As the energy transition gathers momentum, countries and corporates are racing ahead to announce net-zero pledges and decarbonisation targets. Countries with carbon neutrality pledges now account for 75% of LNG demand. The list includes some of Australia’s largest LNG customers—China, Japan and South Korea.
LNG producers are looking for
ways to reduce the environmental impact of the fuel. For example, in October
2020, French oil and gas major Total delivered its first carbon neutral
shipment of LNG from its Ichthys LNG plant in Australia.
It has been estimated that total
LNG supply chain emissions should lie within a range of 2.7g - 32.8g CO₂e per MJ, with a median value of about 13g. However, LNG
requires liquefaction, transportation and then regasification, all of which
raises the median value to about 18g, with the liquefaction process
contributing the largest element of the increase in emissions. Therefore, a key
target for LNG supply chain emissions reductions is the liquefaction process
itself but more specifically what type of compression is used to liquefy gas.
Current LNG Technologies
The technology used in LNG trains
to convert natural gas to LNG has, over the past 30 years, become dominated by
a few key types. These technologies have specific design requirements: driver
power load, LNG purity, capacity and versatility are often the most significant
criteria when considering any LNG technology. The specific power required
varies for each of the different liquefaction processes. The specific power is
the amount of energy required to liquefy a tonne of LNG.

Other factors, such as the ambient temperature, can have an
important role in selecting a liquefaction process. Facilities with lower
seasonal variation or yearly ambient temperatures can receive significant
production boosts, compared to more arid or tropical climates. The processes
outlined constitute the majority of base load capacity and reboiler
technologies using gas compression available today.
C3MR
Process
Air Products and Chemicals Inc (APCI)’s Propane Pre-Cooled Mixed
Refrigerant, or ‘C3MR’, process is the most extensively used process worldwide.
The technology has been used by five of the ten Australian LNG facilities and
utilises a basic design that provides versatility and high-quality LNG. The
process does not discriminate on compressional driver, steam, gas and
electrical drivers, as they have all been successfully integrated into a C3MR
design.

The design provides a versatile liquefaction technology that can
operate at a low specific power across a range of base load capacities. C3MR
requires relatively higher capital outlay than other competing technologies;
despite numerous iterations, the process has remained unchanged in over three
decades.
Optimised Cascade
Process
The Optimised Cascade Process (OCP) was initially created in
1968. The technology has been iteratively improved and continues to be used widely
today. The process utilises a 2-in-1 concept; each circuit has two or more
compressors driven by an individual turbine. This allows the facility to
continue production should a turbine or compressor stop functioning. This
redundancy increases reliability and production flexibility. The technology has
been used by five of the ten Australian mega-projects to maximise the relative
high purity of the feed gas from coal bed methane.
Queensland LNG Facilities
Gladstone LNG is built on the OCP.
The facility uses two 3.9Mtpa LNG trains based on two 33MW, GE gas turbine
drives arranged in the parallel single-mixed refrigerant circuits. Feed gas is
sweetened in a conventional amine plant using a MDEA to remove CO2
and H2S. The warm saturated gas exiting the amine contactor is
cooled using ammonia refrigerant to remove the bulk of the water prior to being
dehydrated in a conventional molecular sieve plant.
Australia Pacific LNG was constructed
using the OCP. The LNG facility’s two initial LNG trains each have a nameplate
capacity of 4.5Mtpa. Meanwhile, Queensland Curtis LNG facility uses the OCP,
whereby three consecutive refrigeration systems cool the gas. The trains use a
two-in-one system, where two independent GE gas turbines drive the refrigerant
compressors. Feed gas pre-treatment begins with reducing pressure and gas heating.
After dehydration, the acid gas removal unit (AGRU) removes CO2
using an activated Di-glycol-amine solvent solution to prevent freezing
complications. QCLNG plant is fitted with GE Dry Low Emissions turbines instead
of heavy-duty turbines which reduces its greenhouse emissions output by about 27%.
Northern Territory LNG Facilities
The Darwin LNG plant uses OCP with
associated storage and marine load-out facilities. The liquefaction technology
employs a two-trains-in-one design for increased reliability and flexibility. The
Ichthys LNG onshore facilities consist of two LNG trains, utilising C3MR, with
four GE gas turbines and eight compressors split equally between the two
trains.
The trains have a capacity of 4.45Mtpa each and treatment facilities
include; acid gas removal, molecular-sieve dehydration and mercury removal
units. The process for cooling the gas, C3MR/Split MR, is one of the lowest
specific power liquefaction technologies used in LNG production; this means the
facility requires less power from the compressional drivers and, therefore,
less gas consumed in the turbines.
Western Australia LNG Facilities
The liquefaction technology at
Prelude FLNG - one of the largest floating offshore facilities in the world, is
based on Shell's proprietary Dual Mixed Refrigerant (DMR) Process. Similar to
the C3MR process, DMR uses a compositionally different mixed refrigerant in the
pre-cooling stage, which results in a lower pre-cooling temperature of -40°C.
The DMR process also requires less equipment which aligns well within the space
constraints of a floating LNG plant.

The North West Shelf LNG plant uses
C3MR. The process uses a mixed refrigerant (MR) that consists of nitrogen,
methane, ethane and propane. The process starts off with the feed gas being
pre-cooled by a separate chiller package down to -35°C. At this intermediate
temperature, the heavier components within the feed are condensed out.
The feed
gas is then sent to the main cryogenic heat exchanger (MCHE). The pre-cooled
refrigerant is further chilled, and pressure reduced in stages before being
used to cool the lighter feed gas within the MCHE. Axial compressors are used
for the low-pressure stage, whereas centrifugal compressors are used for the
high pressure stage, driven by gas turbines.
The Pluto LNG liquefaction stage utilises
the C3MR split propane system developed by Shell and Foster Wheeler. The first
step is propane-pre-cooling down to -30°C. This condenses out LPG and heavier
molecules. The pre-cooled gas then enters the MCHE where is it sub-cooled at
elevated pressures. Once the LNG leaves the MCHE, it is flashed to near storage
tank pressure and subsequently cooled to approximately -161°C.
Gorgon's liquefaction process is C3MR
which features a propane based, pre-cooling stage and a mixed, three
refrigerant main stage. This is the most widely used liquefaction technology
globally.
The Wheatstone LNG plant uses OCP. A
key feature of this process is the use of progressive heat exchangers, starting
with propane, then ethylene and finally methane. The process uses multiple,
parallel compressor circuits at each train that allows ongoing operations in
the case of partial shutdowns for maintenance or repair, avoiding a full plant
shutdown. The Wheatstone trains have a six-turbine driver with internal air humidification.
Each of the turbines has a claimed thermal efficiency of 39% to 43%. A further
consideration for operating costs at this site is the ambient temperature conditions,
which are in the range 15 to 46°C, averaging 25.3°C
with an average humidity of 81.8%.
Electric Compression
Australia is home to some of the
most emission-intensive LNG plants in the world. The average emission intensity
of LNG projects globally is 0.56 t CO2e/t LNG produced and over half of
Australian LNG plants have emissions above this level. To remain globally
competitive Australian LNG producers will be looking to reduce all sources of
emissions. With respect to the liquefaction process which can account for up to
30% of total supply chain emissions, adoption of electric compression is being
given close consideration.
LNG compressors use a large
amount of energy and have traditionally been gas-fired – an obvious solution
given that the gas is already on site. Currently all 10 Australian LNG plants
utilise gas fuelled compression. Raising the efficiency of a compressor, for
example by improving the aerodynamic flow of the gas, reduces the energy input,
emissions and increases the LNG output. However, some developers are now gone
further, intending to use all-electric motor drives to power compressors, pumps
and fans for the next wave of LNG facilities.
For example, US oil major Chevron
estimates that the use of electric drives at its proposed Kitimat LNG plant in Canada
would reduce the plant’s GHG emissions to less than 0.1 metric tons of carbon equivalent
for every ton of LNG produced. Similarly, the Shell-led Canada LNG project will
also use electric drives, sourcing the majority of the electricity required from
local hydro generation. This should allow it to achieve an intensity of 0.15 tons/CO₂e per ton of LNG, below the 0.16 threshold set by British Columbia,
the Canadian province where the plant is being developed. Any higher and the project
would have to pay additional carbon taxes.