Fuel 153 (2015) 257–266

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Fuel

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Effect of temperature on dielectric properties and penetration depthof oil palm shell (OPS) and OPS char synthesized by microwave pyrolysisof OPS

http://dx.doi.org/10.1016/j.fuel.2015.02.1180016-2361/� 2015 Elsevier Ltd. All rights reserved.

⇑ Corresponding authors at: Department of Chemical Engineering, Faculty ofEngineering, University of Malaya, 50603 Kuala Lumpur, Malaysia (J.N. Sahu). Tel.:+60 3 7967 5204/7670 (Office); fax: +60 3 7967 4579.

E-mail addresses: jay_sahu@yahoo.co.in (J.N. Sahu), poo_ganesan@um.edu.my(P. Ganesan).

Manoj Tripathi a, J.N. Sahu b,c,⇑, P. Ganesan a,⇑, T.K. Dey d

a Department of Mechanical Engineering, Faculty of Engineering, University of Malaya, 50603 Kuala Lumpur, Malaysiab Petroleum and Chemical Engineering Programme Area, Faculty of Engineering, Institut Teknologi Brunei, Tungku Gadong, P.O. Box 2909, Brunei Darussalamc Department of Chemical Engineering, Faculty of Engineering, University of Malaya, 50603 Kuala Lumpur, Malaysiad Thermophysical Measurement Laboratory, Cryogenic Engineering Centre, Indian Institute of Technology, Kharagpur 721302, West Bengal, India

h i g h l i g h t s

� Dielectric properties of OPS and OPS char are largely dependent on temperature.� At room temperature, dielectric constant of OPS is higher than that of OPS char.� Increasing the temperature results in the reduction of the dielectric properties.� Rise in temperature & lowering in frequency, both increase the penetration depth.� Boltzmann and Gauss models are suitable for OPS char and OPS respectively.

a r t i c l e i n f o

Article history:Received 25 November 2014Received in revised form 26 February 2015Accepted 26 February 2015Available online 12 March 2015

Keywords:Dielectric propertiesMicrowave pyrolysisOil palm shell (OPS)OPS charPenetration depth

a b s t r a c t

Microwave-assisted pyrolysis is a promising technique for thermochemical treatment of various feed-stocks mainly due to its efficient heating by ‘conversion of electromagnetic energy into heat’.Microwave heating largely depends upon the dielectric properties of the material to be exposed to micro-wave radiation. Temperature dependence of dielectric properties along with penetration depth of oilpalm shell (OPS) and OPS char have been investigated in the temperature range from 30 �C to 600 �Cat 915 MHz and 2450 MHz frequencies. Measurement is made by HP 85070B open-ended coaxial probeattached to computer controlled HP 8722D Vector Network Analyzer (VNA). At room temperature, dielec-tric constant of OPS is higher than that of OPS char for both the frequencies. Dielectric constant anddielectric loss of both these materials decrease with the increase in the temperature. At room tempera-ture, penetration depth corresponding to 2450 MHz is found to be lower as compared to that at 915 MHzfrequency for both the materials, which makes 2450 MHz frequency to be more appropriate for micro-wave heating applications. Rise in temperature is seen to produce an increase of the penetration depthfor both the materials. High dielectric constant and low penetration depth of OPS as compared to OPSchar makes OPS to be more suitable for microwave heating. Experimental results of dielectric constantfor both the materials are fitted to Boltzmann and Gauss models. Gauss model fits the experimental dataof OPS more accurately while Boltzmann model is more suitable for OPS char.

� 2015 Elsevier Ltd. All rights reserved.

1. Introduction

World, today is confronting with the problem of increasing gapbetween demand and supply of energy. Most of the energy

produced around the globe comes from the fossil fuels. It is awell-known fact that these fossil fuel are depleting day by day.The use of fossil fuels causes problems like environmental pollu-tion and global warming. Other than this, the dependency on fewoil exporting nations is also needs to be avoided which leads usto find alternative energy sources. Among different alternaterenewable energy sources like solar, wind, hydro, nuclear, etc., bio-mass is regarded as a potential alternative energy source in therecent past. Production of biofuel has increased from 0.79 mb/d

258 M. Tripathi et al. / Fuel 153 (2015) 257–266

in 2006 to 1.88 mb/d in 2012 and is expected to further increase to2.38 mb/d by 2016 (1 mb/d = 106 barrels per day) [1]. Biomass isadvantageous because of its wide availability, and high energy con-tent [2]. Moreover, the use of biofuel will reduce the emission ofCO2, SO2 and other harmful gasses which help to reduce the prob-lem of environmental pollution and global warming [3,4].

In Malaysia oil palm trees are planted on over 3.8 million hec-tare land and this is expected to increase to 5 million hectare by2020 [5,6]. These palm trees produce a huge amount of oil palmshell (OPS) and a suitable and efficient mechanism is required toutilize it. OPS has a growing acceptability in the constructionindustry since it has been found to be a good replacement for theconventional granite aggregate [7]. Moreover, lightweight concreteincorporating OPS is reported to have a good mechanical propertiesas well as durable performance [8–10]. It is a common practice toutilize OPS for production of bio oil and other energy rich productslike bio char [3,11,12]. In recent past, microwave pyrolysis hasemerged as a potential and efficient technique for thethermochemical conversion of waste material into energy richproducts. Microwave pyrolysis is a process in which biomass isthermally degraded into its constituents using microwave radia-tion in inert/low oxygen environment. Microwave radiation is elec-tromagnetic radiation with frequency ranging between 300 MHzand 300 GHz which correspond to the wavelength rangingbetween 1 m and 1 mm [13].

Microwave pyrolysis of OPS results in three kinds of products:liquid, gas and solid. Bio oil produced by microwave pyrolysis ofOPS carries high energy value and with some treatment it can beused as bio fuel [14]. Bio char produced during microwave pyroly-sis can be used in different areas such as in agriculture, e.g., biochar is very often used for soil procurement [15]. Bio char inhibitsdecomposition of nutrients from soil by enhancing the natural rateof carbon sequestration which in result increases the quality of thesoil [15,16]. Due to high carbon content and its morphology, biochar can be used as asphalt binder modifier [17]. In purificationindustry, it can be used to remove chemicals like phenol [18] andtetracycline [19]. Moreover, it can be used to remove heavy metalssuch as Hg [20], Cr [21], Cd [22], Zn [23], Pb [24], Ni [25].

Conventional heating is often referred to conduction or convec-tion based heating in which some heat loss is inevitable making it aless efficient method. In microwave heating, however, material canbe directly heated without heating the surrounding making it moreefficient and economical [11,26]. Moreover, microwave heatingprovides volumetric heating which is more uniform and rapid ascompared to conventional heating [11,27,28]. Microwave heatingis found to be very effective for wood drying, food processingand material treatment (curing, molding, etc.) [29] and signifi-cantly accelerates chemical reactions [30,31].

Microwave preheating is found to be more effective preheatingtechnique as compared to conventional heating or chemical pre-heating process [32]. Zhu et al. studied the microwave preheatingon rice straw [33,34] & wheat straw [35,36] and established thatmicrowave preheating is very effective for the fermentation andenzymatic hydrolysis. Moretti et al. reported that the pre-treat-ment of sugarcane bagasse in presence of microwave radiationimproved the enzymatic hydrolysis [37]. Microwave pretreatmentis found to be capable of converting few other biomass based feed-stock such as corn stover [38], palm kernel shell [39], empty fruitbunch [40] and municipal waste products [41] into energy richvalue added products. All these studies suggest that microwaveheating is a potential technique for the treatment of biomass.Various studies have shown that interaction of the material withmicrowave radiation depends upon the dielectric properties ofthe material [42–44]. Therefore it is very important to know thetemperature dependence of dielectric behaviour of the materialbefore exposing it to microwave radiation. Motasemi et al. studied

the temperature dependence of dielectric properties of switchgrass[45] and hay [46] between room temperature and 700 �C and thefindings revealed that the increase in temperature produces areduction in the dielectric constant. Navarrete et al. obtained thesimilar results on investigating the dielectric properties ofRosemary in the temperature range of 20–160 �C [47].

Effect of microwave frequency on dielectric properties of OPSand OPS char in different frequency range is studied by Salemaet al. [48] and Tripathi et al. [49] and it was found that the increasein frequency reduces the dielectric constant of OPS. To the bestknowledge of the authors of the present study, the temperaturedependence of dielectric properties of OPS and OPS char is reportedfor the first time. The aim of the present work is to study the tem-perature dependence of dielectric properties of OPs and OPS char.The knowledge of temperature dependence of dielectric propertiesof material is important for designing a microwave processing sys-tem as the dielectric properties determines the microwave-mate-rial interaction and if dielectric properties are changing withtemperature microwave-material interaction will also be affectedby it. Knowing the dielectric properties of bio char may lead toits use in other fields like electronic packaging, sensors, etc. Inthe present study, we have investigated the dielectric propertiesof OPS and OPS char within the temperature ranging from 30 �Cto 600 �C. The knowledge of dielectric properties of these materialswill be very helpful in microwave heating related process. A widetemperature range has been selected so as to get clear informationof the dielectric properties at low as well as at high temperatures.The knowledge of dielectric properties of OPS at high temperaturemay be used to find high temperature applications of OPS.Penetration depth (Dp), which plays a very important role inoptimizing the microwave heating process, has been calculatedfor both the materials. The dielectric properties have been investi-gated for two different frequencies viz. 915 MHz (= f1) and2450 MHz (= f2). These frequencies have been specifically selectedsince these frequencies are reserved for ISM (Industrial, Scientific,Medical) applications to avoid interference with other radio orcommunication service and most of the microwave ovens/instru-ment used for industrial or medical purposes operate at these fre-quencies [50,51].

2. Materials and methods

The oil palm shell (OPS) used in the present study was suppliedby Seri Ulu Langat Palm Oil Mill Dengkil, Selangor, Malaysia.Supplied OPS was in granular form and brown in colour. OPS charused in this study was produced by the microwave pyrolysis of thissupplied oil palm shell.

2.1. OPS char preparation

The OPS was first washed using water to remove any impurities.Afterwards, the washed OPS was dried in a furnace at 80 �C for 24 h.Moisture content of this dried OPS was measured to be 6.7 wt.% ondry basis. The dried OPS was manually grinded and sieved to OPSparticle size <150 lm using wired mesh sieve (100 number). 100 gof OPS was set to undergo pyrolysis process using HAMiLab-C1500microwave muffle system oven. This oven consists of a microwavegenerator which generates microwave radiation of 2450 ± 25 MHzfrequency and adjustable microwave power controller rangingbetween 200 W and 1400 W. This muffle oven has a maximum heat-ing rate up to 50 �C/min. An infrared (IR) pyrometer with tempera-ture measurement range 250–1650 �C with ±0.5% accuracy isprovided in the muffle oven to measure the temperature. Set amount(100 g) of OPS was filled in a quartz tube with ID 28 mm, OD 35 mmand length 500 mm and placed in a cavity in microwave oven and

M. Tripathi et al. / Fuel 153 (2015) 257–266 259

then OPS was exposed to microwave radiation. Nitrogen gas supplywas supplied to create and maintain inert atmosphere inside themuffle oven throughout the pyrolysis process. Nitrogen gas was sup-plied by a nitrogen cylinder and the flow rate was set to 400 cm3/minby flow meter (Dwyer VFA 22-SS-V). A schematic diagram of themicrowave muffle system oven and the synthesizing method ofOPS char is presented in Fig. 1(a). The microwave muffle oven wasset to 1000 W power for 30 min. After the completion of pyrolysisthe quartz tube was allowed to cool down to room temperatureand then it was removed from the oven. The material in the quartztube (i.e., pyrolyzed OPS) was then carefully collected and referredto as OPS char.

2.2. Preparation of sample for dielectric measurement

OPS and OPS-char both were grinded in Fritsch Pulverisette-6planetary high energy ball mill consisting of tungsten carbide(WC) vessel and Zirconium oxide (ZrO2) balls. The ball-to-powderweight ratio (BPR) was 4:1 and the rotation speed was set to300 rpm. The ball mill was programmed to rotate for 20 min andthen to stop for 12 min before next rotation and this process wasrepeated for 12 h. Stoppage time was given to ball mill to avoidthe excess heat produced during the process. The powdered mate-rial was fed to a cylindrical die and then pressed into a cylindricalpellet using a hydraulic press machine applying a pressure of400 kN/m2. Three pellets of each material were made with heightof 5 mm and diameter 20 mm.

2.3. Thermal and compositional characterization of OPS

Thermo gravimetric analysis (TGA) is an analytical techniqueused to determine material thermal stability and its fraction ofvolatile component by monitoring the weight change which is pro-duced by the evaporation of different volatile components anddecomposition of as test sample is heated. Derivative thermogravimetry (DTG) curve gives the rate of decomposition of the testsample in the selected temperature range. The thermal degradationbehavior of OPS was studied by Mettler Toledo model; MettlerToledo TGA/STDA 85e. 18.07 mg OPS sample was placed in an alu-mina crucible. The samples were scanned from 35 �C to 1000 �Cwith a heating rate of 10 �C/min in the N2 atmosphere with N2 flowrate of 100 ml/min. Readings were taken at each 30 s which corre-spond to a temperature interval of 5 �C for successive readings.

Fourier Transform Infrared (FT-IR) spectroscope (Brand: Bruker,Model: IFS66v/S) was used to analyze and determine the differentfunctional groups present in OPS. FT-IR spectrum of OPS wasrecorded between 400 cm�1 and 4000 cm�1 with the spectral res-olution of 2 cm�1.

Ultimate and proximate characterization of OPS and OPS charwas carried out to estimate the compositional constituents of thematerial. Table 1 summarizes the results of ultimate and proxi-mate analysis of OPS and OPS char.

2.4. Dielectric properties measurement

Dielectric properties of the OPS and OPS char was measured for atemperature range of 30–600 �C using HP 85070B open-endedcoaxial probe attached to computer controlled HP 8722D VectorNetwork Analyzer (VNA). The probe measures the dielectric con-stant and dielectric loss value with an accuracy of ±5% and ±0.05%respectively. This analyzer has a wide frequency range from0.05 GHz to 40 GHz with accuracy of ±10 ppm, stability ±7.5 ppmand frequency resolution of 1 Hz. The samples were heated to therequired temperature in an electric oven which was attached to atemperature controller. Once the desired temperature wasachieved the sample was taken out from the oven and was brought

to contact with the analyzing probe to measure the dielectric prop-erties. The time elapsed in taking the sample out of the oven andbringing in contact to the probe was about 3–4 s. Fig. 1(b) gives aschematic diagram of the dielectric measurement techniqueemployed in the present study. The smooth surface of the pelletsensured that there is almost no chance for air to find a placebetween the sample and the probe and affect the measurement.The dielectric measurements were carried out between tempera-tures 30 �C and 600 �C at an interval of 50 �C for 915 MHz and2450 MHz frequencies and each sample was tested three times.

3. Mathematical formulation

3.1. Dielectric properties

Dielectric properties consist of dielectric constant, dielectricloss and tangent loss. Dielectric response of a material is a complexquantity consisting of real and imaginary parts. Symbol e repre-sents the absolute complex dielectric constant of a material in adimensionless form which can be written as Eq. (1) [48]:

e ¼ e0 � je00 ð1Þ

where e0 represents the real part, which is termed as dielectric con-stant, e00 represents the imaginary part, which is known as dielectricloss and jð¼

ffiffiffiffiffiffiffi�1p

Þ is the imaginary unit. e0 is the measure of theability of a material to store electric energy while e00 representshow much energy is dissipated (or lost) during the interaction ofthe material with radiation.

The complex dielectric constant (e) often is shown in Argandplane which is a plot between e0 and e00 with e0 on abscissa and e00on ordinate keeping frequency as a parameter. If any point on thisArgand plane intersects the origin, it will give us the complex con-jugate of e which can be represented by Eq. (2) [48]:

e� ¼ e0 þ je00 ð2Þ

where e⁄ is the complex conjugate of e. The angle, d between theline from a point on the Argand plane to the origin and the abscissais given by Eq. (3) [48]:

tan d ¼ e00=e0 ð3Þ

Herein tand is known as tangent loss and d is called loss angle.Tangent loss is a measure of the ability of the material to convertEM waves into heat at a specific frequency at a particulartemperature.

3.2. Penetration depth

Penetration depth (DP) depth is the distance from the surface ofthe material at which the microwave power is reduced to 1/e of itssurface value [52]. It is the measure of how deep a microwaveradiation can penetrate into a material. It is a very important cri-terion for designing of any microwave heating system.Penetration depth depends upon characteristics of the heatingmaterial and the frequency of microwave radiation that incidenton the material. Studies on dielectric properties of various materi-als have revealed that penetration depth is a function of tempera-ture also [45,46]. Penetration depth is calculated by inserting the e0and e00 values in Eq. (4) [48]:

DP ¼k0

2p 2e0ð Þ1=2 1þ e00

e0

� �2( )1=2

� 1

24

35�1=2

ð4Þ

where k0 ¼ cf refers to the wavelength of microwave in free space, c

is the velocity of light in free space and f is the frequency of micro-wave radiation.

Fig. 1. Schematic representations of synthesis of OPS char from OPS using microwave pyrolysis (a) and dielectric properties measurement (b).

260 M. Tripathi et al. / Fuel 153 (2015) 257–266

Table 1Ultimate and proximate analysis of OPS and OPS char.

Ultimate analysis (wt.% dry basis) Proximate analysis (wt.% dry basis)

C H O N S Moisture Ash Volatile Fixed carbon

OPS 42.73 3.24 53.51 0.43 1.09 7.2 1.3 36.8 54.7OPS Char 59.42 2.76 36.30 0.44 1.08 5.7 2.4 19.9 72.0

M. Tripathi et al. / Fuel 153 (2015) 257–266 261

3.3. Dielectric constant modelling

Regression analysis is carried out for the dielectric constantresults obtained from the present study which have been fittedto two different pre-existing regression models. These models areselected as these models have been used to predict the dielectricconstant values of biomass based materials in previous studiesand have been reported to be successful in predicting the dielectricconstant [45,46]. The first model is called Boltzmann model and itis based on a sigmoidal function and can be presented as [46]:

y ¼ A1 � A2

1þ eðT�T0Þ=dTþ A2 ð5Þ

where y is the dielectric property (dielectric constant/dielectricloss) and T (�C) (which is the only independent variable) is tempera-ture. A1, A2 and dT are the regression coefficient. Second modelwhich is selected to fit in with the experimental results of dielectricconstant is Gauss model which also is a sigmoidal function. Thisfunction is given by [46]:

y ¼ y0 þA

wffiffiffiffiffiffiffiffiffip=2

p !

� eð�2ðT�T0Þ=w2Þ ð6Þ

where y is dielectric constant and T is the temperature. w, y0, A andT0 are regression coefficients. In this equation y is dependent vari-able and T is the only independent variable.

4. Results and discussion

4.1. Thermo gravimetric analysis of OPS

Fig. 2 represents the TG and DTG curve of the OPS. It is evidentfrom the TG curve that increase in the temperature produces anincrease in the weight loss of OPS residue. Weight loss of OPS cal-culated in the temperature range of 25–120 �C is 6 wt%. Thisweight loss is the result of the evaporation of moisture/water pre-sent in the OPS. The weight loss between 250 �C and 450 �C is

Fig. 2. TG and DTG curves of OPS.

found to be 36.63 wt% (OPS residue reduced from 92.40 wt% to55.77 wt%) which corresponds to the release of low molecularvolatiles present in OPS. This result is in agreement to the previousstudies of thermal degradation behavior of different biomassshowing maximum degradation in almost the same temperaturerange [53,54]. Beyond 450 �C the degradation of OPS continuessteadily up to the final temperature (1000 �C) and total weight ofOPS residue at 1000 �C is 8.33 mg which is 46.12 wt% of initialweight. The temperature at which OPS has lost 10% of its initialweight (T10) is selected as the quantitative measure of its thermalstability and is found to be 270 �C.

DTG curve, which is derivative of TG is an indication of thereactivity of the material under investigation [55] shows that therate of weight loss increases sharply as the temperature reaches250 �C. Two sharp minima at 290 �C and 360 �C have been noticed.The rate of weight loss of OPS at these temperatures, are calculatedto be 3.34%/min and 3.98%/min. The shape of DTG curve of OPSimplies that thermal decomposition of OPS is an endothermic pro-cess and the heat is absorbed by OPS during the reaction.

4.2. FT-IR analysis of OPS

FT-IR spectra of OPS and OPS char is presented in Fig. 3. OPS andOPS char both show a strong and broad absorption peak at3454.88 cm�1 and 3520.70 cm�1. These peaks correspond to thestretching of OAH group which indicates the presence of bondedhydroxide in OPS sample. This OAH group is present in the formof moisture in OPS. Another peak seen at 2921.59 cm�1 is assignedto the CAH sp3 stretching of aliphatic compounds. 2357.93 cm�1

also gives a sharp absorption peak which implies the presence ofketone group A(C@O)A. Presence of quinone, which is derivedfrom the aromatic compounds by the conversion of ACH = groupinto AC(@O)A group by any required rearrangement of doublebonds, is indicated by the peak at wavenumber 1642.75 cm�1. Asmall and less intense peak at 1514.38 attributed to the stretchingof C@C aromatic ring. Two other small and less intense peaks areobserved at 1459.87 cm�1 and 1380.26 cm�1 which indicates theCAH bond stretching in methyl group (ACH3 group). A very strongabsorption peak at 1034.21 cm�1 accompanied by another peak at1255.17 cm�1 are seen in the FT-IR spectra of OPS, which impliesthe stretching of functional group CAO. The peaks correspondingto CAO stretching is missing in the FT-IR spectra of OPS char. Aweak absorption peak at 770.78 implies to CAH out of plane bend-ing. Another peak is seen at 531.55 cm�1which correspond to thepresence of CAN bending vibration of nitride group is seen in theFT-IR spectra of OPS while for OPS char no such peak is observed.

4.3. Dielectric properties of OPS and OPS char

Fig. 4 shows the effect of temperature on the dielectric constantof OPS and OPS char for two different frequencies f1 and f2. Theshaded area in Fig. 4 represents the standard deviation of thedielectric loss value while the dash and line within the shaded por-tion represents the mean value. For both the frequencies, thedielectric constant of OPS reduces sharply as the temperatureincreases from 30 �C to 100 �C. Quantitatively the drop is calcu-lated to be 14.9% and 11.9% corresponding to f1 and f2 respectively.

Fig. 3. FT-IR curve of OPS and OPS char.

262 M. Tripathi et al. / Fuel 153 (2015) 257–266

Biomass is associated with some amount of water either in theform of chemically bound water or free water [56–58]. Increasingthe temperature liberates the moisture from OPS and the decreas-ing trend in dielectric constant on increasing the temperature isthe result of the release of free and lightly bound water fromOPS. Dielectric constant remains almost constant between 100 �Cand 300 �C for f1 and between 100 �C and 250 �C for f2. In the tem-perature ranging from 300 �C to 400 �C, dielectric constant is seento drop by 21.6% and 12.8% for f1 and f2 respectively. The sharpreduction in dielectric constant is due to the breaking of chemicalbonds and the release of volatile materials from the OPS. Thechange in dielectric constant is not significant beyond 500 �C forany of the investigated frequencies. FT-IR curve of OPS indicatesthe presence of some polar groups (like OAH, CAO and CAN).These polar groups help in increasing the dielectric properties ofthe material. Some of these polar groups present in the OPS getsvolatized before 500 �C and are absent in OPS char (CAO andCAN) as seen in Fig. 3, causing a reduction in the dielectric con-stant of OPS char. For OPS char, temperature does not influencedielectric constant up to 200 �C. This is because most of the mois-ture content, which is responsible for the dielectric behavior of bio-mass based materials is removed during microwave pyrolysisleaving no/very little amount of moisture in OPS char. Beyond200 �C, a reduction in dielectric constant is observed until 450 �Cand then the value becomes nearly constant for the remainingtemperatures. Between 300 �C and 400 �C dielectric constant ofOPS char is reduced by 17.2% for f1 and 13.2% for f2. Increasing

Fig. 4. Variation of dielectric constant of OPS (a) and OPS char (b) at

temperature provides thermal energy to the dipoles which in turnrandomize them. Thus, less number of dipoles remains aligned inthe direction of electric component of microwave radiation, result-ing in a lower dielectric constant. A further increase in temperatureprovides higher thermal energy to these dipoles which in turncauses more thermal agitation. It increases the number of thosedipoles which does not follow the direction of electric componentof microwave radiation and consequently reduces the dielectricconstant. Moreover, heating the material produces some weightloss which in turn reduces the density of the material. Thisdecrease in density causes a reduction in the number of atomsper unit volume and thus the polarizability per unit volume.Motasemi et al. studied the dielectric properties of switchgrass[45], hay [46] and corn stover [59] between 30 �C and 700 �C. Allthese biomass based materials showed a similar trend of dielectricconstant having a sharp initial reduction in dielectric constant thenan almost constant trend followed by a large decrease in dielectricconstant with some difference in the temperature range. The beha-viour of dielectric constant of OPS and OPS char in the presentstudy is similar to the dielectric behavior of switchgrass [45],hay[46] and corn stover [59] up to nearly 500 �C although the tem-peratures at which the degradation of OPS and OPS char takesplace is slightly different. Above this temperature the dielectricconstant of OPS and OPS char remains almost constant up to600 �C while the previously studied materials show a rise in dielec-tric constant above 500 �C. Different physical and chemicalcomposition of OPS as compared to switchgrass, hay and cornstover may be a reason of this changed behaviour. Fig. 4 also showsthat, at lower temperatures, the dielectric constant of OPS is differ-ent for different frequencies but beyond 450 �C, both the frequen-cies exhibit almost the same value of dielectric constant. A similartrend of dielectric constant for both the frequencies is alsoobserved for OPS char. This infers that at higher temperatures, fre-quency of microwave radiation has no significant effect on thedielectric properties of OPS and OPS char. The maximum differencein dielectric constants corresponding to f1 and f2 is seen at 300 �Cfor both the materials and the difference is calculated to be 0.261for OPS and 0.165 for OPS char.

Fig. 5 shows the temperature dependence of dielectric loss forOPS and OPS char for f1 and f2. The shaded area in Fig. 5 representsthe standard deviation of the dielectric loss value while the dashand solid line within the shaded portion represents the meanvalue. Dielectric loss for both the materials has been found todecrease on increasing the temperature. Initially a sharp declinein dielectric loss is observed up to 100 �C and between 350 �Cand 450 �C. Dielectric loss is seen to be almost constant between100 �C and 350 �C. Beyond 450 �C, no significant dependencedielectric loss on temperature is observed. Dielectric loss of

915 MHz and 2450 MHz frequency as function of temperature.

Fig. 5. Variation of dielectric loss of OPS (a) and OPS char (b) at 915 MHz and 2450 MHz frequency as function of temperature.

Fig. 6. Variation of tangent loss of OPS (a) and OPS char (b) at 915 MHz and 2450 MHz frequency as function of temperature.

M. Tripathi et al. / Fuel 153 (2015) 257–266 263

switchgrass [45] and corn stover [59] also showed a similar trendwith a sudden decrease up to 75 �C followed by a nearly constantvalue of dielectric loss up to 200 �C. Reduction in dielectric losson increasing the temperature is the combined result of random-ization of dipoles at high temperatures and the removal of vola-tized constituents present in the material. For OPS, dielectric lossis higher for f2 for temperature ranging from 30 �C to 400 �C.After 400 �C, both the frequencies are showing nearly the samevalue of dielectric loss till 600 �C. However for OPS char, dielectricloss corresponding to f2 is higher than that of f1 only up to 100 �C.Above 100 �C, dielectric loss of corresponding to f1 is higher withina temperature range from 100 �C to 400 �C. Beyond 400 �C, boththe frequencies exhibit almost same value of dielectric loss. Themaximum difference in the dielectric loss corresponding to f1

and f2 is seen at 50 �C for OPS and at 200 �C for OPS char.Tangent loss which is the ratio of the imaginary and the real

parts of complex dielectric constant is plotted as a function of tem-perature for OPS and OPS char for the f1 and f2 and shown in Fig. 6.For both the materials and frequencies a sudden drop in tangentloss has been observed up to 150 �C temperature while for tem-peratures above 450 �C tangent loss has been found to be indepen-dent of temperature within the investigated region of temperature.Like dielectric loss, tangent loss for OPS is higher for f2 in tempera-ture range from 30 �C to 400 �C after which tangent loss is almostsame for both the frequencies. Moreover, for OPS char, tangent lossis higher for f2 within 30–100 �C while it is higher for f1 between100 �C to 400 �C. Above 400 �C, tangent loss is nearly for both thefrequencies is same.

4.4. Effect of temperature on penetration depth of OPS and OPS char

The variation of penetration depth for OPS and OPS char asfunction of temperature is presented in Fig. 7. Initial rise in thetemperature up to 100 �C exhibit an increase in the penetrationdepth for both the materials on both the frequencies. Removal ofmoisture on increasing the temperature is the reason of decrementof penetration depth in this region [45,46,59]. Between 100 �C and300 �C, the penetration depth is remained almost constant. Above300 �C penetration depth shows a slight reduction and then itincreases and become constant after 500 �C. Increase the tempera-ture liberates the volatile matter present in OPS which in turnincrease the penetration depth. Similar behavior is observed forOPS char. The increase in the penetration depth on increasing thetemperature implies that at higher temperatures OPS and OPS charare not capable of absorbing the microwave radiation as comparedto lower temperatures. A sudden and large reduction occurs inpenetration depth is seen at 450 �C for hay [46] and at 475 �C forswitchgrass [45] and corn stover [59]. In the present study, for boththe studied materials, value of dielectric constant and tangent lossis seen to be almost constant even as the temperature goes beyond500 �C which leads to an almost constant penetration depth athigher temperatures (>500 �C) while the dielectric constant andtangent loss of previously investigated switchgrass [45], hay [46]and corn stover [59] were reported to increase on increasing thetemperature beyond 500 �C. The dielectric constant and dielectricloss of previously studied biomass based materials show a suddenincrease at higher temperatures and the increase in dielectric

Fig. 7. Variation of penetration depth of OPS (a) and OPS char (b) at 915 MHz and 2450 MHz frequency as function of temperature.

Table 2Dielectric properties and penetration depth of OPS and OPS char at 915 MHz and24050 MHz frequencies at room temperature (30 �C).

OPS OPS Char

915 MHz 2450 MHz 915 MHz 2450 MHz

e0 2.982 2.783 2.526 2.451e00 0.234 0.271 0.193 0.198tand 0.078 0.097 0.076 0.080DP (cm) 38.547 12.013 43.013 15.425

264 M. Tripathi et al. / Fuel 153 (2015) 257–266

constant and dielectric loss is linked with the reduction in thepenetration depth. The temperature at which dielectric constantincreases is different for different materials since it depends uponthe physical and chemical composition of the material underinvestigation. If we increase the temperature beyond 600 �C wemay see an increase in dielectric constant and reduced penetrationdepth like the other studied biomass based materials.

At room temperature, penetration depth of OPS is 11.6% and28.4% lower than that of OPS char for f1 and f2 respectively.Lower penetration depth of OPS makes it more suitable to beused in microwave heating as compared to OPS char. For OPS,rise in the temperature from 30 �C to 600 �C increases the pene-tration depth by 15.71 cm for f1 and 33.47 cm for f2. Within thesame temperature range, increase in penetration depth for OPSchar has been found to be 28.28 cm and 11.27 cm for f1 and f2

respectively. Fig. 7 shows that for both the materials, penetrationdepth corresponding to f2 is much lower than that of f1. Itimplies that higher frequency microwave radiation have moretendency to get absorbed within the material while low fre-quency radiation are allowed to pass through the material with-out getting absorbed. Penetration depth is an importantparameter in characterizing temperature distribution in micro-wave heated materials. In general, larger the penetration depth,the more uniform the temperature distributions would be [60]which indicates that f1 will provides more uniform heating ascompared to f2. Higher penetration depth of OPS also impliesthat OPS undergoes more uniform microwave heating as com-pared to OPS char.

4.5. Dielectric properties of OPS and OPS char at room temperature

Knowledge of dielectric properties of OPS and OPS char at roomtemperature is also be required in some applications. Table 2 liststhe summarized results of dielectric properties of OPS and OPS

char at room temperature. At room temperature, dielectric con-stant of OPS is higher by 15.3% for f1 and 11.9% for f2 than that thatof OPS char. Higher dielectric constant value of OPS is mainlybecause the presence of moisture in OPS. Moisture is a good absor-ber of microwave radiation and its presence in the OPS escalates itsdielectric constant value while absence of moisture/vapor in OPSchar is the reason for relatively lower dielectric constant.Dielectric loss and tangent loss both are found to be higher forOPS than that of OPS char for both the investigated frequencies.Penetration depth of OPS is calculated to be lower than that ofOPS char which indicates that OPS absorbs more microwave radia-tion as compared to OPS char. If we compare the dielectric proper-ties corresponding to f1 and f2 at room temperature we find thatdielectric constant for f1 is higher in comparison to f2 for boththe materials. The comparison clearly implies that at room tem-perature OPS does have better dielectric properties as comparedto OPS char for both the frequencies. Penetration depth for f1 alsoshows a higher value than that for f2 for both the studied materials.However, dielectric loss and tangent loss have been found to belower for f1 than that of at f2.

4.6. Modelling of dielectric constant

The experimental results of the dielectric constant of the pre-sent study are fitted to two pre-existing regression models viz.Boltzmann and Gauss function regression models. Fig. 8 presentthe Boltzmann fit and Gauss fit of the dielectric constant of OPSand OPS char for f1 and f2. The regression coefficients ofBoltzmann and Gauss models are listed in Table 3 and Table 4respectively. Coefficient of determination (R2), which indicateshow well the function is fitted to the experimental data points, var-ied in between 0.998 and 0.944 for Boltzmann model while it aranged from 0.985 to 0.947 for Gauss model. For OPS char, R2 valueof Boltzmann fit is 0.998 for f1 and 0.996 for f2 compared to 0.986for f1 and 0.983 for f2 for Gauss model. Very high R2 values forBoltzmann model implies that the predicted value of dielectricconstant by Boltzmann model is in very close agreement to theexperimental findings. For OPS R2 value, corresponding to Gaussmodel is higher than that of Boltzmann model for both the fre-quencies. R2 values obtained by fitting the models to the experi-mental data points suggest that the Boltzmann function predictthe dielectric constant values more accurately while Gauss func-tion is more suitable for the prediction of dielectric constant ofOPS. Prediction of dielectric constants using these models can behelpful to design and simulate large size microwave systems forOPS and OPS char.

Fig. 8. Experimental value of dielectric constant fitted with Boltzmann and Gauss model at 915 MHz and 2450 MHz frequency.

Table 3Regression coefficients and R2 values for Boltzmann model (Eq. (5)) fitted to thedielectric constant experimental results.

Material Frequency (MHz) A1 A2 T0 dT R2

OPS 915 3.263 1.463 241.604 167.647 0.9442450 2.829 1.651 250.768 109.504 0.966

OPS char 915 2.517 1.704 320.949 42.402 0.9982450 2.438 1.689 306.227 46.779 0.996

Table 4Regression coefficients and R2 values for Gauss model (Eq. (6)) fitted to the dielectricconstant experimental results.

Material Frequency(MHz)

y0 T0 w A R2

OPS 915 3.103 586.863 599.282 �1054.922 0.9472450 2.928 554.646 572.257 �868.734 0.969

OPSchar

915 1.643 111.412 351.155 404.377 0.9832450 1.643 101.459 346.132 57.066 0.985

M. Tripathi et al. / Fuel 153 (2015) 257–266 265

5. Conclusion

Dielectric properties along with the penetration depth of OPSand OPS char are investigated for 915 MHz and 2450 MHz frequen-cies for the temperature ranging from 30 �C to 600 �C. The conclu-sions of this study can be summarized as below;

� Thermo gravimetric analysis shows that maximum degradationof OPS takes place between 250 �C and 450 �C. DTG curve

implies that the thermal decomposition rate of OPS is the maxi-mum at 360 �C and the process is endothermic in nature.� The dielectric properties of both the materials are found to be

largely dependent upon temperature. At room temperature,dielectric constant of OPS is 15.3% and 11.9% higher as com-pared to OPS char for 915 MHz and 2450 MHz frequenciesrespectively mainly because of its physical and chemicalcomposition and high moisture content.� For lower temperatures (up to 120 �C), increment in tempera-

ture results in a sharp decrease in dielectric constant anddielectric loss for both the materials while at higher tempera-tures (>500 �C) dielectric constant as well as dielectric lossacquires a constant value and does not change significantly.� At room temperature, penetration depth corresponding to

2450 MHz frequency is lower to that of 915 MHz by 11.6%for OPS and by 28.4% for OPS char implying that 2450 MHzfrequency is more suitable for microwave heatingapplications.� For both the materials, increasing the temperature increase

the penetration depth for lower temperature (up to 100 �C)while for higher temperatures (>500 �C) it remains nearlyconstant.� At room temperature, OPS has a high dielectric constant and

low penetration depth as compared to OPS char which impliesthat OPS is more suitable for microwave heating applicationsas compared to OPS char.� The experimental data of dielectric constant is fitted to

preexisting Boltzmann and Gauss models. Boltzmann modelhas a higher value of R2 for OPS char while Gauss model pro-vides higher R2 value for OPS. These models can be utilized todesign and simulate large scale microwave system for micro-wave heating of OPS and OPS char.

266 M. Tripathi et al. / Fuel 153 (2015) 257–266

Acknowledgements

This research is financially supported by University of Malaya,Ministry of Higher Education High Impact Research (UM.C/HIR/MOHE/ENG/20) and Exploratory Research Grant Scheme (ERGS:ER013-2013A).

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