Dielectric properties and microwave heating of oil

Industrial Crops and Products 50 (2013) 366–374

Contents lists available at ScienceDirect

Industrial Crops and Products journal homepage: www.elsevier.com/locate/indcrop

Dielectric properties and microwave heating of oil palm biomass and biochar Arshad Adam Salema a,c,∗ , You K. Yeow b , Kashif Ishaque c , Farid Nasir Ani d , Muhammad T. Afzal a , Azman Hassan e a

Department of Mechanical Engineering, University of New Brunswick, Head Hall, 15 Dineen Drive, Fredericton, NB E3B 5A3, Canada Faculty of Electrical Engineering, Universiti Teknologi Malaysia, UTM 81310, Johor Bahru, Malaysia c College of Engineering, Karachi Institute of Economics and Technology, Karachi 75190, Pakistan d Faculty of Mechanical Engineering, Universiti Teknologi Malaysia, UTM 81310, Johor Bahru, Malaysia e Faculty of Chemical Engineering, Universiti Teknologi Malaysia, UTM 81310, Johor Bahru, Malaysia b

a r t i c l e

i n f o

Article history: Received 31 May 2013 Received in revised form 29 July 2013 Accepted 2 August 2013 Keywords: Biomass Biochar Microwave Dielectric properties Frequency Heating characteristics

a b s t r a c t The conversion of the electromagnetic energy into heat depends largely on the dielectric properties of the material being treated. Therefore, the fundamental understanding of these properties is necessary for designing industrial microwave processing unit. The objective of this study is to investigate the dielectric properties of oil palm biomass and biochar at varying frequency in the range 0.2–10 GHz. The dielectric properties were measured using a coaxial probe attached to a network analyzer. The results indicate the dielectric constant was found to be inversely proportional to the frequency. However, the biomass in the present study did not obey the famous Debye equation and hence, the loss factor was found to be directly proportional to the frequency. The dielectric properties of oil palm shell (OPS) and its biochar were found to be almost similar and higher than oil palm fiber (OPF). Relaxation time and static dielectric constant were also presented in the paper. Lastly, the heating characteristics under MW irradiation confirmed poor microwave absorbing properties of oil palm biomass. © 2013 Elsevier B.V. All rights reserved.

1. 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

∗ Corresponding author at: Department of Mechanical Engineering, University of New Brunswick, Head Hall, 15 Dineen Drive, Fredericton, NB E3B 5A3, Canada. Tel.: +1 506 447 8111; fax: +1 506 453 5025. E-mail addresses: [email protected], [email protected] (A.A. Salema). 0926-6690/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.indcrop.2013.08.007

and hence, the research work on MW treatment and heating of various agricultural crop residues is increasing rapidly (JankerObermeier 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 (ε ), dielectric loss factor (ε ), and tangent loss (tan ı). 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 (ε*) indicates the charge storing capacity of the material irrespective of the sample dimension (Gabriel et al., 1998). Further, the

A.A. Salema et al. / Industrial Crops and Products 50 (2013) 366–374

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, 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, 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 (), and static dielectric constant (εs ) were determined using plot of −ωεr against εr . 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. 2. 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 equation ε∗ = ε − iε

(1)

where ε is the real part of relative permittivity, the so-called dielectric constant and ε is the imaginary part which is called the loss factor. Then, the loss tangent is given as tan ı =

ε ε

(2)

367

Each term from the above equations represents specific feature of the dielectric material undergoing microwave radiation. For instance, ε represents the ability of the molecule to become polarize under the electric field. It should be noted that the electric field oscillates in its direction at about 4.9 billion times per second at 2.45 GHz frequency (Chatterjee and Misra, 1990). The dielectric constant determines the behavior of the material under the microwave radiation. The ability of material to convert the electromagnetic energy into heat is indicated by ε . Lastly, loss tangent determines the ability of the material to convert electromagnetic energy into heat at a specific frequency and temperature. However, it was found that ε and ε are frequency dependent and the extent to which the material will interact with the microwaves is controlled by their magnitude and hence forms fundamental parameter in determining the dielectric heating of the materials (Hascakir and Akin, 2010). The relation of ε and ε with frequency is shown by famous Debye equation as follow: ε∗ = ε∞ +

(εs − ε∞ ) 1 + iω

(3)

where ε∞ = the limiting high frequency relative dielectric constant; εs = the limiting low frequency relative dielectric constant (static); ω = the angular frequency; = relaxation time. Solving Eq. (3), and separating the real and imaginary parts the following equation can be deduced ε = ε∞ + ε =

(εs − ε∞ ) 1 + ω2 2

(εs − ε∞ )ω 1 + ω2 2

(4)

(5)

When frequency (ω) is zero, the ε will be equal to the static value of the relative permittivity (εs ) from Eq. (4) and ε is equal to zero according to Eq. (5). However, at high frequency (ω » 1), ε will become ε∞ from Eq. (4) and ε will be negligibly small. 3. Materials and methods 3.1. 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 ␮m 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 the biochar in the present study may contain about 80–85% carbon in addition to other inorganic or metallic impurities. The present biomass and biochar material have been used without any pre-treatment. These materials were grinded in order to evaluate the dielectric properties. 3.2. Equipment and method The dielectric properties were measured with the help of HP 85070D open-ended coaxial probe. This was attached to computer controlled HP 8720B Vector Network Analyzer (VNA). The frequency ranged from 0.2 to 10 GHz at 25 ± 1 ◦ C. After calibration, the probe sensor was immersed into the sample to measure its dielectric properties. The measurement of dielectric properties for each material was repeated about 5–6 times to gain confidence in the obtained data. The dielectric properties of the materials in the present paper are the averaged value. The MW heating experiments were carried out in a modified microwave system of 1 kW power and 2450 MHz frequency.

368


Fig. 1. Photos of (a) oil palm fiber, (b) oil palm shell, and (c) palm shell biochar.

The experimental set-up for MW heating can be found elsewhere (Salema and Ani, 2011). The samples (50 g) were placed in a quartz glass tube of 0.1 m inner diameter and 0.15 m height. The samples were heated using 450 W power for about 25 min. Measurement of process temperature was done by K-type metallic thermocouples connected to Pico data acquisition system (temperature accuracy of ±0.5 ◦ C, as many readings as possible per second) acquired from United Kingdom, and further this was linked to personal computer for continuous recording. During the experimental run nitrogen gas was continuously supplied at flow rate of about 8 LPM to maintain the inert environment in the quartz tube. 4. Results and discussion 4.1. Dielectric properties The relative dielectric constant (εr ), dielectric loss factor (εr ) 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 εr decreased gradually up to 7 GHz and thereon almost remained stable. The εr decreased approximately by 16%, 26%, and 17% for OPF, OPS and biochar respectively when the frequency was varied from 0.2 GHz to 10 GHz. This can be explained due to the effect of polarization taking place in the material due to the continuous varying electric field. It is this electric component or field of the MW that is responsible for interaction of material with the electromagnetic waves (Kappe and Stadler, 2005). The decrease in εr along the frequency may also be because of gradual decrease in the dipole movement or change in orientation (Torgovnikov, 1993). Such interaction creates heat in the materials via molecular polarization. This is because the dipoles try to align themselves with rapidly varying electric field of the electromagnetic waves. It was important to investigate the nature of εr as a function of increasing frequency, since dipole may not have enough time to realign itself if the frequencies are too high or may align too fast at low frequencies. Thus, no heating may occur at these conditions (Kappe and Stadler, 2005). The change in wavelength may also play a role in defining the εr profile. Among the biomass, OPS showed

highest decrease in εr which reduced rapidly till 4 GHz frequency and later a gradual decrease was noticed. Variation in dielectric constant εr may also arise due to difference in physical and chemical characteristics of the materials. For biochar, the εr was higher compared to OPS and OPF (Fig. 2a), but close to that of OPS. However, OPS showed higher εr at lower frequency (less than 1 GHz). Another reason might be the presence of moisture or humidity that can alter the εr profiles as investigated by Omar et al. (2011). Moreover, the biomass materials consist of complex chemical components which might also play a role in variation of εr . It has been reported (Afzal et al., 2003) that lower εr and higher values of tangent loss depicts higher MW energy absorption rate in the material. The profiles of εr and tan ı were almost found to be similar. It seems that tan ı of the materials depended largely on the εr . Loss factor for OPF was noticed to increase at low frequencies, but decreased from 6 to 8 GHz and later on increased till 10 GHz. This bell shaped curve for loss factor was noticed in all the materials but at different range of frequencies. For OPS it was observed between 3 and 4 GHz frequencies whereas for biochar it was between 6 and 8 GHz. Almost similar profiles for tan ı can be deduced from Fig. 2c. The increase in direct current conductance may have contributed in increase of εr at lower frequencies (Torgovnikov, 1993). It is clear from Fig. 2b that εr decreases sharply at higher frequencies (>7 GHz) for OPS, but for OPF it was vice versa. This might be because of change in electrical conductivity of the materials. Since loss factor is directly proportional to the conductivity (i.e. εr = /2ε0 f ), the materials undergoing alternating frequency may exhibit difference in conductivity at particular frequencies. The maximum value of εr for OPF, OPS and biochar corresponds to 5.5, 6, and 9 GHz frequency. Oil palm biomass showed maximum εr peak values at approximately similar frequency. It seems that the dielectric properties not only depend on the frequency but also on the types of materials. For example, the dielectric properties of coal were found to be almost independent of the frequency (Marland et al., 2001). However, their study was limited to three frequencies only i.e. 0.615 GHz, 1.413 GHz, and 2.216 GHz. Nevertheless, the types of coal in terms of rank, its chemistry, and mineral content were reported to affect the dielectric properties. The calcined biochar obtained from coal revealed dielectric constant and loss factor of about 10.10 and 2.45 respectively at

Table 1 Physical and chemical characteristics of materials. Materials

Volatile matter, wt.%

Fixed carbon, wt.%

Ash content, wt.%

Moisture content, wt.%

Bulk Density, kg/m3

OPS OPF Biochar

78 72.8 21.4

20 19.2 71.1

2 8 7.5

8.5 10 7

500 100 350

Carbon, wt.% 50.1 45.18 –

Hydrogen, wt.%

Oxygen, wt.%

6.85 5.52 –

41.15 40.72 –


369

Fig. 3. Statistical analysis of dielectric properties for oil palm biomass and biochar.

properties compared to water or other liquid, it is considered to be good MW absorber agent (Challa et al., 1994; Dominguez et al., 2007; Menendez et al., 2010; Salema and Ani, 2011). Statistical analysis of dielectric properties has been presented in Fig. 3. The average values of εr , εr and tan ı for OPF were 1.9, 0.16, and 0.086 respectively. However, these mean values were for frequency ranging from 0.2 to 10 GHz. The average vales of εr , εr and tan ı for OPS were 2.6, 0.35, and 0.133 respectively. This shows that the dielectric properties of OPS were about 37%, 119%, and 55% higher compared to OPF. The average values of εr , εr and tan ı for palm shell biochar was 2.71, 0.3, and 0.11 respectively. Since this biochar was obtained from OPS biomass, the dielectric properties were found to be fairly similar to that of OPS. Furthermore, smaller standard deviation depicted that the average values agreed well with the data obtained from repetitive testing. Higher values of dielectric properties for OPS compared to OPF might be because of higher moisture content in OPS material. The water in the OPS might be present largely in form of intrinsic moisture content rather than absorbed moisture because physically OPS are hard nut type shells as shown in Fig. 1. A similar view was reported for rice husk/PF composite materials (Wee et al., 2009). It was found that moisture content appears to be the dominant factor in increasing the dielectric properties. Since water is good microwave absorber due to its dielectric property and polar nature (Kappe and Stadler, 2005) it can contribute largely to the dielectric properties. Undoubtedly, other chemical constituents present in the biomass materials associated with dipole moments can also play a role in dielectric properties. The profile of rise and decrease in dielectric properties of oil palm biomass and biochar with varying frequency was in agreement with earlier studies that were done on wood material (Kabir et al., 2001; Olmi et al., 2000) and other solid materials (Marzinotto et al., 2007; Paz et al., 2010; Chatterjee and Misra, 1990). Universally, 2.45 GHz MW frequency is used for industrial and domestic heating application. Hence, the dielectric properties for oil palm biomass and biochar at such frequency are depicted in Table 2. OPF and biochar material showed lowest tangent loss which depicts good microwave energy absorption ability compared Fig. 2. (a) Dielectric constant, (b) loss factor and (c) tangent loss for oil palm biomass (OPF and OPS) and biochar (@ 25 ◦ C).

2.45 GHz (Challa et al., 1994). Accordingly, the palm shell biochar dielectric constant and loss factor were about 72% and 90% lower than that of coal biochar. To our knowledge, little has been published about the dielectric properties of the biochar (Omar et al., 2011) in the literature. Even though biochar exhibits low dielectric

Table 2 Dielectric properties of oil palm biomass and char at 2.45 GHz. Materials

Dielectric constant

Loss factor

Tangent loss

Reference

OPF OPS EFB OPS char EFB char

1.99 2.76 6.4 2.83 3.5

0.16 0.35 1.9 0.23 0.47

0.08 0.12 0.3 0.08 0.13

Present study Present study Omar et al., 2011 Present study Omar et al., 2011

370


to OPS. Whilst εr was highest for biochar materials compared to OPF and OPS. In addition to the water molecules in the material, the carbonaceous component in the biochar might play a crucial role to develop high polarization in the material. According the Menendez et al. (2010), the delocalized ␲-electrons in carbon materials are free to move in relatively broad regions which might bring very interesting phenomenon. The kinetic energy of some of these carbon electrons may increase to such an extent that they might create ionization in the surrounding atmosphere. Lastly, the density of the materials also contributes in increasing the dielectric properties (Torgovnikov, 1993). For instance, the density of the OPS and palm shell biochar is higher than OPF and thus, the dielectric properties were also observed to be higher as shown in Table 2, except the tangent loss. Comparatively, empty fruit bunch depicted higher dielectric properties than OPS and OPF biomass. This might be because of high moisture content (about 18 wt%) and also due to presence of remaining palm oil in the EFB biomass (Omar et al., 2011). 4.2. Relaxation time and static permittivity The relaxation time and static permittivity (εs ) were determined by plotting graph as shown in Fig. 4 according to the following equation, and as done by Yeow et al. (2010): εr = −(ωεr ) + εs

(6)

Hence, the slope of the graph in Fig. 4 will represent the relaxation time (), and the point where −ωεr = 0 will give the value of εs . However, Fig. 4 does not show linear plot for the materials, which depicts that they do not follow the well-known Debye equation (3) as mentioned earlier. Therefore, the plot in Fig. 4 contains more than one slope or relaxation time at varying frequencies. However, only one relaxation time is observed in Debye model (Thostenson and Chou, 1999), usually the materials show more than one relaxation time. Furthermore, according to them the relaxation time is also related to the structure of the materials. The time taken by the molecules to realign themselves to their original position once the electromagnetic field is removed from the materials is known as relaxation time. The estimated values of static permittivity and relaxation time at such low and high frequency are revealed in Table 3. It is clear that the relaxation time of the materials is different at low and high frequencies. The highest relaxation time of 0.5 ps was observed for OPS at low frequency of 0–1 GHz, but it decreased drastically with increase in frequency. Overall for OPF and biochar, the relaxation time was found to decrease at low frequency region (0–5 GHz), while it increased in the high frequency region (6–10 GHz). Interestingly, the relaxation time for OPF almost ceased or was zero at frequencies ranging from 9 to 10 GHz indicating that no polarization of molecules took place at such high frequency, which can also be observed from Fig. 2a whereby εr became steady with respect to frequency (7–9 GHz). At such high frequency, negligible or no polarization took place and the corresponding εr is known as optical permittivity (ε∞ ). This phenomenon happens at high frequency because the MW field is very rapid. On the other hand at low or static frequency, the dipoles or molecules align themselves along the slowly alternating fields and thus results in total polarization (Zaky and Hawley, 1970). The loop emerged in Fig. 4(i) for OPF between −6 and −8 (−ωεr ) was due to the bell shaped curve obtained in Fig. 2b between 6 and 9 GHz frequency for OPF.

Fig. 4. Plot of εr versus −ωεr for (i) OPF, (ii) OPS, and (iii) biochar materials to determine the relaxation time.

A.A. Salema et al. / Industrial Crops and Products 50 (2013) 366–374 Table 3 Relaxation time and static permittivity at different frequencies.

Table 4 Penetration depth (cm) of MW in oil palm biomass and biochar at well-known frequencies.

Material

Frequency, GHz

Frequency range, GHz

, ps

εs

OPF

Low

0–1 1–2 2–3 4–5 6–7 7–8 9–10

0.5 0.044 0.029 0.013 0.021 0.012 0

2.15

0–1 1–2 2–3 4–5 6–7 7–8 9–10

0.086 0.051 0.041 0.021 0.022 0.03 0.045

3.21

0–1 1–2 2–3 4–5 6–7 7–8 9–10

0.102 0.036 0.021 0.017 0.021 0.011 0.02

3.02

High

Low

OPS

High

Low

Biochar

High

Materials

0

[{1 + (tan ı)

2 0.5

− 1]

−0.5

0.915

2.45

5.8

67.0 36.0 55.6

24.8 13.4 20.6

10.2 5.5 8.5

for OPB and biochar when frequency was increased from 0.915 to 2.45 GHz. The highest MW penetration depth could be achieved for OPF biomass at 2.45 GHz followed by biochar and OPS. Basically, the amount of biomass would restrict the MW to penetrate into the bed mass. In such case one has to select lower frequency. The practical reason for determining penetration depth is to heat the materials efficiently throughout its interior. If the frequencies are higher, the MW might get absorbed on the surface of the materials, and will penetrate only a short distance. 4.4. Comparison of dielectric properties of various solids

The penetration depth is a very important factor to design the size of the MW cavity, scale-up the MW heating system, and investigate the dissemination of MW energy into the material. Fig. 5 depicts the depth that MW can penetrate the oil palm biomass and biochar material at particular frequencies. The penetration depth was calculated based on following well-known equation (Metaxas and Meredith, 1983): 2(ε )0.5

Frequency, GHz

OPF OPS Biochar

4.3. Penetration depth

Dp =

371

(7)

where Dp is the penetration depth in cm, 0 is the wavelength of the frequency in cm. The penetration depth of MW dropped drastically as the frequency was increased from 0.2 to 2 GHz. These low frequencies exhibit higher penetration depth in the biomass and biochar materials. Beyond 2 GHz there was no significant change in the penetration depth till 10 GHz frequency. The penetration depth at well-known heating frequencies 0.915, 2.45 and 5.8 GHz is presented in Table 4. The penetration depth decreased by around 64%

Fig. 5. Penetration depth of the oil palm biomass and biochar material against varying frequency.

A comparative data of εr and tan ı for some common solids is tabulated in Table 5. These properties were measured at 2.45 GHz frequency and at ±20 to 25 ◦ C. Generally, solids reveals lower εr and considerable variation in tan ı. The εr for wood, biomass and plastic were found to be nearly similar. However, tan ı is much lower in case of plastic, glass, and metals compared to wood and oil palm biomass. Thus, plastics and glass are transparent to microwaves, whereas metals are the reflector. These materials do not absorb the microwave energy and hence cannot convert it into heat. In contrast, wood and biomass materials (Salema and Ani, 2011) are able to absorb the microwave energy to some extent because of moisture content in the materials and convert it into heat. However, this heat is not enough to pyrolyze them and thus, are considered to be poor absorber of microwaves (Krieger, 1994). Our previous (Salema and Ani, 2011) research explains that oil palm shell and oil palm fibers biomass materials can barely reach the temperature of about 125 ◦ C and 95 ◦ C respectively as discussed in subsequent Section Table 5 Dielectric properties of some known solids compared to oil palm biomass. ε r

tan ␦

References

Biomass and bioresource products 2.3 Wood Fir plywood ≈1.5 Particle board ≈2.5 Bark (Aspen) ≈9.4 Bark (Pine) ≈4.4 Oil palm fiber ≈2.0 ≈2.7 Oil palm shell 6.4 EFB OPS char ≈2.8 3.5 EFB char

0.11 0.01–0.05 0.1–1.0 0.22 0.18 0.08 0.13 0.3 0.08 0.13

Torgovnikov (1993) Torgovnikov (1993) Torgovnikov (1993) Torgovnikov (1993) Torgovnikov (1993) This study This study Omar et al. (2011) This study Omar et al. (2011)

Plastics and rubbers Polypropylene Polyethylene Teflon Natural rubber

≈2.2 ≈2.3 ≈2.1 ≈2.1

0.003–0.004 0.001–0.002 0.001–0.002 0.002–0.005

Eccosorb (2011) Eccosorb (2011) Eccosorb (2011) Eccosorb (2011)

Glass Fused quartz Fused silica Borosilicate glass Pyrex glass

≈3.0 ≈3.0 ≈4.0 ≈4.0

0.001–0.002 0.002–0.003 0.001–0.002 0.005–0.01

Eccosorb (2011) Eccosorb (2011) Eccosorb (2011) Eccosorb (2011)

Metals and others Zinc oxide Mica SiC

≈3.0 ≈5.0 10.8

0.1–1.0 0.003–0.004 0.0324

Eccosorb (2011) Eccosorb (2011) Eccosorb (2011)

Materials

372


Fig. 6. Real time temperature profile of OPS biomass under MW irradiation (450 W power @ 2.45 GHz frequency) (a) large particle size and (b) grinded particle, 850 ␮m.

Fig. 7. Real time temperature profile of (a) OPF and (b) biochar under MW irradiation (450 W power @ 2.45 GHz frequency).

4.5 of this paper. The dielectric properties of oil palm biomass and biochar are close to that of wood material (from Table 4), because the physical and chemical properties of biomass materials resemble comparatively to that of wood.

between the particles to particle in the bed might also play a role in MW penetration. Only water evaporation was noticed when considering the temperature from Figs. 6 and 8a. This proves that biomass in general is poor microwave absorber because of their low loss dielectric properties as reported in previous studies (Krieger, 1994; Salema and Ani, 2011; Wan et al., 2009; Dominguez et al., 2007). Certainly, the only factor that contributes to the increase in temperature at this stage was the moisture in the biomass. Since water is good microwave absorber (Zhang and Datta, 2003) due to its dielectric property and polar nature (Kappe and Stadler, 2005) it can generate the heat within the biomass. Hence, once the microwaves encounters the biomass, the moisture absorbs the microwaves and creates a dielectric polarization whereby the water molecules try to align themselves according to the radiation, which finally leads to friction within the molecules generating energy in the form of heat. This phenomenon is so rapid that water molecules attain superheating point in a very short time viz. in seconds. Hence, a sudden increase in temperature is observed the instant microwave is turned ON. In particular, carbonaceous materials such as biochar (Fig. 7b) can even reach temperature of about 1200 ◦ C in just 8 seconds. Another important observation was that the biochar bed inside temperature (T1) was significantly higher than the bed surface temperature (T2) (Fig. 7b). This indicates high penetration rate of MW in the

4.5. MW heating characteristics Fig. 6 depicts the real time temperature profiles of oil palm shell (OPS) under microwave power input of 450 W. It can be observed that OPS biomass can reach a maximum bed (T1) and surface (T2) temperature of about 122 ◦ C and 180 ◦ C, respectively. In case of OPF these temperatures were 95 ◦ C and 93 ◦ C, respectively as shown in Fig. 7. It seems that the penetration of microwaves depends on the types of materials and particle size. Fig. 6 also demonstrates the effect of particle size on the temperature profile of the biomass. The surface (T2) was much higher than the bed inside (T1) temperature when original un-grinded OPS were used (Fig. 6a). The MW was not able to penetrate the material bed due to hard and thick type of OPS biomass. While the bed inside temperature was considerably higher than surface temperature when same OPS biomass particles were grinded to small size (Fig. 6b). The height of the OPS biomass bed was approximately 5 cm, which was enough to penetrate the MW inside the bed according to the calculated penetration depth (13 cm) as shown in Section 4.4 of this paper. Moreover, the cavities


biochar bed. The temperatures in Figs. 6 and 7a were observed to stabilize after certain period of time which might be because of complete drying of biomass. Increase in microwave power beyond 450 W did not show any progress in temperature profiles. However, for OPF at high microwave power 850 and 1000 W, mild vapor generation was observed and the temperature was slightly higher than 450 W. This study confirmed the penetration characteristics of MW by measuring the temperature at surface and inside the biomass bed. It is expected that MW can penetrate the given material provided it does not reach the maximum penetration depth as stated in Table 4 at that particular frequency. MW heating profiles and dielectric properties have confirmed that oil palm biomass is a low loss dielectric material and they cannot reach the desired high temperature (above 200 ◦ C) when heated alone. However, if MW absorbing carbonaceous materials such as char, activated carbon, or graphite is mixed with them than they would certainly attain high temperature and pyrolysis reactions can be induced under MW irradiation. In this study, biochar derived from oil palm shell achieved very high temperature (1000 ◦ C) which proved good MW absorbing properties. The reason is not only the presence of moisture in the biochar but most essential is the carbon content in the material. Certainly, the practical implementation of this study besides high temperature processing such as pyrolysis, and gasification may extend to drying, modeling and simulation of MW processing unit. 5. 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 variation in dielectric properties of biomass could be due to the physical characteristics of the biomass, the moisture content, and the density of the materials. The dielectric properties investigated for oil palm biomass and biochar suggested that these materials are low-loss dielectric materials. This was proved by heating characteristics of oil palm biomass under MW irradiation. On the other hand, biochar are excellent MW absorber and can be used in various MW heating applications. Finally, these data will be useful for practical purposes to design MW processing system for oil palm and other biomass at a large scale. 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. References Abubakar, Z., Salema, A.A., Ani, F.N., 2013. A new technique to pyrolyse biomass in a microwave system: effect of stirrer speed. Bioresource Technology 128, 578–585. Afzal, M.T., Colpitts, B., Galik, K., 2003. Dielectric properties of softwood species measured with an open-ended coaxial probe. In: Proceeding of the 8th International IUFRO Wood Drying Conference, August 24–29, Brasov, Romania, pp. 110–115. Appleton, T.J., Colder, R.I., Kingman, S.W., Lowndes, I.S., Read, A.G., 2005. Microwave technology for energy-efficient processing of waste. Applied Energy 81, 85–113. Bélanger, J.M.R., Paré, J.R.J., Poon, O., Fairbridge, C., Ng, S., Mutyala, S., Hawkins, R., 2008. Remarks on various applications of microwave energy. Journal of Microwave Power and Electromagnetic Energy 42, 24–44.

373

Challa, S., Little, W.E., Cha, C.Y., 1994. Measurement of the dielectric properties of char at 2.45 GHz. Journal of Microwave Power and Electromagnetic Energy 29, 131–137. Chatterjee, I., Misra, M., 1990. Dielectric properties of various ranks of coal. Journal of Microwave Power and Electromagnetic Energy 25, 224–229. Dominguez, A., Fernandez, Y., Fidalgo, B., Pis, J.J., Menendez, J.A.B., 2007. Biogas to syngas by microwave-assisted dry reforming in the presence of char. Energy Fuel 21, 2066–2071. Eccosorb, 2011. Dielectric Chart, www.eccosorb.com/file/1138/dielectric-chart.pdf; 2011 (accessed 30.01.11). Gabriel, C., Gabriel, S., Grant, E.H., Halstead, B.S.J., Mingos, D.M.P., 1998. Dielectric parameters relevant to microwave dielectric heating. Chemical Society Reviews 27, 213–223. Hascakir, B., Akin, S., 2010. Recovery of Turkish oil shales by electromagnetic heating and determination of the dielectric properties of oil shales by an analytical method. Energy Fuel 24, 503–509. Janker-Obermeier, I., Sieber, V., Faulstich, M., Schieder, D., 2012. Solubilization of hemicellulose and lignin from wheat straw through microwave-assisted alkali treatment. Industrial Crops and Products 39, 198–203. Jones, D.A., Lelyveld, T.P., Mavrofidis, S.D., Kingman, S.W., Miles, N.J., 2002. Microwave heating application in environmental engineering—a review. Resources, Conservation and Recycling 34, 75–90. Kabir, M.F., Daud, W.M., Khalid, K., Sidek, H.A.A., 1998. Dielectric and ultrasonic properties of rubber wood: effect of moisture content, grain direction and frequency. Holz- als Roh- und Werkstoff 56, 223–227. Kabir, M.F., Daud, W.M., Khalid, K.B., Sidek, H.A.A., 2001. Temperature dependence of the dielectric properties of rubber wood. Wood and Fiber Science 33, 233–238. Kao, K.C., 2004. Dielectric Phenomena in Solids. Elsevier Academic Press, Amsterdam. Kappe, C.O., Stadler, A., 2005. Microwaves in Organic and Medicinal Chemistry. Wiley-VCH Verlag GmbH & Co, Weinheim. Kol, H.S., 2009. Thermal and dielectric properties of pine wood in the transverse direction. BioResources 4, 1663–1669. Koubaa, A., Perré, P., Hutcheon, R.M., Lessard, J., 2008. Complex dielectric properties of the sapwood of aspen, white birch, yellow birch, and sugar maple. Drying Technology 26 (5), 568–578. Krieger, B.B., 1994. Microwave pyrolysis of biomass. Research on Chemical Intermediates 20, 39–49. Luque, R., Menéndez, J.A., Arenillas, A., Cot, J., 2012. Microwave-assisted pyrolysis of biomass feedstocks: the way forward? Energy & Environmental Science 5 (2), 5481–5488. Macquarrie, D.J., Clark, J.H., Fitzpatrick, E., 2012. The microwave pyrolysis of biomass. Biofuels, Bioproducts and Biorefining 6 (5), 549–560. Marland, S., Merchant, A., Rowson, N., 2001. Dielectric properties of coal. Fuel 80, 1839–1849. Marzinotto, M., Santulli, C., Mazzetti, C., 2007. Dielectric properties of oil palmnatural rubber biocomposites. In: Annual Report Conference on Electrical Insulation and Dielectric Phenomena, October 14–17, Vancouver, B.C., Canada, pp. 584–587. Menendez, J.A., Arenillas, A., Fidalgo, B., Fernandez, Y., Zubizarreta, L., Calvo, E.G., Bermudez, J.M., 2010. Microwave heating process involving carbon materials. Fuel Processing Technology 91, 1–8. Metaxas, A.C., Meredith, R.J., 1983. Industrial Microwave Heating. IEEE power engineering series 4, London, Peter Peregrinus. Navarrete, A., Mato, R.B., Dimitrakis, G., Lester, E., Robinson, J.R., Cocero, M.J., Kingman, S., 2011. Measurement and estimation of aromatic plant dielectric properties. Application to low moisture rosemary. Industrial Crops and Products 33, 697–703. Nelson, S.O., 1991. Dielectric properties of agricultural products: measurements and applications. IEEE Transactions on Electrical Insulation 26, 845–869. Olmi, R., Bini, M., Ignesti, A., Riminesi, C., 2000. Dielectric properties of wood from 2 to 3 GHz. Journal of Microwave Power and Electromagnetic Energy 35, 135–143. Omar, R., Idris, A., Yunus, R., Khalid, K., Aida Isma, M.I., 2011. Characterization of empty fruit bunch for microwave-assisted pyrolysis. Fuel 90, 1536–1544. Pang, F., Xue, S., Yu, S., Zhang, C., Li, B., Kang, Y., 2013. Effects of combination of steam explosion and microwave irradiation (SE–MI) pretreatment on enzymatic hydrolysis, sugar yields and structural properties of corn stover. Industrial Crops and Products 42, 402–408. Paz, A.M., Trabelsi, S., Nelson, S.O., 2010. Dielectric properties of peanut-hull pellets at microwave frequencies. In: Proceeding of Instrumentation and Measurement Technology Conference (I2MTC), May 3–6, Austin, TX, U.S.A, pp. 62–66. Ramasamy, S., Moghtaderi, B., 2010. Dielectric properties of typical Australian woodbased biomass materials at microwave frequency. Energy Fuel 24, 4534–4548. Sahin, H., Ay, N., 2004. Dielectric properties of hardwood species at microwave frequencies. Journal of Wood Science 50, 375–380. Salema, A.A., Ani, F.N., 2012b. Pyrolysis of oil palm empty fruit bunch biomass pellets using multimode microwave irradiation. Bioresource Technology 125, 102–107. Salema, A.A., Ani, F.N., 2011. Microwave induced pyrolysis of oil palm biomass. Bioresource Technology 102, 3388–3395. Salema, A.A., Ani, F.N., 2012a. Microwave-assisted pyrolysis of oil palm shell biomass using an overhead stirrer. Journal of Analytical and Applied Pyrolysis 96, 162–172. Sánchez, C., Serrano, L., Andres, M.A., Labidi, J., 2013. Furfural production from corn cobs autohydrolysis liquors by microwave technology. Industrial Crops and Products 42, 513–519.

374


Singh, A., Bishnoi, N.R., 2012. Optimization of ethanol production from microwave alkali pretreated rice straw using statistical experimental designs by Saccharomyces cerevisiae. Industrial Crops and Products 37 (1), 334–341. Sukari, N., Khalid, K., 2009. Effectiveness of sterilisation of oil palm bunch using microwave technology. Industrial Crops and Products 30, 179–183. Thostenson, E.T., Chou, T.W., 1999. Microwave processing: fundamentals and applications. Composites Part A: Applied Science and Manufacturing 30, 1055–1071. Torgovnikov, G.I., 1993. Dielectric properties of wood and wood-based materials. Springer-Verlag, Berlin. Wan, Y., Chen, P., Zhang, B., Yang, C., Liu, Y., Lin, X., Ruan, R., 2009. Microwave assisted pyrolysis of biomass: catalyst to improve product selectivity. Journal of Analytical and Applied Pyrolysis 86, 161–167. Wee, F.H., Soh, P.J., Suhaizal, A.H.M., Nornikman, H., Ezanuddin, A.A.M., 2009. Free space measurement technique on dielectric properties of agricultural residues at

microwave frequencies. In: International Microwave and Optoelectronics Conference (IMOC 2009), November 3–6, Belem, pp. 178–182. Xiao, W., Han, L., Zhao, Y., 2011. Comparative study of conventional and microwaveassisted liquefaction of corn stover in ethylene glycol. Industrial Crops and Products 34 (3), 1602–1606. Yeow, Y.K., Abbas, Z., Khalid, K., Rahman, M.Z., 2010. Improved dielectric model for polyvinyl alcohol-water hydrogel at microwave frequencies. American Journal of Applied Sciences 7 (2), 270–276. Zaky, A.A., Hawley, R., 1970. Dielectric Solids. Routledge and Kegan Paul Ltd, UK. Zhang, H., Datta, A.K., 2003. Microwave power absorption in single and multiple item foods. Trans. IChemE 8, 257–265. Zhao, X.Q., Song, Z.L., Liu, H.Z., Li, Z.Q., Li, L.Z., Ma, C.Y., 2010. Microwave pyrolysis of corn stalk bale: a promising method for direct utilization of large-sized biomass and syngas production. Journal of Analytical and Applied Pyrolysis 89, 87–94.