FR2914687A1 - METHOD AND SYSTEM FOR INJECTING REDUCING AGENT IN AN EXHAUST LINE - Google Patents

Abstract

The invention relates to a reducing agent injection process in an exhaust line of an engine (1), the reducing agent being intended to be used to reduce chemically during a selective catalytic reduction reaction. , called SCR, nitrogen oxides emitted by the engine (1), the injection being carried out upstream (6) of a catalyst (3) in which the reaction takes place, the process comprising the following steps: determines the distribution of the gas at the inlet (9) of the catalyst SCR, and when this distribution is greater than a predetermined value, reducing agent is injected into the exhaust line.

Description

METHOD AND SYSTEM FOR INJECTING REDUCING AGENT IN A LINE

EXHAUST

  The present invention relates to a reduction agent injection process in the exhaust line of an engine, more particularly to an engine installed in a diesel-type motor vehicle.

  In diesel-type vehicle engines, the combustion of fuel results in the creation of gases such as nitrogen monoxide (NO), nitrogen dioxide (NO2) or nitrous oxide (N20). These gases, known generally as nitrogen oxides (NOx) present a danger, on the one hand for the health of human beings, and on the other hand for the environment since they contribute to the formation pollution fog in cities, and global warming by increasing the greenhouse effect. Consequently, it is necessary to provide solutions for destroying these gases inside the vehicles, before they are emitted into the atmosphere. The treatment of these gases in vehicles is also severely regulated by different standards.

  To chemically destroy these nitrogen oxides before their escape into the atmosphere, it is known to carry out a reduction of the selective catalytic reduction type, known as SCR. For this purpose, it is possible to use different reducing agents. For example, mention may be made of carbon monoxide, hydrocarbons, amides, acids or ammonia. Many solutions using urea as a reducing agent are also known. In fact, urea contains ammonia which reacts with nitrogen oxides in an SCR catalyst to form completely harmless dinitrogen. Most of these solutions use an injection of urea in liquid form. The choice to use urea instead of pure ammonia is that ammonia is a poisonous and corrosive gas, and it is therefore expensive and complicated to provide, in a conventional vehicle, a reservoir. allowing the storage of such a gas safely. Urea, for its part, can be stored in the form of an aqueous solution, which allows easy storage and injection into the exhaust line.

  Thus, in a process for treating nitrogen oxides using urea, the decomposition of urea into ammonia is first observed, followed by a reduction of nitrogen oxides by this ammonia. The reduction of nitrogen oxides using liquid urea implements several successive chemical reactions. A solution of urea injected into an exhaust line undergoes, in the first place, evaporation of the water, thus leading to the formation of solid urea, in the form of tiny particles. This reaction results in the following chemical equation: NH2-CO-NH2 (aqueous) -> NH2-CO-NH2 (soiide). (Eq1)

  Generally, following this evaporation, the solid urea undergoes thermolysis under the effect of the high temperature of the surrounding gases, from 180 C. This thermolysis produces ammonia gas and isocyanic acid during the following reaction: NH2-CO-NH2 (soiide) -> NH3 (gas) + HNCO (gas) (Eq2)

  The final step of the reduction process consists of hydrolysis of isocyanic acid to form gaseous ammonia and carbon dioxide: HNCO (gas) + H2O (gas) -> NH3 (gas) + CO2 (gdz) ( Eq3) Below 180 ° C., the rate of decomposition of the urea particles by thermolysis is slower than the partial polymerization rate of urea. As a result, the urea particles are transformed into biuret. The biuret decomposes rapidly by sublimation above 180 C or, below 180 C, on any catalytic surface that contains acidic sites. The two forms of biuret decomposition can be described in a global manner by the following reaction: H2NCONHCONH2 (solid) + 2 x H2O (gas) -> NH3 (gas) + CO2 (gdz) (Eq4) When the temperature of the urea injected into the exhaust line becomes less than 180 ° C., there is no longer any complete decomposition, and we then see a total polymerization of the urea at high concentration, and the formation of a white solid on the surface of the SCR catalyst or on the walls of the exhaust line. Indeed, as explained above, the thermolysis step of solid urea, represented by equation 2, occurs due to the high temperature of the exhaust gas. When the temperature is lower, a solid cyamelide polymer is formed: NH 2 -CO-NH 2 (solid) -> H NCOX (solid polymer) (Eq 5) To avoid this, the urea injection is generally carried out at temperatures between 180 C and 200 C.

  The nitrogen oxide treatment solutions described in the present application all implement a selective catalytic reduction reaction, in the enclosure of a catalyst, called SCR catalyst, installed in the exhaust line of the engine, in downstream of the engine. In order for this reduction to be as effective as possible, it is necessary for the reducing agent to be sufficiently distributed over all the active zones of the catalyst. Indeed, if certain zones do not contain, or little, reducing agent, it is possible that some nitrogen oxides pass through the catalyst without coming into contact with the reducer, and therefore without being treated. On the other hand, when a large amount of reducing agent is injected into or upstream of the catalyst, saturation of the catalyst may occur if the amount of reducing agent to be stored exceeds the storage capacity of the catalyst. This storage capacity depends, for example, on the age of the catalyst and / or the type of catalyst, depending on whether a microporous structure catalyst of large capacity or a non-porous catalyst is used.

  However, motor vehicles are subject to strict standards with respect to gases released into the atmosphere. Thus, it is not conceivable that a vehicle releases too large quantities of nitrogen oxides or reducing agent. Accordingly, the invention aims to overcome the disadvantages described above, which lead to unacceptable gas emissions.

  More specifically, the invention relates to a method of injecting reducing agent into an exhaust line of an engine, the reducing agent being intended to be used to reduce chemically during a reduction reaction. catalytic selective, called SCR, nitrogen oxides emitted by the engine, the injection being carried out upstream of a catalyst in which the reaction takes place.

  The process comprises the following steps: a value representative of the distribution of all the gases at the inlet of the SCR catalyst is determined, and when this distribution is greater than a predetermined value, reducing agent is injected into the the exhaust line. When the gas distribution at the inlet of the catalyst is not uniform in reducing agent, it is preferable not to inject reducing agent into the exhaust line. In the present case, we are interested in the distribution of the reducing agent among all the gases, on a section of a tube at the inlet of the catalyst. The set of gases comprises the reducing agent and the gases emitted by the engine. A representative value of this distribution is, for example, a distribution ratio λ of between 0 and 1, corresponding, for example, to the ratio, if it is located at an inlet section of a catalyst, between the surface occupied by the reducing agent and the total area of the section. Thus, the value λ = 1 represents a perfect distribution over the entire section, and the value λ = 0 represents an absence of reducing agent among all the gases on the section.

  In an advantageous embodiment of the invention, it will be chosen to inject a reducing agent only in the case where the gas distribution ratio is greater than 0.75.

  A poor distribution of a reducing agent in an exhaust line is, for example, due to the architecture of the line, which may have bends, turns, or areas of expansion and contraction, which have as a consequence of varying the distribution of gas, and sometimes to change the course of the gas. Thus, it sometimes leads to a non-uniform distribution of the reducing agent which causes numerous drawbacks at the level of the exhaust line, among which: premature thermal or chemical aging, inhomogeneous or incomplete distribution and combustion of particles in a particle filter located at the end of the exhaust line, - significant thermal variations, or gradients, along the exhaust line, which can lead to cracks in the filter, and - poor catalytic conversion .

  These gas distribution problems are, moreover, not amplified by the number of elements added in the exhaust line, such as the injection systems and reducing agent tanks, or the probes and sensors used to measure the quantities or temperatures of gas. The introduction of these elements leads to diversions 30 of the gas flow, sometimes preventing a return to a uniform distribution. This problem of distribution in the exhaust line is the main cause of the problems of excess or lack of reducing agent in the catalyst, previously described.

  Several solutions are possible to determine the homogeneity of the gaseous mixture at the inlet of the catalyst, which is equivalent to a distribution. Thus, according to the embodiment chosen, the method comprises one or more of the following steps: the step of measuring the flow of gas in the exhaust line upstream of the catalyst; the step of measuring the temperature of the gases; in the exhaust line, - the step of performing a calculation using laws of the dynamics of the fluids, as a function of a gas flow and / or a gas temperature in the exhaust line.

  The solution of performing a computation of fluid dynamics is particularly advantageous in the case where it is difficult to determine the distribution experimentally, for example by measurement. Indeed, the calculations can be relatively long, since they sometimes last several hours, even days or weeks. Such a calculation, for example implemented by a software product, is also advantageous when it is desired to determine the gas distributions in several types of exhaust lines. Indeed, it can be expensive to physically perform several lines, while it is possible to modify the calculation parameters in software, without additional cost. To perform these calculations it is possible, for example, to use a fluid dynamics calculation software such as Fire. In the case where one chooses to use such a software product, it is necessary to reproduce the exact architecture of the exhaust line, with injectors, particle filters, catalysts, or the diameters and angles of different tubes. Indeed, all these elements have an influence on the distribution of gases in the exhaust line, and any calculation not taking into account one of these parameters would not be exploitable. On the other hand, it was found that the state of the particulate filter, ie the quantity of trapped particles, or the type of fuel, did not influence the distribution of the gases.35 Another possibility considered for determining the gas distribution consists in using predetermined data, and stored in a memory of a processor used for the implementation of the invention. For example, it is possible to measure, in the laboratory, the gas distributions over a complete operating cycle of a vehicle, passing through all possible operating phases. Thus, in one embodiment, the method comprises the following preliminary steps: the distribution of the gases at the inlet of the catalyst is measured for several phases of operation of a vehicle, and it is recorded in a memory of a processor , the measured values, associated with the corresponding operating conditions of the vehicle.

  Since the distribution measurement is made in the laboratory, it is possible to use devices such as infrared cameras or motion or mass sensors to detect the presence and movements of the gases. These steps make it possible to highlight reducing agent injection points safely. We can then choose to perform the injection of reducing agent at the time of the occurrence of these points.

  It is also possible to choose not to perform these measurements over a complete cycle, but only on certain specific operating phases of a vehicle, for example the acceleration and deceleration phases.

  The method then comprises, preferably, the step of measuring, at the time of its implementation, the operating conditions of a vehicle, and using the data recorded in memory to determine the distribution of the gases.

  The data stored in memory may also include maps representing relationships between some of the following parameters: the temperature in the exhaust line, the speed of the vehicle, the speed of movement of the gases, or the concentration of nitrogen oxides.

  Moreover, the amount of reducing agent to be injected depends on numerous parameters, and in particular on the concentration of nitrogen oxides in the exhaust gas. Indeed, the chemical reactions used for the treatment of nitrogen oxides are for example: NH 3 + NO x + O 2 -> N 2 + H 2 O in the case where the reducing agent is ammonia, and HC + NOx + 02 -> CO2 + N2 + H2O in the case where the reducing agent is a hydrocarbon. In order for the reaction to be complete and no reducing agent remaining at the end, it is necessary to inject a correct amount of reducing agent, for example an amount such that the reaction is carried out under stoichiometric conditions. . For this purpose, in one embodiment, the method comprises the step of determining the amount of nitrogen oxides emitted by the engine, and determining the amount of reducing agent necessary to reduce these nitrogen oxides. Thus, during the injection step, an amount of reducing agent is injected equal to this predetermined quantity.

  The invention also relates to a reducing agent injection system in an engine exhaust line, the reducing agent being intended to be used to reduce chemically, during a selective catalytic reduction reaction, called SCR, nitrogen oxides emitted by the engine, the system comprising: - a catalyst seat of the reduction reaction, - a device for measuring the temperature of the gases in the exhaust line, - a device for determining the distribution of the catalyst inlet gas, a device for determining the amount of reducing agent to be injected, a reducing agent reservoir and a reducing agent injector located upstream of the catalyst.

  Other advantages and characteristics of the invention will become apparent with the description of some of its embodiments, this description being made in a nonlimiting manner using the figures in which: FIG. 1 shows the evolution of certain parameters present in a vehicle engine on a known operating cycle, namely an MVEG cycle. FIG. 2 shows a vehicle exhaust line equipped with a liquid urea injector; FIGS. 3a and 3b show examples of gas distribution on an inlet section of an SCR catalyst; FIG. 4 shows a functional diagram of a method according to the invention.

  In Figure 1, there is shown various parameters measured in the motor of a motor vehicle, a normalized operating cycle MVEG. This cycle is currently used for the approval of vehicles in Europe. FIG. 1 shows three curves representing, respectively, the evolution of the vehicle speed (curve 11), the amount of nitrogen oxides released by the engine (curve 12) and the temperature (curve 13), in function of time. The operation of the vehicle according to the MVEG cycle involves a succession of accelerations and decelerations of the vehicle.

  An advantageous embodiment of a process according to the invention is carried out in an exhaust line as shown in FIG. 2. This figure illustrates the case where the reducing agent is ammonia contained in urea. liquid, but similar systems can be used in the case of another reducing agent. This figure shows a vehicle engine 1, which emits nitrogen monoxide NO and NO2 nitrogen dioxide. At the outlet of this engine is an oxidation catalyst 2, used to increase the NO2 / NO ratio of the exhaust gas, thus allowing a better subsequent reduction of the nitrogen oxides in the selective reduction catalyst 3. The gases The treated exhaust gases lastly pass through a particulate filter 4 before being discharged into the atmosphere. In order to allow the implementation of the method, the various elements of the exhaust line are managed by the onboard computer 5 of the vehicle. Thus, for example, from the temperature of the gases, read by the device 7, the computer 5 is able to determine, from experimental data recorded in a memory, the amount of urea to be injected into the 6. The urea is stored in a tank 8. In addition, the exhaust line is provided with two gas detectors 9 and 10 for measuring the amounts of gas present respectively upstream and downstream. downstream of the SCR catalyst 3. The predetermined data contained in a memory of the computer 5 may be in the form of a map, a table or any other set of data.

  Figures 3a and 3b show examples of possible gas distribution on an inlet section 21 of an SCR catalyst installed in a motor vehicle exhaust line. In FIG. 3a, it can be seen that the reducing agent 22 is distributed over almost all of the section 21, while in FIG. 3b the reducing agent 23 is only present on a small part of the section. In these two cases, it is possible to determine a gas distribution coefficient on the section. This coefficient corresponds to the ratio of the area occupied by the gas to the total area of the section. It is, for example of the order of 0.95 in Figure 3a, and of the order of 0.45 in Figure 3b. Thus, in the context of a process according to the invention, if an injection limit threshold λ = 0.95 is chosen, an injection will take place in the case of FIG. 3a, but not in that of FIG. 3b, since the injected element would not be correctly distributed over all the active zones of the catalyst.

  With the aid of FIG. 4, a possible implementation of a process according to the invention will now be described in the case where the injected reducing agent is ammonia contained in urea. It has been found that, at temperatures below 120 ° C., it is unnecessary to inject urea, for the following reasons: at first, at these temperatures, the evaporation of urea is relatively difficult, this compromises the implementation of the first necessary chemical reaction normally leading to a formation of urea particles, and - on the other hand, the SCR catalysts are generally such that they can not be active at less than 120 ° C., and they can not therefore be the site of a decomposition of urea into ammonia. In addition, even if the SCR catalyst already contains ammonia stored in its micropores, the reduction of nitrogen oxides is impossible at such low temperatures.

  As a result, it is necessary to measure the temperature of the gases in the exhaust line. For this purpose, a thermocouple 30 is used, that is to say a device comprising two metals connected by two junctions, and which generates a potential difference depending on the temperature difference between the two junctions. In order to be able to relate the potential difference to a temperature difference, it is therefore necessary to know the thermocouple response as a function of temperature. This response is, for example, stored in a memory processor, so that it can be used during a step of the process.

  Then, during a step 31, the raised temperature is determined to be greater than 120 ° C. If this is not the case, the process stops. On the other hand, if the temperature is sufficient, during a step 32, the distribution of the gases is determined on an inlet section of the catalyst SCR. To make this determination, data stored in processor memory is used in the form of one or more gas distribution mappings 33. If the distribution of the gases, that is to say the proportion of reducing agent among all the gases at the inlet of the catalyst, is less than a predetermined value, for example 0.95, no injection is carried out. 'urea. On the other hand, if the distribution of gases is sufficient, it is possible to inject urea.

  A processor 34 is then used to determine the amount of urea to be injected. To perform this calculation, the processor 34 uses parameters such as the amounts of gas detected in the exhaust line by a set of sensors. Furthermore, the processor 34 can use data 36 stored in memory, for example in the form of maps showing the evolution of parameters such as the temperature or the distribution of gases. The determined quantity is finally injected into the exhaust line via an injector 37.

Claims (8)

  A method of injecting reducing agent into an exhaust line of an engine (1), the reducing agent being intended to be used to reduce chemically, during a selective catalytic reduction reaction, called SCR , oxides of nitrogen emitted by the engine (1), the injection being carried out upstream (6) of a catalyst (3) in which the reaction takes place, the method comprising the following steps: (32) a value representative of the distribution of all the gases at the inlet (9) of the SCR catalyst (3), and when this distribution is greater than a predetermined value, the reducing agent (37) is injected into the line exhaust.   2. Method according to one of the preceding claims comprising the step, for determining the distribution of the gas at the catalyst inlet, to measure the flow of gas in the exhaust line upstream of the catalyst (3).   3. Method according to one of the preceding claims comprising the step of determining the distribution of the gas at the inlet of the catalyst, to measure the temperature of the gases in the exhaust line.   4. Method according to one of the preceding claims comprising the step, for determining the distribution of the gas at the catalyst inlet, to perform a calculation using laws of fluid dynamics, according to a flow rate of gas and / or the temperature of the gases in the exhaust line.   5. Method according to one of the preceding claims comprising the following preliminary steps: - the distribution of the gases at the inlet of the catalyst (3) for several phases of operation of a vehicle is measured, 14 is recorded, in a memory d a processor (5), the measured values, associated with the corresponding operating conditions of the vehicle.   6. The method of claim 5 comprising the following steps: (31) the operating conditions of the vehicle are measured, and - from these operating conditions and the data stored in memory, (32) the distribution of the gases to be determined is determined. the catalyst inlet.   7. Method according to one of the preceding claims, comprising the following steps: - the amount of nitrogen oxides emitted by the engine (1) is determined, the amount of reducing agent necessary to reduce these oxides is determined; nitrogen, during the injection step, this quantity of reducing agent determined is injected.   8. A reducing agent injection system in an engine exhaust line (1), the reducing agent being intended to be used for chemically reducing, during a selective catalytic reduction reaction, so-called SCR, nitrogen oxides emitted by the engine (1), the system comprising: - a catalyst (3) seat of the reduction reaction, a device (7) for measuring the temperature of the gases in the exhaust line, - a device for determining the distribution of gases at the inlet of the catalyst, a device for determining the amount of reducing agent to be injected, - a reservoir (8) of reducing agent and an injector (6) of reducing agent located upstream catalyst (3).