| The resource of coal mine methane(CMM) gas is very abundant in China, but the low utilization efficiency of CMM gas, especially for the low-concentration CMM gas, has resulted in a huge waste of resources and a serious problem of environmental pollution. The study of improving the utilization efficiency of CMM has become a hot issue in the field of energy. Gas hydrate is crystalline compounds formed by water and gas molecules under suitable temperature and pressure conditions. The phase equilibrium data of gas hydrates formed with different gases are different, which is the theoretical basis for the separation of gas mixtures using gas hydrate formation. In recent years, it has been found that employing gas hydrate formation to recover the low-concentration CMM gas has a wide application prospect and is attracting worldwide attentions of scholars. However, long crystallization process of hydrate formation, harsh temperature and pressure conditions of hydrate growth, and low CH4 recovery rate are the key scientific problems that need to be solved at present.In this work, aiming at solving these key scientific problems we carried out experiments of separting CH4 from a low-concentration CMM gas(30.0 mol%CH4, 60.0 mol%N2, 10.0 mol%O2) using gas hydrate formation in a stirred solution system, in a fix bed of silica sand, and in a fixed bed of coal particles respectively, and studied the effects of surfactants(SDS) and the the saturation of porous medium on gas consumption, rate of gas hydrate formation and CH4 recovery.The conclusions of this study are summarized as follows:① The phase equilibrium conditions of gas hydrates formed from the low-concentration CMM gas were measured in the presence of 1.0 mol% THF solution using the observation method. Compared with the phase equilibrium data obtained in pure water, it was found that the promoter of THF is able to reduce the hydrate phase equilibrium pressure of the low-concentration CMM significantly.② According to the measured phase equilibrium data, the hydrate formation experiments for CH4 separation from the low-concentration CMM gas were performed in THF solutions and THF/SDS solutions. The results indicated that the presence of surfactant SDS in THF solutions can greatly shorten the induction time of hydrate nucleation, promote hydrate growth, and improve the CH4 recovery and separation efficiency.③ The kinetic experiments of hydrate formation for CH4 recovery from the low-concentration CMM gas were carried out in a fixed bed of silica sand. Compared with the results obtained in the THF/SDS solution in the stirred reactor, it was found that the induction time was significantly reduced in the fixed bed of silica sand saturated by the THF/SDS solution, and the rate of hydrate formation was promoted. But the gas consumption and the CH4 recovery was lower in the fixed bed of silica sand.④ The kinetic experiments for methane separation from the low-concentration CMM gas were carried out in a fixed bed of coal particles using the hydrate-based separation process. The results indicated that coal particles have a strong adsorption ability of CH4, resulting in great reduction of the hydrate formation process. Gas consumption and the rate of gas consumption were increased with the decrease of the coal particle saturation.Comparing the experimental results obtained in the three different systems(THF solution system, the fix bed of silica sand, and the fixed bed of coal particles), it was found that the highest CH4 recovery was obtained in the THF solution with the presence of SDS(40%), and CH4 in the gas mixture was concentrated from 30 mol% to 50 mol%. Adsorption and hydrate hybrid process occurred in the fixed bed of coal particles. The shortest induction time for hydrate crystallization was obtained at 40% saturation of coal particles, and the largest values for gas consumption and rate of gas consumption were obtained at 40% saturation of coal particles. |
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