Formation and decomposition of methane hydrate in coal

The formation of methane hydrate from water adsorbed on the surface of natural bituminous coal has been studied. The state of the adsorbed water has been examined with DSC and NMR techniques. The isotherm of adsorption of water on coal has been studied. The experiments have shown that the water adsorbed on the coal studied is bound and non-freezing, i.e., no ice formation and melting occurs on cooling and heating of the coal samples containing the adsorbed water. This water undergoes a glass-like transition. The adsorbed water can be divided into strongly bound and weakly bound, in accordance to known models of water sorption by coals. Cooling and heating of containing adsorbed water coal samples under methane pressure results in expressed pressure anomalies corresponding to the formation and decomposition of the methane hydrate. Thus, the non-freezing water is capable of forming gas hydrate. It has been demonstrated that the hydrate is formed only from the weakly bound water. The equilibrium pressure and temperature parameters of this hydrate are identical to those of the bulk hydrate. Thus, it has been shown that at proper pressure and temperature gas hydrates can be formed in coals even in absence of free water. The results obtained make possible to suggest that the hydrophilic areas of the coal surface are active sites of gas hydrate nucleation.