A hydrogen liquefaction system capable of achieving zero emission of replaced gas in a liquid hydrogen storage tank

Abstract

The present invention discloses a hydrogen liquefaction system capable of achieving zero emission of the replaced gas in a liquid hydrogen storage tank, comprising: a first heat exchanger, a second heat exchanger, a third heat exchanger, a first low-temperature adsorber, a second low-temperature adsorber, a first-stage compressor, a second-stage compressor, a first-stage buffer tank, a second-stage buffer tank, a fourth heat exchanger, a liquid hydrogen storage tank, a first-stage cooler, a second-stage cooler, a throttle valve, and a hydrogen expander. When the liquid hydrogen storage tank needs to be replaced, the hydrogen liquefaction system can draw out the recycled hydrogen that has been purified by low-temperature adsorption in the recycled hydrogen compression refrigeration system as the replacement gas, so that the liquid hydrogen storage tank can avoid secondary pollution. The hydrogen containing impurities replaced in the liquid hydrogen storage tank can also return to the recycled hydrogen compression refrigeration system as recycled hydrogen for continued use, enabling the replaced gas to achieve zero emission, thereby reducing production costs.

Description

Technical Field

[0001] The present invention relates to the field of hydrogen liquefaction equipment, and particularly to a hydrogen liquefaction system capable of achieving zero emission of the replacement gas in a liquid hydrogen storage tank. Background Art

[0002] With the repeated filling and pressurization of a liquid hydrogen storage tank, solid oxygen, solid air, and solid nitrogen will precipitate and accumulate in the liquid hydrogen storage tank. In order to extend the service life of the liquid hydrogen storage tank and for safety reasons, it is necessary to regularly replace the liquid hydrogen storage tank with high-purity hydrogen, so as to evaporate impurities such as solid oxygen, solid air, and solid nitrogen in the liquid hydrogen storage tank out of the liquid hydrogen storage tank. When performing the replacement operation on the liquid hydrogen storage tank, the hydrogen gas mixed with impurity gas in the liquid hydrogen storage tank will be discharged into the atmosphere, resulting in waste of this part of hydrogen gas. Summary of the Invention

[0003] The technical problem to be solved by the present invention is to provide a hydrogen liquefaction system capable of achieving zero emission of the replacement gas in a liquid hydrogen storage tank.

[0004] To solve the above problems, the technical solution adopted by the present invention is as follows: a hydrogen liquefaction system capable of achieving zero emission of replacement gas in a liquid hydrogen storage tank, characterized in that it includes: a first heat exchanger, a second heat exchanger, a third heat exchanger, a first low-temperature adsorber, a second low-temperature adsorber, a first-stage compressor, a second-stage compressor, a first-stage buffer tank, a second-stage buffer tank, a fourth heat exchanger, a liquid hydrogen storage tank, a first-stage cooler, a second-stage cooler, a throttle valve, and a hydrogen expander. An A1 passage, an A2 passage, an A3 passage, and an A4 passage are provided in the first heat exchanger; a B1 passage, a B2 passage, a B3 passage, a B4 passage, and a B5 passage are provided in the second heat exchanger, and the inlet of the B5 passage is connected to the B2 passage; a C1 passage and a C2 passage are provided in the third heat exchanger. One end of the raw gas inlet pipe is connected to the inlet of the A1 passage, the outlet of the A1 passage is connected to the inlet of the first low-temperature adsorber through a pipeline, the outlet of the first low-temperature adsorber is connected to the inlet of the B1 passage through a first pipeline, the outlet of the B1 passage is connected to the inlet of the C1 passage through a pipeline, the outlet of the C1 passage is connected to the inlets of the upper inlet pipe and the lower inlet pipe through a pipeline, the outlet of the upper inlet pipe is connected to the upper liquid inlet of the liquid hydrogen storage tank, and the outlet of the lower inlet pipe is connected to the lower liquid inlet of the liquid hydrogen storage tank. The inlet of the first-stage compressor is connected to the outlet of the first-stage buffer tank through a pipeline, the outlet of the first-stage compressor is connected to the inlet of the first-stage cooler through a pipeline, the outlet of the first-stage cooler is connected to the inlet of the second-stage buffer tank through a pipeline, the outlet of the second-stage buffer tank is connected to the inlet of the second-stage compressor through a pipeline, the outlet of the second-stage compressor is connected to the inlet of the second-stage cooler through a pipeline, the outlet of the second-stage cooler is connected to the inlet of the A2 passage through a pipeline, the outlet of the A2 passage is connected to the inlet of the second low-temperature adsorber through a pipeline, the outlet of the second low-temperature adsorber is connected to the inlet of the B2 passage through a pipeline, the outlet of the B2 passage is connected to the inlet of the throttle valve through a pipeline, the outlet of the throttle valve is connected to the inlet of the C2 passage through a pipeline, the outlet of the C2 passage is connected to the inlet of the B3 passage through a pipeline, the outlet of the B3 passage is connected to the inlet of the A4 passage through a pipeline, the outlet of the A4 passage is connected to the inlet of the first-stage buffer tank through a pipeline, the outlet of the B5 passage is connected to the inlet of the expansion end of the hydrogen expander through a pipeline, the outlet of the expansion end of the hydrogen expander is connected to the inlet of the B4 passage through a pipeline, the outlet of the B4 passage is connected to the inlet of the A3 passage through a second pipeline, the outlet of the A3 passage is connected to the inlet of the second-stage buffer tank through a pipeline, the first pipeline and the second pipeline are connected through a third pipeline, a first regulating valve is connected in series on the third pipeline, an upper inlet valve and a second stop valve are connected in series on the upper inlet pipe, a lower inlet valve is connected in series on the lower inlet pipe, the lower liquid discharge port of the liquid hydrogen storage tank is connected to one end of the lower liquid discharge pipe, a fifth stop valve and a sixth stop valve are connected in series on the lower liquid discharge pipe, the inlet of the fourth heat exchanger is connected to the lower liquid discharge pipe through a pipeline in which a seventh stop valve is connected in series, and the connection point is located between the fifth stop valve and the sixth stop valve.The outlet of the fourth heat exchanger is connected to the inlet of the first-stage buffer tank through a pipeline in series with an eighth stop valve. One end of the replacement liquid inlet pipe is connected to the pipeline between the outlet of the A3 passage and the second-stage buffer tank, and the other end of the replacement liquid inlet pipe is connected to the upper liquid inlet pipe, and the connection point is located between the upper liquid inlet valve and the second stop valve. A second regulating valve and a first stop valve are connected in series on the replacement liquid inlet pipe. A first relief pipeline and a BOG recovery pipeline are also connected to the top of the liquid hydrogen storage tank. A tenth stop valve is connected in series on the relief pipeline, and a fourth stop valve is connected in series on the BOG recovery pipeline. One end of the second relief pipeline is connected to the pipeline at the inlet of the first-stage buffer tank, and a third regulating valve is connected in series on the second relief pipeline.

[0005] Further, in the above-mentioned hydrogen liquefaction system capable of achieving zero emission of replacement gas from the liquid hydrogen storage tank: the first heat exchanger and the second heat exchanger are plate heat exchangers.

[0006] Further, in the above-mentioned hydrogen liquefaction system capable of achieving zero emission of replacement gas from the liquid hydrogen storage tank: the fourth heat exchanger is an air-cooled heat exchanger.

[0007] Further, in the above-mentioned hydrogen liquefaction system capable of achieving zero emission of replacement gas from the liquid hydrogen storage tank: a first parallel branch and a second parallel branch are also provided. The two ends of the first parallel branch are respectively connected to the pipeline at the outlet of the first-stage cooler and the pipeline at the inlet of the first-stage buffer tank, and the two ends of the second parallel branch are respectively connected to the pipeline at the outlet of the second-stage cooler and the pipeline at the inlet of the second-stage buffer tank. A cut-off valve is connected in series on each of the first parallel branch and the second parallel branch.

[0008] Further, in the above-mentioned hydrogen liquefaction system capable of achieving zero emission of replacement gas from the liquid hydrogen storage tank: the first heat exchanger, the first low-temperature adsorber, and the second low-temperature adsorber are located in the precooling cold box, and the second heat exchanger, the third heat exchanger, the hydrogen expander, and the throttle valve are located in the hydrogen liquefaction cold box.

[0009] The advantages of the present invention are as follows: when the liquid hydrogen storage tank needs to be replaced, the above-mentioned hydrogen liquefaction system can lead out the recycled hydrogen that has been purified by low-temperature adsorption in the recycled hydrogen compression refrigeration system as the replacement gas, so that the liquid hydrogen storage tank can avoid secondary pollution. The hydrogen containing impurities replaced in the liquid hydrogen storage tank can also return to the recycled hydrogen compression refrigeration system as recycled hydrogen for continued use, so that the replaced gas can achieve zero emission, thereby reducing production costs. BRIEF DESCRIPTION OF THE DRAWINGS

[0010] Figure 1 It is a schematic structural diagram of a hydrogen liquefaction system capable of achieving zero emission of replacement gas from the liquid hydrogen storage tank according to the present invention. DETAILED DESCRIPTION OF THE INVENTION

[0011] The present invention will be further described in detail below in conjunction with specific embodiments and the accompanying drawings.

[0012] Such as Figure 1As shown in the figure, a hydrogen liquefaction system capable of achieving zero emission of replaced gas in a liquid hydrogen storage tank includes: a first heat exchanger 1, a second heat exchanger 2, a third heat exchanger 3, a first low-temperature adsorber 5, a second low-temperature adsorber 6, a first-stage compressor 7, a second-stage compressor 8, a first-stage buffer tank 9, a second-stage buffer tank 10, a fourth heat exchanger 4, a liquid hydrogen storage tank 73, a first-stage cooler 71, a second-stage cooler 72, a throttle valve 74. In the first heat exchanger 1, there are provided an A1 passage 11, an A2 passage 12, an A3 passage 13, and an A4 passage 14. In the second heat exchanger 2, there are provided a B1 passage 21, a B2 passage 22, a B3 passage 23, a B4 passage 24, and a B5 passage 25. The inlet of the B5 passage 25 is connected to the B2 passage 22. In the third heat exchanger 3, there are provided a C1 passage 31 and a C2 passage 32. One end of the raw gas inlet pipe 80 is connected to the inlet of the A1 passage 11. The outlet of the A1 passage 11 is connected to the inlet of the first low-temperature adsorber 5 through a pipeline. The outlet of the first low-temperature adsorber 5 is connected to the inlet of the B1 passage 21 through a first pipeline 77. The outlet of the B1 passage 21 is connected to the inlet of the C1 passage 31 through a pipeline. The outlet of the C1 passage 31 is connected to the inlets of the upper inlet pipe 78 and the lower inlet pipe 79 through a pipeline. The outlet of the upper inlet pipe 78 is connected to the upper liquid inlet of the liquid hydrogen storage tank 73. The outlet of the lower inlet pipe 79 is connected to the lower liquid inlet of the liquid hydrogen storage tank 73. The inlet of the first-stage compressor 7 is connected to the outlet of the first-stage buffer tank 9 through a pipeline. The outlet of the first-stage compressor 7 is connected to the inlet of the first-stage cooler 71 through a pipeline. The outlet of the first-stage cooler 71 is connected to the inlet of the second-stage buffer tank 10 through a pipeline. The outlet of the second-stage buffer tank 10 is connected to the inlet of the second-stage compressor 8 through a pipeline. The outlet of the second-stage compressor 8 is connected to the inlet of the second-stage cooler 72 through a pipeline. The outlet of the second-stage cooler 72 is connected to the inlet of the A2 passage 12 through a pipeline. The outlet of the A2 passage 12 is connected to the inlet of the second low-temperature adsorber 6 through a pipeline. The outlet of the second low-temperature adsorber 6 is connected to the inlet of the B2 passage 22 through a pipeline. The outlet of the B2 passage 22 is connected to the inlet of the throttle valve 74 through a pipeline. The outlet of the throttle valve 74 is connected to the inlet of the C2 passage 32 through a pipeline. The outlet of the C2 passage 32 is connected to the inlet of the B3 passage 23 through a pipeline. The outlet of the B3 passage 23 is connected to the inlet of the A4 passage 14 through a pipeline. The outlet of the A4 passage 14 is connected to the inlet of the first-stage buffer tank 9 through a pipeline. The outlet of the B5 passage 25 is connected to the expansion end inlet of a hydrogen expander 75 through a pipeline. The expansion end outlet of the hydrogen expander 75 is connected to the inlet of the B4 passage 24 through a pipeline. The outlet of the B4 passage 24 is connected to the inlet of the A3 passage 13 through a second pipeline 76. The outlet of the A3 passage 13 is connected to the inlet of the second-stage buffer tank 10 through a pipeline. The first pipeline 77 and the second pipeline 76 are connected through a third pipeline 81. A first regulating valve 82 is connected in series on the third pipeline 81. An upper inlet valve 84 and a second stop valve 83 are connected in series on the upper inlet pipe 78.A lower liquid inlet valve 85 is connected in series on the lower liquid inlet pipe 79. The lower liquid discharge port of the liquid hydrogen storage tank 73 is connected to one end of the lower liquid discharge pipe 86. A fifth stop valve 87 and a sixth stop valve 88 are connected in series on the lower liquid discharge pipe 86. The inlet of the fourth heat exchanger 4 is connected to the lower liquid discharge pipe 86 through a pipe with a seventh stop valve 89 connected in series, and the connection point is between the fifth stop valve 87 and the sixth stop valve 88. The outlet of the fourth heat exchanger 4 is connected to the inlet of the first-stage buffer tank 9 through a pipe with an eighth stop valve 90 connected in series. One end of the replacement liquid inlet pipe 91 is connected to the pipe between the outlet of the A3 passage 13 and the second-stage buffer tank 10, and the other end of the replacement liquid inlet pipe 91 is connected to the upper liquid inlet pipe 78, and the connection point is between the upper liquid inlet valve 84 and the second stop valve 83. A second regulating valve 92 and a first stop valve 93 are connected in series on the replacement liquid inlet pipe 91. A first relief pipeline 94 and a BOG recovery pipeline 95 are also connected to the top of the liquid hydrogen storage tank 73. A tenth stop valve 96 is connected in series on the first relief pipeline 94. A fourth stop valve 97 is connected in series on the BOG recovery pipeline 95. One end of the second relief pipeline 98 is connected to the pipe at the inlet of the first-stage buffer tank 9. A third regulating valve 99 is connected in series on the second relief pipeline 98.,

[0013] In this embodiment, the first heat exchanger 1 and the second heat exchanger 2 are plate heat exchangers. The fourth heat exchanger 4 is an air-cooled heat exchanger.

[0014] A first parallel branch 101 and a second parallel branch 100 are also provided. The two ends of the first parallel branch 101 are respectively connected to the pipe at the outlet of the first-stage cooler 71 and the pipe at the inlet of the first-stage buffer tank 9. The two ends of the second parallel branch 101 are respectively connected to the pipe at the outlet of the second-stage cooler 72 and the pipe at the inlet of the second-stage buffer tank 10. A cut-off valve 102 is connected in series on each of the first parallel branch 101 and the second parallel branch 100. Such a setting is to facilitate the replacement of the first-stage compressor 7 and the second-stage compressor 8.

[0015] The first heat exchanger 1, the first low-temperature adsorber 5, and the second low-temperature adsorber 6 are located in the precooling cold box 61. The second heat exchanger 2, the third heat exchanger 3, the hydrogen expander 75, and the throttle valve 74 are located in the hydrogen liquefaction cold box 62.

[0016] Hydrogen liquefaction process: The raw hydrogen enters the first heat exchanger 1 through the raw gas inlet pipe 80 for precooling. A cold source is introduced into the first heat exchanger 1. Then the raw hydrogen enters the first low-temperature adsorber 5 for purification. Next, the raw hydrogen enters the second heat exchanger 2 for precooling. The cooling capacity in the second heat exchanger 2 is provided by the recycle hydrogen compression refrigeration system. Then the raw hydrogen enters the third heat exchanger 3 for deep cooling so that the raw hydrogen can be liquefied. The cooling capacity in the third heat exchanger 3 is provided by the recycle hydrogen compression refrigeration system. The liquefied raw hydrogen is stored in the liquid hydrogen storage tank 73 through the upper inlet pipe 78 and the lower inlet pipe 79.

[0017] Recycle hydrogen compression refrigeration system: After being purified by the first low-temperature adsorber 5, a part of the raw hydrogen is supplemented into the recycle hydrogen compression refrigeration system through the first regulating valve 82 to become recycle hydrogen. The recycle hydrogen is first pressurized to 2.5 Mpa (A) by the two-stage compressor 8. The normal-temperature recycle hydrogen after pressurization and cooling enters the first heat exchanger 1 for precooling, so that the recycle hydrogen is cooled to 80K and the pressure is 2.43 Mpa (A). Then the recycle hydrogen enters the second low-temperature adsorber 6 for purification, so that oxygen + argon ≤ 0.2PPm, nitrogen ≤ 0.4PPm, total carbon ≤ 0.5PPm and moisture ≤ 0.5PPm in the recycle hydrogen. The purified recycle hydrogen enters the B2 passage 22 of the second heat exchanger 2 to exchange heat with the recycle hydrogen in the B3 passage 23 and the B4 passage 24 for precooling. Then the recycle hydrogen in the B2 passage 22 is divided into two paths. The first path of recycle hydrogen is cooled to 30.6K and the pressure is 2.28 Mpa (A) in the second heat exchanger 2. Then this path of recycle hydrogen enters the throttle valve 74 for throttling expansion and cooling, so that this path of recycle hydrogen can be cooled to 21K and the pressure is 0.125 Mpa (A). The recycle hydrogen after throttling expansion enters the third heat exchanger 3 to exchange heat with the raw hydrogen to liquefy the raw hydrogen. Then this path of recycle hydrogen enters the B3 passage 23 to cool the recycle hydrogen in the B2 passage 22. Then this path of recycle hydrogen enters the A4 passage 14 to provide cooling capacity for the first heat exchanger 1, so that this path of recycle hydrogen can be reheated to normal temperature and the pressure is 0.105 Mpa (A). Then this path of recycle hydrogen enters the one-stage compressor 7 for recompression. After one-stage compression and cooling, this path of recycle hydrogen enters the two-stage compressor 8 for recompression; The second path of recycle hydrogen enters the hydrogen expander 75 for turbine expansion, so that the temperature of this path of recycle hydrogen can drop to 31.97k and the pressure is reduced to 0.34 Mpa (A). Then this path of recycle hydrogen enters the B4 passage 24 to cool the recycle hydrogen in the B2 passage 22. Then this path of recycle hydrogen enters the A3 passage 13 to provide cooling capacity for the first heat exchanger 1, so that this path of recycle hydrogen can be reheated to normal temperature and the pressure is 0.29 Mpa (A). Then this path of recycle hydrogen enters the two-stage compressor 8 together with the first path of recycle hydrogen for compression, thus completing a closed-cycle stroke of the recycle hydrogen.

[0018] After the liquid hydrogen storage tank 73 has been used for a period of time, it needs to be reheated and replaced. The replacement process is as follows:

[0019] (1) Stop filling the liquid hydrogen storage tank 73, close the upper liquid inlet valve 84 and the lower liquid inlet valve 85, open the fifth stop valve 87 and the sixth stop valve 88, and drain the remaining liquid hydrogen in the liquid hydrogen storage tank 73 through the loading system. After the liquid hydrogen is drained, close the sixth stop valve 88.

[0020] (2) Slowly open the tenth stop valve 96 to relieve the pressure in the liquid hydrogen storage tank 73 to 0.105 MPa (A), and then close the tenth stop valve 96.

[0021] (3) The circulating hydrogen of 0.29 Mpa (A) at the inlet of the secondary buffer tank 10 in the circulating hydrogen compression refrigeration system is led out as the replacement gas through the replacement liquid inlet pipe 91, and the second regulating valve 92, the first stop valve 93, and the second stop valve 83 are opened to send the clean replacement gas into the liquid hydrogen storage tank 73. The clean replacement gas at room temperature replaces the liquid hydrogen storage tank 73 from top to bottom and slowly reheats it. The fifth stop valve 87 and the seventh stop valve 89 are opened, and the eighth stop valve 90 is slowly opened to send the replaced hydrogen containing impurity gas into the fourth heat exchanger 4 for reheating until the gas temperature after the outlet of the fourth heat exchanger 4 is greater than -20°C and the inlet pressure of the first compressor 7 is maintained at 0.105 Mpa (A). The amount of replacement hydrogen can be adjusted by the second regulating valve 92 to ensure that the temperature of the replacement hydrogen after passing through the fourth heat exchanger 4 is not too low and the pressure is not overpressured. The hydrogen containing impurities will eventually be added to the circulating hydrogen compression refrigeration system.

[0022] (4) After detecting that the temperature of the replacement hydrogen coming out of the liquid hydrogen storage tank 73 rises from -253°C to -120~-100°C, the replacement hydrogen after passing through the fourth heat exchanger 4 is analyzed and sampled to analyze the oxygen, argon, nitrogen and total carbon therein. After testing that various indicators meet the requirements, it is confirmed that the replacement is qualified.

[0023] (5) After the liquid hydrogen storage tank 73 is replaced and qualified, start precooling the replaced liquid hydrogen storage tank 73, close the second regulating valve 92 and the first stop valve 93, open the upper liquid inlet valve 84, and control the flow of liquid hydrogen into the liquid hydrogen storage tank 73 through the second stop valve 83. The liquid hydrogen enters the liquid hydrogen storage tank 73 and begins to evaporate and absorb heat to ensure that there is no overpressure in the storage tank. The evaporated liquid hydrogen is discharged from the bottom of the liquid hydrogen storage tank 73 and enters the fourth heat exchanger 4 for reheating. The reheated hydrogen enters the inlet of the first compressor 7 to participate in the circulation of the circulating hydrogen compression refrigeration system. After there is no shortage of hydrogen in the circulating hydrogen compression refrigeration system, the excess hydrogen is discharged through the second vent pipe 98.

[0024] After the temperature of the hydrogen coming out of the bottom of the liquid hydrogen storage tank 73 drops below -240°C, confirm that the pre-cooling of the liquid hydrogen storage tank 73 is completed, close the seventh stop valve 89, and open the fourth stop valve 97 at the top of the liquid hydrogen storage tank 73 to send the evaporated BOG to the device for recovery.

[0025] (7)According to the requirements of the production working conditions, open the upper liquid inlet or the lower liquid inlet of the liquid hydrogen storage tank 73 to start re-storing and using liquid hydrogen.

Claims

1. A hydrogen liquefaction system capable of achieving zero emission of replacement gas in a liquid hydrogen storage tank, characterized in that: Including: The first heat exchanger, the second heat exchanger, the third heat exchanger, the first low-temperature adsorber, the second low-temperature adsorber, the first-stage compressor, the second-stage compressor, the first-stage buffer tank, the second-stage buffer tank, the fourth heat exchanger, the liquid hydrogen storage tank, the first-stage cooler, the second-stage cooler, the throttle valve, the hydrogen expander. In the first heat exchanger, there are A1 passage, A2 passage, A3 passage, and A4 passage. In the second heat exchanger, there are B1 passage, B2 passage, B3 passage, B4 passage, and B5 passage. The inlet of B5 passage is connected to the B2 passage. In the third heat exchanger, there are C1 passage and C2 passage. One end of the raw gas inlet pipe is connected to the inlet of A1 passage. The outlet of A1 passage is connected to the inlet of the first low-temperature adsorber through a pipeline. The outlet of the first low-temperature adsorber is connected to the inlet of B1 passage through the first pipeline. The outlet of B1 passage is connected to the inlet of C1 passage through a pipeline. The outlet of C1 passage is connected to the inlets of the upper liquid inlet pipe and the lower liquid inlet pipe through a pipeline. The outlet of the upper liquid inlet pipe is connected to the upper liquid inlet of the liquid hydrogen storage tank. The outlet of the lower liquid inlet pipe is connected to the lower liquid inlet of the liquid hydrogen storage tank. The inlet of the first-stage compressor is connected to the outlet of the first-stage buffer tank through a pipeline. The outlet of the first-stage compressor is connected to the inlet of the first-stage cooler through a pipeline. The outlet of the first-stage cooler is connected to the inlet of the second-stage buffer tank through a pipeline. The outlet of the second-stage buffer tank is connected to the inlet of the second-stage compressor through a pipeline. The outlet of the second-stage compressor is connected to the inlet of the second-stage cooler through a pipeline. The outlet of the second-stage cooler is connected to the inlet of A2 passage through a pipeline. The outlet of A2 passage is connected to the inlet of the second low-temperature adsorber through a pipeline. The outlet of the second low-temperature adsorber is connected to the inlet of B2 passage through a pipeline. The outlet of B2 passage is connected to the inlet of the throttle valve through a pipeline. The outlet of the throttle valve is connected to the inlet of C2 passage through a pipeline. The outlet of C2 passage is connected to the inlet of B3 passage through a pipeline. The outlet of B3 passage is connected to the inlet of A4 passage through a pipeline. The outlet of A4 passage is connected to the inlet of the first-stage buffer tank through a pipeline. The outlet of B5 passage is connected to the expansion end inlet of the hydrogen expander through a pipeline. The expansion end outlet of the hydrogen expander is connected to the inlet of B4 passage through a pipeline. The outlet of B4 passage is connected to the inlet of A3 passage through the second pipeline. The outlet of A3 passage is connected to the inlet of the second-stage buffer tank through a pipeline. The first pipeline and the second pipeline are connected through a third pipeline. A first regulating valve is connected in series on the third pipeline. An upper liquid inlet valve and a second stop valve are connected in series on the upper liquid inlet pipe. A lower liquid inlet valve is connected in series on the lower liquid inlet pipe. The lower liquid discharge port of the liquid hydrogen storage tank is connected to one end of the lower liquid discharge pipe. A fifth stop valve and a sixth stop valve are connected in series on the lower liquid discharge pipe. The inlet of the fourth heat exchanger is connected to the lower liquid discharge pipe through a pipeline with a seventh stop valve connected in series, and the connection point is between the fifth stop valve and the sixth stop valve. The outlet of the fourth heat exchanger is connected to the inlet of the first-stage buffer tank through a pipeline with an eighth stop valve connected in series. One end of the replacement liquid inlet pipe is connected to the pipeline between the outlet of A3 passage and the second-stage buffer tank.The other end of the replacement inlet pipe is connected to the upper inlet pipe, and the connection point is located between the upper inlet valve and the second stop valve. A second regulating valve and a first stop valve are connected in series on the replacement inlet pipe. A first relief pipeline and a BOG recovery pipeline are also connected to the top of the liquid hydrogen storage tank. A tenth stop valve is connected in series on the relief pipeline, and a fourth stop valve is connected in series on the BOG recovery pipeline. One end of the second relief pipeline is connected to the pipeline at the inlet of the first-stage buffer tank, and a third regulating valve is connected in series on the second relief pipeline., 2. The hydrogen liquefaction system capable of achieving zero-emission replacement gas for a liquid hydrogen storage tank according to claim 1, wherein: The first heat exchanger and the second heat exchanger are plate heat exchangers.

3. A hydrogen liquefaction system capable of achieving zero emissions of replacement gas in a liquid hydrogen storage tank according to claim 1 or 2, characterized in that: The fourth heat exchanger is an air-cooled heat exchanger.

4. A hydrogen liquefaction system capable of achieving zero emission of replacement gas in a liquid hydrogen storage tank according to claim 1 or 2, characterized in that: A first parallel branch and a second parallel branch are also provided. The two ends of the first parallel branch are respectively connected to the pipeline at the outlet of the primary cooler and the pipeline at the inlet of the primary buffer tank. The two ends of the second parallel branch are respectively connected to the pipeline at the outlet of the secondary cooler and the pipeline at the inlet of the secondary buffer tank. A cut-off valve is connected in series on each of the first parallel branch and the second parallel branch.

5. A hydrogen liquefaction system capable of achieving zero emission of replacement gas in a liquid hydrogen storage tank according to claim 1 or 2, characterized in that: The first heat exchanger, the first low-temperature adsorber, and the second low-temperature adsorber are located in the pre-cooling cold box, and the second heat exchanger, the third heat exchanger, the hydrogen expander, and the throttle valve are located in the hydrogen liquefaction cold box.

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