Effect of Calcite, Kaolinite, Gypsum, and Montmorillonite on Huadian

Feb 18, 2014 - ABSTRACT: In this paper, anhydrous pyrolysis experiments were performed on Huadian oil shale kerogen with and without different mineral...
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Effect of Calcite, Kaolinite, Gypsum, and Montmorillonite on Huadian Oil Shale Kerogen Pyrolysis Meijuan Hu,† Zhiqiang Cheng,†,‡ Mingyue Zhang,† Mengzhu Liu,† Lihua Song,†,§ Yongqiang Zhang,† and Junfeng Li*,† †

College of Chemistry, Jilin University, Changchun 130012, P. R. China College of Resources and Environment, Jilin Agriculture University, Changchun 130118, P. R. China § Research Institute of Geology and Mineral Resources, The Ministry of Land and Resources, Shenyang 110032, P. R. China ‡

S Supporting Information *

ABSTRACT: In this paper, anhydrous pyrolysis experiments were performed on Huadian oil shale kerogen with and without different minerals (calcite, kaolinite, gypsum, montmorillonite) using a Fischer assay retorting system. The effect of mineral matrixes on the formation of oil was investigated, and their catalytic activities were obtained through pyrolysis experiments. Because of strong catalytic activity, montmorillonite and gypsum promoted the formation of oil products and minimized the formation of residue products. Kerogen with montmorillonite or kaolinite tends to direct the generated hydrocarbons from kerogen into low molecular hydrocarbons (C7−C12), indicating the Lewis acidic activity by montmorillonite and kaolinite. Calcite appears to inhibit the formation of oil. The ratios of isoalkanes/n-alkanes and alkanes/olefins and the content of branched saturated hydrocarbons generally increase in pyrolysis experiments for kerogen mixed with montmorillonite. In addition, the adsorption affinities of hydrocarbons on montmorillonite were obtained through pyrolysis experiments, consistent with computer simulation.

1. INTRODUCTION Oil shale, a fine-grained sedimentary rock, consists of an inorganic mineral matrix containing a cross-linked macromolecular organic matter called kerogen.1,2 It is a valuable source of energy since it can be converted into oil. The production of oil from oil shale is one of the prospective alternative energy resources available in China.3 Oil shale is considered to be one of the largest hydrocarbon reserves among the sources of energy, and its oil equivalent in the world is assumed to be approximate 30 times the reserve of crude oil.4,5 Therefore, for the energy needs of the near future, it is essential to investigate substitute fuel sources such as oil shale.6 To improve the quality of generated oil from oil shale, many researchers have focused on hydrogenation catalysts for oil shale pyrolysis.7−11 However, other researchers have investigated the pyrolysis behavior of single organic compounds, such as propionic acid, with and without clay mineral.12−15 Montmorillonite can adsorb pyrolysis products and promote the conversion of organic matter into oil and gas. It has been found that the influence of calcite on the pyrolysis products generated from kerogen is small,16,17 while other researchers have found that the adsorption effect of the production on calcite is very strong.18,19 However, it is not a definitive conclusion for the effect of other minerals on products generated during pyrolysis. In the present work, the influence of four different minerals on kerogen pyrolysis products is investigated by retorting pyrolysis. On the basis of these Fischer assay experiments, the component distribution of oil products, oil composition, quality, and mineral properties that govern the conversion efficiency of kerogen can be evaluated. The purpose of the present research is to realize an environmentally friendly oil shale retorting pyrolysis process © 2014 American Chemical Society

with high-efficiency and to identify the industrial pyrolysis catalysts which can redirect the reaction pathway.

2. EXPERIMENTAL SECTION 2.1. Materials. The oil shale samples used for this work were collected from the Paleogene-age Huadian strata in the Huadian Basin, China. The shales were sampled, broken into smaller pieces, ultimately ground, and sieved to a size range of 0−0.38 mm, according to the National Standards of China (GB 474-1996), and subsequently stored under a flow of dry N2. Four mineralscalcite, kaolinite, gypsum, and montmorillonite claywere used in these experiments. They were highly pure (>99.99%) according to X-ray diffraction (XRD) results, and the organic carbon contents were less than 0.01 wt %. All the minerals were screened through a 200 mesh sieve after grinding. Kerogen concentrates were prepared as in the previous research20 considering the National Standards of China (GB/T 19144-2010). The four minerals and kerogen samples were dried at 110 °C for 24 h under a vacuum to remove free water. Results of elemental analysis of the oil shale kerogen

Table 1. Elemental Analysis of Huadian Oil Shale Kerogen carbon hydrogen sulfur nitrogen oxygen H/C type

70.52% 9.86% 1.53% 1.28% 9.21% 1.67 I

Received: December 11, 2013 Revised: February 15, 2014 Published: February 18, 2014 1860

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are listed in Table 1. The data of Proximate, Ultimate and Fischer assay analysis of Huadian oil shale are available in Supporting Information. 2.2. Pyrolysis Experiments. 2.2.1. Pyrolysis Apparatus. According to the National Standards of China (GB/T 1341-2007 and GB/T 4802000), a Fischer assay retorting system for oil shale kerogen is shown in Figure 1. An aluminum retort (120 mm height, 67 mm i.d.) was prepared

convenient to obtain the weight of these two absorption materials before and after each experiment. The noncondensable gases were purified and dried by the absorption devices and collected in gas sampling bags. After the pyrolysis experiment, the residue, oil, and water in the device were accurately weighted, respectively. Pyrolysis products generated during pyrolysis were analyzed at qualitative and quantitative aspect using gas chromatography−mass spectrometry (GC−MS). 2.3. Characterization. The pyrolysates obtained from the sequential temperature-programmed pyrolysis experiments were diluted with toluene and identified by an Agilent 7890 gas chromatography (GC) and Agilent 5795N mass spectrum (MS) detector, equipped with an online injector. The operation conditions for Agilent 7890-5795N GC−MS are listed in Table 2.

Table 2. GC−MS Operating Conditions Figure 1. The improved Fischer assay retorting system. 1: Proportion− integration−differentiation (PID) temperature controller; 2: thermocouple; 3: aluminum retort; 4: thermal insulation; 5: electric furnace; 6: exhaust vent; 7: absorption device of oil (absorption felt); 8: absorption device of water (high absorbent resin−sodium polyacrylate); 9: manometer; 10: absorption flask (concentrated sulfuric acid); 11: control unit of gas flow; 12: Tedlar gas sampling bags.

column

HP-5MS (30 m × 0.25 mm × 0.25 μm) fused silica capillary column helium, flow rate 1 mL/min 250 °C

carrier gas injector temperature temperature program

on the basis of the National Standards of China (SH/T 0508-92), and the dried samples were placed in pyrolysis vessel. For each experiment, the retort reactor was heated to 520 °C from room temperature at 10 °C/min using a resistance electric furnace and proportion− integration−differentiation (PID) temperature controller. After the pyrolysis temperature (520 °C) was maintained for about 20 min, the pyrolysis experiment was terminated. 2.2.2. Sample Loading and Pyrolysis Process. All catalytic pyrolysis experiments of kerogen were conducted in the Fischer assay retorting system. Five series of samples of kerogen alone and kerogen mixed with calcite, kaolinite, gypsum, and montmorillonite clay were loaded into the aluminum retort. Approximately 50 g of sample was used in each experiment. For the experiments of oil shale kerogen mixed with mineral, the ratio of kerogen to mineral was 1:9 by weight. Each retorting experiment was performed in duplicate under identical conditions to check the repeatability of the test data. The final data listed below are the average values of the experimental results. During the course of non-isothermal programming, the experimental temperatures are consistent with the specified temperatures. The temperature curves from experimental process are shown in Figure 2.

50 °C for 2 min 10 °C min−1 to 250 °C, isothermal 20 min 10 °C min−1 to 280 °C, isothermal 10.0 min 70 eV 40−400/min

ionization energy mass range

Thermogravimetric analysis was performed on a thermogravimetry (TG) analyzer system (Shimadzu DTG-60, Japan). TG and derivative thermogravimetry (DTG) data were collected automatically by the software. For these analyses, about 10 mg of each sample was heated in the TG instrument at a heating rate of 10 °C/min from room temperature to 900 °C. The pyrolysis tests were conducted in a dry nitrogen atmosphere with a flow rate of 50 mL/min. After the sample was coated with gold using ETD-2000 auto sputter coater (Elaborate Technology Development Co., Ltd., China) with a current of 4 mA for 2 min, the surface morphology was examined using a SSX-550 scanning electron microscope (Shimadzu, Japan). The scanning electron micrographs (SEM) were obtained at an acceleration voltage of 15.0 kV. The SEMs for each experiment are available in the Supporting Information. 2.4. Computational Details. The geometric configuration of montmorillonite was optimized from the cluster having the formula Al2Si6O24H20, having two octahedral aluminums, six tetrahedral silicons, and six free waters.21 All the calculations are carried out with the DMol3 module, with a doubled numerical basis set within p-polarization function for hydrogen and d-polarization functions for other atoms (DNP). This work was completed in the Materials Studio 6.0 of Accelrys Inc. A generalized gradient approximation (GGA) density functional by Perdew and Wang (PW91) was used,22 and the fine quality mesh size is employed for the numerical integration. The global orbital cutoff is set

Table 3. Two Sets of Fischer Assay Analysis Data of Huadian Kerogen with and without Different Minerals Fischer assay mineral/ kerogen (9:1)

recoverya (%)

gas lossb (%)

cokingc (%)

51.6 40.8 50.0 54.8 57.2

9.0 13.1 9.0 29.4 25.4

38.8 45.8 40.8 14.6 16.4

Figure 2. Heating curves from the pyrolysis experiments.

kaolinite + kerogen calcite + kerogen kerogen gypsum + kerogen montmorillonite + kerogen

51.3 40.9 50.3 54.7 57.0

9.2 13.4 9.1 29.6 25.2

38.9 45.5 40.6 14.8 16.7

During the heating process, the products of generated oil and water were formed and trapped into the absorption materials in Figure 1. It is

Recovery (R) is defined as R = moil/mkerogen × 100%. bGas loss (G) is defined as G = mgas/mkerogen × 100%. cCoking (C) is defined as C = mresidue/mkerogen × 100%. a

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Figure 3. Gas chromatograms of oil collected from the pyrolysis of (a) kerogen; (b) kerogen mixed with montmorillonite; (c) kerogen mixed with gypsum; (d) kerogen mixed with kaolinite; (e) kerogen mixed with calcite. LAO: linear α-olefin; non-LAO: nonlinear α-olefin; MAH: monocyclic aromatic hydrocarbon; PAH: polycyclic aromatic hydrocarbon; HC: heteroatomic compound. at 5.0 Å with a Fermi smearing of 0.0005 Ha. The tolerances of energy, displacement, and gradient are 1 × 10−5 au, 6 × 10−3 Å, and 4 × 10−3 au·Å−1, respectively. For hydrocarbons adsorption, adsorption energies (ΔEads) are calculated by subtracting the energies of the hydrocarbon and montmorillonite (MMT) from the energy of the optimized hydrocarbon/cluster complex, as indicated in eq 1, based on this comparison, the greater the absolute value of Eads, the stronger the adsorption.

ΔEads = E(hydrocarbon/MMT) − [E(hydrocarbon) + E(MMT)]

(1)

3. RESULTS AND DISCUSSION 3.1. Fischer Assay Results. Table 3 shows the two sets of retorting assay data for kerogen with and without different 1862

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kerogen mixed with gypsum. However, the amounts of ΣC7−12 in the experiments using montmorillonite and kaolinite mixed samples are higher, and the amounts of ΣC12+ are lower. These results show that the kerogen pyrolysis in the presence of montmorillonite or kaolinite tends to direct the generated hydrocarbons from kerogen into the low molecular hydrocarbons (C7−C12). It is supposed that these results may be attributed to the Lewis acid sites in the minerals of montmorillonite or kaolinite.24,29 These data demonstrate the ratios of the isoalkanes/n-alkanes and alkanes/olefins in the test for kerogen mixed with montmorillonite are higher than for kerogen alone or kerogen mixed with the other minerals used in this study (Table 4). This consequent may be explained due to montmorillonite having acidic sites on its surface.23 Previous studies have indicated that, in petroleum formation process, the branched isoalkanes are formed by the carbonium ion reaction of α-olefins with protons that are enhanced advanced under acidic conditions.30,31 Thus, this phenomenon demonstrates that the pyrolysis reaction is accomplished via the carbonium ion mechanism, which is induced by acidic sites of montmorillonite during the experiment. Furthermore, montmorillonite and kaolinite as clay catalysts can adsorb hydrocarbons on their surface resulting in various organic reactions during kerogen pyrolysis.28 Montmorillonite may possess the highest catalytic activity during the kerogen pyrolysis, due to the highest cation exchange capacity (CEC) (about 80−120 meq/100 g)24 and highest surface area (BET) (264.11 m2/g) among the clay minerals. However, kaolinite is endowed with the relatively fewer Lewis acid sites under a high temperature than montmorillonite because of its lower CEC (2.0 meq/100 g)24 and lower BET (10.07 m2/g); it is a less active catalyst. The gypsum is viewed as a weak Brønsted acid. Furthermore, a lot of organic acids that are formed during the process of kerogen degradation are mainly considered as the heteroatom compounds containing oxygen atoms, and then minerals catalyze the decarboxylation of organic acids.27 So it is likely that a significant decarboxylic reaction occurred in montmorillonite by reaction with organic acids, according to the dramatic decrease in the content of heteroatom compounds containing oxygen in pyrolysis products (Table 4). 3.3. Gas Compositions. Figure 5 shows all the collected gas samples during the pyrolysis process, suggesting that the gas compositions derived from kerogen mixed with different minerals are similar to that from the kerogen alone. These gas compositions consist of the following components: methane, carbon dioxide, ethane, propylene, propane, water, butane, isobutane, pentane, etc. From the different peak areas of water in pyrolysis gas for kerogen with and without different mineral, it can be concluded that the content of crystal water in gypsum is higher than the other minerals used in this work. Because of the inclusion of carbonate in calcite, more carbon dioxide is released during the pyrolysis process for kerogen mixed with calcite. The gas compositions of kerogen mixed with different mineral are redistributed due to their different properties. In these collected gases, acetone peak appears in the experiments for kerogen in the presence of montmorillonite or gypsum at about 8 min, and hexane peaks are more obvious in the experiments for kerogen using montmorillonite, gypsum, and kaolinite at about 13 min. The hydrocarbon proportions of gas are generally highest in the pyrolysis experiment for kerogen mixed with montmorillonite, probably due to its strong Lewis acidity.23

minerals. The Fischer assay yields of the pyrolytic water are not shown in Table 3 due to its negligible amount. These results show that montmorillonite redirect the reaction for the kerogen pyrolysis experiment. Montmorillonite significantly promotes the formation of gas and oil and minimizes the formation of residue during the kerogen pyrolysis, probably due to the strong Lewis acidity.23−25 However, kaolinite indicates less activity, perhaps owing to the lower cation exchange capacity and lower surface area,24 and calcite has a negative influence on the yield of gas and oil. These results show that the amounts of oil increase in the following order: kerogen mixed with calcite < kerogen alone < kerogen mixed with kaolinite < kerogen mixed with gypsum < kerogen mixed with montmorillonite. On the other hand, the amounts of residue increase in the following order: kerogen mixed with gypsum < kerogen mixed with montmorillonite < kerogen mixed with kaolinite < kerogen alone < kerogen mixed with calcite. The decrease in the amount of residue in the experiment for kerogen mixed with gypsum may be attributed to the presence of a large amount of crystal water in gypsum.26 3.2. Ratios of C7−12/C12+, Isoalkanes/n-Alkanes, and Alkanes/Olefins. As shown in Figure 3, the yield and composition of saturated hydrocarbons, especially n-alkanes, depend on which mineral was pyrolyzed with the kerogen. The effect of montmorillonite is most obvious, as shown by a decrease in the amounts of olefin products, an increase in the amounts of saturated hydrocarbons products, and the ratio of lighter hydrocarbons. For the kerogen pyrolysis experiments using gypsum, kaolinite, and calcite, respectively, the amounts of nonLAOs are found relatively higher in Figure 3. These results show the hydrogenation of generated olefins leads to an increase in the amounts of saturated hydrocarbons products in the experiment using montmorillonite. It is possible that the olefins are unstable and easily hydrogenated into the saturated hydrocarbons in the presence of montmorillonite during the anhydrous pyrolysis experiment.27,28 The compositions of generated oil products from GC−MS analysis are summarized in Figure 4. These results show that the

Figure 4. The carbon-number distribution of collected oil component from the pyrolysis of kerogen with and without different minerals.

amounts of ΣC7−12 (7 ≤ carbon number ≤ 12) and ΣC12+ (carbon number >12) are not significantly different in the oil products of kerogen alone, kerogen mixed with calcite, and 1863

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Table 4. Components of Oil Products in the Pyrolysis Experiments aliphatic

aromatic HC

n-alkanes isoalkanes isoalkanes/n-alkanes LAOs non-LAOs alkanes/olefins MAHs PAHs O, N, S

kerogen (%)

MMT + kerogen (%)

gypsum + kerogen (%)

kaolinite + kerogen (%)

calcite + kerogen (%)

26.87 2.21 8.23 24.95 3.93 101 9.87 5.11 7.70

46.53 17.10 36.75 12.56 3.10 406 2.14 0.36 0.51

27.50 2.35 8.86 21.86 5.92 108 7.98 5.95 7.53

35.69 2.27 6.36 21.17 5.65 137 5.91 3.19 7.38

27.47 1.29 4.70 22.27 4.73 107 11.31 6.92 6.06

Figure 5. The gas chromatogram of collected gas from the pyrolysis of (a) kerogen; (b) kerogen mixed with montmorillonite; (c) kerogen mixed with gypsum; (d) kerogen mixed with kaolinite; (e) kerogen mixed with calcite. 1864

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3.4. Mineral Effects. 3.4.1. Adsorption Effect. The difference in molecular structure has an important influence on the sorption affinity. From Table 4, it can be concluded the sorption affinities of hydrocarbons on montmorillonite increase as follows: n-alkanes < isoalkanes < LAOs < non-LAOs < MAHs < PAHs. The sorption affinities of hydrocarbons on gypsum, kaolinite, and calcite increase in the order: alkanes < olefins < MAHs < PAHs. Since the sorption affinities of hydrocarbons on montmorillonite are most obvious than those on gypsum, kaolinite, and calcite, we have only simulated the adsorption of hydrocarbons on montmorillonite. The adsorption behaviors of different hydrocarbons in montmorillonite model21 were researched in this study. All optimized structures are displayed in Figure 6. In this work, we

alkaline earth metal cations, and interlayer structure in the minerals, so the catalytic effect of the montmorillonite seems to be greater than that of gypsum (without interlayer structure) in the pyrolysis reactions. Montmorillonite includes two types of catalytic sites, namely, the Lewis and the Brønsted sites,34 where the main organic reactions are catalyzed. 3.4.3. Kinetic Analysis. As shown in Figure 7, the catalytic effect of montmorillonite is not obvious until the temperature

Figure 6. Hydrocarbons adsorption on montmorillonite cluster models. (a) Pentane; (b) neo-pentane; (c) 1-pentene; (d) 2-methyl-1-butene; (e) toluene; (f) 1-methylnaphthalene.

choose pentane, neo-pentane, 1-pentene, 2-methyl-1-butene, toluene, and 1-methylnaphthalene as the n-alkanes, isoalkanes, LAOs, non-LAOs, MAHs, and PAHs, respectively. After calculation and comparison of the Eads, as shown in Table 5, Table 5. Calculated Etotal (Ha), Eads (eV) for the Montmorillonite Cluster Model with Different Hydrocarbons a b c d e f

Etotal (Ha)

Eads (eV)

−4239.03 −4240.95 −4237.79 −4238.23 −4312.92 −4466.53

2.11 2.25 2.35 2.50 5.47 9.08

Figure 7. Thermogravimetric curves of (a) kerogen and kerogen mixed with different minerals; (b) kerogen alone and kerogen after subtract the loss of mineral itself.

comes to 450 °C or higher. At last, values of weight loss on kerogen alone and kerogen in the presence of montmorillonite, gypsum, kaolinite, and calcite were 59%, 73%, 64%, 57%, and 56%, respectively. Since the value of weight loss reflects the pyrolysis degree, these results demonstrate the catalytic effect on montmorillonite and gypsum and little catalytic effect on calcite. However, the effect on kaolinite was not obviously reflected on the thermogravimetric curve, which will be investigated in-depth in our future work. Through the thermoanalysis, non-isothermal kinetics shows that the decomposition activation energy of gypsum alone is 191.31 kJ·mol−1 with A (pre-exponential factor) of 1.737 × 1011 s−1. However, the activation energy of gypsum reacting with coke is 86.54 kJ·mol−1 with A of 2.408 × 103 s−1. According to the thermodynamic calculation, it is inferred that gypsum can easily react with coke, which results in the coking proportion in the pyrolysis experiment for kerogen mixed with gypsum being lower than the others; this result is consistent with the above-mentioned experimental data (Table 3).

Eads in montmorillonite increases in the order for pentane < neo-pentane