High Temperature Corrosion in Fluidised Bed Environment

Doctoral thesis, 2006

Successful material selection for power plants is the result of understanding the environment in each boiler. The boiler environment is affected by fuel chemistry, co-firing, process control and deposits as well as boiler type, design, size and load. The boiler environment is further complicated by the eroding sand found in fluidized bed techniques. This thesis extends this complexity further by addressing more corrosive erosion corrosion environments in both in-situ field studies and laboratory exposures. The field study concerned the exposure of a full scale loop seal superheater in a wood-chip fired circulating fluidized bed, CFB, in Nässjö, which generates heat and electricity with an efficiency of 26 MW and 9 MW, respectively. The superheater tubes were made of a ferritic stainless steel and three austenitic steels. In addition, several tubes were coated by arc spray, HVOF and laser techniques. The tested material temperature varied between 510 to 540°C, depending on the superheater position. The first exposure lasted one firing season of 5242 hours. For the second firing season, three austenitic steels were reinstalled for a further 6150 hours. Material loss was measured on the uncoated tubes and corrosion products are investigated by microscopy and surface analysis techniques. Overall, the corrosion rate in the loop seal was very low and no severe corrosion attack was observed on the uncoated tubes. Out of the coatings only the carbide-containing ones were totally unable to withstand the environment and partially spalled as a consequence of carbide oxidation. Thermally sprayed Alloy 625 coatings also delaminated after the formation of radial cracks, but this was due to a mechanical failure resulting from thermal expansion. The deposits formed on the tube surfaces strongly reflected the flowing conditions and mainly consisted of CaSO4 and K2SO4, which are not considered to be aggressive. Furthermore, the non-linear material wastage pattern on the tubes, showed that corrosion and not erosion-corrosion is the major degradation mechanism. The laboratory exposures were conducted in an erosion corrosion rig that exposes tubular samples in a simulated fluidised bed. Fe- and Ni-base alloys were isothermally exposed at 550°C in air, air + 50 ppm HCl for three weeks (504 hours). A thermal gradient was then applied in air and air + 50 ppm SO2 environments by internally cooling the tubes, which preserved a material temperature of 550°C while the bed temperature was 750°C. Finally, experiments were done in a furnace to create a comparison to an erosion free atmosphere. Material wastage was measured on the tubes and detailed microstructural studies were performed on selected tubes. When SO2 was added to the isothermal condition, material wastage on the austenitic alloys decreased, whereas the oxide thickness increased compared to air exposed samples. Consequently, the erosion pattern changed from Type A to B for this condition. However, when the samples were cooled this effect was not observed. Furthermore, material wastage increased for the Ni-base alloy and decreased for the austenitic steel, indicating that the maximum wastage temperature is material dependent.