Pathways of lignocellulosic biomass conversion to renewable fuels

The increased worldwide demand for energy, particularly from petroleum-derived fuels has led to the search for a long-term solution of a reliable source of clean energy. Lignocellulosic biomasses appear to hold the key for a continuous supply of renewable fuels without compromising with the increasing energy needs. However, themajor possible solutions to the current energy crisis include ethanol, bio-oils and synthesis gas (syngas) produced from lignocellulosic biomass. Recently, a great deal of research has been made in the fields of biomass conversion through biochemical, hydrothermal or thermochemical pathways to biofuels. However, a broadspectrum assessment of the above pathways is rare in literature in terms of technology used, biofuel yields, potential challenges and possible outcomes. This review paper discusses different routes for biofuel production, particularly ethanol, bio-oil and syngas with the bio-oil upgrading techniques. This review highlights ethanol fermentation and available biomass pretreatment as the biochemical mode, not limiting to the pros and cons of the pretreatments. Supercritical water gasification (hydrothermal pathway) of biomass for syngas production followed by gas-to-liquid technologies (syngas fermentation and Fischer-Tropsch catalysis) has been discussed. In addition, thermochemical pathway dealing with biomass gasification for syngas and pyrolysis for bio-oils has been presented with compositional analysis of bio-oils and their upgrading technologies. The review focuses on various engineering limitations encountered during biomass conversion and bioprocessing with the potential solutions which do not restrict them to different biofuel production pathways.