U.S. patent application number 15/091465 was filed with the patent office on 2016-07-28 for liquefied gas treatment method for vessel.
The applicant listed for this patent is DAEWOO SHIPBUILDING & MARINE ENGINEERING CO., LTD.. Invention is credited to Dong Kyu CHOI, Jeheon JUNG, Dong Chan KIM, Nam Soo KIM, Soon Been KWON, Joon Chae LEE, Young Sik MOON.
Application Number | 20160215929 15/091465 |
Document ID | / |
Family ID | 50544922 |
Filed Date | 2016-07-28 |
United States Patent
Application |
20160215929 |
Kind Code |
A1 |
LEE; Joon Chae ; et
al. |
July 28, 2016 |
LIQUEFIED GAS TREATMENT METHOD FOR VESSEL
Abstract
A liquefied gas treatment method for a vessel is performed by a
liquefied gas treatment system for the vessel including a cargo
tank storing LNG, and a main engine and a sub engine using the LNG
stored in the cargo tank as fuel. The liquefied gas treatment
system includes a compressor line configured to compress BOG
generated in the cargo tank by a compressor and supply the
compressed BOG to the engines as fuel, and a pump line configured
to compress the LNG stored in the cargo tank by a pump and supply
the compressed LNG to the engines as fuel. In a laden condition in
which an amount of the LNG stored in the cargo tank is larger than
in a ballast condition, the BOG generated in the cargo tank is
supplied as fuel to at least one of the engines through the
compressor line.
Inventors: |
LEE; Joon Chae; (Daegu,
KR) ; KWON; Soon Been; (Seoul, KR) ; CHOI;
Dong Kyu; (Seongnam-si, KR) ; MOON; Young Sik;
(Gwangmyeong-si, KR) ; KIM; Dong Chan; (Busan,
KR) ; JUNG; Jeheon; (Seoul, KR) ; KIM; Nam
Soo; (Suwon-si, KR) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
DAEWOO SHIPBUILDING & MARINE ENGINEERING CO., LTD. |
Seoul |
|
KR |
|
|
Family ID: |
50544922 |
Appl. No.: |
15/091465 |
Filed: |
April 5, 2016 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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14692639 |
Apr 21, 2015 |
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15091465 |
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PCT/KR2013/009542 |
Oct 24, 2013 |
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14692639 |
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Current U.S.
Class: |
1/1 |
Current CPC
Class: |
F17C 2221/033 20130101;
Y02T 70/50 20130101; F25J 1/023 20130101; F02M 21/0212 20130101;
F02M 21/023 20130101; F17C 13/082 20130101; Y02T 10/32 20130101;
F17C 7/04 20130101; B63B 25/14 20130101; F02M 21/0221 20130101;
F17C 2265/034 20130101; Y02T 10/30 20130101; F02M 21/0245 20130101;
F02M 21/0248 20130101; F17C 1/002 20130101; Y02T 90/40 20130101;
F17C 2270/0107 20130101; F02M 31/16 20130101; B63H 21/00 20130101;
F02M 21/0215 20130101; F17C 2201/0157 20130101; F17C 2265/066
20130101; B63B 25/08 20130101; F02M 21/0287 20130101; Y02T 90/46
20130101; F17C 7/02 20130101; F25J 1/0202 20130101; Y02T 10/12
20130101; F17C 2223/0161 20130101; F25J 1/0277 20130101; F17C
2201/052 20130101; F17C 2223/033 20130101; F17C 9/02 20130101; F25J
1/0042 20130101; Y02T 70/5209 20130101; B63H 21/38 20130101; Y02T
10/126 20130101; F02M 21/02 20130101; F25J 1/004 20130101; F17C
2265/037 20130101; F25J 1/0025 20130101; B63B 25/16 20130101 |
International
Class: |
F17C 7/04 20060101
F17C007/04; B63B 25/14 20060101 B63B025/14; B63B 25/16 20060101
B63B025/16 |
Foreign Application Data
Date |
Code |
Application Number |
Oct 24, 2012 |
KR |
10-2012-0118241 |
Dec 11, 2012 |
KR |
10-2012-0143522 |
Jun 26, 2013 |
KR |
10-2013-0073731 |
Claims
1. An LNG tank ship comprising: an LNG tank containing therein an
LNG composition in liquid phase and gas phase; a gas fuel
combustion device configured to consume gas phase LNG; a
supercritical fuel injection engine configured to consume
supercritical state LNG; a gas discharge port located inside the
LNG tank at a higher portion thereof and configured to discharge
gas phase LNG from the LNG tank; a gas-to-supercritical pathway for
processing gas phase LNG from the gas discharge port to generate
supercritical state LNG, the gas-to-supercritical pathway
comprising a gas phase LNG transfer line for receiving gas phase
LNG from the gas discharge port, a heat exchanger downstream of the
gas phase LNG transfer line, and a multi-stage compressor
downstream of the heat exchanger, the multi-stage compressor
comprising a plurality of serially connected compressors configured
to pressurize the gas phase LNG for generating supercritical state
LNG having a pressure of 150-400 bara; a supercritical-to-liquid
pathway for processing supercritical state LNG to generate liquid
phase LNG for returning to the LNG tank, the
supercritical-to-liquid pathway comprising a supercritical state
LNG transfer line for receiving supercritical state LNG from the
gas-to-supercritical pathway, an oil filter configured to filter
lubricant oil added to an LNG stream in the multi-stage compressor,
the heat exchanger downstream of the supercritical state LNG
transfer line, a decompressor downstream of the heat exchanger and
a LNG return line downstream of the decompressor; the heat
exchanger configured to heat-exchange between the gas phase LNG of
the gas-to-supercritical pathway and the supercritical state LNG of
the supercritical-to-liquid pathway such that the gas phase LNG of
the gas-to-supercritical pathway is heated while the supercritical
state LNG of the supercritical-to-liquid pathway is cooled
sufficient to form liquid phase LNG, wherein the LNG tank ship does
not comprise a refrigeration cycle of a coolant for cooling the
supercritical state LNG in the supercritical-to-liquid pathway; the
decompressor of the supercritical-to-liquid pathway, being
configured to depressurize the liquid phase LNG from the heat
exchanger; the LNG return line of the supercritical-to-liquid
pathway being in fluid communication with the LNG tank for
returning the depressurized liquid phase LNG to the LNG tank; the
multi-stage compressor of the gas-to-supercritical pathway being in
fluid communication with the supercritical fuel injection engine
for supplying at least part of the supercritical state LNG from the
multi-stage compressor to the supercritical fuel injection engine;
and a valve system configured to control connection of the
gas-to-supercritical pathway to the gas discharge port and
connection of the supercritical-to-liquid pathway to the
gas-to-supercritical pathway for selective operation of one or more
of the gas-to-supercritical pathway and the supercritical-to-liquid
pathway such that the supercritical-to-liquid pathway is to operate
only when the gas-to-supercritical pathway is operating to generate
supercritical state LNG.
2. The ship of claim 1, wherein the plurality of serially connected
compressors comprise at least one non-lubricated compressor
configured to operate without using lubricant oil and at least one
lubricated compressor configured to operate with lubricant oil, the
at least one lubricated compressor disposed downstream of the at
least one non-lubricated compressor such that the heated gas phase
LNG is pressurized by the at least one non-lubricated compressor
and further pressurized by the at least one lubricated compressor
for generating the supercritical LNG to which the lubricant oil is
added.
3. The ship of claim 2, wherein the multi-stage compressor is
configured such that the at least one non-lubricated compressor is
to operate for generating pressurized gas phase LNG which is to be
supplied to the gas fuel combustion device while not operating the
at least one lubricated compressor for generating supercritical
state LNG.
4. The ship of claim 3, wherein no oil filter is provided for
pressurized gas phase LNG generated by the at least one
non-lubricated compressor to be supplied to the gas fuel combustion
device.
5. The ship of claim 2, further comprising: a liquid discharge port
located inside the LNG tank at a lower portion thereof and
configured to discharge liquid phase LNG from the LNG tank; and a
liquid-to-supercritical pathway for processing liquid phase LNG
from the liquid discharge port to generate supercritical state LNG
for supplying to the supercritical fuel injection engine, the
liquid-to-supercritical pathway comprising a pump and a heater
downstream of the pump, wherein, in the liquid-to-supercritical
pathway, the pump is configured to pressurize liquid phase LNG from
the LNG tank to a pressure of 150-400 bara, wherein, in the
liquid-to-supercritical pathway, the heater is configured to heat
the pressurized liquid phase LNG for generating supercritical state
LNG, wherein the heater is in fluid communication with the
supercritical fuel injection engine for supplying at least part of
the supercritical state LNG to the supercritical fuel injection
engine, wherein the valve system is further configured to control
connection of the liquid-to-supercritical pathway to the liquid
discharge port for operation of the liquid-to-supercritical pathway
such that the multi-stage compressor of the gas-to-supercritical
pathway is not to operate for generating supercritical state LNG
while the liquid-to-supercritical pathway is operating to generate
supercritical state LNG.
6. The ship of claim 5, wherein the LNG tank ship is configured
such that while the liquid-to-supercritical pathway is operating to
generate supercritical state LNG, the at least one lubricated
compressor is not to operate for generating supercritical state LNG
but the at least one non-lubricated compressor is to operate for
generating pressurized gas phase LNG which is to be supplied to the
gas fuel combustion device.
7. The ship of claim 1, wherein the decompressor is configured to
provide a liquid-gas mixture of LNG, wherein the LNG return line is
configured to return the liquid-gas mixture to the LNG tank.
8. The ship of claim 1, wherein the decompressor is configured to
provide a liquid-gas mixture of LNG, wherein the
supercritical-to-liquid pathway further comprises a liquid-gas
separator downstream of the decompressor and upstream of the LNG
return line, wherein the liquid-gas separator is configured to
separate liquid phase LNG from the liquid-gas mixture and to supply
the separated liquid phase LNG to the LNG return line for returning
to the LNG tank.
9. The ship of claim 1, wherein the valve system is configured to
control connection of the gas-to-supercritical pathway to the gas
discharge port to selectively operate the gas-to-supercritical
pathway based on a rate of boil-off gas generation within the LNG
tank and further based on a rate of a total amount of LNG
combustion in the LNG tank ship.
10. The ship of claim 9, wherein the valve system is configured to
control connection of the gas-to-supercritical pathway to the gas
discharge port to selectively operate the gas-to-supercritical
pathway when the rate of boil-off gas generation within the LNG
tank is smaller than the rate of the total LNG combustion in the
LNG tank ship.
11. The ship of claim 1, wherein the valve system is configured to
control connection of the gas-to-supercritical pathway to the gas
discharge port to operate the gas-to-supercritical pathway only
during a laden voyage, wherein the valve system is configured to
control connection of the supercritical-to-liquid pathway to the
gas-to-supercritical pathway to operate the supercritical-to-liquid
pathway only during a laden voyage.
12. The ship of claim 1, wherein the gas fuel combustion device
comprises an engine configured to run on gas phase LNG, wherein the
supercritical fuel injection engine comprises an MEGI engine.
13. The ship of claim 1, wherein the LNG tank ship comprises only
one unit of the multi-stage compressor for the gas-to-supercritical
pathway and does not comprise a backup multi-stage compressor for
the gas-to-supercritical pathway.
14. A method of processing LNG in the LNG tank ship of claim 1, the
method comprising: running the gas-to-supercritical pathway, which
comprises the multi-stage compressor comprising the plurality of
serially connected compressors, which comprise at least one
non-lubricated compressor and at least one lubricated compressor
which are disposed, wherein gas phase LNG discharged from the LNG
tank is pressurized by the at least one non-lubricated compressor
and further pressurized by the at least one lubricated compressor
for generating supercritical LNG having a pressure of 150-400 bara,
wherein lubricant oil is added to an LNG stream in the multi-stage
compressor; and running the supercritical-to-liquid pathway, which
comprises cooling supercritical state LNG to form liquid phase LNG,
subsequently depressurizing the liquid phase LNG, and returning at
least part of the liquid phase LNG to the LNG tank, wherein, in the
supercritical-to-liquid pathway, the lubricant oil added to the
supercritical LNG is filtered with the oil filter.
15. The method of claim 14, wherein the at least one non-lubricated
compressor operates for generating pressurized gas phase LNG which
is to be supplied to the gas combustion device while the at least
one lubricated compressor is idle.
16. The method of claim 15, wherein the pressurized gas phase LNG
has a pressure of 6 bara to 10 bara.
17. The method of claim 15, wherein no oil filter is provided for
pressurized gas phase LNG generated by the at least one
non-lubricated compressor for supplying to the gas fuel combustion
device.
18. The method of claim 14, wherein the multi-stage compressor
operates based on a rate of boil-off gas generation within the LNG
tank and further based on a rate of fuel consumption at the
supercritical fuel injection engine.
19. The method of claim 18, wherein when boil-off gas generation is
smaller than fuel consumption at the supercritical fuel injection
engine, the at least one lubricated compressor is not operating to
generate supercritical state LNG but the at least non-lubricated
compressor is operating to generate pressurized gas phase LNG.
20. The method of claim 18, wherein the multi-stage compressor does
not operate when the rate of boil-off gas generation within the LNG
tank is smaller than the rate of fuel consumption at the
supercritical fuel injection engine.
Description
INCORPORATION BY REFERENCE TO ANY PRIORITY APPLICATIONS
[0001] Any and all applications for which a foreign or domestic
priority claim is identified in the Application Data Sheet as filed
with the present application are hereby incorporated by reference
under 37 CFR 1.57.
TECHNICAL FIELD
[0002] The present disclosure relates to a liquefied gas treatment
method for a vessel.
BACKGROUND ART
[0003] Recently, the consumption of liquefied gas, such as
liquefied natural gas (LNG) or liquefied petroleum gas (LPG), has
been rapidly increasing throughout the world. Liquefied gas is
transported in a gas state through onshore or offshore gas
pipelines, or is transported to a remote consumption place while
being stored in a liquefied state inside a liquefied gas carrier.
Liquefied gas, such as LNG or LPG, is obtained by cooling natural
gas or petroleum gas to a cryogenic temperature (in the case of
LNG, about -163.degree. C.). Since the volume of liquefied gas is
considerably reduced as compared to a gas state, liquefied gas is
very suitable for a long-distance marine transportation.
[0004] A liquefied gas carrier such as an LNG carrier is designed
to load liquefied gas, sail across the sea, and unload the
liquefied gas at an onshore consumption place. To this end, the
liquefied gas carrier includes a storage tank (also called "cargo
tank") that can withstand a cryogenic temperature of liquefied
gas.
[0005] Examples of a marine structure provided with a cargo tank
capable of storing cryogenic liquefied gas may include vessels such
as a liquefied gas carrier and an LNG Regasification Vessel (LNG
RV), or structures such as an LNG Floating Storage and
Regasification Unit (LNG FSRU) and an LNG Floating, Production,
Storage and Off-loading (LNG FPSO), and a Barge Mounted Power Plant
(BMPP).
[0006] The LNG RV is a self-propelled, floatable liquefied gas
carrier equipped with an LNG regasification facility, and the LNG
FSRU is a marine structure that stores LNG unloaded from an LNG
carrier on the sea far away from the land and, if necessary,
supplies the LNG to an onshore consumption place by gasifying the
LNG. The LNG FPSO is a marine structure that refines extracted LNG
on the sea, stores the LNG in a storage tank after direct
liquefaction, and, if necessary, transships the LNG to an LNG
carrier. The BMPP is a structure that is equipped with a power
generation facility to produce electricity on the sea.
[0007] The term "vessel" as used herein is a concept including a
liquefied gas carrier such as an LNG carrier, an LNG RV, and
structures such as an LNG FPSO, an LNG FSRU, and a BMPP.
[0008] Since the liquefaction temperature of natural gas is a
cryogenic temperature of -163.degree. C. at ambient pressure, LNG
is likely to be vaporized even when the temperature of LNG is
slightly higher than -163.degree. C. at ambient pressure. In the
case of a conventional LNG carrier, even though an LNG cargo tank
is thermally insulated, external heat is continuously transferred
to LNG. Therefore, during the transportation of LNG by the LNG
carrier, LNG is continuously vaporized within the LNG cargo tank
and boil-off gas (hereinafter, referred to as BOG) is generated
within the LNG cargo tank.
[0009] The generated natural gas may increase the inside pressure
of the cargo tank and accelerate the flow of the natural gas due to
the rocking of the vessel. Therefore, it is necessary to suppress
the generation of BOG
[0010] In order to suppress the generation of BOG within the cargo
tank of the liquefied gas carrier, a method of discharging the BOG
from the cargo tank and burning the BOG, a method of discharging
the BOG from the cargo tank, reliquefying the BOG through a
reliquefaction apparatus, and returning the BOG to the cargo tank,
a method of using the BOG as fuel for a vessel's propulsion engine,
and a method of suppressing the generation of BOG by maintaining an
inside pressure of a cargo tank at a high level have been used
solely or in combination.
[0011] In a vessel equipped with a BOG reliquefaction apparatus,
BOG inside a cargo tank is discharged from the cargo tank and then
reliquefied through a reliquefaction apparatus in order to maintain
a pressure of the cargo tank at an appropriate level. In this case,
the discharged BOG is reliquefied through heat exchange with a
refrigerant (for example, nitrogen, mixed refrigerant, or the like)
cooled to a cryogenic temperature in the reliquefaction apparatus
including a refrigeration cycle, and the reliquefied BOG is
returned to the cargo tank.
[0012] In an LNG carrier equipped with a DFDE propulsion system,
BOG is consumed in such a manner that it is supplied as fuel to the
DFDE after treating BOG by only a BOG compressor and heating,
without installing the reliquefaction facility. Therefore, when an
amount of fuel necessary for an engine is smaller than a generation
amount of BOG, BOG is burnt in a gas combustion unit (GCU) or is
vented to atmosphere.
[0013] Even though an LNG carrier equipped with a reliquefaction
facility and a low-speed diesel engine can treat BOG through the
reliquefaction facility, the control of the entire system is
complicated due to the operation complexity of the reliquefaction
facility using nitrogen gas, and a considerable amount of power is
consumed.
[0014] Consequently, there is a need for continuous research and
development of systems and methods for efficiently treating
liquefied gas, including BOG generated naturally from the cargo
tank.
SUMMARY
[0015] One aspect of the present invention is directed to a
liquefied gas treatment system and method for a vessel, which
includes a cargo tank storing LNG, and an engine supplied with the
LNG stored in the cargo tank and using the LNG as fuel, wherein the
BOG generated in the cargo tank and the LNG stored in the cargo
tank are used in the engine as fuel, thereby achieving the
efficient use of liquefied gas.
[0016] Another aspect of the present invention provides a liquefied
gas treatment method for a vessel, which is performed by a
liquefied gas treatment system for the vessel including a cargo
tank storing liquefied natural gas (LNG), and a main engine and a
sub engine using the LNG stored in the cargo tank as fuel, the
liquefied gas treatment system including a compressor line
configured to compress BOG generated in the cargo tank by a
compressor and supply the compressed BOG to the main engine and the
sub engine as fuel, and a pump line configured to compress the LNG
stored in the cargo tank by a pump and supply the compressed LNG to
the main engine and the sub engine as fuel, the liquefied gas
treatment method including: supplying the BOG generated in the
cargo tank to at least one of the main engine and the sub engine as
fuel through the compressor line in a laden condition in which an
amount of the LNG stored in the cargo tank is larger than in a
ballast condition.
[0017] In the ballast condition, the LNG stored in the cargo tank
may be supplied as fuel to the main engine and the sub engine
through the pump line.
[0018] In the ballast condition, the BOG generated in the cargo
tank may be supplied as fuel to one of the main engine and the sub
engine through the compressor line.
[0019] In the ballast condition, the BOG generated in the cargo
tank may be supplied as fuel to the sub engine through the
compressor line, and the LNG stored in the cargo tank may be
supplied as fuel to the main engine through the pump line.
[0020] In the ballast condition, the BOG generated in the cargo
tank may be intermittently supplied as fuel to at least one of the
main engine and the sub engine through the compressor line, and
when the BOG is not supplied to at least one of the main engine and
the sub engine, the LNG stored in the cargo tank may be supplied as
fuel to at least one of the main engine and the sub engine through
the pump line.
[0021] In the ballast condition, the BOG generated in the cargo
tank and the LNG stored in the cargo tank may be simultaneously
supplied as fuel to the main engine and the sub engine.
[0022] The compressor may include a plurality of compression
cylinders, and the BOG generated in the cargo tank may be
compressed by a part of the plurality of compression cylinders and
is supplied as fuel to the sub engine.
[0023] The BOG generated in the cargo tank and forcibly vaporized
LNG may be supplied to and compressed by the compressor and are
supplied as fuel to at least one of the main engine and the sub
engine.
[0024] When the LNG stored in the cargo tank is supplied to the sub
engine, a heavy hydrocarbon component may be separated from the LNG
so as to adjust a methane number of the LNG to a value necessary
for the sub engine.
[0025] BOG, which is not supplied as fuel to the main engine and
the sub engine among BOG compressed by the compressor, may be
liquefied by exchanging heat with BOG that is discharged from the
cargo tank and transferred to the compressor.
[0026] According to embodiments of the present invention, all BOG
generated during the transportation of cargo (including LNG) in the
LNG carrier can be used as the fuel of the engine, or may be
reliquefied, be returned to the cargo tank and be stored therein.
Therefore, an amount of BOG consumed in the GCU or the like can be
reduced or removed. Furthermore, BOG can be treated by
reliquefaction, without using separate refrigerants such as
nitrogen.
[0027] Therefore, according to the liquefied gas treatment system
and method of embodiments of the present invention, BOG generated
from the cargo tank can be reliquefied without installing a
reliquefaction apparatus consuming a large amount of energy and
requiring excessive initial installation cost, thereby saving
energy consumed in the reliquefaction apparatus.
[0028] In addition, according to the liquefied gas treatment system
and method of the present invention, a part of compressed BOG after
pressurizing BOG discharged from a cargo tank can be supplied to a
high pressure gas injection engine (for example, in a propulsion
system) as fuel. The remaining compressed BOG can be cooled with
cold energy of BOG after discharge from the cargo and before
compression, and returned to the cargo tank.
[0029] In addition, in the liquefied gas treatment system and
method according to embodiments of the present invention, since it
is unnecessary to install the reliquefaction apparatuses using
separate refrigerants (for example, nitrogen-refrigerant
refrigeration cycle, mixed-refrigerant refrigeration cycle, or the
like), facilities for supplying and storing the refrigerants need
not be separately installed. Consequently, it is possible to save
initial installation cost and operation cost for configuring the
entire system.
[0030] In addition, according to the liquefied gas treatment system
and method of embodiments of the present invention, when BOG cooled
and liquefied in the heat exchanger after compression is
decompressed by the expander, wasted energy can be reused because
energy can be generated during expansion
BRIEF DESCRIPTION OF THE DRAWINGS
[0031] FIG. 1 is a schematic configuration diagram illustrating a
liquefied gas treatment system for a vessel according to a first
embodiment of the present invention.
[0032] FIG. 2 is a schematic configuration diagram illustrating a
liquefied gas treatment system for a vessel according to a second
embodiment of the present invention.
[0033] FIGS. 3 and 4 are schematic configuration diagrams
illustrating liquefied gas treatment systems for a vessel according
to modifications of the second embodiment of the present
invention.
[0034] FIG. 5 is a schematic configuration diagram illustrating a
liquefied gas treatment system for a vessel according to a third
embodiment of the present invention.
[0035] FIG. 6 is a schematic configuration diagram illustrating a
liquefied gas treatment system for a vessel according to a fourth
embodiment of the present invention.
[0036] FIGS. 7 and 8 are schematic configuration diagrams
illustrating liquefied gas treatment systems for a vessel according
to modifications of the fourth embodiment of the present
invention.
[0037] FIG. 9 is a schematic configuration diagram illustrating a
liquefied gas treatment system for a vessel according to a fifth
embodiment of the present invention.
[0038] FIGS. 10 to 12 are schematic configuration diagrams
illustrating liquefied gas treatment systems for a vessel according
to modifications of the fifth embodiment of the present
invention.
[0039] FIG. 13 is a schematic configuration diagram illustrating a
liquefied gas treatment system for a vessel according to a sixth
embodiment of the present invention.
DETAILED DESCRIPTION OF EMBODIMENTS
[0040] Embodiments of the present invention will be described below
in detail with reference to the accompanying drawings. These
embodiments are provided so that this disclosure will be thorough
and complete, and will fully convey the scope of the invention to
those skilled in the art. The invention may, however, be embodied
in many different forms and should not be construed as being
limited to the embodiments set forth herein. Throughout the
drawings and description, like reference numerals will be used to
refer to like elements.
[0041] The International Maritime Organization (IMO) regulates the
emission of nitrogen oxides (NOx) and sulfur oxides (SOx) among
exhaust gases of ships and also tries to regulate the emission of
carbon dioxide (CO.sub.2). In particular, the issue of the
regulation of nitrogen oxides (NOx) and sulfur oxides (SOx) was
raised by the Prevention of Marine Pollution from Ships (MARPOL)
protocol in 1997. After eight long years, the protocol met
effectuation requirements and entered into force in May 2005.
Currently, the regulation is in force as a compulsory
provision.
[0042] Therefore, in order to meet such a provision, a variety of
methods have been introduced to reduce the emission of nitrogen
oxides (NOx). As one of these methods, a high-pressure natural gas
injection engine for an LNG carrier, for example, an MEGI engine,
has been developed and used. As compared with the diesel engine of
the same power, the MEGI engine can reduce the emission of
pollutants (carbon dioxide: 23%, nitrogen compound: 80%, sulfur
compound: 95% or more). Hence, the MEGI engine is considered as an
environment-friendly next-generation engine.
[0043] Such an MEGI engine may be installed in a vessel such as an
LNG carrier which transports LNG while storing the LNG in a storage
tank capable of withstanding a cryogenic temperature. The term
"vessel" as used herein includes an LNG carrier, an LNG RV, and
offshore plants such as an LNG FPSO and an LNG FSRU. In this case,
the MEGI engine uses natural gas as fuel and requires a high
pressure of about 150 to 400 bara (absolute pressure) for gas
supply, depending on a load thereof.
[0044] The MEGI can be directly connected to the propeller for
propulsion. To this end, the MEGI engine is provided with a
2-stroke engine rotating at a low speed. In embodiments, the MEGI
engine is a low-speed 2-stroke high pressure natural gas injection
engine.
[0045] In addition, in order to reduce the emission of nitrogen
oxide, a DF engine (for example, DFDG: dual fuel diesel generator)
using a mixture of diesel oil and natural gas as fuel has been
developed and used for propulsion or power generation. The DF
engine is an engine that can burn a mixture of oil and natural gas,
or can selectively use one of oil and natural gas as fuel. Since a
content of sulfur is smaller than that in the case where only oil
is used as fuel, a content of sulfur oxide is small in exhaust
gas.
[0046] The DF engine need not supply fuel gas at a high pressure
like the MEGI engine, and has only to supply fuel gas after
compressing it to about several bar to several tens bara. The DF
engine obtains power by driving a power generator through the
driving force of the engine. This power can be used to drive a
propulsion motor or operate various apparatuses or facilities.
[0047] When supplying natural gas as fuel, it is unnecessary to
match the methane number in the case of the MEGI engine, but it is
necessary to match the methane number in the case of the DF
engine.
[0048] If LNG is heated, methane component having a relatively low
liquefaction temperature is preferentially vaporized. Hence, since
a methane content of BOG is high, the BOG can be directly supplied
as fuel to the DF engine. However, since the methane content of the
LNG is relatively lower than that of the BOG, the methane number of
the LNG is lower than the methane number required in the DF engine.
Ratios of hydrocarbon components (methane, ethane, propane, butane,
and the like) constituting the LNG are different according to
producing areas. Therefore, it is not suitable to vaporize the LNG
as it is and then supply the vaporized LNG to the DF engine as
fuel.
[0049] In order to adjust the methane number, the heavy hydrocarbon
(HHC) component having a higher liquefaction point than methane can
be liquefied and removed by forcibly vaporizing the LNG and
lowering the temperature of the LNG. After the methane number is
adjusted, it is possible to additionally heat natural gas whose
methane number is adjusted according to the temperature condition
required in the engine.
[0050] Hereinafter, configurations and operations of embodiments of
the present invention will be described in detail with reference to
the accompanying drawings. In addition, the following embodiments
can be modified in various forms and are not intended to limit the
scope of the present invention.
[0051] FIG. 1 is a configuration diagram illustrating a liquefied
gas treatment system for a vessel according to a first embodiment
of the present invention. The liquefied gas treatment system of the
present embodiment may be applied to an LNG carrier equipped with
an MEGI engine as a main propulsion engine (that is propulsion
means using LNG as fuel).
[0052] Referring to FIG. 1, the liquefied gas treatment system 100
according to the present embodiment includes a fuel supply line 110
and a BOG line 140. The fuel supply line 110 is configured to
provide a passage for transferring LNG from a cargo tank 1 to a
main engine 3 as a propulsion system. The BOG line 140 is
configured to provide a passage for transferring BOG generated from
the cargo tank 1 to the main engine 3. In addition, the liquefied
gas treatment system 100 using BOG according to the present
embodiment supplies LNG to the main engine 3 as fuel through the
fuel supply line 110 by an LNG pump 120 and an LNG vaporizer 130,
supplies BOG to the main engine 3 as fuel through the BOG line 140
after compressing the BOG by a BOG compressor 150, and supplies
surplus BOG from the BOG compressor 150 to an integrated inert gas
generator/gas combustion unit (IGG/GCU) system 200.
[0053] An MEGI engine usable as the main engine 3 needs to be
supplied with fuel at a high pressure of about 150 to 400 bara
(absolute pressure). Therefore, as the LNG pump 120 and the BOG
compressor 150 according to the present embodiment, a high pressure
pump and a high pressure compressor are used which can compress LNG
and BOG to a pressure necessary for the MEGI engine,
respectively.
[0054] The fuel supply line 110 provides a passage through which
LNG supplied from the LNG cargo tank 1 by the driving of a transfer
pump 2 is transferred to the main engine 3 as fuel, and the LNG
pump 120 and the LNG vaporizer 130 are installed therein.
[0055] The LNG pump 120 is installed in the fuel supply line 110 to
provide a pumping force necessary for transferring the LNG. As an
example of the LNG pump 120, an LNG high pressure (HP) pump may be
used. Like the present embodiment, a plurality of LNG pumps 120 may
be installed in parallel.
[0056] The LNG vaporizer 130 is installed at a rear end of the LNG
pump 120 in the fuel supply line 110 and vaporizes LNG transferred
by the LNG pump 120. As an example, LNG is vaporized by heat
exchange with a heat medium circulated and supplied through a heat
medium circulation line 131. As another example, a variety of
heating means, including heaters, may be used for providing a
vaporization heat of LNG. In addition, the LNG vaporizer 130 may
use a high pressure (HP) vaporizer that can be used at a high
pressure for vaporization of LNG. Meanwhile, as an example of the
heat medium circulated and supplied through the heat medium
circulation line 131, steam generated from a boiler or the like may
be used.
[0057] The BOG line 140 provides a passage for transferring BOG
naturally generated from the cargo tank 1 to the main engine 3.
Like the present embodiment, the BOG line 140 is connected to the
fuel supply line 110 to supply BOG to the main engine 3 as fuel.
Alternatively, the BOG line 140 may provide a passage for directly
supplying BOG to the main engine 3.
[0058] The BOG compressor 150 is installed on the BOG line 140 to
compress BOG passing through the BOG line 140. Although only one
BOG compressor 150 is illustrated in FIG. 1, the system may be
configured such that two BOG compressors of the same specification
are connected in parallel so as to satisfy redundancy requirements
just like the general fuel supply systems. However, like the
present embodiment, when a single BOG compressor 150 is installed
in a branched portion of a surplus BOG line 160 in the BOG line
140, it is possible to obtain additional effects of reducing
burdens on costs for installation of the expensive BOG compressor
150 and burdens on maintenance.
[0059] The surplus BOG line 160 provides a passage for supplying
surplus BOG from the BOG compressor 150 to an integrated IGG/GCU
system 200. The surplus BOG line 160 can supply surplus BOG as fuel
to an auxiliary engine, such as a DF engine, as well as the
integrated IGG/GCU system 200.
[0060] The integrated IGG/GCU system 200 is a system in which an
IGG and a GCU are integrated.
[0061] Meanwhile, the surplus BOG line 160 and the fuel supply line
110 may be connected together by a connection line 170. Therefore,
due to the connection line 170, surplus BOG can be used as the fuel
of the main engine 3, or vaporized LNG can be used as the fuel of
the integrated IGG/GCU system 200. A heater 180 may be installed in
the connection line 170 so as to heat BOG or vaporized LNG passing
therethrough, and a pressure reduction valve (PRV) 190 may be
installed to reduce excessive pressure by adjusting a pressure
caused by BOG or vaporized LNG. Meanwhile, the heater 180 may be a
gas heater using combustion heat of gas. Also, the heater 180 may
use a variety of heating means, including a heat medium
circulation/supply unit providing a heat source for heating by the
circulation of the heat medium.
[0062] The operation of the liquefied gas treatment system
according to the first embodiment of the present invention will be
described below.
[0063] When a pressure inside the cargo tank 1 is equal to or
higher than a set pressure or a large amount of BOG is generated,
BOG is compressed by the driving of the BOG compressor 150 and is
then supplied as fuel to the main engine 3. In addition, when the
pressure inside the cargo tank 1 is lower than the set pressure or
a small amount of BOG is generated, LNG is transferred and
vaporized by the driving of the LNG pump 120 and the LNG vaporizer
130 and is then supplied as fuel to the main engine 3.
[0064] Meanwhile, surplus BOG from the BOG compressor 150 is
supplied to the integrated IGG/GCU system 200 or the auxiliary
engine such as the DF engine through the surplus BOG line 160. The
surplus BOG is consumed or is used for generating inert gas for
supply to the cargo tank 1. Furthermore, the surplus BOG may be
used as the fuel of the auxiliary engine or the like.
[0065] The integrated IGG/GCU system 200 supplied with BOG may
consume BOG continuously generated from the cargo tank 1 by BOG
combustion inside a main body 210 and may, if necessary, generate
combustion gas as inert gas for supply to the cargo tank 1.
[0066] FIG. 2 is a schematic configuration diagram illustrating a
liquefied gas treatment system for a vessel according to a second
embodiment of the present invention.
[0067] Although FIG. 2 illustrates an example in which the
liquefied gas treatment system of embodiments of the present
invention is applied to an LNG carrier equipped with a high
pressure natural gas injection engine capable of using natural gas
as fuel (that is propulsion means using LNG as fuel), the liquefied
gas treatment system of embodiments of the present invention can
also be applied to any type of vessels (LNG carrier, LNG RV, and
the like) and marine plants (LNG FPSO, LNG FSRU, BMPP, and the
like), in which a liquefied gas cargo tank is installed.
[0068] In the liquefied gas treatment system for the vessel
according to the second embodiment of the present invention, NBOG
generated and discharged from a cargo tank 11 storing liquefied gas
is transferred along a BOG supply line L1, is compressed in a
compressor 13, and is then supplied to the high pressure natural
gas injection engine, for example, an MEGI engine. The BOG is
compressed at a high pressure of about 150 to 400 bara by the
compressor 13 and is then supplied as fuel to the high pressure
natural gas injection engine, for example, the MEGI engine.
[0069] The cargo tank 11 has sealing and heat-insulating walls so
as to store liquefied gas such as LNG in a cryogenic state, but
cannot perfectly block heat transferred from the outside.
Therefore, the liquefied gas is continuously vaporized within the
cargo tank 11. In order to maintain the pressure of the BOG at an
appropriate level, BOG is discharged from the cargo tank 11 through
the BOG discharge line.
[0070] A discharge pump 12 is installed within the cargo tank 11 so
as to discharge LNG to the outside of the cargo tank when
necessary.
[0071] The compressor 13 may include one or more compression
cylinders 14 and one or more intercoolers 15 for cooling BOG of
which the temperature is raised. The compressor 13 may be
configured to compress BOG to, for example, about 400 bara.
Although FIG. 2 illustrates the multi-stage compressor 13 including
five compression cylinders 14 and five intercoolers 15, the number
of the compression cylinders and the number of the intercoolers may
be changed when necessary. In addition, a plurality of compression
cylinders may be arranged within a single compressor, and a
plurality of compressors may be connected in series.
[0072] BOG compressed in the compressor 13 is supplied to the high
pressure natural gas injection engine through the BOG supply line
L1. All or part of the compressed BOG may be supplied to the high
pressure natural gas injection engine according to an amount of
fuel necessary for the high pressure natural gas injection
engine.
[0073] In addition, according to the embodiment of the present
invention, when BOG discharged from the cargo tank 11 and
compressed in the compressor 13 (for example, all BOG discharged
from the cargo tank) is a first stream, the first stream of the BOG
may be divided into a second stream and a third stream after
compression. The second stream may be supplied as fuel to the high
pressure natural gas injection engine, and the third stream may be
liquefied and returned to the cargo tank.
[0074] At this time, the second stream is supplied to the high
pressure natural gas injection engine through the BOG supply line
L1. When necessary, the second stream may be supplied as fuel
through a line (for example, the BOG supply line L1) connected to
the high pressure natural gas injection engine after passing
through all of the plurality of compression cylinders 14 included
in the compressor 13, or may be supplied as fuel through a line
(for example, the BOG branch line L8) connected to the DF engine
after passing through a part of the plurality of compression
cylinders 14 included in the compressor 13.
[0075] The third stream is returned to the cargo tank 11 through
the BOG return line L3. A heat exchanger 21 is installed in the BOG
return line L3 so as to cool and liquefy the third stream. In the
heat exchanger 21, the third stream of the compressed BOG exchanges
heat with the first stream of the BOG discharged from the cargo
tank 11 and then supplied to the compressor 13.
[0076] Since a flow rate of the first stream of the BOG before
compression is larger than a flow rate of the third stream, the
third stream of the compressed BOG may be liquefied by receiving
cold energy from the first stream of the BOG before compression. As
such, in the heat exchanger 21, the BOG of the high pressure state
is cooled and liquefied by heat exchange between the BOG of the
cryogenic temperature immediately after being discharged from the
cargo tank 11 and the BOG of the high pressure state compressed in
the compressor 13.
[0077] The LBOG cooled in the heat exchanger 21 and liquefied at
least partially is decompressed while passing through an expansion
valve 22 serving as decompressing means, and is supplied to a
gas-liquid separator 23 in a gas-liquid mixed state. The LBOG may
be decompressed to approximately atmospheric pressure (for example,
decompressed from 300 bar to 3 bar) while passing through the
expansion valve 22. The liquefied BOG is separated into gas and
liquid components in the gas-liquid separator 23. The liquid
component, that is, LNG, is transferred to the cargo tank 11
through the BOG return line L3, and the gas component, that is,
BOG, is discharged from the cargo tank 11 through a BOG
recirculation line L5 and is joined with BOG supplied to the
compressor 13. More specifically, the BOG recirculation line L5
extends from an upper end of the gas-liquid separator 23 and is
connected to a more upstream side than the heat exchanger 21 in the
BOG supply line L1.
[0078] In order to smoothly return the decompressed BOG to the
cargo tank 11 and smoothly join the gas component of the
decompressed BOG to the BOG supply line L1 through the BOG
recirculation line L5, it is advantageous that the pressure of the
BOG after being decompressed by the decompressing means is set to
be higher than the inside pressure of the cargo tank 11.
[0079] For convenience of explanation, it has been described that
the heat exchanger 21 is installed in the BOG return line L3, but
the heat exchanger 21 may be installed in the BOG supply line L1
because the heat exchange is actually performed between the first
stream of the BOG transferred through the BOG supply line L1 and
the third stream of the BOG transferred through the BOG return line
L3.
[0080] Another expansion valve 24 may be further installed in the
BOG recirculation line L5. Therefore, the gas component discharged
from the gas-liquid separator 23 may be decompressed while passing
through the expansion valve 24. In addition, a cooler 25 is
installed in the BOG recirculation line L5 so as to further cool
the third stream by heat exchange between the third stream of the
BOG liquefied in the heat exchanger 21 and supplied to the
gas-liquid separator 23 and the gas component separated from the
gas-liquid separator 23 and transferred through the BOG
recirculation line L5. That is, the cooler 25 additionally cools
the BOG of a high pressure liquid state to natural gas of a low
pressure cryogenic gas state.
[0081] For convenience of explanation, it has been described that
the cooler 25 is installed in the BOG recirculation line L5, but
the cooler 25 may be installed in the BOG return line L3 because
the heat exchange is actually performed between the third stream of
the BOG transferred through the BOG return line L3 and the gas
component transferred through the BOG recirculation line L5.
[0082] Although not illustrated, according to a modification of the
present embodiment, the system may be configured such that the
cooler 25 is omitted. If the cooler 25 is not installed, the total
efficiency of the system may be slightly lowered. However, the pipe
arrangement and the system operation may be facilitated, and the
initial installation cost and the maintenance fee may be
reduced.
[0083] Meanwhile, when it is expected that surplus BOG will be
generated because an amount of BOG generated from the cargo tank 11
is larger than an amount of fuel necessary for the high pressure
natural gas injection engine, BOG having been compressed or being
compressed stepwise in the compressor 13 is branched through the
BOG branch lines L7 and L8 and is then used in BOG consuming means.
Examples of the BOG consuming means may include a GCU, a DF
generator (DFDG), and a gas turbine, each of which can use natural
gas having a relatively lower pressure than the MEGI engine as
fuel. At the middle stage of the compressor 13, the pressure of the
BOG branched through the BOG branch lines L7 and L8 may be about 6
to 10 bara.
[0084] As described above, in the liquefied gas treatment system
and method according to the embodiment of the present invention,
BOG generated during the transportation of cargo (including LNG) in
the LNG carrier may be used as the fuel of the engine, or may be
reliquefied, be returned to the cargo tank and be stored therein.
Therefore, an amount of BOG consumed in the GCU or the like can be
reduced or removed. Furthermore, BOG can be treated by
reliquefaction, without installing reliquefaction apparatuses using
separate refrigerants such as nitrogen.
[0085] In addition, in the liquefied gas treatment system and
method according to the embodiment of the present invention, since
it is unnecessary to install the reliquefaction apparatuses using
separate refrigerants (for example, nitrogen-refrigerant
refrigeration cycle, mixed-refrigerant refrigeration cycle, or the
like), facilities for supplying and storing the refrigerants need
not be separately installed. Consequently, it is possible to save
initial installation cost and operation cost for configuring the
entire system.
[0086] Although FIG. 2 illustrates the example in which the BOG
return line L3 for supplying the compressed BOG to the heat
exchanger 21 is branched at the rear end of the compressor 13, the
BOG return line L3 may be installed to branch the BOG being
compressed stepwise in the compressor 13, like the above-described
BOG branch lines L7 and L8. FIG. 3 illustrates a modification in
which 2-stage compressed BOG is branched by two cylinders, and FIG.
4 illustrates a modification in which 3-stage compressed BOG is
branched by three cylinders. At this time, the pressure of the BOG
branched from the middle stage of the compressor 13 may be about 6
to 10 bara.
[0087] In particular, in the case of using a compressor
(manufactured by Burckhardt company) including five cylinders in
which three cylinders of the front stage are operated in an
oil-free-lubricated method and two cylinders of the rear stage are
operated in an oil-lubricated method, BOG needs to be transferred
while passing through an oil filter when BOG is branched at the
rear stage or 4-stage or more of the compressor. However, it is
advantageous in that the oil filter need not be used when BOG is
branched at 3-stage or less of the compressor.
[0088] FIG. 5 is a schematic configuration diagram illustrating a
liquefied gas treatment system for a vessel according to a third
embodiment of the present invention.
[0089] The liquefied gas treatment system according to the third
embodiment differs from the liquefied gas treatment system
according to the second embodiment in that LNG can be used after
forcible vaporization when an amount of BOG necessary for the MEGI
engine or the DF generator is larger than an amount of BOG
generated naturally in the cargo tank 11. Hereinafter, only a
difference from the liquefied gas treatment system of the second
embodiment will be described in more detail. In addition, the same
reference numerals are assigned to the same elements as those of
the second embodiment, and a detailed description thereof will be
omitted.
[0090] The liquefied gas treatment system for the vessel according
to the third embodiment of the present invention is identical to
that according to the second embodiment in that NBOG generated and
discharged from a cargo tank 11 storing liquefied gas is
transferred along a BOG supply line L1, is compressed in a
compressor 13, and is then supplied to the high pressure natural
gas injection engine, for example, an MEGI engine, or NBOG is
supplied to a DF engine (DF generator) while being multi-stage
compressed in the compressor 13 and is then used as fuel
therein.
[0091] However, the liquefied gas treatment system according to the
third embodiment includes a forcible vaporization line L11 such
that LNG stored in the cargo tank 11 can be vaporized in a forcible
vaporizer 31 and be then supplied to the compressor 13 when an
amount of BOG required as fuel in the high pressure natural gas
injection engine or the DF engine is larger than an amount of BOG
generated naturally in the cargo tank 11.
[0092] When the forcible vaporization line L11 is provided as in
the third embodiment, fuel can be stably supplied even when a small
amount of BOG is generated because a small amount of LNG is stored
in the cargo tank 11, or an amount of BOG required as fuel in
various engines is larger than an amount of BOG generated naturally
in the cargo tank 11.
[0093] FIG. 6 is a schematic configuration diagram illustrating a
liquefied gas treatment system for a vessel according to a fourth
embodiment of the present invention.
[0094] The liquefied gas treatment system according to the fourth
embodiment differs from the liquefied gas treatment system
according to the second embodiment in that an expander 52 instead
of the expansion valve is used as the decompressing means. That is,
according to the fourth embodiment, LBOG cooled in a heat exchanger
21 and liquefied at least partially is decompressed while passing
through the expander 52 and is supplied to a gas-liquid separator
23 in a gas-liquid mixed state. Hereinafter, only a difference from
the liquefied gas treatment system of the second embodiment will be
described in more detail. In addition, the same reference numerals
are assigned to the same elements as those of the second
embodiment, and a detailed description thereof will be omitted.
[0095] The expander 52 produces energy while expanding the high
pressure liquefied BOG at a low pressure. The LBOG may be
decompressed to approximately atmospheric pressure while passing
through the expander 52. The liquefied BOG is separated into gas
and liquid components in the gas-liquid separator 23. The liquid
component, LNG, is transferred to the cargo tank 11 through a BOG
return line L3, and the gas component, BOG, is discharged from the
cargo tank 11 through a BOG recirculation line L5 and is joined
with BOG supplied to the compressor 13. More specifically, the BOG
recirculation line L5 extends from an upper end of the gas-liquid
separator 23 and is connected to a more upstream side than the heat
exchanger 21 in the BOG supply line L1.
[0096] Another decompressing means, for example, an expansion valve
24, may be further installed in the BOG recirculation line L5.
Therefore, the gas component discharged from the gas-liquid
separator 23 may be decompressed while passing through the
expansion valve 24.
[0097] FIGS. 7 and 8 are schematic configuration diagrams
illustrating liquefied gas treatment systems for a vessel according
to modifications of the fourth embodiment of the present
invention.
[0098] In the fourth embodiment illustrated in FIG. 6, the BOG
return line L3 for supplying the compressed BOG to the heat
exchanger 21 is branched at the rear end of the compressor 13.
However, according to the modifications illustrated in FIGS. 7 and
8, as in the BOG branch lines L7 and L8 as described above or the
BOG return line in the modification of the second embodiment as
described with reference to FIGS. 3 and 4, the BOG return line L3
may be installed to branch BOG being compressed stepwise in the
compressor 13.
[0099] FIG. 7 illustrates a modification in which 2-stage
compressed BOG is branched by two cylinders, and FIG. 8 illustrates
a modification in which 3-stage compressed BOG is branched by three
cylinders. In particular, in the case of using a compressor
(manufactured by Burckhardt company) including five cylinders in
which three cylinders of the front stage are operated in an
oil-free-lubricated method and two cylinders of the rear stage are
operated in an oil-lubricated method, BOG needs to be transferred
while passing through an oil filter when BOG is branched at the
rear stage or 4-stage or more of the compressor. However, it is
advantageous in that the oil filter need not be used when BOG is
branched at 3-stage or less of the compressor.
[0100] In addition, referring to the first modification of the
fourth embodiment illustrated in FIG. 7, the liquefied gas
treatment system according to the fourth embodiment may be modified
such that the cooler 25 (see FIG. 6) serving as the heat exchanger
for additionally cooling the BOG cooled and liquefied while passing
through the heat exchanger 21 is omitted. If the cooler 25 is not
installed, the total efficiency of the system may be slightly
lowered. However, the pipe arrangement and the system operation may
be facilitated, and the initial installation cost and the
maintenance fee may be reduced.
[0101] In addition, referring to the second modification of the
fourth embodiment illustrated in FIG. 8, the liquefied gas
treatment system according to the fourth embodiment may be modified
such that the expander 52 and the expansion valve 55 serving as the
decompressing means are arranged in parallel. At this time, the
expander 52 and the expansion valve 55 arranged in parallel are
disposed between the heat exchanger 21 and the gas-liquid separator
23. A bypass line L31, which is branched from the BOG return line
L3 between the heat exchanger 21 and the gas-liquid separator 23
and is configured to bypass the expander 52, is installed so as to
install the expansion valve 55 in parallel and use only the
expander 52 or the expansion valve 55 when necessary. The expansion
valve 55 is closed when the liquefied BOG is expanded by using only
the expander 52, and on-off valves 53 and 54 installed respectively
at the front end and the rear end of the expander 52 are closed
when the liquefied BOG is expanded by using only the expansion
valve 55.
[0102] Like the liquefied gas treatment system and method according
to the foregoing embodiments, in the liquefied gas treatment system
and method according to the fourth embodiment of the present
invention, BOG generated during the transportation of cargo
(including LNG) in the LNG carrier may be used as the fuel of the
engine, or may be reliquefied, be returned to the cargo tank and be
stored therein. Therefore, an amount of BOG consumed in the GCU or
the like can be reduced or removed. Furthermore, BOG can be treated
by reliquefaction, without installing reliquefaction apparatuses
using separate refrigerants such as nitrogen.
[0103] Even when the liquefied gas treatment system and method
according to the fourth embodiment of the present invention is
applied to plants (LNG FPSO, LNG FSRU, BMPP, and the like) as well
as vessels (LNG carrier, LNG RV, and the like), BOG generated from
the cargo tank storing the LNG may be used as the fuel of the
engine (including engines for power generation as well as engines
for propulsion) or may be reliquefied, thereby reducing or removing
the unnecessary waste of BOG.
[0104] In addition, in the liquefied gas treatment system and
method according to the fourth embodiment of the present invention,
since it is unnecessary to install the reliquefaction apparatuses
using separate refrigerants (for example, nitrogen-refrigerant
refrigeration cycle, mixed-refrigerant refrigeration cycle, or the
like), facilities for supplying and storing the refrigerants need
not be separately installed. Consequently, it is possible to save
initial installation cost and operation cost for configuring the
entire system.
[0105] FIG. 9 is a schematic configuration diagram illustrating a
liquefied gas treatment system for a vessel according to a fifth
embodiment of the present invention.
[0106] The liquefied gas treatment system according to the fifth
embodiment differs from the liquefied gas treatment system
according to the second embodiment in that BOG liquefied in the
heat exchanger 21 and then decompressed in the decompressing means
(for example, the expansion valve 22) is returned to the cargo tank
11, without passing through the gas-liquid separator 23.
Hereinafter, only a difference from the liquefied gas treatment
system of the second embodiment will be described in more detail.
In addition, the same reference numerals are assigned to the same
elements as those of the second embodiment, and a detailed
description thereof will be omitted.
[0107] According to the present embodiment, the BOG (two-phase
BOG), which becomes a state in which the gas component (or flash
gas) and the liquid component (or liquefied BOG) are mixed while
being decompressed after liquefaction, is returned to the cargo
tank 11 through the BOG return line L3. The BOG return line L3 may
be configured such that the two-phase BOG returned to the cargo
tank 11 is injected to the bottom of the cargo tank 11.
[0108] The gas component (or flash gas) of the two-phase BOG
injected to the bottom of the cargo tank 11 may be partially melted
into LNG stored in the cargo tank 11, or may be liquefied by cold
energy of LNG. In addition, flash gas (BOG), which is not melted or
liquefied, is discharged from the cargo tank 11 again through the
BOG supply line L1 together with BOG (NBOG) additionally generated
in the cargo tank 11. The flash gas discharged from the cargo tank
11 together with the newly generated BOG is recirculated to the
compressor 13 along the BOG supply line L1.
[0109] According to the present embodiment, since the two-phase BOG
after expansion is injected to the bottom of the cargo tank 11, a
larger amount of BOG is liquefied by the LNG stored in the cargo
tank 11. Furthermore, since the facilities such as the gas-liquid
separator or the like are omitted, installation cost and operation
cost can be saved.
[0110] FIG. 10 is a schematic configuration diagram illustrating a
liquefied gas treatment system for a vessel according to a first
modification of the fifth embodiment of the present invention.
[0111] The first modification of the fifth embodiment illustrated
in FIG. 10 differs from the liquefied gas treatment system
illustrated in FIG. 9 according to the fifth embodiment in that an
expander 52 instead of the expansion valve is used as the
decompressing means. According to the first modification of the
fifth embodiment, LBOG cooled and liquefied in a heat exchanger 21
is decompressed to a gas-liquid mixed state while passing through
the expander 52 and is returned to a cargo tank 11 in a two-phase
state.
[0112] FIG. 11 is a schematic configuration diagram illustrating a
liquefied gas treatment system for a vessel according to a second
modification of the fifth embodiment of the present invention.
[0113] The second modification of the fifth embodiment illustrated
in FIG. 11 differs from the liquefied gas treatment system
illustrated in FIG. 9 according to the fifth embodiment in that a
plurality of compressors (for example, a first compressor 13a and a
second compressor 13b) instead of the multi-stage compressor is
used as the compressing means.
[0114] In the liquefied gas treatment system for the vessel
according to the second modification of the fifth embodiment of the
present invention, NBOG generated and discharged from a cargo tank
11 storing liquefied gas is transferred along a BOG supply line L1
and is then supplied to the first compressor 13a. The BOG
compressed in the first compressor 13a may be compressed at about 6
to 10 bara and then supplied to a demander, that is, a propulsion
system (for example, DFDE) using LNG as fuel, along a fuel supply
line L2. The BOG remaining after being supplied to the DFDE may be
additionally compressed by the second compressor 13b serving as a
booster compressor. Then, as in the above-described fifth
embodiment, the BOG may be liquefied while moving along a BOG
return line L3 and be then returned to the cargo tank 11.
[0115] The first compressor 13a may be a 1-stage compressor
including one compression cylinder 14a and one intercooler 15a. The
second compressor 13b may be a 1-stage compressor including one
compression cylinder 14b and one intercooler 15b. If necessary, the
second compressor 13b may be provided with a multi-stage compressor
including a plurality of compression cylinders and a plurality of
intercoolers.
[0116] The BOG compressed in the first compressor 13a is compressed
at about 6 to 10 bara and then supplied to the demander, for
example, the DF engine (that is, DFDE), through the fuel supply
line L2. At this time, all or part of the compressed BOG may be
supplied to the engine according to an amount of fuel necessary for
the engine.
[0117] In embodiments, when BOG discharged from the cargo tank 11
and supplied to the first compressor 13a (for example, all BOG
discharged from the cargo tank 11) is a first stream, the first
stream of the BOG may be divided into a second stream and a third
stream at a downstream side of the first compressor 13a. The second
stream may be supplied as fuel to the propulsion system, for
example, the DF engine (DFDE), and the third stream may be
liquefied and returned to the cargo tank 11.
[0118] At this time, the second stream is supplied to the DFDE
through the fuel supply line L2, and the third stream is further
compressed in the second compressor 13b, experiences liquefaction
and decompression processes, and is returned to the cargo tank 11
through the BOG return line L3. A heat exchanger 21 is installed in
the BOG return line L3 so as to liquefy the third stream of the
compressed BOG. The third stream of the BOG compressed in the heat
exchanger 21 exchanges heat with the first stream of the BOG
discharged from the cargo tank 11 and then supplied to the first
compressor 13a.
[0119] Since a flow rate of the first stream of the BOG before
compression is larger than a flow rate of the third stream, the
third stream of the compressed BOG may be cooled (that is,
liquefied at least partially) by receiving cold energy from the
first stream of the BOG before compression. As such, in the heat
exchanger 21, the BOG of the high pressure state is cooled
(liquefied) by heat exchange between the BOG of the cryogenic
temperature immediately after being discharged from the cargo tank
11 and the BOG of the high pressure state compressed in the
compressor 13.
[0120] The LBOG cooled in the heat exchanger 21 is decompressed
while passing through an expansion valve 22 (for example, J-T
valve) serving as decompression means, and is then supplied to the
cargo tank 11 in a gas-liquid mixed state. The LBOG may be
decompressed to approximately atmospheric pressure (for example,
decompressed from 300 bar to 3 bar) while passing through the
expansion valve 22.
[0121] Meanwhile, when it is expected that surplus BOG will be
generated because an amount of BOG generated from the cargo tank 11
is larger than an amount of fuel necessary for the DF engine (for
example, at the time of engine stop or during low-speed
navigation), BOG compressed in the first compressor 13a is branched
through the BOG branch line L7 and is then used in BOG consuming
means. Examples of the BOG consuming means may include a GCU and a
gas turbine, each of which can use natural gas as fuel.
[0122] FIG. 12 is a schematic configuration diagram illustrating a
liquefied gas treatment system for a vessel according to a third
modification of the fifth embodiment of the present invention.
[0123] The third modification of the fifth embodiment illustrated
in FIG. 12 differs from the liquefied gas treatment system
illustrated in FIG. 11 according to the second modification of the
fifth embodiment in that an expander 52 instead of the expansion
valve is used as the decompressing means. That is, according to the
third modification of the fifth embodiment, LBOG cooled and
liquefied in a heat exchanger 21 is decompressed to a gas-liquid
mixed state while passing through the expander 52 serving as the
decompressing means and is returned to a cargo tank 11 in a
two-phase state.
[0124] Like the liquefied gas treatment system and method according
to the foregoing embodiments, in the liquefied gas treatment system
and method according to the fifth embodiment of the present
invention, BOG generated during the transportation of cargo
(including LNG) in the LNG carrier may be used as the fuel of the
engine, or may be reliquefied, be returned to the cargo tank and be
stored therein. Therefore, an amount of BOG consumed in the GCU or
the like can be reduced or removed. Furthermore, BOG can be treated
by reliquefaction, without installing reliquefaction apparatuses
using separate refrigerants such as nitrogen.
[0125] Even when the liquefied gas treatment system and method
according to the fifth embodiment of the present invention is
applied to plants (for example, LNG FPSO, LNG FSRU, BMPP, and the
like) as well as vessels (for example, LNG carrier, LNG RV, and the
like), BOG generated from the cargo tank storing the LNG may be
used as the fuel of the engine (including engines for power
generation as well as engines for propulsion) or may be
reliquefied, thereby reducing or removing the unnecessary waste of
BOG.
[0126] In addition, in the liquefied gas treatment system and
method according to the fifth embodiment of the present invention,
since it is unnecessary to install the reliquefaction apparatuses
using separate refrigerants (for example, nitrogen-refrigerant
refrigeration cycle, mixed-refrigerant refrigeration cycle, or the
like), facilities for supplying and storing the refrigerants need
not be separately installed. Consequently, it is possible to save
initial installation cost and operation cost for configuring the
entire system.
[0127] FIG. 13 is a configuration diagram illustrating a liquefied
gas treatment system for a vessel according to a sixth embodiment
of the present invention.
[0128] The liquefied gas treatment system illustrated in FIG. 13
according to the sixth embodiment of the present invention is
configured by integrating the liquefied gas treatment system
illustrated in FIG. 1 according to the first embodiment (hybrid
system including the line through which LNG is compressed by the
high pressure pump 120 and supplied as fuel to the propulsion
system, and the line through which BOG is compressed by the
compressor 150 and supplied as fuel to the propulsion system) and
the liquefied gas treatment system illustrated in FIG. 2 according
to the second embodiment.
[0129] According to embodiments of the present invention, it is
obvious that the liquefied gas treatment systems illustrated in
FIGS. 3 to 13 according to the third to fifth embodiments can also
be integrated with the hybrid system (see L23, L24 and L25 of FIG.
13) as illustrated in FIG. 13.
[0130] The liquefied gas treatment system illustrated in FIG. 13
according to the present invention includes a high pressure natural
gas injection engine (for example, MEGI engine) as a main engine,
and a DF engine (DF generator: DFDG) as a sub engine. Generally,
the main engine is used for propulsion to navigate the vessel, and
the sub engine is used for power generation to supply power to
various apparatuses and facilities installed in the vessel.
However, the present invention is not limited to the purposes of
the main engine and the sub engine. A plurality of main engines and
a plurality of sub engines may be installed.
[0131] The liquefied gas treatment system according to embodiments
of the present invention is configured such that the natural gas
stored in the cargo tank 11 (for example, the BOG of the gas state
and the LNG of the liquid state) can be supplied as fuel to the
engines (for example, the MEGI engine serving as the main engine
and the DF engine serving as the sub engine).
[0132] In order to supply the BOG of the gas state as fuel gas, the
liquefied gas treatment system according to the present embodiment
includes a main BOG supply line L1 serving as a BOG supply line to
supply the main engine with BOG stored in the cargo tank 11, and a
sub BOG supply line L8 branched from the main BOG supply line L1 to
supply the sub engine with BOG. The main BOG supply line L1 has the
same configuration as the BOG supply line L1 of the foregoing
embodiment. However, in the description given with reference to
FIG. 13, this BOG supply line is referred to as the main BOG supply
line L1 so as to distinguish from the BOG supply line for the DF
engine (for example, the sub BOG supply line L8). In addition, the
sub BOG supply line L8 has the same configuration as the BOG branch
line L8 of the foregoing embodiment. However, in the description
given with reference to FIG. 13, this BOG supply line is referred
to as the sub BOG supply line L8 so as to distinguish from the main
BOG supply line L1.
[0133] In order to supply the LNG of the liquid state as fuel gas,
the liquefied gas treatment system according to the present
embodiment includes a main LNG supply line L23 serving to supply
the main engine with LNG stored in the cargo tank 11, and a sub LNG
supply line L24 branched from the main LNG supply line L23 to
supply the sub engine with LNG.
[0134] According to the present embodiment, a compressor 13 for
compressing the BOG is installed in the main BOG supply line L1,
and a high pressure pump 43 for compressing the LNG is installed in
the main LNG supply line L23.
[0135] The NBOG generated in the cargo tank 11 storing liquefied
gas and discharged through the BOG discharge valve 41 is
transferred along the main BOG supply line L1, is compressed in the
compressor 13, and is then supplied to the high pressure natural
gas injection engine, for example, the MEGI engine. The BOG is
compressed at a high pressure of about 150 to 400 bara by the
compressor 13 and is then supplied to the high pressure natural gas
injection engine.
[0136] The cargo tank 11 has sealing and heat-insulating walls so
as to store liquefied gas such as LNG in a cryogenic state, but
cannot perfectly block heat transferred from the outside.
Therefore, the liquefied gas is continuously vaporized within the
cargo tank 11, and BOG is discharged from the cargo tank 11 so as
to maintain the pressure of the BOG at an appropriate level.
[0137] The compressor 13 may include one or more compression
cylinders 14 and one or more intercoolers 15 for cooling BOG of
which the temperature is raised. The compressor 13 may be
configured to compress BOG to, for example, about 400 bara.
Although FIG. 13 illustrates the multi-stage compressor 13
including five compression cylinders 14 and five intercoolers 15,
the number of the compression cylinders and the number of the
intercoolers may be changed when necessary. In addition, a
plurality of compression cylinders may be arranged within a single
compressor, and a plurality of compressors may be connected in
series.
[0138] The BOG compressed in the compressor 13 is supplied to the
high pressure natural gas injection engine through the main BOG
supply line L1. All or part of the compressed BOG may be supplied
to the high pressure natural gas injection engine according to an
amount of fuel necessary for the high pressure natural gas
injection engine.
[0139] The sub BOG supply line L8 for supply fuel gas to the sub
engine (for example, the DF engine) is branched from the main BOG
supply line L1. More specifically, the sub BOG supply line L8 is
branched from the main BOG supply line L1 such that BOG can be
branched in the process of being multi-stage compressed in the
compressor 13. Although FIG. 13 illustrates that the 2-stage
compressed BOG is branched and a part of the BOG is supplied to the
sub engine through the sub BOG supply line L8, this is merely
exemplary. The system can also be configured such that 1-stage
compressed BOG or 3- to 5-stage compressed BOG is branched and then
supplied to the sub engine through the sub BOG supply line. As an
example of the compressor, a compressor manufactured by Burckhardt
company may be used. The compressor manufactured by Burckhardt
company includes five cylinders. It is known that the three
cylinders of the front stage are operated in an oil-free-lubricated
method and two cylinders of the rear stage are operated in an
oil-lubricated method. Therefore, in the case where the compressor
manufactured by Burckhardt company is used as the compressor 13 for
compressing BOG, the BOG needs to be transferred through an oil
filter when the BOG is branched at 4-stage or more of the
compressor. However, it is advantageous in that the oil filter need
not be used when the BOG is branched at 3-stage or less of the
compressor.
[0140] The required pressure of the DF engine (for example, DFDG)
serving as the sub engine is lower than that of the MEGI engine.
Therefore, when the BOG compressed at a high pressure is branched
at the rear end of the compressor 13, it is inefficient because the
pressure of the BOG needs to be lowered again and then supplied to
the sub engine.
[0141] As described above, if LNG is heated, methane component
having a relatively low liquefaction temperature is preferentially
vaporized. Hence, since a methane content of BOG is high, the BOG
can be directly supplied as fuel to the DF engine. Therefore,
separate apparatuses for adjusting methane number need not be
installed in the main BOG supply line and the sub BOG supply
line.
[0142] Meanwhile, when it is expected that surplus BOG will be
generated because an amount of BOG generated from the cargo tank 11
is larger than an amount of fuel necessary for the main engine and
the sub engine, the liquefied gas treatment system of embodiments
of the present invention can reliquefy the BOG and return the
reliquefied BOG to the cargo tank.
[0143] When BOG is generated over the reliquefaction capacity, BOG
having been compressed or being compressed stepwise in the
compressor 13 can be branched through the BOG branch line L7 and be
used in the BOG consuming means. Examples of the BOG consuming
means may include a GCU and a gas turbine, each of which can use
natural gas having a relatively lower pressure than the MEGI engine
as fuel. As illustrated in FIG. 13, the BOG branch line L7 may be
branched from the sub BOG supply line L8.
[0144] Since the process in which at least a part of BOG compressed
in the compressor 13 and then supplied to the high pressure natural
gas injection engine through the BOG supply line L1 is treated
through the BOG return line L3, for example, reliquefied and
returned to the cargo tank 11 is identical to that described with
reference to FIG. 2, a detailed description thereof will be
omitted.
[0145] Although FIG. 13 illustrates the example in which the BOG
return line L3 for supplying the compressed BOG to the heat
exchanger 21 is branched at the rear end of the compressor 13, the
BOG return line L3 may be installed to branch the BOG being
compressed stepwise in the compressor 13, like the above-described
BOG branch line L7 and the sub BOG supply line L8 serving as the
BOG branch line. FIG. 3 illustrates a modification in which 2-stage
compressed BOG is branched by two cylinders, and FIG. 4 illustrates
a modification in which 3-stage compressed BOG is branched by three
cylinders. At this time, the pressure of the BOG branched from the
middle stage of the compressor 13 may be about 6 to 10 bara.
[0146] In particular, in the case of using a compressor
(manufactured by Burckhardt) including five cylinders in which
three cylinders of the front stage are operated in an
oil-free-lubricated method and two cylinders of the rear stage are
operated in an oil-lubricated method, BOG needs to be transferred
while passing through an oil filter when BOG is branched at the
rear stage or 4-stage or more of the compressor. However, it is
advantageous in that the oil filter need not be used when BOG is
branched at 3-stage or less of the compressor.
[0147] A discharge pump 12 and a high pressure pump 43 are
installed in the main LNG supply line L23. The discharge pump 12 is
installed inside the cargo tank 11 and configured to discharge LNG
to the outside of the cargo tank 11. The high pressure pump 43 is
configured to secondarily compress LNG, which is primarily
compressed in the discharge pump 12, to a pressure necessary for
the MEGI engine. The discharge pump 12 may be installed in each
cargo tank 11. Although only one high pressure pump 43 is
illustrated in FIG. 4, a plurality of high pumps may be connected
in parallel when necessary.
[0148] As described above, the pressure of the fuel gas necessary
for the MEGI engine is a high pressure of about 150 to 400 bara
(absolute pressure). In this specification, it should be considered
that the term "high pressure" as used herein refers to a pressure
necessary for the MEGI engine, for example, a pressure of about 150
to 400 bara (absolute pressure).
[0149] The LNG discharged from the cargo tank 11 storing liquefied
gas through the discharge pump 12 is transferred along the main LNG
supply line L23 and is then supplied to the high pressure pump 43.
Then, the LNG is compressed to a high pressure in the high pressure
pump 43, is supplied to the vaporizer 44, and is vaporized in the
vaporizer 44. The vaporized LNG is supplied as fuel to the high
pressure natural gas injection engine, for example, the MEGI
engine. Since the pressure necessary for the MEGI engine is in a
supercritical state, the LNG compressed to the high pressure is a
state that is neither gas nor liquid. Therefore, it should be
considered that the expression "vaporizing the LNG compressed to
the high pressure in the vaporizer 44" means raising the
temperature of the LNG being in the supercritical state up to a
temperature necessary for the MEGI engine.
[0150] The sub LNG supply line L24 for supply fuel gas to the sub
engine (for example, the DF engine) is branched from the main LNG
supply line L23. More specifically, the sub LNG supply line L24 is
branched from the main LNG supply line L23 such that LNG can be
branched before being compressed in the high pressure pump 43.
[0151] Meanwhile, in FIG. 13, the sub LNG supply line L24 is
illustrated as being branched from the main LNG supply line L23 at
the upstream side of the high pressure pump 43. However, according
to the modification, the sub LNG supply line L24 may be branched
from the main LNG supply line L23 at the downstream side of the
high pressure pump 43. However, in the case where the LNG supply
line L24 is branched from the downstream side of the high pressure
pump 43, since the pressure of the LNG has been raised by the high
pressure pump 43, it is necessary to lower the pressure of the LNG
to the pressure necessary for the sub engine by the decompressing
means before supplying the LNG to the sub engine as fuel. Like the
embodiment illustrated in FIG. 13, it is advantageous in that
additional decompressing means need not be installed when the sub
LNG supply line L24 is branched at the upstream side of the high
pressure pump 43.
[0152] A vaporizer 45, a gas-liquid separator 46, and a heater 47
are installed in the sub LNG supply line L24 so as to adjust the
methane number and temperature of LNG supplied as fuel to the value
required in the DF engine.
[0153] As described above, since the methane content of the LNG is
relatively lower than that of the BOG, the methane number of the
LNG is lower than the methane number required in the DF engine.
Ratios of hydrocarbon components (methane, ethane, propane, butane,
and the like) constituting the LNG are different according to
producing areas. Therefore, it is not suitable to vaporize the LNG
as it is and then supply the vaporized LNG to the DF engine as
fuel.
[0154] In order to adjust the methane number, the LNG is heated and
partially vaporized in the vaporizer 45 The fuel gas partially
vaporized to a state in which the gas state and the liquid state
are mixed is supplied to the gas-liquid separator 46 and is
separated into gas and liquid. Since the vaporization temperature
of heavy hydrocarbon (HHC) component having a high calorific value
is relatively high, a ratio of the HHC component is relatively
increased in the LNG of the liquid state that remains without being
vaporized in the partially vaporized BOG. Therefore, the methane
number of the fuel gas can be increased by separating the liquid
component in the gas-liquid separator 46, for example, by
separating the HHC component.
[0155] In order to obtain appropriate methane number, the heating
temperature in the vaporizer 45 can be adjusted considering the
ratio of the hydrocarbon component included in the LNG, the methane
number required in the engine, and the like. The heating
temperature in the vaporizer 45 may be determined in the range of
-80.degree. C. to -120.degree. C. The liquid component separated
from the fuel gas in the gas-liquid separator 46 is returned to the
cargo tank 11 through the liquid-component return line L5. The BOG
return line L3 and the liquid-component return line L5 may extend
to the cargo tank 11 after joining each other.
[0156] The fuel gas, the methane number of which is adjusted, is
supplied to the heater 47 through the sub LNG supply line L24, is
further heated to a temperature required in the sub engine, and is
then supplied as fuel to the sub engine. For example, when the sub
engine is the DFDG, the required methane number is generally 80 or
more. For example, in the case of general LNG (typically, methane:
89.6%, nitrogen: 0.6%), the methane number before separating the
HHC component is 71.3, and a lower heating value (LHV) at that time
is 48, 872.8 kJ/kg (at 1 atm, saturated vapor). When the HHC
component is removed by compressing the general LNG to 7 bara and
heating it to -120.degree. C., the methane number is increased to
95.5 and the LHV at that time is 49, 265.6 kJ/kg.
[0157] According to the present embodiment, there are two passages
through which the fuel gas is supplied to the engines (the main
engine and the sub engine). That is, the fuel gas may be supplied
to the engines after being compressed through the compressor 13, or
may be supplied to the engines after being compressed through the
high pressure pump 43.
[0158] In particular, a vessel, such as LNG carrier or LNG RV, is
used to transport LNG from a producing area to a consumer.
Therefore, when sailing to the producing area, the vessel sails in
a laden condition in which the LNG is fully loaded into the cargo
tank. When returning to the producing area after unloading the LNG,
the vessel sails in a ballast condition in which the cargo tank is
almost empty. In the laden condition, a large amount of BOG is
generated because an amount of LNG is relatively large. In the
ballast condition, a relatively small amount of BOG is generated
because an amount of LNG is small.
[0159] Although there is a difference according to the capacity of
the cargo tank, outside temperature, and the like, an amount of BOG
generated when the capacity of the LNG cargo tank is about 130,000
to 350,000 is 3 to 4 ton/h in the laden condition and is 0.3 to 0.4
ton/h in the ballast condition. In addition, an amount of fuel gas
necessary for the engines is about 1 to 4 ton/h (about 1.5 ton/h on
average) in the case of the MEGI engine and is about 0.5 ton/h in
the case of the DF engine (DFDG). Meanwhile, in recent years, since
a boil-off rate (BOR) has tended to be lowered due to the
improvement in the heat insulation performance of the cargo tank, a
generation amount of BOG has tended to be reduced.
[0160] Therefore, in the case where both the compressor line (for
example, L1 and L8 in FIG. 13) and the high pressure pump line (for
example, L23 and L24 in FIG. 13) are provided like the fuel gas
supply system of the present embodiment, it is preferable that the
fuel gas is supplied to the engines through the compressor line in
the laden condition in which a large amount of BOG is generated,
and the fuel gas is supplied to the engines through the high
pressure pump lines in the ballast condition in which a small
amount of BOG is generated.
[0161] Generally, energy necessary for the compressor to compress
gas (BOG) up to the high pressure of about 150 to 400 bara
(absolute pressure) required in the MEGI engine is considerably
more than energy necessary for the pump to compress liquid (LNG).
The compressor for compressing the gas to the high pressure is very
expensive and occupies a large space. Therefore, it can be
considered that the use of the high pump line alone without any
compression line is cost-effective. For example, 2-MW power is
consumed for supplying fuel to the MEGI engine by driving one set
of the compressor configured with the multi-stage. However, if the
high pressure pump is used, 100-kW power is consumed. However, when
the fuel gas is supplied to the engines by using the high pressure
pump line alone in the laden condition, a reliquefaction apparatus
for reliquefying BOG is necessarily required so as to treat BOG
continuously generated in the cargo tank. When considering energy
consumed in the reliquefaction apparatus, it is advantageous that
both the compressor line and the high pressure pump line are
installed, the fuel gas is supplied through the compressor line in
the laden condition, and the fuel gas is supplied through the high
pressure pump line in the ballast condition.
[0162] Meanwhile, like the ballast condition, when an amount of BOG
generated in the cargo tank is smaller than an amount of fuel
necessary for the MEGI engine, it may be efficient to branch BOG
through the sub BOG supply line L8 in the process of being
multi-stage compressed and use the branched BOG as the fuel of the
DF engine, without compressing BOG in the multi-stage compressor to
the high pressure required in the MEGI. That is, for example, if
BOG is supplied to the DF engine through only the 2-stage
compression cylinders of the 5-stage compressor, the remaining
3-stage compression cylinders run idle. 2-MW power is required when
BOG is compressed by driving the entire 5-stage compressor. 600-kW
power is required when the 2-stage compression cylinders are used
and the remaining 3-stage compression cylinders run idle. 100-kW
power is required when the fuel is supplied to the MEGI engine
through the high pressure pump. Therefore, like the ballast
condition, when a generation amount of BOG is smaller than an
amount of fuel necessary for the MEGI engine, it is advantageous in
terms of energy efficiency to consume all amount of BOG in the DF
engine or the like and supply LNG as fuel through the high pressure
pump.
[0163] However, if necessary, even when a generation amount of BOG
is smaller than an amount of fuel necessary for the MEGI engine,
LNG may be forcibly vaporized and supplied as much as a deficient
amount while supplying BOG as fuel to the MEGI engine through the
compressor. Meanwhile, since a generation amount of BOG is small in
the ballast condition, BOG is not discharged but accumulated until
the cargo tank reaches a predetermined pressure, and is
intermittently discharged and supplied as fuel to the DF engine or
the MEGI engine, instead of discharging and consuming BOG whenever
the BOG is generated.
[0164] In the ballast condition, the engine of the vessel (the DF
engine or the MEGI engine) may be simultaneously supplied with BOG
compressed by the compressor 13 and LNG compressed by the high
pressure pump 43 as fuel. In addition, in the ballast condition,
the engine of the vessel (the DF engine or the MEGI engine) may be
alternately supplied with BOG compressed by the compressor 13 and
LNG compressed by the high pressure pump 43 as fuel.
[0165] In the case of a low-pressure engine, such as a boiler, a
gas turbine, or a low-pressure DF engine, which is supplied with a
low-pressure fuel in use, a fuel supply system has been developed
which uses BOG generated in the storage tank as fuel in a normal
condition, and forcibly vaporizes LNG and uses the vaporized LNG as
fuel together with the BOG when an amount of BOG is smaller than a
necessary amount of fuel. Such a fuel supply system is limited to a
case where only a low-pressure engine is installed in a vessel. The
naturally generated BOG and the forcibly vaporized LNG are
different in the heating value and the methane number. Thus, in a
case where the BOG and the forcibly vaporized LNG is supplied to
one engine in a mixed manner, the engine power is changed as the
component of the fuel, that is, the heating value, is continuously
changed. This makes it difficult to operate the engine. In the case
of a cargo ship such as an LNG carrier, a relatively sufficient
amount of BOG is generated in a laden condition in which the cargo
ship is fully loaded with cargo on voyage. However, in a ballast
condition in which the cargo ship is returned after unloading the
cargo, an amount of BOG is deficient and thus it is necessary to
forcibly vaporize the LNG. Therefore, in the ballast condition
corresponding to about half the total sailing period, a change in
the engine power may continually occur.
[0166] However, the above-described embodiments of the present
invention significantly differ from the fuel supply system mounted
with only the low-pressure engine, in that both of the
high-pressure engine supplied with the fuel at a high pressure (for
example, MEGI engine, about 150 to 400 bara) and the low-pressure
engine supplied with the fuel at a low pressure (for example, DF
engine, about 6-10 bara) are mounted.
[0167] In addition, according to embodiments of the present
invention, when the generation amount of BOG is smaller than the
amount of fuel required in the entire engine, the BOG is supplied
as the fuel to only the low-pressure engine, or the LNG is supplied
as the fuel to both the high-pressure engine and the low-pressure
engine. When a predetermined amount of BOG is accumulated in the
storage tank, the BOG and the LNG are alternately supplied as the
fuel to the engines. Therefore, it is possible to avoid the
situation that the BOG and the forcibly vaporized LNG are supplied
to one engine in a mixed manner.
[0168] However, according to the embodiments of the present
invention, it is obvious that the BOG compressed by the compressor
13 and the LNG compressed by the high pressure pump 43 can be
simultaneously supplied as the fuel to one engine as necessary.
[0169] In addition, in the vessels where it is not easy to repair
and replace equipments, important facilities are required to be
installed by two in consideration of emergency (redundancy). In
embodiments, the redundancy of important facilities is required
such that extra facilities capable of performing the same function
as the main facility, and the extra equipment is set to a standby
state during the normal operation of the main facility and takes
over the function of the main facility when the main facility does
not operate due to malfunction. Examples of the facilities
requiring the redundancy may include rotating facilities, for
example, compressors or pumps.
[0170] As such, various facilities need to be redundantly installed
in the vessel so as to satisfy only the redundancy requirement
while not being used at regular days. The fuel gas supply system
using two compression lines requires much cost and space for the
installation of the compressor. When using the fuel gas supply
system, much energy is consumed. The fuel gas supply system using
two high pressure pump lines may consume much energy in the
treatment (reliquefaction) of BOG. On the other hand, in the fuel
gas supply system of embodiments of the present invention in which
both the compressor line and the high pressure pump line are
installed, even when there occurs a problem in one of the supply
lines, the vessel can continue to sail normally through another
supply line. In the case where only one compression line is
installed, expensive compressors are less used and an optimal fuel
gas supply method can be appropriately selected and used according
to a generation amount of BOG. Therefore, it is possible to obtain
additional effect that can save operation cost as well as initial
shipbuilding cost.
[0171] As illustrated in FIG. 13, when the liquefied gas treatment
system and the hybrid fuel gas supply system are combined according
to the embodiment of the present invention, BOG generated during
the transportation of cargo (including LNG) in the LNG carrier may
be used as the fuel of the engine, or may be reliquefied, be
returned to the cargo tank and be stored therein. Therefore, an
amount of BOG consumed in the GCU or the like can be reduced or
removed. Furthermore, BOG can be treated by reliquefaction, without
installing reliquefaction apparatuses using separate refrigerants
such as nitrogen.
[0172] According to the present embodiment, in spite of the recent
trend in which the generation amount of BOG is increased due to the
increased capacity of the cargo tank and a necessary amount of fuel
is reduced due to the improved performance of the engine, the BOG
remaining after being used as the fuel of the engine can be
reliquefied and returned to the cargo tank, thereby preventing the
waste of BOG.
[0173] In particular, in the liquefied gas treatment system and
method according to the present embodiment, since it is unnecessary
to install the reliquefaction apparatuses using separate
refrigerants (for example, nitrogen-refrigerant refrigeration
cycle, mixed-refrigerant refrigeration cycle, or the like),
facilities for supplying and storing the refrigerants need not be
separately installed. Consequently, it is possible to save initial
installation cost and operation cost for configuring the entire
system.
[0174] While embodiments of the present invention have been
described with reference to the specific embodiments, it will be
apparent to those skilled in the art that various changes and
modifications may be made without departing from the spirit and
scope of the invention as defined in the following claims.
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