The Biofuel Revolution: What Plant-Based Propulsion Means for Novel Aircraft Design

What do you picture when you hear sustainability? Electric cars, green factories, maybe even solar-powered homes? When we think of carbon reduction, many of us rarely think about aviation. Aircraft produce trillions of tons of carbon each year despite most of the world agreeing to decrease greenhouse gases via the passage of UN Sustainable Development Goals 7, 12 and 13. Unlike other industries, the aviation sector has already begun decarbonization. Why? Fuel prices directly influence ticket prices and profitability. The joy of travel has been kept from most of the world because of economics. If there was a way to reduce carbon costs of propulsion, passengers could travel for less while airlines could keep or increase their profit margins. Luckily, there is a solution: biofuel.

What is Biofuel?

Biofuel is fuel made from natural resources. A mix of organic substances forming cellulosic ethanol, a type of biofuel that includes straw and corn, has the potential to emit 86% less greenhouse gases than traditional kerosene [1]. Despite current higher prices, both climate activists and industry professionals are pushing to increase its usage. For example, each of the eight hundred flights that depart from LAX (Los Angeles International Airport) each day use a blend of sustainable aviation fuel (SAF) [2]. With innovation happening daily, it is very likely that prices will be similar to that of kerosene in the coming decades [3]. Currently, the only commercially viable option is to have a low-biofuel density blend with Jet-A fuel, which is a fuel blend traditionally used for turbofan aircraft. With billions in funding and public support for carbon reduction, the call to develop aircraft capable of running entirely on sustainable fuels has expanded. In this article, we will discuss not only how current market trends make sustainable aviation feasible, but also the changes the entire industry must make to achieve a net-zero carbon future.

How Can Biofuel Be Used Today?

Figure 1 – The engine of a Singapore Airlines A350

This aircraft is noted for its efficiency. Reprinted from Ref [6].

Manufacturers today are trying to build more efficient aircrafts [4]. In spite of the innovation over the last decade, fuel prices are still around 20-40% of an airline’s operating costs [5]. Manufacturers have already come up with many ideas to combat fuel burn that would also be useful in a jet that would run solely on SAF. First of all, researchers looked into the part of the aeroplane that actually uses the most energy: the engine (Fig. 1). 

In an airliner’s engine, kerosene is compressed and ignited to create a jet (a rapid stream of liquid or gas forced out of a small opening). However, not all of the air goes through the interior. Much of the surrounding atmosphere is fanned through large turbofans around the centre. The proportion between the amount of air that flows around the core to the air that flows into the core is called the bypass ratio [7]. A higher bypass means that the engine will burn less fuel because its thrust-specific fuel consumption (TSFC) – or the amount of fuel used to accelerate the aircraft – is decreased [8]. This is due to the fact that these larger turbofans can also be used to provide propulsion. If less fuel is needed, biofuels will be able to step up and improve the efficiency further. Because SAF has lower energy density than Jet-A [1], the larger, more economic engines will be able to maintain performance and possibly range. If an aircraft does not need a lot of power or a long range, a smaller engine works best due to its increase in aerodynamics. To achieve a small bypass ratio, the engine compressor will also be cut down [9].

The heaviest part of the aeroplane, however, is the fuselage. More energy will be spent trying to accelerate it than any other part of the aircraft. Therefore, lighter materials for the fuselage have been sought for since the dawn of commercial air travel. Recently, composites made from carbon fibres and resin are the materials of choice for jetliners – fuselages made from these compounds reduce the structural weight by 42% compared to aluminium [10]. Even biofuels that lack the energy density of Jet-A will be able to accelerate and power the aircraft for long periods of time. 

Finally, reducing the fuel consumption that is required to keep a plane flying level would allow biofuels to be used without increasing the size of tanks. Wings with high aspect ratios (the measure of how long and slender a wing is) achieve greater lift and less induced drag [11]. Climbs, cruise, and descent are all easier with these large wings. Fuel efficiency is drastically improved, enabling SAF to have a greater role in the future of aerodynamic propulsion. By flying at lower speeds, the aircraft will be able to align with the runway closer to the airport. If all of these features are combined with modern navigation and biofuel injection systems, carbon emissions could be reduced significantly from traditional aeroplanes.

Future Technology

Despite the clear advancements in sustainable technology in the past few decades, biofuels are not yet feasible options. Both cost and energy density continue to plague any program that deals with SAF. The solutions listed above act only as a bandage over a deep gash in the neck. To make significant progress, radical design changes for aircraft must be made. 

One such idea is the flying wing. Although not a new idea to the aviation world, a wing-fuselage aircraft has only been used by the military. However, a paper from the ICAS 2000 Congress states: “claimed advantages of these configurations are drag reduction, increased useful load, short airfield capability, noise reduction, [and] cuts in direct operating cost” [12]. The reason flying wings are so efficient is because of their inherently large wing structure, which has benefits listed above. It also combines the rear stabilisers with the ailerons, providing even greater weight loss. Nevertheless, issues with control and stability have been reported in the B-2 Spirit, the only version of this concept actually in production.  With heavy research and funding from airlines and aircraft manufacturers, it is likely that these problems will be solved. KLM Royal Dutch Airlines has partnered with Dutch university TU Delft to create the “Flying-V,” which purportedly will use “20% less fuel than the Airbus A350” [13]. 

Other designs have also been proposed, such as the X66-A by Boeing and NASA (Fig. 2). Development was announced in mid-2023 and features a long, truss-braced wing. Despite the project costing over a billion dollars, the sponsors claim a 30% reduction in emissions from leading aircraft today [14]. The X66-A is actually one of the first environmentally focused designs in the famed history of X planes, showing the industry wide commitment to sustainability.

 Figure 2 - X66-A rendition.

This environmentally focused design claims to offer a 30% reduction in emissions compared to other aircraft today.  Reprinted from Ref [15].

What Fuel Should We Use?

Finally, the question of which biofuel to use may be as important as the previously mentioned technological advancements. All novel fuel concepts have their own advantages and disadvantages. The three most common propulsion compounds are ethanol, biodiesel, and hydrogen.

The prime SAF is ethanol. Usually combined with Jet-A fuel in airports, it has been found that US corn ethanol achieves around fifty percent less CO2 emissions when compared to gasoline [16]. Because a majority of this hydrocarbon comes from corn, sustainable agricultural practices “can reduce ethanol-to-jet fuel emissions to 153% lower than petroleum jet fuel, [meaning] biofuels could be not just net-zero, but net-negative [in] carbon emissions” [16]. Because it is so abundant and effective, it is no surprise that ethanol is the leader in SAF. However, kerosene produces around 15,000 more megajoules per metre cubed than ethanol [17]. In addition, it is important to look at whether the ethanol is sourced sustainably. While ethanol is cheaper than Jet-A fuel, its low energy density means that it would actually cost more to travel the same distances.

Another popular substitute is biodiesel, which is a biodegradable fuel made from vegetable oils and animal fats. B20, a variant that combines 20% biodiesel and 80% gasoline, produces 15% less CO2 than pure gasoline [18]. Nonetheless, B20 is more expensive and has around 2% less fuel economy than the gasoline benchmark [18]. 

The most promising alternative fuel is hydrogen, which has potential to be a near-zero greenhouse gas emitter. Found abundantly in water, hydrogen has 2.5 times more energy per kilogram than kerosene [19]. There are many drawbacks to this perceived solution. First of all, the volumetric energy density of hydrogen gas is four times less than that of Jet-A fuel, meaning current fuel tanks would need to be expanded to impossible volumes [20]. A proposed solution comes in the form of compact liquid hydrogen, but comes with another set of challenges. Liquid hydrogen must be kept below -253°C, which creates problems for transport and cooling [19]. Massive changes to airport infrastructure and even more radical design changes must be made to achieve a hydrogen future. However, by 2035, hydrogen airliners would be 2% cheaper to run than kerosene flights after oil taxes [21]. 

However, owing to the fundamental overhaul of the aviation world as we know it, hydrogen fuel will not be the answer in the short term. Climate change is not going anywhere soon; therefore, biofuels seem to be the best answer to achieve net zero emissions by 2050. While ethanol may be cheap, biodiesel’s high energy density means that it will likely carry the flag of sustainable aviation in the coming decades.

Opinions on SAF

Figure 3 – A  private Cessna 172 propeller aircraft flies overhead.

This model of aircraft does not run on biofuels. Many pilots don’t have widespread access to biofuels and remain hesitant about the use of SAFs. Reprinted from Ref [23].

There remain different views from researchers and the pilot community on the benefits of SAF. The United States government has promised to increase production from 4.5 million to three billion gallons of SAF by 2030, which would cause prices to drop significantly [22]. As one of the main concerns to using biofuels is the cost, a reduction in cost would make SAF more appealing to airlines, and emissions would decrease. Some private pilots, on the other hand, seem to be more resistant to the biofuel revolution. The operator of a Cessna 172 (Fig. 3) based in San Carlos Airport (which has very limited access to biofuels) shared his opinion, stating: “as far as I know, aviation doesn’t contribute much to the total greenhouse gas emissions of the U.S. And with [SAF] prices so high, I don’t think it makes much sense [for me] to go green”. Although it is true that the aviation sector does not emit the levels of carbon dioxide that automobile emissions account for, it remains much harder to decarbonize, as high prices and lack of availability are pushing private pilots away from the net-zero future.

To contrast, Shirly Marquardt, director of an Alaskan economic development organisation, believes “there’s opportunity for a kelp [biofuel] industry in Alaska, which could diversify our economic base and add big benefits to coastal communities” [24]. While some kelp is eaten, a vast majority is currently unused. Using abundant natural resources will increase the supply and, thereby, lower the cost of SAF.

Finally, the opinions of those who fly regularly, and those who do not, vary vastly. A frequent flyer in India stated that he “cared more about getting from point A to B” than carbon reductions, while a customer who testified that he seldom travels on an aircraft understood that “aviation is one of the reasons [the planet] is warming up” and should therefore be limited.

These differences in opinion highlight the need for accessible and widespread climate education, and for continued biofuel innovation.

Conclusion

Biofuel innovation is critical towards a more sustainable aviation industry, and - advancing many of the UN Sustainable Development Goals at once - SAF use will continue to expand in the coming years. The aviation industry is already prioritising fuel usage reductions, enabling aircraft with low density propulsion to have the same range and performance as current designs. Biofuels will be able to further reduce carbon emissions, and alongside sustainable farming methods, can possibly reach net zero even with kerosene fuel blends. A push towards a more sustainable aviation industry benefits people and planet, and goes beyond the aviation sector; every country in the world will reap the rewards in both tourism and trade.

If you are interested in the potential of biofuels, what can you do now? Studying chemical or biological engineering will give you a strong footing within the world of novel propellant development. With government and industry support, as well as a large interest in biofuels with today’s youth, it is time to envision a future painted green with plant-based fuels.

Bibliography

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