Corrosion in Fluidized Bed Boilers Burning Potassium and Chlorine Containing Fuels, 1996 Engineering Conference Proceedings

K. Salmenoja & K. Mäkelä

Tampella Power Inc.

Severe corrosion of high alloyed steel DIN 17175 grade X20 CrMoV 12 1 (X20) was discovered in a BFB boiler burning a mixture of bark and pulp mill biosludges. Maximum measured corrosion rate was about 10 mm/year. The steam outlet temperature of the boiler is 530 °C. High potassium chloride concentrations were found in the deposits. In SEM/EDXA analyses of corroded tubes high chlorine concentrations were always found in the interface between the metal and the scale. Another feature was that iron has diffused through the scale and formed a new oxide layer above the original tube surface but below the ash deposit layer. Tubes with failures also showed a different scale structure than other corroded tubes. Mechanism of corrosion was similar to high temperature oxidation, although it occurred at temperatures below 580 °C. Oxidation behavior of iron and X20 in the presence of HCl was studied with simultaneous DTA-TGA analyzer. The rate of oxidation of X20 is very low below 600 °C. However, iron starts to oxidize above 350 °C and rapid oxidation occurs above 600 °C. It was concluded that the deposit has a crucial role in the initiation of the corrosion and the mechanism is some sort of a chlorine enhanced low temperature oxidation.