Farm-scale biomass pelletizer performance for switchgrass pellet production

The impact of feedstock characteristics (moisture content, additive content) and operational parameters (die temperature, pelletizer speed) on pellet quality, plugging tendency, and pelletizer energy use was investigated for a small scale pelletizer, suitable for on-farm use (∼70 kg hr-1 rated output). Ground switchgrass (Panicium virgatum) was prepared in small (<1 kg) batches and run through a flat plate, rotating die pelletizer with 6 mm die holes and a die thickness of 25 mm. The resulting pellet quality and die flow (plugging tendency) were assessed using a subjective pellet quality scale. Results indicate that successful pelleting conditions were most consistently achieved by using a "premix" consisting of ground switchgrass and Distillers Dried Grains in a 3:7 ratio (mass basis) followed by the actual feedstock mixture. The highest quality pellets were obtained from switchgrass with a moisture content ranging from 11 to 17% (wet basis). Adding between 1% and 4% vegetable oil improved pellet quality slightly, while adding starch to the switchgrass feedstock did not yield noticeable improvements. Power consumption by the pelletizer averaged 0.22 kwh per kg for feedstock at 14% moisture and 1% oil, but dropped to 0.16 kwh per kg when the oil content was increased to 5%. Reducing the operating speed of the pelletizer resulted in improved quality of the pellets.