Analysis of functionality distribution and microstructural characteristics of upgraded rice husk after undergoing non-oxidative and oxidative torrefaction

• Oxidative torrefaction modified elemental and compositional characteristics more effectively; • Increasing torrefaction severity lowered the amount of hydrophilic group; • Flue gas was more economical and efficient to strengthen biomass stability and reactivity; • Relative amount of N-5 increased after undergoing flue gas torrefaction; • Hydrophobicity was strengthened for appropriate oxidizing agent addition. Rice husk, a typical agricultural by-product was used for experimental investigation on characterizing the variations in fuel behaviors before and after torrefaction performed under different conditions. The effects of torrefaction temperature (493, 543 and 573 K), atmosphere (argon, air and flue gas) and duration (30 and 60 min) on the compositional variation, microstructural evolution, functional group distribution and surface hydrophobicity were evaluated systemically through the utilization of elemental analysis, FTIR, Raman spectra, XPS and contact angle. Oxidative torrefaction expressed a tendency to improve the torrefaction efficiency and increase the carbon content in torrefied sample due to the simultaneous occurrence of volatile release, carbonization and surface oxidation. Oxidizing agents could accelerate the decomposition or conversion of surface hydrophilic groups during oxidative torrefaction process effectively, strengthening the hydrophobicity of torrefied sample significantly. Furthermore, partial oxidation and thermal degradation tended to promote the crack of condense aromatic structures synergistically, increasing the proportion of active structures in the torrefied rice husk. Simultaneously, some nitrogen atoms initially located inside the aromatic structures with high thermal stability would migrate to the edge rapidly during the oxidative torrefaction (N-Q → N-5 + N-6). In the process of air torrefaction, excessive oxidants would reduce the content of reactive structure and functional group with low thermal stability through severe oxidation reaction, reducing the quality of torrefied sample, especially at high temperatures. The results in this study illustrated that torrefaction performed in flue gas atmosphere at 573 K for 30 min was the optimum torrefaction condition for rice husk upgradation, and employing flue gas as the carrier gas for biomass torrefaction was more effective and economical than N 2 and air in improving the fuel quality of rice husk.