Effect of ultrasonic vibration-assisted pelleting of biomass on biochar properties

Abstract

The objective of this research was to investigate the potential of ultrasonic vibration-assisted (UV-A) pelleting as pretreatment method to produce biochar with desired properties. In this study, poplar (PP), switchgrass (SG), and wheat straw (WS) were pretreated with UV-A pelleting process, then the biochar were produced in a muffle at atmosphere condition with a highest temperature treatment (HTT) at 300, 500, and 700 °C, respectively. Ultrasonic power had an obvious effect on the biochar yield and pH, a higher biochar yield and biochar pH were obtained from PP00, SG50, and WS50 and PP50, SG50, and WS00, respectively. The FTIR spectra showed that O–H stretching, C–H stretching, carbonyl bonds/CO stretching, C–H stretching, C–O–C, and C–C stretching were found in spectra of UV-A pretreated biomass, while disappeared in spectra of biochar, indicating the changes of functional groups. Although UV-A pelleting pretreatment did not show considerable effect on the C, H, N, and S content of biochar (less than 2%) under the same HTT, the formation of multi-layer and porous structure was observed on biochar surface. The specific surface area, micropore surface area, and total pore volume of biochar from UV-A pelleting increased with increasing of ultrasonic power. In addition, HTT and type of biomass also had different functions on the biochar properties. The results of this study showed that UV-A pelleting pretreatment has a great potential to produce biochar with desired chemical structures and physical properties.