Estimating hardness and density of wood and charcoal by near-infrared spectroscopy

Hardness has been considered an important indicator of wood quality and presents a high correlation with density, and, as far as the authors know, there is no specific methodology to evaluate the hardness of charcoal. In this context, it is necessary to develop a reliable and efficient methodology to classify charcoal mechanical properties. Therefore, the aim of this study was to establish multivariate models to estimate dynamic hardness and density of wood and charcoal based on near-infrared spectra. For this, nine wood specimens were examined and pyrolyzed at 300, 450, 600 and 750 degrees C. The density of wood and charcoal was determined according to standard NBR 11941 (2003) and hydrostatic method, respectively. Dynamic hardness of wood and charcoal was determined by an automated portable hardness tester. Materials submitted to thermal treatment have different spectral signatures compared to those obtained by the wood. Charcoals produced at temperatures of 450, 600 and 750 degrees C exhibit similar behaviour, with low absorbance compared to wood and thermally treated material at 300 degrees C, indicating homogenization of charcoal caused by pyrolysis temperature. NIR spectroscopy was able to estimate density and dynamic hardness of wood and charcoal. Charcoal produced at 450 degrees C obtained a model with highest coefficient of determination and smaller errors. The results show the potential of NIR technology for fast and reliable estimation of physical and mechanical properties of charcoal based on spectra recorded on charcoal and even from wood before pyrolysis.