Production

So far, only five manufacturing processes exist for the production of sustainable kerosene made from renewable resources on a large scale. The aviation industry, fuel producers and science are therefore jointly pioneering the research on further processes and technologies.

Current manufacturing processes

The production of fuels from renewable resources not only requires excellence in research and development, but also a good infrastructure. There are currently no refineries for the production of alternative jet fuel in Germany. Several technologies would already be feasible today and their implementation would be extremely sensible. Here, there is an urgent need for increased political support in order to build up the necessary production capacity as quickly as possible.

Hydroprocessed Esters and Fatty Acids (HEFA)

For the production of sustainable fuels based on esters and fatty acids (Hydroprocessed Esters and Fatty Acids, HEFA), all forms of native fat or oil can be used. In addition to used fats and wastes from the food industry, mainly vegetable oils and fatty acids from the refining process of oils and fats are used.

In a first step, the oils and fats are hydrogenated and then refined similar to fossil fuels. This manufacturing process is already mature and has been approved by the International Standardization Authority ASTM since 2011. HEFA kerosene has been used in thousands of test flights and in regular service.

Numerous analyses on greenhouse gas emissions and other environmental effects of HEFA kerosene are available. HEFA can comply with the EU Renewable Energy Directive under certain conditions.

Today, several refineries worldwide specialize in the production of certified vegetable oil fuels, including kerosene from renewable resources.

Fischer-Tropsch (FT)

In the Fischer-Tropsch synthesis, long-chain hydrocarbons are generated from a synthesis gas (carbon monoxide and hydrogen) in a reactor. Thereafter these must be further processed into kerosene (e.g., by hydrocracking and / or isomerization). Since 2009, the original process is ASTM-certified for aviation. In the meantime, an extension of the process was approved separately.

The large-scale feasibility of this technology was successfully demonstrated on the basis of coal decades ago. However, the technology as a whole is extremely demanding. A wide range of biomass is suitable as a starting material. In principle, the two different supply concepts of Gas-to-Liquids and Biomass-to-Liquids must be distinguished.

Gas to liquid (GtL)

Biomass of different origin and composition is first converted into biogas via biochemical processes. In addition to wood, almost all available biomass including waste can be used. Also, in terms of techno-economic aspects the production can be realized in a comparatively small-scaled way. As a result, biomass resources with a relatively low energy content, such as liquid manure, can be opened up which would be uneconomical to transport over long distances. However, this can only be achieved by feeding the biomethane into the existing natural gas network. The collected biomethane can be fed to refineries. In the subsequent Gas-to-Liquid (GtL) process, this biomethane is converted into carbon monoxide (CO) and hydrogen (H2). CO and H2 are then converted by Fischer-Tropsch synthesis in long-chain hydrocarbons - and thus ultimately in sustainable kerosene.

GtL technology has been used for years in conventional refineries. In the Middle East, for example, subsidized fossil natural gas is liquefied and sold on the global energy markets as a high-quality source of energy.

So far, no sustainable aviation fuel has been generated via the GtL process. Since biomethane and fossil methane are chemically identical and the technology for natural gas is already used on a large scale, production is considered to be problem-free. However, there are high costs for fuel production from renewable resources via the GtL process.

Biomass to Liquid (BtL)

For the production of Biomass-to-Liquid fuels, solid biomass is converted directly via a thermo-chemical gasification into a synthesis gas which consists primarily of carbon monoxide and hydrogen. For this purpose, lignocellulosic biomass, e.g. wood, is used. This must be available in considerable quantities directly at the production site, as a BtL plant - due to the "economy of scale" effect - can only be operated economically in the case of a very high product capacity.

After a purification by Fischer-Tropsch synthesis the synthesis gas produced from the biomass is transferred in hydrocarbon chains. Subsequently, kerosene is separated from the resulting hydrocarbon mixture by means of the refinery processes.

Direct Sugar to Hydrocarbons (DSHC)

In DSHC technology, sugar is converted directly into pure unsaturated hydrocarbons by aerobic fermentation of genetically modified yeasts. Subsequently these must be hydrogenated. In principle, even higher sugars (polysaccharides such as cellulose) can be metabolized after digestion. While the fermentation of simple sugars to hydrocarbons has already reached industrial scale, the fermentation of higher sugar which belongs to the future, still needs some development work.

In the current development phase, mainly an unsaturated hydrocarbon with 15 carbon atoms (C15) is produced by the fermentation - the Farnesene. Hydrogenation turns Farnesene into the Farnesane. This will be produced in a first industrial-scale plant in Brazil. This plant is designed for an annual capacity of 30 million liters.

Farnesane received its ASTM certification as Appendix 3 of ASTM 7566 on June 15, 2014. It can be used as a bio-based kerosene and can replace up to 10% of the fossil JET-A1 fuel.

Alcohol to Jet (AtJ)

In alcohol-to-jet technology, hydrocarbons are produced from alcohols. First, an alcohol must be made from the biomass. Therefore, sugary biomass comes into consideration, thus conceivable is the use of lignocellulose.

The actual conversion of the alcohol into a hydrocarbon differs only slightly between the alcohols. First, the oxygen is removed by dehydration and short-chain alkenes are formed. Subsequently, their oligomerization takes place to long-chain alkenes. In the next step, they are hydrogenated to alkanes, which can fall into the kerosene fraction.

Since 2016, AtJ has been approved as aviation fuel based on butanol. Furthermore, there is a process based on ethanol in the approval process.