Method of removing mercury from a mercury-containing flue gas

Details Method of removing mercury from a mercury-containing flue gas Method of removing mercury from a mercury-containing flue gas

1999-12-15

2001-04-10

Dunn, Tom (Department: 1754)

Chemistry of inorganic compounds

Modifying or removing component of normally gaseous mixture

C423S107000, C423S531000, C423S561100, C423S588000, C423S231000, C423S236000, C423S095000, C423S134000

Reexamination Certificate

active

06214304

ABSTRACT:

BACKGROUND OF THE INVENTION
The invention relates to a method of removing mercury from a mercury-containing flue gas that may also contain dust, further heavy metals and further gaseous noxious gas components, especially flue gas from a refuse incinerator; the mercury, using an alkali sulfide solution, especially sodium sulfide solution, and in particular sodium tetrasulfide solution, is converted to mercury sulfide and the mercury sulfide is precipitated out via a dust separator.
Such a method is known from DE 44 37 781 A1, where an aqueous sodium tetrasulfide solution is conveyed to the flue gas immediately after a partial removal of dust from the flue gas in an electrostatic precipitator, i.e. at a relatively high temperature, and thereafter separating off in a fabric filter the mercury sulfide that is formed by precipitation of ionic mercury Hg
2++
and Hg
+
and from metallic mercury Hg
0
. A scrubbing out of aqueous, gaseous noxious components such as HCl and SO
2
is subsequently effected. With the known method, the sodium tetrasulfide solution is sprayed in with the aid of cold atomizing air at room temperature. Relatively large quantities of sodium tetrasulfide were needed for the separation of the mercury. A relatively long contact stretch is required since the sodium tetrasulfide is activated only in the hot flue gas.
It is the object of the present invention to provide a method of the aforementioned type where the mercury separation is improved by means of the introduced alkali sulfide solution, especially sodium sulfide solution.
SUMMARY OF THE INVENTION
This object is realized in that the alkali sulfide solution is introduced into the flue gas accompanied by the simultaneous introduction of heat.
Examples of alkali sulfide solutions can preferably be sodium sulfide solutions and potassium sulfide solutions.
In the specification and the claims, “sodium sulfide solution” means a solution that contains sodium monosulfide and/or sodium polysulfide. A preferred “sodium tetrasulfide solution” that is to be used refers to a solution that essentially contains sodium tetrasulfide, but for technical reasons also contains portions of other polysulfides and possibly monosulfide.
It was surprisingly discovered that by adding heat during the introduction, the separation efficiency of the alkali sulfide, especially the sodium sulfide, in other words the sodium tetrasulfide, is very effective with respect to ionic mercury, and this even at lower flue gas temperatures than exist relative to the known method at the output of the electrostatic precipitator. By using sodium tetrasulfide, the effectiveness is also improved relative to metallic mercury. In addition, with flue gases having high temperatures the contact stretch required for the separation can also be shortened.
The alkali sulfide solution is preferably introduced by means of an auxiliary fluid that is heated prior to being joined with the alkali sulfide solution.
One can proceed on the basis that due to the temperature increase, in the alkali sulfide solution for a relatively short period of time there is accelerated a conversion of the alkali sulfide molecules to substances that absorb mercury, for example in the case of sodium sulfide to the reaction components Na
2
S, H
2
S and S. The temperature increase is preferably in the range of 40-200° C., and preferably about 80-200° C., but could also be in the range of 100-200° C., and further preferably 100-120° C.
The alkali sulfide solution is preferably introduced into the flue gas together with hot steam or vapor as an auxiliary fluid medium, for example via the use of a vapor jet pump or ejector for the introduction of the alkali sulfide solution.
However, it is also conceivable to introduce the alkali sulfide solution into the flue gas together with hot air and/or hot flue gas.
The temperature of the flue gas during introduction of the alkali sulfide solution should be below 300° C., expediently below 250° C., and further preferably below 150° C.
It was furthermore discovered that the effectiveness was improved if the alkali sulfide solution was introduced into a flue gas having increased moisture content at a temperature below 150° C., and furthermore preferably was introduced into a flue gas that was saturated with moisture at a temperature of below 100° C.; if necessary, in the event that the water vapor content of the flue gas is insufficient, the required moisture content is achieved by the additional supply of water.
At low flue gas temperatures, it is expedient to utilize an auxiliary medium having a higher temperature that also acts as a propellant gas. The use of steam as an auxiliary medium is preferred since it has a higher thermal content than do air and flue gas. Furthermore, it was discovered that when steam was used, the formation of the reaction components was improved to a greater extent relative to the use of air as an auxiliary medium. The separation or precipitation of mercury utilizing steam as the auxiliary fluid medium can also be achieved without acidic noxious gas components having to be present in the flue gas.
Therefore, acidic noxious gas components that might be present can be removed from the flue gas, preferably by washing, prior to or after the addition of the alkali sulfide solution and subsequent dust separation. If this washing takes place prior to the addition of the alkali sulfide solution, the preferred moisture saturation is achieved immediately.
For the procedure it is furthermore preferred that prior to the introduction or together with the alkali sulfide solution an oxidizing agent, preferably H
2
O
2
, and/or a sorption agent, preferably activated coke, is added.
If a flue gas having no acidic noxious gas components is provided, it can be expedient to add an acidic gas such as HCl in order to further improve the precipitation.
It is of course also expedient to remove dust from the flue gas prior to the introduction of the alkali sulfide solution. The use of a sodium sulfide solution is preferred, especially the use of a sodium tetrasulfide solution.


REFERENCES:
patent: 5034203 (1991-07-01), Audeh et al.
patent: 5607496 (1997-03-01), Brooks
patent: 44 37 781 A1 (1996-05-01), None
patent: 0 506 132 A1 (1992-09-01), None
patent: 0 709 128 A2 (1996-05-01), None
patent: 6-91129 (1994-05-01), None
patent: WO 89/08493 (1989-09-01), None
patent: WO 94/22563 (1994-10-01), None
Nebel et al; Mercury Control Systems. . . To Go; 1992; pp. 40-49.

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