Multiphysics modeling of ultrasound-assisted biomass torrefaction for fuel pellets production

When an ultrasound wave propagates through a volume of biomass medium, the majority of the energy in the acoustic field is absorbed locally by the biomass, resulting in the generation of heat. This torrefaction effect results in a temperature increase of the biomass, converting biomass into a coal-like intermediate with upgraded fuel properties over the original biomass. However, few analyses can be found in the literature explaining the mechanism of ultrasound-assisted biomass torrefaction. This research aims to model an ultrasound-assisted biomass torrefaction system. The developed multiphysics model depicts the piezoelectric effect of a transducer, the vibration amplitude at the output end of the ultrasonic horn, and the acoustic intensity and temperature distributions in the biomass medium. The vibration amplitude and frequency of the ultrasonic horn were measured by a non-contact capacitive sensor, and it is verified the model can accurately simulate the ultrasonic vibration of the experimental system. The temperature at the center of the biomass was measured to validate the model’s temperature prediction. Both simulation and experiment showed that ultrasound-assisted biomass torrefaction can create torrefied fuel pellet within 60 seconds.