Biodiesel production from microalgae: A comprehensive review on influential factors, transesterification processes, and challenges

•Biodiesel production from third-generation feedstock viz., microalgae.•Pre-treatment methods, extraction methods, and transesterification processes of biodiesel production from microalgae are discussed.•Direct and indirect transesterification processes in the transesterification process are elaborated. The impending depletion of fossil fuels has sparked a global hunt for non-conventional energy sources such as biofuels. Biofuels such as biodiesel can be produced using third-generation feedstock viz., microalgae. It has gained huge attention as it has the potential to achieve the goals of sustainable development. Biodiesel production using microalgae depends upon the microalgal biomass availability and the amount of neutral lipid accumulated inside them. Biomass productivity and lipid accumulation vary from species to species but in general, are influenced by cultivation conditions (mode and method of cultivation) and growth parameters such as nutrients and environmental factors (light, temperature, pH, aeration, and photoperiod). Lipid extracted from microalgae is transesterified into biodiesel. This review discusses several factors, viz., pre-treatment methods, extraction methods, and transesterification processes that influence the quality and quantity of biodiesel production from microalgae. Explaining the difference between direct and indirect transesterification processes, the review elucidates the roles of catalyst(s) in the transesterification process and FAME production. It further discusses about the cost and challenges associated with each step of biodiesel production from microalgae.