13 C-NMR Study on Structure Evolution Characteristics of High-Organic-Sulfur Coals from Typical Chinese Areas

The structure evolution characteristics of high-organic-sulfur (HOS) coals with a wide range of ranks from typical Chinese areas were investigated using 13C-CP/MAS NMR. The results indicate that the structure parameters that are relevant to coal rank include CH3 carbon (f al*), quaternary carbon, CH/CH2 carbon + quaternary carbon (f al), aliphatic carbon (f al), protonated aromatic carbon (f a), protonated aromatic carbon + aromatic bridgehead carbon (f a), aromaticity (f a), and aromatic carbon (f ar). The coal structure changed dramatically in the first two coalification jumps, especially the first one. A large number of aromatic structures condensed, and aliphatic structures rapidly developed at the initial stage of bituminous coal accompanied by remarkable decarboxylation. Compared to ordinary coals, the structure evolution characteristics of HOS coals manifest in three ways: First, the aromatic CH3 carbon, alkylated aromatic carbon (f a), aromatic bridgehead carbon (f a), and phenolic ether (f a) are barely relevant to rank, and abundant organic sulfur has an impact on the normal evolution process of coal. Second, the average aromatic cluster sizes of some super-high-organic-sulfur (SHOS) coals are not large, and the extensive development of cross bonds and/or bridged bonds form closer connections among the aromatic fringes. Moreover, sulfur-containing functional groups are probably significant components in these linkages. Third, a considerable portion of “oxygen-containing functional groups” in SHOS coals determined by 13C-NMR are actually sulfur-containing groups, which results in the anomaly that the oxygen-containing structures increase with coal rank.


Introduction
Sulfur is one of the most hazardous elements in coal, and organic sulfur is more difficult to remove than other forms of sulfur in the conventional coal washing process.Thus, organic sulfur released during coal utilization could have severe adverse effect on the environment and human health.Generally, the coal with >1% organic sulfur is called high-organic-sulfur coal, and the organic sulfur content in super-high-organic-sulfur (SHOS) coal exceeds 4% [1,2].The distribution of SHOS coals in the world is fairly limited.Croatia Raša coal is the representative with an incredible organic sulfur content of 11.4 wt % [3].In addition, Spain Mequinenza lignite [4], New Zealand Charming Creek coal [5], India Tipong coal [6], and some coals from the Australia Gippsland Basin [7] are all SHOS coals.The SHOS coals in China are mainly part of the Late Permian coals in South China and primarily distributed in Guiding in Guizhou Province [8,9], Heshan in Guangxi Province [10,11], Chenxi in Hunan Province [12], Yanshan in Yunnan Province [13], Anxian in Sichuan Province [14], and Yishan in Guangxi Province [15].Currently, it is controversial that the abundant "organic sulfur" in the SHOS coals is fully organically bound [16].The existence of fine-grained pyrites would be expected to increase the organic sulfur content determined by subtraction method [12].Nevertheless, in the present study, these difference values are tentatively regarded as the contents of organic sulfur in coal.With regard to the formation of high organic sulfur and/or pyrite in coal, although seawater has been considered one of the major sources [2,[17][18][19], hydrothermal fluid and special evolution mechanism could play critical roles.The multi-stage hydrothermal activity is one of the reasons for enrichment not only of pyrite [20,21] but also of organic sulfur [22], and the repeating reduction-reoxidation-disproportionation enrichment model proposed by Li and Tang [23] is another explanation for the abnormal enrichment of organic sulfur in coal.In addition, most sulfur in some SHOS coals (e.g., the Yanshan coal [13]) was likely influenced by submarine exhalation, which was carried into the peat swamp and then evenly distributed in the organic matter [13,16].The hydrothermal fluids that have caused highly elevated S in coal are generally of epithermal origin [20] or are closely related to volcanic activities both in the marine (e.g., submarine exhalation) and in the terrestrial environments [24].On the other hand, the SHOS coals in some cases contain highly-elevated critical elements that have a great potential for industrial extraction and utilization (e.g., rare earth elements, Y, V, Se, Mo, U) [3,25,26].
There have been extensive studies on coal structure, organic sulfur structure in coal, and the structure evolution characteristics of ordinary coals.However, systematic studies on the structure of HOS coals are still limited, and the structure evolution characteristics of HOS coals are rarely reported.Besides, several S-containing coal structure models (Figure 1; [61][62][63][64]) have been proposed previously for a better understanding of coal structure, but the impact of abundant organic sulfur on coal structure remains unclear.The present study aims to investigate the compositional variations of aromatic carbons, aliphatic carbons, and oxygen-containing functional groups versus coal rank of HOS coals from typical Chinese areas; in addition, we explored the influence of abundant organic sulfur on the coal structure evolution.It could provide us new perspectives on the relationship between high organic sulfur and coal structure, and be potentially helpful for removing organic sulfur in the HOS coals.
Coal structure is one of the major characteristics of the coal rank that is formed in different conditions including geological setting, type of peat-forming plants, paleoclimate, and paleo-hydrological regime [48].Besides, pressure and temperature play significant roles in modifying the deposition environment and the chemical structure of deposited biopolymers, which leads to the formation of coals with various ranks [65][66][67][68]; whereas the length of the formation time is an additional factor determining coal quality [69].Therefore, it is not surprising that structure differences exist between coals sharing the same rank, and the structure characteristics of certain coals do not correspond to the overall evolution trend.

Samples and Analytical Methods
Nine HOS bench samples across a wide range of coal ranks were collected from different locations in China (Figure 2 and Table 1).The span of their maximum vitrinite/huminite reflectances (from 0.293% to 3.934%; Table 1) covers four coalification jumps (i.e., Rmax around 0.6%, 1.3%, 2.5%, and 3.7%, respectively).The samples from southern China (JJP, CM, 6K, and GH) are all of a Late Permian age; while most of the samples from northern China (WHS, XY, SSP, and WTP) are Late Carboniferous coals, and the only lignite sample YX was collected from a Middle Jurassic coal seam.Each sample was stored in plastic bags to avoid from contamination and oxidation, and all samples were air dried before subsequent analyses.
The maximum reflectance determination was conducted using ASTM Standard D2798-05 [70], and the maceral classification applied in the present study was based on ICCP System 1994 [71,72].Proximate and ultimate analyses were performed using ASTM Standards D3173-03, D3175-02, D3174-04, and D3176-15 [73][74][75][76].The total sulfur and forms of sulfur were determined following ASTM Standards D3177-02 and D2492-02 [77,78], respectively. 13C-NMR analyses were performed using a Bruker AVANCE III-400 MHz spectrometer with the CP/TOSS (cross polarization/ total suppression of spinning sidebands) pulse program (5.8 μs) at the Peking University Analytical Instrumentation Center, Beijing, China.A 13 C resonance frequency of 100 MHz and a static magnetic field of 9.37 T were used for the experiments.The samples were ground to pass through a 200-mesh sieve and packed into a cylindrical zirconia rotor that was 4 mm in diameter.Cross polarization (contact time 3000 μs) with magic angle spinning (MAS) was applied at 5 KHz.5200 scans were required to obtain the 13 C spectra using a double-resonance probe head, and the acquisition time and sweep width were 0.05 s and 300 ppm, respectively.Topspin 3.0 was used as the testing software.

Samples and Analytical Methods
Nine HOS bench samples across a wide range of coal ranks were collected from different locations in China (Figure 2 and Table 1).The span of their maximum vitrinite/huminite reflectances (from 0.293% to 3.934%; Table 1) covers four coalification jumps (i.e., R max around 0.6%, 1.3%, 2.5%, and 3.7%, respectively).The samples from southern China (JJP, CM, 6K, and GH) are all of a Late Permian age; while most of the samples from northern China (WHS, XY, SSP, and WTP) are Late Carboniferous coals, and the only lignite sample YX was collected from a Middle Jurassic coal seam.Each sample was stored in plastic bags to avoid from contamination and oxidation, and all samples were air dried before subsequent analyses.
The maximum reflectance determination was conducted using ASTM Standard D2798-05 [70], and the maceral classification applied in the present study was based on ICCP System 1994 [71,72].Proximate and ultimate analyses were performed using ASTM Standards D3173-03, D3175-02, D3174-04, and D3176-15 [73][74][75][76].The total sulfur and forms of sulfur were determined following ASTM Standards D3177-02 and D2492-02 [77,78], respectively. 13C-NMR analyses were performed using a Bruker AVANCE III-400 MHz spectrometer with the CP/TOSS (cross polarization/ total suppression of spinning sidebands) pulse program (5.8 µs) at the Peking University Analytical Instrumentation Center, Beijing, China.A 13 C resonance frequency of 100 MHz and a static magnetic field of 9.37 T were used for the experiments.The samples were ground to pass through a 200-mesh sieve and packed into a cylindrical zirconia rotor that was 4 mm in diameter.Cross polarization (contact time 3000 µs) with magic angle spinning (MAS) was applied at 5 KHz.5200 scans were required to obtain the 13 C spectra using a double-resonance probe head, and the acquisition time and sweep width were 0.05 s and 300 ppm, respectively.Topspin 3.0 was used as the testing software.Rmax, maximum vitrinite/huminite reflectance, oil immersion. 1Two decimals are usually required for vitrinite/huminite reflectance values, but three decimals are used in the present study to distinguish close values for SHOS coal samples 6K and GH (1.801% and 1.804%, respectively). 2 The Yongxing Mine has only one coal seam.
The chemical shifts and assignment for the 13 C-NMR spectra of coal and its precursor have been summarized in previous studies [57][58][59][60][79][80][81][82][83][84][85], and the reported chemical shifts are mostly identical with only minor differences.The 13 C-NMR structure parameters and assignment used in the present study are given in Table 2 [84, [86][87][88].Prior to deconvolution, the 13 C-NMR spectrum was adjusted using the Nuts Software.Then, the PFM module of the Origin Software was used to subtract the baseline, add peaks manually according to the spectrum shape and assignment of NMR structure  Northern China R max , maximum vitrinite/huminite reflectance, oil immersion.1 Two decimals are usually required for vitrinite/huminite reflectance values, but three decimals are used in the present study to distinguish close values for SHOS coal samples 6K and GH (1.801% and 1.804%, respectively). 2 The Yongxing Mine has only one coal seam.

Basic Petrographic and Chemical Data
Basic data, including maceral composition, proximate analysis, ultimate analysis, and total sulfur and forms of sulfur of the HOS coals are shown in Table 3.The organic sulfur content of Sample WTP (0.95%, Table 3) is a bit below 1%, which is, as mentioned above, taken as the criterion of least organic sulfur content for HOS coals, but this sample is also included in the present study due to its high rank.All samples are dominated by vitrinite/huminite macerals (65.0-96.6%),and inertinite components (0.5-32.7%) come next.The liptinite macerals cannot be observed in most samples, except for Samples YX, WHS, and GH that contain 2.0-2.9%liptinites.The volatile-matter yield, carbon content, hydrogen content, and H/C ratio display clear correlations with maximum reflectance of HOS coals (Figure 3A-D), suggesting that they could be regarded as indicating parameters for coal rank.As expected for lignite, Sample YX has much higher oxygen content than other samples, and the oxygen content generally shows a decreasing trend versus coal rank (Figure 3E).It seems that the nitrogen content has nothing to do with the coal rank due to its variable trend (Figure 3F).
oxygen content than other samples, and the oxygen content generally shows a decreasing trend versus coal rank (Figure 3E).It seems that the nitrogen content has nothing to do with the coal rank due to its variable trend (Figure 3F).Generally, sulfide mineral (e.g., pyrite that contains pyritic sulfur) is one of the causes for coal combustion residual; thus, there should be a positive correlation between pyritic sulfur and ash yield.Meanwhile, pyritic sulfur and organic sulfur are usually negatively correlated to each other, especially when sulfate sulfur is negligible.Therefore, a negative correlation should exist between ash yield and organic sulfur.However, in the present study, the variation of ash yield across HOS coals (except for Samples YX and SSP) is generally compatible to that of organic sulfur content (Figure 4A).A possible explanation to this observation is that the organic sulfur contents derived from subtraction method are not definitively accurate, because a portion of sulfur occurring in fine-grained S-containing minerals that are dispersed in organic matter as inclusions were mistaken as organically associated, especially for the SHOS coals, as suggested by Li et al. [12].The negative correlations between oxygen/nitrogen and organic sulfur (Figure 4B,C) indicate a limited capacity of coal macromolecular structure for heteroatoms.Part of bonding sites that can be assigned to oxygen and nitrogen are taken up by organic sulfur in the HOS coals.

13 C-NMR Spectra and Peak-Fitting Results of HOS Coals
The solid-state 13 C-CP/MS NMR spectra of the HOS coals are shown in Figure 5. From the qualitative perspective, each spectrum exhibits two broad bands that represent aliphatic carbons (<90 ppm) and aromatic carbons (>90 ppm), respectively.With increasing coal rank, aliphatic carbons decrease gradually, and aromatic carbons increase along with the aromatic bands becoming more thin and sharp, indicating an increasing maturity of the coal.Besides, the spectrum of aromatic carbons becomes smoother as coal rank grows, suggesting an increasing homogeneity of chemical structure.The satisfactory peak-fitting effect (Figure 6) is the prerequisite for reliable deconvolution results.Based on the 13 C-NMR structure assignment, each spectrum can be resolved into as many as twelve bands (Table 4).Generally, sulfide mineral (e.g., pyrite that contains pyritic sulfur) is one of the causes for coal combustion residual; thus, there should be a positive correlation between pyritic sulfur and ash yield.Meanwhile, pyritic sulfur and organic sulfur are usually negatively correlated to each other, especially when sulfate sulfur is negligible.Therefore, a negative correlation should exist between ash yield and organic sulfur.However, in the present study, the variation of ash yield across HOS coals (except for Samples YX and SSP) is generally compatible to that of organic sulfur content (Figure 4A).A possible explanation to this observation is that the organic sulfur contents derived from subtraction method are not definitively accurate, because a portion of sulfur occurring in fine-grained S-containing minerals that are dispersed in organic matter as inclusions were mistaken as organically associated, especially for the SHOS coals, as suggested by Li et al. [12].The negative correlations between oxygen/nitrogen and organic sulfur (Figure 4B,C) indicate a limited capacity of coal macromolecular structure for heteroatoms.Part of bonding sites that can be assigned to oxygen and nitrogen are taken up by organic sulfur in the HOS coals.

13 C-NMR Spectra and Peak-Fitting Results of HOS Coals
The solid-state 13 C-CP/MS NMR spectra of the HOS coals are shown in Figure 5. From the qualitative perspective, each spectrum exhibits two broad bands that represent aliphatic carbons (<90 ppm) and aromatic carbons (>90 ppm), respectively.With increasing coal rank, aliphatic carbons decrease gradually, and aromatic carbons increase along with the aromatic bands becoming more thin and sharp, indicating an increasing maturity of the coal.Besides, the spectrum of aromatic carbons becomes smoother as coal rank grows, suggesting an increasing homogeneity of chemical structure.The satisfactory peak-fitting effect (Figure 6) is the prerequisite for reliable deconvolution results.Based on the 13 C-NMR structure assignment, each spectrum can be resolved into as many as twelve bands (Table 4).The unit for chemical shift is ppm.

Discussion
Carbon content and oxygen/carbon ratio were chosen as parameters of coal rank by Zhang [89] to explore the carbon structure evolution characteristics of low-to-medium rank coals using 13 C-NMR.Zhang's results demonstrated that there is a good correlation between aromatic carbon (far C ), protonated aromatic carbon (fa H ), aromatic bridgehead carbon (fa B ), phenolic ethers (fa P ), alkylated aromatic carbon (fa S ), aliphatic carbon (fal C ), aromatic CH3 carbon, and aliphatic CH3 carbon vs. coal rank.In the present study, maximum reflectance of vitrinite/huminite (Rmax) was selected as the coal rank parameter, and the variations in the 13 C-NMR structure parameters versus Rmax are shown in Figure 7. Aliphatic CH3 carbon, CH3 carbon (fal*), quaternary carbon, CH/CH2 carbon + quaternary carbon (fal H ), aliphatic carbon (fal C ), protonated aromatic carbon (fa H ), protonated aromatic carbon + aromatic bridgehead carbon (fa H+B ), aromaticity (fa CP ), and aromatic carbon (far C ) exhibit clear relevance with coal rank.Thus, in comparison with ordinary coals, the structure evolution of the HOS coals does not display an apparent relationship between aromatic CH3 carbon, aromatic bridgehead carbon (fa B ), phenolic ethers (fa P ), and alkylated aromatic carbon (fa S ) vs. coal rank, suggesting that abundant organic sulfur has an impact on coal structure evolution. 13C-NMR structure parameters can be divided into three categories as follows: aliphatic carbons, aromatic carbons, and oxygen-containing functional groups.The unit for chemical shift is ppm.

Discussion
Carbon content and oxygen/carbon ratio were chosen as parameters of coal rank by Zhang [89] to explore the carbon structure evolution characteristics of low-to-medium rank coals using 13  ) exhibit clear relevance with coal rank.Thus, in comparison with ordinary coals, the structure evolution of the HOS coals does not display an apparent relationship between aromatic CH 3 carbon, aromatic bridgehead carbon (f a B ), phenolic ethers (f a P ), and alkylated aromatic carbon (f a S ) vs. coal rank, suggesting that abundant organic sulfur has an impact on coal structure evolution. 13C-NMR structure parameters can be divided into three categories as follows: aliphatic carbons, aromatic carbons, and oxygen-containing functional groups.

Aliphatic Carbon (f al C )
A negative correlation is shown between aliphatic carbon and coal rank (Figure 7J).The variation trend at low-to-medium rank is relatively steep, and it becomes a gentle slope as the rank increases.The maximum at approximately R max 1.404% (Sample CM) could be related to the second coalification jump, and another maximum exists around R max 1.801% (Sample 6K).4.1.2.CH 3 Carbon (f al *) 0-25 ppm is the combined effect scope of aliphatic CH 3 carbons (0-16 ppm) and aromatic CH 3 carbons (16-25 ppm).The aliphatic CH 3 carbons decrease significantly from low to medium rank (R max 0.293-1.80%)and are completely detached at higher rank (Figure 7A).This indicates that the chain structures progressively break off during the coalification process.
The proportion of aromatic CH 3 carbons in lignite (Sample YX) is the largest, and it decreases in anthracite (Sample WTP).It is important to note that the aromatic CH 3 carbons increase with coal rank in the whole bituminous stage (Figure 7B).This variation indicates that there is no shortage of aromatic structures in lignite, and they are highly dispersed.Sufficient aromatic bonding sites lead to high aromatic methyl content.The coal structure suffered immense alteration in the first coalification jump, and the aromatic structures condensed at the initial stage of bituminous coal resulting in a decrease in the quantity of bonding sites for aromatic methyl.The increasing content of aromatic methyl in the bituminous coal stage is due to the extension of aromatic structures.As for anthracite, the aromatic structures experienced severe polycondensation, which resulted in the decrease of aromatic CH 3 carbons.
The combination of aliphatic CH 3 carbons and aromatic CH 3 carbons brought a generally negative correlation between CH 3 carbons (f al *) and coal rank (Figure 7C).Specifically, the increase in aromatic CH 3 carbons is more pronounced in the first half of the bituminous stage, and the decrease in aliphatic CH 3 carbons is dominant in the second half of the bituminous stage.The content of methylene and methyne represents the development degree of the chain structures in coal, and the average length of the aliphatic chains (Table 5) can be calculated through the ratio of the sum of methylene and aliphatic methyl to aliphatic methyl [90].Overall, the average length gradually shortens except for lignite (Sample YX).The large quantity of CH 3 carbons (f al * ) and low content of CH 2 /CH carbons in lignite (Figure 7C,D) suggest that the aliphatic chains in Sample YX are widespread, but they are generally short and undeveloped.The chain structures experienced a large expansion at the beginning of bituminous coal stage; afterwards, the chain structures gradually shortened.When R max reached 1.752% (Sample SSP), the aliphatic chains vanished due to the absence of aliphatic methyl.As an exception, the average length of aliphatic chains in Sample 6K is the longest among all studied samples and is not in line with the general variation trend (Table 5), which warrants further study.In addition, quaternary carbons decrease substantially with coal rank (Figure 7E).At the initial stage of bituminous coal, the increase in CH 2 /CH carbons is more severe than the decrease in quaternary carbons, which results in a small rise in f al H followed by a dramatic decrease (Figure 7D-F).

Aromatic Carbon Evolution Characteristics of HOS Coals
Three main parameters are widely used for coal aromaticity characterization as follows: aromatic carbon (f ar C ), aromaticity (f a CP ), and the ratio of aromatic bridgehead carbon to aromaticity (X BP ).
4.2.1.Aromatic Carbon (f ar C ) and Aromaticity (f a CP ) Aromatic carbons exhibit a positive correlation with R max (Figure 7T); thus, it can be regarded as an alternative parameter for coal rank determination.The aromatic carbon content is slightly lower than expected at R max 1.404% (Sample CM) due to its low inertinite content (6.0%;Table 3), which bears a higher degree of condensation relative to vitrinite/huminite.The aromaticity also displays a positive correlation versus R max (Figure 7R), and its steady variation suggests a gradual quantity growth of aromatic fringes during the coalification process.

Ratio of Aromatic Bridgehead Carbon to Aromaticity (X BP )
The parameter X BP refers to the percentage of aromatic bridgehead carbons in all aromatic carbons, which characterizes the average aromatic cluster size of the coal macromolecule; in general, the value of this parameter increases with coal rank.Aromatization predominates in the low-to-medium metamorphic stage of coal, in other words, the aliphatic chains break off to form aromatic rings; while, condensation is dominant in the medium-to-high metamorphic stage, which means that the simple aromatic rings are condensed into polynuclear structures.
In the present study, the significant variation trend of X BP is dependent on that of aromatic bridgehead carbon due to the gradual change of aromaticity across all samples (Figure 7L,R,U).The gradual variations in f ar C and f a CP indicate that coals with similar ranks possess equivalent amounts of aromatic carbons and aromatic fringes; accordingly, they should have comparable average sizes for aromatic clusters.Therefore, in theory, X BP should also display a gradual variation trend, which is incompatible with the observation made in this study (Figure 7U).Since aromatic fringes are not the causes for this contradiction, it is likely due to the structures between aromatic fringes.The most reasonable explanation for this anomaly is that the high X BP values of Samples JJP, WHS, and GH are abnormal, and their aromatic clusters are not that large; however, the extensive development of cross bonds and/or bridged bonds results in closer connections between aromatic fringes.Besides, the Samples JJP and GH are typical SHOS coals, and sulfur-containing functional groups are likely to be significant components in these linkages.
In addition, both protonated aromatic carbon (f a H ) and aromatic bridgehead carbon (f a B ) show variable trends across all HOS coals (Figure 7K,L), but the sum of them (f a H+B ) exhibits a gradual trend and a positive correlation versus coal rank (Figure 7M).Non-protonated aromatic carbon (f a N ) and aromatic carbon bonded to carbon (f a B+S ) share similar variation trends (Figure 7O,Q).

Oxygen-Containing Functional Group Evolution Characteristics of HOS Coals
Carbonyl/carboxyl carbon (f a C ), phenolic ethers (f a P ), and aliphatic carbon bonded to oxygen (f al O ) are appropriate parameters for characterizing oxygen-containing functional groups in coal [91].
In the present study, the content variations of oxygen-containing functional groups vs. maximum reflectance of vitrinite/huminite are shown in Figure 8.As expected for lignite, Sample YX has the most oxygen-containing structures, which conforms to the characteristics of low-rank coals; specifically, phenolic ethers are the most abundant, and carbonyl/carboxyl carbon comes next and followed by aliphatic carbon bonded to oxygen.From lignite (Sample YX) to bituminous coal (Sample JJP), the significant drop of oxygen-containing structures demonstrates that they suffered from intense destruction during the first coalification jump.Coalification is typically a de-oxidation process; however, the variation of total oxygen-containing structures generally exhibits an increase with coal rank in the bituminous to anthracite stage.Among bituminous coals, the SHOS Samples CM, SSP, 6K, and GH are of abnormally high organic sulfur contents, and they have more oxygen-containing structures than other HOS coals (except Sample JJP).Considering the similar chemical properties and atomic structures of sulfur and oxygen, the most likely explanation for this observation is that a considerable portion of the "oxygen-containing functional groups" determined by 13 C-NMR are actually sulfur-containing functional groups, which leads to the anomalous increase of oxygen-containing structures versus coal rank.This differs from the observations made in previous studies on the structure evolution characteristics of non-HOS coals.In addition, compared to other coals in the present study, the organic sulfur content of Sample WTP (anthracite) is relatively low, and its large proportion of oxygen-containing structures is possibly due to some functional groups being subjected to oxidation.chemical properties and atomic structures of sulfur and oxygen, the most likely explanation for this observation is that a considerable portion of the "oxygen-containing functional groups" determined by 13 C-NMR are actually sulfur-containing functional groups, which leads to the anomalous increase of oxygen-containing structures versus coal rank.This differs from the observations made in previous studies on the structure evolution characteristics of non-HOS coals.In addition, compared to other coals in the present study, the organic sulfur content of Sample WTP (anthracite) is relatively low, and its large proportion of oxygen-containing structures is possibly due to some functional groups being subjected to oxidation.Besides the prominent maxima near the middle of the medium rank that correspond to Samples 6K and SSP, phenolic ethers exhibit a distinct decrease at low to medium rank (Figures 7P and 8), which results from continuous decomposition into CO2 and H2O.

Carbonyl/Carboxyl Carbon (fa C )
Generally, the content of carbonyl/carboxyl carbon decreases and decarboxylation persists in the whole coalification process.It reflects the characteristics of side chains falling off and aromaticity increasing.At first, the -OH groups fall off during decarboxylation, and then, the proportion of -C=O groups increases followed by the destruction of -C=O groups [89].
Due to the negligible contents of carbonyl carbon (0-0.05%), the variation trend of fa C depends on that of the carboxyl carbon (Figure 7S and Table 4).Lignite has the highest content (5.42%) of carboxyl carbon, and the contents of all the bituminous coals are lower and comparable (1.65-2.29%)except Sample GH (4.70%) (Table 4), indicating the remarkable decarboxylation during the first coalification jump.The formation of thionocarboxylic groups is possibly responsible for the abnormally high carboxyl carbon content in Sample GH, which owns the most organic sulfur in the studied samples.

Aliphatic Carbon Bonded to Oxygen (fal O )
The proportion of methoxyl/aromatic methoxyl (51-75 ppm) increases generally with coal rank, with a dip at Sample SSP (Rmax 1.752%) (Figure 7G).By comparison, the variation in the aliphatic carbon bonded to oxygen in cyclic hydrocarbon (75-90 ppm) is much more complicated (Figure 7H).Thus, the variation of fal O exhibits no correlation to coal rank (Figure 7I).In addition, Sample WTP Besides the prominent maxima near the middle of the medium rank that correspond to Samples 6K and SSP, phenolic ethers exhibit a distinct decrease at low to medium rank (Figures 7P and 8 Generally, the content of carbonyl/carboxyl carbon decreases and decarboxylation persists in the whole coalification process.It reflects the characteristics of side chains falling off and aromaticity increasing.At first, the -OH groups fall off during decarboxylation, and then, the proportion of -C=O groups increases followed by the destruction of -C=O groups [89]. Due to the negligible contents of carbonyl carbon (0-0.05%), the variation trend of f a C depends on that of the carboxyl carbon (Figure 7S and Table 4).Lignite has the highest content (5.42%) of carboxyl carbon, and the contents of all the bituminous coals are lower and comparable (1.65-2.29%)except Sample GH (4.70%) (Table 4), indicating the remarkable decarboxylation during the first coalification jump.The formation of thionocarboxylic groups is possibly responsible for the abnormally high carboxyl carbon content in Sample GH, which owns the most organic sulfur in the studied samples.

Aliphatic Carbon Bonded to Oxygen (f al O )
The proportion of methoxyl/aromatic methoxyl (51-75 ppm) increases generally with coal rank, with a dip at Sample SSP (R max 1.752%) (Figure 7G).By comparison, the variation in the aliphatic carbon bonded to oxygen in cyclic hydrocarbon (75-90 ppm) is much more complicated (Figure 7H).Thus, the variation of f al O exhibits no correlation to coal rank (Figure 7I).In addition, Sample WTP (anthracite) has a low aliphatic/aromatic CH 3 carbon content and relatively high f al O content.In combination with the analysis of oxygen-containing structures above, the oxidized structures in Sample WTP can be inferred as methyls.

Conclusions
Each 13 C-NMR spectrum of HOS coal can be resolved into as many as 12 peaks.The CH 3 carbon (f al *), quaternary carbon, CH/CH 2 carbon + quaternary carbon (f al H ), aliphatic carbon (f al C ), protonated aromatic carbon (f a H ), protonated aromatic carbon + aromatic bridgehead carbon (f a H+B ), aromaticity (f a CP ), and aromatic carbon (f ar C ) exhibit clear correlations with coal rank.Compared to ordinary coals, the structure evolution of HOS coals exhibits no correlations between the aromatic CH 3 carbon, aromatic bridgehead carbon (f a B ), phenolic ethers (f a P ), and alkylated aromatic carbon (f a S ) versus rank, which suggests that high organic sulfur has an impact on the regular evolution process of coal structure.The maximum of aliphatic carbon (f al C ) at R max 1.404% is due to the second coalification jump.
There was no shortage of aromatic structures in lignite but were dispersed.The coal structure varied significantly in the first coalification jump, and the aromatic structures condensed at the initial stage of bituminous coal and stretched gradually in the whole bituminous stage.The aliphatic chains in lignite were widespread but generally short.These chains expanded greatly at the beginning of bituminous coal and then became shortened and detached before vanishing at around R max 1.752%.The aromatic fringes of HOS coals increase gradually in the coalification process, but the aromatic cluster size barely exhibits relevance to rank.It is inferred that the aromatic cluster sizes of some SHOS coals are not large, but the extensive development of cross bonds and/or bridged bonds form closer connections among aromatic fringes.In addition, sulfur-containing functional groups are likely to be significant components in these linkages.
For SHOS coals, a considerable portion of the "oxygen-containing functional groups" determined by 13 C-NMR are actually sulfur-containing groups, which results in the anomaly that oxygen-containing structures increase versus coal rank.This indicates the substitution of oxygen by sulfur, which is also supported by the inverse correlation between oxygen and organic sulfur based on the ultimate analysis data.In addition, the decarboxylation is remarkable during the first coalification jump.

Figure 2 .
Figure 2. Sampling locations of high-organic-sulfur coals in the present study.

Figure 2 .
Figure 2. Sampling locations of high-organic-sulfur coals in the present study.

Figure 3 .
Figure 3. Relationships between selected proximate/ultimate parameters and maximum reflectance of vitrinite/huminite (R max ) of high-organic-sulfur (HOS) coals.(A) volatile matter vs. R max ; (B) carbon content vs. R max ; (C) hydrogen content vs. R max ; (D) hydrogen/carbon ratio vs. R max ; (E) oxygen content vs. R max ; (F) nitrogen content vs. R max .

Figure 4 .
Figure 4. Relationships between selected proximate/ultimate parameters and organic sulfur content (So,d) of HOS coals.(A) ash yield vs. So,d; (B) oxygen content vs. So,d; (C) nitrogen content vs. So,d.

Figure 5 .
Figure 5. Solid-state 13 C-NMR spectra of high organic sulfur coals with increasing rank from top to bottom.

Figure 4 .
Figure 4. Relationships between selected proximate/ultimate parameters and organic sulfur content (S o,d ) of HOS coals.(A) ash yield vs. S o,d ; (B) oxygen content vs. S o,d ; (C) nitrogen content vs. S o,d .

Figure 4 .
Figure 4. Relationships between selected proximate/ultimate parameters and organic sulfur content (So,d) of HOS coals.(A) ash yield vs. So,d; (B) oxygen content vs. So,d; (C) nitrogen content vs. So,d.

Figure 5 .
Figure 5. Solid-state 13 C-NMR spectra of high organic sulfur coals with increasing rank from top to bottom.

Figure 5 .
Figure 5. Solid-state 13 C-NMR spectra of high organic sulfur coals with increasing rank from top to bottom.

Figure 6 .
Figure 6.Peak-fitting effect for 13 C-NMR spectra.The test spectra are solid black, and the curve-fitted lines are dotted red.(A) Sample YX; (B) Sample WTP.

Figure 6 .
Figure 6.Peak-fitting effect for 13 C-NMR spectra.The test spectra are solid black, and the curve-fitted lines are dotted red.(A) Sample YX; (B) Sample WTP.

Figure 8 .
Figure 8. Variations in oxygen-containing functional groups versus coal rank of high organic sulfur coals.

Table 1 .
Basic information about vitrinite/huminite reflectance, sampling location, and geological background of high-organic-sulfur coals.

Table 1 .
Basic information about vitrinite/huminite reflectance, sampling location, and geological background of high-organic-sulfur coals.

Table 3 .
Maceral composition, proximate analysis, ultimate analysis, and total sulfur and forms of sulfur in high-organic-sulfur coals.

Table 5 .
Average length of aliphatic chains in high-organic-sulfur coals.
Unit length is the average C-C bond length in aliphatic chains.