Green aircraft fuel and commercial aviation

From used cooking oil to synthetic paraffinic kerosene, sustainable aviation fuel is crucial to hitting global environmental targets, reports Nigel Pittaway

A recent United Nations Intergovernmental Panel on Climate Change (IPCC) report serves to highlight the importance of sustainable aviation fuels and the urgent requirement to reduce carbon global emissions.

The IPCC report released in August 2021 warns the Earth is likely to reach the threshold of 1.5°C warmer before 2035, ten years ahead of originally forecast and the findings form an important lens through which to view sustainable aviation fuel (SAF) and its role in slowing the effects of climate change.

The Air Transport Action Group’s Waypoint 2050 report identified several pathways that aviation could take to meet the sector-wide plan to reduce net CO2 emissions by 50% in 2050 ATAG

In 2019, before air travel was affected by the COVID-19 pandemic, aviation was estimated to contribute 2-3% of the total carbon emissions of the planet. While this does not sound like much, according to experts it represents around 900 million tonnes of carbon dioxide (CO2) annually, or around the same as Germany or Japan.

According to data and analytics company GlobalData, the advantages offered by SAF over conventional jet fuels, coupled with government incentives, will see the increasing adoption of SAF as a way to reduce carbon emissions.

Global imperative
Currently, the approved ratio of SAF to conventional fuel is 50%, but with the aviation sector expected to recover from the pandemic and resume a growth trajectory of at least 3% per year until 2050 or later, the sector is under growing pressure to increase this.

Nicolas Jeuland, ‘Future Fuels’ expert for aerospace group Safran, warned: “Air transportation will experience massive growth in the years to come and that’s the main issue.

“We have to place aviation on a trajectory that will lead us to conformity with all the environmental constraints. We will not be treated differently.”

The aviation industry experiences very long cycles of technological development, often taking a decade or more to design, test, develop and certify new aircraft, engines and technologies. If the climate predictions in the IPCC report are realised, this is time the industry may no longer have.

Jeuland explained: “We have an industry that is intrinsically difficult to decarbonise and, if we want to decarbonise it, it’s clear that there is an urgency to develop this [SAF] technology now. It’s not a question of waiting for a hypothetical technology to come to the market in ten years. Given the lifecycle of our industry, we have to develop this technology right now.”

While there are new technologies (at least in terms of aviation) being developed, such as hydrogen fuels or electric and hybrid-electric propulsion, the replacement of the vast fleet of aircraft in service globally today will not occur quickly, even if these new technologies are proven viable across the spectrum.

Therefore, it would seem the future of aviation will rely on a mixture of current and developing technologies, but SAFwill remain in that mix for many years yet.

Jeuland added: “The only solution to massively reduce the CO2 footprint is to develop SAF. It’s not a question of choice, it’s not a simple strategy, we don’t have any other solution.

Sean Newsum, Boeing’s director of environmental strategy, agrees: “We view SAF not as our only strategy, but certainly the point of emphasis for the actions we need to be taking now to reduce the long-term emissions of aviation,” he said. “This is because of the inherent advantage of the sustainable fuels approach to address the emissions of the existing fleet, in addition to the future aeroplanes we’re developing now and will be in the not too distant future.”

Defining SAF
There are seven different pathways to the production of SAF, but not all are equal and only five of those seven processes currently lead to a jet fuel that can be mixed with Jet A/Jet A1 up to a maximum blend of 50% without any modification of the engine or airframe. Three of those are being actively commercialised and are referred to as drop-in SAFs, because they require no aircraft or engine modifications.

Alaska Airlines
As part of Boeing’s ecoDemonstrator programme, a new Alaska Airlines 737-9 MAX is testing a range of enhancements aimed at reducing the impact on the environment Boeing

To define what a SAF is, GE Aviation’s fuel additives expert Gurhan Andac likes to describe what SAF is not: “SAF is not something other than Jet A/Jet A1. There is an assumption that it’s another fuel, something exotic, but it is actually Jet A/Jet A1. It’s the same kerosene-range hydrocarbon mixture, the same molecules. It just comes from a different feedstock, a different source,” he explained.

While the emissions from the exhaust of a gas turbine engine burning SAF are not less than an equivalent engine burning Jet A/Jet A1, its environmental impact is measured by the carbon footprint of its entire lifecycle, including the amounts sequestered by the feedstock up until it is processed.

Andac explained: “SAF blend is the synthetic blend component blended with a conventional jet fuel component and, if the synthetic component is sustainable, the end product is SAF.”

Therefore, consideration must be given to such things as the emissions generated during the production process and during transportation of the biomass to the refinery, as well as the traditional considerations inherent with the production of Jet A/Jet A1.

The processes involved
Although similar to the refinery procedure Jet A/Jet A1 undergoes, the processes for SAF differ according to the feedstock. One example of the current pathways is the hydroprocessed esters and fatty acids (HEFA) synthetic paraffinic kerosene (SPK) pathway, using feedstocks typified by waste cooking oil, fat and grease.

The HEFA SPK pathway is similar to that which produces bio-diesel for the road transportation sector, which has been growing at a significant rate for the past decade and is the most mature, albeit with some future limitations around adequate supply of feedstock. An emerging alternative is typified by the cellulosic process pathways, which use municipal, forestry or agricultural waste as the feedstock and can possibly use either the Fischer-Tropsch (FT) SPK process, or one under development by Fulcrum BioEnergy, which combines gasification technology with an FT process.

A further strategy is the Alcohol to Jet (AtJ) SPK fuel pathway, which uses a gasification process and then converts the gas into jet fuel by passing it through alcohol (either isobutenol or ethanol) as an intermediate, ultimately producing a drop-in Jet A/Jet A1 replacement.

Future technologies such as the PtL (Power to Liquid, sometimes referred to as E-Fuel) FT pathway, which requires electricity, water and a concentrated source of CO2 to produce, may also reduce the reliance on adequate stocks of biomass.

According to Boeing’s Sean Newsum, the combination of these different SAF pathways is the key to the future. He explained: “The combination of those technologies with agricultural waste, forestry waste and municipal waste from around the world has tremendous potential in respect to the volumes of SAF it can produce. The combination of the HEFA, FT and ATJ [alcohol to jet] pathways could collectively produce upwards of five million tonnes of SAF per year, which is enough to meet the long-term demand for aviation jet fuel to 2050. It’s not a question of whether there is enough feedstock out there, but whether you can actually make it practical and commercially viable.”

A trilogy of challenges
Safran’s Nicolas Jeuland explains that the overall pathway to the use of SAF has challenges that can be categorised under the broad headings of economic challenges, biomass availability and sustainability. Jeuland said: “It seems there are no limitations to developing the SAF pathways, but we have close to zero SAF currently on the market. It’s only a matter of market creation, but for the moment the production pathways are at a minimum of two to three times higher in cost than conventional fuels.”

The process behind PtL (Power to Liquid), sometimes referred to as E-Fuel Airbus

While it would be simple to close the gap in price by imposing levies or taxes, Jeuland warns that the aviation industry is an international one and this strategy, if applied locally, would result in market distortion: “This is an important problem that has to be solved at international level, will rely upon the voluntary adoption by each state and will take some time,” he added.

The issue of biomass availability has to be considered in conjunction with the needs of surface transportation, which has been a leader in the field of biofuel uptake for more than a decade. Emerging technological advances such as hydrogen fuels and electric propulsion have the potential to take the pressure off biomass availability, but Jeuland cautions these are not yet developed sufficiently for long-range commercial aviation and the solution must therefore be tailored to the individual needs of each transportation sector.

Jeuland added: “We have a sufficient biomass for aviation, but we have to decide the best way to decarbonise the entire transportation sector.

“In aviation we have a lot of constraints, but the fact is we don’t have a lot of solutions. Road transportation for example has other solutions available, such as electrification or the easier application of hydrogen fuels or biogas, etc.”

The third problem, that of sustainability, is linked to the availability of biomass, and has to take into consideration the entire carbon lifecycle, including competition with food production, in terms of land use and water resources, etc. “That’s why we should focus on municipal waste or forest residue but, while it’s not the most expensive feedstock it is the most difficult to transform into SAF,” he explained.

Developing a sustainable future
With the future of viable SAF likely to be a combination of drop-in and non-drop-in fuels, including combinations within each of those categories, the need for an international standard is critical. GE Aviation’s Gurhan Andac also chairs a task force within the American Society for Testing and Materials (ASTM) International, as part of an industry-wide effort to determine what 100% will look like in the future.

SAF Taxi
A rendering of an AirTaxi Hub: Boeing sees SAF as a key component of initiatives like this Boeing

Andac explained: “There will be drop-in SAFs that will be fleet-wide compatible and, possibly, there will be SAF that is only compatible with a subset of that fleet for various reasons – it might be more environmentally friendly or perhaps there may be local reasons for that fuel to make more sense. There will be local forces and factors that I think will dictate a solution that is multi-faceted. It will be an array of SAF options, I think.”

The roadmap to the future will be a staged process, with progression from the currently approved 50% blend up to 100% SAF as the first step, as Andac highlighted: “The initial efforts will be the approval of 100% SAF, either through just one pathway or by a blending of components and I think we will accomplish this in the next two to three years. Then we will look at the possibility of standardising the non-drop-in SAF and, because it is outside the Jet A/Jet A1 standard, it will not be compatible fleet-wide.”

While the recent UN IPCC report mentioned at the beginning of this article suggests there is a renewed priority on the early reduction of carbon emissions, there is still a lot of work to do to deliver a sustainable aviation transportation industry.

Boeing’s Sean Newsum explained: “We have believed for a while that the two key elements to accelerating SAF production and use are investment and incentives. The fundamental technology is there, the feedstocks exist, we know how to produce the fuels. What we don’t have are the right economic conditions to attract investors to invest in fuel production, so that producers and buyers are incentivised to produce and buy them.”

Newsum said government support and incentives are essential to allow investors, fuel producers and airlines to make the dynamics of the SAF business work and therefore accelerate its growth. He added: “There’s no one person or stakeholder in, or outside, aviation that can cause this to happen by themselves. It’s going to take everybody working together, and we’re committed to continue to work together with everybody across the sector.”

It seems certain therefore, that SAF is not an interim solution, but one which will be key to the future of a sustainable aviation industry even after hydrogen fuels and electric propulsion become a reality.

Safran and GE Aviation RISE up
Engine manufacturers Safran and GE Aviation are each making individual contributions to reducing carbon emissions through the development of SAF, but have also partnered in the CFM Revolutionary Innovation for Sustainable Engines (RISE) programme to develop powerplant technologies that will realise future efficiencies.

Safran, for example, is working towards the use of 100% SAF and will begin flight testing of a helicopter in the third quarter of 2021, followed by an Airbus A320 before the end of the year.

While drop-in SAFs have already been demonstrated, regulatory approval is required to increase the ratio beyond the currently approved limit of 50% and work is under way to explore the use of non-drop-in SAFs, which will require modifications to engines and airframe systems.

Safran’s group expert, ‘Future Fuels’, Nicolas Jeuland said: “By the end of 2021 we will use 10% of SAF in all our activities, especially new engines. By 2025, we have a goal of more than 35% SAF in all our activities. Clearly, this will cost a lot, but we’ve decided to do this to support deployment of SAF. It’s a good message to promote our new-build engines and show that we’re in no doubt about the compatibility of our technology. We don’t see the development of SAF as a constraint; it’s an opportunity to promote highly-efficient technology that is fully compatible and clearly there will be market opportunities.”

GE Aviation’s fuel additives expert Gurhan Andac described the roadmap his company has developed to develop, assess and validate the use of 100% SAF in engines, focusing on non-drop-in SAFs.

Andac explained: “We have a plan laid out for 100% paraffinic SAF when that becomes a reality and we’ve already flown with that type of fuel. For example, we flew with the Boeing ecoDemonstrator using 100% SPK [synthetic paraffinic kerosene] in 2018, with no modifications to the engine whatsoever. During the three-month flight test campaign there were no observed issues, but we do need to do a bit more work to assess and validate across all our platforms and our new and upcoming technologies.”

Under the CFM RISE programme launched in June 2021, GE and Safran aim to explore new technologies to reduce carbon emissions, including the development of Open Fan Architecture (OFA) engines and the study of alternate energy sources. The aim is to reduce fuel consumption and CO2 emissions by 20% compared with today’s engines and it is hoped the programme will begin testing a full demonstrator engine in the mid-2020s.

Boeing: leading by action
Reducing CO2 emissions through the lifecycle of SAFs can be augmented by modifications to airframe and engine design to make them more efficient and reduce fuel burn in the first instance.

LEAP is a high-bypass turbofan produced by CFM International, a 50-50 joint venture between GE Aviation and Safran. It features enhanced fuel consumption GE Aviation / Safran / LufthansaTechnik

As one of the world’s largest manufacturers of commercial aircraft, Boeing is investing in both strategies and, in January 2021, the company announced it has set an ambitious target to ensure its commercial aircraft are capable and certified to fly on 100% SAF by 2030.

Boeing’s director of environmental strategy, Sean Newsum, explained: “We have been improving fuel efficiency at about 2% per annum, going back multiple decades, and that has helped contain the growth of our CO2 emissions footprint, even as our overall business traffic has grown at 4-5% per year. As we get into the COVID-19 recovery on a global scale we expect air travel to recover to a similar level; we want to make sure we’re doing all we can to contain the growth of CO2 emissions as air traffic returns.”

Boeing began working with airlines to conduct SAF (then called biofuel) test flights in 2008, gaining regulatory approval in 2011. The world’s first commercial flight of an SAF airliner (a Boeing 777 freighter belonging to Fedex) was conducted in 2018.

The aircraft was part of Boeing’s ecoDemonstrator programme that has been running since 2012 and various test campaigns have since used a range of the company’s commercial products. The most recent campaign was launched in June 2021 and is being conducted in partnership with Alaska Airlines, using one of the carrier’s new 737-9 MAX aircraft to test a range of enhancements aimed at reducing the impact on the environment.

More recently, the airframe manufacturer announced in July 2021 that it has partnered with SkyNRG to focus on scaling the availability and use of SAFs. It will also invest in SkyNRG Americas’ SAF production plant in the US, which will be the first dedicated production facility in the country, supplying airports and operators on the West Coast.

Newsum added: “I believe we need to lead with action. The [SkyNRG] project isn’t going to make a huge difference in and of itself. We believe, however, that investing where we can, to attract other investors to produce fuels and help send the message that we really believe SAF is the key for decarbonising aviation, will result in a greater swathe of action across other stakeholders.”