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ReaxFF-based molecular dynamics simulation of the initial pyrolysis mechanism of lignite
Fuel Processing Technology 195 (2019) 106147
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Fuel Processing Technology journal homepage: www.elsevier.com/locate/fuproc
Research article
ReaxFF-based molecular dynamics simulation of the initial pyrolysis mechanism of lignite ⁎
T
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Fang Xua,b, Hui Liua, , Qing Wangb, , Shuo Panb, Deng Zhaoa, Qi Liub, 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 Molecular structure Pyrolysis mechanism ReaxFF Molecular dynamics
In this paper, a series of ReaxFF molecular dynamics (ReaxFF-MD) simulations were employed to explore the characteristics of pyrolysis products, transformation behavior of major elements, and thermal decomposition mechanism of lignite. The results suggest that the pyrolysis of lignite mainly undergoes decomposition of macromolecular structure and breakage of bridge bonds at lower temperatures. At relatively high temperatures, further cracking of tar fragments and condensation of aromatic structures occur. The relationship between the main pyrolysis gases and the structural characteristics of lignite has been studied. The results show that the formation of H2O, CO2, and C2H4 is associated with the typical structure of lignite i.e., hydroxyl groups, carboxyl groups, and methylene carbon, respectively. Unlike bituminous coal and subbituminous coal, light tar is the main component of pyrolysis tar, which is due to large amounts of monocyclic and bicyclic structures in lignite. The pyrolysis mechanism of lignite was further analyzed through element migration behavior, and the simulation results are consistent with the above reaction mechanism. This work provides in-depth insight into the initial reaction mechanism of lignite pyrolysis and may be useful for the industrialization of lignite clean utilization.
1. Introduction
chemical bonds and the decomposition of functional groups. Xu et al. [11] reported pyrolysate distributions and kinetics characteristics of Zhaotong lignite. They found the optimal reaction conditions for generation of tar and pyrolysis gas. Meng et al. [12] studied the morphology and chemical structure of low-temperature pyrolysis chars. They found that temperature has an apparent influence on product yields, surface morphology, and the evolution of chemical functional groups of chars. He et al. [13] analyzed the thermal degradation of Shengli lignite using TG–GC–MS. The results indicated that benzene series are the major gaseous products, and small molecular structures are the main components of aliphatic hydrocarbons. According to experimental studies, some progress has been made in understanding the characteristics of pyrolysis products and the chemical reaction mechanism of lignite pyrolysis. However, it is quite difficult to understand the complicated thermal decomposition reactions in depth through experimental methods alone because of the heterogeneity of lignite and the complexity of the pyrolysis process. Furthermore, vast free radicals are generated in an exceedingly short time, and these are difficult to detect in laboratories [14]. Undoubtedly, computational approaches would provide a promising platform for further researching the lignite pyrolysis mechanism.
Coal, which is the principal traditional source of energy, occupies a dominant position in world energy consumption, especially in China [1]. With the amount consumption of high rank coal, lignite has attracted considerable interest due to its vast reserves, low selling price, high chemical reactivity, and low pollution-forming impurities [2,3]. However, self-defection (e.g., low calorific value, high moisture content, and a high tendency of spontaneous combustion) makes it unfit for direct utilization [4]. Upgrading lignite to higher quality products by means of pyrolysis is considered to be a promising way to achieve economically efficient utilization of lignite [5,6]. Solid residues (coke or char) with high calorific value can be utilized for direct combustion or preparation of slurry fuels [7]. Pyrolysis gases contain large amounts of combustible gases, such as H2, CO, and hydrocarbon gas [8]. Tars produced from lignite pyrolysis are suitable for liquid fuels and valuable chemicals [9]. Numerous efforts have been devoted to study the pyrolysis behavior of lignite. Liu et al. [10] investigated the interrelation between lignite structural characteristics and product distributions. They concluded that the formation of pyrolysis products is related to the breakage of
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Corresponding authors. E-mail addresses: [email protected] (H. Liu), [email protected] (Q. Wang).
https://doi.org/10.1016/j.fuproc.2019.106147 Received 22 March 2019; Received in revised form 9 July 2019; Accepted 9 July 2019 0378-3820/ © 2019 Elsevier B.V. All rights reserved.
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Fig. 1. Two-dimensional (2D) unimolecular structural model for Huolinhe lignite.
bituminous coal and subbituminous coal, the molecular structure of lignite contains more oxygen-containing functional groups, aliphatic side chains, and single aromatic ring structures [28]. Published studies on the pyrolysis behavior of lignite are rare [29]. Further research is needed to better understand the complex pyrolysis process, especially the relationship between pyrolysis products and lignite structure. The main purpose of this work is to explore the pyrolysis behavior of lignite using ReaxFF-MD simulations. The paper is organized as follows: First, the characteristics of pyrolysis products, especially tar and gas, are analyzed in detail. Second, the transformation behavior of carbon, hydrogen, and oxygen during the pyrolysis process are revealed. Finally, two typical temperatures (2000 K and 3000 K) are selected for an in-depth investigation of the pyrolysis reaction mechanism. The elucidation of these problems may provide more constructive information of high-efficiency clean technology for converting lignite.
Quantum chemistry (QC) modeling is applicable for studying chemical reactions with high accuracy. However, QC-based methods are computationally expensive and intensive, thus they have rarely been applied in models with more than 100 atoms [15]. Classical molecular dynamics (MD) has the ability to model molecular systems that contain more than 10,000 atoms, but it is not suitable for exploring the cleavage and production of bonds [16]. Fortunately, the Reactive Force Field (ReaxFF), introduced by van Duin et al., can address the chemical interactions of atoms and molecules [17]. ReaxFF-MD combines the advantages of MD and ReaxFF, which can be used to study the chemical reactions of large and complicated molecular systems. Additionally, ReaxFF-MD has been proven to maintain basically the calculation precision of QC but with much reduced computational costs [18,19]. In the last decade, ReaxFF-MD has been successfully used to simulate the combustion and pyrolysis characteristics of complex compounds, such as coal [20], biomass [21], oil shale kerogen [22] and char [23], with the goal of trying to understand the reaction process in depth. Zhan et al. [24] investigated the reaction mechanism of subbituminous coal via ReaxFF-MD. The calculation results indicated that the pyrolysis process is initiated by breaking unstable CeC and CeO bonds, which is followed by intramolecular hydrogen transformation. Castro-Marcano et al. [25] adopted ReaxFF-MD to study the combustion processes of coal char. They found that aromatic structures are more liable to oxidize and combust under fuel lean and higher temperature conditions. Zheng et al. [26] employed ReaxFF-MD to analyze the overall reaction stages for Liulin bituminous coal. The simulation results showed that the pyrolysis processes are roughly divided into four stages according to the breakage behavior of chemical bonds. These four stages are the activation stage, primary pyrolysis stage, secondary pyrolysis stage, and recombination dominated stage. Chen et al. [27] studied the pyrolysis and combustion behavior of biomass using ReaxFF-MD. The simulation results indicated that environment and temperature play a significant influence on the dissociation of chemical bonds and product distributions. Extensive studies of the chemical reaction mechanism using ReaxFF-MD simulations have been reported. Nevertheless, coal type is a critical factor that greatly affects the pyrolysis process. Compared with the structural features of
2. Computational details ReaxFF [17] is an empirical bond-order-based reactive force field and can explicitly describe chemical reactions within complex systems. The relationship between bond order and bond energy plays an important role in ReaxFF. Bond order is got from interatomic distances and is updated continually at every iteration, thus ensuring connectivity changes. The bonded interactions (i.e., bonds, angles, and torsions) are bond-order dependent so that energies related to these terms disappear upon bond dissociation. Moreover, ReaxFF includes non-bonded interactions (van der Waals and Coulomb), which play significant roles in predicting structures and properties [30]. ReaxFF can be utilized in large molecular models containing ~10,000 atoms with high calculation precision close to density functional theory (DFT). Nevertheless, the time scale of ReaxFF is at least 100 times faster than that of DFT. Further detailed information about ReaxFF can be found in the ReaxFF User Manual written by van Duin. A Huolinhe lignite model [31] with the molecular formula C201H195O32N3S1 was constructed previously by our group and was utilized as the initial molecular structure model (Fig. 1). As seen clearly in Fig. 1, the Huolinhe lignite molecular model contains abundant 2
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oxygen-containing functional groups, such as carboxyl, carbonyl, hydroxyl. In addition, a large number of aliphatic side chains and double aromatic rings are typical structural characteristics of Huolinhe lignite. Geometry optimization of the model was obtained using molecular mechanics (MM) and MD simulations [32]. Subsequently, 10 optimized unimolecular models were placed in a periodic box of 81.0 × 81.0 × 81.0 Å3 using the construction function of the MS Amorphous Cell module. The initial density of the macromolecule was set as 0.10 g/cm3 to avoid overlapping of the important functional groups. In this work, all ReaxFF-MD simulations were implemented in the “reaxc” package of LAMMPS. First, energy minimization was performed at 50 K for 20 ps. Then, the system was maintained in a Berendsen thermostat to perform an NVT-MD simulation. To obtain a proper density of lignite, the system was compressed to a density of 0.966 g/ cm3 at 300 K using a constant number, pressure, and temperature (NPT) ensemble at a pressure of 10 MPa. Next, a no-reaction NVT-MD simulation was performed at 300 K to relax the system. In order to avoid any reaction occurrence, the CeO and OeH bond parameters were switched off in the relaxation process. Finally, a set of ReaxFF-MD constant temperature simulations employing the NVT ensemble were carried out at 1600–3000 K for 250 ps with an interval of 200 K. An initial simulation temperature of 1600 K was chosen because nearly no thermal decomposition reaction occurred before that. The velocityVerlet algorithm was applied to integrate Newton's equation of motion with a time step of 0.1 fs, and the temperature was controlled by a damping constant of 100 fs. Molecular species were identified by a bond-order cutoff of 0.3. The parameters of ReaxFF utilized are proposed by Mattsson et al. [33] and have been successfully used to investigate the pyrolysis process of solid fuels [26,34]. Due to the limitation on computing capacity, the simulation time scale (on the order of picoseconds) is far less than the reaction time in experiments (on the order of seconds). Based on Arrhenius equation, increasing the reaction temperature greatly rises the reaction rate, especially for the reaction with higher energy barrier. Therefore, higher temperatures were usually used in previous ReaxFF-MD simulations [35,36]. Despite obvious differences between simulation and experiment in the temperature and time, product distributions and reaction kinetics parameters obtained by simulations show good consistent with the experimental results [22,37,38]. Zheng et al. studied the product evolution and reaction mechanisms of the Liulin coal by ReaxFF MD at 1000–2600 K. They found that the evolution tendencies of naphthalene, methylnaphthalene and dimethyl-naphthalene are in accord with the results of Py-GC/MS [37]. Wang et al. investigated the pyrolysis and combustion of n-dodecane, and found that the activation energies and pre-exponential factors obtained from ReaxFF-MD at high temperatures are in agreement with the experimental values [38].
Fig. 2. Distribution of pyrolysis products in ReaxFF-MD simulations at 1600–3000 K.
reached a maximum value of 29.20% at 3000 K. Remarkably, the yield of organic gas was much higher than that of inorganic gas in the whole simulation temperature range. CO2 and H2O were the predominant inorganic gases, and C2H4 was the major organic gas. The production of main pyrolysis gases (CO2, H2O and C2H4) is closely related to the structural characteristics of Huolinhe lignite, that will be illustrated in detail below. The content of tar first increased with the temperature and reached the peak at 2200 K. In contrast, char content decreased continuously to the minimum production at 2200 K and then increased with the temperature. It can be inferred that 2200 K is very probably the transition temperature of different pyrolysis mechanisms. The snapshots of pyrolysis products intermediate configurations (at 1600 K, 2200 K, and 3000 K) obtained from VMD are displayed in Fig. 3. At the lower temperatures (1600–2200 K), the primary pyrolysis reactions were dominated by the breakage of bridge bonds and the dissociation of the macromolecular network structure, leading to the production of amounts of tar and the decomposition of char [41]. With further increasing temperature from 2200 to 3000 K, the remarkable reduction in the tar yield (46.79%) was accompanied by an increase in the char yield (30.36%). These phenomena suggest that secondary pyrolysis reactions become pronounced at higher temperatures (2200−3000 K). Further cracking of tar, rearrangement, and condensation of aromatic structures occur, contributing to the formation of char along with a reduction of tar yield. The simulation results of product distributions are consistent with previous experimental researches reported by Meng et al. [12] and Zhou et al. [42] They concluded that the gas content increased monotonically as the temperature increasing, but the tar yield first increased and then decreased. Therefore, if it is desirable to get tar as much as possible, a moderate pyrolysis temperature should be selected. Because tar is easy to transport, store, and process, it is considered to be the most desirable pyrolysis product of lignite [43]. Analyzing tar evolution behavior during pyrolysis process is of great significance. Fig. 4 shows that the distributions of heavy tar and light tar at 1600–3000 K for 250 ps. As displayed in Fig. 4a, the yields of heavy tar and light tar have similar evolution trends over the entire simulation temperature range. When the temperature was below 2200 K, the content of heavy tar increased from 4.27% to 47.21%, whereas the yield of light tar presented an increasing trend from 1.06% to 20.50%. The weight percentage of heavy tar was obviously higher than that of light tar at each simulation temperature, especially at 2200 K. Nevertheless, as clearly seen from Fig. 4b, the quantity of light tar fragments was close to that of heavy tar at 1600–2200 K. When the temperature is higher than 2600 K, the quantity of light tar is even higher than that of heavy tar. It can be speculated that lignite pyrolysis tar contains a lot of
3. Results and discussion 3.1. Analysis of lignite pyrolysis products Undoubtedly, temperature is a critical factor that affects the generation and evolution of pyrolysis products. To evaluate the influence of temperature on product distributions, a set of ReaxFF-MD constant temperature simulations were conducted at temperatures 1600–3000 K for 250 ps. To be consistent with previous work [39,40], pyrolysis products from ReaxFF-MD simulations are classed as five types: inorganic gas, organic gas, heavy tar, light tar, and char. Inorganic small molecules (e.g., H2, CO2, and H2O) are regarded as inorganic gas; fragments of C1-C4 are regarded as organic gas; fragments of C5-C13 and C14-C40 with molecular weight of 80–600 amu are light tar and heavy tar, respectively. C40+ compounds with molecular weight greater than 600 amu are considered to be char. As shown in Fig. 2, the yield of gas (inorganic gas and organic gas) kept increasing with an increase in temperature. Gas production 3
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Fig. 3. Snapshots of pyrolysis products intermediate configurations obtained from VMD: (a) configuration at 1600 K, (b) configuration at 2200 K, (d) configuration at 3000 K.
number of heavy tar and light tar fragments both declined obviously with increasing temperature. However, the opposite trend was observed for the number of gas molecules, which increased from 121 to 318. That is to say, further decomposition of tar compounds occurred, resulting in an increase of gas yield at 2200–3000 K. In addition, the quantity of char fragments fluctuated slightly with increasing temperature, but the average molecular weight of char increased greatly, especially at 2800 K and 3000 K (see Fig. 5). It means that the condensation of aromatic structures (tar compounds and char fragments) occur, giving rise to the growth of char fragments. Moreover, the higher the temperature is, the more pronounced the secondary pyrolysis reactions will be. The simulation results are in accord with the experimental studies of Song et al. [45] and Hu et al. [46], who found that volatile-char interactions play an important role during pyrolysis process. Tar can be converted into gas by cracking reactions or translated into char by condensation reactions. The evolution of typical gas products of lignite at different temperatures is presented in Fig. 6. H2O is the main pyrolysis gas of lignite,
light tar, and the difference in the weight percentage between heavy tar and light tar is greatly due to the molecular weight of the fragments. The formation of light tar is attributed to large amounts of monocyclic and bicyclic structures in Huolinhe lignite. As described above, the primary pyrolysis reactions were predominant at 1600–2200 K. The formation of tar was mainly because of the breakage of bridge bonds. According to species analysis, only 8 tar fragments (3C14H10O3, 3C14H12O3, C11H9O, and C13H10O) were observed at 1600 K, which indicated that only weaker chemical bonds (e.g., Cal-Cal, CaleH, CaleO) are liable to fracture at low temperature. Nevertheless, the quantity of tar fragments increased remarkably and achieved a maximum value of 90 at 2200 K, as shown in Fig. 5. A large quantity of tar fragments were produced at relatively high temperature, and this may be associated with the fact that higher temperature can provide more energy to break stronger chemical bonds linked with aromatic structures [44]. With a further increase in temperature from 2200 to 3000 K, the yield of heavy tar dropped significantly from 47.21% to 12.38%, accompanied by a 19.95% reduction of light tar. Furthermore, the 4
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Fig. 6. Evolution of typical gas product with the temperature in ReaxFF-MD simulations at 1600–3000 K.
the yield increased sharply when the temperature was higher than 2000 K, and number of H2O reached as high as 90 at 3000 K. This is mainly due to the existence of a large number of hydroxyl groups in Huolinhe lignite, which react with hydrogen radicals to produce H2O during pyrolysis. Because the decomposition of carboxyl groups is easier at lower temperature, CO2 first appeared at 1600 K. The amount of CO2 continued to increase at 1600–3000 K, which means that high temperature is in favor of producing CO2. Large amounts of C2H4 were generated at 1800 K, and the yield increased rapidly before 2600 K. The formation of C2H4 is attributed to the breakage of weak bonds (Cal-Cal) [47], which is coincident with the high content of methylene carbon in the molecular structure of Huolinhe lignite. It is interesting that the reduction of C2H4 was approximately equivalent to the increase of C2H2, which demonstrates that parts of C2H4 probably were converted to C2H2 with further increasing temperature from 2600 to 3000 K [22]. H2 and CH4 were generated at 2000 K, and their yields raised as a function of temperature. It must be mentioned that the amount of H2 was extraordinarily high at 2800 K and 3000 K. This indicates that high temperature (2800 K and 3000 K) causes the condensation reaction of aromatic structures to be more pronounced, which leads to the production of a high concentration of hydrogen radicals and the release of H2. As the final gas product, C2H2 was generated at 2200 K, and the yield was relatively lower at temperatures between 2200 K and 2600 K. The remarkable increase in C2H2 was related to the dehydrogenation reaction of C2H4 at the higher temperature. The evolution trend of typical gas products is consistent with previous experimental studies [48,49].
Fig. 4. Distributions of tar (heavy tar and light tar) fragments in ReaxFF-MD simulations at 1600–3000 K: (a) weight percentage and (b) number of molecules.
3.2. Transformation behavior of carbon, hydrogen and oxygen Carbon (C), hydrogen (H) and oxygen (O) are the most significant chemical elements. Studying the transformation behavior of C, H, and O during the pyrolysis process is important for gaining an in-depth understanding of the thermal decomposition mechanism. Fig. 7 gives an overview of the distributions of the main elements (C, H, and O) at different temperatures. As seen in Fig. 7a, the content of char-C greatly decreased from 93.73% at 1600 K to 20.05% at 2200 K, along with the increase trends of tar-C (62.58%) and gas-C (10.10%). That is to say, increasing the temperature facilitated carbon transferring from char into tar and gas at temperatures 1600–2200 K, especially tar. At 2200 K, char-C content reached a minimum value of 20.05%, whereas the content of tar-C increased to a maximum value of 68.96%. The significant decrease in char-C content was associated with the cleavage of chemical bonds containing carbon (e.g., CaleH, Cal-Cal and Car-Cal). Meanwhile, the increase in tar-C content is due to the production of
Fig. 5. Number of pyrolysis products fragments and average molecular weight of char in ReaxFF-MD simulations at 1600–3000 K.
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Fig. 8. Time evolution of pyrolysis products in ReaxFF-MD simulations: (a) 2000 K and (b) 3000 K.
speculated that the major proportion of carbon migrated to char by means of condensation reactions of aromatic structures. Only a small fraction of the carbon transferred to gas via further decomposition of tar fragments. The evolution trend of carbon is consistent with the above pyrolysis mechanism. As displayed in Fig. 7b, the char-H yield greatly decreased with increasing temperature, whereas the yield of tar-H and gas-H showed the opposite trend below 2200 K. At temperatures between 2200 K and 3000 K, the apparent decline of tar-H content from 63.23% to 19.33% was accompanied by an increase in char-H content (23.59%) and gas-H (20.31%). Notably, the content of gas-H was higher than that of tar-H in the whole 2200–3000 K temperature range. This means that, unlike carbon, most hydrogen migrated into gas phase at high temperature. This phenomenon is consistent with the above studies of tar pyrolysis behavior, specifically, high temperature caused the condensation reactions of char fragments to be more pronounced, leading to the formation of a high concentration of hydrogen radicals and the increase of gaseH. Gas-H existed mainly in the forms of H2, H2O, C2H4, CH4, and so on. Fig. 7c shows that the yield of char-O decreased from 90.63% at 1600 K to 18.75% at 2200 K, along with an increase in the yield of tar-O (53.69%) and gas-O (17.19%). Nevertheless, oxygen in tar significantly reduced the higher heating value (HHV) of fuels and increased their reactivity, resulting in storage difficulties [50]. Therefore, it is suitable
Fig. 7. Main element distributions in pyrolysis products in ReaxFF-MD simulations at 1600–3000 K: (a) carbon, (b) hydrogen, and (c) oxygen.
large amounts of tar. When further increasing temperature from 2200 to 3000 K, the yield of tar-C presented a linear decrease along with an increase in char-C and gaseC. Compare to 2200 K, the remarkable decrease in the tar-C (45.92%) was accompanied by the increasing tendency of char-C (38.01%) and gas-C (7.91%) at 3000 K. It can be 6
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②
④
④
ĺ
③
ķ
④
③ ③
Fig. 9. Initial pyrolysis mechanism of lignite molecular structure in ReaxFF-MD simulations at 2000 K.
(a) bond1
+
CH3·
+
H·
bond1 bond 2
b)
bond1
bond2
Fig. 10. Formation pathways of typical products C13H13ON, C12H8O4, and CO2.
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+
CO2
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4. Conclusions
to minimize the content of oxygen in tar. With a further increase in temperature from 2200 to 3000 K, the reduction of tar-O was nearly equivalent to the increase of gaseO. This indicates that temperature has a slight influence on char-O content. In addition, the content of gas-O was extremely high at 3000 K, and this was associated with the production of large amounts of CO2 and H2O. The results are in accord with the evolution trend of the typical gases mentioned above.
In this work, the initial pyrolysis mechanism of lignite was investigated using ReaxFF-MD constant temperature simulations at 1600–3000 K for 250 ps. The results show that lignite pyrolysis process begins with the dissociation of the weaker bridge bonds such as Cal-O and Cal-Cal. Moreover, increasing temperature is favorable for the decomposition of macromolecular structure and the breakage of bridge bonds at the lower temperatures (1600–2200 K). At relatively high temperatures (2200–3000 K), the secondary pyrolysis reactions become more pronounced. The opposite evolution tendency of tar and gas indicates that further decomposition of tar fragments into pyrolysis gas are carried out. The remarkable increase of char content and generation of large amounts of H2 are related to the condensation of aromatic structures. Analysis of migration behavior of the main elements showed that the major proportion of carbon migrated to char and most hydrogen migrated into gas phase by means of condensation reactions at high temperatures (2200–3000 K), which is consistent with the above pyrolysis mechanism. H2O, CO2, and C2H4 are the main gas products, and the formation is closely related to the structural characteristics of lignite. Light tar is the main component of pyrolysis tar, which is due to large amounts of monocyclic and bicyclic structures in lignite. This work may aid in understanding the pyrolysis mechanism of lignite and provide theoretical basis for clean coal technologies.
3.3. Analysis of pyrolysis reaction mechanism To study the influence of pyrolysis temperature on reaction mechanism more clearly, it is desirable to focus on the specific pyrolysis temperature. According to the results of product distributions, 2200 K was most probably the transition temperature of different mechanisms. As the highest temperature of the two reaction mechanisms, 2000 K and 3000 K were selected to research the complex chemical reaction mechanism of lignite pyrolysis. Fig. 8 demonstrates products profile tendency with time at 2000 K and 3000 K. The yields of char were very close, but the yields of gas and tar had a great difference at 250 ps. It is supposed that the reaction mechanism of lignite pyrolysis at lower temperatures (below 2200 K) is different from that at higher temperatures (above 2200 K). At 2000 K, gas content slightly raised and tar yield increased obviously by 37.66%, whereas the yield of char decreased greatly by 46.57% over the initial 200 ps. Thereafter, the yields of products (char, tar, and gas) almost remained a constant value, which means the main reaction process was completed within 200 ps. With the aid of simulation trajectories and species analysis, deep insight into the preliminary thermal decomposition mechanism at lower temperatures is possible. Because of a relatively lower dissociation energy and smaller bond order of the Cal-O bond, the lignite pyrolysis process was started by the breakage of the alkyl ether bridge bond (labeled “①”), as shown in Fig. 9. The release of tar species C14H9O3 is the best proof. Breakage of the Cal-Cal bond (labeled “②”) in the lignite molecular structure then occurred, leading to the formation of CH2O. The simulation results are coincident with previous studies that the aliphatic and ether bridge bonds are usually the first breaking sites during the primary thermolysis reactions [26,51]. Decomposition of carboxyl functional groups (labeled “③”) contributed to the formation of CO2. Very early production of CO2 during the pyrolysis process was also found in the experiments by Feng et al. [52]. Subsequently, a large quantity of Cal-Cal and Cal-O (labeled “④”) broke down, generating many pyrolysis products such as C7H5O2, C10H9ON, and C4H5N. At 3000 K, the pyrolysis reactions are approximately grouped into three major stages according to product distributions. The first stage (Stage-I) was from the beginning to 10 ps, as the primary pyrolysis reactions stage. The molecular structure of lignite quickly underwent thermal decomposition, and thereby generated numerous product fragments. Because a temperature of 3000 K is extremely high, enough energy is provided to break the weak bonds of Cal-O and Cal-Cal simultaneously, and 41 fragments were observed as soon as possible. Fig. 10a shows an example of the formation pathway of C13OH13N, in which it is clear that the Cal-O bond (denoted as bond 1) and the Cal-Cal bond (denoted as bond 2) dissociated, resulting in the production of CH3·, C13H13ON, and C12H8O4. Then, CO2 was generated via the decarboxylation reaction of C12H8O4, as shown in Fig. 10b. The interval from 10 to 200 ps was taken as the second stage (Stage-II), which was dominated by the secondary pyrolysis reactions. The yield of tar decreased continuously with an increase in char, which was associated with the condensation reactions of tar compounds and char fragments. The yield of gas increased first and nearly remained a constant value after 100 ps. It is supposed that the cracking reactions of tar compounds were more pronounced during 10–100 ps. The last stage (Stage-III) was from 200 to 250 ps, as the equilibration stage. The weight percentages of char, tar, and gas remained constant at this stage. This also means that using 250 ps as the simulation time is suitable for studying the reaction mechanism of lignite pyrolysis.
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Fuel Processing Technology journal homepage: www.elsevier.com/locate/fuproc
Research article
ReaxFF-based molecular dynamics simulation of the initial pyrolysis mechanism of lignite ⁎
T
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Fang Xua,b, Hui Liua, , Qing Wangb, , Shuo Panb, Deng Zhaoa, Qi Liub, 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 Molecular structure Pyrolysis mechanism ReaxFF Molecular dynamics
In this paper, a series of ReaxFF molecular dynamics (ReaxFF-MD) simulations were employed to explore the characteristics of pyrolysis products, transformation behavior of major elements, and thermal decomposition mechanism of lignite. The results suggest that the pyrolysis of lignite mainly undergoes decomposition of macromolecular structure and breakage of bridge bonds at lower temperatures. At relatively high temperatures, further cracking of tar fragments and condensation of aromatic structures occur. The relationship between the main pyrolysis gases and the structural characteristics of lignite has been studied. The results show that the formation of H2O, CO2, and C2H4 is associated with the typical structure of lignite i.e., hydroxyl groups, carboxyl groups, and methylene carbon, respectively. Unlike bituminous coal and subbituminous coal, light tar is the main component of pyrolysis tar, which is due to large amounts of monocyclic and bicyclic structures in lignite. The pyrolysis mechanism of lignite was further analyzed through element migration behavior, and the simulation results are consistent with the above reaction mechanism. This work provides in-depth insight into the initial reaction mechanism of lignite pyrolysis and may be useful for the industrialization of lignite clean utilization.
1. Introduction
chemical bonds and the decomposition of functional groups. Xu et al. [11] reported pyrolysate distributions and kinetics characteristics of Zhaotong lignite. They found the optimal reaction conditions for generation of tar and pyrolysis gas. Meng et al. [12] studied the morphology and chemical structure of low-temperature pyrolysis chars. They found that temperature has an apparent influence on product yields, surface morphology, and the evolution of chemical functional groups of chars. He et al. [13] analyzed the thermal degradation of Shengli lignite using TG–GC–MS. The results indicated that benzene series are the major gaseous products, and small molecular structures are the main components of aliphatic hydrocarbons. According to experimental studies, some progress has been made in understanding the characteristics of pyrolysis products and the chemical reaction mechanism of lignite pyrolysis. However, it is quite difficult to understand the complicated thermal decomposition reactions in depth through experimental methods alone because of the heterogeneity of lignite and the complexity of the pyrolysis process. Furthermore, vast free radicals are generated in an exceedingly short time, and these are difficult to detect in laboratories [14]. Undoubtedly, computational approaches would provide a promising platform for further researching the lignite pyrolysis mechanism.
Coal, which is the principal traditional source of energy, occupies a dominant position in world energy consumption, especially in China [1]. With the amount consumption of high rank coal, lignite has attracted considerable interest due to its vast reserves, low selling price, high chemical reactivity, and low pollution-forming impurities [2,3]. However, self-defection (e.g., low calorific value, high moisture content, and a high tendency of spontaneous combustion) makes it unfit for direct utilization [4]. Upgrading lignite to higher quality products by means of pyrolysis is considered to be a promising way to achieve economically efficient utilization of lignite [5,6]. Solid residues (coke or char) with high calorific value can be utilized for direct combustion or preparation of slurry fuels [7]. Pyrolysis gases contain large amounts of combustible gases, such as H2, CO, and hydrocarbon gas [8]. Tars produced from lignite pyrolysis are suitable for liquid fuels and valuable chemicals [9]. Numerous efforts have been devoted to study the pyrolysis behavior of lignite. Liu et al. [10] investigated the interrelation between lignite structural characteristics and product distributions. They concluded that the formation of pyrolysis products is related to the breakage of
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Corresponding authors. E-mail addresses: [email protected] (H. Liu), [email protected] (Q. Wang).
https://doi.org/10.1016/j.fuproc.2019.106147 Received 22 March 2019; Received in revised form 9 July 2019; Accepted 9 July 2019 0378-3820/ © 2019 Elsevier B.V. All rights reserved.
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Fig. 1. Two-dimensional (2D) unimolecular structural model for Huolinhe lignite.
bituminous coal and subbituminous coal, the molecular structure of lignite contains more oxygen-containing functional groups, aliphatic side chains, and single aromatic ring structures [28]. Published studies on the pyrolysis behavior of lignite are rare [29]. Further research is needed to better understand the complex pyrolysis process, especially the relationship between pyrolysis products and lignite structure. The main purpose of this work is to explore the pyrolysis behavior of lignite using ReaxFF-MD simulations. The paper is organized as follows: First, the characteristics of pyrolysis products, especially tar and gas, are analyzed in detail. Second, the transformation behavior of carbon, hydrogen, and oxygen during the pyrolysis process are revealed. Finally, two typical temperatures (2000 K and 3000 K) are selected for an in-depth investigation of the pyrolysis reaction mechanism. The elucidation of these problems may provide more constructive information of high-efficiency clean technology for converting lignite.
Quantum chemistry (QC) modeling is applicable for studying chemical reactions with high accuracy. However, QC-based methods are computationally expensive and intensive, thus they have rarely been applied in models with more than 100 atoms [15]. Classical molecular dynamics (MD) has the ability to model molecular systems that contain more than 10,000 atoms, but it is not suitable for exploring the cleavage and production of bonds [16]. Fortunately, the Reactive Force Field (ReaxFF), introduced by van Duin et al., can address the chemical interactions of atoms and molecules [17]. ReaxFF-MD combines the advantages of MD and ReaxFF, which can be used to study the chemical reactions of large and complicated molecular systems. Additionally, ReaxFF-MD has been proven to maintain basically the calculation precision of QC but with much reduced computational costs [18,19]. In the last decade, ReaxFF-MD has been successfully used to simulate the combustion and pyrolysis characteristics of complex compounds, such as coal [20], biomass [21], oil shale kerogen [22] and char [23], with the goal of trying to understand the reaction process in depth. Zhan et al. [24] investigated the reaction mechanism of subbituminous coal via ReaxFF-MD. The calculation results indicated that the pyrolysis process is initiated by breaking unstable CeC and CeO bonds, which is followed by intramolecular hydrogen transformation. Castro-Marcano et al. [25] adopted ReaxFF-MD to study the combustion processes of coal char. They found that aromatic structures are more liable to oxidize and combust under fuel lean and higher temperature conditions. Zheng et al. [26] employed ReaxFF-MD to analyze the overall reaction stages for Liulin bituminous coal. The simulation results showed that the pyrolysis processes are roughly divided into four stages according to the breakage behavior of chemical bonds. These four stages are the activation stage, primary pyrolysis stage, secondary pyrolysis stage, and recombination dominated stage. Chen et al. [27] studied the pyrolysis and combustion behavior of biomass using ReaxFF-MD. The simulation results indicated that environment and temperature play a significant influence on the dissociation of chemical bonds and product distributions. Extensive studies of the chemical reaction mechanism using ReaxFF-MD simulations have been reported. Nevertheless, coal type is a critical factor that greatly affects the pyrolysis process. Compared with the structural features of
2. Computational details ReaxFF [17] is an empirical bond-order-based reactive force field and can explicitly describe chemical reactions within complex systems. The relationship between bond order and bond energy plays an important role in ReaxFF. Bond order is got from interatomic distances and is updated continually at every iteration, thus ensuring connectivity changes. The bonded interactions (i.e., bonds, angles, and torsions) are bond-order dependent so that energies related to these terms disappear upon bond dissociation. Moreover, ReaxFF includes non-bonded interactions (van der Waals and Coulomb), which play significant roles in predicting structures and properties [30]. ReaxFF can be utilized in large molecular models containing ~10,000 atoms with high calculation precision close to density functional theory (DFT). Nevertheless, the time scale of ReaxFF is at least 100 times faster than that of DFT. Further detailed information about ReaxFF can be found in the ReaxFF User Manual written by van Duin. A Huolinhe lignite model [31] with the molecular formula C201H195O32N3S1 was constructed previously by our group and was utilized as the initial molecular structure model (Fig. 1). As seen clearly in Fig. 1, the Huolinhe lignite molecular model contains abundant 2
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oxygen-containing functional groups, such as carboxyl, carbonyl, hydroxyl. In addition, a large number of aliphatic side chains and double aromatic rings are typical structural characteristics of Huolinhe lignite. Geometry optimization of the model was obtained using molecular mechanics (MM) and MD simulations [32]. Subsequently, 10 optimized unimolecular models were placed in a periodic box of 81.0 × 81.0 × 81.0 Å3 using the construction function of the MS Amorphous Cell module. The initial density of the macromolecule was set as 0.10 g/cm3 to avoid overlapping of the important functional groups. In this work, all ReaxFF-MD simulations were implemented in the “reaxc” package of LAMMPS. First, energy minimization was performed at 50 K for 20 ps. Then, the system was maintained in a Berendsen thermostat to perform an NVT-MD simulation. To obtain a proper density of lignite, the system was compressed to a density of 0.966 g/ cm3 at 300 K using a constant number, pressure, and temperature (NPT) ensemble at a pressure of 10 MPa. Next, a no-reaction NVT-MD simulation was performed at 300 K to relax the system. In order to avoid any reaction occurrence, the CeO and OeH bond parameters were switched off in the relaxation process. Finally, a set of ReaxFF-MD constant temperature simulations employing the NVT ensemble were carried out at 1600–3000 K for 250 ps with an interval of 200 K. An initial simulation temperature of 1600 K was chosen because nearly no thermal decomposition reaction occurred before that. The velocityVerlet algorithm was applied to integrate Newton's equation of motion with a time step of 0.1 fs, and the temperature was controlled by a damping constant of 100 fs. Molecular species were identified by a bond-order cutoff of 0.3. The parameters of ReaxFF utilized are proposed by Mattsson et al. [33] and have been successfully used to investigate the pyrolysis process of solid fuels [26,34]. Due to the limitation on computing capacity, the simulation time scale (on the order of picoseconds) is far less than the reaction time in experiments (on the order of seconds). Based on Arrhenius equation, increasing the reaction temperature greatly rises the reaction rate, especially for the reaction with higher energy barrier. Therefore, higher temperatures were usually used in previous ReaxFF-MD simulations [35,36]. Despite obvious differences between simulation and experiment in the temperature and time, product distributions and reaction kinetics parameters obtained by simulations show good consistent with the experimental results [22,37,38]. Zheng et al. studied the product evolution and reaction mechanisms of the Liulin coal by ReaxFF MD at 1000–2600 K. They found that the evolution tendencies of naphthalene, methylnaphthalene and dimethyl-naphthalene are in accord with the results of Py-GC/MS [37]. Wang et al. investigated the pyrolysis and combustion of n-dodecane, and found that the activation energies and pre-exponential factors obtained from ReaxFF-MD at high temperatures are in agreement with the experimental values [38].
Fig. 2. Distribution of pyrolysis products in ReaxFF-MD simulations at 1600–3000 K.
reached a maximum value of 29.20% at 3000 K. Remarkably, the yield of organic gas was much higher than that of inorganic gas in the whole simulation temperature range. CO2 and H2O were the predominant inorganic gases, and C2H4 was the major organic gas. The production of main pyrolysis gases (CO2, H2O and C2H4) is closely related to the structural characteristics of Huolinhe lignite, that will be illustrated in detail below. The content of tar first increased with the temperature and reached the peak at 2200 K. In contrast, char content decreased continuously to the minimum production at 2200 K and then increased with the temperature. It can be inferred that 2200 K is very probably the transition temperature of different pyrolysis mechanisms. The snapshots of pyrolysis products intermediate configurations (at 1600 K, 2200 K, and 3000 K) obtained from VMD are displayed in Fig. 3. At the lower temperatures (1600–2200 K), the primary pyrolysis reactions were dominated by the breakage of bridge bonds and the dissociation of the macromolecular network structure, leading to the production of amounts of tar and the decomposition of char [41]. With further increasing temperature from 2200 to 3000 K, the remarkable reduction in the tar yield (46.79%) was accompanied by an increase in the char yield (30.36%). These phenomena suggest that secondary pyrolysis reactions become pronounced at higher temperatures (2200−3000 K). Further cracking of tar, rearrangement, and condensation of aromatic structures occur, contributing to the formation of char along with a reduction of tar yield. The simulation results of product distributions are consistent with previous experimental researches reported by Meng et al. [12] and Zhou et al. [42] They concluded that the gas content increased monotonically as the temperature increasing, but the tar yield first increased and then decreased. Therefore, if it is desirable to get tar as much as possible, a moderate pyrolysis temperature should be selected. Because tar is easy to transport, store, and process, it is considered to be the most desirable pyrolysis product of lignite [43]. Analyzing tar evolution behavior during pyrolysis process is of great significance. Fig. 4 shows that the distributions of heavy tar and light tar at 1600–3000 K for 250 ps. As displayed in Fig. 4a, the yields of heavy tar and light tar have similar evolution trends over the entire simulation temperature range. When the temperature was below 2200 K, the content of heavy tar increased from 4.27% to 47.21%, whereas the yield of light tar presented an increasing trend from 1.06% to 20.50%. The weight percentage of heavy tar was obviously higher than that of light tar at each simulation temperature, especially at 2200 K. Nevertheless, as clearly seen from Fig. 4b, the quantity of light tar fragments was close to that of heavy tar at 1600–2200 K. When the temperature is higher than 2600 K, the quantity of light tar is even higher than that of heavy tar. It can be speculated that lignite pyrolysis tar contains a lot of
3. Results and discussion 3.1. Analysis of lignite pyrolysis products Undoubtedly, temperature is a critical factor that affects the generation and evolution of pyrolysis products. To evaluate the influence of temperature on product distributions, a set of ReaxFF-MD constant temperature simulations were conducted at temperatures 1600–3000 K for 250 ps. To be consistent with previous work [39,40], pyrolysis products from ReaxFF-MD simulations are classed as five types: inorganic gas, organic gas, heavy tar, light tar, and char. Inorganic small molecules (e.g., H2, CO2, and H2O) are regarded as inorganic gas; fragments of C1-C4 are regarded as organic gas; fragments of C5-C13 and C14-C40 with molecular weight of 80–600 amu are light tar and heavy tar, respectively. C40+ compounds with molecular weight greater than 600 amu are considered to be char. As shown in Fig. 2, the yield of gas (inorganic gas and organic gas) kept increasing with an increase in temperature. Gas production 3
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Fig. 3. Snapshots of pyrolysis products intermediate configurations obtained from VMD: (a) configuration at 1600 K, (b) configuration at 2200 K, (d) configuration at 3000 K.
number of heavy tar and light tar fragments both declined obviously with increasing temperature. However, the opposite trend was observed for the number of gas molecules, which increased from 121 to 318. That is to say, further decomposition of tar compounds occurred, resulting in an increase of gas yield at 2200–3000 K. In addition, the quantity of char fragments fluctuated slightly with increasing temperature, but the average molecular weight of char increased greatly, especially at 2800 K and 3000 K (see Fig. 5). It means that the condensation of aromatic structures (tar compounds and char fragments) occur, giving rise to the growth of char fragments. Moreover, the higher the temperature is, the more pronounced the secondary pyrolysis reactions will be. The simulation results are in accord with the experimental studies of Song et al. [45] and Hu et al. [46], who found that volatile-char interactions play an important role during pyrolysis process. Tar can be converted into gas by cracking reactions or translated into char by condensation reactions. The evolution of typical gas products of lignite at different temperatures is presented in Fig. 6. H2O is the main pyrolysis gas of lignite,
light tar, and the difference in the weight percentage between heavy tar and light tar is greatly due to the molecular weight of the fragments. The formation of light tar is attributed to large amounts of monocyclic and bicyclic structures in Huolinhe lignite. As described above, the primary pyrolysis reactions were predominant at 1600–2200 K. The formation of tar was mainly because of the breakage of bridge bonds. According to species analysis, only 8 tar fragments (3C14H10O3, 3C14H12O3, C11H9O, and C13H10O) were observed at 1600 K, which indicated that only weaker chemical bonds (e.g., Cal-Cal, CaleH, CaleO) are liable to fracture at low temperature. Nevertheless, the quantity of tar fragments increased remarkably and achieved a maximum value of 90 at 2200 K, as shown in Fig. 5. A large quantity of tar fragments were produced at relatively high temperature, and this may be associated with the fact that higher temperature can provide more energy to break stronger chemical bonds linked with aromatic structures [44]. With a further increase in temperature from 2200 to 3000 K, the yield of heavy tar dropped significantly from 47.21% to 12.38%, accompanied by a 19.95% reduction of light tar. Furthermore, the 4
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Fig. 6. Evolution of typical gas product with the temperature in ReaxFF-MD simulations at 1600–3000 K.
the yield increased sharply when the temperature was higher than 2000 K, and number of H2O reached as high as 90 at 3000 K. This is mainly due to the existence of a large number of hydroxyl groups in Huolinhe lignite, which react with hydrogen radicals to produce H2O during pyrolysis. Because the decomposition of carboxyl groups is easier at lower temperature, CO2 first appeared at 1600 K. The amount of CO2 continued to increase at 1600–3000 K, which means that high temperature is in favor of producing CO2. Large amounts of C2H4 were generated at 1800 K, and the yield increased rapidly before 2600 K. The formation of C2H4 is attributed to the breakage of weak bonds (Cal-Cal) [47], which is coincident with the high content of methylene carbon in the molecular structure of Huolinhe lignite. It is interesting that the reduction of C2H4 was approximately equivalent to the increase of C2H2, which demonstrates that parts of C2H4 probably were converted to C2H2 with further increasing temperature from 2600 to 3000 K [22]. H2 and CH4 were generated at 2000 K, and their yields raised as a function of temperature. It must be mentioned that the amount of H2 was extraordinarily high at 2800 K and 3000 K. This indicates that high temperature (2800 K and 3000 K) causes the condensation reaction of aromatic structures to be more pronounced, which leads to the production of a high concentration of hydrogen radicals and the release of H2. As the final gas product, C2H2 was generated at 2200 K, and the yield was relatively lower at temperatures between 2200 K and 2600 K. The remarkable increase in C2H2 was related to the dehydrogenation reaction of C2H4 at the higher temperature. The evolution trend of typical gas products is consistent with previous experimental studies [48,49].
Fig. 4. Distributions of tar (heavy tar and light tar) fragments in ReaxFF-MD simulations at 1600–3000 K: (a) weight percentage and (b) number of molecules.
3.2. Transformation behavior of carbon, hydrogen and oxygen Carbon (C), hydrogen (H) and oxygen (O) are the most significant chemical elements. Studying the transformation behavior of C, H, and O during the pyrolysis process is important for gaining an in-depth understanding of the thermal decomposition mechanism. Fig. 7 gives an overview of the distributions of the main elements (C, H, and O) at different temperatures. As seen in Fig. 7a, the content of char-C greatly decreased from 93.73% at 1600 K to 20.05% at 2200 K, along with the increase trends of tar-C (62.58%) and gas-C (10.10%). That is to say, increasing the temperature facilitated carbon transferring from char into tar and gas at temperatures 1600–2200 K, especially tar. At 2200 K, char-C content reached a minimum value of 20.05%, whereas the content of tar-C increased to a maximum value of 68.96%. The significant decrease in char-C content was associated with the cleavage of chemical bonds containing carbon (e.g., CaleH, Cal-Cal and Car-Cal). Meanwhile, the increase in tar-C content is due to the production of
Fig. 5. Number of pyrolysis products fragments and average molecular weight of char in ReaxFF-MD simulations at 1600–3000 K.
5
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Fig. 8. Time evolution of pyrolysis products in ReaxFF-MD simulations: (a) 2000 K and (b) 3000 K.
speculated that the major proportion of carbon migrated to char by means of condensation reactions of aromatic structures. Only a small fraction of the carbon transferred to gas via further decomposition of tar fragments. The evolution trend of carbon is consistent with the above pyrolysis mechanism. As displayed in Fig. 7b, the char-H yield greatly decreased with increasing temperature, whereas the yield of tar-H and gas-H showed the opposite trend below 2200 K. At temperatures between 2200 K and 3000 K, the apparent decline of tar-H content from 63.23% to 19.33% was accompanied by an increase in char-H content (23.59%) and gas-H (20.31%). Notably, the content of gas-H was higher than that of tar-H in the whole 2200–3000 K temperature range. This means that, unlike carbon, most hydrogen migrated into gas phase at high temperature. This phenomenon is consistent with the above studies of tar pyrolysis behavior, specifically, high temperature caused the condensation reactions of char fragments to be more pronounced, leading to the formation of a high concentration of hydrogen radicals and the increase of gaseH. Gas-H existed mainly in the forms of H2, H2O, C2H4, CH4, and so on. Fig. 7c shows that the yield of char-O decreased from 90.63% at 1600 K to 18.75% at 2200 K, along with an increase in the yield of tar-O (53.69%) and gas-O (17.19%). Nevertheless, oxygen in tar significantly reduced the higher heating value (HHV) of fuels and increased their reactivity, resulting in storage difficulties [50]. Therefore, it is suitable
Fig. 7. Main element distributions in pyrolysis products in ReaxFF-MD simulations at 1600–3000 K: (a) carbon, (b) hydrogen, and (c) oxygen.
large amounts of tar. When further increasing temperature from 2200 to 3000 K, the yield of tar-C presented a linear decrease along with an increase in char-C and gaseC. Compare to 2200 K, the remarkable decrease in the tar-C (45.92%) was accompanied by the increasing tendency of char-C (38.01%) and gas-C (7.91%) at 3000 K. It can be 6
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②
④
④
ĺ
③
ķ
④
③ ③
Fig. 9. Initial pyrolysis mechanism of lignite molecular structure in ReaxFF-MD simulations at 2000 K.
(a) bond1
+
CH3·
+
H·
bond1 bond 2
b)
bond1
bond2
Fig. 10. Formation pathways of typical products C13H13ON, C12H8O4, and CO2.
7
+
CO2
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4. Conclusions
to minimize the content of oxygen in tar. With a further increase in temperature from 2200 to 3000 K, the reduction of tar-O was nearly equivalent to the increase of gaseO. This indicates that temperature has a slight influence on char-O content. In addition, the content of gas-O was extremely high at 3000 K, and this was associated with the production of large amounts of CO2 and H2O. The results are in accord with the evolution trend of the typical gases mentioned above.
In this work, the initial pyrolysis mechanism of lignite was investigated using ReaxFF-MD constant temperature simulations at 1600–3000 K for 250 ps. The results show that lignite pyrolysis process begins with the dissociation of the weaker bridge bonds such as Cal-O and Cal-Cal. Moreover, increasing temperature is favorable for the decomposition of macromolecular structure and the breakage of bridge bonds at the lower temperatures (1600–2200 K). At relatively high temperatures (2200–3000 K), the secondary pyrolysis reactions become more pronounced. The opposite evolution tendency of tar and gas indicates that further decomposition of tar fragments into pyrolysis gas are carried out. The remarkable increase of char content and generation of large amounts of H2 are related to the condensation of aromatic structures. Analysis of migration behavior of the main elements showed that the major proportion of carbon migrated to char and most hydrogen migrated into gas phase by means of condensation reactions at high temperatures (2200–3000 K), which is consistent with the above pyrolysis mechanism. H2O, CO2, and C2H4 are the main gas products, and the formation is closely related to the structural characteristics of lignite. Light tar is the main component of pyrolysis tar, which is due to large amounts of monocyclic and bicyclic structures in lignite. This work may aid in understanding the pyrolysis mechanism of lignite and provide theoretical basis for clean coal technologies.
3.3. Analysis of pyrolysis reaction mechanism To study the influence of pyrolysis temperature on reaction mechanism more clearly, it is desirable to focus on the specific pyrolysis temperature. According to the results of product distributions, 2200 K was most probably the transition temperature of different mechanisms. As the highest temperature of the two reaction mechanisms, 2000 K and 3000 K were selected to research the complex chemical reaction mechanism of lignite pyrolysis. Fig. 8 demonstrates products profile tendency with time at 2000 K and 3000 K. The yields of char were very close, but the yields of gas and tar had a great difference at 250 ps. It is supposed that the reaction mechanism of lignite pyrolysis at lower temperatures (below 2200 K) is different from that at higher temperatures (above 2200 K). At 2000 K, gas content slightly raised and tar yield increased obviously by 37.66%, whereas the yield of char decreased greatly by 46.57% over the initial 200 ps. Thereafter, the yields of products (char, tar, and gas) almost remained a constant value, which means the main reaction process was completed within 200 ps. With the aid of simulation trajectories and species analysis, deep insight into the preliminary thermal decomposition mechanism at lower temperatures is possible. Because of a relatively lower dissociation energy and smaller bond order of the Cal-O bond, the lignite pyrolysis process was started by the breakage of the alkyl ether bridge bond (labeled “①”), as shown in Fig. 9. The release of tar species C14H9O3 is the best proof. Breakage of the Cal-Cal bond (labeled “②”) in the lignite molecular structure then occurred, leading to the formation of CH2O. The simulation results are coincident with previous studies that the aliphatic and ether bridge bonds are usually the first breaking sites during the primary thermolysis reactions [26,51]. Decomposition of carboxyl functional groups (labeled “③”) contributed to the formation of CO2. Very early production of CO2 during the pyrolysis process was also found in the experiments by Feng et al. [52]. Subsequently, a large quantity of Cal-Cal and Cal-O (labeled “④”) broke down, generating many pyrolysis products such as C7H5O2, C10H9ON, and C4H5N. At 3000 K, the pyrolysis reactions are approximately grouped into three major stages according to product distributions. The first stage (Stage-I) was from the beginning to 10 ps, as the primary pyrolysis reactions stage. The molecular structure of lignite quickly underwent thermal decomposition, and thereby generated numerous product fragments. Because a temperature of 3000 K is extremely high, enough energy is provided to break the weak bonds of Cal-O and Cal-Cal simultaneously, and 41 fragments were observed as soon as possible. Fig. 10a shows an example of the formation pathway of C13OH13N, in which it is clear that the Cal-O bond (denoted as bond 1) and the Cal-Cal bond (denoted as bond 2) dissociated, resulting in the production of CH3·, C13H13ON, and C12H8O4. Then, CO2 was generated via the decarboxylation reaction of C12H8O4, as shown in Fig. 10b. The interval from 10 to 200 ps was taken as the second stage (Stage-II), which was dominated by the secondary pyrolysis reactions. The yield of tar decreased continuously with an increase in char, which was associated with the condensation reactions of tar compounds and char fragments. The yield of gas increased first and nearly remained a constant value after 100 ps. It is supposed that the cracking reactions of tar compounds were more pronounced during 10–100 ps. The last stage (Stage-III) was from 200 to 250 ps, as the equilibration stage. The weight percentages of char, tar, and gas remained constant at this stage. This also means that using 250 ps as the simulation time is suitable for studying the reaction mechanism of lignite pyrolysis.
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