Topics: Aviation fuels, greenhouse gas emissions, life cycle analysis
Type: Briefing paper
Publication date: March 2023

Download the full paper or the executive summary [PDF]

Launch

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Authors

Andrea Fantuzzi, Paola A. Saenz Cavazos, Nadine Moustafa, Michael High, Mai Bui, A. William Rutherford, Isabella von Holstein

Summary

The aviation industry is responsible for 2.1% of global CO₂ emissions and represents 12% of CO₂ emissions from all transport sources. Aviation is a particularly difficult sector to decarbonise because alternative fuels are relatively expensive, produce highly distributed greenhouse gas emissions in their production and combustion, and should preferably be compatible with existing aviation infrastructure. Emissions from aviation also include nitrogen oxides (NO_x), water vapour, particulates, carbon monoxide, unburned hydrocarbons, and sulfur oxides (SO_x). These have a 2–3 times greater climate change impact than CO₂ alone. The non-CO₂ emissions of alternative low-carbon aviation fuels can differ significantly from those of kerosene and have not been fully evaluated.

Biofuels

• Bio-jet fuels are currently the most technologically mature option for low-carbon aviation fuels because some of these feedstocks and processes are already deployed at scale for other uses.
• Bio-jet fuels must be blended with kerosene to achieve certification and can then be used with existing aviation infrastructure. This blending proportionally decreases any potential CO₂ emission saving.
• Bio-jet fuels can be made from a range of feedstocks, which are restricted in the UK to waste materials. UK biofuel feedstock availability is sufficient for only a small proportion of UK aviation fuel demand (<20%). With blending, their contribution to CO₂ emissions saving is much less (<10%).
• Life cycle assessment scenarios show very variable impacts on CO₂ emissions for biofuel processes: only some deliver emissions savings compared to fossil fuel kerosene. Calculations for forest residues appear to show consistent savings in CO₂ emissions compared to jet fuel, but these do not take account of the difference in timescale between emission and re-absorption, leading to a major underestimation of emissions. The diversion of agricultural and forestry waste to bio-jet fuel production will have detrimental effects, for example on soil quality.

Power-to-Liquid fuels

• PtL fuels must be blended with kerosene to achieve certification and can then be used with existing aviation infrastructure. This blending proportionally decreases any potential CO₂ emission saving.
• PtL fuels are currently not produced at scale. Significant technological development is required to reduce production costs and increase production scale.
• Use of PtL fuels in aviation would require a very significant increase of UK low-carbon electricity generation and storage capacity to power production of green hydrogen and CO₂ from direct air capture.
• Life cycle assessment scenarios show that PtL fuels could have 3–10 times lower emissions impact than fossil fuel kerosene if renewable electricity and CO₂ from direct air capture are used to produce the fuel.

Hydrogen

• Hydrogen cannot be used as a drop-in fuel for aircraft, and its use will require significant redesign of aviation infrastructure.
• The greenhouse gas emissions impact of hydrogen depends on its mode of production. Currently, global hydrogen production is mostly from fossil fuel sources, with much less than 1% generated from low-carbon sources.
• Increasing low-carbon hydrogen production via electrolysis (green hydrogen) will require the building of additional low-carbon electricity generation capacity.
• Low-carbon hydrogen production via methane reforming with carbon capture and storage (blue hydrogen) should use natural gas obtained from producers with low emissions intensity.

The goal of policy will be to promote whichever technologies achieve the desired sustainability targets. A molecular science and engineering approach combines an understanding of molecular behaviour with a problem-solving mindset derived from engineering. This approach is crucial to the development and the eventual deployment of the fuel technologies discussed in this paper.

Download the full paper or the executive summary [PDF]

Watch the launch [YouTube, 92 min]