Capture Of SO₂and CO₂from The Conversion Of Coal By Ionic Liquids

Coal is one kind of important fossil energy, and large quantities of acidic gases, such as SO2and CO2, are produced during the conversion of coal. Capturing and utilizing the acidic gases is of great realistic significance. This work aims to develop new absorbents for the capture of acidic gases during coal conversion and to study the behaviors of acidic gases absorption into the new absorbents. As one kind of solvents with excellent properties, ionic liquids (ILs) shows great advantages in the field of acidic gases trapping. In this work, a series of basic researches on the synthesis and application of ionic liquid-based absorbents for the capture of acidic gases have been carried out, and the results provide a theoretical basis for applying these new absorbents in chemical industry. The results of this work are shown as follows:1. ILs used for the absorption of SO2are usually with low thermal stability, and ILs with high thermal stability are demanded for the absorption of SO2. In this work, four kinds of ILs, imidazolium-, quaternary ammonium-, guanidinium-and alkanolaminium-based ILs with lactate anion, were synthesized. The relationships between thermal stability and SO2absorption efficiency and structures of the ILs were studied. The result shows that the imidazolium-based ILs have the highest thermal stability, and the decomposition temperature of these ILs is more than230℃. Both of thermal stability and SO2absorption efficiency of ILs can be adjusted by changing the cation. Compared with guanidinium-and alkanolaminium-based ILs, thermal stability and SO2absorption capacity of the imidazolium-based ILs have been greatly improved.2. A new idea that absorption of SO2by ILs at high temperatures has been proposed. According to thermal stability and SO2absorption capacity of ILs,[Bmim]L and [N2222]L were selected for the absorption of SO2at high temperatures. Effects of temperature, SO2partial pressure and water on the absorption of SO2were studied. The result shows that both of the ILs can absorb more than1.0mol SO2/mol IL even at100℃. Furthermore, the absorption mechanism was proposed based on FT-IR,1H NMR and13C NMR characterizations, which confirms that SO2reacts with the anion of ILs.3. IL can absorb water continuously during absorbing SO2, which suggests that strong interactions exist between IL and water. Density and viscosity of1,1,3,3-tetramethylguanidinium lactate and water were measured. Volumetric properties, such as excess molar volumes (VE), partial molar volumes (Vi) and excess partial molar volumes (ViE), and viscosity properties, such as viscosity deviations (Δη), Gibbs energy of activation for viscous flow (AG*) and excess Gibbs energy of activation for viscous flow (AG*E) of the binary mixtures, were also calculated based on the density and viscosity data. The strong interactions between IL and water were revealed at molecular level. Besides, density and viscosity of the binary mixtures with respect to the mixture composition and temperature were correlated by Jouyban-Acree model.4. In order to reduce the energy cost of removing water from ILs, a new idea that absorption of SO2by hydrophobic task-specific ILs has been proposed. In this work, two kinds of hydrophobic task-specific ILs, such as [Et2NEmim][PF6] and [Et2NEmpyr][PF6], were designed and synthesized. Physical properties of the ILs and application of the ILs for SO2absorption were systematically studied. The result shows that both of [Et2NEmim][PF6] and [Et2NEmpyr][PF6] are hydrophobic, and saturated water contents in the ILs are5.4wt%and6.2wt%at30℃, respectively. Compared with [Et2NEmpyr][PF6],[Et2NEmim][PF6] shows much lower viscosity, much higher thermal stability and much higher absorption efficiency. For the absorption of SO2with low concentrations,[Et2NEmim][PF6] shows high absorption capacities up to0.94mol SO2/mol IL. The interactions between hydrophobic ILs and water are much weaker than that between hydrophilic ILs and water. Therefore, the energy cost of removing water from hydrophobic ILs will be surely decreased significantly.5. High cost of ILs hinders their industrial application in large scale.The organic bases corresponding to the organic cations of ILs are expensive, which results in the high cost of ILs. According to the design principles of ILs for the capture of SO2, absorbents with low cost could be obtained by choosing inorganic bases as substitutes for the expensive organic bases. In this work, aqueous calcium lactate (CaL2) solution was selected as absorbent for the capture of SO2. Effects of temperature, CaL2concentration and excessive lactic acid on the absorption of SO2were studied. The reuse of aqueous CaL2solution was also investigated. The results indicate that aqueous CaL2solution can absorb SO2efficiently, and SO2can be recovered by heating the absorbent. Compared with the traditional calcium-based absorbent, there are some advantages, such as reuse of absorbent, recovery of SO2and reduction of by-product. The work suggests that aqueous CaL2solution is a promising recyclable absorbent, which can be used in large scale for the capture of SO2from flue gas.6. High viscosity of ILs seriously hinders the mass transfer when they are used for the absorption of CO2. In this work, N-methyl-2-pyrrolidone (NMP) was selected as additive of [Bmim][BF4] to prepare hybrid absorbents. Physical properties of the hybrid absorbents and solubility of CO2in the hybrid absorbents under high pressures were studied. The result shows that density and viscosity of the hybrid absorbents decrease with the increase of temperature and NMP mass fraction. Solubility of CO2in the hybrid absorbents increases with the increase of CO2pressure and NMP mass fraction, and it also increases with the decrease of temperature. Furthermore, it has been found that solubility of CO2in the hybrid absorbents is smaller than the sum of that in pure solvents at the same conditions.7. Mixing amines into ILs with chloride ion is much helpful to decrease the volatility of amines, but the volatility of amines still cannot be ignored. In this work, ammonium salts containing chloride ion were proposed as CO2absorbent. The absorption of CO2by hybrid absorbents composed of ammonium salts containing chloride ion and ethylene glycol was systematically studied. Furthermore, physical properties of the hybrid absorbents during the absorption of CO2were also investigated. The result shows that ammonium salts containing chloride ion have very low volatility, and the hybrid absorbents have very high CO2absorption capacities. Besides, the viscosity of the hybrid absorbents is not very high during the absorption of CO2. Compared with other ionic liquid-based absorbents, the hybrid absorbents composed of ammonium salts containing chloride ion and ethylene glycol are more suitable for CO2capture.