Thermochemical Characterization of Untorrefied and Torrefied Wood Dust Biomass: Evaluation of Energy Value, Proximate and Elemental Composition, and Surface Morphology

This study investigates the thermochemical properties of untorrefied and torrefied wood dust biomass derived from selected hardwood and softwood species, with the aim of identifying suitable feedstocks for bio-coal production. High Heating Value (HHV) analysis revealed that Cassia fistula and Mangifera indica possessed the highest energy contents among the tested hardwood and softwood samples, respectively. Upon torrefaction at temperatures ranging from 200°C to 300°C, both samples exhibited significant improvements in HHV, with Cassia fistula reaching a maximum value of 29.67 MJ/kg—approaching the energy density of coal. Further analysis demonstrated a corresponding enhancement in energy yield and solid yield, with Cassia fistula exhibiting a higher energy retention capacity. Fiber composition analysis showed a marked increase in lignin content and a reduction in hemicellulose with increasing torrefaction severity, confirming thermal degradation trends. Proximate analysis indicated substantial reductions in moisture and volatile matter, alongside a notable increase in fixed carbon content, improving the combustion quality of the biomass. Elemental analysis and Van Krevelen plotting confirmed a decrease in H/C and O/C atomic ratios, indicating carbonization and improved fuel properties. Surface morphology analysis using Scanning Electron Microscopy (SEM) highlighted structural changes from compact fibrous networks in raw biomass to more porous and fractured structures in torrefied samples. These structural alterations, along with thermochemical enhancements, confirm that torrefaction significantly improves the energy potential, stability, and grindability of biomass, supporting its viability as a renewable solid fuel.