Mercury Oxidation Chemistry
Coal combustion, most notably in utility boilers, has been targeted as a major source of mercury emissions. Mercury levels in coals mined in the U.S. typically range from 0.05 to 0.2 ppm. In the high-temperature regions of coal-fired boilers, mercury in coal is volatilized and converted to its elemental metallic form (Hg0). A complex series of reactions occurs as the flue gas is cooled, converting Hg0 to oxidized (ionic) mercury (Hg2+) and/or mercury compounds that are solid-phase (particle bound) (Hgp).
The partitioning of mercury into its three forms (Hg0, Hg2+, and Hgp) is referred to as mercury speciation, and the degree to which speciation occurs can substantially affect mercury control approaches. Mercuric chloride (HgCl2 or Hg2+) compounds are soluble and can be captured in FGD systems used for sulfur dioxide (SO2) removal. Hgp compounds or Hg compounds adsorbed onto the surface of other particles can be captured to varying degrees using particulate matter control devices such as fabric filters or ESPs. This process may be facilitated by use of additives such as activated carbon. The emissions control device capture points for oxidized and particle-bound mercury are shown schematically in picture below (click more)
Triple play. Mercury capture points for the three forms of mercury in flue gas are illustrated. The gray arrow represents the particulate mercury removed in the particulate control device. The blue arrow represents the oxidized mercury that is removed in the flue gas desulfurization system. At the stack, the “total” arrow (the sum of the solid blue, solid orange, dotted blue, and dotted orange lines) represents the amount of mercury that comes out of the stack without a selective catalytic reduction and flue gas desulfurization system present. The solid blue plus the solid gold lines denote the smaller portion of elemental and oxidized mercury that leaves the stack because the SCR and FGD are present. Source: Cormetech Inc.
It is also well known that catalysts used for selective catalytic reduction of NOx compounds can exhibit the co-benefit of promoting mercury oxidation. In addition to reacting with NOx compounds to form nitrogen and water, titania-based SCR catalysts containing oxides of vanadium and molybdenum or tungsten have been shown to be effective in oxidizing elemental mercury to its Hg2+ form. The SCR process and its key reactions are shown in Figure 2.
SCR process schematic and key reactions. Source: Cormetech Inc.
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