Influence of aging on the rheological behavior and characteristics of bio-oil produced from olive pomace via slow pyrolysis

Biomass-derived pyrolytic oil, bio-oil, has the potential to substitute fossil fuels from a sustainable point of view. The utilization of bio-oil in different applications is limited due to the aging effects on its stability in terms of its compositional, thermal, and rheological changes. Thus, the objective of this work is to investigate the short- and long-term storage effects on bio-oil properties. For this purpose, bio-oil produced from olive pomace using a laboratory-scale slow pyrolysis reactor was aged under two different conditions: at room temperature in a sealed bottle for 7 days and under accelerated aging conditions (80 °C) for 24 and 168 h. The raw and aged bio-oil samples were characterized by elemental analysis, thermogravimetric analysis (TGA), gas chromatography–mass spectrometry (GC–MS), Fourier transform infrared spectroscopy (FT-IR), TGA-FT-IR, and rheometer. Carboxylic acids, esters, and phenols were detected to be the main groups of bio-oil. Long-term and high-temperature storage, known as accelerated aging, affected the rheological behavior of bio-oil while increasing the instability, which is attributed to the polymerization reactions that occurred during storage. The viscosity of this aged bio-oil was measured as 111.2 cP at 20 °C, which is 46.8% higher than that of the fresh bio-oil. The maximum decomposition temperature was shifted to around 300 °C for the 168 h of accelerated aged bio-oil. Overall, this study enables a better understanding of the olive pomace-based bio-oil storage conditions for its possible use as a synthetic fuel and provides data for the development of more feasible biorefinery processes.