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

Abstract

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 °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 °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 °C. However, if the superheater temperature are increased to around 500 °C, these measures will not be as effective due to the formation of molten alkali sulfate−chloride mixtures at temperatures around 500 °C and higher. A comparison of predicted values with experimental data for Na, K, Pb, 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.

Thermodynamic Equilibrium Model Applied To Predict the Fouling Tendency in a Commercial Fluidized-Bed Boiler, Combusting Solid Waste

“…Several studies − reported gas-phase sulfation of alkali chlorides during the biomass combustion, aiming to reduce the amount of sticky alkali chlorides in the flue gas. On the other hand, the formation of low-melting-point acidic alkali sulfates, such as K 2 S 2 O 7 and KHSO 4 , were also reported when the SOx concentrations in the flue gas were high. , In this case, the deposited alkali sulfates (such as K 2 SO 4 and Na 2 SO 4 ) are further sulfated with the presence of SO 3 and lead to the formation of low-melting-point alkali pyrosulfates (Na 2 S 2 O 7 with mp 389 °C, and K 2 S 2 O 7 with mp 404 °C) . However, the solid-state sulfation of alkali chlorides is believed to be slow and is only important in the deposits and not in the fly ashes .…”

“…The Equilib module of Factsage calculates the phase assemblage and phase composition of a multicomponent and multiphase system at the lowest Gibbs energy, based on the specified conditions and included thermodynamic databases. 28 All input data in the equilibrium model are based on experimental data and operational data obtained during the fullscale measurements. Therefore, it is possible to compare the experimental findings with the theoretical calculations and thus evaluate the accuracy of the thermodynamic model for the fouling prediction.…”

“…Thermodynamic equilibrium modeling is based on the Gibbs energy minimization technique. The Equilib module of Factsage calculates the phase assemblage and phase composition of a multicomponent and multiphase system at the lowest Gibbs energy, based on the specified conditions and included thermodynamic databases …”

Advanced Coatings by Thermal Spray Processes

“…Boilers utilizing wastes, prone to significant corrosion and degradation of components in different operating environments, are well-documented [30]. Corrosion from alkali-chlorides is also significant in such boilers and is often further complicated if demolition wood from old buildings or railroad beams, with appreciable content of metals like lead and zinc, is used [31,32]. Similarly, the black liquor recovery boilers (BLRBs) are also beset with problems of corrosion [33].…”