A review on organosolv pretreatment of softwood with a focus on enzymatic hydrolysis of cellulose

A clean and efficient fractionation of biomass into cellulose, hemicellulose and lignin streams is a key step in biofuel and biorefinery industries. Lignin-free cellulose and hemicellulose streams can be enzymatically hydrolysed to sugars for fermentation to different biofuels, high value biochemicals and biopolymers. Towards this objective, organosolv pretreatment is the promising strategy to separate lignin from carbohydrates. Scientific information on the organosolv pretreatment using lignocellulosic biomass in general and various organic wastes is available with a focus on lignin separation and its use. Amongst different lignocellulosic biomass, softwood is a challenging feedstock due to recalcitrance towards enzymatic hydrolysis of cellulose. The aim of this review is to describe technical and research efforts on various organosolv processes developed specifically for softwood as a feedstock and how it influences enzymatic hydrolysis of cellulose. Process severity factor, selection of the solvent and choice of a catalyst in organosolv process are discussed. The differences in conventional pulping versus organosolv pretreatment and physico-chemical changes that occur in organosolv fractionated cellulose, lignin and hemicellulose are explained. Pilot and demonstration scale organosolv treatment plants and the challenges they face going towards commercialisation, as well as a path to the future growth and advancement in softwood organosolv processes, are discussed. Graphical abstract