Article pubs.acs.org/EF
Molecular Dynamics Study of Combustion Reactions in a Supercritical Environment. Part 1: Carbon Dioxide and Water Force Field Parameters Refitting and Critical Isotherms of Binary Mixtures Artem ̈ E. Masunov,*,†,‡,§,∥,⊥,○ Arseniy A. Atlanov,†,# and Subith S. Vasu∇ †
NanoScience Technology Center, ‡Department of Chemistry, §Department of Physics, and ∥Florida Solar Energy Center, University of Central Florida, 12424 Research Parkway, Suite 400, Orlando, Florida 32826, United States ○ South Ural State University, 76 Lenin pr., Chelyabinsk 454080, Russia ⊥ National Research Nuclear University MEPhI, Kashirskoye shosse 31, Moscow 115409, Russia # Department of Chemistry and Biochemistry, Florida State University, 95 Chieftan Way, Dittmer Hall (DLC) Room 118, Tallahassee, Florida 32816, United States ∇ Center for Advanced Turbomachinery and Energy Research (CATER), Department of Mechanical and Aerospace Engineering, University of Central Florida, Orlando, Florida 32816, United States S Supporting Information *
ABSTRACT: The oxy-fuel−carbon dioxide combustion process is expected to drastically increase the energy efficiency and enable easy carbon sequestration. In this technology, the combustion products (carbon dioxide and water) are used to control the temperature and nitrogen is excluded from the combustion chamber, so that nitrogen oxide pollutants do not form. Therefore, in oxy-combustion, carbon dioxide and water are present in large concentrations in their transcritical state and may play an important role in kinetics. The computational chemistry methods may assist in understanding these effects, and molecular dynamics with a reactive force field (ReaxFF) seems to be a suitable tool for such a study. Here, we investigate applicability of the ReaxFF to describe the critical phenomena in carbon dioxide and water and find that several non-bonding parameters need adjustment. We report the new parameter set, capable of reproducing the critical temperatures and pressures. The critical isotherms of CO2/H2O binary mixtures are computationally studied here for the first time, and their critical parameters are reported. distances and are updated every time step. The force field parameters within the ReaxFF model are based on the DFT calculations for the relative energies of the reactants, products, and transition states for a benchmark set of reactive processes. In ReaxFF, both chemical bonds and van der Waals interactions are described by Morse functions, with their parameters depending upon the chemical environment of the interacting atoms.20 The variable atomic charges are calculated at each MD step by the charge equilibration technique and also depend upon the chemical environment. In fact, several MD studies used ReaxFF to investigate the chemical processes in supercritical water.21,22 Unlike ab initio MD, ReaxFF parameters often need to be fitted for a specific system under study. Fortunately, ReaxFF parameters for hydrocarbon oxidation had been published23 and validated.24−28 In this series of papers, we intend to use ReaxFF to investigate effects of the supercritical environment on some of the combustion reactions. Before that can be done, one has to make sure that the parameters of the force field describe the critical phenomena accurately. In our preliminary studies, we found that published ReaxFF does not reproduce the critical temperatures for either carbon dioxide (too low) or water (too high). This is likely to be a consequence of
1. INTRODUCTION Oxy-fuel−carbon dioxide combustion1 uses pure oxygen instead of air and returns the combustion products (CO2 and H2O) into the combustion chamber as diluents to reduce the temperature.2 Consequently, the products can be easily separated, which simplifies the task of carbon sequestration.3 Moreover, the main pollutants (nitrogen oxides) are never formed.4 Under room temperature and high pressures, CO2 exists in the liquid form, which adds a benefit of rapid expansion of the mixture from liquid to supercritical density5 and can significantly increase power plant efficiency.6−8 However, the effects of supercritical CO2 on the combustion kinetics had not been well understood.9 Computational chemistry methods can play an important role in this understanding,10−13 given technical difficulties of high-pressure kinetic studies.14 The computational studies of oxidation kinetics in supercritical water had been published.15−17 Traditional force fields are not well suited to describe the process of chemical bond breaking and forming, and ab initio molecular dynamics (MD) based on density functional theory (DFT) methods are typically used for those purposes.17 However, ab initio MD is computationally expensive, and significant effort was extended to develop force field alternatives. Empirical valence bond (EVB)18 and reactive force field (ReaxFF)19 represent two of them. ReaxFF is an empirical force field based on variable bond order, which permits for the smooth transition from non-bonded to bonded interactions. The bond orders are calculated from interatomic © 2016 American Chemical Society
Received: August 3, 2016 Revised: October 3, 2016 Published: October 4, 2016 9622
DOI: 10.1021/acs.energyfuels.6b01927 Energy Fuels 2016, 30, 9622−9627
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Figure 1. Predicted and experimental isotherms for (a) carbon dioxide and (b) water.
the parameter fitting protocol. In fact, only DFT data on small molecules and reactive complexes were used to fit all (including non-bonding) parameters. No dispersion correction to DFT was used, and no ensemble properties were included in the benchmark data. Clearly, some adjustments of non-bonding parameters are need. In this contribution, we report such adjustment and validate the new parameter set for the pure substances. Next, we investigate the CO2/H2O mixtures and predict their critical behavior.
cations to a minimum, parameters for CC and HH interactions were fixed at their original values. Rvdw for the OO interaction (repulsion) was reduced to reproduce the pressures at high density of CO2, and Rvdw and Dij parameters for the CO interaction were adjusted for the best fit of all densities. Likewise, Rvdw for the OH interaction was increased to fit the pressures at high density of H2O, and parameters for hydrogen bonding were adjusted to best fit all of the densities. The final parameter values are included in the Supporting Information. Their verification demonstrated no significant change in bond-breaking potential energy surfaces and is described in the last part of the Results and Discussion.
2. COMPUTATIONAL DETAILS 3. RESULTS AND DISCUSSION We begin with validation of the new parameter set for the isotherms of the pure substances. The results for CO2 and H2O calculations are reported in panels a and b of Figure 1, respectively. From Figure 1a, one can see that the 500, 330, and 310 K isotherms are predicted to be supercritical, while the 300 K isotherm is subcritical. From these data, one can estimate the predicted critical temperature (Tc) to be 305 K and the critical pressure (Pc) to be 74 atm. The experimental critical 304 K isotherm is flat at the measured critical pressure of 72 atm. It remains between 300 and 310 K isotherms at all densities. Similarly, Figure 1b demonstrates that the 630 K isotherm for H2O is predicted to be subcritical and 660 K is supercritical, while isotherms 640 and 650 K are both nearcritical. They bracket the critical isotherm, predicting Tc = 645 K and Pc = 248 atm. The experimental 647 K isotherm is flat at 15 atm below the calculated 640 K isotherm. Thus, our modified ReaxFF model accurately (within 5 K) reproduces both CO2 and H2O critical temperatures as well as the critical pressure for CO2. However, Pc calculated for H2O is somewhat overestimated (248 atm) compared to its experimental value (218 atm). More accurate isotherms were obtained previously with the rigid molecule force fields TraPPE34 and SPC/E.35 However, predictions with those force fields become less accurate when
All calculations were performed with the LAMMPS software package,29 compiled with the ReaxFF module.30 The system included 216 molecules in cubic boxes of various sizes. The periodical boundary conditions and NVT ensemble were used. The time step of 0.25 fs was found to be necessary to prevent breaking chemical bonds and decomposition of the molecules. TINKER tools31 were used to fill the box randomly with the molecules and for initial optimization to relax steric clashes. MOLDEN was used to visualize the trajectories.32 The systems were optimized for 1000 steps and then gradually heated to the target temperature for 10 ps, followed by 100 ps equilibration and 1 ns data collection. Each isotherm was built with 10 calculations for density of 0.1−1.1 g/mL. The presence of a local minimum on the isotherm was taken as evidence of subcritical behavior at a given temperature, while a monotonic increase of the pressure versus density was considered an evidence of the supercritical temperature. The constant pressure over two or more consecutive segments of the critical isotherm was taken as the critical pressure. Because transcritical behavior for a neat fluid is mostly determined by intermolecular interactions, only parameters for hydrogen-bonding and van der Waals interactions were adjusted. While parameters rhb and phb1 describe the equilibrium length and dissociation energy for hydrogen bonds, the respective properties of pairwise van der Waals interactions are described by Rvdw and Dij parameters (see the Supporting Information from ref 23 for exact definitions). We used one-dimensional optimization to reproduce the experimental isotherms for pure carbon dioxide and water, taken from the National Institute of Standards and Technology (NIST) database.33 The quality of the fit is presented in Figure 1 and described in the Results and Discussion. To keep modifi9623
DOI: 10.1021/acs.energyfuels.6b01927 Energy Fuels 2016, 30, 9622−9627
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Figure 2. Predicted pressure versus density isotherms for binary mixtures of carbon dioxide and water at various molar concentrations: (a) 10% water, (b) 20% water, (c) 30% water, (d) 40% water, (e) 50% water, (f) 60% water, (g) 70% water, (h) 80% water, and (i) 90% water.
molecular flexibility is included in the model.36 ReaxFF not only accounts for molecular flexibility but also describes the
chemical bond breaking. With such a broad spectrum of predictive capabilities, one should expect a compromise on some 9624
DOI: 10.1021/acs.energyfuels.6b01927 Energy Fuels 2016, 30, 9622−9627
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Energy & Fuels of the properties. To the best of our knowledge, this work presents the first attempt to use ReaxFF for predictions of transcritical properties for both water and carbon dioxide. Next, we use the modified ReaxFF parameters to investigate the binary mixtures of carbon dioxide and water. Here, the simulation boxes are built by replacing the fraction of CO2 molecules with H2O molecules in 10 M % steps. The calculated isotherms are shown in Figure 2. The critical temperatures and pressures, estimated from these data, are collected in Table 1 and Figure 3. One can see from this table that the critical Table 1. Predicted Critical Parameters for Carbon Dioxide/ Water Binary Mixtures water (M %)
Tc (K)
Pc (atm)
0 10 20 30 40 50 60 70 80 90 100
305 355 410 480 540 600 650 690 700 670 645
74 85 95 107 122 138 172 203 218 219 248
Figure 4. Relaxed potential energy scan along the reaction coordinate for the HO + CO → H + CO2 reaction (see the text for the definition).
parameters, is presented in Figure 4. One can see that the energy barrier and both dissociation limits obtained with the published and updated ReaxFF parameter sets remain within 2 kcal/mol of each other.
4. CONCLUSION We performed ReaxFF parameter fitting to reproduce critical behavior for pure carbon dioxide and water fluids by the MD method. The updated parameters were then used to build the pressure−density isotherms for the binary mixtures, which is not available from the experiment. The composition with 80 M % water was found to have the critical temperature of 700 K, higher than each of the individual components. On the basis of the performance of the new ReaxFF parameters, we recommend them for computational studies of oxy-combustion processes at a high pressure. To the best of our knowledge, this work presents the first attempt to use ReaxFF for predictions of transcritical properties for both water and carbon dioxide. The current work will aid the development of supercritical CO2 combustion systems, which are expected8 to work at high pressures (∼300 bar).
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ASSOCIATED CONTENT
S Supporting Information *
Figure 3. Predicted critical temperature versus composition for binary mixtures of carbon dioxide and water.
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.energyfuels.6b01927. Updated ReaxFF parameters for C/H/O combustion systems (PDF)
temperature reaches a maximum (700 K) in the mixture with 80 M % water. Although experimental data on CO2/H2O mixtures are not available, a similar behavior was reported for CO2/O237 and CO2/CH438 and other non-ideal binary mixtures. The composition of the mixture at which the maximum is reached is known as the azeotrope. Finally, we validate the updated ReaxFF parameter set for its ability to reproduce the chemical bond-breaking effects on the example of the HO + CO → H + CO2 reaction potential surface. This reaction is one of the most important reactions in combustion chemistry, it proceeds via the association− dissociation mechanism and formation of the short-lived radical intermediate HOCO.39 It is convenient to define the reaction coordinate for this system at having the origin at the equilibrium geometry of this intermediate, with a positive direction along the H−O bond dissociation and a negative direction along the central O−C bond dissociation. Relaxed potential energy scan, obtained with the published and updated ReaxFF
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AUTHOR INFORMATION
Corresponding Author
*Telephone: 1-407-374-3783. E-mail:
[email protected]. Notes
Disclaimer: This report was prepared as an account of work sponsored by an agency of the United States Government. Neither the United States Government nor any agency thereof nor any of their employees makes any warranty, express or implied, or assumes any legal liability or responsibility for the accuracy, completeness, or usefulness of any information, apparatus, product, or process disclosed, or represents that its use would not infringe privately owned rights. Reference herein to any specific commercial product, process, or service by trade name, trademark, manufacturer, or otherwise does not necessarily 9625
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constitute or imply its endorsement, recommendation, or favoring by the United States Government or any agency thereof. The views and opinions of authors expressed herein do not necessarily state or reflect those of the United States Government or any agency thereof. The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was supported in part by the Department of Energy (Grant DE-FE0025260). The authors acknowledge the National Energy Research Scientific Computing Center (NERSC) and the University of Central Florida Advanced Research Computing Center (https://arcc.ist.ucf.edu) for providing computational resources and support. Artëm E. Masunov is grateful to the Russian Science Foundation, Contract 14-4300052, operated by the Center of Photochemistry, Russian Academy of Science, and to the Government of the Russian Federation Act 211, contract #02.A03.21.0011.
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