One difference between three desulphurization processes: additive process, dry process and wet process. The last process is the one most commonly used and consists of the following parts: fluegas washer, limemilk processing and gypsum processing. The fluegases coming from the electrofilter reach the fluegas desulphurization plant over a vacuum system. In the FGD, the fluegas is routed through a prewasher in order to make the process of desulphurization and gypsum production largely independent of the firing material. In the prewasher, most of the acidic fluegas pollutants, e.g. chlorides and fluorides, as well as flyash, is separated. The prewasher is designed as uniflow current washer with two injecting levels and works in the pH range of 1 to 2. Central part is the absorber with absorber sump. In the spray levels in the upper part of the absorber, the flyash is covered with lime suspensions. The SO2 contained in the fluegases reacts with the limestone (CaCO3)initially, to become calcium sulfite which subsequently becomes a gypsum suspension with the injection of calcium sulfite. The air is injected into the absorber sump, in which the lime suspension is contained.
The washer has the task to achieve an optimum desulphurization effect as well as an optimum gypsum quality. Main parameters for the control of processes are the excess of lime, pH value and solids content of the suspensions. In the upper part of the absorber, the drop separators, the steam saturated fluegas is dried. As the fluegas was cooled during washing from 130°C to 50°C, it has to be reheated to 72°C in order to become light enough to be emitted through the chimney. This task is carried out by a REGAUD (regenerative gas warmer) which removes heat from the fluegas prior to entry into the desulphurization plant and heats it again after the fluegas desluphurization stage. In gypsum processing, the gypsum suspension from the absorber sump, produced by crystallization of the flimemilk suspension at a certain pH value, is subjected to a thickening process in the hydro cyclones, to be subsequently drained in filters and centrifuges to a degree that a residue moisture of less than 10% remains in the FGD gypsum. Subsequently, the gypsum is thermically dried and stored in gypsum silos for further use. The gypsum is used in the cement and gypsum industry, The lime suspension (lime milk) required for fluegas desulphurization are produced in settling containers with the addition of process water and lime milk. |
