Hydrothermal liquefaction of soybean straw: Effect of steam explosion pretreatment and reaction media

In this study, soybean straw (SS) was subjected to a steam explosion to produce pretreated soybean straw (PSS). Food waste was used to make a liquor with a lower layer (LL) and an upper layer (UL). Hydrothermal liquefaction (HTL) of SS and PSS was performed in deionized water (DW), LL, and UL at 320 degrees C for 30 min. The highest bio-oil yield was obtained by conducting HTL in the UL, with SS accounting for 31.30 wt% yield and PSS accounting for 31.77 wt% of the bio-oil yield; this result may be due to the high gas content of the UL. Moreover, conducting HTL in the LL produced lowest bio-oil yield, which was only 15.30 wt% bio-oil yield for PSS. Bio-oil is not suitable for direct use as a fuel because of its high nitrogen content. However, bio-oil consists mainly of ketones and phenols and is therefore a promising feedstock for the production of high-value-added chemicals. PSS is high in lignin and therefore produces more hydrochar than SS. The hydrochar produced from PSS has a higher heating value (HHV) ranging from 27 to 28 MJ/kg and can be used as green biocoal. PSS could also be used as a feedstock to reduce the gaseous product yield, especially the water-soluble product yield. The gaseous products mainly consisted of CO2, CO, H2, N2, CH4, and C2H6, among which CO2 and CO accounted for more than 95 %. Compared to the products obtained by HTL from PSS, there were more acetic acid, ketones and fewer phenolic substances in the water-soluble products obtained by HTL from SS.