Dielectric and Thermal Properties of Sweet Sorghum Biomass

Sweet sorghum has recently received a lot of attention as a potential biomass feedstock in the United States. This study investigated the dielectric properties (dielectric constant s’ and dielectric loss factor s’) of sweet sorghum biomass as a function of frequency (around 915 and 2450 MHz, respectively) and water content-WH2O (important for microwave drying and processing operations). Fundamental thermal properties needed in process design calculations (specific heat, thermal conductivity, thermal diffusivity) were also determined using a thermal conductivity probe method. Dielectric constant decreases with frequency at both frequency ranges. At 915 MHz, ε’ ranged from 2.73 at wH2O = 49.5% to 1.1 at wH2O = 0%, whereas at 2450 MHz, ε’ ranged from 2.67 at WH2O = 55.4% to 1.03 at WH2O = 0%. The dielectric loss factor ranged from 0.44 at wH2O = 49.5% (at 915 MHz) and 0.7 at wH2O = 55.4% (at 2450 MHz) to near zero at WH2O = 0% for both frequencies. Thermal conductivity, specific heat, and thermal diffusivity were determined to be, respectively, 0.13±0.003 W•m-1•K-1, 2321.2±43.12 J•kg-1 • K-1, and 0.37±0.01 mm2«s”’. These results can be used for design of microwave processing systems for sweet sorghum biomass, as well as for process design calculations involving various heat-based unit operations.