With the Paris Agreement, the world’s nations agreed to keep global warming below 2 °C and to take additional actions to limit warming to 1.5 °C. Greenhouse gas (GHG) emissions in all sectors must be reduced to zero by 2050 to meet these international commitments within the EU. Within this context, particular attention needs to be paid to the transport sector, where the progress in GHG reduction is limited until now, as defossilization in some areas is quite challenging. This is especially true in aviation, where a “green” liquid fuel with a high energy density is urgently needed (e.g., drop-in biokerosene).

BIOCTANE aims to utilize organic waste streams from agriculture, industry, and municipalities by integrating intelligent biotechnological, thermochemical and catalytic conversion steps to market-ready renewable jet fuel. The respective catalyst-controlled biorefinery conversion cascade is intertwined to maximize the carbon use efficiency of the overall process. Therefore, the proposed BIOCTANE project develops an innovative process to contribute to achieving the EU’s quota in the ReFuelEU of 35 % advanced fuels within the aviation fuel mixture by 2050 by converting organic waste streams in a highly efficient way into advanced drop-in “green” jet-fuel.

Work Package 1 – Biotechnological conversion of organic wastes

WP1 aims to develop and set up a set of biological processes that maximise the conversion of complex organic matter (organic wastes from municipalities, food industry and agricultural sectors, characterized by high-water content) to 2,3-Butanediol (2,3-BDO) and acetoin. Stability and robustness of the biological systems will be optimised to ensure high conversion efficiency and constant productivity. Remaining solids in the effluents will be sent to WP2 for thermochemical conversion. The main operational objective is to make the proof of concept and optimise the coupling of the set of three biological processes, consisting in:

  1. a fermentation process based on mixed microbial cultures (MMC) aiming to convert complex organic waste into a concentrated mixture of carboxylic acids, characterized by a high propionic acid content,
  2. a polishing Microbial Electrolysis Cell (MEC) step based on artificial microbial  aiming to convert by-products (foremost acetic and butyric acids) to H2 and CO2, and enriching the specificity of propionic acid in the effluent, and
  3. a final membrane biofilm-based conversion process with an engineered strain of Cupriavidus necator converting with a high efficiency propionic acid, hydrogen and CO2  to chemical precursors, ie. 2,3-Butanediol and acetoin.

Work Package 2 – Hydrothermal gasification to hydrogen

Photos of the inside of the experimental hydrothermal gasification unit (Konti-C) developed at PSI.

The goal of WP2 is to develop a catalyst for the production of hydrogen from aqueous fermentation residue (produced in WP1), under hydrothermal conditions at moderate temperatures (300 to 450°C) to allow full utilization of wet organic residues. The challenge lies in developing a catalyst that is selective towards the production of hydrogen but also stable towards coking and sintering. The development of catalysts will proceed together with the optimization of process conditions. The final objective is to produce hydrogen from real fermentation residues, along with a small stream containing most of the inorganic and organic salts (short oxygenated carboxylates) extracted from the mainstream thanks to the properties of supercritical water. The potential for the valorization of this brine effluent as fertilizers (K, P) or for conversion to 2.3-BDO in WP1 will be assessed. Ultimately, this will allow for determining the accurate mass and chemical energy balances for WP4 techno-economic assessment.

Work Package 3 – Chemical conversion of 2,3-BDO and acetoin to jet-fuel range hydrocarbons

Both 2,3-butanediol (2,3-BDO) and acetoin, as target platform molecules coming from the biological conversion of biowastes (WP1), can be further catalytically converted into hydrocarbons with suitable properties for jet-fuel formulation. In the case of 2,3-BDO, a typical catalytic sequence comprises the dehydration to C3+ olefins, the oligomerization to C6+ olefins and a final hydrogenation step. On the other hand, acetoin molecules need first to be enlarged by C-C coupling via aldol condensation with additional molecules, in particular ketones and aldehydes derived from biomass (furfural, cyclopentanone, cyclohexanone, etc.). The resultant adducts can then be subjected to hydrodeoxygenation (HDO) to yield the final oxygen-free target hydrocarbons.

Each catalytic step requires the use of specific catalysts, operating conditions (temperature, pressure, WHSV, etc.), as well as reactor configurations, all of them affecting the process from both the technical and economic side. Therefore, the challenge to face in this WP is to simplify the process by integrating at least two chemical steps for each platform molecule in a one-pot reaction system. For the 2,3-BDO route, the reactions to be integrated are the dehydration and the oligomerization; and for acetoin the steps to be integrated are the C-C coupling and the HDO reaction. The use of multifunctional catalysts and the optimization of operation parameters will be key to succeed. Thus, the main challenge will be designing new catalysts, or modifying conventional ones, to get multifunctional and versatile activity with proper resistance to deactivation under the conditions to which they will be subjected; and adapting the reaction setups to operate in conditions that are foreseeably different from the usual ones.

Work Package 4 – Process Evaluation

The objective of WP4 is – based on an extensive process flow modelling – to assess the process efficiency (e.g.material flow, energy flow, carbon-use efficiency) for the individual steps as well as the overall process relative to the main target product as well as the overall product spectrum. These results are the basis for a subsequent economic and environmental assessment carried out with the overarching goal to analyze the overall process among others related to the fuel provision costs for potential markets as well as the resulting GHG emissions.

Based on the work package tasks, it will be evaluated to what extent this process can contribute to GHG reduction compared to fossil and other alternative fuels. Furthermore, it will be analyzed for subsequent developments how the waste feedstock situation will develop in the years to come and thus how high the potential for jet-fuel production will be – based on this process as well as based on other (competing) processes. With the additional techno-economic analysis of the process, the technical potentials, as well as the potential competitiveness to competing processes, will be evaluated. Additionally, scenarios how such a process can be implemented (market ramp-up) will be assessed in parallel to the possible contributions to help the commercial air transportation industry to fulfill their GHG reduction goals in a most cost-efficient way.

Sustainable Aviation Fuel (SAF)

The currently dominating fossil aviation fuels are not the only approved fuel options for the civil aviation industry. Also, non-fossil fuels that can be produced based on different renewable biogenic and non-biogenic feedstocks can be used. These fuels, also referred to as renewable or alternative jet fuels are presently mainly subsumed as “Sustainable Aviation Fuel” (SAF). From a climate perspective, the advantage of their use is that they produce fewer GHG emissions compared to fossil aviation fuels. The generic term “SAF” covers various combinations of non-fossil feedstocks and chemical, biochemical, and thermochemical conversion processes. Furthermore, they can also be used to produce other fuels (e.g., diesel) or other products (e.g., chemicals) in a coupled production. The types of sustainable aviation fuels can be classified into three categories.

Biogenic SAF. Biogenic or biomass-based SAF (biokerosene) covers a broad scope of different SAF options that are produced from oil, fat, starch, or sugar-containing biomass or lignocellulosic (woody and semi-woody) biomass. This includes, for example, plant oils, algae, specific components of energy plants, organic municipal- and industrial waste, or agricultural and forestry residues.

Electricity-based SAF. Electricity-based SAF or Power-to-Liquid (PtL) SAF defines sustainable aviation fuels, which are not produced based on biomass but solely on electricity from renewable sources, water, and CO2.

Hybrid SAF. Hybrid SAF (also called PBtL SAF) origin from hybrid production processes or combined biomass and electricity-based processes. Other than pure electricity-based approaches, water, CO2, and electricity are not the only possible input materials. Besides electricity, an additional carbon source (e.g., biomethane), which already contains a part of the energy of the final aviation fuel, is used for production.

Depending on the biomass used, various pretreatment steps are necessary to produce biogenic SAF to further process the feedstocks into a liquid fuel that conforms to aviation standards. Next, the pretreated feedstock is converted to an intermediate product such as alcohol (e.g., isobutanol or ethanol), synthesis gas, so-called “bio-crude oil”, or other hydrocarbon mixtures. Finally, the intermediate products are converted into biokerosene via another conversion step.

The production of PtL SAF is exclusively based on water, CO2, and electricity. In this process, water is first converted into hydrogen via an electrolysis process by using electrical energy (from renewable energy sources), which is then converted together with CO2 into a synthesis gas (i.e., a gas mixture of carbon monoxide (CO) and hydrogen (H2).

(adopted from “Sustainable Aviation Fuels – Status, Options, Necessary Actions“, 2020, aireg Roadmap for the Deployment of Sustainable Aviation Fuels, with kind permission)