Fuel cold energy utilization system for stepped pressure storage tank of CO2 ship

Abstract

The invention belongs to the technical field of ships, and provides a CO2 ship stepped pressure storage tank fuel cold energy utilization system which comprises a first storage tank, a second storage tank, a third storage tank, a fourth storage tank, a heat exchanger I, a heat exchanger II, a heat exchanger III, a heat exchanger IV, an LNG fuel cabin, a lightering pump, a booster pump, a processing unit and a main machine. According to the storage tank, CO2 is stored in a pressure and temperature gradient change mode, the refrigeration cost generated by maintaining the low temperature of the C-shaped tanks is saved, more importantly, LNG cold energy is sequentially distributed to the C-shaped tanks with different temperatures, gradient utilization of the LNG cold energy is achieved, and the cold energy utilization efficiency of the system is improved. Meanwhile, compared with a traditional mode that all high-pressure C-shaped tanks are adopted, the requirement for the strength of C-shaped tank materials is correspondingly lowered, and the tank body material cost and the manufacturing cost are greatly reduced.

Description

Technical Field

[0001] The present invention belongs to the technical field of ships, and particularly relates to a fuel cold energy utilization system for a stepped pressure storage tank of a CO2 ship. Background Technique

[0002] With the increasing intensification of the global greenhouse effect, the International Maritime Organization has become more and more strict with the carbon emissions of ships. At present, the most effective way to achieve ship carbon emission reduction in the ship field is carbon capture and storage technology (abbreviated as CCS technology). CCS technology refers to capturing CO2 generated by industries and related energy industries through carbon capture technology and then storing it in the seabed of the ocean or places such as Iceland. Since the process of transporting CO2 to the storage site involves maritime transportation, a CO2 transport ship (abbreviated as CO2 ship) is required to achieve this purpose.

[0003] During the process of transporting CO2 by a CO2 ship, in order to increase the weight of CO2 transported per single trip and achieve higher transportation efficiency, CO2 is usually stored in a storage tank on the ship in a liquid state. At present, the semi-cooling and semi-pressure method is generally used to store CO2 in a C-type tank. The semi-cooling and semi-pressure storage method means that the temperature in the C-type tank is controlled between -50°C and -20°C, and the pressure is controlled between 0.7 MPa and 2.1 MPa. However, there are some problems with the C-type tank during the process of storing CO2 due to the storage temperature and pressure: on the one hand, if the internal temperature of the C-type tank is uniformly set to a lower temperature (such as -50°C) in the semi-cooling and semi-pressure storage method, this will cause a large temperature difference between the inside of the C-type tank and the outside. Therefore, the ship refrigeration system needs to continuously work to maintain the low-temperature environment in the C-type tank, which will greatly increase the refrigeration power consumption of the system. On the other hand, if the internal temperature of the C-type tank is uniformly set to a higher temperature (such as -20°C) in the semi-cooling and semi-pressure storage method, although this can reduce the refrigeration power consumption of the system to a certain extent, the pressure in the C-type tank will be relatively high, which requires the C-type tank body to use materials with higher strength to withstand the pressure, thereby greatly increasing the material cost and manufacturing cost of the C-type tank.

[0004] Under the background of strict emission reduction of ship exhaust gas, many marine clean fuels have emerged. Among them, liquefied natural gas (LNG) has received more and more favor on merchant ships due to its good emissions after combustion. The storage temperature of LNG is usually extremely low, about -163°C, and it needs to be heated to about 20°C to 45°C before being supplied to the ship's main engine for combustion. During the process of LNG from -163°C to normal temperature, a large amount of cold energy is released, and LNG changes from high-grade cold energy to low-grade cold energy as the temperature rises. If these cold energies can be efficiently recovered and utilized on the ship, especially the utilization of low-grade cold energy, there will be a high cold energy utilization efficiency and extremely considerable economic benefits.

[0005] Based on the above problems, if a fuel cold energy utilization system for a CO2 ship stepped pressure storage tank can be designed to utilize the cold energy of LNG from low to high temperature in each C-type tank to maintain the low-temperature environment in the C-type tank, and at the same time, not all C-type tanks need to use high-strength materials, this not only saves the refrigeration power consumption of the ship, but also reduces the manufacturing cost of the C-type tank, effectively solving the problems of high energy consumption and high cost of existing CO2 carriers. Summary of the Invention

[0006] To substantially solve the above problems, the present invention provides a fuel cold energy utilization system for a CO2 ship stepped pressure storage tank, which includes: a first storage tank, a second storage tank, a third storage tank, a fourth storage tank, a heat exchanger I, a heat exchanger II, a heat exchanger III, a heat exchanger IV, an LNG fuel tank, a barge pump, a booster pump, a processing unit, and a main engine.

[0007] The first storage tank, the second storage tank, the third storage tank, and the fourth storage tank are used to store liquid CO2. The first storage tank and the second storage tank adopt single C-type tanks, and the third storage tank and the fourth storage tank adopt triple C-type tanks. The heat exchanger I, the heat exchanger II, the heat exchanger III, and the heat exchanger IV are respectively arranged in the first storage tank, the second storage tank, the third storage tank, and the fourth storage tank. The LNG fuel tank is used to store LNG fuel. The barge pump is arranged at the bottom of the LNG fuel tank and is connected to the booster pump, the heat exchanger IV, the heat exchanger III, the heat exchanger II, the heat exchanger I, the processing unit, and the main engine in sequence through pipelines.

[0008] Furthermore, the first storage tank, the second storage tank, the third storage tank, and the fourth storage tank jointly form a stepped pressure storage tank group, with a design temperature of -50°C to -20°C and a design pressure of 0.8 MPa to 2.1 MPa. The design temperature and design pressure in the first storage tank, the second storage tank, the third storage tank, and the fourth storage tank decrease in sequence.

[0009] During the LNG supply process of the CO2 ship, first, the barge pump transports the LNG in the LNG fuel tank to the booster pump, and then the LNG enters the heat exchanger IV after being pressurized by the booster pump, where it exchanges heat with the gas phase space in the fourth storage tank, that is, exchanges heat with gaseous CO2. The heat-exchanged LNG enters the heat exchanger III through the pipeline, where it exchanges heat with the gas phase space in the third storage tank, that is, exchanges heat with gaseous CO2. The heat-exchanged LNG enters the heat exchanger II through the pipeline, where it exchanges heat with the liquid phase space in the second storage tank, that is, exchanges heat with liquid CO2. The heat-exchanged LNG enters the heat exchanger I through the pipeline, where it exchanges heat with the liquid phase space in the first storage tank, that is, exchanges heat with liquid CO2. During the above heat exchange process, the temperatures of the CO2 in the fourth storage tank, the third storage tank, the second storage tank, and the first storage tank reach the design temperatures of their respective storage tanks. Finally, the heat-exchanged LNG enters the processing unit to be pressurized and heated to the pressure and temperature required by the main engine, and is finally sent to the main engine to provide power for the ship.

[0010] Advantages of the present invention:

[0011] 1. The storage tank of the present invention stores CO2 by means of stepwise changes in pressure and temperature, which not only reduces the refrigeration cost for maintaining the low temperature of the C-type tank, but more importantly, the present invention realizes the stepwise utilization of the cold energy of LNG by sequentially distributing the cold energy of LNG to C-type tanks with different temperatures, thereby improving the cold energy utilization efficiency of the system.

[0012] 2. The present invention stores CO2 by means of stepwise reduction of the storage tank pressure. Compared with the traditional method of using all high-pressure C-type tanks (for example, temperature -20°C, pressure 2.1 MPa), the strength requirement for the material of the C-type tank in the present invention is correspondingly reduced, greatly reducing the cost of the tank body material and the manufacturing cost. Description of the drawings

[0013] Figure 1 It is a system diagram of the present invention;

[0014] Figure 2 It is a schematic diagram of the third storage tank inside the ship;

[0015] Figure 3 It is a schematic diagram of the III heat exchanger inside the three C-type tanks;

[0016] Figure 4 It is a schematic diagram of the heat exchanger;

[0017] In the drawings: 1. First storage tank; 2. Second storage tank; 3. Third storage tank; 4. Fourth storage tank; 5. I heat exchanger; 6. II heat exchanger; 7. III heat exchanger; 8. IV heat exchanger; 9. LNG fuel tank; 10. Barge pump; 11. Booster pump; 12. Processing unit; 13. Main engine. Detailed implementation manners

[0018] In order to make the objectives, technical solutions and advantages of the present invention clearer and more understandable, the present invention will be further described in detail below with reference to the accompanying drawings.

[0019] A CO2 ship stepped-pressure storage tank cold energy utilization system using fuel cold energy, as Figure 1 , Figure 2 , Figure 3 shown, the system includes: First storage tank 1, Second storage tank 2, Third storage tank 3, Fourth storage tank 4, I heat exchanger 5, II heat exchanger 6, III heat exchanger 7, IV heat exchanger 8, LNG fuel tank 9, Barge pump 10, Booster pump 11, Processing unit 12, Main engine 13.

[0020] The first storage tank 1, the second storage tank 2, the third storage tank 3, and the fourth storage tank 4 are used to store liquid CO2. Among them, the first storage tank 1 and the second storage tank 2 adopt single C-type tanks, and the third storage tank 3 and the fourth storage tank 4 adopt triple C-type tanks; the I heat exchanger 5, the II heat exchanger 6, the III heat exchanger 7, and the IV heat exchanger 8 are respectively arranged in the first storage tank 1, the second storage tank 2, the third storage tank 3, and the fourth storage tank 4; the LNG fuel tank 9 is used to store LNG fuel; the barge pump 10 is arranged at the bottom of the LNG fuel tank 9 and is connected in sequence with the booster pump 11, the IV heat exchanger 8, the III heat exchanger 7, the II heat exchanger 6, the I heat exchanger 5, the treatment unit 12, and the main engine 13 through pipelines.

[0021] Furthermore, the first storage tank 1, the second storage tank 2, the third storage tank 3, and the fourth storage tank 4 together form a stepped pressure storage tank group, with a design temperature of -50°C to -20°C and a design pressure of 0.8 MPa to 2.1 MPa. Among them, the design temperatures and design pressures in the first storage tank 1, the second storage tank 2, the third storage tank 3, and the fourth storage tank 4 decrease in sequence.

[0022] Furthermore, as Figure 3 and Figure 4 shown, the III heat exchanger 7 and the IV heat exchanger 8 are located above the CO2 liquid level and exchange heat with the gaseous CO2 in the third storage tank 3 and the fourth storage tank 4; the I heat exchanger 5 and the II heat exchanger 6 are located below the CO2 liquid level and exchange heat with the liquid CO2 in the first storage tank 1 and the second storage tank 2.

[0023] Next, one embodiment of the present invention will be described. Assume that the design temperatures of the first storage tank 1, the second storage tank 2, the third storage tank 3, and the fourth storage tank 4 are -20°C, -30°C, -40°C, and -50°C in sequence, and the saturation pressures of CO2 at the above corresponding temperatures are 1.97 MPa, 1.43 MPa, 1.00 MPa, and 0.68 MPa. In order to leave a certain margin, the design pressures of the first storage tank 1, the second storage tank 2, the third storage tank 3, and the fourth storage tank 4 are 2.1 MPa, 1.6 MPa, 1.2 MPa, and 0.8 MPa in sequence.

[0024] During the process of burning LNG on a CO2 ship, first, the barge pump 10 barges the LNG in the LNG fuel tank 9 into the pipeline. Then, the LNG enters the IV heat exchanger 8 after being pressurized by the booster pump 11, and exchanges heat with the gas phase space in the fourth storage tank 4, that is, exchanges heat with gaseous CO2, so as to maintain the temperature in the fourth storage tank 4 at about -50°C. The LNG after heat exchange enters the III heat exchanger 7 through the pipeline and exchanges heat with the gas phase space in the third storage tank 3, that is, exchanges heat with gaseous CO2, so as to maintain the temperature in the third storage tank 3 at about -40°C. The LNG after heat exchange enters the II heat exchanger 6 through the pipeline and exchanges heat with the liquid phase space in the second storage tank 2, that is, exchanges heat with liquid CO2, so as to maintain the temperature in the second storage tank 2 at about -30°C. The LNG after heat exchange enters the I heat exchanger 5 through the pipeline and exchanges heat with the liquid phase space in the first storage tank 1, that is, exchanges heat with liquid CO2, so as to maintain the temperature in the first storage tank 1 at about -20°C. The LNG after heat exchange enters the treatment unit 12, is pressurized and heated to the pressure and temperature required by the main engine 13, and finally is sent into the main engine 13 to provide power for the ship.

[0025] In this embodiment, the design temperatures of the third storage tank 3 and the fourth storage tank 4 are -40°C and -50°C respectively. To prevent the liquid CO2 in the third storage tank 3 and the fourth storage tank 4 from absorbing heat from the external environment and generating CO2 evaporation gas, it is necessary to continuously refrigerate the third storage tank 3 and the fourth storage tank 4. The temperature of the LNG barged out from the LNG fuel tank 9 is extremely low. After the LNG exchanges heat with the CO2 in each C-type tank, the temperature of the LNG gradually rises. Since the temperatures in the fourth storage tank 4 and the third storage tank 3 are relatively low, maintaining the low-temperature environment in the fourth storage tank 4 and the third storage tank 3 requires a large amount of cold energy. Therefore, the LNG is first introduced into the fourth storage tank 4 and the third storage tank 3, and the high-grade LNG cold energy is utilized through the IV heat exchanger 8 and the III heat exchanger 7 to maintain the temperatures of the fourth storage tank 4 and the third storage tank 3. At the same time, to prevent the extremely low-temperature LNG from entering the IV heat exchanger 8 and the III heat exchanger 7 and contacting the liquid CO2 and solidifying into dry ice, as Figure 3 shown, the positions of the III heat exchanger 7 and the IV heat exchanger 8 are set above the CO2 liquid level, so that the LNG cold energy can be utilized to moderately reduce the temperature, and the liquid CO2 can be prevented from condensing into dry ice.

[0026] In this embodiment, the design temperatures of the first storage tank 1 and the second storage tank 2 are -20°C and -30°C respectively. The temperature difference between the first storage tank 1 and the second storage tank 2 and the external environment is small, and the required cold energy is less. Since a large amount of the cold energy of the LNG has been consumed after flowing through the IV heat exchanger 8 and the III heat exchanger 7, the LNG after flowing through the IV heat exchanger 8 and the III heat exchanger 7 is then introduced into the second storage tank 2 and the first storage tank 1. At this time, the remaining cold energy of the LNG is transferred to the CO2 through efficient heat exchange. Since the heat exchange efficiency of the liquid phase space is higher, and the temperature of the LNG at this time is not sufficient to solidify the CO2, the heat exchangers in the first storage tank 1 and the second storage tank 2 are placed below the CO2 liquid level to ensure full utilization of the cold energy.

[0027] As Figure 2 shown, taking the third storage tank 3 as an example, the present invention shows the position of the third storage tank 3 as a three-C type tank inside the ship. During the navigation of the CO2 ship, due to reasons such as wind and waves, the ship will jolt and shake, which in turn causes the liquid CO2 in the C-type tank to slosh. The distance between the liquid CO2 level and the upper space in a single C-type tank is relatively small. When the liquid CO2 in the C-type tank sloshes, the distance between the CO2 liquid level and the heat exchanger becomes smaller, increasing the possibility of the heat exchanger coming into contact with the liquid CO2. If the third storage tank 3 and the fourth storage tank 4 are single C-type tanks, when the ship jolts and shakes, the liquid CO2 in the third storage tank 3 and the fourth storage tank 4 will come into contact with the heat exchanger III 7 and the heat exchanger IV 8. Since the LNG flowing through the heat exchanger III 7 and the heat exchanger IV 8 has an extremely low temperature, the liquid CO2 coming into contact with the heat exchanger will solidify on the outer surface of the heat exchanger, forming a layer of dry ice, which will greatly reduce the heat transfer coefficient of the heat exchanger and is not conducive to efficient heat exchange. If the distance between the heat exchanger and the liquid CO2 is increased by reducing the liquid level, the CO2 transportation volume will be greatly reduced. However, the distance between the liquid CO2 level and the upper space in the three-C type tank is relatively large. Therefore, designing the third storage tank 3 and the fourth storage tank 4 as three-C type tanks can ensure a large distance between the heat exchanger III 7 and the heat exchanger IV 8 and the CO2 liquid level, thereby greatly reducing the possibility of CO2 forming dry ice on the surface of the heat exchanger.

[0028] The above is a detailed introduction of one of the embodiments.

[0029] The beneficial effects of the present invention will be further described below. If the design temperature of all C-type tanks is designed to be -50°C, a large amount of refrigeration energy consumption is required to maintain the low-temperature environment of -50°C. Moreover, after the LNG flows through the heat exchangers 8, 7, and 6, the temperature of the LNG may be higher than -50°C, resulting in the inability to cool the CO2 in the first storage tank 1. If all the C-type tanks storing CO2 are maintained at a relatively high temperature environment of -20°C, the pressure inside the C-type tanks is relatively high at this time. To ensure the safety and stability of the storage tanks, it is necessary to use tank body materials with higher pressure-bearing capacities, which undoubtedly greatly increases the material cost and manufacturing cost of the tank body. Therefore, the present invention innovatively adopts a storage tank group with a stepped pressure and temperature design, which is composed of the first storage tank 1, the second storage tank 2, the third storage tank 3, and the fourth storage tank 4. Their design temperatures are -20°C, -30°C, -40°C, and -50°C in sequence, and their design pressures are 2.1 MPa, 1.6 MPa, 1.2 MPa, and 0.8 MPa in sequence. During the ship's voyage, the LNG flows through the heat exchangers 8, 7, 6, and 5 in sequence, and the cold energy of the LNG is transferred to the C-type tanks with different temperature settings. Since the temperature of the LNG just transferred from the LNG fuel tank 9 is extremely low, the cold energy of the LNG is relatively large at this time, which preferentially meets the requirements of the fourth storage tank 4 and the third storage tank 3 with lower temperature requirements and higher cold energy demand. As the cold energy is gradually consumed, an appropriate amount of cold energy is provided for the first storage tank 1 and the second storage tank 2 subsequently. The present invention distributes the cold energy of the LNG to the C-type tanks with different temperatures in sequence, realizing the cascaded utilization of the cold energy of the LNG, ensuring that the LNG after heat exchange with the liquid CO2 in the first storage tank 1 has a relatively high temperature, and improving the cold energy utilization efficiency of the system. At the same time, since the design pressures and pressure-bearing capacities of the first storage tank 1, the second storage tank 2, the third storage tank 3, and the fourth storage tank 4 decrease in sequence, the requirements for the tank body materials of the first storage tank 1, the second storage tank 2, the third storage tank 3, and the fourth storage tank 4 decrease in sequence. Compared with the method of using all high-pressure C-type tanks (for example, temperature -20°C, pressure 2.1 MPa), the present invention greatly reduces the material cost and manufacturing cost of the tank body.

[0030] The above are only the preferred embodiments of the present invention, but the implementation is not limited by the above embodiments. It should be pointed out that for those of ordinary skill in the art, without departing from the principle of the present invention, several improvements and refinements can be made, and these improvements and refinements should also be regarded as the protection scope of the present invention.

Claims

1. A CO2 ship stepped-pressure storage tank utilization fuel cold energy system, characterized in that: The system includes: a first storage tank (1), a second storage tank (2), a third storage tank (3), a fourth storage tank (4), a heat exchanger I (5), a heat exchanger II (6), a heat exchanger III (7), a heat exchanger IV (8), an LNG fuel tank (9), a barge pump (10), a booster pump (11), a processing unit (12), and a main engine (13); The heat exchanger I (5), heat exchanger II (6), heat exchanger III (7), and heat exchanger IV (8) are respectively arranged in the first storage tank (1), second storage tank (2), third storage tank (3), and fourth storage tank (4); the barge pump (10) is arranged at the bottom of the LNG fuel tank (9) and is connected in sequence through pipelines to the booster pump (11), heat exchanger IV (8), heat exchanger III (7), heat exchanger II (6), heat exchanger I (5), processing unit (12), and main engine (13).

2. The CO2 ship stepped pressure storage tank utilization fuel cold energy system according to claim 1, characterized in that: The first storage tank (1), second storage tank (2), third storage tank (3), and fourth storage tank (4) together form a stepped pressure storage tank group, with a design temperature of -50°C to -20°C and a design pressure of 0.8 MPa to 2.1 MPa. The design temperature and design pressure in the first storage tank (1), second storage tank (2), third storage tank (3), and fourth storage tank (4) decrease in sequence.

3. A CO2 ship stepped-pressure storage tank utilizing fuel cold energy system according to claim 1, characterized in that: The first storage tank (1) and second storage tank (2) adopt single C-type tanks, and the third storage tank (3) and fourth storage tank (4) adopt triple C-type tanks.

4. A CO2 ship stepped pressure storage tank utilizing fuel cold energy system according to claim 1, characterized in that: The heat exchanger III (7) and heat exchanger IV (8) are located above the CO2 liquid level, and the heat exchanger I (5) and heat exchanger II (6) are located below the CO2 liquid level.

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