Chemistry of inorganic compounds – Modifying or removing component of normally gaseous mixture – Organic component
Patent
1993-10-26
1996-10-22
Straub, Gary P.
Chemistry of inorganic compounds
Modifying or removing component of normally gaseous mixture
Organic component
588206, B01J 800, C01B 1124
Patent
active
055674020
DESCRIPTION:
BRIEF SUMMARY
FIELD OF THE INVENTION
The present invention relates to methods of wet cleaning gases containing condensable pollutants, in particular organic pollutants.
HISTORY OF THE RELATED ART
For example in the case of household garbage incineration fumes, on the most modern incinerations after recovery of energy, i.e. at a temperature of about 180.degree. to 250.degree. C., gaseous organic pollutant contents, most often lying in the range 1 to 10 mg/m.sup.3, may be measured (measurement of the total organic carbon by the flame ionization method). Among these organic pollutants, certain compounds form the subject matter of particular investigations due to their inherent toxicity: this is the case for polychlorodibenzo-p-dioxines (PCDD) and polychlorodibenzofuranes (PCDF) for which it is necessary to attain very low contents.
In order to collect these pollutants, several techniques have been proposed. The first technique consists of injecting in the gases an active carbon powder, ensuring a sufficient contact time between the powder and the gases, and then separating this active carbon powder like fly ash, in an electro-filter or in a sleeve filter. This method may be usefully combined with a semi-dry cleaning or a dry cleaning; in the latter case, the active carbon powder may be mixed with slaked lime powder before injection in the gases. This first technique is, however, limited in its application by several factors; the temperature of the gases must remain fairly high to avoid clogging and corrosion by condensation, which limits the possibilities of absorption; the presence of other pollutants consumes the absorption potential of the active carbon powder; and, finally, the mixture of the active carbon powder with the fly ash limits the maximum quantity of active carbon powder which may be injected.
In order to avoid the drawbacks and limitations of this first technique, a second technique has been proposed. It consists of cleaning the gases firstly down to very low values for the principal particulate and gaseous pollutants then in passing the gases to a filter comprising several layers of active carbon in granular form. This second technique involves the production of large complementary installations and it, too, is limited in its application by several factors; the temperature of the gases must not be too high due to the risks of inflammation of the large quantity of active carbon employed; it is necessary to renew the active carbon fairly often by reason of the presence of other pollutants which are absorbed and to limit the risks of inflammation; and, finally, the active carbon must, after use, be treated due to its composition.
In order to avoid these drawbacks and limitations, a third technique has been proposed. It consists of cleaning the gases firstly down to very low values for the principal particulate and gaseous pollutants except for the nitrogen oxides then in passing the gases to a filter constituted by catalysts identical to those used in the known processes of selective catalytic reduction of nitrogen oxides; An injection of NH.sub.3 gas is performed for the selective reduction of the nitrogen oxides, but the quantity of catalysts is increased to allow the absorption and destruction of the dioxines and furanes: A variant consists of adding catalysts specifically designed for the destruction of the dioxines and furanes. This third technique involves the production of large complementary installations and it, too, is limited in its application by several factors: the temperature of the gases must remain fairly high to avoid poisoning of the catalysts by condensation of salt remaining gaseous if the temperature is fairly high; the prior cleaning of the gases must be thorough in order to avoid poisoning of the catalysts by specific elements, arsenic for example; finally, although this third technique has the advantage of destroying the dioxines and furanes, there is a risk of desorption before destruction under the effect of such or such upstream factor which is more or less well controlled.
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English Language Abstract of JP-A-55 022320.
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Vicard Gilles
Vicard Jean-Fran.cedilla.ois
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