Reducing Mercury Pollution
THE MAGAZINE DEVOTED TO NICKEL AND ITS APPLICATIONS
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OUR COVER STORY on 'cleaner' electricity in the November 2004 issue did not address the important issue
of mercury pollution for coal-fired electric generating plants. W. L. Mathay address this issue briefly at
right.
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Nickel Magazine, March 2005 -- Letter to the Editor
RE: ‘Cleaner’ Electricity, Nickel Magazine, November 2004.
Is there a role for nickel in the equipment which will be required to remove mercury from the effluent of
coal-burning power plants?
William L. Larsen, PE
Iowa, U.S.A.
The answer is yes, depending on conditions, but some background information is necessary for a full understanding of the problems encountered with mercury removal.
When coal is burned, it produces particulate mercury, elemental mercury and oxidized mercury. Particulate mercury is trapped with the flyash and removed in a collector such as an electrostatic precipitator. Elemental mercury can only be removed by some type of sorbent unless it is first oxidized. Oxidized mercury can be removed by wet scrubbing.
There is still much to be learned about mercury removal, but wet scrubbers which are used to remove most of the sulphur dioxide from the flue gas also will remove most of the oxidized mercury from the flue gases. Too, selective catalytic reduction units which are used to reduce nitrogen oxide emissions will also oxidize much of the elemental mercury to which they are exposed thus increasing the mercury capture in the scrubbers to the required level. Most of the wet scrubbers that are installed or are being installed are constructed of nickel-containing alloys to resist the corrosive attack from the medium-to-high sulphur coals being burned.
About one-third of the coal being burned today in the U.S. is low-sulphur compliance quality coal which for the most part does not require wet scrubbers. Also, these coals emit mostly elemental mercury which means that a significant amount of mercury cannot be removed by existing equipment.
Legislation to reduce mercury emissions is still being worked out but it appears that reductions of up to 90% will be required by 2007 to 2009. Meanwhile, plans reportedly are underway for the installation of over 100 new wet scrubber installations within 10 years. It will be necessary for mercury control as well as other pollution control devices to be included in these plans.
W. L. Mathay, a consultant to the Nickel Institute.
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