Electrofuels, also known as e-fuels, are a type of more sustainable aviation fuel (SAF) that will be playing an increasing role in the mix as aviation strives to decarbonise its kerosene-powered operations. So what are they? We get to grips with them in the latest of our introductory ab initio primer guides!
In their widest definition, e-fuels are a type of synthetic fuel, produced from two key components: captured carbon plus hydrogen sourced from renewable or sustainable energy.
They are, e-fuel producer Zero’s chief executive officer Paddy Lowe tells us, “carbon-neutral alternatives to fossil fuels. They are clean burning and can be made at scale in the form of petrol, diesel or jet fuel. They provide the same power and performance as fossil fuels and although they do emit carbon dioxide, the same amount emitted is removed from the atmosphere to make more fuel, creating an entirely carbon-neutral lifecycle.”
Within the range of SAF feedstock and processes used for jet kerosene, e-fuels sit alongside fuels produced from waste or biomass feedstocks such as HEFA (hydroprocessed esters and fatty acids), municipal solid waste, or ethanol-to-jet.
Crucially, though, e-fuels scale to the volume requirements that aviation will need, without impinging on either potential food stocks or on arable land that could be used for the human food chain.
Production methods and technologies are different to other SAF pathways
E-fuels are produced in three main steps.
First, carbon — usually carbon dioxide, but potentially carbon monoxide — is extracted from air via a direct air capture process and stored for processing. Industrial-sized fans pull in air form the atmosphere and pass it over liquid or solid solvents that trap the carbon. Using heat, carbon dioxide is then released and stored in tanks.
Second, hydrogen and oxygen are split from water using electrolysis, passing an electric current through the water. To be classified as green hydrogen, this must use sustainably sourced electricity from wind, solar, nuclear or other sources.
Third, the hydrogen and carbon dioxide are combined to create a hydrocarbon fuel, usually through a catalytic method such as the Fischer-Tropsch process. The catalyst and process used determine the type of fuel produced.
Since the e-fuel process is in effect ‘building’ the hydrocarbon kerosene fuel from its component parts, fewer to no trace elements or volatile compounds — such as the sulphur and particulate matter found in fossil kerosene — are present. This is a sustainability advantage both in terms of reducing non-direct emissions from aviation and the effects they have on contrail and cloud formation.
Zero’s Paddy Lowe highlights, in the context of developing science around the direct and indirect emissions from jet kerosene, that e-fuels like the ones the company produces “use only water and carbon dioxide as source materials, both of which are abundant in the atmosphere. Unlike any other form of fossil fuel alternative, they can also offer a carbon-neutral solution for the continued use of legacy vehicles without the need for modification.”
E-fuels offer emission, production and industrialisation benefits — and opportunities
“There have been many different forms of energy developed to replace fossil fuels in vehicles: battery-electric, hydrogen, synthetic fuel and micro nuclear reactors,” Lowe explains.
One of the key benefits of e-fuels is that they are drop-in: in other words, e-fuel kerosene can be blended with fossil kerosene, rather than requiring entirely new infrastructure like battery-electric or hydrogen power sources. This is a major advantage for the substantial part of aviation operations that will still be powered by liquid fuel — in other words, the entire existing airline fleet, and much of the fleet that will be produced for the foreseeable future.
Lowe continues, “of the liquid fuel solutions, synthetic fuel is the most suitable solution due to its scalability. Biofuel is made using biological materials that must be grown in fields or forests and the process is not truly scalable without significant impact on land use and deforestation. Fuels made from waste are not scalable, as waste is a limited resource, and, as it can include fossil fuel materials such as plastics, it is not truly sustainable or renewable.”
One key critique is around terminology: not all synthetic fuels are e-fuels, with some produced from natural gas or other fossil fuel sources. The type of electricity production used at all stages of the process is also important, and calculating the right balance of demand for lower carbon electricity sources like solar, wind and hydro for residential, other industrial and electrically-powered transportation needs will be vital.
Cost is another major question, as European pressure group Transport & Environment highlighted in a 2021 report [PDF], noting that “estimating future cost trajectories is challenging. Some reports suggest under the right conditions e-kerosene can reach price parity with taxed fossil kerosene by the 2030s, others later. Deployment of renewables and new technology in other sectors has shown that with sufficient support, more ambitious falls in price can be achieved.”
Those price reductions are a consequential part of delivering more e-fuels into the drop-in mix, and here action from both industry and from governments is needed to stimulate the economics of their production. Largely, this is on the demand side, particularly around certainty of demand.
Aviation is, on balance, doing a decent job in signalling its future demand for SAF, which gives investors the certainty they need for R&D spending and capital expenditures to build production facilities at the scale airlines will need.
On the government side, e-fuel mandates — also known as blending mandates — that require a certain percentage of e-fuel kerosene to be blended with fossil fuels can help stimulate demand. So can footprint intensity targets, where the overall carbon equivalent warming intensity of a particular supplied fuel must be below a certain threshold.
Together, industry, government and producers can work to stimulate this crucial market to invest in a more sustainable future for aviation.
Author: John Walton
Published 02 November 2023
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