Bioenergy characteristics of microalgae under elevated carbon dioxide

Abstract

Nowadays, biological mitigation of carbon dioxide (CO2) by means of microalgae is one of the promising approaches, as biomass can be used to produce renewable fuels such as biodiesel and crude bio-oil. In this study, the growth ofDesmodesmussp. in response to different CO2levels (ambient air, 5%, 10%, 15%, 20%, 25%, 30%, and 35% CO2) was studied, and then the suitability of the biomass for bio-oil and biodiesel production was evaluated. The studied strain was able to grow efficiently in a broad range of elevated CO2(5–30%,v/v), suggesting its suitability for CO2mitigation. The highest specific growth rate (0.769 d−1) and biomass productivity (0.033 g L−1d−1) were achieved at 15% CO2; therefore, the concentration of 15% CO2was selected as the optimum. Alpha-Linolenic acid (ALA) was the dominant polyunsaturated fatty acids (PUFAs) in studiedDesmodesmusstrain, and its content increased from 31.82% under ambient air to 41.94% and 50.11% under 15% CO and 30% CO2, respectively, suggesting a key role of ALA in response to CO2elevation. Using fatty acid methyl ester (FAME) profiles of cultures in different CO2conditions, biodiesel parameters such as cetane number (CN), saponification value (SV), iodine value (IV), long chain saturation factor (LCSF), oxidative stability (OS), higher heating value (HHV) and degree of unsaturation (DU) were estimated. Overall, the lipid extract was not suitable for biodiesel production due to high PUFA content ranging between 52.91% and 71.14%. Under optimum CO2(15%), saturated fatty acids (SFAs) content increased to 31.91% and PUFA content reduced to 52.91%. This could significantly improve the CN to 38.07, which was 11% and 13% higher than theair and 30% CO2. Pyrolysis gas chromatography-mass spectrometry (Py-GC-MS) was used to analyze the potential of microalgal biomass for bio-oil production upon CO2changes. The pyrolysate contained 40–47% acid content due to the high content of fatty acids. The pyrolysate quality declined under 15% CO2due to the higher accumulation of acidic and nitrogen-containing compounds and reduced hydrocarbon proportion.In conclusion, the optimum CO2could enhance biodiesel parameters such as CN, but demonstrated negative effects on bio-oil quality.