Procedure and device for the purification of exhaust gas

Abstract

The invention relates to a procedure for purifying exhaust gas in the exhaust gas duct of a combustion engine with an exhaust gas purification system and a SCR-catalyzer that is arranged in the exhaust gas purification system for a selective catalytic reduction of nitrous gases (SCR), whereby reducing agents for the selective catalytic reduction of nitrous gases can be added to the exhaust gas in direction of the flow before the SCR-catalyzer. According to the invention it is provided that the exhaust gas is led through a catalyzer for converting nitrous gases with hydrocarbons and subsequently through the SCR-catalyzer and that the operating parameters are adjusted in a first operating phase in such a way, that nitrous gases are mostly converted at the catalyzer for converting nitrous gases with hydrocarbons and that the operating parameters are adjusted in a second operating phase in such a way, that nitrous gases are mostly converted at the SCR-catalyzer. The adjustments for the first operating phase are selected at the start of the combustion engine. The catalyzer is heated to its operating temperature in this operating phase for the conversion of nitrous gases with hydrocarbons. Because it is arranged before the SCR-catalyzer and a particle filter that is usually arranged in front of it, its heating takes place very fast and the output of nitrous gases in the starting phase of the combustion engine can be reduced as opposed to the state of the art. The normal operation of the combustion engine is characterized by a lean mixture composition and therefore by an oxygen-rich exhaust gas. This is adjusted in the second operating phase, in which nitrous gases are converted in the SCR-catalyzer that is now heated to its operating temperature.

1 . A method of purifying an exhaust gas in an exhaust gas duct of a combustion engine with an exhaust gas purification system and a SCR-catalyzer that is arranged in the exhaust gas purification system for a selective catalytic reduction of nitrous gases, wherein at least one reducing agent for the selective catalytic reduction of the nitrous gases is added to the exhaust gas in a direction of flow before the SCR-catalyzer, the method comprising:
 leading the exhaust gas first through a catalyzer for converting nitrous gases with hydrocarbons and second through the SCR-catalyzer, wherein one or more operating parameters are adjusted in a first operating phase whereby the nitrous gases are more substantially converted at the catalyzer, and wherein the one or more operating parameters are adjusted in a second operating phase whereby the nitrous gases are more substantially converted at the SCR-catalyzer.   
   
   
       2 . A method according to  claim 1 , further comprising adjusting the one or more operating parameters in the first operating phase to reduce an oxygen amount in the exhaust gas in comparison to a normal operation of the combustion engine, wherein an exhaust gas temperature at the catalyzer for converting nitrous gases with hydrocarbons is raised to at least 250° C. and in that the concentration of unburned hydrocarbons in the exhaust gas is increased. 
   
   
       3 . A method according to  claim 2 , further comprising reducing the oxygen amount in the exhaust gas in the first operating phase to less than or equal to 5%, preferably between 0.6% and 0.8%. 
   
   
       4 . A method according to  claim 1 , further comprising adjusting the one or more operating parameters in the second operating phase to operate the combustion engine in a normal operation with a lean fuel-air mixture and to reduce the concentration of unburned hydrocarbons in comparison to the first operating phase. 
   
   
       5 . A method according to  claim 1 , further comprising switching from the first into the second operating phase when the SCR-catalyzer has reached a default operating temperature. 
   
   
       6 . A device configured for purifying an exhaust gas in an exhaust gas duct of a combustion engine with an exhaust gas purification system and a SCR-catalyzer that is arranged in the exhaust gas purification system for a selective catalytic reduction of nitrous gases, wherein at least one reducing agent for the selective catalytic reduction of the nitrous gases is added to the exhaust gas in a direction of flow before the SCR-catalyzer, comprising:
 a catalyzer for converting nitrous gases with hydrocarbons, wherein the catalyzer is arranged in the exhaust gas duct in close proximity to the combustion engine, and wherein the SCR-catalyzer is arranged in a direction of flow of the exhaust gas after the catalyzer for converting nitrous gases with hydrocarbons.   
   
   
       7 . The device of  claim 6 , wherein a particle filter is arranged in the exhaust gas duct between the catalyzer for converting nitrous gases with hydrocarbons and the SCR-catalyzer.