Abstract
Corrosion of water walls in fossil fueled boilers and gasifiers has traditionally been considered the result of gaseous corrodants such as H2S and HCl, reacting with the heat exchanger tube surfaces. Under reducing conditions these corrodants prevent the formation of a protective oxide scale, leading to increased metal loss. Recent field experience in boilers, using staged combustion systems have shown much greater corrosion rates than predicted by simple gas/solid corrosion processes. The presence of large quantities of unoxidized iron sulfide in deposits in areas where high corrosion rates were found suggests that deposits play a role as well. Subsequent laboratory corrosion studies found that the presence of FeS can indeed lead to very high corrosion rates, but only under oxidizing conditions. Since FeS usually deposits only where reducing conditions are present, the accelerated corrosion observed requires alternating reducing and oxidizing conditions. Such conditions may exist in areas in staged boilers where the overfire air mixes with substoichiometric fluegas from the burner zone. It will further be shown that chlorine corrosion may be caused or at least accelerated by chloride containing deposits in fossil fueled boilers instead of or in addition to corrosion due to HCl in the fluegas. Due to the high sulfur content of fossil fuels, chloride deposits can form only under reducing conditions. However, once formed they are highly corrosive under mildly reducing and oxidizing conditions. It is therefore likely that the basic chlorine corrosion mechanisms in fossil fueled boilers are the same as those in waste incinerators.
