Morphological and chemical differences within superheater deposits from different locations of a black liquor recovery boiler

The effects of two deposit ageing mechanisms were identified by analysing superheater ash deposits from a kraft recovery boiler. Local differences in deposit morphology and chemical composition were identified under the electron microscope. Temperature-gradient-induced diffusion of alkali chloride vapours toward the steel was evident. Two deposit types were identified, based on local chemical compositions: Type 1 deposits had an innermost porous layer of fine, sintered fume particles enriched in K and Cl, that deposited after homogeneous condensation in the gas phase. Type 2 deposits formed via sulfation of initially deposited fume particles rich in K and Cl. Thus the innermost layer was enriched in K and S, while concurrently depleted in Cl. Differences in the local first melting temperature (T0) within the innermost regions of the two deposit types were identified. T0 reached a minimum within the innermost region of Type 1 deposits, implying an increased risk for melt formation and corrosion. Whereas for Type 2 deposits, T0 was increased closest to the steel, reducing the risk for melt formation and superheater corrosion. The presented results provide a better understanding of intradepositional changes taking place after initial deposition, helping assess risks of deposit-related operational problems in the boiler.

Automated summary

By analyzing superheater ash deposits from a kraft recovery boiler, the effects of two deposit ageing mechanisms were identified. Local differences in deposit morphology and chemical composition were observed under the electron microscope. The diffusion of alkali chloride vapours towards the steel due to temperature gradient was evident. The presented results provide a better understanding of intradepositional changes and help assess risks of deposit-related operational problems in the boiler.