Review on the ionic liquids affecting the desulfurization of coal by chemical agents

A large amount of SOx will be produced in the combustion process of coal without desulfurization , which is the main factor causing air pollution, environmental damage and PM2.5 increase. Here, desulfurization of coal by chemical agents are summarized. Acid/alkali desulfurization and occur mainly through the chemical reaction of an acid/alkali with a metal oxide or sulfate (such as calcium oxide and ferric sulfate) to remove sulfur from coal, which can only remove a small amount of inorganic sulfur and organic sulfur. Moreover, the combination of an organic solvent with an acid/alkali for desulfurization can transform the organic sulfur molecule into an inorganic sulfur salt in an acid/alkali environment due to cyclic polar compounds containing oxygen or nitrogen that react with coal molecules and dissolve part of the coal molecules. Furthermore, microwave combined with chemical reagents for desulfurization shows better desulfurization efficiency and economy, but most of them were aimed at the removal of oil and model sulfur, and there were few studies on sulfur removal from coal. In view of the advantages of low volatility and high recycling rate of ionic liquids (ILs), the research progress and mechanism of action of ionic liquids in coal desulfurization are reviewed. It is found that acidic ionic liquids can be combined with hydrogen in coal to effectively dissolve organic sulfur. While the acidic ionic liquid was applied in combination with the oxidant hydrogen peroxide, the desulfurization effect appears more significant. The –SH and -S-S- functional groups in coal could be better oxidized by hydrogen peroxide to sulfone and sulfoxide, which was due to the oxidation promoted by acidic ionic liquids.