Bioresource Technology 101 (2010) 4584–4592

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Bioresource Technology

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Investigation on thermochemical behaviour of low rank Malaysian coal,oil palm biomass and their blends during pyrolysis via thermogravimetricanalysis (TGA)

Siti Shawalliah Idris a,*, Norazah Abd Rahman a, Khudzir Ismail b, Azil Bahari Alias a, Zulkifli Abd Rashid a,Mohd Jindra Aris a

a Faculty of Chemical Engineering, Universiti Teknologi MARA Malaysia, 40450 Shah Alam, Selangor, Malaysiab Faculty of Applied Sciences, Universiti Teknologi MARA Perlis, 02600 Arau, Perlis, Malaysia

a r t i c l e i n f o

Article history:Received 16 September 2009Received in revised form 12 January 2010Accepted 14 January 2010Available online 12 February 2010

Keywords:DevolatilisationOil palm wastesSub-bituminous coalNon-isothermal TG analysisCoal/biomass blends

0960-8524/$ - see front matter � 2010 Elsevier Ltd. Adoi:10.1016/j.biortech.2010.01.059

* Corresponding author. Tel.: +60 3 55436312; fax:E-mail address: shawal075@salam.uitm.edu.my (S

a b s t r a c t

This study aims to investigate the behaviour of Malaysian sub-bituminous coal (Mukah Balingian), oilpalm biomass (empty fruit bunches (EFB), kernel shell (PKS) and mesocarp fibre (PMF)) and their respec-tive blends during pyrolysis using thermogravimetric analysis (TGA). The coal/palm biomass blends wereprepared at six different weight ratios and experiments were carried out under dynamic conditions usingnitrogen as inert gas at various heating rates to ramp the temperature from 25 �C to 900 �C. The deriva-tive thermogravimetric (DTG) results show that thermal decomposition of EFB, PMF and PKS exhibit one,two and three distinct evolution profiles, respectively. Apparently, the thermal profiles of the coal/oilpalm biomass blends appear to correlate with the percentage of biomass added in the blends, thus, sug-gesting lack of interaction between the coal and palm biomass. First-order reaction model were used todetermine the kinetics parameters for the pyrolysis of coal, palm biomass and their respective blends.

� 2010 Elsevier Ltd. All rights reserved.

1. Introduction

Renewable energy has become more important globally espe-cially with the current fuel and economic crisis. In Malaysia, thegovernment has encouraged the use of renewable energy throughits Five Fuels Policy plan in 1999 with the estimation of 5% utilisa-tion of renewable energy in the energy mix for year 2008 and thisusage will increase to 35% in 2030 (Bernama, 2008). Malaysia iswell positioned amongst the ASEAN countries to promote the useof biomass as a renewable energy source in her national energymix since she is a major agricultural commodity producer in theregion. The favourable climate conditions that prevail throughoutthe year are an advantage for palm oil cultivation. Evidently,Malaysia is the world’s largest producer and exporter of palm oil,replacing Nigeria as the chief producer since 1971 (Yusoff, 2006).At present, there are more than 3.88 million hectares of land underoil palm cultivation and it is the main contributor to biomass re-sources in Malaysia. Approximately 368 palm oil mills are operat-ing in the country to date, thus producing substantial amount oflignocellulosic biomass in the form of empty fruit bunches (EFB),palm kernel shell (PKS) and palm mesocarp fibre (PMF). It was esti-mated that the amount of solid wastes produced could reach 39

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+60 3 554356300..S. Idris).

million tons by the year 2020 (Yusoff, 2006). Even though thereare a number of researches investigating the ways of utilising thesewastes there is still a large fraction of the residues abundantlyunattended (Yusoff, 2006). Currently, most of the wastes are incin-erated or utilised as boiler fuels for steam generation to the palmoil mills. They have also been used as mulching around the plantsso as to prevent excessive evaporation or erosion, producing med-ium density fibre board in furniture and used as material for mat-tresses, seats, insulation and paper making industries (Chuah et al.,2006; Kalam et al., 2004; Kelly-Yong et al., 2007).

Oil palm biomasses are highly potential materials for energy re-sources. The fact that they are renewable and abundantly availableare amongst the attractive reasons of employing them as the majorsource for renewable energy (Yusoff, 2006). Furthermore, it ap-pears to have soundly positive environmental properties resultingin net zero releases of carbon dioxides and very low sulphur con-tent. In practice, about half of the agricultural residues are con-sumed for energy generation. This amount contributes to about20% of the primary energy demand of industries in Malaysia. Therole of biomass is presently limited in power development, butopportunities exist for increasing its share. It is estimated that by2050 biomass could provide nearly 38% of the world’s direct fueluse and 17% of the world’s electricity (Bernama, 2008). Even so,with respect to power production, it appears that, a total replace-ment of coal for biomass requires further development.

Table 1Ultimate and proximate analyses of Mukah Balingian coal, PMF, PKS and EFB.

Analyses Coal Biomass

Mukah Balingian EFB PMF PKS

Proximate analysis (db) (wt.%)Volatile matter 42.2 70.5 68.8 69.2Fixed carbon 48.8 15.4 15.2 16.0Ash 5.8 4.5 10.2 10.5

Ultimate analysis (daf) (wt.%)Carbon 54.37 40.93 43.19 41.33Hydrogen 5.29 5.42 5.24 4.57Nitrogen 1.75 1.56 1.59 0.99Sulphur 0.25 0.31 0.19 0.09Oxygena 38.34 51.78 49.79 53.02Calorific value (MJ/kg) 24.6 16.8 19.0 16.3

db – Dry basis; daf – dry ash free basis.a Calculated by difference.

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Combustion of fossil fuels such as coal contributes to CO2 emis-sion that causes global warming effect to the environment. Ameans of reducing the CO2 emissions is by minimizing coal com-bustion by implementing coal-biomass co-combustion and it isone of the promising short term alternatives for the use of renew-able fuels. Moreover, the use of biomass may represent an alterna-tive to the disposal of waste and its exploitation as energy source.Coal availability worldwide and price stability, are essential com-ponents in the long term for continuing utilisation around theworld. In Malaysia, coal reserve stands at about 1712 million tonesof various ranks ranging from lignite to anthracite. Apparently,Malaysia has moved towards utilising coal through National Min-eral Policy for power production. On a long term, coal will have agreater contribution in Malaysia’s energy mix at about 40–45% incomparison to that of natural gas with contribution of less than50% (Rahman Mohamed and Lee, 2006).

Pyrolysis of coal, biomass and coal/biomass blends are relativelynew area of research and several researchers have investigatedtheir pyrolysis and combustion behaviour (Caballero et al.,1997a; Caballero et al., 1997b; Ismail et al., 2005b; Kastanakiet al., 2002; Pan et al., 1996). Recently, Vamvuka et al. (2003)developed a kinetic modelling for the volatile matter released dur-ing the pyrolysis of several biomass (i.e. olive kernel, forest andcotton residues) blends with lignite using thermogravimetry(TG). Their findings revealed that the biomass possess higher ther-mochemical reactivity with shorter devolatilisation times in com-parison to the lignite. In another work, Moghtaderi et al. (2004)investigated the pyrolytic behaviour of coal/woody biomass (i.e.pine dust) blends over a wide range of heating rates and tempera-tures using tubular reactors in an attempt to simulate the pulver-ized fuel boilers. Most important, they found that the total yield ofthe major pyrolysis products were linearly proportional to theblending ratio, indicating no synergistic effect between coal andbiomass. Likewise, Vuthaluru (2004) also reported the same obser-vation during the investigation on the thermal behaviour duringco-pyrolysis of coal/biomass (i.e. wood waste and wheat straw)using thermogravimetry.

Up to date, the study of Malaysian coal and oil palm biomass co-pyrolysis has not yet been reported. Nevertheless, few studies per-taining to pyrolysis of oil palm biomass were presented. Yang et al.(2004) had investigated the pyrolysis of oil palm biomass namelyPMF, PKS, and EFB using TGA-FTIR analyser. The investigationwas carried out to study the effects of particle size and heatingrates on the evolution of gases during pyrolysis process. From thisstudy, they found out that the particle size between 250 lm and2 mm has no significant influence on pyrolysis process and thatthe pyrolysis kinetics reaction for the palm biomass is of first-orderreaction. Luangkiattikhun et al. (2008) in their work, had modelledthe kinetics of pyrolysis of palm kernel, palm mesocarp fibre andpalm kernel shell using non-isothermal thermogravimetric meth-od. They proposed two parallel reaction model to describe pyroly-sis reaction of palm kernel shell and mesocarp fibre while one stepglobal kinetic to describe pyrolysis of palm kernel.

In the development of converting coal-biomass blends intorenewable energy sources, understanding of the chemical compo-sition, thermal behaviour and reactivity of oil palm biomass andtheir blends with coal during pyrolysis, is of paramount important.This is because pyrolysis or solid devolatilisation is always a firststep in the thermochemical conversion studies towards producingthis renewable energy sources. Therefore the aim of this work is toinvestigate the thermal behaviour and reactivity of Mukah Balin-gian coal, PMF, PKS, EFB, coal/PMF, coal/PKS and coal/EFB blendsduring pyrolysis process using thermogravimetry analyser. Thethermogravimetric technique was used to identify the thermalevolution profiles occurred during pyrolysis and to obtain the ki-netic parameter of these materials during the process. The effect

of varying heating rates on the reactivity of coal, PMF, PKS, andEFB and their respective blends on the kinetic parameters will alsobe reported.

2. Methods

2.1. Materials

Malaysian low rank coal (Mukah Balingian) originated from Sar-awak and oil palm biomass namely EFB, PMF and PKS obtainedfrom oil palm mill located in Nilai, Negeri Sembilan, were selectedin this study. After air-drying for 2–3 days, the samples werecrushed and sieved to desired particle size (<212 lm) using RetschSM2000 heavy duty cutting mill and Endecott Shaker respectively.Finally, the samples were dried in a vacuum oven set at 80 �C for24 h and kept in a tightly screw cap bottle. The characterisationsof coal and biomass samples were shown in Table 1. Proximateanalysis was carried out according to ASTM standards (ASTM D5142 – 02a) using thermobalance TGA/SDRA51e manufacturedby Mettler Toledo, while ultimate analysis was performed usinga Thermo Finnigan Flashed 1112 analyzer according to ASTM D5373 – 02. The calorific value was achieved based on ASTM D2015 – 96 via Ika-works C5000 calorimeter. The analysis on min-eral matter content in each samples were done using X-ray Fluo-rescence (XRF) model Bruker S4-Explorer X-ray Fluorescence(1 kW) and the results were reported in Table 2.

2.2. Thermogravimetry analysis

A thermobalance TGA/SDRA51e was used for the pyrolysisexperiments. Its precision of temperature measurementwas ±0.5 K; microbalance sensitivity was less than 1.0 lg, whileheating rate could be changed from 1 to 100 �C/min. Prior to thethermal investigation, the instrument is calibrated using indium/aluminium check method. The two compounds of approximately10 mg each were inserted in a 150 ll alumina crucible next to eachother as such the two will not touch. The calibration was per-formed based on the set heating profiles from the manufacturer.The furnace and sample temperature was tested to be within thespecified allowable specification. The pyrolysis investigations werecarried out using nitrogen as carrier gas, under dynamic conditionsat heating rates of 10, 20, 40 and 60 �C/min. The pyrolysis temper-ature was raised from room temperature to 900 �C. The biomasswas added to coal at weight ratios of 0:100, 20:80, 40:60, 50:50,60:40, 80:20 and 100:0. To avoid the problem of mass and heattransfer, a sample size of 20 mg was selected for this experiment.The weighted sample were inserted directly into 150 ll ceramiccrucible and the temperature was kept isothermal for one minute

Table 2XRF analysis for Mukah Balingian coal, PMF, PKS and EFB (wt.%).

Samples Al Ca Cl Fe K Mg P S Si Sr Ti Y Zn

MB coal 16.60 9.77 1.10 24.82 2.89 0.63 3.76 18.10 17.90 0.52 2.51 0.81 0.64EFB 0.45 15.40 8.83 4.84 61.53 1.40 1.48 1.41 3.96 0.04 0.17 0.00 0.22PMF 1.96 14.60 5.19 38.56 18.00 0.38 2.20 2.66 16.00 0.00 1.15 0.00 0.43PKS 0.84 58.96 0.00 33.71 1.98 0.19 0.67 0.38 2.51 0.11 0.37 0.00 0.10

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until a steady condition was obtained before ramping to the de-sired temperature. Each individual samples was pyrolysed at leasttwice but more repetitions were carried out in case some variabil-ity was observed.

3. Results and discussion

3.1. Thermal decomposition of single fuel: coal, oil palm biomass

The results of TGA analysis are shown in Fig. 1(a) and (b) whichshow the weight loss curves (TG) and derivative thermogravimet-ric (DTG) evolution profiles respectively, as a function of reactiontemperature, for coal and oil palm biomass using heating rate of10 �C/min. It is worth noting that the DTG evolution profile corre-sponds to the region where the slope of TG curve is constant (Is-mail et al., 2005a; Vuthaluru, 2004). Thermal decomposition of

(a)

Wei

ght (

%)

20

30

40

50

60

70

80

90

100

900 25 100 200 300 400 500 600 700 800

Temperature (oC)

Mukah Balingian Coal

(MB)

Palm Kernel Shell (PKS)Empty Fruit

Brunch (EFB)

Palm Mesocarp Fibre (PMF)

900

(b)

Der

ivat

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Wei

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%/m

in)

- 0.08

- 0.07

- 0.06

- 0.05

- 0.04

- 0.03

- 0.02

- 0.01

0.0

0.01

0.02

25 100 200 300 400 500 600 700 800

Temperature (oC)

Mukah Balingian Coal (MB)

Palm Kernel Shell (PKS)

Palm Mesocarp Fibre (PMF)

Empty Fruit Brunch (EFB)

Fig. 1. (a) TG curves and (b) DTG curves for Mukah Balingian coal and oil palmbiomass materials at heating rate of 10 �C/min.

Mukah Balingian coal results in one major peak in the high temper-ature range of 380–580 �C. This peak was attributed to the releaseof carbon containing volatile matter (Ismail et al., 2005b). As forbiomass, its thermal degradation can be divided into three stages;moisture drying, main devolatilisation and continuous slightdevolatilisation (Munir et al., 2009). The moisture drying region(temperature range of less than 150 �C) corresponds to the firstpeak in the DTG evolution profiles of the oil palm biomass as indi-cated in Fig. 1(b). The main devolatilisation regime of the threepalm biomass samples showed different characteristics. As canbe observed, the PMF starts to degrade first (at temperature of180 �C) followed by EFB and PKS at temperature of �200 �C, andthe weight loss starts to decrease promptly after that temperature.The three oil palm biomasses have less observable evolution pro-files between temperature ranges of 350–500 �C, indicated by thenon-zero value on DTG curve after the main peak of devolatilisa-tion. There is no obvious weight loss observed beyond temperatureof 550 �C for both PMF and EFB. Nevertheless, the PKS continues todecompose until beyond temperature of 750 �C with maximumdecomposition at temperature of 704 �C. Oil palm biomass is of lig-nocellulosic material which consist mainly hemicellulose, celluloseand lignin. Apparently, the thermal decomposition of EFB showedthe presence of only one major DTG evolution profile at maximumtemperature (Tmax) of about 300 �C. Abdullah and Gerhauser(2008) reported the thermal evolution profile of EFB appeared atmuch higher temperature, i.e. �355 �C and was assigned to thethermal decomposition of cellulose (ca. 297–327 �C) componentappeared in the samples (Várhegyi et al., 1997; Yang et al.,2007). As expected, the lower temperature shoulder appeared inDTG curve of palm mesocarp fibre (PMF) represents the decompo-sition of hemicellulose material and the higher temperature peakassigns to decomposition of cellulose material (Kastanaki et al.,2002). With regards to the thermal decomposition of PKS, the firstand second DTG evolution profiles is expected to correspond to thedevolatilisation of hemicellulose and cellulose materials, respec-tively (Kastanaki et al., 2002; Vamvuka et al., 2003). The non-ob-servable peak exists in the three biomass samples was attributedby the decomposition of lignin which is known to decomposeslowly over a wide range of temperature (137–667 �C) (Vamvukaet al., 2003). Nonetheless, the third peak which appeared to becompletely isolated at temperature over �650 �C is not reportedby other investigations on similar samples. The repetitivenessoccurrence of this peak is further analysed via TG/MS (Mass Spec-trometer) analysis. The DTG curve with relative intensities for PKSthermal decomposition is plotted as in Fig. 2. It appeared that, onlyrelative intensities of mass/charge ratio (m/z) of 2, 12, 16, 44 and45 showed significant peak at temperature range between 600 �Cand 750 �C. The co-existence of both peak having m/z of 16 and12 was assigned for the presence of methane, while the peak cor-respond to m/z of 2 showed that hydrogen being released at thistemperature range, both as a result of charring process (Gómezet al., 2005). The mass spectrometry intensity for m/z of 12, 44and 45 were assigned for the release of carbon dioxide duringthe course of pyrolysis of PKS. This numerous high-temperaturemass losses with the release of CO2 are indicative of the decompo-sition of mineral carbonates contained in PKS. Moreover, the

Fig. 2. DTG curve with MS intensities for decomposition of palm kernel shell (PKS)at heating rate of 10 �C/min.

S.S. Idris et al. / Bioresource Technology 101 (2010) 4584–4592 4587

results on mineral analysis using XRF reported in Table 2, provedthat, the mineral content particularly calcium present in enormousamount as compared to other elements in EFB and PMF. The highamount of calcium carbonates present in PKS samples is due tothe addition of this substance during the final separation processin the oil palm mill. Similar observation was reported elsewhereon thermal decomposition of biogran (Biagini et al., 2002), lignite(Vamvuka et al., 2003), and chicken litter (Whitely et al., 2006).

The peak position and height in the DTG evolution profiles rep-resent the reactivity of the samples i.e. the temperature corre-sponding to peak height is inversely proportional to reactivitywhile the peak height is directly proportional to the reactivity(Vamvuka et al., 2003). Table 3 shows the reactivity and its corre-sponding temperature at maximum rate of decomposition for coaland oil palm biomass materials at heating rate of 10 �C/min. The

Table 3The maximum temperature and reactivity of Mukah Balingian coal, EFB, PKS, PMF, and th

Biomass materials BLEND MB:BM TEP 1 TEP 2

Tmax R Tmax

EFB 100:080:20 30060:40 30150:50 30040:60 30020:80 3010:100 301

PMF 100:080:20 277 0.06 33660:40 279 0.08 33750:50 277 0.13 33740:60 275 0.15 33620:80 276 0.20 3360:100 277 0.25 336

PKS 100:080:20 284 0.06 35360:40 284 0.13 35250:50 284 0.14 35240:60 283 0.20 35320:80 284 0.25 3520:100 282 0.39 352

TEP = thermal evolution profile; BM = biomass material; PMF = palm mesocarp fibreTmax = temperature of maximum weight loss (�C); VM = volatile matter released (wt.%).

peaks present in the DTG curves of decomposition of the coaland biomass materials is denoted as TEP (thermal evolution pro-file), with TEP 1 and 2 indicate the evolution profiles due to decom-position of biomass components, while TEP 3 and 4 indicate theevolution profile for decomposition of coal and inorganic materialspresent in PKS, respectively. It was observed that the highest rateof decomposition was obtained for EFB, while the temperature cor-responding to the maximum pyrolysis rate was lower for PMF,indicating its higher reactivity among the samples (Kastanakiet al., 2002). On the other hand, the thermal decomposition of Mu-kah Balingian (MB) coal starts at about 250 �C, temperature whichis higher than that of biomass materials. The maximum pyrolysisrate was achieved at temperature of 432 �C at a mean reactivityof 6 times lower than that of biomass materials, thus as expected,this signify that MB coal is less reactive. The release of volatile mat-ter in the MB coal ends in wide temperature interval in comparisonto biomass, as indicated by the non-zero value of the DTG curves.In addition, its volatile matter amounted to 32 wt.% on dry basis,which is much lower with comparison to biomass samples withthe highest amount of 72 wt.% released by PKS decomposition.

3.2. Thermal decomposition of coal/PMF, coal/PKS and coal/EFB blends

Fig. 3 represents the DTG curves for the thermal decompositionof MB coal/oil palm biomass for various blends at heating rate of10 �C/min. Each of the oil palm biomass to coal blends shows a dis-tinctive DTG evolution profiles. The coal/EFB blends (Fig. 3a) re-vealed two evolution profiles with the first peak at temperature(Tmax) of 300 �C, while the second peak at temperature �430 �C.Three thermal evolution profiles showed in the thermal decompo-sition of coal/PMF blend (Fig. 3b), with the first, second and thirdpeaks at temperatures of 277 �C, 336 �C and 428 �C, respectively.As for coal/PKS blend (Fig. 3c), four thermal evolution profiles pres-ent during the pyrolysis reaction with the first to fourth peak tem-perature of 284 �C, 353 �C, 433 �C and 699 �C, respectively.

The comparison between the single biomass and coal fuel withthat of the coal/biomass blends for the three oil palm biomasssamples were shown in Fig. 3. One may regards the first peak incoal/EFB blends corresponds to the EFB decomposition, while the

eir respective blends at heating rate of 10 �C/min.

TEP 3 TEP 4 Rm VM

R Tmax R Tmax R

432 0.24 0.06 38.700.18 431 0.14 0.09 42.550.52 429 0.09 0.19 51.170.68 425 0.06 0.24 54.520.78 426 0.05 0.27 56.861.05 433 0.02 0.35 60.711.60 0.53 69.82

432 0.24 0.06 38.810.14 428 0.11 0.09 43.330.30 427 0.07 0.13 49.040.41 428 0.05 0.18 53.590.50 432 0.04 0.21 56.650.63 428 0.02 0.26 62.870.60 0.27 69.87

432 0.24 0.06 38.810.19 433 0.09 679 0.04 0.10 46.050.36 431 0.06 691 0.07 0.17 52.350.41 428 0.04 695 0.08 0.18 51.310.52 426 0.03 695 0.10 0.24 57.210.68 418 0.02 699 0.14 0.30 61.450.76 704 0.18 0.38 71.15

; PKS = palm kernel shell; EFB = empty fruit bunch; R: peak height (mg/min);

Increasing wt% of biomass Increasing peak height (R)

Increasing wt% of biomass Increasing peak height (R)

(a)

(b)

(c)

MB (20): PMF (80)

MB (40): PMF (60)

MB (50): PMF (50)

MB (60): PMF (40)

MB (80): PMF (20)

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MB (20): PKS (80)

PKS 100

MB (50): PKS (50)

MB (40): PKS (60)MB100

MB (80): PKS (20)

MB (60): PKS (40)

100 200 300 400 500 600 700 800 90050

90050 100 200 300 400 500 600 700 800

EFB

MB(20): EFB(80)

MB(40): EFB(60)

MB(50): EFB(50)

MB(60): EFB(40)

MB(80): EFB (20)

MB

-0.075

-0.05

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0.00

Increasing wt% of biomass Increasing peak height (R)

Fig. 3. DTG curves of (a) coal/EFB, (b) coal/PMF and (c) coal/PKS for various blends at a heating rate of 10 �C/min.

4588 S.S. Idris et al. / Bioresource Technology 101 (2010) 4584–4592

second peak due to that of coal decomposition. Similarly, the firsttwo peaks present in coal/PMF blends belongs to the PMF presentin the blends while peak at higher temperature was as a results ofthe coal present in the blend. The same characteristic is observedin the coal/PKS blends with the fourth peak observed in the blendswas attributed by the inorganic carbonates material in the biomasssample. Likewise, the observed thermal evolution profiles in thecoal/biomass blends could be attributed to the decomposition ofpolymers of hemicellulose, cellulose and lignin present in biomassfuels (Jones et al., 2005; Kastanaki et al., 2002; Vamvuka et al.,2003).

It was found that the DTG peak height (R) for the first evolutionprofile increased with increasing percentage of the biomass in theblends as depicted in Table 3. The same behaviour was observedwith the second evolution profile in both coal/PKS and coal/PMFblends, and in the fourth evolution profile of coal/PKS blend. It ap-peared that no obvious shift in pyrolysis temperature in the blendswith respect to that observed for the individual fuel. Furthermore,the increment in peak height appears to correlate with the mass ra-tios of biomass in the fuel blends. Nevertheless, it is interesting to

note that fourth evolution profile for the co-pyrolysis of coal/PKSseem affected with the presence of coal in the blends. The evolu-tion of this peak appeared at lower temperature than that of theraw PKS, which could be due to the catalytic effect as a result ofthe presence of considerable amount of inorganic carbonates inthe sample, as indicated in the XRF analysis (Table 2). Yet the ther-mal evolution profile due to the release of volatile matter from coaldecomposition in the blends showed a slight decreased in peakheight with increasing in the percent of biomass in the blendswhich signify that the presence of oxygenated species releasedby the biomass pyrolysis does not affect the pyrolysis of coal(Biagini et al., 2006). The higher yield of volatiles, in biomass mate-rials are attributed to stronger effects of temperature and heatingrate in the depolymerisation reaction, which leads to rapid evolu-tion of volatile content of the biomass materials. On contrary, thelower volatile release in coal is most likely due to the strongerbonding within the molecular structure of coal (Meesri andMoghtaderi, 2002).

In an attempt to investigate possible synergy in the coal/bio-mass blends, the amount of volatile matter released from the

MB coal

calc. 50: 50 MB/PMF

0

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0 100 200 300 400 500 600 700 800 900 1000Temperature, oC

Nor

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ised

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calc. 50: 50 MB/EFB

EFB

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90

100

wei

ght l

oss,

%

S.S. Idris et al. / Bioresource Technology 101 (2010) 4584–4592 4589

co-pyrolysis reaction of coal and palm biomass were plottedagainst the proportion of biomass in the blends as shown inFig. 4. The lack of synergy between the two fuels during co-pyroly-sis is indicated by the linear relationship between the volatile mat-ter release and the percentage of biomass added to the mixture(Jones et al., 2005; Vuthaluru, 2004) and this condition was ob-served on all the blends. It was also possible to verify this behav-iour using the weighted average of normalised weight loss of theblend, as shown in Eq. (1)

Wblend ¼ xMBWMB þ xOPWWOPW ð1Þ

where xMB and xOPW is the percentage of MB coal and oil palmwastes in the blend, respectively, and WMB and WOPW is the norma-lised weight loss of MB coal and oil palm wastes, respectively, underthe same operative conditions. Calculated and experimental weightloss curves were compared as shown in Fig. 5. The slight differencesshown were within the experimental error range. The results agreereasonably well with those of similar studies done by otherresearchers who co-pyrolysed coal with cotton residue, olive kernel,forest residue, rice husk, sugarcane bagasse, pine saw dust and pine(Biagini et al., 2002; Ismail et al., 2005a; Jones et al., 2005; Vamvukaet al., 2003; Vuthaluru, 2004).

PMF

exp.50: 50 MB/PMF

0

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60

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80

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100

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mal

ised

wei

ght l

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%

MB coal

PKS

exp. 50: 50 MB/PKS

calc. 50: 50 MB/PKS

0 100 200 300 400 500 600 700 800 900 1000Temperature, oC

Temperature, oC0 100 200 300 400 500 600 700 800 900 1000

(c)

0

10

20

30

40N

orm

alis

ed

3.3. Effect of heating rates on reactivity of coal, biomass materials andtheir blends

Table 4 shows the Tmax, and peak height (R) at various heatingrates (i.e. 10, 20, 40, 60 �C/min) for oil palm biomass materials(PMF, PKS and EFB), Mukah Balingian coal and coal/biomass blendsat 50 wt.%. Evidently, Tmax and R for the evolution profiles of ther-mal decomposition of palm biomass materials, coal and theirblends, increase with increasing heating rates during the pyrolysisprocess, indicating an increase in reactivity. One may observe thisbehaviour from the thermograms of EFB, PKS and PMF, and coal,where the peak was systematically shifted to higher temperatureregime as the heating rate increases. Fig. 6 describes the behaviourof thermograms of PKS at various heating rates. Similar trendswere observed with PMF, Mukah Balingian coal and 50:50 coal/palm biomass blends.

The reason for these temperature shifts is that less heat is re-quired for the cracking of the solid fuel particles into products.This process is reached later at higher temperatures since the heattransfer is not as effective and efficient as they were at lowerheating rates. At lower heating rates, the heating of biomass par-ticles occurred more gradually leading to an improved and moreeffective heat transfer to the inner portions and among the parti-

Percentage of biomass in coal/biomass blend (wt%)

Vol

atile

mat

ter

rele

ased

(w

t%)

MB/PKSMB/PMFMB/EFBLinear (MB/PMF)Linear (MB/PKS)Linear (MB/EFB)

Fig. 4. Volatile matter released from coal/biomass blends at different percentage ofbiomass in coal blends.

Fig. 5. Comparison of calculated and experimental TG curves for (a) MB/EFB blend,(b) MB/PMF blend and (c) MB/PKS blend at heating rate 10 �C/min.

cles (Biagini et al., 2006). Therefore cracking process was moreeffective which caused more weight loss in the form of volatilescomponents. As the heating rate is decreased, the residue at theend of pyrolysis reactions is also decreased. The Tmax, which isthe point at which maximum weight loss takes place, shifts tohigher values with an increase in the heating rates. This shiftshave been reported by researcher on other biomass materials(Biagini et al., 2008; Luangkiattikhun et al., 2008; Vamvukaet al., 2003). In this study, it was found that the amount of percentvolatile matter released (VM) differs between the biomass andcoal, but not much difference among the biomass fuels and thisshows that heating rates have less impact on the release of vola-tile matter, thus the char yield. The same observation was re-ported by Luangkiattikhun et al. (2008) on pyrolysis of PKS andPMF at various heating rates.

Table 4Peak temperature (Tmax) and peak height at heating rates of 10, 20, 40 and 60 �C/min for biomass materials, coal and coal/biomass (50:50) blends.

FEED HR TEP 1 TEP 2 TEP 3 TEP 4 VM

Tmax R Tmax R Tmax R Tmax R

EFB 10 301 1.60 69.820 311 2.89 63.440 320 6.23 64.560 325 9.04 64.0

PMF 10 277 0.25 336 0.6 69.920 288 0.64 348 1.09 69.940 298 1.17 358 1.81 66.660 304 2.00 377 2.4 67.3

PKS 10 282 0.39 352 0.76 704 0.18 71.220 295 0.70 363 1.26 724 0.36 69.040 306 1.40 375 2.12 745 0.62 65.560 313 2.11 382 2.87 754 0.92 66.8

MB 10 432 0.24 38.720 442 0.62 38.340 453 1.04 37.760 460 1.63 38.5

MB:EFB (50:50) 10 300 0.68 425.5 0.06 54.520 311 1.33 437.4 0.14 51.240 320 2.22 446.1 0.27 50.560 326 3.89 453.4 0.31 49.9

MB:PMF (50:50) 10 277 0.13 337 0.41 428 0.05 53.620 289 0.25 348 0.75 437 0.1 52.340 298 0.43 358 1.23 450 0.19 51.760 305 0.84 361 1.75 455 0.28 52.4

MB:PKS (50:50) 10 284 0.14 352 0.41 428 0.04 695 0.08 51.320 295 0.28 365 0.72 440 0.08 711 0.18 49.840 308 0.62 377 1.31 451 0.17 736 0.36 52.960 316 1.10 385 2.00 456 0.21 751 0.60 54.8

TEP = thermal evolution profile; HR = heating rate; BM = biomass material; PMF = palm mesocarp fibre; PKS = palm kernel shell; EFB = empty fruit bunch; R: peak height (mg/min); Tmax = temperature of maximum weight loss (�C); VM = volatile matter released (wt.%).

Temperature, oC

Der

ivat

ive

Wei

ght (

%/m

in)

-35 25

-30

-25

-20

-15

-10

-5

0

5

900 100 200 300 400 500 600 700 800

10oC/min

20oC/min

40oC/min

60oC/min

Fig. 6. DTG curves of PKS at heating rate of 10, 20, 40 and 60 �C/min.

4590 S.S. Idris et al. / Bioresource Technology 101 (2010) 4584–4592

3.4. Kinetic analysis

The activation energy and pre-exponential factor of coal andbiomass pyrolysis were determined by differential method (Bam-ford and Tipper, 1980). The fundamental rate equation used in allkinetic studies is expressed as Eq. (2);

dadt¼ kðTÞf ðaÞ ¼ A exp

�Ea

RT

� �f ðaÞ ð2Þ

where k is the rate constant and f(a) is the reaction model, a func-tion depending on the actual mechanism. For non-isothermal ther-

mogravimetric analysis at constant heating rate, Hr = dT/dt, thusEq. (2) may be written as Eq. (3);

dadT¼ A

Hrexp

�Ea

RT

� �f ðaÞ ð3Þ

At the peak temperature (Tmax), where the maximum rate ofdecomposition is reached, the second derivative of Eq. (2) is equalto zero and is given by Eq. (4)

d2adt2 ¼

EaHr

RT2max

!þ A exp

�Ea

RTmax

� �f 0ðaÞ

" #dadt¼ 0 ð4Þ

where f0(a) is the derivative form of f(a). Thus after simple rear-rangement of Eq. (4), the equation becomes Eq. (5)

Hr

T2max

!¼ AR

Ea

� �exp

�Ea

RTmax

� �f 0ðaÞ ð5Þ

It is transformed into linear function as in Eq. (6) by taking nat-ural log on both sides of Eq. (5)

lnHr

T2max

!¼ �Ea

R

� �1

Tmax

� �þ ln

AREa

nð1� aÞn�1� �

ð6Þ

In this study, it is assumed that solid fuel pyrolysis is a first-or-der reaction (Biagini et al., 2002). Although it is not always thecase, but the simplicity and the number of parameters of thefirst-order model allow the direct comparison of different fuels toprovide preliminary parameters for further and more accuratemodelling. Thus, Eq. (6) may be written as Eq. (7)

lnHr

T2max

!¼ �Ea

R

� �1

Tmax

� �þ ln

AREa

� �ð7Þ

Table 5The activation energy (Ea), pre-exponential factor (A) and calorific value of coal, biomass materials and their respective blends.

Feed Blend Ea

(kJ/mol)A (s�1) R2 (%) Ea

(kJ/mol)A (s�1) R2 (%) Ea

(kJ/mol)A (s�1) R2 (%) Ea

(kJ/mol)A (s�1) R2 (%) LHV (MJ/kg)

TEP 1 TEP 2 TEP3 TEP 4

MB: EFB 100:0 272.88 1.74E + 18 99.77 24.0280:20 181.55 4.01E + 14 99.82 266.61 6.54E + 17 99.68 23.0960:40 189.92 2.20E + 15 99.86 263.60 4.39E + 17 99.67 21.5350:50 194.11 5.47E + 15 99.89 262.95 4.78E + 17 99.62 20.6940:60 207.12 9.40E + 16 99.73 243.24 1.45E + 16 99.98 19.8820:80 226.85 5.80E + 18 99.81 239.13 4.86E + 15 99.87 18.190:100 209.68 1.45E + 17 99.93 16.58

MB: PMF 100:0 272.88 1.74E + 18 99.77 24.0280:20 157.71 9.49E + 12 89.55 198.19 1.01E + 15 99.16 258.71 1.87E + 17 97.83 23.5360:40 161.48 2.20E + 13 99.42 202.59 2.30E + 15 98.30 271.37 1.87E + 18 99.43 22.0850:50 162.55 2.82E + 13 98.78 206.97 5.94E + 15 98.45 266.04 7.02E + 17 99.72 21.3840:60 163.83 4.19E + 13 98.53 202.62 2.60E + 15 98.78 263.66 4.23E + 17 99.24 20.4920:80 165.64 6.13E + 13 99.87 205.55 4.56E + 15 99.23 262.84 4.05E + 17 99.92 19.220:100 166.24 6.55E + 13 99.80 216.10 3.54E + 16 98.78 17.80

MB: PKS 100:0 272.88 1.74E + 18 99.77 24.0280:20 138.97 9.28E + 10 98.56 174.35 3.01E + 12 99.09 254.93 7.86E + 16 99.99 234.74 3.51E + 10 97.87 23.2360:40 137.84 7.47E + 10 99.52 176.43 4.93E + 12 99.93 264.34 4.61E + 17 99.92 251.29 2.17E + 11 99.97 21.6250:50 142.72 2.27E + 11 99.91 176.43 4.93E + 12 99.98 261.69 3.07E + 17 99.26 245.79 1.01E + 11 99.65 20.7440:60 144.66 3.54E + 11 99.79 187.64 4.46E + 13 99.92 233.97 2.83E + 15 98.29 255.61 3.18E + 11 98.45 19.8020:80 149.59 1.09E + 12 99.91 186.35 3.53E + 13 99.97 223.72 5.56E + 14 98.28 260.58 5.39E + 11 99.64 18.120:100 150.13 1.16E + 12 99.79 192.12 1.12E + 14 99.98 277.11 3.72E + 12 99.66 16.72

TEP = thermal evolution profile; PMF = palm mesocarp fibre; PKS = palm kernel shell; EFB = empty fruit bunch; R2 = correlation coefficient; Ea = activation energy (kJ/mol);A = pre-exponential factor (s�1), LHV = low heating value (MJ/kg).

S.S. Idris et al. / Bioresource Technology 101 (2010) 4584–4592 4591

where a = fraction decomposition, A = pre-exponential factor,Ea = activation energy, R = gas constant, n = reaction order, T = tem-perature at peak, Hr = heating rate. Eq. (6) is the common ‘‘model-free” kinetics equation that is first developed by Kissinger (Kissing-er, 1956). In the Kissinger method, lnðHr=T2

maxÞ is plotted against (1/Tmax) for a series of experiments performed using TGA at differentheating rates with peak temperature obtained from the DTG curve.In this experiment, activation energy, Ea, pre-exponential factor, Aand correlation coefficient (R2) obtained from the pyrolysis andco-pyrolysis of PMF, PKS, EFB, coal and their respective blends arepresented in Table 5.

In general, the value of activation energies and pre-exponentialfactors for the first thermal evolution profile (TEP 1), increases withthe addition of biomass in the coal blends. Similar trend was ob-served in the second thermal evolution profile (TEP 2) for coal/PMF and coal/PKS blends with an exception for coal/EFB mixtures.In comparison, the activation energies due to the decomposition ofcellulose (TEP 2) present in biomass showed the highest value(174–227 kJ/mol), than that due to the decomposition of hemicel-lulose (TEP 1) (ca. 140–166 kJ/mol). These values fall within therange of that reported elsewhere on olive kernel (Kastanaki et al.,2002), rice husk (Ismail et al., 2005a) and corn cob (Fisher et al.,2002).

Evidently, the activation energy of the co-pyrolysis reaction dueto the evolution of volatile matter in coal (TEP 3) is minimum at80 wt.% of biomass in the coal feed stream for both coal/EFB andcoal/PKS. As of coal/PMF blend, the minimum value of activationenergy is reported for 20 wt.% of PMF in the coal blend. The activa-tion energy of the coal blends rises to a peak of 266.6 kJ/mol,271.4 kJ/mol and 264.3 kJ/mol at 20 wt.% biomass for coal/EFBmixture and 40 wt.% biomass content for both coal/PKS and coal/PMF mixtures, respectively. The present trend in the activationenergies and pre-exponential factors for the coal/biomass blendsseems to be in good agreement with the ones obtained by otherresearchers (Ismail et al., 2005a; Vuthaluru, 2004). As the energybarrier, activation energy provides the information of critical en-ergy needed to start a reaction. Thus, to ensure lower activation en-

ergy and so lower temperatures required for promotion of forwardpyrolysis reaction, the mixture with lowest activation energy isrecommended. However, the heating value of the fuel blend shouldbe considered to be over 20 MJ/kg as to ensure auto-thermal com-bustion (Biagini et al., 2002). Based on this limitation, it can be con-cluded that, the optimum blends based on the lowest activationenergy on coal thermal evolution profile and the acceptable heat-ing value limit, are 50: 50 for coal/EFB and 80:20 for both coal/PMF and coal/PKS. Comparing activation energy data has been acommon method of analysis for the suitability of various biomasstypes to be converted into renewable fuel source and has beenused by various researchers on coal/wood wastes (Vuthaluru,2004) and rice husk (Mansaray and Ghaly, 1999).

Apparently, the kinetic parameters reported for the forth ther-mal evolution profiles (TEP 4) in pyrolysis of coal/PKS were slightlyhigher values as compared to the values found in the literature (Liuet al., 2003; Zhao et al., 2001). These differences could possibly dueto the use of different model that describe the decompositionbehaviour of calcium carbonates (Ioannou et al., 2009; Maciejew-ski, 2000). However, further studies have to be carried out in orderto understand this behaviour.

4. Conclusions

Thermal behaviour of Malaysian sub-bituminous coal and threeoil palm biomass materials have been investigated using thermo-gravimetric analysis. Evidently, the three palm biomass wastesand coal showed distinct behaviour upon pyrolysis. As expectedthe pyrolysis of the PMF, PKS and EFB occurred at lower tempera-ture in comparison to coal sample. The mean reactivity is in the or-der of EFB > PKS > PMF > MB coal. Apparently, the coal/oil palmbiomass blends appear to undergo an independent thermal degra-dation without any synergistic interaction as such its behaviourduring co-pyrolysis can be estimated from those of parent fuels.Nevertheless, the presence of biomass in the coal blend representscarbon neutrality for further development of co-firing system inMalaysia.

4592 S.S. Idris et al. / Bioresource Technology 101 (2010) 4584–4592

Acknowledgements

The authors would like to thank the Research ManagementInstitute (RMI), University Technology MARA, Faculty of ChemicalEngineering and Biomass and Fossil Fuel Research Team (UiTMPerlis) for their support.

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