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.