Dielectric properties and microwave heating of oil palm biomass and biochar
Introduction
Oil palm (Elaeis guineensis) covers the largest agricultural land in Malaysia. More than 450 palm oil industries run on this plantation. Therefore, every year large amount of oil palm biomass is generated which needs proper utilization and disposal. Microwave (MW) technology can provide an alternative form of energy to convert this biomass into useful value added products. Further, MW technology has been used for drying, food processing, curing, cooking and chemical synthesis (Bélanger et al., 2008) at commercial level. It is also applied to treat the waste materials (Appleton et al., 2005, Jones et al., 2002) including biomass pyrolysis (Luque et al., 2012, Macquarrie et al., 2012). Basically, in pyrolysis process the material is decomposed into gas, liquid and biochar in the absence of oxygen. Interestingly, MW energy is capable of carrying out pyrolysis process in a most efficient and effective way (Zhao et al., 2010). MW offers several advantages over the conventional heating systems and hence, the research work on MW treatment and heating of various agricultural crop residues is increasing rapidly (Janker-Obermeier et al., 2012, Pang et al., 2013, Sánchez et al., 2013, Singh and Bishnoi, 2012, Xiao et al., 2011). But prior to MW treatment or heating the fundamental understanding of dielectric properties is necessary for designing industrial microwave processing unit.
Since MW are non-ionizing waves, the generation of heat in the material takes place due to vibrational or rotational motion of the molecules present in the materials, also known as dipole polarization. This provides rapid and volumetric heating rather than surface or conductive heating. However, prior to application of MW in heating applications, the fundamental understanding of interaction between MW and materials is important. This includes knowledge of dielectric properties such as dielectric constant (ɛ′), dielectric loss factor (ɛ″), and tangent loss (tan δ). These properties not only help to scrutinize the MW and material interaction but also define the heating characteristics of the materials. The efficient use of MW energy depends on these properties. Moreover, these properties are also found to depend on parameters such as frequency, temperature, types of materials, etc. Selection of right frequency is important either to increase the penetration depth of MW or reduce the energy requirement. Overall, the complex dielectric constant (ɛ*) indicates the charge storing capacity of the material irrespective of the sample dimension (Gabriel et al., 1998). Further, the author also reported that the permittivity of the material is frequency dependent and it decreases as the frequency is increased from radio to MW region.
Basically, MW is responsible to produce polarization and magnetization phenomenon in the dielectric materials and further this depends on the strength of external applied field (Kao, 2004). Large amount of materials comprises of dielectric molecules in them, however they may differ in the way they absorb the microwave energy. This surely affects their overall MW heating characteristics (Navarrete et al., 2011). Since molecular arrangement depends on the physical nature of the materials whether they are solid, liquid, or gas, the dielectric properties may also differ accordingly. Consequently, the dielectric polarization depends on the dipole moment present in the compounds (Gabriel et al., 1998). In case of gas and liquid, high dielectric constant can be observed due to rapid molecular rotation. In contrast, the molecular rotation in solid materials is restricted and hence there is less contribution of electric field toward the dielectric properties.
Even though research work on MW heating of oil palm biomass (Salema and Ani, 2011, Salema and Ani, 2012a, Salema and Ani, 2012b, Abubakar et al., 2013) was found in the literature, but little attention has been paid on the dielectric properties of oil palm biomass (Omar et al., 2011). They characterized dielectric properties of empty fruit bunch biomass. Sukari and Khalid (2009) detected dielectric properties of fresh oil palm fruit bunch at different moisture contents. No published article has yet revealed the details on dielectric properties of oil palm shell, oil palm fiber and palm shell biochar. Nevertheless, dielectric properties of wood (Kabir et al., 1998, Kabir et al., 2001, Ramasamy and Moghtaderi, 2010, Sahin and Ay, 2004, Olmi et al., 2000, Kol, 2009, Koubaa et al., 2008) and other solid materials (Marzinotto et al., 2007, Wee et al., 2009) can be found in the literature. Some of these studies reported that the dielectric properties are influenced by varying the frequency. Since biomass can be considered as woody material, some comparison can be made with the dielectric properties of the wood.
The aim of the present study was to determine the dielectric properties of oil palm fiber, oil palm shell and palm shell biochar. The dielectric properties were measured from 0.2 to 10 GHz frequency. In addition to this, the relaxation time (τ), and static dielectric constant were determined using plot of against These data will be useful both practically and theoretically to estimate the amount of MW energy absorbed in oil palm biomass and the dependence of dielectric properties on the frequency. This is because the loading of oscillators and the design of the MW power is dependent on the dielectric properties of the materials to be heated under MW energy (Nelson, 1991). In the later section of the paper some work on MW heating characteristics of oil palm biomass is also presented in order to prove its low loss material property.
Section snippets
Microwave dielectric theory
The ability of the materials to absorb and generate the heat on interaction with microwaves is defined by its dielectric properties and specified by complex dielectric constant. Thus, the relative complex permittivity of the material is presented by well-known equationwhere ɛ′ is the real part of relative permittivity, the so-called dielectric constant and ɛ″ is the imaginary part which is called the loss factor. Then, the loss tangent is given as
Each term from the above
Materials
Two oil palm biomass (oil palm fiber, oil palm shell) and palm shell biochar were selected to determine the dielectric properties, as illustrated in Fig. 1. Oil palm biomass was obtained from the local palm oil mill situated in Malaysia. Palm shell biochar produced from fast pyrolysis of OPS at 500 °C in a fluidized bed system with size of about 150 μm was used in this study. The proximate and ultimate analysis of oil palm biomass and biochar is presented in Table 1. Hence, it can be assumed that
Dielectric properties
The relative dielectric constant dielectric loss factor and tangent loss for OPF, OPS and biochar at varying frequency and at ambient condition (room temperature 25 °C) are presented in Fig. 2. It can be observed that dielectric constant for all the materials decreased with increasing frequency (Fig. 2a). In case of OPF, the decreased gradually up to 7 GHz and thereon almost remained stable. The decreased approximately by 16%, 26%, and 17% for OPF, OPS and biochar
Conclusions
In this article, the dielectric properties of oil palm fiber (OPF), oil palm shell (OPS) and palm shell biochar were investigated in the frequency range 0.2–10 GHz and at room temperature of 25 °C. The results revealed that dielectric properties largely depend on the frequency. The dielectric constant decreased with increasing frequency while loss factor had a vice versa effect against the frequency. Dielectric properties of OPS and its biochar were observed to be higher compared to OPF. The
Acknowledgements
The authors thank the Ministry of Higher Education (MOHE), Malaysia for providing financial support under Fundamental Research Grant no. 78200 and 78561. The authors would also like to extent their appreciation to Universiti Putra Malaysia for allowing us to use their facilities.
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