating the load command signal or by measuring the load change within a set time window, such as through real-time monitoring of the boiler's dispatch commands or load prediction model output. Its main purpose is to quantify the dynamic changing trend of the boiler's operating conditions, providing input for proactive control. The thermal inertia parameters of each stage of series heat exchangers in a flue gas waste heat recovery system refer to the response speed of each heat exchanger to temperature changes or its internal heat storage capacity. These parameters can be characterized or calculated using physical quantities such as the heat capacity, heat transfer coefficient, structural dimensions, internal medium type, and flow rate of the heat exchanger. For example, parameter identification can be performed by establishing a dynamic thermodynamic model of each stage of the heat exchanger. This is mainly to identify and quantify the differences in thermal response characteristics of each stage of the heat exchanger under dynamic operating conditions. Specifically, this can be achieved by dynamically modeling the heat exchanger and then obtaining the relevant thermal inertia parameters through simulation experiments based on the model. For example, refer to the appendix... Figure 2 and attached Figure 3 The heat exchanger consists of an evaporator and a condenser. The evaporator uses a finned tube heat exchanger, and the condenser uses a plate heat exchanger. The fluid flows counter-currently through the heat exchanger. The heat exchanger model is based on the finite volume method. To simplify the model and improve computational efficiency, it is necessary to make appropriate simplification assumptions, as follows:

[0038] a. The fluid inside the pipe is considered as one-dimensional flow. The flow of fluid is actually a three-dimensional process, but there is a clear dominant flow direction between the fluid inlet and outlet. Therefore, the flow of fluid can be assumed to be one-dimensional.

[0039] b. Pressure drop is negligible. With water as the secondary fluid, pressure has a very weak effect on its properties, so the pressure drop has a very small impact on the heat transfer performance on the water side. For the working fluid side, due to the short inlet and outlet paths and the large flow area between the plates, the pressure drop is also very small and can be ignored.

[0040] c. Viscous stress is negligible. Viscous dissipation is less than the dominant energy transfer between the heat exchanger wall and the working fluid and the water on the other side; therefore, viscous stress dissipation can be completely ignored.

[0041] d. Neglect axial heat transfer. The working fluid state, water, and the wall temperature gradient that varies along the length of the heat exchange plate cause heat to diffuse axially, while high flow rates and high Pe numbers (i.e., Peclay number, which represents the ratio of convective heat or mass transfer rate to internal conduction rate, quantifying the relative effects of heat convection and heat diffusion modes) can negligible axial heat conduction.

[0042] e. Thermal resistance is negligible. Metallic materials have a large thermal conductivity and a large heat transfer area, resulting in very low lateral thermal resistance. The primary effect is radial heat conduction; therefore, only radial heat conduction is considered. Furthermore, the dynamic heat capacity of the wall material must be taken into account.

[0043] Based on the above simplifying assumptions, a dynamic model of the heat exchanger is performed. For example... Figure 4 As shown, the working fluid in the heat exchanger flows from left to right, and its thermodynamic parameters change gradually during the heat exchange process along the plate. The secondary fluid flows from right to left in a counter-current manner, exchanging heat with the working fluid through the wall units. The state can be represented by the thermodynamic parameters at the center point of the unit (as shown by the black dots in the figure), which is the average value of the state parameters at the inlet and outlet of the unit. The heat transfer process between units can be represented by the thermodynamic state parameters of the unit boundaries; therefore, there are n+1 boundaries (as shown by the red dots in the figure), and thermodynamic parameters are transferred between the boundaries of adjacent units through these boundaries.

[0044] The pre-adjustment amount of cooling medium flow rate refers to the amount of advance or coordinated adjustment of the cooling medium flow rate of each stage of heat exchanger based on the predicted dynamic response of the system before or at the beginning of a load command change. It can be calculated using methods such as model predictive control, feedforward control, or expert system rules, for example, through a preset lookup table or dynamic simulation model. Its main purpose is to achieve forward-looking and coordinated control of the cooling medium flow rate in order to cope with changes in flue gas parameters. For any two-stage heat exchangers, the heat exchanger with lower thermal inertia has a lower pre-adjustment of cooling medium flow rate compared to the heat exchanger with higher thermal inertia. This limits the reduction in cooling medium flow rate for the heat exchanger with lower thermal inertia, while increasing the reduction in cooling medium flow rate for the heat exchanger with higher thermal inertia. This means that under the trend of decreasing flue gas temperature due to load reduction, the pre-adjustment of cooling medium flow rate for heat exchangers with faster thermal response is set to a relatively small value. This implies that the reduction in cooling medium flow rate is intentionally limited. In certain situations, to maintain the overall flue gas temperature gradient of the system, the reduction in cooling medium flow rate is even reduced. This is mainly to enable the heat exchanger with lower thermal inertia to continuously absorb relatively more heat in the early stage of rapid load reduction through differentiated control, thereby suppressing the rise in its inlet flue gas temperature and avoiding the "temperature mismatch" phenomenon. Closed-loop feedback control refers to a control method that automatically adjusts the flow rate of the cooling medium based on the deviation between the actual operating parameters of the heat exchanger and the set value, in order to eliminate the deviation and maintain the operation of the system. It can be implemented by using PID controllers, fuzzy logic controllers, or adaptive controllers. For example, by monitoring the heat exchanger outlet temperature in real time and comparing it with the target value, its main purpose is to correct the pre-adjusted flow rate in real time, make up for the model prediction deviation, and ensure control accuracy and system stability.

[0045] The working principle of this method lies in predicting the load change trend of the gas-fired industrial boiler and implementing feedforward coordinated control by combining the inherent thermal inertia differences of each stage of the heat exchanger in the flue gas waste heat recovery system. This method pre-adjusts the cooling medium flow rate of each stage of the heat exchanger before the actual change in flue gas parameters, effectively avoiding the problem of abnormal rise in downstream flue gas temperature caused by the mismatch in the dynamic response of each stage of the heat exchanger when the load changes rapidly. This improves the heat recovery efficiency and operational stability of the system under dynamic operating conditions.

[0046] The core innovation of this invention lies in introducing the rate of change of load command of gas-fired industrial boiler and the thermal inertia parameters of each series heat exchanger as inputs for feedforward control. Based on this, the pre-adjustment amount of cooling medium flow rate of each heat exchanger is determined. In particular, for heat exchangers with smaller thermal inertia, the reduction of their cooling medium flow rate is limited or increased. Thus, under the condition of rapid load reduction, the heat load distribution of each heat exchanger is actively and collaboratively regulated, solving the "temperature mismatch" phenomenon caused by the difference in thermal inertia of each level. This achieves the effects of ensuring the normal operation of the low-temperature heat exchanger, maintaining the latent heat recovery efficiency, and reducing the thermal stress of the equipment.

[0047] Specifically, this method first acquires the rate of change of the load command of the gas-fired industrial boiler to perceive and quantify the dynamic changes in the boiler's operating conditions in real time, especially the changes in flue gas flow and temperature when the load drops rapidly, providing external disturbance information for subsequent pre-adjustment of the cooling medium flow. Simultaneously, it acquires the thermal inertia parameters of each stage of the series heat exchanger in the flue gas waste heat recovery system to identify and quantify the differences in the response speed of each heat exchanger to temperature changes, which is the root cause of the "temperature mismatch" phenomenon. By comprehensively considering the rate of change of the external load and the inherent thermal inertia of each stage of the internal heat exchanger, the system can proactively determine the pre-adjustment amount of the cooling medium flow for each stage of the heat exchanger. This pre-adjustment is feedforward in nature, aiming to coordinately adjust the cooling medium flow before the actual changes in flue gas parameters, thereby avoiding the lag and misalignment caused by passive response. The strategy of this method lies in its targeted handling of differences in thermal inertia: for any two stages of heat exchangers, the pre-adjustment of the cooling medium flow rate is set lower for the heat exchanger with lower thermal inertia compared to the one with higher thermal inertia. This means that the reduction in its cooling medium flow rate is limited, and in certain cases, the reduction in its cooling medium flow rate is even increased to maintain the flue gas temperature gradient of the entire system. This differentiated pre-adjustment allows the heat exchanger with lower thermal inertia to continue absorbing relatively more heat in the initial stage of rapid load reduction, suppressing the rise in its inlet flue gas temperature, thereby avoiding the occurrence of "temperature mismatch" and ensuring the normal operation and latent heat recovery function of the low-temperature section heat exchanger. After calculating the pre-adjustment amount, the system immediately executes this forward-looking adjustment, adjusting the cooling medium flow rate of each stage of heat exchanger. Simultaneously, to address the complexity of actual operating conditions and potential deviations in model predictions, this method further integrates the original closed-loop feedback control of each stage of heat exchanger. The closed-loop feedback control can correct based on real-time outlet parameters, compensating for the deficiencies of the pre-adjustment and ensuring that the final cooling medium flow rate adapts to the actual operating conditions. This combined control strategy of pre-adjustment and feedback correction enables thermal management of multi-stage heat exchangers under dynamic operating conditions. It has both the foresight to cope with rapid changes and the real-time correction capability to ensure control, thereby solving the "temperature mismatch" problem when the load drops rapidly, ensuring the safe operation of the system and improving efficiency.

[0048] Through the above solution, this invention solves the "temperature mismatch" phenomenon caused by the differences in thermal inertia and independent control of each stage of the multi-stage heat exchanger in the flue gas waste heat recovery system of a gas-fired industrial boiler under conditions of rapid load reduction. Specifically, by proactively acquiring the load command change rate and the thermal inertia parameters of each stage of the heat exchanger, and accordingly coordinating and pre-adjusting the cooling medium flow rate, especially by limiting or increasing the flow rate reduction for heat exchangers with low thermal inertia, the heat exchangers at each stage can respond more coordinatedly when flue gas parameters change, avoiding the rise of flue gas inlet temperature in the low-temperature section heat exchanger. This ensures the normal operation of the low-temperature section heat exchanger, maintains the recovery of latent heat of the flue gas, and thus improves the thermal efficiency of the waste heat recovery system. At the same time, it avoids additional thermal stress caused by undesigned temperature shocks, extends the service life of the equipment, reduces system operating risks, and improves the operational stability and reliability of the gas-fired industrial boiler.

[0049] In some embodiments, the step of determining the pre-adjustment amount of the cooling medium flow rate for each stage of the heat exchanger, based on the rate of change of the load command and the thermal inertia parameters of each stage of the heat exchanger, includes:

[0050] Based on the rate of change of the load command and the thermal inertia parameters of each heat exchanger, calculate the initial cooling medium flow pre-adjustment amount for each heat exchanger.

[0051] The influence of the initial cooling medium flow rate pre-adjustment on the flue gas temperature distribution of the multi-stage heat exchanger is determined based on the initial cooling medium flow rate pre-adjustment and the thermal connection relationship between the multi-stage heat exchangers.

[0052] Based on the determined impact results, the initial cooling medium flow rate pre-adjustment amount is adjusted to obtain the final cooling medium flow rate pre-adjustment amount for each stage of heat exchanger, thereby maintaining the flue gas temperature gradient between the multi-stage heat exchangers.

[0053] Specifically, the initial cooling medium flow rate pre-adjustment refers to the adjustment amount of the cooling medium flow rate initially calculated based on the rate of change of the load command and the thermal inertia parameters of each stage of the heat exchanger. This can be achieved using empirical formulas, lookup tables, or simplified model calculations. The thermal connection relationship between multi-stage heat exchangers refers to the mutual influence and coupling between series heat exchangers at each stage of the flue gas waste heat recovery system due to flue gas flow and heat transfer. This can be obtained by establishing a system thermodynamic model, conducting experimental tests, or analyzing historical operating data. The impact of the initial cooling medium flow rate pre-adjustment on the flue gas temperature distribution in the multi-stage heat exchanger system refers to the temperature change trend and distribution state of the flue gas in the entire multi-stage heat exchanger system after the cooling medium flow rate of each stage of the heat exchanger is adjusted according to the initial pre-adjustment. This can be determined using numerical simulation, real-time monitoring, or predictive algorithms. The flue gas temperature gradient refers to the gradual decrease in temperature of flue gas along the flow direction as it flows through multiple heat exchangers. It reflects the heat exchange effect of each heat exchanger and the rationality of the system's energy cascade utilization. It can be maintained by setting a reasonable range or temperature drop target for the outlet flue gas temperature of each heat exchanger.

[0054] This method implements thermal control of multi-stage heat exchangers for flue gas waste heat recovery in gas-fired industrial boilers under conditions of rapid load changes, particularly rapid load drops. First, based on the rate of change of the boiler load command and the thermal inertia parameters of each heat exchanger stage, the initial cooling medium flow rate pre-adjustment is calculated. This preliminary calculation is based on the independent thermal response characteristics of each heat exchanger stage, providing a foundation for subsequent fine-tuning. For example, the adjustment range of the cooling medium flow rate for heat exchangers with lower thermal inertia is limited to avoid excessively rapid responses that could lead to abnormal downstream flue gas temperatures. Building upon this, the method further considers the inherent thermal connections between the multi-stage heat exchangers. Specifically, based on the calculated initial cooling medium flow rate pre-adjustment and the thermal connections between the multi-stage heat exchangers, the system determines the potential impact of this initial adjustment on the overall flue gas temperature distribution within the multi-stage heat exchanger system. This step is crucial because it simulates how the flue gas temperature changes as it flows through each stage of the series heat exchanger under the influence of the initial adjustment, thus revealing potential "temperature mismatch" or temperature gradient imbalance problems. For example, if the initial adjustment results in an excessively high flue gas outlet temperature at a certain stage, it will affect the inlet flue gas temperature at the next stage. Finally, based on the determined impact, the initial cooling medium flow rate pre-adjustment is corrected and optimized to obtain the final cooling medium flow rate pre-adjustment for each stage of the heat exchanger. This correction is no longer isolated to a single heat exchanger, but rather from the perspective of the entire multi-stage system, aiming to maintain the flue gas temperature gradient between the heat exchangers. Through this iterative or feedback adjustment, it can be ensured that the final cooling medium flow rate pre-adjustment not only considers the characteristics of a single-stage heat exchanger, but more importantly, the interaction between stages. This allows for more precise control of the temperature drop curve of the flue gas in each stage of the heat exchanger during rapid load changes, avoiding local overheating or undercooling. This method, combined with the basic pre-adjustment scheme, elevates thermal management from simple estimation to optimization based on the overall thermodynamic behavior of the system. It can more effectively address the "temperature mismatch" problem caused by differences in thermal inertia between heat exchangers at different stages and solve the problem of local overheating or undercooling caused by inter-stage interactions, thereby improving the overall thermal efficiency and operational stability of the system.

[0055] In a specific embodiment, this method can be implemented as follows: First, a pre-adjustment calculation module can be set up in the control system of the gas-fired industrial boiler. This module can calculate the initial cooling medium flow pre-adjustment amount of each stage of the heat exchanger based on the real-time load command change rate, for example, obtained by differentiating the load command signal, and combined with pre-stored thermal inertia parameters of each stage of the heat exchanger, such as the response time constants of each stage of the heat exchanger obtained through offline testing or historical data fitting. For example, a simplified mathematical model can be used, taking the load change rate and the thermal inertia parameters of each stage as inputs, and quickly estimating the initial flow adjustment range through linear or nonlinear functional relationships. Second, in order to determine the impact of the initial cooling medium flow pre-adjustment amount on the flue gas temperature distribution of the multi-stage heat exchanger, a dynamic thermodynamic simulation model of the multi-stage heat exchanger can be constructed. This model can include the geometric structure, material properties, and physical properties of the flue gas and cooling medium of each stage of the heat exchanger. When the initial cooling medium flow rate pre-adjustment is input into the simulation model, the model can simulate the flow and heat transfer process of flue gas throughout the entire series heat exchanger system, thereby predicting the transient flue gas outlet temperature of each stage of the heat exchanger under conditions of rapid load reduction. By comparing the predicted flue gas temperature distribution with the ideal or target flue gas temperature gradient, the impact of the initial pre-adjustment can be quantified, for example, identifying which heat exchangers may experience flue gas temperature deviations from the target value. Finally, based on the impact results determined by the simulation model, an optimization algorithm can be used to adjust the initial cooling medium flow rate pre-adjustment. For example, an optimizer based on model predictive control (MPC) can be designed. This optimizer uses maintaining the flue gas temperature gradient between multiple stages of heat exchangers as the objective function and the initial cooling medium flow rate pre-adjustment as the optimization variable. Through iterative calculation, it finds a set of optimal cooling medium flow rate pre-adjustments. This optimization process can consider deviations in the flue gas temperature distribution and make incremental or decremental corrections to the initial pre-adjustment until the predicted flue gas temperature distribution meets the preset temperature gradient requirements. The final adjustment amount will be sent to the cooling medium flow actuator of each stage of the heat exchanger, thereby achieving precise thermal control of the multi-stage heat exchanger.

[0056] In summary, this method, by incorporating consideration of the thermal connections between multi-stage heat exchangers, can more accurately determine the pre-adjustment amount of the cooling medium flow rate. Specifically, based on the initial calculation of the initial adjustment amount, its impact on the flue gas temperature distribution throughout the multi-stage heat exchanger is further analyzed, and corrections are made based on the analysis results to obtain the final pre-adjustment amount of the cooling medium flow rate. This method can effectively maintain the flue gas temperature gradient between multi-stage heat exchangers, avoiding flue gas temperature mismatch caused by inter-stage interactions under conditions of rapid load reduction, thereby eliminating local overheating or undercooling problems. This helps ensure stable operation of each stage of the heat exchanger during dynamic transitions, improves the overall thermal efficiency of the flue gas waste heat recovery system, and guarantees the reliability of equipment operation.

[0057] In some embodiments, the step of calculating the initial cooling medium flow pre-adjustment amount for each stage of the heat exchanger based on the rate of change of the load command and the thermal inertia parameters of each stage of the heat exchanger includes:

[0058] The system cooling medium flow rate adjustment reference amount is determined based on the rate of change of the load command;

[0059] Based on the thermal inertia parameters of each heat exchanger, calculate the flow rate adjustment weight factor for each heat exchanger; the flow rate adjustment weight factor is proportional to the thermal inertia parameters.

[0060] The initial cooling medium flow rate pre-adjustment amount for each stage of heat exchanger is obtained by multiplying the system cooling medium flow rate adjustment benchmark amount with the flow rate adjustment weight factor of each stage of heat exchanger.

[0061] Specifically, when determining the baseline amount for adjusting the system cooling medium flow rate, this baseline amount is a system-wide reference value for cooling medium flow rate adjustment. Its purpose is to reflect the overall scale of cooling medium flow rate adjustment required by the entire flue gas waste heat recovery system under conditions of rapid load reduction. This baseline amount can be obtained from pre-established mapping relationships, lookup tables, or dynamic models that correlate the rate of change of load commands with the corresponding system-level cooling medium flow rate adjustment magnitude. For example, the faster the rate of change of load commands, the larger the corresponding baseline amount for system cooling medium flow rate adjustment may be.

[0062] Furthermore, when calculating the flow adjustment weighting factor for each stage of heat exchanger, this factor is a dimensionless proportionality coefficient used to indicate the relative share of each heat exchanger in the total system adjustment. The introduction of this factor aims to quantify the relative importance of each stage of heat exchanger in the dynamic response, particularly their thermal inertia characteristics. Since the flow adjustment weighting factor is proportional to the thermal inertia parameter, this means that heat exchangers with higher thermal inertia will be assigned a larger weight, while those with lower thermal inertia will be assigned a smaller weight. This proportional relationship helps manage the "temperature mismatch" phenomenon by avoiding premature or excessive flow adjustments to heat exchangers with faster response times. This weighting factor can be obtained by normalizing the thermal inertia parameters of each stage of heat exchanger to ensure that the sum of all weighting factors is one, thereby achieving a reasonable allocation of the total system adjustment.

[0063] Subsequently, by multiplying the determined system cooling medium flow rate adjustment baseline with the flow rate adjustment weighting factor of each heat exchanger, the initial cooling medium flow rate pre-adjustment of each heat exchanger can be obtained. This multiplication operation is a mathematical operation that organically combines the system-level adjustment requirements with the characteristics of each heat exchanger, ensuring that the initial pre-adjustment of each heat exchanger can respond to overall load changes while also reflecting its unique dynamic response requirements.

[0064] The present invention, when determining the pre-adjustment amount of the cooling medium flow rate for each stage of heat exchanger, first calculates the initial pre-adjustment amount of the cooling medium flow rate for each stage of heat exchanger, and then adjusts it according to its influence on the flue gas temperature distribution. To more effectively calculate the initial pre-adjustment amount of the cooling medium flow rate, the present invention introduces a systematic and proportionally coordinated method. Specifically, firstly, a system cooling medium flow rate adjustment benchmark is determined based on the rate of change of the load command of the gas-fired industrial boiler. This benchmark reflects the overall scale of the cooling medium flow rate adjustment required by the entire flue gas waste heat recovery system under conditions of rapid load decline, ensuring that subsequent adjustments match the overall operational needs of the gas-fired industrial boiler. Secondly, for each stage of heat exchanger, a corresponding flow rate adjustment weight factor is calculated based on its respective thermal inertia parameter. This weight factor is proportional to the thermal inertia parameter, meaning that heat exchangers with high thermal inertia are assigned a greater weight, while heat exchangers with low thermal inertia are assigned a smaller weight. This design quantifies the relative importance of each heat exchanger stage in the dynamic response, allowing heat exchangers with high thermal inertia to achieve a greater reduction in cooling medium flow rate during load decreases to accommodate their slow temperature drop characteristics. Simultaneously, heat exchangers with low thermal inertia have their cooling medium flow rate reduction limited to avoid receiving upstream overheated flue gas due to premature or excessive flow rate reduction. Finally, the determined system cooling medium flow rate adjustment baseline is multiplied by the respective flow rate adjustment weighting factor for each heat exchanger stage to obtain the initial pre-adjustment of the cooling medium flow rate for each stage.

[0065] Through this product operation, the overall system adjustment requirements are proportionally allocated to each stage of heat exchanger, fully combining the macroscopic system requirements with the individual thermal inertia differences of each heat exchanger. This method ensures that the calculated initial cooling medium flow rate pre-adjustment can respond to rapid load changes while also taking into account the differentiated requirements of each heat exchanger under dynamic operating conditions. Compared to simply considering the load command change rate and thermal inertia parameters, this invention provides a more detailed and forward-looking initial adjustment amount by introducing a system cooling medium flow rate adjustment benchmark and a flow rate adjustment weighting factor, and performing a product operation. This initial adjustment amount lays a solid foundation for subsequent detailed adjustments based on the thermal connection relationship between multiple heat exchangers, enabling the subsequent adjustment process to maintain the flue gas temperature gradient between multiple heat exchangers more efficiently and accurately. It is precisely because of this initial adjustment strategy that combines systemic and individual differences that this invention can fundamentally alleviate or even avoid the "temperature mismatch" problem caused by the thermal inertia differences of each heat exchanger, thereby effectively supporting the coordinated thermal management of multiple heat exchangers and improving the system's operational stability and thermal efficiency under dynamic operating conditions.

[0066] As a preferred implementation, in scenarios where the load of a gas-fired industrial boiler is rapidly decreasing, the load command signal of the boiler can be monitored first, and differential or filtered processing can be applied to obtain the real-time rate of change of the load command. For example, as the load command decreases from 100% to 50%, the rate of change can be calculated as the megawatts decrease per minute. Based on this rate of change, a pre-established database or a mathematical model can be consulted to determine the baseline amount for adjusting the system cooling medium flow rate. The database can store the baseline amounts corresponding to different load change rates, or the mathematical model can be a linear or nonlinear function that maps the load change rate to the required total system adjustment.

[0067] Furthermore, for each stage of heat exchanger in the flue gas waste heat recovery system, such as the high-temperature superheater, medium-pressure evaporator, and low-pressure economizer, its thermal inertia parameters can be pre-determined or obtained through simulation models. These thermal inertia parameters can be stored in a configuration table. Subsequently, a calculation module can calculate the flow adjustment weighting factor for each stage of the heat exchanger based on these thermal inertia parameters. This ensures that the flow adjustment weighting factor is proportional to the thermal inertia parameter, and that the sum of all weighting factors is 1, thereby achieving a reasonable allocation of the total system adjustment.

[0068] Ultimately, a computing unit in a central controller or distributed control system receives the determined baseline amount for adjusting the system cooling medium flow rate and the flow rate adjustment weighting factors for each stage of the heat exchanger. This computing unit multiplies the baseline amount by the weighting factor for each stage. By performing this multiplication operation on all heat exchangers, the initial pre-adjustment amount of the cooling medium flow rate for each stage of the heat exchanger under the current load change rate can be obtained. These initial adjustments can then be passed to subsequent optimization modules for further fine-tuning to ensure the maintenance of the flue gas temperature gradient.

[0069] This invention, by introducing a baseline amount for adjusting the system's cooling medium flow rate and a flow rate adjustment weighting factor, and performing a product operation, achieves a systematic and proportionally coordinated calculation of the initial cooling medium flow rate pre-adjustment for each stage of heat exchangers. This effectively combines the overall system adjustment requirements with the individual differences (especially thermal inertia) of each stage of heat exchangers, ensuring that the obtained initial adjustment amount can both fully respond to rapid load changes and take into account the relative cooling medium flow rate requirements of each stage of heat exchangers under dynamic operating conditions. This provides a reasonable starting point for subsequent detailed adjustments based on thermal connection relationships and is forward-looking, thereby simplifying the subsequent adjustment process and accurately maintaining the flue gas temperature gradient between multiple stages of heat exchangers. Ultimately, this invention alleviates the "temperature mismatch" problem caused by differences in thermal inertia among heat exchangers at each stage, helps prevent low-temperature heat exchangers from receiving undesigned over-temperature flue gas, thereby improving the operational stability and thermal efficiency of the flue gas waste heat recovery system under conditions of rapid load reduction, and protecting equipment from temperature shocks and thermal stress.

[0070] In some embodiments, the step of determining the impact of the initial cooling medium flow rate pre-adjustment on the flue gas temperature distribution of the multi-stage heat exchanger, based on the initial cooling medium flow rate pre-adjustment and the thermal connection relationship between the multi-stage heat exchangers, includes:

[0071] The dynamic thermo-coupling characteristics of a multi-stage heat exchanger under rapid load reduction conditions are obtained; the dynamic thermo-coupling characteristics characterize the transient heat transfer response relationship between each stage of the heat exchanger.

[0072] Obtain the nonlinear thermo-mechanical coupling characteristics of a multi-stage heat exchanger under different operating conditions; the nonlinear thermo-mechanical coupling characteristics characterize the heat transfer coefficients on the flue gas side and the cooling medium side, as well as the nonlinear relationship between fluid physical parameters and temperature and flow rate.

[0073] Based on the initial cooling medium flow pre-adjustment, dynamic thermo-coupling characteristics, and nonlinear thermo-coupling characteristics, the transient flue gas temperature distribution of a multi-stage heat exchanger during a rapid load decrease is predicted.

[0074] Based on the predicted transient flue gas temperature distribution, the influence of the initial cooling medium flow pre-adjustment on the flue gas temperature distribution of the multi-stage heat exchanger is determined.

[0075] Dynamic thermo-coupling characteristics refer to the transient heat transfer response between different stages of a multi-stage heat exchanger under conditions of rapid load reduction, caused by differences in thermal inertia. These characteristics can be obtained using dynamic models based on system identification, lumped parameter models, or distributed parameter models. Nonlinear thermo-coupling characteristics refer to the nonlinear relationships between the heat transfer coefficients on the flue gas side and the cooling medium side, as well as the fluid properties, with temperature and flow rate under different operating conditions. These characteristics can be obtained using experimental data fitting, CFD (Computational Fluid Dynamics) simulations, or mechanistic nonlinear models. Transient flue gas temperature distribution refers to the instantaneous temperature change trajectory of flue gas flowing through a multi-stage heat exchanger during a rapid load reduction process. These characteristics can be predicted using numerical simulation methods, finite element analysis, or the finite volume method.

[0076] The present invention aims to accurately predict the impact of the initial cooling medium flow rate pre-adjustment on the transient flue gas temperature distribution during a rapid load decrease by comprehensively considering the dynamic and nonlinear thermo-coupling characteristics of a multi-stage heat exchanger system. This provides an accurate basis for subsequent flow rate adjustments and effectively solves the "temperature mismatch" problem. The method first obtains the dynamic thermo-coupling characteristics of the multi-stage heat exchanger under rapid load decrease conditions. These characteristics capture the mutual influence and response time delay of each stage of the heat exchanger during the transient process, laying the foundation for subsequent transient temperature prediction. Simultaneously, the nonlinear thermo-coupling characteristics of the multi-stage heat exchanger under different operating conditions are obtained. These characteristics accurately reflect the complexity of heat exchange efficiency and heat transfer in actual operation, avoiding prediction deviations caused by simplified models or linear assumptions. Subsequently, based on the initial cooling medium flow rate pre-adjustment, the obtained dynamic thermo-coupling characteristics, and the nonlinear thermo-coupling characteristics, a comprehensive prediction model is constructed to predict the transient flue gas temperature distribution of the multi-stage heat exchanger during a rapid load decrease. This prediction process uses the initial cooling medium flow rate pre-adjustment as the control variable to be evaluated. Combined with the accurate capture of transient responses by dynamic thermo-coupling characteristics and the realistic reflection of complex property changes and heat transfer mechanisms by nonlinear thermo-coupling characteristics, it can simulate the instantaneous temperature change trajectory of flue gas flowing through a multi-stage heat exchanger, obtaining the transient flue gas temperature distribution of the entire system. Finally, based on the predicted transient flue gas temperature distribution, the specific impact of the initial cooling medium flow rate pre-adjustment on the flue gas temperature distribution of the entire multi-stage heat exchanger system can be clearly assessed, including identifying potential overheating or undercooling regions and whether the temperature gradient is effectively maintained. By introducing the acquisition and application of dynamic and nonlinear thermo-coupling characteristics, this invention elevates the evaluation of the impact of the initial cooling medium flow rate pre-adjustment from rough empirical judgments or simplified models to accurate predictions based on the actual physical behavior of the system. This precise predictive capability enables more accurate identification and correction of "temperature mismatch" when adjusting the initial cooling medium flow rate, thereby effectively maintaining the flue gas temperature gradient between multi-stage heat exchangers, significantly improving waste heat recovery efficiency, and ensuring equipment operation safety.

[0077] In a specific embodiment, to determine the impact of the initial cooling medium flow rate pre-adjustment on the flue gas temperature distribution of a multi-stage heat exchanger, the dynamic thermo-mechanical coupling characteristics of the multi-stage heat exchanger under rapid load reduction conditions can be obtained first through historical operating data analysis and system identification methods. For example, a dynamic model based on a transfer function or state-space model can be established, which can describe the response relationship between the outlet flue gas temperature and the cooling medium outlet temperature of each stage of the heat exchanger when the flue gas flow rate and temperature change transiently. Simultaneously, the nonlinear thermo-mechanical coupling characteristics of the multi-stage heat exchanger under different operating conditions can be obtained. This can include obtaining heat transfer coefficient curves of flue gas and cooling medium at different temperatures and flow rates through experimental testing or CFD simulation, as well as a database of nonlinear variations in fluid properties (e.g., density, specific heat capacity, viscosity, thermal conductivity, etc.) of water, steam, and flue gas with temperature and pressure. Subsequently, based on the calculated initial cooling medium flow rate pre-adjustment, and combined with the aforementioned dynamic thermodynamic coupling characteristic model and nonlinear thermodynamic coupling characteristic database, a transient thermodynamic simulation model can be constructed to predict the transient flue gas temperature distribution of a multi-stage heat exchanger during a rapid load decrease. For example, this simulation model can employ piecewise linearization or iterative solutions. Within each time step, based on the current flue gas inlet conditions and cooling medium flow rate, and considering the dynamic response and nonlinear heat transfer characteristics of each heat exchanger stage, the instantaneous flue gas outlet temperature of each heat exchanger stage can be calculated. Through continuous time-step calculations, the trajectory of the flue gas outlet temperature of each heat exchanger stage over time during the entire load decrease process can be obtained, thus forming the transient flue gas temperature distribution. Finally, based on the predicted transient flue gas temperature distribution, the specific impact of the initial cooling medium flow rate pre-adjustment on the flue gas temperature distribution of the multi-stage heat exchanger can be quantitatively analyzed. For example, the deviation of the flue gas outlet temperature of each heat exchanger stage during the entire load decrease process can be calculated, or it can be assessed whether the flue gas temperature exceeds or falls below the design range, and whether the temperature gradient between each heat exchanger stage remains within a reasonable range. These analytical results will directly guide further optimization and adjustment of the initial cooling medium flow rate pre-adjustment.

[0078] This invention acquires the dynamic thermodynamic coupling characteristics of a multi-stage heat exchanger under rapid load reduction conditions, as well as its nonlinear thermodynamic coupling characteristics under different operating conditions. Based on these characteristics, it predicts the transient flue gas temperature distribution of the multi-stage heat exchanger during rapid load reduction, accurately capturing the system's true thermodynamic behavior under dynamic and nonlinear conditions. This makes the assessment of the impact of the initial cooling medium flow rate pre-adjustment on the flue gas temperature distribution more accurate and comprehensive, avoiding prediction biases caused by simplified models or empirical judgments. Therefore, it can provide a reliable basis for subsequent cooling medium flow rate adjustments, effectively avoiding the occurrence of "temperature mismatch," thereby maintaining the flue gas temperature gradient between multi-stage heat exchangers and ensuring the operational stability and efficiency of the waste heat recovery system.

[0079] In some embodiments, the step of predicting the transient flue gas temperature distribution of a multi-stage heat exchanger during a rapid load decrease, based on the initial cooling medium flow pre-adjustment, dynamic thermo-coupling characteristics, and nonlinear thermo-coupling characteristics, includes:

[0080] Obtain the operating parameters of the multi-stage heat exchanger on the flue gas side and the cooling medium side during a rapid load decrease; the operating parameters include temperature, pressure, and flow rate;

[0081] Based on the operating parameters, determine whether the multi-stage heat exchanger is in or close to an off-design operating condition; off-design operating conditions include the critical point of fluid phase change or the critical point of heat exchange mechanism transformation.

[0082] When a multi-stage heat exchanger is in or close to an off-design operating condition, the correction amount of the nonlinear thermo-mechanical coupling characteristics under the off-design operating condition is determined based on the degree of deviation between the operating parameters and the off-design operating condition.

[0083] Based on the correction amount, the nonlinear thermo-coupling characteristics are adjusted to obtain the adjusted nonlinear thermo-coupling characteristics;

[0084] Based on the initial cooling medium flow pre-adjustment, dynamic thermo-coupling characteristics, and adjusted nonlinear thermo-coupling characteristics, the transient flue gas temperature distribution of a multi-stage heat exchanger during a rapid load decrease is predicted.

[0085] Off-design operating conditions refer to the critical points of fluid phase transitions at specific temperatures and pressures, such as the boiling point of water changing from liquid to gas or the condensation point of water vapor condensing from gas to liquid, or the dew point of water vapor condensing in flue gas. These can be determined using preset phase diagram data, empirical formulas, or real-time calculations. Alternatively, they refer to the critical points of heat transfer mechanism transitions where the dominant heat transfer mode changes significantly during heat exchange, such as the transition from single-phase convection heat transfer to boiling heat transfer, or from forced convection to natural convection dominance. These can be identified using empirical criteria based on dimensionless parameters such as Reynolds number and Nusselt number, or using fluid dynamics models. Deviation refers to the quantitative difference between current operating parameters (such as temperature, pressure, and flow rate) and the critical parameters of off-design operating conditions. This can be expressed as an absolute difference, a relative percentage, or a weighted value based on a specific function mapping. The correction amount refers to the adjustment value introduced to make the nonlinear thermo-coupling characteristic model more accurately reflect the actual physical behavior under non-design operating conditions. It can be determined by methods such as regression analysis based on historical data, real-time optimization algorithms, or preset correction curves. The adjusted nonlinear thermo-coupling characteristics refer to the set of characteristic parameters updated or calibrated based on the correction amount on the basis of the original nonlinear thermo-coupling characteristics. It can be obtained by directly modifying the model coefficients, updating the lookup table data, or through an iterative optimization process.

[0086] This invention introduces a dynamic correction mechanism for nonlinear thermo-coupling characteristics to ensure that the predictive model maintains high accuracy even under rapidly changing operating conditions during periods of rapid load reduction. This provides a more reliable basis for subsequent flow rate adjustments and effectively maintains the flue gas temperature gradient. Specifically, firstly, the system continuously acquires operating parameters of the multi-stage heat exchanger on both the flue gas and cooling medium sides during rapid load reduction. These parameters include temperature, pressure, and flow rate, providing a real-time data foundation for subsequent judgment and correction. Secondly, based on these real-time operating parameters, the system determines whether the multi-stage heat exchanger is in or approaching an off-design operating condition, particularly identifying fluid phase change critical points or heat transfer mechanism transition critical points. This judgment mechanism can provide early warning of special operating conditions where traditional models may fail. When the system identifies an off-design operating condition, it accurately calculates the correction amount for the nonlinear thermo-coupling characteristics under the current operating condition based on the deviation of the current operating parameters from the off-design operating condition. This quantification process ensures the targetedness and accuracy of the correction. Subsequently, the system dynamically adjusts the pre-calibrated nonlinear thermo-coupling characteristics based on the calculated correction amount, thereby obtaining adjusted characteristics that more accurately reflect the actual heat exchange behavior under the current extreme operating conditions. Finally, based on the initial cooling medium flow pre-adjustment amount, dynamic thermo-coupling characteristics, and dynamically adjusted nonlinear thermo-coupling characteristics, the system can predict the transient flue gas temperature distribution of the multi-stage heat exchanger during a rapid load decrease.

[0087] This dynamic correction mechanism, closely integrated with the overall thermal management method proposed in this invention, forms a more comprehensive solution. In the basic scheme, predicting the flue gas temperature distribution in multi-stage heat exchangers is a key step in determining the impact of the initial cooling medium flow rate pre-adjustment on the flue gas temperature distribution. Through the real-time correction of the nonlinear thermo-coupling characteristics by this invention, the accuracy of the prediction model is improved, especially under transient conditions where flue gas and cooling medium parameters change rapidly. This high-precision prediction result can more reliably guide the optimization of the initial cooling medium flow rate pre-adjustment, enabling the cooling medium flow rate adjustment of each stage of the heat exchanger to respond more accurately to load changes, thereby effectively avoiding the "temperature mismatch" phenomenon caused by prediction deviations. Therefore, this invention not only solves the problem of decreased accuracy of traditional prediction models under extreme conditions, but also further enhances the adaptability and effectiveness of the entire thermal management method in maintaining the flue gas temperature gradient in multi-stage heat exchangers, ensuring the safe and stable operation and waste heat recovery efficiency of gas-fired industrial boilers during rapid load reduction.

[0088] In practical implementation, to obtain the operating parameters of the multi-stage heat exchanger on both the flue gas and cooling medium sides during rapid load reduction, various sensors can be used for real-time data acquisition. For example, temperature data can be obtained by thermocouples or resistance temperature detectors (RTDs) arranged in the flue gas and cooling medium pipelines; pressure data can be obtained by pressure sensors installed on the flue gas and cooling medium pipelines; and flow rate data can be measured by orifice flow meters, vortex flow meters, or ultrasonic flow meters. The data collected by these sensors can be periodically read by the data acquisition system and transmitted to the central control unit or computing platform.

[0089] Based on the acquired operating parameters, a series of critical thresholds can be preset to determine whether a multi-stage heat exchanger is in or approaching an off-design operating condition. For example, for the cooling medium water, its saturation temperature and pressure curve can be preset as the critical point of fluid phase change; for the change in heat exchange mechanism, the critical range of Reynolds number or Nusselt number can be set based on experience or theoretical calculations. The control unit can compare the acquired operating parameters with these preset critical thresholds in real time. When any parameter approaches or reaches the critical threshold, the system can determine that it is in or approaching an off-design operating condition.

[0090] When a multi-stage heat exchanger is determined to be operating at or near its design operating condition, the correction amount for the nonlinear thermo-coupling characteristics under this condition can be determined by using a lookup table, interpolation, or a machine learning model, based on the deviation of the current operating parameters from the critical threshold. For example, a multidimensional lookup table can be established, with inputs being the deviations of temperature, pressure, flow rate from the critical point, and outputting correction factors for nonlinear thermo-coupling characteristics (such as heat transfer coefficients and physical property parameters). The greater the deviation, the larger the correction amount can be.

[0091] When adjusting nonlinear thermo-coupling characteristics based on a determined correction amount, the correction amount can be applied to the original nonlinear thermo-coupling characteristic model. For example, if the nonlinear thermo-coupling characteristic model is based on empirical formulas or curves, the correction amount can be used as a multiplication factor or additive term to directly modify the calculated values ​​of heat transfer coefficients or physical property parameters. If the model is based on a lookup table, the relevant data in the lookup table can be updated in real time or interpolated based on the correction amount.

[0092] Finally, based on the initial cooling medium flow rate pre-adjustment, dynamic thermo-coupling characteristics, and adjusted nonlinear thermo-coupling characteristics, the transient flue gas temperature distribution of the multi-stage heat exchanger during a rapid load decrease is predicted. This can be achieved using a numerical simulation program based on the finite volume method or finite difference method. This program iteratively calculates the flue gas outlet temperature of each stage of the heat exchanger during the rapid load decrease by stepping through time, thereby obtaining the flue gas temperature distribution of the entire transient process.

[0093] This invention acquires the operating parameters of a multi-stage heat exchanger in real time during a rapid load decrease and determines whether it is in or approaching a non-design operating condition, such as a fluid phase change critical point or a heat transfer mechanism transition critical point. This allows for the identification of special cases where traditional pre-calibration models may fail. When such conditions are identified, the invention dynamically determines and applies corrections to the nonlinear thermo-coupling characteristics based on the degree of deviation between the operating parameters and the non-design operating conditions, thereby adjusting the nonlinear thermo-coupling characteristics in real time. This dynamic correction mechanism ensures that the prediction model accurately reflects the actual physical behavior when facing extreme transient conditions with rapid changes in flue gas and cooling medium temperature and flow rate, improving the accuracy of transient flue gas temperature distribution prediction. Therefore, this invention effectively solves the problem of decreased prediction accuracy of pre-calibrated nonlinear thermo-coupling characteristics under extreme transient conditions, providing a more reliable basis for subsequent cooling medium flow rate adjustments. This effectively maintains the flue gas temperature gradient of the multi-stage heat exchanger during a rapid load decrease, avoids "temperature mismatch," ensures safe equipment operation, and improves waste heat recovery efficiency.

[0094] In some embodiments, the transient flue gas temperature distribution of a multi-stage heat exchanger during a rapid load decrease is predicted according to the following steps:

[0095] Obtain the geometric parameters and material thermophysical parameters of each stage of heat exchanger;

[0096] Based on geometric parameters, material thermophysical parameters, dynamic thermo-mechanical coupling characteristics, and nonlinear thermo-mechanical coupling characteristics (including nonlinear thermo-mechanical coupling characteristics before or after adjustment), the transient energy balance relationship between the flue gas side and the cooling medium side of each stage of heat exchanger is established.

[0097] The initial cooling medium flow rate pre-adjustment amount is used as the flow input on the cooling medium side, and combined with the flue gas inlet flow rate and temperature change trend during the rapid load decrease process, it is used as the inlet boundary condition on the flue gas side.

[0098] Using a time-stepping method, within each time step, the flue gas outlet temperature of each stage of heat exchanger is iteratively calculated based on the transient energy balance relationship, the flow input on the cooling medium side, and the inlet boundary conditions on the flue gas side. By continuously performing time-stepping calculations until the rapid load reduction process ends, the transient flue gas temperature distribution of the multi-stage heat exchangers during the rapid load reduction process is obtained.

[0099] Transient energy balance refers to the dynamic equilibrium maintained between the accumulation, input, output, and conversion of energy within a system at a given moment. It can be established using a system of differential equations or difference equations. For example, by performing energy conservation analysis on the control volume of the flue gas side and cooling medium side of a heat exchanger, considering sensible heat, latent heat, wall heat storage, and heat transfer, the system's heat changes over time can be described. Time-stepping simulation involves discretizing the continuous dynamic process into a series of small time intervals, performing calculations within each interval, and using the results of the previous interval as the initial conditions for the current interval, thus progressively advancing the simulation process. This can be implemented using explicit or implicit difference schemes, such as the Euler method, Runge-Kutta method, or Croncke-Nicholson method, to capture the system's dynamic response over time.

[0100] This invention provides a detailed and physically robust method for accurately predicting the transient flue gas temperature distribution in a multi-stage heat exchanger during a rapid load decrease. The prediction process begins by acquiring the geometric and thermophysical parameters of each stage of the heat exchanger. These parameters quantify the inherent properties of the heat exchanger and are crucial for accurately describing its heat storage and transfer characteristics, especially under dynamically changing operating conditions. They reflect the heat exchanger's response speed and capability to temperature changes, laying the foundation for establishing an accurate physical model. Based on this, and combining dynamic and nonlinear thermophysical coupling characteristics (including before and after adjustment), a transient energy balance relationship between the flue gas side and the cooling medium side of each stage of the heat exchanger is established. Establishing this transient energy balance relationship is the core of dynamic prediction; it captures the heat accumulation and transfer process of the system over time, rather than just steady-state equilibrium. By introducing geometric and thermophysical parameters, the energy balance model ensures that it accurately reflects the physical characteristics of the heat exchanger itself. Simultaneously, by incorporating dynamic thermophysical coupling characteristics, the mutual influence between each stage of the heat exchanger during the transient process can be considered, which is crucial for solving the coordination problem between heat exchangers with different thermal inertia. The introduction of nonlinear thermo-coupling characteristics (including before and after adjustment) ensures that the model can accurately handle the nonlinear relationship between the physical properties of flue gas and cooling medium and the changes in temperature and flow rate, especially under non-design operating conditions, which plays a decisive role in improving prediction accuracy.

[0101] Furthermore, the initial cooling medium flow rate pre-adjustment is used as the flow input on the cooling medium side, and the flue gas inlet flow rate and temperature change trends during the rapid load decrease process are used as the inlet boundary conditions on the flue gas side. Using the initial cooling medium flow rate pre-adjustment as input allows the prediction model to directly assess the impact of specific flow rate adjustment schemes on the system's transient temperature distribution. Simultaneously, combining the actual flue gas inlet flow rate and temperature change trends as boundary conditions ensures that the simulation process matches the real operating conditions of a gas-fired industrial boiler experiencing a rapid load decrease, providing an accurate external driver for prediction.

[0102] Finally, a time-stepping approach was adopted. Within each time step, the flue gas outlet temperature of each heat exchanger stage was iteratively calculated based on the transient energy balance relationship, the flow input on the cooling medium side, and the inlet boundary conditions on the flue gas side. This time-stepping calculation was continued until the rapid load reduction process ended, thus obtaining the transient flue gas temperature distribution of the multi-stage heat exchangers during the rapid load reduction process. The time-stepping approach is a standard method for solving transient problems. It allows the model to advance step by step at discrete time points, thereby capturing continuous dynamic changes. Iterative calculations within each time step ensured the convergence and accuracy of the calculations. By continuously calculating until the rapid load reduction process ended, a complete flue gas temperature distribution curve for the entire transition phase could be obtained. This enabled the system to comprehensively understand the temperature state of each heat exchanger stage at different times, thereby accurately identifying potential over-temperature or temperature mismatch risks and providing a solid data foundation for subsequent detailed control and optimization.

[0103] This detailed prediction method provides a solid foundation for determining the impact of the initial cooling medium flow rate pre-adjustment on the flue gas temperature distribution of the multi-stage heat exchangers in the aforementioned scheme. By accurately simulating the transient flue gas temperature distribution, this invention can accurately assess the impact of different cooling medium flow rate pre-adjustments on the temperature gradient of each stage of the heat exchanger. This allows subsequent flow rate adjustments to more accurately maintain the flue gas temperature gradient between the multi-stage heat exchangers, effectively avoiding the "temperature mismatch" problem caused by differences in thermal inertia, and thus improving the effectiveness and reliability of the entire thermal control strategy.

[0104] In a specific embodiment, the prediction of transient flue gas temperature distribution during a rapid load decrease in a multi-stage heat exchanger can be achieved as follows: First, the geometric parameters and material thermophysical parameters of each stage of the heat exchanger are obtained. This can be done by consulting the heat exchanger design drawings, equipment nameplates, or supplier datasheets. For example, the diameter, length, arrangement, heat exchange area of ​​the tube bundle, as well as the specific heat capacity, thermal conductivity, and density of the tube and shell materials can be obtained.

[0105] Next, based on these acquired geometric and material thermophysical parameters, and combined with dynamic and nonlinear thermophysical coupling characteristics, the transient energy balance relationship between the flue gas side and the cooling medium side of each stage of the heat exchanger is established. This can be established using a numerical model based on the finite volume method or the finite difference method, dividing each stage of the heat exchanger into multiple control volumes, and discretizing the energy conservation equations for the flue gas, cooling medium, and wall within each control volume. For example, for the flue gas side, the energy input and output brought about by the flue gas flow, and the convective heat transfer with the wall can be considered; for the cooling medium side, the energy input and output brought about by the cooling medium flow, and the convective heat transfer with the wall can be considered; for the wall, its heat storage and heat transfer with the fluids on both sides can be considered.

[0106] Subsequently, the initial cooling medium flow rate pre-adjustment is used as the flow input on the cooling medium side, and combined with the flue gas inlet flow rate and temperature change trends during the rapid load reduction process, it serves as the inlet boundary condition on the flue gas side. The initial cooling medium flow rate pre-adjustment can be provided by the upper-level control system or a preset strategy. For example, when the load decreases by 50%, the cooling medium flow rate pre-adjustment of a certain stage heat exchanger can be set to a certain percentage of the current flow rate. The flue gas inlet flow rate and temperature change trends can be obtained from the operating data of the gas-fired industrial boiler or historical load reduction curves. For example, during the load reduction process, the flue gas inlet flow rate can linearly decrease from a certain initial value to a certain target value, and the temperature can also decrease from a certain initial value to a certain target value.

[0107] Finally, a time-stepping method is adopted. Within each time step, the flue gas outlet temperature of each stage of the heat exchanger is iteratively calculated based on the established transient energy balance relationship, the flow input on the cooling medium side, and the inlet boundary conditions on the flue gas side. A suitable time step can be set, such as 0.1 seconds to 1 second, to balance calculation accuracy and efficiency. Within each time step, the discretized transient energy balance equations can be solved using the Newton-Raphson method or the Gauss-Seidel iterative method until the temperature of each control volume converges. By continuously performing time-stepping calculations until the rapid load decrease process ends, for example, for 300 seconds, the transient flue gas temperature distribution curves of the multi-stage heat exchangers during the rapid load decrease process are obtained.

[0108] This invention acquires the geometric and thermophysical parameters of each stage of the heat exchanger, and establishes a transient energy balance relationship based on these parameters, dynamic thermo-coupling characteristics, and nonlinear thermo-coupling characteristics. Combined with the initial cooling medium flow rate pre-adjustment and flue gas inlet boundary conditions, and employing time-stepping and iterative calculations, it can accurately predict the transient flue gas temperature distribution of a multi-stage heat exchanger during a rapid load decrease. This solves the problems of existing prediction methods being insufficiently detailed or lacking a physical basis, accurately capturing the dynamic changes in flue gas temperature and potential "temperature mismatch" phenomena during rapid load decreases. Therefore, it provides an accurate basis for subsequent cooling medium flow rate adjustments, effectively avoiding energy waste and equipment operation risks caused by inaccurate predictions, thereby improving the accuracy and reliability of thermal management of multi-stage heat exchangers.

[0109] Reference Appendix Figure 5 This invention provides a multi-stage heat exchanger thermal management system for flue gas waste heat recovery, applied to gas-fired industrial boilers, comprising:

[0110] The first acquisition module 100 is used to acquire the rate of change of the load command of the gas-fired industrial boiler.

[0111] The second acquisition module 200 is used to acquire the thermal inertia parameters of the series heat exchangers at each stage in the flue gas waste heat recovery system.

[0112] The determining module 300 is used to determine the pre-adjustment amount of the cooling medium flow rate of each heat exchanger based on the rate of change of the load command and the thermal inertia parameters of each heat exchanger. Specifically, for any two heat exchangers, the heat exchanger with lower thermal inertia has a lower pre-adjustment amount of the cooling medium flow rate than the heat exchanger with higher thermal inertia, so that the heat exchanger with lower thermal inertia is limited in its reduction of cooling medium flow rate, and the heat exchanger with higher thermal inertia is allowed to have its reduction of cooling medium flow rate increased.

[0113] The control module 400 is used to adjust the cooling medium flow rate of each stage of heat exchanger according to the pre-adjustment amount of the cooling medium flow rate, and to correct the cooling medium flow rate by combining the closed-loop feedback control of each stage of heat exchanger, so as to realize thermal management and control of multi-stage heat exchangers.

[0114] In this document, relational terms such as first and second are used only to distinguish one entity or operation from another entity or operation, without necessarily requiring or implying any such actual relationship or order between these entities or operations.

[0115] The above description is merely an embodiment of the present invention and is not intended to limit the scope of protection of the present invention. For those skilled in the art, the present invention can have various modifications and variations. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the scope of protection of the present invention.

Claims

1. A method for thermal control of a multi-stage heat exchanger for flue gas waste heat recovery, applied to a gas-fired industrial boiler, characterized in that, Includes the following steps: Obtain the rate of change of load commands for gas-fired industrial boilers; Obtain the thermal inertia parameters of each series heat exchanger in the flue gas waste heat recovery system; Based on the rate of change of the load command and the thermal inertia parameters of each heat exchanger, the pre-adjustment amount of the cooling medium flow rate for each heat exchanger is determined. Specifically, for any two heat exchangers, the heat exchanger with lower thermal inertia has a lower pre-adjustment amount of cooling medium flow rate compared to the heat exchanger with higher thermal inertia, so that the reduction of the cooling medium flow rate of the heat exchanger with lower thermal inertia is limited, while the reduction of the cooling medium flow rate of the heat exchanger with higher thermal inertia is increased. Based on the pre-adjustment amount of the cooling medium flow rate, the cooling medium flow rate of each stage of the heat exchanger is adjusted, and combined with the closed-loop feedback control of each stage of the heat exchanger, the cooling medium flow rate is corrected to achieve thermal management of the multi-stage heat exchanger.

2. The thermal control method for a multi-stage heat exchanger for flue gas waste heat recovery according to claim 1, characterized in that, The steps for determining the pre-adjustment amount of the cooling medium flow rate for each stage of the heat exchanger, based on the rate of change of the load command and the thermal inertia parameters of each stage of the heat exchanger, include: Based on the rate of change of the load command and the thermal inertia parameters of each heat exchanger, calculate the initial cooling medium flow pre-adjustment amount for each heat exchanger. The influence of the initial cooling medium flow rate pre-adjustment on the flue gas temperature distribution of the multi-stage heat exchanger is determined based on the initial cooling medium flow rate pre-adjustment and the thermal connection relationship between the multi-stage heat exchangers. Based on the determined impact results, the initial cooling medium flow rate pre-adjustment amount is adjusted to obtain the final cooling medium flow rate pre-adjustment amount for each stage of the heat exchanger.

3. The thermal control method for a multi-stage heat exchanger for flue gas waste heat recovery according to claim 2, characterized in that, The steps for calculating the initial cooling medium flow pre-adjustment for each stage of heat exchanger, based on the rate of change of the load command and the thermal inertia parameters of each stage of heat exchanger, include: The system cooling medium flow rate adjustment reference amount is determined based on the rate of change of the load command; Based on the thermal inertia parameters of each heat exchanger, calculate the flow rate adjustment weight factor for each heat exchanger; the flow rate adjustment weight factor is proportional to the thermal inertia parameters. The initial cooling medium flow rate pre-adjustment amount for each stage of heat exchanger is obtained by multiplying the system cooling medium flow rate adjustment benchmark amount with the flow rate adjustment weight factor of each stage of heat exchanger.

4. The thermal control method for a multi-stage heat exchanger for flue gas waste heat recovery according to claim 2, characterized in that, The steps for determining the impact of the initial cooling medium flow rate pre-adjustment on the flue gas temperature distribution of the multi-stage heat exchanger, based on the initial cooling medium flow rate pre-adjustment and the thermal connection relationship between the multi-stage heat exchangers, include: To obtain the dynamic thermo-mechanical coupling characteristics of a multi-stage heat exchanger under conditions of rapid load reduction; Obtain the nonlinear thermo-mechanical coupling characteristics of a multi-stage heat exchanger under different operating conditions; Based on the initial cooling medium flow pre-adjustment, dynamic thermo-coupling characteristics, and nonlinear thermo-coupling characteristics, the transient flue gas temperature distribution of a multi-stage heat exchanger during a rapid load decrease is predicted. Based on the predicted transient flue gas temperature distribution, the influence of the initial cooling medium flow pre-adjustment on the flue gas temperature distribution of the multi-stage heat exchanger is determined.

5. The thermal control method for a multi-stage heat exchanger for flue gas waste heat recovery according to claim 4, characterized in that, Based on the initial cooling medium flow pre-adjustment, dynamic thermo-coupling characteristics, and nonlinear thermo-coupling characteristics, the steps for predicting the transient flue gas temperature distribution of a multi-stage heat exchanger during a rapid load decrease include: Obtain the operating parameters of the multi-stage heat exchanger on the flue gas side and the cooling medium side during the rapid load reduction process; Based on the operating parameters, determine whether the multi-stage heat exchanger is in or close to non-design operating conditions; When a multi-stage heat exchanger is in or close to an off-design operating condition, the correction amount of the nonlinear thermo-mechanical coupling characteristics under the off-design operating condition is determined based on the degree of deviation between the operating parameters and the off-design operating condition. Based on the correction amount, the nonlinear thermo-coupling characteristics are adjusted to obtain the adjusted nonlinear thermo-coupling characteristics; Based on the initial cooling medium flow pre-adjustment, dynamic thermo-coupling characteristics, and adjusted nonlinear thermo-coupling characteristics, the transient flue gas temperature distribution of a multi-stage heat exchanger during a rapid load decrease is predicted.

6. The thermal control method for a multi-stage heat exchanger for flue gas waste heat recovery according to claim 5, characterized in that, Operating parameters include temperature, pressure, and flow rate.

7. The thermal control method for a multi-stage heat exchanger for flue gas waste heat recovery according to claim 5, characterized in that, Non-design operating conditions include the critical point of fluid phase change or the critical point of heat transfer mechanism transformation.

8. The thermal control method for a multi-stage heat exchanger for flue gas waste heat recovery according to claim 4 or 5, characterized in that, Predict the transient flue gas temperature distribution of a multi-stage heat exchanger during a rapid load decrease using the following steps: Obtain the geometric parameters and material thermophysical parameters of each stage of heat exchanger; Based on geometric parameters, material thermophysical parameters, dynamic thermo-mechanical coupling characteristics, and nonlinear thermo-mechanical coupling characteristics, the transient energy balance relationship between the flue gas side and the cooling medium side of each stage of heat exchanger is established. The initial cooling medium flow rate pre-adjustment amount is used as the flow input on the cooling medium side, and combined with the flue gas inlet flow rate and temperature change trend during the rapid load decrease process, it is used as the inlet boundary condition on the flue gas side. Based on the transient energy balance relationship, the flow input on the cooling medium side, and the inlet boundary conditions on the flue gas side, the transient flue gas temperature distribution of the multi-stage heat exchanger during the rapid load reduction process is obtained.

9. The thermal control method for a multi-stage heat exchanger for flue gas waste heat recovery according to claim 8, characterized in that, Based on the transient energy balance relationship, the flow input on the cooling medium side, and the inlet boundary conditions on the flue gas side, the steps to obtain the transient flue gas temperature distribution of a multi-stage heat exchanger during a rapid load decrease include: Using a time-stepping method, within each time step, the flue gas outlet temperature of each stage of heat exchanger is iteratively calculated based on the transient energy balance relationship, the flow input on the cooling medium side, and the inlet boundary conditions on the flue gas side. By continuously performing time-stepping calculations until the rapid load reduction process ends, the transient flue gas temperature distribution of the multi-stage heat exchangers during the rapid load reduction process is obtained.

10. A multi-stage heat exchanger thermal control system for flue gas waste heat recovery, applied to a gas-fired industrial boiler, characterized in that, include: The first acquisition module is used to acquire the rate of change of the load command of the gas-fired industrial boiler. The second acquisition module is used to acquire the thermal inertia parameters of the series heat exchangers at each stage in the flue gas waste heat recovery system; The determination module is used to determine the pre-adjustment amount of the cooling medium flow rate of each heat exchanger based on the rate of change of the load command and the thermal inertia parameters of each heat exchanger. Specifically, for any two heat exchangers, the heat exchanger with lower thermal inertia has a lower pre-adjustment amount of the cooling medium flow rate than the heat exchanger with higher thermal inertia, so that the heat exchanger with lower thermal inertia is limited in the reduction of its cooling medium flow rate, and the heat exchanger with higher thermal inertia is allowed to reduce its cooling medium flow rate by a larger margin. The control module is used to adjust the cooling medium flow rate of each stage of the heat exchanger according to the pre-adjustment amount of the cooling medium flow rate, and to correct the cooling medium flow rate in combination with the closed-loop feedback control of each stage of the heat exchanger, so as to realize thermal management and control of multi-stage heat exchangers.

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