influence of process conditions on the pyrolysis of pakistani oil shales

Influence of process conditions on the pyrolysis of Pakistani oil shales

Paul T. Williams*, Nasir Ahmad1

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

Received 16 July 1998; accepted 27 October 1998

Abstract

Oil shale samples of two geological ages, Infracambrian and Eocene from two regions of Pakistan were pyrolysed in a thermogravimetricanalyser (TGA) and a fixed bed reactor to determine the influence of temperature and heating rate on the thermal degradation of the samples.The heating rates investigated in the TGA were, 58C–408C min21 to a final temperature of 9508C. It was found that for the oil shale samplesanalysed in the TGA, increasing the heating rate shifted the reaction to higher temperatures. The main region of weight loss corresponding tohydrocarbon oil and gas release was between 2008C and 6208C, and at higher temperatures, significant weight loss was attributed tocarbonate decomposition. Two of the oil shale samples were also investigated in a fixed bed reactor to determine the influence of heatingrate and temperature of pyrolysis on the yield of products and composition of the gases evolved. The pyrolysis reactor was 200 cm3 volume,constructed of stainless steel and externally electrically heated. The samples were heated to 5208C at heating rates similar to those of theTGA; in addition, the influence of pyrolysis temperature between 4008C and 6508C was also investigated. The maximum oil yield wasoptimised in terms of heating rate at 208C min21 and in terms of temperature at 5008C. Maximum oil yields were between 12.0 and 16.5 wt%for the two oil shales used representing a considerable potential source of liquid hydrocarbons for Pakistan. The main gases produced wereH2, CO, CO2 and CH4 and also minor concentrations of alkene and alkane gases. Gaseous yield increased linearly with both increasingheating rate and increased pyrolysis temperature. There was a corresponding decrease in spent shale. Alkene/alkane gas ratios weredetermined and were linked to secondary reactions, which were discussed in relation to the formation of oil.q 1999 Elsevier ScienceLtd. All rights reserved.

Keywords:Oil shale; Pyrolysis; Pakistan; Shale oil; TGA

1. Introduction

The exploitation of oil shales represents for many coun-tries a valuable potential source of liquid hydrocarbons andenergy [1]. Pakistan has a high reliance on imported energy,estimated at 25% of the total energy used [2]. Although oilshale deposits are known in Pakistan, their development hasnot occurred, due in part to a poor knowledge of their fullextent and characteristics [1]. However, Raza et al., [3] havesuggested that the deposits are of a high grade. For theenergy potential of any oil shale to be maximised theconversion process of the oil shale to oil should be under-taken under optimal process conditions. A range of processconditions have been investigated in the pyrolysis of oilshale to ensure the maximum and efficient extraction ofthe oil. These parameters include pyrolysis temperature,heating rate, pyrolysis atmosphere, particle grain size, etc.

Heating rate and pyrolysis temperature was shown to

have the most influence on yield and composition of oil inoil shale pyrolysis studies. Research on the influence oftemperature has concentrated on the temperature range ofbetween 4508C and 6508C, with the optimum temperature ofmaximum oil yield dependent on the properties of the oilshale, particle grain size, and heating rate [4–6]. For exam-ple it was shown that an optimum temperature for the maxi-mum yield of oil is generally between 5008C and 5308C [7–9]. In addition, heating rate influences not only the oil yieldbut also the composition of the derived oil [4–6]. Shen et al.[7] concluded that the entire oil shale pyrolysis mechanismsand derived oil composition are heating rate dependent, withthe number of kerogen decomposition pathways being morediverse at higher heating rates. They also conclude however,that oil yield and the extent of any oil cracking wasgoverned by the peak pyrolysis temperature.

Thermogravimetric analysis (TGA) of oil shale sampleshave been extensively used as a means of determining thecharacteristics of devolatilisation [8–14] TGA records theloss of weight of a sample as the temperature is raised at auniform rate. A number of researchers have reported theinfluence of heating rate and final pyrolysis temperature

Fuel 78 (1999) 653–662

0016-2361/99/$ - see front matterq 1999 Elsevier Science Ltd. All rights reserved.PII: S0016-2361(98)00190-2

* Corresponding author. Tel.: 0044 0032332504; fax: 0044 1132440572.1 On leave from the Institute of Geology, Punjab University, Lahore,

Pakistan.

on oil shale decomposition using TGA. For example,Drescher et al. [13] and Rajeshwar [14] showed a distinctsystematic shift of the region of maximum rate of weightloss to higher temperatures as the heating rate was increasedfor the analysis of oil shale using TGA. Dogan and Uysal [9]and Haddadin and Mizyed [8] have investigated the influ-ence of pyrolysis temperature and shown higher weightlosses for higher temperatures of pyrolysis.

In this article, oil shale samples from two regions ofPakistan, were pyrolysed in a TGA in relation to heatingrate and temperature. Two oil shale samples were pyrolysedin a fixed bed reactor to determine the influence on oil andgas yield and gas composition in relation to heating rate andfinal pyrolysis temperature.

2. Experimental

2.1. Oil shales

The oil shales were obtained from the Kohat basin areaand Salt Range areas of northern Pakistan. The Kohat basinis approximately 350 km to the west of Islamabad andcontains rock sequences from Eocene to recent in age andare folded and faulted to produce a complex formation. Thearea contains the Kark, Malgeen and Dharangi oil shalesexamined in this paper and which are of Eocene age. Thethree oil shales are type localities representing a facieschange from north to south with a systematic increase incarbonate deposition. The Kark oil shales are variablein thickness from 0.5 to 6.0 m and are green to dark greyin appearance, medium hard and compact, brittle and thinlylaminated in character. The Kohat basin is characterized bycomplex geological structures showing east–west orienta-tion, the area is folded and faulted and is also influenced bysalt diapirism. The Eocene strata generally occur in thecores of tight, narrow and thrust bounded anticlines sepa-rated by broad synclines.

The Salt Range area contains rocks, which range in age

from Precambrian to Pleistocene, which occur as exposedscarps rising out of an alluvial plain. The sequence containsthe Salt Range oil shale formation exposed in the Khewragorge approximately 120 km south of Islamabad. The age ofthe sequence is Precambrian, although the exact age is notclear as there are no diagnostic fauna and is therefore termedInfacambrian. The Salt Range oil shales vary in thicknessfrom 3.0 to 5.0 m and are black in color with a shale char-acteristic. Two different Salt Range samples were collected,I and II, which differed in their physical appearance, the SaltRange II sample appeared distinctly darker and moreorganic rich than the Salt Range I sample. Table 1 showsan analysis of the five shale types investigated in this paper.The organic carbon and hydrogen contents of the Pakistanioil shales are higher than many oil shales found throughoutthe world.

2.2. Major and minor element analysis of the oil shales

The major and minor element components of the Kark,Dharangi and Salt Range I oil shale samples were analysedby X-ray fluorescence (XRF). The samples of oil shale wereprepared by pressing approximately 1 g of fine grained shaleinto a pellet under pressure and placed in an aluminiumholder adopting the method of Brown and Brindley [15].The system used was an ARL 9400 sequential spectrometerfitted with a 3 kW end window Rh target X-ray tube. Theanalysis was on a semi-quantitative basis. Six samples fromeach oil shale formation were taken for analysis.

2.3. Thermogravimetric analysis

Thermogravimetric analysis (TGA) of the shale sampleswere investigated using a Shimadzu Model-50 Series TGanalyser. In this work the sample was heated to 9508C at58C, 108C, 208C and 408C min21 heating rate using nitrogenas the purge gas. The TGA apparatus provides for thecontinuous measurement of sample weight as a functionof temperature and provision is made for an electronic

P.T. Williams, N. Ahmad / Fuel 78 (1999) 653–662654

Table 1Characteristics of the oil shale samples (wt%)

Eocene Infracambrian

Kark Dharangi Malgeen Salt Range I Salt Range II

Moisture 5.5 2.5 1.0 5.0 6.3Volatile matter 34.4 30.7 29.0 36.1 56.4Fixed carbon 4.1 5.7 3.2 10.3 2

Ash 61.3 63.6 67.7 53.6 50.2Total carbon 24.4 30.0 32.2 29.6 29.6Organic carbon 22.0 12.0 10.0 2 2

H 3.0 3.0 2.9 3.5 3.9N 0.08 0.52 0.34 0.84 0.6S 0.4 2 2 1.5 2

CV (MJKg21) 9.6 5.7 2 13.0 14.1Density (kg m3) 1.63 2 2 1.16 2

differentiation of the weight signal to give the rate of weightloss (DTG).

2.4. Pyrolysis reactor

A fixed bed gas purged pyrolytic reactor was used topyrolyse the two samples of Salt Range oil shale – SaltRange I in relation to heating rate and Salt Range II inrelation to final pyrolysis temperature. The influence ofthese process conditions was determined with respect toproduct yield and gas composition. The reactor consistedof a 200 cm3 stainless steel reactor externally heated by anelectric ring furnace with nitrogen as the carrier gas at afixed metered flow rate to sweep the evolved productsquickly from the reaction zone, to minimise secondary reac-tions such as thermal cracking, repolymerisation and recon-densation [16]. The furnace was controlled by aprogrammable temperature controller which enabledcontrolled heating rates which in this work for the SaltRange I sample was 58C, 108C, 208C, 308C, 408C and608C min21. The final pyrolysis temperature used was5208C and the sample was held at that temperature for onehour. The particle size of the oil shale was, 0.5 mm. Forthe Salt Range II oil shale sample investigated in relation to

P.T. Williams, N. Ahmad / Fuel 78 (1999) 653–662 655

Fig. 1. Schematic diagram of the fixed bed reactor.

Table 2Element Concentrations of Pakistani oil shales and comparison with other oil shales of the World

Elements KKa DHGb SRNG Ic Alum [19] Rundle [20] Chattanooga [21] World [22] average

Major elements (wt%)SiO2 29.0 28.0 31.0 45.0 36–60 2 58.5Al2O3 8.5 7.7 2.8 6.6 7–16 7.0 15FeO 4.0 4.0 5.4 6.0 2–10 3.0 2

MgO 3.6 5.5 7.2 0.5 1–9 0.7 2.5CaO 4.6 9.1 1.1 0.7 0.5–20 0.3 3.1Na2O 0.6 0.3 1.0 0.2 0.5–1.5 0.1 1.3K2O 1.9 2.0 1.0 4.0 1–2.5 3.0 3.2TiO2 0.4 0.4 0.2 0.4 0.4–1.1 0.2 0.8P2O5 0.1 0.1 0.1 0.7 0.4 2 0.2Minor elements (ppm)Sc 12 13 6 2 8–16 15 13V 236 276 9330 680 60–120 150 130Cr 83 91 1504 94 35–70 70 90Zn 78 212 264 150 25–85 , 300 95Ba 97 133 359 500 150–450 300 580Sr 278 183 89 2 100–750 2 300Co 22 26 24 , 50 5–35 20 19Ni 46 54 98 160 5–70 100 68Cu 21 42 14 190 10–50 100 45Y 12 12 7 40 2 30 2

Zr 65 71 36 110 2 100 2

Nb 9 9 4 2 2 2 2

Pb 12 19 69 140 32 15 30 20Th 11 11 8 2 3–7 2 12U 2 2 58 2060 0–7 50 4Se 10 10 18 2 0–3 2 1As 9 10 16 2 2 2 2

Br 8 12 2 2 3–40 2 4

a KK � Kark oil shale of Pakistan.b DHG� Dharangi oil shale of Pakistan.c SRNGI� Salt Range I oil shale of Pakistan.

final pyrolysis temperature, the heating rate was fixed at108C min21, particle grain size 0.5–1.00 mm, to finaltemperatures of 4008C, 4508C, 5008C, 5208C, 5508C,6008C and 6508C. The liquid oil phase was trapped in aseries of cold traps. Fig. 1 shows a schematic diagram ofthe fixed bed reactor.

2.5. Gas analysis

The evolved pyrolytic gases were sampled at intervals bymeans of a gas syringe and were analysed off line by packedcolumn gas chromatography. The gases were analysed forCO, H2, and CH4 using a molecular sieve SA 60–80 columnwith argon as the carrier gas and a thermal conductivitydetector. Nitrogen, which was the carrier gas used in thereactor, was also determined on this column and the volu-metric flow rates of the derived gases were calculated bycomparison with the nitrogen flow rate. Carbon dioxide wasdetermined using a silica column and argon as the carrier

gas with a thermal conductivity detector. Gaseous hydro-carbons up to C4 were determined on a Porosil C 80–100column with nitrogen as the carrier gas, using a flame ioni-sation detector.

2.6. Surface area analysis

The specific surface area of the spent oil shale samplesafter pyrolysis in relation to heating rate was undertaken bythe nitrogen adsorption method of Brunaer, Emmet andTeller (BET) [17] by means of a technique developed byNelson and Eggertsen [18]. This technique is based on thesame theory as the BET method and uses the BET equationand graphical method for calculation. However, the amountof adsorbed gas is determined by concentration measure-ment in a static system. The system used was a Quantasorb,manufactured by Quantachrome (USA). A pre-weighed drysample cell was filled with a suitable amount of sampleproviding an adequate room for the unimpeded flow ofgas above the sample surface. The sample cell wasimmersed in liquid nitrogen and the adsorbing nitrogenmixed with inert helium flowing through the system wasadsorbed onto the sample. The concentration of nitrogenadsorbed was determined from thermal conductivitymeasurements. Desorption of the nitrogen occurs when theliquid nitrogen coolant is removed and the sample warmsup, desorption being measured via the change in nitrogenconcentration in the off-gases. The three point BET theorywas used to calculate the surface area [17,18].

3. Results and discussion

3.1. Major and minor element analysis of the oil shales

Table 2 shows the major and minor element analysis ofthe Kark and Dharangi oil shale samples of Eocene age andSalt Range I oil shale of Infracambrian age analysed byX-ray fluorescence. Also shown are example analyses ofdifferent oil shale samples for comparison [19–22]. TheKark and Dharangi oil shales of Eocene age have similarmajor element compositions; however, there are somedifferences such as CaO and MgO, which are significantlydifferent between the two samples. The Infracambrian, SaltRange I oil shale contains significantly different majorelement concentrations compared to the Eocene samples,for example, Al2O3, CaO, K2O, and TiO2 are lower, whereasSiO2 FeO, Na2O and MgO are higher. In addition, the K2O/Na2O ratios in the Kark and Dharangi samples are 3–6 timeshigher than in the Salt Range I sample and the Al2O3/Na2Oratios are between 14 and 24 times higher. The SiO2/Al 2O3

ratio is lower in the Kark and Dharangi samples compared tothe Salt Range I sample.

Minor element differences between the oil shales from thetwo geological ages are shown by the very much differentconcentrations of vanadium, chromium, barium, strontium,lead and uranium found in the Kark and Dhrangi oil shales


Fig. 2. Rate of weight loss (DTG) of the Kark oil shales in relation toheating rate.

Fig. 3. Rate of weight loss (DTG) of the Dharangi oil shales in relation toheating rate.

compared to the Salt Range I sample. Most strikingly differ-ent are the very much higher concentrations of vanadiumand chromium between the two areas.

3.2. Thermogravimetric analysis

Figs. 2–5 show the differential weight loss curves (DTG)in relation to heating rate to a final pyrolysis temperature of9508C for four of the oil shale samples, the Kark, Dharangi,and Malgeen oil shales of Eocene age and the Salt Range Isample of Infracambrian age. All four oil shale samplesexhibited distinctly different patterns of thermal decompo-sition as the pyrolysis temperature was increased.

Lower temperature thermal decomposition of the oilshale, up to about 2208C produced weight loss which wasattributed to the loss of moisture, loss of interlayer waterfrom clay minerals and also decomposition of the mineralnahcolite (NaHCO3) [23] Haddadin and Mizyed [8] havealso attributed the lower temperature weight loss to physical

changes in the kerogen prior to decomposition to pyrolyticbitumen, these changes being in the form of softening of thekerogen, molecular rearrangement accompanied by therelease of gas. These lower temperature weight losseswere most significant in the Salt Range I sample and wereless for the Eocene samples of oil shale. The main region ofweight loss from about 2208C to 6008C was because of theloss of hydrocarbon material and the evolution of gases andoil vapour. The Eocene oil shale samples, i.e., the Kark,Dharangi and Malgeen exhibit a one step thermal decom-position in the main weight loss area suggesting a one stepevolution of hydrocarbon volatiles from the oil shale;whereas the Salt Range I sample exhibits a two stagedecomposition in the range 2508C–6008C, representing atwo stage evolution of hydrocarbon material. The twostage decomposition has also been observed by other work-ers for different oil shales, for example, UK oil shale [10],Jordanian oil shale [8], Turkish oil shale [9] and GreenRiver oil shale [24]. The determination of whether thedecomposition is single stage or two stage depends on thetype of oil shale [9,10] as is shown in this work. The decom-position of kerogen to oil, gas and char products were attrib-uted to a two stage process involving decomposition of thekerogen to pyrolytic bitumen and then decomposition toproducts [8,10]. The lower temperature weight loss for theSalt Range I oil shale between 2508C and 3808C is attributedto the first stage kerogen to pyrolytic bitumen decomposi-tion, followed by decomposition of the pyrolytic bitumenbetween 4008C and 6008C. The single or two stage decom-position process for the oil shales in this work is an overallweight loss recorded by the TGA, but, the actual mechanismof oil shale decomposition is a much more complex reactioninvolving a series of parallel reactions [10,23,25].

Higher temperature decomposition between approxi-mately 6008C and 9508C was attributed to the decomposi-tion of carbonate minerals such as calcite, dolomite andankerite. The Kark oil shale showed a small weight loss inthis range, whereas the Dharangi and Malgeen samplesshow a high rate of weight loss because of the presence ofcarbonate minerals. The Salt Range I sample shows only alow level of carbonates present.

Table 3 shows the data of weight loss in the low, mediumand high temperature regions discussed in terms of Figs. 2–5, but in terms of percentage weight loss. The table empha-sizes the significant weight loss because of hydrocarbons inthe medium temperature region of 2008C–6208C. In addi-tion, the high weight loss because of carbonate decomposi-tion for the Dharangi and Malgeen oil shale samplesbetween 6208C and 9508C is shown.

Table 4 shows the analysis of the thermogravimetric DTGdate in relation to heating rate in terms of the onset of weightloss, the temperature where maximum devolatilisationoccurs and the maximum rates of weight loss in thetemperature range of 2008C–9508C. Fig. 6 defines thenomenclature used in Table 4.

With an increase in the heating rate there was a lateral


Fig. 4. Rate of weight loss (DTG) of the Malgeen oil shales in relation toheating rate.

Fig. 5. Rate of weight loss (DTG) of the Salt Range I oil shales in relationto heating rate.

shift to higher temperatures forTonset, Tmax1, Tmax2 andTmax3

for the four oil shale samples as the heating rate wasincreased (see Table 4). The lateral shift is also illustratedin Figs. 2–5 for the DTG curves. The rate of weight loss alsoreflects the lateral shift with an increase in the rate asthe heating rate was increased from 58C min21 to408C min21. The lateral shift to higher temperatures forthe maximum region of weight loss was observed by otherworkers using TGA to investigate the pyrolysis of oil shale.For example, Drescher et al. [13] showed a lateral increasein the maximum rate of weight loss of about 608C as the


Table 3Weight loss in relation to pyrolysis temperature range for different heatingrates and oil shale types

Weight loss (wt%)

Temperature range (8C) 25–200 200–620 620–950 Total

KarkHeating rate (8C min21)5 5.2 25.8 4.4 35.410 4.3 25.5 7.7 37.420 4.8 26.2 4.2 33.240 4.3 26.9 8.2 39.4DharangiHeating rate (8C min21)5 2.4 11.2 18.0 31.610 2.4 12.5 15.0 29.920 2.4 11.6 16.8 30.840 2.4 11.7 15.5 29.6MalgeenHeating rate (8C min21)5 2 2 2 2

10 1.2 9.7 28.2 39.120 1.0 9.1 26.9 37.040 0.8 8.2 27.2 36.2Salt Range IHeating rate (8C min21)5 6.5 22.5 10.0 39.010 6.0 17.5 10.0 33.020 6.0 18.2 8.8 33.040 6.0 19.5 8.5 34.0

Table 4Comparison of thermogravimetric data in relation to heating rate and oil shale type

Temperature (8C) Rate of weight loss (wt% min21)

Tonset Tmax1 Tmax2 Tmax3 Rmax1 Rmax2 Rmax3

KarkHeating rate (8C min21)5 332 2 454 705 2 1.3 0.210 360 2 476 742 2 2.5 0.620 390 2 490 780 2 5.1 0.840 408 2 507 787 2 10.7 2.7DharangiHeating rate (8C min21)5 362 2 448 684 2 0.5 0.810 370 2 468 705 2 1.3 1.120 387 2 471 711 2 2.3 2.740 408 2 490 769 2 5.1 4.4MalgeenHeating rate (8C min21)10 395 2 478 831 2 0.8 3.120 410 2 490 847 2 1.3 5.740 442 2 500 865 2 2.0 11.3Salt Range IHeating rate (8C min21)5 230 302 445 2 0.3 0.5 2

10 2 2 453 2 2 1.4 2

20 259 325 478 2 1.1 2.2 2

40 278 337 491 2 2.2 3.6 2

Fig. 6. Nomenclature for Table 4.

heating rate was increased from 0.668C to 508C min21 forGreen River oil shale. Thakur and Nuttall [26] and Herralland Arnold [27] also showed a lateral shift in the maximumrate of weight loss for the TGA of oil shale samples.

3.3. Fixed bed reactor

3.3.1. Influence of heating rateTable 5 shows the yield of oil, gases, water and spent

shale from the pyrolysis of the Salt Range I oil shales inrelation to heating rate from 58C to 608C min21 to a finalpyrolysis temperature of 5208C in the fixed bed reactor.There was an increase in oil from 58C–208C min21 afterwhich the yield of oil decreased as the heating rate wasincreased to 608C min21. The yield of gases increasedprogressively throughout as the heating rate increasedfrom 58C to 608C min21, while there was a correspondingprogressive decrease in water yield and derived spent shale.

It is clear that the results from the fixed bed reactor inrelation to product yield reflect the results obtained anddiscussed earlier in relation to the TGA. The total weightloss from the TGA experiments which can be obtained fromTable 3 are similar to the total volatile loss of oil, gases andwater from Table 5 for the fixed bed reactor for the sameheating rates. However, the TGA results are in some casesslightly higher in weight loss because of the higher opera-tional temperature of the TGA and therefore would alsoproduce some weight losses because of carbonate decom-position in the 6208C–9508C temperature range.

Burnham [4] and Campbell et al. [5] have shown adistinct influence of heating rate, higher heating rates upto about 108C min21 producing increased yields of oil.Low oil yield at low heating rates they attributed to the

formation of a carbonaceous residue or coke via secondaryreactions of the liquid oil. They suggest that heating rateinfluences the formation of the autogeneous intraparticle gaswhich sweeps the oil vapours from the oil shale, higherheating rates generate greater intraparticle gas sweep ratesand less oil degradation. Table 5 shows that at heating ratesabove 208C min21 the oil yield decreases slightly.Weitkamp and Gutberlet [28] and Thiele [29] suggest thatthe pyrolysis process may be a diffusion-limited processcontrolled by heat and product diffusion. They suggestthat the extent of diffusion control increases at high heatingrates because products are generated faster than they candiffuse out of the pores, consequently secondary cokingreactions will occur. The extent of diffusion controldiminishes during the course of the reaction as the shalechanges from impervious rock to porous ash as the pyrolysisproducts leave the matrix.

It was shown previously for the TGA results that increas-ing the heating rate produces a shift to higher temperaturesfor the region of main hydrocarbon evolution. It may there-fore be summarized that at low heating rates, heat is trans-ferred into the particle generating oil products, whichdiffuse out of the sample under a low internal autogeneousgas sweep rate. This allows secondary reactions resulting inincreased coke formation and lower oil yield. Secondary gasphase cracking reactions may also result, but to a lesserextent, because the oil evolved from the particle is atlower temperatures. At higher heating rates the pyrolysisvapours are produced at a higher rate than they can escapefrom the pores in which they are located as porosity has nothad sufficient time to develop, consequently oil coking willoccur and oil yield reduced. Also as the oil diffusing out ofthe particle will meet higher temperatures, increased gasphase cracking reactions will occur, resulting in a decreasedoil yield. This gives an optimum heating rate of about 208Cmin21 in the case of the sample and reactor conditions foundin this work. Table 6 shows the surface area of the spentshale samples in relation to heating rate. Increasing the heat-ing rate to 208C min21 produces the maximum surface area,which coincides with the maximum yield of oil. In thiscontext secondary coking reactions involve the liquid oiland result in the fusion of two or more molecular speciesto form a mainly carbonaceous product. Secondary crackingreactions involve oil vapour, gas phase, bond fission reac-tions leading mainly to the formation of smaller molecularweight gaseous species [5]. Table 5 also shows that the totalgas yield increases as the heating rate is increased and thatthe spent shale decreases. This may also be linked to thesecondary coking and cracking reactions of pyrolysisvapours.

Table 7 shows the detailed analysis of the gases derivedfrom the pyrolysis of the Salt Range I oil shale in relation toheating rate. Detailed analysis of the gas composition fromthe pyrolysis of oil shale revealed the main gases werehydrogen, carbon monoxide, carbon dioxide, methane,ethane, ethene, propane, propene, with lower concentrations


Table 5Influence of heating rate on the product yield from the pyrolysis of SaltRange I, Pakistan oil shales (wt%)a

Heating rate (8C min21)

Product 5 10 20 30 40 60

Oil 8.7 10.5 12.0 11.8 11.7 11.4Gases 5.1 6.2 7.0 8.1 8.6 9.1Water 18.1 17.0 16.0 15.7 15.5 15.5Spent shale 66.2 65.0 64.4 64.2 64.0 63.6Total 98.1 98.7 99.2 99.8 99.8 99.6

a All results are the mean of three experiments.

Table 6BET surface areas of the Salt Range I spent shales in relation to heating rate

Heating rate (8C min21) Surface area (m2g21)

Raw sample 12.05 14.010 18.020 23.040 22.060 21.5

of iso-butane, butane and butene as was reported bymany authors [5,6,30,31]. The increase in the overallgas yield with increasing heating rate from 58C to608C min21 is reflected in increasing concentrations ofindividual gases.

Table 7 shows that there are increased concentrations ofhydrocarbons and hydrogen as the heating rate wasincreased indicating that significant secondary reactionsmay be occurring. In particular, the cracking of oil vapourleading to the formation of increased hydrocarbons anddecreased oil yield. Campbell et al. [31] also observedincreased hydrogen and methane concentrations inrelation to increasing heating rate for the pyrolysis ofoil shale.

Table 7 also shows the ethene/ethane, propene/propane,butene/butane ratios and the total alkene/alkane ratios. Theratio of alkene to alkane gases in the evolved pyrolysis gaseshave been used to determine reaction mechanisms and

indicate pyrolysis conditions [6,31–36]. For example, Burn-ham and Taylor [35] and Raley [34] related increasedethene/ethane and propene/propane ratios to increasedvapour phase cracking and decreased oil yield. In addition,Campbell et al. [6] reported that ethene/ethane and Propene/propane ratios were increased with increasing heating rate inthe range of 0.38C – 128C min21. Table 7 shows that therewere no conclusive trends in ethene/ethane, propene/propane and butene/butane ratios in relation to heatingrate, although alkene/alkane ratios for the lower heatingrate tended to be higher than those at the higher heatingrates. Whilst it was shown that low alkene ratios arefound at low heating rates [6], linked to secondary cokingreactions, increased alkene/alkane ratios may result as theoil emerges from the particle and is subject to subsequentsecondary gas phase reactions [32]. Similarly at high heat-ing rates, both coking which tends to reduce the alkene/alkane ratio and cracking which increases the alkene/alkaneratio may occur. Consequently for the heating rate rangeused in this work, no conclusive trend of alkene/alkaneratio was observed.

3.3.2. Influence of final pyrolysis temperatureTable 8 shows the yield of oil, total gas, water and spent

shale in relation to temperature for the Salt Range II oilshale. At the lowest pyrolysis temperature examined of4008C, the oil yield was low at only 2.5 wt%, this datareflecting incomplete pyrolysis. As the temperature wasincreased the oil yield increased until it reached a maximumat 5008C after which there was a decrease in oil yield at6508C. There was a progressive increase in gas yield from


Table 7Yield of gases from the pyrolysis of Salt Range I, Pakistan oil shales inrelation to heating rate (wt%)

Gas Heating rate (8C min21)

5 10 20 30 40 60

Non-Hydrocarbon GasesHydrogen 0.06 0.08 0.10 0.11 0.12 0.12Carbon monoxide 0.34 0.33 0.28 0.24 0.20 0.15Carbon dioxide 2.50 2.54 2.60 2.72 3.06 4.51Total 2.90 2.95 2.98 3.07 3.38 4.84Hydrocarbon GasesMethane 0.25 0.36 0.48 0.59 0.61 0.56Ethane 0.12 0.22 0.38 0.49 0.52 0.48Propane 0.10 0.12 0.17 0.20 0.23 0.24Isobutane 0.01 0.01 0.01 0.01 0.02 0.05Butane 0.03 0.03 0.04 0.05 0.06 0.07Ethene 0.06 0.07 0.09 0.10 0.11 0.11Propene 0.11 0.14 0.18 0.21 0.22 0.21Butene 0.03 0.04 0.04 0.04 0.04 0.05Total Alkanes (C1–C4) 0.76 0.74 1.08 1.34 1.44 1.40Total Alkenes 0.20 0.25 0.31 0.35 0.37 0.37Ethene/Ethane 0.50 0.32 0.24 0.20 0.21 0.22Propene/Propane 1.10 1.17 1.06 1.05 0.97 0.87Butene/Butane 1.00 1.11 1.00 0.80 0.67 0.71Alkenes/Alkanes 0.26 0.34 0.29 0.26 0.26 0.26

Table 8Influence of temperature on he product yield from the pyrolysis of SaltRange II, Pakistan oil shales (wt%)a

Product Temperature (8C)

400 450 500 520 550 600 650

Oil 2.5 14.1 16.5 16.1 15.7 14.3 12.6Gases 4.1 5.9 6.8 6.8 7.2 8.8 9.7Water 7.5 8.7 9.0 9.0 9.0 8.9 8.8Spent shale 85.0 70.4 68.0 67.5 67.4 67.2 67.0Total 99.1 99.1 99.3 99.4 99.3 99.2 98.1

a All results are the mean of at least two experiments.

Table 9Yield of gases from the pyrolysis of Salt Range II, Pakistan oil shales inrelation to pyrolysis temperature (wt%)

Product Temperature (8C)

400 450 500 520 550 600 650

Non-Hydrocarbon GasesHydrogen 0.04 0.09 0.14 0.19 0.25 0.30 0.30Carbon monoxide 0.26 0.18 0.17 0.25 0.34 0.40 0.41Carbon dioxide 2.14 2.20 2.10 2.24 2.81 3.30 3.52Total 2.44 2.47 2.41 2.68 3.40 4.00 4.23Hydrocarbon GasesMethane 0.08 0.19 0.60 0.75 0.96 1.15 1.10Ethane 0.06 0.10 0.22 0.30 0.44 0.69 0.68Propane 0.04 0.05 0.10 0.14 0.18 0.19 0.23Isobutane 0.00 0.01 0.05 0.09 0.12 0.10 0.10Butane 0.02 0.02 0.06 0.06 0.06 0.05 0.05Ethene 0.03 0.06 0.15 0.22 0.36 0.72 0.70Propene 0.04 0.07 0.14 0.21 0.30 0.42 0.58Butene 0.02 0.02 0.07 0.07 0.08 0.08 0.09Total Alkanes (C1–C4) 0.13 0.19 0.37 0.50 0.68 0.94 0.95Total Alkenes 0.09 0.15 0.35 0.50 0.74 1.22 1.36Ethene/Ethane 0.47 0.57 0.67 0.72 0.83 1.04 1.05Propene/Propane 1.11 1.26 1.41 1.56 1.70 2.14 2.46Butene/Butane 1.05 1.09 1.10 1.09 1.22 1.57 1.67Alkenes/Alkanes 0.69 0.79 0.94 1.00 1.09 1.30 1.43

4008C to 6508C, while the product water appeared to firstincrease then decrease as the temperature of pyrolysisincreased.

Other workers have examined the influence of tempera-ture on the yield of products from pyrolysis of oil shale. Forexample, Carter and Taulbee [37] and Taulbee and Khan[38] pyrolysed oil shale and showed a decrease in oil yieldcoupled with an increase in gas yield as the temperature ofpyrolysis was increased which was attributed to gas phasecracking reactions to yield increased hydrocarbons. At lowtemperatures, below 4508C, oil yields are reduced becauseof the coking reactions of the oil via conversion of the liquidoil to solid product [39]. Below about 4008C oil yields aredecreased because of incomplete pyrolysis [28]. There isconsequently an optimum temperature where maximumoil yields are obtained. For example, Jiying and Changshan[40] pyrolysed oil shales in a fixed bed reactor at 108Cmin21 heating rate and showed that the maximum oilyield was obtained at 5208C final pyrolysis temperature.In the present work, the maximum oil yield was foundfor a temperature of 5008C at a heating rate of 108Cmin21.

The influence of temperature on the yield of oil from thepyrolysis of oil shales can therefore be summarised as attemperatures below the maximum oil yield there is cokingof the liquid oil or incomplete pyrolysis and consequentdecrease in oil yield. At temperatures higher than the maxi-mum, reduced oil yields are because of secondary cokingand cracking reactions of the oil vapour to yield increasedgases.

Detailed analysis of the gas composition from the pyro-lysis of the salt Range II oil shale in relation to temperatureof pyrolysis is shown in Table 9. In all cases as the pyrolysistemperature was increased, the weight percentage of thegases increased.

Table 9 also shows that the alkene/alkane gas ratiosincreased with an increase in the temperature of pyrolysis.The increased alkene/alkane ratios indicate that increasedsecondary gas phase cracking reactions were occurring asthe pyrolysis temperature was increased. A number of work-ers [33–36] have shown that increasing ethene/ethane,propene/propane and butene/butane ratios are related toincreasing temperature of pyrolysis and in turn linked toincreased secondary reactions.

The results have shown that both heating rate and finalpyrolysis temperature influence the yield and compositionof the products of Pakistani oil shales. Of more significanceis the high yield of oil obtained from these oil shales,between 12.0 and 16.5 wt% oil depending on oil shaletype and process conditions. While there is very littleknown about Pakistani oil shales [1], this work has shownthat the deposits are potentially a viable source of oil for acountry highly dependent on imported petroleum oil andderived petroleum refined products. Consequently, furtherdetailed geological mapping and assessment of the hydro-carbon potential of the oil shales identified would highlight

the large potential of this energy source and its contributionto the Pakistan economy.

4. Conclusions

Thermogravimetric analysis of Pakistani oil shales inrelation to heating rate and final pyrolysis temperature hasshown that the maximum range of weight loss because ofhydrocarbon formation is between 2008C and 6208C. Athigher temperatures there was significant weight loss fromsome samples because of mineral carbonate decomposition.

Pyrolysis of two of the oil shale samples in a fixed bedreactor in relation to heating rate and final pyrolysistemperature was examined. Oil yield was related to bothheating rate and final pyrolysis temperature with an opti-mum heating rate of 208C min21 and optimum final pyro-lysis temperature of 5008C. The role of diffusion of heat intothe sample and products out of the sample was also high-lighted in determining oil yield in relation to secondarycracking reactions.

Analysis of the gases derived from the pyrolysis showedthat hydrogen, carbon monoxide, carbon dioxide and hydro-carbons were produced which increase in concentration asboth the heating rate and final temperature of pyrolysis wasincreased. The alkene/alkane ratio which was used as ameasure of the degree of secondary reactions were shownto be related to reduced oil yields in the case of pyrolysistemperature but were inconclusive in respect of heating rate.

Acknowledgements

The authors would like to acknowledge the support of theUniversity of Leeds technical staff, Peter Thompson, EdWoodhouse, Alan Wheeler and Rod Holt. The award of aPakistan Government Scholarship to N. Ahmad is alsogratefully acknowledged.

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influence of process conditions on the pyrolysis of pakistani oil shales

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