The effect of fireside deposits on kraft recovery boiler superheater corrosion

CIM Bulletin, Vol. 77, No. 872, 1984

HOC NGHIA IRAN, DOUGLAS W. REEVE and DAVID BARHAM, Department of Chemical Engineering and Applied Chemistry, University of Toronto

In kraft pulp mills, the spent pulping liquor is concentrated and used as fuel in the recovery boiler. Extensive deposits are formed in the upper section of the boiler due to the very high ash content of the fuel and the volatile nature of the ash. Deposits are composed of Na2SO4, Na2CO3, NaCl, some potassium salts and reduced sulphur compounds. Chloride and potassium salts are enriched in the deposits compared to the smelt due to their greater volatility. Severe corrosion of the superheater tubes in a recovery boiler has been traced to three factors: a non-uniform temperature profile in the flue gas, relatively high operating steam temperatures and high chloride and potassium concentrations in the fireside deposits. The role of chloride and potassium in superheater corrosion has been explained by increased liquid phase in deposits. Together with corrosion studies, this information has shown that kraft recovery boiler superheater corrosion due to the chloride and potassium enriched deposits can be avoided by proper boiler operation and by judicious superheater and furnace design.

Mots Clés: Corrosion, Fireside deposits, Recovery boilers, Kraft pulp mills, Chloride, Potassium