Oxidative Torrefaction of Sunflower Husk Pellets in the Kaolin Layer

Abstract

The aim of this work was to experimentally study the oxidative torrefaction of sunflower husk pellets in a kaolin layer. The kaolin layer was used to limit oxygen access to the biomass to prevent it from self-ignition. The changes in mass yield, volatiles, ash content, high calorific value (HHV), hydrophobicity, and morphology of the biomass were investigated at torrefaction temperatures 240, 260, and 280 °C, process durations 30–60 min, and kaolin layer heights 3–5 cm. The Van Krevelen diagram and the CHO index based on atomic C, H, and O data were also used for explanation. As a result of oxidative torrefaction in the kaolin layer, the atomic O/C and H/C ratios decreased and the sunflower husk position shifted towards peat. During torrefaction at 260 °C for 1 h (kaolin layer height of 3 cm) the O/C decreased by 26.9% and the H/C by 18.8%. Reducing the height of kaolin layer, as well as increasing the temperature and time of torrefaction, led to a decrease in mass yield and an increase in ash content. Torrefaction increased the FC/VM ratio from 50 to 225%, which improved the fuel properties of the biomass. A 16.5% increase in HHV was achieved compared to untreated sunflower husk. As the torrefaction intensity increased, drier fuel was formed, the moisture absorption of which decreased from 213 to 25.8%.