Experimental Study On The Kinectics Of Hydrate-based Gas Separation Process For CO₂/H₂ Gas Mixture Separation

China is a big coal-consuming country. CO₂ emission due to coal burning accounts for appromimately 40% of the total CO₂ emission in the country, so reducing CO₂ emission from coal-fired power plants is a key issue that should be solved for CO₂ capture. In recent years, the application of integrated gas combined cycle power generation（IGCC） system has been considered as one typical pre-combustion CO₂ capture technology as well as an important developing direction of clean and efficient coal-fired power generation technology in the future. However, the efficient separation of fuel gas（CO₂/H₂） will influence the efficiency of energy-saving and power generation in the IGCC power plants. The hydrate based gas separation（HBGS） process is a promising technology for gas mixture separation due to the difference of hydrate phase equilibrium conditions of different gas molecules, and the advantages of this process for CO₂ capture from fuel gas（CO₂/H₂） are energy saving, environmental protection and high separation efficiency. Recently, it has attracted the attentions of researchers around the world. However, increasing CO₂ separation efficiency and the rate of gas consumption are the key issues that should be solved to improve this novel technology. Aiming to solve these problems, in this work we carried out experimental investigations on HBGS process for CO₂ capture in the stirred reactor and in the porous media system, respectively. The results obtained in this research will significantly promote the development of the HBGS technology for CO₂ capture from fuel gas.The research contents and conclusions obtained in this research are given as follows:（1） The mixture of tetrahydrofuran（THF） and cyclohexane（CH） was used as a new thermodynamic promoter to reduce the phase equilibrium pressures of gas hydrates formed from CO₂/H₂ mixture for the first time. The phase equilibrium conditions of gas hydrates formed in the 1.0 mol% THF + 10 vol% CH solution were determined by the isochoric pressure search method. In contrast to 1.0 mol% THF solution system, lower equilibrium pressure and higher CO₂ recovery and separation efficiency were obtained in the 1.0 mol% THF+10% CH solution at the same temperature.（2） In order to improve the kinetics of hydrate formation such as gas consumption, CO₂ recovery, and rate of hydrate formation, we performed kinetic experiments of hydrate-based CO₂ capture from the mixed CO₂/H₂ gas in the fixed bed of coal particles.（1） Impact of driving force on gas consumption and CO₂ separation efficiency was studied. It was found that higher gas consumption was obtained at a larger driving force, but CO₂ recovery and separation factor was reduced.（2） The influence of liquid saturation（100%, 40%, 0%） of the coal filled fixed bed on the separation of CO₂/H₂ was studied, and the reaction mechanism in the fixed bed of coal particles was discussed. It was found that CO₂/H₂ separation in the fixed bed of coal particles was a hybrid adsorption-hydration formation process as coal particles have a strong adsorption capacity for CO₂. It was also found that the higher the saturation of the fixed bed, the higher CO₂ recovery and the separation factor.（3） The influence of THF concentration（1, 3, 5.6 mol%） on the separation of CO₂/H₂ was studied in the fixed bed of coal particles. It was found that increasing THF concentration can promote hydrate formation and increase gas consumption per mole of water, but the total gas consumption and CO₂ separation efficiency decreased. The main reason was that H₂ can enter the hydrate cavities of the structure II hydrate in the presence of THF, so increasing THF concentration is not conducive to separation CO₂ from H₂.（4） The effect of particle size（0.1-0.5mm, 0.5-1mm, 1-3mm） on the kinetics of hydrate formation from CO₂/H₂ mixture was studied. It was found that large-sized particles created large spaces between the particles, which were not suitable for hydrate nucleation at the gas-liquid interface, causing a small rate of hydrate formation. In addition, small-sized coal particles（0.1-0.5mm） contained a number of fine coal powders that were unable to remove, which hindered the diffusion of gas molecules in the fixed bed, resulting in the reduction of gas consumption.（3） The kinetics of hydrate formation from CO₂/H₂ obtained in different systems（THF, TBAB, CP, silica sand, W/O emulsions, etc.） were compared and evaluated. The comparison shows that comparing to the stirred system the fixed bed filled with porous media（silica sand, coal particles） could shorten the induction time and reaction process and improve the water converstion to gas hydrates. At present, the highest gas consumption（51.9 mmol of gas/mol of water） was obtained at 6.0 MPa and 1.0 mol% THF in the coal particle system, while the highest CO₂ recovery（57%） and CO₂/H₂ separation factor（42.5） was obtained at 3.0 MPa and 1.0 mol% THF in the coal particle system.