Drop-in aviation and marine fuels from upgraded direct thermochemical liquefaction products
Fast Pyrolysis and Hydrothermal Liquefaction are two thermochemical conversion routes that transform biomass into liquid intermediates, which require subsequent upgrading to meet fuel specifications.
In fast pyrolysis, dry lignocellulosic biomass is rapidly heated in the absence of oxygen, producing pyrolysis oil that is acidic, oxygen-rich, and not directly compatible with conventional fuels.
Hydrothermal liquefaction, in contrast, processes wet biomass such as algae, sewage sludge, or food waste in hot compressed water, yielding a biocrude with lower oxygen content but often higher nitrogen levels.
In both cases, hydrotreatment is essential to (i) remove so-called ‘heteroatoms’ – the non-carbon and non-hydrogen atoms in a molecule – such as oxygen (O), nitrogen (N), and sulphur (S), (ii) stabilize the resulting fuels, and (iii) produce hydrocarbon mixtures suitable for distillation into final products such as naphtha, jet, and marine fractions.
The report “Drop-in aviation and marine fuels by upgrading of products from Direct Thermochemical Liquefaction” by IEA Bioenergy Task 34 (Direct Thermal Liquefaction) provides a comprehensive overview of the production, properties, and certification challenges of drop-in aviation (SAF) and marine fuels (SMF) derived from two DTL pathways: fast pyrolysis (FP) and hydrothermal liquefaction (HTL). The focus of the research lies on producing upgraded products through hydrotreatment of fast pyrolysis bio-oil (FPBO) and HTL biocrude and investigating their suitability as advanced biofuels for aviation and maritime applications.
The resulting jet fuels from FP are characterized by high cyclo-paraffin content, relatively high density, and high energy density, while HTL-derived jet fuels exhibit a broader composition, including n- and iso-paraffins, cyclo-paraffins, and aromatics. A key distinction is the presence of nitrogen in HTL-derived fuels, particularly when using protein-rich feedstocks, which poses
challenges for upgrading and certification. Both FP and HTL fuels can achieve favourable cold flow properties and energy densities comparable to conventional jet fuels.
Positively, both FP- and HTL-derived jet fuels largely comply with the ASTM D7566 fuel quality specifications for synthetic aviation fuels, however none of the existing annexes are applicable to describe these new fuels.(Each approved/qualified SAF production pathway is described in the ASTM D7566 annexes). Due to significant differences in composition, feedstock origin, and upgrading processes, separate certification pathways and dedicated annexes will be required for both FP and HTL technology, with extensive testing to establish the relevant fuel specifications. In particular, nitrogen management in HTL fuels and the high cyclo-paraffinic nature of FP fuels present specific certification challenges. The properties of FP jet fuels are complementary to e.g. HEFA and Fischer Tropsch fuels and thus open a pathway to 100% sustainable aviation fuel.
For marine fuels, both FP and HTL pathways offer flexibility in producing a wide range of fuel qualities, from distillates to residual fuels, depending on hydrotreatment severity and distillation strategy. These fuels can be tailored to meet ISO 8217 specifications, although aspects such as acidity, ignition quality, and blend compatibility must be carefully managed.
In conclusion, DTL-derived fuels represent a viable pathway toward sustainable aviation and marine fuels, offering the potential for true drop-in compatibility. However, significant technical and regulatory challenges remain, particularly in fuel upgrading, standardization, and certification. Continued development and alignment with existing fuel standards will be essential to enable large-scale deployment.
Key findings for policymakers
- Fast pyrolysis (FP) and hydrothermal liquefaction (HTL) are conversion pathways can produce drop-in aviation and marine fuels from diverse (resp. dry lignocellulosic or wet) biomass feedstocks, with properties comparable to conventional fuels after upgrading
- Significant differences in composition (e.g. cyclo-paraffins in FP, nitrogen in HTL) prevent alignment with approved ASTM pathways for synthetic aviation fuels and separate certification is required which could enable 100% sustainable aviation fuels, and
- The produced DTL marine fuels can meet ISO 8217 specifications, but certification, standardisation and scale-up remain key challenges requiring policy support.
