A sustainable integrated in situ transesterification of microalgae for biodiesel production and associated co-product-a review

Microalgae has large scale cultivation history particularly in aquaculture, pigments and nutraceutical production. Despite the advantages of microalgal oil as feedstock for biodiesel production, algal biodiesel is still at laboratory scale due to technical challenges required to be overcome to make it economical and sustainable. Indeed, complete drying of microalgae is energy intensive and significantly increases the cost of algae pre-treatment. In situ transesterification is more water tolerant due to excess methanol to oil molar ratio required by such production route. However, the need to remove unreacted methanol (>94% of it) from the product streams certainly requires distillation heat load which increases the operating cost. This article reviews the key process variables affecting efficiency of in situ transesterification. These include alcohol to oil molar ratio, moisture, stirring rate, reaction time, temperature, microalgal cell wall and catalyst type. Potential solutions of improving the efficiency/economy are discussed. Overall, an integrated approach of in situ dimethyl ether (DME) production along with the desired biodiesel synthesis during in situ transesterification would substantially reduce the volume of unreacted methanol thereby reduces operating cost. Use of resulting microalgal residue for biogas (methane) production can provide energy for biomass production/separation from the dilute algae˗water mixture. Use of bio˗digestate as nutrients for supporting microalgal growth is among the probable solutions suggested for reducing the production cost of in situ transesterification.