Fischer-Tropsch Synthetic Paraffinic Kerosene (FT-SPK) is an approved pathway for sustainable aviation fuel production from a range of biomass feedstocks

Summary

Fischer-Tropsch Synthetic Paraffinic Kerosene (FT-SPK) was the first sustainable aviation fuel to achieve technical certification for blending with petroleum-derived kerosene in 2009. This fuel is produced from syngas, a mixture of carbon monoxide and hydrogen, which can be sourced from various feedstocks including biomass, municipal solid waste, or through renewable energy processes. The production involves multiple steps: gasification to create syngas, cleaning to remove impurities, Fischer-Tropsch synthesis to convert syngas into hydrocarbons, fuel upgrading through hydrotreating and hydrocracking, and finally product fractionation to separate the desired aviation fuel components.

---

Fischer-Tropsch Synthetic Paraffinic Kerosene (FT-SPK) was the first sustainable aviation fuel (SAF) to achieve ASTM D7566 technical certification for 50% blending with petroleum-derived kerosene back in 2009 ¹. The primary feedstock of FT-SPK is syngas, which is a mixture of carbon monoxide (CO), hydrogen (H₂). Syngas can be produced from a number of feedstocks including lignocellulosic biomass, but also from other sources of carbon-based matter such as municipal solid waste (MSW). Syngas could also be produced utilizing renewable energy via hydrogen electrolysis and direct air capture of carbon dioxide (CO₂). The main steps for the production of FT-SPK are the initial gasification to produce syngas, syngas cleaning, Fischer-Tropsch (F-T) synthesis, fuel upgrading (hydrotreating or hydrocracking and isomerization), and separation to produce synthetic kerosene.

Gasification

Syngas (short for synthesis gas) is produced through a thermochemical process known as gasification. Gasification involves the thermal degradation of organic matter in a controlled oxygen environment to produce a mixture of predominantly carbon monoxide (CO) and hydrogen (H₂). This is achieved by heating predried organic material to above 1652°F (900°C) in a controlled oxygen environment that does not allow full combustion of the material ¹. Gasification of carbon matter first involves a short pyrolysis step where the organic matter is heated to produce solid carbon (C) or liquid pyrolysis oil, depending on the feedstock, as well as other gases such as carbon monoxide and hydrogen and carbon dioxide. Next, the actual gasification process can take place which consumes the solid carbon and volatile gases to produce more carbon monoxide and hydrogen as shown below.

$$\begin{align*} \mathrm{C + CO_2 \leftrightarrow 2CO} \end{align*}$$ $$\begin{align*} \mathrm{C + H_2O \leftrightarrow CO + H_2} \end{align*}$$

Syngas cleaning

The goal of gasification is to produce as pure a stream as possible of carbon monoxide and hydrogen. However, some inorganic elements such as carbon dioxide and oxygen (O₂) gas, organic materials such as benzene and tar, and other impurities that contain sulfur and chlorine such as hydrogen sulfide (H₂S) and hydrogen chloride (HCl) are contained in the syngas stream ². These impurities can damage equipment and impact the effectiveness of the Fischer-Tropsch conversion at later stages and need to be removed. In the case of organic impurities, they can be further broken down through a process called 'cracking' into more carbon monoxide and hydrogen ². In the case of inorganic impurities, they can be further removed using reactants (as is the case with deoxidants for oxygen removal) or by passing the syngas through scrubbers and filters ².

Fischer-Tropsch (F-T) synthesis

Fischer-Tropsch synthesis is a broad term that refers to a number of catalytic processes for converting clean syngas into liquid hydrocarbons. For the synthesis to occur, syngas is passed through reactors at elevated temperatures of around 446°F (230°C) for low temperature synthesis and 644°F (340°C) for high temperature ². The choice of catalyst is dependent on temperature and the reactor design, but are typically iron (Fe), cobalt (Co), ruthenium (Ru) or nickel (Ni) ². Because the FT synthesis involves a number of different processes that all are occurring at the same time, the actual output of the process is a mixture of different sized hydrocarbons that form according to reactor conditions in a probabilistic nature. For example, some of the reactions for the formation of octene (C₈H₁₆) and octane (C₈H₁₈) molecules and water (H₂O) byproduct are shown below ²:

$$\begin{align*} \mathrm{17H_2 + 8CO \xrightarrow{\text{catalyst}} C_8H_{18}+ 8H_2O} \end{align*}$$ $$\begin{align*} \mathrm{16H_2 + 8CO \xrightarrow{\text{catalyst}} C_8H_{16}+ 8H_2O} \end{align*}$$

Fuel Upgrading (hydrotreating, hydrocracking, and isomerization)

Hydrotreating

Depending on the feedstock and Fischer-Tropsch reactor design, a series of different upgrading processes can be employed to produce the desired amount of aviation fuel that meets the ASTM D7566 standard. The first process is known as hydrotreating, which is used to convert any remaining olefins (hydrocarbons that contain a double carbon bond) and oxygenates (hydrocarbons that contain an oxygen atom such as alcohols) that were produced in the Fischer-Tropsch synthesis into paraffins (sometimes called alkanes i.e. hydrocarbons that contain only carbon and hydrogen atoms) ³. This process is similar to the hydrogenation process used in Alcohol-to-Jet Synthetic Paraffinic Kerosene (ATJ-SPK) and involves introducing hydrogen gas in the presence of a catalyst. Below an example is shown between a hydrogen molecule (H₂) and an octene molecule (C₈H₁₆) to produce a nonbonded olefin molecule, in this case octane (C₈H₁₈):

$$\begin{align*} \mathrm{C_8H_{16} + H_2 \xrightarrow{\text{catalyst}} C_8H_{18}} \end{align*}$$

Hydrocracking & isomerization

As the Fischer-Tropsch synthesis process is probabilistic, many long-chain hydrocarbons (C22+) are produced that are not suitable for aviation fuel. These chains can be broken down through a process known as hydrocracking which operates similarly to hydrotreating, but at slightly different temperatures and pressures. In the presence of hydrogen gas and a catalyst, long chain paraffins are cleaved into smaller paraffins, as shown below with a paraffin chain of 30 carbon atoms (C30H62):

$$\begin{align*} \mathrm{C_{30}H_{62} + H_2 \xrightarrow{\text{catalyst}} C_{16}H_{34} + C_{14}H_{30}} \end{align*}$$

Isomerization

After hydrocracking, further processing such as isomerization can be used to create a fuel that has other desirable characteristics such as a lower freezing point. This is often achieved in the same reactor where the hydrocracking occurs, and involves rearranging the paraffin molecule while maintaining the same number of carbon and hydrogen atoms ³. The goal of isomerization is to form methyl branches (-CH₃) that are attached to the main carbon chain.

Product Fractionation (distillation)

The final step after fuel upgrading is the final product separation. First, any remaining hydrogen from the fuel upgrading process is removed, for example, using a hydrogen membrane. Next, the lighter components with low carbon chain length are removed through a process called steam stripping. Finally, the remaining product is passed through a distillation column that separates out the majority of the desired hydrocarbons between the C9 and C16 range that meet the ASTM D7566 standard ³.

Sources

Footnotes

1. Naik, S. N., Goud, V. V., Rout, P. K., & Dalai, A. K. (2010). Production of first and second generation biofuels: A comprehensive review. Renewable and Sustainable Energy Reviews, 14(2), 578–597. https://doi.org/10.1016/j.rser.2009.10.003 ↩ ↩²

2. Hu, J., Yu, F., & Lu, Y. (2012). Application of Fischer–Tropsch Synthesis in Biomass to Liquid Conversion. Catalysts, 2(2), 303-326. https://doi.org/10.3390/catal2020303 ↩ ↩² ↩³ ↩⁴ ↩⁵ ↩⁶

3. Nyholm, F., Toppinen, S., & Saxén, H. (2025). Sustainable power-to-liquids aviation fuels: Modelling and comparison of two Fischer-Tropsch upgrading process concepts. Energy Conversion and Management, 342, 120153. https://doi.org/10.1016/j.enconman.2025.120153 ↩ ↩² ↩³