Changes in chlorine content over time-Probe deposit sampling in a Finnish kraft recovery boiler

Probe deposit samples of 1, 5, and 8 weeks exposure time were collected from the superheater area of a Finnish kraft recovery boiler. Deposit cross-sections were analysed regarding their morphology and chemical composi-tion using SEM/EDXA. Morphological differences along the probe's circumference were observed due to different prevailing deposition mechanisms. Overall enrichment in the deposit Cl content with increasing exposure time was observed. The Cl enrichment proceeds via diffusion of Cl-vapours from the flue gas into the deposit. Two deposit ageing mechanisms were identified within the deposit samples. First, alkali chloride layers formed on the furnace-facing side of deposit particles and also directly on the steel surface as a result of deposit ageing. Second, within the 5 and 8-week probe samples, sulfation of alkali chloride within the inner regions of the deposits was observed. Analysis of the local first melting temperature of the deposits showed a significant decrease within the Cl and K-rich layer adjacent to the steel surface, implying an increased risk for melt formation, and thus corrosion, in the direct vicinity of the steel.

Automated summary

Deposit samples of different exposure times were collected from a Finnish kraft recovery boiler. Analyzing the morphology and chemical composition of the deposits revealed differences due to varying deposition mechanisms. Increasing exposure time led to enrichment in Cl content, which occurred through diffusion of Cl-vapours from the flue gas. Two aging mechanisms were identified within the deposits, involving alkali chloride layers and sulfation. Analysis of melting temperature showed a higher risk of corrosion in the Cl and K-rich layer adjacent to the steel surface.