Parametric study on the wood sawdust flow in circular dies through capillary rheometry

Wood fuel pellets are currently one of the ecological alternatives to fossil fuels for heat and power generation. These fuel pellets are produced through the densification/pelletizing process of a preconditioned sawdust material inside huge pellet mill presses, with dies made of multiple small cylindrical channels. While the industrial aspects of the fuel pellets are well understood and controlled, little knowledge was acquired on the flow of wood granular systems (sawdust) through circular dies. In this paper, such a flow was investigated by means of a capillary rheometer, in the case of a pretreated isotropic sawdust. Influences of the feedstock (water content and particle size distribution), extrusion parameters (piston speed, temperature), and die design (entry-cone angle, length of the cylindrical section) on the pellet density and aspect at the die exit were particularly discussed. Water contents were tested uncommonly until 40 wt%. Careful attention was paid to forces applied on the piston as an indication of power requirement in order to determine the best extrusion conditions. Low temperature and high moisture content appear to be the conditions that require lower power, while long capillaries increase density by 20% and give rise to a better aspect.