More circular and climate-neutral societies will require responsible utilisation of residues and wastes, as well as large volumes of sustainable fuels for transport sectors like aviation and shipping, where direct electrification is not viable.
To address these challenges, CIRCULAIR embraces four high-level objectives.

Develop and demonstrate a cost-effective pathway to biofuel production from abundant feedstock
  • Hydrothermal liquefaction is a feedstock-flexible process for the conversion of various organic feedstock into liquid fuels. HTL conversion is increasingly perceived as a prime option for the conversion of wet residues and wastes.
  • Manure and straw are widely available as agricultural residues. Feedstock costs can be low, especially in areas with regional manure surplus.
  • The problem of handling HTL process water – one of the biggest hurdles to its commercialisation – is addressed by integrating HTL conversion with wet oxidation of the HTL process water.
  • Thereby, the HTL process water is cleaned-up and thermal integration with exothermic wet oxidation reduces the process heat demand of HTL. CIRCULAIR plans to demonstrate autothermal operation of a continuous HTL process with integrated wet oxidation at pilot-scale.
  • The effluent gasses from HTL and wet oxidation contain a high concentration of CO₂ that can serve as cost-effective carbon source for power-to-X schemes. CIRCULAIR aims at the synthesis of methanol with renewable H₂. At the HTL plant site, CO₂ provision costs are expected to be below competing power-to-X schemes that utilise carbon capture from the atmosphere or from less concentrated flue gas streams.

Produce a high share of on-specification jet fuel from HTL biocrudes
  • Biocrudes that result from HTL conversion are energy-dense intermediate products (heating values of 32-38 MJ/kg depending on feedstock) that may be upgraded to liquid hydrocarbon fuels by hydrotreating processes
  • Specific challenges for the upgrading towards jet fuel specification include the residual nitrogen content (CIRCULAIR’s target: < 10 ppm, if possible < 2 ppm) and the aromaticity of the product as well as the valorisation of distillation residues
  • CIRCULAIR develops process and reactor solutions for processing HTL biocrudes through pre-treatment and continuous hydrotreatment. The aim is to obtain a high share of on-specification jet fuel through hydrocracking and targeted hydrofinishing
  • By continuous processing of HTL biocrudes to jet fuel under industrially relevant conditions and analysis of resulting product properties, CIRCULAIR will contribute to the future approval of HTL-derived jet fuels for civil aviation

Nearly-complete biomass utilisation by coupling with green hydrogen
  • The carbon content of the feed biomass is transferred to several product phases that result from HTL conversion. CIRCULAIR develops suitable technologies to derive marketable by-products from all HTL conversion products. The target is to transfer > 95% of the initial carbon content to various product streams.
  • HTL conversion yields a biocrude as main product. CIRCULAIR envisages the utilisation of green hydrogen for the further upgrading of HTL biocrude to hydrocarbon fuels.
  • HTL process water contains another large fraction of the initial carbon content. In CIRCULAIR, the majority of these carbon species are transferred to CO₂ during wet oxidation. Furthermore volatile fatty acids are recovered from the process water after wet oxidation.
  • CIRCULAIR will develop tailored recovery and purification strategies for CO₂ streams resulting from HTL conversion and the wet oxidation of HTL process water.
  • Purified CO₂ is combined with green hydrogen for methanol synthesis. Methanol can be marketed as marine fuel or as a renewable commodity chemical.
  • Finally, a solid phase is evolved. CIRCULAIR investigates the use of HTL chars for soil application and carbon sequestration.

Enable negative contributions to the GHG balance of HTL fuel production
  • The CIRCULAIR concept minimizes major emission drivers of state-of-the art HTL conversion schemes: Integration of wet oxidation minimizes or removes external process heat demand for HTL conversion, utilisation of green hydrogen minimizes emission associated with biocrude upgrading.
  • CIRCULAIR investigates opportunities to generate negative contributions to the GHG balance of HTL fuel production, potentially enabling carbon-negative fuel production.
  • In particular, the performance of HTL solids for soil amendment is investigated and their long-term carbon sequestration potential is quantified. Furthermore, negative contributions may also be attributed to the feedstock of HTL conversion, e.g. based on avoided burdens in comparison with current manure handling practices.