Characterization of chars pyrolyzed from oil palm stones for the preparation of activated carbons

Journal of Analytical and Applied Pyrolysis46 (1998) 113–125

Characterization of chars pyrolyzed from oil palm stonesfor the preparation of activated carbons

Jia Guo *, Aik Chong Lua

Di6ision of Thermal and Fluids Engineering, School of Mechanical and Production Engineering,Nanyang Technological Uni6ersity, Nanyang A6enue, Singapore, 639798, Singapore

Received 17 March 1998; accepted 28 May 1998

Abstract

Chars pyrolyzed from oil palm stones, an abundant palm-oil mill solid waste, for thepreparation of activated carbons were characterized in this paper. An ultra-pycnometer anda mercury intrusion porosimeter were used to measure the solid and apparent densities of thesamples, respectively and a thermogravimetric analyzer was used for proximate analysis. Theeffects of pyrolysis temperature and retention time on the char yield and BET surface areaof the chars were investigated. The optimum condition for pyrolysis was found to be at apyrolysis temperature of 800°C for a retention time of 3 h. The SEM micrographs of the charsurfaces verified the presence of porosity. The experimental results showed that it wasfeasible to prepare chars with sufficient densities and relatively high porosities from oil palmstones for the further preparation of activated carbons. © 1998 Elsevier Science B.V. Allrights reserved.

Keywords: Char; Pyrolysis; Characterization; Oil palm solid waste; Porosity

1. Introduction

Activated carbons are used in a variety of applications, such as air pollutioncontrol, wastewater treatment and catalyst support, due to their high adsorptivecapacities, adequate pore size distributions and relatively high mechanical strengths.Activated carbons can be prepared from a large number of carbonaceous rawmaterials (e.g. coal, lignite, wood, coconut shell and some agricultural wasteproducts) by either a physical method or a chemical method [1]. The latter may

* Corresponding author. Tel.: +65 7995913; fax: +65 7924062.

0165-2370/98/$ - see front matter © 1998 Elsevier Science B.V. All rights reserved.PII S0165-2370(98)00074-6

114 J. Guo, A. Chong Lua / J. Anal. Appl. Pyrolysis 46 (1998) 113–125

generate a secondary environmental pollution due to the use of some chemicals (e.g.zinc chloride ZnCl2 [2], potassium hydroxide KOH [3], phosphoric acid H3PO4 [4]or sulfuric acid H2SO4 [5]) as activating agents, so it is not used as commonly as thephysical method. The physical process normally involves two stages, i.e. pyrolysis(also called carbonization) and activation. In the first stage, the starting materialsare pyrolyzed at a moderate temperature (�700–900°C) to remove the volatilematters and produce chars with rudimentary pore structures. Subsequently, in thesecond stage, the resulting chars are subjected to a partial gasification at a highertemperature (usually \900°C) with oxidizing gases, such as steam, carbon dioxide,air or a mixture of these, to produce final products with well-developed andaccessible internal porosities [6]. The adsorptive capacity of the activated carbon,which is related to its pore structure and pore size distribution, is largely pre-deter-mined by the nature of the starting material and the history of pyrolysis. Thepurposes of activation are only to enlarge the diameters of the pores that arecreated during pyrolysis and to create some new porosities [7].

Pastor–Villegas et al. studied the chemical–physical properties of chars preparedfrom raw and extracted rockrose, a typical Mediterranean plant widely used in theperfume industry [8]. Chars were prepared by heating raw materials in nitrogen flowfrom room temperature to 600°C at a heating rate of 10°C min−1 and holding thistemperature for 120 min. The highest BET surface areas of the chars from raw andextracted rockrose were 357 and 379 m2 g−1, respectively. It was found that theextraction had an influence on the pyrolysis of this woody material since theextraction delayed the kinetics of the pyrolysis process and decreased the ashcontent of the chars obtained at high temperatures and the loss originated in theirsurface area, microporosity, and total porosity. Gergova and Eser characterizedchars pyrolyzed at 750, 800 and 850°C for 2 h from apricot stones [9]. Theirexperimental results showed that the pyrolysis temperature did not have a signifi-cant effect on the pore structure of the resulting chars. Rodriguez–Mirasol et al.also investigated the carbonization of chars from eucalyptus kraft lignin at differenttemperatures and characterized the structure of the microporous chars [10]. Theyfound that the presence of ash content (inorganic matter) in the starting materialwas an important feature to avoid partial fusion and swelling in the carbonizationstage. Therefore, a new pre-treatment method had been established to wash theinorganic matter with diluted acidic solutions after previous carbonization.

However, characterization of chars pyrolyzed from oil palm stones, a cheap andabundant solid agricultural waste, for the preparation of activated carbons has notbeen reported in the literature. In Malaysia, which is the largest palm oil producerin the world, greater than one million tons of oil palm stones are estimated to begenerated annually [11]. The preparation of activated carbons from oil palm stonesis supposed to be an economical way to utilize these agricultural by-products [12].Shamsuddin and Williams studied the pyrolysis of oil palm stones by thermogravi-metric analysis for the conversion of these solid wastes into useful fuels and foundthat two groups of reactions were evident in the pyrolysis process, which wascharacterized by the presence of two peaks in the derivative thermogravimetrycurves. It was also found that the char yields were affected by both the particle sizeof the stones and the maximum pyrolysis temperature [13].

115J. Guo, A. Chong Lua / J. Anal. Appl. Pyrolysis 46 (1998) 113–125

The aim of this paper was to investigate the influences of pyrolysis temperatureand retention time on the properties of chars. For characterization, an ultra-pyc-nometer and a mercury intrusion porosimeter were used to measure the solid andapparent densities of the raw material and chars, respectively, and a thermogravi-metric analyzer was used to carry out the proximate analysis. An acceleratedsurface area and porosimetry system was used to obtain adsorption isotherms forthe determination of the BET surface areas of the chars. In addition, a scanningelectron microscope was used to observe the char surfaces in order to verify thepresence of porosity.

2. Experimental

As-received oil palm stones were first crushed and sieved. The size fraction of1.0–2.0 mm was used in this study. Table 1 shows the characteristics of the rawmaterial-oil palm stone. These agricultural solid wastes appeared to be suitablematerials for dense and high-quality activated carbons because of their inherenthigh solid density and relatively high fixed-carbon content but low ash content.

Pyrolysis of the pre-treated oil palm stones was performed in a stainless-steelvertical reactor which was placed in a tube furnace (818P, LENTON). The furnacehas programmable temperature controls such as heating rate, pyrolysis temperatureand retention time. About 15 g of raw materials were placed on a 120 mm metalmesh which was located at the bottom of the reactor. Purified nitrogen (99.9995vol% purity) with a flow rate of 150 cm3 min−1 was used as the inert gas flushingthrough the reactor right from the beginning of the pyrolysis process. The experi-ments were carried out from room temperature to 400, 500, 600, 700, 800 or 900°Cfor retention times of 1, 2, 3 and 4 h at each pyrolysis temperature, and a heatingrate of 10°C min−1 was used.

An ultra-pycnometer (UPY-1003, QUANTACHROME) and a mercury intrusionporosimeter (Poresizer-9320, Micromritics) were used to measure the solid andapparent densities of the samples, respectively. For known solid density rs andapparent density ra, the porosity o can be calculated as follows:

o= (rs−ra)/rs×100% (1)

A thermogravimetric analyzer (TA-50, SHIMADZU) was used to carry out theproximate analysis which was expressed in terms of moisture, volatile matter, fixedcarbon and ash contents [14]. About 20 mg of samples were heated from roomtemperature to 110°C in nitrogen until complete dehydration was accomplished,followed by decomposition at 850°C for 7 min to determine the quantity of volatilematters. The atmosphere was then changed to be oxidizing. The sample was cooledto 800°C and maintained at this temperature until its weight remained unchanged.The weight loss during this period was due to the reaction of the fixed carbon withoxygen and the remaining residue was ash.

Adsorption characterization of the chars was determined by nitrogen adsorptionat −196°C with an accelerated surface area and porosimetry system (ASAP-2000,


Tab

le1

Cha

ract

eris

tics

ofra

woi

lpa

lmst

ones

Pro

xim

ate

anal

ysis

(wt.

%)

Den

sity

and

poro

sity

Surf

ace

area

(m2

g−1)

App

aren

tde

nsit

y(g

cm−

3)

Solid

dens

ity

(gcm

−3)

Por

osit

y(%

)M

oist

ure

Vol

atile

mat

ter

Fix

edca

rbon

Ash

BE

TM

icro

pore

1.47

3.9

5.3

76.5

1.53

16.4

1.8

1.5

0.3


Micromeritics). The BET surface area was calculated from the adsorption isothermsby using the Brunauer–Emmett–Teller (BET) equation [15]. The cross-sectionalarea for nitrogen molecule was assumed to be 0.162 nm2.

In addition, a scanning electron microscope (S360, Cambridge Instruments) wasused to verify the presence of porosity on the char surfaces. The magnification ofthe SEM was selected as ×1000 in this study.

3. Results and discussion

3.1. Density and porosity

The density of the chars depends on not only the nature of the starting materialbut also the pyrolysis process [16]. The solid and apparent densities of the charsprepared at 400–900°C for various retention times are shown in Table 2. As seenfrom Table 2, for the same retention time of 3 h, increasing the pyrolysistemperature up to 800°C would increase the solid densities of the chars anddecrease their apparent densities, resulting in development of porosities from 8.3 to24.0%. However, when the temperature increased to 900°C, the apparent density ofthe char increased and the porosity decreased. This was probably due to thesintering effect occurred at post-softening and swelling temperatures, accompaniedby the shrinkage of the char particle, which resulted in the realignment of the charstructure and consequently narrowing or even closing some pores.

For the pyrolysis temperature of 800°C, a maximum porosity was obtained at aretention time of 3 h since a relatively long retention time was needed to enhanceporosites as well as to clear the blocked entrances of pores, but if retention time wastoo long, the porosity decreased also due to the shrinkage of chars, which leaded tocompression of the char and subsequent reduction of the pores.

Table 2Densities and porosities of the chars prepared at different temperatures for various times

Apparent densitySolid densityPyrolysis tempera- PorosityRetention timeture (°C) (g cm−3)(h) (g cm−3) (%)

400 3 1.57 1.44 8.312.51.401.60500 3

1.35 17.2600 1.633700 19.51.321.643

1.271.67 24.038001.31 22.5900 1.693

1800 1.58 1.33 15.82 20.2800 1.301.63

1.271.67 24.038004800 1.68 1.29 23.2


Table 3Proximate analyses of the chars prepared at different temperatures for various times

Volatile matter (%) Fixed carbon (%) Ash (%)Pyrolysis condition Moisture (%)

31.0 3.8400°C-3 h 63.61.648.3400°C-3 h 1.2 4.645.9

5.962.9600°C-3 h 30.40.876.6700°C-3 h 0.5 15.4 7.583.4800°C-3 h 0.4 7.9 8.388.5 9.1900°C-3 h 2.20.2

8.081.8800°C-1 h 9.60.683.0800°C-2 h 0.3 8.5 8.283.4800°C-3 h 0.4 7.9 8.3

6.2 8.5800°C-4 h 85.10.2

3.2. Proximate analysis

A starting material of the activated carbon is expected to be high in carbon andvolatile contents but low in ash content. The proximate analyses of chars pyrolyzedat different pyrolysis temperatures for a retention time of 3 h and at 800°C forvarious retention times are listed in Table 3.

As the pyrolysis temperature increased from 400 to 900°C, the volatile content ofthe chars decreased progressively from 63.6 to 2.2% whilst both the fixed carbonand the ash content increased. This was to be expected because increaseddevolatilization during pyrolysis resulted in the char being predominantly carbon.For the chars pyrolyzed at 800°C for different retention times, changes in thevolatile and fixed carbon contents were relatively small, because large amounts ofvolatiles had already been released before 800°C. Therefore, chars pyrolyzed fromoil palm stones are of high fixed carbon and low ash contents which are favorablefor the preparation of activated carbons.

3.3. Yield of char

The yield of chars can be calculated from the sample weight after pyrolysis to itsinitial weight. Fig. 1 shows the yield of chars versus pyrolysis temperature fordifferent retention times. The trend was decreasing char yield as the pyrolysistemperature was increased for a particular retention time. The differences in charyields became less for increasing retention time since more volatiles were released,leaving only small quantities of volatiles available for evolution at the end of longretention times. At each retention time, increasing the pyrolysis temperaturereduced the yield of char progressively. However, it was noteworthy that there weredistinct effects of retention time on the yield of char for different pyrolysistemperatures. At a low pyrolysis temperature, the effect of retention time on thechar yield was more significant than at a high pyrolysis temperature. For instance,at 400°C, the yield of char dropped 7.0% as the retention time increased from 1 to


4 h. However, at 900°C, the yield showed only 1.6% change for the same increasein retention time. This was because at a high pyrolysis temperature, most of thevolatile matters were released during the first hour, and therefore subsequentretention times produced very little further decreases in yields. But, at a lowpyrolysis temperature, there was an insufficient amount of heat energy available torelease all the volatile matter within the first hour, and therefore the amount ofvolatiles released was time-dependent. Summarizing, either increasing pyrolysistemperature or increasing retention time resulted in decreasing the yield of chars.

3.4. Adsorption isotherm

The isotherms of nitrogen adsorption at −196°C for chars pyrolyzed at tempera-tures 600 to 900°C for 3 h are shown in Fig. 2. The shape of the adsorptionisotherm can provide qualitative information on the adsorption process and theextent of the surface area available to the adsorbate. Based on an extensiveliterature survey, Brunauer et al. [17] found that all adsorption isotherms would fitinto one of five basic types (types I to V). All isotherms in Fig. 2 are of type Iisotherms which are associated with microporous structures. For a constant reten-tion time of 3 h, increasing the pyrolysis temperature from 600 to 800°C increasedthe volume of adsorbed nitrogen. The amount of adsorbed nitrogen is indicative ofthe adsorptive capacity of the chars. However, at the highest pyrolysis temperatureinvestigated here i.e. 900°C, the chars had a lower adsorptive capacity possibly dueto the sintering effect which sealed off some of the pores and reduced theaccessibility of the N2 molecules during the adsorption process.

Fig. 1. Yields of the chars pyrolyzed at 400–900°C for various retention times.


Fig. 2. Adsorption isotherms of the chars pyrolyzed at 600–900°C for retention time of 3 h.

3.5. BET surface area

The most important property of the activate carbon is its adsorptive capacity,which is related to the specific surface area. Generally, the higher the surface areaof the activated carbon, the larger is its adsorptive capacity [18]. The effects ofpyrolysis temperature and retention time on the BET surface areas of the chars are

Fig. 3. BET surface areas of the chars pyrolyzed at 400–900°C for various retention times.


shown in Fig. 3. When the pyrolysis temperature was 400°C, pyrolysis reac-tions had just commenced, thereby producing very small BET surface areaseven though the retention time was increased up to 4 h. This phenomenonwas again due to the inadequacy of heat energy to drive away any substan-tial amounts of volatiles. As the temperature was increased from 500 to700°C, increasingly greater volatile matters were released progressively duringpyrolysis thereby resulting in the development of some new porosities, andhence the BET surface areas increased progressively. With further increasesof temperature to 800°C, the surface area increased with retention time upto a maximum value at 3 h and thereafter decreased. This surface area de-crease was due to some of the pores being sealed off as a result of sinteringat excessive time duration. Generally, a longer retention time is needed toenhance porosity as well as to clear blocked pore entrances before detrimen-tal effects set in at prolonged times. However, at a high temperature of900°C, the trend was reversed. From an initial high surface area, it deterio-rated with increasing retention time. This might be due to a sintering effectat such high temperatures, followed by a shrinkage of the char, and realign-ment of the char structure which resulted in reduced pores. The optimumconditions for pyrolysis of oil palm stones to derive maximum BET surfaceareas were found to be at a pyrolysis temperature of 800°C and a retentiontime of 3 h.

3.6. SEM micrograph

Fig. 4(a) shows the surface of the oil palm stone. The surfaces of charspyrolyzed for retention time of 3 h and pyrolysis temperatures of 600, 800and 900°C are shown in Fig. 4(b, c, d), respectively. It can be seen fromFig. 4(a) that the surface of the raw oil palm stone was dense and planarwithout any cracks and crevices. This would account for the poor or negligi-ble adsorptive capacity of the raw material. For the chars pyrolyzed at600°C with a retention time of 3 h (Fig. 4b), the micrograph showed somepits on the surface and even several hollows at the right hand corner, signi-fying an under-pyrolyzed pore structure. The micrograph of the char py-rolyzed at 800°C for 3 h (Fig. 4c) showed that there were many orderlypores over the surface, forming a system of advanced pore structures. Dueto this well-developed pores, the chars possessed high BET surface area andadsorptive capacity. But, at a higher pyrolysis temperature of 900°C, thechars had lower BET surface areas due to the shrinkage of chars at post-softening and swelling temperatures, resulting in narrowing or closing pores.This could be seen in Fig. 4(d) in which there were less pores but more so-lidified areas in appearance. The cracks appeared at the left hand also confi-rmed the over-pyrolyzed structure due to severe thermal treatment.


Fig. 4. SEM micrographs: (a) raw oil palm stone; (b) char pyrolyzed at 600°C for 3 h; (c) char pyrolyzedat 800°C for 3 h; (d) char pyrolyzed at 900°C for 3 h.


Fig. 4. (Continued)


4. Conclusions

The pyrolysis results showed that it was feasible to prepare chars with relativelyhigh BET surface areas from oil palm stones with high solid density and high fixedcarbon but low ash contents, which are favorable for preparing activated carbons.

Increasing the pyrolysis temperature for a certain retention time decreased theyield of chars as increasing volatiles were evolved. For lower temperatures, increas-ing retention time decreased char yield progressively at a fixed temperature, due tocontinuous evolution of volatiles. However, at a high temperature, the effect ofretention time on the char yield was less significant.

The pyrolysis temperature and the retention time at this temperature are two veryimportant parameters for the pyrolysis process. At a low pyrolysis temperature of400°C, pore development was poor due to insufficient energy to release the volatilematter. However, at a high pyrolysis temperature of 900°C, a sintering effect andshrinkage of the char reduced the pore areas. For the pyrolysis temperature of800°C, a prolonged retention time of 4 h also reduced the BET surface area becauseof sealing of some pores as a result of sintering effect. Overall, high pyrolysistemperature and long retention time had detrimental effects on the development ofmicropore areas due to a shrinkage effect on the char microstructures.

The optimum conditions for pyrolysis was heating to a final temperature of800°C and holding at this temperature for 3 h, producing chars with a maximumBET surface area of 318 m2 g−1. In addition, SEM micrographs of the charsurfaces verified the presence of porosities.

Appendix A. Nomenclature

Greek symbolso (rs−ra)/rs, porosity of the sample (%)

solid density (g cm−3)rs

apparent density (g cm−3)ra

References

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Characterization of chars pyrolyzed from oil palm stones for the preparation of activated carbons