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Sustainable Aviation Fuels: Carbon-Free Flying?

What are the chances of guilt-free (carbon-free) flying in the near future? Associate Editor, Jane Whaley, discusses more in her latest Energy Transition column.
This article appeared in Vol. 19, No. 2 - 2022

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Sustainable Aviation Fuels: Carbon-Free Flying?

For the sake of the planet, I am happy to consider an electric car, to change my electricity supplier and to install solar panels or a heat pump – but I have to confess that I find it difficult to consider giving up flying to far-flung parts of the world. There are just so many wonderful places out there I have always wanted to visit. So, what are the chances of guilt-free flying in the near future?

The global aviation industry produced just over 2% of all human-induced carbon emissions and 12% of those from all transport sectors in 2019 (Covid and the resulting grounding of many flights mean that annual figures after that date are not truly representative). It doesn’t sound a huge amount but to put it in perspective: a return flight from London to San Francisco emits around 5.5 tonnes of CO2 equivalent per person – more than the emissions produced by a family car in a year, and about half the average carbon footprint of someone living in Britain. Planes also affect the concentration of other gases and pollutants in the atmosphere, resulting in a long-term decrease in ozone as well as emissions of water vapour, soot and sulphur.

  • Achieving a place in the Guinness Book of World Records, the first flight by an aircraft powered entirely by synthetic fuel was undertaken by the Royal Air Force and Zero Petroleum Ltd in Kemble, Cirencester, UK, on 2 November 2021. Credit: Zero Petroleum/RAF.

Rising Demand = Rising Emissions

The aviation industry has committed to reducing carbon emissions by 50% from their 2005 level by 2050. One positive factor is the increasing efficiency of plane engines, which has been rising faster than associated emissions have been doing, but this is wiped out by the rapid increase in passenger numbers, projected to double in the next 20 years. Efficiency alone will not be enough; blending lower-carbon sustainable aviation fuel (SAF) with fossil jet fuel (meaning that no changes are required in aircraft or engine fuel systems) will be essential to meeting the goal. This is not a totally new idea; in fact, the first test flight using blended biofuel was back in 2008, and in 2011 jet fuels blended with up to 50% biofuels were allowed for commercial flights.

It is proving a hard area to decarbonise at scale, however, and we are still some way from finding the innovative solutions that will ‘green’ air transport.

An aviation fuel needs to be stable in temperatures ranging from -40°C to over 40°C, with good combustion and flow characteristics, and have sufficient energy density to supply the high energy demand of long-haul flights. This means that gaseous biofuels and electrification are not very appropriate, especially for long-haul flights, and liquid biofuels derived from either plants or waste are considered the most viable low-CO2 option for substituting kerosene in aeroplanes.

Plants, Algae and Household Waste

A number of different processes can be used to convert the carbon content of source material into the chemical components needed to fly a plane. These can be divided into biological processes, such as fermentation and microbial conversion, and thermo-chemical processes, including gasification, torrefaction, pyrolysis and catalytic conversion. 

Source material for aviation biofuel includes plants such as jatropha, babassu and palm oil, most of which do not compete with food crops or natural forest, although there are environmental questions over the use of palm oil. Jatropha oil, for example, is an inedible plant of the Euphorbiaceae family, native to central America but found in many tropical and subtropical areas throughout the world, where it thrives on marginal land where few other plants survive. It has already been used as an SAF and is estimated to lower CO₂ emissions by 50–80% compared to jet fuel. This type of biofuel is the only technically mature and commercialised SAF at the moment, but it has a low energy density compared to fossil fuels and therefore requires a significantly larger volume of source material to generate the same amount of energy.

Research into processing solid biomass derived from wood and industrial, agricultural and household waste using pyrolysis to make SAF is underway. Since these feedstocks are more abundant and generally cost less than specially grown plants and also have a low water footprint, this could prove to be a more cost-effective source for SAF. Technologies to convert these to aviation biofuel do exist but they have not yet been taken to a commercial level. The organic portion of municipal solid waste is another environmentally friendly potential source for SAF and experiments have shown it works and has the potential to lower greenhouse gas emissions by 85–95% compared to traditional fossil-based jet fuel, as well as reduce the amount of waste going to landfill. However, the process is expensive, there are issues with the safe transport and storage of the waste and little research has gone into it as yet.

More efficient again would be biofuels derived from photosynthetic algae, which have no food value, provide high yields with virtually no land requirement, are relatively cheap to grow and have a very low carbon output. However, the technology for large-scale production remains immature.

Totally Green Fuel

A very promising technology is the creation of synthetic SAF using fossil-free electricity to produce hydrogen from water by electrolysis, which is combined with atmospheric or recycled CO2 by an advanced version of the well-known Fischer–Tropsch method to directly manufacture fuel suitable for aeroplanes. The first flight totally powered by a synthetic fuel of this type was undertaken in late 2021, and in February this year, KLM operated the world's first passenger flight powered partially by synthetic kerosene.

  • The seeds of Jatropha curcus contain up to 40% oil, which can be processed to make SAF. Ton Rulkens, CC BY-SA 2.0

Also called power-to-liquid or e-fuels, these synthetic fuels are considered a potential long-term sustainable option, due to their low lifecycle emissions and other environmental impacts. They are especially attractive environmentally if captured CO2 is used, an area which is of particular interest to geoscientists. If the technique can be made economically viable, with the scale up of both renewable power generation and captured carbon, it is possible that e-fuel could provide a carbon neutral circular solution to the problem of aviation emissions.

Investment Needed

We still have a very long way to go; despite the research being undertaken throughout the world and the achievements so far, estimated current production of sustainable aviation fuels is only 0.05% of current aviation fuel consumption. In 2019 IATA (the International Air Transport Association) announced that it was aiming for a 2% penetration of ‘green’ fuels by 2025. This is a modest but achievable goal, with a number of major airlines, including United, Quantas, SAS, British Airways, Virgin Atlantic and Lufthansa, already mixing some SAF with their jet fuel.

A major issue at the moment is distribution, with only five airports – Bergen, Brisbane, Los Angeles, Oslo and Stockholm – having the facilities for regular biofuel distribution. However, the fact that aviation fuelling is very centralised, with less than 5% of all airports handling 90% of international flights, means that a small expansion in the number of airports supplying SAF could cover a lot of demand.

Further investment is needed, not just in research into the techniques to create SAF, but also in associated green technologies like renewable electrical sources. At the moment, SAF costs between three and nine times more than jet fuel. Scaling up from pilot projects to commercially-sized facilities may well require subsidies or similar assistance to prove the viability of SAF and to transform it from being a long-term goal to a near-term solution.

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