Current status of chemical- or enzyme-assisted ultrasonic pre-treatment processes for lignocellulosic biomass to assess industrialization progress: A review

Global warming and rising pollution levels require a paradigm shift from fossil fuels to renewable feedstock. The valorization of lignocellulose, a virtually endless resource, implies the selective extraction of the three main components, cellulose, hemicellulose and lignin, to then treat them separately. Among the methods of pretreatment/preferential dissolution of biomass, low-frequency ultrasound (US) has shown to be a promising disruptive technology. Eager to be combined with physical technologies, chemical agents or enzymes, many examples under low-frequency US exist at the lab scale. However, examples of scaling-up of US-processing of biomass remain yet scarce. It appears quite challenging to design ultrasonic equipment that allows sufficient and homogeneous energy powers in large volumes, although recent pioneering work shows considerable progress. This review aims at highlighting the latest works on biomass pretreatment under chemically or enzymatically assisted ultrasonic irradiation on both lab and pilot/semi-industrial scales together with future directions to enable scale-up of ultrasonic processes for biomass valorization.