Browsing by Author "Balint, Roland"

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    Equilibrium model approach to predict local chemical changes in recovery boiler deposits
    (Elsevier Ltd, 2024-10-15) Niemi, Jonne; Balint, Roland; Engblom, Markus; Lehmusto, Juho; Lindberg, Daniel; Hupa, Leena; Department of Chemical and Metallurgical Engineering; Metallurgical Thermodynamics and Modelling; Åbo Akademi University
    A step-by-step equilibrium model approach was developed to estimate and describe the enrichment of K and Cl within recovery boiler deposits. The model predicts free melt movement towards colder temperatures within deposits. Due to a temperature gradient, the melt amount and composition differ across the deposit. The melt movement affects the local composition, which leads to changes in the local phase composition. The model predicts how the deposit profile changes due to melt migration into pores within the deposits. Changes in the deposit composition profile also affect the melting behavior locally, resulting in lower local melting temperatures, which can be detrimental to heat exchanger materials. The modeling results were compared to earlier published laboratory and full-scale boiler measurements, and there is a good agreement between the results. The model predicts a local decrease in the first melting temperature of recovery boiler deposits by ∼30 °C. These findings closely align with experimental results, shedding light on the intricate mechanisms of melt percolation and intra-deposit aging processes. The proposed step-by-step model offers a means to achieve more accurate estimations of locally prevalent first melting temperatures in recovery boiler deposits.
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    Morphological and chemical differences within superheater deposits from different locations of a black liquor recovery boiler
    (Elsevier Ltd, 2023-03-15) Balint, Roland; Engblom, Markus; Niemi, Jonne; Lindberg, Daniel; Saarinen, Timo; Rautala, Jaakko; Hupa, Mikko; Hupa, Leena; Åbo Akademi University; Department of Chemical and Metallurgical Engineering; Metsä Fibre Oy; Department of Chemical and Metallurgical Engineering
    The effects of two deposit ageing mechanisms were identified by analysing superheater ash deposits from a kraft recovery boiler. Local differences in deposit morphology and chemical composition were identified under the electron microscope. Temperature-gradient-induced diffusion of alkali chloride vapours toward the steel was evident. Two deposit types were identified, based on local chemical compositions: “Type 1” deposits had an innermost porous layer of fine, sintered fume particles enriched in K and Cl, that deposited after homogeneous condensation in the gas phase. “Type 2” deposits formed via sulfation of initially deposited fume particles rich in K and Cl. Thus the innermost layer was enriched in K and S, while concurrently depleted in Cl. Differences in the local first melting temperature (T0) within the innermost regions of the two deposit types were identified. T0 reached a minimum within the innermost region of Type 1 deposits, implying an increased risk for melt formation and corrosion. Whereas for Type 2 deposits, T0 was increased closest to the steel, reducing the risk for melt formation and superheater corrosion. The presented results provide a better understanding of intra-depositional changes taking place after initial deposition, helping assess risks of deposit-related operational problems in the boiler.
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    Temperature gradient induced changes within superheater ash deposits high in chlorine
    (PERGAMON-ELSEVIER SCIENCE LTD, 2021-07-01) Balint, Roland; Engblom, Markus; Niemi, Jonne; Silva da Costa, Daniel; Lindberg, Daniel; Yrjas, Patrik; Hupa, Leena; Hupa, Mikko; Department of Chemical and Metallurgical Engineering; Metallurgical Thermodynamics and Modelling; Abo Akademi University; CENIBRA
    Cross-sections of kraft recovery boiler superheater deposits were analyzed using scanning electron microscopy (SEM) and energy dispersive X-ray analysis (EDX). The observed deposit morphology can be explained by temperature gradient induced time-dependent processes such as diffusional transport of alkali chloride vapours, temperature gradient zone melting, formation of melt enriched in Cl and K, and movement of this enriched melt towards the steel. These processes have recently been identified on a laboratory scale under well-controlled conditions, and are now for the first time identified to take place also in actual boiler superheater deposits. The identified processes alter the local deposit composition and melting behaviour close to the steel. The local first melting temperature (T0) close to the steel is lower by 30 °C compared to that of the deposit bulk T0. The observations made in this work give new insight into the melting and ageing behaviour of superheater deposits, relevant for superheater corrosion.
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    Temperature-Gradient-Driven Aging Mechanisms in Alkali-Bromide- and Sulfate-Containing Ash Deposits
    (American Chemical Society, 2019-07) Niemi, Jonne; Balint, Roland; Engblom, Markus; Lehmusto, Juho; Lindberg, Daniel; Åbo Akademi University; Department of Chemical and Metallurgical Engineering
    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-drivenalkali 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.