Parametric study and fuel quality assessment of biofuel from hydrothermal liquefaction of microalgae grown in municipal wastewater

Hydrothermal liquefaction (HTL) of algal biomass is a promising approach for renewable biofuel production. The actual study investigates the effects of reaction temperature (225-325 °C), residence time (15-60 min), algae-to-water mass ratio (1:5-1:20), and pressure on the yield and quality of biofuel derived from municipal wastewater-grown mixed algal-cyanobacterial biomass. Eleven HTL experiments were conducted, and the resulting products were separated into gas, liquid, and solid phases for thermal and chemical analyses. Selected biofuel samples were characterized using gas chromatography-mass spectrometry (GC-MS), elemental analysis, and thermogravimetric analysis (TGA). The biofuels contained complex mixtures of aliphatic hydrocarbons, aromatics, phenolics, carboxylic acids, esters, and nitrogen-containing compounds, classified into biogasoline, bio-jet fuel, biodiesel, and motor oil fractions. Optimal yields of biofuel, gas, and solid residues were 16.86%, 26.14%, and 40.43%, respectively, achieved at a 1:10 algae-to-water ratio, 30 min reaction time, and 250 °C. The biofuel composition comprised 11.37% gasoline, 29.41% kerosene, 9.71% diesel, with a heating value of 42.93 MJ·kg⁻¹. A higher fraction of gasoline, kerosene and diesel-range compounds enhances energy density and combustion stability, while lower oxygen and nitrogen content improves storage and fuel properties. Solid residues exhibited uniform physical properties but were unsuitable for high-grade biochar due to low carbon and high inorganic content. These findings demonstrate that HTL of municipal wastewater-grown microalgae is a viable route for sustainable biofuel production, integrating resource recovery with renewable energy generation, while systematically evaluating key operational parameters and characterizing the resulting biofuel for downstream applications.