A waste heat recovery system based on supercritical carbon dioxide Brayton cycle

Abstract

The application discloses a waste heat recovery system based on supercritical carbon dioxide Brayton cycle, and relates to the field of waste heat recovery systems.The waste heat recovery system comprises a waste heat recovery unit, a reverse osmosis unit and a seawater preheating unit.The waste heat recovery unit is used for supercritical carbon dioxide Brayton cycle of high-temperature and high-pressure carbon dioxide, and the electric energy generated by the supercritical carbon dioxide Brayton cycle is used for reverse osmosis separation of seawater, and the low-temperature waste heat in the waste heat recovery unit is used for preheating of seawater.The reverse osmosis unit is used for reverse osmosis separation of seawater, so that the electric energy generated by the supercritical carbon dioxide cycle in the waste heat recovery unit is used for driving the reverse osmosis system, and the low-temperature waste heat is used for preheating of the inlet seawater of the reverse osmosis system, thereby realizing efficient utilization of waste heat and improving the efficiency of the waste heat recovery system.

Description

Technical Field

[0001] The present application relates to the field of waste heat recovery technology, and more specifically, to a waste heat recovery system based on a supercritical carbon dioxide Brayton cycle. Background Art

[0002] Gas turbines fueled by natural gas are efficient and clean power generation systems. However, since most of the heat energy is discharged into the environment in the form of waste heat, the thermal efficiency of a single gas turbine is still limited. Waste heat recovery of gas turbines has become an important means to improve the efficiency of gas turbines.

[0003] Supercritical carbon dioxide refers to a carbon dioxide fluid with a temperature and pressure above the critical value. As a working fluid in thermal energy circulation, supercritical carbon dioxide fluid has the following advantages: (1) The critical density of 0.448g / m3 is close to that of liquid and 2 orders of magnitude greater than that of gas, with high heat transfer efficiency and strong work capacity; (2) The viscosity is close to that of gas and 2 orders of magnitude less than that of liquid; it has strong fluidity, is easy to diffuse, and has low system circulation loss; (3) The critical temperature and pressure are low, and it is easy to reach the supercritical state, which is convenient for engineering applications; (4) The density is high and the compressibility is good compared to the commonly used inert gas supercritical fluid, and the system equipment structure is compact and small in size; (5) The corrosiveness is less than that of water vapor; (6) It is non-toxic, non-flammable, stable, has no damage to the ozone layer, and is cheap and easy to obtain.

[0004] Carbon dioxide is used as a working fluid in power cycles and can transfer a large amount of energy in a very small volume. The supercritical carbon dioxide cycle used as a working fluid for power generation has the characteristics of being environmentally friendly, highly thermally efficient, and economical. The Brayton cycle is a typical thermodynamic cycle that uses gas as a working fluid and achieves efficient energy conversion through four processes: adiabatic compression, isobaric heat absorption, adiabatic expansion, and isobaric cooling. Compared with the traditional steam Rankine cycle, the Brayton cycle has a higher cycle efficiency, and when the working fluid is in a supercritical state, its cycle efficiency can be greatly improved because the change in the working fluid phase is avoided and the consumption of compression work is reduced.

[0005] The reverse osmosis process has become the most widely used process due to its simple layout and high efficiency. The reverse osmosis desalination process is a high-energy consumption process that requires a large amount of electricity.

[0006] Therefore, how to provide a waste heat recovery system based on the supercritical carbon dioxide Brayton cycle to achieve the combination of waste heat recovery and reverse osmosis, reduce energy loss, and achieve efficient energy utilization is a technical problem that needs to be solved. Summary of the Invention

[0007] The present invention provides a waste heat recovery system based on a supercritical carbon dioxide Brayton cycle to solve the technical problems of low waste heat recovery efficiency and high energy consumption in the reverse osmosis process in the prior art. The system comprises:

[0008] A waste heat recovery unit is used to perform a supercritical carbon dioxide Brayton cycle on high-temperature and high-pressure carbon dioxide, and to use the electricity generated by the supercritical carbon dioxide Brayton cycle for reverse osmosis separation of seawater, and to use the low-temperature waste heat in the waste heat recovery unit for preheating seawater;

[0009] A reverse osmosis unit is used for reverse osmosis separation of seawater.

[0010] In some embodiments of the present application, the waste heat recovery unit includes a turbine, a heat exchanger, a cooler, a compressor, a low-temperature heater and a high-temperature heater.

[0011] In some embodiments of the present application, the reverse osmosis unit includes a preheater, a high-pressure pump, a reverse osmosis membrane, a water tank, and an energy recovery turbine.

[0012] In some embodiments of the present application, the turbine is respectively connected to the high-temperature heater and the heat exchanger, the heat exchanger is respectively connected to the cooler and the high-temperature heater, the cooler is respectively connected to the compressor and the high-pressure pump, the compressor is respectively connected to the heat exchanger and the low-temperature heater, and the low-temperature heater is respectively connected to the preheater and the high-temperature heater.

[0013] In some embodiments of the present application, the high-pressure pump is connected to the cooler, the reverse osmosis membrane and the preheater respectively, and the reverse osmosis membrane is connected to the water tank and the energy recovery turbine respectively.

[0014] In some embodiments of the present application, the waste heat recovery unit is specifically used to:

[0015] The high-temperature and high-pressure carbon dioxide is discharged as a working fluid after being expanded in a turbine to generate electricity;

[0016] The working fluid flows into the heat exchanger and the cooler in sequence to release heat, and the working fluid after releasing heat flows into the compressor;

[0017] The compressor compresses the working fluid after releasing heat into high pressure and generates a high-pressure compressed working medium, wherein the high-pressure compressed working medium includes a first-path compressed working medium and a second-path compressed working medium;

[0018] The first compressed working fluid flows into the low-temperature heater and absorbs heat from the flue gas, and the second compressed working fluid flows into the heat exchanger and absorbs heat from the low-pressure carbon dioxide;

[0019] The first compressed working fluid flowing out of the low-temperature heater and the second compressed working fluid flowing out of the heat exchanger are mixed and flow into the high-temperature heater together to absorb heat, thereby completing the supercritical carbon dioxide Brayton cycle.

[0020] In some embodiments of the present application, the reverse osmosis unit is specifically used for:

[0021] The seawater is preheated through a preheater and a cooler, and the preheated seawater is input into a high-pressure pump;

[0022] The high-pressure pump is used to perform high-pressure treatment on the seawater, and the reverse osmosis separation is performed on the seawater through the reverse osmosis membrane;

[0023] The water produced by reverse osmosis separation is transported to a water tank for storage, and the brine produced by reverse osmosis separation is discharged after passing through an energy recovery turbine.

[0024] In some embodiments of the present application, the cooler is a two-stage cooler, and the cooler is specifically used for:

[0025] The cooler receives seawater, cools the carbon dioxide through the seawater, and preheats the seawater through the carbon dioxide;

[0026] A predetermined volume of seawater is discharged through the cooler, and the remaining seawater continues to be preheated.

[0027] By applying the above technical solution, the waste heat recovery system includes: a waste heat recovery unit, which is used to perform a supercritical carbon dioxide Brayton cycle on high-temperature and high-pressure carbon dioxide, and use the electricity generated by the supercritical carbon dioxide Brayton cycle for reverse osmosis separation of seawater, and use the low-temperature waste heat in the waste heat recovery unit for preheating seawater; a reverse osmosis unit, which is used to perform reverse osmosis separation of seawater, so that the electricity generated by the supercritical carbon dioxide cycle in the waste heat recovery unit is used to drive the reverse osmosis system, and at the same time, the low-temperature waste heat is used to preheat the inlet seawater of the reverse osmosis system, thereby realizing efficient utilization of waste heat and improving the efficiency of the waste heat recovery system. BRIEF DESCRIPTION OF THE DRAWINGS

[0028] In order to more clearly illustrate the technical solutions in the embodiments of the present application, the following briefly introduces the drawings required for use in the description of the embodiments. Obviously, the drawings described below are only some embodiments of the present application. For those skilled in the art, other drawings can be obtained based on these drawings without creative work.

[0029] Figure 1 A schematic structural diagram of a waste heat recovery system based on a supercritical carbon dioxide Brayton cycle proposed in an embodiment of the present invention is shown;

[0030] Description of labels:

[0031] 11. Turbine; 12. Heat exchanger; 13. Cooler; 14. Compressor; 15. Low-temperature heater; 16. High-temperature heater; 17. Preheater; 18. High-pressure pump; 19. Reverse osmosis membrane; 20. Water tank; 21. Energy recovery turbine. DETAILED DESCRIPTION

[0032] The following will be combined with the drawings in the embodiments of this application to clearly and completely describe the technical solutions in the embodiments of this application. Obviously, the embodiments described are only part of the embodiments of this application, not all of the embodiments. Based on the embodiments in this application, all other embodiments obtained by ordinary technicians in this field without making creative efforts are within the scope of protection of this application.

[0033] In the description of this application, it should be understood that the terms "center", "up", "down", "front", "back", "left", "right", "vertical", "horizontal", "top", "bottom", "inside", "outside", etc., indicating the orientation or position relationship, are based on the orientation or position relationship shown in the accompanying drawings, and are only for the convenience of describing this application and simplifying the description, and do not indicate or imply that the device or element referred to must have a specific orientation, be constructed and operated in a specific orientation, and therefore cannot be understood as a limitation on this application.

[0034] The terms "first" and "second" are used for descriptive purposes only and should not be construed as indicating or implying relative importance or implicitly specifying the number of the technical features being referred to. Thus, a feature specified as "first" or "second" may explicitly or implicitly include one or more of such features. Throughout this application, unless otherwise specified, "plurality" means two or more.

[0035] In the description of this application, it should be noted that, unless otherwise expressly specified or limited, the terms "mounted," "connected," and "connected" should be understood in a broad sense. For example, they can refer to fixed connections, detachable connections, or integral connections; mechanical connections or electrical connections; direct connections or indirect connections through an intermediate medium; and internal connections between two components. Those skilled in the art will understand the specific meanings of the above terms in this application based on the specific circumstances.

[0036] The present application provides a waste heat recovery system based on a supercritical carbon dioxide Brayton cycle, such as Figure 1 As shown, the system includes:

[0037] A waste heat recovery unit is used to perform a supercritical carbon dioxide Brayton cycle on high-temperature and high-pressure carbon dioxide, and to use the electricity generated by the supercritical carbon dioxide Brayton cycle for reverse osmosis separation of seawater, and to use the low-temperature waste heat in the waste heat recovery unit for preheating seawater;

[0038] A reverse osmosis unit is used for reverse osmosis separation of seawater.

[0039] like Figure 1 As shown, this solution combines waste heat recovery with reverse osmosis. The waste heat recovery system in this solution uses both electricity and heat to drive reverse osmosis to produce fresh water. Typically, the high-pressure pump in a reverse osmosis unit is electrically driven. The performance of a reverse osmosis system improves with increasing operating temperature. The electricity generated by the supercritical carbon dioxide Brayton cycle in this system is used to drive the reverse osmosis system, while the low-temperature waste heat is used to preheat the influent seawater. The seawater enters the reverse osmosis system in two separate routes, where it is simultaneously preheated by the waste heat from the supercritical carbon dioxide cycle and the low-temperature flue gas.

[0040] In addition, compared to water vapor, supercritical carbon dioxide is more efficient as a working fluid at temperatures above 400°C. At temperatures of 550°C, the efficiency of converting thermal energy into output electrical energy in a supercritical carbon dioxide power generation system can generally reach 45%. Supercritical carbon dioxide does not require very high circulation temperatures to achieve ideal conversion efficiency. At the same time, supercritical carbon dioxide has stable chemical properties and corrodes metal pipe equipment at a slower rate than high-temperature and high-pressure water vapor, so the requirements for the material of pressure-bearing equipment are relatively low. At the same time, the power generation system of this hydropower cogeneration system does not require a supporting water treatment system, saving a large amount of water resources and the consumption of water treatment chemicals. In addition, due to the physical properties of low viscosity and high density, supercritical carbon dioxide has typical advantages such as good fluidity, high heat transfer efficiency, and low compressibility. Therefore, the system's key components such as the compressor and turbine are small in size and compact in structure, taking up little space.

[0041] To further illustrate the waste heat recovery unit, in some embodiments of the present application, the waste heat recovery unit includes a turbine, a heat exchanger, a cooler, a compressor, a low-temperature heater, and a high-temperature heater.

[0042] To further illustrate the reverse osmosis unit, in some embodiments of the present application, the reverse osmosis unit includes a preheater, a high-pressure pump, a reverse osmosis membrane, a water tank, and an energy recovery turbine.

[0043] In order to ensure the normal operation of the waste heat recovery unit, in some embodiments of the present application, the turbine is respectively connected to the high-temperature heater and the heat exchanger, the heat exchanger is respectively connected to the cooler and the high-temperature heater, the cooler is respectively connected to the compressor and the high-pressure pump, the compressor is respectively connected to the heat exchanger and the low-temperature heater, and the low-temperature heater is respectively connected to the preheater and the high-temperature heater.

[0044] The relationship between the components in the waste heat recovery unit is as follows Figure 1 As shown, the flue gas circulation sequence is based on the process of g1-g2-g3-g4, and the supercritical carbon dioxide Brayton cycle process in this scheme starts from the turbine and follows Figure 1 Cycle in the direction of the arrows numbered 1-6.

[0045] To further illustrate the reverse osmosis unit, in some embodiments of the present application, the high-pressure pump is connected to the cooler, the reverse osmosis membrane and the preheater respectively, and the reverse osmosis membrane is connected to the water tank and the energy recovery turbine respectively.

[0046] In order to ensure the normal operation of the supercritical carbon dioxide Brayton cycle, in some embodiments of the present application, the waste heat recovery unit is specifically used to:

[0047] The high-temperature and high-pressure carbon dioxide is discharged as a working fluid after being expanded in a turbine to generate electricity;

[0048] The working fluid flows into the heat exchanger and the cooler in sequence to release heat, and the working fluid after releasing heat flows into the compressor;

[0049] The compressor compresses the working fluid after releasing heat into high pressure and generates a high-pressure compressed working medium, wherein the high-pressure compressed working medium includes a first-path compressed working medium and a second-path compressed working medium;

[0050] The first compressed working fluid flows into the low-temperature heater and absorbs heat from the flue gas, and the second compressed working fluid flows into the heat exchanger and absorbs heat from the low-pressure carbon dioxide;

[0051] The first compressed working fluid flowing out of the low-temperature heater and the second compressed working fluid flowing out of the heat exchanger are mixed and flow into the high-temperature heater together to absorb heat, thereby completing the supercritical carbon dioxide Brayton cycle.

[0052] In this embodiment, high-temperature, high-pressure carbon dioxide expands in a turbine to generate electricity. The exhausted working fluid then flows into a heat exchanger and a cooler in sequence to release heat. Afterwards, the working fluid that has released heat flows into a compressor, which compresses the working fluid that has released heat under high pressure. The high-pressure compressed working fluid is divided into two parts (a first-path compressed working fluid and a second-path compressed working fluid): the first-path compressed working fluid flows into a low-temperature heater to absorb heat from the flue gas; the second-path compressed working fluid enters a heat exchanger to absorb heat from the low-pressure carbon dioxide. The first-path compressed working fluid and the second-path compressed working fluid are mixed at point 6, and then absorb heat in a high-temperature heater to complete the supercritical carbon dioxide Brayton cycle.

[0053] In order to ensure the normal operation of reverse osmosis of seawater, in some embodiments of the present application, the reverse osmosis unit is specifically used to:

[0054] The seawater is preheated through a preheater and a cooler, and the preheated seawater is input into a high-pressure pump;

[0055] The high-pressure pump is used to perform high-pressure treatment on the seawater, and the reverse osmosis separation is performed on the seawater through the reverse osmosis membrane;

[0056] The water produced by reverse osmosis separation is transported to a water tank for storage, and the brine produced by reverse osmosis separation is discharged after passing through an energy recovery turbine.

[0057] In this embodiment, Figure 1 As shown, numerals 01-07 represent the reverse osmosis process for seawater. The seawater is preheated by circulating flue gas and waste heat from carbon dioxide, respectively. The seawater is supplied from both the preheater and the cooler. The preheater is preheated by circulating flue gas, while the cooler is preheated by waste heat from the circulating carbon dioxide. The preheated seawater is pumped to a high pressure by a high-pressure pump and passed through a reverse osmosis membrane for reverse osmosis separation. The produced water is collected in a tank, and the brine is discharged after passing through an energy recovery turbine.

[0058] This embodiment uses both electricity and heat to drive reverse osmosis to produce fresh water. Typically, the high-pressure pump in a reverse osmosis unit is electrically driven, and the unit's performance improves with increasing operating temperature. In this system, the electricity generated by the supercritical carbon dioxide cycle is used to drive the reverse osmosis unit, while low-temperature waste heat is used to preheat the influent seawater. The seawater enters the reverse osmosis system in two ways and is simultaneously preheated by the waste heat from the supercritical carbon dioxide cycle and the low-temperature flue gas, fully utilizing the waste heat from the carbon dioxide cycle and reducing energy loss.

[0059] In order to fully utilize the waste heat of seawater, in some embodiments of the present application, the cooler is a two-stage cooler, which is specifically used to:

[0060] The cooler receives seawater, cools the carbon dioxide through the seawater, and preheats the seawater through the carbon dioxide;

[0061] A predetermined volume of seawater is discharged through the cooler, and the remaining seawater continues to be preheated.

[0062] In this embodiment, the cooler is a two-stage cooler, in which the feed seawater first flows into the cooler to cool the carbon dioxide. Then, a portion of the seawater is discharged directly, and the other portion of the seawater is further preheated to a higher temperature. When the state point is far away from the critical point, the heat capacity of the carbon dioxide is almost constant. However, as the state point gets closer and closer to the critical point, the heat capacity increases rapidly. In contrast, the heat capacity of the cooling water is almost constant. If a traditional heat exchanger is used as a cooler, when the flow rate of the cooling water is small, the heat exchange temperature difference may be very small. On the other hand, when the flow rate of the cooling water is large, the outlet temperature of the seawater may not be able to be preheated to a given temperature. In order to overcome this problem, a two-stage cooler is used. By discharging most of the feed seawater in the middle of the cooler, the average temperature difference of the cooler can be controlled within an acceptable range, and the remaining seawater can be fully preheated.

[0063] Therefore, a two-stage cooler is used to maximize the CO2 preheating temperature. Due to the unique thermodynamic properties of CO2, when the operating temperature approaches the critical point, the specific heat capacity of high-pressure CO2 is much higher than that of low-pressure CO2. Therefore, a portion of the CO2 is diverted to the low-temperature heater to match the heat capacity of the recuperator. Clearly, there is an optimal split ratio at which the heat capacities on both sides of the heat exchanger are perfectly matched. If the split ratio is greater than this optimal value, the CO2 on the cold side of the heat exchanger will not be fully preheated. If the split ratio is less than this optimal value, the waste heat of the CO2 on the hot side of the heat exchanger will not be fully recovered.

[0064] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present application, rather than to limit them. Although the present application has been described in detail with reference to the aforementioned embodiments, those skilled in the art should understand that they can still modify the technical solutions described in the aforementioned embodiments, or make equivalent replacements for some of the technical features therein. However, these modifications or replacements 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 the present application.

Claims

1. A waste heat recovery system based on supercritical carbon dioxide Brayton cycle, characterized in that: The system comprises: A waste heat recovery unit is used to perform a supercritical carbon dioxide Brayton cycle on high-temperature and high-pressure carbon dioxide, and to use the electricity generated by the supercritical carbon dioxide Brayton cycle for reverse osmosis separation of seawater, and to use the low-temperature waste heat in the waste heat recovery unit for preheating seawater; Reverse osmosis unit, used for reverse osmosis separation of seawater; Wherein, the waste heat recovery unit includes a turbine, a heat exchanger, a cooler, a compressor, a low-temperature heater and a high-temperature heater; The reverse osmosis unit includes a preheater, a high-pressure pump, a reverse osmosis membrane, a water tank and an energy recovery turbine; The turbine is connected to the high-temperature heater and the heat exchanger respectively, the heat exchanger is connected to the cooler and the high-temperature heater respectively, the cooler is connected to the compressor and the high-pressure pump respectively, the compressor is connected to the heat exchanger and the low-temperature heater respectively, and the low-temperature heater is connected to the preheater and the high-temperature heater respectively; The high-pressure pump is connected to the cooler, the reverse osmosis membrane and the preheater respectively, and the reverse osmosis membrane is connected to the water tank and the energy recovery turbine respectively; The waste heat recovery unit is specifically used for: The high-temperature and high-pressure carbon dioxide is discharged as a working fluid after being expanded in a turbine to generate electricity; The working fluid flows into the heat exchanger and the cooler in sequence to release heat, and the working fluid after releasing heat flows into the compressor; The compressor compresses the working fluid after releasing heat into high pressure and generates a high-pressure compressed working medium, wherein the high-pressure compressed working medium includes a first-path compressed working medium and a second-path compressed working medium; The first compressed working fluid flows into the low-temperature heater and absorbs heat from the flue gas, and the second compressed working fluid flows into the heat exchanger and absorbs heat from the low-pressure carbon dioxide; The first compressed working fluid flowing out of the low-temperature heater and the second compressed working fluid flowing out of the heat exchanger are mixed and flow into the high-temperature heater together to absorb heat, thereby completing the supercritical carbon dioxide Brayton cycle.

2. The waste heat recovery system according to claim 1, characterized in that: The reverse osmosis unit is specifically used for: The seawater is preheated through a preheater and a cooler, and the preheated seawater is input into a high-pressure pump; The high-pressure pump is used to perform high-pressure treatment on the seawater, and the reverse osmosis separation is performed on the seawater through the reverse osmosis membrane; The water produced by reverse osmosis separation is transported to a water tank for storage, and the brine produced by reverse osmosis separation is discharged after passing through an energy recovery turbine.

3. The waste heat recovery system according to claim 2, characterized in that: The cooler is a two-stage cooler, and the cooler is specifically used for: The cooler receives seawater, cools the carbon dioxide through the seawater, and preheats the seawater through the carbon dioxide; A predetermined volume of seawater is discharged through the cooler, and the remaining seawater continues to be preheated.

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