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

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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 composition
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.
Original languageEnglish
Article number127599
Publication statusE-pub ahead of print - 15 May 2023
MoE publication typeA1 Journal article-refereed


  • Ash deposits
  • Probe sampling
  • Temperature gradient
  • Aging mechanisms


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