Pyrolysis of<i> Chlorella</i><i> vulgaris:</i> Kinetic analysis, advanced characterization, and bio-oil optimization

Pyrolysis of Chlorella vulgaris was investigated to evaluate its thermal degradation behavior, kinetic parameters, and potential for producing biochar and bio-oil as sustainable energy sources. Thermogravimetric analysis revealed that the main active pyrolysis occurred between 200 and 500 degrees C, corresponding to a significant release of volatiles. The resulting biochar exhibited improved carbon content (80.8 wt%), a surface area of 161.3 m2/g, and a higher heating value (HHV) of 31.6 MJ/kg. Kinetic modeling using model-free methods (Flynn-Wall-Ozawa, Kissinger-Akahira-Sunose, Starink, and Tang) estimated activation energies within the range of 138.7-141.5 kJ/mol, while thermodynamic analysis indicated an endothermic and non-spontaneous process. Gaseous products such as CO2, H2O, and CH4 were identified via TGA-FTIR/MS. Py-GC/MS analysis of the bio-oil showed a complex composition, including 42.6 wt% oxygenated compounds, 25.4 wt% nitrogenous species, and light hydrocarbons. Process optimization using the Box-Behnken design identified the heating rate as the most influential factor for maximizing bio-oil yield. The optimum conditions were determined to be a pyrolysis temperature of 500 degrees C, a heating rate of 100 degrees C/min, and a nitrogen flow rate of 50 mL/min, under which the maximum bio-oil yield of 32.08 wt% was achieved. The upgraded bio-oil exhibited enhanced fuel properties (H/C = 1.18, O/C = 0.28, HHV = 30.4 MJ/kg). In conclusion, the present study suggested that C. vulgaris appeared to be a promising renewable biomass for biofuel production using pyrolysis technology.