Superheater fireside deposits and corrosion in kraft recovery boiler

Superheater fireside deposits are formed by the impingement of particles ofblack liquor residue entrained in combustion gase-s (carryover), and by condinsation of chemicals vaporized in the lower furnace (vaporization-condensation). Deposits are principally sodium, sulfate, and carbonate, but there are also significant concentrations of chloride and potassium. The enrichment of chloride and potassium salts depends strongly on the temperature of the deposit, with Iower temperatures favoring enrichment andhigher temperatures resulting in depletion. Enriched deposits have lowermelting points, of which the first melting point and the radical melting pointare particularly important. The former allows accelerated corrosion caused bythe liquid phase, and the latter is the temperature at which both slagging andmore aggressive liquid-phase corrosion occurs. Laboratory corrosion resultsshow that potassium can cause an enhanced attack on mild steels as well assuperalloys. Enrichment of chloride and potassium salts in fireside depositsand thus increased corrosion of superheater tubes is not limited to the closed cycle mill.This work has been generously supported by Erco Industries, Ltd., Great Lakes Forest Products, Ltd., and the Natural Sciences and Engineering Research Counci