The experimental data illustrate how balancing sulfur and chlorine contents in the fuels can help prevent high-temperature, chlorine-based corrosion. Additional considerations with respect to corrosion include: (1) sulfates and other non-chlorine containing species can be corrosive (though rarely as corrosive as chlorine-containing species); (2) local chlorine enrichment may cause corrosion even if the overall stoichiometry suggests that it will not occur (see below); and (3) superheater materials selection strongly impacts corrosion rates.
The formation of local conditions that may lead to corrosion even when overall conditions do not favor it is common in combustion systems. For example, biomass is commonly fired in only a few of the burners during cofiring. Most boilers do not mix gases from separate burners thoroughly. Therefore, the chlorine to sulfur ratios in each burner should be considered when calculating the potential for corrosion. An illustration of this lack of mixing is indicated in the computational fluid dynamics result below, where a plume of low-oxygen gas is seen to persist throughout the furnace and convection pass. Such lack of mixing is a common characteristic of most boilers.