HYDROCARBON
ENGINEERING
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following the guidelines of the liquefaction technology
provider, is working with the EPC contractor’s joint
venture, between CB&I, Zachry and Chiyoda, to
incorporate additional provisions in the design that
would allow the pre-cooldown process to be performed
without flaring hydrocarbons. By doing so, the amount
of natural gas required to be flared will be substantially
reduced.
Reduction of fugitive emissions
With the purpose of reducing VOC emissions,
Freeport LNG upgraded around 3600 manual valves in
propane, mixed refrigerant and ethylene services to
bellows seal ‘zero-leak’ gate valves, for which the body
bonnet welds hermetically seal the valve, preventing the
undesired fugitive emissions. It is estimated that by
converting conventional packing-type valves to bellows
seal-type valves will reduce fugitive emissions by
6.7 tpy.
In addition to this, the piping design of the
refrigeration system minimises the use of bolted flanges,
which further reduces the fugitive emissions.
Boil-off gas recovery for fuel gas
systems
Boil-off gas (BOG) is generated due to the flashing of
high pressure LNG when stored at low pressures, heat
input from the environment, heat added by submerged
cryogenic motors, etc.
To minimise the BOG generated, the liquefaction
trains produce slightly subcooled LNG that is stored at
0.5 psig. LNG hydraulic turbines are used to recover
energy from the pressure letdown operation and to
increase efficiency. LNG in liquefaction facilities is
stored at lower pressures than in regasification facilities,
meaning that the flashing effect during LNG carrier
loading operations is minimised.
The BOG produced at the liquefaction facility will
be compressed and transported via a 12 in. pipeline to
the pretreatment facility, where it will be used as fuel
gas to primarily run a GE Frame 7EA gas turbine
generator (GTG). The heat from the exhaust of the GTG
system will be recovered using the gas turbine waste
heat recovery unit (WHRU). The WHRU will recover a
nominal 445 million Btu/hr, which is enough energy to
supply heating to all three trains of the PTF.
Control of exhaust emissions from the GTG is
achieved primarily by the turbine’s dry low NO
X
combustion system, and for further abatement of
emissions the WHRU is furnished with CO-reduction
catalyst and selective catalytic reduction (SCR), using
19% (by weight) aqueous ammonia.
Hot oil is used as a heating medium and, to increase
the energy efficiency, the hot oil is cascaded from the
outlet of the higher temperature users to the inlet of
the lower temperature users. Backup fired heaters will
be used to supply heat when the gas turbine is not in
service.
During LNG carrier loading operations (normal
loading or cooldown operations), excess BOG that
cannot be used at the PTF will be recycled back to the
inlet of the liquefaction trains for reprocessing (no
flaring will be required). Figure 3 shows an overall flow
diagram with the BOG system and fuel gas users.
High destruction efficiency ground
flares
There are obvious reasons for avoiding burning natural
gas, or releasing it into the atmosphere, and that is why
flaring activities during normal operations are not
contemplated. During startup or upset conditions, it will
be necessary to flare in order to bring the process to a
stable condition or to prevent unsafe situations. The
release of natural gas or refrigerants directly into the
atmosphere without proper destruction of the different
components is not contemplated in the Freeport LNG
liquefaction project.
Freeport LNG chose the flare technology that has
the least impact on the surrounding community, while
also having higher levels of destruction efficiency and
safe reliability. The multi-point, multi-stage, pressure
assisted ground flare, designed by Zeeco, consists of
multiple stage headers that are normally closed, but
which will open depending on the pressure on the
main flare header, providing a greater turndown
capability while maintaining the stringent requirements
of smokeless waste gas. By having multiple stages, the
discharge velocity at the burner tip ensures clean
flaring. A 60 ft tall radiation fence will ensure that
flames are not visible from the outside in any of the
relieving cases. The radiation fence, while allowing air
inspiration for the proper combustion at the burner
tip, also limits the radiation to the outside of the
fence. By comparison, an elevated flare designed for
this project would require a structure of approximately
500 ft in height. A 99% minimum destruction of
methane, ethane and propane, and 98% minimum
destruction of VOC, will be achieved with the
multi-point ground flares.
In addition to the typically installed flare knock-out
drum, in-train cold drain drums are provided to recover
liquid hydrocarbons from pressure relief headers to be
reinjected into the system, which would otherwise end
up being flared.
Conclusion
As concerns over air emissions have increased over the
past decades, the demand for natural gas continues to
grow. Natural gas and, therefore, LNG are currently the
preferred alternative source of energy as opposed to
other less environmentally friendly fossil fuels. Gas
pretreatment and liquefaction processes require large
amounts of energy and handling of hydrocarbon
refrigerants, such as methane, propane and ethylene.
With a focus on minimising the impact to the
surrounding community and the environment,
Freeport LNG is designed to be the largest 'all-electric'
baseload LNG liquefaction facility in the world,
operating with the lowest emission levels per metric
tonne of LNG produced.
T&T
