Improved selective catalytic reduction system and method

Abstract

A selective catalytic reduction (SCR) system for treating exhaust gases in an exhaust passage is provided. The system comprises a first catalyst for converting urea to ammonia, the first catalyst being located in the passage and having an upstream face and a downstream face. The upstream face is at a right angle to an axial flow path (A) through the passage. The system further comprises a second catalyst for converting NOx to nitrogen gas and water in the presence of ammonia, with the second catalyst being located in the passage downstream of the first catalyst. A diesel exhaust fluid dosing unit is located upstream of the first catalyst. The dosing unit comprises a nozzle arranged so as to inject DEF directly onto the upstream face of the first catalyst.

1 . A selective catalytic reduction (SCR) system for treating exhaust gases in an exhaust passage, the system comprising:
 a first catalyst for converting urea to ammonia, the first catalyst being located in the passage and having an upstream face and a downstream face, wherein the upstream face is at a right angle to an axial flow path through the passage;   a second catalyst for converting NOx to nitrogen gas and water in the presence of ammonia, the second catalyst being located in the passage downstream of the first catalyst; and   a diesel exhaust fluid (DEF) dosing unit located upstream of the first catalyst, the dosing unit comprising a nozzle arranged so as to inject DEF directly onto the upstream face of the first catalyst.   
     
     
         2 . The system of  claim 1 , wherein the first catalyst comprises a permeable metallic substrate. 
     
     
         3 . The system of  claim 1 , wherein the first catalyst comprises a permeable ceramic substrate. 
     
     
         4 . The system of  claim 2 , wherein the substrate is coated with a hydrolysis catalyst. 
     
     
         5 . The system of  claim 1 , wherein the nozzle is substantially co-axial with the axial flow path through the passage. 
     
     
         6 . The system of  claim 1 , further comprising an exhaust gas mixer located in the passage downstream of the first catalyst. 
     
     
         7 . The system of  claim 6 , wherein the exhaust gas mixer is close-coupled to the downstream face of the first catalyst. 
     
     
         8 . A method of treating exhaust gases in an exhaust passage, the method comprising the steps of:
 locating a first catalyst for converting urea to ammonia in the passage, the first catalyst having an upstream face and a downstream face, wherein the catalyst is located such that the upstream face is at a right angle to an axial flow path through the passage;   locating a second catalyst in the passage for converting NOx to nitrogen gas and water in the presence of ammonia, the second catalyst being located in the passage downstream of the first catalyst;   locating a diesel exhaust fluid (DEF) dosing unit upstream of the first catalyst, the dosing unit comprising a nozzle; and   injecting DEF from the nozzle directly onto the upstream face of the first catalyst.   
     
     
         9 . The method of  claim 8 , wherein the first catalyst comprises a permeable metallic substrate. 
     
     
         10 . The method of  claim 8 , wherein the first catalyst comprises a permeable ceramic substrate. 
     
     
         11 . The method of  claim 9 , further comprising the step of coating the substrate with a hydrolysis catalyst. 
     
     
         12 . The method of  claim 8 , wherein the step of locating the dosing unit comprises locating the nozzle such that it is co-axial with the axial flow path through the passage. 
     
     
         13 . The method of  claim 8 , wherein the upstream face of the first catalyst has a first surface area, and the nozzle injects the DEF such that a DEF spray impinges upon a second surface area of the upstream face which is at least 50% of the first surface area. 
     
     
         14 . The method of  claim 8 , further comprising the step of locating an exhaust gas mixer in the passage downstream of the first catalyst. 
     
     
         15 . The method of  claim 14 , wherein the step of locating the exhaust gas mixer in the passage comprises close-coupling the exhaust gas mixer to the downstream face of the first catalyst.