A BOG heating and utilization and LNG regasification system and control method thereof

Abstract

The present invention discloses a BOG heating and utilization and LNG regasification system, comprising an LNG storage tank, a heat exchange medium circulation subsystem and a cold energy recovery subsystem, wherein the heat exchange medium circulation subsystem comprises a heat exchange medium, a first heat exchanger and a second heat exchanger, wherein the heat exchange medium sequentially exchanges heat through the first heat exchanger and the second heat exchanger, wherein the cold energy recovery subsystem comprises a third heat exchanger and air, wherein the heat exchange medium exchanges heat with the air through the third heat exchanger after passing through the second heat exchanger to obtain low-temperature air, wherein the LNG storage tank is provided with a BOG outlet connected to the first heat exchanger, and a submersible pump for extracting LNG connected to the second heat exchanger is provided inside the LNG storage tank. By adding a heat exchange medium for heat exchange transition, the heat exchange medium undergoes a phase change in the heat exchanger, absorbs or releases a large amount of latent heat, realizes the exchange of air heat energy with LNG and BOG cold energy, and simultaneously completes BOG heating, LNG regasification and cold energy recovery, thereby avoiding energy waste, and the system is simple and efficient, with high economic benefits.

Description

Technical Field

[0001] The invention relates to natural gas production, and in particular to a BOG heating and utilization and LNG regasification system and a control method thereof. Background Art

[0002] With the rapid increase in demand for natural gas (NG) and the development of the liquefied natural gas (LNG) industry chain, the application of offshore LNG floating storage and regasification units (FSRU) has become increasingly widespread. At present, a complete industrial system has gradually been formed from production, storage, transportation, shipping to receiving, regasification, cold utilization, peak load regulation, etc. During the LNG storage and transportation process, under the action of wind, waves and currents, the FSRU will slosh, and the LNG storage tank or insulation tank will inevitably exchange heat with the outside world, which will cause the LNG to continuously vaporize and produce a large amount of flash steam (Boil Off Gas, BOG). The continuous generation of BOG will cause the internal pressure of the storage tank to increase. If it is not effectively handled, the continuous increase in pressure will lead to serious safety hazards.

[0003] At present, the following solutions are adopted in the market to solve the BOG gas problem: first, BOG gas is burned and directly discharged, which not only pollutes the environment but also wastes energy; second, additional storage tanks are added to store BOG gas for BOG gas utilization, but BOG has low storage density, low economic value per unit volume, and the storage tanks occupy a large space, resulting in high cost and cumbersome operation; third, BOG is liquefied by refrigeration technology, but due to technical limitations, the BOG gas is re-liquefied and recovered by pressurizing and cooling, which will cause huge energy consumption and complex process flow.

[0004] Collecting BOG gas, heating BOG using heat exchange technology, recovering the cold energy released, and directly utilizing BOG gas is more in line with the current trend of energy conservation and environmental protection, and the process is simple. At the same time, LNG needs to be re-vaporized before being transported to the NG user end. During this process, LNG needs to absorb a large amount of heat and release cold energy. In the prior art, different heat sources are used to act on the two processes of BOG heating and utilization and LNG re-vaporization, making the entire heat exchange system cumbersome and costly. Moreover, during the heat exchange process, the heat source directly exchanges heat with BOG and LNG, which can easily cause the heat exchange medium to freeze or incompletely vaporize. Summary of the invention

[0005] Purpose of the invention: In view of the above shortcomings, the present invention provides a BOG heating and utilization and LNG regasification system that shares a heat source and exchanges heat between a heat exchange medium and the heat source.

[0006] Technical solution: To solve the above problems, the present invention adopts a BOG heating utilization and LNG regasification system, including an LNG storage tank, a heat exchange medium circulation subsystem and a cold energy recovery subsystem, the heat exchange medium circulation subsystem includes a heat exchange medium, a first heat exchanger and a second heat exchanger, the heat exchange medium passes through the first heat exchanger and the second heat exchanger in sequence for heat exchange, the cold energy recovery subsystem includes a third heat exchanger and air, the heat exchange medium passes through the second heat exchanger and then passes through the third heat exchanger, the heat exchange medium exchanges heat with the air passing through the third heat exchanger, and low-temperature air is formed after heat exchange through the third heat exchanger, the LNG storage tank is provided with a BOG outlet, and a submersible pump for extracting LNG is provided inside the LNG storage tank, the BOG outlet is connected to the first heat exchanger, and the output end of the submersible pump is connected to the second heat exchanger.

[0007] Furthermore, the heat exchange working fluid circulation subsystem also includes a heat exchange working fluid buffer tank arranged between the second heat exchanger and the third heat exchanger, and a gas-liquid separator arranged between the first heat exchanger and the second heat exchanger, the gas outlet of the gas-liquid separator is connected to the inlet of the second heat exchanger, and the liquid outlet of the gas-liquid separator is connected to the inlet of the heat exchange working fluid buffer tank.

[0008] Furthermore, the heat exchange medium circulation subsystem also includes a first three-way valve arranged between the first heat exchanger and the gas-liquid separator, and the three ends of the first three-way valve are respectively connected to the first heat exchanger, the gas-liquid separator, and the heat exchange medium buffer tank.

[0009] Furthermore, a circulating pump is provided at the output end of the heat exchange working medium buffer tank, and the heat exchange working medium is propane.

[0010] Furthermore, the cold energy recovery subsystem also includes an air pressure regulating valve arranged at the outlet of the third heat exchanger.

[0011] Furthermore, the cold energy recovery subsystem also includes an air purifier and an air compressor, the output end of the air compressor is connected to the input end of the air purifier, and the output end of the air purifier is connected to the inlet end of the third heat exchanger.

[0012] Furthermore, the first heat exchanger and the second heat exchanger are microchannel heat exchangers, and the third heat exchanger is a fin heat exchanger.

[0013] Furthermore, the first heat exchanger and the second heat exchanger are both 3D printed structures.

[0014] Furthermore, a BOG gas compressor is arranged between the BOG outlet and the first heat exchanger.

[0015] The present invention also adopts a control method for a BOG heating and utilization and LNG regasification system, comprising the following steps:

[0016] S1: Select the working mode of the system: working mode 1 is to heat BOG alone, and when working mode 1 is selected, go to step S2; working mode 2 is to heat BOG and regasify LNG at the same time, and when working mode 2 is selected, go to step S3;

[0017] S2: Open the BOG outlet of the LNG storage tank, and then the BOG enters the first heat exchanger (4) to exchange heat with the heat exchange medium circulation subsystem;

[0018] At the same time, the working process of the heat exchange medium circulation subsystem is as follows: the heat exchange medium exchanges heat with the air through the third heat exchanger (14), then enters the first heat exchanger (4) to exchange heat with BOG, the first three-way valve (16) in the heat exchange medium circulation subsystem is switched, and the heat exchange medium enters the first heat exchanger (4) and then enters the heat exchange medium buffer tank (12) through the first three-way valve (16) to perform the next cycle;

[0019] Meanwhile, the working process of the cold energy recovery subsystem is as follows: air enters the third heat exchanger (14) to exchange heat with the heat exchange medium to obtain low-temperature air, which is then sent to the cold energy utilization end;

[0020] S3: opening the BOG outlet of the LNG storage tank and turning on the submersible pump (7), so that the BOG enters the first heat exchanger (4) and the LNG enters the second heat exchanger (9), and both the first heat exchanger (4) and the second heat exchanger (9) exchange heat with the heat exchange medium circulation subsystem;

[0021] At the same time, the working process of the heat exchange medium circulation subsystem is as follows: the heat exchange medium exchanges heat with the air through the third heat exchanger (14), then enters the first heat exchanger (4) to exchange heat with BOG, switches the first three-way valve (16) in the heat exchange medium circulation subsystem, and after entering the first heat exchanger (4), the heat exchange medium enters the second heat exchanger (9) through the first three-way valve (16) to exchange heat with LNG, and then enters the heat exchange medium buffer tank (12) to carry out the next cycle;

[0022] At the same time, the working process of the cold energy recovery subsystem is as follows: air enters the third heat exchanger (14) to exchange heat with the heat exchange medium to obtain low-temperature air, and the low-temperature air is sent to the cold energy utilization end.

[0023] Beneficial effects: Compared with the prior art, the significant advantage of the present invention is that the heat exchange transition is performed by adding a heat exchange medium (propane) circulation subsystem. The heat exchange medium undergoes phase change in each heat exchanger, absorbs or releases a large amount of latent heat, and realizes the exchange of air heat energy with LNG or BOG cold energy, thereby avoiding energy waste. At the same time, the three process flows of BOG heating and utilization, LNG regasification, and low-temperature air preparation are completed. The system is simple, easy to operate, has high heat exchange efficiency, and high economic benefits. It avoids direct heat exchange between air and LNG and BOG. ​​Direct heat exchange between air is single-phase heat exchange with low heat transfer efficiency, and water vapor in the air is easy to freeze and clog the heat exchanger. Liquid propane is phase-changed into gaseous propane to absorb a large amount of heat from the air, and then the heat in the gaseous propane is transferred to the low-temperature BOG gas through the first heat exchanger to complete BOG heating. During the BOG heating process, there is no phase change in BOG and the heat absorbed is relatively small. Therefore, when BOG heating and LNG regasification are carried out simultaneously, propane does not undergo phase change when exchanging heat with BOG, retaining a large amount of latent heat. The gaseous propane then transfers the heat to LNG through the second heat exchanger, and the propane is liquefied, completing the LNG regasification. BRIEF DESCRIPTION OF THE DRAWINGS

[0024] Figure 1 It is a structural schematic diagram of the system of the present invention. DETAILED DESCRIPTION

[0025] like Figure 1 As shown, in this embodiment, a BOG heating and utilization and LNG regasification system includes a BOG heating and utilization subsystem, an LNG regasification subsystem, a heat exchange medium circulation subsystem and a cold energy recovery subsystem. The cold energy recovery subsystem can heat BOG and LNG at the same time as an indirect heat source, and recycle the cold energy of BOG and LNG. The cold energy recovery subsystem heats BOG and LNG through the heat exchange medium circulation subsystem as a heat exchange medium. The heat exchange medium circulation subsystem and the cold energy recovery subsystem perform heat exchange through the third heat exchanger 14 (fin heat exchanger); the BOG heating and utilization subsystem and the LNG regasification subsystem perform heat exchange with the heat exchange medium circulation subsystem through the first heat exchanger 4 (microchannel heat exchanger) and the second heat exchanger 9 (microchannel heat exchanger) respectively.

[0026] This system is suitable for FSRU or FLNG, integrating three process flows including LNG regasification, BOG heating and utilization, and cold energy recovery. It also ensures the simplicity and efficiency of the system, and can effectively avoid the limitations of limited space in FSRU or FLNG, improve space utilization, and create higher economic benefits.

[0027] A control method for a BOG heating and utilization and LNG regasification system can be divided into two working modes according to working condition requirements: one is to simultaneously perform BOG heating and utilization and LNG regasification. Before LNG is transported to an NG user end, LNG regasification must be performed first. During this process, BOG will continue to be generated. Usually, a large amount of BOG will remain in the LNG storage tank and cannot be completely discharged. The system meets the requirements of simultaneously performing BOG heating and utilization and LNG regasification, thereby avoiding resource waste and maximizing economic benefits; the second is to perform BOG heating and utilization alone. In the case of LNG storage and transportation, LNG regasification is not required, but a large amount of LNG stored and transported will continue to vaporize and generate a large amount of BOG gas. The system meets the requirements of performing BOG heating and utilization alone, handles BOG in a timely manner, avoids resource waste, and reduces safety hazards.

[0028] The heat exchange working fluid circulation subsystem includes a heat exchange working fluid buffer tank 12, a circulation pump 13, a third heat exchanger 14, a first heat exchanger 4, a first three-way valve 16, a gas-liquid separator 17, a second heat exchanger 9, and a propane pressure regulating valve 15, which are sequentially connected by pipelines. In this embodiment, propane is selected as the heat exchange working fluid in the heat exchange working fluid circulation subsystem. Propane has a high heat transfer coefficient and a low standard boiling point. Liquid propane is colorless, slightly toxic, and has stable and safe chemical properties. It is a hydrocarbon. Compared with traditional heat exchange working fluids such as Freon, it will not damage the atmospheric ozone layer, will not cause a greenhouse effect, and is green and environmentally friendly. In addition, propane is easy to obtain and has a low cost. It is gradually replacing traditional heat exchange working fluids and becoming the most mainstream heat exchange working fluid. Liquid propane exchanges heat with air through the third heat exchanger 14, and liquid propane vaporizes, generating phase change to store a large amount of latent heat. Then, it exchanges heat with BOG and LNG in succession through the pipeline, and propane liquefies, releasing a large amount of latent heat. The heat exchange efficiency is high, which can meet the needs of BOG heating and utilization and LNG re-vaporization.

[0029] The pressure of propane in the heat exchange medium buffer tank 12 is 0.1-0.3 MPa and the temperature is -45°C--15°C. The frequency of the circulating pump 13 is adjusted to increase the propane pressure and control the volume flow rate of liquid propane at 3.5-4.5 m 3 / h. The pressurized liquid propane enters the finned heat exchanger 12 and exchanges heat with the air. The propane absorbs heat and vaporizes, and the temperature rises. The vaporized propane enters the microchannel heat exchanger 4 through the propane pressure regulating valve 15 to exchange heat with BOG. ​​The opening of the propane pressure regulating valve 15 is adjusted, and the operating pressure of the propane is controlled at 0.4-0.8MPa. The pressure and temperature of the propane before and after the heat exchange with BOG are read by the pressure transmitter and the temperature transmitter before and after the microchannel heat exchanger 4. Before the heat exchange with BOG, the propane pressure value is 0.4-0.8MPa, and the temperature value is -5℃~20℃; after the heat exchange with BOG, the propane pressure value is 0.2-0.6MPa, and the temperature value is -30℃~10℃. Calculate the saturation temperature of propane before and after the heat exchange with BOG under the current pressure, so as to calculate the superheat or subcooling of the propane before and after the heat exchange with BOG. When BOG is heated and utilized alone, the superheat of propane before heat exchange with BOG is controlled to 3-5°C to ensure that propane is in a completely gaseous state before entering the microchannel heat exchanger 4; the subcooling of propane after heat exchange with BOG is controlled to 3-5°C to ensure that the phase change process from gas phase to liquid phase of propane is mainly carried out in the microchannel heat exchanger 4, making full use of the huge latent heat in the phase change process and improving the efficiency of the heat exchanger. When BOG is heated and utilized and LNG is re-vaporized at the same time, the superheat of propane before heat exchange with BOG is controlled to 5-10°C, and the superheat of propane after heat exchange with BOG is controlled to 3-5°C to ensure that the gaseous propane does not undergo phase change in the microchannel heat exchanger 4, and still contains sufficient heat after heat exchange with BOG for subsequent LNG re-vaporization. After heat exchange with BOG, the propane is divided into two flows through the first three-way valve 16. One is that when BOG is heated and utilized alone, liquid propane flows to the heat exchange medium buffer tank 12 through the first three-way valve 16 for the next cycle; the other is that when BOG is heated and utilized and LNG is re-vaporized at the same time, propane flows into the gas-liquid separator 17 through the first three-way valve 16. When propane and BOG are heat exchanged, delays and errors will inevitably occur in measurement, calculation and control, thereby generating a small amount of liquid propane. The liquid propane returns to the heat exchange medium buffer tank 12 through the liquid phase port of the gas-liquid separator 17, and most of the gaseous propane flows into the microchannel heat exchanger 9 through the gas phase port for heat exchange with LNG. The LNG is re-vaporized and the propane is liquefied. The pressure and temperature of the gaseous propane are read by the pressure transmitter and temperature transmitter after the microchannel heat exchanger 9, respectively. The pressure value is 0.1-0.3MPa, and the temperature value is -45°C--15°C. The saturation temperature of propane after the microchannel heat exchanger 9 under the current pressure is calculated, so as to calculate and control the supercooling degree of propane after the microchannel heat exchanger 9 to be 3-5°C, so as to ensure that the phase change process of propane from gas phase to liquid phase is mainly carried out in the microchannel heat exchanger 9, and make full use of the huge latent heat in the phase change process to improve the efficiency of the heat exchanger. The propane after heat exchange in the microchannel heat exchanger 9 returns to the heat exchange medium buffer tank 12 for the next cycle.

[0030] The third heat exchanger 14 adopts a finned heat exchanger, which has good and stable heat exchange performance. Liquid propane circulates in the tube, and the cold energy of propane is transferred to the air flowing through the fins through the fins tightly wound on the steel tube, so as to achieve the effect of cooling the air and heating the propane. In order to prevent the possible condensation of residual moisture in the air in the heat exchanger, resulting in a decrease in the heat exchange rate, the finned heat exchanger is set as a standby. In this embodiment, the microchannel heat exchanger 4 and the microchannel heat exchanger 8 are both microchannel heat exchangers based on 3D printing structure. Compared with the traditional heat exchanger, this type of heat exchanger greatly improves the heat exchange efficiency while effectively reducing the volume size of the heat exchanger, greatly reducing the space occupied by the entire system, so as to meet the needs of arranging the system in the limited space of the FSRU. The steel structure has a large bearing pressure and can also meet the high-pressure operation requirements of the system. In addition, the microchannel heat exchanger using a 3D printing structure is easy to assemble in a modular system.

[0031] The heat exchange working medium circulation subsystem has high controllability. According to whether LNG regasification is required, the propane flow direction is controlled by adjusting the first three-way valve 16, and two working modes can be realized. One is to heat and utilize BOG and regasify LNG at the same time; the other is to heat and utilize BOG alone. According to the working conditions, the system can be adjusted simply and flexibly to make full use of the heat of gaseous propane and maintain a high heat exchange efficiency. By adjusting the circulation pump 13 to increase the pressure and mass flow of propane, the speed of propane circulation is controlled, that is, the speed at which propane transfers the heat of air to LNG is controlled to match the requirements of BOG heating and LNG regasification.

[0032] The cold energy recovery subsystem includes an air intake valve 18, an air compressor 19, an air purifier 20, a finned heat exchanger 14, and an air pressure regulating valve 21, which are sequentially connected through pipelines. After the air intake valve 18 is opened, air enters the air compressor 19 for compression. The air temperature entering the air intake valve 18 is 5°C to 30°C. The frequency of the air compressor 19 is adjusted to control the air volume flow rate to 1000 to 1200 m / s. 3 / h, increase the air pressure, adjust the opening of the air pressure regulating valve 21, and control the operating pressure of the air at 0.5-1.5MPa. The compressed air enters the air purifier 20, and the purified (dust-removed and dried) air enters the finned heat exchanger 14 to exchange heat with liquid propane. The air temperature drops to obtain low-temperature air, and the air temperature is -20-0°C. The low-temperature air is transported to the cold energy utilization end through the air pressure regulating valve 21. When the BOG heating amount and the LNG re-vaporization amount are small, adjust the air intake valve 18 to reduce the air intake amount, adjust the frequency of the air compressor 19 to reduce the air volume flow rate, and adjust the air compressor 19 and the air pressure regulating valve 21 to reduce the air pressure; when the BOG heating amount and the LNG vaporization amount are large, the adjustment method is opposite. The mass flow rate of the air is controlled by adjusting the frequency of the air compressor 19, and the air pressure is controlled by adjusting the opening of the air pressure regulating valve 21. The controllability is high, so it is easy to adjust the operating parameters of the air to quickly match the required requirements of BOG heating utilization and LNG re-vaporization to achieve the best heat exchange effect under the required working conditions.

[0033] The air purifier 20 includes an air dust collector and an air dryer. External air enters the subsystem through the air intake valve 18, and after being pressurized by the air compressor 19, enters the air purifier 20 to remove moisture and various micro-particle impurities in the air. The purified air enters the finned heat exchanger 14 for heat exchange with propane. The cold energy recovery subsystem recovers the cold energy released in the two processes of BOG heating and LNG regasification, and stores the collected cold energy in low-temperature air. The produced low-temperature air is transported to the cold energy utilization end along the pipeline. The low-temperature air can directly generate economic value. The low-temperature air can be used for the cold energy user end of FSRU, such as the oil cooler of the turbine, ventilation of the engine pump room, refrigeration of the warehouse and cooling of the flue gas, so as to realize the comprehensive utilization of energy.

[0034] The BOG heating and utilization subsystem includes an LNG storage tank 1, a BOG air intake valve 2, a BOG gas compressor 3, a microchannel heat exchanger 4, and a second three-way valve 6, which are sequentially connected through pipelines. The pressure of the BOG gas in the LNG storage tank 1 is 0.1-0.3 MPa, and the temperature is -162°C--120°C. After the BOG air intake valve 2 is opened, the BOG generated in the LNG storage tank 1 enters the pipeline, and then enters the BOG gas compressor 3. The frequency of the BOG gas compressor 3 is adjusted to increase the BOG pressure, and the BOG volume flow rate is controlled at 600-1000m 3 / h, adjust the opening of the BOG pressure regulating valve 5 to control the operating pressure of BOG at 0.5-1.5MPa. After being pressurized by the BOG gas compressor 3, it enters the microchannel heat exchanger 4 to exchange heat with gaseous propane to complete BOG heating. The heated BOG is divided into two flows through the second three-way valve 6. One is that when BOG is heated and utilized alone, BOG is transported to the fuel gas user end, such as the ship's main engine, ship's generator, etc. through the second three-way valve 6; the other is that when BOG is heated and utilized and LNG is re-vaporized at the same time, BOG is merged into the NG produced by LNG re-vaporization through the second three-way valve 6 and sent to the NG user end along the transmission pipeline. When BOG is heated and utilized alone, BOG is heated to 0-10℃; when BOG is heated and utilized and LNG is re-vaporized at the same time, BOG is heated to -30℃~-10℃. The pipeline for collecting BOG is connected to the LNG storage tank 1 to ensure that the pipeline inlet is above the LNG liquid level to prevent LNG from entering the BOG gas compressor 3 and causing damage to it, thereby extending the service life of the compressor.

[0035] The LNG regasification subsystem includes an LNG storage tank 1, a submersible pump 7, an LNG inlet valve 8, a microchannel heat exchanger 9, and a regulating valve 10, which are sequentially connected by pipelines. In this embodiment, the submersible pump 7 is a low-temperature submersible pump, which is placed at the bottom of the LNG storage tank. After opening the LNG inlet valve 8, the frequency of the submersible pump 7 is adjusted to control the LNG volume flow rate at 3 to 3.5 m3 / h, increase the LNG pressure, and adjust the opening of the LNG pressure regulating valve 10 to control the operating pressure of LNG at 7 to 13 MPa. The submersible pump 7 transports the LNG in the LNG storage tank 1 to the microchannel heat exchanger 9, exchanges heat with the separated gaseous propane, and the LNG is regasified into NG. The pressure transmitter and temperature transmitter after the microchannel heat exchanger 9 read the pressure of NG respectively, and the pressure value of NG is 6 to 12 MPa and the temperature value is -30°C to -10°C. The high-pressure NG is reduced to 0.1 to 0.7 MPa through the pressure-reducing orifice plate 11 and sent to the NG user end along the delivery pipeline. The submersible pump 6 is placed at the lowest position in the LNG storage tank 1 to ensure that the submersible pump 6 is always below the liquid surface, and is used to pressurize the LNG and provide power to the LNG regasification subsystem.

[0036] During the storage and transportation of LNG, LNG regasification is not required, but a large amount of LNG stored and transported will continue to vaporize and produce a large amount of BOG gas. If it is not handled in time, the safety hazards will be greatly increased. This system can realize the separate heating and utilization of BOG through the first three-way valve 16, handle a large amount of BOG in time, reduce safety hazards, and make full use of resources to avoid waste. All equipment in this system is connected by stainless steel pipes through welding or flange connection, so it can meet the high pressure requirements inside the system, and can be flexibly installed according to the on-site conditions. This system solves the technical problems of BOG heating and utilization and LNG regasification system, has practical engineering significance, and can be used as a reference for relevant engineering personnel. In the embodiments and the drawings of the specification, only the core equipment and devices in a BOG heating and utilization and LNG regasification system are emphasized. In addition, a large number of auxiliary devices and instruments are not emphasized in this patent and the drawings, such as liquid level, flow, pressure and temperature transmitters at corresponding positions, auxiliary valves, PLC control systems, etc.

Claims

1. A control method for a BOG heating and utilization and LNG regasification system, the BOG heating and utilization and LNG regasification system comprising an LNG storage tank (1), characterized in that: It also includes a heat exchange medium circulation subsystem and a cold energy recovery subsystem. The heat exchange medium circulation subsystem includes a heat exchange medium, a first heat exchanger (4) and a second heat exchanger (9). The heat exchange medium sequentially passes through the first heat exchanger (4) and the second heat exchanger (9) for heat exchange. The cold energy recovery subsystem includes a third heat exchanger (14) and air. The heat exchange medium passes through the second heat exchanger (9) and then passes through the third heat exchanger (14). The heat exchange medium exchanges heat with the air passing through the third heat exchanger (14). After heat exchange through the third heat exchanger (14), low-temperature air is formed. The LNG storage tank (1) is provided with a BOG outlet, and a submersible pump (7) for extracting LNG is provided inside the LNG storage tank (1). The BOG outlet is connected to the first heat exchanger (4), and the output end of the submersible pump (7) is connected to the second heat exchanger (9). The heat exchange working medium circulation subsystem further comprises a heat exchange working medium buffer tank (12) arranged between the second heat exchanger (9) and the third heat exchanger (14), and a gas-liquid separator (17) arranged between the first heat exchanger (4) and the second heat exchanger (9), wherein the gas outlet of the gas-liquid separator (17) is connected to the inlet of the second heat exchanger (9), and the liquid outlet of the gas-liquid separator (17) is connected to the inlet of the heat exchange working medium buffer tank (12); The heat exchange medium circulation subsystem further comprises a first three-way valve (16) arranged between the first heat exchanger (4) and the gas-liquid separator (17), wherein three ends of the first three-way valve (16) are respectively connected to the first heat exchanger (4), the gas-liquid separator (17), and the heat exchange medium buffer tank (12); A circulating pump (13) is provided at the output end of the heat exchange working medium buffer tank (12), and the heat exchange working medium is propane; The cold energy recovery subsystem further comprises an air pressure regulating valve (21) arranged at the outlet of the third heat exchanger (14); The control method comprises the following steps: S1: Select the working mode of the system: working mode 1 is to heat BOG alone, and when working mode 1 is selected, go to step S2; working mode 2 is to heat BOG and regasify LNG at the same time, and when working mode 2 is selected, go to step S3; S2: Open the BOG outlet of the LNG storage tank, and then the BOG enters the first heat exchanger (4) to exchange heat with the heat exchange medium circulation subsystem; At the same time, the working process of the heat exchange medium circulation subsystem is as follows: the heat exchange medium exchanges heat with the air through the third heat exchanger (14), then enters the first heat exchanger (4) to exchange heat with BOG, the first three-way valve (16) in the heat exchange medium circulation subsystem is switched, and the heat exchange medium enters the first heat exchanger (4) and then enters the heat exchange medium buffer tank (12) through the first three-way valve (16) to perform the next cycle; At the same time, the working process of the cold energy recovery subsystem is as follows: air enters the third heat exchanger (14) to exchange heat with the heat exchange medium to obtain low-temperature air, and the low-temperature air is sent to the cold energy utilization end; S3: opening the BOG outlet of the LNG storage tank and turning on the submersible pump (7), so that the BOG enters the first heat exchanger (4) and the LNG enters the second heat exchanger (9), and both the first heat exchanger (4) and the second heat exchanger (9) exchange heat with the heat exchange medium circulation subsystem; At the same time, the working process of the heat exchange medium circulation subsystem is as follows: the heat exchange medium exchanges heat with the air through the third heat exchanger (14), then enters the first heat exchanger (4) to exchange heat with BOG, switches the first three-way valve (16) in the heat exchange medium circulation subsystem, and after entering the first heat exchanger (4), the heat exchange medium enters the second heat exchanger (9) through the first three-way valve (16) to exchange heat with LNG, and then enters the heat exchange medium buffer tank (12) to carry out the next cycle; At the same time, the working process of the cold energy recovery subsystem is as follows: air enters the third heat exchanger (14) to exchange heat with the heat exchange medium to obtain low-temperature air, and the low-temperature air is sent to the cold energy utilization end.

2. The control method of the BOG heating utilization and LNG regasification system according to claim 1, characterized in that: The cold energy recovery subsystem further comprises an air purifier (20) and an air compressor (19), wherein the output end of the air compressor (19) is connected to the input end of the air purifier (20), and the output end of the air purifier (20) is connected to the inlet end of the third heat exchanger (14).

3. The control method of the BOG heating utilization and LNG regasification system according to claim 1, characterized in that: The first heat exchanger (4) and the second heat exchanger (9) are microchannel heat exchangers, and the third heat exchanger (14) is a finned heat exchanger.

4. The control method of the BOG heating utilization and LNG regasification system according to claim 3, characterized in that: The first heat exchanger (4) and the second heat exchanger (9) are both 3D printed structures.

5. The control method of the BOG heating utilization and LNG regasification system according to claim 1, characterized in that: A BOG gas compressor (3) is arranged between the BOG outlet and the first heat exchanger (4).

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