Modelling alkali chloride superheater deposition and its implications

Alkali chlorides are often involved in superheater tube fouling and corrosion problems, especially during biofuel combustion. We have carried out a modelling study on fume particle and condensable alkali chloride vapour behaviour in superheater tube boundary layers. Two models of boundary layer aerosol behaviour are presented: one is a general computational fluid dynamics (CFD)-based model and the other one is a one-dimensional boundary layer theory-based model for the stagnation point. Simulations of NaCl behaviour in recovery boiler panel heat exchangers indicated that both (1) boundary layer condensation and subsequent particle thermophoretic deposition and (2) direct vapour deposition are significant deposition mechanisms for NaCl. In the first panel tube boundary layer, NaCl is clearly supersaturated, while further in the panel section boundary layer, it seems to be closer to the thermodynamic equilibrium. As the surface temperature of the ash deposit layer increases during a soot-blowing cycle from 480 to 670 °C, the overall deposition rate of NaCl remains fairly constant (increases only by 52%) due to boundary layer aerosol dynamics. Homogeneous nucleation within boundary layers may take place at certain conditions. Additionally, we have made estimates of the subsequent behaviour of deposited alkali chlorides. The rates of deposit sulphation and generation of HCl(g) do not seem to be sensitive to alkali chloride deposition rates provided that SO2 levels are sufficiently low.