Arrangement for Reducing Nitrogen Oxides in Exhaust Gases

Abstract

An arrangement and method for reducing the nitrogen oxide content in the exhaust gas of an internal combustion engine with the aid of ammonia and / or ammonia-releasing reduction agents, whereby ammonia and / or ammonia-containing reduction agent is added to the exhaust gas stream upstream of a catalyst combination composed of an SCR catalyst and a subsequent NH3-oxidation catalyst in such a way that a homogeneous mixture of exhaust gas and ammonia is present upstream of the SCR catalyst. To optimize the reaction or conversion of nitrogen oxides, disposed downstream of the combination of a first SCR catalyst and a first NH3-oxidation catalyst is at least one second catalyst having SCR activity in order in this way to reduce the nitrogen oxides formed at the first NH3-oxidation catalyst due to insufficient selectivity of the catalyst to nitrogen with not yet oxidized NH3.

Description

[0001]The instant application should be granted the priority date of Jul. 8, 2006 the filing date of the corresponding German patent application 10 2006 031 659.2.BACKGROUND OF THE INVENTION[0002]The present invention relates to an arrangement and a method for reducing the nitrogen oxide content in the exhaust gas of an internal combustion engine with the aid of ammonia and / or ammonia-releasing reduction agents.[0003]Nitrogen oxides belong to the limited exhaust gas components that are produced during combustion processes and the permissible emissions of which are continuously being lowered. In this connection, the reduction of the nitrogen oxides generally occurs with the aid of catalysts. In oxygen-rich exhaust gas, a reduction agent is additionally necessary in order to raise the selectivity and the NOx conversions. These methods have become known under the general term SCR processes, whereby SCR stands for “Selective Catalytic Reduction”. They have been used for many years in th...

AI Technical Summary

Benefits of technology

[0017]A further advantageous improvement of the system is to provide a second NH3-oxidation catalyst downstream of the second SCR catalyst in order to similarly oxidize the NH3 quantities that still occur downstream of the SCR catalyst. This alternative arrangement of the SCR catalyst and / or NH3-oxidation catalyst can be repeated as often as desired and advantageously contributes to the stability of the overall system.

[0023]To reduce the cost of the inventive catalyst arrangement, it is furthermore advantageous to apply the alternatingly arranged SCR catalyst and NH3-oxidation catalyst, with their different catalyst combinations, on a common support or substrate.

Problems solved by technology

The SCR process causes special problems with the reduction of nitrogen oxides of internal combustion engines, and here in particular in vehicles, since care must be taken that there is no emission of unused ammonia.

In contrast to the situation with power plants, in vehicles, no adequately precise and stable exhaust gas sensors are available for regulating the system and hence for avoiding NH3 emissions when an overdosing of reduction agent occurs.

In addition, the use of V2O5 is problematic since at temperatures over 650° C. it sublimes, so that in recent times zeolite having the active substituents iron and / or copper and / or cobalt have been used.

The intended task of the NH3-oxidation catalyst of oxidizing excess NH3 to nitrogen can be realized only inadequately in practice due to the too low selectivity of the active substituents, for example platinum-containing substituents, so that the oxidation, as shown in the following equations, rather than ending at the oxidation state [0] ends at the oxidation states [+1], [+2] or even only at [+4] and thus again nitrogen oxides result.

In addition, the platinum metals (platinum, palladium, rhodium, iridium. osmium, ruthenium) as well as their oxides, that are used as active material for the NH3-oxidation catalyst, are very expensive and rare.

For this reason, the NH3-oxidation catalysts are generally made very small, which means that they are often overloaded, so that there is no complete NH3 conversion.

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