Evolution retrospective for alternative fuels: First to fourth generation

The ever increasing worldwide demand for liquid and gaseous fuels along with overwhelming environmental concerns for greenhouse gas emissions have driven scientists and technologists to consider different alternative energy sources. In past decades, several biomass sources have been identified with increasing potential to be used as new alternative sources of energy - the "Biofuels". The evolution of biofuels is classified into four different generations.In this article an overview of the systematic evolution of different biofuel generations with their advantages and disadvantages has been presented. The advancements in technology, reduction in greenhouse gas emission and assessment of commercial production cost of each generation of biofuel have also been highlighted. Finally this review provides an outlook for a better future generation biofuel.