The Greening of Aviation: Which Sustainable Fuels Hold the Most Potential?

February 08, 2023

The aviation industry’s carbon footprint accounts for 2.1% of all carbon emissions and 2.5% of greenhouse gases. Aviation has proven one of the hardest industries to decarbonize, but it’s important to keep trying, as commercial and freight aviation collectively made up 3.5% of greenhouse gas emissions in 2021. In other carbon-heavy transportation industries – railroad, shipping, and trucking – hydrogen power and electrification have proven sufficient but do not provide the amount of high-density energy needed to power mid-to-long-range flights with the currently available and affordable technology. 

Despite these challenges, the aviation industry is currently putting a large amount of time and investment into finding greener alternatives to traditional carbon-emission-producing fuel. 

Green hydrogen and sustainable biofuel are two areas of investigation that could lead to a significant reduction in aviation emissions. At first glance, green hydrogen seems like the superior solution as it has nearly limitless potential and could not only contribute to reduced greenhouse gas emissions but also create an entirely new segment of the energy industry. However, the current obstacles facing widespread green hydrogen usage make it unlikely to become aviation’s go-to energy source for quite some time, leaving sustainable aviation fuel to stand out as the current, most realistic, and actionable solution. The International Air Transit Association (IATA) estimates that sustainable fuels could lead to a potential decrease of 65% in carbon emissions by 2050.

Green Hydrogen: An excellent choice…someday

Green hydrogen is completely carbon-emission free. Not only does green hydrogen boast an eco-friendly fuel source, it also holds the potential to create a new energy market entirely as early-stage research indicates the fuel source could offer benefits far beyond the aviation industry. However, the price tag on electrolysis products in conjunction with an immature market and a lack of renewable energy is an undeniable deterrent to the fuel source’s potential use today. Not to mention the challenge of storage and transportation. 

Today, hydrogen is used globally to power coal combustion and natural gas extraction. These two hydrogen fuels – named black and gray respectively – are sourced from coal or natural gas with or without a carbon capturing storage unit, (CCS). A CCS captures the carbon dioxide emissions created as a result, making this form of fuel slightly more environmentally friendly. While black hydrogen heavily contributes to greenhouse gas emissions. Its harmful side effects are well known, but the raw market materials and inexpensive cost keep it as a main player today. 

Unlike preceding hydrogen fuels (e.g., “black” and “gray” hydrogen), green hydrogen is made through the process of electrolysis – electrochemical water splitting – which uses electricity to decompose water to oxygen and hydrogen gas. This process uses renewable energy (solar, wind, hydro), to power the electricity used in the decomposition process, making this fourth generation of hydrogen fuel more circular and sustainable. 

Organizations with a long-term commitment to sustainable fuels are investing heavily in green hydrogen as a possible “platform” technology of the future (see Figure 1). 

Figure 1: R&D activity in Green Hydrogen, 2018-2022. Source: Wellspring Scout

This investment has ramped up significantly in the past few years, and the sudden onrush of interest and activity has created an unusual pattern, in which we see everything from early-stage scientific grants to downstream patent activity ramping up sharply all at once. Not only has public appetite for decarbonization passed the tipping point into mainstream interest, but also there are various signs of technical progress that have revealed at least a vision of how green hydrogen could reach large-scale feasibility. For such reasons, large players including the US Department of Energy have made large moves into space, in hopes of catalyzing breakthroughs. 

Nonetheless, the green hydrogen market has multiple obstacles downstream – cost, storage, transportation – that must be tested, tweaked, perfected, and scaled before the fuel source is a viable option. The price of energy is high, but the price of sustainable energy today is higher still. Sustainably powering electrolysis production with green energy is currently not feasible for the industry, and in order to scale production the cost must diminish greatly. Additionally, the physical equipment required for electrolysis has a high price tag. 

Unfortunately, price isn’t the only argument against green hydrogen, as the element’s liquid form has specific thermodynamic requirements that current high-pressure tank storage capabilities do not meet. Further, due to the substance’s low volumetric density, scalable transportation is another obstacle that remains to be overcome. 

Sustainable Aviation Fuel: the next best thing 

Traditional jet propellant is made from fossil-fuel-based refined oil, which produces 3.16 kg of carbon dioxide per 1 kg of fuel burned. Sustainable Aviation Fuel (SAF) is made from sustainable feedstocks. To date, there are four generations of these types of biofuel, but the latter two (third and fourth) have proven themselves the strongest contenders as potential options for airline decarbonization. As a result, the R&D activities surrounding these technologies have begun to increase steeply in the past few years (see Figure 2).

Figure 2: R&D activity in Sustainable Aviation Fuel, 2018-2022. Source: Wellspring Scout

The first-generation of biofuel stemmed from perennial grass, fast growing trees, and used cooking oils. This phase of SAF has been around the longest and therefore has a well-defined, affordable process of production but it greatly contributes to fossil-fuel pollution and disrupts the agricultural industry. Second-generation biofuel burns 90% less carbon than the traditional fossil-based fuel; its energy source stems from wood and forestry waste, eliminating both the large pollution and agricultural issues. However, these sources are limited, and the current rate of biofuel production will quickly cause a shortage of supply. 

Third-generation biofuel stems from a combination of organic and synthetic sources, specifically the growth of micro and macro-organisms such as algae and kelp respectively. Algae holds three key scalable properties – it requires a small growth space, it has a high yield potential, and it has genetic modifiability. Algae produces vitamins, feedstock, and as of recently, biofuel. Algal biomass is unicellular, and it contains hydrogen, oxygen, nitrogen, and carbon dioxide. The current emerging technologies behind its cultivation fall to biochemical conversion, chemical reactions, direct combustion, and thermochemical conversion. 

Eni – an Italian oil and gas company received €10 million research grant from the Karlsruher Institute for Science in 2021 with three main objectives: to focus on sustainable feedstock sources, to prove the potential for algal biomass to fuel commercial jets, and to reassure EU policymakers that this third generation of biofuel is a sustainable solution for reducing the airline industry’s carbon emissions. 

In July of 2022, Eni went on to publish a patent focused on advancing third generation biofuels through an integrated process for the cultivation of algae/plants and the production of electric energy. The six step process involves a luminescent solar concentrator to trigger a photovoltaic cell capturing electric energy to queue solar radiation within a greenhouse, artificially triggering photosynthesis. The algal biomass is then captured from the cultivated aqueous suspension. 

Genetic engineering is becoming increasingly popular with each new evolution of sustainable biomass. The fourth generation of biofuel (FGB) is made from genetically modified organisms – namely algae’s oil content. Biomass must be changed chemically and physically before it can be of use. Previously, algae submerged in the Earth has naturally been exposed to high temperatures and high pressure, which produces oil. The Pacific Northwest National Laboratory mimicked this process by taking algae and putting it under these same artificial conditions for just over an hour, which successfully produced crude oil, water, and other reusable byproducts. The finished hydrocarbon product combined with gasoline is a sustainable replacement for traditional engine fuel. The GMO process is a key performer in improving oil accumulation. The most advantageous aspect of FGB is that the process not only replaces fossil fuels, but captures and stores carbon dioxide as well. 

Through pyrolysis, gasification, and digestion, the carbon dioxide undergoes carbon sequestration: moving the greenhouse gasses to depleted oil fields, unmineable coal seams, or saline aquifers to store – potentially for millennia. Then, the fuel is upgraded through liquefaction and gas cleaning. The final product is an extremely clean carbon-negative fuel. This output is more circular than other forms of alternative green energy as wind and solar power are net zero at best, and in practice traditionally carbon positive, albeit in small amounts. 

Today, there are exciting opportunities and technologies globally investigating where fourth-generation sustainable biofuel can shape the future of flight. One organization that is actively changing the limited resource narrative is Metafuels, a Swiss aviation company committed to changing the aviation industry through SAF. They’ve developed Aerobrew, the technology behind the conversion of green methanol to SAF, or as they call it, e-kerosene. 

In conjunction with the Paul Scherrer Institute, Metafuel has created a process that captures carbon dioxide and hydrogen through biogenic methods and air capturing. The hydrogen runs through water electrolysis which is powered by green energy – namely wind turbines – and processes the element turning it into green methanol and then to SAF. Overall, Aerobrew reduces air fuel carbon emissions by over 80%. 

The ability to control the growth of algae and yeast, in conjunction with their minimal-price source and unlimited supply, moves third and fourth-generation aviation fuel to the top of the list of contenders for sustainable alternative fuel. While the potential power for green hydrogen is exciting and potentially limitless, the price point, lack of appropriate technology, and inability to store the gas once produced will limit its viability for at least some time to come. 



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