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Formation of sulfur-rich melt near the floor tubing of kraft recovery boilers.

Application: In the floor tubes of kraft recovery boilers, conditions are conducive to stress-corrosion cracking in composite metal when smelt that is rich in sulfur enters a molten phase.

This study is part of a research project on the conditions that favor the formation of stress-corrosion cracks while a recovery boiler is operating. The project includes full-scale probing and measurements, laboratory studies, and theoretical chemical modeling. Here, we concentrate on sulfur-rich melts that can form near floor tubes.

Sampling and analysis

Smelt bed samples were taken near the floor tubing in two kraft recovery boilers. The inner layers of the samples had higher concentrations of sulfur, chlorine, and potassium. Model calculations of melting behavior

According to model calculations, the first melting point of a carbonate-sulfide mixture falls from 760[degrees]C to 510[degrees]C when chlorine and potassium are present. It decreased to 320[degrees]C when the mixture has polysulfldes.

On-line electrical conductivity measurements of the smelt layer adjacent to the floor tubing showed sudden drops in the resistivity, indicating the beginning of the molten phase. Laboratory measurements confirmed that the low resistivity values relate to the molten phase.


The composition of the smelt bed close to the floor tubes differs considerably from the average composition of the smelt bed because sulfur, chlorine, and potassium become enriched in the layer closest to the floor tubes. The enrichment of these three elements and the formation of a sulfur-rich melt may accelerate stress-corrosion cracking in the composite metal on the floor tubes in some kraft recovery boilers. However, sulfur enrichment varies widely, even within the same boiler.

Backman, Hupa, and Kymalainen are researchers with Abo Akademi University, FI-20500 Abo, Finland. Pulliainen is with Savcor Group Ltd. Oy, FI-50100 Mikkeli, Finland. Address correspondence to Backman by email at:
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Title Annotation:Recovery boiler corrosion: summary of peer-reviewed paper
Author:Pulliainen, Martti
Publication:Solutions - for People, Processes and Paper
Date:Feb 1, 2002
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