Structural and chemical modifications of typical South African biomasses during torrefaction

•Different fibre composition between woods and sweet sorghum bagasse.•Higher surface area increases after torrefaction for bagasse than wood.•Torrefaction reduced significantly hemicelluloses than cellulose.•Formation of acid-resistant was observed with torrefaction of bagasse.•Torrefaction treatment increases average crystallite diameter of biomass.

Torrefaction experiments were carried out for three typical South African biomass samples (softwood chips, hardwood chips and sweet sorghum bagasse) to a weight loss of 30 wt.%. During torrefaction, moisture, non-structural carbohydrates and hemicelluloses were reduced, resulting in a structurally modified torrefaction product. There was a reduction in the average crystalline diameter (La) (XRD), an increase in the aromatic fraction and a reduction in aliphatics (substituted and unsubstituted) (CPMAS 13C NMR). The decrease in the aliphatic components of the lignocellulosic material under the torrefaction conditions also resulted in a slight ordering of the carbon lattice. The degradation of hemicelluloses and non-structural carbohydrates increased the inclusive surface area of sweet sorghum bagasse, while it did not change significantly for the woody biomasses.