Kiln shell corrosion

The corrosion of 12 suspension calciner kilns with a tertiary air duct has been studied, and corrosion scales from the kiln transition zone examined. A common feature of the scales was the layered structure in which high-oxygen low-sulphur layers alternate in a variable manner with high-sulphur low-oxygen layers. Varying amounts of chlorides were present and distributed quite homogeneously. Oxygen-deficient layers were less frequent where larger quantities of alkalis were present. The scales were acidic, water extracts showing pH/spl sime/3.5. Water extracts from mechanically cleaned kiln shells contained iron and chloride and had similar pH. The observations collected indicate that kiln shell corrosion may be understood as the combination of ash-deposit corrosion and hot corrosion of the same type observed on the fireside of boiler walls. Thermodynamic calculations have been used to suggest why calciner kilns with tertiary air ducts are so vulnerable to this type of corrosion. There are two strategies for preventing the corrosion; either process changes to change the corrosion conditions or selection of corrosion resistant materials. Following strategy one, it is recommended to increase the kiln valve for chlorides by introducing a by-pass or significantly to reduce the burning zone temperature. Following strategy two, many companies are offering surface treatments of the kiln shell to improve the corrosion resistance. The authors believe the last strategy to be the most promising, and are currently engaged in field testing of a number of multi-component treatments.