Chemistry of inorganic compounds – Modifying or removing component of normally gaseous mixture
Patent
1998-09-09
2000-07-18
Straub, Gary P.
Chemistry of inorganic compounds
Modifying or removing component of normally gaseous mixture
423240R, 423240S, 42324308, 42324407, 4232451, 423555, 422168, 422187, 422234, B01D 4700
Patent
active
060903554
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND OF THE INVENTION
The invention relates to a method and a device for cleaning flue gases of furnaces, whereby the flue gases are subjected to a wet scrubbing process with addition of activated carbon.
Such a method is known from German patent application 41 23 258. Under the conditions of wet scrubbing, the activated carbon particles adsorb the heavy metals and also organic contaminates, such as dioxins and furanes, contained in the flue gases. The removal of heavy metals, especially also of mercury, and of the organic components thus is performed during the desulfurization step without additional cleaning steps being required. The expenditure with regard to the facility and operational measures is thus comparatively minimal.
Furthermore, a relatively small amount of activated carbon particles can be employed. However, this still requires a considerable expenditure because activated carbon is a relatively expensive material.
It is an object of the invention to reduce the expenditures by minimizing the required amount of activated carbon particles.
SUMMARY OF THE INVENTION
As a solution to this object the method according to the invention is characterized in that the activated carbon particles are mixed with particles of slaked lime and/or limestone and that the particle mixture is introduced into the flue gases upstream of the wet scrubbing step. Surprisingly, it was found that this relatively simple measure results in a reduction of the activated carbon consumption by more than 25%. The invention is based on the recognition that the acidic components of the flue gases such as SO.sub.2, SO.sub.3, HCl and HF during the wet scrubbing process have the tendency to be adsorbed at the activated carbon particles. This tendency is counteracted by the invention. Ca(OH).sub.2 or CaCO.sub.3 reacts already in the gas phase with the acidic components of the flue gases so that the activated carbon particles during the wet scrubbing process are available fully for their actual function, that is, for the adsorption of primarily Hg.sup.0 and Hg.sup.2+ as well as dioxins and furanes. The additional material expenditure is comparatively minimal because slaked lime or limestone is a comparatively inexpensive material. Also, the additional facility expenditure is limited. Thus, a considerable cost advantage remains.
Inasmuch as the wet scrubbing step has upstream thereof a flue gas heat exchanger, the particle mixture is introduced advantageously upstream of the flue gas heat exchanger into the flue gases. In the flue gas heat exchanger the temperature falls below the dew point of sulfuric acid, so that H.sub.2 SO.sub.4 will condense and can react to a greater extent with the particles of slaked lime or limestone. When a regenerative heat exchanger is used, the passage of contaminates to the clean gas side is prevented.
An especially favorable method results when the activated carbon particles are separated directly from the suspension resulting from the wet scrubbing process, preferably by flotation, whereby it is advantageous to subject the suspension before this step to a partial dewatering process.
Moreover, in a further development of the invention it is suggested, to recirculate at least a portion of the activated carbon particles into the wet scrubbing process. This method step is based on the recognition that the activated carbon particles, even though used in minimal amounts, are not yet spent after a single passage through the wet scrubbing process and still retain an activatable adsorption potential.
It is advantageous in this context to subject a portion of the recirculated activated carbon particles, before introduction into the wet scrubbing process, to a thermal desorption process. In this manner, it is possible to guide the activated carbon particles in circulation.
The thermal desorption of the activated carbon particles results in a drying of the material so that, advantageously, it is possible to recirculate the desorbed activated carbon particles to be mixed with the particles of slaked lime o
REFERENCES:
patent: 5306475 (1994-04-01), Fichtel et al.
patent: 5352647 (1994-10-01), Suchenwirth
patent: 5569436 (1996-10-01), Lerner
patent: 5618508 (1997-04-01), Suchenwirth et al.
patent: 5811066 (1998-09-01), Winkler et al.
Neumann Marion
Winkler Hermann
Steag Aktiengesellschaft
Straub Gary P.
Vanoy Timothy C
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