Method and system for preventing corrosion of heat exchange tube of flue gas cooler

Abstract

The invention relates to the technical field of unit operation safety, in particular to a method and system for preventing corrosion of a heat exchange tube of a flue gas cooler, and the method comprises the following steps: carrying out interlocking protection inspection on a flue gas cooler recirculation system, so that the interlocking protection operation of the flue gas cooler recirculation system is normal; the flue gas cooler recirculation system is checked before being put into operation, so that the flue gas cooler recirculation system is put into operation normally; the flue gas cooler recirculation system is subjected to operation maintenance, and normal operation of the flue gas cooler recirculation system is monitored and adjusted; by stopping the system, protective forced stopping is carried out on the recirculation system of the flue gas cooler, and operation is quitted; and accident treatment is carried out on the flue gas cooler recirculation system. The method has the beneficial effects that frequent starting and stopping caused by small load fluctuation are reduced through the set dead zone range, and the operation efficiency of the system is improved. Low-temperature corrosion is avoided by maintaining the water temperature of the inlet of the flue gas cooler within a safe range.

Description

Technical Field

[0001] The invention relates to the technical field of unit operation safety, and in particular to a method and a system for preventing corrosion of a heat exchange tube of a flue gas cooler. Background Art

[0002] In modern power systems, flue gas coolers are an important heat exchange equipment. Their main function is to use cooling water to absorb heat from flue gas, improve the thermal efficiency of the boiler by lowering the flue gas temperature, and reduce thermal pollution to the environment. In order to ensure the effective operation of the flue gas cooler, the inlet water temperature needs to be maintained at 72°C to ensure heat exchange efficiency and equipment safety. However, in actual operation, when the unit load is lower than 500MW, the heat load of the flue gas cooler is reduced due to the reduction in the amount of flue gas generated by the boiler. In this case, the temperature of the mixed water is often difficult to reach the required 72°C, which cannot meet the minimum operating temperature requirements of the flue gas cooler. If the inlet water temperature of the flue gas cooler is lower than 72°C for a long time, it will cause corrosion risk to the heat exchange tubes. This low-temperature corrosion will gradually weaken the material of the heat exchange tubes, increase the maintenance cost and downtime risk of the equipment. Once the heat exchange tubes of the flue gas cooler corrode, it will not only reduce the heat exchange efficiency of the equipment, but also may cause leakage of the heat exchange tubes. Leaking heat exchange tubes will further pollute the flue gas and affect the subsequent operation of environmental protection facilities, such as electrostatic precipitators and desulfurization systems. In addition, leaking heat exchange tubes may also cause ash blockage inside the flue gas cooler, which will further reduce the heat exchange efficiency, increase the resistance to flue gas flow, and may even cause complete failure of the flue gas cooler. Summary of the invention

[0003] In view of the above problems or problems existing in the prior art, the present invention is proposed.

[0004] Therefore, the purpose of the present invention is to provide a method for preventing corrosion of flue gas cooler heat exchange tubes, which can solve the problem that when the unit load is lower than 500MW, the temperature of the mixed water is lower than 72°C for a long time, causing the heat exchange tubes to be corroded, resulting in reduced efficiency of the heat exchange tubes, or leakage of the heat exchange tubes.

[0005] In order to solve the above technical problems, the present invention provides the following technical solutions: a method for preventing corrosion of a flue gas cooler heat exchange tube, comprising: performing an interlock protection inspection on a flue gas cooler recirculation system to ensure that the interlock protection of the flue gas cooler recirculation system operates normally;

[0006] By conducting a pre-commissioning inspection on the flue gas cooler recirculation system, the flue gas cooler recirculation system can be put into operation normally;

[0007] Monitor and adjust the normal operation of the flue gas cooler recirculation system by operating and maintaining the system;

[0008] By shutting down the system, the flue gas cooler recirculation system is forced to shut down for protection and take it out of operation;

[0009] Carry out accident handling on the flue gas cooler recirculation system.

[0010] As a preferred embodiment of the method for preventing corrosion of the heat exchange tubes of the flue gas cooler of the present invention, the flue gas cooler recirculation system is subjected to an interlock protection check, including the start-up permission conditions of the flue gas cooler recirculation pump, the interlock start conditions of the main pump and the standby pump, the interlock shutdown conditions, the protection stop interlock conditions, the interlock opening and closing logic of the outlet electric door, and the interlock logic of the No. 7 low-pressure inlet to the flue gas cooler regulating valve.

[0011] As a preferred embodiment of the method for preventing corrosion of the heat exchange tubes of the flue gas cooler of the present invention, the starting permission conditions of the flue gas cooler recirculation pump include: when the electric door from the #7 low-pressure heater outlet to the flue gas cooler is fully open, and the electric door from the flue gas cooler to the #6 low-pressure heater inlet is fully open, if the unit load is lower than 570MW, the flue gas cooler recirculation pump is started.

[0012] As a preferred solution of the method for preventing corrosion of flue gas cooler heat exchange tubes of the present invention, the main pump interlock start-up conditions of the flue gas cooler recirculation system include: when the system interlock is put into "IN" and the system selects a pump as the main pump and puts it into "AUTO", if the unit load is less than 500MW and the dead zone is 2MW, the main pump interlock start-up of the flue gas cooler recirculation system.

[0013] As a preferred solution of the method for preventing corrosion of flue gas cooler heat exchange tubes of the present invention, the interlocking start-up conditions of the flue gas cooler recirculation system standby pump include: when the unit load is less than 570MW and the dead zone is 5MW, the standby pump is started; if the system main pump trips and delays for 80s, the interlocking start-up of the flue gas cooler recirculation system standby pump is started.

[0014] As a preferred solution of the method for preventing corrosion of flue gas cooler heat exchange tubes of the present invention, the interlocking shutdown conditions of the flue gas cooler recirculation system include: when the system interlock is put into operation, if the unit load is greater than 570MW and the dead zone is 5MW, the flue gas cooler recirculation system is interlocked and shut down.

[0015] As a preferred solution of the method for preventing corrosion of heat exchange tubes of flue gas coolers of the present invention, the protection and stop interlocking conditions of the recirculation pump of the flue gas cooler include:

[0016] When the recirculation pump runs for 90 seconds, the recirculation pump outlet electric door open feedback is lost and the closed feedback is received, the smoke cooler recirculation pump protection stop interlock is delayed for 30 seconds;

[0017] When the recirculation pump inverter operation signal is lost and no stop command is issued, there is a delay of 5s and the smoke cooler recirculation pump protection stop interlock.

[0018] In order to solve the above technical problems, the present invention also provides the following technical solutions: a system for preventing corrosion of heat exchange tubes of a flue gas cooler, comprising a cooling module for cooling the flue gas in the heat exchange tubes; and

[0019] The circulation module is used to increase the water inlet temperature of the cooling module.

[0020] As a preferred solution of the system for preventing corrosion of heat exchange tubes of flue gas coolers of the present invention, the cooling module includes a flue gas cooler, which absorbs flue gas heat through cooling water to reduce flue gas temperature and improve boiler thermal efficiency.

[0021] As a preferred solution of the system for preventing corrosion of flue gas cooler heat exchange tubes of the present invention, the circulation module includes two recirculation pumps, the recirculation pumps include a running pump and a standby pump, the recirculation pumps adopt frequency conversion control, and the inlet water temperature of the flue gas cooler is increased by recirculating hot water.

[0022] Beneficial effects of the present invention: The present invention realizes effective management of the flue gas cooler recirculation system through interlocking protection inspection and operation monitoring. By setting specific start-up permission conditions, interlocking start-up conditions of the main pump and the standby pump, interlocking shutdown conditions and protection shutdown interlocking conditions, the safety and stability of the flue gas cooler recirculation system under various operating conditions are ensured. The set dead zone range reduces frequent starts and shutdowns caused by small load fluctuations, improves the operating efficiency of the system, and reduces energy consumption. By maintaining the inlet water temperature of the flue gas cooler within a safe range, low-temperature corrosion is avoided, thereby extending the service life of the heat exchange tube. BRIEF DESCRIPTION OF THE DRAWINGS

[0023] In order to more clearly illustrate the technical solutions of the embodiments of the present invention, the following briefly introduces the drawings required for describing the embodiments. Obviously, the drawings described below are only some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without creative work. Among them:

[0024] Figure 1 Schematic diagram of the overall system structure of the method for preventing corrosion of flue gas cooler heat exchange tubes.

[0025] Figure 2 Schematic diagram of the change of inlet water temperature of the unit under low load condition after the modification of the system to prevent corrosion of the flue gas cooler heat exchange tubes. DETAILED DESCRIPTION

[0026] In order to make the above-mentioned objects, features and advantages of the present invention more obvious and easy to understand, the specific implementation methods of the present invention are described in detail below in conjunction with the accompanying drawings.

[0027] In the following description, many specific details are set forth to facilitate a full understanding of the present invention, but the present invention may also be implemented in other ways different from those described herein, and those skilled in the art may make similar generalizations without violating the connotation of the present invention. Therefore, the present invention is not limited to the specific embodiments disclosed below.

[0028] Secondly, the term "one embodiment" or "embodiment" as used herein refers to a specific feature, structure, or characteristic that may be included in at least one implementation of the present invention. The term "in one embodiment" that appears in different places in this specification does not necessarily refer to the same embodiment, nor does it refer to a separate or selective embodiment that is mutually exclusive with other embodiments.

[0029] Example 1

[0030] Reference Figure 1-2 , which is the first embodiment of the present invention, provides a method for preventing corrosion of the flue gas cooler heat exchange tube, which can solve the problem that when the unit load is lower than 500MW, the temperature of the mixed water is lower than 72°C for a long time, causing the heat exchange tube to be corroded, resulting in reduced efficiency of the heat exchange tube, or leakage of the heat exchange tube.

[0031] Specifically, by performing an interlock protection check on the flue gas cooler recirculation system, the interlock protection of the flue gas cooler recirculation system is ensured to operate normally;

[0032] By conducting a pre-commissioning inspection on the flue gas cooler recirculation system, the flue gas cooler recirculation system can be put into operation normally;

[0033] Monitor and adjust the normal operation of the flue gas cooler recirculation system by operating and maintaining the system;

[0034] By shutting down the system, the flue gas cooler recirculation system is forced to shut down for protection and take it out of operation;

[0035] Carry out accident handling on the flue gas cooler recirculation system.

[0036] It should be noted that after the recirculation pump was put into use to heat the flue gas cooler inlet water temperature, the flue gas cooler inlet water temperature was improved accordingly, avoiding the situation where the mixed water temperature was difficult to reach the required 72°C when the unit load was lower than 500MW. The comparison of the on-site measurement values ​​before and after the improvement is shown in the following table:

[0037] time Before smoke cooler modification After smoke cooler modification load 495MW 493MW Condensate flow 1317t / h 1318t / h Smoke temperature protection flow 168t / h 177t / h Smoke cooler inlet water temperature 70.8℃ 73.1℃

[0038] Data comparison before and after smoke cooler transformation

[0039] It can be seen from the table that under the same load and condensate flow rate, the inlet water temperature of the smoke cooler before the modification is significantly lower than the inlet water temperature of the smoke cooler after the modification. Figure 2 It can be seen that when the load is lower than 500MW, the recirculation pump of the smoke cooler starts automatically, and the inlet water temperature of the smoke cooler rises rapidly after 1 minute. During the continuous low-load operation, the inlet cooling water of the smoke cooler can be maintained at more than 72℃ and within a higher temperature range.

[0040] In summary, the present invention realizes effective management of the flue gas cooler recirculation system through interlock protection inspection and operation monitoring, and ensures the safety and stability of the flue gas cooler recirculation system under various operating conditions by setting specific start-up permission conditions, interlock start conditions of the main pump and the standby pump, interlock shutdown conditions, and protection stop interlock conditions. The set dead zone range reduces frequent starts and shutdowns caused by small load fluctuations, improves the operating efficiency of the system, and reduces energy consumption. By maintaining the inlet water temperature of the flue gas cooler within a safe range, low-temperature corrosion is avoided, thereby extending the service life of the heat exchange tube.

[0041] Example 2

[0042] Reference Figure 1 , which is the second embodiment of the present invention, provides a method for preventing corrosion of the flue gas cooler heat exchange tube, which can solve the problem that when the unit load is lower than 500MW, the temperature of the mixed water is lower than 72°C for a long time, causing the heat exchange tube to be corroded, resulting in reduced efficiency of the heat exchange tube, or leakage of the heat exchange tube.

[0043] Specifically, by performing an interlock protection check on the flue gas cooler recirculation system, the interlock protection of the flue gas cooler recirculation system is ensured to operate normally;

[0044] By conducting a pre-commissioning inspection on the flue gas cooler recirculation system, the flue gas cooler recirculation system can be put into operation normally;

[0045] Monitor and adjust the normal operation of the flue gas cooler recirculation system by operating and maintaining the system;

[0046] By shutting down the system, the flue gas cooler recirculation system is forced to shut down for protection and take it out of operation;

[0047] Carry out accident handling on the flue gas cooler recirculation system.

[0048] Furthermore, the interlock protection check of the flue gas cooler recirculation system includes the start-up permission conditions of the flue gas cooler recirculation pump, the interlock start conditions of the main pump and the standby pump, the interlock shutdown conditions, the protection stop interlock conditions, the interlock opening and closing logic of the outlet electric door, and the interlock logic of the No. 7 low-pressure inlet to the flue gas cooler regulating valve.

[0049] Furthermore, the start-up permission conditions of the flue gas cooler recirculation pump include: when the electric door from the #7 low-pressure heater outlet to the flue gas cooler is fully open, and the electric door from the flue gas cooler to the #6 low-pressure heater inlet is fully open, if the unit load is lower than 570MW, the flue gas cooler recirculation pump is started.

[0050] Furthermore, the smoke cooler recirculation system main pump interlock start-up conditions include: when the system interlock is put into "IN", and the system selects a pump as the main pump and puts it into "AUTO", if the unit load is less than 500MW and the dead zone is 2MW, the smoke cooler recirculation system main pump interlock is started.

[0051] Furthermore, the interlocking start-up conditions of the smoke cooler recirculation system standby pump include: when the unit load is less than 570MW and the dead zone is 5MW, the standby pump starts; if the system main pump trips and delays for 80s, the smoke cooler recirculation system standby pump interlocking start-up.

[0052] Furthermore, the interlocking shutdown conditions of the smoke cooler recirculation system include that when the system interlock is put into operation, if the unit load is greater than 570MW and the dead zone is 5MW, the smoke cooler recirculation system interlocking shutdown.

[0053] It should be noted that the overall operation logic of the flue gas cooler circulating pump is: when the load of unit #2 is less than 500MW, the standby pump will be automatically started;

[0054] When the load of unit #2 is greater than 570MW, the running pump will be automatically shut down.

[0055] Furthermore, the smoke cooler recirculation pump protection stop interlock conditions include:

[0056] When the recirculation pump runs for 90 seconds, the recirculation pump outlet electric door open feedback is lost and the closed feedback is received, the smoke cooler recirculation pump protection stop interlock is delayed for 30 seconds;

[0057] When the recirculation pump inverter operation signal is lost and no stop command is issued, there is a delay of 5s and the smoke cooler recirculation pump protection stop interlock.

[0058] It should be noted that the automatic adjustment logic of the smoke cooler recirculation pump is to obtain temperature data through on-site measuring points, and then automatically take the average and center value through the DCS system module, that is, the inlet water temperatures of #1, #2, and #3 smoke coolers are averaged, and the inlet water temperatures of #4, #5, and #6 smoke coolers are averaged at the same time. The two sets of averaged values ​​are the output values, and the output values ​​are averaged.

[0059] Specifically, the DCS system module automatically takes the middle and average values ​​including:

[0060] The DCS system collects real-time temperature data from the on-site #1, #2, #3 and #4, #5, #6 smoke cooler inlet water temperature measurement points;

[0061] Sort the collected water temperature data at the inlet of each smoke cooler to identify the highest and lowest values;

[0062] For the three temperature data #1, #2, and #3, remove the highest value and the lowest value to reduce the impact of measurement errors or outliers on the final control parameters, and use the middle value as the representative value of this group;

[0063] Repeat the above-mentioned centering operation for the data measured at the #4, #5, and #6 smoke cooler inlet water temperature measuring points;

[0064] The middle values ​​obtained after taking the two groups are averaged to obtain the final output value;

[0065] The DCS system continuously monitors changes in temperature data and continuously adjusts the centering and averaging calculations based on real-time data to achieve precise control of the smoke cooler recirculation pump.

[0066] In summary, the present invention realizes effective management of the flue gas cooler recirculation system through interlock protection inspection and operation monitoring, and ensures the safety and stability of the flue gas cooler recirculation system under various operating conditions by setting specific start-up permission conditions, interlock start conditions of the main pump and the standby pump, interlock shutdown conditions, and protection stop interlock conditions. The set dead zone range reduces frequent starts and shutdowns caused by small load fluctuations, improves the operating efficiency of the system, and reduces energy consumption. By maintaining the inlet water temperature of the flue gas cooler within a safe range, low-temperature corrosion is avoided, thereby extending the service life of the heat exchange tube.

[0067] Example 3

[0068] Reference Figure 2 , which is the third embodiment of the present invention, provides a system for preventing corrosion of flue gas cooler heat exchange tubes, which can solve the problem that when the unit load is lower than 500MW, the temperature of the mixed water is lower than 72°C for a long time, causing the heat exchange tubes to be corroded, resulting in reduced efficiency of the heat exchange tubes, or leakage of the heat exchange tubes.

[0069] Specifically, it includes a cooling module for cooling the flue gas in the heat exchange tube; and

[0070] The circulation module is used to increase the water inlet temperature of the cooling module.

[0071] Furthermore, the cooling module includes a flue gas cooler, which absorbs flue gas heat through cooling water to reduce the flue gas temperature and improve the thermal efficiency of the boiler.

[0072] Furthermore, the circulation module includes two recirculation pumps, the recirculation pumps include a running pump and a standby pump, the recirculation pumps are controlled by frequency conversion, and the inlet water temperature of the flue gas cooler is increased by recirculating hot water.

[0073] Specifically, Figure 2 As shown, the green curve is the condensate temperature at the flue gas cooler inlet, the red curve is the unit load, the white straight line is the frequency converter of the flue gas cooler recirculation pump, and the yellow straight line box is the current of the flue gas cooler recirculation pump.

[0074] After adding the recirculation pump, when the unit was put into operation, the condensate temperature at the flue gas cooler inlet ranged from 60 to 80°C, with the lowest being 70.97°C and the highest being 75.82°C. In addition, excluding the start and end times, the condensate temperature at the flue gas cooler inlet was always higher than 72°C during operation. The unit load range was 400 to 1000 MW, with the lowest value being 452 MW during operation. The frequency converter of the flue gas cooler recirculation pump was 0 to 20 Hz. Figure 2 The current of the flue gas cooler recirculation pump is 0~33A. Figure 2 Shown in Figure 1 is constant at 28A.

[0075] In summary, the present invention maintains the inlet water temperature of the flue gas cooler within a safe range, thereby avoiding low-temperature corrosion, resulting in reduced efficiency of the heat exchange tube, or leakage of the heat exchange tube, thereby extending the service life of the heat exchange tube. At the same time, by setting the operating pump and the standby pump, the stability and reliability of the entire flue gas cooling system are improved, the downtime caused by equipment failure is reduced, and the continuity and economy of power production are improved.

[0076] Importantly, it should be noted that the construction and arrangement of the present application shown in a plurality of different exemplary embodiments are only exemplary. Although only a few embodiments are described in detail in this disclosure, it should be readily understood by those who refer to this disclosure that many modifications are possible without substantially departing from the novel teachings and advantages of the subject matter described in the application (e.g., mounting arrangements, use of materials, color, changes in orientation, etc.). For example, the element shown as integrally formed may be composed of a plurality of parts or elements, the position of the element may be inverted or otherwise changed, and the nature or number or position of the discrete element may be altered or changed. Therefore, all such modifications are intended to be included within the scope of the present invention. The order or sequence of any process or method steps may be changed or reordered according to an alternative embodiment. In the claims, any "bracket plus function" clause is intended to cover the structure of the execution function described herein, and is not only structurally equivalent but also equivalent structure. Without departing from the scope of the present invention, other replacements, modifications, changes and omissions may be made in the design, operating conditions and arrangement of the exemplary embodiments. Therefore, the present invention is not limited to a specific embodiment, but extends to a variety of modifications that still fall within the scope of the appended claims.

[0077] Additionally, in an effort to provide a concise description of example embodiments, all features of an actual implementation may not be described.

[0078] It should be understood that in the development of any actual implementation, as in any engineering or design project, numerous implementation-specific decisions may be made. Such a development effort may be complex and time-consuming, but for those of ordinary skill having the benefit of this disclosure, the development effort will be a routine task of design, fabrication, and production without undue experimentation.

[0079] It should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention rather than to limit it. Although the present invention has been described in detail with reference to the preferred embodiments, those skilled in the art should understand that the technical solutions of the present invention may be modified or replaced by equivalents without departing from the spirit and scope of the technical solutions of the present invention, which should all be included in the scope of the claims of the present invention.

Claims

1. A method for preventing corrosion of heat exchange tubes of a flue gas cooler, characterized in that: By checking the interlock protection of the flue gas cooler recirculation system, the interlock protection of the flue gas cooler recirculation system is operating normally; By conducting a pre-commissioning inspection on the flue gas cooler recirculation system, the flue gas cooler recirculation system can be put into operation normally; Monitor and adjust the normal operation of the flue gas cooler recirculation system by operating and maintaining the system; By shutting down the system, the flue gas cooler recirculation system is forced to shut down for protection and take it out of operation; Carry out accident handling on the flue gas cooler recirculation system.

2. The method for preventing corrosion of flue gas cooler heat exchange tubes according to claim 1, characterized in that: The interlock protection check of the flue gas cooler recirculation system includes the start-up permission conditions of the flue gas cooler recirculation pump, the interlock start conditions of the main pump and the standby pump, the interlock shutdown conditions, the protection stop interlock conditions, the interlock opening and closing logic of the outlet electric door, and the interlock logic of the No. 7 low-pressure inlet to the flue gas cooler regulating valve.

3. The method for preventing corrosion of flue gas cooler heat exchange tubes according to claim 2, characterized in that: The start-up permission conditions of the flue gas cooler recirculation pump include: when the electric door from the #7 low-pressure heater outlet to the flue gas cooler is fully open, and the electric door from the flue gas cooler to the #6 low-pressure heater inlet is fully open, if the unit load is lower than 570MW, the flue gas cooler recirculation pump will start.

4. The method for preventing corrosion of flue gas cooler heat exchange tubes according to claim 2, characterized in that: The main pump interlock start-up conditions of the smoke cooler recirculation system include: when the system interlock is put into "IN", and the system selects a pump as the main pump and puts it into "AUTO", if the unit load is less than 500MW and the dead zone is 2MW, the main pump interlock of the smoke cooler recirculation system will be started.

5. The method for preventing corrosion of flue gas cooler heat exchange tubes according to claim 2, characterized in that: The interlocking start-up conditions of the smoke cooler recirculation system standby pump include: when the unit load is less than 570MW and the dead zone is 5MW, the standby pump starts; if the system main pump trips and delays for 80s, the smoke cooler recirculation system standby pump interlocking start-up.

6. The method for preventing corrosion of flue gas cooler heat exchange tubes according to claim 2, characterized in that: The interlocking shutdown conditions of the smoke cooler recirculation system include: when the system interlock is put into operation, if the unit load is greater than 570MW and the dead zone is 5MW, the smoke cooler recirculation system interlocking shutdown.

7. The method for preventing corrosion of flue gas cooler heat exchange tubes according to any one of claims 1 to 6, characterized in that: The smoke cooler recirculation pump protection stop interlock conditions include: When the recirculation pump runs for 90 seconds, the recirculation pump outlet electric door open feedback is lost and the closed feedback is received, the smoke cooler recirculation pump protection stop interlock is delayed for 30 seconds; When the recirculation pump inverter operation signal is lost and no stop command is issued, there is a delay of 5s and the smoke cooler recirculation pump protection stop interlock.

8. A system for preventing corrosion of heat exchange tubes of flue gas coolers, used in a method for preventing corrosion of heat exchange tubes of flue gas coolers, characterized in that: include, A cooling module, used for cooling the flue gas in the heat exchange tube; The circulation module is used to increase the water inlet temperature of the cooling module.

9. The method for preventing corrosion of heat exchange tubes of a flue gas cooler according to claim 8, characterized in that: The cooling module comprises a flue gas cooler, which absorbs flue gas heat through cooling water to reduce the flue gas temperature and improve the thermal efficiency of the boiler.

10. The method for preventing corrosion of heat exchange tubes of a flue gas cooler according to claim 9, characterized in that: The circulation module includes two recirculation pumps, which include a running pump and a standby pump. The recirculation pumps are controlled by frequency conversion and the inlet water temperature of the flue gas cooler is increased by recirculating hot water.

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