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.

Automated summary

In this study, steam explosion was used to pretreat soybean straw (SS) and produce pretreated soybean straw (PSS). Hydrothermal liquefaction (HTL) of SS and PSS was conducted in deionized water (DW), lower layer (LL), and upper layer (UL). The UL yielded the highest bio-oil, with SS and PSS accounting for 31.30 wt% and 31.77 wt% of the bio-oil yield respectively, possibly due to the high gas content of the UL. Bio-oil is not suitable as a fuel, but can be used for high-value-added chemical production.