9. Bio-based aviation fuel

 9. Bio-based aviation fuel

There are many different methods of biofuel production. Some are already mature and in use, others are still at an experimental stage or in the trial phase. In the future, synthetic fuels based on abiotic renewable energy sources (e.g. renewable electricity) will become increasingly important.

What you should know

• A number of processes for the production of Sustainable Aviation Fuels (SAF) have already been approved by the International Civil Aviation Organization (ICAO), including HEFA, Alcohol-to-Jet and Fischer-Tropsch synthesis
• SAFs are not used in their pure form, but are blended with up to 50% fossil kerosene
• The production of kerosene using genetically modified yeasts (SIP) is also being tested 

9.1 Which bio-based fuels are already available?

Bioethanol and biodiesel are among the most widely used biofuels and are mostly used in road vehicles or blended with conventional fossil-based fuel. Bioethanol is produced via a fermentation process with subsequent distillation. Currently, the main raw materials used are corn (in the USA) or sugar cane (in Brazil). Depending on the production process and raw material basis, a distinction is made between FAME (fatty acid methyl ester) and HVO (hydrognenated vegetable oil) or, synonymously, HEFA (hydroprocessed esters and fatty acids) in the case of biodiesel. FAME has a different chemical composition to fossil diesel and is therefore not suitable in its pure form for conventional diesel engines and is therefore only blended in. HVO and HEFA, on the other hand, are fully compatible. If you want to learn more about the different production processes and raw materials, you can take a look at chapter 2 of the educational material on the topic of future-oriented fuels on the HOOU platform.

Classification of biofuels

Biofuels can be divided into 3 types, which are also referred to as generations. The characterization depends on the raw materials used and the level of their technological maturity:

1st generation fuels

• Conventional biofuels
• are based on sugar, starch or vegetable oil, i.e. biomass that is also used as food
• the production process is well mastered and is already widely used and commercialized

2nd generation fuels

• are based on cellulosic biomass (e.g. wood, straw or maize straw)
• do not compete directly with food production, but still require land resources
• the processing technology is sometimes immature and costly, as cellulose-containing material must be pre-treated before conversion

3rd generation fuels

• use special microorganisms such as algae as raw materials
• Algae are a large group of photosynthesizing organisms with around 30,000 known species
• Biofuel production from algae is well established on a small scale, but not yet economically feasible on an industrial scale

There is now also talk of a 4th generation of fuels. This refers to biofuels that are produced from genetically modified algae or plants. Genetic modification can eliminate the disadvantages of “normal” plants, such as a low fat content or low growth rates.

In some cases, hydrogen from renewable sources, biomethane, synthetic biofuels and fuels from renewable electricity, sometimes in combination with carbon capture and storage, are also referred to as fourth-generation fuels.

Status quo production

Im Jahr 2021 wurden 159 Milliarden Liter Biokraftstoffe produziert (IEA, 2022). Große Mengen der derzeit verwendeten Biokraftstoffe werden aus Pflanzenölen gewonnen. Bioethanol ist der weltweit am häufigsten verwendete Biokraftstoff. Im Jahr 2021 wurden weltweit ca. 103 Mrd. Liter produziert. USA, Brasilien und die Europäische Union sind die größten Erzeugerländer (Renewable Fuels Association, 2022: ).

Global production of biodiesel and biokerosene is significantly lower.Around 54 billion liters were produced in 2021. The production of biodiesel through transesterification (FAME) accounted for around 80% of this volume, while hydrogenated biodiesel (HVO and HEFA) only accounted for 17%.

 HEFA technology is currently used in most cases for the production of SAF (Shahriar & Khanal, 2022: ).Waste fats, oils and animal fats are used as raw materials. Biodiesel production using the alcohol-to-jet process from municipal waste or forestry and agricultural residues is much lower (Shahriar & Khanal 2022).

In 2021, however, only 0.1% of global kerosene demand was covered by SAF. According to the IEA (2022), the share is expected to increase to 5% by 2030.

Aviation fuels

Aviation fuels must meet very specific requirements in terms of combustion quality and safety.

The fuel must:

• not ignite uncontrollably
• not form deposits
• not become electrically charged
• not freeze at temperatures below -30 °C

Of course, these requirements also apply to bio-based aviation fuels.

The term SAF - Sustainable Aviation Fuel - has been used for some time by the aviation industry for alternative fuels based on biogenic raw materials and also includes electricity-based fuels. At present, however, these SAFs are not used in their pure form, but are blended with up to 50% fossil kerosene.

 9.2 How are SAFs manufactured?

The basic principles of the most important, already approved production processes for the manufacture of bio-based SAF are explained below. A complete and up-to-date overview of the production processes approved to date by the American Society for Testing and Materials (ASTM) can be found at the ICAO. 

HEFA process

All vegetable oils or animal fats can be used to produce the fuel. However, a process based on algae has also already been approved. On a large scale, used cooking oils (e.g. frying fat) and animal fats from slaughterhouse waste are already being used to produce biodiesel or biokerosene, particularly by the Finnish company Neste.

Process steps

Flow chart HEFA process 

Fischer-Tropsch synthesis from biogas

In order to be able to produce a fuel from municipal waste or forestry and agricultural residues, the solids must first be converted into a gas. This process takes place in biogas plants and was explained in detail in chapter 8 of this learning offer.

The further steps involved in liquefying the gas are briefly explained in the following table.

Alcohol-to-jet

In order to produce aviation fuel from alcohol, ethanol or butanol must first be produced from sugar. This is done by means of fermentation (bioethanol production), as explained in detail in chapter 2.1 of the HOOU platform's Future Fuels learning offer.

The actual steps of the alcohol-to-jet process are briefly explained in the following table.

Synthesized iso-paraffins from hydrogenated fermented sugar (SIP)

The production process is based on the metabolization of sugars to long-chain hydrocarbons by genetically modified yeasts. The C-15 alkenes produced are called farnesenes. Various enzymes and fungi are required for this. Basically, this is also a fermentation process that takes place under aerobic conditions, i.e. under the influence of oxygen. The long-chain farnesenes are separated from the fermenter broth and converted into farnesanes via hydrogenation. Sugar-based synthesized iso-paraffins are currently only produced in small quantities, as the process is still in the demonstration phase. It is questionable whether this type of kerosene will actually be used in large quantities.

It is interesting to note that the anti-malarial agent artemisinin, which occurs naturally in annual mugwort, can be biosynthesized as a by-product of the process. The only manufacturer of SIP at present is therefore concentrating more on bio-based chemical products than on kerosene.

Further information on artemisinin and other active plant substances can be found in the book Pflanzliche Arzneimittel.

9.3 From research

• Project report on the use of renewable fuel at Leipzig/Halle Airport
• The use of SAF is being investigated: https://alight-aviation.eu/ 
• A consortium funded by the EU is currently planning the first alcohol-to-jet production plant.