While greening the world’s road transport has been at the top of the climate agenda for more than a decade, air travel is just now coming into focus. Part of the reason why air travel has mostly flown under the climate radar is its limited role in our personal carbon emissions — with the vast majority of people being infrequent flyers at most. In fact, aviation is not included under the Paris Agreement.
But with air travel responsible for roughly 12% of all CO2 emissions from transport, and stricter international regulation on the horizon, the industry is increasingly seeking sustainable alternatives to petroleum-based fuel.
We’ve put together an overview of the most common alternative aero fuels, and a few examples of promising projects:
AERO FUELS 101
- What’s used today? Planes mainly run on two forms of fuel: aviation gasoline and aviation kerosene. The former is limited to smaller airplanes that have spark ignition engines, such as smaller commercial or private planes; while kerosene, or “jet fuel,” is used to power medium-to-large turbine engines. Both these fuels are made from non-renewable petroleum derivatives, providing an estimated yearly 177 billion liters of aviation fuel per year.
- The alternatives. Although the aviation industry has been slow to face the music of the new environmental era, research projects for alternative fuels have grown significantly in the last decade. The majority of these focus on the development of viable biofuels — meaning any liquid fuel derived from biological material such as trees, agricultural wastes, crops, or grass. Other emerging fuels include hydrogen and synthetic kerosene.
PROS
- Climate. The most promising aero-fuel alternative biofuels could, if derived from the right types of biomass and with sustainable conversion processes, drastically reduce CO2 emissions. Some estimates suggest up to an 80% reduction. It also limits other atmospheric pollution such as sulphur and nitrogen oxides.
- Costs. Fuel costs are a major share of the total expenditures of aviation companies, and diversifying into alternative fuel sources could offer a hedge against fluctuating oil prices. In addition, increasingly stringent climate regulation is bound to drive up the future costs of fossil-based fuels.
- Energy Security. A broader and geographically decentralized mix of viable aero fuels would undermine dependence on foreign oil in many countries — alleviating geopolitical tensions while reducing the risk of resource scarcity.
- Social benefits. The aviation industry could contribute to growing the sustainable energy sector — creating new job opportunities and advancing research towards a greener future.
CONS
- Production cost. As sustainable aero fuels are still in their nascent stages, the use of biofuels in aviation still suffers from high production costs — mostly due to lack of infrastructure and complex approval protocols.
- Compatibility. Not all current alternatives are fully compatible with existing storage conditions or transport methods, and there’s also the issue of engine compatibility. For example, the combustion engine in a car typically has low energy demand, while airplane engines tend to be in high demand 80-90% of the time. This entails strict requirements for the characteristics of aviation fuels, such as low explosion risk and good thermal stability.
- Environmental challenges. The legitimacy of climate-friendly fuel alternatives of course hinges on sustainable practices throughout the production supply chain. For example, the harvesting of edible biomass should be regenerative and involve limited use of fertilizers and insecticides. Both agricultural sourcing and synthetic production must include responsible stewardship of water resources, while all chemical processes should maximize circular use of toxic materials.
THE MAIN AERO-FUEL ALTERNATIVES
Alcohol-to-jet (ATJ)
Alcohol-to-jet ATJ allows for the conversion to jet fuel from alcohols such as methanol, ethanol, butanol, and long-chain fatty alcohols. Of these, ethanol is currently viewed as the most promising aero-fuel alternative due to relatively low cost, high global production and extensive logistical infrastructure.The process begins with the production of alcohols from biomass feedstocks, like sugar cane or fermented corn grain, which is then turned into jet fuel by de-oxygenation and purification.
Project highlight California-based Byogy Renewables has built one of the world's largest ATJ demonstration facilities in Japan where cellulosic bioethanol will be converted into sustainable jet fuel. The company’s plan is to use a net-zero, closed-loop process to deliver jet fuel that doesn’t require any blending with standard fuel.
Food-to-fuel.
Another developing way of leveraging biomass to green the aviation industry is turning food waste like cooking oil, animal manure and wastewater sludge into jet fuel. This technique would sap greenhouse gas emissions both by diverting methane-emitting waste away from landfill while replacing polluting jet fuels — potentially lowering the carbon footprint by 165%, according to a recent study. Wet waste is energy-rich, and research suggests it could be converted into volatile fatty acids that can then be upgraded to jet fuel.
Project highlight. Researchers from University of Dayton have teamed up with the U.S.
National Renewable Energy Laboratory to take another step toward turning wet food waste into a sustainable aviation fuel.
The research demonstrates that U.S. wet waste has enough energy content to
replace about 20% of the country’s jet fuel consumption while meeting quality
standards. The team aspires to someday blend up to 70% of its sustainable
product with conventional jet fuel.
Hydrogen.
Airline manufacturers are also plugging into the breakthroughs in the hydrogen energy sector, with research suggesting that the abundant resource has the potential to match the flight distances and payload of the current fossil fuel aircraft. If derived from renewable resources like sun and wind power, hydrogen — with an energy-density almost three times higher than gasoline or diesel — could work as a fully sustainable aviation fuel that emits only water into the atmosphere.
Project highlight. Californian fuel-cell
specialist HyPoint has unveiled the first operable prototype version of its turbo air-cooled hydrogen system to power
electric aircrafts. The system will be able to achieve up to 2,000 watts per
kilogram of specific power, equivalent to two to four hours of flight time
depending on plane size and hydrogen storage capacity. HyPoint expects the
system will be ready for testing at the beginning of 2022 and commercialized in
2023.
Synthetic kerosene.
While the kerosene currently used to power jet engines is derived from petroleum, it’s also possible to make sustainable, synthetic kerosene by mixing hydrogen with
CO2 . Earlier this year, the world’s first commercial passenger flight partly powered by sustainable synthetic kerosene flew from Amsterdam to Madrid. The fuel was produced in Shell’s Amsterdam-based research center, where green hydrogen was mixed with captured
CO2 and converted into synthetic wax which is then cut into tiny molecules used as building blocks for the sustainable kerosene.
Project highlight. ENGIE has joined forces with German electrolysis company Sunfire and leading partners from the aviation sector
to build a French subsidiary to produce synthetic kerosene. The product will be made from locally sourced renewable electricity, water and biogenic CO2. The first production unit will be located in Normandy, and a share of the synthetic kerosene will be used for research and certification purposes by French aviation industry stakeholders.
