Ammonia-powered ship fuel leakage treatment system

Abstract

The invention belongs to the technical field of ships, and particularly relates to a system for treating excess BOG (boil-off gas) by using fuel cold energy for an LNG (liquefied natural gas) ship. The system comprises an N2 heat exchanger, a compressor, an expansion machine, a precooling heat exchanger, a BOG compressor, an N2 compressor, a seawater heat exchanger, a low-temperature heat exchanger, a cold energy release heat exchanger, an ethane supply valve, an ethane circulating pump, a supply valve, a stop valve, a cold energy storage heat exchanger and an LNG cargo hold. In order to solve the problem that when an LNG ship sails at a low speed or in a maneuvering mode or sails in a stormy day, a ship main engine cannot completely consume BOG generated in an LNG cargo hold, the cold energy of BOG used for burning of the main engine is used for liquefying the excessive BOG in the LNG cargo hold, waste of the cold energy of BOG used for burning of the ship main engine is reduced, power consumption of reliquefaction of the excessive BOG is reduced, and the energy consumption of the LNG cargo hold is reduced. And the method has good economical efficiency and practicability.

Description

Technical Field

[0001] The invention belongs to the technical field of ships, and in particular relates to a system for an LNG ship to process excess BOG by utilizing cold energy of fuel. Background Art

[0002] As the demand for liquefied natural gas (LNG) continues to grow, the volume of LNG trade between countries is also increasing. Among them, ships specially used to transport liquefied natural gas are called liquefied natural gas carriers (LNG ships for short).

[0003] LNG is usually stored in a liquid form at -163℃ in LNG cargo holds. However, during the transportation process of LNG ships, there is a temperature difference of about 200℃ between the LNG in the cargo hold and the surrounding environment, resulting in a large amount of heat exchange, thereby absorbing external heat to produce boil-off gas (BOG for short). Taking the "Changheng Series" LNG ship with a loading capacity of 174,000 cubic meters as an example, the daily evaporation rate of LNG cargo on this ship is about 0.08% (there are fluctuations in different seasons, with a higher evaporation rate in summer and a lower evaporation rate in winter). Based on this calculation, the volume of BOG that the ship can generate per day is about 140m 3 (The mass is about 60t). In addition, during the voyage, the LNG ship may also be affected by natural factors such as wind and waves and undergo multi-degree-of-freedom swaying motion. At this time, part of the mechanical energy contained in the LNG in the cargo hold will be converted into heat energy, exacerbating the generation of BOG, resulting in more BOG generated during windy and wavey sailing. If the generated BOG is discharged into the atmosphere, it will not only cause environmental pollution, but also waste resources, so this part of BOG needs to be processed.

[0004] The BOG in the LNG cargo tank can be used as fuel for the main power unit of the LNG ship, providing power for the ship's navigation. At this time, the BOG in the LNG cargo tank that is sent to the ship's main engine for fuel is called the main engine fuel BOG. ​​In traditional LNG ships, most of the BOG generated in the LNG cargo tank is sent to the ship's boiler for fuel. The steam generated by the boiler drives the steam turbine, and the steam turbine generates power for propeller propulsion. However, the thermal efficiency of the boiler-steam turbine is low, about 29% to 32%. At present, LNG ships usually use dual-fuel main engines, using dual-fuel diesel engines to directly drive propellers for propulsion. The thermal efficiency of the diesel engine reaches about 50%. Compared with the boiler-steam turbine propulsion mode, the amount of fuel used by the diesel engine is greatly reduced.

[0005] When a diesel-powered ship is in normal navigation, the ship's main engine can usually completely consume the BOG generated in the LNG cargo tank; however, when the ship is in low-speed navigation or maneuvering navigation, the propulsion power required by the ship is relatively low, and the main engine burns less BOG, but the BOG generated in the LNG cargo tank at this time is not much different from the BOG generated during normal navigation; when the ship is sailing in windy and wavey days, the propulsion power required by the ship is also relatively low, but the BOG generated in the LNG cargo tank at this time is more than the BOG generated during normal navigation. Therefore, in these two cases, the BOG consumed by the ship is far less than the BOG generated in the cargo tank, resulting in the phenomenon that the BOG in the LNG cargo tank is not burned by the ship's main engine. At this time, the BOG that is not burned in the LNG cargo tank is called excess BOG. ​​The treatment of excess BOG includes two methods: combustion and reliquefaction. One is to burn the excess BOG in the LNG cargo tank through a gas combustion unit (GCU for short), but this method has a serious problem of resource waste; the other is to directly reliquefy the BOG through a refrigeration unit. Since the liquefaction temperature of BOG is about -163℃, it is impossible to liquefy BOG through a single-stage refrigeration cycle, and generally a multi-stage refrigeration cycle is required. Therefore, BOG liquefaction requires a large cooling capacity, and the power consumption of the refrigeration system is very large.

[0006] However, the temperature of BOG coming out of the LNG cargo tank is extremely low, about -140℃~-150℃. Before BOG is supplied to the ship's main engine as fuel, it needs to be heated to about 40℃ before it can be used normally. This process not only consumes a large amount of heat source, but also leads to a large waste of BOG's cold energy.

[0007] Based on the above problems, the present invention proposes a system for LNG ships to use fuel cold energy to treat excess BOG, which collects the cold energy supplied to the main engine to burn BOG and uses it to liquefy excess BOG in the LNG cargo hold. This avoids the waste of a large amount of BOG cold energy resources, reduces refrigeration power consumption, and reduces the liquefaction cost of BOG. Summary of the invention

[0008] The first object of the present invention is to propose a system for LNG ship to treat excess BOG using fuel cold energy based on the above problem. The system includes an LNG fuel supply system, a N2 circulation system, a BOG cold energy storage system and an excess BOG liquefaction system.

[0009] Among them, the LNG fuel supply system includes: LNG cargo tank, supply valve, N2 heat exchanger, compressor, temperature control system, and main engine.

[0010] The N2 circulation system includes: N2 heat exchanger, expander, low-temperature heat exchanger, N2 compressor, and pre-cooling heat exchanger.

[0011] The BOG cold energy storage system includes: a cold energy release heat exchanger, an ethane supply valve, a cold energy storage heat exchanger, and an ethane circulation pump.

[0012] The excess BOG liquefaction system includes: pre-cooling heat exchanger, BOG compressor, seawater heat exchanger, low-temperature heat exchanger, cold energy release heat exchanger, stop valve, cold energy storage heat exchanger, and LNG cargo tank.

[0013] In the LNG fuel supply system, the LNG cargo tank is connected to the supply valve, N2 heat exchanger, compressor, temperature control system and main engine in sequence through pipelines. The main engine fuel BOG in the LNG cargo tank is transported to the N2 heat exchanger through the supply valve for heat exchange and releases cold energy. After passing through the compressor and temperature control system to make the BOG reach the pressure and temperature of the engine, it is used for fuel of the ship's main engine.

[0014] In the N2 circulation system, the N2 heat exchanger is connected to the expander, the low-temperature heat exchanger, the N2 compressor, and the pre-cooling heat exchanger in sequence through pipelines, and the pre-cooling heat exchanger is connected to the N2 heat exchanger through pipelines to form a closed loop, and the medium in the closed pipeline is flowing N2.

[0015] In the BOG cold energy storage system, the cold energy storage heat exchanger is connected to the ethane supply valve, the cold energy release heat exchanger, and the ethane circulation pump in sequence through pipelines. The ethane circulation pump is connected to the cold energy storage heat exchanger through pipelines, also forming a closed loop. The medium in the closed pipeline is flowing ethane.

[0016] In the excess BOG liquefaction system, the LNG cargo tank is connected to the stop valve, the cold energy storage heat exchanger, the pre-cooling heat exchanger, the BOG compressor, the seawater heat exchanger, the cold energy release heat exchanger, and the low-temperature heat exchanger in sequence through pipelines, and the low-temperature heat exchanger is connected to the LNG cargo tank through pipelines to form a circulation pipeline.

[0017] The working mechanism of the excess BOG liquefaction system is that the excess BOG in the LNG cargo tank can enter the cold energy storage heat exchanger through the stop valve to exchange heat with ethane, so that the temperature of the excess BOG increases and the temperature of ethane decreases; then the excess BOG passes through the pre-cooling heat exchanger, where the N2 is pre-cooled initially, and the temperature of the excess BOG increases; the excess BOG is compressed by the BOG compressor to increase the temperature and pressure, and then passes through the seawater heat exchanger to use seawater to initially cool the excess BOG; then it enters the cold energy release heat exchanger to exchange heat with ethane, so that the excess BOG is cooled again; finally, it enters the low-temperature heat exchanger to exchange heat with N2, the cold energy of which comes from the cold energy of BOG burned by the main engine, so that the temperature of the excess BOG is reduced to about -163°C for liquefaction, and finally it is sent to the LNG cargo tank through a pipeline for storage.

[0018] Beneficial effects of the present invention:

[0019] 1. The present invention aims to solve the problem that the main engine of the ship cannot completely consume the BOG generated in the LNG cargo hold when the LNG ship is sailing at a low speed, maneuvering or in a stormy day. The present invention rationally utilizes the cold energy of the main engine burning BOG to liquefy the excess BOG in the cargo hold, thereby reducing the waste of the cold energy of the main engine burning BOG.

[0020] 2. When the main engine of the ship cannot completely consume the BOG generated in the LNG cargo tank, the present invention uses the cold energy of the BOG burned by the main engine to liquefy the excess BOG in the LNG cargo tank, which has lower power consumption than completely using the refrigeration system to liquefy the excess BOG. BRIEF DESCRIPTION OF THE DRAWINGS

[0021] Figure 1 is a system diagram of the present invention;

[0022] Figure 2 This is a schematic diagram of the LNG cargo tank in the present invention;

[0023] In the attached figure: 1. N2 heat exchanger; 2. compressor; 3. expander; 4. pre-cooling heat exchanger; 5. BOG compressor; 6. N2 compressor; 7. seawater heat exchanger; 8. low-temperature heat exchanger; 9. cold energy release heat exchanger; 10. ethane supply valve; 11. ethane circulation pump; 12. supply valve; 13. stop valve; 14. cold energy storage heat exchanger; 15. LNG cargo hold. DETAILED DESCRIPTION

[0024] In order to make the purpose, technical solutions and advantages of the present invention more clearly understood, the present invention is further described in detail below in conjunction with the accompanying drawings and embodiments.

[0025] A system for LNG ships to treat excess BOG using fuel cold energy, such as Figure 1 As shown in FIG. 1 , the system includes an LNG fuel supply system, an N2 circulation system, a BOG cold energy storage system, and an excess BOG liquefaction system. Figure 2 As shown, an LNG ship usually has a plurality of LNG cargo tanks 15. Figure 1 A system diagram of only one of the LNG cargo tanks 15 is shown.

[0026] The LNG fuel supply system includes: N2 heat exchanger 1, compressor 2, supply valve 12, LNG cargo tank 15, temperature control system, and main engine.

[0027] The N2 circulation system includes: an N2 heat exchanger 1, an expander 3, a precooling heat exchanger 4, an N2 compressor 6, and a low-temperature heat exchanger 8.

[0028] The BOG cold energy storage system includes: a cold energy releasing heat exchanger 9 , an ethane supply valve 10 , an ethane circulation pump 11 , and a cold energy storage heat exchanger 14 .

[0029] The excess BOG liquefaction system includes: a precooling heat exchanger 4, a BOG compressor 5, a seawater heat exchanger 7, a low-temperature heat exchanger 8, a cold energy release heat exchanger 9, a stop valve 13, a cold energy storage heat exchanger 14, and an LNG cargo tank 15.

[0030] In the LNG fuel supply system, the LNG cargo tank 15 is connected to the supply valve 12, the N2 heat exchanger 1, the compressor 2, the temperature control system, and the main engine in sequence through pipelines. The main engine fuel BOG in the LNG cargo tank 15 is transported to the N2 heat exchanger 1 through the supply valve 12 for heat exchange and releases cold energy. After that, the BOG is heated to the engine inlet temperature by the compressor 2 and the temperature control system, and then is used for the ship's main engine. The BOG used for the main engine is called the main engine fuel BOG; the BOG in the LNG cargo tank 15 that is not used by the main engine is called excess BOG.

[0031] In the N2 circulation system, the N2 heat exchanger 1 is connected to the expander 3, the low-temperature heat exchanger 8, the N2 compressor 6, and the precooling heat exchanger 4 in sequence through pipelines. The precooling heat exchanger 4 is connected to the N2 heat exchanger 1 through pipelines to form a closed loop, and the closed pipeline is filled with flowing N2. The function of the N2 heat exchanger 1 is to absorb the cold energy of the BOG burned by the host and temporarily store the cold energy; the precooling heat exchanger 4 performs preliminary precooling of the N2 here, and the temperature of the excess BOG increases; the function of the low-temperature heat exchanger 8 is to use N2 to cool the excess BOG again, so as to liquefy it.

[0032] In the BOG cold energy storage system, the cold energy storage heat exchanger 14 is connected to the ethane supply valve 10, the cold energy release heat exchanger 9, and the ethane circulation pump 11 in sequence through pipelines. The ethane circulation pump 11 is connected to the cold energy storage heat exchanger 14 through pipelines, and a closed loop is also formed. The closed pipeline contains ethane. The function of the cold energy release heat exchanger 9 is to release the cold energy of the excess BOG absorbed by ethane and perform secondary cooling on the excess BOG; the function of the cold energy storage heat exchanger 14 is to use ethane to perform heat exchange with the excess BOG, thereby temporarily storing the cold energy of the excess BOG.

[0033] In the excess BOG liquefaction system, the LNG cargo tank 15 is connected to the shut-off valve 13, the cold energy storage heat exchanger 14, the precooling heat exchanger 4, the BOG compressor 5, the seawater heat exchanger 7, the cold energy release heat exchanger 9, and the low-temperature heat exchanger 8 in sequence through pipelines. The low-temperature heat exchanger 8 is connected to the LNG cargo tank 15 through pipelines to form a circulation pipeline. The working mechanism of the excess BOG liquefaction system is that the excess BOG in the LNG cargo tank 15 enters the cold energy storage heat exchanger 14 through the shut-off valve 13, exchanges heat with ethane, and increases the temperature of the excess BOG to about -30°C and reduces the temperature of ethane to about -115°C; then the excess BOG passes through the precooling heat exchanger 4, where N2 is preliminarily precooled, and the temperature of the excess BOG is increased to about 5°C; the excess BOG is compressed by the BOG compressor 5 and then the temperature and pressure are increased, so that the temperature of the excess BOG reaches 8 8℃; then it passes through the seawater heat exchanger 7 to use seawater to cool the excess BOG to about 40℃; then it enters the cold energy release heat exchanger 9 to exchange heat with ethane, so that the excess BOG is cooled to about -85℃ again; finally it enters the low-temperature heat exchanger 8 to exchange heat with N2, the cold energy of which comes from the cold energy of BOG burned by the main engine, so that the temperature of the excess BOG is reduced to about -163℃ and liquefied, and finally it is sent to the LNG cargo tank 15 through a pipeline for storage.

[0034] When the ship is in low-speed navigation, maneuvering navigation or sailing in windy and wavey days, the propulsion power required by the ship is relatively low. At this time, the BOG in the LNG cargo tank 15 cannot be completely burned by the ship's main engine. For the excess BOG in the LNG cargo tank 15, the excess BOG liquefaction system can be used to liquefy the excess BOG in the LNG cargo tank 15 by using the cold energy of the main engine burning BOG. ​​Compared with completely using the refrigeration system to liquefy BOG, the power consumption is lower and the liquefaction cost of the excess BOG is reduced. Since the time of a ship sailing at low speed, maneuvering navigation or sailing in windy and wavey days usually occupies a certain proportion of the total time of the entire voyage, for example, the total sailing time of the ship is 30 days. According to navigation experience, the total time of the ship sailing at low speed, maneuvering navigation and sailing in windy and wavey days is about 6 to 8 days. Therefore, the present invention has good practical application significance.

[0035] When the ship is in normal navigation, the BOG generated in the LNG cargo tank 15 can be completely burned by the ship's main engine, and the excess BOG liquefaction system can be deactivated.

[0036] In addition, the amount of BOG generated by the LNG cargo tank 15 will fluctuate in different seasons. When the ship is sailing in summer, the evaporation rate of LNG cargo is higher, and more BOG is generated. When the main engine power remains unchanged, there is more excess BOG in the LNG cargo tank 15. If a refrigeration system is used to liquefy the excess BOG, more power consumption is required and the cost is higher. At the same time, before the LNG ship supplies BOG as fuel to the ship's main engine, its temperature needs to be raised from -140℃ to -150℃ to about 40℃ before it can be used normally. This heating process wastes a lot of cold energy of the main engine burning BOG. ​​The present invention uses the cold energy of the main engine burning BOG to liquefy the excess BOG in the LNG cargo tank 15, which reduces the cost of BOG reliquefaction, avoids the waste of cold energy of the main engine burning BOG, and reasonably utilizes resources.

[0037] The above is only a preferred embodiment of the present invention. The specific embodiments described herein are only used to explain the present invention and are not used to limit the present invention. It should be pointed out that for ordinary technicians in this technical field, several improvements and modifications can be made without departing from the principle of the present invention, and these improvements and modifications should also be regarded as the protection scope of the present invention.

Claims

1. A system for treating excess BOG on an LNG ship by utilizing fuel cold energy, characterized in that: The system comprises an N2 heat exchanger (1), a compressor (2), an expander (3), a precooling heat exchanger (4), a BOG compressor (5), an N2 compressor (6), a seawater heat exchanger (7), a low-temperature heat exchanger (8), a cold energy release heat exchanger (9), an ethane supply valve (10), an ethane circulation pump (11), a supply valve (12), a stop valve (13), a cold energy storage heat exchanger (14), and an LNG cargo tank (15). The LNG cargo tank (15) is connected to the supply valve (12), the N2 heat exchanger (1), the compressor (2), the temperature control system, and the main engine in sequence through pipelines. The N2 heat exchanger (1) is connected to the expander (3), the low-temperature heat exchanger (8), the N2 compressor (6), and the precooling heat exchanger (4) in sequence through pipelines, and the precooling heat exchanger (4) is connected to the N2 heat exchanger (1) through pipelines to form a closed loop. The cold energy storage heat exchanger (14) is connected to the ethane supply valve (10), the cold energy release heat exchanger (9), and the ethane circulation pump (11) in sequence through pipelines, and the ethane circulation pump (11) is connected to the cold energy storage heat exchanger (14) through pipelines to form a closed loop. The LNG cargo tank (15) is connected to the stop valve (13), the cold energy storage heat exchanger (14), the precooling heat exchanger (4), the BOG compressor (5), the seawater heat exchanger (7), the cold energy release heat exchanger (9), and the low-temperature heat exchanger (8) in sequence through pipelines, and the low-temperature heat exchanger (8) is connected to the LNG cargo tank (15) through pipelines to form a circulation pipeline.

2. The system for treating excess BOG by utilizing cold energy of fuel on an LNG ship according to claim 1, characterized in that: The heat exchange medium in the N2 heat exchanger (1) is N2, which absorbs cold energy of BOG burned by the main engine, thereby temporarily storing the cold energy of BOG burned by the main engine; the heat exchange medium in the low-temperature heat exchanger (8) is N2, and the low-temperature heat exchanger (8) cools down and liquefies excess BOG.

3. The system for treating excess BOG by utilizing fuel cold energy on an LNG ship according to claim 1, characterized in that: When the main engine combustion BOG flows through the N2 heat exchanger (1), N2 exchanges heat with the main engine combustion BOG, and the temperature of N2 decreases after absorbing the cold energy of the main engine combustion BOG. ​​The N2 is throttled and depressurized through the expander (3), and the temperature is further reduced. The N2 then enters the low-temperature heat exchanger (8), and the N2 exchanges heat with the excess BOG, and the temperature of N2 increases. The pressure is increased through the N2 compressor (6), and the temperature of N2 increases at this time. The N2 then enters the pre-cooling heat exchanger (4), and the N2 exchanges heat with the excess BOG again, thereby reducing the temperature of N2. Finally, the N2 flows back to the N2 heat exchanger (1), forming an N2 cycle.

4. The system for treating excess BOG by utilizing fuel cold energy on an LNG ship according to claim 1, characterized in that: The excess BOG in the LNG cargo tank (15) exchanges heat with ethane through the cold energy storage heat exchanger (14), and its temperature increases; it exchanges heat with N2 through the precooling heat exchanger (4), and its temperature increases; the excess BOG is compressed and passes through the BOG compressor (5), and its temperature further increases; then the excess BOG passes through the seawater heat exchanger (7), and exchanges heat with seawater to reduce its temperature; then it enters the cold energy release heat exchanger (9), and exchanges heat with ethane, and its temperature decreases again; finally, the excess BOG enters the low-temperature heat exchanger (8), and exchanges heat with N2, and its temperature further decreases and it is liquefied, and finally the liquefied excess BOG flows back to the LNG cargo tank (15) for storage.

5. The system for treating excess BOG by utilizing cold energy of fuel on an LNG ship according to claim 1, characterized in that: The main engine fuel BOG is the BOG in the LNG cargo tank (15) that is delivered to the main engine of the ship for fuel; and the excess BOG is the BOG in the LNG cargo tank (15) that is not fueled.

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