Effects of raw material moisture content, densification pressure and temperature on some properties of Norway spruce pellets

In order to study the pelletising process, Norway spruce sawdust pellets were produced under strictly controlled conditions on a laboratory scale. The aim of the work was to investigate how the moisture content of raw material and the densification parameters, pressure and temperature, affect compression strength, dry density and moisture uptake of the formed pellets. In the experiments performed, temperature (26-144 degrees C), moisture content (6.3-14.7 wt.% of d.b.) and pressure (46-114 MPa) were the factors which varied according to a prescribed central composite design. The relationships between the factor settings and the responses (dry density, moisture uptake and compression strength) were evaluated by multiple linear regressions. In the present study, it was found that high compression strength was strongly correlated with the density of the pellets. High temperature (at least up to 144 degrees C) and low moisture content at the start of compression (down to 6.3 wt.% of d.b.) increased the dry density of the pellets. Remarkably compression force had very little effect in the tested range of 46-114 MPa, indicating that pressure in the die does not need to be higher than 50 MPa. Similarly, compression force had very little effect on moisture uptake in the pellets. The least moisture uptake occurred when the pellets were produced at 90 degrees C. (c) 2005 Elsevier B.V. All rights reserved.