A system and method for cooling a burner with induced draft fan outlet flue gas

Abstract

The application discloses a system and method for cooling a burner by using flue gas from an induced draft fan, wherein a plurality of layers of burners are arranged on a front wall and / or a back wall of a boiler furnace, and a plurality of burners are arranged on each layer; the burners are connected with coal mills through a pulverized coal pipeline, and the coal mills are in one-to-one correspondence with the burners; a flue gas main pipe at an outlet of the induced draft fan is connected with a flue gas main pipe at an outlet of the induced draft fan, a plurality of flue gas main pipes at outlets of burners are connected with the flue gas main pipe at the outlet of the induced draft fan, and the flue gas main pipes at the outlets of the burners are in one-to-one correspondence with the burners; a plurality of flue gas branch pipes are connected with each flue gas main pipe at the outlet of the burner, and the flue gas branch pipes are in one-to-one correspondence with the burners of the corresponding layer; and an adjusting door and an isolation door are arranged on each flue gas branch pipe. The application can improve the boiler efficiency under full load conditions, expand a new way for flue gas waste heat utilization, improve the combustion stability and steam temperature characteristics under low load, and accurately control the flue gas cooling amount.

Description

Technical Field

[0001] This invention belongs to the field of energy-saving retrofit technology for coal-fired power generating units, specifically relating to a system and method for cooling burners using flue gas from the outlet of an induced draft fan. Background Technology

[0002] In coal-fired boilers, the burner is the core equipment for achieving pulverized coal combustion. To prevent the burner nozzles from being burned by high-temperature flue gas when the boiler is shut down, cooling air is usually introduced for protection. Current technologies mostly use on-site air intake, that is, directly introducing cold air from the boiler room atmosphere and sending it through pipes to the shut-down burner nozzles. This method has the following problems: 1. Unorganized air leakage: The cooling air is not heated by the air preheater and is considered unorganized air intake. This reduces the amount of air that can pass through the air preheater in an organized manner, resulting in increased flue gas temperature and decreased boiler efficiency.

[0003] 2. Uncontrollable air volume: The cooling air duct is only equipped with an electric door, which causes unorganized air leakage under the negative pressure of the furnace. It is impossible to accurately adjust the air volume according to actual needs, resulting in unnecessary cold air entering the furnace and increasing the heat loss of flue gas.

[0004] 3. Significant Energy Waste: Taking a 1000MW bituminous coal-fired boiler as an example, the boiler is equipped with 6 coal mills, each corresponding to 8 burners in one layer. Under the low-load peak-shaving condition of 40% THA, typically 3 coal mills are operating and 3 are shut down. The 24 burners corresponding to the shut-down coal mills require a continuous supply of cooling air. The cooling air leakage of a single burner is approximately 3.0 t / h, and the total cooling air leakage reaches approximately 72 t / h. This portion of cold air enters the furnace directly without being heated by the air preheater, resulting in increased flue gas temperature, reduced boiler efficiency, and significant energy waste.

[0005] In the prior art, Chinese utility model patent CN206846704U discloses a pulverized coal burner cooling air system using boiler hot primary air. This system uses hot primary air (approximately 320°C) from the air preheater as the cooling medium. The cooling air is connected to the burner via a manually operated isolation door in the pulverized coal pipeline through a main cooling air duct, branch ducts, and sub-ducts. The cooling air is switched on and off using an interlocking control between the quick-opening / closing door of the cooling air system and the quick-opening / closing door of the pulverized coal pipeline. While this technical solution can avoid heat loss caused by cold air directly entering the furnace to some extent, it still has the following shortcomings: (1) The temperature of the primary hot air is as high as about 320°C. As a cooling medium, the temperature is too high, the cooling effect is limited, and the high temperature hot air entering the furnace will change the local combustion conditions and affect the combustion stability. (2) When the cooling air branch pipe is connected to the pulverized coal pipeline, the connection between the cooling air and the pulverized coal pipeline is complicated when the coal mill is in operation standby or maintenance standby, the system reliability is reduced, and there is a risk of pulverized coal leakage. (3) Based solely on the static pressure value of the main pipe, a rough on / off quantity or graded adjustment cannot be made, which makes it difficult to achieve continuous and precise control of the cooling air volume and adapt to the cooling requirements under different load conditions. (4) Failed to make full use of the waste heat of flue gas, and did not consider the improvement effect on furnace temperature and steam temperature characteristics under low load conditions. Summary of the Invention

[0006] The technical problem to be solved by the present invention is to provide a system and method for cooling burners using flue gas from the outlet of an induced draft fan, thereby solving the problems of low boiler efficiency, uncontrollable cooling air volume, insufficient utilization of waste heat, and lack of clear calculation basis for branch pipe air volume adjustment in the prior art. This invention fully utilizes the clean waste heat of the flue gas from the outlet of the induced draft fan, which can solve the burner cooling problem and improve the boiler's performance under wide load conditions.

[0007] According to the technical solution of this invention, this invention provides a system for cooling burners using flue gas from an induced draft fan outlet, including a boiler furnace. Multiple layers of burners are arranged on the front and / or rear walls of the boiler furnace, with multiple burners arranged on each layer. The burners are connected to coal mills via pulverized coal pipelines, and the number of coal mills corresponds to the number of burner layers, with each layer corresponding to the previous one. A dust collector and an induced draft fan are sequentially connected to the flue gas outlet end of the boiler furnace, and the outlet of the induced draft fan is connected to an induced draft fan outlet flue gas header. The exhaust flue gas header is connected to the induced draft fan outlet cooling flue gas header, which in turn connects to multiple parallel burner cooling flue gas headers. The number of burner cooling flue gas headers corresponds to the number of burner layers and is one-to-one with each layer. Each burner cooling flue gas header is connected to multiple parallel cooling flue gas branch pipes, the number of which corresponds to the number of burners in the corresponding layer and is one-to-one with each other. Each cooling flue gas branch pipe is equipped with a regulating damper and an isolation damper.

[0008] In some implementations, a control system is also included, which is connected to each coal mill to obtain the start-stop status of each coal mill, and is also connected to each regulating gate and each isolation gate to perform control.

[0009] In some implementations, the regulating door is an electrically operated regulating door, and the isolation door is a pneumatically operated isolation door; and / or, the isolation door is located on the side of the regulating door closer to the burner.

[0010] In some implementations, pressure measuring points and flow measuring points are installed on each burner cooling flue gas header.

[0011] In some implementations, the outlet of the induced draft fan is connected to the desulfurization tower via an induced draft fan outlet flue gas header, and the connection point between the induced draft fan outlet cooling flue gas header and the induced draft fan outlet flue gas header is located on the upstream side of the desulfurization tower.

[0012] According to the technical solution of the present invention, the present invention also provides a method for cooling a burner using flue gas from the outlet of an induced draft fan, which employs the system of the present invention for cooling a burner using flue gas from the outlet of an induced draft fan, and includes the following steps: Step S1: Obtain the start-up and shutdown status of each coal mill in real time; Step S2: When the coal mill is in operation, keep all isolation doors on the corresponding cooling flue gas branch pipes closed; when a coal mill switches from operation to shutdown, open the corresponding isolation door to allow cooling flue gas to enter, and set the opening of the corresponding regulating door to the preset initial opening. Then, adjust the opening of the regulating door in real time to ensure that the air volume of the cooling flue gas meets the cooling requirements; when the shutdown coal mill switches back to operation, close the corresponding isolation door.

[0013] In some implementations, in step S2, when a coal mill switches from the running state to the shutdown state, the corresponding N burners in the layer are supplied with cooling flue gas through the corresponding burner cooling flue gas main pipe and the N cooling flue gas branch pipes thereon. In step S2, the process of adjusting the opening degree of the regulating door in real time includes the following steps: Step S21: Obtain the pressure P of the corresponding burner cooling flue gas header and the boiler furnace negative pressure P. f And the total flow rate Q of the corresponding burner cooling flue gas header. total ; First, confirm the total flow Q. total Does it fall within the total required air volume range [N×Q]? min , N×Q max Within, Q min Q is the minimum cooling airflow for the branch pipe. max This represents the maximum cooling airflow of the branch pipe; if Q total <N×Q min If Q increases, then the opening of each regulating valve is increased; if Q... total >N×Q max If Q decreases, then the opening of each regulating valve is reduced; when Q... total To fall within the total demand air volume range [N×Q] min , N×Q max Afterwards, the target air volume Q for each cooling flue gas branch pipe is calculated using the following formula. set : Q set = Q total / N; Then, the actual air volume Q in each cooling flue gas branch pipe is calculated using the following formula: , Where k is the flow coefficient, A(θ) is the flow area of ​​the regulating valve when the opening is θ, ρ is the flue gas density, and ΔP is the pressure difference, ΔP = P - P f ; Step S22: Calculate the actual air volume Q and the target air volume Q using the following formula. set The deviation e, and the rate of change of deviation ec: e = Q set – Q, ec = de / dt; When |e|>e1, a large-step adjustment method is adopted, and the adjustment amount Δθ of the gate opening is: Δθ = K1•e, Where K1 is the large step gain coefficient, and e1 is the preset large deviation threshold; When e2 < |e| ≤ e1, the PID control method is used, and the gate opening adjustment amount Δθ is: , Where Kp, Ki, and Kd are the proportional coefficient, integral coefficient, and derivative coefficient, respectively, and e2 is the preset small deviation threshold. When |e| ≤ e2, keep the opening of the regulating gate unchanged.

[0014] In some embodiments, in step S22, when the boiler load is below 50% of the rated load, the proportional coefficient Kp is 2.0 to 3.0; when the boiler load is above 70% of the rated load, the proportional coefficient Kp is 0.5 to 1.2; when the boiler load is between 50% and 70% of the rated load, the proportional coefficient Kp is determined by linear interpolation between 2.0 and 0.5.

[0015] In some implementations, step S23 is also included: after step S22 is completed, if the corresponding coal mill is still in a stopped state, the process returns to step S21 and repeats; otherwise, the adjustment of the opening of the regulating gate ends.

[0016] In some implementations, the temperature range of the flue gas in the flue gas header at the exhaust fan outlet is 85°C to 140°C.

[0017] Compared with the prior art, the beneficial technical effects of the present invention are as follows: 1. Improve boiler efficiency under full load conditions: Utilize waste heat from flue gas to replace ambient air as cooling air, reduce heat loss caused by cold air absorbing heat, and lower flue gas temperature, which can improve boiler efficiency by 0.2% to 0.4%.

[0018] 2. Expanding new ways of waste heat utilization: This invention breaks with conventional industry knowledge and successfully applies the clean flue gas after dust removal to burner cooling, providing a new approach for the in-depth utilization of flue gas waste heat.

[0019] 3. Significantly improves combustion and steam temperature characteristics at low loads: Compared to ambient cold air, flue gas at 85℃~140℃ enters the furnace, which can effectively increase the furnace temperature, improve the ignition conditions of pulverized coal, and help maintain reheat steam temperature and superheat steam temperature at low loads, so as to achieve efficient operation of the unit under wide loads.

[0020] 4. Precise and controllable flue gas volume: The volume of cooling flue gas can be precisely controlled through regulating valves and corresponding controllers, avoiding unorganized air leakage and reducing heat loss from exhaust.

[0021] 5. Convenient system modification: The cooling flue gas branch pipe is directly connected to the burner without the need to connect to the pulverized coal pipeline. It does not affect the normal operation and standby switching of the coal mill. It utilizes the existing flue and burner interface, and only requires the addition of flue gas pipelines and control devices, making it suitable for the modification of various coal-fired boilers.

[0022] 6. High degree of automation: The control system can be linked with the start and stop signals of the coal mill to automatically complete the switching and adjustment of cooling flue gas without manual operation, reducing the workload of operators.

[0023] 7. The control strategy is innovative: This invention proposes an air volume calculation model based on the main pipe pressure difference and the branch pipe valve opening, as well as a hierarchical adjustment strategy that combines large-step rapid approximation with PID incremental fine adjustment. This solves the problems of uneven air volume distribution and adjustment lag when multiple branches are connected in parallel, and significantly improves the response speed and steady-state accuracy of the control system. Attached Figure Description

[0024] Figure 1 This is a schematic diagram of the system structure of the present invention.

[0025] Figure 2 This is a flowchart of the first part of the method of the present invention.

[0026] Figure 3 This is a flowchart of the second part of the method of the present invention.

[0027] Figure 4 This is a flowchart of the third part of the method of the present invention.

[0028] Explanation of reference numerals in the attached figures: 1. Boiler furnace; 2. Burner; 3. Coal mill; 4. Induced draft fan; 5. Desulfurization tower; 6. Burner cooling flue gas header; 7. Cooling flue gas branch pipe; 8. Regulating valve; 9. Isolation valve; 10. Control system; 11. Pressure measuring point; 12. Flow measuring point; 13. Induced draft fan outlet flue gas header; 14. Dust collector; 15. Induced draft fan outlet cooling flue gas header. Detailed Implementation

[0029] This invention provides a system and method for cooling burners using flue gas from the induced draft fan outlet, thereby improving boiler efficiency and enhancing low-load operating characteristics. In a typical embodiment, the system mainly includes a burner cooling flue gas header connected to the induced draft fan outlet, multiple cooling flue gas branch pipes, and a control system. Regulating valves and isolation valves are installed on the cooling flue gas branch pipes, and pressure and flow measurement points are provided on the burner cooling flue gas header. The control system controls the opening and closing of the isolation valves based on the pulverizer start / stop signal, and adjusts the regulating valve opening using a variable-parameter PID strategy based on the pressure difference between the cooling flue gas header pressure and the furnace negative pressure, as well as flow feedback. This invention effectively utilizes the low-temperature flue gas from the induced draft fan outlet, treated with dust removal, to replace ambient air as the burner cooling medium, overcoming the technical prejudice that flue gas is unsuitable as a cooling medium. While reducing exhaust gas temperature and improving boiler efficiency, it significantly improves combustion stability and steam temperature characteristics under low-load conditions.

[0030] Please see Figure 1 This invention discloses a system for cooling burners using flue gas from an induced draft fan. The system includes a boiler furnace 1, burners 2, a coal mill 3, an induced draft fan 4, etc. Multiple layers of burners 2 are installed on the front and / or rear walls of the boiler furnace 1, with multiple burners in each layer. The burners 2 are connected to the coal mill 3 via pulverized coal pipes. The number of coal mills 3 corresponds to the number of layers of burners 2, with each layer corresponding to one other layer. For example, the boiler furnace 1 may have three layers (layer A, layer B, and layer C) of burners on both the front and rear walls, for a total of six layers. Figure 1 Only the three layers of the front wall are shown; there are three coal mills 3 corresponding to the three layers of the front wall (coal mill A, coal mill B, and coal mill C), and a total of six coal mills 3; each layer has eight burners, for a total of 48 burners; each layer of eight burners 2 is connected to the same coal mill 3, and the coal mill 3 is connected to eight branch pipes of the pulverized coal pipeline, which delivers pulverized coal to the corresponding eight burners 2. When the coal mill 3 is stopped, the corresponding eight burners 2 need to be cooled.

[0031] A dust collector 14 and an induced draft fan 4 are sequentially connected to the flue gas output end of the boiler furnace 1. The outlet of the induced draft fan 4 is connected to an induced draft fan outlet flue gas header 13, and an induced draft fan outlet cooling flue gas header 15 is connected to the induced draft fan outlet flue gas header 13; in other words, the induced draft fan outlet cooling flue gas header 15 is a pipe that draws flue gas from the induced draft fan outlet flue gas header 13, and the drawn flue gas is the flue gas after dust removal. Multiple parallel burner cooling flue gas headers 6 are connected to the induced draft fan outlet cooling flue gas header 15, the number of which is the same as the number of layers in the burner 2 and corresponds one-to-one with each layer. Multiple parallel cooling flue gas branch pipes 7 are connected to each burner cooling flue gas header 6. Figure 1The diagram only shows, by way of example, one cooling flue gas branch pipe 7 of the burner cooling flue gas main pipe 6. The number of cooling flue gas branch pipes 7 is the same as the number of burners 2 in the corresponding layer, and they are connected one-to-one. This allows the flue gas from the induced draft fan outlet to be delivered to the burner 2 that needs cooling after passing sequentially through the induced draft fan outlet cooling flue gas main pipe 15, the cooling flue gas main pipe 6, and the cooling flue gas branch pipes 7. Each cooling flue gas branch pipe 7 is equipped with a regulating valve 8 and an isolation valve 9. The regulating valve 8 is used to regulate the cooling flue gas flow rate (or air volume), and the isolation valve 9 is used to control the switching of cooling flue gas.

[0032] Furthermore, it also includes a control system 10, which is specifically a distributed control system (DCS). The control system 10 is connected to each coal mill 3 to obtain the start and stop status of each coal mill 3, and the control system 10 is also connected to each regulating gate 8 and each isolation gate 9 to perform control.

[0033] Furthermore, the regulating door 8 is preferably an electrically operated regulating door to precisely control the cooling airflow of each burner. The isolation door 9 is preferably a pneumatically operated isolation door to achieve rapid and reliable opening for cooling and closing for sealing. The isolation door 9 is preferably located on the side of the regulating door 8 closer to the burner 2, so that when the burner 2 is in operation, the isolation door 9 is closed, which can, to a certain extent, block high temperatures and reduce the impact on the regulating door 8 and its control circuit.

[0034] Furthermore, pressure measuring points 11 and flow measuring points 12 are installed on each burner cooling flue gas main pipe 6 for real-time monitoring of operating parameters. In this scheme, only the pressure and airflow of the burner cooling flue gas main pipe 6 are monitored. The pressure of each cooling flue gas branch pipe 7 is the same as (or substantially the same as) the pressure of the burner cooling flue gas main pipe 6, and the airflow of each cooling flue gas branch pipe 7 is evenly distributed, thus enabling centralized monitoring and control. More specifically, pressure measuring points 11 and flow measuring points 12 are respectively equipped with corresponding detection devices such as sensors, and the detection devices are connected to the control system 10 to transmit detection data.

[0035] As a supplementary explanation, the outlet of the induced draft fan 4 is connected to the desulfurization tower 5 through the induced draft fan outlet flue gas header 13. The connection point between the induced draft fan outlet cooling flue gas header 15 and the induced draft fan outlet flue gas header 13 is located on the upstream side of the desulfurization tower 5. That is, the flue gas used for cooling is specifically the flue gas after dust removal and before desulfurization.

[0036] Please see Figures 2 to 4 Based on the above-described system for cooling a burner using flue gas from an induced draft fan, this invention provides a method for cooling a burner using flue gas from an induced draft fan, comprising the following steps.

[0037] Step S1: Obtain the start / stop status of each coal mill 3 in real time.

[0038] Step S2: When the coal mill 3 is in operation, the burner 2 of the corresponding layer is working normally, and the isolation doors 9 on the corresponding cooling flue gas branch pipe 7 are all closed to prevent flue gas from entering the normally operating burner 2 and affecting the combustion conditions. When a coal mill 3 switches from operating to shut-down, the corresponding burner 2 on the same floor is also shut down. The corresponding isolation door 9 is opened to allow cooling flue gas to enter (for example, the control system 10 receives the shutdown signal of the coal mill 3 and automatically controls the isolation door 9 of each burner 2 on that floor to open). The opening of the corresponding regulating door 8 is a preset initial opening (for example, 50%, which can be determined according to the on-site debugging results). Then, the opening of the regulating door 8 is adjusted in real time to ensure that the air volume of the cooling flue gas meets the cooling requirements. In this way, the flue gas from the induced draft fan outlet enters the shut-down burner 2 through the burner cooling flue gas main pipe 6 and the cooling flue gas branch pipe 7 to cool and protect the nozzle of the burner 2. When the shut-down coal mill 3 is switched back to the running state, the corresponding isolation door 9 is closed, and the regulation of the regulating door 8 is also ended.

[0039] The following provides a more specific working process.

[0040] In step S2, when a coal mill 3 switches from operating to shut-down mode, the corresponding N burners 2 are supplied with cooling flue gas through a corresponding burner cooling flue gas main pipe 6 and its N cooling flue gas branch pipes 7. The supply of cooling flue gas signifies that the mill is put into operation. Here, N is both the number of cooling flue gas branch pipes and the number of burners; both values ​​are the same. This parameter is defined to explain the subsequent calculation process.

[0041] In step S2, the process of adjusting the opening of the regulating gate 8 in real time includes the following steps.

[0042] Step S21: Obtain the pressure P of the corresponding burner cooling flue gas header 6 and the boiler furnace negative pressure P. f And the total flow rate Q of the corresponding burner cooling flue gas header 6 total For example, the required parameters are obtained by the control system 10 through corresponding measuring points.

[0043] First, confirm the total flow Q. total Does it fall within the total required air volume range [N×Q]? min , N×Q max Within, Q min Q is the minimum cooling airflow for the branch pipe. max This represents the maximum cooling airflow of the branch pipe. If Q... total <N×Q minThis indicates that the air supply capacity of the main pipe is limited. Therefore, the opening of the regulating valves 8 (of the branches in operation) should be appropriately increased (e.g., by 5%–10% simultaneously) to improve the main pipe pressure and total flow. If Q total >N×Q max Then reduce the opening of the regulating valve 8 (of each branch pipe in operation). When Q total To fall within the total demand air volume range [N×Q] min , N×Q max After ensuring the total air supply meets the demand, the target air volume for each branch pipe is set according to the principle of equal distribution. That is, the target air volume Q of each cooling flue gas branch pipe 7 is calculated using the following formula. set : Q set = Q total / N.

[0044] Then, the actual air volume Q in each cooling flue gas branch pipe 7 is calculated using the following formula: , Where k is the flow coefficient, which can be pre-calibrated through field tests; A(θ) is the flow area of ​​the regulating valve when the opening is θ, which is determined by the valve's inherent flow characteristics, usually an equal percentage characteristic or a linear characteristic, and its functional relationship can be pre-determined and stored in the control system; ρ is the flue gas density, for example, calculated from the flue gas temperature and pressure; ΔP is the pressure difference, ΔP = P - P f As a supplementary explanation, the pressure difference ΔP is the driving force that propels the cooling flue gas from the main pipe through each branch pipe into the boiler furnace, and it is the same for all the cooling flue gas branch pipes 7 that are in operation at the same time; therefore, the air volume of each cooling flue gas branch pipe 7 depends only on the opening degree of its respective regulating valve 8.

[0045] Step S22: Calculate the actual air volume Q and the target air volume Q using the following formula. set The deviation e, and the rate of change of deviation ec: e = Q set – Q, ec = de / dt.

[0046] Furthermore, a graded adjustment strategy is adopted to determine the opening adjustment amount of each branch pipe regulating valve 8, and the specific rules are as follows.

[0047] (1) Large-step rapid approach stage: When |e|>e1, a large-step adjustment method is adopted to make the air volume rapidly approach the target value; the adjustment amount Δθ of the regulating door opening is: Δθ = K1•e, Where K1 is the large step gain coefficient, for example, K1 = 0.05% / (t / h); e1 is the preset large deviation threshold, for example, e1 = 0.3Q. set .

[0048] (2) PID incremental fine-tuning stage: When e2 < |e| ≤ e1, PID control mode (PID incremental control) is adopted, and the adjustment amount Δθ of the control gate opening is: , Where Kp, Ki, and Kd are the proportional coefficient, integral coefficient, and derivative coefficient, respectively; e2 is a preset small deviation threshold, for example, e2 = 0.05Q. set ; More specifically, the proportional coefficient is determined according to the boiler load as follows: when the boiler load is below 50% of the rated load, the proportional coefficient Kp is 2.0 to 3.0; when the boiler load is above 70% of the rated load, the proportional coefficient Kp is 0.5 to 1.2; when the boiler load is between 50% and 70% of the rated load, the proportional coefficient Kp is determined by linear interpolation between 2.0 and 0.5.

[0049] (3) Dead zone holding stage: When |e| ≤ e2, it is determined that the air volume of the branch pipe has reached the steady-state accuracy requirement, and the opening of the regulating valve 8 is kept unchanged to avoid frequent adjustment leading to system oscillation.

[0050] Furthermore, step S23 is also included. After step S22, if the corresponding coal mill 3 is still in a stopped state, the process returns to step S21 and repeats; otherwise, the adjustment of the opening of the regulating valve 8 is terminated. Specifically, when the stopped coal mill 3 is restarted, the control system 10 receives a start signal, quickly closes the corresponding isolation door 9, stops supplying cooling flue gas to the burner 2 of that layer, and simultaneously terminates the adjustment of the opening of the regulating valve 8. Terminating the adjustment of the opening of the regulating valve 8 means resetting the regulating valve 8 of each branch pipe of that layer to the initial zero position (preset initial opening) or keeping the current opening in memory (to be adjusted to the preset initial opening again when it is put into operation next time).

[0051] Cyclic operation refers to periodically executing the aforementioned adjustment logic for the regulating valve opening during burner cooling, dynamically adjusting to ensure long-term cooling requirements are met. For example, a control cycle occurs every 2 seconds. Within each control cycle, the control system 10 first reads the main pipe pressure P and total flow rate Q. total and boiler furnace negative pressure P f Calculate ΔP and the required target air volume Q. set Then, the actual air volume and deviation are estimated for each branch pipe in operation in sequence; finally, the corresponding adjustment action is performed according to the deviation level of each branch pipe. Furthermore, if the deviation of a branch pipe exceeds the preset threshold (e.g., ±10%), the regulating valve of that branch pipe is adjusted individually; if the deviation of all branch pipes is within the preset threshold, the opening of each regulating valve is kept unchanged.

[0052] Preferably, the above-mentioned calculation, processing and control processes are all automatically completed by the control system 10.

[0053] As a supplementary explanation, due to long-standing concerns about dust in the flue gas causing burner nozzle blockage and low-temperature corrosion, those skilled in the art generally believe that flue gas should not be directly introduced into the burner system as a cooling medium. However, after the flue gas at the induced draft fan outlet is treated by dust collector 14, the dust concentration is extremely low (typically below 20 mg / Nm³). 3 The flue gas has a high cleanliness level, with a temperature range of 85℃ to 140℃ in the flue gas header 13 at the induced draft fan outlet. This fully meets the burner cooling requirements and will not cause nozzle blockage due to dust. Furthermore, the flue gas temperature of 85℃ to 140℃ is higher than that of ambient cold air. Using this flue gas as a cooling medium will not lower the furnace temperature after entering the boiler furnace, resulting in outstanding performance in stable combustion under low load. It can also increase the steam temperature under low load conditions, helping to maintain the furnace temperature, enabling the unit to operate efficiently across a wide load range, reducing coal consumption, and improving combustion stability and steam temperature characteristics under low load conditions. This provides technical support for flexible peak shaving and safe and stable operation of the unit.

[0054] In summary, this invention utilizes waste heat from flue gas to replace ambient air as cooling air, reducing heat loss caused by cold air absorbing heat and lowering the flue gas temperature, thereby improving boiler efficiency by 0.2% to 0.4%. This invention breaks with conventional industry understanding by successfully applying clean flue gas after dust removal to burner cooling, providing a new approach for the deep utilization of waste heat from flue gas. Compared to ambient cold air, flue gas at 85℃ to 140℃ entering the furnace effectively increases the furnace temperature, improves pulverized coal ignition conditions, and helps maintain reheat steam temperature and superheat steam temperature under low loads, enabling efficient operation of the unit across wide load ranges. Precise control of the cooling flue gas volume can be achieved through regulating valves and corresponding controllers, avoiding unorganized air leakage and reducing flue gas heat loss. The cooling flue gas branch pipe is directly connected to the burner, eliminating the need for pulverized coal pipelines and ensuring uninterrupted operation and standby switching of the coal mill. Utilizing existing flue ducts and burner interfaces, only additional flue gas pipelines and control devices are required, making it suitable for retrofitting various types of coal-fired boilers. The control system can be linked with the coal mill start / stop signals to automatically switch and regulate the cooling flue gas, eliminating the need for manual operation and reducing the workload of operators. This invention proposes an airflow calculation model based on the main pipe pressure difference and branch pipe valve opening, as well as a hierarchical regulation strategy combining large-step rapid approximation with PID incremental fine regulation. This solves the problems of uneven airflow distribution and regulation lag when multiple branch pipes are connected in parallel, significantly improving the response speed and steady-state accuracy of the control system.

Claims

1. A system for cooling a burner using flue gas from an induced draft fan outlet, characterized in that, The boiler includes a furnace (1), with multiple layers of burners (2) installed on the front and / or rear walls of the furnace (1), and multiple burners (2) installed on each layer; the burners (2) are connected to the coal mills (3) via pulverized coal pipes, and the number of coal mills (3) is the same as the number of layers of burners (2) and corresponds one-to-one with each layer; a dust collector (14) and an induced draft fan (4) are sequentially connected at the flue gas output end of the boiler furnace (1), and the outlet of the induced draft fan (4) is connected to the induced draft fan outlet flue gas header (13); an induced draft fan outlet is connected to the induced draft fan outlet flue gas header (13). The exhaust cooling flue gas header (15) is connected to multiple parallel burner cooling flue gas headers (6). The number of burner cooling flue gas headers (6) is the same as the number of layers of burners (2) and corresponds one-to-one with each layer. Multiple parallel cooling flue gas branch pipes (7) are connected to each burner cooling flue gas header (6). The number of cooling flue gas branch pipes (7) is the same as the number of burners (2) in the corresponding layer and is connected one-to-one. Each cooling flue gas branch pipe (7) is equipped with a regulating valve (8) and an isolation valve (9).

2. The system for cooling a burner using flue gas from an induced draft fan according to claim 1, characterized in that, It also includes a control system (10), which is connected to each coal mill (3) to obtain the start and stop status of each coal mill (3). The control system (10) is also connected to each regulating gate (8) and each isolation gate (9) to perform control.

3. The system for cooling a burner using flue gas from an induced draft fan according to claim 1, characterized in that, The regulating door (8) is an electric regulating door, and the isolation door (9) is a pneumatic isolation door; and / or, the isolation door (9) is located on the side of the regulating door (8) closer to the burner (2).

4. The system for cooling a burner using flue gas from an induced draft fan according to claim 1, characterized in that, Pressure measuring point (11) and flow measuring point (12) are set on each burner cooling flue gas header (6).

5. The system for cooling a burner using flue gas from an induced draft fan according to any one of claims 1 to 4, characterized in that, The outlet of the induced draft fan (4) is connected to the desulfurization tower (5) through the induced draft fan outlet flue gas header (13). The connection point between the induced draft fan outlet cooling flue gas header (15) and the induced draft fan outlet flue gas header (13) is located on the upstream side of the desulfurization tower (5).

6. A method for cooling a burner using flue gas from an induced draft fan outlet, characterized in that, The system employing the method of cooling the burner with flue gas from the induced draft fan outlet according to any one of claims 1 to 5 includes the following steps: Step S1: Real-time acquisition of the start-up and shutdown status of each coal mill (3); Step S2: When the coal mill (3) is in operation, keep the isolation doors (9) on the corresponding cooling flue gas branch pipe (7) closed. When a coal mill (3) switches from operation to shutdown, open the corresponding isolation door (9) to allow cooling flue gas to enter. The opening of the corresponding regulating door (8) is the preset initial opening. Then, adjust the opening of the regulating door (8) in real time to ensure that the air volume of the cooling flue gas meets the cooling requirements. When the shutdown coal mill (3) switches back to operation, close the corresponding isolation door (9).

7. The method for cooling a burner using flue gas from an induced draft fan according to claim 6, characterized in that, In step S2, when a coal mill (3) switches from the running state to the shutdown state, the corresponding N burners (2) of the corresponding layer are supplied with cooling flue gas through the corresponding burner cooling flue gas main pipe (6) and the N cooling flue gas branch pipes (7). In step S2, the process of adjusting the opening of the regulating gate (8) in real time includes the following steps: Step S21: Obtain the pressure P of the corresponding burner cooling flue gas header (6) and the boiler furnace negative pressure P. f And the total flow rate Q of the corresponding burner cooling flue gas header (6) total ; First, confirm the total flow Q. total Does it fall within the total required air volume range [N×Q]? min , N×Q max Within, Q min Q is the minimum cooling airflow for the branch pipe. max This represents the maximum cooling airflow of the branch pipe; if Q total < N×Q min If Q increases the opening of each regulating valve (8), then Q increases the opening of each regulating valve (8). total > N×Q max If Q decreases, then reduce the opening of each regulating gate (8); when Q... total To fall within the total demand air volume range [N×Q] min , N×Q max Afterwards, the target air volume Q of each cooling flue gas branch pipe (7) is calculated using the following formula. set : Q set = Q total / N; Then, the actual air volume Q in each cooling flue gas branch pipe (7) is calculated using the following formula: ， Where k is the flow coefficient, A(θ) is the flow area of ​​the regulating valve when the opening is θ, ρ is the flue gas density, and ΔP is the pressure difference, ΔP = P - P f ; Step S22: Calculate the actual air volume Q and the target air volume Q using the following formula. set The deviation e, and the rate of change of deviation ec: e = Q set – Q, ec = de / dt; When |e| > e1, the large-step adjustment method is adopted, and the adjustment amount Δθ of the gate opening is: Δθ = K1•e, Where K1 is the large step gain coefficient, and e1 is the preset large deviation threshold; When e2 < |e| ≤ e1, the PID control method is used, and the gate opening adjustment amount Δθ is: ， Where Kp, Ki, and Kd are the proportional coefficient, integral coefficient, and derivative coefficient, respectively, and e2 is the preset small deviation threshold. When |e| ≤ e2, keep the opening of the regulating gate (8) unchanged.

8. The method for cooling a burner using flue gas from an induced draft fan according to claim 7, characterized in that, In step S22, when the boiler load is below 50% of the rated load, the proportional coefficient Kp is 2.0 to 3.0; when the boiler load is above 70% of the rated load, the proportional coefficient Kp is 0.5 to 1.2; when the boiler load is between 50% and 70% of the rated load, the proportional coefficient Kp is determined by linear interpolation between 2.0 and 0.

5.

9. The method for cooling a burner using flue gas from an induced draft fan according to claim 7, characterized in that, It also includes step S23. After step S22 is completed, if the corresponding coal mill (3) is still in a stopped state, the process returns to step S21 and repeats. Otherwise, the adjustment of the opening of the regulating gate (8) is terminated.

10. The method for cooling a burner using flue gas from an induced draft fan according to claim 6, characterized in that, The temperature range of the flue gas in the flue gas header (13) at the outlet of the induced draft fan is 85℃~140℃.

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