Dielectric properties of biomass by-products generated from wood and agricultural industries in Finland

Knowledge of the dielectric properties (complex permittivities) of biomasses is critical for understanding their behaviors in a microwave field and for designing large-scale microwave systems. The present research was focused on determining the dielectric properties of different types of biomasses (sawdust, bark, fiber reject, grass, and straw) at temperatures from 25 to 700 °C and frequencies in the range of 397 to 2985 MHz, using cavity perturbation technique. The dielectric properties decreased during the drying (25 to 200 °C) and the pyrolysis stages (200 to 400 °C), but sharply increased during the biochar formation stage (400 to 700 °C). At 912 MHz, straw, grass, and fiber reject exhibited the greatest half-power depths at approximately 300 °C, and sawdust and bark at approximately 350 °C, suggesting that from room temperature to 350 °C, larger material volumes can reduce costs; above 500 °C, the sample size must not exceed the microwave half-power depth to prevent hot spots or uneven heating. The interaction mechanisms of microwaves with biomass can be explained as follows, during biomass drying, the dielectric changes are driven by dipolar polarization of water molecules; during pyrolysis, by polar molecules and functional groups; and during carbonization, by scattering and interface polarization within the biochar. Furthermore, the addition of the produced biochar to the raw biomass could increase the loss tangent up to 400 °C, enabling faster heating and reducing energy consumptions and residence times. The dielectric properties data provided in this study can be used to design large-scale microwave systems, including selection of column diameter, sample size, and microwave frequency.