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EU project HyFlexFuel converted sewage sludge and other biomasses into kerosene by hydrothermal liquefaction (HTL); SAF

The EU-funded research project HyFlexFuel recently successfully produced biocrudes via hydrothermal liquefaction (HTL) from a variety of biomasses, including sewage sludge, food waste, manure, wheat straw, corn stover, pine sawdust, miscanthus and microalgae in a pilot-scale continuous HTL plant at Aarhus University (Denmark).


HyFlexFuel, Process overview. Copyright: Bauhaus Luftfahrt

Biocrudes from three representative feedstocks—Spirulina (micro-algae), sewage sludge, and wheat straw—were further upgraded to a mixture of hydrocarbon fuels at Aalborg University (Denmark).

Thanks to the expertise of several project partners, HyFlexFuel proved that HTL biocrudes can be successfully upgraded to drop-in fuels in an industrially-relevant environment, achieving hundreds of hours of continuous operations.

—Daniele Castello of Aalborg University

Analyses of the kerosene fractions of the upgraded biocrude show promising compositions for a use as aviation fuel.

The production of HTL fuels from three different feedstock classes shows the flexibility of the process. Meeting jet fuel specifications is an appropriate target to validate that highperformance transportation fuels can indeed be produced from a broad range of residue and waste streams via hydrothermal liquefaction.

—project coordinator Valentin Batteiger of Bauhaus Luftfahrt (Germany)

The decarbonization of the transportation sector will require large volumes of renewable fuels. So far, renewable diesel and jet fuels are mainly derived from plant oils, but the EU Renewable Energy Directive limits the use of biofuel from food and feed crops since they do not meet sustainability requirements when produced at large scale. For the future, it will be important to commercialize advanced biofuel conversion technologies, which utilize a broader and more sustainable feedstock base.

Hydrothermal liquefaction (HTL) can produce transportation fuels from a broad variety of bio-wastes and other biomasses. HTL has several key-advantages, of which the most important are:

  • Flexible production potential: The HTL conversion technology taps into a huge global bio-resource with local variety of primary biomasses. The technology is compatible with a broad variety of organic wastes and residues, lignocellulosic energy crops or aquatic biomasses and can adapt to specific regional feedstock availabilities.

  • Cost-effectiveness: It can produce advanced biofuels, from marine fuels to kerosene, potentially at lower cost than most competing renewable fuel pathways.

  • Sustainability: The HTL technology has the potential to produce fuels with a low carbon footprint over the entire life cycle, without competing with food and feed production. It has the potential to recycle waste streams and thereby contribute to a more circular economy.

The pilot-scale HTL plant processes aqueous biomass slurries (~20% dry matter content) at temperatures up to 350 °C and pressures around 200 bar, where water does not boil but remains in a liquid state. Under these conditions, biomass is converted into a crude bio-oil, which is separated from the process water behind the reactor.

In a second step, the HTL biocrude is upgraded to transportation fuel products via catalytic treatment with hydrogen at high temperature and pressure (hydrotreating). Thereby, oxygen and nitrogen are removed from the biocrude, which is in turn converted into a mixture of hydrocarbons. Distillation of the upgraded HTL biocrudes yields drop-in capable fuels in the gasoline, kerosene and diesel range.

Within the EU-project HyFlexFuel all key steps along a HTL fuel production chain are investigated at the premises of several European research institutions and companies.

  • The potential availability of residues and waste streams across Europe is analysed by DBFZ, the German institute for biomass research, using a geospatial approach.

  • Aarhus University further develops and optimizes HTL conversion at lab-scale and transfers the results to a pilot-scale continuous HTL plant, which also produced all samples for the downstream processes.

  • Aalborg University, supported by Haldor Topsøe (Denmark), implemented the upgrading of various biocrudes to transportation fuels via catalytic hydrotreatment.

  • Eni (Italy) investigated the perspective of co-processing HTL biocrudes in conventional crude oil refineries.

  • Solid particles and process waters, which also evolve during HTL conversion, contain a significant fraction of the carbon and the nutrients from the initial biomass. Two options to produce biogas from the organics in these process waters are investigated by Paul Scherrer Institute (Switzerland), via catalytic hydrothermal gasification on their Energy System Integration Platform, and by OWS (Belgium), via anaerobic digestion.

  • The recovery of phosphorus is particularly important for the HTL of sewage sludge in order to close nutrient cycles. University of Hohenheim (Germany) has demonstrated the precipitation of struvite, a fertilizer product, from HTL solids and process waters.

  • Bauhaus Luftfahrt coordinates the project and analyses the economic and environmental impact of HTL fuel production.

  • ARTTIC Innovation GmbH (Germany) serves as a project management partner and supports dissemination activities.

Approval of HTL kerosene for civil aviation. So far, seven alternative fuel production pathways have been approved as a blend component for civil aviation by the ASTM D-7566 specification. The HyFlexFuel consortium is preparing the approval of HTL kerosenes in collaboration with key partners in Europe and the US.

Kerosene samples from the HyFlexFuel project were analysed by the H2020 project JETSCREEN (coordinated by DLR, Germany) and the University of Dayton. The results show that all basic physicochemical properties of jet fuel, such as energy density or cold flow properties are already met, while the concentration of specific trace components needs to be further reduced to comply with stringent jet fuel specifications.


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