FR2914688A1 - ADDITIVE TO IMPROVE THE FUNCTIONING OF NITROGEN OXIDE REDUCTION SYSTEMS THROUGH THE EXHAUST GAS UHEATION OF DIESEL ENGINES - Google Patents

Abstract

This additive for promoting the after-treatment of the exhaust gases of an internal combustion engine, and more particularly of diesel engines, comprises an aqueous solution of urea intended to be vaporized in said exhaust gas.The aqueous solution contains ammonium nitrate alone or in combination with other additives.

Description

ADDITIVE TO IMPROVE THE FUNCTIONING OF REDUCTION SYSTEMS

  NITROGEN OXIDES THROUGH THE UREA OF EXHAUST GASES FROM 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.

  More particularly, it relates to the post-treatment of said exhaust gases in order to reduce the nitrogen oxides present therein.

  PRIOR ART In the known manner, the exhaust gases of diesel engines contain a relatively large proportion of nitrogen oxides (NOx). Other industrial sources also emit such nitrogen oxides.

  In the general framework of the preservation of the environment, a standard issued by the European Commission, known as 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 (Selective Catalyst Reduction) system, comprising a catalyst for reducing said oxides to transform them into nitrogen gas N2.

  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 C, the urea hydrolyses according to the following reaction:

NH2-CO-NH2 + H20 E2NH3 + CO2

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

  However, when the temperature in the exhaust gas is between 200 and 250 ° C., and in particular less than 300 ° C., corresponding to the configuration of the urban buses and other skips. 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 moreover is relatively chemically stable at these temperatures, its decomposition occurring only above 350 C, and still slowly. This cyanuric acid is formed by decomposition of the urea according to the following reaction: 3NH 2 -CO-NH 2 E 3 NH 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 greater than 400 ° C., or conversely at idle at temperatures Under these conditions, the small amounts of cyanuric acid formed are generally rapidly removed by thermal decomposition, each time 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.

  Indeed, for this type of configuration, 85% of the engine operating time generates exhaust gas of less than 200 C, 14% between 200 and 300 C 25 and less than 1% at a temperature greater than 300 C. Due to the majority of operation at a temperature below the decomposition temperature of cyanuric acid, it accumulates and causes 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 gas.

  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 of about 500 ° C and is long. In doing so, the change or the regeneration of the catalytic system proves to be labor intensive, time consuming, and therefore expensive, 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 (NH4NO3). This ammonium nitrate is therefore introduced simultaneously with the urea in the exhaust gas.

  The ammonium nitrate droplets will preferentially wet the crystals of cyanuric acid that might 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 ° C., which renders 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 temperature of ammonium nitrate is close to 170 C.

  Therefore, when an aqueous solution containing urea and ammonium nitrate is vaporized in the exhaust gas, and when these are at a temperature of about 200 C, there is therefore evaporation of the water of the aqueous solution, and corollary formation of urea crystals. The ammonium nitrate is above its melting point: 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 of spraying a solution containing ammonium nitrate into the exhaust gases.

  Since the temperature of the exhaust gas exceeds 200 ° C, the ammonium nitrate reacts with either cyanuric acid or unfortunately also with urea or both. We have the reactions:

  3NH4NO3 + 2CNOH-CNOH-CNOH 9H2O + 6N2 + 6C0 + heat 3NH4NO3 + NH2-CO-NH2E CO2 + 8 H2O + 5 N2 This decomposition reaction 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 ammonium nitrate decomposes either nitrous oxide (N2O) and water vapor, either 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 (N2) 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 urea consumption by ammonium nitrate, it may be advantageous to further concentrate the aqueous solutions of urea commercially available to date in order to maintain the full effectiveness of said solution. comprising at least 32.5% urea. This concentration having been chosen, it is under 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-free additives (without ash, of mineral or organic nature, functionalized with alcohol groups, amine, 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 a minimum temperature between 200 and 300 C, c that is to say, allowing the vaporization of the aqueous solution, the hydrolysis of urea and the decomposition of the complete ammonium nitrate. Advantageously, on engines equipped with exhaust brakes, it is sufficient to action this braking device and hold 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 is obtained after vaporization of the water, ammonium nitrate crystals which become liquid almost instantaneously since as recalled above, the temperature of ammonium nitrate is close to 170.degree.

  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 C, thus causing the decomposition of the cyanuric acid formed according to the relation below:

  3NH4NO3 + 2CNOH-CNOH-CNOH> 9H2O + 6N2 + 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 the post-treatments of the exhaust gases of internal combustion engines and in particular diesel engines.

Claims (6)

  An additive for promoting the after-treatment of the exhaust gases of an internal combustion engine, and more particularly of diesel engines, comprising an aqueous solution of urea intended 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 acidic type, amine, amide, nitrile or other hetero atoms.   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.   6. Process for regenerating a catalytic reducing system of 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 included between 5 and 100 percent.