A pulverizing system and control method for coal-fired units operating at high variable load rates

Abstract

The present invention belongs to the field of pulverized coal boiler equipment in coal-fired power plants, and discloses a pulverizing system and control method for coal-fired units operating at high variable load rates. A pulverized coal bin system is connected in parallel to the primary air-powder duct at the outlet of the coal mill. Under stable load, the pulverized coal bin system does not work, and the original direct-blowing pulverizing method is adopted. The pulverized coal at the outlet of the coal mill is directly transported to the boiler burner through the primary air-powder duct. When the unit needs to change load, the coal mill receives the boiler fuel quantity instruction as usual and performs pulverizing response, but the air-powder mixture at the outlet of the coal mill is completely switched into the fine powder separator, and the pulverized coal bin is fed through the pulverizer feeder. At the end of the unit's load change process, when the total output of the coal mill matches the fuel quantity demand on the boiler side and the operation is stable, the pulverized coal bin system is switched back to the coal mill to supply pulverized coal to the boiler, and the pulverized coal bin system stops working. At the same time, a control strategy for the switching sequence of the pulverized coal bin system and a control loop for important operating parameters of the pulverizing system under different operating conditions are also designed.

Description

Technical Field

[0001] The present invention belongs to the field of pulverized coal boiler equipment in coal-fired power plants, and in particular relates to a pulverizing system and a control method for a coal-fired unit operating at a high variable load rate. Background Art

[0002] Currently, large-capacity units generally adopt direct-blowing pulverizing systems due to their simple structure and easy maintenance. However, the severe coupling between the pulverizer and the boiler in direct-blowing pulverizing systems has become a major factor restricting the increase in the variable load rate of coal-fired units. This is mainly manifested in the following: On the one hand, when the unit receives a variable load instruction, there is difficult-to-eliminate inertia and delay in the process of the coal feeder feeding the raw coal into the pulverizer, the pulverizer grinding the raw coal into pulverized coal, and the primary air drying and conveying the pulverized coal. This results in the direct-blowing pulverizing system's insufficient responsiveness to the unit's variable load, thus affecting the unit's higher-rate variable load operation. In response to this, many scholars have proposed optimizing the control of direct-blowing pulverizing systems through the use of advanced control strategies such as neural networks to address the large delay and high inertia issues. Others have proposed improving the pulverizing system's responsiveness to a certain extent by additionally blowing out the pulverized coal stored in the pulverizer. On the other hand, when the unit undergoes a large-scale load change, the pulverizer's own output limit requires multiple manual start and stop operations. In the case of high-rate load changes, the higher the rate of load increase or decrease, the less time the operator has to perform start and stop operations. At the same time, the less time the unit's operating parameters have to overcome disturbances and return to a stable state. Therefore, the existing direct-blowing pulverizing system is difficult to achieve large-scale, high-rate (such as 5% / min) load change operation of the unit due to the start and stop operations of the pulverizer. To this end, it is necessary to modify the direct-blowing pulverizing system to reduce the degree of coupling between the pulverizer and the boiler, and in particular to achieve decoupling of the pulverizer and the boiler during the unit's rapid load change.

[0003] Through the above analysis, the problems and defects of the existing technology are as follows:

[0004] 1. The direct-blowing pulverizing system has a large delay and inertia in the coal grinding and transportation process, and its ability to track boiler load response is insufficient, which restricts the unit's higher-speed variable load operation.

[0005] 2. When the unit's load changes over a large range, the coal mill needs to be started and stopped multiple times due to its own output limitation. When the load change rate is very high (such as 5% / min), the time required for the unit's load change process is very short, but the time required for the coal mill to start and stop normally is necessary. This causes the direct-blowing pulverizing system to seriously affect the unit's large-scale, high-load rate operation due to the start and stop operations of the grinding group. Summary of the Invention

[0006] To address the problems of the prior art, the present invention provides a pulverizing system and control method for coal-fired units operating at high variable load rates. The system modifies the medium-speed mill's positive-pressure, direct-blowing pulverizing system by connecting a pulverized coal silo system in parallel to the primary air-to-powder duct at the mill outlet. Under stable load, the pulverized coal silo system is inoperative, and the original direct-blowing pulverizing method is adopted. The pulverized coal at the mill outlet is directly transported to the boiler burner through the primary air-to-powder duct. When the unit needs to vary its load, the mill continues to receive the boiler's fuel quantity command and performs pulverizing. However, the air-to-powder mixture at the mill outlet is completely switched into a fine powder separator, where it is fed from the pulverized coal silo via a pulverizer feeder. At the end of the unit's load variation process, when the mill's total output matches the boiler's fuel quantity demand and operation stabilizes, the system switches back to pulverizing the boiler, and the pulverized coal silo system stops operating. Simultaneously, based on the structure and operation of this novel pulverizing system, a control strategy for the switching sequence of the pulverized coal silo system under different operating conditions is designed, as well as control loops for the system's key operating parameters. Thus, the novel pulverizing system and control method proposed in this invention achieves decoupling of the pulverizer and boiler under variable load conditions. On the one hand, compared to direct-blowing pulverizing systems, this system directly responds to changes in boiler fuel demand by changing the pulverizer feeder speed. This reduces the time required to transport raw coal to the feeder outlet and the time required for pulverizing coal by the pulverizer, significantly improving the pulverizing system's responsiveness to boiler load changes. On the other hand, during unit load changes, the pulverizer's start and stop operations do not affect the operating status of the pulverized coal bin. The pulverized coal bin and pulverizer feeder provide pulverized coal throughout the load change process, effectively avoiding the impact of the pulverizer's start and stop on boiler load changes and enabling flexible operation of the coal-fired unit. Furthermore, because this system only serves variable load conditions, the required capacity is minimal, making it feasible to apply intermediate storage pulverizing methods to large-capacity units (300MW and above). This system combines the advantages of both direct-blowing and intermediate storage pulverizing systems, enabling large-capacity units to operate over a wide range and at high variable load rates.

[0007] The present invention adds a silo system to the direct-blowing pulverizing system. This silo is connected in parallel with the primary air-powder pipe at the pulverizer outlet. One route of the air-powder mixture at the pulverizer outlet goes directly to the boiler, while the other route goes to a fine powder separator and then into the silo. The silo then supplies pulverized coal to the boiler via a pulverizer feeder. This achieves a two-way, separate supply of pulverized coal to the boiler. Under stable operating conditions, the pulverized coal at the pulverizer outlet is directly fed to the boiler. Under variable load conditions, the pulverized coal at the pulverizer outlet enters the silo, and the entire pulverized coal required by the boiler is supplied by the silo. Simultaneously, a new pipeline is introduced from the primary air main pipe into the primary air-powder pipe, providing sufficient conveying air and drying capacity for the pulverizer feeder. This system is equivalent to a central storage pulverizing system using hot air pulverization. Exhaust gas separated by the fine powder separator is directly fed into the boiler's burnout air nozzle, where a portion of the accompanying pulverized coal (approximately 10%) is burned in the furnace. Due to negative pressure leakage in the silo system, a portion of the exhaust gas needs to be separated and recirculated to the pulverizer inlet.

[0008] The present invention is implemented as follows: a pulverizing system for a coal-fired unit operating at a high variable load rate adopts a plurality of fine powder separators, pulverized coal bins arranged on the front and rear walls, and 18 pulverized coal feeders connected to coal hoppers, providing a configuration of pulverizing, storing, and feeding for the coal-fired unit operating at a high variable load rate; ensuring that when the unit is operating at a variable load, the air-pulverized coal mixture at the pulverizer outlet can be quickly transferred to the fine powder separator and stored in the pulverized coal bin in an orderly manner, and then accurately fed through multiple pulverized coal feeders to meet the operating requirements under different load conditions.

[0009] Further, it includes: a coal feeder, a gate, a pulverized coal separator, a coal mill, a pulverized coal distributor, a fine powder separator, a pulverized coal silo, a pulverized coal feeder, a pulverized coal mixer, a moisture absorption pipe, a pulverized coal exhaust fan, a burnout air nozzle, a burner, a boiler, a three-compartment rotary air preheater, a primary fan, a first air lock, a second air lock, a first explosion-proof door, a second explosion-proof door, an exhaust gas bellows, a primary bellows, a first valve, a second valve, a valve for the exhaust gas to the burnout air nozzle, a hot gas recirculation damper, a third valve, a fourth valve, a fifth valve, a sixth valve, a seventh valve, and an eighth valve;

[0010] The coal feeder is connected to the pulverizer inlet through a powder dropping pipe. There are gate plates and the first air lock on the powder dropping pipe. The pulverizer is connected to the coal powder separator. The coal powder separator outlet is divided into two paths through the coal powder distributor: one path is connected to the burner through the first valve and the coal powder mixer; the other path is connected to the fine powder separator through the second valve and the moisture absorption pipe. The upper end of the fine powder separator is provided with a first explosion-proof door. The lower end of the fine powder separator is connected to the coal powder bin through the second air lock. The coal powder bin has a moisture absorption pipe connected to the fine powder separator inlet pipe. The upper end outlet of the fine powder separator is connected to the powder exhaust fan. The powder exhaust fan outlet is divided into two paths: one path goes to the pulverizer inlet powder dropping pipe, which is a recirculation pipe, and the baffle on the pipe is for hot gas recirculation. The exhaust gas outlet is divided into two routes, one route passes through the three-compartment air preheater, and then is divided into two routes, one route passes through the fourth valve to connect to the primary air box, and after distribution, it passes through the third valve to connect to the first valve and the middle section of the pulverized coal mixer pipeline, and the other route passes through the sixth valve and the eighth valve to connect to the pulverizer; the other route of the primary air fan outlet does not pass through the three-compartment air preheater, and is divided into two routes, one route passes through the fifth valve to connect to the primary air box, and the other route passes through the seventh valve to connect to the middle section of the pulverized coal mixer pipeline.

[0011] Furthermore, the fine powder separator is provided with a powder level monitoring device and an air-powder temperature monitoring device, the air-powder temperature measuring device has no less than 4 temperature measuring points, and the powder level detection device has no less than 3 measuring points; a first explosion-proof door is also provided;

[0012] The pulverized coal bin is equipped with a pulverized coal level monitoring device and an air-powder temperature monitoring device. The air-powder temperature measuring device has no fewer than four temperature measurement points, and the pulverized coal level detection device has no fewer than three measurement points. It is also equipped with a second explosion-proof door, a moisture absorption pipe with an electric isolation valve, and an inlet pipe interface for fire extinguishing, which can be used to introduce fire extinguishing media such as nitrogen or carbon dioxide.

[0013] The coal mill, coarse powder separator and pulverized coal distributor are built in one body. The pulverized coal distributor has 6 pulverized coal outlet pipes, called primary air pulverized coal pipes.

[0014] The boiler furnace is an opposed-firing furnace, divided into a front wall and a rear wall. Each wall has three layers of burners and two layers of burnout air nozzles, with six burners on each layer, for a total of 36 burners and 24 burnout air nozzles.

[0015] There are 6 coal mills, namely A mill, B mill, C mill, D mill, E mill and F mill, among which F mill is a spare coal mill; the powder outlet pipes of A mill, B mill and C mill are respectively connected to the lower, middle and upper layers of the front wall of the boiler furnace; the powder outlet pipes of D mill, E mill and F mill are respectively connected to the lower, middle and upper layers of the rear wall of the boiler furnace.

[0016] Furthermore, there are two fine powder separators and two pulverized coal silos, and they are respectively arranged on the front and rear walls. Mills A, B, and C are connected to the boiler front wall burner through one line, and to the front wall fine powder separator through another line. The lower end of the fine powder separator is connected to the front wall pulverized coal silo, and the upper end is connected to the burnout air nozzles on the front wall after distribution through the exhaust gas wind box. Mills D, E, and F are connected to the boiler rear wall burner through one line, and to the rear wall fine powder separator through another line. The lower end of the fine powder separator is connected to the rear wall pulverized coal silo, and the upper end is connected to the burnout air nozzles on the rear wall after distribution through the exhaust gas wind box.

[0017] There are 18 coal hoppers at the lower end of the pulverized coal bin, each of which is connected to a pulverized coal feeder. There are a total of 18 pulverized coal feeders, which are connected to 18 pulverized coal mixers respectively.

[0018] Furthermore, to avoid clogging due to coal dust accumulation, the port of the primary air duct where the third valve is located should be as close as possible to the location of the first valve;

[0019] The total effective capacity of the pulverized coal silo is set according to the amount of fuel required by the boiler during the unit's load variation range of 20% to 100%. Taking a 660MW supercritical unit with a load variation rate of 5% / min as an example, considering the safety margin, a total effective pulverized coal storage capacity of approximately 150t is required. The capacity of the front and rear wall pulverized coal silos is 3:2, that is, the front wall pulverized coal silo has a pulverized coal storage capacity of approximately 90t, and the rear wall pulverized coal silo has an effective pulverized coal storage capacity of 60t.

[0020] The system's overall operating process is as follows: Under stable load, the pulverized coal silo system is inactive, and the original direct-blowing pulverizing method is used. The pulverized coal at the pulverizer outlet is directly delivered to the boiler burner through the primary air-to-powder duct. When the unit needs to change load, the pulverizer receives the boiler's fuel quantity command and performs pulverizing as usual, but the air-to-powder mixture at the pulverizer outlet is completely switched to the fine powder separator, where it is fed from the pulverized coal silo via the pulverizer feeder. At the end of the unit's load change process, when the pulverizer's total output matches the boiler's fuel quantity demand and operation stabilizes, the pulverized coal silo system is switched back to the pulverizer supplying pulverized coal to the boiler, and the pulverized coal silo system is deactivated.

[0021] Another object of the present invention is to provide a pulverizing system control method for the coal-fired unit operating at a high variable load rate, which controls the switching process of the pulverizing silo system in a sequential control manner, specifically:

[0022] Step 1: Start the powder silo system control sequence:

[0023] Permitted conditions:

[0024] (1) Load change instruction comes;

[0025] (2) The fine powder separator, powder feeder and powder exhaust fan stop working;

[0026] (3) The pulverized coal level in the pulverized coal bin is normal;

[0027] (4) The second valve is fully closed;

[0028] (5) The third valve is fully closed;

[0029] (6) The hot gas recirculation damper is fully closed;

[0030] (7) The first valve is fully open;

[0031] Start the sequential control sequence:

[0032] (1) Open the exhaust gas to burnout air nozzle valve;

[0033] (2) The hot gas recirculation damper is automatically turned on;

[0034] (3) Start the powder exhaust fan;

[0035] (4) Start the fine powder separator;

[0036] (5) Open the powder feeder outlet valve;

[0037] (6) Start the powder feeder;

[0038] (7) The second valve opening is set to 100%;

[0039] (8) The first valve opening is set to 0;

[0040] (9) The third valve is put into automatic operation;

[0041] (10) Sequential control completed;

[0042] Step 2: Powder silo system stops sequential control:

[0043] Permitted conditions:

[0044] (1) No variable load instruction;

[0045] (2) The coal feeder's coal quantity is stable and matches the total fuel quantity instruction;

[0046] Stop sequence:

[0047] (1) The second valve opening is set to 0;

[0048] (2) The first valve opening is set to 100%;

[0049] (3) Stop the powder feeder;

[0050] (4) The third valve is manually switched on and the opening is set to 0;

[0051] (5) Stop the fine powder separator;

[0052] (6) Stop the powder exhaust fan;

[0053] (7) Close the hot gas recirculation damper;

[0054] (8) Close the exhaust gas to burnout air nozzle valve;

[0055] (9) Sequential control is completed.

[0056] Furthermore, the powder level control method of the pulverized coal silo is as follows: under stable operating conditions, when the powder level feedback is lower than the lower limit of the powder level, powder is stored by controlling the opening of the second valve. It is ensured that there is sufficient powder silo storage margin to respond to the entire load change process of the unit before the load needs to be changed and the powder silo is switched to supply powder. The specific control logic is that under stable load conditions, the three pulverized coal silo powder level measurement points obtain the pulverized coal silo powder level feedback through the median selector, and the difference between it and the powder level setting value is obtained through a broken line function to obtain the opening instruction of the second valve, which is transmitted to the actuator after rate limiting. If the powder silo powder level feedback value is higher than the set upper limit value, the second valve opening instruction is switched to a constant value of 0, that is, the second valve is closed.

[0057] Furthermore, if there is no excess output to store pulverized coal in the pulverized coal bin due to the maximum output limit of the working coal mill, the pulverized coal storage operation can be carried out on the non-working coal mill.

[0058] Furthermore, the feeder coal quantity control method is as follows: the feeder coal quantity, after a delay, is added to the hot air recirculation coal quantity to obtain the mill outlet coal quantity; the mill outlet coal quantity is multiplied by the correction coefficient formed by the opening of the second valve to obtain the mill outlet coal quantity to the primary air pulverized coal pipe. If the pulverized coal feeder is operating automatically under variable load, this multiplication coefficient is not required; the difference between the mill outlet coal quantity to the primary air pulverized coal pipe and the mill outlet coal quantity is multiplied by the fine powder separator separation coefficient to obtain the exhaust gas pulverized coal quantity; the exhaust gas pulverized coal quantity is multiplied by the correction coefficient formed by the exhaust gas deburnout air nozzle valve and the hot gas recirculation damper opening to obtain the exhaust gas deburnout air nozzle pulverized coal quantity; the difference between the exhaust gas pulverized coal quantity and the exhaust gas deburnout air nozzle pulverized coal quantity is the hot air recirculation coal quantity. The sum of the mill outlet coal quantity to the primary air pulverized coal pipe, the exhaust gas deburnout air nozzle coal quantity, and the fuel oil equivalent coal quantity is multiplied by the coal quality correction coefficient to obtain the actual total fuel quantity. The total fuel quantity instruction from the boiler master control is used as the set value to compare with the actual fuel quantity. The deviation is processed by PID calculation and manual / automatic station to form the fuel master control instruction for parallel control of the coal feeder speed. Taking into account the different number of coal feeders put into automatic operation, resulting in different control gains of the control loop, the number of automatic coal feeders is passed through two different broken line functions to obtain different proportional gains and integral times of the PID controller.

[0059] Furthermore, the pulverizer feeder speed control method is as follows: the feeder coal quantity, after a delay, is added to the hot air recirculation coal quantity to obtain the mill outlet coal quantity. The mill outlet coal quantity is multiplied by a correction coefficient formed by the valve opening relationship to obtain the mill outlet coal quantity for the primary air pulverized coal pipe, the exhaust gas pulverized coal quantity for the overburning air nozzle, and the hot air recirculation coal quantity. The calculations for this are similar to those for the feeder coal quantity control loop above. The sum of the mill outlet coal quantity for the primary air pulverized coal pipe, the exhaust gas pulverized coal quantity for the overburning air nozzle, the feeder coal quantity, and the fuel oil equivalent coal quantity is multiplied by the coal quality correction coefficient to obtain the actual fuel quantity for the burner. The total fuel command is used as the set value, compared with the actual fuel quantity, and then subjected to PID calculation and speed limiting to obtain the pulverizer feeder speed command. Similarly, to account for the varying number of automatic pulverizer feeders, the proportional gain and integral time of the PID controller need to be adjusted. When the pulverizer silo system removes the sequential control command or the pulverized coal silo level reaches the lower limit, the pulverizer feeder speed command is switched to 0.

[0060] Furthermore, the primary air volume control in the primary air powder pipe is achieved by adjusting the third valve. The control method is as follows: the sum of the coal volume going to the primary air powder pipe at the pulverizer outlet and the coal feeder is converted into the primary air volume demand set value of the primary air powder pipe through a function converter, and then compared with the actual air volume of the primary air powder pipe, and then the third valve opening instruction is obtained after PID calculation, manual-automatic switcher, and speed limit. In order to improve the valve opening response speed, the first valve opening feedback is used as the feedforward of the PID controller.

[0061] Another object of the present invention is to provide a computer device, which includes a memory and a processor, wherein the memory stores a computer program, and when the computer program is executed by the processor, the processor executes the steps of the method for controlling the pulverizing system of the coal-fired unit operating at a high variable load rate.

[0062] Another object of the present invention is to provide a computer-readable storage medium storing a computer program. When the computer program is executed by a processor, the processor executes the steps of the method for controlling a pulverizing system of a coal-fired unit operating at a high variable load rate.

[0063] Another object of the present invention is to provide an information data processing terminal, which is used to implement the pulverizing system of the coal-fired unit with high variable load rate operation.

[0064] In combination with the above technical solutions and the technical problems solved, the advantages and positive effects of the technical solutions to be protected by the present invention are as follows:

[0065] First, compared to the existing technology, the present invention has the following beneficial effects: it achieves decoupling of the pulverizer and boiler in the pulverizing system under variable load conditions. On the one hand, compared to the direct-fired pulverizing system, this system directly responds to changes in boiler fuel demand by changing the pulverizer speed, saving the time required to transport raw coal to the feeder outlet and the time required to grind pulverized coal in the pulverizer, significantly improving the pulverizing system's ability to respond to boiler load changes. On the other hand, during the unit's load variation, the pulverizer's start and stop operations do not affect the working state of the pulverized coal bin. The pulverized coal bin and pulverizer respond to the pulverized coal supply throughout the load variation process, effectively avoiding the impact of the pulverizer's start and stop on boiler load changes and achieving flexible operation of the coal-fired unit. At the same time, because this system only serves the unit's variable load conditions, the required capacity is very small, making it feasible to apply the mid-storage pulverizing method to large-capacity units (300MW and above).

[0066] Second, the pulverizing system and control method for high variable load rate operation of coal-fired units provided by the present invention can improve the variable load response capability of the pulverizing system, and effectively solve the problem that the direct-blowing pulverizing system cannot perform large-scale high-rate variable load operation of the unit due to the start-stop mill operation.

[0067] Third, currently, the calculation method for the dynamic process variable load rate of coal-fired units is

[0068]

[0069] Where: V N is the load change rate of the unit during the transient process, % / min, P0, P1, and P2 are the rated power, initial power, and target power of the unit respectively, in MW; Δτ is the time of the load change transient process, in min.

[0070] In this method, P1 and P2 do not involve changes in the number of coal mills in operation, that is, the load rate measurement for the load change process of the coal-fired unit is based on the load range between the start and stop points of the coal mill. However, in reality, for the direct-blowing pulverizing system, when the unit changes load over a wider load range, due to the output limitation of the coal mill itself, it is necessary to go through the grinding group switching process, which will limit the coal-fired unit from achieving high load rate operation. In order to achieve the true load rate of the coal-fired unit, that is, the total load rate within any load range, the present invention decouples the coal mill from the boiler, and switches the direct-blowing pulverizing system to a central storage pulverizing system with hot air powder feeding during the load change process, so that the unit can operate at a high load rate within any load range and is no longer affected by the start and stop of the coal mill. At the same time, the present invention directly supplies powder to the boiler by adjusting the speed of the powder feeder, which effectively improves the responsiveness of the pulverizing system to changes in boiler load.

[0071] Fourth, the significant technological advancements brought about by each claim are as follows:

[0072] 1. Improvement of infrastructure and processes:

[0073] This system introduces specific pulverized coal handling and storage structures, such as fine powder separators, pulverized coal bins, multi-way connections and controls, so that coal-fired units can handle and supply pulverized coal more flexibly and efficiently when operating at high variable load rates.

[0074] The recycling of fine powder and the utilization of exhaust gas are increased, thus improving the energy efficiency of the system.

[0075] 2. Innovation in front and rear wall layout and optimization of coal powder handling:

[0076] By arranging different fine powder separators and pulverized coal bins on the front and rear walls, the handling, storage and distribution of pulverized coal are made more uniform and efficient, and the stability of the system is enhanced.

[0077] The setting of multiple coal hoppers and pulverized coal feeders realizes precise control of coal pulverization supply, ensuring that the coal pulverization supply can meet the demand under different operating conditions.

[0078] 3. Optimize coal powder accumulation and storage strategies:

[0079] According to the changes in unit load, the effective capacity of the pulverized coal bin is optimized to ensure that the system can effectively respond to the storage and supply needs of pulverized coal during the load change process.

[0080] A working strategy for the pulverized coal bunker when the unit changes load is proposed, making the system more efficient and stable when the load changes.

[0081] In summary, each claim proposes an innovative solution to the problems that may be encountered when coal-fired units operate at high variable load rates, making the handling, storage and supply of pulverized coal more flexible, efficient and stable, thereby improving the operating efficiency and stability of the coal-fired units. BRIEF DESCRIPTION OF THE DRAWINGS

[0082] In order to more clearly illustrate the technical solutions of the embodiments of the present invention, the following briefly introduces the drawings required for use in the embodiments of the present invention. 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 any creative work.

[0083] Figure 1 This is a schematic diagram of the connection structure of a single coal mill, a pulverized coal bin, and a burner provided by an embodiment of the present invention;

[0084] Figure 2 Schematic diagram of the connection structure between the pulverized coal bin and different coal mills provided in an embodiment of the present invention;

[0085] Figure 3 This is a coal pulverized bin powder level control logic diagram provided by an embodiment of the present invention;

[0086] Figure 4 This is a coal feeder coal quantity control logic diagram provided by an embodiment of the present invention;

[0087] Figure 5 This is a logic diagram for controlling the coal feeding amount and primary air volume of a pulverized coal feeder provided in an embodiment of the present invention;

[0088] Figure 6 This is a load reduction test data diagram provided by an embodiment of the present invention, wherein the powder supply rate of change is 0.4t / h / s;

[0089] Figure 7 This is a load-raising test data diagram provided by an embodiment of the present invention, wherein the powder supply rate of change is 0.4 t / h / s;

[0090] In the figure: 1. Coal feeder; 2. Gate; 3. Pulverized coal separator; 4. Coal mill; 5. Pulverized coal distributor; 6. Fine powder separator; 7. Pulverized coal silo; 8. Pulverized coal feeder; 9. Pulverized coal mixer; 10. Moisture absorption pipe; 11. Pulverized coal exhaust fan; 12. Burnout air nozzle; 13. Burner; 14. Boiler; 15. Three-compartment rotary air preheater; 16. Primary fan; 17. First air lock; 18. Second air lock; 19. First explosion-proof door; 20. Second explosion-proof door; 21. Exhaust gas bellows; 22. Primary bellows; 23. First valve; 24. Second valve; 25. Exhaust gas to burnout air nozzle valve; 26. Hot gas recirculation damper; 27. Third valve; 28. Fourth valve; 29. ​​Fifth valve; 30. Sixth valve; 31. Seventh valve; 32. Eighth valve. DETAILED DESCRIPTION

[0091] In order to make the purpose, technical solutions and advantages of the present invention more clearly understood, the present invention is further described in detail below in conjunction with the embodiments. It should be understood that the specific embodiments described herein are only used to explain the present invention and are not intended to limit the present invention.

[0092] In view of the problems existing in the prior art, the present invention provides a pulverizing system and control method for a coal-fired unit operating at a high variable load rate. The present invention is described in detail below with reference to the accompanying drawings.

[0093] Taking a 660MW ultra-supercritical unit with a variable load rate of 5%Pe / min as an example, the mill switching plan for the unit under different load conditions is as follows: between 100% and 75% THA, keep five mills in operation; between 75% and 50% THA, keep four mills in operation; between 50% and 40% THA, keep three mills in operation; and below 30% THA, keep two mills in operation. According to this plan, under variable load conditions with a variable load rate of 5%Pe / min, the ideal time to reduce the load from 100% THA to 20% THA is only 16 minutes. Within 16 minutes, the three upper mills must be shut down, and after 16 minutes, all system parameters must be stable. This requires that one mill be shut down every 5 minutes and that the air and powder gas levels must stabilize quickly. However, the control level of the coal mills is insufficient, and the boiler operator is still required to control the start and stop timing of the grinding group. Therefore, the existing pulverizing system's response to the unit's 5%Pe / min load rate is largely limited by the start and stop of the grinding group. In fact, in order for the unit to achieve a 5%Pe / min load rate, the pulverizing system's pulverizing rate must change more rapidly. Preliminary simulation verification shows that the pulverizing system's pulverizing rate must change by more than 0.4t / h / s to achieve a 5%Pe / min load rate. This is equivalent to starting and stopping three grinding mills within 8 minutes, which is unattainable with the existing pulverizing system. Therefore, to avoid the impact of the pulverizer's start and stop operation on the combustion side, the present invention combines the characteristics of a direct-blowing pulverizing system and a central storage pulverizing system. When the unit needs to change load, the pulverizer output is completely cut to the pulverizing bin pipeline, and the pulverizing bin supplies pulverized coal to the boiler through the pulverizing feeder.

[0094] refer to Figure 1 The pulverizing system of the present invention, which is suitable for high-variable load rate operation of a coal-fired unit, includes a coal feeder 1, a gate plate 2, a pulverized coal separator 3, a coal mill 4, a pulverized coal distributor 5, a fine powder separator 6, a pulverized coal bin 7, a pulverized coal feeder 8, a pulverized coal mixer 9, a moisture absorption pipe 10, a pulverized coal exhaust fan 11, a burnt air nozzle 12, a burner 13, a boiler 14, a three-compartment rotary air preheater 15, a primary fan 16, a first air lock 17, a second air lock 18, a first explosion-proof door 19, a second explosion-proof door 20, an exhaust gas wind box 21, a primary wind box 22, a first valve 23, a second valve 24, an exhaust gas to burnt air nozzle valve 25, a hot gas recirculation damper 26, a third valve 27, a fourth valve 28, a fifth valve 29, a sixth valve 30, a seventh valve 31, and an eighth valve 32.

[0095] The coal feeder 1 is connected to the inlet of the coal mill 4 through a powder dropping pipe. There are a gate 2 and a first air lock 17 on the powder dropping pipe. The coal mill 4 is connected to the coal powder separator 3. The outlet of the coal powder separator 3 is divided into two paths through the coal powder distributor 5: one path is connected to the burner 13 through the first valve 23 and the coal powder mixer 9; the other path is connected to the fine powder separator 6 through the second valve 24 and the moisture absorption pipe 10. A first explosion-proof door 19 is provided at the upper end of the fine powder separator 6, and the lower end of the fine powder separator 6 is connected to the pulverized coal bin 7 through the second gas lock 18. The pulverized coal bin 7 has a moisture absorption pipe 10 connected to the inlet pipe of the fine powder separator 6. The outlet of the upper end of the fine powder separator 6 is connected to the pulverized coal fan 11. The outlet of the pulverized coal fan 11 is divided into two routes: one route is to the inlet powder drop pipe of the pulverizer 5, which is a recycling pipe, and the baffle on the pipe is the hot gas recycling damper 26; the other route is distributed by the exhaust gas wind box 21 and then connected to the burnout air nozzle 12 through the exhaust gas to burnout air nozzle valve 25. The lower end of the pulverized coal bin 7 is connected to the pulverized coal feeder 8, and the pulverized coal feeder 5 is connected to the pulverized coal mixer 9. The outlet of the primary fan 16 is divided into two routes, one route passes through the three-compartment air preheater 15, and then splits into two routes, one route passes through the fourth valve 28 to connect to the primary wind box 22, and after distribution, passes through the third valve 27 to connect to the first valve 23 and the middle section of the coal powder mixer 9, and the other route passes through the sixth valve 30 and the eighth valve 32 to connect to the pulverizer 4; the other route of the primary fan 16 outlet does not pass through the three-compartment air preheater 15, and is split into two routes, one route passes through the fifth valve 29 to connect to the primary wind box 22, and the other passes through the seventh valve 31 to connect to the middle section of the pipeline between the sixth valve 30 and the eighth valve 32.

[0096] like Figure 1 As shown, the pulverizing system for coal-fired units operating at high variable load rates provided by the embodiment of the present invention has the following specific working process:

[0097] Under stable operating conditions, second valve 24 and third valve 27 are closed, fine powder separator 6, pulverized coal exhaust fan 11, and pulverized coal feeder 8 are inoperative, and the electric isolation valve of the moisture absorption pipe of pulverized coal bin 7 is closed. First valve 23 remains open, and the pulverized coal ground in pulverizer 4 is directly fed into burner 13 for combustion via hot primary air.

[0098] When a high-rate load change command is issued, the air-coal mixture at the outlet of the pulverized coal separator 5 is completely switched to the fine powder separator 6 at a constant rate. The required fuel for the boiler 14 is supplied from the pulverized coal silo 7 via the pulverized coal feeder 8, using hot air delivery. The exhaust gas separated by the fine powder separator 6 is discharged by the pulverized coal exhaust fan 11. A portion of the exhaust gas passes through the hot gas recirculation damper 26 and enters the coal drop pipe at the outlet of the coal feeder 1. The remaining portion is distributed by the exhaust gas wind box 21 and enters the various burnout air nozzles 12.

[0099] like Figure 2As shown, the embodiment of the present invention provides a pulverizing system for a coal-fired unit operating at a high variable load rate, wherein the boiler furnace is an opposed combustion furnace, which is divided into a front wall and a rear wall, each wall having 3 layers of burners and 2 layers of burnout air nozzles, 6 burners per layer, a total of 36 burners, and 24 burnout air nozzles.

[0100] There are 6 coal mills, namely A mill, B mill, C mill, D mill, E mill and F mill. Among them, F mill is a spare coal mill. The powder outlet pipes of A mill, B mill and C mill are respectively connected to the lower, middle and upper layers of the front wall of the boiler furnace; the powder outlet pipes of D mill, E mill and F mill are respectively connected to the lower, middle and upper layers of the rear wall of the boiler furnace.

[0101] There are two fine powder separators and two pulverized coal silos, and they are located on the front and back walls respectively. Another route of the powder outlet pipeline of mills A, B and C is connected to the fine powder separator on the front wall. The lower end of the fine powder separator is connected to the pulverized coal silo on the front wall, and the upper end is connected to the burnout air nozzles on the front wall after distribution through the exhaust gas wind box; another route of the powder outlet pipeline of mills D, E and F is connected to the fine powder separator on the back wall. The lower end of the fine powder separator is connected to the pulverized coal silo on the front wall, and the upper end is connected to the burnout air nozzles on the back wall after distribution through the exhaust gas wind box.

[0102] There are 18 coal hoppers at the lower end of the pulverized coal bin, each of which is connected to a pulverized coal feeder. There are a total of 18 pulverized coal feeders, which are connected to 18 pulverized coal mixers respectively.

[0103] The control method of the pulverizing system of a coal-fired unit operating at a high variable load rate provided by the embodiment of the present invention controls the switching process of the pulverizing silo system in a sequential control manner, specifically: the pulverizing silo system startup sequence control:

[0104] Permitted conditions:

[0105] 1. Variable load instruction

[0106] 2. The fine powder separator, powder feeder and powder exhaust fan stop working. 3. The powder level in the pulverized coal bin is normal.

[0107] 4. The second valve 24 is fully closed

[0108] 5. The third valve 27 is fully closed

[0109] 6. Hot gas recirculation damper 26 is fully closed

[0110] 7. The first valve 23 is fully open

[0111] Start the sequential control sequence:

[0112] 1. Open the exhaust gas to burnout air nozzle valve 25

[0113] 2. Hot air recirculation damper 26-position automatic

[0114] 3. Start the powder exhaust fan

[0115] 4. Start the fine powder separator

[0116] 5. Open the powder feeder outlet valve

[0117] 6. Start the powder feeder

[0118] 7. The opening of the second valve 24 is set to 100%

[0119] 8. The opening of the first valve 23 is set to 0

[0120] 9. The third valve 27 is put into automatic

[0121] 10. Sequential control completed

[0122] Powder silo system stop sequence control:

[0123] Permitted conditions:

[0124] 1. No variable load instruction

[0125] 2. The coal feeder's coal supply is stable and matches the total fuel quantity instruction stop sequence:

[0126] 1. The opening of the second valve 24 is set to 0

[0127] 2. The opening of the first valve 23 is set to 100%

[0128] 3. Stop the powder feeder

[0129] 4. The third valve 27 is switched to manual mode and the opening is set to 0.

[0130] 5. Stop the fine powder separator

[0131] 6. Stop the powder exhaust fan

[0132] 7. Close the hot gas recirculation damper 26

[0133] 8. Close the exhaust gas to burnout air nozzle valve 25

[0134] 9. Sequential control completed

[0135] like Figure 3As shown, the control method of the pulverizing system of the coal-fired unit operating at a high variable load rate provided by the embodiment of the present invention, the pulverized coal bin powder level control scheme is: under stable working conditions, when the pulverized coal level feedback is lower than the lower limit of the pulverized coal level, the pulverized coal level is stored by controlling the opening of the second valve 24. It is ensured that there is sufficient pulverized coal bin storage margin to respond to the entire load change process of the unit before the load needs to be changed and the pulverized coal bin is switched to supply pulverized coal. The specific control logic is that under stable load conditions, the three pulverized coal bin powder level measurement points obtain the pulverized coal bin powder level feedback through the median selector, and the difference between the pulverized coal bin powder level feedback and the pulverized coal bin powder level setting value is obtained through a broken line function to obtain the opening instruction of the second valve 24, which is transmitted to the actuator after rate limiting. If the pulverized coal bin powder level feedback value is higher than the set upper limit value, the opening instruction of the second valve 24 is switched to the constant value 0, that is, the second valve 24 is closed.

[0136] If there is no excess output to store pulverized coal in the pulverized coal bin 7 due to the maximum output limit of the working coal mill 4, the non-working coal mill can be activated to perform the pulverized coal storage operation.

[0137] like Figure 4 As shown, the control method of the pulverizing system of the coal-fired unit operating at a high variable load rate provided by the embodiment of the present invention has the following control logic for the coal feed amount of the coal feeder: the coal feed amount of the coal feeder is added to the hot air recirculation coal amount after a lag to obtain the coal amount at the pulverizer outlet; the coal amount at the pulverizer outlet is multiplied by the correction coefficient formed by the opening of the second valve 24 (if it is in the variable load and the pulverizer feeder is automatic, this coefficient does not need to be multiplied) to obtain the coal amount going to the primary air and pulverized coal pipe at the pulverizer outlet; the difference between the coal amount going to the primary air and pulverized coal pipe at the pulverizer outlet and the coal amount at the pulverizer outlet is multiplied by the separation coefficient of the fine powder separator to obtain the amount of pulverized coal carried in the exhaust gas; the amount of pulverized coal carried in the exhaust gas is multiplied by the correction coefficient formed by the opening of the exhaust gas de-burning air nozzle valve 25 and the hot gas recirculation damper 26 to obtain the pulverized coal amount of the exhaust gas de-burning air nozzle, and the difference between the pulverized coal amount in the exhaust gas and the pulverized coal amount in the exhaust gas de-burning air nozzle is the hot air recirculation coal amount. The actual total fuel quantity is calculated by multiplying the sum of the coal quantity flowing to the primary air pulverizer pipe at the mill outlet, the coal quantity flowing to the burnout air nozzle by the exhaust gas, and the fuel oil equivalent by the coal quality correction factor. The total fuel quantity command from the boiler master control is used as the setpoint and compared with the actual fuel quantity. The deviation is then processed through PID calculation and the manual / automatic switch to form the fuel master control command for parallel control of the feeder speed. Considering that the control loop gain varies depending on the number of feeders in automatic operation, the proportional gain and integral time of the PID controller are derived by applying two different broken line functions to the number of feeders in automatic operation.

[0138] like Figure 5As shown, the control method for a pulverizing system suitable for coal-fired units operating at high variable load rates, described in the present invention, includes the following control logic for the pulverizer speed: the feeder coal quantity, after a delay, is added to the hot air recirculation coal quantity to obtain the mill outlet coal quantity; the mill outlet coal quantity is multiplied by a correction coefficient formed by the opening relationship between the first valve 23 and the second valve 24 to obtain the mill outlet coal quantity for the primary air pulverizer pipe, the exhaust gas pulverized coal quantity for the overburning air nozzle, and the hot air recirculation coal quantity, as described above in the feeder coal quantity control loop. The sum of the mill outlet coal quantity for the primary air pulverizer pipe, the exhaust gas pulverized coal quantity for the overburning air nozzle, the pulverizer coal quantity, and the fuel oil equivalent coal quantity is multiplied by the coal quality correction coefficient to obtain the actual fuel quantity for the burner. The total fuel command is used as the set value, compared with the actual fuel quantity, and then subjected to PID calculation and speed limiting to obtain the pulverizer speed command. Similarly, considering the different number of automatic pulverizer feeders, the proportional gain and integral time of the PID controller need to be corrected. When the powder bin system cuts off the sequential control instruction or the powder level in the pulverized coal bin reaches the lower limit, the powder feeder speed instruction switches to 0.

[0139] like Figure 5 As shown, the control method of the pulverizing system of the coal-fired unit operating at a high variable load rate provided by the embodiment of the present invention, the primary air volume control in the primary air powder pipe is achieved by adjusting the third valve 27, and its control logic is: the sum of the coal volume going to the primary air powder pipe at the pulverizer outlet and the coal feeder is converted into the primary air volume demand set value of the primary air powder pipe by a function unit, and is compared with the actual air volume of the primary air powder pipe, and then the opening instruction of the third valve 27 is obtained after PID calculation, manual-automatic switcher, and speed limit. In order to improve the valve opening response speed, the opening feedback of the first valve 23 is used as the feedforward of the PID controller.

[0140] An application embodiment of the present invention provides a computer device, which includes a memory and a processor. The memory stores a computer program. When the computer program is executed by the processor, the processor executes the steps of a method for controlling a pulverizing system of a coal-fired unit operating at a high variable load rate.

[0141] An application embodiment of the present invention provides a computer-readable storage medium storing a computer program. When the computer program is executed by a processor, the processor executes the steps of a method for controlling a pulverizing system of a coal-fired unit operating at a high variable load rate.

[0142] An application embodiment of the present invention provides an information data processing terminal, which is used to realize a pulverizing system for a coal-fired unit operating at a high variable load rate.

[0143] The present invention is further described below in conjunction with the experimental process and effects of the present invention:

[0144] 1) To explore the extent to which the coal supply rate on the pulverizing system side can change so that the unit can achieve a 5% load change rate, a unit load change test was conducted on the constructed simulation unit model.

[0145] The test steps are:

[0146] ① Modify the boiler master control logic and add a speed limit module before generating the boiler master control command to specify the fuel command change rate;

[0147] ② Switch the boiler master control to manual mode, i.e., to turbine follower mode, while the other control circuits remain unchanged;

[0148] ③ Carry out load reduction and load increase tests respectively. The load reduction test uses the rated load of 665.5MW and the fuel volume of 272.5t / h as the initial operating conditions, and the load increase test uses the rated load of 344.4MW and the fuel volume of 145.9t / h as the initial operating conditions.

[0149] ④ In the load reduction test, the boiler master control command was set to 206t / h, and the rate of change was reduced at 0.2t / h / s, 0.4t / h / s, and 0.5t / h / s respectively. In the load increase test, the boiler command was set to 206t / h, and the rate of change was increased at 0.2t / h / s, 0.4t / h / s, and 0.5t / h / s respectively.

[0150] The load test curve for the coal supply rate change of 0.4t / h / s is shown in Figure 6-7. Figure 6 For load reduction test, the powder supply rate is changed at 0.4t / h / s; Figure 7 This is a load increase test, and the powder supply change rate is 0.4t / h / s.

[0151] The test results show that without any modification, in the process of small-scale load variation, the unit can only achieve a 5% load variation rate when the coal supply change rate reaches about 0.4t / h / s. Under the 100% THA stable operating condition of a certain 660MW supercritical unit, the required fuel amount (design coal type) is about 272t / h, and under the 20% THA stable operating condition, it is about 70t / h. If the rate changes at 0.4t / h / s, the change from 272t / h to 70t / h can be completed in less than 10 minutes. In the existing mill group operation plan, multiple mill stop operations are required to complete the change, so the start and stop operation of the mill group will seriously limit the unit's operation in a large range of high-speed (5%) load variations.

[0152] 2) Therefore, for the new pulverizing system with an additional small powder silo, its structural design and operation plan need to solve the coupling problem between the coal mill and the boiler during the variable load process. To this end, the present invention proposes to connect the powder silo system in parallel with the primary air-powder pipeline at the outlet of the coal mill, and adopt a direct-blowing pulverizing method under stable working conditions. Under variable load conditions, it switches to the powder silo to supply powder to the boiler, thereby realizing the decoupling of the coal mill and the boiler. The pulverizing system and control method proposed in the present invention have great advantages over the original medium-speed mill positive pressure direct-blowing pulverizing system.

[0153] Based on the above-mentioned method for controlling a pulverizing system for a coal-fired unit operating at a high variable load rate, two specific embodiments and implementation schemes thereof are provided below:

[0154] Example 1: Real-time monitoring based on sensors

[0155] 1. Implementation plan:

[0156] Sensors are installed at key locations such as the pulverized coal bin, fine powder separator, and coal feeder to monitor key parameters such as the storage volume, flow rate, and temperature of the pulverized coal in real time.

[0157] A central control system is used to aggregate the data collected by each sensor and display it in real time.

[0158] Based on real-time data, the opening degree of each valve and the operating status of the equipment are automatically adjusted to meet the real-time needs of the coal-fired unit.

[0159] When the sensor detects that the coal powder storage level is too low or the temperature is abnormal, the system will automatically send an alarm and take corresponding measures according to the preset program.

[0160] Example 2: Optimization based on Model Predictive Control (MPC)

[0161] 1. Implementation plan:

[0162] Using the historical operating data of the unit, a mathematical model of the pulverizing system is established to describe the dynamic relationship between its various parts.

[0163] The model predictive control algorithm is implemented in the central control system to predict the unit operating status in the future.

[0164] Based on the prediction results, the opening degree of each valve and the operating status of the equipment are optimized to achieve the best operating efficiency.

[0165] During actual operation, the system will continuously update model parameters based on real-time data to ensure the accuracy of the prediction.

[0166] The two embodiments respectively emphasize the importance of real-time monitoring and predictive optimization. In practical applications, the appropriate embodiment can be selected according to the specific situation or the two methods can be combined to achieve the best operating effect.

[0167] The above description is only a specific embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Any modifications, equivalent substitutions and improvements made by any technician familiar with this technical field within the technical scope disclosed by the present invention and within the spirit and principles of the present invention should be covered by the scope of protection of the present invention.

Claims

1. A pulverizing system for coal-fired units operating at high variable load rates, characterized in that: The system utilizes several fine powder separators, pulverized coal silos arranged on the front and rear walls, and 18 pulverized coal feeders connected to coal hoppers, providing a configuration for pulverized coal production, storage, and feeding for coal-fired units operating at high variable load rates. This ensures that during variable load operation, the air-pulverized coal mixture at the pulverizer outlet can be quickly transferred to the fine powder separator and orderly stored in the pulverized coal silo. Multiple pulverized coal feeders then precisely feed the mixture, meeting operational requirements under varying load conditions. include: The coal feeder is connected to the pulverizer inlet through a powder dropping pipe. There are gate plates and the first air lock on the powder dropping pipe. The pulverizer is connected to the coal powder separator. The coal powder separator outlet is divided into two paths through the coal powder distributor: one path is connected to the burner through the first valve and the coal powder mixer; the other path is connected to the fine powder separator through the second valve and the moisture absorption pipe; the upper end of the fine powder separator is provided with a first explosion-proof door, and the lower end of the fine powder separator is connected to the coal powder bin through the second air lock. The coal powder bin is connected to the fine powder separator inlet pipeline by the moisture absorption pipe, and the upper end outlet of the fine powder separator is connected to the powder exhaust fan. The powder exhaust fan outlet is divided into two paths: one path goes to the pulverizer inlet powder dropping pipe, which is a recirculation pipe, and the baffle on the pipe is for hot gas recirculation The exhaust gas outlet is divided into two routes, one route passes through the three-compartment air preheater, and then is divided into two routes, one route passes through the fourth valve to connect to the primary air box, and after distribution, it passes through the third valve to connect to the first valve and the middle section of the pulverized coal mixer pipeline, and the other route passes through the sixth valve and the eighth valve to connect to the pulverizer; the other route of the primary air fan outlet does not pass through the three-compartment air preheater, and is divided into two routes, one route passes through the fifth valve to connect to the primary air box, and the other route passes through the seventh valve to connect to the middle section of the pulverized coal mixer pipeline.

2. The pulverizing system for coal-fired units operating at high variable load rates according to claim 1, characterized in that: There are two fine powder separators and two pulverized coal silos, located on the front and rear walls respectively. Mills A, B, and C are connected to the boiler's front wall burner through one line, and to the front wall fine powder separator through the other line. The lower end of the fine powder separator is connected to the front wall pulverized coal silo, and the upper end is distributed through the exhaust gas windbox and then connected to the various burnout air nozzles on the front wall. Mills D, E, and F are connected to the boiler's rear wall burner through one line, and to the rear wall fine powder separator through the other line. The lower end of the fine powder separator is connected to the rear wall pulverized coal silo, and the upper end is distributed through the exhaust gas windbox and then connected to the various burnout air nozzles on the rear wall. There are 18 coal hoppers at the lower end of the pulverized coal bin, each of which is connected to a pulverized coal feeder. There are a total of 18 pulverized coal feeders, which are connected to 18 pulverized coal mixers respectively.

3. The pulverizing system for coal-fired units operating at high variable load rates according to claim 1, characterized in that: The total effective capacity of the powder silo is set according to the amount of fuel required by the boiler during the unit's load range of 20% to 100%.

4. A method for controlling a pulverizing system of a coal-fired unit operating at a high variable load rate, applied to the pulverizing system of a coal-fired unit operating at a high variable load rate according to any one of claims 1 to 3, characterized in that: Sequential control is used to accurately manage the switching process of the pulverized coal silo system. Through clear start and stop conditions and corresponding sequential control steps, it is ensured that the pulverizing system can efficiently and stably meet the needs of the coal-fired units under high variable load rate operation, while improving the safety and response speed of the system.

5. The method for controlling a pulverizing system of a coal-fired unit operating at a high variable load rate according to claim 4, wherein: The powder silo system feeding and cutting process is controlled in a sequential control mode, specifically: Step 1: Start the powder silo system control sequence: Permitted conditions: (1) Load change instruction comes; (2) The fine powder separator, powder feeder and powder exhaust fan stop working; (3) The pulverized coal level in the pulverized coal bin is normal; (4) The second valve is fully closed; (5) The third valve is fully closed; (6) The hot gas recirculation damper is fully closed; (7) The first valve is fully open; Start the sequential control sequence: (1) Open the exhaust gas to burnout air nozzle valve; (2) The hot gas recirculation damper is automatically turned on; (3) Start the powder exhaust fan; (4) Start the fine powder separator; (5) Open the powder feeder outlet valve; (6) Start the powder feeder; (7) The second valve opening is set to 100%; (8) The first valve opening is set to 0; (9) The third valve is put into automatic operation; (10) Sequential control completed; Step 2: Powder silo system stops sequential control: Permitted conditions: (1) No variable load instruction; (2) The coal feeder's coal quantity is stable and matches the total fuel quantity instruction; Stop sequence: (1) The second valve opening is set to 0; (2) The first valve opening is set to 100%; (3) Stop the powder feeder; (4) The third valve is manually switched on and the opening is set to 0; (5) Stop the fine powder separator; (6) Stop the powder exhaust fan; (7) Close the hot gas recirculation damper; (8) Close the exhaust gas to burnout air nozzle valve; (9) Sequential control is completed.

6. The method for controlling a pulverizing system of a coal-fired unit operating at a high variable load rate according to claim 5, wherein: The powder level control method of the pulverized coal silo is as follows: under stable operating conditions, when the powder level feedback is lower than the powder level lower limit, powder is stored by controlling the opening of the second valve; ensuring that there is sufficient powder silo storage margin to respond to the entire load change process of the unit before the load needs to be changed and the powder silo is switched to supply powder; the specific control logic is that under stable load conditions, the three pulverized coal silo powder level measurement points obtain the pulverized coal silo powder level feedback through the median selector, and the difference between it and the powder level set value is obtained through a broken line function to obtain the opening instruction of the second valve, which is transmitted to the actuator after rate limiting; if the powder silo powder level feedback value is higher than the set upper limit value, the second valve opening instruction is switched to a constant value of 0, that is, the second valve is closed.

7. The method for controlling a pulverizing system of a coal-fired unit operating at a high variable load rate according to claim 6, wherein: If there is no extra output to store pulverized coal in the pulverized coal bin due to the maximum output limit of the working pulverizer, the pulverized coal storage operation can be carried out by the non-working pulverizer; The method for controlling the coal feeding amount of the coal feeder is as follows: the coal feeding amount of the coal feeder is added to the hot air recirculation coal amount after a lag to obtain the coal feeding amount at the pulverizer outlet; the coal feeding amount at the pulverizer outlet is multiplied by the correction coefficient formed by the opening of the second valve to obtain the coal feeding amount going to the primary air powder pipe at the pulverizer outlet. If it is in a variable load and the pulverizer is automatic, it is not necessary to multiply by this coefficient; the difference between the coal feeding amount going to the primary air powder pipe at the pulverizer outlet and the coal feeding amount at the pulverizer outlet is multiplied by the separation coefficient of the fine powder separator to obtain the amount of pulverized coal in the exhaust gas; the amount of pulverized coal in the exhaust gas is multiplied by the correction coefficient formed by the opening of the exhaust gas to overburn air nozzle valve and the hot gas recirculation damper to obtain the amount of pulverized coal in the exhaust gas to overburn air nozzle. The difference between the amount of coal powder going to the overburned air nozzle with exhaust gas is the amount of hot air recirculation coal; the sum of the coal amount going to the primary air powder pipe at the mill outlet, the coal amount going to the overburned air nozzle with exhaust gas, and the coal amount converted to fuel oil is multiplied by the coal quality correction coefficient to obtain the actual total fuel amount; the total fuel amount instruction from the boiler master control is used as the set value to compare with the actual fuel amount, and the deviation is formed after PID calculation and manual / automatic station to form the fuel master control instruction for parallel control of the coal feeder speed. Taking into account the different number of coal feeders put into automatic operation, resulting in different control gains of the control loop, the number of automatic coal feeders is passed through two different broken line functions to obtain different proportional gains and integral times of the PID controller.

8. The method for controlling a pulverizing system of a coal-fired unit operating at a high variable load rate according to claim 7, wherein: The control method for the pulverizer speed is as follows: the coal feed rate of the pulverizer is added to the hot air recirculation coal rate after a delay to obtain the coal flow at the pulverizer outlet; the coal flow at the pulverizer outlet is multiplied by the correction coefficient formed by the relationship between the valve and the valve opening, and the coal flow to the primary air pulverized coal pipe at the pulverizer outlet, the pulverized coal flow to the overburning air nozzle with exhaust gas, and the hot air recirculation coal flow are obtained in the same way as the pulverizer feed rate control loop; the sum of the coal flow to the primary air pulverized coal pipe at the pulverizer outlet, the pulverized coal flow to the overburning air nozzle with exhaust gas, the pulverizer feed rate, and the fuel oil equivalent coal flow is multiplied by the coal quality correction coefficient to obtain the actual fuel flow to the burner; the total fuel command is used as the set value, compared with the actual fuel flow, and then subjected to PID calculation and speed limit to obtain the pulverizer speed command; also considering the different number of automatic pulverizer feeders, the proportional gain and integral time of the PID controller need to be corrected; when the pulverizer silo system cuts off the sequential control command or the pulverized coal silo powder level reaches the lower limit, the pulverizer speed command is switched to 0.

9. The method for controlling a pulverizing system of a coal-fired unit operating at a high variable load rate according to claim 8, wherein: The primary air volume control in the primary air powder pipe is achieved by adjusting the third valve. The control method is as follows: the sum of the coal volume going to the primary air powder pipe at the pulverizer outlet and the coal feeder is converted into the primary air volume demand set value of the primary air powder pipe through a function converter. After comparison with the actual air volume of the primary air powder pipe, the third valve opening instruction is obtained after PID calculation, manual-automatic switcher, and speed limit. In order to improve the valve opening response speed, the first valve opening feedback is used as the feedforward of the PID controller.

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