WO1999001648A1 - Alveolate catalyst and method for cleaning exhaust gas from an air excess-driven combustion engine - Google Patents

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 catalyst volume (7) and the hydraulic diameter of its flow channels (9) to the engine capacity (3) of the combustion engine (1) enables the exhaust gas back pressure to be reduced due to a high contaminant conversion rate and a mitigated occlusion trend, with no engine capacity loss.

Description

description

Honeycomb catalyst and method for purifying an exhaust gas from an internal 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, which 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 a 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. For example, to reduce nitrogen oxides in the exhaust gas of such an internal combustion engine, the use of a so-called DeNOx catalytic converter is known, which uses the selective catalytic reduction (SCR) process to convert the nitrogen oxides into environmentally friendly nitrogen and water using a suitable reducing agent, usually ammonia implements.

Various considerations have also been made with regard to the geometry of the particular catalyst used to avoid a blockage of the catalytic converter by the unburned particles or soot particles contained in the exhaust gas of an internal combustion engine working with excess air.

In order to reduce so-called SOF contained in the exhaust gas of a diesel engine, ie organic components soluble in organic solvents, a cleaning process is described in EP 0 382 434 B1, in which the exhaust gas of the diesel engine is generated by a honeycomb-like catalyst that runs parallel to the cells Has flow direction of the exhaust gas is directed. The individual, continuous cells have a hydraulic diameter of 1.00 to 2.00 mm, the cross section of the inflow surface 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 catalyst is only suitable for use in the exhaust gas of a diesel engine, in which primary engine measures are already taken on the engine side to reduce particle emissions, e.g. in the form of soot particles. Furthermore, the specified catalyst causes an increased exhaust gas back pressure, which leads to a reduction in engine performance and to an increase in fuel consumption.

The invention has for its object to remove the pollutants contained therein by means of a versatile, honeycomb-shaped catalyst from the exhaust gas of an internal combustion engine operated with excess air. Effective catalytic degradation of the pollutants is also to take place in the exhaust gas of a conventional diesel engine, but clogging of the catalytic converter and exhaust gas back pressure, which would greatly reduce the engine output, should be practically avoided. The object is achieved by a honeycomb

Catalyst with a number of parallel flow channels for cleaning an exhaust gas of an internal combustion engine working with excess air with a predetermined displacement, the ratio of the volume of the catalyst to the displacement of the internal combustion engine between 0.5 and 8.0, the hydraulic diameter of the flow channels between 1 and 4 mm and the percentage of the open frontal area at the inlet of the catalyst is between 45 and 95%.

In the corresponding method, the exhaust gas of an internal combustion engine working with excess air and having a predetermined displacement with a temperature between 100 and 700 ° C. is passed through a honeycomb-shaped catalyst with a number of parallel flow channels, the open, frontal area at the inlet of the catalyst being at most 95% and is at least 45%, the flow channels have a hydraulic diameter between 1 and 4 mm and the ratio of the catalyst volume to the cubic capacity of the internal combustion engine is between 0.5 and 8.0.

In principle, 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 means 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 a catalyst of this type, the volume of which is specifically adapted to the displacement of the internal combustion engine, achieves a high pollutant conversion rate with a low exhaust gas back pressure and a low tendency to clog. Because of the low exhaust back pressure does not reduce the engine output of the internal combustion engine and the use of such a catalyst does not result in additional fuel consumption.

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 reduce the reduction in engine performance, the ratio of the catalytic converter volume to the cubic capacity of the internal combustion engine is between 0.5 and 8.0. The ratio is preferably between 1.0 and 5.0. If the ratio is less than 0.5, a relatively large amount of exhaust gas must flow through a small volume of the catalytic converter, so that the exhaust gas back pressure on the catalytic converter increases. In addition, the degree of separation for the pollutants contained in the exhaust gas deteriorates. With a ratio of over 8.0, the honeycomb-shaped catalyst becomes too large to be used in particular in the case of non-stationary applications in the passenger car sector.

If the percentage of the open, frontal area of the inlet side of the catalytic converter is below 45%, 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 catalytic converter is above 95% due to the thin wall thickness of the individual flow channels is severely restricted. 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 1 and 4 mm. With a hydraulic diameter of less than 1 mm, the flow channels tend to clog with soot particles contained in the exhaust gas and thus reduce the catalytic activity of the catalytic converter. With a hydraulic diameter of the flow channels 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. 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 controllable nozzle can be provided. The honeycomb-shaped catalytic converter can be connected to conventional internal combustion engines operated with excess air, in particular to diesel engines or to lean-mix petrol engines. The invention has the advantage that no impairment of the engine performance is to be expected due to the low exhaust 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 still 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 an exhaust gas.

The catalyst 7 is designed as a DeNOx catalyst based on Ti0 ₂ , which comprises one or more of the substances V ₂ 0 ₅ , Wo0 ₃ and Mo0 ₃ as catalytically active material. Such a DeNOx catalytic converter reduces the nitrogen oxides contained in the exhaust gas according to the SCR process 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 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, a metering device 4 is installed in the exhaust pipe 2 in front of the catalytic converter 7. 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 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 ratio of the volume of the catalyst 7 to the cubic capacity 3 of the diesel engine is 4.0.

The exhaust gas flowing from the diesel engine 1 passes through the inflow surface 11 of the catalyst 7, which is perpendicular to the flow direction 6, the size of which results from the volume ratio. The percentage of the inflow surface 11 through which exhaust gas and reducing agent can flow is 80%.

The parallel flow channels 9 through which the exhaust gas and the reducing agent can flow have a hydraulic diameter of 2 mm.

Claims

claims 1. Honeycomb-shaped catalyst (7), with a number of parallel flow channels (9), for cleaning an exhaust gas of an internal combustion engine (1) working with excess air with a predeterminable cubic capacity (3), characterized in that the ratio of the volume of the catalyst (7) to the displacement (3) of the internal combustion engine (1) between 0.5 and 8.0, the hydraulic diameter of the flow channels (9) between 1 and 4 mm and the percentage of the open frontal area at the inlet of the catalyst between 45 and 95%. 2. Catalytic converter according to claim 1, so that the ratio of the volume of the catalyst (7) to the cubic capacity (3) of the internal combustion engine (1) is between 1.0 and 5.0. 3. Catalyst according to claim 1 or 2, 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 1.2 and 2.5 mm. 4. A catalyst according to any one of claims 1 to 3, that the percentage of the open frontal area is between 60 and 85%. 5. Catalyst according to one of claims 1 to 4, that the catalyst (7) is a DeNOx catalyst. 6. A method for cleaning an 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 passes through at a temperature between 100 and 700 ° C the catalyst (7) is passed, the ratio of the volume of the catalyst (7) to the displacement (3) of the internal combustion engine (1) between 0.5 and 8.0, the hydraulic diameter of the flow channels (9) between 1 and 4 mm and the percentage of the open frontal area is at least 45%. 7. The method according to claim 6, so that the ratio of the volume of the catalyst (7) to the displacement (3) of the internal combustion engine (1) is between 1.0 and 5.0. 8. The method according to claim 6 or 7,; d a d u r c h g e k e n n e e c e n t that the hydraulic diameter of the flow channels (9) is between 1.2 and 2.5 mm. 9. The method according to any one of claims 6 to 8, such 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, so that a reducing agent is metered into the exhaust gas before it is passed through the DeNOx catalytic converter.