Chemistry of inorganic compounds – Modifying or removing component of normally gaseous mixture – Halogenous component
Reexamination Certificate
2001-05-09
2003-07-08
Silverman, Stanley (Department: 1754)
Chemistry of inorganic compounds
Modifying or removing component of normally gaseous mixture
Halogenous component
Reexamination Certificate
active
06589495
ABSTRACT:
BACKGROUND OF THE INVENTION
Field of the Invention
The invention relates to a catalyst body for reducing the levels of halogenated hydrocarbons, in particular of dioxins and/or furans. The invention also relates to a process for reducing the levels of halogenated hydrocarbons, in particular of dioxins and/or furans, in which a gas stream which contains the halogenated hydrocarbons is passed over the catalyst body.
While many such halogenated compounds contain oxygen as well as hydrogen and carbon, the collective term “halogenated hydrocarbon” has become conventional for these in technical, toxicological and regulatory literature and is so used in the present specification and claims.
The off-gas from an industrial plant, such as for example a power plant which is operated with a fossil fuel, a refuse incineration plant or a sintering plant in which metal ore is made accessible to smelting by being sintered, contains not inconsiderable levels of halogenated hydrocarbons. Examples of halogenated hydrocarbons are chlorobenzenes, chlorophenols, polychlorinated biphenyls, and also polyhalogenated cyclic ethers (furans) and cyclic diethers (the actual dioxins). Many of these halogenated hydrocarbons, in particular the polychlorinated dibenzodioxins (PCDDs) and the polychlorinated dibenzofurans (PCDFs) are highly toxic to humans. For this reason, the permissible emission levels from an industrial plant for halogenated hydrocarbons of this type are subject to strict statutory guidelines.
To enable the statutory guidelines to be fulfilled, catalyst bodies are used to reduce the levels of halogenated hydrocarbons in the off-gas from the abovementioned industrial plants. For example, the Siemens company brochure “Diox-Katalysatoren zerstören Dioxine in den Rauchgasen von Müllverbrennungsanlagen” [Diox catalysts destroy dioxins in the flue gases from refuse incineration plants], order no. A96001-U11-A293, 1995, has disclosed a catalyst body through which off-gas can flow and which has an active material which is based on titanium dioxide and, in a temperature range of the off-gas to be treated of between 200 and 400° C., breaks down dioxins contained in the off-gas to form carbon dioxide, water and hydrogen halides.
In many cases, however, the temperature of the off-gas to be treated lies below 250° C., and consequently the catalytic activity of the abovementioned catalyst body becomes too low to still achieve effective reduction of the levels of halogenated hydrocarbons. In particular, low temperatures of this nature occur in the off-gas from a refuse incineration plant or a sintering plant. It is also known that at temperatures which lie below 200° C., the so-called DeNovo synthesis in the presence of organic carbon compounds, alkali metal or alkaline-earth metal chlorides and metal compounds which act as catalysts leads to dioxins and/or furans being formed again in the off-gas during cooling. In this case too, treatment of the off-gas in a temperature range in which the catalytic activity of the abovementioned catalyst body is no longer sufficient is required.
Therefore, other measures have to be taken in order to treat low-temperature off-gases. For this purpose, it is known from W. Weiss: “Minderung der PCDD/PCDF-Emissionen an einer Eisenerz-Sinteranlage” [Reducing the levels of PCDD/PCDF emissions from an iron ore sintering plant], VDI-Berichte No. 1298, (1996), pages 249 ff, to add a mixture of calcium hydroxide Ca(OH
2
) and carbon in the form of half-furnaced coke or activated carbon as an additive to the off-gas from a sintering plant after it has flowed through an electrostatic filter, to remove the additive which is partially laden with dioxins and/or furans again by means of dust filtering and to recycle this additive once again. In this case, the carbon serves as an adsorbing agent for the dioxins/furans, while the calcium hydroxide reduces the risk of spontaneous ignition of the carbon.
Furthermore, it is known from G. Mayer-Schwinning et al.: “Minderungstechniken zur Abgasreinigung für PCDD/F” [Techiques for reducing the levels of PCDD/F for off-gas cleaning], VDI-Berichte No. 1298, (1996), pages 191 ff, to add zeolites, as an additive which adsorbs dioxins/furans, to the off-gas from a refuse incineration plant after conventional off-gas cleaning with the aid of a fluidized-bed reactor. The zeolites which are partially laden with dioxins/furans and are situated in the off-gas are once again removed by means of dust filtering and fed back to the fluidized-bed reactor. From this process too it is known to add the zeolites to the off-gas together with calcium hydroxide.
It is a drawback that the adsorption capacity of the additive becomes exhausted over time, so that the consumed additive which is laden with dioxins/furans has to be landfilled and/or made non-hazardous by a thermal treatment or by being incorporated in a slag. Also, both the zeolites and the calcium hydroxide which is imperative when using carbon as additive are relatively expensive, so that observing emission limits for halogenated hydrocarbons in an industrial plant in this way entails high costs.
Furthermore, it is proposed in German published patent application DE 195 04 597 A1 to feed used DeNOx catalysts in powder form to the off-gas from an industrial plant in order to reduce the levels of the halogenated hydrocarbons contained therein. The DeNOx catalysts which are laden with the halogenated hydrocarbons are once again removed from the off-gas by means of dust filtering and fed back to the off-gas. In this process, both the adsorption capacity of the DeNOx catalyst and their activity with regard to oxidation of the halogenated hydrocarbons become fully exhausted. As base material, the specified DeNOx catalysts comprise titanium dioxide, and have additions of tungsten trioxide, vanadium pentoxide and molybdenum trioxide.
However, a drawback is the fact that the oxidation activity of the used DeNOx catalysts for breaking down the halogenated hydrocarbons is relatively low, so that a high mean contact time of the off-gas with the used DeNOx catalysts of several days is required. This is achieved by frequent recirculation of the DeNOx catalysts which have been added to the off-gas. Therefore, when using the used DeNOx catalysts as an additive, a complex recirculation process is required in order to treat the off-gas from an industrial plant.
WO 91/17828 has disclosed a catalyst body based on an ion-exchanged zeolite for breaking down halogenated hydrocarbons in a temperature range between 150 and 450° C. As metal ions, the ion-exchanged zeolite contains vanadium, chromium, cobalt, nickel, copper, iron, molybdenum or manganese and is additionally impregnated with titanium, vanadium, chromium, manganese, iron, cobalt, nickel, copper, molybdenum, palladium or platinum. The catalyst body is used in the form of pellets and oxidizes the halogenated hydrocarbons in the presence of oxygen, to form carbon dioxide and halo acids. A drawback is that a catalyst body of this type based on an ion-exchanged zeolite is relatively expensive. However, the discussed problem of complex recirculation when using an adsorbed additive is eliminated.
SUMMARY OF THE INVENTION
It is accordingly an object of the invention to provide a catalyst body and process for reducing the levels of halogenated hydrocarbons that overcomes the above-mentioned disadvantages of the prior art methods and devices of this general type, which is relatively inexpensive, is easy to use and is suitable for effective reduction of the levels of halogenated hydrocarbons even at temperatures below 250° C. A further object of the invention is to provide a simple and inexpensive process for reducing the levels of halogenated hydrocarbons from the off-gas from an industrial plant. Also, this process is to be suitable in particular for reducing the levels of halogenated hydrocarbons in a low-temperature off-gas, i.e. at a temperature below 250° C.
With the foregoing and other objects in view there is provided, in accordance with t
Fischer Stefan
Pajonk Günther
Witzel Frank
Greenberg Laurence A.
Johnson Edward M.
Mayback Gregory L.
Siemens Aktiengesellschaft
Silverman Stanley
LandOfFree
Process for reducing the levels of halogenated hydrocarbons does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Process for reducing the levels of halogenated hydrocarbons, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Process for reducing the levels of halogenated hydrocarbons will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-3022771