*3.3. Optimized Process Conditions*

The regression equations given in Table 3 were further optimized to find the optimum pelleting process conditions that can maximize bulk density and durability and minimize the pellet moisture content for the different blend of ratios of pine + switchgrass tested. Tables 4–6 indicate the optimized conditions obtained using the hybrid genetic algorithm optimization developed by Tumuluru and

McCulloch [48]. It is clear from these tables that optimized process conditions identified for the three tested blends were different in terms of L/D ratio of the pellet die but lower pellet moisture content of about 20% (w.b.) was necessary to maximize bulk density and durability and minimize the pellet moisture content. For maximizing bulk density and durability, a maximum L/D ratio and minimum moisture content were desirable for all three of the blends tested. To minimize blend pellet moisture content, a lower blend moisture content was desirable for all three of the blends tested, but the L/D ratio in the pellet die was different. For 50% 2-inch top pine residue + 50% switchgrass, a lower L/D ratio of 1.58 was needed; for the 75% 2-inch top pine residue + 25% switchgrass, a higher L/D ratio of 2.55 was necessary; and for the 25% 2-inch top pine residue + 75% switchgrass, a medium L/D ratio of 2.01 was desirable to reduce the moisture content in the blended pellets. Among the three blends tested, the blend with 75% switchgrass + 25% 2-inch top pine residue and the blend with the 50% switchgrass + 50% 2-inch top pine residue produced blended pellets with durability values of about 95%, while the pellets produced with 75% 2-inch top pine residue + 25% switchgrass produced pellets with durability values of about 98.11%. In regard to bulk density, the maximum of 591 kg/m<sup>3</sup> was observed for the pellets produced with the 25% 2-inch top pine residue + 75% switchgrass, whereas the other two combinations produced pellets with durability values of 583 kg/m<sup>3</sup> and 554 kg/m3.

**Table 4.** Optimum process conditions for maximizing density and durability and minimizing pellet moisture content for 50% milled 2-inch top pine residue + 50% milled switchgrass blends.


**Table 5.** Optimum process conditions for maximizing bulk density and durability and minimizing pellet moisture content for 75% milled 2-inch top pine residue + 25% milled switchgrass blends.


**Table 6.** Optimum process conditions for maximizing bulk density and durability and minimizing pellet moisture content for 25% milled 2-inch top pine residue + 75% milled switchgrass blends.

