An Alternative for Carbon Dioxide Emission Mitigation: in situ Methane Hydrate Conversion

In situ conversion of naturally-occurring methane hydrate to carbon dioxide hydrate is a very attractive way of CO2 sequestration. Though this possibility has been experimentally confirmed [1] and the process patented [2], its feasibility will hinge on the exact microscopic mechanisms governing the conversion. We have used MD simulations to investigate a system initially containing methane hydrate in contact with pure carbon dioxide that ran for a total time of 62 nanoseconds at 200 K. The system contained 3680 water molecules, 640 methane molecules, and 1052 CO2 molecules. Intermolecular interactions were treated by a combination of Coulomb and Lennard-Jones potentials. Temperature was controlled by a Nose-Hoover thermostat. It has been shown that CO2 is able to replace methane in the large hydrate cages, resulting in conversion from methane to mixed CO2-methane hydrate. It appeared that CO2 was unable to enter a cage until its current occupier, a methane molecule, exited. Moreover, there existed a not-insignificant time lapse between methane molecule leaving and CO2 entering. One might conclude that even though surrounding CO2 molecule may contribute to methane's exit, it's not always the molecules that pushed it out of the cage that enters. We have also found evidence of CO(2)s leaving the cages and which then are occupied by another CO2 molecule. No methane re-entry was observed.