Microwave dielectric properties of biomass in combustion and pyrolysis environment

Introduction: In recent years, microwave (MW) heating has become one of the emerging technologies to valorise the biomass materials specifically for thermo-chemical processes (pyrolysis and gasification). Several biomass materials such as oil palm shell [1], empty fruit bunch [2], rice husk [3], wood [4] and others [5-7] have been processed using MW technology. Most of these research works is conducted under inert environment or pyrolysis process. Very recently, some limited work on biomass gasification [8] and combustion [9] under microwave irradiation are published. The change in process or gas environment (air/oxygen or nitrogen) will impact the interaction of biomass with microwave as well as the dielectric properties. The comparison of microwave dielectric properties of biomass under oxygen/air and nitrogen environment has not been reported yet in the literature, that provide some crucial information such as the change in MW absorption characteristics of the material during processing, the penetration depth, and how well the material will absorb or reflect MW energy at particular temperatures. Dielectric properties are also considered as the most important factors for designing large scale MW systems [10], and lack of proper understanding about the dielectric properties may hinder the development of MW technology [6].
Therefore, for the first time a research work on the microwave dielectric characterization of oil palm biomass during the combustion process has been presented and compared with the data from the pyrolysis process. The dielectric properties (dielectric constant (DC), and loss factor (LF)) were measured from room temperature to ∼750 °C and at two industrial frequencies (912 and 2450 MHz) using a cavity perturbation equipment. The loss tangent (LT) and penetration depth (PD) were calculated based on the measured dielectric properties data.
Method: The oil palm shell (OPS) and Empty Fruit Bunch (EFB) biomass samples were pulverized to particle size of ~ 100 to 600 μm and the dielectric measurement were done on pelletized (formed by uniaxial pressing at ∼130MPa in a die lined with tungsten carbide) samples. The dielectric properties were measured during combustion (air) and pyrolysis (nitrogen) using the cavity perturbation technique developed by Microwave Properties North, Canada [11] and the details are provided by previous articles [12]. The cavity perturbation system measures between 400 MHz to 3000 MHz and has a temperature range up to 1400 °C. The size of the sample is small (~0.1 cm3), but the measurement system is sensitive enough for low loss material such as biomass. Unlike the pyrolysis process, very small material (ash) was left after the combustion process.
Results: The values of DC in the combustion environment for EFB and OPS had almost similar profile (Fig. 1) when the temperature was increased from room to 700 °C. The value of DC varied between 3.5 to 1.4. A peak was observed around 175 °C (just after the drying stage) indicating the MW response to the degrading structure and the mobile ions created during the initial combustion and pyrolysis reaction. A significant drop (~60 to 65 %) in the DC was observed in the temperature region from 175 °C to 500 °C. The values of DC almost remained plateau beyond 500 °C up to 700 °C in case of OPS biomass, while it increased at 600 °C for EFB. The EFB ash formed at 700 °C showed highest DC and LF, followed by OPS at both frequencies. The profiles (Fig. 1) of LF were almost similar to those of DC except that for EFB that showed a sudden increase in the LF beyond 500 °C. The LF and Loss Tangent (LT) profiles showed a peak at 275 °C for EFB biomass that corresponds to the main combustion DTG peak between 225 and 300 °C. The DC and LF did not showed much strong dependency of frequency during combustion process.