Influence of deposit aging on superheater corrosion

Recovery boiler fireside deposits consist of various alkali salt mixtures. The chemical composition and physical properties of these deposits change over time. Some variations may be explained by changes in the operational conditions of the recovery boiler but chemical reactions, such as sulfation, may also give rise to the changes in the deposit composition.

Alkali chlorides in recovery boiler deposits are volatile at high temperatures and the mobility of volatile species can lead to enrichment or depletion of chlorides over time in various locations within the deposits. Enrichment of alkali chlorides close to the superheater material may dramatically increase the corrosivity of such deposits.

This paper presents a laboratory method developed to study the chemical and physical behavior of ash deposits in a temperature gradient. The setup involves an air-cooled probe that is inserted into a tube furnace, where the probe temperature simulates superheater temperatures and the furnace temperature simulates the flue gas temperature close to the superheaters. Synthetic deposits are applied on probe rings made from superheater materials. Advanced electron microscopy is used to study the cross-section of the deposits and the corrosion layers in the superheater materials.

Experiments with synthetic alkali salt mixtures similar to recovery boiler deposits show that alkali chlorides evaporate from hotter particles in the deposit and condense on colder particles closer to the cooled metal surface or even condense on the metal surface. Formation of a partially or completely molten layer in the outer hotter region closer to the hot gas is also observed in the experiments. The effect of time is shown to be significant for the enrichment of chlorides as longer experiment time leads to higher amounts of vaporization, transportation and condensation within the deposits. These effects are quantitatively verified using Computational Fluid Dynamics modeling. The transportation of alkali chloride vapors becomes negligible if the deposit and metal temperature is cold enough. An enrichment of alkali chlorides towards the cooled metal surface occurs and can increase chlorine-induced corrosion of superheaters as the deposits mature over time.