Dielectric Properties of Biomass/Biochar Mixtures at Microwave Frequencies

Candice Ellison,Samir Trabelsi,Dorin Boldor,Murat Mckeown

doi:10.3390/en10040502

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 GHz 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.

Highlights

Biomass resources offer a plentiful, renewable energy alternative to fossil fuels and can reduceCO2 emission due to the potential of net zero emissions [1]
To effectively design microwave reactors for processing biomass feedstocks, an accurate knowledge of the dielectric properties of biomass materials is necessary to evaluate the dielectric response of materials in an applied electric field [8]
This study aims to fill this knowledge gap by characterizing the dielectric properties of biomass/biochar mixtures for four different biomass feedstocks readily available in Louisiana and southeastern United States: energy cane bagasse, pine sawdust (Pinus sp.), live oak (Quercus sp.), and Chinese tallow tree wood (Triadica sebifera (L.))

Summary

Introduction

Biomass resources offer a plentiful, renewable energy alternative to fossil fuels and can reduceCO2 emission due to the potential of net zero emissions [1]. Lignocellulosic biomass materials can be converted to energy-dense products via thermochemical processes (namely pyrolysis and gasification) [2,3]. During these conversion processes, the biomass feedstock is heated to temperatures in the range of 400–700 ◦ C, usually by conventional heating methods, i.e., conduction and convection [4]. In attempts to improve heating efficiency, recent studies have applied dielectric heating to these thermochemical conversion processes [5,6,7]. To effectively design microwave reactors for processing biomass feedstocks, an accurate knowledge of the dielectric properties of biomass materials is necessary to evaluate the dielectric response of materials in an applied electric field [8].

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