Relations Between Superheater Corrosion in Recovery and Power Boilers in a Modern Pulp Mill, 1998 International Chemical Recovery Conference Proceedings

The behavior of potassium and chlorine was studied with deposit probe tests in a pulp mill, having both a recovery (RB) and a bubbling fluidized bed (BFB) boiler. The aim was to find out the differences between the corrosion behavior of the two boilers. The BFB boiler has suffered from severe corrosion in the hottest superheater section. The recovery boiler, on the other hand, has been running over five years without corrosion. The steam outlet temperature of the two boilers was 480 °C during the experiments. The enrichment of potassium was found to be very high in the BFB boiler superheater deposits. Chlorine was not enriched in the BFB boiler. In the recovery boiler, typical enrichment of both potassium and chlorine was observed. Enrichment factor for potassium and chlorine in the recovery boiler precipitator dust was 1.5 and 2.5, respectively. The only major difference between the two boilers found in this study was the sulfation of alkali chlorides in the deposits. This was observed in the BFB boiler, but not in the recovery boiler. Suggested corrosion mechanism in the BFB boiler is chlorine induced corrosion due to the sulfation of the deposits. Calculated first melting temperature (T0 ) for the recovery boiler deposit is 582 °C, which is above the highest tube metal temperatures. Corresponding T0 for the BFB boiler deposit is 543 °C, which is over the highest metal temperature. Therefore, molten phase corrosion seems not to be responsible for the enhanced corrosion rates encountered in the BFB boiler. According to this study, the steam outlet temperature of the recovery boiler could be raised up to 530 °C without enhanced corrosion.