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Reactive force field molecular dynamics (ReaxFF MD) simulation of coal oxy-fuel combustion
PTEC-14556; No of Pages 12 Powder Technology xxx (2019) xxx
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Reactive force field molecular dynamics (ReaxFF MD) simulation of coal oxy-fuel combustion Yu Qiu a,b, Wenqi Zhong a,b,⁎, Yingjuan Shao a,b, Aibing Yu b,c a b c
Key Laboratory of Energy Thermal Conversion and Control of Ministry of Education, School of Energy and Environment, Southeast University, Nanjing 210096, China Centre for Simulation and Modelling of Particulate Systems, Southeast University - Monash University Joint Research School, Suzhou 215123, China ARC Research Hub for Computational Particle Technology, Department of Chemical Engineering, Monash University, Clayton, Vic 3800, Australia
a r t i c l e
i n f o
Article history: Received 14 April 2019 Received in revised form 27 July 2019 Accepted 30 July 2019 Available online xxxx Keywords: Oxy-fuel combustion Reaction pathways Molecular dynamics ReaxFF
a b s t r a c t Understanding the reaction mechanism of oxy-coal combustion is fundamentally important to the CO2 capture and storage in coal-fired industrial processes. In this paper, the microscopic reactive behaviors of brown coal combustion in O2/CO2 atmosphere were simulated by combining the molecular dynamics (MD) simulations and the reactive force field (ReaxFF). With thoroughly tracing the motion trajectories of atoms, as well as the formation, transition and breaking of bonds between atoms, the generation pathways of CO2 and the effects of temperature (1600–2000 K) and oxygen concentration (21%~30%) on the coal oxy-fuel combustion process were studied. Results showed that the main process of CO2 generation consists of the decomposition, oxygenation, and dehydrogenation. The atmosphere with a high concentration of CO2 can accelerate the reaction rate of coal combustion and reduce the consumption of O2. The higher temperature will promote the production of major intermediates with more fragments being released earlier, and finally increase the generation of CO2. The increase of O2 concentration obviously hastens the decomposition of coal molecule and the generation of H2O, but shows very weak influence on the distributions of other major products and intermediates. © 2019 Elsevier B.V. All rights reserved.
1. Introduction Oxy-fuel combustion as one of the most promising technologies to capture CO2 is attracting increasing attention in coal-fired industrial processes [1–4]. In this technology, a mixture of O2 and recycled flue gas (mainly CO2) is used as the oxidant instead of O2/N2 to obtain the glue gas with high concentration of CO2 (usually N90 vol%), which is convenient for CO2 separation and storage at the end of the combustion process [5,6]. Researchers around the world have been doing a variety of studies on theoretical analysis, experiments and numerical simulation to develop the coal oxy-fuel combustion and a wealth of macroscopic reaction information and rules, for example, the ignition and flame behaviors [7–12], pollutant emission characteristics [13–15], effects of the atmosphere on the combustion [16–19] and so on have been comprehensively figured out. However, up to now the understanding on the microscopic reaction behaviors of coal oxy-fuel combustion is very insufficient. Many fundamental and interesting mechanisms are still not clear, including the migration characteristics of carbon between coal and CO2, the generation pathways of CO2 and pollutants in O2/ CO2 atmosphere and so on. Clarifying these essential mechanisms is of
⁎ Corresponding author at: Sipailou 2#, Nanjing 210096, Jiangsu, China. E-mail address: [email protected] (W. Zhong).
significant importance to develop and improve the oxy-coal combustion technology. Reactive Molecular Dynamics simulation has been proved very effective to analyze the detailed mechanism of chemical reactions in the atomistic level. The MD method can display the dynamic behavior of atoms or molecules on the computer screen, so as to understand the evolution of the system intuitively. By calculating the interaction between the atoms with Newtonian equation of motion [20], obtaining atoms' trajectories and describing the formation, transition and breaking of chemical bonds [21,22], the Molecular Dynamics method based on ReaxFF can efficiently simulate the molecular behaviors in chemical reaction in large-scale system with the high accuracy close to quantum chemical method at the much lower computational cost. This ReaxFF molecular dynamics (ReaxFF MD) simulation method has been widely used in coal combustion, oxidation and pyrolysis process to obtain the mechanism of chemical reactions and pollutant characteristics [23–28]. Castro-Marcano et al. [29] investigated the coal char combustion by constructing a large scale model of Illinois No.6 coal char and found that the initial oxidation process of char included the thermal decomposition of char structure and dehydrogenation reaction. Chen et al. [30] found the initial reaction of spontaneous combustion of lignite coal was the formation of ∙HO2 radicals caused by the hydrogen abstraction reaction, which was the main reason for the generation of H2O2 and H2O molecules. Bhoi et al. [31] compared the potential energy in the spontaneous
https://doi.org/10.1016/j.powtec.2019.07.103 0032-5910/© 2019 Elsevier B.V. All rights reserved.
Please cite this article as: Y. Qiu, W. Zhong, Y. Shao, et al., Reactive force field molecular dynamics (ReaxFF MD) simulation of coal oxy-fuel combustion, Powder Technol., https://doi.org/10.1016/j.powtec.2019.07.103
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Fig. 1. Brown coal model (C39H37NO10S): (a) molecular structure and (b) optimized geometry model with oxygen-containing and other reactive functional groups.
combustion and pyrolysis process of brown coal under different conditions and found that the initial reaction of the combustion process was thermal decomposition, forming small fragments. Yan et al. [32] simulated the oxidation of brown coal at high temperature with ReaxFF MD simulations and found that the initial reaction of the oxidation was the hydrogen abstraction reaction by oxidants or the decomposition of coal molecule. Li et al. [33,34] studied the influences of O2 on the mechanism of N transfer and S transfer and the results showed that O2 reacted with intermediates to form HNO molecules, HNCO molecules, and sulfur oxide intermediates, which were then oxidized to NO and sulfur-containing gas molecules, respectively. Zheng et al. used Liulin coal model to study the influence of model scale on the pyrolysis [35] and found that the reaction diversity of coal pyrolysis was easier to be obtained in larger coal model system. Zheng et al. [36] revealed the four stages of Liulin coal pyrolysis, namely the activation stage, the primary pyrolysis stage, the second pyrolysis stage, and the recombination dominated stage. Zheng and his team also used GPU enabled ReaxFF MD (GMD-Reax) program [37–39] to significantly improve the computing performance and developed visualization and analysis of reactive molecular dynamics (VARMD) [38] to analyze the reaction of the trajectories from ReaxFF MD, which is based on cheminformatics analysis. However, previous MD studies mainly focused on coal combustion or oxidation in pure oxygen or coal pyrolysis. Up to now, few studies have been conducted on the microscopic reaction behaviors of coal combustion in O2/CO2 atmosphere, where the high concentration of CO2 would significantly affect the combustion process and CO2 would react with the coal molecule. In this study, the ReaxFF MD simulation was applied to study the microscopic molecular reaction characteristics of coal combustion in O2/CO2 atmosphere, with the emphasis on exploring the generation pathways of CO2 and the effects of operating conditions. Coal combustion in pure O2 and O2/N2 were also conducted for
comparison. Snapshots of the reactions, including the structural transformation and the dissociation and formation of the chemical bond were displayed by using the visualization software to analyze the atomic trajectories and thus to obtain the generation mechanism of CO2 and the distribution of major intermediates and products.
2. Computational methods 2.1. ReaxFF MD models In the MD simulation scheme, the movement of the atom is determined by the second Newton's law of motion, with which the potential energy (Eqs. (1)–(2)) and the force (Eq. (3)) on the molecule are first calculated at each time step. U ¼ U VDW þ U int
U VDW ¼ u12 þ u13 þ ⋯ þ u1n þ u23 þ u24 ⋯ ¼
ð1Þ
n −1 X
n X
uij r ij
ð2Þ
i¼1 j¼iþ1
where U is the total potential energy of the system, consisting both of Van der Waals interactions between atoms and intramolecular potential energy, UVDW is the potential energy of Van der Waals interactions, and Uint is the intramolecular potential energy, which is the sum of various types of potential energy (e.g. bond stretching, angle bending, and torsion angle…). uij is the potential energy of Van der Waals interactions between i atom and j atom, rij is the radius between i atom and j atom,
Table 1 Functional groups and major intermediates/products.
Please cite this article as: Y. Qiu, W. Zhong, Y. Shao, et al., Reactive force field molecular dynamics (ReaxFF MD) simulation of coal oxy-fuel combustion, Powder Technol., https://doi.org/10.1016/j.powtec.2019.07.103
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Fig. 3. The reaction pathway of C atom in –COOH to generate CO2: intramolecular structural reorganization and hydrogen abstraction by O2.
broken with all the forces and potential energies related to bond order being zero. On the other hand, Van der Waals force and Coulomb force between atoms are also taken into consideration and are calculated by Electronegativity Equilibration Method (EEM) or charging equilibration (QEq) methods. As ReaxFF MD simulation is driven by the potential energy of the system, there is no necessary to pre-define the reaction path, which makes the ReaxFF widely applicable for most systems with complicated chemical reactions, such as coal combustion, oxidation or pyrolysis. The energy function of the system takes the following form [21]: Esystem ¼ Ebond þ Elp þ Eover þ Eunder þ Eval þ Epen þ Etors þ Econj þ EvdWaals þ ECoulomb
Fig. 2. Reaction system of coal molecule in O2/CO2 atmosphere (Vol. 21% O2) with periodic boundary.
n is the number of atoms. * ∂ * ∂ * ∂ * þ j þk U F i ¼ −∇i U ¼ − i ∂xi ∂yi ∂zi
ð3Þ
*
where F i is the force of an atom, ∇ is Hamiltonian. Then, the acceleration, position and velocity of each atom are obtained by Eqs. (4)–(6), respectively. *
*
ai ¼ *
Fi mi *
ð4Þ *
vi ¼ v0i þ ai δt * * r i ¼ r 0i
*
þ v0i δt þ
ð5Þ 1* 2 a δt 2 i
where Esystem is the total energy of the system. Ebond is the bond energy. Elp is the energy of long pair electrons. Eover and Eunder are the correction term for coordination energy. Eval, Epen, Etors, Econj, EvdWaals and ECoulomb represent the valence angle term, penalty energy, torsion energy, conjugation effects to molecular energy, non-bonded Van der Waals interaction and Coulomb interaction, respectively [22]. h pbe;2 i ππ −Dπe BOπij −Dππ Ebond ¼ −Dσe BOσij exp pbe;1 1− BOσij e BOij ð8Þ where Dσe , Dπe , and Dππ e are the force field parameters of single, double and triple bond related to bond energy, pbe, 1 and pbe, 2 are temporary parameters, BO'ij is the bond order between two atoms, and the original bond order can be expressed as pbo2 pbo4 0 r ij rij þ exp pbo3 π BOij ¼ BOσij þ BOπij þ BOππ ij ¼ exp pbo1 σ ro ro pbo6 r ij þ exp pbo5 ππ ro
ð9Þ
ð6Þ
* * where ai, mi, vi, and ri are the acceleration, mass, velocity, and position of * * an atom, respectively. v0i and r 0i are the initial velocity and initial position
of the atom. δt is time variation. Based on the above information at every time step, the trajectories of the atoms can be obtained. Reactive force field (ReaxFF) is a bond order-based force field developed by Van Duin and his co-workers [21]. In ReaxFF model, the relative distance between two atoms is used to define the bond order of them (Eq. (9)), which determines whether there are the covalent or hydrogen bonding interactions between the atom pair. Bond order is used for the recognition of molecular fragments in the chemical reaction, and its value increases with the relative distance and when it exceeds the set bond-order cutoff radius, the bond of the atom pair is considered to be
where pbo, 1~pbo, 6 are the empirical parameters regressed by ReaxFF force field, rσo , rπo, and rππ o are the equilibrium distance of single, double and triple bond, respectively. 2.2. Combustion simulations Coal has complex carbonaceous structures and N130 kinds of chemical structures have been proposed over the past 70 years [29,40,41]. In this study, a brown coal molecule as shown in Fig. 1a was chosen, which is a small-scale model. Although the diversity of chemical reactions cannot be well presented, the reaction pathways of CO2 can be very useful to obtained. The coal molecule was built with ChemSketch software and its geometry model was optimized on the Forcite Module in the Materials Studio (MS) with the sites of the functional groups being
Table 2 Construction and simulation conditions of the reaction system. Environment
O2
CO2
N2
Temperature/K
Size/Å3
Density (g/cm3)
100%O2 21%O2/79%N2 21%O2/79%CO2 25%O2/75%CO2 30%O2/70%CO2
46 46 46 55 66
0 0 173 164 153
0 173 0 0 0
1600–2000 1600–2000 1600–2000 1800 1800
22.949 × 22.949 × 22.949 33.887 × 33.887 × 33.887 37.851 × 37.851 × 37.851 37.711 × 37.711 × 37.711 37.539 × 37.539 × 37.539
0.3 0.3 0.3 0.3 0.3
Please cite this article as: Y. Qiu, W. Zhong, Y. Shao, et al., Reactive force field molecular dynamics (ReaxFF MD) simulation of coal oxy-fuel combustion, Powder Technol., https://doi.org/10.1016/j.powtec.2019.07.103
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Fig. 4. The reaction pathways of the C atom connected with\ \O\ \to generate C2O3 intermediate and CO2: (a) twice oxidation by O2 and HO2, and dissociation, (b) once oxidation by O2 and dissociation, (c) twice hydrogen abstraction, oxidation by OH and HO2, and dissociation, (d) breaking of two C\ \O bond.
highlighted in Fig. 1b. The specific form of functional groups of the above geometry structure was shown in Table 1. The above coal molecule and a certain amount of O2 and CO2 molecules was placed in a cubic box with all boundaries being periodic to avoid surface effects. Fig. 2 shows the two-dimensional view of the reactive system. There is only one coal molecule used in the system to demonstrate the evolution of the chemical reaction process more clearly. In this system, the density (ratio of the total mass of the coal molecule and gases in the system to the volume) of the initial system was
Fig. 5. The reaction pathways of the phenolic C atom connected with –OH to generate C2O3 intermediate and then CO2: (a) oxidation, intramolecular structural reorganization and decomposition, (b) hydrogen abstraction by O2, oxidation and hydrogen abstraction and oxidation and the decomposition.
set at a low level (0.5 g/cm3) to avoid the overlapping effect. During the reaction, the system was initially minimized with the NVE ensemble (namely the atom number, volume and energy being constant) for 500 ps at the temperature of 1000 K. Then, the specific operating conditions for reaction simulation were set in Large-scale Atomic/Molecular Massively Parallel Simulator (LAMMPS) software. In this step, the combustion process would be simulated with the constant atom number, volume and temperature conditions (the NVT ensemble) for 2 ns with the time step of 0.2 fs. The Berendsen thermostat was used to control the temperature with a 0.1 ps damping constant. The force field parameters for C, H, O, N, and S elements in the ReaxFF were directly utilized in this study [29]. The constant temperature MD simulations were performed on each system in the “reax” package in LAMMPS software. All the reaction conditions involved in this study were displayed in Table 2 with the reaction temperature varying from 1600 K to 2000 K and the volume concentration of O2 in O2/CO2 atmosphere between 21% and 30%, respectively. Some of the important structures during the reaction process, such as reactants, intermediates and products are also displayed in Table 1. A simulated temperature of 1600–2000 K was used in the ReaxFF MD simulations and such high temperature can accelerate the chemical reaction and simultaneously shorten the computing time in coal combustion process. This strategy is well grounded by the timetemperature superposition (TTS) theory [42] and can obtain a good agreement with experiments on simulating molecular motions [27,29,31].
Fig. 6. The reaction pathway of C atom in –CO to generate CO: oxidation by O2, hydrogen abstraction and decomposition.
Please cite this article as: Y. Qiu, W. Zhong, Y. Shao, et al., Reactive force field molecular dynamics (ReaxFF MD) simulation of coal oxy-fuel combustion, Powder Technol., https://doi.org/10.1016/j.powtec.2019.07.103
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Fig. 7. The reaction pathways of the C atom in –CH3 to generate CO2: (a) decomposition, stepwise dehydrogenation and oxidation, (b) hydrogen abstraction, dissociation and oxidation. Fig. 8. Reaction mechanism of the generation of CO2 and CO.
3. Results and discussion In the current simulation study, there are many back-and-forth reactions during the ReaxFF MD simulation, and if these reactions occur repeatedly in a very short time, they will not be considered as a reaction because that is an unstable state of energy vibration [33]. 3.1. Elementary reactions in the oxy-fuel combustion system One macro chemical reaction process contains a myriad of elementary reactions and to clarify these reactions is the foundation for the understanding of the mechanism of oxy-coal combustion. In the molecular structure of coal, C atom exists in different forms, and some are in aromatic ring and some are in functional groups, which makes them undergo different pathways to generate CO2. Because of a higher reactivity of the functional groups, the bonds between C atoms that connect with the functional groups and the main body of coal molecule (for example, the bond 1 and bond 2 in Fig. 1b) are easier to break. The coal molecule firstly decomposed into smaller molecules or fragments at the site of oxygen-containing moieties, as shown in Fig. 1b. These small fragments were released by the process of aromatic-ring opening or carbon skeleton fracture, and then were attacked by oxidants and finally dehydrogenated to formed C2O3, CO and CO2. There are four kinds of functional groups and one active methyl group in a coal molecule, and correspondingly five different reaction pathways were found to generate CO2 in the current simulation study. The detailed reaction processes are illustrated as follows. (1) Carboxylic group: –COOH There are two carboxyl groups (−COOH) in the coal structure and both of them finally decomposed to CO2 (Fig. 3). Previous studies have demonstrated that direct decarboxylation contributes to the formation
of CO2 in the process of pyrolysis and oxidation [32,43]. The reaction pathway obtained in this study shows that the generation process of CO2 was initialized by intermolecular structural reorganization of – COOH, forming one ether group and one carbonyl group. Further, the dehydrogenation reaction happened and caused the fragment to lose the H atom and to form the ∙H radical, which is an essential intermediate in the combustion process. Finally, the remaining structure decomposed to release the CO2 molecule. (2) Ether group:\\O\\ There are three ether groups (\\O\\) in this coal structure. One of them is connected to a methyl group (−CH3) and will be analyzed later. The four reaction pathways of the remaining two ether groups are shown in Fig. 4. One C\\O bond in the ether group dissociated, followed by the attack of the O2 molecule to form carbon-oxygen complex. Next, there were two ways to generate CO2. One was the bond-breaking with neighboring C under the existence of CO2, leading to the formation of carbon trioxide (C2O3) and then being oxidized to CO2. The other way was that the fragment was attacked by another O2 molecule, followed by intramolecular structural rearrangement and finally, the CO2 molecule was released by the cleavage of ether group. (3) Hydroxyl group: –OH The two hydroxyl groups (−OH) in the coal structure connected to the aromatic rings and were recognized as phenolic hydroxyl. The trajectories showed that the neighboring carbons of the two phenolic hydroxyl groups ultimately transformed to CO2 and CO, respectively (Fig. 5). First, one O2 molecule or ∙O radical attacked the neighboring C carbon and gradually replaced the hydroxyl group and formed C\\O bond. Next, the new structure decomposed into a small fragment,
Please cite this article as: Y. Qiu, W. Zhong, Y. Shao, et al., Reactive force field molecular dynamics (ReaxFF MD) simulation of coal oxy-fuel combustion, Powder Technol., https://doi.org/10.1016/j.powtec.2019.07.103
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Fig. 9. Distribution of major products at different temperatures of (a) 1600 K, (b) 1800 K and (c) 2000 K with pure O2.
which was attacked by the CO2 molecule to generate carbon trioxide (C2O3). Finally, the active C2O3 decomposed to generate CO2 and CO. (4) Carbonyl group: –CO There is one –CO group in the coal molecule, which first dissociated from the structure and then was attacked by the O2 molecule, generating CO2 (Fig. 6). The two C − C bonds in the carbonyl group broke and the newly generated small fragment was attacked by the O2 molecule to form the ester group (−COO−). Finally, CO2 was released after the break of the bond between the newly added O atom and the molecular structure. (5) Methyl group: –CH3 In the simulation process, both of the two carbon atoms in methyl groups were converted to CO, although they were in two different fragments. One methyl group was connected to an O atom to form the ether group, while the other was connected to an aromatic ring. The reaction pathways were shown in Fig. 7. The methyl group initially dissociated from the ether group and form –CH3 group, which was then stepwise oxidized and dehydrogenized to generate formaldehyde (CH2O), aldehyde group (−CHO) and CO.
In addition, there was a small difference in the reaction pathways at different simulated temperatures. Only at 2000 K, the methyl carbon could dissociate from the aromatic ring to form CO, while at lower simulation temperature, the methyl carbon still connected to the decomposed fragment. However, the methyl group connected to the ether bond was separated from the decomposed fragment at any temperature. The only difference was that, at 2000 K, the methyl carbon was finally transformed into CO, at 1800 K, the methyl group kept unchanged in the simulation and at 1600 K, even the C–O bond did not break. But at 1600 K, the methyl group was finally transformed into – CHO together with the neighboring O atom of the ether group. It can be concluded that temperature influenced the distribution and the generation pathway of products. Different reaction pathways and different products in the system indicated that combustion reaction could be accelerated by improving the temperature. Comparing the reaction processes of the methyl group connected to O atom at 1800 K and 2000 K, it was found that the small fragment experienced a complete reaction and finally formed CO2. Snapshots in Figs. 3–7 show the chemical reactions in coal oxy-fuel combustion with the processes of bond breaking and bond formation. The reaction pathways were demonstrated according to the types of the functional group in the coal molecule. Finally, a complete
Fig. 10. Time evolution of (a) Vol. O2% and (b) Vol. CO2% at different temperatures with pure O2.
Please cite this article as: Y. Qiu, W. Zhong, Y. Shao, et al., Reactive force field molecular dynamics (ReaxFF MD) simulation of coal oxy-fuel combustion, Powder Technol., https://doi.org/10.1016/j.powtec.2019.07.103
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Fig. 11. Distribution of major products at different temperatures of (a) 1600 K, (b) 1800 K and (c) 2000 K with 21%O2/79%N2.
mechanism of the generation of CO2 or CO was obtained based on these five microscopic reaction pathways (Fig. 8). The main generation mechanism can be concluded as the process: the decomposition of macromolecules into small fragments, the oxidation reactions of fragments and the dehydrogenation reaction to form CO2 or CO. 3.2. Comparison of coal combustion between pure O2, O2/N2 and O2/CO2 environment A series of ReaxFF MD simulations of coal combustion under O2 and O2/N2 conditions were also conducted for comparison at temperature ranging from 1600 K to 2000 K for 2 ns, with the purpose of underlining the effect of high concentration of CO2 in oxy-fuel combustion atmosphere. 3.2.1. Simulation of coal combustion in pure O2 Simulations of coal combustion in pure O 2 atmosphere with stoichiometric composition at temperature range of 1600–2000 K were performed. Fig. 9 shows the time evolution of major intermediates and combustion products (H2 O, ∙CHO, ∙HO 2 , ∙OH, C 2 O 3 ,
and CO2) formed in the combustion process at different temperatures. C-CO 2 represented the newly-generated CO 2 from coal molecule. It can be seen that the generation of CO2 and H2O molecules was promoted with the temperature. During brown coal combustion process, ∙HO 2 radical was found to be a key intermediate but at high temperature the formation of ∙HO 2 radical was unstable and the number decreased slightly. During the combustion process, a small amount of some important intermediates were also generated, like ∙CHO and ∙OH radicals. The existence of C 2O3 was greatly influenced by the temperature, and the formation of C 2O 3 was favored at a higher temperature. The time evolutions of the O2 and CO2 concentration with temperature were displayed in Fig. 10. The adequate oxygen present in the system took part in the oxidation reaction and decomposed the coal molecule easily. From Fig. 10, it was observed that the concentration of CO2 increased with elevated temperature whereas the concentration of O2 decreased. At the initial stage of simulation (from 250 ps to 500 ps), both the decrease of O2 concentration and the increase of CO2 concentration changed rapidly.
Fig. 12. Time evolution of (a) Vol. O2% and (b) Vol. CO2% at different temperatures with 21%O2/79%N2.
Please cite this article as: Y. Qiu, W. Zhong, Y. Shao, et al., Reactive force field molecular dynamics (ReaxFF MD) simulation of coal oxy-fuel combustion, Powder Technol., https://doi.org/10.1016/j.powtec.2019.07.103
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Fig. 13. Distribution of major products at different temperatures of (a) 1600 K, (b) 1800 K and (c) 2000 K with Vol. 21%O2.
3.2.2. Simulation of coal combustion in O2/N2 A series of traditional coal combustion in O2/N2 atmosphere were simulated at temperatures ranging from 1600 K to 2000 K for 2 ns. The time evolution of the number of major intermediates and combustion products (H2O, ∙CHO, ∙HO2, ∙OH, C2O3, and CO2) in simulations are shown in Fig. 11. C-CO2 represented the newly-generated CO2 from coal molecule. From Fig. 11, it can be seen that the production and generation rate of CO2 and H2O molecules increased with the increase in temperature, while the number of ∙HO2 radical decreased and then reached a plateau. Several ∙CHO radical and a very small amount of ∙OH radical were found in the reaction system. The time evolutions of the O2 and CO2 concentration with temperature were displayed in Fig. 12. The distributions of O2 concentration during the combustion of coal at different temperatures in the O2/N2 atmosphere were displayed in Fig. 12a. For these simulations, the O2 molecule consumed largely. The concentration of O2 increased with the increase of temperature, because high temperature promoted the decomposition of coal molecule and more gaseous products were generated in the system. Fig. 12b displays the distributions of CO2 concentration during these simulations of coal combustion in O2/N2 atmosphere at different temperatures. With the increase of temperature, the concentration of CO2 increased as time went on.
3.3. Effect of temperature in the combustion The effect of temperature on the coal oxy-fuel combustion process was investigated by a series of ReaxFF MD simulations at temperatures ranging from 1600 K to 2000 K for 2 ns. The time evolution of the number of major intermediates and combustion products (H2O, ∙CHO, ∙HO2, ∙OH, C2O3, and CO2) in simulations are shown in Fig. 13. C-CO2 represented the newly-generated CO2 from coal molecule. At each temperature, the amount of H2O among these products was always the largest in the system. The H2O molecule was produced when ∙H radical collided and combined with an ∙OH radical, i.e., H + OH → H2O. However, the H2O molecule was not stable in the system and could dissociate through the reverse reaction, i.e., H2O
→ H + OH, releasing ∙H radical. There were also a few ∙HO2 radicals during the reaction process, which was generated when ∙H radical was absorbed by one O2 molecule. However, ∙HO2 radicals were very unstable, and the number of ∙HO2 fluctuated drastically. The reason is that ∙HO2 radical was a kind of reactive intermediates in chemical reaction and would be consumed soon after being generated in the reaction process. There was also a small amount of other free radicals, such as ∙OH radical, ∙CHO radical and C2O3. The amount of hydroxyl group (∙OH) was less than H2O molecule and ∙HO2 radical, but larger than ∙CHO radical and C2O3. The ∙OH radical is a very important oxidant, which could attack the small fragments, which were generated from the decomposition of coal molecule. The aldehyde group (∙CHO) radical was very few in the reaction process because ∙CHO radical was mainly generated from the two –CH3 functional groups in the coal structure. The existence of C2O3 depended heavily on temperature. As the intermediate consisted of CO2 and CO, it was very unstable and easily changed back to CO2 and CO molecules in the reaction process. Among the five major intermediates, the first generated product was ∙HO2 radical, and subsequently the H2O molecule and ∙OH radical, and finally ∙CHO radical and C2O3 molecule. Fig. 13 shows that the distributions of the main products were different at each temperature. At higher temperature, the amount of all the five kinds of products, especially the C2O3 molecule, became larger and were generated earlier. For example, at 2000 K, ∙HO2 radical was observed even at the beginning of the NVT simulation. The amount and the generation time of CO2 released by chemical reactions were shown in Fig. 13. The yield of CO2 increased with the temperature, being 2, 2 and 8 at 1600 K, 1800 K and 2000 K, respectively. With the increase of temperature, the CO2 molecule was produced earlier, and the first CO2 almost appeared at 579.6 ps, 391.6 ps and 229.4 ps, respectively. The distributions of O2 concentration during the combustion of coal at different temperatures in the O2/CO2 atmosphere with 21% vol. O2 were displayed in Fig. 14a. Results suggest that the O2 concentration in the flue gas decreased gradually as time went by. As one of the major oxidants in the combustion system, O2 molecule provided ∙O radical to attack large coal molecule and small fragments or combined with ∙H radical to form ∙OH radical. When the temperature became higher,
Please cite this article as: Y. Qiu, W. Zhong, Y. Shao, et al., Reactive force field molecular dynamics (ReaxFF MD) simulation of coal oxy-fuel combustion, Powder Technol., https://doi.org/10.1016/j.powtec.2019.07.103
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Fig. 14. Time evolution of (a) Vol. O2% and (b) Vol. CO2% at different temperatures with Vol. 21%O2.
the concentration of O2 was smaller. Hence, the consumption of O2 was promoted. The major purpose of oxy-fuel combustion is to control and utilize CO2. Therefore, to analyze the effect of temperature on the generation of CO2 is very important. Fig. 14b displays the distributions of CO2 concentration during these simulations at different temperatures and the volume of O2 was 21%. In the whole simulation process, the concentration of CO2 grew at the initial stage, then declined, and increased again at the final stage. The concentration of CO2 did not monotonically vary in the simulation process. The instability of the concentration of CO2 is to the result of reverse reactions of CO2 as the system has a high concentration of CO2, the CO2 molecule would take part in the reaction at high temperature. At 1600 K, the concentration of CO2 was the lowest but the O2 concentration was the highest among the three cases. At lower temperature, the decomposition of coal molecule was more difficult and thus less CO2 were generated at 1600 K. Also, less O2 took part in the oxidation reaction or generated ∙O and ∙OH radicals, leading to an increase in the total amount of gas molecules in the flue gas at low temperature.
In present simulations, carbon-oxygen complex tended to be generated in the form of carbon trioxide (C2O3) at higher temperature. Coal decomposed easier at the higher temperature and more gaseous molecules would be produced in the flue gas. These are the reasons that the concentration of CO 2 at 2000 K was lower than that at 1800 K. As shown in Figs. 9 and 13, the rate of CO2 generation was lower in O2/CO2 reaction environment as compared to O2 condition. This is because of the high concentration of CO2 in oxy-fuel combustion system, which inhibits the reaction of producing CO2. It can be seen that for constant temperature, the number of H2O molecule was much larger in oxy-fuel combustion than in pure O2 combustion. As compared to the combustion simulation in O2/N2 environment, the evolution and equilibrium quantity of major products and intermediates in oxy-fuel combustion system were approximately the same except CO2, which was also due to the high concentration of CO2 in the reaction system (Figs. 9 and 13). However, the generation times of ∙HO2 radical and H2O were both delayed in O2/CO2 atmosphere. The ∙HO2 radical was found to be the first intermediate formed during simulations of the coal combustion in O2/N2 atmosphere. The formation of
Fig. 15. Distribution of major products at 1800 K of (a) Vol. 21% O2, (b) Vol. 25% O2 and (c) Vol. 30% O2.
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Fig. 16. Time evolution of (a) Vol. O2% and (b) Vol. CO2% at 1800 K with different Vol. O2%.
∙HO2 radical was caused by the hydrogen abstraction reaction from coal macromolecule.
be obtained, the original concentration of O2 in the simulation atmosphere determined the final concentration of O2 and CO2.
3.4. Effect of oxygen concentration in the combustion
4. Conclusions
To figure out the effect of O2 concentration on oxy-coal combustion, we conducted a series of simulations at 1800 K in the O2/CO2 atmosphere with different oxygen concentrations, i.e., 21%, 25%, and 30%, respectively. Fig. 15 shows the time evolution of the number of major intermediates and combustion products (H2O, ∙CHO, ∙HO2, ∙OH, C2O3 and CO2) during simulations in different O2 concentration atmosphere. When the system had 21% volume of O2, the ∙HO2 radical was first generated and followed by the generation of H2O molecule and ∙OH radical. While in the other two systems with 25% and 30% O2 concentration, the H2o molecule was first generated and then the ∙HO2 and ∙OH radicals. The generation of the first H2O molecule was earlier at higher O2 concentration atmosphere and the formation time were 300 ps, 260 ps and 160 ps with the O2 concentration being 21%, 25%, and 30%, respectively. The number of generated ∙OH radical was less with higher O2 concentration due to the reaction between CO2 and ∙H radical being prohibited. In O2/CO2 atmosphere, the concentration of CO2 was high, so more ∙OH radicals were generated by the chemical reaction of CO2 + H → CO + OH. The generation time of ∙OH radical and ∙HO2 radical with 21% O2 concentration were close to that with 25% O2 concentration, almost at the time of 250 ps. However, ∙OH was generated later while ∙HO2 radical was generated earlier in the 30% vol. O2 reaction system. The amount of O2 molecule was larger, and ∙H radical was easier to be absorbed by O2 molecule, so the ∙HO2 radical was generated easily. ∙OH radical could be generated by the CO2 involved reaction, and the number was inhibited by a reduction of CO2. The ∙CHO radical and C2O3 intermediate were hardly generated in the oxy-fuel combustion process. Results also showed that with the increase of O2 concentration, the decomposition of coal molecule became faster and more completely. The amount of CO2 released by chemical reactions was shown in Fig. 15. With the increase of O2 concentration, the release of newly generated CO2 became much earlier first and then was delayed. High O2 concentration could lead to more CO2 molecules, but when the concentration rose to 25% and 30%, the effect of O2 concentration on the number of generated CO2 was not obvious as the system reached an equilibrium. The distributions of O2 concentration at the temperature of 1800 K with different initial O2 concentrations were shown in Fig. 16a. The original O2 concentrations determined the final gas distributions in the flue gas. Data analysis showed that the amount of consumed O2 was almost the same in different combustion systems, and thus leading to an increase in the final O2 concentration at higher O2 concentration. Fig. 16b shows the distribution of CO2 concentration during oxy-fuel combustion simulations at 1800 K with different O2 concentrations. As can
In this study, the brown coal combustion in O2/CO2 atmosphere was simulated with ReaxFF Molecular Dynamics (ReaxFF MD) method and the microcosmic reaction behaviors of the oxy-coal combustion were investigated in atomic level with the emphasis laid on the generation pathways of CO2 and the effects of temperature and the O2 concentration. The main findings are as follows. (1) The generation of CO2 or CO began with the bond-breaking of oxygen-containing moieties in coal molecule and the whole process could be summarized as the decomposition process of coal molecular structure to form smaller molecules, the hydrogen abstraction reaction of fragments, and the oxygenation reaction by the attack of oxidants (like O2, ∙O radical, ∙HO2 radical and so on). The atmosphere with a high concentration of CO2 benefited the decomposition of coal and the generation of oxidants like ∙O radical, and thus accelerated the reaction rate of coal combustion and reduced the consumption of O2. (2) The rate of CO2 generation was lower in O2/CO2 environment than that in pure O2 and O2/N2 conditions at constant temperature. The production of H2O molecule was much larger than that in pure O2 environment and was slightly lower than that in O2/N2 environment. (3) The temperature showed obviously positive effects on the generation of intermediate products and the decomposition of CO2. With the higher temperature, the production of major intermediates and CO2 was promoted with more fragments or molecules being released earlier. However, the concentration of CO2 in the whole system increased first and then decreased as the CO2 tended to exist in the format of C2O3 in the higher temperature. (4) The increase of the concentration of O2 hastened the decomposition of coal molecule and the generation of H2O. The number of generated CO2 increased first and then remained stable with the rise of O2 concentration, and the release time was advanced first and then slightly delayed. The concentration distributions of CO2 in the flue gas were determined by the initial reaction atmosphere. The O2 concentration had the very weak influence on the distributions of other major products and intermediates.
Nomenclature * ai Acceleration of an atom (m/s2) BOσij Bond order of sigma bond BOπij Bond order of one π bond BOππ Bond order of the second π bond ij
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Dσe
The force field parameters of single bond related to bond energy Dπe The force field parameters of double bond related to bond energy Dππ The force field parameters of triple bond related to bond e energy Ebond Bond energy (kcal/mol) Econj Energy of conjugation effects (kcal/mol) ECoulomb Energy of Coulomb interaction (kcal/mol) Elp Energy of long pair electrons (kcal/mol) Eover Over-coordinated energy (kcal/mol) Epen Penalty energy (kcal/mol) Esystem Total energy of the system (kcal/mol) Etors Torsion energy (kcal/mol) Eunder Under-coordinated energy (kcal/mol) Eval Energy of valence angle term (kcal/mol) EvdWaals Energy of Van Der Waals interaction (kcal/mol) fs Femtosecond (10−15s) * Fi Force of an atom (N) mi Mass of an atom (kg) ns Nanosecond (10−9s) pbe, 1, pbe, 2 Temporary parameter pbo, 1~pbo, 6 Empirical parameters regressed by ReaxFF ps Picosecond (10−12s) * r 0i Initial position of an atom (m) * ri Position of an atom (m) rij Radius between i atom and j atom (m) rσo The equilibrium distance of single bond (m) rπo The equilibrium distance of double bond (m) rππ The equilibrium distance of triple bond (m) o uij Potential energy of Van Der Waals interactions between i atom and j atom U Total potential energy of the system (J) Uint Intramolecular potential energy (J) U Potential energy of Van Der Waals interactions (J) *VDW v0i Initial velocity of an atom (m/s) * vi Velocity of an atom (m/s) Greek letters ∇ Hamiltonian δt Time variation (s) Superscripts σ Single bond π Double bond ππ Triple bond Acknowledgments The work was financially supported by the Key Project of the National Natural Science Foundation of China (No. 51736002) and the Key Project of the National Natural Science Foundation of China (No. 51876037). References [1] F. Ahmed, Ghoniem, needs, resources and climate change: clean and efficient conversion technologies, Prog. Energy Combust. Sci. 37 (2011) 15–51. [2] Lei Chen, Yong Sze Zheng, F. Ghoniem Ahmed, Oxy-fuel combustion of pulverized coal: Characterization, fundamentals, stabilization and CFD modeling, Prog. Energy Combust. Sci. 38 (2012) 156–214. [3] B.J.P. Buhre, L.K. Elliott, C.D. Sheng, R.P. Gupta, et al., Oxy-fuel combustion technology for coal-fired power generation, Prog. Energy Combust. Sci. 31 (2005) 283–307. [4] Maja B. Toftegaard, Brix Jacob, A. Jensen Peter, Glarborg Peter, et al., Oxy-fuel combustion of solid fuels, Prog. Energy Combust. Sci. 36 (2010) 581–625. [5] Lunbo Duan, Sun Haicheng, Zhao Changsui, Zhou Wu, et al., Coal combustion characteristics on an oxy-fuel circulating fluidized bed combustor with warm flue gas recycle, Fuel 127 (2014) 47–51.
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Reactive force field molecular dynamics (ReaxFF MD) simulation of coal oxy-fuel combustion Yu Qiu a,b, Wenqi Zhong a,b,⁎, Yingjuan Shao a,b, Aibing Yu b,c a b c
Key Laboratory of Energy Thermal Conversion and Control of Ministry of Education, School of Energy and Environment, Southeast University, Nanjing 210096, China Centre for Simulation and Modelling of Particulate Systems, Southeast University - Monash University Joint Research School, Suzhou 215123, China ARC Research Hub for Computational Particle Technology, Department of Chemical Engineering, Monash University, Clayton, Vic 3800, Australia
a r t i c l e
i n f o
Article history: Received 14 April 2019 Received in revised form 27 July 2019 Accepted 30 July 2019 Available online xxxx Keywords: Oxy-fuel combustion Reaction pathways Molecular dynamics ReaxFF
a b s t r a c t Understanding the reaction mechanism of oxy-coal combustion is fundamentally important to the CO2 capture and storage in coal-fired industrial processes. In this paper, the microscopic reactive behaviors of brown coal combustion in O2/CO2 atmosphere were simulated by combining the molecular dynamics (MD) simulations and the reactive force field (ReaxFF). With thoroughly tracing the motion trajectories of atoms, as well as the formation, transition and breaking of bonds between atoms, the generation pathways of CO2 and the effects of temperature (1600–2000 K) and oxygen concentration (21%~30%) on the coal oxy-fuel combustion process were studied. Results showed that the main process of CO2 generation consists of the decomposition, oxygenation, and dehydrogenation. The atmosphere with a high concentration of CO2 can accelerate the reaction rate of coal combustion and reduce the consumption of O2. The higher temperature will promote the production of major intermediates with more fragments being released earlier, and finally increase the generation of CO2. The increase of O2 concentration obviously hastens the decomposition of coal molecule and the generation of H2O, but shows very weak influence on the distributions of other major products and intermediates. © 2019 Elsevier B.V. All rights reserved.
1. Introduction Oxy-fuel combustion as one of the most promising technologies to capture CO2 is attracting increasing attention in coal-fired industrial processes [1–4]. In this technology, a mixture of O2 and recycled flue gas (mainly CO2) is used as the oxidant instead of O2/N2 to obtain the glue gas with high concentration of CO2 (usually N90 vol%), which is convenient for CO2 separation and storage at the end of the combustion process [5,6]. Researchers around the world have been doing a variety of studies on theoretical analysis, experiments and numerical simulation to develop the coal oxy-fuel combustion and a wealth of macroscopic reaction information and rules, for example, the ignition and flame behaviors [7–12], pollutant emission characteristics [13–15], effects of the atmosphere on the combustion [16–19] and so on have been comprehensively figured out. However, up to now the understanding on the microscopic reaction behaviors of coal oxy-fuel combustion is very insufficient. Many fundamental and interesting mechanisms are still not clear, including the migration characteristics of carbon between coal and CO2, the generation pathways of CO2 and pollutants in O2/ CO2 atmosphere and so on. Clarifying these essential mechanisms is of
⁎ Corresponding author at: Sipailou 2#, Nanjing 210096, Jiangsu, China. E-mail address: [email protected] (W. Zhong).
significant importance to develop and improve the oxy-coal combustion technology. Reactive Molecular Dynamics simulation has been proved very effective to analyze the detailed mechanism of chemical reactions in the atomistic level. The MD method can display the dynamic behavior of atoms or molecules on the computer screen, so as to understand the evolution of the system intuitively. By calculating the interaction between the atoms with Newtonian equation of motion [20], obtaining atoms' trajectories and describing the formation, transition and breaking of chemical bonds [21,22], the Molecular Dynamics method based on ReaxFF can efficiently simulate the molecular behaviors in chemical reaction in large-scale system with the high accuracy close to quantum chemical method at the much lower computational cost. This ReaxFF molecular dynamics (ReaxFF MD) simulation method has been widely used in coal combustion, oxidation and pyrolysis process to obtain the mechanism of chemical reactions and pollutant characteristics [23–28]. Castro-Marcano et al. [29] investigated the coal char combustion by constructing a large scale model of Illinois No.6 coal char and found that the initial oxidation process of char included the thermal decomposition of char structure and dehydrogenation reaction. Chen et al. [30] found the initial reaction of spontaneous combustion of lignite coal was the formation of ∙HO2 radicals caused by the hydrogen abstraction reaction, which was the main reason for the generation of H2O2 and H2O molecules. Bhoi et al. [31] compared the potential energy in the spontaneous
https://doi.org/10.1016/j.powtec.2019.07.103 0032-5910/© 2019 Elsevier B.V. All rights reserved.
Please cite this article as: Y. Qiu, W. Zhong, Y. Shao, et al., Reactive force field molecular dynamics (ReaxFF MD) simulation of coal oxy-fuel combustion, Powder Technol., https://doi.org/10.1016/j.powtec.2019.07.103
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Fig. 1. Brown coal model (C39H37NO10S): (a) molecular structure and (b) optimized geometry model with oxygen-containing and other reactive functional groups.
combustion and pyrolysis process of brown coal under different conditions and found that the initial reaction of the combustion process was thermal decomposition, forming small fragments. Yan et al. [32] simulated the oxidation of brown coal at high temperature with ReaxFF MD simulations and found that the initial reaction of the oxidation was the hydrogen abstraction reaction by oxidants or the decomposition of coal molecule. Li et al. [33,34] studied the influences of O2 on the mechanism of N transfer and S transfer and the results showed that O2 reacted with intermediates to form HNO molecules, HNCO molecules, and sulfur oxide intermediates, which were then oxidized to NO and sulfur-containing gas molecules, respectively. Zheng et al. used Liulin coal model to study the influence of model scale on the pyrolysis [35] and found that the reaction diversity of coal pyrolysis was easier to be obtained in larger coal model system. Zheng et al. [36] revealed the four stages of Liulin coal pyrolysis, namely the activation stage, the primary pyrolysis stage, the second pyrolysis stage, and the recombination dominated stage. Zheng and his team also used GPU enabled ReaxFF MD (GMD-Reax) program [37–39] to significantly improve the computing performance and developed visualization and analysis of reactive molecular dynamics (VARMD) [38] to analyze the reaction of the trajectories from ReaxFF MD, which is based on cheminformatics analysis. However, previous MD studies mainly focused on coal combustion or oxidation in pure oxygen or coal pyrolysis. Up to now, few studies have been conducted on the microscopic reaction behaviors of coal combustion in O2/CO2 atmosphere, where the high concentration of CO2 would significantly affect the combustion process and CO2 would react with the coal molecule. In this study, the ReaxFF MD simulation was applied to study the microscopic molecular reaction characteristics of coal combustion in O2/CO2 atmosphere, with the emphasis on exploring the generation pathways of CO2 and the effects of operating conditions. Coal combustion in pure O2 and O2/N2 were also conducted for
comparison. Snapshots of the reactions, including the structural transformation and the dissociation and formation of the chemical bond were displayed by using the visualization software to analyze the atomic trajectories and thus to obtain the generation mechanism of CO2 and the distribution of major intermediates and products.
2. Computational methods 2.1. ReaxFF MD models In the MD simulation scheme, the movement of the atom is determined by the second Newton's law of motion, with which the potential energy (Eqs. (1)–(2)) and the force (Eq. (3)) on the molecule are first calculated at each time step. U ¼ U VDW þ U int
U VDW ¼ u12 þ u13 þ ⋯ þ u1n þ u23 þ u24 ⋯ ¼
ð1Þ
n −1 X
n X
uij r ij
ð2Þ
i¼1 j¼iþ1
where U is the total potential energy of the system, consisting both of Van der Waals interactions between atoms and intramolecular potential energy, UVDW is the potential energy of Van der Waals interactions, and Uint is the intramolecular potential energy, which is the sum of various types of potential energy (e.g. bond stretching, angle bending, and torsion angle…). uij is the potential energy of Van der Waals interactions between i atom and j atom, rij is the radius between i atom and j atom,
Table 1 Functional groups and major intermediates/products.
Please cite this article as: Y. Qiu, W. Zhong, Y. Shao, et al., Reactive force field molecular dynamics (ReaxFF MD) simulation of coal oxy-fuel combustion, Powder Technol., https://doi.org/10.1016/j.powtec.2019.07.103
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Fig. 3. The reaction pathway of C atom in –COOH to generate CO2: intramolecular structural reorganization and hydrogen abstraction by O2.
broken with all the forces and potential energies related to bond order being zero. On the other hand, Van der Waals force and Coulomb force between atoms are also taken into consideration and are calculated by Electronegativity Equilibration Method (EEM) or charging equilibration (QEq) methods. As ReaxFF MD simulation is driven by the potential energy of the system, there is no necessary to pre-define the reaction path, which makes the ReaxFF widely applicable for most systems with complicated chemical reactions, such as coal combustion, oxidation or pyrolysis. The energy function of the system takes the following form [21]: Esystem ¼ Ebond þ Elp þ Eover þ Eunder þ Eval þ Epen þ Etors þ Econj þ EvdWaals þ ECoulomb
Fig. 2. Reaction system of coal molecule in O2/CO2 atmosphere (Vol. 21% O2) with periodic boundary.
n is the number of atoms. * ∂ * ∂ * ∂ * þ j þk U F i ¼ −∇i U ¼ − i ∂xi ∂yi ∂zi
ð3Þ
*
where F i is the force of an atom, ∇ is Hamiltonian. Then, the acceleration, position and velocity of each atom are obtained by Eqs. (4)–(6), respectively. *
*
ai ¼ *
Fi mi *
ð4Þ *
vi ¼ v0i þ ai δt * * r i ¼ r 0i
*
þ v0i δt þ
ð5Þ 1* 2 a δt 2 i
where Esystem is the total energy of the system. Ebond is the bond energy. Elp is the energy of long pair electrons. Eover and Eunder are the correction term for coordination energy. Eval, Epen, Etors, Econj, EvdWaals and ECoulomb represent the valence angle term, penalty energy, torsion energy, conjugation effects to molecular energy, non-bonded Van der Waals interaction and Coulomb interaction, respectively [22]. h pbe;2 i ππ −Dπe BOπij −Dππ Ebond ¼ −Dσe BOσij exp pbe;1 1− BOσij e BOij ð8Þ where Dσe , Dπe , and Dππ e are the force field parameters of single, double and triple bond related to bond energy, pbe, 1 and pbe, 2 are temporary parameters, BO'ij is the bond order between two atoms, and the original bond order can be expressed as pbo2 pbo4 0 r ij rij þ exp pbo3 π BOij ¼ BOσij þ BOπij þ BOππ ij ¼ exp pbo1 σ ro ro pbo6 r ij þ exp pbo5 ππ ro
ð9Þ
ð6Þ
* * where ai, mi, vi, and ri are the acceleration, mass, velocity, and position of * * an atom, respectively. v0i and r 0i are the initial velocity and initial position
of the atom. δt is time variation. Based on the above information at every time step, the trajectories of the atoms can be obtained. Reactive force field (ReaxFF) is a bond order-based force field developed by Van Duin and his co-workers [21]. In ReaxFF model, the relative distance between two atoms is used to define the bond order of them (Eq. (9)), which determines whether there are the covalent or hydrogen bonding interactions between the atom pair. Bond order is used for the recognition of molecular fragments in the chemical reaction, and its value increases with the relative distance and when it exceeds the set bond-order cutoff radius, the bond of the atom pair is considered to be
where pbo, 1~pbo, 6 are the empirical parameters regressed by ReaxFF force field, rσo , rπo, and rππ o are the equilibrium distance of single, double and triple bond, respectively. 2.2. Combustion simulations Coal has complex carbonaceous structures and N130 kinds of chemical structures have been proposed over the past 70 years [29,40,41]. In this study, a brown coal molecule as shown in Fig. 1a was chosen, which is a small-scale model. Although the diversity of chemical reactions cannot be well presented, the reaction pathways of CO2 can be very useful to obtained. The coal molecule was built with ChemSketch software and its geometry model was optimized on the Forcite Module in the Materials Studio (MS) with the sites of the functional groups being
Table 2 Construction and simulation conditions of the reaction system. Environment
O2
CO2
N2
Temperature/K
Size/Å3
Density (g/cm3)
100%O2 21%O2/79%N2 21%O2/79%CO2 25%O2/75%CO2 30%O2/70%CO2
46 46 46 55 66
0 0 173 164 153
0 173 0 0 0
1600–2000 1600–2000 1600–2000 1800 1800
22.949 × 22.949 × 22.949 33.887 × 33.887 × 33.887 37.851 × 37.851 × 37.851 37.711 × 37.711 × 37.711 37.539 × 37.539 × 37.539
0.3 0.3 0.3 0.3 0.3
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Fig. 4. The reaction pathways of the C atom connected with\ \O\ \to generate C2O3 intermediate and CO2: (a) twice oxidation by O2 and HO2, and dissociation, (b) once oxidation by O2 and dissociation, (c) twice hydrogen abstraction, oxidation by OH and HO2, and dissociation, (d) breaking of two C\ \O bond.
highlighted in Fig. 1b. The specific form of functional groups of the above geometry structure was shown in Table 1. The above coal molecule and a certain amount of O2 and CO2 molecules was placed in a cubic box with all boundaries being periodic to avoid surface effects. Fig. 2 shows the two-dimensional view of the reactive system. There is only one coal molecule used in the system to demonstrate the evolution of the chemical reaction process more clearly. In this system, the density (ratio of the total mass of the coal molecule and gases in the system to the volume) of the initial system was
Fig. 5. The reaction pathways of the phenolic C atom connected with –OH to generate C2O3 intermediate and then CO2: (a) oxidation, intramolecular structural reorganization and decomposition, (b) hydrogen abstraction by O2, oxidation and hydrogen abstraction and oxidation and the decomposition.
set at a low level (0.5 g/cm3) to avoid the overlapping effect. During the reaction, the system was initially minimized with the NVE ensemble (namely the atom number, volume and energy being constant) for 500 ps at the temperature of 1000 K. Then, the specific operating conditions for reaction simulation were set in Large-scale Atomic/Molecular Massively Parallel Simulator (LAMMPS) software. In this step, the combustion process would be simulated with the constant atom number, volume and temperature conditions (the NVT ensemble) for 2 ns with the time step of 0.2 fs. The Berendsen thermostat was used to control the temperature with a 0.1 ps damping constant. The force field parameters for C, H, O, N, and S elements in the ReaxFF were directly utilized in this study [29]. The constant temperature MD simulations were performed on each system in the “reax” package in LAMMPS software. All the reaction conditions involved in this study were displayed in Table 2 with the reaction temperature varying from 1600 K to 2000 K and the volume concentration of O2 in O2/CO2 atmosphere between 21% and 30%, respectively. Some of the important structures during the reaction process, such as reactants, intermediates and products are also displayed in Table 1. A simulated temperature of 1600–2000 K was used in the ReaxFF MD simulations and such high temperature can accelerate the chemical reaction and simultaneously shorten the computing time in coal combustion process. This strategy is well grounded by the timetemperature superposition (TTS) theory [42] and can obtain a good agreement with experiments on simulating molecular motions [27,29,31].
Fig. 6. The reaction pathway of C atom in –CO to generate CO: oxidation by O2, hydrogen abstraction and decomposition.
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Fig. 7. The reaction pathways of the C atom in –CH3 to generate CO2: (a) decomposition, stepwise dehydrogenation and oxidation, (b) hydrogen abstraction, dissociation and oxidation. Fig. 8. Reaction mechanism of the generation of CO2 and CO.
3. Results and discussion In the current simulation study, there are many back-and-forth reactions during the ReaxFF MD simulation, and if these reactions occur repeatedly in a very short time, they will not be considered as a reaction because that is an unstable state of energy vibration [33]. 3.1. Elementary reactions in the oxy-fuel combustion system One macro chemical reaction process contains a myriad of elementary reactions and to clarify these reactions is the foundation for the understanding of the mechanism of oxy-coal combustion. In the molecular structure of coal, C atom exists in different forms, and some are in aromatic ring and some are in functional groups, which makes them undergo different pathways to generate CO2. Because of a higher reactivity of the functional groups, the bonds between C atoms that connect with the functional groups and the main body of coal molecule (for example, the bond 1 and bond 2 in Fig. 1b) are easier to break. The coal molecule firstly decomposed into smaller molecules or fragments at the site of oxygen-containing moieties, as shown in Fig. 1b. These small fragments were released by the process of aromatic-ring opening or carbon skeleton fracture, and then were attacked by oxidants and finally dehydrogenated to formed C2O3, CO and CO2. There are four kinds of functional groups and one active methyl group in a coal molecule, and correspondingly five different reaction pathways were found to generate CO2 in the current simulation study. The detailed reaction processes are illustrated as follows. (1) Carboxylic group: –COOH There are two carboxyl groups (−COOH) in the coal structure and both of them finally decomposed to CO2 (Fig. 3). Previous studies have demonstrated that direct decarboxylation contributes to the formation
of CO2 in the process of pyrolysis and oxidation [32,43]. The reaction pathway obtained in this study shows that the generation process of CO2 was initialized by intermolecular structural reorganization of – COOH, forming one ether group and one carbonyl group. Further, the dehydrogenation reaction happened and caused the fragment to lose the H atom and to form the ∙H radical, which is an essential intermediate in the combustion process. Finally, the remaining structure decomposed to release the CO2 molecule. (2) Ether group:\\O\\ There are three ether groups (\\O\\) in this coal structure. One of them is connected to a methyl group (−CH3) and will be analyzed later. The four reaction pathways of the remaining two ether groups are shown in Fig. 4. One C\\O bond in the ether group dissociated, followed by the attack of the O2 molecule to form carbon-oxygen complex. Next, there were two ways to generate CO2. One was the bond-breaking with neighboring C under the existence of CO2, leading to the formation of carbon trioxide (C2O3) and then being oxidized to CO2. The other way was that the fragment was attacked by another O2 molecule, followed by intramolecular structural rearrangement and finally, the CO2 molecule was released by the cleavage of ether group. (3) Hydroxyl group: –OH The two hydroxyl groups (−OH) in the coal structure connected to the aromatic rings and were recognized as phenolic hydroxyl. The trajectories showed that the neighboring carbons of the two phenolic hydroxyl groups ultimately transformed to CO2 and CO, respectively (Fig. 5). First, one O2 molecule or ∙O radical attacked the neighboring C carbon and gradually replaced the hydroxyl group and formed C\\O bond. Next, the new structure decomposed into a small fragment,
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Fig. 9. Distribution of major products at different temperatures of (a) 1600 K, (b) 1800 K and (c) 2000 K with pure O2.
which was attacked by the CO2 molecule to generate carbon trioxide (C2O3). Finally, the active C2O3 decomposed to generate CO2 and CO. (4) Carbonyl group: –CO There is one –CO group in the coal molecule, which first dissociated from the structure and then was attacked by the O2 molecule, generating CO2 (Fig. 6). The two C − C bonds in the carbonyl group broke and the newly generated small fragment was attacked by the O2 molecule to form the ester group (−COO−). Finally, CO2 was released after the break of the bond between the newly added O atom and the molecular structure. (5) Methyl group: –CH3 In the simulation process, both of the two carbon atoms in methyl groups were converted to CO, although they were in two different fragments. One methyl group was connected to an O atom to form the ether group, while the other was connected to an aromatic ring. The reaction pathways were shown in Fig. 7. The methyl group initially dissociated from the ether group and form –CH3 group, which was then stepwise oxidized and dehydrogenized to generate formaldehyde (CH2O), aldehyde group (−CHO) and CO.
In addition, there was a small difference in the reaction pathways at different simulated temperatures. Only at 2000 K, the methyl carbon could dissociate from the aromatic ring to form CO, while at lower simulation temperature, the methyl carbon still connected to the decomposed fragment. However, the methyl group connected to the ether bond was separated from the decomposed fragment at any temperature. The only difference was that, at 2000 K, the methyl carbon was finally transformed into CO, at 1800 K, the methyl group kept unchanged in the simulation and at 1600 K, even the C–O bond did not break. But at 1600 K, the methyl group was finally transformed into – CHO together with the neighboring O atom of the ether group. It can be concluded that temperature influenced the distribution and the generation pathway of products. Different reaction pathways and different products in the system indicated that combustion reaction could be accelerated by improving the temperature. Comparing the reaction processes of the methyl group connected to O atom at 1800 K and 2000 K, it was found that the small fragment experienced a complete reaction and finally formed CO2. Snapshots in Figs. 3–7 show the chemical reactions in coal oxy-fuel combustion with the processes of bond breaking and bond formation. The reaction pathways were demonstrated according to the types of the functional group in the coal molecule. Finally, a complete
Fig. 10. Time evolution of (a) Vol. O2% and (b) Vol. CO2% at different temperatures with pure O2.
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Fig. 11. Distribution of major products at different temperatures of (a) 1600 K, (b) 1800 K and (c) 2000 K with 21%O2/79%N2.
mechanism of the generation of CO2 or CO was obtained based on these five microscopic reaction pathways (Fig. 8). The main generation mechanism can be concluded as the process: the decomposition of macromolecules into small fragments, the oxidation reactions of fragments and the dehydrogenation reaction to form CO2 or CO. 3.2. Comparison of coal combustion between pure O2, O2/N2 and O2/CO2 environment A series of ReaxFF MD simulations of coal combustion under O2 and O2/N2 conditions were also conducted for comparison at temperature ranging from 1600 K to 2000 K for 2 ns, with the purpose of underlining the effect of high concentration of CO2 in oxy-fuel combustion atmosphere. 3.2.1. Simulation of coal combustion in pure O2 Simulations of coal combustion in pure O 2 atmosphere with stoichiometric composition at temperature range of 1600–2000 K were performed. Fig. 9 shows the time evolution of major intermediates and combustion products (H2 O, ∙CHO, ∙HO 2 , ∙OH, C 2 O 3 ,
and CO2) formed in the combustion process at different temperatures. C-CO 2 represented the newly-generated CO 2 from coal molecule. It can be seen that the generation of CO2 and H2O molecules was promoted with the temperature. During brown coal combustion process, ∙HO 2 radical was found to be a key intermediate but at high temperature the formation of ∙HO 2 radical was unstable and the number decreased slightly. During the combustion process, a small amount of some important intermediates were also generated, like ∙CHO and ∙OH radicals. The existence of C 2O3 was greatly influenced by the temperature, and the formation of C 2O 3 was favored at a higher temperature. The time evolutions of the O2 and CO2 concentration with temperature were displayed in Fig. 10. The adequate oxygen present in the system took part in the oxidation reaction and decomposed the coal molecule easily. From Fig. 10, it was observed that the concentration of CO2 increased with elevated temperature whereas the concentration of O2 decreased. At the initial stage of simulation (from 250 ps to 500 ps), both the decrease of O2 concentration and the increase of CO2 concentration changed rapidly.
Fig. 12. Time evolution of (a) Vol. O2% and (b) Vol. CO2% at different temperatures with 21%O2/79%N2.
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Fig. 13. Distribution of major products at different temperatures of (a) 1600 K, (b) 1800 K and (c) 2000 K with Vol. 21%O2.
3.2.2. Simulation of coal combustion in O2/N2 A series of traditional coal combustion in O2/N2 atmosphere were simulated at temperatures ranging from 1600 K to 2000 K for 2 ns. The time evolution of the number of major intermediates and combustion products (H2O, ∙CHO, ∙HO2, ∙OH, C2O3, and CO2) in simulations are shown in Fig. 11. C-CO2 represented the newly-generated CO2 from coal molecule. From Fig. 11, it can be seen that the production and generation rate of CO2 and H2O molecules increased with the increase in temperature, while the number of ∙HO2 radical decreased and then reached a plateau. Several ∙CHO radical and a very small amount of ∙OH radical were found in the reaction system. The time evolutions of the O2 and CO2 concentration with temperature were displayed in Fig. 12. The distributions of O2 concentration during the combustion of coal at different temperatures in the O2/N2 atmosphere were displayed in Fig. 12a. For these simulations, the O2 molecule consumed largely. The concentration of O2 increased with the increase of temperature, because high temperature promoted the decomposition of coal molecule and more gaseous products were generated in the system. Fig. 12b displays the distributions of CO2 concentration during these simulations of coal combustion in O2/N2 atmosphere at different temperatures. With the increase of temperature, the concentration of CO2 increased as time went on.
3.3. Effect of temperature in the combustion The effect of temperature on the coal oxy-fuel combustion process was investigated by a series of ReaxFF MD simulations at temperatures ranging from 1600 K to 2000 K for 2 ns. The time evolution of the number of major intermediates and combustion products (H2O, ∙CHO, ∙HO2, ∙OH, C2O3, and CO2) in simulations are shown in Fig. 13. C-CO2 represented the newly-generated CO2 from coal molecule. At each temperature, the amount of H2O among these products was always the largest in the system. The H2O molecule was produced when ∙H radical collided and combined with an ∙OH radical, i.e., H + OH → H2O. However, the H2O molecule was not stable in the system and could dissociate through the reverse reaction, i.e., H2O
→ H + OH, releasing ∙H radical. There were also a few ∙HO2 radicals during the reaction process, which was generated when ∙H radical was absorbed by one O2 molecule. However, ∙HO2 radicals were very unstable, and the number of ∙HO2 fluctuated drastically. The reason is that ∙HO2 radical was a kind of reactive intermediates in chemical reaction and would be consumed soon after being generated in the reaction process. There was also a small amount of other free radicals, such as ∙OH radical, ∙CHO radical and C2O3. The amount of hydroxyl group (∙OH) was less than H2O molecule and ∙HO2 radical, but larger than ∙CHO radical and C2O3. The ∙OH radical is a very important oxidant, which could attack the small fragments, which were generated from the decomposition of coal molecule. The aldehyde group (∙CHO) radical was very few in the reaction process because ∙CHO radical was mainly generated from the two –CH3 functional groups in the coal structure. The existence of C2O3 depended heavily on temperature. As the intermediate consisted of CO2 and CO, it was very unstable and easily changed back to CO2 and CO molecules in the reaction process. Among the five major intermediates, the first generated product was ∙HO2 radical, and subsequently the H2O molecule and ∙OH radical, and finally ∙CHO radical and C2O3 molecule. Fig. 13 shows that the distributions of the main products were different at each temperature. At higher temperature, the amount of all the five kinds of products, especially the C2O3 molecule, became larger and were generated earlier. For example, at 2000 K, ∙HO2 radical was observed even at the beginning of the NVT simulation. The amount and the generation time of CO2 released by chemical reactions were shown in Fig. 13. The yield of CO2 increased with the temperature, being 2, 2 and 8 at 1600 K, 1800 K and 2000 K, respectively. With the increase of temperature, the CO2 molecule was produced earlier, and the first CO2 almost appeared at 579.6 ps, 391.6 ps and 229.4 ps, respectively. The distributions of O2 concentration during the combustion of coal at different temperatures in the O2/CO2 atmosphere with 21% vol. O2 were displayed in Fig. 14a. Results suggest that the O2 concentration in the flue gas decreased gradually as time went by. As one of the major oxidants in the combustion system, O2 molecule provided ∙O radical to attack large coal molecule and small fragments or combined with ∙H radical to form ∙OH radical. When the temperature became higher,
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Fig. 14. Time evolution of (a) Vol. O2% and (b) Vol. CO2% at different temperatures with Vol. 21%O2.
the concentration of O2 was smaller. Hence, the consumption of O2 was promoted. The major purpose of oxy-fuel combustion is to control and utilize CO2. Therefore, to analyze the effect of temperature on the generation of CO2 is very important. Fig. 14b displays the distributions of CO2 concentration during these simulations at different temperatures and the volume of O2 was 21%. In the whole simulation process, the concentration of CO2 grew at the initial stage, then declined, and increased again at the final stage. The concentration of CO2 did not monotonically vary in the simulation process. The instability of the concentration of CO2 is to the result of reverse reactions of CO2 as the system has a high concentration of CO2, the CO2 molecule would take part in the reaction at high temperature. At 1600 K, the concentration of CO2 was the lowest but the O2 concentration was the highest among the three cases. At lower temperature, the decomposition of coal molecule was more difficult and thus less CO2 were generated at 1600 K. Also, less O2 took part in the oxidation reaction or generated ∙O and ∙OH radicals, leading to an increase in the total amount of gas molecules in the flue gas at low temperature.
In present simulations, carbon-oxygen complex tended to be generated in the form of carbon trioxide (C2O3) at higher temperature. Coal decomposed easier at the higher temperature and more gaseous molecules would be produced in the flue gas. These are the reasons that the concentration of CO 2 at 2000 K was lower than that at 1800 K. As shown in Figs. 9 and 13, the rate of CO2 generation was lower in O2/CO2 reaction environment as compared to O2 condition. This is because of the high concentration of CO2 in oxy-fuel combustion system, which inhibits the reaction of producing CO2. It can be seen that for constant temperature, the number of H2O molecule was much larger in oxy-fuel combustion than in pure O2 combustion. As compared to the combustion simulation in O2/N2 environment, the evolution and equilibrium quantity of major products and intermediates in oxy-fuel combustion system were approximately the same except CO2, which was also due to the high concentration of CO2 in the reaction system (Figs. 9 and 13). However, the generation times of ∙HO2 radical and H2O were both delayed in O2/CO2 atmosphere. The ∙HO2 radical was found to be the first intermediate formed during simulations of the coal combustion in O2/N2 atmosphere. The formation of
Fig. 15. Distribution of major products at 1800 K of (a) Vol. 21% O2, (b) Vol. 25% O2 and (c) Vol. 30% O2.
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Fig. 16. Time evolution of (a) Vol. O2% and (b) Vol. CO2% at 1800 K with different Vol. O2%.
∙HO2 radical was caused by the hydrogen abstraction reaction from coal macromolecule.
be obtained, the original concentration of O2 in the simulation atmosphere determined the final concentration of O2 and CO2.
3.4. Effect of oxygen concentration in the combustion
4. Conclusions
To figure out the effect of O2 concentration on oxy-coal combustion, we conducted a series of simulations at 1800 K in the O2/CO2 atmosphere with different oxygen concentrations, i.e., 21%, 25%, and 30%, respectively. Fig. 15 shows the time evolution of the number of major intermediates and combustion products (H2O, ∙CHO, ∙HO2, ∙OH, C2O3 and CO2) during simulations in different O2 concentration atmosphere. When the system had 21% volume of O2, the ∙HO2 radical was first generated and followed by the generation of H2O molecule and ∙OH radical. While in the other two systems with 25% and 30% O2 concentration, the H2o molecule was first generated and then the ∙HO2 and ∙OH radicals. The generation of the first H2O molecule was earlier at higher O2 concentration atmosphere and the formation time were 300 ps, 260 ps and 160 ps with the O2 concentration being 21%, 25%, and 30%, respectively. The number of generated ∙OH radical was less with higher O2 concentration due to the reaction between CO2 and ∙H radical being prohibited. In O2/CO2 atmosphere, the concentration of CO2 was high, so more ∙OH radicals were generated by the chemical reaction of CO2 + H → CO + OH. The generation time of ∙OH radical and ∙HO2 radical with 21% O2 concentration were close to that with 25% O2 concentration, almost at the time of 250 ps. However, ∙OH was generated later while ∙HO2 radical was generated earlier in the 30% vol. O2 reaction system. The amount of O2 molecule was larger, and ∙H radical was easier to be absorbed by O2 molecule, so the ∙HO2 radical was generated easily. ∙OH radical could be generated by the CO2 involved reaction, and the number was inhibited by a reduction of CO2. The ∙CHO radical and C2O3 intermediate were hardly generated in the oxy-fuel combustion process. Results also showed that with the increase of O2 concentration, the decomposition of coal molecule became faster and more completely. The amount of CO2 released by chemical reactions was shown in Fig. 15. With the increase of O2 concentration, the release of newly generated CO2 became much earlier first and then was delayed. High O2 concentration could lead to more CO2 molecules, but when the concentration rose to 25% and 30%, the effect of O2 concentration on the number of generated CO2 was not obvious as the system reached an equilibrium. The distributions of O2 concentration at the temperature of 1800 K with different initial O2 concentrations were shown in Fig. 16a. The original O2 concentrations determined the final gas distributions in the flue gas. Data analysis showed that the amount of consumed O2 was almost the same in different combustion systems, and thus leading to an increase in the final O2 concentration at higher O2 concentration. Fig. 16b shows the distribution of CO2 concentration during oxy-fuel combustion simulations at 1800 K with different O2 concentrations. As can
In this study, the brown coal combustion in O2/CO2 atmosphere was simulated with ReaxFF Molecular Dynamics (ReaxFF MD) method and the microcosmic reaction behaviors of the oxy-coal combustion were investigated in atomic level with the emphasis laid on the generation pathways of CO2 and the effects of temperature and the O2 concentration. The main findings are as follows. (1) The generation of CO2 or CO began with the bond-breaking of oxygen-containing moieties in coal molecule and the whole process could be summarized as the decomposition process of coal molecular structure to form smaller molecules, the hydrogen abstraction reaction of fragments, and the oxygenation reaction by the attack of oxidants (like O2, ∙O radical, ∙HO2 radical and so on). The atmosphere with a high concentration of CO2 benefited the decomposition of coal and the generation of oxidants like ∙O radical, and thus accelerated the reaction rate of coal combustion and reduced the consumption of O2. (2) The rate of CO2 generation was lower in O2/CO2 environment than that in pure O2 and O2/N2 conditions at constant temperature. The production of H2O molecule was much larger than that in pure O2 environment and was slightly lower than that in O2/N2 environment. (3) The temperature showed obviously positive effects on the generation of intermediate products and the decomposition of CO2. With the higher temperature, the production of major intermediates and CO2 was promoted with more fragments or molecules being released earlier. However, the concentration of CO2 in the whole system increased first and then decreased as the CO2 tended to exist in the format of C2O3 in the higher temperature. (4) The increase of the concentration of O2 hastened the decomposition of coal molecule and the generation of H2O. The number of generated CO2 increased first and then remained stable with the rise of O2 concentration, and the release time was advanced first and then slightly delayed. The concentration distributions of CO2 in the flue gas were determined by the initial reaction atmosphere. The O2 concentration had the very weak influence on the distributions of other major products and intermediates.
Nomenclature * ai Acceleration of an atom (m/s2) BOσij Bond order of sigma bond BOπij Bond order of one π bond BOππ Bond order of the second π bond ij
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Dσe
The force field parameters of single bond related to bond energy Dπe The force field parameters of double bond related to bond energy Dππ The force field parameters of triple bond related to bond e energy Ebond Bond energy (kcal/mol) Econj Energy of conjugation effects (kcal/mol) ECoulomb Energy of Coulomb interaction (kcal/mol) Elp Energy of long pair electrons (kcal/mol) Eover Over-coordinated energy (kcal/mol) Epen Penalty energy (kcal/mol) Esystem Total energy of the system (kcal/mol) Etors Torsion energy (kcal/mol) Eunder Under-coordinated energy (kcal/mol) Eval Energy of valence angle term (kcal/mol) EvdWaals Energy of Van Der Waals interaction (kcal/mol) fs Femtosecond (10−15s) * Fi Force of an atom (N) mi Mass of an atom (kg) ns Nanosecond (10−9s) pbe, 1, pbe, 2 Temporary parameter pbo, 1~pbo, 6 Empirical parameters regressed by ReaxFF ps Picosecond (10−12s) * r 0i Initial position of an atom (m) * ri Position of an atom (m) rij Radius between i atom and j atom (m) rσo The equilibrium distance of single bond (m) rπo The equilibrium distance of double bond (m) rππ The equilibrium distance of triple bond (m) o uij Potential energy of Van Der Waals interactions between i atom and j atom U Total potential energy of the system (J) Uint Intramolecular potential energy (J) U Potential energy of Van Der Waals interactions (J) *VDW v0i Initial velocity of an atom (m/s) * vi Velocity of an atom (m/s) Greek letters ∇ Hamiltonian δt Time variation (s) Superscripts σ Single bond π Double bond ππ Triple bond Acknowledgments The work was financially supported by the Key Project of the National Natural Science Foundation of China (No. 51736002) and the Key Project of the National Natural Science Foundation of China (No. 51876037). References [1] F. Ahmed, Ghoniem, needs, resources and climate change: clean and efficient conversion technologies, Prog. Energy Combust. Sci. 37 (2011) 15–51. [2] Lei Chen, Yong Sze Zheng, F. Ghoniem Ahmed, Oxy-fuel combustion of pulverized coal: Characterization, fundamentals, stabilization and CFD modeling, Prog. Energy Combust. Sci. 38 (2012) 156–214. [3] B.J.P. Buhre, L.K. Elliott, C.D. Sheng, R.P. Gupta, et al., Oxy-fuel combustion technology for coal-fired power generation, Prog. Energy Combust. Sci. 31 (2005) 283–307. [4] Maja B. Toftegaard, Brix Jacob, A. Jensen Peter, Glarborg Peter, et al., Oxy-fuel combustion of solid fuels, Prog. Energy Combust. Sci. 36 (2010) 581–625. [5] Lunbo Duan, Sun Haicheng, Zhao Changsui, Zhou Wu, et al., Coal combustion characteristics on an oxy-fuel circulating fluidized bed combustor with warm flue gas recycle, Fuel 127 (2014) 47–51.
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Please cite this article as: Y. Qiu, W. Zhong, Y. Shao, et al., Reactive force field molecular dynamics (ReaxFF MD) simulation of coal oxy-fuel combustion, Powder Technol., https://doi.org/10.1016/j.powtec.2019.07.103