Linking torrefaction mechanisms to combustion kinetics and thermodynamics of hardwood logging residues

Thermochemical pretreatment techniques have been widely applied to improve the fuel properties of biomass. In this study, thermogravimetric-differential scanning calorimetry (TGA-DSC) was employed to investigate the effects of three torrefaction methods on the combustion characteristics, reaction kinetics, and thermodynamic properties of red maple logging residues. Proximate and ultimate analyses, elemental analysis, Fourier-transform infrared spectroscopy (FTIR), and X-ray diffraction (XRD) were used to examine the fundamental physicochemical properties of both torrefied and untorrefied samples. The results showed that inert torrefaction (NT) produced the highest solid yield (91.42%), thereby retaining the largest amount of carbon. Although hydrothermal torrefaction (HT) resulted in the highest carbon content (65.21%), its low yield (39.60%) led to substantial carbon loss. Combustion analysis revealed that weight loss behavior varied with heating rate. As the heating rate increased from 10 °C/min to 40 °C/min, the first-stage weight loss of NT, oxidative torrefaction (OT), and untorrefied (UT) samples ranged from 59.9% to 74.0%, while second-stage losses ranged from 22.0% to 37.9%. In contrast, the HT sample exhibited first-stage losses between 37.7% and 41.9%, and second-stage losses between 53.0% and 57.9%. The activation energies calculated using the Kissinger-Akahira-Sunose (KAS) and Flynn-Wall-Ozawa (FWO) methods were comparable on average. In the first stage, the activation energy followed the order HT > OT > UT ≈ NT; in the second stage, it followed HT > OT > UT > NT. This study provides new insights into the thermochemical pretreatment and conversion of hardwood logging residues and contributes to the development of sustainable strategies for renewable energy and material applications. • Evaluated thermochemical pretreatments for hardwood logging residues. • Inert torrefaction retained most solids while hydrothermal raised C % at low yield. • KAS and FWO gave consistent Ea with hydrothermal highest in both stages. • Findings inform biomass pretreatment for efficient and sustainable energy.