Effect of Torrefaction on the Physicochemical Properties and Pyrolysis Behavior of Raw and Torrefied Varieties of Selected Biomass Fuels

Abstract

This study investigates the effect of torrefaction on the physicochemical properties, elemental composition, thermal stability, and fuel upgrading potential of five widely available Nigerian biomasses: rice husk (RH), corncob (CC), groundnut shell (GNS), palm kernel shell (PKS), and sawdust (SD). Raw samples were characterized to establish baseline values for moisture, ash, volatile matter, fixed carbon, and heating value, revealing significant compositional differences driven by lignin, cellulose, and ash distributions. Torrefaction was conducted at 230°C, 260°C, and 290°C to evaluate changes in mass yield, energy yield, higher heating value (HHV), and elemental ratios. The results show a consistent reduction in mass yield coupled with substantial increases in HHV across all feedstocks, confirming effective devolatilization and carbon densification. GNS and PKS achieved the highest upgraded HHVs, reaching 27.1 MJ/kg and 26.2 MJ/kg, respectively, at 290°C, while RH exhibited the least improvement due to its high silica-rich ash content. Van Krevelen analysis indicated marked decreases in O/C and H/C ratios, demonstrating progressive deoxygenation shift toward coal-like characteristics. Thermogravimetric analysis further revealed enhanced thermal stability, upward shifts in devolatilization onset temperatures, and increased char residue in torrefied samples, particularly for GNS and PKS. Overall, the study confirms that torrefaction significantly enhances the energy quality and thermochemical suitability of Nigerian agricultural residues. GNS and PKS show strong potential as high-grade solid biofuels, while CC and SD present viable options for pelletization and medium-scale combustion. Although RH requires blending due to its high ash content, the collective findings highlight the role of torrefied biomass in supporting sustainable energy production and advancing Nigeria’s bioenergy sector.