Process for the purification of contaminated exhaust gases from

Details Process for the purification of contaminated exhaust gases from Process for the purification of contaminated exhaust gases from

1993-05-26

1995-10-10

Langel, Wayne

Chemistry of inorganic compounds

Modifying or removing component of normally gaseous mixture

4232391, 423240S, 42324401, 42324403, 4232453, 423522, 423523, 423525, 423531, 588206, C01B 1774, B01D 5314

Patent

active

054568919

DESCRIPTION:

BRIEF SUMMARY
BACKGROUND OF THE INVENTION

1. Field of the Invention
The invention relates to a process, using regenerable adsorption materials, for purifying exhaust gases that have been contaminated with at least SO.sub.2, a heavy metal, in particular mercury, and other toxic gases, in particular dioxins and furans, and that originate from incineration plants, in particular waste-incineration plants.
2. Description of Related Art
Exhaust gases from waste incineration plants contain HCl, HF, SO.sub.x Hg and NO.sub.x, as well as highly toxic organic PCDD/PDCF compounds, known as dioxins and furans. In order to maintain clean air quality, the contaminants from the exhaust gases need to be eliminated and as far as possible reclaimed as re-usable materials.
Double or multi-stage lime-based wet-processes are known for the separation of HCl, HF and SO.sub.2, which produce mainly CaCl.sub.2 and CaSO.sub.4 as reaction products. These reaction products must be taken and stored in waste disposal sites. A further disadvantage of these processes is that they produce contaminated waste waters that must be either subjected to purification or thermally disposed of. Only after such waste waters have been purified can they be conducted into a receiving canal or into the sewage system. This whole process is expensive, and damaging to the environment, because of the storage of the reaction products in waste disposal sites. There are known designs (cf. Chem.-Ing. Tech. 60, 1988, Pages 247-255) by which the heavy metals extracted during the purification in the wet-cleaning process can in principal be reprocessed and recovered. Since these processes are very expensive, with an unfavorable cost-benefit ratio, underground disposal is regarded as economically sensible and ecologically permissible. In the recovery of heavy metals in a large power plant, 2000 tons of salts per year are produced, which must then be subjected to controlled release into the sea. As for furans and dioxins, these can be easily destroyed at moderate temperatures by recycling the gases contaminated with them back through the incinerator.
There are proposals known for using a caustic soda solution as a neutralization medium to recover NaCl as an industrially marketable product, and thus reduce the amount of disposable waste material. This procedure is expensive and would only be feasible, given the high purity requirements for NaCl used in chlorine-alkali-electrolysis, if the HCl contained in the exhaust gas had previously been thoroughly separated from all remaining contaminants, something which is not possible using the known processes for producing NaCl.
There are also dry sorption processes known, that operate dry or almost dry and are also lime-based. The resulting reaction products are disposed of, along with the flue-dust from the incineration, as waste. In this process a further problem arises, since the dioxins and furans contained in the exhaust gas are not eliminated.
In both the dry and wet processes there is the problem of separating the mercury, present in gaseous form, from the exhaust gas. In the dry sorption process this separation is impossible, and in the wet-wash process it is at best unreliable.
In DE 37 06 131 A1 a process is known for removing contaminants from the flue gas, according to which the gas is previously treated in a wet scrubber and passed through a series of adsorbent beds. This process takes advantage of the fact that contaminants accumulate in the adsorber with a specific selectivity, so that smaller molecules are expelled from the adsorbent when it reaches saturation. The gas treatment provides for the removal of such components from the exhaust gas, since otherwise they could impede a subsequent catalytic denitration. In a first adsorber layer heavy metals, in particular mercury, are adsorbed. In one or two further layers SO.sub.2 and HCl are adsorbed. Later, the contaminated adsorption material is passed through a further adsorption stage, in which excess ammonia from the denitration stage, contained in the gas, is adsorbed. The

REFERENCES:
patent: 4889698 (1989-12-01), Moller
patent: 5173286 (1992-12-01), Audeh
patent: 5294409 (1994-03-01), Cohen et al.
"Production of Sulfuric (and Nitric) Acid with a Modified Nitrogen-Oxide Sulphuric Acid Process" by V. Fattinger Proc. Brit. Sulphur Corp.; 3rd Int. Conf. London; Nov. 79 p. XXVI-9.
"Energie Spektrum", Jul. 1989, pp. 13-16.
"Von der Schadstoffquelle zur Schadstoffsenke--neue Konzepte der Mullverbrennung", Chem.-Ing. Tech., 60, (1988) Nr. 4, S., pp. 247-255. Apr. 1988.
"Sulphuric acid", Neue Zurcher Zeitung, Nr. 141, Jun. 21, 1989, p. 65.

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