Hydrothermal liquefaction of green microalga Kirchneriella sp. under sub- and super-critical water conditions

Hydrothermal liquefaction (HTL) is one of the promising and reliable thermochemical conversion processes capable of converting wet biomass feedstock into renewable bio-oils. In this study, microalga Kirchneriella sp. was liquefied under hydrothermal conditions in a stainless-steel batch reactor. Various process parameters such as reaction temperature, pressure, biomass solid loading, and reaction duration were varied from 200 to 375 °C, 9–25 MPa, 10–20%, and 15–60 min, respectively. A one-factor-at-a-time approach was employed, and comprehensive experimental runs were further performed at 10% solid loading and a reaction time of 30 min. The maximum bio-crude yield (45.5%) was obtained at 300 °C, 9 MPa, with 10% solid loading and 30 min reaction duration. Fresh algal biomass, bio-oil and biochar samples were characterized by the ultimate and proximate analyses. The bio-oil and bio-char samples obtained at 300 °C, 9 MPa, with 10% solid loading and 30 min reaction duration have a higher heating value of 37.52 and 23.48 MJ kg⁻¹, respectively. The HTL aqueous phase was analyzed for potential co-products by spectrophotometric techniques and is rich in soluble carbohydrates, dissolved ammoniacal nitrogen and phosphates. The metal impurities in the algae, bio-oil, and biochar were identified by ICP-OES where algae and biochar contain a large proportion of phosphorous and magnesium.