Multi-stage power generation system utilizing LNG (Liquefied Natural Gas) cold energy and working method of multi-stage power generation system

Abstract

The invention discloses an LNG cold energy multi-stage power generation system and a working method thereof. The LNG cold energy multi-stage power generation system comprises 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. According to different grades of needed cooling capacity, the cooling capacity released in the LNG gasification process is directly used for 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 LNG cold energy, and gasified natural gas is directly expanded to generate power. By arranging the multi-stage power generation system, the overall power generation capacity is remarkably improved, the temperature difference between the power generation systems is reduced, and compared with a conventional LNG cold energy power generation system, the efficiency and the heat efficiency are better improved; besides, the solar heat collector is arranged to dynamically regulate and control the temperature of combustion products, heat sources of different temperature levels are generated while the stability of the system is considered, the oxygen-enriched combustion products are liquefied and recycled after being subjected to multi-level cooling, and zero carbon emission is achieved.

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, due to the growing demand for energy supply and carbon dioxide emission reduction, natural gas is one of the most widely used energy sources at home and abroad due to its high hydrogen-carbon ratio and low carbon emission coefficient. In order to facilitate the large-scale storage and transportation of natural gas and improve the efficiency of natural gas use, after natural gas is mined, it will be treated with impurities and liquefied to convert natural gas into LNG. The temperature of liquefied natural gas at atmospheric pressure is -162°C. During the regasification process, 823kJ / kg of cold energy can be released. If this part of energy is recovered and used for power generation, good economic and social benefits will be obtained. The cold capacity of LNG is the energy that can be obtained when liquefied natural gas changes to a state of equilibrium with the environment. It can be evaluated by the parameter exergy. Exergy efficiency is an indicator to measure the efficiency of cold energy conversion.

[0003] The basic methods of LNG cold energy power generation mainly include direct expansion method, Rankine cycle using intermediate cooling medium, Brayton cycle method, Kalina cycle method and multi-stage combined cycle method. However, the single-stage cold energy power generation system has the problems of large initial investment, unstable system operation and low efficiency.

[0004] In the prior art, there have been means of cascade utilization of LNG cold energy, such as patent CN106150579A, but it mainly applies LNG cold energy in a two-stage Rankine cycle, and the temperature of the gasified natural gas is -38°C, which does not meet the requirements of 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, which utilizes the cold energy of LNG in an organic Rankine power generation unit, an air conditioning refrigeration unit, and a direct expansion cycle power generation unit. Although the cold energy of LNG is fully utilized, the temperature span during the release of 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] Although the current LNG cold energy power generation technology has made cascade use of LNG cold energy, it has not made reasonable use of each cascade of LNG cold energy. The inlet and outlet temperature difference of the heat exchanger is large, which ultimately leads to low exergy efficiency of the system. Different power generation methods are suitable for cold and heat sources in different temperature ranges. The supercritical carbon dioxide recompression Brayton cycle subsystem needs to operate under high temperature conditions to heat the medium, while the two-stage Rankine cycle system relies on a low temperature environment to achieve the liquefaction of the circulating medium. Generally, the Rankine cycle uses seawater as a heat source. Due to the low temperature of seawater, the circulating medium in the Rankine cycle generally cannot utilize the part of LNG cold energy with a temperature 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 view of 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 adopts 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: A multi-stage power generation system utilizing LNG cold energy, specifically comprising: 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 comprises: 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; The supercritical carbon dioxide recompression Brayton cycle subsystem is used for high temperature power generation, and comprises: Heat exchanger 2, heat exchanger 3, compressor 1, compressor 2, pump 3, primary heat recovery heat exchanger, secondary heat recovery 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 produce high-temperature and high-pressure gas by oxygen-enriched combustion of fuel; The two-stage Rankine cycle subsystem is used as an intermediate system between the liquefied natural gas direct expansion power generation subsystem and the supercritical carbon dioxide Brayton cycle subsystem to transfer heat and cold, and includes: a primary Rankine cycle and a secondary Rankine cycle; the primary Rankine cycle is connected to the primary multi-stream heat exchanger through a pipeline, and the secondary Rankine cycle is connected to the secondary multi-stream heat exchanger through 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; The direct cooling subsystem is connected to a three-stage multi-stream heat exchanger through a pipeline, and 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.

[0010] Furthermore, in the liquefied natural gas direct expansion power generation subsystem: 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 connected in sequence through pipelines, and 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 connected to expander 1, heat exchanger 2, and heat exchanger 3 in sequence 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; 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-aiding agent outlet of the fourth-stage multi-stream heat exchanger is connected to the combustion chamber through a pipeline for conveying the combustion-aiding agent thereto.

[0011] Furthermore, in the two-stage Rankine cycle subsystem: The primary Rankine cycle is used to utilize high-quality cold energy, and includes a pump 1, a heat exchanger 4, and an expander 2 connected in sequence through pipelines; the inlet of the pump 1 and the outlet of the expander 2 are respectively connected to the primary multi-stream heat exchanger; The secondary Rankine cycle is used to utilize low-grade cold energy, and comprises a pump 2, a heat exchanger 5, and an expander 3 connected in sequence through pipelines; the inlet of the pump 2 and the outlet of the expander 3 (24) are respectively connected to the secondary multi-stream heat exchanger; Circulating medium flows through both the primary Rankine cycle and the secondary Rankine cycle.

[0012] 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.

[0013] 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.

[0014] Furthermore, in the supercritical carbon dioxide recompression Brayton cycle subsystem: 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 compressor is connected to the inlet of the secondary heat exchanger through a pipeline; the outlet of the secondary heat exchanger is connected to the inlets of the second turbine and the first heat exchanger through pipelines respectively; the outlet of the compressor 2 is connected to the inlet of the primary heat exchanger through a pipeline; the outlet of the heat exchanger 1 is connected to the solar collector and the combustion chamber through pipelines respectively, the outlet of the solar collector is connected to the inlet of the heat exchanger 1 through a pipeline, and the outlet of the combustion chamber is connected to the inlet of the first turbine through a pipeline; the first turbine, the secondary heat exchanger, the primary heat exchanger, and the heat exchanger 5 are connected in sequence through pipelines.

[0015] 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.

[0016] Furthermore, 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, the low-temperature liquefied natural gas LNG is sequentially passed 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, and then output from the fuel outlet of the fourth-stage multi-stream heat exchanger into two branches; the natural gas in branch 1 is directly expanded by expander 1 to generate electricity, and the expanded natural gas is sequentially heated by heat exchanger 2 and heat exchanger 3, and the heated natural gas is transported to the natural gas pipeline network, and the natural gas in branch 2 is directly transported to the gas chamber to serve as fuel; 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 to the combustion chamber through the combustion-supporting agent outlet of the fourth-stage multi-stream heat exchanger. The cryogenic liquefied natural gas and cryogenic liquid oxygen provide cooling capacity for the two-stage Rankine cycle subsystem.

[0017] Furthermore, the working method of the two-stage Rankine cycle subsystem is specifically as follows: The primary Rankine cycle utilizes high-grade cold energy, and the secondary Rankine sub-cycle utilizes low-grade cold energy. The circulating medium in the two sub-cycles is first cooled by the primary multi-stream heat exchanger and the secondary multi-stream heat exchanger respectively, and then respectively input into pump one and pump two for pressurization after cooling. After pressurization, it flows into heat exchanger four and heat exchanger five for heating and temperature rise. The heated circulating medium flows into expander two and expander three for expansion and power generation, and the expanded circulating medium flows into the primary multi-stream heat exchanger and the secondary multi-stream heat exchanger again for cooling.

[0018] Furthermore, 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 and a combustion chamber; the solar collector is heated to obtain circulating CO2, which is used to inject into the combustion chamber to control its temperature, and the mass and flow rate of the circulating CO2 are dynamically adjusted by controlling the temperature of the solar collector heat source, thereby controlling 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; The circulating medium first expands in the steam turbine to generate electricity, and the expanded exhaust gas is cooled in turn by the secondary heat exchanger, the primary heat exchanger, the fifth heat exchanger, and the fourth heat exchanger. After cooling, it flows into the gas-liquid separator for gas-liquid separation, and 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; The circulating CO2 is compressed, including multi-stage compression and single-stage compression, specifically: Multi-stage compression: a part 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 heat exchangers in sequence for heating and temperature increase and then flows into steam turbine 2 for expansion and power generation. 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 multi-stage compressed circulating CO2 at the outlet of the second turbine and flows into the first heat exchanger for heating and then flows into the combustion chamber. 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.

[0019] More specifically, 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 is in the range of 0.3-0.7.

[0020] Based on the above technical solution, the present invention has the following beneficial effects: 1. By setting up a supercritical carbon dioxide recompression Brayton cycle subsystem as a high-temperature power generation subsystem, a two-stage Rankine cycle as a medium-temperature power generation subsystem, and a liquefied natural gas direct expansion subsystem as a low-temperature power generation subsystem, not only the overall power generation is significantly improved, but also the temperature difference between the various power generation subsystems and the inlet and outlet media of each component of each system is reduced, thereby improving the exergy efficiency and thermal efficiency of the entire system. This design realizes the efficient use of energy, optimizes the energy conversion process, and has important energy-saving and environmental protection significance.

[0021] 2. Inject a certain amount of CO2 into the natural gas combustion chamber, and dynamically adjust the mass flow rate of the injected CO2 according to the changes in the heat source of the solar collector, so as to control 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.

[0022] 3. The use of low-temperature liquefied natural gas can not only provide the necessary cooling function for the power generation process, but also provide sufficient cooling capacity for other application units that require cold energy. This efficient energy utilization method realizes the diversified supply of energy and improves the overall energy utilization efficiency.

[0023] 4. Energy is generated through oxygen-enriched combustion of natural gas, followed by multi-stage cooling of the combustion products to achieve liquefaction. This process not only improves energy efficiency, but also achieves zero carbon emissions, which is of great significance to environmental protection.

[0024] 5. The CO2 supercritical Brayton cycle technology is adopted. The CO2 compression process is divided into two stages: multi-stage compression and single-stage compression. The ratio of CO2 in these two stages is precisely controlled to effectively adjust the temperature of the pressurized CO2. This CO2 intercooling and recompression system not only reduces the energy consumption in the compression process, but also reduces the demand for the calorific value of the heat source, thereby improving the energy efficiency and economy of the entire power generation process. BRIEF DESCRIPTION OF THE DRAWINGS

[0025] Figure 1 This is a general schematic diagram of a multi-stage power generation system utilizing LNG cold energy proposed by the present invention; Figure 2 It is a schematic diagram of the supercritical carbon dioxide Brayton cycle subsystem in the present invention; Figure 3 It is a schematic diagram of the liquefied natural gas direct expansion subsystem in the present invention; Figure 4 It is a schematic diagram of a two-stage Rankine cycle subsystem in the present invention; Figure 5 It is a schematic diagram of the direct cooling subsystem in the present invention.

[0026] The accompanying drawings in the figure are marked as follows: 1. LNG cryogenic pump; 2. primary multi-stream heat exchanger; 3. secondary multi-stream heat exchanger; 4. tertiary multi-stream heat exchanger; 5. quaternary 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 heat recovery heat exchanger; 13. secondary heat recovery 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. cold unit; 26. water storage tank; 27. gas-liquid separator; 28. liquid oxygen pump; 29. ​​liquid carbon dioxide storage tank. DETAILED DESCRIPTION

[0027] In order to make the purpose, technical solution and advantages of the present invention more clearly understood, the present invention is further described in detail below in conjunction with 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 used to limit the present invention.

[0028] Although the steps in the present invention are arranged with numbers, they are not used to limit the order of the steps. Unless the order of the steps is clearly stated or the execution of a certain 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 in this article involves and covers any and all possible combinations of one or more of the associated listed items.

[0029] 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; 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: 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; 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 connected in sequence through 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 connected to expander 1 6, heat exchanger 2 7, and heat exchanger 3 8 in sequence 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; 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.

[0030] like Figure 1 , Figure 4 As shown, the two-stage Rankine cycle subsystem is used as an intermediate system between the liquefied natural gas direct expansion power generation subsystem and the supercritical carbon dioxide Brayton cycle subsystem to transfer heat and cold, and includes: a primary Rankine cycle and a secondary Rankine cycle; the primary Rankine cycle is connected to the primary multi-stream heat exchanger 2 through a pipeline, and the secondary Rankine cycle is connected to the secondary multi-stream heat exchanger 3 through 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; As a preferred embodiment, the primary Rankine cycle is used to utilize high-grade cold energy, and includes a pump 19, a heat exchanger 20, and an expander 21 connected in sequence through pipelines; the inlet of the pump 19 and the outlet of the expander 21 are respectively connected to the primary multi-stream heat exchanger 2; The secondary Rankine cycle is used to utilize low-grade cold energy, and includes a pump 22, a heat exchanger 5 23, and an expander 3 24 connected in sequence through pipelines; the inlet of the pump 22 and the outlet of the expander 3 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.

[0031] like Figure 1 , Figure 5 As shown, the direct cooling subsystem is connected to the three-stage multi-stream heat exchanger 4 through a pipeline, and 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, and its outlet 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.

[0032] In this example, the working fluids of the cooling units are liquid ammonia and water. The direct cooling subsystem uses the extra cold energy generated by the power generation process to provide sufficient cooling capacity for other cooling units that need cold energy. This efficient energy utilization method realizes the diversified supply of energy and improves the overall energy utilization efficiency.

[0033] like Figure 1 , Figure 2 As shown, the supercritical carbon dioxide Brayton cycle subsystem is used for high temperature power generation, including: 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 through the combustion chamber 17; the combustion chamber 17 is used to produce high-temperature and high-pressure gas by oxygen-enriched combustion of fuel; 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 of the gas-liquid separator 27 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 1 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.

[0034] 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 heat exchanger 2 7, the heat exchanger 3 8, the heat exchanger 1 14, the heat exchanger 4 20, the heat exchanger 5 23, the first-stage regenerator 12, the second-stage regenerator 13 and the heat exchanger 1 14 include hot side channels and cold side channels, and the heat exchange medium includes circulating CO2, 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.

[0035] As for the above-mentioned multi-stage power generation system, this embodiment also provides its working method, which is specifically as follows: A working method of a multi-stage power generation system utilizing LNG cold energy, 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; The working method of the liquefied natural gas direct expansion power generation subsystem is specifically as follows: The low-temperature liquefied natural gas LNG is pressurized by the LNG cryogenic pump 1, and the LNG pressure at the outlet of the LNG cryogenic pump 1 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 in sequence for heat exchange and gasification, and then is divided into two branches by the fuel outlet of the fourth-stage multi-stream heat exchanger 5; the natural gas in branch 1 is directly expanded by the expander 1 6 to generate electricity, and the expanded natural gas is heated and heated by the heat exchanger 2 7 and the heat exchanger 3 8 in sequence, and the heated and heated natural gas is transported to the natural gas pipeline network, and the natural gas in branch 2 is directly transported to the gas chamber 17 to serve as fuel; In this embodiment, the natural gas pressure at the outlet of the expander 1 6 is 74 bar, and the expanded natural gas flows to the cold side channels of the heat exchanger 2 7 and the heat exchanger 3 8 in turn 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.

[0036] 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; 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.

[0037] The working method of the two-stage Rankine cycle subsystem is as follows: 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 of LNG and LO2, and its circulating medium is a mixture of ethylene and isopentane with a mixing ratio of 0.89:0.11. The circulating medium is pressurized in pump 19, and 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, and the circulating medium at the outlet of the cold side channel of heat exchanger 4 20 flows to expander 21, and 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, and 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; 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 oC, 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.

[0038] In this embodiment, the heated circulating medium flows into the expander 2 21 and the expander 3 24 respectively for expansion and power generation, and the expanded circulating medium flows into the primary multi-stream heat exchanger 2 and the secondary multi-stream heat exchanger 3 respectively for cooling, thereby making full use of the cold energy and heat energy in the overall system.

[0039] The working method of the supercritical carbon dioxide recompression Brayton cycle subsystem is 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, and the mass and flow rate of the circulating CO2 are dynamically adjusted by controlling the temperature of the heat source of the solar collector 16, thereby controlling 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 temperature of the circulating medium is 750 o C, the pressure is 250bar, 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 74bar, the expanded exhaust gas provides heat for the two-stage Rankine cycle subsystem, and at the same time, it passes through the secondary heat exchanger 13, the primary heat exchanger 12, the heat exchanger 5 23, and the heat exchanger 4 20 in sequence for cooling, and 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; The circulating CO2 is compressed, including multi-stage compression and single-stage compression. The proportion of the circulating CO2 used for multi-stage compression is in the range of 0.3~0.7 of the whole. The specific process is as follows: Multi-stage compression: a part of the circulating CO2 flows into the heat exchanger 3 8 for cooling and then flows into the compressor 1 9 for pressurization. The outlet pressure of the compressor 1 9 is 150 bar. The pressurized CO2 flows into the heat exchanger 2 7 for cooling and then flows into the pump 3 11 for re-pressurization. The CO2 pressure at the outlet of the pump 3 11 is 300 bar. The re-pressurized CO2 flows into the primary heat exchanger 12 and the secondary heat exchanger 13 in sequence for heating and temperature increase and then flows into the steam turbine 2 15 for expansion and power generation. The CO2 pressure at the outlet of the steam turbine 2 15 is 250 bar. Single-stage compression: another part of the circulating CO2 flows into compressor 2 10 for compression, and 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 in turn for heating, and after heating, it is combined with the circulating CO2 at the outlet of turbine 2 15 that has been compressed in multiple stages and flows into heat exchanger 14 for heating and then flows into combustion chamber 17; 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 minimum under different parameter settings, such as the mass flow rate of liquefied natural gas (LNG), the natural gas combustion pressure, and the minimum temperature of CO2 before compression by adjusting the ratio of carbon dioxide (CO2) in the two.

[0040] The remaining CO2 in the combustion products directly 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.

[0041] In summary, in the multi-stage power generation system using 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 the two-stage Rankine cycle to complete the process of heating the circulating medium; the cold energy of LNG and LO2 is used to complete the process of condensing the circulating medium in the two-stage Rankine cycle, to complete the excess CO2 generated by the liquefied combustion chamber, and to meet the cold demand of the cold unit; the present invention not only significantly improves 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 utilization of energy, it optimizes the energy conversion process, which has important energy-saving and environmental protection significance.

[0042] It should be noted that it is obvious 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 features of the present invention. Therefore, the embodiments should be regarded as exemplary and non-restrictive from any point of view, and the scope of the present invention is defined by the appended claims rather than the above description, and it is intended that all changes falling within the meaning and scope of the equivalent elements of the claims are included in the present invention. Any reference numerals in the claims should not be regarded as limiting the claims involved.

[0043] In addition, it should be understood that although the present specification is described according to implementation modes, not every implementation mode contains only one independent technical solution. This description 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 may also be appropriately combined to form other implementation modes 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 comprises: 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), an expander 1 (6), a heat exchanger 2 (7), a heat exchanger 3 (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 comprises: 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 generate high-temperature and high-pressure gas by oxygen-enriched combustion of fuel; The two-stage Rankine cycle subsystem is used as an intermediate system between the liquefied natural gas direct expansion power generation subsystem and the supercritical carbon dioxide Brayton cycle subsystem to transfer heat and cold, and includes: a primary Rankine cycle and a secondary Rankine cycle; the primary Rankine cycle is connected to the primary multi-stream heat exchanger (2) through a pipeline, and the secondary Rankine cycle is connected to the secondary multi-stream heat exchanger (3) through 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; The direct cooling subsystem is connected to a three-stage multi-stream heat exchanger (4) via a pipeline, and 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 connected in sequence through 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 connected to the expander 1 (6), the heat exchanger 2 (7), and the heat exchanger 3 (8) in sequence 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); 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 via pipelines, and the combustion-aiding agent outlet of the fourth-stage multi-stream heat exchanger (5) is connected to the combustion chamber (17) via a pipeline for conveying the combustion-aiding agent thereto.

3. A multi-stage power generation system utilizing LNG cold energy according to claim 1, characterized in that: In the two-stage Rankine cycle subsystem: The primary Rankine cycle is used to utilize high-quality 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 primary multi-stream heat exchanger (2); The secondary Rankine cycle is used to utilize low-grade cold energy, and comprises a pump 2 (22), a heat exchanger 5 (23), and an expander 3 (24) connected in sequence through pipelines; the inlet of the pump 2 (22) and the outlet of the expander 3 (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. A 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 using LNG cold energy according to claim 1, characterized in that: The direct cooling subsystem comprises a cold unit (25) and a four-stage multi-stream heat exchanger (5); the cold unit (25) comprises 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. A 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 inlet 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) The outlet of the compressor (10) 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 second turbine (15) and the first heat exchanger (14) through pipelines; the outlet of the compressor (10) is connected to the inlet of the first heat recovery exchanger (12) through a pipeline; the outlet of the heat exchanger (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 (14) through a pipeline, and the outlet of the combustion chamber (17) is connected to the inlet of the first turbine (18) through a pipeline; the first turbine (18), the secondary heat recovery exchanger (13), the first heat recovery exchanger (12), and the fifth heat exchanger (23) are connected in sequence through pipelines.

7. A method for operating 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 is sequentially passed 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, and then the fuel outlet of the fourth-stage multi-stream heat exchanger (5) is output into two branches; the natural gas in branch 1 is directly expanded by an expander 1 (6) to generate electricity, and the expanded natural gas is sequentially heated by a heat exchanger 2 (7) and a heat exchanger 3 (8), and the 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 after pressurization, it 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, and 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 primary Rankine cycle utilizes high-quality cold energy, and the secondary Rankine cycle utilizes low-quality cold energy. The circulating medium in the two Rankine cycles is first cooled by the primary multi-stream heat exchanger (2) and the secondary 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 into the primary multi-stream heat exchanger (2) and the secondary multi-stream heat exchanger (3) again 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 spray into the combustion chamber (17) to control its temperature, and the mass and flow rate of the circulating CO2 are dynamically adjusted by controlling the temperature of the heat source of the solar collector (16), thereby controlling the temperature of the combustion product; 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 1 (18) to generate electricity, and the expanded exhaust gas is sequentially cooled through the secondary heat exchanger (13), the primary heat exchanger (12), the fifth heat exchanger (23), and the fourth heat exchanger (20). 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 CO2 generated by the combustion of natural gas; The circulating CO2 is compressed, including multi-stage compression and single-stage compression; specifically: Multi-stage compression: a portion of the circulating CO2 flows into the heat exchanger three (8) to be cooled and then flows into the compressor one (9) to be pressurized. The pressurized CO2 flows into the heat exchanger two (7) to be cooled and then flows into the pump three (11) to be pressurized again. The re-pressurized CO2 flows into the first regenerative heat exchanger (12) and the second regenerative heat exchanger (13) in sequence to be heated and then flows into the steam turbine two (15) to be expanded and generate electricity. 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) in sequence for heating. After the temperature is increased, it is combined with the circulating CO2 that has been compressed in multiple stages at the outlet of the second turbine (15) and flows into the first heat exchanger (14) for heating and then flows into the combustion chamber (17). The remaining CO2 in the combustion product 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 working method of a multi-stage power generation system using 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 is in the range of 0.3-0.7.

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