Using Propane Or Methylcyclohexane Hydrate As Hydrogen Storage Material: MD Simulation

As a new energy source,hydrogen is considered to be the most ideal energy source due to its high heating value and no pollution after combustion.However,due to the low density of hydrogen,its storage has become a bottleneck for the application of hydrogen.The hydrate hydrogen storage technology is favored by researchers because of its low cost and high safety.However,pure hydrogen hydrate can only be produced at 270K and 250MPa.How to efficiently store hydrogen under mild conditions has become a problem for hydrate storage.As a research method,molecular dynamics simulation can break through the limitations of macro experiments and intuitively reveal the process as well as influencing factors of hydrate hydrogen storage.In this paper,molecular dynamics simulation was used to compare the simultaneous generation and induced generation of hydrogen hydrates in the presence of promoter.The hydrogen storage process and hydrogen storage effect of s II and s H hydrate promoter were investigated.The result provided certain data and theoretical guidance for the application of hydrate technology.The simultaneous formations of propane-hydrogen binary hydrate and methylcyclohexane-hydrogen binary hydrate were simulated at 240-260K&50-70MPa and 230K&110MPa respectively.Low temperature was beneficial to improve the hydrogen storage effect of propane hydrate.The highest hydrogen storage capacity of propane-hydrogen binary hydrate was1.128wt%under 60MPa,240K.The hydrogen storage capacity of s H hydrate was not as good as that of s II hydrate and the hydrogen storage density of methylcyclohexane-hydrogen hydrate was 1.08wt%under 230K,110MPa.The induced formation of propane hydrate and methylcyclohexane were simulated and studied under 250-270K&30-70MPa and 240-260K&70-110MPa,respectively.The simulation results of propane hydrate showed that the hydrogen temperature should not be higher than260K,when the hydrogen storage capacity was 0.893-1.455wt%;above 270K,the hydrogen storage capacity of propane hydrate was 0.099-1.124wt%.This was because at 270K,hydrogen molecules were only stable in 5¹² cages;when the temperature was lower than 270K,hydrogen molecules can be stabilized in 5¹²6⁴ cages under specific pressure condition,which leaded to two hydrogens to occupy 5¹²6⁴cages at the same time.This phenomenon was beneficial to improve the space utilization of hydrates.The simulation results of methylcyclohexane hydrate showed that hydrogen storage capacity of hydrates was mainly affected by temperature when above certain level of pressure.Specifically,the hydrogen storage capacity of hydrates was low??1.0wt%?at 250-260K and was greatly improved??1.6wt%?at 240K.At 250-260K,hydrogen molecules only entered the cavities that were near the interface,which seems that 5¹² cages layers obstructed the hydrogen molecules.The results showed that the induced hydrogen formation required lower conditions than the simultaneously hydrogen storage formation,and the hydrogen storage effect was better.