A turbine expansion liquefaction system utilizing natural gas pressure differential and its liquefaction method

Abstract

This invention discloses a turbine expansion liquefaction system and liquefaction method utilizing natural gas pressure differential. The outlet of a high-pressure natural gas pipeline is connected to a first compressor and the inlet of a pressure differential power generation device via pipelines. A pressure sensor is installed on the pipeline from the high-pressure natural gas pipeline outlet to the pressure differential power generation device inlet. The outlet of the pressure differential power generation device is connected to the inlet of a low-pressure output client via a pipeline through a heat exchange device. The outlet of the first compressor is connected to the heat exchange device via a pipeline. The heat exchange device is connected to a pressure replenishment device and a liquefied natural gas storage tank via pipelines. The liquefied natural gas storage tank is connected to the outlet of the high-pressure natural gas pipeline via a pipeline. A pressure replenishment device is installed on the pipeline from the liquefied natural gas storage tank to the high-pressure natural gas pipeline outlet, and the pressure replenishment device is electrically connected to the pressure differential power generation device. This invention utilizes the natural gas pressure differential to obtain cooling energy and store energy for cooling and liquefaction. By recovering the energy of natural gas during the expansion process, overall energy consumption is reduced and system efficiency is improved.

Description

Technical Field

[0001] This invention relates to the field of natural gas production technology, and in particular to a turbine expansion liquefaction system that utilizes natural gas pressure differential. Background Technology

[0002] Because urban natural gas consumption is unstable, fluctuating periodically between peak and off-peak periods, city gas companies typically use gas storage devices to store some natural gas during off-peak times and then transport the stored gas into the pipeline network for peak shaving during peak periods. However, as natural gas consumption increases, the required gas storage and peak shaving devices not only require significant investment but also occupy large amounts of land, resulting in high peak shaving costs. To reduce peak shaving costs, city gas companies building their own natural gas liquefaction plants is an effective way to lower these costs. High-pressure natural gas in upstream pipelines needs to be depressurized through city distribution stations, gate stations, and pressure regulating stations. Traditional pressure regulation methods mainly rely on throttling valves for depressurization, which leads to a significant waste of pressure and heat energy. Therefore, this invention discloses a turbine expansion liquefaction system utilizing natural gas pressure differentials, organically combining the utilization of pipeline pressure energy with the gate station pressure regulation requirements to maximize energy efficiency. Summary of the Invention

[0003] To address the shortcomings of existing technologies, this invention proposes a turbine expansion liquefaction system that utilizes natural gas pressure differential to depressurize high-pressure natural gas from pipelines before delivering it to the urban pipeline network, while simultaneously utilizing the cold energy generated by the expansion of natural gas to obtain liquefied natural gas.

[0004] The technical solution adopted in this invention is:

[0005] This invention includes a high-pressure natural gas pipeline, a pressure sensor, a first compressor, a differential pressure power generation device, a pressure replenishment device, a heat exchange device, a liquefied natural gas storage tank, and a low-pressure output client.

[0006] The outlet of the high-pressure natural gas pipeline is connected to the inlet of the first compressor and the differential pressure generator via pipelines. A pressure sensor is installed on the pipeline from the outlet of the high-pressure natural gas pipeline to the inlet of the differential pressure generator. The outlet of the differential pressure generator is connected to the inlet of the low-pressure output client via a heat exchange device. The outlet of the first compressor is connected to the heat exchange device via a pipeline. The heat exchange device is connected to the pressure replenishment device and the liquefied natural gas storage tank via pipelines. The liquefied natural gas storage tank is connected to the outlet of the high-pressure natural gas pipeline via a pipeline. A pressure replenishment device is installed on the pipeline from the liquefied natural gas storage tank to the outlet of the high-pressure natural gas pipeline. The pressure replenishment device is electrically connected to the differential pressure generator.

[0007] The pressure replenishment device includes a phase change energy storage device, a gas mixing tank, and a second compressor. The heat exchange device is connected to the gas mixing tank and the liquefied natural gas storage tank via pipelines. The gas mixing tank, the second compressor, and the phase change energy storage device are sequentially installed on the pipeline from the liquefied natural gas storage tank to the outlet of the high-pressure natural gas pipeline. The phase change energy storage device is electrically connected to the differential pressure power generation device.

[0008] The differential pressure power generation device includes an expander and a generator. The outlet of the high-pressure natural gas pipeline is connected to the first compressor and the inlet of the expander through pipelines. The expander is connected to the generator via a drive, and the generator is connected to the phase change energy storage device via an electrical connection. The outlet of the expander is connected to the inlet of the low-pressure output client through a pipeline via a heat exchange device.

[0009] The heat exchange device includes a heat exchanger and a gas-liquid mixing tank. The heat exchanger is provided with a first heat exchange tube and a second heat exchange tube. The gas-liquid mixing tank has openings at the upper part, top and bottom of its inner cavity, and a third heat exchange tube is arranged inside the tank, directly penetrating the inner cavity.

[0010] The outlet of the first compressor is connected to the inlet of the first heat exchange tube in the heat exchanger via a pipeline. The outlet of the first heat exchange tube is connected to the upper opening of the inner cavity of the gas-liquid mixing tank via a pipeline. The top opening of the inner cavity of the gas-liquid mixing tank is connected to the gas mixing tank via a pipeline. The bottom opening of the inner cavity of the gas-liquid mixing tank is connected to the liquefied natural gas storage tank via a pipeline. The outlet of the expander is connected to the inlet of the third heat exchange tube in the gas-liquid mixing tank via a pipeline. The outlet of the third heat exchange tube is connected to the inlet of the second heat exchange tube in the heat exchanger via a pipeline. The outlet of the second heat exchange tube is connected to the inlet of the low-pressure output client via a pipeline.

[0011] The outlet of the high-pressure natural gas pipeline is connected to the inlet of the first compressor and the differential pressure power generation device via a three-way valve. A pressure sensor is installed on the pipeline from the outlet of the high-pressure natural gas pipeline to the three-way valve.

[0012] Shut-off valves are installed on the pipelines between the gas-liquid mixing tank and the gas mixing tank, the pipelines between the gas mixing tank and the second compressor, and the pipelines between the liquefied natural gas storage tank and the gas mixing tank. A throttling valve is installed on the pipeline between the gas-liquid mixing tank and the liquefied natural gas storage tank.

[0013] A heater is installed on the pipe between the second heat exchange tube and the low-pressure output terminal.

[0014] A turbine expansion liquefaction method utilizing natural gas pressure differential:

[0015] A pressure threshold is set, and the pressure sensor detects the pressure of the high-pressure natural gas flowing through it.

[0016] If the pressure sensor detects that the high-pressure natural gas pressure exceeds the set threshold, the control shut-off valve will be closed, and the system will operate as if the high-pressure natural gas pressure is sufficient.

[0017] When the pressure sensor detects that the high-pressure natural gas pressure exceeds or is below or equal to the threshold, the control shut-off valve opens, and the system operates as if the high-pressure natural gas pressure is insufficient.

[0018] The specific steps for operating under sufficient high-pressure natural gas pressure are as follows:

[0019] High-pressure natural gas is first divided into two paths by a three-way valve through a high-pressure natural gas pipeline. The first path of high-pressure natural gas flows into the first compressor through the pipeline for pressurization, and then is transported through the pipeline to the first heat exchange tube in the heat exchanger for the first heat exchange. After that, it is fed into the gas-liquid mixing tank through the pipeline.

[0020] The second high-pressure natural gas expands and depressurizes through an expander to obtain cooling. During the operation of the expander, it coaxially drives a generator to generate electricity. The generator drives a phase change energy storage device to store thermal energy. Then, the natural gas output from the expander is transported through a pipeline to the third heat exchange tube in the gas-liquid mixing tank. The natural gas input from the expander into the third heat exchange tube of the gas-liquid mixing tank undergoes a second heat exchange with the high-pressure natural gas input from the first heat exchange tube into the gas-liquid mixing tank. This causes more of the high-pressure natural gas input from the first heat exchange tube to be converted into liquid. The natural gas output through the third heat exchange tube is transported through a pipeline to the second heat exchange tube for the first heat exchange treatment. Then, it is heated by a heater to the temperature required by the customer and delivered to the low-pressure output customer.

[0021] In the first route, the high-pressure natural gas fed into the gas-liquid mixing tank through the first heat exchange pipe is transported to the liquefied natural gas storage tank for storage through the pressure regulation of the throttle valve.

[0022] The specific steps for operating under insufficient high-pressure natural gas pressure are as follows:

[0023] In addition to the same processing as when operating under sufficient high-pressure natural gas pressure, the following processing is also implemented:

[0024] The gaseous portion of the high-pressure natural gas in the first heat exchanger tube enters the gas-liquid mixing tank and flows out from the top opening of the inner cavity of the gas-liquid mixing tank. It is then transferred to the gas mixing tank via a shut-off valve. The gaseous portion of the liquefied natural gas storage tank is transferred to the gas mixing tank via a shut-off valve. Subsequently, the natural gas in the gas mixing tank flows through a shut-off valve to the second compressor for pressurization. The pressurized high-pressure natural gas flows into the phase change energy storage device to exchange heat with its stored heat energy and is heated to the same temperature as the high-pressure natural gas in the high-pressure natural gas pipeline. It is then transported to the pipeline between the high-pressure natural gas pipeline and the three-way valve for pressurization.

[0025] The beneficial effects of this invention are:

[0026] 1. This invention uses a turbo expander instead of a traditional throttle valve to generate electricity using the pressure difference of high-pressure natural gas, thereby achieving the liquefaction and capture of natural gas;

[0027] 2. This invention utilizes a turbine expander to obtain cooling capacity, which is then exchanged with high-pressure natural gas for heat, thereby improving liquefaction efficiency. It has the advantage of reducing energy waste and achieving better energy-saving effects.

[0028] 3. The present invention adds a phase change heat storage device to achieve effective temperature regulation and control and improve gas supply stability. Attached Figure Description

[0029] Figure 1 This is a schematic diagram illustrating the principle of the present invention when the high-pressure natural gas is under sufficient pressure.

[0030] Figure 2 This is a schematic diagram illustrating the principle of the present invention when the high-pressure natural gas pressure is insufficient.

[0031] Figure reference numerals: 1. High-pressure natural gas pipeline; 2. Pressure sensor; 3. First compressor; 4. Expander; 5. Generator; 6. Phase change energy storage device; 7. Heat exchanger; 8. Gas-liquid mixing tank; 9. Liquefied natural gas storage tank; 10. Gas mixing tank; 11. Second compressor; 11. Three-way valve S1; 12. Shut-off valve F2; 13. Shut-off valve F3; 14. Throttling valve J1; 15. Heater; 16. Low-pressure output terminal; 17. Detailed Implementation

[0032] The present invention will be further described below with reference to the accompanying drawings and embodiments. The embodiments of the present invention include, but are not limited to, the following embodiments.

[0033] A turbine expansion liquefaction system utilizing natural gas pressure differentials, such as Figure 1 As shown, it includes a high-pressure natural gas pipeline 1, a pressure sensor 2, a first compressor 3, a differential pressure power generation device, a pressure replenishment device, a heat exchange device, a liquefied natural gas storage tank 9, and a low-pressure output client 13;

[0034] The outlet of the high-pressure natural gas pipeline 1 is connected to the first compressor 3 and the inlet of the differential pressure generator via pipelines. A pressure sensor 2 is installed on the pipeline from the outlet of the high-pressure natural gas pipeline 1 to the inlet of the differential pressure generator. The outlet of the differential pressure generator is connected to the inlet of the low-pressure output client 13 via a heat exchange device. The outlet of the first compressor 3 is connected to the heat exchange device via a pipeline. The heat exchange device is connected to the pressure replenishing device and the liquefied natural gas storage tank 9 via pipelines. The liquefied natural gas storage tank 9 is connected to the outlet of the high-pressure natural gas pipeline 1 via a pipeline. A pressure replenishing device is installed on the pipeline from the liquefied natural gas storage tank 9 to the outlet of the high-pressure natural gas pipeline 1. The pressure replenishing device is electrically connected to the differential pressure generator.

[0035] The liquefied natural gas storage tank 9 is equipped with an LNG loading and unloading interface and is connected to an external LNG loading and unloading device for transferring LNG.

[0036] The pressurization device includes a phase change energy storage device 6, a gas mixing tank 10, and a second compressor 11. The heat exchange device is connected to the gas mixing tank 10 and the liquefied natural gas storage tank 9 via pipelines. The gas mixing tank 10, the second compressor 11, and the phase change energy storage device 6 are sequentially installed on the pipeline from the liquefied natural gas storage tank 9 to the outlet of the high-pressure natural gas pipeline 1. The phase change energy storage device 6 is electrically connected to the differential pressure power generation device.

[0037] The differential pressure power generation device includes an expander 4 and a generator 5, specifically a turbine expander. The outlet of the high-pressure natural gas pipeline 1 is connected to the first compressor 3 and the inlet of the expander 4 via pipelines. The working shaft of the expander 4 is connected to the rotating shaft of the generator 5 via a drive. The generator 5 is electrically connected to the phase change energy storage device 6. The outlet of the expander 4 is connected to the inlet of the low-pressure output client 13 via a pipeline through a heat exchange device.

[0038] Low-pressure output client refers to the user's natural gas load equipment.

[0039] The heat exchange device includes a heat exchanger 7 and a gas-liquid mixing tank 8. The heat exchanger 7 is provided with a first heat exchange tube and a second heat exchange tube. The upper part, top and bottom of the inner cavity of the gas-liquid mixing tank 8 are all provided with openings. A third heat exchange tube is arranged directly through the inner cavity of the tank. The third heat exchange tube is not connected to the inner cavity of the tank and plays the role of mutual heat exchange.

[0040] The outlet of the first compressor 3 is connected to the inlet of the first heat exchange tube in the heat exchanger 7 via a pipeline. The outlet of the first heat exchange tube is connected to the upper opening of the inner cavity of the gas-liquid mixing tank 8 via a pipeline. The top opening of the inner cavity of the gas-liquid mixing tank 8 is connected to the gas mixing tank 10 via a pipeline. The bottom opening of the inner cavity of the gas-liquid mixing tank 8 is connected to the liquefied natural gas storage tank 9 via a pipeline. The outlet of the expander 4 is connected to the inlet of the third heat exchange tube in the gas-liquid mixing tank 8 via a pipeline. The outlet of the third heat exchange tube is connected to the inlet of the second heat exchange tube in the heat exchanger 7 via a pipeline. The outlet of the second heat exchange tube is connected to the inlet of the low-pressure output client 13 via a pipeline.

[0041] The outlet of the high-pressure natural gas pipeline 1 is connected to the first compressor 3 and the inlet of the differential pressure power generation device via a three-way valve S1. A pressure sensor 2 is installed on the pipeline from the outlet of the high-pressure natural gas pipeline 1 to the three-way valve S1.

[0042] Shut-off valves F1, F2, and F3 are installed on the pipelines between the gas-liquid mixing tank 8 and the gas mixing tank 10, the pipelines between the gas mixing tank 10 and the second compressor 11, and the pipelines between the liquefied natural gas storage tank 9 and the gas mixing tank 10. Specifically, a first shut-off valve is installed on the pipeline between the gas-liquid mixing tank 8 and the gas mixing tank 10; a second shut-off valve is installed on the pipeline between the liquefied natural gas storage tank 9 and the gas mixing tank 10. A third shut-off valve is installed on the pipeline between the gas mixing tank 10 and the second compressor 11. Pressure sensor 2 is electrically connected to shut-off valves F1, F2, and F3 to control the opening and closing of shut-off valves F1, F2, and F3. A throttle valve J1 is installed on the pipeline between the gas-liquid mixing tank 8 and the liquefied natural gas storage tank 9 to regulate the pressure of the liquefied natural gas storage tank 9.

[0043] A heater 12 is installed on the pipeline between the second heat exchange tube and the low-pressure output client 13 to regulate the temperature of the natural gas input to the low-pressure output client 13.

[0044] The present invention has two states: a state with sufficient high-pressure natural gas pressure and a state with insufficient high-pressure natural gas pressure.

[0045] A pressure threshold is set, and pressure sensor 2 detects the pressure of the high-pressure natural gas flowing through it.

[0046] If pressure sensor 2 detects that the high-pressure natural gas pressure exceeds the set threshold, it considers this state to be a state of sufficient high-pressure natural gas pressure, and then controls the shut-off valves F1, F2, and F3 to close, and operates according to the state of sufficient high-pressure natural gas pressure.

[0047] When pressure sensor 2 detects that the high-pressure natural gas pressure exceeds or is below or equal to the threshold, it considers this state to be a state of insufficient high-pressure natural gas pressure. Then, control shut-off valves F1, F2, and F3 are opened, and the system operates according to the state of insufficient high-pressure natural gas pressure.

[0048] The natural gas pressure differential energy utilization turbine expansion liquefaction system involves two main processes. A portion of the high-pressure natural gas passes through a turbine expander, generating electricity. Excess electricity is then converted into heat and stored. Another portion of the high-pressure natural gas passes through a compressor and exchanges heat with the turbine-expanded, cryogenic natural gas, liquefying it and storing it in an LNG tank. The low-pressure loop natural gas, after heat exchange, then enters the low-pressure natural gas pipeline, achieving highly efficient energy utilization. Furthermore, when the high-pressure natural gas supply pressure is insufficient, the high-pressure pipeline can be pressurized using the LNG tank and phase change energy storage device.

[0049] Specifically, such as Figure 1 As shown in the attached diagram, the bolded lines represent the pipelines or lines used in this state. Specifically, this is done under conditions of sufficient high-pressure natural gas pressure:

[0050] High-pressure natural gas is first divided into two paths by the high-pressure natural gas pipeline 1 through the three-way valve S1. The first path of high-pressure natural gas flows into the first compressor 3 through the pipeline for pressurization, and then is transported through the pipeline to the first heat exchange tube in the heat exchanger 7 for the first heat exchange and cooling to become a gas-liquid two-phase state. Subsequently, it is input into the gas-liquid mixing tank 8 through the pipeline.

[0051] The second high-pressure natural gas expands and depressurizes through expander 4 to obtain cooling. During the operation of expander, it coaxially drives generator 5 to generate electricity. Generator 5 drives phase change energy storage device 6 to store thermal energy. Then, the natural gas output from expander 4 is transported through pipeline to the third heat exchange tube in gas-liquid mixing tank 8. The natural gas input from expander 4 into the third heat exchange tube of gas-liquid mixing tank 8 undergoes a second heat exchange with the high-pressure natural gas input from the first heat exchange tube into gas-liquid mixing tank 8. This secondary heat exchange causes the high-pressure natural gas input from the first heat exchange tube into gas-liquid mixing tank 8 to be converted into more liquid. At this time, the natural gas in gas-liquid mixing tank 8 is in a gas-liquid two-phase state. After the second heat exchange, the temperature of the natural gas in the third heat exchange tube initially rises. Then, it flows through heat exchanger 7 for preheating. The natural gas output from the third heat exchange tube is transported through pipeline to the second heat exchange tube for the first heat exchange treatment. Then, it is heated by heater 12 to the temperature required by the client and transported to the low-pressure output client 13.

[0052] In the first path, the high-pressure liquid portion of natural gas, which enters the gas-liquid mixing tank 8 through the first heat exchange pipe, expands to the storage pressure through the pressure regulation of the throttle valve J1 and is then transported to the liquefied natural gas storage tank 9 for storage.

[0053] like Figure 2 As shown, the specific operation under insufficient high-pressure natural gas pressure is as follows:

[0054] In addition to the same processing as when operating under sufficient high-pressure natural gas pressure, the following processing is also implemented:

[0055] The gaseous portion of the high-pressure natural gas in the first heat exchanger tube input gas-liquid mixing tank 8 flows out from the top opening of the inner cavity of the gas-liquid mixing tank 8, and is then transferred to the gas mixing tank 10 via the first shut-off valve. The gaseous portion of the liquefied natural gas storage tank 9 is transferred to the gas mixing tank 10 via the second shut-off valve. Subsequently, the natural gas in the gas mixing tank 10 flows through the third shut-off valve to the second compressor 11 for pressurization. The pressurized high-pressure natural gas flows into the phase change energy storage device 6 to exchange heat with its stored heat energy, and is heated to the same temperature as the high-pressure natural gas in the high-pressure natural gas pipeline 1. It is then transported to the pipeline between the high-pressure natural gas pipeline 1 and the three-way valve S1 for pressurization.

[0056] When the pressure is replenished to a sufficient level, continue to operate as if the high-pressure natural gas pressure is sufficient.

Claims

1. A turboexpansion liquefaction system utilizing natural gas pressure differential, characterized by: The system comprises a high-pressure natural gas pipeline (1), a pressure sensor (2), a first compressor (3), a pressure differential power generation device, a pressure supplementing device, a heat exchange device, a liquefied natural gas storage tank (9) and a low-pressure output client (13). The outlet of the high-pressure natural gas pipeline (1) is connected with the first compressor (3) and the inlet of the pressure differential power generation device through pipelines, and the pipeline from the outlet of the high-pressure natural gas pipeline (1) to the inlet of the pressure differential power generation device is provided with the pressure sensor (2); the outlet of the pressure differential power generation device is connected with the inlet of the low-pressure output client (13) through a pipeline passing through the heat exchange device; the outlet of the first compressor (3) is communicated with the heat exchange device through a pipeline; the heat exchange device is connected with the pressure supplementing device and the liquefied natural gas storage tank (9) through pipelines; the liquefied natural gas storage tank (9) is connected with the outlet of the high-pressure natural gas pipeline (1) through a pipeline, and the pipeline from the liquefied natural gas storage tank (9) to the outlet of the high-pressure natural gas pipeline (1) is provided with the pressure supplementing device; and the pressure supplementing device is electrically connected with the pressure differential power generation device. The pressure supplementing device comprises a phase change energy storage device (6), a gas mixing tank (10) and a second compressor (11), the heat exchange device is connected with the gas mixing tank (10) and the liquefied natural gas storage tank (9) through pipelines, and the pipeline from the liquefied natural gas storage tank (9) to the outlet of the high-pressure natural gas pipeline (1) is sequentially provided with the gas mixing tank (10), the second compressor (11) and the phase change energy storage device (6); and the phase change energy storage device (6) is electrically connected with the pressure differential power generation device. The pressure differential power generation device comprises an expander (4) and a generator (5), the outlet of the high-pressure natural gas pipeline (1) is connected with the first compressor (3) and the inlet of the expander (4) through pipelines, the expander (4) is drivingly connected with the generator (5), the generator (5) is electrically connected with the phase change energy storage device (6), and the outlet of the expander (4) is connected with the inlet of the low-pressure output client (13) through a pipeline passing through the heat exchange device. The heat exchange device comprises a heat exchanger (7) and a gas-liquid mixing tank (8), the heat exchanger (7) is provided with first heat exchange pipes and second heat exchange pipes, and the inner cavity of the gas-liquid mixing tank (8) is provided with openings in the upper portion, the top and the bottom, and the third heat exchange pipes are arranged in the inner cavity and directly pass through the inner cavity. The outlet of the first heat exchange pipes is connected with the inlet of the gas-liquid mixing tank (8) through a pipeline, the outlet of the first heat exchange pipes is connected with the upper opening of the inner cavity of the gas-liquid mixing tank (8) through a pipeline, the top opening of the inner cavity of the gas-liquid mixing tank (8) is connected with the gas mixing tank (10) through a pipeline, the bottom opening of the inner cavity of the gas-liquid mixing tank (8) is connected with the liquefied natural gas storage tank (9) through a pipeline, the outlet of the expander (4) is connected with the inlet of the third heat exchange pipes in the gas-liquid mixing tank (8) through a pipeline, the outlet of the third heat exchange pipes is connected with the inlet of the second heat exchange pipes in the heat exchanger (7) through a pipeline, and the outlet of the second heat exchange pipes is connected with the inlet of the low-pressure output client (13) through a pipeline.

2. A turboexpansion liquefaction system utilizing natural gas pressure differential according to claim 1, characterized in that: The outlet of the high-pressure natural gas pipeline (1) is connected with the first compressor (3) and the inlet of the pressure differential power generation device through a three-way valve (S1), and the pipeline from the outlet of the high-pressure natural gas pipeline (1) to the three-way valve (S1) is provided with the pressure sensor (2).

3. A turboexpansion liquefaction system utilizing natural gas pressure differential according to claim 2, characterized in that: A stop valve is arranged on the pipeline between the gas-liquid mixing tank (8) and the gas mixing tank (10), on the pipeline between the gas mixing tank (10) and the second compressor (11), and on the pipeline between the liquefied natural gas storage tank (9) and the gas mixing tank (10).

4. A turboexpansion liquefaction system utilizing natural gas pressure differential according to claim 3, wherein: A heater (12) is arranged on the pipeline between the second heat exchange pipe and the low-pressure output client (13).

5. The liquefaction method of the turbine expansion liquefaction system utilizing natural gas pressure difference according to claim 4, characterized in that: A pressure threshold is set, and the pressure sensor (2) detects the pressure of the high-pressure natural gas flowing through the pressure sensor (2): When the pressure sensor (2) detects that the pressure of the high-pressure natural gas exceeds the set threshold, the stop valve is controlled to be closed, and the system works in the high-pressure natural gas sufficient state; When the pressure sensor (2) detects that the pressure of the high-pressure natural gas is less than or equal to the threshold, the stop valve is controlled to be opened, and the system works in the high-pressure natural gas insufficient state.

6. The liquefaction method of the turbine expansion liquefaction system utilizing natural gas pressure difference according to claim 5, characterized in that: The working in the high-pressure natural gas sufficient state specifically includes: The high-pressure natural gas is first divided into two paths by the three-way valve (S1) on the high-pressure natural gas pipeline (1), the first path of the high-pressure natural gas flows into the first compressor (3) through the pipeline for pressure increase, and then is transported to the first heat exchange pipe in the heat exchanger (7) for first heat exchange, and then is input into the gas-liquid mixing tank (8) through the pipeline; The second path of the high-pressure natural gas is expanded by the expander (4) to obtain cold energy, the expander (4) drives the generator (5) to generate electricity during operation, the generator (5) drives the phase change energy storage device (6) to store heat energy, and then the natural gas output by the expander (4) is transported to the third heat exchange pipe in the gas-liquid mixing tank (8) through the pipeline, the natural gas input from the expander (4) into the third heat exchange pipe in the gas-liquid mixing tank (8) is subjected to second heat exchange with the high-pressure natural gas input into the gas-liquid mixing tank (8) through the first heat exchange pipe, so that the high-pressure natural gas input into the gas-liquid mixing tank (8) through the first heat exchange pipe is more converted into liquid, the natural gas output through the third heat exchange pipe is transported to the second heat exchange pipe through the pipeline for first heat exchange treatment, and then is heated to the required temperature of the client by the heater (12) and is transported to the low-pressure output client (13); The high-pressure natural gas input into the gas-liquid mixing tank (8) through the first heat exchange pipe in the first path is transported to the liquefied natural gas storage tank (9) through the pressure regulation of the throttle valve (J1) for storage.

7. The method of claim 6, wherein the pressure difference is provided by a natural gas source. The working in the high-pressure natural gas insufficient state specifically includes: On the basis of the same treatment as the working in the high-pressure natural gas sufficient state, the following treatment is further arranged: The gaseous part of the high-pressure natural gas in the first heat exchange pipe input gas-liquid mixing tank (8) flows out from the top opening of the inner cavity of the gas-liquid mixing tank (8), and is then transmitted to the gas mixing tank (10) through a stop valve. The gaseous part of the liquefied natural gas in the liquefied natural gas storage tank (9) is transmitted to the gas mixing tank (10) through a stop valve. Then, the natural gas in the gas mixing tank (10) flows through a stop valve and is transmitted to the second compressor (11) for pressure increase. The high-pressure natural gas after pressure increase flows into the phase change energy storage device (6) to exchange heat with the stored energy of the phase change energy storage device (6), is heated to the same temperature as the high-pressure natural gas in the high-pressure natural gas pipeline (1), and is delivered to the pipeline between the high-pressure natural gas pipeline (1) and the three-way valve (S1) for pressure compensation.

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