Insight into the effect of hydrogenation on efficiency of hydrothermal liquefaction and physico-chemical properties of biocrude oil

• Hydrogenation increases HTL efficiency on biocrude oil yield and high heat values.
• Hydrogenated biocrudes have smaller molecular weight and polydispersity index.
• Hydrogenation HTL enhances stabilization of produced radicals and oil quality.
• Yield and quality of biocrudes are better from N. salina than from vermicompost.

Hydrothermal liquefaction of Nannochloropsis salina (N. salina) and larvae-vermicompost were conducted under both non-hydrogenating and hydrogenating subcritical conditions using H2 and Ni–Mo/Al2O3. Hydrogenation raised biocrude yields from 33.2% to 43.5% (vermicompost) and 55.6% to 78.5% (N. salina), whereas high heat values increased from 32.89 to 34.24 MJ/kg (vermicompost) and 36.30 to 37.53 MJ/kg (N. salina). Compared with the non-hydrogenated HTL process, the contents of acids, amides, phenols, and alcohols decreased, whereas hydrocarbons content increased. More branched cyclic nitrogenous compounds were detected in the hydrogenated biocrudes, whereas the aromatic/hetero-aromatic functionality was somewhat decreased. Smaller molecular weights and polydispersity index of the hydrogenated biocrudes were also detected. Results show that hydrogenation enhanced the removal of hydrophilic functional groups and the stabilization of radicals, thereby leading to the inhibition of loss of mass toward liquid and gaseous products and the upgrading of oil quality.