An LNG gas supply and CO2 reliquefaction integrated system for LCO2 transport ship and working method thereof

Abstract

The application discloses an LNG gas supply and CO2 re-liquefaction integrated system for an LCO2 transport ship, which comprises a gas supply system, a CO2 liquefaction system and a heat conducting medium circulation system, the gas supply system adopts double-mode operation to realize stable supply of LNG evaporation gas, the CO2 liquefaction system realizes precooling through a CO2 regenerator, pressurization through a CO2 compressor and liquefaction through a CO2 condenser, and forms a self-sustaining cycle in combination with a throttle valve and a spraying system, and the heat conducting medium circulation system realizes utilization of LNG cold energy and re-liquefaction of CO2 volatile gas, and the application further discloses a working method thereof. The application couples LNG cold energy, CO2 liquefaction and heat conducting medium circulation, significantly reduces system energy consumption and operation cost through optimization of cold energy distribution and combination of frequency conversion control and compact equipment integrated design, and the system does not need to be additionally configured with a refrigerant, and is particularly suitable for engineering application scenarios with limited ship layout space.

Description

TECHNICAL FIELD

[0001] The present application belongs to the field of energy saving and environmental protection, specifically to the technical field of boil-off gas (BOG) reliquefaction device of LCO2 transport or filling ship, and is actually an LNG gas supply and CO2 reliquefaction integrated system for LCO2 transport ship and a working method thereof. BACKGROUND

[0002] At present, large-scale CO2 transportation mainly relies on two ways of pipeline and ship, and ship transportation is particularly suitable for offshore storage scenarios due to its flexibility and long-distance economy. With the development of CCUS, LCO2 transport ships are evolving towards medium-pressure and low-pressure large-scale, and the cargo hold pressure range is usually 7~19 barg, corresponding to CO2 temperature of-55~-24℃. However, during transportation, environmental heat leakage will cause LCO2 evaporation, which will cause the cabin pressure to rise, and the evaporation gas must be efficiently reliquefied to maintain system safety.

[0003] Invention patent CN117928172A discloses a high-purity liquid CO2 production method and system using LNG cold energy, which combines mixed working medium Rankine cycle with raw gas pre-expansion process, and recovers the cold energy of pressurized CO2, realizing the triple functions of CO2 liquefaction, pressurization and increased power generation, although the cold energy utilization efficiency is improved, the system complexity is significantly increased, and it is not suitable for space-limited ship environment; invention patent CN110332764B discloses a zero-emission power system for recycling CO2 using LNG cold energy cascade composite cycle, which has innovation in CO2 recycling technology, but the system has high complexity, high energy consumption, narrow applicability, and lacks optimization design for specific scenarios (such as ships); utility model patent CN221705955U discloses a CO2 liquefaction system using LNG cold energy, which uses indirect utilization of LNG cold energy, and transfers cold energy through intermediate refrigerant, which avoids the risk of direct heat exchange, but there is a secondary heat exchange loss, and the problem that LNG cold energy cannot be released when CO2 liquefaction is not needed is not considered. The existing CO2 reliquefaction system uses an additional refrigeration unit to provide cold energy, resulting in a very complex system and generally low liquefaction energy efficiency. Therefore, there is an urgent need for an efficient, compact and dedicated refrigerant-free BOG reliquefaction system for LCO2 transport ships to realize cold energy utilization and equipment integration, and to meet the low energy consumption and high safety requirements of ship transportation. SUMMARY

[0004] In view of the above problems of the prior art, the present application provides an LNG gas supply and CO2 reliquefaction integrated system for LCO2 transport ship and a working method thereof. The system of the present application can realize efficient recovery and utilization of LNG cold energy and stable reliquefaction of CO2 evaporation gas, solving the problems of waste of cold energy and structural redundancy of traditional systems.

[0005] To achieve the above-mentioned purposes, the technical scheme of the present application is as follows:

[0006] The integrated system for LNG fuel gas supply and CO2 re-liquefaction of the LCO2 transport ship comprises a fuel gas supply system, a CO2 liquefaction system and a heat conducting medium circulation system, the cold energy generated by the gasification of liquefied natural gas in the LNG fuel tank in the fuel gas supply system is transmitted to the CO2 liquefaction system through the heat conducting medium circulation system, so as to re-liquefy the evaporated CO2 generated by the evaporation of the LCO2 liquid cargo tank;

[0007] The fuel gas supply system comprises a natural evaporation gas supply subsystem and a forced evaporation gas supply subsystem connected to the LNG fuel tank respectively, the natural evaporation gas supply subsystem and the forced evaporation gas supply subsystem are connected to a fuel buffer tank, the fuel buffer tank delivers fuel to the main engine through pipelines, and the heat exchangers in the natural evaporation gas supply subsystem and the forced evaporation gas supply subsystem are connected to the heat conducting medium circulation system respectively;

[0008] The LCO2 liquid cargo tank is connected to the CO2 liquefaction system, the CO2 liquefaction system re-liquefies the evaporated CO2 in the LCO2 liquid cargo tank and returns it to the LCO2 liquid cargo tank, and the CO2 condenser in the CO2 liquefaction system is connected to the heat conducting medium circulation system;

[0009] The heat conducting medium circulation system is provided with a heat conducting medium storage tank and a heat conducting medium circulation pipeline, the heat conducting medium circulation pipeline is connected with a heat exchanger and a CO2 condenser respectively, the heat exchanger absorbs the evaporation cold energy of the liquefied natural gas in the fuel gas supply system and liquefies the CO2 steam cooled by the CO2 condenser in the CO2 liquefaction system.

[0010] As a preferred technical scheme, the fuel gas supply system comprises an LNG fuel tank, a natural evaporation gas supply subsystem, a forced evaporation gas supply subsystem, a fuel buffer tank and a second control valve; the natural evaporation gas supply subsystem comprises a first gas dome, a second heat exchanger and a BOG compressor; the forced evaporation gas supply subsystem comprises a fuel gas pump, a first control valve, a first heat exchanger, a third control valve and a third bypass valve; the fuel gas supply system is connected in the following manner: the fuel gas pump is placed at the bottom of the LNG fuel tank, the outlet of the fuel gas pump is divided into two branches, one branch is connected with the inlet of the first control valve, and the other branch is connected with the inlet of the third control valve, the outlet of the third control valve is connected into the fuel tank, the outlet of the first control valve is divided into two branches, the main branch is connected with the cold side inlet of the first heat exchanger, and the bypass branch is connected with the inlet of the third bypass valve, the cold side outlet of the first heat exchanger and the outlet of the third bypass valve are connected in parallel, the volatile gas in the LNG fuel tank is connected with the cold side inlet of the second heat exchanger through the first gas dome, the cold side outlet of the second heat exchanger is connected with the inlet of the BOG compressor, the outlet of the BOG compressor, the cold side outlet of the first heat exchanger and the outlet of the third bypass valve are connected in parallel and then connected with the inlet of the fuel buffer tank, the outlet of the fuel buffer tank is connected with the inlet of the second control valve, and the outlet of the second control valve is connected with the main engine.

[0011] As a preferred technical scheme, the CO2 liquefaction system comprises an LCO2 liquid cargo tank, a second gas dome, a CO2 regenerator, a CO2 compressor, a cooler, a CO2 condenser, a throttle valve and a spraying system; the CO2 liquefaction system is connected in the following manner: the second gas dome is installed at the top of the LCO2 liquid cargo tank, the inlet of the second gas dome is connected with the inside of the LCO2 liquid cargo tank, the outlet of the second gas dome is connected with the cold side inlet of the CO2 regenerator, the cold side outlet of the CO2 regenerator is connected with the inlet of the CO2 compressor, the outlet of the CO2 compressor is connected with the inlet of the cooler, the outlet of the cooler is connected with the hot side inlet of the CO2 regenerator, the hot side outlet of the CO2 regenerator is connected with the hot side inlet of the CO2 condenser, the hot side outlet of the CO2 condenser is connected with the inlet of the throttle valve, and the outlet of the throttle valve is connected with the spraying pipe system in the LCO2 liquid cargo tank through the second gas dome.

[0012] As a preferred technical scheme, the heat-conducting medium circulation system comprises a heat-conducting medium storage tank, a heat-conducting medium pump, a three-way valve, a first heat exchanger, a second heat exchanger, a steam heat exchanger, a CO2 condenser, a first bypass valve, a second bypass valve and a fourth bypass valve; the heat-conducting medium circulation system is connected in the following manner: the heat-conducting medium pump is placed at the bottom of the heat-conducting medium storage tank, the outlet of the heat-conducting medium pump is connected with the inlet of the three-way valve, the outlet a of the three-way valve is connected with the hot side inlet of the second heat exchanger, the outlet b of the three-way valve is connected with the hot side inlet of the first heat exchanger, the hot side outlet of the second heat exchanger and the hot side outlet of the first heat exchanger are combined and connected with the cold side inlet of the steam heat exchanger, the cold side outlet of the steam heat exchanger is connected with the cold side inlet of the CO2 condenser, the cold side outlet of the CO2 condenser is connected with the heat-conducting medium storage tank, the second bypass valve is a bypass valve of the steam heat exchanger, the inlet of the second bypass valve is connected with the cold side inlet of the steam heat exchanger, the outlet of the second bypass valve is connected with the cold side outlet of the steam heat exchanger, the first bypass valve is a bypass valve of the CO2 condenser, the inlet of the first bypass valve is connected with the cold side inlet of the CO2 condenser, the outlet of the first bypass valve is connected with the cold side outlet of the CO2 condenser, and steam is connected with the hot side inlet of the steam heat exchanger through the fourth bypass valve.

[0013] As a preferred technical scheme, the heat-conducting medium circulation system adopts heat-conducting oil as the circulating medium, the working temperature range of which covers-160℃ to 40℃, so as to realize the heat transfer demand of LNG cold energy recovery; the heat-conducting oil is selected from synthetic alkyl benzene type or silicon oil type heat-conducting oil; the heat-conducting medium pump adopts frequency conversion control, and the flow rate is adjusted in real time according to the system cold load; when the CO2 condenser cold side outlet temperature is lower than the set value, the pump speed is automatically reduced, otherwise the pump speed is increased to reduce the temperature.

[0014] As a preferred technical scheme, the CO2 compressor adopts a single-stage centrifugal compressor made of low-temperature stainless steel with a temperature resistance of-55℃, and the pressure ratio is within the range of 1.5-2.2.

[0015] As a preferred technical scheme, the CO2 condenser cools the compressed CO2 gas to a subcooling temperature; the CO2 condenser cold side inlet temperature is between-40℃ and-60℃.

[0016] As a preferred technical scheme, the first bypass valve can adjust the flow rate of the cold side heat-conducting medium of the CO2 condenser, and control the final liquefaction temperature of the CO2 gas; when the CO2 condenser cold load is low, part of the heat-conducting medium directly bypasses through the first bypass valve, so as to avoid the icing of the throttling valve caused by CO2 subcooling.

[0017] According to the working method of the LNG gas supply and CO2 reliquefaction integrated system for the LCO2 transport ship, the specific steps include the following steps:

[0018] The gas supply system adopts dual-mode operation: in the natural evaporation mode, the LNG evaporation gas in the fuel tank is collected by the first gas dome, enters the second heat exchanger to exchange heat with the heat conducting medium, is pressurized by the BOG compressor, and is delivered to the fuel buffer tank; in the forced evaporation mode, the LNG liquid is extracted by the gas pump, enters the first heat exchanger to exchange heat with the heat conducting medium, and the third control valve maintains the stable pressure of the fuel tank by adjusting the backflow.

[0019] Meanwhile, the CO2 evaporation gas generated in the liquid cargo tank is first introduced through the second gas dome, then pre-cooled by the CO2 regenerator by exchanging with the high-pressure CO2 gas from the cooler, pressurized by the CO2 compressor, cooled by the cooler, pre-cooled again by the CO2 regenerator, cooled to a supercooling temperature below by the CO2 condenser using the cold energy of the heat conducting oil, and finally expanded and cooled by the throttling valve and injected back into the liquid cargo tank through the spraying system to form a closed loop self-sustaining cycle.

[0020] The heat conducting medium in the heat conducting medium circulation system is driven by the heat conducting medium pump to flow in two ways: one way is to enter the second heat exchanger to exchange heat with the LNG evaporation gas, and the other way is to enter the first heat exchanger to exchange heat with the forced evaporation LNG, and the two ways are combined to flow through the steam heat exchanger for temperature adjustment, and then enter the CO2 condenser to release the cold energy of the LNG. In this process, the first bypass valve serves as a bypass valve for the CO2 condenser, which automatically opens when the outlet temperature of the CO2 condenser is detected to be too low, and part of the heat conducting medium is directly returned to the storage tank to prevent CO2 from being too cold. The second bypass valve serves as a heat conducting medium bypass valve for the steam heat exchanger, which, when the CO2 re-liquefaction system is disabled, opens the valve to make the heat conducting medium directly return to the circulation pipeline. The steam heat exchanger can independently provide auxiliary heating for the gas supply system. Compared with the prior art, the present application has at least the following beneficial effects:

[0021] (1) The present application realizes efficient utilization of LNG cold energy. Through the coordinated operation of the heat conducting medium circulation system and the dual-mode gas supply system, the LNG evaporation gas is used to pre-cool the heat conducting medium, and the CO2 gas is deeply cooled by the CO2 condenser, realizing LNG cold energy recovery and CO2 gas re-liquefaction, and using heat conducting oil as an intermediate medium to avoid the generation of CO2 dry ice caused by direct heat exchange between LNG and CO2.

[0022] (2) The present application realizes self-sustaining circulation liquefaction of the LCO2 transport ship evaporation gas. Through the coordinated process of CO2 regenerator pre-cooling, low-temperature compressor pressurization and CO2 condenser deep cooling, combined with electronic expansion valve intelligent throttling and spraying system uniform injection, the CO2 evaporation gas is efficiently liquefied and returned to the liquid cargo tank to form a closed loop self-sustaining cycle, without the need for additional refrigerant, significantly reducing operation and maintenance costs.

[0023] (3) This invention realizes the integrated design of the system and improves the safety and reliability performance. By adopting compact equipment such as high-efficiency aluminum plate-fin CO2 condenser and stainless steel brazed plate heat exchanger, the overall system volume is reduced by 25%, which is particularly suitable for application scenarios with limited space on ships. Attached Figure Description

[0024] Figure 1 This is a schematic diagram of the working process of an integrated LNG gas supply and CO2 reliquefaction system for an LCO2 transport ship, as described in this application.

[0025] The specific explanations of the reference numerals in the attached figures are as follows:

[0026] 1. Gas pump; 2. First control valve; 3. First heat exchanger; 4. Fuel buffer tank; 5. Second control valve; 6. Third control valve; 7. Second heat exchanger; 8. BOG compressor; 9. Heat transfer medium pump; 10. Heat transfer medium storage tank; 11. CO2 condenser; 12. Steam heat exchanger; 13. CO2 regenerator; 14. Three-way valve; 15. Cooler; 16. CO2 compressor; 17. Throttling valve; 18. First gas dome; 19. Second gas dome; 20. Spray system; 21. First bypass valve; 22. Second bypass valve; 23. Third bypass valve; 24. Fourth bypass valve; 25. LNG fuel tank; 26. LCO2 liquid cargo tank. Detailed Implementation

[0027] To make the objectives, technical solutions, and advantages of the present invention clearer, the present invention is described below with reference to specific embodiments shown in the accompanying drawings. However, it should be understood that these descriptions are merely exemplary and not intended to limit the scope of the present invention. Furthermore, descriptions of well-known structures and technologies are omitted in the following description to avoid unnecessarily obscuring the concept of the present invention. To better understand the technical solutions of the present invention, the present invention will be described in detail below with reference to the accompanying drawings.

[0028] Example 1, as Figure 1 As shown, this invention is an integrated system for LNG fuel supply and CO2 reliquefaction in LCO2 transport vessels. The LCO2 transport vessel is equipped with an LNG fuel tank, an LCO2 cargo tank, and a main engine. The integrated system includes a fuel supply system, a CO2 liquefaction system, and a heat transfer medium circulation system. The cold energy generated by the vaporization of liquefied natural gas in the LNG fuel tank through the fuel supply system is transferred to the CO2 liquefaction system via the heat transfer medium circulation system to reliquefy the evaporated CO2 generated in the LCO2 cargo tank.

[0029] Specifically, the above-mentioned fuel gas supply system is used to provide stable and reliable LNG fuel gas for a ship main engine, and release LNG cold energy for downstream systems (heat conducting medium circulation system and CO2 liquefaction system) to use. The fuel gas supply system comprises a natural evaporation gas supply subsystem and a forced evaporation gas supply subsystem connected with an LNG fuel tank respectively, and the natural evaporation gas supply subsystem and the forced evaporation gas supply subsystem are connected to a fuel buffer tank which delivers fuel to the main engine through a pipeline, and heat exchangers in the natural evaporation gas supply subsystem and the forced evaporation gas supply subsystem are connected to the heat conducting medium circulation system respectively;

[0030] The CO2 liquefaction system is responsible for processing the evaporation gas CO2 in the LCO2 liquid cargo tank, and then realizing CO2 re-liquefaction and tank pressure stability control through a closed loop self-sustaining circulation. The LCO2 liquid cargo tank is connected with the CO2 liquefaction system, the CO2 liquefaction system returns the re-liquefied evaporation CO2 in the LCO2 liquid cargo tank to the LCO2 liquid cargo tank, and a CO2 condenser in the CO2 liquefaction system is connected to the heat conducting medium circulation system;

[0031] The heat conducting medium circulation system acts as an energy transmission center and undertakes the core task of recovering, transporting and accurately distributing the cold energy of the fuel gas supply system. The heat conducting medium circulation system is provided with a heat conducting medium storage tank and a heat conducting medium circulation pipeline, the heat conducting medium circulation pipeline is respectively connected with a heat exchanger and a CO2 condenser, the heat exchanger absorbs the evaporation cold energy of the liquefied natural gas in the fuel gas supply system and cools and liquefies the CO2 vapor passing through the CO2 condenser in the CO2 liquefaction system.

[0032] In the integrated system of the present application, the fuel gas supply system, the CO2 liquefaction system and the heat conducting medium circulation system are coupled through an energy transmission chain, operation logic and equipment deep integration to realize high-efficiency cooperation.

[0033] Energy transmission chain: the fuel gas supply system acts as a cold energy "producer" and releases cold energy in the LNG evaporation / gasification process; the heat conducting medium circulation system acts as a "mover" and captures cold energy through heat conducting oil and transports it to the CO2 condenser; the CO2 liquefaction system acts as a "consumer" and converts the cold energy into CO2 liquefaction power to form a cascade utilization chain of "LNG cold energy→heat conducting medium→CO2 liquefaction".

[0034] Operation logic coupling: when the high-load operation of the main engine triggers the forced evaporation mode, the increased LNG flow of the fuel gas pump synchronously improves the cold energy gain of the heat conducting oil, enhances the CO2 liquefaction capacity to cope with possible tank pressure rise; when the ambient temperature is extremely low, causing LNG cold energy surplus, the first bypass valve of the heat conducting system is automatically opened to shunt, preventing the CO2 condenser from being over-cooled, and the electronic expansion valve of the CO2 liquefaction system dynamically adjusts the liquefaction rate according to the tank pressure to realize cross-system load linkage.

[0035] Device and control integration: CO2 condenser as a physical hub, directly connecting the heat-conducting medium circulation system and the CO2 liquefaction system; three-way valve and bypass valve constitute a flexible cold energy distribution network. Finally, the collaborative design synchronously solves the fuel supply and CO2 re-liquefaction demand in the limited space of the ship, reduces energy consumption with zero additional refrigerant consumption, and highlights the advantages of integrated innovation.

[0036] In embodiment 2, on the basis of embodiment 1, in the system design and implementation, the fuel gas supply system comprises an LNG fuel tank 25, a natural evaporation gas supply subsystem, a forced evaporation gas supply subsystem, a fuel buffer tank 4, and a second control valve 5; the natural evaporation gas supply subsystem comprises a first gas dome 18, a second heat exchanger 7, and a BOG compressor 8; the forced evaporation gas supply subsystem comprises a gas pump 1, a first control valve 2, a first heat exchanger 3, a third control valve 6, and a third bypass valve 23. The fuel gas supply system adopts an intelligent dual-mode operation mechanism: when the main engine load is low (main engine load < 60%), the natural evaporation mode is preferentially started, the LNG evaporation gas (BOG) in the fuel tank is collected through the first gas dome, is heated and warmed through the second heat exchanger and the heat-conducting medium, and is pressurized and delivered to the fuel buffer tank by the BOG compressor for use by the main engine; when the main engine is accelerated, extremely cold weather or the tank pressure is too low, the system automatically switches to the forced evaporation mode, liquid LNG is extracted by the gas pump, the flow is adjusted by the first control valve, and the LNG is gasified by heat exchange with the heat-conducting medium in the first heat exchanger, while the third control valve dynamically adjusts the LNG return flow to maintain the stability of the fuel tank pressure.

[0037] The fuel gas supply system described above is coupled with the heat-conducting medium circulation system through the cold energy release link (first heat exchanger / second heat exchanger), transfers the cold energy of -160℃ LNG, provides a core cold source for CO2 re-liquefaction, and solves the problem of cold energy waste in traditional gas supply systems. The connection mode and working process of the fuel gas supply system are as follows: the gas pump 1 is placed at the bottom of the LNG fuel tank 25, the outlet of the gas pump 1 is divided into two paths, one path is connected with the inlet of the first control valve 2, and the other path is connected with the inlet of the third control valve 6, the outlet of the third control valve 6 is connected into the fuel tank 25, the outlet of the first control valve 2 is divided into two paths, the main path is connected with the cold side inlet of the first heat exchanger 3, and the bypass path is connected with the inlet of the third bypass valve 23, the cold side outlet of the first heat exchanger 3 and the outlet of the third bypass valve 23 are connected in parallel, the volatile gas in the tank is connected with the cold side inlet of the second heat exchanger 7 through the first gas dome 18, the cold side outlet of the second heat exchanger 7 is connected with the inlet of the BOG compressor 8, the outlet of the BOG compressor 8, the cold side outlet of the first heat exchanger 3, and the outlet of the third bypass valve 23 are connected in parallel and then connected with the inlet of the fuel buffer tank 4, the outlet of the fuel buffer tank 4 is connected with the inlet of the second control valve 5, and the outlet of the second control valve 5 is connected with the main engine.

[0038] Embodiment 3, on the basis of Embodiments 1 and 2, in the system design implementation, the CO2 liquefaction system comprises an LCO2 liquid cargo tank 26, a second gas dome 19, a CO2 regenerator 13, a CO2 compressor 16, a cooler 15, a CO2 condenser 11, a throttle valve 17, and a spray system 20; the heat conducting medium circulation system comprises a heat conducting medium storage tank 10, a heat conducting medium pump 9, a three-way valve 14, a first heat exchanger, a second heat exchanger, a steam heat exchanger 12, the CO2 condenser 11, a first bypass valve 21, a second bypass valve 22, and a fourth bypass valve 24. The CO2 liquefaction system is connected as follows: the second gas dome 19 is installed on the top of the LCO2 liquid cargo tank 26, the second gas dome 19 inlet is connected to the inside of the LCO2 liquid cargo tank 26, the second gas dome 19 outlet is connected to the cold side inlet of the CO2 regenerator 13, the cold side outlet of the CO2 regenerator 13 is connected to the inlet of the CO2 compressor 16, the outlet of the CO2 compressor 16 is connected to the inlet of the cooler 15, the outlet of the cooler 15 is connected to the hot side inlet of the CO2 regenerator 13, the hot side outlet of the CO2 regenerator 13 is connected to the hot side inlet of the CO2 condenser 11, the hot side outlet of the CO2 condenser 11 is connected to the inlet of the throttle valve 17, and the outlet of the throttle valve 17 is connected to the spray pipe system 20 in the LCO2 liquid cargo tank 26 through the second gas dome 19.

[0039] The above-mentioned CO2 liquefaction system working process is divided into four steps: first, the CO2 in the LCO2 liquid cargo tank is collected through the second gas dome and then enters the CO2 regenerator to exchange heat with high-pressure CO2 gas from the cooler; then, the pre-cooled gas is pressurized to 1.5-2.2 times by a low-temperature stainless steel centrifugal CO2 compressor, and then cooled by the cooler; next, the gas is deeply pre-cooled by the CO2 regenerator and then enters the CO2 condenser, where it is cooled to a CO2 supercooling temperature and liquefied by the LCO2 cold energy transferred by the heat conducting medium; finally, the liquid CO2 is throttled and expanded by an electronic expansion valve (the opening degree is controlled by the tank pressure signal in real time), and then uniformly injected into the liquid cargo tank through the annular porous atomizing spray system. The CO2 liquefaction system completely relies on the cold energy provided by the LNG gas supply system, does not need external refrigeration units, and realizes zero-emission CO2 self-sustaining liquefaction circulation through intelligent throttling and spray control.

[0040] In the system design implementation, the heat conducting medium circulation system is connected as follows: the heat conducting medium pump 9 is placed at the bottom of the heat conducting medium storage tank 10, the outlet of the heat conducting medium pump 9 is connected with the inlet of the three-way valve 14, the outlet a of the three-way valve 14 is connected with the hot side inlet of the second heat exchanger 7, the outlet b of the three-way valve 14 is connected with the hot side inlet of the first heat exchanger 3, the hot side outlet of the second heat exchanger 7 and the hot side outlet of the first heat exchanger 3 are combined and connected with the cold side inlet of the steam heat exchanger 12, the cold side outlet of the steam heat exchanger 12 is connected with the cold side inlet of the CO2 condenser 11, the cold side outlet of the CO2 condenser 11 is connected with the heat conducting medium storage tank 10, the second bypass valve 22 is a bypass valve of the steam heat exchanger 12, the inlet of the second bypass valve 22 is connected with the cold side inlet of the steam heat exchanger 12, the outlet of the second bypass valve 22 is connected with the cold side outlet of the steam heat exchanger 12, the first bypass valve 21 is a bypass valve of the CO2 condenser 11, the inlet of the first bypass valve 21 is connected with the cold side inlet of the CO2 condenser 11, the outlet of the first bypass valve 21 is connected with the cold side outlet of the CO2 condenser 11, and the steam is connected with the hot side inlet of the steam heat exchanger 12 through the fourth bypass valve 24.

[0041] The heat conducting medium circulation system in the embodiment uses synthetic alkyl benzene or silicon oil as the working medium, which has excellent low temperature performance and is not easy to crystallize under low temperature working conditions and has no corrosive effect on the system metal materials. The heat conducting oil is driven by a variable frequency heat conducting medium pump and is divided into two paths through a three-way valve: one path enters the second heat exchanger to absorb the cold energy of the naturally evaporated LNG, and the other path enters the first heat exchanger to capture the cold energy of the forced gasification LNG. The two paths of low temperature heat conducting oil are combined and then flow through the steam heat exchanger for temperature regulation, and finally enter the CO2 condenser to release the cold energy to liquefy the CO2 gas. The heat conducting medium circulation system realizes efficient utilization of cold energy through multiple dynamic controls: the first bypass valve automatically adjusts the cold energy input according to the outlet temperature of the CO2 condenser to prevent CO2 from icing; the second bypass valve is opened when the heat conducting oil temperature is too low (such as when the LNG cold energy is excessive) to avoid equipment freezing damage; and the variable frequency pump adjusts the flow rate in real time based on the cold load. The core function of the heat conducting medium circulation system is to solve the risk of dry ice generation caused by direct heat exchange between LNG and CO2 in the form of intermediate medium, and to realize safe transmission of cross-system cold energy.

[0042] The heat conducting medium pump 9 adopts variable frequency control and can adjust the flow rate in real time according to the system cold load. When the cold side outlet temperature of the CO2 condenser is lower than the set value, the pump speed is automatically reduced, and vice versa to reduce the temperature.

[0043] The BOG compressor 8 and the CO2 compressor 16 are single-stage centrifugal compressors made of low-temperature-resistant stainless steel. The impeller of the compressor is specially dynamically balanced to meet the compression requirements of the CO2 medium. The compression ratio of the compressor inlet and outlet is controlled within the range of 1.5-2.2, and the speed can be adjusted in real time by the frequency converter according to the change of the liquid cargo tank pressure.

[0044] The CO2 condenser 11 adopts a high-efficiency aluminum plate-fin structure and can cool CO2 gas to a subcooling temperature. The throttling valve 17 is an electronic expansion valve, the opening degree of which is controlled by a liquid cargo tank pressure sensor signal, and the opening degree is automatically increased when the pressure is higher than the set value, and vice versa. The spray system 20 adopts a multi-hole atomizing nozzle design, and the nozzles are arranged in a ring shape at the top of the liquid cargo tank to ensure uniform spraying of liquefied CO2.

[0045] The first heat exchanger 3, the second heat exchanger 7, and the CO2 recuperator 13 all adopt stainless steel brazed plate heat exchangers, and the surface of the heat exchange plate is specially treated to adapt to the heat exchange requirements of LNG and the heat conducting medium.

[0046] The heat conducting medium circulation system is provided with temperature-pressure interlocking protection device, which automatically starts the emergency heating circuit when detecting that the medium temperature is lower than -40℃ or the pressure is higher than 1.5MPa.

[0047] A filter is arranged on the pipeline between the cold side outlet of the steam heat exchanger 12 and the cold side inlet of the CO2 condenser 11, which can filter impurities in the heat conducting medium. A pressure gauge and a temperature sensor are arranged on the pipeline between the hot side outlet of the CO2 condenser 11 and the inlet of the throttling valve 17, which can monitor the pressure and temperature at the inlet of the throttling valve in real time.

[0048] Liquid level sensors and pressure sensors are arranged in the LNG fuel tank 25 and the LCO2 liquid cargo tank 26 to monitor the liquid level and pressure of the fuel tank and the liquid cargo tank in real time, ensuring safe operation of the system. Temperature sensors and liquid level sensors are arranged in the heat conducting medium storage tank 10 to monitor the temperature and liquid level of the heat conducting medium in real time. The capacity of the heat conducting medium storage tank in the heat conducting medium circulation system is designed according to the system requirements to ensure that the circulation amount of the heat conducting medium meets the heat exchange requirements of the LNG cold energy and the CO2 liquefaction system.

[0049] The fuel buffer tank 4 is equipped with a double safety valve design, and the main safety valve is set to 1.8MPa, and the secondary safety valve is set to 2.0MPa.

[0050] A check valve is arranged on the pipeline between the heat conducting medium storage tank 10 and the heat conducting medium pump 9 to prevent backflow of the heat conducting medium. An emergency shut-off valve is arranged on the pipeline between the fuel tank 25 and the fuel buffer tank 4 to quickly cut off the fuel supply in an emergency.

[0051] In example 5, a working method based on the above-mentioned LNG gas supply and CO2 re-liquefaction integrated system is provided, which is used on an LCO2 transport ship, and the working process includes the following steps:

[0052] When the system is working, the gas supply system adopts double mode operation: in the natural evaporation mode, the LNG evaporation gas in the LNG fuel tank 25 is collected by the first gas dome 18, then enters the second heat exchanger 7 to exchange heat with the heat conducting medium, and is heated, and then is delivered to the fuel buffer tank 4 through the BOG compressor 8 after being pressurized; in the forced evaporation mode, the LNG liquid is extracted by the gas pump 1, enters the first heat exchanger 3 to exchange heat with the heat conducting medium, and is gasified, and at the same time, the third control valve 6 maintains the pressure in the LNG fuel tank 25 stable by adjusting the backflow.

[0053] At the same time, the CO2 evaporation gas generated by the LCO2 liquid cargo tank 26 is first introduced through the second gas dome 19, then is pre-cooled and exchanged with the high-pressure CO2 gas from the cooler 15 through the CO2 regenerator 13, and then is pressurized through the CO2 compressor 16, and then is cooled through the cooler 15 after being pressurized, and then is further pre-cooled by flowing through the CO2 regenerator 13 again, and then is deeply cooled to a critical temperature below by using the LNG cold energy through the CO2 condenser 11, and finally is liquefied by expansion through the throttling valve 17 and is injected back into the LCO2 liquid cargo tank 26 through the spraying system 20, to form a closed loop self-sustaining cycle.

[0054] The heat conducting medium in the heat conducting medium circulating system is driven by the heat conducting medium pump to flow in two ways, one way is to enter the second heat exchanger 7 to exchange heat with the LNG evaporation gas, and the other way is to enter the first heat exchanger 3 to exchange heat with the forced evaporation LNG, and the two ways are combined to flow through the steam heat exchanger 12 for temperature adjustment, and then enter the CO2 condenser 11 to absorb the LNG cold energy. In this process, the first bypass valve 21 serves as a bypass valve of the CO2 condenser 11, and automatically opens when it is detected that the outlet temperature of the CO2 condenser 11 is too low, so as to return part of the heat conducting medium directly to the heat conducting medium storage tank 10, to prevent CO2 from being too cold; the second bypass valve serves as a heat conducting medium bypass valve of the steam heat exchanger, and when the CO2 re-liquefaction system is disabled, the valve is opened to make the heat conducting medium return directly to the circulating pipeline; the steam heat exchanger can independently provide auxiliary heating for the gas supply system. In the conventional working condition: the heat conducting medium flows through the steam heat exchanger, and is heated by steam to prevent the temperature from being too low (such as when the cold energy is excessive); in the independent operation mode of the gas system (when the CO2 re-liquefaction system is disabled): the second bypass valve is opened, and the heat conducting medium bypasses the steam heat exchanger, to avoid invalid heating; the steam heat exchanger can provide auxiliary heating for the gas system at this time (steam needs to be supplied).

[0055] Although the above embodiments have been specifically described for the present application, it should be understood by those of ordinary skill in the art that modifications or improvements can be made based on the disclosure of the present application without departing from the spirit and scope of the present application, and these modifications and improvements are within the spirit and scope of the present application.

Claims

1. An integrated system for LNG fuel supply and CO2 reliquefaction in an LCO2 transport vessel, characterized in that, The integrated system includes a gas supply system, a CO2 liquefaction system, and a heat transfer medium circulation system. The cold energy generated by the vaporization of liquefied natural gas in the LNG fuel tank in the gas supply system is transferred to the CO2 liquefaction system through the heat transfer medium circulation system to reliquefy the evaporated CO2 generated by the evaporation of the LNG cargo tank. The gas supply system includes a natural evaporation gas supply subsystem and a forced evaporation gas supply subsystem, which are respectively connected to the LNG fuel tank. Both the natural evaporation gas supply subsystem and the forced evaporation gas supply subsystem are connected to a fuel buffer tank, which supplies fuel to the main unit through pipelines. The heat exchangers in the natural evaporation gas supply subsystem and the forced evaporation gas supply subsystem are respectively connected to the heat transfer medium circulation system. The LCO2 cargo tank is connected to a CO2 liquefaction system, which liquefies the evaporated CO2 in the LCO2 cargo tank and returns it to the LCO2 cargo tank. The CO2 condenser in the CO2 liquefaction system is connected to the heat transfer medium circulation system. The aforementioned heat transfer medium circulation system includes a heat transfer medium storage tank and a heat transfer medium circulation pipeline. A heat exchanger and a CO2 condenser are connected to the circulation pipeline. The heat exchanger absorbs the evaporative cooling energy of the liquefied natural gas in the gas supply system and cools and liquefies the CO2 vapor that has passed through the CO2 condenser in the CO2 liquefaction system. The gas supply system includes a natural evaporation gas supply subsystem, a forced evaporation gas supply subsystem, a fuel buffer tank, and a second control valve. The natural evaporation gas supply subsystem includes a first gas dome, a second heat exchanger, and a BOG compressor. The forced evaporation gas supply subsystem includes a gas pump, a first control valve, a first heat exchanger, a third control valve, and a third bypass valve. The gas pump is located at the bottom of the LNG fuel tank, and its outlet is split into two paths: one connected to the inlet of the first control valve, and the other connected to the inlet of the third control valve. The outlet of the third control valve leads into the fuel tank. The outlet of the first control valve is split into two paths. The main path is connected to the cold side inlet of the first heat exchanger, and the bypass path is connected to the inlet of the third bypass valve. The cold side outlet of the first heat exchanger and the outlet of the third bypass valve are connected in parallel and merged. The volatile gas in the LNG fuel tank is connected to the cold side inlet of the second heat exchanger through the first gas dome. The cold side outlet of the second heat exchanger is connected to the inlet of the BOG compressor. The three pipelines of the BOG compressor outlet, the cold side outlet of the first heat exchanger, and the outlet of the third bypass valve are connected to the inlet of the fuel buffer tank. The outlet of the fuel buffer tank is connected to the inlet of the second control valve. The outlet of the second control valve is connected to the main unit.

2. The integrated LNG fuel supply and CO2 reliquefaction system for an LCO2 transport ship according to claim 1, characterized in that, The CO2 liquefaction system includes a second gas dome, a CO2 regenerator, a CO2 compressor, a cooler, a CO2 condenser, a throttle valve, and a spray system. The second gas dome is installed on top of the LCO2 cargo tank. The inlet of the second gas dome is connected to the inside of the LCO2 cargo tank. The outlet of the second gas dome is connected to the cold-side inlet of the CO2 regenerator. The cold-side outlet of the CO2 regenerator is connected to the inlet of the CO2 compressor. The outlet of the CO2 compressor is connected to the inlet of the cooler. The outlet of the cooler is connected to the hot-side inlet of the CO2 regenerator. The hot-side outlet of the CO2 regenerator is connected to the hot-side inlet of the CO2 condenser. The hot-side outlet of the CO2 condenser is connected to the inlet of the throttle valve. The outlet of the throttle valve is connected to the spray pipe system inside the LCO2 cargo tank through the second gas dome.

3. The integrated LNG fuel supply and CO2 reliquefaction system for an LCO2 transport ship according to claim 1, characterized in that, The heat transfer medium circulation system includes a heat transfer medium storage tank, a heat transfer medium pump, a three-way valve, a first heat exchanger, a second heat exchanger, a steam heat exchanger, a CO2 condenser, a first bypass valve, a second bypass valve, and a fourth bypass valve. The heat transfer medium pump is placed at the bottom of the heat transfer medium storage tank. The outlet of the heat transfer medium pump is connected to the inlet of the three-way valve. Outlet a of the three-way valve is connected to the hot-side inlet of the second heat exchanger, and outlet b of the three-way valve is connected to the hot-side inlet of the first heat exchanger. The hot-side outlets of the second and first heat exchangers merge and connect to the cold-side inlet of the steam heat exchanger. The cold-side outlet of the heat exchanger is connected to the cold-side inlet of the CO2 condenser, and the cold-side outlet of the CO2 condenser is connected to the heat transfer medium storage tank. The second bypass valve is a bypass valve for the steam heat exchanger. The inlet of the second bypass valve is connected to the cold-side inlet of the steam heat exchanger, and the outlet of the second bypass valve is connected to the cold-side outlet of the steam heat exchanger. The first bypass valve is a bypass valve for the CO2 condenser. The inlet of the first bypass valve is connected to the cold-side inlet of the CO2 condenser, and the outlet of the first bypass valve is connected to the cold-side outlet of the CO2 condenser. Steam is connected to the hot-side inlet of the steam heat exchanger through the fourth bypass valve.

4. The LNG fuel gas supply and CO2 reliquefaction integrated system for an LCO2 transport ship as described in claim 1, characterized in that, The heat transfer medium circulation system uses heat transfer oil as the circulation medium, with an operating temperature range covering -160℃ to 40℃, to meet the heat transfer requirements of LNG cold energy recovery; the heat transfer oil is selected from synthetic alkylbenzene type or silicone oil type heat transfer oil; the heat transfer medium pump adopts frequency conversion control, and adjusts the flow rate in real time according to the system cold load. When the cold side outlet temperature of the CO2 condenser is lower than the set value, the pump speed is automatically reduced, and vice versa to reduce the temperature.

5. An integrated system for LNG fuel supply and CO2 reliquefaction for an LCO2 transport ship as described in claim 2, characterized in that, The CO2 compressor is a single-stage centrifugal compressor made of stainless steel resistant to low temperatures of -55℃, with a pressure ratio in the range of 1.5-2.

2.

6. The integrated LNG fuel supply and CO2 reliquefaction system for an LCO2 transport ship as described in claim 1, characterized in that, The CO2 condenser cools the compressed CO2 gas to below the subcooling temperature; the cold side inlet temperature of the CO2 condenser is between -40 and -60°C.

7. The LNG fuel gas supply and CO2 reliquefaction integrated system for an LCO2 transport ship as described in claim 3, characterized in that, The first bypass valve can adjust the flow rate of the heat transfer medium on the cold side of the CO2 condenser and control the final liquefaction temperature of the CO2 gas. When the cooling load of the CO2 condenser is low, part of the heat transfer medium is directly bypassed through the first bypass valve to avoid the CO2 from being too cold and causing the throttle valve to freeze.

8. A method for operating an integrated LNG fuel supply and CO2 reliquefaction system for an LCO2 transport ship according to any one of claims 1-7, characterized in that, Specifically, the following steps are included: The gas supply system operates in two modes: in the natural evaporation mode, the LNG vapor in the fuel tank is collected by the first gas dome and then enters the second heat exchanger to exchange heat with the heat transfer medium and be heated. It is then pressurized by the BOG compressor and delivered to the fuel buffer tank. In forced evaporation mode, the gas pump draws LNG liquid through the first control valve into the first heat exchanger to exchange heat with the heat transfer medium and vaporize. At the same time, the third control valve maintains the fuel tank pressure by adjusting the return flow. Meanwhile, the CO2 vapor generated in the cargo tank is drawn out through the second gas dome. It first undergoes pre-cooling exchange with the high-pressure CO2 gas from the cooler through the CO2 regenerator, and then enters the CO2 compressor for pressurization. After being cooled by the cooler, the high-temperature CO2 gas flows through the CO2 regenerator again for further pre-cooling. It then enters the CO2 condenser and is cooled to below the subcooling temperature by the cooling energy of the heat transfer oil. Finally, it expands and cools through the throttle valve and is reinjected into the cargo tank through the spray system, forming a closed-loop self-sustaining cycle. In the heat transfer medium circulation system, the heat transfer medium pump drives the working medium to flow in two paths. One path enters the second heat exchanger to exchange heat with the LNG vaporized gas, and the other path enters the first heat exchanger to exchange heat with the forced-evaporation LNG. After the two paths merge, they flow through the steam heat exchanger for temperature regulation, and then enter the CO2 condenser to release the LNG's cold energy. During this process, the first bypass valve acts as a bypass valve for the CO2 condenser. When the outlet temperature of the CO2 condenser is detected to be too low, it automatically opens, diverting a portion of the heat transfer medium directly back to the storage tank to prevent the CO2 from becoming too cold. The second bypass valve acts as a bypass valve for the heat transfer medium in the steam heat exchanger. When the CO2 reliquefaction system is shut down, this valve is opened to allow the heat transfer medium to return directly to the circulation pipeline. The steam heat exchanger independently provides auxiliary heating for the gas supply system.

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