Flue gas low heat recovery system based on reverse brayton cycle heat pump energy storage and control method thereof

Abstract

The present application relates to a flue gas low-heat recovery system based on reverse Brayton cycle heat pump energy storage and a control method thereof, and belongs to the technical field of heat pump energy storage peak shaving of heat engine units. The system comprises a boiler module, a high-heat cycle module and a low-heat cycle module, and realizes the hierarchical utilization of high-grade and low-grade flue gas low-heat through collaborative operation. The flue gas of the boiler module is coupled with the high-heat cycle module through a molten salt heat exchanger, and the low-heat cycle module utilizes the low-heat of the flue gas to generate power through an organic Rankine cycle. The high-heat cycle module stores electric energy and thermal energy during off-peak hours and releases energy during peak hours, thereby improving the peak shaving capacity of the system. The present application realizes the deep integration of efficient recovery of flue gas low-heat and energy storage technology, significantly improves the energy utilization efficiency, system flexibility and economy, and has environmental protection and sustainable development benefits.

Description

Technical Field

[0001] This invention relates to the field of thermal power unit pump heat storage peak shaving technology, and in particular to a flue gas low heat recovery system and its control method based on reverse Brayton cycle heat pump energy storage. Background Technology

[0002] With the profound transformation of the global energy structure and the comprehensive advancement of the "dual carbon" goal, the efficient utilization and sustainable development of traditional energy sources have become core challenges urgently needing to be addressed in the energy sector. Coal-fired power generation, as a crucial pillar of the current power system, often directly releases the low- and medium-temperature flue gas it generates into the atmosphere, resulting in a significant waste of low-grade heat energy. Simultaneously, the large-scale grid integration of renewable energy sources places higher demands on the stability and flexibility of the power system, necessitating improvements in the peak-shaving and frequency regulation capabilities of coal-fired units. Therefore, achieving efficient recovery of low-heat flue gas and deep integration with energy storage technologies to construct a flexible and efficient energy utilization system has become a key path to solving these problems.

[0003] Among numerous energy storage solutions, compressed carbon dioxide energy storage, with its high energy density, fast response speed, and environmental friendliness, has become an important technological direction for improving the peak-shaving and frequency regulation performance of coal-fired units and realizing low-heat recovery from flue gas. Meanwhile, the organic Rankine cycle, as a bottom-cycle, can effectively absorb flue gas or react with low-heat. However, how to achieve deep coupling of various systems to realize efficient utilization of low-heat flue gas, while simultaneously reducing system operating costs and improving the peak-shaving and frequency regulation capabilities of the units, remains a pressing technical challenge in this field. Summary of the Invention

[0004] To address the problems existing in the prior art, this invention provides a flue gas low-heat recovery system and its control method based on reverse Brayton cycle heat pump energy storage, specifically:

[0005] A flue gas low-heat recovery system based on reverse Brayton cycle heat pump energy storage includes: a boiler module, a high-heat cycle module and a low-heat cycle module;

[0006] In the boiler module, the flue gas outlet of the superheater in the boiler is connected to the first inlet of the mixer via a first valve and a high-heat circulation module; the flue gas outlet of the air preheater is connected to the second inlet of the mixer via a second valve; and the outlet of the mixer is connected to the boiler module via a low-heat circulation module.

[0007] Furthermore, the high-temperature cycle module includes: a thermal energy module and a pressure energy module;

[0008] The first molten salt heat exchanger, on its molten salt side, is connected in series with the high-temperature storage tank, the third valve, the second molten salt heat exchanger, the low-temperature storage tank, and the fourth valve to form a circuit, which is the thermal energy module. The second molten salt heat exchanger, on its working fluid side, is connected in series with the first expander, the low-pressure storage tank, the fifth valve, the compressor, the high-pressure storage tank, and the sixth valve to form a circuit, which is the pressure energy module.

[0009] Furthermore, the flue gas temperature at the outlet of the superheater is 500-700℃. After heat exchange in the first molten salt heat exchanger, the temperature drops to about 150-250℃, and it is mixed with the flue gas from the air preheater at 50-150℃ in the mixer.

[0010] Furthermore, the heat storage medium in both the high-temperature and low-temperature storage tanks is solar salt, with a temperature range set at 220-550℃.

[0011] Furthermore, the pressure of the high-pressure storage tank is set to 8-12 MPa, and the pressure of the low-pressure storage tank is set to 5-2 MPa.

[0012] Furthermore, the compressor is a positive displacement compressor.

[0013] Furthermore, the first expander is a turbine expander, and the second expander is a screw expander.

[0014] Furthermore, the low-heat cycle module is as follows: the outlet of the first condenser working fluid side is connected to the inlet of the regenerator working fluid side via a pump; the outlet of the regenerator working fluid side is connected in series with the evaporator working fluid side, the second expander, and the inlet of the regenerator medium side; the outlet of the regenerator medium side is connected to the inlet of the first condenser working fluid side; condensate is introduced into the inlet of the first condenser medium side; the outlet of the first condenser medium side is connected to a low-temperature heater; the outlet of the mixer is connected to the inlet of the evaporator medium side; and the outlet of the evaporator medium side is connected to the dust removal device in the boiler.

[0015] Furthermore, in the waste heat circulation module, the organic working fluid is R245fa, and the temperature difference between the ends of each heat exchanger is specified to be 5-10℃.

[0016] On the other hand, the present invention also provides a control method for controlling any of the above-mentioned low-heat flue gas recovery systems, comprising:

[0017] Energy storage process: During off-peak electricity hours, the first, second, fourth, and fifth valves are opened, and the third and sixth valves are closed. The excess power from the generator set is used to drive the compressor to compress the carbon dioxide from the low-pressure storage tank to a high-temperature and high-pressure state and store it in the high-pressure storage tank. The molten salt in the low-temperature storage tank exchanges heat with the high-temperature flue gas in the first molten salt heat exchanger. The high-temperature molten salt after absorbing heat is stored in the high-temperature storage tank, and the energy storage process ends.

[0018] Energy release process: During peak power periods, the second, third, and sixth valves are opened, and the first, fourth, and fifth valves are closed. High-pressure carbon dioxide from the high-pressure storage tank absorbs heat from the high-temperature molten salt in the high-temperature storage tank through the second molten salt heat exchanger and then enters the first expander to expand and do work. After the heat exchange is completed, the molten salt re-enters the low-temperature storage tank, and the low-pressure carbon dioxide re-enters the low-pressure storage tank to prepare for the next energy storage. The energy release process ends.

[0019] The low-heat cycle module serves as the bottom cycle, where the low-temperature, high-pressure working fluid absorbs the low-heat of the flue gas in the evaporator and then performs work in the second expander.

[0020] This invention relates to a flue gas low-heat recovery system and its control method based on reverse Brayton cycle heat pump energy storage. It is a coal-fired flue gas low-heat recovery system based on a cascade heat pump. Through the synergistic effect of low-temperature stage and high-temperature stage heat pump cycles, it achieves graded utilization of low-grade and low-grade flue gas low-heat, making full use of both high-heat and low-heat energy. By storing energy during off-peak hours and releasing energy during peak hours, it optimizes the spatiotemporal allocation of energy, significantly improves the overall energy utilization efficiency, effectively recovers low-grade flue gas low-heat, further expands the unit's peak-shaving range, and smooths load fluctuations. Attached Figure Description

[0021] To more clearly illustrate the technical solutions in the embodiments of this application, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of this application. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0022] Figure 1 This is a schematic diagram of a flue gas low-heat recovery system based on reverse Brayton cycle pump thermal energy storage according to the present invention.

[0023] In the diagram: 1. Boiler, 2. Superheater, 3. Air preheater, 4. First valve, 5. First molten salt heat exchanger, 6. High-temperature storage tank, 7. Third valve, 8. Second molten salt heat exchanger, 9. Low-temperature storage tank, 10. Fourth valve, 11. First expander, 12. Low-pressure storage tank, 13. Fifth valve, 14. Compressor, 15. High-pressure storage tank, 16. Sixth valve, 17. Second valve, 18. Mixer, 19. Condenser, 20. Pump, 21. Regenerator, 22. Evaporator, 23. Second expander. Detailed Implementation

[0024] The present invention will now be described in further detail with reference to the accompanying drawings and specific embodiments.

[0025] Coal-fired power generation, as a crucial pillar of the current power system, often directly releases low- and medium-temperature flue gas into the atmosphere during operation, resulting in a significant waste of low-grade heat energy. Simultaneously, the large-scale grid integration of renewable energy sources places higher demands on the stability and flexibility of the power system, necessitating improvements in the peak-shaving and frequency regulation capabilities of coal-fired units. Therefore, this invention provides a low-heat recovery system for flue gas based on reverse Brayton cycle pump thermal energy storage and its control method, such as… Figure 1 As shown, the system includes: a boiler module, a high-heat circulation module, and a low-heat circulation module; the output end of the boiler module is connected to the heat exchange components of the high-heat circulation module and the low-heat circulation module.

[0026] In the boiler module, the flue gas outlet of the superheater 2 in boiler 1 is connected to the first inlet of the mixer 18 via the first valve 4 and the high-heat circulation module; the flue gas outlet of the air preheater 3 is connected to the second inlet of the mixer 18 via the second valve 17; and the outlet of the mixer 18 is connected to the boiler module via the low-heat circulation module.

[0027] Preferably, the high-temperature cycle module is a carbon dioxide energy storage module, which preferably includes a thermal energy module and a pressure energy module; wherein, the molten salt side of the first molten salt heat exchanger 5 is connected in series with the high-temperature storage tank 6, the third valve 7, the molten salt side of the second molten salt heat exchanger 8, the low-temperature storage tank 9, and the fourth valve 10 to form a loop, which is the thermal energy module; the working fluid side of the second molten salt heat exchanger 8 is connected in series with the first expander 11, the low-pressure storage tank 12, the fifth valve 13, the compressor 14, the high-pressure storage tank 15, and the sixth valve 16 to form a loop, which is the pressure energy module.

[0028] Preferably, the low-heat cycle module includes: the working fluid side outlet of the first condenser 9 is connected to the working fluid side inlet of the regenerator 21 via a pump 20; the working fluid side outlet of the regenerator 21 is connected in series with the working fluid side of the evaporator 22, the second expander 23, and the medium side inlet of the regenerator 21; the medium side outlet of the regenerator 21 is connected to the working fluid side inlet of the first condenser 9; condensate is introduced into the medium side inlet of the first condenser 9; the medium side outlet of the first condenser 9 is externally connected to a low-temperature heater (for example, connected to a certain stage low-temperature heater, replacing part of the extracted steam); the outlet of the mixer 18 is connected to the medium side inlet of the evaporator 22; and the medium side outlet of the evaporator 22 is connected to the dust removal device in the boiler 1.

[0029] Preferably, the flue gas temperature at the outlet of the superheater 2 is about 600°C. After heat exchange in the first molten salt heat exchanger 5, the temperature drops to about 200°C. In the mixer 18, it is mixed with flue gas from the air preheater 3 at about 100°C, thereby improving the low heat recovery rate and optimizing the overall operating performance of the system.

[0030] Preferably, the heat storage medium in the high-temperature storage tank 6 and the low-temperature storage tank 9 is solar salt, and the temperature range is set to 220-550℃ to achieve efficient heat storage and release, while ensuring the stability and economy of the system over a wide temperature range.

[0031] Preferably, the pressure of the high-pressure storage tank 15 is set to 8-12 MPa, and the pressure of the low-pressure storage tank is set to 5-2 MPa, so as to balance the energy density and safety of the carbon dioxide energy storage module and ensure the efficient and stable operation of the system.

[0032] Preferably, the compressor 14 is a positive displacement compressor, the first expander 11 is a turbine expander, and the second expander 23 is a screw expander.

[0033] Preferably, in the waste heat circulation module, the organic working fluid is R245fa, and the temperature difference between the ends of each heat exchanger is specified to be 5-10℃ to ensure heat exchange efficiency and safe and stable operation.

[0034] On the other hand, the present invention provides a control method for controlling any of the above-mentioned low-heat flue gas recovery systems, comprising:

[0035] 1. Energy storage process: During off-peak electricity hours, open the first valve 4, the second valve 17, the fourth valve 10 and the fifth valve 13, and close the third valve 7 and the sixth valve 16. Use the excess power of the generator set to drive the compressor 14 to compress the carbon dioxide from the low-pressure storage tank 12 to a high-temperature and high-pressure state and store it in the high-pressure storage tank 15. The molten salt in the low-temperature storage tank 9 exchanges heat with the high-temperature flue gas in the first molten salt heat exchanger. The high-temperature molten salt after absorbing heat is stored in the high-temperature storage tank 6, and the energy storage process ends.

[0036] 2. Energy release process: During peak power periods, the second valve 17, the third valve 7, and the sixth valve 16 are opened, while the first valve 4, the fourth valve 10, and the fifth valve 13 are closed. The high-pressure carbon dioxide from the high-pressure storage tank 15 absorbs the heat from the high-temperature molten salt in the high-temperature storage tank 6 through the second molten salt heat exchanger and then enters the first expander 11 to expand and do work. After the heat exchange is completed, the molten salt re-enters the low-temperature storage tank 9, and the low-pressure carbon dioxide re-enters the low-pressure storage tank 12 to prepare for the next cycle. The energy release process ends.

[0037] 3. The low-heat circulation module serves as the bottom circulation. The low-temperature and high-pressure working fluid absorbs the low-heat of the flue gas in the evaporator 22 and then does work in the second expander 23.

[0038] The beneficial effects of this invention are:

[0039] 1) This invention provides a flue gas low-heat recovery system and its control method based on reverse Brayton cycle heat pump energy storage. It is a coal-fired flue gas low-heat recovery system based on cascade heat pump, which can effectively recover low-grade flue gas low-heat, further expand the unit's peak-shaving range, and smooth load fluctuations.

[0040] Specifically, in this energy storage process, electrical energy and low-grade heat energy are converted into high-grade heat energy and pressure energy for storage; in this energy release process, high-grade heat energy and pressure energy are converted into low-grade heat energy and electrical energy for release; while the low-heat cycle module continues to operate during both energy storage and energy release processes, absorbing waste heat. Through the synergistic effect of the low-temperature stage and high-temperature stage heat pump cycles, the graded utilization of high-grade and low-grade flue gas heat is achieved, and both high-heat and low-heat energy are fully utilized. Furthermore, by storing energy during off-peak hours and releasing energy during peak hours, the spatiotemporal allocation of energy is optimized, significantly improving overall energy utilization efficiency.

[0041] 2) This invention enhances the system's flexibility and economy. The high-heat circulation module can store electrical and thermal energy during periods of low electricity demand and release energy during periods of high electricity demand, while also responding quickly to frequency fluctuations in the power system, thus improving system flexibility. Furthermore, by recovering low-heat from flue gas and utilizing low-priced electricity during off-peak hours, the system's operating costs are reduced, improving its economic efficiency.

[0042] 3) This invention also has significant environmental and sustainable development benefits. By efficiently recovering the low-heat of coal-fired flue gas and reducing steam extraction from the unit, it reduces energy waste in coal-fired power plants and indirectly reduces carbon emissions. The low-heat cycle module uses environmentally friendly working fluid R245fa, reducing negative environmental impacts. Furthermore, the system achieves the recycling of low-heat flue gas, electrical energy, and thermal energy, which aligns with the concept of sustainable development.

[0043] The above control method is based on the flue gas low-heat recovery system and will not be described in detail here. The above embodiments are only used to illustrate the technical solutions of this application and are not intended to limit them; although this application has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some of the technical features; and these modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the spirit and scope of the technical solutions of the embodiments of this application, and should all be included within the protection scope of this application.

Claims

1. A flue gas low-heat recovery system based on reverse-Brayton cycle heat pump energy storage, characterized in that: Comprise: a boiler module, a high-temperature cycle module and a low-temperature cycle module; in the boiler module, the superheater flue gas side outlet in the boiler is connected with the first valve, the high-temperature cycle module and the first inlet of the mixer through the second valve; the air preheater flue gas side outlet is connected with the second inlet of the mixer; the outlet of the mixer is connected with the low-temperature cycle module and the boiler module; the high-temperature cycle module comprises a thermal energy module and a pressure energy module, wherein: the first molten salt heat exchanger molten salt side is connected with the high-temperature storage tank, the third valve, the second molten salt heat exchanger molten salt side, the low-temperature storage tank and the fourth valve in series to form a loop for the thermal energy module; the second molten salt heat exchanger working medium side is connected with the first expander, the low-pressure storage tank, the fifth valve, the compressor, the high-pressure storage tank and the sixth valve in series to form a loop for the pressure energy module; the low-temperature cycle module comprises a first condenser working medium side outlet connected with a pump and a regenerator working medium side inlet, a regenerator working medium side outlet connected with an evaporator working medium side, a second expander and a regenerator medium side inlet in series, and a regenerator medium side outlet connected with a first condenser working medium side inlet; the flue gas low-temperature recovery system is controlled by the following methods: energy storage process: during the valley electricity period, the first valve, the second valve, the fourth valve and the fifth valve are opened, the third valve and the sixth valve are closed, the compressor is driven by the excess power of the generator set, the carbon dioxide from the low-pressure storage tank is compressed to a high-temperature and high-pressure state and stored in the high-pressure storage tank, and the molten salt in the low-temperature storage tank is heated in the first molten salt heat exchanger with the high-temperature flue gas, the high-temperature molten salt after heat absorption is stored in the high-temperature storage tank, and the energy storage process is completed; energy release process: during the peak electricity period, the second valve, the third valve and the sixth valve are opened, the first valve, the fourth valve and the fifth valve are closed, the high-pressure carbon dioxide from the high-pressure storage tank is heated by the second molten salt heat exchanger after absorbing the heat of the high-temperature molten salt in the high-temperature storage tank, and then enters the first expander to do work, the molten salt after heat exchange reenters the low-temperature storage tank, and the low-pressure carbon dioxide after work reenters the low-pressure storage tank for the next energy storage, and the energy release process is completed; the low-temperature cycle module serves as a bottom cycle, and the working medium at low temperature and high pressure does work in the second expander after absorbing the low-temperature heat of the flue gas in the evaporator; the flue gas temperature of the superheater flue gas side outlet is about 600℃, which is reduced to about 200℃ after heat exchange in the first molten salt heat exchanger, and then mixed with the flue gas at about 100℃ from the air preheater in the mixer.

2. The low level heat recovery system of flue gases according to claim 1, characterized in that: The heat storage medium in the high-temperature storage tank and the low-temperature storage tank is solar salt, and the temperature range is set to 220-550℃.

3. The low level heat recovery system of flue gases according to claim 2, characterized in that: The pressure of the high-pressure storage tank is set to 8-12MPa, and the pressure of the low-pressure storage tank is set to 5-2MPa.

4. The low level heat recovery system of flue gases according to claim 3, characterized in that: The compressor is a positive displacement compressor.

5. The low level heat recovery system of flue gases according to claim 4, characterized in that: The first expander is a turbine expander, and the second expander is a screw expander.

6. The low level heat recovery system of flue gases according to any of claims 1-5, characterized in that: The low-temperature cycle module further comprises a first condenser medium side inlet connected with condensing water, a first condenser medium side outlet connected with a low-temperature heater, a mixer outlet connected with an evaporator medium side inlet, and an evaporator medium side outlet connected with a dust removal device in the boiler.

7. The low level heat recovery system of flue gases according to claim 6, characterized in that: In the waste heat cycle module, the organic working medium is R245fa, and the temperature difference of each heat exchanger is set to 5-10℃.

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