Predicting the energy properties of torrefied debarked pine pellets from torrefaction temperature and residence time

Abstract

In this study, high quality radiata pine pellets were torrefied at a range of temperatures (T = 210, 240, 270 and 300 °C) and residence times (t = 20, 40 and 60 min). The effects of these treatments on mass yield (MY), ash content (A), volatile matter (VM) and fixed carbon (FC), high heating value on a dry basis (HHVdb) and low heating value on a wet basis (LHVwb) were investigated, and models were constructed to predict these energy properties from T and t. Untreated pellets were analysed as control samples. Threshold conditions of 240 °C 40 min yielded significant increases in HHVdb and carbon content and a significant decrease in oxygen content. This combination significantly increased A and FC and significantly decreased VM. Logistics models using T, t, t × T and T2 were able to predict A accurately, with no bias, whereas FC values tended to be underestimated. The variation in HHVdb and LHVwb was explained by linear models with t and t × T as variables. Use of the models enabled definition of torrefaction severity that ensures an A below 0.7% and a LHVwb value 30% higher than obtained with raw pine pellets. Torrefaction conditions of T = 260–270 °C and t = 50–60 min are proposed