Experimental Study on the Molecular Hydrogen Release Mechanism

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Experimental Study on the Molecular Hydrogen Release Mechanism during Low-temperature Oxidation of Coal Yongyu Wang, Jianming Wu, Sheng Xue, Junfeng Wang, and Yulong Zhang Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.6b02460 • Publication Date (Web): 06 Mar 2017 Downloaded from http://pubs.acs.org on April 23, 2017

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Experimental Study on the Molecular Hydrogen Release Mechanism during Low-temperature Oxidation of Coal a

Yongyu Wang , Jianming Wu a, Sheng Xue b, Junfeng Wang a,*, Yulong Zhang a a

College of Mining and Technology, Taiyuan University of Technology, Taiyuan 030024, People’s Republic of China

b

CSIRO Energy, PO Box 883, Kenmore, QLD4069, Australia

Abstract: Although H2 gas is used in coal mines as an important indicator to reflect the state of coal spontaneous combustion, the gas production of H2 at low temperature has been scarcely reported in the literature. In this paper, the modes and release mechanism of molecular hydrogen were investigated for three different coal ranks below 200 oC. Batch reactor tests were performed in combination with chromatographic analysis of the coal oxidation process. The experimental results showed that molecular hydrogen release mainly originated from coal oxidation rather than thermal decomposition of inherent hydrogen-containing groups. The amount of hydrogen released increased with the coal rank. The H2 release process during low-temperature oxidation typically proceeds in two phases namely, H2 slow release (T < 100 oC) and H2 accelerated release (T > 100 o C) phases. Experiments with model compounds revealed aldehyde compounds to noticeably produce H2. Coal plays an positive role in promoting the aldehyde groups to release H2 and CO2, but an opposite trend was observed in the case of CO. As revealed by Fourier transform infrared FTIR) spectroscopy, the amount of aliphatic structures significantly decreased with the oxidation intensity, and a drastic increase in the aldehyde content was found at temperatures above 120 oC. Additionally, the path for the formation of H2 during low-temperature oxidation of coal was provided. Keywords: Coal; low-temperature oxidation; molecular hydrogen; precursor; pathway. 1 Introduction Spontaneous combustion of coal, a process of coal oxidation, is a major hazard seriously affecting safety in underground coal mines[1, 2]. The transit from low-temperature oxidation to spontaneous combustion involves a series of changes in the microscopic (e.g., pore structure, active functional groups, and elemental composition) and macroscopic (e.g., mass change, heat release, and formation of oxidation products) characteristics of coal[3]. Among these, oxidation products, especially gaseous products usually change to a large extent as coal temperature rises[4, 5]. The factors governing the formation of gas products during self-heating process have been investigated in many studies. In fact, some of these gases are usually chosen as coal combustion index gases to detect coal spontaneous combustion. These gaseous products mainly include carbon dioxide, carbon monoxide, methane, molecular hydrogen, ethane, ethylene, and other higher hydrocarbons[6]. The composition and concentration of these gaseous products have a close relationship with temperature. P. Lu et al.[7] divided the coal oxidation process into four stages. They concluded that CO could be used as an index gas at coal temperatures reaching 50 oC, and its concentration rapidly increased at temperatures exceeding 130 oC. Katerina Derychova et al.[8] heated coal samples at 50–200 oC and found that the CO/CO2 concentration ratio was strongly dependent on the temperature and could therefore be used for studying coal temperature. Additionally, Graham combined the CO/O2(R1), CO2/O2(R2), and CO/CO2(R3) concentration ratio parameters as composite indicators to systematically predict the coal temperature[9]. Although carbon oxides have been playing a positive role in combustion prediction, the prediction value could be easily affected and become inaccurate because carbon

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oxides can be produced at ambient mine temperature in coal seam. Thus, Ghosh and Banerjee introduced the carbon to available hydrogen ratio along with oxygen consumption values to determine the coal combustion process[10, 11]. Wang et al. investigated the oxidation behavior of low-rank coal and found the carbon to effective hydrogen ratio to be directly related with the coal temperature[12]. Molecular hydrogen generated during coal self heating is also chosen as one of the index gases in US mines. It is well known that H2 is one of the main gaseous products formed during coal spontaneous combustion. It is generated in large amounts at high temperatures (T > 400 oC) by coal pyrolysis. Krzysztof Stanczyk et al.[13] applied the underground coal gasification technique at temperatures above 700 oC to produce a hydrogen-rich gas. Li et al.[14] conducted open-system non-isothermal pyrolysis experiments on selected immature and mature carbonaceous coals and found considerable amounts of H2 being released at temperatures up to 1200 oC. However generation mechanism and release rule responsible for H2 generation at low temperature(T SD > ZB at coal temperatures lower than 100 oC, and this order changed to SD > XS > ZB at 100–200 oC. These results indicated that the H2 release rate during low-temperature coal oxidation was controlled by the coal rank and the temperature. 3.2.2 Dynamic characteristics of molecular hydrogen According to the Arrhenius equation, Eq (3) can be obtained by calculating the logarithm as follows: ln K = − Ea / RT + ln A

(3)


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where K is the reaction rate and Ea is the apparent activation energy. Ea can be obtained by lnk vs 1/T for the H2 release process from the three coal samples, as shown in Fig. 4. Two different segments rather than a straight line were used to fit the experimental data. Similar changing rules were observed between lnk and 1/T for the three coal samples. These results infer that the H2 release process during low-temperature oxidation can be divided into two stages (i.e., 60–100 ºC and 100–200 ºC) in terms of the value of the apparent activation energy for H2 release. Combined with the trend of H2 emission rate with increasing coal temperature (Fig. 5), it was found that the H2 release process was closely related to the critical temperature 100 ºC. In this sense, the first stage corresponds to the slow release phase at coal temperatures below 100 ºC while the second stage is the accelerated release phase with similar exponential form at coal temperatures of 100–200 ºC. Meanwhile, the H2 release rate of the three coal samples below 100 ºC showed a 10-10 order of magnitude in all cases (i.e., nearly 2 orders of magnitude lower as compared with the CO emission rate[22]), so the amount of H2 released was quite low and difficult to monitor. However, the amount of H2 released increased significantly at temperatures above 100 ºC, and this can explain why many scholars began to find H2 released gradually when coal temperature was over 100 ºC [2, 24].

-24

-1

-1

ZB coal SD coal XS coal

-26

-28

H 2 em ission rate (m ol g -1 m in -1 )

1.50E-008

-22

lnk (mol g min )

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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ZB coal SD coal XS coal

1.20E-008

R=0.98

9.00E-009

R=0.99 Stage 2 y=A[exp(-Bx)-exp(-Cx)]

6.00E-009

R=0.99 Stage 1 y=ax+b

3.00E-009

0.00E+000

0.0020 0.0022 0.0024 0.0026 0.0028 0.0030 -1

40

1/T (k )

60

80 100 120 140 160 180 200 220 Coal temperature ( )

Fig. 4 lnK vs 1/T plots for the H2 release process from three coals. Fig. 5 H2 release rate as a function of the coal temperature from the three coals. Table. 4 Apparent activation energy for the H2 release process in the two stages. Apparent activation energy (kJ mol-1)

Stages ZB coal

SD coal

XS coal

First stage

33.01

26.77

24.69

Second stage

58.11

58.01

53.77

Table 4 shows the apparent activation energy for the H2 release process in the two stages. As a whole, the apparent activation energy for H2 emission from the three coals increased with the coal temperature. With regard to the reaction mechanism for coal oxidation, chemical adsorption played a dominant role in coal oxidation at coal temperatures below 100 ºC, while chemical reaction prevailed during coal oxidation at temperatures over 100 ºC (i.e., more energy is needed for the intense reaction at this stage). These results indicate that the stability of the precursor compound gradually enhanced and significantly larger amounts of energy would be needed to release H2 with increasing the coal temperature. Meanwhile, the apparent activation energy of each stage for H2 release from the three coals followed the order: ZB > SD > XS, which was consistent with the coal rank trend. It was inferred that bituminous coal released H2 more easily as compared to lignite when heated under air. This finding was consistent with the earlier


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experimental results. Additionally, it should be noted that the difference among the apparent activation energies for H2 release from the three coals was not large. For example, in the second stage the ZB coal showed an activation energy only 0.1 kJ mol-1 larger than the SD coal, and 4.34 kJ mol-1 larger than the XS coal. These results indicated that the functional group serving as a H2 release precursor may be same for the three coal samples, but organic compounds in coal containing the functional groups may be slightly different for the three coals. 3.3 Model compound experiments 5 -C=O

4 Absorbance

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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SD coal

3 2

-OH Aliphatic C-H

1 0 4000 3500 3000 2500 2000 1500 1000 -1 Wavenumber (cm )

500

Fig.6 The FTIR spectra of SD coal The release of hydrogen molecules from coal involves hydrogen free radical conversion and reaction steps. Based on the absorption peaks of the coal infrared spectrum (Fig. 6), the hydrogen-containing functional groups mainly included OH stretching vibration from 3100 to 3600 cm-1, aliphatic C–H stretching vibration from 2800 to 3000 cm-1, and -CHO and -COOH attached to C=O stretching vibration from 1500 to 1800 cm-1[25,26]. Table 5 summarizes the experimental results of the reaction between several hydrogen-containing chemical compounds and oxygen during the model compound experiments. It can be found chemical compounds containing carboxyl, hydroxyl, and C–H functional groups did not release H2. However, four kinds of aldehyde group compounds produced noticeable amounts of molecular hydrogen after 3 h of oxidation at 60 and 100 oC. It is therefore believed that H2 was directly generated by oxidation of aldehyde group compounds, and aldehyde group is the functional group to produce molecular hydrogen under air environments[20]. It also can be seen that CO and CO2 were also produced by reaction of aldehyde groups with O2, and that the amounts of the three kinds of gaseous products increased with the temperature. Table 5 Gasesous products produced by hydrogen-containing chemical compounds. Samples

-CHO

-COOH

-OH

H2 (ppm) o

60 C

CO (ppm) o

100 C

o

60 C

CO2 (ppm) o

o

100 C

60 C

100 oC

acetaldehyde

33

81

96

416

2009

4891

formaldehyde

76

301

65

244

778

2236

benzaldehyde

31

55

76

298

1739

3449

glutaraldehyde

217

1167

342

2924

5351

9849

acetic acid

0

0

44

68

6799

11100

phenylacetic acid

0

0

31

61

1127

2354

alcohol

0

0

15

23

314

592

phenylcarbinol

0

0

15

18

497

867

tertiary butanol

0

0

6

8

668

971


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-CH-

diphenylmethane

0

0

10

17

221

344

dichloromethane

0

0

7

9

154

221

5000 Compounds alone Coal mixed with compounds

3000 2000 1000 0

Compounds alone Coal mixed with compounds

50000 CO2 amount (ppm)

4000 H2 amount (ppm)

40000 30000 20000 10000 0

acetaldehyde formaldehyde benzaldehyde glutaraldehyde


Compounds

Compounds

(a)

(b) 3000

Compounds alone Coal mixed with compounds

2500 CO amount (ppm)

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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2000 1500 1000 500 0


Compounds

(c) Fig.7 The amounts of gases produced by compounds alone and compounds absorbed by SD coal at 100 oC,a-H2, b-CO2, c-CO.

The amounts of H2, CO, and CO2 produced by functional compounds with and without SD coal at 100 ºC are shown in Fig. 7. The amounts of gases produced by the SD coal mixed with the model compounds was obtained by subtracting the amounts produced by the SD coal itself. First, the amounts of H2 produced by the coal sample mixed with aldehyde group compounds were significantly larger than those of the compounds alone. For example, the amounts of H2 produced by coal mixed with formaldehyde were 12 times larger than that produced by formaldehyde alone, and the amounts of H2 produced by coal mixed with benzaldehyde were 13 times larger than that produced by benzaldehyde alone. The same phenomenon was observed in the amounts of CO2. For example, the amounts of CO2 produced by coal mixed with formaldehyde were almost 20 times larger than those obtained by formaldehyde alone (6 times in the case of acetaldehyde). However, CO followed a different trend as compared to H2 and CO2. Although the amounts of CO produced by coal mixed with benzaldehyde were nearly the same as those produced by benzaldehyde alone, the amounts of CO produced by coal mixed with the other three kinds of aldehyde group compounds all were noticeably lower than those generated by the compounds alone. Therefore, it can be concluded that coal played a positive role in promoting the release of H2 and CO2 by aldehyde group , whereas this trend was opposite with CO. These results may indicate that coal can catalyze and promote C–H bond breaking reactions in the aldehyde group by the attack of molecular oxygen to produce H radicals[20]. The H radicals will subsequently


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combine into H2. Meanwhile, C=O bonds are prone to being further oxidized to CO2 rather than producing CO directly. Besides, these results may also be caused by the reactions between aldehyde groups and some specific organic components in coal. The carbonyl carbon is attacked by hydroxyl radicals resulting in an intermediate species[27]. These intermediate species further form into carboxylate, which is accompanied with the production of hydrogen gas. Therefore, aldehyde groups have a direct and close relationship with molecular hydrogen release during coal oxidation according to the H2 formation paths. 3.4 FTIR spectroscopy measurements -1

Absorbance

8

1800-1500 cm

o

7

40 C o 80 C o 120 C o 160 C

o

40 C o 80 C o 120 C o 160 C

6 Absorbance

10

6 -1

3000-2800 cm

4

5 4 3 2

2 1 0

0

4000 3500 3000 2500 2000 1500 1000 -1 Wavenumber (cm )

500

4000 3500 3000 2500 2000 1500 1000 -1 Wavenumber (cm )

(a) 5 4

500

(b)

o

40 C o 80 C o 120 C o 160 C

3 2 1 0 4000 3500 3000 2500 2000 1500 1000 -1 Wavenumber (cm )

500

(c) Fig. 8 FTIR spectra of the three coals at different temperatures. a-ZB coal; b-SD coal; and c-XS coal. SD coal XS coal

0.34 Peak

0.32 Absorbance

Absorbance

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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0.30

Peak

0.28 0.26 2700 2710 2720 2730 2740 2750 2760 -1 Wavenumber (cm )

Fig. 9 FTIR spectra of -CH- of the aldehyde group between 2700 and 2757 cm-1.


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As shown in Fig. 8, the FTIR spectra clearly showed that the content of active functional groups was in the change increased with temperature. Since the aldehyde groups stretching vibration can be overlapped by the ketone group signal, it is preferable to determined the aldehyde content based upon the analysis of the -CH- stretching vibration of the aldehyde group at 2700–2757cm-1[28], as shown in Fig. 9. Fig. 10 shows the evolution of the aldehyde content for the three coals with the coal temperature. The aldehyde content changed slowly and even decreased with temperature up to 120 oC as a result of constant consumption and reaction. At temperatures above 120 oC, a drastic increase in the aldehyde content was found, since the reaction between coal and oxygen intensified with the coal temperature. This resulted in increasing rates for the generation of aldehyde groups versus consumption, thereby drastically increasing the amount of H2 released. Moreover, the aldehyde content in coal followed the order: XS > SD > ZB, thereby indicating that a larger number of aldehyde groups in the XS coal were involved in coal oxygen reactions. Thus, larger amounts of H2 were released by XS coal as compared to the SD and ZB coals, which was consistent with oxidation experiment results. 0.5 ZB coal SD coal XS coal

3.5

1670， 1640（ Highly 1610（ Aromatic C=C） conjugated C=O） 1580, 1554（ COO-, 1705（ Aromatic COOH） aromatic ring stretch）

3.0

0.4 Adsorbance

2.5

Peak Area

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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0.3 0.2

1761， 1735（ Esters, aliphatic COOH）

2.0 1.5 1.0 0.5

0.1

0.0

80

100

120

140

160 o

Temperature ( C)

180

200

1800

1750

1700 1650 1600 -1 Wavenumber (cm )

Fig. 10

1550

1500

Fig.11

Fig. 10. Evolution of the intensity of aldehyde groups adsorption with coal temperature. Fig. 11. Curve-fitted FTIR spectrum in the 1800–1500 cm -1 region for the ZB coal.

Besides noticeably changes were observed in the 2800–3000 cm-1(aliphatic C–H stretching) region, and the aliphatic hydrogen C–H content to the aromatic carbon C=C ratio (Hal/Car) was usually used to provide a measurement of the evolution of the aliphatic structures during coal oxidation due to the fact that the C=C content remains invariable below 200 oC[29]. The 1800–1500 cm-1 region of the FTIR spectra was studied by curve-fitting analysis and the C=C stretching vibration was near 1610 cm-1. An example of the curve-fitted spectra for ZB coal is shown in Fig. 11. Table 6 shows the evolution of the Hal/Car ratio (2800–3000 cm-1 zone/1610 cm-1 band) of the three coals. It can be observed that this ratio decreased with temperature, indicating a significant disappearance of aliphatic structures when increasing the oxidation intensity, in agreement with the reported mechanisms for coal oxidation[29–31]. When fresh coal was exposed to air, oxygen adsorption occurred initially on aliphatic C–H forming intermediate products such as peroxide or hydroperoxide. These intermediate products are unstable and easy to convert into oxygenated functional groups such as hydroxyl, carboxyl, and aldehyde. Meanwhile, hydroxyl in coal can also be further oxidized into aldehyde[32,33]. The aldehyde contents gradually increased with the oxidation intensity, resulting in releasing H2 by two pathways: (i) direct oxidation of aldehyde;


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and (ii) reactions between aldehyde groups and some organic components in coal. Based on these results, the H2 formation path for low-temperature oxidation of coal can be depicted as shown in Fig. 12. Table 6. Evolution of the aliphatic structures derived from the curve-fitting analysis of the FTIR spectra. Hal/Car

80oC

120oC

160oC

200oC

ZB coal

0.584

0.576

0.510

0.485

SD coal

0.696

0.646

0.629

0.501

XS coal

1.69

1.58

1.51

1.36

.

Ar CH Ar'+O2 Chemical adsorption

Ar CH Ar' +Ar'' H . O O

Pathway 2:

Ar CH + Ar' OH O

Ar CH Ar' O OH

Pathway 1:

Generation of aldehyde groups

Ar CH Ar' . O O . Ar CH Ar'+ Ar'' O OH

.

Ar CH Ar'+ Ar'' O OH

.

Ar CH Ar'+ OH O Ar''

.

Ar CH Ar' + OH

Ar CH Ar' OH

Ar'+ O2 Pathway 1: Ar C H + H C O

Molecular hydrogen release

Pathway 2:

O

.

Ar CH + OH O O Ar CH OH

Ar CH + Ar' H O

H2 +CO2+ CO + Ar Ar' OH Ar CH O . O Ar C O + H2

Fig.12 Molecular hydrogen release pathways during low-temperature oxidation of coal. Ar is an aromatic structure such as the benzene ring.

4. Conclusions A set of isothermal batch reactors were used to investigate the modes and kinetics of molecular hydrogen release from three ranks of coals below 200 ºC. Molecular hydrogen release was mainly derived from the reaction between certain functional groups of coal with oxygen rather than thermal decomposition of inherent hydrogen-containing groups. The amount of H2 released during oxidation process remarkably increased with the coal rank when similar amounts of O2 are consumed. H2 emission rates at different oxidation temperature were obtained and the activation energies for the molecular hydrogen emission process were therefore studied. The results showed that the first stage (i.e., H2 slow release phase) was followed when coal temperature was below 100 ºC, while the second stage (i.e., H2 accelerated release phase) was followed in similar exponential form when the coal temperature was above 100 ºC. Model compound experiments showed that aldehyde group compounds noticeably released molecular hydrogen after 3 h of oxidation at 60 and 100 oC. It is therefore believed that H2 was directly generated by oxidation of aldehyde group compound. Coal promoted the release of H2 and


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CO2 by aldehyde group, and an opposite trend was observed in the case of CO. The aldehyde groups showed a direct and close relationship with the molecular hydrogen release process during coal oxidation. The aldehyde contents gradually increased with the oxidation intensity, resulting in higher amounts of H2 released by two pathways: (i) direct oxidation of aldehyde; and (ii) reactions between aldehyde groups and some organic components in coal. Acknowledgments The authors gratefully acknowledge financial support from the National Natural Science Foundation of China (51274146) and the National Basic Research Program of China (2012CB214902). Author Information Corresponding Author *E-mail: [email protected] References [1] Yibo Tang, Sheng Xue. Laboratory Study on the Spontaneous Combustion Propensity of Lignite Undergone Heating Treatment at low Temperature in Insert and Low-Oxygen Environments. Energy and Fuel 2015, 29, 4683-4689. [2] Jun Xie, Sheng Xue, Weimin Cheng, Gang Wang. Early detection of spontaneous combustion of coal in underground coal mines with development of an ethylene enriching system. International Journal of Coal Geology 2011, 85, 123-127. [3]Zhengfeng Li, Yulong Zhang, Xiaoxia Jing, et.al. Insight into the intrinsic reaction of brown coal oxidation at low temperature: Differential scanning calorimetry study. Fuel Processing Technology 2016, 147, 64-70. [4] Wang, H. H., Dlugogorski, B. Z., & Kennedy, E. M. Pathways for production of CO2 and CO in low-temperature oxidation of coal. Energy and Fuel 2003, 17, 150-158. [5] Kongvui Yip, Esther Ng, Chun-Zhu Li, et al. A mechanistic study on kinetic compensation effect during low-temperature oxidation of coal chars. Proceedings of the Combustion Institute 2011, 33, 1755–1762. [6] B. Basil Beamis, Ahmet Arisoy. Effect of mineral matter on coal self-heating rate. Fuel 2008, 87, 125-130. [7] P. Lu, G.X. Liao, J.H. Sun, P.D. Li. Experimental research on index gas of the coal spontaneous at low-temperature stage. Journal of Loss Prevention in the Process Industries 2004, 17, 243-247. [8] Katerina Derychova, MichaelaPerdochova, Hana Veznikova et al. The composition of gaseous products of low-temperature oxidation of coal mass and biomass depending on temperature. Journal of Loss Prevention in the Process Industries 2016, 43, 203-211 [9] Xincheng Hu, Shengqiang Yang, Xiuhong Zhou et al. Coal spontaneous combustion prediction in gob using chaos analysis on gas indicators from upper tunnel. Journal of Natural Gas Science and Engineering 2015, 26, 461-469. [10] Ghosh, A.K., Banerjee, D.D. Use of carbon–hydrogen ratio as an index in the investigation of explosions and underground fires. Journal of Mines, Metals and Fuels 1967, 15, 334–340. [11] Ghosh, A.K., Banerjee, D.D., Banerjee, B.D., Sen, S.K. Assessment of the seat of heating inside a sealed off area with a view to combat and control. Proc. Silver Jubilee Seminar on Combating Coal Fires 1980, 6–10. [12] H. Wang, B.Z. Dlugogorski, E.M. Kennedy. Theoretical analysis of reaction regimes in low-temperature oxidation of coal. Fuel 1999, 78, 1073–1081. [13] Krzysztof Stan´czyk, Krzysztof Kapusta, Marian Wiatowski et al. Experimental simulation of hard coal


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Experimental Study on the Molecular Hydrogen Release Mechanism

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