Imperial College briefing paper on low-carbon fuels for aviation

A paper has been published by Imperial College, on low-carbon fuels for aviation. The authors looked carefully at the various fuels that the sector is hoping to use in future, to enable it to continue with its expansion plans, flying ever more people each year. The Imperial scientists concluded that hydrogen is impractical and will not contribute significantly as jet fuel in the foreseeable future. They looked at fuels made from various wastes, and their real lifecycle costs, including manufacture and emissions when burned in a jet engine (“well to wake”),and concluded that the scope for production of such fuels, that genuinely offer a CO2 advantage, on a large enough scale, is unlikely.  For fuels made from plant material, it is important to look at the timescale of carbon absorption by plants, and its emissions when burned.  Ignoring the time lag makes these fuels look unrealistically positive. Looking at “power to liquid” fuels, ie. those made using surplus renewably-generated electricity, they conclude that there will not be enough of this electricity available to make jet fuels in sufficient quantity. They appreciate that it is important that novel fuels to not have other negative environmental impacts. All the novel fuels come with serious problems of scalability and dubious carbon savings.


Imperial College, London

Briefing Paper No 9
March 2023

Low-carbon fuels for aviation

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


A few extracts from the  32 page paper:


• 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.


Sustainable aviation fuels could potentially make a big contribution to meeting the UK’s net zero emissions targets. However, for all three fuel types considered, biofuels, power-to-liquid (PtL) fuels and hydrogen, a significant scale-up from current capacity would be needed to meet aviation fuel demand.

Scale-up of all three fuel types share common challenges of minimising energy use, water use and land use, and there are additional challenges which are specific each fuel type.

The total quantity of biomass available to convert to bio-jet fuels is limited and is now further restricted by mandate in the UK to use “waste” feedstocks only.  Bio-jet fuel production can therefore fulfil only a small fraction of low-carbon fuel demand.

Consequently, reduction of carbon emissions from aviation will have to be achieved by other means.

The physical and chemical properties of bio-jet fuels and PtL are closer to those of kerosene, so these are certified for use as blends with kerosene in current engine types.  Improvements of production processes could generate fuels which match kerosene’s properties more closely, enabling the use of 100% (i.e. unblended) low-carbon fuel.

If achieved by improving fractionation, this will reduce the yield from the feedstock and will therefore be difficult to scale up. Ideally, innovating the chemistry of the conversion process would optimise fuel composition without reducing yield.  The wide biochemical range of biofuel feedstock types could be an advantage here.

The physical and chemical properties of hydrogen are significantly different from kerosene. Hydrogen-powered aviation will require the development of new aircraft, new fuel storage systems and infrastructures. This introduces a significant time lag for development, safety testing and regulatory approval.

It also needs to be integrated with commercial fleet replacement cycles. So, hydrogen-powered aviation is unlikely to contribute to carbon emissions reduction by aviation
in the short to medium term.

Short- to medium-term development of low-carbon fuels must be considered in the context of current global challenges (e.g. high energy prices, geopolitical tensions, supply chain
constraints, and changing water availability). High fossil fuel prices could be beneficial to the development of low-carbon fuels, as they make investing in low-carbon energy generation
more economically attractive.

The production of hydrogen and PtL fuels is very energy intensive, so they will require
a significant investment in increased low-carbon energy production and storage.

Policy recommendations from a molecular science and engineering perspective

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 eventual deployment of the fuel technologies discussed in this paper.
The use of hydrogen, PtL fuels and bio-jet fuels in aircraft requires new development or adaption of manufacturing technologies, catalysts, storage facilities, transport facilities, engines, aircraft, and airports.

The goal of policy will be to promote whichever technologies achieve in the desired sustainability targets. This requires a considerable research effort.

We make the following policy recommendations:

For all low-carbon fuel types:

• Implement policy support only where a low-carbon fuel technology has been demonstrated to achieve the following criteria:

(i) to provide at least 50% CO2 emissions saving when deployed at scale vs the kerosene
baseline, in line with the UK Sustainable Aviation Fuel Mandate;11

(ii) where there is enough feedstock for its production at a meaningful scale;

(iii) its use will not have a negative environmental impact.

• Support the development of infrastructure (e.g. aircraft, airports, fuel transport, fuel storage, operational practices) for low carbon fuels, when these fulfil the criteria in the previous point.
• Implement more rigorous life cycle analyses for low-carbon fuels, and update standardised methodologies such as CORSIA52 to:

– Include evaluation of the secondary impacts of resource use choices (burden-shifting)
relative to current fossil fuels.
– Take into account the CO2 emissions from fuel production as well as its combustion (well-to-wake approach).
– Include non-CO2 effects of both production and combustion in assessments of the
impact of low-carbon fuels.

• Develop aircraft fuel systems which do not require the presence of aromatics in the fuel.
• Collaborate with commercial entities in the sector, especially those who own infrastructure, to generate momentum for change.

• Build systems to promote information-sharing between commercial entities and the independent research sector to help define research priorities and enable research
projects, while protecting IP appropriately.

For bio-jet fuel technologies:

• Standardised life cycle analysis methodologies, such as CORSIA, should address the scalability of the benefits of the co-products and land-use change, and also the time delay between CO2 emission and photosynthetic reabsorption.
• Improve existing bio-jet fuel production processes to optimise fuel composition and reduce the need for blending.
• Improve existing bio-jet fuel production processes to improve yield of conversion and reduce resource pressure.
• For municipal waste and other heterogeneous sources of feedstock material, develop robust processes that can efficiently convert this to fuel.

For PtL fuel technologies:

• Increase UK production of low-carbon electricity and energy storage. Reduce and stabilise the price of low-carbon electricity.
• Scale up production of green hydrogen.
• Assess the scalability of direct air capture using existing technologies.
• Develop novel solid adsorbents and membranes to reduce direct air capture cost.
• Develop mechanisms to reduce the price of PtL fuel relative to fossil fuel kerosene.
• Develop novel affordable catalysts to improve the efficiency of fuel production, preferably in a single step reaction and at low temperature.
• Develop novel fuel production pathways (in addition to Fischer-Tropsch) that meet certification requirements.
• Develop PtL fuels which can be used as a 100% replacement for kerosene.
• Promote and support the commercial development and implementation of improved technologies for all stages of PtL fuels production chain.

For hydrogen technologies:

• Increase UK production of low-carbon electricity and energy storage. Reduce and stabilise the price of low-carbon electricity.
• Scale up production of green hydrogen.
• Scale up carbon capture and storage (for blue hydrogen production) by developing improved absorbents, adsorbents and membranes in scaled-up industrial CO2 capture units with lower energy demands.
• Develop advanced materials for cost-effective electrolysers to enhance both performance and durability.
• Develop new pressurising and cooling infrastructure for efficient storage and refuelling of hydrogen,
• Redesign aircraft to locate fuel tanks in the fuselage.

Major technical improvements are required before any of the fuels discussed here can be considered as a viable replacement for jet fuel in terms of sustainability and cost.

The Institute for Molecular Science and Engineering will work to identify solutions that will overcome existing limitations by using the expertise available at Imperial College London.

Sustainable aviation fuel work at Imperial College London

Imperial hosts a number of researchers and institutes whose work is relevant to the sustainable aviation fuels challenge:

Details at