Understanding the effects of sulfur on mercury capture from coal-fired utility flue gases

Abstract

Coal combustion continues to be a major source of energy throughout the world and is the leading contributor to anthropogenic mercury emissions. Effective control of these emissions requires a good understanding of how other flue gas constituents such as sulfur dioxide (SO2) and sulfur trioxide (SO3) may interfere in the removal process. Most of the current literature suggests that SO2 hinders elemental mercury (Hg0) oxidation by scavenging oxidizing species such as chlorine (Cl2) and reduces the overall efficiency of mercury capture, while there is evidence to suggest that SO2 with oxygen (O2) enhances Hg0 oxidation by promoting Cl2 formation below 100 °C. However, studies in which SO2 was shown to have a positive correlation with Hg0 oxidation in full-scale utilities indicate that these interactions may be heavily dependent on operating conditions, particularly chlorine content of the coal and temperature. While bench-scale studies explicitly targeting SO3 are scarce, the general consensus among full-scale coal-fired utilities is that its presence in flue gas has a strong negative correlation with mercury capture efficiency. The exact reason behind this observed correlation is not completely clear, however. While SO3 is an inevitable product of SO2 oxidation by O2, a reaction that hinders Hg0 oxidation, it readily reacts with water vapor, forms sulfuric acid (H2SO4) at the surface of carbon, and physically blocks active sites of carbon. On the other hand, H2SO4 on carbon surfaces may increase mercury capacity either through the creation of oxidation sites on the carbon surface or through a direct reaction of mercury with the acid. However, neither of these beneficial impacts is expected to be of practical significance for an activated carbon injection system in a real coal-fired utility flue gas.