Net zero aviation fuels

With air travel expected to increase, alternative fuels need to be truly beneficial, according to a new report by The Royal Society.

Aviation is a contributor to global warming, including through the emissions of carbon dioxide and the formation of contrails high up in the atmosphere, says the Royal Society in its new report: Net zero aviation fuels: resource requirements and environmental impacts. 

Globally, it says, save for the few years of the pandemic, air travel is expected to continue to grow in the future, increasing the impact on climate change unless a close to net zero form of flying can be developed or any residual emissions offset by removals. Other than reducing the amount of air travel or relying on long-term offsets, the options are limited and revolve around replacing fossil aviation fuel with a low or zero carbon energy source. 

That source must consider the following parameters:  a high enough energy density to give the range needed for up to long haul flights, can be produced at scale and implemented around the world, is cost competitive, can be implemented safely in the timescale required (net zero by 2050). This includes any modifications to / replacement of airframes and ground support facilities. 

Four options

The Royal Society briefing looks at four options: hydrogen, ammonia, synthetic fuels (efuels) and biofuels. 

Aircraft solely powered by batteries were not considered in this report as battery technologies are unlikely to have been developed to give the energy density required for most commercial flights in the timescale available to reach net zero by 2050. Hybrid systems utilising batteries to support one of the other options might be a potential solution. 

Overall, says the Royal Society, the results of this analysis are uncertain and there is no clear or single net zero alternative to jet fuel. One of the problems encountered is that the parameters are difficult to measure and are interconnected, so for example, hydrogen can be produced using low- carbon generated electricity, which reduces the carbon footprint but increases the cost. Many parameters require further research, for example the formation of contrails from hydrogen-powered engines. 

Benefits and limitations

  • Biofuels: CO2 would be produced from the aircraft engines. Only some biofuels
    can be described as net low-carbon and he scale and availability of feedstock is
    a restriction (perhaps with the exception of sewage). However, it has the benefit of requiring little modification to aircraft or support infrastructure and to an extent can be introduced quickly. 
  • Hydrogen: No CO2 would be produced by the aircraft. Low-carbon hydrogen can be produced but at higher cost and might need to be imported to get the scale required. Producing the amount of renewable electricity to create the green hydrogen required would be a challenge and substantial modification and replacement of aircraft and supporting infrastructure would be needed. Safety would have to be proven and further work would be required to confirm improvement in non-CO2 climate and environmental impacts. 
  • Synthetic efuels: CO2 would be produced from the aircraft engines. Few modifications to existing systems would be needed and could be quickly used in aviation. There would likely be some improvement in non- CO2 climate and environmental impacts, however costs would be higher. For efuels to be considered ‘net zero’, the development of green hydrogen feedstock (as above) and direct air capture (DAC) of CO2 at scale would be needed. An alternative solution to match fossil fuel use to DAC might be attractive but also has question marks regarding future fossil fuel availability, DAC energy consumption and continuing non-CO2 climate and environmental impacts. 
  • Ammonia: No CO2 would be produced by the aircraft. Low-carbon ammonia can be produced but at a higher cost. Production will depend upon generating green hydrogen at scale and substantial modification and replacement of aircraft and supporting infrastructure might be needed. Safety would have to be proven and further work would be required to confirm improvement in non-CO2 climate and environmental impacts. 

Depending upon the fuel used, says the Royal Society, changes to aircraft operations, ground handling systems and airport layouts might be required. In addition, aviation relies upon trained, qualified and regularly refreshed staff in key roles who are licenced to carry out their jobs. Alternative low carbon jet fuel technologies cannot be introduced effectively without updating skills, training, and professional standards. 

Global acceptance

The Royal Society adds that selected solutions need to be globally accepted and each of the options considered in a holistic manner, both to provide the best solution now and for the coming years. The options available now offer some carbon savings but are not ideal:

“Further research and development will be needed to produce better alternative fuels, including accessing sustainable feedstocks, and the development of the efficient production, storage and use of green hydrogen, ammonia and efuels. Some of the solutions will require the substantial redesign of airframes and support infrastructure. R&D is also needed to understand and mitigate the non-CO2 climate impacts of all the fuel options.”

To read the full report, click here

Air travel is expected to continue to grow in the future, increasing the impact on climate change unless a close to net zero form of flying can be developed or any residual emissions offset by removals. Other than reducing the amount of air travel or relying on long-term offsets, the options are limited and revolve around replacing fossil aviation fuel with a low or zero carbon energy source.

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