WO1999001647A1

WO1999001647A1 - Alveolate catalyst and method for cleaning exhaust gas from an air excess-driven combustion engine

Info

Publication number: WO1999001647A1
Application number: PCT/EP1998/004085
Authority: WO
Inventors: Ronald Neufert; Wieland Mathes; Klaus Wenzlawski; Dietmar Weisensel
Original assignee: Siemens Aktiengesellschaft
Priority date: 1997-07-04
Filing date: 1998-07-02
Publication date: 1999-01-14
Also published as: EP0993544A1

Abstract

In order to eliminate contaminants in exhaust gas from an air excess-driven combustion engine (1) with a given engine capacity (3), an alveolate catalyst (7) is disclosed as well as a method consisting in feeding said exhaust gas via an exhaust pipe (2) into the catalyst (7). Adapting the surface of approach (11) to the engine capacity (3) enables, with a hydraulic flow channel (9) diameter of 2 to 4 mm and a front area aperture at the catalyst inlet (7) of 45 to 95 %, a high contaminant conversion rate to be obtained with no engine capacity reduction.

Description

description

Honeycomb-shaped catalyst and method for purifying an exhaust gas from a combustion engine operated with excess air

The invention relates to a honeycomb-shaped catalyst and a method for the catalytic removal of pollutants from an exhaust gas of an internal combustion engine operated with excess air, for example a diesel engine or a gasoline engine with mixed engines. Such pollutants are e.g. Nitrogen oxides, sulfur oxides, carbon monoxide and hydrocarbons, but also dioxins, furans and other organic compounds.

When operating an internal combustion engine, the pollutants mentioned can arise to a not inconsiderable extent. They are released into the environment via the exhaust gas and can cause damage there. An internal combustion engine that works with excess air, for example a diesel engine or a lean-mix petrol engine, releases such pollutants into the environment when its fuel is burned.

A large number of catalysts have been developed in the past in order to reduce the pollutants released into the environment by an internal combustion engine operating with excess air. To reduce nitrogen oxides in the exhaust gas of such an internal combustion engine, for example, the use of a so-called DeNOx catalyst is known, which uses the method of selective catalytic reduction (SCR) to reduce the nitrogen oxides with a suitable reducing agent, usually ammonia, to environmentally friendly nitrogen and to water implements.

Various considerations have also been made with regard to the geometry of the particular catalyst used a clogging of the catalyst due to the in the exhaust gas of the

Avoid excess combustion internal combustion engine contained unburned particles or soot particles.

For example, EP 0 382 434 B1 describes a cleaning process for reducing so-called SOF contained in the exhaust gas of a diesel engine, ie organic components soluble in organic solvents. In this method, the exhaust gas from the diesel engine is passed through a honeycomb-like catalyst which has cells parallel to the flow direction of the exhaust gas. The individual, continuous cells have a hydraulic diameter of 1.00 to 2.00 mm, the cross section of the inflow area of the catalyst being 10 to 100 cm ² per liter of cubic capacity of the diesel engine. The percentage of the open area of the inflow area through which the exhaust gas can flow is 40 to 95%.

Disadvantageously, however, the specified catalytic converter is only suitable for use in the exhaust gas of a diesel engine, in which primary measures for reducing the particle emissions, e.g. in the form of soot particles. Furthermore, the specified catalyst causes a relatively high exhaust gas back pressure, which leads to a reduction in engine performance and to increased fuel consumption.

The object of the invention is to provide a versatile, honeycomb-shaped catalyst and a method for removing the pollutants from the exhaust gas of an internal combustion engine operated with excess air. Effective catalytic degradation of the pollutants should also take place in the exhaust gas of a conventional diesel engine with a low tendency to clog the catalytic converter, whereby an increased exhaust gas back pressure, which would greatly reduce the engine output, should be practically avoided. The object relating to the catalyst is achieved according to the invention by a honeycomb-shaped catalyst with a number of parallel flow channels for cleaning the exhaust gas of an internal combustion engine working with excess air with a predetermined displacement, the inflow area on the inlet side ^' - of the catalyst being between 100 and 500 cm ² per liter of displacement Combustion engine is, and wherein the flow channels have a hydraulic diameter between 2 and 4 mm and the percentage of the open frontal area at the inlet of the catalyst is between 45 and 95%.

The object of the method is achieved by a method for purifying the exhaust gas from an excess air internal combustion engine with a predetermined displacement, comprising passing the exhaust gas through a honeycomb-shaped catalyst with a number of parallel flow channels, the percentage of the open, frontal area at the inlet of the catalyst is at most 95%, the exhaust gas according to the invention being passed through the catalyst at a temperature between 100 and 700 ° C., the inflow area on the inlet side of the catalyst being between 100 and 500 cm ² per liter of displacement of the internal combustion engine, and wherein the hydraulic diameter of the flow channels between 2 and 4 mm and the percentage of the open frontal area is at least 45%.

The “inflow area” is understood to mean the total area of the inflow side of the catalytic converter lying perpendicular to the flow direction of the exhaust gas and the “percentage of the open frontal area” is the ratio of the inflow area and the total cross-sectional area of the flow channels in percent. The "hydraulic diameter" is defined as four times the cross-sectional area of a flow channel divided by its circumference. The invention is based on the consideration that such a catalytic converter, the flow area of which is specifically adapted to the displacement of the internal combustion engine, results in a high

Pollutant turnover rate with low exhaust gas back pressure and low clogging tendency is achieved. ^~ Up - due to the low exhaust gas back pressure is not reduced, the engine output of the internal combustion engine, and the use of such a catalyst is not drawing fuel consumption to himself.

The honeycomb-shaped catalyst can be designed as an oxidation catalyst with a known composition for the oxidation of unburned hydrocarbons, carbon monoxide or of dioxins or furans. The honeycomb-shaped catalyst can also be designed as a reduction catalyst of known composition for removing nitrogen oxides.

In order to minimize the exhaust gas back pressure on the catalytic converter and thus keep the engine output low, the inflow area is between 100 and 500 cm ² per liter of displacement of the internal combustion engine. The inflow area is preferably 100-300 cm ² per liter of cubic capacity.

Below 100 cm ^{2 of} inflow area per liter of displacement, the cross-sectional area through which the exhaust gas flows is so small on the inlet side of the catalytic converter that an exhaust gas counterpressure arises at the catalytic converter, which reduces the performance of the internal combustion engine. The small cross-sectional area on the inlet side of the catalyst also leads to an increase in the tendency to clog. If the inflow area per liter of cubic capacity is more than 500 cm ² , the honeycomb-shaped catalyst becomes too large to be used in particular in non-stationary applications, in particular in the motor vehicle sector. Is the percentage of the open, frontal area of the

Inlet side of the catalyst below 45%, so the exhaust gas back pressure increases due to the then reduced open cross-sectional area of the flow channels, while the mechanical stability of the catalyst is severely restricted at a percentage above 95% due to the thin wall thickness of the individual flow channels. The percentage of the open, frontal area of the inlet side of the catalyst is advantageously chosen between 60 and 85%.

In order to further reduce the pressure drop across the catalytic converter, the parallel flow channels have a hydraulic diameter between 2 and 4 mm.

The hydraulic diameter is preferably 2 to 3 mm.

With a hydraulic diameter of less than 2 mm, the flow channels tend to clog with soot particles contained in the exhaust gas and thus reduce the catalytic activity of the catalyst. If the flow channels have a hydraulic diameter of more than 4 mm, the catalytic conversion of pollutants in the catalytic converter also decreases, since the frequency of contact of the pollutants carried in the exhaust gas decreases with the catalytic converter surface.

For an effective breakdown of nitrogen oxides contained in the exhaust gas, it is advantageous if the honeycomb-shaped catalyst is designed as a DeNOx catalyst on which, among other things, Nitrogen oxides contained in the exhaust gas can be catalytically converted to nitrogen and water using a reducing agent, for example ammonia, previously introduced into the exhaust gas by the selective catalytic reduction (SCR) method.

Substances which release the actual reducing agent only after being introduced into the exhaust gas can of course also be used as the reducing agent. Egg- Such a substance is, for example, urea, from which ammonia is released.

In the flow direction of the exhaust gas upstream of the DeNOx catalytic converter there is advantageously a metering device for metering the reducing agent into the exhaust gas stream. The reducing agent can be injected, injected or blown in. For this purpose, an appropriately designed, controllable or regulatable nozzle can be provided.

The honeycomb-shaped catalyst can be connected to conventional internal combustion engines operated with excess air, in particular to diesel engines or to lean-mix petrol engines.

The use of the catalyst according to the invention has the advantage that no impairment of the engine performance is to be expected due to the low exhaust gas back pressure. The installation of the honeycomb-shaped catalyst therefore leads to negligible additional fuel consumption by the internal combustion engine, while the additional fuel consumption in systems according to the prior art is between 5 and 20%.

An exemplary embodiment of the invention is explained in more detail below with reference to a figure which shows a schematic illustration of a diesel engine with an associated honeycomb-shaped catalytic converter.

In the figure, as a combustion engine 1 working with excess air, a diesel engine is shown, which is connected via an exhaust pipe 2 to a honeycomb-shaped catalytic converter 7, which has a number of flow channels 9, which are aligned parallel to the flow direction 6 of the exhaust gas.

The catalyst 7 is designed as a DeNOx catalyst based on Ti0 ₂ , which is a catalytically active material or more of the substances V ₂ 0 ₅ , W0O3 and M0O3 comprises. Such a DeNOx catalytic converter reduces the nitrogen oxides contained in the exhaust gas according to the SCR method of selective catalytic reduction with the aid of a reducing agent, for example ammonia, to nitrogen and water.

Depending on the operating state of the engine, the exhaust gas generated in the diesel engine flows through the exhaust gas pipe 2 into the catalytic converter 7 at a temperature of 100 to 700 ° C. Relative to the direction of flow of the exhaust gas upstream of the catalytic converter 7, a metering device 4 is fitted in the exhaust pipe 2. The metering device 4 comprises a reducing agent tank 4A and a controllable nozzle 4B connected to it, through which the reducing agent located in the reducing agent tank 4A is injected into the exhaust gas flowing through the exhaust pipe 2. This can be done using compressed air, for example. The reducing agent is intimately mixed with the exhaust gas.

Urea is used as the reducing agent, which partially converts to ammonia under the conditions prevailing in the exhaust gas. To support the conversion, a hydrolysis catalytic converter (not shown in more detail) can be arranged between the metering device 4 and the DeNOx catalytic converter 7.

After the metering device 4, the exhaust pipe 2 opens into the catalytic converter 7, which has an inflow surface 11 on its inlet side. The inflow surface 11 perpendicular to the flow direction 6 of the exhaust gas is here 200 cm ² per liter of cubic capacity 3 of the internal combustion engine 1. The percentage of the inflow surface 11 through which the exhaust gas and reducing agent can flow is 70%. The parallel flow channels 9 through which the exhaust gas and the reducing agent can flow have a hydraulic diameter of 2 mm.

The honeycomb-shaped catalytic converter 7 of this exemplary embodiment has a high pollutant conversion rate as well as a low tendency to clog and a low exhaust gas back pressure and thus does not impair the performance of the internal combustion engine 1.

Claims

claims

1. A honeycomb catalyst (7) with a number of parallel flow channels (9) for cleaning the exhaust gas of an operating with excess air, the internal combustion engine (1) having egg ^g NEM predetermined displacement (3), characterized in that its inflow area (11) on the inlet side between 100 and 500 cm ² per liter of displacement (3) of the internal combustion engine (1) is that the hydraulic diameter of the flow channels (9) is between 2 and 4 mm, and that the percentage of its open frontal area at the inlet is between 45 and 95% is.

2. Catalyst (7) according to claim 1, characterized in that its inflow surface (11) on the inlet side is between 100 and 300 cm ² per liter of displacement (3) of the internal combustion engine (1).

3. Catalyst (7) according to claim 1 or 2, so that the hydraulic diameter of the flow channels (9) is between 2 and 3 mm.

4. Catalyst (7) according to one of claims 1 to 3, d a d u r c h g e k e n n z e i c h n e t that the percentage of the open frontal area is between 60 and 85%.

5. Catalyst (7) according to any one of claims 1 to 4, d a d u r c h g e k e n n z e i c h n e t that it is a DeNOx catalyst.

6. A method for cleaning the exhaust gas from an internal combustion engine (1) working with excess air and having a predetermined displacement (3), comprising passing the exhaust gas through by a honeycomb-shaped catalyst (7) with a number of parallel flow channels (9), the percentage of the open frontal area at the inlet of the catalyst (7) being at most 95%, characterized in that the exhaust gas is at a temperature between 100 and 700 ° C is passed through the catalytic converter (7), the inflow area (11) on the inlet side of the catalytic converter (7) being between 100 and 500 cm ² per liter of cubic capacity (3) of the internal combustion engine (1), and the hydraulic diameter of the flow channels ( 9) between 2 and 4 mm and the percentage of the open frontal area is at least 45%.

7. The method according to claim 6, characterized in that the inflow surface (11) on the inlet side of the catalyst (7) is between 100 and 300 cm ² per liter of displacement (3) of the internal combustion engine (1).

8. The method according to claim 6 or 7, d a d u r c h g e k e n n z e i c h n e t that the hydraulic diameter of the flow channels (9) is between 2 and 3 mm.

9. The method according to any one of claims 6 to 8, so that the percentage of the open frontal area is between 60 and 85%.

10. The method according to any one of claims 6 to 9, that the exhaust gas is passed through a DeNOx catalytic converter.

11. The method according to claim 10, characterized in that the exhaust gas

Passing through the DeNOx catalyst, a reducing agent is added.