DE19636041A1 - Waste gas purification process - Google Patents

Abstract

Waste gas purification process comprises feeding the waste gas of a self-ignition I.C. engine to a NOx storage suitable for storing NOx from the waste gas under first operating conditions, and releasing the stored NOx to reduction under second operating conditions. The novelty is that the waste gas, before it is fed to the NOx storage under the first operating conditions, is fed through a converter (5), in which a NOx/NO ratio is increased in the waste gas and/or at \- 230 deg C, \-50% of the NO contained in the waste gas and fed with the waste gas into the converter is converted to NO2.

Description

The invention relates to an exhaust gas purification method for a self-igniting Internal combustion engine according to the preamble of claim 1 and a Self-igniting internal combustion engine according to claim 22.

The reversible storage of nitrogen oxides and sulfur oxides is known from US Pat. No. 4,755,499 e.g. B. known from exhaust gases from motor vehicles, the absorber by heating in a reducing atmosphere is regenerated. At the same time, there is a reduction in Nitrogen oxides.

Such a storage catalytic converter is used in EP 0 580 389 A Motor vehicles described in more detail, with high temperatures (above 500 ° C) for regeneration of the absorber is necessary. This is the use of Storage catalytic converter only possible in motor vehicles that have a high exhaust gas temperature have, d. H. especially in motor vehicles with an Otto engine. However, here too use is only possible to a limited extent, since under certain operating conditions the Internal combustion engine, such as are given for example in city traffic the acceleration phases result in high nitrogen oxide emissions, but not high ones Temperature is reached, which is essential for the regeneration of the absorber Sulfur oxides, is required.

EP 0 560 991 A describes a gasoline engine with a storage catalytic converter and one each upstream and downstream three-way catalyst known. Due to the arrangement close to the engine of the three-way catalytic converter upstream of the absorber heats up after a Cold start of the gasoline engine very quickly, so that it early its catalytic activity reached. After reaching its catalytic activity, the upstream one sets Three-way catalyst that increases during the warm-up phase of the gasoline engine HC and CO present, while NOx is reduced at the same time. This ensures  that during a warm-up phase of the gasoline engine an NOx reduction in Exhaust gas takes place even though the NOx storage is not yet the one for storing NOx has reached the necessary temperature. This arrangement and procedure is only for Otto engines make sense, since diesel engines do not work even during the warm-up phase emit enough CO and HC to reduce the NOx content of the exhaust gases sufficiently. Apart from the better exhaust gas cleaning during the warm-up phase, the Otto-Mo shows Tor with pre-catalyst no improved NOx reduction. Furthermore are from this Publication of two methods for NOx reduction in diesel engines known, on the one hand on a throttling of the air supply to the Diesel engine and on the other hand based on a fuel injection. Diesel engines, however, which are equipped with such NOx stores, show at higher exhaust gas temperatures a significant decrease in NOx storage in the NOx storage.

The object of the invention is one for use with diesel internal combustion engines suitable absorbers for nitrogen oxides and a corresponding process places with which a high NOx reduction should be possible overall.

In the method described in the introduction, this object is achieved with the characterizing measures of claim 1, with respect to the internal combustion engine solved the problem with the features of claim 22.

The subclaims show preferred embodiments, with which in particular also at very low exhaust gas temperatures, such as those with direct injection Internal combustion engines are present, an early use of the absorber function after a Cold start is possible.

In the exhaust gas method according to the invention on a self-igniting Internal combustion engine (diesel engine) are the exhaust gases of the Internal combustion engine is supplied to a NOx accumulator that operates under first operating conditions NOx from the supplied exhaust gas is stored. The first operating conditions are especially those in which the net oxidizing conditions (λ <1 and in particular λ <1.1), the temperature of the exhaust gas stream being above 150 ° C. and in particular should be above 200 ° C. Under second operating conditions by  the NOx accumulator gives the first operating conditions stored NOx again, this reducing in particular immediately or immediately becomes. The second operating conditions are particularly those in which the exhaust gas a sufficient amount for the reduction of the stored nitrogen oxides Reducing agent with it. This is particularly the case with a λ (stoichiometric air Fuel ratio) 1.05 and in particular λ 1.0 the case. According to the invention found that the NOx reduction can be increased significantly if the A converter is connected upstream of the NOx storage in the exhaust gas flow Temperature 230 ° C at least 50% of the NO contained in the exhaust gas flow to NO₂ implements. The converter preferably already achieves this degree of implementation at one Temperature 200 ° C and especially at the temperature 180 ° C. Usually such converters achieve an at least 90% conversion of the NO at one Temperature 250 ° C.

Alternatively, but in particular in addition, the converter is used in the exhaust gases Diesel engine present NO₂ / NO ratio increased, which the subsequent storage especially in the warm-up phase of the diesel engine less NO is supplied.

For such NOx treatments, all methods are suitable that increase the NO₂- Share in the nitrogen oxides, for example, electrical discharges in the Exhaust system, preferably barrier discharges, and catalytic processes, in particular Oxidation catalysts. These include, in particular, oxidation catalysts an element of the platinum group and here again platinum itself is particularly preferred. Such catalysts are principally used as exhaust gas aftertreatment catalysts Diesel engines known.

In contrast to the use of a three-way catalytic converter upstream of the NOx storage in Otto engines it is in the method according to the invention in the emission control of Diesel engines do not need the converter for the reducing agent Nitric oxide reduction must be supplied, the minimum amounts of CO and HC, that are present in the diesel exhaust gases are not harmful. The method according to the invention Rather, it is based on the fact that the NOx storage has a low NO flow NO₂-rich exhaust gas the physisorption (adsorption) of NO at relatively low  NOx storage temperatures, i. H. especially at temperatures of 200 ° C and is reduced especially at temperatures of 180 ° C. According to the invention, it was found that the NO partially blocks the NOx storage by chemisorption (Absorption) of NO₂ is reduced by the deposited NO. By the NOx storage upstream converter is achieved in the method according to the invention, that the physical absorption phase of NO present in diesel engines when cold started is strong is reduced.

The significant NOx reduction according to the invention from the diesel engine the surrounding exhaust gas is presumably caused by the following reaction sequence reached:

• - from about 150 ° C, the NOx storage absorbs NO₂;
• - The NOx storage NO is stored up to approx. 180 ° C;
• - From about 200 ° C on the NOx storage there is an oxidation of NO to NO₂, provided that NOx storage has a precious metal coating (platinum);
• - The NO storage blocks NO₂ storage, which, especially at high Space velocities, the NOx storage capacity of the NOx storage decreases;
• - At temperatures above approx. 250 ° C, desorption of NO takes place, possibly below Oxidation of the NO to NO₂, with a significant part of the desorbed NOx because of partial blocking of the memory cannot be immediately saved as NO₂ again can and is emitted outdoors;
• - The converter located close to the engine quickly reaches its NO to NO₂- Conversion temperature of approx. 200 ° C before the NOx accumulator is out NO storage temperature reached, as a result, the NOx storage is almost no NO supplied in the phase in which the NOx accumulate NO but not NO₂ can implement.

Due to the different mode of operation of Otto engines, this occurs according to the invention described problem does not arise.

The specified temperatures can vary by 20 to depending on the version of the NOx storage Deviate 50 K or more. The temperatures refer to the exemplary Response sequence to at least 50% implementation or storage.

According to the invention, therefore, by avoiding NO storage in the NOx storage reduces NOx emissions. This can be seen, for example, in an MVEG Test with a direct-injection 66 kW diesel engine with a displacement of 1.9 l. There is a strong desorption in the without using a NO / NO₂ converter first part of the EUDC section, this effect does not occur with a pre-catalyst. Through the Relocation of the NO oxidation to the pre-catalyst is at the NOx storage at higher Temperatures and space velocities also have a higher NOx (NO₂) storage rate achieved because the NOx storage is no longer sluggish in addition to the comparative one Sorption reaction also requires the oxidation of NO to NO₂.

According to the invention, the converter is close to the exhaust gas outlet of the diesel engine arranged, d. H. advantageously at a distance of 1 m and in particular at a distance  70 cm.

The converter is particularly advantageously designed as a metal converter, i. H. the carrier for the catalytically active layer is made from a metal foil. Is preferred here a metal foil with a thickness of 50 microns and in particular with a thickness of  40 µm used, which means that the converter heats up particularly quickly Operating temperature is guaranteed. It has also been found according to the invention found that the converter preferably has a total volume of 10 to 25% and has in particular from 15 to 20% of the engine displacement of the diesel internal combustion engine, because optimal NOx cleaning values can be achieved with this ratio. Still has the converter preferably has a platinum load of 60 g / ft³. This dimensioning of the The converter is significantly different from that usually used in diesel engines Oxidation catalysts used in the exhaust gas flow, which are usually loaded with platinum of significantly less than 50 g / ft³ on ceramic substrates. The absorption memory has however, advantageously a lower platinum occupancy, i. H. especially with 50 g / ft³ platinum.  

As already stated, the invention is particularly suitable for use with direct-injection diesel engines, because at their exhaust gas temperatures with the inventive method the highest cleaning effect is achieved.

According to the invention, the usual absorbent materials can be used, such as for example in US 4,755,499, but also in EP 0 580 389 A or WO 94- 04258 are described. All these storage materials have in common that they are one have an increased working temperature, especially when regenerating (especially an even higher temperature is required when removing the sulfur oxides). Both Most storage media of this type have temperatures in the range of 150 ° C to 700 ° C, especially temperatures above 300 ° C required. Such temperatures occur in vehicles with Otto engines usually, but are in Diesel vehicles and especially direct injection Diesel engines are relatively rare.

The preferred NOx storage materials are characterized in that they are under net oxidizing conditions (stoichiometric excess of oxidizing agents), such as they are present in the exhaust gas of diesel engines, temporarily store nitrogen oxides and at one Reduction of excess oxygen can reduce. For this are the NOx storage catalytic converters usually also coated with precious metals, especially with the usual precious metal coatings for three-way catalysts. The regeneration of the Storage material loaded with NOx then advantageously takes place at λ 1 in one Regeneration phase.

Different reactions usually run on the NOx storage catalytic converters successively until at the same time, the most important reactions

• - Oxidation of the NO in the exhaust gas to NO₂
• - Storage of the NO₂ as nitrate
• - decomposition of the nitrate  
• - Reduction of the NO₂ re-formed to nitrogen and oxygen

are.

As described above, the course of the reactions depends, among other things, on the Temperature of the catalyst, but also from the concentration of the reactants on active center of the catalyst and the flow rate of the gas.

With different factors that can be combined with each other, it is also with only little effort possible to optimize the exhaust gas absorber, especially for direct-injection diesel engines. The main features are:

• - Reduction of the wall thickness of the support body on which the absorption layer is applied to 160 microns, especially 140 microns;
• - Use of metal supports, preferably with a wall thickness of 50 microns, preferably  40 µm and especially 30 µm; and or
• - Heating the absorber to a temperature above the temperature of the exhaust gas stream.

It has been shown that when using thin-walled ceramic supports for the Absorption layer, d. H. in particular of support bodies with a wall thickness of 0.14 mm, not only a faster temperature rise of the absorption layer is possible, but a thicker absorption layer can also be used. This will do two things achieved: on the one hand, short high-temperature phases can be regenerated can be exploited because the storage layer assumes the higher temperature faster, and on the other hand, a thicker absorption layer can be used to apply a higher one Storage capacity can be achieved, so that the longer storage capacity of the Absorbers during the operation of the internal combustion engine for a longer period of time can elapse until the memory is to be regenerated, so that despite the rarer occurring temperature peaks in the exhaust gas flow from consumption-optimized Internal combustion engines no breakthrough of the memory (reaching the Saturation limit).  

In particular, absorbers with a carrier body made of metal foil are suitable, the Metal foil can advantageously be switched as resistance heating, so that too at low exhaust gas temperatures, the absorber to the necessary Regeneration temperature by passing an electrical current through the Metal support body can be brought. In addition, when using a Metal carrier body the channels that are coated with the absorption layer design differently so that, for example, turbulence (turbulent flow) the exhaust gas flow in the channels can be specifically adjusted.

The absorption layer has an enlarged layer to achieve particularly good sales Surface area of at least 20 m² / g, in particular at least 40 m²1g. Has the advantage Absorbent layer preferably a pore volume of at least 0.2 cm³1g and in particular at least 0.4 cm³ / g, with a bimodal pore size distribution is suitable with micropores and macropores. This is done, for example, by choosing one certain particle size for the formation of the absorber surface reached, also Mixtures or certain distributions of different particle sizes are suitable.

Γ-Alumina is particularly suitable as the absorption surface, which is coated with an or several elements from the group of alkali metals, alkaline earth metals, rare earths and / or lanthanum is loaded. Copper and manganese are also suitable elements. The Elements are usually present as an oxide, but also as carbonate or nitrate, the Storage effect is achieved by the formation of appropriate nitrates and sulfates, which then under the appropriate reaction conditions to oxides or carbonates be transferred. This makes it possible to in particular NOx and / or SOx from one Absorb exhaust gas containing at least 1% oxygen.

As described, the absorbed substances are particularly increased Temperatures and released in a reducing atmosphere. For this it is advantageous if the oxygen concentration is determined in the exhaust gas, then the Oxygen concentration or one known to the oxygen concentration standing size for controlling the absorption or desorption process can be used. The same applies to the temperature of the Exhaust gas flow, the decisive factor being the temperature of the absorption layer is determined directly or indirectly. For example, the temperature can be  Measurement of the temperature of the exhaust gas flow or the carrier body can be measured; is also a temperature determination via a map of the internal combustion engine possible.

The absorption layers are preferably of a thickness of at least 50 μm, in particular at least 70 µm and particularly advantageously at least 90 µm (Average layer thickness of a cross-section; values apply to ceramics and metal apply half the values) whereby this layer thickness of the absorption layer overlaps preferably at least 50% and in particular at least 80% of the absorber extends. Such layer thicknesses enable a compared to conventional absorbers higher storage capacity and thus the longer intervals described above up to Regeneration.

As for the release and implementation of the NOx from the memory and the release of the Sulfur oxides from storage different temperatures are necessary (at the latter higher), can also be operated so that a desorption of the Sulfur oxides (which are present in particular as sulfate) in longer periods or at Need is made so that the memory is only occasionally on the high Temperatures are heated, which are necessary for a desorption of the sulfur oxides. Also this counteracts premature aging of the memory, so that a particularly good long-term stability of the absorber is achieved.

Contains the diesel engine with exhaust gas purification belonging to the invention advantageous the features described above.

The invention is described below using an exemplary embodiment and drawings described in more detail.

Show it:

Fig. 1 is a diesel engine with exhaust gas purification; and

Fig. 2 shows a NOx storage behavior with and without a converter.

The internal combustion engine shown in FIG. 1 1 (1.9 l, 4 cyl., Diesel direct injection, 66 kW) has an air intake passage 2 and an exhaust system 3. An exhaust gas recirculation line 4 leads from the exhaust system 3 to the air inlet 2 , by means of which overall a reduction in the raw NOx emissions takes place.

A converter 5 is arranged in the exhaust system 3 close to the engine and has a volume of 15% of the displacement of the diesel internal combustion engine 1 . The distance between the exhaust gas outlet 6 and the converter 5 is approximately 20 cm. In addition, a conventional NOx storage catalytic converter 7 is arranged in the exhaust system 3 about 70 cm after the converter 5 , after which the exhaust gases are released.

The converter 5 has a metal foil carrier body on which a γ-aluminum oxide washcoat with a platinum loading of 70 g / ft³ is applied. The NOx storage catalytic converter is made up of a honeycomb-shaped ceramic carrier on which a γ-alumina washcoat with barium, lanthanum and sodium is applied. The washcoat storage catalytic converter also has a platinum loading of 46 g / ft³.

In FIG. 2, the NOx storage performance of the NOx storage catalytic converter 7 is shown, with and without the use of the converter. 5 The storage capacity was recorded in the MVEG test. It shows that the NOx storage behavior of the storage catalytic converter 7 is about ² / ₃ the test time without using the converter 5 is slightly higher than when using the converter, which is due to the NO adsorption of the NOx storage catalytic converter. However, as soon as higher exhaust gas temperatures are reached in the test, the storage catalytic converter 7 shows stagnation 8 during NOx storage. This stagnation 8 is probably a blockage of the NO₂ absorption at relatively high space velocities by adsorbed NO. With a further increase in the NOx storage catalyst temperature and a further increase in the space velocity of the exhaust gases, there is a sharp drop in the NOx storage behavior, resulting in a decrease 9 in the stored NOx. This can presumably be attributed to a desorption of NO, which can then no longer be oxidized to NO₂ and stored due to the high space velocities and due to the still existing storage blockage due to the fact that NO is still attached.

By using the pre-catalyst 5 , an increase in NOx storage is obtained even at high exhaust gas temperatures and high space velocities.

Claims (24)

1. Exhaust gas purification method in a self-igniting internal combustion engine, in which the exhaust gases of the self-igniting internal combustion engine are supplied to a NOx storage device which is suitable for storing NOx from the supplied exhaust gas under first operating conditions and for releasing the stored NOx from it under second operating conditions in order to reduce it is characterized in that the exhaust gas, before it is supplied to the NOx storage under the first operating conditions, is passed through a converter in which a NO₂ / NO ratio present in the exhaust gases is increased and / or in which at a temperature 230 ° C at least 50% of the NO contained in the exhaust gas and led into the converter with the exhaust gas is converted to NO₂. 2. The method according to claim 1, characterized in that as a converter Oxidation catalyst, in particular a platinum-containing catalyst is used. 3. The method according to claim 2, characterized in that the converter is a Has a platinum loading of 60 g / ft³. 4. The method according to any one of the preceding claims, characterized in that the converter close to the exhaust gas outlet of the self-igniting internal combustion engine, is arranged in particular at a distance of 1 m. 5. The method according to any one of the preceding claims, characterized in that the stored NOx is reduced at λ 1.05. 6. The method according to any one of the preceding claims, characterized in that the converter has a metal carrier body.   7. The method according to claim 6, characterized in that the metal carrier body a metal foil with a thickness of 50 microns is constructed. 8. The method according to any one of the preceding claims, characterized in that the converter has a total volume of 10 to 25% of the cubic capacity has self-igniting internal combustion engine. 9. The method according to any one of the preceding claims, characterized in that the self-igniting internal combustion engine is a direct-injection diesel engine. 10. The method according to any one of the preceding claims, characterized in that the converter during a warm-up phase of the auto-ignition Internal combustion engine achieves a conversion rate of NO to NO₂ of at least 50%, before the NOx storage has reached a temperature at which it is among the first Operating conditions the NO₂ content of the exhaust gas supplied to the NOx storage reduced by at least 75%, in particular by at least 50%. 11. The method according to any one of the preceding claims, characterized in that the NOx storage has an increased surface area of at least 20 m² / g, in particular has at least 40 m² / g and particularly advantageously has at least 100 m² / g, based on the storage mass achievable by the exhaust gas flow. 12. The method according to any one of the preceding claims, characterized in that the NOx storage contains an aluminum oxide, in particular γ-aluminum oxide. 13. The method according to any one of the preceding claims, characterized in that the NOx storage element from the group of alkali metals, alkaline earth metals, contains rare earths, lanthanum, titanium, copper and / or manganese. 14. The method according to claim 13, characterized in that the NOx storage contains at least one of the elements barium, sodium, potassium.   15. The method according to any one of the preceding claims, characterized in that the NOx trap absorbs NOx and SOx from the exhaust gas when there is an excess of oxygen. 16. The method according to any one of the preceding claims, characterized in that the NOx storage NOx and / or SOx in a reduced atmosphere and / or at releases low oxygen concentration. 17. The method according to claim 15 and / or 16, characterized in that a Oxygen concentration determination device that measures the oxygen concentration or a variable containing the oxygen concentration is determined, is provided the oxygen concentration or the quantity containing it as an input quantity gives to the controller that causes loading or unloading of the NOx storage. 18. The method according to any one of the preceding claims, characterized in that the NOx storage releases NOx and / or SOx at elevated temperature. 19. The method according to claim 18, characterized by a temperature Determination device which contains the temperature or a temperature Size of the gas flow and / or the NOx storage determined and the temperature or the variable containing this as an input variable to the controller that a Loading or unloading of the NOx storage causes. 20. The method according to claim 17 and 19, characterized in that the control the Oxygen concentration and the temperature or the quantities containing them as Has input variables. 21. The method according to any one of the preceding claims, characterized in that an oxidation catalytic converter, in particular a three-way catalytic converter is connected downstream as an independent unit. 22. Self-igniting internal combustion engine with an exhaust system that has a NOx storage contains, which is suitable under the first operating conditions NOx from a stored exhaust gas of the self-igniting internal combustion engine and save the stored NOx for reduction under the second operating conditions  the same is releasable, characterized in that between the NOx storage and an exhaust gas outlet of the internal combustion engine arranged a converter which is a total volume in the range of 10 to 25% of the cubic capacity Internal combustion engine has, and that the converter uses at least 60 g / ft³ platinum is. 23. Self-igniting internal combustion engine according to claim 22, characterized in that that the converter has a metal support body. 24. Self-igniting internal combustion engine according to claim 22 or 23, characterized characterized in that it is a direct injection diesel engine.