Fractional distillation of pine needle bio-oil: comprehensive characterization and rheological properties across temperature gradients

The study investigated the fractional distillation of bio-oil obtained from the pyrolysis of pine needles under reduced pressure (vacuum) conditions. The fractions were characterized in terms of energy content, viscosity, pH, elemental composition, and chemical composition to assess the efficacy of the distillation process in segregating components and enhancing product properties. The findings revealed substantial yields (by weight) of heavy fractions (boiling point 200–250 °C at 1 bar and 0.5 bar). The heating values, especially for light and middle distillates, increased notably under both vacuum pressures. Chemical analysis revealed the distribution of aromatic and oxygenated compounds across the light and middle fractions, with only a few phenolic compounds concentrated in these fractions, thereby enhancing the higher heating value (HHV) and viscosity of the fractions. Various rheological models were applied to analyze the rheological properties of different bio-oil fractions. It was observed that bio-oil viscosity decreased significantly as temperature rose from 20 to 80 °C, emphasizing temperature’s impact on viscosity. The Vogel–Fulcher–Tammann (VFT) model provided the most accurate description of the temperature-viscosity relationship. Additionally, the viscosity of all four fractions closely matched that of high-speed diesel (HSD). The distillation process efficiently segregated the components, resulting in a product with enhanced fuel properties and composition, rendering it suitable for further utilization as fuel in compression ignition (CI) engines.