The coal sample was taken from Shanxi. The ultimate and sulfur composition analysis are shown in Table 1. Organic sulfur is the most important form of sulfur in coal, and it accounts 68.75% of the total sulfur. In order to reduce the interference of inorganic sulfur in spectroscopy characterization, coking coal was selected as the research object. The 6~13 mm particle size coals were subjected to floatation test and density classification.

Table 1

Table 1

Table 1 Ultimate, Sulfur composition of Shanxi coking coal Ultimate analysis/(Wt%) Sulfur composition analysis/(Wt%) C H N S O Ss Sp So 78.23 4.87 1.42 3.03 12.45 9.17 22.08 68.75

Table 1 Ultimate, Sulfur composition of Shanxi coking coal

1.2.1 XPS, XANES and RAMAN analysis

To ensure homogeneity, the coal powder was ball milled before being pressed and sieved into particles with diameters between 75 and 100 μ m.

Coal samples were subjected to XPS detection. The XPS experiments were carried out at room temperature in an ultra-high vacuum (UHV) system with the surface analysis system (EscaLab250Xi). The data processing was fittedwith XPS Peak fit v4.1 software.

The XANES of samples was performed for studying sulfur forms, at 4W1B endstation, Beijing synchrotron Irradiation Facility. The XANES data of the standard samples and model samples were processed by interpolating and normalizing, fitted by the Athena software.

This study was conducted using a LabRAM HR Evolution Raman spectrometer, equipped with 532 nm laser and 600 grooves· mm^-1 grating. The configuration provided a spectral resolution of 2.0 cm^-1, repeat rate better than 0.2 cm^-1.

1.2.2 Microwave irradiation experiment

The model compounds were subjected to microwave irradiation experiments at 2 450 MHz(MCR-3 Microwave generator)and 750~950 MHz. The microwave irradiation experimental platform of 750~950 MHz consists of vector network analyzer(Agilent E5071C), microwave reactor (BDS7595-200) and frequency-reconfigurable reaction chamber.

1.2.3 Density functional theory (DFT) calculation

The structural models of the thiophene sulfur model compounds were constructed and optimized by the Visualizer and Doml3 module respectively in the Materials Studio. The molecular structure and properties of the compounds were obtained by density functional quantum chemical calculation.

Selected 162167 eV in the XPS scan spectrums of the coal samples to fit the organic sulfur fitting. The results are shown in Fig.1.

Fig.1

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	Fig.1 XPS fitting spectration of organic sulfur in different densities coking coals

In Fig.1, the coal samples with density of +1.8 cannot be baselined and have no fitted curve of organic sulfur. High mineral contentsin high density coals have large interference to the organic sulfur spectrum. It is the main reason for the spectral fitting failure of density +1.8. Pusz believes that pyrite is mainly distributed in high density coal^[18]. According to this, the morphological sulfur analysis is carried out on coal samples with density of +1.8. The results are shown in Table 2. The content of iron and sulfur exceeds 50% and the inorganic sulfur contents account for 82.01% of total sulfur, which can explain the failure of coal sample organic sulfur XPS with density of +1.8 reasonably.

Table 2

Table 2

Table 2 The speciation analysis data of sulfur in density of +1.8 Form sulfur/(Wt%), daf Form sulfur accounts for the total sulfur ratio/% St Ss Sp So Ss Sp So 9.06 2.38 5.05 1.63 26.27 55.74 17.99

Table 2 The speciation analysis data of sulfur in density of +1.8

The forms of sulfur in other densities coal samples include thiol (ether), (sub) sulfone and thiophene. Among them, thiophene sulphur is the most important endowment form. The relative contents of three organic sulfur forms in high densities coals tend to be average. The quantitative analysis results are shown in Fig.2.

Fig.2

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	Fig.2 XPS analysis of sulfur occurrence of different densities coking coals

XANES tests were carried out with samples of 1.4~1.5, which are more samples in coking coal. Seventeen substances such as DMSO were selected as the sulfur standard to analyze the occurrence characteristics of sulfur in coking coal by XANES. The absorption edge position XANES spectrum of standard sulfur compounds are shown in Fig.3. The K-side XANES measured spectrum and fitting spectrum of sulfur in coal is shown in Fig.4.

Fig.3

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	Fig.3 Standard sulfur compounds XANES spectral absorption edge dosition 1: ZnSO4; 2: Na2S2O3; 3: Sublimated Sulphur; 4: C12H8S; 5: FeS2; 6: C3H7NO2S; 7: C12H19N3O7S; 8: K2S2O8; 9: Tetrathionate; 10: Na2SO3; 11: NH4Fe3(SO4)2(OH)6; 12: KFe3(SO4)2(OH)6; 13: DMSO; 14: (C6H5)2SO2; 15: CH3CH2SH; 16: C2H6S; 17: S

Fig.4

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	Fig.4 The XANES spectrogram of K edge in the coking coal

The XANES measured spectrum has four absorption peaks with electron binding energy of 2.474, 2.476, 2.482 and 2.485 keV, respectively. Compared with the standard library, Na₂S₂O₃, KFe₃(SO₄)₂(OH)₆, C₁₂H₈S and C₃H₇NO₂S were selected for the sulfur K-side XANES spectrum fitting. The results are shown in Table 3. Thiophene is still the main form of organic sulfur in coal and accounts for more than 50%, followed by thiol (ether), again inorganic sulfur.

Table 3

Table 3

Table 3 The analysis of sulfur forms in the the coking coal of XANES Sulfur groups Sulfate Thiophene thiol (ether) Relative contents 19.20 53.58 27.22

Table 3 The analysis of sulfur forms in the the coking coal of XANES

The 915 and 2 450 MHz are the most commonly used civilian microwave frequencies. The coal samples with density of 1.4~1.5 were also selected for radiating by microwave. The XPS analytical spectrum of the sample after irradiation are shown in Fig.5, and the types and contents of organic sulfur are shown in Fig.6. There is no change in the types of organic sulfur in coal after microwave irradiation. However, the content of thiophene sulphide and (sub) sulfone increases, and the thiol (ether) content decreases. After 915 and 2 450 MHz microwave radiation, the relative content of thiol (ether) in coal decreased from 31% to 15%, 22%. It can be seen that microwave radiation has an effect on the structure of organic sulfur in coal, which changes the relative content of organic sulfur components. Thiophene is the most stable structure of organic sulfur in coal. The Sulfur-containing bonds energy in thiol (ether) is lower than that of sulfone and thiophene. Therefore, microwave energy is relatively easy to change the chemical structure of thiol (ether).

Fig.5

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	Fig.5 XPS fitting spectration of organic sulfur in coal after microwave irradiation (a): 915 MHz microwave frequency radiation; (b): 2 450 MHz microwave frequency radiation

Fig.6

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	Fig.6 The Change of organic sulfur components in coal by microwave irradiation (a): Raw coal; (b): 915 MHz irradiation; (c): 2 450 MHz irradiation

According to the results of coal structure analysis, thio-phene is the most important organic sulfur component in raw coal and coal after microwave treatment. Benzothiophene and 3-thiophenecarboxylic acid were selected for microwave irradiation experiments at 750~950 and 2 450 MHz to study the mechanism of action of microwave on the structure of thiophene. The test conditions are shown in Table 4. Rapid heating and selective heating are special effects of microwave fields. Since the power and frequency of the two microwave reactors are different, the same temperature difference was chosen. The microwave irradiation time was controlledin order to avoid sample evaporation by excessive temperature.

Table 4

Table 4

Table 4 The testconditions of model compounds in microwave irradiation Thiophene model compound Power/W Microwave absorption frequency/MHz Radiation time/s Elevated temperature/℃ Benzothiophene 208 800 910 2 450 600 77 36 36 3-Thiophenecarboxylic acid 206 800 853 2 450 478 93 32 32

Table 4 The testconditions of model compounds in microwave irradiation

The Raman spectrum of the two model compounds before and after microwave irradiation are shown in Figures 7 and 8. Raman spectrum information is contained in the peaks and Raman absorption of common sulfur structure has three main regions. The wave numbers are 430~550, 630~780 and 1 050~1 100 cm^-1. The functional groups are S— S, aliphatic carbon-sulfur bonds and aromatic carbon-sulfur bonds respectively^[19].

Fig.7

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	Fig.7 The Raman spectrum of Benzothiophene microwave activity

Fig.8

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	Fig.8 The Raman spectrum of beta-thiophenic acid microwave activity

The vibration peaks of sulfur-containing bonds of benzothiophene and 3-thiophenecarboxylic acid are red-shifted by microwave action. The vibrational peaks of carbon-sulfur bond in 3-thiophenecarboxylic acid and benzothiophene are red-shifted 26, 20 cm^-1 by 853 MHz and 21, 26 cm^-1 by 2 150 MHz respectively. The movement of the Raman spectral vibration peaks indicate that the energy bands and structure have changed. In the thiophene model compounds, the sulfur-containing vibration peaks shift toward the long wavelength direction. It indicates that the wave number is reduced, the energy of the photons lower, the band gap is smaller and the groups are more unstable. Therefore, the energy of microwave changes the microscopic environmental structure of the model compounds molecules. As a result, the lattice vibration frequency of the model compound and the relative motion of the atom increases, and the vibration recovery force and the molecular stability decreases.

The essence of microwave action on the medium is that the medium responds to microwave energy and changes the chemical structure. The above experiments have shown that sulfurcontaining structure in coal and thiophene model compounds respond to microwaves. In order to study the influence of response on structural parameters, the simulated three-dimensional microwave energy field was added using the Materials Studio simulation platform^[20]. The change in structural property parameters of benzothiophene and 3-thiophenecarboxylic acid was calculated.

According to the directivity of microwave transmission and the conversion relationship between microwave energy and electric field strength, setted electricfield 0.000 27, 0.000 20, 0.000 15 a.u. and electric field 0.000 20, 0.000 15, 0.000 27 a.u. as the energy and direction parameters of the two simulated energy fields. The configuration of the model compounds after the action of the simulated energy fields are shown in Fig.9, and the structural parameters are shown in Table 5.

Fig.9

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	Fig.9 The model compound configuration after simulated energy fields

Table 5

Table 5

Table 5 Changes of structural parameters of model compounds after simulated energy fields Model compounds Sulfur containing bonds Original parameters electric_field 0.000 20, 0.000 15, 0.000 27 a.u. electric_field 0.000 27, 0.000 20, 0.000 15 a.u Bond length /Å Key level Molecular dipole moment/D Bond length /Å Key level Molecular dipole moment/D Bond length /Å Key level Molecular dipole moment/D Benzothio phene C3-S9 C8-S9 1.751 1.744 0.805 0.824 0.445 1 1.751 1.745 0.805 0.824 0.536 6 1.751 1.744 0.805 0.824 0.539 6 3-thiophen- ecarboxylic acid C1-S5 C4-S5 1.736 1.714 0.846 0.901 2.028 8 1.736 1.714 0.846 0.901 2.020 4 1.736 1.714 0.846 0.901 2.107 6

Table 5 Changes of structural parameters of model compounds after simulated energy fields

The bond length and bond level of sulfur containing chemical bonds in the benzothiophene and 3-thiophenecarboxylic acid are not change substantially after the action of the simulated energy field. Therefore, the energy field has a very limited effect on the stretching and torsion of chemical bonds. It does not have the ability to break the sulfur containing bonds in the thiophene structure. This is consistent with the stability of the thiophene. However, the bond angle and dipole moment in the molecular structureare changed in different directions energy field. The key angle always has a tendency to shift to the most stable form. The dipole moment is an important parameter for characterizing the spatial structure of a molecule and it is used to determine the polarity of a molecule. It can be seen that the simulated microwave energy field can change the molecular configuration and molecular polarity of the thiophene sulfur model compounds.