Modeling of Temperature Gradient-Induced Melt Movement within Kraft Recovery Boiler Ash Deposits

Ash deposits on boiler tubes exposed to temperature gradients undergo chemical and physical changes over time. The three main mechanisms affecting the local chemical composition and morphology of a deposit have been identified: 1) vaporization and recondensation of volatile species, 2) liquid phase sintering and melt movement induced enrichment of species, and 3) melt migration within a solid phase due to temperature gradient zone melting (TGZM) type migration. The prior mentioned phenomena have been observed to occur both in laboratory and boiler environments. The TGZM mechanism has not been studied in detail in the context of ash deposits. However, the phenomenon is understood on a qualitative level. The TGZM mechanism induces melt migration toward the hotter temperature. The migration occurs due to the concentration gradient in the liquid phase, induced by the temperature gradient over the deposit. In practice, the mechanism has resulted in alkali chloride migration toward the outer/hotter edge of the deposit. The present study introduces a modeling approach to estimate how the migration phenomenon can induce local changes in the chemical composition of ash deposits. The modeling approach focuses on internal changes occurring within the deposits and does not consider external mechanisms that affect the deposit composition, e.g., continuous deposit build-up and soot-blowing cycles. The modeling approach considers local phase equilibria and species migration within the molten phase. The temperature gradient-induced concentration gradient over the melt drives the species migration. The results from the modeling approach are compared to earlier published laboratory experiment data conducted with the KCl-NaCl-K2SO4–Na2SO4 chemical system, relevant for kraft recovery boilers.