The present invention relates to a method for the aftertreatment of exhaust gases of internal combustion engines, in particular for reducing the proportion of NOx in the exhaust gases, NOx being reduced in the presence of O2 via an SCR (Selective Catalytic Reduction) catalytic converter using a reducing agent. Furthermore, the present invention relates to a device for the aftertreatment of exhaust gases of internal combustion engines, in particular for executing the above-mentioned method.
Conventional systems for aftertreatment of exhaust gases are discussed in “Selective Catalytic Reduction,” Dieselnet Technology Guide, August 2000 (2000-06), pages 1-12, XPOO2163384, pages 1-11. According to this article, the oxidizing converter is used for producing NO2 and for removing hydrocarbons from the exhaust gas of the internal combustion engine prior to the exhaust gases entering the SCR catalytic converter. According to this article, vaporized ammonia or an aqueous ammonia solution is to be introduced into the exhaust tract as a reducing agent.
Another system for exhaust emission control using an SCR catalytic converter is discussed in German Patent No. 100 21 693, in which the exhaust gases are activated prior to entering the SCR catalytic converter using non-thermal gas discharge plasma. According to this system, the hydrocarbons contained in the exhaust gas are initially absorbed to the greatest possible extent before the exhaust gas is exposed to the non-thermal gas discharge plasma. An ammonia-based reducing agent is then added to the exhaust gas pre-treated in this manner for the selective catalytic reduction of nitrogen oxides in the downstream SCR catalytic converter.
U.S. Pat. No. 5,893,267 discusses an exhaust emission control system in which a plasma reactor is arranged upstream from an SCR catalytic converter. In addition to exhaust gas, fuel is also supplied to the plasma reactor. According to this patent, the plasma oxidizes NO to NO2 in a first step and breaks down the original hydrocarbons of the fuel into smaller molecules. In a second step, the smaller molecules should significantly increase the activity of the SCR catalytic converter. This should result in a significant synergy effect through the combination of the plasma with the SCR catalytic converter. Compared to using ammonia as a reducing agent, the use of hydrocarbons as a reducing agent is presented in this publication as being more practical, more cost-effective, and environmentally friendlier.
It is an object of the present invention to provide a method and a device which make effective NOx conversion possible without the use of ammonia as a reducing agent.
This object may be achieved by a method in which fuel as a reducing agent is introduced downstream from the oxidizing converter and upstream from the SCR catalytic converter.
In addition, this object may be achieved by using a system in which fuel as a reducing agent is supplied via an introduction device.
The present invention provides a method and a device for the aftertreatment of exhaust gases of internal combustion engines, in particular for reducing the proportion of NOx in the exhaust gases, the NOx being reduced in the presence of O2 and a reducing agent. Such methods are referred to by the term SCR (Selective Catalytic Reduction) technology.
The method according to the present invention is based on the recognition that in SCR technology, when reducing agents are used which have no high selectivity, NO2 is reduced while NO is only marginally influenced or not at all. Therefore, according to the present invention, the exhaust gases are pre-conditioned prior to reduction via SCR technology in such a manner that the largest possible proportion of NOx or NO is converted into NO2. Subsequent to the conversion of NO into NO2, the proportion of NO in the exhaust gases is consequently lower and the proportion of NO2 is correspondingly higher. Since NO2 is converted in the subsequent reduction, the proportion of NOx in the exhaust gases is reduced overall due to the prior conversion of NO into NO2.
Fuel is used as a reducing agent in the present invention. Using fuel provides the advantage that fuel is available in the vehicle for supplying the internal combustion engine. Despite the relatively low selectivity of the fuel as a reducing agent, the proportion of NO2 may be reduced using SCR technology due to the conversion of a large proportion of NO2 of the exhaust gases.
NOx or NO is converted into NO2 using the oxidizing converter before the exhaust gases are supplied to the SCR catalytic converter. This has the advantage that fuel as a reducing agent having low selectivity may also be used for the aftertreatment of the exhaust gases and, in particular, for reducing the proportion of NOx in the exhaust gases.
Using fuel provides the advantage that fuel is available in the vehicle. A different reducing agent, ammonia for example, must be stocked up separately as another operating supply item.
Although fuel as a reducing agent in SCR catalytic converter 16 has low selectivity, it has been found that fuel reduces NO2. Due to the definite increase of the proportion of NO2 in the exhaust gas due to the utilization of oxidizing converter 12, the overall proportion of NOx is definitely reduced due to the large proportion of NO2. In the ideal case, oxidizing converter 12 oxidizes the entire NO to NO2. This would result in a largely complete reduction of the proportion of NOx in the SCR catalytic converter using fuel as a reducing agent.
Using fuel as a reducing agent without utilizing oxidizing converter 12 would result in only the relatively small proportion of NO2 in exhaust gas A being reduced. Due to low selectivity of the fuel, only a small proportion of NO is converted. According to the present invention, the proportion of NO2 of the NOx emission is consequently increased prior to the addition of fuel, to finally achieve an overall improved NOx conversion due to the good NO2 conversion using fuel as a reducing agent.
The proportion of platinum in the SCR catalytic converter should be as low as possible to prevent a conversion of NOx into N2O to the greatest possible extent.
Number | Date | Country | Kind |
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101 59 849.1 | Dec 2001 | DE | national |
Filing Document | Filing Date | Country | Kind |
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PCT/DE02/04286 | 11/21/2002 | WO |