Method for the cleaning of exhaust gas from a compression ignition engine

Abstract

A method for the cleaning of exhaust gas from a compression ignition engine, comprising the steps of injecting a first amount of an aqueous urea solution into the gas; in a first mode of operation and at an exhaust gas temperature of between 150 and 220 C. hydrolysing the first amount of urea to ammonia reducing agent in presence of a first catalyst comprising vanadium oxide supported on titania and subsequently removing part of nitrogen oxides contained in the exhaust gas by contacting the gas mixed with the ammonia reducing agent through a second catalyst comprising platinum on titania and/or alumina; and in a second mode of operation and at an exhaust gas temperature above 220 C. removing a part of the nitrogen oxides in presence of the first catalyst and the ammonia reducing agent and subsequently oxidising hydrocarbons, carbon monoxide and remaining amount of the ammonia reducing agent further contained in the exhaust gas by passing the gas through the second catalyst.

Claims

1. A method for the cleaning of an exhaust gas from a compression ignition engine, comprising the steps of injecting an aqueous urea solution into the exhaust gas upstream of a first catalyst, which is a selective catalytic reduction (SCR) catalyst comprising vanadium oxide supported on titania, said first catalyst is close coupled with a second catalyst, which is a diesel oxidation (DOC) catalyst comprising platinum on titania and/or alumina, and said first catalyst is arranged close to the compression ignition engine such that exhaust gas therefrom heats the first catalyst to a first temperature resulting in catalytic hydrolysis of aqueous urea into ammonia and then heats the first catalyst to a second temperature resulting in catalytic reduction of NOx, whereby in a first mode of operation the exhaust gas comprising the aqueous urea solution reaches an exhaust gas temperature of between 150 and 220 C. and (a) the aqueous urea solution is catalytically hydrolysed to an ammonia reducing agent by contacting with the first catalyst and (b) a part of the nitrogen oxides contained in the exhaust gas are subsequently removed in the exhaust gas mixed with the ammonia reducing agent by contacting with the second catalyst arranged downstream of the first catalyst; and in a second mode of operation the exhaust gas comprising the aqueous urea solution reaches an exhaust gas temperature above 220 C. and (a) the aqueous urea solution is decomposed to the ammonia reducing agent and a part of the nitrogen oxides contained in the exhaust gas is removed by reacting the part of the nitrogen oxides with the ammonia reducing agent in presence of the first catalyst and (b) subsequently oxidising hydrocarbons, carbon monoxide, and ammonia in the exhaust gas leaving the first catalyst by contacting the exhaust gas with the second catalyst.

2. The method of claim 1, wherein the first catalyst further comprises tungsten oxide.

3. The method of claim 1, wherein the second catalyst further comprises palladium.

4. The method of claim 1, wherein an ammonia oxidation catalyst is layered on at least part of the second catalyst.

5. The method of claim 4, wherein the ammonia oxidation catalyst comprises platinum and zeolites promoted with iron and/or copper.

6. The method of claim 1, wherein the first catalyst further comprises a zeolite SCR catalyst.

7. The method of claim 6, wherein the zeolite SCR catalyst comprises at least one of Y-zeolite, beta-zeolite, SAPO-zeolites and chabazites.

8. The method of claim 1, wherein the first catalyst and second catalyst are arranged in a single catalyst unit.

Description

(1) The invention is described in more detail by reference to the drawings, in which

(2) FIG. 1A shows schematically a conventional diesel exhaust cleaning system and FIG. B a diesel exhaust cleaning system according to the invention;

(3) FIG. 2 is a graph depicting the efficiency of the vanadium oxide/TiO.sub.2 in the hydration of urea to ammonia at different temperatures; and

(4) FIGS. 3a and b show the activity of the close couplet vanadium oxide/TiO.sub.2 (SCR) and Pt/TiO.sub.2 (DOC) catalyst according to the invention at two different temperature intervals.

(5) As shown in FIG. 1, the combined catalyst SCR/DOC in system B used in the method according to invention replaces the oxidation catalyst DOC in the conventional exhaust gas cleaning system A. The SCR/DOC catalyst has dual functions, one as close coupled SCR catalyst with vanadium oxide on TiO.sub.2 and low temperature SCR using the Pt/TiO.sub.2 of the second catalyst at low temperatures and then also using the vanadium oxide/TiO.sub.2 catalyst for hydrolyzing urea to ammonia at low temperatures, which enables the use of urea as precursor of the ammonia reducing agent at lower temperatures down to 150 C.

(6) FIG. 2 shows the graph of hydration of urea to ammonia and HNCO by contact with a vanadium oxide/TiO.sub.2 catalyst. The catalyst was impregnated with a 32.5 vol % urea/water solution.

(7) The catalyst was heated at a rate of 10 K/min and swept with 70 ml/min with argon and a humidifier.

(8) As apparent from the FIG. 2, the vanadium oxide catalyst has a high activity in the hydration of urea to ammonia at a temperature between 150 and 220 C.

(9) FIGS. 3a and 3b show the activity of the close couplet SCR/DOC catalyst according to the invention at two different temperature intervals.

(10) As apparent from FIG. 3a, in a temperature range of between 150 and 220 C., a certain amount of urea is hydrolysed over the first vanadium oxide/TiO.sub.2 (SCR) catalyst and ammonia is formed. Ammonia reacts then with NOx on the second Pt/TiO.sub.2 (DOC) catalyst, which is SCR active in that temperature range. Both the NOx and ammonia concentration is reduced in the oxidation catalyst due to the SCR reaction.

(11) When the temperature increases to 220 C. or higher as shown in FIG. 3b, the first SCR catalyst becomes more active. The reaction zone is moved towards the front of the catalyst and no SCR reaction occurs on the DOC catalyst. The formed ammonia concentration is reacted with NOx and both reactants are reduced due to the SCR reaction.