A Temperature Model for Synchronized Ultrasonic Torrefaction and Pelleting of Biomass for Bioenergy Production

Low energy and volumetric density of cellulosic biomass has been a challenge hindering its large-scale utilization as a bioenergy resource. Torrefaction is a thermochemical pretreatment process that can significantly enhance the properties of biomass as a fuel by increasing the heating value and thermal stability of biomass materials. Densification of cellulosic biomass by pelleting can greatly increase the volumetric density of biomass to improve its handling efficiency. Currently, torrefaction and pelleting are processed separately, which consumes a great amount of time and energy. In addition, it is more difficult to pellet torrefied biomass than untreated biomass. Synchronized ultrasonic torrefaction and pelleting has been developed to address these challenges. Synchronized ultrasonic torrefaction and pelleting can produce pellets of high energy and volumetric density in a single step, which tremendously reduces the time and energy consumption compared to that by the prevailing multi-step method. This novel fuel upgrading process can increase biomass temperature to 473–573 K within tens of seconds to realize torrefaction. Studying the temperature distribution is a crucial key to understand the fuel upgrading mechanism since pellet energy density, thermal stability, volumetric density, and durability are all highly related to temperature. In this research, a physics-based temperature model is developed to explain torrefaction temperatures measured experimentally and to provide guidelines to optimize process variables to produce high quality pellets that can be used as a sustainable fuel.