Combined Effects of Torrefaction and Binders on the Quality of Pellets Produced from Oat Straw

This study was carried out to investigate the combined effects of torrefaction and binders on the quality of pellets produced from oat straw. Torrefaction pretreatment was performed with the aid of a lab-scale microwave oven at temperatures in the range of 200–300 °C and a retention time of 5–9 min in an inert environment to ascertain the effect of these two process variables on the physiochemical, thermal, and mechanical properties of the torrefied biomass. Torrefaction liquid (TL) and sawdust were introduced during pelletization as binding agents in the pellet formulation. From the results in this study, the physiochemical and thermal properties of the oat straw pellet improved after torrefaction except for the tensile strength. The higher heating value (HHV), particle density, and bulk density of the torrefied oat straw improved from 16.84 to 24.23 MJ/kg, 882.23 to 1374.87 kg/m3, and 127.87 to 80.23 kg/m3, respectively. The tensile strength of the pellet after torrefaction ranged between 0.35 and 1.02 MPa. The introduction of the binder during pelletization improved the pellet tensile strength to about 1.32–2.28 MPa. Although ash content increased after torrefaction from 5.32 to 10.22%, there was no significant increase in the ash content after the addition of binders as the value ranged between 8.88 and 9.00% for the torrefied pellet sample used. The addition of binder further improved the HHV, pellet density, dimensional stability, and moisture uptake rate from approximately 21 to 24 MJ/kg, 1032 to 1393 kg/m3, 2.23 to 1.02%, and 16 to 9%, respectively for the torrefied pellet sample used. The energy efficiency of this system was improved by the addition of the substance lost during torrefaction as a heat source.