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, SO 2 , in the boiler. The addition of SO 2 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, Cl 2.The following paper is part of an ongoing work to understand the effect of the SO 2 /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 SO 2 /HCl ratio as well as the water concentration. For tests operated at 5% H 2 O, corrosion was reduced by more than 40% as the SO 2 /HCl ratio was increased from 1/8 to 5/8. These results were obtained in the absence of chloride salts, suggesting that SO 2 can decrease chlorine corrosion by interacting with HCl. Tests conducted at 15% H 2 O however demonstrated a constant corrosion rate, with magnitudes comparable to 5% H 2 O at 1/8 SO 2 /HCl. Further work is needed to relate the SO 2 /HCl relationship to water and to elucidate the mechanism associated with mitigating corrosion.
High temperature corrosion via chlorine is a key factor in the degradation of boiler tubes in waste-to-energy (WTE) plants. Corrosion rates are particularly high in the superheater where material temperatures may exceed 450°C and where carbon or low alloy steels may be used. Although increasing sulfur, in the form of SO2, in WTE flue gas has been shown in previous works to have potential for decreasing the corrosion of these materials, the inhibitive effect is not well understood. This work investigated the corrosion of SA178A, a low carbon steel alloy (0.07 wt% C), and NSSER-4, a stainless steel (17.3Cr-13.1Ni-2.5 Si-Fe), via exposure under various well-defined environments, SO2:HCl ratios between 1:8 to 2:1 (HCl fixed at 800 ppm), 8% O2, 12% CO2, 0 and 15% H2O, N2 (balance) at 500°C in a horizontal tube furnace for 50 hours. Additional coupon testing was performed on NSSER-4 after application of 4 mg/cm2 ± 10% NaCl or Na2SO4 at 500 and 700°C for 24 hours to assess the impact of higher SO2 in the against both deposit and gaseous corrosion. Specimen preparation and corrosion assessment followed ASTM method G1-03. Experiments demonstrated little to no trends in corrosion rates at SO2:HCl ratios between 1:8–2:1 under mixed gas environment. However corrosion reduction was observed when SO2:HCl was increased from a reference condition of 1:8 to greater than 1.4:1 in tests with NaCl present, which was also not observed under dry conditions. These results suggest that one possible explanation for the reduction of boiler materials corrosion rate with higher concentrations of SO2 may be largely attributed to the conversion of metal chlorides to sulfates.
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