Study the microwave dielectric properties and pyrolysis mechanism of lignin and lignin/steel dust mixtures using dual-mode cylindrical cavity

The steel industry's dependence on carbon-based reductants leads to large carbon dioxide (CO 2 ) emissions; lignin, a bioethanol production by-product, offers a greener alternative with lower pyrolysis temperature and CO 2 emissions. This study investigates the dielectric properties of lignin, electric arc furnace (EAF) dust, ferrochrome converter (CRC) dust, and their mixtures using a dual-mode microwave cylindrical cavity to measure dielectric properties with simultaneous microwave heating of the sample. The dielectric constant, loss tangent, half-power depth, cooling behavior, and absorbed power were analyzed. Density functional theory (DFT) calculations were used to explain the lignin pyrolysis mechanisms and reduction reactions. The dielectric properties of lignin showed a peak at 110 °C then decreased owing to moisture removal, and a substantial increase above 350 °C due to biochar formation. DFT confirmed that pyrolysis mainly follows CH 3 -O bond cleavage and radical-driven rearrangements. EAF and CRC showed low dielectric properties, at room temperature and gradually increased with increasing temperatures due to increase sample conductivity. For EAF2 and CRC2, the dielectric properties sharply increase from 300 °C to 700 °C, owing to biochar formation and reduction of zinc oxide and zinc ferrite into zinc vapor and iron metal. For EAF2, the formation of metallic iron caused the dielectric constant to become negative near 800 °C and sharply reduced penetration depth from 50 cm to 0.19 cm. During cooling, lignin remained a strong microwave absorber and EAF2 maintained non-dielectric behavior. In terms of absorbed power, lignin enabled rapid low-temperature absorption and accelerated heating of mixtures. • Dielectric properties of lignin and lignin/steel dust mixtures were studied. • Effect of temperature and frequency on dielectric properties was investigated. • Density function theory was used to explain the pyrolysis mechanism of lignin. • Dual-mode cylindrical cavity was used for dielectric properties measurements.