The main goal of this work was to determine the differences in the composition of the surface and bulk of lignocellulosic feedstock subjected to torrefaction. Miscanthus × giganteus and Willow were used as widely available types of second generation of biomass. The surface of the samples was primarily characterized by time-of-flight secondary ion mass spectrometry. The bulk of the investigated biomass was analyzed by Fourier transform infrared spectroscopy, thermogravimetry and classical chemical methods. The results obtained show that the destruction of the surface of both Miscanthus × giganteus and Willow begin at lower temperature than that observed for the bulk. Moreover, in the case of Miscanthus × giganteus the possibility of the partial surface decomposition of not only the hemicellulose and cellulose but also lignin structure is pointed out. The observed differences between the behavior of the uppermost and deeper layers of the studied biomass samples indicate that the efficiency of their thermal degradation is different and should be taken into account when discussing the torrefaction process.
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