*2.2. Experimental Plan*

In the present study, milled switchgrass + milled 2-inch top pine residue blend ratio, blend moisture content, and pellet die compression (L/D) ratio were selected as the process variables. Table 1 indicates the lower and upper limits of the process variables and the experimental plan for the pelleting tests, which were conducted using 4.76 mm (3/16-inch) grind switchgrass and 2-inch top pine residue blends. Pelleting tests were conducted at each L/D ratio in the pellet die at three different blend moisture contents. The pelleting tests were conducted at 60 Hz rotational speed of the pellet die. The diameter of the pellet die used for the present study was 6 mm. The pellets produced were used to measure physical properties, such as pellet moisture content (%, w.b.), bulk density (kg/m3), and durability (%). LabVIEW software was used to log the power data. These data were further used to calculate the specific energy consumption of the pelleting process.


**Table 1.** Experimental conditions used for pelleting of milled 2-inch top pine residue and switchgrass blends.

Note: Both switchgrass and 2-inch top pine residue were ground in a hammer mill fitted with a 3/16-inch (4.8 mm) screen size.

### *2.3. Flat Die Pellet Mill*

In the present study, an ECO-10 flat die pellet mill was used to perform pelleting tests, as shown in Figure 3 [21–23,40,42,44]. Figure 4 provides a look at the various pellet dies with the different L/D ratios that were tested in this study. The pellet mill was provided with a 10 HP motor. At 60 Hz, the rotational speed of the pellet die is 350 rpm [21]. It has a hopper and screw feeder, which feed the pellet mill continuously. Flexible heating tapes are provided to the screw feeder and hopper, which can help to preheat the biomass before pelleting. A variable frequency drive is provided to the pellet mill to vary the rotational speed of the die. A horizontal pellet cooler is provided to cool the warm pellets coming out of the pellet die. Power consumption during pelleting was measured using the power meter provided to the pellet mill.

**Figure 3.** Flat die pellet mill at Idaho National Laboratory.

**Figure 4.** *Cont.*

**Figure 4.** Flat die pellet mill dies with different length-to-diameter (L/D) ratios.
