Sulfur-free liquid fuel production via continuous catalytic hydrotreating of crude bio-oils from straws

In this study, four crude bio-oils (CBOs) obtained from hydrothermal liquefaction of soybean straw (SS), peanut straw (PS), corn straw (CS) and rice straw (RS) were hydrotreated with various amounts of n-hexane. Then the obtained oil was separated into light oil (LO) and heavy oil (HO) by centrifugation. The LO yields increased from 53.89 to 59.15 wt% to 76.25–78.54 wt% when the mass ratio of n-hexane to CBO (w_nH/w_CBO) increased from 1:1–3:1. Despite containing the least asphaltenes, the hydrotreating of SS-CBO with n-hexane produced the lowest LO yield and the highest solid yield. The obtained LOs were mainly composed of saturated hydrocarbons, aromatics, and phenolic compounds, which showed obviously higher saturation and HHVs, also lower O and N contents than the corresponding HOs. Further hydrotreating of LOs resulted in significantly higher yields of upgraded light oils (ULO), falling in a narrower range of 92.04–93.48 wt%. The N and S content (26–503 ppm and <1 ppm, respectively) of the four ULOs all showed a sharp decline relative to the corresponding LOs. Saturated hydrocarbons (51.08–61.48%) and aromatics (31.98–43.00%) accounted for the vast majority of the identified compounds in the four ULOs, indicating the conversion of phenolic compounds in LOs to benzene and cyclohexane and their alkyl derivatives in the further hydrotreating process.