Sustainable biofuels: opportunities and challenges

Global energy demand is expected to increase by 48% in the next 20 years owing to the precipitous increase in the global population. Currently, 80% of the energy demand is met by fossil fuels. However, rapidly depleting fossil fuel reserves coupled with the negative environmental impacts from its combustion has prompted significant interest in sustainable biofuels. This will aid in the transition toward a carbon-neutral bio-economy. Several feedstocks have been identified as possible substrates for biofuel production. Agricultural residues have shown significant potential since they are environmentally benign, abundant, and low cost. Nevertheless due to its structural complexity, an appropriate pretreatment is required to enhance enzymatic and microbial conversion. Currently, first-generation biofuels such as bioethanol do not require intensive pretreatments; however, the major drawback is the utilization of food crops, thus contributing to the food versus fuel debate. Additionally, greenhouse gas emissions associated with first-generation biofuels are another obstacle. Second-generation biofuels such as bioethanol, biohydrogen, and biomethane appear to be most promising owing to its bioconversion from waste material. A major bottleneck in this process is the requirement of costly pretreatments and subsequent effluent treatment. Third-generation biofuels such as bioethanol from microalgae also show potential since process optimization could significantly enhance yields. Fourth-generation biofuels aim to utilize genetically optimized feedstocks that are designed to enhance capture of carbon dioxide; however, carbon capture and sequestration technology has limited the commercialization of this process. Integrated biorefineries have the potential to produce several generations of biofuels in one process, thereby completing valorizing the feedstock and enhancing the life cycle and techno-economic assessment of the bioprocess.