Study on Density, Hardness, and Moisture Uptake of Torrefied Wood Pellets

Torrefied pellets, a transportable renewable energy source, have a higher energy density than the regular wood pellets (control pellets). The quality of torrefied pellets is determined mainly by the density, hardness, and the hygroscopicity or moisture uptake. In this study, the density and the hardness of torrefied pellets were systematically examined by using torrefied samples prepared at different conditions in a press machine. The hygroscopicity of prepared torrefied pellets was evaluated in a humidity chamber by measuring the moisture uptake rate of control and torrefied pellets. The results showed that the density and the hardness of torrefied pellets mainly depended on the densification die temperature and the weight loss of torrefied samples. To make strong torrefied pellets of high density and low moisture uptake from 30 wt % weight loss torrefied samples, a die temperature of 230 C or above was needed. Preconditioning torrefied samples to a moisture content of similar to 10% can improve the quality of torrefied pellets. The moisture uptake of torrefied pellets was more sensitive to the weight loss of torrefaction and the relative humidity of the storage environment. The saturated moisture uptake of torrefied pellets made from 30 wt % weight loss torrefied samples was at least 40% lower than the control pellets.