Effect of pellet diameter on pellet quality and torrefaction efficiency in corn stalk pellets at high torrefaction temperature

In this study, 6 mm (D6) and 8 mm (D8) diameter corn stalk pellets were produced under identical conditions and subjected to torrefaction at the upper limit temperature (320 °C). An evaluation was conducted on the fundamental physical indicators, macro-microscopic examination, fuel properties, and FTIR spectra. An increase in pellet diameter facilitated pellet production but led to a deterioration in pellet density and mechanical durability. In relation to this, it is statistically significant that D8 pellets are more sensitive to the torrefaction process than D6 pellets due to their lower physical stability. Additionally, calorific values increased from 19.7 MJ/kg to 22.5 MJ/kg for D6 pellets and 19.8 MJ/kg to 24.0 MJ/kg for D8 pellets. While there is a satisfactory increase in calorific value as a result of pellet torrefaction, the deterioration in physical properties should not be overlooked. As a result, the energy density values of pellets, both before and after torrefaction, decreased from 14.7 GJ/m³ to 7.3 GJ/m³ for D6 and from 11.9 GJ/m³ to 6.1 GJ/m³ for D8 pellets. Ultimately, these findings underscore the trade-off between energy density and mechanical durability, suggesting that optimal torrefaction conditions are crucial for enhancing fuel quality without compromising physical stability. Notably, the larger diameter of D8 pellets contributed to their increased calorific value but also heightened their susceptibility to degradation during torrefaction.