Molecular dynamic investigation on hydrogen production by polycyclic aromatic hydrocarbon gasification in supercritical water

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Molecular dynamic investigation on hydrogen production by polycyclic aromatic hydrocarbon gasification in supercritical water Hui Jin*, Yue Wu, Liejin Guo, Xiaohui Su State Key Laboratory of Multiphase Flow in Power Engineering, Xi'an Jiaotong University, Xianning West Road, Xi'an 710049, PR China

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abstract

Article history:

The decomposition of polycyclic aromatic hydrocarbon (PAH) is the rate-determining step

Received 3 November 2015

for coal gasification in supercritical water (SCW). Anthracene, which is the simplest PAH,

Received in revised form

was selected as the model compound to investigate the gasification characteristics. The

8 December 2015

reactive force field method combined with the method of density functional theory was

Accepted 3 January 2016

utilised to investigate the SCW gasification process of anthracene, and the process was also

Available online xxx

compared with steam gasification and pyrolysis. Compared with pyrolysis, SCW effectively weakened the C(ring)-C(ring) bond energy in anthracene and decreased the energy barrier

Keywords:

of the ring-opening reaction by 558.22 kJ/mol. The effect of SCW on the anthracene gasi-

Anthracene

fication was revealed. This effect proved that supercritical water accelerated the gasifica-

Supercritical water gasification

tion rate and increased the hydrogen yield. The SCW molecule was converted into H

ReaxFF

radical-rich water clusters, which contributed to the main source of H2 production.

DFT

Copyright © 2016, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights

PAH

reserved.

Introduction The utilization method based on coal burning causes serious pollution problems. The efficient and clean utilization of coal is a major issue that is yet to be resolved at present [1,2]. Over the past few decades, the supercritical water (SCW) gasification of coal has become a research hotspot. This method is considered effective and clean with high carbon conversion and can convert coal into small molecules of gas [3,4]. Many researchers have studied the process of SCW gasification and have mainly focused on various feed stocks, the reactor design, and major thermodynamic analysis. The group of Bi [5e10] completed a series of pyrolysis experiments based on a

variety of feedstock [5e7,9,10] gasification in SCW They developed coal gasification in SCW with a short-running process, but the carbon conversion was higher than that in pyrolysis in N2. The group of Guo [3,4,11e13] contributed to optimization of the reactor and proposed a SCW fluidized bed reactor to eliminate the plugging problem and therefore realise continuous and stable operation. They also investigated and numerically simulated various reactors based on gasification kinetics. The group of Vostrikov [14e17] established a semi-continuous reactor for coal gasification and investigated the kinetic characteristics based on a homogenous non-reactive core and porous pore model. These above investigations showed that the product distributions of coal gasification in SCW are obviously different from those of

* Corresponding author. Tel.: þ86 2982660876. E-mail address: [email protected] (H. Jin). http://dx.doi.org/10.1016/j.ijhydene.2016.01.007 0360-3199/Copyright © 2016, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved. Please cite this article in press as: Jin H, et al., Molecular dynamic investigation on hydrogen production by polycyclic aromatic hydrocarbon gasification in supercritical water, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/ j.ijhydene.2016.01.007

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Fig. 1 e Molecular dynamics calculation flow chart.

steam gasification and pyrolysis. A SCW environment can effectively increase the yield of small molecules, especially H2 [17]. SCW has been reported to act as solvent and as reactant in the reaction process [18e20]. However, the effects of SCW on the reactions are yet to be comprehensively determined because of the unique characteristics of SCW. Hence, we need to be cautious in improving the yield of hydrogen in the coal gasification process in SCW. Liu and his colleagues [21] used a series of quantitative chemical calculation methods to study the possible reaction pathways between the carboxyl aromatic ring and hydroxyl aromatic ring with a brown coal model. However, their results did not involve SCW. The group of Zhang [22e25] simulated the molecular dynamics to study the effects of SCW on the cracking of a linear chain, aromatic ring openings and H2 production with the three-dimensional Wiser model. These above-mentioned contributions helped to reveal the mechanism and to promote the optimization of the reactor based on the SCW reaction environment. Polycyclic aromatic hydrocarbons (PAHs) are the main intermediate products of the biomass, petroleum and coal chemistry industries [26,27]. PAHs are also believed to be the rate-determining step in the thermal chemistry conversion process. As a typical and the simplest PAH, anthracene shares numerous common characteristics with coal and oil [22,28e35]. Thus, anthracene is chosen as the model

compound of PAH in this study. The method of reactive force field (ReaxFF) coupled with density functional theory (DFT) is used to investigate SCW gasification process of anthracene, and the process is also compared with traditional pyrolysis and steam gasification to investigate the effects of SCW on the gasification process.

Computational method Simulation details The calculations were performed using the DFT and ReaxFF methods of the Amsterdam Density Functional (ADF) software supplied by SCM Inc. And the overall approach was shown in Fig. 1. Firstly, the anthracene model and the water model were optimized using the ADF module in the ADF software. The model optimization and thermodynamic analysis of anthracene were conducted using DFT. The DFT calculations were at the level of the generalized gradient approximation using the BeckeeLeeeYaneParr function [22,36]. The DZP basis set was applied in this study. For comparison, three reaction systems were designed. These systems are listed in Table 1. The initial structures of the steam gasification system, supercritical water gasification system and pyrolysis system were built using the ReaxFF module in ADF software. Periodic boundary

Table 1 e The parameter setting of three simulation systems. System Pyrolysis Steam gasification SCW gasification

Anthracene (molecules)

Water (molecules)

Density/g$cm 3

Pressure/MPa

50 50 50

0 500 500

0.58 0.039 0.38

0.1 0.1 25

Please cite this article in press as: Jin H, et al., Molecular dynamic investigation on hydrogen production by polycyclic aromatic hydrocarbon gasification in supercritical water, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/ j.ijhydene.2016.01.007

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conditions in all directions were applied to eliminate possible surface effects. The effect of temperature on the reaction rate is much higher than that of the activation energy according to the Arrhenius formula [22]. Reactions with high energy barrier are more accelerated than the reactions with low energy barrier at high temperatures [34]. Many researchers tend to use an increased simulation reaction temperature to study a variety of reaction systems rather than the experimental temperature because of the limitation of the computational simulation time [37e43]. The group of Goddard tested the difference

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between the simulation and experimental temperatures of the pyrolysis of brown coal, and they concluded that the simulation temperature may affect the distribution of the reaction product, but not the path of the reaction [37,44]. Thus, the simulated temperature was set to 1873 K to ensure that the reaction can be observed in the range of computable time. Geometric relaxation was conducted on the systems as follows [45]: Firstly, the temperature of the system was slowly raised from 0 K to 300 K at a rate of 5 K/ps. Then, the temperature was increased from 300 K to 600 K and the NVT ensemble (i.e., constant atomic number, volume and

Fig. 2 e The ring-opening reaction of anthracene in different environment. Please cite this article in press as: Jin H, et al., Molecular dynamic investigation on hydrogen production by polycyclic aromatic hydrocarbon gasification in supercritical water, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/ j.ijhydene.2016.01.007

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Table 2 e Products of the three systems after 250 ps ReaxFF reactive dynamics simulation.

temperature) was used for 100 ps. The equilibrium structures were obtained after the NVT-MD simulation was configured as the initial models for the following simulations. Finally, a 20 ps uniform heating process was carried out from 600 K to 1873 K and the ReaxFF reactive molecular dynamics simulations were carried out in NPT ensemble (i.e., constant atomic number, pressure and temperature) for 500 ps. The time step was set as 0.25 fs for all calculations. All the simulations were performed with the ReaxFF module in the ADF software.

Results and discussion Bond cracking energy of anthracene The opening reaction of the aromatic substance is the ratedetermining step in the thermalchemical conversion process of coal [46,47]. The structure of coal consists of variety of small fragments after linear chain breaking. We used anthracene to substitute these small fragments in simulation. We were able to observe the main reaction paths in the ReaxFF reactive molecular dynamics simulations, then we proceed to DFT calculations to do the thermodynamic analysis, which led us to get the energy barrier of the reactions. So we got the following results through observation and calculation: the first bond cracking reaction occurs at the C(9)eC(12) bond (Fig. 2)in the pyrolysis system and that the cracking energy is 776.70 kJ/mol. Furthermore, in the SCW gasification system, the first bond cracking reaction occurs at the C(12)eC(13) bond (Fig. 2) and the cracking energy is 218.48 kJ/mol. But in steam gasification system, there are two main reaction paths. The one is same to the reaction path in pyrolysis system and the other one is showed in Fig. 2. The first bond cracking reaction occurs at the C(3)eC(6) bond (Fig. 2) and the cracking energy is 236.30 kJ/mol. Notably, the energy barrier of the ring-opening reaction of anthracene decreases by 558.22 kJ/mol in the SCW gasification system compared with the pyrolysis system, and SCW has a promoting effect on the opening reaction of the aromatic substance.

Fig. 3 e H2 molecules formed in the SCW gasification, steam gasification and pyrolysis systems in different reaction times. Please cite this article in press as: Jin H, et al., Molecular dynamic investigation on hydrogen production by polycyclic aromatic hydrocarbon gasification in supercritical water, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/ j.ijhydene.2016.01.007

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Effect of SCW on the aromatic ring opening The effects of SCW on the ring-opening reaction of the aromatic substance were investigated by comparing the SCW gasification and pyrolysis conditions. The processes involved in the gasification of PAH, consisted of structural distortion, change in the aromatic ring, linear chain composition and destruction into small molecules [22]. In particular, the C(9)C(12) bond was broken after a preliminary structural distortion in the pyrolysis system. In the steam gasification system, the reactions were a little complicated. For the first 50 ps during the reaction, the bond break is the same as it is in the pyrolysis system. Afterwards, H radicals in the system were added to the C(3), and then the C(3)-C(6) bond was broken. In the SCW gasification system, a hydrogen atom in the SCW was added to the C(12). The C(12)-C(13) bond was then broken, and the OH in SCW was connected to the C(13) at the same time. Afterwards, the C(8)eC(10) bond in the aromatic ring opened, and simple structures might have been produced. This step is the key for the ring-opening reaction of anthracene. After cracking and bonding, the structure under went further twisting, until the middle benzene ring was completely destroyed. Several experimental studies showed that coal gasification in a SCW reaction environment can produce more gas products than that in steam or N2 [5,7,48,49]. The gas yield was calculated to be significantly increased in SCW, which is consistent with the above mentioned experiments. This result was due to SCW, which acted as both reactant and catalyst to decrease the energy barrier of the aromatic ring-opening reaction and to promote the cracking of the aromatic ring into small molecules. The products of the three systems after 250 ps by ReaxFF reactive dynamics calculation at 1873 K are illustrated in Table 2. The products of the SCW gasification system were all small molecules in the same reaction time. These results indirectly reflected that SCW had a role in promoting the ring-opening reactions of PAHs and in preventing unwanted condensation polymerisation reactions.

Effect of SCW on hydrogen production One advantage of coal in SCW gasification is the high yield of hydrogen [4,12]. The reaction pathways were investigated to show the effects of SCW on the hydrogen production process in coal gasification. The comparison of the hydrogen molecules formed in the SCW gasification, steam gasification and pyrolysis systems at 1873 K by simulation is shown in Fig. 3.

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The numbers of hydrogen molecules in the three systems after 500 ps were 496,288 and 145. Thus, compared with pyrolysis and steam gasification, SCW accelerated the gasification rate and increased the yield of hydrogen. Notably, the number of H2 molecule for pyrolysis system is larger than that for steam gasification system when time is less than 120 ps, but after that the trend reverses. While analysing the reasons to this phenomenon, we could use the ring-opening reaction paths of anthracene in steam gasification as reference. Within 50 ps, H radicals were added to C(3) in anthracene in steam gasification system, which resulted in no H2 production in the system. After 50 ps, anthracene which have added H radicals were broken down and generated a lot of H radicals. And then H2 was starting to produce. In the simulation, atomic tracing was used to observe that most of the hydrogen atoms derive from SCW. The H2 production pathway was speculated after the cracking of the aromatic rings in anthracene. Several SCW molecules were converted into SCW clusters under high temperature and high pressure. OH radicals were then isolated from SCW clusters and were provided to the cyclic structures. Afterwards, SCW clusters became H radical-rich. Finally, H radicals in the SCW clusters interacted with each other to produce hydrogen. In addition, some H radicals would be release from OH on intermediates and they would become H radicals-rich water after combining with water. Then, H radicals-rich water would become H2 and water. This reaction is the main pathway to produce hydrogen in the SCW gasification system and this process is shown in Fig. 4.

Conclusion Anthracene was selected as the model compound to investigate the rate determining step in the coal gasification process in SCW, compared with the pyrolysis and steam gasification process. The method of ReaxFF combined with DFT was implemented. The simulation results showed that SCW serves as both reactant and catalyst to weaken the C(ring)-C(ring) bond cracking energy of the aromatic ring in anthracene from 776.70 kJ/mol to 218.48 kJ/mol compared with that in pyrolysis process. SCW accelerates the aromatic ring-opening reaction to produce more small molecules, such as H2, CH4, CO and CO2. SCW accelerates the gasification process and increases the hydrogen yield. H2 is produced from the H radicalrich water clusters. SCW water gasification is demonstrated to be an effective method of coal gasification.

Fig. 4 e The main reaction paths for H2 production in the SCW gasification system. Please cite this article in press as: Jin H, et al., Molecular dynamic investigation on hydrogen production by polycyclic aromatic hydrocarbon gasification in supercritical water, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/ j.ijhydene.2016.01.007

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Acknowledgement This work was financially supported by the National Natural Science Foundation of China (Contract No. 51306145 and 51236007) and the National Basic Research Program of China (Contract No. 2012CB215303).

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