DE102005055845A1 - Method for selective catalytic reduction of nitrogen oxide contained in exhaust gas of combustion engine, involves drying of primary air contained in carrier-gas stream while removing fresh air or circulating current conduction - Google Patents

Abstract

The invention relates to a process for the selective catalytic reduction (SCR) of nitrogen oxides contained in the exhaust gas of an internal combustion engine, fed as a reducing agent urea in solid form an SCR catalyst and converted to ammonia and carbon dioxide, wherein the urea is taken from a urea supply and by a metering device is introduced to form a urea aerosol in a carrier gas stream and fed through this a catalyst system.
According to the invention, the transport air contained in the carrier gas stream is dried by being taken from the fresh air and / or circulation flow guidance of the passenger air conditioning system.

Description

The The invention relates to a process for the selective catalytic Reduction (SCR) of contained in the exhaust of an internal combustion engine Nitrogen oxides in which as a reducing agent urea in solid form fed to an SCR catalyst and converted to ammonia and carbon dioxide, the urea taken from a urea supply and by means of a metering device forming a urea aerosol in a carrier gas stream introduced and fed through this a catalyst system.

A high population growth, Increased industrialization and rising traffic volumes are increasing a concentration of pollutants in the ambient air, meanwhile takes on questionable proportions. Of particular importance in this context are nitrogen oxide emissions, which largely depends on the Combustion of gasoline and Diesel fuels in fuels. nitrogen oxide emissions contribute to increased Ozone concentrations near the bottom at.

Around counteract this questionable development and due to repeated intensification The pollutant limits by the legislature are from the car manufacturers Efforts are made to reduce the nitric oxide concentrations produced during operation to reduce. Promising is the application of the industrial plants already known SCR process in which ammonia is added to the exhaust gas stream. The ammonia reacts with the nitrogen oxides to form nitrogen, carbon dioxide and water.

by virtue of of the potential hazard that emanates from ammonia is the entrainment of Ammonia in a motor vehicle problematic. It is known, ammonia exactly in the for to produce the chemical conversion necessary amount of urea.

From the DE 40 38 054 A1 It is known to carry an aqueous urea solution in a container in the motor vehicle and spray onto a hydrolysis catalyst, whereby a urea decomposition in ammonia and carbonic acid is achieved. The hydrolysis catalyst is followed by the actual SCR catalyst, which is followed by an oxidation catalyst in which excess ammonia is oxidized as completely as possible to form nitrogen and water.

It has been shown to be compatible with the use of an aqueous urea solution in practice significant problems with winter operation of vehicles due to the relatively high freezing point of the urea solution as well in terms of relatively high, to be carried in the vehicle Weight are connected. About that changes out solubility with temperature of urea in the water. additionally withdraws the injected aqueous urea solution during the evaporation of your Water content of the already mostly too cold catalyst heat. moreover tends the injection system for applying rigid urea encrustations when the supply the urea solution frequently is interrupted.

In the EP 0 615 777 A1 It is proposed to carry the urea in solid form in the vehicle and to meter it into the exhaust gas as urea dust aerosol. For this purpose, the urea is carried in the form of prills or microprills and supplied to a carrier gas flow with the aid of a precise dosing device. In this way, although the above-mentioned disadvantages of an aqueous urea solution are completely avoided, it is necessary for an exact metering and charging of the urea prills to keep the material flowing. This is made difficult by the fact that urea is hydroscopic and tends to flow under loads, which results in the risk of caking of the prills. To solve this problem is according to the EP 0 551 777 A1 provided that the prills are airtight packed in plastic containers under the exclusion of atmospheric moisture and the plastic containers are locally pierced after introduction into the reservoir for removing the prills. On the one hand, this procedure is very complicated and, on the other hand, the service life of a suitably equipped vehicle depends on the amount of prills contained in a plastic container. However, this must not be too large, otherwise moisture can penetrate the punctured site in the plastic container over time, with which the problems mentioned are connected. The moisture can occur on the one hand by the water content of the outside air or by splashing water, on the other hand by partial condensation of the high water vapor content in the combustion exhaust gas. It is thus necessary to frequently replenish the urea supply manually by inserting a new plastic container, which is inconvenient and less user-friendly.

To overcome these disadvantages, the DE 197 54 135 A1 to carry the urea in solid monolithic structure. Depending on the need, the corresponding amount of urea is continuously removed from a block using a removal device, optionally the urea finely zer grind, if the particles are still too large, and the powdered urea then fed to transport a carrier gas stream. The removal device consists of a rotating, with bristles, abrasive grains, knives or milling tools occupied disc or roller. By changing the feed rate of the removal device relative to the urea block, the metered amount can be varied.

With Although this approach solves the problem of caking, others Problems remain, however. So here is the preparation of the Urea to monolithic blocks necessary, which is associated with an appropriate effort in advance is. Another disadvantage arises from the use of the described Ablation. When removing the urea block will be inevitable Produced urea particles of different sizes. This leads to, that supplied to the system Urea quantity dependent the particle varies. An exact dosage according to the current Need is, if any, then only within wide limits possible.

The DE 197 54 135 A1 also proposes the use of an additional grinder in which the particles removed from the urea block are comminuted to powder form. However, this has the disadvantage that the conversion of the urea block to powdered urea is another upstream process step, which adversely affects the reaction time of the entire system, ie, the system is too slow. This leads to the fact that the demand for ammonia caused by load changes can not be satisfied for a short time or that there is a shortage of oversupply.

In front This is the object of the present invention in it, a procedure for the selective catalytic reduction (SCR) of in the exhaust gas of an internal combustion engine to create contained nitrogen oxides, which is a simple, to the respective Operating conditions adapted and reliable dosage of the reducing agent allows.

These The object is achieved by a method having the features of the patent claim 1 solved. Advantageous training and further education, which individually or in combination can be used together are the subject of dependent Claims.

The inventive method for the selective catalytic reduction (SCR) of in the exhaust gas of an internal combustion engine contained nitrogen oxides, in which as a reducing agent urea supplied in solid form to an SCR catalyst and converted to ammonia and carbon dioxide provides that the urea is taken from a urea supply and by means of a metering device to form a urea aerosol in a carrier gas stream introduced and fed through this a catalyst system, where in the carrier gas stream contained conveying air dried by passing the fresh air or / and recirculating flow guide the Passenger air conditioning is removed. It is advantageous Use urea in a form that is already in large quantities to disposal stands. So will be in big Scope urea for fertilization Applied to agricultural land. The urea from the fertilizer industry corresponds in its composition, form and condition to the Requirements of the invention, d. h., he is in spherical or spheroidal Shape ahead. Such urea is also referred to as a pellet, whereby thereby also granular Material can be meant.

Basically as a target size for the individual Pellets of any size possible. With decreasing size of the pellets let yourself However, achieve a finer gradation in the dosage. To The pellets are first isolated and to a carrier gas stream passed, which accomplished the further transport. The speed the separation and transport is decisive for the dosage.

It It is preferred that the removal of the conveying air after the coolant evaporator in fresh air or / and passenger compartment air conditioning.

alternative is provided, the removal of the conveying air after the coolant evaporator and after the heat exchanger the heating system of fresh air or / and passenger compartment air conditioning perform.

Advantageous is that taken from the fresh air and / or passenger compartment air conditioning conveying air to dry continuously.

It is preferred that the conveying air through a membrane dryer or a regenerable adsorption dryer is dried continuously.

Furthermore, it is preferred that the conveying air when exceeding a certain limit for the water content in the conveying air is dried.

Preferably becomes the conveying air only if the value falls below a certain limit for the water content in the conveying air taken.

From The advantage is, the Reduktionsmitteldosierung only when falling below a certain limit for the water content in the conveying air perform.

It It is preferred that the water content still contained in the conveying air by means of temperature measurement of the saturation water vapor quantity after the coolant evaporator determined and with the limit values which are exceeded or fallen short of the water content in the conveying air determine is compared.

advantageously, is the water content of the conveying air by a humidity sensor, which after the cooling evaporator and / or after the heat exchanger and / or after the conveying air extraction is arranged determined.

Preferably Fulfills those for promotion the conveying air used apparatus at standstill the function of a shut-off.

It is preferred that the drying of the conveying air before and / or after the apparatus for conveying air supply carried out becomes.

A preferred embodiment of the invention is described below with reference to 1 which illustrates the critical moisture range of the reducing agent carrier.

object of an experiment with which the invention is explained are the hygroscopic Properties of the reducing agent carrier urea. First related experiences depending on urea conditioning and environmental conditions to a critical limit for the humidity. For the operational safety of the dosing unit and a promotion to ensure the pellets permanently The relative humidity must always be below the critical limit stay.

The to investigate the critical humidity of urea used samples were defined in a climate chamber relative Humidities and temperatures are exposed and timed Intervals optically examined for a clumping.

Tabelle 1: Spezifikation der Harnstoffproben

Table 1: Specification of urea samples

1 shows the critical humidity range of the examined samples as a function of temperature. As expected, urea dust represented by graph 3 is the most sensitive sample due to its high specific surface area. At 10 ° C, the critical relative humidity is 40% and decreases below 10 as the temperature increases (50 ° C) %. Graphs 1 and 2 of the pellet samples (Sample 1 + 2) show differences due to the different accessibility of the urea in the containers. As the temperature increases, the critical humidity of the pellets in the watch glass (Graph 2) drops due to the greater contact with moisture compared to the beaker (Graph 1). At room temperature, this influence is expressed by a difference in critical humidity of about 10%. The simultaneous clumping of the pellet form at 10 ° C and 70% relative humidity is classified as measurement error.

The tests have shown that the relative humidity of the conveying air, when using urea pellets to ensure the reliability of the dosing system in a temperature range of 10-50 ° C is possible to keep below 20%. A urea dust formation within the dosing system, for example by pellet abrasion on Zellenraddosierer should be avoided.

1

beaker + Pellets sample 12

2

watch glass + Pellets sample 2

3

beaker + Dust sample 3

Claims (12)

Process for the selective catalytic reduction (SCR) of nitrogen oxides contained in the exhaust gas of an internal combustion engine, fed as a reducing agent urea in a solid form SCR catalyst and converted to ammonia and carbon dioxide, the urea taken from a urea supply and by means of a metering device to form a urea aerosol is introduced into a carrier gas stream and fed through it to a catalyst system, characterized in that the conveying air contained in the carrier gas flow is dried by being taken from the fresh air and / or circulation flow guidance of passenger air conditioning. Method according to claim 1, characterized in that that the removal of the conveying air after the coolant evaporator in fresh air or / and passenger compartment air conditioning. Method according to claim 1, characterized in that that the removal of the conveying air after the coolant evaporator and after the heat exchanger the heating system of fresh air or / and passenger compartment air conditioning he follows. Method according to one of the preceding claims, characterized characterized in that the fresh air or / and passenger compartment air conditioning removed conveying air is dried continuously. Method according to one of the preceding claims, characterized characterized in that the conveying air through a membrane dryer or a regenerable adsorption dryer is dried continuously. Method according to one of the preceding claims, characterized characterized in that the conveying air when crossing a certain limit for the water content in the conveying air is dried. Method according to one of the preceding claims, characterized characterized in that the conveying air only if the value falls below a certain limit for the water content in the conveying air is removed. Method according to one of the preceding claims, characterized characterized in that the reducing agent dosage only when falling below a certain limit for the water content is carried out in he conveying air. Method according to one of the preceding claims, characterized characterized in that the water content still contained in the conveying air by means of temperature measurement of the saturation water vapor quantity after the coolant evaporator determined and with the limit values which are exceeded or fallen short of the water content in the conveying air determine is compared. Method according to one of the preceding claims, characterized in that the water content of the conveying air is determined by a moisture sensor, the after the cooling evaporator and / or after the heat exchanger and / or after the conveying air extraction is arranged is determined. Method according to one of the preceding claims, characterized characterized in that to promote the conveying air used apparatus at standstill the function of a shut-off Fulfills. Method according to claim 11, characterized in that that the drying of the conveying air before and / or after the apparatus for conveying air supply is performed.