Construction ofa macromolecular structuralmodelofChinese lignite and analysis of its low-temperature oxidation behavior☆

Xianliang Meng ,Mingqiang Gao ,Ruizhi Chu ,*,Zhenyong Miao ,Guoguang Wu ,Lei Bai,Peng Liu ,Yuanfang Yan ,Pengcheng Zhang

1 Key Laboratory of Coal-based CO2 Capture and Geological Storage,Jiangsu Province(China University of Mining&Technology),Xuzhou 221116,China

2 School of Chemical Engineering and Technology,China University of Mining&Technology,Xuzhou 221116,China

3 Department of Chemical and Biomedical Engineering,West Virginia University,Morgantown 26506,United States

1.Introduction

Fire due to spontaneous combustion is a perennial danger in the global coal mining industry and its storage and transportation process causing many problems such as the difficulties in long distance transportation, fire safety concerns and storage issues[1–4].In particular,coal spontaneous combustion will pollute the environment and endanger human health by releasing a large number of SOX,NOXand other harmful gases in the combustion process.

The spontaneous combustion of coal is a very complicated process.Over the years,many domestic and foreign scholars have researched the process of coal spontaneous combustion,and the research methods have been developed on the macro-to microscopic scales;as a result,great progress has been made.Low-temperature oxidation is an important stage for coal spontaneous combustion,which controls the occurrence and development of spontaneous combustion[5].As a result,research targeting the low-temperature oxidation of coal plays a key role in the prevention of self-heating hazards.

Coal is well known as a complex organic fossil fuel consisting of similar but different molecular structures.Generally,coal is considered as a porous polymer with a large internal surface area[6].Given this property,direct investigation into the internal structural changes of coal during low-temperature oxidation is often difficult.Therefore,a reasonable structure of coal molecules must be established before it is possible to study the mechanism of coal spontaneous combustion at the molecular level.Due to its complexity,many modern experimental techniques are adopted to study the structure of coal,such as ultimate analysis,X-ray diffraction(XRD),UV/Vis spectra,solvent extraction and adsorption,Fourier transform infrared spectroscopy(FTIR)and nuclear magnetic resonance spectroscopy(NMR)[7–12].The micro crystal structure,the size and arrangement of the aromatic structure,the bond length and the distribution of atoms are all obtained by X-ray methods.From the FTIR and NMR,the distribution of functional groups,the distribution ofaromaticity,and the distribution ofhydrocarbon atoms are obtained.Finally,a structural model of coal is inferred from these methods.

With the developmentofcomputer simulation technology,more and more attention has been paid to the study of chemical mechanism by using simulation software.DFT is now very successful in calculating related parameters of molecular models because of its less computational and the consideration of electronic correlation effect in the calculation process.More researchers have used DFT and other quantum chemical methods to calculate some reactions of model compounds[13,14].

In this work,which is focused on Chinese lignite,the macromolecular structural model of lignite from the Xiaolongtan coal mine was successfully established.The parameters(the ratio of hydrogen to carbon and the aromatic carbon ratio were calculated by ultimate analysis,the aromatic hydrogen ratio was calculated by FTIR analysis)were calculated for building an aromatic structural unit of the Xiaolongtan lignite.The types and the content of the functional groups of the lignite structure were obtained by analyzing the FTIR and XPS spectrum of lignite.Based on that,the aromatic structural units and the heteroatomic structures were further determined,respectively,and finally the structural unit of lignite was constructed.GaussViewwas used to construct a macromolecular structure model of lignite,and the model was submitted to Gaussian09 for calculation.The macromolecular structure model was optimized and the vibrational frequencies were calculated using DFT/B3LYP methods with the 6–31+G(d,p)basis set[15].Macromolecular structure models were constructed by changing the spatial structure,and structure optimization and frequency calculations were carried out.Finally,a functional group activity analysis was made on the low-temperature oxidation process of lignite based on the model,and the evolution law of the functional groups in the process of low-temperature oxidation was obtained.

2.Samples and Experiments

2.1.Proximate and ultimate analysis

The lignite sample was taken from the Xiaolongtan coal mine in the Yunnan province of China.Before the experiment,the sample was crushed,ground,and then sieved using a 0.074-mm sieve.The pretreated sample was stored in a nitrogen atmosphere for later use.The results of the proximate and ultimate analysis are shown in Table 1.

Table 1Proximate and ultimate analysis of lignite sample from Xiaolongtan

As seen from the Table 1,the moisture content of the lignite is relatively high,and the volatile mass fraction of the samples is as high as 52.39%.These results show that the coal samples belong to low-grade metamorphic coal,and the degree of condensation of the aromatic rings in the coal sample is low.From the ultimate analysis data,we can see that the oxygen content of the coal sample is up to 23.56%,which indicates that the coal sample consists of oxygen-containing functional groups.This property makes the coal sample more prone to spontaneous combustion.The content of sulfur in the coal sample is less than 2%,so the effect of sulfur on the coal oxygen recombination process is small,and it can be ignored.

2.2.FTIR analysis

A sample-KBr mixture pellet was prepared for FTIR analysis.A reference spectrum was obtained by pure ground KBr.The mixture of2 mg of coalsample and 300 mg ofKBr were ground in an agate mortar,then the mixture was pressed under a pressure of 10 MPa for a few seconds.The FTIR analysis were completed with NICOLET-Nenus380 FTIR spectrometer(UK)and the spectra of 32 scans were recorded in the range of wavenumbers from 450 to 4000 cm?1at a resolution of 4 cm?1.In addition,PeakFitv4.12 was used to separate peaks in the FTIR spectrogram to obtain the peak area of each absorption peak[16].

In this paper,the effects of oxygen-containing functional groups,aromatics and aliphatic hydrocarbons on low-temperature oxidation of lignite were studied using qualitative analysis and semi-quantitative analysis of the FTIR spectral analysis[17–20].As shown in Fig.1,the infrared spectrum of raw lignite is divided into four parts for the fitting analysis[21–24];namely,3700? 3000 cm?1is attributed to the hydroxyl groups[25];3000?2700 cm?1is attributed to the aliphatic hydrocarbon groups[26,27];1800? 1000 cm?1is attributed to the oxygen-containing functional groups[28]and 900?700 cm?1is attributed to the substitutions about the aromatic ring[29].

Fig.1.Curve- fitted FTIR spectrum of the coal sample from Xiaolongtan.a(3700?3000 cm?1),b(3000?2700 cm?1),c(1800?1000 cm?1),d(900?700 cm?1).

As seen from Table 2(3700?3000 cm?1),the FTIR spectra from lignite reveals a large number of phenolic hydroxyl groups and alcohol hydroxyl groups,and their ratio is 2:1.Due to the presence of moisture in the lignite,the free hydroxyl group and hydroxyl bond are observed in the infrared spectrum of lignite.Thus,this signal does not help to build a molecular structure of the lignite,and it was ignored for the remainder of the analysis.

Table 2FTIR spectrum parameters of coal sample from Xiaolongtan

Combined with Fig.1(b)and Table 2(3000?2700 cm?1),we can see that the band number of 3000? 2700 cm?1in the absorption peak mainly corresponds to the methyl and methylene.This area is very important in understanding the structure of coal.The composition of the corresponding peaks in the wavenumber range of 3000?2700 cm?1is almost entirely derived from the methyl and methylene vibrations in the aliphatic chains or rings.Thus,the peak of this area is used to characterize the content of aliphatic hydrocarbons in coal,which is one of the important parameters needed to construct a basic structural unit of coal.As shown in Table 2,Xiaolongtan lignite contains a large number of methylene anti symmetric stretching vibrations and methylene symmetric stretching vibration,and their ratio is 2:1.In addition,the content of both methyl symmetric stretching vibrations and methylantisymmetric stretching vibrations is relatively low in the coal.In summary,the ratio of methyl and methylene is approximately 2:7.

Oxygen-containing functional groups in coal include carboxyl,hydroxyl,quinone,methoxyl and ether groups.These oxygen-containing functional groups appeared in this region,except the hydroxyl group.Thus,it is very important to determine the oxygen-containing functional groups in coal.As seen from Fig.1(c)and Table 2(1800?1000 cm?1),C--O on the phenyl ether is the most important structure,followed by the C--O on the aliphatic ether,and finally C=O(O--C=O).The ratio of C--O on the phenyl ether and C--O on the aliphatic ether is approximately 5:3.

The position of the absorption peak of the region can be speculated to replace the aromatic ring.In addition,the peak area of the region can also be used to characterize the content of aromatic hydrogen in the coal.The ratio of the aromatic hydrogen to the total hydrogen atom is the aromatic hydrogen ratio and is essentialforthe construction of basic structural units.As seen from Table 2(900?700 cm?1),the aromatic ring of the lignite has two,three and four substitutions.Among them,the content of triply substituted aromatics was the highest,reaching 87%,followed by four substitutions at approximately 13%.Doubly substituted aromatics were the least common.

2.3.XPS analysis

The coal sample was analyzed using an ESCALAB250 X-ray photoelectron spectrometer(Thermo VG Scientific,UK).Al Kαanode was employed and the power was 200 W.The full scanning transmission energy was 150 eV with 0.5 eV of step size;the narrow scanning transmission was 60 eV with 0.5 eV of step size[10,30].The basic vacuum is 10?7Pa.Take C1s(284.6 eV)as a calibration standard to conduct the check.Origin software was used for peak fitting.

Carbon exists on the coal surface structure in four morphologies shown in Fig.2 and Table 3.Among them,the content of C--H and C--C make up 71.34%of the total,indicating that the carbon in coal is primarily composed of aromatic structures and substituted alkanes.The C--C content is 18.88%,which includes hydroxyl and ether oxygen bonds.The remaining carbon atoms are present in the form of carbonyl and carboxyl groups,and they make up a relatively small in proportion.Comprehensive consideration of the three types of oxygen-containing functional groups leads to a ratio of 9:1:2.

Fig.2.XPS C1s and N1s spectra of the coal sample from Xiaolongtan.

As shown in Fig.2 and Table 3,nitrogen exists on the coal surface structure in four morphologies.Pyrrolic nitrogen is the most common form of nitrogen at 68.74%followed by quaternary nitrogen and pyridine nitrogen accounting for 26.11%and 3.78%,respectively.The content of nitrogen oxides is the lowest accounting for 1.37%,and their existence may be due to oxidation of pyridinic nitrogen and pyrrolic nitrogen in the air.Together,the content of pyrrole nitrogen and quaternary nitrogen has been added up to 95%.Thus,only these forms of nitrogen are considered in the construction of the model,and their ratio is approximately 3:1.

The relative higher contentofpyrrolic nitrogen may be related to the chloroplasts found in coal-forming plants,as chlorophyll structure contains quite a few pyrrole rings.The pyrrole rings have aromatic conjugated systems with higherstability,and thus can be retained during long-term coal formation.

Table 3XPS C1s and N1s fitting results of the coal sample from Xiaolongtan

3.Results and Discussion

3.1.Macromolecular structure of lignite and its model

3.1.1.Basic structural parameters

To build an aromatic structural unit of the Xiaolongtan lignite,we need to calculate the parameters of the basic structure.These parameters include the ratio of hydrogen to carbon,the aromatic hydrogen ratio,and the aromatic carbon ratio.

(1)The ratio of hydrogen to carbon,H/C

The ratio of hydrogen to carbon is calculated from the ultimate analysis.

The calculated ratio of atomic carbon and hydrogen for the Xiaolongtan lignite is 0.97.

(2)The aromatic hydrogen ratio,

The aromatic hydrogen ratio is the ratio of the number of hydrogen atoms present in the aromatic compounds to the total number hydrogen atoms.In the calculation,it is assumed that the hydrogen atoms exist as aromatic hydrogens(Har)or aliphatic hydrogens(Hal).The absorption peak area in the range of 2970?2800 cm?1corresponds to the aliphatic hydrogen,and the absorption peak area in the range of 900?700 cm?1corresponds to the aromatic hydrogen.

The calculated aromatic hydrogen ratio for the Xiaolongtan lignite is 0.286.

(3)The aromatic carbon ratio,fa.

The aromatic carbon ratio represents the ratio of the number of carbon atoms in the aromatic compounds to the total number of carbon atoms.In the calculation,it is assumed that the carbon atoms exist in the form ofaromatic carbons(Car)or aliphatic carbons(Cal).

whereCal/Cis the ratio ofaliphatic carbons to the totalnumber of carbons,Hal/His the ratio of aliphatic hydrogens to the total number of hydrogens,andHal/Calis the ratio of the number of aliphatic hydrogen atoms to the number of aliphatic carbon atoms.For the Xiaolongtan lignite,the value ofHal/Calis 0.614.

3.1.2.Construction of an aromatic structural unit

Previous studies indicated thataromatic units oflignite are dominated by 1–2 rings[31].The condensation degree of lignite is low[32],so few carbon atoms are included in the model.In this paper,the basic structural unit is constructed with 45 carbon atoms.As reported above,the ratio of atoms of carbon and hydrogen in the Xiaolongtan lignite was 0.97,and the aromatic hydrogen ratio and the aromatic carbon ratio were 0.286 and 1.8,respectively.Based on the ultimate analysis,the atomic ratio of C:H:O:N:S is 1:0.966:0.261:0.020:0.012.

3.1.3.Heteroatomic structure

Based on the ultimate analysis and the number of C atoms in the Xiaolongtan macromolecular structure model,the numbers of O,N and S atoms were determined to be 12,1 and 0,respectively.

Oxygen-containing functional groups in coal include carboxyl,hydroxyl,quinone,methoxyl and ether groups[33].Combined with the FTIR and XPS results,it can be determined that the Xiaolongtan lignite model should contain seven oxygen atoms in ether groups,one oxygen atom in a carbonyl group,and two oxygen atoms in hydroxyl groups.

Pyrrolic nitrogen,pyridinic nitrogen and quaternary nitrogen are the main forms of nitrogen in the coal structure.The XPS results show that pyrrolic nitrogen is significantly dominant in the Xiaolongtan lignite sample.Thus,the nitrogen atom in the structural model is in the form of pyrrolic nitrogen.

The XPS results also demonstrate that the organic sulfur in coal sample includes the forms of thiophene type sulfur,sulfone type sulfur,sulfoxide type sulfur,and mercaptan thiophenol.The sulfur content in this sample is only 1.45%,and the number of atoms in the model is limited.Thus,no sulfur atom is considered in construction of this model.

On this basis,a simplified structural unit of the lignite was constructed,and the modelwas tested and modified to determine the structure of the model.The molecular formula is C45H47O12N(molecular mass:793).

3.1.4.The construction of the coal macromolecular structure model

GaussViewwas used to construct a macromolecular structure model of lignite,and the model was submitted to Gaussian09 for calculation.The macromolecular structure model was optimized,and the vibrational frequencies were calculated using DFT/B3LYP methods with the 6–31+G(d,p)basis set[15].Macromolecular structure models were constructed by changing the spatial structure,and structure optimization and frequency calculations were carried out.By calculating the most stable structure,the imaginary frequency is 0.

After modification and calculation,the most stable structure of the basic structural unit is shown in Fig.3.

The macromolecular structure model of Xiaolongtan lignite is calculated using a frequency calculation,and the FTIR spectrum of the model and the corresponding vibration mode of each absorption peak can be obtained.The infrared vibrational results of the theoretical calculation are adjusted and compared with the experimental results.The results are shown in Table 4.

As can be seen from the Table 4,the experimental measurement of the hydroxyl absorption peak is wider,which is due to the formation of strong hydrogen bond between the hydroxyl groups.At the same time,the existence of hydrogen bond makes the frequency shift to the low wave number,which leads to the gap between the experimental and calculated values.Exceptthe hydroxylabsorption peak,the theoretical calculation of the absorption peak range of the other functional groups is smaller than that of the experimental results,which proves that the basic structural unit is reasonable.

3.2.Application of the model in low-temperature oxidation of lignite

To analyze the evolution process of various lignite functional groups during the process of low-temperature oxidation,the chemical bond lengths were calculated for the model,and the calculated results were compared with the results of the FTIR analysis.

3.2.1.Bond length analysis

The bond lengths in the macromolecular structure model is shown in the appendix.Generally,the longer the bond length,the more easily it is broken in a chemical reaction[34].Excluding hydrogen bonds,the bond lengths ranked from long to shortwere as follows:C--C of aliphatic chain＞C--C formed by the connection of aromatic ring and α carbon＞C--O formed by the connection of hydroxyl or ether in aliphatic hydrocarbons with α carbon＞C=C of aromatic ring＞C--O formed by the connection of the aromatic ring＞C=O＞O--H.Thus,C--C and C--O are easy to break in the low-temperature oxidation process,and C=O and H--O are more stable.For the oxygen-containing functional groups,bond length from long to short were as follows:C--O in the alcohol hydroxyl group＞C--O in aliphatic ether＞the C=O in the carbonyl group＞the C=O in the carboxyl group.In addition,it can be observed from appendix that the bond length of the oxygen-containing functional groups connected to an aliphatic chain is larger than that of the oxygen-containing functional groups connected to an aromatic ring.

Fig.3.The Xiaolongtan lignite macromolecular structure model.In the image above,carbon is dark gray,hydrogen is light gray,oxygen is red,and nitrogen is blue.

Table 4Xiaolongtan lignite FTIR comparison between experimental value and theoretical calculation value

Through bond length analysis,we predictthat the methyl group,the methylene of the aliphatic chain,the hydroxyl and the other active groups will be oxidized first to generate relatively stable groups,such as carbonyl and carboxyl groups,in the process of low-temperature oxidation.Ethers will also be oxidized at low temperatures.With an increase in temperature,the C=O and C--O in the carboxyl,aliphatic ethers and carbonyl groups will be broken to generate CO and CO2gases[35].Next,C=C,C--O and C=O will react as the temperature continues to increase.In general,the main reactive groups in the lowtemperature oxidation of coal are alkanes,hydroxyl groups,and ethers.They reactwith oxygen to release heat,which willspeed up the reaction of coal and oxygen.

Fig.4.The FTIR spectrum of Xiaolongtan lignite and the peak area of the functional groups at different temperatures.A-raw coal,B-40 °C,C-60 °C,D-80 °C,E-100 °C,F-120 °C,G-140 °C,H-160 °C,I-180 °C,J-200 °C.

3.2.2.FTIR analysis of the low-temperature oxidation of Xiaolongtan lignite

The FTIR of Xiaolongtan lignite under different oxidation temperature,and the trends of changes in the functional groups during lowtemperature oxidation are shown in Fig.4.

The wavenumbers of 3200?3700 cm?1in Fig.4 correspond to the absorption peak of the hydroxyl group.The figure shows that with an increase in the oxidation temperature,the alcohol hydroxyl groups and phenolic hydroxylgroups are reduced.The bond length calculations show that the bond length of the C--O on the alcohol hydroxyl group is 0.145 nm,and the bond length of C--O on the phenolic hydroxyl group is 0.140 nm.This observation means that the carbon–oxygen bond of the alcohol hydroxyl group is more active.The experimental results are in agreement with the theoretical calculation results.The hydroxyl group has the longest bond of the four main oxygen functional groups connected to carbon atoms,so the hydroxyl groups react at lower temperatures to generate stable carbonyl and carboxyl groups.

The wavenumbers of 2850?2950 cm?1in Fig.4 correspond to the absorption peaks of methyl and methylene.The changes in the methyl groups and the methylene groups were roughly the same;they decreased with increasing temperature.As the content of the methyl groups in the coal sample is already quite low,almost no methyl groups remain at 200°C.

The wavenumbers of 1700?1750 cm?1in Fig.4 correspond to the absorption peak of C=O on carbonyl and carboxyl groups.It can be observed from the figure that the C=O bond is relatively stable at low temperature,and the content of C=O bond increases only after the temperature exceeds 140°C.The C--C in the aliphatic hydrocarbon decreases and takes a small portion of the C=O on the aliphatic chain,causing it to slightly lower at lower temperatures.The bond length analysis shows that C=O is the most stable of the major oxygencontaining functional groups,so some unstable structures will be transformed into C=O during the process of low-temperature oxidation.These unstable structures are generally alkyl side-chains,alcohols and ethers,and the resulting C=O increases with increasing temperature.As seen from Fig.4,the C=O content at approximately 100°C has already begun to increase.At200°C C=Ocontentwould presumably continue to rise;then,at a certain temperature,the C=O groups would generate a large amount of gas.

The wavenumbers of 1500?1600 cm?1in Fig.4 correspond to the absorption peak of C=C in the aromatic rings.The aromatic rings are stable,and their content changes little with increasing temperature.From the perspective of bond length,C=C in aromatic rings are between C--C and C=O,implying they are notespecially stable.However,due to the aromatic ring,it is relatively stable at low temperatures.When the temperature rises,the C=C starts to break.The aromatic ring structure of the coal sample is destroyed,and the voids in the coal begin to increase in temperature.Once above 1,a large amount of oxygen diffuses into the coal.The C=C groups are then oxidized into stable structures,such as carbonyl and carboxyl groups,generating a large amount of CO,CO2and other gases.

The ether in the coal sample mainly has two forms:aromatic ethers and aliphatic ethers.The wavenumber of aromatic ethers is approximately 1200 cm?1in the infrared,and thatofaliphatic ethers is approximately 1100 cm?1.As seen from Fig.4,arylether is stable as its content slightly increases with the increase in temperature during the lowtemperature oxidation process.Aliphatic ether content,in contrast,clearly changes during the process of low-temperature oxidation.When the temperature is low,its content decreased with increasing temperatures.After 100°C began to gradually increase,reaching a maximum at 180°C before decreasing.The reactive activity of the aliphatic ether is second only to the hydroxyl group for the oxygen-containing functional groups,and the content of the aliphatic ether decreased at the initial stages of low-temperature oxidation because of the reaction between the aliphatic ether and oxygen.However,a small amount of ether bonds are generated with the increase in temperature,which may be due to the methyl,methylene and hydroxyl groups being transformed into more stable ether bonds at higher temperatures.The content of the aliphatic ether increases when the temperature reaches 100 °C,and the maximum is reached at 180 °C,which means that a large number of ether bonds are generated.

The active structure of Xiaolongtan lignite changed slightly,but the methyl,methylene and ether groups in the aliphatic hydrocarbons and the hydroxyl groups in various forms react with oxygen.These groups have a large reaction activity before the oxidation temperature is 100°C.Low-temperature oxidation led to slightly decreased carbonyl and carboxyl content followed by an increase as the reaction progressed.C=C and ether on the aromatic ring has changed little in the low temperature due to their stable structures.At a temperature of 120 to 160°C,the content of methyl,methylene and hydroxyl group continue to decrease.As a reaction product,the content of aliphatic ether,carbonyl and carboxyl group increased.C=C and ether on the aromatic ring increased slowly.After 160°C,the changes in the active structure are more obvious,and the hydroxyl content continues to decrease,causing the absorption peak of methyl and methylene to gradually disappear.The C=C on the aromatic ring is broken by high temperatures beyond 180°C and the content of the aromatic ring is reduced.At this point,the content of the aliphatic ether reaches its maximum value.Thereafter,the content of C--O decreased.C=O,due to its stability,is not reduced even when the temperature reached 200°C.Through this analysis,we can see that the change tendency of the functional groups in the process of low-temperature oxidation is consistent with the model.

4.Conclusions

Through proximate analysis,ultimate analysis,and FTIR and XPS experimentalresults,the structuralparameters ofthe modelare obtained.On the basis of this information,a modelof the macromolecular structure was constructed usingGauss View,and the quantum chemistry calculation of the model was performed byGaussian09.

The macromolecular structure model was optimized using DFT.Bond length analysis of the macromolecular structure model was carried out.The results show that the main reactant of coal low-temperature oxidation in addition to the active aliphatic chain,andαcarbon atom,hydroxyl and ether of the aromatic ring,the main products are the more stable carbonyl and carboxyl groups.

The accuracy of the modelwas verified by infrared spectrum analysis of the low-temperature oxidation process of raw coal.It is feasible to study the evolution law of active structures during low-temperature oxidation process of lignite by analyzing our proposed structural model for coal.

Supplementary Material

Supplementary data to this article can be found online at http://dx.doi.org/10.1016/j.cjche.2017.07.009.

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