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

Abstract

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.

Ash deposition behavior of a high-alkali coal in circulating fluidized bed combustion at different bed temperatures and the effect of kaolin

“… 5,6 These inorganic vapors deposited on the heating surface and fly ash particles via condensation, thermophoresis and chemical reactions. 7–9 For instance, Li et al 10 systematically investigated the characteristics of ash deposits formed on an air-cooled steel probe simulating superheater surfaces at different temperatures. They reported that fine ash rich in Na, Ca, S, and Mg on the probe was responsible for the severe ash deposition.…”

Online Corrosion Measurements in Combination with Deposit and Aerosol Analysis during the Co-firing of Straw with Coal in Electrically Heated, Small-Scale Pulverized Fuel and Circulating Fluidized Bed Systems

“…Furthermore, the deposit layer causes deteriorated heat transfer, which in turn reduces the steam production efficiency of power stations. The main mechanisms leading to the formation of deposit layers on superheater tubes are inertial impaction, thermophoresis of small particles due to a temperature gradient, diffusion of aerosols, heterogeneous condensation of inorganic vapors, and chemical reactions of such vapors with the deposit . Due to the lower content of vaporized inorganics during coal combustion, deposit build-up takes usually longer when compared to the combustion of straw or other high-alkali biomass materials …”

“…The main mechanisms leading to the formation of deposit layers on superheater tubes are inertial impaction, thermophoresis of small particles due to a temperature gradient, diffusion of aerosols, heterogeneous condensation of inorganic vapors, and chemical reactions of such vapors with the deposit. 21 Due to the lower content of vaporized inorganics during coal combustion, deposit build-up takes usually longer when compared to the combustion of straw or other high-alkali biomass materials. 22 In a pilot-scale study, Nielsen et al 17 showed that the initial deposits formed during pure straw combustion are mainly composed of condensed KCl and K 2 SO 4 .…”

Investigation on Sodium Fate for High Alkali Coal during Circulating Fluidized Bed Combustion

“…The formation of a sticky layer, coming from condensed vapors, is expected to be crucial for particle deposition in the early stage. 41 Normally, three types of condensation were proposed: 42 (1) directly condensing on the heating exchanging surfacecalled direct condensation, (2) condensing on the particle surface called heterogeneous condensation, and (3) form new aerosol particlescalled nucleation. In this work, nucleation condensation is neglected because of the high furnace temperature.…”

“…21,22 The saturation vapor pressure of the condensing species in eq 4 may be calculated via eq 7. 42 Coefficients A 0 , B 0 , C 0 for varied sodium species are obtained from Kleinhans's work. 42…”