Organic Matter in Yilan Oil Shale: Characterization ... - ACS Publications

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Organic Matter in Yilan Oil Shale: Characterization and Pyrolysis with or without Inorganic Minerals Xiaosheng Zhao,† Xiaoliang Zhang,† Zhenyu Liu, Zhenghua Lu, and Qingya Liu* State Key Laboratory of Chemical Resource Engineering, Beijing University of Chemical Technology, Beijing 100029, People’s Republic of China S Supporting Information *

ABSTRACT: To better understand the effect of inherent minerals in oil shale on the pyrolysis behavior of organic matter, Chinese Yilan oil shale is used as the raw material and its minerals are removed sequentially by stepwise acid treatment. The resulting samples are subjected to pyrolysis experiment on a thermogravimetric analyzer coupled with a mass spectrometer (TG− MS), and the effect of acid treatments on the organic matter structure is investigated by Fourier transform infrared (FTIR) and 13 C nuclear magnetic resonance (NMR). It is found that the acid treatments have little effect on the structure of organic matter, except oxidation of a tiny amount of aliphatic carbons to carboxyl by HNO3. Inherent calcite, iron oxide, and pyrite in the minerals, with contents of about 0.64, 2.63, and 0.60 wt %, respectively, have obvious catalytic effects on the decomposition of the organic matter. Acid treatment by HF + HCl increases the mass loss of organic matter during pyrolysis but decreases the formation of C2H4, C3H8, C4H10, C6H6, C7H8, and C6H6O. Quartz and kaolinite removed by HF + HCl treatment were reported to have little effect on the pyrolysis process; therefore, inherent montmorillonite may promote the reaction of organic volatiles to form coke and gas.

1. INTRODUCTION It is well-known that oil shale is composed of organic matter and inorganic minerals. The inorganic minerals generally account for 50−85 wt % of oil shale and include mainly silicates, carbonates, quartz, and pyrite.1,2 Pyrolysis is the major technology to convert the organic portion of oil shale to liquid and gas, during which the inorganic minerals were found to have some effects.3−13 Yan et al.3 reported that the minerals in Huadian oil shale promoted the mass loss and gas formation during pyrolysis in a thermogravimetric analyzer coupled with Fourier transform infrared (TG−FTIR), but the role of each of the minerals was not identified. Espitalié et al.4 found that the kerogen of Cameroon oil shale yielded more liquid hydrocarbons than raw oil shale during pyrolysis in a fixed-bed reactor. Borrego et al.5 showed that inherent montmorillonite in Spanish oil shale lowered the initiation temperature of kerogen cracking. Karabakan et al.6 reported the catalytic effect of alkali and alkaline earth carbonates because the activation energy decreased by 1.2−8.9% for Göynük oil shale and 5.4−22.1% for Green River oil shale when carbonates were removed by a HCl treatment. They also reported the inhibition effect of silicates on pyrolysis because the volatile yield increased by 20 wt % for Göynük oil shale and 40 wt % for Green River oil shale when silicates were removed by a HF treatment. Ballice and Sert et al.7−9 investigated pyrolysis of two Turkey oil shales (Beypazari and Göynük) in a fixed-bed reactor at 450−650 °C and reported 4.0−7.3 wt % decreases in the volatile hydrocarbon yield by a HCl treatment and 2.3−8.0 wt % increases in the volatile hydrocarbon yield by a further HF treatment. Al-Harahsheh et al.10 found increases in the total oil and gas yields when a Jordan oil shale (El-lajjun) was subjected to a HCl or HF treatment, from 18.0 to around 30.4 and 24.7 wt %, respectively. The result of HCl treatment was different from those discussed above. Hu et al.11 introduced montmorillonite, © XXXX American Chemical Society

gypsum, calcite, and kaolinite into Huadian oil shale at the mineral/kerogen ratio of 9 and studied their effects using an aluminum retort. They reported that montmorillonite increased oil and gas yields by 7.0 and 16.3 wt %, respectively, gypsum increased oil and gas yields by 4.6 and 20.5 wt %, respectively, calcite decreased the oil yield by 9.3 wt % and increased the gas yield by 4.2 wt %, and kaolinite had little catalytic effect, which are different from those reported above. The role of pyrite in oil shale pyrolysis does not seem clear as well. Ballice and Sert et al.7−9 reported that inherent pyrite had little influence on pyrolysis of the organic matter of Turkey oil shale. Al-Harahsheh et al.10 found a decrease in the oil yield from 13.5 to 5.9 wt % by a HNO3 treatment on a Jordan oil shale, suggesting a promoting effect of inherent pyrite. Bakr et al.12 reported that pyrite at loadings of 10−30 wt % could promote pyrolysis of kerogen. Gai et al.13 reported that the addition of 8 wt % pyrite to Longkou oil shale increased the oil and gas yields at 500 °C by 4.5 and 3.3 wt %, respectively. The above contradictive results regarding the effect of minerals on oil shale pyrolysis may be attributed to differences in type and quantity of organic and inorganic matter in different oil shale. It may also be attributed to possible changes in the organic matter of oil shale during the acid treatments, which were performed differently in different studies. These suggest that the effect of the mineral on pyrolysis of oil shale should be studied in more detail to avoid simplified generalization. Yilan oil shale is a major oil shale resource in China but was rarely studied in terms of organic structure and the role of minerals in pyrolysis. Therefore, the present paper examines the Received: December 21, 2016 Revised: March 6, 2017 Published: March 9, 2017 A

DOI: 10.1021/acs.energyfuels.6b03404 Energy Fuels XXXX, XXX, XXX−XXX

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corresponding to a relative standard deviation of less than 1.6 and 0.7%, respectively. The volatile products generated in the TG were swept to the MS through a hot stainless-steel capillary at about 180 °C. To eliminate the baseline effect, the ion intensity of each product was divided by that of Ar (m/z 40) and the corrected intensities of products were normalized on the basis of the daf sample mass. 2.4. Characterization of Oil Shale Samples. X-ray diffraction (XRD) of the oil shale samples was carried out on a D8FOCUS X-ray diffractometer (Brook) equipped with Cu Kα1 (λ = 0.154 06 nm) radiation operated at 40 kV and 200 mA. The samples were scanned over a 2θ range of 10−80° at a frequency of 2° min−1. The content of every mineral component in the oil shale samples was determined by Xray fluorescence (XRF) spectrometry (S4-Explorer, Bruker, Germany). Fourier transform infrared (FTIR) analysis was carried out on a Nicolet 6700 infrared spectrometer. The sample was mixed with KBr at a mass ratio of 1:200, ground adequately under an infrared lamp, and pressed to form a pellet. All of the spectra were acquired with a spectral range of 4000−400 cm−1, a scanning frequency of 32, and a resolution of 4 cm−1. Cross-polarization magic angle spinning (CP/MAS) 13C nuclear magnetic resonance (NMR) was performed on a Bruker AV-300 spectrometer at a resonance frequency of 75.47 MHz at room temperature. The contact time, MAS rotation speed, and recycle delay time were 1 ms, 12.0 kHz, and 0.5−1.5 s, respectively.

structure change of organic matter in Yilan oil shale during various acid treatments as well as the effects of inorganic minerals on pyrolysis of the organic matter using modern characterization methods.

2. EXPERIMENTAL SECTION 2.1. Raw Material. Proximate analysis indicated that the contents of ash and fixed carbon (FC) in the raw Yilan oil shale (termed YLOS-R) on a dry basis are 53.5 and 18.3 wt %, respectively, and the volatile content is 62.7 wt % on a dry and ash-free (daf) basis. The Fischer assay indicated that the yields of char, oil, water, and gas on a dry basis are 80.44, 10.00, 3.38, and 6.18 wt %, respectively. 2.2. Demineralization by Acid Treatments. The raw Yilan oil shale was ground and sieved to 60−100 mesh and then demineralized by acid treatments.14,15 The acids used include HCl (18.5 wt %), HNO3 (20 wt %), and a mixture of HF (40 wt %) and HCl (18.5 wt %) at a volume ratio of 1 (termed as HCl + HF). All of the acids are purchased from Beijing Chemical Plant (China) and in analytical grade. The HCl treatment mainly removed carbonates and metal oxides; the HCl + HF treatment removed clay and quartz; and the HNO3 treatment removed pyrite. Figure 1 shows the acid treatment procedure and sample codes,

3. RESULTS AND DISCUSSION 3.1. Effect of the Acid Treatment on the Structure of Organic Matter. FTIR analysis is a common practice to differentiate the difference in organic matter. As seen in Figure 2,

Figure 1. Acid treatment procedure of oil shale and sample codes.

where YLOS-C is the HCl-treated sample, YLOS-CN is the sample treated successively by HCl and HNO3, YLOS-CF is the sample treated by HCl and HCl + HF, and YLOS-CNF is the sample treated successively by HCl, HNO3, and HCl + HF. All of the acid treatments were carried out at a ratio of oil shale/acid of 1 g/10 mL in a nitrogen atmosphere under stirring. The HCl and HCl + HF treatments were carried out at 60−70 °C, while the HNO3 treatment was carried out at room temperature. The acid-insoluble matter in each step was separated from the acid solution using a Büchner funnel equipped with three layers of quantitative filter papers with pore sizes of 1−3 μm, washed with deionized hot water (60 °C) to neutral, and finally dried at 60 °C under a vacuum for 24 h. 2.3. TG−MS Experiment. The pyrolysis experiment was performed in a thermogravimetric analyzer (TG, Setsys Evolution 24, Setaram) coupled online with a mass spectrometer (MS, Oministar QMS 200, Balzers). In each experiment, approximately 30 mg of sample was heated in the TG at a rate of 10 °C min−1 from room temperature to 110 °C and stayed at this temperature for 30 min to remove moisture. The sample was then heated at the same rate to 900 °C and stayed at the temperature for 20 min to remove volatiles. All of the above procedures were carried out in an argon atmosphere at a flow rate of 100 mL min−1. Finally, a gas stream containing 10% oxygen and 90% argon was introduced to the TG for 1 h to allow for complete combustion of the carbon residue at 900 °C. The mass loss in 110−900 °C is the volatile content; the mass loss in the combustion process is the fixed carbon (FC) content; and the residual mass is the ash content. Repeated experiments on YLOS-R, YLOS-C, and YLOS-CF showed that the standard deviations of the volatile and ash contents are less than 0.7 and 0.2%, respectively,

Figure 2. FTIR spectra of YLOS-R and demineralized oil shale samples.

YLOS-R shows several absorption peaks, including −OH of crystalline hydrate or hydroxyl in minerals (3700 and 3620 cm−1), asymmetrical and symmetrical alkyl −CH2 stretching (2928 and 2847 cm−1, respectively), CO stretching in carboxyl and/or carbonyl (the weak shoulder peak at around 1710 cm−1), CO stretching of quinones bridged to acidic hydroxyl or C C stretching of olefins and aromatic rings (about 1607 cm−1), and asymmetrical bending of −CH3 and −CH2 or carbonates (1450 cm−1).16,17 B


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Energy & Fuels The acid treatments by HCl or HCl followed by HNO3 did not alter the absorption peaks significantly (see YLOS-C or YLOSCN), except a slight decrease at around 1450 cm−1 for carbonates. The subsequent HCl + HF treatment for YLOSCN or YLOS-C samples eliminated the peaks at 3700 and 3620 cm−1 (see YLOS-CNF and YLOS-CF), suggesting removal of minerals containing hydrate or hydroxyl. The HCl + HF treatment did not change the peak positions at 2928, 2847, 1710, 1607, and 1450 cm−1 but obviously increased the peak intensities, which suggests enrichment of organic matter. These phenomena indicate that the acid treatments did not alter the organic matter significantly if the peak position is the main criterion. To further understand the effect of acid treatments on the structure of organic matter, the 13C NMR spectrum of YLOS-R is compared in Figure 3 to that of YLOS-CNF, a sample subjected

Table 1. Carbon Composition of YLOS-R and YLOS-CNF Determined from Data in Figure 3

the acid treatments had little effect on the aromatic structure of organic matter but introduced a few oxygen to the aliphatic chains. This introduction may be attributed to oxidation aliphatic carbons by HNO3 as reported in the literature20 and is supported by the increased sample mass after the HNO3 treatment. Except for this change, the effect of acid treatment on the main structure of organic matter may be insignificant, which agrees with the observation of Maciel et al.21 Table 3 shows the ultimate analysis results of YLOS-R and YLOS-CNF samples. In comparison to YLOS-R, YLOS-CNF shows similar H, N, and S contents, higher C content, and lower O content, which seems to suggest alteration of the organic matter by the acid treatments and is somewhat different from the 13 C NMR result. In view of the operating procedure of the ultimate analysis, the difference may be ascribed to decomposition of crystalline hydrate and hydroxyl in some minerals as well as carbonates in YLOS-R at high temperatures, which generate water as well as carbon dioxide. These water and carbon dioxide were miscounted as H, O, and C in the organic matter. If this is the case, the ultimate analysis data of YLOS-R in Table 3 should be corrected to remove the contribution of the minerals to show the true elemental composition of the organic matter. The correction performed is based on the findings of the next subsection and included in the Supporting Information. The corrected data are listed in Table 3 under the heading of YLOS-Rb. It can be seen that the data are very similar to those of YLOS-CNF, indicating little alteration in the organic matter by the acid treatments. 3.2. Characterization of Minerals. On the basis of the weight change before and after the acid treatments, it is found that the mineral content in YLOS-R is about 58.2 wt %, which is higher than its ash content in Table 3 as a result of the reason

Figure 3. Solid-state 13C NMR spectra of YLOS-R and YLOS-CNF.

to all of the acid treatments and with the least amount of minerals. It is apparent that both samples show two major peaks at 29 and 127 ppm, indicating that aliphatic carbon (0−90 ppm) is mainly composed of straight-chain methylene and methyne carbons, while aromatic carbon (90−165 ppm) is mainly composed of protonated and bridgehead aromatic carbons. However, the chemical shift of carbonyl and carboxyl carbons in the range of 165−220 ppm is extremely weak, indicating a very small amount of them. The different types of carbons are quantified by integrating the peak areas using the method reported in the literature,18,19 and the results are shown in Tables 1 and 2. It can be seen that the organic matter in YLOS-R contains 50.43% aliphatic carbons, with 73% of them (36.67 in 50.43) being aliphatic C(2) and methylene carbons, 46.85% aromatic carbons, with 33% of them being multi-ring aromatic carbons (15.67 in 46.85), and 2.72% carbonyl and carboxyl carbons. In comparison to YLOS-R, YLOS-CNF shows a composition slightly lower in aliphatic carbons (48.21%), similar in aromatic carbons (46.15%), and slightly higher in carbonyl and carboxyl carbons (5.63%), suggesting transformation of some aliphatic carbons to carbonyl or carboxyl carbons during the acid treatments. The average methylene chain lengths of both YLOSR and YLOS-CNF range from 5 to 6. These results suggest that C


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Energy & Fuels Table 2. Structure Parameter of Organic Matter in YLOS-R and YLOS-CNF value structural parameter

definition O1

aromaticity, far (%) aliphaticity, fal (%) average methylene chain length, Cn substitute degree of aromatic ring, δ (%)

O2

farS

C−C

far = far + far + far + + far fal = fal1 + fala + fal2 + fal3 + fal4 + falO Cn = ( fal2 + fal3)/farS δ = ( farO1 + farO2 + farO3 + farS)/far

ultimate analysis (wt %, daf) sample

C

H

N

St

Oa

ash (wt %, dry)

YLOS-R YLOS-CNF YLOS-Rb

68.6 74.4 75.8

6.8 6.4 6.4

1.2 1.6 1.3

1.4 1.6 1.6

22.0 16.0 14.9

53.5 1.6 53.5

YLOS-CNF

46.85 50.43 5.2 38.74

46.15 48.21 6.0 43.16

(see YLOS-CN). The further HCl + HF treatment eliminated diffraction peaks of quartz, kaolinite, and montmorillonite and yielded a broad weak peak around 15−25° (see YLOS-CNF) for organic matter, indicating nearly complete removal of all of the minerals. This phenomenon is consistent with the ash contents in Table 3, 53.5 wt % in YLOM-R versus 1.6 wt % in YLOMCNF. It is noted that the sample without the HNO3 treatment, YLOS-CF, shows obvious diffraction peaks of pyrite (termed P) in addition to that of the organic matter, suggesting the existence of a small amount of pyrite in YLOS-R and YLOS-C. The absence of pyrite diffraction in YLOS-R and YLOS-C may be attributed to the large scale used, which also results in the absence of organic matter. The detailed composition of minerals in the oil shale samples were determined by XRF. However, the results are presented in the form of oxides (see Table S1 of the Supporting Information), and they are approximately ash composition of the samples. Therefore, the contents of every component in minerals were estimated by multiplying the ash content of each sample and the corresponding data in Table S1 of the Supporting Information, and the results are shown in Table 4. It should be noted that these data are different from those of actual minerals because some minor portions of minerals are in the form of carbonates, hydrates, pyrite, and sulfates. It can be seen that the mineral in YLOS-R mainly consists of Si and Al compounds, accounting for around 85 wt % [(34.00 + 11.37)/53.5]. The contents of Fe and Ca compounds are significantly decreased, and those of Al and Mg are slightly decreased by the HCl treatment. Because pyrite evidenced by XRD is not soluble in HCl solution, this result indicates the existence of some Fe2O3 in the minerals. The decreased value of Fe2O3 approximates its content in YLOS-R, about 2.63 wt %. Assuming that calcite is the only form of calcium in YLOS-R, its content is about 0.64 wt % (0.36/56 × 100). Removal of a few Al and Mg by the HCl treatment suggests the presence of a small portion of Al2O3 and MgO in the mineral besides kaolinite and montmorillonite. Acid treatment of YLOS-C by HNO3 only slightly decreases the contents of Fe and S, although the HNO3 treatment was aimed to remove pyrite, while the subsequent treatment by HCl + HF further decreases the contents of Fe and S obviously. These phenomena indicate the presence of both pyrite (soluble in HNO3) and silicate iron (soluble in HCl + HF) in the minerals of YLOS-R, and the content of pyrite is approximately 0.60 wt % [(1.37 − 0.97)/160 × 120 × 2]. Acid treatment of YLOS-CN by

Table 3. Ultimate Analyses of YLOS-R and YLOS-CNF

a

H

YLOS-R

By difference. bCorrected data.

discussed above: decomposition of crystalline hydrate and hydroxyl in some minerals as well as carbonates in YLOS-R during proximate analysis. To identify the form of minerals, Figure 4 shows XRD spectra of YLOS-R and all of the acid-

Figure 4. XRD patterns of YLOS-R and demineralized oil shale samples (Q, quartz; K, kaolinite; M, montmorillonite; C, calcite; and P, pyrite).

treated samples. It can be seen that YLOS-R contains quartz (SiO2, termed Q), kaolinite [Al2Si2O5(OH)4, termed K], and small amounts of montmorillonite {(Al,Mg)2[Si4O10](OH)2· nH2O, termed M} and calcite (CaCO3, termed C). The HCl treatment removed calcite (see YLOS-C), but the subsequent HNO3 treatment resulted in little change in the XRD pattern

Table 4. Contents of Total Ash and Each Component in Minerals of Oil Shale Samples content (wt %) sample

ash (wt %)

SiO2

Al2O3

Fe2O3

SO3

K2O

TiO2

CaO

MgO

Na2O

others

YLOS-R YLOS-C YLOS-CN YLOS-CNF

53.5 50.4 47.5 1.6

34.00 34.55 33.22 0.03

11.37 10.53 9.69 0.03

4.00 1.37 0.97 0.19

1.80 2.02 1.84 0.83

0.64 0.65 0.64 0.00

0.41 0.43 0.42 0.14

0.36 0.06 0.05 0.03

0.27 0.18 0.16 0

0.06 0 0 0

0.59 0.59 0.52 0.36

D


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Energy & Fuels HCl + HF significantly decreases the contents of Si, Al, K, and Mg to about zero, which is consistent with the XRD result: disappearance of diffraction peaks of quartz, kaolinite, and montmorillonite. Their total content is approximately 45 wt %. 3.3. Effect of Minerals on Pyrolysis of Organic Matter. 3.3.1. TG/DTG Analysis. Figure 5 shows the mass loss (TG) and

such as water (see H2O release curves shown in Figure S1 of the Supporting Information) and carbon dioxide released from calcite at temperatures higher than 600 °C,26 the daf-based mass losses have to be corrected to compare the differences in pyrolysis of organic matter in these samples. According to the difference between mineral and ash contents of YLOS-R and the contents of calcite and pyrite, it is estimated that carbon dioxide released from calcite is approximately 0.28 wt % and the content of water released from kaolinite and montmorillonite is about 4.22 wt % (the detailed calculation is shown in the Supporting Information). The corrected mass loss data are also shown in Table 5. Before discussion, it should be pointed out that pyrolysis includes decomposition of the organic matter and reaction of organic volatiles, which should be distinguished. It is clear that the corrected mass loss of organic matter (TG) decreases from 52.1 wt % of YLOS-R to 47.1 wt % of YLOS-C, suggesting the catalytic effect of calcite and/or iron oxide on decomposition of the organic matter as reported in the literature.6−9 The lower corrected mass loss of YLOS-CN (45.6 wt %) than YLOS-C (47.1 wt %) and the lower corrected mass loss of YLOS-CNF (51.4 wt %) than YLOS-CF (55.5 wt %) may suggest a catalytic effect of inherent pyrite on decomposition of the organic matter, although its content is very low (0.60 wt %), which is consistent with the observation of Al-Harahsheh et al.10 The corrected mass loss of YLOS-CNF (51.4 wt %) is obviously higher than that of YLOS-CN (45.6 wt %), and the corrected mass loss of YLOS-CF (55.5 wt %) is higher than that of YLOS-C (47.1 wt %), which indicate that quartz, kaolinite, or montmorillonite decrease the mass losses as reported in the literature.6−10 This result may not be due to their inhibition effect on decomposition of the organic matter because quartz is inert to most reactions, kaolinite was reported to have little catalytic effect,11 and montmorillonite is a common catalytic component.5,27,28 It is very possible that montmorillonite promotes the reaction of organic volatiles to form coke and gas as reported by Espitalié et al.,4 which causes the decreased mass loss and will be evidenced by the MS results. The slightly higher mass loss of YLOS-R than YLOS-CNF suggests that the overall effect of the minerals on pyrolysis of organic matter is insignificant for Yilan oil shale. The effects of different minerals on pyrolysis of organic matter may be seen more clearly from the shape and peak depth of DTG curves in Figure 5. It is clear that the mass loss of all of the samples is mainly in the range of 350−520 °C, similar to that of many oil shales,3,4,9,10 and peaks at around 450 °C, slightly lower than that of Huadian29 and Jimsar30 oil shale at the same heating rate. YLOS-CF shows the largest DTG peak depth, while YLOSCN shows the smallest DTG peak depth. These behaviors again indicate the catalytic effect of pyrite on pyrolysis of the organic matter and the negative effect of montmorillonite on the mass loss. The difference in DTG peak depth between YLOS-R and

Figure 5. TG and DTG curves of YLOS-R and demineralized oil shale samples.

differential mass loss (DTG) curves of the oil shale samples on a daf basis, and Table 5 summarizes the TG data. It is clear that YLOS-R shows a maximum apparent total mass loss of 61.8%, which is followed sequentially by YLOS-C, YLOS-CF, YLOSCN, and YLOS-CNF. Because some of the minerals decompose in the temperature range of the TG analysis and yield gases,22−25

Table 5. Pyrolysis Behavior of YLOS-R and Demineralized Oil Shale Samples

a

sample

mass loss (wt %, daf)

corrected mass lossa (wt %, daf)

DTG peak depth (wt %/min, daf)

mineral matter

YLOS-R YLOS-C YLOS-CN YLOS-CNF YLOS-CF

61.8 55.6 53.6 51.4 55.5

52.1 47.1 45.6 51.4 55.5

5.75 5.14 3.58 4.83 6.17

Q, K, C, M, P, and TiO2 Q, K, M, P, and TiO2 Q, K, M, and TiO2 TiO2 P and TiO2

The mass losses of organic matter during pyrolysis. E


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Figure 6. Light products detected by MS during pyrolysis of YLOS-R and YLOS-CNF.

YLOS-C again indicates the catalytic effect of calcite and iron oxide on pyrolysis of the organic matter.

The effect of acid treatments on the oil shale samples can also be seen in the DTG curves. The samples subjected to HNO3 F


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Figure 7. Light products detected by MS during pyrolysis of YLOS-CN and YLOS-CF.

C3H8, C4H10, C6H6, C7H8, and C6H6O with mass-to-charge ratios (m/z) of 2, 15, 26, 29, 43, 78, 91, and 94, respectively. The high-boiling-point volatile products are not measurable by the MS as a result of condensation in the system. As seen in Figure 6, the release curves of each product of the two samples almost overlap, indicating insignificant overall effects of minerals on pyrolysis of organic matter, which is consistent with the TG results. It can also be seen in Figure 6 that the peak temperatures of C3H8, C4H10, C6H6, C7H8, and C6H6O are close to the DTG peak temperature of around 450 °C, while those of CH4, C2H4, and H2 are higher than the DTG peak temperature. Furthermore, the CH4 release range is obviously wider than the main mass loss range in the DTG curve. These behaviors suggest that the former

treatment, YLOS-CN and YLOS-CNF, yield more mass loss at around 250 °C in comparison to other samples, indicating oxidation of the sample by HNO3 and loss of oxygen in pyrolysis at the temperature. The samples subjected to the HCl + HF treatment, YLOS-CF and YLOS-CNF, yield less mass loss in a temperature range of 480−650 °C in comparison to other samples, indicating removal of hydrate or hydroxyl water by HF. The mass loss peak observed in a range of 750−850 °C for the raw sample, YLOS-R, indicates decomposition of calcite, which is not seen in all of the HCl-treated samples. 3.3.2. MS Analysis. Figure 6 shows the major light volatile products detected by MS during pyrolysis of YLOS-R (with the highest mineral content) and YLOS-CNF (with the lowest mineral content). The products detected include H2, CH4, C2H4, G


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Energy & Fuels compounds (C3H8, C4H10, C6H6, C7H8, and C6H6O) are mainly the primary products, while the latter compounds (CH4, C2H4, and H2) are more influenced by the secondary reaction, such as decomposition of C3H8 for C2H4 and demethylation of primary volatiles followed by reaction with hydrogen. The evolution curve of H2 of YLOS-R is similar to that reported for low-rank coals and shows two peaks, a small peak at around 485 °C and a large peak at around 700 °C.31 Disappearance of the small H2 peak for YLOS-CNF may indicate that some of the minerals are able to catalyze cleavage of the bond between aliphatic carbon and hydrogen. It may also be related with the oxidation of some aliphatic C−H bonds by HNO3 to form benzene carboxylic acid (mono-Car-COOH, easy to crack in pyrolysis) as indicated by NMR analysis and appearance of a C6H6 peak at around 280 °C. Besides the small C6H6 peak at 280 °C, each of the C6H6 curves shows a major peak at around 450 °C and another small peak at around 550 °C. This phenomenon is also observed in coal pyrolysis, where the major peak was ascribed to hydrogenation of the substituent of the phenyl moiety, while the small peak was ascribed to dehydrogenation of cyclic aliphatic compounds.32 Figure 7 shows the MS curves of C2H4, C3H8, C4H10, C6H6, C7H8, and C6H6O of YLOS-CN (with the smallest DTG peak depth) and YLOS-CF (with the largest DTG peak depth). It is clear that the peaks of YLOS-CN are obviously higher than those of YLOS-CF, which supports the above deduction that montmorillonite promotes reaction of organic volatiles to form coke and gas. These gas release phenomena also suggest that the catalytic effect of inherent pyrite in the YLOS-CF sample on pyrolysis may mainly be ascribed to the catalytic decomposition of the organic matter.

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AUTHOR INFORMATION

Corresponding Author

*Telephone: +86-10-64421077. E-mail: [email protected]. ORCID

Zhenyu Liu: 0000-0002-3525-273X Qingya Liu: 0000-0003-0354-9026 Author Contributions †

Xiaosheng Zhao and Xiaoliang Zhang contributed equally to this work. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors thank the State Basic Research Development Program of China (2014CB744301) for financial support.

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REFERENCES

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4. CONCLUSION (1) The organic matter in Yilan oil shale contains approximately 50.43% aliphatic carbons, 46.85% aromatic carbons, and 2.72% carbonyl carbons. The aliphatic carbons include 73% aliphatic C(2) and methylene carbons, while the aromatic carbons include 33% multi-ring aromatics. The average methylene chain length is 5−6. The acid treatments have little influence on the structure of organic matter, except that a few aliphatic carbons are oxidized to the carboxyl group by HNO3 treatment. (2) The minerals in the oil shale are mainly quartz and kaolinite, accounting for about 85 wt %, with some montmorillonite, calcite, iron oxide, pyrite, and TiO2. Calcite, iron oxide, and pyrite, with contents of 0.64, 2.63, and 0.60 wt %, respectively, promote decomposition of the organic matter, which results in an around 6.5 wt % increase in the mass loss. HCl + HF treatment increases the mass loss by 5.8−8.4 wt % but decreases the formation of C2H4, C3H8, C4H10, C6H6, C7H8, and C6H6O, which indicates the catalytic effect of montmorillonite on the reaction of organic volatiles to form coke and gas. (3) The formation of C3H8, C4H10, C6H6, C7H8, and C6H6O is mainly from the primary reaction, while the formation of CH4, C2H4, and H2 is affected more by reactions of volatiles.

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montmorillonite during pyrolysis, determination of mass losses of organics (the corrected mass loss) in Table 5, and correction of the ultimate analysis result in Table 3 (PDF)

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.energyfuels.6b03404. XRF analysis results of YLOS-R and the acid-treated oil shale samples (Table S1), H2O release curves detected by MS during pyrolysis of YLOS-R and YLOS-CNF (Figure S1), estimation of water released from kaolinite and H


Article

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Organic Matter in Yilan Oil Shale: Characterization ... - ACS Publications

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