Temperature gradient induced changes within superheater ash deposits high in chlorine

Cross-sections of kraft recovery boiler superheater deposits were analyzed using scanning electron microscopy (SEM) and energy dispersive X-ray analysis (EDX). The observed deposit morphology can be explained by temperature gradient induced time-dependent processes such as diffusional transport of alkali chloride vapours, temperature gradient zone melting, formation of melt enriched in Cl and K, and movement of this enriched melt towards the steel. These processes have recently been identified on a laboratory scale under well-controlled conditions, and are now for the first time identified to take place also in actual boiler superheater deposits. The identified processes alter the local deposit composition and melting behaviour close to the steel. The local first melting temperature (T₀) close to the steel is lower by 30 °C compared to that of the deposit bulk T₀. The observations made in this work give new insight into the melting and ageing behaviour of superheater deposits, relevant for superheater corrosion.