A multi-stage power generation system utilizing LNG cold energy and its working method

Abstract

The present invention discloses an LNG cold energy multi-stage power generation system and its working method, which includes a liquefied natural gas direct expansion power generation subsystem, a supercritical carbon dioxide recompression Brayton cycle subsystem, a two-stage Rankine cycle subsystem and a direct cooling subsystem. Depending on the required cooling quality, the cooling energy released during the LNG gasification process is directly used in the two-stage Rankine cycle and the direct cooling subsystem. The supercritical carbon dioxide recompression Brayton cycle is coupled with the two-stage Rankine cycle to indirectly utilize the LNG cold energy, and the gasified natural gas is directly expanded to generate electricity. By setting up a multi-stage power generation system, the overall power generation capacity is significantly improved, the temperature difference between the power generation systems is reduced, and the exergy efficiency and thermal efficiency are further improved compared to conventional LNG cold energy power generation systems. In addition, a solar collector is set up to realize dynamic regulation of the temperature of the combustion products, while taking into account the stability of the system, generating heat sources of different temperature levels. The oxygen-rich combustion products are also liquefied and recovered after multi-stage cooling, achieving zero carbon emissions.

Description

Technical Field

[0001] The present invention relates to the technical field of LNG cold energy power generation, and in particular to a multi-stage power generation system utilizing LNG cold energy and a working method thereof. Background Art

[0002] In recent years, the demand for energy supply and carbon dioxide emissions reduction has continued to grow. Natural gas, due to its high hydrogen-to-carbon ratio and low carbon emission coefficient, has become one of the most widely used energy sources both domestically and internationally. To facilitate large-scale storage and transportation of natural gas and improve its efficiency, natural gas is removed and liquefied after extraction, converting it into LNG. Liquefied natural gas has a temperature of -162°C at atmospheric pressure and releases 823 kJ / kg of cold energy during the regasification process. Recovering this energy for power generation would yield significant economic and social benefits. The cooling capacity of LNG is the energy gained by bringing liquefied natural gas to equilibrium with the environment. This can be evaluated using the parameter exergy, which measures the efficiency of cold energy conversion.

[0003] Basic methods for LNG cold power generation include direct expansion, the Rankine cycle using an intermediate cooling medium, the Brayton cycle, the Kalina cycle, and multi-stage combined cycles. However, single-stage cold power generation systems suffer from high initial investment, unstable operation, and low efficiency.

[0004] In the existing technology, there have been means of using LNG cold energy in a cascaded manner, such as patent CN106150579A. However, it mainly applies LNG cold energy to a two-stage Rankine cycle. The temperature of the gasified natural gas is -38°C, which does not meet the requirements for combustion or direct connection to the natural gas pipeline network.

[0005] Patent CN117268005A proposes a combined refrigeration system based on LNG cold energy power generation. This system utilizes the LNG cold energy in a cascaded manner in an organic Rankine power generation unit, an air conditioning refrigeration unit, and a direct expansion cycle power generation unit. Although this system fully utilizes the LNG cold energy, the temperature span during the release of the LNG cold energy is large, and the temperature difference between the inlet and outlet of the heat exchanger in the Rankine power generation cycle is too large, resulting in large exergy losses in the system.

[0006] While current LNG cold energy power generation technology utilizes LNG cold energy in a tiered manner, it doesn't rationally utilize each tier. Large temperature differences between the inlet and outlet of the heat exchanger ultimately lead to low system exergy efficiency. Furthermore, different power generation methods are suited to different temperature ranges for cold and heat sources. The supercritical CO2 recompression Brayton cycle subsystem requires high-temperature operation to heat the medium, while the two-stage Rankine cycle system relies on low temperatures to liquefy the circulating medium. The Rankine cycle typically uses seawater as a heat source. Due to the low temperature of seawater, the circulating medium in the Rankine cycle is generally unable to utilize the portion of LNG cold energy above -40°C.

[0007] Therefore, it is urgent to propose a comprehensive power generation system that utilizes LNG cold energy to solve the problems of insufficient exergy efficiency and heat source temperature in the existing technology. Summary of the Invention

[0008] In response to the problems existing in the above-mentioned prior art, the present invention proposes a multi-stage power generation system utilizing LNG cold energy, which combines multiple power generation methods such as supercritical carbon dioxide Brayton cycle, two-stage Rankine cycle, and LNG direct expansion, and uses solar collectors and natural gas combustion chambers as common heat sources to increase the overall power generation while reducing the temperature difference between the inlet and outlet media of each power generation system and each component in each power generation system. Compared with conventional power generation systems utilizing LNG cold energy, it also has better exergy efficiency and thermal efficiency.

[0009] In order to achieve the above technical objectives, the present invention provides the following technical solutions:

[0010] A multi-stage power generation system utilizing LNG cold energy, specifically comprising:

[0011] LNG direct expansion subsystem, supercritical CO2 recompression Brayton cycle subsystem, two-stage Rankine cycle subsystem and direct cooling subsystem;

[0012] The liquefied natural gas direct expansion power generation subsystem is used for low-temperature power generation and includes:

[0013] LNG cryogenic pump, first-stage multi-stream heat exchanger, second-stage multi-stream heat exchanger, third-stage multi-stream heat exchanger, fourth-stage multi-stream heat exchanger, expander 1, heat exchanger 2, heat exchanger 3, combustion chamber, liquid oxygen pump;

[0014] The supercritical carbon dioxide recompression Brayton cycle subsystem is used for high-temperature power generation and includes:

[0015] Heat exchanger 2, heat exchanger 3, compressor 1, compressor 2, pump 3, first-stage regenerative heat exchanger, second-stage regenerative heat exchanger, heat exchanger 1, steam turbine 2, solar collector, combustion chamber, steam turbine 1, heat exchanger 4, heat exchanger 5, water storage tank, gas-liquid separator; and connected to the liquefied natural gas direct expansion subsystem through a pipeline through the combustion chamber; the combustion chamber is used to burn the fuel with oxygen-enriched combustion to produce high-temperature and high-pressure gas;

[0016] The two-stage Rankine cycle subsystem serves as an intermediate system between the liquefied natural gas direct expansion power generation subsystem and the supercritical carbon dioxide Brayton cycle subsystem, and is used to transfer heat and cold. It includes: a primary Rankine cycle and a secondary Rankine cycle; the primary Rankine cycle is connected to the primary multi-stream heat exchanger via a pipeline, and the secondary Rankine cycle is connected to the secondary multi-stream heat exchanger via a pipeline; the primary multi-stream heat exchanger and the secondary multi-stream heat exchanger are used to exchange heat between natural gas and oxygen and the circulating medium in the two-stage Rankine cycle subsystem;

[0017] The direct cooling subsystem is connected to a three-stage multi-stream heat exchanger via a pipeline. The three-stage multi-stream heat exchanger is used to exchange heat between natural gas and oxygen and a circulating medium of the direct cooling subsystem.

[0018] Furthermore, in the liquefied natural gas direct expansion power generation subsystem:

[0019] The LNG cryogenic pump, the first-stage multi-stream heat exchanger, the second-stage multi-stream heat exchanger, the third-stage multi-stream heat exchanger, and the fourth-stage multi-stream heat exchanger are sequentially connected by pipelines. The outlet of the fourth-stage multi-stream heat exchanger is divided into a fuel outlet and an oxidant outlet. The fuel outlet is divided into two branches. Branch 1 is sequentially connected to expander 1, heat exchanger 2, and heat exchanger 3 through a pipeline, and branch 2 is connected to the combustion chamber through a pipeline. Branch 1 is used to transport natural gas into the natural gas pipeline network, and branch 2 is used to transport fuel to the combustion chamber.

[0020] The liquid oxygen pump, the first-stage multi-stream heat exchanger, the second-stage multi-stream heat exchanger, the third-stage multi-stream heat exchanger, and the fourth-stage multi-stream heat exchanger are connected in sequence through pipelines, and the combustion-supporting agent outlet of the fourth-stage multi-stream heat exchanger is connected to the combustion chamber through a pipeline for conveying the combustion-supporting agent thereto.

[0021] Furthermore, in the two-stage Rankine cycle subsystem:

[0022] The first-stage Rankine cycle is used to utilize high-grade cold energy, and includes a first pump, a fourth heat exchanger, and a second expander connected in sequence through pipelines; the inlet of the first pump and the outlet of the second expander are respectively connected to the first-stage multi-stream heat exchanger;

[0023] The secondary Rankine cycle is used to utilize low-grade cold energy, and comprises a second pump, a fifth heat exchanger, and a third expander connected in sequence through pipelines; the inlet of the second pump and the outlet of the third expander (24) are respectively connected to the secondary multi-stream heat exchanger;

[0024] Circulating medium flows through both the primary Rankine cycle and the secondary Rankine cycle.

[0025] More specifically, the circulating medium in the primary Rankine cycle is a mixture of ethylene and isopentane, with a mixing ratio of 0.89:0.11; the circulating medium in the secondary Rankine cycle is a mixture of ethylene and ethane, with a mixing ratio of 0.72:0.28.

[0026] Furthermore, the direct cooling subsystem includes a cold unit and a four-stage multi-stream heat exchanger; the cold unit includes a cold storage and a refrigeration and air-conditioning system, the outlet of which is connected to the inlet of the four-stage multi-stream heat exchanger through a pipeline, and the output of the four-stage multi-stream heat exchanger is used as input.

[0027] Furthermore, in the supercritical carbon dioxide recompression Brayton cycle subsystem:

[0028] The outlet of the heat exchanger five is connected to the inlet of the heat exchanger four through a pipeline; the outlet of the heat exchanger four is connected to the inlet of the gas-liquid separator through a pipeline; a liquid outlet of the gas-liquid separator is connected to the water storage tank through a pipeline, and the gas outlet is connected to the inlet of the three-stage multi-stream heat exchanger, the heat exchanger three, and the compressor two through three pipelines respectively; the outlet of the three-stage multi-stream heat exchanger is connected to the liquid carbon dioxide storage tank through a pipeline; the heat exchanger three, the compressor one, the heat exchanger two, and the pump three are connected in sequence through pipelines; the outlet of the pump three is connected to the inlet of the first-stage heat recovery heat exchanger through a pipeline, and the first-stage heat recovery heat exchanger The outlet of the regenerator is connected to the inlet of the secondary regenerative heat exchanger through a pipeline; the outlet of the secondary regenerative heat exchanger is connected to the inlets of steam turbine 2 and heat exchanger 1 through pipelines respectively; the outlet of compressor 2 is connected to the inlet of the primary regenerative heat exchanger through a pipeline; the outlet of heat exchanger 1 is connected to the solar collector and combustion chamber through pipelines respectively, the outlet of the solar collector is connected to the inlet of heat exchanger 1 through a pipeline, and the outlet of the combustion chamber is connected to the inlet of steam turbine 1 through a pipeline; the steam turbine 1, the secondary regenerative heat exchanger, the primary regenerative heat exchanger, and the heat exchanger 5 are connected in sequence through pipelines.

[0029] In addition, the present invention also provides a working method of the multi-stage power generation system as described above, specifically including: a working method of a liquefied natural gas direct expansion power generation subsystem, a working method of a two-stage Rankine cycle subsystem, and a working method of a supercritical carbon dioxide recompression Brayton cycle subsystem.

[0030] Furthermore, the working method of the liquefied natural gas direct expansion power generation subsystem is specifically as follows:

[0031] After being pressurized by an LNG cryogenic pump, cryogenic liquefied natural gas (LNG) passes through a first-stage, second-stage, third-stage, and fourth-stage multi-stream heat exchanger for heat exchange and gasification. The gas is then output from the fuel outlet of the fourth-stage multi-stream heat exchanger and divided into two branches. The natural gas in branch 1 is directly expanded by expander 1 to generate electricity. The expanded natural gas is then heated and delivered to the natural gas pipeline network through heat exchangers 2 and 3. The natural gas in branch 2 is then directly delivered to the gas chamber to serve as fuel.

[0032] The cryogenic liquid oxygen LO2 flows through a liquid oxygen pump for pressurization, and then passes through a first-stage multi-stream heat exchanger, a second-stage multi-stream heat exchanger, a third-stage multi-stream heat exchanger, and a fourth-stage multi-stream heat exchanger for heat exchange and gasification. The oxygen after heat exchange and gasification flows into the combustion chamber through the combustion-supporting agent outlet of the fourth-stage multi-stream heat exchanger.

[0033] The cryogenic liquefied natural gas and cryogenic liquid oxygen provide cooling capacity for the two-stage Rankine cycle subsystem.

[0034] Furthermore, the working method of the two-stage Rankine cycle subsystem is specifically as follows:

[0035] The first-level Rankine cycle utilizes high-grade cold energy, and the second-level Rankine sub-cycle utilizes low-grade cold energy. The circulating medium in the two sub-cycles is first cooled by the first-level multi-stream heat exchanger and the second-level multi-stream heat exchanger respectively. After cooling, it is respectively input into pump one and pump two for pressurization. After pressurization, it flows into heat exchanger four and heat exchanger five for heating. The heated circulating medium flows into expander two and expander three respectively for expansion and power generation. The expanded circulating medium flows into the first-level multi-stream heat exchanger and the second-level multi-stream heat exchanger again for cooling.

[0036] Furthermore, the working method of the supercritical carbon dioxide recompression Brayton cycle subsystem is specifically as follows:

[0037] The supercritical carbon dioxide recompression Brayton cycle subsystem is heated by a solar collector and a combustion chamber; the solar collector heats the combustion chamber to generate circulating CO2, which is injected into the combustion chamber to control its temperature. By controlling the temperature of the solar collector heat source, the mass and flow rate of the circulating CO2 are dynamically adjusted to control the temperature of the combustion products; the oxygen-enriched combustion of natural gas in the combustion chamber produces a CO2 / H2O mixture; the CO2 / H2O mixture and the circulating CO2 constitute the circulating medium of the supercritical carbon dioxide recompression Brayton cycle subsystem;

[0038] The circulating medium first expands in the first steam turbine to generate electricity. The expanded exhaust gas then passes through the second regenerative heat exchanger, the first regenerative heat exchanger, the fifth heat exchanger, and the fourth heat exchanger to cool down. After cooling, it flows into the gas-liquid separator for gas-liquid separation. The separated liquid water flows into the water storage tank. The separated CO2 includes the circulating CO2 and the remaining CO2 in the combustion products.

[0039] The circulating CO2 is compressed, including multi-stage compression and single-stage compression, specifically:

[0040] Multi-stage compression: A portion of the circulating CO2 flows into heat exchanger 3 for cooling and then flows into compressor 1 for pressurization. The pressurized CO2 flows into heat exchanger 2 for cooling and then flows into pump 3 for re-pressurization. The re-pressurized CO2 flows into the first and second regenerative heat exchangers in sequence for heating and then flows into steam turbine 2 for expansion and power generation.

[0041] Single-stage compression: The other part of the circulating CO2 flows into the second compressor for compression and then flows into the first and second regenerators for heating. After heating, it is combined with the circulating CO2 at the outlet of the second turbine that has undergone multi-stage compression and flows into the first heat exchanger for heating and then flows into the combustion chamber.

[0042] The remaining CO2 in the combustion products flows into the three-stage multi-stream heat exchanger for cooling and condensation after gas-liquid separation, and then flows into the liquid carbon dioxide storage tank.

[0043] More specifically, in the working method of the supercritical carbon dioxide recompression Brayton cycle subsystem,

[0044] The expanded exhaust gas provides heat for the two-stage Rankine cycle subsystem;

[0045] The proportion of the circulating CO2 used for multi-stage compression to the whole CO2 ranges from 0.3 to 0.7.

[0046] Based on the above technical solution, the present invention has the following beneficial effects:

[0047] 1. By employing a supercritical carbon dioxide recompression Brayton cycle subsystem as the high-temperature power generation subsystem, a two-stage Rankine cycle as the medium-temperature power generation subsystem, and a liquefied natural gas direct expansion subsystem as the low-temperature power generation subsystem, the system significantly increases overall power generation while reducing temperature differences between the various power generation subsystems and between the inlet and outlet media of each component, thereby improving the exergy efficiency and thermal efficiency of the entire system. This design achieves efficient energy utilization and optimizes the energy conversion process, with significant energy-saving and environmental benefits.

[0048] 2. A certain amount of CO2 is injected into the natural gas combustion chamber. The mass flow rate of the injected CO2 is dynamically adjusted according to the changes in the heat source of the solar collector, thereby controlling the temperature of the combustion products. This can not only generate heat energy at different temperature levels, but also maintain the stability of the system. This adjustment method can meet the specific requirements of the heat source temperature for various equipment.

[0049] 3. The use of low-temperature liquefied natural gas not only provides the necessary cooling function for the power generation process, but also provides sufficient cooling capacity for other application units that require cooling energy. This efficient energy utilization method realizes the diversified supply of energy and improves the overall energy utilization efficiency.

[0050] 4. Energy is generated through oxygen-enriched combustion of natural gas, followed by multi-stage cooling of the combustion products, ultimately leading to liquefaction. This process not only improves energy efficiency but also achieves zero carbon emissions, which is of great significance to environmental protection.

[0051] 5. Utilizing CO2 supercritical Brayton cycle technology, the CO2 compression process is divided into two stages: multi-stage compression and single-stage compression. By precisely controlling the CO2 ratio in these two stages, the temperature of the pressurized CO2 is effectively regulated. This CO2 intercooling and recompression system not only reduces energy consumption during the compression process but also lowers the demand for heat source calorific value, thereby improving the energy efficiency and economics of the entire power generation process. BRIEF DESCRIPTION OF THE DRAWINGS

[0052] Figure 1 This is an overall schematic diagram of the multi-stage power generation system utilizing LNG cold energy proposed in the present invention;

[0053] Figure 2 Schematic diagram of the supercritical carbon dioxide Brayton cycle subsystem in the present invention;

[0054] Figure 3 This is a schematic diagram of the liquefied natural gas direct expansion subsystem of the present invention;

[0055] Figure 4 Schematic diagram of the two-stage Rankine cycle subsystem in the present invention;

[0056] Figure 5 Schematic diagram of the direct cooling subsystem in the present invention.

[0057] The accompanying drawings are marked as follows: 1. LNG cryogenic pump; 2. Primary multi-stream heat exchanger; 3. Secondary multi-stream heat exchanger; 4. Third-stage multi-stream heat exchanger; 5. Fourth-stage multi-stream heat exchanger; 6. Expander 1; 7. Heat exchanger 2; 8. Heat exchanger 3; 9. Compressor 1; 10. Compressor 2; 11. Pump 3; 12. Primary regenerative heat exchanger; 13. Secondary regenerative heat exchanger; 14. Heat exchanger 1; 15. Steam turbine 2; 16. Solar collector; 17. Combustion chamber; 18. Steam turbine 1; 19. Pump 1; 20. Heat exchanger 4; 21. Expander 2; 22. Pump 2; 23. Heat exchanger 5; 24. Expander 3; 25. Refrigeration unit; 26. Water storage tank; 27. Gas-liquid separator; 28. Liquid oxygen pump; 29. ​​Liquid carbon dioxide storage tank. DETAILED DESCRIPTION

[0058] In order to make the purpose, technical solutions and advantages of the present invention more clearly understood, the present invention will be further described in detail below with reference to the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are only used to explain the present invention and are not intended to limit the present invention.

[0059] Although the steps in the present invention are arranged with numbers, they are not intended to limit the order of the steps. Unless the order of the steps is clearly stated or the execution of a step requires other steps as a basis, the relative order of the steps can be adjusted. It is understood that the term "and / or" used herein refers to and covers any and all possible combinations of one or more of the associated listed items.

[0060] like Figure 1-5 As shown, the present invention proposes a multi-stage power generation system utilizing LNG cold energy, which includes a liquefied natural gas direct expansion power generation subsystem, a supercritical carbon dioxide recompression Brayton cycle subsystem, a two-stage Rankine cycle subsystem and a direct cooling subsystem;

[0061] like Figure 1 、 Figure 3 As shown, the liquefied natural gas direct expansion power generation subsystem is used for low-temperature power generation, which includes:

[0062] LNG cryogenic pump 1, first-stage multi-stream heat exchanger 2, second-stage multi-stream heat exchanger 3, third-stage multi-stream heat exchanger 4, fourth-stage multi-stream heat exchanger 5, expander 1 6, heat exchanger 2 7, heat exchanger 3 8, combustion chamber 17, liquid oxygen pump 28;

[0063] More specifically, the LNG cryogenic pump 1, the first-stage multi-stream heat exchanger 2, the second-stage multi-stream heat exchanger 3, the third-stage multi-stream heat exchanger 4, and the fourth-stage multi-stream heat exchanger 5 are sequentially connected by pipelines. The outlet of the fourth-stage multi-stream heat exchanger 5 is divided into a fuel outlet and an oxidant outlet. The fuel outlet is divided into two branches. Branch 1 is sequentially connected to expander 1 6, heat exchanger 2 7, and heat exchanger 3 8 through a pipeline, and branch 2 is connected to the combustion chamber 17 through a pipeline. Branch 1 is used to transport natural gas into the natural gas pipeline network, and branch 2 is used to transport fuel to the combustion chamber 17.

[0064] The liquid oxygen pump 28, the first-stage multi-stream heat exchanger 2, the second-stage multi-stream heat exchanger 3, the third-stage multi-stream heat exchanger 4, and the fourth-stage multi-stream heat exchanger 5 are connected in sequence through pipelines, and the combustion aid outlet of the fourth-stage multi-stream heat exchanger 5 is connected to the combustion chamber 17 through a pipeline for conveying the combustion aid thereto.

[0065] like Figure 1 、 Figure 4 As shown, the two-stage Rankine cycle subsystem serves as an intermediate system between the liquefied natural gas direct expansion power generation subsystem and the supercritical carbon dioxide Brayton cycle subsystem, and is used to transfer heat and cold. It includes: a primary Rankine cycle and a secondary Rankine cycle; the primary Rankine cycle is connected to the primary multi-stream heat exchanger 2 via a pipeline, and the secondary Rankine cycle is connected to the secondary multi-stream heat exchanger 3 via a pipeline; the primary multi-stream heat exchanger 2 and the secondary multi-stream heat exchanger 3 are used to exchange heat between natural gas and oxygen and the circulating medium in the two-stage Rankine cycle subsystem;

[0066] As a preferred embodiment, the first-stage Rankine cycle is used to utilize high-grade cold energy, and includes a pump 19, a heat exchanger 4 20, and an expander 21 connected in sequence by pipelines; the inlet of the pump 19 and the outlet of the expander 21 are respectively connected to the first-stage multi-stream heat exchanger 2;

[0067] The secondary Rankine cycle is used to utilize low-grade cold energy, and includes a second pump 22, a fifth heat exchanger 23, and a third expander 24 connected in sequence by pipelines; the inlet of the second pump 22 and the outlet of the third expander 24 are respectively connected to the secondary multi-stream heat exchanger 3;

[0068] Circulating medium flows through both the primary Rankine cycle and the secondary Rankine cycle.

[0069] like Figure 1 、 Figure 5As shown, the direct cooling subsystem is connected to the three-stage multi-stream heat exchanger 4 through a pipeline. The three-stage multi-stream heat exchanger 4 is used to exchange heat between natural gas and oxygen and the circulating medium of the direct cooling subsystem; it specifically includes a cold unit 25 and a four-stage multi-stream heat exchanger 5; the cold unit 25 includes a cold storage and a refrigeration and air-conditioning system, the outlet of which is connected to the inlet of the four-stage multi-stream heat exchanger 5 through a pipeline, and the output of the four-stage multi-stream heat exchanger 5 is used as input.

[0070] In this example, the working fluids in the cooling units are liquid ammonia and water, respectively. The direct cooling subsystem utilizes the excess cooling energy generated during the power generation process to provide sufficient cooling capacity for other cooling units requiring cooling energy. This efficient energy utilization method achieves a diversified energy supply and improves overall energy efficiency.

[0071] like Figure 1 、 Figure 2 As shown, the supercritical carbon dioxide Brayton cycle subsystem is used for high-temperature power generation, including:

[0072] Heat exchanger 2 7, heat exchanger 3 8, compressor 1 9, compressor 2 10, pump 3 11, primary regenerative heat exchanger 12, secondary regenerative heat exchanger 13, heat exchanger 1 14, steam turbine 2 15, solar collector 16, combustion chamber 17, steam turbine 1 18, heat exchanger 4 20, heat exchanger 5 23, water storage tank 26, gas-liquid separator 27; and connected to the liquefied natural gas direct expansion power generation subsystem through a pipeline via the combustion chamber 17; the combustion chamber 17 is used to generate high-temperature and high-pressure gas by oxygen-enriched combustion of fuel;

[0073] More specifically, the connection relationship between the various components is as follows: the outlet of the heat exchanger five 23 is connected to the inlet of the heat exchanger four 20 through a pipeline; the outlet of the heat exchanger four 20 is connected to the inlet of the gas-liquid separator 27 through a pipeline; a liquid outlet of the gas-liquid separator 27 is connected to the water storage tank 26 through a pipeline, and the gas outlet is connected to the inlets of the three-stage multi-stream heat exchanger 4, the heat exchanger three 8, and the compressor two 10 through three pipelines respectively; the outlet of the three-stage multi-stream heat exchanger 4 is connected to the liquid carbon dioxide storage tank 29 through a pipeline; the heat exchanger three 8, the compressor one 9, the heat exchanger two 7, and the pump three 11 are connected in sequence through pipelines; the outlet of the pump three 11 is connected to the inlet of the first-stage heat recovery heat exchanger 12 through a pipeline, and the gas outlet is connected to the water storage tank 26 through a pipeline. The outlet of the first-stage heat recovery exchanger 12 is connected to the inlet of the second-stage heat recovery exchanger 13 through a pipeline; the outlet of the second-stage heat recovery exchanger 13 is connected to the inlets of turbine 2 15 and heat exchanger 14 through pipelines respectively; the outlet of compressor 2 10 is connected to the inlet of the first-stage heat recovery exchanger 12 through a pipeline; the outlet of heat exchanger 14 is connected to the solar collector 16 and the combustion chamber 17 through pipelines respectively, the outlet of the solar collector 16 is connected to the inlet of heat exchanger 14 through a pipeline, and the outlet of the combustion chamber 17 is connected to the inlet of turbine 18 through a pipeline; the steam turbine 18, the second-stage heat recovery exchanger 13, the first-stage heat recovery exchanger 12, and the heat exchanger 5 23 are connected in sequence through pipelines.

[0074] In addition, in this embodiment, among the above-mentioned components: the first-stage multi-stream heat exchanger 2, the second-stage multi-stream heat exchanger 3, the third-stage multi-stream heat exchanger 4, the fourth-stage multi-stream heat exchanger 5, the second heat exchanger 7, the third heat exchanger 8, the first heat exchanger 14, the fourth heat exchanger 20, the fifth heat exchanger 23, the first-stage regenerative heat exchanger 12, the second-stage regenerative heat exchanger 13 and the first heat exchanger 14 include hot side channels and cold side channels, and the heat exchange medium includes circulating CO2, a CO2 / H2O mixture, natural gas, oxygen and ORC circulating medium; the combustion chamber 17 includes an oxygen inlet, a natural gas inlet, and a CO2 inlet; the gas-liquid separator 27 includes a CO2 / H2O mixture inlet, a bottom liquid water outlet, and a top gaseous CO2 outlet.

[0075] Regarding the above-mentioned multi-stage power generation system, this embodiment also provides its working method, which is specifically as follows:

[0076] A method for operating a multi-stage power generation system utilizing LNG cold energy, including a method for operating a liquefied natural gas direct expansion power generation subsystem, a method for operating a two-stage Rankine cycle subsystem, and a method for operating a supercritical carbon dioxide recompression Brayton cycle subsystem;

[0077] The working method of the liquefied natural gas direct expansion power generation subsystem is specifically as follows:

[0078] Low-temperature liquefied natural gas (LNG) is pressurized by LNG cryogenic pump 1, with the outlet pressure of LNG at 250 bar. After pressurization, it passes through the first-stage multi-stream heat exchanger 2, the second-stage multi-stream heat exchanger 3, the third-stage multi-stream heat exchanger 4, and the fourth-stage multi-stream heat exchanger 5 for heat exchange and gasification. The fuel outlet of the fourth-stage multi-stream heat exchanger 5 is then divided into two branches. The natural gas in branch 1 is directly expanded by expander 1 6 to generate electricity. The expanded natural gas is then heated and delivered to the natural gas pipeline network. The natural gas in branch 2 is directly delivered to the gas chamber 17 for use as fuel.

[0079] In this embodiment, the natural gas pressure at the outlet of expander 1 6 is 74 bar, and the expanded natural gas flows to the cold side channels of heat exchanger 2 7 and heat exchanger 3 8 in sequence for heat exchange. The temperature of the natural gas after heat exchange can reach 10 o C, meeting the requirements of natural gas pipeline network.

[0080] The cryogenic liquid oxygen LO2 flows through the liquid oxygen pump 28 for pressurization. The outlet pressure of the cryogenic pump 28 is 250 bar. After pressurization, it passes through the first-stage multi-stream heat exchanger 2, the second-stage multi-stream heat exchanger 3, the third-stage multi-stream heat exchanger 4 and the fourth-stage multi-stream heat exchanger 5 for heat exchange and gasification. The outlet oxygen temperature of the multi-stream heat exchanger 5 is 15 o C, the oxygen after heat exchange and gasification flows to the combustion chamber 17 through the combustion agent outlet of the four-stage multi-stream heat exchanger 5;

[0081] In addition, the cryogenic liquefied natural gas and cryogenic liquid oxygen in the liquefied natural gas direct expansion power generation subsystem also provide cooling capacity for the two-stage Rankine cycle subsystem.

[0082] The working method of the two-stage Rankine cycle subsystem is as follows:

[0083] In this example, the two-stage Rankine cycle is a two-stage series Rankine cycle with the first-stage Rankine cycle as the primary cycle and the second-stage Rankine cycle as the bottom cycle. The first-stage Rankine cycle uses high-grade cold energy from LNG and LO2. Its circulating medium is a mixture of ethylene and isopentane in a ratio of 0.89:0.11. The circulating medium is pressurized in pump 19. The pressure of the circulating medium at the outlet of pump 19 is 20 bar and the temperature is -86°C. The pressurized circulating medium flows to the cold side channel of heat exchanger 4 20 for heat exchange. The circulating medium at the outlet of the cold side channel of heat exchanger 4 20 flows to expander 21. The circulating medium pressure at the outlet of expander 21 is 1.1 bar. The expanded circulating medium flows to the hot side channel of the first-stage multi-stream heat exchanger 2 for heat exchange. The low-temperature circulating medium at the outlet of the hot side channel of the first-stage multi-stream heat exchanger 2 flows to pump 19.

[0084] The secondary Rankine cycle uses low-grade cold energy. Its circulating medium is a mixture of ethylene and ethane with a mixing ratio of 0.72:0.28. The circulating medium is pressurized in pump 22. The pressure of the circulating medium at the outlet of pump 22 is 20 bar and the temperature is -84 o C, the pressurized circulating medium flows to the cold side channel of heat exchanger five 23 for heat exchange, and the circulating medium at the outlet of the cold side channel of heat exchanger five 23 flows to expander three 24. The circulating medium pressure at the outlet of expander three 24 is 1.1 bar. The expanded circulating medium flows to the hot side channel of the secondary multi-stream heat exchanger 3 for heat exchange, and the low-temperature circulating medium at the outlet of the hot side channel of the secondary multi-stream heat exchanger 3 flows to pump two 22.

[0085] In this embodiment, the heated circulating medium flows into the second expander 21 and the third expander 24 for expansion and power generation, and the expanded circulating medium flows again into the first-stage multi-stream heat exchanger 2 and the second-stage multi-stream heat exchanger 3 for cooling, thereby fully utilizing the cold energy and heat energy in the overall system.

[0086] The working method of the supercritical carbon dioxide recompression Brayton cycle subsystem is as follows:

[0087] The supercritical carbon dioxide recompression Brayton cycle subsystem is heated by a solar collector 16 and a combustion chamber 17. The solar collector 16 heats the supercritical carbon dioxide to produce circulating CO2, which is injected into the combustion chamber 17 to control its temperature. By controlling the temperature of the heat source of the solar collector 16, the mass and flow rate of the circulating CO2 are dynamically adjusted to control the temperature of the combustion products. The oxygen-enriched combustion of natural gas in the combustion chamber 17 produces a CO2 / H2O mixture. The CO2 / H2O mixture and the circulating CO2 constitute the circulating medium of the supercritical carbon dioxide recompression Brayton cycle subsystem.

[0088] The temperature of the circulating medium is 750 o C, the pressure is 250 bar, the circulating medium first expands through the turbine 18 to generate electricity, the pressure of the mixture at the outlet of the turbine 18 is 74 bar, and the expanded exhaust gas provides heat for the two-stage Rankine cycle subsystem. At the same time, it passes through the secondary heat exchanger 13, the primary heat exchanger 12, the fifth heat exchanger 23, and the fourth heat exchanger 20 to cool down. After cooling, it flows into the gas-liquid separator 27 for gas-liquid separation, and the separated liquid water flows into the water storage tank 26; the separated CO2 includes the circulating CO2 and the remaining CO2 in the combustion products;

[0089] The circulating CO2 is compressed, including multi-stage compression and single-stage compression. The proportion of the circulating CO2 used for multi-stage compression to the whole ranges from 0.3 to 0.7. The specific process is as follows:

[0090] Multi-stage compression: A portion of the circulating CO2 flows into heat exchanger 3 8 for cooling and then flows into compressor 1 9 for pressurization. The outlet pressure of compressor 1 9 is 150 bar. The pressurized CO2 flows into heat exchanger 2 7 for cooling and then flows into pump 3 11 for re-pressurization. The CO2 pressure at the outlet of pump 3 11 is 300 bar. The re-pressurized CO2 flows into the first and second regenerative heat exchangers 12 and 13 in sequence for heating and then flows into steam turbine 2 15 for expansion and power generation. The CO2 pressure at the outlet of steam turbine 2 15 is 250 bar.

[0091] Single-stage compression: Another portion of the circulating CO2 flows into compressor 2 10 for compression. The CO2 pressure at the outlet of compressor 10 is 250 bar. After compression, it flows into the first-stage regenerator 12 and the second-stage regenerator 13 for heating. After the temperature is increased, it is combined with the circulating CO2 at the outlet of turbine 2 15 that has undergone multi-stage compression and flows into heat exchanger 14 for heating before flowing into combustion chamber 17.

[0092] In this embodiment, the design of multi-stage compression and single-stage compression aims to effectively control the energy consumption in the CO2 compression process to the lowest level under different parameter settings, such as the mass flow rate of liquefied natural gas (LNG), the natural gas combustion pressure, and the lowest temperature of CO2 before compression, by adjusting the ratio of carbon dioxide (CO2) in the two.

[0093] The remaining CO2 in the combustion products flows directly into the three-stage multi-stream heat exchanger 4 for cooling and condensation after gas-liquid separation, and then flows into the liquid carbon dioxide storage tank 29.

[0094] In summary, in the multi-stage power generation system utilizing LNG cold energy proposed by the present invention, the heat generated by the solar collector and the combustion chamber is used for the direct expansion process in the supercritical carbon dioxide Brayton cycle subsystem and for heating the circulating medium in the two-stage Rankine cycle. The cold energy of LNG and LO2 is used to condense the circulating medium in the two-stage Rankine cycle, liquefy the excess CO2 produced by the combustion chamber, and meet the cooling needs of the cooling unit. This not only significantly increases the overall power generation, but also reduces the temperature difference between the various power generation subsystems and the inlet and outlet media of each component of each system, thereby improving the exergy efficiency and thermal efficiency of the entire system. While achieving efficient energy utilization, it also optimizes the energy conversion process, which has important energy-saving and environmental protection significance.

[0095] It should be noted that it will be apparent to those skilled in the art that the present invention is not limited to the details of the exemplary embodiments described above, and that the present invention can be implemented in other specific forms without departing from the spirit or essential characteristics of the present invention. Therefore, the embodiments should be considered in all respects as illustrative and non-restrictive, and the scope of the present invention is defined by the appended claims, not the foregoing description, and it is intended that all variations within the meaning and range of equivalents of the claims be encompassed within the present invention. Any reference signs in the claims should not be construed as limiting the claim to which they relate.

[0096] In addition, it should be understood that although this specification is described in terms of implementation methods, not every implementation method contains only one independent technical solution. This narrative method of the specification is only for the sake of clarity. Those skilled in the art should regard the specification as a whole. The technical solutions in each embodiment can also be appropriately combined to form other implementation methods that can be understood by those skilled in the art.

Claims

1. A multi-stage power generation system utilizing LNG cold energy, characterized in that: include: LNG direct expansion subsystem, supercritical CO2 recompression Brayton cycle subsystem, two-stage Rankine cycle subsystem and direct cooling subsystem; The liquefied natural gas direct expansion power generation subsystem is used for low-temperature power generation and includes: LNG cryogenic pump (1), a first-stage multi-stream heat exchanger (2), a second-stage multi-stream heat exchanger (3), a third-stage multi-stream heat exchanger (4), a fourth-stage multi-stream heat exchanger (5), a first expander (6), a second heat exchanger (7), a third heat exchanger (8), a combustion chamber (17), and a liquid oxygen pump (28); The supercritical carbon dioxide recompression Brayton cycle subsystem is used for high-temperature power generation and includes: Heat exchanger 2 (7), heat exchanger 3 (8), compressor 1 (9), compressor 2 (10), pump 3 (11), primary heat recovery heat exchanger (12), secondary heat recovery heat exchanger (13), heat exchanger 1 (14), steam turbine 2 (15), solar collector (16), combustion chamber (17), steam turbine 1 (18), heat exchanger 4 (20), heat exchanger 5 (23), water storage tank (26), gas-liquid separator (27); and connected to the liquefied natural gas direct expansion power generation subsystem through a pipeline via the combustion chamber (17); the combustion chamber (17) is used to burn the fuel with oxygen-enriched combustion to generate high-temperature and high-pressure gas; The two-stage Rankine cycle subsystem serves as an intermediate system between the liquefied natural gas direct expansion power generation subsystem and the supercritical carbon dioxide Brayton cycle subsystem, and is used to transfer heat and cold. The two-stage Rankine cycle subsystem includes a first-stage Rankine cycle and a second-stage Rankine cycle. The first-stage Rankine cycle is connected to the first-stage multi-stream heat exchanger (2) via a pipeline, and the second-stage Rankine cycle is connected to the second-stage multi-stream heat exchanger (3) via a pipeline. The first-stage multi-stream heat exchanger (2) and the second-stage multi-stream heat exchanger (3) are used to exchange heat between natural gas and oxygen and the circulating medium in the two-stage Rankine cycle subsystem. The direct cooling subsystem is connected to a three-stage multi-stream heat exchanger (4) via a pipeline. The three-stage multi-stream heat exchanger (4) is used to exchange heat between natural gas and oxygen and the circulating medium of the direct cooling subsystem.

2. A multi-stage power generation system utilizing LNG cold energy according to claim 1, characterized in that: In the liquefied natural gas direct expansion power generation subsystem: The LNG cryogenic pump (1), the first-stage multi-stream heat exchanger (2), the second-stage multi-stream heat exchanger (3), the third-stage multi-stream heat exchanger (4), and the fourth-stage multi-stream heat exchanger (5) are sequentially connected via pipelines, and the outlet of the fourth-stage multi-stream heat exchanger (5) is divided into a fuel outlet and an oxidant outlet; the fuel outlet is divided into two branches, branch 1 is sequentially connected to the expander 1 (6), the heat exchanger 2 (7), and the heat exchanger 3 (8) via a pipeline, and branch 2 is connected to the combustion chamber (17) via a pipeline; the branch 1 is used to transport natural gas into the natural gas pipeline network, and the branch 2 is used to transport fuel to the combustion chamber (17); The liquid oxygen pump (28), the first-stage multi-stream heat exchanger (2), the second-stage multi-stream heat exchanger (3), the third-stage multi-stream heat exchanger (4), and the fourth-stage multi-stream heat exchanger (5) are connected in sequence through pipelines, and the combustion-supporting agent outlet of the fourth-stage multi-stream heat exchanger (5) is connected to the combustion chamber (17) through a pipeline for conveying the combustion-supporting agent thereto.

3. The multi-stage power generation system utilizing LNG cold energy according to claim 1, characterized in that: In the two-stage Rankine cycle subsystem: The first-stage Rankine cycle is used to utilize high-grade cold energy, and comprises a pump 1 (19), a heat exchanger 4 (20), and an expander 2 (21) connected in sequence through pipelines; the inlet of the pump 1 (19) and the outlet of the expander 2 (21) are respectively connected to the first-stage multi-stream heat exchanger (2); The secondary Rankine cycle is used to utilize low-grade cold energy, and comprises a second pump (22), a fifth heat exchanger (23), and a third expander (24) connected in sequence through pipelines; the inlet of the second pump (22) and the outlet of the third expander (24) are respectively connected to the secondary multi-stream heat exchanger (3); Circulating medium flows through both the primary Rankine cycle and the secondary Rankine cycle.

4. The multi-stage power generation system utilizing LNG cold energy according to claim 3, characterized in that: The circulating medium in the primary Rankine cycle is a mixture of ethylene and isopentane, with a mixing ratio of 0.89:0.11; the circulating medium in the secondary Rankine cycle is a mixture of ethylene and ethane, with a mixing ratio of 0.72:0.

28.

5. The multi-stage power generation system utilizing LNG cold energy according to claim 1, characterized in that: The direct cooling subsystem includes a cold storage unit (25) and a four-stage multi-stream heat exchanger (5); the cold storage unit (25) includes a cold storage and a refrigeration and air-conditioning system, the outlet of which is connected to the inlet of the four-stage multi-stream heat exchanger (5) through a pipeline, and the output of the four-stage multi-stream heat exchanger (5) is used as input.

6. The multi-stage power generation system utilizing LNG cold energy according to claim 1, characterized in that: In the supercritical carbon dioxide recompression Brayton cycle subsystem: The outlet of the heat exchanger five (23) is connected to the inlet of the heat exchanger four (20) through a pipeline; the outlet of the heat exchanger four (20) is connected to the inlet of the gas-liquid separator (27) through a pipeline; a liquid outlet of the gas-liquid separator (27) is connected to the water storage tank (26) through a pipeline, and the gas outlet is connected to the inlets of the three-stage multi-stream heat exchanger (4), the heat exchanger three (8), and the compressor two (10) through three pipelines respectively; the outlet of the three-stage multi-stream heat exchanger (4) is connected to the liquid carbon dioxide storage tank (29) through a pipeline; the heat exchanger three (8), the compressor one (9), the heat exchanger two (7), and the pump three (11) are connected in sequence through pipelines; the outlet of the pump three (11) is connected to the inlet of the first-stage heat recovery heat exchanger (12) through a pipeline, and the first-stage heat recovery heat exchanger (12) is connected to the inlet of the first-stage heat recovery heat exchanger (12). The outlet of the compressor is connected to the inlet of the secondary heat recovery exchanger (13) through a pipeline; the outlet of the secondary heat recovery exchanger (13) is connected to the inlet of the steam turbine 2 (15) and the heat exchanger 1 (14) through pipelines; the outlet of the compressor 2 (10) is connected to the inlet of the primary heat recovery exchanger (12) through a pipeline; the outlet of the heat exchanger 1 (14) is connected to the solar collector (16) and the combustion chamber (17) through pipelines, the outlet of the solar collector (16) is connected to the inlet of the heat exchanger 1 (14) through a pipeline, and the outlet of the combustion chamber (17) is connected to the inlet of the steam turbine 1 (18) through a pipeline; the steam turbine 1 (18), the secondary heat recovery exchanger (13), the primary heat recovery exchanger (12), and the heat exchanger 5 (23) are connected in sequence through pipelines.

7. The operating method of a multi-stage power generation system utilizing LNG cold energy according to any one of claims 1 to 6, characterized in that: Including the working method of the liquefied natural gas direct expansion power generation subsystem, the working method of the two-stage Rankine cycle subsystem and the working method of the supercritical carbon dioxide recompression Brayton cycle subsystem; The working method of the liquefied natural gas direct expansion power generation subsystem is specifically as follows: After being pressurized by an LNG cryogenic pump (1), the low-temperature liquefied natural gas (LNG) passes through a first-stage multi-stream heat exchanger (2), a second-stage multi-stream heat exchanger (3), a third-stage multi-stream heat exchanger (4), and a fourth-stage multi-stream heat exchanger (5) in sequence for heat exchange and gasification, and then is output from the fuel outlet of the fourth-stage multi-stream heat exchanger (5) into two branches; the natural gas in branch 1 is directly expanded by an expander (6) to generate electricity, and the expanded natural gas is heated and heated by heat exchangers (7) and (8) in sequence, and the heated and heated natural gas is transported to a natural gas pipeline network, and the natural gas in branch 2 is directly transported to a gas chamber (17) to serve as fuel; The cryogenic liquid oxygen LO2 flows through a liquid oxygen pump (28) for pressurization, and then passes through a first-stage multi-stream heat exchanger (2), a second-stage multi-stream heat exchanger (3), a third-stage multi-stream heat exchanger (4), and a fourth-stage multi-stream heat exchanger (5) for heat exchange and gasification. The oxygen after heat exchange and gasification flows to a combustion chamber (17) through an oxidant outlet of the fourth-stage multi-stream heat exchanger (5); The cryogenic liquefied natural gas and cryogenic liquid oxygen provide cooling capacity for the two-stage Rankine cycle subsystem.

8. The operating method of a multi-stage power generation system utilizing LNG cold energy according to claim 7, characterized in that: The working method of the two-stage Rankine cycle subsystem is specifically as follows: The first-stage Rankine cycle utilizes high-grade cold energy, and the second-stage Rankine cycle utilizes low-grade cold energy. The circulating medium in the two Rankine cycles is first cooled by the first-stage multi-stream heat exchanger (2) and the second-stage multi-stream heat exchanger (3), respectively. After cooling, it is respectively input into pump one (19) and pump two (22) for pressurization. After pressurization, it flows into heat exchanger four (20) and heat exchanger five (23) for heating. The heated circulating medium flows into expander two (21) and expander three (24) for expansion and power generation. The expanded circulating medium flows again into the first-stage multi-stream heat exchanger (2) and the second-stage multi-stream heat exchanger (3) for cooling.

9. The operating method of a multi-stage power generation system utilizing LNG cold energy according to claim 7, characterized in that: The working method of the supercritical carbon dioxide recompression Brayton cycle subsystem is specifically as follows: The supercritical carbon dioxide recompression Brayton cycle subsystem is heated by a solar collector (16) and a combustion chamber (17); the solar collector (16) heats to obtain circulating CO2, which is used to inject into the combustion chamber (17) to control its temperature. By controlling the temperature of the heat source of the solar collector (16), the mass and flow rate of the circulating CO2 are dynamically adjusted to control the temperature of the combustion products; the oxygen-enriched combustion of natural gas in the combustion chamber (17) produces a CO2 / H2O mixture; the CO2 / H2O mixture and the circulating CO2 constitute the circulating medium of the supercritical carbon dioxide recompression Brayton cycle subsystem; The circulating medium first expands through the steam turbine (18) to generate electricity. The expanded exhaust gas then passes through the secondary heat exchanger (13), the primary heat exchanger (12), the fifth heat exchanger (23), and the fourth heat exchanger (20) to cool down. After cooling down, the exhaust gas flows into the gas-liquid separator (27) for gas-liquid separation. The separated liquid water flows into the water storage tank (26). The separated CO2 includes the circulating CO2 and the CO2 generated by the combustion of natural gas. Compression of circulating CO2 includes multi-stage compression and single-stage compression; specifically: Multi-stage compression: a portion of the circulating CO2 flows into heat exchanger three (8) for cooling and then flows into compressor one (9) for pressurization. The pressurized CO2 flows into heat exchanger two (7) for cooling and then flows into pump three (11) for re-pressurization. The re-pressurized CO2 flows into the first regenerative heat exchanger (12) and the second regenerative heat exchanger (13) in sequence for heating and then flows into steam turbine two (15) for expansion and power generation. Single-stage compression: Another part of the circulating CO2 flows into the second compressor (10) for compression and then flows into the first regenerator (12) and the second regenerator (13) for heating. After the temperature is increased, it is combined with the circulating CO2 at the outlet of the second turbine (15) that has undergone multi-stage compression and flows into the first heat exchanger (14) for heating and then flows into the combustion chamber (17). The remaining CO2 in the combustion products flows into the three-stage multi-stream heat exchanger (4) for cooling and condensation after gas-liquid separation, and then flows into the liquid carbon dioxide storage tank (29).

10. The operating method of a multi-stage power generation system utilizing LNG cold energy according to claim 9, characterized in that: In the working method of the supercritical carbon dioxide recompression Brayton cycle subsystem, The expanded exhaust gas provides heat for the two-stage Rankine cycle subsystem; The proportion of the circulating CO2 used for multi-stage compression to the whole CO2 ranges from 0.3 to 0.7.

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