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-dr...
Highlights
Waste-derived fuels and cheap biomass fuels have become increasingly interesting for energy conversion applications
The aim of this paper is to recognize fundamental phenomena prevailing in synthetic ash deposits, and to test the hypothesis that alkali bromide−alkali sulfate mixtures behave in a similar fashion as alkali chloride−alkali sulfate mixtures when exposed to temperature gradients
Alkali bromides are not commonly found in recovery boilers, the results of this study further indicate which mechanisms are responsible for the enrichment of alkali chlorides in cold temperatures, i.e., gas phase migration and liquid phase sintering
Summary
Waste-derived fuels and cheap biomass fuels have become increasingly interesting for energy conversion applications. Waste fractions can include high amounts of alkali metals (Na, K), heavy metals (Pb, Zn), and halides (Cl, Br), all of which are often connected to rapid corrosion of boiler materials.[3−9] Alkali-chloride-induced high-temperature corrosion has been studied extensively, and it is recognized as one of the prominent reasons for corrosion in biomass-fired boilers. The detailed corrosion mechanisms are still being discussed.[2] Heavy metals in combination with Cl have been studied to some extent, and several studies discussing the corrosion effects and mechanisms have been published.[3,4,8−11] The existing corrosion results with alkali bromides are similar to those with alkali chlorides. To the best of our knowledge, alkali bromide corrosion has not been studied in the presence of a temperature gradient across the ash deposit
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