Development of a process for combined removal of mercury and nitric oxides fromflue gas

摘要

Continuing concern about the effects of mercury in the environment may lead torequirements for the control of mercury emissions from coal-fired power plants. If such controls are mandated, the use of existing flue-gas cleanup systems, such as wet scrubbers currently employed for flue-gas desulfurization, would be desirable. Such scrubbers have been shown to be effective for capturing oxidized forms of mercury, but cannot capture the very insoluble elemental mercury (Hg(sup 0))that can form a significant fraction of the total emissions. At Argonne National Laboratory, we have proposed and tested a concept for enhancing removal of Hg(sup 0), as well as nitric oxide, through introduction of an oxidizing agent into the flue gas upstream of a scrubber, which readily absorbs the soluble reaction products. When a dilute solution of this agent was introduced into a gas stream containing Hg(sup 0) and other typical flue-gas species at 300 degrees F, we found that about 100% of the mercury was removed from the gas phase and recovered in process liquids. At the same time, approximately 80% of the nitric oxide was removed. The effect of sulfur dioxide on this process was also investigated and the results showed that it had only minor negative effects. Preliminary economic projections based on the results to date indicate that the chemical cost for nitric oxide oxidation could be less than $5,000/ton removed, while for Hg(sup 0) oxidation it would be less than $25,000/lb removed. Testing of the process at a pilot-plant scale is needed to further refine the concept and address a number of engineering questions. Such tests have been proposed in collaboration with an industrial partner and could start in 2001.