Study of Nitrogen Compound Migration during the Pyrolysis of Longkou Oil Shale with Thermal Bitumen as the Intermediate

Abstract

Oil shale is an unconventional energy resource with high nitrogen content. In this study, XPS, GC–MS and ESI FT-ICR MS were carried out to investigate the nitrogen compound migration during pyrolysis, with thermal bitumen as the intermediate. The results showed that the yield of thermal bitumen was highest when the reaction temperature was 380 °C. In the process of pyrolysis, amines and some nitrides generate ammonia gas due to the hydrogen transfer process, or they generate low-grade amines, which subsequently dissolve in pyrolysis water due to bond breakage during the pyrolysis process. As determined by GC–MS analyses, the basic components in shale oil are mainly quinoline compounds. Benzocarbazole and dibenzocarbazole compounds, such as C1-benzocarbazoles, C2-benzocarbazoles and C3-benzocarbazoles, were detected via ESI FT-ICR MS in thermal bitumen.

1. Introduction

Oil shale, a kind of unconventional energy resource with rich reserves, is regarded as a valuable supplement and alternative to conventional oil [1,2,3]. The organic matter in oil shale is called kerogen, and it can be converted into shale oil and shale retort gas. The solid product produced after pyrolysis is called semi-coke. The most effective method of processing shale oil resources produces fuel oil and fine chemicals [4,5,6]. The yield of shale oil is also the most direct indicator of whether oil shale has exploitation value or whether an oil shale retorting unit is efficient. Currently, shale oil is mainly used for marine fuel or hydrofining to produce fuel oil and chemical products. Nevertheless, the high concentration of nitrogen compounds in oil shale results in the emission of nitrogen-containing pollutants during processing, and the elevated nitrogen content, particularly the basic nitrogen content, poses a limitation to the processing efficiency of shale oil.

Despite the abundance of alkanes and aromatics in both shale oil and petroleum, shale oil processing differs considerably from that of petroleum, primarily due to the high nitrogen component content in shale oil. For example, shale oil is not suitable for processing with catalytic cracking technology because the nitrogen components in shale oil, especially basic nitrogen components, are prone to coke and cause the deactivation of the catalysts [7,8]. Therefore, it is very important to study the migration characteristics of the nitrogenous compounds in the process of oil shale pyrolysis for the utilization of oil shale resources. Generally, the composition of oil shale pyrolysis products is influenced by various factors, such as the original material composition of the oil shale, which is different depending on the origin of the shale, and the different pyrolysis methods [9]. Nevertheless, the oil shale pyrolysis process can be divided into two steps: The first step is the pyrolysis of organic macromolecules present in the kerogen of oil shale into solvent-soluble thermal bitumen which is the primary pyrolysis product during the thermal decomposition of organic matter in oil shale and is more solid-like in nature, possessing limited fluidity, during which small amounts of shale oil and distillate gas will be generated [10]. The second phase involves the further decomposition of thermal bitumen, resulting in the formation of shale oil, gas and semi-coke [11].

Similar to other energy resources, the compositions of oil shale and shale oil are also extremely complex, making it difficult to characterize them in detail [12,13,14]. Today, with progress in characterization technologies, several breakthroughs have been reported in the molecular determination of nitrogen components in oil shale. Traditional analytical techniques, such as Fourier transform infrared (FT-IR) spectroscopy [15,16], nuclear magnetic resonance (NMR) spectroscopy [17,18], GC–nitrogen, GC–sulfur chemiluminescence detection [19,20] and gas chromatography–mass spectrometry (GC–MS), have been used for the molecular characterization of kerogen, thermal bitumen and shale oil [21,22]. Many researchers have determined the nitrogen components in oil shale samples using negative-ion ESI (Electrospray Ionization) FT-ICR MS, GC–MS and XPS. Prior studies have shown that the nitrogen compounds in shale oil comprise aliphatic amines, aniline, pyridine, quinoline, pyrrole, indole and carbazole substances, with pyridine, indole and carbazole as the main substances. He Lu compared the composition of nitrogen compounds in shale oil heated using a microwave and a Fischer assay-type retort by means of ESI FT-ICR MS and other methods and proposed the migration and transformation rules of nitrogen compounds in oil shale during two pyrolysis processes. The results showed that using microwave heating to prepare shale oil can effectively reduce the content of nitrogen compounds in the shale oil [23]. Cui Da used a gas chromatography nitrogen chemiluminescence detector (GC–NCD) and ESI FT-ICR MS to analyze the content and composition of basic nitrogen compounds in shale oil at different temperatures. The results showed that the pyrolysis temperature has a significant effect on the relative abundance, unsaturated (double bond equivalent: DBE) value distribution and carbon number distribution of basic nitrogen compounds in shale oil [24]. However, there are still few studies of the migration and transformation of nitrogen components during the pyrolysis of oil shale with thermal bitumen as the intermediate pyrolysate.

In this research, Longkou (Shandong, China) oil shale, which has the highest oil content in China, was selected, and the distribution of its pyrolysis products was analyzed. The structure of nitrogen components in the products was analyzed via XPS, GC–MS, ESI FT-ICR MS and other methods, and then the migration law of nitrogen components (with thermal bitumen as the intermediate product) in the pyrolysis of oil shale was hypothesized. Therefore, the experimental findings from this study hold significance since they offer a theoretical understanding of the nitrogen compounds’ composition in the products resulting from oil shale pyrolysis. In addition, the results could contribute to generating new nitrogen removal treatments by providing a theoretical basis for optimizing the pyrolysis process, improving the oil quality and enhancing the comprehensive utilization value of shale oil.

2. Experimental Section

2.1. Materials

This paper examines oil shale samples collected from Longkou City, Shandong Province, China, which had an oil content of 16.42%. Before conducting the experiments, the samples were crushed and screened to a size of 100 mesh. To remove any extracted bitumen, the samples were subjected to a 2-day extraction process using a Soxhlet extractor and chloroform (CHCl₃) as the solvent at a temperature of 65–70 °C. The proximate and ultimate analyses of the oil shale are presented in

Table 1. Proximate, ultimate and Fischer assay analyses of Longkou.

Proximate Analysis wtad/%				Ultimate Analysis wtad/%					Fischer Assay wtad/%
M	V	A	FC	C	H	O	N	S	oil	water	semi-coke	gas
1.76	53.83	38.82	5.59	30.81	3.04	12.91	0.66	1.27	16.42	4.77	68.24	10.57

ad: dry basis; M: moisture; V: volatile matter; A: ash; FC: fixed carbon.

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2.2. Oil Shale Pyrolysis and Thermal Bitumen Preparation

For each experiment, a 50-g oil shale sample was placed in a stainless-steel reactor for pyrolysis in a Fischer assay-type retort. To ensure a uniform temperature distribution in the sample, the heating rate was set at 2 °C·min⁻¹. Once the final pyrolysis temperature of 520 °C was reached, the reactor was rapidly cooled to room temperature. Subsequently, the pyrolysates, shale oil and semi-coke were weighed. Thermal bitumen was extracted from the semi-coke using a Soxhlet extractor with chloroform as the solvent (CHCl₃) at 65–70 °C for 2 days. Each experiment was performed three times to ensure the accuracy of the experiments. The yields of thermal bitumen at different temperature are shown in Figure 1. The results showed that the yield of the thermal bitumen is the highest at a reaction temperature of 380 ℃, so thermal bitumen obtained at 380 °C was selected as the research object in this experiment.

2.3. Kerogen Preparation

Kerogen was obtained via the acid washing method. The inorganic substances in oil shale and semi-coke are mainly carbonate and silicate, so HCl and HF acid were used to remove the inorganic salts. First, a certain quantity of oil shale or semi-coke was weighed in a polytetrafluoroethylene beaker, 20% HCl was added to create a mixture with an oil shale/hydrochloric acid ratio of 1:10, the mixture was stirred in a 40 °C water bath for 2 h, and then the filtrate was washed with deionized water until it was neutralized. This process removed the majority of carbonate components from the oil shale. Then, a mixed acid was prepared with a 20% HCl to 40% HF volume ratio of 1:2, which was then mixed with oil shale (oil shale/acid 1:10), and the mixture was stirred for 2 h and then washed with deionized water until the filtrate was neutralized. This process removed the majority of silicate components in the oil shale. Then, 10% hydrochloric acid was reacted with the oil shale for 2 h. After treatment, the mixture was washed with deionized water until the filtrate was neutralized. This process removed the fluoride generated in the previous reaction. The final product was dried to constant weight in the oven, sealed and then placed in the oven until further analyses.

2.4. Acid Base Extraction of Shale Oil and Thermal Bitumen

In this experiment, acid and base components in thermal bitumen and shale oil were obtained via acid–base extraction. An amount of 15 g of the sample was first dissolved in n-hexane. The n-hexane-soluble and -insoluble substances, which can be separated into alkaline, neutral and acidic components, were separated. The separation method was as follows:

(1)

Dichloromethane was added to the component to dissolve it completely, and then it was extracted with 50 mL of 3 mol/L sodium hydroxide solution three times to obtain a water phase and an oil phase;

(2)

An amount of 50 mL of n-hexane was used to back extract the aqueous phase, and then 6 mol/l hydrochloric acid was used to adjust the pH to 1–2. Subsequently, 150 mL of methylene chloride was used to extract the aqueous phase after adjusting the pH three times to obtain the acidic component;

(3)

The oil phase in (1) was extracted with a 150-mL 6 mol/L hydrochloric acid solution three times to obtain new neutral oil and water phases;

(4)

The aqueous phase in (3) was back extracted with 50 mL of n-hexane, adjusted to a pH of 11 with sodium hydroxide solid and then extracted with 150 mL methylene chloride three times to obtain the alkaline component.

2.5. GC–MS Analysis

The nitrogen component compositions in n-hexane-soluble matter was analyzed via GC–MS performed with an ACION TQ (Bruker, Germany) with an HP-5MS quartz capillary column to determine the composition of thermal bitumen and shale oil. The oven temperature was initially held at 20 °C for 5 min and then gradually raised to 280 °C at a rate of 10 °C min⁻¹. Subsequently, the pyrolysis temperature was maintained for 20 min. For analysis, a Qual Browser and Technology mass spectral library search was employed [11]. To perform qualitative and quantitative analysis of the peaks in the total ion chromatogram, the curves were compared with spectra available in the National Institute of Standards and Technology (NIST) database.

2.6. XPS Analysis

The bonding modes of nitrogen in oil shale kerogen, thermal bitumen and semi-coke were determined via XPS. The XPS instrument adopted an ESCALab 250 Xi photoelectron spectroscopy analyzer (Bruker, Germany). The test conditions were a working power of 200 W, a basic vacuum of 3 × 10⁻⁸ Pa, a monochromated Al Kα X-ray excitation source, a passing energy during narrow scanning of 30 eV (steps of 0.05) and a passing energy during wide scanning of 50 eV (steps of 0.5). The parameters of different element peaks were fitted and established with XPSPEAK software (Thermo Fisher Scientific, Waltham, MA, USA), version 4.1, the analysis results were quantitative, and the content of nitrogen compounds was calculated via normalization [11].

2.7. ESI FT-ICR MS Analysis

The nitrogen components in thermal bitumen and shale oil were characterized by a Bruker Apex ultra 9.4 T FT-ICR MS. An amount of 20 μL of each solution was further diluted with 1 mL of toluene/methanol (1:3, v/v) solution. Each solution was injected into an Apollo II electrospray source at 250 μL/h with a syringe pump. The test conditions were a sample injection flow rate of 180 µL/h, a capillary inlet voltage of 5000 V, a polarization voltage of 4000 V, an RF voltage of 300 Vpp, a quadrupole Q1 of 100 Da, an ion source hexapole DC voltage of 2.4 V, a capillary outlet voltage of 320 V and an RF of 140 Vpp. The enrichment time was 4 s, the helium flow rate in the collision pool was 0.3 L/s, the collision energy was −1.5 V, and the collection mass range was 115–700 Da. The optimized mass for quadrupole 1 (Q1) was 250 Da. Hexapoles of the Qh interface were operated at 5 MHz and 400 Vp–p RF amplitude, in which ions accumulated for 0.001 s. The delay was set to 1.0 ms to transfer ions to an ICR cell by electrostatic focusing of transfer optics. The ICR was operated at 11.75 (or 13.5) db of attenuation, 150–650 (or 150–1000) Da mass range and 4 M acquired data size. A total of 64 scans were accumulated [23,25]. The data processing was carried out using the software independently prepared by the State Key Laboratory of Heavy Oil, China University of Petroleum (Beijing), and was described elsewhere [26].

3. Analysis of Experimental Results

3.1. Nitrogen Distribution during the Pyrolysis of Oil Shale

To explore the migration and transformation law of nitrogen components during the pyrolysis of Longkou oil shale, thermal bitumen was obtained at 380 °C, and shale oil, gas and semi-coke were obtained at 380 °C and 520 °C, respectively. First, a nitrogen element analysis was carried out on each sample. The analysis results and nitrogen element distribution are shown in Figure 2.

In this experiment, thermal bitumen was used as the pyrolysis intermediate to investigate the distribution of nitrogen in different samples. Figure 2 lists the yield and nitrogen content of each product during oil shale pyrolysis. It can be seen from the nitrogen balance diagram during oil shale pyrolysis that most nitrogen compounds were still present in the semi-coke at 520 °C, and a small number of organic nitrogen compounds became solvent-soluble thermal bitumen after pyrolysis. When the final pyrolysis temperature was reached, half of the nitrogen components were gasified into shale oil or gas, and the other half were stored in semi-coke. Previous studies have shown that the nitrogen compounds in oil shale are mainly organic nitrogen compounds, including basic nitrogen compounds, such as pyridine, quinoline, aniline, etc., and non-basic nitrogen compounds, such as pyrrole, indole, carbazole, etc. [23,24,25,26,27]. In the process of pyrolysis, amines and some nitrogen components generate ammonia gas due to the hydrogen transfer process, or they generate low-grade amines, which dissolve in the pyrolysis water due to bond breaking during pyrolysis. For aliphatic or aromatic nitrogen compounds, the pyrolysis process includes complex bond breaking, polymerization, aromatization and other processes. Thus, smaller molecules enter the shale oil after gasification, while larger molecules remain in the semi-coke.

3.2. XPS Analysis

XPS is an analysis and detection technology that can detect the element composition and content, chemical state, molecular structure, chemical bonds and other information in solid samples. Therefore, XPS was used to analyze the structure and composition of nitrogen compounds in solid samples of kerogen, thermal bitumen and semi-coke. Figure 3 shows the XPS peak separation diagram of the sample, and the sample information is provided in

Table 2. XPS results of N1s and relative contents.

Group Type	BE (eV)	Relative Peak Area (%)
		Kerogen	Thermal Bitumen	Semi-Coke
Pyrrolic	399.47	49.46	53.41	40.68
Pyridinic	398.07	30.56	18.58	32.59
Aniline	400.29	16.06	21.92	22.78
Quaternary	401.45	3.92	6.09	3.95

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Nitrogen-containing compounds are mainly composed of pyrrolic, pyridinic, aniline and quaternary nitrogen compounds. The results showed that the contents of pyrrolic and pyridinic compounds were the highest in these three samples. The contents of aniline and quaternary nitrogen in kerogen were lower than those of thermal bitumen and semi-coke because, during the pyrolysis process, the content of aromatic compounds increases, while the branched chain compounds easily decompose at a lower temperature. In addition, a certain amount of ammonia or ammonia oxides will be generated during the pyrolysis process, which also causes a decrease in the aniline compound content during the pyrolysis process.

3.3. Composition Analysis of Nitrogen Components

GC–MS results were compared with a chromatogram of a standard sample according to the different residence times of different substances in the chromatographic column to conduct qualitative and quantitative analyses of the tested sample. In this paper, the composition of treated shale oil was detected via GC–MS and compared with spectra in the NIST database. The total ion chromatograms of the acidic, alkaline and neutral components are shown in Figure 4.

It can be seen from Figure 4 that the GC–MS chromatogram signals of each component in the shale oil have good peak-splitting effects. Through GC–MS analysis, the composition of shale oil nitrogen components was determined. However, due to the limitations of the GC–MS detection method, only small molecules with fewer branches were detected, while large molecules with more branches were not easily detected. The total ion peaks of nitrogen components obtained using different mass nucleus ratios are shown in Figure 5.

In the total ion flow diagram, pyrrole, indole, pyridine (or aniline), quinoline and acridine compounds can be observed at m/z = 67 + 14n, m/z = 143 + 14n, m/z = 79 + 14n, 129 + 14n and 179 + 14n, respectively. Nitrogen compounds mainly appear in the form of heterocycles. The content of various nitrogen compounds is shown in

Table 3. Various nitrogen component contents in shale oil.

Relative Amount/%	Corresponding Compound	Relative Amount/%	Corresponding Compound
15.29	Pyrrole	14.92	C2-quinoline
9.33	Indole	13.43	C3-quinoline
4.28	C1- aniline	7.13	C4-quinoline
3.01	C2- pyridine	4.95	Acridine
9.03	C3- pyridine	9.18	C1-acridine
2.84	C4- pyridine	3.51	C2-acridine
0.89	Quinoline	0.31	C3-acridine
1.90	C1- quinoline

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It can be seen from Table 3 that the nitrogen components in shale oil are mainly quinoline compounds, followed by pyridine compounds and then acridine compounds with the lowest content because the molecular weights of acridine compounds are large and may not be completely detected. From the branched chain number of various compounds, the maximum number of branched chain carbons of pyridine compounds and quinoline compounds is four, while the maximum number of branched chain carbons of acridine compounds is three. The latter compounds also have low content, which is also due to a limitation in GC–MS detection.

To further study the molecular weight information of nitrogen compounds in oil shale pyrolysis products, ESI(+) FT-ICR MS was used to analyze the basic N class species, and ESI(−) FT-ICR MS was used to analyze the neutral N in thermal bitumen and shale oil, with the results shown in Figure 6. In the presence of nitrogen compounds, the response of sulfide is very low; thus, N₁ compounds are dominant in the mass spectra. Figure 6 shows the ESI FTICR MS results of relative abundance of N class species in Longkou thermal bitumen produced at 380℃ and shale oil produced at 520℃. The bar chart in Figure 6 shows that nitrogen classes mainly include N₁, N₁O₁, N₁O₂, N₂, N₂O₁ and N₁S₁ classes. Among them, the N₁, N₁O₁ and N₂ classes were dominant.

Figure 6A shows the relative abundance of nitrogen-containing species in basic N class in thermal bitumen and shale oil. In thermal bitumen, N₁ and N₂ species were dominant, followed by N₁O₁, N₂O₁ and N₁O₂ species. In shale oil, N₁ was dominant, followed by N₁O₁, N₂, N₁O₂ and N₂O₁ species. In Figure 6B, the composition is ranked by abundance as N_1, N₁O₁, N₂, N₁O₂ and N₂O₁ species.

Based on the exact masses gained from ultrahigh-resolution mass spectrometry, a formula assignment is possible. These molecular formulas are then used to calculate the DBEs, a measure of hydrogen deficiency. This information, combined with the information gained from FT ICR results and the knowledge of ionization efficiency in the different ionization modes, allows for speculating about the structures of the molecules as presented in Figure 7 and Figure 8. It is evident that the composition of nitrogen compounds in thermal bitumen is more complex than in shale oil. DBE was used to characterize the unsaturation of compounds. For N₁ species (Figure 7a, Figure 8a), the nitrogen atom was determined to be a pyridine in the positive-ion mode and pyrrole in the negative-ion mode. The N₁ class species in shale oil had DBE values of 4–11 and carbon numbers of 8–28. The contents of N₁ class species with DBE values of 4 and 5 were higher and were likely to be pyridine and quinoline. Additionally, for the N₁ class species with DBE values of 4–14 and carbon numbers of 11–22 in thermal bitumen, the most common values were 5, 6, 8 and 12, likely to be pyridobenzene and benzocarbazole compounds, respectively. The N₁ compounds with a DBE value of 8 and carbon numbers of 17, 18 and 19 were dominant, assigned as C1–C3 benzocarbazoles.

For N₁O₁-type compounds (Figure 7b, Figure 8b), the compositions in the thermal bitumen were much more complex than those in shale oil. The DBE values of thermal bitumen were concentrated between 4 and 21, and the carbon number distribution was between 15 and 35. Additionally, the DBE value of shale oil was concentrated between 4 and 13, and the carbon number distribution was between 10 and 22, indicating derivatives of indoles or quinolines. Based on the GC–MS analysis, it is speculated that these components may be derivatives of furan. N₂ compounds in thermal bitumen and shale oil (Figure 7c, Figure 8c) may be pyridazine or pyrazine derivatives, which might be present as two pyridine rings or a pyridine ring and a pyrrole ring.

Compared with oil shale in other regions, Longkou oil shale has a high maturity and nitrogen content [27,28,29]. Therefore, the composition of nitrogen compounds in intermediate products is relatively complex. As condensation and aromatization reactions occur during the pyrolysis of oil shale, the molar mass of the components increases, and the shale becomes less volatile; at the end of the pyrolysis reaction, a large number of nitrogen components will remain in the semi-coke, as confirmed by the elemental analysis results, in which more than 50% of the nitrogen components were concentrated in the semi-coke.

4. Conclusions

Oil shale is classified as an unconventional energy source capable of being subjected to pyrolysis to generate shale oil. Throughout the pyrolysis process, the presence of nitrogen compounds within the oil shale not only contributes to environmental pollution but also restricts the performance and efficiency of shale oil production. In this study, to investigate the migration behavior of nitrogen compounds during the pyrolysis of Longkou oil shale, we utilized thermal bitumen, obtained as an intermediate product at a pyrolysis temperature of 380 °C. Notably, the composition of nitrogen compounds within the thermal bitumen was found to be more intricate compared to that of shale oil, primarily comprising pyrrolic, pyridinic, aniline and quaternary nitrogen compounds. Upon exposure to high temperatures, the long-chain molecules undergo fragmentation, enabling the ingress of small molecule nitrogen compounds into the shale oil. However, due to the processes of aromatization and polycondensation inherent within pyrolysis, nitrogen compounds tend to show a higher propensity for retention within the semi-coke residue. Through the conclusive findings of this research, it is evident that a thorough understanding of nitrogen compound migration offers valuable theoretical foundations and technical guidelines for the development of more sustainable and environmentally friendly shale oil production methods, with reduced levels of organic nitrogen compounds.