MS Analyzable and

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Separation and composition analysis of GC/MS analyzable and unanalyzable parts from coal tar Ming Sun, Dan Zhang, Qiuxiang Yao, Yongqi Liu, Xiaoping Su, Charles Q Jia, Qingqing Hao, and Xiaoxun Ma Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.8b01054 • Publication Date (Web): 15 Jun 2018 Downloaded from http://pubs.acs.org on June 24, 2018

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GC/MS unanalyzable parts (the pyrolysis residues by TG at 300oC)

The composition of GC/MS unanalyzable parts from coal tar by chemical degradation analysis

TG

Py-GC/MS The quantitative determination of GC/MS analyzable parts from coal tar

Coal tar

Alkanes

Aromatics

Acidic

Alkenes

Oxygenated

Others

100

(1)

CnH2n+n

CnH2n+H2

(2)

CnH2n+n

CmH2m+ CkH2k+2

(may be)

m+k=n

(may be)

The same temperature programme of TG and GC/MS

Percentage Content(%)

GC/MS

LTCT L-PE HTCT H-PE

60

40

Distribution of compounds (%)

80

80

OH (3)

60

R

40

0 oxygenated

nitrogen

OH

OHOH

R -H2O

OH

O

OH

others

ACS Paragon Plus Environment

R1

R

R

(may be)6

R

OH

OH O

O R

R O

OH

0 0 0 0 0 0 0 0 0 0 0 0 -I-80 -I-80 -40 -40 -50 -50 -60 -60 -70 -70 -80 -80 TCT TCT TCT-S CT-STCT-S CT-STCT-S CT-STCT-S CT-STCT-S CT-S 300L 300H 300L 300HT 300L 300HT 300L 300HT 300L 300HT 300L 300HT

acidic

-H2O -HCHO

R

(4)

aliphatic

R

20

0

aromatic

acid

O -H2O

HO

R1

20

OH OH HCHO

O CH H ka li al

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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R1

O

R1

R

+

(may be)

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Separation and composition analysis of GC/MS analyzable and unanalyzable parts from coal tar Ming Sun1*, Dan Zhang1, Qiuxiang Yao1, Yongqi Liu1, Xiaoping Su1, Charles Q. Jia2, Qingqing Hao1, Xiaoxun Ma1* 1. School of Chemical Engineering, Northwest University, International Scientific and Technological Cooperation Base for Clean Utilization of Hydrocarbon Resources, Chemical Engineering Research Center of the Ministry of Education for Advance Use Technology of Shanbei Energy, Shannxi Research Center of Engineering Technology for Clean Coal Conversion, Collaborative Innovation Center for Development of Energy and Chemical Industry in Northern Shaanxi, Xi'an 710069, Shannxi, China; 2. Department of Chemical Engineering and Applied Chemistry, University of Toronto, Toronto, Ontario M5S 3E5, Canada.

Abstract: Composition analysis of coal tar remains a challenging task because of its complex components. In this paper, the compositions of low temperature coal tar (LTCT) and the wash oil fraction of high temperature coal tar (HTCT) were studied. The thermogravimetric analyzer (TG) combined with gas chromatography-mass spectrometry (GC/MS) with the same temperature program was put forward to analyze the quantitative determination of the GC/MS analyzable part of coal tar, and the composition and distribution of the GC/MS unanalyzable part (300LTCT and 300HTCT obtained from TG at the final temperature of 300oC) was investigated by a pyrolysis gas chromatography-mass spectrometer (Py-GC/MS). Results reveal that light compositions can be more effectively extracted by petroleum ether (PE) than heavy compositions. PE soluble fractions of LTCT and HTCT cannot be totally gasified by GC/MS and the remaining parts at above 300 oC are 6.51 w% and 4.99 w%,


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respectively. GC/MS combined with TG can accurately analyze the composition of ≤300 oC fractions in coal tar. Four dehydrogenation reactions were presented in the fast pyrolysis process of coal tar. An intermolecular association occurs in 300HTCT. 300LTCT is mainly composed of phenols, aliphatics, and aromatics. The composition analysis of 300LTCT and 300HTCT by Py-GC/MS indicates that there are some bridge bonds in macromolecular structure of coal tar and they have broken down to produce small molecular weight of phenolic compounds and aromatic hydrocarbons during pyrolysis.

1. INTRODUCTION Coal tar is a kind of organic compound with very complex compositions, which is composed of ten thousands of compounds 1. The research history of coal tar has more than 300 years 2, but due to the complexity of the coal tar component, there are many components that are not clearly detected. Up to now, about 500 kinds of compounds have been identified from coal tar 3. Coal tar is a complex mixture of aromatic compounds with different functional groups and a wide molecular weight distribution. How to quantitatively analyze the coal tar components accurately is a worldwide problem 4-5, which also limits the processing, pollution control and utilization of coal tar. In other words, it is necessary to analyze and identify coal tar so as to separate it effectively. Analytical instruments of gas chromatography, UV/fluorescence spectroscopy, infrared spectroscopy and gas chromatography-mass spectrometry (GC/MS) are commonly used. At present, GC/MS is the most effective method to analyze the composition and structure of coal tar. It can usually provide the information of composition and structure of the compounds which relative molecular mass is less than 500 u and the


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boiling point is less than 300 oC 6-8. This part is the main processing part of coal tar at present, such as phenol, naphthalene and pyridine, which has an important guiding significance to the industrial processing of coal tar. Samples of coal tar must be vaporized at the injection of gas chromatography. Some compounds which boiling point is more than 300 oC are presented in coal tar, such as carbazole, phenanthrene and pyrene. These components cannot be vaporized into chromatographic column at the injection of gas chromatography when using GC/MS to analyze their compositions. But actually they have been dectected by GC/MS. Because the vaporazation contains two forms: evaporation and boiling. These compounds maybe evaporated into chromatographic column, which makes some compounds (such as carbazole, phenanthrene and pyrene) with boiling point more than 300 oC detected. When complex mixtures are analyzed by GC/MS, the accurate content of compounds into the detector is uncertain, and how many components have been vaporized into chromatographic column at the injection of gas chromatography is also uncertain, so the content of each compound analyzed by GC/MS is not very accurate. Many researchers have done a lot of experiments, such as distillation extraction

12-13

and chromatography

9-11

, solvent

14-15

, etc., to separate the GC/MS analyzable and

unanalyzable fractions from coal tar, in order to achieve an accurate quantitative analysis of coal tar composition. Leonard et al.

16

used the solvent extraction method

to classify the tar acids in the petrochemical process. Aliphatic hydrocarbons and aromatic compounds were identified by GC/MS. Sun et al.

17

used petroleum ether

(PE) extraction to separate the low temperature coal tar into GC/MS analyzable and unanalyzable fractions, and have identified more than 300 compounds from the GC/MS analyzable part. Luo et al.

18

used GC/MS to analyze the cyclohexane-ether

extracts of high temperature coal tar. More than 200 kinds of compounds from the


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extracts were identified. In addition, the researchers also tried supercritical extraction 19

and dense gas extraction 20-21 to separate coal tar and similar substances, such as oil

and liquefied coal oil and so on. Some advanced analytical instruments, such as size-exclusion chromatography, matrix assisted laser desorption/ionization post source decay mass spectrometry, nuclear magnetic resonance spectroscopy, UV-fluorescence spectrum, etc., were applied to study the range of molecular weight and the composition and structure 22-25. However, the present study shows that there is no effective way to separate GC/MS analyzable and unanalyzable fractions of coal tar. That is to say, the current separation method and GC/MS analysis can't get the accurate quantitative results of the low boiling point compounds in coal tar. Usually, physical instrument analysis

26

and chemical degradation analysis

27

were

used to analyze the heavy components (including GC/MS unanalyzable fractions) from coal tar or oil. Chemical degradation analysis can transform the heavy components of coal tar into smaller molecular fragments, which is easy to realize the conventional instrument analysis and to obtain the molecular structure information of compounds in the heavy components. The pyrolysis-gas chromatography/mass spectrometry (Py-GC/MS) has been widely employed in coal 28, oil 29 and biomass 30 to analyze the thermal cracking and cracking products. In this study, the TG combined with GC/MS was put forward to realize the quantitative determination of the GC/MS analyzable part of coal tar, and the composition and distribution of the GC/MS unanalyzable part of coal tar was investigated by Py-GC/MS. This analysis method can be used not only in the quantitative analysis of coal tar and its distillate compositions, but also in the quantitative analysis of oil sand, oil shale and biomass pyrolysis and their separated fractions.


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2. EXPERIMENTAL DETAILS Table 1. The characteristics of LTCT and HTCT used in this study Density

Moisture

Ash

Toluene insoluble

Sample (20oC/g.mL-1)

(w%)

(w%)

(w%)

LTCT

1.05

7.84

0.06

0.92

HTCT

1.15

4.77

1.09

1.29

2.1. Materials. Low temperature coal tar (LTCT) and the wash oil fraction of high temperature coal tar (HTCT) were provided by Shaanxi Yanchang Petroleum Anyuan Co., Ltd. and Shaanxi Heimao Coking Co., Ltd. in Shaanxi Province of China, respectively. The elemental compositions of LTCT and HTCT were C: 75.44 w%, H: 6.90 w%, N: 0.89 w%, S: 1.06 w%, O: 15.71 w% and C: 88.70 w%, H: 4.56 w%, N: 1.01 w%, S: 0.55 w%, O: 5.18 w%, respectively. The LTCT sample has higher oxygen content compared with the HTCT sample. Some properties of the samples according to Chinese national standards were listed in Table 1. 2.2 Sample preparation and experimental process. The flow path of the experimental process was shown in Figure 1. The dehydration and deslag of LTCT and HTCT were treated as described in the literature 31. A 5 g sample of coal tar was extracted with PE (the boiling point is range from 60 oC to 90 oC) under ultrasonic irradiation to separate into PE soluble fractions and PE insoluble fractions. The extraction conditions are as follows: Extract for 10 times, one time for 30 s. PE soluble fractions of LTCT and HTCT were obtained and defined as L-PE and H-PE, respectively. LTCT, HTCT, L-PE and H-PE were analyzed by GC/MS and by TG. The real compound contents of LTCT, HTCT, L-PE and H-PE were defined as R-LTCT, R-HTCT, R-L-PE and R-H-PE, respectively. In addition, the GC/MS


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unanalyzable fractions of LTCT and HTCT obtained from the pyrolysis residues of LTCT and HTCT at 300 oC by TG were defined as 300LTCT and 300HTCT, respectively. 300LTCT and 300HTCT were multi-step pyrolyzed (The samples were not taken out of the pyrolysis tube at different pyrolysis temperatures) by Py-GC/MS, and the pyrolysis volatiles obtained at the temperature sequence of 400 oC, 500 oC, 600 oC, 700 oC and 800oC were defined as 300LTCT-S-400, 300LTCT-S-500, 300LTCT-S-600,

300LTCT-S-700,

300HTCT-S-500,

300HTCT-S-600,

300LTCT-S-800, 300HTCT-S-700

and

300HTCT-S-400, 300HTCT-S-800,

respectively. At the same time, 300LTCT and 300HTCT were single-step pyrolyzed to 800 oC by Py-GC/MS, and the pyrolysis volatiles obtained were defined as 300LTCT-I-800 and 300HTCT-I-800, respectively. All the experiments were repeated at least three times to make sure that the results were reproducible. In this paper, the result is an average of the three times. Crude coal tar atmospheric distillation dehydration

Waterless coal tar THF dissolution, centrifugation

THF removal by atmospheric distillation

Coal tar extraction by PE Just ≥300oC fraction of coal tar

Coal tar-PE

same temperature

GC/MS

program

TG

Py-GC/MS Temperature

Real contents of ≤300 oC parts in coal tar-PE and coal tar

sequence

The composition of ≥300 oC fraction in coal tar

The instruction of scientific research and application processing of coal tar

Figure 1. The flow path of the experimental process.


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2.3 GC/MS analysis. GC/MS was performed by using a Shimadzu (Japan) GCMS-QP2010 plus instrument operating in an electron ionization (EI, 70 eV) mode. Separation was achieved using a 30 m × 0.25 mm i. d., 0.25 µm, Rtx-5 ms capillary column (Restek). The initial oven temperature was set at 60 oC and held for 1 min at this temperature, then the temperature increases at 3 oC/min to 90 oC (held for 1 min), then 3 oC /min to 170 oC (held for 1 min), and then 3 oC /min to 300 oC (held for 8 min). The carrier gas flow rate in column was 1 mL/min. The GC injector temperature was set at 300 oC, with a split ratio of 20: 1. The ion source temperature was set at 230 o

C. The spectrometer was set to scan between 30 m/z and 500 m/z. The injected

volume of the sample was 0.4 µL. 2.4 TG analysis. TG experiments of samples were performed using a thermal analyzer system (DSC/DTA-TG STA 449 F3 Jupiter®, Netzsch, Germany). The programming temperature is the same with GC/MS analysis. Approximately 10 mg of sample was analyzed under nitrogen atmosphere of 70 mL/min. 2.5 Py-GC/MS analysis. The fast pyrolysis of samples were performed using a CDS 5200 pyrolyzer (CDS Analytical Inc., USA) and the pyrolysis gaseous products were analyzed by online GC/MS (QP2010 plus, Shimadzu Inc., Japan). The probe was a computer controlled resistively heated element which held an open ended quartz tube. 300LTCT and 300HTCT were in turns placed in the quartz tube, in which a quartz rod was fitted to support the samples 32, and were dropped into the pyrolysis furnace. The furnace was directly attached to the injection port of the GC/MS. In the process of multi-step pyrolysis of 300LTCT and 300HTCT, the residence time of the furnace was held at 400 oC, 500 oC, 600 oC, 700 oC and 800 oC for 60 min to desorb the preservative components presented in the samples. At the same time, the pyrolysis products were on-line synchronous analyzed by GC/MS. The sample was raised and


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maintained at room temperature under helium (99.999%) gas condition. Another, 300LTCT and 300HTCT were single-step fast pyrolyzed to 800 oC under helium by this carrier gas to purge the pyrolysis gaseous products. And the products were brought into the GC/MS analyzer through the transfer line at 300 oC. The flow rate of carrier gas is 40 mL/min and the spilt ratio is 100:1. The GC/MS conditions are the same with the literature 33. 2.6 Calculation of the contents of coal tar components. GC/MS and TG have the same temperature programs and the same final temperatures to analyze coal tar samples. The relative contents of coal tar detected by GC/MS multiplied with final conversion by TG analysis can be more accurate. The real component content of coal tar and its fractions can be calculated by Eq. (1) and Eq. (2).

E real content =C weight loss rate × E content

(1)

E real content =C weight loss rate × E content × E extract

(2)

In the equations, Ereal content denotes the real component content of coal tar or its PE extract fractions, w%; Cweight loss rate denotes the weight loss rate of coal tar or its PE extract fractions during analysis by TG analysis, w%. Econtent denotes the component content of coal tar or its PE extract fractions obtained by using GC/MS analysis, w%. Eextract denotes the extraction yield of coal tar with PE, w%. 3. RESULTS AND DISCUSSION 3.1 GC/MS analysis of LTCT, HTCT, L-PE and H-PE. The extraction yields of L-PE and H-PE were 39.60 w% and 14.15 w%, respectively. The result shows that some of the compounds in LTCT and HTCT have been extracted with PE. The GC/MS total ion chromatograms of LTCT, HTCT, L-PE and H-PE were showed in Figure 2, and main components identified were listed in Table S1 (see supplementary material). As shown in Table S1, the main component types detected include free


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alkanes, phenols, naphthalenes, fluorenes, anthracenes, phenanthrenes, etc. in LTCT and L-PE. LTCT and L-PE contain considerable phenols and free alkanes. The same compound types have been detected in LTCT and L-PE, but their contents are different. The contents of phenol, methyl phenols, naphthalene and phenanthrene are 3.50 w%, 8.31 w%, 1.63 w% and 0.84 w% in LTCT, and their contents are 1.24 w%, 3.93 w%, 1.69 w% and 1.05 w% in L-PE, respectively. The PE extraction method was still used for separation of light components and GC/MS was also applied for the composition analysis. These methods were employed in this paper to contrast with the combined GC/MS and TG analytical method to determine the compositions of coal tar. 100 80 60 40

Intensity(%)

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20 0

LT C T HTCT L -PE H -P E 10

20

30

40

50

60

70

80

90

R e te ntio n tim e (m in)

Figure 2. GC/MS total ion chromatograms of LTCT, HTCT, L-PE and H-PE. Compared with LTCT and L-PE, HTCT and H-PE mainly contain aromatics compound (naphthalenes, fluorenes, anthracenes, phenanthrenes, triphenylenes, fluoranthenes and chrysenes). The contents of naphthalene, biphenyl, triphenylene and chrysene are 10.77 w%, 0.77 w%, 2.39 w% and 2.32 w% in HTCT, and their contents are 13.02 w%, 1.17 w%, 1.91 w% and 1.79 w% in H-PE, respectively. No


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phenolic compounds were detected in HTCT and H-PE. The result reveals that the compounds with a boiling point greater than 300 oC were detected, such as anthracene, phenanthrene and chrysene, which is mainly due to the evaporation and boiling of vaporization.

80

Percentage Content(%)

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LTCT L-PE HTCT H-PE

60

40

20

0 aromatic

aliphatic

acidic

oxygenated

nitrogen

others

Figure 3. The group compositions of LTCT, L-PE, HTCT and H-PE. Figure 3 shows the comparison of all tested compounds in LTCT, L-PE, HTCT and H-PE by GC/MS. As shown in Figure 3, PE can effectively extract aliphatic hydrocarbons and oxygen compounds from LTCT. The extraction of phenols in LTCT by PE is not obvious. The relative content of aromatic hydrocarbons in H-PE is higher than that in HTCT. By comparing LTCT and L-PE, HTCT and H-PE, it can be found that light compositions can be extracted more effectively than heavy compositions by PE. Compared LTCT and L-PE with HTCT and H-PE, the component contents are considerable different. As reported in the literatures of 6, 15, 17, the component contents of coal tar or its fractions are not accurate. 3.2 TG analysis and the component contents of the GC/MS analyzable part of LTCT, HTCT, L-PE and H-PE. TG analyses of LTCT, HTCT, L-PE and H-PE were presented in Figure 4. In this paper, the sample final weight loss rate at 300 oC


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needs to be focus on. As shown in Figure 4, the final weight loss rate of LTCT and HTCT are 86.78 w% and 46.63 w%, respectively. Compared with the GC/MS detection method, TG can illustrate the gasification quantity of coal tar at 300 oC. The final weight loss rate of L-PE and H-PE are 93.85 w% and 95.01 w%, respectively. The gasification of L-PE and H-PE are not complete, and their remaining components at above 300 oC are 6.51 w% and 4.99 w%, respectively. Therefore, the quantitative results of PE extracts by GC/MS are not accurate. Samples of coal tar must be vaporized at the injection of gas chromatography. Using TG to simulate the heating process of the GC column, the GC vaporization was maximized.

100

HTCT 80

Weight Loss(%)

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LTCT 60

H-PE 40

L-PE 20

0 50

100

150

200

250

300

o

Temperature( C)

Figure 4. The TG curves of LTCT, HTCT, L-PE and H-PE at the final temperature of 300 oC. The accurate contents of ≤300 oC parts in LTCT and HTCT were calculated as the Section 2.6 described according to Eq. (1), and the results were shown in Table S2 (see supplementary material). As shown in Table S2, compared with the results by GC/MS, the accurate contents of detected compounds in LTCT and HTCT decrease.


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According to Eq. (2), the accurate contents L-PE and H-PE were listed in Table S2. Compared with LTCT and HTCT, the accurate contents of L-PE and H-PE are significant different. Figure 5 shows the contents of phenol, naphthalene, naphthalenol, phenanthrene, C16 alkane and C24 alkane in LTCT, L-PE, R-LTCT and R-L-PE. The order of total contents of these compounds is LTCT > R-LTCT > L-PE > R-L-PE according to Figure 5. The compositions with a boiling point over 300 oC in LTCT will be kept in the injection of GC/MS. LTCT is considered completely vaporization in the injection of GC/MS, which results in the increase of the compound contents. LTCT can't be completely extracted with PE to obtain the GC/MS analyzable part. What is more, the compositions of GC/MS unanalyzable part are mixed in L-PE. Because of the characteristics of PE, the extraction abilities of compounds are different, which leads to the higher content of some compounds as shown in Table S1. R-LTCT is obtained by GC/MS and TG. TG cannot completely simulate LTCT gasification process in the capillary. The compound contents in LTCT calculated by this method are more accurately compared with others. The same is true of HTCT. In this study, GC/MS combined with TG can accurately analyze ≤ 300 oC fraction from coal tar and this method can be trusted. LTCT R-LTCT L-PE R-L-PE

3.5 3.0 2.5

content/%

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2.0 1.5 1.0 0.5 0.0

nol phe

l ne ne lene leno thre alka htha htha nan C16 nap nap phe

ne alka C24


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Figure 5. The compound contents in LTCT, L-PE, R-LTCT and R-L-PE. 3.3 The composition analysis of 300LTCT and 300HTCT by Py-GC/MS. Multi-step fast pyrolysis of the samples can obtain the compositions of the pyrolysis products under different temperature conditions, and can avoid the low temperature pyrolysis products escaping. It is advantageous to analyze the compositions of GC/MS unanalyzable part indirectly. In this paper, the five-stage processes of the compositions of volatiles generated during 300LTCT and 300HTCT pyrolysis by Py-GC/MS were investigated. The total ion chromatograms of single-step pyrolysis and multi-step pyrolysis of 300LTCT and 300HTCT by Py-GC/MS were shown in Figure 6 and Figure 7, respectively. The components of 300LTCT-S-400, 300LTCT-S-500,

300LTCT-S-600,

300LTCT-S-700,

300LTCT-S-800,

300HTCT-S-400,

300HTCT-S-500,

300HTCT-S-600,

300HTCT-S-700,

300HTCT-S-800, 300LTCT-I-800 and 300HTCT-I-800 were listed in Table S3, Table S4 and Table S5 (see supplementary material), respectively. 100 80 60 40

Intensity(%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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20 0

300LTCT-S-800 300LTCT-S-700 300LTCT-S-600 300LTCT-S-500 300LTCT-S-400 300LTCT-I-800 5

10

15

20

25

30

35

Retention Time(min)

Figure 6. The total ion chromatograms of single-step pyrolysis and multi-step pyrolysis of 300LTCT by Py-GC/MS.


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100 80 60 40

Intensity(%)

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20 0

300HTCT-S-800 300HTCT-S-700 300HTCT-S-600 300HTCT-S-500 300HTCT-S-400 300HTCT-I-800 5

10

15

20

25

Retention Time(min)

30

35

Figure 7. The total ion chromatograms of single-step pyrolysis and multi-step pyrolysis of 300HTCT by Py-GC/MS. The composition distributions of multi-step fast pyrolysis products of 300LTCT and 300HTCT at the temperature of 400 oC, 500 oC, 600 oC, 700 oC and 800 oC by Py-GC/MS were shown in Figure 8 (Figure 8 is based on Table S3, Table S4 and Table S5). It can be known that volatiles of GC/MS analyzable part are released at 400 oC and 500 oC in 300LTCT pyrolysis, but they are not released at 600 oC, 700 oC and 800 oC. It is indicated that the molecular weights of the compounds in 300LTCT are low, and it may be suggested that there are many heat-sensitive substances, such as phenolic compounds, which results in the occurrence of thermal condensation

17

.

The main volatiles of 300LTCT-S-400 include alkanes, aromatics and acidic compounds. The contents of alkanes and aromatics are 50.18 w% and 24.63 w%, respectively. As temperature rises, a large amount of alkenes appear in the pyrolysis products of 300LTCT-S-500. Compared with 300LTCT-S-400, the contents of alkanes and aromatics are reduced to 44.73 w% and 11.46 w%, respectively. The content of acidic compounds is rapidly reduced to 17.27 w%. The more abundant light materials (alkane or alkene) tends to mask the identification of the features of the less


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abundant heavy fractions (atromatics) 7. No phenolic compounds were found in the acidic compounds during 300LTCT fast pyrolysis at 400 oC and 500 oC as shown in Figure 6 and Table S3 (see supplementary material). According to the above results, it can be explained that the thermal decomposition of alkanes mainly produces short chain alkanes or alkenes. Acidic compounds may be responsible for the breakdown of ester compounds. The aromatic hydrocarbons did not have an alkyl reaction at this temperature range, which can only occur at a higher temperature 34. It is not possible to eliminate the possibility of polycondensation of phenols and other compounds. The difference is that the product compositions of 300LTCT fast pyrolysis at 800 o

C are more complex. As shown in Figure 8, the content of aromatics is 76.13 w%, the

content of alkanes is 5.85 w%, the content of acidic compounds is 3.99 w% and the content of the others is 12.21 w% in the volatiles. Low boiling point phenolic compounds are found in 300LTCT-I-800, and the contents of phenol and methyl phenol are 2.19 w% and 2.50 w%, respectively. This shows that when 300LTCT has been rapidly pyrolysis at 400 oC and 500 oC, the phenols may have a condensation reaction. Some phenolic compounds with a high boiling point and a large molecular weight (especially for the phenolic compounds with bridge bonds, such as phenolphthalin) were volatilized and broken down. The phenolic compounds didn't have a condensation reaction at 800 oC in the process of fast pyrolysis. Pyrenols is the most complex phenols which are found in low temperature coal tar by GC/MS

15

.

300LTCT may have some phenolic compounds with a molecular weight higher than that of pyrenols. Compared with 300LTCT-S-400 and 300LTCT-S-500, the contents of alkanes and alkenes are drastically reduced which may be caused by more gases generated at 800 oC 35.


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Alkanes

Aromatics

Acidic

Alkenes

Oxygenated

Others

100

Distribution of compounds (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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80

60

40

20

0 00 00 00 00 00 00 00 00 00 00 00 00 -I-8 -I-8 -S-4 -S-4 -S-5 -S-5 -S-6 -S-6 -S-7 -S-7 -S-8 -S-8 TCT0HTCT LTCT HTCT LTCT HTCT LTCT HTCT LTCT HTCT LTCT HTCT L 0 0 0 0 0 0 30 30 30 300 300 30 300 30 300 30 300 30

Figure 8. The distribution of organic volatile products produced from 300LTCT and 300HTCT during the single-step fast pyrolysis at 800 oC and the multi-step fast pyrolysis at 400 oC, 500 oC, 600 oC, 700 oC and 800 oC by Py-GC/MS. As shown in Figure 8, the compounds of GC/MS analyzable part have been detected in the temperature range from 400 oC to 800 oC by the multi-step fast pyrolysis of 300HTCT. The compositions of HTCT and LTCT are different. HTCT is produced by high temperature coking and it has experienced strong secondary reactions. HTCT has more aromatics and less alkanes, and almost no phenolic compounds. Compared with 300LTCT, the compound distribution of 300HTCT is more regular in the temperature range from 400 oC to 800 oC. It can be found that the molecular weight of aromatic compounds in 300HTCT is larger than that in 300LTCT. The thermal stability of the compounds in 300HTCT is better. The content of aromatic hydrocarbons gradually increases, and the content of other compounds gradually decreases with the cracking temperature goes up. As shown in Figure 8 and Table S4, the order of the compounds with the minimum molecular weight is biphenylene (in 300HTCT-S-400) > methyl naphthalene (in 300HTCT-S-500) >


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indane (in 300HTCT-S-600). However, the order of the compounds with the minimum molecular weight is benzo[]phenanthrene (in 300HTCT-S-800) > fluoranthene (in 300HTCT-S-700). Biphenylene, methyl naphthalene, fluorene, perylene, 8H indeno[]phenanthrene and D12 perylene are selected to investigate the gas release rule of the rapid cracking in the process of 300HTCT fast pyrolysis. It can be found that methyl naphthalene, fluorene and biphenylene are only in the volatiles of 300HTCT-S-400, 300HTCT-S-500 and 300HTCT-S-600. Large molecular weight compounds have been detected in pyrolysis products at different temperatures, such as perylene, 8H indeno[]phenanthrene and D12 perylene. It may be that the intermolecular association leads to the duplication of products

36

. In addition,

300HTCT is composed of aromatic structure units with different structures of bridge bonds, which results in the release of the same compounds at different pyrolysis temperatures. Among the products of 300HTCT by fast pyrolysis at 400 to 800 oC, it can be found that the compound composition of 300HTCT-S-800 is the summation of the others. The smallest compound found in 300HTCT-S-400 is biphenylene. This indicates that compounds, such as biphenylene and acenaphthene, released in 300HTCT-S-400 may be due to the intermolecular association. Indene is the smallest compound detected in 300HTCT-I-800, and it may be the product of 300HTCT cracking. 3.4 Possible formation mechanisms of 300LTCT and 300HTCT fast pyrolysis. In this study, it is almost impossible to determine how many products have been transformed into GC/MS analyzable part in 300LTCT and 300HTCT pyrolysis by Py-GC/MS. However, we can get the valuable information about the composition and structure of the samples

34, 37

. The pyrolysis mechanisms of 300LTCT and 300HTCT


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were discussed. In addition, the possible chemical structures of 300LTCT and 300HTCT were speculated according to the compositions of the pyrolysis products by Py-GC/MS. (1)

CnH 2n+n

CnH2n+H2

(2)

CnH 2n+n

CmH 2m+ CkH 2k+2

OH (3) R

OH OH HCHO acid

(may be)

O

(may be) OH

OHOH R

OH OH

-H2O

OH

(4)

(may be)

The ord er of the pyrolysis products with the minimum molecular weight is biphenylene (in 300HTCT -S-400) > methyl naphthalene (in 300HTCT -S-500) > indane ( in 300HT CT-S-600) as shown in T able S4, and the release of the same compounds at d if f erent pyrolysis temperatures that maybe due to the present of the aromatic structure units with d if f erent structures of bridge bonds. Low boiling point phenolic compounds were f ound in 300LTCT -I-800. Phenolic compounds with a high boiling p oint and a large molecular weight (especially f or the phenolic compounds connected with bridge bonds, such as phenolphthalin) were volatilized and broken down. I n summary, Reaction (4) may take p laced in the pyrolysis process.

OH O

O R

R O

OH

R1

(may be)6

N o phenolic compounds were f ound in 300LT CT-S-400 and 300LTCT-S-500 (as shown in T able S3), but in 300LT CT-I-800 (as shown in Table S5). The p henolic p olymerization of p henolic hydroxyl groups may have taken place by Reaction ( 3).

R

R

O

R

R1

Some alkenes detected in 300LT CT-S-400 and 300LT CT-S-500 ( as shown in T able S3) and 300HTCT S-400 ( as shown in Table S4) that may be generated by Reaction ( 1). Some alkanes such as C9 alkane and C10 alkane were detected in 300LT CT-S-400 and 300HT CT-S-400 as shown in Table S3 and Table S4, respectively. These alkanes with less than 28 carbon atoms in the main chain ( as shown in Table S1) may be generated by Reaction ( 2).

-HCHO R

HO

m+k=n

-H 2O

-H2O

R

O CH H kal i al

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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R1

O

R1

R

+

Figure 9. Possible formation mechanisms of 300LTCT and 300HTCT pyrolysis and possible compound structures in 300LTCT and 300HTCT. Possible formation mechanisms of 300LTCT and 300HTCT pyrolysis, and possible compound structures in 300LTCT and 300HTCT are shown in Figure 9. As shown in Figure 9, the four reactions are listed, including dehydrogenation of alkane (1), alkane cracking (2), polymerization (3) and bridge bond cracking (4). According to the composition analysis of 300LTCT and 300HTCT by Py-GC/MS in the Section 3.3, the composition distribution of alkenes is obvious in 300LTCT-S-500, while it is not detected in 300HTCT-S-500. It may be that the amount of alkanes in 300HTCT is small and their chains are short. Low boiling point phenolic compounds were found in 300LTCT-I-800, but they were not detected in 300LTCT-S-400 or 300LTCT-S-500. They may be fixed by the condensation reaction (Figure 9 (3)). The reasonable explanation is that there are some phenols with a high molecular weight in 300LTCT. These phenolic compounds were volatilized and cracked, but they didn't have the


Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

condensation reaction at 800 oC (Figure 9 (4)). The TG curves (as shown in Figure S1 of the supplementary material) of LTCT and HTCT at the final temperature of 800 oC can show that condensation reactions have happened in 300LTCT and 300HTCT at different fast pyrolysis conditions by Py-GC/MS. In 300HTCT, the compounds may have an intermolecular association according to the analysis of 300HTCT-S-400 and 300HTCT-S-500. In the case of phenols and aromatics, the results of the Section 3.3 the composition analysis of 300LTCT and 300HTCT by Py-GC/MS indicate that some bridge bonds are exist, and these bonds have broken down to produce a small molecular weight of phenolic compounds and aromatic hydrocarbons. The above analyses show that: 300LTCT is mainly composed of phenols, aliphatics, and aromatics, and 300HTCT is mainly composed of aliphatics and aromatics. The carbon chain length of aliphatics was larger than 28. The dehydrogenation of alkanes has taken place as shown in Figure 9 Reaction (1). The structure of some phenols was more complicated than that of phenyphenol and methyl naphthalenol. These phenols may be composed of small molecular phenols connected by bridge bonds in 300LTCT. The structure of aromatics was more complicated than that of perylene and indeno[]pyrene. They maybe made up of small molecular aromatics connected by bridge bonds. And these bridge bonds may be cracked by Reaction (4) as shown in Figure 9. 4. CONCLUSIONS In this paper, the TG combined with GC/MS was put forward to realize the quantitative determination of the GC/MS analyzable part of coal tar and the composition and distribution of the GC/MS unanalyzable part of coal tar was investigated by Py-GC/MS. Possible formation mechanisms of fast pyrolysis and the composition of GC/MS unanalyzable part of coal tar were discussed. The main


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conclusions are as follows: (1) Compared with the GC/MS detection method, TG can illustrate the gasification quantity of coal tar at 300 oC. L-PE and H-PE cannot be gasified totally by GC-MS and the remaining parts at above 300 oC are 6.51 w% and 4.99 w%, respectively. (2) GC/MS combined with TG can accurately analyze the compositions of ≤300 oC fractions in coal tar. This method also can be used in the quantitative analysis of oil sand, oil shale and biomass pyrolysis and their separated fractions. (3) The four reactions of dehydrogenation of alkane, alkane cracking, polymerization and bridge bond cracking were proposed in the fast pyrolysis process of coal tar. The phenolic compounds from 300LTCT may have a condensation reaction at 400 oC and 500 oC in the process of multi-step pyrolysis. (4) The composition analyses of 300LTCT and 300HTCT by Py-GC/MS indicate that they have some bridge bonds, and these bonds have broken down to produce a small molecular weight of phenolic compounds and aromatic hydrocarbons.

AUTHOR INFORMATION Corresponding Author *Tel.: +86 029 88302633. To whom correspondence should be addressed. E-mail: [email protected]; [email protected] Notes The authors declare no competing financial interest. ACKNOWLEDGMENTS This work was financed by the project supported by the Joint Funds of the National Natural Science Foundation of China (21536009; 21776229), Science and


Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Technology Plan Projects of Shaanxi Province (2017ZDCXL-GY-10-03), The Young Science and Technology Star Project of Shaanxi Province (2017KJXX-62), Project Supported by Natural Science Basic Research Plan in Shannxi Province of China (2017JQ2040) and Foundation of Outstanding Young Academic backbone Supporting Program of Northwest University (2015). REFERENCES (1) Dong, J.; Li, F.; Xie, K. C. J. Hazard. Mater. 2012, 243, 80-85. (2) Gao, J. S.; Zhang, D. X.; Yu. J. Coal Chem. Ind. 2004, 32, 4-9. (3) Shui, H. F.; Zhang, D. X.; Zhang, C. Q. Separation and purification of coal tar; Chemical Industry Press: Beijing, 2007; p4. (4) Zander, M. Fuel 1991, 70, 563-565. (5) Boenigk, W.; Haenel, M. W.; Zander, M. Fuel 1990, 69, 1226-1232. (6) Sun, M.; Ma, X. X.; Lv, B.; Dai, X. M.; Yao, Y.; Liu, Y. Y.; He, M.; Zhao, X. L. Fuel 2015, 160, 16-23. (7) Islas, C. A.; Suelves, I.; Carter, J. F.; Li, W.; Morgan, T. J.; Herod, A. A.; Kandiyoti, R. Rapid Commun. Mass Spectrom. 2002, 16, 774-784. (8) Sun, M.; Lv, B.; Dai, X. M.; Ma, X. X.; Zhao, X. L. Energ. Sour. Part A 2016, 38, 2282-2289. (9) Sun, M.; Chen, J.; Dai, X. M.; Ma, X. X.; Zhao, X. L.; Liu, K. Coal Convers. 2015, 38, 58-63. (10) Morgan, T. J.; George, A.; Alvarez, P.; Millan, M.; Herod, A. A.; Kandiyoti, R. Energy Fuels 2008, 22, 3275-3292. (11) Wang, Y. G.; Jiang, G. C.; Zhang, S. J.; Zhang, H. Y.; Lin, X. C.; Huang, X.; Fan, M. H. Fuel Proc. Technol. 2016, 149, 313-319. (12) Wang, X. L.; Shen, J.; Niu, Y. X.; Sheng, Q. T.; Liu, G.; Wang, Y. G. J. Clean.


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MS Analyzable and

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