Scale formation in wet scrubbers and the current state of anti-scaling and softening methods for hard waters: A review

Flue-gas desulphurization (FGDs) systems were developed primarily to remove sulphur dioxide (SO2) from industrial gas streams. The main techniques adopted involve wet scrubbing, particularly lime/limestone scrubbing. These techniques are relatively simple, cost less, and have thus, become applicable to other types of plants that emit SO2 such as refineries and smelters. In lime and limestone scrubbing, calcium compounds are added as slurry to the scrubber liquid, and this can result in the formation of solid calcium salts. As a result, the efficiency of such wet calcium-containing scrubbing systems is limited by the solubility of calcium salts in the slurry. As the concentrations of calcium ions build up in the water, the solubility limit of calcium sulphate (CaSO4) and many other compounds is often exceeded. In such systems, the formation of supersaturated CaSO4 causes scaling or deposition in the scrubber. The scaling problem further results in numerous plant downtimes due to the need to open lines and descale. The current review looks into existing and potential methods that have and can be developed to prevent or remove the formation of scaling in FGD systems such as scrubbers. A particular focus of this review is given to approaches such as chemical “softening” of hard waters using ion-exchange and precipitation using lime and soda ash. This review also looks into a contentious physical water treatment – magnetic water treatment (MWT) - as a potential alternative and environmentally conscious solution for the prevention of scale in scrubbers and hard waters in general.