Cleaner Kerosene for Cleaner Air and Climate

18 Apr, 2024

The health impacts of air pollution

Did you know that air pollution from aircraft can cause a variety of serious health impacts?

This is caused by very small “ultrafine particles” (UFPs) that are emitted from jet engines. These particles are 1000 times smaller than the diameter of a human hair. If we breathe them in, they can enter our blood and internal organs – leading to increased risk of disease in our lungs, heart and brain:

Diagram from ITF Factsheet on Airport Pollution


Air pollution around airports has a direct impact on the health of aviation workers. In particular, ground crew, who are regularly exposed to aircraft emissions. It also affects communities living around the airport, who may often be the friends and family of airport workers.

In 2010, a study was conducted at Copenhagen Airport to test the levels of UFPs and the exposure to employees working close to aircraft. The results were alarming. The investigation showed that the levels of UFPs at one testing station were almost four times higher than than on a busy street in the city centre during rush hour:

The good news is that by reducing certain chemicals within jet fuel – it appears possible to significantly reduce these health impacts, while also reducing aviation’s environmental impact.

One proposal is to remove these chemicals from fossil jet fuel during production, so that it burns more cleanly.

Let’s explain how this could work…


Watch a recording of our Safe Landing organised webinar on this topic (30 minutes presentation, followed by 30 minutes discussion):


Potential solutions

There are a variety of operational changes that an airline can initially make to reduce its pollution and environmental impact:

  • Operate the latest, more fuel efficient aircraft, rather than older, more polluting aircraft.
  • Select engines with “Lean Burn Combustors” which produce lower NOx and particulate emissions.
  • Optimise flightpaths to avoid contrails where possible, and minimise fuel burn e.g. via continuous (rather than stepped) descent.
  • Reduce aircraft taxi times through logistical planning.
  • Optimise engine start/pushback procedures to minimise pollution at other aprons.
  • Reduce engine running during taxiing through single-engine taxi (rather than running all engines) operations, and use an electric tug where possible.
  • Reduce Auxiliary Power Unit (APU) running on board the aircraft by connecting to electrical ground power as quickly as possible once the aircraft is parked.
  • Using electric-powered ground equipment and ground support vehicles (e.g. baggage carts) where possible.

Some of these latter options have already been implemented at Copenhagen Airport.

Regardless of how efficient you make the aircraft operations though, there will still be air pollution created.

Until we are operating electric or hydrogen aircraft with zero tailpipe emissions, the other option is to burn cleaner jet fuel.

Burning conventional jet fuel

Jet fuel, or kerosene, is conventionally made from fossil fuels. This is a hydrocarbon, which means it’s chemically composed of chains of hydrogen and carbon. However, there are often impurities in the fuel, for instance: sulphur and aromatic compounds like naphthalene.  When fuel containing these compounds is combusted in a jet engine, it burns less completely and additional harmful emissions are created such as sulphur dioxide, and soot particles. Fuels with a high concentration of aromatics, and especially naphthalenes, cause higher particulate emissions because aromatics burn slower than other hydrocarbons.

These substances do not improve the combustion process, but can be damaging to jet engine components (due to e.g. corrosion), are toxic to humans, and can increase the formation of contrails when aircraft are flying at altitude, which can increase global warming.

If the atmosphere is sufficiently cold and humid, soot particles will act as condensation points for water droplets which then freeze into ice crystals. These crystals can remain for several hours and form contrail cirrus clouds. These artificially generated clouds can remain in the sky for long periods of time and trap heat in the Earth’s atmosphere. Contrails are responsible for a large chunk of aviation’s climate impact, and it might even be double the CO2 emissions. Therefore, by reducing soot particle emissions we could also reduce the climate impact of contrails.


Burning alternative jet fuel

Engine emission tests in a test cell have shown that running a common airline engine on 32% HEFA biofuel (produced from waste oils/fats) can reduce the number of particles emitted by up to 60% vs. running the engine on 100% Jet A-1 (kerosene). The greatest effect is during ground idle, while e.g. the aircraft taxis around the airport:

Figure 1

Similar findings have also been observed in real world emissions tests at Copenhagen Airport that measured a reduction in the emission of ultrafine particles by about 30% when the engine was running on 34% bio-based fuel in the tank while it was taxiing between the runway and the gate at the airport.

The reason for this is that most alternative jet fuels or so-called “Sustainable Aviation Fuels” (or “SAF”), like biofuels or electro-fuels, have zero-aromatic content. As they haven’t been produced from fossil fuel, they can be free of the impurities which can be present in crude oil. While some “SAF” can be produced with aromatics, the most common industrially produced and commercially supplied “SAF” is HEFA and has no aromatics and sulphur content because it is produced by hydrotreatment (source, page 38).


Scaling alternative jet fuels

So why don’t we simply switch to these alternative jet fuels or “SAF”, rather than fossil kerosene?

The problem is the time it will take to scale up this fuel production. In the EU, “SAF” targets start with 2% in 2025, reaching 5% in 2030, 20% in 2035, 32% in 2040 and 38% in 2045. Even in 2050, a large % of jet fuel used is still planned from fossil kerosene. This means that as we scale “SAF”, we will continue to mostly burn conventional kerosene for decades:

The other problem is that as lower-aromatic kerosene can be more expensive to produce, refineries may increase the aromatic content of conventional kerosene as it is blended with “SAF”. This would mean that the potential non-CO2 health and climate benefits of blending alternative jet fuels could be reduced or eliminated, even as they are scaled (source, page 4).

So is it possible to clean up the existing supply of conventional kerosene?


Cleaner Kerosene

The other option is to reduce the chemical compounds present in conventional fossil kerosene which are responsible for soot and UFP emissions.

It is possible to lower the aromatic content in kerosene by further processes at the refinery such as hydrotreatment or extractive distillation. These processes are well understood and are already used to reduce the sulphur content of diesel for road vehicles. In fact, as refineries prioritise diesel production, and there is an overlap between diesel and kerosene products, some hydrotreated kerosene is already produced.

Jet fuel standards set a maximum limit for aromatics of 25% by volume and also a minimum limit for aromatics of 8% by volume (as these compounds can assist with seal swelling in the fuel system of older engines). There is a maximum limit of naphthalene of 3% by volume.

A review of jet fuel supply data (source, page 34) shows an average of 18.7% aromatics by volume, with a range between 8% and 25%. For naphthalene an average of 0.81% was found, with some samples very close to zero and very few samples above 2%.

This demonstrates that there is a wide range of jet fuel compositions being used already, probably due to a global variation in crude oil and refinery operations. It also means that we could selectively purchase cleaner kerosene today from existing supply.


Potential Benefits

The key benefits we have identified to a ‘cleaner kerosene’ supply are that it would be:

● Quick: it can be implemented this decade, compared to waiting a few decades for the scale-up of “SAF”. There is already the potential to selectively purchase cleaner kerosene today from existing supply, and perform trials to better quantify the benefits (and understand issues).
● Effective: Measurements show that cleaner jet fuel can improve air quality. Flight tests have shown that if reduced it can also significantly reduce soot emissions, ice crystals and therefore should reduce contrail formation. EASA has also proposed this solution (page 89).
● Cheap: compared to “SAF” (which is several times more expensive than kerosene), hydrotreating jet fuel is very low cost and would be expected to increase fuel price by only a few percent (page 57).

There are also some other potential benefits which are worth exploring:

● Reduced maintenance costs: the improvement in fuel quality is likely to lead to reduced engine/fuel system maintenance costs, and the savings from these may well pay back any additional fuel costs. For example, there could be a potential reduction in sulfidation corrosion within jet engine turbines. There could also be a potential reduction in aircraft/engine fuel system pipe coking/lacquering issues (when carbon soot builds up inside pipes above certain temperatures).
● Improved system performance: the increased energy density of the fuel could mean less volume of fuel would be required for a given engine thrust, so the fuel system pumps don’t need to work as hard.


Potential Issues

The are are a number of potential issues with cleaner kerosene that would need to be assessed:

Carbon Emissions: the additional refinery processes require hydrogen, and currently most hydrogen used at refineries is ‘Grey Hydrogen’ (Grey H2) produced from fossil gas. Increased hydrogen use could therefore lead to increased CO2 emissions. One solution to this problem is to mandate that ‘Green Hydrogen’ (Green H2) produced from renewable electricity is used.

Graphic showing the green, grey and blue hydrogen.

The different types of hydrogen production and their relative carbon emissions

Competition for Green Hydrogen: if Green H2 was used, there could be concerns that this will utilise relatively scarce quantities of the resource, which could instead be used for other purposes. On the flip-side, all Grey H2 at refineries needs to be replaced with Green H2 as soon as possible, and this represents a logical and efficient initial use for early Green H2 production. On the other hand, most “SAF” production (including biofuels and particularly electro-fuels) would use a relatively massive amount of Green H2.

Diverting attention: a potential concern is that hydrotreating kerosene could distract us from more important activities, e.g. reducing fossil fuel use and scaling-up “SAF” production. We think this could help, rather than hinder, the business case for building out Green H2 electrolyser capacity at refineries / industrial clusters, which is already needed to decarbonise other processes and will be needed for any type of “SAF” production. So the objectives could actually be aligned.

Delaying alternative jet fuels: we don’t want to prolong the use of fossil fuels and delay the switch to alternative, cleaner jet fuels. It’s important to note that we can never eliminate all pollution from kerosene, and it needs to be phased out rapidly to reduce CO2 emissions. However, we also need to recognise that for industry roadmaps show continued kerosene use even beyond 2050. An increased price on fossil fuel kerosene due to improved fuel quality, would reduce the cost gap to alternatives and should be an incentive to continue developing alternative jet fuels / “SAF”, rather than a disincentive.

Increased costs for airlines: airlines will oppose this as it adds a small additional cost to the fuel, and clearly airline shareholders do not bear the in-direct costs of health impacts for workers and communities living/working around airports. It’s worth noting that kerosene hydrotreatment is estimated to only increase fuel prices by a few percent (source, page 57). This additional cost is well within the current jet fuel price fluctuations during a normal year. A very low percentage fuel price change is relatively inconsequential compared to the high costs of all other options for reducing aviation environmental and health impacts.

Difficulty certifying or implementing a fuel change: any change in fuel specification has to be done internationally at ICAO level, and approved by nationally e.g. by the FAA (in the US), EASA (in Europe), CAA (in UK) etc. – due to international nature of aviation. There are a number of potential risks of changing the fuel specification which would need to be worked through. Having talked to some fuel certification specialists, they think by far the easiest approach would be to hydrotreat kerosene to remove sulphur and most naphthalenes, while keeping aromatic contents within existing fuel specifications (8-25%) – meaning no additional fuel certification would be required.

Sulphur Emissions: while sulphur emissions (e.g. sulphur dioxide) is harmful to human health, and can increase particulate emissions – sulphate emissions from aviation can reflect sunlight which can contribute to a global cooling effect during day time. This is something that would need to be assessed and factored into any decision on the level of hydrotreatment necessary to reduce air pollution, without making other issues significantly worse. It should be noted that alternative jet fuels / “SAF” will have low/zero sulphur content, so we need to address this eventually for “SAF” anyway and doing this early through kerosene hydrotreatment trials could be useful.

NOx Emissions: if a difference in energy density of the fuel means that jet engine combustor temperatures are higher, this could increase NOx emissions. However, we think that the higher energy content of the fuel means the engine fuel pumps will supply less fuel to the engines to deliver the same thrust, while keeping combustor temperatures constant. This has been confirmed with tests that have reported “The total NOx (NO and NO2) emissions were minimally affected during operation with the various fuels” (source, page 12). As with sulphur, it should be noted that we’ll need to address the increased energy density of “SAF” on NOx emissions anyway, and it’s useful to be able to understand this as early as possible.


Get Involved

The are are a number of ways that our community can get involved with exploring, and potentially pushing for, cleaner kerosene:

● We have created a draft trade union motion to support trials for hydrotreating kerosene and have produced a supporting presentation pack. If you’re in a trade union, you can use or modify these for your own purposes to present the idea. We would also be happy to join any meeting as guests to help the discussion.

● We have created this webpage and resources for sharing with colleagues in your company, friends/family or anybody impacted by aviation air pollution and well-placed to develop the idea or push for testing.

● We would welcome any feedback on this page, and also contributions towards improving it. If you’d like to work on progressing this issue please get in touch!

● Sign up to our newsletter and join our community, to talk to others about this issue and many others!

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