Coal Mine Methane Gas Recovery by Hydrate Formation in a Fixed Bed of Silica Sand Particles

In the present work, the separation of CH4 from low-concentration coal mine methane gas (30 mol % CH4/N-2) through hydrate crystallization was investigated in a fixed bed of silica sand particles. The influence of the additive tetrahydrofuran (THF) on hydrate equilibrium conditions and kinetics of CH4 separation was studied as well. The incipient hydrate equilibrium conditions at 1 mol % THF were determined using the isothermal pressure search method. It was found that the presence of THF significantly reduced the hydrate equilibrium conditions as compared to those obtained in liquid water with the same gas mixture. CH4 recovery in the water-saturated silica sand bed was considerably low (similar to 12.0%) because N-2 molecules might compete with CH4 molecules to enter the hydrate crystals under high pressure conditions. The addition of THF to the bed of silica sand particles reduced the nucleation time of gas hydrate formed from the 30 mol % CH4/N-2 gas and increased the CH4 recovery (similar to 21.4%) significantly. The comparison of CH4 separation between the silica sand bed and the stirred reactor in the presence of THF indicated that CH4 recovery was approximately the same, but the conversion of water to hydrate in the THF solution-saturated silica sand bed was largely increased.