Effect of H2O on Acidic Sulphate Corrosion in Kraft Recovery Boilers, 1995 Engineering Conference Proceedings

Honghi Tran, Nurez Mapara and David Barham - University of Toronto

Acidic sulphates, mainly NaHSO4 and Na2 S2 O7 , are highly corrosive when molten. Liquid NaHSO4 is formed as a result of a direct reaction between Na2 SO4 in the deposit, and SO, and H2 O in the flue gas. Once formed, it decomposes rapidly into solid Na2 S2 O7 and H2 O Solid Na2 S2 O7 , which is non-corrosive, but may revert to corrosive liquid Na2 S2 O7 if the H2 O content in the flue gas exceeds a critical value. This critical H2 O content increases with temperature, consistent with a regression relationship, log PH 2 O (atm) = 3.598 - 2.300 x l0 3 /T, suggested by Ingraham and Hotz. The re-formed liquid NaHSO4 will cause tube wastage if the deposits at the tube surface do not contain sufficient Na2 CO3 to neutralize it. The risks of acidic sulphate corrosion are higher for low pressure boilers, particularly boilers that are operated at low bed temperatures, with low solids firing, high sulphur input and high sootblowing steam consumption.