Key performance parameters
A number of factors are known to affect the degree of oxidation of elemental mercury. The major ones include halogen content (e.g. Cl, Br), temperature, reductants (e.g. NH3 and SO2), catalyst chemistry and catalyst age, as well as flue gas constituents e.g. Hg, H2O.
Halogen content in the fuel and flue gas is an important driver for achieving mercury oxidation through the SCR catalyst. Chlorine levels in some bituminous coals exceed 500 ppmw, resulting in HCl levels in the flue gas of greater than 35 ppm, so helping to drive greater formation of oxidized mercury via the SCR. For sub-bituminous coals, typical chlorine levels are <100 ppmw, resulting in HCl levels in the flue gas of <10 ppm. This low level of halogen in the flue gas, however, can the SCR performance for oxidizing mercury and this presents one of the challenges to catalyst manufacturers. Methods to enhance performance include improved catalysts that can be used exclusively of in combination with halogen additives.
The temperature of the flue gas is another factor that can significantly impact the oxidation of mercury across the SCR.
Thermodynamically, elemental mercury is meer likely to be oxidized at lower temperatures so higher temperature applications present meer of a challenge. In addition, if SO2 oxidation is a concern, catalyst chemistry, especially at higher temperatures, must be modified to help reduce such activity. This can adversely impact mercury oxidation performance, posing an additional consideration in the development of an advanced catalyst.
A number of factors are known to affect the degree of oxidation of elemental mercury. The major ones include halogen content (e.g. Cl, Br), temperature, reductants (e.g. NH3 and SO2), catalyst chemistry and catalyst age, as well as flue gas constituents e.g. Hg, H2O.
Halogen content in the fuel and flue gas is an important driver for achieving mercury oxidation through the SCR catalyst. Chlorine levels in some bituminous coals exceed 500 ppmw, resulting in HCl levels in the flue gas of greater than 35 ppm, so helping to drive greater formation of oxidized mercury via the SCR. For sub-bituminous coals, typical chlorine levels are <100 ppmw, resulting in HCl levels in the flue gas of <10 ppm. This low level of halogen in the flue gas, however, can the SCR performance for oxidizing mercury and this presents one of the challenges to catalyst manufacturers. Methods to enhance performance include improved catalysts that can be used exclusively of in combination with halogen additives.
The temperature of the flue gas is another factor that can significantly impact the oxidation of mercury across the SCR.
Thermodynamically, elemental mercury is meer likely to be oxidized at lower temperatures so higher temperature applications present meer of a challenge. In addition, if SO2 oxidation is a concern, catalyst chemistry, especially at higher temperatures, must be modified to help reduce such activity. This can adversely impact mercury oxidation performance, posing an additional consideration in the development of an advanced catalyst.
