Impact of the Temperature, Pressure, and Particle Size on Tar

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Impact of the Temperature, Pressure, and Particle Size on Tar Composition from Pyrolysis of Three Ranks of Chinese Coals Xiaomin Gong,†,‡ Ze Wang,† Shuang Deng,§ Songgeng Li,*,† Wenli Song,† and Weigang Lin† †

State Key Laboratory of Multi-phase Complex Systems, Institute of Process Engineering, Chinese Academy of Sciences, Beijing 100190, People’s Republic of China ‡ University of Chinese Academy of Sciences, Beijing 100049, People’s Republic of China § Chinese Research Academy of Environmental Sciences, Beijing 100012, People’s Republic of China ABSTRACT: Tar compositions from pyrolysis of three ranks of Chinese coals of Hailaer lignite, Fugu sub-bituminous coal, and Liulin bituminous coal under different temperatures and pressures were analyzed via a pyrolyer combined with gas chromatography/mass spectrometry (Py−GC/MS). A nuclear magnetic resonance (NMR) analysis was performed to characterize the coals. It was found that the relative content of polycyclic aromatic hydrocarbons (PAHs) is not as sensitive to temperature as monocyclic aromatic hydrocarbons (MAHs), which increased with increasing the pyrolysis temperature. A decreasing trend of the content of acids/esters was observed for all three ranks of coals. However, the three coals exhibited quite different trends with the temperature in terms of the relative content of aliphatic hydrocarbons. MAHs and PAHs were predominant components in the tar from Liulin bituminous coal. The tars from Hailaer lignite and Fugu sub-bituminous coal were rich in phenolics, which can be detected at a low temperature of 400 °C. In contrast, the phenolics were much less in the tar from Liulin bituminous coal and can merely remarkably be detected at a higher temperature of 600 °C. The contents of phenolics and aliphatic hydrocarbons decreased with increasing the pressure for all three coals, and the contents of MAHs and PAHs showed inverse trends with the pressure. This could result from the enhanced reactions of cyclizations of aliphatics and scissions of the C−OH bond in phenolics by elevated pressure. C19 alkane and p-cresol as model compounds of aliphatics and phenolics were pyrolyzed under high pressures to verify this point. For a larger particle size of Fugu sub-bituminous and Liulin bituminous coals, the contents of aliphatics decreased remarkably, while the contents of MAHs and PAHs were distinctly augmented. This could be attributed to the intraparticle secondary reactions.

1. INTRODUCTION

However, knowledge on this aspect is still limited, although much effort has been made. In China, bituminous coal, sub-bituminous coal, and lignite comprise up to 85% of the coal reserves. Accordingly, three typical Chinese coals (bituminous coal, sub-bituminous coal, and lignite, respectively) are selected in this work for pyrolysis tests. A pyrolyzer combined with gas chromatography/mass spectrometry (Py−GC/MS)19−24 is employed to perform the pyrolysis tests. Tar compositions from pyrolysis of the three ranks of coals under different pyrolysis conditions (temperature, pressure, and particle size) are compared in an attempt to establish a correlation between tar composition and coal rank. A nuclear magnetic resonance (NMR) analysis is conducted to characterize the coal, and a selection of relevant model compounds are pyrolyzed to elucidate further the reactions contributing to the overall thermal conversions of coal.

Coal is the largest fossil fuel energy resource in China, accounting for 68% of the primary energy consumption. In comparison to natural gas and oil, coal emits more pollutants, such as SO2, NOx, and particulate matters, during combustion, leading to more severe environmental impact. Thus, an efficient and clean use of coal is urgently needed for sustainable economic development of China. As the first step of coal thermal conversion, pyrolysis can be applied to produce gas, tar, and char simultaneously. Various processes for the production of tar based on pyrolysis in the absence of oxygen have been developed by many research institutions and coal companies in China, such as rotary kiln pyrolysis,1 downer-moving bed coupling pyrolysis,2 and downer pyrolysis.3 The influence parameters, such as the temperature,4−7 heating rate,8 pressure,9−12 and particle size,13−15 have been studied extensively to optimize the process. In addition to these processes, the nature of coal plays an important role in the determination of the compositions and yields of tar. Primary tar composition can strongly be correlated with coal properties.16−18 Tar composition may exhibit different trends with pyrolysis conditions for different types of coal. A comprehensive understanding of the correlation would help with the selection of a suitable pyrolysis technique for a specified coal feedstock because it is not possible for one single pyrolysis process to efficiently deal with different types of coal. © 2014 American Chemical Society

2. EXPERIMENTAL SECTION 2.1. Coal Samples. Three Chinese coals were used in the experiments, which were obtained from Hailaer (HLE) of Inner Mongolia and Fugu (FG) and Liulin (LL) of Shanxi. The proximate and ultimate analyses of the coals are given in Table 1. HLE coal has high volatile content, which is classified as lignite. LL and FG coals can Received: April 30, 2014 Revised: June 27, 2014 Published: July 8, 2014 4942

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Table 1. Proximate and Ultimate Analyses of Coals proximate analysis (wt %, dry) coal sample

volatiles

LL FG HLE

20.8 35.0 50.3

ultimate analysis (wt %, daf)

ash

fixed carbon

C

H

N

S

O

11.4 4.5 10.1

67.8 60.5 39.6

88.5 81.2 74.2

4.8 5.0 5.6

0.9 1.3 0.1

0.5 0.3 0.2

5.3 12.2 19.9

be grouped into bituminous and sub-bituminous coals, respectively, because of their relatively low contents of volatile matters and high contents of fixed carbon. The coal samples were ground and sieved to 120−140 μm particle size and dried at 105 °C for 1 h prior to experimentation if no other indication is made. 2.2. Apparatus. Pyrolysis was carried out at a platinum filament pyrolyzer (CDS 5200, Oxford, PA) directly connected to a GC/MS analyzer (Trace GC, ISQ MS, Thermo Scientific Co.). Py−GC/MS can be operated in two modes: Py mode and trap mode. In the Py mode, the volatiles released from coal directly go to GC/MS through a transfer line heated at 300 °C with nitrogen as the carrying gas. In the trap mode, the volatiles are first captured by an adsorption trap at 40 °C and then desorbed at an elevated temperature (300 °C) for analysis. Pressurized pyrolysis can only be operated in the trap mode because of the instrument limitation. For each run, a coal sample of 0.5 ± 0.015 mg was loaded in the pyrolyzer and then heated to a desired temperature at a rate of 20 °C/ms under a N2 atmosphere, holding for 15 s (except for studying the particle size effect) at a set temperature ranging from 400 to 800 °C. The pyrolysis at a heating rate of 20 °C/ ms can be categorized into flash pyrolysis. The tar compositions derived from flash pyrolysis can give more valuable information on the coal structure from which it originates.25 To study the effect of the particle size, the coal particle size ranging between 0.9 and 1 mm was used for comparison. In this case, the holding time is prolonged to 4 min to ensure the complete pyrolysis. To eliminate the pyrolysis time effect, the same holding time was also applied to the small particle (120−140 μm) pyrolysis in studying the particle size effect. All of the experiments were conducted 3 times. The relative errors are less than ±5%. A WAX column (30 m × 0.25 mm × 0.25 μm) is applied in GC/ MS. The column temperature is programmed as follows: start at 40 °C, with holding for 3 min, increase to 100 °C by 6 °C/min, with holding for 1 min, then increase to 180 °C by 6 °C/min, with holding for 2 min, further increase to 240 °C by 6 °C/min, with holding for 5 min, and finally increase to 280 °C by 6 °C/min, with holding for 1 min. The flow rate of the carrier gas is 1.2 mL/min with a split ratio of 115:1. The interface temperature between GC and MS is kept to 280 °C. The ion source temperature is 250 °C with an electron ionization (EI) energy of 70 eV. The mass range is set from 2 to 500. The peaks can be identified by matching with data in the National Institute of Standards and Technology (NIST) library. 2.3. 13C Cross-Polarization/Total Suppression of Sidebands (CP/TOSS) NMR Analysis. 13C CP/TOSS NMR spectra of coal samples were obtained on a Bruker Avance III 400 NMR spectrometer. It operated at a 13C frequency of 100.37 MHz and a magic angle spinning rate of 5 kHz. Pulse widths of 3.75 ms were used with a 1 s recycle time and a 3 ms contact time.

Figure 1. 13C NMR spectra of HLE, LL, and FG coals.

carboxyl (165−188 ppm), and carbonyl (188−220 ppm). The bridgehead aromatic carbon atoms are distributed around 129− 137 ppm. In the aliphatic region (0−90 ppm), the methylene group in saturated alkyl chains is around 30 ppm. The signal at 15 ppm is caused by the methyl group. It can be seen from Figure 1 that, in comparison to HLE lignite and FG sub-bituminous coal, the aromatic peaks of LL bituminous coal are narrower, with the loss of shoulders around 160 ppm (Ar−O). A higher aliphatic peak intensity can be observed on the spectra of HLE lignite and FG sub-bituminous coal, especially at 30 ppm (methylene group). For LL, FG, and HLE coals, the numbers of oxygen-bound carbons per 100 atoms are 12, 20, and 25, respectively. Most of the oxygen atoms are bound to aromatic carbons. Fractions of aliphatic carbon and aromatic carbon are shown in Table 2. As expected, LL bituminous coal has the highest aromatic carbon (fa) and the lowest aliphatic carbon ( fal). Aromatic carbon and aliphatic carbon in FG sub-bituminous coal are nearly equivalent to those in HLE lignite. The amount of bridgehead carbon (faB) exhibits a strong correlation with the coal rank. It increases as the rank of the coal increases. The mole ratio of aromatic bridgehead carbon to the aromatic carbon in an aromatic ring (χb = faB/fa′) is an important parameter in the description of the aromatic structure rings in coal because it can be used to estimate the aromatic cluster size.26 The χb value of 0.28 (LL bituminous coal) indicates an average cluster size of 14 carbon atoms, corresponding to 3 rings. In HLE and FG samples, their average cluster sizes are 7 and 10 carbon atoms, corresponding to 1 and 2 rings, respectively. 3.2. Effect of the Temperature. The effect of the temperature on tar composition is shown in Figure 2. The compounds can be classified into five groups: aliphatic hydrocarbons, phenolics, monocyclic aromatic hydrocarbons (MAHs), polycyclic aromatic hydrocarbons (PAHs), and acids/ esters. After pyrolysis at 400 °C, very few products were detected by GC/MS for all three coals. Only a trace amount of phenolics was detected for HLE lignite. No phenolics were found for LL and FG coals at 400 °C. The results at 400 °C are not given in Figure 2 because few compounds are released. All of the tested coals exhibit the same trend with the temperature in terms of phenolics content in tar. The content of phenolics in tar significantly increases with an increase in the temperature in the relatively low temperature range (400−600

3. RESULTS AND DISCUSSION 3.1. Characterization of Coals by 13C CP/TOSS NMR. 13 C CP/TOSS NMR analyses were performed for LL, FG, and HLE coal to examine their chemical structures. Figure 1 presents the 13C CP/TOSS NMR spectra of the three coal samples. The spectra of coal can clearly be divided into two regions: aliphatic carbon (0−90 ppm) and aromatic carbon (90−165 ppm). Oxygen-linked carbon species include oxygenated aliphatic carbon (50−90 ppm) and oxygenated aromatic carbon, such as phenols/ether (150−165 ppm), 4943

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Table 2. Aliphatic Carbon and Aromatic Carbon Distributions and H/C Ratio of the Samplesa sample

fa

fa ′

fac

fap

fa B

χb

fal

falH

fal*

falO

H/C

LL FG HLE

0.76 0.66 0.65

0.72 0.61 0.57

0.04 0.05 0.08

0.04 0.08 0.1

0.20 0.11 0.09

0.28 0.18 0.15

0.24 0.34 0.35

0.13 0.22 0.22

0.07 0.06 0.06

0.04 0.06 0.07

0.65 0.74 0.91

a fa, total aromatic carbon; fa′, aromatic carbon in an aromatic ring; fac, carboxyl COO, ketones, quinones, and aldehyydes; fap, O-subsitituted aromatic C in phenols and aromatic ethers; faB, aromatic bridgehead carbon; fal, total aliphatic carbon; fal*, terminal methyl carbon; falH, CH or CH2; and falO, O-substituted aliphatic carbon.

Figure 2. Effect of the temperature on tar compositions at atmospheric pressure.

Figure 3. Variation of the amount of acids/esters with the temperature at atmospheric pressure.

°C). With a further increase in the temperature, phenolics appear to level off or slightly decrease. This could be the result of the competition between the release and the cracking reactions of phenolics. The rapid increase of cracking reaction rates at high temperatures results in the decrease of phenolics.27 The phenolics content for the tested coals is in the order of HLE > FG > LL. This is in agreement with the results from NMR analysis, which shows that HLE lignite has the largest number of phenolic carbon atoms, followed by FG subbituminous coal and LL bituminous coal. The phenolic products from pyrolysis of HLE lignite at 500 °C mainly consist of phenol, alkylphenol, methoxyphenol, indenol, naphthalenol, and catechol, while phenol and methoxyphenols do not appear in the FG volatiles. In the volatiles released from LL bituminous coal, only alkylphenols (methylphenol, dimethylphenol, and methylethylphenol) are detected over 600 °C. Almost no phenolics are found in pyrolysis of LL bituminous coal even at 500 °C. According to Nali et al.,28 monocyclic aromatic segments linked by either one aryl or two aryl-ether bridge(s) could be the precursors of phenolics. Thus, we may speculate that Ar−O−CH2 may be the origin of phenolics from FG sub-bituminous coal and HLE lignite. The phenolics from LL bituminous coal may be originated from the Ar−O−Ar structure.28 Higher energy is needed to break this bond compared to Ar−O−CH2. This could explain that the phenolics from LL coal are released at a relatively higher temperature in comparison to those from HLE and FG coals. The content of acids/esters shows a general declining trend with an increasing temperature. HLE lignite has the highest content of acids/esters in tar, followed by FG sub-bituminous coal and LL bituminous coal. However, their absolute amounts (expressed as integrated intensity, shown in Figure 3) change very little with the temperature, indicating that acids/esters can be completely released at the low temperature of 400 °C.

According to the host/guest theory,29,30 coal can be described as consisting of a mobile fraction and a macromolecular carbon skeleton. The mobile fraction consists of relatively small molecules physically trapped in the macromolecular network. The mobile fraction is partly extractable in organic solvents and tends to be released at low temperatures of 300−400 °C.7 Accordingly, we speculate that most of the acids/esters exist in the mobile fraction beyond the macromolecular network because most of them are released at 400 °C. MAHs and PAHs are dominant in the tar from LL bituminous coal. The PAHs from HLE lignite and FG subbituminous coal are less than those from LL bituminous coal. There are almost no 3-ring PAHs found in the tar from HLE lignite. This observation could be explained by the abovementioned results from NMR. The average aromatic cluster size in LL coal is 3 rings. The cluster size in the samples of FG and HLE is no more than 2 rings. For all three coals, the content of the MAHs increases with the increasing temperature. The increase of MAHs may result from the conversions of oxygen-containing aromatics, such as phenolics and phenyl ethers at high temperatures.31 The decrease of the phenolics content at high temperatures may indicate this point. The content of PAHs is not as sensitive to the temperature as MAHs. The contents of aliphatic hydrocarbons exhibit quite different trends for the three coals. For HLE, the aliphatics content increases with an increase in the temperature, while the reverse is true for LL coal. Little change is found for FG coal. GC/MS results show that alkanes and alkenes are predominant in aliphatics for HLE and FG coals. The ratio of alkenes to alkanes for HLE is higher than that for FG, as shown in Figure 4. The ratio increases with the temperature. The increasing extent for HLE is greater than that for FG. Alkenes and low-molecularweight alkanes are most likely derived from the cleavage of C− C of the aliphatic side chains, such as the bond of Car−Cal or 4944

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Figure 4. Ratio of alkenes to alkanes from HLE and FG at atmospheric pressure.

Figure 6. Variation of the amount of cyclanes from LL coal with the temperature at atmospheric pressure.

Cal−Cal. Large-molecular-weight alkanes (>C19) may come from the mobile fraction of coal, as indicated by nearly invariation of the amount of C > 19 (expressed as integrated intensity) with the temperature, as shown in Figure 5. The

Figure 7. Effect of the pressure on tar compositions at different pyrolysis temperatures. Figure 5. Variation of the amount of alkanes (C > 19) with the temperature at atmospheric pressure.

is speculated that the increase of MAHs may be due to cyclizations of aliphatics and scission of phenolics. To confirm this point, C19 alkane and p-cresol selected as model compounds of aliphatics and phenolics were pyrolyzed at a temperature of 800 °C under high pressures (shown in Figure 8). It can be seen that benzene (RT = 3.59 min), toluene (RT = 5.26 min), and phenol (RT = 25.77 min) as major products from pyrolysis of p-cresol are identified. Their amounts are more distinct under 9 atm than that under 5 atm. In the pyrolysates of C19 alkane under 5 atm, a variety of compounds, including benzene (RT = 3.40 min), toluene (RT = 5.29 min), dimethylbenzene (RT = 7.18 min), styrene (RT = 10.19 min), C11−C15 alkenes (RT = 7.71, 10.07, 12.39, 14.84, and 17.27 min, respectively), naphthalene (RT = 21.01 min), and methylnapathalene (RT = 23.17 min), are identified. When pyrolyzed under 9 atm, all C19 alkanes (RT = 24.43 min) are almost decomposed. The possible reaction scheme is shown in Figure 9. For the pyrolysis of C19 alkane, the molecule initially cracks to two radical fragments via C−C bond scission and then the chain radical species further converts to cyclanes, benzenes, and naphthalenes through sequential cyclization, dehydration, and polymerization reactions. Comparatively, the pyrolysis of pcresol is more complicated. Toluene and phenol are two major

increase of aliphatics for HLE coal can be attributed to the significant increase of alkenes. Cyclanes and alkanes are major components of aliphatics for LL coal. Figure 6 gives the variation of the absolute amount of cyclanes from LL coal with the temperature. It is seen that it initially increases with the temperature and appears to level off after the temperature of 600 °C, indicating that almost all cyclanes are released at 600 °C. The decreasing trend of the aliphatics content in the LL coal tar is because more other compounds come out at high temperatures and dilute the aliphatics concentration. The order of content of aliphatics in the tars from the three coals is FG > HLE > LL. 3.3. Effect of the Pressure. Figure 7 presents the effect of the pressure on tar composition at different temperatures. It can be seen that the effect of the pressure is not significant at the low temperature of 500 °C, especially for FG coal. At the high temperature of 800 °C, the pressure effect is more profound. For the three coals, the contents of phenolics and aliphatic hydrocarbons decrease with the increasing pressure. The contents of MAHs and PAHs increase with the pressure. It 4945

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Figure 9. Possible reaction mechanisms.

products generated by removal of −OH and −CH3 from the mother molecule, respectively. Some small compounds, such as methane, ethylene, and cyclopentadiene, are also generated via deep pyrolysis. p-Cresol may also isomerize to the intermediate of cyclohepta-2,4,6-trienone first and then convert to the stable product of benzene via the loss of CO. Further combination or polymerization reactions may take place among the obtained products that lead to dimethylbenzene or double-ring aromatic compounds. 3.4. Effect of the Particle Size. Figure 10 gives the effect of the particle size on tar composition. It is interesting to note that the tar from large particles has high contents of MAHs and PAHs and low contents of aliphatics compared to that from small particles for both FG and LL coals. This variation can be attributed to the intraparticle secondary reactions. The volatiles generated during primary pyrolysis experience the migration from the internal of a coal particle to the outer surface. During this migration, they may crack, condense, and polymerize. An increase in the particle size increases the residence time of primary volatiles within the particle. From another perspective, a large coal particle has a relatively lower heating rate compared to a small coal particle. It is generally acknowledged that the volatile yield increases with the heating rate.6 With a decreasing heating rate, the intraparticle residence time of coal volatiles is increased because a small amount of volatiles evolves from coal. The increase of the intraparticle residence time of coal volatiles enhances the intraparticle secondary reactions. The particle size has a quite different effect on the phenolics content in tar from FG and LL coals. For FG sub-bituminous coal, the phenolics content in the tar from pyrolysis of large coal particles is higher than that from small particles, while the reverse is true for LL bituminous coal. Encinar et al.32 also found that phenols vary irregularly with the particle size. Further studies are necessary to give an explanation.

4. CONCLUSION On the basis of the experimental work, major conclusions can be drawn as follows: (1) The tars from HLE lignite and FG sub-bituminous coal are rich in phenolics, which can even be detected at a temperature of 400 °C. The phenolics in the tar from LL bituminous coal can merely be remarkably detected at a higher temperature of 600 °C. (2) Acids/esters can be nearly completely released at 400 °C for all three coals. Acids/esters may come from the mobile fraction of the coal, which is physically trapped in the macromolecular carbon skeleton

Figure 8. Chromatogram of the pyrolysates of p-cresol and C19 alkane at 800 °C: (a) p-cresol at 5 atm, (b) p-cresol at 9 atm, (c) C19 at 5 atm, and (d) C19 at 9 atm. 4946

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that the average aromatic cluster size in LL bituminous coal is 3 rings and the cluster size in both HLE lignite and FG subbituminous coal is no more than 2 rings. (4) The content of aliphatic hydrocarbons shows different trends with the pyrolysis temperature for the three coals. Alkanes and alkenes are predominant in aliphatics from FG sub-bituminous coal and HLE lignite. Cyclanes and alkanes are main components of aliphatics from LL bituminous coal. (5) The contents of phenolics and aliphatic hydrocarbons decrease with increasing the pressure. Inverse trends are observed for MAHs and PAHs. This can be attributed to cyclizations of aliphatics and scission of phenols under high pressures. This kind of effect is more pronounced at high temperatures. (6) The tar from large particles has high contents of MAHs and PAHs and low contents of aliphatics because of intraparticle secondary reactions.

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AUTHOR INFORMATION

Corresponding Author

*Telephone: 86-10-82544815. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work is supported by the joint program of the Natural Science Foundation of China and the Shenhua Group Corporation Limited (51174284), the Chinese Academy of Sciences (CAS)/State Administration of Foreign Expert Affairs (SAFEA) International Partnership Program for Creative Research Teams, and the “Strategic Priority Research Program” of the Chinese Academy of Sciences (XDA07010200). The authors thank Dr. Lufei Jia from Natural Resources Canada’s CanmetENERGY for his contribution to this work.

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Figure 10. Effect of the particle size on tar compositions at atmospheric pressure: (a) FG at 500 °C, (b) FG at 800 °C, (c) LL at 500 °C, and (d) LL at 800 °C.

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