WO2010100352A1 - Device and method for processing nitrogen oxides contained in exhaust gases - Google Patents

Abstract

The invention relates to a device for processing exhaust gases generated by the internal combustion engine of an automobile, including an exhaust line with an ammonia reduction catalytic converter (11) and an injector for an ammonia precursor or for a reducing agent capable of forming an ammonia precursor, mounted upstream from the reduction catalytic converter (11). The device further includes a static mixer (13) mounted between the injector (12) and the reduction catalyst (11), said static mixer (13) including a catalytic converter for forming ammonia from the ammonia precursor.

Description

 Device and method for treating nitrogen oxides contained in exhaust gases

The present invention relates to a device and a method for treating the exhaust gas discharged by an internal combustion engine. In particular, the present invention relates to a device and a method for treating nitrogen oxides contained in exhaust gases. In order to meet the lower thresholds for emissions of gaseous pollutants from motor vehicles, increasingly complex gas after-treatment systems are arranged in the exhaust line. These aftertreatment systems make it possible in particular to reduce the emissions of nitrogen oxides in addition to carbon monoxide, unburned hydrocarbons and particles.

More specifically, the invention relates to the treatment of NOx nitrogen oxides, such as NO and NO ₂ . Current NOx treatment devices include, for example, NOx traps, reduction catalysts (SCRs), or 3-way catalysts. Reduction catalysts reduce NOx in the presence of a reducing agent. The reducing agent, for example urea or ammonia, is introduced into the exhaust line upstream of the reduction catalyst. In order to optimize the NOx treatment efficiency, a mixing system is arranged upstream of the reduction catalyst, so that the mixture of exhaust gas and of the reducing agent is as homogeneous as possible. It is thus possible to use an additional element in the exhaust line, in order to create turbulence. Another solution is to increase the distance between the point of injection of the reducing agent and the reduction catalyst. However, increasing the distance increases heat loss, which greatly reduces the efficiency of the catalyst.

On the other hand, in the case of the injection of a urea solution for example, the ammonia is formed in the exhaust line in two stages. At first, urea undergoes thermolysis for give ammonia and isocyanic acid. Then, the isocyanic acid is hydrolyzed to ammonia and carbon dioxide. However, the hydrolysis of isocyanic acid is sometimes carried out on the upstream part of the reduction catalyst, in particular for temperatures below 220 ^° C., which reduces the effective volume of the reduction catalyst for treating NOx, and therefore the efficiency of NOx treatment.

There are NOx treatment devices including static mixers, for example a grid or mixer with fins. These mixers are intended to homogenize the gaseous mixture comprising the exhaust gas and the reducing agent. However, these static mixers do not include catalysts.

An object of the invention is to improve the treatment of the exhaust gas. In particular, an object of the invention is to increase the NOx treatment efficiency by the reduction catalyst, and in particular to reduce the heat losses upstream of the reduction catalyst.

Another object of the invention is to allow the use of the entire volume of the reduction catalyst.

For this purpose, in one embodiment, there is provided a device for treating exhaust gases emitted by an internal combustion engine of a motor vehicle, comprising an exhaust line with an ammonia reduction catalyst. and an injector of an ammonia precursor or a reducing agent capable of forming an ammonia precursor, mounted upstream of the reduction catalyst. The device also includes a static mixer mounted between the injector and the reduction catalyst, the static mixer comprising a catalyst for forming ammonia from the precursor of ammonia.

The term "precursor of ammonia" is understood to mean a chemical molecule that can, by chemical reaction, allow the formation of ammonia. Urea is an example of a precursor of ammonia since the thermolysis reaction of urea makes it possible to form ammonia (and isocyanic acid). Similarly, isocyanic acid is also a precursor of ammonia since it allows, by hydrolysis reaction with water, to form ammonia (and carbon dioxide).

In addition, the term "ammonia forming catalyst" means a catalyst for the chemical reaction which makes it possible to form ammonia from the precursor. In the case of urea, the catalyst for the formation of ammonia will be a catalyst for the thermolysis reaction. In the case of isocyanic acid, the catalyst for ammonia formation will be a catalyst for the hydrolysis reaction.

The static mixer makes it possible to prepare the gaseous mixture supplying the reduction catalyst, so as to improve the operation of the latter. In particular, the static mixer makes it possible, on the one hand, to homogenize the mixture of exhaust gas and reducing agent, and on the other hand to produce, within the gaseous mixture, the ammonia used by the reduction catalyst. to treat NOx. In particular, in the case where the reducing agent is an aqueous solution of urea, the thermolysis of urea makes it possible to produce only one mole of ammonia and one mole of isocyanic acid per mole of urea, while the combination of thermolysis and hydrolysis gives two moles of ammonia per mole of urea. Thus, the isocyanic acid is a precursor of ammonia and the static mixer may comprise a catalyst for the hydrolysis reaction of isocyanic acid so that the gas mixture leaving the static mixer is immediately ready to be treated by the reduction catalyst, and does not require a fraction of the reduction catalyst to effect the hydrolysis of isocyanic acid. The reduction catalyst then serves only to treat the NOx. Moreover, the static mixer also makes it possible to reduce the distance between the reducing agent injector and the reduction catalyst, thus limiting the thermal losses of the exhaust gases during their circulation in the exhaust line.

Preferably, the precursor of ammonia comprises isocyanic acid and the ammonia forming catalyst comprises a hydrolysis catalyst comprising iron, copper and / or titanium oxide. In particular, the hydrolysis catalyst comprises TiO4, an iron-associated zeolite as a promoter and / or a copper-associated zeolite as a promoter.

The catalyst is selected to have significant catalytic activity for the hydrolysis reaction of isocyanic acid. In particular, a very active catalyst will eventually reduce the volume of the static mixer, and thus further reduce heat losses, as well as pressure drops.

The static mixer may comprise a foam having open pores and coated with the ammonia forming catalyst. The foam with open pores is, for example, a ceramic foam or a metal foam.

The foam with open pores makes it possible to efficiently mix the exhaust gas and the reducing agent while limiting the pressure drop in the exhaust line.

According to another embodiment, the static mixer may comprise at least two foams provided with open pores and arranged in an adjoining manner. In addition, foams with open pores can be separated by spacers. The use of several foams may make it possible to modify the characteristics of the static mixer, for example its specific surface area and / or its volume.

Preferably, the reducing agent is urea.

In another aspect, there is provided a method of treating exhaust gases emitted by an internal combustion engine of a motor vehicle, wherein:

an ammonia precursor or a reducing agent capable of forming a precursor of ammonia is injected, and then

the exhaust gas and the ammonia precursor are mixed, and the ammonia precursor is reacted to form the ammonia, and then

a catalytic reduction of the gases by ammonia is carried out. Advantageously, the reducing agent is urea and thermolysis of the urea is carried out before hydrolyzing the isocyanic acid to form the ammonia. Other advantages and characteristics will appear on examining the detailed description of an embodiment given by way of non-limiting example, and of the appended figure in which is illustrated, in section, an example of a device for processing data processing. exhaust gas.

In the attached figure, there is shown very schematically the general structure of an internal combustion engine 1 and an exhaust gas treatment device 2. The internal combustion engine 1 comprises, for example, at least one cylinder 3, an intake manifold 4, an exhaust manifold 5, an exhaust gas recirculation circuit 6 provided with an exhaust gas recirculation valve 7, and an exhaust system. turbo compression 8.

The exhaust gas treatment device 2 comprises, for example, an exhaust line, an oxidation catalyst 9, a particulate filter 10 mounted downstream of the oxidation catalyst 9, and a reduction catalyst (in particular English: Selective Catalytic Reduction SCR) 1 1 mounted downstream of the particulate filter 10. In other words, the two catalysts 9, 1 1 are connected in series on the exhaust line, with a particulate filter 10 mounted between the two.

Furthermore, a reducing agent injector 12 and a static mixer 13 are mounted on the exhaust line between the particulate filter 10 and the reduction catalyst 1 1. The reducing agent injector 12 is mounted upstream. of the static mixer 13. The oxidation catalyst 9 is used to treat, in particular, carbon monoxide CO and unburned hydrocarbons HC contained in the exhaust gases. In particular, the catalyst 9 makes it possible to oxidize carbon monoxide to CO ₂ carbon dioxide, and unburned hydrocarbons to carbon dioxide and water. The reduction catalyst 11 is designed to effectively treat the NOx contained in the exhaust gas. In particular, the reduction catalyst 11 may be selected to catalyze the NOx reduction reaction with ammonia. For example, the catalyst 11 may be copper based on a porous zeolite matrix, or else based on iron on a matrix porous zeolite, based on vanadium or based on mixed oxides.

In order to improve the NOx treatment efficiency by the reduction catalyst 11, the injector 12 introduces a reducing agent into the exhaust line. The purpose of the reducing agent is to facilitate the reduction of NOx by the reduction catalyst 1 1. The reducing agent may be, for example, urea, ammonia or any other chemical capable of forming ammonia. when introduced into the exhaust line. Among the compounds capable of forming ammonia are urea and isocyanic acid. Insofar as urea also forms isocyanic acid, it is considered, for the rest of the description, that the reducing agent is urea. When injected into the exhaust line, the urea (stored in liquid form) vaporizes on contact with the exhaust gas, then undergoes a thermolysis reaction when the temperature is above 152 ^° C. The reaction of thermolysis is written in the form:

(NH ₂ ) ₂ CO → NH ₃ + HNCO. Thus, one mole of urea makes it possible to produce one mole of ammonia and one mole of isocyanic acid.

Moreover, at a temperature above 128 ^° C. and in the presence of a catalyst, the isocyanic acid can undergo, in a second step, a hydrolysis reaction in the form of:

HNCO + H ₂ O → NH ₃ + CO ₂ . When a solution of urea is injected into the exhaust gas upstream of a reduction catalyst, the urea solution evaporates and undergoes thermolysis. However, the hydrolysis reaction occurs most often only at the reduction catalyst, due to a temperature in the exhaust line too low. This prevents on the one hand the treatment of NOx by the part of the catalyst which catalyzes the hydrolysis reaction. On the other hand, at the inlet of the reduction catalyst, the urea solution has not yet produced half the moles of ammonia that can be formed from the moles of urea. There is therefore a lower efficiency of the reduction catalyst. A static mixer 13 is mounted downstream of the injector 12 and upstream of the reduction catalyst 1 1. The static mixer 13 makes it possible on the one hand to homogenize the exhaust gas / ammonia mixture, and on the other hand to catalyze the hydrolysis reaction of isocyanic acid. An homogeneous mixture comprising in particular two moles of ammonia per one mole of urea injected is then obtained at the inlet of the reduction catalyst 11. The reduction catalyst can then catalyze only the NOx treatment reactions, and is no longer useful for the hydrolysis reaction of isocyanic acid. Furthermore, thanks to the efficiency of the static mixer, the distance required between the injector 12 and the reduction catalyst 1 1 can be reduced, which reduces the thermal losses of the gases in the exhaust line and therefore increases the efficiency of NOx reduction reactions. The static mixer 13 may comprise an open pore foam which, on the one hand, allows the exhaust gas to pass through it and, on the other hand, creates turbulence leading to the homogenization of the gases. In particular, a compromise between efficiency and pressure drop must be chosen. This compromise may depend in particular on the volume of open-pore foam, the technical characteristics of the foam (such as the porosity, the average pore diameter, the pore distribution, the specific surface area), and the pore impregnation mass. . Thus, for example, it is possible to opt for a static mixer comprising a plurality of open-pore foams arranged adjacently in the direction of circulation of the exhaust gases. These foams can also be separated by spacers or not.

The open pore foam is impregnated with catalyst for the hydrolysis of isocyanic acid which is injected by the injector 12 or which is formed by thermolysis of the urea injected by the injector 12. The catalyst may be based on copper on a porous zeolite matrix, based on iron on a porous matrix of zeolite, based on vanadium or based on mixed oxides such as TiO4. Furthermore, the impregnation will be chosen so as to withstand the operating conditions, in particular so as not to sinter under normal operating conditions. The material of the foam is chosen according to the operating conditions to which it must be resistant, in particular the temperature reached, the corrosion, the vibrations, etc. The foam can thus comprise materials of the ceramic type, or even metal.

In the case of a metal foam, it is also possible to use the static mixer as a heating means. Thus, the metal foam may be electrically powered, for example by an electronic control means (not shown), so as to increase the temperature of the gases passing through the foam. It is thus possible to increase the efficiency of the reduction catalyst 11 or to provide the heat required for the compounds capable of forming ammonia, for example by the reaction of thermolysis and hydrolysis of the urea, when the temperature of the gases it's not enough. Alternatively, the exhaust gas treatment device 2 may also have a different layout of the processing means. Thus, the particulate filter 10 may be disposed downstream of the reduction catalyst 11, or it may be realized, with the reduction catalyst 11, in the form of a single element noted SCRF (in English: "Selective Catalytic Reduction

Filter "). In both cases, the injector 12 and the static mixer 13 are mounted downstream of the oxidation catalyst 9, and upstream of the reduction catalyst and the particulate filter.

In addition, it is also possible to increase the thermal insulation of the exhaust line, in order to maintain a sufficient temperature in the reduction catalyst 1 1, especially during periods of idling. Similarly, the use of substrates with low thermal inertia, for the oxidation catalyst 9, the particulate filter 10 and / or the reduction catalyst 1 1, can reduce the heating time of the reduction catalyst.

1 1.

The exhaust gas treatment device 2 thus allows improved treatment of NOx, while maintaining a small footprint and a low cost of use.

Claims

An exhaust gas treatment device emitted by an internal combustion engine of a motor vehicle, comprising an exhaust line with a reduction catalyst

(1 1) with ammonia and an injector (12) of an ammonia precursor or a reducing agent capable of forming an ammonia precursor, mounted upstream of the reduction catalyst (1 1), characterized in that the device also comprises a static mixer (13) mounted between the injector (12) and the reduction catalyst

(1 1), the static mixer (13) comprising a catalyst for forming ammonia from the precursor of ammonia.

The device of claim 1 wherein the ammonia precursor comprises isocyanic acid and wherein the ammonia catalyst comprises a hydrolysis catalyst comprising iron, copper and / or titanium oxide.

3. Device according to the preceding claim wherein the hydrolysis catalyst comprises TiO4, an iron-associated zeolite as a promoter and / or a zeolite associated with copper as a promoter.

4. Device according to one of the preceding claims wherein the static mixer (13) comprises a foam provided with open pores and coated with the catalyst for forming ammonia.

5. Device according to the preceding claim wherein the foam provided with open pores is a ceramic foam or a metal foam.

6. Device according to claim 4 or 5 wherein the static mixer (13) comprises at least two foams provided with open pores and disposed adjacently.

7. Device according to the preceding claim wherein the foams provided with open pores are separated by spacers.

8. Device according to one of the preceding claims wherein the reducing agent is urea.

9. A method of treating exhaust gases emitted by an internal combustion engine of a motor vehicle, wherein:

an ammonia precursor or a reducing agent capable of forming a precursor of ammonia is injected, and then

the exhaust gas and the ammonia precursor are mixed, and the ammonia precursor is reacted to form the ammonia, and then

a catalytic reduction of the gases by ammonia is carried out.

10. The method of claim 9 wherein the reducing agent is urea and wherein urea is thermolyzed prior to hydrolyzing isocyanic acid to form ammonia.