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.

The development and utilization of biomass can not only address the demand for low-carbon energy and reduce environmental pollution, but can also facilitate the achievement of carbon neutrality. However, there are many factors justifying the case for low utilization of agricultural residues. These factors could be well controlled by producing top-quality pellets. Production of pellets is generally accompanied by the problems of high energy consumption and serious mold wearing. To eliminate these deficiencies, pretreatment has attracted scholars’ attention. In this review, the effects of four pretreatments on the properties of pellets were assessed. Thermal pretreatment can improve the hydrophobicity of pellets, and optimize their properties, while degradation of diverse extractives is noteworthy. Hydrothermal pretreatment improves the physical properties of pellets, through the increase of polar functional groups on the surface of the biomass. Ultrasonic vibration-assisted (UV-A) pelleting produces pellets under low pressure without a heating process; however, it is still not applied to large-scale production. Supercritical fluid extraction can achieve the graded utilization of extracts and bioactive substances in biomass, and the residues can be subsequently utilized as pellet feedstock. Mild hydrothermal treatment is a promising approach to improving the quality of agricultural pellets. Additionally, the effects of process parameters on the physical and chemical properties of pellets should be systematically analyzed.

Top-quality pellets can significantly increase density and durability of agricultural residues, reducing logistic costs. However, these pellets depend on numerous parameters, including feedstock properties and production conditions. To ensure high-quality pellets, a single-factor experiment and the response surface methodology were used to investigate the effects of particle size, moisture content, molding pressure, pelleting time, ultrasonic power, and interaction effects between variables on density and durability of pellets for ultrasonic vibrationassisted pelleting of corn stover. The response surface models between variables and response were established. The results showed that all variables affect the density and durability of pellets.An optimal condition for density and durability was obtained, and a further experiment was conducted to validate the values. The results suggested that desirability (0.999) under optimal conditions confirmed the validation of models. The optimal combination of process parameters included particle size of 1.5 mm, moisture content of 10 %, molding pressure of 379 kPa, pelleting time of 80 s, ultrasonic power of 250 W, with values of 1,381.14 kg m -3 and 97.58 % for density and durability of pellets, respectively.

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