Green Fossil Fuels?

'Green LNG' is an unclear term, referring to emissions-neutral LNG produced from a nominally emissions-neutral facility. For 'Green LNG', emissions produced along all or some of the supply chain must be reduced or offset by investments in carbon neutral projects outside the sector. However, there are no specific guidelines formalising 'Green LNG'.   

LNG’s competitive advantage over other fuels is significant, whether it be over coal in the realm of carbon emissions, or over renewables in the area of energy storage, but it must continue to develop to retain that advantage.

15 green LNG cargoes were reported worldwide between 2019 and July 2021. However, the suppliers had offset the emissions released by some fraction the full life-cycle by investing in external carbon offsets, rather than decarbonising production.

The methods for examining what the exact emissions are and whether suppliers included both ‘Well-to-Tank (of user)’ stages and ‘Tank-to-Wheel’ use stages varied. The UK government estimates emissions of 0.9tCO2e/tLNG for ‘Well-to-Tank’ extraction, liquefaction and transport stages and 2.54tCO2e/tLNG for ‘Tank-to-Wheel’ stages.

Often, methane emissions are excluded from existing emissions accounting, despite being significantly worse than CO2 emissions in the short term.

When examining the life-cycle of natural gas, there are at least three general stages of emissions in the LNG supply chain:

• Direct emissions from the supply chain, from purification, liquefaction, transport and storage.

• Indirect emissions from the power generation during these purification and liquefaction stages.

• Emissions from production of natural gas feedstock.

When considering the ‘green-ness’ of a fuel, it must include the full life cycle from production or extraction all the way to combustion. Current variability of accounting methods offers many cracks which can conceal flaws in the tools used to tally emissions.

Shades of Green

There are three key types of green LNG – biomethane, synthetic methane, and carbon capture offset. Each type of green LNG has a different production process, but ultimately shares the same fundamental composition of primarily methane. Commercial LNG is typically 85% or greater methane, 5-10% ethane with elements of propane and other gases such as CO2, oxygen, or nitrogen.

Biomethane, or biogas, is produced through biological breakdown of organic matter. This is usually done as capture on organic waste but can also be captured from livestock waste emissions or artificially induced bacterial slurry. Biogas is typically initially 40-60% methane, with the remainder being largely CO2 and traces of other organic compounds such as H2S.

The gas is then purified to keep the biomethane. This purification can generate significant excess CO2 waste, which requires capture and storage, but is significantly easier to capture than ‘free’ CO2 emissions. Biomethane offers all the environmental advantages of drilled natural gas, but also avoids the natural methane emissions of decaying organic matter in order to capture the fuel.

Methane can also be formed from hydrogen and carbon dioxide via thermal catalysis, electrocatalysis or photocatalysis. Capturing carbon from the air allows a net-neutral paradigm. Synthetic methane formed in this way releases oxygen, but has decreased energy efficiency due to the need for energy input to produce the methane.

Gotta Catch It All

Direct air capture (DAC) is used to collect CO2 from the atmosphere. Hydrogen is produced using electrolysis powered by renewable sources to avoid any emissions in production. Conversion of CO2 and H₂ to synthetic methane of high purity follows. The gas is then liquefied to produce liquefied synthetic methane (LSM).

Unfortunately, while the cost of liquefaction is near identical to natural gas, estimated costs for DAC with an amine catalyst at the end of 2021 amounted to US$114/tCO2 produced at a 1MtCO2pa scale. This is a significant cost and without improvement LSM will fail to compete with coal seam gas and LNG. Technological improvement is already occurring in this area. This may make LSM not only viable but superior to LNG over time, particularly if the cost of carbon rises and LSM can maintain a zero-emissions production status.

Carbon capture and storage (CCS) as an offset is the least multipurpose of the options presented here. It is, however, the simplest and requires the least infrastructure to establish an effective partial offset to the emissions of LNG production. Sequestration and storage are currently used to minimise emissions and create a differentiation with competitors to attract customer bases looking for Green LNG.

CCS has had significant investment in recent years, with both successes and failures. Gorgon LNG ‘s CCS program has achieved under 25% (1.25Mtpa) of its agreed 5Mtpa carbon capture targets with the West Australian government, at an estimated cost of A$3.1bn (US$2.2bn).

Carbon sequestration is generally done in salt caverns or in older gas reservoirs that no longer offer any usable products. This allows reuse of the infrastructure to lower costs. DAC, as opposed to upstream or mid-stream production capture, is much more expensive due to lower concentrations of carbon gases.

Green LNG will continue to play a significant role in the energy transition, but the most favoured form of green LNG is yet to be determined. Likely a mix of all sources will be necessary with CCS being the most expensive, but remaining the easiest to implement and be applied to existing infrastructure.