Free energies of carbon dioxide sequestration and methane recovery in clathrate hydrates

Classical molecular dynamics simulations are used to compare the stability of methane, carbon dioxide, nitrogen, and mixed CO2/N-2 structure I (sI) clathrates under deep ocean seafloor temperature and pressure conditions (275 K and 30 MPa) which were considered suitable for CO2 sequestration. Substitution of methane guests in both the small and large sI cages by CO2 and N-2 fluids are considered separately to determine the separate contributions to the overall free energy of substitution. The structure I clathrate with methane in small cages and carbon dioxide in large cages is determined to be the most stable. Substitutions of methane in the small cages with CO2 and N-2 have positive free energies. Substitution of methane with CO2 in the large cages has a large negative free energy and substitution of the methane in the large cages with N-2 has a small positive free energy. The calculations show that under conditions where storage is being considered, carbon dioxide spontaneously replaces methane from sI clathrates, causing the release of methane. This process must be considered if there are methane clathrates present where CO2 sequestration is to be attempted. The calculations also indicate that N-2 does not directly compete with CO2 during methane substitution or clathrate formation and therefore can be used as a carrier gas or may be present as an impurity. Simulations further reveal that the replacement of methane with CO2 in structure II (sII) cages also has a negative free energy. In cases where sII CO2 clathrates are formed, only single occupancy of the large cages will be observed. (C) 2007 American Institute of Physics.