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ARS Home » Southeast Area » Athens, Georgia » U.S. National Poultry Research Center » Quality and Safety Assessment Research Unit » Research » Publications at this Location » Publication #340508

Title: Dielectric properties of biomass/biochar mixtures at microwave frequencies

item ELLISON, CANDACE - Louisiana State University
item MCKEOWN, MURAT - University Of Georgia
item Trabelsi, Samir
item BOLDER, DORIN - Louisiana State University

Submitted to: Energies
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 4/4/2017
Publication Date: 4/9/2017
Citation: Ellison, C., Mckeown, M., Trabelsi, S., Bolder, D. 2017. Dielectric properties of biomass/biochar mixtures at microwave frequencies. Energies. 10(4):502.

Interpretive Summary: Biomass resources offer a plentiful, renewable energy alternative to fossil fuels and through their use CO2 emissions can be reduced also. Lignocellulosic biomass materials can be converted to energy-dense products by thermochemical processes such as pyrolysis and gasification. For these conversion processes, the biomass feedstock is heated to temperatures of 400 to 700 °C, usually by conventional heating methods involving conduction and convection. In attempts to improve heating efficiency, recent studies have used dielectric heating for these thermochemical conversion processes. Microwave heating offers potential advantages over conventional methods, including contactless energy transfer, volumetric energy absorption and dissipation, and selective heating in samples composed of different materials. Microwave heating of materials depends upon their dielectric properties, so these properties of biomass materials must be known in order to study the use of microwave heating for efficient processing of such materials. The rate of microwave heating of biomass materials can be accelerated by mixing biochar materials with them before exposure to microwave processing. Therefore, biochar, produced by pyrolysis of pine sawdust, was used in this study and dielectric properties of four biomass materials, with different amounts of biochar were measured over the frequency range from 500 MHz to 20 GHz. The four biomass materials studied were energy cane bagasse, pine sawdust, and wood of the live oak, and Chinese tallow tree, all ground and sieved to desired particle sizes. The dielectric properties were measured for known moisture contents and bulk densities of the materials, and results are presented graphically in the paper. The dielectric properties of biomass and biochar mixtures were measured from 500 MHz to 20 GHz at room temperature. Results from this study determined the dependence of dielectric properties on frequency, biomass type, and biomass/biochar mixture ratio. Dielectric properties increased quadratically with increasing biochar content for all biomasses. Dry biomass materials require a considerable amount of microwave energy to reach high processing temperatures because of low dielectric loss. Biochar, a byproduct of biomass pyrolysis, was found to be a good microwave absorber and can be used as an additive to biomass feedstocks to increase microwave absorption in the bulk material and accelerate heating rates. The dielectric properties data presented in this study are important for the design, simulation, and scale-up of microwave reactors for high temperature microwave processing of biomass materials. Biomass and biochar mixture ratios can be optimized for a given microwave processing design.

Technical Abstract: Material dielectric properties are important for understanding their response to microwaves. Carbonaceous materials are considered good microwave absorbers and can be mixed with dry biomasses, which are otherwise low- loss materials, to improve the heating efficiency of biomass feedstocks. In this study, dielectric properties of pulverized biomass and biochar mixtures are presented from 0.5 to 20 GHz at room temperature. An open-ended coaxial-line dielectric probe and vector network analyzer were used to measure dielectric constant and dielectric loss factor. Results show a quadratic increase of dielectric constant and dielectric loss with increasing biochar content. In measurements on biochar, a strong dielectric relaxation is observed at 8 GHz as indicated by a peak in dielectric loss factor at that frequency. Biochar is found to be a good microwave absorber and mixtures of biomass and biochar can be utilized to increase microwave heating rates for high temperature microwave processing of biomass feedstocks. These data can be utilized for design, scale-up and simulation of microwave heating processes of biomass, biochar, and their mixtures.