**3. Results**

### *3.1. Physical Properties of the Milled 2-Inch Top Pine Residue and Milled Switchgrass Blends*

Edmunds et al. [11] discussed the physical properties of 2-inch and 6-inch top pine residue, switchgrass, and blends of 2 and 6-inch top pine residue and switchgrass in detail. The average values of milled switchgrass, milled 2-inch top pine residue and blends particle size information, bulk, particle and tap densities, compressibility, and Hausner ratio are given in Table 2. The bulk and tapped density of the blends of the 2-inch top pine residue + switchgrass indicated that d50 increased with an increase in pine percentage, whereas the span reduced with an increase in the pine percentage. The trends were similar for bulk, tap, and particle densities where higher pine percentage increased the density values. Edmunds et al. [11] reasoned that a higher span of switchgrass particles could be due to an elongated nature and a higher aspect ratio of the switchgrass grind. Also, the elongated nature of the switchgrass particles can result in entanglements of the particle, which increase void spaces and reduce density. The calculated flow properties, such as the Hausner ratio, were calculated using physical properties data. The Hausner ratio of the blends was in a range between 1.26 and 1.33 [11].


**Table 2.** Physical characteristics of grinds from switchgrass and 2-inch top pine residue blends [11].

Figure 5 shows the pellets made from 2-inch top pine residue + switchgrass blends at different blend moisture contents, and L/D ratio in the pellet die. The pelleting experiments were conducted based on the experimental design provided in Table 1. Some of the key results of blending 25% switchgrass + 75% 2-inch top pine residue, 75% switchgrass+25% 2-inch top pine residue, and 50%

Note: CM: compressibility; and HR: Hausner ratio.

switchgrass + 50% 2-inch top pine residue that helped to achieve the durability and bulk density (> 95% and > 550 kg/m3) are provided in Figures 6 and 7. The pelleting process conditions that resulted in bulk density > 550 kg/m<sup>3</sup> and durability > 95% were an higher L/D ratio of 2.6 and a blend moisture content of 20% (w.b.) for all the three blend ratios tested.

**Figure 5.** Blend pellets made using milled 2-inch top pine + milled switchgrass blends at different moisture contents and L/D ratios of the pellet die.

**Figure 6.** Durability of the pellets produced using a blend of milled 2-inch top pine residue + milled switchgrass.

**Figure 7.** Bulk density of the pellets produced using a blend of milled 2-inch top pine residue + milled switchgrass.

### *3.2. Response Surface Models and Plots*

Table 3 indicates the models developed for the blends of 2-inch top pine residue + switchgrass based on the experimental data obtained. Coefficient-to-determination values, which were in the range of 0.60 to 0.98, sugges<sup>t</sup> that the models have described the pelleting process reasonably well with respect to the process variables tested. The statistical significance of the models developed for these different blends was evaluated based on their *p* values. For the 50% 2-inch top pine residue +50% switchgrass, the models developed pellet moisture content, bulk density and specific energy consumption were found to be statistically non-significant, whereas for durability it was found to be significant (*p* < 0.01). In the case of the 75% 2-inch top pine residue + 25% switchgrass, the models developed for pellet moisture content and durability were as found to be statistically non-significant, whereas the bulk density and specific energy consumption were found to be statistically significant at *p* < 0.05. Finally, in the case of the 25% 2-inch top pine residue + 75% switchgrass blend models, the pellet moisture content, bulk density, and durability were found to be statistically significant (*p* < 0.05, *p* < 0.01), whereas the specific energy consumption model was found to be statistically non-significant. Using these equations, response surface plots were developed. The significance of response surface plots is that they assist in understanding the interactive effect of the process variables (i.e., 2-inch top pine residue + switchgrass blend, blend moisture content, and compression ratio or L/D ratio of the pellet die) on product quality (i.e., blend pellet moisture content, bulk density, and durability) and the specific energy consumption of the pelleting process.



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; x1: L/D ratio of the pellet die; x2: Blend moisture content (%, w.b.).

### 3.2.1. Blend Ratio: 50% 2-inch Milled Pine Top Residue + 50% Milled Switchgrass

The moisture content of the pellet decreased with a decrease in blend moisture content. The lowest pellet moisture content of < 14% (w.b.) was observed at an L/D ratio of 1.5 to 2.6 for 20% (w.b.) blend moisture content tested, as shown in Figure 8. The L/D ratio of the die did not have a significant effect on moisture loss during pelleting. This observation corroborated with earlier studies on a pilot-scale pellet mill for corn stover feedstock [21–23]. The bulk density of the blend pellets decreased at higher blend moisture content and lower L/D ratio in the pellet die. Higher bulk densities of > 580 kg/m<sup>3</sup> were observed at an L/D ratio of 2.6 and a blend moisture content of about 20–22% (w.b.), as shown in Figure 9. In the case of pellet durability, the L/D ratio had a significantly greater effect than blend moisture content. A lower to medium moisture content of 20–25% and a higher L/D ratio of 2.6 resulted in durability values of > 94%, as shown in Figure 10. Specific energy consumption decreased with a decrease in the L/D ratio in the pellet die. A higher L/D ratio in the pellet die (about 2.4–2.6) and a lower moisture content of 20–22% (w.b.) resulted in a higher specific energy consumption of > 140 kWh/ton, whereas lowering the L/D ratio to 1.5 at the same moisture content resulted in a lower specific energy consumption of < 102 kWh/ton, as shown in Figure 11.

**Figure 8.** Effect of blend moisture and L/D ratio on pellet moisture content of 50% 2-inch top pine residue + 50% switchgrass.

**Figure 9.** Effect of blend moisture and L/D ratio on pellet bulk density of 50% 2-inch top pine residue + 50% switchgrass.

**Figure 10.** Effect of blend moisture and L/D ratio on pellet durability of 50% 2-inch top pine residue + 50% switchgrass.

**Figure 11.** Effect of blend moisture and L/D ratio on specific energy consumption of 50% 2-inch top pine residue + 50% switchgrass.

3.2.2. Blend Ratio: 75% 2-inch Milled Pine Top Residue and 25% Milled Switchgrass

Lowering the blend moisture content and increasing the L/D ratio decreased pellet moisture content. An L/D ratio of 2.6 and a lower blend moisture content of 20% (w.b.) resulted in pellets with a moisture content of < 14.5% (w.b.), whereas the same moisture content with a lower L/D ratio resulted in higher moisture content in the blended pellets (16.5%), as shown in Figure 12. Bulk density of the blended pellet increased with an increase in the L/D ratio in the pellet die (to 2.6) and a decrease in blend moisture content to 20% (w.b.). The highest and lowest bulk density values observed were > 540 and < 364 kg/m3, as shown in Figure 13. The durability values of the produced pellets using pine and switchgrass blends were positively influenced by the L/D ratio but negatively influenced by the moisture content of the blends. Increasing the L/D ratio to 2.6 and decreasing the moisture content to 20% produced pellets with durability values of > 98%, while increasing the moisture content to 30% and decreasing the pellet die L/D ratio to 1.5 reduced the durability values to < 78%, as observed in Figure 14. The lower specific energy consumption of < 78 kWh/ton was observed for the L/D ratio of 2.2 to 2.6 at a lower blend moisture content of 20% (w.b.), as shown in Figure 15.

**Figure 12.** Effect of blend moisture and L/D ratio on pellet moisture content of 75% 2-inch top pine residue + 25% switchgrass.

**Figure 13.** Effect of blend moisture and L/D ratio on pellet bulk density of 75% 2-inch top pine residue + 25% switchgrass.

**Figure 14.** Effect of blend moisture and L/D ratio on durability of 75% 2-inch top pine residue + 25% switchgrass.

**Figure 15.** Effect of blend moisture and L/D ratio on specific energy consumption of 75% 2-inch top pine residue + 25% switchgrass.

3.2.3. Blend Ratio: 25% 2-inch Milled Pine Top Residue + 75% Milled Switchgrass

Lowering the blend moisture content to 20% (w.b.) and the L/D ratio to 1.6 to 2.4 reduced the pellet moisture content to < 14% (w.b.), whereas a higher moisture content in the blend at 30% (w.b.) resulted in a higher moisture content in the produced pellets, as observed in Figure 16. The loss of moisture was greater with a corresponding increase in the initial moisture content of the blend. For example, about 8–10% moisture loss was seen at 30% (w.b.) initial moisture content, whereas at 20% (w.b.) initial moisture content, the loss of moisture observed during pelleting was only about 6–7% (w.b.). The bulk density increased with an increase in the L/D ratio and a decrease in blend moisture content. The maximum bulk density observed at 20% moisture content and an L/D ratio

of 2.6 was 580 kg/m3, whereas at an L/D ratio of 1.5 and a blend moisture content of 30% (w.b.), the lowest bulk density of < 404 kg/m<sup>3</sup> was observed, as shown in Figure 17. The L/D ratio had a more significant effect on the durability values as compared to the blend moisture content. The maximum durability observed was > 92% at a blend moisture content of 20% and an L/D ratio of 2.6. At a lower L/D ratio of 1.5, the predicted durability values were in the range of 76–78% for the different blend moisture contents tested, as seen in Figure 18. A higher moisture content and a lower L/D ratio reduced the specific energy consumption. At an L/D ratio of 1.5 and a blend moisture content of 30% (w.b.), the specific energy consumption observed was 80 kWh/ton. At a higher L/D ratio of 2.6 and a lower moisture content of 20% (w.b.), the specific energy consumption was > 180 kWh/ton, as seen in Figure 19.

**Figure 16.** Effect of blend moisture and L/D ratio on pellet moisture content of 25% 2-inch top pine residue + 75% switchgrass.

**Figure 17.** Effect of blend moisture and L/D ratio on pellet bulk density of 25% 2-inch top pine residue + 75% switchgrass.

**Figure 18.** Effect of blend moisture and L/D ratio on durability of 25% 2-inch top pine residue + 75% switchgrass.

**Figure 19.** Effect of blend moisture and L/D ratio on specific energy consumption of 25% 2-inch top pine residue + 75% switchgrass.
