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. 2023 Jun 2;8(23):20823–20833. doi: 10.1021/acsomega.3c01525

High-Temperature Pyrolysis of N-Tetracosane Based on ReaxFF Molecular Dynamics Simulation

Xiaowen Yu 1, Chunhua Zhang 1,*, Hanwen Wang 1, Yangyang Li 1, Yujia Kang 1, Ke Yang 1
PMCID: PMC10268645  PMID: 37332798

Abstract

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In order to further understand the high-temperature reaction process and pyrolysis mechanism of hydrocarbon fuels, the high-temperature pyrolysis behavior of n-tetracosane (C24H50) was investigated in this paper via the reaction force field (ReaxFF) method-based molecular dynamics approach. There are two main types of initial reaction channels for n-heptane pyrolysis, C–C and C–H bond fission. At low temperatures, there is little difference in the percentage of the two reaction channels. With the temperature increase, C–C bond fission dominates, and a small amount of n-tetracosane is decomposed by reaction with intermediates. It is found that H radicals and CH3 radicals are widely present throughout the pyrolysis process, but the amount is little at the end of the pyrolysis. In addition, the distribution of the main products H2, CH4, and C2H4, and related reactions are investigated. The pyrolysis mechanism was constructed based on the generation of major products. The activation energy of C24H50 pyrolysis is 277.19 kJ/mol, obtained by kinetic analysis in the temperature range of 2400–3600 K.

1. Introduction

Hydrocarbon fuels play an important role in industrial production and our daily activities, the high-temperature combustion process is usually accompanied by pyrolysis. So, detailed studies of pyrolysis behavior are essential to deeply investigate the application of fuel, improve the utilization efficiency, and reduce the production of harmful species.1−6 Pyrolysis is a very complex process, usually involving thousands of reactions, and the products of pyrolysis can also be reused to relieve the pressure of the present energy shortage.7−9 However, it is impractical to characterize the pyrolysis process of more and more complex compounds by using only experimental and theoretical methods.

Molecular simulation is a computational chemistry method developed with the advance of computer technology and quantum chemistry to study the microscopic properties of molecules from the atomic level.10,11 In order to make molecular dynamics simulations of large-scale chemical reaction systems practical, Van Duin et al.12 developed a reaction force field (ReaxFF) system and it was shown that ReaxFF calculations are faster and the results are more accurate for small-molecule hydrocarbons. This also officially opened the application of ReaxFF method in the field of hydrocarbon microreactions. With the more intensive application of computational simulations, ReaxFF methods based on molecular dynamics (ReaxFF-MD) simulations have increasingly been used to study the reaction mechanisms of various fuels.13−16 Chenoweth et al.17 extended the ReaxFF training set in order to study the initiation reactions of hydrocarbon oxidation at high temperatures, and their study showed that the ReaxFF method can simulate the reaction process of compounds without pretreatment, which provides a new method to study the initiation reactions of hydrocarbon oxidation under extreme conditions. Page18 and Lummen19 used ReaxFF-MD to simulate the thermal degradation behavior of methane at low and high temperatures. This further proves the universality of the hydrocarbon reaction force field proposed by van Duin. Xin et al.20 studied the thermal decomposition mechanism of 11 typical hydrocarbons (isobutane, isopentane, isohexane, n-butane, n-pentane, n-hexane, cyclobutane, cyclopentane, cyclohexane, benzene, and toluene) using the ReaxFF method based on molecular dynamics simulations and the density functional theory (DFT) method and analyzed in detail the initiating reactions of hydrocarbon pyrolysis, the effects of temperature and molecular structure on the rate of pyrolysis, and the distribution of major fragments. Ding et al.21 explored the high-temperature pyrolysis mechanism of n-heptane by a series of reactive molecular dynamics simulations and calculated the apparent activation energy of n-heptane, which provides new insights into the application of fossil fuels. Liu et al.22 used reaction molecular dynamics to analyze the detailed process of methylcyclohexane (MCH) pyrolysis and presented the main hydrocarbon species generated by MCH pyrolysis, initiation mechanism, intermediate reaction processes, and related kinetic behaviors. These studies prove that the ReaxFF method is outstanding in exploring the pyrolysis process and detailed reaction mechanism of low-carbon molecular structure.

However, there are many hydrocarbon fuels with long carbon chain structures. Longer carbon chain structures imply more complex reaction mechanisms, and studies have shown that the ReaxFF method is still able to describe the microscopic reaction behavior of compounds with long carbon chains in detail. Wang et al.23 studied the initiation mechanism and kinetic behavior of n-dodecane combustion process using the ReaxFF-MD simulation and chemical kinetic modeling; their results showed that ReaxFF provides a reasonable oxidation reaction pathway for n-dodecane. Li et al.24 studied the pyrolysis reaction of n-tetradecane using ReaxFF-MD simulations, which provides new ideas for understanding the effects of thermal cracking of underground crude oil. Chen et al.25 studied the thermal cracking behavior of hexadecane at high temperatures and pressures based on ReaxFF-MD, developed the initial reaction mechanism of hexadecane, and linearly fitted the reaction activation energy of hexadecane by Arrhenius formula. Li et al.26 studied the microscopic reaction mechanism and main products of n-eicosane pyrolysis and combustion based on the reactive force field molecular dynamics and proposed its detailed reaction mechanism. These studies demonstrate that ReaxFF can provide important theoretical guidance for kinetic reaction processes of hydrocarbons and simulate complex chemical processes; however, few studies have explored the microscopic reaction processes of hydrocarbons with carbon chain lengths above 20 using the ReaxFF method.

Long-chain alkanes are the main components of crude oils27−29 as well as products produced by processing of crude oils (transportation fuels—usually below 20 carbon atoms,30 lubricants—usually 20–40 carbon atoms,31 heavy oils—usually 20–70 carbon atoms,32 etc.). The study of the kinetics of alkanes with more than 20 carbon atoms is important to guide the application of lubricants or heavy oils. N-Tetracosane is a common structure in crude oil and lubricants.33,34 Jin et al.35 explored the reasons for the variation in carbon isotope values of natural gas from oil and gas reservoirs in the Tarim Basin based on the n-tetracosane cracking gas formation experiment. In addition, n-tetracosane (melting point of 51 °C) achieves renewable thermal energy storage and release as a phase change material and is a potential candidate for achieving high-temperature management.36,37 No studies have completely described the detailed reaction mechanism about n-tetracosane.

Therefore, in order to provide an additional explanation of the detailed reaction mechanism of n-tetracosane, molecular dynamics simulations of the n-tetracosane at various temperatures were carried out using the ReaxFF method. Simulation results of the pyrolysis of multimolecular n-tetracosane are given, and the initial initiation reactions, intermediate and final product distributions, and pyrolysis kinetics of n-tetracosane pyrolysis are analyzed. In particular, the intermediate reaction mechanism and the detailed reaction mechanism, which cannot clearly be analyzed under the conditions of current experimental and theoretical methods, are studied in detail. The results are expected to establish the understanding of n-tetracosane pyrolysis from the atomic scale and provide a reliable reference for lubricant and crude oil cracking as well as practical high-temperature thermal management applications.

2. Computational Details

In this study, we performed simulations to investigate the pyrolysis of n-tetracosane using the ReaxFF method. The ReaxFF method can be used to describe chemical reaction processes in complex systems, and it describes the bond fission and formation by the bond order. We applied the CHO reaction force field proposed by van Duin,17 which has been proven to be appropriate for application in the study of hydrocarbon systems in numerous studies.18−26

All simulations are repeated three times. The calculations in this study are divided into three parts. The first part is to place 20 C24H50 molecules in a cubic periodic cell through the Amorphous Cell module of Materials Studio. The size is 24.14 Å *24.14 Å *24.14 Å, the initial density of the system is 0.7991 g/cm3, and the model establishment is shown in Figure 1. Later, the structure was optimized by the forcite module, and the optimization process used the COMPASS force field, which is proved to be suitable for the property prediction of alkane structures.38

Figure 1.

Figure 1

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Model building process.

The second part is the simulation calculation of the constructed model by the ReaxFF method in LAMMPS39 for predicting the pyrolysis behavior of n-tetracosane. Before the simulation, the initial equilibrium model was obtained by static optimization methods, and minimizations used a conjugate gradient method with a force tolerance of 1 × 10–6 kcal/mol/Å and energy tolerance of 1 × 10–6 kcal/mol. The structure was first relaxed at 300 K for 10 ps, and at 300 K, n-tetracosane did not cleave. The simulations were performed under the NVT (fixed number of atoms N, specified temperature T, constant volume V) ensemble, and the temperature was controlled by the Berendsen method. First, annealing simulations are performed for 1000 ps in the temperature range of 1000–3500 K to observe the pyrolysis of n-tetracosane in the overall temperature range. Then, simulations are performed in the temperature range of 2000–3600 K with 400 K as the interval. The final reaction mechanism is obtained from the 500 ps simulation at 3600 K. The timestep is 0.1 fs, the temperature damping coefficient is 0.05, and the critical value of the bond order for molecular recognition is set to 0.3, which is proved to be feasible in hydrocarbon-related simulations.40

The third part is the product distribution and mechanism analysis of LAMMPS simulation results, and the analysis was done using the ChemTrayzer (Chemical Trajectory Analyzer) code, which is based on the python code.41

3. Results and Discussion

3.1. Multimolecular Annealing Simulation Results

In order to provide a macroscopic description of the pyrolysis of n-tetracosane, multimolecular annealing simulations are performed in the temperature range of 1000–3500 K from 0 ps to 1000 ps. The variation of temperature and major fragment distribution with simulation time is shown in Figure 2. The pre-equilibrated n-tetracosane molecule begins to decompose at 430 ps at a temperature of around 2021 K. The whole decomposition process of n-tetracosane lasts about 271 ps. According to the different decomposition rates of n-tetracosane, the pyrolysis of n-tetracosane can be divided into three stages as the temperature increases. The first stage is in the temperature range of 2091–2574 K (430–622 ps), where approximately 20% of the molecules decompose. The second stage is from 622 to 671 ps, where the temperature rises to 2654 K. The decomposition rate of n-tetracosane is maximum at this stage, with about 75% of the molecules decomposing. The final stage of decomposition is from 671 to 701 ps, and this stage has the slowest rate of decomposition. After 701 ps, there are no n-tetracosane molecules that have not yet been decomposed.

Figure 2.

Figure 2

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Result of annealing simulation.

Throughout the pyrolysis process, many species are produced, in which C2H4 is the highest, followed by H2 and CH4, which is consistent with previous studies,26 and as the simulation continues, C2H4 exhibits a pattern of first increasing and then decreasing. During the reduction of C2H4, the production of small-molecule species such as H2 and CH4 as well as CH3 radicals is still on the rise. However, at the end of the simulation, 980–996 ps, the amount of H2 shows a brief decrease, which may be due to the partial consumption of H2 by the reaction of H2 with the intermediate products, the main relevant reaction equation of which can be obtained in Figure 9. The amount of H2 starts to rise slowly again at 996–1000 ps. A large number of unstable intermediates and free radicals, short-life and long-life compounds, are also observed, and these intermediates are an important source for generating small-molecule species. This suggests that small molecules, such as H2 and CH4, and important radicals, such as •H and •CH3, are partly derived from secondary decomposition.

Figure 9.

Figure 9

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Intermediate reactions associated with H2.

3.2. Initial Cleavage Reaction Mechanism of N-Tetracosane

The initiating reactions are the basis of complex pyrolysis processes and determine the distribution of intermediate reactions and products. Particularly for long-chain alkanes, different initial conditions may form different initiation reactions. Therefore, a detailed description of the initiation reaction channel is necessary to establish the mechanism of pyrolysis of n-tetracosane. To investigate the different initial reactions of n-tetracosane at different temperatures, simulations were performed with 300 ps at 400 K intervals in the temperature range of 2000–3600 K. The variation of the number of n-tetracosane molecules with time at different temperatures is shown in Figure 3. With the increase of temperature, the consumption rate of C24H50 molecules increases significantly. As the temperature increases, the decomposition of C24H50 starts earliest, and the higher the temperature, the shorter the time for complete decomposition. As shown in Figure 4, the decomposition of C24H50 can be divided into three stages, the first stage is the stable period of C24H50 without decomposition, the second stage is the process of C24H50 decomposition, and at the third stage, C24H50 is completely decomposed and the pyrolysis process start. At 2000 K, after 300 ps of simulation, only three C24H50 molecules are completely decomposed. At 2400 K, there are 19 C24H50 molecules completely decomposed. At 2800, 3200, and 3600 K, the C24H50 molecule is completely decomposed at 48.6 ps, 17.4 ps, and 5.5 ps, respectively.

Figure 3.

Figure 3

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Change of the number of C24H50 with time at different temperatures.

Figure 4.

Figure 4

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Different pyrolysis stages of C24H50.

The way of initial decomposition reaction of n-tetracosane at different temperatures is shown in Figure 5. The initial decomposition channel of n-tetracosane is mainly the fission of C–C bond and C–H bond. At the lower temperature of 2000 K, the ratio of the two initiation types, C–C bond fission and C–H bond fission, is not much different, and with the increase of temperature, C–C bond fission dominates. The reaction initiated by C–H fission is mainly C24H50 → •C24H49 + •H, where all positions of C–H bond fission are shown in Figure 6. C–H bond fission occurs between the 2nd to 12th carbon atom with its connected H atom. A more specific initiation reaction occurs at higher temperatures, where the nonreacted C24H50 reacts with the formed reaction intermediates to form new reaction intermediates and small molecules. It is noteworthy that the reaction C24H50 + C24H50 → •C18H37 + •C24H49 + C6H12 + H2 reaches about 4% at 3600 K. In general, under high temperatures, most of C24H50 is pyrolyzed by breaking C–C bonds to form intermediates and small molecules, a small portion of C24H50 forming •C24H49 and •H by breaking C–H bonds, and the remaining small amount of still unreacted C24H50 is reacted with already formed radicals or small molecules to further produce reaction intermediates and small molecules.

Figure 5.

Figure 5

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Way of initial decomposition reaction of n-tetracosane at different temperatures.

Figure 6.

Figure 6

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Position C–H bond fission.

3.3. Main Product Analysis

3.3.1. Effect of Temperature on Product Species

By analyzing the n-tetracosane pyrolysis products, the distribution of several major small-molecule products at different temperatures is summarized. Due to the effect of simulation temperature and simulation time, n-tetracosane is not completely decomposed at 2000 K. Therefore, in this section, only the distributions of small molecules at four temperatures of 2400, 2800, 3200, and 3600 K are listed, as shown in Figure 7. H2, CH4, C2H2, C2H4, C2H6, and C3H6 are six major small-molecule products observed in this study, which are also observed during the cleavage of n-alkanes of different chain lengths.20,22,26 It is obvious from the comparison that the production of C2H4 is the largest and the production rate is the fastest at different temperature conditions, and its production rate increases with the increase of temperature, and the higher the temperature is, the faster the production of C2H4 reaches its peak. At 2400 and 2800 K, only the peak stabilization of C2H4 is observed. At the high temperatures of 3200 and 3600 K, C2H4 shows an obvious trend of first increasing and then decreasing, which is attributed to the further reaction of C2H4 with intermediates and the decomposition of C2H4 itself. At 3600 K, the amount of C2H4 decreases to a certain extent and then begins to stabilize. In addition, H2 and CH4 show higher production with increasing temperature, and a further increase in the production of H2 and CH4 occurs under high-temperature conditions when C2H4 decreases, which indicates that some of H2 and CH4 production is related to the consumption of C2H4. It is obvious that during the decomposition of n-tetracosane, small molecules such as C2H4 and C3H6 are produced preferentially, and the time that H2 and C2H4 start to be produced does not differ much, CH4 generally starts to be produced after H2, and C2H2 and C2H6 molecules always start to appear at the latest moment.

Figure 7.

Figure 7

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Distribution of main products at different temperatures.

3.3.2. Effect of Temperature on the Production of Major Products

At different temperatures, H2, CH4, and C2H4 are the maximum products that are produced by the pyrolysis process. The variation of H2, CH4, and C2H4 production with simulation time at different temperatures is shown in Figure 8.

Figure 8.

Figure 8

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Effect of temperature on the production of major products.

Figure 8a shows the variation of H2 production. At 2400 K, H2 starts to be produced only at 135 ps and its production is very low. At the pyrolysis temperatures of 2800, 3200, and 3600 K, the production of H2 starts at 15, 3, and 3 ps, respectively, and reaches the maximum values of 21, 51, and 75 at 255, 267, and 388 ps. Its production increases steadily with the increase of temperature. At 3200 and 3600 K, the amount of H2 shows a fluctuation down state after reaching the maximum value, which is due to the reaction between H2 and the intermediate products, where H2 loses H atom and transforms into free-radical H, which can be confirmed by the subsequent reactions related to H2 during the reaction.

Figure 8b shows the variation of CH4 production. At 2400 K, the first appearance of CH4 is at 125 ps; with the increase of temperature, the moment of first appearance of CH4 becomes significantly earlier, and the production of CH4 is positively proportional to both temperature and time.

Figure 8c shows the variation of C2H4 production. At 2400 K, the first appearance of C2H4 is at 9 ps. As the temperature increases, C2H4 appears at the beginning of the reaction around 3 ps. The rate of C2H4 production gradually increases with increasing temperature. At 3600 K, C2H4 reaches a peak at 22 ps and then declines, which can be explained by the C2H4-related reactions where C2H4 reacts with intermediates or performs secondary decomposition. C2H4 production stabilizes as the simulation proceeds to about 250 ps, which is the same trend as that observed previously in the annealing simulation.

3.4. Detail Analysis of N-Tetracosane Pyrolysis Intermediate Reaction

Although the pyrolysis reactions of various hydrocarbon structures have been studied in detail, the intermediate reactions for the cracking of long-chain alkanes are still difficult to describe directly. In this section, we analyze the intermediate reactions of n-tetracosane pyrolysis at the atomic level.

3.4.1. Small-Molecule Intermediate Reactions and Distribution

A total of 2065 stable and unstable intermediates are observed during the high-temperature pyrolysis of n-tetracosane simulated by ReaxFF-MD. This subsection provides statistics on the relationship between the dependence of several major small-molecule production amounts on the simulation time. The production and consumption of several major products are counted at a time interval of 20 ps to explore their relevant reaction paths and reaction type distribution.

H2 is the simplest small-molecule product in the pyrolysis of n-tetracosane, and it has been found in many types of reactions. It is important to understand the change rule of H2 in the pyrolysis process to illustrate the pyrolysis process of n-tetracosane. The variation of H2 production and consumption at 3600 K with time is shown in Figure 9a. The process of H2 changes through two growth stages. The first stage is from 0 to 60 ps where H2 production is significantly higher than consumption, and at this time, H2 is in the growth stage. The second stage is 61–130 ps where the consumption of H2 increases significantly, gradually equal or even higher than production, and at this time, H2 is in the stable fluctuation stage. After 130 ps, H2 production is again higher than consumption, and the number of H2 increases significantly. The main paths of H2 production and consumption are shown in Figure 9b. The main path of H2 production is CH4 + •H → •CH3 + H2, and its reverse reaction is the most important production path of H2 consumption. The main pathway of the production process is H addition reaction, and H radicals are mainly derived from alkane and olefin radicals, such as •C2H5, •C4H7, etc.

CH4 is the simplest and most abundant alkane produced during the pyrolysis of n-tetracosane; the variation of CH4 is essential for an in-depth understanding of the pyrolysis mechanism of n-tetracosane. The time dependence of CH4 production and consumption at 3600 K is shown in Figure 10a. A large amount of CH4 is produced from 0 to 80 ps. During this period, the production is greater than the consumption, but due to the higher reaction temperature, the production of CH4 gradually decreases and the consumption gradually increases. From 81 to 250 ps, the consumption rate of CH4 becomes obviously larger. After 250 ps, the production increases significantly, the production and consumption gradually reach dynamic equilibrium, and the total number of CH4 varies within a certain range. This change in CH4 is mainly influenced by the methyl-related reaction pathway. The main paths of CH4 production and consumption are shown in Figure 10b. CH4 is formed mainly by the combination of •CH3 with •H and the cleavage of alkyl or alkyl radicals. The reverse reaction of •CH3 + •H → CH4 is also the main pathway for CH4 consumption. These pathways suggest that the changing conditions of CH4 are closely related to methyl radicals. During the pyrolysis process, the general trend of CH4 is that the production is first greater than the consumption, and after some fluctuations in the middle, it finally reaches a dynamic balance between production and consumption.

Figure 10.

Figure 10

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Intermediate reactions associated with CH4.

C2H4 is the most abundant olefin produced during the pyrolysis of n-tetracosane. The variation of C2H4 production and consumption at 3600 K with simulation time is shown in Figure 11a. The process of C2H4 change can be divided into three stages. The first stage is 0–20 ps, where C2H4 accumulates rapidly and its production reaches a peak in a short period. The second stage is 21–180 ps, when the consumption rate of C2H4 increases significantly and the production rate decreases significantly, and this stage corresponds to the further involvement of C2H4 in the reaction and its production decreases significantly. The third stage is after 180 ps, where the production and consumption reactions decrease with time, and the production is slightly larger than the consumption. The main reactions of the species concerned can explain the different trends of C2H4 production and consumption. In the first stage, C2H4 is mainly generated from the dissociation of long-chain alkanes and unsaturated alkanes. Most of these reactive species come from the initial reaction channel of n-tetracosane pyrolysis discussed in Section 3.2, and therefore, the production of C2H4 may be closely related to the initial reactants. As can be seen from Figure 11b, the main source of C2H4 production is the fission of C–C bonds of larger hydrocarbon groups, such as •C5H11 → •C3H7 + C2H4, •C6H13 → •C2H5 + C2H4 + C2H4, •C8H17 → C3H6 + C2H4 + C2H4 + •CH3, •C12H25 → •C8H17 + C2H4 + C2H4, etc. H addition reactions such as •C2H3 + •H → C2H4 are also the main reactions producing C2H4. The main reactions that consume C2H4 are CH4 + C2H4 → •C2H5 + •CH3, where the reversible reactions, such as C2H4 + •CH3 → •C3H7, C2H4 + •H → •C2H5, also have a large proportion in the process of C2H4 consumption, and the number of C2H4 reaching dynamic equilibrium in the late stage is closely related to these reversible reactions.

Figure 11.

Figure 11

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Intermediate reactions associated with C2H4.

3.4.2. Free-Radical Distribution

Many free radicals are present during the pyrolysis of n-tetracosane. Small-molecule radicals play an important role in the conduct of the pyrolysis reaction. Based on the analysis of the distribution of small-molecule products in Section 3.4.1, we investigate the time dependence of the production and consumption distribution of H, CH3, and C2H5 radicals, respectively.

H and CH3 radicals have a high participation in the pyrolysis process, and the presence of radicals promotes further reactions of intermediates. Figure 12a,b shows the dependence of •H and •CH3 production and consumption distributions on time, respectively. At the high temperature of 3600 K, the production of •H is slightly higher than the consumption. The reactions related to •H do not vary much throughout the pyrolysis process, •H is mainly produced by the decomposition reactions of alkanes or olefins, and only small amounts of H radicals are preserved as the reactions proceed. In the first 20 ps of the reaction, the production of •CH3 is higher than the consumption, and the consumption reaction gradually increases as the simulation proceeds. •CH3 appears more throughout the pyrolysis process, and most of •CH3 is obtained by the •H extraction reaction of CH4, a small portion of •CH3 is obtained by the direct cracking of alkanes or olefins, and •CH3 is still present in the final product.

Figure 12.

Figure 12

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Small free-radical distribution of n-tetracosane pyrolysis.

Figure 12c shows the distribution of •C2H5 production and consumption dependence on time. Most •C2H5 production reactions occur at the beginning of the reaction, and the main channels for •C2H5 production are the •H elimination decomposition reactions of C2H6 (such as C2H6 + •H → •C2H5 + H2) and the related reactions of intermediate reactants (such as CH4 + C3H8 + •C3H7 → CH4 + C4H10 + •C2H5). A small amount of •C2H5 is produced by C–C bond fission (such as C4H10 → •C2H5 + C2H4 + •H). The changes in the production and consumption reactions of •C2H5 proceed in parallel, so that no significant amount of •C2H5 exists at the end of pyrolysis.

3.5. Detailed Analysis of the Pyrolysis Mechanism

It is shown that setting higher temperature is only to speed up the simulation process and shorten the simulation time.24−26Section 3.3 analyzes the variation of the main products with the simulation temperature, and it can be seen that the temperature has a greater effect on the number of products generated, and the higher the temperature, the earlier the pyrolysis products appear. The effect of temperature variation on the type of pyrolysis products is not obvious, so that, according to the simulation results, the detailed reaction mechanism of n-tetracosane pyrolysis process at 3600 K is shown in Figure 13. It describes the detailed reaction process from the initial reactants to the most common products H2, CH4, and C2H4. The pyrolysis of C24H50 is similar to the basic long-chain alkane pyrolysis initiation reaction, which is mainly initiated by the fission of C–C bond. Many intermediates are formed during the pyrolysis process, such as C10H20 is a relatively important intermediate. These intermediates further react to form the main products. As can be seen from the reaction mechanism map, the main source of C2H4 is produced by the decomposition of long-chain hydrocarbon intermediates, or the H radical elimination reaction of C2H5 radicals. The main source of CH4 is the H addition reaction of CH3 radicals. The main source of H2 is the H addition reaction of H radicals. We expect this reaction mechanism can be used to construct relevant chemical kinetic models.

Figure 13.

Figure 13

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Detailed reaction mechanism of n-tetracosane pyrolysis.

3.6. Kinetic Analysis of N-Tetracosane Pyrolysis

To investigate the kinetic characteristics of n-tetracosane pyrolysis, the reaction kinetics of n-tetracosane was described by the consumption rate of n-tetracosane at temperature intervals of 400 K in the temperature range of 2400–3600 K.42

In our study, the concentration of n-tetracosane is simply replaced by the number of n-tetracosane molecules. The initial cleavage reaction rate (k) of n-tetracosane at each temperature was obtained by exponential fitting the number of decomposed n-tetracosane molecules. The reaction rate is calculated by eq 1

3.6. 1

Where N0 is the number of initial reactions of n-tetracosane molecules (N0 = 20), t0 is the reaction start time, and Nt is the number of molecules of n-tetracosane at the moment t.

The expression of Arrhenius formula is given by

3.6. 2

Taking the logarithm gives the reaction rate k versus the activation energy Ea as a function of

3.6. 3

where T is the temperature, R is the gas constant, and A is the pre-exponential factor.

The Arrhenius relationship for the pyrolysis of n-tetracosane was obtained by fitting ln(k) to 1000/T as shown in Figure 14. The calculation shows that the activation energy Ea = 277.19 kJ/mol.

Figure 14.

Figure 14

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Kinetic analysis of n-tetracosane pyrolysis.

4. Conclusions

In this study, the pyrolysis of n-tetracosane at high temperatures is studied based on ReaxFF-MD, and the initial reaction, intermediate reaction mechanism, and product distribution associated with the pyrolysis of n-tetracosane are investigated, and the reaction activation energy was fitted by Arrhenius formula.

In order to describe the initial reaction process of n-tetracosane in detail, we discuss the initial reaction channels of n-tetracosane cleavage at different temperatures. There are two main types of initial decomposition of n-tetracosane: C–C bond and C–H bond fission. At lower temperatures, the ratios of the two initiating types, C–C bond fission and C–H bond fission, are not very different, and as the temperature increases, C–C bond fission begins to dominate. For high temperatures, a small amount of n-tetracosane is decomposed by reaction with intermediates due to the rapid reaction rate.

The main small-molecule products of n-tetracosane pyrolysis are H2, CH4, C2H2, C2H4, C2H6, and C3H6. C2H4 is the most abundant olefin, and the main source of C2H4 is the fission of C–C bond of the larger hydrocarbon group, and C2H4 reaches its peak value after the completion cleavage of C24H50, and then the content of C2H4 starts to decrease until it is stable. The main source of CH4 is the H addition reaction of •CH3, and the main source of H2 is the H addition reaction of H radicals. The H2 and CH4 production continues to rise during the C2H4 decline, suggesting that some of the H2 and CH4 formation is associated with the C2H4 reaction, which is also observed in the intermediate product-related reaction pathway. The other products such as C3H6 appears not far from the moment of appearance of H2, C2H2 and C2H6 appear later, and C2H6 does not exist at 2000 K, which indicates that the high temperature is favorable for the formation of this product.

The apparent activation energy of n-tetracosane is 277.19 kJ/mol in the temperature range of 2400–3600 K fitted by Arrhenius formula. This study shows that ReaxFF is able to describe the high-temperature thermal decomposition of n-tetracosane which is expected to contribute to the thermal safety of solid hydrocarbon fuels.

Acknowledgments

This study was supported by the Key Research and Development Program of Ningxia Hui Autonomous Region (2018BDE02057), the Innovation Capability Support Program of Shaanxi (2021TD-28), and the Special Fund for Basic Scientific Research of Central Colleges, Chang’an University (300102222502, 300102383501, 300102383502).

The authors declare no competing financial interest.

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