Coal Mine Methane Gas Recovery by Hydrate Formation in a Fixed Bed of Silica Sand Particles Dong-Liang Zhong,* ,†,‡ Nagu Daraboina, § and Peter Englezos § † College of Power Engineering, Chongqing University, Chongqing, 400044, China ‡ State Key Laboratory of Coal Mine Disaster Dynamics and Control, Chongqing University, Chongqing, 400044, China § Department of Chemical and Biological Engineering, University of British Columbia, Vancouver, British Columbia, V6T 1Z3, Canada ABSTRACT: In the present work, the separation of CH 4 from low-concentration coal mine methane gas (30 mol % CH 4 /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 CH 4 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. CH 4 recovery in the water-saturated silica sand bed was considerably low (∼12.0%) because N 2 molecules might compete with CH 4 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 % CH 4 /N 2 gas and increased the CH 4 recovery (∼21.4%) significantly. The comparison of CH 4 separation between the silica sand bed and the stirred reactor in the presence of THF indicated that CH 4 recovery was approximately the same, but the conversion of water to hydrate in the THF solution-saturated silica sand bed was largely increased. 1. INTRODUCTION The safety of coal mining operations requires continuous separation and recovery of coal mine methane (CMM) from a mixture of methane with air. CH 4 is also a strong greenhouse gas and a source of energy. 1-3 Recently, attention has been paid to the use of gas hydrate formation for gas mixture separation. 4-7 Thus, the separation of CH 4 from model coal mine methane gas (CH 4 /N 2 or CH 4 /N 2 /O 2 ) using gas hydrate crystallization has been considered. Although methane is preferentially incorporated into the hydrate phase compared to other components such as N 2 and O 2 in the CMM gas, the hydrate formation pressures at given temperatures are very high. 8 Some additives such as tetrahydrofuran (THF), tetra-n- butyl ammonium bromide (TBAB), and cyclopentane (CP) are known to promote hydrate formation thermodynamically by shifting hydrate formation conditions to lower pressures and higher temperatures. 9-13 Zhang and Wu 14 reported incipient equilibrium data of gas hydrate formed with a low- concentration CMM gas in the presence of THF. Sun et al. 15 formed TBAB semiclathrate hydrate from a simulated CMM gas (46.3 mol % CH 4 /N 2 ) and concluded that CH 4 concentration can be increased from 46.3 to 67.9 mol % through a single-stage hydrate separation in the presence of TBAB. Zhong and Englezos 16 measured the incipient hydrate formation conditions for gas hydrate formed with a simulated low-concentration CMM gas (30 mol % CH 4 /N 2 ) in the presence of TBAB and investigated the kinetics of CH 4 separation in a small scale stirred vessel. Zhong et al. 17 also reported the phase equilibrium data of gas hydrate synthesized from the same gas mixture in the presence of CP and found that the CH 4 recovery was significantly increased as compared to that obtained in the presence of TBAB. It should be noted that all these investigations were carried out in stirred reactors which is a frequently used arrangement for laboratory hydrate studies. However, the agitation of reactor contents would consume a significant proportion of energy that is needed for system operation when the stirred reactors are scaled up for industrial application. 18 Recently, studies on gas hydrate formation in a fixed bed of porous media have been reported, including phase equilibrium investigations 19-23 and kinetic property studies. 24-26 Kang and Lee 27 investigated the kinetics of natural gas hydrate formed in the fixed bed of porous silica gel. They concluded that the hydrate formation rate and gas storage capacity were enhanced without mechanical stirring. Li and Zhang 28 studied the dissociation behavior of methane hydrate formed in a fixed bed of silica gel and reported the effect of formation pressure, environmental temperature, and pore size on the rate of methane released from the hydrates. Linga et al. 29 formed methane hydrate in a fixed bed of silica sand particles instead of using a stirred vessel and found that the conversion of water to hydrate was significantly increased as compared to that in the stirred vessel, indicating that more hydrate can be obtained in the porous media system. They also found that the rate of hydrate formation was enhanced in a silica sand bed compared to a stirred reactor. 30 Zanjani et al. 31 formed gas hydrates using natural gas and a mixture of methane, ethane, and propane in the presence of a small amount of silica-based porous media and concluded that the addition of the silica-based porous media can considerably increase the gas storage capacity of the hydrate phase. However, Received: April 15, 2013 Revised: July 15, 2013 Published: July 16, 2013 Article pubs.acs.org/EF © 2013 American Chemical Society 4581 dx.doi.org/10.1021/ef400676g | Energy Fuels 2013, 27, 4581-4588