Abstract: Sulfur dioxide (SO2) is a kind of colorless and intensely irritating gas. When dispersing in the air, SO2 will threaten the human health, do harm to the environment, and deteriorate air quality. The main sources of man-made SO2 pollution are fuel combustion, industrial production, transportation and so forth. Among them, fuel combustion accounts for 70%. Therefore, minimizing and controlling SO2 emission from fuel combustion are primary research directions in the field of energy utilization and environmental protection in China. Flue gas desulfurization (FGD) is considered as an effective way to remove SO2. Currently, the wet flue gas desulfurization is the most popular method for flue gas treatment. Wet flue gas desulfurization installation accounts for 85% of the total installed capacity of flue gas desulfurization units in the world, and the amount of flue gas treated by wet flue gas desulfurization accounts 80% of the total treatment capacity. There are mainly practical techniques for the wet flue gas desulfurization, including calcium, organic amine and seawater desulfurization. Among them, calcium desulfurization can remove SO2 efficiently and has strong adaptability to different kinds of coal. However, there are large amounts of by-product CaSO4 formed in the process, which leads to the low economic efficiency. The organic amine desulfurization is of less system corrosion, and the byproduct can be used to produce sulfuric acid, but the amine is volatile, resulting in loss of absorbent and second pollution. Seawater desulfurization is a simple and reliable method for SO2 removal, whereas its application is limited by the geographical location and has an adverse impact on environment. Ionic liquid (IL) is an emerging green medium with the advantages of environment protective, reproducible and structure tunable which provides a new method for solving the pollution problems of traditional processes. Especially, ILs have a promising application prospect in gas separation, because there is no weight loss for ILs during the absorption process and the cycle of gas absorption and desorption can be conducted at a lower temperature. This excellent SO2 absorption performance of ILs endows them with great potential for SO2 capture. Currently, researchers have synthesized a series guanidinium, imidazoles, alkanolamines and pyridine-based ILs, for SO2 absorption. The SO2 absorption performance and mechanism of ILs have been also explored. Based on the tunable structure, functional groups, such as cyano, ether, amino and halogen are introduced into the anion and cation of ILs to synthesized the ILs with specific ability, aiming at making desulfurization process efficient, reversible and energy-saving. In this article, the properties and mechanism of SO2 absorption by various types of ILs in recent years are reviewed, which provides a systematic understanding of the ILs application in the field of SO2 absorption. The effects of anion and cation species, functionalization, especially acidity and alkalinity on the absorption of SO2 in ILs are emphasized. It is of great value to adjust the acidity and basicity of ILs, to design the structure of ILs, to explore the mechanism of SO2 absorption by ILs, and to improve the absorption properties of SO2. Finally, the problems existing in the current research are pointed out and the prospects for the synthesis of new type ILs for SO2 absorption are also discussed.
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2018, Vol. 32 
