FR2690358A1 - Denitrification of exhaust gas contg. oxygen esp. from internal combustion engine - Google Patents

Abstract

The invention relates to a method for reducing nitrogen oxides in exhaust gases containing oxygen. This process, which operates by reaction on a zeolite catalyst for the reduction of nitrogen oxides contained in exhaust gases containing oxygen, is characterized by the fact that oxocylic hydrocarbons are made to arrive as a reducing agent for the gases of 'exhaust in the direction of flow, upstream of the zeolite catalyst. Application, in particular, to the reduction of nitrogen oxides in the exhaust gases of internal combustion engines.

Description

 The present invention relates to a process for reducing nitrogen oxides present in oxygen-containing exhaust gases, in particular exhaust gases from internal combustion engines, in which the exhaust gases are reacted by means of passing on a zeolitic catalyst after the supply of reducing hydrocarbons. This process is then particularly directed to the removal of oxides of nitrogen from oxygen-containing exhaust gases from internal combustion engines as used for driving motor vehicles.

 A method of this kind is known from DE-OS 36 42 018, BOLS 53/36. A fundamental advantage of such a process operating with zeolite catalysts is that it avoids the use of ammonia as a reducing agent, which must be considered as dangerous to health.

 While the aforementioned document indicates a multiplicity of zeolites that can be used for the construction of the catalyst, no detailed information is provided regarding the hydrocarbons that can be supplied.

 Consequently, the problem on which the invention is based is to design a process of the kind mentioned, retaining its advantages as regards the choice of the hydrocarbons brought, so as to ensure an effective reduction of the nitrogen oxides within the limits of a realistic temperature range even for internal combustion engines.

 The solution that the invention brings to this problem lies in the fact that we add hydrocarbons of the oxocyclic hydrocarbon type whose thermal decomposition of the molecules is accompanied by the formation of at least one acetaldehyde-type molecule. an intermediate reaction product.

 Other features and advantages of the present invention will become apparent from the detailed description which follows.

 It should be added here that in the context of the process according to the invention, it is also possible to use zeolites as already mentioned in DE-OS 36 42 018 mentioned above, that is to say for example zeolites in which copper is introduced by exchange, especially in combination with at least one other element of the fourth period, natural or synthetic zeolites, type X zeolites or type Y or also zeolites of the mordenite type.

 The oxocyclic hydrocarbons added according to the invention are cyclic heteroalkanes or heteroalkenes, that is to say saturated or unsaturated cyclic hydrocarbons which contain as hetero atoms in the carbon nucleus at least one oxygen atom. The compounds can be monocyclic or they can also be bicyclic or polycyclic.

 During the thermal decomposition of a molecule of these compounds, at least one molecule containing acetaldehyde of an intermediate reaction product must be formed. Hydrocarbons which are to be considered as oligomerized acetaldehyde are preferably used for this purpose, the number of acetaldehyde molecules in the hydrocarbon molecule being equal to or greater than one.

1,4-Dioxanne 2,4,6-Trimfithyl-1,3,5-Trioxarme Te'trahydrofuranne (Paraldéhyde) tTI1F) - Résultats (Abaissement de la proportion de NOx en %)

Examples
With simple representatives of this class of substances, nitrogen oxide reduction tests were carried out under the following conditions: Catalyst: H-ZSM5 (module 90) at 2%
W03, treated twice by exchange
with copper - Composition of the gas: NO: 1500 vpm
O2: 6.5% by volume
N2: the rest - Experimental conditions: Spatial velocity: 23 800 h ~ 1
Temperature range: 300 7b0C - Hydrocarbons added

1,4-Dioxan 2,4,6-Trimethyl-1,3,5-Trioxarme Te'trahydrofuran (Paraldehyde) tTI1F) - Results (Decrease of NOx proportion in%)

<tb><SEP> Dioxane <SEP> TF <SEP> Paraldehyde
<tb> 350C <SEP> 5 <SEP> 30
<tb> 400 <SEP> 40 <SEP> 30
<tb> 450 <SEP> 40 <SEP> 35 <SEP> 50
<tb> 500 <SEP> 40 <SEP> 65 <SEP> 80
<tb> 550 <SEP> 45 <SEP> 75 <SEP> 85
<tb> 600 <SEP> 50 <SEP> 75
<tb> 650 <SEP> 45 <SEP> 70
<tb> 700
<Tb>
The single figure of the appended drawing represents an internal combustion engine - in this case, a diesel engine comprising devices that allow the implementation of the method according to the invention.

The control apparatus 2, to which signals are sent from a temperature detector 3 in a catalyst 5 disposed in the muffler 4 of the engine and in a speed detector 6, is associated with the internal combustion engine. , designated as a whole by 1, whose construction is known and therefore does not require a more detailed description. The characteristic curves of the motor and a characteristic curve of the emission of the oxides
NOx are stored in the control unit 2. The latter delivers to the compressor 7 and the nozzle 8, by using the detector signals as well as load signals, control signals derived from the characteristic curves for the hydrocarbon feed. oxocyclic in the exhaust gas reservoir 9 in the conduit 4 in the forward flow direction of the zeolitic catalyst 5. As a result, there is thus a controlled addition of this hydrocarbon to the exhaust gases in front of to be treated, depending on the emission of NOx oxides from the engine.

 As is apparent from the results table presented above, the invention creates a process of the defined kind which is distinguished by a high degree of conversion of the nitrogen oxides in the exhaust gas.

Claims (5)

 A method of reducing nitrogen oxides present in oxygen-containing exhaust gases, especially internal combustion engines, wherein the exhaust gas is reacted by passage over a zeolitic catalyst after supply. of reducing hydrocarbons, characterized in that hydrocarbons of the oxocyclic hydrocarbon type are added whose thermal decomposition of the molecules is accompanied by the formation at least of an acetaldehyde-type molecule of an intermediate product of the reaction .  2. Process according to claim 1, characterized in that hydrocarbons of the type of cyclic heteroalkanes are added.  3. Process according to claim 1, characterized in that hydrocarbons of the cyclic heteroalkene type are added.  4. Process according to any one of claims 1 to 3, characterized in that oxocyclic hydrocarbons of the type of an oligomerized acetaldehyde are added, for which the number of acetaldehyde molecules in the hydrocarbon molecule is equal to or greater than has a.  5. Process according to any one of claims 1 to 4, characterized in that the exhaust gas is reacted by passing over a catalyst containing a type H-ZSM5 zeolite (Module 90) containing about 2% of WO 3 treated twice by exchange with copper.