Fossil aviation fuel can be cheaply upgraded to mitigate climate-heating contrails and improve air quality

by | 05 Jun 2023 | Information

Written by Eric Lombard

While the aviation sector and governments promise a new era of “sustainable” aviation fuels (SAF), we know that it will divert much-needed resources away from other sectors and take decades to happen, if at all. Yet there is an efficient way to rapidly and significantly reduce the non-CO2 effects of aviation, and by that its total climate footprint, by treating conventional jet fuel with limited quantities of hydrogen. This deserves more attention. 

The European Union is about to approve the Fit for 55 ReFuelEU aviation regulation. Its main outcome is a roadmap for the progressive introduction of SAF, both biofuels and e-fuels. The plan targets a 6% blend ratio in 2030, 34% in 2040 and 70 % in 2050, half of it would be e-fuels in 2050. This means that by 2040 the fuel in aircraft tanks would still be more than two-thirds fossil kerosene!

The intended benefit of SAF is to reduce CO2 emissions, but there are others too. As SAF are free of aromatics, naphthalene and sulphur (ANS)(1), they are expected to produce less soot when burned and thus reduce the climate impact of contrail-cirrus, and to improve air quality at airports. These additional benefits could nevertheless be easily achieved much earlier by reducing ANS in current fossil kerosene.

That is why some members of the European parliament have filed amendments aimed at monitoring the ANS content of aviation fuels and mandating the European Commission to write a report and prepare a legislative proposal. These amendments are part of the final text, but the latter provision will not happen before the next revision of the regulation in 2026-27. There are nevertheless strong grounds to act as soon as possible to reduce ANS in kerosene rather than just to start monitoring fuel composition.

Hydrotreating fossil kerosene: the best use of the scarce amount of green hydrogen available for aviation

Recent flight tests with SAF blends have confirmed that reducing aromatics in aviation fuel can significantly reduce contrail-cirrus by reducing soot emissions (2). The same result could be achieved with fossil jet fuel, provided it is processed to remove aromatic compounds. This can be achieved by hydrotreatment (reacting it with hydrogen), a process commonly used in refineries for other fuels. Reducing jet-fuel aromatics is indeed one of the measures that EASA proposed to the EC in 2020 to reduce contrails (3). The CO2 penalty of about 2% associated with producing grey hydrogen in refineries can be avoided by using green hydrogen, as it will have to be the case for making e-fuels.

Hydrotreating conventional kerosene would also drastically reduce air pollution by fine and ultrafine particles of soot and sulphate at airports hydrotreating fuel implies hydrodesulfurization, which means that not only soot but also sulphate particles would be drastically reduced. This would improve air quality and reduce health impacts for airport customers, workers, and communities living nearby.

The aviation sector plans to use massive amounts of green hydrogen as a raw material to make alternative fuels, biofuels and e-fuels. But green hydrogen will be in very short supply for decades. It would be more efficient to first use the available green hydrogen to hydrotreat conventional fossil kerosene than to use it to produce SAF. For the same small quantity of hydrogen, the total climate impact of jet fuel could be reduced a lot more by hydrotreating kerosene than by producing some SAF and blending it with non-hydrotreated kerosene. 

In a first step, the hydrotreatment would have to be limited to 50%, whereby only half of the fuel aromatics are converted, because older aircraft still need aromatics to protect rubber seals. At this rate, CO2 emissions could be reduced by 1% and the radiative forcing of contrail-cirrus by 10 to 20%. If the same quantity of green hydrogen was used to make e-fuels, the quantity produced would be enough to make, at most, 1% blends with fossil kerosene (4), resulting in the same CO2 reduction, but in a much lower reduction of the radiative forcing of contrail-cirrus.

It would be possible to further reduce aromatics in fossil fuels when all aircraft can bear it, and hence further reduce contrail-cirrus radiative forcing, once refinery processes are adapted to allow deep hydrogenation (5).

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The reduction of contrails is key to reducing climate heating. To understand why, let’s compare the atmosphere with a tank about to overflow. The tank is fed by CO2 emissions that accumulate inside it. If you reduce the CO2 emissions, you only reduce the filling rate, in the case of our first example by 1%, which means that 99% of the CO2 will continue to fill the tank. But about half of the current content of the tank are contrail-cirrus. If you reduce contrails, as their lifetime is less than one day, the tank will empty: their quantity will reduce by 10-20%, in our example if 50% hydrotreated fuel is used, by less than 1% with a 1% e-fuel blend (6). This suggests that green hydrogen should be used as a priority to hydrotreat kerosene.

A very favourable cost-benefit balance

A Social Cost-Benefit Analysis (SCBA) carried out by CE Delft in the frame of the EU Jetscreen project was completed in 2020 but only published on Dec 5th 2022 (7) after a series of misfortunes (8). It shows a significant global benefit of 8 billion euros for a full desulphurisation and a reduction of aromatics from 17 to 7% (-60%). In fact, the benefit might be even larger because, in particular, the health benefits of improving air quality at airports seem to have been greatly underestimated.

The aviation sector still denies non-CO2 effects

The sector is hiding behind the still significant uncertainty about the magnitude of non-CO2 effects to allow them to do nothing, neither to account for them, nor more importantly to reduce them. And yet, as one of its experts recently pointed out, addressing the effects of contrail-cirrus would give large and immediate environmental benefits. So why this “wait and see” attitude? We see two reasons:

  • Recognising non-CO2 effects would more than double the climate impact of the sector, so it would no longer be able to say it’s only responsible for 2.5% of global emissions.
  • Very few managers and policy makers understand the science of non-CO2 effects and draw the right conclusions.

Hydrotreating fossil kerosene: an efficient but insufficient step

Hydrotreating fossil kerosene can do a lot to reduce the non-CO2 impact of aviation but is totally insufficient to reduce CO2 emissions. Even if hydrotreating is pushed to 100%, the CO2 reduction would hardly reach 2%. This means that air traffic would still need to degrow in the next 10 to 30 years in order to follow the pace required from all sectors to meet climate targets (See Greenwashing Fact Sheet #6 Net Zero & Carbon Neutrality).

See also:

Air transport can stop increasing its climate impact very quickly without waiting for a hypothetical “green” plane

Hydrogen requirements for various aromatics-free jet fuels (Hydrotreated fossil fuel, biofuel, e-fuel)


(1) Aromatics is a class of hydrocarbons present in jet fuels that produce more soot than other classes of hydrocarbons when burned. Naphthalene is the aromatic molecule that produces the most soot. Jet fuels also contain small quantities of sulphur (less than 0.1%) that produce SO2 and sulphate particles when burned.

(2) C. Voigt et al. (2021): Cleaner burning aviation fuels can reduce contrail cloudiness

(3) EASA (2020): Updated analysis of the non-CO2 effects of aviation, p. 89

(4) The EU goal is to substitute 0.7% of fossil jet-fuels by 2030.

(5) Alain Quignard (2022): Non-CO2 effects from aviation decreasing sulfur and aromatic content in jet fuel

(6) Sources for the Table:

  • Hydrogen requirements for a 50% hydrotreated fuel: 4.6 kg H2/ton fuel: calculation based on stoichiometry
  • Hydrogen requirements to make e-fuels: 560-685 kg H2/ton e-fuel. CONCAWE (2019): A look into the role of e-fuels in the transport system in Europe (2030–2050) (literature review)
  • C02 reduction for a 50% hydrotreated fuel: calculation based on stoichiometry
  • C02 reduction for a 1% e-fuel blend: 1%, assuming e-fuel is 100% decarbonised
  • Ice number reduction: derived from Fig. 3c of C. Voigt et al. (2021) (see ref #2). A 50% HT fuel is expected to contain about 14.2% H. A 100% HT fuel is expected to contain about 14.6% H.
  • Contrail RF reduction: derived from U. Burkhardt et al. (2018): Mitigating the contrail cirrus climate impact by reducing aircraft soot number emissions

(7) Jetscreen (2022): Socio-Economic Benefits of Reducing Sulphur & Aromatics (Note: a corrigendum was issued for the original report to correct several calculation errors (see next footnote)

(8) The cost/benefit analysis was completed in 2020 but withheld from publication by Airbus and the Commission. It would have remained unpublished if a member of Stay Grounded had not publicly challenged Airbus to release it. A copy was finally made available for distribution just days before the RefuelEU Aviation vote in the EU Parliament plenary. That was too late particularly as the Commission (DG Move) had separately lobbied the largest parliamentary group – the EPP – against the proposed amendment. Anyway, the analysis showed no net benefit. Challenging the unexpected outcome, another SG member examined the still unpublished report and came to the conclusion that some significant mistakes had been made which he shared with CE Delft in October. After a round of discussion, they accepted a large part of his remarks and published it with a corrigendum on their website on Dec 5th 2022. It should be noted that the cost/benefits of using green hydrogen for the hydrotreatment have not been assessed.