With the climate crisis accelerating and the EU aiming to reach net-zero emissions by 2050, every industry that emits greenhouse gas emissions must be targeted ­— and the consensus is that aviation will be the toughest sector to decarbonise.

Air travel is responsible for 2% of worldwide carbon emissions — last year accounting for 915 million tonnes of CO2 — and, pre-coronavirus, passenger numbers were expected to shoot up some 70% by mid-century.

Globally, airlines flew 38.1 million flights in 2018, consuming 95 billion gallons (360 billion litres) of jet fuel — with less than 0.1% of that (about 15 million litres) being classed as sustainable aviation fuel (SAF).

Decarbonising all those flights might seem like an impossible task, but it has not prevented the International Air Transport Association (IATA) — the trade body representing the world’s airlines — promising to reduce the sector’s emissions by 50% (compared to 2005 levels) by 2050.

So how can this be achieved, and what role will renewables play in the decarbonisation of air travel?

Is decarbonisation of aviation possible?

When the UK Sustainable Aviation coalition committed in February to net-zero carbon emissions by 2050, it grabbed headlines around the world.

But the group’s Decarbonisation Roadmap has led to charges of “creative accounting”. The plan involves cutting 36.3% of emissions through carbon offsetting, 33% from more fuel-efficient planes, 4.4% from “improved operations” such as less circling, 6% from “the effect of carbon price on demand”, and only 20.3% from sustainable aviation fuel (SAF).

Others are more optimistic that the fuel itself — the source of the emissions — can be fully decarbonised.

The international think-tank Energy Transitions Commission (ETC) argues that fossil-fuel jet fuel — refined kerosene — can be replaced with carbon-neutral technology by 2050.

The ETC highlights four options — all of which are powered by renewable energy: electric batteries and green hydrogen (both for short-haul flights only); synthetic fuels produced from green hydrogen (made from splitting water molecules into hydrogen and oxygen using renewable electricity) — also known as electrofuels or e-fuels; and biofuels.

Battery-powered flight

While road transport can be decarbonised with battery or fuel-cell electric vehicles — powered by renewable energy or green hydrogen — it will not be so easy for gravity-defying airplanes.

According to a calculation by the University of Pennsylvania, a Boeing 747 flying from New York to London would require energy from 3,881 tonnes of batteries to complete the journey — more than ten times the plane’s maximum take-off weight of 333 tonnes.

The ETC says that the energy density of batteries would need to improve by five to ten times to make electrification feasible for long-distance aviation, something that would require “fundamental changes in battery chemistry”.

But despite the problem with the energy-to-weight ratio, it is widely agreed that battery-powered planes — charged using renewable energy — will be used for short-haul flights within the next ten to 20 years.

The International Civil Aviation Organization (ICAO), a UN agency, lists no less than 27 electric planes currently in development by companies such as Boeing and Airbus, as well as five hybrid-electric models that would be partly propelled by electric motors.

So far, only a small number of prototype light aircraft have been built, but this has not stopped the likes of Easyjet vowing to run a fleet of battery-powered 180-seat planes to cover its short-haul routes by 2030.

Easyjet’s partner in this mission, US start-up Wright Electric, aims to have its 186-seat Wright 1 aircraft, which will have a range of at least 560km, in commercial service by 2030, with flight testing of the airplane’s electric motor due to begin as early as 2023.

Green hydrogen

A European Commission-funded €4m ($4.3m) project called ENABLEH2 — which stands for “ENABLing cryogenic Hydrogen” — is investigating the use of liquid hydrogen (LH2) as an aviation fuel, with a view to providing “comprehensive roadmaps for the introduction of LH2 for civil aviation” in 2021.

The project — led by the UK’s Cranfield University — has some big names behind it, with an advisory board including Airbus, Rolls Royce, Siemens, Total, Air Liquide and Heathrow Airport.

However, airlines can only fly with fuels that have been approved by the industry under the auspices of US-based standards organisation ASTM International. So far, these have been limited to biofuels and e-fuels that match kerosene almost molecule for molecule. Fossil-fuel aviation fuel — “Jet A-1” and “Jet A” — also contain aromatic hydrocarbons such as benzene (due to their presence in crude oil), which biofuels and e-fuels do not have.

According to a recent scientific paper from the German Aerospace Centre, for novel fuels to be approved for commercial use, ASTM would require up to 235,000 gallons (890,000 litres) of the fuel to be tested, “which seems a too large amount of new fuel to be produced just for R&D purposes”.

However, producing 890,000 litres of liquefied hydrogen would require only 63 tonnes of hydrogen, according to Recharge calculations — 69 million tonnes of grey hydrogen (derived from unabated fossil fuels) are currently being produced every year. And this grey H2 could be used instead of green hydrogen for R&D testing as they are chemically identical.

Nevertheless, most airlines and green aviation advocates are focused on so-called “drop-in fuels” that can act as a direct replacement for conventional jet fuel.

“In long-distance shipping and aviation, the fact that the likely route to full decarbonization entails the use of zero-carbon fuels within existing engines means that the longevity of shipping and aviation engines is not a constraint on the pace of transition, which will instead be determined by the relative costs of zero-carbon versus conventional fuels,” says the ETC.

E-fuels

It is possible to create synthetic fuel that is physically identical to Jet A-1 (minus the aromatics) by combining green hydrogen with captured carbon dioxide. This would produce synthetic methanol (CH3OH) that can then be further refined into carbon-neutral jet fuel using well-established refining processes.

According to a 2017 study by the international campaign group Transport & Environment, the short-term cost of electro jet fuel was likely to be €3,000 per tonne — six times higher than Jet A-1.

“The cost of electricity is the dominant term in electrofuel production cost,” says the report. “At €0.05/kWh, it contributes €1,200/tonne of electrofuel for a facility with 50% conversion efficiency of electricity to fuel. For electricity at €0.10/kWh — around current EU average grid electricity prices to large industrial consumers — this doubles to €2,400/tonne. Without low-cost renewable electricity supply, electrofuels simply cannot expect to compete with other fuel alternatives.”

Sarah Wilkin, the founder of the Fly Green Alliance lobby group, tells Recharge: “[E-fuel] is viable, but right now it’s too expensive to process it. The processing price needs to be brought down.”

And as the T&E study explains: “As with any new industrial process, there is great opportunity to reduce investment costs in particular over time, both by applying the experience of operating first-of-a-kind facilities to reduce overnight capital costs, and by reducing the cost of capital assuming that the technology can be successfully demonstrated.”

Despite the misgivings over the cost of e-fuels, companies including EDF, Orsted and industrial giant Thyssenkrupp have joined forces to work on an ambitious German project that aims to get a 700MW green-hydrogen plant powered by a dedicated offshore wind farm up and running by 2030 at the Heide oil refinery, where the H2 would be combined with CO2 to create e-jet fuel.

The Westküste 100 project, named for its location on the west coast of Schleswig-Holstein state, has been identified by energy minister Peter Altmaier as one of Germany’s “real-world laboratories of the energy transition” and is set to receive close to €100m of government funding.

It aims to start with a five-year pilot project with 30MW of electrolysers powered by cheap wind energy that would otherwise be curtailed — a major problem in the grid-constrained windy state — and initially use that green H2 to replace the grey hydrogen (produced from unabated fossil fuels) that the oil refinery needs to remove contaminants such as sulphur from crude oil.

After that, the plan is to build a 700MW electrolysis unit to produce large amounts of green hydrogen that would then be combined with CO2 captured at a nearby cement factory to create the e-methanol, which would then be refined into synthetic jet fuel.

Whether the project will happen or not will largely depend on the German government — which is currently developing a national hydrogen strategy — as current power market mechanisms do not allow local electricity prices to fall when the supply of wind power is high. This is because wind farms are paid a set amount for the power they generate, regardless of whether that electricity is allowed onto the grid — resulting in annual subsidies of about €500m for wind power that is curtailed. A higher price on CO2 would also help project economics.

A similar project is being planned at Rotterdam The Hague airport in the Netherlands, where the CO2 will be captured directly from the air, although this seems to be at a far less developed stage, with the consortium partners — led by German mechanical engineers EDL Anlagenbau — still conducting studies to “define the concept and basic engineering”.

The partners hope to build a demonstration plant at the airport to produce 1,000 litres of “renewable jet fuel from air” per day, but no date or cost targets for such a development have been announced.

According to calculations by research associate Oscar Serpell at the University of Pennsylvania, e-fuel produced with green hydrogen and CO2 from direct-air capture would increase the fuel cost per mile (1.6km) of a Boeing 747 from about $8.55 using A-1 to about $15.52. On the New York to London flight, this would increase each passenger’s fuel costs from $73.60 to $133.60.

But Serpell points out that this extra $60 only accounts for an extra 8.5% on a $700 Transatlantic airline ticket.

“Compared to the doubling of utility bills or gasoline costs, this makes aviation a much more accommodating sector to the use of synthetic fuels than home heating or automobiles,” he says. “This cost structure also means that decarbonisation of commercial air travel is not a far-off possibility dependent on ground-breaking advancements in energy storage, but an actionable and pragmatic approach to achieving global emissions targets.

“In addition, by seizing this opportunity to decarbonise aviation using synthetic fuel, the world can continue to develop and improve electrolysis and DAC [direct-air capture] technologies, both of which will play a vital role in the future of climate action.”

The ETC has made similar calculations, arguing that “using bio jet fuel or synthetic jet fuel, which cost 50-100% more than conventional jet fuel, would only add $40-80 or 10-20% to the price of a long-distance economy ticket”.

Despite a seemingly modest price increase per flight, airlines seem reluctant to increase ticket costs, says Wilkin, pointing out that the commercial aviation sector is highly competitive, with slim margins and tough price competition.

“Airlines have said they cannot afford to buy SAF,” she tells Recharge. “We do not think at this stage that airlines believe their customers will pay any extra to support [greener fuels]. We know that there are customers that are happy to pay for sustainability and to reduce emissions and are speaking with them. And that has been demonstrated with Tesla [electric cars], with buying organic food, using tote bags instead of plastic bags, there are many examples.”

She adds, however, that some airlines — including Ryanair, KLM and Lufthansa — already offer to offset passengers’ flight emissions at the point of sale for an additional fee, but that few customers take up this opportunity.

Biofuels

While e-fuels are still at the development stage, aviation biofuel has been more or less commercialised, despite costing 50-400% more than conventional jet fuel (depending on the processes used and the fluctuating oil price). Since 2016, more than 200,000 flights have taken off with SAF, but only when blended in small amounts with regular fuel.

There are five methods of producing jet biofuel that have been approved by the ASTM for blending with standard jet fuel, but only one is said to be technically mature and commercialised — the process that uses vegetable oils, waste oils and fats (often used cooking oil) as its feedstock, known as HEFA-SPK (hydroprocessed esters and fatty acids synthetic paraffinic kerosene). More than 95% of biofuel flights to date have used HEFA-SPK fuel.

“Therefore, HEFA‑SPK is anticipated to be the principal aviation biofuel used over the short to medium term,” says International Energy Agency (IEA) analyst Pharoah Le Feuvre.

However, he adds: “Ongoing research and development is needed to support the commercialisation of novel advanced aviation biofuels which can unlock the potential to use agricultural residues and municipal solid wastes. These feedstocks are more abundant and generally cost less than the waste oils and animal fats commonly used by HEFA-SPK, and can therefore facilitate greater SAF production.”

The greatest competition to HEFA-SPK are biofuels made using the Fischer-Tropsch process, which typically uses high temperatures and high pressures to break down carbon-containing materials and steam into syngas — a mixture of carbon monoxide and hydrogen. The syngas is then passed through a metal catalyst which causes “polymerisation” — the joining of the carbon and hydrogen molecules into hydrocarbons. Further refinement creates a hydrocarbon that is very similar to conventional jet fuel.

The fact that any biomass material — and even old plastic — can be gasified in this process means that that there is a huge number of potential feedstocks — with the favourites being waste and calorific non-food crops that can grow on non-farmland, such as jatropha and camelina.

“Of course, waste makes the most sense from a circular economy and sustainability point of view — waste is truly wasted,” says Wilkin. “That would always be what we'd like to use first, that's the ideal scenario. The next best scenario is non-food-based crops.”

Many of the leading players in the SAF market — such as Finland’s Neste and Dutch company SkyNRG — recycle used cooking oils, as well as animal and fish fat waste from the food industry.

Neste currently produces 100,000 tonnes of SAF annually in the US and Europe, but when the expansion of its Singapore refinery is completed in 2022, it will be able to produce more than one million tonnes (about 1.25bn litres) a year.

SkyNRG is building a plant in the Netherlands that will produce 100,000 tonnes of SAF annually from 2022 onwards, with 75% of that to be bought by Dutch airline KLM.

US-based Fulcrum Bioenergy, however, is ploughing a new furrow by producing SAF from municipal solid waste (MSW) — literally, rubbish collected from homes and businesses.

“The organic material found in our household garbage is rich in hydrogen and carbon, the building blocks for jet fuel and diesel,” the company explains. “And unlike other biomass feedstocks, MSW does not have to be grown and has no competing uses — it is a true waste product located in the same population centres that demand low-cost, low-carbon transportation fuels.”

Notably, the company also claims its products will be cost-competitive with conventional fuels. This is due to its cheap feedstock, the fact that the heat used in the process generates the plant’s electricity, and because — unlike conventional jet fuel — its products contain no sulphur, which is expensive to remove.

Fulcrum is currently building a Fischer-Tropsch “renewable fuel” plant in Nevada that will turn about 175,000 tonnes of MSW feedstock a year into 11 million gallons (41.6 million litres) of “renewable synthetic crude oil”, which will then be processed by Marathon Petroleum into renewable diesel or jet fuel. The proportion of those two fuels will depend on demand.

On top of that, Fulcrum is “actively developing projects that will have the capacity to produce hundreds of millions of gallons of low-cost, low-carbon transportation fuel across North America”, the company says.

It adds that it has already secured offtake agreements with Cathay Pacific, United Airlines, BP, Marathon and Fortune 500 company World Fuels Services, which among other things, supplies jet fuel to airports.

United, for example, has said it will purchase up to 10 million gallons of “cost-competitive sustainable aviation fuel” from Fulcrum over the next two years.

The company also points out that it has secured “long-term access to MSW feedstock totalling about 4% of the garbage landfilled annually in the United States”.

Carbon-negative jet fuel

An outlier in the SAF industry, and a potential game changer, is the innovative carbon-negative jet fuel being worked on by Denmark’s Stiesdal Fuel Technologies, led by wind-power pioneer Henrik Stiesdal.

The start-up’s SkyClean technology, which is on track to be commercialised in 2025, is unique in the SAF field because the more its aviation fuel is used, the more net carbon is removed from the atmosphere.

SkyClean could be described as a combination of biofuel and e-fuel, which uses a pyrolysis process that removes carbon from agricultural matter and stores it in the form of biochar, a carbon-based material similar to charcoal.

In simple terms, a pyrolysis oven is used to heat agricultural waste in the absence of oxygen, converting the biomass into three components: biooil, syngas and biochar. The biooil and syngas are then combined to create biomethanol, which is then synthesised into aviation fuel with the addition of green hydrogen.

The carbon-negative fuel would be affordable, Stiesdal tells Recharge, because the SkyClean process has four potential revenue streams — the fuel itself, biochar (which can be used to improve soil quality), waste heat and carbon credits. These three products effectively subsidise the cost of the aviation fuel, enabling it to reach price parity with A-1 jet fuel if the company received the equivalent of €65 per tonne of sequestered CO2, according to Stiesdal’s calculations.

Volumes: Scaling up

While progress is being made on SAF production, replacing billions of litres of conventional jet fuel with SAF will be a mammoth task.

According to T&E calculations, decarbonising half of the EU aviation sector by 2050 with e-fuels would require 24% of all the electricity currently being generated in Europe, with 8 million hectares of land — the size of the Czech Republic — required to host the required wind turbines and solar panels to generate the green hydrogen needed.

Meeting the same target using biofuels would require 33 million hectares of farmland — the size of Finland — it adds.

Yet the sector is heading in the right direction.

According to Wilkin, 30 million litres of SAF were produced in 2019 — double the volume of the previous year — and production is set to grow substantially in the coming years.

The IEA says long-term off-take agreements between airlines and biofuel producers “cumulatively cover six billion litres” over the coming years, and that production facilities are being built around the world to meet these commitments.

If this could be expanded to 7.2 billion litres per year, jet biofuels would get on a path to cost-competitiveness with their fossil-fuel equivalent, according to Le Feuvre.

“Meeting 2% of annual jet fuel demand from international aviation with SAF could deliver the necessary cost reduction for a self-sustaining aviation biofuel market thereafter,” he says, adding that this would require 20 new specialist refineries and investment of about $10bn.

To be in line with the IEA’s Sustainable Development Scenario, which would keep the planet to a temperature increase of 1.8C, SAF would need to account for 5% of total jet fuel demand by 2025, he adds. This would require the consumption of SAF to be subsidised to the tune of $6.5bn due to its higher price, but he points out that this figure is a drop in the ocean compared to the $143bn of support spent on renewable power generation globally in 2017.

Of course, like any new green technology — including wind turbines, solar panels and batteries — costs only come down with economies of scale, which are usually kick-started by government intervention such as subsidies.

National mandates and targets, as well as higher carbon prices, would clearly be beneficial to the development of jet biofuels, says Le Feuvre.

Norway introduced the world’s first national mandate for jet biofuel in 2018, calling on all refuelling in the country to use at least 0.5% SAF from sustainable non-food sources from 2020. The government aims to increase this mandate to 30% by 2030.

Similar mandates have been proposed in Sweden, Spain and France, but not officially adopted. In 2016, Indonesia announced a mandate for 3% biofuels in jet fuel by 2020 and 5% by 2030, but this does not seem to have been enforced.

German environment minister Svenja Schulze recently suggested setting a mandate for 2% green-hydrogen content in jet fuel, arguing that this would create a substantial ready-made market for the gas. But as a member of the junior party in Germany’s coalition government, it is hard to say how much support her views have in Berlin.

At the same time, organisations such as Fly Green Alliance are lobbying Brussels for green aviation to be specifically targeted in the European Commission’s forthcoming Green Deal. The initial draft, unveiled in December, only mentions aviation in passing, saying that fossil-fuel subsidies and tax exemptions for airlines should be eliminated, while the number of free EU Emissions Trading System allowances allocated for free to airlines should be reduced. There were no specific targets for SAF that could help support a move to cleaner aviation fuel.

What will the future bring?

Ultimately, the few targets that do exist for SAF are quite unambitious. Wilkin tells Recharge that due to the sheer scale of volumes involved, the green aviation sector will consider a 10% SAF blend in jet fuel a significant achievement.

But at some point in the future, the industry may face a technical barrier. Currently, ASTM standards do not allow airlines to fly with more than 50% biofuel or e-fuel in their tanks. And there is good reason for this.

As the German Aerospace Centre study reports: “As things stand currently, blending shares above 50% may not be safe as synthetic fuels do not contain any aromatics. Those aromatics, included in kerosene, are not required by the engines but by current O-Rings [rubber seals] to function properly.”

These seals are designed to expand — a reaction to the aromatics in conventional jet fuel, which can make up 8-22% of the hydrocarbon content. Flying without aromatics may cause the O-Rings to shrink, resulting in fuel leaks.

Yet synthetic rubber O-Rings are already on the market that can solve this problem and allow 100% biofuel use.

And 100% biofuel flight has already happened. Boeing flew the world’s first 100% biofuel test flight back in 2018, using its ecoDemonstrator 777 freighter plane.

Scott Fenwick, chairman of the ASTM fuels committee, tells Recharge that “it is very possible that under certain conditions, these type of fuels may eventually be used in their pure (100%) form”, explaining that “individual flights may need to consider the exact type of renewable fuel and all of its performance properties — and the type of engine and aircraft it is used in — to ensure compatibility and proper performance.”

And, of course, to fully decarbonise the aviation industry, the entire value chain and production processes used to manufacture SAF need to be 100% carbon-free.

But it does seem that this is possible — even with the volumes required.

In a “working paper” produced last year, the ICAO wrote that analysis by one of its sub-committees found that by 2050, “it would be physically possible to meet 100% of international aviation jet fuel demand with sustainable aviation fuels”.

“However, this level of fuel production could only be achieved with extremely large capital investments in sustainable aviation fuel production infrastructure, and substantial policy support.”

So it does seem that a fully decarbonised aviation system — propelled by renewable energies — is possible by 2050 using existing solutions.

Like many other green technologies, it is merely a case of bringing down costs — with the help of government policy — and then increasing volumes as quickly as possible.

As soon as airlines recover from the coronavirus impact, green aviation should finally start taking off.