Two important challenges need to be addressed to realize a practical biorefinery for the conversion of biomass to fuels and chemicals: (i) effective methods for the degradation and fractionation of lignocelluloses and (ii) efficient and robust chemical methods for the conversion of bio-feeds to target products via highly selective catalytic reactions. Ultrasonic energy promotes the pretreatment and conversion process through its special cavitational effects. In this review, recent progress and methods for combining and integrating sonication into biomass pretreatment and conversion for fuels and chemicals are critically assessed. Ultrasonic energy combined with proper solvents allows destruction of the recalcitrant lignocellulosic structure, fractionation of biomass components, and then assists many thermochemical and biochemical reactions, with increased equilibrium yields of sugars, bio-ethanol and gas products by 10-300%. Sonication promotes hydrolysis, esterification and transesterification in biodiesel synthesis and leads to reduced reaction time by 50-80%, lower reaction temperature, less amounts of solvent and catalyst than comparable unsonicated reaction systems. For algal biomass, sonication benefits the disruption, lysis and content release of macro and microalgae cells, and reduces the time required for subsequent extraction and chemical/biochemical reactions, with efficiencies typically being improved by 120-200%. High-frequency ultrasound of 1-3 MHz allows harvesting of microalgae, liquid product separation and in-situ process monitoring of biomass reactions, while highintensity ultrasound at 20-50 kHz activates heterogeneous and enzymatic catalysis of the biomass reactions. The use of ultrasound in conversion of biomass to biofuels provides a positive process benefit.
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