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Pore-scale Investigation of Gas Mixing Phenomenon Using CFD Methods For Underground Hydrogen Storage

Ghanbari Masir, Mohadeseh | 2024

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  1. Type of Document: M.Sc. Thesis
  2. Language: Farsi
  3. Document No: 57975 (06)
  4. University: Sharif University of Technology
  5. Department: Chemical and Petroleum Engineering
  6. Advisor(s): Mahani, Hassan; Ayatollahi, Shahaboddin
  7. Abstract:
  8. The increasing trend of greenhouse gas production and the limited resources of fossil fuels has forced industries to find cleaner energy resources. Since hydrogen has the highest amount of energy per unit mass compared to other used fuels, it is expected to play an important role as an energy carrier. Underground hydrogen storage (UHS) is proposed to solve the imbalance between supply and demand of clean energy. UHS in abandoned gas reservoirs is more favorable because the remaining gas in place can play as the cushion gas for more hydrogen recovery. This technique increases the efficiency of hydrogen production however, the purity of the produced hydrogen decreases because of its mixing with the cushion gas. Therefore, it is important to investigate the conditions that maintain higher hydrogen purity through minimum mixing with the cushion gas to optimize the efficiency of the injection and production cycles. In this research work, computational fluid dynamics (CFD) is used to investigate the mixing of hydrogen and cushion gas at the pore scale of porous rocks. This scenario which can be followed by an upscaling technique leads to estimate dispersion parameters which are the main input for field scale simulators. For this purpose, COMSOL software has been used to solve mass and momentum conservation equations using the finite element method. For this purpose, Hydrogen dispersion in a capillary tube containing nitrogen gas has been simulated using different initial and boundary conditions. The results of this study demonstrate that increasing the purity of injected hydrogen—ranging from 10 to 100% by mass—in a continuous injection from one side leads to an increase in the dispersion coefficient and a deviation from a Fickian dispersion state. It was observed that the quality of the velocity definition for the equivalent model, even if the velocity changes in the system seem negligible, has a significant effect on the dispersion and Peclet power in dispersion vs Peclet plot. For continuous injection of pure hydrogen from one side and defining the velocity for the equivalent model as a function of time, dimensionless dispersion values and Peclet power we 0.01 and 1.8, respectively. Simulations were performed to investigate the effect of the density gradient term in the continuity equation. Simulations were conducted to examine the impact of the density gradient term in the continuity equation and the resulting modified stresses in the compressible velocity equation. It was also observed that the presence of density gradient in the flow direction in the system under consideration, where the movement is from lower to higher density, results in a reduction of the velocity in the mixing zone. This effect gradually decreases with increasing velocity, so that with an increase in velocity from 0.05 m/s to 0.2 m/s, the relative fluid velocity (velocity at each point divided by average inlet velocity) in the mixing zone increases from 0.75 to about 0.92 of the injection velocity at a dimensionless time of 0.17
  9. Keywords:
  10. Underground Hydrogen Storage ; Gas Mixing ; Pore Scale Simulation of Fluid Flow ; Dispersion ; Computational Fluid Dynamics (CFD) ; COMSOL Software

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