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

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.