A BOG treatment system and method for an LNG and liquid hydrogen combined carrier

Abstract

The application discloses a BOG processing system and method for LNG and liquid hydrogen combined transport ship, and belongs to the ship technical field.The system comprises an LNG cargo hold, a liquid hydrogen cargo hold, an LNG boil-off gas valve, a liquid hydrogen boil-off gas valve, a supply valve, a supply compressor, a heating unit, a carbon capture unit, a methanol synthesis unit, a hydrogen compressor, a BOG heat exchanger, a methanol valve, a methanol storage tank, a check valve, a liquefaction compressor, a separation unit and a liquid return valve.The application combines liquid hydrogen and LNG for transport, sends BOG in the LNG cargo hold to the ship main engine for fueling, makes CO2 generated by the fueling react with the BOG of the liquid hydrogen after being captured to synthesize methanol, and re-liquefies the BOG in the LNG cargo hold which is not used for fueling of the ship main engine by using the cold energy of the liquid hydrogen BOG when the ship sails at a low speed, and the methanol is pumped to the land for recycling when the ship is berthed.The application not only avoids the problems of liquefaction of the captured CO2 and storage of the ship, but also solves the BOG processing problems in the LNG cargo hold and the liquid hydrogen cargo hold, and makes the cold energy of the liquid hydrogen BOG be fully utilized, and has high practical value.

Description

Technical Field

[0001] This invention belongs to the field of marine technology, specifically relating to a BOG handling system and method for LNG and liquid hydrogen combined transport vessels. Background Technology

[0002] With the increasing global demand for clean energy, the international trade volume of natural gas and hydrogen, as pollution-free and highly efficient clean energy sources, has been rising year by year. Shipping, as the most cost-effective way to transport large quantities of goods over long distances, plays an important role in the international trade of natural gas and hydrogen. To increase the volume of natural gas and hydrogen transported by sea, both are stored and transported in liquid form (liquid hydrogen and LNG). Ships used to transport liquid hydrogen and LNG are respectively called liquid hydrogen carriers and LNG carriers.

[0003] Since liquid hydrogen has a temperature of -253℃ and LNG has a temperature of -163℃, even if the liquid hydrogen and LNG cargo tanks on liquid hydrogen and LNG carriers are equipped with insulation layers, the extremely low temperatures of liquid hydrogen and LNG will inevitably lead to heat exchange with the external environment and the generation of bulky air (BOG). As the ship sails, the BOG content in the cargo tanks will gradually increase. For safety reasons, the generated BOG needs to be dealt with in a timely manner.

[0004] Generally, LNG carriers handle Balance of Goods (BOG) in two ways. The first is to send the BOG generated in the LNG cargo tanks to the ship's main engine for combustion, providing power. However, this method still produces a large amount of CO2 after combustion. To meet the requirements of international maritime conventions to reduce ship CO2 emissions, the emitted CO2 must be captured, liquefied, and stored. The CO2 reliquefaction process consumes significant energy and requires high-pressure, low-temperature storage tanks. The second method is to reliquefy the BOG generated in the cargo tanks and return it. However, LNG BOG has an extremely low liquefaction temperature, resulting in significant energy consumption during the liquefaction process. For liquid hydrogen carriers, the first method would be uneconomical due to the high price of liquid hydrogen, leading to high fuel system costs. The second method, with its even lower liquefaction temperature, consumes significantly more energy to liquefy BOG compared to LNG BOG. This shows that both LNG carriers and liquid hydrogen carriers have certain drawbacks in their methods of handling BOG (Body Gas) in the cargo hold.

[0005] Furthermore, since the density of liquid hydrogen is only 70 kg / m³ 3Liquid hydrogen has a very small mass per unit volume. Even when fully loaded, liquid hydrogen carriers still need to inject a large amount of ballast water into their ballast tanks to lower the ship's center of gravity and improve its stability. This means that liquid hydrogen carriers transport a large amount of worthless ballast water on each voyage, making them economically inefficient. LNG, on the other hand, has a density of 420 kg / m³. 3 The density of LNG is about 6 times that of liquid hydrogen, and its mass per unit volume is much greater than that of liquid hydrogen. In addition, the physicochemical properties of LNG and liquid hydrogen are similar. LNG and liquid hydrogen can be loaded and unloaded at the same port. Countries such as Australia are important exporters of both LNG and liquid hydrogen, which makes it possible to transport LNG and liquid hydrogen together.

[0006] Currently, captured CO2 can be treated by preparing high-value-added chemicals. Among these methods, CO2 hydrogenation to methanol technology is relatively mature, and the produced methanol is relatively stable and easy to store. If this technology can be applied to the CO2 treatment problem in the first BOG treatment method of the aforementioned ships, it will effectively solve the problems of high energy consumption for reliquefaction of captured CO2 and difficulty in ship storage.

[0007] Based on this, the present invention proposes a BOG (Boiled Air Gathering) handling system and method for LNG and liquid hydrogen combined transport vessels. The system utilizes a vessel simultaneously transporting LNG and liquid hydrogen. During navigation, a portion of the BOG in the LNG cargo tank is sent to the ship's main engine for combustion. The generated CO2 is captured and then chemically synthesized with the BOG in the liquid hydrogen cargo tank to form methanol. Methanol is easily stored on board the vessel, and when the ship is at berth, it is pumped ashore for recycling. The remaining BOG in the LNG cargo tank is reliquefied using the cold energy from the BOG in the liquid hydrogen cargo tank and returned to the LNG cargo tank. This method avoids the difficulties of CO2 reliquefaction and ship storage, and effectively handles the BOG from both LNG and liquid hydrogen on LNG-liquid hydrogen combined transport vessels, demonstrating high practical value. Summary of the Invention

[0008] The first objective of this invention is to address the aforementioned problems by proposing a BOG handling system for LNG and liquid hydrogen combined transport vessels, comprising: a BOG supply system, a methanol synthesis system, and a cold energy utilization system.

[0009] The BOG supply system includes: LNG cargo tank, LNG vapor valve, supply valve, supply compressor, and heating unit.

[0010] The methanol synthesis system includes: a liquid hydrogen cargo tank, a liquid hydrogen evaporation valve, a carbon capture unit, a methanol synthesis unit, a hydrogen compressor, a methanol valve, and a methanol storage tank.

[0011] The cold energy utilization system includes: LNG cargo tank, liquid hydrogen cargo tank, LNG vapor valve, liquid hydrogen vapor valve, hydrogen compressor, BOG heat exchanger, check valve, liquefaction compressor, separation unit, and return valve.

[0012] In the BOG supply system, the LNG cargo tank is connected in sequence to the LNG vapor valve, supply valve, supply compressor, heating unit, and ship main engine via pipelines.

[0013] In the methanol synthesis system, the ship's main engine is connected to the carbon capture unit and the methanol synthesis unit via pipelines; the liquid hydrogen cargo tank is connected in sequence to the liquid hydrogen evaporation valve, hydrogen compressor, BOG heat exchanger and methanol synthesis unit via pipelines; and the methanol synthesis unit is connected in sequence to the methanol valve and methanol storage tank via pipelines.

[0014] In the cold energy utilization system, the LNG cargo tank is connected in sequence to the LNG evaporation valve, check valve, liquefaction compressor, BOG heat exchanger and separation unit via pipelines. The bottom of the separation unit is connected in sequence to the return valve and LNG cargo tank via pipelines. The separation unit is also provided with a branch pipeline that connects to the pipeline between the check valve and the liquefaction compressor.

[0015] The second objective of this invention is to propose a BOG handling method for LNG and liquid hydrogen combined transport vessels, based on the aforementioned BOG handling system for such vessels. The purpose of this method is to handle the BOG of LNG and liquid hydrogen under different operating conditions in LNG and liquid hydrogen combined transport vessels.

[0016] During normal navigation, once the pressure in the LNG cargo tank reaches the set pressure of the LNG vaporizer valve, the valve opens, opening the supply valve. Boiled gas (BOG) from the LNG cargo tank passes through the LNG vaporizer valve and supply valve to the supply compressor. The supply compressor pressurizes the BOG and sends it to the heating unit, where it is heated before being supplied to the ship's main engine for combustion. The flue gas emitted by the ship's main engine enters the carbon capture unit, where CO2 is captured and separated. The captured CO2 is then sent to the methanol synthesis unit. Simultaneously, once the pressure in the liquid hydrogen cargo tank reaches the set pressure of the liquid hydrogen vaporizer valve, the valve opens. Liquid hydrogen-containing BOG, pressurized by the hydrogen compressor, passes through the BOG heat exchanger and enters the methanol synthesis unit. In the methanol synthesis unit, CO2 reacts with H2 to synthesize methanol. The methanol valve opens, and the synthesized methanol is transported to a methanol storage tank for storage. When the ship docks, the methanol in the storage tank is pumped ashore for recycling.

[0017] When a ship is about to arrive at port or encounters rough seas or other weather conditions, it operates at a low load, resulting in slower speeds and lower main engine power. The main engine cannot fully utilize the BOG (Boiled Gas) in the LNG cargo tank, necessitating the reliquefaction of the remaining BOG. ​​In this situation, the check valve is opened, allowing the BOG in the LNG cargo tank to pass through the LNG vapor valve and the check valve to the liquefaction compressor. The compressor pressurizes the BOG and sends it to the BOG heat exchanger, where it exchanges heat with the liquid hydrogen BOG passing through the heat exchanger. The resulting gas-liquid mixture enters the separation unit, and the return valve is opened. The liquefied BOG flows back to the LNG cargo tank through a pipe below the separation unit. Unliquefied BOG flows out of the separation unit and into the pipe between the check valve and the liquefaction compressor, mixing with the BOG in the LNG cargo tank. Under the action of the liquefaction compressor, it flows back into the BOG heat exchanger for further liquefaction.

[0018] The beneficial effects of this invention are:

[0019] 1. This invention ingeniously utilizes captured CO2 and hydrogen-based bulk gas (BOG) to produce methanol on LNG and liquid hydrogen combined transport vessels. This solves the problem of difficult BOG handling in liquid hydrogen cargo holds and also allows for the efficient processing of captured CO2, avoiding the high energy consumption and storage difficulties associated with CO2 reliquefaction.

[0020] 2. This invention utilizes the cold energy of liquid hydrogen BOG to reliquefy BOG in the LNG cargo tank when the ship is sailing under low load. This not only avoids the waste of the cold energy of liquid hydrogen BOG, but also solves the problem of BOG waste when the BOG in the LNG cargo tank cannot be completely burned by the ship's main engine. Attached Figure Description

[0021] Figure 1 System diagram of the invention;

[0022] Figure 2 Schematic diagram of the cargo hold layout on a ship;

[0023] In the attached diagram: 1. LNG cargo tank; 2. Liquid hydrogen cargo tank; 3. LNG vapor valve; 4. Liquid hydrogen vapor valve; 5. Supply valve; 6. Supply compressor; 7. Heating unit; 8. Carbon capture unit; 9. Methanol synthesis unit; 10. Hydrogen compressor; 11. BOG heat exchanger; 12. Methanol valve; 13. Methanol storage tank; 14. Check valve; 15. Liquefaction compressor; 16. Separation unit; 17. Return valve. Detailed Implementation

[0024] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be further described in detail below with reference to the accompanying drawings and embodiments.

[0025] A BOG handling system for LNG and liquid hydrogen combined transport vessels, such as Figure 1As shown, the system includes a BOG supply system, a methanol synthesis system, and a cold energy utilization system.

[0026] The BOG supply system includes: LNG cargo tank 1, LNG vapor valve 3, supply valve 5, supply compressor 6, and heating unit 7.

[0027] The methanol synthesis system includes: liquid hydrogen cargo tank 2, liquid hydrogen evaporation valve 4, carbon capture unit 8, methanol synthesis unit 9, hydrogen compressor 10, methanol valve 12, and methanol storage tank 13.

[0028] The cold energy utilization system includes: LNG cargo tank 1, liquid hydrogen cargo tank 2, LNG evaporation valve 3, liquid hydrogen evaporation valve 4, hydrogen compressor 10, BOG heat exchanger 11, check valve 14, liquefaction compressor 15, separation unit 16, and return valve 17.

[0029] In the BOG supply system, the LNG cargo tank 1 is connected in sequence to the LNG vapor valve 3, supply valve 5, supply compressor 6, heating unit 7, and ship main engine via pipelines.

[0030] In the methanol synthesis system, the ship's main engine is connected in sequence to the carbon capture unit 8 and the methanol synthesis unit 9 via pipelines. The liquid hydrogen cargo tank 2 is connected in sequence to the liquid hydrogen evaporation valve 4, the hydrogen compressor 10, the BOG heat exchanger 11, and the methanol synthesis unit 9 via pipelines. The methanol synthesis unit 9 is connected in sequence to the methanol valve 12 and the methanol storage tank 13 via pipelines. The LNG evaporation valve 3 and the liquid hydrogen evaporation valve 4 are set to a pressure of 0.1 bar. The methanol synthesis unit 9 can produce methanol from the CO2 captured by the ship and the BOG in the liquid hydrogen cargo tank under the action of a methanol synthesis catalyst.

[0031] In the cold energy utilization system, the LNG cargo tank 1 is connected in sequence via pipelines to the LNG evaporation valve 3, check valve 14, liquefaction compressor 15, BOG heat exchanger 11, and separation unit 16. The separation unit 16 is connected in sequence via pipelines to the return valve 17 and the LNG cargo tank 1. The separation unit 16 also has a bypass pipeline connected to the pipeline between the check valve 14 and the liquefaction compressor 15. The BOG heat exchanger 11, separation unit 16, and compressor 15 are connected in sequence via pipelines to form a closed loop. The check valve 14 prevents BOG backflow, and the separation unit 16 separates the gas-liquid mixture into liquid and gaseous states.

[0032] On an LNG-liquid hydrogen combined transport vessel, there are three LNG cargo tanks (1) and two liquid hydrogen cargo tanks (2). The LNG cargo tanks (1) and liquid hydrogen cargo tanks (2) are arranged alternately from bow to stern on the vessel, and are sequentially numbered as Cargo Tank NO.1, Cargo Tank NO.2, Cargo Tank NO.3, Cargo Tank NO.4, and Cargo Tank NO.5. Cargo Tanks NO.1, NO.3, and NO.5 are LNG cargo tanks (1), and Cargo Tanks NO.2 and NO.4 are liquid hydrogen cargo tanks (2). This arrangement aims to ensure a uniform overall mass distribution of the vessel, preventing large bending moments from occurring at any particular point on the hull when subjected to wind, waves, or other factors. Figure 2 As shown.

[0033] During normal navigation, due to the significant temperature difference between the cargo in LNG cargo tank 1 and liquid hydrogen cargo tank 2 and the external environment, even with insulation, some cargo will still evaporate and form bulk gas (BOG). As the ship sails, BOG is continuously generated in LNG cargo tank 1 and liquid hydrogen cargo tank 2. When the pressure in LNG cargo tank 1 reaches the set pressure of LNG vaporization valve 3, LNG vaporization valve 3 opens, opening supply valve 5. The BOG in LNG cargo tank 1 passes through LNG vaporization valve 3 and supply valve 5 to supply compressor 6. Supply compressor 6 pressurizes the BOG and sends it to heating unit 7. Heating unit 7 heats the BOG to the temperature required for the ship's main engine, and then sends it to the ship's main engine for combustion. The flue gas emitted by the ship's main engine enters carbon capture unit 8, where CO2 is captured and separated. The remaining exhaust gas is discharged into the atmosphere, and the captured CO2 is sent to methanol synthesis unit 9. Simultaneously, once the pressure inside the liquid hydrogen cargo tank 2 reaches the set pressure of the liquid hydrogen evaporation valve 4, the valve opens. The liquid hydrogen BOG, pressurized by the hydrogen compressor 10, passes through the BOG heat exchanger 11 and enters the methanol synthesis unit 9. In the methanol synthesis unit 9, CO2 reacts with the hydrogen BOG to synthesize methanol. The methanol valve 12 is then opened, and the synthesized methanol is transported to the methanol storage tank 13 for storage. When the ship docks, the methanol in the storage tank 13 is pumped ashore for recycling.

[0034] When a ship is about to arrive at port or encounters stormy weather, it sails at a low load. At this time, the ship's speed is slow and the power of the main engine is low. The main engine cannot fully consume the BOG in the LNG cargo tank 1. If the remaining BOG is directly discharged into the atmosphere or sent to the ship's gas combustion unit (GCU unit) for combustion, it will result in energy waste. Therefore, it is necessary to reliquefy and recycle this remaining BOG. At this time, check valve 14 is opened, and BOG in LNG cargo tank 1 reaches liquefaction compressor 15 through LNG vapor valve 3 and check valve 14. After being pressurized by liquefaction compressor 15, it is sent to BOG heat exchanger 11, where it exchanges heat with liquid hydrogen BOG passing through BOG heat exchanger 11. Part of the LNG BOG is liquefied and turns into liquid. After the heat exchange is completed, the gas-liquid mixture enters separation unit 16. Separation unit 16 separates the gas-liquid mixture and opens return valve 17. The liquefied BOG flows back to LNG cargo tank 1 through the pipe below separation unit 16. Unliquefied BOG enters the pipe between check valve 14 and liquefaction compressor 15, mixes with the BOG in LNG cargo tank 1, and flows back into BOG heat exchanger 11 for heat exchange and liquefaction under the action of liquefaction compressor 15.

[0035] Due to the high price of hydrogen and its extremely low liquefaction temperature, handling liquid hydrogen-based bulk cargo (BOG) on ships is very difficult. However, using CO2 hydrogenation to produce methanol to treat BOG in liquid hydrogen cargo tank 2 on ships, the produced methanol, as a high-value-added chemical, avoids the waste of liquid hydrogen energy. Furthermore, if CO2 captured by ships is reliquefied and stored, the reliquefaction process generates significant energy consumption, and liquefied CO2 requires high-pressure, low-temperature storage tanks, resulting in high storage costs. Methanol, on the other hand, has stable physical properties and low storage requirements on ships, requiring only ordinary storage tanks, making its storage cost on ships far lower than that of CO2. Therefore, this invention avoids the difficulties of CO2 reliquefaction and ship storage, and effectively handles BOG in liquid hydrogen cargo tank 2, possessing high practical value.

[0036] This invention can fully utilize the abundant cold energy of liquid hydrogen BOG. ​​Before the BOG in the liquid hydrogen cargo tank 2 is sent to the methanol synthesis unit 9, it exchanges heat with the BOG in the LNG cargo tank 1 in the BOG heat exchanger 11, so that the liquefied BOG of LNG is finally returned to the LNG cargo tank 1. This not only avoids the waste of the cold energy of liquid hydrogen BOG and greatly reduces the power consumption of LNG BOG reliquefaction, but also solves the problem that the BOG in the LNG cargo tank 1 cannot be completely used by the main engine.

[0037] Furthermore, since liquid hydrogen and LNG have similar physicochemical properties and can be loaded and unloaded at the same port, this invention utilizes an LNG-liquid hydrogen combined transport vessel to simultaneously transport LNG and liquid hydrogen. This requires minimal modifications to the ship's structure; simply arranging the LNG cargo tank 1 and the liquid hydrogen cargo tank 2 alternately is sufficient. Figure 2As shown. At the same time, relying on the weight of LNG itself, the overall center of gravity of the ship is lowered, and the ship does not need to carry a large amount of worthless ballast water, or even any ballast water at all, which has good economic benefits.

[0038] The above description is merely a preferred embodiment of the present invention. The specific embodiments described herein are only for explaining the present invention and are not intended to limit the present invention. It should be noted that those skilled in the art can make several improvements and modifications without departing from the principle of the present invention, and these improvements and modifications should also be considered within the scope of protection of the present invention.

Claims

1. A BOG handling system for a combined LNG and liquid hydrogen transport vessel, characterized in that: The system includes an LNG cargo tank (1), a liquid hydrogen cargo tank (2), an LNG vapor valve (3), a liquid hydrogen vapor valve (4), a supply valve (5), a supply compressor (6), a heating unit (7), a carbon capture unit (8), a methanol synthesis unit (9), a hydrogen compressor (10), a BOG heat exchanger (11), a methanol valve (12), a methanol storage tank (13), a check valve (14), a liquefaction compressor (15), a separation unit (16), and a return valve (17). The LNG cargo tank (1) is connected in sequence to the LNG vapor valve (3), supply valve (5), supply compressor (6), heating unit (7), ship main engine, carbon capture unit (8), and methanol synthesis unit (9) via pipelines. The liquid hydrogen cargo tank (2) is connected in sequence to the liquid hydrogen vapor valve (4), hydrogen compressor (10), BOG heat exchanger (11), and methanol synthesis unit (9) via pipelines. The methanol synthesis unit (9) is connected in sequence to the methanol valve (12) and methanol storage tank (13) via pipelines. The LNG evaporation valve (3) is connected in sequence to the check valve (14), the liquefaction compressor (15), the BOG heat exchanger (11), and the separation unit (16) via pipelines. The bottom of the separation unit (16) is connected in sequence to the return valve (17) and the LNG cargo tank (1) via pipelines. A side branch pipeline is also provided to connect the pipeline between the check valve (14) and the liquefaction compressor (15).

2. The BOG handling system for a combined LNG and liquid hydrogen transport vessel according to claim 1, characterized in that: The separation unit (16) separates the gas-liquid mixture into liquid and gas, with the liquid product flowing out through the lower pipe and the gaseous product flowing out through the side pipe.

3. A BOG handling system for a combined LNG and liquid hydrogen transport vessel according to claim 1, characterized in that: The LNG evaporation valve (3) and the liquid hydrogen evaporation valve (4) are set to a pressure of 0.1 bar.

4. A BOG handling method for a combined LNG and liquid hydrogen transport vessel based on the system described in claim 1, characterized in that: During normal navigation, the BOG in the LNG cargo tank (1) is supplied to the ship's main engine for combustion, providing propulsion for the ship's navigation. When a ship is about to arrive at port or encounters stormy weather, the BOG cold energy in the liquid hydrogen cargo tank (2) is used to reliquefy the remaining BOG in the LNG cargo tank (1) that has not been used by the ship's main engine.

Images