Evoking research attention back on liquid hot water pretreatment: a comprehensive review

Abstract

Refining lignocellulose into energy, chemicals, and materials offers a possible approach to replace petrochemicals. After biomass chipping, pretreatment is the first and foremost step in biomass processing, as it causes successful fractionation of cellulose, hemicellulose, and lignin to realize their individual conversion. Numerous solvents have been proposed as media for biomass pretreatment, which generally utilize chemicals to accelerate biomass dissolution and inevitably cause environment pollution. In this regard, green liquid hot water pretreatment (LHWP) using water as the sole medium seems to be a perfect technology. However, research on LHWP has diminished in recent years owing to the emergence of other advanced solvents. To draw attention back on LHWP, this review provides a comprehensive discussion of its mechanism, application potential, and advances compared to other solvents. In particular, the structural changes of the biomass during LHWP are discussed in detail which has been seldom considered in other reviews. Finally, the challenges and perspectives of LHWP are also analyzed. Overall, the authors believe that LHWP should be considered not only for bioethanol production but also as a route for biomass fractionation, which could contribute to establishing a modern biorefinery for various coproducts based on cellulose, hemicellulose, and lignin.