Temperature-Gradient-Driven Aging Mechanisms in Alkali-Bromide- and Sulfate-Containing Ash Deposits

Abstract

The aging of alkali-bromide-containing ash deposits was studied by applying premixed alkali bromide–alkali sulfate mixtures on a laboratory-scale temperature gradient probe. The probe temperature was kept at 500 °C, while the furnace air temperature was measured to be 800 °C, simulating a heat exchanger ash deposit temperature profile. Deposits of ∼5 mm thick were aged in the furnace for 2–8 h and subsequently rapidly cooled to room temperature. The deposit cross-sections were analyzed and characterized using SEM/EDX. The deposits were observed to form multilayered structures, where the furnace-facing region was dense and sintered, while the steel-facing region was porous. Within the porous region, a gas phase migration of alkali bromides toward the colder temperature was observed. The alkali bromide migration toward the colder steel temperature observed in the experiments was quantified and compared to modeling results. The modeling results were calculated by modifying an existing temperature-gradient-driven alkali chloride intradeposit migration model for alkali bromides. The model is in agreement with the experimental results, validating an enrichment mechanism for alkali bromides. Because of their relatively high saturation pressures, alkali bromide migration was observed to be significantly faster than the earlier reported migration of alkali chlorides. Enrichment of alkali bromides in colder temperatures in boiler deposits could lead to significant changes in the local composition of the deposit, possibly leading to an enhanced corrosion rate of the tube material and/or densification of the deposit structure.

Superheater Ash Deposit Ageing – Impact of Melt Fraction on Morphology and Chemistry

“…Nevertheless, local element enrichment within the molten phase has also been reported for binary systems [15,24,25]. The composition of the molten phase approaches the eutectic composition toward the interface between the molten and porous layer [15,24,25]. However, this does not change the local first melting temperature and no significant decrease in the thickness of the porous layer has been reported when using binary salts.…”

“…For binary salts, a local change in deposit composition due to element enrichment does not affect T 0 due to their eutectic nature. Nevertheless, local element enrichment within the molten phase has also been reported for binary systems [15,24,25]. The composition of the molten phase approaches the eutectic composition toward the interface between the molten and porous layer [15,24,25].…”

“…In addition, smaller amounts of various inorganics originating either from the raw wood [14] or introduced into the liquor during various process steps [14] can be found in the deposits, further affecting T 0 . In addition, corrosion products originally formed at the steel surface [15] can be found within deposits, which can form low melting eutectics and increase the risk of melt getting in direct contact with the steel. For kraft recovery boiler deposits, the melting behaviour is mainly influenced by the deposit K and Cl content.…”

“…Recent studies carried out at Åbo Akademi University have shown that temperature gradient-induced processes can also cause changes in the local chemical composition and morphology of deposits [15,24,25]. Due to the temperature difference between the hotter flue gas and the cooler steel surface, deposits are exposed to a temperature gradient.…”

“…Due to the temperature difference between the hotter flue gas and the cooler steel surface, deposits are exposed to a temperature gradient. In laboratory experiments utilising synthetic ash deposits, several temperature gradient-induced mechanisms affecting the local deposit chemistry and morphology have been identified [15,24,25]. Such mechanisms are commonly referred to as deposit ageing, as they take place after the deposit has formed.…”