Local Structure and Dynamic Studies of mixed CH4-CO2 Gas Hydrates via Computational Simulation and Neutron Scattering

• Main Author: Cladek, Bernadette Rita
• Format: Text
• Language: unknown
• Published: TRACE: Tennessee Research and Creative Exchange 2020
• Subjects: Neutron Scattering; Diffraction; Clathrates; Gas Hydrates; Molecular Simulations; Engineering Physics; Geology; Materials Chemistry; Mineral Physics; Other Materials Science and Engineering; Thermodynamics; Arctic; permafrost
• Online Access: https://trace.tennessee.edu/utk_graddiss/6062; https://trace.tennessee.edu/cgi/viewcontent.cgi?article=7067&context=utk_graddiss

Permeated throughout the ocean floor and arctic permafrost, natural gas hydrates contain an estimated 3000 trillion cubic meters, over three times that of traditional shale deposits, of CH4 that is accessible for extraction. Gas hydrates are a crystal structure in which water molecules form a cage network, the host, through hydrogen bonds while trapping a guest molecule such as CH4 in the cavities. These compounds form naturally where the appropriate low temperature and high pressure conditions occur. A promising and tested method of methane recovery is through exchange with CO2, which energetically takes place of the methane when pressurized into hydrate deposits. When CH4 is replaced with CO2 in the hydrate structure, the stability temperature is increased. Currently, hydrate deposits are at risk of releasing CH4,, and potent greenhouse gas, into the oceans and atmosphere. Recovery of CH4 via CO2 exchange presents natural gas hydrates as a potential fuel source and carbon sequestration medium while mitigating the risk of CH4 release. This work studies the molecular level structure and properties of gas hydrates with CH4, CO2, and mixed CH4 and CO2 occupying the cage structure in order to better understand how CO2 stabilizes hydrates, the effectiveness of altering a deposit with a mixed CH4-CO2 result, and how each guest molecule type affects interactions in the hydrate framework. A combined approach of computational simulations and neutron scattering is used to characterize how altering the guest molecule composition with CH4 and CO2 impacts the guest-host, host-host, and guest-guest interactions in hydrates. Carried out over temperature ranges, this work provides insight to show that in mixed CH4-CO2 and pure CO2 hydrate structures the CO2 guest interacts strongly with the surrounding cages and guest molecules to stabilize the hydrate.