High Efficiency Waste-to-Energy Plants - Effect of Ash Deposit Chemistry on Corrosion at Increased Superheater Temperatures

Thermodynamic modeling has been used to study the ash deposit chemistry in waste-fired power plants and the implications for corrosion for increased superheater temperatures. Melting of ash particles and deposits can increase the corrosion of the superheater tubes due to molten salt corrosion. Predictions were made for various waste fuel mixtures with varying contents of Na, K, Pb, Zn, Cl, and S at superheater temperatures (350-550 degrees C). The results show that the formation of a low-melting, chloride-rich melts containing lead and zinc is expected for all of the waste fuel mixtures in the present study at 350-450 degrees C, which are typical superheater temperatures for waste-fired power plants. A molten phase in deposits can increase molten salt corrosion of superheaters. Predictions show that addition of calcium or firing fuel mixtures with a molar ratio of (Na + K)/(Cl + 2S) > 1 may decrease the tendency of the ash to form the corrosive melts in superheater deposits below 500 degrees C. However, if the superheater temperature are increased to around 500 degrees C, these measures will not be as effective due to the formation of molten alkali sulfate-chloride mixtures at temperatures around 500 degrees C and higher. A comparison of predicted values with experimental data for Na, K, Pd, and Zn in the gas phase in combustion of different waste mixtures at different temperatures showed that for several cases the general trends were satisfactorily predicted taking into account experimental and modeling uncertainties. However, in some cases the differences between the measured and predicted values were considerable.