Chapter 9 - Mercury Removal

Abstract

The predominant forms of mercury in coal-fired flue gas are elemental and oxidized. During combustion, the mercury in the coal is transformed into three species that include particle-bound mercury, vapor-phase elemental mercury, and vapor-phase oxidized mercury, primarily in the form of HgCl2. For the best possible removal of mercury from flue gas, a high level of oxidation is helpful. Because HgCl2 is water soluble, it can be removed during the wet flue gas desalifurization process. Particle-bound (Hgp) is easily removed by dust control equipment, such as baghouse filters and electrostatic precipitators. Existing air pollution control devices (APCDs) are capable of removing mercury to some extent. Existing APCDs include electrostatic precipitators (ESPs), fabric filters (FF or baghouse), flue gas desulfurization (FGD), and selective catalytic reduction (SCR). ESPs are designed to reduce fly ash emissions by creating an ionized field that removes charged particles. They have low operating costs, but a limited capacity to remove mercury because it is not generally adsorbed in the fly ash at combustion temperatures. SCR technology is used to reduce emissions of nitrogen oxides by installation of a fixed catalyst bed with reductant injection to reduce NOx to N2. In conjunction with SCR, low NOx burners create a fuel-rich primary combustion zone. This reduces the amount of thermal and fuel NOx created during combustion.