DE19753572A1 - Catalytic purification of exhaust gases from diesel engine over wide temperature range - Google Patents

Abstract

The engine exhaust gases enter a precatalyst unit where they encounter a high-temperature catalyst and a relatively smaller low-temperature catalyst. The gases continue into the main catalyst unit, via a low temperature catalyst. In the low temperature section of the precatalyst unit, when the high temperature catalyst is operational, predetermined hydrocarbon slippage is effected. Preferred features: At high exhaust temperatures, hydrocarbon slip sets in, the precatalyst low temperature section being inadequate to eliminate it.

Description

According to the preamble of claim 1, the invention relates to Process for catalytic exhaust gas purification in diesel engines, the exhaust gas over a plurality of catalytic converters arranged in an exhaust line with under different temperature windows for nitrogen oxide reductions.

As is known, catalysts for diesel exhaust have the serious disadvantage that that their DeNOx activity is limited to a narrow temperature window. Purpose For this purpose, a serial arrangement of catalysts for increasing is moderate the overall reduction of nitrogen oxides is provided, whereby according to the respective Ab gas temperature the front or rear catalytic converter is in the appropriate range. Serial arrangements of catalysts with a difference are also known Lichen temperature windows to the total temperature range in which a Re production of nitrogen oxides is possible to spread even further.

It is also known for combinations of high and low temperature catalysts Ren diesel fuel as a reducing agent either in the exhaust line upstream of one To provide pre-catalyst or directly in the combustion chamber. This results in one Combination of a low-temperature catalytic converter in the pre-cat position with one High temperature catalyst in the main catalytic converter has the disadvantage that at high  Temperatures at which the main catalyst has its maximum NOx reduction effect could unfold, this is no longer supplied with reducing agent, because of the before lying highly active low-temperature cat hydrocarbons in that temperature area almost completely oxidized.

With a reverse combination with a high temperature catalyst in the Pre-cat position and a low-temperature catalyst in the main cat position results there is the disadvantage that when the high-temperature cat is arranged close to the engine at low At lower exhaust gas temperatures, no reducing agent can be injected, since un break through hydrocarbons through both catalysts below the light-off. The low-temperature catalytic converter is then so far away from the engine that it is in it likewise no conversion took place. This means there is no reduction in NOx Engine load possible, HC emissions also increase.

The lack of reducing Koh resulting from the first catalyst combination hydrogen at the high-temperature catalytic converter in the main cat position the usually larger volume of the high temperature catalyst in the main To have fully available cat position for NOx reduction, in front of this High-temperature catalyst suitable fuel injection into the exhaust system must be provided to supply the high-temperature catalyst with Ensure hydrocarbons. Such systems are in terms of their Ko most and the technical risks disadvantageous.

With the second catalyst combination there is a significant NOx reduction in diesel typical temperature ranges excluded, in addition there are HC-Pro bleme.

The invention has for its object the generic method for ka To improve talytical exhaust gas cleaning in diesel engines so that with ge the least construction effort and without the introduction of additional reducing agents the kataly table exhaust gas cleaning is qualitatively improved.  

This object is achieved with the patent claim in that the exhaust gas downstream of the diesel engine in a pre-cat device in the flow direction via a High temperature catalyst and a relatively smaller low temperature catalyst and then in a main cat facility via a low-temperature catalyzer sator is performed, with the pre-cat low-temperature catalyst at the active High temperature catalyst a predetermined hydrocarbon (HC) slip is effected.

This hydrocarbon slip of the pre-cat low-temperature Catalyst in an embodiment of the invention about its active size at high Ab generated gas temperatures.

With the combination made according to the invention in the pre-cat device is a improved process for catalytic exhaust gas cleaning in diesel engines enough, this low-temperature catalyst is designed in terms of size and oxidation activity that it reduces HC and at low exhaust temperatures its DeNOx effect develops, but a carbon at high temperatures causes hydrogen slip. This hydrocarbon slip serves at higher ones Exhaust gas temperatures as a reducing agent for the pre-in the main cat facility see low-temperature catalyst to achieve a full DeNOx effect.

In summary, this advantageously results at low exhaust gas temperatures full NOx reduction and HC reduction in the low-temperature catalyst of the pre-cat device and at high exhaust gas temperatures a full NOx re duction both in the high-temperature catalyst of the pre-cat device and in Low temperature catalytic converter of the main catalytic converter, the hydrocarbon Slip an additional fuel injection from the pre-cat device Pre-cat device and main cat device are advantageously dispensed with. At Skilful selection and dimensioning of the low-temperature catalytic converter of the pre-cat Establishment is a secure supply of hydrocarbons from coal hydrogen slip as a reducing agent for the low-temperature catalyst Main cat facility reached.  

The invention is described below. In a catalytic process Exhaust gas cleaning in diesel engines, the exhaust gas over several in one Ab Catalysts with different temperatures arranged in a gas line windows for nitrogen oxide reductions.

For the qualitative improvement of catalytic exhaust gas cleaning with a simple opening construction of the device and avoiding the introduction of additional Reduk tion medium is proposed according to the invention that the exhaust gas downstream of the selmotors in a pre-cat device in the direction of flow over a high temperature rature catalyst and a relatively smaller low-temperature catalyst as well closing in a main catalyst facility via a low-temperature catalyst leads, being over the pre-catalyst low-temperature catalyst High temperature catalyst a predetermined hydrocarbon (HC) slip is effected. This hydrocarbon slip of the pre-cat low-temperature catalyst is generated via its active size at high exhaust gas temperatures.

Claims (2)

1. Process for catalytic exhaust gas purification in diesel engines,

• the exhaust gas being passed over a plurality of catalytic converters distributed in an exhaust line with different temperature windows for nitrogen oxide reductions, characterized in that
• - That the exhaust gas downstream of the diesel engine in a pre-cat device in the direction of flow via a high-temperature catalyst and a relatively smaller low-temperature catalyst and
• - Is then performed in a main catalyst device over a low-temperature catalyst, wherein
• - A predetermined hydrocarbon (HC) slip is effected via the pre-cat low-temperature catalyst with the pre-cat high-temperature catalyst acting.

2. The method according to claim 1, characterized in that the coals hydrogen (HC) slip of the pre-cat low-temperature catalyst over its active size is generated at high exhaust gas temperatures.