Chemical characterisation and durability assessment of torrefied radiata pine (Pinus radiata) wood chips

Wood chips from New Zealand grown radiata pine were torrefied at 220, 260, and 300 °C with the aim of understanding the fundamentals behind the enhanced durability against major decay fungi. Chemical analysis methods, including high-resolution synchrotron-based X-ray diffraction, were used to ascertain the mode of chemical changes in wood after various torrefaction levels. Compositional analysis showed that the carbohydrates and lignin in 220 °C samples remained at about the same levels as in the control, with a noticeable drop in the relative ratio of carbohydrates levels and an increase in lignin levels at 260 °C, and a steep drop in carbohydrates and a sharp increase in lignin concentrations at 300 °C. Hemicelluloses were the most severely affected carbohydrate component, particularly with 300 °C treatment. Analysis with FTIR showed similar spectra for control and 220 °C treated chips, a broad shoulder around 1610 cm−1 at 260 °C, and several changes at 300 °C involving carbonyl at 1705 cm−1 and cc and c-c absorptions. Synchrotron-based X-ray diffraction showed a slight contraction of peak d(200) for torrefied samples, which is related to increasing hydrophobic cellulose crystallinity. The information obtained suggests that the durability enhancement achieved against Oligoporus placenta and Trametes versicolor at 260 °C can be attributed mainly to depletion of hemicelluloses, and complete resistance at 300 °C is likely to be related to changes in all cell wall components, particularly hemicelluloses and lignin.