The aviation passenger count is expected to recover to pre-pandemic levels by 2025, with about four billion passengers and a potential surge to eight billion annually thereafter. However, without significant intervention, this increase would lead to a dramatic rise in CO₂ emissions from 2bn t/y today to 3bn t/y, according to our report.

To address this vast rise in emissions, global initiatives, led by the International Air Transport Association (IATA), are aiming for net zero emissions by 2050, with a raft of measures to achieve climate neutrality.

To put the issue in perspective, Lux Research’s analysis uses British Airways as an example of an airline operator which has a significant global presence, with over 250 aircraft and flights to over 200 destinations worldwide. Like its counterparts, British Airways must implement strategies to meet the net zero target by 2050.

Operators like British Airways and its peers face a formidable challenge. British Airways emits nearly 20mn t/y CO₂. Although there was a temporary dip in emissions during the pandemic, projections suggest a speedy rebound to previous levels, heightening the urgency to devise and implement strategies to drastically reduce its carbon footprint.

Basically, there are three different pathways for reducing the carbon footprint of airline operations: electric aviation, hydrogen aviation and sustainable aviation fuel (SAF).

Electric aviation  
The electric aviation industry can be segmented into three very different target applications:

• Air taxi and flying car initiatives targeting intra-urban transit applications (range ~200 km), with rotor-based architectures for vertical take-off and landing configurations that require no landing strips. Hybrids for longer distances, which may use rotors for take-off and landing but switch to fixed-wing operations for horizontal flight.
• Small regional aircraft for island- or fjord-hopping and accessing rural locations (range ~400km), seating nine to 20 passengers or carrying cargo. Electric powertrains could enable more aerodynamic designs due to the absence of hot exhaust. Hybrid electric aircraft may reach the market quicker.
• Commercial aircraft targeting long-distance international travel with seats for over 100 passengers (range ~700km).

The main challenge hindering widespread adoption of electric aircraft is the energy density of batteries. Today’s electric planes chiefly rely on lithium-ion batteries, which have low energy density compared to using kerosene. To achieve the same range as conventional aircraft, an electric plane would require a prohibitively heavy battery.

Although next generation battery technologies like solid-state are being developed, they still do not offer a viable solution. As a result, hybrid configurations, combining batteries with hydrogen fuel cells or jet fuel, are likely to be a more feasible interim solution.

Today, there are few commercial electric aircraft. Instead, start-ups like Wright Electric are leading the charge. Wright is working on an ambitious project to launch a commercial-scale electric aircraft with a 186-seat capacity, aiming for a range of 800 miles by 2030. This venture is partnering with EasyJet. Such an aircraft could have an impact on regional travel. Even though the range of electric planes is limited currently, they could service key European destinations like Paris and Frankfurt, covering a substantial portion of British Airways' network.

Hydrogen aviation  
There are two different propulsion technologies for hydrogen aviation. Hydrogen fuel cell companies are developing unmanned aerial vehicles and small to medium-size airliners. Hydrogen combustion developers are mostly targeting commercial applications flying passengers on medium to long-haul routes. But development is currently at the conceptual and test stage.

The largest fuel cell-electric aircraft flown to date is a 40-seat regional airliner (range ~500 km). Proton exchange membrane fuel cells are preferred as they operate at low temperatures. Fuel cells may often be used in hybrid configuration with batteries or combustion engines. But they will face challenges at high altitudes due to low temperature, pressure and oxygen concentrations.

ZeroAvia has developed a 600 kW fuel cell electric propulsion system as part of the UK government-funded HyFlyer II programme. It aims to launch a 19-seater plane flying 300 miles by 2025, and a 200-seater plane capable of flying 2,000 miles by 2030.

The hydrogen combustion turbofan engine could have a range of 3,700 km. Planes will use liquid hydrogen. Modifications to the combustor will be required, including a new cryogenic fuel delivery system. A major challenge is flame stability, as flame flashback could lead to an explosion. Hydrogen also has lower energy density compared with kerosene. Despite liquid hydrogen offering a higher density, it still falls short of kerosene’s energy levels. This is a significant hurdle in maintaining the performance standards of existing flight operations. Commercial deployment is likely to be some decades away.

In 2021, aircraft manufacturer Airbus launched ZEROe and unveiled three concepts for aircraft powered by hydrogen combustion. The designs include blended-wing body and turbofan planes, including a 200-seater plane with a 2,000-mile flying range. But there’s still a long way to go before Airbus is likely to bring these aircraft into commercial service – possibly some time after 2040.

Theoretically, British Airways could use hydrogen planes for intercontinental flights, to reach international hubs like Dubai or New York City. This would enable airlines to maintain a large portion of their existing networks, especially within Europe, and also to access long-haul destinations. Nonetheless, the transition to hydrogen-powered aviation is currently speculative and awaits further developments and concrete timelines from existing airframe suppliers and other innovators in the industry.

Takeoff of the 19-seat test Dornier 228 aircraft, one of whose propellor engines is driven by a motor powered by a hydrogen fuel cell. Flight range is 300 nautical miles.
Photo: ZeroAvia

Sustainable aviation fuel   
SAF is a drop-in aviation fuel made from low-carbon feedstock, which, technically speaking, is blended into fossil jet fuel. The types of SAF can be segmented by feedstock. Bio-based SAF is made from bioresources like biomass or sugars, which is thermochemically converted or biologically converted to fuel. Synthetic SAF is made from CO₂ and hydrogen gas, and thermochemically converted into fuel. The SAF market is expanding, with post-pandemic growth expected to reach 800,000 to 1mn t/y by 2024, despite high costs and cautious policy support in the US and Europe.

SAF will remain the best option to decarbonise aviation. While recent advances in electric and hydrogen aviation are pushing the limits of alternative propulsion technologies, inherent limitations like energy density will limit the range of an electric or hydrogen airplane.

British Airways and other airline operators can leverage SAF’s similarity to conventional jet fuel potentially to power their entire network, including their longest flights, in alignment with environmental goals. However, the switch to SAF also presents challenges – notably in securing sufficient and sustainable feedstocks.

The primary method of production, by refining hydrogenated esters and fatty acids (HEFA), depends on limited waste feedstocks and poses scalability issues. Catalytic hydro-treatment of oil-based feedstock in the presence of hydrogen removes oxygen, and is followed by hydrocracking into renewable diesel, naphtha and SAF. The HEFA process uses vegetable oil or waste fat, oil or grease as feedstock, which requires pre-treatment to remove impurities. The typical jet fuel fraction in a HEFA facility is 15%, but can be raised to 80% for maximum output.

Neste has developed NEXBTL for the hydrotreatment of bio-oil for renewable fuels. Biorefineries in the Netherlands and Singapore had a combined capacity of 3mn t/y of renewable fuels.

Other pathways, such as Fischer-Tropsch (FT) and alcohol-to-jet (ATJ), face hurdles in terms of complexity, high capital cost and sustainability. In FT, synthetic gas (syngas – hydrogen and carbon monoxide) is catalytically converted into synthetic crude, which is then refined into diesel, jet fuel, naphtha and waxes. The syngas can be produced from gasification of biomass or municipal waste (MSW), or from hydrogen and CO₂ through the reverse water-gas shift reaction. The typical jet fuel fraction in an FT facility is 10–15%, although early estimates indicate it can be increased up to 50–60%. Fulcrum Bioenergy launched the first demonstration facility for SAF from MSW in 2021, using Johnson Matthey’s FT technology, with a capacity of 10mn gallons per year (g/y).

ATJ is a three-step catalytic conversion of alcohol to jet fuel. The alcohol is dehydrated into alkene, then oligomerised into long-chain hydrocarbons before hydrogenation to jet fuel. ATJ is not yet commercial; the first demonstration facility converting ethanol to SAF was launched by LanzaJet in January 2024. The plant has a capacity of 10mn g/y and uses first generation ethanol. ATJ uses inexpensive, recyclable and stable catalysts with high selectivity towards paraffinic compounds.

The high cost of SAF, especially from vegetable oils, challenges widespread adoption, as it remains significantly more expensive than fossil fuels. That affects airlines’ operational costs and broader uptake, without further technological or policy advances. Currently, cost parity with fossil jet fuel isn’t attainable without heavy subsidies.

Work is also under way on development of methanol-to-jet, which promises to be a scalable technology, using abundant and sustainable feedstock, at relatively low cost with robust developers. It produces SAF by synthesising methanol from widely-available CO₂ and hydrogen.

Key takeaways  
SAF is, and will remain, the best option to decarbonise aviation. While recent advances in electric and hydrogen aviation are pushing the limits of alternative propulsion technologies, inherent limitations like energy density will limit the range of an electric or hydrogen airplane.

Airport capacity will be a limiting factor to electric and hydrogen aviation. Even if the levelised cost of flying all-electric or all-hydrogen aircraft is cheaper than SAF, there isn’t enough space in airports to accommodate the larger number of such craft needed.

Finally, methanol-to-jet looks to be the most promising opportunity in SAF, as CO₂ and hydrogen are not inherently limited, and the technology is less complex than FT.

Registration has opened to the Farnborough International Air Show, 22–26 July 2024 in Hampshire. The biannual event alternates with the Paris Air Show to showcase developments in military and civilian aerospace.

• Further reading: ‘Tees’ green kerosene’. Alfanar Energy plans to build in Teesside what it claims will be Europe’s largest advanced second-generation facility in the SAF supply chain. 
• Find out about the challenges that remain in terms of scalability and cost for SAF.