Effect of dry water on methane separation and recovery from coal mine gas based on hydrate

Coal mine gas (CMG) is a form of unconventional natural gas and its reserves are abundant. However, a large proportion of coal mine gas cannot be used owing to the low concentration of the extraction gas. The hydrate-based method can be used for the separation and recovery of methane from coal mine gas. To devise an economic and efficient method for the separation of mine gas using hydrates, dry water (DW) was used as the carrier for separation under conditions in which the initial pressure was fixed at 10 MPa and the temperature was set at 274 K. On this basis, repeated gas hydrate separation experiments were carried out three times. A pure-water system, a stirring system, and a system using a compound solution of 1 mol L^-1 THF + 500 mg L^-1 SDS were used in control experiments. The spectral characteristics of the gas hydrates in pure water were determined by Raman spectroscopy, and the hydration index was calculated. The concentrations of gas components in the equilibrium gas phase were determined by chromatography. Moreover, the hydrate formation rate, methane recovery rate, distribution coefficient and separation factor were also calculated. The results indicate that the average gas hydrate formation rate was up to 6.85 × 10^-4 mol min^-1 in the presence of the THF + SDS solution. The maximum average methane recovery rate was 38.15%, the average distribution coefficient was up to 1.99, the average separation factor reached 2.47 and the highest methane concentration in the hydrate phase was 32.2% in DW. In the experimental range, the efficiency of DW in the recovery of methane by gas hydrate separation was greater than that of the stirring system and the system using the THF + SDS compound solution.