Patent Number: 043839690
Section: description

DETAILED DESCRIPTION OF THE INVENTION In accordance with the invention, the radioactive carbon compounds contained in the exhaust gas are oxidized to .sup.14 CO.sub.2 and are, subsequently, removed from the exhaust gas stream by CO.sub.2 -- absorption and retention methods known per se. The equipment required for this purpose can be inserted, referring to the drawings, for instance, after the customary recombination arrangement R or also in the feed line leading to the exhaust air stack. The equipment serving for oxidation, designated with O, are in part similar to the recombination arrangement R. The latter consists customarily of palladium contact bodies which are heated to 300.degree. C. These bodies consist of solid supports or carriers, for instance of Al.sub.2 O.sub.3, and are generally of spherical or annular shape which are usually superficially coated with palladium. Such a known recombination arrangement R is suitable, in addition to the hydrogen-oxygen recombination, for oxidizing carbon monoxide to carbon dioxide, if a slight excess, usually about 2-8% above the amount stoichiometrically required, of oxygen is present. However, this is no longer possible with alkanes such as CH.sub.4, which are substantially harder to oxidize. The R arrangement acts as an oxidizing device O only if the operating temperature of the palladium contact bodies is increased to above 450.degree. C. An oxidizing device meeting these purposes may be constructed from platinum contact bodies which, with a slight excess of oxygen, have an operating temperature of about 500.degree. C. The contact bodies may also consist of copper oxide, but then, an operating temperature of at least 750.degree. C. is necessary. The oxygen excess, which is recommended here, too, ensures continuous regeneration of the CuO. It should be pointed out at this point that the recombination device R and the oxidizing device O can be provided as separate, as well as uniform, devices which can meet both purposes. This conversion of the radioactive compounds into CO.sub.2 is the first step of the method according to the invention. The second step is to remove this CO.sub.2 from the main gas stream. For this purpose, various techniques known per se are available. Thus, the CO.sub.2 -containing gas can be conducted through precipitation apparatus C which is filled with sodium or potassium hydroxide solution as well as with a small amount of barium chloride, and is connected to a filtering station. In the process, the CO.sub.2 is precipitated as solid Ba.sup.14 CO.sub.3 and separated in the filtering station. It is also possible to introduce the exhaust gas directly into a Ba(OH).sub.2 -solution, from which barium carbonate BaCO.sub.3 precipitate can be separated. Instead of liquid reagents, soda lime which represents a technical product of NaOH+CaO, sodium asbestos or similar technical products with comparable "alkaline" absorption effect for CO.sub.2 can be used if the exhaust gas still has a small moisture content. Ensuring the latter is no problem since so-called water ring pumps are generally used for moving the exhaust gas. These use water as the sealing liquid, so that thereby humidification of the transported exhaust gas takes place automatically. The kind of absorbents, liquid or solid, used, depends on the design and the pressure relationships of the respective purification loops and also on the physical capability of the plant to install this apparatus. The precipitation apparatus and filter C are well known from chemical engineering, so that no further explanations are necessary on this point. The last step for freeing the exhaust gas of radioactive carbon is then merely to remove the filter residues or the precipitates from the precipitation apparatus C, to dry them and, as shown, for instance, in FIG. 1 by the dash-dotted line, to feed them to the final concentration plant and filling into barrels, which are then stored in an ultimate storage facility, without danger to the environment. It should further be pointed out that only extremely small amounts of carbon contents to be separated in the exhaust gas are involved. An improvement of the degree of separation can therefore be achieved by first admixing an additional carrier gas which contains carbon but is not radioactive. As suitable for this purpose has been found methane gas in an amount of about 0.1% by volume of the exhaust gas. The extra amount of oxygen required is insignificant and can be taken into account in the oxygen dosing in the recombination device R. To evaluate the practical importance of this method, it should be mentioned that the liberation of .sup.14 C amounts to about 10 to 15 Ci/year per 1300-MW power plant and can be reduced to almost zero by the method proposed here.