Article pubs.acs.org/EF
Influence of Temperature on the Product Distribution during the Fast Pyrolysis of Indonesian Oil Sands and the Relationships of the Products to the Oil Sand Organic Structure Fan Nie,† Demin He,† Jun Guan,† Han Bao,† Kaishuai Zhang,† Tao Meng,‡ and Qiumin Zhang*,†,§ †
School of Chemical Engineering and §State Key Laboratory of Fine Chemicals, Dalian University of Technology, No. 2 Linggong Road, Dalian 116024, P. R. China ‡ Beijing Guodian Longyuan Environmental Engineering Co., Ltd., Building No. 1, Yard No. 16, Xisihuan Mid Road, Haidian District, Beijing 100039, P. R. China S Supporting Information *
ABSTRACT: The influence of temperature on the product distributions of oil sand fast pyrolysis was studied by a combined pyrolyzer-gas chromatography/mass spectrometry (Py-GC/MS) technique. Characteristics of the organic structure in bitumen deduced from the pyrolytic products and given by 1H/13C nuclear magnetic resonance (NMR) spectrometry were compared as well. The oil sand sample was pyrolyzed at temperatures from 300 to 650 °C in intervals of 50 °C in an inert atmosphere (helium gas), and more than 200 types of compounds were detected, including carbon dioxide, aliphatics (alkanes, cycloalkanes, olefins, dialkenes, cycloolefins), aromatics (alkyl benzenes, alkyl naphthalenes, alkyl indenes), oxygen-containing compounds, and sulfurcontaining compounds. From the evolution of the product yields, it was clearly observed that temperature affected both the primary and secondary reactions during fast pyrolysis. Major thermal cracking took place until about 400 °C, as evidenced by a dramatic increase in product species and yields. However, temperatures higher than 600 °C were beneficial for generating smaller molecules as products. Among the pyrolytic products, alkanes and olefins were predominant and were mainly derived from the thermal cracking of abundant polymethylene substituents linking to the aromatic cores. It was found that the yields of alkanes and olefins decreased with increasing carbon number, and more olefins were generated at higher temperatures. Monoaromatics with more alkyl or alkenyl multisubstituent groups appeared above 400 °C, but the substituent groups were no longer than isopropyl. In the range of 300−650 °C, few polycyclic aromatic hydrocarbons were observed. Higher temperatures also obviously enriched the species of naphthalene, indene, and compounds with heteroatoms. In addition, the raw aliphatic sulfur in the sample tended to be converted into sulfur-containing heterocycles during fast pyrolysis in an inert atmosphere. The results of this study show that both the NMR and Py-GC/MS methods can provide information on the organic structures in oil sand. However, NMR spectrometry is able to present an overview of the structure of hydrocarbons, whereas Py-GC/MS can help to deduce some characteristics of the macromolecules in oil sand organics.
1. INTRODUCTION Global primary energy consumption reached 13147.3 million tonnes of oil equivalents in 2015.1 It is predicted that the world’s energy demands will continue to grow and increase by more than one-third between 2013 and 2040, mostly due to developing countries.2 During the past several decades, conventional oil and gas played a predominant role in global energy consumption. However, considering the prospective energy demands and complicated supply situation, researchers are actively seeking fuel alternatives and corresponding conversion technologies. Unconventional fossil fuels, such as oil shale, shale gas, heavy oil, and oil sands, are thought to be complements for today’s huge energy supply. However, given the concerns about environmental problems and global warming, cleaner and more efficient utilization of unconventional fossil fuels is attracting attention. A better understanding of the properties and evolution during processing of unconventional fossil fuels might provide benefits in terms of better development of these resources. Oil sand, also called tar sand or bituminous sand, is a sedimentary rock (consolidated or unconsolidated) containing © XXXX American Chemical Society
bitumen (solid or semisolid hydrocarbons) or other viscous forms of petroleum. Bitumen occurring in oil sand deposits is notoriously immobile at formation temperatures and therefore requires some stimulation (usually by thermal means) to ensure recovery.3 Depending on the mining technique and type of oil sand, bitumen can be recovered by hot water extraction,4−7 organic solvent extraction,3 thermal treatment or direct combustion,8−10 or other underground mining methods (in situ oil mining).11,12 As a thermal treatment method, oil sand pyrolysis is thought to be a promising means of achieving bitumen recovery without prior separation. It has been reported that oil sand bitumen consists of core segments comprising aliphatic and aromatic units bonded together by polymethylene bridges. In addition, sulfide linkages predominately exist between core segments.13 Through pyrolysis, bitumen is not only recovered but is also upgraded as an overhead product without the formation of excessive amounts of coke (as small Received: October 14, 2016 Revised: December 20, 2016 Published: January 10, 2017 A
DOI: 10.1021/acs.energyfuels.6b02667 Energy Fuels XXXX, XXX, XXX−XXX
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Energy & Fuels amounts of fixed carbon were found in the solid residues).3,13 In addition, pyrolysis is more effective than hot water extraction in recovering bitumen from oil-wet types of oil sands.14 Since 1920, many studies have been performed on the pyrolysis of oil sands, most of which aimed at obtaining reaction schemes with associated kinetics to describe thermal cracking quantitatively.15 A typical examples, Barbour et al.,16 Murugan et al.,17 and Phillips et al.18 developed pseudocomponent reaction models for explaining the process of bitumen thermal cracking. It is known that temperature significantly affects oil sand pyrolysis and determines the cracking extent of bitumen. Generally, bitumen does not show appreciable thermal cracking below 300 °C, and the main pyrolysis temperature range (in which thermal conversion increases most) is 350−500 °C.19 Moreover, the effects of temperature on the composition of pyrolytic products have been studied in fixed and rotary kilns and fluidized-bed reactors.20−23 It has been reported that changes in temperature result in different yields and constituents of pyrolysis products. Meng et al.20 observed that the composition of the liquid fraction changed with changing temperature in the range of 430−600 °C in a fixed-bed reactor. As the temperature increased, saturates tended to decrease, whereas aromatics and resin exhibited increasing trends. Hanson et al.21 conducted continuous-flow pyrolysis experiments in the range of 475−575 °C in a rotary kiln and found a decrease of the liquid yield (C5+) and an increase of the gas yield (C1−C4) with increasing temperature. However, the pyrolysis mechanism, including the effect of temperature, is still far from being fully understood. As a result of the pyrolysis temperature, the evolution of the product yields and constituents with varying temperature could indicate the pyrolysis process. Nevertheless, the secondary reactions of primary volatiles during pyrolysis could strongly impact the final composition of pyrolytic products. Therefore, a method of conducting pyrolysis under conditions resulting in lower extents of secondary reactions and allowing coupling with in situ analysis will be beneficial for exploring the effect of temperature on pyrolysis. Recently, pyrolyzer-gas chromatography (Py-GC) and pyrolyzer-gas chromatography/mass spectrometry (Py-GC/ MS), as simple and fast methods for performing both pyrolysis and in situ analysis of volatiles, have become widely used for the study of pyrolysis processes and the characterization of macromolecules with low solubilities or insoluble substances. Ramljak et al.24 observed the characteristic behaviors of structurally different bitumen fractions by Py-GC. They found that maltenes, compared with the polyaromatic structures of asphaltenes, achieved higher ratios of unsaturated to saturated light hydrocarbons in pyrolytic fragments (C3−C5). Calemma et al.25 also compared the differences in the features of asphaltenes given by 1H/13C nuclear magnetic resonance (NMR) spectrometries and Py-GC/MS. In contrast to the calculated average structural parameters based on NMR spectrometry, longer aliphatic chain but smaller aromatic systems were predominantly observed by Py-GC/MS. Zhao et al.26 also investigated bitumen pyrolysis behavior by Py-GC/ MS. They showed that the molecular weight of evolved gas varied with temperature. Unfortunately, no additional details were reported for the changes in specific compounds. As explained by Kapadia et al., 15 further studies on the characterization of bitumen and observation of changes in its physical properties and chemical structure during pyrolysis are
needed to improve bitumen recovery and conversion by pyrolysis. To explore the influence of temperature on the formation and distribution of the products of oil sand fast pyrolysis, PyGC/MS was applied for pyrolysis and the online analysis of volatiles. Simultaneously, the possible pyrolytic pathways for the formation of the major products are discussed. Furthermore, characteristics of the organic structure deduced from the pyrolytic products and given by 1H/13C NMR analysis were compared.
2. MATERIALS AND METHODS 2.1. Materials. The oil sand sample was collected from Buton, Indonesia. Initially, the sample was crushed and sieved below 200 mesh and then preserved in a dryer. Considering the possibility of devolatilization when the oil sand bitumen was dried in the oven, the proximate analysis, other than for moisture content, was performed according to Chinese standard GB/T 212-2008. Furthermore, we determined the content of carbonates upon the release of CO2 (Chinese standard GB/T 218-1996), which could be used to subtract the CO2 content from the value for volatiles. The moisture and components were determined by Dean−Stark extraction in toluene and n-heptane, respectively.27 The components of maltene were measured by separating it into saturates, aromatics, and resins.28 The results of proximate and component analyses are listed in Table 1.
Table 1. Proximate Analysis and Components of Oil Sand Sample property
value
Proximate Analysis (wt %, Dry Basis) ash 52.78 volatiles 13.49 CO2 25.69 fixed carbon 8.04 Oil Sand Components (wt %, As Recevied) bitumena 19.33 maltenesb 13.72 asphaltenesc 5.61 moisture 2.38 solid dregs 78.29 Maltene Components (wt %) saturates 12.29 aromatics 42.86 resins 44.85 a
Toluene-soluble. bn-Heptane-soluble. cn-Heptane-insoluble.
Ultimate analysis (Table 2) was carried out on a varioEL III element analyzer (Elementar). Minerals derived from the Dean−Stark extraction were analyzed by X-ray fluorescence (Bruker SRS3400), the results of which are reported in Table 3. 2.2. XPS and 1H/13C NMR Spectrometry. X-ray photoelectron spectroscopy (XPS) analysis of raw oil sands was undertaken using a
Table 2. Ultimate Analysis of Oil Sand and Its Bitumen content (wt %)
a
B
component
oil sand (daf)
bitumen
C H N S Oa
75.13 7.06 0.66 6.12 11.03
75.90 8.35 0.39 6.95 8.42
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amu. Chromatographic peaks were identified by means of the NIST mass spectral data library. According to the results of thermogravimetric analysis in our previous study,30 samples were pyrolyzed at temperatues from 300 to 650 °C in 50 °C incremements covering the main oil-producing stage during bitumen pyrolysis. Because of the tiny amount and volume of the sample used for analysis, the heating could be uniform and fast in the furnace of the pyrolyzer. However, considering the complexity of the thermal cracking products, the substance corresponding to each peak could not be quantitatively determined. In this work, the peak area per sample weight, as relative intensity, was employed for comparison. Meanwhile, because of the great area variety of tar constituents within the studied temperature range, the logarithm of the value was taken as
Table 3. X-ray Fluorescence Results for Minerals Derived from Dean−Stark Extraction mineral
content (wt %)
mineral
content (wt %)
CaO SiO2 Al2O3 Fe2O3 SO3 MgO
76.3 11.5 3.97 2.93 1.96 1.80
K2O Na2O SrO TiO2 BaO P2O5
0.483 0.380 0.271 0.241 0.141 0.102
Thermo ESCALAB250 surface analysis instrument with Al Kα radiation as the X-ray source. The sample was also crushed and sieved below 200 mesh. The size of the analyzed area was 6 × 6 mm2. In addition, the analyzer was operated in constant-analyzer-energy (CAE) mode with 1 eV energy steps at a pass energy of 100 eV. 1 H and 13C NMR spectra were recorded to investigate the details of the aliphatic and aromatic structures. To facilitate NMR analysis, the bitumen was extracted from the oil sands with a Soxhlet apparatus in toluene for 8 h. The resulting solution was suction-filtered and evaporated on a vacuum-rotary apparatus at 55 °C. The ultimate analysis of the extracted bitumen is also listed in Table 2. The discrepancy between the raw oil sand and the extracted bitumen might be because some preasphaltene in the bitumen was not able to be completely extracted with toluene.29 However, the content of preasphaltene in oil sands is generally far less than that of bitumen. Therefore, the characteristics of the toluene extracts could serve as the main organic structure of the oil sands. Judging from the results of gel permeation chromatography (GPC), the average molecular weight of the bitumen was 1179 g/mol. One-dimensional 1H and 13C NMR spectra were obtained on a Bruker AVANCE III 500 MHz spectrometer operating at 500 and 125.7 MHz, respectively. The bitumen was prepared by dissolving about 90 mg in 1 mL of deuterated chloroform (CDCl3). Experiments were carried out at ambient temperature. The 13C NMR spectrum was collected using a pulse program of zgpg30 and a relaxation delay (D1) of 1, and 12800 scans were employed to improve the signal-to-noise ratio sufficiently. 2.3. Py-GC/MS. An EGA/PY-3030D multishot pyrolyzer with a selective sampler (SS-1010E), a carrier gas selector (CGS-1050Ex), and a microjet cryo-trap (MJT-1030Ex; Frontier Laboratories Ltd., Fukushima, Japan) was used for pyrolysis. Meanwhile, a gas chromatograph−mass spectrometer (Trace GC Ultra-ISQ, Thermo Scientific, Waltham, MA) was equipped for the online analysis of volatiles. The preparation of samples was the same as in section 2.1. Initially, about 0.80 mg of prepared sample was placed in a sample cup and then moved into the pyrolyzer with a sampler tool. The pyrolyzer was set in single-shot mode, and a constant furnace temperature was employed in this study. The reaction time was 0.5 min, which could be achieved with a selective sampler to control the pressure of the purge gas. To avoid the condensation of volatiles before entering the gas chromatograph, the temperature of the interface was autocontrolled up to 300 °C. A carrier gas of pure He at 1.0 mL/min was selected to enter from the pyrolyzer for an inert atmosphere and carried the volatiles through a weak-polarity stainless steel capillary column (Ultra alloy+-5, 30.0 m × 0.25 mm × 0.25 μm, Frontier Laboratories Ltd., Fukushima, Japan) for mixture separation. To maintain substances with lower boiling points, before and during the pyrolysis, a microjet cryo-trap system was applied to condense the volatiles by cooled pure nitrogen gas jetting at the start of the column. The cooled pure nitrogen gas was at about −190 °C and provided cooling through a thermal exchange coil in a container filled with liquid nitrogen. After the reaction was completed, the oven temperature started at 40 °C, was increased at a rate of 4 °C/min to 200 °C, and was held at this value for 5 min. Finally, the oven temperature was programmed from 200 to 300 °C at 3 °C/min and held at the latter temperature for 15 min. The ion-source and transfer-line temperatures were both 300 °C. Each component was identified by MS detector with an electron ionization source (70 eV) in full scan mode ranging from 40 to 400
relative intensity = log(peak area/sample weight)
(1)
An average of at least three parallel measurements was calculated. Meanwhile, considering the superiority of MS in identifying the species of substances but its difficulties in accurately clarifying isomers and the positions of substituents, we took compounds with the same weight and the same types and numbers of substituents as only one statistical unit. For example, 1,2,3-trimethylbenzene, 1,3,5-trimethylbenzene, and (1-methylethyl)-benzene were treated as “alkyl C3benzene” (C9H12).
3. RESULTS AND DISCUSSION 3.1. Characterization of Oil Sand Sample. Oil sands are mixtures of complex organic matters and various minerals, whose constituents are determined by different formations and geological conditions. The organic matter in oil sands is composed of complex substances with aromatic and hydroaromatic cores in a wide range of configurations of aliphatic subgroups. Other heteroatoms, including O, N, and S, are incorporated into organic matter in a variety of forms.31,32 The results listed in Tables 1−3 reveal the basic components of the raw oil sand sample. As shown in Table 1, the organic matter of the oil sand sample comprised nearly 20 wt %, and most of it consisted of n-heptane-soluble maltenes. According to the ultimate analysis, shown in Table 2, the content of sulfur on a dry, ash-free basis was greater than 6 wt %, generally higher than the sulfur contents in coal, oil shale, biomass, and other types of hydrocarbon resources. Minerals contain amounts of carbonates. Measured by the release of CO2, the carbonate content exceeded 25% of the total weight. Simultaneously, as analyzed by X-ray fluorescence (Table 3), calcium was most prominent, followed by silica and aluminum. Figure 1 shows an XPS survey spectrum of the raw oil sand powder, with the species corresponding to the obvious peaks labeled. However, no obvious peaks were found in the range of nitrogen, possibly because of its lower content. In this work, in particular, the species of carbon and sulfur were quantitatively determined by the deconvolution of the C 1s and S 2p spectra according to the works of Lamb et al.33 and Ma et al.34 The results of the peak fitting and deconvolution are shown in Figure 2 and Table 4. Induced by oxygen-containing functional groups, the C 1s photoelectron peak had an asymmetric component appearing on the high-binding-energy side and accounting for about 17% of the total peak area. This indicates that approximately 83% of the carbon atoms on the surface were in unoxidized states. Because of the presence of component peaks for both CC and CC species in the sample, the peak width was 1.9 eV, which was broader than that of clean polyethylene in the same region.33 As shown in Figure 2b, every species in the S 2p region was deconvolved into two peaks, represented by S 2p1/2 and S 2p3/2. According to the C
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Table 4. Contents of Carbon and Sulfur Species Obtained by the Deconvolution of XPS Spectra species
binding energy (eV)
fwhm
content (atom %)
1.90 1.90 1.90
82.90 10.20 6.90
1.90 2.00 1.25 1.40
69.89 19.76 7.80 2.55
C 1s CC CO CO
284.6 286.2 288.4
aliphatic sulfur aromatic sulfur sulfone inorganic sulfur
162.9, 164.4, 168.4, 170.9,
S 2p 164.1 165.6 169.4 171.6
which aromatic sulfur is predominant.35 The high content of aliphatic sulfur reveals abundant sulfide linkage and mercaptan subgroups but relatively little incorporated into the aromatic cores. The chemical structure, especially for hydrocarbons, was determined from the 1H and 13C NMR spectra of the extracted bitumen. According to the works of Sheremata et al.36 and Michon et al.,37 the spectral regions of protons and carbons corresponding to the NMR chemical shifts are listed in Table 5.
Figure 1. XPS survey spectrum of a raw oil sand powder.
Table 5. NMR Spectrometry Chemical Shifts of Proton and Carbon Spectral Regions region
chemical shift (ppm)
HP1 HP2 HP3 HA1 HA2
1.00−0.50 2.00−1.00 2.30−1.90 7.05−6.00 9.00−7.05
CP CA
55.00−42.70 145.00−115.0
structural type Proton γ-CH3 aliphatic CH2 α-CH3, CH2, CH monoaromatic CH polyaromatic CH Carbon aliphatic carbon aromatic carbon
content (mol %) 23.31 53.94 15.41 4.62 2.72 62.10 37.90
It was found by 13C NMR spectrometry that aromatic carbon in bitumen accounted for only about 38% of the total. However, the aliphatic structure was abundant, and the high value of HP2 resulted in a large proportion of internal methylene chains. According to the relatively lower content of aliphatic groups in maltenes (listed in Table 1), the aliphatic structure can be derived from aliphatic substituents linking to aromatic cores. To better understand the chemical structures of the hydrocarbons, the average structural parameters were calculated based on the results of NMR spectrometry, molecular weight, and ultimate analysis.38−40 In addition, two assumptions were applied during the calculations:41 (1) The number of hydrogens attached to Cα (HP3) is equal to the number of hydrogens attached to Cβ (HP2). (2) The overall ratio of hydrogen to carbon for substituted chains is 2. Based on these two assumptions, we can write HP1 + HP2 + HP3 HP1 + HP2 + HP3 = HP1 =2 HP1 HP2 HP3 (HP2 + HP3) + x + y + 3 (2) 3 x
Figure 2. Fitted curves of XPS spectra for C 1s and S 2p.
A description of the other structural parameters and their values are provided in Table 6. Among them, the aromaticity values ( fA) obtained from 1H NMR calculations and the integral of the 13 C NMR spectrum are close, which provides a good reliability
results listed in Table 4, more than 97% of the total sulfur was organic sulfur. Interestingly, about 70% was aliphatic sulfur, whereas aromatic sulfur accounted for less than 20%. This is a characteristic quite different from that of Alberta oil sand, in D
DOI: 10.1021/acs.energyfuels.6b02667 Energy Fuels XXXX, XXX, XXX−XXX
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Energy & Fuels Table 6. Description of Structural Parameters and Their Values parameter CT HT fA
meaning
formula for calculation CT = [Mn × (C weight percentage)]/1201 HT = [Mn × (H weight percentage)]/100.8
total number of carbons per mole total number of hydrogens per mole aromaticity
C T/HT − (HP1/3 + HP2/x + HP3/y)/HT C T / HT
value 74.51 97.67
by 1H NMR spectrometry
fA =
by 13C NMR spectrometry
fA = CA/100
0.38
atomic hydrogen/carbon ratio of hypothetical unsubstituted aromatic structure
HAU (HA1 + HA2) + HP3/y = CA C T/HT − (HP1/3 + HP2/x + HP3/y)HT
0.53
σ
ratio of substituted carbons to peripheral aromatic carbons
σ=
CA*
number of aromatic carbons per unit
1+ HAU = CA
n Mun C*T H*T C*S C*α C*AP C*I C*F R*A R*T R*N C*N C*P fN fP l BI
number of units per mole unit molecular weight number of carbons per unit number of hydrogens per unit number of saturated carbons per unit number of α-carbons per unit number of peripheral aromatic carbons per unit number of internal aromatic carbons per unit number of fused aromatic carbons per unit number of aromatic rings per unit total number of rings per unit number of naphthenic rings per unit number of naphthenic carbons per unit number of paraffinic carbons per unit naphthenic fraction paraffinic fraction average chain length of substituted alkanes branching index
n = CT fA/CA* Mun = Mn/n C*T = CT/n HT* = HT/n CS* = CT* − CA* Cα* = HP3 × HT*/x C*AP = C*α + (HA1 + HA2)H*T CI* = CA* − CAP * CF* = 6 + CA* − 2CAP * R*A = C*I /2 + 1 R*T = (CT − HT/2 + 1 − CA/2)/n RN* = RT* − RA* CN* = 3RN* C*P = C*T − C*N − C*A f N = C*N/C*T f P = 1 − fA − f N l = 3CP*/HP1 BI = HP1/HP2
HAU CA
check.40 The number of repeating units per mole (n) was determined for the average chemical structure of bitumen. According to the numbers of peripheral (C*AP), internal (C*I ), and fused (C*F ) carbons, the aromatic core in a unit was formed with about six pericondensed aromatic rings, whereas the number of naphthenic rings was about three. Also, as determined by 1H NMR spectrometry, the average aliphatic chain length was about four. Simultaneously, the peak area of the 13C NMR spectrum near 29 ppm was about 11% of the total, which appears with the internal methylene groups more than three carbon atoms away from a branching point and more than four carbon atoms away from a terminal point.42 Multiplied by the total number of carbons per unit, the number of methylene groups in straight side chain was about seven; therefore, the length of the straight chain per unit was about 12. In terms of the structural parameters, the probable average hydrocarbon structure of the bitumen is shown in Figure 3. 3.2. Product Distribution of Oil Sand Fast Pyrolysis. As for coal or biomass, vapors generated during the fast pyrolysis of oil sands consist of gases (such as H2, CH4, CO, CO2), volatile compounds, and nonvolatile oligomers.43 Figure 4 presents an example of a total ion chromatogram (TIC) of fast pyrolysis products at 600 °C. About 200 types of organic compounds were identified by the means of NIST mass spectral data library, including aliphatics (alkanes, cycloalkanes, olefins, dialkenes, cycloolefins), aromatics (alkyl benzenes, alkyl
HP3/x (HA1 + HA2) + HP3/x
0.39
0.54
6CA* − 11 CA*
23.2 1.26 935.39 59.1 77.5 35.9 6.6 12.3 10.9 4.6 6.4 9.6 3.1 9.3 26.6 0.16 0.45 4.4 0.43
Figure 3. Average hydrocarbon structure of oil sand bitumen.
naphthalenes, alkyl indenes), oxygen-containing compounds, and sulfur-containing compounds. Nitrogen-containing compounds were in a minority and mainly consisted of amines and amides. Among organic species, the most evident peaks were for alkanes and olefins. Of these, C3−C5 species were detected only as olefins, whereas no evident peaks of alkanes were found with the same numbers of carbon atoms. Above C6, alkanes and olefins appeared remarkably in doublet peaks (evident in the inset graph of Figure 4, alkane following olefin with the same number of carbons), and the peaks corresponding to the identified carbon numbers are marked in Figure 4. To compare the total yield of volatiles at each temperature, the peak areas per sample weight at each temperature were E
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Figure 4. Total ion chromatogram (TIC) of fast pyrolytic products at 600 °C.
dissociative fragments or fragments with lower-energy bonds exist in the bitumen of oil sands, and they will volatilize at lower temperatures.26 However, depending on their bond energies, higher temperature promoted deep thermal cracking to generate more fragments so that new groups appeared with a rise of existing groups above 350 °C. The yield of alkanes started to decline above 450 °C. However, the relative intensity of other species increased stably from 450 to 600 °C and then exhibited a slight decrease up to 650 °C. Considering the low weight of sample, pyrolysis with a carrier gas, vacuum by MS, and microjet cryo-trap at the start of the column, the residence time for volatiles in the furnace was short, which weakened the secondary reactions. Therefore, almost all of the relative intensity still remained stable when the temperature was above 550 °C compared to the decrease of tar yield in a fixed reactor above 500 °C.44 As shown in Figure 6c, carbon dioxide was observed with increasing intensity from 350 to 650 °C. Generally, carbon dioxide can come from the decomposition of carboxyl groups in bitumen (above about 300 °C) and carbonates in minerals (above about 600 °C). Because of the high level of carbonates in the oil sands, we cannot exclude the decomposition of carbonates at higher temperature. However, the lack of an evident rise in the carbon dioxide intensity above 600 °C might indicate a very small change in the carbonates. In addition, another route for carbon dioxide is possibly from the reaction with oxygen, which can absorb in the sample or in the dead zone of the sample cup. The line labeled “Others” in Figure 6c is mainly the intensity of unidentified peaks and very a few nitrogen-containing compounds. Unidentified peaks generally have strong ion-fragment intensities at higher amu level, but no similar data was found in the NIST mass spectral data library. We thought that there might be some fragments with high masses or oligomers whose relative intensities decreased with the intensification of thermal cracking with rising temperature. In the following sections, the evolutions of typical compounds as a function of temperature are discussed. 3.3. Aliphatics. Aliphatics, also known as nonaromatics, are organic compounds without aromatic-ring configurations of hydrocarbons, such as alkanes, olefins, and naphthenes.45 Identified aliphatic compounds included alkanes, cycloalkanes, olefins, dialkenes, and cycloolefins. Figure 7 shows the distributions of alkanes and olefins at each temperature. The dashed lines in the figure connecting adjacent points are meant
summed, as shown in Figure 5. With increasing temperature, the total detected peak area increased and achieved the highest
Figure 5. Peak intensity (area/sample weight) at each temperature.
intensity at 600 °C. However, it decreased at 650 °C, which might be related to the limits of detection (less than 40 amu), as thermal cracking is strengthened and more small molecules are generated at higher temperatures.19 Among species of detected volatiles, aliphatics comprised a predominant proportion (more than 50%) at temperatures greater than 450 °C. To clarify the evolution of each group with temperature, the relative intensity of each species is shown Figure 6. In terms of organic groups, alkanes and olefins are the major pyrolysis products of the oil sands, especially above 450 °C. Below 350 °C, cycloalkanes, alkyl naphthalenes, alkyl indenes, and oxygencontaining compounds were rarely detected, but they all exhibited dramatic increases from 350 to 400 °C. Other groups also increased in the same temperature range, indicating that the organic structure in oil sands would be visibly damaged above 350 °C. This evident distinction from 350 to 400 °C agrees with the two-stage model of oil sand pyrolysis, as the oilproducing process can be divided by temperature into devolatilization-dominant and thermal-cracking-dominant stages.17,20 For the devolatilization-dominant stage, some F
DOI: 10.1021/acs.energyfuels.6b02667 Energy Fuels XXXX, XXX, XXX−XXX
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Figure 7. Alkane and olefin distributions by carbon number at each temperature.
to about 400 °C. Moreover, the relative intensity above 450 °C appeared to be an approximately linear relationship with the number of carbon atoms and continuously decreased with increasing carbon number. Generally, oil sand pyrolysis without a catalyst is considered to occur through free-radical reactions, which have much lower activation energies than carbonium-ion reactions.46 Olefins and alkanes have been reported to be generated from the disproportionation of alkyl radicals.13,47 Although the total yield of alkanes or olefins reached its highest value at 600 °C, as shown in Figure 7, the highest intensity was not the same for every carbon number. Alkanes higher than C17 and olefins higher than C21 had lower intensities at 600 °C than at 550 °C. When the temperature was increased to 650 °C, the identified alkanes had narrowed to C25, and the yields were all lower than those at 600 °C. However, as shown in Figure 7b, propylene (C3H6), which can be easily derived from the breaking of long alkyl chains at higher temperatures,48 was detected only when the temperature was above 550 °C. It thus appears that higher temperatures promote the thermal cracking of aliphatic substituents from aromatic cores to generate more alkyl
Figure 6. Intensities of groups as a function of temperature.
only to distinguish the distributions at various temperatures conveniently. Based on the instrument and test method, alkanes from C5 to C27 were able to be detected, whereas the olefins detected were from C3 to C26. Above 400 °C, however, both alkanes and olefins had wider ranges of carbon numbers, which again illustrates that major thermal cracking could take place up G
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naphthalene, indene, and hydroindene. The evolutions of all of the aromatics are shown in Figure 9. As shown in Figure 9a,
radicals, as alkanes and olefins have wider carbon number distributions with relatively higher yields for pyrolysis temperatures above 450 °C. However, secondary reactions are simultaneously strengthened, so that the alkyl radicals break into smaller pieces, which produce more lightweight volatiles above 600 °C. Meanwhile, it has been reported that higher temperatures are also beneficial for chain-scission and dehydrogenation reactions. Possible routes are shown in eqs 3 and 4.49 Because of the higher energy of the CH bond, eq 4 might be much easier than eq 3 at the same temperature. In addition, long alkanes are less thermally stable, which also favors chain scission (eq 4) rather than dehydrogenation. Therefore, higher ratios of unsaturated to saturated hydrocarbons have ben found above 500 °C. Figure 8 specifically
Figure 8. Evolutions of C11H22 (olefin) and C11H24 (alkane) with temperature.
shows the evolutions of C11H22 (olefin) and C11H24 (alkane) from 300 to 650 °C, which illustrates that the relative intensities of olefins and alkanes vary with the temperature. The yield of C11H22 exceeded the yield of C11H24 at temperatures above 450 °C, which coincides with the trends in the total yields of olefins and alkanes shown in Figure 6a. In addition, the unsaturated-to-saturated ratio generally increased with increasing carbon number.
Figure 9. Evolutions of aromatics with temperature.
alkyls and alkenyls up to C8-substituted benzenes were detected. However, the substituent groups were no longer than isopropyl, which means that the derivatives were multisubstituted benzenes with C1−C3 alkyl and alkenyl groups. With increasing temperature, each compound exhibited a trend similar to that of the summed intensity of benzenes in Figure 4b. At 300 °C, only benzenes with alkyls from C1 to C3 were found. Higher temperatures diversified the derivatives of benzene observed, with alkyl C4−Ph appearing at 350 °C, alkyl C6/C8−Ph appearing at 500 °C, and alkenyl C4/C5−Ph appearing at 400 °C. As shown in Figure 9b, alkyl-substituted naphthalenes, indene, and 1H-indene were observed until 400 °C, whereas alkyl-substituted hydronaphthalenes were detected at about 450 °C. Similarly to the aliphatics, lower temperatures were suitable for the devolatilization only of substances with low molecular weight, which is generally because of the weak bonds connecting to the cores.17 Sufficient energy will promote these fragments to be disaggregated by breaking the connections combined with aromatics. Therefore, above 500 °C, more derivatives were generated. Meanwhile, higher temperatures benefit polycondensation and dehydrogenation.48
The intensity of cycloolefins was lower than those of olefins and alkanes. The identified cycloolefins mainly contained C5 or C6 rings, such as cyclopentadiene, cyclopentene, cyclohexadiene, and cyclohexene. Cycloalkanes and dialkenes were detected as well. Alkyl-substituted cyclopentane and cyclohexane were predominant among the cycloalkanes, whereas cycloalkanes substituted with long chains up to C18 and cycloalkyls were also detected. Interestingly, all of the dialkenes were identified as pentadiene or its homologues. 3.4. Aromatics. Generally, the amounts and polymerization degrees of aromatics in the raw bitumen of oil sands are lower than for those in coal.13 The identified aromatics included alkyland alkenyl-substituted monoaromatics, naphthalene, hydroH
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Energy & Fuels Parts of aromatics can come from the dehydrogenation of cycloalkyl species. Regarding indene, some believe that it can be generated from hydronaphthalene.50 However, few polycyclic aromatic hydrocarbons (PAHs) were observed from 300 to 650 °C. This was mainly because the sample in the pyrolyzer was heated and reached the predetermined temperature swiftly. Then, the carrier gas rapidly carried the volatiles out of the hot region, which weakened the secondary reactions and decreased the formation of PAHs.51 3.5. Sulfur-Containing Compounds and OxygenContaining Compounds. The sulfur-containing compounds identified among the pyrolytic products included carbon disulfide (CS2), thiocyanate, thioether, thiophene, benzo[b]thiophene (BT), dibenzo[b,d]thiophene (DBT), and their alkyl derivatives. Higher temperatures obviously enriched the hydrocarbon compounds containing heteroatoms. In addition, thiophene and benzo[b]thiophene were in the majority among the sulfur compounds above 400 °C, as shown in Figure 10a. Within the range from 300 to 650 °C, not all of the identified species other than thiophene could be detected. Thiocyanate
and CS2 were observed in the low temperature range (from 300 to 400 °C), which indicates that they were possibly generated from relatively unstable structures in the bitumen. However, their intensities were relatively low, so they might be obscured by neighboring peaks when the further extent of cracking triggered the explosion of volatiles above 400 °C. Thioether was found at low and decreasing intensity from 450 to 550 °C. Regarding thiophenic sulfur species, the intensity of thiophene exhibited an obvious rise from 350 to 450 °C. Benzo[b]thiophene and dibenzo[b,d]thiophene were detected from 400 and 450 °C. When the temperature was above 600 °C, the intensities of thiophene and benzo[b]thiophene decreased slightly, whereas that of dibenzo[b,d]thiophene remained almost stable from 550 to 650 °C. As can be seen in Figure 10b, oxygen-containing compounds including alcohols, ethers, esters, ketones, and carboxylic acid were identified above 400 °C. Among them, carboxylic acid could be detected with a weak intensity only below 550 °C, as it decomposed into carbon dioxide at relatively lower temperatures. Alcohols had the strongest intensity among oxygencontaining compounds and were found from 500 to 600 °C with carbon chains as long as C17. However, the intensity decreased after 600 °C mainly because of elimination reactions into unsaturated hydrocarbons. Esters exhibited a trend similar to that of alcohols in the range of 300−650 °C. Ethers were found above 450 °C. No evident trends with increasing temperature can be seen in Figure 10b for ethers. Ketones maintained a relatively stable group structure as the intensity continued to increase within the 300−650 °C temperature range. 3.6. Relationships of Temperature, Pyrolytic Products, and Organics in Oil Sands. Pyrolytic products can provide insight into the organic structures in oil sands. The temperature of fast pyrolysis, which controls the extent of thermal cracking of organics in bitumen, determines the species and yields of pyrolytic products. Because of the homology of the components and organics in oil sand, the species of pyrolytic products are similar above 400 °C. Although the amounts of saturates that can be separated from bitumen are quite lower than the amounts of aromatics or resins (polar aromatics), alkanes and olefins, which are mainly generated from the aliphatic-chain substituents connecting to the aromatic cores, account the largest amounts among the total volatiles. According to NMR results and average unit model of the oil sand bitumen, the length of the straight chain per unit is about 12. However, aliphatics could be detected with the carbon numbers up to C27, which indicates that the polymethylene chains in the organic macromolecules of oil sand should be C27 at least. The aromatics in pyrolytic products are also different from what should be derived from the average unit, whose condensed aromatic cores should have six rings. Under the test conditions, aromatic products mainly had single or double rings, which can hardly be generated from the ringcleavage reactions of aromatic cores. This indicates the existence of aromatic substituent groups with fewer rings in the raw organic structure. In addition, the hydroaromatics in pyrolytic products can be generated from cyclization reactions with aliphatic-substituted rings. Sulfur and oxygen are the major heteroatoms as heterocyclic or bridge species in the bitumen of oil sands. The sulfur content of bitumen was found to be higher than 6 wt % and mainly of the aliphatic type. However, the yields of aliphatic sulfur were much lower than the yields thiophenics. By contrast, the yield
Figure 10. Evolutions of sulfur- and oxygen-containing compounds with temperature. I
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decreased, except for those of ketones and dibenzothiophene, which was possibly due to the relatively stable structures of ketones and dibenzothiophene within the tested temperature range.
of thiophenics exhibited a trend that was similar to or even higher in intensity than that of the yield of aromatics. It is possible that aromatic fragments are generated along with thiophenics. Moreover, thiophene, benzo[b]thiophene, and dibenzo[b,d]thiophene were found to be much steadies than thioether at high temperatures, especially for sulfur from aromatic rings for the resonance stabilization of thiophenic rings.52 Generally, the energy of breaking SC (methylene) bonds is lower than the energy of breaking cores. Without the presence of a catalyst and a hydrogen supply, the sulfide linkages connected to aromatic cores probably undergo ring closure to form conjugated structures and then crack into volatiles. In addition, the cyclization of thioether might also produce thiophenics.
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.energyfuels.6b02667. Relative intensities of compounds in each group generated by oil sand fast pyrolysis at 300, 350, 400, 450, 500, 550, 600, and 650 °C (PDF)
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4. CONCLUSIONS Both NMR spectrometry and Py-GC/MS can provide information on the organic structures in oil sands. The average model deduced from NMR spectrometry can provide an overview of the structure of hydrocarbons, whereas Py-GC/MS can help to reveal some characteristics of the macromolecules in oil sand organics. The products of oil sand fast pyrolysis are varied. More than 200 compounds were detected under our test method, including aliphatics (alkanes, cycloalkanes, olefins, dialkenes, cycloolefins), aromatics (alkyl benzenes, alkyl naphthalenes, alkyl indenes), oxygen-containing compounds, and sulfurcontaining compounds. The temperature of fast pyrolysis affected the species and yields of products through both primary and secondary reactions. Major thermal cracking took place until about 400 °C as evidence by dramatic increases in numbers of species and peak intensities. However, temperatures higher than 600 °C were beneficial for generating smaller molecules as products. Alkanes and olefins were found to be the predominant products of fast pyrolysis and are mainly generated from the aliphatic substituents linking to the aromatic cores. The relative intensities of identified alkanes and olefins decreased with increasing carbon number. The ratio of olefins to alkanes for the same carbon number also varied with temperature. Higher temperature contributed more olefins, especially those with lower molecular weights. C1−C3 alkyl- and alkenyl-multisubstituted monoaromatics comprised the majority of the aromatic products of fast pyrolysis from 300 to 650 °C. Higher temperatures promoted the generation of aromatics. In addition, monoaromatics with more alkyl and alkenyl substituent groups appeared above 400 °C, but the substituent groups were no longer than isopropyl. Naphthalenes, indene, and 1H-indene were identified at temperatures up to 400 °C, whereas hydronaphthalenes were first identified at about 450 °C. Few PAHs were observed within the temperature range of 300−650 °C. Higher temperature enriched the compounds containing heteroatoms of sulfur and oxygen, which appeared dramatically at temperatures above above 400 °C. Although aliphatic sulfur was predominant in raw oil sand, the generated sulfurcontaining compounds mainly were sulfur-containing heterocycles of thiophene, benzo[b]thiophene, and their alkyl substituents. This reveals that sulfur tends to be converted into more thermally stable forms during pyrolysis under an inert atmosphere. Alcohols comprised a great portion of oxygen-containing compounds, but their yields decreased above 600 °C. The yields of other species with heteroatoms also
AUTHOR INFORMATION
Corresponding Author
*Tel.: +86 411 84986150. Fax: +86 411 84986151. E-mail:
[email protected]. ORCID
Fan Nie: 0000-0003-1013-5235 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS Support from the National Energy Administration of China (NY 20130302513-1) is gratefully acknowledged. REFERENCES
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