Bioresource Technology 158 (2014) 193–200

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Bioresource Technology

journal homepage: www.elsevier .com/locate /bior tech

Microwave-enhanced CO2 gasification of oil palm shell char

http://dx.doi.org/10.1016/j.biortech.2014.02.0150960-8524/� 2014 Elsevier Ltd. All rights reserved.

⇑ Corresponding author. Tel.: +60 4 593 7788; fax: +60 4 594 1025.E-mail addresses: mezainal@yahoo.com, mezainal@eng.usm.my (Z.A. Zainal).

Pooya Lahijani a, Zainal Alimuddin Zainal a,⇑, Abdul Rahman Mohamed b, Maedeh Mohammadi c

a Biomass and Bioenergy Laboratory, School of Mechanical Engineering, Universiti Sains Malaysia, 14300 Nibong Tebal, Pulau Pinang, Malaysiab Low Carbon Economy (LCE) Research Group, School of Chemical Engineering, Engineering Campus, Universiti Sains Malaysia, 14300 Nibong Tebal, Pulau Pinang, Malaysiac Faculty of Chemical Engineering, Babol Noushirvani University of Technology, 47148 Babol, Iran

h i g h l i g h t s

�Microwave heating system wasdeveloped to convert CO2 to CO viaBoudouard reaction.� Superior performance of microwave

over thermal driven gasification wasobserved.� Formation of hot spot in microwave

heating pronouncedly improved charreactivity.� Ea of 74 and 247 kJ/mol was obtained

for microwave and thermal CO2 chargasification.� CO2 conversion of 99% was achieved

in microwave gasification of Fe-catalyzed char.

g r a p h i c a l a b s t r a c t

a r t i c l e i n f o

Article history:Received 5 December 2013Received in revised form 4 February 2014Accepted 6 February 2014Available online 17 February 2014

Keywords:CO2 gasificationMicrowave heatingBoudouard reactionBiomass charActivation energy

a b s t r a c t

CO2 gasification of oil palm shell (OPS) char to produce CO through the Boudouard reaction (C + CO2 -M 2CO) was investigated under microwave irradiation. A microwave heating system was developed tocarry out the CO2 gasification in a packed bed of OPS char. The influence of char particle size, temperatureand gas flow rate on CO2 conversion and CO evolution was considered. It was attempted to improve thereactivity of OPS char in gasification reaction through incorporation of Fe catalyst into the char skeleton.Very promising results were achieved in our experiments, where a CO2 conversion of 99% could be main-tained during 60 min microwave-induced gasification of iron-catalyzed char. When similar gasificationexperiments were performed in conventional electric furnace, the superior performance of microwaveover thermal driven reaction was elucidated. The activation energies of 36.0, 74.2 and 247.2 kJ/mol wereobtained for catalytic and non-catalytic microwave and thermal heating, respectively.

� 2014 Elsevier Ltd. All rights reserved.

1. Introduction

Advances in microwave heating systems have opened up newavenues for prompt and effective thermal processing of materials.This emerging technology has empowered both researchers in aca-demic communities and engineers in industry to engage in pro-cesses with reduced energy consumption and short processing

time. Microwave heating systems are considered energy efficient,wherein rapid and selective material heating is achieved in onlyfew minutes with instantaneous start-up and close-down of theprocesses (Kasin, 2006). Nowadays, the exploitation of microwaveenergy in heating applications is becoming popular due to the en-hanced chemical reaction and improved yield attainable in micro-wave heating systems over conventional heating.

The mechanism of heating differs greatly between microwaveand conventional thermal heating. Convection, conduction andradiation are the three well-known mechanisms for transfer of

Table 1The ultimate analysis and proximate analysis of OPS.

Ultimate analysis (wt%) Proximate analysis (wt%)

C H N Oa Moisture FC Volatile Ash

49.65 6.43 8.25 35.67 7.0 21.3 69.9 1.8

a By difference.

194 P. Lahijani et al. / Bioresource Technology 158 (2014) 193–200

thermal energy to the heating material in conventional heating.The principle of microwave heating relies on the transformationof microwave electromagnetic energy to thermal energy through-out the entire volume of the substance which is being subjectedto radiation (Will et al., 2004). Microwave heating is consideredas a sub-category of dielectric heating in which, the interactionof the electric field component of electromagnetic wave withcharged particles in the material (not in every material) concludesto the heating up of the material (Menendez et al., 2010). Thisinteraction and response to the applied electric field depends ontwo fundamental properties of the material known as dielectricconstant (e0) and dielectric loss (e00). The dielectric constant repre-sents the ability of the material to store the electromagnetic energyand is an indication of real permittivity. The dielectric loss is ameasure of the ability of the material to dissipate the microwaveenergy as heat and is an indication of imaginary permittivity. Thesetwo parameters are linked together through dielectric loss tangent(tan d = e00/e0) which is a measure of the ability of a material to beheated under microwave irradiation (Fernandez et al., 2011; Idriset al., 2004). A high value of tan d indicates a high absorbency ofmicrowave energy.

Carbon materials are known as excellent microwave receptorswith dielectric loss tangents around 0.02–2.95 (Menendez et al.,2010). The principal heating mechanism in the case of carbonand other solid materials with charge carriers (ions, electron,etc.) is explained by the movement of charged particles througha delimited region of the material under the influence of electro-magnetic field which induces current. Since the charge carrierscannot couple with the phase changes of the electric field, theyaccumulate on the surface and dissipate energy in the form of heat.This heating mechanism is known as the interfacial or Maxwell–Wagner polarization which differs from the dipolar polarizationmechanism that creates heat in polar organic-solvents and water(Menendez et al., 2010; Will et al., 2004). Considering that charand other carbonaceous materials are very good microwaveabsorbers, it is expected that high temperature heterogeneousgas–solid reactions would be favored under microwave irradiation.

CO2 gasification of char, based on Boudourad reaction (C + CO2 -M 2CO) is one of the high temperature gas-solid reactions whichhas long been the topic of interest, especially in the area of coalgasification. Recently, a lot of hope has been pinned on the utiliza-tion of lignocellulosic biomass residues as the source of char to car-ry out the Boudourad reaction. This would offer an opportunity forconversion of biomass wastes into value-added products in anenvironmentally benign and green manner, where the most noto-rious greenhouse gas, CO2, is reduced to fuel gas, CO. Gasification ofbiomass-derived char with CO2 as an evolving field of sustainabil-ity has been considered recently. Extensive studies have been con-ducted on the kinetics of biomass-char gasification using CO2

(Ahmed and Gupta, 2011; Mani et al., 2011; Ollero et al., 2003)and some attempts have been made to improve the reactivity ofbiomass-char in CO2 gasification reaction using catalysts (Huanget al., 2009; Lahijani et al., 2012, 2013; Mitsuoka et al., 2010). How-ever, very few studies have focused on the implementation ofmicrowave heating to conduct the high temperature CO2 gasifica-tion reaction.

So far, many attempts have been made to implement micro-wave irradiation for pyrolysis of biomass (Abubakar et al., 2013;Huang et al., 2013; Li et al., 2013; Zhao et al., 2012); yet, little isknown about microwave gasification. Menendez et al. (2006) re-ported some evidences of char gasification with CO2 during themicrowave induced pyrolysis of coffee hulls. Kasin (2006) patentedthe development of an apparatus for microwave induced pyrolysisand gasification of municipal wastes and sludge from differentplants. Brzeski (2013) also published a patent regarding the devel-opment of a microwave furnace for gasification of substances to

extract fuel gas. Very recently, Hunt et al. (2013) performed theCO2 gasification of activated charcoal in microwave and discussedon the heating mechanism and thermodynamics of the reactionextensively.

This paper reports the results of an investigation specifically fo-cused on the CO2 gasification of biomass-derived char undermicrowave irradiation. To the best of authors’ knowledge, similardata are very scarcely found in the open literature. In this work,oil palm shell (OPS) as an abundant lignocellulosic biomass wastein Malaysia was utilized to prepare the char. The average crudepalm oil production in Malaysia was reported around 18.8 milliontonnes in 2012 (www.mida.gov.my, 2012) and considering thatpalm shell constitutes about 6% of fresh fruit bunch, the generatedOPS is estimated around 5.37 million tonnes annually. In the cur-rent investigation, the char produced from the OPS was gasifiedusing CO2 under microwave irradiation and the influence of charparticle size, temperature and gas flow rate on the evolution ofCO was considered. Moreover, it was attempted to enhance thereactivity of the char during gasification reaction through theimplementation of iron-catalyzed char. To prove the salient fea-tures of microwave heating system, similar experiments were per-formed under conventional electric furnace heating and the resultswere compared. The activation energies for catalytic and non-cat-alytic microwave and thermal gasification reactions were calcu-lated and compared.

2. Methods

2.1. Raw material and char preparation

Oil palm shell (OPS) was obtained from a local palm oil mill. Theshells were sun dried, crushed and sieved into grains of 2–3 mm.The ground shells were washed with tap water to remove impuri-ties then dried at 105 �C for 48 h. The ultimate analysis and prox-imate analysis of OPS are provided in Table 1.

The char preparation experiments were conducted in a benchscale rig designed for pyrolysis experiments. The specifications ofthis carbonization system have been described elsewhere (Lahijaniet al., 2012). The shells were carbonized at 900 �C for 90 min to ob-tain OPS char. The so-obtained chars were pulverized and sievedinto four different size distributions with particle diameters (dp,in lm) of dp < 150; 150 < dp < 425; 425 < dp < 850 and dp > 850,using ASTM sieves, from no. 20 to 100. The ground chars werestored in desiccator.

2.2. CO2 gasification of OPS char in microwave

The experimental set-up used to carry out the CO2 char gasifica-tion consisted of gas cylinders (N2 and CO2), mass flow meters,tubular reactor packed with OPS char, microwave chamber, tem-perature controlling unit and gas cooling train. In order to developa microwave heating system for gasification, a commercial micro-wave oven (Cornell, CMO-EL17L) with maximum output power of1150 W and frequency of 2.45 GHz was modified to accommodatea quartz annular reactor inside the microwave cavity. The annularreactor was a double-walled quartz tube with inner and outerdiameters of 1.5 and 3.5 cm and height of 30 cm. The inner tube

P. Lahijani et al. / Bioresource Technology 158 (2014) 193–200 195

with perforated bottom was packed with OPS char plugged be-tween quartz wool at both ends.

For measurement of the intrinsic temperature of the bedthroughout the experiment, a type K thermocouple, 45 cm longwith sheath diameter of 1.5 mm, was used. Considering that elec-tromagnetic waves of microwave create a non-uniform heating,the thermocouple tip was always inserted into the char bed. Toavoid generation of spark in the electromagnetic field, the thermo-couple was connected to the ground wire. The temperature datawere collected using a data logger (Digi-sense scanning thermom-eter). In a typical experiment, N2 (100 ml/min) was introduced tothe bottom of the reactor. Then, the microwave oven was turnedon and the OPS char was heated up to the desired gasification tem-perature. Since biomass char is an excellent microwave absorber,the char bed temperature rose rapidly upon microwave heatingand the desired gasification temperature was achieved withinfew minutes. After reaching the pre-set gasification temperature,the bed was maintained at the corresponding temperature underN2 for 10 min to achieve a stable condition, then a pure streamof CO2 was flowed into the reactor for gasification reaction. Tomaintain the temperature constant during the gasification reac-tion, the power supply circuit of the microwave oven was modifiedand equipped with a PID controller. The microwave heating appa-ratus (magnetron) was modulated so that it could work in an on-off cycle to maintain a constant temperature.

In microwave CO2 gasification experiments, the effect of charparticle size (four size distributions), temperature (750–900 �C)and CO2 flow rate (50–125 ml/min) on the CO2 conversion andCO evolution was investigated. In each experiment, the conversionof CO2 and evolution of CO through the Boudourad reaction wascalculated by analyzing the outlet gas stream. The effluent gaswas passed through a gas sampling train which consisted of a ser-ies of glass bottles partially filled with cold water and a chamber ofsilica gel; finally, the clean gas was collected in Tedlar gas samplingbags (Cole–Parmer) for Gas Chromatography (GC) analysis. Thefirst sample was collected 2 min after the start-up of the gasifica-tion reaction to assure that N2 was purged out from the reactorand sampling lines. The gasification run time was 60 min and sam-ples were taken at intervals of 2, 3 and 5 min. The collected sam-ples were analyzed with GC and the acquired data were used tocalculate the CO2 conversion and CO production. The effluent gasof the reactor was analyzed in a GC (Agilent technology, 4890)equipped with a thermal conductivity detector (TCD) and a packedcolumn (Carboxene 1000 (15 ft � 1/8 in, 80/100 mesh), Supelco,USA). Helium at the flow rate of 45 ml/min was used as the carriergas. The oven temperature was set at 150 �C and then increased to210 �C at the rate of 20 �C/min and maintained for 7 min. The injec-tor and detector temperatures were set at 150 and 220 �C,respectively.

It was attempted to improve the yield of CO produced duringthe gasification reaction through the implementation of catalyzedchar. Our previous investigation carried out in a thermogravimetricanalyzer (TGA) showed that iron species, especially Fe (NO3)3, wereeffective in promoting the gasification reactivity of OPS char (Lahi-jani et al., 2012). In the current study, 5% Fe (NO3)3 loaded OPS charwas utilized to study the catalytic CO2 gasification of char undermicrowave irradiation at different temperatures. Moreover, toprove the effectiveness of the microwave heating compared to con-ventional heating, similar gasification experiments at selected con-ditions were performed in an electric furnace. A stainless steelreactor surrounded by a vertical tubular furnace (Lindberg/BlueM) was used to perform the CO2 gasification experiments of theOPS char. The operation condition and sampling were the sameas described for microwave operation. Table 2 summarizes theoverall gasification runs conducted in the microwave oven andelectric furnace.

3. Results and discussion

3.1. Effect of particle size

The effect of char particle size on the CO2 conversion in gasifica-tion reaction was investigated at four size distributions, at constanttemperature of 850 �C and gas flow rate of 100 ml/min and the re-sults are projected in Fig. 1a. As far as the CO2 conversion was con-cerned, reduction of particle size enhanced the CO2 conversion. Theaverage CO2 conversion improved from 46% for particles >850 lmto 62% for particles in the range of 150–425 lm. Enhancement ofchar gasification reactivity with reduction of the char particle sizecould be expected due to the effective heat and mass transfer tak-ing place in small particles. In large particle fuels (>850 lm), theremight be two limitations contributing to the lower CO2 conversion.The first limitation (physical limitation) can be related to diffu-sional resistances, emanating from temperature or CO2 concentra-tion gradient inside the char particles. The second limitation(chemical limitation) may originate from inhibition effect imposedby CO; as the Boudourad reaction proceeds the concentration of COinside the char pores can reach an appreciable level to cause inhi-bition effect. However, it can be observed in the figure that the CO2

conversion for particles >850 lm pronouncedly improved from32% to 46% after 25 min. It could be speculated that as the reactionproceeded to higher extent, the char particle started to shrink andthus the higher bulk density and enhanced intraparticle contactarea resulted in higher reactivity of diminishing char particlesand improved the CO2 conversion. As the char particle was reducedto <850 lm, the chemical reaction was more likely to become thegoverning mechanism to control the reaction and resulted in thehigher CO2 conversions. However, while the char particle sizewas lowered to <150 lm, the CO2 conversion dropped to 55%. Thisresult was not in agreement with findings of Gomez-Barea et al.(2006) who reported the highest char reactivity for biomass-charparticles <150 lm (mean value 60 lm) and Mani et al. (2011)who obtained the highest char conversion with biomass-char par-ticles <60 lm (average size) in CO2 gasification reaction. The lowerCO2 conversion attained with fine particles (<150 lm) was proba-bly attributed to the different source of heating used in our exper-iments, i.e., microwave heating. As depicted in Fig. 1b, the fine charparticles showed the lowest heating rate among the investigatedparticle size distributions. This shows that very fine particles werenot able to absorb microwave as well as large particles. Moreover,this sample likely dissipated the absorbed heat faster than largeparticles as evidenced by the consistent fluctuation in temperatureprofile of this sample (data not shown). Another plausible explana-tion for the observed low CO2 conversion could be due to the defi-cient number of hot spots (high-temperature microscopic spots)formed within the bed. Particle size is known as one of the param-eters that impinge the generation of hot spots (Horikoshi et al.,2013). In our experiments, generation of hot spots was rarely ob-served when the size of char was <150 lm, a phenomenon whichwas visually observed during the gasification of larger particles.The creation of hot spot in microwave and the heating mechanismis addressed in Section 3.5.

3.2. Effect of gasification temperature

Fig. 2 illustrates the effect of gasification temperature on theCO2 conversion with char particle size of 150–425 lm and gas flowrate of 100 ml/min. With increase of the gasification temperaturefrom 750 to 900 �C, the CO2 conversion improved from an averagevalue of 28–74%. This implies the strong effect of temperature onthe extent of completion of the gasification reaction; wherein,CO2 conversion at 900 �C was almost 2.6 times that of 750 �C.

Table 2Operating conditions for CO2 gasification of OPS char.

Heating system Mass of sample (g) Particle size (lm) Temperature (�C) CO2 flow rate (ml/min)

MH 8.0 <150 850 100MH 8.0 150–425 850 100MH 8.0 425–850 850 100MH 8.0 >850 850 100MH 8.0 150–425 900 100MH 8.0 150–425 800 100MH 8.0 150–425 750 100MH 8.0 150–425 900 50MH 8.0 150–425 900 75MH 8.0 150–425 900 125MH 8.4a 150–425 900 100MH 8.4a 150–425 850 100MH 8.4a 150–425 800 100MH 8.4a 150–425 750 100MH 8.4a 150–425 900 50CH 8.4a 150–425 850 100CH 8.0 150–425 900 100CH 8.0 150–425 850 100CH 8.0 150–425 800 100CH 8.0 150–425 750 100

MH: microwave heating, CH: conventional heating.a Catalyst loaded char.

Time (min)

0 10 20 30 40 50 60

CO

2 c

onve

rsio

n (%

)

0

20

40

60

<150 µm

150-425 µm

425-850 µm

>850 µm

(a)

Particle size (µm)

no char <150 150-425 425-850 >850

Incr

ease

rat

e of

tem

pera

ture

(o C

/s)

0

5

10

15

20

25(b)

Fig. 1. (a) Effect of char particle size on CO2 conversion and (b) rate of increase oftemperature within the char bed at the temperature of 850 �C and CO2 flow rate of100 ml/min.

Time (min)

0 10 20 30 40 50 60

CO

2 co

nver

sion

(%

)

0

20

40

60

80

100

900 oC 850 oC800 oC 750 oC

Fig. 2. Effect of gasification temperature on CO2 conversion with char particle sizeof 150–425 lm and CO2 flow rate of 100 ml/min.

196 P. Lahijani et al. / Bioresource Technology 158 (2014) 193–200

Consistent with the highly endothermic nature of Boudouardreaction (DH = 172 kJ/mol at 298 K), high temperatures shift theequilibrium towards CO production and result in higher CO2 con-versions. At high temperatures (typically >700 �C) when the large

positive entropic term (TDS) prevails the enthalpic component,the Gibbs free energy becomes negative (DG = DH�TDS) and theBoudourad reaction proceeds spontaneously in forward directionto produce CO. It can be confidently assumed that progress of theBoudouard reaction and evolution of CO is more facile in micro-wave compared to conventional heating at the same gasificationcondition. Unlike the conventional heating wherein the furnacecavity should reach the operating temperature to commence thegasification, microwave energy is directly delivered to carbon char.Thus, in microwave heating one would expect an enhanced rate ofreaction within the volume of char particle, especially at hot spots.As a proof of concept, similar CO2 gasification experiments wereperformed in conventional electric furnace and the results werecompared, as discussed in Section 3.5.

3.3. Effect of gas flow rate

The effect of gas flow rate on progress of the Boudouard reac-tion and evolution of CO was investigated at constant temperatureof 900 �C with char particles of 150–425 lm. Fig. 3a and b depict

Time (min)

0 10 20 30 40 50 60

CO

2 co

nver

sion

(%

)

0

20

40

60

80

100

50 ml/min75 ml/min100 ml/min125 ml/min

(a)

Time (min)

0 10 20 30 40 50 60

Cum

ulat

ive

CO

pro

duct

ion

(mm

ol)

0

100

200

300

400

50 ml/min 75 ml/min 100 ml/min 125 ml/min

(b)

Fig. 3. Effect of gas flow rate on (a) CO2 conversion and (b) CO production at thetemperature of 900 �C and char particle size of 150–425 lm.

Time (min)0 10 20 30 40 50 60

CO

2 co

nver

sion

(%

)

0

20

40

60

80

100

Catalytic microwave heatingMicrowave heating

Fig. 4. Comparison of CO2 conversion of 5% Fe-loaded char and pristine char at thetemperature of 900 �C and CO2 flow rate of 50 ml/min.

P. Lahijani et al. / Bioresource Technology 158 (2014) 193–200 197

the CO2 conversion and CO production at CO2 flow rates of 50–125 ml/min, respectively. The influence of gas flow rate on CO2

conversion followed the expected trend, where the highest conver-sion of 93% was achieved at CO2 flow rate of 50 ml/min; while theflow rate was increased to 125 ml/min, the CO2 conversiondropped to 58%. The production of CO from gasification of charwith CO2 was quite linear with time as observed in Fig. 3b.Although increase of the CO2 flow rate decreased the retentiontime of the gas inside the carbon bed and thus lowered the CO2

conversion; however, higher flow rates through the bed improvedthe CO production. The CO production rate at the CO2 flow rate of125 ml/min (6.83 mmol/min) was almost twice that of 50 ml/min(3.74 mmol/min); there was only a slight difference in CO produc-tion rate between the flow rates of 75 and 100 ml/min.

In microwave gasification experiments, the interaction ofmicrowave with carbon char creates high temperatures whichencourage char gasification in the presence of CO2; this results inacute loss of mass. Such mass loss should be considered in spacevelocity calculations. Gas hourly space velocity (GHSV), which isdefined as the volume of the reactant gas per hour per volume ofthe reaction zone, changes during the course of gasification asthe char is gasified to CO. The GHSV for flow rates of 50, 75, 100and 125 ml/min was obtained in the range of 339.70–424.63,509.55–679.40, 679.41–893.95 and 849.26–933.25 h�1, respec-tively. The higher GHSVs correspond to shorter retention timesconsistent with lower CO2 conversions achieved at higher flowrates.

3.4. Effect of catalyst

The effect of Fe catalyst on promoting the gasification reactivityof OPS char with CO2 was investigated under microwave irradia-tion. Fig. 4 compares the CO2 conversion of 5% Fe-loaded charand pristine char gasified in the microwave at constant tempera-ture of 900 �C and CO2 flow rate of 50 ml/min. Use of catalyst in-creased the average CO2 conversion from 93% in pristine char to99% in Fe-catalyzed char. The promotion in char reactivity wasattributed to the significant role of catalyst in increasing the reac-tion centers and active sites on the surface of the char. It has beenreported that in the absence of catalyst, the rate of diffusion of CO2

through the pores of char is faster than the intrinsic chemical reac-tion (Moon and Sahajwalla, 2006). However, while a catalyst isincorporated into the char skeleton, the rate of chemical reactionis chiefly enhanced which contribute to the higher gasificationreactivity obtained with catalyzed chars. The effect of catalyst onenhancing the reactivity of char and improving the CO2 conversionwas not as great as that of gasification temperature; however, itwas expected that the role of catalyst would be more highlightedat low temperatures.

3.5. Effect of source of heating

A comparison of the effect of microwave irradiation andconventional electric furnace heating on the CO2 gasification per-formance of OPS char was made at the temperature range of750–900 �C. Both sets of experiments were performed under iden-tical conditions of char particle size (150–425 lm), CO2 flow rate(100 ml/min) and mass of char sample (8 g). It was also attemptedto match the dimensions of the implemented annular reactors andother reaction conditions in the two systems as closely as possible.A comparison of the results achieved in microwave (catalyzed andun-catalyzed char) and thermal reactions is presented in Fig. 5. Asdeduced from the thermodynamics of the Boudouard reaction,high temperatures favor CO production and this is consistent withthe trends observed in both microwave and conventional heatingsystems. Interestingly, production of CO in microwave gasificationwas remarkably higher than that of thermal gasification, especiallyat low gasification temperatures. The average conversion of CO2 at750 and 800 �C was only 2.1% and 6.8% in electric furnace whichwas considerably lower than the corresponding values of 28.3%and 41.5% for un-catalyzed and 60.0% and 68.7% for catalyzed charin microwave. The comparison of results obtained for gasification

Time (min)

0 10 20 30 40 50 60

Cum

ulat

ive

CO

pro

duct

ion

(mm

ol)

0

100

200

300

400900 oC850 oC800 oC750 oC

Temperature (oC)750 800 850 900

Com

posi

tion

of o

utle

t gas

str

eam

(%

)

0

20

40

60

80

100

COCO2

(a)

Time (min)0 10 20 30 40 50 60

Cum

ulat

ive

CO

pro

duct

ion

(mm

ol)

0

100

200

300

400 900 oC850 oC800 oC750 oC

Temperature (oC)

750 800 850 900

Com

posi

tion

of o

utle

t gas

str

eam

(%

)

0

20

40

60

80

100

COCO2

(b)

Time (min)

0 10 20 30 40 50 60

Cum

ulat

ive

CO

pro

duct

ion

(mm

ol)

0

100

200

300

400900 oC850 oC800 oC750 oC

750 800 850 900

Com

posi

tion

of o

utle

t gas

str

eam

(%

)

0

20

40

60

80

100

CO CO2

(c)

Temperature (oC)

Fig. 5. Production of CO and ratio of CO:CO2 in (a) catalytic microwave, (b) non-catalytic microwave and (c) conventional electric furnace heating at CO2 flow rate of 100 ml/min and char particle size of 150–425 lm.

198 P. Lahijani et al. / Bioresource Technology 158 (2014) 193–200

of catalyzed and pristine OPS char also confirmed our expectationsabout the pronounced effect of catalyst in low temperaturegasification. The moles of CO produced by catalyzed char at thetemperatures of 750, 800, 850 and 900 �C were, respectively 2.1,1.7, 1.3 and 1.1 times that of pristine char in microwave-inducedgasification.

As clearly inferred from the results obtained for CO2 conversionand CO evolution, microwave heating had an immenseperformance over conventional electric heating. Such superior

performance of microwave is believed to be related to the mecha-nism of microwave heating. While a carbon-material is exposed toelectromagnetic field, delocalized p-electrons start to movethrough broad regions of the material. These induced currents heatup the material as a result of electrical resistances within the mate-rial. In cases where the electromagnetic and thermal properties ofthe material which is subjected to heating are non-linearly depen-dent to temperature, transitory hot spots are formed within thematerial. If the rate of absorption of microwave energy by the

1/T(K) (10-4)

9.789.328.908.53

Ln(k

)

-10

-9

-8

-7

-6

-5

-4

Temperature (oC)750 800 850 900

Microwave heatingCatalytic microwave heatingConventional heating

Ea= 247.2 kJ/mol

Ea= 74.2 kJ/mol

Ea= 36.0 kJ/mol

(a)

Temperataure (oC)

750 800 850 900

CO

MH/C

OC

H

0

2

4

6

8

10

12

14

16(b)

Fig. 6. (a) Arrhenius plot for catalytic and non-catalytic microwave and thermaldriven Boudouard reaction and (b) production of CO in microwave (MH) overthermal heating (CH) after 60 min.

P. Lahijani et al. / Bioresource Technology 158 (2014) 193–200 199

material increases non-linearly with temperature (faster than lin-ear increase), a non-uniform heating is created within the materialand regions of very high temperature known as hot spot forms(Reimbert et al., 1996).

It has been reported that use of carbon-materials can give riseto hot spot formation (Fernandez et al., 2011). In microwave heat-ing of carbonaceous materials, it is possible that over increase ofkinetic energy of some electrons make them jump out of the bulkand ionize the surrounding atmosphere of the material (Menendezet al., 2010). Macroscopically, such phenomenon can be observedas small sparks or electric arcs; however, these hot spots couldbe regarded as plasma at microscopic level. Since these plasmascan be perceived as very tiny sparks lasting for a few minutes, theyare considered as microplasmas (Fernandez et al., 2011; Menendezet al., 2010). The hot spots formed during microwave heating areknown to be responsible for enhancement of the gasification reac-tion rate, as the temperatures at these spots are considerably high-er than the bulk of material (Fidalgo et al., 2008). Generation ofmicroplasmas was regularly observed in our microwave gasifica-tion experiments; however, direct detection of these high temper-ature microscopic spot was rather difficult.

3.6. Kinetic studies

The results obtained from CO2 gasification of OPS char in micro-wave and conventional electric heating system were used to studythe kinetics of the gasification reaction. It is assumed that char gas-ification reaction rate follows a first-order reaction (Dominguezet al., 2008):

dmdt¼ �km ð1Þ

where m is the mass of char and k is the reaction rate constant. Inte-gration of Eq. (1) yields:

ln ðm=m0Þ ¼ �kt ð2Þ

The char conversion is defined as:

X ¼ m0 �mm0

ð3Þ

where m0 and m represent the initial and instantaneous mass ofchar at time t. Combination of Eqs. (2) and (3) gives:

ln ð1� XÞ ¼ �kt ð4Þ

From the plot of ln (1�X) vs t, the value of the first-order reac-tion rate constant (k) is obtained for each temperature. Since theinstantaneous weight of char could not be recorded directly, thefollowing equation was used to calculate the char conversion:

XðtnÞ ¼Xi¼n

i¼1

M0;CO2 XCO2 ;ti

ðm0=12Þ ; n ¼ 0; 1; 2; ::: ð5Þ

where X (tn) represents the char conversion at reaction time of tn,XCO2;ti

is the conversion of CO2 at time ti and M0;CO2 represents theinitial moles of CO2 introduced to the char bed. Then, the Arrheniusplot can be developed to obtain the kinetic parameters based on thefollowing equation:

k ¼ A expð�Ea=RTÞ ð6Þ

where A is the pre-exponential factor, Ea represents the activationenergy, R is the universal gas constant and T denotes the gasificationtemperature. Fig. 6a shows the Arrhenius plot for catalytic and non-catalytic microwave and thermal driven gasification reactions. Ahigh regression coefficient was obtained for catalytic and non-cata-lytic microwave (R2 = 0.996) and furnace (R2 = 0.994) gasificationexperiments. The activation energies were obtained from the slope

of the plots which were 36.0, 74.2 and 247.2 kJ/mol for catalytic andnon-catalytic microwave and thermal heating, respectively. Theactivation energy of the Boudourad reaction in thermal driven gas-ification varies in the range of 170–370 kJ/mol from study to study(Moon and Sahajwalla, 2006). The results of our experiments clearlyshow that under microwave irradiation, the activation energy wasconsiderably lower than that of conventional electric furnace heat-ing. It can also be inferred that use of catalyst had a significant rolein reducing the activation energy, where the activation energy formicrowave gasification of catalyzed char reduced to half of un-cat-alyzed char. A summary of kinetic parameters and reaction ratesunder different gasification conditions is presented in Table 3. Therates of char reaction and CO evolution increased systematicallywith increase of temperature. These rates were highest for catalyticmicrowave reaction followed by non-catalytic microwave and ther-mal heating. The large difference between the rates of microwaveinduced and thermal reaction suggests that the rate constants musthave fundamentally different dependencies on temperature whichare manifested in the Arrhenius parameters.

A comparison of the moles of CO produced in the Boudouardreaction through the two different heating routes is illustrated inFig. 6b. The rate of microwave driven reaction at low temperatureswas pronouncedly higher than that of thermal heating, where the

Table 3Kinetic parameters, CO evolution rate and char reaction rate under microwave and thermal heating.

Microwave Catalytic microwave Conventional heatingArrheniusparameters

Ea = 74.22 kJ/molA = 3.91 � 10�1 s�1

Ea = 36.04 kJ/molA = 5.79 � 10�3 s�1

Ea = 247.24 kJ/molA = 8.39 � 106 s�1

T (�C) CO production rate(mmol/min)

Char reaction rate(mmol/min)

CO production rate(mmol/min)

Char reaction rate(mmol/min)

CO production rate(mmol/min)

Char reaction rate(mmol/min)

750 2.226 1.458 4.667 2.638 0.173 0.152800 3.300 1.722 5.510 3.056 0.571 0.388850 4.970 2.667 6.540 3.263 1.973 1.111900 6.515 3.306 7.545 3.931 4.825 2.611

200 P. Lahijani et al. / Bioresource Technology 158 (2014) 193–200

microwave induced reaction was 13.7 times faster than thermaldriven reaction at 750 �C; this points to the high effectiveness ofmicrowave heating at lower temperature. The difference in ratesfollowed a decreasing trend and reached to 1.3 at 900 �C. By equat-ing the Arrhenius equations for the microwave and thermal reac-tions, the temperature of 926 �C was found to be the point atwhich an equal rate of CO production could be expected throughthe two routes.

4. Conclusion

CO2 gasification of OPS char was successfully performed inmicrowave heating system. The effect of several operating parame-ters as well as use of catalyzed char on CO2 conversion and CO pro-duction was investigated. A high CO2 conversion of 99% wasachieved in microwave gasification which could be sustained for60 min. The microwave gasification results were compared to thatof thermal driven experiments; the CO production in microwavedriven reaction was 13.7 times that of thermal gasification at750 �C. The pronounced difference between microwave and thermalgasification results is believed to be related to the heating mecha-nism of microwave and formation of transitory microplasmas.

Acknowledgements

The authors gratefully acknowledge the Universiti SainsMalaysia and the Ministry of Science, Technology and InnovationMalaysia for funding this project in the form of the USM-ERGSGrant (203/PMEKANIK/6730064), Long Term Research GrantScheme (LRGS) (203/PKT/6723001) and RUT Grant (1001/PJKI-MIA/854001), respectively.

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