Effect of final temperature on charcoal stiffness and its correlation with wood density and hardness

Mechanical performance is important for charcoal used in blast furnaces as charcoal layers may support the load of iron ore within blast furnaces without breaking. Pyrolysis temperature influences charcoal quality and its chemical composition; however, the effect of temperature on physical-mechanical properties is still unknown. Thus, the aim of this study was to establish the effect of pyrolysis temperature on the density, stiffness and hardness of charcoal and the correlations among them. Four pyrolysis processes were performed at final temperatures of 300, 450, 600 and 750 °C using wood specimens from nine different tree species. Stiffness of wood and charcoal was determined by ultrasound and dynamic hardness by a portable hardness tester. In short, density, dynamic hardness and stiffness of charcoal tend to decrease with increasing temperature. Pyrolysis at 450 °C decreases charcoal stiffness by approximately 30%, while hardness is reduced from 29 MPa in wood specimens to approximately 3 MPa in charcoal pieces produced from the same specimens. Considering the wood, the highest values of stiffness were presented by the same materials which presented highest density, confirming high positive correlation between wood properties. The correlation between density and stiffness in charcoal is higher than in wood. However, correlation between density and dynamic hardness or stiffness and dynamic hardness is higher for wood than for charcoal. Ultrasound was able to determine differences in stiffness between the materials at different pyrolysis temperatures. These findings are useful to identify the best production temperature for industrial charcoal with adequate mechanical properties.