WO2008129203A1

WO2008129203A1 - Additive for improving the performance of nitrogen oxide reducing systems by urea of the exhaust gases of diesel engines

Info

Publication number: WO2008129203A1
Application number: PCT/FR2008/050432
Authority: WO
Inventors: Jean-Claude Fayard
Original assignee: Jean-Claude Fayard
Priority date: 2007-04-04
Filing date: 2008-03-13
Publication date: 2008-10-30
Also published as: FR2914688A1

Abstract

This additive designed to promote exhaust gas post-treatment of an internal combustion engine, and particularly diesel engines, in temperatures between 200 and 300 °C, comprises an aqueous solution to be vaporized in said exhaust gases. Said aqueous solution contains ammonium nitrate alone or in combination with other additives.

Description

ADDITIVE TO IMPROVE THE OPERATION OF NITROGEN OXIDE REDUCTION SYSTEMS THROUGH THE EXHAUST GAS UHEA OF DIESEL ENGINES

FIELD OF THE INVENTION

The invention relates to the field of exhaust gas treatment of internal combustion engines, and more particularly to diesel engines.

It relates more particularly to the post-treatment of said exhaust gas for the purpose of reducing the nitrogen oxides present therein.

PRIOR STATE OF THE ART

In a known manner, the exhaust gases of diesel engines contain a relatively large proportion of nitrogen oxides (NO _x ). Other industrial sources are also emitting such nitrogen oxides.

In the general framework of the preservation of the environment, a standard issued by the European Commission, called "EURO 4", has fixed in a precise way the allowed tolerances within the framework of the discharges of these nitrogen oxides.

In order to reduce their emission, it is known to set up a so-called SCR system ("Selective Catatyst Reduction"), comprising a catalyst for reducing said oxides in order to transform them into nitrogen gas N ₂ .

In order to promote this reduction reaction, an aqueous urea solution, which is more convenient to use and less dangerous than the use of ammonia NH ₃ , is injected into the exhaust gas, this injection being carried out upstream of the catalyst. Above 200 ⁰ C, the urea hydrolyses according to the following reaction:

NH ₂ -CO-NH ₂ + H ₂ OO 2NH ₃ + CO ₂

In doing so, NH ₃ ammonia gas is evolved which reacts with the aforementioned catalyst and the nitrogen oxides to give nitrogen gas and water vapor:

2NH ₃ + 3NO O N ₂ + 3H ₂ O

4NH ₃ + 3NO 2 O N ₂ + 6H ₂ O However, when the temperature in the exhaust gas is between 200 and 250 ⁰ C, and especially less than 300 ⁰ C, corresponding to the configuration of urban buses and other dumpsters in which the engine is never loaded, a parasitic reaction occurs in which part of the urea is converted into cyanuric acid. This acid is solid and furthermore is relatively chemically stable at these temperatures, its decomposition occurring only above 350 ^° C., and still in a slow manner. This cyanuric acid is formed by decomposition of the urea according to the following reaction:

3NH ₂ -CO-NH ₂ O 3NH ₃ + CNOH-CNOH-CNOH

If this parasitic reaction is still present, it is not a problem when the engine is operating at full load, that is to say with exhaust gas temperatures that are typically above ^400.degree. C., or conversely at an idle speed. temperatures below 200 ^° C. In fact, under these conditions, the small amounts of cyanuric acid formed are generally rapidly removed by thermal decomposition, whenever the temperature of the exhaust gas exceeds 350 ^° C.

However, this reaction and in particular the formation of cyanuric acid becomes of concern when the engines operate unloaded, and typically in the context of urban bus routes or dumpsters.

For these types of configuration, it can be observed that 85% of the operating time of the engine generates exhaust gases of less than 200 ^° C., 14% between 200 and 300 ^° C. and less than 1% at a temperature of temperature above 300 ^° C. Due to the majority of operation at a temperature below the decomposition temperature of cyanuric acid, it accumulates and causes the clogging of the catalytic system. The deposition of the acid on said catalytic system is in the form of lumps or agglomerates, the acid gradually agglomerating on the system, so that its removal by heating becomes more and more difficult, even when the engine is at full load because it has a small contact surface with the gases.

In this case, it becomes necessary to change the entire catalytic system, either by replacing it with a new system, or to achieve its regeneration. The regeneration process is carried out at a typical temperature close to 500 ⁰ C, and is long. In doing so, the change or regeneration of the catalytic system is labor-intensive, time consuming, and therefore ultimately, particularly expensive.

There is currently no technical solution to overcome this drawback, unacceptable, especially for operators of bus fleets, or dumpsters.

SUMMARY OF THE INVENTION

The present invention aims to solve this problem simply, efficiently and inexpensively.

According to this, it consists in adding to the aqueous solution of urea a proportion of ammonium nitrate (NH ₄ NO ₃ ). This ammonium nitrate is therefore introduced simultaneously with the urea in the exhaust gas.

The droplets of ammonium nitrate will preferentially wet crystals of cyanuric acid that could have formed.

Ammonium nitrate has the particularity of completely solubilizing cyanuric acid and thus already changing its appearance by making it more friable. The other interest of ammonium nitrate is related to its theoretical decomposition temperature of 210 ⁰ C which makes it reactive with respect to all reducing substances. In fact, cyanuric acid has a reducing character. In doing so, the cyanuric acid / ammonium nitrate combination is completely decomposed from this relatively low temperature, effectively eliminating the formation of said acid.

It is also recalled that the melting point of ammonium nitrate is close to 170 ^° C.

Therefore, when an aqueous solution containing urea and ammonium nitrate is vaporized in the exhaust gas whose temperature is in a range from 200 to 300 ⁰ C, there is therefore evaporation of the water of the aqueous solution, and corollary formation of urea crystals. Ammonium nitrate is above its melting temperature: it is therefore in the form of droplets that wet the walls and other solids and preferably cyanuric acid if it is formed.

In other words, the invention consists in vaporizing a solution containing ammonium nitrate in the exhaust gases.

As soon as the temperature of the exhaust gas exceeds 200 ^° C., the ammonium nitrate reacts either with cyanuric acid, or unfortunately also with urea, or with both.

We have the reactions:

3NH ₄ NO ₃ + 2CNOH-CNOH-CNOH O 9H ₂ O + 6N ₂ + 6 CO + heat

3NH ₄ NO ₃ + NH ₂ -CO-NH ₂ O CO ₂ + 8 H ₂ O + 5 N ₂

This reaction of decomposition of both urea and ammonium nitrate is complete and has a high reaction kinetics as soon as the temperature exceeds 210 ^° C. It is specified that depending on the decomposition temperature, the nitrate of Ammonium decomposes either into nitrous oxide (N ₂ O) and water vapor, or into nitrogen oxide NO, or directly into nitrogen gas and water.

The nitrogen oxides, when they come into contact with the specific denitrification catalyst, in the presence of ammonia, are automatically reduced and converted into nitrogen gas (N ₂ ) after catalysis. It has been demonstrated that by adding a proportion of between 0.1 and a few percent by weight of ammonium nitrate, relative to the amount of pure urea, it leads firstly to the complete operation of the catalytic system, which is already known, but especially to the suppression of the formation of cyanuric acid, the disadvantages of which have been shown in the preamble. Because of the low consumption of urea by ammonium nitrate, it may be advantageous to further concentrate the aqueous solutions of urea available to date commercially to allow to maintain the full effectiveness of said solution, this one comprising at least 32.5% urea. This concentration having been retained, it is in these conditions that the freezing temperature is the lowest, -11 ° C. Incidentally, the addition of ammonium nitrate lowers this freezing temperature. Advantageously, when it is desired to make the blocking reaction of cyanuric acid even more selective, it is possible to add to the solution other nitrogen-containing additives (without ash, of mineral or organic nature, functionalized with alcohol, amine groups, amide, nitrile or other hetero atoms, to be soluble in water).

Thus, the other additives that may be added to the aqueous solution are of a nitrogenous nature and are constituted by nitrates or nitrites of organic compounds which may or may not be amine, or amides of organic acids which may or may not be associated with functional groups of the type alcohol or acid, amine, amide, nitrile or other hetero atoms.

Some of these products may make the thermal decomposition reaction of cyanuric acid formed with ammonium nitrate more selective, or promote the wetting of the cyanuric acid powder deposited by the ammonium nitrate droplets by acting as surfactant, surfactant or wetting agent.

In the case of obstruction of the catalytic system by cyanuric acid, it is possible to use a solution of concentrated ammonium nitrate, alone or in combination with urea directly on the vehicle, that is to say without having to deposit said catalyst system. Under this hypothesis, the catalytic system is regenerated by injecting the solution of pure ammonium nitrate or urea-associated with the same device that is usually used to inject urea.

It is advantageous to add a proportion of urea of between 5 and 100% of the ammonium nitrate contained in the aqueous solution in order to avoid the discharges of nitrogen oxides which may occur by the thermal decomposition of ammonium nitrate. the urea associated with the solution reacts on the catalyst with the nitrogen oxides to form nitrogen gas.

In order to achieve a satisfactory efficiency, it is then sufficient to search on the stationary vehicle for the engine operating conditions which guarantee a temperature of the exhaust gas sufficient to reach at least a temperature of between 200 and 300 ⁰ C, that is, allowing vaporization of the aqueous solution, hydrolysis of the urea and decomposition of the complete ammonium nitrate. Advantageously, on engines equipped with exhaust brakes, it is sufficient to put this braking device into action and to keep the engine accelerated to obtain an exhaust gas temperature greater than 300 ^° C. Thus, when the aqueous solution of ammonium nitrate and urea is sprayed in the form of aerosol in the exhaust gas, it is obtained after vaporization of the water, ammonium nitrate crystals which become liquid almost instantaneously since as previously recalled the melting point of the ammonium nitrate is close to 170 ^° C.

These droplets of ammonium nitrate nitrate are deposited preferentially in the exhaust duct and also at the place where the cyanuric acid is likely to form. The ammonium nitrate instantly solubilizes the cyanuric acid as soon as the temperature is higher than 210 ⁰ C, thus causing the decomposition of the cyanuric acid formed according to the relationship below:

3NH ₄ NO ₃ + 2CNOH-CNOH-CNOH O 9H ₂ O + 6N ₂ + 6CO + heat

Because of the exothermic nature of this reaction, the decomposition of cyanuric acid is itself accelerated. Thus, the higher the temperature of the exhaust gas, the greater the oxidative character of the ammonium nitrate is powerful and thus its effectiveness vis-à-vis the improved cyanuric acid.

In fact, all the advantages of the additive according to the invention are conceived in the context of post-treatments of the exhaust gases of internal combustion engines and in particular diesel engines.

Claims

An additive for promoting the after-treatment of the exhaust gases of an internal combustion engine, and more particularly of diesel engines, in a temperature range of between 200 and 300 ^° C., comprising an aqueous solution of urea intended for to be vaporized in said exhaust gas, characterized in that said aqueous solution contains ammonium nitrate alone or in combination with other additives.

2. Additive for promoting the aftertreatment of the exhaust gas of an internal combustion engine according to claim 1, characterized in that the concentration of ammonium nitrate is between 0.1 and a few percent by weight by relative to the pure urea contained in said solution.

3. Additive to promote the aftertreatment of the exhaust gas of an internal combustion engine according to one of claims 1 and 2, characterized in that the other additives are devoid of ash, and have a surfactant character , surfactant or wetting.

4. Additive for promoting the aftertreatment of the exhaust gas of an internal combustion engine according to claim 3, characterized in that the other additives which may be added to the aqueous solution are of a nitrogenous nature and are constituted by nitrates or nitrites of organic compounds which may or may not be amine, or amides of organic acids optionally associated with functional groups of the alcohol or acid, amine, amide, nitrile or other hetero atom type.

5. A method for regenerating a catalytic reducing system of nitrogen oxides present in the exhaust gas of an internal combustion engine, in particular a diesel engine characterized in that it consists in vaporizing in the exhaust gas an aqueous solution of urea and ammonium nitrate and raise the temperature of said exhaust gas to a temperature greater than or equal to 210 ^° C. Process for regenerating a reducing catalytic system of the nitrogen oxides present in the exhaust gas of an internal combustion engine, characterized in that the concentration of urea relative to the ammonium nitrate in the aqueous solution is between 5 and 100 percent.