Analysis of Volatile Organic Pyrolysis Products of Bituminous and

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Analysis of volatile organic pyrolysis products of bituminous and anthracite coals with SPI-TOFMS and GC/MS Jingying Xu, Jiankun Zhuo, Yanan Zhu, Yang Pan, and Qiang Yao Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.6b02335 • Publication Date (Web): 13 Dec 2016 Downloaded from http://pubs.acs.org on December 21, 2016

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Analysis of volatile organic pyrolysis products of bituminous and anthracite coals with SPI-TOFMS and GC/MS Jingying Xu,

†

Jiankun Zhuo,

†

Yanan Zhu,

‡

Yang Pan,

‡

and Qiang Yao

∗, †

†Key

Laboratory for Thermal Science and Power Engineering of Ministry of Education, Department of Thermal Energy Engineering, Tsinghua University, Beijing 100084, China ‡National Synchrotron Radiation Laboratory, University of Science and Technology of China, Hefei 230029, China E-mail: [email protected]

Phone: +86-010-62782108

Abstract The volatile organic compounds (VOCs) emissions from the pyrolysis of anthracite and bituminous coals are studied by using both the online single-photon ionization time-of-ight mass spectrometry (SPI-TOFMS) and the oine gas chromatographmass spectrometry (GC/MS) techniques. It is found that the combination of the online and oine techniques is eective in the VOCs analysis. It reveals that the dominant pyrolysis products of both anthracite and bituminous coals are comprised of a mixture of non-aromatic hydrocarbons, mainly alkanes and light alkenes, aromatic compounds and their corresponding isomers. Rank variation of coal samples has important impacts on the release of pyrolysis products. High volatile bituminous coal releases more VOCs than low volatile anthracite coal under the same temperature. The relative contents of the pyrolysis products of the two coals also dier. SH coal releases more short chain 1

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aliphatic and oxygen-containing compounds, while XT coal releases more aromatic and sulfur-containing compounds. Temperature has an important inuence on the release of VOCs from coal pyrolysis. The relative signal intensities of most products increase as temperature increases. According to the time-evolved spectra, the higher the temperature, the earlier and faster the release of VOCs will be, regardless of the coal type. For both coals, the release of most VOCs is favored at higher temperatures. For bituminous coal, at a xed temperature, almost all alkenes and dienes started to release at approximately the same time except propylene, which has two peaks, indicating that there may be two steps in the release of this compound. The aromatics were released a little delayed especially at low temperatures.

1. Introduction

Volatile organic compounds (VOCs) from pulverized coal combustion were determined to be negligible comparing to other sources. 1 However, interest in VOCs from pulverized coal combustion seems to be revived recently due to the growing concern on its health eect and environmental issues. 2 VOCs react with NOx forming ground-level ozone ( O3 ) in the presence of sunlight. As a highly reactive component, ozone will irritate the lungs and eventually result in asthma and other pulmonary diseases. 37 In addition, VOCs will also contribute to the formation of particulate matter and secondary organic aerosols. 7,8 Consequently, many countries have established stringent regulations on VOCs emission control. 2 Pulverized coal combustion power plant, as a major electricity supplies worldwide, is one of the VOCs emission sources. 2 The VOCs released from coal combustion was reported to be majorly organic acids and some polycyclic aromatic hydrocarbons (PAHs) and alkanes. 9 Its emission is found to be favored by the inorganic matter. 10 In addition, VOCs formation in pulverized coal combustion is closely related to the in situ plant operation. 9,1114 During coal combustion, the coal particles rst go through the process of devolatilisation (pyrolysis), followed by tar combustion and char combustion. The volatiles, derived from the 2


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pyrolysis stage, are recognized as the precursor of soot 15 and organic aerosols 16 due to their similarity in chemical compositions. To identify the mechanism and characteristics of VOCs emission from coal combustion, therefore, one has to have a comprehensive understanding of the species in the volatiles generated from the coal pyrolysis step. However, there are very few studies in this area, 1618 most of which merely focus on the total volatiles or light gases yield, 1921 and failed to further distinguish the categories of the volatiles. Jia

et al. 17

conducted an online analysis of the volatile products pyrolyzed from two bituminous coals in a low-pressure environment using Synchrotron Vacuum Ultraviolet Photoionization Mass Spectrometry (SVUV-PIMS) and demonstrated the capability of the SVUV-PIMS system. They found that the volatiles are dominated by aromatic compounds, with some aliphatic products. The HN coal has more condensed aromatic structures than YM coal. The results imply that the volatile species are coal type correlated. There is still a wide gap of knowledge in VOCs formation mechanisms and the characteristics of the volatile species formed from other coals. To ll the blank, this study investigated the formation and compositions of the VOCs from pyrolysis of one anthracite and one bituminous coal. To reveal the formation mechanisms of VOCs from coal pyrolysis, one has to know the in-situ compositions of the volatile and the accurate species information. In this work, the single-photon ionization time-of-ight mass spectrometry (SPI-TOFMS) and GC/MS are used to full this goal. In comparison to the traditional "hard" electron ionization (EI) method used in commercial mass spectrometers, the SPI-TOFMS system uses the nearthreshold "soft" photoionization, which is able to obtain the fragment-free and/or fragmentcontrollable mass spectra of the pyrolysis products online. However, due to the abundant isomers of high molecular weight primary pyrolysis products, it is quite challenging to distinguish and identify them. 22 It is more dicult when they have close ionization energy (IE). 17 Therefore, an oine coal pyrolysis products collection system combined with GC/MS technique was adopted. This method lacks the capacity for real-time analysis, however, with a higher IE, it has the capacity to further conrm low molecular weight alkanes, distinguish

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high molecular weight isomers, as well as achieve the concentration of each product. Thus it could identify species in a more accurate way. Based on the above mentioned online and oine technologies, this study addressed the formation mechanisms and characteristics of the VOCs pyrolyzed from dierent ranked coals, as well as the eects of pyrolysis temperature and time.

2 Materials and methods

2.1 Coal samples One bituminous coal and one anthracite coal, namely SH coal and XT coal, which are both widely used in utility boilers for electric power generation in China, were selected to study the eect of coal rank variation on VOCs formation during pyrolysis. The proximate and ultimate analysis of the coal samples are listed in Table 1. Prior to use, both samples underwent the same drying process in the furnace for more than 8 hours. 20 mg sieved coal samples, with particle diameter ranging from 57 to 90 µm and the median particle size of about 50 µm were used for each experiment. Table 1: Proximate and ultimate analysis of SH and XT coals Coal Sample

Proximate analysis ( wt%, ad) Ultimate analysis( wt%, ad) M V A FC C H O* N S SH coal 1.96 25.61 18.65 53.78 63.84 3.84 10.31 0.68 0.72 XT coal 1.18 7.23 22.54 69.05 68.89 2.98 2.35 0.91 1.15 ad: air dry basis M: moisture. V: volatiles. A: ash. FC: xed carbon. O*: by dierence

2.2 The oine coal pyrolysis products collection system The oine coal pyrolysis products collection system was shown in Figure 1. The tubular furnace and the pyrolysis conditions are the same as the online system. However, the gas-phase pyrolysis products generated from coal samples that introduced by carrier gas will ultimately 4


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be collected by a gas bag with a capacity of 10 L after owing through a transmission line and a U-shape condensation tube, which is immersed in ice water with a temperature of about 0◦ C. The diluted gas-phase pyrolysis products collected by the gas bag will then be analyzed utilizing GC/MS technique. To avoid secondary reactions and ensure accuracy of the species measurement, the overall dilution ratio of the concentrations detected by the oine system to the ones measured by the online system is estimated to be 150.

Figure 1: Schematic diagram of the online coal pyrolysis SPI-TOFMS analysis system and the oine coal pyrolysis products collection system

2.3 The online coal pyrolysis SPI-TOFMS analysis system The SPI-TOFMS, located at the National Synchrotron Radiation Laboratory, University of Science and Technology of China, was used to conduct the online mass spectrometric analysis of the VOCs from coal pyrolysis. The schematic diagram of the system is shown 5


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in Figure 1. The detailed description of the system can be found in the work of Wang

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et

al 23 and Pan et al . 24,25 The device consists of a tubular furnace used for coal pyrolysis, a preheated transmission line to prevent condensation of volatile products, a photoionization chamber designed for introducing vacuum ultraviolet (VUV) light as well as ionizing pyrolysis products and a TOFMS. The pressure in the ionization chamber and the TOFMS chamber were 0.75 Pa and 1.5 × 10−5 Pa respectively. A direct current excited vacuum ultraviolet discharge lamp (Krypton lamp) was installed perpendicular to the sampling capillary in the ionization chamber, while a pair of ring-shaped electrodes was assembled above the lamp. When the volatile products entered the region between the two electrodes, they would be ionized by the vacuum ultraviolet light. The ion beam generated was then driven by the electric eld into the skimmer and focused by a set of einzel lens into the reectron TOFMS chamber. Inside the TOFMS chamber, the ions were accelerated, ied, reected by a reector and ied until they arrived at the detector. The detector consisted of a pair of microchannel plates (MCPs) and a collector. The collision between the ions and the MCPs would generate electrons, which could be multiplied when collided on the inner wall of the channel, producing a current. The current signal was then converted to a voltage signal to be detected. During the experiment, the temperature of the furnace was measured by a K-type thermocouple and controlled by a temperature controller. 99.999 % high purity nitrogen was used as carrier gas to introduce the pyrolysis products into the photoionization region from the pyrolysis region at a constant ow rate of 200 standard cubic centimeters per minute (SCCPM). Coal samples were introduced quickly into the midpoint of the furnace by a quartz boat when the furnace was heated to the target temperature in order to achieve a more realistic simulation of fast pyrolysis environment. Meanwhile, the photon energy of 10.6 eV, a moderate photon energy that ionizes most organic compounds while minimizes fragmentation, was picked to analyze the pyrolysis products evolved from both coals. Before each experiment, a blank sample was measured at the same condition. Then the background signal would be deducted afterwards. Meanwhile, the intensity of the VUV

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light would attenuate gradually during the experiment, the average peak area of ethylene (99.999% purity) with a ow rate of 6 SCCPM before and after each experiment was regarded as the symbol of light intensity. Thereby comparing the nal mass spectrometry signal of the pyrolysis products could be achieved on the basis of comparing the average peak area of ethylene on the spectrum. Two types of spectra, namely cumulative spectrum and time-evolved spectrum, can be obtained by the online SPI-TOFMS analysis system. The cumulative spectrum illustrates the species distribution at a given pyrolysis temperature, while the time-evolved proles of the pyrolysis products reveals the in situ signal intensities during the whole pyrolysis process under that temperature. Each time-evolved spectrum at a certain temperature is recorded continuously with an interval of 4s.

2.4 Thermogravimetric analysis of SH bituminous coal and XT anthracite coal The thermogravimetric analysis of the coal samples was conducted in a thermogravimetric analyzer. The heating rate was set to 10 ◦ C/min for all the experiments. Figure 2 presents the thermogravimetric (TG) and dierential thermogravimetric (DTG) curves of SH and XT coal. As it is shown, the corresponding temperature at which weight loss begins and ends of the XT coal is higher than that of the SH coal. It is due to the increased stability of the basic coal structure as coal rank increases. 2628 Meanwhile, the maximum weight loss rate corresponds to a temperature of about 460 ◦ C for SH bituminous coal, while this temperature increases to about 550 ◦ C for XT anthracite coal. This work concentrated on mass spectra at temperatures higher than this characteristic temperature, for the reason that extremely limited signal can be observed until the temperature rose up to this particular temperature. Although the weight loss of the two coals was relatively stable at temperatures above 800 ◦ C, higher temperatures might lead to serious thermal fragmentation, which would create difculties for distinguishing parent compounds and ionic fragments. 17 Thus, temperatures of 7


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500◦ C, 550◦ C, 600◦ C, 700◦ C and 800◦ C were chosen in the case of SH coal while 600 ◦ C, 700◦ C and 800◦ C were selected for XT coal. The eects of coal type and temperature on the pyrolysis products were illustrated according to the mass spectra.

Figure 2: Thermogravimetric analysis of the SH bituminous coal (black) and XT anthracite coal (blue).

Results and discussion

3.1 Main pyrolysis products of SH coal at 600 C and XT coal at 800 C ◦

◦

According to the cumulative spectrum of the two coals (shown in section 3.2), the main pyrolysis products of SH coal are not all released until 600 ◦ C, and as for XT coal, that temperature increased to 800 ◦ C. Therefore, 600◦ C and 800◦ C are selected as the characteristic temperatures of SH and XT coal accordingly to launch the oine coal pyrolysis products measurements. Since the result of oine measurement could determine the exact substance represented by a certain mass to charge ratio, a more in-depth understanding of the result of online measurement could be acquired by combining the two approaches.

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Over a hundred kinds of organic compounds with a wide mass range were observed in the SH and XT coal pyrolysis products by GC/MS. Table 2 presents the types and the concentrations of the main VOCs released from the pyrolysis of SH coal at 600 ◦ C and XT coal at 800◦ C analyzed by GC/MS in accordance with the order of their molecular weight. As it is shown, for the volatile organic pyrolysis products of SH coal, light alkenes and dienes such as ethylene, propylene and butene are among the species of remarkable concentration. While benzene, toluene, ethylbenzene, xylene, trimethylbenzene, diethylbenzene and their corresponding isomers are the main aromatic compounds. In addition, the existence of low molecular weight alkanes such as ethane, propane and butane, high molecular weight alkanes(e.g., undecane, n-nanone), oxygen-containing compounds (e.g., hexanal, acetone), nitrogen-containing compounds (e.g., acetonitrile) and halogen-containing compounds (e.g., 1,2-dichloroethane 1,4-dichlorobenzene and bromoform) is proved by using the oine coal pyrolysis products analysis system. Table 2: List of the main pyrolysis products of SH coal at 600 ◦ C and XT coal at 800 ◦ C Molecular Weight

Name

Formula

SH 600 ◦ C

XT 800◦ C

(µg/g-coal)

(µg/g-coal)

28

Ethylene

C 2 H4

415.69

314.00

30

Ethane

C 2 H6

743.24

122.28

41

Acetonitrile

C 2 H3 N

132.98

27.76

42

Propylene

C 3 H6

420.47

271.59

44

Propane

C 3 H8

300.04

18.94

56

1-butene

C 4 H8

140.13

26.90

56

Acrolein

C 3 H4 O

128.28

44.78

58

acetone

C 3 H6 O

1348.76

246.89

58

N-butane

C4 H10

172.19

289.09

58

Propionaldehyde

C 3 H6 O

100.41

16.61

58

Isobutane

C4 H10

61.24

117.94

9


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70

Methyl vinyl ketone (MVK)

C 4 H6 O

99.45

29.64

70

Methacrolein

C 4 H6 O

87.91

10.47

72

2-butanone (MEK)

C 4 H8 O

82.24

16.20

72

Butyral

C 4 H8 O

77.45

14.66

78

benzene

C 6 H6

116.65

101.16

84

Cyclohexane

C6 H12

3.23

129.51

85

Dichloromethane

CH 2 Cl2

94.54

140.78

92

Toluene

C 7 H8

558.98

419.54

99

1,2-dichloroethane

C 2 H4 Cl2

211.26

102.98

100

Hexanal

C6 H12 O

1721.88

1171.88

106

Ethylbenzene

C 8 H10

300.73

469.43

106

P / m - xylene

C8 H10

217.68

318.95

106

Ortho-xylene

C 8 H10

203.96

309.25

113

1,2-dichloropropane

C 3 H6 Cl2

83.74

88.79

114

N-octane

C8 H18

79.14

67.99

120

1,2,4-trimethylbenzene

C 9 H12

266.52

363.48

120

1,2,3-trimethylbenzene

C 9 H12

210.54

290.63

120

Mesitylene

C9 H12

88.93

124.29

120

3-ethyl-toluene

C 9 H12

122.95

172.77

120

2-ethyl-toluene

C 9 H12

65.44

92.68

128

N-nonane

C9 H20

292.57

401.14

134

1,4-diethylbenzene

C 10 H14

1891.55

2758.67

142

Decane

C10 H22

65.93

77.34

147

1,4-dichlorobenzene

C 6 H4 Cl2

209.67

254.30

147

1,2-dichlorobenzene

C 6 H4 Cl2

76.13

77.11

147

1,3-dichlorobenzene

C 6 H4 Cl2

69.27

68.91

156

Undecane

C11 H24

1128.91

1170.35

10


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166

Tetrachlorethylene

C 2 Cl4

104.45

53.84

253

Bromoform

CHBr 3

312.30

368.77

For the volatile organic pyrolysis products of XT coal, the table conrms the ndings that ethylene and propylene are the main alkenes, while benzene, toluene, ethylbenzene, xylene, trimethylbenzene, diethylbenzene and their corresponding isomers are the main aromatics. Moreover, ethane, hexanal and bromoform are the indicators for light alkanes, oxygen-containing compounds and halogen-containing compounds accordingly. Thus the categories of these species are almost the same as that of the pyrolysis products of SH coal at 600◦ C. However, it seems that the total mass concentration of the pyrolysis products of XT coal is much lower comparing to that of SH coal even at a higher temperature of 800◦ C. In addition, the relative content of oxygen-containing compounds is fairly low, while the relative content of aromatics is fairly high, which suggest that XT coal may have higher fraction of aromatic rings and less aliphatic bridges and chains in its original coal structure 29 or the temperature is not high enough for full decomposition.

3.2 Cumulative spectra of SH and XT coal The mass spectra of the volatile compounds of SH coal pyrolysis products at dierent temperatures, including 500 ◦ C, 600◦ C, 700◦ C and 800◦ C, are presented in Figure 3. The horizontal axis is the mass to charge ratio (m/z) of the ion, which corresponds to the molecular weight of the parent molecule, while the vertical axis is its signal intensity. For the same species, when the experimental light intensity is corrected, the signal intensity of the species can reect its mole fraction. According to the mass spectrum results of 600 ◦ C, suitable for representing the cumulative spectra of SH coal as discussed in Section 3.1, a series of peaks at m/z 17, 28, 34, 42, 56, 70, 78, 84, 92, 108, 122, 136, 142 etc. are detected. The peak at m/z 28 actually has a bimodal distribution on the mass spectra. Since the previous and the latter peak in this case can be easily separated, and the inuence of nitrogen (carrier gas, previous 11


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peak) could be disregarded after the deduction of background signal, the peak at m/z 28 here only refers to ethylene. The peaks at m/z 17 and 34 indicate the presence of ammonia and hydrogen sulde, respectively. The peaks at 28, 42, 56, 70 and 84 originate from non-aromatic compounds, mainly alkenes and dienes. The peaks at 78, 92 108, 122, 136 and 142 result from aromatic compounds, mainly benzene, toluene and xylene, known as BTX. Thus, other than the light alkanes (IE > 10.6 eV), which cannot be detected by the online analysis system SPI-TOFMS, the dominant pyrolysis products of the SH coal are comprised of a mixture of the non-aromatic hydrocarbons, mainly light alkenes, dienes and aromatic compounds. Comparison between Figure 3 and Table 2 indicates partial similarities of the compositions of the pyrolysis products detected by the oine and the online approaches. The distinctions are caused mainly by the dierent ionization energy the two systems used. In general, by the combination of the oine and online methods, it could be determined that the VOCs produced during the pyrolysis of SH coal mainly consist of non-aromatic hydrocarbons, particularly alkanes and light alkenes, aromatic compounds and a few kinds of oxygen-containing compounds, nitrogen-containing compounds and halogen-containing compounds. Figure 3 also shows the mass spectrum of pyrolysis products of XT coal at temperatures of 500◦ C, 600◦ C, 700◦ C and 800◦ C. According to the cumulative spectrum of the volatile pyrolysis products of XT coal at 800 ◦ C, which is suitable for representing the cumulative spectra of XT coal, the eect of rank variation of coal samples on the type and intensity of pyrolysis products could be qualitatively to semi-qualitatively studied. In comparison with the mass spectrum of SH coal in Figure 3, XT coal has almost the same kinds of pyrolysis products, with non-aromatic hydrocarbons and aromatic compounds as the principal VOCs. Nevertheless, the dissimilarities between SH coal and XT coal cannot be ignored. First of all, the shape of the cumulative spectrum of XT coal is completely dierent from that of SH coal. For XT coal, the peak at m/z 34 (H 2 S) becomes dominant at all temperatures, whereas species with high molecular weight can hardly be detected. In addition, the peak

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Figure 3: Cumulative spectra of SH (left) and XT (right) coals at dierent temperatures. Note dierent scales.

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at m/z 64 is relatively stronger. This peak could be assigned to sulfur dioxide (SO 2 ) and/or sulfur dimer (S2 ). However, since the IE of SO 2 (12.3 eV) exceeds the photon energy used in this work (10.6 eV), 30 this peak could only be ascribed to S 2 . This phenomenon reveals that the relative content of sulfur dimer in the VOCs of XT coal exceeds that of SH coal because of higher sulfur in the XT coal. Furthermore, the signal intensity of peak at m/z 106 which can be identied as dimethylbenzene and/or ethylbenzene is greater than that of the peak at m/z 108 corresponding to cresol, which is because the relative content of oxygen in XT coal is less than that in SH coal. Moreover, the peaks at m/z 70 and 84 are almost invisible. Those results are highly consistent with the ultimate analysis listed in Table 1 and conrm the eect of coal rank on VOCs formation. On the other hand, it can be seen clearly that the signal intensities for almost all the volatile matters of XT coal are not as obvious as those of SH coal. Previous research has shown a relationship between the volatiles released and the volatile matter content of the coal. 29 The coal rank of XT coal is higher than that of SH coal, which is reected in the volatile matter content as presented in Table 1. It is quite understandable that the volatiles released from XT coal are much less. Furthermore, greater degree of coalication and carbonation in the high ranked XT coal results in lower moisture content, higher ratio of C/H and C/O. Because of those, one would expect a lower volatile matters release from a high rank XT coal pyrolysis.

3.2.1 Eects of temperature on cumulative spectrum of SH coal It can be seen that temperature apparently aects the signal intensity of pyrolysis products. When the temperature increases, a consequent increase in the signal intensity of nearly all pyrolysis products occurs. Some species, of which signal intensities are too low to be distinguished from the background signal, can be observed at higher temperatures. For instance, when the temperature increases from 500 ◦ C to 600◦ C, the data in Figure 3 shows that peaks at m/z 17, 28, 34, 78, 92 increases markedly, especially the peak at m/z 17 which is hardly visible at 500◦ C. The peak at m/z 54, 68, as asterisked in Figure 3, increases signicantly as the temperature increases from 700 ◦ C to 800◦ C, with the peak at m/z 54 increasing by over

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200 percent, which can be seen more clearly in Figure 4. This gure, on the basis of the curve areas on the spectrum, and treating the peak area under 800 ◦ C as a standard for normalization, illustrates the tendencies of major coal pyrolysis products as temperature increases. More specically, for most of the products such as inorganic compounds and aromatic compounds, their relative signal intensities increase as temperature increases, which is consistent with the thermogravimetric analysis (see Figure 1). This implies that the increase of temperature will promote the breaking of chemical bond. However, with regard to low molecular weight alkenes, especially ethylene, two peaks are observed. The relative intensity rst rises from 500◦ C to 550◦ C, then decreases in the range of 550 ◦ C to 600◦ C, then rises again to the maximum at 800◦ C. Correspondingly, the relative intensities of those high molecular weight alkenes, especially m/z 210, show a small decline accompanied by some uctuations. That correlation between the heavy and the light alkenes may be explained in two eects: the secondary reactions occurred at higher temperatures and the oligomerization reactions of light alkenes. 23 According to the overall downward trend of high molecular weight alkenes, the rst eect is dominant, which means that the alkenes with relatively larger molecular weight would undergo the secondary reactions and decompose into light hydrocarbons under higher temperatures. Although previous studies have indicated that the yield of total volatiles and overall gas will increase by increasing the ultimate pyrolysis temperature, 21,31 there is few research on the sensitivity of each specic substance to the temperature. The discovery of this phenomenon would be very valuable to modeling studies in the future.

3.2.2 Eects of temperature on cumulative spectrum of XT coal Analogously, signicant dierences can be observed between the signal intensities in the mass spectrum when the pyrolysis temperature of XT coal increases as shown in Figure 3. When the temperature increases from 600 ◦ C to 700◦ C, almost all species marked on the spectrum of 600◦ C increase accordingly, except the peak at m/z 34. The growth of the peaks at m/z 28, 42, 78, 92 is quite noticeable. When the temperature increased to 800 ◦ C, the signal intensities of these pyrolysis products grow further along with. It is also worth pointing out

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Figure 4: Eects of temperature on major pyrolysis products of SH coal. Relative intensity represents the ratio of the peak area of a certain species under a certain temperature on the cumulative spectrum to that of the same species under 800 ◦ C.

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that although extremely low signal intensities of the peaks at m/z 54, 68, 196, 210 could be found at lower temperatures, their intensities become fairly manifest under the temperature of 800◦ C. These behaviors, dierent from the complex changes of SH coal, indicate that the temperature required for pyrolysis of XT coal is much higher than that of SH coal, which is well consistent with the thermogravimetric analysis presented in Figure 1. This could be explained based on the dierent oxygen content of XT and SH coals. The oxygencontaining radicals may promote the decomposition of organic materials, 17,26 therefore, it is comprehensible that the pyrolysis of XT coal, with less oxygen content and more stable structure, would be more dicult.

3.3 Time-evolved spectra of SH and XT coal 3.3.1 Time-evolved spectra of SH coal The time-evolved proles of the pyrolysis products could be obtained from the online analytical system. Figure 5 illustrates the time-evolved proles of selected major pyrolysis products of SH coal at temperatures of 600 ◦ C, 700◦ C and 800◦ C. The spectra at a certain temperature was recorded continuously with an interval of 4s during the whole pyrolysis process. For all time-evolved spectra of SH coal, the intensity was normalized based on the spectra under 600◦ C. Despite that the behaviors of all materials detected could be monitored in real time, only some representative substances will be discussed in this work due to space limitation. These selected pyrolysis products can be classied into alkenes (m/z 42, 56, 196), dienes (m/z 54, 68) and aromatics (m/z 78, 92, 106). As presented in Figure 5, the higher the reaction temperature is, the faster the reaction rate will be. This conclusion is expected and can be reected in terms of two aspects. First, the starting point of the decomposition becomes earlier at higher temperatures. On the other hand, the time that the whole pyrolysis process takes becomes much shorter. For instance, most alkenes began to release at about 20s after the coal sample was placed in the furnace at 600 ◦ C, and the main pyrolysis process lasted for roughly 80s. At 800 ◦ C, however, the alkenes started to release in 10s after placing the sam17


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ple and the total reaction time was shortened to 20s. At a xed temperature, the evolution curves of almost all olens and aromatics exhibit only one peak and almost all olens were formed at approximately the same time, while the aromatics were released a little delayed especially at low temperatures. Thus if the conjecture about the conversion of light olens and heavy olens is correct as discussed in the former section, the reactions will take place at nearly the same time. It is dierent from the previous experimental results that the evolution curves of the initial pyrolysis products may present several peaks according to Zou

et al., 18

possibly because of dierent heating rates in this work. The substance (m/z 42), most likely propylene, however, does seem to be dierent from the release behavior of other substances. The time prole of alkenes in Figure 5 reveals that this substance has two peaks, among which, the rst one appears very early, the second one occurs slightly later than most of the olen. This property is particularly evident at low temperatures, indicating that there may be two steps in the release of propylene. According to the "host-guest" model proposed by Solomon et al., 32 coal consists of a solid matrix and a mobile fraction. As illustrated in the previous work, 10,33 the solid matrix can be represented by clusters of many fused aromatic rings connected with other clusters via ether, thioether and aliphatic bridges or loops. The mobile fraction consists of a series of "guest molecules" exhibiting relatively lower molecular weights, which are physically trapped in the three-dimensional structure of coal and can be thermovaporized from the coal by moderate heat. 34 At higher temperatures, on the one hand, an enormous variety of products could be formed from the macromolecular network structure of the coal. Since the aromatic bonds are very stable compared to the aliphatic bridges and loops, the aliphatic bridges in coal macromolecule break up rst during the heating process and form smaller fragments. On the other hand, longer-chanied aliphatics, resulted from thermovaporization from the mobile fraction of the coal, might suer cleavages of C-H and C-C bonds to form shorter-chained unsaturated aliphatics containing one double bond, such as propylene. The low molecular weight fragments escape away from coal particle as tar while high molecular weight fragments remain in the char with a liquid or solid state

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as metaplast. At an even higher temperature, ring formation processes will occur, leading to aromatics from long-chained saturated aliphatics. 21 The curve of the substance (m/z 78) does not decline rapidly after reaching the maximum, exactly indicating the presence of cyclization reaction.

Figure 5: Time-evolved proles of major pyrolysis products of SH coal at dierent temperatures. Note dierent scales.

3.3.2 Time-evolved spectrum of XT coal The signal intensity of XT coal is generally low, suggesting low signal to noise ratio and poor repeatability, which is not suitable for the collection of time-evolved spectrum. Therefore, although same approaches were adopted for the time-evolved spectrum of XT coal, there are no regularity results except that the conclusions concerning the variation of reaction rate with temperature share several similarities with SH coal. In order to better understand the pyrolysis process of dierent coals, the pyrolysis model should be introduced to complement the experimental results, which is left for future work.

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4 Conclusions

The VOCs released from pyrolysis of both bituminous and anthracite coals was studied by the combination of online and oine test methods. The mass spectra data exhibit that rank variation of coal samples has important impacts on the intensity of pyrolysis products. High volatile bituminous coal releases more VOCs than low volatile anthracite coal under the same temperature. The dominant pyrolysis products of both coals are comprised of a mixture of non-aromatic hydrocarbons, mainly alkanes, light alkenes and aromatic compounds, such as BTX, trimethylbenzene, ethylbenzene, diethylbenzene and their corresponding isomers. The relative contents of the pyrolysis products of the two coals also dier. SH coal releases more short chain aliphatic and oxygen-containing compounds, while XT coal releases more aromatic and sulfur-containing compounds. Temperature has an important inuence on the release of VOCs from coal pyrolysis. The relative signal intensities of most products increase as temperature increases. According to the time-evolved proles, the higher the temperature, the earlier and faster the release of VOCs is regardless of the coal type. For bituminous coal, at a xed temperature, almost all alkenes and dienes started to release at approximately the same time except the substance (m/z 42), most likely propylene, which has two peaks, indicating that there may be two steps in the release of propylene. The aromatics were released a little delayed especially at low temperatures. As for anthracite coal, the release of VOCs is almost undetectable under low temperatures.

Acknowledgement

This work was supported by the National Key Basic Research Program of China (No. 2013CB228506). The authors appreciate the support of the National Synchrotron Radiation Laboratory, University of Science and Technology of China, for the online VOCs analysis.

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Analysis of Volatile Organic Pyrolysis Products of Bituminous and

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