Oxidative torrefaction of woody biomass: introducing a modified severity factor

Torrefaction experiments of Rubberwood and Gliricidia were conducted at 250–300 °C for 30–60 min in a 3–9% oxygen environment to study the oxidative torrefaction behavior. The higher heating value of the torrefied Rubberwood increased from 18.9 to 24.68 MJ/kg and from 19.46 to 23.19 MJ/kg for Gliricidia under the most severe oxidative conditions. Effects of torrefaction conditions on the solid yield, VM removal, C enhancement, HHV enhancement, and energy yield were modeled using response surface methodology, and temperature and oxygen concentration mainly affected the torrefied biomass properties. Rubberwood recorded a significantly greater energy mass co-benefit index (EMCI) than Gliricidia. EMCI of oxidative torrefaction of Gliricidia showed no significant difference from that of inert conditions. A new severity factor was introduced for oxidative torrefaction, and the normalized severity factor showed a linear correlation with torrefied biomass properties, which could facilitate oxidative torrefaction modeling.