Upgrading rice husk via oxidative torrefaction: Characterization of solid, liquid, gaseous products and a comparison with non-oxidative torrefaction

In this study, oxidative torrefaction and non-oxidative torrefaction of rice husks were performed under different severities, and the effects of temperature (220-300 degrees C) and O-2 concentration (0-15 vol%) on the properties of the solid, liquid, and gas products were investigated. Oxidation of hydrocarbons play an important role in thermal degradation of samples, and higher O-2 concentration promotes the severity of torrefaction. Oxygen content in rice husk decreased whether the torrefaction temperature increased or the O-2 concentration increased. Consequently, the H/C and O/C ratios as well as the mass and energy yield of the sample remarkably decreased, even at small O-2 concentrations of 2%. Gas yield in the oxidative torrefaction process was higher than that in non-oxidative torrefaction. CO2 was the dominant gas, and volume fraction of CO2 further increased during oxidative torrefaction. This may be contributed to enhanced thermal degradation of rice husk and to the combustion reaction between CO and O-2, especially at a higher torrefaction temperature and with a higher concentration of O-2. In the case of liquid products, acids and phenols were the dominant organic components. However, formation of water and acetic acid was promoted, whereas the presence of O-2 inhibited generation of levoglucosan. On the basis of products distribution and the evaluation of O-2 that participates in torrefaction, it was concluded that O-2-containing atmosphere could be a potential agent to replace a pure N-2 atmosphere, which may reduce operational costs and make biomass torrefaction more commercially feasible.