Dielectric Properties of Biomass/Biochar Mixtures at Microwave Frequencies

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.

Tailoring product characteristics in the carbonisation of brewers’ spent grain through solvent selection

“…That water shows lower conversion than the alcohol solvents can be ascribed to the fact that it has, even under hydrothermal conditions, a more extensive hydrogen bond network, which in turn results in an anomalously high dielectric constant; ~30 F m -1 at 250 °C [35]. A higher dielectric constant will reduce the capacity of the solvent to solubilise large organic molecules arising from the degradation of BSG which itself has an expected dielectric constant of ~2 F m -1 [36]. At elevated temperatures the dielectric constant of methanol is 6.02 F m -1 (240 °C), while those of ethanol and 2-propanol are 4.20 (240 °C) and 3.75 F m -1 (230 °C) respectively [37].…”

Electrical conductivity of beech sawdust using graphite catalytic coating: unlocking the microwave-assisted thermolysis efficiency of lignocellulosic biomass

“…wood sawdust) exhibit low dielectric properties limiting direct microwave-assisted thermolysis. 10,18 Previous work reported that the pine sawdust has a loss tangent (dT) of 0.01 which is ten times lower than water (dT ¼ 0.11) at 25 C and 2.45 GHz. 18 To overcome its low microwave absorbing properties, the biomass is generally mixed with microwave susceptors such as biochar, activated carbon or silicon carbide.…”

Dielectric characterization of white birch–activated biochar composites: A sustainable alternative to radar-absorbing materials

“…[12][13][14][15] Since carbon materials are good microwave absorbers, biochar can be used to make more biochar by mixing it with biomass and submitting it to microwave heating. 16 Because of this property, activated biochar may be a sustainable and cost-beneficial alternative to graphene, and it may find use as electromagnetic shielding, microwave antenna, resonant sensor, and polymeric solar cell. 17,18 The activation process intends to improve the characteristics of regular carbon, such as electrical conductivity, sorption efficiency, and porosity.…”