The Future Of Aviation: Sustainable Alternatives To Jet Fuel

Sustainable aviation fuel (SAF) is a proven technology that could replace jet fuel and help the aviation industry achieve its target of net-zero carbon emissions by 2050. SAFs are biofuels that can be produced from plant or animal sources, or using electrochemical reactions between water and captured carbon. SAFs have similar chemistry to conventional jet fuel, and can be blended with fossil fuels and used on conventional planes without needing any new onboard technology. However, the production of alternative fuels remains minuscule due to the time, investment, and technology required to swap out a fuel as ubiquitous as kerosene.

Sustainable Aviation Fuel (SAF)

SAF is compatible with existing aircraft and infrastructure. It can be used in existing aircraft and infrastructure when blended with conventional Jet A. SAF is also compatible with modern aircraft as it is a drop-in fuel that can be directly blended into existing fuel infrastructure at airports.

SAF is produced from non-petroleum-based renewable feedstocks, including the food and yard waste portion of municipal solid waste, woody biomass, fats/greases/oils, and other feedstocks. SAF production is in its early stages, with a few known commercial producers. SAF production pathways include Fischer-Tropsch (FT) Synthetic Paraffinic Kerosene (SPK), Hydroprocessed Esters and Fatty Acids (HEFA-SPK), Hydroprocessed Fermented Sugars to Synthetic Isoparaffins (HFS-SIP), FT-SPK with Aromatics (FT-SPK/A), Alcohol-to-Jet Synthetic Paraffinic Kerosene (ATJ-SPK), Catalytic Hydrothermolysis Synthesized Kerosene (CH-SK or CHJ), Hydrocarbon-Hydroprocessed Esters and Fatty Acids (HC-HEFA-SPK), and Fats, Oils, and Greases (FOG) Co-Processing.

SAF is essential to achieving the aviation industry's target of net-zero carbon emissions by 2050. It is estimated that SAF could contribute around 65% of the reduction in emissions needed by aviation to reach this goal. Government policy plays an important role in SAF deployment, and incentives should be used to accelerate its adoption.

SAF technology faces challenges due to feedstock constraints, particularly the limited supply of hydrotreated esters and fatty acids (HEFA), which are crucial for SAF production. However, SAF developers are exploring more readily available feedstocks such as woody biomass and agricultural and municipal waste to produce lower-carbon jet fuel more sustainably and efficiently.

Synthetic paraffinic kerosene (SPK)

SPK can be produced in two main ways: using solid biomass or with an alcohol-to-jet (ATJ) process. Processing solid biomass using pyrolysis can produce oil or syngas, which is then processed into Fischer-Tropsch Synthetic Paraffinic Kerosene (FT-SPK). The ATJ process takes alcohols like ethanol or butanol and de-oxygenates and processes them into jet fuels.

SPK has several advantages over conventional jet fuel. It is non-toxic, with low sulphur and aromatic content, and has a high cetane number due to its high alkane composition. Its combustion is generally clean, without emitting nitrogen oxides and with reduced particulate emissions. SPK is also a "drop-in" fuel, meaning it is fully compatible with existing aircraft and fuel infrastructure.

While SPK shows promise as a sustainable alternative jet fuel, it also faces some challenges. The feedstocks necessary for SPK production, such as hydrotreated esters and fatty acids (HEFA), are in limited supply as demand increases. Additionally, SPK production is more expensive than conventional jet fuel, and the technology is still maturing. Nevertheless, SPK is an important part of the aviation industry's efforts to reduce carbon emissions and achieve sustainability goals.

Alcohol-to-jet (ATJ) pathway

The alcohol-to-jet (ATJ) pathway is a process for producing jet fuel from sugary, starchy, and lignocellulosic biomass (such as sugarcane, corn grain, and switchgrass) via fermentation of sugars to ethanol or other alcohols.

ATJ is one of four major aviation biofuel technologies that are currently technically feasible. The others are Fischer-Tropsch (F-T), hydroprocessed renewable esters and fatty acids (HEFA), and direct liquefaction (pyrolysis).

ATJ has several advantages over the HEFA process, which is currently the most common SAF supply pathway. Ethanol yield per acre is six times higher than oils used in HEFA, and US ethanol production is also much larger. However, several barriers are limiting the uptake of ATJ, including higher costs and the need to ensure a low enough carbon intensity of the ethanol source.

Despite these challenges, ATJ is still a viable technology for sustainable aviation fuel (SAF) due to the scale of ethanol production and the prospect of decreasing demand for ethanol-based gasoline. As a result, projects are forging ahead, with support from major investors like Bill Gates.

ATJ technology is continuously advancing, and with ongoing investments, innovative approaches, and supportive incentives, it could become increasingly competitive.

Fischer-Tropsch Synthetic Paraffinic Kerosene (FT-SPK)

The aviation industry's experience with FT-SPK has been positive, and it is now generically approved for use worldwide. FT-SPK has the potential to be "greened," meaning it could become mandatory in the future if legislation requires a minimum proportion of renewable carbon atoms in jet fuel. This type of fuel also provides an opportunity to diversify aviation fuel supply and meet world demand, especially as crude oil prices increase and resources deplete.

FT-SPK has been found to have excellent cold flow properties, which are critical for ensuring effective operation of aircraft at high altitudes. Additionally, FT-SPK has a high cetane number, which is a measure of the ignition delay of diesel during compression ignition. While FT-SPK does not currently contain aromatics, synthetic aromatics are on the horizon and are being considered for certification.

Overall, FT-SPK is a viable alternative to traditional jet fuel that can help reduce greenhouse gas emissions and improve air quality. With the aviation industry targeting net-zero carbon emissions by 2050, investment in sustainable aviation fuels is essential, and FT-SPK is a proven drop-in technology that can play a key role in achieving these goals.

Power-to-liquid (PtL) or sun-to-liquid (StL) processes

• Using renewable energy to power electrolysers and produce green hydrogen.
• Converting climate-neutral CO2, captured through methods such as Direct Air Carbon Capture, into carbon feedstock.
• Synthesising carbon feedstocks with green hydrogen through processes like Fischer-Tropsch to generate liquid hydrocarbons, which are then converted into a synthetic equivalent of kerosene.

Capturing and storing CO2 are integral to PtL production, as the recaptured CO2 is reused to create fuel, reducing "well-to-wheel" emissions by up to 90% compared to fossil fuels. PtL can also be transported and distributed via existing fossil fuel infrastructure, including pipelines and filling stations, and blended with conventional kerosene. However, PtL is currently produced at a high cost and small scale, which is expected to change as the green hydrogen ecosystem expands. Experts predict PtL will be ready for wider use between 2025 and 2030.

StL, on the other hand, directly converts solar energy into liquid fuel. The process involves concentrating sunlight using parabolic mirrors to reach temperatures of 1500°C, at which point water and CO2 are transformed into a synthesis gas of hydrogen and carbon monoxide. Advantages of StL include fewer conversion losses compared to PtL and simpler mechanics. However, StL relies solely on solar energy and requires complex mechanics to continuously adjust mirrors to follow the sun's path.

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