The Effect of Urea Pretreatment Combined with Ultrasonic Vibration-Assisted Pelleting on Pellet Solid Density and Durability

Abstract

Pellets, as a clean and renewable energy source, can overcome the shortcomings of high moisture content, expensive transportation costs, and non-uniform sizes for agricultural residues. However, traditional pelleting methods are associated with high energy consumption and severe mold abrasion due to the application of high temperature and pressure. To address these issues while improving pellet solid density and durability, urea pretreatment combined with ultrasonic vibration-assisted (UV-A) pelleting is investigated in the present research. Comparative experiments were initially conducted to verify the feasibility of the approach, followed by a central composite rotatable design (CCRD) to investigate the relative contributions and interactions of tested variables on pellet solid density and durability during pretreatment. The results revealed that combining urea pretreatment with UV-A pelleting could enhance pellet solid density and durability. Urea content, temperature, waste soybean flour (WSF) content, and distilled water significantly impacted pellet solid density, and all variables except distilled water had a significant effect on pellet durability. The optimal conditions were determined and a subsequent experiment was conducted to verify the agreement between experimental data and predicted results. The optimal conditions consisted of 42% distilled water, temperature of 45 °C, 10% urea content, and 12% WSF content resulting in pellet solid density and durability values of 1438.28 kg m−3 and 98.67%, respectively.