corn stover

enzymatic digestibility

pretreatment

hydrothermal carbonization

lignocellulosic biomass

bioethanol production

glucose yield

cellulose hemicellulose

Chemosphere

The pretreatment of lignocellulosic biomass enhances the conversion efficiency to produce biofuels and value-added chemicals, which have the potential to replace fossil fuels. Compared to physicochemical and other pretreatment techniques, the hydrothermal methods are considered eco-friendly and cost-effective. This paper reviews the strengths, weaknesses, opportunities and threats of steam explosion and subcritical water hydrolysis as the two promising hydrothermal technologies for the pretreatment of lignocellulosic biomass. Although the principle of the steam explosion in depolymerizing the lignin and exposing the cellulose fibers for bioconversion to liquid fuels is well known, its underlying mechanism for solid biofuel production is less identified. Therefore, this review provides an insight into different operating conditions of steam explosion and subcritical water hydrolysis for a wide variety of feedstocks. The mechanisms of subcritical water hydrolysis including dehydration, decarboxylation and carbonization of waste biomass are comprehensively described. Finally, the role of microwave heating in the hydrothermal pretreatment of biomass is elucidated. • Hydrothermal pretreatments such as steam explosion and subcritical water are elaborated. • Properties, mechanism, pros and cons of these hydrothermal pretreatments are explained. • Microwave-assisted pretreatments are more advantageous than conventional techniques. • These pretreatments are economical and efficient to depolymerize lignin and hemicellulose. • Biomass degradation products e.g., furfurals and phenolics can inhibit fermentation process.

Hydrothermal pretreatment technologies for lignocellulosic biomass: A review of steam explosion and subcritical water hydrolysis