Chemical apparatus and process disinfecting – deodorizing – preser – Chemical reactor – Waste gas purifier
Reexamination Certificate
1999-03-31
2001-09-04
Griffin, Steven P. (Department: 1754)
Chemical apparatus and process disinfecting, deodorizing, preser
Chemical reactor
Waste gas purifier
C422S172000, C422S186220, C422S186220, C422S234000
Reexamination Certificate
active
06284199
ABSTRACT:
FIELD AND BACKGROUND OF THE INVENTION
The present invention relates generally to the field of combustion and flue gas cleanup methods and apparatus and, in particular, to a new and useful method and apparatus for removing mercury from the flue gases generated during the combustion of fossil fuels such as coal, or solid wastes, through the use of hydrogen sulfide.
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 and waste to energy plants. 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
) form, principally as mercuric chloride, (HgCl
2
), or in its elemental (Hg
0
) form as vaporous metallic mercury. 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 various other factors.
The search for industrially acceptable methods for the capture of mercury from industrial flue gases has included a significant effort to determine how much mercury can be removed by existing, conventional air pollution control equipment. One device used in air pollution control is the wet scrubber, which is designed for the capture of sulfur oxides and other acid gases. Tests have been performed on several commercial scale and pilot scale wet scrubbers. These tests have produced some expected and some surprising results. It was generally expected that the oxidized mercury would be easily captured and the elemental mercury would be difficult to capture. These expectations were based on the high solubility of mercuric chloride in water and the very low solubility of elemental mercury in water. This expectation was generally fulfilled.
The surprising result concerned elemental mercury. Repeated tests during which the concentration of elemental mercury in the flue gas was measured revealed that more elemental mercury was leaving the wet scrubber than was entering.
One postulate proposed to explain the cause of the elemental mercury generation in the wet scrubber is described for example, by the following general reactions:
M
e
0
+Hg
+2
→M
e
+2
+Hg
0
2M
e
+
Hg
+2
2M
e
+2
+Hg
0
M
e
is any number of transition metals such as Fe, Mn, Co, Sn, . . .
Transition metal ions are generally present in wet scrubber slurries as impurities in the industrial applications of concern. Thus, as the mercuric chloride is absorbed, a portion reacts with and becomes reduced by trace levels of transition metals and metal ions and because of its low solubility the elemental mercury is stripped from the liquid and returned to the flue gas.
Most of the recent efforts to capture and remove mercury from the flue gas produced by coal-fired units have concentrated on gas-phase reactions with introduced reagents such as activated carbon.
The subject of mercury emissions by the utility and waste to energy industries is a new area being investigated by both the DOE and EPA.
SUMMARY OF THE INVENTION
The present invention provides a means in the wet scrubber to rapidly precipitate the oxidized mercury at the gas/liquid interface in the wet scrubber before it can be reduced by the transition metals. One of the most insoluble forms of mercury is mercuric sulfide, which in mineral form is cinnabar. One means for supplying a source of sulfide for the oxidized mercury to react with is hydrogen sulfide. Thus, at the gas/liquid interface in the scrubber, the following reaction is proposed for the absorption and precipitation of ionized (oxidized) mercury:
H
2
S(g)+HgCl
2
(g)→HgS(s)+2HCl(aq)
HgS has a solubility product of 3×10
−52
and therefore precipitates essentially completely. There is good reason to expect that the precipitation reaction proceeds faster than the reduction reactions. Specifically, in the case of the precipitation reaction, both reactants are well mixed in the gas phase. Thus, as they diffuse from the gas to the gas/liquid interface both reactants can react instantly at that interface. By contrast, the reduction reactions require that the reactants, i.e., the Hg
+2
and the transition metal ion, diff-use in the liquid phase to a reaction plane in the liquid. Liquid phase diffusion is orders of magnitude slower than gas phase diffusion. Therefore, the oxidized mercury will rapidly precipitate as cinnabar in the scrubber and thereby prevent the reduction of that mercury back to vaporous elemental mercury. The precipitation of mercury as cinnabar has a distinct advantage over other mercury sequestering methods in that it converts mercury to a very insoluble form. In this way, the mercury should be inert and effectively removed from the food chain.
Accordingly, one aspect of the present invention is drawn to an improvement in a method using a wet scrubber for receiving and scrubbing an industrial gas containing mercury with a wet scrubber slurry, the improvement comprising: adding hydrogen sulfide to the industrial gas; and scrubbing the industrial gas in the wet scrubber. The method according to the present invention is particularly suited to the task of reducing mercury emissions in an industrial process which burns coal in a furnace to produce an exhaust flue gas, including conveying the exhaust flue gas through a dust collector and adding hydrogen sulfide to the flue gas before it enters the wet scrubber, or within the wet scrubber.
Another aspect of the present invention is drawn to an apparatus using a wet scrubber for receiving and scrubbing an industrial gas containing mercury with a wet scrubber slurry, and particularly the improvement comprising: means for generating hydrogen sulfide; and means for supplying the hydrogen sulfide to the industrial gas upstream of the wet scrubber. The present invention is again particularly suited to utility installations which burn fossil fuels such as coal, or solid wastes, and which use, in addition to the wet scrubber, an electrostatic precipitator or a fabric filter and other conventional components for reducing emissions to the atmosphere.
Particularly, the present invention contemplates provision of a hydrogen sulfide generating system which produces the hydrogen sulfide from a reaction of adding an acid to a solution of aqueous sodium and/or potassium sulfide to generate the hydrogen sulfide. Advantageously, the hydrogen sulfide generating system may use equipment and methods wherein the acid is added to green liquor from the Kraft pulping process to generate the hydrogen sulfide.
The system has an inherent safety advantage in that no gas phase H
2
S is accumulated or stored. All H
2
S that is generated is immediately injected.
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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patent: 3331732 (1967-07-01), Venemark
patent: 3855387 (1974-12-01), Brockmiller et al.
patent: 3892837 (1975-07-01), Uchiyama et al.
patent: 3981972 (1976-09-01), Hishinuma et al.
patent: 4094879 (1978-06-01), Bates et al.
patent: 4098697 (1978-07-01), DeAngelis et al.
patent: 4190709 (1980-02-01), Hodgkin
patent: 4230183 (1980-10-01), Kalfoglou
patent: 4273
Bailey Ralph T.
Downs William
Baraona R. C.
Griffin Steven P.
Marich Eric
McDermott Technology Inc.
Vanoy Timothy C
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