Thermochemical conversion of low-lipid microalgae for the production of liquid fuels: challenges and opportunities

The development of renewable biomass energy sources has attracted attention because of the potential for a sustainable fuel with a low carbon intensity. Microalgae are considered as a third generation biofuel, and have a notable advantage over other biomass in that they do not compete with food or cropland resources. The conversion of algal biomass into liquid fuels provides a long-term sustainable option for fuels production, which can be achieved in an environmentally compatible manner. Among the microalgal conversion methods, thermochemical conversion, which can make full use of all components in the algae, is viewed as one of the best conversion methods, especially for low-lipid microalgae. This article reviews recent developments in the field of algal biomass conversion into liquid fuels, with particular attention focused on the thermochemical conversion of low-lipid microalgae. We start with a brief introduction of microalgae and its biochemical components. After an overview of the main strategies involved in algal biomass conversion, we focus on the thermochemical conversion of algae, including pyrolysis and hydrothermal liquefaction and compare the two methods in detail. In addition, the catalytic upgrading of algae-derived crude bio-oil was also examined. An assessment is made of the challenges and opportunities of a commercial-scale microalgae-to-fuels process in light of mitigating technical, environmental, and logistical issues. This critical review provides an investigation elaborated by recent references on conversion of low-lipid microalgae into bio-oil via pyrolysis and hydrothermal liquefaction, and the catalytic upgrading of algal-derived bio-oil was examined.