Sustainable Aviation Fuel: Part 1
Introduction to SAF
What if current transportation fuels could be replaced with alternatives that can be burned in existing engines but don’t add more planet-warming gases to the atmosphere? It may sound too good to be true, but the technology to produce sustainable fuels already exists. The question is whether it can be cost-effective, energy-efficient, and scaled to meet the rising global demand.
Over the next few months, this new blog series from Prime Movers Lab will explore the emerging developments and technologies to produce sustainable fuel, in particular sustainable aviation fuel (SAF). In Part 1, we explain how sustainable fuels work. We introduce the advantages of SAF and offer a holistic picture of a sustainable fuel lifecycle, which will set the stage for subsequent posts detailing the ways to produce SAF, their shortcomings, and how new startups are innovating to make sustainable fuel a reality at scale.
Why SAF and Why Now
As governments and consumers signal greater demand for sustainability in every part of the economy, aviation is no exception. Major airlines and industry trade associations have set targets of net-zero carbon emissions for air travel by 2050, primarily by switching to SAF such as biofuels or synthetic kerosene. Government incentives are also making the space attractive for investment. The 2022 Inflation Reduction Act (IRA), for example, offers tax credits of up to $1.75 per gallon (compared to jet fuel costs ranging from $1 to $4) depending on the extent of reduction in CO2 emissions.
Yet global SAF production was only 80 million gallons in 2022, less than 0.1% of the ~100 billion gallons of aviation fuel consumed annually. More than 130 SAF projects have now been announced that could expand annual production to as much as 18 billion gallons in 2028, but sustained growth and innovation are clearly needed to achieve net zero emissions targets.
Why not transition away from hydrocarbon fuel like today’s kerosene jet fuel and towards hydrogen or battery-powered aircraft? A big reason is energy density. The space and weight constraints of commercial airliners require a power source with high energy density, especially for long-haul flights which contribute the bulk of the industry’s CO2 emissions. Hydrocarbon fuels are inherently advantaged in this regard. Compared to kerosine jet fuel, today’s batteries supplying equivalent energy would weigh 40 times as much. Liquid hydrogen would take up 4 times the amount of space and would have to be stored at a temperature of -250°C.
Hydrogen propulsion also faces a chicken-and-egg problem. Airlines need to invest in new aircraft which still require decades of development, but they won’t do it until the infrastructure is ready for all of the airports where they might be used. And hydrogen may not be the clean-combusting fuel some imagine. Since it burns so hot, it can form more environment-damaging NOX emissions than hydrocarbon fuels do. Companies like ZeroAvia are paving the way for hydrogen-powered aviation with solutions like H2-electric engines, and we are watching and hoping for success. However, there is value in enabling decarbonization with existing aircraft and infrastructure.
That’s not to say all the advancements and investments in clean energy and green hydrogen don’t have a role to play in clean air travel now. On the contrary, hydrogen is a key ingredient in most of the approaches to produce SAF, and recent innovations and incentives to reduce the cost of electrolysis are key enablers for why SAF has been receiving more attention. In that sense, hydrocarbon fuels can be thought of as a kind of “liquid battery” that is especially stable, transportable, and dense. But to put it in perspective, one gallon of kerosene would have the energy capacity of nearly 10,000 AA batteries (although electric motors are more efficient than turbine engines at using that energy).
Carbon-based Fuel as Part of a Circular Energy Economy
To see how SAF like biofuels or synthetic kerosene can be a part of a circular energy economy, it helps to think a bit about the chemistry of fuels and combustion. Transportation fuels are hydrocarbons, molecules containing primarily hydrogen and carbon atoms bonded to each other in chains of various sizes and shapes. When hydrocarbon fuel burns in an engine, it reacts with oxygen in the air. The hydrogen and carbon atoms break apart from each other and form new bonds with oxygen, producing water (H2O), carbon dioxide (CO2), and lots of heat (energy). The expansion of hot gases can be harnessed to, e.g., spin a turbine, push a piston connected to a drivetrain, or propel an aircraft from the thrust of the gases.
The path to sustainable fuel is exactly the reverse of combustion: energy is harnessed to convert the ingredients of water and CO2 back into fuel while putting oxygen back into the air. Whether this process happens inside a photosynthesizing plant or using electricity, the process is remarkably similar. Green plants and algae use the energy of the sun to split water, using hydrogen to fix CO2 into carbon-containing biomass like wood, starches, or oils, which can be converted into biofuels. Electricity can be used instead of sunlight to split water and captured CO2, first forming intermediates like hydrogen gas (H2) and carbon monoxide (CO) or methanol and then ultimately synthesizing a kerosene equivalent fuel. This latter process is often called power-to-liquid (PtL). In either case, H2O supplies the hydrogen content, CO2 supplies the carbon content, oxygen is removed, and energy is required.
The overall process could be circular and sustainable if all carbon-containing compounds in the process originated from atmospheric CO2. This would mean all chemical reactors and other equipment in the production process would run on zero-emissions electricity and/or burn only fuels that are likewise sustainable. In practice, there are still lifecycle emissions inherent in all currently available renewable energy sources. Every step in a SAF production process must therefore be highly efficient, as any inefficiencies impact the yield and require overbuilding the feedstock or clean energy supply. Other barriers to sustainability (and cost) include whether the fuel composition of SAF can truly allow drop-in replacement and how to address other warming effects from aviation contrails related to fuel composition.
In the next few months, we’ll dive into these challenges and the technologies and tradeoffs of different SAF production methods in future posts. In the meantime, we at Prime Movers Lab are surveying the landscape and would love to hear from startups innovating in the SAF space. If you are a company developing sustainable fuels or adjacent technologies, please reach out at mikeb@primemoverslab.com.
Prime Movers Lab invests in breakthrough scientific startups founded by Prime Movers, the inventors who transform billions of lives. We invest in seed-stage companies reinventing energy, transportation, infrastructure, manufacturing, human augmentation, and agriculture.
