Hydrotreating of bio-oil from updraft pyrolysis of straw and its miscibility with ultra-low-sulfur fuel oil

Bio-oil produced by biomass pyrolysis has strong potential as a renewable marine fuel but requires upgrading to meet fuel specifications and ensure compatibility with marine fuels. In this study, bio-oil from updraft pyrolysis of wheat straw was hydrotreated in a batch reactor using a sulfided NiMo/Al₂O₃ catalyst. Twenty-four experiments were performed across temperatures of 150–360 °C, hydrogen pressures of 70–130 bar, stirring speeds of 400–1500 rpm, and reaction times of 2–12 h to determine the severity required for miscibility with ultra-low-sulfur fuel oil (ULSFO) and to quantify changes in chemical and physical properties.

Higher temperature and hydrogen pressure increased deoxygenation and hydrogenation, while higher stirring speeds reduced mass-transfer limitations and suppressed coke formation. Complete miscibility with ULSFO was achieved only under high hydrogen availability (130 bar), sufficient mixing (1000–1500 rpm), and extended reaction time (2–8 h). The fully miscible products exhibited H/C ratios of ~1.5, O/C ratios near 0.05, and micro carbon residue values of 0.1–1.2 wt%. Despite these improvements, the upgraded oils did not meet the ISO 8217 marine fuel specifications for flashpoint (>60 °C) or total acid number (<2.5 mg KOH g⁻¹), with acid numbers up to 11 mg KOH g⁻¹, indicating the need for additional steps such as mild distillation or further hydrotreatment. However, in a 15:85 wt% hydrotreated-oil to ULSFO blend ratio, as used in these experiments, the acid number would remain below the 2.5 mg KOH g⁻¹ threshold.

Two hydrotreatment experiments were made with wood-derived fast-pyrolysis bio-oil, to compare oil yields and properties with the updraft bio-oil derived from straw. The updraft oil showed higher carbon recovery after hydrotreatment (~90 wt% vs. 79 wt% C) and a larger fraction of high-boiling compounds (>140 °C) at 68 wt% vs. 62 wt% than the fast pyrolysis oil, consistent with its more high-molecular-weight composition. However, relative to the dry biomass, the fast-pyrolysis oil achieved a higher overall oil yield after hydrotreatment (18 wt% vs. 6 wt%) due to a higher bio-oil yield in the fast pyrolysis process compared to the updraft process (65 vs. 15 wt% of dry biomass). The hydrotreated updraft oil samples showed elevated nitrogen concentration (~2 wt%) due to a higher nitrogen content in the straw biomass. This may contribute to increased NOₓ emissions from the ship if blended with marine fuels.

Overall, hydrotreatment substantially improves the fuel properties of straw-derived updraft bio-oil and can enable its use as a marine fuel blend component when optimized conditions and appropriate post-treatment steps are applied.