Keywords:Methane Hydrate, Carbon dioxide Capture and Storage , Replacement of CH4 by CO2, Molecular Dynamics Simulation
Methane (CH4) is the major component of natural gas and exists as clathrate hydrate occurring in the deep seafloor sediment around Japanese Islands. The total amount of CH4 hydrate around Japanese Islands is so abundant that it is considered to be a potential future energy resource. Carbon Dioxide (CO2) is considered to be responsible for global warming effect. The conception of replacement of CH4 by CO2 in clathrate hydrate has been conceived resulting in an innovative solution. Replacement of CH4 by CO2 from gas hydrate can recover CH4 as a potential future energy resource meanwhile sequestrate CO2. Further, the formation of CO2 hydrate leads to geological stabilization after decomposition of CH4 hydrate. Recently, replacement of CH4 by CO2 has been studied both from experiments and simulations. However, the detailed mechanism of replacement process is still poorly known. Although the experiments using mixtures of CO2 and various additives have been conducted, little work has been done with Hydrogen sulfide (H2S) on the replacement process. H2S as a concomitant of CO2, produced by developing oil or natural gas fields. Given H2S not negatively influence the replacement reaction, the mixture of CO2 and H2S can be injected into clathrate hydrate with the advantage of lowering the overall cost and disposing of pollutants. Here, classical molecular dynamics (MD) simulations were performed to understand the effect of additive H2S imposed on the replacement process. MD simulations can investigate the physical interactions between different atoms and molecules on the basis of Newton’s law and represent the relevant movements of the particles in the evolution, of which results can be visualized later on by using computational way. In this study, in order to investigate the microscopic mechanism of replacement process, we conducted simulations in three cases. 1) CH4 hydrate + CO2, 2) CH4 hydrate + H2S, 3) CH4 hydrate + CO2 + H2S, and investigated the influence of H2S on the replacement process of CH4 by CO2. The temperature and pressure were set to 280 K and 6 MPa, respectively. Under this condition, CH4 hydrate is decomposed and subsequently CO2 and H2S hydrates are formed. Step interval was 1 femtosecond and the total simulation time was 200 ns in all cases. Within MD simulations, CO2 molecules are trapped by water-forming cages. As for H2S, H2S molecules also replace with CH4 and are trapped by cages as well. On the other hand, in all cases, replacement of CH4 by CO2 or H2S does not occur in the center of CH4 hydrate crystals. This implies that it is difficult for CO2 and H2S molecules to go deeper into bulk clathrate hydrate. In addition, the results show that H2S molecules decrease the total number of CO2 molecules trapped by cages in comparison with injection of pure CO2 phase. The replacement reaction of CH4 and CO2 occurs preferably in large cages, whereas, H2S molecules can replace CH4 molecules in both small and large cages. Consequently, it is possibly ascribed to that CO2 and H2S molecules compete with each other during the replacement progress of CH4 hydrate. The formation rate of H2S hydrate was faster than that of CO2 hydrate, which may due to stronger coulomb interaction between H2S and H2O molecules since they are polar molecules. Also owing to this, H2S becomes more competent over CO2 during formation process of clathrate hydrate in the presence of mixture of H2S and CO2. From our results, the coulomb interaction between H2S and H2O may lead to stabilization of H2S hydrate and decrease the formation efficiency of CO2 hydrate. The recovery rate of CH4 is decelerated by H2S, which suggests the additive can impede the replacement progress of CH4 hydrate. All of the results suggest that the addition of H2S weaken the storage capability of CH4 hydrate for CO2.