Energy Conversion and Management 50 (2009) 1163–1171

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Energy Conversion and Management

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Influence of inherent minerals and pyrolysis temperature on the yieldof pyrolysates of some Pakistani coals

Tauqeer Ahmad a, Iftikhar A. Awan a, Jan Nisar a, Imtiaz Ahmad b,*

a National Center of Excellence in Physical Chemistry, University of Peshawar, Peshawar 25120, Pakistanb Institute of Chemical Sciences, University of Peshawar, Peshawar, Pakistan

a r t i c l e i n f o

Article history:Received 19 July 2007Received in revised form 28 June 2008Accepted 27 January 2009Available online 28 February 2009

Keywords:Coal pyrolysisDemineralizationTemperature rangeGas chromatography

0196-8904/$ - see front matter � 2009 Elsevier Ltd. Adoi:10.1016/j.enconman.2009.01.031

* Corresponding author. Tel./fax: +92 091 9216652E-mail address: patwar2001@yahoo.co.in (I. Ahma

a b s t r a c t

An analytical method has been developed to quantitize tar, liquids and gaseous products resulting fromthe flash pyrolysis of sub-bituminous Makarwal coal. The method involves the thermal decomposition of200 mg of 85-mesh size coal at 690 �C under the flow of nitrogen using Shimadzu PYR-2A open tubularpyrolyzer. The resulting tar and liquid fractions were separated using two traps at the exit of the pyro-lyzer while the gaseous products leaving the traps were on line injected to gas chromatograph equippedwith porapak Q column and flame ionization detector for the analysis. Effect of demineralization on theyields of products was investigated by treating raw samples with 2 M HCl. Removal of inherent mineralsfrom coal by acid wash decreased the yield of total volatiles indicating catalytic properties of mineralunder the condition used in present study. The influence of pyrolysis temperature on the yield of pyrol-ysates and hydrocarbon gases, resulting from raw coal samples, was studied over the temperature rangeof 500–770 �C.

� 2009 Elsevier Ltd. All rights reserved.

1. Introduction

Energy sources are the need of modern civilization. Petroleumand petroleum based oils are the key sources used for power gen-eration in the premium market i.e. as traffic fuels. Other alternativefuels are being researched recently to reduce the reliance on petro-leum based oils. However; they will be in the center of the energyscene in far future. Keeping in view the heralding oil crises, there isa need to focus on carbonaceous candidate materials for conver-sion in to highly demanding liquid fuels. Coal is the most plentifuland versatile fuel available on earth which becomes more impor-tant both as an energy source and as the source of organic feed-stock in the 21st century. There are several coal conversionavenues to get chemicals and synthetic fuels [1–5]. Pyrolysis isone of the avenues of coal conversion to get useful products [6–8]. Merely, by heating, coal decomposes to char, tar liquids andgases. Certain properties of coal and process variables influencethe process efficiency and product quality. These include coal type,properties of coal, mineral constituents, caking ability and operat-ing conditions [9–13]. Among several coal pyrolysis methods, flashpyrolysis is a promising process for producing hydrocarbons andchemicals such as benzene, toluene and xylene [14].

The catalytic role of inorganic elements on the distribution ofproduct of many coal conversion processes is well established

ll rights reserved.

.d).

[15–18]. It is also confirmed from the available literature thattheir role is deleterious if external catalysts are used [19]. Theoccurrence of inorganic elements in coal is strongly dependanton the geological location of the coal seam and hence varies fromcoal to coal. Moreover, because of the variation in compositionand properties for different coals, there were many discrepanciesobserved in the results obtained by different workers for differ-ent coal samples. Pakistani coals have high mineral contents[20] and their beneficial and/or deleterious role on product dis-tribution of pyrolysate needs to be evaluated. In continuationof our work on pyrolysis studies of Pakistani coals [21–23], thework reported here is aimed to investigate the effect of inher-ently present inorganic elements on the yield of tar, liquid andgaseous products obtained from the pyrolysis of Makarwal coalsamples. The pyrolysis products were divided into three frac-tions; tar, liquid and gas. Tar and liquid fractions were collectedin two traps while the gaseous fraction leaving these traps wasdirectly injected to a gas chromatograph equipped with flameionization detector.

2. Experimental

2.1. Source of materials

Four representative coal samples labeled as MC-1, MC-2, MC-3and MC-4 were collected from Makerwal coalfield. The sampleswere used for different pyrolysis studies.

Fig. 1. Experimental setup for the flash pyrolysis of coal samples.

Table 1Elemental analyses of raw coal samples.

S. no. Sample name C O Al Si S K Ca Ti Fe

1 MC-1 raw Weight% 72.11 20.91 0.98 1.35 2.82 – 0.32 0.65 0.852 MC-2 raw Weight% 53.99 33.16 2.65 3.25 3.37 0.16 0.16 0.69 2.573 MC-3 raw Weight% 53.00 33.97 2.90 4.66 2.96 0.35 0.37 0.28 1.504 MC-4 raw Weight% 55.43 32.77 2.56 2.80 4.08 0.44 0.27 – 1.65

MC: Makarwal coal.

Table 2Elemental analyses of demineralized coal samples.

S. no. Sample name C O Al Si S K Ca Ti Fe

1 MC-1 (DM) Weight% 72.93 21.44 0.88 0.94 2.70 – 0.13 0.69 0.292 MC-2 (DM) Weight% 59.44 30.65 2.62 3.05 2.31 0.10 – 0.54 1.293 MC-3 (DM) Weight% 55.87 33.88 2.71 3.81 2.42 0.26 – 0.20 0.854 MC-4 (DM) Weight% 57.76 32.30 2.14 2.52 3.73 0.30 0.19 – 1.06

DM: demineralized.

0

10

20

30

40

50

60

70

MC-1 Raw MC-2 Raw MC-3 Raw MC-4 Raw

Coal Type

Yie

ld (

%W

eigh

t)

Char

tar

liquid

gas

Fig. 2. Yield of char, tar, liquid and gaseous products from raw coals at 690 �C.

1164 T. Ahmad et al. / Energy Conversion and Management 50 (2009) 1163–1171

T. Ahmad et al. / Energy Conversion and Management 50 (2009) 1163–1171 1165

2.2. Chemicals

All chemicals used were Merck grade. Standard hydrocarbongases used were obtained from Matheson Company.

2.3. Preparation of coal samples for pyrolysis

The coal samples collected in plastic bags were opened in thelaboratory air and transferred to labeled bottles. These sampleswere crushed and sieved to 85-mesh size, dried at 150 �C for 8 hin an oven and cooled to room temperature in a desiccator. Thesamples were then stored in capped bottles and used for pyrolysisstudies.

2.4. Preparation of demineralized samples

To study the effect of inorganic elements on the yield of pyroly-sis products, acid washed coals were prepared from the raw coalsby leaching 10 g of each sample with 100 mL of 2 M HCl at roomtemperature. After addition of the acid, the samples were kept invacuum cupboard and held there for 3 weeks. The samples werethen filtered, washed several times with deionized water to re-move even last traces of acid from the treated coal samples.Demineralized samples were dried and used for pyrolysis studies.

2.5. Elemental analyses

The elemental analyses were carried out by using ScanningElectron Microscope (JEOL Model No. JSM-5910) coupled with En-ergy Dispersive X-rays Analyzer (EDX Model No. INCA200, OxfordInstruments Company), at Centralized Resource Laboratories, Uni-versity of Peshawar.

2.6. Pyrolysis–gas chromatography system

A Shimadzu PYR-2A microfurnace pyrolyzer was directly cou-pled to Shimadzu GC-7AG gas chromatograph equipped with aflame ionization detector (FID). The conditions used for the analy-sis of pyrolytic products of coal samples were the following.

The analyses of the products from all coal samples were carriedout on a spiral, stainless steel column (6 feet � 1/8 in.), packedwith porapak Q (100–120 mesh). For all the pyrolytic studies, col-umn temperature was programmed from 60 to 150 �C at the rate of32 �C/min with initial time 2 min. The nitrogen gas was used as a

0

20

40

60

MC-1 DM MC-2 DM

Coal T

Yie

ld (

%W

eigh

t)

Fig. 3. Comparison of the yields of different fragments f

carrier at a flow rate of 40 mL/min. Analysis detector was FID withrange 103. Air pressure was 0.5 kg/cm2 and hydrogen pressure was1.0 kg/cm2. The injector and detector were maintained at 170 and150 �C, respectively. Pyrolysis of each coal sample was carried outat 690 �C. Total pyrolysis time was kept 3 min.

2.7. Pyrolysis–gas chromatography procedure

In a typical run, the quartz pyrolysis tube was cleaned bysoaking overnight in chromic acid cleansing solution and rinsingwith distilled water. The tube was dried at 150 �C for 1 h andwas positioned in the furnace. There were two traps betweenPyrolyzer and GC i.e. Trap-1 (U-shaped) Pyrex glass tube contain-ing quartz wool and placed in cold water (�20 �C), while Trap-2was placed in methanol–nitrogen slush bath (�93.9 �C). Thequartz pyrolysis tube, Trap-1 and Trap-2 were weighed beforeand after pyrolysis. In order to get consistent results, the sampleswere allowed to achieve moisture-equilibrium with the labora-tory air. Then, a small portion (20 mg) of coal sample was pre-cisely weighed in a small piece of aluminum foil usingShimadzu Electronic Balance with an accuracy of ±0.1 mg (ModelAX120). Then, these samples were placed on platinum boat at-tached at the tip of the operation rod. The rod was connectedto the pyrolysis furnace unit and the valve was kept closed. Thevalve was then opened and the operation rod was pushed insidethe furnace unit slowly in order to purge the air. The rod wasintroduced rapidly inside the furnace unit, which was alreadyheated, to the desired temperature. After 3 min, the operationrod was pulled out and the stop valve was closed. For one exper-iment, 10 such samples were pyrolyzed. The same process wasused for the pyrolysis of the entire coal samples understudy.The recording of the chromatograms and the measurements ofthe areas of different peaks were carried out by Spectra PhysicsModel SP-4600 Data Jet Integrator attached to the gas chromato-graph. Pyrolysis products were identified by comparing the reten-tion times of the peaks in pyrogram with those of the purecompounds. The schematic of the process is shown in Fig. 1.

3. Results and discussion

3.1. Leaching of coal samples

The inorganic elements present in original and demineralizedcoal samples were identified by elemental analyses performed by

MC-3 DM MC-4 DM

ype

Char

tar

liquid

gas

rom the pyrolysis of demineralized coals at 690 �C.

1166 T. Ahmad et al. / Energy Conversion and Management 50 (2009) 1163–1171

SEM-EDX analyzer. The results are given in Tables 1 and 2. It can beobserved that the original coal samples contain inorganic elementslike Al, Si, K, Ca, Ti and Fe. Among the inorganic elements studied,the concentration of Al and Si is high compared to K, Ca, Ti and Fein all four samples studied. The same samples were then deminer-alized in order to leach out the inorganic elements. The inorganicelements were then determined in the leached coal samples. Thedata is provided in Table 2. Demineralization caused removal ofinorganic elements and reduction in weight% of each elementcan be seen. Al and Si were removed in high concentration com-pared to the rest of the elements in case of all the four samplesstudied.

0

10

20

30

Methane Ethylene Ethane

Pr

Pea

k A

rea

x E

5(uV

.S)

Fig. 5. Effect of demineralization on the yields of gaseous (C

0

10

20

30

40

50

Methane Ethylene EthanePro

Pea

k A

rea

x E

5(uV

.S)

Fig. 4. Effect of demineralization on the yields of gaseous (C

3.2. Influence of demineralization on the product distribution

Inherently present inorganic elements have been reported toinfluence the pyrolytic reactions of coal [24]. To measure the effectof demineralization, all of the four coal samples (MC-1, MC-2, MC-3 and MC-4) were pyrolyzed in their original and demineralizedform at 690 �C using the process outlined in Fig. 1. The yields ofchar, tar, liquid and gaseous products from pyrolysis of raw coalsat 690 �C are given in Fig. 2. It can be seen that all coal samplesshowed conversion to desirable products like tar and liquid. It isalso observed that conversion to gaseous products is also profound.The yields of char in case of all four samples studied are significant.

Propene Butene/Butane n-Pentane/1-

Pentene oduct

MC-2 Raw

MC-2 Dm

1–C5) hydrocarbons obtained from MC-2 coal at 690 �C.

Propene Butene/Butane n-Pentane/1-

Pentene ducts

MC-1Raw

MC-1DM

1–C5) hydrocarbons obtained from MC-1 coal at 690 �C.

T. Ahmad et al. / Energy Conversion and Management 50 (2009) 1163–1171 1167

In order to view the influence of demineralization on the prod-uct distribution, pyrolysis of the demineralized samples was per-formed. The results have been compiled in Fig. 3. Uponcomparing the results with those obtained in case of original coalsamples, it can be observed that inorganic elements have greatlyinfluenced the product distribution of pyrolysate. The tar and li-quid yields decreased but the gas yield increased in case of demin-eralized coal samples. The decline in tar yield was 3.9%, 4.75%,4.15% and 4.65% for MC-1, MC-2, MC-3 and MC-4, respectively.The decrease in liquid yield was 0.8%, 2.6%, 4.75% and 0.15% forMC-1, MC-2, MC-3 and MC-4 samples, respectively. On contrary,the gas yield increased by 1.55%, 1.1%, 5.1% and 1.05% for MC-1,MC-2, MC-3 and MC-4 samples, respectively. The results indicatethat demineralization caused a deleterious effect on the yields of

0

10

20

30

Methane Ethylene Ethane

Products

Pea

k A

rea

x E

5(uV

.S)

Fig. 7. Effect of demineralization on the yields of gaseous (C

0

10

20

30

Methane Ethylene EthaneProd

Pea

k A

rea

x E

5(uV

.S)

Fig. 6. Effect of demineralization on the yields of gaseous (C

tar and liquid. The yields are quite appreciable in case of originalcoal samples. This can be attributed to high hydrogen transfercapabilities associated with inorganic elements. High hydrogentransfer favours greater product release and minimizes resolidifi-cation and is responsible for the formation of tar or liquid duringpyrolysis [25,26]. In the absence of inorganic elements, free radi-cals generated due to thermal shock could not be effectivelycapped, disproportionate and stabilized and hence recombine ret-rogressively to yield char [27].

The formation of gaseous products increased in case of demin-eralized samples. This is attributed due to the fact that most ofthese elements and particularly their oxides are non porous. More-over, presence of inorganic elements may alter some of the coalproperties like its softening and swelling behavior [28]. Removal

Propene Butene/Butane n-Pentane/1-

Pentene

MC-4 Raw

MC-4 DM

1–C5) hydrocarbons obtained from MC-4 coal at 690 �C.

Propene Butene/Butane n-Pentane/1-Pentene ucts

MC-3 Raw

MC-3 DM

1–C5) hydrocarbons obtained from MC-3 coal at 690 �C.

1168 T. Ahmad et al. / Energy Conversion and Management 50 (2009) 1163–1171

of non porous inclusions may affect the porosity. The diffusion lim-itations are more likely to occur in case of non porous matrix com-pared to porous. Hence, with demineralization, diffusionlimitations were avoided, leading to expulsion of gaseous products.In addition, the coal can swell significantly in the absence of inor-ganic elements under pyrolysis conditions. Highly swollen coalshave high porosity. High swollen coals usually yield more productswhen pyrolyzed [29].

The gaseous fractions were analyzed by using gas chromato-graph under similar conditions as used for original coal. It was ob-served that total yield of C1–C5 hydrocarbons decreased up to27.18%, 4.14%, 23.14% and 8.82%, respectively, for MC-1, MC-2,MC-3 and MC-4 coal samples. The comparison of gaseous productyields of original and demineralized coals are shown in Figs. 4–7. Itis observed that the yields were decreased after demineralization.

0

10

20

30

40

50

60

70

80

450 500 550 600

Pyrolysis

Yie

ld (

%W

eigh

t)

Char

Tar

Liquid

Gas

Fig. 9. Dependence of product yield on temper

0

20

40

60

80

100

120

140

160

MC-1 MC-2

Coa

Tota

l Are

a x

E5(

uV.S

)

Fig. 8. Effect of demineralization on total peak ar

The comparison of the total peak area of volatile hydrocarbons ob-tained in case of all coal samples in their original and demineral-ized forms is graphically represented in Figs. 8–12. The observedhigher pyrolysate yield in case of raw coal compared to acid lea-ched coal samples indicates that the inorganic elements acted ascatalyst for the secondary decomposition of tar molecules to lowmolecular weight gaseous hydrocarbons.

3.3. Effect of pyrolysis temperature on product yield

To study the effect of pyrolysis temperature on the yield ofpyrolysate, two raw form coal samples MC-1 (72.11% C) and MC-3 (53% C) were selected on the basis of their carbon content. Thesesamples were pyrolyzed over the temperature range of 500–770 �Cand the quantitative results are shown in Figs. 9 and 10. It was

650 700 750 800

Temperature(°C)

ature from the pyrolysis of MC-1 raw coal.

MC-3 MC-4

l Type

Raw

DM

ea from the pyrolysis of each coal at 690 �C.

T. Ahmad et al. / Energy Conversion and Management 50 (2009) 1163–1171 1169

observed that total pyrolysate increased with the increase in tem-perature. A similar trend was also observed for tar and liquid frac-tions which attained a peak yield at 600 and 700 �C, respectively.The compositional analysis of gaseous fractions was carried outby GC–FID. The major peaks were of methane, ethane, ethylene,propane + propene, n-butane + 1-butene and n-pentane + 1-pen-tene. The results were graphically represented in Figs. 11 and 12,respectively. It was observed that the yield of each component in-creased with increase in temperature. The overall increase wasmuch pronounced above 650 �C. It was found that the total hydro-carbon yield was increased from 6.88% to 35.60% and 6.55% to

0

10

20

30

40

50

60

70

550 600 650 700

Pyrolysis Temperatu

Pea

k A

rea

x E

5(uV

.S)

Fig. 11. Dependence of product yield on temper

0

10

20

30

40

50

60

70

80

450 500 550 600

Pyrolysis Te

Yie

ld (

%W

eigh

t)

Char

Tar

Liquid

Gas

Fig. 10. Dependence of product yield on temper

32.63%, respectively, for MC-1 and MC-3 coal samples when thetemperature was increased from 570 to 770 �C.

It was also observed that the yield of total pyrolysates increasedwith the increase in temperature. In case of both coals, the maxi-mum tar yield was obtained at 600 and 570 �C, respectively, forMC-1 and MC-3. From gaseous analysis using GC–FID, it was ob-served that among all the hydrocarbons (C1–C5), methane yieldwas high and increased with temperature. This can be attributedto the fact that the coal samples used are lignitic to sub-bitumi-nous in rank. Due to high oxygen contents, such coals are suscep-tible to thermal shock leading to fragmentation [30]. Rupture of

750 800

re(°C)

Methane

Ethylene

Ethane

Propene/Propane

Butene/Butane

n-pentane/1-pentene

ature from the pyrolysis of MC-1 raw coal.

650 700 750 800

mperature(°C)

ature from the pyrolysis of MC-3 raw coal.

0

10

20

30

40

50

60

550 600 650 700 750 800

Pyrolysis Temperature(°C)

Pea

k A

rea

x E

5(uV

.S)

Methane

Ethylene

Ethane

Propene/Propane

Butene/Butane

n-pentane/1-pentene

Fig. 12. Dependence of product yield on temperature from the pyrolysis of MC-3 raw coal.

1170 T. Ahmad et al. / Energy Conversion and Management 50 (2009) 1163–1171

methylene, ethylene, propylene, and butylenes linkages adjoiningthe aromatic lamellaes is another reason for formation of pyroly-sates particularly in the form of gaseous products. In addition,most of the aromatic moieties present in coals having substituentsmostly in the form of methyls [31]. Upon heating, dealkylationreactions are favoured leading to formation of gaseous productsmostly enriched in methane.

The yields of other hydrocarbons were also observed to increasewith temperature. This is due to secondary thermal cracking of vol-atiles as long as these stay in the reactor. Products of pyrolysis aremore susceptible towards cracking compared to original coal. Thisis also called post oil generation phase in which increase in gas vs.decrease in oil products is observed owing to the secondary crack-ing of oil at higher temperatures [32].

4. Conclusions

The inherent mineral matter has a profound effect on thebehavior of coal and the yield of hydrocarbons which is decreasedas these minerals are washed out from the coal samples. This isindicative of the fact that these minerals have catalytic role duringpyrolysis of coal. It was also observed that the weight of total vol-atile yield was not significant as compared with char content incase of each coal sample at all temperatures. The results showedthat the yield of pyrolysates increased with increase intemperature.

Acknowledgement

The authors wish to thank Higher Education Commission,Islamabad for financial support.

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