Flying higher

The aviation sector emits around 2-3% of global greenhouse gas emissions. But while airlines have promised to halve their carbon footprint by 2050, passengers and global cargo are expected to increase significantly over the next 30 years, Anthony King reports

 ‘Aviation emissions are growing at an exponential rate as more people are flying more often,’ says Nikita Pavlenko, aviation fuels expert at the International Council on Clean Transportation. Over 21bn gallons/year of jet fuel are consumed in the US alone, and demand is expected to double by 2050.

Aviation is forecast to become the second highest residual emitter in 2050, according to a UK government report, and is one of the most challenging sectors to decarbonise. ‘When we talk about medium and long-haul aviation, there are no alternatives to liquid hydrocarbons,’ says Ehsan Alborzi, a mechanical engineer at the Sustainable Aviation Fuels Innovation Centre at the University of Sheffield, UK. Petroleum-based jet fuel must be substituted with liquid hydrocarbons that do not use fossil fuels, so-called sustainable aviation fuels (SAFs).

Seven approved pathways are presently approved to generate SAFs, some with more potential than others. First, there are near-term commercially available SAFs, especially made by HEFA (hydrotreating of esters and fatty acids) that taps waste, fats, oils and grease, as well as virgin vegetable oil. The length of the carbon chains favours the production of diesel-length molecules, and further processing is required to obtain a higher fraction of carbon in the SAF product. This can make diesel from HEFA comparatively greener than jet fuel; 2m gallons of HEFA aviation fuel was produced in 2018, but around 300m gallons of renewable diesel.

Nonetheless, HEFA production could reach over 1bn gallons/year. The two main producers are Neste in Europe and World Energy in the US. Neste describes its fuel as 100% made from waste and residues, with an 80% reduction in greenhouse gas emissions, compared with fossil jet fuel. HEFA can also be made from oil-rich crops such as camelina. Such fuels typically cost two to three times that of fossil jet fuel.

But there is a limit to waste oils availability, and growing oil crops for fuel flags up sustainability issues. ‘Bio-based aviation fuels are not the solution,’ adds Morten Birkved, an environmental chemist at the University of Southern Denmark. ‘They’re part of the problem because we have limited biomass available in the world.’

Future SAF

Many other SAF processes are technically more challenging than fats and oils, but in theory available in greater quantities and more suitable for achieving long-term decarbonisation goals. Ordinary jet fuel consists of a mixture of hydrocarbon distillates with eight to 16 carbons, some linear, some circular, some unsaturated. ‘Sustainable aviation fuels are quite similar in terms of chemical composition,’ says Alborzi. But jet fuel has a variety of functions; to cool the engine, gauge fuel levels, and to swell seals.

Any new SAF must therefore go through a lengthy qualification process before approval as a reliable ‘drop-in’ replacement for conventional fuel. ‘One of the most important criteria is backward compatibility with existing aircraft,’ says Alborzi. The current blend limit for specific SAFs is 50% or 10%, with fossil aviation fuel, depending on the process to produce the fuel. It takes years to produce an aircraft, ‘and most aircraft then have a service life of 30 years or more,’ says Joshua Heyne, an aerospace engineer at the University of Dayton, Ohio, US. ‘There are going to be old aircraft out there that could be exposed to these new fuels, which could cause horrendous safety issues.’ Based on current announcements, he estimates 7% of total aviation fuel capacity could be SAF by 2030.

The biggest producers now are the HEFA processes. But the first approved for blending with regular jet fuel was Fischer-Tropsch synthesised isoparaffinic kerosene (FT-SPK) in 2009. This involves coal, natural gas or biomass being gasified into a syngas of hydrogen and carbon monoxide, to be catalytically converted into a liquid hydrocarbon in an FT reactor. And there are plenty of wet wastes available for such processes. The US is estimated to generate enough food waste to produce 2.5bn gallons/year of jet fuel, while animal manure could add over 4bn gallons and wastewater sludge almost 2bn gallons. Food waste contributes 6% to greenhouse emissions, so turning it to fuel has additional environmental benefits.

Fulcrum in Reno, Nevada, US, gasifies municipal solid waste and puts it through an FT process to generate renewable transportation fuels. It completed a plant in July 2021 that will turn 175,000t of waste into around 11m gallons of synthetic crude, which can be upgraded to SAF. Another US company, Red Rock Biofuels in Colorado, plans to turn woody forest waste into SAFs using FT and hydro-processing technologies.

Such processes are often inefficient, especially if directly compared with jet fuel made from petroleum. The FT process was invented in the 1920s, and today is usually used to convert coal or natural gas into synthetic petroleum products. ‘The Fischer-Tropsch processes are not particularly efficient, especially at a small scale, which is the scale at which biogenic feedstocks are produced,’ says Michael Wolcott, Director of the Aviation Sustainability Center (ASCENT), co-led by MIT and Washington State University in the US. What is required, he explains, is a more efficient gasification process at smaller scales and for the FT process itself to be more efficient at such scales, which might require new catalysts. Many see potential in this route, nonetheless. Sheffield’s SAF innovation centre is focused on capturing CO2 from combustion and putting it through an efficient FT reactor.

The molecules present in jet fuel matter. One major challenge is that most SAFs are either devoid of or light on aromatics. ‘And aromatics provide a variety of functions within jet fuels in terms of swelling of O-ring seals in certain engine parts,’ explains Wolcott. The aromatics go into the material and maintain its shape and size. Crucially, there are O-rings all over aircraft as sealants. ‘Not all these O-rings are replaced routinely, maybe ever,’ says Heyne. ‘The O-rings can shrink when they are exposed to some of these SAFs.’ Shrinkage can mean leaks, an unacceptable hazard. ‘The major production routes for SAF do not meet the swelling capacity,’ warns Heyne. The situation could be improved for newer aircraft, but there will still be quite old aircraft that could be exposed to these fuels. This is one of the concerns about pure SAFs.

There have been recent test flights powered by 100% SAFs. In December, a United Airlines commercial jet with 100 passengers flew from Chicago to Washington DC. This blended in aromatic kerosene components made using a biosynthetic process by Virent in Madison, Wisconsin. ‘There’s a lot of work beginning on what a 100% SAF would like look,’ says Wolcott. Safety in aviation is paramount though, and progress slow.

An upside from the low aromatic content of SAFs is that it cuts soot formation and aerosol emissions by 50 to 70%. This is especially significant because soot is possibly the main initiator of aircraft contrails, which are estimated to have a larger warming impact than aviation-emitted CO2 alone. ‘In terms of the climate impact of aviation, it is estimated that two-thirds are from non-CO2 emissions,’ says Heyne.

There is the possibility that a synthetic aromatic component could generate fewer particles, improving local air quality, and reducing contrails.

[Sustainable aviation fuel] is really a drop in the bucket compared with overall fuel consumption. There are small demonstration projects for publicity and not necessarily meaningful structural change.
Nikita Pavlenko aviation fuels lead, International Council on Clean Transportation, Washington DC, US.

Under pressure

Airlines are feeling the pressure on sustainability. A ‘flight shame’ movement began in Sweden in 2018. ‘More and more people are sensitive to the carbon impact of their flight,’ says Heyne. His centre has received almost 100 novel SAF fuel compositions from several dozen institutes in Europe and North America for tests, such as compatibility with materials on aircraft and compositions for high blend ratios or lower contrail formation.

Others say aviation needs a shove. ‘[SAF] is really a drop in the bucket, compared with overall fuel consumption,’ says Pavlenko. ‘There are small demonstration projects for publicity and not necessarily meaningful structural change.’

But if airlines do not act, governments will. In Denmark, ‘our prime minister just promised that all domestic flights will be carbon neutral SAFs by 2025,’ says Birkved.

Ultimately, however, someone will need to make up the cost difference between SAFs and fossil fuel. Efforts are under way to encourage a transition. ‘The EU introduced a SAF mandate, which is a really promising step forward,’ says Pavlenko. The UK ran a ‘jet zero consultation’ in 2021 and has set up a partnership between industry and government with the aim of zero emissions on transatlantic flights within a generation.

California, meanwhile, has adopted policies to encourage decarbonisation of aviation. However, Pavlenko thinks a proposed US tax credit will divert existing biofuels from the road sector to aviation, at taxpayer’s expense. Another option would be to tax kerosene the same as diesel and petrol, but this would require international consensus and looks unlikely.

Future flights will certainly consume more SAF. Prices may go up, and demand for flying may go down. ‘People will still fly around, but to a somewhat lesser extent than today. And maybe we will be more selective on when you fly, due to price increases,’ Birkved concludes.

The other key issue, meanwhile, is availability, adds Karen Thole, head of mechanical engineering at Penn State University, US. ‘We used about 100m gallons of SAF in 2021, but there is 14bn gallons pre-ordered by the airlines right now. The big challenge: can we produce enough?’