Investigation of hydrogen diffusion in supercritical water: A molecular dynamics simulation study

Supercritical water gasification of coal is an efficient method for hydrogen production. The diffusion of hydrogen is the key to completely gasification at moderate condition. The researches have been done about hydrogen diffusion in liquid water and there is little information about hydrogen diffusion in supercritical water, especially for typical supercritical water gasification conditions. In this article, Molecular dynamics (MD) simulation of hydrogen diffusion in water is carried at the temperature ranging from ambient to supercritical. The results show that at ambient condition, the calculation of diffusion coefficient is 0.37 × 10⁻⁸ m²/s, which is reliable compared with references. However, the diffusion coefficient is 217.80 × 10⁻⁸ m²/s at 973 K, 250 atm where the temperature is far away from the critical point that most empirical equations do not fit our calculation well due to the unique of supercritical water. It is revealed that the diffusion coefficient at supercritical conditions quite obeys the Arrhenius equation. Our calculated activation energy is 19.41 kJ/mol. Effects of water density and viscosity are discussed. These results are expected to provide fundamentals for optimization of supercritical water gasification process.