Fuel 256 (2019) 115884
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Full Length Article
Study of non-isothermal pyrolysis mechanism of lignite using ReaxFF molecular dynamics simulations ⁎
T
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Fang Xua,b, Hui Liua, , Qing Wangb, , Shuo Panb, Deng Zhaoa, Ying Liub a b
School of Energy Science and Engineering, Harbin Institute of Technology, Harbin 150001, PR China School of Energy and Power Engineering, Northeast Electric Power University, Jilin 132012, PR China
A R T I C LE I N FO
A B S T R A C T
Keywords: Lignite Pyrolysis mechanism Heating rate Reactive force field
Compared with the isothermal pyrolysis, non-isothermal pyrolysis is more accurate and efficient, and closer to the actual reaction process. To better understand the pyrolysis mechanism of lignite, a series of ReaxFF molecular dynamics (ReaxFF-MD) simulations were performed within the temperature range of 300–3000 K at different heating rates. The results indicated that the secondary reaction of tar took place when the temperature was above 2400 K at a low heating rate (2 K/ps). In comparison with light tar, more heavy tar was converted to gas or char through further decomposition or condensation reaction. The influence of heating rate on pyrolysis reaction was investigated at the heating rates of 2, 10, 50 and 100 K/ps. In the low heating rate range (below 10 K/ps), the heating rate exhibited a prominent influence on the product distribution. Moreover, more tar and pyrolysis gas were generated at low temperatures (below 2400 K) compared to those at high heating rates. The reaction mechanisms of typical lignite pyrolysis products (C2H4 and phenols) were explored. The formation of C2H4 was attributed to the presence of large amounts of methylene carbon in lignite. Possible formation and transformation pathways of phenols have been proposed, and the simulation results were found to be consistent with the evolution of gases.
1. Introduction
product constitution. The structural characteristics of char have been investigated using various advanced technologies, such as FTIR [13], 13 C NMR [14], XPS [15], and XRD [16]. Additionally, the evolution mechanisms of tar and gas have been studied using GC–MS [17] and TG-FTIR [18], respectively. Based on a number of experimental studies, some achievements have been made on the pyrolysis processes and product characteristics. Nevertheless, the detailed reaction mechanism of lignite pyrolysis is still vague, which is due to the diversity of lignite structures and the complexity of pyrolysis processes. Compared with the experimental methods, molecular modeling is a promising method for elucidating the reaction mechanism of lignite pyrolysis. As a principal method for studying the chemical reaction, quantum mechanics (QM) can accurately address the transition states and chemical reaction pathways. However, computational costs of QM are too high to be used for macromolecule models [19]. Based on classical principles, molecular dynamics (MD) incurs much lower computational costs and can simulate dynamic processes for large-scale models containing more than 10,000 atoms. Unfortunately, it fails to solve the issues involving the generation and breakage of chemical bonds [20]. The Reactive Force Field (ReaxFF), proposed by van Duin et al., can provide accurate description of the details of chemical reaction
Globally, coal occupies a vital position in the field of energy applications [1]. In China, the coal consumption was 4490 million tons standard coal in 2017, accounting for 60.40% of the total energy consumption [2]. Among various types, lignite has gained special attention due to its huge storage, low selling price, high chemical reactivity, and low pollution-forming impurities [3]. Unfortunately, obvious drawbacks, such as the presence of large amounts of moisture, low heat content, and a high tendency of spontaneous combustion restrict the extensive utilization of lignite [4,5]. Therefore, upgrading lignite is essential and urgent to safe and efficient utilization of lignite. Pyrolysis is one of the most prospective upgrading approaches. It can not only overcome the self-defection of lignite, but also generate available products (char, tar, and pyrolysis gas) [6]. Pyrolysis reaction requires considerable heat to retain or increase the reaction temperature [7]. Heat transfer from reactor to coal is strongly influenced by the operating conditions. To date, various efforts have been devoted to investigate the influences of the type of reactor [8], final pyrolysis temperature [9], heating rate [10], reaction environment [11], and particle size [12] on the pyrolysis behavior and
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Corresponding authors. E-mail addresses:
[email protected] (H. Liu),
[email protected] (Q. Wang).
https://doi.org/10.1016/j.fuel.2019.115884 Received 20 April 2019; Received in revised form 16 June 2019; Accepted 24 July 2019 0016-2361/ © 2019 Elsevier Ltd. All rights reserved.
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structural characteristics of lignite was constructed based on multiple technologies (including element analysis, 13C NMR, XPS, and FTIR). The optimum configuration (see Fig. 1b) was obtained by molecular mechanics (MM) and MD calculations using Dreiding force field in Materials Studio [36]. Then, a macromolecule containing 10 unimolecular structures was assembled into a periodic box (81.0 × 81.0 × 81.0 Å3). The lower initial density was helpful to prevent stacking of major functional groups [26]. ReaxFF MD simulations were initiated by minimizing the system energy at 50 K for 20 ps. Then, the system was equilibrated at 300 K under constant atoms number, volume, and temperature (NVT) ensemble. In order to approach the actual density of lignite, the density of macromolecule was adjusted to 0.966 g/cm3 at 300 K and 10 MPa using constant atoms number, pressure, and temperature (NPT) ensemble. Afterthat, the system was relaxed using no-reaction NVT-MD simulation at 300 K. Finally, a set of non-isothermal simulations were performed using NVT ensemble in the temperature range of 300–3000 K at the heating rates of 2, 10, 50 and 100 K/ps, and then kept the temperature at 3000 K for 10 ps. To avoid the randomness of simulation results effectively, all simulations progresses were implement for three times, and the final results were the average value of three simulations. The velocity Verlet approach was used to solve Newton’s equation of motion with a time step of 0.1 fs, whereas a Berendsen thermostat was used to regulate the temperature with a damping constant of 100 fs. The product fragments were recognized through a bond-order cutoff of 0.3. Furthermore, C/H/O/N/S/B force field parameters [25] utilized in the present work were verified to be suitable for studying the reaction mechanism of coal [37]. All the ReaxFF-MD simulations were performed using “reaxc” package in LAMMPS platform. In order to ensure that the simulation process can be completed in an acceptable time scale, the simulation temperature was increased deliberately. This strategy was widely used in previous works, and was in good accordance with the experimental results [38,39].
processes [21]. ReaxFF-MD, as the combination of MD and ReaxFF, is one of the most suitable choices for exploring the complex reaction mechanisms of large molecular models [22]. It is worth mentioning that the calculation precision of ReaxFF-MD is close to that of QM, however the computational efficiency is much better than the QM [23]. ReaxFF-MD has been widely utilized to simulate the reaction mechanism of large-scale systems. By employing ReaxFF-MD simulations, Salmon et al. [24] reported thermal decomposition processes of Morwell Brown coal with several functional models. They systematically studied the main maturation processes of coal, such as defunctionalization, depolymerization and structural rearrangement. Castro-Marcano et al. [25] adopted ReaxFF-MD to investigate the influences of environment and temperature on the reaction processes and product composition. They found that the char oxidation processes were initiated by either the degradation of char structures or hydrogen abstraction reactions. Hong et al. [26] analyzed the pyrolysis processes of Zhundong coal using ReaxFF-MD and showed that the secondary reaction of tar has an important effect on the pyrolysis mechanism and product distribution. Chen et al. [27] investigated the hydrogenation reaction of coal model compounds using ReaxFF-MD, and concluded that the aromatic ortho-position was prone to hydrogenation, and the initial hydrogenation reaction was dominated by kinetics. As far as is known, a great majority of studies on pyrolysis simulation were carried out using isothermal method. Nevertheless, compared with the isothermal simulation, non-isothermal simulation has numerous advantages. The gradual heating process is closer to the actual pyrolysis reaction. Moreover, non-isothermal simulation is computationally more efficient. Zhang et al. [28] compared the isothermal and non-isothermal pyrolysis behaviors of lignin and reported that, when the heating rate was 10 K/ps, the calculation efficiency of non-isothermal pyrolysis was five times higher than that of the isothermal pyrolysis. Additionally, non-isothermal pyrolysis simulation has the ability to explore the influence of temperature in a large range with just a single simulation process. In this work, non-isothermal pyrolysis behavior of lignite was investigated using ReaxFF-MD simulations. The overall pyrolysis processes were grouped into three stages according to product profiles. The decomposition mechanisms and product distributions of each stage were investigated. Then, the influence of heating rate on the pyrolysis reaction was studied. Finally, the formation and transformation mechanisms of typical lignite pyrolysis products were explored. These studies may provide valuable theoretical guidance for efficient and clean utilization of lignite.
3. Results and discussion 3.1. Analysis of non-isothermal pyrolysis behavior As is known, high heating rate shortens the reaction time at each temperature. In order to gain comprehensive understanding of the evolution of pyrolysis products, non-isothermal simulations were implemented within the temperature range from 300 to 3000 K at the heating rate of 2 K/ps. To be accordance with the previous studies [40,41], pyrolysis products can be classified as inorganic gas, organic gas, heavy tar, light tar, and char. Inorganic gas contains H2, CO2, H2O, and so on. The gas molecules including 1–4 carbon atoms are considered as organic gas, while the fragments including 5–13 carbon atoms and 14–40 carbon atoms are regarded as light tar and heavy tar, respectively. Compounds including more than 40 carbon atoms are regarded as char. Based on the evolution of products as shown in Fig. 2, the lignite pyrolysis processes can be approximately grouped into three stages. The snapshots of initial structure and intermediate configurations (at 1500 K, 2400 K, and 3000 K) obtained from VMD are displayed in Fig. 3. The initial pyrolysis stage (Stage I) was at temperatures between 300 K and 1500 K, where the molecular model of lignite showed a slight weight loss and only few small radicals (such as CH3%,OH%, and H%) were generated. In this stage, no obvious thermal decomposition reaction occurred. However, based upon the analysis of trajectories, the conformation adjustment of macromolecular structure was observed. The simulation results were in accord with the study reported by Zheng et al. [42] The second pyrolysis stage (Stage II) occurred within the temperature range of 1500–2400 K. In this stage, the macromolecule structure of lignite decomposed obviously, especially at high temperatures (above 1800 K). At 2400 K, the yield of solid product approached its minimum value. In contrast, the production of heavy tar and light tar
2. Computational methods ReaxFF is a general reactive force field on the base of bond order principle [23]. The parameters in ReaxFF are derived from the comprehensive training set of energetic and conformational data obtained from substantial QM calculations, using a certain optimization procedure [29]. Therefore, ReaxFF has the capability to provide detailed information of the generation and breakage of bonds for complex reaction systems. Compared with the traditional non-reactive force field, the connectivity in ReaxFF depends on bond orders, which are calculated from interatomic distances for each iteration. Notably, bonded interactions and non-bonded interactions in ReaxFF are computed independently, and have no influence on each other [30]. All the bonded interaction terms, such as bond, angle, and torsion, are bond-order dependent, and their contributions to bonded energy terms diminish when the bond breaks. The non-bonded interactions (van der Waals and Coulomb) are obtained using Morse and Coulomb potentials between every pair of atoms regardless of their connectivity [31]. More information about the ReaxFF can be obtained from previous works [32–34]. A Huolinhe lignite model (C201H195O32N3S1) [35] was used as the preliminary model (see Fig. 1a). The molecular model with remarkable 2
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Fig. 1. Molecular model of Huolinhe lignite. (a) Initial two-dimensional molecular structure, (b) Three-dimensional structure after geometry optimization.
However, high temperature accelerated the decomposition of macromolecular structure at Stage II of 1800–2400 K, and the yield of solid product achieved the minimum value at 2400 K. Therefore, the second pyrolysis stage can be further classified into two sub-stages (Stage II-A and Stage II-B) based on linear fitting method. The corresponding fitting equations are provided in Fig. 4 with good regression coefficients (R2) of 0.95843 and 0.98439, respectively. The slope of Stage II-B was higher than four times that of Stage II-A, which indicated that the thermal decomposition reaction mainly took place in Stage II-B. In Stage II-A, the linear reduction of char yield as the temperature increasing was associated with the breakage of weak bonds and the dissociation of macromolecular network structures. As illustrated in Figs. 2 and 5, apart from char, heavy tar was the major pyrolysis product, along with a low proportion of light tar and gas in Stage II-A. It indicated that the molecular structure of coal mainly decomposed into
increased continuously with increasing temperature and reached the peak values of 25.49% and 11.13% at 2400 K, respectively. Meanwhile, the yield of gas increased slightly with increasing temperature. The last pyrolysis stage (Stage III) occurred at temperatures from 2400 to 3000 K. The content of char increased significantly from 18.31% to 40.33% accompanied by 33.73% reduction in tar production. Additionally, the gas yield continued to increase with increasing temperature. The evolution trend of char, tar, and gas can be regarded as a sign of the occurrence of secondary reaction in this temperature range. The evolution of pyrolysis products of lignite, obtained from ReaxFFMD simulation, was found to be consistent with the results reported in previous works [43,44]. As shown in Fig. 2, the weight loss of macromolecular model less than 10% at Stage II of 1500–1800 K. It suggested that the rate of lignite pyrolysis reaction was relatively slow in this temperature range. 3
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Fig. 4. Stage II divided into two sub-stages based on the evolution of solid product.
Fig. 2. Evolution of pyrolysis products at the heating rate of 2 K/ps within the temperature range from 300 to 3000 K.
hydroxyl groups are present in the molecular model of lignite. The cleavage of Cal-Cal with relatively low dissociation energy led to the formation of C2H4 and C3H6 [45]. Furthermore, CO2 was the only inorganic gas, and its generation may be attributed to the decomposition of carboxyl functional groups [46]. As mentioned above, Stage II-B was the main pyrolysis stage. The
char and heavy tar at relatively low temperatures (1500–1800 K). Table 1 lists the detailed composition of pyrolysis products at 1800 K. Char mainly existed in the form of high-molecular-weight fragments. Alkylnaphthols were the predominant tar compounds, which was in accordance with the fact that plenty of bicyclic structures and phenolic
Fig. 3. Snapshots of initial structure and intermediate configurations of Huolinhe lignite obtained from VMD: (a) initial configuration, (b) intermediate configuration at 1500 K, (c) intermediate configuration at 2400 K, (d) intermediate configuration at 3000 K. 4
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significant role in the formation and evolution of pyrolysis products. In order to further study the influence of heating rate, a number of nonisothermal simulations were implemented from 300 to 3000 K at various heating rates of 2, 10, 50, and 100 K/ps respectively, and then kept the temperature at 3000 K for 10 ps. As seen clearly in Fig. 7, when the heating rate increased from 2 to 10 K/ps, the yield of tar increased dramatically by 33.32%. Meanwhile, char production decreased significantly from 40.09% to 17.40%, along with a 6.62% reduction of gas yield. Interestingly, when the heating rate further increased from 10 to 100 K/ps, tar production increased by only 14.13%, while the char and gas production changed slightly. This phenomenon showed that increasing the heating rate greatly promotes the production of tar and suppresses the formation of gas at lower heating rates (below 10 K/ps). However, the influence of heating rate on pyrolysis products was relatively small with further increasing the heating rate. Based on these observations, it can be inferred that heating rate has an important influence on pyrolysis mechanism. Two specific heating rates (2 and 50 K/ps) were chosen to study the influence of heating rate on reaction mechanism. Fig. 8 shows the evolution of pyrolysis products at the heating rates of 2 and 50 K/ps. The initial decomposition temperatures of macromolecular structure were 1500 K and 1800 K at 2 and 50 K/ps, respectively. Increasing the heating rate increased the outset temperature of pyrolysis reaction, which was mainly associated with the thermal hysteresis effect. Furthermore, compared with higher heating rate, more product fragments, especially tar and gas, were generated at the lower heating rate under low temperatures (below 2400 K). For example, at 2300 K, the number of product fragments were 189 and 20 at the heating rates of 2 and 50 K/ps, respectively. The simulation results were consistent with previous studies reported by Zhang et al. [47], who found that the thermal decomposition took place earlier and produced more fragments at the lower heating rate. Therefore, low temperature pyrolysis at low heating rate is a promising thermal conversion method for lignite. At the heating rate of 2 K/ps, tar production first increased and reached the maximum value of 67.06% at 2400 K, and then, it decreased significantly with further increasing temperature. However, the char yield showed the opposite evolution trend and attained the minimum value at 2400 K. In addition, gas production continued to increase with the increase of temperature. The evolution of char, tar, and gas indicated that the secondary reaction was prominent when the temperature was above 2400 K. The condensation reaction of aromatic structures and further decomposition of tar fragments occurred, resulting in a significant reduction in the yield of tar. At the heating rate of 50 K/ps, the yield of tar kept on increasing with the increase of temperature and achieved the maximum value of 68.73% at 3000 K. Obviously, high heating rate shortens the reaction time, which inhibits the secondary reaction of char compounds and tar fragments. Therefore, it can be concluded that the influence of heating rate on nonisothermal pyrolysis process is closely related to the secondary reaction. Fig. 9 displays the evolution of potential energy of lignite pyrolysis within the temperature range from 300 to 3000 K at the heating rates of 2 and 50 K/ps. It is well known that pyrolysis is an endothermic reaction. Moreover, the higher the temperature is, the more the required energy will be. The potential energy values were close to each other at the heating rates of 2 and 50 K/ps within the temperature range of
Fig. 5. Evolution of molecule number of product fragments within the temperature range from 300 to 3000 K.
dramatic decline in the yield of char was observed along with an apparent increase in the production of tar (see Fig. 2). Furthermore, the amount of tar and gas increased significantly with the increase of temperature (see Fig. 5). In this stage, high temperature accelerated the decomposition of char into small molecular fragments (heavy tar, light tar, and gas). Meanwhile, parts of heavy tar was further converted to light tar and gas. In this reaction process, heavy tar may be either the intermediate or final product. There is a competitive relationship between the formation and decomposition of heavy tar, whereas the composition of product greatly depends on the reaction temperature. As seen clearly in Fig. 5, the quantity of light tar fragments was even higher than that of heavy tar fragments when the temperature was higher than 2300 K. It indicated that high temperature facilitates the formation of light tar. In other words, pyrolysis tar obtained at high temperatures exhibits better quality. Secondary reaction was dominant in Stage III at temperatures between 2400 K and 3000 K. The yield of char increased greatly by 25.79%, while the number of molecules changed slightly. The average molecular weight of char exhibited an obvious increase from 835 amu to 1609 amu. Furthermore, significant reductions in the yields of heavy tar (28.01%) and light tar (10.25%) were accompanied by a sharp increase in gas production (11.71%) within this temperature range. It can be concluded that the condensation reaction of tar fragments and char compounds took place, giving rise to the increase of in the size of aromatic clusters. Meanwhile, some of the tar was further decomposed into gas molecules. The number of light tar fragments was higher than that of heavy tar fragments in the whole Stage III. It can be inferred that more heavy tar was converted into gas or char through further decomposition or condensation reaction than light tar. According to the evolution mechanism of product, non-isothermal pyrolysis processes of lignite can be simplified, as shown in Fig. 6. 3.2. Effect of heating rate Heating rate is a critical factor in pyrolysis processes and plays a Table 1 Detailed composition of pyrolysis products at 1800 K. Char
C201H193O32N3S C183H176O29N3S C95H95O12NS
C199H188O32N3S C146H155O21N2S C45H44O9
C187H184O29N3S C170H161O26N3S
C185H181O29N3S C134H124O22NS
C184H181O27N3S C101H92O19N2
Heavy tar Light tar Inorganic gas Organic gas
3 C14H10O3 C13H14ON 9 C2H4 CO2
3 C14H9O3 C10H9ON C3H6
C14H12O3
C19H23ON
C14H11O3
4 CH2O
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Fig. 6. Non-isothermal pyrolysis processes of lignite in the temperature range of 300–3000 K.
Fig. 7. Final distribution of pyrolysis products obtained at 3000 K at the heating rates of 2, 10, 50, and 100 K/ps.
300–2400 K. However, the potential energy at the heating rate of 2 K/ps became higher than that at 50 K/ps at temperatures between 2400 K and 3000 K. This indicated that the secondary reaction requires more energy and also suggests that the influence of heating rate on pyrolysis reaction is mainly related to the secondary reaction. 3.3. Formation and transformation mechanisms of typical pyrolysis products The composition and distribution of pyrolysis products largely depend on the structural characteristics of coal. Lignite, as a typical lowrank coal, has abundant aliphatic structures. Solid state 13C NMR results showed that the content of aliphatic carbon was 34.32%, while the content of methylene carbon was as high as 17.60% in the Huolinhe lignite [35]. The breakage of Cal-Cal gave rise to the production of large amounts of C2H4. As shown in Fig. 10, C2H4 was produced at 1500 K, and the yield of C2H4 first increased with the increase of temperature,
Fig. 8. The evolution of pyrolysis products at temperatures between 300 K and 3000 K at the heating rates of 2 and 50 K/ps.
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(such as, H% and CH3%) to form naphthol or methyl naphthol. Therefore, it can be concluded that the formation of phenols depends on many factors, such as the type of oxygen-containing functional groups, aromatic structures and small free radicals. According to the analysis of species, it can be found that the yield of phenols reached the maximum value at 2400 K, after which, it gradually decreased with the increase of temperature. This result was consistent with that reported in a previous experimental study [48], which reported that the production of phenols first increased and achieved the maximum value at 800 ℃, and then, decreased with further increasing temperature. It can be inferred that phenols are transformed at high temperatures. Two possible transformation pathways of phenols are shown in Fig. 11c. They are: i) when the energy of the system was more than the bond energy itself, Car-O in C10H8O broke down and a phenolic hydroxyl radical was generated. The phenol hydroxyl radical reacted with hydrogen radical to produce water. The energy of the system increased with the increase of temperature, which caused more Car-O to break, and thereby more water was obtained. As shown in Fig. 10, water was generated mainly beyond 2400 K and the yield of water increased with increasing temperature. The transformation mechanism of phenols was consistent with the evolution behavior of water. ii) The condensation reaction occurred between the fragments C18O2H16 and C6OH6, in which hydrogen atom combined with the phenolic hydroxyl groups to form water. The simulation results were in accordance with those reported by Jiang et al. [49], who found that the dehydration and condensation reactions took place during the pyrolysis of phenolic resin at high temperatures.
Fig. 9. The evolution of potential energy in the temperature range of 300–3000 K at the heating rates of 2 and 50 K/ps.
4. Conclusions In this work, non-isothermal pyrolysis of lignite was studied using ReaxFF-MD simulations within the temperature range of 300–3000 K at different heating rates. At a relatively low heating rate (2 K/ps), the pyrolysis processes can be classified as: the structural adjustment stage, the main pyrolysis stage, and the secondary reaction stage. In the main pyrolysis stage, some of the heavy tar was further decomposed into light tar and gas. The formation and decomposition of heavy tar largely depend on the reaction temperature. In the secondary reaction stage, compared with the light tar, more heavy tar was converted to gas or char by decomposition or condensation reaction. The amount of heavy tar fragments was lower than that of the light tar in this stage. The influence of heating rate on pyrolysis reaction was explored at the heating rates of 2, 10, 50, and 100 K/ps. The heating rate has a significant influence on the product distribution at low heating rates (below 10 K/ps), which is mainly associated with the secondary reaction. Furthermore, more tar and pyrolysis gas were generated at low temperatures (below 2400 K) compared to high heating rates. Since C2H4 and phenols were the major products of lignite pyrolysis, their reaction pathways have been studied. The formation of C2H4 was associated with the cleavage of Cal-Cal, while the generation of phenols depends on many factors, such as the type of oxygen-containing functional groups, aromatic structures and small free radicals. Two possible conversion pathways of phenols have been proposed: (1) dissociation of the phenolic hydroxyl from tar fragment that reacted with hydrogen radical to produce water; (2) condensation between the phenolic hydroxyl and hydrogen atom. The simulation results were in accordance with the evolution of typical pyrolysis gases.
Fig. 10. The evolution of typical gases in the temperature range of 300–3000 K.
and then, decreased when the temperature was above 2600 K. The reduction of C2H4 production is probably due to the fact that high temperature is conducive to the dehydrogenation reaction. The obvious increase in C2H2 also verified that C2H4 may be converted to C2H2 at high temperatures. An example of the generation pathway of C2H4 is displayed in Fig. 11a. The breakage of Cal-Cal (marked as bond 1) led to the production of free-radical fragments 1 and 2. Then, the Car-Cal (marked as bond 2) in fragment 2 broke down to generate C2H4 and C10H8O%. The breaking sequence of the bonds (bond 1 and bond 2) was in accord with the strength of bonds. Phenols were the main components of pyrolysis tar for lignite. It is well known that lignite has a low degree of coalification with many oxygen-containing functional groups, such as carboxyl, carbonyl, hydroxyl, and ether bond. The formation of phenols was largely related to phenolic hydroxyl groups and aromatic ethers. Two possible generation pathways for phenols are displayed in Fig. 11b. These are: i) Alkyl-aryl ether bond (marked as bond 3) fractured to produce free-radical fragment C14H9O3%, which is a strong hydrogen acceptor and can easily get a hydrogen radical to generate phenols. Because the Cal-O bond has lower dissociation energy and can crack easily at low temperatures, phenols can be observed at 1800 K. The simulation results were consistent with the results in previous experimental studies [46]. ii) The breakage of Cal-Car (marked as bond4) generated free-radical fragment C10H7O%. The unstable C10H7O% combined with small free radicals
Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
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Fig. 11. Reaction pathways of typical pyrolysis products for lignite.
Acknowledgments
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