Specific energy consumption and quality of wood pellets produced using high-moisture lodgepole pine grind in a flat die pellet mill

In the present study a Box-Behnken experimental design was used to understand the effect of the moisture content of lodgepole pine grind (33-39%, w.b.), die speed (40-60 Hz) and preheating temperature (30-90 °C) on the pellet quality and specific energy consumption. The partially dried pellets produced had high-moisture content in the range of 19-28% (w.b.), and were further dried to <9% (w.b.) in a mechanical oven set at 70 °C for 3 h. Dried pellets were further evaluated for pellet moisture content, unit, bulk, tapped density, and durability. Response surface models developed for the product properties have adequately described the process based on coefficient of determination values. Surface plots developed indicated higher unit, bulk, and tapped density (1050, 520, 560 kg/m3) are achievable at 33-35% (w.b.) moisture content of the lodgepole pine grind, die speed of 60 Hz and preheating temperature of 30-60 °C. Higher moisture content of 39% (w.b) reduced unit, bulk, and tapped density to <912, 396, and 452 kg/m3. Higher durability values of >95% were obtained at 33-35% (w.b.) at lower preheating temperatures of 30-50 °C and higher die speed of >50 Hz. At 33% (w.b.) moisture content of the lodgepole pine grind, preheating temperature of 90 °C, and die speed of 60 Hz, the observed specific energy consumption was <116 kW h/ton. Scanning electron microscope studies indicated that lignin crosslinking is the primary reason for binding of the lodgepole pine grind at high-moisture content.