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Hydrogen Transfer Route During Hydrothermal Treatment Lignite Using Isotope Tracer Method and Improving Pyrolysis Tar Yield Lanlan Wang, Tieying Pan, Peng Liu, and Dexiang Zhang Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.6b00281 • Publication Date (Web): 05 May 2016 Downloaded from http://pubs.acs.org on May 10, 2016
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Hydrogen Transfer Route During Hydrothermal Treatment Lignite Using Isotope Tracer Method and Improving Pyrolysis Tar Yield Lanlan Wang1, Tieying Pan1,*, Peng Liu2, Dexiang Zhang2,* 1
Analysis and Research Center, East China University of Science & Technology, 130 Meilong Rd, Shanghai 200237, China 2
Key Laboratory of Coal Gasification and Energy Chemical Engineering of Ministry of Education, Department of Chemical Engineering for Energy Resources, East China University of Science & Technology, 130 Meilong Road, Shanghai 200237, China
ABSTRACT: Hydrothermal treatment was used to upgrade Inner Mongolia lignite (IML) before pyrolysis. The hydrogen transfer route during hydrothermal treatment lignite by isotope tracer method was carried out in an autoclave. Typical experiments of D2O substituted for pure water under four treatment temperatures (180, 220, 260, and 300 oC) were also performed. The pyrolysis tar composition was analyzed by 1H nuclear magnetic resonance (NMR) and gas chromatography and mass spectrometry (GC-MS). Four kinds of typical substance were studied in detail in tar from D2O treated lignite and the deuterated extent (D1, D2, D3 and D4) were quantitated by selected peaks in mass spectra. The results showed that the deuterium atom was more prone to incorporate into aromatic ring with respect to aliphatic carbon chains. The value of D2, D3 and D4 were all increased with the increase of treatment temperature. Toluene and phenol analysis showed that deuterium atoms incorporated into aromatic ring were distinct by different substituents. The route of hydrogen transfer during hydrothermal treatment was well investigated by deuterium tracer method.
1. INTRODUCTION Coal provides 30.1% of global primary energy needs and generates over 40% of the world's electricity and is also used in the production of over 70% of the world’s steel in 2014.1 It will still be an important energy resource in the foreseeable future. Lignite which accounts for abundant of China’s total coal resource will also be an important resource even if its high moisture and low heating value.2 Hydrothermal treatment is one of effective non-evaporative processes for dewatering and upgrading lignite. It can not only remove the water irreversibly but also alter the structure and chemical composition of the lignite.3,4 A significant body of studies5-10 showed that hydrothermal treatment have effect on liquefaction reactivity and solvent extraction of coal. Previous work11 showed that tar yield of Inner Mongolia lignite increased after hydrothermal treatment and reached maximum 17% at 260 oC, but the mechanism on improving tar yield should be invested further. Isotopic tracer is a method to have a
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more complete understanding of reaction mechanisms and widely used by researchers. The deuterium tracer techniques have been exploited to provide a detailed understanding of mechanistic pathways of hydrogen transfer reactions operating at the molecular level during coal liquefaction.12-18 Deuterated methane (CD4) was used to investigate the formation mechanism of coal tar in the integrated process through analyzing the chemical compounds of coal tar with gas chromatography and mass spectrometry (GC-MS).19 Mixture of Loy Yang lignite with different content of deuterium-labeled water (D2O) was studied to investigate the exchange of hydrogen between water and coal during coal pyrolysis.20 Isotope tracer was used in the study to explore hydrogen transfer route during hydrothermal treatment on improving lignite tar yield. In the present study, hydrothermal treatment of Inner Mongolia lignite was carried out in an autoclave. Deuterium oxide (D2O) was used to substitute pure water (H2O) to conduct the experiments at the same treatment condition. The pyrolysis tar was analyzed by 1H nuclear magnetic resonance (NMR) and GC-MS techniques. The hydrogen transfer route during hydrothermal treatment on improving lignite tar yield was investigated carefully.
2. EXPERIMENTAL SECTION 2.1 Materials IML from Inner Mongolia of China was ground to less than 0.2mm and stored under cryogenic environment. The proximate and ultimate analysis of the raw and H2O treated lignite were determined and shown in Table 1. The raw lignite was dried under vacuum at 50 oC for 8 h before the tests.
2.2 Hydrothermal Treatment Hydrothermal treatment of lignite was performed in a 500ml autoclave at various temperatures. The autoclave was equipped with an electrically heated furnace, a magnetic stirrer, and a controller. In each run, mixtures of lignite and water by 5:3 were placed into the autoclave. The reaction was started by heating the autoclave at a heating rate of 4 oC/min to a certain temperature (180, 220, 260, or 300 oC) and maintained for 30min. The stirring speed was maintained by 200 r/min and the valve was not screwed up below 60 oC. Subsequently, the autoclave was cooled to room temperature using tap water. Then the treated coal was filtered to remove excess water, and then dried under vacuum at 50 oC for 8 h.
2.3 Pyrolysis The pyrolysis experiments were carried out in a quartz tubular reactor. The reactor was made of quartz tube with an internal diameter of 20 mm and an overall length of 340 mm. The gas-liquid separator was connected with the reactor through ground glass. 15g lignite was placed in the reactor and heated from ambient temperature to 600 oC at the rate of 5 oC/min and maintained for 15 min by a temperature-controlled electric furnace. The volatile compounds from the reactor entered into the liquid
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collector which was immerged in an ice-water bath unit. The tar could be separated from moisture by means of centrifugation, and then was stored in liquid nitrogen to be measured.
2.4 1H-NMR Analysis 1
H-NMR spectroscopy were conducted on a Bruker AVANCE III 500 NMR spectrometer operating at 20 oC, equipped with a 5 mm BBO probe, observing 1H at 500.13 MHz in zg pulse sequence. 1H NMR spectra were acquired using spectral width of 10.0 KHz; 32,768 data points; pulse width of 4 us; relaxation delay of 2.0 s; acquisition time of 1.638 s and 32 scans. Spectra were processed by applying an exponential line broadening of 0.3 Hz for sensitivity enhancement before fourier transforms and were accurately phased and baseline adjusted. Tar received from pyrolysis of different experiments were dissolved in deuterated chloroform(CDCl3) using TMS as an internal reference.
2.5 GC-MS Analysis Tar was also analyzed by GC-MS, a technique that couples the high resolving power of capillary gas chromatography and the structural elucidation ability of mass spectrometry. All detection were performed using an Agilent 7980A gas chromatograph coupled with an Agilent 5975C mass spectrometer operating in the electron impact ionization mode and the voltage was 70 eV. Split injection mode was introduced to the experiments and the split ratio with injection volume was 100:1 and 0.1µL, respectively. The temperature of quadrupole and ion source is 150 oC and 230 o C respectively. The oven temperature was programmed as follows: initial column temperature was 50 oC and increased by 10 oC/min to 100 oC and remains 1 min, then from 100 to 310 oC at a rate of 8 oC/min and finally held at 310 oC for 2 min. The total run time was about 34 min. The carrier gas was ultrapure He (more than 99.999%) at a current speed of 1.0 m L/min. The chemical workstation NIST08 (US National Institute of Standards and Technology) standard spectrum library searching and area normalization method was used for qualitative and quantitative of the components, respectively. The deuterated extent in compounds were expressed by value of D1, D2, D3 and D4. The calculating formula were shown in Eq.(1-4). Similar techniques have been used in other mass spectral investigations using deuterium as a tracer.21,22
D1 =
A[ M + 1] A[ M ] + A[ M + 1] + A[ M + 2] + A[ M + 3] + A[ M + 4]
(Eq.1)
D2 =
A[ M + 2] A[ M ] + A[ M + 1] + A[ M + 2] + A[ M + 3] + A[ M + 4]
(Eq.2)
D3 =
A[ M + 3] A[ M ] + A[ M + 1] + A[ M + 2] + A[ M + 3] + A[ M + 4]
(Eq.3)
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D4 =
A[ M + 4] A[ M ] + A[ M + 1] + A[ M + 2] + A[ M + 3] + A[ M + 4]
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(Eq.4)
3.1 Effects of D2O Hydrothermal Treatment on Tar Component Four kinds of typical substance were studied in detail and their mass spectra were showed in Figure 1 to Figure 4 and they were represented for monocyclic aromatic hydrocarbons, polycyclic aromatic hydrocarbons, phenols and aliphatic hydrocarbons, respectively. Compared to the standard mass spectra in spectral library, it was observed that there were significant changes in D2O hydrothermal treatment mass spectra. The base peak was even one larger than the standard substance with the increase of temperature. Table 2 listed the deuterated extent of these studied compounds, it only showed the deuterated species from D1 to D4 and more extensively deuterated forms were not detected. The composition of tar can be categorised into five groups according to their molecular structure and chemical properties23-25 as shown in Figure 5. The relative amount of each group was obtained by peak area normalization method. The peak area of all the compounds were set to 100% in each total ionic chromatogram (TIC) and then the relative amounts of each group was obtained. The TIC of tar were shown in Figure 6. Figure 5 revealed that phenols and aromatic hydrocarbons were the major components of tars. It may be explained that the ether link (-O-) in lignite is activity and then makes it easily convert to OH during pyrolysis.26,27 The relative percentage of phenols decreased while aromatic hydrocarbons with short branched chains was obviously increased after the hydrothermal treatment. This would improve the quality of tar as phenols can increase the carbon residual, foul smell, discoloration corrosion, bad combustion conditions and other negative effects for fuel oil. The change of phenols dues to the decrease of O content in decarboxylic reaction through hydrothermal treatment11 and the result were shown in Table 1. Methyl benzene, ethyl benzene and TMB were characteristic components of monocyclic aromatic hydrocarbons and owing to the fracture of side chains beside aromatic ring. The source of alkenes and alkanes in coal pyrolysates was proposed to be free hydrocarbons, aromatic alkyl side chains, or polymethylene bridges between aromatic nuclei presented in the coal,28,29 and the alkenes and alkanes could also arise from the conversion of oxygen-containing compounds such as carboxylic acids and esters.30 It was observed that aliphatic hydrocarbons were decreased from 9.59% to 8.12%. This may due to further splitting decomposition of long carbon chains under the hydrothermal treatment. Whether the fracture of aromatic side chains or long carbon chains, both of the two processes can release some small gas molecules, such as CH4, C2H4 etc. Moreover, the relative contents of heterocyclic compounds were decreased slightly (from 5.80% to 5.34%). This was due to removal of heteroatom during hydrothermal treatment process. The total ionic chromatogram of GC-MS from the tar showed that a number of lower molecular weight components eluting from the chromatographic column in the initial 10 min, including phenols, monocyclic aromatic hydrocarbons, pyridines and
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thiophenes, then followed naphthalenes and a regular series of n-alkanes and n-alkenes for their higher boiling point. Table 3 showed the matching quality of compounds obtained from spectral library search report of the chemical workstation NIST08 at different treatment temperatures. Nearly all of the compounds matching quality were decreased obviously with the increase of treatment temperature. This owing to the emergence of more isotopic peaks around the main fragment peaks. Moreover, it was also verified the incorporation of deuterium in D2O treatment experiment as showed above. Likewise, matching quality of aliphatic hydrocarbons only had a slight reduce and the result was consistent with their deuterated extent.
3.2 Comparison of Proton Distribution of Tar After Hydrothermal Treatment Using D2O and H2O The assignments of proton chemical shifts in 1H-NMR were list in Table 4 31, 32. The H-NMR spectra were integrated and their percentage from the total signal was estimated. According to Table 4 the 1H-NMR integrals results of tar at different treatment conditions were presented in Table 5. Table 5 showed that the percentage integral areas of Har, Hα and Hγ increased significantly while Hβ decreased after hydrothermal treatment which indicated that hydrothermal treatment could make part of bridge bonds (-CH2-O-) in lignite cracked. As a result, molecular structures in tar became aromatic hydrocarbons with short branched chains and smaller molecule alkanes. Because of the shorten of branched chains, the percentage integral areas of Hβ decreased from 40.78 % to 30.58% correspondingly. Furthermore, the ratio of Har/Hali increased after hydrothermal treatment and reached the maximum 0.397 when the pretreatment temperature was 260 oC and this result was consistent with Figure 5. That dues to pyrolysis of lignite is a free radical reaction process and there exist some weak covalent bonds in lignite structure. These weak covalent bonds are easy to be broken to form free radicals during pyrolysis 33. With the increasing of temperature, the strength of intra-and inter-molecular hydrogen bonds and Van der Waals force were weakened and then weakening their interactions with aromatic side chains during the hydrothermal treatment. As a result, aromatic side chains were more easily broken to form small molecular gaseous products during pyrolysis, thus, the proportion of aliphatic protons decreased and aromatic protons increased correspondingly. Meanwhile, the free radical reactions were more active and brings out the increase of tar yield and tar quality improved. However, the Har/Hali had a slight decrease and Hα had a slight increase when pretreatment temperature reached 300 oC. That was attribute to further fracture of covalent in lignite causing the proportion of aromatic compounds increase, while α position was difficult to break for its large bond-dissociation energy. In addition, alkyl radicals on aromatic rings were mainly produced by the crack of β position during pyrolysis process. The weaken intra-and inter interactions made carbons between α and β more easily to crack and decreased the proportion of Hβ consequently. Deuterium oxide (D2O, 99.9% atom grade) was used to substitute for pure water to study proton transfer route during hydrothermal 1
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treatment and the experiment condition was the same as above. The result of D2O experiment was basically consistent with H2O experiment; nevertheless, each part had obvious difference. Previous work11 had showed that tar yield of Inner Mongolia lignite increased most and reached maximum 17% at 260 oC. Therefore, three samples of tar produced by IML, H2O-260 oC and D2O-260 oC were selected to analysis and were shown in Figure 7. If the percentage integral area is reduced, then there is an incorporation of isotope deuterium, and vice versa. The percentage integral area of polycyclic aromatic protons (δ=9.0~7.2) 12.1% decreased and this suggest that isotope deuterium in D2O was incorporated into polycyclic aromatic rings during hydrothermal treatment. Moreover, the content of monocyclic aromatic protons (δ=7.2~6.0) 15.33% increased while Har was lower than that of D2O-260 oC. This indicated that isotope deuterium tended to incorporated into polycyclic aromatic rings in terms of Har. For Hali part, the percentage integral areas of Hβ and Hγ reduced somewhat relatively. It indicated that isotope deuterium also incorporated into these positions and demonstrate that the hydrothermal treatment really has function on Har, β position and γ position of aromatic side chains.
3.3 Analysis of Hydrogen Transfer Route During Hydrothermal Treatment It was evident from the mass spectra of standard and H2O treatment experiment that toluene had a significant [M-1]+. 91 fragment peak while in D2O treatment experiment m/z=92 had the maximum abundance gradually in Figure 1. Fragments of C5H5+(m/z=65) and C3H3+(m/z=39) will form in the wake of further disintegrating of toluene by losing C2H2. A notable difference was that around these characteristic peaks existed more vary abundance cluster of peaks in D2O experiment mass spectra and the abundance were enhanced simultaneously. The m/z=92 peak was almost equal with m/z=91 in D2O-180 oC and m/z=93 was also enhanced. This indicated that a certain amount of toluene contained one deuterium atom and small amount contained two deuterium atoms. From D2O-220 oC m/z=92 had the maximum abundance and the abundance of other isotopic peaks (m/z=93,94,95) were enhanced unceasingly. Peak of m/z=93 was higher than m/z=91 when processing temperature reached 260 oC. This was attributed to free deuterium instead of hydrogen being incorporated into the structure of lignite during the hydrothermal treatment process. This means that in the pyrolysis of lignite fracture process, benzyl methyl radicals were combined with deuterium to form stable compounds. Data of the deuterated extent in toluene were shown in Table 2. The abundance value of peaks m/z=92, 93, 94, 95 and 96 can be read from Figure 1 were integrated and normalized to 1. It was noticed that toluene has strong [M-1] (m/z=91) peak and the calculated value of 93 was stand for the extent of one deuterium incorporated into toluene to a certain extent, viz., D1. Similarly, m/z=94, 95 and 96 represented D2, D3 and D4, respectively. The toluene molecular was mainly replaced by D1 (0.17) at 180
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o
C and the value of D1, D2, D3 and D4 were increased with the increase of treatment temperature. This indicated that the increase of treatment temperature can be propitious to the incorporation of deuterium. Considering the data of covalent bond dissociation energy of C6H5CH2-H (356KJ·mol-1) and C6H5-H (461KJ·mol-1) in Table 6 26. We could estimate that deuterium atoms prefer to incorporate into the methyl carbon rather than aromatic ring. However, -CH3 can make the ortho and para hydrogen activated to be further substituted for deuterium as -CH3 was weak activated group. Proposed reaction schemes of toluene and its fragmentation mechanism were shown schematically in Eq. (5-8) . + . + CH3 . +
M
. + CH2
(M-1)
. +
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-C 2H2
m/z 91
. +
(Eq.5)
m/z 65
CH2
(Eq.6)
CH2
+ 2D
CH2D + D
CHD2 + D
2
CH2D
CHD2 + H
(Eq.7)
CD3 + H
(Eq.8)
The mass spectra of naphthalene in Figure 2 showed the similar changes with
toluene. The extent of incorporation of deuterium into naphthalene was increased with the increase of treatment temperature. The calculated peaks were [M], [M+1], [M+2], [M+3] and [M+4] (m/z=128, 129, 130, 131 and 132). The relative amounts of the deuterated species were shown in Table 2. Peaks of m/z=130, 131 and 132 enhanced obviously from 260 oC indicated that there were also two and more deuterium atoms incorporated into certain amount of naphthalene and this result can be reflected from Table 2. The value of D1 was increased first and then decreased while D2, D3 and D4 were increased with the increase of treatment temperature. It was worth mentioning that all deuterium atoms were incorporated into the aromatic ring and this was the biggest difference with toluene. This result was consistent with the change of proton in aromatic rings in 1H-NMR. α substituted structure of naphthalene has two resonance types and both of them has one complete benzene ring while β substituted only has one complete benzene ring. Therefore, the substitution reaction of naphthalene was more likely to occur in the α position. Typically, substituents in the α position for α carbon was more activity than β carbon, however, this was controlled by kinetics and α carbon substituents will be transferred to the β carbon at high temperature. This result can be seen that the abundance of
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peaks m/z=133 and 134 enhanced from 260 oC as well. Possible reaction scheme of naphthalene was put forward in Eq.9. HO
O D
+ H 2O
(Eq.9)
Heat + HDO + CO 2
H
+D
D
D
(Eq.9)
+
+H
The mass spectra in Figure 3 showed the changes of phenol that in H2O and D2O
experiments. Phenol formed molecular ion peak [M]+. (m/z=94) by losing an electron firstly and its abundance was rather strong. The [M]+. was prone to further fragmentation to form [M-28]+. (m/z=66) and [M-29]+. (m/z=65) by losing neutral molecule CO or CHO. Figure 3 showed that spectra had the same molecular ion peak of m/z=94 in addition to D2O-300 oC. Similarly, peaks of [M], [M+1], [M+2], [M+3] and [M+4] (m/z=94, 95, 96, 97 and 98) were integrated and normalized to 1 and the data were recorded in Table 2. However, its value of D4 were all very low (
