Formation of Aromatic Structures during the Pyrolysis of Bio-oil

The pyrolysis of biomass to produce bio-oil is a very effective way of biomass use. Bio-oil undergoes drastic structural changes as it is upgraded into biofuels or used as a fuel for gasification/combustion. The evolution of aromatic ring systems in bio-oil is a key consideration in bio-oil use. A bio-oil sample produced from the fast pyrolysis of mallee wood at 500 °C, its lignin-derived oligomers, and pure cellulose have been pyrolyzed in a novel two-stage fluidized-bed/fixed-bed reactor at temperatures between 350 and 850 °C. The product tars were characterized with ultraviolet (UV) fluorescence spectroscopy. Our results indicate that significant portions of aromatic ring systems in the bio-oil could turn/polymerize into solids not soluble in CHCl3 + CH3OH during the pyrolysis at relatively low temperatures, e.g., 350–400 °C. This process can be enhanced by the presence of cellulose-/hemicellulose-derived species in the bio-oil, which are reactive and produce radicals to enhance the polymerization reactions. The pyrolysis of cellulose-derived species in the bio-oil tended to form additional very large aromatic ring systems at temperatures higher than 700 °C