EVALUATION OF DIFFERENT METAL-ALLOYS FOR WASTE-WATER TREATMENT OF FLUE-GAS DESULFURIZATION PLANTS

Desulfurization of flue gases from coal-fired power plants is carried out by wet scrubbing processes in 90% of the cases. Using milk of lime or lime stone suspensions as a sorbent, said processes not only remov e sulfur dioxide but also all other flue gas pollutants. The sulfur co mpounds are mostly separated in the form of calcium sulfate (gypsum) i n the solid-liquid separation process whereas the water-soluble compon ents, mainly calcium chloride are discharged with the scrubber solutio ns. Since the salt load of effluents to be discharged into municipal s ewer systems or surface waters is limited gy legislation, the FGD scru bber solutions must be treated by evaporation, salt modification or re source recovery. For evaporation crystallization of these extremely co rrosive FGD scrubber solutions which - depending on the process - can lead all the way to a product suitable for depositing, an economic equ ipment concept based on metallic construction materials was to be inve stigated. In a parameter study, electrochemical test methods were used to determine the influence of the pH value, the Fe3+ F- and SO2 conte nts in a solution containing 200 g/l CaCl2 + 30 g/l NaCl at 80 degrees C on the resistance to pitting corrosion of a stainless steel and var ious NiCrMo alloys. In addition, electrochemical and wet corrosion tes ts (e.g. crevice corrosion) were conducted using original FGD scrubber solutions with 43-50% dry substance from the power plants Reuter-West , Oberhavel and Rudow of the BEWAG, Berlin. Of the materials tested, N iCrMo allays 2.4605 (NiCr 23 Mo 16 Al, Alloy 59), 2.4602 (NiCr 21 Mo 1 4 W, Alloy 22) and 2.4819 (NiMo 16 Cr 15 W, Alloy C-276) proved to be resistant under the conditions of the third evaporation crystallizatio n stage, with the corrosion resistance decreasing in the above order. The stainless steel 1.4529 (X 2 NiCrMoCu 25 20 6) and the NiCrMo alloy 2.4856 (NiCr 22 Mo 9 Nb, Alloy 625) experienced pitting corrosion in nearly all test solutions at redox potential. Also the alloy 2.4610 (N iMo 16 Cr 16 Ti, Alloy C-4) was not always sufficiently resistant. In the evaporation crystallization process, care should be taken to ensur e that the pH does not fall below 5, that the redox potential of the F GD scrubber solutions remains below the transpassive dilution or pitti ng potential and that crevices are avoided, if possible.