Biomass is a promising renewable solid fuel, but its low energy density and poor grindability limit its direct utilization. In this study, wood pellets were torrefied at 220–300 °C to evaluate fuel upgrading and identify an effective torrefaction condition. Physicochemical properties, solid and energy yields, grindability, and TG–DTG characteristics were analyzed together with optical petrography and fluorescence. The mean fluorescence intensity decreased markedly from approximately 139 at 220 °C to 84 at 240 °C and further to 69 at 300 °C, indicating progressive structural transformation during torrefaction. In contrast, optical intensity decreased up to 280 °C and subsequently increased at 300 °C, demonstrating distinct optical responses to structural evolution. The integrated assessment provides a basis for selecting an effective torrefaction condition by balancing fuel upgrading, energy retention, grindability, and structural transformation.
 

Torrefaction, Biochar, Petrography, Fuel upgrading