The formation of carbon dioxide hydrate from water sorbed by coals

Experiments on the formation and dissociation of carbon dioxide gas hydrate formed from water that was adsorbed in two sub-bituminous coals and anthracite were carried out. Preliminarily, the coal samples were studied using various physicochemical methods. It was demonstrated that the conditions of the hydrate decomposition in coal do not differ substantially from the equilibrium conditions of carbon dioxide hydrate. In all the cases, the conversion of water to hydrate increases with increasing the pressure of the hydrate former and the initial humidity of coal samples. At the same time, it turned out that the amount of unreacted water in the sample also increases with an increase in the initial humidity of coal sample. A phenomenological model explaining the observed features of the hydrate formation in coal was proposed on the basis of the data obtained. In this model, the hydrate formation in coal is considered as a two-stage process: (1) a partial displacement of coal-sorbed water from the pore space into macropores and on the coal surface due to competitive sorption of carbon dioxide, and (2) the formation of hydrate from water droplets formed in the first stage.