A start-stop method for a supercritical carbon dioxide cycle system

Abstract

The application provides a start-stop method of a supercritical carbon dioxide cycle system, and belongs to the technical field of power generation. The method comprises the following steps: the shutdown process of an S-CO2 Brayton cycle power generation system is mainly divided into three stages: the first stage is the process of reducing the system load from 100% to 10%, the load reduction of the system is realized by using valve storage coupling control, and the thermal efficiency of the system is maintained at a high value in a wide load range; the second stage is the process of reducing the system load from 10% to 0, the process is realized by reducing the inlet temperature of the turbine, so that the circulation flow can be ensured not to be reduced during the load reduction process, and the possible pipe explosion accident of the boiler during low load operation can be avoided; the third stage is the process of stopping each device, and the potential over-temperature pipe explosion risk of the gas cooling wall when the working medium flow in the boiler is too low and the cavitation erosion risk of the compressor when the minimum circulation pressure is lower than the critical pressure are considered. The method can ensure the safety and rapidity during the start-stop process of the system.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of power generation, and particularly relates to a start-stop method of a supercritical carbon dioxide cycle system. BACKGROUND

[0002] In recent years, a supercritical carbon dioxide Brayton cycle (SCBC) system has advantages of high efficiency, compact structure and wide heat source application range, and is considered as one of the most promising next-generation power conversion systems. As a closed cycle, the SCBC has a dramatic change in physical properties of the working medium at the near critical point. The start-stop process as an important transition stage needs to design a corresponding control strategy to ensure the safety and rapidity of the system in the start-stop process. SUMMARY

[0003] In order to overcome the deficiencies of the prior art, the application provides a start-stop method of a supercritical carbon dioxide cycle system.

[0004] In order to achieve the above-mentioned purpose, the application provides the following technical scheme:

[0005] A start-stop method of a supercritical carbon dioxide cycle system, comprising:

[0006] When the supercritical carbon dioxide cycle system is reduced from 100% load to 10% load:

[0007] If the generator speed is greater than 3000 rpm, the opening of the turbine inlet valve V20 towards the turbine side is reduced; if the generator speed is less than 3000 rpm, the opening of the turbine inlet valve V20 towards the turbine side is increased; when the opening of the turbine inlet valve is less than 80%, the CO2 discharge valve V17 is opened and the CO2 charging valve V16 is closed, and the opening of the turbine inlet valve is increased; when the opening of the turbine inlet valve is greater than 80%, the CO2 charging valve V16 is opened and the CO2 discharge valve V17 is closed, and the opening of the turbine inlet valve is reduced;

[0008] When the supercritical carbon dioxide cycle system is reduced from 10% load to 0 load:

[0009] The variable load of the supercritical carbon dioxide cycle system is adjusted by reducing the inlet temperature of the turbine;

[0010] When the equipment of the supercritical carbon dioxide cycle system is closed:

[0011] When the inlet temperature of the turbine is below 100℃, the turbine inlet valve V20 is opened to connect the bypass valve V21, and closed to connect the turbine; the bypass valve V6 is opened, the boiler is stopped when the flow rate of the working medium through the boiler is less than 30% of the rated flow rate, the opening of the three-way valve V4 through the boiler is reduced under the premise that the reduction speed of the boiler heat load is greater than the reduction speed of the flow rate of the working medium through the boiler; after the boiler is completely stopped, the opening of the three-way valve V4 through the boiler is reduced to 0, and the bypass valve V6 is adjusted to ensure that the minimum circulating pressure of the supercritical carbon dioxide cycle system is 8MPa.

[0012] The re-compressor and the main compressor in the supercritical carbon dioxide cycle system are closed.

[0013] Further, the closing of the re-compressor and the main compressor comprises:

[0014] The speed of the re-compressor is reduced, and the three-way valve V1 is opened to connect the three-way valve V2, and the speed of the re-compressor is reduced to 0.

[0015] The speed of the main compressor is reduced, and the CO2 discharge valve V17 is opened; when the speed of the main compressor is reduced to 0, the system inventory working medium is exhausted from the CO2 discharge valve V17, and the protective gas is filled from the CO2 charging valve V16.

[0016] Further, when the speed of the re-compressor is reduced, the compressor speed reduction rate and the opening of the backflow valve V8 are adjusted to prevent the re-compressor from surging.

[0017] When the speed of the main compressor is reduced, the speed reduction rate of the main compressor, the opening of the valve V17, and the opening of the backflow valve V13 are controlled to prevent the main compressor from surging.

[0018] Further, when the supercritical carbon dioxide cycle system is loaded, if the circulation split coefficient of the supercritical carbon dioxide cycle system is less than 0.318, the speed of the main compressor is reduced.

[0019] If the circulation split coefficient of the supercritical carbon dioxide cycle system is greater than 0.318, the speed of the main compressor is increased.

[0020] Further, when the supercritical carbon dioxide cycle system is loaded:

[0021] If the actual pressure value at the inlet of the main compressor is not equal to the set pressure value at the inlet of the main compressor, the re-compressor inlet pressure deviation signal is input to the outer loop controller PI2, and the re-compressor flow rate set value is obtained by using the outer loop controller PI2.

[0022] The actual value of the re-compressor flow rate is measured, and the re-compressor flow rate deviation signal is obtained according to the actual value of the re-compressor flow rate and the re-compressor flow rate set value.

[0023] The deviation signal of the re-compressor flow is input to the inner loop controller PI3, and the re-compressor speed is controlled by the inner loop controller PI3, so that the main compressor inlet pressure is controlled at 7.6 MPa by the cascade control system of the inner and outer loops.

[0024] Further, when the supercritical carbon dioxide cycle system is started:

[0025] The system protection gas is pumped from the CO2 discharge valve V17, and the CO2 charging valve V16 is opened;

[0026] The main compressor is started and the main compressor speed is increased, and the low-temperature regenerator and the pre-cooler are started;

[0027] After the main compressor, the low-temperature regenerator and the pre-cooler are stably operated, the re-compressor is started, and the speed of the re-compressor is gradually increased;

[0028] When the re-compressor outlet pressure is higher than the main compressor outlet pressure, the opening of the three-way valve V1 is changed to transfer the CO2 at the re-compressor outlet from the three-way valve V2 to the three-way valve V3, and the opening of the bypass valve V6 is adjusted to ensure that the compressor inlet pressure is 8 MPa;

[0029] The speed of the main compressor and the re-compressor is increased, and when the circulation flow of the supercritical carbon dioxide cycle system is greater than 30% of the rated flow, the three-way valve V4 is opened, and the heat load of the boiler is increased;

[0030] When the turbine inlet working medium temperature reaches 100℃ or above, the bypass valve V21 is closed.

[0031] Further, when the supercritical carbon dioxide cycle system is started from 0 load to 10% load:

[0032] If the generator speed is greater than 3000 rpm, the opening of the turbine inlet valve V20 is reduced;

[0033] If the generator speed is less than 3000 rpm, the opening of the turbine inlet valve V20 is increased;

[0034] When the opening of the turbine inlet valve is less than 80%, the CO2 discharge valve is opened and the CO2 charging valve is closed, and the opening of the turbine inlet valve is increased;

[0035] When the opening of the turbine inlet valve is greater than 80%, the CO2 charging valve is opened and the CO2 discharge valve is closed, and the opening of the turbine inlet valve is reduced;

[0036] When the load of the supercritical carbon dioxide cycle system is full load, the inlet temperature control target of the turbine is set to 600℃; wherein the temperature rise rate of the pipe wall is 10℃ per minute.

[0037] The supercritical carbon dioxide cycle system start-stop method has the following beneficial effects:

[0038] The S-CO2 Brayton cycle power generation system shutdown process of the application mainly includes three stages: the first stage is a load following process in which the system is reduced from 100% load to 10% load, the load of the system is reduced by using valve storage coupling control system, and the heat efficiency of the system is maintained at a high value in a wide load range; the second stage is a load reduction process in which the system is reduced from 10% load to 0, and the process is realized by reducing the inlet temperature of the turbine, so that the circulation flow is not reduced during the load reduction process, and the possible pipe explosion accident of the boiler during low load operation is avoided; the third stage is the process of stopping each device, which considers the potential over-temperature pipe explosion risk of the gas cooling wall when the working medium flow in the boiler is too low and the cavitation risk of the compressor when the minimum pressure of the cycle is lower than the critical pressure. The problem that the start-stop process control of the S-CO2 Brayton cycle system in the prior art cannot consider safety and rapidity is solved. BRIEF DESCRIPTION OF DRAWINGS

[0039] In order to more clearly illustrate the embodiments of the application and the design scheme thereof, the drawings required by the embodiments will be briefly introduced as follows. The drawings in the following description are only part of the embodiments of the application, and other drawings can be obtained by those skilled in the art without creative labor on the basis of these drawings.

[0040] Figure 1 It is a schematic diagram of the supercritical carbon dioxide cycle system of the application;

[0041] Figure 2 It is a schematic diagram of the supercritical carbon dioxide cycle system of the application;

[0042] Figure 3 It is a schematic diagram of the system split coefficient and minimum pressure control strategy of the application;

[0043] Figure 4 It is a schematic diagram of the valve storage coupling variable load control strategy of the application;

[0044] Figure 5 It is a schematic diagram of the S-CO2 Brayton cycle system shutdown control strategy of the application;

[0045] Figure 6 It is a schematic diagram of the S-CO2 Brayton cycle system main compressor inlet temperature and pressure control strategy in the embodiments of the application;

[0046] Figure 7 It is a schematic diagram of the S-CO2 Brayton cycle system main and re-compressor anti-surge control strategy in the embodiments of the application. DETAILED DESCRIPTION

[0047] In order to better understand the technical solutions of the present application and to be able to implement them, the present application will be described in detail below in conjunction with the accompanying drawings and specific embodiments. The following embodiments are only used to more clearly illustrate the technical solutions of the present application, and cannot be used to limit the protection scope of the present application.

[0048] Embodiments:

[0049] The present application provides a start-stop method of a supercritical carbon dioxide cycle system, specifically as shown in the accompanying drawings, the system of the present application comprises: Figures 1-5 The system comprises:

[0050] A CO2 storage tank, whose output end is connected with the input ends of a pre-cooler and a re-compressor; the pre-cooler, whose output end is connected with the input end of a main compressor; the main compressor, whose output end is connected with the input end of the CO2 storage tank; a low-temperature regenerator, whose first input end is connected with the output end of the main compressor; a high-temperature regenerator, whose first input end is connected with the first output end of the low-temperature regenerator and the output end of the re-compressor; whose first output end is connected with the input end of a boiler; the boiler, whose input end is connected with the first output end of the high-temperature regenerator; a turbine, whose input end is connected with the output end of the boiler; whose output end is connected with the second input end of the high-temperature regenerator; the second output end of the high-temperature regenerator is connected with the second input end of the low-temperature regenerator; the second output end of the low-temperature regenerator is connected with the input ends of the pre-cooler and the re-compressor; a backflow valve V8, whose two ends are respectively connected with the input and output ends of the re-compressor; a turbine inlet valve V20, which is arranged on the connecting pipeline between the boiler and the turbine; a bypass valve V21, whose two ends are connected with the turbine inlet valve V20 and the output end of the turbine.

[0051] A CO2 discharge valve V17, which is arranged on the input end of the CO2 storage tank.

[0052] A CO2 charging valve V16, which is arranged on the output end of the CO2 storage tank.

[0053] A three-way valve V4 and a three-way valve V3 connected in series, which are arranged on the connecting pipeline between the first input end of the high-temperature regenerator and the first output end of the low-temperature regenerator.

[0054] A three-way valve V2 and a three-way valve V5 connected in series, which are arranged on the connecting pipeline between the second output end of the high-temperature regenerator and the second input end of the low-temperature regenerator.

[0055] A bypass valve V6, whose two ends are connected with the three-way valve V4 and the three-way valve V5.

[0056] A three-way valve V1, whose three ends are respectively connected with the three-way valve V2, the three-way valve V3 and the output end of the re-compressor.

[0057] The start-stop method of the application comprises: when the supercritical carbon dioxide cycle system is reduced from 100% load to 10% load: if the generator speed is greater than 3000 rpm, reducing the opening of turbine inlet valve V20 towards the turbine side; if the generator speed is less than 3000 rpm, increasing the opening of turbine inlet valve V20 towards the turbine side; when the opening of turbine inlet valve is less than 80%, opening CO2 discharge valve V17 and closing CO2 charging valve V16, increasing the opening of turbine inlet valve; when the opening of turbine inlet valve is greater than 80%, opening CO2 charging valve V16 and closing CO2 discharge valve V17, reducing the opening of turbine inlet valve; when the supercritical carbon dioxide cycle system is reduced from 10% load to 0 load: adjusting the variable load of the supercritical carbon dioxide cycle system by reducing the inlet temperature of the turbine; when the equipment of the supercritical carbon dioxide cycle system is closed: waiting for the inlet temperature of the turbine to be reduced to below 100 DEG C, opening the turbine inlet valve V20 connected to the bypass valve V21 side, and closing the turbine inlet valve V20 connected to the turbine side; opening the bypass valve V6, and when the flow rate of the working medium through the boiler is less than 30% of the rated flow rate, stopping the boiler to ensure that the reduction speed of the boiler heat load is greater than the reduction speed of the flow rate of the working medium through the boiler, and reducing the opening of the three-way valve V4 through the boiler side; after the boiler is completely stopped, reducing the opening of the three-way valve V4 through the boiler side to 0, and adjusting the opening of the bypass valve V6 to ensure that the minimum circulating pressure of the supercritical carbon dioxide cycle system is 8 MPa; closing the re-compressor and the main compressor in the supercritical carbon dioxide cycle system.

[0058] The following is the implementation details of the application:

[0059] 1S-CO2 Brayton cycle load following control strategy

[0060] In order to realize the fast, efficient, safe and stable load following of the S-CO2 Brayton cycle power generation system, the application designs a system minimum pressure control strategy, a system split coefficient control strategy, a valve inventory joint control strategy, a turbine inlet temperature control strategy, a system minimum temperature control strategy and a compressor anti-surge control strategy.

[0061] Among them, the turbine inlet temperature control strategy and the cycle minimum temperature control strategy are to ensure that the cycle efficiency is maintained at a high value during the system variable load process.

[0062] The cycle minimum pressure control strategy and the compressor surge control strategy are to ensure that the system can operate safely and stably during the variable load process.

[0063] The valve inventory joint control strategy is to realize the fast and efficient variable load of the system.

[0064] The system split coefficient control strategy, the cycle minimum pressure control strategy and the valve inventory joint control strategy will be described in detail below.

[0065] 1.1 System split coefficient and cycle minimum pressure control strategy:

[0066] The split coefficient of the S-CO2 recompression Brayton cycle system has a great influence on the cycle thermal efficiency. With the increase of the split ratio, the cycle efficiency presents a trend of first increasing and then decreasing, and there is an optimal split ratio for different pressure ratios. In this paper, the optimal split ratio corresponding to the design pressure ratio is taken as 0.318.

[0067] The system split coefficient control strategy is shown in Figure 3 to ensure that the system has high cycle efficiency during variable load process. Specifically:

[0068] When the cycle split coefficient is less than 0.318, the speed of the main compressor is reduced, so that the flow of the working medium through the main compressor is reduced, and the flow through the recompression machine is increased, thereby increasing the cycle split coefficient; otherwise, the speed of the main compressor is increased to reduce the cycle split coefficient. When the minimum cycle pressure, i.e. the inlet pressure of the main compressor, approaches the critical point pressure, the cycle can obtain high thermal efficiency.

[0069] In this paper, a cycle minimum pressure control strategy is designed to maintain the inlet pressure of the main compressor at 7.6 MPa. Specifically:

[0070] When the inlet pressure of the main compressor is not equal to the target value, the inlet pressure deviation signal of the recompression machine is obtained through the outer loop controller PI2 to get the recompression machine flow set value, and then the recompression machine flow deviation signal is controlled through the inner loop controller PI3 to control the recompression machine speed, and through the inner and outer loop cascade control system, the inlet pressure of the main compressor is controlled at 7.6 MPa.

[0071] 1.2. Valve inventory joint control strategy:

[0072] To realize the variable load of S-CO2 power generation system quickly and efficiently in a wide load range, a load following control strategy coupling valve control and inventory control is proposed, as shown in Figure 4 The fundamental advantage of this control strategy is that the variable load of the power generation system is realized through inventory control, but in order to make up for the slow variable load speed of inventory control, the load of the system is quickly adjusted to the target value by using inventory control, and the load change realized by valve control is gradually changed to inventory control, thereby avoiding the low efficiency of valve control itself. It can be seen that the valve inventory coupling variable load control strategy realizes the rapid variable load of the system through valve control, and ensures that the thermal efficiency of the system is maintained at a high value in a wide load range through inventory control.

[0073] To meet the requirement of the power generation frequency, the target value of the generator speed is set to 3000 rpm. When the generator speed is greater than 3000 rpm, the opening of the turbine inlet valve is reduced, so that the turbine inlet pressure is reduced, the turbine output torque is reduced, and the generator speed is reduced; otherwise, the opening of the turbine inlet valve is increased, so that the generator speed is increased.

[0074] In addition, considering the following factors: 1. Too small turbine inlet valve opening will cause too large throttling loss; 2. Turbine inlet valve opening as an index to evaluate whether the system power generation matches the grid demand, it needs to ensure that the valve has upward adjustable margin, the target value of the turbine inlet valve opening is selected as 80% in this paper. The reason why the valve control selects the turbine inlet valve instead of the turbine bypass valve as the main actuator is that the opening of the turbine inlet valve can reflect whether the load of the power generation side and the grid side matches in both directions, while the opening of the bypass valve can only reflect in one direction. When the opening of the turbine inlet valve is less than 80%, the CO2 exhaust valve is opened and the CO2 charging valve is closed to reduce the CO2 inventory of the system, thereby reducing the circulating flow of the system, so that the turbine power generation decreases and the turbine inlet valve opening increases; otherwise, the CO2 charging valve is opened and the CO2 exhaust valve is closed, so that the turbine inlet valve opening decreases.

[0075] 2. S-CO2 Brayton cycle start-stop control strategy

[0076] 2.1 S-CO2 Brayton cycle shutdown control strategy:

[0077] To realize safe, fast and efficient shutdown of the S-CO2 cycle power generation system, the shutdown strategy of the system is mainly divided into three stages.

[0078] Among them, the first stage is the load following process of the system from 100% load to 10% load, this stage adopts the valve inventory coupled variable load control strategy introduced in section 1, the control strategy of this stage mainly includes main compressor inlet pressure control, main compressor temperature control, circulating split coefficient control, turbine inlet temperature control, compressor anti-surge control and valve inventory coupled load following control strategy.

[0079] The second stage of load reduction is the load following process of the system from 10% load to 0, when the system still adopts the same load following control strategy, there will be corresponding problems,

[0080] The main points are as follows: 1. When the valve inventory control is still used to realize load reduction in this stage, the flow of the cycle will be further reduced, thereby causing the flow of the working medium in the boiler to be less than 30% of the rated flow, which does not meet the requirements for safe and stable operation of the boiler; 2. When the cycle operates at a lower flow, the risk of surge of the main compressor and the re-compressor increases; 3. When the power generation load of the system is further reduced, the maximum pressure of the cycle has been reduced to operate below 10 MPa, and the minimum pressure control of the cycle is difficult to realize, thereby causing the working medium to enter a trans-critical state of operation. Therefore, for the load reduction process from 10% to 0, the inlet temperature of the turbine is reduced, so that the flow of the cycle is not reduced during the load reduction process, and the possible pipe explosion accident of the boiler during low load operation of the system can be avoided.

[0081] The third stage of system shutdown is the process of shutting down each device, which mainly includes the following steps:

[0082] 1. Further reduce the thermal load of the boiler, and when the inlet temperature of the turbine is reduced by 100℃ or less, open the bypass valve V21, close the three-way valve flowing through the turbine, so that CO2 no longer flows through the turbine to do work; 2. Gradually open the bypass valve V6, and when the flow of the working medium flowing through the boiler is less than 30% of the rated flow, shut down the boiler, and gradually reduce the opening of the three-way valve V4 flowing through the boiler; attention should be paid to the fact that the reduction speed of the thermal load of the boiler should be greater than the reduction speed of the flow of the working medium flowing through the boiler, so as to ensure sufficient cooling of the working medium to the pipe wall surface; 3. After the boiler is completely shut down, the opening of the three-way valve V4 flowing through the boiler is reduced to 0, and the minimum pressure of the cycle is ensured to be about 8 MPa by adjusting the opening of the V6 valve; 4. Gradually reduce the speed of the re-compressor, and the reduction of the speed of the re-compressor means the reduction of the flow of the compressor, attention should be paid to the fact that the speed reduction rate of the compressor and the opening of the backflow valve V8 are adjusted to prevent the re-compressor from surging; 5. Change the opening of the three-way valve V1, so that the CO2 flowing out of the re-compressor flows into V2, and the speed of the re-compressor is further reduced to 0; 6. Reduce the speed of the main compressor, and open the CO2 discharge valve V17, attention should be paid to the fact that the speed reduction rate of the main compressor, the opening of the V17 valve and the opening of the backflow valve V13 are controlled to avoid the surge of the compressor; 7. The main compressor is reduced to 0, the system inventory working medium is pumped out by the compressor, and the protective gas is filled in, so as to prevent corrosion and oxidation caused by long-term shutdown of the system. Unlike the first stage, the safety and rapidity should be focused on in the second and third stages of system shutdown, and the efficiency can be appropriately ignored.

[0083] 2.2S-CO2Brayton cycle startup control strategy:

[0084] In order to realize the safe, rapid and efficient startup of the S-CO2 cycle power generation system, the startup strategy of the system is mainly divided into three stages in this section.

[0085] The first stage is the process of the system from the auxiliary power to the power generation load to 0%. The first stage is mainly divided into the following steps:

[0086] 1. The system protection gas is evacuated, and the CO2 filling valve V16 is opened; 2. The main compressor is started and the speed is gradually increased. Note that the main compressor speed increasing rate, the V16 valve opening degree and the backflow valve V13 opening degree are controlled to avoid the compressor from surging and the CO2 at the main compressor inlet from entering the subcritical state. At this time, the system uses the main compressor, the low-temperature regenerator and the pre-cooler three devices; 3. After the above devices are stably operated, the re-compressor is started and the speed is gradually increased. Note that the re-compressor speed decreasing rate and the backflow valve V8 opening degree are adjusted to prevent the re-compressor from surging and the CO2 at the re-compressor inlet from entering the subcritical state; 4. When the re-compressor outlet pressure is higher than the main compressor outlet pressure, the opening degree of the three-way valve V1 is changed so that the CO2 at the re-compressor outlet flows into V3 from V2. Note that the V6 valve opening degree is adjusted to ensure that the compressor inlet state is about 8 MPa; 5. The main and re-compressor speeds are further increased. When the circulating flow is greater than 30% of the rated flow, the V4 valve is opened to flow through the boiler, and the boiler heat load is gradually increased. Note that the boiler heat load increasing speed should be less than the flow through the boiler working medium flow increasing speed to ensure that the working medium is fully cooled to the pipe wall surface; 6. When the turbine inlet working medium temperature reaches 100°C or above, the bypass valve V21 is gradually closed, and the working medium enters the turbine to start working. Through the series of steps in the first stage, the S-CO2 cycle power generation system completes the starting process, and the system starting process enters

[0087] The second stage is the load increasing process from 0 to 10%. Considering the safety of the system operation, the second stage is improved on the basis of the valve inventory coupled variable load control strategy introduced in the first section. The improvement mainly reflects that the turbine inlet temperature control target changes with the load. The load increasing in the second stage is mainly realized by increasing the heat load of the heater. After the second stage is completed, the turbine inlet temperature is increased to 600°C and 620°C, respectively. The second stage limits the pipe wall temperature increasing rate while ensuring that the system load increasing rate is high.

[0088] The third stage of the system starting process is the load following process from 10% load to 100% load. The control strategy of the third stage mainly includes the main compressor inlet pressure control, the main compressor temperature control, the circulating split coefficient control, the turbine inlet temperature control, the compressor anti-surge control and the valve inventory coupled load following control strategy. The S-CO2 Brayton cycle starting control strategy is shown in Figure 5 .

[0089] The control strategy of the S-CO2 Brayton cycle system is as follows:

[0090] 1. Main compressor inlet state control strategy:

[0091] When the cycle working medium enters the subcritical state operation will cause the compressor cavitation, serious reduce the service life of the compressor, so the need to design the corresponding control system to ensure that the main compressor inlet temperature and pressure are greater than the critical point state. This paper designs the main compressor inlet temperature and pressure control strategy, as shown in Figure 6 .

[0092] The main compressor inlet temperature control logic can be expressed as: when the main compressor inlet temperature is greater than the set value, increase the cooling water pump speed to increase the flow of cooling water to reduce the main compressor inlet temperature; otherwise, reduce the speed of the cooling water pump to increase the main compressor inlet temperature. In the process of adjusting the cooling water pump speed to ensure that the speed is changed in the safe range of 500r·min -1 ~2000r·min -1 In addition, the main compressor inlet pressure control logic can be expressed as: when the main compressor inlet pressure and target value are not equal, the re-compressor inlet pressure deviation signal is obtained through the outer loop controller PI4 to get the re-compressor flow set value, and then the re-compressor flow deviation signal is controlled through the inner loop controller PI5 to control the re-compressor speed. Through the inner and outer loop cascade control system, the main compressor inlet pressure is controlled near the set target value fluctuation, to ensure the safe and efficient operation of the system.

[0093] 2、Compressor anti-surge control strategy:

[0094] The compressor is the key equipment of the S-CO2 re-compression Brayton cycle, and its safety is crucial for the stable operation of the system. The common security risk of the compressor is the surge problem. When the speed, pressure ratio and flow of the compressor cannot be coordinated, the flow will appear strong oscillation, and will emit "roar" sound. This phenomenon will cause damage to important components such as impeller, main shaft, bearing and blade of the equipment, and sometimes even cause the entire unit to be scrapped. Therefore, the main and re-compressor anti-surge control system must be designed during the system operation. The main and re-compressor anti-surge control strategy designed in this paper is shown in Figure 7 . The main compressor anti-surge control logic can be expressed as:

[0095] When the difference between the relative flow and the relative pressure ratio of the main compressor and the surge point, i.e. the surge margin, is greater than 0.2, it indicates that the compressor does not surge, and the backflow valve of the main compressor is closed; when the difference between the relative flow and the relative pressure ratio of the main compressor and the surge point, i.e. the surge margin, is less than 0.2, it indicates that the compressor is at risk of surging, and the backflow valve of the main compressor needs to be opened to increase the flow of the compressor to prevent the occurrence of the surging problem. The anti-surge control logic of the re-compressor can be expressed as: when the difference between the relative flow and the relative pressure ratio of the re-compressor and the surge point, i.e. the surge margin, is greater than 0.2, it indicates that the compressor does not surge, and the backflow valve of the re-compressor is closed; when the difference between the relative flow and the relative pressure ratio of the re-compressor and the surge point, i.e. the surge margin, is less than 0.2, it indicates that the re-compressor is at risk of surging, and the backflow valve of the re-compressor needs to be opened to increase the flow of the compressor to prevent the occurrence of the surging problem.

[0096] The above-described embodiments are merely preferred specific embodiments of the present application, and the protection scope of the present application is not limited thereto. Any simple change or equivalent replacement of the technical solutions within the technical range disclosed by the present application, which can be obviously obtained by those skilled in the art, shall fall within the protection scope of the present application.

Claims

1. A method for starting and stopping a supercritical carbon dioxide cycle system, characterized in that, The system includes: a CO2 storage tank, the output of which is connected to the input of a precooler and a recompressor; a precooler, the output of which is connected to the input of a main compressor; a main compressor, the output of which is connected to the input of the CO2 storage tank; a low-temperature regenerator, the first input of which is connected to the output of the main compressor; a high-temperature regenerator, the first input of which is connected to the first output of the low-temperature regenerator and the output of the recompressor; the first output of which is connected to the input of a boiler; a boiler, the first input of which is connected to the first output of the high-temperature regenerator; and a turbine, the input of which is connected to... The boiler's output end is connected; its output end is connected to the second input end of the high-temperature regenerator; the second output end of the high-temperature regenerator is connected to the second input end of the low-temperature regenerator; the second output end of the low-temperature regenerator is connected to the input ends of the precooler and the recompressor; reflux valve V8, whose two ends are connected to the input and output ends of the recompressor respectively; turbine inlet valve V20, located on the connecting pipe between the boiler and the turbine; bypass valve V21, whose two ends are connected to the turbine inlet valve V20 and the turbine output end; reflux valve V13, whose two ends are connected to the input and output ends of the main compressor respectively; CO2 discharge valve V17 is located at the inlet end of the CO2 storage tank; CO2 filling valve V16 is located at the output end of the CO2 storage tank; Three-way valves V4 and V3, connected in series, are installed on the connecting pipe between the first input end of the high-temperature regenerator and the first output end of the low-temperature regenerator. Three-way valves V2 and V5, connected in series, are located on the connecting pipe between the second output end of the high-temperature regenerator and the second input end of the low-temperature regenerator. Bypass valve V6, with its two ends connected to three-way valve V4 and three-way valve V5; Three-way valve V1, whose three ends are respectively connected to three-way valve V2, three-way valve V3, and the output end of the compressor; The method includes: When the supercritical carbon dioxide cycle system is reduced from 100% load to 10% load: If the generator speed is greater than 3000 rpm, reduce the opening of the turbine intake valve V20 towards the turbine side; if the generator speed is less than 3000 rpm, increase the opening of the turbine intake valve V20 towards the turbine side; when the turbine intake valve opening is less than 80%, open the CO2 discharge valve V17 and close the CO2 charging valve V16, increasing the turbine intake valve opening; when the turbine intake valve opening is greater than 80%, open the CO2 charging valve V16 and close the CO2 discharge valve V17, decreasing the turbine intake valve opening. When the supercritical carbon dioxide cycle system is reduced from 10% load to 0% load: The variable load of the supercritical carbon dioxide cycle system is adjusted by lowering the turbine inlet temperature. When shutting down the equipment of the supercritical carbon dioxide cycle system: Once the turbine inlet temperature drops below 100℃, open the turbine inlet valve V20 connected to the bypass valve V21 side and close the turbine inlet valve V20 connected to the turbine side. Open the bypass valve V6. When the flow rate of the working fluid flowing through the boiler is less than 30% of the rated flow rate, shut down the boiler. Ensure that the rate of decrease in boiler heat load is greater than the rate of decrease in the flow rate of the working fluid flowing through the boiler, and reduce the opening of the three-way valve V4 flowing through the boiler. After the boiler is completely shut down, reduce the opening of the three-way valve V4 flowing through the boiler to 0, and adjust the opening of the bypass valve V6 to ensure that the minimum circulating pressure of the supercritical carbon dioxide circulation system is 8MPa. Shut down the recompressor and main compressor in the supercritical carbon dioxide cycle system.

2. The start-up and shutdown method for a supercritical carbon dioxide cycle system according to claim 1, characterized in that, The shutdown of the re-compressor and main compressor includes: Reduce the speed of the recompressor; open the three-way valve V1 connected to the three-way valve V2 side to reduce the speed of the recompressor to 0; Reduce the speed of the main compressor and open the CO2 discharge valve V17; when the speed of the main compressor drops to 0, evacuate the system's stock of working fluid from the CO2 discharge valve V17 and charge protective gas from the CO2 charging valve V16.

3. The start-up and shutdown method for a supercritical carbon dioxide cycle system according to claim 2, characterized in that, When reducing the speed of the recompressor, surge in the recompressor is prevented by adjusting the compressor deceleration rate and the opening of the return valve V8. When reducing the speed of the main compressor, control the main compressor speed reduction rate, valve V17 opening degree and return valve V13 opening degree to prevent the main compressor from surging.

4. The start-up and shutdown method for a supercritical carbon dioxide cycle system according to claim 1, characterized in that, Also includes: When the supercritical carbon dioxide circulation system is under variable load, if the circulation split coefficient of the supercritical carbon dioxide circulation system is less than 0.318, the speed of the main compressor should be reduced. If the circulation split coefficient of the supercritical carbon dioxide circulation system is greater than 0.318, increase the speed of the main compressor.

5. The start-up and shutdown method for a supercritical carbon dioxide cycle system according to claim 1, characterized in that, When the supercritical carbon dioxide cycle system experiences a change in load: If the actual pressure value at the main compressor inlet is not equal to the set pressure value at the main compressor inlet, the recompressor inlet pressure deviation signal will be input to the outer loop controller PI2, and the recompressor flow set value will be obtained by using the outer loop controller PI2. Measure the actual value of the recompressor flow rate, and obtain the deviation signal of the recompressor flow rate based on the actual value of the recompressor flow rate and the set value of the recompressor flow rate; The deviation signal of the recompressor flow rate is input into the inner loop controller PI3, which controls the speed of the recompressor. The main compressor inlet pressure is controlled at 7.6MPa through the cascade control system of inner and outer loops.

6. The start-up and shutdown method for a supercritical carbon dioxide cycle system according to claim 1, characterized in that, When the supercritical carbon dioxide cycle system is turned on: Evacuate the system protective gas from CO2 discharge valve V17 and open CO2 charging valve V16; Start the main compressor and increase its speed; start the cryogenic regenerator and precooler. After the main compressor, low-temperature regenerator and precooler are running stably, start the recompressor and gradually increase the speed of the recompressor. When the recompressor outlet pressure is higher than the main compressor outlet pressure, change the opening of the three-way valve V1 to transfer the CO2 at the recompressor outlet from the three-way valve V2 to the three-way valve V3, and adjust the opening of the bypass valve V6 to ensure that the compressor inlet pressure is 8MPa. Increase the speed of the main compressor and re-compressor. When the circulation flow of the supercritical carbon dioxide circulation system is greater than 30% of the rated flow, open the three-way valve V4 to increase the boiler's heat load. When the temperature of the working fluid at the turbine inlet reaches 100°C or higher, close the bypass valve V21.

7. The start-up and shutdown method for a supercritical carbon dioxide cycle system according to claim 6, characterized in that, When a supercritical carbon dioxide cycle system increases from 0 load to 10% load: If the generator speed is greater than 3000 rpm, reduce the opening of the turbine intake valve V20; If the generator speed is less than 3000 rpm, increase the opening of the turbine intake valve V20; When the turbine intake valve opening is less than 80%, open the CO2 exhaust valve and close the CO2 charging valve to increase the turbine intake valve opening. When the turbine intake valve opening is greater than 80%, open the CO2 charging valve and close the CO2 exhaust valve to reduce the turbine intake valve opening. When the supercritical carbon dioxide cycle system is at full load, the turbine inlet temperature control target is set to 600℃; wherein, the heating rate of the control tube wall is 10℃ per minute.

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