Microwave dielectric characterization of hay during pyrolysis

•Microwave dielectric properties of hay were analyzed during pyrolysis.•Dielectric properties are greatly influenced by the pyrolysis temperature.•The microwave absorbing property of the biomass increases in the char region.•Some modelling work was presented on dielectric properties.•The penetration depth at 2450 MHz and 700 °C was about 2 mm.

In this study, the microwave dielectric properties of hay were investigated at two frequencies (915 and 2450 MHz) from room temperature to ∼700 °C in an inert environment. These properties were scrutinized in three distinct stages; namely drying (from room temperature to ∼200 °C), pyrolysis (from ∼200 °C to ∼450 °C), and char region (from ∼450 °C to ∼700 °C). The dielectric properties were found to decrease during drying and pyrolysis stages, while increased significantly in the char region. The penetration depth of the microwave in hay is presented against the increasing temperature. Overall, the microwave absorption capability of the biomass material improved significantly after pyrolysis process, i.e. in the char region. Hay in its original form can be considered as a low loss dielectric material. However, its char product showed a good microwave absorbing capability. The experimental data were fitted using regression fit and based on this the dielectric properties model related to the temperature was developed. The result from this study can be useful to design and develop microwave system for hay.