Hydrothermal liquefaction of barks into bio-crude – Effects of species and ash content/composition

•Bark-containing ash can be efficiently removed by diluted nitric acid treatment.•Bark conversion and bio-crude yield differ significantly with bark species.•The K and Ca compounds in bark ash catalyze bark conversion into bio-crude.•The bio-crude oils are aromatic/phenolic nature and have HHVs of 25.5–38.9MJ/kg.•The bark-derived bio-crude oils have lower molecular wrights (Mw=800–1700). Liquefaction of barks of white pine, white spruce and white birch was performed in ethanol–water (50:50, v/v) co-solvents under the initial N2 pressure of 2.0MPa at 300°C for 15min. It was found that the liquefaction efficiency, in terms of bark conversion and bio-crude yield, varied significantly with bark species and ash content/composition. As far as the bark conversion was concerned, the order follows: white spruce bark (92%)>white birch bark (89%)>white pine bark (68%), which is also in good agreement with the ash content of the barks: white spruce bark (3.07%)>white birch bark (2.68%)>white pine bark (1.07%). Bio-crude yield reduced in the following order: white birch bark (67%)>white spruce bark (58%)≫white pine bark (36%). Effects of ash content on bark liquefaction were investigated by comparing the liquefaction efficiencies between crude bark and de-ashed bark. Deashing pre-treatment of barks in 0.5M nitric acid efficiently decreased ash content in barks of white pine, white spruce and white birch form 1.07%, 3.07%, 2.68% to 0.67%, 0.33% and 0.32%, respectively. The de-ashing of bark decreased both bark conversion rate and bio-crude yield for all barks, suggesting that the ash of the bark play catalytic roles in the bark liquefaction, which was confirmed by the addition of K2CO3 and Ca(OH)2 into de-ashed barks. The bark-derived bio-crude oils are aromatic/phenolic in nature with HHVs of 25–39MJ/kg. The obtained bio-crude oils also have relatively lower molecular weights (Mn=320–600g/mol, Mw=800–1700g/mol, PDI=2.2–2.9), which makes the bio-crude oils promising in the applications for either bio-fuel or as a phenol substitute in bio-phenolic resins.