DE19801410C1 - Device to reduce nitrogen oxides present in the exhaust gas of a diesel engine or a lean mix spark ignition engine - Google Patents

Abstract

An exhaust gas cleaning system for an internal combustion engine includes a device (4) containing metal salts and oxide to remove nitrogen oxide. Downstream from this device (4) there is container (5) which collects condensed water arising from device (4). A further device recovers and returns salts in the condensed water back to the nitrogen oxide removal device (4). Preferably the metal salts/oxide absorb the nitrogen oxide and reduce it. They consist of compounds of alkali metals, alkaline earth metals or rare earth metals and include an alkaline carbonate. The absorbed nitrogen oxide may be decomposed electrochemically. The salts are returned through a return conduit (6) into the exhaust pipe (2) through a nozzle upstream of the nitrogen oxide removal device (4).

Description

The invention relates to an exhaust gas purification system for an internal combustion engine according to the preamble of claim 1.

The emission of nitrogen oxides (NO _x ) from internal combustion engines, especially those used in road traffic, represents a considerable environmental impact, which is being increasingly restricted by the relevant laws. While Otto engines with three-way catalytic converters, which are operated with a fuel-air mixture of λ = 1, meet all current emissions regulations, this is for the Otto Mager engines, which are much cheaper in terms of fuel consumption, due to the high excess air in the Combustion is not the case. Vehicles with a diesel engine currently have higher limit values, but lower limit values are also provided here. A special NO _x removal is required here. For this purpose, so-called Denox catalysts are used, which, however, only reach a maximum degree of conversion of 50%.

So-called NO _x storage catalysts have been developed to reduce the NO _x emission in Otto-Mager engines. B. ceramic or metal, a porö se coating, for. B. an alumina coating, aufwei sen, the one hand metal salts or oxides, such as alkali, alkaline earth or rare earth metal salts or oxides, which absorb NO _x from, and on the other hand contains noble metals, such as platinum. During normal operation of the Otto-Lean engine, that is, with oxygen-rich exhaust gas, NO _x z. B. absorbed by the alkaline earth metal oxide, while carbon monoxide (CO) and Kohlenwas serstoff (HC) are oxidized by the platinum to carbon dioxide and water. By intermittent short-term rich operation of the otherwise lean-burn gasoline engine, the NO _x absorbed on the catalyst is reduced to nitrogen by the reducing gases, such as HC, and the catalyst is thus regenerated (EP 0 645 173 B1).

In addition, according to the German patent application 196 51 492.4 to reduce the NO _x emission of diesel and Otto-Lean engines, it is proposed as a NO _x removal device a porous absorber body soaked with an alkali metal carbonate melt to absorb the NO _x as nitrate or nitrite use in which electrodes for electrochemical decomposition of the nitrate and nitrite to nitrogen are divided in pairs. The liquid alkaline electrolyte is formed by an alkali carbonate melt.

The NO _x storage catalyst and the electrochemical NO _x nitrogen oxide removal device according to the German patent application 196 51 492.4 lead to high degrees of conversion. However, their effectiveness decreases noticeably during the long operating time of a motor vehicle.

The object of the invention is therefore an exhaust gas purification system high effectiveness and long durability in particular especially for diesel and lean Otto engines.

This is according to the invention characterized in that in claim 1 Neten invention achieved. In the subclaims are before reproduced partial embodiments of the invention.

The known NO _x removal devices in emission control systems, ie the NO _x storage catalyst as well as the electrochemical device according to the German patent application 196 51 492.4, are characterized in that they contain metal salts or oxides to the NO _x to absorb from the exhaust.

Fuel combustion produces carbon dioxide and water as the main product. During a cold start or a cold exhaust gas cleaning system, however, what was formed during combustion can condense in the exhaust gas cleaning system as liquid water. As could be determined, this condensed water dissolves the metal salts or oxides in the NO _x removal device, in particular also the nitrates and nitrites formed therefrom after absorption of NO _x . As a result, the absorbent is gradually washed out, whereby the effectiveness of the NO _x removal device deteriorates over time.

This is countered according to the invention in that behind, that is, downstream of the exhaust gas, the NO _x -Entfernungsein direction is a collecting container for the saline condensed water that emerges from the NO _x removal device. The salt in the condensed water in the receiver is then returned while the condensed water is evaporated.

The salt in the condensate in the collecting container can thus be the metal salt absorbent of the NO _x -Dickering device or z. B. from such a metal salt or Me talloxid formed metal nitrate or nitrite, but also z. B. from a metal oxide and the carbon dioxide of the Abga ses formed metal carbonate.

To return the salt, a line is preferably provided through which the salt-containing condensed water flows back to the NO _x removal device.

The return line preferably opens into the exhaust pipe on the inlet side, that is to say upstream of the nitrogen oxide removal device. When the exhaust gas cleaning system has reached the operating temperature, the water evaporates, while the salt particles are taken up again by the NO _x removal device.

The promotion of condensed water in the return line can, for. B. done by an electrical or mechanical pump. However, the mouth of the return line in the exhaust pipe is preferably designed as a nozzle oriented in the exhaust gas flow direction. The high exhaust gas volume flows create a suction effect (water jet pump principle) through which the condensed water is sucked out of the collecting container. At the same time, the nozzle atomizes the condensed water in the exhaust gas (injector principle), whereby the condensed water evaporates and the fine salt particles are regularly distributed in the NO _x removal device.

Instead, the condensate collecting container can also be arranged above the NO _x removal device, so that the condensate is returned by gravity.

The evaporation of the condensed water can also be carried out with a Ver evaporator trough, which is arranged above the NO _x removal device. The evaporator pan preferably has at least one opening. If the temperature of the evaporator trough is sufficiently high, the condensate evaporates, where the salts contained therein remain and, when the melting temperature is reached, flow through the opening in the evaporator trough into the NO _x removal device.

The return with the evaporator trough therefore presupposes that the salts in the condensed water have a melting point which is below the operating temperature of the NO _x removal device. This requirement is met in particular in the NO _x removal device according to German patent application 196 51 492.4, in which the absorbed nitrogen oxides are electrochemically decomposed.

This means that this NO _x removal system has a porous absorber body with a liquid, alkaline electrolyte for the absorption of the NO _x as nitrate and nitrite and electrodes distributed in pairs in the absorber body for electrochemical decomposition of the nitrate or nitrite to nitrogen. The liquid alkaline electrolyte is preferably formed by a molten salt which at least partially consists of alkali carbonate. The alkali carbonate is preferably a mixture of the carbonates of lithium, sodium and potassium. A mixture of alkali and / or alkaline earth carbonate and an alkali and / or alkaline earth nitrate and / or nitrite is preferably used as the salt. The porous absorber body can consist of lithium aluminate, optionally lithiated cerium oxide or zirconium oxide or lithium zirconate. Since the NO _x removal device according to German patent application 196 51 492.4 represents a particularly preferred NO _x removal device for the exhaust gas purification system according to the invention, reference is also made in full to this German patent application.

In addition, the exhaust gas purification system according to the invention can be used before geous with NO _x storage catalysts, that is, catalysts which have a porous coating, for. B. made of aluminum oxide, which contains metal salts and / or oxides on the one hand, which absorb nitrogen oxides from the exhaust gas of diesel engines and gasoline engines, and on the other hand noble metals such as platinum. During normal operation of the diesel or Otto-Lean engine, the NO _x formed is absorbed by the metal salt or oxide, while CO and HC are oxidized to carbon dioxide and water by the noble metal catalyst. By intermittent operation with richer exhaust gas, the NO _x z absorbed by the metal salts or oxides is reduced. B. reduced to nitrogen by HC or CO and thus regenerates the sorbent.

The invention based on the drawing is an example explained in more detail. Each shows schematically:

. Figure 1 is an exhaust gas purification system; and

Fig. 2 and 3, a condensate evaporation device according to a first or second embodiment.

Referring to FIG. 1, an internal combustion engine 1, an exhaust gas washer system on catcher 5 are arranged in succession in an exhaust pipe 2, that is in the exhaust gas flow direction, an oxidation catalyst 3, a NO _x -Entfernungseinrichtung 4 and a Kondenswasserauf. The collecting container 5 is connected via a condensate return line 6 to the NO _x removal device 4 . The oxidation catalyst 3 can, for example, be a noble metal catalyst, in particular a platinum catalyst, with which the CO and HC are oxidized, and NO _x essentially to NO ₂ .

The NO _x removal device 4 can be designed as an electrochemical NO _x removal device according to the German patent application 196 51 492.4. For this purpose, a battery 7 is seen before, which is connected to the electrodes of this electrochemical NO _x removal device 4 via the dashed electrical line.

Referring to FIG. 2, the mouth of the condensate return line 6 in front of the NO _x -Entfernungseinrichtung 4 arranged in the exhaust pipe 2 at. The mouth of the line 6 is designed as a nozzle 9 directed in the direction of the exhaust gas flow 8 . The high speed of the exhaust gas stream 8 creates a suction effect (water jet pump principle) through which the condensed water is sucked out of the collecting container 5 . The condensation water is atomized and evaporated while the salts contained therein finely and regularly in the NO _x -absor-absorbing material of the NO _x -Entfernungsreinrichtung 4 and are thus received by it again.

According to FIG. 3, the NO _x has -Entfernungseinrichtung 4 Ver an evaporator pan 10, which is formed in the cross-sectional shape of a funnel having an average opening 11. The condensed water return line 6 opens above the evaporator pan 10 at a distance from the opening 11 . The condensed water can be supplied by gravity, that is to say by arranging the collecting container 5 in the exhaust pipe 2 above the NO _x removal device 4 .

If, in the embodiment according to FIG. 2 or 3, the NO _x removal device 4 is at a temperature below the dew point of the water vapor, the condensed water flows back from the collecting container 5 via the NO _x removal device 4 to the collecting container.

At a higher, in particular operating temperature of the exhaust gas cleaning system, the water in the embodiment according to FIG. 2 evaporates after entering the exhaust pipe 2 or according to FIG. 3 when it hits the evaporator pan 10 .

Thus, in the embodiment according to FIG. 3, the salts separate out in the condensation water on the evaporator pan 10 . Upon reaching the melting temperature of the liquid salts then flow back through the opening 11 in the lower part of the NO _x -Entfernungseinrichtung 4 to the NO _x absorbent mate rial.

Claims (13)

1. Exhaust gas cleaning system for an internal combustion engine with egg ner device for nitrogen oxide removal, which contains metal salts and / or oxides, characterized by a from the gas downstream of the nitrogen oxide removal device ( 4 ) arranged collecting container ( 5 ) for receiving the from the nitrogen oxide removal device ( 4 ) escaping condensate and a device for returning salts in the condensate of the collecting container ( 5 ) to the nitrogen oxide removal device ( 4 ) 2. Emission control system according to claim 1, characterized records that the metal salts and oxides for absorption of nitrogen oxides from the exhaust gas of diesel and Otto lean Motors are trained. 3. Emission control system according to claim 2, characterized records that the metal salts and oxides for reduction of the absorbed nitrogen oxides formed in the rich exhaust gas are. 4. Emission control system according to claim 2, characterized in that the nitrogen oxide removal device ( 4 ) has electrodes for electrochemical decomposition of the absorbed nitrogen oxides. 5. Emission control system according to claim 1 and 4, characterized ge indicates that the metal salt by a molten salt is formed.   6. Emission control system according to one of the preceding claims che, characterized in that the metal salts and -oxides by alkali, alkaline earth and / or rare earth metal loxides and / or salts are formed. 7. emission control system according to claim 5 and 6, characterized ge indicates that the metal salt at least partially Alkali carbonate exists. 8. Exhaust gas purification system according to claim 1, characterized in that the device for returning the salts from the collecting container ( 5 ) to the nitrogen oxide removal device ( 4 ) is formed by a return line ( 6 ) for the condensed water. 9. Exhaust gas purification system according to claim 8, characterized in that the return line ( 6 ) opens upstream of the nitrogen oxide removal device ( 4 ) into the exhaust pipe ( 2 ). 10. Exhaust gas cleaning system according to claim 9, characterized in that the mouth of the return line ( 6 ) as in the exhaust gas flow direction ( 8 ) directed nozzle ( 9 ) is ausgebil det. 11. Exhaust gas purification system according to claim 1, characterized in that the collecting container ( 5 ) for returning the salts by gravity is arranged higher than the nitrogen oxide removal device ( 4 ). 12. Exhaust gas purification system according to claim 1 or 11, characterized in that the return line ( 6 ) opens onto an evaporator pan ( 10 ) which is arranged above the nitrogen oxide removal device ( 4 ). 13. Exhaust gas purification system according to claim 12, characterized in that the evaporator pan ( 10 ) has at least one opening ( 11 ) for the passage of liquid to the nitrogen oxide removal device ( 4 ).