Nano-Fe2O3 Assisted Torrefaction of Pine Wood Waste: Structural Characterization, Thermal Stability, and Gaseous Hydrocarbon Analysis

The effect of nano-Fe2O3 on the structural transformation, thermal behavior, product distribution, and gas-phase hydrocarbon composition of pine wood waste during torrefaction was investigated. Torrefaction experiments were performed in a fixed-bed tubular reactor at 300 °C for 5 min under a continuous nitrogen atmosphere at 4 atm. Experiments were conducted using pine wood waste without the iron oxide additive and after homogeneous mixing with nano-Fe2O3 at a loading of 7 wt.% relative to the initial biomass mass. The structural changes in the biomass and recovered solid products were investigated by Fourier-transform infrared spectroscopy (FT-IR), while the morphology and elemental composition of the iron oxide-containing material were examined by scanning electron microscopy coupled with energy-dispersive X-ray spectroscopy (SEM–EDS). The thermal behavior of the biomass was evaluated by thermogravimetric and differential thermal analysis (TGA/DTG/DTA), and the detected gas-phase hydrocarbons were analyzed by gas chromatography. The results showed that torrefaction caused pronounced changes in the oxygen-containing and aliphatic functional groups of pine wood waste, with differences observed between the non-catalytic and nano-Fe2O3-assisted treatments. TGA/DTG/DTA analysis indicated that the principal thermal transformation of the biomass occurred within the 200–380 °C range, with the maximum overall mass-loss rate observed at 322.8 °C. The addition of nano-Fe2O3 was associated with a decrease in the recovered solid fraction from 72.17 ± 0.47 to 66.80 ± 0.98 wt.% and an increase in the gaseous fraction from 27.83 ± 0.47 to 33.20 ± 0.98 wt.%. The gas-phase chromatographic results, expressed as normalized chromatographic peak-area percentages (Norm. %), showed that the relative contribution of methane increased from 34.2% to 39.8%. The relative contributions of the C2, C3, C4, C5, and C6 fractions decreased from 15.7%, 14.5%, 14.9%, 11.9%, and 5.4% to 13.4%, 11.8%, 12.5%, 9.7%, and 4.8%, respectively, whereas the C6⁺ fraction increased from 3.4% to 8.0%. The results indicate that nano-Fe2O3 influenced the structural transformation and solid–gas product distribution of pine wood waste and modified the relative composition of the detected gas-phase hydrocarbon fractions under the applied torrefaction conditions. The principal novelty of this study lies in the combined evaluation of the structural, thermal, and gas-phase compositional changes associated with nano-Fe2O3 -assisted torrefaction of pine wood waste.