Co-processing of Hydrothermal Liquefaction Sewage Sludge Biocrude with a Fossil Crude Oil by Codistillation: A Detailed Characterization Study by FTICR Mass Spectrometry

Co-processing renewable feedstock in existing refineries could be a prompt and ready-to-use approach to decarbonize the transportation sector, without large modifications to current processing infrastructures. In this study, we explore the possibility of codistilling a blend of fossil crude with hydrothermal liquefaction (HTL) biocrude from primary sewage sludge. HTL biocrude is indeed gaining an increasingly relevant role, because it can be produced from a huge variety of biomass feedstock, including wet byproducts, with no competition with food or feed applications. Despite the highly valuable properties of HTL in comparison with other bio-oils (high heating value, relatively low heteroatoms content, etc.), its introduction in a refinery distillation unit can still be problematic, because of its high acidity and inorganics content. Therefore, partial hydrotreatment was performed prior to blending with a low-sulfur fossil oil, which allowed a blending ratio of 1:4. Codistillation tests were compared with an analogous test with pure fossil oil, in order to assess the contribution of the biomass feed. The obtained distilled cuts were fully analyzed, and a petroleomic approach employing FTICR mass spectrometry was used for a more-detailed characterization at the molecular level. Results showed that biocrude mostly contributes to the high boiling point fractions, especially diesel and residue, although a significant contribution can be also observed to the kerosene range. However, significant amounts of nitrogen were found in the distilled fractions, corresponding to compounds recalcitrant to hydrotreating, resulting in a different carbon number and double-bond equivalent (DBE) distribution. This issue could be controlled by reducing the blending ratio or with specific upgrading treatments. Therefore, codistillation of HTL biocrude with fossil oil is a promising route for the introduction of renewables in the existing refineries.