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.