Moisture content and storage time influence the binding mechanisms in biofuel wood pellets

In a pelletizing experiment the three factors sawdust moisture content, steam conditioning of the sawdust, and storage time of the raw material, were varied in a full factorial design with a total of 34 experiments to evaluate the influences on the pelletizing process and the pellet quality when producing biofuel wood pellets from pine sawdust. Moisture content of the sawdust was found to be the dominant factor for the bulk density and for the pelletizer motor current, both showing low values at high moisture contents due to the lubricating property of water that lowers the friction in the pelletizing process. More complex models were obtained for mechanical durability and the amount of fines, where all factors and most interactions and squared terms were significant. In order to explain the effect of the sawdust moisture content and the storage time on these response factors, a qualitative model for the binding mechanism is suggested. In this model water is supposed to be actively involved in the binding mechanism as hydrogen bonded bridges. The increase in binding strength with storage time is explained by the reduction of extractive content which contain molecules that can block binding sites on the material surface. Optimum pellet quality was obtained when the storage time exceeded 120 days and within a range of sawdust moisture content of 11–13%.