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Molecular dynamics investigation on the gasification of a coal particle in supercritical water
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Molecular dynamics investigation on the gasification of a coal particle in supercritical water Jian Chen a, Xiong Pan a, Hanqing Li b, Hanhui Jin c,*, Jianren Fan b a
School of Energy and Power Engineering, University of Shanghai for Science and Technology, Shanghai, 200093, China b State Key Laboratory of Clean Energy Utilization, Zhejiang University, Hangzhou, 310027, China c School of Aeronautics and Astronautics, Zhejiang University, Hangzhou, 310027, China
highlights The whole SCW gasification process of a coal particle is studied with ReaxFF. The carbon structure evolution and the generation mechanism of H2 are studied. The two stages of gasification process are disclosed. The temperature can significantly influence pyrolysis of the organic fragments. The transition of N in the coal particle is revealed.
article info
abstract
Article history:
Coal gasification technology in supercritical water provides a clean and efficient way to
Received 24 September 2019
convert coal to H2. In the present paper, the whole supercritical water（SWC）gasification
Received in revised form
process of a coal particle is studied with the reactive force field (ReaxFF) molecular dy-
26 November 2019
namics (MD) method for the first time. First, the detailed reaction mechanism which can’t
Accepted 1 December 2019
be clearly illustrated in experiments, such as the evolution of the carbon structure during
Available online xxx
the gasification process and the detailed reaction mechanism of the main products, is obtained. According to the generation mechanism of H2, it is found that the supercritical
Keywords:
water gasification process of a coal particle can be divided into two stages with different
Coal
reaction mechanisms, namely the rapid reaction stage and the stable reaction stage. Then,
Gasification
the effects of temperature and coal concentration in the reaction system on the yield of H2
Supercritical water
are studied. Finally, the transition of N in the coal particle is revealed, in which the pre-
H2 production
cursors of NH3 such as CN, CHN, and CHON are the basic molecular structures for nitrogen
ReaxFF
atoms during the gasification process at high temperature. © 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
Introduction Coal is one of the most important energy sources which supplies one-quarter of the world’s primary energy [1,2].
Direct combustion is still the dominant way of coal utilization. However, the burning process produced a lot of harmful substances, such as NOX and SOX, which are the main pollution sources of the environment [3,4]. Coal gasification
* Corresponding author. E-mail address: [email protected] (H. Jin). https://doi.org/10.1016/j.ijhydene.2019.12.002 0360-3199/© 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved. Please cite this article as: Chen J et al., Molecular dynamics investigation on the gasification of a coal particle in supercritical water, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.12.002
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technology provides an alternative way for clean and efficient coal utilization. The main goal of the coal gasification technology is to produce hydrogen， which is a promising secondary non-pollution, efficient renewable energies [5,6]. During the past three decades, coal gasification in supercritical water has become one of the valuable technologies to convert coal to hydrogen. When water is in the supercritical state (both the temperature T > 374.3 C and the pressure P > 22.1 MPa), it presents some special physical properties such as high diffusivity [7e9], low viscosity and salvation [10e12]. Benefiting from the variation in the physical properties, the supercritical water gasification shows great advantages over traditional gasification. In the 1970s, Modell [13,14] was the pioneer who firstly applied supercritical water to convert organic materials like coal, biomass into useful gaseous products. From then on, the researchers all over the world started the relative investigation. In the 1980s and 1990s, some researchers [15e17] studied the extraction of low-rank coal in supercritical water at the temperature of 647e773 K experimentally, and they found that supercritical water was not only a solvent but also a reactant. Since the beginning of this century, people have begun to apply supercritical water to the coal gasification for hydrogen-rich gas production. Some researches focused on the influences of the operating parameters. Such as Bi’s group [18] conducted experiments to investigate the effect of temperature, water density and reaction time on the gas yields, and found the temperature was an important factor. Yamaguchi [19] investigated the non-catalytic gasification of Victorian brown coal in quartz batch reactors and found the significant effect of running temperature and feed concentration. These conclusions were also confirmed by Jin et al. [20,21]. Some researches focus on the improvement by using catalyst additions. Such as Lin et al. [22,23] studied the gasification of Taiheiyo coal in SCW with Ca(OH)2 and a small proportion of NaOH. Wang et al. [24] investigated the catalytic effect of calcium hydroxide and found the improvement of coal gasification with a decrease in the amount of residual char. Some researchers such as Guo’s group successfully developed a novel continuous-flow system for coal gasification in supercritical water to ensure the continuous and stable gasification [25,26], the hydrogen yield of their system reached 77.5 Mol/(kg coal) [27]. A pilot-scale demonstration plant driven by solar concentration system was even established by them [28]. Due to the complexity of the reaction processes in supercritical water coal gasification, it is difficult to illustrate the detailed reaction mechanism only through the experimental method. Nowadays, the reactive force field (ReaxFF) [29] molecular dynamics (MD) has become a useful tool to study complex reactions of hydrocarbons. With this method, researchers can numerically investigate the evolution of formation, transition, and dissociation of chemical bonds and obtain results with certain precision at the cost of a small amount of calculation. The effectiveness of ReaxFF MD has been proved in the coal pyrolysis and combustion [30,31], thermal decomposition of polymer [32], catalysis [33e35], gasification [36,37], and formation [38e40], etc. Jin et al. [34] investigated the supercritical water gasification process of anthracene macromolecule with both ReaxFF MD and density
functional theory (DFT) methods, and the process was compared with steam gasification and pyrolysis. They found that supercritical water effectively weakened the C(ring)C(ring) bond energy in anthracene, decreased the energy barrier of the ring-opening reaction, accelerated the gasification rate, and increased the hydrogen yield. Zhang’s group [37] studied the pyrolysis and hydrogen production of a coal macromolecule in supercritical water using the ReaxFF force field method combined with the DFT method. Results demonstrated that the main source of hydrogen molecules in the SCW-coal system was H radical-rich water clusters. Overall, these MD researches mainly focused on the decrease of the bond cracking energy due to the effect of the supercritical water molecule and structure changes of aromatic rings, and only the initial stage of the coal gasification was studied, in which no more than eight hydrogen molecules were generated in the simulations. Our group [41] recently studied the whole SCW gasification process of a coal macromolecule with the ReaxFF MD method, in which the detailed chemical reactions and pathways of H2 generation were disclosed. Although the macromolecule contains 76%e90% C content of a coal particle [42], the apparent difference between the macromolecule and the coal particle can’t be ignored because a real coal particle generally has a great deal of medium and little fragments. These fragments can influence the reaction process and the final products significantly. Hence, it is essential to carry out researches on the reaction mechanism of a real coal particle which is actually encountered in the gasification system. The objective of this paper is to investigate the SCW gasification process of a real coal particle with different fragments using the ReaxFF force field method, in which both the evolution of the carbon structure and the mechanism of H2 generation are studied. Meanwhile, the transition of the nitrogen inside the coal particle, which can’t be obtained in the studies of a coal macromolecule, is also investigated. In Section MD simulation, simulation methods and details are described. In Section Simulation validation, the comparison between the MD results and experimental data is conducted to validate the MD simulation. In Section Evolution of the carbon structure and Section Mechanism of H2 production, the reaction mechanism including the carbon structure evolution and the mechanism of H2 generation during the SCW gasification process is discussed. In Section Effect of temperature and Section Effect of coal concentration, the effect of temperature and coal concentration, which are the operating conditions that appeared in the gasification process, is studied. In Section Transition of nitrogen, the transition of the nitrogen is disclosed.
MD simulation Reactive force field method (ReaxFF) The ReaxFF proposed by Van Duin can continuously simulate the reaction process based on the normal MD method, utilizing the inter-atomic potential to model the interaction between atoms with a bond-order formalism, where the bond order is calculated empirically from the interatomic distances.
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In the ReaxFF, the potential energy function is usually given in the following form: Esystem ¼ Ebond þ Eover þ Eunder þ Eval þ Epen þ Etors þ Econj þ EvdWaals þ ECoulomb where Ebond is the bond energy, Eover and Eunder represent the over- and under-coordinated atoms in the energy contribution. Eval , Epen , Etors , Econj , EvdWaals and ECoulomb are the valence angle term, penalty energy term, torsion angle term, conjugation effects to molecular energy, non-bond van der Waals interactions, and Coulombic interactions, respectively. More details about the ReaxFF can also refer to Ref. [29,33].
Coal-water reaction system and simulation details To build the coal-water reaction system, a coal particle is set up based on the atomic structure first. In the present paper, the 3D molecular model of a coal particle is constructed following the way used by Zheng [30]. The Wiser model of the coal macromolecule is adopted to build the coal particle, together with 15 kinds of small molecular fractions. Both the Wiser model and the number of each molecular fraction are adjusted to fit the experimental data of the chemical compositions based on the averaged data of four kinds of coals [43e45]. Before the final 3D structure of the coal particle is obtained in a stable state with the minimum free energy, it is optimized in a DREIDING force field with the Materials Studio
software, then several times of anneal cycles are carried out. The final coal particle contains 19638 atoms in a cube with an edge length of 6.06 nm (as shown in Fig. 1). The detailed information on constructing the coal particle can also refer to Ref. [46]. To build the supercritical coal-water reaction system, the established coal particle is placed at the center of the box and surrounded H2O molecules randomly by MATLAB. Four reaction systems of S1, S2, S3, and S4 are set up to represent four different kinds of coal concentrations, respectively (as shown in Table 1). To maintain a stable pressure, the volume density of the H2O molecules in the systems is set as 0.2 g/cm3. Six different cases are built to investigate the effect of coal concentration and temperature on the gasification with LAMMPS software. Among these cases, Case2, Case3, and Case4 are used to investigate the effect of temperature. Case1, Case3, Case5, and Case6 are used to disclose the effects of coal concentration. The specific parameters of the 6 cases are listed in Table 2. In all the cases of simulation, the time step is set to 0.2 fs. The NVT ensemble is adopted, and the temperature of the system is corrected with the Berendsen thermostat at every time step. During the simulation, the temperature is increased from 100 K to 300 K firstly at the rate of 10 K/ps so that the initial reaction system can be fully relaxed and reach the equilibrium state at the low temperature. After the system reaches its equilibrium state, the temperature starts to be improved from 300 K to 2500 K at a faster rate of 22 K/ps to reduce the time cost of simulation. After the system temperature reaches about 2500 K, chemical reactions start to appear. The temperature is then increased slowly at the rate of 10 K/ps until the target temperature. After the temperature of the reaction system reaches at designated temperature (3000 K, 3500 K and 4000 K for different cases, respectively), different reaction periods are set for different temperatures so that the reactions of the gasification are fully developed (as shown in Table 2). In this period, the temperature keeps constant throughout the reaction process. Such a high temperature is used to improve the reaction rate so that the simulation of the reaction can be finished in a reasonable period of time.
Results and discussions Simulation validation
Fig. 1 e Coal particle model with formula C9570H8637O1147N99S18.
H2 is the desired and main product of coal gasification in SCW, and its yield is the main objective of concern in the process. The comparison of the yield of H2 (YH2) at different coal concentrations between the MD results and the four experimental
Table 1 e Parameters of different reaction systems. System S1 S2 S3 S4
Coal particle
Number of water molecules
Density of water molecules (kg/m3)
Concentration of coal (mass%)
Dimension(A)
1 1 1 1
57863 36738 26631 18252
0.2 0.2 0.2 0.2
12% 18% 25% 32%
210*210*210 184*184*184 172*172*172 155*155*155
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Table 2 e Parameters of the MD cases. Case
System
Coal concentration
Case1 Case2 Case3 Case4 Case5 Case6
S1 S2
12% 18%
S3 S4
25% 32%
Reaction Temperature 3500 3000 3500 4000 3500 3500
results is shown in Fig. 2 [20,21,25,47]. The operation parameters of four experiments are shown in Table 3. The MD results agree with the experimental results in tendency, showing that the coal concentration can effectively influence the yield of H2. However, the yield of H2 of the MD is obviously higher than that of the experimental results, this is due to the reaction temperature of the MD (3500 K) is higher than that of the reaction temperature of experiments (853e1223 K). The temperature is an important factor that could effectively improve H2 production. Salmon et al. [48,49] proved that such a difference in reactive temperature between simulation and experiment wouldn’t effectively influence the reaction process in the thermal decomposition of brown coal, in spite of the possible effect on the product distributions. In their studies, although the high reaction temperature led to the disappearance of C4 hydrocarbon in the gas phase during the simulation, the processes related to the formation of low molecular weight products and sequences of products formation agree well with previous experimental results. Therefore, the ReaxFF MD
Fig. 2 e Comparison of the YH2 between the MD results and the four experimental results at different coal concentrations.
K K K K K K
Reaction period 800 ps 850 ps 800 ps 1380 ps 800 ps 800 ps
simulation in the present paper is feasible to investigate the SCW gasification of a coal particle.
Evolution of the carbon structure The temporal evolutions of the number of the C atoms contained in major intermediate products C1-2, C3-13, C14-39, and C40þ in Case 4 is presented in Fig. 3. It can be found that the number of C atoms contained in C40þ fragments starts to decrease at t z 110 ps when the temperature T ¼ 2280 K. Along with the rapid degradation of C40þ fragments, the number of C1-2, C3-13, and C14-39 fragments rise rapidly. C40þ fragments disappear and C14-39 fragments reach the maximum at T ¼ 2959 K around t ¼ 170 ps. Subsequently, the number of the C14-39 fragments decreases rapidly during the period t ¼ 170~270 ps, accompanied by the obvious increase of the C12 fragments. The number of C3-13 fragments reaches the peak value and then starts to decrease during this period. Such a sequence of the variation reflects the pyrolysis process of the coal particle. Similar to the process of the coal combustion, the pyrolysis occurs first before the gasification reaction of the coal particle with the SCW. The amount of the C1-2 fragments
Fig. 3 e Temporal evolution of the carbon structures in Case 4.
Table 3 e Operation parameters of the four experiments.
Exp Exp Exp Exp
1 2 3 4
Temperature
Coal
Residence time
Catalyst
853 K 973 K 1253 K 973 K
Liangzhuang coal Hongliulin Bituminous Coal Yimin lignite Yimin lignite
5.0 kg/h 10 min 2 min 5 min
K2CO3 K2CO3 Non-catalyst K2CO3
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reaches the peak value at the moment t z 390 ps after which the number of the C3-13 fragments gradually turns to be stable. Since the C14þ fragments almost have been completely exhausted, such a stable state for C3-13 means the quasiequilibrium state of the reversible pyrolysis reaction between C3-13 and C1-2. As the gasification reaction of C1-2 with the SCW proceeds and the number of the C1-2 decreases, the number of C3-13 fragments decreases slightly. The amount of the C1-2 fragments remains to be stable until the new equilibrium state is obtained after about 1000 ps.
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Fig. 4 shows the probability density distribution of the number of C atoms in C1eC200 molecules at different times. It demonstrates the transition of the C structures inside the particle as the gasification process proceeds. The general tendency of the C atoms’ distribution moving from the large carbon structures to the small carbon structures can be observed clearly. Meanwhile, coking processes arise in the early stage. Obviously, the appearance of C194 is the product of coking because the largest C contained macromolecule is C191 in the initial coal particle. Tracking of the C atoms shows that
Fig. 4 e Distribution of C atoms in the C1eC200 molecules at different times in Case4.
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the emergence of some C158 and C177 fragments at t ¼ 120, 130 ps also results from the coking process. It can be also found that the duration of the coking process is very short due to the high temperature; the coking products are soon broken into small fragments as the temperature keeps on increasing. The much more uniform distribution of C atoms can be observed at t ¼ 150 ps.
Mechanism of H2 production It is of great significance to understand the mechanism of the H2 generation and consumption during the gasification process. Fig. 5 illustrates the temporal variation of the number of the H2 molecules produced in the system. According to the generation rate of H2, the gasification process can be divided into two different stages, namely the rapid reaction stage
(Stage 1) and the stable reaction stage (Stage 2). At Stage 1, the amounts of H2, CO2, C1, and C2 molecules increase rapidly from t ¼ 120 ps to t ¼ 350 ps (as shown in Fig. 5(A)). Then at Stage 2, the numbers of H2, CO2, and C1 increase slowly until the growth rates decrease to zero at t z 1000 ps. Fig. 5(C) shows the quantitative relationship between H2 and CO2. It can be found that the growth rate of H2 is much greater than CO2 with a variable proportion between them at Stage 1, however, the proportion between the amount of H2 and CO2 comes to keep in a fixed value at Stage 2. It implies the pyrolysis of the coal particle is the non-negligible source of H2 generation at Stage 1, in which the proportion between H2 and CO2 depends on the evolvement and the distribution of C structures during the pyrolysis. At Stage 2, H2 and CO2 mainly come from the reactions with the C1-2 fragments so that the proportion between H2 and CO2 keeps in the fixed value. The
Fig. 5 e Temporal variation of products during the gasification process in Case4. (A) Temporal variation of H2, CO2, C1, and C2; (B) Temporal variation of C, H, and O content; (C) Quantitative relationship between H2 and CO2. Please cite this article as: Chen J et al., Molecular dynamics investigation on the gasification of a coal particle in supercritical water, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.12.002
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detailed reaction listed in Tables 4e5 can also prove the conclusion, which will be discussed in the following chapter. Fig. 5(B) demonstrates the temporal variation of C, H and O contents in the organic compounds. Before t ¼ 290 ps, the percentage of the C and H contents is decreasing and that of the O content is rising apparently. It results from the oxidation of organic compounds and the departure of H. The slight rise of the H content after t ¼ 290 ps results from the increasingly generation of intermediate CHO compounds during the gasification process. In order to obtain the detailed chemical reaction mechanism, the bond order information, which reflects the interaction between different atoms, is examined at a certain interval of time by MATLAB. The detailed chemical reactions during the time interval thus can be obtained based on the variation of the bonds for every atom. Meanwhile, the occurring times of every reaction during the time interval are used to obtain the occurrence frequency, which is defined as: FRi ¼
n X nRi;j tSI Dt tSL j¼1
where: tSI is the statistical sampling interval in which all the samples are picked, tSI ¼ 2 ps is selected in the present paper, tSL is the sampling duration for every sample, tSL ¼0.4 ps is selected in the present paper, nRij is the times of the reaction i that happens in the sample j, Dt is the period that every time of sampling lasts. The occurrence frequency represents the activity of the reaction.
Rapid reaction stage (stage 1) Table 4 lists the main reaction paths to generate H2, CO2, OH radicals and H radicals at Stage 1 from 100 ps to 350 ps, together with the corresponding occurrence frequencies. As listed in Table 4, the forward reaction (generation) FRi ðpsÞ1 represents occurrence frequencies of forward reaction in which the target product is generated, reverse reaction (consumption) FRi ðpsÞ1 represents occurrence frequencies of reverse reaction in which the target product is consumed as the reactant, the net production FRi ðpsÞ1 represents occurrence frequencies of the reaction generating the net production. Based on the FRi ðpsÞ1 of the net production, the numbers of H2 and CO2 molecules generated at Stage 1 can be calculated to be 5855 and 775, respectively. They are basically consistent
Table 4 e Reaction path and occurrence frequency of each reaction path at Stage 1 in Case4. Production
H2
CO2
OH
H
Path
Total total (carbon related) 1e1 1e2 2 3
Total 1e1 1e2 2 Total 1 total (carbon related) 2e1 2e2 3e1 3e2 Total 1 total (carbon related) 2e1 2e2 3e1 3e2
Characteristic reaction
C12 þ H2 O4H2 þ C12 O þ OH C12 þ H2 O4H2 þ （C12 þ HO）=C12 O Ccps 4H2 þ Ccps ðH3 OÞ=ðH2 O þHÞ4H2 þ OH H3 O þ H2 O4H2 þ H3 O2 H3 O=H þ H4ðH2 Þ=ðH2 þH2 OÞ C12 O 4CO2 þ H C12 O 4CO2 þ C12 C3þ O 4CO2 þ Ccps Ccps O þ H2 O4CO2 þ Ccps ðH3 OÞ=ðH2 O þHÞ=H2 O4ðH2 =HÞ þ OH H3 O2 4H2 O þ OH
C12 þ H2 O4C12 þ OH C3þ þ H2 O4C3þ þ OH C12 O 4C12 þ OH C3þ O 4C3þ þ OH H3 O=ðH2 þHOÞ4H þ H2 O H2 O4H þ OH
C12 4C12 =CO2 þ H C3þ 4C3þ þ H C12 þ H2 O4H þ C12 O =CO2 C3þ þ H2 O4H þ C3þ O
Forward Reaction (Generation) FRi ðpsÞ1
Reverse Reaction (Consumption) FRi ðpsÞ1
Net Production FRi ðpsÞ1
85.55 30.87
62.12 18.90
23.42 11.97
6.67 3.77 8.70 38.81 4.70 6.57 6.27 1.78 1.42 1.17 1.00 235.17 41.65 114.61 66.07
3.47 1.42 5.57 30.73 3.29 4.62 3.17 1.74 0.41 0.27 0.42 232.85 32.38 114.04 73.12
3.2 2.35 3.12 8.09 1.41 1.95 3.10 0.04 1.01 0.90 0.57 2.32 8.34 0.57 7.05
21.65 8.32 20.15 10.72 87.32 43.94 3.43 32.95
19.87 7.97 24.47 14.55 85.90 50.21 3.41 25.27
1.77 0.35 4.32 3.82 1.43 6.27 0.02 7.67
13.32 8.80 2.45 1.85
11.32 4.95 1.37 0.62
2.00 3.85 1.07 1.22
C12 , C1 molecules and C2 molecules;C3þ , C3þ molecules; Ccps , Organic molecules; C12 O , C1 molecules and C2 molecules with higher oxygen content;C3þ O , C3þ molecules with higher oxygen content; Ccps O , Organic molecules with higher oxygen content.
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Table 5 e Reaction paths and occurrence frequency of each reaction path at Stage 2 in Case4. Production
H2
CO2
OH
H
Path
Total 1
total (carbon related) 2 3 4 Total 1 2 3 4 Total 1 total (carbon related) 2 3 Total 1 total (carbon related) 2 3 4
Characteristic reaction
ðH3 OÞ=ðH2 O þHÞ4H2 þ OH H3 O þ H2 O4H2 þ H3 O2 H3 O=H þ H4ðH2 Þ=ðH2 þH2 OÞ
C12 þ H2 O4H2 þ C12 O þ OH C12 þ H4H2 þ C12 O C12 4H2 þ C12 O C12 O þ H2 O4CO2 þ C12 C12 4CO2 þ H C12 O 4CO2 þ C12 =H2 O C12 O þ OH4CO2 þ C12 =H2 O CO2 4C0 ðH3 OÞ=ðH2 O þHÞ=H2 O4ðH2 =HÞ þ OH H3 O2 4H2 O þ OH
C12 O 4C12 þ OH C12 O þ H2 O4C12 þ OH H3 O=ðH2 þOHÞ4H þ H2 O H2 O4H þ OH
C12 4C12 =CO2 þ H C12 þ H2 O4C12 þ H C12 þ OH4C12 þ H
Forward Reaction (Generation) FRi ðpsÞ1
Reverse Reaction (Consumption) FRi ðpsÞ1
Net Production FRi ðpsÞ1
501.04 283.83 29.90 55.03 108.19
497.29 282.85 29.68 54.94 105.74
3.76 0.98 0.22 0.10 2.45
34.66 18.87 27.16 66.76 23.25 16.27 16.02 2.6 0.01 688.80 277.31 352.32 41.23
32.84 18.32 27.34 64.04 19.98 10.52 18.8 4.73 1.79 688.86 277.27 353.76 39.83
1.82 0.55 0.18 2.72 3.26 5.75 2.78 2.13 1.78 0.06 0.04 1.42 1.40
34.23 3.83 210.89 126.40 9.5 63.49
33.43 3.22 211.21 129.56 9.42 60.76
0.80 0.60 0.12 3.16 0.08 2.84
29.54 15.24 2.81
27.21 14.73 2.48
2.33 0.51 0.33
C12 , C1 molecules and C2 molecules; C12 O , C1 molecules and C2 molecules with higher oxygen content.
with the MD results, which are 5608 and 898 respectively. It proves the correctness of the FRi ðpsÞ1 calculation. The reactions to produce H2 are mainly classified into 3 reaction paths. Path 1 is the reactions between organic fragments and water molecules, including the reactions between C1-2 molecules and H2O molecules (Path 1-1) and the reactions between C3þ molecules and H2O molecules (Path 1e2). Path 1 presents the gasification reactions of the organic C structures to generate H2, and it occupies 1/4 of the H2 net production. Path 2 is the reaction between organic fragments or pyrolysis of single organic fragments which occupies 1/8 of the H2 net production. Path 3 is the reactions independent of the carbon. In these reactions, an H radical usually reacts with an inorganic fragment (mainly water) to form an H2 molecule which occupies about 1/2 the H2 net production. Apparently, H radicals are the most important source of H2 at this stage, it is necessary to discuss the generation of H radicals. Path 2, including the pyrolysis of C1-2 molecules (Path 2-1) and the pyrolysis of C3þ molecules (Path 2-2), is the most important source to produce H radicals at this stage and it occupies about 3/4 of the H radicals net production. Path 3 is the reactions between organic fragments s and water molecules, and it accounts for about 1/4 of the H radicals net production. Therefore, Together with Path 3 in H2 generation, it can be easily concluded that the pyrolysis reaction of the organic C structures is the largest source of H2 generation at Stage 1, which
accounts for about 1/2 of H2 generation. Meanwhile, the gasification reaction of the organic C structures with the water is also an important source of H2 generation (about 3/8). A similar classification method is used to manage the reactions generating CO2, OH radicals (as shown in Table 4). The pyrolysis is the dominant source of CO2 and all the CO2 molecules are released due to the breakage of large organic fragments in the early process when the temperature is relatively low. It can partly explain the high proportion of CO2 in the gas product under the conditions of the low temperature [25,26] in the experiment. The OH radicals are mainly generated in the reactions of paths 1 and 2-1 and mainly consumed in the reactions of paths 3-1 and 3-2. This demonstrates that the OH radicals come from water, and are consumed in the reactions with the organic fragments. As an intermediate product, the OH radicals play an important role in the hydrolysis of coal particles. It can explain that the addition of basic catalyst in the experiments can catalyze the generation of H2 at a lower temperature [50,51], in which the catalyst was used to promote the generation of OH radicals and the reverse oxidation reaction of organic fragments.
Steady reaction stage (stage 2) Table 5 lists the main reaction paths to generate H2, CO2, OH radicals and H radicals at Stage 2 from 350 ps to 1000 ps, together with the corresponding occurrence frequencies. The
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Fig. 6 e Effect of temperature on the evolution of the carbon structure （A）C1-2; (B) C3-13; (C) C14-39; (D) C40þ.
most obvious difference between Stage 2 and Stage 1 is the disappearance of the C3þ fragments in Table 5. It is because the number of C3þ fragments tends to be invariant at such low levels at stage 2 that C3þ molecules related reactions can no more be observed. Because of the disappearance of C3þ fragments, the generation rate of H radicals decreases apparently. It makes the reactions in Path 1 no more the dominant source of H2 generation, which occupies only about 1/3 at Stage 2 instead of about 50% at Stage 1. Path 2, the dehydrogenation and gasification of C1-2 molecules, comes to be the leading source and occupies about 50% of the H2 generation. Unlike at Stage 1, the pyrolysis reaction of the organic C structures is no more the largest source of H2 generation at Stage 2, which accounts for about 3/10 of H2 generation. The gasification reaction between the organic C structures and the water comes to be the largest source of H2 generation, which accounts for about 3/5 of H2 generation. It can be observed that the gasification and dehydrogenation of C1-2 small molecules are still the main sources of CO2. Because of the improvement of the CO2 and H2, two paths (paths 3 and 4) consuming CO2 can be found. It implies the reversible reactions are gradually close to the equilibrium state in the case. The negative net production in Path 1 for OH radical generation means the OH radicals can prevent the generation of H radicals and H2 by reacting with them to produce water at Stage 2. Such a role of OH radicals is quite different from that at Stage 1, in which the OH radicals oxidize the organic fragments and subsequently can accelerate the generation of H2. The change of the role of OH radicals in H2 generation implies the change of generation mechanism of H2.
Fig. 7 e Effect of temperature on the C, H and O contents in organic molecules.
Please cite this article as: Chen J et al., Molecular dynamics investigation on the gasification of a coal particle in supercritical water, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.12.002
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Fig. 8 e Effect of temperature on the yield of CO2 and H2 (A) H2; (B) CO2.
Effect of temperature Fig. 6 shows the temporal evolution of the carbon structure at different temperatures. No obvious difference can be observed
for the amount of evolution of the C40þ fragments between the three different temperature settings. The temporal evolution shows that the pyrolysis of the C40þ fragments proceeds in the heating process and finishes before the final reaction
Fig. 9 e Effect of coal concentration on the evolution of the carbon structure (A) C1-2; (B) C3-13; (C) C14-39; (D) C40þ. Please cite this article as: Chen J et al., Molecular dynamics investigation on the gasification of a coal particle in supercritical water, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.12.002
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Fig. 10 e Effect of coal concentration on the C, H and O contents in organic molecules.
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temperature. However, reduction in amounts of the C14-39 and C3-13 fragments at T ¼ 3000 K is significantly slower than those at T ¼ 3500 K and 4000 K. And the final amounts of the C1-2 and C3-13 fragments at T ¼ 3000 K are apparently higher than those at T ¼ 3500 K and 4000 K. It denotes the gasification reaction of the fragments with the SCW at T ¼ 3000 K is not as adequate as those at T ¼ 3500 K and 4000 K. Fig. 7 shows the variation of the C, H and O contents in organic fragments with temperature. The H content experiences two periods, it decreases quickly first and then increases fast. The decrease of the H content can be explained as the dehydrogenation in the pyrolysis of the organic fragments in coal particle. Because the temperature is relatively low in this period, the effect of the gasification is not very apparent. Temperature can effectively influence the pyrolysis of the organic fragments, it can be easily observed in the figure the lowest H content at t z 250 ps when the temperature is 4000 K, and highest when the temperature is 3000 K. As the temperature rises with time, the gasification reactions come to be more active. According to the H2 generation mechanism in Section Evolution of the carbon structure, two different kinds of reactions are included in gasification reactions: OH oxidation of organic fragments to generate CHO fragments and reactions between the organic fragments and the water to generate H2. The intensified OH oxidation can effectively improve the H content. Meanwhile, the O content is also improved quickly together with the decrease of the C content as the OH oxidation proceeds. H2 generation mechanism at Stage 2 shows the declination of OH oxidation in the late period of the gasification process. It denotes the reactions between the organic fragments and the water to generate H2 and CO2 come to be more important. The increase in temperature can enhance this kind
Fig. 11 e Effect of coal concentration on the yields of CO2 and H2 (A) CO2; (B) H2. Please cite this article as: Chen J et al., Molecular dynamics investigation on the gasification of a coal particle in supercritical water, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.12.002
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of reaction so that the products of H2 and CO2 are apparently improved (as shown in Fig. 8). It can be found the discrepancy of the final amount of the C1-2 in Fig. 6 molecules between the different temperatures is basically equivalent to the discrepancy of CO2 in Fig. 8 (B).
Effect of coal concentration Figs. 9e11 illustrate the effect of the coal concentration on the process. It is observed that the effect of the coal concentration is much less on the evolution of the carbon structure evolution than the temperature so that it is almost negligible. However, the productions of H2 and CO2 at the low coal concentrations are much higher than those at the high concentration. The change of the reaction results from the coal concentration is also reflected in the variation of the C, H and O contents in organic fragments. While the coal concentration decreases from 32% to 18%, the O content increases evidently. However, the C and H contents decrease (as shown in Fig. 10). The reason is the promotion of the reaction process by the decrease of coal concentration. The gasification reactions in SCW are reversible chemical reactions, and the equilibrium will move toward further gasification when the water fraction inside the system is improved. As a result, the products of H2 and CO2 are improved obviously (as shown in Fig. 11).
Transition of nitrogen Thecoal combustion processisknown asone ofthemain sources of nitrogen oxides (NOX) production which causes the atmosphere pollution such as photochemical smog and acid rain. Table 6 shows the detailed molecular formula containing N and the corresponding number of the molecules at t ¼ 1600 ps when the reaction is nearly finished and the new steady equilibrium state has been obtained. According to the number of the molecules, it can be found that CN, CHN, and CHON are the dominant product containing N (>70% of all N-containing molecules), which are the precursors of NH3. Tracking all the N atoms discloses that every N atoms experience either of two basic molecular formulas of CN and CHN. This is consistent with the
Table 6 e List of N-containing product molecules in Case4. Molecular formula CHN CN CHON CH2N CON N2 CH3N ON CH2O2N C3H2O2N H2N CH2ON C 2N C2H3N C2O2N C2H2N
Number of molecules 35 26 10 6 4 3 2 2 1 1 1 1 1 1 1 1
Fig. 12 e Temporal variation in the quantity of CN, CHN, and CHON in Case4.
experimental results that CHN is a primary product [52]. Fig. 12 shows a change in the quantity of CN, CHN, and CHON with the time throughoutthe entire gasification process inthesimulation.
Conclusions In the present paper, the ReaxFF MD investigation on the full gasification process of a coal particle in SCW is conducted. The detailed reaction mechanism, the effect of temperature and coal concentration, and the transition of N are disclosed. (1) The detailed reaction mechanism of the full gasification is obtained. According to the generation of H2, the gasification process can be divided into two different stages. At Stage 1, the pyrolysis reaction is the largest source of H2 generation and the OH radicals can improve the oxidation of organic fragments. While at Stage 2, the gasification reaction comes to be the largest source and the OH radicals prevent the production of H radicals and H2. (2) The temperature can significantly influence the evolution of the carbon structure and the pyrolysis of the organic fragments. Meanwhile, it can also intensify OH oxidation of organic fragments and reactions between the organic fragments and the water to generate H2 and CO2. (3) The decreasing the coal concentration in the reaction system can significantly raise the yields of H2 and other products, but no obvious effect can be observed on the temporal evolution of the carbon structure. (4) The transition of the N in a coal particle is revealed. CN, CHN, and CHON are the basic molecular structures for nitrogen atoms during the gasification process at high temperature, which are the precursors of NH3.
Acknowledgments The authors gratefully acknowledge the financial support of the National Key Research and Development Program of
Please cite this article as: Chen J et al., Molecular dynamics investigation on the gasification of a coal particle in supercritical water, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.12.002
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China (Grant No: 2016YFB0600101) and the Natural Science Fund of China (Grant No: 91741103).
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Molecular dynamics investigation on the gasification of a coal particle in supercritical water Jian Chen a, Xiong Pan a, Hanqing Li b, Hanhui Jin c,*, Jianren Fan b a
School of Energy and Power Engineering, University of Shanghai for Science and Technology, Shanghai, 200093, China b State Key Laboratory of Clean Energy Utilization, Zhejiang University, Hangzhou, 310027, China c School of Aeronautics and Astronautics, Zhejiang University, Hangzhou, 310027, China
highlights The whole SCW gasification process of a coal particle is studied with ReaxFF. The carbon structure evolution and the generation mechanism of H2 are studied. The two stages of gasification process are disclosed. The temperature can significantly influence pyrolysis of the organic fragments. The transition of N in the coal particle is revealed.
article info
abstract
Article history:
Coal gasification technology in supercritical water provides a clean and efficient way to
Received 24 September 2019
convert coal to H2. In the present paper, the whole supercritical water（SWC）gasification
Received in revised form
process of a coal particle is studied with the reactive force field (ReaxFF) molecular dy-
26 November 2019
namics (MD) method for the first time. First, the detailed reaction mechanism which can’t
Accepted 1 December 2019
be clearly illustrated in experiments, such as the evolution of the carbon structure during
Available online xxx
the gasification process and the detailed reaction mechanism of the main products, is obtained. According to the generation mechanism of H2, it is found that the supercritical
Keywords:
water gasification process of a coal particle can be divided into two stages with different
Coal
reaction mechanisms, namely the rapid reaction stage and the stable reaction stage. Then,
Gasification
the effects of temperature and coal concentration in the reaction system on the yield of H2
Supercritical water
are studied. Finally, the transition of N in the coal particle is revealed, in which the pre-
H2 production
cursors of NH3 such as CN, CHN, and CHON are the basic molecular structures for nitrogen
ReaxFF
atoms during the gasification process at high temperature. © 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
Introduction Coal is one of the most important energy sources which supplies one-quarter of the world’s primary energy [1,2].
Direct combustion is still the dominant way of coal utilization. However, the burning process produced a lot of harmful substances, such as NOX and SOX, which are the main pollution sources of the environment [3,4]. Coal gasification
* Corresponding author. E-mail address: [email protected] (H. Jin). https://doi.org/10.1016/j.ijhydene.2019.12.002 0360-3199/© 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved. Please cite this article as: Chen J et al., Molecular dynamics investigation on the gasification of a coal particle in supercritical water, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.12.002
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technology provides an alternative way for clean and efficient coal utilization. The main goal of the coal gasification technology is to produce hydrogen， which is a promising secondary non-pollution, efficient renewable energies [5,6]. During the past three decades, coal gasification in supercritical water has become one of the valuable technologies to convert coal to hydrogen. When water is in the supercritical state (both the temperature T > 374.3 C and the pressure P > 22.1 MPa), it presents some special physical properties such as high diffusivity [7e9], low viscosity and salvation [10e12]. Benefiting from the variation in the physical properties, the supercritical water gasification shows great advantages over traditional gasification. In the 1970s, Modell [13,14] was the pioneer who firstly applied supercritical water to convert organic materials like coal, biomass into useful gaseous products. From then on, the researchers all over the world started the relative investigation. In the 1980s and 1990s, some researchers [15e17] studied the extraction of low-rank coal in supercritical water at the temperature of 647e773 K experimentally, and they found that supercritical water was not only a solvent but also a reactant. Since the beginning of this century, people have begun to apply supercritical water to the coal gasification for hydrogen-rich gas production. Some researches focused on the influences of the operating parameters. Such as Bi’s group [18] conducted experiments to investigate the effect of temperature, water density and reaction time on the gas yields, and found the temperature was an important factor. Yamaguchi [19] investigated the non-catalytic gasification of Victorian brown coal in quartz batch reactors and found the significant effect of running temperature and feed concentration. These conclusions were also confirmed by Jin et al. [20,21]. Some researches focus on the improvement by using catalyst additions. Such as Lin et al. [22,23] studied the gasification of Taiheiyo coal in SCW with Ca(OH)2 and a small proportion of NaOH. Wang et al. [24] investigated the catalytic effect of calcium hydroxide and found the improvement of coal gasification with a decrease in the amount of residual char. Some researchers such as Guo’s group successfully developed a novel continuous-flow system for coal gasification in supercritical water to ensure the continuous and stable gasification [25,26], the hydrogen yield of their system reached 77.5 Mol/(kg coal) [27]. A pilot-scale demonstration plant driven by solar concentration system was even established by them [28]. Due to the complexity of the reaction processes in supercritical water coal gasification, it is difficult to illustrate the detailed reaction mechanism only through the experimental method. Nowadays, the reactive force field (ReaxFF) [29] molecular dynamics (MD) has become a useful tool to study complex reactions of hydrocarbons. With this method, researchers can numerically investigate the evolution of formation, transition, and dissociation of chemical bonds and obtain results with certain precision at the cost of a small amount of calculation. The effectiveness of ReaxFF MD has been proved in the coal pyrolysis and combustion [30,31], thermal decomposition of polymer [32], catalysis [33e35], gasification [36,37], and formation [38e40], etc. Jin et al. [34] investigated the supercritical water gasification process of anthracene macromolecule with both ReaxFF MD and density
functional theory (DFT) methods, and the process was compared with steam gasification and pyrolysis. They found that supercritical water effectively weakened the C(ring)C(ring) bond energy in anthracene, decreased the energy barrier of the ring-opening reaction, accelerated the gasification rate, and increased the hydrogen yield. Zhang’s group [37] studied the pyrolysis and hydrogen production of a coal macromolecule in supercritical water using the ReaxFF force field method combined with the DFT method. Results demonstrated that the main source of hydrogen molecules in the SCW-coal system was H radical-rich water clusters. Overall, these MD researches mainly focused on the decrease of the bond cracking energy due to the effect of the supercritical water molecule and structure changes of aromatic rings, and only the initial stage of the coal gasification was studied, in which no more than eight hydrogen molecules were generated in the simulations. Our group [41] recently studied the whole SCW gasification process of a coal macromolecule with the ReaxFF MD method, in which the detailed chemical reactions and pathways of H2 generation were disclosed. Although the macromolecule contains 76%e90% C content of a coal particle [42], the apparent difference between the macromolecule and the coal particle can’t be ignored because a real coal particle generally has a great deal of medium and little fragments. These fragments can influence the reaction process and the final products significantly. Hence, it is essential to carry out researches on the reaction mechanism of a real coal particle which is actually encountered in the gasification system. The objective of this paper is to investigate the SCW gasification process of a real coal particle with different fragments using the ReaxFF force field method, in which both the evolution of the carbon structure and the mechanism of H2 generation are studied. Meanwhile, the transition of the nitrogen inside the coal particle, which can’t be obtained in the studies of a coal macromolecule, is also investigated. In Section MD simulation, simulation methods and details are described. In Section Simulation validation, the comparison between the MD results and experimental data is conducted to validate the MD simulation. In Section Evolution of the carbon structure and Section Mechanism of H2 production, the reaction mechanism including the carbon structure evolution and the mechanism of H2 generation during the SCW gasification process is discussed. In Section Effect of temperature and Section Effect of coal concentration, the effect of temperature and coal concentration, which are the operating conditions that appeared in the gasification process, is studied. In Section Transition of nitrogen, the transition of the nitrogen is disclosed.
MD simulation Reactive force field method (ReaxFF) The ReaxFF proposed by Van Duin can continuously simulate the reaction process based on the normal MD method, utilizing the inter-atomic potential to model the interaction between atoms with a bond-order formalism, where the bond order is calculated empirically from the interatomic distances.
Please cite this article as: Chen J et al., Molecular dynamics investigation on the gasification of a coal particle in supercritical water, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.12.002
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In the ReaxFF, the potential energy function is usually given in the following form: Esystem ¼ Ebond þ Eover þ Eunder þ Eval þ Epen þ Etors þ Econj þ EvdWaals þ ECoulomb where Ebond is the bond energy, Eover and Eunder represent the over- and under-coordinated atoms in the energy contribution. Eval , Epen , Etors , Econj , EvdWaals and ECoulomb are the valence angle term, penalty energy term, torsion angle term, conjugation effects to molecular energy, non-bond van der Waals interactions, and Coulombic interactions, respectively. More details about the ReaxFF can also refer to Ref. [29,33].
Coal-water reaction system and simulation details To build the coal-water reaction system, a coal particle is set up based on the atomic structure first. In the present paper, the 3D molecular model of a coal particle is constructed following the way used by Zheng [30]. The Wiser model of the coal macromolecule is adopted to build the coal particle, together with 15 kinds of small molecular fractions. Both the Wiser model and the number of each molecular fraction are adjusted to fit the experimental data of the chemical compositions based on the averaged data of four kinds of coals [43e45]. Before the final 3D structure of the coal particle is obtained in a stable state with the minimum free energy, it is optimized in a DREIDING force field with the Materials Studio
software, then several times of anneal cycles are carried out. The final coal particle contains 19638 atoms in a cube with an edge length of 6.06 nm (as shown in Fig. 1). The detailed information on constructing the coal particle can also refer to Ref. [46]. To build the supercritical coal-water reaction system, the established coal particle is placed at the center of the box and surrounded H2O molecules randomly by MATLAB. Four reaction systems of S1, S2, S3, and S4 are set up to represent four different kinds of coal concentrations, respectively (as shown in Table 1). To maintain a stable pressure, the volume density of the H2O molecules in the systems is set as 0.2 g/cm3. Six different cases are built to investigate the effect of coal concentration and temperature on the gasification with LAMMPS software. Among these cases, Case2, Case3, and Case4 are used to investigate the effect of temperature. Case1, Case3, Case5, and Case6 are used to disclose the effects of coal concentration. The specific parameters of the 6 cases are listed in Table 2. In all the cases of simulation, the time step is set to 0.2 fs. The NVT ensemble is adopted, and the temperature of the system is corrected with the Berendsen thermostat at every time step. During the simulation, the temperature is increased from 100 K to 300 K firstly at the rate of 10 K/ps so that the initial reaction system can be fully relaxed and reach the equilibrium state at the low temperature. After the system reaches its equilibrium state, the temperature starts to be improved from 300 K to 2500 K at a faster rate of 22 K/ps to reduce the time cost of simulation. After the system temperature reaches about 2500 K, chemical reactions start to appear. The temperature is then increased slowly at the rate of 10 K/ps until the target temperature. After the temperature of the reaction system reaches at designated temperature (3000 K, 3500 K and 4000 K for different cases, respectively), different reaction periods are set for different temperatures so that the reactions of the gasification are fully developed (as shown in Table 2). In this period, the temperature keeps constant throughout the reaction process. Such a high temperature is used to improve the reaction rate so that the simulation of the reaction can be finished in a reasonable period of time.
Results and discussions Simulation validation
Fig. 1 e Coal particle model with formula C9570H8637O1147N99S18.
H2 is the desired and main product of coal gasification in SCW, and its yield is the main objective of concern in the process. The comparison of the yield of H2 (YH2) at different coal concentrations between the MD results and the four experimental
Table 1 e Parameters of different reaction systems. System S1 S2 S3 S4
Coal particle
Number of water molecules
Density of water molecules (kg/m3)
Concentration of coal (mass%)
Dimension(A)
1 1 1 1
57863 36738 26631 18252
0.2 0.2 0.2 0.2
12% 18% 25% 32%
210*210*210 184*184*184 172*172*172 155*155*155
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Table 2 e Parameters of the MD cases. Case
System
Coal concentration
Case1 Case2 Case3 Case4 Case5 Case6
S1 S2
12% 18%
S3 S4
25% 32%
Reaction Temperature 3500 3000 3500 4000 3500 3500
results is shown in Fig. 2 [20,21,25,47]. The operation parameters of four experiments are shown in Table 3. The MD results agree with the experimental results in tendency, showing that the coal concentration can effectively influence the yield of H2. However, the yield of H2 of the MD is obviously higher than that of the experimental results, this is due to the reaction temperature of the MD (3500 K) is higher than that of the reaction temperature of experiments (853e1223 K). The temperature is an important factor that could effectively improve H2 production. Salmon et al. [48,49] proved that such a difference in reactive temperature between simulation and experiment wouldn’t effectively influence the reaction process in the thermal decomposition of brown coal, in spite of the possible effect on the product distributions. In their studies, although the high reaction temperature led to the disappearance of C4 hydrocarbon in the gas phase during the simulation, the processes related to the formation of low molecular weight products and sequences of products formation agree well with previous experimental results. Therefore, the ReaxFF MD
Fig. 2 e Comparison of the YH2 between the MD results and the four experimental results at different coal concentrations.
K K K K K K
Reaction period 800 ps 850 ps 800 ps 1380 ps 800 ps 800 ps
simulation in the present paper is feasible to investigate the SCW gasification of a coal particle.
Evolution of the carbon structure The temporal evolutions of the number of the C atoms contained in major intermediate products C1-2, C3-13, C14-39, and C40þ in Case 4 is presented in Fig. 3. It can be found that the number of C atoms contained in C40þ fragments starts to decrease at t z 110 ps when the temperature T ¼ 2280 K. Along with the rapid degradation of C40þ fragments, the number of C1-2, C3-13, and C14-39 fragments rise rapidly. C40þ fragments disappear and C14-39 fragments reach the maximum at T ¼ 2959 K around t ¼ 170 ps. Subsequently, the number of the C14-39 fragments decreases rapidly during the period t ¼ 170~270 ps, accompanied by the obvious increase of the C12 fragments. The number of C3-13 fragments reaches the peak value and then starts to decrease during this period. Such a sequence of the variation reflects the pyrolysis process of the coal particle. Similar to the process of the coal combustion, the pyrolysis occurs first before the gasification reaction of the coal particle with the SCW. The amount of the C1-2 fragments
Fig. 3 e Temporal evolution of the carbon structures in Case 4.
Table 3 e Operation parameters of the four experiments.
Exp Exp Exp Exp
1 2 3 4
Temperature
Coal
Residence time
Catalyst
853 K 973 K 1253 K 973 K
Liangzhuang coal Hongliulin Bituminous Coal Yimin lignite Yimin lignite
5.0 kg/h 10 min 2 min 5 min
K2CO3 K2CO3 Non-catalyst K2CO3
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reaches the peak value at the moment t z 390 ps after which the number of the C3-13 fragments gradually turns to be stable. Since the C14þ fragments almost have been completely exhausted, such a stable state for C3-13 means the quasiequilibrium state of the reversible pyrolysis reaction between C3-13 and C1-2. As the gasification reaction of C1-2 with the SCW proceeds and the number of the C1-2 decreases, the number of C3-13 fragments decreases slightly. The amount of the C1-2 fragments remains to be stable until the new equilibrium state is obtained after about 1000 ps.
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Fig. 4 shows the probability density distribution of the number of C atoms in C1eC200 molecules at different times. It demonstrates the transition of the C structures inside the particle as the gasification process proceeds. The general tendency of the C atoms’ distribution moving from the large carbon structures to the small carbon structures can be observed clearly. Meanwhile, coking processes arise in the early stage. Obviously, the appearance of C194 is the product of coking because the largest C contained macromolecule is C191 in the initial coal particle. Tracking of the C atoms shows that
Fig. 4 e Distribution of C atoms in the C1eC200 molecules at different times in Case4.
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the emergence of some C158 and C177 fragments at t ¼ 120, 130 ps also results from the coking process. It can be also found that the duration of the coking process is very short due to the high temperature; the coking products are soon broken into small fragments as the temperature keeps on increasing. The much more uniform distribution of C atoms can be observed at t ¼ 150 ps.
Mechanism of H2 production It is of great significance to understand the mechanism of the H2 generation and consumption during the gasification process. Fig. 5 illustrates the temporal variation of the number of the H2 molecules produced in the system. According to the generation rate of H2, the gasification process can be divided into two different stages, namely the rapid reaction stage
(Stage 1) and the stable reaction stage (Stage 2). At Stage 1, the amounts of H2, CO2, C1, and C2 molecules increase rapidly from t ¼ 120 ps to t ¼ 350 ps (as shown in Fig. 5(A)). Then at Stage 2, the numbers of H2, CO2, and C1 increase slowly until the growth rates decrease to zero at t z 1000 ps. Fig. 5(C) shows the quantitative relationship between H2 and CO2. It can be found that the growth rate of H2 is much greater than CO2 with a variable proportion between them at Stage 1, however, the proportion between the amount of H2 and CO2 comes to keep in a fixed value at Stage 2. It implies the pyrolysis of the coal particle is the non-negligible source of H2 generation at Stage 1, in which the proportion between H2 and CO2 depends on the evolvement and the distribution of C structures during the pyrolysis. At Stage 2, H2 and CO2 mainly come from the reactions with the C1-2 fragments so that the proportion between H2 and CO2 keeps in the fixed value. The
Fig. 5 e Temporal variation of products during the gasification process in Case4. (A) Temporal variation of H2, CO2, C1, and C2; (B) Temporal variation of C, H, and O content; (C) Quantitative relationship between H2 and CO2. Please cite this article as: Chen J et al., Molecular dynamics investigation on the gasification of a coal particle in supercritical water, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.12.002
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detailed reaction listed in Tables 4e5 can also prove the conclusion, which will be discussed in the following chapter. Fig. 5(B) demonstrates the temporal variation of C, H and O contents in the organic compounds. Before t ¼ 290 ps, the percentage of the C and H contents is decreasing and that of the O content is rising apparently. It results from the oxidation of organic compounds and the departure of H. The slight rise of the H content after t ¼ 290 ps results from the increasingly generation of intermediate CHO compounds during the gasification process. In order to obtain the detailed chemical reaction mechanism, the bond order information, which reflects the interaction between different atoms, is examined at a certain interval of time by MATLAB. The detailed chemical reactions during the time interval thus can be obtained based on the variation of the bonds for every atom. Meanwhile, the occurring times of every reaction during the time interval are used to obtain the occurrence frequency, which is defined as: FRi ¼
n X nRi;j tSI Dt tSL j¼1
where: tSI is the statistical sampling interval in which all the samples are picked, tSI ¼ 2 ps is selected in the present paper, tSL is the sampling duration for every sample, tSL ¼0.4 ps is selected in the present paper, nRij is the times of the reaction i that happens in the sample j, Dt is the period that every time of sampling lasts. The occurrence frequency represents the activity of the reaction.
Rapid reaction stage (stage 1) Table 4 lists the main reaction paths to generate H2, CO2, OH radicals and H radicals at Stage 1 from 100 ps to 350 ps, together with the corresponding occurrence frequencies. As listed in Table 4, the forward reaction (generation) FRi ðpsÞ1 represents occurrence frequencies of forward reaction in which the target product is generated, reverse reaction (consumption) FRi ðpsÞ1 represents occurrence frequencies of reverse reaction in which the target product is consumed as the reactant, the net production FRi ðpsÞ1 represents occurrence frequencies of the reaction generating the net production. Based on the FRi ðpsÞ1 of the net production, the numbers of H2 and CO2 molecules generated at Stage 1 can be calculated to be 5855 and 775, respectively. They are basically consistent
Table 4 e Reaction path and occurrence frequency of each reaction path at Stage 1 in Case4. Production
H2
CO2
OH
H
Path
Total total (carbon related) 1e1 1e2 2 3
Total 1e1 1e2 2 Total 1 total (carbon related) 2e1 2e2 3e1 3e2 Total 1 total (carbon related) 2e1 2e2 3e1 3e2
Characteristic reaction
C12 þ H2 O4H2 þ C12 O þ OH C12 þ H2 O4H2 þ （C12 þ HO）=C12 O Ccps 4H2 þ Ccps ðH3 OÞ=ðH2 O þHÞ4H2 þ OH H3 O þ H2 O4H2 þ H3 O2 H3 O=H þ H4ðH2 Þ=ðH2 þH2 OÞ C12 O 4CO2 þ H C12 O 4CO2 þ C12 C3þ O 4CO2 þ Ccps Ccps O þ H2 O4CO2 þ Ccps ðH3 OÞ=ðH2 O þHÞ=H2 O4ðH2 =HÞ þ OH H3 O2 4H2 O þ OH
C12 þ H2 O4C12 þ OH C3þ þ H2 O4C3þ þ OH C12 O 4C12 þ OH C3þ O 4C3þ þ OH H3 O=ðH2 þHOÞ4H þ H2 O H2 O4H þ OH
C12 4C12 =CO2 þ H C3þ 4C3þ þ H C12 þ H2 O4H þ C12 O =CO2 C3þ þ H2 O4H þ C3þ O
Forward Reaction (Generation) FRi ðpsÞ1
Reverse Reaction (Consumption) FRi ðpsÞ1
Net Production FRi ðpsÞ1
85.55 30.87
62.12 18.90
23.42 11.97
6.67 3.77 8.70 38.81 4.70 6.57 6.27 1.78 1.42 1.17 1.00 235.17 41.65 114.61 66.07
3.47 1.42 5.57 30.73 3.29 4.62 3.17 1.74 0.41 0.27 0.42 232.85 32.38 114.04 73.12
3.2 2.35 3.12 8.09 1.41 1.95 3.10 0.04 1.01 0.90 0.57 2.32 8.34 0.57 7.05
21.65 8.32 20.15 10.72 87.32 43.94 3.43 32.95
19.87 7.97 24.47 14.55 85.90 50.21 3.41 25.27
1.77 0.35 4.32 3.82 1.43 6.27 0.02 7.67
13.32 8.80 2.45 1.85
11.32 4.95 1.37 0.62
2.00 3.85 1.07 1.22
C12 , C1 molecules and C2 molecules;C3þ , C3þ molecules; Ccps , Organic molecules; C12 O , C1 molecules and C2 molecules with higher oxygen content;C3þ O , C3þ molecules with higher oxygen content; Ccps O , Organic molecules with higher oxygen content.
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Table 5 e Reaction paths and occurrence frequency of each reaction path at Stage 2 in Case4. Production
H2
CO2
OH
H
Path
Total 1
total (carbon related) 2 3 4 Total 1 2 3 4 Total 1 total (carbon related) 2 3 Total 1 total (carbon related) 2 3 4
Characteristic reaction
ðH3 OÞ=ðH2 O þHÞ4H2 þ OH H3 O þ H2 O4H2 þ H3 O2 H3 O=H þ H4ðH2 Þ=ðH2 þH2 OÞ
C12 þ H2 O4H2 þ C12 O þ OH C12 þ H4H2 þ C12 O C12 4H2 þ C12 O C12 O þ H2 O4CO2 þ C12 C12 4CO2 þ H C12 O 4CO2 þ C12 =H2 O C12 O þ OH4CO2 þ C12 =H2 O CO2 4C0 ðH3 OÞ=ðH2 O þHÞ=H2 O4ðH2 =HÞ þ OH H3 O2 4H2 O þ OH
C12 O 4C12 þ OH C12 O þ H2 O4C12 þ OH H3 O=ðH2 þOHÞ4H þ H2 O H2 O4H þ OH
C12 4C12 =CO2 þ H C12 þ H2 O4C12 þ H C12 þ OH4C12 þ H
Forward Reaction (Generation) FRi ðpsÞ1
Reverse Reaction (Consumption) FRi ðpsÞ1
Net Production FRi ðpsÞ1
501.04 283.83 29.90 55.03 108.19
497.29 282.85 29.68 54.94 105.74
3.76 0.98 0.22 0.10 2.45
34.66 18.87 27.16 66.76 23.25 16.27 16.02 2.6 0.01 688.80 277.31 352.32 41.23
32.84 18.32 27.34 64.04 19.98 10.52 18.8 4.73 1.79 688.86 277.27 353.76 39.83
1.82 0.55 0.18 2.72 3.26 5.75 2.78 2.13 1.78 0.06 0.04 1.42 1.40
34.23 3.83 210.89 126.40 9.5 63.49
33.43 3.22 211.21 129.56 9.42 60.76
0.80 0.60 0.12 3.16 0.08 2.84
29.54 15.24 2.81
27.21 14.73 2.48
2.33 0.51 0.33
C12 , C1 molecules and C2 molecules; C12 O , C1 molecules and C2 molecules with higher oxygen content.
with the MD results, which are 5608 and 898 respectively. It proves the correctness of the FRi ðpsÞ1 calculation. The reactions to produce H2 are mainly classified into 3 reaction paths. Path 1 is the reactions between organic fragments and water molecules, including the reactions between C1-2 molecules and H2O molecules (Path 1-1) and the reactions between C3þ molecules and H2O molecules (Path 1e2). Path 1 presents the gasification reactions of the organic C structures to generate H2, and it occupies 1/4 of the H2 net production. Path 2 is the reaction between organic fragments or pyrolysis of single organic fragments which occupies 1/8 of the H2 net production. Path 3 is the reactions independent of the carbon. In these reactions, an H radical usually reacts with an inorganic fragment (mainly water) to form an H2 molecule which occupies about 1/2 the H2 net production. Apparently, H radicals are the most important source of H2 at this stage, it is necessary to discuss the generation of H radicals. Path 2, including the pyrolysis of C1-2 molecules (Path 2-1) and the pyrolysis of C3þ molecules (Path 2-2), is the most important source to produce H radicals at this stage and it occupies about 3/4 of the H radicals net production. Path 3 is the reactions between organic fragments s and water molecules, and it accounts for about 1/4 of the H radicals net production. Therefore, Together with Path 3 in H2 generation, it can be easily concluded that the pyrolysis reaction of the organic C structures is the largest source of H2 generation at Stage 1, which
accounts for about 1/2 of H2 generation. Meanwhile, the gasification reaction of the organic C structures with the water is also an important source of H2 generation (about 3/8). A similar classification method is used to manage the reactions generating CO2, OH radicals (as shown in Table 4). The pyrolysis is the dominant source of CO2 and all the CO2 molecules are released due to the breakage of large organic fragments in the early process when the temperature is relatively low. It can partly explain the high proportion of CO2 in the gas product under the conditions of the low temperature [25,26] in the experiment. The OH radicals are mainly generated in the reactions of paths 1 and 2-1 and mainly consumed in the reactions of paths 3-1 and 3-2. This demonstrates that the OH radicals come from water, and are consumed in the reactions with the organic fragments. As an intermediate product, the OH radicals play an important role in the hydrolysis of coal particles. It can explain that the addition of basic catalyst in the experiments can catalyze the generation of H2 at a lower temperature [50,51], in which the catalyst was used to promote the generation of OH radicals and the reverse oxidation reaction of organic fragments.
Steady reaction stage (stage 2) Table 5 lists the main reaction paths to generate H2, CO2, OH radicals and H radicals at Stage 2 from 350 ps to 1000 ps, together with the corresponding occurrence frequencies. The
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Fig. 6 e Effect of temperature on the evolution of the carbon structure （A）C1-2; (B) C3-13; (C) C14-39; (D) C40þ.
most obvious difference between Stage 2 and Stage 1 is the disappearance of the C3þ fragments in Table 5. It is because the number of C3þ fragments tends to be invariant at such low levels at stage 2 that C3þ molecules related reactions can no more be observed. Because of the disappearance of C3þ fragments, the generation rate of H radicals decreases apparently. It makes the reactions in Path 1 no more the dominant source of H2 generation, which occupies only about 1/3 at Stage 2 instead of about 50% at Stage 1. Path 2, the dehydrogenation and gasification of C1-2 molecules, comes to be the leading source and occupies about 50% of the H2 generation. Unlike at Stage 1, the pyrolysis reaction of the organic C structures is no more the largest source of H2 generation at Stage 2, which accounts for about 3/10 of H2 generation. The gasification reaction between the organic C structures and the water comes to be the largest source of H2 generation, which accounts for about 3/5 of H2 generation. It can be observed that the gasification and dehydrogenation of C1-2 small molecules are still the main sources of CO2. Because of the improvement of the CO2 and H2, two paths (paths 3 and 4) consuming CO2 can be found. It implies the reversible reactions are gradually close to the equilibrium state in the case. The negative net production in Path 1 for OH radical generation means the OH radicals can prevent the generation of H radicals and H2 by reacting with them to produce water at Stage 2. Such a role of OH radicals is quite different from that at Stage 1, in which the OH radicals oxidize the organic fragments and subsequently can accelerate the generation of H2. The change of the role of OH radicals in H2 generation implies the change of generation mechanism of H2.
Fig. 7 e Effect of temperature on the C, H and O contents in organic molecules.
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Fig. 8 e Effect of temperature on the yield of CO2 and H2 (A) H2; (B) CO2.
Effect of temperature Fig. 6 shows the temporal evolution of the carbon structure at different temperatures. No obvious difference can be observed
for the amount of evolution of the C40þ fragments between the three different temperature settings. The temporal evolution shows that the pyrolysis of the C40þ fragments proceeds in the heating process and finishes before the final reaction
Fig. 9 e Effect of coal concentration on the evolution of the carbon structure (A) C1-2; (B) C3-13; (C) C14-39; (D) C40þ. Please cite this article as: Chen J et al., Molecular dynamics investigation on the gasification of a coal particle in supercritical water, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.12.002
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Fig. 10 e Effect of coal concentration on the C, H and O contents in organic molecules.
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temperature. However, reduction in amounts of the C14-39 and C3-13 fragments at T ¼ 3000 K is significantly slower than those at T ¼ 3500 K and 4000 K. And the final amounts of the C1-2 and C3-13 fragments at T ¼ 3000 K are apparently higher than those at T ¼ 3500 K and 4000 K. It denotes the gasification reaction of the fragments with the SCW at T ¼ 3000 K is not as adequate as those at T ¼ 3500 K and 4000 K. Fig. 7 shows the variation of the C, H and O contents in organic fragments with temperature. The H content experiences two periods, it decreases quickly first and then increases fast. The decrease of the H content can be explained as the dehydrogenation in the pyrolysis of the organic fragments in coal particle. Because the temperature is relatively low in this period, the effect of the gasification is not very apparent. Temperature can effectively influence the pyrolysis of the organic fragments, it can be easily observed in the figure the lowest H content at t z 250 ps when the temperature is 4000 K, and highest when the temperature is 3000 K. As the temperature rises with time, the gasification reactions come to be more active. According to the H2 generation mechanism in Section Evolution of the carbon structure, two different kinds of reactions are included in gasification reactions: OH oxidation of organic fragments to generate CHO fragments and reactions between the organic fragments and the water to generate H2. The intensified OH oxidation can effectively improve the H content. Meanwhile, the O content is also improved quickly together with the decrease of the C content as the OH oxidation proceeds. H2 generation mechanism at Stage 2 shows the declination of OH oxidation in the late period of the gasification process. It denotes the reactions between the organic fragments and the water to generate H2 and CO2 come to be more important. The increase in temperature can enhance this kind
Fig. 11 e Effect of coal concentration on the yields of CO2 and H2 (A) CO2; (B) H2. Please cite this article as: Chen J et al., Molecular dynamics investigation on the gasification of a coal particle in supercritical water, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.12.002
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of reaction so that the products of H2 and CO2 are apparently improved (as shown in Fig. 8). It can be found the discrepancy of the final amount of the C1-2 in Fig. 6 molecules between the different temperatures is basically equivalent to the discrepancy of CO2 in Fig. 8 (B).
Effect of coal concentration Figs. 9e11 illustrate the effect of the coal concentration on the process. It is observed that the effect of the coal concentration is much less on the evolution of the carbon structure evolution than the temperature so that it is almost negligible. However, the productions of H2 and CO2 at the low coal concentrations are much higher than those at the high concentration. The change of the reaction results from the coal concentration is also reflected in the variation of the C, H and O contents in organic fragments. While the coal concentration decreases from 32% to 18%, the O content increases evidently. However, the C and H contents decrease (as shown in Fig. 10). The reason is the promotion of the reaction process by the decrease of coal concentration. The gasification reactions in SCW are reversible chemical reactions, and the equilibrium will move toward further gasification when the water fraction inside the system is improved. As a result, the products of H2 and CO2 are improved obviously (as shown in Fig. 11).
Transition of nitrogen Thecoal combustion processisknown asone ofthemain sources of nitrogen oxides (NOX) production which causes the atmosphere pollution such as photochemical smog and acid rain. Table 6 shows the detailed molecular formula containing N and the corresponding number of the molecules at t ¼ 1600 ps when the reaction is nearly finished and the new steady equilibrium state has been obtained. According to the number of the molecules, it can be found that CN, CHN, and CHON are the dominant product containing N (>70% of all N-containing molecules), which are the precursors of NH3. Tracking all the N atoms discloses that every N atoms experience either of two basic molecular formulas of CN and CHN. This is consistent with the
Table 6 e List of N-containing product molecules in Case4. Molecular formula CHN CN CHON CH2N CON N2 CH3N ON CH2O2N C3H2O2N H2N CH2ON C 2N C2H3N C2O2N C2H2N
Number of molecules 35 26 10 6 4 3 2 2 1 1 1 1 1 1 1 1
Fig. 12 e Temporal variation in the quantity of CN, CHN, and CHON in Case4.
experimental results that CHN is a primary product [52]. Fig. 12 shows a change in the quantity of CN, CHN, and CHON with the time throughoutthe entire gasification process inthesimulation.
Conclusions In the present paper, the ReaxFF MD investigation on the full gasification process of a coal particle in SCW is conducted. The detailed reaction mechanism, the effect of temperature and coal concentration, and the transition of N are disclosed. (1) The detailed reaction mechanism of the full gasification is obtained. According to the generation of H2, the gasification process can be divided into two different stages. At Stage 1, the pyrolysis reaction is the largest source of H2 generation and the OH radicals can improve the oxidation of organic fragments. While at Stage 2, the gasification reaction comes to be the largest source and the OH radicals prevent the production of H radicals and H2. (2) The temperature can significantly influence the evolution of the carbon structure and the pyrolysis of the organic fragments. Meanwhile, it can also intensify OH oxidation of organic fragments and reactions between the organic fragments and the water to generate H2 and CO2. (3) The decreasing the coal concentration in the reaction system can significantly raise the yields of H2 and other products, but no obvious effect can be observed on the temporal evolution of the carbon structure. (4) The transition of the N in a coal particle is revealed. CN, CHN, and CHON are the basic molecular structures for nitrogen atoms during the gasification process at high temperature, which are the precursors of NH3.
Acknowledgments The authors gratefully acknowledge the financial support of the National Key Research and Development Program of
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China (Grant No: 2016YFB0600101) and the Natural Science Fund of China (Grant No: 91741103).
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Please cite this article as: Chen J et al., Molecular dynamics investigation on the gasification of a coal particle in supercritical water, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.12.002