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Structural characterization and oxidation study of a Chinese lignite with the aid of ultrasonic extraction
Journal of the Energy Institute xxx (2014) 1e8
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Structural characterization and oxidation study of a Chinese lignite with the aid of ultrasonic extraction Xiaoxing Zhong a, b, Manman Wang c, Guolan Dou a, b, *, Deming Wang a, b, Yun Chen a, Yuxin Mo a, Yi Zhang d a
School of Safety Engineering, China University of Mining & Technology, Xuzhou 221116, China Key Laboratory of Gas and Fire Control for Coal Mines, Xuzhou 221116, China Sanquan Medical College, Xinxiang 453003, China d Xuzhou Anyun Mining Technology Inc., Xuzhou 221116, China b c
a r t i c l e i n f o
a b s t r a c t
Article history: Received 2 September 2014 Received in revised form 20 November 2014 Accepted 24 November 2014 Available online xxx
Chinese lignite coal from the Beizao mine which located in Shandong Province of China was studied using a combination of ultrasonic extraction under mild conditions (which generate minimal chemical changes) in conjunction with modern analytical instrumental techniques. This procedure allowed the development of a structural model. Calculations with the Gaussian 09W package generated an optimized structural geometry and also determined the activities of various functional groups. The initial stages of oxidation were found to consist of the oxidation of hydroxyl groups, the rupture of CeO and CeC bonds, and the dehydrogenation of carboxyl groups. In contrast, both the decarboxylation and decarbonylation reactions were determined to require significant energy inputs and thus occur only at high temperatures. In situ Fourier transform infrared analyses were used to confirm the results of the theoretical calculations. © 2014 Energy Institute. Published by Elsevier Ltd. All rights reserved.
Keywords: Ultrasonic extraction Coal Spontaneous combustion Chemical structure Oxidation activity
1. Introduction Coal is an increasingly important and valuable energy source with numerous uses. A fundamental hazard associated with coal is selfheating, a phenomenon that has been of practical importance for more than a century [1]. Low rank coal, accounting for 50e60% of the total coal resources in China, is primarily found in the northwest Jurassic coal-bearing basin, the Cenozoic down-faulted basin in the northeast, and the northern carboniferous-Permian coal-accumulating basin in the west. Low rank coal is composed mostly of lignite and low metamorphic bituminous coal, with lignite accounting for 5.74% of China's coal reserves. These reserves are widely distributed in the northwest, northeast and eastern regions of the country. It is well known that low-rank coal is prone to spontaneous combustion and so, in these areas, coal seams have undergone spontaneous combustion to generate fires that have seriously impacted the ability to safely produce coal. It is therefore important to study the oxidation characteristics of lignite as a means of predicting or mitigating such hazards. Of particular interest in recent years has been the study of the correlations between the chemical structures of coals and spontaneous combustion [2,3]. As such, various analytical techniques, such as Fourier transform infrared (FTIR) spectroscopy, nuclear magnetic resonance (NMR), and X-ray diffraction (XRD) have been used in the study of chemical structures of coals. Ibarra et al. [4] used FTIR spectroscopy to study a series of coals varying in rank from peat to semi-anthracite, and found that this technique can be successfully used to determine aromaticity, hydrogen distribution and oxygen-containing species in coals. Thermogravimetry (TG)eFTIR has also been used to determine the oxygen content of coals [5]. Carbon-13 (13C) NMR spectra, which provide direct measurements of the chemical structure of organic matter, were obtained for a large number of New Zealand coals ranging from peat to semi-anthracite, and four NMR parameters (fa, Sox, fCO2H and fCOH) were examined [6]. Knicker et al. [7] found that solid-state 15N NMR spectroscopy may add substantially to our understanding of the nature and diagenesis of nitrogen structures in coals. XRD analysis is also a useful means for evaluating carbon stacking structures [8].
* Corresponding author. School of Safety Engineering, China University of Mining &Technology, Xuzhou 221116, China. E-mail address: [email protected] (G. Dou). http://dx.doi.org/10.1016/j.joei.2014.11.004 1743-9671/© 2014 Energy Institute. Published by Elsevier Ltd. All rights reserved.
Please cite this article in press as: X. Zhong, et al., Structural characterization and oxidation study of a Chinese lignite with the aid of ultrasonic extraction, Journal of the Energy Institute (2014), http://dx.doi.org/10.1016/j.joei.2014.11.004
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X. Zhong et al. / Journal of the Energy Institute xxx (2014) 1e8
However, these studies only provide the rough structures of coals and do not allow the more detailed structural analyses needed in the study of coal oxidation. A combination of solvent extraction under mild conditions, which causes little chemical change to coals, combined with modern analytical instrumental techniques may provide an essential and effective approach to the investigation of coal compositions and structures [9e12]. This method provides valuable information concerning coal structures, although both the time required for experimental trials and limited extraction efficiencies remain problematic with regard to coal oxidation studies. Ultrasonic irradiation is a powerful tool in modern chemistry and has attracted significant attention because of its advantages of convenience, mild conditions, short reaction times, and high efficiency. This process has become increasingly popular in recent years [13,14]. Ultrasonic technology has also been applied to the extraction of coal [15e17] and has been shown to enhance extraction yields. Many investigators [18e20] report that relatively small molecules with masses less than 500 Da may represent about 30e40% of coal, and so the small molecules that are obtained by extraction may be representative of the overall reactivity of coal samples, at least to some extent. Therefore, in the present work, Beizao (BZ) coal, a typical lignite coal from China, was extracted using various organic solvents, together with the application of ultrasonic irradiation. The raw coal, extracted coal, and extraction residues were all characterized by FTIR and 1H NMR to determine the structure of BZ coal in detail. Based on the data obtained, the oxidation activity of the coal was analyzed using the Gaussian 09W software package. In addition, a series of in situ diffuse reflectance FTIR studies were used to examine the low-temperature oxidation of BZ coal to validate the results obtained from Gaussian 09W. 2. Materials and methods 2.1. Sample origin and preparation The coal used in this study was a lignite coal obtained from the Beizao mine in Shandong Province, China. The coal was ground to a particle size of 0.18e0.38 mm and then dried overnight under vacuum at 313 K prior to experimental trials. The results of proximate and ultimate analyses of the coal are summarized in Table 1. 2.2. Organic solvents The organic solvents used in this work included methanol (CH3OH), tetrahydrofuran (THF), pyridine (Py), carbon disulfide (CS2) and Nmethyl pyrrolidone (NMP). All chemicals were analytical grade. 2.3. Ultrasonic extraction of coal samples Prior to ultrasonic extractions, a 5-g sample of dried coal was combined with 20 mL of an organic solvent in a flask, with mixing, and then extracted for 8 h under ultrasonic (40 Hz) irradiation at room temperature. The mixtures were subsequently kept overnight and then filtered. The solid filtrate was then extracted with fresh solvent in the same manner as described previously. 2.4. FTIR measurements In situ FTIR spectra were acquired using a Nicolet 6700 spectrometer, providing spectra in units of cm1. A KBr powder background was collected prior to sample analysis as a baseline reference. Each ground coal sample was placed in the reaction chamber, the chamber dome was put in place, and dry air at a flow rate of 100 mL/min was allowed into the reaction chamber, entering from its base and exiting from the top. A temperature controller was connected to the reaction chamber and the chamber was heated to 220 C at a rate of 1 C/min. The region from 650 to 4000 cm1 was scanned with 4 cm1 resolution, adding 64 scans per spectrum. For each sample, a series of spectra were collected at 30 s intervals. 2.5. Liquid-state NMR experiments 1 H NMR spectra were recorded on a Varian NMR system (UNITY INOVA 400). The chemical shifts were reported in parts per million (d) relative to the trimethylsilane internal standard (at 0 ppm) in CDCl3. The peak patterns are indicated as follows: s, singlet; d, doublet; dd, doublet of doublets; t, triplet; m, multiplet; q, quartet. The coupling constants, J, are reported in Hertz (Hz).
3. Results and discussion 3.1. FTIR results Fig. 1 shows the FTIR spectra of the BZ coal. In total, there are 16 peaks in the spectrum, and these are summarized in Table 2. The 3650e3100 cm1 region corresponds to the eOH stretching vibrations (n) of water, alcohols, phenols, carboxylic acids and similar groups. Of Table 1 Proximate and ultimate analyses of BZ coal. Proximate analysis, % Moisture/Mad 19.13
Ash/Aad 13.32
Volatile/Vad 31.30
Fixed carbon/FCad 36.26
Calorific power, kJ/g
Elemental analysis, %
Qnet, ad 23.78
Had 4.49
Real density, g/cm3 Std 0.66
r 1.54
Please cite this article in press as: X. Zhong, et al., Structural characterization and oxidation study of a Chinese lignite with the aid of ultrasonic extraction, Journal of the Energy Institute (2014), http://dx.doi.org/10.1016/j.joei.2014.11.004
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Fig. 1. Fourier transform infrared spectra of a representative coal sample.
these, peaks at lower wavenumbers correspond to eOH groups with a significant degree of association. The peak at 3054 cm1 corresponds to the AreCH stretching vibration, while the aromatic ring C]C stretching vibration is located at 1609 cm1. The 2925 cm1 peak corresponds to the eCH3 asymmetrical stretching vibration, and the peak at approximately 2855 cm1 represents the eCH2e symmetrical stretching vibration. The bending vibration peaks of CeH are located at 1455 and 1375 cm1. The 1455 cm1 peak area is much larger than that of the 1375 cm1 peak, indicating that BZ coal contains abundant methylene groups. In general, these results demonstrate that this coal contains abundant hydroxyl, methyl, methylene, carboxyl (eCOOH), carbonyl (C]O), ether (CeOeC) and substituted benzene groups. 3.2. NMR results In this work, various organic solvents, including methanol (CH3OH), tetrahydrofuran (THF), pyridine (Py), carbon disulfide (CS2) and Nmethyl pyrrolidone (NMP), were used to extract the coal. In initial trials, we found that the spectra of CH3OH and THF extracts obtained over either 6 or 12 h were similar, indicating that the extraction time had little effect on the results. For this reason, extractions with Py and CS2/ NMP were performed only for 6 h. In the 1H NMR spectra, the aromatic peaks were well resolved (6.5e7.1 ppm) from the aliphatic peaks (0.8e4.8 ppm), and thus these spectra could be reliably interpreted. It was found that the 1H NMR spectra of CH3OH, THF, Py and CS2/NMP extracts exhibited peaks at d values of 0.80e3.75, 0.86e7.10 and 0.86e3.40 ppm, respectively. The 1H NMR spectra of the extracted residues showed peaks at d values of 0.84e4.81 ppm. Significant information concerning the aliphatic hydrogens was obtained from these spectra. The protons responsible for the aliphatic peaks are usually further divided into three types (a, b and g) according to their respective positions on the aromatic ring, although wide bands often result from overlapping of several peaks, allowing much less accurate estimation of the peak positions. According to the literature, peaks at 0.5e1.0, 1.0e1.85 and 1.85e4.5 ppm correspond to a, b and g protons, although these divisions are approximate, because the effects of heteroatoms are neglected. Taking these factors in consideration, along with the observed splitting and chemical shifts of the peaks, the spectra of the coal extracts and residues were analyzed, and the possible assignments of representative signals are listed in Table 3. From the above it is evident that the coal contained phenolic hydroxyl (PheOH), alcoholic hydroxyl (ReOH) and methoxy (CH3O) groups, as well as methyl groups connected to aromatic rings (AreOCH3, AreCH3), methylidyne groups connected to aromatic rings (AreCHe), alicyclic hydrogens, ester groups (COOC2H5) and carboxyl groups (eCOOH). 3.3. Structural model During low temperature oxidation, the effects of aromatic rings on the active groups are negligible. Thus, in order to reduce the necessary calculations, we used simplified aromatic structures to model more complicated aromatics when investigating the low temperature oxidation of coal. The development of a structural model for coal on the basis of the above data remained challenging, however, because the Table 2 Attribution and analysis of the Fourier transform infrared spectra of Beizao coal. Peaks (cm1)
Identified functional groups
Peaks (cm1)
Identified functional groups
3648.19 3619.66 3358.50 3054.00 1609.16 2925.73 2855.50 1455.18 1375.84
eOH
1705.22 1265.17 1058.91 872.24 815.33 753.94 696.89
Carboxyl or carbonyl CeOeC SieOeSi or SieOeC Substituted benzene
AreCH C]C in aromatic rings Methyl and methylene
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X. Zhong et al. / Journal of the Energy Institute xxx (2014) 1e8 Table 3 Assignment of peaks in 1H NMR spectra (referenced to trimethylsilane ¼ 0 ppm). Sample
Chemical shift range/ppm
Assignment
First CH3OH extract
3.75e3.70 (q, 2H)
CH2 in CH3CH2Oe or
O
Second CH3OH extract
First THF extract
2.12e2.07 (m, 3H) 2.17 (s, 3H) 3.73 (s, 3H) 2.17 (s, 3H) 1.25 (s, 16H) 3.73 (q, 2H)
CH3 in CH3CH2Oe CH3 in AreCH3 Methoxyl (CH3O) CH3 in AreCH3 CH2 in aliphatic chains CH2 in CH3CH2Oe or
O
Second THF extract Py extract CS2/NMP extract
Residue
2.17 (s, 3H) 2.01e1.86 (m, 14H) 1.25 (s, 30H) 0.94e0.80 (m, 15H) 3.75e3.70 (m, 2H) 2.34 (t, 1H) 7.10e6.99 (m, 3H), 6.57e6.54 (m, 1H) 1.26 (s, 10H), 0.88 (t, J ¼ 8.0 Hz, 3H) 3.38 (t, 2H) 2.85 (s, 3H) 2.38 (t, 2H) 4.81 (br, s, 2H) 3.74e3.71 (m, 6H) 2.17 (s, 3H) 1.85 (t, 2H)
CH3 in AreCH3 Aliphatic H in methyl or methylene groups
CH2 in CH3CH2Oe CH connected with methylene Aromatic H Aliphatic H in methyl and methylene groups CH2 in eCH2CH2OCH3 in AreCH3 CH2 in ArCH2CH2e ReOH or PheOH Methoxy CH3 in AreCH3 CH2 in ArCH2CH2e
structure of coal is very complex. Shi et al. [2] have reported results for the active groups on coal surfaces. Among these active groups, the most common and most active during coal oxidation are secondary hydroxyl groups. During the present study, we found that these groups are also present in BZ coal, and so our structural model was developed based on these hydroxyl groups together with various aryl groups. The other functional groups identified in the above spectra were also taken into account, making reference to a previously reported model [21], to develop the BZ molecular model shown in Fig. 2. 3.4. Analysis of oxidative activity by quantum chemistry Geometric optimization of the structure model was achieved by performing computerized calculations based on density functional theory (DFT) at the B3LYP/6-31G level (E(B3LYP) ¼ 1724.6519 Eh), resulting in the structure presented in Fig. 3. In addition, the bond lengths associated with the structural model were calculated, with the results summarized in Table 4. From Table 4, it is evident that the longest bond is the carbonecarbon single bond connecting the aryl group and the hydroxyl group (C7eC8), indicating that the associated moiety is the site most readily split and thus is most likely to generate free radicals. The carbonecarbon single bonds in the long-chain alkyl groups are also relatively long and easy to split (C25eC26 and C27eC29). In contrast, the carbonecarbon single bonds associated with the carboxyl and ester groups are relatively short (C11eC30 and C6eC17), suggesting that the decarboxylation and reaction of coal will be limited during low-temperature oxidation. To allow more detailed study of these active groups, the frontier orbitals of the optimized coal model were calculated using the Gaussian 09W package, with the results shown in Table 5. From Table 5, it is evident that the order of the electron densities of the functional groups is O9 > C17 > C30 > O32 > O33 > C23 > C26, C29 > C16 > O28 > C34 > O18. According to molecular orbital theory, the oxidation reactions leading to the spontaneous combustion of coal will involve atoms at which the charge density is high. Thus, the hydroxyl, carboxyl, ester and phenolic hydroxyl groups will all readily undergo reactions, and the alkyl, carbonyl and epoxy groups will all be relatively less active, in that order. However, as a result of the higher degree of bond polarization in the carbonyl group and the sp2 hybridization effect, the bond energy of the C]O bonds will be twice that of the CeO bonds, such that the carboxyl and ester groups will be much more stable. Therefore, in the initial phase of coal oxidation, these groups may preferentially undergo CeO bond rupture and dehydrogenation. Based on the above analysis of bond lengths, it is apparent that the reactions that will take place most readily in the initial stage of coal oxidation will include the oxidation of hydroxyl groups, the rupture of CeO and CeC bonds and the dehydrogenation of carboxyl groups. In addition, decarboxylation reaction may take place at high temperatures, at which the systems has accumulated sufficient thermal energy. 3.5. In situ diffuse reflection FTIR To verify the above results, the oxidation of BZ coal was further assessed by in situ series diffuse reflection FTIR, generating 3D (KubelkaeMunk/time/wavenumber) data. The resulting 3D data are presented in Fig. 4. Please cite this article in press as: X. Zhong, et al., Structural characterization and oxidation study of a Chinese lignite with the aid of ultrasonic extraction, Journal of the Energy Institute (2014), http://dx.doi.org/10.1016/j.joei.2014.11.004
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Fig. 2. Structure model for BZ coal.
Fig. 3. Geometrical optimization of the structural model.
Table 4 Calculated bond lengths for the structural model. Series
Bond
Bond length
Series
Bond
Bond length
1 2 3 4 5 6 7 8 9
C5eC7 C7eC8 C8eC10 C6eC17 C11eC30 C12eC34 C22eC23 C25eC26 C2eC16
1.5274 1.5525 1.5203 1.4966 1.4650 1.5208 1.5459 1.5220 1.5078
10 11 12 13 14 15 16 17
C27eC29 C7eO9 C16eO32 C17eO24 C13eO20 C11eO21 C22eO21 O33eH64
1.5311 1.4161 1.2244 1.3626 1.3582 1.3831 1.4283 0.9971
Table 5 Frontier orbital analysis of the optimized coal structure. Orbital
eOH
eCH2eCH2eCH3
Electron density
Electron density
HOMO 0.24725
O9 0.789 O33 0.730
C24 0.492 C25 0.461 C26 0.700
Methoxy and epoxy
Carboxyl and ester group
Electron density
Electron density
Electron density
C16 0.542 C27 0.549 C29 0.700 O28 0.526
C19 0.359 O18 0.495 C34 0.522 C35 0.135 O36 0.508
C30 0.768 O31 0.587 O32 0.759 C17 0.781 C22 0.142 C23 0.719 O20 0.543 O21 0.560
O
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Fig. 4. 3D in situ series Fourier transform infrared data obtained for the Beizao coal oxidation process.
Fig. 5. Variations in AreCH Fourier transform infrared absorbance in a Beizao coal sample over time.
The 3D data were analyzed to determine the peak intensity at specific wavenumbers as well as changes in intensity at several of these wavenumbers with variations in time or temperature. The wavenumbers that were monitored during these trials were: 2960 and 2875 cm1 (corresponding to n of the CH3 group), 2925 and 2855 cm1 (assigned to CH2 stretching), 1125e1110 cm1 (corresponding to the antisymmetric CeOeC stretching vibrations of ethers), 3040e3030 cm1 (corresponding to n of AreCH groups) and 3650e3200 cm1 (specific to eOH stretching). The variations in the functional groups over time are shown in Figs. 5e8. The data collected by in situ series FTIR spectroscopy demonstrate that the concentration of CeO bonds gradually increases during the oxidation process. It also can be seen that the KubelkaeMunk values of the hydroxyl groups decrease in the initial stage of oxidation. This results from the decomposition of hydroxyl groups, generating carbonyl groups and water and leading to the observed increase in the concentration of carbonyl groups with time. From Fig. 5, it is also evident that the concentration of AreCH gradually decreases with time during the oxidation process. This may result from the rupture of CeC bonds in AreCH groups, generating new alkyl moieties. Considering these in situ FTIR results, it is apparent that the low-temperature oxidation of hydroxyl groups, along with the rupture of CeC and CeO bonds, are the primary causes of the self-heating and subsequent spontaneous combustion of coal. These experimental results are also consistent with the results of theoretical calculations using Gaussian 09W. Based on the results above, an outline reaction steps for the oxidation of the active coal groups is given in Schemes 1 and 2. Among them, the activation energy of the reaction listed in Scheme 1 was calculated by Shi et al. [2]. The activation energy is only 4.75 kJ/mol, which means this group is very active with oxygen.
Fig. 6. Variations in eOH Fourier transform infrared absorbance in a Beizao coal sample over time.
Please cite this article in press as: X. Zhong, et al., Structural characterization and oxidation study of a Chinese lignite with the aid of ultrasonic extraction, Journal of the Energy Institute (2014), http://dx.doi.org/10.1016/j.joei.2014.11.004
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Fig. 7. Variations in C]O Fourier transform infrared absorbance in a Beizao coal sample over time.
Fig. 8. Variations in CeO Fourier transform infrared absorbance in a Beizao coal sample over time.
4. Conclusion Based on solvent extraction under mild conditions, combined with modern analytical instrumental techniques such as FTIR and liquid state 1H NMR, Chinese lignite BZ coal was studied with regard to its low-temperature reactions. From the data obtained during this study, the following conclusions can be made. (1) A structural model was developed for BZ coal, containing alkyl, methoxy, carbonyl, carboxyl, ester, epoxy and hydroxyl groups. (2) The reactivity of these functional groups was assessed, based on calculated bond lengths, bond energies and the highest occupied molecular orbital electron densities provided by Gaussian 09W. The results indicated that the oxidation of hydroxyl groups and the rupture of CeO and CeC bonds, as well as the dehydrogenation of carboxyl groups, all take place in the initial stage of oxidation, while the decarboxylation and degreasing reactions require more energy and thus only occur at higher temperatures.
Scheme 1. The reaction step for hydroxyl group of coal oxidation.
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X. Zhong et al. / Journal of the Energy Institute xxx (2014) 1e8
Scheme 2. The reaction step for the oxidation of long-chain alkyl group of coal.
(3) The in situ FTIR spectra of BZ coal samples during the oxidation process showed that the oxidation of hydroxyl groups and the rupture of CeO and CeC bonds readily occur, and that the concentrations of carbonyl groups increase with time. These data are consistent with the calculation results, and confirm the information obtained from ultrasonic extraction and Gaussian 09W computations. An outline reaction step of the active groups has been postulated. In summary, ultrasonic extraction combined with modern analytical instrumental techniques represents a suitable means of studying the structure of coals. On the basis of this analytical process and related calculations, we are able to offer a theoretical model for the oxidation-based spontaneous combustion of coal. Acknowledgments The authors wish to express their gratitude for joint funding by the National Natural Science Foundation of China and the Shenhua Group Corporation, Limited (Nos. 51134020 and U1361213), as well as financial support from the National Natural Science Foundation of China Youth Science Foundation (No. 51204171), a General Financial Grant from the China Postdoctoral Science Foundation (No. 2013M531429), the Foundation Research Project of Jiangsu Province (The Natural Science Fund No. BK20141132), the New Century Excellent Researcher Award Program from the Ministry of Education of the People's Republic of China (No. NCET-13-1021) and the Priority Academic Program Development of Jiangsu Higher Education Institutions. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21]
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Please cite this article in press as: X. Zhong, et al., Structural characterization and oxidation study of a Chinese lignite with the aid of ultrasonic extraction, Journal of the Energy Institute (2014), http://dx.doi.org/10.1016/j.joei.2014.11.004
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Structural characterization and oxidation study of a Chinese lignite with the aid of ultrasonic extraction Xiaoxing Zhong a, b, Manman Wang c, Guolan Dou a, b, *, Deming Wang a, b, Yun Chen a, Yuxin Mo a, Yi Zhang d a
School of Safety Engineering, China University of Mining & Technology, Xuzhou 221116, China Key Laboratory of Gas and Fire Control for Coal Mines, Xuzhou 221116, China Sanquan Medical College, Xinxiang 453003, China d Xuzhou Anyun Mining Technology Inc., Xuzhou 221116, China b c
a r t i c l e i n f o
a b s t r a c t
Article history: Received 2 September 2014 Received in revised form 20 November 2014 Accepted 24 November 2014 Available online xxx
Chinese lignite coal from the Beizao mine which located in Shandong Province of China was studied using a combination of ultrasonic extraction under mild conditions (which generate minimal chemical changes) in conjunction with modern analytical instrumental techniques. This procedure allowed the development of a structural model. Calculations with the Gaussian 09W package generated an optimized structural geometry and also determined the activities of various functional groups. The initial stages of oxidation were found to consist of the oxidation of hydroxyl groups, the rupture of CeO and CeC bonds, and the dehydrogenation of carboxyl groups. In contrast, both the decarboxylation and decarbonylation reactions were determined to require significant energy inputs and thus occur only at high temperatures. In situ Fourier transform infrared analyses were used to confirm the results of the theoretical calculations. © 2014 Energy Institute. Published by Elsevier Ltd. All rights reserved.
Keywords: Ultrasonic extraction Coal Spontaneous combustion Chemical structure Oxidation activity
1. Introduction Coal is an increasingly important and valuable energy source with numerous uses. A fundamental hazard associated with coal is selfheating, a phenomenon that has been of practical importance for more than a century [1]. Low rank coal, accounting for 50e60% of the total coal resources in China, is primarily found in the northwest Jurassic coal-bearing basin, the Cenozoic down-faulted basin in the northeast, and the northern carboniferous-Permian coal-accumulating basin in the west. Low rank coal is composed mostly of lignite and low metamorphic bituminous coal, with lignite accounting for 5.74% of China's coal reserves. These reserves are widely distributed in the northwest, northeast and eastern regions of the country. It is well known that low-rank coal is prone to spontaneous combustion and so, in these areas, coal seams have undergone spontaneous combustion to generate fires that have seriously impacted the ability to safely produce coal. It is therefore important to study the oxidation characteristics of lignite as a means of predicting or mitigating such hazards. Of particular interest in recent years has been the study of the correlations between the chemical structures of coals and spontaneous combustion [2,3]. As such, various analytical techniques, such as Fourier transform infrared (FTIR) spectroscopy, nuclear magnetic resonance (NMR), and X-ray diffraction (XRD) have been used in the study of chemical structures of coals. Ibarra et al. [4] used FTIR spectroscopy to study a series of coals varying in rank from peat to semi-anthracite, and found that this technique can be successfully used to determine aromaticity, hydrogen distribution and oxygen-containing species in coals. Thermogravimetry (TG)eFTIR has also been used to determine the oxygen content of coals [5]. Carbon-13 (13C) NMR spectra, which provide direct measurements of the chemical structure of organic matter, were obtained for a large number of New Zealand coals ranging from peat to semi-anthracite, and four NMR parameters (fa, Sox, fCO2H and fCOH) were examined [6]. Knicker et al. [7] found that solid-state 15N NMR spectroscopy may add substantially to our understanding of the nature and diagenesis of nitrogen structures in coals. XRD analysis is also a useful means for evaluating carbon stacking structures [8].
* Corresponding author. School of Safety Engineering, China University of Mining &Technology, Xuzhou 221116, China. E-mail address: [email protected] (G. Dou). http://dx.doi.org/10.1016/j.joei.2014.11.004 1743-9671/© 2014 Energy Institute. Published by Elsevier Ltd. All rights reserved.
Please cite this article in press as: X. Zhong, et al., Structural characterization and oxidation study of a Chinese lignite with the aid of ultrasonic extraction, Journal of the Energy Institute (2014), http://dx.doi.org/10.1016/j.joei.2014.11.004
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However, these studies only provide the rough structures of coals and do not allow the more detailed structural analyses needed in the study of coal oxidation. A combination of solvent extraction under mild conditions, which causes little chemical change to coals, combined with modern analytical instrumental techniques may provide an essential and effective approach to the investigation of coal compositions and structures [9e12]. This method provides valuable information concerning coal structures, although both the time required for experimental trials and limited extraction efficiencies remain problematic with regard to coal oxidation studies. Ultrasonic irradiation is a powerful tool in modern chemistry and has attracted significant attention because of its advantages of convenience, mild conditions, short reaction times, and high efficiency. This process has become increasingly popular in recent years [13,14]. Ultrasonic technology has also been applied to the extraction of coal [15e17] and has been shown to enhance extraction yields. Many investigators [18e20] report that relatively small molecules with masses less than 500 Da may represent about 30e40% of coal, and so the small molecules that are obtained by extraction may be representative of the overall reactivity of coal samples, at least to some extent. Therefore, in the present work, Beizao (BZ) coal, a typical lignite coal from China, was extracted using various organic solvents, together with the application of ultrasonic irradiation. The raw coal, extracted coal, and extraction residues were all characterized by FTIR and 1H NMR to determine the structure of BZ coal in detail. Based on the data obtained, the oxidation activity of the coal was analyzed using the Gaussian 09W software package. In addition, a series of in situ diffuse reflectance FTIR studies were used to examine the low-temperature oxidation of BZ coal to validate the results obtained from Gaussian 09W. 2. Materials and methods 2.1. Sample origin and preparation The coal used in this study was a lignite coal obtained from the Beizao mine in Shandong Province, China. The coal was ground to a particle size of 0.18e0.38 mm and then dried overnight under vacuum at 313 K prior to experimental trials. The results of proximate and ultimate analyses of the coal are summarized in Table 1. 2.2. Organic solvents The organic solvents used in this work included methanol (CH3OH), tetrahydrofuran (THF), pyridine (Py), carbon disulfide (CS2) and Nmethyl pyrrolidone (NMP). All chemicals were analytical grade. 2.3. Ultrasonic extraction of coal samples Prior to ultrasonic extractions, a 5-g sample of dried coal was combined with 20 mL of an organic solvent in a flask, with mixing, and then extracted for 8 h under ultrasonic (40 Hz) irradiation at room temperature. The mixtures were subsequently kept overnight and then filtered. The solid filtrate was then extracted with fresh solvent in the same manner as described previously. 2.4. FTIR measurements In situ FTIR spectra were acquired using a Nicolet 6700 spectrometer, providing spectra in units of cm1. A KBr powder background was collected prior to sample analysis as a baseline reference. Each ground coal sample was placed in the reaction chamber, the chamber dome was put in place, and dry air at a flow rate of 100 mL/min was allowed into the reaction chamber, entering from its base and exiting from the top. A temperature controller was connected to the reaction chamber and the chamber was heated to 220 C at a rate of 1 C/min. The region from 650 to 4000 cm1 was scanned with 4 cm1 resolution, adding 64 scans per spectrum. For each sample, a series of spectra were collected at 30 s intervals. 2.5. Liquid-state NMR experiments 1 H NMR spectra were recorded on a Varian NMR system (UNITY INOVA 400). The chemical shifts were reported in parts per million (d) relative to the trimethylsilane internal standard (at 0 ppm) in CDCl3. The peak patterns are indicated as follows: s, singlet; d, doublet; dd, doublet of doublets; t, triplet; m, multiplet; q, quartet. The coupling constants, J, are reported in Hertz (Hz).
3. Results and discussion 3.1. FTIR results Fig. 1 shows the FTIR spectra of the BZ coal. In total, there are 16 peaks in the spectrum, and these are summarized in Table 2. The 3650e3100 cm1 region corresponds to the eOH stretching vibrations (n) of water, alcohols, phenols, carboxylic acids and similar groups. Of Table 1 Proximate and ultimate analyses of BZ coal. Proximate analysis, % Moisture/Mad 19.13
Ash/Aad 13.32
Volatile/Vad 31.30
Fixed carbon/FCad 36.26
Calorific power, kJ/g
Elemental analysis, %
Qnet, ad 23.78
Had 4.49
Real density, g/cm3 Std 0.66
r 1.54
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Fig. 1. Fourier transform infrared spectra of a representative coal sample.
these, peaks at lower wavenumbers correspond to eOH groups with a significant degree of association. The peak at 3054 cm1 corresponds to the AreCH stretching vibration, while the aromatic ring C]C stretching vibration is located at 1609 cm1. The 2925 cm1 peak corresponds to the eCH3 asymmetrical stretching vibration, and the peak at approximately 2855 cm1 represents the eCH2e symmetrical stretching vibration. The bending vibration peaks of CeH are located at 1455 and 1375 cm1. The 1455 cm1 peak area is much larger than that of the 1375 cm1 peak, indicating that BZ coal contains abundant methylene groups. In general, these results demonstrate that this coal contains abundant hydroxyl, methyl, methylene, carboxyl (eCOOH), carbonyl (C]O), ether (CeOeC) and substituted benzene groups. 3.2. NMR results In this work, various organic solvents, including methanol (CH3OH), tetrahydrofuran (THF), pyridine (Py), carbon disulfide (CS2) and Nmethyl pyrrolidone (NMP), were used to extract the coal. In initial trials, we found that the spectra of CH3OH and THF extracts obtained over either 6 or 12 h were similar, indicating that the extraction time had little effect on the results. For this reason, extractions with Py and CS2/ NMP were performed only for 6 h. In the 1H NMR spectra, the aromatic peaks were well resolved (6.5e7.1 ppm) from the aliphatic peaks (0.8e4.8 ppm), and thus these spectra could be reliably interpreted. It was found that the 1H NMR spectra of CH3OH, THF, Py and CS2/NMP extracts exhibited peaks at d values of 0.80e3.75, 0.86e7.10 and 0.86e3.40 ppm, respectively. The 1H NMR spectra of the extracted residues showed peaks at d values of 0.84e4.81 ppm. Significant information concerning the aliphatic hydrogens was obtained from these spectra. The protons responsible for the aliphatic peaks are usually further divided into three types (a, b and g) according to their respective positions on the aromatic ring, although wide bands often result from overlapping of several peaks, allowing much less accurate estimation of the peak positions. According to the literature, peaks at 0.5e1.0, 1.0e1.85 and 1.85e4.5 ppm correspond to a, b and g protons, although these divisions are approximate, because the effects of heteroatoms are neglected. Taking these factors in consideration, along with the observed splitting and chemical shifts of the peaks, the spectra of the coal extracts and residues were analyzed, and the possible assignments of representative signals are listed in Table 3. From the above it is evident that the coal contained phenolic hydroxyl (PheOH), alcoholic hydroxyl (ReOH) and methoxy (CH3O) groups, as well as methyl groups connected to aromatic rings (AreOCH3, AreCH3), methylidyne groups connected to aromatic rings (AreCHe), alicyclic hydrogens, ester groups (COOC2H5) and carboxyl groups (eCOOH). 3.3. Structural model During low temperature oxidation, the effects of aromatic rings on the active groups are negligible. Thus, in order to reduce the necessary calculations, we used simplified aromatic structures to model more complicated aromatics when investigating the low temperature oxidation of coal. The development of a structural model for coal on the basis of the above data remained challenging, however, because the Table 2 Attribution and analysis of the Fourier transform infrared spectra of Beizao coal. Peaks (cm1)
Identified functional groups
Peaks (cm1)
Identified functional groups
3648.19 3619.66 3358.50 3054.00 1609.16 2925.73 2855.50 1455.18 1375.84
eOH
1705.22 1265.17 1058.91 872.24 815.33 753.94 696.89
Carboxyl or carbonyl CeOeC SieOeSi or SieOeC Substituted benzene
AreCH C]C in aromatic rings Methyl and methylene
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X. Zhong et al. / Journal of the Energy Institute xxx (2014) 1e8 Table 3 Assignment of peaks in 1H NMR spectra (referenced to trimethylsilane ¼ 0 ppm). Sample
Chemical shift range/ppm
Assignment
First CH3OH extract
3.75e3.70 (q, 2H)
CH2 in CH3CH2Oe or
O
Second CH3OH extract
First THF extract
2.12e2.07 (m, 3H) 2.17 (s, 3H) 3.73 (s, 3H) 2.17 (s, 3H) 1.25 (s, 16H) 3.73 (q, 2H)
CH3 in CH3CH2Oe CH3 in AreCH3 Methoxyl (CH3O) CH3 in AreCH3 CH2 in aliphatic chains CH2 in CH3CH2Oe or
O
Second THF extract Py extract CS2/NMP extract
Residue
2.17 (s, 3H) 2.01e1.86 (m, 14H) 1.25 (s, 30H) 0.94e0.80 (m, 15H) 3.75e3.70 (m, 2H) 2.34 (t, 1H) 7.10e6.99 (m, 3H), 6.57e6.54 (m, 1H) 1.26 (s, 10H), 0.88 (t, J ¼ 8.0 Hz, 3H) 3.38 (t, 2H) 2.85 (s, 3H) 2.38 (t, 2H) 4.81 (br, s, 2H) 3.74e3.71 (m, 6H) 2.17 (s, 3H) 1.85 (t, 2H)
CH3 in AreCH3 Aliphatic H in methyl or methylene groups
CH2 in CH3CH2Oe CH connected with methylene Aromatic H Aliphatic H in methyl and methylene groups CH2 in eCH2CH2OCH3 in AreCH3 CH2 in ArCH2CH2e ReOH or PheOH Methoxy CH3 in AreCH3 CH2 in ArCH2CH2e
structure of coal is very complex. Shi et al. [2] have reported results for the active groups on coal surfaces. Among these active groups, the most common and most active during coal oxidation are secondary hydroxyl groups. During the present study, we found that these groups are also present in BZ coal, and so our structural model was developed based on these hydroxyl groups together with various aryl groups. The other functional groups identified in the above spectra were also taken into account, making reference to a previously reported model [21], to develop the BZ molecular model shown in Fig. 2. 3.4. Analysis of oxidative activity by quantum chemistry Geometric optimization of the structure model was achieved by performing computerized calculations based on density functional theory (DFT) at the B3LYP/6-31G level (E(B3LYP) ¼ 1724.6519 Eh), resulting in the structure presented in Fig. 3. In addition, the bond lengths associated with the structural model were calculated, with the results summarized in Table 4. From Table 4, it is evident that the longest bond is the carbonecarbon single bond connecting the aryl group and the hydroxyl group (C7eC8), indicating that the associated moiety is the site most readily split and thus is most likely to generate free radicals. The carbonecarbon single bonds in the long-chain alkyl groups are also relatively long and easy to split (C25eC26 and C27eC29). In contrast, the carbonecarbon single bonds associated with the carboxyl and ester groups are relatively short (C11eC30 and C6eC17), suggesting that the decarboxylation and reaction of coal will be limited during low-temperature oxidation. To allow more detailed study of these active groups, the frontier orbitals of the optimized coal model were calculated using the Gaussian 09W package, with the results shown in Table 5. From Table 5, it is evident that the order of the electron densities of the functional groups is O9 > C17 > C30 > O32 > O33 > C23 > C26, C29 > C16 > O28 > C34 > O18. According to molecular orbital theory, the oxidation reactions leading to the spontaneous combustion of coal will involve atoms at which the charge density is high. Thus, the hydroxyl, carboxyl, ester and phenolic hydroxyl groups will all readily undergo reactions, and the alkyl, carbonyl and epoxy groups will all be relatively less active, in that order. However, as a result of the higher degree of bond polarization in the carbonyl group and the sp2 hybridization effect, the bond energy of the C]O bonds will be twice that of the CeO bonds, such that the carboxyl and ester groups will be much more stable. Therefore, in the initial phase of coal oxidation, these groups may preferentially undergo CeO bond rupture and dehydrogenation. Based on the above analysis of bond lengths, it is apparent that the reactions that will take place most readily in the initial stage of coal oxidation will include the oxidation of hydroxyl groups, the rupture of CeO and CeC bonds and the dehydrogenation of carboxyl groups. In addition, decarboxylation reaction may take place at high temperatures, at which the systems has accumulated sufficient thermal energy. 3.5. In situ diffuse reflection FTIR To verify the above results, the oxidation of BZ coal was further assessed by in situ series diffuse reflection FTIR, generating 3D (KubelkaeMunk/time/wavenumber) data. The resulting 3D data are presented in Fig. 4. Please cite this article in press as: X. Zhong, et al., Structural characterization and oxidation study of a Chinese lignite with the aid of ultrasonic extraction, Journal of the Energy Institute (2014), http://dx.doi.org/10.1016/j.joei.2014.11.004
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Fig. 2. Structure model for BZ coal.
Fig. 3. Geometrical optimization of the structural model.
Table 4 Calculated bond lengths for the structural model. Series
Bond
Bond length
Series
Bond
Bond length
1 2 3 4 5 6 7 8 9
C5eC7 C7eC8 C8eC10 C6eC17 C11eC30 C12eC34 C22eC23 C25eC26 C2eC16
1.5274 1.5525 1.5203 1.4966 1.4650 1.5208 1.5459 1.5220 1.5078
10 11 12 13 14 15 16 17
C27eC29 C7eO9 C16eO32 C17eO24 C13eO20 C11eO21 C22eO21 O33eH64
1.5311 1.4161 1.2244 1.3626 1.3582 1.3831 1.4283 0.9971
Table 5 Frontier orbital analysis of the optimized coal structure. Orbital
eOH
eCH2eCH2eCH3
Electron density
Electron density
HOMO 0.24725
O9 0.789 O33 0.730
C24 0.492 C25 0.461 C26 0.700
Methoxy and epoxy
Carboxyl and ester group
Electron density
Electron density
Electron density
C16 0.542 C27 0.549 C29 0.700 O28 0.526
C19 0.359 O18 0.495 C34 0.522 C35 0.135 O36 0.508
C30 0.768 O31 0.587 O32 0.759 C17 0.781 C22 0.142 C23 0.719 O20 0.543 O21 0.560
O
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Fig. 4. 3D in situ series Fourier transform infrared data obtained for the Beizao coal oxidation process.
Fig. 5. Variations in AreCH Fourier transform infrared absorbance in a Beizao coal sample over time.
The 3D data were analyzed to determine the peak intensity at specific wavenumbers as well as changes in intensity at several of these wavenumbers with variations in time or temperature. The wavenumbers that were monitored during these trials were: 2960 and 2875 cm1 (corresponding to n of the CH3 group), 2925 and 2855 cm1 (assigned to CH2 stretching), 1125e1110 cm1 (corresponding to the antisymmetric CeOeC stretching vibrations of ethers), 3040e3030 cm1 (corresponding to n of AreCH groups) and 3650e3200 cm1 (specific to eOH stretching). The variations in the functional groups over time are shown in Figs. 5e8. The data collected by in situ series FTIR spectroscopy demonstrate that the concentration of CeO bonds gradually increases during the oxidation process. It also can be seen that the KubelkaeMunk values of the hydroxyl groups decrease in the initial stage of oxidation. This results from the decomposition of hydroxyl groups, generating carbonyl groups and water and leading to the observed increase in the concentration of carbonyl groups with time. From Fig. 5, it is also evident that the concentration of AreCH gradually decreases with time during the oxidation process. This may result from the rupture of CeC bonds in AreCH groups, generating new alkyl moieties. Considering these in situ FTIR results, it is apparent that the low-temperature oxidation of hydroxyl groups, along with the rupture of CeC and CeO bonds, are the primary causes of the self-heating and subsequent spontaneous combustion of coal. These experimental results are also consistent with the results of theoretical calculations using Gaussian 09W. Based on the results above, an outline reaction steps for the oxidation of the active coal groups is given in Schemes 1 and 2. Among them, the activation energy of the reaction listed in Scheme 1 was calculated by Shi et al. [2]. The activation energy is only 4.75 kJ/mol, which means this group is very active with oxygen.
Fig. 6. Variations in eOH Fourier transform infrared absorbance in a Beizao coal sample over time.
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Fig. 7. Variations in C]O Fourier transform infrared absorbance in a Beizao coal sample over time.
Fig. 8. Variations in CeO Fourier transform infrared absorbance in a Beizao coal sample over time.
4. Conclusion Based on solvent extraction under mild conditions, combined with modern analytical instrumental techniques such as FTIR and liquid state 1H NMR, Chinese lignite BZ coal was studied with regard to its low-temperature reactions. From the data obtained during this study, the following conclusions can be made. (1) A structural model was developed for BZ coal, containing alkyl, methoxy, carbonyl, carboxyl, ester, epoxy and hydroxyl groups. (2) The reactivity of these functional groups was assessed, based on calculated bond lengths, bond energies and the highest occupied molecular orbital electron densities provided by Gaussian 09W. The results indicated that the oxidation of hydroxyl groups and the rupture of CeO and CeC bonds, as well as the dehydrogenation of carboxyl groups, all take place in the initial stage of oxidation, while the decarboxylation and degreasing reactions require more energy and thus only occur at higher temperatures.
Scheme 1. The reaction step for hydroxyl group of coal oxidation.
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X. Zhong et al. / Journal of the Energy Institute xxx (2014) 1e8
Scheme 2. The reaction step for the oxidation of long-chain alkyl group of coal.
(3) The in situ FTIR spectra of BZ coal samples during the oxidation process showed that the oxidation of hydroxyl groups and the rupture of CeO and CeC bonds readily occur, and that the concentrations of carbonyl groups increase with time. These data are consistent with the calculation results, and confirm the information obtained from ultrasonic extraction and Gaussian 09W computations. An outline reaction step of the active groups has been postulated. In summary, ultrasonic extraction combined with modern analytical instrumental techniques represents a suitable means of studying the structure of coals. On the basis of this analytical process and related calculations, we are able to offer a theoretical model for the oxidation-based spontaneous combustion of coal. Acknowledgments The authors wish to express their gratitude for joint funding by the National Natural Science Foundation of China and the Shenhua Group Corporation, Limited (Nos. 51134020 and U1361213), as well as financial support from the National Natural Science Foundation of China Youth Science Foundation (No. 51204171), a General Financial Grant from the China Postdoctoral Science Foundation (No. 2013M531429), the Foundation Research Project of Jiangsu Province (The Natural Science Fund No. BK20141132), the New Century Excellent Researcher Award Program from the Ministry of Education of the People's Republic of China (No. NCET-13-1021) and the Priority Academic Program Development of Jiangsu Higher Education Institutions. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21]
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Please cite this article in press as: X. Zhong, et al., Structural characterization and oxidation study of a Chinese lignite with the aid of ultrasonic extraction, Journal of the Energy Institute (2014), http://dx.doi.org/10.1016/j.joei.2014.11.004