WO2008071727A1

WO2008071727A1 - Method and system for metering an aqueous ammonia precursor solution into the exhaust gases of an engine

Info

Publication number: WO2008071727A1
Application number: PCT/EP2007/063764
Authority: WO
Inventors: Francois Dougnier; Joel Op De Beeck; Jules-Joseph Van Schaftingen
Original assignee: Inergy Automotive Systems Research (Société Anonyme)
Priority date: 2006-12-14
Filing date: 2007-12-12
Publication date: 2008-06-19
Also published as: WO2008071730A1

Abstract

Method for metering an aqueous ammonia precursor solution into the exhaust gases of an internal combustion engine and consisting in determining the total amount of ammonia available in the solution in the form of precursor, ammonia and ammonium hydroxide and in calculating the volume of the solution to inject as a function of this amount.

Description

METHOD AND SYSTEM FOR METERING AN AQUEOUS AMMONIA PRECURSOR SOLUTION INTO THE

EXHAUST GASES OF AN COBUSTION ENGINE

The present invention relates to a method and system for metering an aqueous ammonia precursor solution into the exhaust gases of an engine. The change in legislation relating to emissions from diesel vehicles especially stipulates a significant reduction in releases of nitrogen oxide in 5 exhaust gases. The SCR (Selective Catalytic Reduction) process in which nitrogen oxides (NO_x) are reduced by ammonia is one of the technologies of choice for achieving this pollution control objective. An aqueous solution of an ammonia precursor (generally of urea or of a mixture of urea and ammonium formate) is then injected into the exhaust line upstream of the SCR catalyst. The

10 operation of this catalyst requires precise metering of the amount of solution sprayed. This is because both overmetering (leading to a release of ammonia into the atmosphere) and undermetering (leading to an expulsion of untreated nitrogen oxides into the atmosphere) are harmful.

However, these solutions are not stable over time, as the urea (and the

15 formate, where appropriate) decompose in water (and even more so when the temperature is high) into ammonia and into carbon dioxide. Therefore, certain systems of the prior art provide a correction of the amount of solution injected as a function of the change in the urea concentration as a function of the time, temperature and pressure. Such a system is, for example, described in Patent US

20 6,408,619 and Application US 2003/0033799 which describe the fact of correcting the volume of solution injected as a function of its effective urea concentration following its ageing. However, the methods described in these documents do not take into account the ammonia dissolved in said solution (from the decomposition of the urea or of the formate and partially in the form of

25 ammonium hydroxide) and that yet also participates in the reaction of reduction.

In US '799, the calculation of the urea concentration is based on the fact that the urea decomposition is expressed by a pressure increase that depends on the vapor pressure of the resultant ammonia, plus the vapor pressure of the water and that is measured by a pressure sensor. Since some ammonia is dissolved in

30 the water, and some of it decomposes in the water, the partial pressure of every nitrogen component is calculated and used to deduce the amount of urea decomposed and hence, the very urea concentration.

Therefore these methods/systems from the prior art induce an overmetering of ammonia that is bad for the environment. The present application aims to solve this problem by providing a more accurate metering method and in particular that avoids overmetering.

Therefore, the present invention relates to a method for metering an aqueous ammonia precursor solution into the exhaust gases of an internal combustion engine, said method consisting in determining the total amount of ammonia in the solution in the form of precursor, ammonia and ammonium hydroxide and in calculating the volume of the solution to inject as a function of this amount.

Any ammonia precursor in aqueous solution may be suitable within the context of the invention. The invention gives good results with eutectic solutions of urea for which there is a standard quality: for example, according to the standard DIN 70070, in the case of the AdBlue^® solution (commercial solution of urea), the urea content is between 31.8% and 33.2% (by weight) (i.e. 32.5 +/- 0.7% by weight) hence an available amount of ammonia between 18.0% and 18.8%. The invention may also be applied to the urea/ammonium formate mixtures sold under the trade name Denoxium™ and of which one of the compositions (Denoxium-30) contains an equivalent amount of ammonia to that of the Adblue^® solution. The latter have the advantage, with respect to urea, of only freezing from -30⁰C onwards (as opposed to -11°C), but have the disadvantages of corrosion problems linked to the possible release of formic acid.

The present invention may be applied to any internal combustion engine likely to generate NO_x in its exhaust gases. It may be an engine with or without a fuel return line (that is to say, a line returning the surplus fuel not consumed by the engine to the fuel tank). It is advantageously applied to diesel engines, and in particular to vehicle diesel engines and particularly preferably to the diesel engines of heavy goods vehicles.

The method according to the invention consists in taking into account all the forms of ammonia available directly or indirectly in the solution. It takes into account the fact that: - urea hydrolyses according to the following overall reaction:

(NH₂)₂CO + H₂O -> CO₂ + 2NH₃ (1) - the ammonia released is found partly in the atmosphere above the solution, partly dissolved in the latter and reacts partly with water to form ammonium hydroxide according to the reaction:

NH₃ + H₂O = NH₄OH (2) - the ammonia and the ammonium hydroxide present in the solution are injected with it and participate in the reaction with the NO_x: 2NH₃ + NO + NO₂ -> 2N₂ + 3H₂O (3) the equilibrium reaction (2) being displaced towards the left as the ammonia is consumed. Similar reactions take place with the ammonium formate which, in solution, may release ammonia and formic acid and which decomposes at high temperature to form ammonia, carbon monoxide and carbon dioxide.

According to a preferred variant of the invention, with a view to limit the number of parameters influencing the ageing (and thus the number of measurements to do) the solution of precursor is stored in a ventilated tank (i.e. vented at the atmospheric pressure) and/or of which the pressure is calibrated so that only the duration and the temperature have to be considered as parameters. A calibrated overpressure gives good results.

In the method according to the invention, the concentrations of precursor (urea and/or ammonium formate for example), of ammonia and of ammonium hydroxide, may be determined in any known manner. They can be determined by means of suitable instruments integrated in the additivation system (for instance conductivity probe for urea and formate, and ammonia probe for NH₃ and NH₄OH). Alternatively and in a preferred manner, these measurements are first done for reference solutions that have been intentionally left to age at various temperatures (and optionally, pressures), so as to obtain nomograms that make it possible to deduce the amount of ammonia available as a function of the history (ageing) of the solution.

According to one variant, the total nitrogen is metered by an analytical chemistry method and the corresponding number of moles of NH₃ per litre of solution are deduced therefrom. Methods which are particularly suitable for this purpose are the Kjeldahl method and a pyrometry/chemiluminescence combination such as applied, for example, by analysers of the ANTEK 9000 NS type. These methods are suitable for establishing the nomograms as explained hereabove. According to another variant, separate determinations of the urea and/or formate content (for example, by a conductivity method such as described in the aforementioned Patent US '619) and the content of ammoniacal nitrogen derived from its decomposition (for example, using an ammonia probe or by a colorimetric method, for example based on indophenol blue) are carried out, and the amounts OfNH₃ available via these two sources are added together. The conductivity and ammonia probes are suitable both for in situ measurements and for establishing the nomograms, but their cost is such that the second variant (nomograms) is preferred. Whichever variant is used, either the calculated value is compared with the theoretical value given before (around 18% for the AdBlue^® or Denoxium -30™ solutions) and a correction factor is deduced therefrom for the volume of solution to be injected; or the method according to the invention makes use of a computer which directly uses the calculated concentration value to deduce therefrom the vo lume to be inj ected.

According to an advantageous variant which will be described in greater detail later on, the method according to the invention uses a computer designed to be able to combine the conditioning times when the temperature of the solution varies over time (or in other words: that it can jump from one nomogram to another when the temperature varies and this with a view to combining the thermal ageing effects at the various temperatures). Alternatively, the tank could be equipped with a conditioning system that ensures a constant temperature in the tank, but this variant is more difficult and expensive to carry out in practice. The present invention also relates to a system for metering an aqueous ammonia precursor solution and injecting it into the exhaust gases of an internal combustion engine, said system comprising at least one tank intended for storing the solution; a line for injecting the solution into the exhaust gases; and a computer that makes it possible to calculate the amount of solution to be injected into said gases as a function of at least one operating parameter of the engine and of the total amount of ammonia available in the solution.

The system according to the invention comprises at least one tank intended for storing the additive (precursor solution). This tank may be made from any material, preferably one that is chemically resistant to the additive in question. In general, this is metal or plastic. Polyolefin resins, in particular polyethylene (and more particularly HDPE or high-density polyethylene), constitute preferred materials. The system according to the invention generally comprises a pump that is used to bring the additive solution to the pressure required for metering and injecting it. Various types of pumps may be suitable for the application: gear pump, piston pump, diaphragm pump, etc. This pump may be located in the additive tank (with the advantage of forming, with it, a compact and integrated module) or, considering the corrosive environment, be located outside of the additive tank. Its constituent materials will preferably be chosen from corrosion- resistant metals (especially certain grades of stainless steel and aluminium). The use of copper, even for connection components, is undesirable. Preferably, as explained hereabove and with a view to limit the number of parameters and nomograms to establish, where appropriate, the pressure in the tank is calibrated by means of a suitable device. The latter may consist of a simple calibrated valve. Preferably, this valve can direct vapours that create an overpressure in the tank towards a canister so that these vapours are not expelled as they are into the atmosphere but are first purified by the ammonia. This variant (with overpressure) also has the advantage that the solubility of the ammonia is increased and that therefore, a large part of the ammonia derived from the degradation of the precursor in the end remains available for the SCR reaction. The system according to the invention also comprises an injection line intended to bring the additive to the exhaust pipe of the engine and in order to do this connecting the tank and an injector generally located at the end of the injection line that opens into or near the exhaust pipe. This injector may be of any known type. It may be a so-called "active" injector, i.e. that includes a metering function, or a so-called "passive" injector then coupled to an additional metering device, such as a metering valve for example. It is advantageously a passive injector, and in particular a nozzle or spray gun making it possible to obtain drops of solution having a diameter between 5 and 100 μm. Such a nozzle is advantageously equipped with an orifice having a diameter of around 150 μm - 250 μm. This orifice is preferably supplied by a system of narrow channels (3-4) producing a "swirl" phenomenon (vortex) of the solution upstream of the nozzle. Clogging could be avoided by the purge which removes the last droplets of urea; there is therefore no crystallization by evaporation.

In this variant according to the invention, the amount of solution is preferably metered by regulating the opening frequency and duration of the metering valve. This valve may be a piezoelectric or solenoid valve, the regulation of which may be electronic.

The system according to the invention comprises a computer connected to the metering device and enabling the amount of additive required, as a function of the state of ageing of the solution and of at least one engine operating parameter which may be chosen from the emission level and degree of conversion of the NO_x; temperature and/or pressure; engine speed and/or load, etc., to be brought to the injector.

In a preferred variant of the invention, this computer has a memory, in which are stored nomograms that give the change, as a function of time, of the concentration of precursor and of ammonia/ammonium hydroxide of the solution (expressed as they are or in the form of total available ammonia) for which the system is intended and this at various temperatures or even at various pressures. So as to use these nomograms in operation, the computer must receive at least an indication of the temperature and optionally of the pressure in the tank. For this purpose, the tank is advantageously equipped with a temperature sensor that advantageously may also fulfil another role in the system (for example, for thermal conditioning of the solution) and/or with a pressure sensor. When the tank is pressure-calibrated as explained previously, nomograms at different temperatures and one pressure (i.e. the calibration pressure) are sufficient enough, and one can do without the pressure sensor. Optionally several nomograms corresponding to different calibration pressures may be established, so that the system may operate with different calibration valves.

The present invention is illustrated, in a non- limiting manner, by Figure 1 which illustrates a system according to a preferred variant of the invention. The flow direction of the exhaust gases is indicated therein by an arrow marked "G".

This system comprises a tank (1) containing the ammonia precursor solution. This tank is equipped with a system for maintaining an overpressure (in a range generally extending from 100 mbar to 1 bar, with a preferred interval between 150 and 500 mbar and preferably at around 250 mbar) (2) and a sensor (3) enabling the temperature of the solution to be measured. The overpressure has the advantage of increasing the solubility of the ammonia produced by degradation of the precursor.

The ammonia precursor is conveyed by the action of a pump (4) towards a nozzle (6) located in the line (9) for discharging the exhaust gases of the engine, upstream of an SCR catalyst (8). The amounts injected are controlled by a metering valve (5), the opening of which is operated by an electronic control unit or ECU (7). A temperature sensor (3) supplies data for input into the control unit

(V).

The changes in the concentration of urea and of ammonia/ammonium hydroxide of the precursor solution as a function of time, determined in addition (for example by using the analytical methods described previously on solutions intentionally aged in the same system and from which samples are regularly withdrawn for analysis), are introduced into the ECU, considering the temperature as a parameter. The temperature of the precursor solution is recorded over time. For given temperature levels, the conditioning times are combined. The ECU then refers to the changes in the concentration of urea and of ammonia/ammonium hydroxide and establishes the equivalent amount of ammonia available in the precursor solution at any point in the ageing. This equivalent amount is then compared to the initial amount, for which there is a standard quality: for example, according to the standard DIN 70070, in the case of the AdB lue^® solution, the urea content is between 31.8% and 33.2% (by weight), hence an amount of available ammonia between 18.0% and 18.8%, and the correction factor for the volume to be injected is thus determined.

Claims

C L A I M S

1 - Method for metering an aqueous ammonia precursor solution into the exhaust gases of an internal combustion engine, said method consisting in determining the total amount of ammonia available in the solution in the form of precursor, ammonia and ammonium hydroxide and in calculating the volume of solution to inject as a function of this amount.

2 - Method according to the preceding claim, characterized in that the aqueous ammonia precursor solution is a eutectic solution of urea.

3 - Method according to any one of the preceding claims, characterized in that the precursor solution is stored in a ventilated and/or pressure calibrated tank.

4 - Method according to the preceding claim, characterized in that it uses nomograms obtained from previous measurements of reference solutions aged at various temperatures and that make it possible to deduce the amount of ammonia available in the solution as a function of the history (ageing) of said solution.

5 - Method according to the preceding claim, characterized in that the amount of ammonia available in the reference solutions is determined by an analytical metering of the total nitrogen present in the solution.

6 - Method according to claim 4, characterized in that the amount of ammonia available in the reference solutions is determined based on the precursor content and the ammoniacal nitrogen content, determined independently by specific methods.

7 - Method according to any one of the claims 4 to 5, characterized in that it uses a computer designed to be able to combine the conditioning times when the temperature of the solution varies over time.

8 - System for metering an aqueous ammonia precursor solution and injecting it into the exhaust gases of an internal combustion engine, said system comprising at least one tank intended for storing the solution, a line for injecting the solution into the exhaust gases, and a computer that makes it possible to calculate the amount of solution to be injected into said gases as a function of at least one operating parameter of the engine and of the total amount of ammonia available in the solution in the form of precursor, ammonia and ammonium hydroxide.

9 - System according to the preceding claim, characterized in that the tank comprises a calibrated valve capable of directing vapours that create an overpressure in the tank towards a canister.

10 - System according to claim 8 or 9, characterized in that the tank comprises a temperature sensor, and in that the computer has a memory in which are stored nomograms that give the evolution as a function of time, of the concentration in precursor and in ammonia/ammonium hydroxide of the solution (expressed as such or in the form of the total ammonia available) at different temperatures.