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Industrial Crops and Products 50 (2013) 366– 374

Contents lists available at ScienceDirect

Industrial Crops and Products

journa l h om epa ge: www.elsev ier .com/ locate / indcrop

ielectric properties and microwave heating of oil palm biomassnd biochar

rshad Adam Salemaa,c,∗, You K. Yeowb, Kashif Ishaquec, Farid Nasir Anid,uhammad T. Afzala, Azman Hassane

Department of Mechanical Engineering, University of New Brunswick, Head Hall, 15 Dineen Drive, Fredericton, NB E3B 5A3, CanadaFaculty of Electrical Engineering, Universiti Teknologi Malaysia, UTM 81310, Johor Bahru, MalaysiaCollege of Engineering, Karachi Institute of Economics and Technology, Karachi 75190, PakistanFaculty of Mechanical Engineering, Universiti Teknologi Malaysia, UTM 81310, Johor Bahru, MalaysiaFaculty of Chemical Engineering, Universiti Teknologi Malaysia, UTM 81310, Johor Bahru, Malaysia

r t i c l e i n f o

rticle history:eceived 31 May 2013eceived in revised form 29 July 2013ccepted 2 August 2013

eywords:

a b s t r a c t

The conversion of the electromagnetic energy into heat depends largely on the dielectric properties ofthe material being treated. Therefore, the fundamental understanding of these properties is necessary fordesigning industrial microwave processing unit. The objective of this study is to investigate the dielectricproperties of oil palm biomass and biochar at varying frequency in the range 0.2–10 GHz. The dielectricproperties were measured using a coaxial probe attached to a network analyzer. The results indicate the

iomassiocharicrowaveielectric propertiesrequencyeating characteristics

dielectric constant was found to be inversely proportional to the frequency. However, the biomass in thepresent study did not obey the famous Debye equation and hence, the loss factor was found to be directlyproportional to the frequency. The dielectric properties of oil palm shell (OPS) and its biochar were foundto be almost similar and higher than oil palm fiber (OPF). Relaxation time and static dielectric constantwere also presented in the paper. Lastly, the heating characteristics under MW irradiation confirmedpoor microwave absorbing properties of oil palm biomass.

. Introduction

Oil palm (Elaeis guineensis) covers the largest agricultural land inalaysia. More than 450 palm oil industries run on this plantation.

herefore, every year large amount of oil palm biomass is gener-ted which needs proper utilization and disposal. Microwave (MW)echnology can provide an alternative form of energy to converthis biomass into useful value added products. Further, MW tech-ology has been used for drying, food processing, curing, cookingnd chemical synthesis (Bélanger et al., 2008) at commercial level.t is also applied to treat the waste materials (Appleton et al., 2005;ones et al., 2002) including biomass pyrolysis (Luque et al., 2012;

acquarrie et al., 2012). Basically, in pyrolysis process the materials decomposed into gas, liquid and biochar in the absence of oxygen.

nterestingly, MW energy is capable of carrying out pyrolysis pro-ess in a most efficient and effective way (Zhao et al., 2010). MWffers several advantages over the conventional heating systems

∗ Corresponding author at: Department of Mechanical Engineering, University ofew Brunswick, Head Hall, 15 Dineen Drive, Fredericton, NB E3B 5A3, Canada.el.: +1 506 447 8111; fax: +1 506 453 5025.

E-mail addresses: arshad.salema@unb.ca, arshadsalema@gmail.comA.A. Salema).

926-6690/$ – see front matter © 2013 Elsevier B.V. All rights reserved.ttp://dx.doi.org/10.1016/j.indcrop.2013.08.007

© 2013 Elsevier B.V. All rights reserved.

and hence, the research work on MW treatment and heating ofvarious agricultural crop residues is increasing rapidly (Janker-Obermeier et al., 2012; Pang et al., 2013; Sánchez et al., 2013; Singhand Bishnoi, 2012; Xiao et al., 2011). But prior to MW treatment orheating the fundamental understanding of dielectric properties isnecessary for designing industrial microwave processing unit.

Since MW are non-ionizing waves, the generation of heat in thematerial takes place due to vibrational or rotational motion of themolecules present in the materials, also known as dipole polariza-tion. This provides rapid and volumetric heating rather than surfaceor conductive heating. However, prior to application of MW inheating applications, the fundamental understanding of interactionbetween MW and materials is important. This includes knowledgeof dielectric properties such as dielectric constant (ε′), dielectricloss factor (ε′′), and tangent loss (tan ı). These properties not onlyhelp to scrutinize the MW and material interaction but also definethe heating characteristics of the materials. The efficient use ofMW energy depends on these properties. Moreover, these prop-erties 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 reducethe energy requirement. Overall, the complex dielectric constant(ε*) indicates the charge storing capacity of the material irrespec-tive of the sample dimension (Gabriel et al., 1998). Further, the

ops an

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A.A. Salema et al. / Industrial Cr

uthor also reported that the permittivity of the material is fre-uency dependent and it decreases as the frequency is increasedrom radio to MW region.

Basically, MW is responsible to produce polarization and mag-etization phenomenon in the dielectric materials and further thisepends on the strength of external applied field (Kao, 2004). Largemount of materials comprises of dielectric molecules in them,owever they may differ in the way they absorb the microwavenergy. This surely affects their overall MW heating characteristicsNavarrete et al., 2011). Since molecular arrangement depends onhe physical nature of the materials whether they are solid, liquid,r gas, the dielectric properties may also differ accordingly. Conse-uently, the dielectric polarization depends on the dipole momentresent in the compounds (Gabriel et al., 1998). In case of gas and

iquid, high dielectric constant can be observed due to rapid molec-lar rotation. In contrast, the molecular rotation in solid materials

s restricted and hence there is less contribution of electric fieldoward the dielectric properties.

Even though research work on MW heating of oil palm biomassSalema and Ani, 2011, 2012a; Salema and Ani, 2012b; Abubakart al., 2013) was found in the literature, but little attention has beenaid on the dielectric properties of oil palm biomass (Omar et al.,011). They characterized dielectric properties of empty fruit bunchiomass. Sukari and Khalid (2009) detected dielectric properties ofresh oil palm fruit bunch at different moisture contents. No pub-ished article has yet revealed the details on dielectric properties ofil palm shell, oil palm fiber and palm shell biochar. Nevertheless,ielectric properties of wood (Kabir et al., 1998, 2001; Ramasamynd Moghtaderi, 2010; Sahin and Ay, 2004; Olmi et al., 2000; Kol,009; Koubaa et al., 2008) and other solid materials (Marzinottot al., 2007; Wee et al., 2009) can be found in the literature. Somef these studies reported that the dielectric properties are influ-nced by varying the frequency. Since biomass can be considered asoody material, some comparison can be made with the dielectricroperties of the wood.

The aim of the present study was to determine the dielectricroperties of oil palm fiber, oil palm shell and palm shell biochar.he dielectric properties were measured from 0.2 to 10 GHz fre-uency. In addition to this, the relaxation time (�), and staticielectric constant (ε′

s) were determined using plot of −ωε′′r against

′r . These data will be useful both practically and theoretically tostimate the amount of MW energy absorbed in oil palm biomassnd the dependence of dielectric properties on the frequency. Thiss because the loading of oscillators and the design of the MWower is dependent on the dielectric properties of the materialso be heated under MW energy (Nelson, 1991). In the later sectionf the paper some work on MW heating characteristics of oil palmiomass is also presented in order to prove its low loss materialroperty.

. Microwave dielectric theory

The ability of the materials to absorb and generate the heat onnteraction with microwaves is defined by its dielectric proper-ies and specified by complex dielectric constant. Thus, the relativeomplex permittivity of the material is presented by well-knownquation

∗ = ε′ − iε′′ (1)

here ε′ is the real part of relative permittivity, the so-called dielec-ric constant and ε′′ is the imaginary part which is called the loss

actor. Then, the loss tangent is given as

an ı = ε′′

ε′ (2)

d Products 50 (2013) 366– 374 367

Each term from the above equations represents specific fea-ture of the dielectric material undergoing microwave radiation.For instance, ε′ represents the ability of the molecule to becomepolarize under the electric field. It should be noted that the elec-tric field oscillates in its direction at about 4.9 billion times persecond at 2.45 GHz frequency (Chatterjee and Misra, 1990). Thedielectric constant determines the behavior of the material underthe microwave radiation. The ability of material to convert the elec-tromagnetic energy into heat is indicated by ε′′. Lastly, loss tangentdetermines the ability of the material to convert electromagneticenergy into heat at a specific frequency and temperature. How-ever, it was found that ε′ and ε′′ are frequency dependent and theextent to which the material will interact with the microwavesis controlled by their magnitude and hence forms fundamentalparameter in determining the dielectric heating of the materials(Hascakir and Akin, 2010). The relation of ε′ and ε′′ with frequencyis shown by famous Debye equation as follow:

ε∗ = ε′∞ + (ε′

s − ε′∞)1 + iω�

(3)

where ε′∞ = the limiting high frequency relative dielectric con-stant; ε′

s = the limiting low frequency relative dielectric constant(static); ω = the angular frequency; � = relaxation time.

Solving Eq. (3), and separating the real and imaginary parts thefollowing equation can be deduced

ε′ = ε′∞ + (ε′

s − ε′∞)1 + ω2�2

(4)

ε′′ = (ε′s − ε′∞)ω�

1 + ω2�2(5)

When frequency (ω) is zero, the ε′ will be equal to the staticvalue of the relative permittivity (ε′

s) from Eq. (4) and ε′′ is equalto 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 palmshell biochar were selected to determine the dielectric properties,as illustrated in Fig. 1. Oil palm biomass was obtained from the localpalm oil mill situated in Malaysia. Palm shell biochar produced fromfast pyrolysis of OPS at 500 ◦C in a fluidized bed system with size ofabout 150 �m was used in this study. The proximate and ultimateanalysis of oil palm biomass and biochar is presented in Table 1.Hence, it can be assumed that the biochar in the present studymay contain about 80–85% carbon in addition to other inorganicor metallic impurities. The present biomass and biochar materialhave been used without any pre-treatment. These materials weregrinded in order to evaluate the dielectric properties.

3.2. Equipment and method

The dielectric properties were measured with the help of HP85070D open-ended coaxial probe. This was attached to computercontrolled HP 8720B Vector Network Analyzer (VNA). The fre-quency ranged from 0.2 to 10 GHz at 25 ± 1 ◦C. After calibration,the probe sensor was immersed into the sample to measure itsdielectric properties. The measurement of dielectric properties foreach material was repeated about 5–6 times to gain confidence in

the obtained data. The dielectric properties of the materials in thepresent paper are the averaged value.

The MW heating experiments were carried out in a modi-fied microwave system of 1 kW power and 2450 MHz frequency.

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

) oil p

T(gwpcoUfwt

4

4

aaFaεsOiovMtatSir

ira(a

TP

Fig. 1. Photos of (a) oil palm fiber, (b

he experimental set-up for MW heating can be found elsewhereSalema and Ani, 2011). The samples (50 g) were placed in a quartzlass tube of 0.1 m inner diameter and 0.15 m height. The samplesere heated using 450 W power for about 25 min. Measurement ofrocess temperature was done by K-type metallic thermocouplesonnected to Pico data acquisition system (temperature accuracyf ±0.5 ◦C, as many readings as possible per second) acquired fromnited Kingdom, and further this was linked to personal computer

or continuous recording. During the experimental run nitrogen gasas continuously supplied at flow rate of about 8 LPM to maintain

he inert environment in the quartz tube.

. Results and discussion

.1. Dielectric properties

The relative dielectric constant (ε′r), dielectric loss factor (ε′′

r )nd tangent loss for OPF, OPS and biochar at varying frequency andt ambient condition (room temperature 25 ◦C) are presented inig. 2. It can be observed that dielectric constant for all the materi-ls decreased with increasing frequency (Fig. 2a). In case of OPF, the′r decreased gradually up to 7 GHz and thereon almost remainedtable. The ε′

r decreased approximately by 16%, 26%, and 17% forPF, OPS and biochar respectively when the frequency was var-

ed from 0.2 GHz to 10 GHz. This can be explained due to the effectf polarization taking place in the material due to the continuousarying electric field. It is this electric component or field of theW that is responsible for interaction of material with the elec-

romagnetic waves (Kappe and Stadler, 2005). The decrease in ε′r

long the frequency may also be because of gradual decrease inhe dipole movement or change in orientation (Torgovnikov, 1993).uch interaction creates heat in the materials via molecular polar-zation. This is because the dipoles try to align themselves withapidly varying electric field of the electromagnetic waves.

It was important to investigate the nature of ε′r as a function of

ncreasing frequency, since dipole may not have enough time to

ealign itself if the frequencies are too high or may align too fastt low frequencies. Thus, no heating may occur at these conditionsKappe and Stadler, 2005). The change in wavelength may also play

role in defining the ε′r profile. Among the biomass, OPS showed

able 1hysical and chemical characteristics of materials.

Materials Volatilematter,wt.%

Fixedcarbon,wt.%

Ash content,wt.%

Moisturecontent, w

OPS 78 20 2 8.5

OPF 72.8 19.2 8 10

Biochar 21.4 71.1 7.5 7

alm shell, and (c) palm shell biochar.

highest decrease in ε′r which reduced rapidly till 4 GHz frequency

and later a gradual decrease was noticed. Variation in dielectric con-stant ε′

r may also arise due to difference in physical and chemicalcharacteristics of the materials. For biochar, the ε′

r was higher com-pared to OPS and OPF (Fig. 2a), but close to that of OPS. However,OPS showed higher ε′

r at lower frequency (less than 1 GHz). Anotherreason might be the presence of moisture or humidity that can alterthe ε′

r profiles as investigated by Omar et al. (2011). Moreover, thebiomass materials consist of complex chemical components whichmight also play a role in variation of ε′

r . It has been reported (Afzalet al., 2003) that lower ε′

r and higher values of tangent loss depictshigher MW energy absorption rate in the material.

The profiles of ε′′r and tan ı were almost found to be similar.

It seems that tan ı of the materials depended largely on the ε′′r .

Loss factor for OPF was noticed to increase at low frequencies, butdecreased from 6 to 8 GHz and later on increased till 10 GHz. Thisbell shaped curve for loss factor was noticed in all the materials butat different range of frequencies. For OPS it was observed between3 and 4 GHz frequencies whereas for biochar it was between 6 and8 GHz. Almost similar profiles for tan ı can be deduced from Fig. 2c.The increase in direct current conductance may have contributedin increase of ε′′

r at lower frequencies (Torgovnikov, 1993). It isclear from Fig. 2b that ε′′

r decreases sharply at higher frequencies(>7 GHz) for OPS, but for OPF it was vice versa. This might be becauseof change in electrical conductivity of the materials. Since loss fac-tor is directly proportional to the conductivity (i.e. ε′′

r = �/2�ε0f ),the materials undergoing alternating frequency may exhibit differ-ence in conductivity at particular frequencies. The maximum valueof ε′′

r for OPF, OPS and biochar corresponds to 5.5, 6, and 9 GHzfrequency. Oil palm biomass showed maximum ε′′

r peak values atapproximately similar frequency.

It seems that the dielectric properties not only depend on thefrequency but also on the types of materials. For example, thedielectric properties of coal were found to be almost independentof the frequency (Marland et al., 2001). However, their study waslimited 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 chem-istry, and mineral content were reported to affect the dielectricproperties. The calcined biochar obtained from coal revealed dielec-tric constant and loss factor of about 10.10 and 2.45 respectively at

t.%Bulk Density,kg/m3

Carbon,wt.%

Hydrogen,wt.%

Oxygen,wt.%

500 50.1 6.85 41.15100 45.18 5.52 40.72350 – – –

A.A. Salema et al. / Industrial Crops and Products 50 (2013) 366– 374 369

F(

2dtl2

oil palm biomass and biochar at such frequency are depicted inTable 2. OPF and biochar material showed lowest tangent losswhich depicts good microwave energy absorption ability compared

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

Materials Dielectricconstant

Lossfactor

Tangentloss

Reference

OPF 1.99 0.16 0.08 Present studyOPS 2.76 0.35 0.12 Present study

ig. 2. (a) Dielectric constant, (b) loss factor and (c) tangent loss for oil palm biomassOPF and OPS) and biochar (@ 25 ◦C).

.45 GHz (Challa et al., 1994). Accordingly, the palm shell biocharielectric constant and loss factor were about 72% and 90% lower

han that of coal biochar. To our knowledge, little has been pub-ished about the dielectric properties of the biochar (Omar et al.,011) in the literature. Even though biochar exhibits low dielectric

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

properties compared to water or other liquid, it is considered tobe 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 presentedin Fig. 3. The average values of ε′

r , ε′′r and tan ı for OPF were 1.9,

0.16, and 0.086 respectively. However, these mean values were forfrequency ranging from 0.2 to 10 GHz. The average vales of ε′

r , ε′′r

and tan ı for OPS were 2.6, 0.35, and 0.133 respectively. This showsthat the dielectric properties of OPS were about 37%, 119%, and 55%higher compared to OPF. The average values of ε′

r , ε′′r and tan ı for

palm shell biochar was 2.71, 0.3, and 0.11 respectively. Since thisbiochar was obtained from OPS biomass, the dielectric propertieswere found to be fairly similar to that of OPS. Furthermore, smallerstandard deviation depicted that the average values agreed wellwith the data obtained from repetitive testing.

Higher values of dielectric properties for OPS compared to OPFmight be because of higher moisture content in OPS material. Thewater in the OPS might be present largely in form of intrinsic mois-ture content rather than absorbed moisture because physically OPSare hard nut type shells as shown in Fig. 1. A similar view wasreported for rice husk/PF composite materials (Wee et al., 2009).It was found that moisture content appears to be the dominantfactor in increasing the dielectric properties. Since water is goodmicrowave absorber due to its dielectric property and polar nature(Kappe and Stadler, 2005) it can contribute largely to the dielec-tric properties. Undoubtedly, other chemical constituents presentin the biomass materials associated with dipole moments can alsoplay a role in dielectric properties. The profile of rise and decrease indielectric properties of oil palm biomass and biochar with varyingfrequency was in agreement with earlier studies that were done onwood material (Kabir et al., 2001; Olmi et al., 2000) and other solidmaterials (Marzinotto et al., 2007; Paz et al., 2010; Chatterjee andMisra, 1990).

Universally, 2.45 GHz MW frequency is used for industrial anddomestic heating application. Hence, the dielectric properties for

EFB 6.4 1.9 0.3 Omar et al., 2011OPS char 2.83 0.23 0.08 Present studyEFB char 3.5 0.47 0.13 Omar et al., 2011

370 A.A. Salema et al. / Industrial Crops an

d Products 50 (2013) 366– 374

to OPS. Whilst ε′r was highest for biochar materials compared to OPF

and OPS. In addition to the water molecules in the material, the car-bonaceous component in the biochar might play a crucial role todevelop high polarization in the material. According the Menendezet al. (2010), the delocalized �-electrons in carbon materials arefree to move in relatively broad regions which might bring veryinteresting phenomenon. The kinetic energy of some of these car-bon electrons may increase to such an extent that they might createionization in the surrounding atmosphere.

Lastly, the density of the materials also contributes in increas-ing the dielectric properties (Torgovnikov, 1993). For instance, thedensity of the OPS and palm shell biochar is higher than OPF andthus, the dielectric properties were also observed to be higher asshown in Table 2, except the tangent loss. Comparatively, emptyfruit bunch depicted higher dielectric properties than OPS and OPFbiomass. This might be because of high moisture content (about18 wt%) and also due to presence of remaining palm oil in the EFBbiomass (Omar et al., 2011).

4.2. Relaxation time and static permittivity

The relaxation time and static permittivity (εs) were determinedby plotting graph as shown in Fig. 4 according to the followingequation, and as done by Yeow et al. (2010):

ε′r = −(ωε′′

r )� + εs (6)

Hence, the slope of the graph in Fig. 4 will represent the relax-ation time (�), and the point where −ωε′′

r = 0 will give the value ofεs. However, Fig. 4 does not show linear plot for the materials, whichdepicts that they do not follow the well-known Debye equation (3)as mentioned earlier. Therefore, the plot in Fig. 4 contains morethan one slope or relaxation time at varying frequencies. However,only one relaxation time is observed in Debye model (Thostensonand Chou, 1999), usually the materials show more than one relax-ation time. Furthermore, according to them the relaxation time isalso related to the structure of the materials. The time taken by themolecules to realign themselves to their original position once theelectromagnetic field is removed from the materials is known asrelaxation time.

The estimated values of static permittivity and relaxation timeat such low and high frequency are revealed in Table 3. It is clearthat the relaxation time of the materials is different at low and highfrequencies. The highest relaxation time of 0.5 ps was observed forOPS at low frequency of 0–1 GHz, but it decreased drastically withincrease in frequency. Overall for OPF and biochar, the relaxationtime 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 waszero at frequencies ranging from 9 to 10 GHz indicating that nopolarization of molecules took place at such high frequency, whichcan also be observed from Fig. 2a whereby ε′

r became steady withrespect to frequency (7–9 GHz). At such high frequency, negligibleor no polarization took place and the corresponding ε′

r is known asoptical permittivity (ε∞). This phenomenon happens at high fre-quency because the MW field is very rapid. On the other hand atlow or static frequency, the dipoles or molecules align themselvesalong the slowly alternating fields and thus results in total polar-

ization (Zaky and Hawley, 1970). The loop emerged in Fig. 4(i) forOPF between −6 and −8 (−ωε′′

r ) was due to the bell shaped curveobtained in Fig. 2b between 6 and 9 GHz frequency for OPF.

Fig. 4. Plot of ε′r versus −ωε′′

r for (i) OPF, (ii) OPS, and (iii) biochar materials todetermine the relaxation time.

A.A. Salema et al. / Industrial Crops and Products 50 (2013) 366– 374 371

Table 3Relaxation time and static permittivity at different frequencies.

Material Frequency,GHz

Frequencyrange, GHz

�, ps εs

OPF Low 0–1 0.5 2.151–2 0.0442–3 0.0294–5 0.013

High 6–7 0.0217–8 0.0129–10 0

OPS Low 0–1 0.086 3.211–2 0.0512–3 0.0414–5 0.021

High 6–7 0.0227–8 0.039–10 0.045

Biochar Low 0–1 0.102 3.021–2 0.0362–3 0.0214–5 0.017

4

stdbwa

D

wt

qempws

Fv

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

Materials Frequency, GHz

0.915 2.45 5.8

poor absorber of microwaves (Krieger, 1994). Our previous (Salemaand Ani, 2011) research explains that oil palm shell and oil palmfibers biomass materials can barely reach the temperature of about125 ◦C and 95 ◦C respectively as discussed in subsequent Section

Table 5Dielectric properties of some known solids compared to oil palm biomass.

High 6–7 0.0217–8 0.0119–10 0.02

.3. Penetration depth

The penetration depth is a very important factor to design theize of the MW cavity, scale-up the MW heating system, and inves-igate the dissemination of MW energy into the material. Fig. 5epicts the depth that MW can penetrate the oil palm biomass andiochar material at particular frequencies. The penetration depthas calculated based on following well-known equation (Metaxas

nd Meredith, 1983):

p = �0

2�(ε′)0.5[{1 + (tan ı)2 0.5 − 1]

−0.5(7)

here Dp is the penetration depth in cm, �0 is the wavelength ofhe frequency in cm.

The penetration depth of MW dropped drastically as the fre-uency was increased from 0.2 to 2 GHz. These low frequenciesxhibit higher penetration depth in the biomass and biochar

aterials. Beyond 2 GHz there was no significant change in the

enetration depth till 10 GHz frequency. The penetration depth atell-known heating frequencies 0.915, 2.45 and 5.8 GHz is pre-

ented in Table 4. The penetration depth decreased by around 64%

ig. 5. Penetration depth of the oil palm biomass and biochar material againstarying frequency.

OPF 67.0 24.8 10.2OPS 36.0 13.4 5.5Biochar 55.6 20.6 8.5

for OPB and biochar when frequency was increased from 0.915 to2.45 GHz. The highest MW penetration depth could be achieved forOPF biomass at 2.45 GHz followed by biochar and OPS. Basically,the amount of biomass would restrict the MW to penetrate intothe bed mass. In such case one has to select lower frequency. Thepractical reason for determining penetration depth is to heat thematerials efficiently throughout its interior. If the frequencies arehigher, 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

A comparative data of ε′r and tan ı for some common solids is

tabulated in Table 5. These properties were measured at 2.45 GHzfrequency and at ±20 to 25 ◦C. Generally, solids reveals lower ε′

r andconsiderable variation in tan ı. The ε′

r for wood, biomass and plasticwere found to be nearly similar. However, tan ı is much lower incase of plastic, glass, and metals compared to wood and oil palmbiomass. Thus, plastics and glass are transparent to microwaves,whereas metals are the reflector. These materials do not absorb themicrowave energy and hence cannot convert it into heat. In con-trast, wood and biomass materials (Salema and Ani, 2011) are ableto absorb the microwave energy to some extent because of mois-ture content in the materials and convert it into heat. However, thisheat is not enough to pyrolyze them and thus, are considered to be

Materials ε′r tan � References

Biomass and bioresource productsWood 2.3 0.11 Torgovnikov (1993)Fir plywood ≈1.5 0.01–0.05 Torgovnikov (1993)Particle board ≈2.5 0.1–1.0 Torgovnikov (1993)Bark (Aspen) ≈9.4 0.22 Torgovnikov (1993)Bark (Pine) ≈4.4 0.18 Torgovnikov (1993)Oil palm fiber ≈2.0 0.08 This studyOil palm shell ≈2.7 0.13 This studyEFB 6.4 0.3 Omar et al. (2011)OPS char ≈2.8 0.08 This studyEFB char 3.5 0.13 Omar et al. (2011)

Plastics and rubbersPolypropylene ≈2.2 0.003–0.004 Eccosorb (2011)Polyethylene ≈2.3 0.001–0.002 Eccosorb (2011)Teflon ≈2.1 0.001–0.002 Eccosorb (2011)Natural rubber ≈2.1 0.002–0.005 Eccosorb (2011)

GlassFused quartz ≈3.0 0.001–0.002 Eccosorb (2011)Fused silica ≈3.0 0.002–0.003 Eccosorb (2011)Borosilicate glass ≈4.0 0.001–0.002 Eccosorb (2011)Pyrex glass ≈4.0 0.005–0.01 Eccosorb (2011)

Metals and othersZinc oxide ≈3.0 0.1–1.0 Eccosorb (2011)Mica ≈5.0 0.003–0.004 Eccosorb (2011)SiC 10.8 0.0324 Eccosorb (2011)

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

Fp

4btc

4

(tttFteswnOhwbM(

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

.5 of this paper. The dielectric properties of oil palm biomass andiochar are close to that of wood material (from Table 4), becausehe physical and chemical properties of biomass materials resembleomparatively to that of wood.

.5. MW heating characteristics

Fig. 6 depicts the real time temperature profiles of oil palm shellOPS) under microwave power input of 450 W. It can be observedhat OPS biomass can reach a maximum bed (T1) and surface (T2)emperature of about 122 ◦C and 180 ◦C, respectively. In case of OPFhese temperatures were 95 ◦C and 93 ◦C, respectively as shown inig. 7. It seems that the penetration of microwaves depends on theypes of materials and particle size. Fig. 6 also demonstrates theffect of particle size on the temperature profile of the biomass. Theurface (T2) was much higher than the bed inside (T1) temperaturehen original un-grinded OPS were used (Fig. 6a). The MW wasot able to penetrate the material bed due to hard and thick type ofPS biomass. While the bed inside temperature was considerablyigher than surface temperature when same OPS biomass particles

ere grinded to small size (Fig. 6b). The height of the OPS biomass

ed was approximately 5 cm, which was enough to penetrate theW inside the bed according to the calculated penetration depth

13 cm) as shown in Section 4.4 of this paper. Moreover, the cavities

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

between the particles to particle in the bed might also play a rolein MW penetration.

Only water evaporation was noticed when considering the tem-perature from Figs. 6 and 8a. This proves that biomass in general ispoor microwave absorber because of their low loss dielectric prop-erties as reported in previous studies (Krieger, 1994; Salema andAni, 2011; Wan et al., 2009; Dominguez et al., 2007). Certainly,the only factor that contributes to the increase in temperature atthis stage was the moisture in the biomass. Since water is goodmicrowave absorber (Zhang and Datta, 2003) due to its dielectricproperty and polar nature (Kappe and Stadler, 2005) it can generatethe heat within the biomass. Hence, once the microwaves encoun-ters the biomass, the moisture absorbs the microwaves and createsa dielectric polarization whereby the water molecules try to alignthemselves according to the radiation, which finally leads to frictionwithin the molecules generating energy in the form of heat. Thisphenomenon is so rapid that water molecules attain superheatingpoint in a very short time viz. in seconds. Hence, a sudden increasein 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. Anotherimportant observation was that the biochar bed inside tempera-ture (T1) was significantly higher than the bed surface temperature(T2) (Fig. 7b). This indicates high penetration rate of MW in the

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iochar bed. The temperatures in Figs. 6 and 7a were observed totabilize after certain period of time which might be because ofomplete drying of biomass. Increase in microwave power beyond50 W did not show any progress in temperature profiles. However,or OPF at high microwave power 850 and 1000 W, mild vapor gen-ration was observed and the temperature was slightly higher than50 W. This study confirmed the penetration characteristics of MWy measuring the temperature at surface and inside the biomassed. It is expected that MW can penetrate the given material pro-ided it does not reach the maximum penetration depth as statedn Table 4 at that particular frequency.

MW heating profiles and dielectric properties have confirmedhat oil palm biomass is a low loss dielectric material and they can-ot reach the desired high temperature (above 200 ◦C) when heatedlone. However, if MW absorbing carbonaceous materials such ashar, activated carbon, or graphite is mixed with them than theyould certainly attain high temperature and pyrolysis reactions

an be induced under MW irradiation. In this study, biochar derivedrom oil palm shell achieved very high temperature (1000 ◦C) whichroved good MW absorbing properties. The reason is not only theresence of moisture in the biochar but most essential is the carbonontent in the material. Certainly, the practical implementation ofhis study besides high temperature processing such as pyrolysis,nd gasification may extend to drying, modeling and simulation ofW processing unit.

. Conclusions

In this article, the dielectric properties of oil palm fiber (OPF),il palm shell (OPS) and palm shell biochar were investigated inhe frequency range 0.2–10 GHz and at room temperature of 25 ◦C.he results revealed that dielectric properties largely depend onhe frequency. The dielectric constant decreased with increasingrequency while loss factor had a vice versa effect against the fre-uency. Dielectric properties of OPS and its biochar were observedo be higher compared to OPF. The variation in dielectric prop-rties of biomass could be due to the physical characteristics ofhe biomass, the moisture content, and the density of the materi-ls. The dielectric properties investigated for oil palm biomass andiochar suggested that these materials are low-loss dielectric mate-ials. This was proved by heating characteristics of oil palm biomassnder MW irradiation. On the other hand, biochar are excellentW absorber and can be used in various MW heating applications.

inally, these data will be useful for practical purposes to designW processing system for oil palm and other biomass at a large

cale.

cknowledgements

The authors thank the Ministry of Higher Education (MOHE),alaysia for providing financial support under Fundamental

esearch Grant no. 78200 and 78561. The authors would also like toxtent their appreciation to Universiti Putra Malaysia for allowings to use their facilities.

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