36
LNG
INDUSTRY
APRIL
2016
plant design is dependent upon a detailed analysis of the range of
components in the feed gas. Coal bedmethane (CBM) feedstock, for
example, does not contain heavy hydrocarbons and, therefore, a unit
is not required. Careful consideration of the range of possible heavy
hydrocarbon contaminants is necessary for locations using pipeline
specification gas. The designmay include a turboexpander as well as
a scrub column. The auxiliary steamboiler burns any resultant heavy
liquids, which, alternatively, may be exported for sales.
Ammonia refrigeration plant
The ammonia refrigeration is used to precool the dry feed gas to
approximately 18°F (-8°C) prior to entering the liquefaction plant.
The ammonia system is comprised of a single or two-stage closed
loop refrigeration cycle (depending on plant capacity), utilising two
parallel steam turbine driven compressors powered by steam from
the waste heat recovery plant. The ammonia refrigeration improves
the output and efficiency of the SMR process. It also provides stable
operation of the plant, since it dampens the impact of variations
in ambient air temperatures on theMR gas turbines, which would
otherwise greatly affect plant operation and capacity. Optimising the
temperature of the ammonia refrigerant tunes overall performance
of the plant consistent with the operating environment and plant
capacity.
The ammonia refrigerant cools a number of units around the
LNG train, including the following:
Precooling dry feed gas prior to entering the liquefaction unit.
MR in the ammonia/MR precooler.
Inlet air for the gas turbines.
Wet gas exiting the amine contactor.
Cooling requirements, when necessary, within the heavy
hydrocarbon liquid removal system.
Liquefaction plant
The liquefaction plant cools and liquefies the feed gas from
approximately 18°F exiting the ammonia precoolers to -260°F
(-162°C). The liquefaction process comprises a single-stage high
pressure vapour compression cycle using a mix of refrigerants,
providing a close fit of cooling curves in themain plate-fin heat
exchanger (cold box). Themain liquefaction exchanger is a
multi-core brazed aluminiumplate-fin exchanger using a minimal
number of exchanger streams. Enhancement of main exchanger
performance results from the ammonia precooling refrigerant, which
cools theMR in the ammonia/MR precooler as noted previously.
This allows cooler low pressuremixed refrigerant (LPMR) to exit the
cold box. The cooler LPMR feeding theMR compressor improves its
performance.
Within each train, two separate independent parallel
refrigeration circuits each include anMR compressor, MR air cooler,
ammonia/MR precooler, cold box, and suction scrubber. The dual
parallel refrigeration/liquefaction circuits provide added reliability
and availability, while allowing use of commonly available
equipment sizes. The high pressure precooledMR is further cooled in
a cold box pass, and is then flashed to low pressure across a
Joule-Thomson expansion valve, producing a very cold two-phase
liquid-vapour refrigerant. A liquid knockout separator is used to
provide consistent remixing of the two-phaseMR refrigerant stream,
which is then fed back into the cold box to liquefy the feed gas. The
precooled dry feed gas itself splits into two streams, feeding the two
cold boxes in parallel and exits the cold box as LNG. This liquefied
gas is then flashed to low pressure to achieve the final cryogenic
temperature of -260°F (-162°C) as it flows into the storage tanks.
The MR for each cold box is compressed by a single-stage
centrifugal compressor directly driven by a highly fuel efficient, low
emissions gas turbine. Air coolers remove the heat of compression
from the MR prior to ammonia precooling, while inlet air to the gas
turbines is cooled to approximately 44°F (7°C) using an
ammonia-to-air exchanger, to increase the power output and
efficiency of the gas turbine, particularly at high ambient
temperature conditions.
BOG system
The BOG system for a 4 x 2 million tpy LNG plant would typically
comprise three to five low pressure gas compressors to recover
flash gas, BOG and ship vapour from the LNG tank, and a simple
reliquefaction and nitrogen rejection system to ensure the
required LNG composition is met. The compressed BOG vapour is
reliquefied in the cold box and sent to the liquid methane separator,
where it is separated with the liquid methane stream returning to
the LNG storage tank. The lean vapour flash gas from the liquid
methane separator, containing a high proportion of nitrogen and
some methane, is used as lean low pressure fuel gas in theWaste
Heat Recovery (WHR) and steam plant auxiliary boiler.
Normally, only one BOG compressor is used to handle BOG
from one LNG train. However, during ship loading, additional BOG
compressors are used to recover the additional BOG generated.
WHR and steam plant
TheWHR and steam plant is comprised of the following:
WHR from the two gas turbines using OTSGs.
Two steam turbines for the auxiliary refrigeration plant
(ammonia) compressor drives.
An auxiliary boiler for start-up and supplemental control steam.
Process and utility steam heating system.
Air cooled condensers.
All associated systems required for aWHR and steam plant.
Ammonia compression power and heat for the plant is provided
by waste heat from the gas turbine exhausts as well as from the
auxiliary boiler, which is fuelled by three sources: feed gas in the
plant; lean flash gas from the methane separator in the BOG
system; and heavy hydrocarbon waste stream. The high pressure
steam powers the two ammonia refrigeration steam turbines. A
portion of this steam is attemperated and used as low pressure
process heat for the amine reboiler, fuel gas heater and other
process duties.
The ammonia auxiliary refrigeration plant is sized to consume all
available power that can be generated from the waste heat and
lean flash gas. During ship loading, generation of additional BOG
occurs, thereby producing additional low pressure fuel gas, which
reduces the use of feed gas used in the auxiliary boiler. This
alleviates the need for flaring of BOG during ship loading.
Conclusion
The efficiency of LNG Ltd’s liquefaction process technology results
from integrating proven, highly efficient gas turbine drives for the
main refrigerant compressors with combined cycle heat and power
technology, and efficient ammonia refrigeration. Gas turbine inlet
air cooling and low pressure BOG reliquefaction are employed. The
innovative configuration of these proven technologies results in
competitive capital and process efficiencies. The compact modular
design strategy and mid scale production targets are also features
of the process technology.
