Dielectric properties of biomass and biochar mixtures for bioenergy applications

Authors: ELLISON, CANDICE - Louisiana State University; Trabelsi, Samir

Submitted to: Microwave Power Symposium Proceedings
Publication Type: Proceedings
Publication Acceptance Date: 5/15/2015
Publication Date: 6/16/2015
Citation: Ellison, C., Trabelsi, S. 2015. Dielectric properties of biomass and biochar mixtures for bioenergy applications. Microwave Power Symposium Proceedings. p. 77-78.

Interpretive Summary: Biomass is an abundant and renewable energy resource, which may be converted into energy-dense products through thermochemical processes such as pyrolysis and gasification. Since microwave heating of biomaterials may be useful in these processes, the characteristics of biomass materials that influence microwave heating are important. Dielectric properties are the electrical characteristics of materials that determine how materials interact with electric fields such as those of microwave electromagnetic energy. Therefore, the dielectric properties of materials determine how rapidly they will heat in microwave ovens or other microwave heating equipment. Before designing a microwave system suitable for thermochemical processing, an understanding of the dielectric properties (dielectric constant and dielectric loss factor) of the biomass materials is necessary since they are unique to individual materials and govern the response to microwave heating. Dry biomass is a low dielectric loss material, and therefore cannot absorb enough energy to reach the high temperatures required by thermochemical conversion processes. Heating efficiency may be improved by mixing the biomass feedstock with a microwave energy absorber, which is a material characterized by high dielectric loss. Biochar was investigated for microwave energy absorbing capabilities in this study; it is a byproduct of the thermochemical conversion of biomass, making it a cheap and convenient microwave absorber for this application. Results of the research on Southern pine, Chinese tallow tree, live oak, and energy cane biomass revealed the dielectric properties of several different mixtures of biomass and biochar and showed that the dielectric properties are highly dependent on frequency of the microwaves, density of the material, and the amount of biochar in the material. The dielectric loss factor increased with the amount of biochar in the material and with the density of the biomass-biochar mixture. The data showed that adding biochar to biomass and compressing the material increased the microwave heating efficiency of biomass materials. The new dielectric properties data may be used to design microwave systems for thermochemical processing of biomass materials.

Technical Abstract: Biomass is an abundant and renewable energy resource, which may be converted into energy-dense products through thermochemical processes such as pyrolysis and gasification. Since microwave heating depends on the dielectric properties of the biomass material, these properties were measured at frequencies from 500 MHz to 4 GHz on samples of Southern pine, Chinese tallow tree, live oak, and energy cane biomass of 12% moisture content and particle sizes less than 500 µm. Because dry biomass is a low dielectric loss materials, biochar materials were mixed with the biomass materials to increase the susceptibility to microwave heating, and the dielectric properties measurements were taken at 20 °C on biomass/biochar mixtures containing 0% to 100% of biochar in increments of 25%, by weight, over ranges of bulk density with an open-ended coaxial-line probe and network analyzer. The Landau and Lifshitz, Looyenga mixture equation was used to normalize the densities of the mixtures to 0.30 g/cm3. Both dielectric constant and loss factor increased with increased percentage of biochar in the mixture and with increasing bulk density. Dielectric constant and loss factor data at 2.45 GHz were fitted with quadratic equations as a function of bulk density for a given mixture of biomass and biochar. The new dielectric properties data may be used in the design of microwave systems for thermochemical processing of biomass materials.