Thermal diffusion of hydrogen-containing gas mixtures: applications to underground hydrogen storage

Abstract

The modeling of underground hydrogen (H₂) storage (UHS) requires understanding the thermodynamics of H₂-containing gas mixtures as they approach local equilibrium during storage while subjected to temperature gradients and gravity segregation. Previous investigations using a model based on irreversible thermodynamics have shown the need for experimental measurements of hydrogen thermal diffusion in natural gas to better understand hydrogen composition versus depth during UHS. This work presents thermal diffusion measurements for H₂ in methane (CH₄) at varying temperatures and compositions. The effect on thermodynamic modeling is discussed, and the effect of other cushion gases such as carbon dioxide (CO₂) is also explored. For the H₂–CH₄ system, it was found that the thermal diffusion factor (α_T) increases as a function of composition and temperature, with values ranging from α_T = 0.22–0.36 for H₂ mole fractions ranging from x_H2 = 0.3−0.7. At a fixed composition of 50% H₂ and 50% CH₄, α_T ranged from 0.21 to 0.29 for a median temperature ranging from 250 K to 450 K. Using these values, a reference ideal gas enthalpy of 3.5 kJ mol⁻¹ for CH₄ while setting the reference ideal gas enthalpy of H₂ to 0 kJ mol⁻¹ is needed to properly match the model with the experimental observations at a constant median temperature. For experiments at varying median temperature, a correlation is needed between the enthalpy of the reference ideal gas of CH₄ and the departure of the median temperature from the reference state temperature to match adequately the model with the experimental values. The effect of adding these thermal considerations leads to a more homogeneous mix of H₂ with its cushion gas than previously anticipated. Further study of UHS operations could include the effects of shut-in time to determine gas purity during production cycles.