Investigating pyrolysis characteristics of Shengdong coal through Py-GC/MS

: Shengdong coal was single-step and multi-step pyrolyzed by pyrolysis-gas chromatography/mass spectro-metry. In the single-step pyrolysis process, Mono-ring aro-matics contents reach a maximum value at 800°C, while phenolic contents are signi ﬁ cantly reduced, which might be attributed to the dehydroxylation of phenolics. Naphthalene content also reaches a maximum value at 800°C in the single-step pyrolysis process; however, naphthalene was not detected in the multi-step pyrolysis process. So naphthalene might come from the ring-opening of multi-ring aromatics or the cyclization of mono-ring aromatics in the single-step pyrolysis process. Only phenol and o -cresol were detected, but no p -cresol in the multi-step pyrolysis process; however, p -cresol content reaches a maximum value at 800°C, meanwhile, phenol content was decreased and o -cresol was disappeared in the single-step pyrolysis process, which suggests that p - cresol may be derived from isomerization of o -cresol or alkylation of phenol. It provides an idea for strengthening more basic research related to the rapid pyrolysis of coal.


Introduction
China is relatively rich in coal resources [1,2].As one of the eight largest coalfields in the world, Shengdong (SD) coal is easy to mine and it has excellent quality [3].With the increase in environmental pressures, it is imperative to clean and efficiently use coal [4].Coal pyrolysis, as the first step in all coal conversion processes, can obtain coal char, coal tar, and fuel gas, which provides an alternative source to the petrochemical industry for the production of liquid fuels and chemicals [5].
Fast pyrolysis is an attractive coal conversion with higher tar yield [6]. Unfortunately, none of fast pyrolysis technologies has been commercialized.It seems necessary to strengthen the basic research related to the rapid pyrolysis of coal [7].The destructive pyrolysis technique combined with GC/MS is the most effective method to analyze the compositions of organic volatiles during coal pyrolysis and has the advantages of high precision and simplicity, provides not only information about the coal structure but also helps to explain coal pyrolysis process [8].
Pyrolysis-gas chromatography/mass spectrometry (Py-GC/MS) can be used for direct analysis of organic solids without lengthy extractions or derivatizations, saving time and cost [9], which has been widely used in biomass [10], lignin [11], asphaltenes [12], polymer [13], and coal [14][15][16].The formation of light aromatics during coal pyrolysis was studied by Li et al. [17] and found that the main source of light aromatics was the thermal cracking of coal macromolecular skeleton.Xie et al. [18,19] studied the source of polycyclic aromatic hydrocarbons (PAHs) during coal pyrolysis by Py-GC/MS, and found that the release of PAHs against temperature variation first increased and then decreased.The formation of phenols during coal pyrolysis using in-situ Py-GC/MS were studied by Li et al. [20,21].The results showed that the main source of phenols came from the thermal decomposition of the oxygen-containing structures in coal.Meanwhile, the emission of thiophenic sulfur compounds during coal pyrolysis were studied by Li et al. [22] and found that the amounts of benzothiophene and dibenzothiophene reached a maximum value at 800°C.Wang et al. [23] studied the release characteristics of N-containing compounds, more than one hundred N-containing compounds have been found.Liu et al. [24] systematically studied the composition of oils produced during pyrolysis at the consecutive temperature ranges.The results showed that mono-ring aromatics were only found in oils at 400-500°C, bicy-ring aromatics were mainly found in oils at 400-500°C.
In summary, pyrolysis characteristics of light aromatics, PAHs, phenols, S-containing and N-containing compounds were investigated.However, there is no report on the comprehensive study of pyrolysis characteristics of many compounds.To our knowledge, most studies used single-step pyrolysis mode of Py-GC/MS, the studies on the multi-step coal pyrolysis of Py-GC/MS were few.Generally, GC/MS is the result of qualitative and semi-quantitative analysis of volatile organic compounds by matching with NIST mass spectrometry library and using area normalization method [25].The semi-quantitative results represent the relative contents, so maybe the trend of the relative contents is quite different from their actual contents [26][27][28][29].In literature reports, relative content was replaced by absolute peak area [17] and integrated intensity/mg coal [3,4].Here, the relative content was first modified by multiplying the pyrolysis rate with the relative content.
In this paper, SD coal was single-step pyrolyzed by in-line Py-GC/MS and the pyrolysis temperatures were 300-1,000°C, with a total of eight temperatures at intervals of 100°C.The pyrolysis rate at each temperature was calculated and used to modify the relative content.A multi-step program was used to investigate the multi-step pyrolysis of SD coal at different temperature ranges, a series of important information about the amounts and species of organic volatiles with temperature was obtained, and the organic volatiles evolution mechanism during coal pyrolysis was analyzed.

Coal sample
Shengdong subbituminous coal (SD coal) distributed in the Shanxi province of China was using in this study.The SD coal was pulverized, sieved to pass the 80 mesh standard sieve and dried at 110°C in a vacuum for 4 h before analysis.Proximate and ultimate analyses were determined by Chinese standard methods as described elsewhere [8].Size distribution was measured according to international standard ISO13320-1 by a Laser particle size analyzer (HYL-2076, China) [30,31].A summary of the proximate and ultimate analyses as well as the size distribution of SD coal were shown in Table 1.

Pyrolysis experiments
The destructive pyrolysis technique can provide detailed molecular information because the organic volatiles are related to the original coal structure.Fast pyrolysis of SD coal was performed with directly pyrolysis mode on the CDS 5200 pyrolyzer (CDS Analytical Inc., USA) [4].In order to detect the chemical composition of complex organic volatiles from SD coal pyrolysis, CDS 5200 pyrolyzer was direct connected to a GC/MS (QP2010 plus, Shimadzu Inc., Japan) and the organic volatiles were identified by the standard mass spectra library used the method of probabilitybased match algorithm [32].The detailed parameters have be described in the references [33].Fast pyrolysis of SD coal was performed by in-line Py-GC/MS (Figure 1).

Single-step pyrolysis
In a single-step run, each run corresponds to a sample and a pyrolysis condition.SD coal was single-step pyrolyzed by in-line Py-GC/MS at a heating rate up to 20°C/ms to the final pyrolysis temperature from 300 to 1,000°C at intervals of 100°C and the pyrolysis time at each temperature was 20 s.The organic volatiles produced in sample chamber during coal pyrolysis were transferred to GC by helium purge gas with 40 mL/min.The sample (about 2 mg) was placed at the center of the filament for pyrolysis and held in the place using small plugs of quartz wool.
One hundred thousand molecule-accurate weighing balance (ESJ80-5, China) was used to calculate the pyrolysis rate at different pyrolysis temperatures by equation (1).
The corrective content of organic volatiles was obtained by equation (2). (1) In the equations, R s is the pyrolysis rate in the singlestep mode, wt%, W 1 , W 2 , W 3 and W 4 are the mass of before sample addition, after sample addition, before pyrolysis and after pyrolysis, 0.01 mg, respectively.C c denotes the corrective content of organic volatiles in the single-step pyrolysis, wt%.C r denotes the relative content of organic volatiles, wt%.

Multi-step pyrolysis
A sample was put into the pyrolysis probe and analyzed for many times, which was completed by establishing the sequence method.SD coal was multi-step pyrolyzed (the only difference from single-step pyrolysis is that the sample was not taken out of the pyrolysis tube at different pyrolysis temperatures) by in-line Py-GC/MS at the temperature sequence of 300, 400, 500, 600, 700, 800, 900 and 1,000°C [33].The pyrolysis rate during sequence pyrolysis temperatures was calculated by equation ( 3  corrective content of organic volatiles in multi-step pyrolysis was obtained by equation ( 4).
where n is 300, 400, 500, 600, 700, 800 and 900°C, the value of R s can be obtained based on equation (1).R m is the pyrolysis rate during sequence pyrolysis temperatures, wt%.
3 Results and discussion

Organic volatiles during single-step pyrolysis
When analyzing pyrolytic volatiles by Py-GC/MS, the corrective content was calculated by the pyrolysis rate multiplied by the relative content.Figure 2 shows the pyrolysis rates (R s ) of SD coal during the single-step pyrolysis process.The value of R s increased with the increase of pyrolysis temperature.The structure of coal is very complex, which is composed of different covalent bonds with different bond energies.The primary driving force of coal pyrolysis comes from the thermal cleavage of these bonds.Generally, covalent bonds with lower bond energies crack at a lower temperature, while covalent bonds with higher bond energies crack at a higher temperature [34].The amounts of volatile organic compounds in coal pyrolysis process can be attributed to the devolatilization of fragments produced by covalent bonds breakage.As the pyrolysis temperature increased, the destroyed covalent bonds were increased.
The pyrolysis rate (R m ) of SD coal during multi-step pyrolysis process (Figure 2) was calculated by subtracting the pyrolysis rate (R s ) based on equation (3).The pyrolysis rate of SD coal is higher in the medium-low temperature range.The two temperature regions with higher pyrolysis rates are <400°C and 600-700°C, which can be attributed to the contents of low molecular compounds, carboxyl groups and aromatic branched chains in SD coal [35].More than 50% of the components in SD coal can't be thermal fractured, and the emission temperature of organic volatiles during coal pyrolysis are mainly concentrated before 800°C.The pyrolysis rate distribution (R m ) of SD coal in different temperature ranges can reflect the distribution of fracture difficulty in the structure.
SD coal was single-step pyrolyzed to 300, 400, 500, 600, 700, 800, 900 and 1,000°C by Py-GC/MS in-situ respectively and the individually total ion chromatograms were shown in Figure 3.The organic volatiles were identified by the standard mass spectra library used the method of probability-based match algorithm [32].Coal pyrolysis products are enormous, for comparison conveniently, all identified components were categorized as aliphatics, aromatics, Ocompounds and others [36].
Figure 4(a) is the relative contents of each component in the single-step pyrolysis process.The total peak number and peak intensity of volatile organic compounds are different at different pyrolysis temperatures.Consequently, the treads of the relative contents of organic volatiles will affect each other, which would be different from the treads of their actual contents [26].The relative contents of aliphatics and O-compounds (Figure 4(a)) reach a maximum value at 600°C, and the relative content of aromatics reaches a maximum value at 800°C.Without considering other reactions, the higher the pyrolysis temperature, the  higher the degree of thermal decomposition in coal.Therefore, the higher the temperature, the higher the contents of organic volatile matter.But as the pyrolysis temperature increasing, the relative contents of organic volatiles change irregularly.After modified by equations ( 1) and ( 2), the corrective contents of group compositions during singlestep pyrolysis process were obtained (Figure 4(b)).As the pyrolysis temperature increasing, the change regularity of the corrective contents of organic volatiles was obviously enhanced, which can better explain the content changes of organic volatiles during coal pyrolysis at different pyrolysis temperatures.
The number of carbons in the chains of aliphatics concentrates on 11 and 32, and the other compounds are mainly N-containing compounds.With the increase of the single-step pyrolysis temperature, the contents of organic volatiles should increase successively.When the singlestep pyrolysis temperature is higher than 600°C, the content of aliphatics did not continue to increase.Possible reasons are as follows: at high temperature, the pyrolysis rate of coal increases, releasing more organic volatiles, which leads to the overload of gas chromatography column and cannot effectively separate and detect the products [18].The released organic volatiles affect each other.For example, when single-step pyrolysis temperature is higher than 600°C, a large amount of aromatics are released, resulting in a high peak strength of aromatics, which may cover the peak strength of aliphatics.

Emission characteristics of aromatics during singlestep pyrolysis process
Aromatic hydrocarbons are subdivided into monocyclic, ice ring, and polycyclic structures (Figure 5a).With the pyrolysis temperature increasing from 300 to 1,000°C, the content of mono-ring aromatics first increases and reaches a maximum value (23.3 wt%) at 800°C and then decreases, which gives a single peak trend.This result indicates that with the increase in pyrolysis temperature, more aromatic ring macromolecule structures in SD coal were broken into mono-ring fragments.However, too high temperature may cause the mono-ring fragments to be cracked into aliphatics or condensed into multi-ring aromatics [9], which can be inferred from the increases in the contents of aliphatics and multi-ring aromatics at a high temperature.With the increase in pyrolysis temperature, the content of bicy-ring aromatics increases in general.The change in the content of bicy-ring aromatics is caused by its own ring-opening reaction and the cracking of multi-ring aromatics [19].The content of multi-ring aromatics disappears at 800°C, which can be attributed to the cracking of multi-ring aromatics, the interaction between organic volatiles or the limitation of the GC/ MS column in the detection of multi-ring aromatics [18].The non-fuel use of coal, for example, the raw materials for obtaining chemicals, is an important utilization method.The content of typical compounds is different at different pyrolysis temperatures, which can provide guidance for the directional transformation of coal [37].Figure 5b shows the typical aromatics during the single-step pyrolysis process.The contents of benzene, toluene, and naphthalene reach the maximum values in the organic volatiles at 800°C, while o-xylene, ethylbenzene, and phenanthrene disappear.It is speculated that the ring-opening reaction of phenanthrene, the dealkylation, and cyclization of o-xylene and ethylbenzene may be responsible.With the further increase of pyrolysis temperature, the contents of benzene, toluene, and naphthalene decrease, while the content of phenanthrene increases, indicating that aromatic ring condensation is dominant at a high temperature [19].When the pyrolysis temperature is less than 700°C, the content of phenolics increases with the increase in pyrolysis temperature.At 800°C, together with a decrease in phenolics, a maximum value of mono-ring aromatics has been observed (Figure 5).These trends suggest that one possible pathway for the formation of mono-ring aromatics is the dehydroxylation of phenolics.At 400℃, the content of released carboxylic acid reached the maximum (1.9 wt%).At a higher pyrolysis temperature, the content of carboxylic acids decreases.The reasons are as follows: on the one hand, the structure of most carboxylic acids in SD coal has broken at a low temperature and its amount will not continue to increase with the increase in pyrolysis temperature, while the contents of other compounds increase with the increase in pyrolysis temperature, which has a great influence on the peak strength of the carboxylic acids [26].On the other hand, carboxylic acids are decarboxylated or hydrogenated to aromatics or phenolics, which reduce their contents [38].
Figure 6b shows typical phenols in the one-step pyrolysis process.Typical phenols appear when the pyrolysis temperature is higher than 400℃.For example, at 900℃, the content of phenol reaches the maximum (4.1 wt%), that of p-cresol reaches the maximum (3.1 wt%) at 800℃, and that of o-cresol disappears at 800℃.So it can be speculated that o-cresol is easier to be dehydroxylated to form aromatics than phenol and p-cresol.

Organic volatiles during multi-step pyrolysis
SD coal was multi-step pyrolyzed to 300, 400, 500, 600, 700, 800, 900, and 1,000°C by in-line Py-GC/MS, and the ion chromatograms are shown in Figure 7. Multi-step pyrolysis is a batch of cleavage reactions of a sample, which is used to analyze the difference or difficulty degree of coal pyrolysis at different temperature intervals [2].For example, different from the single-step pyrolysis at 500°C, the organic volatiles from multi-step pyrolysis at 500°C are actually the release of organic volatiles in the temperature range of 400-500°C.Figure 8(a) shows the relative content of group compositions during the multi-step pyrolysis process.After modified by equations ( 3) and ( 4), during the multi-step pyrolysis process, the corrected content of composition was obtained (Figure 8(b)).In different pyrolysis temperature ranges, with the increase in pyrolysis temperature, the composition of pyrolysis volatile matter is irregular.The corrective contents (Figure 8(b)) are close to the real values.Aliphatics and aromatics (Figure 8(b)) were released at each pyrolysis temperature range.The highest content of aliphatics was obtained at 300-400°C and the temperature range of 600-800°C is the main temperature for the release of aromatics, which is in accord with the finding [19].The highest content of O-compounds was obtained at the pyrolysis temperature range of less than 300°C, and the O-compounds are mainly carboxylic acids.The higher content of O-compounds was obtained at 500-600°C, and the O-compounds are mainly phenolics [36].The O-compounds disappear when the pyrolysis temperature is higher than 700°C, which indicates that the O-containing structure in SD coal can be completely broken before 700°C.Other compounds are mainly N-containing compounds, and the main pyrolysis temperature is below 300°C.From the main pyrolysis temperature range of different compounds, it can be inferred that the order of the difficulty of compounds escaping from SD coal is carboxylic acids, N-containing compounds < aliphatics < phenolics < aromatics.

Emission characteristics of aromatics during multistep pyrolysis process
Figure 9a shows the correct content of aromatic hydrocarbons during the multi-step pyrolysis.With the pyrolysis temperature increasing from 400 to 800°C during multi-step pyrolysis, the content of mono-ring aromatics increases and reaches a maximum value (3.5 wt%) at 800°C and its main release temperature range is concentrated at 600-800°C, which indicate that pyrolysis at a high temperature is beneficial to the formation of mono-ring aromatics.Bicy-ring aromatics would be cracked into mono-ring aromatics at higher temperatures, so the content of mono-ring aromatics increases and the content of bicy-ring aromatics decreases.When the multi-step pyrolysis temperature is higher than 800°C, the residual semi-coke is polycyclic aromatic hydrocarbons, which makes it difficult to thermally crack [20].Compared with the single-step pyrolysis, multi-step pyrolysis was taken place in the temperature range [37], which is the cutting temperature of organic volatiles in the single-step pyrolysis.The total amount of volatile organic compounds decreased, and the interaction between volatile organic compounds weakened.Figure 9b shows the corrective content of typical aromatic compounds during multi-step pyrolysis process.Benzene is produced when the multi-step pyrolysis temperature is higher than 600°C [39].The favorable pyrolysis temperature range for toluene and o-xylene is mainly at 500-600°C, while their contents continue to increase at a high temperature during the singlestep pyrolysis process (Figure 5b), which are caused by dehydroxylation, alkylation, and ring opening reactions.There is no naphthalene in the volatile products during multi-step pyrolysis, which suggests that naphthalene may mainly derived from the chemical reactions between pyrolysis volatiles during the single-step pyrolysis.

Emission characteristics of O-compounds during multi-step pyrolysis process
Figure 10 shows the corrective content of O-compounds during multi-step pyrolysis process.With the multi-step pyrolysis temperature increasing from 300 to 700°C, the content of phenolics first increases and reaches a maximum value (1.5 wt%) at 600°C and then decreases, which gives a single peak trend, and this result is consistent with the literature [21].With the increase in multi-step pyrolysis temperature, the content of carboxylic acids decreases gradually and the main release temperature range of aliphatic chain carboxylic acids is concentrated at <300°C.At a higher temperature, carboxylic acids are mainly aromatic chain carboxylic acids.So the cracking order of O-containing bonds in coal is C]O bonds with aliphatic side chains > aromatic C-O bonds > C]O bonds with aromatic side chains [17,34].Typical O-compounds during multi-step pyrolysis include phenol and o-cresol without p-cresol, which suggests that p-cresol in the single-step pyrolysis volatiles (Figure 6b) may be derived from isomerization of o-cresol or alkylation of phenol.The initial escape temperature ranges of o-cresol and phenol are 400-500 and 500-600°C, respectively, which indicates that oxo-aromatic structure containing substituted alkyl groups is easier to be broken from the SD coal structure.

Pyrolysis process and mutual reactions of organic volatiles
Py-GC/MS can provide molecular information and the pyrolysis volatiles are related to the original macromolecular structure of coal.The multi-step pyrolysis can be used to analyze the pyrolysis volatiles at different temperature intervals.Compared with multi-step pyrolysis, the total amount of volatiles is more and the mutual reactions between organic volatiles become easier.Figure 11 shows the pyrolysis process and mutual reactions of organic volatiles.Compared with coal skeletons, low molecular weight compounds have higher activity in the process of coal pyrolysis [17].When the multi-step pyrolysis temperature is lower than 400°C, the main released volatiles are the low molecular compounds [40], mainly including polycyclic aromatic hydrocarbons [38], carboxylic acids, and N-containing compounds.Carboxylic acids are alkyl carboxylic acids and aryl carboxylic acids.Alkyl carboxylic acids are concentrated in C14-C18 alkyl acids.Aryl carboxylic acid is mainly phthalic acid.The typical N-containing compound is oleamide.400-600°C is the main pyrolysis temperature range during the multi-step pyrolysis process, and at this temperature range, large amounts of phenolics and monoring aromatics and a small amount of multi-ring aromatics were produced.When the multi-step pyrolysis temperature is higher than 600°C, the degree of aromatic ring condensation in the residual semi-coke increases and multi-ring aromatics are released.Meanwhile, there is a dealkylation reaction and the benzene is released.
During the multi-step pyrolysis process, only phenol and o-cresol were found in the pyrolysis products, but no p-cresol.However, the content of a single pyrolysis product of p-cresol is obvious, reaching the maximum at 800℃, indicating that p-cresol mainly comes from the interaction of organic volatiles, rather than the direct fracture of SD coal structure.Corresponding to the decrease in the phenol content and the disappearance of o-cresol at 800°C, it is presumed that p-cresol may come from the alkylation of phenol or isomerization of o-cresol [41].In the multi-step pyrolysis process, toluene is mainly released at 400-600℃, but at higher temperature, its content continues to increase, reaching the maximum at 800℃, and then decreases.So toluene may be produced by dehydroxylation of phenolics and dealkylation of mono-ring aromatics [21].Benzene is mainly released at 500-800°C during the multi-step pyrolysis process and reaches a maximum value at 800°C and then decreases, which matches well with the previous study [9].O-xylene disappears at 800°C during multi-step pyrolysis.When the single-step pyrolysis temperature is higher than 800°C, the content of o-xylene and ethylbenzene is obvious, while the content of benzene and toluene decreases.It is indicated that the dealkylation reaction was dominant at 800°C and the alkylation reaction was dominant at >800°C.There is no naphthalene in the multi-step pyrolysis products, and the content of naphthalene reaches a maximum value at 800°C during the single-pyrolysis process.Phenanthrene and pyrene were released below 800°C during the multiple pyrolysis process and disappeared at 800°C.It is speculated that naphthalene may be derived from the ring-opening of polycyclic aromatics and the cyclization of monocyclic aromatics.

Conclusion
It is necessary to strengthen basic research on the rapid pyrolysis of coal.In this study, SD coal was single-step and multi-step pyrolyzed by in-line Py-GC/MS at the temperature range of 300-1,000°C.Using the proposed modified method of pyrolysis rate multiplying relative content, the change regularity of the corrective content of pyrolysis products with temperatures during single-step pyrolysis was obviously enhanced.400-600°C is the main pyrolysis temperature region during the multi-step pyrolysis process.In this temperature range, large amounts of phenolic and mono-ring aromatics, as well as a small amount of multi-ring aromatics, were produced.One possible pathway for the formation of mono-ring aromatics is the dehydroxylation of phenolics.Naphthalene may mainly be derived from the chemical reactions (the ring-opening of multiring aromatics and the cyclization of mono-ring aromatics) between pyrolysis volatiles during single-step pyrolysis.p-cresol in the organic volatiles during the single-step pyrolysis process is mainly derived from the mutual reactions of organic volatiles rather than the direct fracture of SD coal structure and it is presumed that p-cresol may come from the alkylation of phenol or isomerization of o-cresol.

Figure 2 :
Figure 2: Pyrolysis rates of SD coal during single-step and multi-step pyrolysis process.

Figure 4 :
Figure 4: Group compositions of organic volatiles during single-step pyrolysis process (a) relative content, (b) corrective content.

Figure 5 :Figure 6 :
Figure 5: Corrective content of aromatics (a) and the typical compounds (b) during single-step pyrolysis process.

Figure 9 :Figure 10 :
Figure 9: Corrective content of aromatics (a) and the typical compounds (b) during the multi-step pyrolysis process.

Figure 11 :
Figure 11: Pyrolysis process and mutual reactions of organic volatiles.