Dielectric response and pyrolysis mechanisms during biomass microwave pyrolysis: Intrinsic coupling among heat/mass transfer, microstructural evolution, and microwave energy conversion

Biomass often experiences uneven heating and low energy efficiency during microwave (MW) pyrolysis. This study investigates the dielectric response and pyrolysis mechanisms to improve the utilization of MW energy and product quality. The temperature rise behavior was assessed under varying pyrolysis temperature, power density, moisture content, and vertical distance, and the microstructures of the product were characterized using scanning electron microscopy (SEM), Raman spectroscopy, terahertz time-domain spectroscopy (THz-TDS), and two-dimensional Fourier transform infrared spectroscopy with correlation infrared spectroscopy (2D-FTIR-COS). The MW dielectric properties of tobacco stems (TS) were tested using the cavity perturbation method. The results showed that low temperatures mainly remove volatile compounds, medium temperatures promote condensation, and high temperatures drive aromatization, whereas the optimal conditions (vertical distance of 5–10 cm, power density of 47–93 W/g, and moisture content of 20–30 wt %) enhanced biochar graphitization and pore development. The development of micropores and mesopores facilitated the accumulation of charges at the interface, thereby enhancing the dielectric response. At 20 wt % moisture content, the loss tangent reached 0.17 (2450 MHz, experimental value), thereby improving the absorption of MW energy and heating efficiency. The MW pyrolysis process of TS involves dehydration, deoxygenation, condensation and aromatization. 2D-FTIR-COS revealed a sequential temperature response of biochar functional groups (O–H → C=O → C–O–C/C–O → C–H → aromatic C C). The MW dielectric response was attributed to interfacial, dipolar and ionic polarization, producing bulk heating at the macro scale and localized hotspots at the micro scale. Overall, these findings elucidate the MW dielectric response and pyrolysis mechanisms, identify the key factors governing the heating uniformity and energy efficiency, and provide a theoretical basis for the structural regulation of biochar and for achieving efficient energy conversion.