Hydrotreatment of bio-oil distillates produced from pyrolysis and hydrothermal liquefaction of duckweed: A comparison study

A comprehensive comparison of hydrothermal liquefaction (HTL) to the pyrolysis of duckweedwas conducted to determine the yields and components of the crude bio-oils and their distillates. The upgrading behaviors of the distillates were thoroughly investigated with the use of used engine oil as a solvent. With all other variables fixed, HTL produced crude bio-oilwith a lowerH/C ratio (1.28 +/- 0.03) than pyrolysis did (1.45 +/- 0.04). However, its distillates had a higher H/C ratio (1.60 +/- 0.05) and total yield (66.1 +/- 2.0 wt%) than pyrolysis (1.46 +/- 0.04 and 47.2 +/- 1.4 wt%, respectively). Phenolics and nitrogenous heterocycles constituted relatively major proportions of the two crude bio-oils and most of their distillates. Obvious differences in molecular composition between the two crude bio-oils and their distillates were ascribed to the distinct impacts of HTL and pyrolysis and were affected by the distillate temperature. Co-hydrotreating with used engine oil (UEO) provided the upgraded bio-oils much higher H/C ratios (similar to 1.78 +/- 0.05) and higher heating values (similar to 45.5 +/- 1.4 MJ . kg(-1)), as well as much lower contents of N, O and S compared to their initial distillates. Aromatics and alkanes constituted a large proportion in most of upgraded bio-oils. N removal from the pyrolysis distillates was easier than from the HTL distillates. Distinct differences in yields and molecular compositions for the upgraded bio-oilswere also attributed to the different influences associated with the two conversion routes. (C) 2018 Elsevier B.V. All rights reserved.