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Bio-oil from the fast pyrolysis of biomass can be converted to solid carbon materials, chemicals and syngas by various thermochemical conversion methods. As a first step in all of these processes, bio-oil undergoes drastic components changes due to its exposure to the elevated temperature. Understanding the effects of heating rate on bio-oil transformation during its pyrolysis is therefore crucial for effective utilization of bio-oil. In this study, a bio-oil sample produced from the fast pyrolysis of rice husk at 500 °C was pyrolyzed in a fixed-bed reactor at temperatures between 300 and 800 °C at three different heating rates: fast (≈200 °C/s), medium (≈20 °C/s), and slow (≈0.33 °C/s). In addition to the quantification of coke and tar yields, the tar was characterized with an ultraviolet (UV) fluorescence spectroscopy, a gas chromatography/mass spectrometer (GC/MS) and a Fourier transform ion cyclotron resonance mass spectrometer (FT-ICR MS). Our results indicate that slow heating rates promote polymerization of bio-oil components, particularly at low temperatures (<500 °C), resulting in higher primary coke yields than that of the fast heating rates. Decomposition reaction was found to be pronounced at fast heating rates, causing decreases in the tar yields and abundance of light compounds. The increases in the yields of the secondary coke, the formations of more condensed aromatic structures and macromolecules ( m / z  > 500) were also promoted at fast heating rates via the more intense secondary reactions. [ABSTRACT FROM AUTHOR]

[Display omitted] • Polymerization electro-initiates on cathode surface, and propagates in bio-oil bulk phase. • Large molecules (MW > 800 Da) originally existed in bio-oil contributes to polymerization. • Aromatics, furans and levoglucosan are important polymerization reactants. • Increasing reaction time and current density can enhance polymerization of bio-oil. • Polymer layer forms on anode surface by polymerization. The electrochemical method provides a green route to upgrade bio-oil, but it is still challenged by the issue of bio-oil polymerization which restrains its application. This study investigates the polymerization behaviors of the bio-oil during its electrochemical upgrading under various current densities and different reaction time using macrophotography and multiple characterizations. The results show that the polymerization is not only electro-initiated on the cathode surface, but also electro-propagated in the bulk solution. The large molecules (MW > 800 Da) originally existed in the bio-oil are consumed in the polymerization. The small molecules (MW < 800 Da) can be produced from the polymerization of aromatics with 1–3 rings, furans and levoglucosan through cycloaddition, dehydration and demethoxylation. These small molecules can directly form solid products (coke) rather than leading to the increase of the large molecules. The polymerization can be enhanced by the increasing current density and reaction time. The rate of bio-oil polymerization can surpass the rate of coke formation as the reaction proceeds. The polymer layer also forms on the anode surface by polymerization during the electrochemical upgrading of bio-oil. [ABSTRACT FROM AUTHOR]