A coal-fired unit low-load efficiency improving system and an operation method thereof

Abstract

A coal-fired unit low-load efficiency improving system and its operation method, outside the main circulation loop of the coal-fired unit, a steam re-compression branch is added. The branch includes a steam extraction pipeline, a first feedwater heater, a first-stage steam compressor, a second feedwater heater and a second-stage steam compressor. When the unit is in low-load operation, a part of steam is extracted from the medium-pressure cylinder exhaust pipe of the steam turbine; the high-pressure feedwater of the two-stage feedwater heater is used to cool the steam to a slightly superheated state; then the steam is compressed to a pressure higher than the exhaust pressure of the high-pressure cylinder of the steam turbine by the two-stage steam compressor; finally, the compressed steam is delivered to the boiler reheater inlet, and then re-enters the thermal cycle to do work after being combined with the main stream. The present application greatly reduces the steam flow and the heat load of the condenser, thereby significantly reducing the highest proportion of cold-end loss, effectively improving the operation efficiency and power supply economy of the unit under low-load working conditions.

Description

Technical Field

[0001] This invention relates to the field of coal-fired power generation technology, specifically to a low-load coal-fired unit efficiency improvement system and its operation method, which is particularly suitable for the retrofitting and upgrading of existing units and the construction of new units that need to meet the requirements of deep peak shaving and high-efficiency operation under low load. Background Technology

[0002] Currently, coal-fired power units experience a decline in boiler and turbine efficiency when operating at low loads, and the thermal system deviates from its design conditions, leading to a deterioration in overall economic efficiency. For units operating under pure condensing conditions, their power generation efficiency is typically less than 50%, and "cold end losses," which are emitted into the environment through the condenser, account for over 70% of all heat losses. Therefore, reducing cold end losses is the most effective way to improve unit efficiency, especially at low loads.

[0003] In existing technologies, methods to improve the efficiency of units under low load in pure condensing conditions are mostly focused on boiler combustion optimization, turbine flow path modification, and feedwater heating system optimization. Although these methods have some effect, their effect on reducing cold end losses is limited, and there is not much room for improvement in low load thermal efficiency. Summary of the Invention

[0004] The present invention aims to provide a system and operating method that can directly act on the cold end of the thermal cycle, significantly reduce the heat load of the condenser, and thus comprehensively improve the efficiency of the unit under low load operation.

[0005] To achieve the above-mentioned objectives, the present invention provides a low-load efficiency improvement system for coal-fired power units, including a main circulation pipeline, characterized in that: a steam recompression branch is connected to the exhaust pipe of the intermediate-pressure cylinder of the turbine in the main circulation pipeline, which is used to control the steam flow rate entering the low-pressure cylinder and the condenser, and at the same time, the recompressed steam is sent back to the main circulation pipeline.

[0006] Preferably, the vapor recompression branch includes: The first feedwater heater has its shell-side inlet connected to the exhaust pipe of the intermediate pressure cylinder of the steam turbine, and its tube-side inlet connected to the main feedwater pipe at the inlet of the high-pressure heater in the main circulation pipeline. A steam compressor is provided, with its inlet connected to the shell-side outlet of the first feedwater heater; The second feedwater heater has its shell-side inlet connected to the outlet of the first-stage steam compressor, its tube-side inlet connected to the tube-side outlet of the first feedwater heater, and its tube-side outlet connected to the main feedwater pipeline between the outlet of the high-pressure heater and the inlet of the boiler economizer in the main circulation pipeline. The two-stage steam compressor has its inlet connected to the shell-side outlet of the second feedwater heater, and its outlet connected to the boiler reheater inlet steam pipe in the main circulation pipeline. The first-stage steam compressor and the second-stage steam compressor are driven by electric motors.

[0007] Preferably, a first electric valve is provided on the main feedwater pipeline at the inlet of the high-pressure heater, a second electric valve is provided on the inlet pipeline of the first feedwater heater, a third electric valve is provided on the outlet pipeline of the second feedwater heater, a fourth electric valve is provided on the inlet pipeline of the shell side of the first feedwater heater, a fifth electric valve is provided on the inlet pipeline of the low-pressure cylinder of the steam turbine, and a sixth electric valve is provided on the pipeline from the outlet of the second-stage steam compressor to the inlet of the boiler reheater.

[0008] Preferably, the first and second feedwater heaters are surface heat exchangers.

[0009] Preferably, the first-stage steam compressor and the second-stage steam compressor are centrifugal or axial flow compressors.

[0010] Preferably, the motor is a variable frequency motor.

[0011] A method for operating a low-load efficiency improvement system for a coal-fired power unit, characterized by comprising the following steps: S1: Open the vapor recompression branch; S2: A portion of superheated steam is drawn from the exhaust pipe of the intermediate pressure cylinder of the steam turbine and enters the first feedwater heater; S3: A portion of high-pressure feedwater is diverted from the main feedwater pipeline into the tube side of the first feedwater heater to cool the superheated steam in the shell side to a temperature higher than the saturation temperature under the shell-side working pressure of the first feedwater heater. S4: The cooled steam enters a steam compressor and is compressed, pressurized, and heated. S5: Steam from the outlet of a steam compressor enters the second feedwater heater and is cooled again by the diverted high-pressure feedwater to a saturation temperature higher than the working pressure on the shell side of the second feedwater heater. S6: The cooled steam enters the second-stage steam compressor and is compressed to a pressure higher than the exhaust pressure of the high-pressure cylinder of the steam turbine; S7: The steam from the outlet of the second-stage steam compressor is transported to the inlet of the boiler reheater, where it mixes with the exhaust steam from the high-pressure cylinder and enters the boiler reheater for heating. It then enters the intermediate-pressure cylinder of the turbine to perform work.

[0012] Preferably, the steam flow rate drawn out by S2 is adjusted according to the unit load and efficiency optimization target. Compared with the prior art, the beneficial effects of the present invention are as follows:

[0013] 1. Significantly Reduces Cold-End Losses and Improves Low-Load Efficiency: The core of this invention lies in "retaining" a portion of the steam that would otherwise enter the low-pressure cylinder for condensation and returning it to the high-temperature, high-pressure thermodynamic cycle through recompression. This directly and significantly reduces the steam flow rate and condenser heat load entering the condenser. Since the thermodynamic losses during steam recompression are far lower than the irreversible losses from releasing latent heat of vaporization to liquid water during steam condensation, it can significantly reduce the largest proportion of cold-end losses. This invention system can effectively help units, especially existing retrofitted units and newly built units, to control the increase in coal consumption for power supply at 30% rated load to within 25% to 15% of the increase at 100% rated load.

[0014] 2. Ensuring safe and stable boiler operation: The recompressed steam is returned to the boiler reheater, compensating for the insufficient reheater steam flow that may be caused by reduced low-pressure cylinder flow. This ensures the cooling requirements of the reheater tube walls, avoids the risk of boiler overheating due to excessively low reheat steam flow under low load, and also helps meet the minimum stable combustion load requirements of the boiler.

[0015] 3. Optimize the regenerative system to achieve energy cascade utilization: The parallel feedwater heater added in this invention uses high-pressure feedwater to intermediately cool the steam during the recompression process. This process not only improves the compressor efficiency (compressing near-saturated steam is more energy-efficient than compressing superheated steam), but also uses the heat of compression to heat the feedwater, reducing the amount of steam that originally needed to be extracted from the turbine to heat the feedwater. This allows more steam to expand and do work in the turbine, further improving the cycle efficiency.

[0016] 4. Enhanced Unit Regulation Flexibility: By controlling the start-up and shutdown of the recompression branch and the steam flow, the unit's power output and heat distribution can be flexibly adjusted. This provides a new auxiliary regulation method for the unit to participate in deep peak shaving and rapid load changes, helping the unit meet multiple indicators of efficient regulation in the implementation plan for the new generation of coal-fired power plant upgrading project. Attached Figure Description

[0017] Figure 1 This is a schematic diagram of the system of the present invention; Figure 2 This is a schematic diagram of the system status of a coal-fired power unit during normal operation; Figure 3 This is a schematic diagram of the system state when the vapor recompression branch is open; The components include: 1. Boiler; 2. High-pressure cylinder of steam turbine; 3. Intermediate-pressure cylinder of steam turbine; 4. Low-pressure cylinder of steam turbine; 5. Economizer of boiler; 6. Superheater of boiler; 7. Reheater of boiler; 8. Condenser; 9. Condensate pump; 10. Low-pressure heater; 11. Deaerator; 12. Boiler feedwater pump; 13. High-pressure heater; 14. First feedwater heater; 15. Second feedwater heater; 16. Electric motor; 17. First-stage steam compressor; 18. Second-stage steam compressor; 19. First electric valve; 20. Second electric valve; 21. Third electric valve; 22. Fourth electric valve; 23. Fifth electric valve; 24. Sixth electric valve. Detailed Implementation

[0018] To enable those skilled in the art to better understand the technical solutions of the present invention, preferred embodiments of the present invention are described below in conjunction with specific examples. However, it should be understood that the accompanying drawings are for illustrative purposes only and should not be construed as limiting the present patent. It is understandable that some well-known structures and their descriptions may be omitted in the drawings for those skilled in the art. The positional relationships described in the accompanying drawings are for illustrative purposes only and should not be construed as limiting the present patent.

[0019] Reference Figures 1-3 This embodiment takes a 660MW ultra-supercritical coal-fired unit as an example, selects its 30%THA (heat consumption acceptance) pure condensing condition as the design benchmark, and describes the specific integration method, operating parameters and energy efficiency improvement effect of the system of the present invention.

[0020] 1. System Integration and Design Baseline Operating Conditions The steam recompression branch described in this invention is integrated into the thermal system of a conventional 660MW ultra-supercritical unit. The main circulation loop of this benchmark unit includes a boiler 1, a high-pressure turbine cylinder 2, a boiler reheater 7, a medium-pressure turbine cylinder 3, a low-pressure turbine cylinder 4, a condenser 8, a condensate pump 9, a regenerative system consisting of a low-pressure heater 10 and a high-pressure heater 13, a deaerator 11, a boiler feedwater pump 12, and connecting pipelines.

[0021] The design and calculations in this embodiment are based on data from a 660MW ultra-supercritical unit operating under 30% THA pure condensing conditions. Under these conditions, the unit's power generation is 198001.5 kW, and its heat consumption is 7903.5 kJ / kWh. The main steam flow rate is approximately 499.36 t / h, the reheat steam flow rate entering the intermediate-pressure cylinder is approximately 437.44 t / h, and the final steam flow rate entering the low-pressure cylinder is approximately 386.18 t / h.

[0022] The steam recompression branch includes a first feedwater heater 14, a second feedwater heater 15, an electric motor 16, a first-stage steam compressor 17, and a second-stage steam compressor 18. The shell-side inlet of the first feedwater heater 14 is connected to the exhaust pipe of the intermediate-pressure cylinder 3 of the turbine, and its tube-side inlet is connected to the main feedwater pipe at the inlet of the high-pressure heater 13 in the main circulation pipeline. The inlet of the first-stage steam compressor 17 is connected to the shell-side outlet of the first feedwater heater 14. The shell-side inlet of the second feedwater heater 15 is connected to the outlet of the first-stage steam compressor 17, and its tube-side inlet is connected to the tube-side outlet of the first feedwater heater 14. The tube-side outlet of the second feedwater heater 15 is connected to the main feedwater pipe between the outlet of the high-pressure heater 13 and the inlet of the economizer 5 in the main circulation pipeline. The inlet of the second-stage steam compressor 18 is connected to the shell-side outlet of the second feedwater heater 15, and its outlet is connected to the inlet of the boiler reheater 7 in the main circulation pipeline. The first-stage steam compressor 17 and the second-stage steam compressor 18 are driven by the electric motor 16.

[0023] A first electric valve 19 is provided on the main feedwater inlet pipe of the high-pressure heater 13; a second electric valve 20 is provided on the inlet pipe of the first feedwater heater 14; a third electric valve 21 is provided on the outlet pipe of the second feedwater heater 15; a fourth electric valve 22 is provided on the inlet pipe of the shell side of the first feedwater heater 14; a fifth electric valve 23 is provided on the inlet pipe of the low-pressure cylinder of the steam turbine; and a sixth electric valve 24 is provided on the pipe from the outlet of the second-stage steam compressor 18 to the inlet of the boiler reheater 7.

[0024] The first feedwater heater 14 and the second feedwater heater 15 are surface heat exchangers. The first-stage steam compressor 17 and the second-stage steam compressor 18 are centrifugal or axial flow compressors. The electric motor 16 is a variable frequency motor.

[0025] 2. Setting operating parameters for the steam recompression branch When the unit is operating at 30% THA or lower and requires improved low-load efficiency, the steam recompression branch should be activated. Specific operating parameters are set as follows: Steam extraction point and flow rate: A portion of steam is extracted from the exhaust pipe of the intermediate pressure cylinder 3 of the steam turbine through the fourth electric valve 22. The steam extraction rate is set to 100 t / h, the temperature is approximately 291℃, and the pressure is 0.156 MPa.a.

[0026] Feedwater heater parameters: A portion of the high-pressure feedwater is diverted from the main feedwater pipeline before the inlet of the high-pressure heater 13, flowing sequentially through the tube sides of the first feedwater heater 14 and the second feedwater heater 15. The 100 t / h medium-pressure exhaust steam first enters the shell side of the first feedwater heater 14 and is cooled by the diverted high-pressure feedwater. The cooling target is to reduce the steam temperature to a slightly superheated state, approximately 15°C higher than the saturation temperature at the shell-side operating pressure of the first feedwater heater 14, to improve the efficiency of the subsequent compressor. After the first stage of compression, the steam enters the shell side of the second feedwater heater 15 and is again cooled by feedwater to the same slightly superheated state. The two-stage heaters utilize the heat of compression and sensible heat of the steam to increase the feedwater temperature, reducing the need for steam extraction from the turbine. This portion of the regenerative power is 15196 kW.

[0027] Steam compressor parameters: The two-stage compressor is driven by electric motor 16. The first-stage steam compressor 17 increases the steam pressure from the first feedwater heater. The second-stage steam compressor 18 further increases the steam pressure from the second feedwater heater to 1.75 MPa.a, ensuring its outlet pressure is higher than the turbine high-pressure cylinder exhaust pressure of 1.7 MPa.a, thus ensuring smooth injection into the inlet pipe of the boiler reheater 7. The total power consumption of the two-stage compressor is 19688 kW.

[0028] Steam reinjection: The steam, after being pressurized by the second-stage steam compressor, is delivered to the inlet of the boiler reheater 7 through the outlet pipeline of the second-stage steam compressor and the sixth electric valve 24. It mixes with the exhaust steam from the high-pressure cylinder of the steam turbine and enters the boiler reheater 7 together to be heated. Then it enters the intermediate-pressure cylinder 3 of the steam turbine to continue to expand and do work.

[0029] 3. The operation process is as follows: First, open electric valves 19, 20, 21, 22, 23, and 24, and start electric motor 16 and two steam compressors 17 and 18.

[0030] Steam of 100 t / h and 0.156 MPa.a drawn from the exhaust pipe of the intermediate pressure cylinder 3 of the steam turbine enters the first feedwater heater 14 and is cooled by the feedwater from the inlet of the high pressure heater 13.

[0031] The cooled steam enters a steam compressor 17, where it is compressed, pressurized, and heated.

[0032] Steam from the outlet of the first-stage steam compressor 17 enters the second feedwater heater 15 and is cooled again by the feedwater.

[0033] The cooled steam enters the second-stage steam compressor 18 and is compressed to a pressure higher than the exhaust pressure of the high-pressure cylinder 2 of the steam turbine.

[0034] Finally, the steam from the outlet of the second-stage steam compressor 18 is injected into the inlet of the boiler reheater 7 and merges with the mainstream.

[0035] By adjusting the opening of the fourth electric valve 22, the steam flow rate can be precisely controlled within the range of 0 to 100 t / h, thereby flexibly adjusting the degree of reduction in the steam flow rate of the low-pressure cylinder 4 of the steam turbine and realizing dynamic control of cold end losses.

[0036] By adjusting the opening of the first electric valve 19 and the second electric valve 20, the change in high-pressure feedwater flow can be precisely controlled to ensure that the temperature drop of the steam recompression branch meets the requirement of slight superheating of the inlet steam temperature of the two-stage steam compressor.

[0037] 4. Analysis of Energy Efficiency Improvement Effect After the system was put into operation under the 30% THA reference condition, the thermodynamic cycle changed significantly: As 100 t / h of steam is extracted from the exhaust pipe of the intermediate pressure cylinder 3 of the steam turbine and then compressed back into the thermodynamic cycle, the steam flow rate entering the low pressure cylinder 4 of the steam turbine is reduced accordingly. The heat load (cold end loss) of the condenser 8 is directly reduced, and the condensate flow rate and main steam flow rate are also reduced accordingly.

[0038] The compression process consumes 19,688 kW of electrical power, but at the same time, the system undergoes four changes: 1) The reduced flow rate of feedwater into the boiler decreases the heat absorption of the steam-water system in the boiler, thus reducing the amount of coal burned; 2) The decrease in main steam flow leads to a reduction in the work capacity of the high-pressure cylinder 2 of the steam turbine; 3) The recompressed steam returns to the intermediate pressure cylinder 3 of the turbine to continue doing work, increasing the turbine's output power; 4) The two-stage feedwater heater recovers the heat of compression, reduces the steam consumption of the turbine extraction steam, and allows more steam to be used for work. According to simulation calculations, after considering the power consumption of the compressor, the net output power of the system is 157,917 kW, at which point the actual power supply load rate of the unit is 23.93%. Based on the heat consumed by the boiler, the calculated heat consumption of the unit is 8138.98 kJ / kWh. The original unit's heat consumption rate under 23.93% THA conditions was 8165.07 kJ / kWh; after adopting the system of this invention, the heat consumption rate decreased by 26.09 kJ / kWh. With further improvements in the manufacturing process and efficiency of the steam compressor, the energy consumption during the compression process is expected to decrease further. This energy efficiency improvement can reduce the unit's coal consumption for power supply under low load conditions, thereby improving the unit's economic efficiency.

[0039] This embodiment is illustrated using a 30% THA (Total Heat Dissipation) operating condition and a fixed induced steam flow rate. In actual operation, the operation of the steam recompression branch and the induced steam flow rate can be dynamically adjusted according to the unit load, grid dispatch instructions, and the principle of optimal efficiency. For example, it can be operated at 50% THA or lower loads, and the optimal induced steam flow rate varies with the main steam flow rate. The system has good load regulation flexibility, which helps to meet the requirements of deep peak shaving and rapid load change ancillary services.

[0040] The above description is merely a preferred embodiment of the present invention and is not intended to limit the invention. Any modifications, equivalent substitutions, or improvements made within the spirit and principles of the present invention should be included within the scope of protection of the present invention.

Claims

1. A low-load efficiency improvement system for coal-fired power units, comprising a main circulation pipeline, characterized in that: A steam recompression branch is connected to the exhaust pipe (25) of the turbine intermediate pressure cylinder (3) in the main circulation pipeline to control the steam flow rate entering the turbine low pressure cylinder (4) and condenser (8), and at the same time, the recompressed steam is sent back to the main circulation pipeline.

2. The low-load efficiency improvement system for coal-fired power units according to claim 1, characterized in that: The vapor recompression branch includes: The first feedwater heater (14) has its shell-side inlet connected to the exhaust pipe of the intermediate pressure cylinder (3) of the steam turbine, and its tube-side inlet connected to the inlet of the high-pressure heater (13) of the main feedwater pipe in the main circulation pipeline. A steam compressor (17) is connected at its inlet to the shell-side outlet of the first feedwater heater (14); The second feedwater heater (15) has its shell-side inlet connected to the outlet of the first-stage steam compressor (17), its tube-side inlet connected to the tube-side outlet of the first feedwater heater (14), and the tube-side outlet of the second feedwater heater (15) connected to the main feedwater pipeline between the outlet of the high-pressure heater (13) and the inlet of the boiler economizer (5) in the main circulation pipeline. The two-stage steam compressor (18) has its inlet connected to the shell-side outlet of the second feedwater heater (15), and its outlet connected to the inlet of the boiler reheater (7) in the main circulation pipeline.

3. The low-load efficiency improvement system for coal-fired power units according to claim 2, characterized in that: The first-stage steam compressor (17) and the second-stage steam compressor (18) are driven by an electric motor (16).

4. The low-load efficiency improvement system for coal-fired power units according to claim 2, characterized in that: A first electric valve (19) is provided on the main feedwater pipe at the inlet of the high-pressure heater (13), a second electric valve (20) is provided on the inlet pipe of the first feedwater heater (14), a third electric valve (21) is provided on the outlet pipe of the second feedwater heater (15), a fourth electric valve (22) is provided on the inlet pipe of the shell side of the first feedwater heater (14), a fifth electric valve (23) is provided on the inlet pipe of the low-pressure cylinder of the steam turbine, and a sixth electric valve (24) is provided on the pipe from the outlet of the second-stage steam compressor (18) to the inlet of the boiler reheater (7).

5. The low-load efficiency improvement system for coal-fired power units according to claim 2, characterized in that: The first water heater (14) and the second water heater (15) are surface heat exchangers.

6. The low-load efficiency improvement system for coal-fired power units according to claim 2, characterized in that: The first-stage steam compressor (17) and the second-stage steam compressor (18) are centrifugal or axial flow compressors.

7. A low-load efficiency improvement system for coal-fired power units according to claim 2, characterized in that: The motor (16) is a variable frequency motor.

8. The operating method of any one of the low-load efficiency improvement systems for coal-fired power units according to claims 2-7, characterized in that, Includes the following steps: S1: Open the vapor recompression branch; S2: A portion of superheated steam is drawn from the exhaust pipe of the intermediate pressure cylinder of the steam turbine and enters the first feedwater heater (14). S3: A portion of the high-pressure water is diverted into the tube side of the first water heater (14) in the main water supply pipeline to cool the superheated steam in the shell side to a temperature higher than the saturation temperature under the shell side working pressure of the first water heater (14); S4: The cooled steam enters the first-stage steam compressor (17) and is compressed, pressurized, and heated; S5: The steam from the outlet of the first-stage steam compressor (17) enters the second feedwater heater (15) and is cooled again by the diverted high-pressure feedwater to a saturation temperature higher than the shell-side working pressure of the second feedwater heater (15); S6: The cooled steam enters the second-stage steam compressor (18) and is compressed to a pressure higher than the exhaust pressure of the high-pressure cylinder (2) of the steam turbine; S7: The steam from the outlet of the two-stage steam compressor (18) is transported to the inlet of the boiler reheater (7), mixed with the exhaust steam from the high-pressure cylinder (2) of the steam turbine, and then enters the boiler reheater (7) for heating. After that, it enters the intermediate-pressure cylinder (3) of the steam turbine to do work.

9. The operating method of the low-load efficiency improvement system for a coal-fired power unit as described in claim 8, characterized in that: The steam flow rate drawn from S2 is adjusted according to the unit load and efficiency optimization targets.

Images