Evaluating fractional pyrolysis for bio-oil speciation into holocellulose and lignin derived compounds

Abstract

Fractional pyrolysis of lignocellulosic biomass, by staged thermal treatment, has been assessed as an in-situ speciation method of the bio-oil components that could be highly beneficial for extracting valuable compounds or for their subsequent catalytic upgrading. Wheat straw and pine woodchips were used as representative biomasses. Based on the results of TG analyses in an inert atmosphere, 350 and 700 °C were selected, respectively, as operational temperatures for the fractional pyrolysis. Compared to single-step pyrolysis, fractional thermal treatment of both biomasses led to some reduction of the bio-oil yield but with improved properties due to their lower oxygen content. Sharp differences were observed in the bio-oil composition obtained at the two steps of fractional pyrolysis. GC-MS analyses revealed that most of the compounds detected in the bio-oil obtained at 350 °C were products formed by the decomposition of polysaccharides, such as carboxylic acids, furans, sugars, and light oxygenates. In contrast, the organic liquid phase obtained during the subsequent treatment at 700 °C was rich in aromatic oxygenated compounds, coming from the lignin conversion. The content of oligomeric and heavy species, not detected by GC-MS, was much higher in the bio-oils obtained in the high-temperature step of fractional pyrolysis, denoting that they are largely formed from lignin. Significant changes were also observed in the relative contribution of the deoxygenation pathways during the two steps of fractional pyrolysis. Thus, dehydration was the predominant deoxygenation route during the degradation of the holocellulose biopolymers at the low-temperature step, whereas the decomposition of the lignin-rich solid at the high-temperature treatment proceeded with a significant contribution of decarbonylation and decarboxylation. These results evidence the great potential of lignocellulose fractional pyrolysis to generate bio-oil streams with high speciation of the components, facilitating sharply their further processing and upgrading.