Understanding low temperature corrosion in Kraft Recovery Boilers – Implications for Increased Energy Recovery, 2016 Pulping, Engineering, Environmental, Recycling, Sustainability (PEERS) Conference Proceedings

Our measurements in Kraft Recovery boilers, a sodium sulfite boiler and biomass boilers have all shown that there is no sulfuric acid in the stack gases of these boilers. However, hygroscopic salts coat the steel surfaces in boilers. It is absorption of water by these hygroscopic salts that can cause low temperature corrosion. When these salts absorb water, a concentrated salt solution is formed on the surface of the salt. This solution leads to corrosion of the steel surface. In this work we have measured the corrosion of carbon steel coupons under salts, held at different temperatures and different volume % water in the gas. We have tested the salts Na2SO4, Na2CO3, NaCl, KCl, NaHSO4, salt mixtures and ashes from boilers. By testing at different temperatures and volume% H2O, we have been able to find a transition from visible corrosion to no visible corrosion. Because corrosion of steel in contact with these electrolyte solutions is quite rapid, we ran corrosion experiments for 4 or 24 hours. The experimental set-up is shown in Figure 1. These results indicate that flue gas temperatures can be lowered significantly without corrosion, resulting in improved efficiency of these boilers. 

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