Process for cleaning a lean exhaust gas and catalyst system therefor

The present invention relates to a method for Cleaning of a lean exhaust gas from internal combustion engines and a catalyst system therefor.

The exhaust gases of internal combustion engines contain as main pollutants carbon monoxide CO, unburnt Hydrocarbons HC, nitrogen oxides NO_x and particles. Depending on Type of internal combustion engine vary the relative Emission of play and the level of residual oxygen. Depending on the oxygen content differentiates fat Exhaust, exhaust gas stoichiometric composition and lean Exhaust. Stoichiometric exhaust gas is present when the reducing and oxidizing components of the exhaust gas a full implementation of the contaminants to water, carbon dioxide enable and nitrogen. In rich exhaust outweigh the reducing components (hydrocarbons and carbon monoxide) and in the lean exhaust gas, the oxidizing components (oxygen). As a measure of the Exhaust gas composition is the air ratio λ used. In the Air ratio λ is the on stoichiometric Conditions normalized air / fuel ratio. The Air / fuel ratio states how many kilograms of air for complete combustion of one kilogram of Fuel are required. In conventional gasoline engine fuels is the stoichiometric air / fuel ratio at a value of 14.6.

Stoichiometric conditions exist in air ratios λ = 1 in front. Values ​​below 1 characterize a rich exhaust gas and Values ​​above 1 a lean exhaust gas.

In conventional gasoline engines the oxygen content in the exhaust gas by means of a so-called lambda probe in the controlled such that stoichiometric conditions (λ = 1) present. This corresponds to an oxygen content of Exhaust gas from about 0.5% by volume. includes the lambda control However, periodic fluctuations of the air ratio to the stoichiometric value is not enough.

In order to reduce the fuel consumption of gasoline engines so-called lean-burn engines have been developed which a lean have exhaust air speeds of over 1.3. There are Lean-burn engines with a constant exhaust lean and lean-burn engines, for example, during acceleration phases in Change the exhaust rich area. Diesel engines work during the predominant operating period with a lean exhaust gas, which contains 6 to 10 vol% oxygen.

Lean-burn engines have a high potential for the Reduction in fuel consumption. This is particularly for direct-injection lean-burn engines, the theoretically Reduction in consumption of up to 25% compared to gasoline engines in stoichiometric operation permit.

The different exhaust gas composition of the various Engine types requires specially developed exhaust gas cleaning concepts. The exhaust gas from stoichiometrically operated gasoline engines is with so-called three-way catalysts cleaned. These catalysts are capable of the three Pollutants carbon monoxide, hydrocarbons and nitrogen oxides at stoichiometric exhaust gas conditions almost quantitatively to water, carbon dioxide and nitrogen implement.

The exhaust gas treatment of lean-burn engines and diesel engines hand prepares considerable difficulties. While the oxidizable components of the exhaust gas relatively lean can be readily reacted with oxidation catalysts, requires the reduction of nitrogen oxides Special Reduction catalysts. These so-called DeNOx catalysts reduce nitrogen oxides and use it the hydrocarbons and carbon monoxide contained in the exhaust gas as a reducing agent. If not enough Reducing agent in the exhaust gas are present, must appropriate amounts of reducing agents to the exhaust gas before Contact with the reducing catalyst may be added. This may be for example by unburned fuel or act to ammonia. Provided set an adequate supply of reducing agent the DeNOx catalysts, the nitrogen oxides continuously to and are therefore in the following as permanent reduction catalysts designated.

The conversion rates of a catalyst for the individual Pollutants are strongly dependent on the exhaust temperature dependent. With increasing exhaust gas temperature initially constitutes Oxidation of the hydrocarbons and carbon monoxide and achieved within a temperature interval of about 150 to 175 ° C oxidation rates of over 90%. For further increasing temperature remains the implementation of hydrocarbons constant. The exhaust gas temperature at which a Conversion rate of 50% for the respective pollutant is reached, than the light-off temperature for this Emission called.

The rate of conversion of permanent Reduktionskatalaysatoren for nitrogen oxides followed by the conversion rate of hydrocarbons. However, it does not increase monotonically, but passes through at temperatures at which the oxidation of hydrocarbons has about reached its maximum value, a Maximum and then decreases with increasing temperature back up to almost zero. Optimum conversion rates for the nitrogen oxides So only in a narrow temperature window reached. The maximum conversion rate for nitrogen oxides are, depending on the proportion of hydrocarbons to nitrogen oxides in the exhaust gas (HC / NO_xRatio) at about 50 to 70%, which is generally significantly lower than those for other pollutants in car exhaust.

depend The reaction curves for the individual pollutants of the formulation of the respective catalyst. The also applies to the nitrogen oxides, location and width of the Temperature window and the window in this maximum achievable degree of conversion will be the catalyst formulation set. There are so-called Low temperature reduction catalysts which their maximum nitrogen oxide conversion at temperatures between 150 reach and 250 ° C. At high temperature reducing catalysts the maximum of the nitrogen oxide conversion above 300 ° C.

A typical representative of a low-temperature reduction catalyst is in the unpublished DE 196 14 540 discloses. He has a temperature window between 150 and 300 ° C.

The DE 38 41 990 C2 discloses a high-temperature reduction catalyst for the selective reduction of Nitrogen oxides in flue gases by means of ammonia. Of the Catalyst consists essentially of an iron- and / or copper exchanged zeolites of the mordenite and possesses optimum conversion rates for the nitrogen oxides between 350 and 500 ° C. The US 5,185,305 describes a copper exchanged zeolites ZSM5 for cleaning the exhaust gases of a lean-burn engine. Another high temperature reducing catalyst is disclosed in EP B1 0577879 described. It is made of a catalyst Iridium in the state of metal or an oxide or a mixture of these on a support of metal carbide or Metal nitrides. The amount of iridium on the support is from 0.005 to 10.0 wt .-%, based on the Total amount of iridium and the carrier. This Catalyst has its maximum nitrogen oxide conversion at about 450 ° C on.

To broaden the temperature window for the nitrogen oxide reduction be in accordance with DE 40 32 085 A1, a low and a high-temperature-reducing catalyst connected in series. Activity and selectivity of two series-connected Permanent reduction catalysts but insufficient. In particular, the conversion of pollutants of permanent-reduction catalysts during brief, stoichiometric exhaust gas conditions, compared with conventional three-way catalysts, inadequate.

Specifically, the exhaust gas purification of lean-burn engines Therefore nitrogen oxide storage catalysts developed (eg EP 0562516 A1), in the excess of oxygen (λ> 1) the Nitrogen oxides as nitrates of alkali or alkaline earth elements store and this in enrichment of the exhaust gas (λ <1) to reduce elemental nitrogen. The necessary Anfettungen in which high concentrations of reducing agents, at the same time greatly lowered oxygen content made available, occur either in Acceleration phases, in which high loads and speeds required, or must lean partial load operation by the engine electronics in precisely calculated intervals in the form of short, periodic Anfettungsspitzen (Λ <1) be made. The latter requires a complicated control concept for the engine and a very precise tuning of the driving operation on the catalyst. A further disadvantage is that the periodic enrichment phases for regeneration of the storage catalyst to a cause increased fuel consumption, with the lean-burn engines achievable fuel savings diminish.

Opposite the permanent-reduction catalysts, the Nitrogen oxides continuously under lean exhaust gas conditions implement, this only happens when storage catalysts during the periodic enrichment phases.

Nitrogen oxide storage catalysts generally operate effectively in a temperature range of 200 to 500 ° C. In addition to their good nitrogen oxide storage capacity in the lean Exhaust show nitrogen oxide storage catalysts also good Conversion rates for all three types of pollutants in stoichiometric operation of the lean-burn engine, of which the conventional three-way get close.

For the permissible pollutant emissions from motor vehicles certain limits were established. The emissions of Motor vehicles during statutory Driving cycles determined. was from the European Commission therefor the MVEG-A cycle set. In compliance with current valid limits must a vehicle with a gasoline engine during the MVEG-A cycle is not more than 2.7 g CO / km, not more than 0.34 g HC / km and not more than 0.25 g NO_x/ km emit. For diesel engines exist separate Limits. 1999 and 2005 are further tightening of Limits for Otto engines and diesel engines planned, roughly halving the applicable limits include.

The known exhaust gas cleaning systems for lean exhaust gas compositions on the basis of permanent-reduction catalysts are not capable of the future to meet emission limits. In the case of storage catalysts must due to the periodic Anfettungen an additional fuel consumption for regeneration their storage capacity to be accepted, which the theoretical savings from lean-burn engines reduced. In addition, a cleaning method requires using of storage catalysts a complicated motor control system, that the duration of the enrichment phases Calculate and enrichment phases must perform.

It is therefore the object of the present invention to provide a improved method for the purification of exhaust gas lean specify. In particular, the method should also through the periodic enrichment phases related fuel consumption largely avoided by storage catalysts.

This object is achieved in that the exhaust gas first over a permanent reduction catalyst and then is passed over a nitrogen oxide storage catalyst. This has a series connection of the two catalysts the advantage that the good even with permanent-reduction catalysts always present slip of more than 30% of Nitrogen oxides from the downstream nitrogen oxide storage catalyst is intercepted. Since this is only around a portion of the entire cargo nitrogen oxide of the exhaust gas is, the storage capacity of storage catalytic converter relieved, so that it less frequently enrichment of the exhaust gas must be regenerated. This has a reduced fuel consumption. In normal urban driving conditions, the enrichment phases even largely eliminated, as noted in the richer gas mixture while the recurring acceleration phases fully sufficient. At constant lean operation in Partial load range, the light-off temperature of the permanent reduction catalyst exceeded. He then makes largely alone for the conversion of nitrogen oxides. The Frequency of the periodic enrichment phases for Regeneration of the storage catalyst may accordingly reduced or it may on the Anfettungen even completely be waived.

Permanent reduction catalyst and the storage catalyst complement each other in their catalytic effects. This including operating phases of the lean-burn engine with stoichiometric exhaust gas conditions. Under these Conditions have permanent reduction catalysts only poor conversion of pollutants, while storage catalysts still a good three-way activity at λ = 1 exhibit.

The known storage catalysts work optimally in a temperature range between 200 and 450 ° C. direct dialing a permanent reduction catalyst with a Temperature window for the nitrogen oxide reduction between 300 and 600 ° C, therefore, the temperature window for the Overall system to the range between 200 and 600 ° C be extended. Particularly suitable for this is the Use of an iridium catalyst as a permanent Reduction catalyst for nitrogen oxides. Alternatively, also exchanged with copper ZSM-5 zeolite be used.

A further improvement of the conversion of pollutants can be achieved if the permanent reduction catalyst a small-sized motor near three-way is connected upstream to the cold-start phase of the vehicle Carbon monoxide, hydrocarbons and nitrogen oxides implement. The activity of this catalyst was low be because it still after reaching the operating enough slip of hydrocarbons and carbon monoxide as a reducing agent for the downstream permanent reduction catalyst must admit.

1 shows the schematic structure of a for Inventive suitable exhaust gas purification system. The lean burn engine 1 is provided with an exhaust system 2 provided that two converters 3 and 4 has. converter 3 includes a permanent reduction catalyst, while Converter 4 includes the nitrogen oxide storage catalyst.

Both catalysts may in principle in a converter housing be accommodated. The shown here Accommodation in two separate converter housings but allows the exhaust gas temperature on the storage catalyst during the predominant duration of operation in cheap for the storage catalytic converter temperature range to lay. This can in a simple manner by Adjustment of the exhaust gas line between the two happen converter housings, since experience shows that the exhaust when flowing through the exhaust pipe per meter cable length cooled to about 50 to 100 ° C.

Converter housing 5 is located close to the engine and contains an optional pre-catalyst to improve Cold starting of the emission control system.