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Comparing product distribution and desulfurization during direct pyrolysis and hydropyrolysis of Longkou oil shale kerogen using reactive MD simulations
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Comparing product distribution and desulfurization during direct pyrolysis and hydropyrolysis of Longkou oil shale kerogen using reactive MD simulations Zhijun Zhang a, Liting Guo a, Hanyu Zhang b, Jin-Hui Zhan c,* a
School of Chemical and Environmental Engineering, China University of Mining and Technology, Beijing 100083, People’s Republic of China b Department of Chemical and Materials Engineering, University of Alberta, Edmonton, Alberta, T6G 1H9, Canada c State Key Laboratory of Multiphase Complex System, Institute of Process Engineering, Chinese Academy of Sciences, Beijing 100190, People’s Republic of China
highlights Hydropyrolysis and direct pyrolysis of oil shale were studied using ReaxFF MD. Hydropyrolysis is helpful for increasing light oil yield. Hydropyrolysis has positive effects on the desulfurization. Hydropyrolysis contributes to the production of H2O molecules. The transfer of sulfur to the gas requires the participation of H2O molecules.
article info
abstract
Article history:
The product distribution and organic sulfur removal during direct pyrolysis and hydro-
Received 21 May 2019
pyrolysis of oil shale kerogen were investigated via reactive molecular dynamics (RMD)
Received in revised form
simulations with reactive force field (ReaxFF). Two structural models for direct pyrolysis
1 August 2019
and hydropyrolysis of kerogen were constructed about kerogen extracted from Longkou oil
Accepted 6 August 2019
shale to investigate the impact of H2 at different temperatures on the product distribution
Available online 31 August 2019
and reaction processes of oil shale. The experimental results show that hydropyrolysis could increase light shale oil (the most important product in shale oil industry), and
Keywords:
improve the removal rate of organic sulfur simultaneously. It was found that comparing to
Oil shale kerogen
the direct pyrolysis, hydropyrolysis can provide more H free radicals to participate in the
Hydropyrolysis
reaction and therefore promoting the pyrolytic reaction of kerogen. In addition, hydro-
Desulfurization
pyrolysis greatly promoted the desulfurization due to the contribution to the production of
ReaxFF molecular dynamics
H2O molecules, and the transfer of sulfur to the gas products requires the participation of H2O molecules. This work is an intensive study on hydropyrolysis mechanism at different temperatures at the atomic level. These conclusions could be helpful for the clean utilization technology of oil shale industry. © 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
* Corresponding author. No. 1 Zhongguancun North Second Street, Haidian District, Beijing 100190, People’s Republic of China. E-mail address: [email protected] (J.-H. Zhan). https://doi.org/10.1016/j.ijhydene.2019.08.036 0360-3199/© 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
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Introduction With the continuously increasing demand for energy resources, the development and utilization of oil shale as an unconventional oil and gas resources has received great attention from the whole world. Because of its huge reserves and valuable chemical composition of oil shale [1,2], it has become the focus of energy development strategies in various countries. Oil shale is a flammable sedimentary rock rich in organic matter. Its combustible organic matter is mainly kerogen and it can be converted into various oil and gas products by pyrolysis [3]. Therefore, thermal processing is one of the main forms of exploitation of shale oil for the development of oil shale resources. Studies have been carried out to investigate the factors affecting the pyrolysis over the past several decades. Hydropyrolysis refers to pyrolysis under H2 atmosphere in the energy industry. As a helpful method for promoting the conversion of fossil fuels into liquid and gas products [4], hydropyrolysis of biomass [5e7] and coal [8e10] have attracted a lot of research attention. Various studies have been conducted by scholars from the whole world to investigate the effects of minerals [11], metal ion [12], temperature [13] and pressure [14] on hydropyrolysis. Due to the limitations of experimental method, such as the difficulty in observing and confirming intermediates, the effects of hydrogen on the product distribution and chemical mechanisms during hydropyrolysis of kerogen have not yet been discovered. Computational methods would be extremely useful to explore the hydropyrolysis mechanisms and model the chemical reactions. Recently, molecular dynamic (MD) simulations based on the reactive force field (ReaxFF) which is developed by van Duin et al. [15] have been successfully applied to simulate the complex reactions for large scale systems. With this computational method, we could obtain bond cleavage, bond formation, changes of species, and further elementary reactions in the complex thermal reactions. This method has been successfully applied to simulate the complex reactions for a lot of materials, such as lignite [16], pyridine [17] and lignocellulose [18]. Moreover, this
method was extended to improve the understanding of direct pyrolysis and hydropyrolysis reactions for coal and oil shale kerogen macromolecules [19e24]. In this work, the reaction mechanisms during direct pyrolysis and hydropyrolysis of oil shale kerogen were investigated by a series of ReaxFF MD simulations to reveal the effects of H2 on the production of organic gases and shale oil as well as desulfurization. Firstly, the molecular structural model of Longkou oil shale kerogen was constructed. After that, two models were constructed based on the molecular structure to represent the pyrolysis and hydropyrolysis processes of Longkou oil shale kerogen, respectively. The influence of temperature on the product distribution and the effect of desulfurization were studied.
Computational methods Molecular structure model A two-dimensional (2D) molecular model with an empirical formula C341H481O40N5S6 was taken from literature [25] and shown in Fig. 1a. According to the 2D molecular model construction, the geometric optimization was carried out based on the 2D structural model of kerogen to obtain a threedimensional (3D) structural model (Fig. 1b). First, the Material Studio software was used to perform the energy minimization and annealing. The parameters were set as below, iterative steps: 5000 steps; Convergence criteria: energy deviation 0.0001 kcal/mol, atomic standard mean square force 0.005 kcal/mol/ A; using the charge balance method to obtain the net charge of the atom. Atomic states were taken in both Coulomb’s and van der Waals’ calculations. Molecular dynamics calculations were performed to anneal the molecular model: The system was heated from 300 K to 1500 K and then cooled to 300 K. The number of the annealing cycles was 10 and the time of each cycle is 2 ps while the total is 20 ps, energy minimization was used to optimize the model after each cycle. The force field used in the model construction is dreiding.
Fig. 1 e Molecular structure model of Longkou oil shale kerogen. (a) 2-dimension model and (b) 3-dimension model.
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Reactive molecular dynamics simulation details The ReaxFF program developed by van Duin et al. [5] was used to perform ReaxFF simulations. H/C/O/N/S/B force field parameters [24] were used in the present study. To simulate the pyrolysis process of kerogen, two simulation systems need to be constructed for the simulation of the direct pyrolysis and hydropyrolysis of Longkou oil shale kerogen, the system of direct pyrolysis is putting the previously optimized threedimension structural model of Longkou oil shale kerogen (model A) into a 30 30 30 A box, and the system of hydropyrolysis is putting the optimized structural model and 400 hydrogen molecules (model B) into a 30 30 30 A box at the same time. Fig. 2a, b show the initial model of the above two systems built with graphical user interface (GUI) of ReaxFF program, respectively. ReaxFF MD simulations in the present work were processed using a velocity Verlet approach, with a time step of 0.25 fs. A Berendsen thermostat with a damping constant of 100 fs was used for temperature control [18]. Firstly, the initial system of model B was energy minimized using a low temperature (10 K) molecular dynamics for 10 ps. Thereafter, the system was kept in a Berendsen thermostat with a damping constant of 100 fs to perform an NVT (constant number, constant volume and constant temperature) MD simulation. The temperature was increased from 10 K to 300 K with a rising rate of 14.5 K/ps and the system was equilibrated at 300 K for 50 ps with a damping constant of 500 fs. After equilibration, this system was then compressed by no-reaction NPT MD simulation at 300 K and a pressure of 500 MPa for 20 ps. The density of the systems after compression was 0.94 kg/dm3, which is in accordance with the experimental kerogen density. Then, the system was relaxed at 300 K using NVT ensemble for 50 ps. The kerogen molecular model with no additive (model A) was performed in the same way and used as control trials. Finally, ReaxFF MD simulation using an NVT ensemble of 1 ns at 1600, 1800, 2000, 2200, 2400 and 2800 K were performed to exploit product distributions and reaction mechanism of Longkou oil shale kerogen. Due to the limitation on computing capacity, the time scale of the simulation was 1000 ps, which is many magnitudes
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lower than that employed in the experiments (several hours). Both barrier and temperature are in the exponential part of the Arrhenius equation but the barrier is system dependent and cannot be changed. Hence temperature is the controllable parameter to accelerate the reactions in the system. Many researchers chose a much higher temperature instead of the experimental temperature to accelerate reaction in simulations and investigate their reaction systems [19,20,22]. Despite the difference between simulation and experiment in the temperature and time, the previous work showed that the reaction products and pathways of the simulation were in agreement with the experiments [26]. Accordingly, the simulated temperature in this paper was set to be above 1500 K to enable chemical reactions to be observable at the simulated time scale. All the simulations were carried out with ReaxFF program in ADF software.
Simulation results and discussion Major products analysis Fig. 3 displays the number of molecules during direct pyrolysis (Fig. 3a) and hydropyrolysis (Fig. 3b) of the oil shale kerogen at 6 different temperatures. It can be seen from the figure that the total number of molecules increased with the increasing temperature. It is due to the fact that the temperature has an important effect on the speed and sufficiency of the pyrolytic reaction. The total number of molecules during hydropyrolysis was much larger than that during direct pyrolysis, nearly doubled the number of the direct pyrolysis, revealing the fact that hydropyrolysis can help with the sufficiency of the reaction. Another phenomenon can be noticed was that the total number of molecules was nearly the same at the temperature of 2200 K, 2400 K and 2600 K during direct pyrolysis, which was also the maximum value of the direct pyrolysis. However, the process of hydropyrolysis witnessed a different trend, the total number of molecules kept increasing with increasing temperature. Therefore, high temperature can help with the sufficiency of the reaction in the process of
Fig. 2 e Initial reaction system of (a) direct pyrolysis and (b) hydropyrolysis.
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Fig. 3 e Time evolution of total number of molecules during ReaxFF MD at 6 different temperatures in the process of (a) direct pyrolysis and (b) hydropyrolysis.
hydropyrolysis while making no assistance to the direct pyrolysis process. In this work, the pyrolysis of kerogen was simulated for 1000 ps. Fig. 4 shows the distribution of different types of products when the kerogen was directly pyrolyzed (Fig. 4a) and hydropyrolyzed (Fig. 4b) for 1000 ps at different temperatures. The product distributions for direct pyrolysis and hydropyrolysis for 250 ps (Fig. S1), 500 ps (Fig. S2), and 750 ps (Fig. S3) are provided in Supporting Information. The product molecules can be divided into four categories according to the number of carbon atoms. C1eC4 compounds are considered as gas, C5eC13 compounds are considered as light shale oil, C14eC40 compounds are considered as heavy shale oil, C40þ compounds are considered as char. This classification is based on molecular weights of experimental data [27] as well as shown in the literature [28]. As shown in Fig. 4, it can be noted that the high-molecular-weight species are dominant at lower temperatures in both two systems and the number of C40þ compounds gradually dropped with increasing the temperatures, which indicate that high temperatures favor the decomposition of the C40þ compounds. However, no residual C40þ of high-molecular-weight structures was observed at high temperatures during
hydropyrolysis but there was still some C40þ compounds left at the same temperature during direct pyrolysis. This phenomenon indicates that there is polymerization occurring at high temperatures to generate C40þ compounds when the temperature rises to a certain extent. If there is the presence of H in the system, the coupling between H and C40- fragments can prevent the binding among large free radical fragments and inhibit the generation of C40þ heavy byproducts. With the increasing temperature, C40þ compounds gradually decomposed into smaller molecules, while the total number of shale oil products firstly increased and then decreased with the increasing temperature during hydropyrolysis. It can be explained by the fact that light shale oil (C5eC13) and heavy shale oil (C14eC39) are both intermediates and they will be produced by the cleavage reactions of char (C40þ) or by the cross-linking reactions of gas (C1eC4). On the other hand, the proportion of gas products (C1eC4) and light shale oil (C5eC13) account for a much larger proportion during hydropyrolysis, so it can be concluded that hydropyrolysis help with the decomposition of both heavy shale oil and char fragments and it contributes to a more sufficient pyrolytic reaction. Compared with the direct pyrolysis, hydropyrolysis contributes more to the yield of light shale oil. As light shale
Fig. 4 e Composition evolutions in ReaxFF MD at different temperatures for 1000 ps of Longkou oil shale kerogen in the process of (a) direct pyrolysis and (b) hydropyrolysis.
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Fig. 5 e Typical products obtained from ReaxFF MD simulation in the process of (a) direct pyrolysis and (b) hydropyrolysis at 2000 K.
oil is our target products during pyrolysis, the temperature should be controlled around 2000 K. The chemical molecules at 2000 K were listed in Table S1 and Table S2 for variation over the simulation time during direct pyrolysis and hydropyrolysis provided in Supporting Information. In addition, we tried to analyze the information of CeC bond length for C5-13 components at 2000 K. The mean values of CeC bond length were 1.4712 and 1.5006 A for direct pyrolysis and hydropyrolysis, respectively. The difference of CeC bond length
illustrated that the saturation of CeC bonds increased with the presence of H2, indicating that H atom may play a role in decreasing the aromaticity of C5-13 components. Fig. 5 shows the time evolution of the number of typical gas species (CO2, CH4, C2H4, and H2O) obtained from the ReaxFF MD simulation of the kerogen model during direct pyrolysis and hydropyrolysis at the temperature of 2000 K. It can be discovered from the graph that the number of C2H4 is consistently higher than most of the other products
Fig. 6 e Formative path of the first CH4 molecule during (a) direct pyrolysis and (b) hydropyrolysis. The arrows located the bond breaking.
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Fig. 7 e Formative path of the first H2O molecule during (a) direct pyrolysis and (b) hydropyrolysis. The arrows located the bond breaking.
during direct pyrolysis, reaching its maximum number of 19 at the end of the pyrolysis, which is consistent with the light gas FTIR analysis reported by Fletcher [29], while the number of C2H4 during hydropyrolysis decreases by 30%, only has a maximum number of 13, indicating that hydropyrolysis
prevent the production of C2H4. The presence of hydrogen is favorable to the elimination reaction of free radicals and suppress the breaking reactions of long chain free radicals. The number of H2O ranked the first during hydropyrolysis, reaching the maximum number of 30 at the end of
Fig. 8 e Formative path of the first C2H4 molecule during (a) direct pyrolysis and (b) hydropyrolysis. The arrows located the bond breaking.
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Fig. 9 e Main reaction sites of Longkou oil shale kerogen during (a) direct pyrolysis and (b) hydropyrolysis at 2000 K. Arrows indicate the cleavages of representative bonds.
the pyrolysis, while the number of H2O remained stable at the number of 5 during direct pyrolysis. So, it can be concluded that hydropyrolysis has a great effect on the production of H2O molecules. Another noticeable product is CH4, which showed a different trend in two systems respectively. The number of CH4 remained at a small production of 5 during direct pyrolysis. On the other hand, the number of CH4 kept growing up during hydropyrolysis, reaching the maximum number of 14 at the end of the pyrolysis, it can be concluded that hydropyrolysis also contribute to the production of CH4. The number of C2H4 and CH4 increased rapidly and became the main component of the major organic gases. Apart from C2H4, CH4 and H2O, most of the other products remained a small production during pyrolysis. CO2, for example, kept remaining at very small number among all the products at all times, nearly unchanged with pyrolysis time. The time evolutions of the number of CH4, H2O and C2H4 molecular fragments in ReaxFF MD simulation at different temperatures of Longkou oil shale kerogen in the process of direct pyrolysis and
hydropyrolysis are provided as shown in Fig. S4 in Supporting Information.
Typical reaction pathways After the investigations on the time evolution of gas products (CH4, H2O and C2H4) during direct pyrolysis and hydropyrolysis, the formation path of these gas products was also studied. We selected the first generated gas product during direct pyrolysis and hydropyrolysis. Fig. 6a shows the formative path of the first CH4 molecule during direct pyrolysis. It can be seen from the figure that the CH4 is produced by methyl group at the end of the macromolecule. The methyl free radicals shedding from the kerogen reacts with the hydroxyl group at the end of the other chain to obtain a hydrogen atom on the hydroxyl group (H802 in the figure) to generate CH4. The first CH4 molecule is derived from carbon atom (C342 in the figure) and hydrogen atoms in the middle of the fatty chain during hydropyrolysis (Fig. 6b). At first, the CeO bond is
Fig. 10 e Number of hydrogen radicals in the ReaxFF simulation in the process of (a) direct pyrolysis and (b) hydropyrolysis.
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Fig. 11 e Time evolutions of the number of H2S molecular fragments in ReaxFF MD at different temperatures in the process of (a) direct pyrolysis and (b) hydropyrolysis.
broken, the shedding of C342 and O from the fragment simultaneously resulted in the weakening of the CeC bond where C342 was located. Meanwhile, C342 reacts with H2 molecules to obtain a supersaturated structure. This structure weakens the CeO bond and promotes its cleavage, forming methyl free radicals and finally caused the formation of CH4. The first H2O molecule generated during direct pyrolysis (Fig. 7a) is derived from ether oxygen and hydrogen of NeH bond. Firstly, the CeO bond on one side is broken, obtaining a hydrogen radical to form a hydroxyl group, and then generating H2O molecule with another hydrogen radical. The first H2O molecule produced during hydropyrolysis (Fig. 7b) is derived from the oxygen of the carboxyl group and the H2 molecule. The double bond oxygen obtains the hydrogen radical contained in the H2 molecule, forming the hydroxyl group firstly, and then captures another hydrogen radical to generate H2O molecule. The first C2H4 molecule generated during direct pyrolysis (Fig. 8a) is derived from a sulfur-containing chain, the CeS bond is broken firstly, and then the CeC bond is broken to form C2H4. The first C2H4 molecule generated during hydropyrolysis (Fig. 8b) is derived from an oxygen-containing aliphatic chain, firstly a CeO bond is broken, and then the CeC bond is broken to form C2H4. It can be noted that hydropyrolysis affects the formation process of the first C2H4 molecule, which is not the product of the CeS bond breakage. The kerogen macromolecules are composed of complex chemical structures. It is important to study the reaction
pathways of the pyrolysis process to understand the reaction mechanism. Fig. 9 shows the initial bond breaking situation during the pyrolysis of Longkou oil shale kerogen macromolecules. In the process of direct pyrolysis, kerogen macromolecules will split into several molecular fragments. The arrow in the figure refers to the position of the first bond in the reaction process. It can be seen from Fig. 9a that the C]C bond between the tertiary carbon and the secondary carbon in the kerogen macromolecule under pyrolysis can easily break. In addition, the CeC bond between the ether bond and secondary carbon and the CeO bond in oxygen-containing six-membered ring also break to decompose the original macromolecule. Fig. 9b shows the initial bond breaking situation of the hydropyrolysis of Longkou oil shale kerogen macromolecules. For hydropyrolysis, the position of the initial bond is the C]C connected to the tertiary carbon and the secondary carbon, except for the bond between the carbon and the heteroatom. It can be noted that the hydropyrolysis promotes the transfer of heteroatoms.
Desulfurization mechanism Fig. 10 shows the number of hydrogen radicals in the ReaxFF simulation in the process of direct pyrolysis (Fig. 10a) and hydropyrolysis (Fig. 10b), respectively. It can be noticed that there are far less hydrogen radicals observed during direct pyrolysis than during hydropyrolysis. Obviously, hydropyrolysis can provide more hydrogen free radicals to
Table 1 e Elemental atomic number compositions of Gas(C4e), Shale oil (C5e13), Shale oil (C14e39) and Char (C40þ) for direct pyrolysis and hydropyrolysis of Longkou oil shale kerogen. Original molecule
Direct pyrolysis
Hydropyrolysis
Gas (C4e) Shale oil (C5e13) Shale oil (C14e39) Gas (C4e) Shale oil (C5e13) Shale oil (C14e39)
C
H
O
N
S
39.06
55.10
4.58
0.57
0.69
29.43 40.66 46.44 29.08 39.41 41
60.38 53.48 49.85 69.13 59.11 58
9.06 3.3 3.1 0.77 1.06 1
0.75 0.73 0 0 0 0
0.38 1.83 0.61 1.02 0.42 0
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participate in the reaction and therefore promoting the pyrolysis of kerogen. The sulfur atom transferred to the gas products during the pyrolysis is the process of desulfurization of liquid products, the gas product produced by sulfur in pyrolysis is mainly H2S, and H2S can be easily absorbed by the sulfurfixing agent. Therefore, the production of H2S is used to evaluate desulfurization. The time evolutions of the number of H2S molecular fragments at different temperatures are shown in Fig. 11.
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Fig. 11 shows the relationship between the number of H2S molecules and the reaction time when the Longkou oil shale kerogen undergoes direct pyrolysis (Fig. 11a) and hydropyrolysis (Fig. 11b) at different temperatures. From Fig. 11a, it can be seen that the yield of H2S is very low during direct pyrolysis, and the maximum production is obtained at the temperature of 2600 K of only two H2S molecules. However, different trend was observed during hydropyrolysis. The curves in Fig. 11b are very dense, indicating that hydropyrolysis is relatively easy to generate
Fig. 12 e Reactive path of sulfur in thiophene during (a) direct pyrolysis and (b) hydropyrolysis. The arrows located the bond breaking.
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H2S molecules in comparison to direct pyrolysis. At the simulated temperatures of 2400 K and 2600 K, the number of H2S molecules reached 6 (there are only 6 S atoms existing in the molecular model, the generation of 6H2S molecules means all S atoms is removed). It can be explained by the fact that the increasing temperature favors desulfurization. ReaxFF MD simulations can also provide us with the elemental compositions at the atomic level of the pyrolytic products after the hydropyrolysis and direct pyrolysis of Longkou oil shale kerogen. Elemental compositions of
hydropyrolysis simulation and direct pyrolysis simulation system at 2000 K are listed in Table 1. Compared with direct pyrolysis, a sharp decline was witnessed in the contents of O atoms for hydropyrolysis in all categories while the contents of H atoms slightly increase in all categories, all these changes in the elemental atomic number compositions contribute to the large production of H2O in the process of hydropyrolysis. The contents of S atoms increase in C1eC4 fragments and decrease in shale oil contents. It can be concluded from this phenomenon that heteroatoms tend to aggregate in the form
Fig. 13 e Reactive path of sulfur in sulfoxide during (a) direct pyrolysis and (b) hydropyrolysis. The arrows located the bond breaking.
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of C1eC4 fragments and inorganic gases such as H2S to achieve desulfurization. All of the N atoms are transferred to the inorganic gases for there is no contents of N atoms observed in all categories in the table. The analysis of element transformation further illustrates that hydropyrolysis has a significant effect on desulfurization. In order to explore the mechanism of hydropyrolysis to promote desulfurization, this work also traced the reaction pathway of sulfur in kerogen. The original molecular structure of sulfur exists in the form of thiophene and sulfoxide. Fig. 12 and Fig. 13 show the pathways for the formation of H2S from thiophene sulfur and sulfoxide sulfur during the process of direct pyrolysis and hydropyrolysis, respectively. Fig. 12 shows the reaction pathway of thiophene sulfur in the process of direct pyrolysis and hydropyrolysis. It can be noticed from Fig. 12a that the CeS bond (the bond between C248 and S249 in the figure) breaks to cause the ring-opening reaction, and after the ring-opening reaction, sulfur gets a hydrogen radical. Due to the reaction with the aldehyde radicals, it then falls off from the carbon chain. After that, it reacted with H2O molecule to form supersaturated intermediates to produce H2S. Fig. 12b describes the reaction pathway of thiophene sulfur in the hydropyrolysis process. Under the effect of a large number of H2 molecules, the thiophene structure still occur the ring-opening reaction. After the ring-opening, the sulfur radical reacts with CO2 and falls off from the carbon chain. Afterwards, the sulfur atom attracts the H2 molecule to react, resulting in a supersaturated intermediate and promote the direct bond cleavage between C235 and S142, supersaturating the internal atoms of the intermediate. Then, the rearrangement of the supersaturated intermediate internal atoms was followed to form H2S. Fig. 13a shows the reaction pathway of sulfur in sulfoxide in the process of direct pyrolysis. It can be seen from the figure that the interior of the sulfoxide structure changes first and the rearrangement of atoms results in the structure shown in the second figure. After the SeO bond breaks, sulfur radicals are formed at the end of the carbon chain. The sulfur radicals firstly capture H radicals and then react with H2O to produce H2S. In contrast to direct pyrolysis, during hydropyrolysis as shown in Fig. 13b, the O atom in sulfoxide firstly reacts with H2 to generate H2O, and it sheds in macromolecules, resulting in the weakening of the CeS bonds in the original sulfoxide structure. The sulfur-bonded bond breaks and forms sulfur radicals at the end of the carbon chain. The free radicals react with H2O and fall off from the macromolecules to obtain a supersaturated intermediate. From the above analysis of the reaction path, it can be noticed that the formation of H2S is aided by the formation of H2O first. According to the previous analysis, it can be known that pyrolysis with adding H2 molecules can generate a large amount of H2O molecules, and the higher the temperature, the greater the number of H2O molecules generated, the more active the H2O molecules were in the reaction. In conclusion, both increasing temperature and hydropyrolysis promote the formation and reaction of H2O molecules. H2O molecules play an important role in the transfer of sulfur to the gas products. Therefore, hydropyrolysis or increasing the pyrolysis’s
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temperature is of great importance to the removal of sulfur from shale oil products.
Conclusion In this work, the process of pyrolysis and hydropyrolysis based on a Longkou oil shale kerogen model with a density of 0.77 kg/dm3 was simulated via ReaxFF reactive molecular dynamics at different temperatures. The simulated temperature was set to be above 1600 K to enable chemical reactions to be observed at the simulated time scale. The product distributions, reaction processes and organic sulfur removal during hydropyrolysis of Longkou oil shale kerogen were investigated in detail and compared with direct pyrolysis. The main conclusions are as follows: (1) The addition of H2 during pyrolysis could promote the yield of light shale oil and help with the reaction sufficiency comparing with direct pyrolysis in the temperature range of 1600e2600 K, the yield of CH4, H2O, and H2S was especially enhanced. (2) Hydropyrolysis is helpful for increasing light shale oil yield while the direct pyrolysis contributes more to the production of both light shale oil and heavy shale oil, which can be explained by the fact that it can promote the formation of small molecules and prevent the polymerization of small molecules at the same time. (3) Comparing to the direct pyrolysis, hydropyrolysis can provide more hydrogen free radicals to participate in the reaction and therefore promoting the pyrolytic reaction of kerogen. (4) Hydropyrolysis greatly promotes the desulfurization for the reason that hydropyrolysis contributes to the production of H2O molecules, and the transfer of sulfur to the gas is aided by H2O molecules.
Acknowledgments The authors would like to thank the financial support from National Natural Science Foundation of China (51704300, 21875255), Beijing Natural Science Foundation (2192046) and Yue Qi Distinguished Scholar Project, China University of Mining & Technology, Beijing.
Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi.org/10.1016/j.ijhydene.2019.08.036.
references
[1] Gwyn JE. Oil from shale as a viable replacement of depleted crude reserves: processes and challenges. Fuel Process Technol 2001;70(1):27e40.
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[2] Tong J, Han X, Wang S, Jiang X. Evaluation of structural characteristics of Huadian oil shale kerogen using direct techniques (solid-state 13C NMR, XPS, FT-IR, and XRD). Energy Fuel 2011;25(9):4006e13. [3] Tong J, Jiang X, Han X, Wang X. Evaluation of the macromolecular structure of Huadian oil shale kerogen using molecular modeling. Fuel 2016;181:330e9. [4] Rocha JD, Brown SD, Love GD, Snape CE. Hydropyrolysis: a versatile technique for solid fuel liquefaction, sulphur speciation and biomarker release. J Anal Appl Pyrolysis 1997;40:91e103. [5] Chang Z, Duan P, Xu Y. Catalytic hydropyrolysis of microalgae: influence of operating variables on the formation and composition of bio-oil. Bioresour Technol 2015;184:349e54. [6] Machado MA, He S, Davies TE, Seshan K, da Silva VT. Renewable fuel production from hydropyrolysis of residual biomass using molybdenum carbide-based catalysts: an analytical Py-GC/MS investigation. Catal Today 2018;302:161e8. [7] Rocha JD, Luengo CA, Snape CE. The scope for generating biooils with relatively low oxygen contents via hydropyrolysis. Org Geochem 1999;30(12):1527e34. [8] Strugnell B, Patrick JW. Rapid hydropyrolysis studies on coal and maceral concentrates. Fuel 1996;75(3):300e6. [9] Yan L, He B, Hao T, Pei X, Li X, Wang C, Duan Z. Thermogravimetric study on the pressurized hydropyrolysis kinetics of a lignite coal. Int J Hydrogen Energy 2014;39(15):7826e33. lu Z, Canel E, Bozkurt PA. Distribution and [10] Canel M, Mısırlıog comparing of volatile products during slow pyrolysis and hydropyrolysis of Turkish lignites. Fuel 2016;186:504e17. [11] Chen H, Li B, Zhang B. Effects of mineral matter on products and sulfur distributions in hydropyrolysis. Fuel 1999;78(6):713e9. [12] Huang J, Bai Z, Guo Z, Li W, Bai J. Effects of chromium ion on sulfur removal during pyrolysis and hydropyrolysis of coal. J Anal Appl Pyrolysis 2012;97:143e8. lu Z, Sınag A. Hydropyrolysis of a Turkish [13] Canel M, Mısırlıog lignite (Tunc¸bilek) and effect of temperature and pressure on product distribution. Energy Convers Manag 2005;46(13e14):2185e97. [14] Nelson PF, Hu¨ttinger KJ. The effect of hydrogen pressure and aromatic structure on methane yields from the hydropyrolysis of aromatics. Fuel 1986;65(3):354e61. [15] Van Duin AC, Dasgupta S, Lorant F, Goddard WA. ReaxFF: a reactive force field for hydrocarbons. J Phys Chem A 2001;105(41):9396e409. [16] Chen B, Diao ZJ, Lu HY. Using the ReaxFF reactive force field for molecular dynamics simulations of the spontaneous combustion of lignite with the Hatcher lignite model. Fuel 2014;116:7e13.
[17] Liu J, Guo X. ReaxFF molecular dynamics simulation of pyrolysis and combustion of pyridine. Fuel Process Technol 2017;161:107e15. [18] Rismiller SC, Groves MM, Meng M, Dong Y, Lin J. Water assisted liquefaction of lignocellulose biomass by ReaxFF based molecular dynamic simulations. Fuel 2018;215:835e43. [19] Qian Y, Zhan JH, Lai D, Li M, Liu X, Xu G. Primary understanding of non-isothermal pyrolysis behavior for oil shale kerogen using reactive molecular dynamics simulation. Int J Hydrogen Energy 2016;41(28):12093e100. [20] Liu X, Zhan JH, Lai D, Liu X, Zhang Z, Xu G. Initial pyrolysis mechanism of oil shale kerogen with reactive molecular dynamics simulation. Energy Fuel 2015;29(5):2987e97. [21] Wang H, Feng Y, Zhang X, Lin W, Zhao Y. Study of coal hydropyrolysis and desulfurization by ReaxFF molecular dynamics simulation. Fuel 2015;145:241e8. [22] Liang YH, Wang F, Zhang H, Wang JP, Li YY, Li GY. A ReaxFF molecular dynamics study on the mechanism of organic sulfur transformation in the hydropyrolysis process of lignite. Fuel Process Technol 2016;147:32e40. [23] Castro-Marcano F, Kamat AM, Russo Jr MF, van Duin AC, Mathews JP. Combustion of an Illinois No. 6 coal char simulated using an atomistic char representation and the ReaxFF reactive force field. Combust Flame 2012;159:1272e85. [24] Bhoi S, Banerjee T, Mohanty K. Molecular dynamic simulation of spontaneous combustion and pyrolysis of brown coal using ReaxFF. Fuel 2014;136:326e33. [25] Zhang Z, Chai J, Zhang H, Guo L, Zhan J-H. Structural model of Longkou oil shale kerogen and the evolution process under steam pyrolysis based on ReaxFF molecular dynamics simulation. Energy Source Part A Recov Util Environ Eff 2019. https://doi.org/10.1080/15567036.2019.1624879. [26] Salmon E, van Duin AC, Lorant F, Marquaire PM, Goddard III WA. Thermal decomposition process in algaenan of Botryococcus braunii race L. Part 2: molecular dynamics simulations using the ReaxFF reactive force field. Org Geochem 2009;40(3):416e27. [27] Fletcher TH, Kerstein AR, Pugmire RJ, Solum MS, Grant DM. Chemical percolation model for devolatilization. 3. Direct use of carbon-13 NMR data to predict effects of coal type. Energy Fuel 1992;6(4):414e31. [28] Zheng M, Li X, Liu J, Wang Z, Gong X, Guo L, Song W. Pyrolysis of Liulin coal simulated by GPU-based ReaxFF MD with cheminformatics analysis. Energy Fuel 2013;28(1):522e34. [29] Fletcher TH, Gillis R, Adams J, Hall T, Mayne CL, Solum MS, Pugmire RJ. Characterization of macromolecular structure elements from a Green River oil shale, II. Characterization of pyrolysis products by 13C NMR, GC/MS, and FTIR. Energy Fuel 2014;28(5):2959e70.
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Comparing product distribution and desulfurization during direct pyrolysis and hydropyrolysis of Longkou oil shale kerogen using reactive MD simulations Zhijun Zhang a, Liting Guo a, Hanyu Zhang b, Jin-Hui Zhan c,* a
School of Chemical and Environmental Engineering, China University of Mining and Technology, Beijing 100083, People’s Republic of China b Department of Chemical and Materials Engineering, University of Alberta, Edmonton, Alberta, T6G 1H9, Canada c State Key Laboratory of Multiphase Complex System, Institute of Process Engineering, Chinese Academy of Sciences, Beijing 100190, People’s Republic of China
highlights Hydropyrolysis and direct pyrolysis of oil shale were studied using ReaxFF MD. Hydropyrolysis is helpful for increasing light oil yield. Hydropyrolysis has positive effects on the desulfurization. Hydropyrolysis contributes to the production of H2O molecules. The transfer of sulfur to the gas requires the participation of H2O molecules.
article info
abstract
Article history:
The product distribution and organic sulfur removal during direct pyrolysis and hydro-
Received 21 May 2019
pyrolysis of oil shale kerogen were investigated via reactive molecular dynamics (RMD)
Received in revised form
simulations with reactive force field (ReaxFF). Two structural models for direct pyrolysis
1 August 2019
and hydropyrolysis of kerogen were constructed about kerogen extracted from Longkou oil
Accepted 6 August 2019
shale to investigate the impact of H2 at different temperatures on the product distribution
Available online 31 August 2019
and reaction processes of oil shale. The experimental results show that hydropyrolysis could increase light shale oil (the most important product in shale oil industry), and
Keywords:
improve the removal rate of organic sulfur simultaneously. It was found that comparing to
Oil shale kerogen
the direct pyrolysis, hydropyrolysis can provide more H free radicals to participate in the
Hydropyrolysis
reaction and therefore promoting the pyrolytic reaction of kerogen. In addition, hydro-
Desulfurization
pyrolysis greatly promoted the desulfurization due to the contribution to the production of
ReaxFF molecular dynamics
H2O molecules, and the transfer of sulfur to the gas products requires the participation of H2O molecules. This work is an intensive study on hydropyrolysis mechanism at different temperatures at the atomic level. These conclusions could be helpful for the clean utilization technology of oil shale industry. © 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
* Corresponding author. No. 1 Zhongguancun North Second Street, Haidian District, Beijing 100190, People’s Republic of China. E-mail address: [email protected] (J.-H. Zhan). https://doi.org/10.1016/j.ijhydene.2019.08.036 0360-3199/© 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
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Introduction With the continuously increasing demand for energy resources, the development and utilization of oil shale as an unconventional oil and gas resources has received great attention from the whole world. Because of its huge reserves and valuable chemical composition of oil shale [1,2], it has become the focus of energy development strategies in various countries. Oil shale is a flammable sedimentary rock rich in organic matter. Its combustible organic matter is mainly kerogen and it can be converted into various oil and gas products by pyrolysis [3]. Therefore, thermal processing is one of the main forms of exploitation of shale oil for the development of oil shale resources. Studies have been carried out to investigate the factors affecting the pyrolysis over the past several decades. Hydropyrolysis refers to pyrolysis under H2 atmosphere in the energy industry. As a helpful method for promoting the conversion of fossil fuels into liquid and gas products [4], hydropyrolysis of biomass [5e7] and coal [8e10] have attracted a lot of research attention. Various studies have been conducted by scholars from the whole world to investigate the effects of minerals [11], metal ion [12], temperature [13] and pressure [14] on hydropyrolysis. Due to the limitations of experimental method, such as the difficulty in observing and confirming intermediates, the effects of hydrogen on the product distribution and chemical mechanisms during hydropyrolysis of kerogen have not yet been discovered. Computational methods would be extremely useful to explore the hydropyrolysis mechanisms and model the chemical reactions. Recently, molecular dynamic (MD) simulations based on the reactive force field (ReaxFF) which is developed by van Duin et al. [15] have been successfully applied to simulate the complex reactions for large scale systems. With this computational method, we could obtain bond cleavage, bond formation, changes of species, and further elementary reactions in the complex thermal reactions. This method has been successfully applied to simulate the complex reactions for a lot of materials, such as lignite [16], pyridine [17] and lignocellulose [18]. Moreover, this
method was extended to improve the understanding of direct pyrolysis and hydropyrolysis reactions for coal and oil shale kerogen macromolecules [19e24]. In this work, the reaction mechanisms during direct pyrolysis and hydropyrolysis of oil shale kerogen were investigated by a series of ReaxFF MD simulations to reveal the effects of H2 on the production of organic gases and shale oil as well as desulfurization. Firstly, the molecular structural model of Longkou oil shale kerogen was constructed. After that, two models were constructed based on the molecular structure to represent the pyrolysis and hydropyrolysis processes of Longkou oil shale kerogen, respectively. The influence of temperature on the product distribution and the effect of desulfurization were studied.
Computational methods Molecular structure model A two-dimensional (2D) molecular model with an empirical formula C341H481O40N5S6 was taken from literature [25] and shown in Fig. 1a. According to the 2D molecular model construction, the geometric optimization was carried out based on the 2D structural model of kerogen to obtain a threedimensional (3D) structural model (Fig. 1b). First, the Material Studio software was used to perform the energy minimization and annealing. The parameters were set as below, iterative steps: 5000 steps; Convergence criteria: energy deviation 0.0001 kcal/mol, atomic standard mean square force 0.005 kcal/mol/ A; using the charge balance method to obtain the net charge of the atom. Atomic states were taken in both Coulomb’s and van der Waals’ calculations. Molecular dynamics calculations were performed to anneal the molecular model: The system was heated from 300 K to 1500 K and then cooled to 300 K. The number of the annealing cycles was 10 and the time of each cycle is 2 ps while the total is 20 ps, energy minimization was used to optimize the model after each cycle. The force field used in the model construction is dreiding.
Fig. 1 e Molecular structure model of Longkou oil shale kerogen. (a) 2-dimension model and (b) 3-dimension model.
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Reactive molecular dynamics simulation details The ReaxFF program developed by van Duin et al. [5] was used to perform ReaxFF simulations. H/C/O/N/S/B force field parameters [24] were used in the present study. To simulate the pyrolysis process of kerogen, two simulation systems need to be constructed for the simulation of the direct pyrolysis and hydropyrolysis of Longkou oil shale kerogen, the system of direct pyrolysis is putting the previously optimized threedimension structural model of Longkou oil shale kerogen (model A) into a 30 30 30 A box, and the system of hydropyrolysis is putting the optimized structural model and 400 hydrogen molecules (model B) into a 30 30 30 A box at the same time. Fig. 2a, b show the initial model of the above two systems built with graphical user interface (GUI) of ReaxFF program, respectively. ReaxFF MD simulations in the present work were processed using a velocity Verlet approach, with a time step of 0.25 fs. A Berendsen thermostat with a damping constant of 100 fs was used for temperature control [18]. Firstly, the initial system of model B was energy minimized using a low temperature (10 K) molecular dynamics for 10 ps. Thereafter, the system was kept in a Berendsen thermostat with a damping constant of 100 fs to perform an NVT (constant number, constant volume and constant temperature) MD simulation. The temperature was increased from 10 K to 300 K with a rising rate of 14.5 K/ps and the system was equilibrated at 300 K for 50 ps with a damping constant of 500 fs. After equilibration, this system was then compressed by no-reaction NPT MD simulation at 300 K and a pressure of 500 MPa for 20 ps. The density of the systems after compression was 0.94 kg/dm3, which is in accordance with the experimental kerogen density. Then, the system was relaxed at 300 K using NVT ensemble for 50 ps. The kerogen molecular model with no additive (model A) was performed in the same way and used as control trials. Finally, ReaxFF MD simulation using an NVT ensemble of 1 ns at 1600, 1800, 2000, 2200, 2400 and 2800 K were performed to exploit product distributions and reaction mechanism of Longkou oil shale kerogen. Due to the limitation on computing capacity, the time scale of the simulation was 1000 ps, which is many magnitudes
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lower than that employed in the experiments (several hours). Both barrier and temperature are in the exponential part of the Arrhenius equation but the barrier is system dependent and cannot be changed. Hence temperature is the controllable parameter to accelerate the reactions in the system. Many researchers chose a much higher temperature instead of the experimental temperature to accelerate reaction in simulations and investigate their reaction systems [19,20,22]. Despite the difference between simulation and experiment in the temperature and time, the previous work showed that the reaction products and pathways of the simulation were in agreement with the experiments [26]. Accordingly, the simulated temperature in this paper was set to be above 1500 K to enable chemical reactions to be observable at the simulated time scale. All the simulations were carried out with ReaxFF program in ADF software.
Simulation results and discussion Major products analysis Fig. 3 displays the number of molecules during direct pyrolysis (Fig. 3a) and hydropyrolysis (Fig. 3b) of the oil shale kerogen at 6 different temperatures. It can be seen from the figure that the total number of molecules increased with the increasing temperature. It is due to the fact that the temperature has an important effect on the speed and sufficiency of the pyrolytic reaction. The total number of molecules during hydropyrolysis was much larger than that during direct pyrolysis, nearly doubled the number of the direct pyrolysis, revealing the fact that hydropyrolysis can help with the sufficiency of the reaction. Another phenomenon can be noticed was that the total number of molecules was nearly the same at the temperature of 2200 K, 2400 K and 2600 K during direct pyrolysis, which was also the maximum value of the direct pyrolysis. However, the process of hydropyrolysis witnessed a different trend, the total number of molecules kept increasing with increasing temperature. Therefore, high temperature can help with the sufficiency of the reaction in the process of
Fig. 2 e Initial reaction system of (a) direct pyrolysis and (b) hydropyrolysis.
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Fig. 3 e Time evolution of total number of molecules during ReaxFF MD at 6 different temperatures in the process of (a) direct pyrolysis and (b) hydropyrolysis.
hydropyrolysis while making no assistance to the direct pyrolysis process. In this work, the pyrolysis of kerogen was simulated for 1000 ps. Fig. 4 shows the distribution of different types of products when the kerogen was directly pyrolyzed (Fig. 4a) and hydropyrolyzed (Fig. 4b) for 1000 ps at different temperatures. The product distributions for direct pyrolysis and hydropyrolysis for 250 ps (Fig. S1), 500 ps (Fig. S2), and 750 ps (Fig. S3) are provided in Supporting Information. The product molecules can be divided into four categories according to the number of carbon atoms. C1eC4 compounds are considered as gas, C5eC13 compounds are considered as light shale oil, C14eC40 compounds are considered as heavy shale oil, C40þ compounds are considered as char. This classification is based on molecular weights of experimental data [27] as well as shown in the literature [28]. As shown in Fig. 4, it can be noted that the high-molecular-weight species are dominant at lower temperatures in both two systems and the number of C40þ compounds gradually dropped with increasing the temperatures, which indicate that high temperatures favor the decomposition of the C40þ compounds. However, no residual C40þ of high-molecular-weight structures was observed at high temperatures during
hydropyrolysis but there was still some C40þ compounds left at the same temperature during direct pyrolysis. This phenomenon indicates that there is polymerization occurring at high temperatures to generate C40þ compounds when the temperature rises to a certain extent. If there is the presence of H in the system, the coupling between H and C40- fragments can prevent the binding among large free radical fragments and inhibit the generation of C40þ heavy byproducts. With the increasing temperature, C40þ compounds gradually decomposed into smaller molecules, while the total number of shale oil products firstly increased and then decreased with the increasing temperature during hydropyrolysis. It can be explained by the fact that light shale oil (C5eC13) and heavy shale oil (C14eC39) are both intermediates and they will be produced by the cleavage reactions of char (C40þ) or by the cross-linking reactions of gas (C1eC4). On the other hand, the proportion of gas products (C1eC4) and light shale oil (C5eC13) account for a much larger proportion during hydropyrolysis, so it can be concluded that hydropyrolysis help with the decomposition of both heavy shale oil and char fragments and it contributes to a more sufficient pyrolytic reaction. Compared with the direct pyrolysis, hydropyrolysis contributes more to the yield of light shale oil. As light shale
Fig. 4 e Composition evolutions in ReaxFF MD at different temperatures for 1000 ps of Longkou oil shale kerogen in the process of (a) direct pyrolysis and (b) hydropyrolysis.
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Fig. 5 e Typical products obtained from ReaxFF MD simulation in the process of (a) direct pyrolysis and (b) hydropyrolysis at 2000 K.
oil is our target products during pyrolysis, the temperature should be controlled around 2000 K. The chemical molecules at 2000 K were listed in Table S1 and Table S2 for variation over the simulation time during direct pyrolysis and hydropyrolysis provided in Supporting Information. In addition, we tried to analyze the information of CeC bond length for C5-13 components at 2000 K. The mean values of CeC bond length were 1.4712 and 1.5006 A for direct pyrolysis and hydropyrolysis, respectively. The difference of CeC bond length
illustrated that the saturation of CeC bonds increased with the presence of H2, indicating that H atom may play a role in decreasing the aromaticity of C5-13 components. Fig. 5 shows the time evolution of the number of typical gas species (CO2, CH4, C2H4, and H2O) obtained from the ReaxFF MD simulation of the kerogen model during direct pyrolysis and hydropyrolysis at the temperature of 2000 K. It can be discovered from the graph that the number of C2H4 is consistently higher than most of the other products
Fig. 6 e Formative path of the first CH4 molecule during (a) direct pyrolysis and (b) hydropyrolysis. The arrows located the bond breaking.
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Fig. 7 e Formative path of the first H2O molecule during (a) direct pyrolysis and (b) hydropyrolysis. The arrows located the bond breaking.
during direct pyrolysis, reaching its maximum number of 19 at the end of the pyrolysis, which is consistent with the light gas FTIR analysis reported by Fletcher [29], while the number of C2H4 during hydropyrolysis decreases by 30%, only has a maximum number of 13, indicating that hydropyrolysis
prevent the production of C2H4. The presence of hydrogen is favorable to the elimination reaction of free radicals and suppress the breaking reactions of long chain free radicals. The number of H2O ranked the first during hydropyrolysis, reaching the maximum number of 30 at the end of
Fig. 8 e Formative path of the first C2H4 molecule during (a) direct pyrolysis and (b) hydropyrolysis. The arrows located the bond breaking.
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Fig. 9 e Main reaction sites of Longkou oil shale kerogen during (a) direct pyrolysis and (b) hydropyrolysis at 2000 K. Arrows indicate the cleavages of representative bonds.
the pyrolysis, while the number of H2O remained stable at the number of 5 during direct pyrolysis. So, it can be concluded that hydropyrolysis has a great effect on the production of H2O molecules. Another noticeable product is CH4, which showed a different trend in two systems respectively. The number of CH4 remained at a small production of 5 during direct pyrolysis. On the other hand, the number of CH4 kept growing up during hydropyrolysis, reaching the maximum number of 14 at the end of the pyrolysis, it can be concluded that hydropyrolysis also contribute to the production of CH4. The number of C2H4 and CH4 increased rapidly and became the main component of the major organic gases. Apart from C2H4, CH4 and H2O, most of the other products remained a small production during pyrolysis. CO2, for example, kept remaining at very small number among all the products at all times, nearly unchanged with pyrolysis time. The time evolutions of the number of CH4, H2O and C2H4 molecular fragments in ReaxFF MD simulation at different temperatures of Longkou oil shale kerogen in the process of direct pyrolysis and
hydropyrolysis are provided as shown in Fig. S4 in Supporting Information.
Typical reaction pathways After the investigations on the time evolution of gas products (CH4, H2O and C2H4) during direct pyrolysis and hydropyrolysis, the formation path of these gas products was also studied. We selected the first generated gas product during direct pyrolysis and hydropyrolysis. Fig. 6a shows the formative path of the first CH4 molecule during direct pyrolysis. It can be seen from the figure that the CH4 is produced by methyl group at the end of the macromolecule. The methyl free radicals shedding from the kerogen reacts with the hydroxyl group at the end of the other chain to obtain a hydrogen atom on the hydroxyl group (H802 in the figure) to generate CH4. The first CH4 molecule is derived from carbon atom (C342 in the figure) and hydrogen atoms in the middle of the fatty chain during hydropyrolysis (Fig. 6b). At first, the CeO bond is
Fig. 10 e Number of hydrogen radicals in the ReaxFF simulation in the process of (a) direct pyrolysis and (b) hydropyrolysis.
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Fig. 11 e Time evolutions of the number of H2S molecular fragments in ReaxFF MD at different temperatures in the process of (a) direct pyrolysis and (b) hydropyrolysis.
broken, the shedding of C342 and O from the fragment simultaneously resulted in the weakening of the CeC bond where C342 was located. Meanwhile, C342 reacts with H2 molecules to obtain a supersaturated structure. This structure weakens the CeO bond and promotes its cleavage, forming methyl free radicals and finally caused the formation of CH4. The first H2O molecule generated during direct pyrolysis (Fig. 7a) is derived from ether oxygen and hydrogen of NeH bond. Firstly, the CeO bond on one side is broken, obtaining a hydrogen radical to form a hydroxyl group, and then generating H2O molecule with another hydrogen radical. The first H2O molecule produced during hydropyrolysis (Fig. 7b) is derived from the oxygen of the carboxyl group and the H2 molecule. The double bond oxygen obtains the hydrogen radical contained in the H2 molecule, forming the hydroxyl group firstly, and then captures another hydrogen radical to generate H2O molecule. The first C2H4 molecule generated during direct pyrolysis (Fig. 8a) is derived from a sulfur-containing chain, the CeS bond is broken firstly, and then the CeC bond is broken to form C2H4. The first C2H4 molecule generated during hydropyrolysis (Fig. 8b) is derived from an oxygen-containing aliphatic chain, firstly a CeO bond is broken, and then the CeC bond is broken to form C2H4. It can be noted that hydropyrolysis affects the formation process of the first C2H4 molecule, which is not the product of the CeS bond breakage. The kerogen macromolecules are composed of complex chemical structures. It is important to study the reaction
pathways of the pyrolysis process to understand the reaction mechanism. Fig. 9 shows the initial bond breaking situation during the pyrolysis of Longkou oil shale kerogen macromolecules. In the process of direct pyrolysis, kerogen macromolecules will split into several molecular fragments. The arrow in the figure refers to the position of the first bond in the reaction process. It can be seen from Fig. 9a that the C]C bond between the tertiary carbon and the secondary carbon in the kerogen macromolecule under pyrolysis can easily break. In addition, the CeC bond between the ether bond and secondary carbon and the CeO bond in oxygen-containing six-membered ring also break to decompose the original macromolecule. Fig. 9b shows the initial bond breaking situation of the hydropyrolysis of Longkou oil shale kerogen macromolecules. For hydropyrolysis, the position of the initial bond is the C]C connected to the tertiary carbon and the secondary carbon, except for the bond between the carbon and the heteroatom. It can be noted that the hydropyrolysis promotes the transfer of heteroatoms.
Desulfurization mechanism Fig. 10 shows the number of hydrogen radicals in the ReaxFF simulation in the process of direct pyrolysis (Fig. 10a) and hydropyrolysis (Fig. 10b), respectively. It can be noticed that there are far less hydrogen radicals observed during direct pyrolysis than during hydropyrolysis. Obviously, hydropyrolysis can provide more hydrogen free radicals to
Table 1 e Elemental atomic number compositions of Gas(C4e), Shale oil (C5e13), Shale oil (C14e39) and Char (C40þ) for direct pyrolysis and hydropyrolysis of Longkou oil shale kerogen. Original molecule
Direct pyrolysis
Hydropyrolysis
Gas (C4e) Shale oil (C5e13) Shale oil (C14e39) Gas (C4e) Shale oil (C5e13) Shale oil (C14e39)
C
H
O
N
S
39.06
55.10
4.58
0.57
0.69
29.43 40.66 46.44 29.08 39.41 41
60.38 53.48 49.85 69.13 59.11 58
9.06 3.3 3.1 0.77 1.06 1
0.75 0.73 0 0 0 0
0.38 1.83 0.61 1.02 0.42 0
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participate in the reaction and therefore promoting the pyrolysis of kerogen. The sulfur atom transferred to the gas products during the pyrolysis is the process of desulfurization of liquid products, the gas product produced by sulfur in pyrolysis is mainly H2S, and H2S can be easily absorbed by the sulfurfixing agent. Therefore, the production of H2S is used to evaluate desulfurization. The time evolutions of the number of H2S molecular fragments at different temperatures are shown in Fig. 11.
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Fig. 11 shows the relationship between the number of H2S molecules and the reaction time when the Longkou oil shale kerogen undergoes direct pyrolysis (Fig. 11a) and hydropyrolysis (Fig. 11b) at different temperatures. From Fig. 11a, it can be seen that the yield of H2S is very low during direct pyrolysis, and the maximum production is obtained at the temperature of 2600 K of only two H2S molecules. However, different trend was observed during hydropyrolysis. The curves in Fig. 11b are very dense, indicating that hydropyrolysis is relatively easy to generate
Fig. 12 e Reactive path of sulfur in thiophene during (a) direct pyrolysis and (b) hydropyrolysis. The arrows located the bond breaking.
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H2S molecules in comparison to direct pyrolysis. At the simulated temperatures of 2400 K and 2600 K, the number of H2S molecules reached 6 (there are only 6 S atoms existing in the molecular model, the generation of 6H2S molecules means all S atoms is removed). It can be explained by the fact that the increasing temperature favors desulfurization. ReaxFF MD simulations can also provide us with the elemental compositions at the atomic level of the pyrolytic products after the hydropyrolysis and direct pyrolysis of Longkou oil shale kerogen. Elemental compositions of
hydropyrolysis simulation and direct pyrolysis simulation system at 2000 K are listed in Table 1. Compared with direct pyrolysis, a sharp decline was witnessed in the contents of O atoms for hydropyrolysis in all categories while the contents of H atoms slightly increase in all categories, all these changes in the elemental atomic number compositions contribute to the large production of H2O in the process of hydropyrolysis. The contents of S atoms increase in C1eC4 fragments and decrease in shale oil contents. It can be concluded from this phenomenon that heteroatoms tend to aggregate in the form
Fig. 13 e Reactive path of sulfur in sulfoxide during (a) direct pyrolysis and (b) hydropyrolysis. The arrows located the bond breaking.
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of C1eC4 fragments and inorganic gases such as H2S to achieve desulfurization. All of the N atoms are transferred to the inorganic gases for there is no contents of N atoms observed in all categories in the table. The analysis of element transformation further illustrates that hydropyrolysis has a significant effect on desulfurization. In order to explore the mechanism of hydropyrolysis to promote desulfurization, this work also traced the reaction pathway of sulfur in kerogen. The original molecular structure of sulfur exists in the form of thiophene and sulfoxide. Fig. 12 and Fig. 13 show the pathways for the formation of H2S from thiophene sulfur and sulfoxide sulfur during the process of direct pyrolysis and hydropyrolysis, respectively. Fig. 12 shows the reaction pathway of thiophene sulfur in the process of direct pyrolysis and hydropyrolysis. It can be noticed from Fig. 12a that the CeS bond (the bond between C248 and S249 in the figure) breaks to cause the ring-opening reaction, and after the ring-opening reaction, sulfur gets a hydrogen radical. Due to the reaction with the aldehyde radicals, it then falls off from the carbon chain. After that, it reacted with H2O molecule to form supersaturated intermediates to produce H2S. Fig. 12b describes the reaction pathway of thiophene sulfur in the hydropyrolysis process. Under the effect of a large number of H2 molecules, the thiophene structure still occur the ring-opening reaction. After the ring-opening, the sulfur radical reacts with CO2 and falls off from the carbon chain. Afterwards, the sulfur atom attracts the H2 molecule to react, resulting in a supersaturated intermediate and promote the direct bond cleavage between C235 and S142, supersaturating the internal atoms of the intermediate. Then, the rearrangement of the supersaturated intermediate internal atoms was followed to form H2S. Fig. 13a shows the reaction pathway of sulfur in sulfoxide in the process of direct pyrolysis. It can be seen from the figure that the interior of the sulfoxide structure changes first and the rearrangement of atoms results in the structure shown in the second figure. After the SeO bond breaks, sulfur radicals are formed at the end of the carbon chain. The sulfur radicals firstly capture H radicals and then react with H2O to produce H2S. In contrast to direct pyrolysis, during hydropyrolysis as shown in Fig. 13b, the O atom in sulfoxide firstly reacts with H2 to generate H2O, and it sheds in macromolecules, resulting in the weakening of the CeS bonds in the original sulfoxide structure. The sulfur-bonded bond breaks and forms sulfur radicals at the end of the carbon chain. The free radicals react with H2O and fall off from the macromolecules to obtain a supersaturated intermediate. From the above analysis of the reaction path, it can be noticed that the formation of H2S is aided by the formation of H2O first. According to the previous analysis, it can be known that pyrolysis with adding H2 molecules can generate a large amount of H2O molecules, and the higher the temperature, the greater the number of H2O molecules generated, the more active the H2O molecules were in the reaction. In conclusion, both increasing temperature and hydropyrolysis promote the formation and reaction of H2O molecules. H2O molecules play an important role in the transfer of sulfur to the gas products. Therefore, hydropyrolysis or increasing the pyrolysis’s
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temperature is of great importance to the removal of sulfur from shale oil products.
Conclusion In this work, the process of pyrolysis and hydropyrolysis based on a Longkou oil shale kerogen model with a density of 0.77 kg/dm3 was simulated via ReaxFF reactive molecular dynamics at different temperatures. The simulated temperature was set to be above 1600 K to enable chemical reactions to be observed at the simulated time scale. The product distributions, reaction processes and organic sulfur removal during hydropyrolysis of Longkou oil shale kerogen were investigated in detail and compared with direct pyrolysis. The main conclusions are as follows: (1) The addition of H2 during pyrolysis could promote the yield of light shale oil and help with the reaction sufficiency comparing with direct pyrolysis in the temperature range of 1600e2600 K, the yield of CH4, H2O, and H2S was especially enhanced. (2) Hydropyrolysis is helpful for increasing light shale oil yield while the direct pyrolysis contributes more to the production of both light shale oil and heavy shale oil, which can be explained by the fact that it can promote the formation of small molecules and prevent the polymerization of small molecules at the same time. (3) Comparing to the direct pyrolysis, hydropyrolysis can provide more hydrogen free radicals to participate in the reaction and therefore promoting the pyrolytic reaction of kerogen. (4) Hydropyrolysis greatly promotes the desulfurization for the reason that hydropyrolysis contributes to the production of H2O molecules, and the transfer of sulfur to the gas is aided by H2O molecules.
Acknowledgments The authors would like to thank the financial support from National Natural Science Foundation of China (51704300, 21875255), Beijing Natural Science Foundation (2192046) and Yue Qi Distinguished Scholar Project, China University of Mining & Technology, Beijing.
Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi.org/10.1016/j.ijhydene.2019.08.036.
references
[1] Gwyn JE. Oil from shale as a viable replacement of depleted crude reserves: processes and challenges. Fuel Process Technol 2001;70(1):27e40.
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[2] Tong J, Han X, Wang S, Jiang X. Evaluation of structural characteristics of Huadian oil shale kerogen using direct techniques (solid-state 13C NMR, XPS, FT-IR, and XRD). Energy Fuel 2011;25(9):4006e13. [3] Tong J, Jiang X, Han X, Wang X. Evaluation of the macromolecular structure of Huadian oil shale kerogen using molecular modeling. Fuel 2016;181:330e9. [4] Rocha JD, Brown SD, Love GD, Snape CE. Hydropyrolysis: a versatile technique for solid fuel liquefaction, sulphur speciation and biomarker release. J Anal Appl Pyrolysis 1997;40:91e103. [5] Chang Z, Duan P, Xu Y. Catalytic hydropyrolysis of microalgae: influence of operating variables on the formation and composition of bio-oil. Bioresour Technol 2015;184:349e54. [6] Machado MA, He S, Davies TE, Seshan K, da Silva VT. Renewable fuel production from hydropyrolysis of residual biomass using molybdenum carbide-based catalysts: an analytical Py-GC/MS investigation. Catal Today 2018;302:161e8. [7] Rocha JD, Luengo CA, Snape CE. The scope for generating biooils with relatively low oxygen contents via hydropyrolysis. Org Geochem 1999;30(12):1527e34. [8] Strugnell B, Patrick JW. Rapid hydropyrolysis studies on coal and maceral concentrates. Fuel 1996;75(3):300e6. [9] Yan L, He B, Hao T, Pei X, Li X, Wang C, Duan Z. Thermogravimetric study on the pressurized hydropyrolysis kinetics of a lignite coal. Int J Hydrogen Energy 2014;39(15):7826e33. lu Z, Canel E, Bozkurt PA. Distribution and [10] Canel M, Mısırlıog comparing of volatile products during slow pyrolysis and hydropyrolysis of Turkish lignites. Fuel 2016;186:504e17. [11] Chen H, Li B, Zhang B. Effects of mineral matter on products and sulfur distributions in hydropyrolysis. Fuel 1999;78(6):713e9. [12] Huang J, Bai Z, Guo Z, Li W, Bai J. Effects of chromium ion on sulfur removal during pyrolysis and hydropyrolysis of coal. J Anal Appl Pyrolysis 2012;97:143e8. lu Z, Sınag A. Hydropyrolysis of a Turkish [13] Canel M, Mısırlıog lignite (Tunc¸bilek) and effect of temperature and pressure on product distribution. Energy Convers Manag 2005;46(13e14):2185e97. [14] Nelson PF, Hu¨ttinger KJ. The effect of hydrogen pressure and aromatic structure on methane yields from the hydropyrolysis of aromatics. Fuel 1986;65(3):354e61. [15] Van Duin AC, Dasgupta S, Lorant F, Goddard WA. ReaxFF: a reactive force field for hydrocarbons. J Phys Chem A 2001;105(41):9396e409. [16] Chen B, Diao ZJ, Lu HY. Using the ReaxFF reactive force field for molecular dynamics simulations of the spontaneous combustion of lignite with the Hatcher lignite model. Fuel 2014;116:7e13.
[17] Liu J, Guo X. ReaxFF molecular dynamics simulation of pyrolysis and combustion of pyridine. Fuel Process Technol 2017;161:107e15. [18] Rismiller SC, Groves MM, Meng M, Dong Y, Lin J. Water assisted liquefaction of lignocellulose biomass by ReaxFF based molecular dynamic simulations. Fuel 2018;215:835e43. [19] Qian Y, Zhan JH, Lai D, Li M, Liu X, Xu G. Primary understanding of non-isothermal pyrolysis behavior for oil shale kerogen using reactive molecular dynamics simulation. Int J Hydrogen Energy 2016;41(28):12093e100. [20] Liu X, Zhan JH, Lai D, Liu X, Zhang Z, Xu G. Initial pyrolysis mechanism of oil shale kerogen with reactive molecular dynamics simulation. Energy Fuel 2015;29(5):2987e97. [21] Wang H, Feng Y, Zhang X, Lin W, Zhao Y. Study of coal hydropyrolysis and desulfurization by ReaxFF molecular dynamics simulation. Fuel 2015;145:241e8. [22] Liang YH, Wang F, Zhang H, Wang JP, Li YY, Li GY. A ReaxFF molecular dynamics study on the mechanism of organic sulfur transformation in the hydropyrolysis process of lignite. Fuel Process Technol 2016;147:32e40. [23] Castro-Marcano F, Kamat AM, Russo Jr MF, van Duin AC, Mathews JP. Combustion of an Illinois No. 6 coal char simulated using an atomistic char representation and the ReaxFF reactive force field. Combust Flame 2012;159:1272e85. [24] Bhoi S, Banerjee T, Mohanty K. Molecular dynamic simulation of spontaneous combustion and pyrolysis of brown coal using ReaxFF. Fuel 2014;136:326e33. [25] Zhang Z, Chai J, Zhang H, Guo L, Zhan J-H. Structural model of Longkou oil shale kerogen and the evolution process under steam pyrolysis based on ReaxFF molecular dynamics simulation. Energy Source Part A Recov Util Environ Eff 2019. https://doi.org/10.1080/15567036.2019.1624879. [26] Salmon E, van Duin AC, Lorant F, Marquaire PM, Goddard III WA. Thermal decomposition process in algaenan of Botryococcus braunii race L. Part 2: molecular dynamics simulations using the ReaxFF reactive force field. Org Geochem 2009;40(3):416e27. [27] Fletcher TH, Kerstein AR, Pugmire RJ, Solum MS, Grant DM. Chemical percolation model for devolatilization. 3. Direct use of carbon-13 NMR data to predict effects of coal type. Energy Fuel 1992;6(4):414e31. [28] Zheng M, Li X, Liu J, Wang Z, Gong X, Guo L, Song W. Pyrolysis of Liulin coal simulated by GPU-based ReaxFF MD with cheminformatics analysis. Energy Fuel 2013;28(1):522e34. [29] Fletcher TH, Gillis R, Adams J, Hall T, Mayne CL, Solum MS, Pugmire RJ. Characterization of macromolecular structure elements from a Green River oil shale, II. Characterization of pyrolysis products by 13C NMR, GC/MS, and FTIR. Energy Fuel 2014;28(5):2959e70.