Strategies to improve methane content in biogas by cultivation of oleaginous microalgae and the evaluation of fuel properties of the microalgal lipids

Abstract

This study aimed to improve methane content in biogas by feeding biogas through the culture of oleaginous microalgae. As oleaginous microalgae have ability to mitigate CO2 into lipids and accumulate at the content >20%, this process not only contributes to CO2 removal from biogas but also producing microalgal biofuel. Among the species tested, Scenedesmus sp. and marine Chlorella sp. are suitable for this purpose. However, Scenedesmus sp. was selected due to the higher CO2 removal ability and the optimized conditions were as follows: gas flow rate of 0.3 L h−1 per 1 L- microalgal culture inoculated with 107 microalgal cells mL−1, added with KNO3 0.8 g L−1 as nitrogen source and illuminated at 5.5 klux light intensity. Under these conditions, methane content in biogas was increased from 60% up to >90% coupled with lipid productivity of 88.57 mg L−1 day−1. With the strategy of stepwise-increasing gas flow rate to support the increasing biomass, the final biomass and lipid productivity were 1.25 and 1.79 folds increased. CO2 removal rate was as high as 5.097 g-CO2 day−1 per 1 L-microalgal culture. Fuel properties calculated based on fatty acid composition indicated high oxidation stability and high ignition quality of the microalgal lipids-derived biodiesel.