Experimental investigation of feedstock layer thickness and die-roller gap in wood pelleting

Operational parameters as feedstock layer thickness and gap size between die and roller were studied using a 15 kW
ring die pellet press. The feedstock layer was tested at three different feedstock mass flow rates, 90 kg/h, 120 kg/h,
and 150 kg/h, and three peripheral die speeds, 3.06 m/s, 4.08 m/s, and 5.10 m/s. Tests were also performed with three
different roller gaps of 0.2 mm, 0.9 mm, and 1.6 mm.
The specific energy consumption of the pellet press was measured during the tests, and the durability of the pellet
was measured using a pellet tumbler. By using image processing on photos of pellet samples, the length distributions
of the pellets were analysis.
The results shows that increasing the thickness of the feedstock layer, decreases the specific energy consumption
and increases the pellet durability. Increasing the size of the roller gap, increases the specific energy consumption and
decreases the pellet durability. Analysis of the pellet length distributions, shows that the longest pellets were produced
with a roller gap of 0.2 mm and the thickest feedstock layer, which also were the most durable pellets.
In addition to the experimental tests, two empirical expressions for the specific energy consumption and pellet
durability as a function of the feedstock layer are presented.