Effect of hydrothermal pre-treatment on fluidised bed pyrolysis of wood

Hydrothermal carbonisation (HTC) of lignocellulose could improve its pyrolytic behaviour, but to date, few studies have evaluated the effect of HTC (150 °C and 200 °C) on fast pyrolysis. To fill this gap, this paper investigated how HTC affects the chemical composition of fir sawdust and, in turn, how these changes affect bio-oil yield and composition. HTC significantly alters the chemical properties of the solid material, affecting the content and acetylation degree of hemicellulose while removing inorganics (primarily K 2 O and CaO). Whereas the relative mass yield of pyrolysis products was not strongly affected by HTC, the pyrolytic behaviour was influenced by the HTC pre-treatment. HTC alters the rheological behaviour of the reacting material during fast pyrolysis, yielding hydrochar that displays an apparently melting and bed agglomeration, similarly to pure lignin, with ultimate fluidisation impairment. Fast pyrolysis products were characterised with an array of analytical techniques, allowing for the identification of changes in pyrolysis pathways and providing the tentative identification of novel holocellulose-derived oligomers in the aqueous phase portion of bio-oil. Beyond the obvious effects related to hemicellulose removal/deacetylation, results showed a meaningful change in bio-oil composition, with a strong increase in the share of anhydrosugars, which may be due to ash removal. Pyrolytic lignin was the least affected fraction, presenting only a slight reduction in molecular weight and a slight demethoxylation of its components. Results show that HTC at 150 °C/200 °C extracts or hydrolyses part or all of the hemicellulose and converts biomass into a material with different pyrolytic behaviour. In particular, hydrochar obtained at 200 °C behaves similarly to a heterogeneous mixture of pure lignin and cellulose, highlighting the approach’s potential (increased selectivity toward anhydrosugars) and pitfalls (in-bed melting of the material).