Hydrothermal liquefaction of barley straw to bio-crude oil: Effects of reaction temperature and aqueous phase recirculation

Hydrothermal liquefaction (HTL) of barley straw with K2CO3 at different temperatures (280–400 C) was conducted and compared to optimize its process conditions; the aqueous phase as a co-product from this process was recycled to explore the feasibility of implementing wastewater reuse for bio-crude oil production. Results showed that low temperature favored the formation of bio-crude oil, with a maximum yield of 34.9 wt% at 300 C. Contrarily, at high temperature, the bio-crude oil had better qualities in terms of less oxygen content and higher heating values (HHVs). The compounds identified in bio-crude oil were mainly phenolics, carboxylic acids, aldehydes and alcohols, among which the relative contents of phenolics and carboxylic acids decreased with increasing temperature. In the recirculation studies, bio-crude yield was enhanced gradually with aqueous phase addition at 300 C, and reached 38.4 wt% after three cycles. The HHVs of bio-crude oil from HTL with aqueous phase were 28.4–29.4 MJ/kg, slightly higher than those from HTL with fresh water. While no obvious differences in elemental distribution can be found after aqueous phase recirculation. In conclusion, this study gives a detailed insight into the HTL behavior of barley straw, and offers potential opportunities and benefits for bio-crude oil production through the reuse of aqueous phase.