Résumé
•Хylans are less stable than mannans during torrefaction of wood samples.•Cellulose in hardwoods is more resistant to torrefaction than in softwoods.•Torrefaction at 250 and 300°C increases the total crystallinity index of cellulose.•Torrefaction increases the relative content of aromatic structures in samples.•Deconvolution of TGA curves is the rapid method to trace the changes in wood samples.
Torrefaction – the process of soft pyrolysis (200–300°С) in inert atmosphere – is considered to promote the usage of lignocellulosic biomass in various technologies. The initial raw material is not uniform in composition and we compared the effect of torrefaction on the samples of hardwood (birch) and softwood (pine). The major differences between the torrefied samples were observed between 225 and 250°С and were largely connected with different behavior of hemicelluloses. Monosaccharide analysis revealed the decrease in detectable xylose from 26% to 1% (250°С) of the raw sample in birch, and from 11% to 1%—in pine. Mannans were more resistant to degradation. Comparison of data from HPAEC, thermal analysis and IR-spectroscopy revealed that hemicelluloses are modified during torrefaction at 225–250°С, rather than fully degraded and removed from the sample. This may lead to considerable modification of wood properties, more pronounced in hardwoods. The relative content of aromatic structures went up during torrefaction, part of the effect was due to condensation of modified carbohydrate units. Index of cellulose crystallinity increased in torrefied samples. The content of cellulose in birch samples remained the same as in raw sample up to 250°С, while in pine it dramatically decreased after the torrefaction at 250°С. Torrefaction at 300°С made the samples of hardwood and softwood very much alike. The perspectives of usage of hardwoods and softwoods torrefied at different temperatures are discussed.