Production technologies often rely on a source of carbon dioxide as a feedstock to produce sustainable fuels
EASY SOLUTION
Lower carbon alternatives to jet fuel can be used with today’s aeroplanes and infrastructure
STICKING POINT
A shortage in sustainable feedstocks currently available could slow the pace of growth
KEY QUOTE
If we want to make sustainable aviation fuels that are as sustainable as possible, it goes back to designing policies that reward those that deliver the highest emissions reductions
Air travellers seeking to lower their carbon footprints have a few alternatives. These include simply travelling less or “offsetting” the impact of their trip by helping to fund tree planting and other projects aimed at cutting emissions or increasing carbon storage capacity—but this is not an exact science. Increasingly, however, airline passengers are being offered the option of purchasing a lower-emission version of traditional jet kerosene known as sustainable aviation fuel (SAF). Just a drop in the bucket today—accounting for just 0.1% of all jet fuel in 2021—SAF lies at the centre of most decarbonisation roadmaps for the aviation sector. “Unlike the road sector, it’s hard to put a battery on a plane because of weight and range issues. While there’s been a lot of research in batteries and hydrogen planes, that’s further [in the future]”, says Jane O’Malley of the International Council on Clean Transportation (ICCT), a non-profit scientific advisory body. Aviation accounts for about 3% of global carbon dioxide (CO2) emissions. As air traffic rebounds after the Covid-19 pandemic and travel restrictions, further growth will continue in the coming decades. Emissions are seen doubling or even tripling by 2050 in business-as-usual scenarios. This means aviation’s percentage of overall emissions would balloon as progress is made in other sectors where the route to net zero is clearer. There is no universal definition for SAF, but it is generally considered to be a lower-emission “drop-in” alternative to jet fuel. The International Air Transport Association (IATA), an industry body, estimates that SAF could contribute about 65% of the emissions reduction needed for the sector to reach net zero in 2050. The International Energy Agency’s (IEA) net-zero scenario, one pathway for decarbonising the energy sector, sees 75% of all aviation energy demand being supplied with SAFs that year.
QUICK SOLUTION
SAFs are currently blended with conventional jet kerosene. The long lifespan of planes does complicate decarbonisation, but SAF needs neither new aircraft nor infrastructure. Once the fuel is available, the rollout can be rapid. Today almost all sustainable aviation fuel falls under the category of “hydro-processed esters and fatty acids” (HEFA) and is made with feedstocks like used cooking oil and waste animal fat. Given that SAF production today is so low it is unsurprising that its sustainability credentials are generally good. With similar chemical and physical characteristics to conventional fuels, planes using SAF still have some CO2 tailpipe emissions, as well as other harmful gases. The carbon used for SAF is “recycled” from organic waste carbon or crops that absorb CO2 as they grow, so emissions from the combustion of these fuels can be considered carbon neutral. Given that burning fuel is not the only source of greenhouse gas emissions, SAF cannot be considered emission-free. Emissions also come from the cultivation of crops used to make biofuels for aviation, for example, as well as from oil extraction, processing and transportation of alternative fuels. However, the life-cycle emissions of SAFs used today are reported to be as much as about 80% lower than fossil kerosene. 
CARROTS AND STICKS
With progress made in the decarbonisation of power grids, lawmakers are turning their attention to reducing emissions in hard-to-abate sectors like aviation. When it comes to flying, this has been seen clearly in the significant growth in mandates and incentives for SAFs. The European Union’s proposed ReFuelEU Aviation regulation is expected to set an initial, binding target of 2% SAF to be supplied by fuel distributors in EU airports in 2025, rising to 5% in 2030 and reaching 63% in 2050. Sub-mandates are also being set for e-kerosene, using green hydrogen and a carbon source as feedstocks. E-kerosene is expected to account for an initial 0.7% of total aviation fuels in 2030 and reach 28% in 2050. ReFuelEU stipulates that SAF must provide greenhouse gas emissions reductions of at least 65% compared to conventional jet fuel. The use of crops reconverted from food or animal feed use will not be allowed, although they would be unlikely to meet the emissions reduction requirement in any case. In the United States in 2021, the Biden administration unveiled the “Sustainable Aviation Fuel Grand Challenge” with the objective of producing three billion gallons (about 11.4 billion litres) per year of SAF domestically in 2030, equivalent to about 10% of forecast demand that year, and 35 billion gallons in 2050. The 2022 Inflation Reduction Act went further, providing sliding tax credits for the production of SAF that yields at least a 50% reduction in greenhouse gas emissions compared to fossil kerosene. The tax credit is offered through 2027 and starts at $1.25 a gallon, rising by $0.01 for every percentage increase over 50%, potentially up to a level of $1.75 a gallon. “If we want to make sustainable aviation fuels that are as sustainable as possible, it goes back to designing policies that reward those that deliver the highest emissions reductions,” says Praveen Bains of the IEA.
SCALING UP
Airlines around the world have also been unveiling their own SAF targets, which are often more ambitious than those imposed by lawmakers. Irish carrier Ryanair aims to fuel 12.5% of flights with SAF by 2030, while Air France-KLM, Delta and International Airline Group (IAG), the owner of airlines like British Airways and Iberia, have all set a target for 10% SAF in the same timeframe and have announced procurement agreements with SAF suppliers. IATA estimates that SAF production reached 300-450 million litres in 2022 compared to 100 million in 2021 and sees a tipping point at 30 billion litres by 2030, some 100 times current capacity, with the right supporting policies. It argues that incentives should be provided like those for biogas and biodiesel. Neste, the Finnish refiner that ranks as the world’s largest SAF supplier, has provided some indication of the speed and the scale at which production must be increased. It has pledged to increase its annual SAF production capacity from 100,000 tonnes in 2022 (125 million litres) to 1.5 million tonnes (1.875 billion litres) by the end of 2023 and is expanding its facilities that produce SAF in Rotterdam and Singapore. 
NEXT STEPS
Biofuels made using HEFA technology are widely expected to remain the main source of SAF throughout this decade before other production technologies and feedstocks gain ground. McKinsey, a US-based market analysis firm, calculates that converting waste and residue lipids into jet fuel could meet about 5% of total 2030 jet fuel demand. The figure could increase further with HEFA fuels made from purposely growing oil trees on degraded land and oilseed-bearing herbs used as cover crops, the consultancy notes. No single feedstock will be able to satisfy aviation’s thirst for cleaner fuel. Sylvain Verdier of Topsoe, a Danish provider of emissions reduction technologies, stresses that feedstock availability and regulations will be key to what types of SAF develop and where, as well as the pace of SAF development. “The idea is to have as many sustainable paths as possible,” he says. Alongside HEFA, one way to produce SAF involves alcohol-to-jet technology using a biomass feedstock that can be transformed into ethanol. Another involves the gasification of feedstock to produce a “syngas” that is fed into a Fischer-Tropsch reactor, where it is combined into a mix of hydrocarbons with the use of a catalyst. Both production processes can use similar biomass feedstocks although gasification, also known as the Fischer-Tropsch route, can also use municipal waste. Woody feedstocks like forestry residues and wood waste could provide a low-emission source of SAF, says Bains. “The biggest obstacle is that it’s difficult to break down this feedstock to convert into an oil, so the challenge is to commercialise the technologies that can unlock this feedstock,” she adds. The use of municipal solid waste as a feedstock is “interesting but at a similarly lower technology readiness level,” adds Bains. “From an emissions reduction point of view, it has to have a high organic component rather than a lot of fossil fuel plastic,” she stresses. Bains believes that ethanol plant owners in the US could begin to look into producing aviation fuels as a result of the Inflation Reduction Act (IRA), particularly as the use of ethanol in road transport decreases with the rise of electric vehicles. Although ethanol plants need to reach a minimum emissions threshold to receive support from the IRA, she notes that putting carbon capture in place at these plants is straightforward and would allow more facilities to be eligible.
E-KEROSENE POTENTIAL
The potential for the production of e-kerosene aviation fuel through a power-to-liquid (PtL) process using green hydrogen and a sustainable carbon feedstock is significant, says Kate Macfarlane of Linde, an industrial gases and engineering firm. “There is a lot of discussions now about the source of CO2 and what makes a CO2 source sustainable,” she says. The IEA’s net zero scenario, for instance, only considers biogenic sources of CO2 and direct air capture (DAC) as sustainable origins, excluding point source emissions from commercial & industrial facilities. McKinsey says if carbon is used as a PtL feedstock, the industrial facility or power plant originating CO2 should not be considered carbon neutral. “Only DAC and BECSS (bioenergy with carbon capture and storage) carbon fully avoid sustainability concerns around double-claiming emission reductions and unintended incentives for continued carbon emissions generation,” it explained in its Clean Skies for Tomorrow report. At the same time, carbon sourced from industrial or power plants could also be a bridging technology, it added. E-kerosene opens up the possibility of a decline in CO2 emissions of as much as 100% compared to fossil jet fuel if the supply chain is completely decarbonised. It could also avoid some of the land-use issues associated with biofuels. Macfarlane says that challenges in developing an e-kerosene industry include scaling up capacity for electrolysers, significantly ramping up renewable energy capacity to power the electrolysers and bringing down costs, which tower above those for producing SAF via the HEFA route. “You need to increase the scale of these plants to bring down Capex,” she says. Oskar Meijerink of SkyNRG, a Dutch manufacturer of SAF, pointed out in Topsoe’s Fuel for Thought podcast that it would require some 25 gigawatts (GW) in offshore wind plants to provide the green hydrogen needed to fuel all flights taking off from Amsterdam’s Schipol airport alone assuming traffic at pre-Covid levels. This is more than the 2030 offshore wind target for the Netherlands of 21 GW. Bains notes that the deployment of carbon capture technologies is also essential to the development of e-kerosene, particularly given the limited supply of biomass feedstock. In the IEA’s net zero scenario, e-kerosene ramps up after 2035.
ENSURING SUSTAINABILITY
There is no universally agreed definition for SAF and what makes it sustainable. This could potentially present problems as production increases. “Everything has a climate impact, so you need to minimise it,” says Verdier. O’Malley from ICCT highlights the risk of both direct and indirect land-use changes tied to the production of biofuels for SAF, particularly on carbon-rich land. While waste is less problematic, it may also come with substitution risks. For instance, if a sawmill uses residue wood chips to fuel its boiler and these wood chips become a feedstock for SAF, the sawmill might instead begin using natural gas in the boiler, she explains. “The biggest thing is to make sure we don’t repeat the same mistakes (with SAF) that we did with the on-road sector,” says O’Malley. “A lot of money was funnelled to these crop and waste-based sectors of which there wasn’t all that much out there and that comes with all these inherent risks,” she says. While one benefit of aviation biofuels is that they can cut lifecycle CO2 emissions, one risk is that lifecycle CO2 emissions could be high if “for example, a significant amount of land is required to grow the biomass used to make the biofuel”, the Clean Air Task Force (CATF), a US-based non-profit, cautioned. Sustainability may also touch on non-emission aspects such as socioeconomic and labour issues and biodiversity. 
CLEAR SKIES
Using SAF’s is a key pillar of the aviation industry’s decarbonisation plans
BRIDGING THE GAP
One of the challenges of developing SAF will be to bring down prices, which may be roughly double that of conventional jet fuel and can be as much as eight times as expensive, depending on the feedstock and the production process. While the US is providing tax incentives to SAF, the German government’s H2Global initiative is seeking to help kick-start the development in non-EU countries of power-to-X technologies, including what it calls e-SAF. In December 2022, it announced a tender for ten-year supplies of the fuel, with the first delivery due in 2024. To help overcome the problem of high upfront investment costs, the H2Global tender intermediary—the Hydrogen Intermediary Network Company (HINT.CO)—will conclude long-term purchase contracts on the supply side and short-term contracts on the demand side, compensating the difference between prices for the production and transport of fuels and their demand prices with grant funding from the German government. The mechanism is similar to the Contracts for Difference (CfD) mechanisms that have been used to support electricity from renewable energy sources.
TOTAL SAF
Currently, SAF is blended with conventional jet fuel and current fuel standards limit its percentage to 50%, although test flights have been done with 100% SAF. The reasons for the blending requirement are largely tied to safety, as aromatics in traditional jet fuel lead seals in fuel tanks to swell, reducing the risk of leakage. While aromatics can also be added to SAF, researchers are looking into the feasibility of using fuels with reduced or no aromatics. “100% SAFs containing reduced or no aromatics reduce non-volatile particulate matter emissions, which are linked to contrail formation, and contrails are suggested to contribute more to aviation radiative forcing than CO2 emissions,” noted a report in Frontiers in Energy Research published in January 2022. Pure SAF also has low levels of sulphur, leading to low levels of sulphur oxide emissions, it noted. “Thus 100% SAF would reduce aviation’s GHG (greenhouse gas) production, contrails and SOX emissions, all significant environmental benefits.”
OUTSIZED TOOL
As aviation looks to cut emissions operational and technical efficiency of aircraft is also expected to remain a focus. However, the IEA points out that aircraft fuel efficiency improved by 2.4% a year from 2000 to 2010 and by 1.9% from 2010 to 2019, showing that further incremental improvements are becoming difficult. Growth in demand for flying has also outpaced efficiency improvements. Limiting air traffic is key to moderating demand for fuel but may also prove difficult. After the Covid-19 pandemic, the International Civil Aviation Organisation revised its forecast annual growth to 2050 downwards, but it still sees robust growth of 3.6%. In an article published in Nature Sustainability in January 2023, researchers from the International Renewable Energy Agency (IRENA), the University of California Irvine and the Colorado School of Mines outlined an ambitious scenario in which annual demand growth would be kept to an average of 1% a year, but warned that this “implies a sudden and drastic divergence in the historical relationship between aviation demand and expected population and economic growth.” Electric and hydrogen-fuelled aircraft are expected to have a part to play as well, particularly on short-haul flights. IATA estimates electric and hydrogen planes will allow it to meet 13% of its net zero ambitions, offsets and carbon capture 19%, and infrastructure and operational efficiencies 3%. New drop-in fuels are not the only tool available for decarbonising aviation, but SAF looks set to play the biggest role given the difficulties getting to carbon neutrality through other means.
TEXT Heather O’Brian IMAGES Jose Lebron, Unsplash & Suhyeon Choi, Unsplash