Growth of microalgae using CO2 enriched air for biodiesel production in supercritical CO2

• Microalgae strains were growth in CO2 enriched air.
• Growth kinetics was modeled using Haldane model.
• Effects of environment stressing on overall lipid productivity and CO2 biofixation rate were determined.
• Biodiesel was enzymatically produced from extracted microalgae lipids in supercritical CO2 medium.

The optimum conditions for lipids productivity and CO2 fixation of two freshwater strains, namely Chlorella sp. and Pseudochlorococcum sp. and a marine strain; namely Nannochlorpsis sp. have been determined in this work. The species were grown autotrophically under aeration with different CO2 concentrations, ranging from 0.04 to 2% (v/v). The growth was tested in nitrogen sufficient and deficient media at different salinities (0.49–680 mM) and temperatures of 27 and 31 °C. The optimum CO2 enrichment was found to be 1% (v/v) in both media. Nitrogen starvation resulted in an increase in lipid contents, but at lower growth rate, which resulted in a lower overall lipid productivity. The experimental data were used to determine the kinetic parameters of Haldane model. The Chlorella sp. grew well at salinity levels of up to 460 mM. The highest CO2 biofixation rate was observed when Chlorella sp. was grown at 27 °C in seawater (230 mM NaCl).Lipids were extracted from harvested marine strain, Nannochlorpsis sp., and enzymatically transesterified to produce biodiesel in supercritical CO2 (SC–CO2) medium. It was found that the conversion of biodiesel produced from microalgae lipids was 35% higher than that achieved using lamb fat in a similar system.