Influence of wood density and final temperature of pyrolysis on the mechanical properties of charcoal

Abstract

Hardness, stiffness, porosity and crystallinity have been considered important indicators of mechanical quality of many materials. However, there is a lack of specific methodologies to evaluate mechanical properties of charcoal, making it difficult to standardize the tests. New techniques for properly classifying charcoal are necessary and non-destructive methods can be an efficient alternative to classifying charcoal in terms of quality. The hypothesis of this study is that hardness and stiffness obtained by non-destructive methods can be used as a new criterion for classifying quality of charcoal in terms of mechanical performance in blast furnaces. Therefore, the aim of this study was to determine to what extent wood density and final pyrolysis temperature affect hardness, stiffness, surface area and crystallinity of charcoal for steel use. For this, nine plant materials were pyrolyzed at final temperatures of 300, 450, 600 and 750°C, were analysed: stiffness, by ultrasound; surface area, by BET method; crystallinity, by X-Ray diffraction; dynamic hardness, by Portable Hardness Tester and classification of density and hardness by Near Infrared Spectroscopy. Mean dynamic hardness of untreated wood was 29.8 MPa, the stiffness was 3,920 MPa and the density was 0.791 g.cm -3 . In general, pyrolysis temperature decreases mechanical properties of material. Increase temperature from 300 to 750°C decreases hardness, 10.89 to 3.05 MPa, the stiffness from 2,794 to 1,278 MPa and density from 0.634 to 0.444 g.cm -3 . Charcoals produced at temperature at 450, 600 and 750°C exhibit similar behaviour, with low absorbance compared to wood and thermally treated material at 300°C, indicating homogenization of charcoal caused by pyrolysis temperature. NIR spectroscopy models were successfully developed for estimating pyrolysis final temperature, charcoal density and its dynamic hardness. The statistics of the models indicate that NIR spectroscopy is an efficient solution to quickly estimate density and hardness of charcoal. Considering the genetic materials, the highest values of wood stiffness were presented by C. citriodora and E. deglupta, same materials presented highest density and dynamic hardness, confirming high positive correlation between wood properties. Correlation between density and stiffness was R²=0.87, for charcoal R²=0.99. Correlation between density and hardness was R²=0.93, for charcoal R² = 0.69. Correlation between stiffness and hardness of wood was R²=0.79, final pyrolysis temperature negatively affects correlation between these properties. The surface area of pyrolyzed samples at 300 and 450°C were close to zero, while the pyrolyzed samples at 600 and 750°C were 358 and 327 m²/g, respectively. In general, charcoals presented a typical behaviour of micro pores. Spectra obtained by X-ray diffraction show a change in spectral pattern of materials submitted to different pyrolysis temperatures, increased pyrolysis temperature increases formation of graphite crystals. At temperature of 750°C, similarity of spectra obtained by X-ray diffraction of different species, as well as similarity in adsorption curves of N2 indicate the efficiency of pyrolysis process in charcoal homogenization.