Type of Pellet Mill and Die Specification

Pellet mill has two types of pellet presses: ring die and flat die. Ring die is a more common type of pellet mill used in commercial unit. For the ring die pellet mill, the die remains stationary and the rollers rotate. Some models have the dies to rotate and the rollers remain stationary during pellet production. The die of a pelletizer is made of hardened steel that is perforated allowing the ground wood particles to be forced through by the rotating die or rollers. As the die revolves, the friction driven rollers force the feed through holes in the die (steps A and B, Fig. 5.4). A layer of biomass (step C, Fig. 5.4) is needed to develop between the roller and the flat die in order to start flowing the particles into the die channels (step D, Fig. 5.4) to push the materials to extrude through the flat die. Cut-off knives mounted on the swing cover cut the pellets when they extruded from the die (step E, Fig. 5.4). The advantage of the ring die type pellet mill allows a higher production throughput compared to other types of presses (e. g., piston and screw presses), while maintaining the power consumption in the range of 15-40 kW h/t [53].

Flat die pellet mill is widely used for producing animal feed. Similar to the ring die pellet mill, the ground particles are loaded into the densiflcation area. The rollers keep rotating in a clockwise direction in a vertical plane while the flat die is rotating anticlockwise in a horizontal plane. A layer of biomass powder is needed to develop between the roller and the flat die to start flowing into the die channels. Pressure is built up due to friction in the press channel and exponential correlation between press channel length and pressure is found [55,57]. Finally, the materials are extruded through the flat die. A cut-off knife is installed at the exit of the flat die to cut the pellets into a certain specific length.

According to Tumuluru’s review [54], die geometry refers to the size and shape of the die. The die geometry determines the pellet dimensions and the resulted density and durability. The aspect ratio (length to diameter of the pellet) can be a metric for the degree of compression during pelletization. An increase in pelletizing pressure increases the length of the pellet, whereas an increase in pellets diameter decreases the pelletizing pressure. Hence, the geometry of the die has a significant determination on the pressure applied onto the resulted pellet. A mathematical model for hardwood and softwood was developed to show how the variation in the model parameters (sliding friction coefficient, the ratio of compression, and the material — specific parameters, such as elastic modulus and Poisson ratio) significantly changes the necessary pelletizing pressure along the press channels of the matrix [55, 56]. Later, the co-relations of processing conditions between single die and commercial pellet mill was attempted [57].

Fig. 5.4 Mechanisms of pelletization by a ring die

The inner diameter (D) and the effective length (L) of the die determine the pellet density. The effective length is the die thickness that actually performs work on the feed. L/D ratio is the effective length divided by the inner diameter of the die. High L/D ratios provide high pellet die resistance as feed moves through the die. Low L/D ratios provide less resistance. Each material has an L/D ratio requirement to form the material into a pellet. The durability of the pellets improves when a smaller die with higher L/D ratios is used [58]. However, if a longer die length is used, which will introduce a higher friction with no more improvement in the pellet durability. This will consume excess energy for production. Therefore, there should have optimum L/D ratios of the die for different types of biomass to produce durable pellets.