U.S. patent application number 14/006656 was filed with the patent office on 2014-02-27 for system for supplying fuel to high-pressure natural gas injection engine having excess evaporation gas consumption means.
This patent application is currently assigned to DAEWOO SHIPBUILDING & MARINE ENGINEERING CO., LTD.. The applicant listed for this patent is Dong Kyu Choi, Je Heon Jung, Seung Kyo Jung, Jung Han Lee, Sung Jun Lee, Hyun Jun Shin. Invention is credited to Dong Kyu Choi, Je Heon Jung, Seung Kyo Jung, Jung Han Lee, Sung Jun Lee, Hyun Jun Shin.
Application Number | 20140053600 14/006656 |
Document ID | / |
Family ID | 49773111 |
Filed Date | 2014-02-27 |
United States Patent
Application |
20140053600 |
Kind Code |
A1 |
Jung; Seung Kyo ; et
al. |
February 27, 2014 |
SYSTEM FOR SUPPLYING FUEL TO HIGH-PRESSURE NATURAL GAS INJECTION
ENGINE HAVING EXCESS EVAPORATION GAS CONSUMPTION MEANS
Abstract
Provided is a fuel supply system for a high-pressure natural gas
injection engine. The fuel supply system includes: a BOG
compression unit configured to receive BOG, which is generated in a
storage tank, from the storage tank and compress the received BOG
to a pressure of 12 to 45 bara; a reliquefaction apparatus
configured to receive and liquefy the BOG compressed by the BOG
compression unit; a high-pressure pump configured to compress the
BOG liquefied by the reliquefaction apparatus; a high-pressure
gasifier configured to gasify the BOG compressed by the
high-pressure pump and supply the gasified BOG to the high-pressure
natural gas injection engine; and an excess BOG consumption unit
configured to consume excess BOG corresponding to a difference
between an amount of BOG generated in the storage tank and an
amount of BOG required as fuel for the high-pressure natural gas
injection engine.
Inventors: |
Jung; Seung Kyo;
(Busanjin-gu, KR) ; Jung; Je Heon; (Geoje-si,
KR) ; Lee; Jung Han; (Geoje-si, KR) ; Lee;
Sung Jun; (Jungnang-gu, KR) ; Shin; Hyun Jun;
(Yongdeungpo-gu, KR) ; Choi; Dong Kyu; (Geoje-si,
KR) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Jung; Seung Kyo
Jung; Je Heon
Lee; Jung Han
Lee; Sung Jun
Shin; Hyun Jun
Choi; Dong Kyu |
Busanjin-gu
Geoje-si
Geoje-si
Jungnang-gu
Yongdeungpo-gu
Geoje-si |
|
KR
KR
KR
KR
KR
KR |
|
|
Assignee: |
DAEWOO SHIPBUILDING & MARINE
ENGINEERING CO., LTD.
Seoul
KR
|
Family ID: |
49773111 |
Appl. No.: |
14/006656 |
Filed: |
December 20, 2011 |
PCT Filed: |
December 20, 2011 |
PCT NO: |
PCT/KR2011/009820 |
371 Date: |
November 18, 2013 |
Current U.S.
Class: |
62/611 |
Current CPC
Class: |
F25J 1/0045 20130101;
F25J 1/0025 20130101; F17C 2221/035 20130101; Y02T 10/32 20130101;
F25J 2205/30 20130101; Y02T 10/30 20130101; F25J 1/023 20130101;
F17C 2265/034 20130101; F17C 2221/033 20130101; F25J 2290/62
20130101; F25J 1/0278 20130101; F25J 2230/08 20130101; F25J 1/0277
20130101; F02M 21/0215 20130101; F25J 2230/60 20130101; F25J
2235/60 20130101; F25J 1/0291 20130101; F17C 2265/036 20130101;
Y02T 10/36 20130101; F02M 25/08 20130101; F17C 2223/0161 20130101;
F17C 2265/066 20130101; F25J 2220/62 20130101; F25J 2230/30
20130101; F25J 1/0097 20130101; F02M 21/0245 20130101; F25J 1/0052
20130101; F17C 2270/0105 20130101; F17C 2265/037 20130101; F17C
2223/0153 20130101; F17C 2223/033 20130101; F25J 1/0212 20130101;
F02M 21/0287 20130101; F25J 1/0254 20130101; F02D 19/0605
20130101 |
Class at
Publication: |
62/611 |
International
Class: |
F25J 1/00 20060101
F25J001/00 |
Foreign Application Data
Date |
Code |
Application Number |
Mar 22, 2011 |
KR |
10-2011-0025397 |
Sep 23, 2011 |
KR |
10-2011-0096466 |
Oct 19, 2011 |
KR |
10-2011-0107235 |
Claims
1. A fuel supply system for a high-pressure natural gas injection
engine, comprising: a boil-off gas (BOG) compression unit
configured to receive BOG, which is generated in a storage tank,
from the storage tank and compress the received BOG to a pressure
of 12 to 45 bara; a reliquefaction apparatus configured to receive
and liquefy the BOG compressed by the BOG compression unit; a
high-pressure pump configured to compress the BOG liquefied by the
reliquefaction apparatus; a high-pressure gasifier configured to
gasify the BOG compressed by the high-pressure pump and supply the
gasified BOG to the high-pressure natural gas injection engine; and
an excess BOG consumption unit configured to consume excess BOG
corresponding to a difference between an amount of BOG generated in
the storage tank and an amount of BOG required as fuel for the
high-pressure natural gas injection engine.
2. The fuel supply system according to claim 1, wherein the excess
BOG consumption unit is a gas combustion unit configured to receive
flash gas through a fuel gas supply line and consume the received
flash gas as fuel.
3. The fuel supply system according to claim 1, wherein the excess
BOG consumption unit is a dual-fuel diesel engine configured to
receive BOG through a branch line branching in the middle of the
BOG compression unit and consume the received BOG as fuel.
4. The fuel supply system according to claim 1, wherein the excess
BOG consumption unit is a gas turbine configured to receive BOG
through a branch line branching from a rear end of the BOG
compression unit and consume the received BOG as fuel.
5. The fuel supply system according to claim 1, further comprising:
an LBOG return line configured to return the excess BOG to the
storage tank; and a gas-liquid LBOG separator installed at the LBOG
return line and configured to separate BOG including flash gas into
a liquid component and a gaseous component and return only the
liquid component to the storage tank, the flash gas being generated
in a decompression process when the excess BOG is returned to the
storage tank.
6. The fuel supply system according to claim 5, further comprising
an LBOG expansion valve installed at the LBOG return line and
configured to decompress the excess BOG.
7. The fuel supply system according to claim 6, further comprising
a valve installed at the fuel gas supply line and configured to
decompress the gaseous component separated by the gas-liquid LBOG
separator.
8. The fuel supply system according to claim 2, further comprising
a branch line branching from the fuel supply line supplying fuel to
the high-pressure natural gas injection line, and connected to the
fuel gas supply line supplying the fuel to the dual-fuel diesel
engine, such that the fuel is additionally supplied to the gas
combustion unit.
Description
TECHNICAL FIELD
[0001] The present invention relates to a fuel supply system for a
high-pressure natural gas injection engine, and more particularly,
to a fuel supply system for a high-pressure natural gas injection
engine having an excess boil-off gas (BOG) consumption unit, which
can consume excess BOG corresponding to a difference between an
amount of BOG generated in the storage tank and an amount of BOG
required as fuel for the high-pressure natural gas injection
engine, when a larger amount of BOG than required as fuel for the
high-pressure natural gas injection engine is generated.
BACKGROUND ART
[0002] Recently, the consumption of natural 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 gaseous state through onshore or offshore gas
pipelines, or 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 gaseous state, liquefied gas
is very suitable for a long-distance marine transportation
[0003] A liquefied gas 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 hold") that can withstand a
cryogenic temperature of liquefied gas.
[0004] Examples of a marine structure provided with a storage 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).
[0005] 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 offshore 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 term "marine structure" as used herein is a concept
including vessels, such as a liquefied gas carrier and an LNG RV,
and structures, such as an LNG FPSO and an LNG FSRU.
[0006] 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 storage tank
is thermally insulated, external heat is continuously transferred
to LNG. Therefore, LNG is continuously vaporized and boil-off gas
is generated within the LNG storage tank during the transportation
of LNG by the LNG carrier.
[0007] The generated natural gas may increase the internal pressure
of the storage tank and accelerate the flow of the natural gas due
to the rocking of the vessel, causing structural problems.
Therefore, it is necessary to suppress the generation of BOG.
[0008] Conventionally, in order to suppress the generation of BOG
within the storage tank of the liquefied gas carrier, a method of
discharging the BOG from the storage tank and burning the BOG, a
method of discharging the BOG from the storage tank, reliquefying
the BOG through a reliquefaction apparatus, and returning the BOG
to the storage 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 internal pressure of a storage
tank at a high level have been used solely or in combination.
[0009] In the case of a conventional marine structure equipped with
a BOG reliquefaction apparatus, BOG inside a storage tank is
discharged from the storage tank and then reliquefied through a
reliquefaction apparatus in order to maintain a pressure of the
storage tank at an appropriate level. In this case, before a
reliquefaction process, the BOG is compressed to a low pressure of
about 4 to 8 bara and then supplied to the reliquefaction
apparatus. The compressed BOG is reliquefied through heat exchange
with nitrogen cooled to a cryogenic temperature in the
reliquefaction apparatus including a nitrogen refrigeration cycle,
and the liquefied BOG is returned to the storage tank.
[0010] BOG may be compressed to a high pressure in order to
increase the BOG reliquefaction efficiency. However, the LNG stored
in the storage tank is maintained at an ambient pressure state, and
therefore, if a pressure of the liquefied BOG is excessively high,
flash gas may be generated when the BOG is returned to the storage
tank. Consequently, the BOG needs to be compressed to the
above-mentioned low pressure of about 4 to 8 bara, in spite of low
reliquefaction efficiency.
[0011] Conventionally, as illustrated in FIG. 1, BOG generated in a
storage tank, that is, NBOG, is supplied to a BOG compressor and is
compressed to a low pressure of about 4 to 8 bara. Then, the
low-pressure BOG is supplied to a reliquefaction apparatus using
nitrogen gas as a refrigerant (the detailed description of Korean
Patent Application Publication No. 10-2006-0123675 discloses that
the BOG is compressed at about 6.8 bara, and the detailed
description of Korean Patent Application Publication No.
10-2001-0089142 (relevant U.S. Pat. No. 6,530,241) discloses that
the BOG is compressed at about 4.5 bara). Flash gas may be
generated while the BOG liquefied in the reliquefaction apparatus,
that is, LBOG, is returned to the storage tank. Hence, the BOG
compressor necessarily compresses the BOG at a low pressure.
[0012] As a result, according to a typical BOG processing method,
BOG generated in a storage tank is reliquefied through a
reliquefaction apparatus and then returned to the storage tank.
Till now, a basic concept for suppressing the generation of flash
gas after the reliquefaction of BOG as much as possible is not to
increase a pressure of BOG to be reliquefied.
[0013] A BOG reliquefaction apparatus uses a nitrogen refrigeration
cycle disclosed in International Patent Publication Nos. WO
2007/117148 and WO 2009/136793 and Korean Patent Application
Publication Nos. 10-2006-0123675 and 10-2001-0089142, and also uses
other mixed refrigerant cycles. As described above, it is general
that the conventional BOG reliquefaction apparatus reliquefies BOG
by compressing the BOG to a pressure of about 4 to 8 bara. Also, it
is well known in the art that it is technically inappropriate to
compress BOG to a pressure higher than the above-mentioned
pressure.
[0014] This is because if BOG is reliquefied at a high pressure,
the pressure of the BOG is lowered to about ambient pressure when
the BOG is returned later to the tank, and thus, a large amount of
flash gas (BOG) is generated.
[0015] Meanwhile, since the nitrogen refrigeration cycle uses
nitrogen gas (N.sub.2) as a refrigerant, the liquefaction
efficiency is low. Also, the mixed refrigerant cycle uses a
refrigerant mixed with nitrogen and hydrocarbon gases as a
refrigerant, the stability is low.
[0016] More specifically, a conventional offshore LNG
reliquefaction apparatus for a vessel or an offshore plant
reliquefies BOG by implementing a turbo-expander-type nitrogen
reverse Brayton cycle. A conventional onshore LNG liquefaction
plant liquefies natural gas by implementing a Joule-Thomson
refrigeration cycle using a mixed refrigerant. The nitrogen reverse
Brayton cycle used for the offshore LNG liquefaction apparatus is
relatively simple in the configuration of the apparatus and thus is
advantageous to a limited vessel or offshore plant, but has low
efficiency. The mixed-refrigerant Joule-Thomson refrigeration cycle
used for the onshore LNG liquefaction plant has relatively high
efficiency but is complicated in the configuration of the apparatus
because a separator needs to be used for separating a mixed
refrigerant when a gaseous state and a liquid state coexist due to
the feature of the mixed refrigerant. However; such a
reliquefaction method has still been widely used.
[0017] Moreover, in the case of a marine structure equipped with a
storage tank configured to store liquefied gas such as LNG, there
is a need for extensive research and development of methods for
efficiently processing BOG continuously generated in a storage tank
and suppressing the generation of flash gas.
DISCLOSURE
Technical Problem
[0018] An aspect of the present invention is directed to a fuel
supply system for a high-pressure natural gas injection engine
having an excess boil-off gas (BOG) consumption unit, which can
consume excess BOG corresponding to a difference between an amount
of BOG generated in the storage tank and an amount of BOG required
as fuel for the high-pressure natural gas injection engine, when a
larger amount of BOG than required as fuel for the high-pressure
natural gas injection engine is generated.
[0019] The applicant of the present patent application developed a
fuel supply technology in which LNG was compressed (pumped) by a
high-pressure pump, gasified and then supplied as fuel, instead of
fuel supply by gas compression, which was proposed by MAN B&W
Diesel Ltd as a conventional fuel supply method for a high-pressure
gas injection engine. The applicant of the present patent
application filed a patent application in Korea on May 8, 2007
(Korean Patent Application No. 10-2007-0044727), and this
technology made a great apple to ship owners and MAN B&W Diesel
Ltd.
[0020] Hamworthy Gas Systems slightly modified the above-described
technology proposed by the applicant of the present patent
application and filed an international patent application
(International Patent Publication No. WO 2009/136793). However,
even after the development of such technology, there was a concern
in the art about the generation of flash gas when liquefied BOG is
returned to a storage tank. Therefore, when BOG was reliquefied,
the BOG was compressed in a low pressure range (4 to 8 bara), and
the compression of BOG at a pressure higher than the
above-mentioned pressure range has not been considered at all.
[0021] When actually applying the basic technology for
high-pressure pumping of LNG, the applicant of the present patent
application found, in the process of developing the technology for
using BOG generated in an LNG storage tank as fuel, that unlike the
conventional reliquefaction technology for reliquefying BOG by
compressing the BOG to a pressure of 4 to 8 bara, energy consumed
in the reliquefaction was considerably reduced if BOG was
compressed in a medium pressure range (12 to 45 bara) higher than a
conventional reliquefaction pressure and then reliquefied. Based on
such discovery, the applicant of the present patent application
completed the present invention.
[0022] Also, the applicant of the present patent application found
that the present invention had advantages that could reduce power
consumption of a high-pressure pump configured to compress LNG,
which was compressed in a medium pressure range after
reliquefaction, to a high pressure, as well as the considerable
reduction in the reliquefaction energy. Moreover, the applicant of
the present patent application found that the present invention had
advantages in that subcooling needed not be performed because BOG
was compressed by the high-pressure pump after the
reliquefaction.
[0023] The objects and effects of the present invention are
disclosed herein for the first time.
Technical Solution
[0024] According to an aspect of the present invention, a fuel
supply system for a high-pressure natural gas injection engine
includes: a boil-off gas (BOG) compression unit configured to
receive BOG, which is generated in a storage tank, from the storage
tank and compress the received BOG to a pressure of 12 to 45 bara;
a reliquefaction apparatus configured to receive and liquefy the
BOG compressed by the BOG compression unit; a high-pressure pump
configured to compress the BOG liquefied by the reliquefaction
apparatus; a high-pressure gasifier configured to gasify the BOG
compressed by the high-pressure pump and supply the gasified BOG to
the high-pressure natural gas injection engine; and an excess BOG
consumption unit configured to consume excess BOG corresponding to
a difference between an amount of BOG generated in the storage tank
and an amount of BOG required as fuel for the high-pressure natural
gas injection engine.
[0025] The excess BOG consumption unit may be a gas combustion unit
configured to receive flash gas through a fuel gas supply line and
consume the received flash gas as fuel.
[0026] The excess BOG consumption unit may be a dual-fuel diesel
engine configured to receive BOG through a branch line branching in
the middle of the BOG compression unit and consume the received BOG
as fuel.
[0027] The excess BOG consumption unit may be a gas turbine
configured to receive BOG through a branch line branching from a
rear end of the BOG compression unit and consume the received BOG
as fuel.
[0028] The fuel supply system may further include: an LBOG return
line configured to return the excess BOG to the storage tank; and a
gas-liquid LBOG separator installed at the LBOG return line and
configured to separate BOG including flash gas into a liquid
component and a gaseous component and return only the liquid
component to the storage tank, the flash gas being generated in a
decompression process when the excess BOG is returned to the
storage tank.
[0029] The fuel supply system may further include an LBOG expansion
valve installed at the LBOG return line and configured to
decompress the excess BOG.
[0030] The fuel supply system may further include a valve installed
at the fuel gas supply line and configured to decompress the
gaseous component separated by the gas-liquid LBOG separator.
[0031] The fuel supply system may further include a branch line
branching from the fuel supply line supplying fuel to the
high-pressure natural gas injection line, and connected to the fuel
gas supply line supplying the fuel to the dual-fuel diesel engine,
such that the fuel is additionally supplied to the gas combustion
unit.
[0032] The fuel supply method according to the present invention
may reduce BOG liquefaction energy because the BOG recycles
liquefaction energy of the liquefied BOG by heat exchange between
the BOG before liquefaction and the liquefied BOG before
gasification. Also, before compressing the BOG generated in the
liquefied gas storage tank, the BOG generated in the storage tank
may be preheated by heat exchange with the compressed BOG or the
nitrogen refrigerant heated in the nitrogen refrigeration cycle of
the reliquefaction apparatus. The cold heat recovery or the
preheating of the BOG may use the technologies disclosed in
International Patent Publication Nos. WO 2007/117148 and WO
2009/136793, Korean Patent Application Publication No.
10-2006-0123675, and Korean Patent Registration No. 0929250.
Although the cold heat recovery from the liquefied BOG is described
in the present disclosure, LNG stored in an LNG storage tank may be
used when an amount of liquefied BOG is smaller than an amount of
fuel required in a high-pressure natural gas injection engine. In
this case, the cold heat may be recovered from the LNG supplied
from the LNG storage tank.
[0033] Examples of the marine structure may include vessels, such
as a liquefied gas carrier and an LNG RV, or structures, such as an
LNG FSRU and an LNG FPSO.
[0034] The fuel supply method may supply all of the liquefied BOG
to the high-pressure natural gas injection engine during the fuel
supply. That is, the high-pressure natural gas injection engine may
require a larger amount of fuel than an amount of the BOG generated
in the LNG storage tank for a considerable period of time during
the voyage of the marine structure. In this case, all of the
liquefied BOG is supplied to the high-pressure natural gas
injection engine, thereby preventing the generation of flash gas
when the liquefied BOG is returned to the LNG storage tank.
[0035] According to another aspect of the present invention, when
the high-pressure natural gas injection engine requires a larger
amount of fuel than an amount of BOG generated in the LNG storage
tank during the voyage of the marine structure, all or a
considerable amount of the BOG may be supplied to the high-pressure
natural gas injection engine. In this case, if an amount of fuel is
insufficient, the LNG stored in the LNG storage tank may be used as
fuel.
Advantageous Effects
[0036] The present invention may provide a fuel supply system for a
high-pressure natural gas injection engine having an excess
boil-off gas (BOG) consumption unit, which can consume excess BOG
corresponding to a difference between an amount of BOG generated in
the storage tank and an amount of BOG required as fuel for the
high-pressure natural gas injection engine, when a larger amount of
BOG than required as fuel for the high-pressure natural gas
injection engine is generated.
[0037] As opposed to the related art in which the BOG is compressed
to a low pressure of about 4 to 8 bara, the fuel supply method for
the high-pressure natural gas injection engine according to the
present invention compresses the BOG to a medium pressure of about
12 to 45 bara and then reliquefied. As the pressure of the BOG
increases, the liquefaction energy decreases. Therefore, the
liquefaction energy consumed in reliquefaction may be reduced.
[0038] Also, in the fuel supply system for the high-pressure
natural gas injection engine according to the present invention,
since the pressure of the BOG in the BOG reliquefaction is a medium
pressure higher than that of the related art, the liquefying point
of the BOG increases. Therefore, thermal stress applied to a heat
exchanger for reliquefaction is reduced, and a heat duty of a
high-pressure gasifier is reduced, leading to a reduction in the
size of the apparatus.
[0039] Also, since the liquefied BOG compressed to a medium
pressure is compressed to a high pressure, power of a high-pressure
pump is reduced.
[0040] Also, in the fuel supply system for the high-pressure
natural gas injection engine according to the present invention, a
nonflammable mixed refrigerant is used as a refrigerant of a
reliquefaction apparatus for the BOG reliquefaction. Therefore, the
fuel supply method according to the present invention is more
efficient than a conventional nitrogen refrigeration cycle, and can
reliquefy the BOG more safely than a conventional mixed refrigerant
cycle.
[0041] The fuel supply method of the fuel supply system may supply
all of the liquefied BOG to the high-pressure natural gas injection
engine during the operation of the high-pressure natural gas
injection engine. That is, the high-pressure natural gas injection
engine may require a larger amount of fuel than an amount of the
BOG generated in the LNG storage tank for a considerable period of
time during the voyage of the marine structure. In this case, all
of the liquefied BOG is supplied to the high-pressure natural gas
injection engine, thereby preventing the generation of flash gas
when the liquefied BOG is returned to the LNG storage tank. Also,
it is possible to considerably reduce energy that is consumed by
subcooling for reducing the generation of flash gas when the
liquefied BOG is returned to the LNG storage tank. A conventional
Mark III reliquefaction apparatus of Hamworthy Gas Systems (the
technology disclosed in International Patent Publication No. WO
2007/117148) compresses the BOG to a pressure of 8 bara and
liquefies the BOG at a temperature of -159.degree. C. In this case,
since the saturation temperature of the BOG is about -149.5.degree.
C., the BOG is subcooled by about 9 to 10.degree. C. The BOG needs
to be subcooled by such a degree in order to prevent the generation
of flash gas when the liquefied BOG is returned to the LNG storage
tank. However, since the liquefied BOG is compressed by the
high-pressure pump while the liquefied BOG is supplied as fuel for
the high-pressure natural gas injection engine, the LBOG saturated
by the increased pressure can stably maintain the overcooled state
later. Therefore, according to the present invention, the liquefied
BOG may be liquefied by overcooling as much as 0.5 to 3.degree. C.,
preferably about 1.degree. C., as compared to the saturation
temperature at the corresponding pressure, and then supplied as
fuel.
[0042] Also, in the fuel supply system for the high-pressure
natural gas injection engine according to the present invention, if
necessary, the DFDE may be mounted such that fuel remaining after
fuel supply to the high-pressure natural gas injection engine or
flash gas generated during decompression is consumed while being
used as the fuel of the DFDE. That is, BOG exceeding the amount of
fuel required by the high-pressure natural gas injection engine may
be compressed to a pressure of about 4 to 8 bara and directly
supplied from the LNG storage tank to the DFDE without
medium-pressure reliquefaction.
DESCRIPTION OF DRAWINGS
[0043] FIG. 1 is a schematic block diagram illustrating a method
for processing BOG through BOG reliquefaction according to the
related art.
[0044] FIG. 2 is a schematic block diagram illustrating a method
for processing BOG through fuel supply according to the present
invention.
[0045] FIG. 3A is a configuration diagram illustrating a fuel
supply system for a high-pressure natural gas injection engine
according to a first embodiment of the present invention.
[0046] FIG. 3B is a configuration diagram illustrating a fuel
supply system for a high-pressure natural gas injection engine
according to a modified example of the first embodiment of the
present invention.
[0047] FIG. 4A is a graph illustrating the freezing points and
boiling points of components contained in a nonflammable mixed
refrigerant according to the present invention.
[0048] FIG. 4B is a graph illustrating the freezing points and
boiling points of components contained in a hydrocarbon mixed
refrigerant.
[0049] FIG. 4C is a graph illustrating a liquefaction temperature
of natural gas according to a compression pressure.
[0050] FIG. 5 is a graph illustrating the boiling points of
components constituting a nonflammable mixed refrigerant.
[0051] FIGS. 6A to 6C are graphs illustrating comparison of power
consumption when a BOG reliquefaction apparatus uses a nitrogen
refrigeration cycle, a nonflammable mixed refrigerant refrigeration
cycle, and a Single Mixed Refrigerant (SMR) refrigeration
cycle.
[0052] FIG. 7A is a configuration diagram illustrating a fuel
supply system for a high-pressure natural gas injection engine
according to a second embodiment of the present invention.
[0053] FIG. 7B is a configuration diagram illustrating a fuel
supply system for a high-pressure natural gas injection engine
according to a modified example of the second embodiment of the
present invention.
[0054] FIG. 8A is a configuration diagram illustrating a fuel
supply system for a high-pressure natural gas injection engine
according to a third embodiment of the present invention.
[0055] FIG. 8B is a configuration diagram illustrating a fuel
supply system for a high-pressure natural gas injection engine
according to a modified example of the third embodiment of the
present invention.
[0056] FIG. 9A is a configuration diagram illustrating a fuel
supply system for a high-pressure natural gas injection engine
according to a fourth embodiment of the present invention.
[0057] FIG. 9B is a configuration diagram illustrating a fuel
supply system for a high-pressure natural gas injection engine
according to a modified example of the fourth embodiment of the
present invention.
[0058] FIG. 10A is a configuration diagram illustrating a fuel
supply system for a high-pressure natural gas injection engine
according to a fifth embodiment of the present invention.
[0059] FIG. 10B is a configuration diagram illustrating a fuel
supply system for a high-pressure natural gas injection engine
according to a modified example of the fifth embodiment of the
present invention.
[0060] FIG. 11 is a configuration diagram illustrating a fuel
supply system for a high-pressure natural gas injection engine
according to a sixth embodiment of the present invention.
DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS
[0061] Exemplary embodiments of the present invention will be
described below in more detail with reference to the accompanying
drawings. The present invention may, however, be embodied in
different forms and should not be construed as limited to the
embodiments set forth herein. Rather, these embodiments are
provided so that this disclosure will be thorough and complete, and
will fully convey the scope of the present invention to those
skilled in the art.
[0062] The International Maritime Organization (IMO) regulates the
emission of nitrogen oxides (NO.sub.X) and sulfur oxides (SO.sub.X)
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 (NO.sub.X) and sulfur oxides
(SO.sub.X) 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.
[0063] Therefore, in order to meet such a provision, a variety of
methods have been introduced to reduce the emission of nitrogen
oxides (NO.sub.X). As one of these methods, a high-pressure natural
gas injection engine for an LNG carrier, for example, an ME-GI
engine, has been developed and used.
[0064] Such an ME-GI engine may be installed in a marine structure
such as an LNG carrier which transports LNG while storing the LNG
in a storage tank capable of withstanding a cryogenic temperature.
The term "marine structure" as used herein includes vessels, such
as an LNG carrier and an LNG RV, and offshore plants, such as an
LNG FPSO and an LNG FSRU. In this case, the ME-GI 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.
[0065] Even in the case of a marine structure equipped with a
high-pressure natural gas injection engine such as an ME-GI engine,
a reliquefaction apparatus is still required for processing BOG
generated in an LNG storage tank. A conventional marine structure
equipped with both the high-pressure natural gas injection engine
such as an ME-GI engine and the reliquefaction apparatus for
processing the BOG can select whether to use BOG as fuel or to use
heavy fuel oil (HFO) as fuel while reliquefying BOG and
transferring the liquefied BOG to a storage tank, depending on the
change in gas and fuel oil prices and the intensity of regulation
of exhaust gases. In particular, when passing through an area of
the sea which is specially regulated, the marine structure can be
fueled by simply gasifying LNG. In addition, the marine structure
is considered as a next-generation environment-friendly engine and
has an efficiency of up to 50%. Therefore, it is expected that the
marine structure will be used as a main engine of an LNG carrier in
the near future.
[0066] FIG. 2 is a schematic block diagram illustrating a fuel
supply method according to the present invention. According to the
fuel supply method of the present invention, BOG generated in a
storage tank, that is, NBOG, is supplied to a BOG compressor and
then compressed to a medium pressure of about 12 to 45 bara. Then,
the medium-pressure BOG is supplied to a reliquefaction apparatus
using a mixed refrigerant (e.g., a nonflammable mixed refrigerant,
a single mixed refrigerant (SMR), etc.) or nitrogen gas as a
refrigerant. BOG reliquefied in the reliquefaction apparatus, that
is, LBOG, is compressed in a fuel supply system to a pressure
required by an ME-GI engine (for example, a high pressure of about
400 bara) and then supplied to the ME-GI engine as fuel. According
to the present invention, since the LBOG supplied from the
reliquefaction apparatus to the fuel supply system is not returned
to the storage tank, it is possible to prevent the generation of
flash gas, which is a problem of the related art. Therefore, the
BOG compressor may compress the BOG to a medium pressure.
[0067] In the present specification, the "high pressure" range
represents a pressure of about 150 to 400 bara, which is a fuel
supply pressure required by the high-pressure natural gas injection
engine. The "medium pressure" range represents a pressure of about
12 to 45 bara, at which the BOG compressor 13 compresses BOG. The
"low pressure" range represents a pressure of about 4 to 8 bara, at
which BOG is compressed for supply to the reliquefaction apparatus
in the related art.
[0068] As compared to the conventional low-pressure reliquefaction,
the reliquefaction after compression in the medium pressure range
results in considerable reliquefaction energy reduction in the
cases of FIGS. 6A and 6B, in which a nitrogen refrigerant and a
nonflammable mixed refrigerant are used, and the case of FIG. 6C,
in which an SMR is used.
[0069] Data shown in FIGS. 6A and 6B are results obtained using a
Hysys process model (manufactured by Aspentech). As can be seen
from these results, in the case of a Mark III reliquefaction
apparatus of Hamworthy Gas Systems which uses nitrogen gas as a
refrigerant (the technology disclosed in International Patent
Publication No. WO 2007/117148), power consumption required for
reliquefaction is about 2,776 kW when the pressure of the BOG
compressor is 8 bara, but power consumption required for
reliquefaction is rapidly reduced to 2,500 kW when the pressure of
the BOG compressor increases to 12 bara. Also, when the pressure of
the BOG compressor is 12 bara or more, power consumption required
for reliquefaction is gradually reduced.
[0070] A graph of FIG. 6C illustrates a variation in power
consumption when a hydrocarbon SMR is used as a refrigerant. As can
be seen from the result of FIG. 6C, even when the SMR is used as a
refrigerant, power consumption required for reliquefaction is
rapidly reduced in the case where the pressure of the BOG
compressor is 12 bara, as compared to the case where the pressure
of the BOG compressor is 8 bara. Also, when the pressure of the BOG
compressor is 12 bara or more, power consumption required for
reliquefaction is gradually reduced.
[0071] The composition of the SMR by liquefaction pressure was
adjusted for efficiency optimization, as shown in Table 1
below.
TABLE-US-00001 TABLE 1 Refrigerant Composition (mol %) 8 bara 12
bara 30 bara 45 bara Nitrogen 11.91 5.55 0.00 0.00 Methane 45.11
48.54 45.81 36.63 Ethane 17.68 18.66 22.84 30.74 Propane 10.57
11.30 13.70 13.05 i-Pentane 14.74 15.95 17.65 19.58
[0072] In the case of the reliquefaction apparatus using the
nonflammable mixed refrigerant described herein (NFMR, composition
of Table 4 below), energy required for reliquefaction is further
reduced than the case of the reliquefaction apparatus using the
nitrogen refrigerant.
[0073] According to the present invention, the preferable pressure
range of the BOG is a medium pressure range of about 12 to 45 bara.
A pressure lower than 12 bara is not preferable because power
consumption required for reliquefaction is not greatly reduced.
Also, a pressure higher than 45 bara is not preferable because
energy required for reliquefaction is not greatly reduced.
First Embodiment
[0074] FIG. 3A is a configuration diagram illustrating a fuel
supply system for a marine structure such as an LNG carrier having
a high-pressure natural gas injection engine, for example, an ME-GI
engine, according to a first embodiment of the present invention.
FIG. 3A illustrates an example in which the fuel supply system for
the high-pressure natural gas injection engine according to the
present invention is applied to the LNG carrier equipped with the
ME-GI engine capable of using natural gas as fuel. However, the
fuel supply system for the high-pressure natural gas injection
engine according to the present invention may also be applied to
any type of marine structures equipped with a liquefied gas storage
tank. Examples of the marine structures may include vessels, such
as an LNG carrier and an LNG RV, and offshore plants, such as an
LNG FPSO and an LNG FSRU.
[0075] According to the fuel supply system for the marine structure
having the high-pressure natural gas injection engine according to
the first embodiment of the present invention, NBOG generated in
and discharged from a liquefied gas storage tank 11 is compressed
to a medium pressure of about 12 to 45 bara (absolute pressure) by
a BOG compression unit 13 and then supplied to a reliquefaction
apparatus 20. LBOG reliquefied in the reliquefaction apparatus 20
by receiving reliquefaction energy (i.e., cold heat) is compressed
to a high pressure of about 150 to 400 bara by a high-pressure pump
33 and then supplied to a high-pressure gasifier 37. Then, the LBOG
is gasified by the high-pressure gasifier 37 and then supplied as
fuel to the high-pressure natural gas injection engine, for
example, the ME-GI engine.
[0076] Since the liquefied BOG (i.e., LNG) compressed to a high
pressure by the high-pressure pump 33 is in a supercritical
pressure state, it is actually difficult to distinguish a liquid
phase from a gaseous phase. In the present specification, heating
the liquefied BOG to ambient temperature (or temperature required
in the high-pressure natural gas injection engine) in a
high-pressure state is referred to as gasification, and a unit
configured to heat the liquefied BOG to ambient temperature in a
high-pressure state is referred to as a high-pressure gasifier.
[0077] The storage tank includes a sealing and insulating barrier
to store liquefied gas such as LNG in a cryogenic state. However,
the storage tank cannot completely interrupt heat transmitted from
the outside. Accordingly, liquefied gas is continuously boiled off
in the storage tank 110. In order to maintain the pressure of BOG
in the storage tank 11, at a suitable level, the BOG is discharged
through a BOG discharge line L1.
[0078] The discharged BOG is supplied to the BOG compression unit
13 through the BOG discharge line L1. The BOG compression unit 13
includes one or more BOG compressors 14 and one or more
intermediate coolers 15 configured to cool the BOG; a temperature
of which increases while being compressed by the BOG compressors
14. A five-stage BOG compression unit 13 including five BOG
compressors 14 and five intermediate coolers 15 is exemplarily
illustrated in FIG. 3A.
[0079] The BOG compressed by the BOG compression unit 13 is
supplied to the reliquefaction apparatus 20 through a BOG supply
line L2. The BOG supplied to the reliquefaction apparatus 20 is
cooled and reliquefied by a refrigerant while passing through a
cold box 21 of the reliquefaction apparatus 20. The reliquefaction
apparatus 20 may have any configuration as long as the
reliquefaction apparatus 20 can liquefy BOG generated from the
liquefied gas such as LNG.
[0080] The BOG reliquefied through heat exchange in the cold box 21
is separated into a gaseous state and a liquid state in a buffer
tank 31. Only the liquefied BOG of a liquid state is supplied to
the high-pressure pump 33 through a fuel supply line L3. A
plurality of high-pressure pump 33 (for example, two high-pressure
pumps) may be installed in parallel.
[0081] The high-pressure pump 33 compresses the liquefied BOG to a
fuel supply pressure required in the high-pressure natural gas
injection engine (e.g., an ME-GI engine). The liquefied BOG
supplied from the high-pressure pump 33 has a high pressure of
about 150 to 400 bara (absolute pressure).
[0082] The reliquefaction apparatus 20 exemplarily illustrated in
FIG. 3A includes a cold box 21 configured to reliquefy BOG through
heat exchange with a refrigerant, one or more gas-liquid
refrigerant separators 22 configured to separate a refrigerant,
which is heated and partially gasified by the cold box 21, into a
gaseous refrigerant and a liquid refrigerant, one or more
refrigerant compressors 23 configured to compress the gaseous
refrigerant separated by the gas-liquid refrigerant separators 22,
a refrigerant cooler 24 configured to cool the refrigerant
compressed by the refrigerant compressors 23, a refrigerant
expansion valve 25 configured to drop the temperature of the
refrigerant by expanding the refrigerant compressed by the
refrigerant compressors 23 and cooled by the refrigerant cooler 24,
and a refrigerant pump 26 configured to supply the liquid
refrigerant separated by the gas-liquid refrigerant separators 22
to the refrigerant expansion valve 25.
[0083] The refrigerant supplied to the refrigerant expansion valve
25 through the refrigerant pump 26 may be mixed with the
refrigerant supplied to the refrigerant expansion valve 25 after
passing through the refrigerant cooler 24 in an upstream side of
the refrigerant expansion valve 25.
[0084] Meanwhile, the refrigerant supplied to the refrigerant
expansion valve 25 may exchange heat with the refrigerant that
passes through the cold box 21 before expansion and has a cryogenic
state after expansion.
[0085] In addition, the refrigerant cooled by the refrigerant
cooler 24 may be supplied to another gas-liquid refrigerant
separator and separated into a gaseous refrigerant and a liquid
refrigerant. To this end, although the liquefaction apparatus 20
including two gas-liquid refrigerant separators 22, two refrigerant
compressors 23, two refrigerant coolers, and two refrigerant pumps
26 is exemplarily illustrated in FIG. 3A, the present invention is
not limited thereto. The number of the respective components
included in the liquefaction apparatus 20 may be increased or
decreased, depending on a design thereof.
Modified Example of First Embodiment
[0086] FIG. 3B illustrates a fuel supply system according to a
modified example of the first embodiment of the present invention.
Since the modified example of the first embodiment is partially
different from the first embodiment in terms of the configurations
of a BOG compression unit 13 and a liquefaction apparatus 20, only
the difference therebetween will be described below.
[0087] The modified example of the first embodiment illustrated in
FIG. 3B is substantially identical to the first embodiment
illustrated in FIG. 3A in that the BOG compressing unit 13 includes
five BOG compressors 14, but is different from the first embodiment
in that an intermediate cooler 15 is not disposed between the first
and second BOG compressors and between the second and third BOG
compressors included in the BOG compression unit 13. According to
the present invention, the intermediate cooler 15 may or may not be
disposed between every two BOG compressors 14.
[0088] Also, the liquefaction apparatus 20 according to the
modified example of the first embodiment illustrated in FIG. 3B
includes a cold box 21 configured to exchange heat between a
refrigerant and BOG, a compression unit configured to compress a
refrigerant that is heated and at least gasified by the cold box
21, and an expansion unit configured to expand the compressed
refrigerant to drop the temperature thereof.
[0089] More specifically, the liquefaction apparatus 20 according
to the modified example of the first embodiment illustrated in FIG.
3B includes a cold box 21 configured to reliquefy BOG by heat
exchange between a refrigerant and BOG, a first gas-liquid
refrigerant separator 22a configured to separate the refrigerant,
which is heated and partially gasified by the cold box 21, into a
gaseous refrigerant and a liquid refrigerant, a first refrigerant
compressor 23a configured to compress the gaseous refrigerant
separated by the first gas-liquid refrigerant separator 22a, a
first refrigerant cooler 24a configured to cool the refrigerant
compressed by the first refrigerant compressor 23a, a second
gas-liquid refrigerant separator 22b configured to secondarily
separate the refrigerant, which is cooled by the first refrigerant
cooler 24a, into a gaseous refrigerant and a liquid refrigerant, a
second refrigerant compressor 23b configured to compress the
gaseous refrigerant separated by the second gas-liquid refrigerant
separator 22b, a second refrigerant cooler 24b configured to cool
the refrigerant compressed by the second refrigerant compressor
23b, a first refrigerant pump 26a configured to supply the liquid
refrigerant, which is separated by the first gas-liquid refrigerant
separator 22a, to the second refrigerant cooler 24b, a second
refrigerant pump 26b configured to supply the liquid refrigerant,
which is separated by the second gas-liquid refrigerant separator
22b, to the second refrigerant cooler 24b, a third gas-liquid
refrigerant separator 22c configured to tertiarily separate the
refrigerant, which is cooled by the second refrigerant cooler 24b,
into a gaseous refrigerant and a liquid refrigerant, a refrigerant
expansion valve 25 configured to expand the liquid refrigerant
separated by the third gas-liquid refrigerant separator 22c to drop
the temperature thereof, and a third refrigerant pump 26c
configured to supply the liquid refrigerant from the third
gas-liquid refrigerant separator 22c to the refrigerant expansion
valve 25.
[0090] The liquid refrigerants supplied from the first and second
gas-liquid refrigerant separators 22a and 22b to the second
refrigerant cooler 24b may be joined together. Then, the joined
refrigerants may be mixed with the gaseous refrigerant supplied
from the second refrigerant compressor 23b to the second
refrigerant cooler 24b and then be supplied to the second
refrigerant cooler 24b. In addition, the gaseous refrigerant
separated by the third gas-liquid refrigerant separator 22c may be
mixed with the liquid refrigerant supplied to the refrigerant
expansion valve 25 by the third refrigerant pump 26c. Moreover, the
refrigerant supplied to the refrigerant expansion valve 25 may
exchange heat with the refrigerant that passes through the cold box
21 before expansion and has a cryogenic state after expansion.
[0091] The reliquefaction apparatus 20 of FIG. 3B is merely
exemplary and does not limit the present invention. The
configuration of the reliquefaction apparatus, if necessary, may be
modified depending on a design thereof.
[0092] (Nonflammable Mixed Refrigerant)
[0093] According to the present invention, as the refrigerant
circulating within the reliquefaction apparatus 20, a nonflammable
mixed refrigerant including R14 may be used, as opposed to the
related art. The nonflammable mixed refrigerant prepared by mixing
a plurality of nonflammable refrigerants has a mixture composition
ratio such that the refrigerant is not condensed even at a
liquefaction temperature when BOG compressed to a medium pressure
is reliquefied.
[0094] A refrigeration cycle using a phase change of the mixed
refrigerant has higher efficiency than a nitrogen refrigeration
cycle using only nitrogen as a refrigerant. A conventional mixed
refrigerant has a safety problem because a flammable refrigerant is
mixed therein. However, the nonflammable mixed refrigerant
according to the present invention has high safety because the
nonflammable mixed refrigerant is prepared by mixing nonflammable
refrigerants.
[0095] The nonflammable mixed refrigerant according to the present
invention may make it possible to apply a mixed-refrigerant
Joule-Thomson refrigeration cycle to an offshore LNG reliquefaction
apparatus. Meanwhile, it is known that the mixed refrigerant is
used in a conventional onshore LNG liquefaction plant. Since such a
mixed refrigerant is an explosive hydrocarbon (HC) mixed
refrigerant, it has been difficult to treat the mixed refrigerant.
However, the nonflammable mixed refrigerant according to the
present invention is non-explosive because the nonflammable mixed
refrigerant includes argon, a hydrofluorocarbon (HFC) refrigerant,
and a fluorocarbon (FC) refrigerant.
[0096] As the HFC/FC refrigerants, refrigerants listed in Table 2
below may be used. In Table 2 below, argon is also added.
TABLE-US-00002 TABLE 2 Boiling Point Refrigerant No. Chemical
Formula Mole. weight (NBP)(.degree. C.) Ar Ar 39.95 -185.9 R14 CF4
88 -128.1 R23 CHF3 70.01 -82.1 R116 CF3CF3 138.01 -78.2 R41 CH3F
34.03 -78.1 R32 CH2F2 52.02 -51.7 R125 CHF2CF3 120.02 -48.1 R143a
CH3CF3 84.04 -47.2 R161 CH3CHF2 48.06 -37.1 R218 CF3CF2CF3 188.02
-36.6 R134a CH2FCF3 102.03 -26.1 R152a CH3CHF2 66.05 -24 R227ea
CF3CHFCF3 170.03 -15.6 R236fa CF3CH2CF3 152.04 -1.4 R245fa
CHF2CH2CF3 134.05 15.1
[0097] In addition to the refrigerants listed in Table 2 above,
refrigerants prepared by mixing these refrigerants may be used,
with separate refrigerant numbers (R400 and R500) assigned thereto.
These HFC/FC mixed refrigerants are listed in Table 3 below.
TABLE-US-00003 TABLE 3 Chemical Formula Boiling Point Refrigerant
No. (mass ratio) Mole. weight (NBP)(.degree. C.) R410A
R32/125(50/50) 72.58 -51.6 R410B R32/125(45/55) 75.57 -51.5 R507
R125/143a(50/50) 98.86 -47.1 R407B R32/125/134a(10/70/20) 102.94
-46.8 R404A R125/143a/134a(44/52/4) 97.6 -46.6 R407A
R32/125/134a(20/40/40) 90.11 -45.2 R407C R32/125/134a(23/25/52)
86.2 -43.8 R407E R32/125/134a(25/15/60) 83.78 -42.8 R407D
R32/125/134a(15/15/70) 90.96 -39.4
[0098] On the other hand, as illustrated in FIGS. 4A and 4B, the
HFC/FC refrigerants may not be used as a refrigerant when LNG is
reliquefied, because the freezing points of the HFC/FC refrigerants
are higher than a general temperature (-163.degree. C.). However,
the inventors of the present patent application developed a
reliquefaction apparatus that could reliquefy BOG generated in an
LNG storage tank of a marine structure by a high-efficiency, safe
HFC/FC mixed refrigerant (i.e., nonflammable mixed refrigerant)
Joule-Thomson refrigeration cycle, based on the knowledge that the
liquefaction (or reliquefaction) temperature increases as the
pressure of natural gas (or BOG) increases. In other words,
according to the present invention, by compressing BOG to a medium
pressure of about 12 to 45 bara before reliquefaction, the BOG can
be reliquefied at a temperature higher than the reliquefaction
temperature of the BOG at ambient pressure, that is, the freezing
point of the nonflammable mixed refrigerant.
[0099] The nonflammable mixed refrigerant according to the present
invention is prepared by mixing a variety of components, such that
the boiling points are equally distributed between the liquefaction
temperature of the natural gas (or the reliquefaction temperature
of the BOG) and room temperature and thus a wide phase change range
can be used. Refrigerants having similar boiling points are
classified into five groups, and the nonflammable mixed refrigerant
according to the present invention may be prepared by selecting one
or more components from each group. That is, the nonflammable mixed
refrigerant according to the present invention may be prepared by
selecting at least one component from each of the five groups.
[0100] As illustrated in FIG. 5, the group I includes argon (Ar)
having the lowest boiling point among the refrigerants, and the
group II includes R14. The group III includes R23, R116, and R41,
and the group IV includes R32, R410A, R410B, R125, R143a, R507,
R407B, R404A, R407A, R407C, R407E, R407D, R161, R218, R134a, R152a,
and R227ea. The group V includes R236fa and R245fa.
[0101] When considering the easy supply of refrigerants and the
costs thereof, the nonflammable mixed refrigerant according to the
present invention, which is prepared by selecting one or more
refrigerants from each of the five groups, may have components as
shown in Table 4 below. It is preferable in terms of efficiency to
determine the composition ratio of the nonflammable mixed
refrigerant such that a difference in temperature between a
high-temperature fluid (i.e., BOG) and a low-temperature fluid
(i.e., the nonflammable mixed refrigerant) in the heat exchanger
(i.e., the cold box 21) exchanging heat with the BOG is maintained
as constantly as possible.
TABLE-US-00004 TABLE 4 Component Composition (% mole) Ar 20 to 55
R14 15 to 30 R23 5 to 15 R410a 10 to 15 R245fa 15 to 20
[0102] In the case where the nonflammable mixed refrigerant is
used, power consumption (kW) can be reduced to improve the
reliquefaction efficiency, as compared to the related art in which
BOG is reliquefied using the nitrogen refrigerant.
[0103] More specifically, according to the present invention, the
BOG reliquefaction is achieved by compressing BOG at a medium
pressure of about 12 to 45 bara, which is relatively higher than
the BOG reliquefaction pressure used in the conventional
reliquefaction apparatus. Therefore, power consumption for the BOG
reliquefaction can be reduced. In particular, in the case where the
nonflammable mixed refrigerant having the above-mentioned
composition is used, the reliquefaction apparatus can maintain the
highest efficiency when BOG has a pressure of about 12 to 45
bara.
[0104] Also, when the pressure of BOG is 12 bara, the
reliquefaction temperature is about -130.degree. C. In order to
cool the BOG to the reliquefaction temperature, the temperature of
the nonflammable mixed refrigerant is lowered to about -155.degree.
C. The nonflammable mixed refrigerant having the above-mentioned
composition may be frozen at a temperature of below -155.degree. C.
Thus, if the pressure of the BOG is lower than 12 bara, it may be
difficult to configure the refrigeration cycle using the
nonflammable mixed refrigerant.
[0105] Also, if the pressure of the BOG exceeds 45 bara, it is not
preferable because the reduction of liquefaction energy is not
great as compared to the increase of power consumption necessary to
compress the BOG.
[0106] Referring to FIG. 6A, since the present invention is
characterized by the medium pressure, that is, the pressure range
of about 12 to 45 bara (based on 4.3 ton/h of BOG), the present
invention has effects in both the nitrogen refrigerant and the
nonflammable mixed refrigerant. However, as compared to the
reliquefaction apparatus using the nitrogen refrigerant, the
reliquefaction apparatus using the nonflammable mixed refrigerant
having the above-mentioned composition according to the present
invention can further reduce power by about 10 to 20%.
[0107] FIG. 6B is a graph illustrating comparison of required power
in the condition of the conventional reliquefaction apparatus (that
is, in the case where the refrigerant used in the reliquefaction
apparatus is nitrogen gas (N.sub.2) and the pressure of BOG
supplied to the reliquefaction apparatus is 8 bara) and required
power in the condition of the reliquefaction apparatus using the
nonflammable mixed refrigerant (NFMR) according to the present
invention (that is, in the case where the refrigerant used in the
reliquefaction apparatus is the nonflammable mixed refrigerant
(NFMR) and the pressure of the BOG supplied to the reliquefaction
apparatus is 12 to 45 bara). Referring to FIG. 6B, the
reliquefaction apparatus according to the present invention can be
operated with 50 to 80% of power consumed in the conventional
reliquefaction apparatus (refrigeration cycle) using the nitrogen
refrigerant. As such, since the reliquefaction apparatus according
to the present invention can be operated with relatively low power
as compared to the related art, the capacity of a power generator
can be reduced, making it possible to miniaturize the power
generator.
[0108] Meanwhile, the reliquefaction apparatus according to the
present invention uses the Joule-Thomson valve as a refrigerant
expansion unit, the entire system is simplified and cost-effective,
as compared to a conventional nitrogen (N.sub.2) compander.
[0109] Moreover, although not listed in Table 2 above, the
nonflammable mixed refrigerant according to the present invention
may contain a slight amount of nonflammable refrigerant components
other than the components listed in Table 2 above.
Second Embodiment
[0110] FIG. 7A is a configuration diagram illustrating a fuel
supply system for a marine structure having a high-pressure natural
gas injection engine (e.g., an ME-GI engine) according to a second
embodiment of the present invention. The second embodiment
illustrated in FIG. 7A is different from the first embodiment only
in that before a reliquefaction apparatus reliquefies BOG
compressed to a medium pressure, the fuel supply system preheats
the compressed BOG by heat exchange with LNG supplied from a
high-pressure pump 33 to a high-pressure gasifier 37. Thus, the
following description will be focused on the difference from the
first embodiment.
[0111] As illustrated in FIG. 7A, the liquefied BOG compressed to a
high pressure by a high-pressure pump 33 exchanges heat with the
BOG supplied to a reliquefaction apparatus 20, in a heat exchanger
35 before supply to a high-pressure gasifier 37. Since the
liquefied BOG supplied to the high-pressure gasifier 37 is lower in
temperature than the BOG supplied to the reliquefaction apparatus
20, it can reduce the temperature of the BOG supplied to the
reliquefaction apparatus 20 while passing through the heat
exchanger 35, thus making it possible to reduce reliquefaction
energy in the reliquefaction apparatus 20. In addition, the
liquefied BOG supplied to the high-pressure gasifier 37 is heated
while passing through the heat exchanger 35, thus making it
possible to reduce gasification energy in the high-pressure
gasifier 37.
[0112] The BOG compressed in a BOG compressor 13 is supplied
through a BOG supply line L2 to the reliquefaction apparatus 20.
The heat exchanger 35 is installed in the middle of the BOG supply
line L2. As described above, in the heat exchanger 35, the
higher-temperature compressed BOG exchanges heat with the
lower-temperature liquefied BOG discharged from the high-pressure
pump 33. The BOG cooled while passing through the heat exchanger 35
is cooled and reliquefied by a refrigerant while passing through a
cold box 21.
Modified Example of Second Embodiment
[0113] FIG. 7B is a configuration diagram illustrating a fuel
supply system according to a modified example of the second
embodiment of the present invention. As described in the modified
example of the first embodiment, the modified example of the second
embodiment is partially different from the second embodiment in
terms of the configurations of a BOG compression unit 13 and a
reliquefaction apparatus 20.
[0114] That is, the modified example of the second embodiment is
substantially identical to the second embodiment in that the BOG
compression unit 13 includes five BOG compressors 14, but is
different from the second embodiment in that an intermediate cooler
15 is not disposed between the first and second BOG compressors and
between the second and third BOG compressors included in the BOG
compression unit 13. According to the present invention, the
intermediate cooler 15 may or may not be disposed between every two
BOG compressors 14.
[0115] Like the reliquefaction apparatus 20 according to the
modified example of the first embodiment illustrated in FIG. 3B,
the reliquefaction apparatus 20 according to the modified example
of the second embodiment includes a cold box 21 configured to
exchange heat between a refrigerant and BOG, a compression unit
configured to compress the refrigerant that is heated and at least
partially gasified by the cold box 21, an expansion unit configured
to expand the compressed refrigerant to reduce the temperature
thereof, and a gas-liquid refrigerant separator configured to
separate the gaseous refrigerant and the liquid refrigerant.
[0116] In particular, like the reliquefaction apparatus 20
according to the modified example of the first embodiment
illustrated in FIG. 3B, the reliquefaction apparatus 20 according
to the modified example of the second embodiment includes a
plurality of gas-liquid refrigerant separators 22a, 22b and 22c. A
gaseous refrigerant and a liquid refrigerant separated by the
gas-liquid refrigerant separators 22a and 22b disposed at the
upstream side are mixed and then supplied to the gas-liquid
refrigerant separator 22c disposed at the most downstream side. The
gaseous refrigerant separated by the gas-liquid refrigerant
separators 22a and 22b disposed at the upstream side may be
compressed by refrigerant compressors 23a and 23b and cooled by
refrigerant coolers 24a and 24b before supply to the gas-liquid
refrigerant separator 22c disposed at the most downstream side. The
liquid refrigerant separated by the gas-liquid refrigerant
separators 22a and 22b disposed at the upstream side is mixed with
the gaseous refrigerant before the gaseous refrigerant is supplied
to the gas-liquid refrigerant separator 22c disposed at the most
downstream side, specifically before the gaseous refrigerant is
cooled by the refrigerant cooler 24b.
Third Embodiment
[0117] FIG. 8A is a configuration diagram illustrating a fuel
supply system for a marine structure having a high-pressure natural
gas injection engine (e.g., an ME-GI engine) according to a third
embodiment of the present invention. The third embodiment
illustrated in FIG. 8A is different from the first embodiment only
in that the fuel supply system preheats BOG before compression.
Thus, the following description will be focused on the difference
from the first embodiment.
[0118] As illustrated in FIG. 8A, in the fuel supply system for a
marine structure having a high-pressure natural gas injection
engine according to the third embodiment of the present invention,
natural boil-off gas (NBOG) generated and discharged from a
liquefied gas storage tank 11 is compressed by a BOG compression
unit 13 to a medium pressure of about 12 to 45 bara. The compressed
BOG is supplied to a BOG preheater 41 installed at the upstream
side of a BOG compression unit 13, before supply to a
reliquefaction apparatus 20. The BOG compressed by the BOG
compression unit 13 to about 12 to 45 bara and cooled by an
intermediate cooler 15 to about 40.degree. C. is cooled by heat
exchange with cryogenic BOG discharged from the liquefied gas
storage tank 11 to the BOG preheater 41, and then is supplied to
the reliquefaction apparatus 20.
[0119] According to the third embodiment, the temperature of the
BOG to be supplied to the reliquefaction apparatus 20 can be
reduced through the BOG preheater 41, so that a heat load on the
cold box 21 can be reduced. Also, the cryogenic BOG supplied to the
BOG compression unit 13 and the higher-temperature BOG compressed
by the BOG compression unit 13 exchange heat with each other in the
BOG preheater 41 located at the upstream side of the BOG
compression unit 13, so that the temperature of the BOG supplied to
the BOG compression unit can be increased and the inlet temperature
of the BOG compression unit (i.e., the BOG compressor) can be
maintained constant.
[0120] As in the fuel supply system according to the first
embodiment, BOG compressed by the BOG compression unit 13 and then
passing through the BOG preheater 41 is supplied to the
reliquefaction apparatus 20. Liquefied BOG (LBOG) reliquefied by
liquefaction energy (i.e., cold heat) in the reliquefaction
apparatus 20 is compressed by a high-pressure pump 33 to a high
pressure of about 150 to 400 bara and then is supplied to a
high-pressure gasifier 37. BOG gasified by the high-pressure
gasifier 37 is supplied as fuel to a high-pressure natural gas
injection engine (e.g., an ME-GI engine).
Modified Example of Third Embodiment
[0121] FIG. 8B is a configuration diagram illustrating a fuel
supply system according to a modified example of the third
embodiment of the present invention. The modified example of the
third embodiment is partially different from the third embodiment
in terms of the configuration of a reliquefaction apparatus 20.
[0122] That is, like the reliquefaction apparatus 20 according to
the modified example of the first embodiment illustrated in FIG.
3B, the reliquefaction apparatus 20 according to the modified
example of the third embodiment includes a cold box 21 configured
to exchange heat between a refrigerant and BOG, a compression unit
configured to compress the refrigerant that is heated and at least
partially gasified by the cold box 21, an expansion unit configured
to expand the compressed refrigerant to reduce the temperature
thereof, and a gas-liquid refrigerant separator configured to
separate the gaseous refrigerant and the liquid refrigerant.
[0123] In particular, like the reliquefaction apparatus 20
according to the modified example of the first embodiment
illustrated in FIG. 3B, the reliquefaction apparatus 20 according
to the modified example of the third embodiment includes a
plurality of gas-liquid refrigerant separators 22a, 22b and 22c. A
gaseous refrigerant and a liquid refrigerant separated by the
gas-liquid refrigerant separators 22a and 22b disposed at the
upstream side are mixed and then supplied to the gas-liquid
refrigerant separator 22c disposed at the most downstream side. The
gaseous refrigerant separated by the gas-liquid refrigerant
separators 22a and 22b disposed at the upstream side may be
compressed by refrigerant compressors 23a and 23b and cooled by
refrigerant coolers 24a and 24b before supply to the gas-liquid
refrigerant separator 22c disposed at the most downstream side. The
liquid refrigerant separated by the gas-liquid refrigerant
separators 22a and 22b disposed at the upstream side is mixed with
the gaseous refrigerant before the gaseous refrigerant is supplied
to the gas-liquid refrigerant separator 22c disposed at the most
downstream side, specifically before the gaseous refrigerant is
cooled by the refrigerant cooler 24b.
Fourth Embodiment
[0124] FIG. 9A is a configuration diagram illustrating a fuel
supply system for a marine structure having a high-pressure natural
gas injection engine (e.g., an ME-GI engine) according to a fourth
embodiment of the present invention. The fourth embodiment
illustrated in FIG. 9A is different from the third embodiment in
that the fuel system further includes an excess BOG consumption
unit (e.g., a Dual-Fuel Diesel Engine (DFDE)) configured to process
excess BOG and a stable fuel supply unit (e.g., an LNG supply
line). Thus, the following description will be focused on the
difference from the third embodiment.
[0125] Herein, the excess BOG is the BOG that is more than the
amount of liquefied BOG required by the high-pressure natural gas
injection engine. The excess BOG may be generated when a large
amount of BOG is generated, even when the high-pressure natural gas
injection engine is in operation. Also, the excess BOG may be
generated when the high-pressure natural gas injection engine
operates at a low speed or does not operate (e.g., when entering
into port or passing through a canal).
[0126] In the fuel supply system for a marine structure having a
high-pressure natural gas injection engine according to the fourth
embodiment of the present invention, when a load on the
high-pressure natural gas injection engine decreases, or when an
excess amount of BOG is generated, excess LBOG is decompressed
through an LBOG expansion valve 51 installed at an LBOG return line
L4 branching at a rear end of a buffer tank 31 from a fuel supply
line L3. LBOG including flash gas generated in the decompression
process is separated by a gas-liquid separator into a liquid
component (LBOG) and a gaseous component (flash gas), and the
liquid component is returned through the LBOG return line L4 to a
storage tank 11.
[0127] Specifically, the LBOG decompressed through the LBOG
expansion valve and including flash gas is supplied to a gas-liquid
LBOG separator 53 and divided by the gas-liquid LBOG separator 53
into a liquid component and a gaseous component. The gaseous
component (i.e., flash gas) separated by the gas-liquid LBOG
separator 53 is supplied as fuel through a fuel gas supply line L6
to an excess BOG consumption unit (e.g., a DFDE) that may be
installed in the marine structure for power generation. The
pressure of fuel gas supplied to the DFDE may be controlled by a
pressure control valve that is installed at the downstream side of
the gas-liquid LBOG separator 53 in the middle of the fuel gas
supply line L6. Also, by a fuel gas heater 55 installed in the
middle of the fuel gas supply line L6, the temperature of the fuel
gas may be heated up to a temperature required by the DFDE. The
liquid component separated by the gas-liquid LBOG separator 53 is
returned through the LBOG return line L4 to the storage tank.
[0128] Since the fuel gas supply pressure of the DFDE is generally
about 5 to 8 bara, the pressure of the liquid component separated
by the gas-liquid LBOG separator 53 may still be higher than
ambient pressure. In this case, the liquid component (i.e., LBOG)
separated by the gas-liquid LBOG separator 53 is additionally
decompressed through another LBOG expansion valve 52. The
decompressed liquid component is supplied to another gas-liquid
LBOG separator 54 and separated by the gas-liquid LBOG separator 54
into a liquid component (LBOG) and a gaseous component (flash gas).
The ambient-pressure liquid component separated by the gas-liquid
LBOG separator 54 is returned through the LBOG return line L4 to
the storage tank 11. The gaseous component separated by the
gas-liquid LBOG separator 54 may be supplied to and consumed by a
gas combustion unit (GCU) serving as another excess BOG consumption
unit.
[0129] On the other hand, when insufficient fuel is supplied to the
DFDE, additional fuel may be supplied to the DFDE through a branch
line L5 that branches from the fuel supply line L3 supplying fuel
to the high-pressure natural gas injection engine (e.g., ME-GI
engine) and connects with the fuel gas supply line L6 supplying
fuel to the DFDE. A pressure drop valve is installed at the branch
line L5.
[0130] Also, when the BOG reliquefaction apparatus does not operate
or when a small amount of BOG is generated in the storage tank 11,
LNG stored in the storage tank 11 may be supplied to the buffer
tank 31 through an LNG supply line L7 and an LNG supply pump 57
installed in the storage tank 11.
[0131] In this manner, the DFDE functions as a flash gas processing
unit that can process flash gas that may be generated from LBOG in
the process of returning to the storage tank 11 due to a pressure
difference.
[0132] Although not illustrated in the drawings, the gaseous
component separated by the gas-liquid LBOG separator 53 may be
supplied and used as fuel for a consumption unit such as a gas
turbine or a boiler, instead of the DFDE. Also, the gaseous
component may be supplied to and processed by a gas discharge
device discharging natural gas into the atmosphere or a gas
combustion device (e.g., a flare tower) combusting natural gas in
the atmosphere. In this case, the DFDE, the gas turbine, the
boiler, the gas discharge device, or the flare tower may be
included in the excess BOG consumption unit (or the flash gas
processing unit), and the gaseous component supplied to the excess
BOG consumption unit may be heated by the fuel gas heater 55.
[0133] When the BOG compressed by the BOG compression unit 13 to a
medium pressure of about 12 to 45 bara and then liquefied by the
reliquefaction apparatus 20 is not completely consumed by the
high-pressure natural gas injection engine such as an ME-GI engine,
the medium-pressure liquefied BOG needs to be returned to the
storage tank 11. Since the storage tank 11 is in the state of
ambient pressure, the pressure of the liquefied BOG needs to be
reduced before the liquefied BOG is supplied to the storage tank.
However, flash gas is generated in the process of reducing the
pressure. Therefore, the inventors of the present invention
invented the fuel supply system including the excess BOG
consumption unit capable of processing flash gas. The present
invention provides the fuel supply system including the excess BOG
consumption unit capable of processing flash gas. Therefore, the
BOG compressed to a medium pressure of about 12 to 45 bara can be
supplied to the reliquefaction apparatus. Accordingly, the energy
consumption in reliquefaction can be reduced.
Modified Example of Fourth Embodiment
[0134] FIG. 9B is a configuration diagram illustrating a fuel
supply system according to a modified example of the fourth
embodiment of the present invention. The modified example of the
fourth embodiment is partially different from the fourth embodiment
in terms of the configuration of a reliquefaction apparatus 20 and
is also different from the fourth embodiment in that the fuel
supply system processes excess BOG through a line branching from a
BOG compression unit 13 or the end of a downstream side
thereof.
[0135] Like the reliquefaction apparatus 20 according to the
modified example of the first embodiment illustrated in FIG. 3B,
the reliquefaction apparatus 20 according to the modified example
of the fourth embodiment includes a cold box 21 configured to
exchange heat between a refrigerant and BOG, a compression unit
configured to compress the refrigerant that is heated and at least
partially gasified by the cold box 21, an expansion unit configured
to expand the compressed refrigerant to reduce the temperature
thereof, and a gas-liquid refrigerant separator configured to
separate the gaseous refrigerant and the liquid refrigerant.
[0136] In particular, like the reliquefaction apparatus 20
according to the modified example of the first embodiment
illustrated in FIG. 3B, the reliquefaction apparatus 20 according
to the modified example of the fourth embodiment includes a
plurality of gas-liquid refrigerant separators 22a, 22b and 22c. A
gaseous refrigerant and a liquid refrigerant separated by the
gas-liquid refrigerant separators 22a and 22b disposed at the
upstream side are mixed and then supplied to the gas-liquid
refrigerant separator 22c disposed at the most downstream side. The
gaseous refrigerant separated by the gas-liquid refrigerant
separators 22a and 22b disposed at the upstream side may be
compressed by refrigerant compressors 23a and 23b and cooled by
refrigerant coolers 24a and 24b before supply to the gas-liquid
refrigerant separator 22c disposed at the most downstream side. The
liquid refrigerant separated by the gas-liquid refrigerant
separators 22a and 22b disposed at the upstream side is mixed with
the gaseous refrigerant before the gaseous refrigerant is supplied
to the gas-liquid refrigerant separator 22c disposed at the most
downstream side, specifically before the gaseous refrigerant is
cooled by the refrigerant cooler 24b.
[0137] Also, in the fuel supply system according to the modified
example of the fourth embodiment, when an excess amount of BOG is
generated, the excess BOG may be supplied to a DFDE serving as an
excess BOG consumption unit, through a second branch line L8
branching from a BOG compression unit 13. In this case, since the
BOG is cooled to about 40.degree. C. by an intermediate cooler 15
included in the BOG compression unit 13, a separate heater
configured to control the temperature of BOG supplied to the DFDE
may be omitted.
[0138] Alternatively, the excess BOG may be supplied to a gas
turbine serving as another excess BOG consumption unit, through a
third branch line L9 branching from a rear end of the BOG
compression unit 13. Likewise, in this case, a separate unit
configured to control the temperature of BOG supplied to the gas
turbine may be omitted.
[0139] Also, unlike the fuel supply system according to the fourth
embodiment, the fuel supply system according to the modified
example of the fourth embodiment includes an LBOG expansion valve
and a gas-liquid LBOG separator installed at an LBOG return line
L4. However, if necessary, the fuel supply system according to the
modified example of the fourth embodiment may further include
another LBOG expansion valve 52 and another gas-liquid LBOG
separator 54 like the fuel supply system according to the fourth
embodiment.
Fifth Embodiment
[0140] FIG. 10A is a configuration diagram illustrating a fuel
supply system for a marine structure having a high-pressure natural
gas injection engine (e.g., an ME-GI engine) according to a fifth
embodiment of the present invention. The fifth embodiment
illustrated in FIG. 10A is different from the third embodiment in
that the fuel supply system further includes an excess BOG
consumption unit (e.g., a GCU) configured to consume excess BOG and
a stable fuel supply unit (e.g., an LNG supply line). Also, the
fifth embodiment is different from the third embodiment in that the
fuel supply system includes a unit (e.g., a DFDE) configured to
branch and consume a portion of BOG before reliquefaction to
prevent the generation of excess BOG. Thus, the following
description will be focused on the difference from the third
embodiment.
[0141] In the fuel supply system for a marine structure having a
high-pressure natural gas injection engine according to the fifth
embodiment of the present invention, when a load on the
high-pressure natural gas injection engine decreases, or when an
excess amount of LBOG is expected to be generated due to the
generation of a large amount of BOG, BOG compressed or being
compressed by a BOG compression unit 13 is shunted through a branch
line to an excess BOG consumption unit.
[0142] That is, the excess BOG may be supplied to a DFDE serving as
an excess BOG consumption unit, through a second branch line L8
branching from the BOG compression unit 13. In this case, since the
BOG is cooled to about 40.degree. C. by an intermediate cooler 15
included in the BOG compression unit 13, a separate heater
configured to control the temperature of BOG supplied to the DFDE
may be omitted.
[0143] Alternatively, the excess BOG may be supplied to a gas
turbine serving as another excess BOG consumption unit, through a
third branch line L9 branching from a rear end of the BOG
compression unit 13. Likewise, in this case, a separate unit
configured to control the temperature of BOG supplied to the gas
turbine may be omitted.
[0144] On the other hand, even when the amount of BOG supplied to a
reliquefaction apparatus 20 is reduced, if the amount of BOG
supplied as fuel is more than the amount of BOG required by the
high-pressure natural gas injection unit, the excess BOG is
processed in the same manner as in the fourth embodiment.
[0145] That is, the excess LBOG is decompressed through an LBOG
expansion valve 51 installed at an LBOG return line L4 branching at
a rear end of a buffer tank 31 from a fuel supply line L3. LBOG
including flash gas generated in the decompression process is
separated by a gas-liquid LBOG separator 53 into a liquid component
(LBOG) and a gaseous component (flash gas), and the liquid
component is returned through the LBOG return line L4 to a storage
tank 11. The gaseous component (i.e., flash gas) separated by the
gas-liquid LBOG separator 53 is supplied as fuel through a fuel gas
supply line L6 to a GCU serving as an excess BOG combustion
unit.
[0146] On the other hand, the excess BOG may be additionally
supplied to the GCU through a branch line L5 that branches from the
fuel supply line L3 supplying fuel to the high-pressure natural gas
injection engine (e.g., ME-GI engine) and connects with the fuel
gas supply line L6. A pressure drop valve is installed at the
branch line L5.
[0147] Also, as in the fourth embodiment, when the BOG
reliquefaction apparatus does not operate or when a small amount of
BOG is generated in the storage tank 11, LNG stored in the storage
tank 11 may be supplied to the buffer tank 31 through an LNG supply
line L7 and an LNG supply pump 57 installed in the storage tank
11.
[0148] In the fourth and fifth embodiments, the DFDE (in the fourth
embodiment) and the GCU (in the fifth embodiment) configured to
process generated flash gas, and the DFDE (in the fifth embodiment)
and the GCU (in the fifth embodiment) configured to consume excess
BOG before reliquefaction in order to prevent the generation of
flash gas, may be commonly called a flash gas suppression unit
because they can suppress the generation of flash gas. All of these
units may also be called an excess gas consumption unit because
they can consume excess BOG that is more than the amount of fuel
required by the high-pressure natural gas injection unit.
Modified Example of Fifth Embodiment
[0149] FIG. 10B is a configuration diagram illustrating a fuel
supply system according to a modified example of the fifth
embodiment of the present invention. The modified example of the
fifth embodiment is partially different from the fifth embodiment
in terms of the configuration of a reliquefaction apparatus 20.
[0150] Like the reliquefaction apparatus 20 according to the
modified example of the first embodiment illustrated in FIG. 3B,
the reliquefaction apparatus 20 according to the modified example
of the fifth embodiment includes a cold box 21 configured to
exchange heat between a refrigerant and BOG, a compression unit
configured to compress the refrigerant that is heated and at least
partially gasified by the cold box 21, an expansion unit configured
to expand the compressed refrigerant to reduce the temperature
thereof, and a gas-liquid refrigerant separator configured to
separate the gaseous refrigerant and the liquid refrigerant.
[0151] In particular, like the reliquefaction apparatus 20
according to the modified example of the first embodiment
illustrated in FIG. 3B, the reliquefaction apparatus 20 according
to the modified example of the fifth embodiment includes a
plurality of gas-liquid refrigerant separators 22a, 22b and 22c. A
gaseous refrigerant and a liquid refrigerant separated by the
gas-liquid refrigerant separators 22a and 22b disposed at the
upstream side are mixed and then supplied to the gas-liquid
refrigerant separator 22c disposed at the most downstream side. The
gaseous refrigerant separated by the gas-liquid refrigerant
separators 22a and 22b disposed at the upstream side may be
compressed by refrigerant compressors 23a and 23b and cooled by
refrigerant coolers 24a and 24b before supply to the gas-liquid
refrigerant separator 22c disposed at the most downstream side. The
liquid refrigerant separated by the gas-liquid refrigerant
separators 22a and 22b disposed at the upstream side is mixed with
the gaseous refrigerant before the gaseous refrigerant is supplied
to the gas-liquid refrigerant separator 22c disposed at the most
downstream side, specifically before the gaseous refrigerant is
cooled by the refrigerant cooler 24b.
Sixth Embodiment
[0152] FIG. 11 is a configuration diagram illustrating a fuel
supply system for a marine structure having a high-pressure natural
gas injection engine (e.g., an ME-GI engine) according to a sixth
embodiment of the present invention. The sixth embodiment
illustrated in FIG. 11A is different from the first to fifth
embodiments in that the fuel supply system uses a recondenser
instead of the buffer tank.
[0153] In the fuel supply system for a marine structure having a
high-pressure natural gas injection engine according to the sixth
embodiment of the present invention, natural boil-off gas (NBOG)
generated and discharged from a liquefied gas storage tank 110 is
compressed by a BOG compression unit 113 to a medium pressure of
about 12 to 45 bara and then is supplied to a reliquefaction
apparatus 120. Liquefied BOG (LBOG) reliquefied by liquefaction
energy (i.e., cold heat) in the reliquefaction apparatus 120 is
compressed by a high-pressure pump 133 to a high pressure of about
150 to 400 bara and then is supplied to a high-pressure gasifier
137. BOG gasified by the high-pressure gasifier 137 is supplied as
fuel to a high-pressure natural gas injection engine (e.g., an
ME-GI engine).
[0154] The storage tank includes a sealing and insulating barrier
to store liquefied gas such as LNG in a cryogenic state. However,
the storage tank cannot completely interrupt heat transmitted from
the outside. Accordingly, liquefied gas is continuously boiled off
in the storage tank 110. In order to maintain the pressure of BOG
in the storage tank 110, at a suitable level, the BOG is discharged
through a BOG discharge line L11.
[0155] The discharged BOG is supplied through the BOG discharge
line L11 to the BOG compression unit 113. The BOG compression unit
113 includes at least one BOG compressor 114. Although not
illustrated, the BOG compression unit 113 may include at least one
intermediate cooler configured to cool the BOG heated by being
compressed by the BOG compressor 114. FIG. 11 illustrates a
three-stage BOG compression unit 113 including three BOG
compressors 114.
[0156] The BOG compressed by the BOG compression unit 113 is
supplied through a BOG supply line L12 to the reliquefaction
apparatus 120. The BOG supplied to the reliquefaction apparatus 120
is cooled and reliquefied by a refrigerant while passing through a
main cryogenic heat exchanger 121 (i.e., a cold box) of the
reliquefaction apparatus 120.
[0157] The reliquefaction apparatus 120 may have any configuration
that can reliquefy BOG generated from liquefied gas such as LNG.
For example, the reliquefaction apparatus 120 may be a
reliquefaction system using a nonflammable mixed refrigerant, as
described in the first to fifth embodiments and the modified
examples thereof. Also, the reliquefaction apparatus 120 may be a
conventional reliquefaction system using a nitrogen refrigerant.
For example, the reliquefaction apparatus 120 may be any one of the
reliquefaction apparatuses disclosed in International Patent
Publication Nos. WO 2007/117148 and WO 2009/136793.
[0158] The BOG reliquefied by heat exchange in the cold box 121 is
supplied to and temporarily stored in a recondenser 131. According
to this embodiment, the liquefied BOG and the liquefied gas (i.e.,
LNG) supplied from the liquefied gas storage tank 110 are
temporarily stored in the recondenser 131, and a portion or all of
the BOG supplied from the liquefied gas storage tank 110 to the
reliquefaction apparatus 120 is bypassed to and recondensed by the
recondenser 131. Accordingly, the inflow of BOG into the
reliquefaction apparatus 120 is reduced or removed, thereby
improving the total system efficiency. As described below in
detail, the recondenser 131 recondenses a portion or all of the
generated BOG by using cold heat from one of the liquefied BOG
reliquefied by the reliquefaction apparatus 120 and then supplied
to and temporarily stored in the recondenser 131 and the liquefied
gas (i.e., LNG) supplied from the storage tank 110 directly to the
recondenser 131.
[0159] Like the buffer tank in the above-described embodiments, the
recondenser 131 may also separate a gaseous component and a liquid
component. Therefore, the liquefied gas temporarily stored in the
recondenser 131 are separated into a gaseous state and a liquid
state, and only the gaseous liquefied gas is supplied through a
fuel supply line L13 to a high-pressure pump 133. A plurality of
high-pressure pumps 133, for example, two high-pressure pumps 133
may be installed in parallel.
[0160] The high-pressure pump 133 compresses the liquefied gas to a
fuel supply pressure required by a high-pressure natural gas
injection engine (e.g., an ME-GI engine). The liquefied gas
outputted from the high-pressure pump 133 has a high pressure of
about 150 to 400 bara.
[0161] In order to provide a sufficient Net Positive Suction Head
(NPSH) in the high-pressure pump 133, a booster pump 132 may be
installed between the high-pressure pump 133 and the recondenser
131 on the fuel supply line L13, if necessary.
[0162] Also, as in the second embodiment, the system may be
configured such that the liquefied gas compressed by the
high-pressure pump 133 to a high pressure exchanges heat with the
BOG supplied from the reliquefaction apparatus 120, in a heat
exchanger 135 before supply to a high-pressure gasifier 137. The
liquefied gas supplied to the high-pressure gasifier 137 is lower
in temperature than the BOG supplied to the reliquefaction
apparatus 120. Therefore, the BOG supplied to the reliquefaction
apparatus 120 can be cooled while passing through the heat
exchanger 135, so that the reliquefaction energy in the
reliquefaction apparatus 120 can be reduced. Also, the liquefied
gas supplied to the high-pressure gasifier 137 can be heated while
passing through the heat exchanger 135, so that the gasification
energy in the high-pressure gasifier 137 can be reduced.
[0163] If necessary, the liquefied BOG recondensed by and
temporarily stored in the recondenser 131 may be returned through
an LBOG return line L14 to the storage tank 110. Although not
illustrated in FIG. 11, the expansion valve and the gas-liquid
separator of the fourth and fifth embodiments and the modified
examples thereof described with reference to FIGS. 9A to 10B may be
installed at the LBOG return line L14.
[0164] However, according to the fuel supply system of the sixth
embodiment, all of the BOG generated in the storage tank is used as
fuel for the high-pressure natural gas injection engine for most
period of time during the voyage of the marine structure.
Accordingly, the liquefied gas returning through the LBOG return
line L14 to the storage tank 110 can be removed. The LBOG return
line L14 may be used to return the LBOG from the recondenser 131 to
the storage tank 110 only in exceptional cases where the power
consumption of the high-pressure natural gas injection engine is
much smaller than the amount of BOG generated in the storage tank,
such as a case where the marine structure is towed to come
alongside the quay, a case where the marine structure passes
through a canal, and a case where the marine structure is sailing
at low speed. The LBOG return line L14 may also be used to return
the LBOG, which remains in the recondenser 131 during the
malfunction or the maintenance of the recondenser 131.
[0165] According to this embodiment, all of the LBOG can be used in
the engine for most period of time during the voyage of the marine
structure without returning to the storage tank. In this case,
since the returning LBOG itself is removed, it is possible to
prevent the generation of flash gas that may be caused by a
pressure difference while the LBOG is returned to the storage tank
110. In this specification, the expression "removing flash gas"
means consuming generated flash gas to prevent flash gas from being
supplied into the storage tank 110, and also means preventing the
return of liquefied BOG to the storage tank to block and prevent
the generation of flash gas itself that may be caused by the
returning LBOG.
[0166] Also, the expression "the fuel consumption of the
high-pressure natural gas injection engine" in the sentence "the
fuel consumption of the high-pressure natural gas injection engine
is larger or smaller than the amount of BOG generated in the
storage tank" means the sum of the fuel consumption of the
high-pressure natural gas injection engine and the fuel consumption
of other engines (e.g., a DFDE and a gas turbine) using BOG in the
marine structure as fuel. When only the high-pressure natural gas
injection engine among various engines uses BOG as fuel, the
expression "the fuel consumption of the high-pressure natural gas
injection engine" means the fuel consumption of only the
high-pressure natural gas injection engine.
[0167] When the amount of BOG generated in the storage tank 110 is
smaller than the amount of fuel required by the high-pressure
natural gas injection engine, the LNG stored in the storage tank
110 may be supplied through the LNG supply line L17 directly to the
recondenser 131. A submerged pump 157 is installed at one end of
the LNG supply line L17, i.e., the start point of the LNG supply
line L17 located in the storage tank, so that the LNG stored in the
storage tank 110 can be supplied directly to the recondenser 131.
According to this embodiment, since the internal pressure of the
recondenser 131 (or the buffer tank 31 in the first to fifth
embodiments and the modified examples thereof) is substantially
equal to the pressure of the BOG compressed by the BOG compression
unit 130 to a medium pressure of about 12 to 45 bara, there may be
a limit in using only the submerged pump 157 to compress the
liquefied gas stored in the storage tank 110 at about ambient
pressure, to a medium pressure. Thus, it may be preferable that a
booster pump 158 is installed in the middle of the LNG supply line
L17 to compress the liquefied gas discharged by the submerged pump
157 to the outside of the storage tank, to a pressure substantially
equal to the internal pressure of the recondenser 131 (or the
buffer tank).
[0168] When the amount of BOG generated in the storage tank 110 is
larger than the amount of fuel required by the high-pressure
natural gas injection engine and thus excess LBOG is expected to be
generated, the BOG gradually compressed or compressed by the BOG
compression unit 113 is shunted through a BOG branch line L18 and
used in a BOG consumption unit. The BOG consumption unit may be a
gas turbine or a DFDE that can use lower-temperature natural gas
than the ME-GI engine.
[0169] As described above, in order to reduce a load on the
reliquefaction apparatus 120 or to completely stop an operation of
the reliquefaction apparatus 120, the fuel supply system according
to this embodiment may include a BOG bypass line L21 that branches
from the BOG supply line L12 to supply a portion or all of the BOG
compressed by the BOG compression unit, to the recondenser 131 by
bypassing the reliquefaction apparatus 120.
[0170] Specifically, it may be preferable that the BOG bypass line
L21 branches from the downstream side of the heat exchanger 135 of
the BOG supply line L12 and connects with the recondenser 131. If
necessary, a pressure control valve 161 may be installed at the BOG
bypass line L21 to control the pressure of the recondenser 131.
[0171] When the amount of BOG generated in the storage tank 110 is
smaller than the amount of fuel required by the high-pressure
natural gas injection engine, the LNG in the storage tank 110 is
supplied to the recondenser 131 to supplement an insufficient fuel
amount. In this case, a portion of the BOG supplied to the
reliquefaction apparatus 120 is supplied through the BOG bypass
line L21 to the recondenser 131 and is recondensed by being mixed
with LNG. Accordingly, a load on the reliquefaction apparatus 120
can be reduced.
[0172] Hereinafter, with reference to FIG. 11, a description will
be give of a method of using the recondenser 131 to operate the
fuel supply system of the sixth embodiment, when the fuel supply
system is installed in, for example, an LNG carrier.
[0173] The fuel supply system of the sixth embodiment includes the
recondenser 131. Therefore, all of the BOG generated in the storage
tank 110 is not supplied to the cold box 121 of the reliquefaction
apparatus 120, and at least a portion of the generated BOG is
bypassed to the recondenser 131. Accordingly, a load on the
reliquefaction apparatus 120 with large energy consumption can be
reduced, or an operation of the reliquefaction apparatus 120 can be
completely stopped in some cases.
[0174] During the ballast voyage with the storage tank 110 emptied,
the amount of generated BOG is relatively small. In this case, the
amount of fuel required by the high-pressure natural gas injection
engine cannot be satisfied by only the BOG naturally generated in
the storage tank 110. Therefore, the LNG stored in the storage tank
110 is supplied through the LNG supply line L17 directly to the
recondenser 131.
[0175] Also, the BOG discharged from the storage tank 110 is
compressed by the BOG compression unit 113 to a medium pressure of
about 12 to 45 bara, and is cooled by the heat exchanger 135. All
of the cooled BOG is supplied through the BOG bypass line L21 to
the recondenser 131.
[0176] Since the amount of BOG generated during ballast voyage is
small, all of the generated BOG can be supplied to and recondensed
by the recondenser 131. That is, since all of the BOG generated in
the storage tank 110 is recondensed by the recondenser 131 for most
period of time during ballast voyage, an operation of the
reliquefaction apparatus 120 may be stopped. However, in a case
where the high-pressure natural gas injection engine operates at
low speed or stops operating, such as a case where the marine
structure is being towed during the ballast voyage, the
high-pressure natural gas injection engine consumes no fuel or a
very small amount of fuel. Therefore, all of the BOG generated in
the storage tank 110 cannot be recondensed and used as fuel, and a
portion of the generated BOG can be reliquefied by the
reliquefaction apparatus 120. However, this is a very exceptional
case during the ballast voyage.
[0177] Since the LNG supplied through the LNG supply line L17 to
the recondenser 131 is subcooled, all of the BOG supplied through
the BOG bypass line L21 may be condensed by receiving cold heat
from the LNG while being mixed with the LNG subcooled in the
recondenser 131.
[0178] According to the fuel supply system of this embodiment, all
of the BOG generated during ballast voyage can be recondensed in
the recondenser 131 and used as fuel for the high-pressure natural
gas injection engine. Accordingly, no LBOG returns to the storage
tank 110.
[0179] Also, since all of the generated BOG can be processed in the
recondenser 131, the reliquefaction apparatus 120 using much energy
due to large power consumption cannot be operated at all. Thus, a
considerable amount of energy can be saved.
[0180] On the other hand, when the storage tank 110 is laden with
gas cargo during voyage, the amount of generated BOG is relatively
large. In this case, since all of the BOG naturally generated in
the storage tank 110 cannot be processed by the recondenser 131,
the reliquefaction apparatus 120 is operated to reliquefy the BOG.
If necessary, a portion of the generated BOG is bypassed through
the BOG bypass line L21 to the recondenser 131 to reduce a
reliquefaction load on the reliquefaction apparatus 120, thereby
saving energy.
[0181] It may be inefficient to cool BOG to a subcooling
temperature lower than a saturation temperature in order to subcool
the BOG in the reliquefaction apparatus 120. However, when the BOG
is subcooled to a saturation temperature, the saturated LBOG may be
regasified while passing along a pipe. Therefore, it may be
preferable to cool the BOG to a subcooling temperature at a
corresponding pressure when liquefying the BOG in the
reliquefaction apparatus 120.
[0182] In the case of the conventional reliquefaction apparatuses
disclosed in International Patent Publication Nos. WO 2007/117148
and WO 2009/136793, since LBOG reliquefied by the reliquefaction
apparatus is returned to an LNG storage tank, BOG is subcooled to a
temperature much lower than a saturation temperature at a pressure
of about 4 to 8 bara, in accordance with the internal temperature
(about -163.degree. C.) of the LNG storage tank.
[0183] However, according to the fuel supply system of the present
invention, since the LBOG is supplied and used as fuel to the
high-pressure natural gas injection engine, the BOG is compressed
to a pressure of about 12 to 45 bara and the reliquefaction
apparatus is operated with a reliquefaction temperature that is
only about 1.degree. C. lower than a saturation temperature at a
corresponding pressure.
[0184] According to the present invention, since the LBOG
reliquefied by the reliquefaction apparatus is not returned to the
storage tank, it is unnecessary to consider the temperature and
pressure of the LNG stored in the storage tank. In the conventional
method, a pipe for transferring LBOG to a storage tank is
relatively long. However, in the case of the present invention,
since a pipe for transferring LBOG to a storage tank while
maintaining a subcooled state of the LBOG is relatively short, it
is unnecessary to subcool the BOG to a temperature excessively
lower than the saturation temperature.
[0185] Therefore, the present invention operates the reliquefaction
apparatus 120 while setting the liquefaction temperature of BOG to
a temperature slightly lower than the saturation temperature (e.g.,
subcooled by about 0.5.degree. C. to 3.degree. C., preferably by
about 1.degree. C.), thereby reducing the power consumption of the
reliquefaction apparatus.
[0186] In this case, the pressure control valve 161 installed on
the bypass line L21 is on/off controlled such that the BOG cooled
by heat exchange in the heat exchanger 135 can flow into the
recondenser 131. Accordingly, the pressure of the recondenser 131
can be suitably controlled.
[0187] According to this embodiment, even when BOG is subcooled to
a temperature about 1.degree. C. lower than the saturation
temperature and then is supplied to the recondenser 131, the BOG is
compressed by the booster pump 132 and the high-pressure pump 133
while being supplied as fuel to the high-pressure natural gas
injection engine. Therefore, the saturated LBOG can stably maintain
a subcooled state due to a temperature increase.
[0188] As compared to the conventional fuel supply system, the fuel
supply systems for a marine structure having a high-pressure
natural gas injection engine according to the first to sixth
embodiments and the modified examples thereof have the following
advantages.
[0189] In general, BOG may be compressed to a high pressure in
order to increase the BOG reliquefaction efficiency. However, in a
conventional method, BOG is reliquefied by a reliquefaction
apparatus and returned to a storage tank, and the LNG stored in the
storage tank is maintained at an ambient pressure state. Therefore,
if a pressure of the liquefied BOG is excessively high, flash gas
may be generated when the BOG is returned to the storage tank.
Therefore, the BOG needs to be compressed to a low pressure of
about 4 to 8 bara, in spite of low reliquefaction efficiency.
[0190] As compared to the convention method, according to the
present invention, since the BOG discharged from the storage tank
is used as fuel for the high-pressure natural gas injection engine,
the BOG can be compressed to a higher pressure and reliquefied
without causing the generation of flash gas, thus increasing
reliquefaction efficiency.
[0191] According to the present invention, since liquefied BOG is
supplied as a fuel for a high-pressure natural gas injection engine
(e.g., an ME-GI engine), the liquefied BOG need not be returned to
the storage tank for re-storage in the storage tank, thus making it
possible to prevent the generation of flash gas that may be caused
when the BOG is returned to the storage tank. Also, since the
generation of flash gas is suppressed, the BOG can be compressed to
a higher pressure (i.e., a medium pressure of about 12 to 45 bara)
for reliquefaction as compared to the conventional method. Since
the BOG is compressed to a medium pressure and then reliquefied,
reliquefaction efficiency can be improved regardless of the types
of refrigerant. In particular, reliquefaction efficiency can be
increased when a nonflammable mixed refrigerant is used instead of
a nitrogen refrigerant. That is, as compared to the conventional
reliquefaction apparatus using a nitrogen refrigerant, the
reliquefaction apparatus using a nonflammable mixed refrigerant
according to the present invention can reliquefy the BOG by
considerably low energy and supply the liquefied BOG as fuel for
the engine.
[0192] The reliquefaction apparatus 20/120 may have any
configuration that can reliquefy BOG generated from liquefied gas
such as LNG. For example, the reliquefaction apparatus 20/120 may
be a reliquefaction system using a nonflammable mixed refrigerant,
as described in the first to sixth embodiments and the modified
examples thereof. Also, the reliquefaction apparatus 20/120 may be
a conventional reliquefaction system using a mixed refrigerant or a
nitrogen refrigerant. For example, the reliquefaction apparatus
20/120 may be any one of the reliquefaction apparatuses disclosed
in International Patent Publication Nos. WO 2007/117148 and WO
2009/136793 and Korean Patent Application Publication Nos.
10-2006-0123675 and 10-2001-0089142.
[0193] According to the fuel supply system according to the present
invention, all of the BOG generated in the storage tank is used as
fuel for the high-pressure natural gas injection engine for most
period of time during the voyage of the marine structure.
Accordingly, the liquefied gas returning through the LBOG return
line L4/L14 to the storage tank 11/110 can be removed. The LBOG
return line L4/L14 may be used to return the LBOG from the buffer
tank 31 or the recondenser 131 to the storage tank 11/110 only in
exceptional cases where the power consumption of the high-pressure
natural gas injection engine is much smaller than the amount of BOG
generated in the storage tank, such as a case where the marine
structure is towed to come alongside the quay, a case where the
marine structure passes through a canal, and a case where the
marine structure is sailing at low speed. The LBOG return line
L4/L14 may also be used to return the LBOG, which remains in the
buffer tank during the malfunction or the maintenance of the buffer
tank.
[0194] According to the present invention, all of the LBOG can be
used in the engine for most period of time during the voyage of the
marine structure without returning to the storage tank. In this
case, since the returning LBOG itself is removed, it is possible to
prevent the generation of flash gas that may be caused by a
pressure difference while the LBOG is returned to the storage tank
11. In this specification, the expression "removing flash gas"
means consuming generated flash gas to prevent flash gas from being
supplied into the storage tank 11, and also means preventing the
return of liquefied BOG to the storage tank to block and prevent
the generation of flash gas itself that may be caused by the
returning LBOG.
[0195] In the case of the conventional reliquefaction apparatuses
disclosed in International Patent Publication Nos. WO 2007/117148
and WO 2009/136793 and Korean Patent Application Publication Nos.
10-2006-0123675 and 10-2001-0089142, since LBOG reliquefied by the
reliquefaction apparatus is returned to an LNG storage tank, BOG is
subcooled to a temperature much lower than a saturation temperature
at a pressure of about 4 to 8 bara, in accordance with the internal
temperature (about -163.degree. C.) of the LNG storage tank.
[0196] For example, a conventional Mark III reliquefaction
apparatus of Hamworthy Gas Systems (the technology disclosed in
International Patent Publication No. WO 2007/117148) compresses the
BOG to a pressure of 8 bara and liquefies the BOG at a temperature
of -159.degree. C. In this case, since the saturation temperature
of the BOG is about -149.5.degree. C., the BOG is subcooled by
about 9 to 10.degree. C. The BOG needs to be subcooled by such a
degree in order to prevent the generation of flash gas when the
liquefied BOG is returned to the LNG storage tank. However, since
the liquefied BOG is compressed by the high-pressure pump while the
liquefied BOG is supplied as fuel for the high-pressure natural gas
injection engine, the LBOG saturated by the increased pressure can
stably maintain the overcooled state later. Therefore, according to
the present invention, the liquefied BOG may be liquefied by
overcooling as much as 0.5 to 3.degree. C., preferably about
1.degree. C., as compared to the saturation temperature at the
corresponding pressure, and then supplied as fuel. As the level of
subcooling in the reliquefaction apparatus decreases,
reliquefaction energy reduction effect increases. For example, the
energy for subcooling by 1.degree. C. at a temperature of
-150.degree. C. is 1% of the total power required for
reliquefaction.
[0197] According to the fuel supply system of the present
invention, since the LBOG is supplied and used as fuel for the
high-pressure natural gas injection engine, the BOG is compressed
to a pressure of about 12 to 45 bara and the reliquefaction
apparatus is operated with a reliquefaction temperature that is
about 0.5.about.3.degree. C. (preferably 1.degree. C.) lower than a
saturation temperature at a corresponding pressure.
[0198] According to the present invention, since the LBOG
reliquefied by the reliquefaction apparatus is not returned to the
storage tank, it is unnecessary to consider the temperature and
pressure of the LNG stored in the storage tank. In the conventional
method, a pipe for transferring LBOG to a storage tank is
relatively long. However, in the case of the present invention,
since a pipe for transferring LBOG to a storage tank while
maintaining a subcooled state of the LBOG is relatively short
(e.g., the length between the reliquefaction apparatus (e.g., the
gas-liquid separator) and the high-pressure pump), it is
unnecessary to subcool the BOG to a temperature excessively lower
than the saturation temperature.
[0199] That is, the high-pressure natural gas injection engine may
require a larger amount of fuel than an amount of the BOG generated
in the LNG storage tank for a considerable period of time during
the voyage of the marine structure. In this case, all of the
liquefied BOG is supplied to the high-pressure natural gas
injection engine, thereby preventing the generation of flash gas
when the liquefied BOG is returned to the LNG storage tank.
[0200] Therefore, the present invention operates the reliquefaction
apparatus 120 while setting the liquefaction temperature of BOG to
a temperature slightly lower than the saturation temperature (e.g.,
subcooled by about 0.5.degree. C. to 3.degree. C., preferably by
about 1.degree. C.), thereby reducing the power consumption of the
reliquefaction apparatus.
[0201] According to this embodiment, even when BOG is subcooled to
a temperature about 0.5.about.3.degree. C. lower than the
saturation temperature and then is supplied to the buffer tank 31,
the BOG is compressed by the high-pressure pump 33 while being
supplied as fuel to the high-pressure natural gas injection engine.
Therefore, the saturated LBOG can stably maintain a subcooled state
due to a temperature increase.
[0202] Also, since the LBOG supplied to the high-pressure pump is
compressed to a medium pressure, the power required to pump the
LBOG by the high-pressure pump can be reduced.
[0203] In a marine structure having a conventional reliquefaction
apparatus, since BOG is reliquefied, keeping it mind to return the
BOG to a storage tank, the BOG is compressed to a low pressure of
about 4 to 8 bara in order to suppress the generation of flash gas
when the BOG is returned to the storage tank. However, in the fuel
supply system of the present invention, since the BOG is
reliquefied and all of the BOG is used as fuel for the
high-pressure natural gas injection engine, the BOG is compressed
to a relatively high pressure of about 12 to 45 bara. This is the
novel and inventive concept of the present invention, which was not
conceived in the conventional method of returning BOG to a storage
tank after reliquefaction of the BOG.
[0204] In the conventional method, flash gas is generated by
decompression during the return of LBOG to a storage tank, and the
flash gas is returned to a reliquefaction apparatus, thus degrading
the efficiency of the reliquefaction apparatus. However, the
present invention uses all of the LBOG as fuel for the
high-pressure natural gas injection engine without decompression
(performing compression instead), thus improving the efficiency of
the reliquefaction apparatus as compared to the conventional
method.
[0205] According to the fuel supply system of the present
invention, since the BOG is reliquefied and all of the liquefied
BOG is used as fuel for the high-pressure natural gas injection
engine, the BOG can be compressed to a relatively high pressure of
about 12 to 45 bara. Accordingly, as described with reference to
FIG. 6B, the reliquefaction apparatus according to the present
invention can be operated with 50 to 80% of power consumed in the
conventional reliquefaction apparatus (refrigeration cycle). As
such, since the reliquefaction apparatus according to the present
invention can be operated with relatively low power as compared to
the related art, the capacity of a power generator can be reduced,
thus making it possible to miniaturize the power generator.
[0206] The conventional reliquefaction apparatus consumes a power
of about 1 to 1.5 MW for operation in a standby state. However,
according to the present invention, as described in the sixth
embodiment, since an operation of the reliquefaction apparatus can
be stopped for most period of time during ballast voyage, the power
consumption of the reliquefaction apparatus can be reduced. For
example, assuming that an annual ballast voyage period is 150 days
and a diesel power generator with a power consumption of 183 g/kWh
is used to operate the reliquefaction apparatus, 660 to 923 tons of
HFO can be saved annually. Since the price of HFO in Singapore is
671 USD/ton as of mid September 2011, 0.4 to 0.6 mil USD can be
saved annually.
[0207] It has been described above that that the fuel supply system
and method of the present invention are applied to a marine
structure such as an LNG carrier. However, it will be readily
understood that the fuel supply system and method of the present
invention are also applicable to fuel supply to a high-pressure
natural gas injection engine on land.
[0208] While the embodiments of the present invention has 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.
* * * * *