Chemistry of inorganic compounds – Modifying or removing component of normally gaseous mixture
Reexamination Certificate
1999-03-31
2001-12-11
Griffin, Steven P. (Department: 1754)
Chemistry of inorganic compounds
Modifying or removing component of normally gaseous mixture
Reexamination Certificate
active
06328939
ABSTRACT:
FIELD AND BACKGROUND OF THE INVENTION
The present invention relates generally to the field of flue gas cleanup methods and apparatus and, in particular, to a method for removing mercury from the flue gas generated during the combustion of fossil fuels or solid wastes, through the use of a chelating agent.
In recent years, the U.S. Department of Energy (DOE) and the U.S. Environmental Protection Agency (EPA) have supported research to measure and control the emissions of Hazardous Air Pollutants (HAPs) from coal-fired utility boilers. The initial results of several research projects showed that the emissions of heavy metals and volatile organic carbons (VOCs) are very low, except for mercury (Hg). Unlike most of the other metals, most of the mercury remains in the vapor phase and does not condense onto fly ash particles at temperatures typically used in electrostatic precipitators and fabric filters. Therefore, it cannot be collected and disposed of along with fly ash like the other metals. To complicate matters, mercury can exist in its oxidized (Hg
+2
) or elemental (Hg
0
) form and each is affected differently by subsequent downstream pollution control equipment. In a conventional wet scrubber Hg
+2
is relatively easy to capture while capturing Hg
0
is difficult. The relative amount of each species appears to depend on several factors such as fuel type, boiler combustion efficiency, the type of particulate collector installed, and several other factors. As for the type of particulate collector installed, it has been shown that an electrostatic precipitator (ESP), as is used in the majority of utility applications, affects the process chemistry so that Hg
+2
is converted to Hg
0
within a downstream wet scrubber, also commonly used in utility applications to reduce SO
2
emissions. The Hg
0
is then emitted with the flue gas.
Most of the recent efforts to capture and remove mercury from flue gas have concentrated on gas-phase reactions with introduced reagents such as activated carbon.
The subject of mercury emissions by the utility industry is a new area being investigated by both the DOE and EPA.
SUMMARY OF THE INVENTION
The present invention is a method to adjust wet scrubber chemistry to prevent the reduction of Hg
+2
to Hg
0
and thereby increase the mercury removal efficiency of wet scrubber systems. The invention increases the mercury removal efficiency of conventional wet scrubber systems, especially those preceded by an ESP.
Accordingly, one aspect of the present invention is to provide, in an industrial process using a wet scrubber for receiving an industrial gas containing mercury, a method for reducing the mercury content in the industrial gas exiting from the wet scrubber, comprising: adding a chelating agent to the industrial gas; and scrubbing the industrial gas in the wet scrubber with the chelating agent.
Advantageously, the chelating agent comprises at least one of ethylenediaminetetraacetic acid (EDTA), hydroxyethylenediaminetetraacetic acid (HEDTA), diethylenetriaminepentaacetic acid (DTPA or pentetic acid), and nitrilotriacetic acid (NTA). Pilot-scale testing employed EDTA, and an amount of about twice the stoichiometric ratio of chelating agent to the transition metals (presumed to be iron, Fe) was shown to be effective.
Another aspect of the present invention is drawn to a method for reducing elemental mercury concentration in industrial gases exiting from a wet scrubber which scrubs the industrial gases with a slurry, the industrial gases containing mercury in oxidized (Hg
+2
) and elemental (Hg
0
) forms, the wet scrubber containing at least one transition metal which converts the Hg
+2
into the Hg
0
form, comprising the steps of: supplying a chelating agent in the slurry an amount sufficient to reduce the degree to which the at least one transition metal converts the Hg
+2
into the Hg
0
form; and scrubbing the industrial gases with the slurry containing the chelating agent.
In certain aspects of the present invention, the method may comprise determining the amount of the at least one transition metal in the wet scrubber slurry and supplying the chelating agent into the slurry in an amount sufficient to reduce the degree to which the at least one transition metal converts the Hg
+2
into the Hg
0
form based upon such determination.
In other aspects of the present invention, the method may comprise determining the concentration of oxidized (Hg
+2
) and elemental (Hg
0
) forms of mercury in the industrial gases entering and exiting from the wet scrubber and supplying the chelating agent into the slurry in an amount sufficient to reduce the degree to which the at least one transition metal converts the Hg
+2
into the Hg
0
form based upon such determination.
Yet still another aspect of the present invention is drawn to a method of operating a wet scrubber to reduce gaseous emissions of oxidized (Hg
+2
) and elemental (Hg
0
) mercury in industrial gases exiting from the wet scrubber, comprising: scrubbing the industrial gases within the wet scrubber with a slurry containing an amount of chelating agent sufficient to reduce the degree to which the at least one transition metal in the wet scrubber slurry converts the Hg
+2
into the Hg
0
form.
A still further aspect of the present invention is to improve removal of mercury from flue gas in a process which burns pulverized coal.
The various features of novelty which characterize the invention are pointed out with particularity in the claims annexed to and forming a part of this disclosure. For a better understanding of the invention, its operating advantages and specific benefits attained by its uses, reference is made to the accompanying drawings and descriptive matter in which a preferred embodiment of the invention is illustrated.
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J. Peterson et al., “Mercury Removal by Wet Limestone FGD Systems: EPRI HSTC Test Results,” 94-RP114B.01, presented at the 87th Annual Meeting & Exhibition of Air & Waste Managegment Association, Cincinnati, Ohio, Jun. 19-24, 1994, pp. 1-16.
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Baraona Robert C.
Griffin Steven P.
Marich Eric
McDermott Technology Inc.
Vanoy Timothy C
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