Assessment of Bark Properties from Various Tree Species in Terms of Its Hydrophobicity and Energy Suitability

Abstract

Wood bark is a by-product of the forestry industry with significant quantitative potential. Due to the higher heating value and hydrophobic character (dictated by the content of hydrophobic extracts in the chemical composition of the bark) facilitating long-term storage and transport, bark is considered a valuable material to be integrated into the energy industry as a substrate for combustion and co-combustion. However, the heterogeneous structure of the biomass causes significant differences in these parameters between different types of bark. For this reason, this study aimed to analyze the energy usefulness and hydrophobicity of 14 species of bark occurring in large amounts across Europe. Bark’s higher heating value has been shown to range from 17.239 MJ·kg−1 ± 0.318 MJ·kg−1 to 21.618 MJ·kg−1 ± 0.375 MJ·kg−1, and most species are extremely hydrophobic in the WDPT (water drop penetration time; 70–10,495 s) and MED (molarity ethanol droplet; 8.5–24%) tests. Nevertheless, the statistically significant differences between the species in the hydrophobicity tests indicate that the storage of individual types of bark should be adapted to the species and preceded by an analysis of its hydrophobicity. It was also shown that the WDPT test seems to be more suitable for the analysis of highly hydrophobic biomasses, due to the possibility of better differentiation of the level of hydrophobicity between species showing the same degree of hydrophobicity. In addition, the best type of bark that could be implemented in the power industry from the point of view of hydrophobicity and combustion characteristics was Silver Birch bark.