Deposit sintering in modern Kraft recovery boilers − The role of NaOH?

Modern Kraft recovery boilers have become larger compared to earlier designs and burn higher black liquor dry solids (DS), leading to a more intense burning process and, therefore, increased sodium hydroxide (NaOH) concentration in the upper furnace. Indications of rapid deposit build-up and plugging issues in these boilers, specifically downstream of the superheaters, have been observed at lower temperatures than typically expected, which could be a consequence of the increased NaOH. Thermodynamic calculations were performed to explore the formation of NaOH(g) in boilers with high carbonate content in the fly ash particles and the effect of the presence of NaOH on the melting properties of the deposits. The results indicated that at low temperatures (<400 °C), almost no NaOH(g) is expected to be present in the flue gases. However, we propose that rapid cooling of flue gases across superheaters kinetically freezes the carbonation of NaOH(g), resulting in higher NaOH(g) concentrations than predicted by equilibrium calculations alone. At low temperatures, the remaining NaOH(g) vapors condense on the fly ash and form a melt, potentially leading to the sintering of deposits and plugging issues. To confirm this theory, sintering tests were carried out at 300–350 °C by doping electrostatic precipitator (ESP) ash from a recovery boiler with NaOH powder. It was observed that the doped pellets sintered and were much harder than the reference ones across various experimental conditions. The single particle reactor (SPR) test results showed that the pellets started to sinter immediately upon heating up to 350 °C, suggesting that NaOH melt-induced sintering is a rapid mechanism. The findings of this study may also have implications for boilers with a high sodium (Na) content.