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2022-09-01
Research Sustainable Aviation Fuel: Clean Way Forward for Green Air Travel
In the battle to reach net-zero by 2050, one thing has become clear: there is no single solution to get us there. In this three part research series, first we will discuss about possibilities of SAF fuels, then in the October issue hydrogen as a future fuel source and in the November issue about the third candidate that has so far been hiding in plain sight: ammonia.
At a time when the world is rapidly embracing green solutions to tackle threatening levels of carbon dioxide (CO2) emissions in varied sectors, sustainable aviation fuel (SAF) serves as a blessing on a crucial front — commercial aviation. SAF is a biofuel used to power aircraft that has similar properties to conventional jet fuel but with a smaller carbon footprint.
Commercial aviation is responsible for about 13 per cent of transportation greenhouse gas (GHG) emissions (U.S. EIA 2020b, 202). Airlines have committed to carbon-neutral growth for international commercial aviation beginning 2021 and reducing carbon dioxide (CO2) emissions by 50 per cent in 2050 compared to 2005 levels (Airlines for America 2020; IATA 2020).
While airlines are squeezing efficiency through improved aircraft and flight logistics, decoupling carbon growth will require non-fossil-sourced fuel, which is referred to as SAF.
SAF made from renewable biomass and waste resources have the potential to deliver the performance of petroleum-based jet fuel but with a fraction of its carbon footprint, giving airlines solid footing for decoupling greenhouse gas (GHG) emissions from flight.
The ground transportation fuel market (motor gasoline and diesel) is 183 billion gallons per year. The jet fuel market is different from the gasoline market in size, consumers, property variance, and demand locations. The global jet fuel market size exceeds 81 billion gallons. Passenger demand is projected to double over the next 20 years (IATA 2016).
In contrast to the gasoline market, the jet fuel market has a smaller number of customers (individual airlines and a modest number of fuel suppliers), who purchase the bulk of their fuel using negotiated long-term purchase agreements, and some small-volume customers (such as corporate fleets and private airplane owners) who secure fuel at retail prices.
Airlines are very price-sensitive because jet fuel accounts for approximately 20 per cent –30 per cent of their operating costs.
Some studies have suggested that a price increase of US$1.00 per barrel of crude oil results in roughly US$425 million of additional expense for the airline industry.
Storage Advantage
Purchasers have voluntary goals to decouple passenger growth and carbon emission growth. They view SAF as being critical to their future. Hence, there is a strong market pull for SAF. Jet fuels are delivered to airports via pipelines, stored in common tankage, and delivered to planes through hydrant systems at major airports.
SAFs are delivered as fully fungible fuels and once they enter the airport storage are in the same storage and delivery systems as all other jet fuels. Hence, SAF fits into current infrastructure.
The jet fuel market, although smaller than gasoline and diesel fuel markets, exceeds 81 billion gallons (3.4 exajoules of energy, or greater than three quadrillion British thermal units, or Btu). Market growth is expected to double over the next 20 years, while gasoline markets are not expected to grow.
The jet fuel market matches biomass availability. As biomass availability is expected to increase, so does the size of the jet fuel market. Unlike cars, the commercial jet fleet currently cannot be fully electrified with battery technologies. SAF will continue to be important into the 21st century.
Cost Factor
Fuel properties needed in SAF must meet three general requirements: Performance, operability, and drop-in compatibility. These requirements are essential for safety, general usage, and execution of commercial and military missions. The bulk properties of jet fuel are derived from the hydrocarbon classes that make up the fuel, which include n-alkanes, iso-alkanes, cycloalkanes, and aromatics. Aromatics have lower heats of combustion, do not burn cleanly—particularly in older hardware—and are responsible for up to 90 per cent of particulate emissions, which contribute to wear on combustor liners.
SAF fractions today are highly diluted with the conventional petroleum fuel and aromatic concentrations in the composite fuel, providing adequate swelling character. This means there is little reason for renewable fuels to contain aromatics to ensure seal swelling.
Cost cannot be understated in importance. SAFs with low (or no) aromatics offer greater performance attributes in many ways. However, current projected performance increases do not eclipse the increase in costs. High-quality SAFs must be near price parity to sell and scale in volume.
Sustainable Feedstocks
An estimated one billion dry tonnes of biomass can be collected sustainably each year in the United States, enough to produce 50–60 billion gallons of low-carbon biofuels. These resources include: Corn grain, oil seeds, algae, other fats, oils, and greases, agricultural residues, forestry residues, wood mill waste, municipal solid waste streams, wet wastes and dedicated energy crops.
This vast resource contains enough feedstock to meet the projected fuel demand of the U.S. aviation industry, additional volumes of drop-in low carbon fuels for use in other modes of transportation, and produce high-value bioproducts and renewable chemicals.
Multiple Benefits
Growing, sourcing, and producing SAF from renewable and waste resources can create new economic opportunities in farming communities, improve the environment, and even boost aircraft performance.
By growing biomass crops for SAF production, farmers can earn more money during off seasons by providing feedstocks to this new market, while securing benefits for their farms like reducing nutrient losses and improving soil quality.
Biomass crops can control erosion and improve water quality and quantity. They can increase biodiversity and store carbon in the soil, which can deliver on-farm benefits and environmental benefits. Producing SAF from wet wastes, like manure and sewage sludge, reduces pollution pressure on watersheds, while keeping potent methane gas—a key contributor to climate change—out of the atmosphere.
Many SAFs contain fewer aromatic components, which enables them to burn cleaner in aircraft engines. This means lower local emissions of harmful compounds around airports during take-off and landing. Aromatic components are precursors to contrails, which can exacerbate the impacts of climate change.
Variety of Technologies
SAF can be made with a variety of technologies, which use physical, biological, and chemical reactions to break down biomass and waste resources and recombine them into energy-dense hydrocarbons. Like conventional jet fuel, the blend of hydrocarbons in SAF must be tuned to achieve key properties needed to support safe, reliable aircraft operation.
In partnership with biorefiners, aviation companies, and farmers, researchers are developing novel pathways for producing SAFs from renewable and waste feedstocks that meet strict fuel specifications for use in airplanes and infrastructure.
Laboratory and industry partners are working to develop new SAF pathways and fuel formulations in order to enable testing and certification required to ensure these fuels are fully compatible with existing aircraft and infrastructure.
SAF from Wet Waste
Drawing on stores of carbon energy in cheap, widely available food waste, animal manure, and other wastes with high water content, SAF from wet waste is a carbon-negative fuel.
Every year, millions of tonnes of food waste is hauled to landfills across the world. Once there, it rots and produces methane, a greenhouse gas over 20 times more potent than carbon dioxide. Eliminating food waste as a source of methane can be a highly effective way at driving down landfill emissions. It can also be effective at producing fuel.
That task got a burst of energy with the publication of a new paper on carbon-negative SAF by scientists at the National Renewable Energy Laboratory (NREL), the University of Dayton, Yale University, and Oak Ridge National Laboratory.
Published in Proceedings of the National Academy of Sciences, the article outlines a biorefining process using the untapped energy of food waste and other wet waste to produce SAF both compatible with existing jet engines and capable of supporting net-zero-carbon flight.
GHG emissions created from jet fuel combustion are zeroed out by lifecycle GHG emissions removed or diverted from the atmosphere when producing the fuel.
“If our refining pathway is scaled up, it could take as little as a year or two for airlines like Southwest to get the fuel regulatory approvals they need to start using wet waste SAF in commercial flights,” said NREL scientist Derek Vardon, the corresponding author of the paper. “That means net-zero-carbon flights are on the horizon earlier than some might have thought.”
The result is a fuel refining process that offers airlines a quick win on reducing emissions in the short-term with smaller volumes of SAF, while also providing a long-term blueprint for higher SAF blends that enable deeper emissions reductions.
Polycyclic Alkane SAF
If upgraded with ultraviolet light and catalysts, bio-acetone made from a range of biomass resources, like corn stover or bioenergy crops, can yield SAF with 12 per cent more energy than conventional jet fuel.
Funded by the Bioenergy Technologies Office through the Chemical Catalysis for Bioenergy Consortium, chemists Andrew Sutton (now at Oak Ridge National Laboratory), Cameron Moore, and their team at Los Alamos National Laboratory (LANL) have found a solution for increasing the energy content of fuel derived from bio-based feedstocks.
As published in Sustainable Energy and Fuels, Sutton’s team members started with acetone, which can be produced efficiently from biomass via numerous biochemical routes. Three acetone molecules are bound together to make isophorone, which is a cyclic molecule.
Starting with two individual isophorone molecules, the scientists then used ultraviolet (UV) light to produce a more complex molecule that contains a strained cyclobutane. This process essentially traps the UV energy in the molecule, resulting in an overall energy increase. Sutton and his colleagues used chemical catalysts to remove the oxygen atoms to make polycyclic alkane molecules, which are suitable for jet fuel.
The LANL-developed bio-based polycyclic alkane molecules resulted in a sustainable fuel with a 12 per cent higher energy content than Jet-A, which fuels modern commercial airliners.
A renewable fuel that has more energy could make it possible for a plane to carry more cargo or fly further with the same amount of fuel.
3D Printing
A patented process for converting alcohol sourced from renewable or industrial waste gases into jet or diesel fuel is being scaled up at the U.S. Department of Energy’s Pacific Northwest National Laboratory (PNNL) with the help of partners at Oregon State University and the carbon-recycling experts at LanzaTech.
A single-step chemical conversion streamlines what is currently a multi-step process. The new PNNL-patented catalyst converts biofuel (ethanol) directly into a versatile “platform” chemical called n-butene. A microchannel reactor design reduces costs while delivering a scalable modular processing system.
The process would provide a more efficient route for converting renewable and waste-derived ethanol to useful chemicals. Currently, n-butene is produced from fossil-based feedstocks using the energy-intensive cracking — or breaking down — of large molecules. The technology reduces emissions of carbon dioxide by using renewable or recycled carbon feedstocks.
Using sustainably derived n-butene as a starting point, existing processes can refine the chemical for multiple commercial uses, including diesel and jet fuels, and industrial lubricants.
In a leap towards commercialisation, PNNL is partnering with long-time collaborators at Oregon State University to integrate the patented chemical conversion process into microchannel reactors built using newly developed 3D printing technology. Also called additive manufacturing, 3D printing allows the research team to create a pleated honeycomb of mini-reactors that greatly increase the effective surface-area-to-volume ratio available for the reaction.
UAE Initiative
The UAE has demonstrated its capacity to dream big and deliver on its promises, from the launch of its mission to Mars, to becoming a global leader in energy investment and infrastructure.
The UAE is ranked third in the world in terms of both Revenue-Tonne-Kilometres (RTKs) and Revenue-Passenger-Kilometres (RPKs), two of the most important metrics for calculating the impact of aviation, just behind the U.S. and China.
From an internal perspective, the UAE has invested an estimated US$ 270 billion (AED 1 trillion) in its aviation sector, which includes improvements to airport infrastructure and a fleet of 884 modern commercial aircraft.
The country is taking initiatives in developing SAF. Masdar, Siemens Energy and TotalEnergies signed a collaboration agreement in January to act as co-developers for a demonstrator plant project, which will be established at Masdar City, Abu Dhabi’s flagship sustainable urban development.
Masdar announced ahead of ADSW 2021 last year that it was collaborating with Abu Dhabi Department of Energy, Etihad Airways, Lufthansa Group, Khalifa University of Science and Technology, Siemens Energy, and Marubeni Corporation on the demonstrator plant initiative. The aim is that TotalEnergies will offer its expertise in SAF production, offtake and supply the partner airlines.
Since January 2021, the partners in the initiative have completed a range of evaluations on technology suppliers, feasibility studies and conceptual designs, while working closely with regulators on compliance issues. The aim is to proceed to the front-end engineering design (FEED) stage later this year.
By leveraging their respective areas of expertise across the energy spectrum, and their local and global market reach, the co-developers believe that the demonstrator project can pave the way to commercial production of SAF, helping to reduce production costs and making it commercially viable.
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