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

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 (0.49-680mM) 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 460mM. The highest CO₂ biofixation rate was observed when Chlorella sp. was grown at 27°C in seawater (230mM 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.