DE212015000170U1 - Exhaust after-treatment system for a diesel engine - Google Patents

Abstract

An exhaust aftertreatment system for a diesel engine, the system comprising: a selective catalytic reduction (SCR) converter section, an ammonia slip catalyst (ASC) section, a diesel oxidation catalyst (DOC) section and a diesel particulate filter (DPF) section, characterized in that the system is set up so that both urea and diesel are injected before the SCR-ASC-DOC-DPF system.

Description

The present invention relates to a new exhaust aftertreatment system for a diesel engine, the system comprising a selective catalytic reduction (SCR) converter section, an ammonia slip catalyst (ASC) section, a diesel oxidation catalyst (DOC) section, and a diesel particulate filter (DPF) section , The invention thus relates to a system for reducing nitrogen oxide (NOx) and particulate matter present in the exhaust gases of a diesel engine. In particular, the system according to the invention provides improved NOx reduction during cold start of the engine.

The system according to the invention is of particular interest to vehicles such as trucks and passenger cars with diesel post-injection options.

Legislation in the field of engine exhaust requires very low emissions from HD (Heavy Duty) trucks and passenger cars. At the same time, the test cycles used become colder. It is usually difficult to reduce NOx in these cold cycles, especially as so-called aftertreatment systems with many different functions such as hydrocarbon oxidation, soot filtration, NOx reduction and ammonia slip reduction become increasingly complicated. The most common and practicable system consists of components in the following order: oxidation catalyst, soot filter, NOx reduction catalyst, and ammonia slip catalyst.

At the heart of the Euro 6 standard stand exhaust aftertreatment systems that reduce NOx emissions. Selective catalytic reduction SCR systems that use a reducing agent (urea / water solution) that is injected in a targeted manner and evenly distributed in the exhaust system have the potential for high conversion rates. This broadens the scope for the treatment of raw nitrogen oxide emissions and is likely to have a corresponding positive impact on fuel economy and CO ₂ emissions. Total exhaust aftertreatment can be significantly improved by increasing the exhaust gas temperature upstream of the DOC and the SCR catalysts.

Several factors can affect the efficiency of an SCR system. This includes important limiting operating conditions, such as Example, the evaporation quality of the urea / water solution or the degree of uniform distribution of ammonia formed by the urea / water solution upstream of the SCR catalyst, as well as criteria that affect the catalysts themselves, including the DOC and the SCR catalyst ,

The conventional exhaust system of modern cars with lean-burn engines is with a. Sequence of an oxidation catalyst, a particulate filter and a catalyst for the selective catalytic reduction of NOx (SCR) equipped in the presence of a reducing agent.

Oxidation catalysts active in the oxidation of volatile organic compounds and carbon monoxide and SCR catalysts are known in the art and disclosed in numerous publications.

Commonly used particulate filters are the so-called wall-flow filters with a multiplicity of input and output channels. The input channels are closed at their output and the output channels are closed at their entrance so that the gas flowing into the filter is forced through the porous walls defining the channels, filtering particulate matter out of the gas.

In the SCR treatment, ammonia is usually used as the reducing agent. Ammonia is a detrimental compound and it is preferred to produce ammonia in situ by thermal decomposition of a urea solution injected into the hot exhaust as the ammonia precursor upstream of the SCR catalyst.

Although urea is harmless and relatively easy to store in a car, the use of a liquid urea solution as a precursor to an ammonia reducing agent is problematic, especially in the cold start phase of the engine, i. H. if the exhaust gas temperature is below 200 ° C.

When urea is injected as a liquid solution into the exhaust gas, it does not decompose to ammonia until the temperatures reach approximately 200 ° C in sufficient amounts for SCR.

The most difficult to handle emission is NOx, and to achieve a good cycle result, the NOx reduction must be started early in the certification cycle. In the conventional exhaust gas purification systems described above, the SCR catalyst is placed as the third exhaust purification unit in the systems, and it takes some time after the engine is started for this catalyst to reach its reaction temperature.

The present invention provides the second generation of the High Efficiency Exhaust Aftertreatment System (High Efficiency Exhaust Aftertreatment, HEAT) of the applicant, which was developed a few years ago. The system is designed for Euro 6 and beyond, which means it includes components for NOx reduction (DeNOx), ammonia slip removal, hydrocarbon and CO oxidation, and particulate filtering.

1 shows the conventional Euro 6 system, in which the particulate filtering is located upstream of the DeNOx components, while the in 2 shown first generation of the HEAT system is characterized in that the particle filtration is arranged after the DeNOx components.

The present invention, ie the second generation of the HEAT system, is disclosed in U.S. Pat 3 shown. The main difference between the first generation of the HEAT system (HEAT-1) and the present invention (HEAT-2) is that in HEAT-1, urea is injected between the engine and the muffler, while the diesel is injected at a point between SCR and ASC and DOC-DPF is injected. In HEAT-2, both urea and diesel are injected upstream of the SCR-ASC-DOC-DPF system.

A number of prior art publications describe exhaust aftertreatment systems. Thus, the applicant's HEAT-1 system is in EP 2 399 011 discloses a method for purifying exhaust gases of a diesel engine by removing particulates, incompletely combusted hydrocarbons, carbon monoxide and nitrogen oxides (NOx), with a focus on a simple but effective method of filter regeneration.

In US 6,863,874 For example, there is described a method by which impurities in an exhaust gas are removed by oxidation, followed by a filter where carbon black is oxidized by nitrogen dioxide and oxygen. Further downstream, a reducing agent is injected into a NOx absorber, and then a three-way catalyst or catalyst for selective reduction is installed.

US 8,484,954 describes a method and apparatus for selectively purifying the exhaust gas of an internal combustion engine using an SCR catalytic converter, wherein a reductant is metered into the exhaust gas prior to entering the catalytic SCR converter. An overdose of the reductant is added to the exhaust which, after leaving the SCR catalytic converter, is fed in part to a second catalytic SCR converter through a catalytic oxidation converter and partly via a bypass bypassing the catalytic oxidation converter becomes. In the catalytic oxidation converter, one-half of the excess NH ₃ discharged from the first SCR catalytic converter is converted to NO x and reacted to be harmless after mixing with the other half of the excess NH ₃ discharged unchanged by diverting To form nitrogen.

US 2011/0047994 discloses an exhaust purification device including an exhaust pipe, a first oxidation catalyst, a selective catalytic reduction catalyst, an oxidation-reduction catalyst, and a urea-water supply device. Exhaust gas is passed through the first oxidation catalyst disposed in the exhaust passage. The selective catalytic reduction catalyst is disposed downstream of the first oxidation catalyst. The oxidation-reduction catalyst is disposed downstream of the selective catalytic reduction catalyst. The oxidation-reduction catalyst has reducing properties and oxidizing properties that are influenced by the temperature, wherein the oxidizing properties of the oxidation-reduction catalyst are greater than the reducing properties of the oxidation-reduction catalyst at a temperature higher than a temperature at which the reducing properties of the oxidation-reduction catalyst are larger as the oxidizing properties of the oxidation-reduction catalyst. The urea / water supply device supplies urea / water upstream of the selective catalytic reduction catalyst.

Out DE 10 2004 049 289 For example, an exhaust aftertreatment system for a diesel engine is known. The system includes in series a selective nitrogen oxides reduction section, an oxidation catalyst section, and a diesel particulate filter section. Supply lines for supplying aids, such. As ammonia and urea, are arranged in the flow direction of the exhaust gas upstream of the SCR catalyst section. The SCR is disposed upstream of the oxidation catalyst portion, which in turn is installed in the flow direction of the exhaust gas upstream of the particulate filter portion. The system configuration is thus urea-SCR-DOC-DPF in the flow direction of the exhaust gas.

As already mentioned, the configuration of the system according to the invention (HEAT-2) differs from conventional Euro 6 systems with particle filtering upstream of the DeNOx components as well as of the first generation HEAT system (HEAT-1) with the Particle filtering downstream of DeNOx components.

In the conventional system, exhaust gas from the engine flows to an oxidation catalyst (DOC). Then the oxidized exhaust gas is directed to the diesel particulate filter (DPF). Thereafter, a reducing agent such. For example, urea, usually an aqueous solution of urea, is injected into the gas and the mixed gas is then introduced into a catalyst for selective catalytic reduction (SCR), from where the exhaust gas passes through an ammonia slip catalyst (ASC) and the system as a purified Gas leaves.

The DOC is a diesel oxidation catalyst containing Pt for NO ₂ formation and the DPF is a catalyzed soot filter that may also contain Pt for NO ₂ formation. The SCR catalyst is typically a zeolite-based catalyst.

The HEAT-1 system has the reverse configuration, that is, compared with the conventional system. H. the SCR-ASC before the DOC-DPF, and offers advantages over the conventional system, especially with regard to effective soot filter regeneration. It also ensures high NOx conversion during cold start.

• (1) Alle Abgassystemkomponenten (”Bausteine”) können direkt hintereinander eingebaut werden, d. h. SCR-ASC-DOC-DPF.
• (2) Die Bausteine können zusammen eingeschlossen, d. h. in das gleiche Gehäuse eingebaut werden. Die folgenden Bedingungen (3) und (4) müssen erfüllt sein, damit die Bedingungen (1) und (2) zutreffen:
• (3) Sowohl Harnstoff als auch Diesel können zwischen dem Motor und dem Schalldämpfer eingespritzt werden.
• (4) Die SCR kann Diesel-Kohlenwasserstoffe, die gelegentlich eingespritzt werden müssen um die Abgastemperatur zur DPF-Regenerierung zu erhöhen, aushalten und teilweise oxidieren.

The HEAT-2 system of the present invention is novel in several respects both compared to conventional systems and to HEAT-1:

• (1) All exhaust system components ("building blocks") can be installed one after the other, ie SCR-ASC-DOC-DPF.
• (2) The devices can be enclosed together, ie installed in the same housing. The following conditions (3) and (4) must be satisfied for conditions (1) and (2) to be satisfied:
• (3) Both urea and diesel can be injected between the engine and the muffler.
• (4) The SCR can withstand and partially oxidize diesel hydrocarbons which occasionally need to be injected to increase the exhaust gas temperature for DPF regeneration.

• (5) Der DOC kann entfernt werden, womit ein vereinfachtes System verbleibt, das nur aus SCR-ASC-DPF besteht.

If sufficient oxidation of hydrocarbons and carbon monoxide is achieved via the SCR and ammonia slip catalyst, it is potentially possible to avoid, ie remove, the DOC. This means:

• (5) The DOC can be removed, leaving a simplified system consisting only of SCR-ASC-DPF.

• – ein bedeutend reduzierter Systemdruckabfall ΔP,
• – ein bedeutend reduziertes Systemvolumen,
• – eine erhöhte Kaltstartleistung verglichen mit herkömmlichen Euro 6-Systemen,
• – bedeutend reduzierte Kosten aufgrund einfacher Herstellung und potenzieller Entfernung des Pt-enthaltenden DOC, und
• – die Möglichkeit, das System als Erweiterung oder Modifikation von SCR-Systemen gemäß Euro 4/5 basierend auf V₂O₅ zu verwenden.

The system according to the invention offers the following advantages over its predecessor (HEAT-1) as well as the conventional systems adapted to Euro 6:

• A significantly reduced system pressure drop ΔP,
• A significantly reduced system volume,
• An increased cold-start performance compared to conventional Euro 6 systems,
• Significantly reduced costs due to ease of preparation and potential removal of the Pt-containing DOC, and
• - the possibility of using the system as an extension or modification of Euro 4/5 SCR systems based on V ₂ O ₅ .

QUOTES INCLUDE IN THE DESCRIPTION

This list of the documents listed by the applicant has been generated automatically and is included solely for the better information of the reader. The list is not part of the German patent or utility model application. The DPMA assumes no liability for any errors or omissions.

Cited patent literature

• EP 2399011 [0016]
• US 6863874 [0017]
• US 8484954 [0018]
• US 2011/0047994 [0019]
• DE 102004049289 [0020]

Cited non-patent literature

• Euro 6 standard [0004]

Claims (4)

An exhaust aftertreatment system for a diesel engine, the system comprising: a selective catalytic reduction (SCR) converter section, an ammonia slip catalyst (ASC) section, a diesel oxidation catalyst (DOC) section and a diesel particulate filter (DPF) section, characterized in that the system is set up so that both urea and diesel are injected before the SCR-ASC-DOC-DPF system. The system of claim 1, wherein all exhaust system components are mounted directly one after another as a contiguous SCR-ASC-DOC-DPF configuration. An exhaust aftertreatment system for a diesel engine, the system consisting of a selective catalytic reduction (SCR) converter section, an ammonia slip catalyst (ASC) section and a diesel particulate filter (DPF) section, characterized in that the system is arranged to use both urea and Also, diesel are injected upstream of the SCR-ASC-DPF system and sufficient oxidation of hydrocarbons and carbon monoxide is achieved via the SCR converter section and the ammonia slip catalyst. The system of claim 3, wherein all exhaust system components are mounted directly one after another as a contiguous SCR-ASC-DPF configuration.

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