A Study of Softwood Torrefaction and Densification: for the Production of High Quality Wood Pellets

Canada has become a major producer and exporter of wood pellets in the world. But the low water resistivity and the high transportation cost impede the market development. Torrefaction may provide a solution. The present study developed the torrefaction kinetics, studied the torrefaction reaction conditions in two bench-scale reactors, and identified the suitable conditions for making durable torrefied pellets in a press machine. The weight loss of softwood significantly depended on the torrefaction temperature, the residence time, the particle size, and the oxygen concentration in the carrier gas. A two-component and one-step first order reaction kinetic model gave a good agreement with the weight loss range of 0 to 40% at the temperature of 260-300 oC. Considering the quality of torrefied pellets, the optimal torrefaction conditions appeared to correspond to a weight loss of about 30%, which gave a 20% increase in pellet heating value. The suitable densification conditions corresponded to a die temperature of 230 oC, or over 110 oC for torrefied samples conditioned to 10% moisture content.