Energy &Fuels 1987,1,477-483
477
Formation of Soluble Products from Thermal Decomposition of Colorado and Kentucky Oil Shales Francis P. Miknis,* Thomas F. Turner, Gretchen L. Berdan, and Paul J. Connt Western Research Institute, Laramie, Wyoming 82071 Received June 8, 1987. Revised Manuscript Received July 29, 1987
Isothermal pyrolysis studies have been conducted on a Green River Formation oil shale from Colorado and a New Albany oil shale from Kentucky. The conversion of kerogen to bitumen, oil, gas, and residue products was obtained for different reaction times in the temperature range 375-425 O C . In addition, elemental analyses, NMR carbon aromaticities, and molecular weights were obtained for the produced oil and bitumen. The maximum amount of extractable bitumen in the New Albany shale was 10% or less of the original kerogen at any given temperature, indicating that direct conversion of kerogen to oil, gas, and residue products is a major pathway of conversion of this shale during pyrolysis. In contrast, a significant fraction of the Colorado oil shale kerogen was converted to the intermediate bitumen during pyrolysis. The bitumen data imply that the formation of soluble intermediates may depend on the original kerogen structure and may be necessary for producing high yields by pyrolysis. When the pyrolysis reaction was quenched after 20 min at 425 "C, 95% of the kerogen in Colorado oil shale was converted to soluble products in the form of oil and bitumen. In general, the hydrogen-to-carbon ratios, carbon aromaticities, and molecular weights of the oils from each shale were fairly constant at all times and temperatures, whereas for the bitumen these properties varied with time and temperature. However, there were differences in the oil properties between the two shales that reflect the differences in the original kerogen structures.
Introduction Because oil shale must be heated to produce synthetic liquid fuels, pyrolysis is an important first step in any conversion process. Although the pyrolysis of oil shale has been studied for decades, relatively little is known about the chemical reactions that occur during pyrolysis. Generally, previous studies have measured the overall rate of shale weight loss or rates of oil and/or gas production without regard to the chemical functionalities in the kerogen. While the results have been useful for modeling retorting processes, they have not been adequate in predicting product yields for different types of oil shale. Since the kerogen in oil shales exists in a variety of structural assemblages, understanding the chemistry of kerogen pyrolysis requires investigations of the pyrolysis kinetics of several different types of kerogens. For example, eastern U.S. oil shales have a more aromatic kerogen structure than western U.S.oil shales and produce about half as much liquid product during pyrolysis as western oil shales for the same weight of kerogen.' Therefore, understanding differences in the pyrolysis chemistry is important for the development of a general chemical kinetics model of kerogen decomposition. In general, oil shale thermal decomposition is considered to be a first-order rate process, proceeding through different intermediate stages to final products. Various decomposition schemes have been proposed to model the decomposition process.2 One of the most frequently used decomposition models is shown in eq 1,where k l and k , kerogen
kl
bitumen
k2
oil
+ gas + residue
(1)
are the rate constants for the kerogen and bitumen decomposition, respectively. Although this type of kinetic model may be satisfactory for a global or macroscopic description of kerogen decomposition, it does not provide insight into which chemical functionalities in the kerogen 'Permanent address: Jardine Rte. Box 71, Gardner, MT 59030.
0887-0624/87/2501-0477$01.50/0
structure are responsible for the product yields and distributions. Kerogen is defined as that portion of the organic matter in oil shale that is insoluble in common organic ~olvents.~ It is essentially a "black box" from which products evolve upon application of heat. Bitumen is defined as the benzene-soluble organic material formed during the heating period that is nonvolatile and remains in the shale sample. Organic residue is defined as the benzene-insolubleportion of the kerogen remaining in the spent shale. A common feature of oil shale decomposition models is the bitumen intermediate."12 Although bitumen is usually considered an important intermediate in kerogen decomposition, its chemical and physical properties have not been thoroughly investigated. Therefore, the role of a bitumen intermediate in oil shale decomposition is not satisfactorily understood. Historically, models such as eq 1were proposed for oil shales containing type 1 kerogen, which is highly aliphatic and which exhibits high conversions to oil during pyrolysis. These kerogens-which include Colorado and Baltic oil shales, Australian torbanites, and sapropelic coals-show similar first-order rate constants for kerogen (1)Miknis, F.P.;Netzel, D. A.; Smith, J. W.; Mast, M. A.; Maciel, G. E.Geochim. Cosmochim. Acta 1982,46,977-984. (2)Rajeshwar, K.; Nottenburg, R.; DuBow, J. J. Mater. Sci. 1979,14, 2025-2052. (3)Hubbard, A. B.; Robinson, W. E.USBM Rep. Inuest.-US. Bur. Mines 1950.No. 4744. (4)Allred, V. D. Chem. Eng. Prog. 1966,62,55-60. (5)Braun, R. L.;Rothman, A. J. Fuel 1975,54,129-131. (6)Johnson, W. F.; Welston, D. V.; Keller, H. H.; Couch, E.J. Q. Colo. Sch. Mines 1975,70, 237-272. (7)Drescher, E. A,; Bassil, C. A.; Rolinski, E.J. In Alternative Energy Sources u. Part D BiomasslHydrocarbons/Hydrogen; Veziroglu, T. N., Ed.; Elsevier: Amsterdam, 1983;pp 307-324. (8) Rajeshwar, K. Thermochim. Acta 1981,45,254-263. (9)Wallman, P.H.; Ta", P. W.; Spars, B. G. ACS Symp. Ser. 1981, NO.163,93-113. (10)Wen, C. S.;Kobylinski, T.P. Fuel 1983,62,1269-1273. (11)Shen, M. S.;Fan, L. S.;Castleton, K. H. Prepr.-Am. Chem. SOC., Diu. Pet. Chem. 1984,29(1),127-134. (12)Richardson, J. H.; Huss, E.B.; Ott, L.L.; Clarkson, J. E.;Bishop, M. 0.; Taylor, J. R.; Gregory, L. J.; Morris, C. J. Lawrence Liuermore Lab., [Rep] UCID 1982, UCID-19548.
0 1987 American Chemical Society
Miknis et al.
478 Energy & Fuels, Vol. 1, No. 6,1987
Table I. Material Balance Fischer Assay Data for Colorado and Kentucky Oil Shale Fischer Assay Fischer assav oil yield, gal/ton oil, wt 70 gas, wt %
Colorado
Kentucky
52.2 19.3 4.9
13.4 5.3 2.9
Fischer assay water, wt 70 spent shale, wt %
Colorado
Kentucky
1.4 74.4
1.7 89.5
Elemental Composition raw shale Colorado Kentucky carbon, wt % hydrogen, wt % nitrogen, wt % sulfur, wt % carbonate carbon, wt % organic carbon, wt % a
26.9 3.4 0.8 0.5 4.3 22.6
spent shale Colorado Kentucky
13.4 1.5 0.5 5.2 0.3 13.1
11.1 0.3 0.6 0.3 5.5 5.6
oil
gas
Colorado
Kentucky
83.7 11.6 2.1 0.5
84.8 10.2 3.3 1.4
58.9 21.4 0.5
46.1
83.1
84.8
58.9O
30.4"
8.2 0.5 0.6 3.9 0.3 1.9
Colorado
Kentucky 30.4 11.5
CO and C02 in gas are considered to evolve from the organic portion of the shale.
decomposition.6 Eastern Devonian and Mississippian shales have a more aromatic kerogen structure than do the Tertiary oil shales from the Green River F0rmati0n.l~ Little attention has been paid to whether the bitumen is a significant reaction intermediate in the pyrolysis of these more aromatic shales. There is a paucity of published data on the time dependence of the chemical and physical properties of oil shale pyrolysis products. In particular, there is very little information on the bitumen intermediate. For a long time, the data of Hubbard and Robinson3 represented the most comprehensive set of data on the thermal decomposition of Colorado oil shale. However, even in that study, chemical and physical properties of the reaction products were not measured. Instead, overall decomposition data were reported, which subsequently have been used by others4+ to develop global models of kerogen decomposition. For eastern oil shales, comprehensive isothermal decomposition data have not been reported, and the significance of the bitumen intermediate has not been addressed for these shales. As a first step in a systematic approach toward understanding the relationship between kerogen structure and its conversion during pyrolysis, isothermal decomposition studies have been conducted on a Colorado and a Kentucky oil shale in the temperature range 368-425 "C. The goal of this work is to relate the chemistry of kerogen thermal decomposition to the kerogen structure. By understanding these relationships, it may be possible to improve the efficiency of the conversion processes or to develop novel approaches for greater conversion of kerogen to oil. This understanding is particularly important for eastern oil shales, which have high organic matter contents but produce little oil by conventional pyrolysis methods.
Experimental Section Oil Shale Samples. Material balance Fischer assay data for the Colorado and Kentucky oil shales are reported in Table I. The Colorado oil shale was a Piceance Creek Basin, Mahogany Zone oil shale obtained from the Anvil Points mine in Garfield County, CO. T h e 52 gal/ton richness was the result of cutting homogenous bands of shale from a larger block of shale. The Kentucky oil shale was a Clegg Creek member, New Albany shale from Bullitt County, KY. Both oil shales were crushed and sized to obtain a 20/45 mesh particle size distribution for the decomposition studies. The initial bulk sample of this size distribution was thoroughly mixed and then successively riffled to obtain aliquots of approximately 22 g. Pyrolysis studies were conducted on 20-g samples taken from these aliquots. ~
~~
(13) Miknis, F. P.;Smith, J. W. Org. Geochem. 1984,5,193-201.
P r r r r u r s Relief
Brooks F l o w
3 / 0 " l o 1/4" Swogelock Reducer Unions
3 / 0 " O.D. R e o d o r Tube
ne2 0 x 4 5 M e s h S h d e (209.)
/ 3 - W a r Ball Valve
Fluidized Sand
Control Thermocoupl
Heating Elements
-Fluidization
Air
Figure 1. Fluidized sand-bath reactor system. Apparatus. T o satisfy requirements of rapid heatup and quenching, temperature stability, and efficient product collection, several different reactor configurations were tested.I4 A heated sand-bath reactor system (Figure 1) was found to best meet these requirements and was used to generate most of the data in this report. The reactor was built around a Techne Model SBL-2D sandbath heater equipped with a proportional temperature controller, which maintained the temperature in the working volume of the sand bath constant to 1 "C. As shown in Figure 1,20 g of shale was placed inside a 3/8 in. 0.d. stainless-steel U-tube reactor. The shale temperature was measured with a in. 0.d. stainless-steel sheathed thermocouple, which extended into the inlet leg of the shale bed. The outlet line was heat traced to maintain all portions hotter than 250 OC to prevent condensation and refluxing in the outlet line. Twenty-gram samples were quickly heated to reaction temperature by immersing the tubular reactor containing the shale sample into the preequilibrated, fluidized sand bath. The reaction was quenched by removing the reactor from the sand bath and spraying liquid COz on its surface. A helium sweep gas flow rate of 30 cm3/min was used to remove the products from the reactor. Liquid product was collected in a dry ice cooled trap while gaseous products were analyzed by gas chromatography, either by collecting the total gaseous product in an evacuated vessel or by analyzing the product gas on-line. T h e amount of bitumen was determined by extracting the residue shale from each pyrolysis experiment with benzene for 24 h in a Soxhlet apparatus. However, the bitumen was unstable and formed an intractable material with time. Therefore, analytical measurements on the bitumen were obtained as soon as possible after the extraction. Typical heat-up and cool-down characteristics of the sand bath reactor are shown in Figure 2 for 30-min isothermal runs at 400 (14) Conn, P. J.; Rollison, H. J.; Miknis, F. P. Report DOE/FE/ 60177-1791; Western Research Institute: Laramie, WY, 1984.
Thermal Decomposition of Oil Shale
Energy & Fuels, Vol. 1, No. 6,1987 479
soa
400 '0 pi
300
!I E
r
6o
200
100
20
-
:
:
4 0
6
'0
100
200
300
400
500
600
700
800
b)
Kdo Colorado
100K e n t200 ucky
400
500
600
I
c
1
I
I
800 0 J
100
200
300
400
500
600
700
800
20
40
60
80
100
120
140
160
300
700
I 0
I
1
I
I
I
I
I
A
0
Time, min
Figure 3. Plots of the bitumen formation from Colorado and Kentucky oil shales versus time a t (a) 375, (b) 400, and (c) 425 "C. will be reported in future publications.
(15) Miknis, F. P.; Conn, P. J.; Turner, T. F. Report DOE/FE/ 60177-2288; Western Research Institute: Laramie, WY, 1985.
Results and Discussion Bitumen Formation. One objective of this study was to investigate the transient nature of bitumen during thermal decomposition of oil shales. Comparisons of the bitumen concentration from the Colorado and Kentucky oil shales are shown in Figure 3. These data are plotted as a percentage of original organic carbon in the raw shale to facilitate comparison. The data for the Colorado oil shales at 368 and 394 "C were obtained on an tubular reactor heated by an electrical furnace.14 The 425 OC data and all data for the Kentucky shale were obtained by using the sand-bath-heated reactor. These data show that in the temperature range 375-425 "C the bitumen concentration in the Kentucky New Albany shale is less than 10% of the original kerogen. However, the Colorado oil shale shows a sizeable bitumen concentration, reaching a maximum value of about 58 % at 425 "C. In addition, for the Colorado oil shale (which contains 25% aromatic carbon), 69% of the kerogen carbon is converted to oil during Fischer assay, whereas for the New Albany shale (which contains 49% aromatic carbon), only 33% of the kerogen carbon is converted to oil during Fischer assay. The data indicate that (1)only a portion of the New Albany shale kerogen is capable of forming a soluble intermediate, the remaining portion forming residue, oil, and gas directly from the kerogen, or (2) bitumen formation is a major pathway for kerogen conversion and the low bitumen.concentration results from the low relative rate of formation compared to decomposition of the bitumen. Oil shale decomposition models have been proposed that include a bitumen intermediate (Table 11). The maximum concentration of bitumen and the time at which this occurs have been calculated for each of the models at a temperature of 425 "C by using reported values for the overall rate constants, k, and k2, of the kerogen and bitumen decomposition. The temperature at which the rate constants kl and k2 are equal has also been calculated. Braun and Rothman's model5 is the only one based on experi-
Miknis et al.
480 Energy & Fuels, Vol. 1, No. 6,1987
Table 11. Summary of Oil Shale Decomposition Models
model"
KLG+B+R
rate constants at 425 "C, s-* kl kz
fraction of kerogen max % converted bitumenb to bitumen at 425 "C Colorado Oil Shale 0.62-1.0 44-72
time for bitumen to reach max; s 336
temp at which k , = k,,d "C
ref
487
5
356
8
364
9
673
10
0.0067
0.010
1.283
8.033
1
11
0.0022
0.0005
0.24
15.4
1'
55
le
36 58
468 1200
431
62
192
481
B%G+O+R
KLB
BSG+O+R K LLHC + B + G B L HO + G + c
K L B + G + O + C31.03
B h G + o + c + PO PO % ! G+0 C not given
12.05
0.30 882 0.048
+
0.0021
0.0022
12
f
Kentucky Oil Shale
KLG+O+B BSG+O+R
0.0110
0.0020
0.90
not given
0.0071
0.0033
le
11
51
204 274 12 300 f 'Key: K, kerogen; B, bitumen; C, coke; G, gas; R, residue; 0, oil; HO, heavy oil; LHC, light hydrocarbons; PO, polyoil. Max. bitumen = ( k l / k 2 ) k Z / ( k 2 - k lCTime ). of bitumen max = [In ( k 2 / k l ) ] / ( k 2- k , ) . T ("C) = (E, - E2)/R In (A,/A2)- 273, where E = activation energy, A = frequency factor, and R = gas constant. e Authors' assumed value. fThis work. 10
mental data for which the bitumen was actually measured. However, there are discrepancies for all models between the properties of the bitumen predicted from the rate constant data and those measured in this study. For the Colorado oil shale, the data a t 425 "C (Figure 3) show that the maximum concentration of bitumen is about 58%, and this occurs at about 20 min. This means that the bitumen decomposition is the rate-controllingstep for oil production at this temperature. Moreover, the data indicate that direct conversion of kerogen to oil, gas, and residue does occur, but to a lesser extent. Braun and R ~ t h m a n using , ~ Hubbard and Robinson's data,3 have shown that for Colorado oil shale, the bitumen decomposition is rate controlling for oil production up to 487 "C, i.e., the temperature at which kl = k,. At 425 "C, their rate constant data predict maximum bitumen values between 44% and 72%, depending upon what initial fraction of kerogen, f l , is considered to form bitumen. Their analysis indicates that this value lies between 0.62 and 1.0. A value of f i = 0.80 provides excellent agreement with our data for the Colorado oil shale, except for the time for the bitumen to reach ita maximum value. Rajeshwars used nonisothermal TGA methods to study the decomposition of Colorado oil shale kerogen. Using the average values of the Arrhenius parameters reported in that study, the maximum bitumen concentration is predicted to be only 11%and the time at which this occurs is only 0.005 min. In addition, the temperature at which the bitumen decomposition ceases to be rate controlling is 356 "C. The data reported here are clearly in disagreement with both of these predicted values from Rajeshwar's data. Wallman et al? used a fluidized-bed reactor to study the decomposition of Colorado oil shale in the temperature range 482-538 "C. These data predict that bitumen decomposition becomes rate controlling at a temperature above 364 "C. The reason for this is that in their study the activation energy for the kerogen decomposition was found to be greater than that of the bitumen decomposition. This behavior is the opposite of that reported by Braun and Rothman5 and Rajeshwar.8 Wallman et al.'s datagpredict a maximum yield of bitumen of about 15% because only 24% of the initial kerogen forms a bitumen
intermediate in their model. However, their predicted time at which the maximum bitumen occurs agrees more closely with our data. Wen and Kobylinski'O used TGA and DSC measurements to propose a thermal decomposition model for Colorado oil shale that involves an intermediate bitumen and an intermediate polyoil (Table 11). Their reported activation energy and frequency factor data predicted a maximum bitumen concentration of 55% (assuming all the kerogen converts to bitumen). However, their data also predicted that this would occur at a reaction time of about 50 ms, and the temperature at which k l = k2 is predicted to be 673 "C. For the Kentucky New Albany oil shale, no data have been reported on bitumen formation during isothermal pyrolysis. However, Shen et al?l have employed a bitumen intermediate to fit their TGA weight loss curves in their nonisothermai decomposition study of a Kentucky Sunbury oil shale. Their data indicate that the bitumen decomposition is rate controlling up to 480 "C, a value close to that of Braun and Rothman5 for the Colorado oil shale (Table 11). In addition, 90% of the kerogen decomposed to form bitumen, and only 10% of the initial kerogen forms gas and residue by direct conversion. Their data show that the maximum concentration of bitumen should be 62% at a reaction time of about 3 min. The results of our study show a maximum bitumen value of 10% at about 5 min, indicating that less than 90% of the initial kerogen forms the bitumen intermediate or that the temperature at which the bitumen ceases to be rate controlling is much lower than 480 "C in eastern oil shales. The differences in kerogen structure and conversion to oil between the Kentucky Sunbury shale and the Kentucky New Albany shale are not substantial enough to account for this discrepancy.13 The data of Shen et al.ll can be reconciled with the data reported here if 27% or less of the initial kerogen forms an intermediate bitumen; the remaining 73% forms gas, oil, and residue directly from the kerogen without passing through the intermediate state. Because the overall conversion to oil is about 33% for the New Albany shale, direct residue formation could be a major pathway for the kerogen decomposition, and at least two-thirds of the oil
Thermal Decomposition of Oil Shale 40
r
0
Energy &Fuels, Vol. 1, No. 6,1987 481
Kentucky
100
I
8
I
1
I
I
I
200
300
400
500
600
700
800
r 5
f
0
601
b)
0
I
I
I
I
I
I
20
40
60
80
100
120
Time, min
Figure 5. Sum of the oil and bitumen produced from Colorado and Kentucky oil shales at 425 "C.
0
20
40
I
I
1
,
I
1
60
80
100
120
140
160
Time,min
Figure 4. Plota of the oil generation from Colorado and Kentucky oil shales versus time at (a) 375, (b) 400,and (c) 425 "C.
would be generated directly from the kerogen as well. Given the high aromatic carbon content of New Albany shale, it is possible that a certain portion of the kerogen is already "residue". Unfortunately, there is no known way to discriminate between a residue that is formed and one that is already part of the original kerogen. The conversion data for the Kentucky shale indicate that (1) residue formation is a major path for the bitumen decomposition, (2) residue formation is a major path for the kerogen decomposition, or (3) some combination of 1 and 2 is occurring. Any of these three possibilities can account for the low overall conversion to oil from the Kentucky shale. However, with the data reported here, it is not possible to ascertain the relative importance of each of these reaction paths. Nonetheless, the conversion data of both shales, coupled with the differences in the parent kerogen structures, suggest that the original kerogen structure may a priori determine how much bitumen and oil will be generated during pyrolysis. The caveat of the preceding discussion is that caution must be exercised when kinetic data are reported for oil shale decomposition models that include an intermediate, particularly if experimental data are not obtained for the intermediate to support the kinetic model. Indeed, oil production5J5and weight loss datal1 can be fit by models incorporating an intermediate bitumen, yet the same kinetic parameters do not reliably predict the thermal behavior of the bitumen.15J6 Moreover, as will be discussed later, the chemical composition of the bitumen varies with time and temperature, which makes it an unsatisfactory entity for reaction modeling. It is our opinion that the exact role of bitumen during oil shale thermal decomposition has not been satisfactorily explained and that it warrants additional investigations. (16) Miknis, F. P.; Conn, P. J.; Turner, T. F.; Berdan, G. L. Report DOE/FE/60177-2213; Western Research Institute: Laramie, WY, 1985.
Oil Generation. Comparisons of the oil productions from the Colorado and Kentucky oil shales are shown in Figure 4. In all cases, the ultimate oil production from the Colorado oil shale is greater than that for the Kentucky oil shale. However, in all cases, the initial rate of oil production from the Kentucky oil shale is greater than that for the Colorado oil shale. These data are consistent with the bitumen data shown in Figure 3. For example, at 425 "C (Figure 3c) there is a sufficient amount of bitumen remaining in the Colorado oil shale to generate oil for times longer than 20 min. For the New Albany shale, most of the oil production has occurred by this time, and what little bitumen is formed has generated all of its oil by this time. If the oil generation and bitumen formation were plotted on the same graph, the maximum rates of oil production would not occur at the same times that the bitumens reach their maximum values. This means that oil and bitumen are produced simultaneously from the kerogen decomposition as in some of the models shown in Table 11. This effect is most noticeable for the Kentucky oil shale. A particularly significant result from this work is illustrated in Figure 5 where the sum of the oil and bitumen is plotted versus time at 425 "C for both shales. For the Colorado oil shale, the data show that after 20 min at 425 "C, about 95% of the kerogen carbon is converted to carbon in soluble material in the form of bitumen (58%) and oil (37%). Thus, by quenching the reaction at the time the bitumen reaches its maximum, practically all the insoluble kerogen is recoverable as soluble products by pyrolysis. The fact that values significantly less than 95% are obtained during oil shale retorting processes is a result of what happens to the bitumen during its decomposition to the products: oils, gas,and residue. The residue carbon is a result of coking of the bitumen, which agrees with the recently reported results of Adams and Mahajan." Therefore, the ultimate oil yield will always be less than that shown by the maximum in the oil + bitumen curve for processes in which the bitumen is allowed to completely decompose. Gas generation also contributes to the lower oil yields. The data show that the bitumen + oil curve approaches the oil curve at long reaction times (Figure 5). At 120 min, the total conversion of kerogen carbon to oil carbon is 74%. This is significantly lower than the 95% conversion at 20 min and is in the range of Fischer assay conversions. For the Kentucky shale the time dependence of the oil and bitumen (Figure 5) show that the amount of bitumen formed is not sufficient to produce a maximum in the oil (17)Adams, D.C.;Mahajan, 0.P. Energy Fuels 1987,1, 23-28.
Miknis et al.
482 Energy &Fuels, Vol. 1, No. 6,1987 1000 1600
-0
a)
800 0
400 600
1200
200
-5a r"
/Oil
-
-e-'.
0
800
Y B i t u men
1
0
"
20
'
40
I
60
80
I
t
I
I
100
120
140 160
120
140 160
Time, min 400
Aliphatic C a r b o n
t
01
'
I
I
40
I
1
1
120
80
Time, min
0.0
-
Aromatic C a r b o n
0.2
0
20
40
60
80
100
Time. min
08
2.0
r
,Oil
E
0.01
I
I
I
I
40
80
I
3
0 .05 0
I
120
20
40
2'o
a
1.8
-
Oil
*
:
120
140 160
Table 111. Average Properties of Kentucky and Colorado Shale Oils
I
Kentucky Colorado
< IO o
100
Molecular weights (a),carbon fractions (b), and hydrogen to carbon ratios (E) of oils and bitumens versus time at 425 O C for Kentucky oil shale.
r
,. -
80
Time, min
Time, min
.-9
60
40
80
120
lime, min
Figure 6. Molecular weights (a), carbon fractions (b), and hydrogen to carbon ratios (c) of oils and bitumens versus time at 425 "C for Colorado oil shale.
+
bitumen curve. As a result, no improvement in conversion to total soluble products can be obtained by quenching the reaction at a specific time as was done for the Colorado oil shale. However, it has been demonstrated that yields greater than Fischer assay can be obtained from eastern shales by hydropyroly~is,'~J~ donor solvents,20superfluid extractioq21 and rapid heating.12p22 From our (18)Feldkirchner, H.L.: Janka, J. C. Synth. Fuels Oil Shale, S -y m-n Pap. 1979 1980,489-524. (19)Kahn, D.R.;Falk, A. Y.; Varey, M. P. Proceedings: I983 Eastern Oil Shale Symposium: Institute for Mining and Minerals Research. University d f Kentucky: Lexington, KY, 19B3;pp 225-233. (20)Cronauer, D. C.; Solash, J.; Danner, D. A.; Galya, L. G. Proceedings: 1983 Eastern Oil Shale Symposium; Institute for Mining and Minerals Research, University of Kentucky: Lexington, KY, 1983;pp 17-23.
H/C
carbon aromaticity
1.50 i 0.06 1.71 f 0.03
0.42 f 0.04 0.20 f 0.02
mol w t 257 f 16 358 f 40
resulta, it would appear that these increased oil yields are produced by effectively blocking the residue formation steps. However, more detailed studies are needed to understand the mechanisms for these improved yields. Other Properties of the Oil and Bitumen. The use of isothermal methods allowed for other measurements of the oils and bitumens as a function of reaction time. In this study, the elemental compositions, carbon aromaticities, and molecular weights were determined for the oils and bitumens. Results of these measurements are shown in Figures 6 and 7 for the 425 "C experiments. Similar results were obtained at the lower temperatures.15J6 For the shale oils the H/C ratios, carbon aromaticities, and molecular weights were remarkably constant at all times and temperatures for each type of shale oil. Average values from all the time/temperature data are given in Table 111. The differences in average properties of the two shale o h reflect the differences in the original kerogen (21)McKay, J. F.; Chong, S. L. Proceedings: 1983 Eastern Oil Shale Symposium; Institute for Mining and Minerals Research, University of Kentucky: Lexington, KY, 1983: pp 235-239. (22)Carter, S.;Taulbee, D.; Collins, D. Proceedings: 1983 Eastern Oil Shale Symposium; Institute for Mining and Minerals Research, University of Kentucky: Lexington KY, 1983;pp 113-122.
Thermal Decomposition of Oil Shale
structure, Le., the more aromatic nature of the Kentucky oil shale kerogen. Chemical properties measurements of the bitumen were limited by the small amounts of bitumen extracted from the 20-g samples of shale. This was especially true for the Kentucky oil shale in which little bitumen was extracted throughout the decomposition and for the Colorado oil shale at longer reaction times where little bitumen remained. In general, the composition and properties of the bitumen change during the course of the reaction. This is most evident from the molecular weight data in Figures 6a and 7a. These data indicate that the bitumen molecular weight passes through a maximum during the decomposition. This effect had been noted previously by MityureP in his analysis of the thermal decomposition of Baltic shales. Because of this, Mityurev concluded that a kinetic intermediate whose composition changes with time is untenable in a reaction sequence. Instead, Mityurev considers bitumen to be a mixture of tar and unconverted kerogen. The changing composition of the bitumen is attributed to the variation in the amount of unconverted kerogen in the tar at any given time. Allred’s postulate4 of an autocatalytic reaction between the bitumen and kerogen is also a consequence of the changing composition of the bitumen. More recently, Sunberg et al.24125have shown that the molecular weights of the bitumens differ significantlyfrom the oils produced during rapid pyrolysis of Colorado oil shale. They suggest that oil production is partly vaporization controlled and that current models are inadequate for predicting oil release under conditions of rapid heating. The reason for this is that the details of the reactions for formation and destruction of the bitumen are not yet satisfactorily understood, and may be related to the nature of the cross-links in kerogen.2e This is a neoteric concept in oil shale studies although in coal research, the nature of the cross-links has been shown to play a significant role in the amounts of tars and extractables formed during pyrolysis.*’ The differences in the product distribution (including the bitumen) between the Colorado and Kentucky oil shale may be governed by the nature of the cross-links in the parent kerogen. For example, the low oil yields from the Kentucky shale may be the result of a more highly cross-linked kerogen, which upon heating produces more char than oil. Summary Despite the large number of studies that have been conducted on oil shale thermal decomposition, there still exists a paucity of data on the composition of the reaction products. In particular, there is little information on the (23) Mityurev, A. K.In Chemistry and Technology of Combustible Shale and Their Research Products; Geller, G., Ed.; Israel Programs for Scientific Translation: Jerusalem. 1962: DD 205-221. (24) Suuberg, E.;Sherman, J.; Lilly, W. D.Prepr.-Am. Chem. SOC., Diu. Pet. Chem. 1987, 32(1),138. (25) Suuberg, E.; Sherman, J.; Lilly, W. D. Fuel, in press. (26) Suuberg, E., private communication. (27) Suuberg, E.; Unger, P. E.; Larsen, J. W. Energy Fuels 1987, I, 305. (28) Mention of specific brand names or models of equipment is for information only and does not imply endorsement of any particular brand.
Energy & Fuels, Vol. 1, No. 6, 1987 483
nature of the intermediate bitumen, which, although often used in kinetic models of oil shale decomposition, is seldom measured. To bridge this gap, the isothermal decomposition of a Colorado and a Kentucky New Albany oil shale has been studied in the range 368-425 OC. Because the two oil shales have significant differences in their kerogen structures, the results obtained from this study allow for some relevant comparisons to be made on the thermal decomposition of the two shales. For both shales it was found that oil is produced both directly from the kerogen and from the bitumen intermediate. However, the composition of the oil remained relatively constant throughout the decomposition, indicating little change in the decomposition chemistry as the dominant source of oil shifted from kerogen to bitumen. The amount of extractable bitumen appears to be inversely proportional to the carbon aromaticity of the kerogen. This study showed that the New Albany shale, which has a high carbon aromaticity, produced small amounts of bitumen, whereas the Colorado oil shale, which has a low carbon aromaticity, produces significantly more bitumen during pyrolysis. The oil and bitumen evolution data, coupled with the differences in the oil shale kerogen structures, suggest that the ability to form bitumen is related to the parent kerogen structure. However, additional studies are needed on other shales of varying carbon aromaticities before definitive relationships can be uncovered. Recovery of the bitumen intermediate can lead to enhanced recovery of products. If the reaction is quenched after 20 min at 425 OC and the bitumen and oil are recovered, 95% of the kerogen in Colorado oil shale can be recovered as soluble products. This is a significant improvement over the 65-70% conversion of kerogen to oil obtained during most pyrolysis processes. The low bitumen concentration of the Kentucky shale indicates that quenching the reaction at the time bitumen attains its maximum value will not significantly enhance the yield of total soluble products as it does for Colorado oil shale. The molecular weights, carbon aromaticities, and H/C ratios of the produced oils were fairly independent of time and temperature. The New Albany shale oils had lower molecular weights and H/C ratios than the Colorado shale oils produced under similar conditions, and they had higher carbon aromaticities. These differences in shale oil properties reflect the corresponding differences in the original kerogen structure of the shales, i.e., the more aromatic nature of the New Albany shale. In contrast, the composition and properties of the bitumens changed during the course of the reaction. Acknowledgment. The authors gratefully acknowledge support of this work by the US.Department of Energy under Cooperative Agreement DE-FC21-83FE60177. The Analytical Services Division of the Western Research Institute is acknowledged for the numerous elemental analyses provided for this report. The authors are also grateful to Dr. Dan Netzel of the Western Research Institute for providing the liquid-state NMR data. E. Suuberg is also acknowledged for his constructive review of this paper.
