Study of a Vitrinite Macromolecular Structure Evolution Control

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Study of a Vitrinite Macromolecular Structure Evolution Control Mech-anism of the Energy Barrier in Hydrocarbon Generation Wu Li, Yanming Zhu, Yu Song, and Meng Wang Energy Fuels, Just Accepted Manuscript • Publication Date (Web): 19 Dec 2013 Downloaded from http://pubs.acs.org on December 20, 2013

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Study of a Vitrinite Macromolecular Structure Evolution Control Mechanism of the Energy Barrier in Hydrocarbon Generation Wu Li †,‡,§, Yanming Zhu *,†,‡, Yu Song†, Meng Wang†,‡ †

School of Resources and Earth Science, China University of Mining & Technology, Xuzhou 221116, China

‡

Key Laboratory of Coalbed Methane Resource & Reservoir Formation Process, Ministry of Education, Xu-

zhou 221116, China §

School of Chemical Engineering, the University of Queensland, Brisbane, 4072, Australia

ABSTRACT: An energy barrier mechanism exists in the hydrocarbon generation of vitrinite. Traditional coal geochemistry is unable to explain the mechanism of the macromolecular structure evolution in the process of hydrocarbon generation. This paper studies the hydrocarbon generation characteristics by thermal simulation experiments to obtain the control mechanism of the vitrinite macromolecular structure evolution control on hydrocarbon generation. The vitrinite structure characteristics were studied by Fourier transform infrared spectroscopy (FTIR) and Carbon-13 nuclear magnetic resonance (13C NMR), and the structural parameters of vitrinite were calculated. Based on building the model of the macro-molecular structure in a vitrinte sample, the coupling mechanism between hydrocarbon generation and the evolution of the vitrinite structure was determined through quantum chemistry. These results are important and practical for the coalification theory and coalbed methane (CBM), shale gas and other unconventional gases. The results showed that the hydrocarbon production rate increased along with increasing maturity. Gaseous hydrocarbon consists of methane and heavy hydrocarbon alkanes and alkenes. The C2-5/C1-5 ratio decreases linearly with increasing maturity. The intensity of the vitrinite functional group absorption peak decreases. Aliphatic hydroACS Paragon Plus Environment

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carbons have an absorption peak before 430°C, which then declines to periodic variation characteristics. The intensity of the absorption peak due to the C=O moiety of aromatic hydrocarbons (1600 cm-1) decreases. The response of the intensity of substituted aromatic hydrocarbons is weak. A polyester reaction occurs at 450°C. The aromatic carbon rate change is divided into three stages. The average molecular potential energy decreases with the pyrolysis process. Vitrinite removed the methyl macromolecular structure first and then the aliphatic hydrocarbons, aliphatic chain rings and other bonds. Key Words: Vitrinite; Macromolecular structure; Hydrocarbon generation; Energy barrier; Structure evolution 1. INTRODUCTION Coalification is an important proposition of coal geological science. Recently, scholars have conducted related research works. Three aspects of the results have been achieved. First, there are four jumps in the coalification of coals, whereas five or six jumps lie in high rank coals.1 Second, the changing trends of the structural parameters were defined, and the functional groups and aromatic carbon rate in the coalification were qualitatively and quantitatively analyzed.2,3 Third, an average representative coal molecular structure model in different coalification stages was established, as well as a coalification mechanism.4,5 For the corresponding relations between the hydrocarbon generation quantity and the structure evolution, as well as the coalification jump mechanism, further analysis is needed, especially study on the single maceral vitrinite. There is an energy barrier phenomenon in the vitrinite pyrolysis hydrocarbon generation process. The coal structure changed along with the coal rank in coalification. This paper refactored the vitrinite molecular structure model by various structure characterization methods to understand the relationship between molecular structure evolution and the hydrocarbon 2

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generation mechanism. This work is of great theoretical and practical significance for coal hydrocarbon generation kinetics and super molecular engineering, as well as the theory of coalification and CBM, shale gas and other hot research fields in unconventional gas. At present, the thermal simulation of hydrocarbon generation includes three different systems with different experimental purposes.6 Domestic and overseas studies are mainly concentrated on building the three dimensional coal molecular structure and its features and have only minimally reported on the corresponding relation between the vitrinite macromolecular structure evolution and hydrocarbon generation change and control mechanisms to the energy barrier.7-10 A molecular structure was built and applied in hydrocarbon generation kinetics research in this study through years of work. Gas samples and solid samples were from the same chemical reaction to avoid complicated geological processes, which provided more reliable analysis methods. The control mechanism of the molecular structure evolution to the energy barrier was discussed, and a vitrinite macromolecular structure evolution model was obtained in the process of coalification based on the study of the molecular structure of vitrinite and the variation characteristics of hydrocarbon products. 2. SAMPLES AND EXPERIMENTAL There are nineteen solid samples and eighteen gas samples. The vitrinite sample SLV was separated from the coal sample collected from the Sela coal mine in Inner Mongolia, China. The eighteen solid vitrinite samples collected by the pyrolysis were named by SLV-350 to SLV-520 every 10°C .The working procedure for separating vitrinite from coal was performed strictly according to the standard protocol (MT/T807-1999). The purity of vitrinite samples was greater than 95%. The elemental characteristics of the vitrinite are shown in Table 1. Table 1 Samples

Elemental analysis of sample and residue

Ro,max(%)

C (%)

H (%)

3


O (%)

N (%)

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SLV

0.58

78.21

4.15

16.69

0.77

SLV-350

-

85.67

6.56

6.54

1.23

SLV-430

-

87.27

6.08

5.34

1.31

SLV-480

-

91.23

2.93

3.45

1.39

Note: SLV-350 means the vitrinite separated from Sela mine sample at 350°C by heating at 30 °C /h.

Improved MSSV, closed-system pyrolysis was applied for the vitrinite pyrolysis experiment. The experiment includes three parts. First, 15-20 mg vitrinite and a little silica wool were placed in a preparative glass tube, and the tube was sealed. Second, 18 vacuum glass tubes were placed in a pyrolysis apparatus, which was connected to power with temperature programmed heating at a heating rate of 30 °C/h. Third, the samples were removed from 350°C to 520°C every 10 °C, and the samples were processed. The gas and solid products were collected by a gas collecting device and a plastic bottle for analysis of the gas composition and the vitrinite structure.Using the closed-system pyrolysis technique, pyrolysis using microscale sealed vessel (MSSV) was performed at the Guangzhou Institute of Geochemistry, Chinese Academy of Sciences. All of the analyses were conducted in accordance with ISO or ASTM standard procedures. The gas product was analyzed using a 2010puls gas chromatography (GC) equipped with a 1.0 m×1 mm ID capillary column coated with Shin-Carbon ST 100/120. The data were acquired and processed using GC solution software. The structural characterization by Fourier transform infrared spectroscopy (FTIR) and Carbon-13 nuclear magnetic resonance (13C NMR) was performed at the Guangzhou Institute of Geochemistry, Chinese Academy of Sciences and Nanjing University. Figure 1 shows the experimental process.

4


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Figure 1.

Schematic drawing of the experimental setup

FTIR Measurement: Powdered coal (0.9 mg, under 200 mesh), was initially ground with 80 mg of KBr for 20 min in an agate mortar. The mixture was molded into a disc. Then, pressing this powder into a transparent sheet for 10 min using a tablet machine. Pure ground KBr was used to obtain a reference spectrum. The discs were analyzed by FTIR using instrument (model VERTEX-70, made by Bruker in Germany), and the spectra were recorded in the range of wave numbers 400-4000 cm-1 at a resolution of 4 cm-1.11 Solid state cross-polarization magic angle spinning (CP/MAS)

13

C NMR Measurement:

The 13C NMR spectra of vitrinite were performed on a Bruker Avance III 400 spectrometer. All experiments were run in double-resonance probe heads using 4-mm sample rotors. Semi-quantitative compositional information was obtained with good sensitivity using a 13C CP/MAS NMR technique in conjunction with the TOSS technique. (MAS=4 kHz, contact time time=1 ms, the recycle delay=1 s).12,13 3. Results and discussion

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Thermal simulation of the hydrocarbon production rate test showed that the products mainly include olefin and paraffin. The FTIR and 13C NMR spectrums of the solid residue are collected. 3.1. Hydrocarbon generation characteristics. The pyrolysis products were mainly gaseous hydrocarbons, including methane, ethane, propane, butane and some olefin. The butane consisted of normal butane and isobutene. The methane and total hydrocarbon production rate increased with increasing temperature. The methane content percent was 50% at 350°C and increased gradually to the maximum 80%. The change laws of the total hydrocarbon yield and methane production rate were similar. As shown in Figure 2, the production of methane can be divided into five stages. 350-430°C is the initial slow growth. 430-440°C is the stunted growth stage. 440-480°C is the hydrocarbon rapidly increasing stage. 480-500°C is the second generation stagnation phase. The hydrocarbon content continues to increase after 500°C. The five phases have different characteristics, on behalf of the four evolutionary stages in the hydrocarbon generation process, which will be analyzed in details in the next chapter.

Figure 2.

Methane yield and generation rate

Figure 3 shows that the yield of heavy hydrocarbon and C2-5/C1-5. The heavy hydrocarbon gas content increases first and then remains constant as the temperature rises. There are two stages. In the first stage, coal organic matter is translated into hydrocarbons because of heating. In the second stage, with the increasing temperature, liquid hydrocarbon and gaseous hydrocarbon turn into heavy 6


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hydrocarbon gas by pyrolysis. At the same time, the heavy hydrocarbon gases of small molecules crack into methane.14 The cracking rate is greater than the generation rate of the heavy hydrocarbon gases before 450°C. The content of the heavy hydrocarbon gas remains constant and the generation rate of the heavy hydrocarbon gas is similar to the cracking rate after 450°C. Methane, cracked from C2-5 heavy hydrocarbon gas, supplied the conversion rate. Pyrolyzed heavy hydrocarbon gas showed a negative linear trend with the increase of temperature because, in the process of heating, organic matter fully pyrolyzed, leading to a greater generation rate of dry gas than wet gas, and with the increase of temperature, wet gas cracks into dry gas.

Figure 3.

Distribution diagram of the hydrocarbon yield and C2-5/C1-5

3.2. Structure characterization. FTIR. Figure 4a shows the FTIR spectra of the samples before pyrolysis. The 3178 cm-1 and 2361 cm-1 peaks represent hydrogen bonding absorption, the 2921 cm-1 peak represents CH2 absorption, the 2851 cm-1 and 1375 cm-1 peaks represent CH3 absorption, the 1697 cm-1 peak represents carboxyl absorption, the 1684 cm-1 peak represents carbonyl absorption, the 1577 cm-1 peak represents C=C absorption, the 1559 cm-1 peak represents aromatic hydrocarbon absorption, the 1236 cm-1 peak represents C=O absorption, and the 877 cm-1 and 812 cm-1 peaks represents CH absorption of aromatic hydrocarbons. The positions of these samples’ functional groups were at 3200-3600 7


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cm-1, 3000-3100 cm-1, 2800-3000 cm-1 and 1000-1800 cm-1. The 3200-3600 cm-1 position indicates hydroxyl radical absorption region.15 The 3000-3100 cm-1 position represents the aromatics absorption region. The 2800-3000 cm-1 position indicates the absorption region of an aliphatic hydrogen structure. The 1000-1800 cm-1 position is the absorption region of oxygen-containing functional groups. The content of the functional groups of vitrinite decreased with the increasing temperature. The hydrocarbon fat absorption peak is obvious before 430°C and then decreased, showing periodic variation characteristics. The intensity of the 1600 cm-1 peak (aromatics C=O with oxygen substituted) decreased along with the increasing temperature. The intensity of the substituted aromatics remained constant. There was a sharp strong absorption peak (1400 cm-1) at 450°C that represents the lipid groups in Figure 4b. This indicates that 450°C is the initiation temperature of the polyester reaction that forms aromatic lipids and the content begins to increase.

Figure 4.

FTIR spectra of vitrinite samples

Overlapping peaks were distinguished by Origin Software. The relative contents of the different functional groups were determined using the transmission peak area in Table 2, which revealed the pattern of functional groups in vitrinite during the hydrocarbon-generating process in different coal ranks. With the increase of maturity, the absorption peak intensity decreased by varying degrees. The 8


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content of the structural unit functional groups decreased and transformed into the formation of hydrocarbons and inorganic gases in the pyrolysis leading to the average molecular weight of shrinking. Table 2.

Intensity of different absorption peaks Wave/cm-1 (Functional Groups)

Sample

Ro,max(

3000~3100 1600

No

1705

%) C=C

Carbonyl

700~900

2800~3000 Aromatic

Substituted

CH

Aromatic

Aliphatic CH

2920

2950

CH2

CH3

SLV

0.58

10.71

1.07

2.22

1.57

0.71

0.55

0.26

SLV-350

0.82

10.25

0.94

0.92

0.13

1.46

0.41

0.08

SLV-360

1.05

4.99

0.48

0.35

0.05

0.68

0.09

0.04

SLV-370

-

5.01

0.35

0.36

0.06

0.78

0.10

0.04

SLV-380

1.18

7.24

0.77

0.98

0.10

1.43

0.37

0.10

SLV-390

-

1.69

0.16

0.15

0.03

0.45

0.07

0.01

SLV-400

1.22

6.21

0.62

1.11

0.08

1.24

0.41

0.17

SLV-410

-

4.07

0.60

1.35

0.05

1.26

0.51

0.32

SLV-420

1.37

3.49

0.14

0.40

0.07

1.43

0.13

0.05

SLV-430

-

3.63

0.10

0.52

0.07

1.45

0.19

0.06

SLV-440

1.38

1.70

0.03

0.20

0.13

1.03

0.08

0.01

SLV-450

-

3.14

0.05

0.22

0.05

1.67

0.09

0.01

SLV-460

1.40

1.45

0.02

0.09

0.04

0.76

0.02

0.01

SLV-470

-

1.23

0.02

0.11

0.04

1.15

0.01

0.01

SLV-480

1.71

0.91

0.01

0.15

0.03

0.84

0.06

0.01

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SLV-490

-

0.99

0.02

0.14

0.04

1.12

0.04

0.01

SLV-500

-

1.15

0.02

0.08

0.05

1.26

0.01

4.00 ×10-2

SLV-510

-

0.49

0.01

0.06

0.03

0.80

0.03

4.00×10-2

SLV-520

-

0.47

0.02

0.17

0.02

0.52

0.07

0.02

The intensity of the 1600 cm–1 functional group (aromatic ring carbon skeleton C=C double bond) decreased because the degree of metamorphism for vitrinite pyrolysis is a dehydrogenation deoxidization process and chemical bond rupturing and restructuring form gaseous hydrocarbon (Figure 5). The intensity of vitrinite at 350°C is similar to the sample without pyrolysis. However, the intensity decreases after 350°C, mainly because C=C bond breaking requires more energy, which indicates that detection limit of hydrocarbons is 300-350°C. We found that the 1705 cm-1 peak represented carbonyl absorption, and it decreased along with temperature for the carbonyl structure decomposition and weak bond rupture reaction. The intensity of the absorption peak at 3000-3100 cm-1(aromatics) gradually decreased. The intensity mutation of the aliphatic carbon absorption peak (2800-3000 cm-1) occurred to 420°C and can be divided into two stages. In the second stage (after 420°C), the area of the absorption peak becomes smaller. It also can be seen from the Figure 2 that he content of methane sharply increased at 420°C. Therefore, the gaseous methane mainly comes from the carbon of aliphatic hydrocarbon (2800-3000 cm-1).

Figure 5.

The intensity of the 1600 cm–1 functional group changes with increasing maturity 10


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The values of CH2/CH3, I1, I2, A, and C, obtained from the FTIR spectra, are listed in Table 3. The CH2/CH3 (2920 cm-1/2950 cm-1) ratio is used to estimate the length and degree of branching aliphatic side chains.16 The ratio of integrated areas of 3000-2800 cm-1 to 1600 cm-1 (I1) and to 900-700 cm-1 (I2) can be used to compare the relative abundance of aliphatic and aromatic functional groups. The A factor (3000-2700 cm-1/3000-2700 cm-1 + 1600 cm-1) and C factor (1705 cm-1/1705 cm-1 + 1600 cm-1), which represent the intensity of aliphatics relative to aromatic peaks, is used for describing kerogen type and maturation levels.17 The highest CH2/CH3 factor was measured from SLV-450 (8.88) and the lowest from SLV-410 (1.60). HAR/HAL factors from most of the samples were under 0.5, and elevated HAR/HAL factors were measured from SLV, SLV-440 and SLV-500. The highest I1 factor was measured from SLV-520 (0.35). The highest I2 factor was measured from SLV (3.11). The CH2/CH3 ratios of samples are higher than 1.00, which indicates that the contents of the CH2 of the aliphatic side chains are greater than that of the CH3. Previous research suggests that the aliphatics of vitrinite are mainly short chain aliphatics. Through calculation, samples have lower values of CH2 and CH3 in coalification, suggesting that the relative concentration of aliphatic groups in vitrinite decreases. The CH2 and CH3 values of vitrinite increased and decreased periodically, which is in contrast from previous results, indicating that the aliphatic concentration of vitrinite increases at the stage of branched chain breaking and decreases at the stage of aromatic ring opening. The I1 factor values of the samples have a similar relationship with the CH2 and CH3. The ratios of HAR/HAL of the samples was less than 1.0 and increased with maturity, showing that the intensities of the aliphatic CH stretching absorption of these samples are greater than that of the aromatic CH stretching absorption. The I2 factor reduces along with the increasing maturity. The C factor maintains the same values before 420°C, indicating that the aromatic concentration of samples does not reduce. However, it reduces significantly after 420°C due to aromatic cracking. Finally, the C factor 11


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is slightly higher for intermediate aromatic association product formation. The carbonyl concentration decreased significantly between 420°C and 440°C, corresponding to the first lag phase of methane generation, because the oxygen containing functional groups connecting the aromatic break to CO2 generation and hydrocarbon generation is restricted.18 Table 3.

FTIR structure parameters of the samples

Samples

CH2/CH3

HAR/HAL

I1

I2

A

C

SLV

2.12

0.65

0.21

3.11

0.18

0.09

SLV-350

5.32

0.15

0.09

0.63

0.08

0.08

SLV-360

2.60

0.14

0.07

0.51

0.06

0.09

SLV-370

2.54

0.17

0.07

0.46

0.07

0.07

SLV-380

3.66

0.10

0.14

0.69

0.12

0.10

SLV-390

5.66

0.20

0.09

0.34

0.08

0.09

SLV-400

2.49

0.07

0.18

0.90

0.15

0.09

SLV-410

1.60

0.03

0.33

1.07

0.25

0.13

SLV-420

2.66

0.18

0.11

0.28

0.10

0.04

SLV-430

2.95

0.14

0.14

0.36

0.12

0.03

SLV-440

5.85

0.65

0.12

0.19

0.11

0.01

SLV-450

8.88

0.07

0.07

0.13

0.07

0.02

SLV-460

3.25

0.40

0.06

0.12

0.06

0.01

SLV-470

3.33

0.37

0.09

0.10

0.09

0.02

SLV-480

4.14

0.23

0.16

0.17

0.14

0.01

SLV-490

3.63

0.27

0.14

0.12

0.12

0.02

SLV-500

2.15

0.60

0.07

0.06

0.04

0.01

SLV-510

8.08

0.48

0.12

0.07

0.11

0.02

SLV-520

3.30

0.11

0.35

0.32

0.26

0.04

In conclusion, the differences of the chemical bonds and the order of different functional groups cracking off result in some stages in the pyrolysis of vitrinite. The cracking order is fatty branched chains, oxygen containing functional groups, aromatic hydrocarbons, and replacing aromatic hydro12


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carbons. The A factor has the highest values at 410°C, which is the start of hydrocarbon generation. The A factor increases for aromatics translates into aliphatic. However, the result should be further discussed. The hydrocarbon content remains constant between 410°C and 480°C, which indicates no conversion occurred from aliphatic and aromatics to gaseous hydrocarbon for they need more energy to continue the conversion. Solid 13C NMR characterization. Solid State CP/MAS 13C NMR Measurement can reveal the carbon types of the molecular structure in vitrinite. The spectra were clearly divided into two regions, an aromatic band from 100 to 170 ppm and an aliphatic band from 0 to 90 ppm.19 The

13

C

NMR measurement of vitrinite and its different maturity vitrinite samples shows that the intensity of the aromatic band is far greater than the aliphatic band. Two sharp absorption peaks indicate that the carbon types are minimal (Figure 6). The relative intensity of the aliphatic bond decreases along with the increasing maturity.

Figure 6.

13

C NMR spectra of vitrinite samples

The structure parameter was used to study the carbon types in the pyrolysis. The computing method of the structure parameter is based on the reference.20 The main carbon types of sample SLV 13


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are protonated and aromatic C, followed by CH3 or quaternary C, CH or CH2, aromatic bridgehead C, alkylated aromatic C, CH3 or quaternary C, aromatic C bonded to hydroxyl or ether oxygen, and aliphatic C bonded to oxygen.21 The content of carbonyl group or carboxyl group C is minimal. The relative content of aliphatics, including CH3, CH2 and CH, is 40 percent in sample SLV. During pyrolysis, the content of the aromatic carbon increased by 0.25 from 0.594 to 0.85 at 350°C (Table 4). It reached 0.9 after 440°C. The aromatic carbon rate increases as the aliphatic carbon decreases, which leads to a more stable vitrinite structure. In the high coal rank, the aromatic carbon content increases for the macro-molecular basic structural units in coal. The order degree of vitrinite increases with the evolution. It reaches the highest aromatic carbon rate value of 1.00. The carbon skeleton of vitrinite has experienced a condensation reaction, aromatization and making-up along with increasing maturity, which leads to the stage characteristics of the aromatic carbon rate. Table 4.

13

C NMR structure parameters of vitrinite

Sample

fa

fac

f’a

fa H

faN

faP

faS

faB

fal

fal*

falH

falO

SLV

0.594

0.01

0.54

0.34

0.20

0.03

0.06

0.11

0.45

0.08

0.29

0.08

SLV-350

0.85

0.01

0.84

0.40

0.45

0.08

0.16

0.21

0.15

0.06

0.08

0.01

SLV-360

0.83

0.05

0.77

0.35

0.43

0.08

0.16

0.19

0.17

0.06

0.11

0.01

SLV-380

0.83

0.03

0.80

0.45

0.34

0.04

0.11

0.18

0.17

0.07

0.08

0.02

SLV-400

0.84

0.01

0.84

0.47

0.37

0.05

0.12

0.20

0.16

0.06

0.09

0.01

SLV-420

0.85

0.02

0.83

0.47

0.37

0.05

0.12

0.20

0.15

0.07

0.07

0.01

SLV-440

0.91

0.05

0.86

0.50

0.36

0.03

0.12

0.20

0.09

0.04

0.05

0.00

SLV-470

0.92

0.03

0.89

0.52

0.38

0.04

0.12

0.22

0.08

0.03

0.05

0.00

SLV-490

0.92

0.04

0.88

0.58

0.30

0.01

0.08

0.21

0.08

0.04

0.03

0.01

SLV-510

0.90

0.02

0.88

0.48

0.40

0.04

0.14

0.22

0.10

0.03

0.06

0.02

14


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Note： fa－total sp2 hybridized C；fal－total sp3 hybridized C；fac－carbonyl group or carboxyl group C(δ＞165)；fa ’－aromatic C； faH－protonated and aromatic C；faN－nonprotonated and aromatic C；faP－aromatic C bonded to hydroxyl or ether oxygen (δ＝ 150~165)；faS－alkylated aromatic C (δ＝135~150)；faB－aromatic bridgehead C；fal*－CH3 or quaternary C；falH－CH or CH2； falo－aliphatic C bonded to oxygen (δ=50~90).

3.3 Hydrocarbon generation and structure evolution model of vitrinite. Based on the hydrocarbon generation characteristics and the structure characterization, this chapter mainly discusses the influence of the molecular structure on hydrocarbon generation. The hydrocarbon yield, the content of aliphatic hydrocarbon, the aromatic carbon rate and the aromatic bridgehead C were used to indicate hydrocarbon generation and the structure evolution model of vitrinite. Experimental simulation of the gaseous hydrocarbon generation yield and numerical modeling of the molecular structure indicate a relationship between the structure evolution characteristics in the process with the coal structure parameters.

Figure 7.

Mode chart of vitrinite structure evolution along with hydrocarbon generation

Figure 7 indicates that the changing rule of the structure parameters included HAR/HAL and fa. The total hydrocarbon yield, increasing along with the maturity, can be divided into two stages. The total hydrocarbon production rate is low, and the growth is slow in the first stage between 350°C and 440°C. The total hydrocarbon production rate increased rapidly after 440°C in the second stage. The HAR/HAL value increased rapidly at 440°C, indicating that the content of aromatic hydrocarbon decreased at this stage, which also indicates that the hydrocarbon generation of vitrinite is mainly pol15


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ycondensation. The aromatic carbon rate (fa), increasing along with temperature, was divided into three stages. Before 350°C, the aromatic carbon rate increased rapidly for some fatty carbon loss. It remained constant at 0.82 between 350°C and 440°C and finally increased after 440°C. 3.4 Building the structure model of vitrinite. With a carbon content of 78.21% and an aromatic carbon rate of 0.594, the value of XBP calculated for the SLV sample was 0.2128. The ratio of aromatic bridge carbon to aromatic peripheral carbon, XBP=faB/ (faH+faP+ faS), is an important parameter that can be used to calculate the aromatic cluster size.22 Based on this and the fact that the ACD/CNMR Predictor software can only be used with molecules containing no more than 255 atoms (excluding H), we adopted the types and numbers of aromatic structures for this model shown in Table 5. Previous studies indicated that aromatic units are dominated by 2-4 rings when the maximum vitrinite reflectance is 0.5-2.0%. As reported above, the ratio of aromatic bridge carbon to aromatic peripheral carbon (XBP) was 0.2128. This ratio is 0.25 in naphthalene (condensation degree of 2), which is greater than that determined for the SLV sample. Therefore, it was determined that benzene would be the main aromatic compound used in modeling, followed by naphthalene and anthracene. The number of aromatic carbon atoms is 99. The total carbon number is 167, and the number of aliphatic carbon atoms is 68. The number of hydrogen and oxygen is determined by elemental analysis. Nitrogen is mainly present as pyrrole nitrogen and pyridine nitrogen. Table 5. Aromatic structure

Aromatic structure units of sample SLV

H N

N

Number

1

1

2

5

2

chemical shift (ppm)

107,118

124,136,150

128

126,126,134

125,126,128,132

units

According to the proximate and ultimate analyses, the types of structural units in the model as 16


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Energy & Fuels

described above, and other structural parameters, such as the H/C and O/C ratios and the degree of aromaticity, the macromolecular structure model of SLV sample were obtained. The ACD/CNMR Predictor software was applied to compute the 13C NMR spectra for the macromolecular structure. The comparison between the computed 13C NMR and the experimental spectra is shown in Figure 8.

Figure 8.

Experimental and calculated 13C NMR spectrum of the SLV sample

The structural parameter constant was used to modify the macromolecular structure continuously to make it match the experimental

13

C NMR spectra. The final structure model is shown in

Figure 9a. The final structural model was the basis of the hydrogen saturation simulations using Materials Studio 4.0 software to conduct molecular mechanics and molecular dynamics computations to obtain the minimum energy configuration of this model (Figure 9b).

Figure 9.

Model of the macro-molecular structure in the SLV coal sample 17


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The energy of the vitrinite and pyrolysis sample at 350°C and the macromolecular structure are shown in table 6. The total energy E of the Dreiding force field consists of EV (valence energy) and EN (non-bond energy). EV includes EB (bond energy), EA (angle energy), ET (torsion energy) and EI (inversion energy), and EN includes Evan (van der Waals energy), EE (electrostatic energy) and EH (hydrogen bond energy).23 The total energy decreases along with the maturity. The increasing ET (torsion energy) and EN, including Evan (van der Waals energy), indicates that the vitrinite macromolecular structure goes through polycondensation during coalification. Table 6

Energy of the vitrinite macro-molecular structure α

Sample

E

EV

EB

EA

ET

EI

EN

EH

Evan

EE

SLV

484.16

268.46

83.77

91.12

92.23

1.35

215.69

-5.64

354.75

-133.42

SLV-350

458.87

235.02

67.88

64.86

100.23

2.06

223.85

-8.66

289.83

-57.32

α

E, Ev, EB, EA, ET, EI, EN, EH, EVAN, EE, kcal/mol; E= Ev+ EN; Ev= EB+EA+ET+EI; EN=EH+EVAN+EE

Figure 10.

Density dynamics simulation curve and molecular model with minimizing energy

The Amorphous Cell Module in Materials Studio Suite was used to apply periodic boundary conditions to the optimum energy geometric configuration of the SLV coal structure model. Molecular mechanics and annealing dynamics simulations were performed to search for the optimum configuration under the periodic boundary conditions.24 The unit cell size was varied to determine the 18


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Energy & Fuels

potential energy variations under different periodic boundary conditions, yielding plots of potential energy versus density, as shown in Figure 10. The unit cell dimensions are: 15.1 Å×15.1 Å×15.1 Å. Figure 10 shows that there is the lowest potential energy, 368.12 kJ/mol, when the density is 1.25 g/cm3. 3.5. Structure evolution of vitrinite macromolecule. The model of sample SLV-350°C was also obtained. In this model, the activity of the macromolecular structure can be qualitatively described in terms of the bond length. The simulation of the bond length parameter of the molecular structure indicates that the greater the bond length, the smaller the bond energy and the lower the bond order. The relationship between the molecular structure and cracking can be determined by the activity sites of the chemical bond rupture. The bond lengths of the SLV model structure are shown in Table 7. Table 7. Bond Chemical Bond

Bond length parameters of SLV Bond

Chemical Bond length

Bond Chemical Bond

length

Bond Chemical Bond

length

length

C15-C1

9.08

C75-C74

1.541

C94-C93

1.428

C70-C68

1.408

C93-C12

4.890

C144-C143

1.540

C87-C85

1.428

C73-C72

1.407

C26-C15

3.815

C54-C49

1.526

C49-C42

1.427

C51-C50

1.407

C134-C131

3.777

C57-C43

1.514

C47-C46

1.427

C27-C26

1.407

C146-C144

3.362

C36-C31

1.512

C100-C99

1.426

C71-C70

1.407

C40-C34

3.194

C142-C140

1.512

C88-C87

1.426

C89-C88

1.406

C64-C55

2.564

C145-C141

1.504

O78-C75

1.426

C65-C64

1.406

C74-C59

2.546

C154-C150

1.503

C20-C18

1.425

C101-C99

1.406

C85-C82

2.546

C107-C5

1.503

C148-C147

1.425

C72-C64

1.403

19


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C11-C7

2.529

C13-11

1.500

C133-C131

1.425

C24-C23

1.403

C50-C46

2.499

C121-C69

1.499

C48-C41

1.425

C97-C95

1.403

C29-C26

2.489

C79-C48

1.498

C96-C94

1.424

C23-C21

1.403

C7-C3

2.481

C58-C54

1.496

C44-C42

1.422

C153-C151

1.403

C46-C41

2.477

C28-C27

1.495

C86-C85

1.422

C53-C51

1.401

C158-C156

2.464

C109-C6

1.494

C141-C140

1.422

C98-C97

1.401

C41-C40

2.450

C77-C60

1.492

C33-C27

1.422

C10-C8

1.399

C99-C12

2.443

C172-C62

1.489

C31-C30

1.421

C103-C12

1.399

C59-C58

2.401

C115-C105

1.488

C25-C19

1.419

C45-C40

1.397

C171-C132

1.578

C147-C146

1.488

C186-C135

1.419

C90-C89

1.396

C155-C154

1.574

C161-C154

1.488

C22-C20

1.418

C165-C164

1.393

C188-C158

1.571

C84-C22

1.487

C4-C1

1.418

C167-C165

1.388

C132-C77

1.570

C18-C17

1.486

C95-C93

1.415

O81-C1

1.374

C55-C54

1.567

C122-C29

1.483

C60-C59

1.415

O149-C148

1.371

C179-C178

1.565

C56-C44

1.483

C128-C86

1.415

C131-C78

1.364

C189-C188

1.565

C76-C59

1.482

C104-C11

1.415

O114-C64

1.363

C174-C121

1.564

C125-C84

1.482

C152-C147

1.415

N61-C59

1.360

C190-C188

1.563

C19-C16

1.482

C151-C150

1.415

O14-C8

1.359

C191-C189

1.563

C118-C13

1.480

C52-C50

1.414

O173-C85

1.356

C169-C142

1.562

C180-C179

1.479

C111-C50

1.414

O113-C111

1.353

C143-C142

1.562

C181-C180

1.461

C66-C65

1.414

O127-C125

1.350

C176-C174

1.562

O156-C155

1.454

C135-C134

1.414

O117-C115

1.349

C157-C155

1.562

C160-C156

1.449

C32-C60

1.414

O130-C128

1.349

20


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Energy & Fuels

C35-C34

1.561

C193-C192

1.447

C138-C137

1.414

O120-C118

1.348

C105-C28

1.561

C192-C190

1.447

C43-C40

1.414

N166-C164

1.346

C34-C29

1.561

C39-C38

1.435

C184-C102

1.413

O163-C161

1.344

C175-C174

1.561

C136-C131

1.433

C12-C11

1.413

C168-C166

1.342

C177-C176

1.557

C102-C94

1.433

C30-C26

1.412

O124-C122

1.342

C183-C181

1.556

C137-C136

1.432

C9-C7

1.412

O187-C186

1.256

C37-C36

1.555

O38-C37

1.432

C91-C70

1.412

O185-C184

1.252

C178-C175

1.554

C82-C25

1.432

C6-C2

1.411

O112-C111

1.251

C108-C107

1.554

C42-C41

1.432

C21-C18

1.411

O129-C128

1.250

C106-C15

1.553

O170-C146

1.430

C8-C7

1.411

O126-C125

1.250

C196-C108

1.552

C150-C148

1.430

C68-C66

1.410

O116-C115

1.250

C195-C191

1.552

C5-C3

1.430

C164-C139

1.410

O162-C161

1.249

C17-C16

1.551

C32-C31

1.429

C139-C138

1.408

O119-C118

1.248

C159-C157

1.548

C83-C82

1.429

C63-C62

1.408

O123-C122

1.248

C110-C108

1.546

C67-C66

1.429

C92-C88

1.408

O182-C180

1.225

C80-C79

1.543

C69-C67

1.429

C140-C134

1.408

O194-C192

1.220

Analysis of the bond length data shows that the lengths of the C=C bonds are greater than the lengths of the C–O bonds due to the greater electronegativity of the O atom. According to the length of bonds, the C=C bond will break at the C15-C1, C93-C12, C26-C15, C134-C131, C146-C144, and C40-C34 positions and the C=O bond will break at the O38-C37, O170-C146, O78-C75, O81-C1, O149-C148, O114-C64, and O14-C8 positions. The research indicates that the weaker position of the chemical bond is oxygen-containing functional groups, such as ether bonds and aliphatic bonds, in the coal macromolecular structure for different chemical environments and stability. 21


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The change of the SLV molecular structure during the pyrolysis process is shown in the Figure 11. Through the analysis of SLV, chemical bond breaking occurred in the oxygen containing functional groups, such as aliphatic chains, ethers and carboxylics. After releasing the C1- C3 fatty hydrocarbons, the vitrinite is composed of mainly polycyclic aromatic hydrocarbons and monocyclic aromatics, monocyclic double ring aromatics and biphenyl.

Figure 11. Geometry optimized structural conformations of SLV during pyrolysis 3.6. Control of the molecular structure evolution on the hydrocarbon energy barrier. The first continuous hydrocarbon generation cannot objectively describe the hydrocarbon generation history of the organic matter. Research on the inheritance transformation and the new nature of the vitrinite secondary hydrocarbon is non-existent. The energy barrier phenomenon exists in the second hydrocarbon generation. The meaning of the energy barrier is that organic matter evolution requires a much higher temperature than the last termination temperature to continue. More activation energy can spur further maturation of organic matter. Therefore, we believe that there is an energy barrier phenomenon in the second vitrinite structure evolution, which is controlled by the vitrinite macromolecular structure, especially the reaction activation energy. The coupling mechanism between hydrocarbon generation and the evolution of the vitrinite structure was determined by quantum chemistry. The effect on methane value from structure parameter (fa) of vitrinite evolution can be seen in a 22


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Page 23 of 27

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comparison of the curve from Figures 2 and 7 as shown in Table 4. The turning point has been found in curves of the total hydrocarbon yield and fa. These identical means the vitrinite macromolecular structure controls the hydrocarbon generation quantity and hysteresis phenomenon of the secondary hydrocarbon generation. The secondary hydrocarbon generation needs more energy than the activation energy at the end of the continuous hydrocarbon generation. The types of hydrocarbon generation and yield are controlled by the different functional groups breaking, the values of the different types of carbon, and the supply rate of energy by the system. 4. CONCLUTIONS Based on the pyrolysis simulation experiments, the ultimate analysis, and the

13

C CP/MAS

NMR data of vitrinite samples, a macromolecular structural model was constructed. The final structural model was the basis of the hydrogen saturation simulations using Materials Studio 4.0 software to conduct molecular mechanics and molecular dynamics computations to obtain the minimum energy configuration of this model. We believe that there is an energy barrier phenomenon in the second vitrinite structure evolution, which is controlled by the vitrinite macromolecular structure, especially the reaction activation energy. The results show that the hydrocarbon production rate increases along with the maturity. Gaseous hydrocarbon consists of methane and heavy hydrocarbon alkanes and alkenes. The C2-5/C1-5 ratio decreases linearly with increasing maturity. The intensity of the vitrinite functional group absorption peaks decreases. Fatty hydrocarbons have an obvious absorption peak before 430°C, which then declines to periodic variation characteristics. The absorption peak intensity of the aromatic hydrocarbon C=O (1600 cm-1) decreased. The intensity of the substituted aromatic hydrocarbon changes minimally. A polyester reaction occurs at 450°C. The aromatic carbon rate change is divided into three stages. The average molecular potential energy decreases as pyrolysis progresses. Vitrinite first 23


Energy & Fuels

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removed the methyl macromolecular structure evolution and aliphatic, aliphatic chain rings and other bonds. These results are important and practical for the coalification theory and coal gas, shale gas and other unconventional gas. AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]. Notes The authors declare no competing financial interest. ACKNOWLEDGMENTS The authors also gratefully acknowledge the support by the National Natural Science Foundation of China (41072117), the Research and Innovation Project for College Graduates of Jiangsu Province (No.CXZZ12_0945) and the Fundamental Research Funds for the Central Universities (No.2012LWB77). The authors also thank Prof Fangui ZENG for using the Materials studio 4.0 and ACD/CNMR Predictor. REFERENCES (1) Li, M. F.; Zeng, F. G.; Sun, B. L.; Qi, F. H. Evolution kinetics of hydrogen generation from low rank coal pyrolysis and its relation to the first coalification jump. Acta Phys. -Chim. Sin 2009, 25(12), 2597-2603 (in Chinese). (2) Li, W.; Zhu, Y. M.; Chen, S. B.; Zhou, Y. Research on the structural characteristics of vitrinite in different coal ranks. Fuel 2013, 107, 647-652. (3) Wu, D.; Liu, G. J.; Sun, R. Y.; Fan, X. Investigation of structural characteristics of thermally metamorphosed coal by FTIR spectroscopy and X‑ray diffraction. Energy Fuels 2013, 27, 5823-5830. 24


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Study of a Vitrinite Macromolecular Structure Evolution Control

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