Exhaust gas-purifying apparatus and exhaust gas-purifying method

Abstract

An exhaust gas purifying apparatus, including an NO x occlusion reduction catalyst ( 2 ) and a filter catalyst ( 3 ) arranged in series, is used, and the exhaust gas is allowed to flow from the NO x occlusion reduction catalyst ( 2 ) to the filter catalyst ( 3 ) in a normal flow process, and, in a recovery process for allowing exhaust gas added with a reducing agent to flow, the flow direction of the exhaust gas is reversed toward the NO x occlusion reduction catalyst ( 2 ) from the filter catalyst ( 3 ). Since the exhaust gas is heated by reaction heat of the filter catalyst ( 3 ), the NO x occlusion reduction catalyst ( 2 ) can recover from sulfur poisoning in the exhaust gas at a low temperature. Thereby, overheating of the filter catalyst ( 3 ) is prevented. Therefore, recovery from sulfur poisoning can be improved, and also deterioration and breakage of the filter catalyst can be prevented.

1 . An exhaust gas purifying apparatus, comprising:
 a reducing agent supply device supplying a reducing agent to exhaust gas;   a first catalyst comprising an NO x  occlusion reduction catalyst obtained by forming a catalyst layer on a surface of a honeycomb substrate having a straight flow structure, the catalyst layer including a porous oxide support and an NO x  occluding material and noble metal supported thereon;   a second catalyst obtained by forming a catalyst layer at least on a surface of a filter substrate having a wall flow structure and including a porous oxide support and at least a noble metal supported thereon;   a container having at least the first catalyst and the second catalyst arranged in series; and   a direction change device changing a flow direction of exhaust gas in the container between a normal flow direction and a reverse flow direction, wherein the normal flow direction, the first catalyst is provided upstream side of an exhaust gas flow direction and the second catalyst is provided downstream side thereof, and in the reverse flow direction, the second catalyst is provided upstream side of the exhaust gas flow direction and the first catalyst is provided downstream side thereof.   
     
     
         2 . The apparatus according to  claim 1 , wherein the second catalyst further comprises an NO x  occluding material supported on the catalyst layer. 
     
     
         3 . The apparatus according to  claim 1 , wherein, in the container, the first catalyst, the second catalyst, and a third catalyst are arranged in sequence, the third catalyst obtained by forming a catalyst layer on a surface of a honeycomb substrate having a straight flow structure, the catalyst layer including a porous oxide support and a noble metal supported thereon, and
 the direction change device changes the flow direction of the exhaust gas between a normal flow direction in a sequence of the first catalyst, the second catalyst, and then the third catalyst and a reverse flow direction in a sequence of the third catalyst, the second catalyst, and then the first catalyst.   
     
     
         4 . The apparatus according to  claim 3 , wherein the third catalyst is at least one selected from among an NO x  occlusion reduction catalyst, a three-way catalyst, and an oxidation catalyst. 
     
     
         5 . The apparatus according to  claim 3 , wherein the first catalyst comprises an oxidation catalyst or a three-way catalyst disposed at an upstream side thereof and a NO x  occlusion reduction catalyst disposed at a downstream side thereof when the first catalyst is provided upstream side of the exhaust gas flow direction, and
 the third catalyst comprises an oxidation catalyst or a three-way catalyst disposed at an upstream side thereof and an NO x  occlusion reduction catalyst disposed at a downstream side thereof when the third catalyst is provided upstream side of the exhaust gas flow direction.   
     
     
         6 . An exhaust gas purifying method using the exhaust gas purifying apparatus of  claim 1 , comprising a normal flow process for allowing exhaust gas to normally flow and a recovery process for allowing exhaust gas in a high-temperature rich atmosphere added with a reducing agent to flow in a pulsing manner so that a sulfur-poisoned NO x  occluding material is reduced to thereby recover an NO x  occlusion function, in which a flow direction of the exhaust gas in the normal flow process is changed in the recovery process. 
     
     
         7 . The method according to  claim 6 , further comprising a regeneration process for allowing the exhaust gas in a lean atmosphere added with the reducing agent to flow in a pulsing manner to thereby generate combustion heat which is then used to combust PM accumulated in the second catalyst, thus regenerating a PM capture function, in which the flow direction of the exhaust gas in the normal flow process is changed in the regeneration process. 
     
     
         8 . The method according to  claim 7 , wherein the regeneration process precedes the recovery process.