Oxidation of mercury during selective catalytic reduction of nitric oxide

A promising method for mercury removal from the gaseous products of coal combustion is the catalytic oxidation of elemental mercury to water-soluble mercuric chloride during selective catalytic reduction of NOx by ammonia, followed by removal of the oxidized mercury during wet flue gas desulfurization.;Measurements of the mercury species distribution and sulfate formation at the outlet of a 310 mm long square cell monolithic V2O 5-WO3/TiO2 commercial SCR catalyst showed the following trends: (1) Mercury oxidation was highly sensitive to HCl at the low levels characteristic of Powder River Basin subbituminous coals (0-5 ppmv). (2) Mercury oxidation was inhibited by NH3, and promoted by NO in the absence of NH3. The inhibitory effect of NH3 increased with decreasing HCl, increased on approach to stoichiometric NH 3/NO, and increased markedly in the presence of excess NH3. (3) CO had a strong inhibitory effect on mercury oxidation at low levels of HCl, which decreased with increasing HCl levels. Variation in the CO content of flue gas when burning low chlorine coals is thus a possible source of variability and uncertainty in the extent of mercury oxidation in SCR. This is regarded as one of the most significant findings of the work and addressed a specific research need identified by Presto et al. (2006) and Presto and Granite (2006).;Implementation of the Tennessee Valley Authority's thermal mercury reduction system for the determination of total mercury resulted in a great improvement in data quality and a marked decrease in the time required to identify steady-state conditions, resulting in a higher probability of success in each test run attempted.;Measurements of sulfate formation made in conjunction with the mercury oxidation measurements showed little change in SO2 oxidation to SO3 with increasing HCl volume fraction in the absence of NH 3. However, the addition of NH3 inhibited the oxidation of SO2 to SO3, regardless of HCl content. CO had little effect on the oxidation of SO2 to SO3.;A one-dimensional mass transfer and chemical reaction model was developed for three of the important processes in SCR: NO reduction by NH3, mercury oxidation by HCl, and SO2 oxidation by O2.;Keywords: Mercury oxidation, SCR, Flue gas, Carbon monoxide, Sulfate.