Anaerobic conversion of microalgal biomass to sustainable energy carriers – A review

► Microalgal biomass is amenable to anaerobic energy carrier production. ► The highest energy yields have been reported for ethanol and CH4. ► The highest butanol and H2 fermentation yields are still relatively low. ► Simultaneous and sequential production of several energy carriers is also considered. ► Energy yields from microalgae are similar to those from other feedstocks. This review discusses anaerobic production of methane, hydrogen, ethanol, butanol and electricity from microalgal biomass. The amenability of microalgal biomass to these bioenergy conversion processes is compared with other aquatic and terrestrial biomass sources. The highest energy yields (kJg−1drywt. microalgal biomass) reported in the literature have been 14.8 as ethanol, 14.4 as methane, 6.6 as butanol and 1.2 as hydrogen. The highest power density reported from microalgal biomass in microbial fuel cells has been 980mWm−2. Sequential production of different energy carriers increases attainable energy yields, but also increases investment and maintenance costs. Microalgal biomass is a promising feedstock for anaerobic energy conversion processes, especially for methanogenic digestion and ethanol fermentation. The reviewed studies have mainly been based on laboratory scale experiments and thus scale-up of anaerobic utilization of microalgal biomass for production of energy carriers is now timely and required for cost-effectiveness comparisons.