Chemical Properties of Superfine Pulverized Coal Particles. 3. Nuclear


Jul 6, 2016 - 3. Nuclear Magnetic Resonance Analysis of Carbon Structural Features .... proximate analysis (mass %) (ad), ultimate analysis (mass %) (...
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Chemical Properties of Superfine Pulverized Coal Particles. 3. Nuclear Magnetic Resonance Analysis of Carbon Structural Features Jiaxun Liu, Lei Luo, Junfang Ma, Hai Zhang, and Xiumin Jiang* School of Mechanical Engineering, Shanghai Jiao Tong University, Shanghai 200240, China ABSTRACT: The novel superfine pulverized coal combustion technology shows plenty of advantages, and a complementary description of the representative molecular structures plays a paramount role in better understanding its utilization processes. In this work, the carbon skeletal features of superfine pulverized coal were elucidated through 13C NMR analysis. The changes of the coal chemical properties after the demineralization treatment were characterized. Furthermore, the influence of particle size on coal molecular structures was focused on, and the detailed evolution mechanisms were discussed based on the structural and lattice parameters. The final results indicate that decreasing particle size engenders the local coal maturation due to the thermal and mechanical strain effects. The oxygen-substituted aromatic carbon increases in smaller coal fractions at the expense of oxygenated aliphatic carbon. Additionally, the oxidation effect of atmospheric oxygen during the superfine comminution is confirmed. Furthermore, the acid treatment promotes the cleavage of certain chemical bonds, which imposes a significant influence on the attachments and oxygen-containing groups in coal. In all, this research provides some new insights into the role of mechanochemical effect during the coal comminution and is helpful for better understanding the coal chemical structures at a molecular level. The data obtained here will promote the development of the representative molecular models of superfine pulverized coal, and improve the prediction of its behavior during practical application.

1. INTRODUCTION Coal is an important fossil fuel around the world due to its enormous reserves, effective availability, and sophisticated utilization techniques.1,2 Especially in China, coal as the primary energy resource accounts for 67% of the total domestic energy supply. The serious environmental problems caused by coal consumption have become notable restrictions to the development of society and the economy. Therefore, the R&D (research and development) of clean coal technologies have attracted increasing attention on the more efficient and cleaner usage of coal.3 The novel superfine pulverized coal combustion technology shows plenty of advantages for flame stability, combustion efficiency, fusion characteristics, and alleviation of NOx/SO2/CO2 emissions, etc.4−6 Thus, a complementary description of the representative molecular models plays a paramount role in better understanding its utilization processes such as chemical/physical adsorption, swelling, coking, pyrolysis, gasification, liquefaction, and combustion.7 The influence of particle size on coal’s physical and chemical properties should be emphasized. However, due to the extreme complexity, heterogeneity, and diversity of coal properties, a thorough elucidation of coal structures remains challenging, and the validity of various molecular models is still open to debate.8 Consequently, a comprehensive study of the fundamental physiochemical coal nature is required,9 especially for the characterization of superfine pulverized coal with the combination of various advanced analytical techniques. A previous series of our research on the physical properties such as surface morphology, microstructure, and pore networks has been conducted through scanning electronic microscopy (SEM),10 atomic force microscopy (AFM),11 and synchrotron radiation-induced small-angle X-ray scattering (SAXS) beamline,12 etc. The chemical properties of superfine pulverized coal will be the © 2016 American Chemical Society

focus in this series. The free radical features were investigated through electron paramagnetic resonance (EPR), and the detailed occurrence modes were obtained in parts 1 and 2.13,14 The results confirmed that new types of free radicals were formed during the superfine comminution due to the cleavage of covalent bonds in coal macromolecular phases. Therefore, the mechanochemical effect due to the extensive mechanical force exists during the grinding process, which exerts great impacts on the chemical properties of coal particles. Moreover, it has been generally acknowledged the three-dimensional molecular network of coal consists of aromatic moieties with substituted side chains, which are linked by aliphatic bridges and loops. Therefore, it is of great significance to obtain the elaborate information on carbon skeletons in coal organic structures.15 Nuclear magnetic resonance (NMR) spectrometry is a powerful tool for the organic compound structure analysis.16 It can probe the microscopic environment around the nuclei of an atom, and subtle variations in the microstructure can be identified through detecting the electromagnetic state. The resonance happens when the nucleus is subjected to the right combination of magnetic field and electromagnetic radiation. Because of its sensitive and nondestructive nature, NMR has been widely applied in obtaining the structural information on fossil fuels. Ever since VanderHart and Retcofsky estimated coal aromaticity in 1976,17 solid-state 13C NMR spectroscopy has been extensively adopted to obtain the quantitative structural information on coal and coal-derived carbonaceous substances. Solum et al. summarized 12 structural parameters from 13C Received: April 29, 2016 Revised: June 30, 2016 Published: July 6, 2016 6321

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Table 1. Ultimate and Proximate Analysis of Coal Samplesa

NMR analysis, which can realize the quantitative evaluation of carbon skeletons.18 The lattice parameters such as the aromatic cluster size (Xb), the total attachments per cluster (i.e., coordination number, σ + 1), and the fraction of intact bridges and loops, derived from the NMR spectrum, are also widely applied.19,20 Liu et al.21 investigated the difference in carbon structures of lignite through solid 13C NMR spectroscopy and found the aromatic carbon increased with the coalification while the aliphatic chains decreased. Mi et al.22 drew a similar conclusions about coal maturity, combining NMR, vitrinite reflectance, and elemental analyses. A linear relationship between the aromaticity and H/C ratio of bituminous coals was verified through NMR technique by Maroto-Valer et al.23 In addition, there are also numerous studies concerning the carbon skeletal features of coal-derived materials. Li and Zhu characterized the molecular structures of vitrinite using NMR spectroscopy, and discussed their changes during pyrolysis.24 Wang et al.25 employed multiple techniques to investigate the chemical structures of two components associated with coals, i.e., barkinite and vitrinte. In Ashida et al.’s work,26 the thermal and chemical features of six distinct fractions separated from a brown coal using solvent extraction were characterized. The aromaticities were estimated from NMR spectra, and different reactions were identified during the separated fractionation procedure. Furthermore, the NMR technique has also been extensively applied in developing the coal molecular models. The molecular modeling of Upper Freeport coal and its extracts were thoroughly studied by Takanohashi and co-workers27,28 using the molecular dynamics and 13C NMR chemical shift calculations. Mathews et al.29,30 developed the largest molecular model of coals with the aid of various analytical techniques such as NMR, X-ray diffraction (XRD), and transmission electron microscope with high resolution (HRTEM). On the other hand, the quantitative evaluation of coal carbon by NMR technique has its limitations.31,32 The accuracy of the calculated aromaticity has been debated for decades. Especially, the presence of inherent paramagnetic centers in coal significantly impacts the NMR analysis, which broadens the spectra and covers the signals of specific functional groups.7,33 The coal demineralization process can alleviate or eliminate this deficiency effectively.7,19 Above all, among the extensive literature concerning the application of NMR, few reported the influence of particle size on coal chemical structures, not to mention systematic studies of superfine pulverized coal. Due to the emergence of mechanochemical effect, it would be interesting to reveal the changes of coal molecular structures during the superfine comminution and its influences on the subsequent utilization processes. In this work, the carbon skeletal features of superfine pulverized coal were elucidated through 13C NMR analysis. The changes of the coal chemical properties after the demineralization treatment were characterized. Furthermore, the influence of particle size on coal molecular structures was focused on, and the detailed evolution mechanisms were discussed based on the structural and lattice parameters.

proximate analysis (mass %) (ad)

a

SH

moisture volatile ash fixed carbon

11.5 24.22 10.7 53.58

NMG

moisture volatile ash fixed carbon

14.72 35.69 10.64 38.95

TF

moisture volatile ash fixed carbon

5.82 30.30 22.65 41.23

ultimate analysis (mass %) (ad) C H O N S C H O N S C H O N S

63.13 3.62 9.94 0.70 0.41 54.82 4.39 14.58 0.63 0.22 55.69 3.88 10.62 0.75 0.59

ad, on an air-dried basis. O content is calculated by difference.

particles, varying between 10 and 55 μm. The Shenhua coal samples were denoted as SH_14.7, SH_17.4, SH_21.3, and SH_44.2 in the work, indicating the corresponding mean particle sizes were 14.7, 17.4, 21.3, and 44.2 μm, respectively. Similarly, the Nei Mongol coals were labeled as NMG_12.5, NMG_14.9, NMG_25.8, and NMG_52.7, while the Tiefa samples were TF_6.9, TF_11.3, TF_18.9, and TF_33.7. During the whole comminution process, the samples were not sieved to guarantee that no segmentation behavior occurred, and the properties were comparable to the parent lump coal. 2.2. Preparation of Demineralized Coal Samples. Acid washed coal samples were adopted here to alleviate the effect of paramagnetic centers on the NMR spectra evaluation, and the influence of demineralization on the coal chemical properties was addressed. Detailed HCl/HF preparation procedures followed the Chinese standard (GB/T 7560-2001), and most of the predominant coal minerals such as quartz, carbonates, clay, and silicates were removed during this process. For further elimination of inorganic matter such as pyrite and sulfates, HNO3 treatment was employed. Detailed preparation procedures can be found elsewhere.14 2.3. Solid-State 13C CP/MAS/TOSS NMR Technique. Solid-state 13 C NMR spectroscopy has been widely utilized to directly characterize the carbon skeletal structures of fossil fuels. A highresolution Bruker Advance 400 MHz NMR spectrometer (Rheinstetten, Germany) was adopted, and all of the experiments were conducted at a frequency of 100.63 MHz. The solid-state 13C NMR technique can provide better quantitative measurements, combing a line narrowing method of magic angle spinning (MAS) and a signal/ noise enhancing technique of cross-polarization (CP).31 In addition, a TOSS (total sideband suppression) sequence of four-π pulses was employed to avoid the problem of spinning sidebands. The CP contact time was set to 2 ms with a relaxation delay of 4 s, while the spinning frequency was kept at 5 kHz. Due to the complexity, diversity and heterogeneity of coal structures, 13C NMR spectra vary significantly among different types and ranks, or diverse basins. Commonly, the carbon chemical shift can be roughly divided into four categories: aliphatic (10−50 ppm), carbohydrate (50−110 ppm), aromatic (110−160 ppm), and carboxylic or carbonyl carbon (160−220 ppm). Based on an extensive survey of previous studies,1,2,8,34−36 the assignment of chemical shifts detected in our experimental ranges was summarized in Table 2. A deconvolution technique was applied to interpret the envelope NMR spectra, wherein multiple individual peaks were superimposed with one another induced by different carbon functionalities. The curve fitting process was performed automatically through Peakfit software. The multiplicative Gaussian/Lorentzian sum area function was employed to deconvolute the complex NMR spectra, which is defined as

2. MATERIALS AND METHODS 2.1. Preparation of Raw Coal Samples. Three typical Chinese coals with distinct maturity obtained from different geographic seams, Nei Mongol, Tiefa, and Shenhua, were investigated in this work. The ultimate and proximate analyses are displayed in Table 1, which suggest that all samples belong to medium-volatile bituminous coals with different maturation. All the coal samples were ground into fine 6322

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(1)

width, height, and shape index, etc.). The final results in our experiments indicated that the application of ca. 20 Gaussian (or close to Gaussian) peaks normally generated acceptable approximations, which gave the best correlation coefficients (larger than 0.999). Accordingly, the structural and lattice parameters were determined by introducing the resolved peak areas in corresponding chemical shift regions. For example, the carbon aromaticity ( fa) represents the fraction of aromatic carbons (i.e., peaks with chemical shifts larger than 90 ppm), while the sp3-hybridized aliphatic carbons ( fal) are ascribed to ranges smaller than 90 ppm. The aromatic region can be subdivided into the main aromatic (corrected aromaticity f′a, sp2-hybridized carbon in aromatic ring) and carbonyl region ( fCa , >165 ppm). The presence of alkylated aromatic carbon ( fSa ) induces the resonance around 135− 150 ppm. The peaks located in 150−165 ppm are for the oxygensubstituted aromatics ( fPa , e.g., phenolic or phenolic ether). Moreover, the chemical shift regions of 50−90 ppm are assigned to the oxygenated aliphatic carbon ( fOal ), while the protonated aliphatic carbons belong to the ranges of 22−50 ppm ( fHal , e.g., methylene and methine groups). Then, the lattice parameters can be calculated on the basis of this structural information. For instance, similar to carbon aromaticity, the hydrogen aromaticity (Ha = (C/H)atom fHa ) is also employed to evaluate the coal maturity. Another important lattice parameter Xb(fBa /f′a), representing the fraction of aromatic bridgehead carbons, has been widely adopted to estimate the size of aromatic clusters. The detailed descriptions of the structural and lattice parameters deduced from NMR analysis can be found in Solum et al.’s literature.18,37

where a0 represents the area under the absorption curve, a1 is the center of the curve, and a2 represents the full-width at half-maximum. The parameter a3 is the shape index, which varies from 0 to 1, with 1 indicating a pure Gaussian line and 0 being a pure Lorentzian. The NMR spectra were fitted by a superposition of serveral Gaussian− Lorentzian curves according to the assignment of chemical shifts in Table 2, applying different peak parameters (i.e., curve center, line

3. RESULTS AND DISCUSSION 3.1. Influences of Coal Maturity and Particle Size on Carbon Skeletal Features. The CP/MASS/TOSS 13C NMR spectra of NMG, TF, and SH raw coals are shown in Figures 1−3, with multiple superimposed curves being resolved. Two

Table 2. Assignment of Chemical Shift Ranges for 13C NMR Spectra chemical shift range (ppm) 14−16 16−22 22−36 36−50 50−56 56−75 75−90 90−100 100−129 129−137 137−148 148−164 164−185 185−205 205−220

y=

assignment terminal methyl aromatic methyl methylene, methine, naphthenic bridge quaternary, α carbon attached to aromatic ring methoxyl, oxy-methylene α carbon attached to ether, alcohol or some amino acids oxygen attached carbon (oxy-methine or oxy-quaternary), carbohydrate-derived groups anomeric carbon of carbohydrates, aromatics protonated aromatics bridgehead and inner aromatics alkyl-substituted aromatics oxygen-substituted aromatics carbonyl in carboxyl, ester and amide carbonyl in ketone, aldehyde and quinone carbonyl in alkane ketone

a0 2 ⎡ ⎡ x − a1 ⎤2 1⎡x − a ⎤ ⎤ 1 + a3⎣⎢ a ⎦⎥ exp⎢(1 − a3) 2 ⎣⎢ a 1 ⎦⎥ ⎥ 2 2 ⎦ ⎣

Figure 1. Deconvoluted multicomponent structures of 13C NMR spectra for SH raw coals. 6323

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Figure 2. Deconvoluted multicomponent structures of 13C NMR spectra for NMG raw coals.

Figure 3. Deconvoluted multicomponent structures of 13C NMR spectra for TF raw coals.

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Energy & Fuels Table 3. Structural and Lattice Parameters Derived from 13C NMR Spectra of Raw Coal Samples raw coals

fa (%)

fal (%)

fa′ (%)

fCa (%)

fHa (%)

fNa (%)

fPa (%)

fSa (%)

fBa (%)

fal* (%)

fHal (%)

fOal (%)

Xb

Ca

σ+1

NMG_12.5 NMG_14.9 NMG_25.8 NMG_52.7 SH_14.7 SH_17.4 SH_21.3 SH_44.2 TF_6.9 TF_11.3 TF_18.9 TF_33.7

62.3 57.0 55.6 59.7 72.7 77.9 71.4 68.1 54.1 62.1 71.1 69.9

44.7 48.6 49.4 52.4 26.9 25.1 31.3 31.9 45.9 37.9 28.9 40.4

54.1 52.4 52.5 55.9 56.2 55.6 54.0 49.9 43.0 45.7 49.4 53.6

8.28 4.61 3.07 3.78 16.5 22.3 17.5 18.2 11.1 16.4 21.7 16.2

12.6 6.76 17.4 17.7 19.0 20.3 18.3 13.6 17.2 14.6 16.4 19.1

41.5 45.6 35.1 38.2 37.2 35.4 35.6 36.2 25.8 31.1 33.0 34.5

5.36 3.62 7.41 4.15 2.89 1.85 2.15 1.68 2.93 1.51 3.44 3.04

6.74 6.72 6.04 11.7 9.00 5.35 4.79 5.31 4.16 7.68 7.14 7.88

29.4 31.1 21.6 22.3 25.3 28.2 28.7 29.3 18.7 21.9 22.4 23.6

35.5 21.8 27.8 21.9 7.64 11.1 8.10 8.12 34.4 13.6 12.2 10.4

9.16 26.8 21.6 30.5 19.2 14.0 23.2 23.7 11.5 24.3 16.7 29.9

8.58 9.61 14.3 11.4 2.34 2.45 3.04 4.05 10.4 4.61 2.85 3.65

0.54 0.59 0.41 0.39 0.45 0.50 0.53 0.58 0.43 0.47 0.45 0.44

29 36 21 20 22 25 28 35 21 24 22 21

6.48 7.10 5.38 5.67 4.65 3.23 3.59 4.90 3.46 4.82 4.71 4.27

main broad bands can be observed for all of the samples, representing the aliphatic and aromatic functional groups, separately. The peak widths of aromatics are broader, indicating there are more types of aromatic groups compared to aliphatic ones. For TF and SH coals with higher maturation, the fractions of aromatics are larger than the corresponding aliphatic functionalities. In addition, it is worth noticing the presence of a third broad peak over 165 ppm, which is attributed to the carbonyl groups.1,2,8 On the other hand, for NMG coals with lower coalification degrees, the proportion of aliphatic groups is comparable to the aromatic carbon. No apparent carbonyl peaks can be distinguished in the spectra, while a distinctive bump in the chemical shift range of 50−90 ppm emerges, indicating the presence of oxygenated aliphatic carbon.8 In the aliphatic region, all of the samples show an apparent resonance peak around 34 ppm, originating from methylene/ methine groups in aliphatic chains/rings.38 Additionally, a shoulder peak (around 20 ppm) adjacent to the methylene/ methine band is for the α carbons attached to aromatic rings.36 Therefore, the shoulder peaks are more obvious for TF and SH coals that have larger quantities of aromatic moieties. In the aromatic range, the maximum peaks of all of the samples resonate at around 130 ppm, which are assigned to aromatic bridgehead carbon.36 On the left, two shoulder structures show up in the higher chemical shift regions around 140−160 ppm, induced by the alkyl and oxygen-substituted aromatics.8,36,38 The structural and lattice parameters were employed to better interpret the complex carbon skeletal features of superfine pulverized coal. The overall assemblage of NMR spectra were deconvoluted into multiple individual curves with distinctive line widths, shapes, and intensities. These superimposed components were ascribed to different carbon functional groups according to the chemical shifts. The structural and lattice parameters calculated on the basis of these individual peak areas are summarized in Table 3. The structural characteristics of superfine pulverized coal are discussed from three aspects, which comprise the major units of coal macromolecular networks, i.e., the aromatic ( fa, Ha, and Xb, etc.), aliphatic ( fal, fHal , and fal*, etc.), and oxygen-containing functional groups ( fCa , fPa , and fOal , etc.). First, as for the aromatic structures, Figure 4 reveals the influences of coalification and particle size on the aromatics. With the increase of coal maturation, both carbon and hydrogen aromaticity show increasing trends. SH coals have the highest values, while NMG coals are the lowest, indicating

Figure 4. Analysis of aromatic carbon structural parameters of raw coals.

that aromaticity is inversely proportional to the atomic H/C ratio for bituminous coals, which is consistent with the data of Maroto-Valer et al.23 This is because the aromatic sheets become more condensed during the carbonization process, accompanying the cleavage of the covalent bridges and side chains. In addition, the loss of elemental oxygen content with increasing coal maturation can be observed from the ultimate analysis in Table 1. With the proceeding of coalification, the oxygen substituents are gradually removed from the aromatic rings through demethylation and dihydroxylation reactions and replaced by hydrogen or carbon substituents.36 Therefore, more protonated aromatic groups ( fHa ) exist in coals with higher maturation, while the content of alkyl/oxygensubstituted aromatics ( fPa and fSa ) decreases, which can be validated from Table 3. The coordination numbers (i.e., σ + 1, average number of attachments per cluster) of NMG coals are the largest, followed by TF and SH samples. On the other hand, there are only a few variations in the parameter Xb among the distinctive coal species, as depicted in Figure 4. During the early stages of coal maturation (e.g., lignite, subbituminous, and bituminous coals), the ring closure and aromatization of alkyl side chains are identified as the dominant coalification mechanisms, which increases the quantities of aromatic sheets.38,39 Thus, the increase of carbon aromaticity is attributed to the evolution of polycyclic aromatic ring systems, rather than larger average ring sizes.40 With further proceeding of coal carbonization, the condensation and polymerization reactions gradually prevail in the process. The 6325

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Figure 5. Deconvoluted multicomponent structures of 13C NMR spectra for SH demineralized coals.

long-chain aliphatic groups constituted with methylene and methine carbon are susceptible to fracture, and thus separate from the molecular matrix. The replacement by the aryl groups via the cross-linking and condensation of alkyl chains induces the progressive elimination of the relative abundance of protonated aliphatic carbon. Third, it has been well acknowledged that the oxygencontaining functionalities reduce progressively during coal maturation, whereas the carbon content increases.42,43 Table 3 presents a similar trend that NMG coals have the highest content of oxygenated aromatic/aliphatic carbon (fPa and fOal ), followed by TF and SH coals. The deoxygenating reactions such as the β-O-4 ether cleavage and sequential removal of methoxyl and side-chain hydroxyl groups (via demethylation and dehydroxylation) are responsible for the loss of oxygen content in coal during the coalification.36,39 For NMG and SH coals, there is a mild drop on the proportion of oxygenated aliphatic carbon with the decrease of particle size, while the oxygen-substituted aromatic carbon increases. The β-O-4 bonds are susceptible to rupture under intensive mechanical-energy input, which results in the formation of phenolic−OH and catechol-like structures. The demethylation of methoxyl groups also produces phenolic functionalities. This is consistent with our previous research based on FTIR analysis that the hydroxyl groups increase significantly with the decrease of particle size.44 Therefore, both reactions increase the oxygenated aromatic carbon at the expense of oxygenated aliphatic ones. In addition, the simple reduction of the hydroxyls in side chains to alkyl groups might be another reason that induces the decline of oxygenated aliphatic structures. Furthermore, TF samples show the scattered distribution of oxygenated components due to the

increased condensation and cross-linking of small molecules result in the enlargement of aromatic stacks and thus a significant increase of carbon aromaticity. Therefore, more homogeneously oriented structures and larger condensed aromatic nuclei can be typically observed in higher rank coals. When decreasing the particle size, an increase of carbon and hydrogen aromaticity, i.e., local coal maturation, can be observed. Thermal and mechanical strain effects during the deformation metamorphism process play a key role here.41 The mechanochemical effect induced by intense shear stress during superfine comminution influences the coal physical and chemical structures at a molecular level. The stress-induced bond cleavage and frictional heating lead to the reorientation of aromatic lamellae and local graphitization. The elongation in the diameter of the aromatic sheet (La) and shortening of interlayer spacing (d002) were observed during the deformation process.42 However, a discrepancy is noticed for TF coals that have lower carbon aromaticity for the smaller particles. The presence of numerous paramagnetic structures in TF coal ash (22.65%) leads to the line broadening of 13C NMR spectra especially in aromatic regions, which significantly influences the accuracy of the calculated carbon aromaticity. Second, the aliphatic fraction is compatible with the aromatic moiety. The increase of aromatic carbon results in the gradual decrease of the aliphatic carbon proportion. Coals with lower maturation contain more aliphatic moieties, while the higher rank coals preserve larger fractions of aromatic components,16 as revealed in Table 3. The influence of particle size on the aliphatic carbon is predictable. When decreasing the particle size, the proportion of aliphatic carbon declines, especially for NMG and SH coals. A dramatic drop of the protonated aliphatic carbon (fHal ) is observed in smaller coal fractions. The 6326

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Figure 6. Deconvoluted multicomponent structures of 13C NMR spectra for NMG demineralized coals.

Table 4. Structural and Lattice Parameters Derived from 13C NMR Spectra of Demineralized Coal Samples demineralized coals

fa (%)

fal (%)

f′a (%)

fCa (%)

fHa (%)

fNa (%)

fPa (%)

fSa (%)

fBa (%)

f*al (%)

fHal (%)

fOal (%)

Xb

Ca

σ+1

NMG_12.5 NMG_14.9 NMG_25.8 NMG_52.7 SH_14.7 SH_17.4 SH_21.3 SH_44.2

49.1 42.1 45.6 41.0 75.6 78.3 60.4 63.5

48.5 57.9 54.4 57.3 24.4 21.7 37.1 38.7

45.6 38.9 39.3 33.7 60.5 59.0 50.5 55.2

3.42 3.18 6.28 7.30 15.0 19.3 9.95 8.29

8.05 6.51 12.9 7.10 12.7 9.54 16.6 12.6

37.6 32.4 26.4 26.7 47.8 49.5 33.8 42.6

4.29 1.19 2.85 3.22 6.07 5.10 3.75 5.33

4.29 4.60 4.82 5.72 9.68 7.97 5.49 3.85

30.6 26.6 18.7 23.4 32.8 30.9 30.1 33.4

30.5 29.8 19.7 21.3 14.1 8.96 26.0 16.5

17.9 28.1 34.7 36.1 10.3 12.7 11.1 22.2

19.0 5.05 4.63 4.72 3.24 1.95 5.13 3.85

0.67 0.68 0.48 0.69 0.54 0.52 0.59 0.61

56 60 23 64 29 27 37 39

10.54 8.93 4.49 16.99 7.55 5.98 6.77 6.49

coal chemical properties, which are discussed in this work based on 13C NMR analysis. The CP/MASS/TOSS 13C NMR spectra of SH and NMG demineralized coals are depicted in Figure 5 and Figure 6. All the data processing methods are the same as those for raw samples. About 20 superimposed peaks are resolved according to the assignment of distinctive carbon structures. Two major peaks attributed to aromatic and aliphatic moieties are observed in all of the spectra, which are similar to the parent coals. However, the fractions of each specific group vary significantly compared to raw samples, as is shown in Table 4 which summarizes the structural and lattice parameters calculated according to NMR spectra. The parameters of fa, Ha, and Xb are adopted to evaluate the aromatic structures in demineralized coals, as displayed in Figure 7. Similar to the parent coals, the aromatic and hydrogen aromaticities of SH coals with higher maturation are all larger than those of NMG specimens. Further inspection of Table 4 indicates that all the aromatic aromaticities of demineralized coals are higher than the raw samples. Oxidation reactions

high ash content in the parent coal. On the other hand, a dramatic drop of carboxyl and carbonyl groups has been commonly detected during the coalification process.32,36 However, Table 3 presents an opposite trend that SH coals show the highest fCa , while NMG coals are the lowest. This is attributed to the oxidation effect of atmospheric oxygen during the superfine comminution. The O−alkyl−C structures are susceptible to oxidation even under mild oxidative weathering, leading to the formation of carbonyl groups.45 Further oxidation causes the transformations to carboxyl−C.46 3.2. Influence of Demineralization on Carbon Skeletal Features. The influence of the demineralization procedure on the coal macromolecular structure and reactivity has been a matter of debate. Larsen et al.47 found that HCl/HF demineralization had little effect on the organic structure of coal. However, Ö nal and Ceylan48 believed that HCl/EtONa (sodium ethanolate) promoted the selective cleavage of specific chemical bonds and induced the increase of acidic groups such as carboxylic functionalities. There are few studies about the effect of the HCl/HF/HNO3 demineralization treatment on 6327

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maturation (i.e., the increase of fa) induced by thermal and mechanical strain effects can also be observed in the acid washed coals. Several chemical reactions occur during the acid treatment due to the presence of a strong oxidizing agent (HNO3), including aromatic ring carboxylation (eq 2), aromatic ring nitration (eq 3), and side-chain oxidation (as shown in eq 449).

Generally, the HCl/HF/HNO3 demineralization process has a significant impact on the oxygen-containing groups in coal. The carboxylation reactions increase the carboxyl and carbonyl groups, while the side-chain oxidation leads to the enrichment of ester, aldehyde, and alcohol. However, only SH coals show a noticeable increase of oxygenated aromatic and aliphatic carbon, while the data of oxygen-containing groups in NMG are scattered. Coals with higher coalification extent have more inherent free radicals13 and are more susceptible to the oxidation effect of the acid treatment.

Figure 7. Analysis of aromatic carbon structural parameters of demineralized coals.

induced by the acid washing process promote the cleavage of certain chemical bonds, which may result in the opening of the aromatic rings, and thus the decrease of aromaticity (e.g., carboxylation reaction of aromatic rings in eq 2). In addition, the removal of the paramagnetic structures in coal after the acid treatment narrows the aromatic peak widths of 13C NMR spectra evidently, which also causes the decrease of carbon aromaticity.

4. CONCLUSIONS The 13C NMR technique was successfully utilized to elucidate the chemical properties of superfine pulverized coal in this work. The detailed evolution mechanisms of carbon skeletal features were elucidated at a molecular level, which shed new light on the further development of representative coal models. In addition, the influence of particle size on the coal molecular structures was novelly discussed from the mechanochemical point of view. (1) The overall assemblage of NMR spectra can be deconvoluted into ca. 20 individual curves with distinctive line parameters. When decreasing the particle size, the local coal maturation can be observed, which is attributed to the mechanochemical effects during the deformation metamorphism process. (2) With the decrease of particle size, there is a mild increase in the proportion of oxygen-substituted aromatics at the expense of oxygenated aliphatic carbon. The demineralization process has a significant impact on the oxygen-containing groups where coals with higher coalification extent are more susceptible to the oxidation. Moreover, the oxidation effect of atmospheric oxygen during the superfine comminution is confirmed. (3) The acid washing process results in the opening of aromatic rings, which decreases the aromaticity. The condensed aromatic nuclei after the acid treatment become larger compared to the parent coals. Additionally, the electrophilic substitution reactions are susceptible to happening, which induces a significant increase of the attachments per cluster in the acid washed coals.

However, it is interesting to note from Table 4 that, after the acid treatment, the condensed aromatic nuclei become larger (Xb and Ca) compared to the parent coals. The ionic cross-links may exist with the presence of ions (e.g., Ca2+ and COO−) that connect different macromolecular fragments.47 Upon demineralization, the linkages are destroyed, which promotes the formation of highly reactive aromatic pieces with inherent active sites. The condensation polymerization presumably occurs, leading to the enlargement of aromatic nuclei. For the same reason, the electrophilic substitution reactions are susceptible to occur on these active sites, which results in the significant increase of the attachments per cluster (i.e., coordination number σ + 1) in the demineralized coals. Moreover, an obvious drop of the protonated aromatic structures ( fHa ) in acid washed coals is observed, due to the deprotonation effect during substitution reactions (e.g., the nitration reaction in eq 349).



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Corresponding Author

*Tel.: +86 21 3420 5681. E-mail: [email protected] Notes

On the other hand, the aliphatic fractions of SH coals with higher maturation are all smaller than NMG specimens, which is consistent with the trend of parent coals. Additionally, the decrease of carbon aromaticity inevitably induces the increase of the aliphatic structures ( fal) in demineralized coals. Furthermore, with the decline of particle size, the local coal

The authors declare no competing financial interest.



ACKNOWLEDGMENTS The support of the National Natural Science Foundation of China (Grant Nos. 51306116 and 51376131) is acknowledged. 6328

DOI: 10.1021/acs.energyfuels.6b01029 Energy Fuels 2016, 30, 6321−6329

Article

Energy & Fuels



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DOI: 10.1021/acs.energyfuels.6b01029 Energy Fuels 2016, 30, 6321−6329