Microwave dielectric characterization of Saudi Arabian date palm biomass during pyrolysis and at industrial frequencies

• Dielectric properties of date palm biomass were measured during pyrolysis.
• Temperature was the major factor determining the profile of dielectric properties.
• The influence of frequencies was seen in char stage only.
• Penetration depth was much higher in pyrolysis stage but dropped in char stage.
• Knowledge on drastic change in dielectric properties of the biomass was important.

The knowledge of the dielectric property is essential to design and develop microwave technology for processing biomass materials. This paper focuses on the measurement of dielectric properties of three different Saudi Arabian date palm biomass (seed, leaf and stem) from room temperature to ∼800 °C and at six different frequencies (397–2986 MHz) using a cavity perturbation method. The result showed that all date palm biomass samples were poor microwave absorbers from room temperature till 500 °C. The dielectric constant and loss factor dropped slightly in the drying region due to removal of moisture and was almost constant in pyrolysis region. However, a significant increase in dielectric constant and loss factor occurred in temperature between 500 and 750 °C, except for leaf biomass. Penetration depth and tangent loss had an opposite behavior with the rise in temperature. In conclusion, the dielectric properties might not only depend on the temperature and frequency but also on the physical and chemical characteristics of the biomass material. For instance, it was interesting to know that date leaf showed a sharp decrease in loss factor from temperature 475 °C, which was opposite to that observed in case of date seed and stem. The data can be very useful in designing, modeling and simulation of microwave processing system for date palm biomass.

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