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Molecular composition of soluble organic species in Baiyinhua lignite and their evolution profiles during pyrolysis
Fuel 205 (2017) 192–197
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Molecular composition of soluble organic species in Baiyinhua lignite and their evolution profiles during pyrolysis Yun-Peng Zhao a,⇑, Di Zhang a, You-Jia Tian a, Xin-Fu He b, Hao Lu a, Xing Fan a, Jing-Pei Cao a, Xian-Yong Wei a, Zhi-Min Zong a a b
Key Laboratory of Coal Processing and Efficient Utilization, Ministry of Education, China University of Mining & Technology, Xuzhou 221116, Jiangsu, China School of Chemistry and Chemical Engineering, Xi’an University of Science and Technology, Xi’an, Shaanxi 710054, China
h i g h l i g h t s Most of soluble organic species were released from Baiyinhua lignite (BL) below 500 °C during pyrolysis. Abundant long-chain aliphatic hydrocarbons and acids wrap around the aromatic nucleus of BL. Abundant soluble arenes in BL easily were volatilized during pyrolysis at low temperature. Soluble ketones containing a 1-phenyl-ethanone structure unit in lignite are decomposed into phenols and CO above 400 °C.
a r t i c l e
i n f o
Article history: Received 23 December 2016 Received in revised form 3 May 2017 Accepted 10 May 2017
Keywords: Lignite Molecular composition Soluble organic species Evolution profiles
a b s t r a c t To investigate the molecular composition of soluble organic species in Baiyinhua lignite (BL) and their evolution profiles during pyrolysis, BL was firstly pyrolyzed in a fixed-bed reactor, affording gaseous products, tars and chars, then BL and its chars were thermally dissolved in supercritical methanol affording soluble portions (SPs). The composition and structural characteristics of the SPs and tars were analyzed with a gas chromatograph/mass spectrometer. The results indicate that most of the soluble organic species can be released from BL below 500 °C during pyrolysis. Abundant soluble long-chain aliphatic hydrocarbons and carboxylic acids wrap around the aromatic nucleus in BL. According to the difference between the composition and structural characteristics of the SPs and tars, the evolution profiles of the soluble organic species from BL pyrolysis were discussed in detail. Ó 2017 Elsevier Ltd. All rights reserved.
1. Introduction Lignites account for 40–50% of global total coal reserves [1]. High water and oxygen contents along with the resulting low calorific value limit lignite application in traditional utilization technologies, such as combustion, gasification, and liquefaction, while the high volatile matter and oxygen-bridged bond contents are beneficial to get transportation fuels or the raw materials of high value-added chemicals from lignites under mild conditions [2,3]. Studies on lignite pyrolysis have received considerable attentions [4,5]. Insight into molecular compositional information of organic species in lignites and their evolution profiles during pyrolysis is important to improve pyrolysis technologies, design pyrolysis reactors, and reveal the formation mechanism of the organic species from lignite pyrolysis. ⇑ Corresponding author. E-mail address: [email protected] (Y.-P. Zhao). http://dx.doi.org/10.1016/j.fuel.2017.05.041 0016-2361/Ó 2017 Elsevier Ltd. All rights reserved.
In past decades, the composition of organic species in lignite was investigated based on the analyses of lignites and the products from pyrolysis and liquefaction [6–10]. Nevertheless, the analyses of lignites only demonstrate the element composition and occurrence forms, and the functional groups distribution of organic species in lignites, and the analyses of the products from pyrolysis and liquefaction cannot reflect the original molecular composition of the organic species for the complex reactions occurred during these processes. Based on the two phases structure concept of coals, coal is composed of a large three-dimensional cross-linked macromolecular network structure (MNS) containing polycyclic aromatic clusters connected by bridged bonds (fixed phase) and the soluble part embedded in the MNS (mobile phase) [11]. The thermal dissolution of low rank coals in suitable solvents not only extracted out the mobile phase of organic species, but also destroyed the weak bridged bonds in the MNS of coals [12–14]. Therefore, thermal dissolution in suitable solvents is an effective
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method to isolate the organic species in lignites for further analyzing their molecular composition. Although the pyrolysis behavior and products distribution of lignites have been widely investigated [15–19], few reports focused on the evolution profiles of organic species in lignites during pyrolysis. Some studies have been conducted to investigate the evolution profiles of coal model compounds through experiment and theoretical simulation [20–22]. Nevertheless, the pyrolysis of coal model compounds only demonstrated the dissociation of certain chemical bonds but it is hardly to reveal the evolution profiles of original organic species in lignites [23,24]. Lignite accounts for about 13% of China’s total coal resource and is playing an important role in the energy supply and chemical feedstock with the depletion of petroleum and high-quality coal resources because of their abundance, easy access, and low mining cost [25]. In this work, a typical lignite, Baiyinhua lignite (BL) from Inner Mongolia Autonomous Region, was subjected to pyrolysis at different temperatures to afford gaseous products, tars and chars, then BL and its chars were thermally dissolved in methanol to afford soluble portions (SPs). The tars and the SPs were analyzed by using a gas chromatographic mass spectrometry (GC/MS) in detail for identifying the molecular composition of the soluble organic species in BL and their evolution profiles during pyrolysis.
ditions was repeated at least 3 times to control the errors of yields less than ±5%. 2.3. Thermal dissolution of BL and its chars
2. Experimental
Thermal dissolution experiments were conducted in a 100 mL stainless-steel, magnetically stirred autoclave, in which, 2 g BL or its chars and 40 mL methanol were placed. Then the autoclave was sealed and the air inside the autoclave was replaced with nitrogen. Afterwards, the autoclave was heated to 300 °C at 10 °C/min by an external electric furnace, and held for 1 h. Then the autoclave was cooled down to the room temperature in a water bath. The reaction mixture was taken out as cleanly as possible with methanol from the autoclave, filtrated through a Teflon membrane filter with 0.45 lm of pore size, and repeatedly washed with methanol to afford filtrates and thermal dissolution residue. The filtrates were distilled with a rotary evaporator to remove solvent under reduced pressure to afford SPs. The SP from BL was named as SPBL, while the SPs from the chars were named as SP300-700 based on the temperature of the char obtained. The yields of the SPs from both BL and chars were calculated according to the formula: mSP/ mBL,daf, where mSP and mBL,daf denote the mass of SPBL (or SP300700) and BL. Each experiment under the same conditions was repeated at least 3 times to control the errors of yields less than ±5%.
2.1. Materials
2.4. Characterizations
BL was collected from Baiyinhua 1# basin (118°300 –400 E, 44°7 00 –750 N) in Inner Mongolia Autonomous Region, China and pulverized to pass through a 200-mesh sieve (<74 lm) followed by desiccation in a vacuum at 80 °C for 24 h before use. Table 1 lists the results of the proximate and ultimate analyses of BL. Methanol purchased from Sinopharm Chemical Reagent Co., Ltd is an analytical reagent (>99.8%) and was distilled with a Büchi R-134 rotary evaporator prior to use.
All the tars and the SPs were analyzed with a Hewlett-Packard 7890/5975 gas chromatograph/mass spectrometer (GC/MS), which was equipped with a capillary column coated with HP-5 (cross-link 5% PH ME siloxane, 60 m length, 0.25 mm inner diameter, 0.25 lm film thickness) and a quadrupole analyzer operated in electron impact (70 eV) mode. Compounds were identified by comparing mass spectra with NIST11 library data. The relative content (RC) of each compound was determined by the normalization method of peak area, i.e., the peak area of the compound divided by the sum of the peak areas of all identified compounds in the total ion chromatogram. The composition of gaseous products was analyzed by 5A molecular sieves and GDX502 columns using a 7890T gas chromatograph with a thermal conductive detector.
2.2. Pyrolysis of BL Pyrolysis experiments were carried out in a vertical fixed-bed reactor with the inner diameter of 12 mm and the heating zone of 10 cm. The schematic diagram of apparatus is shown in Fig. S1. The reactor loaded with 5 g of coal sample was heated to the desired temperature (300–700 °C) within about 10 min by a preheated furnace and then kept at that temperature for 30 min. The volatile matter during BL pyrolysis was brought out by highpurity nitrogen from the reactor to a cool trap, where the liquid products (tar plus water) were collected. The residence time of volatile matter in the reactor is about in the range of 1–1.8 s according to the pyrolysis temperature. The solid char was removed from the reactor after the experiment and weighted. The tar and water were separated by the method of American Society for Testing and Materials (ASTM) D95-05e1 (2005). The tar, char and water yields were calculated on the basis of the weights of char, tar, and water, and the gas yield was calculated by difference. The tar and char obtained at 300–700 °C were named as Tar300-700 and Char300-700, respectively. Each experiment under the same con-
3. Results and discussion 3.1. Product distribution of BL during pyrolysis As shown in Fig. 1, the char yields decrease with the increase of pyrolysis temperature, which are ascribed to the more release of organic species or the deeper decomposition of coal MNS at higher temperature. The tar yields increase with temperature at range of 300–600 °C, then decrease with the further increase of temperature. The water yields are on the rising trend with temperature, whereas the water yield at 600 °C is slightly lower than that at 500 °C. Pyrolysis water mainly comes from the decomposition of oxygen-containing functional groups in coals and the polymerization of oxygen-containing radicals [26]. It was reported that the cleavage of bonds between hydrogen and aliphatic carbon (HCal),
Table 1 Proximate and ultimate analyses (wt.%) of BL. Proximate analysis
Ultimate analysis (daf)
Mad
Ad
Vdaf
C
H
N
35.07
17.01
40.60
69.91
5.06
1.44
Mad: moisture (air dried base); Ad: ash (dry base, i.e., moisture-free base); Vdaf: volatile matter (dry and ash-free base);
a
By difference.
S
Oa
1.51
22.08
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80
60
Yield (wt.%, daf)
Yield (wt.%, daf)
8
c ha r
40
gas 20
6
4
2
0 300
tar water 400
500
600
700
0
SPBL SP300 SP400 SP500 SP600 SP700
Temperature ( oC)
Fig. 2. SP yields of BL and its chars.
Fig. 1. Product distribution from BL pyrolysis.
3.3. Group component distribution of tars and SPs and between hydrogen and aromatic carbon (HCar) in the organic species of coals results in the formation of hydrogen radicals during pyrolysis [27]. The cleavage of HCal bonds is mainly ascribed to the degradation of the coal MNS and the dehydrogenation of naphthenic structure at low temperature, while the cleavage of H–Car bonds is originated from the condensation of aromaticrings structures in coals at high temperature [28]. Abundant hydrogen radicals occurred for the condensation of aromatic-ring structures in BL above 500 °C. On the one hand, the mutual combination of these hydrogen radicals accelerated the increase of H2 yield with temperature (Fig. S2a). On the other hand, the oxygencontaining radicals were more easily stabilized by these hydrogen radicals to produce tar rather than polymerized each other to generate water and char above 500 °C. Nevertheless, the enhancement of polymerization for the oxygen-containing radicals at 700 °C resulted in the increase of water yield. The gas yields increase with the increase of temperature. As exhibited in Fig. S2a, both H2 and CH4 yields increase with temperature increasing. CO2 is mainly ascribed to the decomposition of carboxyl and carboxylate groups at low temperature and the thermal dissociation of quinones and oxygen-bearing heterocycles at high temperature, while CO is originated from the cleavage of alkyl and aryl ether bonds at low temperature, and the decomposition of phenolic groups at high temperature during pyrolysis [29,30]. Both CO2 and CO yields increase with temperature increasing from 300 to 600 °C then slightly decrease with further increase of temperature, indicating that most of carboxyl groups and ether bonds have been decomposed or cracked below 600 °C. Fig. S2b shows that the yields of C2 and C3 hydrocarbons increase with temperature increasing, while they are quite lower than the yield of CH4 because the alkyl side-chains of the aromatic nucleus and aliphatic groups are prone to form simple hydrocarbons during pyrolysis [31].
As shown in Fig. 3, the compounds detected by GC/MS in the tars can be classified into alkanes, alkenes, arenes, arenols, ketones, furans and organonitrogen compounds (ONCs). The total relative content (TRC) of aliphatic hydrocarbons and arenes in tars decreases, while that of arenols increases with pyrolysis temperature increasing. The TRC of ketones in Tar300 is more than 11%, whereas that in the tars obtained above 300 °C is lower than 3.5%, indicating that the ketones in Tar300 are possibly originated from the release of free ketones embedded in the MNS of BL, while these free ketones easily decompose into other compounds at high temperature. As exhibited in Fig. 4, the compounds identified in the SPs can be classified into alkanes, alkenes, arenes, arenols, ethers, esters, ketones, ONCs and organosulfur compounds (OSCs). None alkanes and alkenes were detected in the SPs from the chars obtained at above 300 °C, which is consistent with the high TRC of them in Tar300 and Tar400, indicating that most of soluble alkanes and alkenes can be released below 400 °C during pyrolysis. The TRC of soluble arenes in chars increase with pyrolysis temperature increasing, except that the TRC of arenes in SP500 is slightly lower than that in SP400 because of the high TRC of arenols and esters in SP500. The TRC of arenols in the SPs firstly increase with the increase of pyrolysis temperature below 500 °C, then sharply decrease with the pyrolysis temperature further increasing. The aryl ether bonds in the chars were possibly activated during pyrolysis at low temperature, thus more aryl ether bonds in the organic species of BL were cracked to form arenols during the thermal dissolution of the chars [23].
40
3.2. SPs yields of BL and its chars
30 TRC (%)
Fig. 2 shows that the yield of SPBL is higher than the yields of SP300-700, and the yields of SP300-700 decrease with pyrolysis temperature increasing especially below 500 °C, indicating most of the soluble organic species in BL have been released below 500 °C during pyrolysis. The difference between the yield of SPBL (9.1%) and the yield of SP300 (6.27%) is 2.83%, which is lower than the yield of Tar300 (4.69%). The results are possibly ascribed to the activation effect of pyrolysis process at 300 °C on the MNS of BL. Some new organic species were dissolved out during the thermal dissolution of Char300.
ls areno arenes
20
10
0 300
ket
alkane s alkenes on es
ONCs
furans 400 500 600 Temperature (oC)
700
Fig. 3. Group component distribution of the tars.
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100 OSCs ONCs ketones ethers esters arenols arenes alkenes alkanes
TRC (%)
80 60 40 20 0
SPBL SP300 SP400 SP500 SP600 SP700 Fig. 4. Group component distribution of the SPs.
3.4. Evolution profiles of soluble organic species in BL during pyrolysis As shown in Table S1, a series of long-chain n-alkanes and nalkenes were dissolved in SPBL. The similar constitute of alkanes and alkenes between the tars (Table S2 and 3) and SPBL indicates that the soluble aliphatic hydrocarbons in BL are directly volatilized and transferred into the tars during pyrolysis. These longchain alkanes and alkenes possibly wrapped around the aromatic nucleus of BL connecting by non-covalent bonds (hydrogen bond, charge-transfer interaction and p-p interaction), thus they are easily volatilized from BL during pyrolysis at low temperature [32]. Additionally, a part of soluble long-chain alkanes and alkenes in BL are easily decomposed into other long-chain alkanes and alkenes by losing light hydrocarbons at high temperature. Therefore, there are more kinds of alkanes and alkenes in the tars obtained at high pyrolysis temperature than those in SPBL. Only 2 branched chain alkanes and a cycloalkane with a long alkyl side-chains, i.e. 2,6,10-trimethylpentadecane, 9-butyldocosane and heptadecylcyclohexane, were dissolved in SPBL, whereas they were not found in SP300. Meanwhile, 2,6,10-trimethylpentadecane was detected in Tar300, and its RC in the tars decrease with pyrolysis temperature increasing. Additionally, methylcyclohexane, 2,6-dimethylundecane and 2,6-dimethyldodecane in the tars should be originated from the decomposition of soluble 9-butyldocosane and heptadecylcyclohexane in BL, implying these soluble branched chain alkanes and cycloalkanes in BL are easily decomposed into other alkanes and cycloalkanes during pyrolysis even at 300 °C. As shown in Table S4, in total 16 arenes were detected in SPBL while several of them, such as 1,1,2,3,3-pentamethylindane, 4-iso propyl-1,6-dimethylnaphthalene and 7-isopropyl-1-methylphe nanthrene, rapidly disappeared in SP300-700 with pyrolysis temperature increasing. Meanwhile, high relative contents (RCs) of them were identified in Tar300 (Table S5). These results imply that abundant soluble arenes in BL are easily volatilized during pyrolysis at low temperature. The bridged bonds containing an aliphatic carbon such as Cal–Cal, Cal–O, and Cal–S bonds among aromatic rings are easily dissociated at low temperature resulting in the formation of alkyl aromates, followed by the release of arenes [33,34]. The arenes containing two aromatic rings, especially for the methylnaphthalene, account for the major proportion of the arenes both in the SPs and the tars, suggesting that the organic species of BL is rich in bicyclic aromatics connected by weak bridged bonds. The kinds of arenes in the tars obtained at high temperature range are obviously more than those in SPBL, which can be ascribed to three reasons. Firstly, the soluble arenes in BL underwent a cracking reaction at high temperature during pyrolysis. For example, the homologs of naphthalene were decomposed into naphthalene and light hydrocarbons, resulting in the RC of
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naphthalene in the tars increased with the increase of pyrolysis temperature. Secondly, several new arenes were released from BL or trapped in chars because of the dissociation of the strong bridged bonds at high temperature [35]. Therefore, abundant homologs of benzene were detected in the tars obtained at high temperature, and some new soluble arenes were dissolved out from the chars obtained at high temperature. Thirdly, the soluble arenes were polymerized each other to form polycyclic aromatic hydrocarbon (PAHs) during their volatilization from BL causing the RC of 3 or 4 ring PAHs in the tars increased with pyrolysis temperature increasing. As exhibited in Table S6 and S7, a series of arenols were detected in SPBL, while only 3 arenols were detected in Tar300. The arenols released at 300 °C are possibly ascribed to the volatilization of the free arenols trapped in the MNS of BL. Previous researches demonstrated that the arenols are mainly originated from the decomposition of aryl ether bonds, rather than the volatilization of the free arenols in coals during pyrolysis [36,37]. The gradual dissociation of aryl ether bonds in BL with the pyrolysis temperature increasing resulted in the increase of arenols content in the tars. Due to the fast release velocity of volatile matter at higher temperature, a part of alkyl side-chains were retained in alkylarenols during pyrolysis. As a nucleophilic reagent, methanol can attack the aryl ether bonds resulting in the formation of soluble arenols during thermal dissolution of BL and its chars [38]. The aryl ether bonds retaining in the chars possibly were activated during pyrolysis. Therefore, the TRC of soluble arenols in the chars increased with pyrolysis temperature below 500 °C. Only a few of arenols were thermally dissolved out in Char700 indicating most of aryl ether bonds in BL have been cracked below 700 °C. Several soluble methoxybenzenes were detected in the SPs (Table S8), while none of them were detected in the tars. These results are the direct evidence that abundant RCH2OArOCH3 structure units exist in the MNS of BL, where R and Ar represent for alkyl side-chains and aromatic rings, respectively. These RCH2OArOCH3 structure units gradually were decomposed into alkylphenols, methoxyphenols, aliphatic hydrocarbons and CO during pyrolysis. Additionally, a few of soluble naphthols were detected in the SPs, especially for 6,7-dimethylnaphth-1-ol. Correspondingly, naphth2-ol and 2-methylnaphth-1-ol were detected in the tars obtained above 400 °C due to the rupture of alkyl side-chains in the soluble naphthols during pyrolysis. Table S9 exhibits that 4 ketones containing a 1-phenylethanone structure unit were thermal dissolved out from BL and its chars, especially for 1-(2-hydroxy-4,5-xylyl)ethanone with high RC in SPBL (6.22%) and SP300 (13.21%). Nevertheless, only 1 ketone containing the 1-phenyl-ethanone structure unit, i.e. 1-(2hydroxy-4,5-xylyl)ethanone, was detected in Tar300, and its RC in the tars decreased sharply with pyrolysis temperature increasing (Table S10). These results indicated that a plenty of soluble ketones containing the 1-phenyl-ethanone structure unit exist in the MNS of BL, whereas most of them are easily decomposed into phenols and CO above 400 °C rather than released as the ketones during pyrolysis. The RCs of trimethylcyclopent-2-enone, tetramethylcyclopent2-enone and 2,6-dimethylcyclohexanone in SPBL are higher than 1%, indicating there are a certain amount of soluble ketones containing a cyclopent-2-enone or a cyclohexanone structure unit in the MNS of BL. Therefore, dimethylcyclopent-2-enone and cyclohexanone in the tars obtained above 400 °C are originated from the release of the soluble ketones containing the cyclopent-2enone or the cyclohexanone structure unit in BL. A few of trimethylcyclopent-2-enone, tetramethylcyclopent-2-enone, and 2-methylcyclopent-2-enone were still detected in SP600 indicating the cyclopent-2-enone structure units in BL are thermal stability below 600 °C. 2,5-dimethylcyclopentanone in SP400 and SP500 were
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possibly due to the hydrogenation of the cyclopent-2-enone structure units during pyrolysis. The RC of soluble 2,6dimethylcyclohexanone in SP300 and SP400 is similar to that in SPBL suggesting 2,6-dimethylcyclohexanone in BL is thermal stability below 400 °C, whereas it is completely decomposed into cyclohexanone above 500 °C. indan-1-one in the tars obtained above 300 °C is possibly originated from the release of soluble 3,3,5,7tetramethylindan-1-one in BL. It was observed that 3,3,5,7tetramethylindan-1-one was not thermally dissolved in BL but in Char300 due to the activation effect of thermal treatment on the MNS of BL during pyrolysis at 300 °C. A series of soluble long-chain fatty acid methyl esters and methyl benzoates were detected in the SPs, and the kinds of esters in the SPs decrease with the increase of pyrolysis temperature (Table S11). Although lignite is rich in carboxyl and ester groups, none carboxylic acids and esters were identified in the tars. Soluble carboxylic acids in BL are easily translated into carboxylic acid methyl ester through esterification reaction with methanol, and as a nucleophilic reagent methanol can attack the oxygen atoms of ester groups in BL to form carboxylic acid methyl esters during thermal dissolution [38]. Similar to the long-chain alkanes and alkenes, long-chain fatty acids might also wrap around the aromatic nucleus of BL with non-covalent bonds, thus they are easily volatilized from BL and dissociated into alkanes or alkenes, and CO2 during pyrolysis [24]. Several soluble methyl benzoates contain methoxyl or phenolic-OH groups implying that abundant benzonic acid structure units in BL possibly are connected to the aromatic nucleus with oxygen bridged bonds. A part of soluble esters especially for the methyl benzoates were retained in the chars even in Char700. Therefore, the methyl benzoates existing in both SPBL and SP300-700 mainly are originated from the soluble organic species containing ester groups in BL for the high thermal stability of ester groups in coals. As shown in Table S12 a few of benzofuran and its derivatives were detected in the tars obtained above 400 °C, while none of them were detected in the SPs. Therefore, these furans in the tars are possibly ascribed to the complex reactions of the organic species released during pyrolysis. Only a few of soluble ONCs and OSCs were thermally dissolved out from BL and its chars (Table S13). Table S14 shows although only 1 ONCs was detected in Tar300, the kinds of ONCs detected in the tars increased with the pyrolysis temperature. In total 14 ONCs were identified in Tar600 and Tar700, and most of them are nitrogen-containing heterocyclic compounds.
4. Conclusions Abundant soluble long-chain aliphatic hydrocarbons and acids wrap around the aromatic nucleus of BL. These long-chain aliphatic hydrocarbons were directly released from BL, whereas these longchain aliphatic acids were volatilized and decomposed into long chain n-alkanes or n-alkenes, and CO2 during pyrolysis at low temperature. Abundant soluble arenes in BL were volatilized during pyrolysis at low temperature, but they were easily subjected a cracking reaction by losing alkyl side-chains or were polymerized each other into PAHs during pyrolysis at high temperature. The arenols released from BL pyrolysis at 300 °C were ascribed to the volatilization of the soluble phenols embedded in the MNS of BL, while the phenols released at high temperature were mainly originated from the decomposition of aryl ether bonds in the MNS of BL. Soluble ketones containing the 1-phenyl-ethanone structure unit were decomposed into phenols and CO above 400 °C rather than released as ketones during pyrolysis. These findings are beneficial to identify the pyrolysis mechanism of lignites at molecular level and improve the efficiency of lignite utilization.
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Full Length Article
Molecular composition of soluble organic species in Baiyinhua lignite and their evolution profiles during pyrolysis Yun-Peng Zhao a,⇑, Di Zhang a, You-Jia Tian a, Xin-Fu He b, Hao Lu a, Xing Fan a, Jing-Pei Cao a, Xian-Yong Wei a, Zhi-Min Zong a a b
Key Laboratory of Coal Processing and Efficient Utilization, Ministry of Education, China University of Mining & Technology, Xuzhou 221116, Jiangsu, China School of Chemistry and Chemical Engineering, Xi’an University of Science and Technology, Xi’an, Shaanxi 710054, China
h i g h l i g h t s Most of soluble organic species were released from Baiyinhua lignite (BL) below 500 °C during pyrolysis. Abundant long-chain aliphatic hydrocarbons and acids wrap around the aromatic nucleus of BL. Abundant soluble arenes in BL easily were volatilized during pyrolysis at low temperature. Soluble ketones containing a 1-phenyl-ethanone structure unit in lignite are decomposed into phenols and CO above 400 °C.
a r t i c l e
i n f o
Article history: Received 23 December 2016 Received in revised form 3 May 2017 Accepted 10 May 2017
Keywords: Lignite Molecular composition Soluble organic species Evolution profiles
a b s t r a c t To investigate the molecular composition of soluble organic species in Baiyinhua lignite (BL) and their evolution profiles during pyrolysis, BL was firstly pyrolyzed in a fixed-bed reactor, affording gaseous products, tars and chars, then BL and its chars were thermally dissolved in supercritical methanol affording soluble portions (SPs). The composition and structural characteristics of the SPs and tars were analyzed with a gas chromatograph/mass spectrometer. The results indicate that most of the soluble organic species can be released from BL below 500 °C during pyrolysis. Abundant soluble long-chain aliphatic hydrocarbons and carboxylic acids wrap around the aromatic nucleus in BL. According to the difference between the composition and structural characteristics of the SPs and tars, the evolution profiles of the soluble organic species from BL pyrolysis were discussed in detail. Ó 2017 Elsevier Ltd. All rights reserved.
1. Introduction Lignites account for 40–50% of global total coal reserves [1]. High water and oxygen contents along with the resulting low calorific value limit lignite application in traditional utilization technologies, such as combustion, gasification, and liquefaction, while the high volatile matter and oxygen-bridged bond contents are beneficial to get transportation fuels or the raw materials of high value-added chemicals from lignites under mild conditions [2,3]. Studies on lignite pyrolysis have received considerable attentions [4,5]. Insight into molecular compositional information of organic species in lignites and their evolution profiles during pyrolysis is important to improve pyrolysis technologies, design pyrolysis reactors, and reveal the formation mechanism of the organic species from lignite pyrolysis. ⇑ Corresponding author. E-mail address: [email protected] (Y.-P. Zhao). http://dx.doi.org/10.1016/j.fuel.2017.05.041 0016-2361/Ó 2017 Elsevier Ltd. All rights reserved.
In past decades, the composition of organic species in lignite was investigated based on the analyses of lignites and the products from pyrolysis and liquefaction [6–10]. Nevertheless, the analyses of lignites only demonstrate the element composition and occurrence forms, and the functional groups distribution of organic species in lignites, and the analyses of the products from pyrolysis and liquefaction cannot reflect the original molecular composition of the organic species for the complex reactions occurred during these processes. Based on the two phases structure concept of coals, coal is composed of a large three-dimensional cross-linked macromolecular network structure (MNS) containing polycyclic aromatic clusters connected by bridged bonds (fixed phase) and the soluble part embedded in the MNS (mobile phase) [11]. The thermal dissolution of low rank coals in suitable solvents not only extracted out the mobile phase of organic species, but also destroyed the weak bridged bonds in the MNS of coals [12–14]. Therefore, thermal dissolution in suitable solvents is an effective
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method to isolate the organic species in lignites for further analyzing their molecular composition. Although the pyrolysis behavior and products distribution of lignites have been widely investigated [15–19], few reports focused on the evolution profiles of organic species in lignites during pyrolysis. Some studies have been conducted to investigate the evolution profiles of coal model compounds through experiment and theoretical simulation [20–22]. Nevertheless, the pyrolysis of coal model compounds only demonstrated the dissociation of certain chemical bonds but it is hardly to reveal the evolution profiles of original organic species in lignites [23,24]. Lignite accounts for about 13% of China’s total coal resource and is playing an important role in the energy supply and chemical feedstock with the depletion of petroleum and high-quality coal resources because of their abundance, easy access, and low mining cost [25]. In this work, a typical lignite, Baiyinhua lignite (BL) from Inner Mongolia Autonomous Region, was subjected to pyrolysis at different temperatures to afford gaseous products, tars and chars, then BL and its chars were thermally dissolved in methanol to afford soluble portions (SPs). The tars and the SPs were analyzed by using a gas chromatographic mass spectrometry (GC/MS) in detail for identifying the molecular composition of the soluble organic species in BL and their evolution profiles during pyrolysis.
ditions was repeated at least 3 times to control the errors of yields less than ±5%. 2.3. Thermal dissolution of BL and its chars
2. Experimental
Thermal dissolution experiments were conducted in a 100 mL stainless-steel, magnetically stirred autoclave, in which, 2 g BL or its chars and 40 mL methanol were placed. Then the autoclave was sealed and the air inside the autoclave was replaced with nitrogen. Afterwards, the autoclave was heated to 300 °C at 10 °C/min by an external electric furnace, and held for 1 h. Then the autoclave was cooled down to the room temperature in a water bath. The reaction mixture was taken out as cleanly as possible with methanol from the autoclave, filtrated through a Teflon membrane filter with 0.45 lm of pore size, and repeatedly washed with methanol to afford filtrates and thermal dissolution residue. The filtrates were distilled with a rotary evaporator to remove solvent under reduced pressure to afford SPs. The SP from BL was named as SPBL, while the SPs from the chars were named as SP300-700 based on the temperature of the char obtained. The yields of the SPs from both BL and chars were calculated according to the formula: mSP/ mBL,daf, where mSP and mBL,daf denote the mass of SPBL (or SP300700) and BL. Each experiment under the same conditions was repeated at least 3 times to control the errors of yields less than ±5%.
2.1. Materials
2.4. Characterizations
BL was collected from Baiyinhua 1# basin (118°300 –400 E, 44°7 00 –750 N) in Inner Mongolia Autonomous Region, China and pulverized to pass through a 200-mesh sieve (<74 lm) followed by desiccation in a vacuum at 80 °C for 24 h before use. Table 1 lists the results of the proximate and ultimate analyses of BL. Methanol purchased from Sinopharm Chemical Reagent Co., Ltd is an analytical reagent (>99.8%) and was distilled with a Büchi R-134 rotary evaporator prior to use.
All the tars and the SPs were analyzed with a Hewlett-Packard 7890/5975 gas chromatograph/mass spectrometer (GC/MS), which was equipped with a capillary column coated with HP-5 (cross-link 5% PH ME siloxane, 60 m length, 0.25 mm inner diameter, 0.25 lm film thickness) and a quadrupole analyzer operated in electron impact (70 eV) mode. Compounds were identified by comparing mass spectra with NIST11 library data. The relative content (RC) of each compound was determined by the normalization method of peak area, i.e., the peak area of the compound divided by the sum of the peak areas of all identified compounds in the total ion chromatogram. The composition of gaseous products was analyzed by 5A molecular sieves and GDX502 columns using a 7890T gas chromatograph with a thermal conductive detector.
2.2. Pyrolysis of BL Pyrolysis experiments were carried out in a vertical fixed-bed reactor with the inner diameter of 12 mm and the heating zone of 10 cm. The schematic diagram of apparatus is shown in Fig. S1. The reactor loaded with 5 g of coal sample was heated to the desired temperature (300–700 °C) within about 10 min by a preheated furnace and then kept at that temperature for 30 min. The volatile matter during BL pyrolysis was brought out by highpurity nitrogen from the reactor to a cool trap, where the liquid products (tar plus water) were collected. The residence time of volatile matter in the reactor is about in the range of 1–1.8 s according to the pyrolysis temperature. The solid char was removed from the reactor after the experiment and weighted. The tar and water were separated by the method of American Society for Testing and Materials (ASTM) D95-05e1 (2005). The tar, char and water yields were calculated on the basis of the weights of char, tar, and water, and the gas yield was calculated by difference. The tar and char obtained at 300–700 °C were named as Tar300-700 and Char300-700, respectively. Each experiment under the same con-
3. Results and discussion 3.1. Product distribution of BL during pyrolysis As shown in Fig. 1, the char yields decrease with the increase of pyrolysis temperature, which are ascribed to the more release of organic species or the deeper decomposition of coal MNS at higher temperature. The tar yields increase with temperature at range of 300–600 °C, then decrease with the further increase of temperature. The water yields are on the rising trend with temperature, whereas the water yield at 600 °C is slightly lower than that at 500 °C. Pyrolysis water mainly comes from the decomposition of oxygen-containing functional groups in coals and the polymerization of oxygen-containing radicals [26]. It was reported that the cleavage of bonds between hydrogen and aliphatic carbon (HCal),
Table 1 Proximate and ultimate analyses (wt.%) of BL. Proximate analysis
Ultimate analysis (daf)
Mad
Ad
Vdaf
C
H
N
35.07
17.01
40.60
69.91
5.06
1.44
Mad: moisture (air dried base); Ad: ash (dry base, i.e., moisture-free base); Vdaf: volatile matter (dry and ash-free base);
a
By difference.
S
Oa
1.51
22.08
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80
60
Yield (wt.%, daf)
Yield (wt.%, daf)
8
c ha r
40
gas 20
6
4
2
0 300
tar water 400
500
600
700
0
SPBL SP300 SP400 SP500 SP600 SP700
Temperature ( oC)
Fig. 2. SP yields of BL and its chars.
Fig. 1. Product distribution from BL pyrolysis.
3.3. Group component distribution of tars and SPs and between hydrogen and aromatic carbon (HCar) in the organic species of coals results in the formation of hydrogen radicals during pyrolysis [27]. The cleavage of HCal bonds is mainly ascribed to the degradation of the coal MNS and the dehydrogenation of naphthenic structure at low temperature, while the cleavage of H–Car bonds is originated from the condensation of aromaticrings structures in coals at high temperature [28]. Abundant hydrogen radicals occurred for the condensation of aromatic-ring structures in BL above 500 °C. On the one hand, the mutual combination of these hydrogen radicals accelerated the increase of H2 yield with temperature (Fig. S2a). On the other hand, the oxygencontaining radicals were more easily stabilized by these hydrogen radicals to produce tar rather than polymerized each other to generate water and char above 500 °C. Nevertheless, the enhancement of polymerization for the oxygen-containing radicals at 700 °C resulted in the increase of water yield. The gas yields increase with the increase of temperature. As exhibited in Fig. S2a, both H2 and CH4 yields increase with temperature increasing. CO2 is mainly ascribed to the decomposition of carboxyl and carboxylate groups at low temperature and the thermal dissociation of quinones and oxygen-bearing heterocycles at high temperature, while CO is originated from the cleavage of alkyl and aryl ether bonds at low temperature, and the decomposition of phenolic groups at high temperature during pyrolysis [29,30]. Both CO2 and CO yields increase with temperature increasing from 300 to 600 °C then slightly decrease with further increase of temperature, indicating that most of carboxyl groups and ether bonds have been decomposed or cracked below 600 °C. Fig. S2b shows that the yields of C2 and C3 hydrocarbons increase with temperature increasing, while they are quite lower than the yield of CH4 because the alkyl side-chains of the aromatic nucleus and aliphatic groups are prone to form simple hydrocarbons during pyrolysis [31].
As shown in Fig. 3, the compounds detected by GC/MS in the tars can be classified into alkanes, alkenes, arenes, arenols, ketones, furans and organonitrogen compounds (ONCs). The total relative content (TRC) of aliphatic hydrocarbons and arenes in tars decreases, while that of arenols increases with pyrolysis temperature increasing. The TRC of ketones in Tar300 is more than 11%, whereas that in the tars obtained above 300 °C is lower than 3.5%, indicating that the ketones in Tar300 are possibly originated from the release of free ketones embedded in the MNS of BL, while these free ketones easily decompose into other compounds at high temperature. As exhibited in Fig. 4, the compounds identified in the SPs can be classified into alkanes, alkenes, arenes, arenols, ethers, esters, ketones, ONCs and organosulfur compounds (OSCs). None alkanes and alkenes were detected in the SPs from the chars obtained at above 300 °C, which is consistent with the high TRC of them in Tar300 and Tar400, indicating that most of soluble alkanes and alkenes can be released below 400 °C during pyrolysis. The TRC of soluble arenes in chars increase with pyrolysis temperature increasing, except that the TRC of arenes in SP500 is slightly lower than that in SP400 because of the high TRC of arenols and esters in SP500. The TRC of arenols in the SPs firstly increase with the increase of pyrolysis temperature below 500 °C, then sharply decrease with the pyrolysis temperature further increasing. The aryl ether bonds in the chars were possibly activated during pyrolysis at low temperature, thus more aryl ether bonds in the organic species of BL were cracked to form arenols during the thermal dissolution of the chars [23].
40
3.2. SPs yields of BL and its chars
30 TRC (%)
Fig. 2 shows that the yield of SPBL is higher than the yields of SP300-700, and the yields of SP300-700 decrease with pyrolysis temperature increasing especially below 500 °C, indicating most of the soluble organic species in BL have been released below 500 °C during pyrolysis. The difference between the yield of SPBL (9.1%) and the yield of SP300 (6.27%) is 2.83%, which is lower than the yield of Tar300 (4.69%). The results are possibly ascribed to the activation effect of pyrolysis process at 300 °C on the MNS of BL. Some new organic species were dissolved out during the thermal dissolution of Char300.
ls areno arenes
20
10
0 300
ket
alkane s alkenes on es
ONCs
furans 400 500 600 Temperature (oC)
700
Fig. 3. Group component distribution of the tars.
Y.-P. Zhao et al. / Fuel 205 (2017) 192–197
100 OSCs ONCs ketones ethers esters arenols arenes alkenes alkanes
TRC (%)
80 60 40 20 0
SPBL SP300 SP400 SP500 SP600 SP700 Fig. 4. Group component distribution of the SPs.
3.4. Evolution profiles of soluble organic species in BL during pyrolysis As shown in Table S1, a series of long-chain n-alkanes and nalkenes were dissolved in SPBL. The similar constitute of alkanes and alkenes between the tars (Table S2 and 3) and SPBL indicates that the soluble aliphatic hydrocarbons in BL are directly volatilized and transferred into the tars during pyrolysis. These longchain alkanes and alkenes possibly wrapped around the aromatic nucleus of BL connecting by non-covalent bonds (hydrogen bond, charge-transfer interaction and p-p interaction), thus they are easily volatilized from BL during pyrolysis at low temperature [32]. Additionally, a part of soluble long-chain alkanes and alkenes in BL are easily decomposed into other long-chain alkanes and alkenes by losing light hydrocarbons at high temperature. Therefore, there are more kinds of alkanes and alkenes in the tars obtained at high pyrolysis temperature than those in SPBL. Only 2 branched chain alkanes and a cycloalkane with a long alkyl side-chains, i.e. 2,6,10-trimethylpentadecane, 9-butyldocosane and heptadecylcyclohexane, were dissolved in SPBL, whereas they were not found in SP300. Meanwhile, 2,6,10-trimethylpentadecane was detected in Tar300, and its RC in the tars decrease with pyrolysis temperature increasing. Additionally, methylcyclohexane, 2,6-dimethylundecane and 2,6-dimethyldodecane in the tars should be originated from the decomposition of soluble 9-butyldocosane and heptadecylcyclohexane in BL, implying these soluble branched chain alkanes and cycloalkanes in BL are easily decomposed into other alkanes and cycloalkanes during pyrolysis even at 300 °C. As shown in Table S4, in total 16 arenes were detected in SPBL while several of them, such as 1,1,2,3,3-pentamethylindane, 4-iso propyl-1,6-dimethylnaphthalene and 7-isopropyl-1-methylphe nanthrene, rapidly disappeared in SP300-700 with pyrolysis temperature increasing. Meanwhile, high relative contents (RCs) of them were identified in Tar300 (Table S5). These results imply that abundant soluble arenes in BL are easily volatilized during pyrolysis at low temperature. The bridged bonds containing an aliphatic carbon such as Cal–Cal, Cal–O, and Cal–S bonds among aromatic rings are easily dissociated at low temperature resulting in the formation of alkyl aromates, followed by the release of arenes [33,34]. The arenes containing two aromatic rings, especially for the methylnaphthalene, account for the major proportion of the arenes both in the SPs and the tars, suggesting that the organic species of BL is rich in bicyclic aromatics connected by weak bridged bonds. The kinds of arenes in the tars obtained at high temperature range are obviously more than those in SPBL, which can be ascribed to three reasons. Firstly, the soluble arenes in BL underwent a cracking reaction at high temperature during pyrolysis. For example, the homologs of naphthalene were decomposed into naphthalene and light hydrocarbons, resulting in the RC of
195
naphthalene in the tars increased with the increase of pyrolysis temperature. Secondly, several new arenes were released from BL or trapped in chars because of the dissociation of the strong bridged bonds at high temperature [35]. Therefore, abundant homologs of benzene were detected in the tars obtained at high temperature, and some new soluble arenes were dissolved out from the chars obtained at high temperature. Thirdly, the soluble arenes were polymerized each other to form polycyclic aromatic hydrocarbon (PAHs) during their volatilization from BL causing the RC of 3 or 4 ring PAHs in the tars increased with pyrolysis temperature increasing. As exhibited in Table S6 and S7, a series of arenols were detected in SPBL, while only 3 arenols were detected in Tar300. The arenols released at 300 °C are possibly ascribed to the volatilization of the free arenols trapped in the MNS of BL. Previous researches demonstrated that the arenols are mainly originated from the decomposition of aryl ether bonds, rather than the volatilization of the free arenols in coals during pyrolysis [36,37]. The gradual dissociation of aryl ether bonds in BL with the pyrolysis temperature increasing resulted in the increase of arenols content in the tars. Due to the fast release velocity of volatile matter at higher temperature, a part of alkyl side-chains were retained in alkylarenols during pyrolysis. As a nucleophilic reagent, methanol can attack the aryl ether bonds resulting in the formation of soluble arenols during thermal dissolution of BL and its chars [38]. The aryl ether bonds retaining in the chars possibly were activated during pyrolysis. Therefore, the TRC of soluble arenols in the chars increased with pyrolysis temperature below 500 °C. Only a few of arenols were thermally dissolved out in Char700 indicating most of aryl ether bonds in BL have been cracked below 700 °C. Several soluble methoxybenzenes were detected in the SPs (Table S8), while none of them were detected in the tars. These results are the direct evidence that abundant RCH2OArOCH3 structure units exist in the MNS of BL, where R and Ar represent for alkyl side-chains and aromatic rings, respectively. These RCH2OArOCH3 structure units gradually were decomposed into alkylphenols, methoxyphenols, aliphatic hydrocarbons and CO during pyrolysis. Additionally, a few of soluble naphthols were detected in the SPs, especially for 6,7-dimethylnaphth-1-ol. Correspondingly, naphth2-ol and 2-methylnaphth-1-ol were detected in the tars obtained above 400 °C due to the rupture of alkyl side-chains in the soluble naphthols during pyrolysis. Table S9 exhibits that 4 ketones containing a 1-phenylethanone structure unit were thermal dissolved out from BL and its chars, especially for 1-(2-hydroxy-4,5-xylyl)ethanone with high RC in SPBL (6.22%) and SP300 (13.21%). Nevertheless, only 1 ketone containing the 1-phenyl-ethanone structure unit, i.e. 1-(2hydroxy-4,5-xylyl)ethanone, was detected in Tar300, and its RC in the tars decreased sharply with pyrolysis temperature increasing (Table S10). These results indicated that a plenty of soluble ketones containing the 1-phenyl-ethanone structure unit exist in the MNS of BL, whereas most of them are easily decomposed into phenols and CO above 400 °C rather than released as the ketones during pyrolysis. The RCs of trimethylcyclopent-2-enone, tetramethylcyclopent2-enone and 2,6-dimethylcyclohexanone in SPBL are higher than 1%, indicating there are a certain amount of soluble ketones containing a cyclopent-2-enone or a cyclohexanone structure unit in the MNS of BL. Therefore, dimethylcyclopent-2-enone and cyclohexanone in the tars obtained above 400 °C are originated from the release of the soluble ketones containing the cyclopent-2enone or the cyclohexanone structure unit in BL. A few of trimethylcyclopent-2-enone, tetramethylcyclopent-2-enone, and 2-methylcyclopent-2-enone were still detected in SP600 indicating the cyclopent-2-enone structure units in BL are thermal stability below 600 °C. 2,5-dimethylcyclopentanone in SP400 and SP500 were
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possibly due to the hydrogenation of the cyclopent-2-enone structure units during pyrolysis. The RC of soluble 2,6dimethylcyclohexanone in SP300 and SP400 is similar to that in SPBL suggesting 2,6-dimethylcyclohexanone in BL is thermal stability below 400 °C, whereas it is completely decomposed into cyclohexanone above 500 °C. indan-1-one in the tars obtained above 300 °C is possibly originated from the release of soluble 3,3,5,7tetramethylindan-1-one in BL. It was observed that 3,3,5,7tetramethylindan-1-one was not thermally dissolved in BL but in Char300 due to the activation effect of thermal treatment on the MNS of BL during pyrolysis at 300 °C. A series of soluble long-chain fatty acid methyl esters and methyl benzoates were detected in the SPs, and the kinds of esters in the SPs decrease with the increase of pyrolysis temperature (Table S11). Although lignite is rich in carboxyl and ester groups, none carboxylic acids and esters were identified in the tars. Soluble carboxylic acids in BL are easily translated into carboxylic acid methyl ester through esterification reaction with methanol, and as a nucleophilic reagent methanol can attack the oxygen atoms of ester groups in BL to form carboxylic acid methyl esters during thermal dissolution [38]. Similar to the long-chain alkanes and alkenes, long-chain fatty acids might also wrap around the aromatic nucleus of BL with non-covalent bonds, thus they are easily volatilized from BL and dissociated into alkanes or alkenes, and CO2 during pyrolysis [24]. Several soluble methyl benzoates contain methoxyl or phenolic-OH groups implying that abundant benzonic acid structure units in BL possibly are connected to the aromatic nucleus with oxygen bridged bonds. A part of soluble esters especially for the methyl benzoates were retained in the chars even in Char700. Therefore, the methyl benzoates existing in both SPBL and SP300-700 mainly are originated from the soluble organic species containing ester groups in BL for the high thermal stability of ester groups in coals. As shown in Table S12 a few of benzofuran and its derivatives were detected in the tars obtained above 400 °C, while none of them were detected in the SPs. Therefore, these furans in the tars are possibly ascribed to the complex reactions of the organic species released during pyrolysis. Only a few of soluble ONCs and OSCs were thermally dissolved out from BL and its chars (Table S13). Table S14 shows although only 1 ONCs was detected in Tar300, the kinds of ONCs detected in the tars increased with the pyrolysis temperature. In total 14 ONCs were identified in Tar600 and Tar700, and most of them are nitrogen-containing heterocyclic compounds.
4. Conclusions Abundant soluble long-chain aliphatic hydrocarbons and acids wrap around the aromatic nucleus of BL. These long-chain aliphatic hydrocarbons were directly released from BL, whereas these longchain aliphatic acids were volatilized and decomposed into long chain n-alkanes or n-alkenes, and CO2 during pyrolysis at low temperature. Abundant soluble arenes in BL were volatilized during pyrolysis at low temperature, but they were easily subjected a cracking reaction by losing alkyl side-chains or were polymerized each other into PAHs during pyrolysis at high temperature. The arenols released from BL pyrolysis at 300 °C were ascribed to the volatilization of the soluble phenols embedded in the MNS of BL, while the phenols released at high temperature were mainly originated from the decomposition of aryl ether bonds in the MNS of BL. Soluble ketones containing the 1-phenyl-ethanone structure unit were decomposed into phenols and CO above 400 °C rather than released as ketones during pyrolysis. These findings are beneficial to identify the pyrolysis mechanism of lignites at molecular level and improve the efficiency of lignite utilization.
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