Properties and Combustion Characteristics of Bio-Oils from Catalytic Co-Pyrolysis of Grape Seeds, Polystyrene, and Waste Tires

This work aims to study the bio-oils obtained from the catalytic co-pyrolysis of waste polymers and a residual biomass (grape seeds, GS). For that purpose, the organic liquid fractions produced in an auger reactor were thoroughly characterized in two steps, obtaining in the first place their main physicochemical properties as well as their chemical compositions, and second, their droplet combustion behaviors. Both the polymer type (waste tires or polystyrene, WT and PS, respectively) and the nature of the low-cost, calcium-based catalyst used (Carmeuse limestone, calcined dolomite, or an inert material such as sand) were studied. A significant improvement in the physicochemical properties of the bio-oils was obtained when using a catalyst, with lower viscosity, density, and oxygen content. These beneficial effects were more marked for the bio-oil produced with the Carmeuse catalyst, presumably due to the higher prevalence of aromatization and hydro-deoxygenation reactions. When changing the polymer source from WT to PS, a considerable increase in the aromatic content and a viscosity reduction were noted. The droplet combustion tests revealed the consistent occurrence of microexplosions for all of the studied bio-liquids, these bursting events being more violent for the GS-PS oil. Regarding the evaporation behavior, this liquid also yielded significantly higher burning rates during the initial heatup phase, in agreement with its richer composition in volatile compounds such as styrene. These results point to this fuel as the one with the best global combustion behavior from all of the explored bio-oils. The GS-WT liquids showed much closer features among them, although with noticeable differences depending on the catalyst used. A more volatile behavior was observed for GS-WT Carmeuse, followed by GS-WT dolomite and GS-WT sand, strengthening thus the previously reported improvements in physicochemical properties. Finally, the propensity to form soot of these bio-oils was characterized through a soot probe, which revealed a higher soot yield for the bio-liquids produced with the Carmeuse catalyst.