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Characteristics of thermal bitumen structure as the pyrolysis intermediate of Longkou oil shale Jian Shi, Yue Ma, Shuyuan Li, and Lei Zhang Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.7b01542 • Publication Date (Web): 03 Aug 2017 Downloaded from http://pubs.acs.org on August 8, 2017
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Energy & Fuels
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Characteristics of thermal bitumen structure as the
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pyrolysis intermediate of Longkou oil shale
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Jian Shi†, Yue Ma†, Shuyuan Li*,† and Lei Zhang‡
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†
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Heavy Oil Processing , China, Beijing, 102249.
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‡
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ABSTRACT: The effects of pyrolysis temperature on the structure of thermal
8
bitumen as the pyrolysis intermediate of Longkou oil shale are investigated through
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electron paramagnetic resonance spectroscopy, Fourier transform infrared (FTIR)
College of Science, China University of Petroleum-Beijing, State Key Laboratory of
Shandong Energy Longkou Mining Group Co., Ltd., Longkou 265700, China
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spectroscopy,
nuclear
magnetic
resonance
(NMR)
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chromatography–mass spectrometry (GC–MS), and X-ray photoelectron spectroscopy
12
(XPS). Results indicate that the free radical concentration of thermal bitumen
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increases at a maximum temperature of 410 °C and then decreases between 410 °C
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and 450 °C. Moreover, the g factors of thermal bitumen are slightly higher than 2 and
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decrease with increasing temperature because of the aromatization of saturates and
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decarboxylation. FTIR analysis indicates that decarboxylation is completed before the
17
temperature reaches 370 °C. NMR analysis shows that aliphatic and aromatic
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compounds comprise more than 80% and 15%–16% of thermal bitumen, respectively.
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During pyrolysis, the fraction of aliphatic compounds, branched alkane, and the
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average methylene chain length decrease because of secondary cracking, and the C–C
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bond at the branch chainis easily cracked into gas and light oil.
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1. INTRODUCTION
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Declining oil price restricts the development of the oil shale industry; nonetheless,
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researchers still focus on the research and use of oil shale, which is an important
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alternative energy resource, because of energy demand1,
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macromolecular organic matter in oil shale is called kerogen, which can be pyrolyzed
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to produce shale oil and gas through a series of complex chemical reactions. To
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investigate the pyrolysis mechanism of oil shale kerogen, various studies analyzed the
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formation and molecular structure3, 4, pyrolysis product yield5, 6, retorting technologies
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2, 7, 8
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compounds of oil shale12, 13. The pyrolysis of oil shale is affected by numerous factors,
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such as its origin and different pyrolysis modes14. Nevertheless, the pyrolysis
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processes can still be divided into two steps. The first step involves the
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depolymerization of macromolecular organic matter in kerogen into soluble extracts,
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including thermal bitumen and tiny gas products. In this step, macromolecules and
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thermally unstable heterocyclic compounds in kerogen disintegrate into small
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molecules15. The second step involves the further decomposition of thermal bitumen
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into shale oil, gas, and semicoke as the temperature increases.
2
. The complex
, pyrolysis mechanism and kinetics model9-11, and heteroatom-containing
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Previous studies characterized the structure and composition of kerogen through
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thermogravimetry, Fourier transform infrared (FTIR) spectroscopy, nuclear magnetic
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resonance (NMR) spectroscopy, pyrolysis gas chromatography–mass spectrometry
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(Py-GC/MS), and X-ray diffraction16-18. However, the molecular structure of oil shale
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kerogen remains to be completely described because of its complex composition. In
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addition, kerogen from different sources considerably varies in constitution and
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construction, and these factors complicate the analysis of the pyrolysis process.
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Recent studies have focused on the thermal decomposition intermediates (thermal
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bitumen) of oil shale kerogen and explored preliminarily the characteristics of thermal
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bitumen9. However, the influence of pyrolysis temperature on the structure of thermal
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bitumen has yet to be clarified. The formation and decomposition of thermal bitumen
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are continuous during pyrolysis; hence, exploring the composition of thermal bitumen
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obtained at different temperatures can further explain the pyrolysis behavior of
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kerogen.
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The present study measured the yields of pyrolysis products at different
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temperatures and explored the structural features of thermal bitumen at pyrolysis
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temperatures of 330 °C, 370 °C, 410 °C, and 450 °C. Quantitative information about
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the chemical structure and distributions of samples was obtained through electron
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paramagnetic resonance (EPR) spectroscopy, FTIR spectroscopy, NMR spectroscopy,
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GC–MS, and XPS. The information derived from the chromatograms provides new
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insights into the pyrolysis process of oil shale.
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2. EXPERIMENTAL SECTION
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2.1. Materials. Oil shale samples were obtained from Longkou City, Shandong
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Province, China. The oil content was 16.67%. Proximate and ultimate analyses of the
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samples are illustrated in Table 1 and Table 2. The samples were crushed and screened
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to 100 mesh size (particles less than 0.15 mm in size). Soluble bitumen in oil shale
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was extracted for two days by a Soxhlet extractor with chloroform (CHCl3) as solvent
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at 65 °C to 70 °C to ensure that the samples did not contain extracted bitumen. The oil
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shale samples were dried at 50 °C to a constant weight and then stored for use in a
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desiccator.
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2.2. Thermal bitumen preparation and characterization. A retorting reactor
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made of stainless steel was selected and modified based on the Fischer assay-type
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retort. A thermocouple was used to measure the temperature of the sample center. For
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each test, 50g of oil shale was loaded into the retorting reactor, which was then slowly
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heated from room temperature to the pyrolysis temperature at a heating rate of
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2 °C·min−1 to guarantee uniform temperature distribution in the sample19, 20. Upon
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reaching the temperature, the reactor was quickly cooled down to room temperature,
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and then the gas, shale oil, and semicoke were weighed. Thermal bitumen was
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extracted from the semicoke by using the Soxhlet extractor with chloroform solvent
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(CHCl3) at 65−70 °C for 48 h. The chloroform solution was finally evaporated to
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obtain thermal bitumen and then maintained in a cold storage for further analysis.
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EPR measurements were performed by using a Bruker A200 spectrometer with a 100
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kHz field modulation. In this experiment, the scanned parameters were kept at room
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temperature, and the operation conditions were used as follows: sweep time, 4min;
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center field, 323.2 mT; frequency, 9.06 GHz; and microwave power, 0.5 mW. The
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signals from Mn2+ dispersed in MnO2 were used to monitor EPR sensitivity. The g
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values play an important role in reflecting the characteristics of the magnetic field
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within the molecules. The change in g values mainly depends on the coupling effect
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of the spin and orbital motions. The EPR signals were located in experiments by their
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g value defined by
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g=
91
where h is the Planck’s constant, the resonance frequency, the Bohr magneton
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and H the magnetic field at which resonance occurs21. The free radical concentration
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of thermal bitumen served as a quantified reference to the standard curve of
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1,1-diphenyl-2-picrylhydrazyl (DPPH, g=2.0036)22, 23. For each test, 0.05 g of the
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sample was diluted 40 times with methylbenzene, and this step was repeated three
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times to ensure repeatability.
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FTIR was used to analysis the factional group of the thermal bitumen. FTIR
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spectra were obtained using a Bruker IFS 66 V/S FTIR spectrometer (Germany). The
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samples were prepared by grinding 1 mg of thermal bitumen and homogenizing it
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with 100 mg of ground KBr (dried under an infrared lamp). The spectra were
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collected in the mid-IR region from 4000 cm−1 to 400 cm−1.
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13
C and
1
H NMR curves were obtained at 100.62 MHz by a Bruker
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AVANCEIII−400 MHz with a 5 mm probe insert and TMS as the internal standard.
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To guarantee the total solution organic matters, CDCL3 was used as the solvent. The
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acquisition times of
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6.1.1 was used to analyze the NMR spectra and quantify the relative proportion of
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different carbon and hydrogen types in the thermal bitumen samples.
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C and 1H NMR were 3 and 0.2 s, respectively. MestReNova
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GC–MS was performed with ACION TQ (Bruker) to determine the composition of
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thermal bitumen. The type of chromatographic column was HP-5MS (30 m × 0.25
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mm × 0.25 μm) fused silica capillary column. The oven temperature was maintained
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at 20 °C for 5 min and then slowly increased to 280 °C at a rate of 10 °C min−1,
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followed by holding the pyrolysis temperature for 20 min. The analysis was
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performed by Qual Browser and Technology mass spectral library search24-26. To
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qualitatively and quantitatively analyze the peaks in total ion chromatogram, the
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curves was compared with the spectrum in NIST. The chromatographic column was
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cleaned by CHCl3, there was about 10% remained in the column, it revealed that the
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macromolecular structures in thermal bitumen were hardly detected by GC–MS.
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Compared with the spectrum in NIST, only the compositions with a carbon number
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that is smaller than C30 were analyzed in the experiments.
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The existing form of heteroatoms in thermal bitumen was measured using XPS by a
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Kratos Axis Ultra DLD spectrometer with Al Kα X-ray source (hm = 1486.6 eV) at
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15 kV and 150 W. The parameters of different element peaks were fitted and
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established with XPSPEAK41.
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Table 1. Proximate and ultimate analyses of Longkou oil shale (wt.%).
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Table 2. Ultimate Analyses of Longkou oil shale and thermal bitumen (wt.%).
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Figure 1. Separation and structure analyses of thermal bitumen
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3. RESULTS AND DISCUSSION
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3.1. Effects of pyrolysis temperature on pyrolysate yields. The yields of
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semicoke, thermal bitumen, shale oil, and gas as a function of temperature from
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290 °C to 480 °C are shown in Fig.2. The yields of shale oil, water and gas increase,
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whereas that of semicoke significantly decreases as the temperature is increased from
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290 °C to 480 °C and reaches the peak at 480 °C. The yield of thermal bitumen
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reaches the maximum value of 7.45% at 370 °C and then decreases when the
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pyrolysis temperature is further increased. With increasing temperature, kerogen in
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the oil shale breaks down into thermal bitumen, oil, and gas, while the thermal
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bitumen decomposes into oil and retorting gas. The formation of thermal bitumen
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starts at 290 °C, and the rate of thermal bitumen formation is faster than that of
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decomposition at 370 °C. Moreover, the decomposition rate of thermal bitumen
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increases at 370–450 °C and converted to the final pyrolysates after 480 ℃ ,
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completely. This results is consistent with the previous studies27, 28.
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Figure 2. Effect of pyrolysis temperature on pyrolysate yields.
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3.2. EPR analysis. The typical EPR spectra of thermal bitumen are illustrated in
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Fig. 3(a), and the free radical concentration (Ng) of DPPH solution in different
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volumes can be calculated as follows:
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N = (ω × m × 6.02 × 10 )⁄M
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(1)
where m is the mass,ω is the percentage (wt. %), and M is the molecular mass of DPPH solution.
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The relationship of Ng versus signal/marker (area of the DPPH sample absorption
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curve vs. Mn2+ curve) and the standard curve of the radical concentrations based on
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numerical analysis is given by the following:
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Signal⁄marker = 8.887 × 10#$% N − 1.571
(2)
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The Ng and g factors of thermal bitumen at different temperatures are shown in Fig.
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3(b). Similar to the field trend of thermal bitumen, the Ng of thermal bitumen
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increases with increasing temperature, reaches the peak at 410 °C, and then decreases
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between 410 °C and 450 °C. The dynamic change in Ng indicates that the unpaired
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electrons in thermal bitumen are constantly forming and disappearing during
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pyrolysis.
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Figure. 3. Effect of temperature on the radical concentrations and g factors of thermal
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bitumen. (a) EPR spectra of thermal bitumen, (b) radical concentrations and g factors
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of thermal bitumen.
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The original paramagnetic centers in coals mainly exist in aromatic units, and their
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unpaired electrons may be found in C, N, or O atoms26. The gfactors of thermal
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bitumen are slightly higher than 2 and decrease with increasing temperature. Petrakis
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and Grandy29 found that the g values for aliphatic hydrocarbon π radicals are
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2.0025–2.0026, 2.0025 for aromatic hydrocarbon π radicals, 2.0008−2.0014 for
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σ-type oxygen-containing radicals, 2.0038−2.00469 for π-type oxygen-containing
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radicals,
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sulfur-containing radicals. The g factors shown in Fig.3(b) indicate that the unpaired
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electrons in thermal bitumen are mainly localized in aliphatic hydrocarbons, aromatic
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hydrocarbons, and O atoms, and that the reduction of g factors is caused by the
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aromatization of saturates and decarboxylation.
2.0031
for
nitrogen-containing
radicals,
and
2.0080−2.0081
for
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3.3. FTIR analysis. Fig.4 shows the FTIR spectra of thermal bitumen at different
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temperatures. The strong peaks at 2829 and 2850cm−1 are attributed to the asymmetric
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and symmetric stretching of the C−H bond of methylene groups, respectively
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The absorption bands at 1463 and 1384 cm−1 correspond to the symmetric bending of
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methyl groups. The peak at 740 cm−1 is attributed to the in-plane bending vibration of
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-CH2-. As the temperature increases, the FTIR spectra exhibit minimal changes at the
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peak of 1722 cm−1, which is related to the C=O stretching of carbonyl and/or carboxyl
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groups (Fig. 4), and this peak weakens as the temperature increases. Moreover, the
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peaks at 1463 cm−1 and 1384 cm−1 FTIR analysis indicates that thermal bitumen
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mainly consists of aliphatic alkyl groups. The peaks at 1722 cm-1 and 1619 cm-1
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decrease from 410℃ to 450℃, which indicates that the end methyl start decompose
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at this temperature. In addition, the carbonyl and/or carboxyl groups decrease as the
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temperature increases because of decarboxylation. The strong peak at 3425 cm−1 is
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likely contributed by adsorbed water.
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Figure 4. FTIR spectrogram of thermal bitumen at different temperatures.
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3.4. NMR analysis. The 13C NMR and 1H NMR curves of the different samples are
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similar, indicating that the compounds of all samples demonstrate an overall similar
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distribution. Thus, the 13C NMR and 1H NMR techniques are only employed for the
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sample at 370 °C (Fig.5), and the distribution of carbon and hydrogen is illustrated in
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Tables 3 and 4.
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The
13
C NMR spectra exhibit signals at 15, 20, 21–25, 25–36, and 36–50 ppm,
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which are assigned to the carbons of n-alkanes, where as the signals at 109–130,
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130–137, 137–148, and 148–165 ppm correspond to aromatic carbons (Fig. 5(a), the
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signal at 77 ppm is assigned to the solvent of the NMR measurement). The 1H NMR
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spectra are dominated by the signals corresponding to aliphatic protons on saturated
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carbons between 0.4 and 1.9 ppm, and the range of peak between 1.9 and 9.0 ppm is
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caused by the protons in the aromatic carbons (Fig. 5(b), the signal at 7.25 ppm is
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assigned to the solvent of the NMR measurement) 31-33.
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Fig. 5(a) shows that the aliphatic compounds comprise over 80% of the integral
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area in the spectrogram. This phenomenon indicates that the carbon skeletal structure
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of thermal bitumen is mainly composed of aliphatic carbons and aromatic (110–165
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ppm) and carboxyl carbons (188–220 ppm). The structure parameters of the carbon
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skeletal structure can be calculated by using the following formulas:
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. / 0 f)* = f)*+ + f)* + f)* + f)* + f)*
(3)
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2 . 3 2 f)1 = f)1 + f)1 + f)1 + f)1
(4)
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0 B5 = f)* /f)*
(5)
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C8 = 9@:;=9:;
:?
218 219
(6)
:?
Where fal and far refer to the alkyl and aromatic carbons, respectively; BI is the degree of branching, and Cn stands for the average chain length.
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The same result is also supported by the 1H NMR spectra shown in Fig. 5(b), and
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the chemical shift ranges of 0.4–1.0 ppm and 1.0–1.9 ppm are assigned to the methyl
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and methylene groups in the aliphatic structures, respectively, accounting for over 80%
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of the total protons
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formulas:
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H)* = H)* + H)*
β
(7)
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α ) H)1 = H)1 + H)1 + H)1
B
(8)
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The calculation results are shown in Table 4.
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Figure 5. 13C NMR and 1H NMR of the thermal bitumen at 370 °C
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Table 3. Distribution of various carbons in thermal bitumen from 13C NMR.
γ
34-36
. The proton types can be calculated by using the following
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Table 4. Distribution of various hydrogens in thermal bitumen from 1H NMR.
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Table 5. Structural parameters of thermal bitumen.
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The fraction of aliphatic structure decreases and the fraction of aromatic carbon
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increases as the temperature increases (Table 5). Moreover, the pyrolysis processing
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reduces the fraction of branched alkane and the average methylene chain length. In
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consideration of prior research about the composition of oil shale kerogen, shale oil,
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and semicoke, the branched alkane and the average methylene chain length of thermal
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bitumen are less than those of kerogen and significantly greater than those of shale oil
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and semicoke. This result proves that the bitumen is the intermediate of oil shale
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pyrolysis. The formation and decomposition of thermal bitumen simultaneously occur.
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Compared with other compounds, branched alkanes are fewer because the C–C bond
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at the branch point is easier to crack, and the branch group disintegrates and tends to
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form short chains.
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The fraction of aromatic compounds in thermal bitumen is about 15%–16%, and
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about 15–16 aromatic carbons per 100 carbons are calculated. This result indicates the
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presence of 2.5 homologs of benzene or naphthalene and benzene ring compounds.
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During pyrolysis, the fraction of aromatic compounds increases because of the
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aromatization of aliphatic and aromatic compounds composited by the aromatization
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of saturates that accumulate in thermal bitumen.
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3.5. GC–MS analysis. GC–MS analysis indicates that thermal bitumen can be
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classified into aliphatic hydrocarbons, aromatic compounds, and hetero atomic
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compounds according to their structures
. As shown in Fig.6, over 70% of the
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detectable substances in thermal bitumen are straight carbon chained aliphatic
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hydrocarbons mainly ranging from C6–C30. The weak peaks at the front part of the
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GC–MS spectra are possibly caused by phenols, ketones, alkyl benzenes, alkenes, and
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alcohols. Polycyclic aromatic compounds and nitrogen and sulfur compounds appear
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at the adjacent carbon number of aliphatic hydrocarbon chromatographic peaks. Other
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compounds, including naphthalene, anthracene, phenanthrene, carboxylic, and lipid,
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can also be detected by GC–MS.
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260
Figure 6. GC–MS of thermal bitumen at different pyrolysis temperatures
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Table 6. Relative contents of each compound of thermal bitumen.
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Figure 7. Carbon number distribution of n-alkanes
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The constitution of thermal bitumen and the intensive overlap in the gas
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chromatogram are complex. Thus, the quantitative information of different
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compounds from GC–MS may not be reliable. However, the variation tendency of
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compounds with temperature changes can be analyzed by GC–MS. The relative
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contents of n-alkanes, α-alkenes, monocyclic aromatics, polycyclic aromatics, and
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oxygenated compounds are highly variable at 330 °C to 370 °C (Table 6). Compared
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with the GC/MS curves, some low molecular monocyclic aromatics can be detected at
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330 °C, such as phenolic homologues but disappear gradually with increasing
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temperature. Moreover, the relative content of aromatic compounds increases between
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370 °C and 450 °C. This phenomenon indicates that thermal bitumen is mainly
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generated from 330 °C to 370 °C. The low molecular monocyclic aromatics are hardly
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accumulated within thermal bitumen.
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The carbon-number distribution of thermal bitumen is summarized in Fig. 7. The
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relative content of low molecular n-alkanes increases with increasing temperature
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from 330 °C to 370°C and then significantly decreases from 370 °C to 450 °C. The
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amounts of ΣC>25 decrease obviously from 330 ℃ to 370℃ and the relative
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content of low molecular hydrocarbons (
