Sulfur dioxide removal: An overview of regenerative flue gas desulfurization and factors affecting desulfurization capacity and sorbent regeneration

Numerous mitigation techniques have been incorporated to capture or remove SO₂ with flue gas desulfurization (FGD) being the most common method. Regenerative FGD method is advantageous over other methods due to high desulfurization efficiency, sorbent regenerability, and reduction in waste handling. The capital costs of regenerative methods are higher than those of commonly used once-through methods simply due to the inclusion of sorbent regeneration while operational and management costs depend on the operating hours and fuel composition. Regenerable sorbents like ionic liquids, deep eutectic solvents, ammonium halide solutions, alkyl-aniline solutions, amino acid solutions, activated carbons, mesoporous silica, zeolite, and metal-organic frameworks have been reported to successfully achieve high SO₂ removal. The presence of other gases in flue gas, e.g., O₂, CO₂, NOx, and water vapor, and the reaction temperature critically affect the sorption capacity and sorbent regenerability. To obtain optimal SO₂ removal performance, other parameters such as pH, inlet SO₂ concentration, and additives need to be adequately governed. Due to its high removal capacity, easy preparation, non-toxicity, and low regeneration temperature, the use of deep eutectic solvents is highly feasible for upscale utilization. Metal-organic frameworks demonstrated highest reported SO₂ removal capacity; however, it is not yet applicable at industrial level due to its high price, weak stability, and robust formulation.

Formation mechanisms of Caprolactam-tetraalkyl ammonium halide deep eutectic and its hydrate

Deep eutectic solvents (DES) are generally composed of two or three cheap and safe components that are associated with each other via hydrogen bonds. The formation Caprolactam (CPL)-tetraalkyl ammonium halide (TAAX) DES and its hydrate were investigated using React IR. TAAX was introduced to CPL in the molten state at 353.15 K, the intermolecular H-N-C=O⋯H-N-C=O hydrogen bonds (CPL dimer) transfer to X^-⋯H-N-C=O (CPL-TAAX DES). When water was added to CPL-TAAX DES, intermolecular X^-⋯H-N-C=O⋯H-O-H hydrogen bonds appeared, and CPL-TAAX DES hydrate was formed. However, if the ratio of water exceeded the moral fraction of 0.61, CPL-TAAX DES hydrate turned into a CPL-TAAX aqueous solution. The CO (1660 cm^-1) bonds were linearly correlated with hydrogen bonds number and TBAB amount in CPL-TBAB DES. When the molar fraction of CPL-TBAB DES was 0.39 to 0.78, the CO peak redshift has a better linear correlation with water. Halide anions and the alkyl chain length of TAAX have little influence on the redshifts and peak intensity of the CO peak.