Qualification of metallic materials for evaporation of waste water from flue gas desulfurization plants

The ecologically-minded processing of waste water from the wet scrubbing of flue gases of coal-fired power plants to produce environmentally acceptabl e products is carried out in a two-step evaporator operating in closed loop mode. The evaporating process leads to high concentration of chlorides in the two evaporation steps: up to about 100 g/l in the Ist step and up to ab out 350 g/l in the 2nd step. Therefore in case of metallic design of the ev aporation equipment materials of construction with exceptional resistance t o chloride induced pitting are required. The corrosion resistance of the hi gh-alloyed stainless steel Alloy 31 (X1NiCrMoCu32-28-7 - UNS N08031) and of the NiCrMo-alloys AlloyC-276 (NiMo16Cr15W - UNS N10276) and Alloy 59 (NiCr 23Mo16Al - UNS N06059) including their weldments were to be tested for this application both in the:laboratory and in field tests. In addition the beh aviour of Ahoy 59 heat exchanger tubes had to be determined in field tests under heat-transfer service conditions. The critical pitting corrosion temperatures of the 3 materials after having been GTAW welded under uniform conditions with FM 59 (ERNiCrMo- 12) filler were determined in potentiostatic tests in model solutions imitating conce ntrated waste water products as they may occur in practice, using 5 K tempe rature intervals. As to be expected the critical corrosion resistance limit s of the materials he at 85 degrees C at chloride concentrations of 100 g/l CI- for the Alloy 31 and of 300 g/l Cl- for both the Alloy 59 and the Ahoy C-276 respectively. Field tests in waste water evaporation: units of flue gas desulfurization p lants of coal-fired power stations are carried out as immersion tests with the welded materials and as heat-exchange experiments using longitudinally welded tubes of Alloy 59 (2.4605). The immersion tests over a period of 32 months show the Alloy 31 (1.4562) t o be a corrosion resistant construction material for tubes and containers i n the first evaporation step, whereas the Alloy 59 (2.4605) and the Alloy C -276 (2.4819) have to be used for the second evaporation step, where the ch loride contents are much higher. The Alloy 59 is to value as the most resistant material according to its lo wer tendency to crevice corrosion. The heat-exchange experiments over a test period of 9 months cause to expec t the Alloy 59 to be a suitable construction material for heat-exchanger tu bes in both evaporation steps in comparison to graphite which is more succe ptible to mechanical destroying.