**Jaya Shankar Tumuluru**

Idaho National Laboratory, Energy Systems and Technologies Directorate, Bioenergy Technologies Department, 750 MK Simpson Blvd., Idaho Falls, ID 83415-3570, USA; JayaShankar.Tumuluru@inl.gov; Tel.: +1-208-526-0529

Received: 20 February 2019; Accepted: 18 March 2019; Published: 28 March 2019

**Abstract:** The blending of woody and herbaceous biomass can influence pellet quality and the energy consumption of the process. This work aims to understand the pelleting characteristics of 2-inch top-pine residue blended with switchgrass at high moisture content. The process variables tested are blend moisture content, length-to-diameter (L/D) ratio in the pellet die, and the blend ratio. A flat die pellet mill was also used in this study. The pine and switchgrass blend ratios that were tested include: (1) 25% 2-inch top pine residue with 75% switchgrass; (2) 50% 2-inch top pine residue with 50% switchgrass; and (3) 75% 2-inch top pine residue with 25% switchgrass. The pelleting process conditions tested included the L/D ratio in the pellet die (i.e., 1.5 to 2.6) and the blend moisture content (20 to 30%, w.b.). Analysis of experimental data indicated that blending 25% switchgrass with 75% 2-inch top pine residue and 50% switchgrass with 50% 2-inch top pine residue resulted in pellets with a bulk density of > 550 kg/m<sup>3</sup> and durability of > 95%. Optimization of the response surface models developed for process conditions in terms of product properties indicated that a higher L/D ratio of 2.6 and a lower blend-moisture content of 20% (w.b.) maximized bulk density and durability. Higher pine in the blends improved the pellet durability and reduced energy consumption.

**Keywords:** 2-inch top pine residue + switchgrass blends; pelleting process variables; pellet quality; specific energy consumption; response surface models; hybrid genetic algorithm
