Correlation Between Sodium Sulfate Mass Transfer and Low-Temperature Hot Corrosion

Abstract

A mass transfer model is developed that considers diffusive and chemical aspects of sodium sulfate formation and deposition on cooled blades of coal-fired gas turbines. The roles of gas phase condensation of sodium sulfate and multicomponent diffusion across a chemically frozen thin boundary layer are elaborated. A rational procedure is presented for correlating material wastage with laboratory weight gain data obtained by exposing alloy specimens pre-coated with a thin film of salt to SO2-SO3 in an oxygen environment. The sodium sulfate mass transfer model is used in conjunction with the correlation to project blade corrosion and lifetime as a function of gas turbine inlet temperature, blade cooling, and sodium and sulfur contaminant concentration.

A simplistic model to study the influence of film cooling on low temperature hot corrosion rate in coal gas/syngas fired gas turbines

“…For years, corrosion problems associated with molten Na 2 SO 4 have been studied in isolation with the gas turbines. The chemistry involved during the formation of Na 2 SO 4 becomes highly complex in a coal or syngas fired gas turbine as discussed by Ahluwalia and co-researchers [5,6]. Sodium, as an impurity in coal, exists primarily as a halide, alumino-silicate or in combination with the organic matter.…”

“…This paper explores the validity of this assumption. Present study uses the results reported by Ahluwalia and Im [6] along with a simple resistance model for heat and mass transfer to establish the variation of hot corrosion rate along a surface, in the presence of film cooling for various film effectiveness distributions corresponding to different blowing ratios. The paper then identifies whether LTHC gets suppressed or augmented in the presence of film cooling.…”

“…4 schematically shows the relevant transport processes that affect Na 2 SO 4 deposition for different values of T d . For a superheated gas, T 1 ) T d , sodium containing species (NaOH and NaCl) and SO 2 diffuse towards the sub-cooled wall (T w < T d ) thereby aiding sulfidation of the salts, while Na 2 SO 4 diffuses outwards due to the large difference between T 1 and T w (with corresponding difference in partial pressures [6]). As the gas temperature reduces and approaches the dew point (T 1 P T d ), a situation arises where the Na 2 SO 4 starts to diffuse towards the wall, thereby reinforcing the sulfidation of NaOH and NaCl.…”

“…The first two cases discussed above are of primary concern in high temperature gas turbine systems. Ahluwalia and Im [6] developed a Na 2 SO 4 mass transfer model and were able to predict the dependence of hot corrosion on the operating temperatures, sulfur content in the fuel and other parameters, as shown in Fig. 5.…”

“…The dew point temperature (T d ) of Na 2 SO 4 , which is inversely proportional to the sodium concentration [6], marks the onset of Na 2 SO 4 deposition. Fig.…”