EP2776688A1

EP2776688A1 - Storage cartridge for a gaseous reducer for the selective catalytic reduction of nitrogen oxides

Info

Publication number: EP2776688A1
Application number: EP12794378.5A
Authority: EP
Inventors: Mathieu Artault; Clement Grise
Original assignee: Peugeot Citroen Automobiles SA
Current assignee: PSA Automobiles SA
Priority date: 2011-11-10
Filing date: 2012-10-26
Publication date: 2014-09-17
Also published as: FR2982638B1; FR2982638A1; WO2013068669A1

Abstract

The invention relates to a cartridge (1) for storing a gaseous reducer for the selective catalytic reduction of nitrogen oxides, said cartridge comprising: - a tank containing a solid storage material of said gaseous reducer, - at least one means of heating (6) for the storage material and the surrounding area of said material, - a plurality of heat conductors (7) within the storage material and in direct or indirect contact with said means of heating and diffusing the heat within said storage material. The heat conductors are arranged within the storage material in mats that delimit, in combination with the wall 15 of the tank, layers (a, b, c) of storage material, the thickness thereof varying according to the distance to the nearest surrounding means of heating.

Description

GAS REDUCER STORAGE CARTRIDGE FOR SELECTIVE CATALYTIC REDUCTION OF NITROGEN OXIDES

The invention relates to the depollution of the exhaust gas of a combustion engine. More specifically, it relates to the reduction of nitrogen oxides contained in the exhaust gas of an internal combustion engine by selective catalytic reduction (or SCR, according to the acronym of "Selective Catalytic Reduction").

The invention is more particularly concerned with the depollution of exhaust gases from vehicle engines, particularly motor vehicles. The nitrogen oxide reduction technology by selective catalytic reduction is to reduce the nitrogen oxides before they exit the exhaust system of a combustion engine, using a reducing agent (or a reducing agent precursor) introduced into the exhaust line. In the remainder of the present text, the term "reducing agent" will be used indifferently to designate the reducing agent or a precursor of the reducing agent.

[0004] Two types of SCR technology are known. There is the so-called liquid SCR technology, which uses a reducing agent precursor in liquid form, such as an aqueous solution of urea, which can be converted into ammonia when it is injected into the exhaust line. There is also the so-called SCR solid / gas technology, where the ammonia is stored in a solid material capable of adsorbing / adsorbing and then releasing it in a controlled manner, in particular by thermal activation.

The invention is concerned with the solid / gas SCR technology, and more particularly with the absorbent / adsorbent solid material tanks of the reducer, a material which will be designated hereinafter by the term "storage material". for the sake of brevity.

By way of example, the document WO 201 1/1 19735 describes such a reservoir, in the form of a cartridge in which one comes to insert discs all identical to each other and stacked on each other, each disc being consisting of the storage material, based on strontium chloride in the form of pressed powder, which is covered on at least one of the faces of the disc with a conductive coating. There is also a means for heating the cartridge, the heat being transmitted to the at least partly through said conductive coatings to desorb ammonia gas.

This cartridge design remains susceptible of improvements. Indeed, the heat propagates from the external source of heat to the conductive coatings, but it turns out that the time of release of a given quantity of reducing agent is not constant over time as and when the depletion of the storage material (it is understood by exhaustion that the storage material is becoming poorer in reducing gear). Indeed, when the cartridge is new, and the storage material loaded at its maximum rate of reducing agent, it is first of all the layers of the storage material closest to the heat source and / or the layers in direct contact with each other. with the conducting coatings of the discs which release the reducing agent. Then, progressively, the heat must travel a greater distance to reach the strata further and further away from storage material, knowing that the heat propagates poorly in the thickness of the storage material, this salt-type material being weakly thermal conductor. However, this increasing inertia in the release of reducing agent from the cartridge, as and when it wears, given thermal input, is not easy to compensate, and can lead to drifts in the injection process of reducing agent at the appropriate time in appropriate quantities in the exhaust line.

The purpose of the invention is therefore to develop a new type of cartridge which overcomes this disadvantage, and which may make it possible to obtain a more uniform, less variable time-release agent release rate, and in particular, which decreases / little as wear of the cartridge, with identical or similar activation conditions.

The invention therefore relates to a gaseous reducing storage cartridge for the selective catalytic reduction of nitrogen oxides, comprising: - a reservoir containing a solid storage material of said gaseous reductant, - at least one heating means for the storage material and peripheral to said material, - a plurality of thermal conductors within the storage material in direct or indirect contact with said heating means and propagating the heat in said storage material, the thermal conductors being arranged within the layer storage material delineating, in conjunction with the tank wall, layers of storage material whose thickness varies as a function of the distance to the nearest peripheral heating means. In a known manner, and for the sake of brevity, in the present text "storage material" will be understood to mean a material in solid form capable of reversibly trapping / storing a gaseous reducing agent of the ammonia type by adsorption and / or or absorption and / or any other reversible physico-chemical phenomenon. [001 1] The innovation has in fact consisted in taking into account the low thermal conductivity of the storage materials usually used, so as to compensate for the fact that the thermal contribution to activate the release is all the more difficult as the material storage is far from the heat source. Here, advantageously, a thickness of the layers is chosen which decreases with distance to the peripheral heating means: thus, each given point in a layer of storage material will be all the closer to an adjacent thermal conductor that it is away from the heat source. These zones, which previously had a lower / slower release rate with identical thermal input, since they are further away, can also have thermal activation identical or similar to the zones closer to the heat source. It is thus possible to have a reductant release which retains a response time, namely a release time when the thermal activation is triggered, which no longer depends, or which depends less, on the wear of the cartridge.

As a corollary of the thickness variation of the storage material layers, the invention provides that the thermal conductors are preferably arranged within the storage material so that the distance between a given point of the mass storage material in the cartridge and the thermal conductor that is closest to it is even smaller as the distance between said point and the heat source increases.

It can thus have thinned areas in the layers of storage material facing thermal conductors instead thicker locally, especially in the same proportions.

Several embodiments are possible: the thickness of the layers may vary gradually or by discrete jump (s) (s).

Preferably, the peripheral heating means is disposed against the tank wall, the outer side or the inner side of said wall, and all or part of its surface. It may be a thermal resistance which is arranged against the wall, and which is supplied with electricity to generate heat by Joule effect in a known manner. But you can also use other types of heat sources. According to one embodiment, the thermal conductors are conductive sheets extending in layers substantially parallel to each other. They may be, for example, metal, such as aluminum, or graphite.

The cartridge may define, according to an exemplary embodiment, a storage volume with a given height and a given section, the thermal conductors extending in substantially parallel planes between them over the height of the cartridge, so as to to form interleaves separating the storage material into separate pellets. In this configuration, each pellet is either entirely enveloped by a thermal conductor, or wrapped only on one of its faces or on one of its faces and all or part of its edge. In the first case in particular, there are thus pellets that can be manufactured before insertion into the cartridge, and the thickness of storage material is locally or gradually thinned. This reduction in thickness can then advantageously be compensated by an increase, in the same proportion, of the thickness of the conductive sheet in this zone, so as to keep a pellet thickness generally constant, which simplifies their stacking / insertion into the cartridge.

Alternatively, the storage material may also be in the form of a bread shaped to the internal volume of the cartridge, in which it comes to arrange the conductors. Advantageously therefore, the thermal conductors may comprise conductive sheets whose thickness varies and / or comprise conductive sheets partially superimposed or partially folded on themselves so that their overall thickness varies.

The invention also relates to the method of manufacturing such a cartridge, the method providing the following steps: - formation of pellets by encapsulation of storage material fractions by a metal conductor in the form of sheet (s) d non-constant thickness, the pellets once encapsulated having an overall thickness however substantially constant, - stacking of the pellets inside the casing of the cartridge to fill it over most of its height, - equipment of the cartridge shell of a heating means in direct or indirect contact with the metal conductors of the pellets. Naturally, the pellets can be inserted in the form of preconstituted batteries. In addition, and in known manner, it may be provided one or more steps of compressing the storage material which may be initially in powder form. It is also planned, but this step is known and not detailed here, a step of absorbing / adsorbing a predefined amount of reducing agent by the storage material, before the constitution of the pellets in particular.

Preferably, the method according to the invention provides means for identifying the variation in thickness on the metal conductors in the form of sheets. These means can be visual, or be reliefs or marks ...

Preferably, the method according to the invention provides to equip the encapsulated pads with keying means to ensure their stacking with a determined relative positioning. These means may be the same as the locating means mentioned above.

The invention also relates to the device for reducing nitrogen oxides contained in the exhaust gas of an internal combustion engine, which device incorporates the cartridge as described above, as well as, for example , other organs such as a dosing member, an injector member for injecting the reducing gas into the exhaust line, and means of the type of pipes, valves, pressure sensors and / or appropriate temperature.

By way of example, the storage material contained in the cartridge may be a salt chosen from at least one of the following salts: CaCl ₂ , SrCl ₂ , FeCl ₂ , MgCl ₂ and NiCl ₂ .

Other features and advantages of the invention will appear on reading the following detailed description of an embodiment of the invention, given by way of example only, with reference to the following schematic figures:

• Figure 1, a set of reducing agent storage cartridges as used to supply an NOx reduction device of an exhaust line of a combustion engine of a motor vehicle;

FIG. 2, a longitudinal section of a cartridge of the assembly according to FIG. 1;

• Figure 3, a section of the cartridge of Figure 2;

• Figure 4, an enlargement of a portion of the longitudinal section of the cartridge according to Figure 2;

FIGS. 5 to 7, forms of thermal conductor variants of that shown in FIG. 5;

FIG. 8, a graph showing the region of the thermally activated cartridge as a function of the state of loading of the cartridge according to FIGS. 2 to 4; FIG. 9, a graph representing the pressure rise times as a function of the state of loading respectively of a standard cartridge and of the cartridge according to the invention of FIGS. 2 to 4.

These figures are extremely schematic, and the various components shown do not necessarily respect the scale to facilitate reading.

FIG. 1 represents a set of cartridges containing SrCI salt ₂ loaded with ammonia, which set comprises two so-called main cartridges 1, 2, arranged in a thermally insulated support assembly E and a so-called starting cartridge 3, of smaller dimensions. (which could also be arranged in the support assembly E). Mechanical means, not shown, of the type of clamps, are provided to rigidly and removably attach the cartridges 1, 2 to the support assembly, and the cartridge 3 to another fixed element of the vehicle, so as to allow a cartridge change. In the example, the main cartridges 1, 2 are identical, but one can choose tanks of different capacities / properties. The starter cartridge 3 allows a faster / more efficient start of the operation of the aftertreatment system. It is intended to provide each cartridge outlet with a non-return valve (not shown). These different cartridges 1, 2, 3 are connected together by appropriate conduits, in a known manner, so that, on thermal activation of at least one of these cartridges, the latter releases the desired quantity of ammonia, via a common conduit 4 to the three cartridges, to the metering member / injector 5 injecting this ammonia in the exhaust line (not shown) to reduce NOx exhaust, so as to comply with current and future regulations. The thermal activation is obtained, for the main cartridges 1, 2, by an external thermal resistance, and, for the starter cartridge 3, by a plunging resistor. The three cartridges define a volume of substantially cylindrical shape, this form being preferred because it promotes resistance to cycled pressure and facilitates the process of filling the cartridges. The walls of the cartridges are metallic, in particular steel type stainless steel.

Figure 2 is a longitudinal section of the cartridge 1: the cartridge is provided on its outer wall with an electrical resistance 6 on most of its height and a portion of its circumference, as can also be seen from Figure 3, which is a cross section of the cartridge 1. The charged SrCI ₂ salt is placed in the layered cartridge a, b, c, interposed by thermal conductors 7 in the form of aluminum sheets. In the context of the invention, sheets are used which precisely delimit the salt layers a, b, c because they constitute layers coming to establish a direct contact with the steel wall of the cartridge, itself in direct contact with the heating resistor 6: the heat emanating from the resistance 6 by Joule effect when it is supplied with electricity is transmitted by conduction to the sheets 7 via of the wall of the cartridge, which come to heat the salt in their vicinity. The invention proposes to have layers a, b, c of salt whose thickness changes as one moves away from the heating resistor 6.

If we refer to Figure 4, which is an enlargement of Figure 2, we consider two points in the salt layer, arranged on a Y axis perpendicular to the X axis, at different distances from the Heating Resistor 6. The choice of the invention is to make sure that the point furthest from the resistor 6 is closer to the conductor 7 (the conductor adjacent to the layer in question) than the point closest to the resistor 6. the distance d2 is smaller than the distance d1 (the distances being measured parallel to the longitudinal axis X of the cartridge 1). Concretely, we see that the layer has thinner as one moves away from the heating resistor 6, and that the two conductors 7 which delimit it thicken in the same proportions, so as to maintain constant the thickness of salt layer a + its thermal conductors 7 which frame it.

These salt layers can be delimited in two different ways: either, as explained above, they are pellets entirely wrapped by a conductive sheet 7 and inserted into the cartridge 1. In this case, FIG. 4 represents pellets whose thickness is x1, with conductive sheets in contact which belong to two different pellets. The other possibility is that each salt layer has only one associated conductor on one of its faces and possibly all or part of its edge, and the width of the pellet is then x2, the encapsulation of the layer with its associated conductor and that of the adjacent layer. In both cases, the conductor 7 must therefore be thickened. FIG. 4 shows a gradual thickening, which can be obtained by a sheet of non-constant thickness. Other variants may be provided, which may provide thickening to define at least two areas in the layer of different thicknesses. Thus, Figure 5 shows a conductor 7 which has a variation in thickness by a jump approximately located near the longitudinal axis X of the cartridge. It can be obtained by a modification of the thickness of the sheet itself, or by a doubling of the sheet in the thicker zone, or a folding of the sheet, so as to obtain a thickness e in the first area, and a thickness by double example, 2 e, in the second zone. It is possible to provide not two, but three or four or more zones, in particular depending on the size of the cartridge.

Figure 6 is a variant, corresponding to the case where the cartridge 1 is provided not with a single heating resistor 6, but two resistors facing one another. In this case, it is sought to thin the layer in its central portion, and to use a conductor 7 which is correspondingly thickened in its central zone about the axis X. There is then a driver whose section s' approach of a diamond. Another variant with two resistors is shown in Figure 7, where the section of the conductor is the one times closer to an oval. Of course, these representations are very simplified. Figures 8 and 9 confirm the interest of the invention. The graph of FIG. 8 represents: on the abscissa, the state of charge of the cartridge, which can vary between 0%, corresponding to a level of ammonia in the cartridge that is too weak to be released under normal conditions of use, therefore to a cartridge that can be considered as empty, 100%, which is the ammonia charge state of the new cartridge, - ordinate the diameter of the cartridge where the ammonia release reaction occurs by activation thermal via the single heating resistor according to the variant of Figure 4, which varies between 0 and 100%. Thus, the 0% point on the ordinate corresponds to the salt which is closest to the resistor 6, and the 100% point on the ordinate corresponds to the salt which is diametrically opposed to the heating resistor 6. From the curve of the Figure 8, it is verified that the cartridge emptied of its ammonia gradually depleting the salt layers further and further away from the resistor 6.

The graph of Figure 9 shows the abscissa the state of charge of the cartridge as in the graph of Figure 8. In ordinate, is shown the pressure rise time at each thermal activation. Curve C1 represents the response of a cartridge according to the prior art, that is to say using cartridges salt pellets of constant thickness fully encapsulated by sheets of aluminum of constant thickness. Curve C2 is representative of the response of a cartridge according to FIG. 4, with pellets whose salt layer decreases as one moves away from the heating resistance, in favor of a thickening of the aluminum foils. who surround them. From the curve C1, it is seen that the more the cartridge is empty, and the more the pressure rise time increases: the salt, for the same thermal activation, will release more slowly the amount of ammonia expected. As one approaches a zero state of load, one reaches pressure rise times which can become unacceptable. In fact, we can thus be led to replace prematurely the cartridges. From the curve C2, it can be seen that the pressure rise time is almost constant throughout the life of the cartridge: it is thus proved that the pellets modified according to the invention make it possible to completely use the cartridges without penalizing the time. response of the cartridge, which is very advantageous. Note that the invention can be implemented in different ways. It can adapt to different cartridge geometries, to different configurations of the heat source (single source or multiple distributed sources). It can also be used for heating resistors, which, for example, would be plunging into the salt, by arranging conductors like rays from these resistors, conductors whose thickness could increase as and when away from the plunging resistance. It is also possible to envisage a heating resistor that would encase the entire periphery of the cartridge, with, in this case, a layer thickness which is thinner as one approaches the axis of the cartridge. Of course, these different variants can be adapted to cartridges that have a non-circular section.

It can be seen that the invention is in fact a joint modification of the thickness of the salt layers and their associated thermal / electrical conductors, modifications which preferably compensate themselves exactly, which has the advantage of not cause no modification in the usual manufacture of the cartridges, apart, possibly, the establishment of polarizers so that the pellets / salt layers are stacked on top of each other with the thinned salt zones of the layers in coincidence. It should be noted that this coding is superfluous when the modified pellets have a symmetry with respect to the longitudinal axis of the cartridge, which is the case when the heating resistor completely surrounds the cartridge, for example.

In known manner, the cartridge can also incorporate at least one of: a filter that prevents the passage of storage material to the outside of the cartridge, a metering device of the reducer, a check valve , a connection tip, a connection sealing device.

Claims

claims

1. Gaseous reductant storage cartridge (1) for selective catalytic reduction of nitrogen oxides, comprising: - a tank containing a solid storage material of said gaseous reductant, - at least one heating means (6) for the storage material and peripheral to said material, - a plurality of thermal conductors (7) within the storage material in direct or indirect contact with said heating means and propagating heat in said storage material, characterized in that said thermal conductors are disposed at within the layered storage material delineating, together with the wall of the reservoir, layers of storage material (a, b, c) whose thickness varies as a function of the distance to the nearest peripheral heating means.

2. Cartridge (1) according to the preceding claim, characterized in that the thickness of the layers (a, b, c) decreases with the distance to the nearest peripheral heating means.

3. Cartridge (1) according to one of the preceding claims, characterized in that the thickness of the layers (a, b, c) varies gradually or by jump (s).

4. Cartridge (1) according to one of the preceding claims, characterized in that the peripheral heating means (6) is disposed against the tank wall, the outer side or the inner side of said wall, and all or part of of its surface.

5. Cartridge (1) according to one of the preceding claims, characterized in that the thermal conductors are conductive sheets (7) extending in layers substantially parallel to each other.

6. Cartridge (1) according to one of the preceding claims, characterized in that the thermal conductors comprise conductive sheets (7), in particular metal, whose thickness varies and / or comprise conductive sheets partially superimposed or partially folded on themselves so that their overall thickness varies.

7. A method of manufacturing a cartridge (1) according to one of the preceding claims, characterized in that it provides the following steps: - constitution of pellets by encapsulation of fractions of storage material by a metallic conductor in the form of sheet (s) of non-constant thickness (7), the pellets once encapsulated having an overall thickness however substantially constant, - stacking of the pellets inside the envelope of the cartridge to fill it over most of its height, - equipment of the envelope of the cartridge of a heating means (6) in direct or indirect contact with the metal conductors (7) of the lozenges.

8. Method according to the preceding claim, characterized in that it provides means for identifying the variation in thickness on the metal conductors (7) in the form of sheets.

9. Method according to one of claims 7 or 8, characterized in that it provides to equip the encapsulated pads keying or locating means to ensure their stacking with a determined relative positioning.

10. Device for reducing nitrogen oxides contained in the exhaust gas of an internal combustion engine, characterized in that it integrates the cartridge (1) according to one of claims 1 to 6.