FR3066541A1 - EXHAUST GAS POST-PROCESSING SYSTEM OF AN INTERNAL COMBUSTION ENGINE - Google Patents

Abstract

The invention relates mainly to a system for post-treatment (10) of the exhaust gases of an exhaust line (11) of an internal combustion engine, in particular of a motor vehicle, comprising: - an organ an oxidation catalyst (13), - a first catalyst element (16) for selective catalytic reduction of the nitrogen oxides, - a particulate filter (17), - a first injector (14) of a reducing agent positioned upstream. of the first catalyst element (16) for selective catalytic reduction of nitrogen oxides, characterized in that the after-treatment system (10) further comprises: a second catalytic reduction member (21) for catalytic selective reduction of the oxides of nitrogen; nitrogen, and - at least a second injector (24) of reducing agent positioned upstream of the second catalyst member (21) selective catalytic reduction of nitrogen oxides.

Description

EXHAUST GAS AFTER-TREATMENT SYSTEM

OF AN INTERNAL COMBUSTION ENGINE The present invention is in the field of depollution of exhaust gases from an internal combustion engine. More specifically, the invention relates to an after-treatment system for the exhaust gases of an internal combustion engine. The invention finds a particularly advantageous, but not exclusive, application with the exhaust lines of internal combustion engines of the diesel type.

Pollutant emissions from internal combustion engines fitted to motor vehicles are regulated by standards. The regulated pollutants are, depending on the internal combustion engine technology considered, carbon monoxide (CO), unburnt hydrocarbons (HC), nitrogen oxides (NOx), and particles, which are formed during combustion. fuel in the combustion chamber then emitted to the exhaust.

It is known to use a number of pollution control means in the exhaust line of internal combustion engines to limit the emission of pollutants. An oxidation catalyst (or DOC for Diesel Oxidation Catalyst in English) allows the treatment of carbon monoxide, unburnt hydrocarbons, and under certain conditions nitrogen oxides. In addition, a particulate filter can be used for the treatment of soot particles.

For vehicles with diesel engines, selective catalytic reduction technologies (or SCR for Selective Catalytic Reduction in English) are known which consist in reducing nitrogen oxides by introducing a reducing agent (or a precursor of such a reducer) in the exhaust gas by catalyzed reactions. It may for example be a urea solution, the decomposition of which will make it possible to obtain ammonia which will serve as a reducing agent, but also a reducing agent or a precursor of such a reducing agent under gaseous form. The reducing agent generated makes it possible to reduce the nitrogen oxides by reaction in an SCR catalyst member, that is to say a substrate carrying a catalytic impregnation capable of promoting the reduction of nitrogen oxides by the reducing agent.

It is for example known from document FR3021695 an after-treatment system 1 illustrated in FIG. 1, grouping together in the same envelope 2 several pollution control elements which will be successively crossed by the exhaust gases. It is proposed, in particular, a series of depollution organs comprising from upstream to downstream: an oxidation catalyst member 3, an injector 4 of urea-type reducing agent, a mixer 5 whose role is to intimately mix the urea droplets injected into the casing 2 through which the gases pass, so as to decompose into ammonia as homogeneously as possible over the entire cross section of the casing 2, a catalyst member for selective catalytic reduction 6, and a particulate filter 7 impregnated with a catalyst for reducing nitrogen oxides and which thus fulfills both the function of soot filter and reduction of nitrogen oxides (called SCRF thereafter).

It is observed that despite the effectiveness of such a system 1 for the treatment of nitrogen oxides, the fact of having recourse to several depollution organs has drawbacks. Indeed, for the SCRF 7 member located downstream of the SCR 6 catalyst member to function, it is necessary for this member 7 to have stored a quantity of reducing agent, namely ammonia (NH ₃ ). If this is not the case, the nitrogen oxides which pass through it cannot be converted. However, the SCR 6 catalyst member located upstream of the member

SCRF 7 converts nitrogen oxides with ammonia according to the stoichiometry of the following reactions:

NO + O ₂ + 4 NH ₃

NO + NO ₂ + 2 NH ₃

NO ₂ + 8 NH ₃ to 4 N ₂ + 6 H ₂ O to 2 N ₂ + 3 H ₂ O

N ₂ + 12 H ₂ O

Standard SCR

SCR with fast kinetics

High temperature SCR [0007] The treatment efficiency of the SCR 6 catalyst member being very high (greater than 80%), the quantities of residual ammonia leaving this SCR 6 catalyst member and therefore entering the member SCRF 7 are weak or even zero. The SCRF 7 member, which therefore contains little ammonia adsorbed in its catalytic coating, is therefore not able to correctly treat the residual nitrogen oxides originating from the SCR 6 catalyst member and which can be emitted by the Internal combustion engine in large quantities especially during heavy load phases.

To solve this problem, it would be possible to inject more urea upstream of the SCR 6 catalyst member (due to the positioning of the urea injector) to saturate the latter and develop a leak downstream. of ammonia (known as NH ₃ slip in English) enabling the SCRF 7 organ to be filled. However, the SCR 6 catalyst organ having good ammonia storage capacity, the quantities of urea to be injected should be high with a significant risk of ammonia leaking to the exhaust cannula during heavy load or regeneration phases of the particulate filter during which the temperatures are then higher and conducive to thermo-desorption. Indeed, under these conditions, the ammonia storage capacity by the SCRF 7 member is rather low.

Furthermore, under these conditions, the temperature in the SCR 6 catalyst member exceeds 450 ° C to 500 ° C, which promotes the readbn of oxidation of ammonia to nitrogen oxides. There is therefore a risk of wasting urea or even transformation of urea into nitrogen oxides then emitted to the exhaust cannula.

Another solution could be to use a third pollution control device which can take the form of an SCR catalyst or an ASC type catalyst (for Ammonia Slip Catalyst in English) capable of treating residual ammonia emissions from of the SCRF unit 7. However, this induces a significant cost and bulk as well as additional risks of failures.

The invention aims to effectively remedy these drawbacks by proposing an exhaust gas post-treatment system for an exhaust line of an internal combustion engine, in particular of a motor vehicle, comprising:

- an oxidation catalyst,

- a first catalyst member for selective catalytic reduction of nitrogen oxides,

- a particle filter,

- a first injector of a reducing agent positioned upstream of the first catalyst member for selective catalytic reduction of nitrogen oxides, characterized in that the post-treatment system further comprises:

a second catalyst member for selective catalytic reduction of nitrogen oxides, and

- At least one second reducing agent injector positioned upstream of the second catalyst member for selective catalytic reduction of nitrogen oxides.

The invention thus allows, in the case where the nitrogen oxides are not completely treated by the first catalytic reduction selective catalyst member, to activate the second injector to send the reducing agent to the second catalyst member selective catalytic reduction to treat the remaining nitrogen oxides. In addition, the fact of independently piloting the two injectors makes it possible to optimize the use of the depollution organs. For example, if the temperature of the first SCR catalyst member is too high and the risk of oxidizing the ammonia to nitrogen oxides is high, it is possible to switch to the second SCR catalyst member to treat at a lower temperature the nitrogen oxides not treated in the first SCR catalytic organ without risk of ammonia oxidation. Furthermore, another advantage of having two independent injectors is the limitation of the risk of fouling of the exhaust line.

According to one embodiment, the particle filter is provided with a coating for impregnating a catalyst for selective catalytic reduction of nitrogen oxides.

According to one embodiment, a first envelope contains the oxidation catalyst member, the first catalyst member for selective catalytic reduction of nitrogen oxides, and the particle filter, and a second envelope located downstream of the first envelope. contains the second catalyst element for the selective catalytic reduction of nitrogen oxides.

In one embodiment, two nitrogen oxide sensors are positioned upstream and downstream of the first envelope and a third nitrogen oxide sensor is positioned downstream of the second envelope.

In one embodiment, the second envelope is connected by a bend in the exhaust line to the first envelope.

It is understood by elbow the fact that the shape of the section of exhaust line connecting the two envelopes requires the flow of gases leaving the first envelope a change of direction of flow of at least 30 °, in particular of at least 50 °, especially about 90 ° or more.

According to one embodiment, the second reducing agent injector is placed upstream of the elbow of the exhaust line so that a flow of reducing agent is directed against the flow of the exhaust gases towards an outlet. of the particle filter.

In one embodiment, the outlet channels of the particulate filter comprise a material promoting the hydrolysis of the reducing agent.

In one embodiment, the oxidation catalyst member comprises a cold nitrogen oxide adsorber material.

According to one embodiment, a supervisor is able to control the two reducing agent injectors separately.

The invention also relates to a motor vehicle comprising an exhaust gas post-treatment system of an exhaust line of an internal combustion engine as defined above.

The invention will be better understood on reading the description which follows and on examining the figures which accompany it. These figures are given only by way of illustration but in no way limit the invention.

Figure 1, already described, is a schematic representation of an exhaust gas post-treatment system according to the state of the art;

Figure 2 is a schematic representation of an exhaust gas post-treatment system according to the present invention.

More specifically, Figure 2 shows a post-treatment system 10 for the exhaust gases of an internal combustion engine, for example of a motor vehicle. This system 10 is integrated into the exhaust line 11 connected to the collector (not shown) of the engine exhaust gases.

The system 10 comprises, in a first envelope 12 (which can also be designated by the English term canning) and, depending on the direction of flow of the exhaust gases (from upstream to downstream therefore) a oxidation catalyst member 13, a first injector 14 of reducing agent which can inject either a reducing agent, such as ammonia, or a reducing agent such as urea (the term reducing agent therefore includes both the reducer and the reducer precursor), a mixer 15, a catalyst member 16 for selective catalytic reduction of nitrogen oxides (or SCR), and a particle filter 17 (also called SCRF) a catalyst impregnation coating for the selective catalytic reduction of nitrogen oxides. The reducing agent is advantageously a liquid mixture of urea at 32.5% by volume and water.

The first envelope 12, which may for example be metallic, is located as close as possible to the exhaust gas collector, in particular approximately 35 cm from its outlet (for example at most 50 cm from its outlet). It is arranged, in the motor vehicle, in the under-hood space accommodating the internal combustion engine.

The first brick of this post-treatment system 10 is the oxidation catalyst member 13, which oxidizes the reducing species that are carbon monoxide (CO) and unburnt hydrocarbons (HC). The reactions it promotes are as follows:

CO + ^1/2 O ₂ -> CO ₂ (R1) Oxidation reaction of carbon monoxide

CxHy + (x + y / 4) O ₂ -> x CO ₂ + (y / 2) H ₂ O (R2) Oxidation reaction of unburnt hydrocarbons [0030] The oxidation catalyst member 13 consists of a cordierite type honeycomb support on which is deposited an active catalytic phase (washcoat). This phase includes oxides such as alumina doped with different stabilizers (lanthanum, iron, titanium, etc.). Precious metals (platinum, palladium) are deposited on these oxides in order to catalyze the oxidation reactions at low temperature. Acid compounds such as zeolites are also added. Their aptitude for the storage of hydrocarbons at low temperature and their destocking at high temperature makes it possible to improve the treatment of hydrocarbons during cold phases. One can add to these functions (oxidation of carbon monoxide and unburnt hydrocarbons and storage of the latter at low temperature) a function of storage of nitrogen oxides at low temperature. This storage function is ensured by the introduction of cold nitrogen oxide adsorber materials of the simple or mixed oxide type with a basic character for the storage of nitrogen dioxide (NO ₂ ) such as, for example, the oxides of cerium or barium among others or acidic for the storage of nitric oxide (NO) such as for example, crystallized aluminosilicates such as zeolites.

We do not describe in detail here the first urea injector 14, nor the mixer 15 (also called mixing box), already described and known, in particular from document WO2011 / 089330. We just recall that the mixing box 15 supplied by the injector 14, itself supplied by a gauge-pump module which draws urea in aqueous solution from a reservoir, ensures a mixture between the drops of urea and the sufficient exhaust gas for the urea thermolysis reaction (R3) to take place completely and for the hydrolysis reaction (R4) to take place in part before being completed on the SCR 16 catalyst member in order to form ammonia molecules according to the following reactions:

(NH ₂ ) ₂ CO NH ₃ + HNCO (R3) thermolysis of urea

HNCO + H ₂ O NH ₃ + CO ₂ (R4) hydrolysis of isocyanic acid These two stages, and especially the first, require temperatures of at least 180 ° C to 200 ° C, whence l interest that the first injector 14 and the mixing box 15 gas-liquid (urea) are close to the outlet of the internal combustion engine. This step makes it possible to form the reducer essential for the operation of the SCR reduction.

Furthermore, the SCR catalyst member 16 is based on zeolites exchanged with iron, such as β zeolites, iron-ferrierite, ZSM5, and the SCR catalyst of the particle filter 17 is based on copper zeolites, such as Chabazite, iron-ferrierite, ZSM5. Such a configuration allows the SCR catalyst to start quickly at low temperature, and the catalyst of the particulate filter 17 to remain effective even at very high temperature (in particular to resist regeneration of the particulate filter 17). The mechanical support of the SCR catalyst member 16 is made of cordierite, while the porous support of the particle filter 17 is rather made of silicon carbide SiC.

The SCR catalyst member 16 and the particle filter 17 provided with its coating treat the nitrogen oxides by selective catalytic reduction of the nitrogen oxides with ammonia (NH ₃ ) by the SCR coatings according to the following reactions R5 to R7:

NO + O ₂ + 4 NH ₃ -> 4 N ₂ + 6 H ₂ O

NO + NO ₂ + 2 NH ₃ 2 N ₂ + 3 H ₂ O

NO ₂ + 8 NH ₃ -> 7 N ₂ + 12 H ₂ O (R5) Standard SCR (R6) Fast kinetic SCR (R7) Low temperature SCR [0035] The optimal and sought after conversion of nitrogen oxides is obtained by to the reaction (R6) whose kinetics are the fastest but whose stoichiometry imposes a NO ₂ / NOx ratio close to 0.5, especially at low temperatures (that is to say at most 250 ° C) .

In addition, a second envelope 20, for example metallic, located downstream of the first envelope 12 contains a second catalyst member 21 of selective catalytic reduction. Advantageously, the SCR catalyst member 21 has a structure similar to that of the SCR catalyst member 16.

Preferably, the second envelope 20 is located under the floor of the vehicle. The inlet of the second enclosure 20 is connected by an elbow 23 of the exhaust line 11 to the outlet of the first enclosure 12.

In addition, a second injector 24 of reducing agent is placed upstream of the second catalyst member SCR 21, between the first envelope 12 and the second envelope 20.

Advantageously, the second injector 24 is positioned upstream of the elbow 23 of the exhaust line 11, so that the flow of reducing agent is directed against the flow of exhaust gas towards the outlet of the particulate filter 17 This improves the projection configuration of the reducing agent and therefore the homogeneity of the mixture between the ammonia formed via the thermolysis and hydrolysis reactions (R3 and R4) and the residual nitrogen oxides leaving the casing 12 for conversion of the latter into the SCR catalyst member 21 of the casing 20.

Exit channels of the particulate filter 17 may be covered with a material promoting the hydrolysis of urea. The flow of reducing agent reaching this brick thus allows the formation of ammonia with great efficiency. The length of the exhaust line 11 between the first envelope 12 and the second envelope 20 is chosen so as to ensure a good mixture between the ammonia formed and the residual nitrogen oxides which are converted into the catalyst member SCR 21 located in the second envelope 20.

In addition, two sensors 26, 27 of nitrogen oxides are positioned upstream and downstream of the first envelope 12. A third sensor 28 of nitrogen oxides is placed downstream of the second envelope 20. A supervisor 30 makes it possible to control the two injectors 14, 24 of reducing agent separately, in particular as a function of the measurement signals received from the sensors 26, 27.

In a conventional operating case, the first casing 12 is sufficient to treat all of the nitrogen oxides emitted by the internal combustion engine. The SCR catalyst member 21 of the second envelope 20 therefore does not need to convert nitrogen oxides. On the other hand, depending on its state, that is to say the quantities of reducers (ammonia) pre-stored, it is possible to ask the second injector 24 to ensure a suitable level of ammonia loading of the organ. SCR catalyst 21 to prevent nitrogen oxides from passing through this second envelope 20 without being converted. The temperature information used to ensure the efficiency of the storage of the ammonia formed can be obtained by means of a thermocouple or by the implementation of a model.

In the event that the nitrogen oxides are not completely treated by the first casing 12 (heavy load of the engine for example), the sensor 27 located downstream of the latter sends the information back to the supervisor 30 who orders a injection of urea by the second injector 24 on the outlet channels of the particulate filter 17 impregnated with a coating promoting the formation of ammonia (cf. thermolysis and hydrolysis reactions). This ammonia mixed with the exhaust gases processes the nitrogen oxides in the second casing 20. The second injector 24 is cut off when the sensor 27 no longer detects nitrogen oxides at the outlet of the first casing 12. This thus avoids the risk of ammonia leaking into the cannula.

Controlling the two injectors 14, 24 independently makes it possible to optimize the use of the SCR catalyst members 16, 21. For example, if the temperature of the first casing 12 is too high (temperature above 450 ° C. - 500 ° C) and the risk of oxidizing the ammonia to nitrogen oxides is high, it is possible to switch to the SCR catalyst member 21 of the second casing 20 to treat the oxides of nitrogen at a lower temperature. nitrogen not treated in the first envelope 12 without risk of oxidation of the ammonia.

The other advantage of having independent injectors 14, 24 is the limitation of the risk of fouling of the exhaust line 11. In fact, because of the strictness of the standards, a configuration of the mono-injector type can lead to inject large amounts of urea which are not entirely transformed into ammonia and which can crystallize in the form of urea derivatives and degrade the effectiveness of the SCR 16 catalyst member. This induces an additional risk of leakage of ammonia during the hot operating phases of the exhaust line 11 (regeneration of the particulate filter 17, for example). This problem is avoided thanks to the invention which makes it possible to distribute the quantity of urea injected into the exhaust line 11 between the two injectors 14, 24.

The invention also makes it possible to promote the diagnosis of the various bricks allowing the elimination of nitrogen oxides. Indeed, when it comes to verifying the good efficiency of the post-treatment system as required in the context of the new standards, in particular the EOBD standard (for European On Board Diagnostic in 5 English), an over-injection is ordered. reducing agent (beyond stoichiometry) to be certain that the system is at its best for treating nitrogen oxides. Independent injector control then isolates the faulty component.

In some embodiments, it will be possible to install in the exhaust line 11 other catalytic selective catalytic reduction members (in addition to members 16 and 21) each associated with a reducing agent injector located upstream of the corresponding additional catalyst member.

Claims (10)

1. Post-treatment system (10) for the exhaust gases from an exhaust line (11) of an internal combustion engine, in particular of a motor vehicle, comprising: - an oxidation catalyst member (13), - a first catalyst member (16) for the selective catalytic reduction of nitrogen oxides, - a particle filter (17), - a first injector (14) of a reducing agent positioned upstream of the first catalyst member (16) for selective catalytic reduction of nitrogen oxides, characterized in that the post-treatment system (10) further comprises: a second catalyst member (21) for the selective catalytic reduction of nitrogen oxides, and - At least one second injector (24) of reducing agent positioned upstream of the second catalyst member (21) for selective catalytic reduction of nitrogen oxides. 2. Post-treatment system according to claim 1, characterized in that the particle filter (17) is provided with a coating of impregnation catalyst for selective catalytic reduction of nitrogen oxides. 3. Post-treatment system according to claim 1 or 2, characterized in that a first envelope (12) contains the oxidation catalyst member (13), the first catalyst member (16) for selective catalytic reduction of oxides nitrogen, and the particulate filter (17), and a second envelope (20) located downstream of the first envelope (12) contains the second catalyst member (21) for selective catalytic reduction of nitrogen oxides. 4. Post-treatment system according to claim 3, characterized in that two sensors (26, 27) of nitrogen oxides are positioned upstream and downstream of the first casing (12) and a third sensor (28) of nitrogen oxides is positioned downstream of the second envelope (20). 5. Post-treatment system according to claim 3 or 4, characterized in that the second envelope (20) is connected by an elbow (23) of the exhaust line (11) to the first envelope (12). 6. Post-treatment system according to claim 5, characterized in that the second injector (24) of reducing agent is placed upstream of the elbow (23) of the exhaust line (11) so that a flow reducing agent is oriented against the flow of exhaust gases to an outlet of the particulate filter (17). 7. Post-treatment system according to any one of claims 1 to 6, characterized in that the outlet channels of the particle filter (17) comprise a material promoting the hydrolysis of the reducing agent. 8. Post-treatment system according to any one of claims 1 to 7, characterized in that the oxidation catalyst member (13) comprises a material for adsorbing nitrogen oxides when cold. 9. Post-treatment system according to any one of claims 1 to 8, characterized in that a supervisor (30) is able to control the two injectors (14, 24) of reducing agent separately. 10. Motor vehicle comprising a post-treatment system (10) for the exhaust gases from an exhaust line (11) of an internal combustion engine as defined in any one of the preceding claims.