WO2011095734A1

WO2011095734A1 - Exhaust line for an internal combustion engine

Info

Publication number: WO2011095734A1
Application number: PCT/FR2011/050192
Authority: WO
Inventors: Grégory BLOKKEEL; Gilbert Blanchard; Ludovic Thobois; Sylvie Honnet; Karine Pajot
Original assignee: Peugeot Citroën Automobiles SA
Priority date: 2010-02-05
Filing date: 2011-02-01
Publication date: 2011-08-11
Also published as: FR2956155A1; FR2956155B1

Abstract

The invention relates to an exhaust line for an internal combustion engine including: a main channel (1) extending along a longitudinal axis (XX) and moving the exhaust gases produced by the internal combustion engine to a catalyst (3) for selectively reducing nitrogen oxides; a means for injecting a reducing agent into the main channel (1), said means being placed in the exhaust gas flow direction and upstream from the catalyst (3); and a static mixer placed between the injection means and the catalyst (3). Said exhaust line is characterized in that the static mixer includes a ceramic foam (8) that is placed in the main channel (1) between the injection means and the selective reduction catalyst (3). The invention also relates to an internal combustion engine that includes an exhaust line of the invention.

Description

Exhaust line for an internal combustion engine Technical field of the invention

The present invention relates to the field of exhaust devices of an internal combustion engine and more particularly to the depollution by catalytic system.

Technological background

For many years, motor vehicle manufacturers have made great efforts to reduce the emission into the atmosphere of environmentally harmful chemical compounds produced by combustion engines during fuel combustion.

Among these compounds, there are in particular carbon dioxide C0 _{2 as} well as nitrogen oxides, mainly NO monoxide and nitrogen dioxide N0 ₂ , designated together under the abbreviation NO _x . It should be noted that the production of nitrogen oxides is greater for diesel engines than for gasoline engines because of their higher combustion temperature.

To limit the emission of nitrogen oxides in the atmosphere, a solution currently used is to place on the exhaust system of the vehicle an NO _x treatment system, called SCR system (Selective Catalytic Reduction), having as a function of chemically reducing nitrogen oxides to di-nitrogen molecules and water vapor by means of a reducing agent. In practice, the reducing agent is introduced into the exhaust line upstream of a specific catalyst SCR in which the reduction reaction occurs.

There are two SCR channels, one with the aqueous solution of urea and the other with ammonia, NH ₃ , gaseous. SCR with urea has been previously favored for reasons of vehicle implantation. The aqueous solution of urea requiring a reservoir, the liquid supply circuit is relatively simple. This path has the disadvantage of requiring a set of sequential physicochemical processes: the injection of the aqueous solution of urea, the atomization of the urea spray droplets and droplets, evaporation and decomposition chemical of these droplets in NH ₃ and finally the mixture of NH ₃ with the exhaust gases. Moreover, given the space available in the line For the injection / mixing phenomena, the SCR solution with the aqueous urea solution greatly complicates the homogenization of the reducing agent with the oxidant.

The other route explored is SCR with ammonia, NH ₃ gas. In this case, the NH ₃ is injected as a gas directly into the exhaust gas where it must then mix.

It is known to leave an adequate distance between the reducing agent injector and the SCR catalyst to allow obtaining a homogeneous mixture as complete as possible between the exhaust gas and the reducing agent.

In order to improve the homogeneity of this mixture and to shorten the distance required for homogenization, it is known to use a so-called static mixer, that is to say without moving parts, positioned inside. of the exhaust duct, between the point of injection of reducing agent in the exhaust gas and the entry of the catalyst SCR.

For example, document WO2009 / 085641 discloses an exhaust device for an internal combustion engine comprising a main duct bringing the flue gases produced by the internal combustion engine to a selective reduction catalyst for nitrogen oxides comprising a catalytic bread. (Also called brick or monolith and serving as a support for a catalytic coating), an injector for injecting an aqueous solution of urea and disposed, in the direction of flow of the exhaust gas, upstream of the selective reduction catalyst. In addition, the device comprises a static mixer, in other words without moving element, arranged between the injector and the selective reduction catalyst. In the document WO2009 / 085641, the static mixer is based on a mesh of entangled and crimped metal wires.

However, such an exhaust device does not make it possible to obtain a diffusion of gaseous ammonia in the exhaust gases so as to obtain a homogeneous mixture of ammonia and exhaust gas at the inlet of the catalytic bread on the most much of its inlet section, this with a short distance between the injection point of the reducing agent in the exhaust gas and the inlet section of the catalytic bread. The device of the prior art is not compact. Such an arrangement is not suitable when the available space of an exhaust line of a motor vehicle is more and more constrained. The aim of the invention is to overcome the drawback of the prior art by proposing a new exhaust line making it possible to obtain a homogeneous mixture between a reducing agent and exhaust gases over substantially the entire section of the engine. catalytic bread inlet, this with a short distance between the injection point of the reducing agent in the exhaust gas and the inlet section of the catalytic bread.

The invention therefore relates to an exhaust line for an internal combustion engine comprising a main duct extending along a longitudinal axis and bringing the exhaust gases produced by the internal combustion engine to a selective reduction catalyst for the oxides of nitrogen, means for injecting a reducing agent in the main pipe arranged, in the direction of the flow of the exhaust gas, upstream of the catalyst, a static mixer disposed between the injection means and the catalyst characterized in that the static mixer comprises a ceramic foam disposed in the main conduit between the injection means and the selective reduction catalyst.

Furthermore, the invention may comprise one or more of the following features: Preferably, the ceramic foam is based on titanium, alumina, zirconia or silicon carbide, in order to use a common ceramic foam.

Preferably, the ceramic foam has a porosity of between 20% and 80% and more preferably within this range of greater than 60%, in order to benefit from a good compromise between limited pressure losses and efficient stirring of the gases. .

In one variant, the ceramic foam comprises a catalytic coating for the selective reduction of nitrogen oxides by ammonia. From a dimensional point of view, preferably, the ceramic foam has, relative to the longitudinal axis of the main duct, a thickness of between 1 cm and 5 cm.

Also preferably, the ceramic foam comprises an exhaust outlet face and the selective reduction catalyst comprises an inlet face of the exhaust gas so that said faces are spaced relative to the longitudinal axis of the exhaust gas. main duct, first distance between 0.5 cm and 5 cm and more preferably between 0.5 cm and 2 cm, in order to have an even more compact package.

In an alternative embodiment, the reducing agent injection means comprise at least one supply duct opening into the main duct by at least one reducing agent injection orifice, which makes it possible to improve the initial distribution of the reducing agent. reducing agent in the exhaust gas, which helps to reduce the mixing and diffusion distance of the reducing agent. Preferably, the ceramic foam comprises an inlet face of the exhaust gas so that the injection orifice is spaced from said inlet face of the ceramic foam, relative to the longitudinal axis of the main conduit, a second distance of between 1 cm and 5 cm. Furthermore, the invention also relates to an internal combustion engine, characterized in that it comprises an exhaust line of the invention of the combustion gases produced by said engine.

Brief description of the drawings

Other features and advantages will appear on reading the following description of a particular embodiment, not limiting of the invention, with reference to the single figure in which: - Figure 1 is a schematic representation of an exhaust line of the invention.

detailed description

Figure 1 shows an exhaust line portion of burnt gases produced by an internal combustion engine, not shown here. The exhaust line comprises a main duct 1, extending along a longitudinal axis XX, in which circulates a stream 2 of flue gas, said main gas stream and connected to a selective reduction catalyst 3 (or SCR), for the reduction nitrogen oxides NOx present in the exhaust gas, by selective catalytic reaction between the NOx and a reducing agent. By reducing agent is meant an agent that can react chemically with pollutants to transform them into less polluting products. Preferably, the reducing agent is a gaseous reducing agent. More preferably the gaseous reducing agent is ammonia. The catalyst 3 is provided with an inlet face 4 of the exhaust gas substantially perpendicular to the longitudinal axis XX.

Prior to the catalyst 3 SCR, relative to the direction of flow of the exhaust gas, means for injecting the reducing agent comprising a conduit 5 for supplying a stream 6 of reducing agent are arranged. The supply duct 5 passes through the main duct 1 to open into the latter and allow the injection of reducing agent through an injection orifice 7.

According to the invention, provision is made in the main conduit 1 between the injection means and the selective reduction catalyst 3, a ceramic foam 8, acting as a static mixer.

This ceramic foam 8 may have the shape of a cylindrical slice in the direction of the longitudinal axis XX and occupies the entire section of passage of the exhaust gas in the main conduit 1.

The invention is particularly suitable with a gaseous reducing agent such as ammonia. This choice makes it possible to contribute to the overall efficiency of the invention from the point of view of compactness because the overall distance required between the injection means and the catalyst is simply the distance necessary to obtain a homogeneous mixture in the main duct 1 between the exhaust gas and the ammonia.

The ceramic foam wafer 8 has an exhaust gas inlet face 9 and an exhaust gas outlet face 10 and preferably has a thickness e _p , taken between these two faces 9, 10, according to a dimension taken parallel to the longitudinal axis XX, between 1 cm and 5 cm.

In order to use a current and economical ceramic foam 8, preferably, the ceramic foam 8 is based on titanium, alumina, zirconia or silicon carbide. The outlet face 10 of the ceramic foam 8 is disposed at a first distance Di from the inlet side 4 of the exhaust gas into the catalyst 3 SCR, in a dimension taken parallel to the longitudinal axis XX, which can be included between 0.5 cm and 5 cm and preferably between 0.5 cm and 2 cm for even more compactness.

Preferably, the injection orifice 7 of the ammonia is disposed at a second distance D ₂ from the inlet face 9 of the ceramic foam wafer 8, in a dimension taken parallel to the longitudinal axis XX, included between 1 and 5 cm. More preferably, the ceramic foam bread 8 has a porosity P of between 20% and 80% and more preferably between 60% and 80%, in order to obtain a good compromise between the pressure drops and the mixing performance. Exhaust gas / ammonia of the ceramic foam 8. Porosity P is understood to mean a quantity expressed here in% and defined by the following relation:

V - V

vtot ^V m ^. _{1 0} 0

V early, with V _tot , the total volume of a ceramic foam sample 8 and V _mc the volume of ceramic material contained in said sample.

Due to its porosity P, the internal structure of the ceramic foam opposes the flow of exhaust gases and ammonia a multitude of obstacles that promotes not only the homogenization of ammonia with the exhaust but also the diffusion of the current mixture in a radial direction with respect to the longitudinal axis XX.

The invention advantageously makes it possible to obtain a rigid static mixer, of high thermal resistance, chemically inert with respect to the exhaust gases, of homogeneous porosity in a radial direction with respect to the longitudinal axis XX. It allows in combination with the use of ammonia as a reducing agent to make the whole more compact.

The embodiment described in FIG. 1 is not limiting. According to one variant, the device of the invention may comprise several supply ducts 5 opening into the duct 1 main by at least one ammonia injection port, to promote the initial distribution of ammonia in the exhaust gas passage section and further reduce the mixing length and thus the overall distance between the points injection of ammonia and the inlet face 4 of the catalyst 3 SCR.

In a variant, the ceramic foam wafer 8 comprises a catalytic coating for selective reduction of nitrogen oxides by ammonia based on one or more zeolites partially exchanged with either iron or copper or iron and deposited copper. in the porosity of the ceramic foam. The catalytic coating of selective reduction of nitrogen oxides can be deposited in the porosity of the ceramic foam by the deposition methods well known to those skilled in the art, such as the suction coating of the suspension or the dip coating. Thus, in this variant, the ceramic foam 8 acts not only as a mixer between the ammonia and the exhaust gas, as well as the radial diffuser of the current mixture, but also the pre-catalyst for the selective reduction of the oxides of nitrogen.

Claims

claims

1. Exhaust line for an internal combustion engine comprising a main duct (1) extending along a longitudinal axis (XX) and conveying the exhaust gases produced by the internal combustion engine to a selective reduction catalyst (3) nitrogen oxides, means for injecting a reducing agent into the main duct (1) arranged, in the direction of the flow of the exhaust gas, upstream of the catalyst (3), a static mixer arranged between the injection means and the catalyst (3), characterized in that the static mixer comprises a ceramic foam (8) arranged in the main conduit (1) between the injection means and the selective reduction catalyst (3) .

2. Exhaust line according to claim 1, characterized in that the ceramic foam (8) is based on titanium, alumina, zirconia or silicon carbide.

3. Exhaust line according to claim 1 or claim 2, characterized in that the ceramic foam (8) has a porosity of between 20% and 80%.

4. Exhaust line according to claim 3, characterized in that the ceramic foam (8) has a porosity greater than 60%.

5. Exhaust line according to any one of the preceding claims, characterized in that the ceramic foam (8) comprises a catalytic coating of selective reduction of nitrogen oxides by ammonia.

6. Exhaust line according to any one of the preceding claims, characterized in that the ceramic foam (8) has, relative to the longitudinal axis (XX) of the conduit (1) main, a thickness (e _p ) included between 1 cm and 5 cm.

7. Exhaust line according to any one of the preceding claims, characterized in that the ceramic foam (8) comprises an exhaust outlet face (10) and the selective reduction catalyst (3) comprises a face (4) exhaust gas inlet so that said faces (10, 4) are spaced, relative to the longitudinal axis (XX) of the conduit (1) main, a first distance (D ^ included between 0.5 cm and 5 cm and preferably between 0.5 cm and 2 cm.

8. Exhaust line according to any one of the preceding claims, characterized in that the reducing agent injection means comprise at least one supply duct (5) opening into the duct (1) main by at least a reducing agent injection port (7).

9. Exhaust line according to claim 8, characterized in that the ceramic foam (8) comprises an inlet face (9) of the exhaust gas so that the orifice (7) of injection is spaced from said inlet face (9) of the ceramic foam (8) relative to the longitudinal axis (XX) of the main duct (1) by a second distance (D ₂ ) of between 1 cm and 5 cm. cm.

10. Internal combustion engine, characterized in that it comprises an exhaust line of the combustion gases produced by said engine according to any one of the preceding claims.