FR2907026A1 - Nitrogen oxide treatment system for use in e.g. heavy truck, has gaseous ammonia trap with heating unit situated in downstream of urea reservoir to trap ammonia vapor emitted during decomposition of urea in reservoir - Google Patents

Abstract

The system has gaseous ammonia trap (17) situated in downstream of an urea reservoir (16) to trap the ammonia vapor emitted during decomposition of the urea in the reservoir, where the trap has a heating unit (9) permitting desorption of ammonia. The trap has chemical substances e.g. active carbon, zeolite, silica gel, activated carbon, or activated clay. An anti-return valve (13) is situated in downstream of the trap, where the valve permits a gaseous transfer of the trap towards exterior.

Description

1 SYSTEM FOR TREATING NITROGEN OXIDES WITH TRAPPING SYSTEM

  The present invention relates to a treatment system by selective catalytic reduction of nitrogen oxides, said SCR system, in which it is done to limit the release of ammonia into the atmosphere.

  It is known, particularly for heavy vehicles, to use SCR systems for the treatment of nitrogen oxides in the exhaust line of the vehicles.

  The principle of such a system is to chemically reduce NOx nitrogen oxides by adding a reducing agent, such as ammonia NH3, upstream of a specific catalyst, called SCR catalyst. Such a system allows vehicles, especially those equipped with diesel engines, to comply with legally tolerated emission levels, these levels being lower and lower. Such a reduction principle is well known in industry, especially in stationary plants for which there is no problem to store ammonia.

  In contrast, applied to the automobile, such treatment requires storage of ammonia in a specific tank installed in the vehicle, and in which the ammonia is stored in various forms. One of the most common forms of storage is storage in the form of liquid urea in solution.

In this case, the principle of the SCR technology is to inject the urea solution upstream of the SCR catalyst. Thus, in the exhaust, there is first evaporation of the water contained in the solution, then decomposition of urea ammonia. This decomposition reaction of urea takes place in two successive stages; on the one hand, a step of thermolysis of urea in the form of the chemical reaction: (NH 2) 2 CO -> HNCO + NH 3, on the other hand a step of hydrolysis of isocyanic acid HNCO created during the thermolysis, with the chemical reaction: HNCO + H2O → NH3 + CO2. It can be seen from the first chemical reaction that the decomposition of the urea takes place without the addition of additional components. Accordingly, urea in the form of an aqueous solution has the characteristic of being relatively unstable since, depending on the temperature, the decomposition reaction can take place in the urea reservoir, especially if the temperature becomes too high. high. In this case, the urea decomposes to ammonia in the tank, and this ammonia created evaporates in the air of the tank. Ammonia is a toxic and odorous gas, its evaporation in the air represents a significant drawback, especially in the case where a vehicle is exposed to high temperatures, creating a significant release of ammonia. Such clearance is particularly troublesome in the case of a particular vehicle that rolls little. Indeed, when the vehicle is in motion, the gases are rapidly dissolved in the atmosphere, and are therefore little inconvenient for the users of the vehicle, which is not the case when the vehicle is stationary. It is for this reason, in particular, that this possible clearance is not taken into account in SCR systems installed on heavy goods vehicles, since these vehicles are, for the most part, in motion. The invention therefore aims to remedy at least part of this disadvantage by trapping the ammonia resulting from the decomposition of the urea in the tank. In particular, the invention relates to a selective catalytic reduction nitrogen oxide treatment system, known as SCR, intended to be installed in the exhaust line of the engine of a vehicle, the treatment consisting in chemically reducing, in a catalyst, called SCR catalyst, the nitrogen oxides by injecting ammonia in the form of liquid urea, pure or in solution, this liquid urea being stored in a specific reservoir intended to be installed in the vehicle, and the system comprising an ammonia gas trap located downstream of the urea reservoir, for trapping the ammonia vapors emitted during a decomposition of the urea in the tank.

The ammonia trap, located downstream of the tank, can be directly connected with the outside. In this case, it is useful for the system to include a check valve, located downstream of the ammonia trap, and for preventing the entry into the trap of external components, for example water or hydrocarbons, and to allow only a gas transfer of the trap to the outside. Indeed, external components can pollute the trap, and thus make it less effective. Furthermore, in some embodiments of the system, it is desired to reuse the ammonia stored in the trap, and it is therefore necessary that this ammonia is not contaminated by external components. On the other hand, in normal operation, ie when there is no excessive ammonia evaporation in the urea reservoir, it is not necessary to provide a gas transfer to the ammonia trap. For this purpose, in one embodiment, the system is such that gaseous transfer from the ammonia trap via the valve is permitted only when the gas pressure is above a predetermined value, for example 50 millibars. Thus, in normal operation, the valve is closed, and the reservoir is perfectly sealed. As mentioned above, in some embodiments it may be desired to reuse the ammonia enclosed in the trap. For example, it may be desired to inject this ammonia into the engine exhaust line so that it is used for the reduction of nitrogen oxides in the SCR catalyst. For this purpose, in one embodiment, the outlet of the ammonia trap is connected to the exhaust of the vehicle, upstream of the catalyst SCR. The ammonia trap contains materials for adsorbing ammonia molecules, these materials being included in the group comprising: activated carbons, zeolites, activated aluminas, silica gels and activated clays. Each of these materials has characteristics for storing the ammonia molecules: Activated carbons are elements prepared by pyrolysis of a material containing carbon, coal or other plant material. During this pyrolysis, charcoal is created, which is then oxidized by the steam under controlled conditions so as to obtain a microporous structure. There are several hundred grades of activated carbons, which differ according to the precursor, ie the material initially used, and the treatment conditions. There are also so-called chemical active carbons, which are activated at high temperature by the presence of desiccant chemicals, phosphoric acid or zinc chloride. Activated carbons are amorphous organophilic adsorbents, which means that their structure is not regular, and have a continuous pore size distribution whose spreading can reach several orders of magnitude. Zeolites are microporous crystalline alumino-silicates of the general formula (AlO2M, nSiO2), where M is most often an alkali or alkaline earth metal and n> 1. There are more than one hundred species of zeolites, which differ by N value and the crystallographic structure. The presence of cations in the micropores generates electric fields of the order of 1010 V.m 1, which makes these bodies powerful polar adsorbents. The activated aluminas are elements obtained by flash thermolysis of Al (OH) 3 aluminum trihydroxide which leads to a product of approximate composition Al2O3r 0.5H2O. This composition product has a porous structure, resulting from the departure of the water molecules. The surface of the pores is covered with Al-H groups, and the adsorption is preferably carried out by hydrogen bonding. Activated aluminas are amorphous, moderately polar and hydrophilic adsorbents. The silica gels are prepared from Si (OH) 4 in the aqueous phase, obtained by acidification of a sodium silicate, or from a silica sol, or by hydrolysis of an alkoxy silane. . The fluid solution thus obtained polymerizes rapidly, which makes it possible to obtain a gel which retains a loose structure after rinsing and drying. Si-OH groups lead to hydrogen bonds. There are two types of silica gels: microporous gels, which are relatively hydrophilic, or macroporous gels, which are versatile. As their name suggests, these gels differ from each other in pore size. Activated clays, on the other hand, are aluminosilicates having a crude formula close to zeolites, but with a different crystalline structure.

One of the characteristics common to all these materials is the fact that, when heated, there is a phenomenon of desorption, that is to say that the molecules trapped in their pores, for example ammonia molecules. , are released. This feature may be particularly useful for regenerating the trap, ie for emptying it. Indeed, in order for the trap to retain its storage efficiency, it is necessary to regularly remove the ammonia and water stored in the pores of the adsorbent materials present in the trap. For this, in one embodiment, the ammonia trap is equipped with a heating device for desorption of ammonia by the materials present in the ammonia trap, and thus a regeneration of the trap by rejection of the ammonia molecules. . This regeneration of the trap therefore corresponds to a degassing of ammonia.

In the case where the ammonia trap is connected directly to the outside, the regeneration does not have to take place when the vehicle is stationary. Indeed, such a phenomenon would lead to significant discomfort for users, an embarrassment that seeks precisely to avoid the system described here. To avoid unwanted outgassing, the system includes means for the heater to be turned on only when the vehicle is traveling at a speed above a predetermined speed. Indeed, as mentioned above, when the vehicle is traveling fast enough, the released gases are rapidly dissolved in the atmosphere, and are therefore not troublesome for the users of the vehicle. This activation of the heater only when the vehicle is traveling at a certain speed is also useful in the case where the outlet of the ammonia trap is connected to the exhaust line of the engine upstream of the catalyst. Indeed, in this configuration, the regeneration of the ammonia trap is carried out during a vehicle operating phase, that is to say during a phase where nitrogen oxides are present in the exhaust. In this case, the system comprises means for the ammonia resulting from the regeneration to be immediately used for the reduction of nitrogen oxides in the SCR catalyst.

If, on the other hand, the system is such that the outlet of the ammonia trap is connected to the exhaust line of the engine upstream of the catalyst and the regeneration of the ammonia trap is carried out while the vehicle is stationary. no nitrogen oxide is created, and ammonia can not be consumed for the reduction of these oxides. In this case, it is useful to be able to store this ammonia. For this purpose, in one embodiment, the system comprises means for the ammonia from the regeneration to be stored in the SCR catalyst, for example in zeolites, for later use in the reduction of the oxides. nitrogen. The amount of chemical material, such as zeolites, activated charcoal or the like, contained in the ammonia trap and necessary to trap all ammonia from the urea reservoir depends on the trap regeneration frequency. For example, in the case where the trap is never regenerated, it is necessary for the trap to contain about 30 kilograms of chemical material to adsorb the ammonia molecules. Indeed, if it is considered that, under the most severe temperature conditions, urea contained in a 25 liter tank is converted to gaseous ammonia and that the ammonia storage capacity of the zeolite is 290%. order of 20 moles per kilogram, then, taking into account the molar mass of zeolites, it is found that it is necessary to have about 2 kilograms of zeolites to avoid any NH 3 ammonia emission towards the outside. vehicle. On the other hand, in the case where regular regenerations of the ammonia trap are carried out, 500 grams of chemical material may be sufficient to adsorb the ammonia molecules. The invention also relates to a motor vehicle comprising a system as described above. Other characteristics and advantages of the invention will become apparent with the description of some of its embodiments, this description being given in a non-limiting manner with the aid of the figures in which: FIG. 1 represents a processing system of FIG. According to the invention, FIG. 2 shows the evolution of the urea concentration of an aqueous solution of urea, for different temperatures. FIG. 3 shows the storage capacity of a zeolite. depending on the temperature, and - Figure 4 shows the adsorption capacities of different types of materials. FIG. 1 shows a system for treating nitrogen oxides present in the exhaust line 10 of a vehicle engine, particularly of the Diesel type. These nitrogen oxides are directed to a specific catalyst SCR 12, in which a chemical reduction is carried out. For this reduction to occur, it is necessary to add ammonia to the nitrogen oxides, for example contained in liquid urea 14. This liquid urea is usually stored in a specific reservoir. 16 installed in the vehicle. This tank 16 is connected, via a supply duct 18, to a specific injector 19, for injecting urea 14 into the exhaust line 14 of the engine.

When the vehicle in which the system is installed is exposed to high temperatures, for example if it remains parked in the sun for a relatively long time, the temperature inside the tank increases rapidly.

This increase in temperature results in a slow decomposition of urea, particularly illustrated in FIG. 2. In this figure, curves 20 and 22 represent the time course of the concentration of urea in water, when a solution of urea is exposed to temperatures of 60 C and 70 C. These temperatures, which seem high for ambient temperatures, are however easily achievable in a vehicle engine exposed to direct sunlight for several hours.

On these two curves, it can be seen that the concentration of urea in the water gradually decreases over the days; thus, after 5 days at 70 ° C., the concentration has already increased from 6 moles per liter initially to 5 moles per liter; after 10 days at 60 ° C., the concentration is only 25 of 5.50 moles per liter, for the same initial concentration of 6 moles per liter. The decomposed urea 14 is converted into ammonia which evaporates in the air of the tank 16. Since the ammonia is a poisonous and odorous gas, it can be inconvenient to leave the tank 30 in the open air. Thus, in a system according to the invention, there is provided an ammonia trap 17 installed at the outlet of the urea reservoir 16. This ammonia trap contains chemical substances such as activated carbons, zeolites, silica gels, activated alumina or activated clays, these substances having the full capacity to store molecules such as ammonia molecules. Depending on the type of chemical material used, the amounts of adsorbed ammonia may vary. These variations appear in FIG. 4, which represents, for three types of adsorbent chemical materials, the adsorption capacity as a function of the moisture content, ie the quantity of water molecules in the presence. The moisture content is shown as a percentage on the x-axis, and the adsorption capacity is shown in kilograms of water adsorbed per 100 kilograms of adsorbent. In this figure, the curves 40, 41 and 42 correspond to the respective adsorption capacities of a zeolite, a microporous silica gel and a macroporous silica gel. It can be seen that these three materials have relatively different capacities depending on the humidity conditions of the medium. Thus, for a low moisture content, ie less than 20%, the microporous silica gel is the most efficient, whereas for a high moisture content, for example greater than 80%, it is macroporous silica gel which offers the best results. Regarding the zeolite it is a relatively effective material regardless of the moisture content of the medium, and therefore regardless of the amount of water molecules to adsorb.

After the ammonia molecules have been stored in the ammonia trap, it may be useful to be able to regenerate the trap by emptying it. For this, the ammonia trap is provided with a heating device 9. In fact, the chemical materials used to trap ammonia are such that when they are subjected to a high temperature, for example greater than 200 C, there is a desorption, ie a release of the molecules stored in the pores of these materials. Thus, the ammonia molecules are released.

In one embodiment, it may be useful to use these desorbed ammonia molecules for the reduction of nitrogen oxides. In this case, there is provided a return tube 11 connecting the outlet of the ammonia trap to the exhaust line 10 of the engine. In addition, in order to prevent the gases present in this exhaust line 10 from returning to the ammonia trap 17, a non-return valve 13 is provided in the return tube 11.

This valve 13 is such that it opens only in the ammonia trap direction => exhaust line, when the pressure becomes greater than a predetermined value. This value can be, depending on the type of valve used, of the order of 50 millibars or 100 millibars.

The ammonia molecules, transferred into the exhaust line 10, are then trapped in a closed zone between the oxidation catalyst 15 and the SCR catalyst 12. The ammonia is thus naturally directed to the catalyst 12. If the vehicle is in operation at this time, this means that nitrogen oxides are present in the exhaust line 10, and the ammonia is then consumed directly in the catalyst 12, for the reduction of these nitrogen oxides . On the other hand, if the vehicle is stationary, it may be useful to store the ammonia molecules for later use. For this purpose, the catalyst 12 contains zeolites, having the capacity to store ammonia. This storage capacity is illustrated in FIG. 3. In this figure, the curve represents the evolution of the storage capacity, expressed in grams of NH 3 ammonia per liter, as a function of the temperature. Thus, it is found that for a temperature of 150 C, the storage capacity is greater than 0.6 grams per liter. Only dots for temperatures above 150 ° C are shown in this figure, but it is known that zeolites are very effective at storing ammonia at ambient temperatures. As a result, for small amounts of ammonia in the gases, this ammonia is entirely stored in the SCR catalyst.

One of the advantages of the heater 9 is that it is possible to control the moment at which the desorption takes place, and thus to ensure that this desorption takes place during a rolling phase of the vehicle, so that it can be consumed immediately. ammonia released. In some embodiments, it may not be desirable to reuse the ammonia from the urea reservoir. In this case, the ammonia trap 17 is not connected to the exhaust line of the engine, but directly to the outside. In this case, the heating device 9 is generally activated when the vehicle has reached a certain speed, so that the ammonia is released and dispersed immediately into the atmosphere, without creating an inconvenience for them. 20 users of the vehicle or people who may be in an environment close to this vehicle. In this embodiment, it may be useful to place at the exit of the trap a non-return valve, similar to the check valve 13, in order to avoid contamination of the ammonia trap by external elements such as water. or hydrocarbons. 30

Claims (11)

  1. Selective catalytic reduction nitrogen oxide treatment system, known as SCR, intended to be installed in the engine exhaust line of a vehicle, the treatment consisting in chemically reducing, in a catalyst, said SCR catalyst , the oxides of nitrogen by injecting ammonia in the form of liquid urea, pure or in solution, this liquid urea being stored in a specific reservoir intended to be installed in the vehicle, and the system comprising a gaseous ammonia trap located downstream of the urea reservoir, for trapping the ammonia vapors emitted during a decomposition of the urea in the tank.   2. System according to claim 1 comprising a non-return valve, located downstream of the ammonia trap, and intended to prevent the entry into the trap of components outside the urea tank, for example water or hydrocarbons. , and to allow only a gaseous transfer of the trap to the outside.   3. System according to claim 2 wherein the gaseous transfer from the ammonia trap via the valve is allowed only when the pressure of the gas is greater than a predetermined value, for example 50 millibars.   4. System according to one of the preceding claims wherein the outlet of the ammonia trap is connected to the exhaust of the vehicle, upstream of the SCR catalyst.   5. System according to one of the preceding claims, wherein the ammonia trap contains materials for adsorbing the ammonia molecules, these materials being included in the group comprising: activated carbons, zeolites, activated aluminas, silica gels and activated clays. 2907026 14   6. System according to claim 5 wherein the ammonia trap is equipped with a heating device for desorption of ammonia by the materials present in the ammonia trap, and thus a regeneration of the trap by rejecting the molecules of ammonia. ammonia.   The system of claim 6 including means for the heater to be turned on when the vehicle is traveling at a speed above a predetermined speed.   8. System according to claim 6 or 7, comprising means so that, when the outlet of the ammonia trap is connected to the exhaust line of the engine upstream of the catalyst SCR, the ammonia from the regeneration of the ammonia trap is immediately used for the reduction of nitrogen oxides in the SCR catalyst.   9. System according to claim 8 comprising means so that, in the case where the vehicle is stationary, the ammonia resulting from the regeneration of the ammonia trap is stored in the SCR catalyst, for example in zeolites, in for further use for the reduction of nitrogen oxides.   10. System according to one of the preceding claims wherein, in the case where the trap is never regenerated, this trap contains about 2 kilograms of chemical material for adsorbing the ammonia molecules.   11. System according to one of claims 1 to 9 wherein, in the case of carrying out regular regenerations of the ammonia trap, this trap contains about 500 grams of chemical material for adsorbing the ammonia molecules.