Achievements in the Control of Superheater Corrosion in Black Liquor Recovery Boilers, 2001 Engineering / Finishing & Converting Conference Proceedings

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Superheater corrosion in black liquor recovery boilers (BLRB) is currently a continuous headache for both the boiler manufacturers and the operators. Since the BLRB is basically designed to recover the valuable pulping chemicals, unscheduled shutdowns are not usually wanted. Mainly for this reason, the steam parameters in BLRB are still on a quite modest level to avoid shutdowns due to tube failures. In the search for advanced steam parameters, corrosion control in the superheater area is an essential matter.

A good rule of thumb to avoid enhanced superheater corrosion in BLRB’s is to keep the material temperatures below the first melting temperature (FMT) of the deposits. The FMT corresponds to the temperature where the first melt appears in the deposit. Several studies in operating boilers have confirmed the validity of this rule. Presently, there are good tools and methods to estimate the FMT based on the liquor analysis and/or according to the deposit samples. In a greenfield mill, the task to estimate the FMT is more complicated. This involves the knowledge of the behavior of sulfur (S), chlorine (Cl) and alkalis (Na, K) in the lower furnace and in the superheater area. In operating boilers, the evaluation can be made based on the deposit analyses.

The FMT calculated with different methods is generally close to the measured temperature. However, there are also cases where enhanced corrosion starts at a lower temperature than the estimated FMT based on the deposit analyses. This makes the prediction of the corrosion and correspondingly the maximum allowed steam outlet temperature intangible. A proper corrosion control of the superheaters comprises good overall knowledge of the combustion phenomena and expertise in different corrosion mechanisms.