Compact plasma reactor

Details Compact plasma reactor Compact plasma reactor

2002-12-02

2004-09-07

McDonald, Rodney G. (Department: 1753)

Chemistry: electrical and wave energy

Processes and products

Electrostatic field or electrical discharge

C204S169000, C204S170000, C204S171000, C204S173000, C204S174000, C204S177000, C422S186040, C422S186290

Reexamination Certificate

active

06787002

ABSTRACT:

BACKGROUND OF THE INVENTION
Field of the Invention
The invention relates to a device for purifying an exhaust gas. Such a device has at least a first component and a second component, each with a shell and a core, through which the exhaust gas can flow. Each of the components have two end faces, at least one end face of the first component and at least one end face of the second component have a predeterminable profile with elevations and depressions. The elevations of the end face of the first component extend into the depressions in the end face of the second component and vice versa, thereby forming a penetration section. Devices and methods of this type are used in particular to purify exhaust gases that are formed during the operation of motor vehicles.
In virtually all highly motorized countries, the pollutant levels in the exhaust gas from motor vehicles are subject to statutory limits. In particular, the carbon monoxide emitted and partially burnt or unburnt hydrocarbons are limited. Moreover, the exhaust gas contains the oxidation products of nitrogen NO and NO
2
(referred to jointly as NO
x
) and pollutants formed from fuel impurities or additives, such as lead or sulfur compounds.
While effective devices and methods for converting carbon monoxide and hydrocarbons are already known, the conversion of the NO
x
causes problems, in particular during the cold-starting phase of an engine. One reason is that modern engines are predominantly operated in lean-burn mode, i.e. with a high oxygen concentration, that in combination with the high temperatures that occur in the engine and/or the exhaust system promotes the formation of nitrogen oxides.
It is possible to reduce the NO
x
by using known catalytic converters (reduction catalytic converters). However, to do this, reducing components, such as for example molecular hydrogen or carbon monoxide, have to be present in the exhaust gas, while significant oxygen concentrations have to be avoided. The affinity of molecular hydrogen and carbon monoxide is greater for oxygen, so that it is not possible to reduce the nitrogen oxides if high oxygen concentrations are present. Accordingly, it would be necessary to operate the engines with a rich mix, but this is not possible for a prolonged period on account of statutory conditions.
Known methods and devices for deNOxing exhaust gases are primarily based on reacting the nitrogen oxides with ammonia to form molecular nitrogen and steam. Without a catalytic converter, these reactions are carried out with excess ammonia and at temperatures of around 900° C. (SNCR process). With a catalytic converter, it is possible to use less excess ammonia and to lower the reaction temperature to 180 to 450° C. (SCR process).
In an SCR system, ammonia is atomized and in the process wets a catalytically active structure in the exhaust system. On the catalytically active surface of the structure, NO
x
reacts in the presence of the ammonia to form molecular nitrogen and water. These reactions preferably take place in an oxygen-rich environment. Furthermore, the reduction of the nitrogen oxides is greatly dependent on the temperature. Effective reduction takes place only in a temperature range of ±28° C. at a defined oxygen content in the exhaust gas. Below this temperature range, the catalytic activity of the structure drops and the ammonia supplied passes into the environment. If the temperature range is exceeded, the ammonia is preferentially converted into additional nitrogen oxide.
Known catalytic converters are unable to provide the required activity in the presence of a significant excess of oxygen and a sufficient service life on account of the presence of water to ensure suitable reduction of NO
x
. The operation of known catalytic converters within a very narrow temperature window continues to cause additional problems.
Furthermore, there are known processes for the reduction of NO
x
in an oxygen-rich exhaust gas from automobiles that combine an exhaust-gas treatment by plasma and a selective catalytic reduction. The exhaust gas is formed of a multiplicity of atoms and molecules, which are converted into the plasma state when sufficient energy is supplied. In the process, the shells of these constituents are broken open by collision processes and positively charged ions, electrons and reactive free radicals are formed. The induced plasma oxidizes nitrogen monoxide to form nitrogen dioxide and breaks up the complex, partially burnt or unburnt hydrocarbons into smaller molecules. The smaller hydrocarbons significantly increase the activity of the catalytic converter and assist the oxidation of the nitrogen monoxide.
In combination with a catalytic converter, the hydrocarbons help to reduce the nitrogen dioxide to molecular nitrogen. This plasma-enhanced catalytic reaction can be schematically divided into two steps:
1. Plasma+NO+O
2
+HC->NO
2
+HC; and
2. Catalyst+NO
2
+HC->N
2
+CO
2
+H
2
O,
where HC represents a large number of hydrocarbon molecules.
In the automotive industry, plasmas of this type are preferably generated by an electric field that is generated by a high voltage. In particular, it is preferable to generate plasma using a corona discharge or a dielectric barrier discharge. If a corona discharge is desired, a nonuniform orientation of the electric field is possible. For example, there are known devices with a wire that is surrounded by a very strong electric field which, however, drops significantly in the radially outward direction and does not reach the housing surrounding it, with the result that the formation of an arc is suppressed. Alternatively, the formation of arcs can be avoided by pulsed application of voltage. In the case of a dielectric barrier discharge, at least one electrode is coated with a dielectric. The arcs that form are extinguished below the surface of the dielectric material.
Known plasma reactors, for example from International Patent Disclosure WO 95/31271, are very complex and take up a large volume. For effective conversion of the nitrogen monoxide, the plasma has to be formed over a certain reaction distance. The reaction distance increases as the voltage applied becomes smaller and/or as the spacing between the electrodes increases.
SUMMARY OF THE INVENTION
It is accordingly an object of the invention to provide a compact plasma reactor that overcomes the above-mentioned disadvantages of the prior art devices of this general type, which purifies an exhaust gas and brings about improved reduction of the NO
x
in the exhaust gas and has a compact structure.
With the foregoing and other objects in view there is provided, in accordance with the invention, a device for purifying an exhaust gas from an internal combustion engine. The device contains components including at least a first component and a second component. Each of the components has a shell and a core through which the exhaust gas flows. The components each have two end faces and at least one of the end faces of the first component and at least one of the end faces of the second component has a predeterminable profile containing elevations and depressions. The elevations of the end face of the first component extend into the depressions in the end face of the second component and vice versa, thereby forming a penetration section. The first component is disposed so that the first component is electrically insulated from the second component. The components have a potential difference with respect to one another and plasma being generated in the penetration section.
The device according to the invention for purifying the exhaust gas from the internal combustion engine is configured at least with the first component and the second component. Each component has a shell and a core, through which the exhaust gas can flow, with two end faces. At least one end face of the first component and at least one end face of the second component have a predeterminable profile containing elevations and recesses. The components are disposed in such a way

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