High temperature superheater corrosion is a major operating problem in waste-to-energy plants since it is responsible for plant shutdowns and low energy recovery. This has been largely attributed to the chloride content in municipal solid wastes and the subsequent reactions that occur with generated chlorine gas. While several methods have been developed to reduce corrosion, one which has received more interest is increasing the concentration of sulfur dioxide, SO2, in the boiler. The addition of SO2 to WTE post combustion gas streams has been shown at times to have a favorable impact on corrosion and is said to be an effective method of suppressing dioxin formation by reacting with molecular chlorine, Cl2.
The following paper is part of an ongoing work to understand the effect of the SO2/HCl ratio on the corrosion of alloys exposed to superheater temperatures and flue gas compositions. A commercial low carbon steel alloy, SA178A, was exposed under various flue gas compositions at 500°C, while varying the SO2/HCl ratio as well as the water concentration. For tests operated at 5% H2O, corrosion was reduced by more than 40% as the SO2/HCl ratio was increased from 1/8 to 5/8. These results were obtained in the absence of chloride salts, suggesting that SO2 can decrease chlorine corrosion by interacting with HCl. Tests conducted at 15% H2O however demonstrated a constant corrosion rate, with magnitudes comparable to 5% H2O at 1/8 SO2/HCl. Further work is needed to relate the SO2/HCl relationship to water and to elucidate the mechanism associated with mitigating corrosion.