High-Efficiency Removal of Mercury from Plant Emissions

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Iron ore processing and steelmaking are being recognized as potentially significant sources of mercury emissions, even though the ore as mined does not contain a high level of mercury. The mercury emissions are due to the high volatility of metallic mercury and its compounds, which results in the small quantities of mercury present being efficiently vaporized by the heat of processing. A large fraction of this mercury is in the elemental state, which is unreactive, easily vaporized, and can be transported worldwide over a period of years before it finally oxidizes and precipitates as a contaminant.

Project Objectives: Since the absolute concentrations of mercury in the emissions is in the parts-per-billion range, it is highly uneconomical to have a dedicated scrubber that captures only the traces of mercury. It is much more feasible to “piggy-back” the mercury capture capacity with scrubbers for high-volume pollutants, particularly sulfur and carbon dioxide, so that a single unit can remove all of the major pollutants of concern at once. The objective of this project is therefore to study the oxidation states of mercury that are produced by the iron and steel industry, and their interactions with novel scrubbing agents for the capture of sulfur oxides and carbon dioxide. Methods for control of the reduction/oxidation environment of the scrubber will be of particular interest, as operation under sufficiently oxidizing conditions will convert the mercury into the more easily-captured oxidized state. The mercury concentration and leachability in the final scrubber sludge will also be investigated, to ensure that it does not leach into groundwater.

For this project, the investigators will combine a laboratory-scale thermal processing unit with an instrumented fluidized-bed scrubbing reactor. This unit will examine the interactions of mercury with combustion gases and with scrubber chemistries, and will determine the conditions that most thoroughly capture highly dilute mercury vapors while still being compatible with high-efficiency scrubbing of sulfur oxides and carbon dioxide. Initial experiments by the investigators have shown that the mercury-capture effectiveness of a scrubbing agent could be increased by 9.1 times when an oxidizing agent was introduced into the scrubber to oxidize elemental mercury. The objectives of the research are therefore to (1) Determine what minimum oxidation potential is needed to oxidize the mercury so that it can be captured; (2) Examine methods for increasing the oxidation potential in the scrubber; (3) Measure the effectiveness of mercury absorption, and determine whether maximizing mercury capture can be accomplished while also maximizing capture of carbon dixoide, sulfur oxides, and other pollutants.

The primary scrubbing agents examined will be materials, which the investigators have been studying for use in sequestration of carbon dioxide. Use of these materials will make it possible to both capture mercury and sequester carbon dioxide without causing environmental harm due to mining of new minerals, or development of new disposal sites.