OS5A-5:“SELF-PRESERVATION” OF CH4 HYDRATES FOR GAS TRANSPORT TECHNOLOGY: P-T DEPENDENCE AND ICE MICROSTRUCTURES

Andrzej FALENTY1,2, Werner F KUHS2, Michael GLOCKZIN1, Gregor REHDER1
1. Leibniz Institute for Baltic Sea Research Warnemünde, Marine Chemistry, GERMANY; 2. GZG Abt.Kristallographie, Universität Göttingen, GERMANY

    “Self-preservation” is a kinetic anomaly that allows storing a substantial amount of gas locked in gas hydrate structure far outside their thermodynamic stability field for a period of days, weeks or even months at very mild p-T conditions by merely maintaining temperatures below the melting point of ice. A number of attempts to utilize this phenomenon in a form of low cost storage and transportation technology for natural gas turned out to be not yet sufficiently developed in order to be competitive with already existing, well-established methods (e.g. LNG, GTL or pipelines) [1]. Aside from the refinement of numerous engineering and safety aspects a deeper understanding of the “self-preservation” phenomenon is needed in order to promote these technologies. We address the last issue in a series of isothermal-isobaric pVT experiments exploring the kinetics of the dissociation process of pure sI methane hydrates in a wide p-T field potentially applicable for gas hydrate based technology. By means of ex-situ cryo-SEM we correlate the kinetic data with morphology of initially formed ice coatings recovered at various stages of the transformation. The p-T dependence on the “self-preservation” strength is seen as a consequence of complex interplay between 1) ice microstructures (shape, arrangement and size of ice crystals) and 2) annealing rate of the ice coating that acts as a diffusion barrier for escaping gas. Moreover, we recognize a progressing sintering of ice coatings of individual particles while closing to the melting point of ice that in the worst case scenario may lead to the complete consolidation of a granular cargo. The most optimal conditions for the transport and storage where this issue is minimized and the preservation strength is still very high have been found at ~250K. Decreasing dissociation rates as a function of pressure offer an additional tool for a fine tuning of the storage capacity. 

    [1] Rehder et al. (2012), “Methane Hydrate Pellet Transport Using the Self-Preservation
    Effect: A Techno-Economic Analysis”, Energies (5) 2499-2523, doi:10.3390/en5072499