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Alcohol-to-Jet Synthetic Paraffinic Kerosene (ATJ-SPK) is an approved pathway for sustainable aviation fuel production from alcohols that utilizes a number of commonly used fuel upgrading steps

Summary​

Alcohol-to-Jet Synthetic Paraffinic Kerosene (ATJ-SPK) is an approved pathway for sustainable aviation fuel production. The process converts alcohols like ethanol or isobutanol, ideally sourced from lignocellulosic biomass, into synthetic kerosene through a multi-step chemical transformation. The conversion involves four main stages: dehydration to remove oxygen and create alkenes, oligomerization to form longer hydrocarbon chains, hydrogenation to eliminate double bonds and produce paraffins, and fractionation to separate the final product into the correct boiling range. The final synthetic kerosene must meet strict physical requirements including specific boiling curves and viscosity to comply with aviation fuel standards.

Alcohol-to-Jet Synthetic Paraffinic Kerosene (ATJ-SPK) is one of the approved technical pathways in ASTM D7566 for the production of sustainable aviation fuel. The alcohol-to-jet process generally utilizes the alcohols ethanol or isobutanol as its main feedstock, which can be sourced from lignocellulosic biomass and converted to alcohol via biochemical pathways. Afterwards, the aviation fuel is produced through a multistep process involving dehydration, oligomerization, hydrogenation, and fractionation, resulting in synthetic kerosene. The process of synthesis using ethanol as the main feedstock is shown below:

Dehydration​

The dehydration process is required to remove any attached oxygen molecules from the alcohol into a class of molecules known as alkenes (also called olefins) as the ASTM D7566 standard requires that the final kerosene product contains no oxygen 1. Ethanol molecules (C₂H₅OH) contain a single hydroxyl group (OH) which can be removed as a water molecule, leaving behind short-chained olefin molecules such as ethylene (C₂H₄) by heating it to a temperature between 750 and 1022°F (400 – 500°C) at a moderate pressure in the presence of a catalyst 1 2.

C2H5OH→catalystC2H4+H2O\begin{align*} \mathrm{C_2H_5OH \xrightarrow{\text{catalyst}} C_2H_4 + H_2O} \end{align*}

Oligomerization​

The oligomerization process is used to create longer chain hydrocarbons by breaking the double carbon bonds in alkene molecules, which in the case of ethanol feedstock is predominantly ethylene (C₂H₄) but also butene (C₄H₈). The double carbon bonds are broken and reformed into single carbon bonds, creating longer chains of bonded olefins that contain between 8 and 16 carbon atoms. The oligomerization reaction is achieved through the use of a catalyst at moderate temperatures between 212 – 482°F (100 – 250°C) and elevated pressures 1. An example of the oligomerization reaction between 4 ethylene molecules into an octene molecule (C₈H₁₆) that contains 8 carbon atoms with a single double bond is shown below:

4C2H4→catalystC8H16\begin{align*} \mathrm{4C_2H_4 \xrightarrow{\text{catalyst}} C_8H_{16}} \end{align*}

Hydrogenation​

The hydrogenation process is required to remove any remaining double bonds that are contained in the bonded olefin chains to produce paraffins (also called alkanes). This is achieved by introducing hydrogen gas in the presence of a catalyst such as platinum at slightly elevated pressures and ambient temperatures 1. Below is an example of the hydrogenation reaction between a hydrogen molecule (H₂) and an octene molecule (C₈H₁₆) to produce a non-bonded olefin molecule, in this case octane (C₈H₁₈)

C8H16+H2→catalystC8H18\begin{align*} \mathrm{C_8H_{16} + H_2 \xrightarrow{\text{catalyst}} C_8H_{18}} \end{align*}

Fractionation​

Fractionation is the final step of a generalized alcohol-to-jet process where the different sized paraffins are separated by chain length often via distillation to meet the ASTM D7566 standard for jet fuel. The example chemical equations shown above are used to highlight how the different steps of the alcohol-to-jet process work. In practice, the final product known as synthetic kerosene contains a mixture of different sized paraffins with carbon chain lengths greater than 8 carbon atoms. However, to be considered synthetic kerosene, it has to meet certain physical requirements such as being in a specified boiling curve range and having the correct viscosity which in turn is dictated by the composition of the paraffins in the final product 1.

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Footnotes​

  1. Voß, S., Bube, S., & Kaltschmitt, M. (2023). Aviation fuel production pathways from lignocellulosic biomass via alcohol intermediates: A technical analysis. Fuel Communications, 17, 100093. https://doi.org/10.1016/j.jfueco.2023.100093 ↩ ↩2 ↩3 ↩4 ↩5

  2. Geleynse, S., Brandt, K., Garcia-Perez, M., Wolcott, M., & Zhang, X. (2018). The Alcohol-to-Jet Conversion Pathway for Drop-In Biofuels: Techno-Economic Evaluation. ChemSusChem, 11(21), 3728–3741. https://doi.org/10.1002/cssc.201801690 ↩