AU2016426102B2 - Natural gas liquefaction apparatus - Google Patents

Abstract

[Problem] To provide an apparatus configuration suitable for a small natural gas liquefaction apparatus. [Solution] A natural gas liquefaction apparatus is provided with a liquefaction processing device 12 for performing a process that liquefies natural gas and a storage tank 13 for storing liquefied liquid natural gas, wherein a boil off gas (BOG) generated inside the storage tank 13 is pressurized in a boil off gas compression unit 211 provided in a boil off gas line 601 and is supplied as a fuel gas including the boil off gas to a gas engine 4 which drives a generator 41. Electric power generated in the generator 41 is used for driving a coolant compression unit 121, etc. which compresses a coolant gas that cooled the natural gas in the liquefaction processing device 12.

Description

NATURAL GAS LIQUEFACTION APPARATUS

Technical Field

[0001] The present invention relates to a facility

configuration suitable for a small natural gas liquefaction unit.

Background Art

[0002] Natural gas (NG) produced in a gas well or the like is

liquefied in a natural gas liquefaction unit (NG liquefaction

facility) and then shipped as liquefied natural gas (LNG) to a

consumption area through an LNG tanker or a pipeline.

[0003] The NG liquefaction facility includes devices and

auxiliary equipment, for example, various pretreatment devices

configured to remove impurities, such as moisture, contained in

NG, a liquefaction processing unit configured to perform

liquefaction processing of NG, and a storage tank (LNG tank)

configured to store liquefied LNG.

In the liquefaction processing unit, refrigerant is used for

cooling NG and is gasified by heat exchange with the NG, and the

refrigerant gas is re-used for cooling NG through compression using

a compressor and a decrease in temperature in association with

adiabatic expansion.

[0004] Further, the NG liquefaction facility includes power

sources for driving a power generator configured to supply electric

power to an electric motor, such as a pump, and the above-mentioned compressor for refrigerant gas. Those power sources are operated by burning fuel gas containing boil off gas (BOG) generated by vaporization of LNG in the LNG tank.

[00051 Meanwhile, along with the development of various gas

fields in recent years, the construction of small NG liquefaction

facilities having an NG processing capability of 1,000,000 tons

per year or less, as well as large NG liquefaction facilities having

an NG processing capability of millions of tons per year, is

increasing.

However, in actual circumstances, it is difficult to say that

theselectionofapowersourcesuitableforsuchsmallNGliquefaction

facility or the proposal of a fuel gas supply facility suitable

for the selected power source is sufficiently performed.

[00061 In view of the foregoing, as disclosed in Patent

Literature 1, the applicant developed a technology involving

providing a gas engine in an LNG receiving facility configured to

receive LNG transported by an LNG tanker, and operating the gas

engine throughuse ofBOGgeneratedinanLNGtank, to therebygenerate

electric power.

However, the NG liquefaction facility is different from the

LNGreceivingfacilityinconfiguration ofafuelgas supplyfacility

and fuel consumption balance, and hence the technology described

in Patent Literature 1 cannot be directly applied to the NG

liquefaction facility.

Citation List

Patent Literature

[0007] [PTL 1] WO 2015/128903 Al

Summary of Invention

Technical Problem

[0008] The present invention has been made under the

above-mentioned background, and has an object to provide a facility

configuration suitable for a small natural gas liquefaction unit.

Solution to Problem

[0009] According to one embodiment of the present invention,

there is provided a natural gas liquefaction unit, including: a

liquefaction processing unit configured to perform liquefaction

processing of natural gas; a storage tank configured to store the

natural gas liquefied in the liquefaction processing unit; a boil

off gas line including a boil off gas compression unit configured

to increase a pressure of boil off gas generated in the storage

tank; a gas engine configured to drive a power generator by burning

fuelgas; afuelgasline configured to supplythe fuelgas containing

the boiloffgasincreasedinpressurein theboiloffgas compression

unit and delivered from the boil off gas line to the gas engine;

and a refrigerant compression unit, which is driven by electric

powergeneratedin thepowergenerator, andis configured tocompress

refrigerant gas that has cooled the natural gas in the liquefaction processing unit.

[0010] The naturalgas liquefactionunitmayhave the following

features:

(a) the liquefaction processing unit has a natural gas

processing capability within a range of from 100,000 tons per year

to 1,000,000 tons per year;

(b) the fuel gas supplied from the fuel gas line to the gas

engine has a pressure within a range of from 0.6 MPaG to 1.0 MPaG,

and in this case, the natural gas liquefaction unit further includes

another combustion equipment using the fuel gas, and the fuel gas

supplied from the fuel gas line to the another combustion equipment

has a pressure equal to the pressure of the fuel gas supplied to

the gas engine;

(c) the natural gas liquefaction unit further includes: a

dehumidifier, whichis provided on an inlet side of the liquefaction

processingunit, and is configured to remove moisture in the natural

gas supplied to the liquefaction processingunit; an undehumidified

natural gas line configured to supply the natural gas before being

processed in the dehumidifier to the fuel gas line; and a pressure

adjusting gas line configured to supply the naturalgas before being

liquefied in the liquefaction processing unit to the fuel gas line

in order to keep a pressure of the fuel gas line at a pressure set

in advance;

(d) the natural gas liquefaction unit further includes: a

rundown line configured to deliver the liquefied natural gas in the storage tank to a liquefied natural gas tanker; a return gas lineconfiguredtojoinreturngas, whichisgeneratedintheliquefied natural gas tanker along with delivery of the liquefied natural gas fromthe rundownline, andis returned fromthe liquefiednatural gas tanker, to the boil off gas in the liquefied natural gas storage tank; and a reprocessing gas line, which includes a reprocessing gas compression unit configured to draw off part of the fuel gas containing the gas increased in pressure in the boil off gas compression unit as reprocessing gas to be reprocessed in the liquefaction processing unit and to further compress the reprocessing gas, and is configured to supply the reprocessing gas increased in pressure in the reprocessing gas compression unit to an inlet side of the liquefaction processing unit; and

(e) the natural gas liquefaction unit further includes an

exhaust-heat recovery unit configured to recover exhaust heat of

exhaust gas discharged from the gas engine, and the exhaust-heat

recovery unit is configured to supply a heat source to at least

one reboiler selected from a reboiler group consisting of a reboiler

of an acid gas removing unit configured to remove acid gas contained

in the natural gas before being liquefied in the liquefaction

processing unit, and a reboiler provided in each of fractionators

configured to fractionate ethane, propane, and butane separated

from the natural gas in the liquefaction processing unit.

Advantageous Effects of Invention

[0011] Accordingtothepresentinvention,insupplyofelectric

power to be used in the natural gas liquefaction unit, the power

generatoris driven throughuse ofthe gasengine. In the gasengine,

it is not required to compress fuel gas to high pressure unlike

a gas turbine. Further, for example, the gas engine can be stably

operated within a wide outside air temperature range and a wide

load range, and thus the gas engine has satisfactory operability.

Therefore, the device configuration suitable for a small natural

gas liquefaction unit operated on a small scale can be provided.

Brief Description of Drawings

[0012] FIG. 1 is an explanatory diagram for illustrating a

configuration example ofa smallNGliquefaction facility according

to an embodiment of the present invention.

FIG. 2 is an explanatory diagram for illustrating a

configuration example of a large NG liquefaction facility according

to a reference example.

Description of Embodiments

[0013] FIG. 1 is a diagram for illustrating a configuration

example ofa smallNGliquefaction facility including a liquefaction

processing unit 12 of natural gas (NG) . The NG liquefaction facility

of this example includes the liquefaction processing unit 12 having

a processing capability within a range of from 100,000 tons per

year to 1,000,000 tons per year, for example, 300,000 tons per year.

[0014] The natural gas (NG) transported from a wellhead or the

like is subjected to mercury removal, acid gas removal, and the

like by a mercury removing device and an acid gas removing unit

(not shown), and is then subjected to moisture removal using an

absorbingliquid, suchas triethyleneglycol (TEG), inadehumidifier

11. The mercury removing device, the acid gas removing unit, and

the dehumidifier correspond to pretreatment devices provided in

the NG liquefaction facility of this example.

[0015] The NG having moisture removed by the dehumidifier 11

is liquefied by the liquefaction processing unit 12. For example,

the liquefaction processing unit 12 includes devices such as a scrub

column, a main cryogenic heat exchanger (MCHE), a refrigerant

compression unit 121, and an aftercooler. The scrub column is

configured to remove a heavy component contained in the NG. The

main cryogenicheat exchanger (MCHE) is configured to cool, liquefy,

and subcool the NG with mixed refrigerant containing a plurality

of kinds of refrigerant raw materials, such as nitrogen, methane,

ethane, andpropane. The refrigerant compressionunit121is formed

of a compressor configured to compress mixed refrigerant gas

vaporized by heat exchange. The aftercooler is configured to cool

the mixed refrigerant compressed by the refrigerant compression

unit 121.

[0016] In the small NG liquefaction facility, installment of

a precooling heat exchanger configured to precool the NG with

precooling refrigerant containing propane as a main component, the refrigerant compression unit configured to compress precooling refrigerant gas after heat exchange, and the like may be omitted.

However, evenin the liquefactionprocessingunit12 in this example,

the precooling heat exchanger and the precooling refrigerant

compression unit may be provided.

[0017] Further, each of fractionators (ethane fractionator,

propane fractionator, and butane fractionator) configured to

fractionate ethane, propane, and butane separated from the cooled

NG is juxtaposed to the liquefaction processing unit 12.

For convenience of illustration, in FIG. 1, each equipment

and device (scrub column, MCHE, aftercooler, each fractionator,

and the like) forming the liquefaction processing unit 12 are not

illustrated except that the refrigerant compression unit 121 is

illustrated.

[0018] Liquefied natural gas (LNG) obtained by liquefying and

subcooling the NG in the liquefaction processing unit 12 is fed

to and stored in an LNG tank 13. The LNG stored in the LNG tank

13 is fed with an LNG pump 131 and loaded into an LNG tanker 50

through a rundown line 51. After that, the LNG is shipped to a

consumption area.

[0019] TheNGliquefaction facilityhavingthe above-mentioned

configuration includes a power generator 41 configured to supply

electric power to an electric motor, such as a motor 122 configured

todrive therefrigerantcompressionunit121. IntheNGliquefaction

facility of this example, the power generator 41 is driven by a gas engine 4 through use of fuel gas containing boil off gas (BOG) generated in the LNG tank 13.

Further, the fuel gas containing the BOG is used as fuel for

boilers provided in the NG liquefaction facility and fuel for a

flare stack pilot burner also in combustion equipment other than

the gas engine 4. Now, a fuel gas supply facility is described with

reference to FIG. 1.

[0020] In the LNG tank 13, BOG containing nitrogen and methane

as main components is generated due to heat input from an outer

wall, a change in liquid surface height of the LNG, and the like.

The BOG generated in the LNG tank 13 is drawn off to a boil off

gas line 601 and fed to a BOG compression unit 211 that is driven

byamotor212 through theboiloffgas line 601. The BOGcompression

unit 211 is configured to increase the pressure of the BOG to a

range of from 0.6 MPaG to 1.0 MPaG, for example, 0.7 MPaG.

[0021] The BOG increased in pressure by the BOG compression

unit 211 is cooled by a cooling unit 22, for example, an air fin

cooler and fed to a knockout drum 3 configured to separate moisture.

The coolingunit 22 is not limited to the case having a configuration

formed of the air fin cooler, and may have a configuration of cooling

the BOG by another procedure, for example, a water-cooling heat

exchanger.

[0022] An undehumidified natural gas line 605 configured to

supply the natural gas before being processed by the dehumidifier

11 and a pressure adjusting gas line 606 configured to supply the

NG before being liquefied by the liquefaction processing unit 12

are joined to a fuel gas line 602 connecting the cooling unit 22

and the knockout drum 3 to each other.

[0023] The BOG increased in pressure by the BOG compression

unit 211, the undehumidified NG, which is supplied from the

undehumidified natural gas line 605, and the NG supplied from the

make-up gas line 606 are mixed with each otherwhile flowing through

the fuel gas line 602 and the knockout drum 3 and supplied as fuel

gas to the combustion equipment, such as the gas engine 4. From

this point of view, the knockout drum 3 also has a function of mixing

the BOG and the NG with each other.

[0024] When theNGfromthemake-upgasline 606isinsufficient,

the undehumidified NG is supplied from the undehumidified natural

gas line 605, and with this, adjustment of a pressure regulating

valve 242 described later is ensured. Further, the NG from the

make-up gas line 606 is supplied so as to adjust the pressure of

the fuel gas.

In this case, as described above, in the NG liquefaction

facility of this example, the power generator 41 is driven through

use of the gas engine 4, and the gas engine 4 can be operated through

use of fuel gas having a pressure lower than that of a gas turbine

(supply pressure of fuel gas is, for example, 3.0 MPaG). In this

example, the fuel gas having a pressure adjusted to a range of from

0.6 MPaG to 1.0 MPaG, for example, 0.7 MPaG is supplied to the gas

engine 4 and the other combustion equipment.

[0025] In order to perform the above-mentioned pressure

adjustment, the pressure regulating valve 242 is provided to the

make-upgasline 606. Thepressureregulatingvalve242isconfigured

to, forexample, basedondetectionresultsofapressurebyapressure

meter 241 provided to a fuel gas line 603 on an outlet side of the

knockout drum 3, increase or decrease the supply amount of the NG

(NG for pressure adjustment) supplied through the make-up gas line

606 so that the detectedpressure isbrought close to apredetermined

set pressure (0.7 MPaG in this example).

[0026] In this case, in the facility configured to supply fuel

gas having low pressure, the heat quantity of the fuel gas is liable

to change in accordance with a change in generation amount of the

BOG and a change in supply amount of the NG in accordance with the

change in generation amount of the BOG, and there is also a risk

in that the heat quantity may change within a relatively short time

period. In view of the foregoing, the function of suppressing a

rapidheatquantitychange ofthe fuelgasmaybe addedto the knockout

drum 3.

[0027] The fuel gas, which is mixed after having moisture

separated therefromby the knockout drum3, is heated to a temperature

equal to or more than a dew point in a heating unit 23, and is then

to the gas engine 4 through a fuel gas line 604A. Further, fuel

gas having a pressure equal to that of the fuel gas supplied to

the gas engine 4 is supplied to another combustion equipment through

afuelgasline 604Bbranchedfromthe fuelgasline 604A. Theheating unit 23 is not limited to the case having a configuration formed of the heat exchanger configured to heat the fuel gas with a heat medium as illustrated in FIG. 1 and may have a configuration of heating the BOG by another procedure, for example, a heater having a burner.

[0028] The gas engine 4 is configured to drive the power

generator 41 through use of the fuel gas supplied from the

above-mentioned fuel gas supply facility. In the small NG

liquefaction facility in which the liquefaction processing unit

12 has a processing capability within a range of from 100,000 tons

per year to 1,000,000 tons per year, the gas engine 4 having an

output also including an output for backup of, for example, from

about 5 MW to about 20 MW is provided in a plurality of numbers,

for example, from about 1 to about 5 in accordance with the electric

power amount required in the liquefaction facility.

The electric power generated by the power generator 41 is

supplied to various devices using electric power in the NG

liquefaction facility, such as the above-mentioned motors 122 and

212 configured to drive the refrigerant compression unit 121 and

the BOG compression unit 211, respectively.

[0029] The exhaust heat after the fuel gas is burnt in the gas

engine 4 is recovered as steamor the like in an exhaust-heat recovery

unit 401. The recovered heat is supplied as a heat source for at

least one reboiler selected from a reboiler group consisting of

a reboiler configured to heat an absorbing liquid of acid gas at a time of regenerating the absorbing liquid in the above-mentioned acid gas removing unit configured to remove the acid gas contained in the NG, and a reboiler provided in a fractionation column of each of fractionators (ethane fractionator, propane fractionator, andbutane fractionator) configured to fractionate ethane, propane, and butane separated from the NG in the liquefaction processing unit 12.

[00301 In the NG liquefaction facility according to an

embodiment of the present invention having the above-mentioned

configuration, when operation of loading the LNG from the LNG tank

13 into the LNG tanker 50 is performed, hydrocarbon gas is generated

in the LNG tanker 50, and the hydrocarbon gas is returned to the

NGliquefaction facility as return gas. In this example, the return

gas is joined to the BOG flowing through the boil off gas line 601

through a return gas line 52.

[0031] In this case, the amount of the return gas returned from

the LNG tanker 50 may reach several times the generation amount

of the BOG in the LNG tank 13. As described above, in the fuel gas

supply facility configured to supply fuel gas having a low pressure

within a range of, for example, from 0.6 MPaG to 1.0 MPaG, there

is also a risk in that the entire amount of the return gas may not

be completely absorbed.

[0032] In view of the foregoing, the NG liquefaction facility

ofthisexampleincludesreprocessinggaslines541and542configured

to draw off the fuel gas, which has become excessive due to the supply of the return gas, and return the fuel gas to the liquefaction processingunit12 forreprocessing. Areprocessinggas compression unit 531thatis drivenby amotor 532isprovided to the reprocessing gaslines541and542, andthepressureofthefuelgas tobereprocessed

(reprocessing gas) is increased to a receiving pressure of the

liquefaction processing unit 12. Further, the fuel gas is cooled

in a reprocessing gas cooling unit 533 and then returned to an inlet

side of the liquefaction processing unit 12.

[00331 The action of the NG liquefaction facility having the

above-mentioned configuration is described. When the generation

amount of the BOG is reduced due to a decrease in outside air

temperature or due to the fact that a change in liquid surface height

in the LNG tank 13 becomes gentle, the supply amount of the BOG

from the BOG compression unit 211 side to the fuel gas line 602

is reduced. As a result, the pressure of the fuel gas detected by

the pressure meter 241 changes in a direction of decreasing.

Therefore, the pressure regulating valve 242 increases the opening

degree to increase the supply amount of the NG from the make-up

gas line 606 so that the detected pressure is kept at the

above-mentioned set pressure value.

[0034] In contrast, when the generation amount of the BOG in

the LNG tank 13 is increased, the supply amount of the BOG from

theBOGcompressionunit211side to the fuelgasline 602isincreased.

As a result, the pressure detected by the pressure meter 241 changes

in a direction of increasing. Therefore, the pressure regulating valve 242 decreases the opening degree to decrease the supply amount of the NG from the make-up gas line 606 so that the detected pressure is kept at the set pressure value.

[00351 Next, description is made of an example of a procedure

for adjusting fuel gas balance when the operation of loading the

LNG from the LNG tank 13 into the LNG tanker 50 is performed.

First, before the operation of loading the LNG is started,

the reprocessing gas compression unit 531 and the reprocessing gas

cooling unit 533, which are usually stopped, are operated. Then,

fuelgas (reprocessing gas) at a predetermined flow rate is returned

to the inlet side of the liquefaction processing unit 12 through

the reprocessing gas lines 541 and 542, and reprocessing is started.

As a result, the supply amount of the fuel gas to the fuel gas line

603 side is reduced. Therefore, in order to keep the pressure

detected by the pressure meter 241 at a set pressure, the pressure

regulating valve 242 increases the supply amount of the NG from

the make-up gas line 606. Further, when the generation amount of

the fuel gas becomes insufficient, the reception of the

undehumidified NG through the undehumidified natural gas line 605

may be started.

[00361 When the operation of loading the LNGis started in this

state, the hydrocarbon gas generatedin the LNG tanker 50 is returned

through the return gas line 52 and joined as the return gas to the

BOG. As a result, the supply amount of the fuel gas raw material

(mixed gas of the BOG and the return gas) from the BOG compression unit 211side is increased, and the pressure detectedby the pressure meter 241 changes in a direction of increasing. Then, the pressure regulatingvalve 242 reduces the supply amount ofthe NGforpressure adjustment increased in advance along with the draw-off of the reprocessing gas, to thereby adjust the supply balance of the fuel gas in accordance with the reception amount of the return gas.

[0037] The reprocessing gas returned to the liquefaction

processing unit 12 is processed together with the NG and liquefied

again to be stored in the LNG tank 13. The excessive fuel gas, which

cannotbe completelyabsorbedin the fuelgas supply facility through

reception of the return gas, is reprocessed and recovered as the

LNG, with the result that the loss of the LNG can be reduced as

compared to the case in which the excessive fuel gas is discarded

by burning with a flare or the like.

[0038] When the operation of loading the LNG is finished, and

there is no reception of the return gas from the LNG tanker 50,

the reprocessing gas compression unit 531 and the reprocessing gas

cooling unit 533 are stopped to finish the reprocessing of the fuel

gas in the liquefaction processing unit 12. As a result, the

generation balance of the fuel gas in the fuel gas supply facility

is returned to a state before the operation of loading the LNG is

started.

[0039] The control of each of the devices in the fuel gas supply

facility described above is performed by a control unit 8 including

controlends, suchas thepressureregulatingvalve242, adistributed controlsystem (DCS) configuredtocontroltheentireNGliquefaction facility through use of those control ends, and the like.

[0040] TheNGliquefactionfacilityaccordingto theembodiment

of the present invention exhibits the following effects. In supply

of electric power to be used in the NG liquefaction facility, the

power generator 41 is driven through use of the gas engine 4. In

the power generator 41, it is not required to compress fuel gas

to high pressure unlike a gas turbine. Further, for example, the

power generator 41 can be stably operated within a wide outside

air temperature range and a wide load range, and thus the power

generator 41 has satisfactory operability. Therefore, a device

configuration suitable for a small natural gas liquefaction unit

operated on a small scale can be provided.

[0041] In this case, the procedure for reprocessing the return

gas from the LNG tanker 50 in the liquefaction processing unit 12

is not limited to the example of FIG. 1 involving once joining the

return gas to the BOG in the boil off gas line 601, then drawing

off the fuel gas from the fuel gas line 602, and returning the fuel

gas as the reprocessing gas to the inlet side of the liquefaction

processing unit 12. The return gas may be reprocessing gas, for

example, by interposing the reprocessing gas compression unit 531

and the reprocessing gas cooling unit 533 in the return gas line

52 and directly connecting a downstream end thereof to the inlet

side of the liquefaction processing unit 12. Further, the position

to which the reprocessing gas is returned is not limited to the inlet of the liquefaction processing unit 12, and may be an inlet side of any one of the pretreatment devices on an upstream side of the liquefaction processing unit 12. When the content of impurities (mercury, acid gas, and moisture) in the reprocessing gas is sufficiently low and does not influence the processing in the liquefaction processing unit 12, the case of returning the reprocessing gas to the inlet of the liquefaction processing unit

12 can minimize the reprocessing cost.

[0042] Next, as a reference example for understanding the

technical features of the NG liquefaction device according to the

embodiment described above, brief description is made of a

configuration example of a large NGliquefaction facility including

the liquefaction processing unit 12 having a processing capability

of millions of tons per year with reference to FIG. 2.

In FIG. 2, the components having the same functions as those

of the NG liquefaction facility described with reference to FIG.

1 are denoted by the same reference symbols as those used in FIG.

1.

[0043] The largest difference of the NG liquefaction facility

(FIG. 2) according to the reference example from the NGliquefaction

facility (FIG. 1) according to the embodiment resides in that a

plurality of gas turbines 71 and 73 are provided as power sources

for driving a power generator 72 and the refrigerant compression

unit 121, respectively.

Unlike the smallNG liquefaction facility, installment of the precoolingheatexchangerisnotomittedin the largeNGliquefaction facility. Therefore, in the large NG liquefaction facility, a refrigerant compression unit 121 configured to compress precooling refrigerant is provided in addition to the refrigerant compression unit 121 configured to compress mixed refrigerant.

[0044] In the large NG liquefaction facility, it is required

to drive large power equipment including the above-mentioned

pluralityofrefrigerantcompressionunits121. However, atpresent,

there is no gas engine 4 having output comparable to that of a large

gas turbine. Therefore, it is not realistic to make an attempt to

obtain the entire motive power by electric power supply from the

power generator 41 driven by the gas engine 4 because it is required

to install several tens of the above-mentioned gas engines 4 having

an output of from about 5 MW to about 20 MW.

In this respect, the small NG liquefaction facility capable

of supplying motive power through use of several gas engines 4 is

considered to be suitable for employing the gas engines 4.

[0045] Meanwhile, as described above, it is required to supply

high-pressure fuel gas increased in pressure to, for example, 3.0

MPaG to the gas turbines 71 and 73. Therefore, the BOG compression

unit 211 increases the pressure of the BOG to the above-mentioned

pressure, and the pressure regulating valve 242 increases or

decreases the supply amount of the NG for pressure adjustment so

that the pressure detected by the pressure meter 241 is kept at

the above-mentioned pressure.

[0046] The high-pressure fuel gas is supplied from the fuel

gas line 604A (high-pressure fuel gas system) to the gas turbines

71 and 73, and the fuel gas is burnt to drive the power generator

72 and the refrigerant compression unit 121. Combustion exhaust

heat of the gas turbines 71 and 73 is recovered as steam or the

like in exhaust-heat recovery units 711 and 731.

[0047] Meanwhile, the other combustion equipment such as the

boiler and the flare stack pilot burner do not require high-pressure

fuel gas. Therefore, a pressure letdown valve 252 is provided to

the fuel gas line 604B configured to supply the fuel gas to those

combustion equipment, to thereby reduce the fuel gas having a high

pressure of 3.0 MPaG to, for example, 0.7 MPaG. The fuel gas having

pressure reduced is subjected to gas-liquid separation in the

knockoutdrum3Band thensupplied toeachofthe combustionequipment

through a low-pressure fuel gas line 607 (low-pressure fuel gas

system).

As described above, the NG liquefaction facility including

the gas turbines 71 and 73 is different from the NG liquefaction

facility including the gas engine 4 capable of being operated only

in the low-pressure fuel gas supply facility in that it is required

to provide fuel gas supply facilities of two systems, that is,

high-pressure and low-pressure fuel gas supply facilities.

[0048] In this case, thehigh-pressure fuelgas supply facility

has a fuelgas adjustmentmarginlarger than that of the low-pressure

fuel gas supply facility. Therefore, even when the entire amount of the return gas generated at a time of the operation of loading the LNG into the LNG tanker 50 is received, the fuel gas that has become excessive along with the reception of the return gas can be balanced with the consumption amount by reducing the reception amount of the NG for pressure adjustment from the make-up gas line

606.

Therefore, it is not required to provide a facility configured

to return the excessive fuel gas and return gas to the inlet side

of the liquefaction processing unit 12 for reprocessing. Also from

this point of view, the large NG liquefaction facility is different

in configuration from the small NG liquefaction facility in which

it is required to provide a facility configured to reprocess, as

the reprocessing gas, the fuel gas that has become excessive at

a time of the reception of the return gas.

Reference Signs List

[0049] Reference Signs

12 liquefaction processing unit

121 refrigerant compression unit

13 liquefied natural gas (LNG) tank

211 boil off gas (BOG) compression unit

4 gas engine

41 power generator

601 boil off gas line

602, 603, 604A, 604B fuel gas line

8 control unit

Claims (6)

1. A natural gas liquefaction unit, comprising:

a liquefaction processing unit configured to perform

liquefaction processing of natural gas;

a storage tank configured to store the natural gas liquefied

in the liquefaction processing unit;

a boil off gas line including a boil off gas compression unit

configured to increase a pressure of boil off gas generated in the

storage tank;

a gas engine configured to drive a power generator by burning

fuel gas;

a fuel gas line configured to supply the fuel gas containing

theboiloffgasincreasedinpressurein theboiloffgas compression

unit to the gas engine;

a refrigerant compression unit, which is driven by electric

powergeneratedin thepowergenerator, andis configuredtocompress

refrigerant gas that has cooled the natural gas in the liquefaction

processing unit;

a dehumidifier, which is provided on an inlet side of the

liquefaction processing unit, and is configured to remove moisture

in the natural gas supplied to the liquefaction processing unit;

an undehumidified natural gas line configured to supply the

natural gas before being processed in the dehumidifier to the fuel

gas line; and

apressure adjustinggas line configured to supply the natural gas before being liquefied in the liquefaction processing unit to the fuel gas line in order to keep a pressure of the fuel gas line at a pressure set in advance, wherein theboiloffgas compressionunitis drivenbyelectric power generated in the power generator.

2. The natural gas liquefaction unit according to claim 1, wherein

the liquefaction processing unit has a natural gas processing

capability within a range of from 100,000 tons per year to 1,000,000

tons per year.

3. The natural gas liquefaction unit according to claim 1, wherein

the fuel gas supplied from the fuel gas line to the gas engine has

a pressure within a range of from 0.6 MPaG to 1.0 MPaG.

4. The natural gas liquefaction unit according to claim 3, further

comprising another combustion equipment using the fuel gas,

the fuel gas supplied from the fuel gas line to the another

combustion equipment having a pressure equal to the pressure of

the fuel gas supplied to the gas engine.

5. The natural gas liquefaction unit according to claim 1, further

comprising:

a rundown line configured to deliver the liquefied natural

gas in the storage tank to a liquefied natural gas tanker;

a return gas line configured to join return gas, which is generated in the liquefied natural gas tanker along with delivery of the liquefied natural gas from the rundown line, and is returned from the liquefied natural gas tanker, to the boil off gas in the liquefied natural gas storage tank; and a reprocessing gas line, which includes a reprocessing gas compression unit configured to draw off part of the fuel gas containing the gas increased in pressure in the boil off gas compression unit as reprocessing gas to be reprocessed in the liquefaction processing unit and to further compress the reprocessing gas, and is configured to supply the reprocessing gas increased in pressure in the reprocessing gas compression unit to an inlet side of the liquefaction processing unit.

6. The natural gas liquefaction unit according to claim 1, further

comprising an exhaust-heat recovery unit configured to recover

exhaust heat of exhaust gas discharged from the gas engine,

the exhaust-heat recovery unit being configured to supply

a heat source to at least one reboiler selected from a reboiler

groupconsistingofareboilerofanacidgasremovingunitconfigured

toremove acidgascontainedin thenaturalgasbeforebeingliquefied

in the liquefaction processing unit, and a reboiler provided in

each of fractionators configured to fractionate ethane, propane,

and butane separated from the natural gas in the liquefaction

processing unit.