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Molecular Dynamics Simulation of Hydrogen Diffusion into Brine: Implications for Underground Hydrogen Storage

Kalati, Saba | 2024

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  1. Type of Document: M.Sc. Thesis
  2. Language: Farsi
  3. Document No: 56840 (06)
  4. University: Sharif University of Technology
  5. Department: Chemical and Petroleum Engineering
  6. Advisor(s): Ayatollahi, Shahaboddin; Mahani, Hassan; Pourkhiabani, Nahid; Esmaeilbeig, Mohammad Amin
  7. Abstract:
  8. The high consumption of worldwide fossil energy and its detrimental effects on the environment and climate have prompted a transition toward renewable energy sources. The availability of renewable energy sources depends on weather and seasonal changes; therefore, an imbalance between supply and demand would be unavoidable. Hydrogen storage as an energy carrier, can fulfill this energy mismatch. Storing hydrogen in underground formations, such as aquifers, has been the subject of extensive research recently. In these storage reservoirs, hydrogen contacts the resident brine which will be lost through the dissolution-diffusion mechanism. Therefore, successful design of these storages requires in-depth knowledge of the hydrogen transport properties in the reservoir brine. On one hand, experimental data of hydrogen diffusion coefficients in aqueous reservoir solutions are scarce due to the high costs, and the time-consuming nature of the experiments. Additionally, the wide variety of dissolved salts in water and the diversity of existing conditions have further compounded the scarcity of information. Furthermore, there are significant discrepancies between the reported data in the published literature, which makes it necessary to obtain reliable diffusion coefficient data. Molecular dynamic simulations have been applied in recent years to calculate hydrogen diffusivity in water. However, most of those research works are focused on hydrogen diffusivity in pure water or NaCl-based solutions. Although NaCl is the dominant salt component in many aquifer waters, the presence of other salts could influence the hydrogen diffusivity as well. In this study, molecular dynamics simulations under more realistic conditions of hydrogen storage reservoirs were conducted to calculate the hydrogen diffusion coefficient in various brines, containing monovalent (NaCl) and divalent salts (MgCl2 and CaCl2). The simulations were conducted using the LAMMPS software. In addition, to comprehend the results, various analyses, including radial distribution functions, hydrogen bonding, and tracking of the hydrogen molecules were employed. Given that the accuracy of the results depends on the application of suitable force fields between the components, extensive investigations were carried out, and thorough comparisons with published data were conducted to validate the outcomes of the simulations based on the selected force fields. The diffusion coefficients were calculated based on the mean square displacement method. The results show higher diffusivities at elevated temperatures and lower diffusivities in more saline solutions. For instance, for a 1 molal solution of NaCl, the hydrogen diffusion coefficient was found to be 7.29×10-9 m2/s at 323K, increasing to 13.53×10-9 m2/s at 373K. This change is attributed to the higher kinetic energy of molecules and the lower density and viscosity of aqueous solutions. It was also found that the diffusivity is reduced by more than 38 percent at 353 K if the salinity rises from 1 to 5 molal. This diffusivity reduction is also shown to be related to the presence of ions’ hydration shells, which restrict the freedom of hydrogen molecules' movement. Additionally, the hydrogen diffusion coefficient is smaller in solutions with divalent salts. The hydrogen diffusivity in MgCl2 solution is up to 60 % smaller than in NaCl solutions at the same molality, which was pronounced at elevated salinities. By investigating the hydrogen diffusion coefficients at a consistent ionic strength, it appears that ion substitution is possible. Nevertheless, this phenomenon is contingent on various factors, including ion charges. The results of this work provide new insight into mechanisms of hydrogen loss in aquifers or water-saturated caprocks
  9. Keywords:
  10. Underground Hydrogen Storage ; Molecular Dynamic Simulation ; Salt Effect ; Aquifers ; Renewable Energy Resources ; Diffusion Coefficient ; Reservoir Fluid

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