Effect of extractives and storage on the pelletizing process of sawdust

The importance of extractives and storage for sawdust as raw material for wood pellet production was analyzed. Sawdust prepared from eastern white pine (Pinus strobus L.), white oak (Quercus alba L.) and yellow-poplar (Liriodendron tulipifera L.) together with two commercial raw materials were extracted with acetone or stored at 60 °C for 21 days. The effects of the treatments were measured on the pellet strength, and with respect to the energy use requirements of a commercial pellet mill using laboratory dies and methods to measure the different stages of the pelletizing process individually. The results showed that removal of the extractives significantly increased the pellet strength and the energy requirements in all stages of the pelletizing processes corresponding to the samples extractives content, whereas the effect of the storage treatment was indefinite. The study indicates that extractives act as plasticizers and lubricants and thereby decrease the energy requirements for the pelletizing process. On the other hand, extractives seem to prevent close contact between the bonding sites of the lignocellulose particles and thereby decrease the pellet strength. Differences in extractives content can therefore explain differences in pellet mill energy consumption and capacity when raw materials from different wood species are used. The effect of storage with respect to particle surface accumulation of extractives may have little importance compared to the total amount of extractives in the raw material.