investigation of oil-shale pyrolysis processing conditions using thermogravimetric analysis

Investigation of oil-shale pyrolysis processingconditions using thermogravimetric analysis

Paul T. Williams*, Nasir Ahmad1

Department of Fuel and Energy, The University of Leeds, Leeds, LS2 9JT, UK

Accepted 4 July 1999

Abstract

Four oil-shale samples from two regions of Pakistan have been pyrolysed in a thermo-gravimetric analyser (TGA) in relation to heating rate and temperature using non-isothermaland isothermal analysis respectively. The heating rates investigated in the TGA were 5±40 K

minÿ1 to a ®nal temperature of 950�C. The main region of weight loss corresponding tohydrocarbon oil and gas release was between 200 and 620�C, and at higher temperatures,signi®cant weight loss was attributed to carbonate decomposition. It was found that for theoil-shale samples analysed in the TGA, increasing the heating rate shifted the reaction to

higher temperatures. The ®nal temperature of the pyrolysis was investigated using the iso-thermal TGA. The temperature range studied was from 350 to 485�C. The data were analysedto determine the kinetic parameters of activation energy and frequency factor using two

methods, the Arrhenius and Coats±Redfern analyses. The order of reaction was determined asunity. There was no clear relationship between activation energy and heating rate. The Coats±Redfern method of analysis gave consistently lower values of activation energy compared with

the Arrhenius method. Isothermal analysis gave similar results to those obtained using thenon-isothermal analysis. # 2000 Elsevier Science Ltd. All rights reserved.

Keywords: Kinetics; Oil shale; Pyrolysis; TGA

1. Introduction

Oil shales represent an enormous potential of liquid hydrocarbon reserves, whichhas been estimated at over 2.7 � 1014 tonnes of oil [1]. The exploitation of oil-shalesrepresents, for many countries, a valuable potential source of liquid hydrocarbonsand energy. Pakistan is a low economy country with a high reliance on imported

Applied Energy 66 (2000) 113±133

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* Corresponding author. Tel.: +44-01132-332504; fax: +44-01132-4405726.1 On leave from the Institute of Geology, Punjab University, Lahore, Pakistan.

energy, estimated at 25% of the total energy used [2]. Whilst oil-shale deposits areknown in Pakistan, and the deposits are thought to be of a high grade [3], theirdevelopment has not occurred, due in part to a poor knowledge of their full extentand characteristics [1]. For the energy potential of oil-shale to be maximised, theconversion process of the oil-shale to oil should be undertaken under optimal pro-cess conditions. A range of process conditions have been investigated in the pyr-olysis of oil-shale to ensure the maximum and e�cient extraction of the oil. Theparameters identi®ed as being of most signi®cance are heating rate and ®nal pyr-olysis temperature. For example, Shen et al. [4] concluded that the entire shale-pyr-olysis mechanisms and derived oil composition are heating rate dependent, with thenumber of kerogen decomposition pathways being more diverse at higher heatingrates. They also concluded that oil yield and the extent of any oil cracking is gov-erned by the peak pyrolysis-temperature.Thermogravimetric analysis (TGA) of oil-shale samples has been extensively used

as a means of determining the characteristics of devolatilisation and also to deter-mine kinetic parameters [5±11]. A number of researchers have reported the in¯uenceof heating rate and ®nal pyrolysis temperature on oil-shale decomposition usingTGA. For example, Drescher et al. [8] and Rajeshwar [11] showed a distinct sys-tematic shift of the region of maximum rate of weight loss to higher temperatures asthe heating rate was increased for the analysis of oil-shale using TGA. Dogan andUysal [7] and Haddadin and Mizyed [5] have investigated the in¯uence of pyrolysistemperature and shown higher weight losses for higher temperatures of pyrolysis.In order to predict the thermal degradation of oil-shale, accurate values of the

kinetic parameters over the entire range of decomposition temperature are required.There appears a range of values of the kinetic parameters pertaining to oil-shaledecomposition [10±15]. The use of TGA to determine kinetic parameters for thepyrolysis of oil is complicated in that the oil-shales are a complex mixture of kerogenand a wide range of minerals. In addition, the decomposition of the oil-shalesrepresents a large number of reactions in parallel and series, whilst the TGA mea-sures the overall weight loss due to these reactions. Therefore, the TGA providesgeneral information on the overall reaction kinetics rather than individual reactionsand therefore the activation energies derived from TGA should be termed apparentactivation energies. However, the technique has been used by several researchers andgives useful comparative data and was used in this work to examine the in¯uence ofheating rate and ®nal pyrolysis temperature on the apparent activation energies.TGA usually involves a non-isothermal analysis of the oil-shale, whereas most

kinetic data are derived from isothermal studies. In isothermal analyses, the rate ofreaction is determined at constant temperature, whilst in non-isothermal studies,time and temperature are coupled via a constant heating rate. The non-isothermalTGA technique has been preferred by some researchers [7±10] because of itsadvantages over the isothermal method. These advantages include the elimination oferrors due to the thermal induction period; it also permits a rapid scan of the wholetemperature range of interest; and it also more closely simulates conditions expectedin large-scale oil-shale retorting processes. On the other hand, Behnisch et al. [16]have pointed out that a combined kinetic analysis of isothermal and non-isothermal

114 P.T. Williams, N. Ahmad /Applied Energy 66 (2000) 113±133

themogravimetric weight loss data is an e�ective method for determining the mostprobable mechanism for polymer decomposition.There are several approaches to the kinetic analysis of non-isothermal thermo-

gravimetric data to determine the activation energy and frequency factor for thethermal degradation of a sample [7,10,17±20]. Coats and Redfern [19] developed amathematical equation which allowed the use of non-isothermal thermogravimetricanalysis to derive the kinetic parameters of a ®rst-order reaction. Nuttall et al. [21]employed mathematical equations developed by Chen and Nuttal [22], Coats andRedfern [19] and Anthony and Howard [20] to analyse non-isothermal thermo-gravimetric data for oil shales from around the world. Rajeshwar [11] made usedirectly, of the Arrhenius equation, the Coats±Redfern equation [19] and the di�er-ence-di�erential method developed by Freeman and Carroll [23] to analyse thermo-gravimetric data to evaluate pyrolysis kinetics for Green River oil-shale. Skala andco-workers [6,14,24,25] employed an integral method to analyse non-isothermalthermogravimetric data using a method used by Doyle [26] and Gorbachev [27]. Theuse of di�erent approaches of analysis may lead to di�erent values of the kineticparameters and comparison of values therefore becomes di�cult. Particular di�-culties also arise from the fact that oil shales are very di�erent in their, mineralmatter kerogen composition. Comparison of methods for the same sample of oilshale under identical conditions is therefore of interest.In this paper, four oil-shale samples from two regions of Pakistan, have been

pyrolysed in a TGA under non-isothermal conditions in relation to heating rate andalso isothermal TGAs were performed to determine the in¯uence of temperature onthe thermal degradation of the oil-shales. The data were analysed to determine thekinetic parameters using two methods of analysis, the Arrhenius and the Coats±Redfern methods and the results compared.

2. Materials and methods

2.1. Oil shales

The oil-shales were obtained from the Salt Range and Kohat basin areas ofnorthern Pakistan, to the south-south-west and west of Islamabad respectively. TheSalt Range area contains rocks which range in age from Precambrian to Pleistocene,which occur as exposed scarps rising out of an alluvial plain. The sequence containsthe Salt Range oil-shale formation whose age is Precambrian, although the exact ageof the sequence is not clear since there are no diagnostic fauna and it is, therefore,termed Infracambrian. The Salt Range oil-shales vary in thickness from 3.0 to 5.0 mand are black in colour with a shale characteristic. The Kohat basin contains rocksequences from Eocene to Recent in age and are folded and faulted to produce acomplex formation. The area contains the Kark, Malgeen and Dharangi oil-shalesexamined in this paper and which are of Eocene age. The Kark oil-shales vary inthickness from 0.5 to 6.0 m and are green to dark grey in appearance, mediumhard and compact, brittle and thinly laminated in character. The Kohat basin is

P.T. Williams, N. Ahmad /Applied Energy 66 (2000) 113±133 115

characterised by complex geological structures showing an east-west orientation, thearea is folded and faulted and is also in¯uenced by salt diapirism. Table 1 shows ananalysis of the four shale-types investigated in this paper. The organic carbon andhydrogen contents of the Pakistani oil-shales are higher than many oil-shales foundthroughout the world.

2.2. Thermogravimetric analysis

Thermogravimetric analysis (TGA) of the shale samples was undertaken using aShimadzu Model-50 Series TG analyser. In this work, the sample was heated to950�C at 5, 10, 20 and 40 K minÿ1 heating rate using nitrogen as the purge gas. Forisothermal work, the furnace temperature was heated to the set ®nal temperature asquickly as possible and held at that temperature for up to 1.5 h. Inevitably withisothermal work, there is a time lag due to heat-transfer e�ects of the furnace to thesample. In this work, the temperature recorded was that of the sample. The particlegrain size used was 0.5±1.0 mm. The authors have shown previously that particlegrain size also in¯uences the rate of weight loss and the kinetic parameters for thethermal degradation of Pakistani oil-shale [28]. The TGA apparatus provides for thecontinuous measurement of sample weight as a function of temperature and provi-sion is made for an electronic di�erentiation of the weight signal to give the rate ofweight loss (DTG).

3. Results and discussion

3.1. Non-isothermal thermogravimetric analysis

Figs. 1±4 show the di�erential weight-loss curves (DTG) in relation to heating rateto a ®nal pyrolysis temperature of 950�C for the four oil-shale samples, the Salt

Table 1

Characteristics of the oil-shale samples (wt%)

Eocene Infracambrian

Kark Dharangi Malgeen Salt range

Moisture 5.5 2.5 1.0 5.0

Volatile matter 34.4 30.7 29.0 36.1

Fixed carbon 4.12 5.7 3.2 10.3

Ash 61.3 63.6 67.7 53.6

Total carbon 24.4 30.0 32.2 29.6

Organic carbon 22.0 12.0 10.0 ±

H 3.0 3.0 2.9 3.5

N 0.08 0.52 0.34 0.84

S 0.4 ± ± 1.5

CV (MJ kgÿ1) 9.6 5.7 ± 13.0

Density (kg m3) 1.63 ± ± 1.16


Fig. 1. DTG of the Salt Range oil-shales in relation to heating rate.

Fig. 2. DTG of the Kark oil-shales in relation to heating rate.


Fig. 3. DTG of the Malgeen oil-shales in relation to heating rate.

Fig. 4. DTG of the Dharangi oil-shales in relation to heating rate.


Range sample of Infracambrian age and the Kark, Dharangi, and Malgeen oil-shales of the Eocene age. All four oil-shale samples exhibit distinctly di�erent pat-terns of thermal decomposition as the pyrolysis temperature was increased.The lower temperature region of weight loss, up to approximately 200�C, pro-

duced weight loss which has been attributed to the loss of moisture, loss of interlayerwater from clay minerals and also decomposition of the mineral nahcolite(NaHCO3) [29]. Clay minerals present in the oil-shale may release structural waterover a wide range of temperatures up to about 550�C [30]. The weight loss has alsobeen attributed to physical changes in the kerogen prior to decomposition to pyr-olytic bitumen, these changes being in the form of softening of the kerogen, mole-cular rearrangement accompanied by the release of gas [5]. These lower temperatureweight-losses are most signi®cant in the Salt Range sample and are less so for theother Eocene samples of oil-shale.The region of weight loss from about 200 to 620�C is due to the loss of hydro-

carbon material. The Eocene oil-shale samples, i.e. the Kark, Dharangi and Malg-een, exhibit a one-step thermal decomposition in the main weight loss areaattributed to hydrocarbon volatile formation that is, in the temperature range, 350±620�C suggesting a one-step evolution of hydrocarbon volatiles from the oil-shale.However, the Salt Range sample exhibits a two-stage decomposition in the range250±620�C, representing a two-stage evolution of hydrocarbon material. The two-stage decomposition has also been observed by other workers for di�erent oil-shales,for example, UK, Kimmeridge oil-shale [10], Jordanian oil-shale [5], Turkish oil-shale [7] and Green River oil-shale [31]. Whether the decomposition is single-stageor two-stage depends on the type of oil-shale [7,10] as has also been shown in thiswork. There have been suggestions that the decomposition of kerogen to oil, gas andchar products is a two-stage process involving decomposition of the kerogen topyrolytic bitumen and then decomposition to products [5,10]. The lower tempera-ture weight loss for the Salt Range oil-shale in the region of 350�C is attributed tothe ®rst-stage kerogen to pyrolytic bitumen decomposition. Whilst the single-or two-stage TGA process is reported for the oil-shales in this work, the actual mechanismfor the thermal decomposition of oil shales is a much more complex reaction invol-ving a series of parallel reactions [10,13,32].Heating above 600�C produced carbonate decomposition at temperatures between

600 and 950�C due to the presence of the carbonate minerals such as calcite, dolo-mite and ankerite. The Kark oil-shale showed a small weight-loss in this range,whereas the Dharangi and Malgeen samples show a high rate of weight loss due tothe presence of carbonate minerals. The Salt Range sample shows only a low level ofcarbonates present.Table 2 shows the data of weight loss in the low, medium and high temperature

regions discussed previously in terms of Figs. 1±4, but in terms of percentageweight loss.The table emphasises the signi®cant weight loss due to hydrocarbons in the mediumtemperature region of 200 to 620�C. Also, the high weight loss due to carbonatedecomposition for theDharangi andMalgeen oil-shale samples between 620 and 950�C.Table 3 shows the analysis of the TGA data in relation to heating rate in terms of

the onset of weight loss, the temperature where maximum devolatilisation occurs


and the maximum rates of weight loss in the temperature range of 200±950�C. Fig. 5de®nes the nomenclature used in Table 3. As the heating rate is increased, Table 3shows that there was a lateral shift to higher temperatures for Tonset, Tmax1, Tmax2

and Tmax3 for the four oil-shale samples. The lateral shift is also illustrated in Figs.1±4 for the DTG curves. The rate of weight loss also re¯ects the lateral shift with anincrease in the rate, as the heating rate was increased from 5 to 40 K minÿ1. Thelateral shift to higher temperatures for the maximum region of weight loss has alsobeen observed by other workers using TGA to investigate the pyrolysis of oil shales.For example, Drescher et al. [8] showed a lateral increase in the maximum rate ofweight loss of about 60�C as the heating rate was increased from 0.66 to 50 K minÿ1

for Green River oil-shale. Thakur and Nuttall [33] and Herrall and Arnold [15]also showed a lateral shift in the maximum rate of weight loss for the TGA ofoil-shale samples. Rajeshwar [34] suggested that the shift to higher temperaturesof decomposition represented di�erences in the rate of heat-transfer to the sampleas the heating rate was varied. Others have explained the shift in the temperatureof decomposition as being due to the combined e�ects of the heat-transfer at the

Table 2

Weight loss in relation to pyrolysis temperature range for di�erent heating rates and oil-shale types

Weight loss (wt%)

Temperature range (�C) 25±200 200±620 620±950 Total

Salt Range

Heating rate (K minÿ1)5 6.5 22.5 10.0 39.0

10 6.0 17.5 10.0 33.0

20 6.0 18.2 8.8 33.0

40 6.0 19.5 8.5 34.0

Kark


10 4.3 25.5 7.7 37.4

20 4.8 26.2 4.2 33.2

40 4.3 26.9 8.2 39.4

Malgeen

Heating rate (K minÿ1)5 ± ± ± ±

10 1.2 9.7 28.2 39.1

20 1.0 9.1 26.9 37.0

40 0.8 8.2 27.2 36.2

Dharangi


10 2.4 12.5 15.0 29.9

20 2.4 11.6 16.8 30.8

40 2.4 11.7 15.5 29.6


di�erent heating rates and the kinetics of the decomposition resulting in delayeddecomposition [35,36].It is also apparent that, at high heating-rates, the outside surface of the oil-shale

particle will be at a di�erent temperature to the inside and this di�erence in tem-perature will be more pronounced as the heating rate is increased. This temperaturegradient results in reactions taking place on the inside of the particle at lower tem-peratures producing oil and gas, which pass through a higher temperature regimeresulting in secondary reactions. Consequently, although there may be some delay inthe heat transfer from the furnace to the sample which will increase with increasingheating rate, this represents a change in the temperature gradient within the sampleand, therefore, a change in the process parameters of pyrolysis.

3.2. Isothermal thermogravimetric analysis

Isothermal thermogravimetric analysis was performed on the Salt Range, andKark oil-shales as representative Infracambrian and Eocene oil-shales of those studied.Fig. 6 shows the Salt Range and Fig. 7 the Kark oil-shale results for the isothermal

Table 3

Comparison of thermogravimetric data in relation to heating rate and oil shale type

Temperature (�C) Rate of weight loss (wt% minÿ1)

Tonset Tmax1 Tmax2 Tmax3 Rmax1 Rmax2 Rmax3

Salt Range

Heating rate (K minÿ1)5 230 302 445 ± 0.3 0.5 ±

10 ± ± 453 ± ± 1.4 ±

20 259 325 478 ± 1.1 2.2 ±

40 278 337 491 ± 2.2 3.6 ±

Kark

Heating rate (K minÿ1)5 332 ± 454 705 ± 1.3 0.2

10 360 ± 476 742 ± 2.5 0.6

20 390 ± 490 780 ± 5.1 0.8

40 408 ± 507 787 ± 10.7 2.7

Malgeen


20 410 ± 490 847 ± 1.3 5.7

40 442 ± 500 865 ± 2.0 11.3

Dharangi


10 370 ± 468 705 ± 1.3 1.1

20 387 ± 471 711 ± 2.3 2.7

40 408 ± 490 769 ± 5.1 4.4


Fig. 5. Nomenclature for Table 3.

Fig. 6. Isothermal thermogravimetric analysis of the Salt Range oil-shale.


thermogravimetric analyses. Under isothermal conditions, the total time required toachieve pyrolysis is the sum of the heat-up time or induction time required for theoil-shale particles to reach the reaction temperature and the time required for thedecomposition (pyrolysis) of the kerogen at that temperature. The e�ect of increasedtemperature is to decrease the pyrolysis time. In addition, the conversion of kerogenis dependent on the ®nal pyrolysis temperature and the higher the temperature ofpyrolysis the higher the conversion of kerogen, as has also been shown by otherworkers [33,37]. Comparison of Figs. 6 and 7 also shows that the Kark oil-shalesample takes a longer time, under identical experimental conditions to obtain thesame percentage conversion relative to the salt Range sample. Clearly di�erent oil-shales behave di�erently under both isothermal and non-isothermal conditions.Other workers have also shown that the amount of weight loss is related to the

®nal temperature. For example, Skala et al. [6] showed an increase in total weightloss with increasing temperature of pyrolysis for Yugoslavian kerogens using ther-mogravimetric analysis. A similar relationship has also been shown by Dogan andUysal [7] for Turkish oil-shales; they also showed that the rate of decomposition wasindependent of temperature. Moroccan oil-shales exhibited a similar link betweenincreased ®nal pyrolysis temperature and increased weight loss, but showed anincrease in the rate of weight loss with increasing ®nal pyrolysis temperature [33].Galan and Smith [38] undertook an isothermal thermogravimetric analysis of Col-orado oil-shale and reported that the fraction of kerogen decomposed at 500�C wasthree times higher than the fraction decomposed at 300�C under the same experi-mental conditions.

Fig. 7. Isothermal thermogravimetric analysis of the Kark oil-shale.


3.3. Non-isothermal kinetic analysis

The approach adopted in this work for the calculation of activation energy wasthe direct Arrhenius equation used by for example Rajeshwar [11] and the mathe-matical equation developed by Coats and Redfern [19] for the non-isothermal ana-lysis of thermogravimetric data. These equations are derived from the simple rateexpressions. The general expression for the kinetics of a solid-state decompositionreaction has been represented by Blazek [39] as

dx

dt� kf

�x� �1�

where x is the fraction of solid oil-shale decomposed in time t, k is the rate constantand f�x� = (1ÿx) for ®rst order reactions. The rate constant is related to the abso-lute temperature T by the Arrhenius equation;

k � A exp ÿE=RT� � �2�

where A is the frequency factor, E the apparent activation energy, R the gas con-stant, and T the temperature. Substituting for k from Eq. (2) into Eq. (1), the rateexpression can be written;

dx

dt� A exp ÿE=RT� � 1ÿ x� � �3�

For a non-isothermal kinetic experiment with a linear heating rate of b K minÿ1,where, b � dT=dt, Eq. (3) can be modi®ed as;

dx

dT� A=b� � exp ÿE=RT� � �1ÿ x� �4�

For a reaction which may be represented functionally as, f�x� = (1ÿx)n where n isthe reaction order, Eq. (4) can be reduced to

dx

dT� A=b� � exp ÿE=RT� � 1ÿ x� �n �5�

According to Eq. (5), a plot of ln[(dx/dt)/(1ÿx)n] versus 1/T corresponds to astraight line with a slope of ÿE=R� �, which can be used to evaluate the activationenergy. The frequency factor can be determined from the intercept.The second method used in this work is the integral method developed by Coats

and Redfern [19]. The Coats±Redfern equation takes the form;

ln1ÿ 1ÿ x� �1ÿn� �

T2�1ÿ n� � lnAR=bE1ÿ 2RT� �

E

ÿERT

�6�


and for the ®rst order reaction (n � 1), Eq. (6) is reduced to the more commonlyused expression;

ln ÿfln�1ÿ x�gT2

� �� lnAR=bE

1ÿ 2RT

E

� �cÿERT

�7�

Therefore a plot of either ln [{1ÿ(1ÿx) 1ÿn}/{T2 (1ÿn)}] versus 1/T or where, theorder of reaction n � 1, ln[{ln(1ÿx)}/T2] against 1/T yields a straight line of slopeE=R for the correct value of n.The approach adopted by many workers in the kinetic analysis of TGA data for a

variety of materials is to assume [7] or determine [5,10] a ®rst-order rate of reactionfor the process of devolatilisation. In this work, the order of reaction was deter-mined for the heating rate of 5 K minÿ1 for the Kark oil-shale as a representativesingle step decomposition of the oil shale. The order of reaction can be determinedby plotting ln[dx/dT)/(1ÿx)n] versus l/T for various values of n Ð the order ofreaction Ð ranging from 0.5 to 2 using the Arrhenius equation. Fig. 8 shows theresults. The decomposition of oil-shale kerogen in terms of a ®rst-order reactionappears to be a suitable assumption since a value of n � 1 o�ers a best ®t regressionline giving a correlation coe�cient of 0.999.Tables 4±7 show the apparent activation energies for the Salt Range, Kark,

Malgeen and Dharangi oil-shales respectively in relation to heating rate. For theKark, Malgeen and Dharangi oil-shales, a single kinetic expression is valid over thetemperature range of kerogen pyrolysis between the temperatures of about 200 and620�C. However, for the Salt Range oil-shale, two kinetic expressions at each of the

Fig. 8. Arrhenius method of analysis with di�erent values of the order of reaction.


Table 4

In¯uence of heating rate on kinetic parameters for the Arrhenius and Coats±Redfern methods of analysis

for the Salt Range oil-shale

Method Heating rate

(K minÿ1)Kinetic parameters Correlation

coe�cient

E1 kJ molÿ1 E2 kJ molÿ1 A1 minÿ1 A2 minÿ1 r

Arrhenius 5 26 95 212 2.2�106 0.998

10 27 93 ± 9.8�106 0.999

20 36 100 812 7.3�106 0.994

40 26 96 290 1.8�106 0.999

Coats±Redfern 5 26 56 0.999

10 27 74 0.997

20 24 73 0.999

40 24 58

Table 5


for the Kark oil-shale

Method Heating rate


coe�cient

E kJ molÿ1 A minÿ1 r

Arrhenius 5 141 2.7�109 0.998

10 132 7.8�108 0.992

20 136 2.4�109 ±

40 160 3.4�109 ±

Coats±Redfern 5 94 0.997

10 90 0.998

20 102 0.999

40 117 ±

Table 6


for the Malgeen oil-shale

Method Heating rate


coe�cient


Arrhenius 5 136 7.8�107 0.997

20 120 3.7�106 0.998

40 100 9.8�106 ±


20 119 0.993

40 108 0.998


heating rates representing di�erent slopes of the straight line plot re¯ecting two-stages of decomposition. The in¯uence of heating rate on the activation energies wasnot clear, in some cases, for example the Salt Range oil-shale, the activation energieswere very similar irrespective of heating rate. However, in other cases, there weresigni®cant di�erences in the calculated activation energies, in relation to heatingrate, for example, the Kark oil-shale. There appeared to be no signi®cant systematicchange in relation to heating rate.The ®rst-order two-stage decomposition for the Salt Range oil-shale implies that

the rates of the decomposition reactions for this particular oil-shale change beyondsome critical temperature. This change takes place independently of the reactionorder. The average value of this critical temperature was 345�C for the Salt Rangesample, which can be compared to the critical temperature of 325±425�C for a UKoil-shale [10] and 300±350�C for a Turkish oil-shale [7]. The ®rst reaction at lowtemperature proceeds with an average activation energy of 29 kJ molÿ1, while thesecond reaction, which starts at a temperature above 345�C has an average activa-tion energy of 90 kJ molÿ1. The low activation energy E1 for the kerogen decom-position to bitumen compares well with the literature [11,40] and indicates that thedecomposition of kerogen to bitumen involves the breaking of relatively weak che-mical-bonds [40].The average values for the activation energies and frequency factors for the four

oil-shale samples yield the following kinetic expressions;

1� � Salt Range : K1 � 0:46� 103 exp ÿ343=T� � �minÿ1� �8�

K2 � 3:9� 106 exp ÿ115=T� � �minÿ1� �9�

2� � Kark : K � 2:32� 109 exp ÿ147=T� � minÿ1ÿ � �10�

3� � Malgeen : K � 2:68� 107 exp ÿ136=T� � minÿ1ÿ � �11�

Table 7


for the Dharangi oil-shale

Method Heating rate


coe�cient


Arrhenius 5 133 2.9�108 0.999

10 140 1.1�109 ±

20 136 2.6�108 ±

40 152 1.5�109 0.996


10 88 ±

20 89 ±

40 97 ±


4� � Dharangi : K � 7:7� 108 exp ÿ137=T� � minÿ1ÿ � �12�

Comparison of the Arrhenius and Coats±Redfern methods of kinetic analysisrevealed very similar results for the Salt Range oil-shale, but the Kark and Dharangioil-shales gave di�erent results depending on which equation was used. For theKark, Malgeen and Dharangi oil-shales the Coats±Redfern equation gave loweractivation energies compared to the Arrhenius Equation.The values of activation energy in Tables 4±7 may be compared to the literature.

Williams [10] obtained an activation energy of 57.9 kJ molÿ1 for UK, Kimmeridgeoil-shales using isothermal TGA at 280�C. At the higher temperature range of 400±500�C and using non-isothermal TGA, higher activation energies ranging from 209±217 kJ molÿ1 were obtained for a range of oil-shale samples. He also reported anapparent activation energy of 40±43 kJ molÿ1 for the lower temperature thermaldecomposition of the oil-shale at approximately 325�C. Other workers have alsoreported similar apparent activation energies for the main stage of oil-shale decom-position, for example, 217 kJ molÿ1 [41]. Skala et al. [6] performed non-isothermalTGA on Yugoslavian oil-shales and found overall apparent activation energiesranging from 42.9 to 114.7 kJ molÿ1. Dogan and Uysal [7], however, reportedresults for Turkish oil-shales, of approximately 25 kJ molÿ1 for the lower tempera-ture decomposition and up to 43 kJ molÿ1 for the main stage of decomposition.Similarly, Haddadin and Mizyed [5] report apparent activation energies for the two-stage pyrolysis of Jordanian oil-shales with a low temperature (280±405�C) value of17.8 kJ molÿ1 and a higher temperature (405±518�C) value of 44.7 kJ molÿ1.The discrepancies in the activation energy for oil-shale decomposition is not per-

haps surprising, in that variations will occur depending on the type of oil-shale andtype of pyrolysis [7]. For example, isothermal and non-isothermal pyrolysis havebeen shown to produce di�erent activation energies, and the mathematical treat-ment of the TGA data may also in¯uence the quoted activation energy [6,7,11]. Alsothe type of kerogen in the oil-shale has been found to yield di�erent apparent acti-vation energies [6]. Other factors include the size range of the oil-shale particles andthe type and amount of mineral matter [28,42]. The thermogravimetric analysermerely records overall weight-loss. The decomposition of kerogen to oil represents acomplex range of reactions, in series and parallel, and the extent to which eachreaction becomes rate limiting under a particular set of experimental conditions mayremain unresolved.

3.4. Isothermal kinetic analysis

The isothermal thermogravimetric analysis data were analysed to determine thekinetic parameters.For the isothermal decomposition of oil-shale, Eq. (1) is integrated to give

ÿ ln 1ÿ x� �� kt �13�


A plot of ÿ[ln(1ÿx)] versus T gives a straight line with a slope of k. The value of tused in this work included the induction period, t0 as suggested by Braun andRothman [40] and Pan et al [37]. The raw thermogravimetric data were used togenerate values of x at each temperature using the following expression;

x � W0 ÿWt� �W0 ÿWf

ÿ � �14�

where W0 denotes the initial weight of the sample, Wt is the weight of the sample attemperature T and Wf represents the weight of sample after complete pyrolysis ofkerogen.Figs. 9 and 10 show the the typical plots of ln(1ÿx) versus time for the Salt Range

and Kark oil-shales. Nearly straight lines were obtained down to a fractional weightchange that depends on temperature [37]. Kinetic parameters were evaluated fromthe Arrhenius plots for the Kark and Salt Range samples for the isothermal dataand compared to the averaged non-isothermal kinetic data obtained for the com-bined Arrhenius and Coats±Redfern activation energies from Tables 4 and 5. Theresults are shown in Table 8. The results show that both the isothermal and non-isothermal analyses give similar results for the kinetic parameters of activationenergy and frequency factor.The speci®c rate-constants for the isothermal and non-isothermal thermogravi-

metric analyses of the Salt Range and Kark oil-shales were calculated and the resultscompared. Table 9 compares the rate constants at di�erent temperatures for the

Fig. 9. Analysis of isothermal data for Salt Range oil-shale at various temperatures.


Fig. 10. Analysis of isothermal data for Kark oil-shale at various temperatures.

Table 8

Comparison of isothermally and non-isothermally derived kinetic parameters

Method Salt Range Kark

E2 (kJ molÿ1) A2 (minÿ1) E (kJ molÿ1) A (minÿ1)

Isothermal 88 4.7�106 125 4.8�108Non-isothermal (averaged data) 96 3.9�106 122 2.3�109

Table 9

Comparison of rate constants (k) for decomposition of the Salt Range and Kark oil-shales using iso-

thermal and non-isothermal thermogravimetric analysis

Temperature Isothermal data Non-isothermal data (20 K minÿ1)

Salt Range Kark Salt Range Kark

350 ± 0.14 ± ±

375 ± ± ± ±

400 0.08 0.15 0.07 0.13

425 0.15 0.25 0.16 0.24

445 0.31 ± 0.31 ±

450 ± 0.40 0.36 0.43

465 0.50 ± 0.56 ±

475 ± 0.70 0.76 0.76

485 0.90 ± 1.0 ±


non-isothermal thermogravimetric analysis at 20 K minÿ1 heating rate with the iso-thermal data. In the temperature range 400±450�C, the rate constants for isothermaland non-isothermal TGAs are very similar. At temperatures exceeding 450�C, therate constants derived using non-isothermal data were slightly higher than the iso-thermally derived data. This may be due to the increased weight loss during theinduction period for the higher temperatures.

4. Conclusions

The pyrolysis of oil-shales from Pakistan in a thermogravimetric analyser showeddistinctly di�erent thermal decompositions depending on oil-shale type in relation tothe temperature of pyrolysis and the heating rate. There was a shift in the maximumrate of weight loss to higher temperatures as the heating rate was increased.Increased weight loss was found as a result of increasing the ®nal pyrolysis temperature.The data were examined to determine kinetic parameters using two di�erent approa-ches. The derived activation energies were found to be dependent on the mathematicalequation used. An isothermal thermogravimetric analysis gave similar results tothose of non-isothermal analysis for the determination of kinetic parameters.

Acknowledgements

The authors would like to acknowledge the support of Peter Thompson of theUniversity of Leeds. The award of a Pakistan Government Scholarship to N.Ahmad is also gratefully acknowledged.

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investigation of oil-shale pyrolysis processing conditions using thermogravimetric analysis

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