Pressurized Torrefaction: Physiochemical characterization of torrefied miscanthus and beechwood

Pressurized torrefaction (PT) is proposed as an effective pretreatment technology for enhancing the fuel properties of lignocellulosic biogenic residues compared to torrefaction at atmospheric conditions. This study investigated the physical and chemical properties of torrefied miscanthus and beechwood using X-ray diffraction (XRD), Fourier Transform Infrared Spectroscopy (FTIR), surface morphology analysis, and equilibrium moisture content (EMC). Torrefaction was performed in a 600 cm3 batch autoclave reactor under varying conditions: temperatures of 250 °C, pressures of 1.0 MPa, 1.4 MPa, and 1.8 MPa, and residence times of 30 and 60 min. Key performance metrics, including reductions in volatile matter, oxygen content, and enhancements in higher heating value (HHV), as well as carbon enrichment and mass/energy yields, were evaluated. Results indicated a reduction in volatile matter by 10.9–32.5 %, carbon enrichment by 12.4–33.4 %, and oxygen removal by 13.9–37.5 % for both miscanthus and beechwood. The HHV of the torrefied biomass reached 25.6 MJ/kg for miscanthus and 26.2 MJ/kg for beechwood, compared to 18.9 MJ/kg and 19.8 MJ/kg, respectively, for the raw biomass. Pressure was found to influence chemical energy preservation and stabilize energy yield during the torrefaction process. Aromaticity decreased while crystallinity increased with higher pressures and longer residence times, while the EMC of the torrefied biomass was consistently reduced under different relative humidity conditions. Scanning electron microscopy (SEM) showed that higher pressures led to increased surface roughness and altered porosity in the torrefied biomass. This study concludes that pressurized torrefaction is a promising method for upgrading biomass, enhancing its suitability for advanced thermochemical processes and sustainable energy production.