Promising evolution of biofuel generations. Subject review

Biofuels have attracted a lot of attention due to the increasing demand on energy resources as well as elevated concerns about greenhouse gas emissions. In contrast to other green energy resources, biofuels can provide liquid fuels which are essential for transportation. This review reports recent advances in liquid biofuels, focusing on their generations and types. Generally, biofuels are classified into four generations based on the type of the feedstock that is used. First generation biofuels utilize edible biomass which sparked controversy because it competes with global food needs. Second generation biofuels use non-edible biomass but there are still some limitations related to the cost-effectiveness involved in scaling the production to a commercial level. Third generation biofuels use microorganisms as feedstock, while fourth generation biofuel focuses on modifying these microorganisms genetically to achieve a preferable hydrogen to carbon (HC) yield along with creating an artificial carbon sink to eliminate or minimize carbon emissions. These last two generations of biofuel are still in early development stages. This article reviews and summarizes 124 papers, 77% of which were published within the last three years. The aim of this work is to provide an overview of the four liquid biofuel generations as well as the latest development efforts in this field. This review concludes that the current production methods of biofuel in the first and second generations will soon fail to satisfy the increasing demand on biofuel. Therefore, development efforts should be focused on third and fourth generations, specifically the genetic engineering of algae.