Application of infrared spectroscopy to the classification of kerogen types and the thermogravimetrically derived pyrolysis kinetics of oil shales

I~F UTTERWORTH E M A N N

0016-2361(95)00093-3

FuelVol. 74 No. 11, pp. 1618-1623, 1995 Copyright © 1995 Elsevier Science Ltd Printed in Great Britain. All rights reserved 0016-2361/95/$9.50 + 0.00

Application of infrared spectroscopy to the classification of kerogen types and the thermogravimetrically derived pyrolysis kinetics of oil shales Levent Ballice, Mithat YOksel, Mehmet Saglam, Hans Schulz* and Cumhur Hanoglu University of Ege, Faculty of Engineering, Department of Chemical Engineering, 35100 Bomova-lzmir, Turkey *Karlsruhe University, Engler-Bunte-lnstitute, D-7500 Karlsruhe, Germany (Received21 July 1994; revised 11 November 1994) In this study, the kerogen types of G6yniik, Beypazari oil shale from Turkey and Timahdit oil shale from Morocco were defined by infrared spectroscopy. Aliphatic, carboxyl/carbonyl and aromatic compounds were considered in order to classify the kerogen types. The IR-analysis result shows that the samples can be classified in kerogen evaluation path I for G6yniik, Timahdit oil shales and path II for Beypazari oil shale. The kinetics of thermal decomposition of G6yniik, Beypazari and Timahdit oil shales have been studied by non-isothermal thermogravimetry (TG). The weight loss data have been analyzed by Coats-Redfern and Chen-Nuttall combinations. The kinetic parameters for the decomposition of the samples were determined and discussed. (Keywords: oil shale; kerogen; infrared spectroscopy)

In oil shales, the Fisher Assay test is commonly used to define the shale oil potential and optical analysis provides additional information on the nature of the organic materials. Incident light microscopy methods (i.e. vitrinite reflectance measurements and the determination of fluorescence properties of liptinite macerals) are used to assess the maturation level of the organic materials and macerals analysis is used to determine the petrographic composition I . In the first part of this study, a method developed by Ganz and Kalkreut in 1987 was used to classify the kerogen types of oil shales. In this method, the changes in the relative intensities of the aliphatic, carboxyl/carbonyl and aromatic peaks are used to describe kerogen type and maturation level. Considerable research has recently been carried out into the kinetics of oil shale pyrolysis under both isothermal and non-isothermal conditions. They are usually represented by a single first-order reaction or multiple parallel first-order reactions. In the second part of this study, kinetic parameters for the decomposition processes related to the non-isothermal pyrolysis of oil shales taken from different regions were determined using a combination of Chen-Nuttall and Coats-Redfern models 2'3. THEORY

Application of infrared spectroscopy Infrared spectra typically give peaks at 2860 cm -1 and

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Fue11995 Volume 74 Number11

2930cm -1 for CH2 and CH 3 aliphatic groups, at 1710cm -1 for carboxyl and carbonyl groups, and 1630cm -1 for aromatic C--C bonds 1. Changes taking place in the peak intensities of the aliphatic groups were expressed by Ganz and Kalkreut 1 as: 2930cm -j + 2860cm -I A factor = 2930cm_1 + 2860cm_1 + 1630cm_1

(1)

while changes in the peak representing carboxyl/carbonyl groups are expressed as: 1710cm -1 C factor = 1710cm_l + 1630cm_1

(2)

The A and C factors can be used in a similar manner to the traditional H / C - O / C van Krevelen diagram to classify kerogen types and maturation levels. Classification of kerogen types according to A and C factors obtained from infrared spectroscopy is shown in Figure 11 . The curves for types I and III show the evaluation paths of oil shale and coal, respectively. Alginite is the predominant organic material in kerogen type I. Kerogen type I originates mainly from marine or lacustrine organic material and has a high liquid hydrocarbon yield. Type II kerogen has a significant component of terrestrial as well as marine material 14' and

Determination of kerogen type and pyrolysis kinetics of oil shales. L. Ballice et al. in 19662 for the pyrolysis of oil shale:

Table 1 Elemental and Fisher Assay analysis of oil shales Oil shales (wt%) G6ynfik

Beypazari

Timahdit

3.8 47.2 46.3 0.9 3.3 5.8 1.3 2.2

0.6 12.9 7.7 5.2 19.0 1.0 0.3 1.5

3.6 13.9 10.9 3.0 11.0 1.9 0.6 1.2

31.8 9.6 3.6 51.2

6.4 1.1 0.7 91.2

7.0 3.8 1.1 84.5

1.5 10.0 39.1 15.3 34.1

2.7 5.4 52.7 4.0 35.2

3.8 10.8 38.0 16.5 30.9

• gas, oil, coke

Ultimate Moisture C (total) C (organic) C (inorganic) (CO2) H N S (total) Fisher assay analyses Shale oil Gas Decomposition water Residue Composition of gas product Hz CO CO; CH 4 C2-C7

76.1 11.3 1.1 1.5

Elemental analyses of residue C H N S

37.9 1.7 1.5 0.8

79.6 11.4 1.3 1.3

79.4 10.1 1.5 4.3

8.5 0.3 0.03 0.3

9.7 0.8 0.2 0.8

cJ

o

~+ 0.9 0.8

f""~'~'~-

~

0,7 "~E 0.6 U

of~ 0.4

,,

Evalut ion path type I kerogen Evalution path type II kerogen

•

~ 0.3

g

(3)

The rate of decomposition of oil shale was given by Blazek (1973)2: dx d~ = kf(x)

(4)

where X-

W0 -- Wt

(5)

W o - Wf

W0 is the initial weight of the sample (mg), Wt is the sample weight at any time (mg), and Wf is the final sample weight at the end of the pyrolysis of the kerogen (mg); k is the specific rate constant andf(x) = (1 - x) for first-order reactions. If k is substituted in terms of activation energy and frequency factor, this equation can be rewritten as dT dt - A exp ~-~ (1 - x)

Elemental analyses of shale oil C H N S

o

Oil shale E ~A 1• gas, bitumen, carbon E2,k2A2

Evatution path type [II kerogen

0.2

(6)

or dx

(7)

~--f--(b) exp(~T)(l-x)

where b is dT/dt (Ref. 3). The integration of Equation (7) and rearranging gives

ln{E+_2RTln 1 "~ bR \ T2 1 - x ) -- In

E RT

(8)

This equation is linear in form and known as the ChenNuttall Model 3. Coats and Redfern (1964) developed a graphical method to determine the kinetic parameters for oil shale decomposition. According to this model, a plot of - ( l n [ - ( l n ( 1 - x ) ) / T 2 ] ) v e r s u s l / T should result in a straight line of slope E/R 2'3. The Coats-Redfern model can also be derived from Equation (8) with some rearrangement. In the present study, both models were used together in the determination of kinetic parameters. In the first step, the E value was calculated by the CoatsRedfern model and then, for a selected temperature, the found x and calculated E values were substituted in the left side of Equation (8). Next, frequency factor A was determined from the same equation.

o

u_ 0.1 <

0

I

I

I

I

I

I

I

I

I

0.1 0.3 0.5 0.7 0.9 C Factor (1710c~ 1)/(1710 ÷1630c~11

Figure 1 Classification ofkerogen types according to A and C factors obtained from infrared spectroscopy. (Numbers: 1: Green River oil shale, 2: Timahdit oil shale, 3: G6ynfik oil shale, 4: Beypazari oil shale)

shows a wider scatter, indicating transitions for some of the samples into kerogen type III I.

Mathematical procedure in the determination of thermogravimetrically derived kinetics Consider a global reaction scheme proposed by Allred

EXPERIMENTAL

Samples The investigations were performed using the following samples: Grynfik oil shale from Turkey, Beypazari oil shale from Turkey, and Timahdit oil shale from Morocco. The samples used for i.r. analysis and thermogravimetric (t.g.) analysis were prepared by crushing oil shales and then grinding in a jaw-mill until the desired particle size was obtained. The samples was sieved to less than 0.1 mm diameter. The results of elemental analysis of oil shale samples are given in Table 1. Preparation of the kerogen concentrate The kerogen-enriched sample was prepared by the

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Determination of kerogen type and pyrolysis kinetics of oil shales." L. Ballice et al. sample (0.5g) was slurried with 5N HCI and mixed with concentrated HF and maintained for 100 h at room temperature so that the silicates were removed from the mineral matrix, after which the sample was filtered, washed with hot concentrated HC1 (5 N) and hot distilled water until no chlor remains in the solid sample.

Procedure for i.r. analysis Infrared spectra were recorded on KBr discs. Shales of known weight of ~ 0.5mg were hand ground to a fine powder. Each sample was mixed with 200 mg of dry KBr powder, uniformly mixed and reground. The entire sample was transferred to a die and pressed under vacuum in the standard way. Spectra were recorded between 4000 and 400cm -1 using a Shimadzu Model spectrometer. Spectra of oil shales and chemically treated samples of oil shales are shown in Figure 2. Evaluations of the results of the spectra are given in

Table 2. Procedure for t.g. analysis T.g. analyses were performed using a nitrogen flow rate of 15cm3min -1. The powdered sample was nonisothermally pyrolysed in the temperature range 251000°C at a constant heating rate of 5 Kmin -~. The t.g. curve in Figure 3 is a modification of the original experimental curves. The methods used for determination of t.g.-derived kinetic parameters were described in the Theory section.

Z

o_ :E

u') Z

<

k,..

3

RESULTS AND DISCUSSION

Classification of kerogen type by i.r. analysis results Timahdit

• i--r7

__ujC__0 ~_

,

i

.

•

,

•

I

. . . .

2000,0 1t00.0

I

'

'

'

I t

IOOQ,O

/,OO.O

FREQUENCY, cm-1 Figure 2

I.r. spectra of oil shale samples and demineralization steps

following procedure: (1) Oil shale (2.5 g) was added to 5 N HCI (30 ml) and heated in a water bath for 2 h, after which the sample was filtered, washed exhaustively with distilled water and dried at 105°C for 30 min. Carbonate minerals were removed at the end of this operation. (2) Oil shale (1 g), after treatment with acid, was mixed with HNO3 (20 wt%) in order to remove pyrite minerals. This operation was carried out in a water bath for 2 h and then the sample was filtered, washed with distilled water and dried at I05°C for 30min. (3) The oil shale

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Spectra of three samples representing the demineralization steps are shown in Figure 2. The related band intensities at 2930cm- 1, 2860cm- 1 and 1630cm- 1 for calculation of the A factor and band intensities at 1710 cm-l and 1630 cm-I for calculation of the C factor were evaluated from the spectra. The bands at 2930 cm-1 and 2860 cm -1 correspond to asymmetric and symmetric CH2 vibrations respectively. The corresponding modes of carboxyl-carbonyl groups and aromatic C=C bonds are at 1710cm -1 and 1630cm -1. The infrared spectroscopic data for the G6yn(ik, Beypazari and Timahdit kerogen types are summarized and compared with the Green River kerogen sample in Table 2. The A and C factors indicate type I kerogen for Gfyn(ik and Timahdit and type II kerogen for Beypazari oil shale samples. The positions of the A and C factors of G6ynOk and Timahdit oil shales kerogen were shown on Figure I and compared with the value of Green River oil shale kerogen from USA because the n-alkane distribution of G6yniik and Type I Green River shale oil was found to be similar4. The results indicated that the values of the A and C factors of G6yniik and Timahdit oil shales are in good agreement with that of Green River oil shale kerogen. Thus, G6ynfik and Timahdit oil shales can also be classified as kerogen type I. A and C factors of Beypazari oil shale kerogen are in good agreement with the type II kerogen curve, so Beypazari oil shale was classified as kerogen type II. G6yniik oil shale was also investigated by Putun et al. 4 in order to determine the kerogen type and according to the position of the H/C and O/C atomic values on the van Krevelen diagram, G6yniJk oil shale kerogen was classified as kerogen type I.

Determination of kerogen type and pyrolysis kinetics of oil shales." L. Ballice et al. Table 2

Summary of the kerogen samples, kerogen type, total organic carbon contents (TOC), and i.r.-derived factors l.r. spectroscopy-derived values

Sample

Location

Kerogen type

TOC (%)

Factor A

Factor C

1 2

Green River (USA) G6yntik (Turkey)

I I

9.0 46.3

0.79 0.75

0.46 0.43

3

Timahdit (Morocco)

I

10.9

0.76

0.46

4

Beypazari (Turkey)

II

7.7

0.68

0.65

r : MASS LOSS PER MINUTE ON THE BASIS OF 1000 mg OIL SHALE 08

1600 1400

E o

0

I11 OC 11.

o

14

1200

//

R: RESIDUE

>

r-

12 E

ui n-

UJ O

os: OIL SHALE

E

rr"

E

16

nUJ n

~

/f

1

"]'

k

800

UJ I'-"

0~ =_ 0

10

1000

6OO

cO

":'-~'L'-~'--~--'~,,.~dT/dt= 5 K/rain

8

.......... 6

<

4

on-

>Q.

P, Ill

2 100

200 TIME,

Figure 3 shales)

Thermogravimetric analysis curves of oil shale samples (

<~ n,.

30O

min

, G6yniik oil shale; - - -

, Timahdit oil shale; . . . . . .

, Beypazari oil

<> B e y p a z l r i

1 5 . 2 ~-

/k

Tlmhadlt

[]

G0yn{ik

0

<>

¢

A

I.-. A A X

[] 14.2

e"

"7 "E" ¢¢

13.2

0

12.2 1

P

I

1.1

1.2

I

I

I

I

1.3 1.4 1.5 1.6 1.7 -1 TEMPERATURE, 1000xl/T, K

t

1.8

1.9

2

Figure 4 Analysis of t.g. data of G6ynfik, Beypazari and Timahdit oil shales samples by Coats-Redfern/Chen-Nuttall model combinations (r = 0.99 for all lines)

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Determination of kerogen type and pyrolysis kinetics of oil shales: L. Ballice et al. 3 Kineticparametersfor the non-isothermaldecompositionof G6yniik,Beypazariand Timahditoil shales Table

Activationenergy (kJ mo1-1)

Frequencyfactor (rain-1)

Oil shale

E1

E2

AI

A2

G6yn~k Beypazari Timahdit

13.70 26.21 14.30

35.00 66.36 28.00

0.19 1.18 0.20

20.67 3602.90 4.57

T.g. analysis Figure 3 presents the weight loss data, the weight of residue and heating rate versus time. The results obtained by t.g. analysis indicate three different weight loss regions for each of the samples. The weight loss due to the water content of the samples takes place mainly at temperatures of 120-200°C. The thermal decomposition of the kerogen of the G6yniik, Beypazari and Timahdit oil shales takes place above 400°C, with the maximum rate at ca. 430°C. At temperatures above 500°C another endothermic effect was observed as a consequence of the secondary pyrolysis (coking) of the heavy fraction produced during kerogen decomposition for the Beypazari sample and Timahdit oil shales. The kerogen decomposition region for the G6ynfik oil shale also includes this effect but it cannot be seen clearly because of the high organic carbon content with respect to the other shales. The same effect was also observed by Scala et al. 3 during pyrolysis studies and they decided that this could not be related to decomposition of minerals in the oil shale, because the same effect also appeared during the pyrolysis of concentrated kerogen. The carbonate decomposition takes place at 600°C and above depending on the oil shale type and gives a greater weight loss for Beypazari and Timahdit oil shales than for G6yniik because of the higher mineral contents. Kinetic analysis The Coats-Redfern and Chen-Nuttall model combinations were used to evaluate the kinetic parameters for two-step mechanisms. The Coats-Redfern analysis was performed by plotting -((ln[-(ln(1 - x))/T2]) vs. 1/T; the kerogen decomposition was assumed to be first-order with respect to kerogen concentration. The conversion values for all the samples calculated by Equation (5) were used to produce a graph from the Coats-Redfern equation and hence for determining the slope and intercept of the linear parts of the plot. Two sets of lines with different slopes for each oil shale type (Figure 4) indicate that two simultaneous reactions occur in the temperature range 300-500°C. Activation energies were calculated from the slopes of these lines and these activation energy values and a conversion that was calculated at any selected temperature were substituted in Equation (8) in order to calculate the frequency factor. The kinetics parameters computed for G6yniik, Beypazari and Timahdit oil shales are listed in Table 3. The specific rate constants, calculated by using the activation energies and frequency factors for each oil shale sample, are shown in Table 4. The low activation energy (El) for kerogen decomposition indicates that the decomposition of kerogen to bitumen involves the breaking of relatively weak chemical bonds. The activation energy of decomposition of the Beypazari oil shale is greater than that of the other oil

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Fuel 1995 Volume 74 Number 11

4 Specificrate constants for decomposition of oil shales (Kmin-1) Table

Non-isothermaldata (5 K min-l) Temperature (°C)

Gryniik

Beypazari

Timahdit

300 325 350 375 400 425 450 475 500

0.0107 0.0120 0.0135 0.0150 0.0397 0.0500 0.0612 0.0763 0.0900

4.813 x 10-3 6.059 × 10 -3 7.48 X 10 -3 9.1 x 10-3 0.0250 0.0390 0.0570 0.0836 0.1180

9.95 x 10-3 0.0112 0.0126 0.0140 0.0306 0.0360 0.0433 0.0506 0.0590

shales because the diffusion of organic matter through the carbonate matrix required a high temperature and relatively more energy; Berber and Okan also found a lower activation energy value for pyrolysis of decarbonated oil shale than for untreated oil shale in their investigation9. The carbonate content is not only a factor of high activation energy. The pyrolysis reactions and hydrocarbon evolution also have an effect on the value of the activation energy. When considering the n-alkane distribution in shale oil of kerogen type I and II oil shales, the selectivity of C-10-20 n-paraffins in shale oil (for kerogen type II oil shale) is much greater than that of shale oil produced from kerogen type I kerogen4'1°. In other words, kerogen type I oil shales give more highmolecular-weight pyrolysis products than kerogen type II oil shales and this high selectivity of low-molecularweight hydrocarbons also needs relatively more energy. Thermogravimetric data were also analysed by Berber et al. in order to obtain kinetic parameters for the thermal decomposition process of Gryniik oil shale from Turkey9. The experiments were conducted isothermally at five different temperatures in the range 265-460°C for both untreated and decarbonated shales. In the temperature range 265-460°C, they considered the pyrolysis reaction as follows: k Bitumen ~ Hydrocarbon and the activation energy was found for this reaction step to be 7.8kcalmol -I (32.6kJmol-l) 9. When compared with the activation energy of the second step of the pyrolysis reaction for G6ynfik oil shale in our study, the activation energy (E2 = 35 kJ mol -~) in this investigation indicated quite good agreement with the literature data. Our investigation was performed in the temperature range 280-500°C. The temperature range for the decomposition reaction of oil shale was also indicated as 300-500°C in the literature2. Thakur and Nuttall investigated the decomposition process of oil shale by evaluating the t.g. data which were divided into two regions: region 1, corresponding to the first step of the reaction in a temperature range of 300-375°C and region 2 corresponding to the second step at 375-500°C 2. In our investigations, the temperature ranges are about 280-380 and 380-500°C for the first and second step of the reaction, respectively, and in quite good agreement with literature values. The kinetics of thermal decomposition of G6yniik, Beypazari and Timahdit oil shales have been studied by non-isothermal thermogravimetry. Several research groups

Determination of kerogen type and pyrolysis kinetics of oil shales: L. Ballice et al. have also employed the non-isothermal technique for oil shale pyrolysis 11-16. Non-isothermal thermogravimetric analysis offers certain advantages over the isothermal method. First, this method eliminates the errors introduced by the thermal induction period, and second, it permits a rapid scan of the whole temperature range of interest 2.

ACKNOWLEDGEMENTS

CONCLUSIONS

REFERENCES

Oil shales can be classified using an i.r. analysis technique. In this method, aliphatic, carboxyl/carbonyl and aromatic compounds can be used to classify the kerogen types. G6ynfik, Beypazari and Timahdit oil shales were studied using this technique and the types of kerogen were found to be evolution path type I for G6yniJk and Timahdit oil shales and evolution path II for Beypazari oil shale. The combined use of C o a t s - R e d f e r n and C h e n Nuttall models for the determination of the kinetic parameters of oil shales has shown two consecutive reactions with bitumen as an intermediate product. The reactions are first-order. It is clear that the calculation of kinetic parameters using the C h e n - N u t t a l l model needs repeated regression analysis. This method required an initial guess of E in order to calculate the left side of Equation (8) and an iteration until the desired accuracy for E and A (frequency factor) was achieved 3. The determination of activation energy by the Coats-Redfern method is very easy and it provides an accurate assessment of E (activation energy) for the C h e n - N u t t a l l model and there is no need for the iteration steps of the repeated regression technique.

The authors are grateful to the University of Karlsruhe, Engler-Bunte-Institute, Department of Gas, Petroleum and Coal for elemental and thermogravimetric analysis of oil shales and Deutsche Akademische Austauschdienst ( D A A D ) for financial support.

1 2 3 4 5 6 7 8 9 I0

11 12 13 14 15 16

Ganz, H. and Kalkreut, W. Fuel 1987, 66, 708 Thakur, D. S. and Nuttall, H. E. Jr lnd. Eng. Chem. Res. 1987, 26, 1351 Skala, D., Kopsch, H., Sokiq, M., Neumann, H. J. and Jovanoviq, J. Fuel 1987, 66, 1185 Putun,E., Akar, A., Ekinci,E. and Bartle, K. D. Fuel 1988,67, 1106 Skala, D., Kopsch, H., Sokig, M., Neumann, H. J. and Jovanoviq, J. Fuel 1990, 69, 490 Evans,E., Batts, B. and Cant, N. Fuel 1987, 66, 326 Ziyad,M., Garnier, J. P. P. and Halim, M. Fuel 1986, 65, 715 Skala,D., Kopsch, H., Soki~, M., Neumann, J. J. and Jovanovig, J. Fuel 1989, 689 168 Berber,R. and Okan, Y. in Proceedings of International Conference on Alternative Energy Sources VI, (Ed. T. N. Veziropi), Hemisphere Publications, 1985, Vol. 3 Ballice,L., Steen, V. E., Schulz, H. and Yiiksel, M. in Proceedings of 27. Internationales Seminar fiir Forschung und Lehre in Chemieingenieurwesen Technischer und Physikalischer Chemie an der Universitht Karlsruhe (TH), Germany, Wissenschaftliche Abschlussberichte 27. Internationales Seminar Juli 1992, p. 70 Campbell,J. H., Kosnikas, G. and Staut, N. Fuel 1978, 57, 372 Chen,W. J. and Nuttall, H. E. Paper presented at the 86th AIChE National Meeting, Houston, TX, 1979 Granoff,B. and Nuttall, H. E. Fuel 1977, 56, 234 Haddadin,R. A. and Mizyet, F. A. Ind. Eng. Chem. Process Des. Dev. 1974, 13(4), 332 Herrell,A. Y. and Arnold, C. Jr Thermochimica Acta 1976, 17, 165 Rajeshwar,K. Thermochimica Acta 1981, 45, 253

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