Alkali Vapor Condensation on Heat Exchanging Surfaces: Laboratory-Scale Experiments and a Mechanistic CFD Modeling Approach

Inorganic vapors and their condensation can lead to severe operational problems in pulverized fuel systems such as integrated gasification and combined-cycle power plants or a conventional pulverized fuel combustion system. In order to understand these phenomena, a laboratory-scale cooling line for hot gases is used to measure and quantify the deposition of alkali vapors caused by heterogeneous condensation on a horizontal probe. The cooling line consists of two zones, an isothermal evaporation zone for the vaporization of alkali salts and a condensation zone. A condensation probe equipped with steel rings is placed inside the condensation zone with a gradually decreasing temperature. Different concentrations of inorganic vapors are studied under controlled conditions, and condensation rates on a probe, maintained at different temperatures, are quantified. A computational fluid dynamics model is developed and used to validate a heterogeneous condensation model based on Ficks' law of diffusion. Numerical modeling can predict the location and amount of condensed inorganic vapors. The model shows a high sensitivity to the wall temperature which needs to be predicted accurately. A calculation procedure for saturation pressures and diffusion coefficients for gaseous alkali salts is presented and discussed. With this fundamental model it is possible to predict condensation rates of inorganic vapors. The model can also be applied for the calculation of condensation of vapors on existing fly ash particles. The model is essential for the prediction of hot gas cleaning systems for future integrated gasification combined-cycle plants or the formation of an initial layer on a superheater tube in pulverized fuel boilers firing alkali-rich biomass. The present study can serve as a development case for simplified empirical models in which the boundary layer is not resolved with a high number of nodes, e.g., for a model of a full-scale boiler.