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The aviation industry accounts for 2.5% of global CO2 emissions, most of which come from the use of jet gasoline, which is converted into CO2 when burned.
To date, no feasible mitigation strategies to tackle the problem of aviation-related emissions have been developed. However, a recent experimental plant-based fuel may just be the answer… and it comes from the compound responsible for making trees the way they are.
AVIATION AND CLIMATE
While aviation accounts for around 2.5% of global CO2 emissions, its overall contribution to climate change is much, much higher. This is because air travel does not exclusively emit CO2: rather, it affects the climate in a number of more complex ways.
Aside from releasing CO2 from burning jet fuel, planes change the concentration of other gases and pollutants in the atmosphere, causing a short-term increase, but long-term decrease in ozone; a decrease in atmospheric methane; as well as emissions of water vapour, soot, sulfur aerosols and water contrails.
A recent study from 2020 aimed to quantify the overall effect of aviation on global warming, when all of these impacts and atmospheric influences were considered. This entailed calculating radiative forcing, or the difference between incoming energy and the energy radiated back to space.
Simply put, if more energy is absorbed than radiated, the atmosphere becomes warmer.
What the study found was that when combined, aviation accounts for approximately 3.5% of effective radiative forcing. In short, aviation is actually responsible for 3.5% of the warming we experience today.
Although CO2 gets most of the spotlight, it actually accounts for less than half of aviation-related warming. Over two-thirds comes from non-CO2 gases, particularly water vapour trails from aircraft exhausts, which account for the largest share.
Global emissions from aviation have increased a lot over the past half-century. However, air travel volumes have increased even more rapidly: since the 1950’s, aviation emissions increased almost seven-fold.
The much slower growth in emissions means aviation efficiency has seen massive improvements. These improvements have come from changes in the design and technology of aircraft; larger aircraft sizes, allowing for more passengers per flight; and an increase in passenger load factor, or how “full” passenger flights are.
BARRIERS TO CLEAN AIR TRAVEL
Here’s the thing: air travel is particularly hard to decarbonize. While solutions to mitigate emissions for many of the largest emitters, such as energy and road transport, exist and are in the process of being scaled, we don’t have proven solutions to tackle aviation yet.
There are some promising design concepts emerging. Airbus, for example, have announced plans to have the first zero-emission aircraft by 2035, using hydrogen fuel cells as a power source. Electric planes may be a viable concept, but are likely to be limited to very small aircraft due to limitations of battery technologies and capacity.
One thing is certain: innovative solutions may be on the horizon, but they are likely to be far, far, far off in the distance.
TREE FUEL: THE WAY FORWARD?
Researchers from Washington State University recently developed jet fuel derived from lignin, an organic polymer responsible for making trees tough and woody.
This plant-based fuel could potentially increase engine performance and efficiency, while dispensing with aromatics added to conventional fuels, which contribute to pollution.
The researchers examined fuel properties critical to jet engine operation, including seal swell, density, efficiency and emissions. The results? The likelihood that this experimental fuel could be combined with other biofuels in development, to eventually fully replace petroleum-derived fuels.
"When we tested our lignin jet fuel, we saw some interesting results," said Bin Yang, professor with the Washington State University’s Department of Biological Systems Engineering. "We found that it not only had increased energy density and content but also could totally replace aromatics, which are a real problem for the aviation industry."
Aromatics are associated with increased soot emissions and water contrails, which as previously mentioned, contribute more to the climate impact of aviation than CO2. If they’re so bad, why still use them?
Well, aromatics solve problems in jet fuel logistics that simply do not have any other answers, such as a certain density that other sustainable alternatives simply just do not. Most importantly, however, aromatics have an ability to swell the rings used to seal metal-to-metal joints in aircraft effectively.
The researchers’ patented process turns lignin from agricultural waste into bio-based lignin jet fuel, which could help the aviation industry reduce its dependency on increasingly expensive fossil fuels while also meeting higher environmental standards.
What’s most promising is that this experimental fuel’s properties offer increased fuel performance, higher efficiency, reduced emissions and reduced cost. The fuel also shows comparable sealant volume swell with aromatics, opening the door for a sustainable fuel alternative with virtually no aromatic content, low carbon footprint and high-performance characteristics.
Simply put, this process creates a cleaner, more energy-dense fuel, which is exactly the missing puzzle piece that sustainable aviation fuels have been looking for.
