Torrefaction of oil palm kernel shell in the presence of oxygen and carbon dioxide

•Torrefaction of palm kernel shell was done under N2, O2 and CO2 atmospheres.•The severity of torrefaction was in the order of O2 > CO2 > N2.•The van Krevelen plots showed the common trend among the three different atmospheres.•The oxidative torrefaction rate was extracted from the overall torrefaction rate.•The morphological changes were distinctive among the three different atmospheres.

Torrefaction of oil palm kernel shell (PKS), one of the biomass residues from the palm oil industry, was carried out in a fixed-bed tubular reactor in the presence of oxygen and carbon dioxide at concentrations ranging from 0 to 15 vol.% (nitrogen balance). The effects of oxygen and carbon dioxide concentrations (0, 3, 9, 12, and 15 vol.%), temperature (493, 523, and 573 K) and biomass size (0.375 mm and unground) on the solid phase conversion, the energy yield and properties of torrefied biomass were investigated. The solid phase conversion increased with increasing temperature and oxygen or carbon dioxide concentration, but was not significantly affected by biomass size. The energy yield decreased with increasing oxygen or carbon dioxide concentration, but was still more than 70%. The extent of torrefaction was in the order of oxygen > carbon dioxide > nitrogen. The ‘oxidative torrefaction’ rate was extracted from the overall reaction rate. The increase in the oxidative torrefaction rate caused by oxygen was higher than that caused by carbon dioxide. Scanning electron microscope observations of the morphology of the PKS showed distinctive differences for torrefaction in nitrogen, oxygen, and carbon dioxide.